Ion channels in cell membranes, whether voltage-dependent or activated by ligands, make repeated transitions among open and shut states during activity. Information about the number of states and the transitional pathways between then can be obtained from the duration of open and shut intervals, as transitions to states of different lifetimes result in intervals of different mean durations. If there is only one open conformation, or state, then the durations of open intervals would be independent of the durations of adjacent shut intervals. On the other hand, if a channel has two or more open states with different mean lifetimes, and if each open state is entered directly from a different shut state with a different mean lifetime, then the open intervals should be related to the adjacent shut intervals. We now report that the durations of adjacent open and shut intervals for both a Cl channel and a large conductance Ca-activated K channel in skeletal muscle are inversely related; shorter open intervals are adjacent to longer shut intervals. These findings indicate that two or more shut states make direct transitions to two or more open states, and suggest that the lifetimes of adjacent open and shut states are inversely related.
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