Different Fast-Gate Regulation by External Cl− and H+ of the Muscle-Type Clc Chloride Channels

Abstract

The fast gate of the muscle-type ClC channels (ClC-0 and ClC-1) opens in response to the change of membrane potential (V). This gating process is intimately associated with the binding of external Cl− to the channel pore in a way that the occupancy of Cl− on the binding site increases the channel's open probability (Po). External H+ also enhances the fast-gate opening in these channels, prompting a hypothesis that protonation of the binding site may increase the Cl− binding affinity, and this is possibly the underlying mechanism for the H+ modulation. However, Cl− and H+, modulate the fast-gate Po-V curve in different ways. Varying the external Cl− concentrations ([Cl−]o) shifts the Po-V curve in parallel along the voltage axis, whereas reducing external pH mainly increases the minimal Po of the curve. Furthermore, H+ modulations at saturating and nonsaturating [Cl−]o are similar. Thus, the H+ effect on the fast gating appears not to be a consequence of an increase in the Cl− binding affinity. We previously found that a hyperpolarization-favored opening process is important to determine the fast-gate Po of ClC-0 at very negative voltages. This [Cl−]o-independent mechanism attracted little attention, but it appears to be the opening process that is modulated by external H+.