Effect of metabolic inhibition on intracellular Ca2+, phosphorylation of myosin regulatory light chain and force in rat smooth muscle

Extracellular acidification increases uterine contraction in pregnant mouse by increasing intracellular calcium

AIMS

As uterine extracellular pH decreases during the ischemic conditions of labor, but its effects on myometrial contraction are largely unknown, there is a need to elucidate its physiological effects and mechanisms of action. Furthermore, it is not known if any of the effects of extracellular acidification are affected by pregnancy, thus we also determined how gestation affects the response to acidification.

METHODS

Nonpregnant, mid-, and term-pregnant myometrial strips were obtained from humanely killed mice. Contractions were recorded under spontaneous, depolarized, and oxytocin-stimulated conditions. The extracellular pH of the perfusate was changed from 7.4 to 6.9 or 7.9 in HEPES-buffered physiological saline. Intracellular pH was measured using SNARF, and intracellular calcium was measured using Indo-1. Statistical differences were tested using the appropriate t-test.

RESULTS

Extracellular acidification significantly increased the frequency and amplitude of spontaneous contractions in pregnant, but not nonpregnant, myometrium, whereas alkalinization decreased contractions. Intracellular acidification, via Na-butyrate, transiently increased force in pregnant tissue. Intracellular pH was gradually acidified when extracellular pH was acidified, but extracellular acidification increased contractility before any significant change in intracellular pH. If myometrial force was driven by oxytocin or high-K depolarization, then extracellular pH did not further increase force. Intracellular calcium changes mirrored those of force in the spontaneously contracting pregnant myometrium, and if calcium entry was prevented by nifedipine, extracellular acidification could not induce a rise in force.

CONCLUSION

Extracellular acidification increases excitability, calcium entry, and thus force in pregnant mouse myometrium, and this may contribute to increasing contractions during labor when ischemic conditions and acidemia occur.

Modeling and experimental approaches for elucidating multi-scale uterine smooth muscle electro- and mechano-physiology: A review

The uterus provides protection and nourishment (via its blood supply) to a developing fetus, and contracts to deliver the baby at an appropriate time, thereby having a critical contribution to the life of every human. However, despite this vital role, it is an under-investigated organ, and gaps remain in our understanding of how contractions are initiated or coordinated. The uterus is a smooth muscle organ that undergoes variations in its contractile function in response to hormonal fluctuations, the extreme instance of this being during pregnancy and labor. Researchers typically use various approaches to studying this organ, such as experiments on uterine muscle cells, tissue samples, or the intact organ, or the employment of mathematical models to simulate the electrical, mechanical and ionic activity. The complexity exhibited in the coordinated contractions of the uterus remains a challenge to understand, requiring coordinated solutions from different research fields. This review investigates differences in the underlying physiology between human and common animal models utilized in experiments, and the experimental interventions and computational models used to assess uterine function. We look to a future of hybrid experimental interventions and modeling techniques that could be employed to improve the understanding of the mechanisms enabling the healthy function of the uterus.

Analysis of decay kinetics of the cytosolic calcium transient induced by oxytocin in rat myometrium smooth muscle cells

The method of kinetic analysis of the relaxation phase of the mechanical response of the smooth muscle previously proposed by Burdyga and Kosterin was applied to study the dynamics of the decay of oxytocin-induced calcium transients in cytosol of the rat myometrium smooth muscle cell detected by a fluorescence signal generated by a calcium-sensitive probe fluo-4 using a laser scanning confocal microscope. The experimental data were well linearized in the coordinates ln [(F_m - F)/F] vs lnt (F and F_m are the current fluorescence intensity of the calcium probe and the fluorescence intensity at the maximum of the calcium transient, respectively, while t is the time). The empirical parameters n and τ were determined by which the maximal normalized relaxation rate V_n was calculated for five different ROIs (regions of interest) in the myocyte cytosol. It proved to be almost the same for all ROIs. The maximal normalized relaxation rate calculated from the fluorescence intensity was always lower than that calculated from the corresponding calcium concentration, i.e. the cytosolic Ca²⁺ concentration in the relaxation phase decreases faster than the corresponding fluorescence intensity. The value of the maximal normalized relaxation rate calculated both from the fluorescence intensity and from the force of oxytocin-induced contractions of isolated rat uterus longitudinal smooth muscles (according to Tsymbalyuk and Kosterin) was exactly the same. This indicates that in the relaxation phase, the decreasing curves of both the fluorescence intensity and the contraction forces coincide.

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.

Antihypertensive activity of Petroselinum crispum through inhibition of vascular calcium channels in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Parsley (Petroselinum crispum; P. crispum) is among the popular aromatic vegetables and a part of the daily diet in the Mediterranean area. This plant is widely used in alternative medicine as a remedy against hypertension.

THE AIM OF THE STUDY

The aim of the study was to evaluate the antihypertensive activity of the aqueous extract of this plant.

MATERIAL AND METHODS

In the current study, the aqueous extract of the aerial parts of parsley (AEPC) was prepared and its antihypertensive activity was evaluated using in vivo and in vitro studies. In the in vivo investigation, anesthetized L-NAME-hypertensive and normotensive rats received orally AEPC (160 mg/kg) during 6 h for the acute experiment and during seven days for the sub-chronic treatment. Thereafter, systolic, diastolic, mean arterial blood pressure and heart rate were recorded using a tail cuff and a computer-assisted monitoring device. Concerning the in vitro investigation, isolated thoracic aortic rings were suspended in a tissue bath and the tension changes were recorded to a data acquisition system.

RESULTS

The results indicated that AEPC extract decreased the systolic, diastolic, mean arterial blood pressure in normotensive and hypertensive rats. The data revealed that parsley extract exerts its hypotensive effects through vasodilatory properties via an endothelium-independent pathway. More interestingly, the study demonstrated here that the vasorelaxing ability of AEPC is exerted through both Voltage Operated and Receptor Operated Calcium Channels (VOCC and ROCC).

CONCLUSION

The study illustrates the beneficial action of P. crispum as an antihypertensive agent.

A randomized controlled trial of a new treatment for labor dystocia

OBJECTIVE

Labor dystocia is an intransigent, high-profile issue in obstetric care. Amniotic fluid lactate (AFL) reflects the uterine metabolic status. High levels associate with subsequent need for operative intervention due to dystocia. In sports medicine, it is known that lactic acid can affect muscular performance and can be decreased by bicarbonate given orally before physical activity.

MATERIAL AND METHODS

Two hundred dystocic deliveries were included. At the confirmation of dystocia, the AFL-level was analyzed. Deliveries were randomized to an intake of bicarbonate or not. In the "non-bicarbonate-group", stimulation with oxytocin was started immediately. In the "bicarbonate-group", bicarbonate was given; and oxytocin was started 1 hour after the intake. New sampling of AF was performed after 1 hour in both groups.

OUTCOME MEASURED

if an oral intake of bicarbonate changes the AFL levels and enhances delivery outcome in dystocic deliveries.

RESULTS

Bicarbonate decreases the AFL levels (p < .001). The spontaneous vaginal delivery rate after treatment with bicarbonate was increased (p = .007), without affecting the fetal outcome.

CONCLUSIONS

An increase of spontaneous vaginal deliveries resulted from bicarbonate ingestion by dystocic women. A decreased level of AFL-level was shown. This simple, low cost treatment has the potential to improve maternal morbidity and satisfaction worldwide.

Smooth muscle function and myosin polymerization

Smooth muscle is able to function over a much broader length range than striated muscle. The ability to maintain contractility after a large length change is thought to be due to an adaptive process involving restructuring of the contractile apparatus to maximize overlap between the contractile filaments. The molecular mechanism for the length-adaptive behavior is largely unknown. In smooth muscle adapted to different lengths we quantified myosin monomers, basal and activation-induced myosin light chain (MLC) phosphorylation, shortening-velocity, power-output and active force. The muscle was able to generate a constant maximal force over a 2-fold length range when it was allowed to go through isometric contraction/relaxation cycles after each length change (length adaptation). In the relaxed state myosin monomer concentration and basal MLC phosphorylation decreased linearly, while in the activated state activation-induced MLC phosphorylation and shortening-velocity/power-output increased linearly with muscle length. The results suggest that recruitment of myosin monomers and oligomers into the actin filament lattice (where they form force-generating filaments) occurs during muscle adaptation to longer length with the opposite occurring during adaptation to shorter length.

Association of rat thoracic aorta dilatation by astragaloside IV with the generation of endothelium-derived hyperpolarizing factors and nitric oxide, and the blockade of Ca2+ channels

The aim of the present study was to elucidate the roles of endothelium-derived hyperpolarizing factors (EDHFs) and nitric oxide (NO) in mediating the vasodilatation response to astragaloside IV and the effects of astragaloside IV on voltage-dependent Ca²⁺ channels and receptor-operated Ca²⁺ channels in rat thoracic aortic rings precontracted with potassium chloride (KCl; 60 mM) or phenylephrine (PHE; 1 µM). The results showed that astragaloside IV (1×10⁻⁴-3×10⁻¹ g/l) concentration-dependently relaxed the contraction induced by KCl (10–90 mM) or PHE (1×10⁻⁹-3×10⁻⁵ µM) and inhibited concentration-contraction curves for the two vasoconstrictors in the aortic rings. Preincubation with N^ω-nitro-L-arginine methyl ester (L-NAME, 100 µM) significantly attenuated astragaloside IV-induced relaxation in the endothelium-intact and -denuded arterial rings precontracted with PHE. Astragaloside IV, following preincubation with L-NAME (100 µM) plus indomethacin (10 µM), exerted vasodilatation, which was depressed by tetraethtylamine (1 mM) and propargylglycine (100 µM), but not by carbenoxolone (10 µM), catalase (500 U/ml) or proadifen hydrochloride (10 µM). The action mode of astragaloside IV was evident in comparison to nifedipine. Inhibition of PHE-induced contraction by astragaloside IV (100 mg/l) was more potent compared to inhibition of KCl-induced contraction, while inhibition of KCl-induced contraction by nifedipine (100 mg/l) was more potent compared to inhibition of PHE-induced contraction by nifedipine (100 mg/l). In addition, the combination of astragaloside IV and nifedipine exhibited synergistic and additive inhibitory effects on contraction evoked by KCl, which was similar to PHE. In conclusion, astragaloside IV, as a Ca²⁺ antagonist, relaxes the vessels through the blockade of superior receptor-operated Ca²⁺ and inferior voltage-dependent Ca²⁺ channels, which modulate NO from vascular endothelial cells and vascular smooth muscle cells, and EDHFs including K⁺ and hydrogen sulfide.

Insights from physiology into myometrial function and dysfunction

NEW FINDINGS

What is the topic of this review? I focus on clinical aspects of uterine physiology, specifically, myometrial contractility. I bring together and contrast findings using physiological approaches and those using newer techniques, 'omics'. What advances does it highlight? Physiological studies have recently shed light on the myometrium in twin pregnancies, but there have been no 'omic' approaches. In contrast, studies of preterm delivery using newer approaches are generating new research avenues, whereas traditional approaches have not flourished. Finally, I describe significant advances in understanding of 'slow-to-progress' labours, achieved using physiological and clinical approaches. Advances in molecular, genetic and 'omic' technologies are fuelling the thirst for better understanding of the uterus and application of this information to problems in pregnancy and labour. Progress has, however, been limited while we still have an incomplete understanding of some of the basic physiology of uterine smooth muscle (myometrium). In this review and opinion piece, I explore some of the fascinating findings from selected recent studies and see how these may provide new avenues for physiological and clinical research. It is also the case, however, that there is still limited mechanistic understanding about physiological and pathophysiological processes in the myometrium. This lack of understanding limits the usefulness of some findings from genomic and allied studies. By focusing on some key recent findings and relating these to two important clinical problems in childbirth that involve myometrial activity, namely preterm delivery and difficult labours, the interplay between our physiological knowledge and the information provided by newer technologies is explored. My opinion is that physiology has provided much more new mechanistic insight into difficult births and that the newer technologies may lead to breakthroughs in preterm birth research, but that this has not yet happened.

Physiological increases in lactate inhibit intracellular calcium transients, acidify myocytes and decrease force in term pregnant rat myometrium

Key points

Lactate is increased in myometrial capillary blood from women in slow or non‐progressive labour (dystocia), suggesting that it is detrimental to uterine contractions.

There are no studies of the effect of lactate on the myometrium. In the present study, we have investigated its effects and mechanism of action on myometrial strips from term pregnant rats.

We show that lactate significantly decreased spontaneous contractility. Lactatedecreased pH_i and inhibited Ca²⁺ transients. Nulling the decrease in pH_i abolished the effects of lactate effects. If Ca²⁺ entry was enhanced, the effects of lactate were abolished.

The present study suggests that the accumulation of extracellular lactate could contribute to labour dystocia.

Abstract

Lactate is increased in myometrial capillary blood from women in slow or non‐progressive labour (dystocia), suggesting that it is detrimental to uterine contractions. There are, however, no studies of the effect of lactate on the myometrium. We therefore investigated its effects and mechanism of action on myometrial strips from term pregnant rats. The effects on spontaneous and oxytocin‐induced contractility in response to sodium lactate and other weak acids (1–20 mM) were studied. In some experiments, simultaneous force and intracellular Ca²⁺ or pH (pH_i) were measured with Indo‐1 or Carboxy‐SNARF, respectively. Statistical differences were tested using non‐parametric tests. Lactate significantly decreased spontaneous contractility with an EC₅₀ of 3.9 mM. Propionate, butyrate and pyruvate also reduced contractions with similar potency. The effects of lactate were reduced in the presence of oxytocin but remained significant. Lactate decreased pH_i and nulling the decrease in pH_i abolished its effects. We also show that lactate inhibited Ca²⁺ transients, with these changes mirroring those produced on force. If Ca²⁺ entry was enhanced by depolarization (high KCl) or applying the Ca²⁺ channel agonist, Bay K 4644, the effects of lactate were abolished. Taken together, these data show that lactate in the physiological range potently decreases myometrial contractility as a result of its inhibition of Ca²⁺ transients, which can be attributed to the induced acidification. The present study suggests that the accumulation of extracellular lactate will reduce myometrial contractions and could therefore contribute to labour dystocia.

Hypoxia-induced force increase (HIFI) is a novel mechanism underlying the strengthening of labor contractions, produced by hypoxic stresses

For successful birth, contractions need to become progressively stronger. The underlying mechanisms are unknown, however. We have found that a novel mechanism, hypoxia-induced force increase (HIFI), is switched on selectively, at term, and is essential to strengthening contractions. HIFI is initiated as contractions cyclically reduce blood flow and produce repeated hypoxic stresses, with associated metabolic and transcriptomic changes. The increases in contractility are a long-lasting, oxytocin-independent, intrinsic mechanism present only in the full-term pregnant uterus. HIFI is inhibited by adenosine receptor antagonism and blockade of cyclooxygenase-2 signaling, and partially reproduced by brief episodes of acidic (but not alkalotic) pH. HIFI explains how labor can progress despite paradoxical metabolic challenge, and provides a new mechanistic target for the 1 in 10 women suffering dysfunctional labor because of poor contractions.

The importance of complete tissue homogenization for accurate stoichiometric measurement of myosin light chain phosphorylation in airway smooth muscle

The standard method for measuring the phosphorylation of the regulatory myosin light chain (MLC20) in smooth muscle is extraction of the light chain using a urea extraction buffer, urea-glycerol gel electrophoresis of the soluble portion of the extract (supernatant) and Western blot analysis. The undissolved portion of the tissue during extraction (the pellet) is usually discarded. Because the pellet contains a finite amount of MLC20, omission of the pellet could result in inaccurate measurement of MLC20 phosphorylation. In this study we compared the level of tracheal smooth muscle MLC20 phosphorylation in the supernatant alone, with that in the complete tissue homogenate (supernatant and pellet) using the standard method. The supernatant fraction showed the well-known double bands representing phosphorylated and un-phosphorylated MLC20. The dissolved pellet fraction showed varying amounts of un-phosphorylated and phosphorylated MLC20. There was a small but statistically significant overestimation of the percent MLC20 phosphorylation if the pellet was not taken into consideration. The overestimation was 7% ± 2% (mean ± SEM) (p < 0.05) in unstimulated muscle and 2% ± 1% (p < 0.05) in acetylcholine (10(-6) mol/L) stimulated muscle. This finding suggests that for accurate estimation of the stoichiometry of MLC20 phosphorylation it is necessary to consider the contribution from the pellet portion of the muscle tissue homogenate.

Store-operated Ca²⁺ entry and depolarization explain the anomalous behaviour of myometrial SR: effects of SERCA inhibition on electrical activity, Ca²⁺ and force

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SERCA pump inhibition by CPA caused membrane depolarization, activation of action potentials, Ca²⁺ spikes and force.

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Depletion of Ca²⁺ store by agonists leads to membrane depolarization and activation of electrical and mechanical activity.

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Ca²⁺ release/Ca²⁺ entry coupling is playing a key role in control of spontaneous electrical and mechanical activity in rat pregnant myometrium.

In the myometrium SR Ca²⁺ depletion promotes an increase in force but unlike several other smooth muscles, there is no Ca²⁺ sparks-STOCs coupling mechanism to explain this. Given the importance of the control of contractility for successful parturition, we have examined, in pregnant rat myometrium, the effects of SR Ca²⁺-ATPase (SERCA) inhibition on the temporal relationship between action potentials, Ca²⁺ transients and force. Simultaneous recording of electrical activity, calcium and force showed that SERCA inhibition, by cyclopiazonic acid (CPA 20 μM), caused time-dependent changes in excitability, most noticeably depolarization and elevations of baseline [Ca²⁺]_i and force. At the onset of these changes there was a prolongation of the bursts of action potentials and a corresponding series of Ca²⁺ spikes, which increased the amplitude and duration of contractions. As the rise of baseline Ca²⁺ and depolarization continued a point was reached when electrical and Ca²⁺ spikes and phasic contractions ceased, and a maintained, tonic force and Ca²⁺ was produced. Lanthanum, a non-selective blocker of store-operated Ca²⁺ entry, but not the L-type Ca²⁺ channel blocker nifedipine (1–10 μM), could abolish the maintained force and calcium. Application of the agonist, carbachol, produced similar effects to CPA, i.e. depolarization, elevation of force and calcium. A brief, high concentration of carbachol, to cause SR Ca²⁺ depletion without eliciting receptor-operated channel opening, also produced these results. The data obtained suggest that in pregnant rats SR Ca²⁺ release is coupled to marked Ca²⁺ entry, via store operated Ca²⁺ channels, leading to depolarization and enhanced electrical and mechanical activity.

A comparison of the contractile properties of myometrium from singleton and twin pregnancies

Objective

Over half of twin pregnancies in US and UK deliver prematurely but the reasons for this are unclear. The contractility of myometrium from twin pregnancies has not been directly investigated. The objective of this research was to determine if there are differences in the contractile activity and response to oxytocin, between myometrium from singleton and twin pregnancies, across a range of gestational ages. Furthermore, we wished to determine if contractile activity correlates with increasing level of stretch, using neonatal birth weights as a marker of uterine stretch.

Methods

This was an in vitro, laboratory based study of myometrial contractility in women pregnant with one or two babies, using biopsies obtained from non-labouring women undergoing Caesarean section. Spontaneous, oxytocin-stimulated and depolarization induced contractile activity was compared.

Results

Direct measurements of myometrial contractility under controlled conditions show that the frequency of contractions and responses to oxytocin are significantly increased in twins compared to singletons. The duration of contraction however was significantly reduced. We find that contractile activity correlates with increasing levels of stretch, using neonatal birth weights as a surrogate for uterine stretch, with response to oxytocin being significantly positively correlated with birth weight.

Conclusions

We have found significant differences in contractile properties between myometrium from singleton and twin pregnancies and that increasing uterine stretch can alter the contractile properties of myometrium. We discuss the implication of these findings to preterm delivery and future studies.

Isolated small uterine arteries from non-pregnant and term pregnant rats exhibit regulatory responses to elevations in extravascular pressure

An adequate blood supply is essential for the maintenance of uterine function and fetal health during parturition. However, labouring uterine contractions will impart compressive forces on small uterine arteries (SUA). We demonstrate that isolated, pressurised rat SUA arteries, pre-constricted with arginine vasopressin or high potassium solution, exhibit regulatory responses to elevations in extravascular pressure (EVP) which maintain internal diameter constant at EVPs of 0-40 mm Hg. This response is endothelium independent and is not modulated by pregnancy. No regulation was observed in calcium free solution. SUA myogenic responses to elevated EVP likely represents a mechanism for limiting reductions in uterine blood flow during uterine contraction.

A new slow releasing, H₂S generating compound, GYY4137 relaxes spontaneous and oxytocin-stimulated contractions of human and rat pregnant myometrium

Better tocolytics are required to help prevent preterm labour. The gaseotransmitter Hydrogen sulphide (H₂S) has been shown to reduce myometrial contractility and thus is of potential interest. However previous studies used NaHS, which is toxic and releases H₂S as a non-physiological bolus and thus alternative H₂S donors are sought. GYY4137 has been developed to slowly release H₂S and hence better reflect endogenous physiological release. We have examined its effects on spontaneous and oxytocin-stimulated contractility and compared them to NaHS, in human and rat myometrium, throughout gestation. The effects on contractility in response to GYY4137 (1 nM–1 mM) and NaHS (1 mM) were examined on myometrial strips from, biopsies of women undergoing elective caesarean section or hysterectomy, and from non-pregnant, 14, 18, 22 day (term) gestation or labouring rats. In pregnant rat and human myometrium dose-dependent and significant decreases in spontaneous contractions were seen with increasing concentrations of GYY4137, which also reduced underlying Ca transients. GYY4137 and NaHS significantly reduced oxytocin-stimulated and high-K depolarised contractions as well as spontaneous activity. Their inhibitory effects increased as gestation advanced, but were abruptly reversed in labour. Glibenclamide, an inhibitor of ATP-sensitive potassium (K_ATP) channels, abolished the inhibitory effect of GYY4137. These data suggest (i) H₂S contributes to uterine quiescence from mid-gestation until labor, (ii) that H₂S affects L-type calcium channels and K_ATP channels reducing Ca entry and thereby myometrial contractions, (iii) add to the evidence that H₂S plays a physiological role in relaxing myometrium, and thus (iv) H₂S is an attractive target for therapeutic manipulation of human myometrial contractility.

What do we know about what happens to myometrial function as women age?

Much has been written about the effects of aging on reproductive function, especially female fertility. Much less is known about how aging may affect the contractility of the smooth muscle within the uterus, the myometrium. The myometrium is active through a woman’s entire life, not just during pregnancy. Here we will discuss briefly the contractile functions of the uterus and the changes it undergoes throughout the stages of a woman’s life from menstruation and the menopause, before evaluating the evidence for any changes in myometrial contractility and responses as women age, with a particular focus on women of advanced maternal age. We present original contractility analysis for the widest data set for human myometrium so far examined, and determine inherent spontaneous activity as well as responses to depolarisation and stimulation with oxytocin. Our data show that in the non-pregnant state there is a significant decrease in contractility for both spontaneous and depolarised-induced contractions, with age. We suggest that muscle atrophy and down regulation of Ca channels may account for this. Interestingly in pregnant myometrium we found a wide range of contractile ability between women and little evidence for decreased spontaneous activity between the ages of 25–40. Oxytocin responses appear to be more affected by aging, a finding that is consistent with previously reported clinical findings, and may partly be the result of membrane lipids such as cholesterol, increasing as women age. The marked differences between the age-related decline of force beyond age 30 in non-pregnant uterus, and the lack of difference in the pregnant state over this period, shows that the uterus retains its ability to respond to gestational hormones. The growth of the pregnant uterus and increase in content of myofibrillar proteins, may abolish any previous age-related force deficit. This finding is consistent with what is apparent for postmenopausal women in their 50s and 60s; that with the appropriate hormonal stimulation the uterus can allow an embryo to implant, and then without further intervention, carry the foetus to term. It is tempting therefore to speculate that unlike other well documented declines in female reproductive functions with age, the myometrium remains able to function into a woman’s 7th decade.

Poor spontaneous and oxytocin-stimulated contractility in human myometrium from postdates pregnancies

Prolongation of pregnancy i.e. going more than 10 days over the estimated due date, complicates up to 10% of all pregnancies and is associated with increased risk to both mother and fetus. Despite the obvious need for contractions of the uterus to end pregnancy, there have been no studies directly examining the role of uterine smooth muscle, myometrium, in the aetiology of prolonged pregnancy. This study tested the hypothesis that the intrinsic contractile characteristics of myometrium taken from women with prolonged pregnancy (>41 weeks and 3 days) was reduced compared to those delivering at term (39–41 weeks). We recruited women undergoing Caesarean Section (CS) delivery either pre-labour (n = 27) or in labour (n = 66) at term or postdates. The contractile ability of the postdates myometrium, whether spontaneous or elicited by oxytocin or high-K solution, was significantly reduced compared to term myometrium. These differences remained when adjusted for parity and other maternal characteristics. The findings remained significant when expressed per cross sectional area. Histological examination revealed no differences between the two groups. The contractile differences were however related to intracellular Ca transients suggesting an effect of [Ca] on reduced force production in the postdates group. In summary, myometrium from prolonged pregnancies contracts poorly in vitro even when stimulated with oxytocin and in active labour. Responses to high K⁺ and measurements of Ca suggest that alterations in excitation contraction coupling, rather than any histological changes of the myometrium, may underlie the differences between term and postdates myometrium. We show that postdates pregnancy is associated with poor myometrial activity and suggest that this may contribute to increased myometrial quiescence and hence, prolonged gestation.

Mitochondrial regulation of cytosolic Ca²⁺ signals in smooth muscle

The cytosolic Ca²⁺ concentration ([Ca²⁺]c) controls virtually every activity of smooth muscle, including contraction, migration, transcription, division and apoptosis. These processes may be activated by large (>10 μM) amplitude [Ca²⁺]c increases, which occur in small restricted regions of the cell or by smaller (<1 μM) amplitude changes throughout the bulk cytoplasm. Mitochondria contribute to the regulation of these signals by taking up Ca²⁺. However, mitochondria's reported low affinity for Ca²⁺ is thought to require the organelle to be positioned close to ion channels and within a microdomain of high [Ca²⁺]. In cultured smooth muscle, mitochondria are highly dynamic structures but in native smooth muscle mitochondria are immobile, apparently strategically positioned organelles that regulate the upstroke and amplitude of IP₃-evoked Ca²⁺ signals and IP₃ receptor (IP₃R) cluster activity. These observations suggest mitochondria are positioned within the high [Ca²⁺] microdomain arising from an IP₃R cluster to exert significant local control of channel activity. On the other hand, neither the upstroke nor amplitude of voltage-dependent Ca²⁺ entry is modulated by mitochondria; rather, it is the declining phase of the transient that is regulated by the organelle. Control of the declining phase of the transient requires a high mitochondrial affinity for Ca²⁺ to enable uptake to occur over the normal physiological Ca²⁺ range (<1 μM). Thus, in smooth muscle, mitochondria regulate Ca²⁺ signals exerting effects over a large range of [Ca²⁺] (∼200 nM to at least tens of micromolar) to provide a wide dynamic range in the control of Ca²⁺ signals.

Diabetes is associated with impairment of uterine contractility and high Caesarean section rate

AIMS/HYPOTHESIS

The prevalence of births worldwide complicated by diabetes mellitus is increasing. In the UK, for example, <25% of diabetic women have a non-instrumental vaginal delivery. Strikingly, more than half the Caesarean sections (CS) in these patients are non-elective, but the reasons for this are not understood. We have tested the hypothesis that poor myometrial contractility as a consequence of the disease contributes to this high CS rate.

METHODS

We compared spontaneous, high K depolarisation and oxytocin-induced contractions from diabetic and matched control patients having an elective CS. To investigate the mechanism of any differences we measured intracellular Ca, and performed western blotting and compared the tissues histologically.

RESULTS

There was significantly decreased contraction amplitude and duration in uteri from diabetic compared with control patients, even when possible confounders such as BMI were analysed. Reduced intracellular calcium signals and expression of calcium entry channels were found in uteruses from diabetic patients, which, along with a reduction in muscle content found on histological examination, could explain the reduced force. Myometrium from diabetic patients was responsive to oxytocin, but still did not reach the levels found in non-diabetic patients.

CONCLUSIONS/INTERPRETATIONS

These are the first data investigating myometrium in diabetic patients and they support the hypothesis that there is poorer contractility even in the presence of oxytocin. The underlying mechanism is related to reduced Ca channel expression and intracellular calcium signals and a decrease in muscle mass. We conclude that these factors significantly contribute to the increased emergency CS rate in diabetic patients.

Distribution, expression and functional effects of small conductance Ca-activated potassium (SK) channels in rat myometrium

Calcium-activated potassium channels are important in a variety of smooth muscles, contributing to excitability and contractility. In the myometrium previous work has focussed on the large conductance channels (BK), and the role of small conductance channels (SK) has received scant attention, despite the finding that over-expression of an SK channel isoform (SK3) results in uterine dysfunction and delayed parturition. This study therefore characterises the expression of the three SK channel isoforms (SK1-3) in rat myometrium throughout pregnancy and investigates their effect on cytosolic [Ca] and force and compares this with that of BK channels. Consistent expression of all SK isoform transcripts and clear immunostaining of SK1-3 was found. Inhibition of SK1-3 channels (apamin, scyllatoxin) significantly inhibited outward current, caused membrane depolarisation and elicited action potentials in previously quiescent cells. Apamin or scyllatoxin increased the amplitude of [Ca] and force in spontaneously contracting myometrial strips throughout gestation. The functional effect of SK inhibition was larger than that of BK channel inhibition. Thus we show for the first time that SK1-3 channels are expressed and translated throughout pregnancy and contribute to outward current, regulate membrane potential and hence Ca signals in pregnant rat myometrium. They contribute more to quiescence that BK channels.

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.

Sex hormones and excitation-contraction coupling in the uterus: the effects of oestrous and hormones

In this review, we examine how far the increased understanding that we have of the events in excitation contraction can explain the effects of the oestrous cycle and sex hormones on uterine function. Observational studies of electrical and mechanical activity in the rat myometrium have shown a relative quiescence during pro-oestrous, with little propagation of any electrical events. Thus, uterine activity can be said to approximately inversely reflect plasma 17beta-oestradiol concentrations. We show that Ca(2+) signalling and mechanical activity are greatest in metoestrous and dioestrous compared to pro-oestrous and oestrous. These data are discussed in terms of hormonal effects on Ca(2+) and K(+) channels. Finally, the influence of sex hormones on lipid rafts and caveolae are considered and discussed in relation to recent findings on their role in uterine signalling and contractility, and cholesterol levels and obesity.

Contractility and calcium signaling of human myometrium are profoundly affected by cholesterol manipulation: implications for labor?

The authors elucidate cholesterol's effect on human uterine contractility and calcium signaling to test the hypotheses that elevation of cholesterol decreases uterine activity and that oxytocin cannot augment contraction when cholesterol is elevated. The effects of cholesterol extraction with methyl beta-cyclodextrin and enrichment with low-density lipoproteins and cholesterol on contractile activity and intracellular calcium signaling in spontaneous or oxytocin-stimulated myometrium are determined. Force occurring spontaneously and with oxytocin is significantly increased by cholesterol extraction. Cholesterol enrichment profoundly inhibits force production in a dose-dependent manner and could reverse the effects of cholesterol extraction. Qualitatively similar results are found for nonpregnant and pregnant laboring and non-laboring myometrium. These contractile changes are related to changes in intracellular Ca2+ . Thus, elevated cholesterol is deleterious to contractility and Ca2+ signaling in human myometrium. Cholesterol may contribute to uterine quiescence but could cause difficulties in labor in obese/dyslipidemic women, consistent with their increased cesarean delivery rates.

Functional and molecular characterization of voltage-gated sodium channels in uteri from nonpregnant rats

We investigated the function and expression of voltage-gated Na(+) channels (VGSC) in the uteri of nonpregnant rats using organ bath techniques, intracellular [Ca(2+)] fluorescence measurements, and RT-PCR. In longitudinally arranged whole-tissue uterine strips, veratridine, a VGSC activator, caused the rapid appearance of phasic contractions of irregular frequency and amplitude. After 50-60 min in the continuous presence of veratridine, rhythmic contractions of very regular frequency and slightly increasing amplitude occurred and were sustained for up to 12 h. Both the early and late components of the contractile response to veratridine were inhibited in a concentration-dependent manner by tetrodotoxin (TTX). In small strips dissected from the uterine longitudinal smooth muscle layer and loaded with Fura-2, veratridine also caused rhythmic contractions, accompanied by transient increases in [Ca(2+)](i), which were abolished by treatment with 0.1 microM TTX. Using end-point and real-time quantitative RT-PCR, we detected the presence of the VGSC alpha subunits Scn2a1, Scn3a, Scn5a, and Scn8a in the cDNA from longitudinal muscle. The mRNAs of the auxiliary beta subunits Scbn1b, Scbn2b, Scbn4b, and traces of Scn3b were also present. These data show for the first time that Scn2a1, Scn3a, Scn5a, and Scn8a, as well as all VGSC beta subunits are expressed in the longitudinal smooth muscle layer of the rat myometrium. In addition, our data show that TTX-sensitive VGSC are able to mediate phasic contractions maintained over long periods of time in the uteri of nonpregnant rats.

Regulation of the uterine contractile apparatus and cytoskeleton

Parturition at term, the end stage of a successful pregnancy, occurs as a result of powerful, co-ordinated and periodic contractions of uterine smooth muscle (myometrium). To occur in a propitious manner, a high degree of control over the activation of a myometrial cell is required. We review the molecular mechanisms and structural composition of myometrial cells that may contribute to their increased contractile capacity at term. We focus attention on pathways that lead to the activation of filamentous networks traditionally labeled 'contractile' or 'cytoskeletal' yet draw attention to the fact that functional discrimination between these systems is not absolute.

Depletion of membrane cholesterol eliminates the Ca2+-activated component of outward potassium current and decreases membrane capacitance in rat uterine myocytes

Changes in membrane cholesterol content have potent effects on cell signalling and contractility in rat myometrium and other smooth muscles. We have previously shown that depletion of cholesterol with methyl-beta-cyclodextrin (MCD) disrupts caveolar microdomains. The aim of this work was to determine the mechanism underlying the increase in Ca(2+) signalling and contractility occurring in the myometrium with MCD. Patch clamp data obtained on freshly isolated myocytes from the uterus of day 19-21 rats showed that outward K(+) current was significantly reduced by MCD. Membrane capacitance was also reduced. Cholesterol-saturated MCD had no effect on the amplitude of outward current suggesting that the reduction in the outward current was due to cholesterol depletion induced by MCD rather than a direct inhibitory action of MCD on the K(+) channels. Confocal visualization of the membrane bound indicator Calcium Green C18, revealed internalization of the surface membrane with MCD treatment. Large conductance, Ca(2+)-sensitive K(+) channel proteins have been shown to localize to caveolae. When these channels were blocked by iberiotoxin outward current was significantly reduced in the uterine myocytes; MCD treatment reduced the density of outward current. Following reduction of outward current by MCD pretreatment, iberiotoxin was unable to produce any additional decrease in the current, suggesting a common target. MCD treatment also increased the amplitude and frequency of spontaneous rises in cytosolic Ca(2+) level ([Ca(2+)](i) transients) in isolated myocytes. In intact rat myometrium, MCD treatment increased Ca(2+) signalling and contractility, consistent with previous findings, and this effect was also found to be reduced by BK channel inhibition. These data suggest that (1) disruption of cholesterol-rich microdomains and caveolae by MCD leads to a decrease in the BK channel current thus increasing cell excitability, and (2) the changes in membrane excitability produced by MCD underlie the changes found in Ca(2+) signalling and uterine contractility.

In Situ Calcium Signaling: No Calcium Sparks Detected in Rat Myometrium

Controlled uterine smooth muscle activity is essential for our reproductive health. While we understand reasonably well the steps that produce contraction following a rise in intracellular [Ca], the mechanism controlling excitability and thus the rise of Ca, is less well understood. Here we examine the role of the internal Ca sore, the sarcoplasmic reticulum (SR), and its relation to surface membrane ion channels. We show that despite having a well-developed SR, the rat uterus does not produce the elemental and local Ca signals, known as Ca sparks. This in turn has consequences for excitability, as the negative feedback loop between these Ca signals and Ca-activated K (BK) channels on the surface membrane is lost. This may be important for producing the powerful long-lasting contractions of the uterus required during labor.

Studies on cardio-suppressant, vasodilator and tracheal relaxant effects ofSarcococca saligna

Sarcococca saligna is a shrub that is traditionally used for its medicinal properties in Pakistan. In this study we report the cardio-suppressant, vasodilator and tracheal relaxant activities of the aqueous-methanolic extract (Ss.Cr) of the plant. Ss.Cr, that tested positive for the presence of saponins, flavonoids, tannins, phenols, and alkaloids, exhibited a dose-dependent (0.3-5 mg/mL) negative inotropic and chronotropic effect on the isolated guinea-pig atrium which was resistant to atropine (1 microM) and aminophylline (10 microM) pretreatment. In rabbit thoracic aorta, Ss.Cr dose-dependently (0.1-3 mg/mL) relaxed the high K+ (80 mM) and phenylephrine (PE, 1 microM)-induced contractions, indicating a possible Ca++ channel blocking (CCB) effect. When tested against PE (1 microM) control peaks in normal Ca++ and Ca++-free Kreb's solution, Ss.Cr exhibited dose-dependent (0.1-3 mg/mL) inhibition, being more potent in relaxing the PE responses in Ca++-free Kreb's solution, thus indicating specific blockade of Ca++ release from the intracellular stores. Ss.Cr also relaxed the agonist-induced contractions in: a) rat aorta irrespective of the presence of endothelium or nitric oxide synthase inhibitor L-NAME and b) rabbit and guinea-pig tracheal strips. The data shows that Ss.Cr possesses possible Ca++ channel blocking activity which might be responsible for its observed cardio-suppressant, vasodilator and tracheal relaxant effects though more tests are required to confirm this Ca++ channel blocking effect.

Hypoxic inhibition of human myometrial contractions in vitro: implications for the regulation of parturition

BACKGROUND

Insufficient tissue oxygenation is a likely contribution to weak, inco-ordinate human uterine contractile activity characteristic of prolonged, dysfunctional labour. However, the direct effects of hypoxia on human myometrial contractility has, surprisingly, not yet been detailed. Therefore, we report the influence of hypoxia on spontaneous and agonist-induced carbachol, prostaglandin (PGF2alpha), and oxytocin contractions of myometria from nonpregnant and pregnant women.

MATERIALS AND METHODS

Uterine biopsies were obtained from pregnant women at term undergoing elective Caesarean section and nonpregnant women undergoing hysterectomy. Myometrial strips were equilibrated at 37 degrees C in normoxic physiological salt solution (95% air/5% CO(2)) and the influence of hypoxia (95% N(2)/5% CO(2)) on contractility was investigated.

RESULTS

Hypoxia resulted in a significant reduction in spontaneous contractile function; nonpregnant tissue was less resistant to the deleterious effects of hypoxia. Agonist-induced contractions, while being more resistant to hypoxia than spontaneous contractions, were also significantly inhibited. In myometria of pregnant women the PGF2alpha- or oxytocin-induced contractility was more resistant to hypoxia than carbachol. Finally, the inhibitory actions of hypoxia were exacerbated with repeated oxytocin administration with a more severe effect on contractile integral than on initial phasic contraction amplitude.

CONCLUSIONS

We detail, for the first time, the effects of hypoxia on contractility of human myometria from nonpregnant and pregnant women. Physiologically important uterotonic agents are more resistant to the effects of hypoxia than spontaneous contractions although repeated stimulation with oxytocin during hypoxia results in progressively less force. The results indicate that if significant hypoxia occurs in vivo then it is a likely contributory factor to the pathways underlying prolonged dysfunctional labour.

HSP20 phosphorylation and interstitial metabolites in hypoxia-induced dilation of swine coronary arteries

OBJECTIVE

Hypoxia induces coronary artery dilation, but the responsible mechanism is largely unknown. Many stimuli induce arterial smooth muscle relaxation by reducing ser19-myosin regulatory light chain (MLC) phosphorylation. Other stimuli can induce smooth muscle relaxation without reductions in ser19-MLC phosphorylation. This form of relaxation has been termed force suppression and appears to be associated with heat shock protein 20 (HSP20) phosphorylation on ser16. We investigated whether hypoxia-induced sustained dilation in swine coronary arteries was promoted without ser19-MLC dephosphorylation and associated with ser16-HSP20 phosphorylation. Nitroglycerin vasodilation served as control.

METHODS

In a pressure myograph, the tunica media of intact pre-contracted (PGF(2alpha); 10(-5) m) porcine coronary artery segments were cannulated using a microdialysis catheter. Diameter responses and interstitial lactate/pyruvate ratios were studied during 90 min hypoxia, hypoxia + reoxygenation (60 min), nitroglycerin (100 microm, 90 min), and nitroglycerin + wash-out (60 min). The arterial segments were snap-frozen and analysed for ser16-HSP20 phosphorylation and ser19-MLC phosphorylation.

RESULTS

The normalized diameter responses to hypoxia (6.1 +/- 4.3%) and nitroglycerin (12.6 +/- 1.6%) were both significantly greater than normoxic control arteries (-10.5 +/- 1.8%, anova, P < 0.05). Ser16-HSP20 phosphorylation was increased with hypoxia and nitroglycerin treatment and ser16-HSP20 phosphorylation correlated with changes in diameters (n = 29, r2 = 0.64, P < 0.001). Ser19-MLC phosphorylation was not significantly altered by hypoxia. The lactate/pyruvate ratio was significantly increased in hypoxic arteries but did not correlate with diameters or ser16-HSP20 phosphorylation.

CONCLUSION

Ser16-HSP20 phosphorylation is a potential regulator of hypoxia-induced dilation in coronary arteries.

Gestational-dependent changes in the expression of signal transduction and contractile filament-associated proteins in mouse myometrium

OBJECTIVES

The timely onset of powerful uterine contractions during parturition involves the integration of many signaling pathways, the protein components of which may be translationaly regulated throughout gestation. We have utilized the pregnant mouse model to examine gestational-dependent changes in the expression of several proteins implicated in the coordination of myometrial excitation-contraction coupling: caveolins 1-3, rho-associated kinases (ROKalpha and beta), alpha-actin, MLC(20), and h-caldesmon.

METHODS

Protein expression was examined by Western blotting of myometrial homogenates from nonpregnant mice (NP) and compared to that from pregnant mice on gestational days 12, 15, 17, and 19 (term = day 19).

RESULTS

All protein expressions were unchanged during the estrous cycle. alpha-Actin was found to be invariant throughout pregnancy. The expressions of caveolin-1, -2 and -3, when compared to alpha-actin expression, also did not change significantly with mid- to late pregnancy. h-caldesmon: alpha-actin, ROKalpha:alpha-actin, and ROKbeta:alpha-actin ratios were, however, significantly elevated on day 19, whereas MLC(20):alpha-actin was significantly down-regulated on day 12 to day 19. Consistent with elevated ROK expression at term, the ROK inhibitor Y27632 gave a greater reduction of thromboxane-stimulated contractions in myometrium from day 19 mice compared to NP mice.

CONCLUSIONS

These data suggest that in mouse myometrium there is a dynamic regulation of the expression of several proteins implicated in contractile signal integration. This may be important for regulating (1) relative uterine quiescence to ensure pregnancy progression and (2) priming the tissue for requisite contractile effort at term.

Increased cholesterol decreases uterine activity: functional effects of cholesterol alteration in pregnant rat myometrium

Uterine quiescence is essential for successful pregnancy. Cholesterol and triglycerides are markedly increased in pregnancy. Cholesterol is enriched in microdomains of the plasma membrane known as rafts and caveolae. Both lipid rafts and caveolae have been implicated in cellular signaling cascades. The purpose of this work was to investigate whether manipulation of cholesterol content alters uterine contractility. Late pregnancy (19–21 days) rats were humanely euthanized and strips of longitudinal myometrium were then dissected. Force and Ca2+ measurements were simultaneously recorded and cholesterol increased by the addition of 5 mg/ml cholesterol or 0.25 mg/ml low-density lipoproteins (LDLs) or reduced by 2% methyl-β-cyclodextrin (MCD) or 2 U/ml cholesterol oxidase addition to the perfusate. Both LDLs and cholesterol profoundly inhibited spontaneous uterine force production and associated Ca2+ transients; frequency, amplitude, and duration of contraction were all significantly reduced compared with preceding control contractions. Force and Ca2+ were also reduced by cholesterol when 1 nM oxytocin was used to stimulate the myometrium. Uterine activity was significantly increased by cholesterol extraction with MCD or cholesterol oxidase treatment. Electron microscopy confirmed the lipid raft disrupting effect of MCD, as formerly electron microscopy-visible caveolae in the myometrial cell membrane all but disappeared after MCD treatment. These data show that uterine smooth muscle cell cholesterol content is critically important for functional activity. A novel finding of our study is that cholesterol is inhibitory for force generation. It may be one of the mechanisms operating to maintain uterine quiescence throughout gestation and may also contribute to difficulties in labor suffered by obese women.

Ginger lowers blood pressure through blockade of voltage-dependent calcium channels

Ginger (Zingiber officinale Roscoe), a well-known spice plant, has been used traditionally in a wide variety of ailments including hypertension. We report here the cardiovascular effects of ginger under controlled experimental conditions. The crude extract of ginger (Zo.Cr) induced a dose-dependent (0.3-3 mg/kg) fall in the arterial blood pressure of anesthetized rats. In guinea pig paired atria, Zo.Cr exhibited a cardiodepressant activity on the rate and force of spontaneous contractions. In rabbit thoracic aorta preparation, Zo.Cr relaxed the phenylephrine-induced vascular contraction at a dose 10 times higher than that required against K (80 mM)-induced contraction. Ca2+ channel-blocking (CCB) activity was confirmed when Zo.Cr shifted the Ca2+ dose-response curves to the right similar to the effect of verapamil. It also inhibited the phenylephrine (1 microM) control peaks in normal-Ca2+ and Ca2+-free solution, indicating that it acts at both the membrane-bound and the intracellular Ca2+ channels. When tested in endothelium-intact rat aorta, it again relaxed the K-induced contraction at a dose 14 times less than that required for relaxing the PE-induced contraction. The vasodilator effect of Zo.Cr was endothelium-independent because it was not blocked by L-NAME (0.1 mM) or atropine (1 microM) and also was reproduced in the endothelium-denuded preparations at the same dose range. These data indicate that the blood pressure-lowering effect of ginger is mediated through blockade of voltage-dependent calcium channels.

Expression of scaffolding, signalling and contractile-filament proteins in human myometria: effects of pregnancy and labour

Successful parturition requires the co-ordination of numerous myometrial signalling events to allow for timely and efficient uterine contractions. Late pregnancy and labour onset in humans may be associated with changes in the expression of myometrial proteins implicated in such uterine contractile signal integration. Accordingly, in myometria from non-pregnant women and pregnant women, not in labour or in labour, we examined the content of putative plasmalemmal scaffolding proteins (caveolin-1 and -2) and compared these to the proportions of signal transducing rho-associated kinases (ROKalpha and beta) and contractile filament-associated proteins alpha-actin, myosin regulatory light chain (MLC(20)) and h-caldesmon. There was no effect of pregnancy or labour on the proportion of caveolin, ROK betaor alpha-actin. However, pregnancy was associated with a decrease in ROKalpha and MLC(20) such that ROK alpha: alpha-actin and MLC(20): alpha-actin ratios were reduced compared to myometria of non-pregnant women. In contrast, h-caldesmon was up-regulated in pregnancy resulting in an elevated h-caldesmon: alpha-actin ratio. There were, however, no further significant changes in ROK alpha, MLC(20) or h-caldesmon expression with spontaneous or oxytocin-induced labour. These data suggest that the mechanism(s) integrating myometrial signalling events with the onset of human labour does not involve differential alterations of the cellular expressions of caveolins, ROK, alpha-actin, MLC(20) or h-caldesmon.

Mitochondrial modulation of Ca2+ sparks and transient KCa currents in smooth muscle cells of rat cerebral arteries

Mitochondria sequester and release calcium (Ca(2+)) and regulate intracellular Ca(2+) concentration ([Ca(2+)](i)) in eukaryotic cells. However, the regulation of different Ca(2+) signalling modalities by mitochondria in smooth muscle cells is poorly understood. Here, we investigated the regulation of Ca(2+) sparks, Ca(2+) waves and global [Ca(2+)](i) by mitochondria in cerebral artery smooth muscle cells. CCCP (a protonophore; 1 microm) and rotenone (an electron transport chain complex I inhibitor; 10 microm) depolarized mitochondria, reduced Ca(2+) spark and wave frequency, and elevated global [Ca(2+)](i) in smooth muscle cells of intact arteries. In voltage-clamped (-40 mV) cells, mitochondrial depolarization elevated global [Ca(2+)](i), reduced Ca(2+) spark amplitude, spatial spread and the effective coupling of sparks to large-conductance Ca(2+)-activated potassium (K(Ca)) channels, and decreased transient K(Ca) current frequency and amplitude. Inhibition of Ca(2+) sparks and transient K(Ca) currents by mitochondrial depolarization could not be explained by a decrease in intracellular ATP or a reduction in sarcoplasmic reticulum Ca(2+) load, and occurred in the presence of diltiazem, a voltage-dependent Ca(2+) channel blocker. Ru360 (10 microm), a mitochondrial Ca(2+) uptake blocker, and lonidamine (100 microm), a permeability transition pore (PTP) opener, inhibited transient K(Ca) currents similarly to mitochondrial depolarization. In contrast, CGP37157 (10 microm), a mitochondrial Na(+)-Ca(2+) exchange blocker, activated these events. The PTP blockers bongkrekic acid and cyclosporin A both reduced inhibition of transient K(Ca) currents by mitochondrial depolarization. These results indicate that mitochondrial depolarization leads to a voltage-independent elevation in global [Ca(2+)](i) and Ca(2+) spark and transient K(Ca) current inhibition. Data also suggest that mitochondrial depolarization inhibits Ca(2+) sparks and transient K(Ca) currents via PTP opening and a decrease in intramitochondrial [Ca(2+)].

Energy cost of contraction in rat urinary bladder smooth muscle during anoxia

1. The aim of the present study was to investigate the effects of hypoxia on energy metabolism and contraction of rat urinary bladder smooth muscle, thereby gaining insight into the capacity of this smooth muscle to maintain contractile function when rendered hypoxic. 2. Isometric force, oxygen consumption, lactate production, heat production and unloaded shortening velocity were measured in isolated muscle strips under both aerobic and anaerobic conditions. Muscle strips were bathed in physiological saline solution with the anaerobic condition being created by replacing the oxygen bubbling the solution with nitrogen. 3. During contraction under anaerobic conditions, the rate of lactate production was increased 2.5-fold above that observed under aerobic conditions. This, however, only provided for a rate of ATP production of approximately 30% of that measured under aerobic conditions. Despite this, force maintenance was only slightly depressed, indicating that the metabolic cost of contraction was reduced in hypoxia. In support of this, the rate of heat production during contractions in anoxia was only approximately half of that under aerobic conditions, whereas, again, force was only slightly lower. Unloaded shortening velocity was significantly lower in anoxia, suggesting a slower cross-bridge turnover rate. 4. The results indicate that the economy of force maintenance is increased in bladder smooth muscle under hypoxic conditions and that this is due, at least in part, to a reduced rate of cross-bridge cycling. This may help to preserve bladder contractile function during periods of ischaemia that may be associated with bladder filling and emptying.

Vasorelaxant effects of eugenol on rat thoracic aorta

Eugenol is a natural pungent substance and the main component of clove oil, with vasorelaxant action. To elucidate some of the possible mechanisms involved in this action isometric tension was measured in aortic rings from male Wistar rats precontracted with phenylephrine (PHE, 10(-7) M) or KCl (75 mM). Responses to increasing concentrations of eugenol (10(-6)-10(-2) M) were obtained in the presence and absence of endothelium. In the presence of eugenol, dose-response curves to PHE (10(-9) to 10(-4) M) and KCl (5-125 mM) were displaced downwards. Concentration-dependent relaxation was observed in rings precontracted with PHE (10(-7) M) and KCl (75 mM). The tension increment produced by increasing external calcium concentration (0.25-3 mM) was also reduced by eugenol (300 microM) treatment. The inhibitory effects of eugenol (300 microM) were compared to those induced by nifedipine (0.01 microM), a selective Ca(2+) channel blocker, producing similar relaxant effects. Two other protocols were performed. After precontraction with PHE (10(-7) M), increasing concentrations of eugenol (10(-6)-10(-2) M) were used before and after N(w)-nitro-L-arginine (L-NAME, 10(-4) M) and methylene blue (10(-5) M) treatment. Eugenol-induced relaxation was reduced by endothelial damage (rubbing), L-NAME and methylene blue treatments. Results suggested that eugenol produces smooth muscle relaxation resulting from the blockade of both voltage-sensitive and receptor-operated channels that are modulated by endothelial-generated nitric oxide.

Inhibition of rho-associated kinase reduces MLC20 phosphorylation and contractility of intact myometrium and attenuates agonist-induced Ca2+ sensitization of force of permeabilized rat myometrium

The role of rhoA/rho-associated kinase (ROK) signaling pathways in agonist-induced contraction of the rat myometrium was investigated. We measured the [Ca(2+)](i)-force relationship, phosphorylation of myosin regulatory light chains (MLC(20)) in intact tissue and the Ca(2+)-sensitization of force in permeabilized myometrial cells of rat. In measurements of the relationship between [Ca(2+)](i) and tension in intact tissue, Y-27632, a ROK inhibitor, significantly attenuated the carbachol-induced contraction without changing [Ca (2+)](i). Phosphorylation of MLC(20) was increased by carbachol and this increased phosphorylation was blocked by treatment of tissue with Y-27632. In tension measurements of single hyperpermeable cells, carbachol evoked sustained contraction at constant pCa 6.7 and these agonist-induced contractions were decreased by treatment with Y-27632. These results suggest that activation of a ROK-mediated signaling pathway(s) plays an important role in agonist-induced alterations in MLC(20) phosphorylation and force of rat myometrium.

The role of the sarcoplasmic reticulum in neonatal uterine smooth muscle: enhanced role compared to adult rat

Little is known about contractile activity, response to agonists or excitation-contraction coupling in neonatal smooth muscle. We have therefore investigated 10-day rat uterus to better understand these processes, and compared it to adult uterus to elucidate how control of contractility develops. Spontaneous contractions are present in the 10-day neonatal uterus, although they are not as large or as regular as those present in adult tissues. External Ca(2+) entry via L-type Ca(2+) channels is the sole source of Ca(2+) and is essential for the spontaneous activity. The neonatal uterus was responsive to carbachol or prostaglandin F(2alpha) application; it showed a marked stimulation and a clear dissociation between the force and Ca(2+) changes. Such sensitization was not apparent in adult rat myometrium. The sarcoplasmic reticulum (SR) had more releasable Ca(2+) and contributed more to the response to agonists in neonatal compared to adult tissues. Thus, Ca(2+) entry as opposed to SR Ca(2+) release contributed much less to the uterine response to agonists in the neonatal, compared to adult tissues. Inhibition of the SR by cyclopiazonic acid also caused a more vigorous increase in Ca(2+) and contractile activity, particularly frequency, in the neonatal compared to the adult uterus. Taken together these data suggest that: (1) spontaneous activity is already present by day 10, (2) receptor-coupling and excitation-contraction signalling pathways are functional, (3) the SR and Ca(2+) sensitization mechanisms play a more prominent role in the neonate, and (4) there is a shift to a greater reliance on Ca(2+) entry and excitability with development of the myometrium.

Influence of mitochondrial inhibition on global and local [Ca(2+)](I) in rat tail artery

Inhibition of oxidative metabolism is often found to decrease contractility of systemic vascular smooth muscle, but not to reduce global [Ca(2+)](i). In the present study, we probe the hypothesis that it is associated with an altered pattern of intracellular Ca(2+) oscillations (waves) influencing force development. In the rat tail artery, mitochondrial inhibitors (rotenone, antimycin A, and cyanide) reduced alpha(1)-adrenoceptor-stimulated force by 50% to 80%, but did not reduce global [Ca(2+)](i). Less relaxation (about 30%) was observed after inhibition of myosin phosphatase activity with calyculin A, suggesting that part of the metabolic sensitivity involves the regulation of myosin 20-kDa light chain phosphorylation, although no decrease in phosphorylation was found in freeze-clamped tissue. Confocal imaging revealed that the mitochondrial inhibitors increased the frequency but reduced the amplitude of asynchronous cellular Ca(2+) waves elicited by alpha(1) stimulation. The altered wave pattern, in association with increased basal [Ca(2+)](i), accounted for the unchanged global [Ca(2+)](i). Inhibition of glycolytic ATP production by arsenate caused similar effects on Ca(2+) waves and global [Ca(2+)](i), developing gradually in parallel with decreased contractility. Inhibition of wave activity by the InsP(3) receptor antagonist 2-APB correlated closely with relaxation. Furthermore, abolition of waves with thapsigargin in the presence of verapamil reduced force by about 50%, despite unaltered global [Ca(2+)](i), suggesting that contraction may at least partly depend on Ca(2+) wave activity. This study therefore indicates that mitochondrial inhibition influences Ca(2+) wave activity, possibly due to a close spatial relationship of mitochondria and the sarcoplasmic reticulum and that this contributes to metabolic vascular relaxation.

In vivo pH and metabolite changes during a single contraction in rat uterine smooth muscle

1. We have used 31P NMR spectroscopy to measure metabolites and pHi at three periods during a phasic contraction of the uterus, in vivo, to determine whether they change as a consequence of contraction. The regular uterine contractions were recorded via a balloon catheter in the uterine lumen. Each phasic contraction was divided into three parts: the period between contractions (rest), the development of force (up) and the relaxation of force (down). The NMR data were summed separately from each of these three periods over 20-40 successive contractions. 2. Significant changes in ATP, phosphocreatine (PCr) and inorganic phosphate (Pi) occurred during the contraction. [ATP] fell from 2.0 to 1.6 mM and [PCr] from 2.6 to 2.0 mM during the up period, while [Pi] increased from 2.2 to 2.8 mM. Recovery of ATP and PCr occurred during the relaxation part of the contraction, whereas Pi did not fully recover until the contraction was complete. 3. Significant acidification from pH 7.28 +/- 0.02 at rest to 7.16 +/- 0.02, occurred with contraction. This acidification is greater than that previously reported for in vitro uterine preparations. Measurements of uterine blood flow show that it decreased with contraction. Therefore, ischaemia, in addition to the metabolic consequences of contraction, may account for the larger acidification observed in vivo. 4. Lowering pHi in an in vitro uterine preparation by a similar level to that found in vivo produced a significant reduction of the phasic contractions. Thus we propose that these changes, especially the fall in pHi during force development, feed back negatively on the contraction to limit its strength, and may help prevent uterine ischaemia and fetal hypoxia during labour.

Mechanisms of hypoxic vasodilatation of isolated rat mesenteric arteries: a comparison with metabolic inhibition

1. Hypoxia (PO2 < 5 mmHg) decreased vessel tone in isolated rat mesenteric arteries precontracted with either high [K+] or the thromboxane analogue U46619. This response was not altered by N-nitro-L-arginine (L-NA) and indomethacin. 2. Simultaneous measurement of pHi and tension showed that the decrease in vessel tone was accompanied by an intracellular acidification. Similar reductions in tone and pHi were observed with the metabolic inhibitors 2,4-dinitrophenol (DNP) and sodium azide. 3. The presence of the lactate transport inhibitor alpha-cyano-4-hydroxy-cinnamic acid (CHC) increased the magnitude of the acidification and resulted in a significantly faster reduction in tone in response to hypoxia. Addition of CHC to normoxic tissues caused both a vasodilatation and a reduction of pHi. 4. A decrease in pHi induced on washout of ammonium chloride (NH4Cl) resulted in an increase in tone. 5. Relaxation to hypoxia or metabolic inhibition was unaffected when the change in pHi was neutralized by addition of the weak base trimethylamine (TMA). 6. It is concluded that severe hypoxia decreases tone in isolated rat mesenteric arteries by a mechanism which is independent of nitric oxide and prostaglandins. Both severe hypoxia and metabolic inhibition reduced pHi, although this does not appear to be contributing to the changes in tone observed.

Mechanisms mediating the vasorelaxing action of eugenol, a pungent oil, on rabbit arterial tissue

The inhibitory actions of eugenol on intracellular Ca2+ concentration ([Ca2+]i) and the contractions induced by excess extracellular K+ concentration ([K+]o) in rabbit thoracic aorta were investigated. Application of excess [K+]o solution (30-90 mM) produced contraction and increased the intensity of the Ca2+ fluorescence signal. Pretreatment with eugenol (> or =0.1 mM) reduced both the amplitude of contraction and the intensity of the Ca2+ fluorescence signal, but the contraction was more strongly affected than the [Ca2+]i. Application of eugenol (0.3 mM) to tissue precontracted by 90 mM [K+]o solution (immediately after the removal of the 90 mM [K+]o solution) slowed the decay of the [Ca2+]i signal, but it did not change the rate of relaxation. Carbonyl cyanide m-chlorophenylhydrozone (10 microM), a mitochondrial metabolic inhibitor, produced a reduction in tension despite a slight increase in [Ca2+]i when applied to muscle precontracted by 90 mM [K+]o solution. These results indicate that eugenol relaxes the rabbit thoracic aorta while suppressing the Ca2+-sensitivity and both the uptake and extrusion mechanisms for Ca2+. To judge from the similarities between its actions and those of metabolic inhibitors, eugenol may produce its actions at least partly through metabolic inhibition.

Contribution of sarcoplasmic reticular calcium to smooth muscle contractile activation: gestational dependence in isolated rat uterus

1. The contribution of Ca2+ released from the sarcoplasmic reticulum (SR) to smooth muscle contractile activation remains poorly understood. By simultaneously monitoring cytosolic [Ca2+] ([Ca2+]i) and force in isolated rat uterine smooth muscle, we report the influence of SR Ca2+ release on contractility during conditions (a) of altered SR Ca2+ homeostasis and (b) where the only source of activating Ca2+ was derived from the SR. 2. In myometria of non-pregnant rats, ryanodine (1-50 microM), a modulator of calcium-induced calcium release (CICR), had no effect on the spontaneous [Ca2+]i or force transients. However, depletion of SR Ca2+ by inhibiting the SR Ca2+-ATPase (with cyclopiazonic acid (CPA), 20 microM) resulted in an enhancement of spontaneous [Ca2+]i and force transients. 3. In myometria of pregnant rats, although ryanodine had no effect in 40% of tissues studied it produced a small but significant enhancement of the integrated spontaneous [Ca2+]i and force transient in 60% of cases. The potentiating effects of CPA were enhanced in myometria of pregnant rats compared with non-pregnant rats, often resulting in maintained [Ca2+]i increases and contraction. 4. In zero external Ca2+, agonist-induced SR Ca2+ release resulted in transient increases in [Ca2+]i and force. The magnitude of these agonist-induced [Ca2+]i and force changes were significantly enhanced in myometria of pregnant rats. No evidence for agonist-induced Ca2+-independent force production was observed. 5. These results indicate that CICR plays little role in SR Ca2+ release from the myometrium, and that there are gestational-dependent alterations in the ability of SR Ca2+ mobilization to contribute to contractile activation. The implications of these findings for the co-ordination of myometrial [Ca2+]i signalling and contractility are discussed.

Hypoxia and smooth muscle function: key regulatory events during metabolic stress

Hypoxia rapidly reduces force in many smooth muscles and we review recent data that shed light on the mechanisms involved. As many regulated cellular processes are integrated to co-ordinate smooth muscle contractility, the processes responsible for decreased force output with altered metabolism are also likely to be many, acting in concert, rather than the actions of one altered parameter. Nevertheless the aim of this study is to elucidate the hierarchical series of events that contribute to reduced smooth muscle force production during altered metabolism. We conclude that in many phasic smooth muscles the decrease in force can be attributed to impaired electro-mechanical coupling whereby the Ca2+ transient is reduced. A direct effect of hypoxia on the Ca2+ channel may be of key importance. In tonic vascular smooth muscles KATP channels may also play a role in the integrated functional responses to hypoxia. There are also many examples of force being reduced, in tonically activated preparations, without a fall in steady-state Ca2+; indeed it usually increases. We examine the roles of altered [ATP], pH, myosin phosphorylation, inorganic phosphate and proteolytic activity on the [Ca2+]-force relationship during hypoxia. We find no defining force-inhibitory role for any one factor acting alone, and suggest that force most probably falls as a result of the combination of myriad factors.

The mechanism of Ca2+ release from the SR of permeabilised guinea-pig and rat ureteric smooth muscle

Recent work has indicated that there is a major difference in the Ca2+ store of smooth muscle from rat and guinea-pig ureter; with the rat store being agonist-sensitive but ryanodine insensitive and the guinea-pig store being ryanodine sensitive but agonist insensitive [Th. V. Burdyga, M.J. Taggart, S. Wray, J. Physiol. 489 (1995) 327-335]. We have therefore examined directly the mechanism of Ca2+ release from the internal Ca2+ store (SR). Following permeabilisation with alpha-toxin or beta-escin the SR was Ca(2+)-loaded before application of carbachol or caffeine. Only carbachol evoked a transient contraction in rat ureter. The carbachol-induced contraction was blocked by heparin and cyclopiazonic acid (CPA) but not ryanodine. Only caffeine produced contraction in guinea-pig ureter, and this was blocked by ryanodine. Direct application of IP3 caused a small transient contraction in rat but not guinea-pig ureter. We conclude that rat ureter possesses only an IP3 sensitive store while guinea-pig ureter only has a ryanodine sensitive store.