Progestin receptor in the thymus of ovariectomized immature rats

Abstract

ABSTRACT By using the synthetic progestin promegestone (R5020), the location and characteristics of progestin receptors in the thymic cytosols from immature ovariectomized oestrogen-treated rats were determined. Tritiated promegestone bound to the cytosol with high affinity (dissociation constant (Kd) = 2·0± 0·3 nmol/l; promegestone > progesterone > oestradiol > corticosterone ≃testosterone) and low capacity (number of binding sites (Bmax) = 143·0 ±13·5fmol/mg protein). These values were appropriate for progestin receptors. However, an extremely high dose of dexamethasone (10 μmol/l; 1000-fold excess over [3H]promegestone) slightly inhibited the specific binding. Progestin receptors were predominantly located in the reticuloepithelial (RE)-cell fraction, with few in the thymocyte T-cell fraction. The receptor level was raised (24·9 ± 11·3 (s.e.m.) to 143·0 ± 13·5 fmol/mg protein) with increased doses of oestrogen (0–30 μg) administered in vivo. Using sucrose density gradient ultra-centrifugation it was found that the thymic progestin receptor had a sedimentation coefficient of 9 S under low-salt conditions. These results clearly suggest that the thymus of the immature female rat contains a specific progestin receptor which is mainly located in the RE cells. J. Endocr. (1985) 107, 223–229