Adjacent interval analysis distinguishes among gating mechanisms for the fast chloride channel from rat skeletal muscle.

Abstract

The durations of adjacent open and shut intervals, obtained with the patch-clamp technique from fast Cl- channels in tissue-cultured rat skeletal muscle, were analyzed to distinguish among eight previously considered gating mechanisms for the channel which differed in the connections among the states. The findings indicate that at least two open states are connected by independent pathways to different shut states; the open state associated with the fast open component is connected to a shut state (or compound shut state) of longer effective lifetime, and the open state associated with the slow open component is connected to a shut state (or compound shut state) of briefer effective lifetime. Seven of the eight previously considered gating mechanisms were rejected because they did not account for the observed relationships between the durations of adjacent open and shut intervals, when analysed in terms of either conditional open distributions or conditional mean open interval durations. The seven rejected gating mechanisms also did not account for the observed correlations between interval durations, when analysed in terms of correlation coefficients. Adjacent interval and correction analysis thus provided a means to distinguish among the gating mechanisms. A free energy state diagram derived, assuming absolute rate theory, from the rate constants for the one consistent kinetic scheme indicated that the free energy required to overcome the effective transition barriers between states was 11-15 kcal/mol, whereas the free energy difference between connected equilibrium states was equal to only about 0.5-1.5 kcal/mol. Thus, if transitions between states involve the breaking of weak bonds (hydrogen bonds, van der Waals bonds or salt bridges), then about the same equivalent number of bonds are reformed immediately ater each transition.