Subcellular flux of potassium and rubidium in amphibian oocytes

To determine the net Rb+ influx and K+ efflux at a subcellular level, fully grown and a Ringer's solution where K+ was substituted for by Rb+ on a molar bases. For 40 hrs serial samples of the oocytes were cryofixed and cryosectioned for elemental analysis in mM per kg dry weight using electron probe x-ray microanalysis. Oocyte volume remained constant. Net Rb+ influx showed a slow exponential increase into the nucleus, the yolk-free cytoplasm and the yolk platelets. There was significant K+ efflux from the nucleus but not from the yolk-free cytoplasm or the yolk. The Na+ concentration remained unchanged in all compartments during the course of the experiment. There was however a slow but significant increase in the concentration of Cl- in each of the subcellular compartments but this increase was not sufficient to balance the observed increase in the sum of K+ plus Rb+. Thus Rb+ accumulates selectively in all three subcellular compartments indicating that Rb+ is not a good K+ surrogate in the oocytes. That K+ demonstrates efflux from the nucleus but not from the cytoplasmic compartments is interpreted to suggest that some of the nuclear K+ is lost in exchange for Rb+ but that essentially none of the cytoplasmic K+ is-lost in exchange for Rb+. The findings provide strong evidence for adsorption of Rb+ in the cell.