Acute exercise in ozone-polluted air induces apoptosis in rat quadriceps femoris muscle cells via mitochondrial pathway

Ozone (O(3)) pollution can decrease sport performance and induce respiratory toxicity, but relatively few studies have investigated its effects on skeletal muscles. We randomly assigned rats to the following groups based on a 2 ​× ​4 two-factor factorial design: Air+0, Air+10, Air+15, and Air+20, O(3)+0, O(3)+10, O(3)+15, and O(3)+20. The rats in the +0 groups rested, whereas those in the +10, +15, and +20 groups ran on a treadmill (in clean air for Air groups and in air polluted with 0.14 ​parts per million [ppm] O(3) for O(3) groups) at speeds of 10, 15, and 20 ​m/min, respectively, for 1 ​h. Thereafter, key enzyme activities involving the tricarboxylic acid cycle, oxidative phosphorylation, adenosine triphosphate (ATP) content, histopathological changes, oxidative stress, inflammation factors, and apoptosis were assessed in the rat quadriceps femoris samples. Ozone reduced key enzyme activities and ATP contents in the quadriceps femoris regardless of whether the rats exercised. Pathological changes, inflammatory factors, oxidative stress, and mitochondria-dependent apoptosis were only evident under conditions of exercise combined with ozone and increasingly worsened as exercise intensity increased. These findings suggested that acute exercise under ozone exposure could induce damage to the quadriceps femoris, which would negatively affect sport performance. Ozone-induced disrupted energy metabolism might be an early event that becomes more critical as exercise intensity increases. Therefore, care should be taken when exercising in polluted air, even when ozone pollution is mild.