EFFECT OF HEAT-STRESS ON MUSCLE ENERGY-METABOLISM DURING EXERCISE

To examine the effect of heat stress on muscle energy metabolism during submaximal exercise, 12 endurance-trained men cycled on two occasions for similar to 40 min at 70% maximal O-2 uptake in an environmental chamber at either 20 degrees C and 20% relative humidity (T-20) or 40 degrees C and 20% relative humidity (T-40). Trials were conducted greater than or equal to 1 wk apart in random order. No difference in mean O-2 uptake was observed when exercise in T-40 was compared with that in T-20. In contrast, exercise in T-40 resulted in a higher mean heart rate (P < 0.01) and respiratory exchange ratio (P < 0.05) compared with that in T-20. Postexercise rectal and muscle temperatures were also higher (P < 0.01) in T-40 than in T-20. Lower (P < 0.01) postexercise creatine phosphate and higher creatine (P < 0.01) and ammonia P < 0.05) were observed in muscle after exercise in T-40 compared with T-20. In addition, an increased (P < 0.01) muscle glycogenolysis and higher (P < 0.01) postexercise muscle lactate accumulation were observed during exercise in T-40 compared with T-20. In contrast, no differences were observed in postexercise concentrations of total adenine nucleotide pool (ATP + ADP + AMP), ATP/ADP ratio, or inosine 5'-monophosphate (IMP) when T-40 was compared with T-20. These results indicate that the rate of ATP utilization may be increased during exercise in the heat but that this increased energy demand is predominantly met by an increase in anaerobic glycolysis and creatine phosphate hydrolysis, preventing a reduction in total adenine nucleotide pool. In addition, the higher (P < 0.05) postexercise concentration of muscle ammonia observed in T-40, in the absence of any differences in muscle IMP accumulation, suggests that ammonia is produced by sources other than net adenine nucleotide degradation.