Diaphragmatic fiber type specific adaptation to endurance exercise

Abstract

Recent evidence suggests that exercise training results in a significant improvement in the oxidative capacity of the mammalian diaphragm; however, limited data exist concernin g 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 examinedthe 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 (agc 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; days·wk −1) at a work rate equal to ca 55–65%s% V̇ o 2max . 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 typc ( P > 0.05); however, training increased capillary density (capillary No. /CSA) in type lla fibers due to a reduction in cell CSA ifP > 0.05. Further, training resulted in significant ifp > 0.05 1 increases in total diaphragmatic SDH activity ( Δ increase = 17.5%) and an increase in SDH activity in both type I (Δ increase = 14%) and lla fibers ( Δ increase = 17.4%). In contrast, training did not alter ( P > 0.05) SDH activity in type llb fibers. These data support the hypothesis that endurance training results in significant improvements in the oxidative capacity of type I and lla fibers in the costal diaphragm of rodents. However, the increase in relative SDH activity and capillary density in type lla fibers is achieved primarily via a reduction in fiber CSA.