GLUCOSE KINETICS DURING EXERCISE IN TRAINED MEN

Six trained men were studied to examine the relative increases in hepatic glucose output and peripheral glucose uptake during 40 min of exercise at 75% VO2max. Rates of appearance (Ra) and disappearance (Rd) were measured using a primed, continuous intravenous infusion of D-[3-H-3]glucose. Plasma glucose increased (P < 0.05) from 4.8+/-0.2 mmol l(-1) at rest to 6.2+/-0.5 mmol l(-1) after 40 min of exercise. Both Ra and Rd increased (P < 0.05) during exercise, however, during the early phase of exercise, Ra exceeded Rd (P < 0.05). Ra peaked at 42.0+/-3.2 mu mol kg(-1) min(-1) after approximately 15 min of exercise. In contrast, the highest Rd of 33.9+/-4.3 mu mol kg(-1) min(-1) was measured at the end of exercise. In additional experiments, five men were studied during 40 min of exercise at 70-75% VO2max, 2 h after ingestion of the non-selective beta-adrenergic antagonist timolol or a placebo capsule. Subjects were unable to complete the exercise bout following timolol, fatiguing after 28.0+/-4.0 min (P < 0.05). The increase in blood glucose from 4.3+/-0.1 to 4.7+/-0.3 mmol l(-1) (P < 0.05) following 20 min of exercise under control conditions was completely abolished by prior timolol ingestion (4.2+/-0.2 to 4.1+/-0.2 mmol l(-1)). These results demonstrate that during exercise at 75% VO2max in trained men, hepatic glucose output is not always closely matched to peripheral muscle glucose uptake and may be subject to feed-forward regulation. The abolition of the hyperglycaemia with non-selective beta-adrenergic blockade implicates adrenaline in this response.