Catecholamines in prevention of hypoglycemia during exercise in humans.

To assess the role of catecholamines in the prevention of hypoglycemia during moderate exercise (approximately 60% peak O2 consumption for 60 min), normal humans were studied with combined alpha- and beta-adrenergic blockade and with adrenergic blockade while changes in insulin and glucagon were prevented with the islet clamp technique (somatostatin infusion with insulin and glucagon infused at fixed rates). The results were compared with those from an islet clamp alone study. In contrast to a comparison study (saline infusion), adrenergic blockade resulted in a small initial decrease in plasma glucose during exercise, from 5.0 +/- 0.2 to 4.4 +/- 0.2 mmol/l (P less than 0.01), but the level then plateaued. There was a substantial exercise-associated decrement in plasma glucose when insulin and glucagon were held constant, i.e., from 5.5 +/- 0.2 to 3.4 +/- 0.2 mmol/l (P less than 0.0001), but the level again plateaued. However, when insulin and glucagon were held constant and catecholamine actions were blocked simultaneously, progressive hypoglycemia, to 2.6 +/- 0.6 mmol/l (P less than 0.001), developed during exercise. Hypoglycemia was the result of an absent increase in glucose production and an exaggerated initial increase in glucose utilization. Thus we conclude that sympathochromaffin activation plays a minor role when insulin and glucagon are operative, but a catecholamine, probably epinephrine, becomes critical to the prevention of hypoglycemia during exercise when changes in insulin and glucagon do not occur.