PHYSIOLOGICAL CONCENTRATIONS OF INORGANIC-PHOSPHATE AFFECT MGATP-DEPENDENT CA-2+ STORAGE AND INOSITOL TRISPHOSPHATE-INDUCED CA-2+ EFFLUX IN MICROSOMAL VESICLES FROM NONHEPATIC CELLS

1. MgATP-dependent Ca-45(2+) uptake by microsomes obtained from various non-hepatic tissues, namely rat brain, rat solid Morris hepatoma 3924A and human platelets, was measured in the presence of P(i) at low, cytosol-like, concentrations. 2. Increasing P(i) concentrations (0.5-3 mm) caused a progressive enlargement of the Ca-45(2+)-storage capacity of all the microsomal fractions. 3. As a result of P(i) stimulation of Ca2+ uptake, Ca-45(2+) and [P-32]P(i) were co-accumulated by the three microsomal fractions. 4. The time course for Ca-45(2+) and [P-32]P(i) accumulation in brain microsomes revealed a biphasic Ca-45(2+) uptake: a rapid phase was followed by a second, slower, phase, which depended on the presence of P(i). During the P(i)-dependent phase, the uptake of Ca-45(2+) was paralleled by the uptake of [P-32]P(i). 5. The passive efflux of Ca2+ was paralleled by the efflux of P(i) and vice versa. In fact, the inhibition of active Ca2+ uptake by excess EGTA, or lowering the P(i) concentration of the incubation system by dilution, caused the release of Ca-45(2+) and [P-32]P(i) from Ca-45(2+) or [P-32]P(i) pre-loaded brain microsomes. The Ca2+ ionophore A23187 also released Ca-45(2+) and [P-32]P(i). 6. Ca2+ efflux by A23187 was rapid (t1/2 approx. 2 s) and independent of the extent of intravesicular Ca2+ loading, which indicates that Ca2+ and P(i) do not form intravesicular insoluble complexes. 7. The progressive increase in Ca2+ accumulation, depending on P(i) stimulation, resulted in a proportional increase in the amount of Ca2+ releasable by InsP3 in the three non-hepatic microsomal fractions and in digitonin-permeabilized platelets. 8. Concomitantly to Ca2+, microsomal P(i) was also released by InsP3.