Effect of magnesium sulfate on contractile force and intracellular calcium concentration in pregnant human myometrium

Maximum Dose Rate of Intrapartum Oxytocin Infusion and Associated Obstetric and Perinatal Outcomes

OBJECTIVE

Despite lack of evidence for a safety threshold for oxytocin dose rate, many hospital protocols specify a maximum rate. We investigated whether exceeding 20 milliunits/min of oxytocin was associated with adverse outcomes.

METHODS

This is a secondary analysis of a double-blind, single-center, randomized controlled trial of nulliparous patients with singleton gestations at 36 weeks of gestation or later who presented in spontaneous labor randomized 1:1 to either a high-dose oxytocin titration regimen (initial-incremental rate of 6 milliunits/min) or standard-dose titration regimen (initial-incremental rate of 2 milliunits/min) for labor augmentation. A maximum oxytocin dose rate limit was not specified in the study protocol. For this secondary analysis, outcomes of participants who received oxytocin and exceeded a dose rate of 20 milliunits/min at any point in labor were compared with those whose rate remained at 20 milliunits/min or less. In addition, the cumulative proportions of labor and birth outcomes were calculated for each maximum dose rate of oxytocin reached among this study cohort.

RESULTS

Of the 1,003 participants in the parent trial, 955 (95.2%) received oxytocin, as planned, and were included, with 190 (19.9%) exceeding a maximum dose rate of 20 milliunits/min. Those who exceeded 20 milliunits/min were older and were more likely to have rupture of membranes as their trial entry indication, have hypertensive disorders of pregnancy, receive intrapartum magnesium sulfate infusion, and receive oxytocin for longer. Those whose maximum rates exceeded 20 milliunits/min underwent cesarean delivery more frequently, but the majority (74%) still delivered vaginally. In multivariable analyses, there were no significant associations between maximum oxytocin dose rates greater than 20 milliunits/min and cesarean delivery (adjusted odds ratio [aOR] 1.57, 95% CI 1.00-2.46), peripartum infection (aOR 0.69, 95% CI 0.41-1.19), postpartum hemorrhage (aOR 1.37, 95% CI 0.70-2.71), or neonatal intensive care unit (NICU) admission (aOR 1.72, 95% CI 0.89-3.31). Although 85% of spontaneous vaginal deliveries occurred at maximum oxytocin dose rates of 20 milliunits/min or less, vaginal deliveries continued to occur at higher maximum dose rates. The cumulative proportions of NICU admissions and composite severe neonatal morbidity and mortality cases increased with increasing oxytocin dose rates even with maximum oxytocin dose rates at 20 milliunits/min or less.

CONCLUSION

In multivariable analyses, there are no significant differences in maternal or perinatal adverse outcomes based on exceeding 20 milliunits/min of oxytocin. These data suggest that oxytocin dosing should be individualized to each patient and not be based on arbitrary thresholds.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov , NCT02487797.

An inhibitory mechanism for suppressing high salt intake in Drosophila

High concentrations of dietary salt are harmful to health. Like most animals, Drosophila melanogaster are attracted to foods that have low concentrations of salt, but show strong taste avoidance of high salt foods. Salt in known on multiple classes of taste neurons, activating Gr64f sweet-sensing neurons that drive food acceptance and 2 others (Gr66a bitter and Ppk23 high salt) that drive food rejection. Here we find that NaCl elicits a bimodal dose-dependent response in Gr64f taste neurons, which show high activity with low salt and depressed activity with high salt. High salt also inhibits the sugar response of Gr64f neurons, and this action is independent of the neuron's taste response to salt. Consistent with the electrophysiological analysis, feeding suppression in the presence of salt correlates with inhibition of Gr64f neuron activity, and remains if high salt taste neurons are genetically silenced. Other salts such as Na2SO4, KCl, MgSO4, CaCl2, and FeCl3 act on sugar response and feeding behavior in the same way. A comparison of the effects of various salts suggests that inhibition is dictated by the cationic moiety rather than the anionic component of the salt. Notably, high salt-dependent inhibition is not observed in Gr66a neurons-response to a canonical bitter tastant, denatonium, is not altered by high salt. Overall, this study characterizes a mechanism in appetitive Gr64f neurons that can deter ingestion of potentially harmful salts.

Effectiveness of Dietary Interventions in the Treatment of Endometriosis: a Systematic Review

A patients' increasing interest in dietary modifications as a possible complementary or alternative treatment of endometriosis is observed. Unfortunately, the therapeutic potential of dietary interventions is unclear and to date no guidelines to assist physicians on this topic exist. The aim of this study, therefore, was to systematically review the existing studies on the effect of dietary interventions on endometriosis. An electronic-based search was performed in MEDLINE and COCHRANE. We included human and animal studies that evaluated a dietary intervention on endometriosis-associated symptoms or other health outcomes. Studies were identified and coded using standard criteria, and the risk of bias was assessed with established tools relevant to the study design. We identified nine human and 12 animal studies. Out of the nine human studies, two were randomized controlled trials, two controlled studies, four uncontrolled before-after studies, and one qualitative study. All of them assessed a different dietary intervention, which could be classified in one of the following principle models: supplementation with selected dietary components, exclusion of selected dietary components, and complete diet modification. Most of the studies reported a positive effect on endometriosis; they were however characterized by moderate or high-risk bias possibly due to the challenges of conducting dietary intervention trials. According to the available level of evidence, we suggest an evidence-based clinical approach for physicians to use during consultations with their patients. Further well-designed randomized controlled trials are needed to accurately determine the short-term and long-term effectiveness and safety of different dietary interventions.

Association between the prolonged use of magnesium sulfate for tocolysis and fracture risk among infants

In 2013, the U.S. Food and Drug Administration issued a safety warning that cautioned against using magnesium sulfate (MgSO4) injections for more than 5 to 7 days to stop preterm delivery due to the bone problems subsequently observed in infants. However, the warning was mainly based on case reports, and further investigation is necessary to determine whether prolonged MgSO4 use increased infant fractures.To evaluate whether prolonged MgSO4 use for tocolysis increased the risk of subsequent fractures among infants.A retrospective population-based cohort study was conducted with a new-user study design using the National Health Insurance Database in Taiwan. We included pregnant women aged between 12 and 55 years old who delivered a live-born singleton. The enrollment period was from January 1, 2012 to December 31, 2014. The exposure group was defined as pregnant women who received MgSO4 injection for >5 days during pregnancy, while those not receiving any tocolytics were the reference group. The outcome was any bone fracture among the infants during the 2-year follow-up period. Propensity score matching and Cox proportional hazards regression models were used to estimate the hazard of fractures. We further studied the effect of MgSO4 treatment with varied dosages and durations of treatment in the sensitivity analyses.Among the 4092 pregnant women in the database, 693 (16.9%) of them were included in the exposure group. The hazard ratio of infant fractures among prolonged MgSO4 users was not significantly different from that of tocolytic nonusers in adjusted models (adjusted hazard ratio (aHR) = 1.48; 95% confidence interval (CI) = 0.59-3.71). A similar lack of significance was found in the sensitivity analyses (aHR = 1.45; 95% CI = 0.40-5.28 for larger treatment dosage; aHR = 2.52; 95% CI = 0.49-12.98 for longer treatment duration).Prolonged MgSO4 tocolysis use did not increase the risk of infant fractures. Our findings reconfirmed the safety of MgSO4 as a tocolytic treatment.

Landscape of Preterm Birth Therapeutics and a Path Forward

Preterm birth (PTB) remains the leading cause of infant morbidity and mortality. Despite 50 years of research, therapeutic options are limited and many lack clear efficacy. Tocolytic agents are drugs that briefly delay PTB, typically to allow antenatal corticosteroid administration for accelerating fetal lung maturity or to transfer patients to high-level care facilities. Globally, there is an unmet need for better tocolytic agents, particularly in low- and middle-income countries. Although most tocolytics, such as betamimetics and indomethacin, suppress downstream mediators of the parturition pathway, newer therapeutics are being designed to selectively target inflammatory checkpoints with the goal of providing broader and more effective tocolysis. However, the relatively small market for new PTB therapeutics and formidable regulatory hurdles have led to minimal pharmaceutical interest and a stagnant drug pipeline. In this review, we present the current landscape of PTB therapeutics, assessing the history of drug development, mechanisms of action, adverse effects, and the updated literature on drug efficacy. We also review the regulatory hurdles and other obstacles impairing novel tocolytic development. Ultimately, we present possible steps to expedite drug development and meet the growing need for effective preterm birth therapeutics.

Association between Receipt of Intrapartum Magnesium Sulfate and Postpartum Hemorrhage

Objective  The aim of the study is to investigate the association between intrapartum administration of magnesium sulfate in women with hypertensive disorders of pregnancy and postpartum hemorrhage. Study Design  This was a retrospective cohort study of women diagnosed with a hypertensive disorder of pregnancy who delivered singleton gestations >32 weeks at a single, large volume tertiary care center between January 2006 and February 2015. Women who received intrapartum magnesium sulfate for seizure prophylaxis were compared with women who did not receive intrapartum magnesium sulfate. The primary outcome was frequency of postpartum hemorrhage. Secondary outcomes included estimated blood loss, uterine atony, and transfusion of packed red blood cells. Bivariable analyses were used to compare the frequencies of each outcome. Multivariable logistic regression models examined the independent associations of magnesium sulfate with outcomes. Results  Of 2,970 women who met inclusion criteria, 1,072 (36%) received intrapartum magnesium sulfate. Women who received magnesium sulfate were more likely to be nulliparous, publicly insured, of minority race or ethnicity, earlier gestational age at delivery, and undergo labor induction. The frequency of postpartum hemorrhage was significantly higher among women who received magnesium sulfate compared with those who did not (12.4 vs. 9.3%, p  = 0.008), which persisted after controlling for potential confounders. Of secondary outcomes, there was no difference in estimated blood loss between women who did and did not receive magnesium sulfate (250 mL [interquartile range 250-750] vs. 250 mL [interquartile range 250-750], p  = 0.446). However, compared with women who did not receive magnesium sulfate, women who received magnesium sulfate had a greater frequency of uterine atony (8.9 vs 4.9%, p  < 0.001) and transfusion of packed red blood cells (2.0 vs. 0.8%, p  = 0.008). These differences persisted after controlling for potential confounders. Conclusion  Intrapartum magnesium sulfate administration to women with hypertensive disorders of pregnancy is associated with increased odds of postpartum hemorrhage, uterine atony, and red blood cell transfusion.

Gestational and Hormonal Effects on Magnesium Sulfate's Ability to Inhibit Mouse Uterine Contractility

Magnesium sulfate is used as a tocolytic, but clinical efficacy has been seriously questioned. Our objective was to use controlled ex vivo conditions and known pregnancy stages, to investigate how 2 key factors, hormones and gestation, affect magnesium's tocolytic ability. We hypothesized that these factors could underlie the varying clinical findings around magnesium's efficacy. Myometrial strips were obtained from nonpregnant (n = 10), mid-pregnant (n = 12), and term-pregnant (n = 11) mouse uterus. The strips were mounted in organ baths superfused with oxygenated physiological saline at pH 7.4 and 37 °C. The effect of different concentrations of MgSO₄ (2-20 mM) was examined on spontaneous and oxytocin-induced (0.5-1 nM) contractions. Contractile properties (amplitude, frequency, and area under the curve) were measured before and after application of magnesium. Magnesium sulfate had a dose-dependent inhibitory effect on both spontaneous and oxytocin-induced contractions but was less effective in the presence of oxytocin. In spontaneous contractions, magnesium was more potent as gestation progressed (P < .0001). In the presence of oxytocin, however, there were no significant gestational differences in its effects on contraction. The rapid onset and reversal of magnesium's effects suggest an extracellular action on calcium entry. Taken together, we conclude that magnesium's actions are influenced by both gestational state and hormones, such that, at least in mice, it is least effective in early gestation with oxytocin present and most effective at term in the absence of oxytocin. That magnesium is least effective preterm and oxytocin decreases its effectiveness throughout gestation, may explain its disappointing clinical effects as a tocolytic.

Gestational and Hormonal Effects on Magnesium Sulfate's Ability to Inhibit Mouse Uterine Contractility

Magnesium sulfate is used as a tocolytic, but clinical efficacy has been seriously questioned. Our objective was to use controlled ex vivo conditions and known pregnancy stages, to investigate how 2 key factors, hormones and gestation, affect magnesium's tocolytic ability. We hypothesized that these factors could underlie the varying clinical findings around magnesium's efficacy. Myometrial strips were obtained from nonpregnant (n = 10), mid-pregnant (n = 12), and term-pregnant (n = 11) mouse uterus. The strips were mounted in organ baths superfused with oxygenated physiological saline at pH 7.4 and 37°C. The effect of different concentrations of MgSO4 (2-20 mM) was examined on spontaneous and oxytocin-induced (0.5-1 nM) contractions. Contractile properties (amplitude, frequency, and area under the curve) were measured before and after application of magnesium. Magnesium sulfate had a dose-dependent inhibitory effect on both spontaneous and oxytocin-induced contractions but was less effective in the presence of oxytocin. In spontaneous contractions, magnesium was more potent as gestation progressed ( P < .0001). In the presence of oxytocin, however, there were no significant gestational differences in its effects on contraction. The rapid onset and reversal of magnesium's effects suggest an extracellular action on calcium entry. Taken together, we conclude that magnesium's actions are influenced by both gestational state and hormones, such that, at least in mice, it is least effective in early gestation with oxytocin present and most effective at term in the absence of oxytocin. That magnesium is least effective preterm and oxytocin decreases its effectiveness throughout gestation, may explain its disappointing clinical effects as a tocolytic.

Magnesium sulphate can alleviate oxidative stress and reduce inflammatory cytokines in rat placenta of intrahepatic cholestasis of pregnancy model

PURPOSE

In our study, we try to investigate whether magnesium sulphate (MgSO₄) could provide protection against oxidative damage and inflammatory response in rat placenta of intrahepatic cholestasis of pregnancy (ICP) model.

METHODS

The rat model of ICP was established by injecting s.c. 17α-ethinyl estradiol (EE) daily for 5 days. MgSO₄, as an therapeutic drug for ICP, was injected i.p. daily for 3 days. Age-matched pregnant rats served as controls. The level of serum total bile acid (TBA) was measured. The data including the number and weight of offsprings on day 20 of pregnancy were collected. We observed ultrastructural changes of mitochondria and endoplasmic reticulum (ER) in placenta by transmission electron microscope. The antioxidant proteins peroxiredoxin-6 (Prdx6) and nuclear factor erythroid 2-related factor-2 (Nrf2) were analyzed by Western Blot. The inflammatory cytokines including IL-1β, TNF-α and IFN-γ were investigated by real-time PCR (RT-PCR) and enzyme-linked immune-sorbent assay (ELISA).

RESULTS

The weight of offsprings on day 20 of pregnancy increased in ICP rats treated with MgSO₄ (ICP + MG group) compared with that in ICP rats (ICP group). However, the level of TBA was not reduced. The damage of mitochondria and ER was observed in placenta, which was much more slighter in ICP + MgSO₄ group as compared with that in ICP group. Prdx6 and Nrf2 were increased, while the inflammatory cytokines including IL-1β, TNF-α and IFN-γ were decreased in ICP + MgSO₄ group compared with that in ICP group.

CONCLUSIONS

MgSO₄ had beneficial effect on improving growth of offsprings in rat model of ICP. The protective effect of MgSO₄ on alleviating oxidative damage and inflammatory response in placenta may play an important role in the process. MgSO₄ may improve the function of placenta.

Associations of hypertensive disorders of pregnancy and outcomes of labor induction with prostaglandin vaginal inserts

OBJECTIVE

To evaluate the association between hypertensive (HTNsive) disorders of pregnancy and outcomes of labor induction, in two cohorts of women induced with either misoprostol vaginal insert (MVI) or dinoprostone vaginal insert (DVI).

STUDY DESIGN

This investigation was a post-hoc analysis of data from three Phase II and III, multi-center, double blind, randomized controlled trials of women induced with identical efficacy endpoints. A competing risk framework investigated the association between HTNsive disorders of pregnancy and the time-to-event endpoints of onset of active labor and vaginal delivery. We analyzed the overall incidence of the competing risk, cesarean delivery, by logistic regression to identify potential differences between the proportion of patients with cesarean and vaginal deliveries for each cohort.

RESULTS

401 women with HTNsive disorders during pregnancy underwent induction of labor in these studies (175 with DVI and 226 with MVI). Significant differences were noted in the cumulative incidence of vaginal delivery 24 hours following insertion between the non-HTNsive and HTNsive groups for both treatments, (57.1% vs. 47.4% (p=0.023) among MVI patients and 39.9% vs. 27.2% (p=0.017) among DVI patients). However, upon adjusting for potential confounders, the estimated relative rates of vaginal delivery among HTNsive vs. non-HTNsive patients was 0.947 (95% CI (0.637, 1.371), p=0.631) and 0.904 (95% CI (0.735, 1.113) p=0.341) within the MVI and DVI sub-groups respectively.

CONCLUSION

After adjustment for confounders, such as BMI, baseline modified Bishop score and gestational age, time-to-event outcomes for induction of labor using MVI or DVI in HTNsive women are not significantly different from non-HTNsive women.

The combination tocolytic effect of magnesium sulfate and an oxytocin receptor antagonist in myometrium from singleton and twin pregnancies

BACKGROUND

Preterm birth at <37 weeks of gestation is the most common and costly complication of pregnancy and remains the leading cause of neonatal morbidity, death, and reduced achievement in surviving infants. Magnesium sulfate is 1 class of tocolytics for threatened preterm labor; however, its clinical efficacy has been questioned. Twin pregnancies are at increased risk of preterm delivery compared with singleton gestations, which suggests that there is twin-specific risk to preterm delivery in twins. The prevention strategies that are applied to singleton pregnancies, however, have not been shown to be effective in twin pregnancies.

OBJECTIVE

The purpose of this study was to compare the relaxant effect of magnesium sulfate on spontaneous and oxytocin-augmented contractions of human myometrium from singleton and twin pregnancies and to examine whether the effect of oxytocin on magnesium sulfate's potency could be reversed with the use of the oxytocin receptor antagonist, atosiban.

STUDY DESIGN

Myometrium was obtained at the time of prelabor cesarean section (36-40 weeks of gestation) from women with singleton (n=23) or twin (n=12) pregnancy. Isometric tension recordings were made on myometrial strips that were mounted in organ baths that were superfused with physiologic saline solution. Strips were exposed to rising concentrations of magnesium sulfate, and the effect on spontaneous contractions or stimulated with oxytocin (0.5 nmol/L) and in the presence or absence of atosiban (100 nmol/L) was recorded. The contractile characteristics after each application of magnesium sulfate, which included amplitude of contraction and activity integral, were measured. Concentration-response curves were fitted with the use of nonlinear regression and comparison of the negative logarithm of the 50% reduction in activity values.

RESULTS

Magnesium sulfate exerted an equal concentration-dependent inhibitory effect on spontaneous myometrial contractions from both singleton and twin myometrium (P>.05). The application of oxytocin produced a significant rightward shift in the concentration-response curves (P<.0001), but no differences were found between pregnancy groups (P>.05). The addition of atosiban shifted concentration-response curves significantly back to the left for amplitude of contraction and activity integral in singletons (P<.0001). However, only activity integral was significantly reversed in twins (P<.01).

CONCLUSION

Magnesium sulfate is equipotent in suppressing contractions in singleton and twin myometrium. Oxytocin (0.5 nmol/L) significantly reduces the tocolytic potency of magnesium sulfate, which may explain, in part, magnesium sulfate's poor efficacy in vivo; however, this can be reversed partially by the use of an oxytocin receptor antagonist. Combination tocolysis that involves oxytocin receptor antagonists requires further investigation.

Factors affecting maternal serum magnesium levels during long-term magnesium sulfate tocolysis in singleton and twin pregnancy

AIM

Our aim was to determine factors that affect maternal serum magnesium (Mg) levels, to help ensure the safety and efficacy of long-term magnesium sulfate (MgSO4 ) therapy for threatened preterm labor in singleton and twin pregnancies.

MATERIAL AND METHODS

We retrospectively and arbitrarily studied 100 patients (singleton pregnancy, n = 65; twin pregnancy, n = 35) who received i.v. MgSO4 for >48 h for tocolysis of threatened preterm labor. We used multiple regression analysis to investigate the functional relations between the candidate factors and maternal serum Mg levels.

RESULTS

MgSO4 was administered as a loading dose of 3 g for 1 h followed by a maintenance dose of 1.0-2.0 g/h. There were no maternal severe adverse events related to the elevated Mg levels in any of the subjects. The results of multiple regression analysis revealed that total dose of MgSO4 for 24 h before blood collection (g/day), total serum protein level (g/dL), serum total calcium level (mg/dL), serum creatinine level (mg/dL) and maternal bodyweight (kg) significantly affected maternal serum Mg levels in both singleton and twin pregnancies (all P-values were < 0.001). Gestational age (weeks) and period of MgSO4 administration (days) at blood collection had no significant effect in singleton or twin pregnancies.

CONCLUSION

Our study statistically shows that dose of MgSO4 , total serum protein level, serum total calcium level, serum creatinine level and maternal bodyweight are key factors to achieving safe and effective long-term tocolysis with MgSO4 in not only singleton but also twin pregnancies.

Single-cell mechanics and calcium signalling in organotypic slices of human myometrium

Elucidation of cellular mechanisms regulating myometrial contractility is crucial for improvement in management of many obstetric abnormalities, such as premature delivery, uterine dystocia and post-partum haemorrhage. Myometrial contractions are triggered by periodic synchronous rises in intracellular calcium concentration ([Ca(2+)]i) elicited by spontaneously generated action potentials propagating throughout the entire myometrium. During labour, hormones like oxytocin and prostaglandins potentiate uterine contractions by increasing their duration, strength and frequency. The most informative approach to studying the mechanisms underlying hormonal modulation of uterine contractility is to record [Ca(2+)]i responses to hormones in intact myometrial samples that have not been subjected to enzymatic treatment for cell isolation or cell culture conditions. However, the spatio-temporal resolution of such recording is limited due to the motion artifacts occurring in contracting tissue. Here we describe the application of our newly developed motion correction algorithm to investigate the [Ca(2+)]i dynamics in control and oxytocin stimulated slices of human myometrium on a cellular level. We present evidence that oxytocin induces asynchronous [Ca(2+)]i oscillations in individual myocytes within intact myometrium which are similar to those observed in cultured cells. The oscillations occur between synchronous action potential-driven [Ca(2+)]i transients but appear to be unrelated to contractions. Furthermore, the oxytocin-triggered [Ca(2+)]i oscillations wane within 30-50min of hormone application, while the action potential induced [Ca(2+)]i transients remain augmented. We conclude that oxytocin-induced [Ca(2+)]i oscillations are not relevant to the acute regulation of myometrial contractility but may play a role in longer-term regulatory processes, for example, by triggering gene expression.

Factors associated with higher oxytocin requirements in labor

OBJECTIVE

To identify clinical characteristics associated with high maximum oxytocin doses in women who achieve complete cervical dilation.

METHODS

A retrospective nested case-control study was performed within a cohort of all term women at a single center between 2004 and 2008 who reached the second stage of labor. Cases were defined as women who had a maximum oxytocin dose during labor >20 mu/min, while women in the control group had a maximum oxytocin dose during labor of ≤20 mu/min. Exclusion criteria included no oxytocin administration during labor, multiple gestations, major fetal anomalies, nonvertex presentation, and prior cesarean delivery. Multiple maternal, fetal, and labor factors were evaluated with univariable analysis and multivariable logistic regression.

RESULTS

Maximum oxytocin doses >20 mu/min were administered to 108 women (3.6%), while 2864 women received doses ≤20 mu/min. Factors associated with higher maximum oxytocin dose after adjusting for relevant confounders included maternal diabetes, birthweight >4000 g, intrapartum fever, administration of magnesium, and induction of labor.

CONCLUSIONS

Few women who achieve complete cervical dilation require high doses of oxytocin. We identified maternal, fetal and labor factors that characterize this group of parturients.

Fine spatiotemporal activity in contracting myometrium revealed by motion-corrected calcium imaging

Successful childbirth depends on the occurrence of precisely coordinated uterine contractions during labour. Calcium indicator fluorescence imaging is one of the main techniques for investigating the mechanisms governing this physiological process and its pathologies. The effective spatiotemporal resolution of calcium signals is, however, limited by the motion of contracting tissue: structures of interest in the order of microns can move over a hundred times their width during a contraction. The simultaneous changes in local intensity and tissue configuration make motion tracking a non-trivial problem in image analysis and confound many of the standard techniques. This paper presents a method that tracks local motion throughout the tissue and allows for the almost complete removal of motion artefacts. This provides a stabilized calcium signal down to a pixel resolution, which, for the data examined, is in the order of a few microns. As a byproduct of image stabilization, a complete kinematic description of the contraction–relaxation cycle is also obtained. This contains novel information about the mechanical response of the tissue, such as the identification of a characteristic length scale, in the order of 40–50 μm, below which tissue motion is homogeneous. Applied to our data, we illustrate that the method allows for analyses of calcium dynamics in contracting myometrium in unprecedented spatiotemporal detail. Additionally, we use the kinematics of tissue motion to compare calcium signals at the subcellular level and local contractile motion. The computer code used is provided in a freely modifiable form and has potential applicability to in vivo calcium imaging of neural tissue, as well as other smooth muscle tissue.

Characterization of the tissue-level Ca2+ signals in spontaneously contracting human myometrium

In the labouring uterus, millions of myocytes forming the complex geometrical structure of myometrium contract in synchrony to increase intrauterine pressure, dilate the cervix and eventually expel the foetus through the birth canal. The mechanisms underlying the precise coordination of contractions in human myometrium are not completely understood. In the present study, we have characterized the spatio-temporal properties of tissue-level [Ca(2+)](i) transients in thin slices of intact human myometrium. We found that the waveform of [Ca(2+)](i) transients and isotonic contractions recorded from thin slices was similar to the waveform of isometric contractions recorded from the larger strips in traditional organ bath experiments, suggesting that the spatio-temporal information obtained from thin slices is representative of the whole tissue. By comparing the time course of [Ca(2+)](i) transients in individual cells to that recorded from the bundles of myocytes we found that the majority of myocytes produce rapidly propagating long-lasting [Ca(2+)](i) transients accompanied by contractions. We also found a small number of cells showing desynchronized [Ca(2+)](i) oscillations that did not trigger contractions. The [Ca(2+)](i) oscillations in these cells were insensitive to nifedipine, but readily inhibited by the T-type Ca(2+) channel inhibitor NNC55-0396. In conclusion, our data suggest that the spread of [Ca(2+)](i) signals in human myometrium is achieved via propagation of long-lasting action potentials. The propagation was fast when action potentials propagated along bundles of myocytes and slower when propagating between the bundles of uterine myocytes.

The effects of extracellular calcium-sensing receptor ligands on the contractility of pregnant human myometrium in vitro

Ligands for extracellular calcium-sensing (CaS) receptors inhibit oxytocin-induced contractions of the rat's uterus. In this study, we investigated whether the CaS receptor ligands calindol, cinacalcet, and calhex 231 have similar effects on pregnant human myometrium. We compared their effects to those of the calcium-channel blocker nifedipine. In conventional concentration-effect experiments, both the mean contractile force (MCF) and the maximum amplitude of contractions induced by 1 nmol/L oxytocin were inhibited by nifedipine. Calindol and cinacalcet were ineffective as inhibitors, while calhex-231 produced partial inhibition. When single 10 μmol/L doses were applied calhex-231 produced a slowly developing inhibition, reducing the MCF to 38%, and amplitude to 34%, of vehicle controls after 1 hour. In similar experiments, calindol was ineffective while cinacalcet weakly inhibited only the amplitude. Immunohistochemistry revealed sparse expression of CaS receptors in pregnant human myometrium.

Immunohistochemical and functional studies on calcium-sensing receptors in rat uterine smooth muscle

1. Activation of calcium-sensing receptors (CaS) leads to relaxation of vascular smooth muscle. However, the role of CaS in uterine smooth muscle is unknown. Therefore the aim of the present study was to investigate the expression and function of CaS in the uterus. 2. The expression of CaS in the oestrogen-dominated rat uterus was investigated using immunohistochemistry. The effects of putative CaS ligands on oxytocin-induced contractions of longitudinally orientated uterine strips from oestrogen-dominated rats were determined at reduced extracellular Ca²⁺ concentrations using conventional organ bath techniques. 3. Immunohistochemical evidence showed the presence of CaS in the endometrium and smooth muscle layers of the rat uterus. Oxytocin-induced contractions were inhibited by cations (Gd³⁺ > Ca²⁺ = Mg²⁺), polyamines (spermine > spermidine) and the positive allosteric modulators cinacalcet and calindol. However (R)- and (S)-cinacalcet were equipotent, indicating a lack of stereoselectivity, and the negative allosteric modulator calhex-231 also caused dose-dependent relaxation. In addition, although intermediate-conductance calcium-activated potassium channels and cytochrome P450-dependent signal transduction have been implicated in CaS-induced relaxation of vascular smooth muscle, neither Tram-34 nor miconazole (1 μmol/L), which block these pathways, respectively, had any effect on the ability of cinacalcet to inhibit oxytocin-induced contractions. 4. Calcium-sensing receptors are expressed in smooth muscle layers of the rat uterus and their ligands produce potent relaxation of longitudinally orientated uterine strips. However, the pharmacological profile of inhibition of contractility by CaS ligands is not consistent with a role for CaS in the regulation of uterine contractility in the rat.

Tocolytic therapy for acute preterm labor

The pathophysiology leading to preterm labor is not well understood and often multifactorial; initiating factors include intrauterine infection, inflammation, ischemia, overdistension, and hemorrhage. Given these different potential causes, directing therapy for preterm labor has been difficult and suboptimal. To date, no single drug has been identified as successful in treating all of the underlying mechanisms leading to preterm labor. In addition, the methodology of many of the tocolytic studies is limited by lack of sufficient patient numbers, lack of comparison with a placebo, and inconsistent use of glucocorticoids. The limitations in these individual studies make it difficult to evaluate the efficacy of a single tocolytic by meta-analysis. Despite these limitations, the goals for tocolysis for preterm labor are clear: To complete a course of glucocorticoids and secure the appropriate level of neonatal care for the fetus in the event of preterm delivery. The literature demonstrates that many tocolytic agents inhibit uterine contractility. The decision as to which tocolytic agent should be used as first-line therapy for a patient is based on multiple factors, including gestational age, the patient’s medical history, common and severe side effects, and a patient’s response to therapy. In a patient at less than 32 weeks gestation, indomethacin may be a reasonable first choice based on its efficacy, ease of administration, and minimal side effects. Concurrent administration of magnesium for neuroprotection may be given. At 32 to 34 weeks, nifedipine may be a reasonable first choice because it does not carry the fetal risks of indomethacin at these later gestational ages, is easy to administer, and has limited side effects relative to beta-mimetics. In an effort to review a commonly faced obstetrical complication, this article has provided a summary of the most commonly used tocolytics, their mechanisms of action, side effects, and clinical data regarding their efficacy.

Magnesium induces neuronal apoptosis by suppressing excitability

In clinical obstetrics, magnesium sulfate (MgSO(4)) use is widespread, but effects on brain development are unknown. Many agents that depress neuronal excitability increase developmental neuroapoptosis. In this study, we used dissociated cultures of rodent hippocampus to examine the effects of Mg(++) on excitability and survival. Mg(++)-induced caspase-3-associated cell loss at clinically relevant concentrations. Whole-cell patch-clamp techniques measured Mg(++) effects on action potential threshold, action potential peak amplitude, spike number and changes in resting membrane potential. Mg(++) depolarized action potential threshold, presumably from surface charge screening effects on voltage-gated sodium channels. Mg(++) also decreased the number of action potentials in response to fixed current injection without affecting action potential peak amplitude. Surprisingly, Mg(++) also depolarized neuronal resting potential in a concentration-dependent manner with a +5.2 mV shift at 10 mM. Voltage ramps suggested that Mg(++) blocked a potassium conductance contributing to the resting potential. In spite of this depolarizing effect of Mg(++), the net inhibitory effect of Mg(++) nearly completely silenced neuronal network activity measured with multielectrode array recordings. We conclude that although Mg(++) has complex effects on cellular excitability, the overall inhibitory influence of Mg(++) decreases neuronal survival. Taken together with recent in vivo evidence, our results suggest that caution may be warranted in the use of Mg(++) in clinical obstetrics and neonatology.

Prematurity prevention: the role of acute tocolysis

PURPOSE OF REVIEW

The preterm birth rate in the United States remains at an all-time high and continues to rise. Acute tocolysis has potential to delay preterm birth for 48 h, the critical period of antenatal steroid administration, or to arrest an episode of preterm labor, thus delaying birth and improving neonatal outcomes. It is therefore paramount that medical providers remain up-to-date regarding the usefulness, indications and contraindications, and side-effects and adverse effects of all tocolytics.

RECENT FINDINGS

Magnesium sulfate remains the most common tocolyic agent in the United States. Recent evidence comparing oral nifedipine with magnesium sulfate suggests equal efficacy with fewer maternal side-effects, thus supporting this oral medication as first-line treatment. This review will summarize the most common acute tocolytic drugs, their methods of action, and clinical data regarding their utility.

SUMMARY

All tocolytic medications have side-effects, some of them potentially life-threatening. Decisions regarding whether to use a tocolytic and which tocolytic to use require the diagnosis of preterm labor, knowledge of the patient's gestational age, medical conditions, and cost. Once tocolysis is initiated, attention must be paid to the patient's response, side-effects, and adverse events. Larger studies are needed which incorporate, in addition to efficacy, data on safety and side-effect profiles and cost.

High dose magnesium sulfate exposure induces apoptotic cell death in the developing neonatal mouse brain

BACKGROUND

Magnesium sulfate (MgSO4) is often used as a treatment for pre-eclampsia/eclampsia and preterm labor, resulting in the exposure of a significant number of neonates to this drug despite a lack of evidence suggesting that it is safe, or effective as a tocolytic. While there is evidence that MgSO4 may be neuroprotective in perinatal brain injury, recent reviews have suggested that the effects are dependent upon dose, and that higher doses may actually increase neonatal morbidity and mortality. There is a lack of evidence investigating the neurotoxic effects of neonatal magnesium (Mg) exposure on the developing brain, specifically in terms of neurodevelopmental apoptosis, a cell-killing phenomenon known to be potentiated by other drugs with mechanisms of action at Mg-binding sites (i.e. NMDA receptor antagonists such as MK-801, ketamine, and PCP).

OBJECTIVE

To investigate the effects of Mg exposure on the neonatal mouse brain at different postnatal ages to determine whether MgSO4 treatment causes significant cell death in the developing mouse brain.

METHODS

C57Bl/6 mice were treated with four doses of MgSO4 (250 mg/kg) on postnatal days 3 (P3), 7 (P7) or 14 (P14). Caspase-3 immunohistochemistry, cupric silver staining, and electron microscopy techniques were used to examine Mg-treated brains for neurotoxic effects.

RESULTS

Qualitative evaluation using cupric silver staining revealed widespread damage throughout the brain in P7 animals. Results of electron microscopy confirmed that the cell death process was apoptotic in nature. Quantitative evaluation of damage to the cortex, caudate-putamen, hippocampus, thalamus, and cerebellum showed that Mg treatment caused significant brain damage in animals treated on P3 and P7, but not P14.

CONCLUSIONS

Administration of high doses of Mg may be detrimental to the fetal brain, particularly if exposure occurs during critical periods of neurodevelopment.