Glucocorticoid cytosol binding in exercise-induced sparing of muscle atrophy.

Female rats were initially divided into a sedentary or an exercise group that was trained by treadmill running to a final work rate of 31 m/min, 100 min/day, for 13-18 wk. During the last 12 days of training each of these groups were further subdivided into groups that received daily subcutaneous injections of cortisol acetate (CA) (100 mg/kg body wt) or the vehicle (1% carboxymethyl cellulose). Exercise prevented approximately 40% of the gastrocnemius muscle weight loss due to CA treatment. Training did not influence glucocorticoid cytosol-receptor binding concentrations, using [3H]triamcinolone acetonide (TA) as the labeled glucocorticoid in any of the skeletal muscle types investigated. TA-receptor binding capacities were depleted by the multiple injections but were higher in the red fiber types of the CA-treated trained than those in the CA-treated sedentary animals. In a second series of experiments in which receptor depletion and repletion rates were studied using a single injection of cortisol, TA binding capacities 2 h after the cortisol injection were higher in slow-twitch red soleus muscles of trained as compared with sedentary rats (36.4 +/- 2.0 vs. 26.8 +/- 2.5 fmol/mg protein). Similar patterns of TA binding were also observed at 2 h between trained and sedentary animals in the fast-twitch red muscle types, whereas no training related differences were observed in white muscle types. Total and free serum cortisol concentrations also returned to base-line values faster in the trained animals following the single injection protocol.(ABSTRACT TRUNCATED AT 250 WORDS)