The mechanism by which Ag+ induces muscle contracture was elucidated by investigating the effect of external Ca2+ concentration and Ca2+ channel blocker on the maximum tension amplitude in single fibers from frog toe skeletal muscle. Five microM Ag+ induced two different types of contracture in the presence of external Ca2+ more than 0.1 mM, i.e., a phasic and a subsequent tonic contracture. The phasic contracture appeared only in fibers with intact T-tubules immersed in a solution with or without Ca2+ after a lag time of 5.7 +/- 0.9 s (N = 5). The maximum amplitude was 58% of the tetanus tension observed in the same fiber immediately before Ag+ exposure. Diltiazem at high-concentration (100 microM) inhibited the Ag+-induced phasic contracture only to a small extent (17%). The contracture was not affected by 1 microM TTX or 1 mM DAP at all. These results indicate that Na+, K+, and Ca2+ channels on the T-tubular membrane would not be attributed to the phasic tension development induced by Ag+. On the contrary, a tonic contracture did not require intact T-tubules. The amplitude and the rate of rise of the contracture depended on external Ca2+ concentrations and were inhibited by a high concentration of diltiazem. Neither 1 microM TTX nor 1 mM DAP affected them. Therefore, the tonic contracture seems to be triggered by Ca2+ which entered the muscle fiber through the surface but not T-tubular membranes.