Modulation of the Ca2+ channel voltage sensor and excitation-contraction coupling by silver

Ag+ (0.5-10 microM) is known to produce a transient contraction of intact frog skeletal muscle fibers followed by complete inhibition of excitation-contraction (E-C) coupling. We have carried out physiological and biochemical experiments to investigate the basis of this effect. Dihydropyridine (DHP) Ca2+ channel blockers, which inhibit the voltage sensor of the Ca2+ channel, completely inhibit Ag+ contractions. Removal of extracellular Ca2+, or blockade of Ca2+ entry with cadmium, does not inhibit Ag+ contractions. Activation of the Ca2+ channel's voltage sensor with the Ca2+ channel agonists Bay K 8644 or with perchlorate, potentiates the Ag(+)-induced contraction. Ag+ binds to the partially purified rabbit skeletal muscle Ca2+ channel and inhibits DHP binding (IC50 = 1.1 microM) and sulfhydryl (SH) reactivity (IC50 = 0.11 microM) over the concentration range where it inhibits E-C coupling. Oxidation of free SH groups by H2O2 or their reaction with DTNB prevents Ag+ contractions, while DTT reduction of oxidized SH groups restores Ag+ contractions. These results suggest that Ag+ binds to critical SH groups on the DHP receptor Ca2+ channel, resulting in modification of the channel's voltage sensor and the failure of E-C coupling.