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The metabolic response of individuals varying in aerobic fitness to acute hypoxia and acclimatization to moderate altitude Beatty, Cheryl

Abstract

The concentrations of high-energy phosphate metabolites and lactate were monitored during and following exercise in ten male subjects varying in endurance fitness status throughout a 3-week moderate altitude acclimatization (3,800m) protocol, in order to determine (1) if metabolic control is altered with altitude acclimatization, and (2) how the metabolic response at altitude varies between fitness groupings. Venous blood lactate and cardiorespiratory measurements were taken during incremental and submaximal (70% relative V[sub O]₂ max) cycling exercise and recovery at sea-level before (PRE) and after (POST) acclimatization, and at altitude upon immediate exposure (AH) and following acclimatization (ACC).³¹ Phosphorus nuclear magnetic resonance spectroscopy was used to measure relative intramuscular phosphocreatine (PCr) and inorganic phosphate (Pi) concentrations, and pH during and following incremental plantar flexion exercise to fatigue, under normoxic and hypoxic (F₁O₂ = 13.8%) conditions. In the untrained group, PCr levels tended to decline, the rate of PCr recovery was delayed, and blood lactate concentrations and P[sub i] /PCr (estimate of ADP[sub free]) were increased when comparing AH to PRE levels. In ACC, PCr levels, PCr recovery rates, lactate concentrations, and P/PCr returned to PRE values, and lactate levels and P[sub i]/PCr decreased further under POST exercise conditions. In the trained subject group, PCr recovery rates, PCr levels and P/PCr were unaltered between test conditions, and lowered lactate concentrations did not persist under POST conditions, as observed in the untrained group. Furthermore, the untrained group displayed lower heart rates, higher minute ventilation values, and improvements in the rate of lactate recovery and maximal oxidative capacity during exercise in POST compared to PRE, while all cardiorespiratory and metabolic measurements returned to PRE levels in the trained group. These results suggest that following altitude acclimatization, metabolic control improved in the untrained group, resulting in a lowering of glycolytic flux and lactate production during exercise. In contrast, the exposure to altitude had no effect on the trained subject group, suggesting that this group exhibited an 'optimal' level of metabolic control prior to altitude exposure, or the hypoxic stimulus was insufficient to activate further change. In conclusion, the biochemical and/or physiological adjustments that occurred in the untrained subjects during the acclimatization protocol resulted in a tightening of metabolic control during exercise, normally characteristic of changes seen following endurance training, accounting for the lower perturbation of high-energy phosphate metabolites and reduced lactate levels following altitude acclimatization.

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