To examine the effect of heat stress on muscle energy metabolism during submaximal exercise, 12 endurance-trained men cycled on two occasions for ~40 min at 70% maximal O2 uptake in an environmental chamber at either 20°C and 20% relative humidity (T20) or 40°C and 20% relative humidity (T40). Trials were conducted ≥1 wk apart in random order. No difference in mean O2 uptake was observed when exercise in T40 was compared with that in T20. In contrast, exercise in T40 resulted in a higher mean heart rate (P < 0.01) and respiratory exchange ratio (P < 0.05) compared with that in T20. Postexercise rectal and muscle temperatures were also higher (P < 0.01) in T40 than in T20. 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 T40 compared with T20. In addition, an increased (P < 0.01) muscle glycogenolysis and higher (P < 0.01) postexercise muscle lactate accumulation were observed during exercise in T40 compared with T20. 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 T40 was compared with T20. 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 T40, in the absence of any differences in muscle IMP accumulation, suggests that ammonia is produced by sources other than net adenine nucleotide degradation.
|Number of pages||5|
|Journal||Journal of Applied Physiology|
|Publication status||Published - 1 Dec 1994|
- adenine nucleotides
- high-energy phosphates
- muscle temperature