Progesterone receptors in rabbit uterus-II characterization and estrogen augmentation

Progesterone-binding protein in uterine, endometrial and myometrial cytosol of normal and estrogen (diethylstilbestrol)-primed, immature rabbits has been characterized using sucrose gradient centrifugation and charcoal adsorption techniques. Estrogen priming daily for five days increased the concentration of uterine progesterone receptors 32-fold as compared to progesterone binding in serum. The number of receptor binding sites in the uterus reached a maximum on day ten of 51,700 per cell. This result compares favorably with the best progestational response, which occurs on days 10 and 11 in McPhail's[1] bioassay. The mean dissociation constant for progesterone and progesterone receptor complexes was estimated at 3.3 × 10^-10M (n = 18). We observed a higher concentration of progesterone receptors in myometrial than in endometrial cytosol; both levels were less than that in uterine cytosol. In addition to quantitative changes, we observed a qualitative change in the sedimentation constant of progesterone-binding protein from a 4-5S to a 5-6S complex after ten days of estrogen administration; the complex changed irreversibly to the 4-5S form in the presence of 0.4 M KCl. Among the natural steroids tested, the highest relative binding affinity was observed with 5α-pregnan-3,20-dione (15%) when progesterone was 100%. Synthetic progestational compounds showed a wide range of binding affinities for progesterone receptors.