Water permeability of cochlear outer hair cells: Characterization and relationship to electromotility

The distinguishing feature of the mammalian outer hair cells (OHCs) is to e longate and shorten at acoustic frequencies, when their intracellular poten tial is changed. This "electromotility" or "electromechanics" depends criti cally on positive intracellular pressure (turgor), maintained by the inflow of water through yet uncharacterized water pathways. We measured the water volume flow, J(v), across the plasma membrane of isolated guinea pig and r at OHCs after osmotic challenges and estimated the osmotic water permeabili ty coefficient, P-f, to be similar to 10(-2) cm/sec. This value is within t he range reported for osmotic flow mediated by the water channel proteins, aquaporins. J(v) was inhibited by HgCl2, which is known to block aquaporin- mediated water transport. P-f values that were estimated for OHCs from neon atal rats were of the order of similar to 2x10(-3) cm/sec, equivalent to th at of membranes lacking water channel proteins. In an immunofluorescence as say we showed that an anti-peptide antibody specific for aquaporins labels the lateral plasma membrane of the OHC in the region in which electromotili ty is generated. Using patch-clamp recording, we found that water influx in to the OHC is regulated by intracellular voltage. We also found that the mo st pronounced increases of the electromotility-associated charge movement a nd of the expression of OHC water channels occur between postnatal days 8 a nd 12, preceding the onset of hearing function in the rat. Our data indicat e that electromotility and water transport in OHCs may influence each other structurally and functionally.