Differential adaptation to endurance training between heart and gastrocnemius muscle mitochondria in rats.

The effects of endurance training on mitochondrial gene expression and replication in heart and gastrocnemius muscles were studied in rats. Rats were trained for 12 weeks by treadmill running. Although the heart weight was significantly increased by the training, activity of cytochrome oxidase and concentrations of cytochrome b mRNA and mitochondrial DNA in the heart were not altered by the training. On the other hand, those in gastrocnemius muscle were increased about 1.5 fold by the training. When these values were compared between heart and gastrocnemius muscles, they were 10, 3.3, and 1.7 fold, respectively, higher in the former than in the latter. These results suggest that the mitochondrial gene expression and enzyme activity for the oxidative capacity in heart are fairly high compared to those in skeletal muscle, therefore adaptation to the training in heart is attained by increasing the muscle mass, but not mitochondrial gene expression.