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 (γ^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, γ ^app_Na was measured to be 0.08±0.02 and γ ^app_K 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 γ ^app_K was substantially greater than the activity coefficient of K⁺ in the external medium (0.76). The data suggest that the inequality of γ ^app_Na and γ ^app_K in this and probably other cells reflects the development of subcellular compartmentalization of ions. Possible intracellular sites of ionic compartmentalization are considered.