[Estrogen receptors of the rat uterus during pregnancy and lactation]

Uterine estrogen receptors (ER) of the rat during pregnancy and postpartal lactation were assayed. Both the cytoplasmic and nuclear ER were determined by the controlled pore glass (CPG) method as previously described in detail. The cytoplasmic ER level slightly decreased as pregnancy advanced and then increased remarkably during postpartal lactation, with a tendency to decrease at 5 weeks of lactation when the estrous cycle could have recurred despite the nursing behavior of the rats. On the other hand, no remarkable difference in nuclear ER was noted during pregnancy and lactation. A high concentration of plasma progesterone could have reduced the translocation of the cytoplasmic ER to the nucleus and the replenishment of the cytoplasmic ER, despite a high concentration of plasma estrogen during pregnancy. In order to keep an environment of a high plasma concentration of estrogen similar to that of pregnancy, a long-acting estrogen (estradiol dipropionate) was injected in to the proestrous rats. The cytoplasmic ER rapidly translocated to the nucleus, followed by a prolonged maintenance of a low cytoplasmic and a high nuclear ER level until 7 days, with a gradual replenishment of the cytoplasmic Er and a gradual decline of the nuclear ER thereafter. A translocation of the cytoplasmic Er to the nucleus, similarly seen in the hysterectonized-adult rats treated with estradiol, was noted in the rats injected with estradiol at 3 weeks of lactation. In general, low estrogen and high prolactin may be held in plasma during lactation. These results indicate that the fluctuation of uterine ER simply depends on the plasma level of estrogen during pregnancy and postpartal lactation in rats.