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

The properties of large-conductance Ca2+-activated K+ (BKCa) channels were studied in rat hippocampal CA1 pyramidal neurons by using the patch-clamp technique in the excised-inside-out-patch configuration. The lowest [Ca2+](i) in which BKCa channel activities were observed was 0.01 muM with the membrane potential of +20 mV and the [Ca2+](i) at which P-o of the channel is equal to 0.5 was 2 mum. The unitary conductance of the single BKCa channel was 245.4 pS with symmetrical 140 mm K+ on both sides of the excised membrane. With a fixed [Ca2+](i) of 2 muM, 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 +20 mV with 2 muM [Ca2+](i), were required to describe the observed open time distribution of BKCa, channel, suggesting the existence of two distinct open channel states with apparently normal conductance. A BKCa channel occasionally entered an apparent third open channel state with the single channel current amplitude about 45% of the normal amplitude. The properties of BKCa channel, which were found in this study to be more steeply dependent on voltage and more sensitive to [Ca2+](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.