The origin, quantitation, and kinetics of intracellular calcium mobilization by vasopressin and phenylephrine in hepatocytes

The addition of phenylephrine or vasopressin to isolated hepatocytes resulted in an efflux of calcium. The intracellular source of this calcium was determined by measuring the calcium released upon the sequential additions of an uncoupling agent and the Ca2+ ionophore A23187 to control and hormone-treated cells. The release promoted by these agents was used as an estimate of the calcium content of the mitochondria and endoplasmic reticulum, respectively. The validity and limitations of this method are critically evaluated. The source of the calcium mobilized by the hormones was found to depend on the intracellular calcium distribution. When the amount of total cell-releasable Ca2+ was low (less than 0.9 nmol/mg cell dry weight), the endoplasmic reticulum represented the major cellular calcium pool and was also the predominant pool mobilized by the hormone. As the cell calcium content was increased, the endoplasmic reticulum attained its maximum capacity and the mitochondria sequestered increasing amounts of calcium. Under these conditions, the hormones mobilized calcium from the mitochondria with minimal effects on the endoplasmic reticulum calcium pool. These results suggest that more than one hormone-induced Ca2+-releasing agent may be formed. Both the total amount and the rate of calcium released from the cell under the influence of hormones was independent of the cell calcium content. The appearance of hormone-releasable Ca2+ in the extracellular medium showed a lag period of 5 to 10 s, during which a rapid increase of phosphorylase activity was observed. In contrast, the mobilization of a comparable amount of calcium by carbonyl cyanide p-trifluoromethoxyphenylhydrazone showed no significant lag, but the activation of phosphorylase was slower. A kinetic analysis of the hormone-releasable Ca2+ indicated a rapid onset with a peak increase of cytosolic free Ca2+ between 5 and 10 s prior to release of Ca2+ from the cell. The results suggest that an early action of the hormone is the inhibition of the plasma membrane Ca2+ efflux pump.