Fast and slow gating of CLC-1: Differential effects of 2-(4-chlorophenoxy) propionic acid and dominant negative mutations

Our knowledge about ClC-1 muscle chloride channel gating, previously gained from single-channel recording and noise analysis, provides a theoretical basis for further analysis of macroscopic currents. In the present study, we propose a simple method of calculation of open probabilities (P_o) of fast and slow gates from the relative amplitudes of ClC-1 inward current components. With this method, we investigated the effects of 2-(4-chlorophenoxy) propionic acid (CPP), a drug known to produce myotonia in animals, and dominant negative myotonic mutations, F307S and A313T, on fast and slow gating of ClC-1. We have shown that these mutations affected the P_o of the slow gate, as expected from their mode of inheritance, and that CPP predominantly affected the fast gating process. CPP's action on the fast gating of mutant channels was similar to its effect in wild-type channels. Comparison of the effects of CPP and the mutations on fast and slow gating with the effects produced by reduction of external Cl^- concentration suggested that CPP and mutations exert their action by affecting the transition of the channel from its closed to open state after Cl^- binding to the gating site.