Mechanism of exercise-induced hypoglycemia in depancreatized dogs maintained on long-acting insulin

Abstract

Human diabetics on intermediate and long-acting insulin occasionaly become hypoglycemic during exercise. We have shown previously that during exercise, hypoglycemia did not occur in depancreatized insulin-infused dogs because the increments in glucose production and utilization were proportional and of the same magnitude as in normal dogs. Therefore, to elucidate the mechanism of the glucose-lowering effect of strenuous exercise, we measured glucose production and utilization, metabolic clearance of glucose, and serum immunoreactive insulin in postabsorptive depancreatized dogs 8 h after a subcutaneous injection of protamine zinc and crystalline insulin. During rest, plasma glucose was stable, but ranged between hypoglycemia and hyperglycemia. Hyperglycemia was associated with overproduction of glucose, indicating insulin deficiency despite normal or elevated serum immunoreactive insulin. Glucose clearance, as in normal dogs, increased threefold but glucose production increased only marginally (50%) and, consequently, glucose decreased in plasma. The decrease of plasma glucose was directly proportional to the preexercise concentration and production of glucose. The magnitude of inhibition glucose production was not correlated with the serum immunoreactive insulin indicating either that some released insluin was not active or that a moderate immunoreactive insulin increment induced a near-maximal inhibition. It is concluded that in depancreatized dogs injected with protamine zinc insulin, exercise accelerates mobilization of insulin from its injection site presumably because of increased blood and lymph flow. Glucose utilization did not exceed that in normal dogs, but hepatic glucose production failed to increase sufficiently to meet the needs of muscle in exercise.