Myometrial oxytocin receptors and prostaglandin in the parturition process in the rat

Neuromodulation of maternal circuits by oxytocin

Motherhood in mammals involves tremendous changes throughout the body and central nervous system, which support attention and nurturing of infants. Maternal care consists of complex behaviors, such as nursing and protection of the offspring, requiring new mothers to become highly sensitive to infant needs. Long-lasting neural plasticity in various regions of the cerebral cortex may enable the perception and recognition of infant cues, important for appropriate caregiving responses. Recent findings have demonstrated that the neuropeptide oxytocin is involved in a number of physiological processes, including parturition and lactation and dynamically shaping neuronal responses to infant stimuli as well. Here, we review experience-dependent changes within the cortex occurring throughout motherhood, focusing on plasticity of the somatosensory and auditory cortex. We outline the role of oxytocin in gating cortical plasticity and discuss potential mechanisms regulating oxytocin release in response to different sensory stimuli.

Maternal age effects on myometrial expression of contractile proteins, uterine gene expression, and contractile activity during labor in the rat

Advanced maternal age of first time pregnant mothers is associated with prolonged and dysfunctional labor and significant risk of emergency cesarean section. We investigated the influence of maternal age on myometrial contractility, expression of contractile associated proteins (CAPs), and global gene expression in the parturient uterus. Female Wistar rats either 8 (YOUNG n = 10) or 24 (OLDER n = 10) weeks old were fed laboratory chow, mated, and killed during parturition. Myometrial strips were dissected to determine contractile activity, cholesterol (CHOL) and triglycerides (TAG) content, protein expression of connexin-43 (GJA1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caveolin 1 (CAV-1). Maternal plasma concentrations of prostaglandins PGE2, PGF2α, and progesterone were determined by RIA. Global gene expression in uterine samples was compared using Affymetrix Genechip Gene 2.0 ST arrays and Ingenuity Pathway analysis (IPA). Spontaneous contractility in myometrium exhibited by YOUNG rats was threefold greater than OLDER animals (P < 0.027) but maternal age had no significant effect on myometrial CAP expression, lipid profiles, or pregnancy-related hormones. OLDER myometrium increased contractile activity in response to PGF2α, phenylephrine, and carbachol, a response absent in YOUNG rats (all P < 0.002). Microarray analysis identified that maternal age affected expression of genes related to immune and inflammatory responses, lipid transport and metabolism, steroid metabolism, tissue remodeling, and smooth muscle contraction. In conclusion YOUNG laboring rat myometrium seems primed to contract maximally, whereas activity is blunted in OLDER animals and requires stimulation to meet contractile potential. Further work investigating maternal age effects on myometrial function is required with focus on lipid metabolism and inflammatory pathways.

Expression of the prostaglandin F synthase AKR1B1 and the prostaglandin transporter SLCO2A1 in human fetal membranes in relation to spontaneous term and preterm labor

BACKGROUND

Human labor is a complex series of cellular and molecular events that occur at the materno-fetal and uterine levels. Many hypotheses have been proposed for the initiation of human labor, one hypothesis suggests that maturation of the fetus releases a signal in the amniotic fluid that will be transmitted to myometrium via the fetal membranes and initiate uterine contractions. There is strong evidence that prostaglandins (PGs) play a central role in initiation and progression of human labor.

OBJECTIVES

In this study we intended to investigate the expression of prostaglandin F synthase and the prostaglandin transporter in the human fetal membranes and to explore the relationship between cytokines and PGs in the mechanism of human labor.

METHODS

We used fetal membranes obtained before labor at term and after spontaneous labor at term or preterm to identify the changes in prostaglandin F synthase (AKR1B1) and human prostaglandin transporter (SLCO2A1) proteins in relation to parturition. Using fetal membranes explants we tested the effect of cytokines (interleukin-1 and tumor necrosis factor alpha) on PG production and the concomitant changes in cyclooxygenase-2 (PTGS2), AKR1B1 and SLCO2A1 expression.

RESULTS

Expression of PTGS2 and AKR1B1 was upregulated in the fetal membranes in association with term labor while SLCO2A1 was downregulated with advancing gestation and during term labor. Before labor, IL-1 increased the expression of PTGS2, however during labor TNF upregulated PTGS2 and AKR1B1 proteins.

CONCLUSIONS

The prostaglandin F synthase AKR1B1 is upregulated while prostaglandin transporter is downregulated during term labor. The amnion is more responsive than choriodecidua to stimulation with pro-inflammatory cytokines. The mechanisms of term and preterm labor are different.

Effective oxytocin treatment on placental expulsion after foaling in heavy draft mares

The aim of this study was to establish the effectiveness of administration of oxytocin (OT) on placental expulsion after foaling. Four foaling mares with the placentas retained for up 1 hr after foaling received OT (50 IU) administration at 1 hr intervals before expulsion of the placenta. The changes in the plasma concentrations of OT and the PGF2alpha metabolite (PGFM) were investigated, and the influence of OT administration was considered. The results were as follows. The placenta was expelled after one to three OT administrations in all four mares that received OT. In two mares, which expelled the placenta within 30 min after OT administration, the OT concentration increased and remained high. Expulsion of the placenta was delayed in two mares, and one of these mares, which received three doses of OT beginning 1 hr after foaling, showed only a small increase in the OT concentration after the first administration; the other mare did not receive OT until 3 hr after foaling. The OT concentration was increased before placental expulsion in all the mares, and the PGFM concentration also increased in the two mares with retained placentas. In conclusion, we suggest that intramuscular administration of 50 IU of OT at 1-hr intervals beginning 1 hr after foaling is effective for inducing placental expulsion.

Design of peptide oxytocin antagonists with strikingly higher affinities and selectivities for the human oxytocin receptor than atosiban

The peptide oxytocin (OT) antagonist atosiban, approved for tocolytic use in Europe (under the tradename Tractocile), represents an important new therapeutic advance for the treatment of premature labor. This paper presents some new peptide OT antagonists which offer promise as superior tocolytics. The solid phase synthesis is reported of four pairs of L and D-2-naphthylalanine (L/D-2Nal) position-2 modified analogs of the following four oxytocin (OT) antagonists: des-9-glycinamide [1-(beta-mercapto-beta,beta-pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly-NH(2),d(CH(2))(5)[Tyr(Me)(2),Thr(4)]OVT) (A); the Tyr-NH(2) (9) analog of (A), d(CH(2))(5)[Tyr(Me)(2),Thr(4),Tyr-NH(2) (9)]OVT (B); the Eda(9) analog of (A), d(CH(2))(5)[Tyr(Me)(2),Thr(4),Eda(9)]OVT (C); and the retro COCH(2)Ph(4-0H)(10) modified analog of (C), d(CH(2))(5)[Tyr(Me)(2),Thr(4),Eda(9)<-- COCH(2)Ph(4-0H)(10)]OVT (D). The eight new analogs of A-D are (1) desGly-NH(2),d(CH(2))(5)[D-2Nal(2),Thr(4)]OVT, (2) desGly-NH(2),d(CH(2))(5)[2-Nal(2),Thr(4)]OVT, (3) d(CH(2))(5)[D-2Nal(2),Thr(4),Tyr-NH(2) (9)]OVT, (4) d(CH(2))(5)[2Nal(2),Thr(4),Tyr-NH(2) (9)]OVT, (5) d(CH(2))(5)[D-2Nal(2),Thr(4),Eda(9)]OVT, (6) d(CH(2))(5)[2Nal(2),Thr(4),Eda(9)]OVT, (7) d(CH(2))(5)[D-2Nal(2),Thr(4),Eda(9)<-- COCH(2)Ph(4-0H)(10)]OVT, (8) d(CH(2))(5)[2Nal(2),Thr(4),Eda(9)<-- COCH(2)Ph(4-OH)(10)]OVT. Peptides 1-8 were evaluated for agonistic and antagonistic activities in in vitro and in vivo rat bioassays, in rat OT receptor (rOTR) binding assays and in human OT receptor (hOTR) and human vasopressin (VP) vasopressor (V(1a)) receptor (hV(1a)R) binding assays. Also reported are the hOTR and hV(1a)R affinity data for atosiban and for B. None of the eight peptides exhibit oxytocic or vasopressor agonism. Peptides 1-8 exhibit weak antidiuretic agonism (activities in the range 0.014-0.21 U/mg). Peptides 1-6 exhibit potent in vitro (no Mg(2+)) OT antagonism (anti-OT pA(2) values range from 7.63 to 8.08). Peptides 7 and 8 are weaker OT antagonists. Peptides 1-6 are all OT antagonists in vivo (estimated in vivo anti-OT pA(2) values in the range 6.94-7.23). Peptides 1-8 exhibit vasopressor antagonism, anti-V(1a) pA(2) values in the range 5.1-7.65. Peptides 1-8 exhibit high affinities for the rOTR (K(i) values = 0.3-7.8 nM). Peptides 1-4 and B exhibit surprisingly very high affinities for the hOTR; their K(i) values are 0.17, 0.29, 0.07, 0.14 and 0.59 nM, respectively. Peptides 1-4 and B exhibit respectively 449, 263, 1091, 546 and 129 times greater affinity for the hOTR than atosiban (K(i) = 76.4 nM). Peptides 1-4 exhibit high affinities for the hV(1a)R (K(i)s = 1.1 nM, 1.3 nM, 0.19 nM and 0.54 nM, all higher than the hV1(a)R affinities exhibited by atosiban (K(i) = 5.1 nM) and by B (K(i) = 5.26 nM). Because of their strikingly higher affinities for the hOTR than atosiban, peptides 1-4 and B exhibit gains in anti hOT/anti hV(1a) receptor selectivity compared with atosiban of 93, 64, 39, 56 and 127, respectively. These OT antagonists are thus promising candidates for development as potential new tocolytic agents.

Mechanisms responsible for parturition; the use of experimental models

In this review, our knowledge, gleaned from a range of species, of what determines gestation length, how fetal maturation and birth are synchronized and how the uterotonic mechanisms are activated at birth are discussed. Accumulated data indicate that fetal glucocorticoids are involved in, but do not necessarily play a causative role in, the initiation of parturition in eutherian mammals generally. Present observations are consistent with a complex, positive regulatory interaction between estrogens, prostaglandins and oxytocin and are consistent with a role for prostaglandins as the final, common effector in myometrial activation. We are, however, left with the possibility that the initial mechanism for the timing of birth is encoded in the fetal genome and is closely linked to, and activated when, certain prerequisite developmental events have occurred in the fetus. Our understanding of these events in the sheep have led to its extensive use as an experimental model for the study of human clinical correlates of fetal maturation and development and the control of the initiation of parturition.

Uterine responses to exogenous oxytocin before and after pre-partum luteolysis in the cow

The aim of this study was to test the functional status of uterine oxytocin receptors in cows in vivo around parturition. The animals received consecutive, intra-arterial injections of 800, 1,600 and 3,200 mU of oxytocin at three different stages: during late gestation (days 260-274), at 12 h and at 24 h after intramuscular injection of a prostaglandin F2alpha analogue at day 275 to induce parturition. Cows (n = 6) had been provided with myometrial electrodes and a catheter had been installed in the aorta and in a branch of the uterine vein (UV). Regular blood samples were obtained from the UV from 5 min before until 45 min after each oxytocin injection to measure plasma levels of prostaglandin F2alpha (PGF2alpha) and oxytocin. Uterine electromyographic (EMG) activity was registered continuously during each experiment. The increase of oxytocin levels in UV plasma after intra-arterial injections was dose dependent (p < 0.02). Pre- and post treatment oxytocin levels at 24 h after induction of parturition were significantly increased (p = 0.0313). Both during late pregnancy and at 12 h after induction of parturition, oxytocin caused a significant increase in EMG activity (p = 0.022). After the 3,200 mU dose the increase was significantly higher than with the other 2 doses (p = 0.004). After each dose, EMG activity returned to baseline levels within some 15 min. At 24 h after induction of parturition, the pre-treatment level of EMG activity had increased. Doses of 800 mU and 1,600 mU of oxytocin produced a significant (p = 0.022) increment of EMG activity, which was of the same magnitude as during the preceding stages; after 3,200 mU of oxytocin the response was significantly higher than before (p = 0.008). No significant increases of PGF2alpha levels in UV plasma could be measured after oxytocin injections at any of the three stages. It is concluded that the myometrium of the pregnant cow responds in vivo to physiological doses of oxytocin. At 24 h after induction of parturition, when luteolysis has occurred and a parturient pattern of parturient myometrial activity has already started to develop, the response is enhanced. Physiological doses of oxytocin did not evoke a spurt release of PGF2alpha in uterine venous blood during the peripartal period.

Design of oxytocin antagonists, which are more selective than atosiban

We report the solid phase synthesis of four pairs of L- and D-thienylalanine (Thi/D-Thi) position two modified analogues of the following four oxytocin (OT) antagonists: des-9-glycinamide [1-(beta-mercapto-beta,beta-pentamethylene propionic acid), 2-O-methyltyrosine, 4-threonine]ornithine-vasotocin (desGly(NH2)9,d (CH2)5[Tyr(Me)2,Thr4]OVT) (A); the Tyr-(NH2)9 analogue of (A), d(CH2)5[Tyr(Me)2,Thr4,Tyr-(NH2)9]OVT (B); the Eda9 analogue (where Eda = ethylenediamine) of (A), d(CH2)5[Tyr(Me)2, Thr4, Eda9]OVT (C); and the retro Tyr10 modified analogue of (C), d(CH2)5[Tyr(Me)2, Thr4, Eda9<--Tyr10]OVT (D). The eight new analogues of A-D are (1) desGly(NH2),d(CH2)5[Thi2,Thr4]OVT, (2) desGly(NH2),d(CH2)5[D-Thi2,Thr4]OVT, (3) d(CH2)5[Thi2, Thr4,Tyr-(NH2)9]OVT, (4) d(CH2)5[D-Thi2,Thr4,Tyr-(NH2)9]OVT (5) d(CH2)5[Thi2,Thr4Eda9]OVT, (6) d(CH2)5[D-Thi2,Thr4,Eda9]OVT, (7) d(CH2) [Thi2,Thr4,Eda9<--Tyr10]OVT, (8) d(CH2),[D-Thi2,Thr4,Eda9<--Tyr10]OVT. We also report the synthesis of (C). Peptides 1-8 and C were evaluated for agonistic and antagonistic activities in in vitro and in vivo OT assays, in in vivo vasopressor (V1a receptor) assays and in in vivo antidiuretic (V2 receptor) assays. None of the eight peptides nor C exhibit oxytocic or vasopressor agonism. Peptides 1-8 are extremely weak V2 agonists (antidiuretic activities range from < 0.0005 to 0.20 U/mg). Peptide C is a weak mixed V2 agonist/antagonist. Peptides 1-8 and C exhibit potent in intro (no Mg2+) OT antagonism (anti-OT pA2 values range from 7.76 to 8.05). Peptides 1-8 are all OT antagonists in vivo (estimated in vivo anti-OT pA2 values range from 6.54-7.19). With anti-V1a pA2 values of approximately 5-5.80, peptides 1-8 exhibit marked reductions in anti-V1a potencies relative to those of the parent peptides A-D (anti-V1a pA2 range from 6.48 to 7.10) and to l-deamino[D-Tyr(Et)2, Thr4]OVT (Atosiban, trade name Tractocile) (anti-V1a pA2-6.14). Atosiban has recently been approved in Europe for clinical use for the prevention of premature labour (Pharm. J. 264(7-100): 871). Peptides 1-8 exhibit striking gains in in vitro anti-OT/anti-V1a selectivities with respect to the parent peptides A, B, C and D and to Atosiban. Peptides 1-8 exhibit anti-OT (in vitro)/anti-V1a selectivities of 450, 525, 550, 450, approximately 1080, 116, 355, 227 respectively. The corresponding values for A-D and Atosiban are 30, 4.2, 4.3, 2.6 and 37. With the exception of peptide 6, the remaining seven peptides exhibit 3-18-fold gains in anti-OT (in vivo)/anti-V1a selectivity with respect to Atosiban, peptides 1-8 exhibit anti-OT (in vivo)/anti-V1a selectivities of 22, approximately 82, approximately 82, 147, approximately 83, 11, 31 and 42. By comparison, Atosiban exhibits an anti-OT (in vivo)/anti-V1a selectivity = 8. With an estimated in vivo anti-OT pA2 value = 7.19+/-0.06, peptide 4 is equipotent with Atosiban (pA2 = 7.05+/-0.05). However, with its significantly reduced anti-vasopressor potency, pA2 = approximately 5, it is approximately 18 times more selective for OT receptors with respect to VP V1a receptors than Atosiban. Since we have shown that V1a antagonism could be an unwanted side-effect in tocolytics, peptide 4 and some of the OT antagonists reported here have advantages over Atosiban and thus may be suitable candidates for evaluation as potential tocolytic agents for the treatment of preterm labour.

Genetic analysis of fetal development and parturition control in the mouse

The application of targeted gene inactivation methodologies to the study of late fetal development and control of the timing for parturition in mice has yielded insight into the mechanisms that enhance fetal survival. An essential role for glucocorticoids in promoting lung maturation sufficient for viability ex utero before the onset of normal parturition has been demonstrated in corticotropin-releasing hormone-deficient mice. In contrast, maternal deficiency in the prostaglandin synthetic enzyme cyclooxygenase-1 results in the markedly delayed onset of labor and fetal demise because of postdates gestation. The complex interplay of factors that govern the onset of labor is highlighted by mice deficient in both cyclooxygenase-1 and oxytocin. Whereas mice deficient in oxytocin demonstrate normal parturition, simultaneous cyclooxygenase-1 and oxytocin deficiency rescues the delayed onset of labor found in cyclooxygenase-1 knockout mice but results in the prolonged duration of labor. The consequences of complete deficiency of molecules involved in parturition in mice suggest novel interventions for human preterm labor.

Maturation of spontaneous and agonist-induced uterine contractions in the peripartum mouse uterus

This study tested the hypothesis that the uterus achieves maximum contractile capabilities before the onset of labor. Basal and agonist-stimulated contractions were assessed in uterine strips on Day 15 or 18 of pregnancy, the day of parturition, or 1 day postpartum (n = 4-13 per group). Spontaneous contractions were evident in all groups (n = 4-13 per gestational group); contraction frequency was greater in peripartum groups than in virgin controls ( approximately 4.6 versus 2.8/200 sec). Peak amplitude was nearly 9-fold higher on Days 15 and 18 and over 30-fold higher in the postpartum and 1 day postpartum groups than in nonpregnant mice. Maximum frequency and peak amplitude were achieved in response to 10(-6) to 10(-8) M oxytocin or arginine vasopressin (OT(max) or AVP(max)). Frequency of contractions in response to OT(max) peaked on Day 18 and then declined. Contraction amplitude increased 5-fold on Day 15, declined on the day of birth (equivalent to nonpregnant level), then rebounded to peak on postpartum Day 1. AVP(max) similarly increased frequency and amplitude of contractions, except that maximum contraction amplitude occurred postpartum. Thus, an endogenous oscillator, residing in the uterus, sustains high basal and agonist-induced contraction frequency during pregnancy. Although acceleration of this pacemaker occurred before term, the data suggest that peripartum increases in contraction amplitude characterize the transition to the powerful synchronous contractions of parturition.

Effect of oxytocin receptor and beta2-adrenoceptor blockade on myometrial oxytocin receptors in parturient rats

It has been proposed that the rise in myometrial oxytocin receptor (OTR) concentrations at term triggers parturition. In the present study, we have shown that in vivo infusion of the beta2-adrenoceptor (beta2AR) antagonist ICI-118.551 in late pregnant rats prevents the rise in myometrial OTR binding normally seen during delivery. A reduced contractile responsiveness of uterine strips isolated from rats in labor when challenged with oxytocin (OT) and a slight shortening of gestation accompanied this effect. OTR mRNA levels were, however, unaltered after the treatment, suggesting that the effect of beta2AR blockade on myometrial OTR was posttranscriptional or due to influences on extra-myometrial tissue. Infusion of the OTR antagonist atosiban down-regulated OTR binding sites in the parturient myometrium and resulted in an impaired contractile response to OT without affecting gestational length. OTR gene expression did not change, as seen from unchanged OTR mRNA values. Neither atosiban nor ICI-118.551 infusions alone changed fetal mortality. A significant increase in the incidence of fetal deaths was found, however, when rats were treated with a combination of atosiban and ICI-118.551. This treatment also down-regulated myometrial OTR and weakened the contractile response to OT, but it did not change gestational length. We conclude that the timing and onset of a normal parturition as well as a favorable outcome seem to be independent of a rise in OTR. This fact cannot exclude the possibility that an increase in OTR is of importance in the genesis of preterm labor. We suggest that beta2 stimulation up-regulates OTR during delivery. This effect may partly be responsible for the tachyphylaxis seen after the use of beta2 agonists to control preterm labor. We further suggest that OTR stimulation up-regulates OTR during labor. The OTR down-regulation seen after atosiban treatment adds to the direct relaxing effect of atosiban on the myometrium. In view of this, atosiban may prove to be a more useful tocolytic than the traditionally used beta2 agonists.

Mesotocin receptors during pregnancy, parturition and lactation in the tammar wallaby

Mestocin receptor concentrations in membrane preparations from reproductive tissues of the tammar Macropus eugenii throughout gestation and lactation were assessed using [3H]-oxytocin as the ligand. There was a single binding site which bound both mesotocin and oxytocin with high and similar affinities. Mesotocin receptor concentrations in the myometrium were low (708 +/- 199 fmol mg-1 protein) in early and middle gestation but increased significantly on day 23 of pregnancy of the 26-day gestation period to 1921 +/- 552 fmol mg-1 protein. Myometrial receptors reached a peak of 2483 +/- 575 fmol mg-1 protein on days 25 and 26 of gestation, but returned to basal levels about an hour after birth. Receptor concentrations in the contralateral non-gravid uterus were much lower (605 +/- 75 fmol mg-1) and did not significantly increase throughout the period of gestation but dropped one day before birth. Mesotocin receptors were undetectable in the endometrium, the yolk sac placenta and the lateral, median and anterior vagina of all animals tested. In the lactating mammary gland after birth mesotocin receptors were initially high (588 +/- 38 fmol mg-1) but decreased after 200 days and by late lactation were 224 +/- 55 fmol mg-1 protein on day 240, close to the time of weaning. Mesotocin receptors in the ipsilateral non-lactating gland were also high in early lactation (430 +/- 153 fmol mg-1) and declined in late lactation (62 +/- 20 fmol mg-1). The changing concentrations of mesotocin receptors in pregnancy and lactation demonstrate that they are specifically regulated in tammar reproductive tissues. The increase in mesotocin receptors in gravid, but not in the non-gravid myometrium three days before birth may make the uterus responsive to the surge of mesotocin at birth. Since this rise is unilateral and only occurs in the gravid myometrium it must be due to local effects from the ipsilateral ovary or the feto-placental unit. Likewise, the down-regulation of mesotocin receptors in the contralateral, non-gravid myometrium may be due to its proximity to the developing follicle. The changing concentrations in the lactating and the adjacent, non-lactating mammary gland also reflect a differential regulation of mesotocin receptors, probably mediated via the sucking stimulus. Thus, local influences appear to be of primary importance in the regulation of mesotocin receptors during reproduction in this marsupial.

The role of oxytocin receptors and vasopressin V1a receptors in uterine contractions in rats: implications for tocolytic therapy with oxytocin antagonists

OBJECTIVE

The objective of the study was to determine in the rat model whether the uterotonic action of vasopressin is mediated by the vasopressin V1a receptor in the uterus, by the oxytocin receptor, or by both. The purpose is to assess whether the anti-V1a activity of oxytocin antagonists is a desirable pharmacologic property in tocolytic therapy for preterm labor.

STUDY DESIGN

Dose-response characteristics of the uterotonic action of oxytocin and arginine vasopressin were compared and analyzed by the in vitro cumulative dose-response curve technique. A nonselective oxytocin-V1a receptor antagonist, a selective oxytocin receptor antagonist, and a selective V1a receptor antagonist were selected for this study. Their relative effectiveness in inhibiting the uterine contractile responses induced by oxytocin and by arginine vasopressin in the isolated rat uterus was examined.

RESULTS

The uterotonic dose-response curves for oxytocin and arginine vasopressin were parallel and had the same maximal response. The nonselective oxytocin/V1a receptor antagonist and the selective oxytocin receptor antagonist were equally potent in inhibiting the uterine contractions induced by either oxytocin or arginine vasopressin, whereas the selective V1a receptor antagonist had no antiuterotonic activity. Inhibition by the selective oxytocin antagonist caused a similar parallel shift to the right of the dose-response curves for oxytocin and arginine vasopressin.

CONCLUSIONS

The parallel dose-response curves for oxytocin and arginine vasopressin suggest that the uterotonic action of vasopressin is also mediated by the oxytocin receptor. Arginine vasopressin binds to both oxytocin and V1a receptors in the uterus, but the activation of V1a receptors appears not to be a mechanism involved in the uterine-stimulating action of vasopressin. The anti-V1a activity of oxytocin antagonists does not contribute to tocolytic efficacy and may represent an undesirable side effect. By blocking the vascular V1a receptors, it may compromise the patient's ability to maintain arterial blood pressure during hemorrhage.

Effect of chronic hypoxia on myometrial responsiveness in the pregnant rat

Mechanisms involving the timing of normal parturition are not well understood in most animal species. To gain a greater understanding of the mechanisms, we employed hypoxia to perturb the normal system of parturition. The present study was designed to investigate the effects of chronic hypoxia on myometrial contractility in the near-term pregnant rat. Rats were exposed to room air (control) or to continuous hypoxia (10.5% O2) either from experimental days 19 through 21 (2-day exposure) or from experimental days 15 through 21 (6-day exposure). On day 21, blood was collected for hormone assays, and the uterine horns were collected from each dam. One horn was snap-frozen in liquid nitrogen for oxytocin (OT) receptor analysis, and the other was used for in vitro assessment of myometrial contractile responses to cumulative doses of OT or arginine vasopressin (AVP). Hypoxic exposure resulted in approximately 60% reduction of the maximal myometrial contractile response to OT and a significant reduction in OT binding sites from 256.9 +/- 34.9 to 84.9 +/- 21.3 fmol/mg protein (P<0.01). In contrast, the contractile response to AVP was unaffected after exposure to chronic hypoxia (P> 0.05). Additionally, we observed no difference in the plasma concentrations of estrogen, progesterone, and corticosterone. We conclude that chronic hypoxia decreased the effectiveness of OT-specific contractile mechanisms, at least partially through a decrease in OT binding sites.

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.