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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.

Intact living frogs (Rana pipiens) were partially immersed in dilute salt solution labeled with K42 or Na24 or, alternatively, injected with Ringer’s fluid containing the appropriate isotope and then partially immersed in unlabeled dilute salt. Before isotopic equilibrium, the animals were sacrificed and specific activities of K42 and Na24 were determined for medium, skin, plasma, and other tissues. With Na24, entering from the medium or escaping to the medium, specific activities of the skin approach that of the plasma. For K42, entering from the medium, the specific activity exceeds that of the plasma. The results are interpreted as indicating that the exchange rate for Na is greater plasma to skin than medium to skin, with the reverse situation for K. Values are given for average Na, K, and Cl contents of the various organ systems.

Fresh, empty, isolated sacs of leg skin from R. pipiens manufacture a salt solution reasonably balanced with respect to ions and approximating a somewhat dilute Ringer's fluid. Concentrations of ions bear little relationship to concentrations in the external medium even down to 12 mM. An effective regulatory mechanism is indicated whereby the amount of salt transported is adjusted to the amount of water or vice versa, the rate of movement of either salt or water being largely independent of osmotic or ionic gradients (outer fluid to manufactured inner fluid). Concentrations of the inner fluids appear to be regulated to conform to a fairly constant concentration within the skin. Some evidence is presented that a major factor in regulating concentration of the inner fluid is triggered by an initial dilution of the inner fluid, followed by stimulation of salt uptake.