The binding of the synthetic radioactive steroids [3H]-methyltrienolone (MT) (17β-hydroxy-17α-methylestra-4,9,11-trien-3-one) and [3H]-dexamethasone (DEX) (9α-fluoro-11β,17α,21-trihydroxy-16α-methylpregna -1,4-diene-3,20-dione) to cytosol from human skeletal muscle was studied using a charcoal adsorption technique and calculating receptor-bound steroids according to Scatchard. The rate of association was more rapid in the case of MT than in the case of DEX. The maximum specific binding of MT and DEX was reached after about 15 and 22 h at 0-4°C, and lasted for at least 30 and 20 h, respectively. The binding sites were specific for androgens (MT) and glucocorticoids (DEX). In most human muscles analyzed, both receptors were detectable. The apparent equilibrium dissociation constants of the androgen and glucocorticoid receptor-binding were 0.07 to 0.70 nM and 1.7 to 9.7 nM, respectively. The maximum number of binding sites (Bmax) ranged from 55 to 240 fmol/g tissue, 1.1 to 4.0 fmol/mg protein or 60 to 450 fmol/mg DNA in the case of the androgen receptor. The Bmax, of the glucocorticoid receptor varied between 1,200 and 5,600 fmol/g tissue, 23 and 94 fmol/mg protein or 770 and 12,000 fmol/mg DNA. Hence, human skeletal muscle may respond directly to androgens and to glucocorticoids.
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