Kinetic properties of the Ca2+-accumulation system of a rat liver microsomal fraction

1. By using Ca-EGTA buffers, the Km for Ca2+ uptake into rat liver heavy microsomes (microsomal fraction) was found to be 0.2 microM free Ca2+. 2. In the absence of oxalate, these vesicles accumulate about 20 nmol of Ca2+/mg of protein. Efflux of Ca2+ from the vesicles is much faster at pH 7.6 than at pH 6.8, but does not apparently show saturation kinetics or any stringent requirement for external ions. 3. The steady-state distribution of Ca2+ between the microsomes and the medium in the presence of ATP and the absence of oxalate is dependent on Ca2+ load. When the vesicles are loaded to 50% capacity, the external free Ca2+ concentration is 70 nM. 4. The affinity of heavy microsomes for Ca2+ is such that is seems likely that they has a dominant role in the determination of cytoplasmic free Ca2+ concentrations.