Relationship between the Quantity of Progesterone Receptor and the Antagonism of Estrogen-Induced Uterotropic Response*

Abstract

The ability of the uterus to respond to progesterone was examined as a function of cytosol progesterone receptor levels. The response was assessed by measuring the capacity of progesterone to antagonize estradiol-induced uterine fluid accumulation in uteri containing either high or low levels of progesterone receptor. High receptor levels (4-12 pmol/uterus) were induced by estradiol treatment (1.0 μg/rat/day for 2 days) in ovariectomized rats. Low levels (≤2 pmol/uterus) were obtained by injecting estrogen-primed rats with 0.5 mg progesterone and waiting 24 h before subsequent treatment. Progesterone (0.5 mg) was very effective in inhibiting estradiol-induced uterine fluid accumulation in uteri with high levels of receptor, whereas the same dose of progesterone was without effect in uteri with low levels of receptor. These data demonstrate that the level of cytosol progesterone receptors is correlated with the ability of the uterus to respond to progesterone. The reduction in progesterone receptors observ...