Specific mitochondrial responses to running training are induced in each type of rat single muscle fibers.

The effects of training by running (sprint or endurance) on the mitochondrial volume, number, and succinate dehydrogenase (SDH) activity of different types of single muscle fibers, and on the elemental composition of soleus and extensor digitorum longus muscles were studied employing histochemical, electron microscopic, and electron probe-micro analysis (EPMA). Newly weaned male Wistar rats were trained on a motor-driven treadmill endurance exercise for 14 weeks. The relative mitochondrial volume per single muscle fiber of slow-twitch oxidative (SO) fiber was significantly increased following endurance training (p less than 0.01). There was no significant correlation between mitochondrial volume and number of SO fibers following endurance exercise training. Following sprint training, both mitochondrial volume and number of fast-twitch oxidative glycolytic (FOG) fibers increased significantly (p less than 0.01), with significant correlation (r = 0.69) between these parameters. SDH activity was higher in the order of SO, FOG, and fast-twitch glycolytic (FG) fibers following endurance training; however, the activity was higher in the order FOG, SO, and FG fibers following sprint training. The potassium concentration in cytoplasm of the soleus muscle showed a tendency to decrease following both types of training. These results suggest that the oxidative capacity of each type of muscle fiber does not always increase equally following training. Changes in mitochondrial number and volume in response to the two different types of training differed according to the type of fiber.