Characterization of the cytosolic estrogen receptor in rat skeletal muscle

A charcoal assay and the synthetic estrogen [6,7-3H]R 2858 (17 alpha-ethynyl-11 beta-methoxyestradiol-17 beta) were used to show that rat skeletal muscle cytosol contains an estrogen receptor, which was characterized with regard to association and dissociation rate-constants at several temperatures. The degradation kinetics was also studied, and was more rapid in the absence than in the presence of ligand. Association, dissociation and degradation were all temperature-dependent. The apparent equilibrium dissociation constant (Kd), however, was not temperature-dependent, and was about 0.1-1.0 nM, whether calculated from the rate constants or determined by Scatchard analysis. Estradiol-17 beta and R 2858 were compared as ligands for Scatchard analysis; the maximum number of binding sites being about the same (120 and 110 fmol/g tissue, respectively, but the Kd was lower for estradiol-17 beta (0.16 nM) than for R 2858 (0.73 nM). Ligand specificity studies (using [3H]R 2858 as well as [3H]estradiol-17 beta) showed that R 2858 binds to an estrogen receptor in rat skeletal muscle. The estrogen receptor interacted both with heparin and with DNA covalently coupled to agarose, and was eluted from either column by NaCl. Chromatography of crude cytosol on heparin-agarose or DNA-agarose lead to an at least 10-fold or 25-fold purification of the estrogen receptor, respectively. DNA-agarose chromatography or ammonium sulfate precipitation did not separate the estrogen receptor from the androgen or glucocorticoid receptors in rat muscle.