Relaxin Receptor in the Rat Myometrium: Regulation by Estrogen and Relaxin

The concentration and affinity of the receptors for relaxin in the myometrium of rats were compared during the estrous cycle, in pregnancy, after ovariectomy, and after estrogen and/or relaxin administration. The concentration of the myometrial relaxin receptors in cyclic rats appeared to be related to plasma estrogen levels. Ovariectomy of cyclic animals resulted in the disappearance of receptors by 7 days after surgery. Recovery of the receptors was demonstrable upon estrogen administration to these animals. The response of the uterus to estrogen was biphasic and dependent on the time of exposure to the hormone in vivo. Relaxin receptor concentration increased 5- and 30-fold relative to those at estrus and proestrus within 24 h of the administration of 5 and 200 μg of the steroid, respectively, with no significant change in the receptor's affinity constant.

The number of myometrial receptors found in pregnancy were unrelated to the known changes in plasma estrogen levels. However, during late pregnancy, a decrease in the concentration of receptors was observed which was inversely related to the increases in endogenous plasma relaxin at this time. Exogenous porcine relaxin administered to ovariectomized, estrogen-primed rats decreased relaxin receptors in the myometria relative to control animals. Acute administration (3 h) of 20 μg relaxin resulted in a 75% decrease in receptor concentration. This relaxin effect was less pronounced at lower relaxin dose (5 μg) and upon chronic administration of relaxin (24 h). The affinity of the receptors for relaxin was not altered under these conditions.

The results suggest that modulation of relaxin receptors in the uterus by estrogen may represent part of the mechanism by which estrogen enhances the sensitivity of target tissues to relaxin. Conversely, a decrease in the number of relaxin receptors in the uterus by relaxin may be responsible in part for the decreased responsiveness of the uterus to relaxin subsequent to relaxin treatment.