Progesterone Receptors in Normal Mammary Glands of Mice: Characterization and Relationship to Development*

Although progesterone receptors have been characterized in a number of target tissues for progesterone, including some mammary tumors, relatively little is known about these receptors in normal mammary tissues. In the present study using R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione), a synthetic progestin, we have identified and characterized the cytoplasmic progesterone receptors in normal mammary glands of mice. The receptor had a high binding affinity (Kd = 2.8 X 10(-9)M) for R5020 and was specific for progestins. The levels of receptor varied at different stages of mammary gland development; the levels were inversely proportional to the secretory activity of the gland. This negative corelation between receptors and secretion was further observed in studies in which lactational involution was induced experimentally. We propose that the functional significance of the modulation of progesterone receptors in normal mammary glands may be related to the known different effects of progesterone in this tissue.