Effects of exercise on the capillary vasculature of the rat heart.

Effects on the myocardium, particularly those structural properties of the capillary network relevant to tissue oxygenation, were studied morphometrically in rats subjected to moderate exercise, strenuous exercise, and strenuous exercise preceded by a preconditioning period of moderate exercise. These different modalities of exercise were used to test the hypothesis that endurance training leads to capillary proliferation in the heart. The findings indicate that treadmill running produces right ventricular hypertrophy exclusively, which is accompanied by lengthening of the myocytes. Furthermore, a moderate running program results in an increase in the numerical density, luminal surface, and total length of capillaries in the right ventricle. In contrast, strenuous exercise with or without a preconditioning period produces a relative decrease in capillary luminal volume, surface, and numerical density. In the former case the diffusion distance for oxygen is decreased and in the latter it is increased. These data suggest that moderate exercise affects the microvasculature in ways that improve the efficiency of tissue oxygenation in the myocardium, whereas strenuous exercise has the opposite effect.