Properties of large conductance calcium-activated potassium channels in pyramidal neurons from the hippocampal CA1 region of adult rats.

The properties of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels were studied in rat hippocampal CA1 pyramidal neurons by using the patch-clamp technique in the excised-inside-out-patch configuration. The lowest [Ca(2+)](i) in which BK(Ca) channel activities were observed was 0.01 microM with the membrane potential of +20 mV and the [Ca(2+)](i) at which P(O) of the channel is equal to 0.5 was 2 microM. The unitary conductance of the single BK(Ca) channel was 245.4 pS with symmetrical 140 mM K(+) on both sides of the excised membrane. With a fixed [Ca(2+)](i) of 2 microM, P(O) increased e-fold with a 17.0 mV positive change in the membrane potential. Two exponentials, with time constants of 2.8 ms and 19.2 ms at the membrane potential of +120 mV with 2 microM [Ca(2+)](i), were required to describe the observed open time distribution of BK(Ca) channel, suggesting the existence of two distinct open channel states with apparently normal conductance. A BK(Ca) channel occasionally entered an apparent third open channel state with the single channel current amplitude about 45% of the normal amplitude. The properties of BK(Ca) channel, which were found in this study to be more steeply dependent on voltage and more sensitive to [Ca(2+)](i) in adult hippocampal neurons than in cultured or immature hippocampal neurons, may be responsible for the shortened duration of action potential in hippocampal CA1 pyramidal neurons of adult rat.