Burst kinetics of single Ca-activated K channels in excised patches of surface membrane from cultured rat muscle were studied using the patch-clamp technique. Channel activity was separated into bursts using a calculated gap derived from the distribution of shut intervals. Shut intervals greater than the calculated gap were taken as gaps between bursts. The distribution of burst duration was described as the sum of two exponentials with mean durations of about 0.8 and 24 msec (1 μM-Ca(i), +20 mV), suggesting two classes of bursts (short and long). The composition of short and long bursts was determined from comparisons of the distributions of open intervals, unit bursts (bursts of single openings), and openings/burst. Short bursts consisted mainly of single openings to the open channel state of short mean lifetime. Long bursts consisted of one or more openings to the (compound) open-channel state of long mean lifetime, plus, in fewer than 70% of the long bursts, one or more openings to the short open-channel state. The frequency of occurrence of bursts from each class first increased and then decreased with increasing [Ca](i), with the number of long bursts increasing at a greater rate than the number of short bursts. The number of openings/short burst was relatively independent of [Ca](i), while the number of openings/long burst increased, often more than linearly, with increasing [Ca](i). This increase arose almost entirely from an increase in openings to the long open state. These results suggest that openings to the long open state typically require the binding of three or more Ca ions, and openings to the short open state typically require the binding of at least one Ca ion. This is the case whether the openings occur in isolation as bursts of single openings or in bursts composed of both types of openings. An obvious burst of channel activity would occur when the channel opens and closes several times without losing all its bound Ca. The power relationship between [Ca](i) and the percentage of time spent in the open state is accounted for in terms of the effects of [Ca](i) upon mean channel open time, openings/burst, and burst rate. A model is presented that describes quantitatively many features of the burst kinetics of the Ca-activated K channel for constant [Ca](i).
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