Correlation between the Stimulatory Effect of Oxytocin on the Formation of Inositol Phosphates and the Oxytocin Receptor Level in the Pregnant Rabbit Myometrium

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Abstract
Objective: In order to elucidate the roles of inositol trisphosphate (IP3) and oxytocin (OT) receptors in rabbit parturition, the concentration of IP3 induced by OT and the OT receptor levels were determined in rabbit myometria before and after parturition. Methods: The effects of OT on IP3 formation and OT receptor levels were determined in the myometria of non-pregnant rabbits, Days 26, 28 and 30 of pregnancy rabbits, postpartum rabbits within 12 hours and steroid-treated ovariectomized rabbits. Prostaglandins (PGs) levels were also measured in the myometrial and decidual tissues. Results: OT receptors were not detectable in the myometria of non-pregnant rabbits, and OT had no effect on the formation of inositol phosphates (IPs). On Day 28 of pregnancy, OT receptors became detectable, and then OT could induce the formation of IPs. Thereafter, the stimulatory effects of OT on IPs formation and the OT receptor levels dramatically increased toward the end of pregnancy and reduced rapidly after parturition. When the ovariectomized pregnant rabbits were treated with estrogen, OT receptors in the myometrium were induced, and OT acquired the ability to stimulate IP3 formation. However, OT had no effect on the production of cAMP, cGMP, prostaglandin (PG) E2 and F2α in the myometria, even if receptors existed, although PGE2 production in the decidual tissues was markedly stimulated. In addition, an OT receptor antagonist inhibited the stimulatory effects of OT on IP3 formation. Conclusions: These results suggest that the formation of IPs by OT, the OT receptor levels in the myometrium, and the production of PGE2 in the decidua might play crucial roles in parturition.