Localization of relaxin binding sites in the rat uterus and cervix by autoradiography

125I-labeled porcine relaxin was injected into 27-day-old rats treated with pregnant mare's serum gonadotropin (PMSG) and known target tissues for relaxin, the myometrium, endometrium and cervix, and putative control tissues, heart, thigh muscle and duodenum, examined for binding by autoradiography. Specific binding in the target tissues was demonstrated by simultaneous injection of excess unlabeled relaxin. Radioactivity was located and quantified by grain counts predominantly over the inner, circular muscle layer of the myometrium and the cervix and to a lesser extent over the outer longitudinal muscle layer of the myometrium and the endometrium. The route of injection, the circulation time, or counting grains in transverse or longitudinal sections of myometrium made little difference in these results. Ovariectomy decreased, but not significantly, the grain count in all of the target tissues studied and estrogen treatment of ovariectomized animals restored the numbers of grains to approximately that of intact PMSG-treated rats. The degree of binding of the cervix was approximately that of the circular myometrial muscle. This work confirms the presence of specific receptors for relaxin in the rat uterus and cervix of primed rats and it also suggests that the inhibitory action of relaxin upon the myometrium is primarily on the inner circular muscle layer.