Progesterone-induced oestrogen receptors in the rat uterus.

In ovariectomized rats progesterone acts like oestradiol at the endometrial, but not at the myometrial level, by increasing the number of oestrogen receptors in the cytoplasm. After 3 days of progesterone priming, the number of endometrial oestrogen receptors was found to be three times higher (P less than 0.05, Student's t-test) than control values. This progesterone-induced oestrogen receptor molecule appears identical in its physico-chemical properties (sedimentation coefficient, kinetic constants, steroid specificity) to that induced in the endometrium and myometrium under the influence of oestradiol. However, in the myometrium progesterone acts antagonistically reducing significantly (P less than 0.001, Student's t-test) the oestradiol-induced increase in the number of oestrogen receptors.