The Membrane Properties of Cochlear Root Cells are Consistent with Roles in Potassium Recirculation and Spatial Buffering

Auditory transduction, amplification, and hair cell survival depend on the regulation of extracellular [K⁺] in the cochlea. K⁺ is removed from the vicinity of sensory hair cells by epithelial cells, and may be distributed through the epithelial cell syncytium, reminiscent of “spatial buffering” in glia. Hypothetically, K⁺ is then transferred from the epithelial syncytium into the connective tissue syncytium within the cochlear lateral wall, enabling recirculation of K⁺ back into endolymph. This may involve secretion of K⁺ from epithelial root cells, and its re-uptake via transporters into spiral ligament fibrocytes. The molecular basis of this secretion is not known. Using a combination of approaches we demonstrated that the resting conductance in guinea pig root cells was dominated by K⁺ channels, most likely composed of the Kir4.1 subunit. Dye injections revealed extensive intercellular gap junctional coupling, and delineated the root cell processes that penetrated the spiral ligament. Following uncoupling using 1-octanol, individual cells had Ba²⁺-sensitive weakly rectifying currents. In the basal (high-frequency encoding) cochlear region K⁺ loads are predicted to be the highest, and root cells in this region had the largest surface area and the highest current density, consistent with their role in K⁺ secretion. Kir4.1 was localized within root cells by immunofluorescence, and specifically to root cell process membranes by immunogold labeling. These results support a role for root cells in cochlear K⁺ regulation, and suggest that channels composed of Kir4.1 subunits may mediate K⁺ secretion from the epithelial gap junction network.