Effects of antenatal exposure to magnesium sulfate on neuroprotection and mortality in preterm infants: a meta-analysis

OBJECTIVE

To review the evidence regarding neuroprotective effects of antenatal exposure to magnesium sulfate.

DATA SOURCES

We conducted database searches of MEDLINE, the Cochrane Library and Controlled Trials Register, as well as the ClinicalTrials.gov and International Clinical Trials Register websites. Bibliographies of all relevant articles were reviewed.

METHODS OF STUDY SELECTION

Randomized controlled trials comparing magnesium sulfate with placebo/other treatment in patients at risk of preterm delivery were evaluated for inclusion and methodological quality. The primary outcome was death or cerebral palsy by 18-24 months corrected age. Secondary outcomes were death, cerebral palsy, moderate-severe cerebral palsy, and death or moderate-severe cerebral palsy. Separate analyses were performed according to the gestational age (GA) at randomization (less than 32 to 34 weeks and less than 30 weeks) and for studies in which magnesium sulfate was used exclusively for fetal neuroprotection.

TABULATION, INTEGRATION, AND RESULTS

Five randomized controlled trials were included (5,235 fetuses/infants). When analyzed by GA at randomization, in utero exposure to magnesium sulfate at less than 32-34 weeks did not reduce the rate of death or cerebral palsy (relative risk [RR] 0.92, 95% confidence interval [CI] 0.83-1.03). However, cerebral palsy (RR 0.70, 95% CI 0.55-0.89), moderate-severe cerebral palsy (RR 0.60, 95% CI 0.43-0.84), and death or moderate-severe cerebral palsy were significantly reduced, without an evident increase in the risk of death (RR 1.01, 95% CI 0.89-1.14). Similar results were obtained when the GA at randomization was less than 30 weeks. When only neuroprotection trials (four trials, 4,324 fetuses/infants) are analyzed, in utero exposure to magnesium sulfate additionally reduced the primary outcome of death or cerebral palsy. The number needed to treat to prevent one case of cerebral palsy among those who survive until age 18-24 months is 46 (95% CI 26-187) in infants exposed to magnesium sulfate in utero before 30 weeks, and 56 (95% CI 34-164) in infants exposed to magnesium sulfate in utero before 32 to 34 weeks.

CONCLUSION

Fetal exposure to magnesium sulfate in women at risk of preterm delivery significantly reduces the risk of cerebral palsy without increasing the risk of death.

The effect of the oxytocin antagonist atosiban on preterm uterine activity in the human

OBJECTIVE

The purpose of this study was to test the hypothesis that infusion of the oxytocin antagonist atosiban results in decreased preterm uterine activity in the human.

STUDY DESIGN

A randomized, double-blind, placebo-controlled trial was performed. One hundred twenty women from 20 to 36 weeks' gestation with a complaint of labor who had more than four uterine contractions per hour after intravenous hydration but no evidence of cervical changes were randomized to receive a 2-hour intravenous infusion of atosiban at a rate of 300 micrograms/min or placebo. Ond hundred-twelve subjects (56 in each arm) were suitable for analysis of efficacy. Both groups remained at bed rest and received hydration.

RESULTS

The mean percent decrease in contraction frequency was greater in atosiban subjects compared with controls (55.3% +/- 36.3% vs 26.7% +/- 40.4%, mean +/- SD, p < 0.001). A minimal (< 20%) decrease or an increase in contraction frequency was noted in 25 placebo subjects (45%) and seven atosiban subjects (13%). There was no clinically or statistically significant change in maternal blood pressure or heart rate during the infusion. The only adverse experiences possibly related to the drug were nausea and vomiting in one atosiban patient.

CONCLUSION

A 2-hour infusion of the oxytocin antagonist atosiban resulted in a significantly greater decline in contraction frequency compared with controls. Oxytocin appears to play a role in the maintenance of preterm uterine activity in the human.

Elevated maternal plasma corticotropin-releasing hormone levels in pregnancies complicated by preterm labor

OBJECTIVES

We investigated whether maternal plasma levels of the placental hormone corticotropin-releasing hormone are elevated in pregnancies complicated by preterm labor.

STUDY DESIGN

Mean maternal corticotropin-releasing hormone levels were studied in women who met specific criteria for preterm labor and in women with normal pregnancies. Levels were also compared in the latent and active phases during term labor.

RESULTS

In pregnancies complicated by preterm labor, maternal corticotropin-releasing hormone levels were higher than in normal pregnancies; this elevation occurred before labor was diagnosed clinically (p less than 0.05). When preterm labor was associated with infection, the mean levels were not elevated. Mean plasma levels were similar in latent and active phases during labor at term.

CONCLUSION

Maternal plasma corticotropin-releasing hormone levels are elevated in association with preterm labor. This elevation does not appear to be due to labor itself and may reflect an early activation of the placenta before the onset of preterm labor.

A new paradigm for the role of smooth muscle cells in the human cervix

BACKGROUND

Premature cervical remodeling resulting in spontaneous preterm birth may begin with premature failure or relaxation at the internal os (termed "funneling"). To date, we do not understand why the internal os fails or why funneling occurs in some cases of premature cervical remodeling. Although the human cervix is thought to be mostly collagen with minimal cellular content, cervical smooth muscle cells are present in the cervix and can cause cervical tissue contractility.

OBJECTIVE

To understand why the internal os relaxes or why funneling occurs in some cases of premature cervical remodeling, we sought to evaluate cervical smooth muscle cell content and distribution throughout human cervix and correlate if cervical smooth muscle organization influences regional cervical tissue contractility.

STUDY DESIGN

Using institutional review board-approved protocols, nonpregnant women <50 years old undergoing hysterectomy for benign indications were consented. Cervical tissue from the internal and external os were immunostained for smooth muscle cell markers (α-smooth muscle actin, smooth muscle protein 22 calponin) and contraction-associated proteins (connexin 43, cyclooxygenase-2, oxytocin receptor). To evaluate cervical smooth muscle cell morphology throughout the entire cervix, whole cervical slices were obtained from the internal os, midcervix, and external os and immunostained with smooth muscle actin. To correlate tissue structure with function, whole slices from the internal and external os were stimulated to contract with 1 μmol/L of oxytocin in organ baths. In separate samples, we tested if the cervix responds to a common tocolytic, nifedipine. Cervical slices from the internal os were treated with oxytocin alone or oxytocin + increasing doses of nifedipine to generate a dose response and half maximal inhibitory concentration. Student t test was used where appropriate.

RESULTS

Cervical tissue was collected from 41 women. Immunohistochemistry showed cervical smooth muscle cells at the internal and external os expressed mature smooth muscle cell markers and contraction-associated proteins. The cervix exhibited a gradient of cervical smooth muscle cells. The area of the internal os contained 50-60% cervical smooth muscle cells that were circumferentially organized in the periphery of the stroma, which may resemble a sphincter-like pattern. The external os contained approximately 10% cervical smooth muscle cells that were randomly scattered in the tissue. In organ bath studies, oxytocin stimulated the internal os to contract with more than double the force of the external os (1341 ± 693 vs 523 ± 536 integrated grams × seconds, respectively, P = .009). Nifedipine significantly decreased cervical tissue muscle force compared to timed vehicle control (oxytocin alone) at doses of 10(-5) mol/L (vehicle 47% ± 15% vs oxytocin + nifedipine 24% ± 16%, P = .007), 10(-4) mol/L (vehicle 46% ± 16% vs oxytocin + nifedipine -4% ± 20%, P = .003), and 10(-3) mol/L (vehicle 42% ± 14% vs oxytocin + nifedipine -15% ± 18%, P = .0006). The half maximal inhibitory concentration for nifedipine was 1.35 × 10(-5) mol/L.

CONCLUSION

Our findings suggest a new paradigm for cervical tissue morphology-one that includes the possibility of a specialized sphincter at the internal os. This new paradigm introduces novel avenues to further investigate potential mechanisms of normal and premature cervical remodeling.

1-((7,7-Dimethyl-2(S)-(2(S)-amino-4-(methylsulfonyl)butyramido)bicyclo [2.2.1]-heptan-1(S)-yl)methyl)sulfonyl)-4-(2-methylphenyl)piperaz ine (L-368,899): an orally bioavailable, non-peptide oxytocin antagonist with potential utility for managing preterm labor

Modifications to the previously reported spiroindenylpiperidine camphor-sulfonamide oxytocin (OT) antagonist L-366,509 have produced a new series of o-tolylpiperazine (TP) camphor-sulfonamides. A number of analogues in the TP series that incorporate a modified or unmodified L-methionine sulfone amide at the C2 endo position on the camphor ring exhibit high affinity for OT receptors (IC50 = 1.3-15 nM) and good selectivity for binding to OT versus arginine vasopressin V1a and V2 receptors. Several of these analogues were additionally characterized as potent antagonists of OT-stimulated contractions of the isolated and/or in situ rat uterus. Compound 7 (L-368,899) exhibited the best overall profile of OT receptor affinity (IC50 = 8.9 nM, rat uterus; 26 nM, human uterus), potency for inhibition of OT-stimulated contractions of the isolated rat uterus (pA2 = 8.9) and in situ rat uterus (AD50 = 0.35 mg/kg after intravenous (i.v.) administration and 7.0 mg/kg after intraduodenal administration), aqueous solubility (3.7 mg/mL at pH 5.0), and oral bioavailability in several species (35% (rat), 25% (dog), and 21% (chimpanzee) as estimated from radioreceptor determination of drug levels in plasma after oral and i.v. dosing). On the basis of these favorable properties, 7 has begun clinical testing for use as an oral and i.v. tocolytic agent. Molecular modeling alignment studies have provided support for the hypothesis that the TP camphor-sulfonamide portion of the non-peptide structures may serve as a mimetic of the important D-AA2-Ile3 dipeptide (AA = aromatic amino acid) found in many potent OT antagonists from the cyclic hexapeptide and OT analogue structural classes.

Magnesium sulfate suppresses inflammatory responses by human umbilical vein endothelial cells (HuVECs) through the NFkappaB pathway

Dysfunctional endothelial cell activation and cytokines are implicated in preterm labor, a condition commonly treated with the tocolytic agent, magnesium sulfate (MgSO(4)). Based on recent findings showing the inflammatory effects of magnesium deficiency, we examined the effect of MgSO(4) on human umbilical vein endothelial cell (HuVEC) inflammatory responses in vitro. HuVECs isolated from term umbilical cords were incubated with MgSO(4) prior to stimulation with lipopolysaccharide (LPS) and then assessed for endothelial cell activation. Endothelial cell supernatants were assayed for inflammatory mediator production (interleukin-8; IL-8), and endothelial cell-associated intercellular adhesion molecule (ICAM-1) expression was determined. In the absence of LPS stimulation, MgSO(4) had no effect on HuVEC responses. Treatment of HuVECs with MgSO(4) prior to LPS stimulation inhibited inflammatory mediator production (p<0.05) and cell adhesion molecule expression (p<0.05) in a dose-dependent manner. Mechanistic studies showed that MgSO(4) reduced NFkappaB nuclear translocation and protected cytoplasmic IkappaBalpha from degradation in LPS-treated HuVECs. In conclusion, MgSO(4) inhibits endothelial cell activation, as measured by levels of IL-8 and ICAM-1 expression, via NFkappaB. Our results support the hypothesis that MgSO(4) treatment may function as an anti-inflammatory agent during preterm labor.

Ontogenesis of beta-adrenoceptor signaling: implications for perinatal physiology and for fetal effects of tocolytic drugs

G-Protein-coupled receptors play an instrumental role in cellular development and function. In the mature organism, receptor signaling is controlled through the processes of desensitization and down-regulation. Recent evidence suggests that these regulatory mechanisms are not inherent properties, however, but rather are acquired during ontogenesis. This review focuses on beta-adrenoceptors (betaARs), which are found in fetal and neonatal tissues and are effectively linked through adenylyl cyclase (AC) to the production of cAMP. Agonist-induced stimulation of betaARs in the immature organism fails to produce desensitization, and instead, responsiveness increases. The unique mechanisms underlying this anomalous response involve induction of AC, a switch to more catalytically efficient AC isoforms, an increase in the ratio of stimulatory to inhibitory G-proteins, and interference with the expression and/or function of other G-protein-linked receptors that provide offsetting, inhibitory inputs. These adjustments are thus heterologous, influencing signaling mediated by a host of other G-protein-coupled neurotransmitter and hormone receptors. The net effect is to maintain and augment betaAR signaling in the face of continued stimulation, properties that disappear with maturation. The unique regulatory mechanisms for betaAR signaling in the fetus and neonate provide the necessary physiological adjustments required for the perinatal transition from intrauterine to extrauterine life. At the same time, however, the inability to restrict betaAR function may underlie adverse effects of betaAR-agonist tocolytics that are used in the treatment of preterm labor.

In utero beta 2 adrenergic agonist exposure and adverse neurophysiologic and behavioral outcomes

Beta 2 adrenergic receptor overstimulation during critical periods of prenatal development can induce a permanent shift in the balance of sympathetic-to-parasympathetic tone. This is a biologically plausible mechanism whereby beta 2 adrenergic agonists can induce functional and behavioral teratogenesis, which explains their association with increases in autism spectrum disorders, psychiatric disorders, poor cognitive, motor function and school performance, and changes in blood pressure in the offspring. The use of beta 2 adrenergic agonists should be limited to proven indications when alternate drugs are ineffective or unavailable; the risks of untreated disease to the mother and fetus are greater than the risk of the beta 2 adrenergic agonist.

Transvaginal cervical cerclage: evidence for perioperative management strategies

The objective was to review the evidence supporting various perioperative technical and management strategies for transvaginal cervical cerclage. We performed MEDLINE, PubMed, EMBASE, and COCHRANE searches with the terms, cerclage, cervical cerclage, cervical insufficiency, and randomized trials, plus each technical aspect (eg, suture, amniocentesis, etc) considered. The search spanned 1966 through September 2012 and was not restricted by language. Each retrieved manuscript was carefully evaluated, and any pertinent references from the reports were also obtained and reviewed. All randomized trials covering surgical and selected perioperative, nonsurgical aspects of cerclage were included in the review. The evidence was assessed separately for history-, ultrasound-, and physical examination-indicated cerclage. Evidence levels according to the new method outlined by the US Preventive Services Task Force were assigned based on the evidence. There are no grade A high-certainty recommendations regarding technical aspects of transvaginal cervical cerclage. Grade B moderate-certainty recommendations include performing a fetal ultrasound before cerclage to ensure fetal viability, confirm gestational age, and assess fetal anatomy to rule out clinically significant structural abnormalities; administering spinal, and not general, anesthesia; performing a McDonald cerclage, with 1 stitch, placed as high as possible; and outpatient setting. Unfortunately, no other recommendations can be made regarding the other technical aspects of cerclage.

Effect of smooth muscle relaxant drugs on proximal human ureteric activity in vivo: a pilot study

Drugs are increasingly being used to promote stone passage in renal colic. Diclofenac, nifedipine and tamsulosin cause ureteric smooth muscle relaxation in vitro; however, in clinical trials nifedipine and tamsulosin promote stone passage whereas diclofenac has no apparent benefit. We adapted a ureteric pressure transducer catheter in an attempt to compare the human ureteric response to these drugs in vivo. The catheter was inserted into the contralateral ureter following ureteroscopy for stone disease. Contraction frequency, pressure and velocity measurements were recorded at 24 h. Each patient was randomly allocated to receive oral diclofenac, nifedipine or tamsulosin. Measurements were taken following drug administration. Eighteen patients (mean age 50 years) were recruited. Two patients were excluded intraoperatively and three required early removal of the catheter. Prior to drug administration, the mean number of contractions recorded was 0-4.1/min and the peak contraction pressure ranged from 11 to 35 mmHg. Conduction velocity ranged from 1.5 to 2.6 cm/s. Ureteric peristalsis persisted in all patients despite these drugs. Diclofenac and nifedipine produced inconsistent ureteric pressure responses but had little effect on contraction frequency. Tamsulosin significantly reduced ureteric pressure but had no effect on contraction frequency. There are many limitations associated with the use of ureteric catheters, however, they may provide some useful information when used to record the response to an intervention in the same patient. These preliminary results suggest a reduction in pressure generation may be the essential factor in the promotion of stone passage. More work is required but these drugs may work by preventing the increased, uncoordinated muscular activity seen in renal colic whilst maintaining peristalsis, thereby promoting stone passage.

Regulation of uterine function: a biochemical conundrum in the regulation of smooth muscle relaxation

Premature birth accounts for the majority of fetal morbidity and mortality in the developed world and is disproportionately represented in some populations, such as African Americans in the United States. The costs associated with prematurity are staggering in both monetary and human terms. Present therapeutic approaches for the treatment of labor leading to preterm delivery are inadequate and our understanding of the regulation of myometrial smooth muscle contraction-relaxation is incomplete. The ability of nitric oxide to relax smooth muscle has led to an interest in employing nitric oxide-donors in the treatment of preterm labor. Fundamental differences exist, however, in the regulation of uterine smooth muscle relaxation and that of other smooth muscles and constitute a conundrum in our understanding. We review the evidence that nitric oxide-mediated relaxation of myometrial smooth muscle, unlike vascular or gastrointestinal smooth muscle, is independent of global elevation of cyclic guanosine 5'-monophosphate. Applying our current understanding of microdomain signaling and taking clues from genomic studies of pregnancy, we offer a framework in which to view the apparent conundrum and suggest testable hypotheses of uterine relaxation signaling that can explain the mechanistic distinctions. We propose that understanding these mechanistic distinctions in myometrium will reveal molecular targets that are unique and thus may be explored as therapeutic targets in the development of new uterine smooth muscle-specific tocolytics.

The effects of potassium channel openers on isolated pregnant human myometrium before and after the onset of labor: potential for tocolysis

OBJECTIVE

Our purpose was to investigate the effects and pharmacologic properties of potassium channel openers in isolated pregnant human myometrium.

STUDY DESIGN

Biopsy specimens of myometrium obtained from 67 women during pregnancy and labor were used for isometric recording under physiologic conditions.

RESULTS

Levcromakalim and pinacidil, two prototype potassium channel openers, are potent inhibitors of spontaneous and induced (0.5 nmol/L oxytocin and 10 mumol/L phenylephrine) contractions in isolated human pregnant myometrium, obtained before and after the onset of labor. The sulfonylurea glibenclamide is an apparent competitive antagonist of this inhibition. No antagonism was observed with the sulfonylurea tolbutamide. Both potassium channel openers significantly inhibited contractility evoked by low (10 and 20 mmol/L) but not high (40 and 80 mmol/L) concentrations of extracellular potassium chloride.

CONCLUSION

These findings suggest that the relaxant ability of levcromakalim and pinacidil in human pregnant myometrium is because of potassium channel activation. This introduces a potential new approach for tocolysis.

Atosiban and nifedipine in acute tocolysis: a comparative study

OBJECTIVE

The objective was to compare the effectiveness, efficacy, and safety of atosiban and nifedipine in preventing or delaying premature labor.

DESIGN

An interventional, randomized, controlled trial of 63 women experiencing preterm labor varying from 24 to 35 completed weeks of gestation. The women were randomized to receive either atosiban intravenously (group I, n=31), or nifedipine orally (group II, n=32).

RESULTS

There were no significant differences in effectiveness and efficacy of tocolysis between the two groups. Women with a history of preterm labor responded significantly better to atosiban than those with no such history. Those at 28 weeks or less responded significantly better to nifedipine, while those at more than 28 weeks' gestation showed an equal response in the two groups. Nifedipine achieved uterine quiescence in a significantly shorter time than atosiban. The maternal side effects were higher with nifedipine. Neonatal complications were comparable in both groups.

CONCLUSIONS

Both drugs are equally effective and efficacious in acute tocolysis. Subgrouping of patients according to gestational age and history of preterm labor may be applied in selecting the line of treatment. The maternal side effects were higher with nifedipine.

The safety and efficacy of tocolytic agents for the treatment of preterm labor

Pharmacologic inhibition of uterine contractions remains the mainstay of treatment for preterm labor despite the ongoing controversy regarding its effectiveness. A diverse variety of tocolytic medications have been proposed for clinical use, with betamimetics and magnesium sulfate being the common therapeutic agents of choice in the United States today. The clinician using these agents should be aware of the significant maternal and fetal side-effects associated with these particular medications. New classes of pharmacologic agents, including prostaglandin synthetase inhibitors, calcium channel blockers and phosphodiesterase inhibitors, have been proposed as tocolytic agents and are currently undergoing critical clinical evaluation. The purpose of this review is to provide a compilation of the available clinical studies that document the safety and efficacy of these various tocolytic agents.

THG113: a novel selective FP antagonist that delays preterm labor

PGF2alpha is an important smooth muscle contractile agent that exerts significant effects on myometrium and is implicated in labor. THG113 was recently identified as a PGF2alpha receptor (FP) antagonist. We characterized the specificity and selectivity of THG113, tested its effects on PGF2alpha-induced smooth muscle contraction, and assessed its efficacy in a model of endotoxin (LPS)-induced preterm labor. [125I]THG113 bound specifically to FP-expressing but not to native (not expressing FP) HEK293 cells. In FP-expressing HEK293 cells, THG113 markedly reduced PGF2alpha-elicited phosphoinositide hydrolysis (IC50 27 nM). Similarly, PGF2alpha-evoked microvascular (retinal) contraction was noncompetitively blocked (by > 90%) by THG113. In contrast, contraction to agonists of homologous prostanoid receptors EP1 and TP (17-phenyl-trinor PGE2 and U46619) was unaffected (< 1%) by high concentrations of THG113 (100 micromol/L); THG113 (100 micromol/L) also did not affect contraction to numerous other agents including platelet activating factor, endothelin, and angiotensin II. Force and duration of PGF2alpha-evoked contractions of myometrial strips of pig (non-pregnant, luteal phase) and mouse (immediately postpartum) were markedly reduced by THG113. In an endotoxin-induced preterm mouse model, lipopolysaccharide (50 microg intraperitioneal) injection at 16 days' gestation resulted in 100% delivery within 15 h; in contrast, 70% of those treated with THG113 (1 mg/day) delivered > 24 h later (at 18 days' gestation; term: 19 days). In addition, in mice injected with lipopolysaccharide and treated 6 h later with THG113 (0.1 mg bolus followed by 1 mg/day) 40% delivered > 48 h later. Fetuses of pregnant mice treated with THG113 were born alive, had higher birth weights (1.6 +/- 0.1 v 1.4 +/- 0.05 g), and appeared healthy. This study describes an effective and selective noncompetitive FP antagonist, THG113, which significantly delays preterm delivery; this provides the basis for future investigations for its use in tocolysis.

Magnesium sulfate and cerebral palsy in premature infants

This article reviews clinical and experimental evidence as to whether magnesium sulfate, administered soon before premature birth, can reduce the high rate of cerebral palsy in tiny infants. Three observational studies have reported an association of magnesium sulfate with lower rate of cerebral palsy, whether treatment was for maternal preeclampsia or for tocolysis. One of these studies also noted a significant reduction in cognitive disability. In another study, no significant protective effect was seen except in a small subset of infants. Magnesium was neuroprotective in many but not all of a variety of experimental studies and has a variety of biologic effects that might account for benefit. All existing clinical studies had relatively small numbers of very premature infants. Although all attempted to control for possible confounders, such studies cannot provide definitive answers as to possible benefits or risks of magnesium therapy. Only randomized clinical trials are likely to resolve the question of whether a brief exposure to an agent which, in the American experience, is considered safe if administered with appropriate supervision, can reduce longterm neurologic disability in premature infants.

Uterotonic plants and their bioactive constituents

Abnormalities in the process of uterine muscle contractility during pregnancy and birth can have major clinical implications, including preterm labour, which is the single largest cause of maternal and prenatal mortality in the Western world and a major contributor to childhood developmental problems. In contrast, induction of labour may be necessary in certain conditions. Currently used interventional therapies to suppress (tocolytic agents) or to induce (uterotonic agents) uterine contractions lack potency and/or selectivity and can have harmful side effects for mother and baby. Nature's diversity has always been, and still is, one of the biggest resources of therapeutic lead compounds. Many natural products exhibit biological activity against unrelated targets, thus providing researchers with starting points for drug development. In this review we will provide an overview of uterine muscle physiology, describe currently available biological screening procedures for testing of uterotonic plant compounds and will summarise traditionally-used uterotonic plants, their active components and their mechanisms, primarily focusing on uterotonic active circular plant peptides called cyclotides. Finally we will comment on the discovery of novel cyclotide-producing plant species and the possibility for the development of novel plant-derived uterotonic and tocolytic drugs.

Contribution of coupling between human myometrial beta2-adrenoreceptor and the BK(Ca) channel to uterine quiescence

The beta(2)-adrenergic receptor (beta(2)-AR) and the large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel have been shown, separately, to be involved in mediating uterine relaxation. Our recent studies reveal that the levels of both beta(2)-AR and BK(Ca) channel proteins in pregnant human myometrium decrease by approximately 50% after the onset of labor. We present direct evidence in support of a structural and functional association between the beta(2)-AR and the BK(Ca) channel in pregnant human myometrium. Localization of both proteins is predominantly plasmalemmal, with 60% of beta(2)-AR colocalizing with the BK(Ca) channel. Coimmunoprecipitation studies indicate that BK(Ca) and beta(2)-AR are structurally linked by direct protein-protein interactions. Functional correlation was confirmed by experiments of human myometrial contractility in which the BK(Ca) channel blocker, paxilline, significantly antagonized the relaxant effect of the beta(2)-AR agonist ritodrine. These novel findings provide an insight into the coupling between the beta(2)-AR and BK(Ca) channel and may have utility in the application of this signaling cascade for therapeutic potential in the management of preterm labor.

Decreased myometrial beta-adrenoceptors in women receiving beta 2-adrenergic tocolytic therapy: correlation with lymphocyte beta-adrenoceptors

We have determined simultaneously the density of beta-adrenoceptors in human myometria (by (-)-[125I]iodopindolol binding) derived from 36 women undergoing cesarean section and in the corresponding circulating lymphocytes (by (-)-[125I]iodocyanopindolol binding). In myometrial membranes about 80% to 85% of the beta-adrenoceptors were of the beta 2-subtype. The density of myometrial and lymphocyte beta-adrenoceptors in women treated with the beta 2-adrenoceptor agonist hexoprenaline to prevent preterm labor was about 65% to 70% lower than that in nontreated women. Concomitantly, in hexoprenaline-treated women the 10 mumol/L isoproterenol-evoked increase in lymphocyte cyclic adenosine monophosphate content (as index for lymphocyte beta-adrenoceptor responsiveness) was diminished to a similar extent. Combining all data resulted in a significant positive correlation between myometrial and lymphocyte beta-adrenoceptor densities (r = 0.7303; n = 36; p less than 0.001). It is possible that determination of beta-adrenoceptor function in circulating lymphocytes may be a useful model to monitor myometrial beta-adrenoceptor changes during tocolytic therapy.

Risks and complications of tocolysis

In this article, the author has reviewed available information on maternal, fetal, and neonatal risks and complications of tocolytic therapy. Because no ideal tocolytic agent exists, clinicians must be aware of the potential problems that can be encountered from initiating tocolysis. A variety of tocolytic agents may be effective in delaying preterm delivery. However, whether these agents reduce perinatal morbidity and mortality is debatable. This review emphasizes that serious complications occur from the use of these drugs including maternal, fetal, and neonatal death. Clinicians who use tocolysis, even oral terbutaline, must be aware of the significant side effects associated with these agents. Caution should be exercised not only in their initiation but also in their continuation. Additional research is needed and ongoing to limit the use of tocolytic agents and develop drugs with more efficacy, less risk, fewer complications, and more tolerable means of administration.

Progesterone enhances the tocolytic effect of ritodrine in isolated pregnant human myometrium

OBJECTIVE

To evaluate the effect of natural progesterone on the relaxant effect of ritodrine on pregnant human oxytocin-induced myometrial contractility.

STUDY DESIGN

Isometric tension recordings were performed under physiologic conditions on isolated myometrial strips taken from low-risk term pregnant women undergoing elective cesarean section. Cumulative effects of natural progesterone (10 (-11) to 10 (-5) mol/L) on oxytocin-induced myometrial contractility were evaluated. Contractile activity following ritodrine exposure was also investigated in myometrium pretreated with natural progesterone.

RESULTS

Natural progesterone alone exerted a concentration-dependent relaxant effect on myometrial contractions. The concentration-response curve for ritodrine from natural progesterone pretreated myometrium was shifted to the left with a significant reduction ( P < .01) of 50% of the maximal response, contraction amplitude ( P < .05), and frequency ( P < .05). However, there was no significant difference in the mean maximal inhibition achieved ( P = .95).

CONCLUSION

Natural progesterone increased the relaxant effect of ritodrine by reducing 50% of the maximal response, amplitude, and frequency of myometrial contraction, most likely through nongenomic actions. These results suggest that natural progesterone may be beneficial for preventing preterm birth in a low-risk population.

Effective diminution of amniotic prostaglandin production by selective inhibitors of cyclooxygenase type 2

OBJECTIVE

Cyclooxygenase inhibitors are effective tocolytic agents, but significant adverse effects limit their use. We hypothesized that selective inhibitors of the isozyme cyclooxygenase 2 would effectively diminish labor-associated prostaglandin production.

STUDY DESIGN

We analyzed cyclooxygenase type 1 and 2 expression in amnion, chorion, decidua, and myometrium from laboring or nonlaboring women and tested the efficacy of selective cyclooxygenase 2 inhibition in diminishing prostaglandin production.

RESULTS

The expression of cyclooxygenase 2 in amnion from women in labor, either preterm or at term, was significantly higher than in amnion before labor. In contrast, cyclooxygenase 1 expression was unchanged by labor. The enhanced expression of amniotic cyclooxygenase 2 was associated with increased prostaglandin E(2) levels in laboring women. Amniotic prostaglandin E(2) production was effectively diminished by the selective cyclooxygenase 2 inhibitors SC-236 and NS-398 but not by the cyclooxygenase 1 inhibitor SC-560.

CONCLUSION

Selective inhibitors of cyclooxygenase 2 are effective in diminishing prostaglandin production in vitro and may be useful in prevention of preterm deliveries.

Beta-adrenoceptor signaling in the developing brain: sensitization or desensitization in response to terbutaline

Beta(2)-adrenoceptor agonists are commonly used to arrest preterm labor but they also penetrate the placenta to stimulate fetal beta-adrenergic receptors (betaAR), and have been implicated in subsequent neurobehavioral deficits. We administered terbutaline to pregnant rats on gestational days (GD) 17-20 and during two postnatal (PN) periods, PN2-5 and PN11-14, that correspond to third trimester human neurological development. We then examined betaAR binding sites and adenylyl cyclase (AC) signaling in fetal brain or neonatal brain regions. Although fetal terbutaline administration evoked betaAR downregulation, the ability of isoproterenol to stimulate AC was enhanced instead of desensitized. Sensitization occurred at post-receptor signaling proteins, as augmented responses were also seen for stimulants that bypass the receptors to work on G-proteins (NaF) or that stimulate AC directly (forskolin and Mn(2+)). When terbutaline was given on PN2-5, betaAR downregulation was obtained in brainstem, forebrain and cerebellum, but desensitization of the AC response was seen only in the forebrain; the desensitization was heterologous, reflecting decrements in total AC activity rather than specific loss of the betaAR response. With treatment on PN11-14, only the cerebellum showed betaAR downregulation and induction at the level of post-receptor signaling proteins maintained the betaAR-mediated AC response. Our results indicate that, unlike the adult, betaAR signaling in the fetus and neonate is resistant to homologous desensitization by beta-agonists, and in fact, displays heterologous sensitization that sustains or enhances the overall response. The inability to desensitize betaAR responses may lead to disruption of neural cell development as a consequence of tocolytic therapy.

Does pharmacotherapy for preterm labor sensitize the developing brain to environmental neurotoxicants? Cellular and synaptic effects of sequential exposure to terbutaline and chlorpyrifos in neonatal rats

It is increasingly clear that environmental toxicants target specific human subpopulations. In the current study, we examined the effects of prior developmental exposure to a beta(2)-adrenoceptor agonist used to arrest preterm labor, terbutaline, on the subsequent effects of exposure to the organophosphate insecticide, chlorpyrifos (CPF). Neonatal rats were given terbutaline on postnatal day (PN) 2-5, followed by CPF on PN11-14. Although neither treatment affected growth or viability, each elicited alterations in indices of brain cell differentiation and cholinergic innervation in the immediate posttreatment period (PN15), persisting into adulthood (PN60). Biomarkers of brain cell number (DNA concentration and content), cell size (protein/DNA ratio) and neuritic projections (membrane/total protein) were affected by either agent alone, with patterns consistent with neuronal and neuritic damage accompanied by reactive gliosis. The combined exposure augmented these effects by both additive and synergistic mechanisms. Similarly, choline acetyltransferase (ChAT), a constitutive marker for cholinergic nerve terminals, was affected only by combined exposure to both terbutaline and CPF. Indices of cholinergic synaptic activity [hemicholinium-3 and m(2)-muscarinic acetylcholine receptor binding] showed impairment after exposure to either terbutaline or CPF but the effects were more severe when the treatments were combined. These findings suggest that terbutaline, like CPF, is a developmental neurotoxicant, and that its use in the therapy of preterm labor may create a subpopulation that is sensitized to the adverse neural effects of a subsequent exposure to organophosphate insecticides.