A stereospecific binding site regulates the C1- ion channel in rat skeletal muscle

ClC-1 CHLORIDE CHANNEL: MATCHING ITS PROPERTIES TO A ROLE IN SKELETAL MUSCLE

1. ClC-1 is a Cl- channel in mammalian skeletal muscle that plays an important role in membrane repolarization following muscular contraction. Reduction of ClC-1 conductance results in myotonia, a state characterized by muscle hyperexcitability. 2. As is the case for other members of the ClC family, ClC-1 exists as a dimer that forms a double-barrelled channel. Each barrel, or pore, of ClC-1 is gated by its own gate ('fast' or 'single pore' gate), whereas both pores are gated simultaneously by another mechanism ('slow' or 'common' gate). 3. Comparison of the biophysical and pharmacological properties of heterologously expressed ClC-1 with the properties of the Cl- conductance measured in skeletal muscle strongly suggests that ClC-1 is the major Cl- channel responsible for muscle repolarization. However, not all results obtained in experiments on whole muscle or muscle fibres support this notion. 4. In the present review we attempt to bring together the current knowledge of ClC-1 with the physiology of skeletal muscle.

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.

Modulation of the gating of CIC-1 by S-(-) 2-(4-chlorophenoxy) propionic acid

1. Using whole-cell patch-clamping and Sf-9 cells expressing the rat skeletal muscle chloride channel, rCIC-1, the cellular mechanism responsible for the myotonic side effects of clofibrate derivatives was examined. 2. RS-(+/-) 2-(4-chlorophenoxy)propionic acid (RS-(+/-) CPP) and its S-(-) enantiomer produced pronounced effects on CIC-1 gating. Both compounds caused the channels to deactivate more rapidly at hyperpolarizing potentials, which showed as a decrease in the time constants of both the fast and slow deactivating components of the whole cell currents. Both compounds also produced a concentration-dependent shift in the voltage dependence of channel apparent open probability to more depolarizing potentials, with an EC50 of 0.79 and 0.21 mM for the racemate and S-(-) enantiomer respectively. R-(+) CPP at similar concentrations had no effect on gating. RS-(+/-) CPP did not block the passage of Cl- through the pore of rCIC-1. 3. CIC-1 is gated by Cl- binding to a site within an access channel and S-(-) CPP alters gating of the channel by decreasing the affinity of this binding site for Cl-. Comparison of the EC50 for RS-(+/-) CPP and S-(-) CPP indicates that R-(+) CPP can compete with the S-(-) enantiomer for the site but that it is without biological activity. 4. RS-(+/-) CPP produced the same effect on rCIC-1 gating when added to the interior of the cell and in the extracellular solution. 5. S-(-) CPP modulates the gating of CIC-1 to decrease the membrane Cl- conductance (GCl), which would account for the myotonic side effects of clofibrate and its derivatives.