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

Abstract

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 μM with the membrane potential of +20 mV and the [Ca2+]i at which Po of the channel is equal to 0.5 was 2 μM. The unitary conductance of the single BKCa channel was 245.4 ρS with symmetrical 140 mM K+ on both sides of the excised membrane. With a fixed [Ca2+]i of 2 μM, Po 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 μM [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.