Metabolic and thermodynamic responses to dehydration-induced reductions inmuscle blood flow in exercising humans

1. The present study examined whether reductions in muscle blood flow with exercise-induced dehydration would reduce substrate delivery and metabolite and heat removal to and from active skeletal muscles during prolonged exer cise in the heat. A second aim was to examine the effects of dehydration on fuel utilisation across the: exercising leg and identify factors related t o fatigue. 2. Seven cyclists performed two cycle ergometer exercise trials in the heat (35 degrees C; 61 +/- 2% of maximal oxygen consumption rate, (V) over dot (O2, max)), separated by 1 week. During the first trial (dehydration, DE), they cycled until volitional exhaustion (135 +/- 4 min, mean +/- S.E.M.), w hile developing progressive DE and hyperthermia (3.9 +/- 0.3% body weight l oss and 39.7 +/- 0.2 degrees C oesophageal temperature, T-oes). On the seco nd trial (control), they cycled for the same period of time maintaining euh ydration by ingesting fluids and stabilising T-oes at 38.2 +/- 0.1 degrees C. 3. After 20 min of exercise in both trials, leg blood flow (LBF) and leg ex change of lactate, glucose, free fatty acids (FFA) and glycerol were simila r. During the 20 to 135 +/- 4 min period of exercise, LBF declined signific antly in DE but tended to increase in control. Therefore, after 120 and 135 +/- 4 min of DE, LBF was 0.8 +/- 0.2 and 1.0 +/- 0.31 min(-1) lower (P < 0 .05), respectively, compared with control. 4. The lower LBF after 2 h in DE did not alter glucose or FFA delivery comp ared with control. However, DE resulted in lower (P < 0.05) net FFA uptake and higher (P < 0.05) muscle glycogen utilisation (45%), muscle lactate acc umulation (4.6-fold) and net lactate release (52%), without altering net gl ycerol release or net glucose uptake. 5. In both trials, the mean convective heat transfer from the exercising le gs to the body core ranged from 6.3 +/- 1.7 to 7.2 +/- 1.3 kJ min(-1), ther eby accounting for 35-40% of the estimated rate of heat production (similar to 18 kJ min(-1)). 6. At exhaustion in DE, blood lactate values were low whereas blood glucose and muscle glycogen les-els were still high. Exhaustion coincided with hig h body temperature (similar to 40 degrees C). 7. In conclusion, the present results demonstrate that reductions in exerci sing muscle blood flow with dehydration do not impair either the delivery o f glucose and FFA or the removal of lactate during moderately intense prolo nged exercise in the heat. However, dehydration during exercise in the heat elevates carbohydrate oxidation and lactate production. A major finding is that more than one-half of the metabolic heat liberated in the contracting leg muscles is dissipated directly to the surrounding environment. The pre sent results indicate that hyperthermia, rather than altered metabolism, is the main factor underlying the early fatigue with dehydration during prolo nged exercise in the heat.