Molecular Mechanism Underlying Silver Ion-induced Contraction in Skeletal Muscle.

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Effects of external Ca2+ concentration, Ca2+ channel blockers and chemical modification of SH groups in membrane proteins on silver ion (Ag+)-induced contraction were studied with single fibers of frog toe muscle. In the presence of 3mM Ca2+, Ag+ produced phasic (P1) and subsequent tonic (P2) contractions. When Ca2+ were replaced by Mg2+, only P1 occurred upon Ag+. This type of contraction was completely blocked by 10μM nifedipine. P2 was inhibited by application of 10μM nifedipine or 3mM Cd2+. When exposed to 1mM dithiothreitol (DTT) during contraction, the fiber rapidly decreased the tension to the resting level and then tension no longer developed. The fiber pretreated with 0.1% H2O2 for 10min did not respond upon 10μM Ag+. In such a fiber, reduction of SH groups by 10mM DTT for 2min restored the responsiveness of the fiber to Ag+. Ag+-induced contraction was not observed after exposure of the fiber to 20mM Hg2+ for 30min. These findings suggest that Ag+ has 2 sites to induce different types of contraction and that oxidation of SH groups in Ca2+ channels participates in Ag+-induced contraction.
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