[Mechanisms of excitation-contraction coupling and calcium liberation in striated muscles of vertebrates]

Abstract

In vertebrate skeletal muscle, the main part of excitation-contraction coupling occurs at the level of the triad, where membranes of T-system and of junctional SR are facing each other. From place to place, the junctional gap is bridged by "feet" structures which include the SR Ca2+ channel. Half of them are closely apposed to tubular intramembranous structures assumed to be DHP-sensitive voltage-sensors which are similar to tubular Ca2+ channels and act by controlling Ca2+ release from SR. During a twitch, the release of Ca2+ activator from SR is controlled both by voltage-sensors via the feet structures and by a tubular Na+ current via a Na+-induced Ca2+ release mechanism. During long-duration mechanical responses, additional mechanisms are involved: a Ca2+-induced Ca2+ release which can be activated by ICa; the release of Ca2+ from membrane, controlled by the operation of a Na+/Ca2+ exchanger and/or new arrangements of surface membrane charges. An IP3-mediated Ca2+ release could be involved too. All these mechanisms can be regulated by intracellular biochemical or ionic processes.