A PRELIMINARY NOTE ON EXCITATION-CONTRACTION COUPLING

Abstract

When a short-lived action potential passes along the muscle fiber membrane, contraction invari ably follows. The mechanism whereby the contractile system is acti vated up to a distance as great as 50 [mu] from the membrane, and within 30 msec., is unknown. Observations on a series of experiments with frog twitch muscle, together with earlier findings, suggest the following tentative mechanism for excitation-contraction coupling. The process is started by the propagated action potential, which eliminates the potential difference across the fiber membrane and also penetrates radially along the "Z" bands, resulting in changes of charge density in the close vicinity of these structures. This "priming" step is an in dispensable agent in the series of events leading to contraction, but it is not adequate by itself to set up maximum activity, since its effective range is narrow. Assuming a periodic distribution of Ca and assuming that ionized Ca is instrumental in the transformation of a resting to an active muscle, the effect of depolarization may be visualized as the release of ionized Ca in the close vicinity of the fiber membrane and the "Z" bands. The role of the internal currents in this picture is to move ionized Ca with great rapidity over a distance of 2.5 [mu] (i.e., the time limit of Hill for maximum activity). This movement of the Ca ion, a powerful activator of actomyosin (as an ATP-ase), can serve to establish direct contact between this ion and the enzyme. The same may be true for Mg and the "relaxing factors". Although the validity of this picture remains to be proved, it nevertheless appears plausible.