Progesterone, 5α-dihydroprogesterone and R5020 binding in female rat anterior pituitary

Abstract

Progestin binding, which may represent receptor binding, was examined in female rat anterior pituitary cytosol. Specific progesterone, 5α-dihydroprogesterone (5α-DHP) and R5020 binding, as well as cortico-steroid binding globulin (CBG)-like complexes, were assayed in cytosol from five animal models: estradiol benzoate primed-ovariectomized-adrenalectomized (EB-OVX-ADX), ovariectomized-adrenalecto-mized (OVX-ADX), estradiol benzoate primed-ovariectomized (EB-OVX), ovariectomized (OVX) and intact rats. With all three progestins, estrogen priming consistently and significantly enhanced specific binding levels, and cytosol from EB-OVX-ADX rats had the greatest amount of binding. In contrast, presumptive CBG-like levels increased with adrenalectomy but not with EB treatment. Specific progesterone and 5α-DHP complexes dissociated more rapidly during Sephadex LH-20 elution than specific R5020 and CBG-like binding complexes. Maximal values for progesterone and 5α-DHP binding were rapidly attained at 4°C and the binding was decreased in the presence of protease, but not DNase or RNase. 5α-DHP binding had an apparent K d of 0.9 to 2 × 10 −9 M and the number of binding sites increased with EB treatment in the OVX-ADX rat. The 5α-DHP binding exhibited greater specificity for progestins (particularly progesterone and R5020) than for androgens, glucocorticoids or estradiol.