Probing the major skeletal muscle chloride channel with Zn²⁺ and other sulfhydryl-reactive compounds

The sensitivity of the human skeletal muscle Cl^- channel, hClC-1, towards various sulfhydryl-reactive agents was tested with the channel expressed in Xenopus oocytes and in human embryonic kidney cells. External but not internal Zn²⁺, at 1 mM, substantially reduced the current without affecting activation parameters. External Cd²⁺ and Hg²⁺ as well as organic mercurial compounds reduced the Cl^- currents to a similar degree. With the mutant channel hClC-1 D136G, presumed to have a defective voltage sensor, external Zn²⁺ also reduced the current without effect on the altered gating. These findings suggest that hClC-1 contains cysteine residues near the extracellular face that may directly influence ion conduction. Since Zn²⁺ can also bind to histidine side chains, we tested the effect of compounds with either more cysteine- or more histidine-specificity. The results confirm the involvement of cysteine(s) in the observed effects but do not exclude the involvement of histidine(s).