Diaphragmatic fiber type specific adaptation to endurance exercise

Recent evidence suggests that exercise training results in a significant improvement in the oxidative capacity of the mammalian diaphragm; however, limited data exist concerning which diaphragmatic fiber types are metabolically altered due to training. To test the hypothesis that exercise training increases the oxidative capacity of diaphragmatic type I and IIa fibers only, we examined the effects of endurance training on the fiber type specific changes in oxidative capacity, cross-sectional area, and capillarity of the costal diaphragm. Female Fischer-344 rats (age ca 180 days) were divided into either sedentary control group (n = 6) or an exercise training group (n = 6). The trained animals exercised for 10 wks on a motor-driven treadmill (60 min.day-1; 5 days.wk-1) at a work rate equal to ca 55-65% VO2max. Capillaries were identified histologically and fiber types determined using ATPase histochemistry. Fiber cross-sectional area (CSA) and succinate dehydrogenase (SDH) activity in individual fibers were measured using a computerized image analysis system. Compared to control animals, training did not increase the capillary to fiber ratio in any diaphragm fiber type (P greater than 0.05); however, training increased capillary density (capillary No./CSA) in type IIa fibers due to a reduction in cell CSA (P less than 0.05). Further, training resulted in significant (P less than 0.05) increases in total diaphragmatic SDH activity (delta increase = 17.5%) and an increase in SDH activity in both type I (delta increase = 14%) and IIa fibers (delta increase = 17.4%). In contrast, training did not alter (P greater than 0.05) SDH activity in type IIb fibers.(ABSTRACT TRUNCATED AT 250 WORDS)