Sodium selectivity of Reissner's membrane epithelial cells

We determined the molecular and functional expression of candidate cation channels with gene array (GEO GSE6196), RT-PCR, and whole-cell patch clamp. Transcript expression analysis of Reissner's membrane detected no amiloride-sensitive acid-sensing ion channels (ASIC1a, ASIC2a, ASIC2b) nor amiloride-sensitive cyclic-nucleotide gated channels (CNGA1, CNGA2, CNGA4, CNGB3). By contrast, α-,β- and γ-ENaC were all previously reported as present in Reissner's membrane. The selectivity of the benzamil-sensitive cation currents was observed in whole-cell patch clamp recordings under Cl--free conditions where cations were the only permeant species. The currents were carried by Na+ but not K+, and the permeability of Li+ was greater than that of Na+ in Reissner's membrane. Complete replacement of bath Na+ with the inpermeable cation NMDG+ led to the same inward current as with benzamil in a Na+ bath.These results are consistent with the amiloride/benzamil-sensitive absorptive flux of Reissner's membrane mediated by a highly Na+-selective channel that has several key characteristics in common with αβγ-ENaC. The amiloride-sensitive pathway therefore absorbs only Na+ in this epithelium and does not provide a parasensory K+ efflux route from scala media.The inner ear has absorptive pathways for both Na+ and K+ that contribute to the homeostasis of the composition of endolymph, the luminal fluid. The regulation of the ion composition of endolymph is essential for normal hearing [1,2]. Transepithelial K+ efflux through the sensory hair cells in the cochlea is responsible for detection of sound. Parasensory K+ absorption through other cell types is needed to compensate for changes in sensory cell K+ flux due to changes in levels of stimulation from acoustic inputs. The cochlear outer sulcus is an epithelial domain known to participate in absorption of both K+ and Na+ [3].Absorptive mechanisms are needed to remove Na+ from endolymph in order to maintain osmotic balance, to prevent lo