Effects of hyperthermia and acidosis on mitochondrial oxidative phosphorylation.

The intracellular environment of skeletal muscle can develop pronounced hyperthermia and acidosis during strenuous exercise, and these alterations in the typical intracellular conditions have been shown to alter mitochondrial respiration. However, the impact of these conditions on ATP synthesis is poorly understood. We used Thoroughbred racehorses to test the hypothesis that both hyperthermia and acidosis decrease the rate of ATP synthesis, but that athletic conditioning mitigates this loss of phosphorylation capacity. Isolated mitochondria were harvested from skeletal muscle before and after a 9-wk racetrack conditioning program that increased whole body aerobic capacity by 19%, and oxidative phosphorylation capacity was tested ex vivo under normothermic and hyperthermic conditions, as well as normal pH and acidic pH created by the addition of lactic acid. In unfit horses, hyperthermia caused a 30-55% decrease in the rate of ATP synthesis and loss of phosphorylation efficiency (P/O ratio decreased from 4.2 to 1.7 during maximal oxidative phosphorylation). Aerobic conditioning resulted in increased phosphorylation efficiency under hyperthermic conditions. Lactic acidosis had a small negative effect on ATP synthesis in unfit horses, but aerobic conditioning increased the sensitivity of isolated mitochondria to the deleterious effects of lactic acidosis. These data support the prominent role of hyperthermia in skeletal muscle fatigue during exercise, particularly in unfit subjects. However, acidosis may be a more important cause of failure of ATP synthesis in fit subjects. This study reports the use of a novel method for the measurement of ATP synthesis and concurrent mitochondrial respiration. Although acidosis is often considered a leading cause of exercise fatigue, this study shows that tissue hyperthermia, which often occurs concurrently with acidosis, has a larger role in decreasing the maximal rate of skeletal muscle ATP synthesis.