EFFECT OF HEAT-STRESS ON GLUCOSE KINETICS DURING EXERCISE

To identify the mechanism underlying the exaggerated hyperglycemia dur ing exercise in the heat, six trained men were studied during 40 min o f cycling exercise at a workload requiring 65% peak pulmonary oxygen u ptake (VO2peak) on two occasions at least 1 wk apart. On one occasion, the ambient temperature was 20 degrees C [control (Con)], whereas on the other, it was 40 degrees C [high temperature (HT)]. Rates of gluco se appearance and disappearance were measured by using a primed contin uous infusion of [6,6-H-2]glucose. No differences in oxygen uptake dur ing exercise were observed between trials. After 40 min of exercise, h eart rate, rectal temperature, respiratory exchange ratio, and plasma lactate were all higher in HT compared with Con (P < 0.05). Plasma glu cose levels were similar at rest (Con, 4.54 +/- 0.19 mmol/l; HT, 4.81 +/- 0.19 mmol/l) but increased to a greater extent during exercise in HT (6.96 +/- 0.16) compared with Con (5.45 +/- 0.18; P < 0.05). This w as the result of a higher glucose rate of appearance in HT during the last 30 min of exercise. In contrast, the glucose rate of disappearanc e and metabolic clearance rate were not different at any time point du ring exercise. Plasma catecholamines were higher after 10 and 40 min o f exercise in HT compared with Con (P < 0.05), whereas plasma glucagon , cortisol, and growth hormone were higher in HT after 40 min. These r esults indicate that the hyperglycemia observed during exercise in the heat is caused by an increase in liver glucose output without any cha nge in whole body glucose utilization.