Interaction of hydrophobic anions with the rat skeletal muscle chloride channel ClC-1: effects on permeation and gating.

Permeation of a range of hydrophobic anions through the rat skeletal muscle chloride channel, rClC-1, expressed in Sf-9 (a Spodoptera frugiperda insect cell line) cells has been studied using the whole-cell patch-clamp technique. Bi-ionic reversal potentials measured with external application of foreign anions gave the following permeability sequence: Cl- (1) > benzoate (0.15) > hexanoate (0.12) > butyrate (0.09) > propionate (0.047) approximately formate (0.046). Anions with larger hydrophobic moieties were more permeant, which suggested that ClC-1 selectivity for hydrophobic anions is dominated by their interaction with a hydrophobic region in the external mouth of the pore. All anions studied when applied from outside show an apparently paradoxical voltage-dependent block of inward currents; this voltage-dependent block could be qualitatively described by a discrete-state permeation model with two binding sites and three barriers. Effects of the external anions with aliphatic side-chains on the apparent open probability (Po) suggested that they are unable to gate the channel, but can modulate ClC-1 gating, probably, by changing Cl- affinity to the gating site. Effects of internal application of benzoate, hexanoate or propionate mimicked those of increasing internal pH, and similarly depended on the channel protonation from the external side. Results for internal benzoate support the concept of a negatively charged cytoplasmic particle being involved in the ClC-1 gating mechanism sensitive to the internal pH.