Abstract
BackgroundSodium absorption by Reissner's membrane is thought to contribute to the homeostasis of the volume of cochlear endolymph. It was previously shown that the absorptive transepithelial current was blocked by amiloride and benzamil. The most commonly-observed target of these drugs is the epithelial sodium channel (ENaC), which is composed of the three subunits α-,β- and γ-ENaC. However, other less-selective cation channels have also been observed to be sensitive to benzamil and amiloride. The aim of this study was to determine whether Reissner's membrane epithelial cells could support parasensory K+ absorption via amiloride- and benzamil-sensitive electrogenic pathways.ResultsWe 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.ConclusionsThese 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.
Highlights
Sodium absorption by Reissner’s membrane is thought to contribute to the homeostasis of the volume of cochlear endolymph
In view of the high luminal concentration of K+ in the inner ear and the importance of K+ efflux pathways for endolymph homeostasis, we investigated whether Reissner’s membrane (RM) epithelial cells could support parasensory K+ absorption via amiloride-sensitive electrogenic pathways
We have shown in previous studies that Reissner’s membrane in mouse and gerbil absorbs Na+ from the cochlear lumen by electrogenic transepithelial transport, which was apparently mediated by apical epithelial Na+ channel (ENaC), basolateral Na+,K+-ATPase, and basolateral K+ channels [4,5]
Summary
Sodium absorption by Reissner’s membrane is thought to contribute to the homeostasis of the volume of cochlear endolymph. Other less-selective cation channels have been observed to be sensitive to benzamil and amiloride. The aim of this study was to determine whether Reissner’s membrane epithelial cells could support parasensory K+ absorption via amiloride- and benzamilsensitive electrogenic pathways. The inner ear has absorptive pathways for both Na+ and K+ that contribute to the homeostasis of the composition of endolymph, the luminal fluid. From endolymph in order to maintain osmotic balance, to prevent loading of sensory hair cells with Na+ and to maintain functional physical properties of the tectorial membrane. Na+, like K+, is absorbed via nonselective cation channels in the apical membranes of outer sulcus cells. Na+ appears to be absorbed via an amiloride-sensitive pathway in Reissner’s membrane (RM) of the cochlea
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