Effect of progesterone on estrogen receptors in the rabbit uterus

The macromolecular binding of estradiol was investigated in the rabbit uterus under various hormonal states in a search for evidence of progesterone antagonism of the estrogen-receptor complex interaction. Progesterone did not alter the sucrose gradient sedimentation characteristics of the estrogen-receptor complex in estrus or castrate animals. Progesterone treatment of castrated animals did not alter the absolute number of EBS (estrogen-binding sites) per cell, but did result in decreased relative concentration of EBS by increasing the soluble protein content of the cells over that seen in untreated castrates. Castration did not alter the absolute number of EBS per cell but did increase the relative concentration of estrogen binding in the cytosol.