SLOWLY ACTIVATING VOLTAGE-DEPENDENT K+ CONDUCTANCE IS APICAL PATHWAY FOR K+ SECRETION IN VESTIBULAR DARK CELLS

Dark cell epithelium secretes K+ into the lumen of the vestibular laby rinth by a previously unidentified apical transport mechanism. Previou s single-channel patch-clamp studies demonstrated nonselective cation channels and maxi-K+ channels in the apical membrane, but in too low a density to account for transepithelial K+ transport. In this report, we demonstrated with the cell-attached macro-patch-clamp technique an outward apical membrane current at 0-mV pipette voltage, which was sti mulated by elevating bath K+ concentration from 3.6 to 25 mM and inhib ited by 10 mu M bumetanide, similar to their known effects on transepi thelial short-circuit current and K+ secretion. Furthermore, the patch current was activated over several seconds by a sustained depolarizat ion and deactivated over several hundred milliseconds by a hyperpolari zation. Current-voltage relationships from tail currents were obtained with either NaCl or KCl in the pipette. Depolarization from -40 to +4 0 mV led to an increased conductance by a factor of 7.3 +/- 1.7 (n = 7 ) and 19.2 +/- 7.6 (n = 6) for NaCl and KCl, respectively, and to a re versal voltage near the presumed equilibrium potential for K+. The res ults demonstrate that dark cell K+ secretion occurs via K+-selective c hannels with characteristics similar to those associated with the I-sK protein.