Modulation of the gating of ClC-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, rClC-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 ClC-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 rClC-1. 3. ClC-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 rClC-1 gating when added to the interior of the cell and in the extracellular solution. 5. S-(-) CPP modulates the gating of ClC-1 to decrease the membrane Cl^- conductance (G(Cl)), which would account for the myotonic side effects of clofibrate and its derivatives.