Alterations in intracellular calcium compartmentation following inhibition of calcium efflux from isolated hepatocytes

Addition of ATP to the incubation medium of freshly isolated rat hepatocytes causes a marked inhibition of the efflux of Ca²⁺ from the cells, and its accumulation in intracellular compartments. After an initial rise in cytosolic free Ca²⁺ concentration, as indicated by the activation of phosphorylase, Ca²⁺ is preferentially sequestered in the mitochondria, without any apparent contribution by the endoplasmic reticulum. Impairment of mitochondrial Ca²⁺ homeostasis by pyridine nucleotide oxidation associated with tert‐butyl hydroperoxide metabolism, prevents the ATP‐dependent cellular Ca²⁺ accumulation and causes a release of Ca²⁺ from the hepatocytes into the medium. Conversely, maintenance of the mitochondrial pyridine nucleotides in a more reduced state, e. g. in presence of 3‐hydroxybutyrate in the medium, prevents this hydroperoxide‐induced release of intracellular Ca²⁺. Under conditions of impaired mitochondrial Ca²⁺ sequestration, there appears to be a redistribution of a minor fraction of the intracellular Ca²⁺ from the mitochondria to the endoplasmic reticulum. Our results provide additional evidence for the critical involvement of the plasma membrane Ca²⁺‐extruding system in the physiological regulation of the cytosolic free Ca²⁺ concentration in hepatocytes, and suggest that the mitochondria play a more important role than the endoplasmic reticulum in the regulation of the cytosolic free Ca²⁺ level when the plasma membrane Ca²⁺ pump is inhibited.