Studies on ion distribution in living cells. II. Cooperative interaction between intracellular potassium and sodium ions.

New steady levels of K(+) and Na(+) ion in frog sartorius muscle were reached in 72 hr at 25 degrees C in environments containing 100-mM Na(+)-ion and K(+)-ion concentrations varying from near zero to 10 mM. These steady levels follow a pattern predicted by a cooperative adsorption isotherm presented in 1964. From a total of 13 sets of experiments carried out over a five year period, the average concentration of adsorption sites is 109 mumoles/g of fresh cells. The average intrinsic equilibrium constant for the Na --> K exchange is 135, and the average free energy of nearest neighbor interaction is 0.54 kcal/mole. These values are in good agreement with those obtained by Jones and Karreman from studies on canine carotid arteries.