Roles of extracellular and "trigger" calcium ions in excitation--contraction coupling in skeletal muscle.

The experimental observations leading to the development of the "trigger" calcium hypothesis of excitation--contraction (E--C) coupling in skeletal muscle are discussed. Also considered in some detail are the experimental technique problems which interfere with the demonstration of this role for calcium. New findings reported are observations showing that in a zero Ca2+ solution after a delay of about 6--10 min, there is a stimulation of Ca2+ efflux. This is of sufficient size, even in very small toe muscles, to restore the twitch which previously had been reduced in size in the zero Ca2+. In studies with isolated fibre preparations it was demonstrated that depolarization contractures required extracellular Ca2+ ions for E--C coupling whereas twitches could use membrane-bound "trigger" calcium ions. Thus in zero Ca2+ the contractures were eliminated in a few seconds but twitch elimination took a few minutes. Finally, the roles in E--C coupling played by "trigger" and extracellular Ca2+ ions are summarized and discussed.