Exercise is associated with the development of oxidative stress, but the specific source and mechanism of production of pro-oxidant chemicals during exercise has not been confirmed. We used equine skeletal muscle mitochondria to test the hypothesis that hyperthermia and acidosis affect mitochondrial oxygen consumption and production of reactive oxygen species (ROS). Skeletal muscle biopsies were obtained at rest, after an acute episode of fatiguing exercise, and after a 9-week conditioning program to increase aerobic fitness. Mitochondrial oxygen consumption and ROS production were measured simultaneously using high-resolution respirometry. Both hyperthermia and acidosis increased non-phosphorylating (LEAK) respiration (5.8X and 3.0X, respectively, p<0.001) and decreased efficiency of oxidative phosphorylation. The combined effects of hyperthermia and acidosis resulted in large decreases in phosphorylating respiration, further decreasing oxidative phosphorylation efficiency from 97% to 86% (p<0.01). Increased aerobic fitness reduced the effects of acidosis on LEAK respiration. Hyperthermia increased, and acidosis decreased ROS production (2X and 0.23X, respectively, p<0.001). There was no effect of acute exercise, but an aerobic conditioning program was associated with increased ROS production during both non-phosphorylating and phosphorylating respiration. Hyperthermia increased the ratio of ROS production to O2 consumption during phosphorylating respiration, suggesting that high temperature impaired transfer of energy through the electron transfer system despite relatively low mitochondrial membrane potential. These data support the role of skeletal muscle mitochondria in the development of exercise-induced oxidative stress, particularly during forms of exercise that result in prolonged hyperthermia without acidosis.
