Training-induced alterations in young and senescent rat diaphragm muscle

The current study sought to examine the effects of chronic endurance treadmill running on oxidative capacity and capillary density in specific diaphragm muscle fiber types in young (5 mo) and senescent (≥23 mo) female Fischer 344 rats. Both young and senescent animals trained at ~75% of maximal O₂ consumption for 1 h/day 5 days/wk for 10 wk. Plantaris citrate synthase activity was significantly increased (P < 0.01) in both young and old trained groups. Densitometric analysis of succinate dehydrogenase (SDH) activity in diaphragm type I, IIa, and IIb muscle fibers was done using a computerized image-processing system. There were no age-related differences in SDH activity between the young and old groups for any of the fiber types. In addition, SDH activity was found to be significantly increased (P < 0.05) in all three fiber types in both the young and senescent trained animals compared with their sedentary counterparts. Fiber size and capillary density did not differ between young and senescent rats, nor did exercise affect this measure. Each fiber, irrespective of type, had an average of approximately four capillaries in contact with it. However, type IIb fibers had a significantly lower capillary density per unit area than type I or IIa muscle fibers. The results indicate that the senescent costal diaphragm maintains its ability to adapt to an increased metabolic demand brought about by locomotor exercise. Of further interest is the finding that training adaptations occurred in all three fiber types, suggesting that increased work of breathing from moderate exercise leads to recruitment of all three fiber types. Furthermore the costal diaphragm appears to maintain its oxidative capacity and microvascularity with progressing age.