Ca(2+)-activated nonselective cation channel in apical membrane of vestibular dark cells

Patch-clamp recordings were made on cell-attached and excised apical membrane from dark cells of the semicircular canal of the gerbil. These cells are thought to secrete K+ and absorb Na+ from the luminal fluid (endolymph). Single-channel events were identified as being equally conductive (27.6 +/- 0.4 pS; n = 48) for K+, Na+, Rb+, Li+, and Cs+ and 1.4 times more permeable to NH4+ but not permeable to Cl-, Ca2+, Ba2+, nor to N-methyl-D-glucamine. The channels displayed linear current-voltage relations that passed nearly through the origin (intercept: -2.6 +/- 0.5 mV; n = 48) when conductive monovalent cations were present on both sides of the membrane in equal concentrations. Channel activity required the presence of Ca2+ at the cytosolic face; there was no activity at less than or equal to 10(-7) M Ca2+ and full activity at greater than or equal to 10(-5) M Ca2+. Cell-attached recordings had a mean reversal voltage of -36.4 +/- 7.9 mV (n = 7), which was interpreted to reflect the intracellular potential of dark cells under the present conditions. We have identified a nonselective cation channel in the apical membrane of vestibular dark cells that might participate in K+ secretion or Na+ absorption under stimulated conditions, but the density appears to be insufficient to fully account for the transepithelial K+ flux.