[Arterialization of the capillary network in the left ventricular subendocardium in young rats subjected to exercise training].

The increase in cardiac activity requires an increase in oxygen supply to the ventricular tissues. Two adaptational changes in the capillary network seem to increase the oxygen supply; one is the increase in capillary density and the other may be the increase in arteriolar capillaries. To test this hypothesis, double staining method, which was validated by microsphere injection, was used for differentiation of arteriolar and venular capillaries in exercise-trained rats in the present study. Male Wistar rats after weaning (3 weeks old) were subjected to treadmill running 1 hour/day in the evening, 6 days/week. The running speed was increased from 10 m/min without gradient to 25 m/min with a 7% gradient for 6 weeks. The total capillary density and capillary to myocyte ratio increased significantly. Capillary domain area decreased, suggesting neoformation of capillaries. In support of the capillary increase two stimulators for angiogenesis, vascular endothelial growth factor and basic fibroblast growth factor were immunohistochemically stained in the ventricular tissues from exercise-trained rats. Capillary of arteriolar type increased with a decrease in the venular capillary. The arteriolar portion of capillaries became significantly longer and the venular portion shorter than in resting control rats. Branching of capillaries increased both in the arteriolar and venular portions. These results suggest that the arterialization of capillary network proceeded with the increase in the capillary density to facilitate the oxygen transport to cardiac tissues.