Effects of sudden changes in external sodium concentration on twitch tension in isolated muscle fibers

When [Na] was suddenly introduced to single muscle fibers (Xenopus or frog), which had been pretreated with Na-free solution (Tris-substituted), the time-course of twitch recovery was very variable, half-time ranging from less than 1 S to 5 S. The [Na] vs. twitch height relationship was also variable. In small Xenopus fibers, decreases of [Na] to 50% increased the twitch, while in large Xenopus fibers twitch height remained constant or decreased as [Na] was decreased to 50%. The apparent diffusion constant (D') of Na+ or K+, calculated from the time-course of twitch recovery and the [Na] vs. twitch relation, and from the time-course of the slow repolarization upon sudden reduction of [K] was about 1-1.5 X 10(-6) cm2/S. This is one order of magnitude smaller than the diffusion constants in an aqueous solution. Even if the tortuosity factor of the T system is taken into account, there remains a substantial discrepancy. Although our value of D' is subject to various errors, if we accept the value, the twitch recovery is predicted to be either very quick or slow depending upon the variation of [Na]-twitch relation and fiber size. Thus, both quick and slow twitch recoveries can be explained by the diffusion time of Na+ in the T system, and therefore the results are consistent with the idea that the T system is excitable.