MAXI K+ CHANNEL IN APICAL MEMBRANE OF VESTIBULAR DARK CELLS

Recordings were made on excised apical membrane patches from vestibular dark cells from the semicircular canal of gerbils to determine if ion channels could be involved in the process of K+ secretion. Both nonselective cation channels [Am. J. Physiol. 262 (Cell Physiol. 31): C1430-C1436, 1992] and K+-selective channels were found. The K+ channels occurred in only 0.7% of the patches. In symmetrical 145 mM KCl solutions, the current-voltage (I-V) relation of the K+-selective channel was linear, indicating the absence of rectification, and the conductance was 240 +/- 8 pS (n = 8). The Goldman-Hodgkin-Katz equation for current carried solely by K+ could be fitted to the I-V relation in asymmetrical K+ and Na+ solutions and yielded a K+ permeability of 5.78 x 10(-13) cm3/s (n = 12). The channel was shown to be impermeable to Li+, NH4+, N-methyl-D-glucamine, and Cl-. Channel activity increased with depolarization and with increasing free [Ca2+]; for voltages between +40 and -60 mV, the strongest regulation occurred in the range 10(-6) to 10(-5) M Ca2+. Tetraethylammonium (2 x 10(-2) M) had from the cytosolic side no effect on the open probability (P(o)) but completely inhibited activity from the extracellular side. P(o) was reduced by Ba2+ (5 x 10(-3) M), verapamil (10(-4) M), quinine (10(-4) M), and quinidine (10(-4) and 10(-3) M), while lidocaine (5 x 10(-3) M) had no measurable effect on P(o) but decreased the amplitude. Rb+ and Cs+ were either poorly permeable or partially blocked the channel in a voltage-dependent manner. These results, when taken into account with the magnitude of transepithelial K+ flux, suggest that additional pathways must be present in the apical membrane to account for K+ secretion.