Effects of extracellular calcium and other divalent cations on mechanical response of frog skeletal muscle

In order to investigate the mechanism by which elevated extracellular Ca ions decrease twitch and tetanus tension in frog skeletal muscle we made mechanical and electrophysiological measurements on single fibers or small bundles from twitch muscles. High concentration of Ca caused a hyperpolarization and an increase in the duration of action potential. The mechanical threshold, estimated by using the strength-duration curve, was shifted upward by adding Ca ions. These effects were fully reversible. Steady state twitch tension was slightly increased by replacing Mg and Ni with Ca and decreased by elevating their concentrations, although Ba resulted in a marked twitch augmentation and a positive correlation with the ion concentration. By contrast, the strength-duration curve was shifted upward by Ni while Mg and Ba showed no shift. These evidences point to a failure of the early step of excitation-contraction coupling, including the T-membrane depolarization, as the primary mechanism of action of high concentration of Ca, Mg, and Ni ions, whereas Ba ion has an additional intracellular potentiating effect.