Effects of external sodium removal on the contracture of frog skeletal muscle

Effects of removal of the external sodium on the contractility of the frog's skeletal muscle were studied. Sodium-free solution, replaced by choline, strongly augmented the twitch tension before it abolished the action potential. A similar marked potentiation was also observed in the sustained contracture induced by a small depolarization under the current-clamped condition where generation of conductive action potential was blocked. On the other hand, the maximum contracture induced either by electrical stimulation or by high potassium solution was not modified by lowering the external sodium concentration. Furthermore, contraction threshold measured on the electrically induced contracture was shifted to more negative value of the membrane potential. Caffeine contracture, examined by a short-term application of 5 mM caffeine, was first inhibited by sodium-free solution but afterwards restored or even augmented during the sodium-free perfusion. From these observations it is concluded that a removal of external sodium lowered the mechanical threshold of the muscle by affecting some step of the coupling process at the T-SR junction without affecting the contractile machinery.