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Open Access 01-12-2016 | Review

Fluid and ion transfer across the blood–brain and blood–cerebrospinal fluid barriers; a comparative account of mechanisms and roles

Authors: Stephen B. Hladky, Margery A. Barrand

Published in: Fluids and Barriers of the CNS | Issue 1/2016

Abstract

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na⁺-pumps. K⁺ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood–brain barrier lining brain microvasculature, allows passage of O₂, CO₂, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood–brain barrier Na⁺ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood–brain barrier is the most important interface for maintaining interstitial fluid (ISF) K⁺ concentration within tight limits. This is most likely because Na⁺-pumps vary the rate at which K⁺ is transported out of ISF in response to small changes in K⁺ concentration. There is also evidence for functional regulation of K⁺ transporters with chronic changes in plasma concentration. The blood–brain barrier is also important in regulating HCO₃ ⁻ and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood–brain barrier HCO₃ ⁻ transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO₃ ⁻ concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pH_ISF and pH_plasma are similar. The key similarities and differences of the two interfaces are summarized.

Glial endfeet, hexokinase and glucose fluxes across the blood–brain barrier Exchange of glucose across the blood–brain barrier between blood and astrocyte endfeet rather than between blood and interstitial fluid (see Sect. 2.4.1) can lead to larger efflux relative to influx. This is possible because the endfeet express the glucose transporter GLUT1, which allows fluxes in both directions [102, 104, 105]. Thus there could be a large unidirectional flux back out of the endfeet if there is a significant [glucose] within the astrocytes. That in turn would require a relatively slow rate of phosphorylation of glucose by hexokinase. This requirement is consistent with the view, said to originate with Betz [112], that net uptake of glucose into the brain is limited by its rate of phosphorylation by hexokinase.

Balance between total rates of influx and efflux of amino acids Felig [467] lists the differences in arterial and venous plasma concentrations for a number of amino acids in the cerebral circulation of human volunteers. The sum of these differences is 125 µmol l⁻¹. If it is assumed that for a 1400 g brain, the plasma flow is 0.4 l min⁻¹, this corresponds to a net uptake of 35 nmol min^–1 g⁻¹. However, Felig's list notably lacks entries for glutamine and glutamate for which, see Sect. 2.4.2, there are likely to be substantial net releases. Thus these data do not challenge the view that total influx of amino acids is nearly balanced by the total efflux.

Betz and Gilboe [468] list net uptakes for a number of amino acids for perfused dog brains in the presence of methoxyflurane or halothane for which the total uptakes are 5 and −5 nmol min⁻¹ g⁻¹ neither of which is significantly different from zero. Thus these results are also consistent with total influx of amino acids being nearly balanced by total efflux.

It is worth noting that the efflux to blood of glutamine and of glutamate and NH₄ ⁺ produced from them in the endothelial cells is the major route for export of amino-group-derived nitrogen from the brain [120, 469].

Location of Na ⁺-coupled amino acid transporters Smith et al. [470] demonstrated using in situ brain perfusion protocols and a number of radiolabelled neutral amino acids, including glutamine that there was a saturable uptake of amino acids into the brain. Ennis et al. [122] extended the results for glutamine and demonstrated that the influx rate was reduced by 62% when the [Na⁺] in the perfusion fluid was reduced from 138 to 2.4 mM. Based on this dependence they proposed that roughly 60% of the transport across the luminal membrane was via system N, which is a Na⁺-dependent transporter. However, Lee et al. ([471], reviewed in [120, 472, 473]) found using fractionated membrane vesicles that the luminal membrane contained system n, a Na⁺-independent carrier, while system N was present only in the abluminal membrane. There is evidence from studies using hepatocytes that two such systems exist [474‐478]. Furthermore it is now known that there are far more genes for amino acid transporters than were envisaged when the system N, A, L etc. nomenclature was developed [479].

One part of the explanation of the apparent discrepancy concerning the Na⁺ dependence of glutamine transport across the luminal membrane [122, 471] may be that the methods used by Hawkins and coworkers do not completely exclude mixed expression of the transporters on the two sides of the cells. Another part may arise because the unidirectional flux from perfusate into the brain is measured across a cell layer rather than a single membrane.

In order for glutamine to cross from plasma or perfusate to the interstitial fluid of the brain, it must cross the luminal membrane, diffuse across the cell cytoplasm, cross the abluminal membrane and then diffuse across the basement membrane and outwards either through or around the astrocyte endfeet (compare Sect. 5). Only then does it count as part of the unidirectional influx into the brain. Transport across the luminal membrane will be primarily Na⁺-independent, system n, and hence bidirectional while that across the abluminal membrane will be primarily Na⁺-dependent, system N, and hence biased towards movement from basement membrane into the cell cytoplasm. When the perfusate has a normal [Na⁺], the [Na⁺] within the endothelial cell will be substantial (perhaps >30 mM, compare with values in Table 2) and system N will mediate some efflux of glutamine from the cell to the basement membrane (compare [480]), and in addition glutamine may leave the cell via another transporter, system L1, that is known to be present. Some of the glutamine reaching the basement membrane will be transported back into the cell via system N, the rest will escape into the interstitial fluid of the brain and be part of the measured unidirectional flux from perfusate to brain. When the [Na⁺] in the vascular perfusate is greatly reduced, that inside the endothelial cells will also be reduced and there will be a larger gradient of [Na⁺] between the basement membrane and the cell interior. This may decrease glutamine efflux from the cells into the basement membrane via system N and increase glutamine influx in the reverse direction. Both of these effects would reduce the measured unidirectional flux from perfusate to brain. Thus the finding that Na⁺ removal decreases the tracer flux of glutamine into the brain does not in itself prove that there is a Na⁺-dependent amino acid transporter for glutamine in the luminal membrane.

Choroid plexus secretion and blood flow Choroid plexus secretion rate as a percentage of blood flow to the plexuses can be calculated from estimates of the fraction of cerebral blood flow that goes to the plexuses, ~0.01 (see Sect. 2), the volume fraction of plasma in blood, ~0.5, the cerebral blood flow, ~800 ml min⁻¹, and the rate of CSF production ~400 ml day⁻¹ = ~0.3 ml min⁻¹, with the result 0.3 ml min⁻¹/(0.01 × 0.5× 800 ml min⁻¹) = 7.5%.

Solute and water gradients in kidney proximal tubule and choroid plexus When solutes that cannot be absorbed are added to renal proximal tubular fluid in a segment that is blocked at both ends, the epithelial cells initially continue to transport NaCl and water out of the lumen to the blood. A steady-state with constant tubular contents is reached when passive NaCl leak back into the lumen balances active transport out. There is then a gradient of [NaCl], higher outside than inside, but no gradient of osmolality as the difference in osmolality for the impermeant solute balances that of NaCl. There must be active transport of NaCl to generate and maintain its gradient but there is no evidence from these experiments for active transport of water [149, 150]. Similar conclusions had been reached earlier using rat ileum [481], but the arguments used in the experiments on proximal tubules [149, 150] are particularly elegant in that they are based on the steady-state condition in which there are no net fluxes of solutes or water. In particular in this condition because there is no flow and no net transport of NaCl there are no gradients of concentration in any "unstirred layers" (see next paragraph).

Comparable tests assessing the steady-state gradients that can be maintained in the absence of flow appear never to have been carried out for the choroid plexus. The closest to such a test is probably the demonstration by Heisey et al. [482] that the choroid plexus continues to secrete even when the fluid in the ventricles is diluted, a result similar to that seen in the gall bladder where absorption of fluid can proceed when the lumen is hyperosmotic by as much as 80 mOsmol kg⁻¹) [483, 484]. However, as discussed by Spring [151] for the gall bladder, this type of observation is not sufficient to prove "active transport of water" [485‐487]. For the choroid plexus when there is net flow it may be possible to have lower osmolality in the CSF than the blood but at the same time higher osmolality at the apical surface (CSF side) than at the basolateral surface (blood side) of the epithelial cells. The difference is made up by the osmotic gradients across "unstirred layers", presumably primarily between the capillaries and the epithelial cells. Thus water could be moving down the osmotic gradient created by active transport of solute across the epithelial layer while being carried through the non-selective unstirred layers by pressure driven bulk flow of the entire solution. No active transport of water per se would then be required to account for the net secretion. With epithelia uphill water movement has only been seen when there are supporting structures, e.g. connective tissue, that create conditions in which unstirred layers are inevitable [151].

Tracer and net fluxes of Na ⁺ across the choroid plexuses Davson and Segal [173] and Davson and Welch [264] found that the tracer Na⁺ flux towards the CSF in the rabbit was just sufficient to account for the amount of Na⁺ in the secretion leaving no room for a tracer flux in the opposite direction. This is a very surprising result because for a leaky epithelium there should be tracer fluxes in both directions and this has been observed in other studies. For instance the tracer fluxes have been shown to be several fold larger than the net fluxes for proximal tubules of the kidney [488‐491] and the choroid plexuses of bullfrogs [492, 493]. Smith and Rapoport [261] and Murphy and Johanson [265] reported tracer Na⁺ flux towards CSF in the rat of 2.3% min⁻¹ (relative to Na⁺ content of the CSF) but did not report net CSF secretion rates. Their values for the influx are about three times larger than needed to account for the secretion rates (expressed as a percentage of CSF volume) measured by others, using ventriculo-cisternal perfusion: 0.72 or 0.75% min⁻¹ (values in Table 6.2 of [17] calculated from data in [173] and [251] respectively).

Effects of acetazolamide on aquaporins There is considerable interest in finding alternative actions of acetazolamide as its effects in treating acute mountain sickness have still not been adequately explained [494]. On the balance of present evidence, acetazolamide may inhibit water fluxes via the aquaporin AQP4 found in astrocytes. However, AQP4 is not found in the choroid plexus and acetazolamide appears not to inhibit AQP1 that is present [495]. Even for AQP4 acetazolamide only inhibits at concentrations approaching its solubility limit in water, ~1.5 mM [496] (but see also references in [494]).

Osmolality of fluid transferred across the blood–brain barrier: effect of metabolic water production Metabolic water production, ~3.3 mol day⁻¹ (see Sect. 2.2) expressed in terms of volume is 3.3 mol day⁻¹ × 18 ml mol⁻¹ = 60 ml day⁻¹. This is very small compared to the large amounts of water entering and leaving the brain but it is not small compared to the possible net water flow across the blood–brain barrier. As a consequence the osmolality of the secretion across the barrier, defined as amount of solutes secreted divided by amount of water secreted, may be surprisingly large. If the net rate of production of fluid by the combination of the blood–brain barrier and metabolic production of water were 200 ml day⁻¹ and the metabolic production of water 60 ml day⁻¹, then for each 140 ml of water transferred across the blood–brain barrier into ISF, 30 mmol of NaCl would have to be transferred for the final [NaCl] of ISF to be 150 mM. Thus the ratio of the net amount of NaCl transported across the blood–brain barrier to the net amount of water crossing the barrier would be 30 mmol/140 ml = 214 mM which is much greater than the concentration in either the endothelial cells or the ISF. The osmolality of the fluid within the basement membrane separating the endothelial cells from the astrocyte endfeet is considered in Sect. 4.7.

The calculation above assumes that the osmolality of the net outflow of ISF from the parenchyma is determined almost entirely by [NaCl] as would be correct if the composition of ISF were the same as that of CSF (see e.g. Table 2.5 in [17]). However, metabolic wastes may be at higher concentrations in ISF than in CSF. It should be noted that the principal metabolites, e.g. CO₂, amino acids etc., need not be considered as they are at low concentrations as a result of transport across the blood–brain barrier (see e.g. Sect. 2). Similarly as noted in Sect. 6.3, [lactate⁻]_ISF is normally only of the order of 1–2 mM. More generally for this example calculation to be valid, the rate of production of those osmotically active metabolic wastes that are removed from the parenchyma by outflow of ISF must be substantially less than 18 mOsmol day⁻¹.

Evidence obtained in studies on hydrocephalus concerning fluid secretion by the blood–brain barrier (Item 3 in Sect. 4.1 is amplified in the following)

When the cerebral aqueduct is blocked CSF production within the lateral and third ventricles continues but the ventricles do not continue to swell at a rate that would accommodate the CSF added, i.e. there must then be a route of escape from the lateral or third ventricles. This route has been called "ventricular absorption" [220, 221]. The evidence establishing ventricular absorption in non-communicating hydrocephalus still stands. It is important to note that this does not require absorption across the blood–brain barrier as the fluid may pass through the periventricular parenchyma, which is oedematous, to other sites of absorption (see section 4.2.4 in [15]). There is even evidence that some such absorption may occur normally for sucrose [222].

Injection of kaolin into the cisterna magna of cats or dogs produces a long-lasting block of CSF flow from the cisterna magna to the cranial and spinal subarachnoid spaces, an acute elevation of intraventricular pressure, and a sustained ventricular swelling. CSF production within the ventricles is maintained, but over days and weeks the ventricles do not continue to swell to accommodate the added CSF. Thus chronically the CSF must escape. It was assumed by those who developed this model that the route was by ventricular absorption [220, 497, 498] and it was used in attempts to investigate this process. However, it has now been shown convincingly, at least in cats, that in chronic kaolin induced hydrocephalus there is almost no ventricular absorption [223]. Instead (see Fig. 5) CSF flows into the swollen central canal of the spinal cord [499], then across swollen or damaged spinal tissue to the spinal subarachnoid space and finally flows out via spinal nerve roots [223, 224]. Demonstration that ventricular absorption did not account for CSF outflow in this model appears to have discredited the possibility of such absorption despite the compelling evidence for it in non-communicating hydrocephalus described above.

When markers are injected into the cisterna magna of normal subjects or experimental animals they distribute rapidly into the cranial subarachnoid space and less rapidly into the spinal subarachnoid space but not to any observable extent into the lateral and third ventricles. By contrast in communicating hydrocephalus, i.e. hydrocephalus in which the pathways connecting the ventricles and cisterna magna are functional, the markers penetrate and accumulate in the ventricles and to some extent in periventricular parenchyma (see e.g. chapter 4 in [225] and [226, 227]). It is, as if there is a reversed flow of CSF carrying the markers through the cerebral aqueduct in hydrocephalus, but not normally. The reversed net flow implies an important source of CSF other than the choroid plexuses in the lateral and IIIrd ventricles. If the rate of secretion by the choroid plexuses is proportional to choroid plexus mass that from the choroid plexus in the IVth ventricle is not sufficient and there must be a non-choroidal source.

PC-MRI studies in normal subjects and patients with communicating hydrocephalus have revealed that the flow of CSF through the cerebral aqueduct varies with time with a flow that is directed from the IIIrd to the IVth ventricle in systole and from IVth to IIIrd in diastole. If this accounts for the non-random variation in the CSF flow, the net, average flow over time can be calculated by taking an average over a cardiac cycle. The "noise" and variations from one recording to the next have usually been reduced by calculating an average over a number of recordings synchronized (i.e. with recording gated) to the cardiac cycle. However, this second stage of averaging will attenuate any and every component of flow changes that is not synchronous with the cardiac cycle, not just random fluctuations. Dreha-Kulaczewski et al. [500] have reported that the major variations in CSF flow through the aqueduct follow the respiratory rather than the cardiac cycle and it is indeed plausible as they state that these variations in CSF flow would not have been seen in the cardiac-gating studies. Whether changes in CSF flow through the aqueduct in the respiratory cycle distort the calculation of the net flow in the cardiac gating studies depends on whether or not the signal recorded in the individual traces varied linearly with flow over the entire range of flows occurring and whether enough cycles were averaged. If both criteria were met the net flow would still be correct and the inference of reversed net flow, fourth to third ventricle, from the data in these studies would still stand.

Rate of fluid secretion by the blood–brain barrier. Does ISF need to mix with CSF to reach lymph or blood? The available evidence does not exclude a fluid secretion rate across the blood–brain barrier as large as that for the choroid plexuses. However, if the secretion rate were larger than say 50% of choroid plexus secretion rate and outflow of ISF from the parenchyma were to CSF, then the estimates of the rate of CSF production rate would be markedly different if based on ventriculo-cisternal perfusion experiments on the one hand and e.g. collection of CSF from the lumbar sac on the other (see Sect. 3.2). The difference would have been larger than those seen [17]. Thus if the secretion rate is so large, much of the ISF must leave the brain without mixing with CSF that can be obtained from the lumbar sac. Lack of mixing was proposed by Cserr, Bradbury and colleagues [202, 501, 502] based on the relatively small proportion of markers injected into the parenchyma that could be recovered from CSF drawn from the cisterna magna. More recent work has emphasized routes that do not entail mixing of the outflow from the parenchyma with CSF even in the subarachnoid space [206, 459, 460, 466, 503‐506] (see also section 4.1 in [15] for discussion).

Hydrostatic pressure gradient needed to drive fluid movement across the blood–brain barrier If we take a volume transfer of 200 ml day⁻¹ containing 0.15 M NaCl into a 1400 g brain, the required flux, J _req, is 21 µmol g⁻¹ day⁻¹ or 0.25 nmol g⁻¹ s⁻¹. Based on influx of radiolabelled Na⁺ from the blood, the permeability of the blood–brain barrier to Na⁺ times the surface area of the barrier per gram of brain, is PS = 1 to 3 × 10⁻⁵ cm³ g⁻¹ s⁻¹ [16, 261] and the value for Cl⁻ is similar. Using the midpoint of this range, the concentration difference required is then Δc = J _req/PS = 12.5 mM for each of Na⁺ and Cl⁻. As a driving force for water movement a concentration difference corresponds to an osmotic pressure difference

\( {{\Delta }}\pi = RT\mathop \sum \nolimits {{\Delta }}c \)

with RT = 19 mmHg mM⁻¹ and thus the concentration gradient needed to drive the flux of NaCl corresponds to an osmotic pressure difference of 500 mmHg, more than ten times larger than any possible hydrostatic pressure difference across the barrier (this point has been made repeatedly before, see e.g. [507] and for further discussion [15] section 2.7). Put the other way round active transport of solute across the barrier can easily produce solute gradients that would produce osmotic pressure differences far more important than any possible hydrostatic pressure differences.

Effect of reducing [K ⁺ ] _{on influx of ISF} ⁴²K⁺ Bradbury, Segal and Wilson [259] found that reducing [K⁺]_ISF produced a 50% increase in the amount of ⁴²K⁺ from plasma that accumulated in the parenchyma over a 2-h period. They suggested two possible explanations: entry of K⁺ occurs by a mechanism that displays a long-pore effect; and sufficient ⁴²K⁺ accumulates within 2 h for efflux to substantially reduce the net accumulation. Decreasing [K⁺]_ISF would then by decreasing the rate of efflux decrease this effect, leading to an increase in the net amount accumulated.

A long-pore effect occurs when permeation is via a long, multiply-occupied, single-file pore. Well known examples include pores formed by gramicidin A [508, 509] and the delayed rectifier K⁺ channels found in nerve and muscle [510]. In essence the long-pore effect can greatly reduce the unidirectional flux of ions through the pore in the direction counter to the net flow. This explanation is difficult to sustain for ⁴²K⁺ influx across the blood–brain barrier because transport of K⁺ into the endothelial cells across the luminal membrane is thought to be mediated by NKCC1 while transport out of the cells in the abluminal membrane is in the direction of the net flow through the channels (see Sect. 4.5.3).

Explanation in terms of enhanced efflux of ⁴²K⁺ also encounters difficulties. The half-time for accumulation of K⁺ in the brain parenchyma is of the order of 19 h [250] and thus even over a time as long as 2 h, the small increase in [⁴²K⁺]_ISF will not lead to sufficient efflux to reduce the net rate of accumulation to an extent that changes in this efflux would matter. Perhaps, a closely related, potentially larger effect might be sufficient. To enter the brain ⁴²K⁺ must cross the endothelial cell into the basement membrane on the abluminal side and then move onwards either through or around the astrocyte endfeet (see Sect. 5). [⁴²K⁺] in the basement membrane may increase more rapidly and to a greater extent than in the parenchyma as a whole. (A similar effect is considered for amino acids in footnote 3.) However, even this is not a convincing explanation because the astrocyte endfeet contain high densities of K⁺ channels, which would minimize this effect.

As the simple explanations have proved wanting, it would appear to be necessary to consider some form of regulation of the number or activity of K⁺ transporters.

Na ⁺ tracer influx and efflux data at the blood–brain barrier Davson and Welch [264] measured changes in concentration of tracers in CSF and the parenchyma when ²²Na⁺ was infused intravenously and interpreted these using a model for exchanges between blood and CSF, CSF and brain parenchyma, and blood and brain parenchyma. The net flux across the blood–brain barrier was modelled as permeability multiplied by the concentration difference across the barrier. From their data they calculated a PS product (the product of the permeability and the area of barrier, usually per gram of tissue) of 0.074 cm³ h⁻¹ g⁻¹ for Na⁺ influx and a slightly higher value for Cl⁻ influx. Cserr et al. [511] measured the rate of Na⁺ efflux after injecting ²²Na⁺ into the parenchyma of rats, and found a rate constant of 0.43 h⁻¹. For a passive permeability this can be converted to a PS product by multiplying by the extracellular volume per unit weight of tissue, which in effect they took to be c. 0.16 cm³ g⁻¹. Because the resulting estimate of PS based on tracer efflux, 0.43 h⁻¹ × 0.16 cm³ g⁻¹ = 0.069 cm³ h⁻¹ g⁻¹, is within the range of the estimates based on tracer influx, their results confirm that the net flux is too small to measure by these techniques, but the range of possible values is also too large to allow these measurements to be used as an argument against secretion of Na⁺ and fluid by the blood–brain barrier.

Curve fitting of data for influx of ³⁶Cl⁻ into brain cortex Smith and Rapoport [269] measured the uptake of ³⁶Cl⁻ into a volume of brain parietal cortex over a period, T, and divided the average uptake rate (units mmol l⁻¹ s⁻¹ or mM s⁻¹) by the integral of [³⁶Cl⁻] over the same period to obtain a transfer constant (see Eq. 1). This is then multiplied by [Cl⁻]_plasma to obtain the unidirectional Cl⁻ influx over the same period, with results shown in Fig. 10 (Fig. 3b in [269]). These data have been fitted here by non-linear least squares regression assuming that transport has two components:

\( {\text{influx}} = {{V_{max} \left[ {{\text{Cl}}^{ - } } \right]} \mathord{\left/ {\vphantom {{V_{max} \left[ {{\text{Cl}}^{ - } } \right]} {\left( {K_{m} + \left[ {{\text{Cl}}^{ - } } \right]} \right)}}} \right. \kern-0pt} {\left( {K_{m} + \left[ {{\text{Cl}}^{ - } } \right]} \right)}} + P\left[ {{\text{Cl}}^{ - } } \right] \).

The short dashed curve is the fit reported by Smith and Rapoport with P = 0, V _max = 250 ×⁻⁵ mM s⁻¹, K _m = 43 mM, and, as calculated here, a residual sum of squares of 5555 × 10⁻¹⁰ (mM s⁻¹)². The solid curve is the best fit with P = 0.88 × 10⁻⁵ s⁻¹, V _max = 103 × 10⁻⁵ mM s⁻¹, and K _m = 14 mM and a residual sum of squares of 4341 × 10⁻¹⁰ (mM s⁻¹)². For an F test on the improvement in fit provided by allowing P to vary (see [512]), p < 0.03. The long-dashed straight line is for P = 1.2 × 10⁻⁵ s⁻¹, V _max = 56 ×⁻⁵ mM s⁻¹, K _m = 0, and a residual sum of squares of 4918 × 10⁻¹⁰ (mM s⁻¹)². The proportions of the influx for [Cl⁻]_plasma = 118 mM by the unsaturable component are 0, 53 and 72% respectively in the three fits. Two conclusions follow from comparisons of these fits. Firstly the data suggest that more than half of the influx (blood to brain) occurs by an unsaturable mechanism and secondly the data do not determine an accurate value for the K _m of the saturable component. To reach firmer conclusions either more accurate data must be obtained or the data must extend over a larger range of [Cl⁻]_plasma, which might be possible at the lower end of the range.

Calculated net flux if the mechanism of tracer influx is electrodiffusion For electrodiffusion of ions across a barrier the unidirectional fluxes, \( \overrightarrow {J} \) and \( \overleftarrow {J} \) and net flux, \( J_{net} \) can be written terms of rate constants for transfers in the two directions, i.e. for Na⁺ moving between plasma (p) and ISF (i),

\( \begin{aligned} \overrightarrow {J} &= k_{p} \left[ {Na^{ + } } \right]_{p} \quad \quad \quad \overleftarrow {J} = k_{i} \left[ {Na^{ + } } \right]_{i} \\ J_{net} &= \overrightarrow {J} - \overleftarrow {J} = k_{p} \left[ {Na^{ + } } \right]_{p} - k_{i} \left[ {Na^{ + } } \right]_{i} \\ \end{aligned} \)

where, because these equations must reduce to the Nernst equation at equilibrium, the rate constants must obey

\( {{k_{i} } \mathord{\left/ {\vphantom {{k_{i} } {k_{p} }}} \right. \kern-0pt} {k_{p} }} = e^{{\frac{F\Delta V}{RT}}} \).

F is the Faraday, R the gas constant, T the absolute temperature and, \( \Delta V = V_{i} - V_{p} \) is the potential in ISF minus that in plasma. When the Na⁺ concentrations are the same on the two sides and the potential difference is small, this becomes

\( \begin{aligned} J_{net} &= \left[ {Na^{ + } } \right]_{p} k_{p} \left( {1 - \frac{{k_{i} }}{{k_{p} }}} \right) = \left[ {Na^{ + } } \right]_{p} k_{p} \left( {1 - e^{{\frac{F\Delta V}{RT}}} } \right) \\& \cong - \left[ {Na^{ + } } \right]_{p} k_{p} \frac{F\Delta V}{RT} = - \overrightarrow {J} \frac{F\Delta V}{RT} \\ \end{aligned} \)

Water permeabilities It should be noted that the constants considered here describe permeation of just water. They can not be used to describe flow of the entire fluid crossing the blood–brain barrier including solutes in response to a hydrostatic gradient.

The hydraulic permeability of a barrier to water, L _p, is defined as the ratio of the volume flow of water per unit area, J _V, to the net pressure difference, hydrostatic plus osmotic, ΔP _total. Fenstermacher and Johnson [287] measured and reported a filtration constant defined as J _V/Δc where Δc is the concentration difference of impermeant solutes (which for this purpose includes Na⁺ and Cl⁻). The filtration constant equals L _p RT where R is the gas constant, 8.3 J mol⁻¹ K⁻¹ and T is the absolute temperature. At 37 °C, T = 310 K and RT = 2576 J mol⁻¹ = 2576 N m⁻² mM⁻¹ = 19.4 mmHg mM⁻¹. For calculation of the filtration constant they assumed that the surface area of capillaries per gram of parenchyma was 52 cm² g⁻¹. Fenstermacher [288] recalculated the value assuming 100 cm² g⁻¹ with the result LpRT = 1.2 × 10⁻⁶ ml min⁻¹ cm⁻² mM⁻¹. To facilitate comparison with the tracer permeability of water, P _d = (flux of tracer per unit area)/(difference in tracer concentration), the osmotic water permeability is sometimes defined as \( P_{f} = {{L_{p} RT} \mathord{\left/ {\vphantom {{L_{p} RT} {\bar{v}_{w} }}} \right. \kern-0pt} {\bar{v}_{w} }} \) where \( \bar{v}_{w} \) is the partial molar volume of water. Fenstermacher and Johnson's value becomes P _f = 1.1 × 10⁻³ cm s⁻¹ which as stated in Sect. 4.3.6 is well within the range of values found for lipid bilayers.

From the filtration constant and an estimate of the area of membrane per gram of parenchyma, and the expression relating net water flow and concentration difference of impermeant

\( J_{V} = L_{p} RT{{\Delta}}c \)

it is possible to calculate the concentration difference of NaCl that would be needed to drive a net flow of 0.1 µl g⁻¹ min⁻¹ (corresponding to 200 ml day⁻¹ in a human). Using 100 cm² g⁻¹ (the value used in [288]), and 1.2 × 10⁻⁶ ml min⁻¹ mM⁻¹ cm⁻²,

\( \begin{aligned} \Delta \left[ {\text{NaCl}} \right] &=\Delta c/2 = J_{V}/(2L_{p}RT) \\ &= \frac{{ 0. 1 \,\upmu{\text{ l g}}^{ - 1} \,{\text{min}}^{ - 1} \times 10^{ - 3}\, {\text{ml}}\, \upmu {\text{l}}^{ - 1} }}{{2 \times 1.2 \times 10^{ - 6} \,{\text{ml min}}^{ - 1} {\text{mM}}^{ - 1} {\text{cm}}^{ - 2} \times 100{\text{ cm}}^{2} \,{\text{g}}^{ - 1} }} \\ &= 0. 4 {\text{ mM}} \end{aligned}\)

Water cotransport The movement of water by cotransport with ions or other solutes either by direct coupling or by local osmotic effects within a transporter vestibule [160] may be large enough to contribute to the net water flux across the blood–brain barrier. If so, the osmotically driven flow might be decreased or even changed in direction so that the final result is still a fluid close to osmotic equilibrium. Coupled transport of 40 water molecules with each glucose molecule has been proposed for GLUT1 in another context [157]. (This is in addition to the water permeability induced by GLUT1.) At the blood–brain barrier with a net glucose transfer of 0.6 mol day⁻¹ this would mean addition of 24 mol day⁻¹ (440 ml day⁻¹) of water to the brain without accompanying osmotically active solutes (the glucose is consumed). This combined with metabolically produced water would mean that the osmotically driven water flow across the blood–brain barrier would be out of the brain.

The pNPP-ATPase assay for localization of the Na ⁺, K ⁺ -ATPase Ernst [302, 303] introduced a procedure for localizing the pNPP-ATPase activity of the Na⁺-pumps based on the hydrolysis of pNPP (p-nitrophenylphosphate) in a K⁺ dependent step, capture of the phosphate using strontium, and subsequent conversion to a stable lead precipitate which can be seen in the electron microscope, the so-called two-step or indirect procedure. Mayahara et al. [304] introduced a substantial simplification of the procedure to allow omission of the strontium step, the so-called one-step or direct procedure. In both methods the tissue is fixed prior to the enzyme assay step using formaldehyde with or without glutaraldehyde. Prior fixation entails the risk that the ATPase will be inactivated. Mayahara reported that 2% formaldehyde +0.5% glutaraldehyde was the best compromise between adequate fixation and loss of enzyme activity.

The pNPP-ATPase assay is not selective for Na⁺-pumps. It also detects alkaline phosphatase, which can be eliminated from the results by inclusion of a suitable inhibitor, e.g. levamisole. However, even so it is necessary to demonstrate that the activity detected requires the presence of K⁺ and is inhibited by ouabain, usually at 1 mM.

Manoonkitiwongsa et al. [311] investigated the effects of a range of concentrations of formaldehyde and glutaraldehyde. They found that fixation with 2% formaldehyde yielded 1.5 as the ratio of the luminal to abluminal product densities but with 2% formaldehyde plus glutaraldehyde at 0.1, 0.25 or 0.5% the ratio decreased to 0.7, 0.5 or 0.4 [311]. They concluded that glutaraldehyde had an effect to selectively decrease luminal activity. Arguing against this, previous studies that had found strong abluminal predominance include those with (e.g. [295, 305]) and without [308] glutaraldehyde. It may be significant that Manoonkitiwongsa et al. reported that some activity (always in the presence of levamisole to inhibit alkaline phosphatase) persisted in the absence of K⁺ or in the presence of ouabain, but even though the measured hydrolysis thus had to represent more than one type of activity, they still used the total measured activities to compare luminal and abluminal activities.

Net rate of acid extrusion At steady-state [HCO₃ ⁻] and [H⁺] are constant inside the cells and there is no net accumulation of acid within the cells. Thus the net rate of acid extrusion from the cells must be balanced by the net rate of acid production within them as part of metabolism. Most of the acid production is in the form of CO₂ that is extruded from the cells as such. The next most important source is production of lactic acid, but this is extruded as such by MCT1 (see Sect. 6.3). Other contributions can arise from metabolism of fats and protein, but these are at such low rates that they do not affect the conclusions in this section. Thus the net rate of acid extrusion, other than as CO₂ and lactic acid, must be close to zero.

Possible effect of upregulation of luminal membrane K ⁺ channels Another possibility is suggested by the schemes shown in Figs. 17 and 18. Instead of downregulation of NKCC1 that mediates a net flux into the cells, there might be upregulation of K⁺ channels in the luminal membrane that mediate a net flux out of the cells. Tracer that enters via NKCC1 can either return to plasma or continue onwards towards ISF. Increasing the rate of return would decrease the fraction of K⁺ entering that reaches ISF.

Compensation and relative changes in pH _arterial and pH _CSF In metabolic acidosis decreased [HCO₃ ⁻]_arterial and pH_arterial increase ventilation rate over time and decrease pCO₂ which reduces the size of the decrease in [HCO₃ ⁻]_arterial/pCO_2,arterial and hence (see Sect. 6.1.1) reduces the decrease in pH_arterial. This reduction in the size of the change in pH_arterial is called respiratory compensation. Respiratory compensation also reduces the size of the increase in pH_arterial in metabolic alkalosis by increasing pCO₂. Tighter regulation of pH_CSF than of pH_arterial is achieved because while the changes in pCO₂ are similar in CSF and arterial plasma, the change in [HCO₃ ⁻]_CSF is less than that of [HCO₃ ⁻]_arterial as shown in Fig. 21c, d. There would be perfect regulation of pH_CSF if the fold change in [HCO₃ ⁻]_CSF were equal to the fold change in pCO_2,CSF, i.e. if the new values of [HCO₃ ⁻]_CSF and pCO_2,CSF were the same factor times their old values.

In respiratory acidosis increased pCO₂ and decreased pH_arterial increase metabolic production of HCO₃ ⁻ which increases [HCO₃ ⁻]_arterial and in turn reduces the size of the decrease in [HCO₃ ⁻]_arterial/pCO_2,arterial and hence the decrease in pH_arterial. This reduction in the size of the change in pH is called metabolic compensation. Note that pH_arterial is still decreased because in compensated respiratory disturbances the fold change in [HCO₃ ⁻]_arterial is still smaller than the fold change in pCO₂. For there to be tighter regulation of pH_CSF than of pH_arterial in respiratory acidosis it would be necessary for the fold change in [HCO₃ ⁻]_CSF to be closer to the fold change in pCO₂, i.e. the fold change in [HCO₃ ⁻]_CSF would have to exceed the fold change in [HCO₃ ⁻]_arterial. This is not the case in the data for human respiratory acidosis shown in Fig. 21b nor for more recent data (see [185]) where the fold changes in [HCO₃ ⁻]_CSF are similar to or smaller than those in [HCO₃ ⁻]_arterial.

H ⁺- and CO ₂-related species that can carry charge across membranes H⁺ and HCO₃ ⁻ are not the only related species that can carry charge across a membrane. Very rarely in studies of epithelial transport it has been found necessary to consider fluxes of OH⁻ (see e.g. [513]). However, the properties of a transporter that would allow selective, rapidly reversible binding of OH⁻ have not yet been described. Because evidence for OH⁻ transport has not been reported in studies of the choroid plexuses and blood–brain barrier, it has not been considered in this review. Another possible species that may be transported is CO₃ ²⁻. For instance it is very difficult experimentally to distinguish coupled transport of one Na⁺ and one CO₃ ²⁻ from coupled transport of one Na⁺ and two HCO₃ ⁻. Both transfer one negative charge and both require the presence of CO₂ [195, 398].

Stewart's approach to the interdependence of ion concentrations and fluxes in acid-base balance and a better but more difficult alternative The interdependence of ion concentrations and fluxes relevant to the control of pH was emphasized in an approach to the subject presented by Stewart [407, 408, 514]. In this approach, the independent variables that can be altered or controlled are taken to be pCO₂, the strong ion difference (i.e. the sum of charges on ions like Na⁺ and Cl⁻) and the concentration of a single "representative" buffer standing for all buffers present other than CO₂ / HCO₃ ⁻. All other variables like pH and [HCO₃ ⁻] are regarded as dependent and not subject to separate control. A major advantage of Stewart's approach is that it allows many acid-base calculations, even involving transfers across membranes, to be completed without any consideration of membrane potential.

A major disadvantage of Stewart's approach is that it becomes tempting to make statements like: "Hydrogen ion movements between solutions can not affect hydrogen ion concentration; only changes in independent variables can." [514] or "Furthermore there are objections, based on physicochemical principles, to the assumption that HCO₃ ⁻ (or H⁺) is primarily and directly handled by active-transport mechanisms" (pg 127 in [185]). The latter statement is just wrong. It would have been much better had Stewart said "The observation of changes in hydrogen ion concentration implies that there are also changes in pCO₂, the strong ion difference or the concentrations or properties of the buffers." Stewart's proposals were useful in that they stimulated consideration of the consequences for acid-base balance of fluxes of those ions that determine the strong ion difference [185, 389, 393]. However, despite statements to the contrary (see e.g. pp. 120–121 in [185]), Stewart's choice of which variables to consider as independent is only a matter of convenience not one of necessity (compare [383, 390, 515]). Thus if all that is known is that there have been changes in [HCO₃ ⁻] and [Cl⁻] it is not possible to say whether either change caused the other. Stewart's approach is of no help in considering the role of changes in membrane potential.

Rather than defining some variables as always independent and some as always dependent as Stewart did, it is much better not to prejudge which concentrations and fluxes can be varied by external processes (e.g. transport into or out of a region) and instead use the explicit constraints on the concentrations and fluxes described in Sect. 6.1. Indeed, now that computer simulations can keep track of concentrations to the accuracy needed for calculation of the net charge within a region as big as a cell, it is possible to do even better and take a more rigorous approach avoiding the need to assume electroneutrality (a necessary part of Stewart's approach). The analysis should employ a model that considers the electrical potentials produced by the actual net charge within cells, the effects of these potentials and the ion concentrations on transport of ions, and the changes in intracellular net charge and ion concentrations produced by such ion transport. Electroneutrality is not imposed, but if the results of the analysis do not obey approximate electroneutrality (i.e. the membrane potentials fail to take on realistic values) either the model is describing a condition which would destroy the cell or something is wrong with either the model or the computer code used to implement it. Students of electrophysiology will recognize that important elements of this approach, avoiding the need to assume electroneutrality, were used in the classical description of the mechanism for propagation of the action potential along a nerve axon [516]. Changes in charge and potential could be calculated in that study without needing to know the concentrations to great accuracy because the potentials and the current across the membrane were measured directly.

There are, however, challenging aspects to extending this more rigorous approach to the choroid plexuses and blood–brain barrier: it requires detailed description of the actual transport processes occurring including their dependence on membrane potential and it requires sufficiently accurate bookkeeping of ion fluxes to calculate the changes in net charge within the cell.

Physiological buffering of pH _ISF Ahmad et al. [415] used surface pH and Cl⁻ electrodes applied to exposed cortex and reported very rapid, equal but opposite changes in ISF [Cl⁻] and [HCO₃ ⁻] following elevation of pCO₂. For a pCO₂ increase from ~28 to 55 mmHg, [Cl⁻] decreased and [HCO₃ ⁻] increased by 4 mM with half-times of 30–40 s, much as expected for buffering within the cells and exchange of intracellular HCO₃ ⁻ for extracellular Cl⁻. Similarly fast changes in ISF pH following changes in pCO₂ have been seen using microelectrodes inserted into the parenchyma [456, 517] and similarly fast surface pH electrode responses have been correlated with phrenic nerve activity indicating activation of medullary chemoreceptors [517, 518]. See footnote 30 for caveats concerning results obtained in a related study by Ahmad et al. [442] that may complicate interpretation of the results discussed here.

Physiological buffering of ISF is likely to be a major part of the explanation of results for ISF pH obtained using ³¹P NMR by Portman et al. [416]. Goats were ventilated with 0.5–1% halothane in oxygen at rates between 2 and 21 l min⁻¹, which produced values of arterial pH between 7.1 and 7.65. ISF pH and intracellular pH were recorded 10 min after each change in ventilation rate. As percentages of the changes in arterial blood, the changes in ISF and intracellular fluid pH were 56 and 23%. These and other results obtained using ³¹P NMR [519, 520] imply that the intracellular fluid is strongly buffered against changes in pH as expected since this fluid is rich in proteins and phosphate compounds. Because the change in ISF pH was intermediate between those in plasma and intracellular fluid, both of which are regarded as strongly buffered solutions, the results also imply that ISF was strongly buffered even though ISF does not obviously contain sufficient buffers. Physiological buffering provides a plausible explanation. When pCO₂ is increased, intracellular [HCO₃ ⁻] is increased and thus [HCO₃ ⁻]_ISF can be increased by efflux of HCO₃ ⁻ from cells. There could also be transport of HCO₃ ⁻ into ISF from plasma. The increase in [HCO₃ ⁻]_ISF then reduces the change in pH to the level observed.

Arterio-venous differences in concentrations for substrates and metabolites The amount of substance taken up by or removed from a region is often estimated from the differences in the concentrations in the arterial inflow and venous outflow from the region and the blood–flow,

\( {\text{uptake or release }} = \, \left( {C_{\text{arterial}} - C_{\text{venous}} } \right) \, \times {\text{ blood flow}}. \)

For substrates for which most movement is uptake, e.g. glucose and O₂, the uptake is reasonably estimated by this calculation. It is, of course, necessary to arrange to take the venous sample from a location exposed to the venous outflow from the region and to none other. However, for metabolites the assumption that removal from the region occurs only by release into the venous blood emerging from that region is incorrect if some can be removed to lymph. This may be important when trying to estimate the rate of production of lactic acid as it is clear that some of the lactate is removed via perivascular routes to lymph which bypasses the sites at which the venous blood is sampled [57, 434, 521].

Blood–brain barrier permeability for lactate and rapid distribution of lactate via astrocytes The permeability-surface-area products, PS, for lactate transport by MCT1, about 0.06 ml g⁻¹ min⁻¹ in adult rats [429, 430] and 0.1 ml g⁻¹ min⁻¹ in humans [431], and the volume of ISF per gram of tissue, ~0.2 ml g⁻¹, corresponds to a half-time for removal of lactate less than 10 min. (For a different view of the rate of removal see [522]). Some removal of lactate⁻ as such will occur as part of the outflow of ISF from the interstitial spaces of the parenchyma. However, as that has a half-time more than tenfold longer (judged by e.g. removal of markers like sucrose and mannitol, see Sect. 4.1 here and section 4.1.1 in [15]), such outflow will only make a relatively small contribution. However there may be another possible route of elimination of lactate⁻. Lactate⁻ or lactic acid can be removed from its site of production and distributed through the network of astrocytes (see e.g. [106, 521, 523, 524]) with release at sites close to perivascular spaces of larger blood vessels. From these spaces it may be able to leave the brain rapidly to blood or cervical lymph [57, 521]. The factors shown by Lundgaard et al. [434] to affect similarly the rates of removal of lactate and inulin may be acting at this final stage—i.e. on transport via the perivascular spaces. Distribution or spreading out of lactate from its site of production was proposed earlier [433] together with another complicating factor in the experimental studies, partial metabolism of lactate with storage of the radiolabel in metabolic intermediates [56].

In the steady-state the net rate at which lactic acid production and lactate⁻ removal provides H⁺ that can deplete HCO₃ ⁻ is difficult to determine from the available evidence. It is likely to be substantially less than the rate of production of lactic acid until this rate approaches the transport capacity of MCT1 in the endothelial cells but, for severe hypocapnia or hypoxia with high [lactate⁻], depletion of HCO₃ ⁻ and reduction of pH_ISF are likely to be substantial.

Potential differences across membranes and across cell layers Movements of ions across the membranes of either choroid plexus epithelial cells or blood–brain barrier endothelial cells via mechanisms that transport net charge must: (a) be sensitive to the electrical potential differences between the cell interiors and the blood on one side and CSF or ISF on the other and (b) affect the charge within the cells and hence the potentials across their membranes. There is thus no avoiding the need to consider the membrane potentials in any adequate discussion of mechanisms. Unfortunately, there are no available data for how the membrane potentials of the cells in vivo are affected by pCO₂, pH or HCO₃ ⁻ or how the membrane potentials affect the rates of ion transport into or out of the cells. However, the potential difference between CSF (and to some extent ISF) and blood (called "the PD") has been measured in a large number of studies. There are two issues to consider: whether and how pH, [HCO₃ ⁻] or pCO₂ in some way determine the PD and whether and how the PD affects the relation between the plasma and CSF values of pH and [HCO₃ ⁻]. Data for ISF would be more interesting, but most relate to CSF.

In dogs and goats the PD between cisternal CSF and jugular vein blood at pH 7.4 is between 3 and 6 mV (CSF positive) at pH 7.4. The PD changes with a slope of −32 mV (pH unit)⁻¹ when pH is varied by making primary changes in pCO₂ of blood and −43 mV (pH unit)⁻¹ when the primary changes are in [HCO₃ ⁻]_arterial [286]. See [525] (discussion after [401]) [438, 439] and for many further references [185, 388]. From the similar effects of increasing pCO₂ and decreasing [HCO₃ ⁻]_arterial, it is inferred that the variation in PD depends primarily on [H⁺]_arterial rather than on [HCO₃ ⁻]_arterial or pCO₂. (A possible explanation for the difference in slopes is that changes in pCO₂ alter blood flow while those in [HCO₃ ⁻] do not, see [189, 436] and the discussion after [401]). Three possible mechanisms by which pH_arterial could affect the PD have been considered (see [185, 388]). Firstly the blood facing membrane of the barrier that generates the trans-barrier PD may have a higher conductance to H⁺ than to anything else. The lack of variation with pH_CSF is then explained if the CSF facing membrane has a potential difference across it that does not vary with pH on either side. Secondly pH_arterial may affect the permeability of the blood facing membrane or the paracellular transport route to other ions. Finally pH_arterial might somehow alter the rate of an active current-carrying mechanism. The first explanation would require a very high permeability to H⁺ to compensate for its very low concentration. It is difficult to imagine that such a large permeability would have escaped notice in studies such as those discussed in Sects. 3 and 4. The second and third explanations are more easily accepted but lack direct experimental evidence.

There have also been three contenders for the location of the barrier across which the PD is generated: the ependyma and pia, which separate CSF and ISF, the choroid plexuses and the blood–brain barrier. The ependyma and pia are very unlikely sources because the barriers are leaky and non-selective between ions. The choroid plexus might generate a potential difference, but the available evidence indicates that it isn't the main source for three reasons. Firstly, direct measurements of the potential difference across isolated choroid plexus have shown either no PD [526, 527] or a PD that does not show the variation with pH_arterial that is seen in measurements of the PD between cisternal CSF and plasma [440, 492]. (In the dogfish the PD for isolated choroid plexus even has the opposite sign to the in vivo PD [528].) Secondly CSF production, measured by ventriculo-cisternal perfusion, is hardly changed when the PD is changed by altering pH_arterial. In other words active secretion by the choroid plexus and the generation of the PD are not coupled [286]. Thirdly it would be difficult for a current source localized to the choroid plexuses within the ventricles to produce a potential elsewhere [529]. Furthermore if it is correct that the transport processes in the basolateral membranes of the epithelial cells are all electrically neutral [4] (see Sect. 4.3.2), transcellular transport at the choroid plexus cannot carry a current and whatever PD is produced by the choroid plexus can only be a diffusion potential across the paracellular pathway.

There are two principal arguments in favour of the blood–brain barrier as the source of the PD: elimination of the other possibilities and the observation that pial microvessels, which are thought to have similar properties to those in parenchyma, do generate a potential (lumen negative) [285]. The available evidence indicates that transport at both the luminal and abluminal membranes of the endothelial cells does transfer net charge (see Sect. 4).

The consequences of a PD that varies with pH_arterial could be considerable for pH regulation. As indicated in Sect. 8, under normal conditions [HCO₃ ⁻]_CSF and thus presumably [HCO₃ ⁻]_ISF are less than would be at equilibrium with [HCO₃ ⁻]_plasma and the PD. In the so-called "passive" theory for ISF pH regulation advanced by Siesjö and colleagues [417], the active process (which might be lactic acid production) that accounts for this disequilibrium under normal conditions is assumed to be constant irrespective of pH, pCO₂ and [HCO₃ ⁻] in plasma or ISF with all changes in the ratio [HCO₃ ⁻]_ISF/ [HCO₃ ⁻]_plasma resulting directly from changes in passive transport.

If the PD were effective in driving a flux of HCO₃ ⁻ or H⁺ across the blood–brain barrier, more positive PD would increase the ratio [HCO₃ ⁻]_ISF/ [HCO₃ ⁻]_plasma and more negative PD would decrease it. This would provide a simple mechanism for pH regulation in ISF (and hence CSF). If pH_arterial were reduced by increasing pCO₂, there would be an increase in [HCO₃ ⁻]_plasma but a larger increase in [HCO₃ ⁻] _ISF because the PD would be increased. Thus the pH change would be smaller in ISF than in plasma. Similarly if pH_arterial were reduced by decreasing [HCO₃ ⁻]_plasma the decrease in [HCO₃ ⁻]_ISF and pH_ISF would be smaller. Smaller pH changes in ISF than in plasma is the definition of pH regulation of ISF. Probably because this explanation of pH regulation could be tested by experiments (see Sect. 6.4), it attracted a great deal of effort and attention. In many, but not all, studies the variation of [HCO₃ ⁻]_CSF/ [HCO₃ ⁻]_plasma was as expected from the variations in PD (see[185, 388, 417]).

[HCO ₃ ⁻ ] in the primary secretion of the choroid plexus Variations in [HCO₃ ⁻] in the primary secretion of the choroid plexus during hyperventilation or with raised pCO₂ in inspired air were investigated by Ames et al. [141] in cats. Taking samples from droplets of fluid secreted into a layer of Pantopaque oil covering a lateral ventricle choroid plexus, they found during hyperventilation substantial decreases in the rates of secretion of both fluid and HCO₃ ⁻ with no change in [HCO₃ ⁻] (determined from charge balance, see Sect. 6.1.2). With raised pCO₂ there was a substantial increase in both the rate of fluid secretion and [HCO₃ ⁻]. These experiments established that the choroid plexuses do secrete and that the effects of changing pCO₂ on choroid plexus secretion are at least in the correct direction to contribute to pH regulation in the brain. (But note these conditions correspond to respiratory alkalosis and acidosis for which regulation is poor, see Fig. 21.) However it has been suggested that the pantopaque oil used in these experiments may have altered the function of the choroid plexuses because it can be toxic when injected into the ventricles ([217], but see [530] for details).

Exposure of the choroid plexus to this oil was avoided in other experiments but those entailed greater dissection of the brain to enclose a choroid plexus in a chamber that could capture the secreted fluid. Measuring secretion by the change in contents of the chamber Husted and Reed [440] found: (i) little change in the addition of HCO₃ ⁻ to the chamber when [HCO₃ ⁻] in arterial blood was reduced; (ii) little change in pH of the net fluid added to the chamber during hypocapnia and hypercapnia, implying that [HCO₃ ⁻] in the secretion varied in proportion to the change in pCO₂; and (iii) a marked effect of [HCO₃ ⁻] in the chamber on the net addition of HCO₃ ⁻. Increased [HCO₃ ⁻] in the chamber almost eliminated addition of HCO₃ ⁻, while decreased [HCO₃ ⁻] increased the amount added. The variations with [HCO₃ ⁻] in the chamber suggest that there is a substantial backflux of HCO₃ ⁻ from chamber to blood that increases with the concentration in the chamber. It is possible that this represents an artefact produced during the dissection.

All of the changes observed by Husted and Reed are in the correct direction to contribute towards pH regulation. However, in contrast to the earlier results of Ames et al. [141], Husted and Reed [440] found no effect of changes in pCO₂ on the rate of CSF production. This discrepancy remains to be resolved.

Increase in [HCO₃ ⁻] in the secretion when pCO₂ is increased is consistent with what is now known about the transporters present at the choroid plexuses. Within the epithelial cells, increased pCO₂ will increase [HCO₃ ⁻] or decrease pH to maintain the equilibrium between CO₂, H⁺ and HCO₃ ⁻ (carbonic anhydrase is present). Indeed if as favoured by current evidence the route of HCO₃ ⁻ entry across the basolateral membrane is NCBE (see Sects. 4.3.2, 4.3.3), the equilibrium is likely to be maintained primarily by an increase in [HCO₃ ⁻] because any decrease in pH would increase the driving force for H⁺ and Cl⁻ exit and HCO₃ ⁻ entry. Increased [HCO₃ ⁻] within the cells will be accompanied by a decrease in [Cl⁻] (see Eq. 11). These concentration changes will be reflected in HCO₃ ⁻ being a larger proportion of the anions crossing the apical membrane, i.e. to an increase in [HCO₃ ⁻] in the secretion. However, this explanation is called into question by the failure of similar reasoning to explain why an increase in plasma [HCO₃ ⁻] and decrease in plasma [Cl⁻] at constant pCO₂ does not increase [HCO₃ ⁻] in the secretion.

HCO ₃ ⁻ and Cl ⁻ permeabilities estimated from loss from ventricular perfusates Pappenheimer, Fencl and colleagues [351, 352] (see section 5.3.1) used ventriculocisternal perfusion to look at the loss from or gain into the perfusion fluids of HCO₃ ⁻ and Cl⁻ measured after 45 min of perfusion. However, it is now clear that 45 min was not long enough for concentrations in the parenchyma to reach steady-state. This is evident from experiments following radiolabelled K⁺ [251] or Na⁺ [441]. The results for Na⁺ were particularly detailed. For periods much longer than 45 min the loss of tracer from the perfusate across the ependyma corresponded to increases in the concentration within the parenchyma rather than transport to the blood. Even after 4 h of perfusion the rate of loss from the ventricles had not decreased to the steady-state value. As blood–brain barrier permeabilities to Na⁺ and Cl⁻ are now thought to be similar (see Sect. 4.3.2) and both ions are present at relatively high concentrations primarily in the extracellular fluid within the parenchyma, it follows that for Cl⁻, just as for Na⁺, perfusions lasting for hours rather than the 45 min employed would be required to reveal the rate of transfer across the blood–brain barrier. For HCO₃ ⁻, an adequate theoretical treatment of the data is more difficult because the concentrations of HCO₃ ⁻, CO₂ and H⁺ are interrelated and the local pH is buffered by the presence of the cells.

The measured rate of loss of Cl⁻ from the perfusion fluid was ~6 µmol min⁻¹ when the concentration in CSF exceeded the value for zero loss by \( c_{CSF} \) = 40 mM. It is instructive to compare this measured value with estimates of the rate of loss, R, calculated assuming that it represents:

steady-state transfer across the blood–brain barrier, approximately:

\( R \approx PS \times \rho \times A \times \delta \times c_{CSF} \)

diffusion into parenchyma with no transfer across the blood–brain barrier:

\( \begin{aligned} R &= - DA\left( {{{{\text{d}}c} \mathord{\left/ {\vphantom {{{\text{d}}c} {{\text{d}}x}}} \right. \kern-0pt} {{\text{d}}x}}} \right)_{x = 0} = - DA\left( {\frac{{{\text{d}}\left( {c_{CSF} {\text{erfc}}\left( {2\sqrt {Dt} } \right)} \right)}}{{{\text{d}}x}}} \right)_{x = 0} \\& = Ac_{CSF} \sqrt {{{2D} \mathord{\left/ {\vphantom {{2D} {\left( {\pi t} \right)}}} \right. \kern-0pt} {\left( {\pi t} \right)}}} \\ \end{aligned} \)

In these expressions: PS is the permeability-area product for Cl⁻ at the blood–brain barrier, which has been determined by Smith and Rapoport as 1.4 × 10⁻⁵ cm³ s⁻¹ g⁻¹ [261] ρ = 1 g cm⁻³ is the density of the region; A is the surface area of the perfused portion of the ventricles, taken here (a guess) as 35 cm²; δ is an equivalent thickness of the layer of parenchyma from which transfer to the blood occurs, which after 45 min will be less than 2 mm [441]; \( c_{CSF} \) = 40 mM is the concentration excess in the perfusate; erfc(y) is the complement error function [441]; D ~ 5 × 10⁻⁶ cm² s⁻¹ is the diffusion constant, and t = 45 min is the time after the start of the infusion at which the rate of loss is determined.

Inserting values into the expression for loss by steady-state transfer across the blood–brain barrier,

\( \begin{aligned} R &\approx 1.4 \times 10^{ - 5} {\text{cm}}^{3} {\text{s}}^{ - 1} {\text{g}}^{ - 1} \times 35\,{\text{cm}}^{2} \times 0.2\,{\text{cm}} \times 1\,{\text{g}}\,{\text{cm}}^{ - 3} \times 40\,\upmu {\text{mol}}\,{\text{cm}}^{ - 3} \\ &\approx 4 \times 10^{ - 3} \,\upmu {\text{mol}}\,{\text{s}}^{ - 1} = 0.24\,\upmu {\text{mol}}\,{ \hbox{min} }^{ - 1} \\ \end{aligned} \)

This is much smaller than the measured value.

Inserting values into the expression for loss from the ventricles by diffusion into the parenchyma

\( \begin{aligned} R &\approx 35\,{\text{cm}}^{2} \times 40\,\upmu {\text{mol}}\,{\text{cm}}^{ - 3} \times \sqrt {{{2 \times 5 \times 10^{ - 6} {\text{cm}}^{2} {\text{s}}^{ - 1} } /{\left( {3.14 \times 2700\,{\text{s}}} \right)}}} \\ &\approx 0.05\,\upmu {\text{mol}}\,{\text{s}}^{ - 1} = 2.9\,\upmu {\text{mol}}\,{ \hbox{min} }^{ - 1} \\ \end{aligned} \)

This is much closer to the measured value. From this comparison it would appear that the measured rate of loss from the ventricular perfusate after 45 min was more than tenfold greater than the rate of transfer across the blood–brain barrier and could be accounted for by accumulation (or depletion) of Cl⁻ in the parenchyma. These numbers shouldn't be taken too seriously. What was needed in the studies by Pappenheimer, Fencl and colleagues was experimental evidence that the rate of loss or gain had reached a steady-state. No such evidence was presented.

Rates of HCO ₃ ⁻ and Cl ⁻ transport inferred from pH electrode measurements Ahmad and Loeschcke [442] using surface electrodes investigated changes when plasma [HCO₃ ⁻] was abruptly increased from ~16 to ~23.5 mM at nearly constant pCO₂. ISF [HCO₃ ⁻] increased and [Cl⁻] decreased by about 3 mM (pH increase of 0.07) with a half-time of about 20 s. This was interpreted as rapid exchange of HCO₃ ⁻ and Cl⁻ between plasma and ISF. Teppema et al. [443, 444] also saw rapid changes in pH. Davies and Nolan [445] using pH sensitive microelectrodes with pH sensitive tips about 1–2 µm diameter and 40 µm in length found that ISF pH responded to isocapnic, iv infusions of HCl or NaHCO₃ with lags of only a few minutes (ΔpH_ISF was 40–80% of ΔpH_arterial at the end of 30 min with no change in pH_CSF).

These changes are much faster and larger than those seen at constant pCO₂ by Javaheri and colleagues [455, 457]. No satisfactory reconciliation of these results has been provided [389]. The rapid changes might be explained if somehow the electrodes were responding, at least partially, to changes in the pH of plasma or peripheral tissue extracellular fluid. However, how this could have occurred is unknown. Presumably worry about such an artefact explains why Javaheri et al. always looked for a positive Rapoport test (see Sect. 6.4.2).

The rapid changes in [Cl⁻] reported by Ahmad and Loeschcke [442] are not consistent with the reported tracer permeability to this ion. More generally the permeability of the blood–brain barrier to all small ions (see e.g. [16, 261, 269], and thus by inference to HCO₃ ⁻, is difficult to reconcile with the reports of large, rapid changes in ISF pH at constant pCO₂. Furthermore it is very difficult to understand how under these circumstances, given free movement of small ions across the ependyma, rapid and large changes in ISF could occur without some change with a similar time course in [HCO₃ ⁻]_CSF. All agree that no such variation is observed (see e.g. [185, 386, 413, 414].

Rate of HCO ₃ ⁻ transport (or lack of) estimated from measurement of total CO ₂ in the brain Siesjö asserted that the methods used would have quantified a change corresponding to the same concentration change as in plasma in a volume of 5% of the water content of brain and concluded that only a little HCO₃ ⁻, if any, crossed the blood–brain barrier during the experiments. It is now known that the volume of ISF is about 20% of the aqueous volume of the parenchyma, which means a change in [HCO₃ ⁻] in ISF of about 25% of that in plasma might not have been seen. The change expected is about 30% of that in plasma if the change in [HCO₃ ⁻]_ISF is the same as the steady-state change in [HCO₃ ⁻]_CSF [185].

Keep RF, Jones HC. A morphometric study on the development of the lateral ventricle choroid-plexus, choroid-plexus capillaries and ventricular ependyma in the rat. Brain Res Devel Brain Res. 1990;56:47–53.CrossRef

Spector R, Keep RF, Snodgrass SR, Smith QR, Johanson CE. A balanced view of choroid plexus structure and function: focus on adult humans. Exp Neurol. 2015;267:78–86.PubMedCrossRef

Cserr HF. Physiology of choroid plexus. Physiol Rev. 1971;51:273–311.PubMed

Damkier HH, Brown PD, Praetorius J. Cerebrospinal fluid secretion by the choroid plexus. Physiol Rev. 2013;93:1847–92.PubMedCrossRef

Mathiisen TM, Lehre KP, Danbolt NC, Ottersen OP. The perivascular astroglial sheath provides a complete covering of the brain microvessels: an electron microscopic 3D reconstruction. Glia. 2010;58:1094–103.PubMedCrossRef

Daneman R. The blood–brain barrier in health and disease. Ann Neurol. 2012;72:648–72.PubMedCrossRef

Daneman R, Prat A. The blood–brain barrier. Cold Spring Harb Perspect Biol. 2015;7:a020412.PubMedPubMedCentralCrossRef

Armulik A, Genove G, Mae M, Nisancioglu MH, Wallgard E, Niaudet C, He L, Norlin J, Lindblom P, Strittmatter K, et al. Pericytes regulate the blood–brain barrier. Nature. 2010;468:557–61.PubMedCrossRef

Daneman R, Zhou L, Kebede AA, Barres BA. Pericytes are required for blood–brain barrier integrity during embryogenesis. Nature. 2010;468:562–6.PubMedPubMedCentralCrossRef

10.

Helms HC, Abbott NJ, Burek M, Cecchelli R, Couraud PO, Deli MA, Forster C, Galla HJ, Romero IA, Shusta EV, et al. In vitro models of the blood–brain barrier: an overview of commonly used brain endothelial cell culture models and guidelines for their use. J Cereb Blood Flow Metab. 2016;36(5):862–90.PubMedPubMedCentralCrossRef

11.

Brightman MW, Reese TS. Junctions between intimately apposed cell membranes in the vertebrate brain. J Cell Biol. 1969;40:648–77.PubMedPubMedCentralCrossRef

12.

Brightman MW, Klatzo I, Olsson Y, Reese TS. The blood–brain barrier to proteins under normal and pathological conditions. J Neurol Sci. 1970;10:215–39.PubMedCrossRef

13.

Haj-Yasein NN, Vindedal GF, Eilert-Olsen M, Gundersen GA, Skare O, Laake P, Klungland A, Thoren AE, Burkhardt JM, Ottersen OP, Nagelhus EA. Glial-conditional deletion of aquaporin-4 (Aqp4) reduces blood–brainwater uptake and confers barrier function on perivascular astrocyte endfeet. Proc Natl Acad Sci USA. 2011;108:17815–20.PubMedPubMedCentralCrossRef

14.

Nuriya M, Shinotsuka T, Yasui M. Diffusion properties of molecules at the blood–braininterface: potential contributions of astrocyte endfeet to diffusion barrier functions. Cereb Cortex. 2013;23:2118–26.PubMedCrossRef

15.

Hladky SB, Barrand MA. Mechanisms of fluid movement into, through and out of the brain: evaluation of the evidence. Fluids Barriers CNS. 2014;11:26.PubMedPubMedCentralCrossRef

16.

Bradbury MWB. The concept of a blood–brain barrier. Chichester: Wiley; 1979.

17.

Davson H, Segal MB. Physiology of the CSF and blood–brain barriers. Boca Raton: CRC Press; 1996.

18.

Keep RF. The blood–brain barrier. In: Walz W, editor. The neuronal environment: brain homeostasis in health and disease. Totowa: Humana Press; 2002. p. 277–307.

19.

O’Donnell ME. Ion and water transport across the blood–brain barrier. In: Alvarez-Leefmans FJ, Delpire E, editors. Physiology and pathology of chloride transporters and channels in the nervous system: from molecules to diseases. Amsterdam: Elsevier Science; 2009. p. 585–606.

20.

Brinker T, Stopa EG, Morrison J, Klinge PM. A new look at cerebrospinal fluid circulation. Fluids Barriers CNS. 2014;11:10.PubMedPubMedCentralCrossRef

21.

Spector R, Snodgrass SR, Johanson CE. A balanced view of the cerebrospinal fluid composition and functions: focus on adult humans. Exp Neurol. 2015;273:57–68.PubMedCrossRef

22.

Johansson PA, Dziegielewska KM, Liddelow SA, Saunders NR. The blood-CSF barrier explained: when development is not immaturity. BioEssays. 2008;30:237–48.PubMedCrossRef

23.

Engelhardt B, Sorokin L. The blood–brainand the blood-cerebrospinal fluid barriers: function and dysfunction. Semin Immunopath. 2009;31:497–511.CrossRef

24.

Kratzer I, Vasiljevic A, Rey C, Fevre-Montange M, Saunders N, Strazielle N, Ghersi-Egea JF. Complexity and developmental changes in the expression pattern of claudins at the blood-CSF barrier. Histochem Cell Biol. 2012;138:861–79.PubMedPubMedCentralCrossRef

25.

Liddelow SA, Dziegielewska KM, Ek CJ, Habgood MD, Bauer H, Bauer HC, Lindsay H, Wakefield MJ, Strazielle N, Kratzer I, et al. Mechanisms that determine the internal environment of the developing brain: a transcriptomic, functional and ultrastructural approach. PLoS One. 2013;8:e65629.PubMedPubMedCentralCrossRef

26.

Strazielle N, Ghersi-Egea JF. Physiology of blood–brain interfaces in relation to brain disposition of small compounds and macromolecules. Mol Pharm. 2013;10:1473–91.PubMedCrossRef

27.

Engelhardt B, Liebner S. Novel insights into the development and maintenance of the blood–brain barrier. Cell Tissue Res. 2014;355:687–99.PubMedPubMedCentralCrossRef

28.

Saunders NR, Dreifuss J-J, Dziegielewska KM, Johansson PA, Habgood MD, Mollgard K, Bauer H-C. The rights and wrongs of blood–brain barrier permeability studies: a walk through 100 years of history. Front Neurosci. 2014;8:404.PubMedPubMedCentralCrossRef

29.

Ghersi-Egea J-F, Babikian A, Blondel S, Strazielle N. Changes in the cerebrospinal fluid circulatory system of the developing rat: quantitative volumetric analysis and effect on blood-CSF permeability interpretation. Fluids Barriers CNS. 2015;12:8.PubMedPubMedCentralCrossRef

30.

Saunders NR, Dziegielewska KM, Mollgard K, Habgood MD, Wakefield MJ, Lindsay H, Stratzielle N, Ghersi-Egea J-F, Liddelow SA. Influx mechanisms in the embryonic and adult rat choroid plexus: a transcriptome study. Front Neurosci. 2015;9:123.PubMedPubMedCentral

31.

Faraci FM, Mayhan WG, Williams JK, Heistad DD. Effects of vasoactive stimuli on blood flow to choroid plexus. Am J Physiol. 1988;254:H286–91.PubMed

32.

Williams JL, Jones SC, Page RB, Bryan RM Jr. Vascular responses of choroid plexus during hypercapnia in rats. Am J Physiol. 1991;260:R1066–70.PubMed

33.

Williams JL, Shea M, Furlan AJ, Little JR, Jones SC. Importance of freezing time when iodoantipyrine is used for measurement of cerebral blood flow. Am J Physiol. 1991;261:H252–6.PubMed

34.

Ennis SR, Keep RF. The effects of cerebral ischemia on the rat choroid plexus. J Cereb Blood Flow Metab. 2006;26:675–83.PubMedCrossRef

35.

Heisey SR. Blood–brain barrier: Vertebrates. In: Altman PL, Dittmer DS, editors. Respiration and circulation. Bethesda: Federation of American Societies for Experimental Biology; 1971. p. 386–90.

36.

Sweet WH, Selverstone B, Soloway S, Stetten D Jr. Studies of formation, flow and absorption of cerebrospinal fluid. II. Studies with heavy water in the normal man. Surg Forum. 1950;92:376–81.

37.

Sweet WH, Brownell GL, Scholl JA, Bowsher DR, Benda P, Stickley EE. The formation, flow and absorption of cerebrospinal fluid—newer concepts based on studies with isotopes. Res Publ-Assoc Res Nerv Ment Dis. 1954;34:101–59.

38.

Bering EA Jr. Water exchange of central nervous system and cerebrospinal fluid. J Neurosurg. 1952;9:275–87.PubMedCrossRef

39.

Yudilevich DL, De Rose N. Blood–braintransfer of glucose and other molecules measured by rapid indicator dilution. Am J Physiol. 1971;220:841–6.PubMed

40.

Eichling JO, Raichle ME, Grubb RL Jr, Ter-Pogossian MM. Evidence of the limitations of water as a freely diffusible tracer in brain of the rhesus monkey. Circ Res. 1974;35:358–64.PubMedCrossRef

41.

Gjedde A, Andersson J, Eklof B. Brain uptake of lactate, antipyrine, water and ethanol. Acta Physiol Scand. 1975;93:145–9.PubMedCrossRef

42.

Takagi S, Ehara K, Finn RD. Water extraction fraction and permeability-surface product after intravenous injection in rats. Stroke. 1987;18:177–83.PubMedCrossRef

43.

Sokoloff L. Circulation and energy metabolism of the brain. In: Siegel GJ, Albers RW, Katzman R, Agranoff BW, editors. Basic neurochemistry. 2nd ed. Boston: Little Brown and Company; 1989. p. 388–413.

44.

Bulat M, Lupret V, Oreskovic D, Klarica M. Transventricular and transpial absorption of cerebrospinal fluid into cerebral microvessels. Coll Antropol. 2008;32(Suppl 1):43–50.PubMed

45.

Oreskovic D, Klarica M. The formation of cerebrospinal fluid: nearly a 100 years of interpretations and misinterpretations. Brain Res Rev. 2010;64:241–62.PubMedCrossRef

46.

Oreskovic D, Klarica M. A new look at cerebrospinal fluid movement. Fluids Barriers CNS. 2014;11:16.PubMedPubMedCentralCrossRef

47.

Buishas J, Gould IG, Linninger AA. A computational model of cerebrospinal fluid production and reabsorption driven by Starling forces. Croat Med J. 2014;55:481–97.PubMedPubMedCentralCrossRef

48.

Siesjö BK. Brain energy metabolism. Chichester: Wiley; 1978.

49.

Sokoloff L. The metabolism of the central nervous system in vivo. In: Field J, Magoun HW, Hall VE, editors. Handbook of physiology section 1 neurophysiology, vol. 3. Washington: American Physiological Society; 1960.

50.

Sokoloff L. The brain as a chemical machine. Prog Brain Res. 1992;94:19–33.PubMedCrossRef

51.

Johnson DC, Hoop B, Kazemi H. Movement of CO₂ and HCO₃ ⁻ from blood to brain in dogs. J Appl Physiol Respir Environ Exerc Physiol. 1983;54:989–96.PubMed

52.

Mitchell RA, Carman CT, Severinghaus JW, Richardson BW, Singer MM, Shnider S. Stability of cerebrospinal fluid pH in chronic acid-base disturbances in blood. J Appl Physiol. 1965;20:443–52.PubMed

53.

Attwell D, Buchan AM, Charpak S, Lauritzen M, Macvicar BA, Newman EA. Glial and neuronal control of brain blood flow. Nature. 2010;468:232–43.PubMedPubMedCentralCrossRef

54.

Hamilton NB, Attwell D, Hall CN. Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease. Front Neuroenerget. 2010;2:5.CrossRef

55.

Hall CN, Reynell C, Gesslein B, Hamilton NB, Mishra A, Sutherland BA, O’Farrell FM, Buchan AM, Lauritzen M, Attwell D. Capillary pericytes regulate cerebral blood flow in health and disease. Nature. 2014;508:55–60.PubMedPubMedCentralCrossRef

56.

Madsen PL, Cruz NF, Sokoloff L, Dienel GA. Cerebral oxygen/glucose ratio is low during sensory stimulation and rises above normal during recovery: excess glucose consumption during stimulation is not accounted for by lactate efflux from or accumulation in brain tissue. J Cereb Blood Flow Metab. 1999;19:393–400.PubMedCrossRef

57.

Ball KK, Cruz NF, Mrak RE, Dienel GA. Trafficking of glucose, lactate, and amyloid-beta from the inferior colliculus through perivascular routes. J Cereb Blood Flow Metab. 2010;30:162–76.PubMedCrossRef

58.

Mintun MA, Lundstrom BN, Snyder AZ, Vlassenko AG, Shulman GL, Raichle ME. Blood flow and oxygen delivery to human brain during functional activity: theoretical modelling and experimental data. Proc Natl Acad Sci USA. 2001;98:6859–64.PubMedPubMedCentralCrossRef

59.

Purves MJ. The physiology of the cerebral circulation. Cambridge: Cambridge University Press; 1972.

60.

Lindauer U, Leithner C, Kaasch H, Rohrer B, Foddis M, Fuchtemeier M, Offenhauser N, Steinbrink J, Royl G, Kohl-Bareis M, Dirnagl U. Neurovascular coupling in rat brain operates independent of haemoglobin deoxygenation. J Cereb Blood Flow Metab. 2010;30:757–68.PubMedCrossRef

61.

Ainslie PN, Ogoh S. Regulation of cerebral blood flow in mammals during chronic hypoxia: a matter of balance. Exp Physiol. 2010;95:251–62.PubMedCrossRef

62.

Ainslie PN, Shaw AD, Smith KJ, Willie CK, Ikeda K, Graham J, Macleod DB. Stability of cerebral metabolism and substrate availability in humans during hypoxia and hyperoxia. Clin Sci. 2014;126:661–70.PubMedCrossRef

63.

Willie CK, Tzeng Y-C, Fisher JA, Ainslie PN. Integrative regulation of human brain blood flow. J Physiol (Lond). 2014;592:841–59.CrossRef

64.

Powers WJ, Hirsch IB, Cryer PE. Effect of stepped hypoglycemia on regional cerebral blood flow response to physiological brain activation. Am J Physiol. 1996;270:H554–9.PubMed

65.

Siesjö BK, Kjällquist A, Pontén U, Zwetnow N. Extracellular pH in the brain and cerebral blood flow. Prog Brain Res. 1968;30:93–8.PubMedCrossRef

66.

Wahl M, Deetjen P, Thurau K, Ingvar DH, Lassen NA. Micropuncture evaluation of importance of perivascular ph for arteriolar diameter on brain surface. Pflügers Arch. 1970;316:152–63.PubMedCrossRef

67.

Fencl V, Vale JR, Broch JA. Respiration and cerebral blood flow in metabolic acidosis and alkalosis in humans. J Appl Physiol. 1969;27:67–76.PubMed

68.

Ainslie PN, Duffin J. Integration of cerebrovascular CO₂ reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation. Am J Physiol. 2009;296:R1473–95.

69.

Curley G, Kavanagh BP, Laffey JG. Hypocapnia and the injured brain: more harm than benefit. Crit Care Med. 2010;38:1348–59.PubMedCrossRef

70.

Roy CS, Sherrington CS. On the regulation of the blood-supply of the brain. J Physiol (Lond). 1890;11:85–108.PubMedCentralCrossRef

71.

Iadecola C. Regulation of the cerebral microcirculation during neural activity: is nitric oxide the missing link? Trends Neurosci. 1993;16:206–14.PubMedCrossRef

72.

Silver IA. Cellular microenvironment in relation to local blood flow. In: Elliott KAC, Chichester OCM, editors. UK CIBA foundation symposium 56 cerebral vascular smooth muscle and its control. Hoboken: Wiley; 1978. p. 49–67.

73.

Leniger-Follert E. Mechanisms of regulation of cerebral microflow during bicuculline-induced seizures in anaesthetized cats. J Cereb Blood Flow Metab. 1984;4:150–65.PubMedCrossRef

74.

Ances BM. Coupling of changes in cerebral blood flow with neural activity: what must initially dip must come back up. J Cereb Blood Flow Metab. 2003;24:1–6.CrossRef

75.

Chesler M, Kaila K. Modulation of pH by neuronal activity. Trends Neurosci. 1992;15:396–402.PubMedCrossRef

76.

Astrup J, Heuser D, Lassen NA, Nilsson B, Karin N, Siesjö BK. Evidence against H⁺ and K⁺ as main factors for the control of cerebral blood flow: a microelectrode study. In: Elliott KAC, Chichester OCM, editors. CIBA foundation symposium 56 cerebral vascular smooth muscle and its control. New York: Wiley; 1978. p. 313–37.

77.

Makani S, Chesler M. Rapid rise of extracellular pH evoked by neural activity is generated by the plasma membrane calcium ATPase. J Neurophysiol. 2010;103:667–76.PubMedCrossRef

78.

Girouard H, Iadecola C. Neurovascular coupling in the normal brain and in hypertension, stroke, and Alzheimer disease. J Appl Physiol. 1985;2006(100):328–35.

79.

Takano T, Tian G-F, Peng W, Lou N, Libionka W, Han X, Nedergaard M. Astrocyte-mediated control of cerebral blood flow. Nat Neurosci. 2006;9:260–7.PubMedCrossRef

80.

Filosa JA, Bonev AD, Straub SV, Meredith AL, Wilkerson MK, Aldrich RW, Nelson MT. Local potassium signaling couples neuronal activity to vasodilation in the brain. Nat Neurosci. 2006;9:1397–403.PubMedCrossRef

81.

Gordon GRJ, Mulligan SJ, MacVicar BA. Astrocyte control of the cerebrovasculature. Glia. 2007;55:1214–21.PubMedCrossRef

82.

Metea MR, Kofuji P, Newman EA. Neurovascular coupling is not mediated by potassium siphoning from glial cells. J Neurosci. 2007;27:2468–71.PubMedPubMedCentralCrossRef

83.

Carmignoto G, Gomez-Gonzalo M. The contribution of astrocyte signalling to neurovascular coupling. Brain Res Rev. 2010;63:138–48.PubMedCrossRef

84.

Dunn KM, Nelson MT. Potassium channels and neurovascular coupling. Circ J. 2010;74:608–16.PubMedPubMedCentralCrossRef

85.

Girouard H, Bonev AD, Hannah RM, Meredith A, Aldrich RW, Nelson MT. Astrocytic endfoot Ca²⁺ and BK channels determine both arteriolar dilation and constriction. Proc Natl Acad Sci USA. 2010;107:3811–6.PubMedPubMedCentralCrossRef

86.

Filosa JA, Iddings JA. Astrocyte regulation of cerebral vascular tone. Am J Physiol. 2013;305:H609–19.CrossRef

87.

Newman EA. Functional hyperemia and mechanisms of neurovascular coupling in the retinal vasculature. J Cereb Blood Flow Metab. 2013;33:1685–95.PubMedPubMedCentralCrossRef

88.

Witthoft A, Filosa JA, Karniadakis GE. Potassium buffering in the neurovascular unit: models and sensitivity analysis. Biophys J. 2013;105:2046–54.PubMedPubMedCentralCrossRef

89.

Jespersen SN, Ostergaard L. The roles of cerebral blood flow, capillary transit time heterogeneity, and oxygen tension in brain oxygenation and metabolism. J Cereb Blood Flow Metab. 2012;32:264–77.PubMedCrossRef

90.

Rasmussen PM, Jespersen SN, Ostergaard L. The effects of transit time heterogeneity on brain oxygenation during rest and functional activation. J Cereb Blood Flow Metab. 2015;35:432–42.PubMedCrossRef

91.

Crone C. Facilitated transfer of glucose from blood into brain tissue. J Physiol (Lond). 1965;181:103–13.CrossRef

92.

Oldendorf WH. Brain uptake of radiolabeled amino acids, amines, and hexoses after arterial injection. Am J Physiol. 1971;221:1629–39.PubMed

93.

Betz AL, Gilboe DD, Yudilevich DL, Drewes LR. Kinetics of unidirectional glucose transport into the isolated dog brain. Am J Physiol. 1973;225:586–92.PubMed

94.

Lund-Andersen H. Transport of glucose from blood to brain. Physiol Rev. 1979;59:305–52.PubMed

95.

Farrell CL, Pardridge WM. Blood–brain barrier glucose transporter is asymmetrically distributed on brain capillary endothelial lumenal and ablumenal membranes: an electron microscopic immunogold study. Proc Natl Acad Sci USA. 1991;88:5779–83.PubMedPubMedCentralCrossRef

96.

Simpson IA, Vannucci SJ, DeJoseph MR, Hawkins RA. Glucose transporter asymmetries in the bovine blood–brain barrier. J Biol Chem. 2001;276:12725–9.PubMedCrossRef

97.

Simpson IA, Carruthers A, Vannucci SJ. Supply and demand in cerebral energy metabolism: the role of nutrient transporters. J Cereb Blood Flow Metab. 2007;27:1766–91.PubMedPubMedCentralCrossRef

98.

Kubo Y, Ohtsuki S, Uchida Y, Terasaki T. Quantitative determination of luminal and abluminal membrane distributions of transporters in porcine brain capillaries by plasma membrane fractionation and quantitative targeted proteomics. J Pharm Sci. 2015;104:3060–8.PubMedCrossRef

99.

Pardridge WM. Transport of insulin-related peptides and glucose across the blood–brain barrier. Ann NY Acad Sci. 1993;692:126–37.PubMedCrossRef

100.

Cutler RW, Sipe JC. Mediated transport of glucose between blood and brain in the cat. Am J Physiol. 1971;220:1182–6.PubMed

101.

Betz AL, Gilboe DD, Drewes LR. Effects of anoxia on net uptake and unidirectional transport of glucose into the isolated dog brain. Brain Res. 1974;67:307–16.PubMedCrossRef

102.

Barros LF, Bittner CX, Loaiza A, Porras OH. A quantitative overview of glucose dynamics in the gliovascular unit. Glia. 2007;55:1222–37.PubMedCrossRef

103.

Fellows LK, Boutelle MG, Fillenz M. Extracellular brain glucose levels reflect local neuronal activity: a microdialysis study in awake, freely moving rats. J Neurochem. 1992;59:2141–7.PubMedCrossRef

104.

Vannucci SJ, Clark RR, Koehler-Stec E, Li K, Smith CB, Davies P, Maher F, Simpson IA. Glucose transporter expression in brain: relationship to cerebral glucose utilization. Dev Neurosci. 1998;20:369–79.PubMedCrossRef

105.

Jurcovicova J. Glucose transport in brain—effect of inflammation. Endocr Regul. 2014;48:35–48.PubMedCrossRef

106.

Gandhi GK, Cruz NF, Ball KK, Theus SA, Dienel GA. Selective astrocytic gap junctional trafficking of molecules involved in the glycolytic pathway: impact on cellular brain imaging. J Neurochem. 2009;110:857–69.PubMedPubMedCentralCrossRef

107.

Mergenthaler P, Lindauer U, Dienel GA, Meisel A. Sugar for the brain: the role of glucose in physiological and pathological brain function. Trends Neurosci. 2013;36:587–97.PubMedPubMedCentralCrossRef

108.

Jakoby P, Schmidt E, Ruminot I, Gutierrez R, Barros LF, Deitmer JW. Higher transport and metabolism of glucose in astrocytes compared with neurons: a multiphoton study of hippocampal and cerebellar tissue slices. Cereb Cortex. 2014;24:222–31.PubMedCrossRef

109.

Cornford EM, Nguyen EV, Landaw EM. Acute upregulation of blood–brain barrier glucose transporter activity in seizures. Am J Physiol. 2000;279:H1346–54.

110.

Leybaert L, De Bock M, Van Moorhem M, Decrock E, De Vuyst E. Neurobarrier coupling in the brain: adjusting glucose entry with demand. J Neurosci Res. 2007;85:3213–20.PubMedCrossRef

111.

Paulson OB, Hasselbalch SG, Rostrup E, Knudsen GM, Pelligrino D. Cerebral blood flow response to functional activation. J Cereb Blood Flow Metab. 2010;30:2–14.PubMedCrossRef

112.

Betz AL, Gilboe DD, Drewes LR. The characteristics of glucose transport across the blood brain barrier and its relation to cerebral glucose metabolism. Adv Exp Med Biol. 1976;69:133–49.PubMedCrossRef

113.

Kuschinsky W, Paulson OB. Capillary circulation in the brain. Cerebrovasc Brain Metab Rev. 1992;4:261–86.PubMed

114.

Klein B, Kuschinsky W, Schrock H, Vetterlein F. Interdependency of local capillary density, blood flow, and metabolism in rat brains. Am J Physiol. 1986;251:H1333–40.PubMed

115.

Betz AL, Goldstein GW. Polarity of the blood–brain barrier: neutral amino acid transport into isolated brain capillaries. Science. 1978;202:225–7.PubMedCrossRef

116.

Yudilevich DL, De Rose N, Sepulveda FV. Facilitated transport of amino acids through the blood–brain barrier of the dog studied in a single capillary circulation. Brain Res. 1972;44:569–78.PubMedCrossRef

117.

Mann GE, Yudilevich DL, Sobrevia L. Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells. Physiol Rev. 2003;83:183–252.PubMedCrossRef

118.

Smith QR, Stoll J. Bood-brain barrier amino acid transport. In: Pardridge WM, editor. Introduction to the blood–brain barrier methodology, biology and pathology, vol. 1. Cambridge: Cambridge University Press; 1998. p. 188–97.CrossRef

119.

Sacks W, Sacks S, Brebbia DR, Fleischer A. Cerebral uptake of amino acids in human subjects and rhesus monkeys in vivo. J Neurosci Res. 1982;7:431–6.PubMedCrossRef

120.

Hawkins RA, O’Kane RL, Simpson IA, Viña JR. Structure of the blood–brain barrier and its role in the transport of amino acids. J Nutr. 2006;136:218S–26S.PubMed

121.

Benrabh H, Bourre JM, Lefauconnier JM. Taurine transport at the blood–brain barrier: an in vivo brain perfusion study. Brain Res. 1995;692:57–65.PubMedCrossRef

122.

Ennis SR, Kawai N, Ren XD, Abdelkarim GE, Keep RF. Glutamine uptake at the blood–brain barrier is mediated by N-system transport. J Neurochem. 1998;71:2565–73.PubMedCrossRef

123.

Bradbury MW, Davson H. The transport of urea, creatinine and certain monosaccharides between blood and fluid perfusing the cerebral ventricular system of rabbits. J Physiol (Lond). 1964;170:195–211.CrossRef

124.

Welch K, Sadler K, Hendee R. Cooperative phenomena in the permeation of sugars through the lining epithelium of choroid plexus. Brain Res. 1970;19:465–82.PubMedCrossRef

125.

Snodgrass SR, Cutler RW, Kang ES, Lorenzo AV. Transport of neutral amino acids from feline cerebrospinal fluid. Am J Physiol. 1969;217:974–80.PubMed

126.

Preston JE, Segal MB. The steady-state amino acid fluxes across the perfused choroid plexus of the sheep. Brain Res. 1990;525:275–9.PubMedCrossRef

127.

Harik SI, Kalaria RN, Andersson L, Lundahl P, Perry G. Immunocytochemical localization of the erythroid glucose transporter: abundance in tissues with barrier functions. J Neurosci. 1990;10:3862–72.PubMed

128.

Hacker HJ, Thorens B, Grobholz R. Expression of facilitative glucose transporter in rat liver and choroid plexus. A histochemical study in native cryostat sections. Histochemistry. 1991;96:435–9.PubMedCrossRef

129.

Farrell CL, Yang J, Pardridge WM. GLUT-1 glucose transporter is present within apical and basolateral membranes of brain epithelial interfaces and in microvascular endothelia with and without tight junctions. J Histochem Cytochem. 1992;40:193–9.PubMedCrossRef

130.

Gerhart DZ, Leino RL, Taylor WE, Borson ND, Drewes LR. Glut1 and glut3 gene-expression in gerbil brain following brief ischemia—an in situ hybridization study. Mol Brain Res. 1994;25:313–22.PubMedCrossRef

131.

Kumagai AK, Dwyer KJ, Pardridge WM. Differential glycosylation of the glut1 glucose-transporter in brain capillaries and choroid-plexus. Biochim Biophys Acta-Biomembr. 1994;1193:24–30.CrossRef

132.

Vannucci SJ, Maher F, Simpson IA. Glucose transporter proteins in brain: delivery of glucose to neurons and glia. Glia. 1997;21:2–21.PubMedCrossRef

133.

Masuzawa T, Sato F. The enzyme-histochemistry of the choroid-plexus. Brain. 1983;106:55–99.PubMedCrossRef

134.

Keep RF, Ennis SR, Xiang J. The blood-CSF barrier and cerebral ischemia. In: Zheng W, Chodobski A, editors. The blood–cerebrospinal fluid barrier. Boca Raton: Taylor & Francis; 2005. p. 345–60.

135.

Redzic ZB, Segal MB. The structure of the choroid plexus and the physiology of the choroid plexus epithelium. Adv Drug Deliv Rev. 2004;56:1695–716.PubMedCrossRef

136.

Zheng W, Chodobski A, editors. The blood–cerebrospinal fluid barrier. Boca Raton: Taylor & Francis; 2005.

137.

Spector R. Nutrient transport systems in brain: 40 years of progress. J Neurochem. 2009;111:315–20.PubMedCrossRef

138.

Redzic Z. Molecular biology of the blood–brainand the blood-cerebrospinal fluid barriers: similarities and differences. Fluids Barriers CNS. 2011;8:3.PubMedPubMedCentralCrossRef

139.

Liddelow SA. Development of the choroid plexus and blood-CSF barrier. Front Neurosci. 2015;9:32.PubMedPubMedCentralCrossRef

140.

de Rougemont J, Ames A 3rd, Nesbett FB, Hofmann HF. Fluid formed by choroid plexus; a technique for its collection and a comparison of its electrolyte composition with serum and cisternal fluids. J Neurophysiol. 1960;23:485–95.PubMed

141.

Ames A, Higashi K, Nesbett FB. Effects of pCO₂ acetazolamide and ouabain on volume and composition of choroid-plexus fluid. J Physiol (Lond). 1965;181:516–24.CrossRef

142.

Miner LC, Reed DJ. Composition of fluid obtained from choroid plexus tissue isolated in a chamber in situ. J Physiol (Lond). 1972;227:127–39.CrossRef

143.

Nilsson C, Stahlberg F, Gideon P, Thomsen C, Henriksen O. The nocturnal increase in human cerebrospinal fluid production is inhibited by a beta 1-receptor antagonist. Am J Physiol. 1994;267:R1445–14448.PubMed

144.

Vates TSJ, Bonting SL, Oppelt WW. Na-K activated adenosine triphosphatase formation of cerebrospinal fluid in cat. Am J Physiol. 1964;206:1165–72.PubMed

145.

Keep RF, Smith DE. Choroid plexus transport: gene deletion studies. Fluids Barriers CNS. 2011;8:26.PubMedPubMedCentralCrossRef

146.

Hakvoort A, Haselbach M, Wegener J, Hoheisel D, Galla HJ. The polarity of choroid plexus epithelial cells in vitro is improved in serum-free medium. J Neurochem. 1998;71:1141–50.PubMedCrossRef

147.

Angelow S, Wegener J, Galla H-J. Transport and permeability characteristics of the blood-cerebrospinal fluid barrier in vitro. In: Sharma HS, Westman J, editors. Blood–spinal cord and brain barriers in health and disease. Amsterdam: Elsevier Inc.; 2004. p. 33–45.CrossRef

148.

Welch K. Secretion of cerebrospinal fluid by choroid plexus of the rabbit. Am J Physiol. 1963;205:617–24.PubMed

149.

Windhager EE, Whittembury G, Oken DE, Schatzmann HJ, Solomon AK. Single proximal tubules of the Necturus kidney. III. Dependence of H2O movement on NaCl concentration. Am J Physiol. 1959;197:313–8.PubMed

150.

Giebisch G, Klose RM, Malnic G, Sullivan WJ, Windhager EE. Sodium movement across single perfused proximal tubules of rat kidneys. J Gen Physiol. 1964;47:1175–94.PubMedPubMedCentralCrossRef

151.

Spring KR. Fluid transport by gallbladder epithelium. J Exp Biol. 1983;106:181–94.PubMed

152.

Oshio K, Watanabe H, Song Y, Verkman AS, Manley GT. Reduced cerebrospinal fluid production and intracranial pressure in mice lacking choroid plexus water channel Aquaporin-1. FASEB J. 2005;19(1):76–8.PubMed

153.

Johansson PA, Dziegielewska KM, Ek CJ, Habgood MD, Mollgard K, Potter A, Schuliga M, Saunders NR. Aquaporin-1 in the choroid plexuses of developing mammalian brain. Cell Tissue Res. 2005;322:353–64.PubMedCrossRef

154.

Fischbarg J, Kuang KY, Hirsch J, Lecuona S, Rogozinski L, Silverstein SC, Loike J. Evidence that the glucose transporter serves as a water channel in J774 macrophages. Proc Natl Acad Sci USA. 1989;86:8397–401.PubMedPubMedCentralCrossRef

155.

Fischbarg J, Kuang KY, Vera JC, Arant S, Silverstein SC, Loike J, Rosen OM. Glucose transporters serve as water channels. Proc Natl Acad Sci USA. 1990;87:3244–7.PubMedPubMedCentralCrossRef

156.

Zeuthen T. Molecular water pumps. Rev Physiol Biochem Pharmacol. 2000;141:97–151.PubMedCrossRef

157.

MacAulay N, Zeuthen T. Water transport between CNS compartments: contributions of aquaporins and cotransporters. Neuroscience. 2010;168:941–56.PubMedCrossRef

158.

Vannucci SJ. Developmental expression of GLUT1 and GLUT3 glucose transporters in rat brain. J Neurochem. 1994;62:240–6.PubMedCrossRef

159.

Zeuthen T, MacAulay N. Cotransporters as molecular water pumps. Int Rev Cytol. 2002;215:259–84.PubMedCrossRef

160.

Naftalin RJ. Osmotic water transport with glucose in GLUT2 and SGLT. Biophys J. 2008;94:3912–23.PubMedPubMedCentralCrossRef

161.

Zeuthen T, MacAulay N. Cotransport of water by Na⁺–K⁺–2Cl⁻ cotransporters expressed in Xenopus oocytes: NKCC1 versus NKCC2. J Physiol (Lond). 2012;590:1139–54.CrossRef

162.

Lapointe JY. Response to Zeuthen and Zeuthen’s comment to the editor: enough local hypertonicity is enough. Biophys J. 2007;93:1417–9.PubMedCentralCrossRef

163.

Sasseville LJ, Cuervo JE, Lapointe JY, Noskov SY. The structural pathway for water permeation through sodium-glucose cotransporters. Biophys J. 2011;101:1887–95.PubMedPubMedCentralCrossRef

164.

Yu ASL, Cheng MH, Angelow S, Gunzel D, Kanzawa SA, Schneeberger EE, Fromm M, Coalson RD. Molecular basis for cation selectivity in claudin-2-based paracellular pores: identification of an electrostatic interaction site. J Gen Physiol. 2009;133:111–27.PubMedPubMedCentralCrossRef

165.

Krug SM, Gunzel D, Conrad MP, Lee IFM, Amasheh S, Fromm M, Yu ASL. Charge-selective claudin channels. Ann NY Acad Sci. 2012;1257:20–8.PubMedCrossRef

166.

Rosenthal R, Milatz S, Krug SM, Oelrich B, Schulzke J-D, Amasheh S, Gunzel D, Fromm M. Claudin-2, a component of the tight junction, forms a paracellular water channel. J Cell Sci. 2010;123:1913–21.PubMedCrossRef

167.

Liddelow SA, Dziegielewska KM, Ek CJ, Habgood MD, Bauer H, Bauer H-C, Lindsay H, Wakefield MJ, Strazielle N, Kratzer I, et al. Correction: mechanisms that determine the internal environment of the developing brain: a transcriptomic. Functional and ultrastructural approach. PLoS One. 2016;11:e0147680.PubMedPubMedCentralCrossRef

168.

Christensen HL, Nguyen AT, Pedersen FD, Damkier HH. Na(+) dependent acid-base transporters in the choroid plexus; insights from slc4 and slc9 gene deletion studies. Front Physiol. 2013;4:304.PubMedPubMedCentral

169.

Brown PD, Davies SL, Millar ID. Ion transport in choroid plexus. In: Alvarez-Leefmans FJ, Delpire E, editors. Physiology and pathology of chloride transporters and channels in the nervous system: from molecules to diseases. Amsterdam: Elsevier Science; 2009. p. 569–83.

170.

Smith QR, Woodbury DM, Johanson CE. Uptake of ³⁶Cl and ²²Na by the choroid plexus-cerebrospinal fluid system: evidence for active chloride transport by the choroidal epithelium. J Neurochem. 1981;37:107–16.PubMedCrossRef

171.

Johanson CE, Murphy VA. Acetazolamide and insulin alter choroid-plexus epithelial-cell [Na⁺], pH, and volume. Am J Physiol. 1990;258:F1538–46.PubMed

172.

Pollay M, Hisey B, Reynolds E, Tomkins P, Stevens A, Smith R. Choroid-plexus Na⁺/K⁺-activated adenosine-triphosphatase and cerebrospinal-fluid formation. Neurosurgery. 1985;17:768–72.PubMedCrossRef

173.

Davson H, Segal MB. The effects of some inhibitors and accelerators of sodium transport on the turnover of ²²Na in the cerebrospinal fluid and the brain. J Physiol (Lond). 1970;209:131–53.CrossRef

174.

Murphy VA, Johanson CE. Alteration of sodium-transport by the choroid-plexus with amiloride. Biochim Biophys Acta. 1989;979:187–92.PubMedCrossRef

175.

Amin MS, Wang HW, Reza E, Whitman SC, Tuana BS, Leenen FHH. Distribution of epithelial sodium channels and mineralocorticoid receptors in cardiovascular regulatory centers in rat brain. Am J Physiol. 2005;289:R1787–97.

176.

Amin MS, Reza E, Wang H, Leenen FHH. Sodium transport in the choroid plexus and salt-sensitive hypertension. Hypertension. 2009;54:860–7.PubMedCrossRef

177.

Masilamani S, Kim GH, Mitchell C, Wade JB, Knepper MA. Aldosterone-mediated regulation of ENaC alpha, beta, and gamma subunit proteins in rat kidney. J Clin Invest. 1999;104:R19–23.PubMedPubMedCentralCrossRef

178.

Wang H-W, Amin MS, El-Shahat E, Huang BS, Tuana BS, Leenen FHH. Effects of central sodium on epithelial sodium channels in rat brain. Am J Physiol. 2010;299:R222–33.

179.

Millar ID, Brown PD. NBCe2 exhibits a 3 HCO₃ ⁻: 1 Na⁺ stoichiometry in mouse choroid plexus epithelial cells. Biochem Biophys Res Commun. 2008;373:550–4.PubMedCrossRef

180.

Wright EM. Transport processes in the formation of the cerebrospinal fluid. Rev Physiol Biochem Pharmacol. 1978;83:3–34.PubMed

181.

Damkier HH, Aalkjaer C, Praetorius J. Na⁺-dependent HCO₃ ⁻ import by the slc4a10 gene product involves Cl⁻ export. J Biol Chem. 2010;285:26998–7007.PubMedPubMedCentralCrossRef

182.

Parker MD, Musa-Aziz R, Rojas JD, Choi I, Daly CM, Boron WF. Characterization of human SLC4A10 as an electroneutral Na/HCO₃ cotransporter (NBCn2) with Cl⁻ self-exchange activity. J Biol Chem. 2008;283:12777–88.PubMedPubMedCentralCrossRef

183.

Ames A, Sakanoue M, Endo S. Na, K, Ca, Mg, and Cl concentrations in choroid plexus fluid and cisternal fluid compared with plasma ultrafiltrate. J Neurophysiol. 1964;27:672–81.PubMed

184.

Vogh BP, Maren TH. Sodium, chloride, and bicarbonate movement from plasma to cerebrospinal-fluid in cats. Am J Physiol. 1975;228:673–83.PubMed

185.

Fencl V. Acid-base balance in cerebral fluids—section 3: the respiratory system, volume II control of breathing. In: Fishman AP, Cherniack NS, Widdicombe JG, editors. Handbook of physiology. Bethesda: American Physiological Society; 1986. p. 115–40.

186.

Johanson CE, Sweeney SM, Parmelee JT, Epstein MH. Cotransport of sodium and chloride by the adult mammalian choroid-plexus. Am J Physiol. 1990;258:C211–6.PubMed

187.

Smith QR, Johanson CE. Chloride efflux from isolated choroid-plexus. Brain Res. 1991;562:306–10.PubMedCrossRef

188.

Keep RF, Xiang J, Betz AL. Potassium cotransport at the rat choroid plexus. Am J Physiol. 1994;267:C1616–22.PubMed

189.

Abbott J, Davson H, Glen I, Grant N. Chloride transport and potential across the blood-CSF barrier. Brain Res. 1971;29:185–93.PubMedCrossRef

190.

Ridderstrale Y, Wistrand PJ, Holm L, Carter ND. Use of carbonic anhydrase II-deficient mice in uncovering the cellular location of membrane-associated isoforms. In: Chegwidden WR, Carter NC, Edwards VH, editors. In the carbonic anhydrases new horizons. Basel: Birkhäuser Verlag; 2000. p. 143–55.CrossRef

191.

Ridderstrale Y, Wistrand PJ. Carbonic anhydrase isoforms in the mammalian nervous system. In: Kaila K, Ransom BR, editors. pH and brain function. New York: Wiley-Liss; 1998. p. 21–43.

192.

Ridderstrale Y, Wistrand PJ. Membrane-associated carbonic anhydrase activity in the brain of CA II-deficient mice. J Neurocytol. 2000;29:263–9.PubMedCrossRef

193.

Kallio H, Pastorekova S, Pastorek J, Waheed A, Sly WS, Mannisto S, Heikinheimo M, Parkkila S. Expression of carbonic anhydrases IX and XII during mouse embryonic development. BMC Dev Biol. 2006;6:22.PubMedPubMedCentralCrossRef

194.

McMurtrie HL, Cleary HJ, Alvarez BV, Loiselle FB, Sterling D, Morgan PE, Johnson DE, Casey JR. The bicarbonate transport metabolon. J Enzym Inhib Med Chem. 2004;19:231–6.CrossRef

195.

Boron WF. Evaluating the role of carbonic anhydrases in the transport of HCO₃–related species. Biochim Biophys Acta. 2010;1804:410–21.PubMedCrossRef

196.

Wu Q, Delpire E, Hebert SC, Strange K. Functional demonstration of Na⁺ -K⁺ -2Cl(−) cotransporter activity in isolated, polarized choroid plexus cells. Am J Physiol. 1998;275:C1565–72.PubMed

197.

Crum JM, Alvarez FJ, Alvarez-Leefmans FJ. The apical NKCC1 cotransporter debate. FASEB J. 2012;26(881):814.

198.

Plotkin MD, Kaplan MR, Peterson LN, Gullans SR, Hebert SC, Delpire E. Expression of the Na⁺ -K⁺ -2Cl(−) cotransporter BSC2 in the nervous system. Am J Physiol. 1997;41:C173–83.

199.

Crum JM, Alvarez-Leefmans FJ. Choroid plexus epithelial cells use NKCC1 cotransporters as sensors and regulators of cerebrospinal fluid potassium levels http://www.posterhall.org/igert2011/posters/164.

200.

Husted RF, Reed DJ. Regulation of cerebrospinal-fluid potassium by cat choroid-plexus. J Physiol (Lond). 1976;259:213–21.CrossRef

201.

Rosenberg GA, Kyner WT. Gray and white matter brain-blood transfer constants by steady-state tissue clearance in cat. Brain Res. 1980;193:59–66.PubMedCrossRef

202.

Szentistvanyi I, Patlak CS, Ellis RA, Cserr HF. Drainage of interstitial fluid from different regions of rat brain. Am J Physiol. 1984;246:F835–44.PubMed

203.

Groothuis DR, Vavra MW, Schlageter KE, Kang EW-Y, Itskovich AC, Hertzler S, Allen CV, Lipton HL. Efflux of drugs and solutes from brain: the interactive roles of diffusional transcapillary transport, bulk flow and capillary transporters. J Cereb Blood Flow Metab. 2007;27:43–56.PubMedCrossRef

204.

Abbott NJ. Evidence for bulk flow of brain interstitial fluid: significance for physiology and pathology. Neurochem Int. 2004;45:545–52.PubMedCrossRef

205.

Rennels ML, Gregory TF, Blaumanis OR, Fujimoto K, Grady PA. Evidence for a paravascular fluid circulation in the mammalian central nervous system, provided by the rapid distribution of tracer protein throughout the brain from the subarachnoid space. Brain Res. 1985;326:47–63.PubMedCrossRef

206.

Iliff JJ, Wang M, Liao Y, Plogg BA, Peng W, Gundersen GA, Benveniste H, Vates GE, Deane R, Goldman SA, et al. A paravascular pathway facilitates CSF flow through the brain parenchyma and the clearance of interstitial solutes, including amyloid beta. Sci Transl Med. 2012;4:147.CrossRef

207.

Arbel-Ornath M, Hudry E, Eikermann-Haerter K, Hou S, Gregory JL, Zhao LZ, Betensky RA, Frosch MP, Greenberg SM, Bacskai BJ. Interstitial fluid drainage is impaired in ischemic stroke and Alzheimer’s disease mouse models. Acta Neuropathol. 2013;126:353–64.PubMedCrossRef

208.

Smith AJ, Jin B-J, Verkman AS. Muddying the water in brain edema? Trends Neurosci. 2015;38:331–2.PubMedPubMedCentralCrossRef

209.

Bedussi B, van Lier MGJTB, Bartstra JW, de Vos J, Siebes M, VanBavel E, Bakker ENTP. Clearance from the mouse brain by convection of interstitial fluid towards the ventricular system. Fluids Barriers CNS. 2015;12:23.PubMedPubMedCentralCrossRef

210.

Bakker ENTP, Bacskai BJ, Arbel-Ornath M, Aldea R, Bedussi B, Morris AWJ, Weller RO, Carare RO. Lymphatic clearance of the brain: perivascular, paravascular and significance for neurodegenerative diseases. Cell Mol Neurobiol. 2016;36:181–94.PubMedPubMedCentralCrossRef

211.

Bedussi B, van der Wel NN, de Vos J, van Veen H, Siebes M, VanBavel E, Bakker EN. Paravascular channels, cisterns, and the subarachnoid space in the rat brain: a single compartment with preferential pathways. J Cereb Blood Flow Metab. 2016. doi: 10.1177/0271678x16655550.PubMed

212.

Pollay M, Curl F. Secretion of cerebrospinal fluid by the ventricular ependyma of the rabbit. Am J Physiol. 1967;213:1031–8.PubMed

213.

Curl FD, Pollay M. Transport of water and electrolytes between brain and ventricular fluid in the rabbit. Exp Neurol. 1968;20:558–74.PubMedCrossRef

214.

Milhorat TH, Hammock MK, Fenstermacher JD, Rall DP, Levin VA. Cerebrospinal fluid production by the choroid plexus and brain. Science. 1971;173:330–2.PubMedCrossRef

215.

Milhorat TH. Failure of choroid plexectomy as treatment for hydrocephalus. Surg Gynecol Obstet. 1974;139:505–8.PubMed

216.

Milhorat TH. Cerebrospinal fluid and the brain edemas. N.Y.: Neuroscience Society of New York; 1987.

217.

Milhorat TH. Third circulation revisited. J Neurosurg. 1975;42:628–45.PubMedCrossRef

218.

Rekate HL. Recent advances in the understanding and treatment of hydrocephalus. Semin Pediatr Neurol. 1997;4:167–78.PubMedCrossRef

219.

Zhu XL, Di Rocco C. Choroid plexus coagulation for hydrocephalus not due to CSF overproduction: a review. Child’s Nerv Syst. 2013;29:35–42.CrossRef

220.

Bering EA Jr, Sato O. Hydrocephalus: changes in formation and absorption of cerebrospinal fluid within the cerebral ventricles. J Neurosurg. 1963;20:1050–63.PubMedCrossRef

221.

Milhorat TH, Clark RG, Hammock MK. Experimental hydrocephalus. 2. Gross pathological findings in acute and subacute obstructive hydrocephalus in the dog and monkey. J Neurosurg. 1970;32:390–9.PubMedCrossRef

222.

Ghersi-Egea JF, Finnegan W, Chen JL, Fenstermacher JD. Rapid distribution of intraventricularly administered sucrose into cerebrospinal fluid cisterns via subarachnoid velae in rat. Neuroscience. 1996;75:1271–88.PubMedCrossRef

223.

Eisenberg HM, McLennan JE, Welch K. Ventricular perfusion in cats with kaolin-induced hydrocephalus. J Neurosurg. 1974;41:20–8.PubMedCrossRef

224.

Voelz K, Kondziella D, von Rautenfeld DB, Brinker T, Lüdemann W. A ferritin tracer study of compensatory spinal CSF outflow pathways in kaolin-induced hydrocephalus. Acta Neuropathol. 2007;113:569–75.PubMedCrossRef

225.

Milhorat TH. Hydrocephalus and the cerebrospinal fluid. Baltimore: Williams & Wilkins; 1972.

226.

James AE Jr, Strecker EP, Sperber E, Flor WJ, Merz T, Burns B. An alternative pathway of cerebrospinal fluid absorption in communicating hydrocephalus. Transependymal movement. Radiology. 1974;111:143–6.PubMedCrossRef

227.

James AE, Flor WJ, Novak GR, Strecker EP, Burns B, Epstein M. Experimental hydrocephalus. Exp Eye Res. 1977;25(Suppl):435–59.PubMedCrossRef

228.

Kim DS, Choi JU, Huh R, Yun PH, Kim DI. Quantitative assessment of cerebrospinal fluid hydrodynamics using a phase-contrast cine MR image in hydrocephalus. Child’s Nerv Syst. 1999;15:461–7.CrossRef

229.

Baledent O, Gondry-Jouet C, Meyer M-E, De Marco G, Le Gars D, Henry-Feugeas M-C, Idy-Peretti I. Relationship between cerebrospinal fluid and blood dynamics in healthy volunteers and patients with communicating hydrocephalus. Invest Radiol. 2004;39:45–55.PubMedCrossRef

230.

Bateman GA, Brown KM. The measurement of CSF flow through the aqueduct in normal and hydrocephalic children: from where does it come, to where does it go? Child’s Nerv Syst. 2012;28:55–63.CrossRef

231.

Baledent O. Imaging of the cerebrospinal fluid circulation. In: Rigamonti D, editor. Adult hydrocephalus. Cambridge: Cambridge University Press; 2014. p. 121–38.CrossRef

232.

Coben LA, Smith KR. Iodide transfer at four cerebrospinal fluid sites in the dog: evidence for spinal iodide carrier transport. Exp Neurol. 1969;23:76–90.PubMedCrossRef

233.

Hammerstad JP, Lorenzo AV, Cutler RW. Iodide transport from the spinal subarachnoid fluid in the cat. Am J Physiol. 1969;216:353–8.PubMed

234.

Lorenzo AV, Hammerstad JP, Cutler RW. Cerebrospinal fluid formation and absorption and transport of iodide and sulfate from the spinal subarachnoid space. J Neurol Sci. 1970;10:247–58.PubMedCrossRef

235.

Lux WE Jr, Fenstermacher JD. Cerebrospinal fluid formation in ventricles and spinal subarachnoid space of the rhesus monkey. J Neurosurg. 1975;42:674–8.PubMedCrossRef

236.

Welch K. The principles of physiology of the cerebrospinal fluid in relation to hydrocephalus including normal pressure hydrocephalus. Adv Neurol. 1975;13:247–332.PubMed

237.

O’Donnell ME, Tran L, Lam TI, Liu XB, Anderson SE. Bumetanide inhibition of the blood–brain barrier Na-K-Cl cotransporter reduces edema formation in the rat middle cerebral artery occlusion model of stroke. J Cereb Blood Flow Metab. 2004;24:1046–56.PubMedCrossRef

238.

O’Donnell ME, Chen Y-J, Lam TI, Taylor KC, Walton JH, Anderson SE. Intravenous HOE-642 reduces brain edema and Na uptake in the rat permanent middle cerebral artery occlusion model of stroke: evidence for participation of the blood–brain barrier Na/H exchanger. J Cereb Blood Flow Metab. 2013;33:225–34.PubMedCrossRef

239.

O’Donnell ME. Blood–brain barrier Na transporters in ischemic stroke. Adv Pharmacol. 2014;71:113–46.PubMedCrossRef

240.

Chen Y-J, Wallace BK, Yuen N, Jenkins DP, Wulff H, O’Donnell ME. Blood–brain barrier Kca3.1 channels: evidence for a role in brain Na uptake and edema in ischemic stroke. Stroke. 2015;46:237–44.PubMedCrossRef

241.

Mokgokong R, Wang S, Taylor CJ, Barrand MA, Hladky SB. Ion transporters in brain endothelial cells that contribute to formation of brain interstitial fluid. Pflügers Arch. 2014;466:887–901.PubMedCrossRef

242.

Welch K, Sadler K, Gold G. Volume flow across choroidal ependyma of the rabbit. Am J Physiol. 1966;210:232–6.PubMed

243.

Oldendorf WH, Cornford ME, Brown WJ. Large apparent work capability of blood–brain-barrier—study of mitochondrial content of capillary endothelial cells in brain and other tissues of rat. Ann Neurol. 1977;1:409–17.PubMedCrossRef

244.

Bito LZ, Davson H. Local variations in cerebrospinal fluid composition and its relationship to the composition of the extracellular fluid of the cortex. Exp Neurol. 1966;14:264–80.PubMedCrossRef

245.

Hansen AJ. Extracellular potassium concentration in juvenile and adult rat brain cortex during anoxia. Acta Physiol Scand. 1977;99:412–20.PubMedCrossRef

246.

Hansen AJ. Effect of anoxia on ion distribution in the brain. Physiol Rev. 1985;65:101–48.PubMed

247.

Jones HC, Keep RF. The control of potassium concentration in the cerebrospinal-fluid and brain interstitial fluid of developing rats. J Physiol (Lond). 1987;383:441–53.CrossRef

248.

Bradbury MW, Davson H. The transport of potassium between blood, cerebrospinal fluid and brain. J Physiol (Lond). 1965;181:151–74.CrossRef

249.

Greenberg DM, Aird RB, Boelter MDD, Campbell WW, Cohn WE, Murayama MM. A study with radioactive isotopes of the permeability of the blood–cerebrospinal fluid barrier to ions. Am J Physiol. 1943;140:47–64.

250.

Katzman R, Leiderman P. Brain potassium exchange in normal adult and immature rats. Am J Physiol. 1953;175:263–70.PubMed

251.

Cserr H. Potassium exchange between cerebrospinal fluid, plasma, and brain. Am J Physiol. 1965;209:1219–26.PubMed

252.

Katzman R, Graziani L, Kaplan R, Escriva A. Exchange of cerebrospinal fluid potassium with blood and brain. Study in normal and ouabain perfused cats. Arch Neurol. 1965;13:513–24.PubMedCrossRef

253.

Bradbury MW, Kleeman CR. Stability of potassium content of cerebrospinal fluid and brain. Am J Physiol. 1967;213:519–28.PubMed

254.

Bradbury MW, Stulcova B. Efflux mechanism contributing to the stability of the potassium concentration in cerebrospinal fluid. J Physiol (Lond). 1970;208:415–30.PubMedCentralCrossRef

255.

Sachs JR, Welt LG. The concentration dependence of active potassium transport in the human red blood cell. J Clin Invest. 1967;46:65–76.PubMedPubMedCentralCrossRef

256.

Post RL, Merritt CR, Kinsolving CR, Albright CD. Membrane adenosine triphosphatase as a participant in the active transport of sodium and potassium in the human erythrocyte. J Biol Chem. 1960;235:1796–802.PubMed

257.

Sachs JR. Competitive effects of some cations on active potassium transport in the human red blood cell. J Clin Invest. 1967;46:1433–41.PubMedPubMedCentralCrossRef

258.

Schielke GP, Moises HC, Betz AL. Potassium activation of the Na, K-pump in isolated brain microvessels and synaptosomes. Brain Res. 1990;524:291–6.PubMedCrossRef

259.

Bradbury MW, Segal MB, Wilson J. Transport of potassium at the blood–brain barrier. J Physiol (Lond). 1972;221:617–32.CrossRef

260.

Keep RF, Ennis SR, Beer ME, Betz AL. Developmental changes in blood–brain barrier potassium permeability in the rat: relation to brain growth. J Physiol (Lond). 1995;488:439–48.CrossRef

261.

Smith QR, Rapoport SI. Cerebrovascular permeability coefficients to sodium, potassium, and chloride. J Neurochem. 1986;46:1732–42.PubMedCrossRef

262.

Ennis SR, Keep RF, Ren XD, Betz AL. 376. Potassium transport at the luminal membrane of the blood–brain barrier. J Cereb Blood Flow Metab. 1997;17(Suppl. 1):S515.

263.

Quinton PM, Wright EM, Tormey JM. Localization of sodium pumps in the choroid plexus epithelium. J Cell Biol. 1973;58:724–30.PubMedPubMedCentralCrossRef

264.

Davson H, Welch K. The permeation of several materials into the fluids of the rabbit’s brain. J Physiol (Lond). 1971;218:337–51.CrossRef

265.

Murphy VA, Johanson CE. Acidosis, acetazolamide, and amiloride—effects on Na-22 transfer across the blood–brainand blood-CSF barriers. J Neurochem. 1989;52:1058–63.PubMedCrossRef

266.

Betz AL. Sodium transport from blood to brain: inhibition by furosemide and amiloride. J Neurochem. 1983;41:1158–64.PubMedCrossRef

267.

Ennis SR, Ren X-D, Betz AL. Mechanisms of sodium transport at the blood–brain barrier studied with in situ perfusion of rat brain. J Neurochem. 1996;66:756–63.PubMedCrossRef

268.

Nicola PA, Taylor CJ, Wang S, Barrand MA, Hladky SB. Transport activities involved in intracellular pH recovery following acid and alkali challenges in rat brain microvascular endothelial cells. Pflügers Arch. 2008;456:801–12.PubMedCrossRef

269.

Smith QR, Rapoport SI. Carrier-mediated transport of chloride across the blood–brain barrier. J Neurochem. 1984;42:754–63.PubMedCrossRef

270.

Crone C. The blood–brain barrier as a tight epithelium: where is information lacking? Ann NY Acad Sci. 1986;481:174–85.PubMedCrossRef

271.

Crone C, Olesen SP. Electrical-resistance of brain microvascular endothelium. Brain Res. 1982;241:49–55.PubMedCrossRef

272.

Butt AM, Jones HC, Abbott NJ. Electrical resistance across the blood–brain barrier in anaesthetized rats: a developmental study. J Physiol (Lond). 1990;429:47–62.PubMedCentralCrossRef

273.

Gunzel D, Yu ASL. Claudins and the modulation of tight junction permeability. Physiol Rev. 2013;93:525–69.PubMedPubMedCentralCrossRef

274.

Bauer H-C, Krizbai IA, Bauer H, Traweger A. “You Shall Not Pass”-tight junctions of the blood brain barrier. Front Neurosci. 2014;8:392.PubMedPubMedCentralCrossRef

275.

Haseloff RF, Dithmer S, Winkler L, Wolburg H, Blasig IE. Transmembrane proteins of the tight junctions at the blood–brain barrier: structural and functional aspects. Semin Cell Dev Biol. 2015;38:16–25.PubMedCrossRef

276.

Tietz S, Engelhardt B. Brain barriers: crosstalk between complex tight junctions and adherens junctions. J Cell Biol. 2015;209:493–506.PubMedPubMedCentralCrossRef

277.

Frelin C, Barbry P, Vigne P, Chassande O, Cragoe EJ, Lazdunski M. Amiloride and its analogs as tools to inhibit Na⁺ transport via the Na⁺ channel, the Na⁺/H⁺ antiport and the Na⁺/Ca²⁺ exchanger. Biochimie. 1988;70:1285–90.PubMedCrossRef

278.

Kleyman TR, Cragoe EJ Jr. Amiloride and its analogs as tools in the study of ion transport. J Membr Biol. 1988;105:1–21.PubMedCrossRef

279.

Balaban RS, Mandel LJ, Benos DJ. On the cross-reactivity of amiloride and 2,4,6 triaminopyrimidine (TAP) for the cellular entry and tight junctional cation permeation pathways in epithelia. J Membr Biol. 1979;49:363–90.PubMedCrossRef

280.

Kottra G, Fromter E. Functional-properties of the paracellular pathway in some leaky epithelia. J Exp Biol. 1983;106:217–29.PubMed

281.

Martens H, Gabel G, Strozyk B. Mechanism of electrically silent na and cl transport across the rumen epithelium of sheep. Exp Physiol. 1991;76:103–14.PubMedCrossRef

282.

Cremaschi D, Meyer G, Rossetti C, Botta G, Palestini P. The nature of the neutral Na⁺-Cl⁻ coupled entry at the apical membrane of rabbit gallbladder epithelium.1. Na⁺/H⁺, Cl⁻/HCO₃ ⁻ double exchange and Na⁺-Cl⁻ symport. J Membr Biol. 1987;95:209–18.PubMedCrossRef

283.

Weinstein SW, Jones SM, Weinstein RJ. Evidence that alteration of charge modifies proximal tubular shunt pathway permselectivity. Am J Physiol. 1989;257:F1079–86.PubMed

284.

Reuss L. Ion-transport across gallbladder epithelium. Physiol Rev. 1989;69:503–45.PubMed

285.

Revest PA, Jones HC, Abbott NJ. Transendothelial electrical potential across pial vessels in anaesthetised rats: a study of ion permeability and transport at the blood–brain barrier. Brain Res. 1994;652:76–82.PubMedCrossRef

286.

Held D, Fencl V, Pappenheimer JR. Electrical potential of cerebrospinal fluid. J Neurophysiol. 1964;27:942–59.PubMed

287.

Fenstermacher JD, Johnson JA. Filtration and reflection coefficients of the rabbit blood–brain barrier. Am J Physiol. 1966;211:341–6.PubMed

288.

Fenstermacher JD. Volume regulation of the central nervous system. In: Staub NC, Taylor AE, editors. Edema. New York: Raven; 1984. p. 383–404.

289.

Paulson OB, Hertz MM, Bolwig TG, Lassen NA. Filtration and diffusion of water across blood–brain-barrier in man. Microvasc Res. 1977;13:113–23.PubMedCrossRef

290.

Fettiplace R, Haydon DA. Water permeability of lipid membranes. Physiol Rev. 1980;60:510–50.PubMed

291.

Dolman D, Drndarski S, Abbott NJ, Rattray M. Induction of aquaporin 1 but not aquaporin 4 messenger RNA in rat primary brain microvessel endothelial cells in culture. J Neurochem. 2005;93:825–33.PubMedCrossRef

292.

Enerson BE, Drewes LR. The rat blood–brain barrier transcriptome. J Cereb Blood Flow Metab. 2006;26:959–73.PubMedCrossRef

293.

Daneman R, Zhou L, Agalliu D, Cahoy JD, Kaushal A, Barres BA. The mouse blood–brain barrier transcriptome: a new resource for understanding the development and function of brain endothelial cells. PLoS One. 2010;5:e13741.PubMedPubMedCentralCrossRef

294.

Eisenberg HM, Suddith RL. Cerebral vessels have the capacity to transport sodium and potassium. Science. 1979;206:1083–5.PubMedCrossRef

295.

Betz AL, Firth JA, Goldstein GW. Polarity of the blood–brain barrier: distribution of enzymes between the luminal and antiluminal membranes of brain capillary endothelial cells. Brain Res. 1980;192:17–28.PubMedCrossRef

296.

Sanchez del Pino MM, Hawkins RA, Peterson DR. Biochemical discrimination between luminal and abluminal enzyme and transport activities of the blood–brain-barrier. J Biol Chem. 1995;270:14907–12.PubMedCrossRef

297.

Goldstein GW. Relation of potassium transport to oxidative metabolism in isolated brain capillaries. J Physiol (Lond). 1979;286:185–95.CrossRef

298.

Lin JD. Potassium transport in isolated cerebral microvessels from the rat. Jpn J Physiol. 1985;35:817–30.PubMedCrossRef

299.

Spatz M, Kawai N, Merkel N, Bembry J, McCarron RM. Functional properties of cultured endothelial cells derived from large microvessels of human brain. Am J Physiol. 1997;272:C231–9.PubMed

300.

Eisenberg HM, Suddith RL, Crawford JS. Transport of sodium and potassium across the blood–brain barrier. Adv Exp Med Biol. 1980;131:57–67.PubMedCrossRef

301.

Harik SI, Doull GH, Dick APK. Specific ouabain binding to brain microvessels and choroid-plexus. J Cereb Blood Flow Metab. 1985;5:156–60.PubMedCrossRef

302.

Ernst SA. Transport adenosine triphosphatase cytochemistry. I. Biochemical characterization of a cytochemical medium for the ultrastructural localization of ouabain-sensitive, potassium-dependent phosphatase activity in the avian salt gland. J Histochem Cytochem. 1972;20:13–22.PubMedCrossRef

303.

Ernst SA. Transport adenosine triphosphatase cytochemistry. II. Cytochemical localization of ouabin-sensitive, potassium-dependent phosphatase activity in the secretory epithelium of the avian salt gland. J Histochem Cytochem. 1972;20:23–38.PubMedCrossRef

304.

Mayahara H, Fujimoto K, Ando T, Ogawa K. A new one-step method for the cytochemical localization of ouabain-sensitive, potassium-dependent p-nitrophenylphosphatase activity. Histochemistry. 1980;67:125–38.PubMedCrossRef

305.

Firth JA. Cytochemical localization of the K⁺ regulation interface between blood and brain. Experientia. 1977;33:1093–4.PubMedCrossRef

306.

Vorbrodt AW, Lossinsky AS, Wisniewski HM. Cytochemical localization of ouabain-sensitive, K⁺-dependent p-nitro-phenylphosphatase (transport ATPase) in the mouse central and peripheral nervous systems. Brain Res. 1982;243:225–34.PubMedCrossRef

307.

Franceschini V, Del Grande P, Ciani F, Caniato G, Minelli G. Cytochemical localization of alkaline phosphatase and ouabain-sensitive K⁺-dependent p-nitrophenylphosphatase activities in brain capillaries of the newt. Basic Appl Histochem. 1984;28:281–9.PubMed

308.

Kato S, Nakamura H. Ultrastructural and ultracytochemical studies on the blood–brain barrier in chronic relapsing experimental allergic encephalomyelitis. Acta Neuropathol. 1989;77:455–64.PubMedCrossRef

309.

Lazzari M, Franceschini V, Ciani F, Minelli G. Cytochemical localization of alkaline phosphatase and Na⁺, K⁺-ATPase activities in the blood–brain barrier of Rana esculenta. Basic Appl Histochem. 1989;33:113–20.PubMed

310.

Nag S. Ultracytochemical localisation of Na⁺, K⁺-ATPase in cerebral endothelium in acute hypertension. Acta Neuropathol. 1990;80:7–11.PubMedCrossRef

311.

Manoonkitiwongsa PS, Schultz RL, Wareesangtip W, Whitter EF, Nava PB, McMillan PJ. Luminal localization of blood–brain barrier sodium, potassium adenosine triphosphatase is dependent on fixation. J Histochem Cytochem. 2000;48:859–65.PubMedCrossRef

312.

Nag S. Ultra cytochemical studies of the compromised blood–brain barrier. In: Walker JM, editor. The blood–brain barrier biology and research protocols—methods in molecular medicine. Clifton: Humana Press; 2003. p. 145–60.

313.

Maunsbach AB, Skriver E, Jorgensen PL. High-resolution cytochemical-localization of the NaK-ion pump. J Ultrastruct Res. 1982;81:396–7.

314.

Vorbrodt AW. Ultrastructural cytochemistry of blood–brain barrier endothelia. Prog Histochem Cytochem. 1988;18:1–99.PubMedCrossRef

315.

Ernst SA, Palacios JR, Siegel GJ. Immunocytochemical localization of Na⁺, K⁺-ATPase catalytic polypeptide in mouse choroid-plexus. J Histochem Cytochem. 1986;34:189–95.PubMedCrossRef

316.

Zlokovic BV, Mackic JB, Wang L, McComb JG, McDonough A. Differential expression of Na, K-ATPase alpha-subunit and beta-subunit isoforms at the blood–brain-barrier and the choroid-plexus. J Biol Chem. 1993;268:8019–25.PubMed

317.

Betz AL, Ennis SR, Ren XD, Schielke GP, Keep RF. Blood–brain barrier sodium transport and brain edema formation. In: Greenwood J, Begley DJ, Segal MB, editors. New concepts of a blood–brain barrier. New York and London: Plenum; 1995. p. 159–68.CrossRef

318.

Colangelo CM, Chung L, Bruce C, Cheung KH. Review of software tools for design and analysis of large scale MRM proteomic datasets. Methods. 2013;61:287–98.PubMedPubMedCentralCrossRef

319.

Sun D, Lytle C, O’Donnell ME. Astroglial cell-induced expression of Na-K-Cl cotransporter in brain microvascular endothelial cells. Am J Physiol. 1995;269:C1506–12.PubMed

320.

Brillault J, Lam TI, Rutkowsky JM, Foroutan S, O’Donnell ME. Hypoxia effects on cell volume and ion uptake of cerebral microvascular endothelial cells. Am J Physiol. 2008;294:C88–96.CrossRef

321.

Lam TI, Wise PM, O’Donnell ME. Cerebral microvascular endothelial cell Na/H exchange: evidence for the presence of NHE1 and NHE2 isoforms and regulation by arginine vasopressin. Am J Physiol. 2009;297:C278–89.CrossRef

322.

Taylor CJ. Intracellular pH regulation in rat brain endothelial cells. Cambridge: Department of Pharmacology, University of Cambridge; 2004.

323.

Yuen NY, Chen YJ, Boron WF, Praetorius J, Anderson SE, Donnell ME. Blood–brain barrier Na/HCO₃ cotransporters: evidence for a role in ischemia-induced brain Na uptake. FASEB J. 2012;26:1152–62.CrossRef

324.

Betz AL. Sodium transport in capillaries isolated from rat brain. J Neurochem. 1983;41:1150–7.PubMedCrossRef

325.

Lin JD. Effect of osmolarity on potassium transport in isolated cerebral microvessels. Life Sci. 1988;43:325–33.PubMedCrossRef

326.

O’Donnell ME. Role of Na-K-Cl⁻ cotransport in vascular endothelial cell volume regulation. Am J Physiol. 1993;264:C1316–26.PubMed

327.

Vigne P, Farre AL, Frelin C. Na⁺-K⁺-Cl⁻ cotransporter of brain capillary endothelial cells. Properties and regulation by endothelins, hyperosmolar solutions, calyculin A, and interleukin-1. J Biol Chem. 1994;269:19925–30.PubMed

328.

Kawai N, McCarron RM, Spatz M. Effect of hypoxia on Na⁺-K⁺-Cl⁻ cotransport in cultured brain capillary endothelial cells of the rat. J Neurochem. 1996;66:2572–9.PubMedCrossRef

329.

O’Donnell ME, Martinez A, Sun D. Cerebral microvascular endothelial cell Na-K-Cl cotransport: regulation by astrocyte-conditioned medium. Am J Physiol. 1995;268:C747–54.PubMed

330.

Yerby TR, Vibat CRT, Sun D, Payne JA, O’Donnell ME. Molecular characterization of the Na-K-Cl cotransporter of bovine aortic endothelial cells. Am J Physiol. 1997;273:C188–97.PubMed

331.

von Weikersthal SF, Barrand MA, Hladky SB. Functional and molecular characterization of a volume-sensitive chloride current in rat brain endothelial cells. J Physiol (Lond). 1999;516:75–84.CrossRef

332.

Millar ID, Wang S, Barrand MA, Brown PD, Hladky SB. Delayed-rectifying and inwardly-rectifying K⁺ channels are expressed in rat brain endothelial cells. Glasgow: The Physiological Society; 2007. p. 215.

333.

Vigne P, Ladoux A, Frelin C. Endothelins activate Na⁺/H⁺ exchange in brain capillary endothelial cells via a high affinity endothelin-3 receptor that is not coupled to phospholipase C. J Biol Chem. 1991;266:5925–8.PubMed

334.

Hsu P, Haffner J, Albuquerque MLC, Leffler CW. pH_i in piglet cerebral microvascular endothelial cells: recovery from an acid load. Proc Soc Exp Biol Med. 1996;212:256–62.PubMedCrossRef

335.

Sipos I, Torocsik B, Tretter L, Adam-Vizi V. Impaired regulation of pH homeostasis by oxidative stress in rat brain capillary endothelial cells. Cell Mol Neurobiol. 2005;25:141–51.PubMedCrossRef

336.

Taylor CJ, Nicola PA, Wang S, Barrand MA, Hladky SB. Transporters involved in the regulation of intracellular pH (pH_i) in primary cultured rat brain endothelial cells. J Physiol (Lond). 2006;576:769–85.CrossRef

337.

Millar ID, Wang S, Brown PD, Barrand MA, Hladky SB. Kv1 and Kir2 potassium channels are expressed in rat brain endothelial cells. Pflügers Arch. 2008;456:379–91.PubMedCrossRef

338.

Abbott NJ, Revest PA. Single-channel currents recorded from rat brain capillary endothelial cells in culture. J Physiol (Lond). 1990;423:105P.CrossRef

339.

Hoyer J, Popp R, Meyer J, Galla HJ, Gogelein H. Angiotensin-II, vasopressin and GTP gamma-S inhibit inward-rectifying-K⁺ channels in porcine cerebral capillary endothelial-cells. J Membr Biol. 1991;123:55–62.PubMedCrossRef

340.

Popp R, Gogelein H. A calcium and ATP sensitive nonselective cation channel in the antiluminal membrane of rat cerebral capillary endothelial-cells. Biochim Biophys Acta. 1992;1108:59–66.PubMedCrossRef

341.

Popp R, Hoyer J, Meyer J, Galla HJ, Gogelein H. Stretch-activated non-selective cation channels in the antiluminal membrane of porcine cerebral capillaries. J Physiol (Lond). 1992;454:435–49.CrossRef

342.

Popp R, Gogelein H. Outward-rectifying potassium channels in endothelial cells from pig cerebral capillaries. Pflügers Arch. 1993;422:120 (Abstracts).

343.

Van Renterghem C, Vigne P, Frelin C. A charybdotoxin-sensitive, Ca²⁺-activated K⁺ channel with inward, rectifying properties in brain microvascular endothelial-cells—properties and activation by endothelins. J Neurochem. 1995;65:1274–81.PubMedCrossRef

344.

Gögelein H, Popp R, Hoyer J. Patch clamp techniques with isolated brain microvessel membranes. In: Pardridge WM, editor. Introduction to the blood–brain barrier methodology, biology and pathology, vol. 1. Cambridge: Cambridge University Press; 1998. p. 71–8.CrossRef

345.

Csanady L, Adam-Vizi V. Ca²⁺- and voltage-dependent gating of Ca²⁺- and ATP-sensitive cationic channels in brain capillary endothelium. Biophys J. 2003;85:313–27.PubMedPubMedCentralCrossRef

346.

Csanady L, Adam-Vizi V. Antagonistic regulation of native Ca²⁺- and ATP-sensitive cation channels in brain capillaries by nucleotides and decavanadate. J Gen Physiol. 2004;123:743–57.PubMedPubMedCentralCrossRef

347.

Chen YJ, Yuen N, Wallace BK, Wulff H, O’Donnell ME. Blood brain barrier KCa3.1 channels: evidence for a role in brain Na uptake and edema during ischemic stroke. FASEB J. 2012;26:695–713.

348.

Chen Y-J, Yuen N, Wallace BK, Wulff H, O’Donnell ME. Inhibition of the calcium-activated Kca3.1 channel reduces Na plus accumulation and edema formation in the early stages of ischemic stroke. J Vasc Res. 2012;49:12–3.CrossRef

349.

Chen Y-J, Raman G, Bodendiek S, O’Donnell ME, Wulff H. The KCa3.1 blocker TRAM-34 reduces infarction and neurological deficit in a rat model of ischemia/reperfusion stroke. J Cereb Blood Flow Metab. 2011;31:2363–74.PubMedPubMedCentralCrossRef

350.

Schielke GP, Betz AL. Electrolyte transport. In: Bradbury MW, editor. Physiology and pharmacology of the blood–brain barrier. Berlin: Springer-Verlag; 1992. p. 221–43.CrossRef

351.

Pappenheimer JR, Fencl V, Heisey SR, Held D. Role of cerebral fluids in control of respiration as studied in unanesthetized goats. Am J Physiol. 1965;208:436–50.PubMed

352.

Fencl V, Miller TB, Pappenheimer JR. Studies on the respiratory response to disturbances of acid-base balance, with deductions concerning the ionic composition of cerebral interstitial fluid. Am J Physiol. 1966;210:459–72.PubMed

353.

Ghandour MS, Langley OK, Zhu XL, Waheed A, Sly WS. Carbonic anhydrase-IV on brain capillary endothelial-cells—a marker associated with the blood–brain-barrier. Proc Natl Acad Sci USA. 1992;89:6823–7.PubMedPubMedCentralCrossRef

354.

Agarwal N, Lippmann ES, Shusta EV. Identification and expression profiling of blood–brain barrier membrane proteins. J Neurochem. 2010;112:625–35.PubMedCrossRef

355.

Amiry-Moghaddam M, Otsuka T, Hurn PD, Traystman RJ, Haug FM, Froehner SC, Adams ME, Neely JD, Agre P, Ottersen OPT, Bhardwaj A. An alpha-syntrophin-dependent pool of AQP4 in astroglial end-feet confers bidirectional water flow between blood and brain. Proc Natl Acad Sci USA. 2003;100:2106–11.PubMedPubMedCentralCrossRef

356.

Nagelhus EA, Horio Y, Inanobe A, Fujita A, Haug FM, Nielsen S, Kurachi Y, Ottersen OP. Immunogold evidence suggests that coupling of K⁺ siphoning and water transport in rat retinal Muller cells is mediated by a coenrichment of Kir4.1 and AQP4 in specific membrane domains. Glia. 1999;26:47–54.PubMedCrossRef

357.

Connors NC, Adams ME, Froehner SC, Kofuji P. The potassium channel Kir4.1 associates with the dystrophin-glycoprotein complex via alpha-syntrophin in glia. J Biol Chem. 2004;279:28387–92.PubMedCrossRef

358.

Nagelhus EA, Ottersen OP. Physiological roles of aquaporin-4 in brain. Physiol Rev. 2013;93:1543–62.PubMedPubMedCentralCrossRef

359.

Bekaert J, Demeester G. The influence of glucose and insulin upon the potassium concentration of serum and cerebrospinal fluid. Arch Int Physiol. 1951;59:262–4.PubMed

360.

Bekaert J, Demeester G. The influence of the infusion of potassium chloride on the cerebrospinal fluid concentration of potassium. Arch Int Physiol. 1951;59:393–4.PubMed

361.

Stummer W, Keep RF, Betz AL. Rubidium entry into brain and cerebrospinal-fluid during acute and chronic alterations in plasma potassium. Am J Physiol. 1994;266:H2239–46.PubMed

362.

Ames A 3rd, Higashi K, Nesbett FB. Relation of potassium concentration in choroid plexus fluid to that in plasma. J Physiol (Lond). 1965;181:506–15.CrossRef

363.

Klarr SA, Ulanski LJ, Stummer W, Xiang JM, Betz AL, Keep RF. The effects of hypo- and hyperkalemia on choroid plexus potassium transport. Brain Res. 1997;758:39–44.PubMedCrossRef

364.

Keep RF, Ulanski LJ, Xiang JM, Ennis SR, Betz AL. Blood–brain barrier mechanisms involved in brain calcium and potassium homeostasis. Brain Res. 1999;815:200–5.PubMedCrossRef

365.

Poopalasundaram S, Knott C, Shamotienko OG, Foran PG, Dolly JO, Ghiani CA, Gallo V, Wilkin GP. Glial heterogeneity in expression of the inwardly rectifying K⁺ channel, Kir4.1, in adult rat CNS. Glia. 2000;30:362–72.PubMedCrossRef

366.

Higashi K, Fujita A, Inanobe A, Tanemoto M, Doi K, Kubo T, Kurachi Y. An inwardly rectifying K⁺ channel, Kir4.1, expressed in astrocytes surrounds synapses and blood vessels in brain. Am J Physiol. 2001;281:922–31.

367.

Ruiz-Ederra J, Zhang H, Verkman AS. Evidence against functional interaction between aquaporin-4 water channels and Kir4.1 potassium channels in retinal Muller cells. J Biol Chem. 2007;282:21866–72.PubMedCrossRef

368.

Zhang H, Verkman AS. Aquaporin-4 independent Kir4.1 K⁺ channel function in brain glial cells. Mol Cell Neurosci. 2008;37:1–10.PubMedCrossRef

369.

Binder DK, Yao X, Zador Z, Sick TJ, Verkman AS, Manley GT. Increased seizure duration and slowed potassium kinetics in mice lacking aquaporin-4 water channels. Glia. 2006;53:631–6.PubMedCrossRef

370.

Orkand RK, Nicholls JG, Kuffler SW. Effect of nerve impulses on the membrane potential of glial cells in the central nervous system of amphibia. J Neurophysiol. 1966;29:788–806.PubMed

371.

Gardner-Medwin AR. A study of the mechanisms by which potassium moves through brain-tissue in the rat. J Physiol (Lond). 1983;335:353–74.PubMedCentralCrossRef

372.

Gardner-Medwin AR. Analysis of potassium dynamics in mammalian brain-tissue. J Physiol (Lond). 1983;335:393–426.CrossRef

373.

Kofuji P, Newman EA. Potassium buffering in the central nervous system. Neuroscience. 2004;129:1045–56.PubMedPubMedCentralCrossRef

374.

Gardner-Medwin AR. A new framework for assessment of potassium-buffering mechanisms. Ann NY Acad Sci. 1986;481:287–302.PubMedCrossRef

375.

Newman EA. Regional specialization of retinal glial cell membrane. Nature. 1984;309:155–7.PubMedPubMedCentralCrossRef

376.

Newman EA, Frambach DA, Odette LL. Control of extracellular potassium levels by retinal glial cell K⁺ siphoning. Science. 1984;225:1174–5.PubMedPubMedCentralCrossRef

377.

Newman EA. Distribution of potassium conductance in mammalian Muller (glial) cells: a comparative study. J Neurosci. 1987;7:2423–32.PubMed

378.

Orkand RK, Newman EA, Gardner-Medwin AR, Crone C, Abbott NJ, Paulson OB, Hansen AJ, Ransom BR, Dietzel I, Wright EM, et al. General discussion on glial-interstitial fluid exchange. Ann NY Acad Sci. 1986;481:354–6.CrossRef

379.

Mutsuga N, Schuette WH, Lewis DV. The contribution of local blood flow to the rapid clearance of potassium from the cortical extracellular space. Brain Res. 1976;116:431–6.PubMedCrossRef

380.

Nicholson C. Dynamics of the brain cell microenvironment. Neurosci Res Prog Bull. 1980;18:175–322.

381.

Altman PL, Dittmer DS, editors. Respiration and circulation. Bethesda: Federation of American Societies for Experimental Biology; 1971.

382.

Siesjö BK. Symposium on acid-base homeostasis. The regulation of cerebrospinal fluid pH. Kidney Int. 1972;1:360–74.PubMedCrossRef

383.

Chesler M. The regulation and modulation of pH in the nervous-system. Prog Neurobiol. 1990;34:401–27.PubMedCrossRef

384.

Katsura K, Kristian T, Nair R, Siesjo BK. Regulation of intra- and extracellular pH in the rat brain in acute hypercapnia: a re-appraisal. Brain Res. 1994;651:47–56.PubMedCrossRef

385.

Chesler M. Regulation and modulation of pH in the brain. Physiol Rev. 2003;83:1183–221.PubMedCrossRef

386.

Leusen I. Regulation of cerebrospinal-fluid composition with reference to breathing. Physiol Rev. 1972;52:1–56.PubMed

387.

Katzman R, Pappius HM. Brain electrolytes and fluid metabolism. Baltimore: Williams & Wilkins; 1973.

388.

Bledsoe SW, Hornbein TF. Central chemosensors and the regulation of their chemical environment—regulation of breathing part 1. In: Hornbein TF, editor. Lung biology in health and disease volume. New York: Marcel Dekker Inc; 1981. p. 327–428.

389.

Nattie EE. Ionic mechanisms of cerebrospinal-fluid acid-base regulation. J Appl Physiol. 1983;54:3–12.PubMed

390.

Siesjö BK. Acid-base homeostasis in the brain: physiology, chemistry, and neurochemical pathology. Prog Brain Res. 1985;63:121–54.PubMedCrossRef

391.

Brooks CM, Kao FF, Lloyd BB, editors. Cerebrospinal fluid and the regulation of ventilation. Oxford: Blackwell Scientific Publications; 1965.

392.

Cohen PJ. Energy metabolism of the human brain. In: Siesjö BK, Sørensen SC, editors. Ion homeostasis of the brain. Copenhagen: Academic; 1971.

393.

Kazemi H, Johnson DC. Regulation of cerebrospinal-fluid acid-base-balance. Physiol Rev. 1986;66:953–1037.PubMed

394.

Nattie E. Control and disturbances of cerebrospinal fluid pH. In: Kaila K, Ransom BR, editors. pH and brain function. New York: Wiley-Liss; 1998. p. 629–50.

395.

Nattie E. Chemoreceptors, breathing and pH. In: Alpern RJ, Moe O, editors. Seldin and Giebisch’s the kidney: physiology and pathophysiology. London: Academic Press; 2013. p. 1979–93.CrossRef

396.

Obara M, Szeliga M, Albrecht J. Regulation of pH in the mammalian central nervous system under normal and pathological conditions: facts and hypotheses. Neurochem Int. 2008;52:905–19.PubMedCrossRef

397.

Ahmad HR, Loeschcke HH. Transient and steady-state responses of pulmonary ventilation to the medullary extracellular pH after approximately rectangular changes in alveolar pCO₂. Pflügers Arch. 1982;395:285–92.PubMedCrossRef

398.

Romero MF, Fulton CM, Boron WF. The SLC4 family of HCO₃ ⁻ transporters. Pflügers Arch. 2004;447:495–509.PubMedCrossRef

399.

Romero MF, Chen A-P, Parker MD, Boron WF. The SLC4 family of bicarbonate (HCO₃ ⁻) transporters. Mol Asp Med. 2013;34:159–82.CrossRef

400.

Fencl V. Distribution of H⁺ and HCO₃ ⁻ in cerebral fluids. In: Siesjo BK, Sorensen SC, editors. Ion homeostasis of the brain. New York: Academic; 1971. p. 175–85 (Alfred Benzon Symposium).

401.

Sørensen SC. Factors regulating [H⁺] and [HCO₃ ⁻] in brain extracellular fluid. In: Siesjo BK, Sorensen SC, editors. Ion homeostasis of the brain. New York: Academic; 1971. p. 206–17 (Alfred Benzon Symposium).

402.

Ponten U, Siesjo BK. Acid-base relations in arterial blood and cerebrospinal fluid of the unanesthetized rat. Acta Physiol Scand. 1967;71:89–95.PubMedCrossRef

403.

Chazan JA, Appleton FM, London AM, Schwartz WB. Effects of chronic metabolic acid-base disturbances on the composition of cerebrospinal fluid in the dog. Clin Sci. 1969;36:345–58.PubMed

404.

Javaheri S, Kazemi H. Electrolyte composition of cerebrospinal fluid in acute acid-base disorders. Respir Physiol. 1981;45:141–51.PubMedCrossRef

405.

Fencl V, Gabel RA, Wolfe D. Composition of cerebral fluids in goats adapted to high-altitude. J Appl Physiol Respir Environ Exerc Physiol. 1979;47:508–13.PubMed

406.

Nattie EE. Brain and cerebrospinal fluid ionic composition and ventilation in acute hypercapnia. Respir Physiol. 1980;40:309–22.PubMedCrossRef

407.

Stewart PA. Independent and dependent variables of acid-base control. Respir Physiol. 1978;33:9–26.PubMedCrossRef

408.

Stewart PA. How to understand acid-base: a quantitative acid-base primer for biology and medicine. New York: Elsevier North Holland; 1981.

409.

Loeschcke HH, Ahmad HR. Transients and steady-state chloride-bicarbonate relationships of brain extracellular fluid. In: Bauer C, Gros G, Bartels H, editors. Biophysics and physiology of carbon dioxide. New York: Springer-Verlag; 1980. p. 439–48.CrossRef

410.

Boron WF, Boulpaep EL. Medical physiology. Philadelphia: Saunders, Elsevier; 2012.

411.

Pannier JL, Weyne J, Leusen I. The CSF-blood potential and the regulation of the bicarbonate concentration of CSF during acidosis in the cat. Life Sci I Physiol Pharmacol. 1971;10:287–300.

412.

Wichser J, Kazemi H. CSF bicarbonate regulation in respiratory acidosis and alkalosis. J Appl Physiol. 1975;38:504–11.PubMed

413.

Hasan FM, Kazemi H. Dual contribution theory of regulation of CSF HCO₃ in respiratory acidosis. J Appl Physiol. 1976;40:559–67.PubMed

414.

Nattie EE, Romer L. CSF HCO₃ ⁻ regulation in isosmotic conditions—role of brain pCO₂ and plasma HCO₃ ⁻. Respir Physiol. 1978;33:177–98.PubMedCrossRef

415.

Ahmad HR, Loeschcke HH. Fast bicarbonate-chloride exchange between brain-cells and brain extracellular fluid in respiratory-acidosis. Pflügers Arch. 1982;395:293–9.PubMedCrossRef

416.

Portman MA, Lassen NA, Cooper TG, Sills AM, Potchen EJ. Intra- and extracellular pH of the brain in vivo studied by ³¹P-NMR during hyper- and hypocapnia. J Appl Physiol. 1985;1991(71):2168–72.

417.

Siesjö BK, Kjällquist A. A new theory for the regulation of the extracellular pH in the brain. Scand J Clin Lab Invest. 1969;24:1–9.PubMedCrossRef

418.

Mines AH, Morril CG, Sørensen SC. The effect of iso-carbic metabolic acidosis in blood on [H⁺] and [HCO₃ ⁻] in CSF with deductions about the regulation of an active transport of H plus-HCO₃ ⁻ between blood and CSF. Acta Physiol Scand. 1971;81:234–45.PubMedCrossRef

419.

Mines AH, Sørensen SC. Changes in the electrochemical potential difference for HCO₃ ⁻ between blood and cerebrospinal fluid and in cerebrospinal fluid lactate concentration during isocarbic hypoxia. Acta Physiol Scand. 1971;81:225–33.PubMedCrossRef

420.

Kety SS. The general metabolism of the brain in vivo. In: Richter D, editor. Metabolism of the nervous system. London: Pergamon Press; 1957. p. 221–37.CrossRef

421.

Dienel GA, Cruz NF. Nutrition during brain activation: does cell-to-cell lactate shuttling contribute significantly to sweet and sour food for thought? Neurochem Int. 2004;45:321–51.PubMedCrossRef

422.

Kazemi H, Valenca LM, Shannon DC. Brain and cerebrospinal fluid lactate concentration in respiratory acidosis and alkalosis. Respir Physiol. 1969;6:178–86.PubMedCrossRef

423.

Kaasik AE, Nilsson L, Siesjö BK. The effect of asphyxia upon the lactate, pyruvate and bicarbonate concentrations of brain tissue and cisternal CSF, and upon the tissue concentrations of phosphocreatine and adenine nucleotides in anesthetized rats. Acta Physiol Scand. 1970;78:433–47.PubMedCrossRef

424.

Scheller D, Kolb J, Tegtmeier F. Lactate and pH change in close correlation in the extracellular space of the rat brain during cortical spreading depression. Neurosci Lett. 1992;135:83–6.PubMedCrossRef

425.

Poole RC, Halestrap AP. Transport of lactate and other monocarboxylates across mammalian plasma-membranes. Am J Physiol. 1993;264:C761–82.PubMed

426.

Gerhart DZ, Enerson BE, Zhdankina OY, Leino RL, Drewes LR. Expression of monocarboxylate transporter MCT1 by brain endothelium and glia in adult and suckling rats. Am J Physiol. 1997;273:E207–13.PubMed

427.

Halestrap AP, Price NT. The proton-linked monocarboxylate transporter (MCT) family: structure, function and regulation. Biochem J. 1999;343:281–99.PubMedPubMedCentralCrossRef

428.

Oldendorf WH. Carrier-mediated blood–brain barrier transport of short-chain monocarboxylic organic-acids. Am J Physiol. 1973;224:1450–3.PubMed

429.

Daniel PM, Love ER, Moorhouse SR, Pratt OE. The movement of ketone bodies, glucose, pyruvate and lactate between blood and brain of rats. J Physiol (Lond). 1972;221:P22–3.

430.

Drewes LR, Gilboe DD. Glycolysis and the permeation of glucose and lactate in the isolated, perfused dog brain during anoxia and postanoxic recovery. J Biol Chem. 1973;248:2489–96.PubMed

431.

Knudsen GM, Paulson OB, Hertz MM. Kinetic analysis of the human blood–brain barrier transport of lactate and its influence by hypercapnia. J Cereb Blood Flow Metab. 1991;11:581–6.PubMedCrossRef

432.

Pardridge WM, Connor JD, Crawford IL. Permeability changes in the blood–brain barrier: causes and consequences. CRC Crit Rev Toxicol. 1975;3:159–99.PubMedCrossRef

433.

Cruz NF, Adachi K, Dienel GA. Rapid efflux of lactate from cerebral cortex during K⁺-induced spreading cortical depression. J Cereb Blood Flow Metab. 1999;19:380–92.PubMedCrossRef

434.

Lundgaard I, Lu ML, Yang E, Peng W, Mestre H, Hitomi E, Deane R, Nedergaard M. Glymphatic clearance controls state-dependent changes in brain lactate concentration. J Cereb Blood Flow Metab. 2016. doi: 10.1177/0271678x16661202.PubMed

435.

Loeschcke HH. DC potentials between CSF and blood. In: Siesjo BK, Sorensen SC, editors. Ion homeostasis of the brain the regulation of hydrogen and potassium ion concentrations in cerebral intra- and extracellular fluids. Copenhagen: Academic; 1971. p. 77–96 (Alfred Benzon Symposium).

436.

Besson JM, Woody CD, Aleonard P, Thompson HK, Albe-Fessard D, Marshall WH. Correlations of brain d-c shifts with changes in cerebral blood flow. Am J Physiol. 1970;218:284–91.PubMed

437.

Cameron IR, Caronna J, Miller R. The effect of acute hyperkalaemia on the c.s.f.-blood potential difference and the control of c.s.f. pH. J Physiol (Lond). 1973;232:102–3.

438.

Bledsoe SW, Mines AH. Effect of plasma [K⁺] on dc potential and on ion distributions between csf and blood. J Appl Physiol. 1975;39:1012–6.PubMed

439.

Bledsoe SW, Eng DY, Hornbein TF. Evidence of active regulation of cerebrospinal-fluid acid-base-balance. J Appl Physiol. 1981;51:369–75.PubMed

440.

Husted RF, Reed DJ. Regulation of cerebrospinal fluid bicarbonate by the cat choroid plexus. J Physiol (Lond). 1977;267:411–28.CrossRef

441.

Patlak CS, Fenstermacher JD. Measurements of dog blood–braintransfer constants by ventriculocisternal perfusion. Am J Physiol. 1975;229:877–84.PubMed

442.

Ahmad HR, Loeschcke HH. Fast bicarbonate-chloride exchange between plasma and brain extracellular fluid at maintained pCO₂. Pflügers Arch. 1982;395:300–5.PubMedCrossRef

443.

Teppema LJ, Barts P, Folgering HT, Evers JAM. Effects of respiratory and (isocapnic) metabolic arterial acid-base disturbances on medullary extracellular fluid pH and ventilation in cats. Respir Physiol. 1983;53:379–95.PubMedCrossRef

444.

Teppema LJ, Barts P, Evers JAM. Effects of metabolic arterial pHchanges on medullary ecf pH, csf pH and ventilation in peripherally chemodenervated cats with intact blood–brain-barrier. Respir Physiol. 1984;58:123–36.PubMedCrossRef

445.

Davies DG, Nolan WF. Cerebral interstitial fluid acid-base status follows arterial acid-base perturbations. J Appl Physiol. 1982;53:1551–5.PubMed

446.

Sanyal G, Maren TH. Thermodynamics of carbonic anhydrase catalysis. A comparison between human isoenzymes B and C. J Biol Chem. 1981;256:608–12.PubMed

447.

Nattie E, Li AH. Central chemoreceptors: locations and functions. Compr Physiol. 2012;2:221–54.PubMedPubMedCentral

448.

Borison HL, Gonsalves SF, Montgomery SP, McCarthy LE. Dynamics of respiratory VT response to isocapnic pHa forcing in chemodenervated cats. J Appl Physiol Respir Environ Exerc Physiol. 1978;45:502–11.PubMed

449.

Kaehny WD, Jackson JT. Respiratory response to HCl acidosis in dogs after carotid body denervation. J Appl Physiol Respir Environ Exerc Physiol. 1979;46:1138–42.PubMed

450.

Siesjö BK. The relation between the bicarbonate concentration in blood plasma and in brain tissue. Experientia. 1964;20:455–6.PubMedCrossRef

451.

Siesjö BK. Active and passive mechanisms in the regulation of the acid-base metabolism of brain tissue. In: Brooks CM, Kao FF, Lloyd BB, editors. Cerebrospinal fluid and the regulation of ventilation. Oxford: Blackwell; 1965. p. 331–71.

452.

Siesjo BK, Ponten U. Acid-base changes in the brain in non-respiratory acidosis and alkalosis. Exp Brain Res. 1966;2:176–90.PubMedCrossRef

453.

Rapoport SI. Cortical pH and the blood–brain barrier. J Physiol (Lond). 1964;170:238–49.PubMedCentralCrossRef

454.

Rapoport SI, Thompson HK. Effect of intravenous NH₄Cl and NaHCO₃ on the pH of the brain surface, as related to respiration and the blood–brain barrier. Exp Neurol. 1974;42:320–33.PubMedCrossRef

455.

Javaheri S, Clendening A, Papadakis N, Brody JS. Changes in brain surface pH during acute isocapnic metabolic acidosis and alkalosis. J Appl Physiol Respir Environ Exerc Physiol. 1981;51:276–81.PubMed

456.

Cragg P, Patterson L, Purves MJ. The pH of brain extracellular fluid in the cat. J Physiol (Lond). 1977;272:137–66.CrossRef

457.

Javaheri S, De Hemptinne A, Vanheel B, Leusen I. Changes in brain ECF pH during metabolic acidosis and alkalosis: a microelectrode study. J Appl Physiol Respir Environ Exerc Physiol. 1983;55:1849–53.PubMed

458.

Adler S, Simplaceanu V, Ho C. Brain pH in acute isocapnic metabolic acidosis and hypoxia: a ³¹P-nuclear magnetic resonance study. Am J Physiol. 1990;258:F34–40.PubMed

459.

Simon MJ, Iliff JJ. Regulation of cerebrospinal fluid (CSF) flow in neurodegenerative, neurovascular and neuroinflammatory disease. Biochim Biophys Acta-Mol Basis Dis. 2016;1862:442–51.CrossRef

460.

Engelhardt B, Carare RO, Bechmann I, Flugel A, Laman JD, Weller RO. Vascular, glial, and lymphatic immune gateways of the central nervous system. Acta Neuropathol. 2016;132(3):317–38.PubMedPubMedCentralCrossRef

461.

Weed LH. Studies on cerebro-spinal fluid. IV. The dual source of cerebro-spinal fluid. J Med Res. 1914;26:93–113.

462.

Flexner LB. Some problems of the origin, circulation and absorption of the cerebrospinal fluid. Q Rev Biol. 1933;8:397–422.CrossRef

463.

Woollam DHM, Millen JW. Perivascular spaces of the mammalian central nervous system. Biol Rev Camb Philos Soc. 1954;29:251–83.CrossRef

464.

Cserr HF, Ostrach LH. Bulk flow of interstitial fluid after intracranial injection of blue dextran 2000. Exp Neurol. 1974;45:50–60.PubMedCrossRef

465.

Gregory TF, Rennels ML, Blaumanis OR, Fujimoto K. A method for microscopic studies of cerebral angioarchitecture and vascular-parenchymal relationships, based on the demonstration of ‘paravascular’ fluid pathways in the mammalian central nervous system. J Neurosci Methods. 1985;14:5–14.PubMedCrossRef

466.

Zhang ET, Richards HK, Kida S, Weller RO. Directional and compartmentalized drainage of interstitial fluid and cerebrospinal-fluid from the rat-brain. Acta Neuropathol. 1992;83:233–9.PubMedCrossRef

467.

Felig P, Wahren J, Ahlborg G. Uptake of individual amino acids by the human brain. Proc Soc Exp Biol Med. 1973;142:230–1.PubMedCrossRef

468.

Betz AL, Gilboe DD. Effect of pentobarbital on amino acid and urea flux in the isolated dog brain. Am J Physiol. 1973;224:580–7.PubMed

469.

Xiang JM, Ennis SR, Abdelkarim GE, Fujisawa M, Kawai N, Keep RF. Glutamine transport at the blood–brainand blood-cerebrospinal fluid barriers. Neurochem Int. 2003;43:279–88.PubMedCrossRef

470.

Smith QR, Momma S, Aoyagi M, Rapoport SI. Kinetics of neutral amino acid transport across the blood–brain barrier. J Neurochem. 1987;49:1651–8.PubMedCrossRef

471.

Lee W-J, Hawkins RA, Vina JR, Peterson DR. Glutamine transport by the blood–brain barrier: a possible mechanism for nitrogen removal. Am J Physiol. 1998;274:C1101–7.PubMed

472.

Hawkins RA, Vina JR, Peterson DR, O’Kane R, Mokashi A, Simpson IA. Amino acid transport across each side of the blood–brain barrier. In: D’Mello JPF, editor. Amino acids in human nutrition and health. England: CAB International; 2011. p. 191–214.CrossRef

473.

Hawkins RA, Viña JR, Mokashi A, Peterson DR, O’Kane R, Simpson IA, Dejoseph MR, Rasgado-Flores H. Synergism between the two membranes of the blood–brain barrier: glucose and amino acid transport. Am J Neurosci Res. 2013;1:201300168.

474.

Kilberg MS, Handlogten ME, Christensen HN. Characteristics of an amino acid transport system in rat liver for glutamine, asparagine, histidine, and closely related analogs. J Biol Chem. 1980;255:4011–9.PubMed

475.

Fafournoux P, Demigne C, Remesy C, Le Cam A. Bidirectional transport of glutamine across the cell membrane in rat liver. Biochem J. 1983;216:401–8.PubMedPubMedCentralCrossRef

476.

Said HM, Hollander D, Khorchid S. An Na⁺-dependent and an Na⁺-independent system for glutamine transport in rat liver basolateral membrane vesicles. Gastroenterology. 1991;101:1094–101.PubMedCrossRef

477.

Pacitti AJ, Inoue Y, Souba WW. Characterization of Na⁺-independent glutamine transport in rat liver. Am J Physiol. 1993;265:G90–8.PubMed

478.

Haussinger D. Hepatic glutamine transport and metabolism. Adv Enzymol. 1998;72:43–86.PubMed

479.

Schioth HB, Roshanbin S, Hagglund MGA, Fredriksson R. Evolutionary origin of amino acid transporter families SLC32, SLC36 and SLC38 and physiological, pathological and therapeutic aspects. Mol Asp Med. 2013;34:571–85.CrossRef

480.

Chaudhry FA, Reimer RJ, Krizaj D, Barber D, Storm-Mathisen J, Copenhagen DR, Edwards RH. Molecular analysis of system N suggests novel physiological roles in nitrogen metabolism and synaptic transmission. Cell. 1999;99:769–80.PubMedCrossRef

481.

Curran PF, Solomon AK. Ion and water fluxes in the ileum of rats. J Gen Physiol. 1957;41:143–68.PubMedPubMedCentralCrossRef

482.

Heisey SR, Held D, Pappenheimer JR. Bulk flow and diffusion in the cerebrospinal fluid system of the goat. Am J Physiol. 1962;203:775–81.PubMed

483.

Whitlock RT, Wheeler HO. Coupled transport of solute and water across rabbit gallbladder epithelium. J Clin Invest. 1964;43:2249–65.PubMedPubMedCentralCrossRef

484.

Diamond JM. Transport of salt and water in rabbit and guinea pig gall bladder. J Gen Physiol. 1964;48:1–14.PubMedPubMedCentralCrossRef

485.

Curran PF, Macintosh JR. A model system for biological water transport. Nature. 1962;193:347–8.PubMedCrossRef

486.

Weinstein AM, Stephenson JL. Coupled water transport in standing gradient models of the lateral intercellular space. Biophys J. 1981;35:167–91.PubMedPubMedCentralCrossRef

487.

Weinstein AM, Stephenson JL. Models of coupled salt and water transport across leaky epithelia. J Membr Biol. 1981;60:1–20.PubMedCrossRef

488.

Jacobson HR, Seldin DW. Proximal tubular reabsorption and its regulation. Annu Rev Pharmacol Toxicol. 1977;17:623–46.PubMedCrossRef

489.

Kokko JP, Burg MB, Orloff J. Characteristics of NaCl and water transport in the renal proximal tubule. J Clin Invest. 1971;50:69–76.PubMedPubMedCentralCrossRef

490.

Schafer JA, Troutman SL, Andreoli TE. Volume reabsorption, transepithelial potential differences, and ionic permeability properties in mammalian superficial proximal straight tubules. J Gen Physiol. 1974;64:582–607.PubMedPubMedCentralCrossRef

491.

Schafer JA, Patlak CS, Andreoli TE. Fluid absorption and active and passive ion flows in rabbit superficial pars recta. Am J Physiol. 1977;233:F154–67.PubMed

492.

Wright EM. Mechanisms of ion transport across the choroid plexus. J Physiol (Lond). 1972;226:545–71.PubMedCentralCrossRef

493.

Wright EM, Wiedner G, Rumrich G. Fluid secretion by the frog choroid plexus. Exp Eye Res. 1977;25(Suppl):149–55.PubMedCrossRef

494.

Swenson ER. Pharmacology of acute mountain sickness: old drugs and newer thinking. J Appl Physiol. 2016;120:204–15.PubMedCrossRef

495.

Søgaard R, Zeuthen T. Test of blockers of AQP1 water permeability by a high-resolution method: no effects of tetraethylammonium ions or acetazolamide. Pflügers Arch. 2008;456:285–92.PubMedCrossRef

496.

Tanimura Y, Hiroaki Y, Fujiyoshi Y. Acetazolamide reversibly inhibits water conduction by aquaporin-4. J Struct Biol. 2009;166:16–21.PubMedCrossRef

497.

Sahar A, Hochwald GM, Ransohoff J. Alternate pathway for cerebrospinal fluid absorption in animals with experimental obstructive hydrocephalus. Exp Neurol. 1969;25:200–6.PubMedCrossRef

498.

Sahar A, Hochwald GM, Sadik AR, Ransohoff J. Cerebrospinal fluid absorption in animals with experimental obstructive hydrocephalus. Arch Neurol. 1969;21:638–44.PubMedCrossRef

499.

Becker DP, Wilson JA, Watson GW. The spinal cord central canal: response to experimental hydrocephalus and canal occlusion. J Neurosurg. 1972;36:416–24.PubMedCrossRef

500.

Dreha-Kulaczewski S, Joseph AA, Merboldt KD, Ludwig HC, Gartner J, Frahm J. Inspiration is the major regulator of human CSF flow. J Neurosci. 2015;35:2485–91.PubMedCrossRef

501.

Bradbury MWB, Cserr HF, Westrop RJ. Drainage of cerebral interstitial fluid into deep cervical lymph of the rabbit. J Physiol (Lond). 1980;307:P84.

502.

Bradbury MW, Cserr HF, Westrop RJ. Drainage of cerebral interstitial fluid into deep cervical lymph of the rabbit. Am J Physiol. 1981;240:F329–36.PubMed

503.

Kida S, Pantazis A, Weller RO. CSF drains directly from the subarachnoid space into nasal lymphatics in the rat—anatomy, histology and immunological significance. Neuropathol Appl Neurobiol. 1993;19:480–8.PubMedCrossRef

504.

Carare RO, Bernardes-Silva M, Newman TA, Page AM, Nicoll JAR, Perry VH, Weller RO. Solutes, but not cells, drain from the brain parenchyma along basement membranes of capillaries and arteries: significance for cerebral amyloid angiopathy and neuroimmunology. Neuropathol Appl Neurobiol. 2008;34:131–44.PubMedCrossRef

505.

Weller RO, Djuanda E, Yow H-Y, Carare RO. Lymphatic drainage of the brain and the pathophysiology of neurological disease. Acta Neuropathol. 2009;117:1–14.PubMedCrossRef

506.

Iliff JJ, Goldman SA, Nedergaard M. Implications of the discovery of brain lymphatic pathways. Lancet Neurol. 2015;14:977–9.PubMedPubMedCentralCrossRef

507.

Cserr HF, Patlak CS. Regulation of brain volume under isosmotic and anisosmotic conditions. In: Gilles R, Hoffmann EK, Bolis L, editors. Advances in comparative and environmental physiology, vol. 9. Heidelberg: Springer; 1991. p. 61–80.CrossRef

508.

Hladky SB. The mechanism of ion conduction in thin lipid membranes containing gramicidin A. Ph.D., Cambridge: Cambridge University; 1972.

509.

Hladky SB. Gramicidin: conclusions based on the kinetic data. Curr Topics Membr Transp. 1988;33:15–33.CrossRef

510.

Begenisich T, De Weer P. Ionic interactions in the potassium channel of squid giant axons. Nature. 1977;269:710–1.PubMedCrossRef

511.

Cserr HF, Cooper DN, Suri PK, Patlak CS. Efflux of radiolabeled polyethylene glycols and albumin from rat brain. Am J Physiol. 1981;240:F319–28.PubMed

512.

Motulsky H, Christopoulos A. Fitting models to biological data using linear and nonlinear regression. 1st ed. Oxford: Oxford University Press; 2004.

513.

Leem CH, Lagadic-Gossmann D, Vaughan-Jones RD. Characterization of intracellular pH regulation in the guinea-pig ventricular myocyte. J Physiol (Lond). 1999;517:159–80.PubMedCentralCrossRef

514.

Stewart PA. Modern quantitative acid-base chemistry. Can J Physiol Pharmacol. 1983;61:1444–61.PubMedCrossRef

515.

Cameron JN. Acid–base homeostasis: past and present perspectives. Physiol Zool. 1989;62:845–65.CrossRef

516.

Hodgkin AL, Huxley AF. A quantitative description of membrane current and its application to conduction and excitation in nerve. J Physiol (Lond). 1952;117:500–44.CrossRef

517.

Kiley JP, Eldridge FL, Millhorn DE. The roles of medullary extracellular and cerebrospinal-fluid pH in control of respiration. Respir Physiol. 1985;59:117–30.PubMedCrossRef

518.

Eldridge FL, Kiley JP, Millhorn DE. Respiratory effects of carbon dioxide-induced changes of medullary extracellular fluid pH in cats. J Physiol (Lond). 1984;355:177–89.CrossRef

519.

Nishimura M, Johnson DC, Hitzig BM, Okunieff P, Kazemi H. Effects of hypercapnia on brain pH_i and phosphate metabolite regulation by ³¹P-NMR. J Appl Physiol. 1985;1989(66):2181–8.

520.

Kintner DB, Anderson ME, Sailor KA, Dienel G, Fitzpatrick JH, Gilboe DD. In vivo microdialysis of 2-deoxyglucose 6-phosphate into brain: a novel method for the measurement of interstitial pH using ³¹P-NMR. J Neurochem. 1999;72:405–12.PubMedCrossRef

521.

Ball KK, Gandhi GK, Thrash J, Cruz NF, Dienel GA. Astrocytic connexin distributions and rapid, extensive dye transfer via gap junctions in the inferior colliculus: implications for [(14)C] glucose metabolite trafficking. J Neurosci Res. 2007;85:3267–83.PubMedPubMedCentralCrossRef

522.

Weyne J, Leusen I. Lactate in cerebrospinal fluid in relation to brain and blood. In: Cserr HF, Fenstermacher JD, Fencl V, editors. Fluid environment of the brain. New York: Academic Press; 1975. p. 255–76.CrossRef

523.

Cruz NF, Ball KK, Dienel GA. Functional imaging of focal brain activation in conscious rats: impact of [¹⁴C] glucose metabolite spreading and release. J Neurosci Res. 2007;85:3254–66.PubMedCrossRef

524.

Gandhi GK, Cruz NF, Ball KK, Dienel GA. Astrocytes are poised for lactate trafficking and release from activated brain and for supply of glucose to neurons. J Neurochem. 2009;111:522–36.PubMedPubMedCentralCrossRef

525.

Sørensen SC, Severinghaus JW. Effect of cerebral acidosis on the CSF-blood potential difference. Am J Physiol. 1970;219:68–71.PubMed

526.

Welch K, Araki H, Arkins T. Electrical potentials of the lamina epithelialis choroidea of the fourth ventricle of the cat in vitro: relationship to the CSF blood potential. Dev Med Child Neurol Suppl. 1972;27:146–50.PubMed

527.

Welch K, Araki H. Features of the choroid plexus of the cat, studied in vitro. In: Cserr HF, Fenstermacher JD, Fencl V, editors. Fluid environment of the brain. New York: Academic Press; 1975. p. 157–65.CrossRef

528.

Patlak CS, Adamson RH, Oppelt WW, Rall DP. Potential difference of the ventricular fluid in vivo and in vitro in the dogfish. Life Sci. 1966;5:2011–5.CrossRef

529.

Hornbein TF, Sørensen SC. DC potential difference between different cerebrospinal-fluid sites and blood in dogs. Am J Physiol. 1972;223:415–8.PubMed

530.

Clark RG, Milhorat TH, Stanley WC, Di Chiro G. Experimental pantopaque ventriculography. J Neurosurg. 1971;34:387–95.PubMedCrossRef

531.

Pappenheimer JR. On the location of the blood–brain barrier. In: Coxon RV, editor. The proceedings of a symposium on the blood–brain barrier sponsored by the Wates Foundation. Oxford: Truex Press; 1970.

532.

Crone C. The permeability of capillaries in various organs as determined by use of the ‘indicator diffusion’ method. Acta Physiol Scand. 1963;58:292–305.PubMedCrossRef

533.

Crone C, Levitt DG. Capillary permeability to small solutes. In: Renkin EM, Michel CC, Geiger SR, editors. Handbook of physiology section 2 the cardiovascular system vol 4—part 1 microcirculation. Bethesda: American physiological Society; 1984. p. 411–66.

534.

Kety SS, Schmidt CF. The effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men. J Clin Invest. 1948;27:484–92.PubMedPubMedCentralCrossRef

535.

Philp NJ, Yoon HY, Lombardi L. Mouse MCT3 gene is expressed preferentially in retinal pigment and choroid plexus epithelia. Am J Physiol. 2001;280:C1319–26.

536.

Zlokovic BV, Apuzzo MLJ. Strategies to circumvent vascular barriers of the central nervous system. Neurosurgery. 1998;43:877–8.PubMedCrossRef

537.

Kashgarian M, Biemesderfer D, Caplan M, Forbush B 3rd. Monoclonal antibody to Na, K-ATPase: immunocytochemical localization along nephron segments. Kidney Int. 1985;28:899–913.PubMedCrossRef

538.

Praetorius J, Nejsum LN, Nielsen S. A SCL4A10 gene product maps selectively to the basolateral plasma membrane of choroid plexus epithelial cells. Am J Physiol. 2004;286:C601–10.CrossRef

539.

Bouzinova EV, Praetorius J, Virkki LV, Nielsen S, Boron WF, Aalkjaer C. Na⁺-dependent HCO₃ ⁻ uptake into the rat choroid plexus epithelium is partially DIDS sensitive. Am J Physiol. 2005;289:C1448–56.CrossRef

540.

Praetorius J, Nielsen S. Distribution of sodium transporters and aquaporin-1in the human choroid plexus. Am J Physiol. 2006;291:C59–67.CrossRef

541.

Praetorius J. Water and solute secretion by the choroid plexus. Pflügers Arch. 2007;454:1–18.PubMedCrossRef

542.

Damkier HH, Brown PD, Praetorius J. Epithelial pathways in choroid plexus electrolyte transport. Physiology. 2010;25:239–49.PubMedCrossRef

543.

Aspelund A, Antila S, Proulx ST, Karlsen TV, Karaman S, Detmar M, Wiig H, Alitalo K. A dural lymphatic vascular system that drains brain interstitial fluid and macromolecules. J Exp Med. 2015;212:991–9.PubMedPubMedCentralCrossRef

544.

Louveau A, Smirnov I, Keyes TJ, Eccles JD, Rouhani SJ, Peske JD, Derecki NC, Castle D, Mandell JW, Lee KS, et al. Structural and functional features of central nervous system lymphatic vessels. Nature. 2015;523:337–41.PubMedPubMedCentralCrossRef

545.

Romero MF. In the beginning, there was the cell: cellular homeostasis. Adv Physiol Educ. 2004;28:135–8.PubMedCrossRef

Title: Fluid and ion transfer across the blood–brain and blood–cerebrospinal fluid barriers; a comparative account of mechanisms and roles
Authors: Stephen B. Hladky
Margery A. Barrand
Publication date: 01-12-2016
Publisher: BioMed Central
Published in: Fluids and Barriers of the CNS / Issue 1/2016
Electronic ISSN: 2045-8118
DOI: https://doi.org/10.1186/s12987-016-0040-3

At a glance: The STEP trials

Springer Medicine

Fluid and ion transfer across the blood–brain and blood–cerebrospinal fluid barriers; a comparative account of mechanisms and roles

Abstract

At a glance: The STEP trials

Springer Medicine

Abstract

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Other articles of this Issue 1/2016

In vitro characterization of pralidoxime transport and acetylcholinesterase reactivation across MDCK cells and stem cell-derived human brain microvascular endothelial cells (BC1-hBMECs)

Amyloid mis-metabolism in idiopathic normal pressure hydrocephalus

Nonsurgical therapy for hydrocephalus: a comprehensive and critical review

Modeling immune functions of the mouse blood–cerebrospinal fluid barrier in vitro: primary rather than immortalized mouse choroid plexus epithelial cells are suited to study immune cell migration across this brain barrier

Growth-factor reduced Matrigel source influences stem cell derived brain microvascular endothelial cell barrier properties

Early and delayed assessments of quantitative gait measures to improve the tap test as a predictor of shunt effectiveness in idiopathic normal pressure hydrocephalus