EFFECTS OF GLUCOSE-INFUSION ON HORMONE-SECRETION AND HEPATIC GLUCOSE-PRODUCTION DURING HEAVY EXERCISE

Blood-borne metabolic feedback vs. neural feedforward regulation of gl ucose homeostasis during exercise was investigated by infusing glucose and [H-3]glucose for glucose appearance determination intravenously i n rats running for 20 min at 28 m/min [almost-equal-to 85% of maximal 02 consumption (VO2max)]. Infused glucose corresponded to the exercise -induced increase in hepatic glucose production (HGP) found in saline- infused rats. Saline- and glucose-infused resting rats were also studi ed. Arterial blood was sampled for analyses of hormones and metabolite s. Plasma epinephrine, norepinephrine, and insulin were always similar and HGP was initially similar in the two exercising groups, although glucose infusion resulted in higher plasma glucose compared with contr ol (P < 0.05). Late during exercise, high plasma glucose (11.3 +/- 0.4 vs. 9.6 +/- 0.3 mM) and low glucagon (16 +/- 2 vs. 27 +/- 3 pM) in gl ucose- vs. saline-infused rats caused an inhibition of HGP in glucose- infused rats, although never below preexercise levels. In resting rats , glucose infusion resulted in elevated plasma glucose and insulin and , in turn, inhibition of HGP but had no effect on catecholamines, cort icosterone, or glucagon. The findings indicate that during heavy exerc ise, glucose homeostasis is regulated primarily by neural feedforward mechanisms and that blood-borne metabolic feedback mechanisms play a r egulatory role if metabolic error signals are pronounced.