Single channel currents were recorded with microsecond time resolution from large-conductance calcium-activated K+ channels to examine the details of the opening and closings transitions. Analysis of averaged closing transitions indicated that the initial average conductance step for closing was to the 90-95% closed channel current level. Averaged brief closings (~50 μs) reopened from the initial 90-95% level, whereas averaged longer closings (>300 μs) closed completely from this level over the next 50-100 μs. The 90-95% initial closed level in the averaged current records resulted typically from the average of both complete and partial closings. From 45- 80% of the initial closings were complete and 20-55% were to brief lifetime (~50 μs) subconductance levels at 65-90% of the completely closed level. Averaged opening transitions were typically mirror images of averaged closing transitions. To extend the analysis to the very brief conductance changes that underlie the flickers of the single channel current toward the closed current level, flickers, brief closings, and longer closings were averaged separately and their slopes compared. The slopes were similar (within the 3% resolution of the method), suggesting similar initial conductance steps. Similar initial closing properties for both the briefer and longer closings would be expected if the channel first passed through the kinetic and subconductance states associated with the briefer closings (including flickers) before entering the longer closed states. Such transitions would provide an explanation for the observation that openings and closings often occur in two steps.
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