Effect of cyclic AMP-dependent hormones and Ca2+-mobilizing hormones on the Ca2+ influx and polyphosphoinositide metabolism in isolated rat hepatocytes

Abstract

The effect of the interaction between the Ca2+-mobilizing hormone adrenaline, used as alpha-adrenergic agonist, and cyclic AMP-dependent hormones, including beta-adrenergic agonists and glucagon, on the initial 45Ca2+ uptake rate and polyphosphoinositide metabolism were investigated in isolated rat hepatocytes. Each hormone alone increased the initial 45Ca2+ uptake rate. When adrenaline was added without inhibitor, it induced a rise in the initial 45Ca2+ uptake rate larger than the sum of the rises elicited by its alpha and beta components singly. Similarly, when adrenaline was used as an alpha-agonist and added together with glucagon, it enhanced the initial 45Ca2+ uptake rate synergistically. Kinetic analysis of the initial 45Ca2+ uptake rate measured at different Ca2+ concentrations suggested that the increased influx elicited by the combination of adrenaline as alpha-adrenergic agonist and glucagon reflects an activation of the rate of Ca2+ transport via a homogeneous population of Ca2+ channels or carriers. Dose-response curves for the alpha-adrenergic action of adrenaline or glucagon applied in the presence of increasing doses of glucagon or adrenaline showed that each hormone increases the maximal response to the other without affecting its ED50. Measurement of polyphosphoinositide hydrolysis and of the inositol phosphates formed in the presence of adrenaline or vasopressin and/or glucagon showed that Ca2+-mobilizing hormones and glucagon had no synergistic effects on inositol 1,4,5-trisphosphate production. It is therefore proposed that the synergistic action of glucagon and Ca2+-mobilizing hormones on Ca2+ influx occurs at a step that takes place close to the Ca2+ channels or carriers themselves. The Ca2+ gating involved might be mainly controlled by two products, one of them arising from the polyphosphoinositide metabolism, and the other from the increase in internal cyclic AMP.