T-type and L-type calcium currents in freshly dispersed human uterine smooth muscle cells

Assessment of uterine contractions in labor and delivery

Normal labor and delivery are dependent on the presence of regular and effective contractions of the uterine myometrium. The mechanisms responsible for the initiation and maintenance of adequate and synchronized uterine activity that are necessary for labor and delivery result from a complex interplay of hormonal, mechanical, and electrical factors that have not yet been fully elucidated. Monitoring uterine activity during term labor and in suspected preterm labor is an important component of obstetrical care because cases of inadequate and excessive uterine activity can be associated with substantial maternal and neonatal morbidity and mortality. Inadequate labor progress is a common challenge encountered in intrapartum care, with labor dystocia being the most common indication for cesarean deliveries performed during labor. Hereafter, an accurate assessment of uterine activity during labor can assist in the management of protracted labor by diagnosing inadequate uterine activity and facilitating the titration of uterotonic medications before a trial of labor is prematurely terminated. Conversely, the ability to diagnose unwanted or excessive uterine activity is also critical in cases of threatened preterm labor, tachysystole, or patients undergoing a trial of labor after cesarean delivery. Knowledge of uterine activity in these cases may guide the use of tocolytic medications or raise suspicion of uterine rupture. Current diagnostic capabilities are less than optimal, hindering the medical management of term and preterm labor. Currently, different methods exist for evaluating uterine activity during labor, including manual palpation, external tocodynamometry, intrauterine pressure monitoring, and electrical uterine myometrial activity tracing. Legacy uterine monitoring techniques have advantages and limitations. External tocodynamometry is the most widespread tool in clinical use owing to its noninvasive nature and its ability to time contractions against the fetal heart rate monitor. However, it does not provide information regarding the strength of uterine contractions and is limited by signal loss with maternal movements. Conversely, the intrauterine pressure catheter quantifies the strength of uterine contractions; however, its use is limited by its invasiveness, risk for complications, and limited additive value in all but few clinical scenarios. New monitoring methods are being used, such as electrical uterine monitoring, which is noninvasive and does not require ruptured membranes. Electrical uterine monitoring has yet to be incorporated into common clinical practice because of lack of access to this technology, its high cost, and the need for appropriate training of clinical staff. Further work needs to be done to increase the accessibility and implementation of this technique by experts, and further research is needed to implement new practical and useful methods. This review describes current clinical tools for uterine activity assessment during labor and discusses their advantages and shortcomings. The review also summarizes current knowledge regarding novel technologies for monitoring uterine contractions that are not yet in widespread use, but are promising and could help improve our understanding of the physiology of labor, delivery, and preterm labor, and ultimately enhance patient care.

Uterine contractions in rodent models and humans

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

Interrupted versus continuous magnesium sulfate and blood loss at cesarean delivery

OBJECTIVE

Magnesium sulfate is standard of care for prevention of eclampsia in women with preeclampsia with severe features. The American College of Obstetrics and Gynecology endorses its use throughout labor, delivery and the immediate postpartum period. Some providers pause magnesium sulfate infusion preoperatively due to concern for increased risk of uterine atony and postpartum hemorrhage. Using a non-inferiority analysis, we investigated the effect of interrupted versus continuous infusion of magnesium sulfate on postpartum hemorrhage in women with preeclampsia with severe features undergoing cesarean delivery.

STUDY DESIGN

Retrospective non-inferiority cohort study of women with preeclampsia with severe features treated with magnesium sulfate undergoing cesarean delivery with singleton pregnancies at tertiary care hospital from 2013 to 2018. The primary outcome was postpartum hemorrhage. Secondary outcomes included estimated blood loss, change in hematocrit and a composite of postpartum hemorrhage severity, including transfusion of blood products, use of more than one uterotonic and additional surgical interventions.

RESULTS

Of 249 women, magnesium sulfate infusion was interrupted in 171 (69%) and continued in 78 (31%). Women with interrupted magnesium sulfate infusion were more likely to be Caucasian (73% vs 67%, p = .024), have chronic hypertension (23% vs 1%, p < .001), labor prior to cesarean delivery (84% vs 55%, p < .001), undergo primary cesarean delivery (86% vs 67%, p = .005), and experience shorter surgical time (50 vs 55 min, p = .026). The rate of postpartum hemorrhage for those receiving interrupted magnesium sulfate infusion (9.9%) and continuous magnesium sulfate infusion (10.2%) was similar, falling within the non-inferiority margin (absolute difference 0.3%, 95% CI -7.8 to 8.4%, p = .88). There were no significant differences in the secondary outcomes.

CONCLUSION

Interrupted magnesium sulfate infusion is non-inferior to continued magnesium sulfate infusion for rates of postpartum hemorrhage in women with preeclampsia with severe features undergoing cesarean delivery.

Uterine Excitability and Ion Channels and Their Changes with Gestation and Hormonal Environment

We address advances in the understanding of myometrial physiology, focusing on excitation and the effects of gestation on ion channels and their relevance to labor. This review moves through pioneering studies to exciting new findings. We begin with the myometrium and its myocytes and describe how excitation might initiate and spread in this myogenic smooth muscle. We then review each of the ion channels in the myometrium: L- and T-type Ca²⁺ channels, K_ATP (Kir6) channels, voltage-dependent K channels (Kv4, Kv7, and Kv11), twin-pore domain K channels (TASK, TREK), inward rectifier Kir7.1, Ca²⁺-activated K⁺ channels with large (KCNMA1, Slo1), small (KCNN1-3), and intermediate (KCNN4) conductance, Na-activated K channels (Slo2), voltage-gated (SCN) Na⁺ and Na⁺ leak channels, nonselective (NALCN) channels, the Na K-ATPase, and hyperpolarization-activated cation channels. We finish by assessing how three key hormones- oxytocin, estrogen, and progesterone-modulate and integrate excitability throughout gestation.

Effect of Matricaria chamomilla hydro-alcoholic and flavonoids rich extracts on rat isolated uterus

Introduction: Pharmacological studies confirm antispasmodic activities of chamomile (Matricaria chamomilla) extract on intestinal smooth muscles and it has been suggested that chamomile increases uterus tone, but so far there is no scientific studies which support this assumption. Therefore, this study was designed to determine spasmodic and spasmolytic activities of M. chamomilla extracts on rat isolated uterus. Methods: Hydro-alcoholic extract of M. chamomilla was prepared by maceration technique. Flavonoids rich extract was prepared by liquid in liquid extraction technique. The spasmodic effects of the extracts were assessed on spontaneously contracting rat uterus. The myorelaxant effect of M. chamomilla extracts was validated on isolated uterus contractions induced by KCl, acetylcholine (ACh), electrical field stimulation (EFS) and oxytocin. Results: Hydro-alcoholic extract of M. chamomilla (0.8 and 1.6 mg/mL) enhanced spontaneous movement of rat isolated uterus smooth muscle suspended in an organ bath. On the other hand, flavonoids rich fraction only diminished uterus contractile activities. Flavonoids rich extract of the plant at bath concentration ranges of 40 μg/mL to 400 μg/mL attenuated uterus response to ACh, KCl, EFS and oxytocin. The hydro-alcoholic extract of M. chamomilla at higher concentration ranges (250 μg/mL to 1.5 mg/mL) inhibited uterus contractions induced by the above spasmogens. Conclusion: The present study confirms both spasmodic and spasmolytic activities M. chamomilla hydro-alcoholic extract. Therefore, medicinal use of the crude extract of M. chamomilla may initiate uterus contraction which could increase risk of spontaneous miscarriage or premature parturition.

Computational physiology of uterine smooth muscle

Pregnancy can be accompanied by serious health risks to mother and child, such as pre-eclampsia, premature birth and postpartum haemorrhage. Understanding of the normal physiology of uterine function is essential to an improved management of such risks. Here we focus on the physiology of the smooth muscle fibres which make up the bulk of the uterine wall and which generate the forceful contractions that accompany parturition. We survey computational methods that integrate mathematical modelling with data analysis and thereby aid the discovery of new therapeutic targets that, according to clinical needs, can be manipulated to either stop contractions or cause the uterine wall muscle to become active.

The effect of extracellular ATP on rat uterine contraction from different gestational stages and its possible mechanisms of action

BACKGROUND

The mechanisms underlying the onset of labor are not fully understood. Extracellular adenosine 5'-triphosphate (ATP) is known to cause uterine contractions in different species but the exact underlying mechanisms are poorly investigated to date. The aims of this study were to investigate the effect of extracellular ATP on spontaneous uterine contractions from different gestational stages and to elucidate its possible underlying mechanisms.

METHODS

Longitudinal uterine strips were obtained from rats in different gestational stages (nonpregnant, late-pregnant, and term-pregnant). The effects of 1 mM ATP were examined on uterine contractions generated spontaneously, depolarized by high-KCl (60 mM), induced by oxytocin (5 nM), in the presence of high external Ca2+, or in the absence of external Ca2+.

RESULTS

Application of 1 mM extracellular ATP significantly increased the force of spontaneous contraction in uterine strips obtained from all gestational stages with prominent increase in term-pregnant rats compared to other gestations. ATP significantly increased the force induced by depolarization (122%, p=0.010, n=6), oxytocin (129%, p=0.001, n=7), high-Ca2+ (145%, p=0.005, n=6) and it was able to cause transient contraction in the absence of external Ca2+ (33%, p<0.01).

CONCLUSIONS

Extracellular ATP is able to increase the force and frequency of uterine contractions and its effect increases with the progression of pregnancy and it involves Ca2+ influx and release. These findings open a new window for clinicians to consider ATP as a therapeutic target to control the uterine activity during difficult labors.

Tocolysis: Present and future treatment options

In the United States, the generally accepted indication for tocolytic therapy centers on suppression of preterm labor. This may be in the form of preventative therapy with progesterone in women with prior spontaneous preterm birth or as an acute intervention to suppress established uterine contractions associated with cervical change occurring at less than 37 weeks gestation. This article seeks to apply this perspective to tocolytic therapy. Here, we provide a review of current tocolytic options and what the last decade of discovery has revealed about the regulation of myometrial excitability and quiescence. Moving forward, we must incorporate the emerging molecular data that is amassing in order to develop novel and effective tocolytic therapeutic options to prevent preterm labor and spontaneous preterm birth (sPTB).

Biomechanics of the human uterus

The appropriate biomechanical function of the uterus is required for the execution of human reproduction. These functions range from aiding the transport of the embryo to the implantation site, to remodeling its tissue walls to host the placenta, to protecting the fetus during gestation, to contracting forcefully for a safe parturition and postpartum, to remodeling back to its nonpregnant condition to renew the cycle of menstruation. To serve these remarkably diverse functions, the uterus is optimally geared with evolving and contractile muscle and tissue layers that are cued by chemical, hormonal, electrical, and mechanical signals. The relationship between these highly active biological signaling mechanisms and uterine biomechanical function is not completely understood for normal reproductive processes and pathological conditions such as adenomyosis, endometriosis, infertility and preterm labor. Animal studies have illuminated the rich structural function of the uterus, particularly in pregnancy. In humans, medical imaging techniques in ultrasound and magnetic resonance have been combined with computational engineering techniques to characterize the uterus in vivo, and advanced experimental techniques have explored uterine function using ex vivo tissue samples. The collective evidence presented in this review gives an overall perspective on uterine biomechanics related to both its nonpregnant and pregnant function, highlighting open research topics in the field. Additionally, uterine disease and infertility are discussed in the context of tissue injury and repair processes and the role of computational modeling in uncovering etiologies of disease. WIREs Syst Biol Med 2017, 9:e1388. doi: 10.1002/wsbm.1388 For further resources related to this article, please visit the WIREs website.

Evaluating the teratogenicity of the selective ß3-adrenoceptor agonist, CL 316.243 hydrate by employing FETAX (frog embryo teratogenesis assay)

In this study, the frog embryo teratogenesis assay (FETAX - Xenopus) technique was employed to evaluate the potential teratogenicity of the selective ß-adrenoceptor (AR) agonist, CL 316.243. In this context, CL 316.243 was applied to the South African clawed frog (Xenopus laevis) embryos. The media containing the CL 316.24-exposed embryos were monitored and changed/replaced once every 24 hours. Using FETAX, we determined the minimum concentrations to inhibit growth (MCIG) for CL 316.243. The 96-hour no observable adverse effect concentration (NOAEC), the 96-hour lowest observable adverse effect concentration (LOAEC), the 96-hour EC₅₀ (malformation) and the 96-hour LC₅₀ (lethal concentration) for mortality and malformation could not be determined because the used concentrations did not affect viability or the presence of abnormalities. On the other hand, the MCIG of CL 316.243 was determined as 1 mg/L. Our results demonstrated that CL 316.243 administration was associated with no of teratogenic and toxic effects. However, from first concentration we used (1 to 5 mg/L) length of embryos reduced significantly (p < 0.001) when compared to control of Xenopus embryos. Further studies should be conducted with different concentrations in order to investigate the optimal concentrations for treating preterm labor with these substances.

BKCa channel regulates calcium oscillations induced by alpha-2-macroglobulin in human myometrial smooth muscle cells

The large-conductance, voltage-gated, calcium (Ca(2+))-activated potassium channel (BKCa) plays an important role in regulating Ca(2+)signaling and is implicated in the maintenance of uterine quiescence during pregnancy. We used immunopurification and mass spectrometry to identify proteins that interact with BKCain myometrium samples from term pregnant (≥37 wk gestation) women. From this screen, we identified alpha-2-macroglobulin (α2M). We then used immunoprecipitation followed by immunoblot and the proximity ligation assay to confirm the interaction between BKCaand both α2M and its receptor, low-density lipoprotein receptor-related protein 1 (LRP1), in cultured primary human myometrial smooth muscle cells (hMSMCs). Single-channel electrophysiological recordings in the cell-attached configuration demonstrated that activated α2M (α2M*) increased the open probability of BKCain an oscillatory pattern in hMSMCs. Furthermore, α2M* caused intracellular levels of Ca(2+)to oscillate in oxytocin-primed hMSMCs. The initiation of oscillations required an interaction between α2M* and LRP1. By using Ca(2+)-free medium and inhibitors of various Ca(2+)signaling pathways, we demonstrated that the oscillations required entry of extracellular Ca(2+)through store-operated Ca(2+)channels. Finally, we found that the specific BKCablocker paxilline inhibited the oscillations, whereas the channel opener NS11021 increased the rate of these oscillations. These data demonstrate that α2M* and LRP1 modulate the BKCachannel in human myometrium and that BKCaand its immunomodulatory interacting partners regulate Ca(2+)dynamics in hMSMCs during pregnancy.

Why the heart is like an orchestra and the uterus is like a soccer crowd

The human uterus has no pacemaker or motor innervation, yet develops rhythmic, powerful contractions that increase intrauterine pressure to dilate the cervix and force the fetus through the pelvis. To achieve the synchronous contractions required for labor, the muscle cells of the uterus act as independent oscillators that become increasingly coupled by gap junctions toward the end of pregnancy. The oscillations are facilitated by changes in resting membrane potential that occur as pregnancy progresses. Reductions of potassium channels in the myocyte membranes in late pregnancy prolong myocyte action potentials, further facilitating transmission of signals and recruitment of neighboring myocytes. Late in pregnancy prostaglandin production increases leading to increased myocyte excitability. Also late in pregnancy myocyte actin polymerizes allowing actin-myosin interactions that generate force, following myocyte depolarization, calcium entry, and activation of myosin kinase. Labor occurs as a consequence of the combination of increased myocyte to myocyte connectivity, increased depolarizations that last longer, and activated intracellular contractile machinery. During labor the synchronous contractions of muscle cells raise intrauterine pressure to dilate the cervix in a process distinct from peristalsis. The synchronous contractions occur in a progressively larger region of the uterine wall. As the size of the region increases with increasing connectivity, the contraction of that larger area leads to an increase in intrauterine pressure. The resulting increased wall tension causes myocyte depolarization in other parts of the uterus, generating widespread synchronous activity and increased force as more linked regions are recruited into the contraction. The emergent behavior of the uterus has parallels in the behavior of crowds at soccer matches that sing together without a conductor. This contrasts with the behavior of the heart where sequential contractions are regulated by a pacemaker in a similar way to the actions of a conductor and an orchestra.

Uterine Telocytes: A Review of Current Knowledge

Telocytes (TCs), a novel cell type, are briefly defined as interstitial cells with telopodes (Tps). However, a specific immunocytochemical marker has not yet been found; therefore, electron microscopy is currently the only accurate method for identifying TCs. TCs are considered to have a mesenchymal origin. Recently proteomic analysis, microarray-based gene expression analysis, and the micro-RNA signature clearly showed that TCs are different from fibroblasts, mesenchymal stem cells, and endothelial cells. The dynamics of Tps were also revealed, and some electrophysiological properties of TCs were described (such as membrane capacitance, input resistance, membrane resting potential, and absence of action potentials correlated with different ionic currents characteristics), which can be used to distinguish uterine TCs from smooth muscle cells (SMCs). Here, we briefly present the most recent findings on the characteristics of TCs and their functions in human pregnant and nonpregnant uteri.

Progress in understanding electro-mechanical signalling in the myometrium

In this review, we give a state-of-the-art account of uterine contractility, focussing on excitation-contraction (electro-mechanical) coupling (ECC). This will show how electrophysiological data and intracellular calcium measurements can be related to more modern techniques such as confocal microscopy and molecular biology, to advance our understanding of mechanical output and its modulation in the smooth muscle of the uterus, the myometrium. This new knowledge and understanding, for example concerning the role of the sarcoplasmic reticulum (SR), or stretch-activated K channels, when linked to biochemical and molecular pathways, provides a clearer and better informed basis for the development of new drugs and targets. These are urgently needed to combat dysfunctions in excitation-contraction coupling that are clinically challenging, such as preterm labour, slow to progress labours and post-partum haemorrhage. It remains the case that scientific progress still needs to be made in areas such as pacemaking and understanding interactions between the uterine environment and ion channel activity.

Distinct roles of L- and T-type voltage-dependent Ca2+ channels in regulation of lymphatic vessel contractile activity

Lymph drainage maintains tissue fluid homeostasis and facilitates immune response. It is promoted by phasic contractions of collecting lymphatic vessels through which lymph is propelled back into the blood circulation. This rhythmic contractile activity (i.e. lymphatic pumping) increases in rate with increase in luminal pressure and relies on activation of nifedipine-sensitive voltage-dependent Ca(2+) channels (VDCCs). Despite their importance, these channels have not been characterized in lymphatic vessels. We used pressure- and wire-myography as well as intracellular microelectrode electrophysiology to characterize the pharmacological and electrophysiological properties of L-type and T-type VDCCs in rat mesenteric lymphatic vessels and evaluated their particular role in the regulation of lymphatic pumping by stretch. We complemented our study with PCR and confocal immunofluorescence imaging to investigate the expression and localization of these channels in lymphatic vessels. Our data suggest a delineating role of VDCCs in stretch-induced lymphatic vessel contractions, as the stretch-induced increase in force of lymphatic vessel contractions was significantly attenuated in the presence of L-type VDCC blockers nifedipine and diltiazem, while the stretch-induced increase in contraction frequency was significantly decreased by the T-type VDCC blockers mibefradil and nickel. The latter effect was correlated with a hyperpolarization. We propose that activation of T-type VDCCs depolarizes membrane potential, regulating the frequency of lymphatic contractions via opening of L-type VDCCs, which drive the strength of contractions.

Computational modeling of inhibition of voltage-gated Ca channels: identification of different effects on uterine and cardiac action potentials

The uterus and heart share the important physiological feature whereby contractile activation of the muscle tissue is regulated by the generation of periodic, spontaneous electrical action potentials (APs). Preterm birth arising from premature uterine contractions is a major complication of pregnancy and there remains a need to pursue avenues of research that facilitate the use of drugs, tocolytics, to limit these inappropriate contractions without deleterious actions on cardiac electrical excitation. A novel approach is to make use of mathematical models of uterine and cardiac APs, which incorporate many ionic currents contributing to the AP forms, and test the cell-specific responses to interventions. We have used three such models-of uterine smooth muscle cells (USMC), cardiac sinoatrial node cells (SAN), and ventricular cells-to investigate the relative effects of reducing two important voltage-gated Ca currents-the L-type (ICaL) and T-type (ICaT) Ca currents. Reduction of ICaL (10%) alone, or ICaT (40%) alone, blunted USMC APs with little effect on ventricular APs and only mild effects on SAN activity. Larger reductions in either current further attenuated the USMC APs but with also greater effects on SAN APs. Encouragingly, a combination of ICaL and ICaT reduction did blunt USMC APs as intended with little detriment to APs of either cardiac cell type. Subsequent overlapping maps of ICaL and ICaT inhibition profiles from each model revealed a range of combined reductions of ICaL and ICaT over which an appreciable diminution of USMC APs could be achieved with no deleterious action on cardiac SAN or ventricular APs. This novel approach illustrates the potential for computational biology to inform us of possible uterine and cardiac cell-specific mechanisms. Incorporating such computational approaches in future studies directed at designing new, or repurposing existing, tocolytics will be beneficial for establishing a desired uterine specificity of action.

Isolated human uterine telocytes: immunocytochemistry and electrophysiology of T-type calcium channels

Recently, telocytes (TCs) were described as a new cell type in the interstitial space of many organs, including myometrium. TCs are cells with very long, distinctive extensions named telopodes (Tps). It is suggested that TCs play a major role in intercellular signaling, as well as in morphogenesis, especially in morphogenetic bioelectrical signaling. However, TC plasma membrane is yet unexplored regarding the presence and activity of ion channels and pumps. Here, we used a combination of in vitro immunofluorescence and patch-clamp technique to characterize T-type calcium channels in TCs. Myometrial TCs were identified in cell culture (non-pregnant and pregnant myometrium) as cells having very long Tps and which were positive for CD34 and platelet-derived growth factor receptor-α. Immunofluorescence analysis of the subfamily of T-type (transient) calcium channels CaV3.1 and CaV3.2 presence revealed the expression of these ion channels on the cell body and Tps of non-pregnant and pregnant myometrium TCs. The expression in TCs from the non-pregnant myometrium is less intense, being confined to the cell body for CaV3.2, while CaV3.1 was expressed both on the cell body and in Tps. Moreover, the presence of T-type calcium channels in TCs from non-pregnant myometrium is also confirmed by applying brief ramp depolarization protocols. In conclusion, our results show that T-type calcium channels are present in TCs from human myometrium and could participate in the generation of endogenous bioelectric signals responsible for the regulation of the surrounding cell behavior, during pregnancy and labor.

A computational model of the ionic currents, Ca2+ dynamics and action potentials underlying contraction of isolated uterine smooth muscle

Uterine contractions during labor are discretely regulated by rhythmic action potentials (AP) of varying duration and form that serve to determine calcium-dependent force production. We have employed a computational biology approach to develop a fuller understanding of the complexity of excitation-contraction (E-C) coupling of uterine smooth muscle cells (USMC). Our overall aim is to establish a mathematical platform of sufficient biophysical detail to quantitatively describe known uterine E-C coupling parameters and thereby inform future empirical investigations of physiological and pathophysiological mechanisms governing normal and dysfunctional labors. From published and unpublished data we construct mathematical models for fourteen ionic currents of USMCs: currents (L- and T-type), current, an hyperpolarization-activated current, three voltage-gated currents, two -activated current, -activated current, non-specific cation current, - exchanger, - pump and background current. The magnitudes and kinetics of each current system in a spindle shaped single cell with a specified surface area∶volume ratio is described by differential equations, in terms of maximal conductances, electrochemical gradient, voltage-dependent activation/inactivation gating variables and temporal changes in intracellular computed from known fluxes. These quantifications are validated by the reconstruction of the individual experimental ionic currents obtained under voltage-clamp. Phasic contraction is modeled in relation to the time constant of changing . This integrated model is validated by its reconstruction of the different USMC AP configurations (spikes, plateau and bursts of spikes), the change from bursting to plateau type AP produced by estradiol and of simultaneous experimental recordings of spontaneous AP, and phasic force. In summary, our advanced mathematical model provides a powerful tool to investigate the physiological ionic mechanisms underlying the genesis of uterine electrical E-C coupling of labor and parturition. This will furnish the evolution of descriptive and predictive quantitative models of myometrial electrogenesis at the whole cell and tissue levels.

Role of T-type Ca Channels in the Spontaneous Phasic Contraction of Pregnant Rat Uterine Smooth Muscle

Although extracellular Ca(2+) entry through the voltage-dependent Ca(2+) channels plays an important role in the spontaneous phasic contractions of the pregnant rat myometrium, the role of the T-type Ca(2+) channels has yet to be fully identified. The aim of this study was to investigate the role of the T-type Ca(2+) channel in the spontaneous phasic contractions of the rat myometrium. Spontaneous phasic contractions and [Ca(2+)](i) were measured simultaneously in the longitudinal strips of female Sprague-Dawley rats late in their pregnancy (on day 18~20 of gestation: term=22 days). The expression of T-type Ca(2+) channel mRNAs or protein levels was measured. Cumulative addition of low concentrations (<1 microM) of nifedipine, a L-type Ca(2+) channel blocker, produced a decrease in the amplitude of the spontaneous Ca(2+) transients and contractions with no significant change in frequency. The mRNAs and proteins encoding two subunits (alpha1G, alpha1H) of the T-type Ca(2+) channels were expressed in longitudinal muscle layer of rat myometrium. Cumulative addition of mibefradil, NNC 55-0396 or nickel induced a concentration-dependent inhibition of the amplitude and frequency of the spontaneous Ca(2+) transients and contractions. Mibefradil, NNC 55-0396 or nickel also attenuated the slope of rising phase of spontaneous Ca(2+) transients consistent with the reduction of the frequency. It is concluded that T-type Ca(2+) channels are expressed in the pregnant rat myometrium and may play a key role for the regulation of the frequency of spontaneous phasic contractions.

MYOMETRIAL ACTIVATION – COORDINATION, CONNECTIVITY AND CONTRACTILITY

One of the most important stages of pregnancy is the activation of uterine contractions that result in the expulsion of the fetus. The timely onset of labour is clearly important for a healthy start to life but incomplete understanding of the precise mechanisms regulating labour onset have prohibited the development of effective and safe treatments for preterm labour. This review explores the activation of the myometrium at labour onset, focussing on mechanisms of uterine contractility, including those proteins that play an important role in smooth muscle contractility. The review primarily focuses on human work but in the absence of human data describes animal studies. A broad overview of myometrial contraction mechanisms is provided before discussing more detailed aspects and identifying areas where uncertainty remains. Also discussed is the recent application of ‘omics’ based approaches to parturition research, which has facilitated an increase in the understanding of myometrial activation.

Insights into the uterus

A better understanding of the mechanisms that generate and modulate uterine contractility is needed if progress is to be made in the prevention or treatment of problems in labour. Dysfunctional labour describes the condition when uterine contractility is too poor to dilate the cervix, and it is the leading cause of emergency Caesarean sections. Recently, insight has been gained into a possible causal mechanism for dysfunctional labour. Study of the physiological mechanisms that produce excitation in the uterus, the subsequent Ca(2)(+) signals and biochemical pathway leading to contraction has underpinned this progress. In this review, I give an account of excitation-contraction signalling in the myometrium and explore the implications of recent findings concerning lipid rafts for these processes. I also discuss how changes of pH are fundamentally enmeshed in uterine activity and biochemistry and explore the effect that pH changes will have on human myometrium. Finally, I present the evidence that acidification of the myometrium is correlated with dysfunctional labour and suggest the processes by which it is occurring. It is only by gaining a better understanding of uterine physiology and pathophysiology that progress will be made and research findings translated into clinical benefit for women and their families.

Magnesium ion inhibits spontaneous and induced contractions of isolated uterine muscle

AIM

Magnesium sulfate, mainly used in obstetrics to treat eclamptic convulsions, is currently questioned as to its clinical tocolytic effect. We aimed to study the relaxant action (if any) of magnesium sulfate on in vitro pregnant and non-pregnant myometrium.

METHODS

Myometrial strips, harvested from five pregnant women (35-39 gestational weeks) during Cesarean procedures indicated for dystocia or scared uterus and five non-pregnant women during hysterectomy or myomectomy for benign conditions, were placed in a Krebs-Henseleit solution organ bath and the isometric force was registered. We assessed the effect of Mg2+ (magnesium sulfate) at different concentrations (0.50-10 mM) on spontaneous and oxytocin-induced (1 microM) myometrial contractility.

RESULTS

Mg2+ temporarily reduced spontaneous myometrial contractions in a dose-dependent manner, with efficient regimens at 2.0-2.5 mM, and arrested contractility completely at 3 mM. Oxytocin-induced contractions were reduced by 30-40% at 8 mM and decreased further at 9-10 mM. Induced contractions were reduced, in a dose-dependent and time-dependent manner (maximum effect at 20 min), at higher Mg2+ concentrations and with non-significant proportional differences between pregnant and non-pregnant myometrium.

CONCLUSIONS

The present in vitro study suggests a possible benefit of Mg2+ in the inhibition of spontaneous myometrial contractility, but not of uterine-induced hyperactivity.

Characterization of the molecular and electrophysiological properties of the T-type calcium channel in human myometrium

Rises in intracellular calcium are essential for contraction of human myometrial smooth muscle (HMSM) and hence parturition. The T-type calcium channel may play a role in this process. The aim was to investigate the role of the T-type calcium channel in HMSM by characterizing mRNA expression, protein localization, electrophysiological properties and function of the channel subunits Cav3.1(alpha1G), Cav3.2(alpha1H), and Cav3.3(alpha1I). QRT-PCR, immunohistochemistry, electrophysiology and invitro contractility were performed on human myometrial samples from term, preterm, labour and not in labour. QRT-PCR analysis of Cav3.1, Cav3.2 and Cav3.3 demonstrated expression of Cav3.1 and Cav3.2 with no significant change (P>0.05) associated with gestation or labour status. Immunohistochemistry localized Cav3.1 to myometrial and vascular smooth muscle cells whilst Cav3.2 localized to vascular endothelial cells and invading leucocytes. Voltage clamp studies demonstrated a T-type current in 55% of cells. Nickel block of T-type current was voltage sensitive (IC50 of 118.57+/-68.9 microM at -30 mV). Activation and inactivation curves of ICa currents in cells expressing T-type channels overlapped demonstrating steady state window currents at the resting membrane potential of myometrium at term. Current clamp analysis demonstrated that hyperpolarizing pulses to a membrane potential greater than -80 mV elicited rebound calcium spikes that were blocked reversibly by 100 microM nickel. Contractility studies demonstrated a reversible decrease in contraction frequency during application of 100 microM nickel (P<0.05). We conclude that the primary T-type subunit expressed in some MSMCs is Cav3.1. We found that application of 100 microM nickel to spontaneously contracting human myometrium reversibly slows contraction frequency.

Control of uterine Ca2+ by membrane voltage: toward understanding the excitation-contraction coupling in human myometrium

Myometrial contractility is a complex and dynamic physiological process that changes substantially during pregnancy and culminates in childbirth. Uterine contractions are initiated by transient rises in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), which in turn are triggered and controlled by myometrial action potentials. The sequence of events between the action potential generation and the contraction initiation is referred to as excitation-contraction coupling. Hormones and other physiologically active substances affect myometrial contractility by modulating different steps in the excitation-contraction coupling process. It is therefore imperative that we understand that process to understand the regulation of myometrial contractility. The complex action potentials generated by human myometrium result from the activity of many ion channels, transporters, and pumps. Two types of myometrial action potential waveform have been described in the literature: a plateau type and a spike type. Parameters of the myometrial [Ca(2+)](i) transients and contractions differ depending on the type of action potential that triggers them. Some aspects of the excitation-contraction coupling are unique to human myometrium and cannot be found in animal models; some others are common between many species. This article reviews the current state and discusses future directions of physiological research on human myometrial excitation-contraction coupling.

Modeling Myometrial Smooth Muscle Contraction

Existing models of uterine contractions assumed a top-down approach in which the function at the organ or tissue level was explained by the behavior of smaller basic units. A new model of the excitation-contraction process in a single myometrial myocyte was recently developed. This model may be used in a bottom-up approach for the description of the contribution of cellular phenomena to the overall performance of the tissue or organ. In this review, we briefly survey current knowledge of uterine electrophysiology and contractility as well as current modeling techniques, which were successfully used to study the function of various types of muscle cells. In the physiological part of the review, we relate to mechanisms of intracellular Ca(2+) control, Ca(2+) oscillations, and Ca(2+) waves and to the various membranal transport mechanisms regulating ion exchange between the intracellular and extracellular spaces. In addition, we describe the process leading from excitation to contraction. In the modeling part of the review, we present the Hodgkin-Huxley (HH) model of excitation in the squid axon as well as models of Ca(2+) control and the latch-bridge model of Hai and Murphy describing the kinetics of smooth muscle cell (SMC) contraction. We also present integrative models describing more than one of these phenomena. Finally, we suggest how these modeling techniques can be applied to modeling myometrial contraction and thus may significantly contribute to current efforts of research of uterine function.

Identification and Electrophysiological Characteristics of Isoforms of T-type Calcium Channel Cav3.2 Expressed in Pregnant Human Uterus

Electrophysiological characteristics were compared among four cloned human alpha1H isoforms transcripted by alternative splicings of exons 25B and 26 [Delta25B/+26 (native form; alpha1H-a), Delta25B/Delta?6 (alpha1H-b), +25B/Delta26, and +25B/+26] in the intracellular loop between domains III and IV (III-IV linker) of a human T-type calcium channel (Ca(v)3.2). The native isoform Delta25B/+26 predominated in ovary and non-pregnant uterus, while isoform Delta25B/Delta26 (alpha1H-b) predominated in pregnant uterus and testis. Expressions of the newly identified +25B/Delta26 and +25B/+26 isoforms were greater in the uterus at gestation than in the non-pregnant uterus. When expressed in Xenopus laevis oocytes, all isoforms produced transient inward currents with low voltage-dependent activation and inactivation characterized in typical T-type Ca2+ currents. Each isoform possessing exon 25B (+25B/?Delta26 or +25B/+26) showed current activation and inactivation at a more negative membrane potential than the respective isoform (Delta25B/Delta26 or Delta25B/+26) lacking it. Moreover, the current activation and inactivation rates were faster for the two isoforms possessing exon 25B than for the respective isoforms lacking it. By itself, exon 26 seemed not to affect any electrophysiological characteristics. Increasing the net positive charge (relative to the native form), as occurred in isoforms Delta25B/Delta26, +25B/Delta26, and +25B/+26, caused recovery from short-term inactivation to become faster. Our results show that molecular-structure variations within the III-IV linker influence the voltage-dependence and kinetics of both activation and inactivation. Although the role of T-type Ca2+ channels in uterine tissue remains unknown, changes in the uterine expression of these alpha1H isoforms may influence physiological functions during pregnancy.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

Three-dimensional culture of human uterine smooth muscle myocytes on a resorbable scaffolding

The objective of this study was to develop a three-dimensional culture system for the study of human myometrial physiology. Primary cell lines were initiated from human myometrium obtained at the time of term cesarean delivery. After several passages, cells were seeded onto a polyglactin-910 (Vicryl) mesh and maintained in culture. After several days in culture, each mesh was transferred to another culture dish and suspended to avoid contact with the plastic of the dish. Time-lapse videomicroscopy was used to observe cell proliferation and three-dimensional (3-D) fill of the pores of the mesh. Membrane potentials of the cells of this 3-D tissue were measured with a conventional microelectrode. Confocal microscopy was used to assess 3-D morphology. In some experiments, cells were seeded onto two layers of mesh and then cultured as described above. In this two-mesh experiment, force was measured by anchoring one mesh and pulling on the other, using a micrometer-driven strain gauge. In the single-mesh experiment, cells grew into and filled the pores of the mesh by repetitive proliferation, retraction, and proliferation. A confluent, 3-D tissue was obtained within 10 to 14 days of the initial seeding of the mesh. The average membrane potential of the cells within the single mesh was -35 +/- 6 mV. Confocal microscopy demonstrated tissue thickness of 9 to 40 microm (one to eight cells) within the pores of the mesh. In the two-mesh experiment, 2 to 3 weeks in culture yielded confluent 3-D tissues, in which myocytes not only filled the pores of each mesh, but also bridged between the two meshes. The bridging myocytes were able to maintain a tension of 5 g/cm(2) before separation of the two meshes, and coordinated contractions of 40 to 200 cells were observed. We conclude that cultured human myocytes proliferate and form 3-D tissues when supported by Vicryl scaffolding. Tissue grown in 3-D may provide a model system that is sufficient to probe the physiology of cell-to-cell interactions in myometrium.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

Effects of mibefradil on uterine contractility

Mibefradil, a benzimidazolyl tetralol derivative, is a new Ca(2+) channel antagonist which is structurally distinct from other Ca(2+) channel antagonists such as nifedipine, verapamil and diltiazem. It is a very effective antihypertensive agent that is thought to achieve its action via a higher affinity block for low-voltage activated (T) than for high-voltage-activated (L) Ca(2+) channels. Nevertheless, it blocks L-type Ca(2+) channels in several tissues. In the present study, the effects of mibefradil on spontaneous rhythmic contractions and on contractions elicited by CaCl(2) (K(+)-depolarized preparations) and oxytocin (in low Ca(2+)/Ca(2+)-free solutions) were investigated on uterus strips from pregnant and non-pregnant rats. Mibefradil (10(-8)-3 x 10(-6) M) caused concentration-dependent inhibition of spontaneous contractions of uterus strips from pregnant and non-pregnant rats with the IC(50) values of 8.83 x 10(-7) M; 5.94 x 10(-7) M (amplitude) and 1.03 x 10(-6) M; 5.48 x 10(-7) M (frequency), respectively. Mibefradil (3 microM) caused a rightward shift in the concentration-response curves for CaCl(2) in K(+) (40 mM)-depolarized uterus strips taken from both pregnant and non-pregnant rats. Mibefradil (3 microM) was, however, more potent for antagonising CaCl(2) responses in uterus strips obtained from pregnant rats than in those from non-pregnant rats. Mibefradil (3 microM) had no effect on oxytocin-induced contraction in Ca(2+)-free physiological salt solution (PSS) on uterus strips from non-pregnant rats. However, it markedly inhibited oxytocin-induced contraction of pregnant rat uterus strips in Ca(2+)-free PSS. Thus, mibefradil probably antagonizes L-type Ca(2+) channels as well as interferes with the intracellular Ca(2+) release mechanism, which would be helpful in the development of a tocolytic agent.

Drotaverine interacts with the L-type Ca(2+) channel in pregnant rat uterine membranes

The effect of the isoquinoline derivative, drotaverine on the specific binding of [(3)H]nitrendipine and [(3)H]diltiazem to pregnant rat uterine membranes was examined. Drotaverine inhibited the specific [(3)H]nitrendipine and [(3)H]diltiazem bindings with IC(50) values of 5.6 and 2.6 microM, respectively. Saturation studies showed that diltiazem caused a significant increase in the maximum binding density without changing the K(D) of [(3)H]nitrendipine while drotaverine increased both the K(D) and the B(max) of [3H]nitrendipine. The dissociation kinetics of both [3H]nitrendipine and [(3)H]diltiazem were accelerated by drotaverine. These results suggest that drotaverine has a negative allosteric interaction with the binding sites for 1,4-dihydropyridines and 1,5-benzothiazepines on the L-type Ca(2+) channel in pregnant rat uterine membranes, which may have implications as to the potential usefulness of this drug in aiding child delivery.

Sevoflurane inhibits contraction of uterine smooth muscle from pregnant rats similarly to halothane and isoflurane

PURPOSE

The present study was designed to clarify the direct effects of the volatile anesthetics halothane, isoflurane and sevoflurane on oxytocin-induced uterine smooth muscle contraction from pregnant rats.

METHODS

Longitudinal smooth muscle layers were obtained from pregnant rats. Intracellular concentration of free Ca++ ([Ca++](i)) was measured, using a fluorescence technique, simultaneously with muscle tension. Inward Ba++ current (I(Ba)) through voltage-dependent Ca++ channels (VDCCs) was measured using a whole cell patch clamp technique. After incubation with 20 nM oxytocin, halothane, isoflurane or sevoflurane (1, 2, and 3%) was introduced into the tissue bath.

RESULTS

All volatile anesthetics significantly inhibited muscle contraction concomitant with a decrease in [Ca++](i). Volatile anesthetics also inhibited the peak I(Ba). When the anesthetic concentrations were expressed as multiples of minimum alveolar concentrations, there were no differences in the inhibitory potencies of the three volatile agents tested for muscle tension and VDCC.

CONCLUSIONS

Volatile anesthetics halothane, isoflurane and sevoflurane reduce the oxytocin-induced contraction of pregnant uterine smooth muscle. Inhibition of the contraction by the volatile anesthetics is due, at least in part, to the decrease in [Ca++](i), and the decrease in [Ca++](i) may be mediated by inhibition of VDCC activity.

Calcium channel blockers as tocolytics

This article reviews the clinical and basic science investigations regarding the safety and efficacy of calcium channel blockers as tocolytic agents. The authors reviewed the English language literature on the pharmacology and clinical applications of calcium antagonists in obstetrics. A MEDLINE (1966-2000) search was performed with the terms "calcium channel blockers," "randomized controlled trial," "preterm labor," "calcium antagonist," "tocolysis," and "nifedipine." References from these data sources were then used to find additional studies. Animal data and clinical trials in humans were included. The safety of these agents was researched in published data from the nonobstetric as well as obstetric literature. The calcium channel blockers most commonly used as tocolytics are nifedipine and nicardipine. These agents act to inhibit calcium influx across cell membranes, thereby decreasing tone in the smooth muscle of the vasculature. They act as profound vasodilatory agents and have minimal effect on the cardiac conduction system. Numerous randomized clinical trials have shown them to be as effective as beta-mimetics and magnesium in achieving tocolysis. When used for tocolysis, calcium antagonists have fewer maternal side effects than other tocolytics and have no adverse effect on fetal outcome.

Inward calcium currents in cultured and freshly isolated detrusor muscle cells: evidence of a T-type calcium current

PURPOSE

We carefully examined the possible routes of Ca2+ influx, and determined whether cultured cells retain Ca2+ channels and whether the culturing process changes their properties.

MATERIALS AND METHODS

Inward currents were measured under voltage clamp in freshly isolated cells and myocytes from confluent cell cultures of detrusor smooth muscle.

RESULTS

In guinea pig and human cells mean peak inward current density plus or minus standard deviation decreased significantly in cell culture (2.0 +/- 0.9 versus 4.5 +/- 2.2 pA.pF.(-1)) but there was no species variation. In primary cultured and passaged guinea pig cells an inward current was identified as L-type Ca2+ current. In freshly isolated cells another component to the inward current was identified that was insensitive to 20 micromol. l(-1) verapamil and 20 to 50 micromol. l(-1) cadmium chloride but abolished by 100 micromol. l(-1) nickel chloride and identified as T-type Ca2+ current. In addition, total inward current was greater at a holding potential of -100 than -40 mV., also indicating a component of current activated at negative voltage. Steady state activation and inactivation curves of the net inward current were also compatible with a single component in cultured cells but a dual component in freshly isolated cells. The action potential was completely abolished in cultured cells by L-type Ca2+ channel blockers but incompletely so in freshly isolated cells. Outward current depended strongly on previous inward current, suggesting a predominant Ca2+ dependent outward current.

CONCLUSIONS

In freshly isolated guinea pig cells T and L-type Ca2+ current is present but T-type current is absent in confluent cultures.

Gestational changes in uterine L-type calcium channel function and expression in guinea pig

Pregnancy can influence both the resting membrane potential and the ion channel composition of the uterine myometrium. Calcium flux is essential for excitation-contraction coupling in pregnant uterus. The uterine L-type calcium channel is an important component in mediating calcium flux and is purported to play a role in parturition. This study was undertaken to characterize gestational changes in 1) the uterine contractile response to the L-type calcium channel agonist, Bay K 8644; 2) the mRNA expression of channel subunits by semiquantitative reverse transcriptase-polymerase chain reaction; and 3) estimate channel protein levels by measuring (3)H-isradipine binding at the dihydropyridine binding site of the alpha(1c) subunit utilizing saturation binding methods. Sensitivity to Bay K 8644 increases beginning at 0.8 of gestation and persists through term. The change in sensitivity is coincident with an increased mRNA expression of the alpha(1c) and beta(2) subunits but with the least detectable amounts of isradipine binding. The expressed alpha(1c) transcript represents a novel structural variant with a 118-amino acid deletion in the III-IV linker and repeats IVS1-S3 of the protein sequence. The guinea pig uterine L-type calcium channel activity is highly regulated through gestation, but the regulation of mRNA expression may be different from regulation of protein levels, estimated by isradipine binding. The up-regulation of function, alpha(1c) subunit mRNA expression, and isradipine binding at term gestation are consistent with a role for this ion channel in parturition.

Contractile activity, membrane potential, and cytoplasmic calcium in human uterine smooth muscle in the third trimester of pregnancy and during labor

OBJECTIVE

This study was undertaken to investigate in human tissue samples the mechanisms underlying spontaneous and prostaglandin F(2)(alpha)-induced contractions during the final trimester of pregnancy and labor.

STUDY DESIGN

Membrane potential and cytoplasmic calcium were recorded simultaneously with contraction in uterine strips obtained from the lower segment during cesarean delivery.

RESULTS

Between week 28 of gestation and term there was a progressive increase in the frequency of spontaneous contractions and a decrease in the negative potential of the membrane. The response to prostaglandin F(2alpha) was biphasic. The initial excitatory component remained stable toward term. A later inhibitory component, which was underpinned by increased activity of the sodium-potassium adenosine triphosphatase pump, decreased at the time of labor.

CONCLUSIONS

There is a gradual increase in excitability in uterine muscle throughout the third trimester of human pregnancy. The initial component of the prostaglandin response is a large contraction that is kept brief by a subsequent inhibitory component of the response, which ensures that full relaxation occurs between contractions.

Maintenance oral nifedipine for preterm labor: a randomized clinical trial

OBJECTIVE

This study was undertaken to evaluate the efficacy of maintenance oral nifedipine in patients initially treated with intravenous magnesium sulfate for preterm labor.

STUDY DESIGN

Patients with a diagnosis of preterm labor between 24 and 33.9 weeks' gestation were randomly assigned to receive either maintenance tocolytic therapy with oral nifedipine (20 mg every 4-6 hours) or no treatment (control) after discontinuation of magnesium tocolysis. Pregnancy and neonatal outcomes were evaluated. A sample size of 50 patients was required to detect a 10-day difference in mean time gained (beta =.2, alpha =.05). Statistical analyses were based on intent to treat. The t, chi(2), and Fisher exact tests were performed.

RESULTS

Seventy-four patients were randomly assigned to receive either oral nifedipine (n = 37) or no treatment (n = 37). There were no statistically significant differences in age, race, parity, preterm delivery risk factors, enrollment gestational age, results of cervical examination, delivery gestational age, time gained, or neonatal complications between the groups. Delivery gestational age (mean +/- SD) was 35.4 +/- 3.2 weeks for patients randomly assigned to receive nifedipine and 35.3 +/- 3.2 weeks for patients who received no treatment (P =.9). Time gained during pregnancy was 37 +/- 23.9 days in the nifedipine group and 32.8 +/- 20.4 days in the control group (P =.4).

CONCLUSION

Maintenance therapy with oral nifedipine does not significantly prolong pregnancy in patients initially treated with intravenous magnesium sulfate for preterm labor.

Voltage-gated K+ currents in freshly isolated myocytes of the pregnant human myometrium

1. Voltage-gated K+ currents in human myometrium are not well characterized, and were therefore investigated, using the whole-cell patch clamp technique, in freshly isolated myometrial smooth muscle cells from pregnant women at term. 2. Three types of voltage-gated K+ currents were identified. IK1 was a 4-aminopyridine-insensitive current with a negative half-inactivation (V0.5 = -61 to -67 mV) and negative activation characteristics (threshold between -60 and -40 mV) and slow kinetics. IK2 was a 4-aminopyridine-sensitive current (half-maximal block at approximately 1 mM) with relatively positive half-inactivation (V0.5 = -30 mV) and activation characteristics (threshold between -40 and -30 mV) and faster kinetics. IK,A was a 4-aminopyridine-sensitive current with a negative inactivation and very fast inactivation kinetics. 3. Both IK1 and IK2 were sensitive to high concentrations of tetraethylammonium (half-maximal block at approximately 3 mM) and low concentrations of clofilium (half-maximal block by 3-10 microM). 4. IK1 and IK2 were unevenly distributed between myometrial cells, most cells possessing either IK1 (30 cells) or IK2 (24 cells) as the predominant current. 5. The characteristics of these currents suggest a possible function in the control of membrane potentials and smooth muscle quiescence in the pregnant human myometrium.

Calcium antagonistic properties of the cyclooxygenase-2 inhibitor nimesulide in human myometrial myocytes

The non-steroidal anti-inflammatory drug nimesulide is a selective inhibitor of cyclooxygenase-2 which relaxes spontaneously contracting human myometrium in vivo and is potentially a useful tocolytic drug. Part of the relaxant action of nimesulide may be via block of myometrial Ca2+ channels. Here, we describe the Ca2+ channel blocking properties of nimesulide in freshly dispersed human term-pregnant myometrial smooth muscle cells (HMSMCs). Both L- and T-components of the whole cell Ca2+ channel current were inhibited by 100 microM nimesulide (38+/-3 and 35+/-1% block, respectively). At physiological pH inside and outside the cell (pHo/pHi = 7.4/7.2), this block did not depend on the holding or test potential, although a degree of use-dependence was observed during high frequency stimulation at a higher concentration of drug (300 microM). At pHo/pHi = 6.8, under which condition the concentration of the non-ionized form of the drug is increased 3 fold compared to pH 7.4, nimesulide blocked the L-type current more potently (58+/-3% inhibition at 100 microM, P<0.01) compared to physiological pH. Nimesulide caused a 7 mV leftward shift in the availability curve of the current at pH 6.8, suggesting that the affinity of the drug for the inactivated channel is approximately 4 fold higher than its affinity for the closed channel. We speculate that acidification and depolarization of the myometrium during the intense and prolonged contractions of labour might increase the potency of nimesulide as a Ca2+ channel antagonist, promoting its action as a tocolytic agent.

Stimulation of oscillatory uterine contraction by the PCB mixture Aroclor 1242 may involve increased [Ca2+]i through voltage-operated calcium channels

Polychlorinated biphenyls (PCBs) are persistent environmental pollutants associated with spontaneous abortion and shortened gestation length in women and animals. In previous studies, we showed that PCB mixtures and noncoplanar ortho-substituted PCB congeners increased contractions in pregnant rat uterus. In the present study, we hypothesized that the PCB mixture Aroclor 1242 (A1242) stimulates oscillatory uterine contraction in pregnant uterus by increasing intracellular calcium concentration ([Ca2+]i). Pretreatment of uterine strips with ryanodine or thapsigargin, to deplete specific intracellular calcium stores, did not prevent the increased frequency of oscillatory contraction due to 50 microM A1242, whereas thapsigargin effectively blocked carbachol-induced stimulation of uterine contraction. However, 100 microM A1242 was unable to increase contraction in the absence of extracellular calcium or in the presence of the voltage-operated L-type calcium channel blocker nifedipine. A1242 (100 microM) was observed to partially depolarize the cell membrane of myometrial cells from pregnant rats, as measured with a potential-sensitive carbocyanine dye. Changes of [Ca2+]i were monitored in single myometrial cells loaded with the fluorescent calcium-sensitive probe fura-2. Cells exposed to 100 microM A1242 showed a delayed and sustained increase of [Ca2+]i, and this increase was completely blocked in the absence of extracellular calcium or the presence of nifedipine. Therefore, the data suggest that depolarization of the cell membrane by A1242 enabled myometrial cells to increase [Ca2+]i through activation of voltage-operated calcium channels, and the increased [Ca2+]i consequently stimulated contraction of uterine smooth muscle.

Oxytocin increases the [Ca2+]i sensitivity of human myometrium during the falling phase of phasic contractions

Oxytocin is commonly used to induce or augment labor, but its mode of action is uncertain. To address the issue, isometric tension and the intracellular free Ca2+ concentration ([Ca2+]i) were simultaneously recorded from isolated strips of pregnant human myometrium loaded with fura 2. The changes in [Ca2+]i and tension during phasic contractions were indistinguishable in myometrium taken before or after the onset of labor, enabling samples to be pooled. Oxytocin (10 nM) had no effect on basal [Ca2+]i or tension, but it increased both the [Ca2+]i and the tension recorded during phasic contractions. Analysis of the [Ca2+]i-tension relationship revealed that during the falling (relaxation) phase of the contractile response, oxytocin increased the tension recorded at each [Ca2+]i. By manipulating extracellular Ca2+ during phasic contractions, it was possible to ensure that the [Ca2+]i signals were similar in the presence and absence of oxytocin, yet oxytocin still improved the [Ca2+]i-tension relationship. We conclude that 10 nM oxytocin increases the [Ca2+]i sensitivity of the contractile proteins only after a contraction has begun, possibly by causing inhibition of myosin light chain phosphatase.

Hyperpolarization and slowing of the rate of contraction in human uterus in pregnancy by prostaglandins E2 and f2alpha: involvement of the Na+ pump

1. The effects of prostaglandins E2 (PGE) and F2alpha (PGF) on membrane potential and isometric tension and cytoplasmic free calcium concentration ([Ca2+]i) and tension were studied in strips of uterine smooth muscle obtained from women undergoing Caesarean delivery at term and during established labour. 2. Prostaglandins (PGs) evoked a biphasic response. The excitatory component consisted of depolarization of the membrane, which initiated spike action potentials, an increase in [Ca2+]i and tension development. The membrane remained depolarized at -19 +/- 1 mV for about 2 min, then repolarized abruptly, [Ca2+]i promptly returned to basal levels, and tension development ceased. 3. This component of the response to PGE or PGF was followed by a slow hyperpolarization which reached -85 +/- 2 mV (n = 22) at term and -70 +/- 2 mV (n = 9) during labour, and during which spontaneous action potentials and tension development did not occur. 4. Nifedipine (10-6 M) abolished spontaneous activity, abolished PG-induced action potentials and reduced the increase in [Ca2+]i (9 +/- 3 %, n = 6), the depolarization (10 +/- 1 mV, n = 14), the tension (2 +/- 1 %, n = 14) and the hyperpolarization (9 +/- 1 mV, n = 14, at term). 5. A variety of K+ channel blockers were without effect on the peak amplitude of the PG-induced hyperpolarization but the latter did not occur in the presence of ouabain (10-6 M) or in K+-free or low-Na+ solutions, suggesting an involvement of the Na+-K+-ATPase pump. 6. In conclusion, a substantial dependence on Ca2+ influx through voltage-operated Ca2+ channels accounts for the importance of membrane potential in regulating contractions in human uterine smooth muscle. The classical excitatory effect of PGE and PGF is followed by hyperpolarization involving the Na+-K+-ATPase pump. The hyperpolarization restricts the response to a single contraction and decreases the frequency of subsequent contractions. The amplitude of the hyperpolarization decreases during labour, allowing contraction frequency to increase. Its persistence at this time ensures complete relaxation between each single robust contraction, preventing spasm of the uterus that would restrict blood flow to the fetus during delivery.

Human fetal membranes release a Ca++ channel inhibitor

OBJECTIVE

Our purpose was to test the hypothesis that an inhibitor of uterine contractions released by human fetal membranes acts on the dihydropyridine site of the myometrial voltage-dependent Ca++ L channel.

STUDY DESIGN

Initial experiments established the time course of release of the inhibitor from term, fetal membranes. Both a competitive binding assay and a uterine contraction bioassay were used to detect the inhibitor. After optimal time of release of inhibitor was determined, a dose-response experiment was performed with the competitive binding assay. To determine the source of the inhibitor, membranes are separated into component layers to generate inhibitor, and the competitive binding assay was used to measure the inhibitor.

RESULTS

An inhibitor released from fetal membranes competes with 3H-isradipine at the Ca++ L channel dihydropyridine binding site. There is a time-dependent release of the inhibitor from membranes, which is maximal at 20 minutes (P < or = .05, n = 4). A dose effect of the inhibitor is present because greater amounts of inhibitor produce greater competition at the dihydropyridine site (P < or = .005, n = 3). The data are consistent with 1-site binding. Inhibition is restricted to the chorion (64% specific inhibition) and decidua (52% specific inhibition) with little competition seen in amnion alone (4% specific inhibition) (P < or = .03, n = 3).

CONCLUSIONS

These studies support the hypothesis that human chorion/decidua releases an inhibitor of uterine contractions that acts specifically at the dihydropyridine site of the myometrial Ca++ L channel.

Sodium and calcium inward currents in freshly dissociated smooth myocytes of rat uterus

Freshly dissociated myocytes from nonpregnant, pregnant, and postpartum rat uteri have been studied with the tight-seal patch-clamp method. The inward current contains both INa and ICa that are vastly different from those in tissue-cultured material. INa is abolished by Na+-free medium and by 1 microM tetrodotoxin. It first appears at approximately -40 mV, reaches maximum at 0 mV, and reverses at 84 mV. It activates with a voltage-dependent tau of 0.2 ms at 20 mV, and inactivates as a single exponential with a tau of 0. 4 ms. Na+ conductance is half activated at -21.5 mV, and half inactivated at -59 mV. INa reactivates with a tau of 20 ms. ICa is abolished by Ca2+-free medium, Co2+ (5 mM), or nisoldipine (2 microM), and enhanced in 30 mM Ca2+, Ba2+, or BAY-K 8644. It first appears at approximately -30 mV and reaches maximum at +10 mV. It activates with a voltage-dependent tau of 1.5 ms at 20 mV, and inactivates in two exponential phases, with tau's of 33 and 133 ms. Ca2+ conductance is half activated at -7.4 mV, and half inactivated at -34 mV. ICa reactivates with tau's of 27 and 374 ms. INa and ICa are seen in myocytes from nonpregnant estrus uteri and throughout pregnancy, exhibiting complex changes. The ratio of densities of peak INa/ICa changes from 0.5 in the nonpregnant state to 1.6 at term. The enhanced role of INa, with faster kinetics, allows more frequent repetitive spike discharges to facilitate simultaneous excitation of the parturient uterus. In postpartum, both currents decrease markedly, with INa vanishing from most myocytes. Estrogen-enhanced genomic influences may account for the emergence of INa, and increased densities of INa and ICa as pregnancy progresses. Other influences may regulate varied channel expression at different stages of pregnancy.