Oxytocin and the initiation of human parturition. I. Prostaglandin release during induction of labor by oxytocin

Oxytocin: physiology, pharmacology, and clinical application for labor management

Oxytocin is a peptide hormone that plays a key role in regulating the female reproductive system, including during labor and lactation. It is produced primarily in the hypothalamus and secreted by the posterior pituitary gland. Oxytocin can also be administered as a medication to initiate or augment uterine contractions. To study the effectiveness and safety of oxytocin, previous studies have randomized patients to low- and high-dose oxytocin infusion protocols either alone or as part of an active management of labor strategy along with other interventions. These randomized trials demonstrated that active management of labor and high-dose oxytocin regimens can shorten the length of labor and reduce the incidence of clinical chorioamnionitis. The safety of high-dose oxytocin regimens is also supported by no associated differences in fetal heart rate abnormalities, postpartum hemorrhage, low Apgar scores, neonatal intensive care unit admissions, and umbilical artery acidemia. Most studies reported no differences in the cesarean delivery rates with active management of labor or high-dose oxytocin regimens, thereby further validating its safety. Oxytocin does not have a predictable dose response, thus the pharmacologic effects and the amplitude and frequency of uterine contractions are used as physiological parameters for oxytocin infusion titration to achieve adequate contractions at appropriate intervals. Used in error, oxytocin can cause patient harm, highlighting the importance of precise administration using infusion pumps, institutional safety checklists, and trained nursing staff to closely monitor uterine activity and fetal heart rate changes. In this review, we summarize the physiology, pharmacology, infusion regimens, and associated risks of oxytocin.

Induction of parturition in horses - from physiological pathways to clinical applications

Based on the marked variability in physiological equine gestation length, induction of foaling in mares often results in the birth of dysmature foals. Precise prediction of preparedness of the mare for foaling is thus essential. Treatment with glucocorticoids mimics the fetal signal that initiates birth. Repeated daily dexamethasone treatment in late gestation results in birth of mature foals but the time from initiation of treatment to foaling is highly variable and complications such as dystocia have been reported. Contrary to most expectations, treatment of prepartum mares with progestogens does not delay but advances the onset of foaling. Prostaglandin F_2α (PGF_2α) and its analogues are effective to induce foaling but even in mares ready for parturition, foal health remains to some extent unpredictable. This may be caused by a relatively long interval between PGF_2α treatment and birth, exposing the fetus for several hours to uterine contractions. Oxytocin reliably induces foaling towards the end of pregnancy, but when given at high doses is effective also in the pre-viable period of gestation, resulting in birth of premature foals. Recent research has focused on reducing the amount of oxytocin with the aim to induce foaling only in mares prepared for foaling. Mares selected on clinical criteria receive 1 dose of 2.5 to 3.5 IU of oxytocin. Mares not responding to oxytocin are judged not yet ready for foaling and treatment is repeated the earliest after 24 h. This protocol at present is the most reliable and safest way to induce parturition in mares.

Uterine contractions in rodent models and humans

Aberrant uterine contractions can lead to preterm birth and other labour complications and are a significant cause of maternal morbidity and mortality. To investigate the mechanisms underlying dysfunctional uterine contractions, researchers have used experimentally tractable small animal models. However, biological differences between humans and rodents change how researchers select their animal model and interpret their results. Here, we provide a general review of studies of uterine excitation and contractions in mice, rats, guinea pigs, and humans, in an effort to introduce new researchers to the field and help in the design and interpretation of experiments in rodent models.

LimeMap: a comprehensive map of lipid mediator metabolic pathways

Lipid mediators are major factors in multiple biological functions and are strongly associated with disease. Recent lipidomics approaches have made it possible to analyze multiple metabolites and the associations of individual lipid mediators. Such systematic approaches have enabled us to identify key changes of biological relevance. Against this background, a knowledge-based pathway map of lipid mediators would be useful to visualize and understand the overall interactions of these factors. Here, we have built a precise map of lipid mediator metabolic pathways (LimeMap) to visualize the comprehensive profiles of lipid mediators that change dynamically in various disorders. We constructed the map by focusing on ω-3 and ω-6 fatty acid metabolites and their respective metabolic pathways, with manual curation of referenced information from public databases and relevant studies. Ultimately, LimeMap comprises 282 factors (222 mediators, and 60 enzymes, receptors, and ion channels) and 279 reactions derived from 102 related studies. Users will be able to modify the map and visualize measured data specific to their purposes using CellDesigner and VANTED software. We expect that LimeMap will contribute to elucidating the comprehensive functional relationships and pathways of lipid mediators.

The oxytocin-prostaglandins pathways in the horse (Equus caballus) placenta during pregnancy, physiological parturition, and parturition with fetal membrane retention

Despite their importance in mammalian reproduction, substances in the oxytocin-prostaglandins pathways have not been investigated in the horse placenta during most of pregnancy and parturition. Therefore, we quantified placental content of oxytocin (OXT), oxytocin receptor (OXTR), and prostaglandin E2 and F2 alpha during days 90-240 of pregnancy (PREG), physiological parturition (PHYS), and parturition with fetal membrane retention (FMR) in heavy draft horses (PREG = 13, PHYS = 11, FMR = 10). We also quantified OXTR and prostaglandin endoperoxide synthase-2 (PTGS2) mRNA expression and determined the immunolocalization of OXT, OXTR, and PTGS2. For relative quantification of OXT and OXTR, we used western blotting with densitometry. To quantify the prostaglandins, we used enzyme immunoassays. For relative quantification of OXTR and PTGS2, we used RT-qPCR. For immunolocalization of OXT, OXTR, and PTGS2, we used immunohistochemistry. We found that OXT was present in cells of the allantochorion and endometrium in all groups. PTGS2 expression in the allantochorion was 14.7-fold lower in FMR than in PHYS (p = 0.007). These results suggest that OXT is synthesized in the horse placenta. As PTGS2 synthesis is induced by inflammation, they also suggest that FMR in heavy draft horses may be associated with dysregulation of inflammatory processes.

Factors that affect ultrasound-determined labor progress in women undergoing induction of labor

BACKGROUND

The traditional approach to the assessment of labor progress is by digital vaginal examination; however, it is subjective and imprecise. Recent studies have investigated the role of transperineal ultrasonographic assessment of fetal head descent by measuring the angle of progression and head-perineum distance.

OBJECTIVE

The objective of this study was to evaluate factors that affected labor progress, which were defined by the transperineal ultrasonographic parameters, in women who achieved vaginal delivery.

STUDY DESIGN

This was a prospective longitudinal study performed in 315 women with singleton pregnancy who underwent labor induction at term between December 2016 and December 2017. Paired assessment of cervical dilation and fetal head station by vaginal examination and transperineal ultrasonographic assessment of fetal head descent (parasagittal angle of progression and head-perineum distance) were made serially after the commencement of labor induction until full cervical dilation. The researchers were blinded to the findings of the clinical team's vaginal examination and vice versa. The repeated measure data were analyzed by mixed effect models to identify the significant factors (age ≥35 years, obesity, parity, methods of labor induction, and epidural anesthesia) that affected the relationship between parasagittal angle of progression and head-perineum distance against fetal head station and cervical dilation.

RESULTS

The total number of paired vaginal examination and transperineal ultrasonographic assessments among the 261 women (82.9%) with vaginal delivery was 945, with a median of 3 per woman. The median assessment-to-assessment interval was 4.6 hours (interquartile range, 4.3-5.2). Multiparity and mechanical methods of labor induction were associated with a faster rate of fetal head descent, which was determined by head-perineum distance against fetal head station, than nulliparity and the use of a slow-release vaginal pessary, respectively. An additional increase of 0.10 cm in head-perineum distance was observed, for an unit increase in fetal head station in nulliparous women (P=.03) and women who had a slow-release vaginal pessary (P=.02), compared with multiparous women and those who had mechanical methods for labor induction. The use of epidural anesthesia was associated with a slower rate of fetal head descent, which was determined by both parasagittal angle of progression and head-perineum distance, against fetal head station. An additional decrease of 3.66 degrees in parasagittal angle of progression (P=.04) and an additional increase in 0.33 cm in head-perineum distance (P≤.001) were observed for a unit increase in fetal head station in women with the use of epidural anesthesia, compared with those without. Obese women had higher head-perineum distance overall, compared with normal weight women; at different cross-sections of time periods, obesity appeared to be associated with a slower rate of change between head-perineum distance and cervical dilation. Advanced maternal age did not affect transperineal ultrasound-determined labor progress (P>.05).

CONCLUSION

Parity, methods of labor induction, the use of epidural anesthesia, and obesity affect labor progress, which has been illustrated objectively by serial transperineal ultrasonographic assessment of fetal head descent.

Prostaglandin-Endoperoxide Synthase 1 Mediates the Timing of Parturition in Mice Despite Unhindered Uterine Contractility

Cyclooxygenase (COX)-derived prostaglandins stimulate uterine contractions and prepare the cervix for parturition. Prior reports suggest Cox-1 knockout (KO) mice exhibit delayed parturition due to impaired luteolysis, yet the mechanism for late-onset delivery remains unclear. Here, we examined key factors for normal onset of parturition to determine whether any could account for the delayed parturition phenotype. Pregnant Cox-1KO mice did not display altered timing of embryo implantation or postimplantation growth. Although messenger RNAs of contraction-associated proteins (CAPs) were differentially expressed between Cox-1KO and wild-type (WT) myometrium, there were no differences in CAP agonist-induced intracellular calcium release, spontaneous or oxytocin (OT)-induced ex vivo uterine contractility, or in vivo uterine contractile pressure. Delayed parturition in Cox-1KO mice persisted despite exogenous OT treatment. Progesterone (P4) withdrawal, by ovariectomy or administration of the P4-antagonist RU486, diminished the delayed parturition phenotype of Cox-1KO mice. Because antepartum P4 levels do not decline in Cox-1KO females, P4-treated WT mice were examined for the effect of this hormone on in vivo uterine contractility and ex vivo cervical dilation. P4-treated WT mice had delayed parturition but normal uterine contractility. Cervical distensibility was decreased in Cox-1KO mice on the day of expected delivery and reduced in WT mice with long-term P4 treatment. Collectively, these findings show that delayed parturition in Cox-1KO mice is the result of impaired luteolysis and cervical dilation, despite the presence of strong uterine contractions.

Molecular Regulation of Parturition: The Role of the Decidual Clock

The timing of birth is a critical determinant of perinatal outcome. Despite intensive research, the molecular mechanisms responsible for the onset of labor both at term and preterm remain unclear. It is likely that a "parturition cascade" exists that triggers labor at term, that preterm labor results from mechanisms that either prematurely stimulate or short-circuit this cascade, and that these mechanisms involve the activation of proinflammatory pathways within the uterus. It has long been postulated that the fetoplacental unit is in control of the timing of birth through a "placental clock." We suggest that it is not a placental clock that regulates the timing of birth, but rather a "decidual clock." Here, we review the evidence in support of the endometrium/decidua as the organ primarily responsible for the timing of birth and discuss the molecular mechanisms that prime this decidual clock.

Roles of prostaglandin receptors in female reproduction

Prostaglandins (PGs) have long been known to play roles in various processes of female reproduction; however, the molecular mechanisms therein remained unsolved until recently. This review summarizes the recent progress towards understanding the molecular mechanisms underlying PG actions in fertilization and parturition. A series of studies using EP2-deficient mice demonstrated that after ovulation chemokine signalling in the cumulus cells stimulates integrin activation and cumulus extracellular matrix (ECM) assembly through the RhoA/ROCK/actomyosin pathway, although excessive chemokine signalling disturbs sperm penetration. PGE2-EP2 signalling suppresses such a chemokine signalling and stimulates cumulus ECM disassembly, which contributes to successful fertilization. A series of studies using FP-deficient mice revealed that PGF(2α)-FP signalling induces parturition at least by terminating progesterone production; however, some other EP signals are likely to be involved in parturition by inducing myometrial contraction. Therefore, it should be clarified as to which EP and/or FP receptor signals are physiologically essential for myometrial contraction and successful parturition.

The association of maternal intrapartum subfebrile temperature and adverse obstetric and neonatal outcomes

BACKGROUND

Subfebrile intrapartum maternal temperature is very common, yet there is sparse evidence regarding its causes or its effects on perinatal outcomes. We examined whether mild temperature elevation during labour is a risk marker for adverse obstetric and neonatal outcomes.

METHODS

A retrospective cohort analysis including 42 601 term, singleton live-births in two medical centres between 2003 and 2010 was performed. This study compared women who experienced a maximal intrapartum temperature of ≤37°C with women who experienced subfebrile intrapartum temperature (37.1-37.9°C). Adjusted risks for adverse obstetric and neonatal outcomes were calculated by using multivariable logistic regression models.

RESULTS

Compared with maternal temperature ≤ 37°C, subfebrile temperature was associated with higher rates of primary caesarean deliveries {adjusted odds ratios [aOR] = 1.36 [95% confidence interval (CI) 1.25, 1.49])} and assisted vaginal deliveries (aOR = 1.20 [95% CI 1.11, 1.30]), as well as with greater risks of early neonatal sepsis (aOR = 2.66 [95% CI 1.88, 3.77]), neonatal intensive care unit admissions (aOR = 1.40 [95% CI 1.08, 1.83]), and neonatal asphyxia or seizures (aOR = 3.18 [95% CI 1.51, 6.70]). Mildly elevated maternal intrapartum temperature (37.1-37.5°C) was also associated with adverse outcomes.

CONCLUSIONS

Maternal intrapartum subfebrile temperature may be an indicator of operative delivery and neonatal morbidity. Further research is needed to confirm these findings and to reveal underlying mechanisms.

A role of stretch-activated potassium currents in the regulation of uterine smooth muscle contraction

Rates of premature birth are alarming and threaten societies and healthcare systems worldwide. Premature labor results in premature birth in over 50% of cases. Preterm birth accounts for three-quarters of infant morbidity and mortality. Children that survive birth before 34 weeks gestation often face life-long disability. Current treatments for preterm labor are wanting. No treatment has been found to be generally effective and none are systematically evaluated beyond 48 h. New approaches to the treatment of preterm labor are desperately needed. Recent studies from our laboratory suggest that the uterine muscle is a unique compartment with regulation of uterine relaxation unlike that of other smooth muscles. Here we discuss recent evidence that the mechanically activated 2-pore potassium channel, TREK-1, may contribute to contraction-relaxation signaling in uterine smooth muscle and that TREK-1 gene variants associated with human labor and preterm labor may lead to a better understanding of preterm labor and its possible prevention.

High-dose oxytocin is not associated with maternal temperature elevation: a retrospective cohort study of mid-trimester pregnancy with intrauterine fetal demise

BACKGROUND

Maternal intrapartum fever has been associated with an increased incidence of neonatal morbidity. In this retrospective cohort study, we evaluated whether intravenous oxytocin has a fever-inducing effect. Oxytocin augments secretion of prostaglandins E(2) and F(2α) which are inflammatory mediators known to elevate body temperature.

METHODS

Between January 2005 and June 2008, 279 patients were admitted with mid-trimester fetal demise. Patients meeting inclusion criteria included 34 women who received a high-dose intravenous oxytocin regimen and 29 patients who delivered after spontaneous labor without the need for augmentation. Oral temperatures were measured on admission and at delivery.

RESULTS

The median length of oxytocin infusion was 5.3h. The calculated temperature change was -0.14°C in the oxytocin group and +0.12°C in the control group. These findings were confirmed in a model adjusted for patients' white blood cell count and duration of labor. We did not observe an effect of analgesia type, epidural versus intravenous analgesia, on duration of labor.

CONCLUSION

Based on this comparative analysis of pregnant women who received high-doses of oxytocin, we found insufficient evidence to support that high-dose intravenous oxytocin elevates intrapartum maternal temperature.

Induction of labor

The rate of labor induction continues to rise significantly in the United States because of a growing use of labor induction for postterm pregnancies and elective induction of labor. Although different types and doses of prostaglandins used for cervical ripening often initiate uterine activity, the principal role of these agents is to soften the unripe cervix independent of uterine activity. Several systematic reviews with meta-analyses have shown that prostaglandins are superior to placebo and oxytocin alone in ripening of the cervix. Numerous studies and meta-analyses have assessed misoprostol's efficacy and safety as a labor induction agent. The most appropriate dose and route of administration has not yet been confirmed.

Prostaglandins in the prevention and management of postpartum haemorrhage

Myometrial contractility is integral to the delivery of the placenta and the arrest of potential subsequent haemorrhage. The details of this physiological process are patchy but it is clear that there is an important hormonal contribution. Oxytocin, with or without ergometrine, has thus been widely used with a recognized beneficial treatment effect. This practice, however, was never universal. The injectable nature of these agents restricted their wider use, even in societies with average medical services. The availability of the prostaglandin analogue misoprostol has renewed interest in the third stage of labour, has taken its pharmacological management to new frontiers, and has expanded the therapeutic options for the management of postpartum haemorrhage.

Development and clinical experience with the new evidence-based tocolytic atosiban

The incidence of preterm birth has remained unchanged for the last few decades. This is due, in part, to the complex etiology of preterm labor, and the limited ability of tocolytic agents to prolong pregnancy as a result of limited efficacy and poor safety profiles. The recent introduction of the oxytocin antagonist, atosiban, represents a new generation of uterine-specific tocolytics, which are associated with more favorable safety profiles. This paper discusses the rationale behind the development of the oxytocin antagonists and provides a review of the phase II and III trials that have investigated atosiban. Also included is a retrospective analysis of 83 women assessed in the Vienna Medical School, providing an insight into the benefits associated with atosiban in the everyday clinical setting. The introduction of a safer tocolytic agent offers the potential to change the current approach to the management of preterm labor. This includes a prolonged period of treatment at earlier or later gestational ages and possibly an extended use to women with contraindications who would normally have been excluded from treatment, e.g. preterm premature rupture of the membranes.

Pharmacology of oxytocin and prostaglandins

Significant advances have been made in the last 20 years in our knowledge about the subcellular events occurring during myometrial contractions and cervical ripening and in the mechanism of action of oxytocin and prostaglandins. These advances have been instrumental in furthering our understanding of the mechanism of action of inhibitors of uterine contractility and have opened the door to clinical trials of agents such as specific COX-II inhibitors that may have the potential to inhibit labor without serious maternal or fetal side effects. There is still a long way to go, however, before all the complex actions of oxytocin and prostaglandins can be understood at a subcellular level, particularly the mechanism of action of prostaglandins in the process of cervical ripening.

Distribution and function of prostanoid receptors: studies from knockout mice

Recent developments in the molecular biology of the prostanoid receptors has allowed the investigation of the physiological roles of each individual receptor type and subtype. The following article reports the prostanoid receptor distributions deduced from Northern blot and in situ hybridization analyses, summarizes the phenotypes of each receptor knockout mice, and discusses recent studies investigating the effects of each receptor deficiency on the inflammatory response and female reproductive processes. The combination of expression pattern and knockout analyses enabled us to determine which receptor expressed in a particular cell is important for the maintenance of normal and/or pathological physiology. The results from these analyses may be useful in the development of novel therapeutics that can selectively manipulate prostanoid-mediated actions.

Prostanoid receptors: structures, properties, and functions

Prostanoids are the cyclooxygenase metabolites of arachidonic acid and include prostaglandin (PG) D(2), PGE(2), PGF(2alpha), PGI(2), and thromboxne A(2). They are synthesized and released upon cell stimulation and act on cells in the vicinity of their synthesis to exert their actions. Receptors mediating the actions of prostanoids were recently identified and cloned. They are G protein-coupled receptors with seven transmembrane domains. There are eight types and subtypes of prostanoid receptors that are encoded by different genes but as a whole constitute a subfamily in the superfamily of the rhodopsin-type receptors. Each of the receptors was expressed in cultured cells, and its ligand-binding properties and signal transduction pathways were characterized. Moreover, domains and amino acid residues conferring the specificities of ligand binding and signal transduction are being clarified. Information also is accumulating as to the distribution of these receptors in the body. It is also becoming clear for some types of receptors how expression of their genes is regulated. Furthermore, the gene for each of the eight types of prostanoid receptor has been disrupted, and mice deficient in each type of receptor are being examined to identify and assess the roles played by each receptor under various physiological and pathophysiological conditions. In this article, we summarize these findings and attempt to give an overview of the current status of research on the prostanoid receptors.

Treatment of preterm labor with the oxytocin and vasopressin antagonist Atosiban

Oxytocin is involved in the regulation of preterm and term labor but the exact effect mechanisms are not fully understood. A regulatory action by vasopressin may also exist. The concentrations of oxytocin and vasopressin V1a receptors in myometrium from pregnant women are high before and in the beginning of labor both preterm and at term. Atosiban has high affinity for both these receptors and is a competitive oxytocin and vasopressin antagonist. The inhibitory effect of Atosiban on oxytocin induced activity on isolated myometrium correlates significantly with the concentration of the oxytocin receptors. Inhibition of preterm contractions with Atosiban was first reported by Akerlund et al 1987. Goodwin et al compared the effect of Atosiban to placebo in threatening preterm labor and the antagonist was in this trial significantly more effective than placebo in reducing the frequency of contractions (55% vs. 23%, p < 0.001). The same authors also reported successful tocolysis with the drug in actual preterm labor. Atosiban is currently in phase III of clinical development and seems to have the same effectiveness but fewer side-effects compared to beta-mimetics. These properties suggests that Atosiban may offer advantages over existing therapies in acute treatment of preterm labor.

Ethanol and parturition: a role for prostaglandins

A common pattern of birth defects was reported in children born to alcoholic women over 20 years ago. Shortly thereafter the constellation of defects became known as the Fetal Alcohol Syndrome, and reports from around the world served to acknowledge the pervasiveness of the disorder. Simultaneously with the clinical reports, animal models were developed to characterize the full spectrum of the teratogenic effects of ethanol. Not only did these animal models serve to define the actions of ethanol on fetal growth and development at the molecular pharmacological, neuroanatomical, and behavioral level, but unintentionally, they have resulted in renewed scientific interest in the effects of ethanol on pregnancy and parturition itself. The purpose of this review is twofold. First we will consolidate and summarize data from both clinical and basic research that pertains to ethanol and parturition. These data will demonstrate that ethanol consumption during pregnancy results in both delayed as well as premature delivery depending upon the pattern of consumption and timing of exposure. With these data as a background, the second objective will be to present a theoretical case for prostaglandins as possible mediators of ethanol-induced effects on the onset of parturition.

Effects of nomegestrol acetate on spontaneous and sulprostone-induced uterine contractions in pregnant cynomolgus monkeys monitored by telemetry

OBJECTIVE

Our purpose was to study the effects of the progestomimetic compound nomegestrol acetate on spontaneous and sulprostone-induced uterine contractility in pregnant cynomolgus monkeys.

STUDY DESIGN

Intrauterine pressure was continuously monitored with use of an implanted intraamniotic catheter and a telemetric pressure transmitter from day 115 to 135 of gestation (term = 165 days). After surgery the animals received either nomegestrol acetate (5 mg per day orally, n = 3) or vehicle only (controls, n = 3). The intramuscular prostaglandin E2 analog sulprostone (25 micrograms) was administered as a single injection 10 days after amniotic catheter implantation. Spontaneous and sulprostone-induced uterine contractions were compared between nomegestrol acetate- and vehicle-treated animals.

RESULTS

The frequency of spontaneous uterine contractions in control animals demonstrated a 24-hour pattern with a minimum at 12 hours and a maximum at 0 hours. The frequency of spontaneous contractions did not differ between nomegestrol acetate- and vehicle-treated animals. Sulprostone induced an increase in both the frequency and amplitude of contractions, reaching a maximum 12 hours after injection and fading out after 24 hours in vehicle-treated animals. In animals receiving nomegestrol acetate, the frequency of contractions increased moderately and transiently for a total duration of 6 hours only and returned to control levels thereafter.

CONCLUSION

Nomegestrol acetate significantly decreases the contractile response of the pregnant uterus induced by the prostaglandin E2 analog sulprostone.

Role of nitric oxide on oxytocin-evoked contractions and prostaglandin synthesis in isolated pregnant rat uterus

Uterine contractions elicited by oxytocin (OT), possibly linked with uterus prostaglandin (PG) release, are involved in the final pathway of labor. It is known that nitric oxide (NO) may contribute to the maintenance of uterine contractile quiescence during gestation. Therefore in this study the effect of the inhibition of NO synthase (NOS), with N-monomethyl L-arginine (L-NMMA), on the ability of OT to stimulate uterine contractions and PG synthesis was investigated in isolated rat uterus at days 13 and 21 of pregnancy. L-NMMA did not modify the frequency and the force of contractions elicited by OT at day 13. On day 21 the frequency of contractions evoked by OT were better sustained in the presence of L-NMMA. PGs were not affected by OT on day 13. OT stimulated PGF2alpha on day 21 when NOS had been inhibited with L-NMMA, but not in the absence of L-NMMA. NOS activity was stimulated by OT at day 21 of gestation. In summary these findings indicate that near term NO can regulate OT PGF2alpha induced contractions and PG synthesis in isolated pregnant rat uterus.

Regulation of COX-2 gene expression in rat uterus in vivo and in vitro

Prostaglandins are involved in mediating several important processes in mammalian reproduction, including the initiation of parturition. In the present study, we examined the expression in the rat uterus of two-rate limiting enzymes involved in prostaglandin production, cyclooxygenase (COX) 1 and 2. Expression of the COX-2 gene in the pregnant rat uterus gave rise to a single mRNA transcript of approximately 4.4 kb. COX-2 mRNA levels increased 3.5 fold between day 7 of pregnancy and the onset of parturition on day 22. In contrast, COX-1 mRNA levels remained constant during the same period. To investigate factors involved in mediating the regulation of COX-1 and COX-2 gene expression, rat endometrial stromal and epithelial cell lines, were used. In the stroma-derived cell line, CUS-V2, COX-2 gene expression was demonstrated by reverse transcriptase/polymerase chain reaction (RT-PCR) and by immunocytochemistry. In these cells, COX-2 gene expression was inducible by the cytokines interleukin-1 beta and tumor necrosis factor alpha, but not by interleukin-6. The two former cytokines also induced prostaglandin F2 alpha production. In contrast, COX-1 gene expression was constitutive in this cell line. In the endometrial epithelium-derived cell line, CUE-P both COX-1 and COX-2 genes were expressed in a constitutive fashion. In conclusion, the present in vivo and in vitro data indicate that decidual COX-2, but not COX-1, gene expression is regulated during pregnancy and implicate specific cytokines as possible inducers within the decidua.

Expression and localization of human oxytocin receptor mRNA and its protein in chorion and decidua during parturition

Oxytocin (OT) is widely used to induce labor in the clinical setting. However, its physiological role in normal human parturition remains unclear. We demonstrated the enhanced expression of OT receptor (OTR) mRNA in chorio-decidual tissue, using the polymerase chain reaction after the reverse transcriptase reaction (RT-PCR) and Northern blot analysis. OTR gene expression in chorio-decidual tissue increased fivefold during the course of parturition. In situ hybridization of fetal membrane revealed the expression of OTR mRNA in maternally derived decidual cells. The OTR mRNA was also detected in fetally derived chorionic trophoblast cells. Immunohistochemistry, using a newly developed anti-OTR monoclonal antibody, demonstrated the distribution of OTR protein in fetal membrane. The distribution pattern of OTR protein and OTR mRNA was identical, indicating that the regulation of OTR expression occurs mainly at the transcriptional level. These results support the idea that the expression of decidual OTR regulates the initiation and amplification of labor. The implications of these findings with regard to the pathogenesis of preterm labor are also discussed.

Oxytocin antagonists in preterm labour and delivery

Evidence has been gained that an oxytocin receptor antagonist given by intravenous infusion effectively stops uterine contractions in threatened as well as in actual preterm labour. The findings suggest that the increase of oxytocin receptors is aetiologically important in uncomplicated preterm labour. Oxytocin antagonists could therefore be an attractive alternative to currently used drugs by virtue of their high specificity and lack of serious side-effects. Their use in prophylactic and maintenance therapy may be greater when modified analogues have been developed that allow non-parenteral therapy.

Effects of lumbar epidural analgesia on prostaglandin F2 alpha release and oxytocin secretion during labor

The concentrations of plasma oxytocin and prostaglandin F2 alpha metabolite (PGFM) were measured in 10 parturients with and 10 without lumbar epidural analgesia. A blood sample was taken immediately before analgesia and another 60 min later. The control patients were matched for the stage of cervical dilatation at the time of the first blood sample; the second was drawn 60 min later. Plasma PGFM decreased significantly after lumbar epidural anesthesia and increased in controls resulting in a highly significant difference between the groups (P < 0.005). Plasma oxytocin concentrations levels also changed in opposite directions in the two groups but the difference did not reach statistical significance (P < 0.1). Uterine activity increased in the controls and decreased in the analgesia group resulting in a significant difference between the groups (P < 0.05). All subjects delivered vaginally. The total duration of labor was longer in the analgesia group (7.8 +/- 1.0 h vs. 4.7 +/- 0.6 h; P < 0.05) as was the duration after analgesia (5.1 +/- 0.9 h vs. 2.5 +/- 0.8 h; P < 0.05), whereas the duration of the second stage was not significantly different. We conclude that lumbar epidural anesthesia results in suppression of PGF2 alpha release which may be the cause of the diminished uterine activity and the prolonged duration of the first stage of labor.

Synthesis of oxytocin in amnion, chorion, and decidua may influence the timing of human parturition

Despite the widespread clinical use of oxytocin (OT) as a potent and specific stimulant of labor, previous research data have not supported a role for OT in the physiology of normal human parturition. We have demonstrated synthesis of OT mRNA in amnion, chorion, and decidua using Northern blot analysis, ribonuclease protection assays, and in situ hybridization. Probes directed towards both the 3' and 5' ends of the gene have been used. Levels were highest in decidua with considerably less in chorion and amnion and very low levels in placenta. The transcript size in decidua appears to be 60-80 nucleotides smaller than the transcripts in amnion and chorion. OT gene expression in chorio-decidual tissues increased three- to fourfold around the time of labor onset. Estradiol stimulated synthesis of OT mRNA during in vitro incubation. These results support the hypothesis of a paracrine system involving OT and sex steroids within intrauterine tissues wherein significant changes could occur without being reflected in the maternal circulation. Such a paracrine system could rationalize a long-sought role for oxytocin in the physiology of human labor. These data may lead to novel approaches towards prevention or treatment or preterm labor.

Maternal endocrine and fetal metabolic responses to heat stress

Heat-stressed pregnant ewes deliver intrauterine growth-retarded lambs. Selected maternal and fetal changes were investigated during acute heat stress in order to elucidate the mechanism for this growth retardation. Uterine blood flow decreased 20 to 30% in pregnant ewes during 1 degree C increases in core temperature. The decreases were accompanied by 60 and 100% increases in serum oxytocin and antidiuretic hormone, respectively. These effects were mimicked by salt loading or injections of antidiuretic hormone or oxytocin, suggesting a role for either or both hormones in regulating uterine blood flow during pregnancy. Chronically heat-stressed pregnant ewes were delivered by Caesarean section. Their fetuses were approximately 20% smaller than thermoneutral controls. Within each pair of heat-stressed twins, one fetus weighted one-third less than its litter mate. No difference in weights were observed within the control twins. The livers and brains of the heat-stressed fetuses were disproportionate in size. The livers from the small heat-stressed twins contained only one-half the protein of the controls and one-fourth the protein of their litter mates. Muscle protein was decreased in the heat-stressed fetuses, and liver and muscle glycogen were elevated as were liver arginase, glutamate-pyruvate transaminase and muscle creatinine. These results are consistent with the following hypothesis: heat stress stimulates the release of maternal antidiuretic hormone or oxytocin, which reduces uterine blood flow and causes a shift in fetal metabolism from anabolic to catabolic pathways; one fetus of heat-stressed twins is more severely affected than its litter mate.

Induction of labour: the effect of vaginal prostaglandin or i.v. oxytocin--a matter of time only?

Ninety-one pregnant women with unfavourable cervix (Bishop score no higher than 6) were randomly allocated to induction of labour with either prostaglandin E2 suppositories 2.5 mg 1-2 a day or i.v. oxytocin 4-32 mU/min. The induction procedure was carried on for 2 days. For statistical comparison of efficacy, life table analysis and the logrank test were used with vaginal delivery as the aimed 'event'. Prostaglandin suppositories were more efficient after 12 h (p less than 0.025) and 24 h (p less than 0.005), whereas no difference in efficacy was observed after 48 h. Vaginal delivery was obtained within 48 h in 74% of the women in the prostaglandin group and in 70% in the oxytocin group. No difference was observed in methods of delivery or neonatal Apgar scores, though, in neonates delivered vaginally within 2 days, lowered umbilical artery blood pH values were found after prostaglandin E2 suppositories (p less than 0.05). The patients attitude toward the method of induction was highly in favour of the prostaglandin suppositories. Prostaglandin E2 suppositories are considered excellent for induction of labour if delivery has to be within 24 h, whereas the two methods are equally effective after 48 h.

Uterine contractility during pregnancy and the effect of abortifacient drugs

There are a number of compounds in clinical use for termination of pregnancy. Treatment with all of them will result in increased uterine contractility. PGF2 alpha and PGE2 as well as different prostaglandin analogues all have a direct stimulatory effect on the myometrium, while other compounds such as hypertonic saline and Rivanol seem to act mainly through a stimulation of the endogenous production of PGF2 alpha. Treatment with antiprogestins which compete with progesterone at the receptor level or which inhibit progesterone biosynthesis results in an increased uterine contractility probably through a release from progesterone inhibition. If the withdrawal of progesterone also induces an increased endogenous prostaglandin production is unclear. The medical method to induce abortion which best resembles the physiological events during a spontaneous abortion is probably treatment with antiprogestins (receptor blockers or progesterone biosynthesis inhibitors) followed by prostaglandin.

Oxytocin receptor in human fetal membranes at term and during labor

Human fetal membranes, taken from 30 patients submitted to caesarean section during the final stages of gestation and labor, were examined in order to evaluate the presence and characteristics of the oxytocin receptor. The presence of oxytocin receptors in human fetal membranes, both in the amnion and in the chorion-decidua, was demonstrated in this study. The receptor binding to oxytocin showed a significant increase during early and advanced labor compared with before the onset of labor. When the pre-labor level was taken as the normalized form (control = 100) the increase with respect to the control (10 cases) for the amnion in early labor (2.27 times +/- 0.11, mean +/- SEM, P less than 0.001, 10 cases) and in advanced labor (2.53 times +/- 0.15, 10 cases, P less than 0.001) was highly significant. In the chorion-decidua the increase was 1.61 times +/- 0.09, P less than 0.001 in early labor and 1.66 times +/- 0.19, P less than 0.001 in advanced labor. Scatchard analysis showed a single receptor site for oxytocin in amnion and chorion decidua. The dissociation constant (Kd) did not change during the various stages of labor; the mean values found were 0.228 +/- 0.02 (mean +/- SEM) nM in the amnion and 0.193 +/- 0.03 nM in the chorion-decidua respectively. These findings suggest that human fetal membranes are target organs for oxytocin and that they might play a role in the onset of labor through an increase of receptor binding.

Increased myometrial responsiveness to oxytocin during term and preterm labor

The contractile effects of oxytocin on the longitudinal and circular muscle of rat uteri from animals undergoing term and preterm labor were studied in vitro to define the action of the hormone. The maximal tension and slopes of oxytocin dose contractile-response curves, but not the pD2 values (-log ED50), were higher in both muscle layers at term delivery and also during preterm delivery induced with antiprogesterones compared with responses from tissues of nondelivering animals. These data indicate that the myometrium is hyperresponsive and hyperreactive to oxytocin during term and preterm birth but not hypersensitive. The increase in oxytocin responsiveness of the myometrium may be fundamental to the initiation of labor and regulated by the increases in oxytocin receptors and postreceptor events, including gap junctions.

Effect of hydrogen peroxide on prostaglandin production and contractions of the pregnant rat uterus

Although evidence for a role for prostaglandins in parturition is abundant, less is known about how prostaglandin levels are regulated at term. Conditions occurring peripartum in the uteroplacental unit can result in reactive oxygen production. We investigated the effect of one reactive oxygen product, hydrogen peroxide, on in vitro activity of uterine segments from the 18-day-pregnant rat. H2O2 (0.3 mmol/L) was found to elicit rhythmic contractions and increase prostaglandins F2 alpha and E2 release by uterine tissue. Indomethacin blocked both of these effects. We conclude that H2O2 stimulates uterine contractions through a prostaglandin release mechanism. A speculative hypothesis of peripartum regulation of prostaglandin production by reactive oxygen is discussed.

The effect of acetylsalicylic acid and indomethacin on the catecholamine- and oxytocin-induced contractility and prostaglandin (6-keto-PGF1 alpha, PGF2 alpha)-production of human pregnant myometrial strips

The effects of acetylsalicylic acid (ASA) and indomethacin (IND) on the epinephrine and oxytocin stimulated contractility and prostaglandin (6-keto-PGF1 alpha, PGF2 alpha) production of superfused myometrial strips from the pregnant human uterus at term are reported. Without preincubation in ASA or IND epinephrine dose-dependently (10 ng/ml to 1 microgram/ml) stimulated the contractility and significantly increased the PG-release of the myometrial strips. The epinephrine induced increase in contractility was correlated to a higher increase in PGF2a production and a decreased 6-keto-PGF1 alpha/PGF2 alpha ratio (5.4 to 1.8). Superfusion of oxytocin increased myometrial contractions and PGF2 alpha release according to dose (3-12 microU/ml). However, 6-keto-PGF1 alpha production was not affected by oxytocin. Myometrial strips preincubated with ASA (100 micrograms/ml) or IND (10 micrograms/ml) demonstrated little spontaneous activity and the PG production was below the detection limit of the RIA. The stimulating effect of epinephrine and oxytocin on the contractility and PGF2 alpha release of the myometrial strips was inhibited significantly. During continuous superfusion of the ASA and IND preincubated myometrial strips with Tyrode's solution the inhibitory effect on spontaneous, epinephrine-, and oxytocin-stimulated contractility and PGF2 alpha release gradually declined over a period of 2 hours. This decrease of the inhibitory effect was more significant in ASA preincubated specimens. Our results demonstrate that spontaneous, epinephrine-, and oxytocin-stimulated contractility and PG release of human myometrial strips can be inhibited by ASA and IND and that this inhibitory effect is reversible. Furthermore our results suggest that in pregnant human myometrium the inhibition of PGF2 alpha production by ASA and IND is more pronounced than that of 6-keto-PGF1 alpha (PGI2).

Estrogen, progesterone, prolactin, prostaglandin E2, prostaglandin F2 alpha, 13,14-dihydro-15-keto-prostaglandin F2 alpha, and 6-keto-prostaglandin F1 alpha gradients across the uterus in women in labor and not in labor

Before or during labor in humans, changes in peripheral levels of estrogen and progesterone are not evident. Local alterations of estrogen, progesterone, and prolactin concentrations may be present and be accompanied by prostaglandin changes. The purpose of this study was to investigate the differences in concentrations of these hormones across the uterus and to evaluate their interrelationships in patients at term gestation with and without labor. Blood samples were obtained from a radial artery and a uterine vein in 22 women without and in 10 with labor. The difference between levels in the two blood vessels was designated as the gradient. Neither levels nor gradients were different between the two groups for estrone, estradiol, estriol, progesterone, or prolactin. The plasma levels of prostaglandin F2 alpha, 13,14-dihydro-15-keto-prostaglandin F2 alpha, and prostaglandin E2 were significantly increased in labor. Prostacyclin levels, as indicated by the 6-keto-prostaglandin F1 alpha metabolite, were not altered. The gradients for prostaglandin F2 alpha and E2 were significantly increased in labor. The results of the study also suggested that, in gestation at term, serum prolactin is produced mainly by the pituitary and that estrone may originate from peripheral conversion of estradiol. We conclude that in humans prostaglandin gradients of the E and F groups are increased in labor. These increases are not associated with changes in sex steroids or prolactin. Prostacyclin metabolite gradients also appear not to be altered in labor, suggesting that some prostaglandins are selectively increased in early labor either by enhanced production or decreased metabolism or both.

15-Keto-13,14-dihydroprostaglandin F2 alpha and prolactin in maternal and cord blood during prostaglandin E2 or oxytocin therapy for labor induction

15-keto-13,14-dihydroprostaglandin F2 alpha plasma levels were measured in pregnant women following labor induction with either oral PGE2 treatment or intravenous oxytocin, both combined with amniotomy. The median time to start of contractions was 62 minutes in the PGE2 treated group and 45 minutes in the oxytocin treated group (p less than 0.01; median test). The increase in 15-ketodihydro-PGF2 alpha appeared earlier in the PGE2 group but not in the oxytocin group (p less than 0.001 and p = 0.210, respectively). At delivery, the 15-ketodihydro-PGF2 alpha values had further increased in both treatment groups. The increase was significantly higher in the PGE2 treated patients compared with oxytocin treated patients (p = 0.03; contrast test). Despite higher 15-ketodihydro-PGF2 alpha concentrations throughout parturition, PGE2 women did not deliver more rapidly than oxytocin infused women. There was no correlation between 15-ketodihydro-PGF2 alpha blood concentrations and either onset of contractions or labor time. The decrease in maternal serum prolactin concentration during parturition was pronounced (p less than 0.001) in the PGE2 group but occurred also in oxytocin treated patients (p less than 0.02). A single oral dose (0.5 mg) of PGE2 taken by non-pregnant women led to significant (p less than 0.05) increases in 15-ketodihydro-PGF2 alpha levels in blood plasma after 10 minutes. This increase persisted for at least 90 minutes. It is suggested that oral PGE2 may be transformed into PGF2 alpha and/or induce endogenous PGF2 alpha biosynthesis.

Prostaglandin F2 alpha and oxytocin interactions in ovarian and uterine function

The oxytocin-neurophysin gene is expressed in several nontraditional sites within the endocrine system. In the ovary its expression in the corpora lutea is initiated by ovulation. Ovarian oxytocin concentrations reach maximal levels around day 11 of luteal cycle and fall to a nadir at estrus. PGF2 alpha has the capacity to release oxytocin from the corpus luteum, and oxytocin in turn releases PGF2 alpha from the uterine endometrium or decidua. This positive feedback loop between the ovary and the uterus ensures the completion of luteolysis in species that depend on the presence of the uterus for the termination of luteal lifespan. Immunization against oxytocin has been shown to disrupt this loop, resulting in much-prolonged luteal cycles. In primates and other species in which luteal life span is independent of the uterus, an oxytocin PGF2 alpha interaction may take place within the ovary itself. At parturition a related interaction takes place which ensures the expulsion of the fetus and placenta in an orderly manner. Oxytocin of both pituitary and ovarian origin reaches the uterus via its blood supply and binds to two types of receptors: one on myometrial cells, the occupation of which initiates contractions, and the other on decidual cells, the occupation of which initiates prostaglandin generation. This prostaglandin diffuses into the adjacent myometrium and augments the oxytocin-induced contractions. In conjunction with a direct softening effect by prostaglandins on the cervix the augmented contractions achieve the force needed to dilate the cervix and expel the fetus. An additional source of oxytocin during labor may be the placenta, another non-traditional site for the occurrence of oxytocin.

The effect of oxytocin and betamimetic stimulation on prostaglandin release in perfused human fetal placenta

The existing data on prostaglandins indicate that they are involved in human parturition and regulation of fetoplacental blood flow. The interference of endogenous and exogenous oxytocin and prostaglandins and, on the other hand, betamimetics, which are commonly used during pregnancy, in the regulation of these phenomena is poorly understood. The production of prostaglandins by fetal placental cotyledons was studied using an in vitro perfusion technique. Isolated cotyledons were perfused without (control) or with oxytocin (200 pg/ml, 2000 pg/ml) or the betasympathomimetic drug ritodrine (10 micrograms/ml, 50 micrograms/ml). The release of prostaglandin F2 alpha (PGF2 alpha), prostaglandin E2 (PGE2), 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) and thromboxane B2 (TxB2) was measured by radioimmunoassay. The release of prostaglandins was also studied during a recovery period after the drug infusion. Oxytocin at a concentration of 200 pg/ml significantly decreased the release of PGF2 alpha. A higher concentration of oxytocin did not cause any changes in prostaglandin production. During ritodrine infusion the perfusion pressure was decreased, but the addition of ritodrine to the perfusion medium had no effect on prostaglandin release. During the recovery period, after ritodrine infusion, the release of PGF2 alpha was significantly decreased. It is suggested that oxytocin at the physiological concentration may protect the fetus from adverse effects of PGF2 alpha before labor in decreasing the release of PGF2 alpha, yet this small decrease in formation of PGF2 alpha is obviously of minor clinical importance because the perfusion pressure remained constant. Ritodrine had no effect on prostaglandin release and so the decrease in the perfusion pressure is probably the result of beta 2-receptor stimulation.

A comparison of analgesia and suppression of oxytocin release by opiates

The potency of opiates for suppressing oxytocin release relative to their potency as analgesics was tested in lactating rats. Oxytocin release was evoked by the sucking of the young in urethane-anaesthetized and unanaesthetized rats, and was detected by the characteristic behaviour of the young and milk yield respectively. The tail-flick test, using noxious radiant heat, was used to assess analgesia. Intraperitoneal injection of morphine (1 mg kg-1 and 5 mg kg-1) significantly reduced milk yield in unanaesthetized rats. Urethane-anaesthetized rats displayed a pattern of reflex milk-ejection responses similar to that found in conscious rats. This reflex was significantly inhibited in a dose-related, naloxone-reversible manner by buprenorphine (ED50 0.18 mg kg-1), meptazinol (ED50: 14.0 mg kg-1), morphine (ED50: 0.67 mg kg-1), pentazocine (ED50: 15.0 mg kg-1) and pethidine (ED50: 7.9 mg kg-1). Although intraperitoneal injection of morphine (5 mg kg-1) abolished the increase in intramammary pressure occurring at reflex milk-ejection, that evoked by intravenous oxytocin (0.5-1 mu) was unaffected. Each opiate also caused significant, dose-related, naloxone-reversible increases in tail-flick latency. The ED50 doses were buprenorphine (ED50: 0.14 mg kg-1), meptazinol (ED50: 12.5 mg kg-1), morphine (ED50: 5.0 mg kg-1), pentazocine (ED50: 12.5 mg kg-1) and pethidine (ED50: 6.1 mg kg-1). The order of potency for analgesia and for suppression of oxytocin release were identical, namely: buprenorphine greater than morphine greater than pethidine greater than meptazinol greater than pentazocine. The results obtained with lactating rats suggest that secretion of the hormone oxytocin is substantially reduced during opiate-induced analgesia.

Plasma prostaglandin F2 alpha and plasma 13,14-dihydro-15-keto-prostaglandin F2 alpha levels in women during induction of labor with i.v. infusion of prostaglandin F2 alpha in relation to uterine contractions

The concentrations of plasma PGF2 alpha and its main metabolite, 13,14-dihydro-15-keto-PGF2 alpha (PGFM) were measured in serial samples of blood collected in 10 pregnant women at term who were given iv infusions of low doses of PGF2 alpha for induction of labor. Six other women served as controls and were given saline infusions. Uterine contractions began with a mean latency of 62 min in the PGF2 alpha infused women, in controls uterine activity remained unchanged. Plasma PGFM levels had increased significantly 30 min after PGF2 alpha infusion began, rising thereafter in a dose dependent manner. Plasma PGF2 alpha also rose reaching a steady state at 2 hours. No significant changes were observed in the controls. The 6-h infusion resulted in delivery in 5 of the 10 women, in the 5 others the cervical scores increased only by 1.25 points on the average and further treatment was needed to achieve delivery, although prostanoid levels rose to similar levels in all. The data show that when uterine contractions are induced by systemic PGF2 alpha, the levels of PGFM are significantly raised. In spontaneous labor uterine contractions begin long before plasma PGFM rises. Thus, if endogenous PGF2 alpha generation is involved in the initiation of uterine contractions during spontaneous labor, it must be synthetized in the myometrium at quantities too low to raise the levels of circulating PGFM.