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°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 109μmoles/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