Excised uteri from 22 reproductively sound cycling mares were divided into 4 groups according to the stage of the estrous cycle of each mare and analyzed for changes in the number of oxytocin binding sites and their affinity. The number of binding sites in the myometrial layer of the uterus was greater (P<.01) than in the endometrial layer, with an approximate 3-fold difference. The number of oxytocin binding sites was greater (P<.05) during Days 14–17, post-ovulation, in both the endometrial and myometrial uterine tissue samples than during Days 2–5, Days 6–13 or Days 18-1. The affinity constants were greater (P<.01) in the endometrium than myometrium, but no difference (P>.05) was found in the four groups throughout the estrous cycle for endometrium or myometrium. In view of the known hormonal events occurring in the estrous cycle of the mare, along with in vitro data demonstrated with other species and the mare, the increase in oxytocin binding sites during Days 14–17 may enhance the production of the luteolytic hormone, prostaglandin (PFG2α). This may be due to the increasing concentration of estradiol, which has a stimulatory effect on estrogen and oxytocin binding sites and thus promotes increased PGF2α synthesis.
ASJC Scopus subject areas