Mechanical Threshold As a Factor in Excitation-Contraction Coupling

I^-, CH₃SO₄^-, and ClO₄^-, like other previously studied type A twitch potentiators (Br^-, NO₃^-, SCN^-, and caffeine), lower the mechanical threshold in K depolarization contractures of frog skeletal muscle. In potentiated twitches, I^-, Br^-, CH₃SO₄^-, ClO₄, and SCN, as already reported for NO₃^- and caffeine, slightly shorten the latent period (L) and considerably increase the rate of tension development (dP/dt) during the first few milliseconds of the contraction period. Divalent cations (8 mM Ca²⁺, 0.5–1.0 mM Zn²⁺ and Cd²⁺) raise the mechanical threshold of contractures, and correspondingly affect the twitch by depressing the tension output, increasing L, and decreasing the early dP/dt, thus acting oppositely to the type A potentiators. These various results form a broad, consistent pattern indicating that electromechanical coupling in the twitch is conditioned by a mechanical threshold as it is in the contracture, and suggesting that the lower the threshold, in reference to the raised threshold under the action of the divalent cations, the more effective is a given action potential in activating the twitch as regards especially both its early rate and peak magnitude of tension development. The results suggest that the direct action by which the various agents affect the level of the mechanical threshold involves effects on E-C coupling processes of the T tubular and/or the sarcoplasmic reticulum which control the release of Ca for activating contraction.