Effects of dietary manipulations and glucose infusion on glucagon response during exercise in rats

The purpose of the present investigation was to test the hypothesis that blood glucose concentration is not always related to glucagon response during exercise. Three groups of rats were submitted to a prolonged (3-h) swimming exercise. Two groups of rats had their normal food intake restricted by 50% the night before the experiment. One of these two groups of rats was intravenously infused with glucose throughout exercise to maintain euglycemia. The third group of rats swam while under normal dietary conditions. Plasma glucose, sampled in arterial blood, was reduced (P < 0.05) at 75, 105, 150, and 170 min of exercise (from approximately 130 to 110 mg/dl) in the food-restricted animals without glucose infusion, whereas a significant (P < 0.05) increase was measured in the two other groups during exercise. A significant (P < 0.01) difference in the mean integrated areas under the glucose-concentration curve was found only between the fed and the two food-restricted groups. Plasma insulin concentrations decreased (P < 0.05) similarly in all groups during exercise, whereas plasma epinephrine and norepinephrine concentrations increased significantly (P < 0.01) in all groups. Despite differences between groups in plasma glucose response during exercise, and despite the absence of any decrease in exercising blood glucose levels in at least two of the three groups, plasma glucagon responses were increased (P < 0.05) similarly in all groups (from approximately 250 to 550 pg/ml) at the end of the exercise period. The increase in glucagon was significant after 90 min of exercise in the food-restricted groups, with or without glucose infusion, but only after 140 min in the fed group. These results indicate that the glucagon response during exercise is not always linked to the decrease in plasma glucose.