Time-course changes in arteriolar and venular portions of capillary in young treadmill-trained rats

This study was designed to examine the time-course changes in capillarity, especially the distribution of arteriolar and venular capillaries, in skeletal muscles during endurance training by running in young female Wistar rats. Exercise training by running started at the age of 5 weeks and lasted for 5 weeks at 25 m min(-1) on a 25% gradient, 10-60 min day(-1), 5 days per week. Morphological findings were obtained from the soleus (SOL) and deep portions of the medial gastrocnemius (GASd) muscles. In SOL, the training significantly increased the capillary-to-fibre ratio (C:F), total capillary density, and density of venular capillaries at Week 1. Significant increases in the C:F and total capillary density were also observed at Weeks 4 and 5. The capillary domain area (CDA) values for total capillary and arteriolar and venular capillaries were significantly decreased after 4 weeks of training. In GASd, the training significantly increased the C:F at Week 1 and from Week 3 onwards. It also significantly increased cross-sectional area of all fibre types. Total capillary density and the CDA values showed no significant difference between control and training groups at all points of time. Significant increases in succinate dehydrogenase (SDH) activity were first noted after 2 weeks in GASd and after 3 weeks in SOL. These findings suggest that, in young growing rats, adaptive changes in the oxygen transport system occurred within 1 week after the onset of training mainly in highly oxidative muscles. The changes in arteriolar and venular capillary densities may help to clarify the growth pattern of the capillary network associated with endurance running training.