Modulation of the Cytosolic Androgen Receptor in Striated Muscle by Sex Steroids*

We studied the influence of orchiectomy (GDX) and steroid administration on the level of cytosolic androgen receptor in rat levator ani muscle and rat skeletal muscles (tibialis anterior and extensor digitorum longus). Androgen receptor binding to muscle cytosol was measured using [3H]methyltrienolone (R1881) as ligand, a 100-fold molar excess of unlabeled R1881 to assess nonspecific binding, and a 500-fold molar excess of triamcinolone acetonide to prevent binding to glucocorticoid and progestin receptors. Bound and free ligand were separated by column chromatography with Sephadex G-75. In levator ani muscles from intact animals (controls), maximum R1881 binding (Bmax), determined by Scatchard analysis, was 2.5 fmol/mg protein (Kd = 0.68 nM). Thirty days after GDX, Bmax increased to 500% of the control value, with no significant change in Kd (0.96 nM). Using saturating levels of R1881, Bmax was increased to 280% of the control value 12 h post-GDX, 600% at 14 days, 478% at 30 days, and 133% at 44 days. The increase in receptor binding was blocked by cycloheximide. Administration of Silastic capsules containing testosterone propionate 30 days post-GDX resulted in R1881 binding at the control level at 44 days. Surprisingly, administration of 17 beta-estradiol (E2) 30 days post-GDX resulted in increased (480%) R1881 binding. Thus, E2 may cause induction of the cytosolic androgen receptor in levator ani muscle from GDX rats; alternatively, the rate of receptor degradation may be altered. R1881 binding by skeletal muscle cytosol was increased 139% at 12 h, 212% on day 14, 220% on day 30, and 158% on day 44 with respect to the control value. Administration of testosterone propionate at 30 days caused R1881 binding to return to the control value by day 44, whereas E2 was without influence. The differences in response of levator ani and skeletal muscle receptors may account for the differential effects of sex steroids on these muscle types.