The binding of dexamethasone and triamcinolone acetonide to glucocorticoid receptors in rat skeletal muscle.

Specific binding of the synthetic glucocorticoids [l ,2,43H]dexamethasone (Qa-fluoro-llp,17a,Zl-trihydroxy-16amethyl-1,4-pregna-1,4-diene-3,20-dione) and [1,~2,4-~H]triamcinolone acetonide (9cu-fluoro-ll~,l6ol,l7~,Zl-tetrahydroxy-pregna-l,4-diene-3,20-dione-16,17-acetonide) by the cytoplasmic fraction of rat gastrocnemius muscle was studied. The cytosol binding reaction displayed stereospecificity and high affinity of binding. Biologically active glucocorticoids, administered to adrenalectomized rats or present during the in vitro binding reaction markedly depressed the binding of [3H]dexamethasone and [“H]triamcinolone acetonide. The biologically inactive stereoisomer, epicortisol, had no effect on the binding of the labeled hormones. The binding component displayed high affinity for [ aH]dexamethasone and [ 3H]triamcinolone acetonide (& = 1.9 and 1.2 X lo-* M, respectively). The number of binding sites was limited (0.1 pmoles per mg of cytosol protein), and Scatchard analysis suggests that only a single class of binding sites exists for both [3H]dexamethasone and [3H]triamcinolone acetonide. Competition studies indicated that the two glucocorticoids interact with the same binding site. The binding macromolecule appears to be a protein since binding is prevented by nagarse treatment and is dependent on the integrity of -SH groups. The glucocorticoid-protein complexes were characterized on 5 to 20% sucrose gradients. Both complexes sedimented at n 4 S in 0.3 M KCl, but only the [3H]triamcinolone acetonide-receptor complex sedimented at 7 S in the absence of salt. Since the specific binding component has the properties of a physiological glucocorticoid receptor, a direct effect of these hormones on skeletal muscle is suggested.