Possible involvement of Ca2+-induced Ca2+ release mechanism in Ag+-induced contracture in frog skeletal muscle

To determine if an Ag⁺-induced contracture is associated with the Ca²⁺-induced Ca²⁺ release mechanism in the sarcoplasmic reticulum, effects of Ca²⁺-induced Ca²⁺ release modulators on the Ag⁺-induced contracture were studied with single fibers of frog toe skeletal muscle. The fiber treated with 1 mM caffeine contracted significantly much more than controls without caffeine at Ag⁺ concentrations below 1 μM. Procaine shifted the Ag⁺ concentration-tension curve to the right, dose-dependently. When 10 mM procaine was applied to contracting fibers not treated with caffeine, the duration of 5 μM Ag⁺-induced contracture was shortened with a little decrease in tension amplitude, that was different from the effect of procaine on caffeine contracture. In caffeine solution, 0.5 μM Ag⁺ caused a long-lasting contracture with sometimes two peaks. 2 mM procaine led to disappearance of such two peaks, resulting in shortening of the contracture. K⁺ contracture was potentiated by 1 mM caffeine only at lower concentrations of K⁺, and inhibited by 10 mM procaine. These results suggest that the Ag⁺-induced contracture is composed of two components: Ca²⁺-induced Ca²⁺ release-dependent and -independent. 5 μM Ag⁺-induced contracture slowly relaxed with a wavy tension pattern to the resting level when 0.05 mM dithiothreitol was applied around peak of the tension. This relaxation was accelerated by procaine application. These findings may be explained by attributing a portion of Ag⁺-induced contracture to the effect of Ca²⁺ released through the Ca²⁺-induced Ca²⁺ release mechanism in the sarcoplasmic reticulum.