Calcium-sensitive nonselective cation channel identified in the epithelialcells isolated from the endolymphatic sac of guinea pigs

We identified a Ca2+-sensitive cation channel in acutely dissociated epithe lial cells from the endolymphatic sac (ES) of guinea pigs using the patch-c lamp technique. Single-channel recordings showed that the cation channel ha d a conductance of 24.0 +/- 1.3 pS (n = 8) in our standard solution. The re lative ionic permeability of the channel was in the order K+ = Na+ > Ca2+ m uch greater than Cl-. This channel was weakly voltage-dependent but was str ongly activated by Ca2+ on the cytosolic side at a concentration of around I mM in inside-out excised patches. With cell-attached patches, however, th e channel was activated by much lower Ca2+ concentrations. Treatment of the cells, under cell-attached configuration, with ionomycin (10 muM). carbony l cyanide 3-chlorophenylhydrazone (CCCP, 20 muM), or ATP (1 mM), which incr eased intracellular Ca2+ concentration ([Ca2+](i)), activated the channel a t an estimated [Ca2+](i) from 0.6 muM to 10 muM. It is suggested that some activators of the channel were deteriorated or washed out during the format ion of excised patches. Based on this Ca2+ sensitivity, we speculated that the channel contributes to the regulation of ionic balance and volume of th e ES by absorbing Na+ under certain pathological conditions that will incre ase [Ca2+](i). This is the first report of single-channel recordings in end olymphatic sac epithelial cells.