Sodium and Potassium in Frog Muscle*

The potassium content of muscle fibers depends, in part, upon the potassium concentration of the external medium (Fenn and Cobb, 1934). In tissues intact in the body, it has been shown that sodium enters the fibers as potassium leaves, either because of low potassium in the blood (Heppel, 1939, 1940) or because of activity or fatigue (Fenn, 1938). Such findings suggest that the normal accumulation of potassium is due to some type of diffusion equilibrium which changes as the physiological condition of the cell changes and which probably is influenced by several factors or mechanisms. If in isolated muscle such an equilibrium exists, it should be possible to demonstrate its presence first, by showing that a simple relationship exists between sodium and potassium in muscle fibers (the sum of the concentrations of the two elements should be approximately constant) and second, by the ability of the fibers (after they have lost potassium, because of the low concentration outside) to reconcentrate potassium when the external concentration is raised. The experiments reported here show that potassium loss from isolated frog muscles is reversible and that changes in concentration of this element in the cells are accompanied by compensatory changes in sodium.