Interaction of calcium and glucose on glucagon secretion

The interrelationship between calcium ion and glucose on glucagon release was studied in the in vitro perfused pancreas. Spontaneous release during perfusion with glucose-free, calcium-depleted (0.2 mEq/l calcium) medium was completely abolished by ethylene glycol bis (beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA; 0.2 mM). Glucose added to calcium-depleted perfusate caused only partial inhibition of glucagon release, even at concentrations of 500 mg/dl, and there was no evidence of a paradoxical increase in secretion with time. When calcium was added in a series of steps (in the absence of additional secretagogues) more than half of the increased glucagon released was elicited by the first step (0.5 mEq/l). Release patterns at subsequent steps suggested that higher concentrations of calcium may cause mixed stimulation and inhibition. With 70 mg/dl glucose, calcium-stimulated release was partially suppressed at all calcium concentrations up through 9 mEq/l. With 150 mg/dl glucose, addition of the normally stimulating 0.5 mEq/l calcium caused abrupt and complete inhibition of glucagon secretion, and this persisted at all higher calcium concentrations. Insulin release, when high enough to be detected, did not correlate with the glucose/calcium suppression of glucagon. In other experiments, control results and all insulin secretion patterns were qualitatively similar to those reported by other investigators; however, various attempts to demonstrate a paradoxical increase glucagon secretion by glucose during calcium deprivation were unsuccessful. It is concluded that small amounts of calcium are normally required for glucagon secretion, although at higher concentrations the effects become complex. In addition, glucagon suppression by glucose is calcium-requiring. Thus, changes in glucagon secretion caused by addition or depletion of calcium can depend on the relative amount of glucose in the milieu.