1. The uptake and the washout of 45Ca2+ and 32Pi is described in free fat-cells and whole epididymal fat-pads from fed rats. 2. In isolated fat-cells, the uptake of 45Ca2+ proceeds with an initial rapid phase of about 1 min duration, followed by a slower subsequent accumulation. In contrast with the rapid phase, the slow phase is inhibited by 2,4-dinitrophenol, warfarin, oligomycin and verapamil, shows saturation, and presumably represents transport across the plasma membrane. 3. The washout of 45Ca2+ from preloaded cells consists of a rapid (1 min) initial phase and a slow phase which is non-monoexponential, suggesting that the radioactive isotope is released from several cellular pools. 4. When Pi is omitted from the incubation medium, the slow phase of 45Ca uptake is almost abolished, and the washout of 45Ca from preloaded fat-cells is markedly accelerated. At elevated extracellular concentrations of Pi (2,4-6.2mM), the uptake of 45Ca is stimulated by 2-10-fold, and the release of the radioactive isotope from preloaded cells is inhibited. In whole epididymal fat-pads, variations in the extracellular concentration of Pi have no detectable effect on the uptake or the washout of 45Ca. 5. In isolated fat-cells, the accumulation of 32Pi is inhibited by 2,4-dinitrophenol or the omission of glucose from the incubation medium. In a Ca2+-depleted buffer, the uptake of 32Pi is diminished, and hyperosmolarity, which stimulates 45Ca uptake, also accelerates the accumulation of 32Pi. 6. It is concluded that in free fat-cells, the uptake and release of Ca2+ and Pi take place by closely interrelated processes, which are dependent on mitochondrial energy production.

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