The relaxant effect of nifedipine in human uterine smooth muscle and the BK(Ca) channel

Assessing the Potency of the Novel Tocolytics 2-APB, Glycyl-H-1152, and HC-067047 in Pregnant Human Myometrium

The intracellular signaling pathways that regulate myometrial contractions can be targeted by drugs for tocolysis. The agents, 2-APB, glycyl-H-1152, and HC-067047, have been identified as inhibitors of uterine contractility and may have tocolytic potential. However, the contraction-blocking potency of these novel tocolytics was yet to be comprehensively assessed and compared to agents that have seen greater scrutiny, such as the phosphodiesterase inhibitors, aminophylline and rolipram, or the clinically used tocolytics, nifedipine and indomethacin. We determined the IC₅₀ concentrations (inhibit 50% of baseline contractility) for 2-APB, glycyl-H-1152, HC-067047, aminophylline, rolipram, nifedipine, and indomethacin against spontaneous ex vivo contractions in pregnant human myometrium, and then compared their tocolytic potency. Myometrial strips obtained from term, not-in-labor women, were treated with cumulative concentrations of the contraction-blocking agents. Comprehensive dose-response curves were generated. The IC₅₀ concentrations were 53 µM for 2-APB, 18.2 µM for glycyl-H-1152, 48 µM for HC-067047, 318.5 µM for aminophylline, 4.3 µM for rolipram, 10 nM for nifedipine, and 59.5 µM for indomethacin. A single treatment with each drug at the determined IC₅₀ concentration was confirmed to reduce contraction performance (AUC) by approximately 50%. Of the three novel tocolytics examined, glycyl-H-1152 was the most potent inhibitor. However, of all the drugs examined, the overall order of contraction-blocking potency in decreasing order was nifedipine > rolipram > glycyl-H-1152 > HC-067047 > 2-APB > indomethacin > aminophylline. These data provide greater insight into the contraction-blocking properties of some novel tocolytics, with glycyl-H-1152, in particular, emerging as a potential novel tocolytic for preventing preterm birth.

Dual effect of nifedipine on pregnant human myometrium contractility: Implication of TRPC1

Nifedipine, an L‐type voltage‐gated Ca²⁺ channel (L‐VGCC) blocker, is one of the most used tocolytics to treat preterm labor. In clinical practice, nifedipine efficiently decreases uterine contractions, but its efficacy is limited over time, and repeated or maintained nifedipine‐based tocolysis appears to be ineffective in preventing preterm birth. We aimed to understand why nifedipine has short‐lasting efficiency for the inhibition of uterine contractions. We used ex vivo term pregnant human myometrial strips treated with cumulative doses of nifedipine. We observed that nifedipine inhibited spontaneous myometrial contractions in tissues with high and regular spontaneous contractions. By contrast, nifedipine appeared to increase contractions in tissues with low and/or irregular spontaneous contractions. To investigate the molecular mechanisms activated by nifedipine in myometrial cells, we used the pregnant human myometrial cell line PHM1‐41 that does not express L‐VGCC. The in vitro measurement of intracellular Ca²⁺ showed that high doses of nifedipine induced an important intracellular Ca²⁺ entry in myometrial cells. The inhibition or downregulation of the genes encoding for store‐operated Ca²⁺ entry channels from the Orai and transient receptor potential‐canonical (TRPC) families in PHM1‐41 cells highlighted the implication of TRPC1 in nifedipine‐induced Ca²⁺ entry. In addition, the use of 2‐APB in combination with nifedipine on human myometrial strips tends to confirm that the pro‐contractile effect induced by nifedipine on myometrial tissues may involve the activation of TRPC channels.

Nifedipine for primary dysmenorrhoea

BACKGROUND

Dysmenorrhoea (period pain) is a common condition with a substantial impact on the well-being and productivity of women. Primary dysmenorrhoea is defined as recurrent, cramping pelvic pain that occurs with periods, in the presence of a normal uterus, ovaries and fallopian tubes. It is thought to be caused by uterine contractions (cramps) associated with a high level of production of local chemicals such as prostaglandins. The muscle of the uterus (the myometrium) responds to these high levels of prostaglandins by contracting forcefully, causing low oxygen levels and consequently pain. Nifedipine is a calcium channel blocker in widespread clinical use for preterm labour due to its ability to inhibit uterine contractions in that setting. This review addresses whether this effect of nifedipine also helps with relief of the uterine contractions during menstruation OBJECTIVES: To assess the effectiveness and safety of nifedipine for primary dysmenorrhoea.

SEARCH METHODS

We searched for all published and unpublished randomised controlled trials (RCTs) of nifedipine for dysmenorrhoea, without language restriction and in consultation with the Cochrane Gynaecology and Fertility Group (CGF) Information Specialist. The following databases were searched to 25 November 2021: the Cochrane Gynaecology and Fertility Group (CGF) Specialised Register of Controlled Trials, CENTRAL, MEDLINE, Embase, PsycINFO, and CINAHL. Also searched were the international trial registers: ClinicalTrials.gov, and the World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP) search portal, the Web of Science, OpenGrey, LILACS database, PubMed and Google Scholar. We checked the reference lists of relevant articles.

SELECTION CRITERIA

We included RCTs comparing nifedipine with placebo for the treatment of primary dysmenorrhoea.

DATA COLLECTION AND ANALYSIS

The primary outcomes to be assessed were pain, and health-related quality of life. Secondary outcomes were adverse effects, satisfaction, and need for additional medication. The two review authors independently assessed the included trials. There were insufficient data to allow meaningful meta-analysis.

MAIN RESULTS

The evidence assessed was of very low quality overall. We examined three small RCTs, with a total of 106 participants. Data for analysis could be extracted from only two of these trials (with a total of 66 participants); two trials were published in the 1980s, and the third in 1993. Nifedipine may be effective for "any pain relief" compared to placebo in women with primary dysmenorrhoea (odds ratio (OR) 9.04, 95% confidence interval (CI) 2.61 to 31.31; 2 studies, 66 participants; very low-quality evidence). The evidence suggests that if the rate of pain relief using placebo is 40%, the rate using nifedipine would be between 64% and 95%. For the outcome of "good" or "excellent" pain relief, nifedipine may be more effective than placebo; the confidence interval was very wide (OR 43.78, 95% CI 5.34 to 259.01; 2 studies, 66 participants; very low-quality evidence). We are uncertain if the use of nifedipine was associated with less requirement for additional analgesia use than placebo (OR 0.54, 95% CI 0.07 to 4.20, 1 study, 42 participants; very low-quality evidence). Participants indicated that they would choose to use nifedipine over their previous analgesic if the option was available. There were similar levels of adverse effects and menstruation-related symptoms in the placebo and intervention groups (OR 0.94, 95% CI 0.08 to 10.90; 1 study, 24 participants; very low-quality evidence); if the chance of adverse effects with placebo is 80%, the rate using nifedipine would be between 24% and 98%. There were no results regarding formal assessment of health-related quality of life.

AUTHORS' CONCLUSIONS

The evidence is insufficient to confirm whether nifedipine is a possible medical treatment for primary dysmenorrhoea. The trials included in this review had very low numbers and were of low quality. Notably, there was a large imbalance in numbers randomised between placebo and treatment groups in one of the two trials with data available for analysis. While there was no evidence of a difference noted in adverse effects between groups, more data from larger participant numbers are needed for this outcome. Larger, more well-conducted trials are required to elucidate the potential role of nifedipine in the treatment of this common condition, as it could be a useful addition to the therapeutic options available if shown to be well tolerated and effective. The safety of nifedipine in women of reproductive age is well established from trials of its use in preterm labour, and clinicians are accustomed to off-label use for this indication. The drug is inexpensive and readily available. Other options for relief of primary dysmenorrhoea are not suitable for all women; NSAIDs and the oral contraceptive pill (OCP) are contraindicated for some women, and the OCP is not suitable for women who are trying to conceive. In addition, the trials examined suggest there may be a participant preference for nifedipine.

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.

Activation of TRPV4 channels leads to a consistent tocolytic effect on human myometrial tissues

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Specific pharmacological activation of alternative Ca²⁺ conductance.

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Activation of TRPV channels, abolishes the rhythmic contractile activity.

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Tocolysis was consistently induced on human myometrial strips.

Background

Human myometrium is a therapeutic target for labor induction and preterm labor.

Objective

This study aimed to assess the physiological role of alternative calcium conductance on contractions triggered by uterotonic drugs in human myometrium. Membrane conductances, supported by TRPV channels, may provide alternative pathways to control either free intracellular and/or submembrane Ca²⁺-concentration, which in turn will modulate membrane polarization and contractile responses.

Study design

Uterine biopsies were obtained from consenting women undergoing elective caesarean delivery at term without labor (N = 22). Isometric tension measurements were performed on uterine smooth muscle strips (n = 132). Amplitude, frequency, and area under the curve (AUC) of phasic contractions, as well as resting tone, were measured under various experimental conditions. Immuno histo- and cyto-chemistry, as well as Western blot analyses, have been performed with specific antibodies against TRPV1, TRPV3, and TRPV4 proteins. TRPV4 agonists; GSK1016790A, 4αPDD, and 5,6-EET were used to assess the role of TRPV4 channels on rhythmic activity triggered by 30–300 nM oxytocin. 5 μM of ruthenium red was used as an efficient blocker of ionic current through TRPV4 channels. Nanomolar concentrations of iberiotoxin (IbTX) were also used to confirm the downstream involvement of BKCa channels in controlling uterine reactivity and contractility.

Results

The expression of TRPV3 and TRPV4 isoforms has now been demonstrated in human myometrial tissue and cell culture. Nanomolar concentrations of the TRPV4 agonists, (either GSK1016790A or 4αPDD) abolished the rhythmic contractions, resulting in a rapid and consistent tocolytic effect. While 5 μM of ruthenium reversed this tocolytic effect. The addition of IbTX (a BKCa channel blocker) reversed the effects of GSK1016790A. Carvacrol, a TRPV3 agonist, had similar tocolytic effects on rhythmic contractions albeit at higher concentrations. This inhibitory effect was also reversed by ruthenium red.

Conclusion

Collectively, these data suggest that activation of TRPV4 leads to a Ca²⁺ entry and subsequent BKCa channel activation (increase in open state probability), which in turn hyperpolarizes the myometrial cell membrane, inactivating L-type Ca²⁺ channels and efficiently abrogates contractile activity. Consequently, alternative Ca²⁺ conductance supported by TRPV4 plays a physiological role in the modulation of myometrial reactivity.

Justicia flava leaf extract potently relaxes pregnant human myometrial contractility: a lead plant for drug discovery of new tocolytic drugs

NEW FINDINGS

What is the central question of this study? Can Justicia flava leaf extract (JF) inhibit human myometrial contractility as was previously shown in mouse myometrium? What is the main finding and its importance? JF abolished human myometrial contractions and therefore presents as a lead plant in drug discovery studies involving drugs for preterm birth.

ABSTRACT

In the search for new potent therapies for preterm labour, Justicia flava leaf extract (JF) was previously shown to potently inhibit uterine contractility in both pregnant and non-pregnant mouse uterus. This study took the investigation a step further and investigated the activity of JF on pregnant human myometrial contractility. JF potently inhibited human myometrial contractility in a concentration-dependent manner. This pilot study provides evidence that JF should be further investigated as a lead plant in the drug discovery of new uterine relaxants.

Tocolytic activity assessment of the methanol leaf extract of Justicia flava Vahl (Acanthaceae) on mouse myometrial contractility and preliminary mass spectrometric determination of secondary metabolites

ETHNOPHARMACOLOGICAL RELEVANCE

The leaves of Justicia flava are traditionally used in the South of Nigeria to prevent preterm births.

AIM OF THE STUDY

In this study, the activity of the methanol leaf extract of J. flava (JF) was investigated on uterine contractility in non-pregnant and pregnant isolated mouse tissues.

MATERIAL AND METHODS

The effects on spontaneous, oxytocin, and KCl-induced contractions were determined. The effects in calcium-free media were also determined. Possible mechanisms of activity were investigated using receptor and channel modulators. Mass spectrometric analysis was additionally performed on the leaf extract to identify secondary metabolites.

RESULTS

JF was observed to inhibit spontaneous, oxytocin and high KCl-induced uterine contractility. JF also inhibited contractions in Ca²⁺-free media. JF was found to exert its inhibitory effect via interaction with inositol triphosphate and ryanodine receptors and also through modulation of K⁺- channels. Lignans and alkaloids were identified with the lignans being the most abundant in JF.

CONCLUSION

JF has been shown to potently inhibit uterine contractions in non-pregnant and pregnant isolated mouse uterus. The inhibitory activity of JF has been shown to occur via blockade of extracellular and intracellular calcium entry and these effects may be due to the lignans identified in - JF. JF has therefore been shown in this study to be a lead plant in the discovery of new drugs with uterine inhibitory activity.

Bitter taste receptors as targets for tocolytics in preterm labor therapy

Preterm birth (PTB) is the leading cause of neonatal mortality and morbidity, with few prevention and treatment options. Uterine contraction is a central feature of PTB, so gaining new insights into the mechanisms of this contraction and consequently identifying novel targets for tocolytics are essential for more successful management of PTB. Here we report that myometrial cells from human and mouse express bitter taste receptors (TAS2Rs) and their canonical signaling components (i.e., G-protein gustducin and phospholipase C β2). Bitter tastants can completely relax myometrium precontracted by different uterotonics. In isolated single mouse myometrial cells, a phenotypical bitter tastant (chloroquine, ChQ) reverses the rise in intracellular Ca2+ concentration ([Ca2+]i) and cell shortening induced by uterotonics, and this reversal effect is inhibited by pertussis toxin and by genetic deletion of α-gustducin. In human myometrial cells, knockdown of TAS2R14 but not TAS2R10 inhibits ChQ's reversal effect on an oxytocin-induced rise in [Ca2+]i Finally, ChQ prevents mouse PTBs induced by bacterial endotoxin LPS or progesterone receptor antagonist mifepristone more often than current commonly used tocolytics, and this prevention is largely lost in α-gustducin-knockout mice. Collectively, our results reveal that activation of the canonical TAS2R signaling system in myometrial cells produces profound relaxation of myometrium precontracted by a broad spectrum of contractile agonists, and that targeting TAS2Rs is an attractive approach to developing effective tocolytics for PTB management.-Zheng, K., Lu, P., Delpapa, E., Bellve, K., Deng, R., Condon, J. C., Fogarty, K., Lifshitz, L. M., Simas, T. A. M., Shi, F., ZhuGe, R. Bitter taste receptors as targets for tocolytics in preterm labor therapy.

The Synergic In Vitro Tocolytic Effect of Nifedipine Plus Ritodrine on Human Myometrial Contractility

Many pharmacological agents have been investigated to manage preterm labor; we postulate that a combination of tocolytic drugs may achieve a better effect in the prevention of uterine contractions without dose-dependent adverse effects. The aim of this study was to evaluate the inhibitory effect of dual combinations of tocolytics in vitro. Human myometrium was obtained during elective cesarean sections (term without labor; n = 40). Myometrial strips were placed in organ baths for the measurement of isometric tension. Contractile activity was induced by oxytocin (10^-8 mol/L), then a concentration-response curve to single or dual combinations of tocolytics was started. All studied tocolytics (nifedipine, ritodrine, nitroglycerin, atosiban, and NS-1619), when used alone, significantly inhibited myometrial contractions. When combined, nifedipine plus ritodrine produced a significantly greater inhibition of contractility than each drug alone in the midrange of concentrations. The combination of nifedipine plus nitroglycerin or nifedipine plus atosiban produced a significantly greater inhibition than nitroglycerin or atosiban alone but not greater than nifedipine. The combination of nifedipine plus NS-1619 (Ca⁺²-activated K⁺ [BK_Ca] channel opener) reduced the inhibitory effect of each drug. We concluded that a selected combination of tocolytics (nifedipine plus ritodrine) produced a significantly greater inhibitory effect on contractility than each drug alone at intermediate concentrations. Thus, specific combinations of tocolytics with different intracellular signaling pathways may have a synergic effect constituting a provocative new option for preterm labor treatment.

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.

BK channels regulate myometrial contraction by modulating nuclear translocation of NF-κB

The large-conductance Ca(2+)-activated K(+) (BK) channel plays an essential role in maintaining uterine quiescence during pregnancy. Growing evidence has shown a link between the BK channel and bacterial lipopolysaccharide (LPS)-induced nuclear factor-κB (NF-κB) activation in macrophages. In the uterus, NF-κB activation plays an important role in inflammatory processes that lead to parturition. Our objective was to determine whether the BK channel regulates uterine contraction, in part, by modulating NF-κB translocation into the nucleus. We compared the effects of BK channel modulation to those of LPS on NF-κB nuclear translocation and contraction in an immortalized human myometrial cell line (human telomerase reverse transcriptase [hTERT]) and uterine myocytes. Our results showed that BK channel inhibitors paxilline and penitrem A induced translocation of NF-κB into the nucleus in both hTERT cells and uterine myocytes to a similar extent as LPS treatment, and LPS and paxilline similarly reduced BK channel currents. Conversely, neither BK channel openers nor blockade of the small conductance Ca(2+)-activated K(+) channel protein 3 had an effect on NF-κB translocation. Additionally, collagen-based assays showed that paxilline induced contraction of hTERT cells and uterine myocytes. This was dependent upon cyclooxygenase-2 activity. Moreover, paxilline-induced contractility and increased cyclooxygenase-2 expression both depended on availability of free NF-κB. This study suggests that BK channels regulate myometrial contraction, in part, by modulating nuclear translocation of NF-κB.

Calcium channel blockers as tocolytics: principles of their actions, adverse effects and therapeutic combinations

Dihydropyridine Ca²⁺ channel blockers (CCBs) are widely accepted in the treatment of premature labour. Their mechanism of action in tocolysis involves the blockade of L-type Ca²⁺ channels, influenced by the Ca²⁺-activated K⁺ channels, beta-adrenergic receptors (β-ARs) and sexual hormones. In clinical practice, most experience has been gained with the use of nifedipine, whose efficacy is superior or comparable to those of β-agonists and oxytocin antagonists. Additionally, it has a favourable adverse effect profile as compared with the majority of other tocolytics. The most frequent and well-tolerated side-effects of CCBs are tachycardia, headache and hypotension. In tocolytic therapy efforts are currently being made to find combinations of tocolytic agents that yield better therapeutic action. The available human and animal studies suggest that the combination of CCBs with β-AR agonists is beneficial, although such combinations can pose risk of pulmonary oedema in multiple pregnancies and maternal cardiovascular diseases. Preclinical data indicate the potential benefit of combinations of CCBs and oxytocin antagonists. However, the combinations of CCBs with progesterone or cyclooxygenase inhibitors may decrease their efficacy. The CCBs are likely to remain one of the most important groups of drugs for the rapid inhibition of premature uterine contractions. Their significance may be magnified by further clinical studies on their combined use for tocolysis.

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.

17 alpha-hydroxyprogesterone caproate vehicle, castor oil, enhances the contractile effect of oxytocin in human myometrium in pregnancy

OBJECTIVE

The possibility exists that the vehicle for 17-alpha-hydroxyprogesterone caproate, castor oil, exerts an effect on human uterine contractility. The aim of this study was to evaluate its effects on contractility of myometrial preparations that were obtained during pregnancy.

STUDY DESIGN

Myometrial strips were suspended under isometric conditions. Contractility was induced with oxytocin. Strips were incubated in castor oil or physiologic salt solution and suspended for a further oxytocin challenge. Contractile integrals were compared between both groups.

RESULTS

Strips that were exposed to castor oil demonstrated increased contractile activity that was elicited by oxytocin (mean contractility value, 165.53%+/-17.03%; n=8; P=.004), compared with control strips (mean contractility value, 72.57%+/-7.48%; n=8; P=.003). There was a significant increase in contractile activity of the castor oil-exposed strips, compared with those that were exposed to physiologic salt solution (n=8; P<.001).

CONCLUSION

Exposure of human myometrial preparations to castor oil results in enhanced oxytocin-induced contractility.

Modification of the effect of nifedipine in the pregnant rat myometrium: the influence of progesterone and terbutaline

AIMS

The aims of the study were to investigate the effects of nifedipine on potassium chloride (KCl)-evoked rat uterine contractions on different days of pregnancy in vitro, and the alterations in the effects of nifedipine on combination with terbutaline or progesterone.

MAIN METHODS

In rat myometrial rings taken on different days of pregnancy, rhythmic contractions were evoked with KCl in an isolated organ bath.

KEY FINDINGS

The relaxing effect of nifedipine was most expressed in the 25 mM KCl-induced uterine contractions, reaching the maximum on the last day of pregnancy (day 22). This effect was decreased by progesterone pretreatment in vivo. Synergism was observed in the uterus-relaxing effect of nifedipine+terbutaline, though the extent of potentiation depended on the sequence of administration of the two compounds. When terbutaline was added first in a single dose, the maximal inhibitory effect of nifedipine was lower. This decrease in the inhibition was suspended by a Ca(2+)-poor buffer, indicating the role of Ca(2+) channel activating effect of terbutaline.

SIGNIFICANCE

It is concluded that the uterus-relaxing effect of nifedipine is weakened by progesterone and may be enhanced by low concentrations of beta-mimetics. However, the administration of terbutaline cannot precede the administration of nifedipine.

A review of recent insights into the role of the sarcoplasmic reticulum and Ca entry in uterine smooth muscle

The uterine sacroplasmic reticulum (SR) takes up and stores calcium [Ca], using an ATPase (SERCA) and the Ca-buffering proteins, calsequestrin and calreticulin. This stored Ca can be released via IP(3)-gated Ca channels. Decreases in luminal Ca concentration [Ca] have been directly measured following agonist stimulation. During spontaneous contractions however, there appears to be no involvement of the SR, as Ca entry and efflux across the plasma membrane account for these phasic contractions. After over-viewing current knowledge concerning SR structure and function, we highlight three areas of research which suggest new ways of looking at the role of the SR in the uterus, although they may be controversial or speculative at the moment. Firstly, we review the evidence for the function, if any, of Ca-induced SR Ca release channels, the ryanodine receptor (RyR) and the lack of Ca sparks (the elemental release events from RyRs), in the uterus. Secondly, we ask does regulation of SERCA by the accessory protein, phospholamban, occur in the uterus and what is the effect of knocking out phospholamban on uterine activity? Thirdly, we address the question of when and how store-operated Ca entry occurs in the myometrium. By analogy with other, usually less excitable tissues, is there a mechanism that links store Ca depletion to plasma membrane Ca entry in smooth muscle cells within intact uterus and is it physiologically relevant and regulated? Are the recently described proteins ORAI and STIM-1 involved in uterine store-operated Ca entry? We end the review by integrating these new insights with previous data to present a new working model of the SR in the uterus.