Sartorius muscles of R. pipiens may be enriched in sodium and depleted of potassium by prolonged soaking in the cold in K-free or low K Ringer's solution. Such K-depleted muscles take up potassium and extrude sodium when exposed to Ringer's solution containing 10 mM K at ordinary room temperature (ca. 20°C), with no dependence on external Na ion concentration in either phase within fairly wide limits.

Extrusion of excess fiber Na is demonstrated on single muscles using Na24-loaded tissues. Rate of exchange of excess fiber Na does not differ significantly from the rate of exchange of Na normally present in muscle fibers.

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