Fine structure of myocardial mitochondria in rats after exercise for one-half to two hours.

Acute exercise (swimming in water at 24°C) provoked marked changes in the fine structure of the mitochondria of heart muscle in rats. At 60 min after the start of exercise, a large number of imaginations appeared in the mitochondrial wall, suggesting the existence of replication phenomena. In the interior of the invaginations, electron-dense granules considered to be ribosomes were seen. After 90 and 120 min of swimming, mitochondria of unusual size, measuring several sarcomeres in length, were seen. The enlarged mitochondria showed no structural alterations. The planimetric measurement of the micrographs of myocardium of the control animals and of those that exercised showed a statistically significant increase in the whole mitochondrial mass in the latter. On trie basis of these observations it is suggested that acute exercise can produce an increase in the mitochondrial mass of the heart muscle cell in a short time. ADDITIONAL