In vitro binding to an in vivo effect on the cytosol and nuclear progesterone receptors of various progestins, and their relationship to synthesis of uteroglobin in rabbit uterus.

In vitro binding affinities of various progestins to cytosol and nuclear progesterone receptors of rabbit uterus were determined and correlated with the biological potency of these steroids. In addition, cytosol and nuclear progesterone receptor levels were measured after a 5-day administration of different progestins (0.5 mg/kg daily) with variable biologic activities. The receptor levels were compared with the biological response; the induction of uteroglobin synthesis. Cytosol and nuclear progesterone receptors had identical steroid binding properties (r = 0.98). The correlation between the in vitro binding affinity (cytosol or nuclear) and the in vivo biologic activity of the steroids was good (r = 0.73). After a 5-day treatment with progestins, the nuclear receptor concentration correlated n an inverse manner (r = -0.84) with the uterine fluid uteroglobin concentration. A similar, but slightly weaker correlation (r = -0.81) was also found for the cytosol receptor content and uteroglobin secretion. These data indicate that not only nuclear, but also cytosol progesterone receptor levels decrease in the rabbit uterus during chronic hormone action. Decline in the nuclear progesterone receptor content seemed to occur during treatment with all progestational steroids, while only progestins with high biological potency were capable of decreasing the cytosol receptor content.