Interrelationship of Glucagon, Insulin and Glucose: The Insulinogenic Effect of Glucagon

Abstract

Glucagon in doses of 0.25 or 1.0 mg. was injected rapidly, intravenously, into healthy normal subjects. There was a striking rise in arterial immunoreactive insulin levels within one minute of the start ot the injection before any change in arterial glucose concentration occurred Plasma insulin levels reached their peak within ten minutes and began to fall thereafter. In contrast, blood glucose levels were highest twenty to thirty minutes after the injection. A second glucagon injection during the hyperglycemic phase produced a larger rise in plasma insulin than the first. During constant infusions of smaller amounts of glucagon, the insulinogenic effect of glucagon was usually only apparent if blood glucose levels were raised. The effect of consecutive forty-minute intravenous infusions of glucagon (10μg./min.), glucose (0.5 gm./min.) and a glucagon-glucose (5μg. and 250 mg./min., respectively) on plasma insulin and arterio-venous blood glucose differences in six healthy normal subjects was studied. Plasma insulin levels were two to five times higher during glucose infusions alone, despite similar peak arterial blood glucose levels during the infusions. Similar results were obtained in other subjects during separate infusions of glucose and glucagon. Total body glucose clearance, judged by the rate of fall in arterial blood glucose levels, was faster after glucagon- than after glucose-induced hyperglycemia. Forearm A-V blood glucose differences were not obviously greater during glucagon, in spite of the higher plasma insulin levels, than during glucose infusions. Possible reasons for the failure of forearm A-V blood glucose differences to reflect accurately total peripheral glucose assimilation under these experimental conditions are discussed, and it is suggested that the co-existence of hyperglucagonemia and hyperinsulinemia may tend to favor incorporation of glucose into adipose tissue rather than muscle. Plasma glucagon levels were measured by immunoassay and the half-life was about ten minutes. The results provide evidence of a direct effect of glucagon upon insulin secretion. The suggestion is made that the insulinogenic effect of glucagon is physiological importance and is due to accelerated intra-β-cell glycogenolysis.