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 a 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% V(O2,max). 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 > 0.05); however, training increased capillary density (capillary No. /CSA) in type IIa fibers due to a reduction in cell CSA (P < 0.05). Further, training resulted in significant (P < 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 > 0.05) SDH activity in type IIb fibers. These data support the hypothesis that endurance training results in significant improvements in the oxidative capacity of type I and IIa fibers in the costal diaphragm of rodents. However, the increase in relative SDH activity and capillary density in type IIa fibers is achieved primarily via a reduction in fiber CSA.