Kinetics of the Ca(2+)-activated K+ channel in rat hippocampal neurons.

The kinetics of the large-conductance Ca(2+)-activated K+ channel (235 pS in symmetrical 150 mM K+) were examined in the inside-out mode of the patch clamp technique. The open probability of the channel increased when [Ca2+]i, [Sr2+]i, or [Ba2+]i was increased. The [Ca2+]i-response relation was fitted with a Hill coefficient of 2 and half-maximum concentrations of 185, 80, 14.5, and 5.5 microM at -40, -20, +20, and +40 mV, respectively. The channel was blocked by TEA or Ba2+. The open-time histogram showed a single exponential component and the closed-time histogram showed at least two exponential components at various [Ca2+]i. Increasing [Ca2+]i decreased the time constant of the slow component of the closed-time histogram. Cell-attached patch recording revealed activation of the large-conductance Ca(2+)-activated K+ channel (BK channel) during the action potential. The deactivation time course was consistent with the fast after-hyperpolarization. A minimum model of the channel, close(2)-close(1)-open, where the transition from close(2) to close(1) requires the binding of 2 Ca2+, reconstructed quick activation of the channel if [Ca2+]i of 40 microM was assumed.