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

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