Effects of sodium depletion on the caffeine-induced contraction of frog's skeletal muscle.

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Abstract
Effects of depletion of Na ions from the environmental medium on the caffeine-induced contraction were investigated in both intact and skinned fibers prepared from the frog''s fast twitch muscle. In intact muscle fibers, both the twitch and the contracture induced by 5 mM caffeine were enhanced by depleting the external Na at 9-10.degree. C. In contrast, in the mechanically skinned muscle fiber, which was used to examine the capability of the sarcoplasmic reticulum to take up Ca ions from the sarcoplasm and/or the superfusing medium, the contractile tension as well as its fractional area measured by applying 25 mM caffeine were markedly inhibited by Na-dependent at 5.degree. C. The apparent discrepancy between these two conditions is discussed.