1. The properties of large conductance Ca(2+)-activated K+ channels (BK channels) were investigaed in neocortical infragranular pyramidal neurons by the use of inside-out patch recordings. Neurons were acutely isolated from slices of newborn to 28-day-old rats (P0-P28) by using minimal protease exposure followed by trituration with a vibrating glass probe. Two types of BK channels, slow-gating and fast-gating, were observed in immature neurons (P0-P5), whereas only slow-gating BK were found in more mature neurons. Fast-gating BK channels differed in conductance, voltage dependence, and kinetics from the slow-gating ones. 2. The properties of fast-gating channels included a conductance of 145 +/- 12.9 (SE) pS; frequent openings with short mean open times that were relatively voltage-independent, mean closed times that showed a voltage-dependent increase, a voltage-dependent decrease in open probability (Po). The properties of slow-gating channels contrasted with those of the fast-gating ones, in that the former had a conductance of 181 +/- 3.9 pS, longer mean open times that showed a voltage-dependent increase, mean closed times that showed a marked voltage-dependent decrease, a voltage-dependent increase in Po, and slight inward rectification. The significance of these developmental variations in channel properties is discussed.
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