The Role of Cell Calcium in the Contraction of Single Cannulated Muscle Fibers

Abstract

Intracellular injections of the calcium-binding agent, EGTA, into single cannulated fibers of Balanus and Maia were able to suppress, almost completely, the contractions induced by various contractile agents. The amount oE EGTA required in Maia fibers for the suppression of the contractile response produced by caffeine and high-K saline, as has already been reported, was similar to the mean fiber calcium level. In Balanus fibers, however, although the amount of EGTA needed for the suppression of the caffeine-saline response was similar to the estimated level of fiber calcium, the amount required in the raised-K-saline experiments was considerably greater. It has been suggested that membrane depolarization under these conditions allows calcium to enter the fiber from the external saline, the amount entering being related, at least in part, to a large effective sarcolemmal surface area and the presence of binding agent internally. The results of intracellular and plate-electrode stimulation of Belanus fibers also suggested that the fiber under certain conditions could utilize external calcium ions, while the results of plate-electrode stimulation of Maia fibers could be explained most easily in terms of mobilization of mainly intracellular calcium for the process of contraction. Efflux of Sr⁸⁹ ions from Balanus fibers under various conditions suggested that this ion is bound and mobilized internally in a manner similar to calcium. The results are seen not to contradict a chanelled-current theory for e-c coupling of the type proposed for crayfish fibers.