Depletion of [ 3 H]Methyltrienolone Cytosol Binding in Glucocorticoid-Induced Muscle Atrophy

Abstract

The present study was undertaken to determine cytosol binding properties of [³H]methyltrienolone, a synthetic androgen, in comparison with [³H]dexamethasone, a synthetic glucocorticoid, under conditions of glucocorticoid excess in skeletal muscle. Male hypophysectomized rats received either seven daily subcutaneous injections of cortisone acetate (CA) (100 mg·kg^- 1 body wt) or the vehicle, 1% carboxymethyl cellulose. Following treatment, both [³H]dexamethasone and [³H]methyltrienolone-receptor concentrations were decreased from those in vehicle-treated rats by more than 90 and 80%, respectively, in CA-treated animals. Scatchard analysis of [³H]methyltrienolone binding in muscles of vehicle-treated animals became nonlinear at high concentrations of labeled ligand and were reanalyzed by a two-component binding model. The lower affinity, higher capacity component, which was attributed to binding of methyltrienolone to a “dexamethasone” component, disappeared in muscles of CA-treated rats and Scatchard plots were linear. Receptor concentrations of the higher affinity lower capacity “methyltrienolone” component were similar in muscles of vehicle-treated and CA-treated groups. From competition studies, the high relative specificities of glucocorticoids for [³H] methyltrienolone binding in muscles of vehicle-treated animals were markedly reduced by CA treatment. In addition, the binding specificity data also showed strong competition by progesterone and methyltrienolone for [³H]dexamethasone binding and estradiol-17β for [³H]methyltrienolone binding. These results demonstrate that most of the [³H]methyltrienolone binding is eliminated under in vivo conditions of glucocorticoid excess. Furthermore, the competitiveness of various steroids for receptor binding suggests that rat muscle may not contain classic (ligand-specific) glucocorticoid and androgen receptors.