[Role of calcium in the contraction of normal tonic muscle fibers and following denervation].

Studies in the frog isolated tonic muscle fibers revealed that change of the external calcium concentration normally has no effect on the magnitude of the potassium and caffeine contractures, whereas in denervated fibers a decrease of successively evoked potassium and caffeine contractures occurs in calcium-free solution. The adaptive relaxation of potassium contracture in denervated fibers accelerates with increase of potassium ions and decelerates with increase of calcium. Analysis of these phenomena suggests that: 1) the intracellular calcium alone participates in activation of contraction of these fibers, and 2) the effect of changes in external calcium concentration on the character of contraction is based on the calcium action upon the cell membrane rather than on changes of its amount.