Synergistic stimulation of Ca2+ uptake by glucagon and Ca2+-mobilizing hormones in the perfused rat liver. A role for mitochondria in long-term Ca2+ homoeostasis.

Abstract

A perfused liver system incorporating a Ca2+-sensitive electrode was used to study the long-term effects of glucagon and cyclic AMP on the mobilization of Ca2+ induced by phenylephrine, vasopressin and angiotensin. At 1.3 mM extracellular Ca2+ the co-administration of glucagon (10 nM) or cyclic AMP (0.2 mM) and a Ca2+-mobilizing hormone led to a synergistic potentiation of Ca2+ uptake by the liver, to a degree which was dependent on the order of hormone administration. A maximum net amount of Ca2+ influx, corresponding to approx. 3800 nmol/g of liver (the maximum rate of influx was 400 nmol/min per g of liver), was induced when cyclic AMP or glucagon was administered about 4 min before vasopressin and angiotensin. These changes are over an order of magnitude greater than those induced by Ca2+-mobilizing hormones alone [Altin & Bygrave (1985) Biochem. J. 232, 911-917]. For a maximal response the influx of Ca2+ was transient and was essentially complete after about 20 min. Removal of the hormones was followed by a gradual efflux of Ca2+ from the liver over a period of 30-50 min; thereafter, a similar response could be obtained by a second administration of hormones. Dose-response measurements indicate that the potentiation of Ca2+ influx by glucagon occurs even at low (physiological) concentrations of the hormone. By comparison with phenylephrine, the stimulation of Ca2+ influx by vasopressin and angiotensin is more sensitive to low concentrations of glucagon and cyclic AMP, and can be correlated with a 20-50-fold increase in the calcium content of mitochondria. The reversible uptake of such large quantities of Ca2+ implicates the mitochondria in long-term cellular Ca2+ regulation.