EFFECT OF CORTICOSTEROIDS ON DIAPHRAGM FATIGUE, SDH ACTIVITY, AND MUSCLE-FIBER SIZE

The influence of dexamethasone on diaphragm (DIA) fatigue, oxidative capacity, and fiber cross-sectional areas (CSA) was determined in growing hamsters. One group received dexamethasone by daily subcutaneous injection for 21 days (D animals), while pair-weight (P) and free-eating controls (CTL) received saline subcutaneously. Isometric contractile properties of the DIA were determined in vitro by supramaximal direct muscle stimulation in the presence of curare. DIA fatigue resistance was determined through repetitive stimulation at 40 pulses/s for 2 min. A computer-based image-processing system was used to histochemically determine muscle fiber-type proportions, CSA, and succinate dehydrogenase activities. The medial gastrocnemius muscle (MG) was used as a limb muscle control, with histochemical studies being performed on both the superficial (s) and deep/red (r) portions. Dexamethasone markedly attenuated the normal increment in body weight over the 3-wk period. DIA fatigue resistance was significantly reduced in the D compared with CTL and P animals. Dexamethasone had no effect on fiber-type proportions of the DIA or MGr (MGs contained only type II fibers). In the DIA, the CSA of type Il fibers was reduced 33% in D and 18.5% in P animals compared with CTL. Although no significant atrophy was noted in the type I DIA fibers of either D or P animals, a trend toward significance was noted in D animals compared with CTL. In the MGs, the CSA of type II fibers was reduced 33% in D and 16.5% in P animals compared with CTL. Significant atrophy of type I and II fibers of the MGr was noted in D animals compared with CTL (33.8 and 35% reductions, respectively). Succinate dehydrogenase activity in type I and II fibers of the DIA and type II fibers of the MGs was significantly reduced (26-30%) in D compared with both CTL and P animals. We conclude that 1) the diminished DIA fatigue resistance may be explained in part by the reduction in fiber metabolic capacity, 2) despite similar body weights of D and P animals, corticosteroids appeared to have a greater catabolic impact on the DIA in D than in P hamsters, and 3) the nonselective effect of dexamethasone on the CSA of fibers in the MG, compared with the DIA, may be due to the different activation histories of these two muscles.