Activity coefficients of intracellular Na+ and K+ during development of frog oocytes

Summary The chemical activities, (a), of Na⁺ and K⁺ were determined in large mature and in small immature frog oocytes, using open-tipped micropipettes and ionselective microelectrodes. The average chemical concentrations,c, of Na⁺ and K⁺ were determined by spectrophotometry and by electron probe X-ray microanalysis. The apparent activity coefficient (^app) was calculated for each ion as the ratio,a/c.With development, (a_Na/a_K) decreased four to fivefold and (c_Na/c_K) increased six to sevenfold. In the large mature oocytes, _Na^app was measured to be 0.08±0.02 and _K^app lay within the range 1.15±0.03 to 1.29±0.04, constituting the smallest value for Na⁺ and largest value for K⁺, respectively, thus far reported. This intracellular value of _K^app was substantially greater than the activity coefficient of K⁺ in the external medium (0.76). The data suggest that the inequality of _Na^app and _K^app in this and probably other cells reflects the development of subcellular compartmentalization of ions. Possible intracellular sites of ionic compartmentalization are considered.