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.

Activation of inositol phosphate formation by circulating noradrenaline but not by sympathetic nerve stimulation with a similar increase of glucose release in perfused rat liver

“…2). The changes were comparable to those described previously for 10 pM noradrenaline [15], which increased InsP3 within 5 min to around 250% basal level under conditions similar to those used here, and for 10 yM adrenaline in the presence of 10 pM of the p-blocker propranolol [12], which increased InsP,, InsP2 and InsP3 to about 112%, 196% and 26l%, respectively, within 30 s. These findings show that the modified methods were reliable. Glucose output from the liver cells was increased to about 360% by 1 pM noradrenaline and to about 580% by 10 yM noradrenaline (Fig.…”

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

“…Altin and Bygrave (1986), making dose-response measurements with the perfused rat liver, observed that the sensitivity of vasopressin and angiotensin (but not so much that of phenylephrine) was apparently increased by the presence of a low glucagon concentration. It should be noted that Poggioli et al (1986) did not observe an alteration in the sensitivity to the agonists. As found by Morgan et al (1985), the increase in sensitivity is such that the concentrations of hormones that are inducing the effects on Ca2l fluxes is well within the physiological range.…”

“…In kinetic studies of the influence of glucagon on 45Ca inflow induced by Ca2+-mobilizing agonists, Mauger et al (1985) and Poggioli et al (1986) reported that co-addition of glucagon with vasopressin greatly stimulated Ca2+ inflow. The dose-response curves for either glucagon or vasopressin were unaffected by the other agent, i.e.…”

“…Thus not only could the glucagon concentration used be important in determining the second messenger produced, but also the time at which measurements are made may have an effect: longer times of glucagon action may provide information different to that produced at shorter times, especially considering the rapid turnover of Ins(1,4,5)P3 generated by low glucagon concentrations (Wakelam et al, 1986 (Altin and Bygrave, 1986;Poggioli et al, 1986;Mauger and Claret, 1988;Kass et al, 1990) have expressed the view that the synergistic action of glucagon and Ca2+-mobilizing agonists occurs close to Ca2l channels or on the carriers themselves located at the plasma membrane. The suggestions are that control might be effected through cyclic AMP perhaps phosphorylating Ca2+ channels and/or (Poggioli et al, 1986) The other possibility, that cross-talk occurs at or before hormone-induced phosphatidylinositol metabolism, has been proposed by Blackmore and Exton (1986). The results of their experiments, in which the effects of 4,-phorbol 12-myristate 13-acetate and aluminium fluoride on Ca2+ mobilization in rat hepatocytes were examined, led them to suggest that the mobilization of Ca2+, induced by glucagon, results from cyclic AMPdependent phosphorylation of Np, phosphatidylinositol bisphosphate, or the event leading to Ins(1,4,5)P3formation.…”

“…Suggested intracellular sites of action of glucagon are the Ca2+ transporters in mitochondria (Morgan et al, 1985), in the endoplasmic reticulum (Burgess et al, 1986;Mauger et al, 1989) and in the plasma membrane (Poggioli et al, 1986;Altin and Bygrave, 1986;Kass et al, 1990). It is possible that all sites are modified but perhaps to different extents and at different times following glucagon administration.…”