Single-channel analysis of fast transient outward K+ currents in frog skeletal muscle

The patch-clamp method was employed to examine the voltage-dependent gating mechanism of A-type K+ channels, which generate the transient outward K+ currents described previously in a study of vesicles derived from the sarcolemma of frog skeletal muscle. Channels were activated by depolarizing pulses. There is evidence for non-random grouping of records with channel openings and blank records when depolarizations were repeated at brief intervals, suggesting a slow process similar to slow inactivation. Binomial analysis was consistent with independent behaviour of the channels. Ensemble average currents obtained from multichannel patches had kinetics similar to those of the macroscopic A-type K+ current, IA. The rate of activation, fitted to n4 kinetics, was fast and voltage dependent. The rate of inactivation had an exponential time course with a voltage-independent time constant. The mean open time and the probability of a channel being open increased with depolarization. The histograms of latency to first opening revealed the presence of more than two voltage-dependent closed states. Channel openings occurred in bursts and the closed-time histograms could be fitted by the sum of two or three exponentials. These results suggest a gating scheme with at least three closed states, probably two open states, and two inactivated states.