Glucose-stimulated insulin secretion is depressed by training. To further elucidate the beta-cell adaptation to training, a nonglucose secretagogue was applied. Arginine was infused for 90 min to seven trained and seven untrained young men. Arginine and glucose concentrations increased identically in the groups. The insulin response was biphasic and waned despite increasing arginine concentrations. Both these phases as well as C-peptide responses were reduced in trained subjects, whereas proinsulin responses were similar in the groups. Identical increases were found in glucagon, growth hormone, catecholamines, and production and disappearance of glucose; identical decreases were found in free fatty acids, glycerol, and beta-hydroxybutyrate. In conclusion, in men training diminishes both arginine- and glucose-stimulated insulin secretion, indicating a profound beta-cell adaptation. Being enhanced, the effects of insulin on both production and disposal of glucose are changed in the opposite direction to beta-cell secretion by training. The responses of glucagon- and growth hormone-secreting cells to arginine do not change with training.
Molecular Mechanisms of Beta Cell Adaptation to Hyperlipidemia
