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

Abstract

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.