Estrogen binding proteins in rat skeletal and perineal muscles: In vitro and in vivo studies

Abstract

Specific vitro binding of [ 3H]-estradiol was demonstrated in the cytosolic fraction of levator ani/bulbocavernosus (LA/BC) muscles and thigh muscles (TM) of 50-day-old rats. Using a charcoal assay, the apparent association constant ( K a ) for estradiol was found to be 1.6 × 10 9 M −1 in LA/BC and 0.9–2.0 × 10 9 M −1 in female and male TM. The K a was not affected by 24 h of castration but the number of binding sites increased significantly ( P < 0.05) from 7.8 ± 1.7 (SD) to 11.7 ± 2.8 (SD) fmol/mg protein in LA/BC and from 3.4 ± 1.6 (SD) to 5.8 ± 0.5 (SD) fmol/mg protein in male TM. In female TM, the level of binding sites of 24 h ovariectomized rats (4.3 ± 1.0 (SD) fmol/mg protein) was relatively similar to the one found in intact rats in diestrus phase (3.4 ± 0.8 (SD) fmol/mg protein) but higher than in proestrus phase (0.8 ±0.15 (SD) fmol/mg protein). In both types of muscles, this cytosol estradiol binding protein sedimented in the 9–11S region of sucrose density gradients. Competition studies with androgens, progestins and corticosteroids showed that the binding was highly specific for estradiol and was only partially decreased by 5α-androstan-3β, 17β-diol. In vivo binding of estradiol in muscles was also found after intravenous perfusion of [ 3H]-estradiol in castrated and functionally hepatectomized rats. Specific 8–10S binding peaks on sucrose gradients were present in LA/BC and in TM cytosols. Moreover in LA/BC muscles, in vivo nuclear estradiol binding proteins could be detected in a 4–5S binding peak on sucrose gradients obtained with 0.4 M KCl nuclear extract. These results show unequivocally the presence of cytosol estrogen binding proteins in skeletal and perineal muscles and suggest also the existence of nuclear estrogen-binding proteins complexes in LA/BC muscles.