ESTROGEN BINDING-PROTEINS IN RAT SKELETAL AND PERINEAL MUSCLES - INVITRO AND INVIVO STUDIES

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 (Ka) for estradiol was found to be 1.6 × 109 M-1 in LA/BC and 0.9-2.0 × 109 M-1 in female and male TM. The Ka 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. © 1979.