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Department of Physiology, University of California School of Medicine, San Francisco 94143-0444.

1. By the use of whole-cell and excised-patch tight-seal recording techniques, we studied ionic conductances in voltage-clamped solitary hair cells isolated from the bull-frog's sacculus. As a basis for assessing their contributions to hair cell electrical resonance, we developed kinetic models describing voltage-dependent Ca2+ and Ca2+-dependent K+ conductances. 2. A transient K+ current (IA) was activated by steps to potentials positive to -50 mV from holding potentials more negative than -70 mV. In the steady state, the current was fully inactivated at the normal resting potential. Possibly due to the dissipation of a Donnan potential between the pipette's interior and the cell, the voltage dependence of IA inactivation slowly shifted in the negative direction during whole-cell recording. 3. The voltage-gated Ca2+ current (ICa) was isolated by blocking IA with 4-aminopyridine (4-AP) and Ca2+-activated K+ current with tetraethylammonium (TEA). The ICa was activated at potentials more positive than -60 to -50 mV and was maximal at about -10 mV. Its magnitude was highly variable among cells, with an average value of -240 pA at -30 mV. Its activation could be fitted well by a third-order (m3) gating scheme. 4. A Ca2+-activated K+ current (IK(Ca)) was isolated as the component of membrane current blocked by TEA. This current was activated at potentials more positive than -60 to -50 mV and had an average value of 1.5 nA at -30 mV. The Ca2+-activated K+ conductance (gK(Ca)) showed a high apparent voltage dependence, increasing e-fold every 3 mV at potentials between -50 and -40 mV. 5. The Ca2+-activated K+ current displayed rapid activation and deactivation kinetics. The current reached half-maximal activation in 2-4 ms at voltages between -50 and -30 mV, and the tail current decayed exponentially with a time constant of 1.0 ms at -70 mV. The activation rate and magnitude of IK(Ca) were reduced by lowering the extracellular Ca2+ concentration. 6. The open probability of Ca2+-activated K+ channels was estimated by ensemble-fluctuation analysis of whole-cell currents evoked by voltage steps to -30 mV. The average open probability was estimated to be 0.8 at this potential. 7. K+-selective channels with a high conductance (140-200 pS) were examined in excised, inside-out membrane patches. The activity of these channels depended on intracellular Ca2+ and membrane potential. These properties suggest that the channels underlie the whole-cell Ca2+-activated K+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

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