Kinetic analysis of voltage‐ and ion‐dependent conductances in saccular hair cells of the bull‐frog, Rana catesbeiana.

Abstract

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)