THE EFFECT OF EDTA AND SCN ON RADIOCALCIUM MOVEMENT IN FROG RECTUS ABDOMINIS MUSCLE DURING CONTRACTURES INDUCED BY CALCIUM REMOVAL

¹ Department of Pharmacology, Schools of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania

Calcium removal from the slow muscle fibers of the rectus abdominis causes a prolonged, transient, weak contracture, a shift of calcium into the interior of the fiber, and a prolonged slow transient increase in calcium. The relaxation of the contracture appears to be associated with decline of the calcium efflux. One possible mechanism for the weak contracture is that an increase in intracellular ionized calcium caused by either a release of intracellular calcium, or a shift of calcium from the membrane into the fiber, leads to the formation of a Ca-actomyosin complex and the development of tension. The relaxation may be the result of the gradual removal of the ionized calcium by the relaxing factor and the dissociation of the Ca-actomyosin complex. SCN ion potentiates the contracture and increases the amount of calcium released. The potentiation of the contracture and the increased release of calcium may be due to the inhibition of calcium binding by the relaxing factor system.