The stochastic properties of single Ca-activated K channels in excised patches of surface membrane from cultured rat muscle cells were studied using the patch-clamp technique. The distribution of all open intervals was described by the sum of two exponential distributions of short and long mean open time, suggesting at least two major open-channel states. Increasing the concentration of Ca at the inner membrane surface, [Ca](i), increased the mean duration of the long open distribution, while having little effect on the mean duration of the short open distribution. The frequency of openings to each distribution increased with [Ca](i). The rate of increase was a much steeper function of [Ca](i) for openings in the long open distribution than for openings in the short open distribution; about 80% of the openings were to the long open distribution with 0.1 μM-Ca(i), increasing to 97% with 1 μM-Ca(i) (+30 mV). These results suggest that openings in both open distributions are Ca-dependent, with openings in the long open distribution requiring the binding of more Ca ions than openings in the short open distribution. The distribution of all shut intervals at 0.5 μM-Ca(i) and +30 mV was described by the sum of three exponential distributions with mean durations of: 0.21 msec (short shut distribution), 1.90 msec (intermediate shut), and 44 msec (long shut). These results indicate that the channel typically enters at least three closed channel states during normal channel activity. In addition, a few longer shut intervals not accounted for by the above distributions suggested that there was a fourth infrequently occurring inactivated closed-channel state. The mean duration of the distribution of long shut intervals decreased with a power of about 2 with increasing [Ca](i) under conditions where most openings were to the long open state (+30 mV, 0.25-1 μM-Ca(i)). This observation suggests that openings to the long open distribution typically require the binding of two or more Ca ions. The mean intermediate shut interval appeared to increase slightly with increasing [Ca](i) while the mean short shut interval was relatively Ca(i)-independent. The percentage of all shut intervals that were short shut intervals increased with increasing [Ca](i) while the percentage of long shut intervals decreased. This shift gives rise to an increase in the frequency and duration of apparent bursts of channel activity with increasing [Ca](i). These results suggest that the short open distribution is dominated by openings to an open state whose lifetime is short and independent of [Ca](i), and that the long open distribution is dominated by openings to a compound open channel state whose lifetime increases with [Ca](i). Openings to the short open state typically require the binding of one Ca ion, and openings to the long (compound) open state typically require the binding of two Ca ions, with the binding of a third prolonging this state. A model is presented that can account quantitatively for many of the kinetic properties of the Ca-activated K channel.
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