Impaired adrenergic response to prolonged exercise in type I diabetes

Patients with type I diabetes mellitus commonly experience hypoglycemia related to physical activity. We investigated the metabolic and hormonal response to exercise in type I diabetics, normal controls, and controls exercising under hypoglycemic conditions. All subjects exercise for 60 minutes at 60% to 65% of their VO2max while insulin concentrations were clamped at basal or hyperinsulinemic levels. With low-dose insulin infusion, despite similar free insulin levels, diabetics had a greater decrease in plasma glucose concentrations during exercise than controls. Nevertheless, the increments of epinephrine (E) and norepinephrine (NE) during exercise tended to be less in the diabetic subjects. Circulating levels of free fatty acids (FFA) were lower in diabetics, especially during early recovery from exercise. To better compare responses, a group of normal controls exercised during an infusion of insulin, which resulted in a similar decrease in plasma glucose to that of exercising diabetics. While exercising during a similar degree of hypoglycemia, diabetics had a significantly smaller increment of E and NE compared with controls. Increments of glucagon (GL) and growth hormone (GH) were not different. These studies suggest that there is a subnormal catecholamine response to exercise under hypoglycemic conditions in some patients with type I diabetes. The hypoglycemia during and after exercise in these individuals is probably the result of multiple factors, including relative hyperinsulinemia, decreased increment in catecholamines, and decreased availability of FFA.