Influence of physical training on the fuel-hormone response to prolonged low intensity exercise

Influence of physical training on insulin release and glucose utilization by islet cells and liver glucokinase activity in the rat

The effect of physical training on insulin release and glucose utilization by perifused islets and on liver glucokinase activity was examined in rats that exercised spontaneously by running (in wheel cage) up to 4-6 mi/day for 36 +/- 4 days and in sedentary controls kept in standard cages. Perifusion of islets with 4 mM glucose resulted in comparable rates of insulin release from islets obtained from trained and sedentary control rats. In contrast, when the perifusion glucose concentration was raised to 10 mM, the biphasic increase in insulin release was 40-50% lower in the trained rats as compared with untrained rats. This decrease in glucose-stimulated insulin release occurred in the face of comparable rates of glucose utilization by islets from control and trained rats. Glucose phosphorylation by liver homogenates from trained rats was reduced at all concentrations of glucose examined (0.5-100 mM). The calculated glucokinase activity was diminished by 40%, whereas hexokinase activity was decreased by 15% in the livers from trained rats. We conclude that 1) hypoinsulinemia induced by exercise training is due to decreased sensitivity of the beta-cell to the stimulant action of glucose independent of changes in islet cell utilization of glucose, and 2) exercise training results in a diminution of liver glucokinase activity that may be a consequence of the hypoinsulinemia.

Hypoglycemia during prolonged exercise in normal men

Hypoglycemia (blood glucose less than 45 mg per deciliter [less than 2.5 mmol per liter]) occurred in seven of 19 healthy men who exercised to exhaustion on a cycle ergometer at 60 to 65 per cent of maximal aerobic power. The hypoglycemic subjects exercised for 15 to 70 minutes despite blood glucose levels of 25 to 48 mg per deciliter (1.4 to 2.7 mmol per liter), and their exhaustion time (mean +/- S.E.M., 142 +/- 15 minutes) was not significantly different from that of the euglycemic group (165 +/- 11). Plasma epinephrine was inversely related to blood glucose (P less than 0.01) and was three times higher in the hypoglycemic subjects (P less than 0.05). Glucose ingestion (40 or 80 g per hour) prevented the hypoglycemia and resulted in a smaller rise in plasma epinephrine but did not alter perceived exertion or consistently delay exhaustion. We conclude that hypoglycemia occurs in normal subjects during prolonged exercise and results in an exaggerated rise in plasma epinephrine. However, hypoglycemia fails to effect endurance, and its prevention does not consistently delay exhaustion.