Ion exchangers mediating NaCl transport in the renal proximal tubule

Abstract

We have studied the mechanisms of NaCl transport in the mammalian proximal tubule. Studies of isolated brush-border membrane vesicles confirmed the presence of Na⁺−H⁺ exchange and identified Cl⁻-formate and Cl⁻-oxalate exchangers as possible mechanisms of uphill Cl⁻ entry. We found that formate and oxalate each stimulate NaCl absorption in microperfused proximal tubules. Stimulation of NaCl absorption by formate was blocked by the Na⁺−H⁺-exchange inhibitor EIPA, whereas stimulation by oxalate was blocked by omission of sulfate from the perfusion solutions. These observations were consistent with recycling of formate from lumen to cell by H⁺-coupled formate transport in parallel with Na⁺−H⁺ exchange and recycling of oxalate by oxalate-sulfate exchange in parallel with Na⁺-sulfate cotransport. Using isoform-specific antibodies, we found that NHE1 is present on the basolateral membrane of all nephron segments, whereas NHE3 is present on the apical membrane of cells in the proximal tubule and the loop of Henle. The inhibitor sensitivity of Na⁺−H⁺ exchange in renal brush-border vesicles and of HCO₃ ⁻ absorption in microperfused tubules suggested that NHE3 is responsible for most, if not all, apical membrane Na⁺−H⁺ exchange in the proximal tubule. The role of NHE3 in mediating proximal tubule HCO₃ ⁻ absorption and formate-dependent Cl⁻ absorption was confirmed by studies in NHE3 null mice. Finally, we cloned and functionally expressed CFEX, an anion transporter expressed on the apical surface of proximal tubule cells and capable of mediating Cl⁻-formate exchange.