ALTITUDE ACCLIMATIZATION AND ENERGY METABOLIC ADAPTATIONS IN SKELETAL-MUSCLE DURING EXERCISE

To determine whether the working muscle is able to sustain ATP homeostasis during a hypoxic insult and the mechanisms associated with energy metabolic adaptations during the acclimatization process, seven male subjects [23 +/- 2 (SE) yr, 72.2 +/- 1.6 kg] were given a prolonged exercise challenge (45 min) at sea level (SL), within 4 h after ascent to an altitude of 4,300 m (acute hypoxia, AH), and after 3 wk of sustained residence at 4,300 m (chronic hypoxia, CH). The prolonged cycle test conducted at the same absolute intensity and representing 51 +/- 1% of SL maximal aerobic power (VO2max) and between 64 +/- 2 (AH) and 66 +/- 1% (CH) at altitude was performed without a reduction in ATP concentration in the working vastus lateralis regardless of condition. Compared with rest, exercise performed during AH resulted in a greater increase (P < 0.05) in muscle lactate concentration (5.11 +/- 0.68 to 22.3 +/- 6.1 mmol/kg dry wt) than exercise performed either at SL (5.88 +/- 0.85 to 11.5 +/- 3.1) or CH (5.99 +/- 0.88 to 12.4 +/- 2.1). These differences in lactate concentration have been shown to reflect differences in arterial lactate concentration and glycolysis (Brooks et al. J. Appl. Physiol. 71: 333-341, 1991). The reduction in glycolysis at least between AH and CH appears to be accompanied by a tighter metabolic control. During CH, free ADP was lower and the ATP-to-free ADP ratio was increased (P < 0.05) compared with AH. With acclimatization, no change was found in the maximal activity of succinate dehydrogenase measured at SL and CH: 63.9 +/- 6.1 vs. 68.6 +/- 4.9 mmol . kg protein-1 . min-1, respectively. Similarly, acclimatization failed to alter fiber area, the number of capillaries per fiber, and the capillary-to-fiber area ratio. These results suggest, that the reduction in glycolytic flux that occurs with acclimatization cannot be explained by changes in the muscle oxidative potential, muscle fiber size, or muscle fiber capillarization.