The transport of Na+ in mature Eurycea oocytes was studied by quantitative radioautography of 22Na+ using techniques suitable for localization of diffusible solutes, together with conventional extractive techniques. Intracellular Na+ consisted of three kinetic fractions: a cytoplasmic fast fraction of about 8.5 µeq/ml H2O; a cytoplasmic slow fraction of about 58.7 µeq/ml H2O; and a nuclear fast fraction of about 11.1 µeq/ml H2O. A nuclear slow fraction, if it exists, does not exceed 5% of the cytoplasmic. The fast fractions represent freely diffusible Na+ in the two compartments; the nuclear solvent space is 1.3 times the cytoplasmic. The flux of both fast fractions is determined by the permeability of the cortical membrane, with neither the nuclear membrane nor diffusion in the cytoplasm detectably slowing the flux. The cytoplasmic slow fraction is interpreted to represent Na+ bound to nondiffusible constituents which are excluded from the nucleus; these may be yolk platelets, although the widespread observation of Na+ binding in other cells, and the high Na+/K+ selectivity, argues against simple ion-binding to the yolk phosphoprotein.

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