Calcium and voltage dependence of single Ca2+-activated K+ channels from cultured hippocampal neurons of rat

Calcium and voltage dependence of the Ca2+-activated K+ channel, K(Ca), was studied at the single-channel level in cultured hippocampal neurons from rat. The K(Ca) channel has approx. 220 pS conductance in symmetrical 150 mM K+, and is gated both by voltage and by Ca2+ ions. For a fixed Ca2+ concentration at the inner membrane surface, Ca]i, channel open probability, Po, increases e-fold for 14 mV positive change in membrane potential. At a fixed membrane potential (0 mV), channel activity is first observed at Ca]i = 10(-6) M, and increases with Ca2+ concentration approximating an absorption isotherm with power 1.4. The Ca]i required to half activate (Po = 0.5) the channel is 4.10(-6) M. When compared to other preparations, the K(Ca) channel from hippocampal neurons reported here shows the lowest Ca2+ sensitivity and the highest voltage sensitivity. These findings are interpreted in evolutionary terms.