Effects of exercise to exhaustion in rat myocardium. A stereological study.

Stereological methods under light and electron microscopy have been used to study the ventricular myocardium of rats exhausted by swimming exercise. The proportion of myocardial cell/interstitium diminishes with the exercise. This result is interpreted as a consequence of an intercellular edema which becomes more pronounced during the first hour of exercise. In the myocardial cells, neither the relative volume occupied by the myofibrils nor the number of mitochondria per cm3 undergoes significant changes. Nevertheless, a progressive increase of the relative mitochondrial volume is observed, specially during the first two hours of exercise. This increase may mainly be due to swelling of these organules, since a significant increase of the mean mitochondrial volume was noticed. These features confirm the observations made at ultrastructural level.