Catecholamine responses and their interactions with other glucoregulatory hormones.

Abstract

We have investigated catecholamine-glucagon-insulin interactions using three stress models: 1) hypoglycemia; 2) exercise; and 3) epinephrine infusion. Phlorizin caused mild hypoglycemia with hypoinsulinemia. Plasma glucagon increased as did hepatic glucose production. Catecholamines did not increase. Insulin caused severe hypoglycemia. Metabolic counterregulation was due mainly to the 40-fold increase in epinephrine. Glucagon played a role only in the recovery from insulin-induced hypoglycemia, which could reflect increased hepatic sensitivity to glucagon with declining plasma insulin. Glucagon suppression during exercise caused transient hypoglycemia due to an inadequate rise in glucose production. Exaggerated epinephrine release during hypoglycemic exercise prevented severe hypoglycemia by inhibiting glucose utilization and stimulating glucose production, with an associated increase in lactate and free fatty acid levels. Hypoglycemic exercise also caused increased cortisol release. Counterregulation was prevented by a euglycemic clamp. We conclude that, during exercise, glucagon is directly responsible for 80% of the increment of glucose production and controls glucose uptake by the muscle indirectly; thus glucagon spares muscle glycogen by increasing hepatic glucose production. Epinephrine infusion in normal dogs caused a transient increase in glucose production and a sustained inhibition of glucose clearance, resulting in hyperglycemia. Insulin rose transiently, followed by a relative inhibition of secretion. Glucagon suppression did not modify the metabolic effects of epinephrine. In alloxan-diabetic dogs, the glucagon response to epinephrine was augmented, whereas in depancreatized dogs, during subbasal insulin infusion, the hepatic response to glucagon was excessive. Glucagon suppression diminished hepatic responsiveness to epinephrine in both models. Stress-induced diabetic instability could relate to exaggerated glucagon release or to increased hepatic sensitivity to glucagon. Thus, during hypoglycemia, exercise, or epinephrine infusion, prevailing plasma insulin levels govern the relative metabolic roles of epinephrine and glucagon.