MgATP-dependent accumulation of calcium ions and inorganic phosphate in a liver reticular pool

1. MgATP-dependent Ca2+ uptake by rat liver microsomal preparations and permeabilized hepatocytes was measured in the presence or absence of P(i). 2. Monitoring of free Ca2+ in incubation systems with a Ca2+ electrode in the presence of P(i) (2-7 nM) revealed a biphasic Ca2+ uptake, with the onset of a second, P(i)-dependent, Ca2+ accumulation. 3. Increasing P(i) concentrations (up to 10 mM) caused a progressive enlargement of 45Ca2+-loading capacity of microsomal fractions. 4. As a result of P(i) stimulation of active Ca2+ uptake, [32P]P(i) and 45Ca2+ were co-accumulated. 5. Experiments with permeabilized hepatocytes revealed that the amount of Ca2+ releasable by myo-inositol 1,4,5-trisphosphate is unaffected by P(i).