Some features of the electrically induced contracture in the frog skeletal muscle: Reexamination of the post-anodal inhibition.

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Abstract
Some features of the electrically induced contracture were investigated by means of the partition polarization method in the toe muscles of a bullfrog. Under the current clamp condition and in the presence of tetrodotoxin, a biphasic contracture was induced by strong depolarizing pulses, 1 sec, in duration. The contracture was depressed either by lengthening the pulse interval or by reducing the external calcium concentration. Furthermore, the contracture was augmented by applying conditioning depolarization and inhibited by conditioning hyperpolarization without accompanying any noticeable changes in the test depolarization. Caffeine at a concentration of 1 mM depressed the inhibitory effect of pre-hyperpolarization. Strong hyperpolarizing pulses had no effect on the caffeine contracture, whereas a similar pre-hyperpolarization markedly suppressed the phenomenon on the post-contracture potentiation of twitch, induced by a powerful sustained depolarization. From these results it was suggested that calcium ions in the vicinity of the myoplasmic site of the transverse tubular membrane may play some role in generating and regulating the contracture. The possibility of direct contribution of calcium entering from the cell exterior in contracture generation, especially for the slow component, and its regulation was also discussed.