Norepinephrine, Vasopressin, Glucagon, and A23187 Induce Efflux of Calcium from an Exchangeable Pool in Isolated Rat Hepatocytes

Isolated rat hepatocytes do not actively accumulate Ca(2+) during prolonged incubation in vitro; however, these cells do exhibit a limited exchange of intracellular with extracellular Ca(2+). The exchangeable pool represents about 2 nmol of Ca(2+) per mg of protein. In medium containing either a low (20 muM) or high (1 mM) concentration of Ca(2+), the divalent cation ionophore, A23187 (at concentrations of 0.03-0.1 nmol/mg of protein), causes release of (45)Ca(2+) from this exchangeable pool but does not allow net influx of extracellular Ca(2+) detectable by the use of a Ca(2+)-sensitive electrode. Like A23187, the hormones norepinephrine, vasopressin, and glucagon (at concentrations that stimulate gluconeogenesis) each induces a similar net efflux of Ca(2+). Treatment with one hormone decreases the subsequent reponse to the others, whereas treatment with A23187 abolishes the hormonal effects upon both Ca(2+) release and gluconeogenesis. The action of norepinephrine, but not of glucagon, upon Ca(2+) efflux is prevented by the alpha-adrenergic antagonist, phenoxybenzamine. The action of norepinephrine is not prevented by the beta-adrenergic antagonist, propranolol. Together these results indicate that the release of Ca(2+) from a common pool of exchangeable Ca(2+) is important to the action of a variety of hormones on hepatocytes. This Ca(2+) pool in the isolated hepatocyte is characterized as being similar in size and having exchange kinetics that are comparable to those reported for the major intracellular pool of Ca(2+) in the intact liver. The possibility that this pool is intramitochondrial calcium is discussed.