Effects of endurance training on the thickness of the muscle fiber-capillary barrier in soleus muscle of growing rats

The muscle fiber-capillary barrier consists of the capillary endothelium and the interstitium. Thinning of the barrier is physiologically significant for gas exchange in skeletal muscle because it shortens the diffusion distance of gases. The purpose of the present study was to examine the effect of endurance training on the ultrastructure of the muscle fiber-capillary barrier in soleus muscle of growing rats. Seventeen male Fischer 344 rats, aged 5 weeks, were assigned to either an exercise-trained group (ET, n = 5), a paired-weight sedentary group (PWS, n = 6), or a sedentary group (S. n = 6), and matched as closely as possible with regard to body mass and V̇o_2max. The ET group performed a treadmill running program for 5 days/week for 10 weeks. The ET and S rats were freely fed rat chow and water. The PWS rats had their food intake restricted so that their mean body mass would be the same as that of the ET rats. After the training period, the diffusion distance in the ET group was significantly shorter than that in the PWS and S groups. There were no differences in the thicknesses of the capillary endothelium among the three groups, but the interstitium was significantly thinner in the ET group than in the PWS and S groups. The degree of decrease in the interstitium thicknesses in the ET group was almost equal to that of the diffusion distance. Therefore it was identified that endurance training shortens the diffusion distance, which depends on thinning of the interstitium. These results suggest that morphological adaptation to endurance training partly contributes to the improvement of gas exchange in the muscle and aerobic work capacity.