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

The chemical activities, (a), of Na+ and K+ were determined in large mature and in small immature frog oocytes, using open-tipped micropripettes and ion-selective 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 (gammaapp) was calculated for each ion as the ratio, a/c. With development, (aNa/ak) decreased four to fivefold and (cNa/cK) increased six to sevenfold. In the large mature oocytes, gammaapp was measured to be 0.08 +/- 0.02 and gammaappK 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 gammaappK was substantially greater than the activity coefficient of K+ in the external medium (0.76). The data suggest that the inequality of gammaappNa and gammaappK in this and probably other cells reflects the development of subcellular compartmentalization of ions. Possible intracellular sites of ionic compartmentalization are considered.