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      Contractile Efficacy of Various Prostaglandins in Pregnant Rat Myometrium Pretreated With Oxytocin

      1 , 1 , 1 , 2 , 1 , 2
      Reproductive Sciences
      SAGE Publications

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          The oxytocin receptor system: structure, function, and regulation.

          The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.
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            Loss of myometrial oxytocin receptors during oxytocin-induced and oxytocin-augmented labour.

            Oxytocin is used widely for the induction and augmentation of labour, but there is little information about the dynamics of oxytocin receptors in human myometrium during parturition, and the possible effect of oxytocin infusion. This information is important because G protein-coupled receptors, such as the oxytocin receptor, undergo desensitization after prolonged or repeated stimulation. The concentration of myometrial oxytocin receptors and the steady state of its mRNA were measured in patients undergoing Caesarean sections before or during spontaneous or induced labour. The concentration of receptors before labour was 477 (175-641) fmol mg(-1) protein (median, quartile range), and decreased to 140 (72-206; P < 0.05) and 118 (69-75; P < 0.01) fmol mg(-1) protein during prolonged oxytocin-augmented and oxytocin-induced labour, respectively. The corresponding oxytocin receptor mRNA concentrations decreased by 60- and 300-fold, respectively. The decrease in receptor binding and mRNA in women receiving oxytocin infusion indicates that homologous receptor desensitization occurs in vivo.
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              Minimum oxytocin dose requirement after cesarean delivery for labor arrest.

              To estimate the minimum effective intravenous dose of oxytocin required for adequate uterine contraction after cesarean delivery for labor arrest. A randomized single-blinded study was undertaken in 30 parturients undergoing cesarean deliveries under epidural anesthesia for labor arrest despite intravenous oxytocin augmentation. Oxytocin was administered as a slow intravenous bolus immediately after delivery of the infant, according to a biased coin up-down sequential allocation scheme. After assisted spontaneous delivery of the placenta, the obstetrician, blinded to the oxytocin dose, assessed uterine contraction as either satisfactory or unsatisfactory. Additional boluses of oxytocin were administered as required, followed by a maintenance infusion. Data were interpreted and analyzed by a logistic regression model at 95% confidence intervals. All patients received oxytocin infusions at a mean +/- standard deviation of 9.8 +/- 6.3 hours before cesarean delivery (maximum infusion dose 10.3 +/- 8.2 mU/min). The minimum effective dose of oxytocin required to produce adequate uterine response in 90% of women (ED90) was estimated to be 2.99 IU (95% confidence interval 2.32-3.67). The estimated blood loss was 1,178 +/- 716 mL. Women requiring cesarean delivery for labor arrest after oxytocin augmentation require approximately 3 IU rapid intravenous infusion of oxytocin to achieve effective uterine contraction after delivery. This dose is 9 times more than previously reported after elective cesarean delivery in nonlaboring women at term, suggesting oxytocin receptor desensitization from exogenous oxytocin administration during labor. Therefore, alternative uterotonic agents, rather than additional oxytocin, may achieve superior uterine contraction and control of blood loss during cesarean delivery for labor arrest. I.
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                Author and article information

                Journal
                Reproductive Sciences
                Reprod Sci
                SAGE Publications
                1933-7191
                1933-7205
                April 24 2012
                September 2012
                April 26 2012
                September 2012
                : 19
                : 9
                : 968-975
                Affiliations
                [1 ]Department of Anesthesia and Pain Management, Mount Sinai Hospital, University of Toronto, Toronto, ON, Canada
                [2 ]Department of Obstetrics and Gynecology, Mount Sinai Hospital, University of Toronto, Toronto, ON, Canada
                Article
                10.1177/1933719112438971
                22539357
                f5ab2bb4-3e05-4048-8a6f-a57c67915f5c
                © 2012

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