Regulation of glucose kinetics during intense exercise in humans: effects of alpha- and beta-adrenergic blockade.

This study examined the effect of combined alpha- and beta-adrenergic blockade on glucose kinetics during intense exercise. Six endurance-trained men exercised for 20 minutes at approximately 78% of their peak oxygen consumption (Vo(2)) following ingestion of a placebo (CON) or combined alpha- (prazosin hydrochloride) and beta- (timolol maleate) adrenoceptor antagonists (BLK). Plasma glucose increased during exercise in CON (0 minutes: 5.5 +/- 0.1; 20 minutes: 6.5 +/- 0.3 mmol. L(-1), P <.05). In BLK, the exercise-induced increase in plasma glucose was abolished (0 minutes: 5.7 +/- 0.3; 20 minutes: 5.7 +/- 0.1 mmol. L(-1)). Glucose kinetics were measured using a primed, continuous infusion of [6,6-(2)H] glucose. Glucose production was not different between trials; on average these values were 25.3 +/- 3.9 and 30.9 +/- 4.4 micromol. kg(-1). min(-1) in CON and BLK, respectively. Glucose uptake during exercise was greater (P <.05) in BLK (30.6 +/- 4.6 micromol. kg(-1). min(-1)) compared with CON (18.4 +/- 2.5 micromol. kg(-1). min(-1)). In BLK, plasma insulin and catecholamines were higher (P <.05), while plasma glucagon was unchanged from CON. Free fatty acids (FFA) and glycerol were lower (P <.05) in BLK. These findings demonstrate that adrenergic blockade during intense exercise results in a blunted plasma glucose response that is due to enhanced glucose uptake, with no significant change in glucose production.