EFFECTS OF ADRENALECTOMY OR RU-486 ON RAT MUSCLE-FIBERS DURING HINDLIMB SUSPENSION

The purpose of this study was to investigate the effects of a glucocor ticoid antagonist, RU-486, and of adrenalectomy (ADX) on rat skeletal muscle structural properties after 3, 7, and 14 days of hindlimb suspe nsion (H). After H, a significant loss in muscle weight was observed a s early as 3 days in soleus (SOL; -10%) and adductor longus (AL; -14%) muscles. In SOL, after only 7 days, a reduction (-14%) in type I fibe r percent distribution occurred, accompanied by an increase (+129%) in intermediate type I fibers. Fiber type changes increased depending on the duration of H. In AL muscle, no change occurred after H in the fi ber type composition despite a similar degree of muscle atrophy. Treat ment with RU-486 or ADX significantly reduced the loss of SOL weight o bserved after 14 days (-42 and -44%, respectively, vs. -50% for H rats ), delayed the SOL atrophy (from 3 to 7 days), and normalized the shif t in fiber type distribution induced by H. In SOL, administration of R U-486 (but not ADX) partly prevented the reduction in size induced by H of all the fibers. In AL, neither treatment affected the extent of m uscle atrophy, even though the reduction in type IIa fiber size was pr evented by RU-486 but not by ADX after 14 days of suspension. ADX or R U-486 administration did not prevent the extensor digitorum longus wei ght loss observed after 14 days of suspension but allowed a recovery o f its normal fiber type composition. Our data suggest a minor role of glucocorticoid hormones in regulating SOL weight and suggest a major r ole in regulating SOL fiber type composition during H.