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).