Progestin binding sites in the rat hypothalamus pituitary and uterus.

Specific, estrogen-inducible, nuclear radioactivity uptake has been demonstrated in the uterus, pituitary and hypothalamus of immature female rats after injection of a highly potent labelled progestin, R 5020 (17, 21-dimethyl-19-nor-pregna-4, 9-diene-3, 20-dione). A cytoplasmic progestin "receptor" has been characterized in these target tissues by an in vitro Dextran-coated charcoal adsorption method. R 5020 binds with the same intrinsic dissociation constant to the receptor present in these tissues; it dissociates at the same rate from the uterine and pituitary receptor (k-1 = 1 multiplied by 10(-2) min (-1), but about 6 times slower than progesterone. This evidence, together with the similar hormone specificity in the uterus and pituitary, suggests that the progestin "receptor" is a similar entity in central and peripheral target tissues.