Characterization of relaxin binding in the uterus of the marmoset monkey

The ovarian peptide hormone relaxin (RLX) plays an important role in the re gulation of the endometrium both during the cycle and in early pregnancy. R LX interacts with specific receptors on endometrial stromal cells causing t hese to decidualize. In order to characterize the molecules with which RLX interacts in the primate uterus, a methodology based on a fully bioactive p reparation of biotinylated porcine RLX was applied to cryosections of the u terus of female marmoset monkeys. Specific RLX binding was weakly detected in the proliferative phase in isolated endometrial stromal cells. In the se cretory phase, the positively reacting cells increased in staining intensit y and in number and also included some epithelial cells. Further increases occurred in pregnancy, but RLX binding in the endometrium decreased at the end of the cycle if pregnancy did not occur. The myometrium showed weak sta ining which did not vary through the cycle, but increased in pregnancy. Ele ctrophoretic analysis of the RLX-binding moieties in these tissue sections indicated that a protein of similar to 40 kDa was the principal RLX-binding molecule, while minor specific bands were detectable at similar to 100 and similar to 200 kDa. The binding of biotinylated RLX could be specifically suppressed by co-incubation with unlabelled RLX, but not by insulin, IGF-I or biotin. This technique therefore allows the detection and molecular char acterization of specific RLX binding in the primate uterus. In the marmoset monkey, the pattern of specific binding closely reflects the RLX-dependent physiology during implantation and early pregnancy, implying the probable involvement of a specific RLX receptor.