Nonselective cation and BK channels in apical membrane of outer sulcus epithelial cells

The outer sulcus epithelium was recently shown to absorb cations from the l umen of the gerbil cochlea. Patch clamp recordings of excised apical membra ne were made to investigate ion channels that participate in this reabsorpt ive flux. Three types of channel were observed: (i) a nonselective cation ( NSC) channel, (ii) a BK (large conductance, maxi K or K-C alpha) channel an d (iii) a small K+ channel which could not be fully characterized. The NSC channel found in excised inside-out patch recordings displayed a linear cur rent-voltage (I-V) relationship (27 pS) and was equally conductive for Naand K+, but not permeable to Cl- or N-methyl-D-glucamine. Channel activity required the presence of Ca2+ at the cytosolic face, but was detected at Ca 2+ concentrations as low as 10(-7) M (open probability (P-o) = 0.11 +/- 0.0 3, n = 8). Gadolinium decreased P-o of the NSC channel from both the extern al and cytosolic side (IC50 similar to 0.6 mu M). NSC currents were decreas ed by amiloride (10 mu M similar to 1 mM) and flufenamic acid (0.1 mM). The BK channel was also frequently (38%) observed in excised patches. In symme trical 150 mM KCl conditions, the I-V relationship was linear with a conduc tance of 268 pS. The Goldman-Hodgkin-Katz equation for current carried sole ly by K+ could be fitted to the I-V relationship in asymmetrical K+ and Na solutions. The channel was impermeable to Cl- and N-methyl-D-glucamine. P- o of the BK channel increased with depolarization of the membrane potential and with increasing cytosolic Ca2+. TEA (20 mM), charybdotoxin (100 nM) an d Ba2+ (1 mM) but not amiloride (1 mM) reduced P-o from the extracellular s ide. In contrast, external flufenamic acid (100 mu M) increased P-o and thi s effect was inhibited by charybdotoxin (100 nM), Flufenamic acid inhibited the inward short-circuit current measured by the vibrating probe and cause d a transient outward current. We conclude that the NSC channel is Ca2+ act ivated, voltage-insensitive and involved in both constitutive K+ and Na+ re absorption from endolymph while the BK channel might participate in the Kpathway under stimulated conditions that produce an elevated intracellular Ca2+ or depolarized membrane potential.