Activation properties of myosin light chain kinase during contraction/relaxation cycles of tonic and phasic smooth muscles

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.

Myosin light chain kinase regulates intestinal permeability of mucosal homeostasis in Crohn's disease

Introduction: Researchers have investigated the potential role of intestinal permeability in Crohn's disease pathogenesis. Intestinal permeability is usually mediated by cytoskeleton and intercellular junctions. The myosin light chain kinase (MLCK) is an enzyme that activates the myosin light chain to exert its function related to cytoskeleton contraction and tight junction regulation. The correlation between MLCK and Crohn's disease pathogenesis has been consistently proven. Areas covered: This study aims to expand the understanding of the regulation and function of MLCK in Crohn's disease. An extensive literature search in the MEDLINE database (via PubMed) has been performed up to Oct. 2020. The roles of MLCK in tight junction activation, intestinal permeability enhancement, and cell signal regulation are comprehensively discussed. Expert opinion: Targeting the MLCK-related pathways such as TNF-α in CD treatment has been put into clinical use. More accurate targeting such as MLCK and TNFR2 has been proposed to reduce side effects. MLCK may also have the potential to become biomarkers in fields like CD activity. With the application of cutting age research methods and tools, the MLCK research could be accelerated.

Interplay Between Calcium and AMPK Signaling in Human Cytomegalovirus Infection

Calcium signaling and the AMP-activated protein kinase (AMPK) signaling networks broadly regulate numerous aspects of cell biology. Human Cytomegalovirus (HCMV) infection has been found to actively manipulate the calcium-AMPK signaling axis to support infection. Many HCMV genes have been linked to modulating calcium signaling, and HCMV infection has been found to be reliant on calcium signaling and AMPK activation. Here, we focus on the cell biology of calcium and AMPK signaling and what is currently known about how HCMV modulates these pathways to support HCMV infection and potentially contribute to oncomodulation.

Reciprocality Between Estrogen Biology and Calcium Signaling in the Cardiovascular System

17β-Estradiol (E₂) is the main estrogenic hormone in the body and exerts many cardiovascular protective effects. Via three receptors known to date, including estrogen receptors α (ERα) and β (ERβ) and the G protein-coupled estrogen receptor 1 (GPER, aka GPR30), E₂ regulates numerous calcium-dependent activities in cardiovascular tissues. Nevertheless, effects of E₂ and its receptors on components of the calcium signaling machinery (CSM), the underlying mechanisms, and the linked functional impact are only beginning to be elucidated. A picture is emerging of the reciprocality between estrogen biology and Ca²⁺ signaling. Therein, E₂ and GPER, via both E₂-dependent and E₂-independent actions, moderate Ca²⁺-dependent activities; in turn, ERα and GPER are regulated by Ca²⁺ at the receptor level and downstream signaling via a feedforward loop. This article reviews current understanding of the effects of E₂ and its receptors on the cardiovascular CSM and vice versa with a focus on mechanisms and combined functional impact. An overview of the main CSM components in cardiovascular tissues will be first provided, followed by a brief review of estrogen receptors and their Ca²⁺-dependent regulation. The effects of estrogenic agonists to stimulate acute Ca²⁺ signals will then be reviewed. Subsequently, E₂-dependent and E₂-independent effects of GPER on components of the Ca²⁺ signals triggered by other stimuli will be discussed. Finally, a case study will illustrate how the many mechanisms are coordinated to moderate Ca²⁺-dependent activities in the cardiovascular system.

Nitric oxide and hydrogen sulfide: the gasotransmitter paradigm of the vascular system

There are several reviews on NO and hydrogen sulfide (H₂ S) and their role in vascular diseases in the current relevant literature. The aim of this review is to discuss, within the limits of present knowledge, the interconnection between these two gasotransmitters in vascular function. In particular, the review focuses on the role played by the balance between the NO and H₂ S pathways in either physiological or pathological conditions. The distinction between physiology and pathology has been made in order to dissect the molecular basis of this crosstalk, highlighting how and if this balance varies, depending upon the vascular status. Perspectives and possible novel therapeutic approaches are also discussed.

LINKED ARTICLES

This article is part of a themed section on Targeting Inflammation to Reduce Cardiovascular Disease Risk. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.22/issuetoc and http://onlinelibrary.wiley.com/doi/10.1111/bcp.v82.4/issuetoc.

Vascular nitric oxide: Beyond eNOS

As the first discovered gaseous signaling molecule, nitric oxide (NO) affects a number of cellular processes, including those involving vascular cells. This brief review summarizes the contribution of NO to the regulation of vascular tone and its sources in the blood vessel wall. NO regulates the degree of contraction of vascular smooth muscle cells mainly by stimulating soluble guanylyl cyclase (sGC) to produce cyclic guanosine monophosphate (cGMP), although cGMP-independent signaling [S-nitrosylation of target proteins, activation of sarco/endoplasmic reticulum calcium ATPase (SERCA) or production of cyclic inosine monophosphate (cIMP)] also can be involved. In the blood vessel wall, NO is produced mainly from l-arginine by the enzyme endothelial nitric oxide synthase (eNOS) but it can also be released non-enzymatically from S-nitrosothiols or from nitrate/nitrite. Dysfunction in the production and/or the bioavailability of NO characterizes endothelial dysfunction, which is associated with cardiovascular diseases such as hypertension and atherosclerosis.

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

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

Simulating uterine contraction by using an electro-chemo-mechanical model

Contractions of uterine smooth muscle cells consist of a chain of physiological processes. These contractions provide the required force to expel the fetus from the uterus. The inclusion of these physiological processes is, therefore, imperative when studying uterine contractions. In this study, an electro-chemo-mechanical model to replicate the excitation, activation, and contraction of uterine smooth muscle cells is developed. The presented modeling strategy enables efficient integration of knowledge about physiological processes at the cellular level to the organ level. The model is implemented in a three-dimensional finite element setting to simulate uterus contraction during labor in response to electrical discharges generated by pacemaker cells and propagated within the myometrium via gap junctions. Important clinical factors, such as uterine electrical activity and intrauterine pressure, are predicted using this simulation. The predictions are in agreement with clinically measured data reported in the literature. A parameter study is also carried out to investigate the impact of physiologically related parameters on the uterine contractility.

The inwardly rectifying K+ channel KIR7.1 controls uterine excitability throughout pregnancy

Abnormal uterine activity in pregnancy causes a range of important clinical disorders, including preterm birth, dysfunctional labour and post-partum haemorrhage. Uterine contractile patterns are controlled by the generation of complex electrical signals at the myometrial smooth muscle plasma membrane. To identify novel targets to treat conditions associated with uterine dysfunction, we undertook a genome-wide screen of potassium channels that are enriched in myometrial smooth muscle. Computational modelling identified Kir7.1 as potentially important in regulating uterine excitability during pregnancy. We demonstrate Kir7.1 current hyper-polarizes uterine myocytes and promotes quiescence during gestation. Labour is associated with a decline, but not loss, of Kir7.1 expression. Knockdown of Kir7.1 by lentiviral expression of miRNA was sufficient to increase uterine contractile force and duration significantly. Conversely, overexpression of Kir7.1 inhibited uterine contractility. Finally, we demonstrate that the Kir7.1 inhibitor VU590 as well as novel derivative compounds induces profound, long-lasting contractions in mouse and human myometrium; the activity of these inhibitors exceeds that of other uterotonic drugs. We conclude Kir7.1 regulates the transition from quiescence to contractions in the pregnant uterus and may be a target for therapies to control uterine contractility.

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

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

Development and maintenance of force and stiffness in airway smooth muscle

Airway smooth muscle (ASM) plays a central role in the excessive narrowing of the airway that characterizes the primary functional impairment in asthma. This phenomenon is known as airway hyper-responsiveness (AHR). Emerging evidence suggests that the development and maintenance of ASM force involves dynamic reorganization of the subcellular filament network in both the cytoskeleton and the contractile apparatus. In this review, evidence is presented to support the view that regulation of ASM contraction extends beyond the classical actomyosin interaction and involves processes within the cytoskeleton and at the interfaces between the cytoskeleton, the contractile apparatus, and the extracellular matrix. These processes are initiated when the muscle is activated, and collectively they cause the cytoskeleton and the contractile apparatus to undergo structural transformation, resulting in a more connected and solid state that allows force generated by the contractile apparatus to be transmitted to the extracellular domain. Solidification of the cytoskeleton also serves to stiffen the muscle and hence the airway. Oscillatory strain from tidal breathing and deep inspiration is believed to be the counter balance that prevents hypercontraction and stiffening of ASM in vivo. Dysregulation of this balance could lead to AHR seen in asthma.

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

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

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

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

The importance of Rho-associated kinase-induced Ca2+ sensitization as a component of electromechanical and pharmacomechanical coupling in rat ureteric smooth muscle

Ureteric peristalsis, which occurs via alternating contraction and relaxation of ureteric smooth muscle, ensures the unidirectional flow of urine from the kidney to the bladder. Understanding of the molecular mechanisms underlying ureteric excitation-contraction coupling, however, is limited. To address these knowledge deficits, and in particular to test the hypothesis that Ca2+ sensitization via activation of the RhoA/Rho-associated kinase (ROK) pathway plays an important role in ureteric smooth muscle contraction, we carried out a thorough characterization of the electrical activity, Ca2+ signaling, MYPT1 (myosin targeting subunit of myosin light chain phosphatase, MLCP) and myosin regulatory light chain (LC20) phosphorylation, and force responses to membrane depolarization induced by KCl (electromechanical coupling) and carbachol (CCh) (pharmacomechanical coupling). The effects of ROK inhibition on these parameters were investigated. We conclude that the tonic, but not the phasic component of KCl- or CCh-induced ureteric smooth muscle contraction is highly dependent on ROK-catalyzed phosphorylation of MYPT1 at T855, leading to inhibition of MLCP and increased LC20 phosphorylation.

Reduction in TRPC4 expression specifically attenuates G-protein coupled receptor-stimulated increases in intracellular calcium in human myometrial cells

Canonical transient receptor potential (TRPC) proteins may play a role in regulating changes in intracellular calcium ([Ca(2+)](i)). Human myometrium expresses TRPC4, TRPC1 and TRPC6 mRNAs in greatest relative abundance. Contributions of TRPC4 to increases in [Ca(2+)](i) were assessed in PHM1-41 and primary human uterine smooth muscle (UtSMC) cells using short hairpin RNAs (shRNAs). Based on a reporter assay screen, one shRNA was selected to construct an adenoviral expression vector (TC4sh1). TC4sh1 induced both mRNA and protein TRPC4 knockdown in PHM1-41 cells without affecting expression of other TRPCs. Signal-regulated Ca(2+) entry (SRCE), defined as a stimulus- and extracellular Ca(2+)-dependent increase in [Ca(2+)](i), was measured in PHM1-41 cells treated with oxytocin (G-protein coupled receptor (GPCR)-stimulated), thapsigargin (store depletion-stimulated), and OAG (diacylglycerol-stimulated), using Fura-2. Cells infected with TC4sh1 exhibited attenuated oxytocin-, ATP- and PGF2alpha-mediated SRCE, but no change in thapsigargin- or OAG-stimulated SRCE. Similar results were obtained in primary uterine smooth muscle cells. Additionally, cells expressing TC4sh1 exhibited a significantly smaller increase in channel activity in response to oxytocin administration than did cells infected with empty virus. These data show that, in human myometrial cells, knockdown of endogenous TRPC4 specifically attenuates GPCR-stimulated, but not thapsigargin- or OAG-stimulated extracellular calcium-dependent increases in [Ca(2+)](i). These data imply that, in this cellular context, the mechanisms regulating extracellular Ca(2+)-dependent increases in [Ca(2+)](i) are differentially affected by different signaling pathways.

Activation by Ca2+/calmodulin of an exogenous myosin light chain kinase in mouse arteries

Activation of myosin light chain kinase (MLCK) and other kinases was studied in the arteries of transgenic mice that express an optical fluorescence resonance energy transfer (FRET) MLCK activity biosensor. Binding of Ca(2+)/calmodulin (Ca(2+)/CaM) induces an increase in MLCK activity and a change in FRET. After exposure to high external [K(+)], intracellular [Ca(2+)] (fura-2 ratio or fluo-4 fluorescence) and MLCK activity both increased rapidly to an initial peak and then declined, rapidly at first and then very slowly. After an initial peak ('phasic') force was constant or increased slowly (termed 'tonic' force). Inhibition of rho-kinase (Y-27632) decreased tonic force more than phasic, but had little effect on [Ca(2+)] and MLCK activation. Inhibition of PKCalpha and PKCbeta with Gö6976 had no effect. KN-93, an inhibitor of CaMK II, markedly reduced force, MLCK FRET and [Ca(2+)]. Applied during tonic force, forskolin caused a rapid decrease in MLCK FRET ratio and force, but no change in Ca(2+), suggesting a cAMP mediated decrease in affinity of MLCK for Ca(2+)/CaM. However, receptor (beta-adrenergic) activated increases in cAMP during KCl were ineffective in causing relaxation, changes in [Ca(2+)], or MLCK FRET. At the same tonic force, MLCK FRET ratio activated by alpha(1)-adrenoceptors was approximately 60% of that activated by KCl. In conclusion, MLCK activity of arterial smooth muscle during KCl-induced contraction is determined primarily by Ca(2+)/CaM. Rho-kinase is activated, by unknown mechanisms, and increases 'Ca(2+) sensitivity' significantly. Forskolin mediated increases in cAMP, but not receptor mediated increases in cAMP cause a rapid decrease in the affinity of MLCK for Ca(2+)/CaM.

Melatonin synergizes with oxytocin to enhance contractility of human myometrial smooth muscle cells

CONTEXT

Studies have shown that labor occurs primarily in the night/morning hours. Recently, we identified the human myometrium as a target for melatonin (MEL), the neuroendocrine output signal coding for circadian night.

OBJECTIVE

The purpose of this study was to determine the effects of MEL on contractility and the contractile machinery in telomerase-immortalized human myometrial cells.

DESIGN

To ascertain the effect of MEL on myometrial contractility in vitro, we performed gel retraction assays with cells exposed to iodomelatonin +/- oxytocin (OT). The effects of iodomelatonin on gap junctions were also investigated. Additionally, expression levels of the type 2 MEL receptor (MT2R) were assessed in myometrial biopsies from term pregnant women with or without labor.

RESULTS

MEL was found to synergistically enhance OT-induced contractility via the MT2R, which is coupled to a protein kinase C-dependent increase in phosphorylation of the myosin light chain protein. MT2R expression was markedly elevated in samples from pregnant women who had entered labor, as compared to matched nonlaboring pregnant women. MEL increased expression of the gap junction protein, connexin 43. In vitro dye spread assays showed that MEL-treated cells displayed substantially increased intercellular coupling. Increases in connexin 43 mRNA and cell to cell coupling were also found to be mediated via the MT2R in a protein kinase C-dependent manner.

CONCLUSIONS

MEL synergizes with OT to promote myometrial cell contractions and to facilitate gap junction activity in vitro. Such a synergy in vivo would promote coordinated and forceful contractions of the late term pregnant uterus necessary for parturition.

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.

The regulation of uterine relaxation

The regulation of uterine relaxation is poorly understood but research in myometrial tissue and other types of smooth muscle has defined a number of receptors, ion channels and regulatory proteins that are likely to be involved. Some of these proteins are substrates for protein kinases, especially cyclic nucleotide dependent kinases. More research is necessary to identify the key molecules involved in the maintenance of uterine quiescence in pregnancy. The use of tocolytics in preterm labour remains controversial; there is a need to identify better pharmacological targets to provoke safe and selective uterine relaxation and improve neonatal outcome.

Mathematical model of excitation-contraction in a uterine smooth muscle cell

Uterine contractility is generated by contractions of myometrial smooth muscle cells (SMCs) that compose most of the myometrial layer of the uterine wall. Calcium ion (Ca2+) entry into the cell can be initiated by depolarization of the cell membrane. The increase in the free Ca2+concentration within the cell initiates a chain of reactions, which lead to formation of cross bridges between actin and myosin filaments, and thereby the cell contracts. During contraction the SMC shortens while it exerts forces on neighboring cells. A mathematical model of myometrial SMC contraction has been developed to study this process of excitation and contraction. The model can be used to describe the intracellular Ca2+concentration and stress produced by the cell in response to depolarization of the cell membrane. The model accounts for the operation of three Ca2+control mechanisms: voltage-operated Ca2+channels, Ca2+pumps, and Na+/Ca2+exchangers. The processes of myosin light chain (MLC) phosphorylation and stress production are accounted for using the cross-bridge model of Hai and Murphy ( Am J Physiol Cell Physiol 254: C99–C106, 1988) and are coupled to the Ca2+concentration through the rate constant of myosin phosphorylation. Measurements of Ca2+, MLC phosphorylation, and force in contracting cells were used to set the model parameters and test its ability to predict the cell response to stimulation. The model has been used to reproduce results of voltage-clamp experiments performed in myometrial cells of pregnant rats as well as the results of simultaneous measurements of MLC phosphorylation and force production in human nonpregnant myometrial cells.

Modeling Myometrial Smooth Muscle Contraction

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

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

OBJECTIVE

This study was undertaken to evaluate the effects of magnesium sulfate (MgSO4) on contractile force and increases in free intracellular calcium concentration ([Ca2+]i) in human myometrial strips from pregnant women.

STUDY DESIGN

Simultaneous measurements of isometric tension and [Ca2+]i were measured in myometrial strips obtained at the time of cesarean delivery from pregnant nonlaboring women at term with the use of a fluorescence spectrometer equipped with a displacement force transducer. Changes in [Ca2+]i were measured with fura-2, a Ca(2+)-sensitive fluorescent probe. Myometrial strips were exposed to MgSO4 (5 or 10 mmol/L) for 5, 10, 20, and 30 minutes and observed for spontaneous contractions or stimulated with either oxytocin (OT; 0.1 micromol/L) or potassium chloride (KCl; 90 mmol/L).

RESULTS

MgSO4 reduced spontaneous, OT, and KCl-evoked contractions and increases in [Ca2+]i in a time and concentration-dependent manner. After 20 minutes exposure to 5 mmol/L MgSO4, the OT-elicited changes in contractile response and [Ca2+]i were significantly decreased. MgSO4 did not change [Ca2+]i/force relationship of the responses to OT or KCl, or during spontaneous activity.

CONCLUSION

At a pharmacologic concentration (5 mmol/L), MgSO4 inhibits contractile response and [Ca2+]i in pregnant human myometrial strips by a pattern that is consistent with both extra- and intracellular mechanisms. At a suprapharmacologic concentration (10 mmol/L), the more immediate effect of MgSO4 is consistent with an extracellular mechanism. MgSO4 does not appear to interfere at the level of the calcium-calmodulin interface, since the [Ca2+]i/force relationship was not changed.

Ontogenesis of myosin light chain phosphorylation in guinea pig tracheal smooth muscle

Increased airway responsiveness occurs in normal young individuals compared to adults. A maturation of airway smooth muscle (ASM) contractility is likely a mechanism of this juvenile airway hyperresponsiveness. Indeed, we showed in guinea pig tracheal smooth muscle (TSM) that maximum shortening velocity decreases dramatically after the first 3 weeks of life. Because the phosphorylation of the 20-kDa myosin light chain (MLC(20)) was shown to be a key event in ASM contractility, in the present work we sought to investigate it during ontogenesis. In three age groups (1-week-old, 3-week-old, and adult guinea pigs), we assessed the amount of MLC(20) phosphorylation achieved either in TSM crude protein homogenates exposed to Mg(2+) . ATP . CaCl(2) or in tracheal strips during electrical field stimulation (EFS). Phosphorylated and unphosphorylated MLC(20) were separated on nondenaturing 10% polyacrylamide gels, and the ratio of phosphorylation was obtained by densitometric analysis of chemiluminescent Western immunoblots. Maximum MLC(20) phosphorylation (% of total MLC(20)) in TSM tissue homogenate was, respectively, 32.6 +/- 5.7, 32.2 +/- 5.7, and 46.8 +/- 5.8 in 1-week, 3-week, and adult guinea pigs. Interestingly, in nonstimulated intact tracheal strips, we found a substantial degree of MLC(20) phosphorylation: respectively, 42.2 +/- 5.8, 36.5 +/- 7.8, and 46.4 +/- 4.7 in 1-week, 3-week, and adult guinea pigs. Maximal EFS-induced MLC(20) phosphorylation (% increase over baseline) in the 3-week age group was attained after 3 sec of EFS, and was 161.2 +/- 17.6, while in 1-week and adult guinea pigs, it was attained at 1.5 sec of EFS and was, respectively, 133.3 +/- 9.3 and 110.2 +/- 3.9 (P < 0.05). We conclude that MLC(20) phosphorylation in guinea pig intact tracheal strips correlates with ontogenetic changes in shortening velocity and changes in myosin light chain kinase content. These results further suggest that the maturation of ASM contractile properties plays a role in the greater airway responsiveness reported in children and young animals.

Possible role of the protein kinase C/CPI-17 pathway in the augmented contraction of human myometrium after gestation

1. Activation of protein kinase C (PKC) by phorbol 12,13-dibutylate (PDBu, 1 microm) induced sustained contractions with no increase in [Ca2+]i in nonpregnant and pregnant human myometria. The contractile effects of PDBu in pregnant myometrium were much greater than those in nonpregnant myometrium, and the contractions in pregnant myometrium were accompanied by an increase in myosin light chain (MLC) phosphorylation at Ser19. 2. The contraction induced by PDBu in pregnant myometrium was inhibited by the inhibitors of conventional PKC isoforms, bisindolylmaleimides and indolocarbazole, such as Go6976, Go6983, and Go6850 (1 microM). LY333531 (1 microM), a specific inhibitor of PKC beta, also inhibited the PDBu-induced contraction in the pregnant myometrium. 3. In the pregnant myometrium permeabilized with alpha-toxin, PDBu increased the contractions induced at fixed Ca2+ concentration (0.3 microM) both in nonpregnant and pregnant myometria, indicating Ca2+ sensitization of contractile elements. 4. Western immunoblot analysis indicated that pregnant myometrium contained PKC isozymes such as conventional PKC (alpha, beta, gamma), novel PKC (delta, epsilon, theta), and atypical PKC (zeta but not iota and lambda). RT-PCR and real-time RT-PCR analysis indicated that, among the conventional PKC, the levels of mRNA of beta isoform in pregnant human myometrium were greater than those in nonpregnant myometrium. 5. CPI-17 is a substrate for PKC, and the phosphorylated CPI-17 is considered to inhibit myosin phosphatase. The levels of CPI-17 mRNA and protein expression were also greater in the pregnant myometrium. 6. These results suggest that the PKC-mediated contractile mechanism is augmented in human myometrium after gestation, and that this augmentation may be attributable to the increased activity of the beta PKC isoform and CPI-17.

Downregulation of profilin with antisense oligodeoxynucleotides inhibits force development during stimulation of smooth muscle

The actin-regulatory protein profilin has been shown to regulate the actin cytoskeleton and the motility of nonmuscle cells. To test the hypothesis that profilin plays a role in regulating smooth muscle contraction, profilin antisense or sense oligodeoxynucleotides were introduced into the canine carotid smooth muscle by a method of reversible permeabilization, and these strips were incubated for 2 days for protein downregulation. The treatment of smooth muscle strips with profilin antisense oligodeoxynucleotides inhibited the expression of profilin; it did not influence the expression of actin, myosin heavy chain, and metavinculin/vinculin. Profilin sense did not affect the expression of these proteins in smooth muscle tissues. Force generation in response to stimulation with norepinephrine or KCl was significantly lower in profilin antisense-treated muscle strips than in profilin sense-treated strips or in muscle strips not treated with oligodeoxynucleotides. The depletion of profilin did not attenuate increases in phosphorylation of the 20-kDa regulatory light chain of myosin (MLC20) in response to stimulation with norepinephrine or KCl. The increase in F-actin/G-actin ratio during contractile stimulation was significantly inhibited in profilin-deficient smooth muscle strips. These results suggest that profilin is a necessary molecule of signaling cascades that regulate carotid smooth muscle contraction, but that it does not modulate MLC20 phosphorylation during contractile stimulation. Profilin may play a role in the regulation of actin polymerization or organization in response to contractile stimulation of smooth muscle.

Mechanisms of labour--biochemical aspects

The mechanism of labour is not fully understood and further research into this important physiological process is needed. In some species, notably sheep, parturition is due to activation of the fetal hypothalamic-pituitary-adrenal axis. However, in primates, this axis appears to have a supportive, rather than essential role. Successful parturition requires an increase in coordinated uterine contractility together with changes in connective tissue that allow cervical ripening and dilatation. In most mammals, however, these changes are synchronised by a fall in maternal progesterone levels and a rise in oestrogens. This is not the case in women in whom the onset of labour occurs without apparent changes in circulating steroid levels. The basis of uterine contractility is the interaction between actin and myosin in myometrial smooth muscle cells. This is driven by calcium through Ca(2+)-calmodulin-dependent myosin light chain kinase (MLCK) activity. Moreover, calcium sensitisation occurs via activation of Rho kinase, a calcium-independent pathway that promotes contractility by inhibiting myosin phosphatase and probably by phosphorylating myosin on the same site as MLCK. Uterine activity can be modulated by many G-protein coupled receptors (GPCRs). For example, receptors coupled to Galpha(q) (oxytocin-, prostanoid FP and TP, endothelin-receptors) stimulate contractility by activating the phospholipase C/Ca(2+) pathway; receptors coupled to Galpha(s) (beta(2)-adrenoceptors, prostanoid EP2 and IP, some 5-hydroxytryptamine receptors e.g. 5-HT(7)) relax the uterus by increasing myometrial cyclic AMP levels; and receptors coupled to Galpha(i) (alpha(2)-adrenoceptors, muscarinic, 5-HT(1)) potentiate contractility, probably by inhibiting cAMP production. Because of its relative abundance in pregnant uterine tissue, the oxytocin receptor is an obvious target for tocolytic therapy. Oxytocin antagonists have been introduced into clinical practice for the management of preterm labour and offer the advantage of uterine selectivity and fewer side effects than conventional beta-agonist therapy.

Ca2+-activated K+ channel blockers induce PKC modulated oscillatory contractions in guinea pig trachea

Mechanisms underlying the Ca2+-activated K+ channel (K(Ca)) blockers-induced oscillatory contractions were investigated in guinea pig tracheal smooth muscle. The mean oscillatory frequencies induced by charybdotoxin (ChTX; 100 nM) and iberiotoxin (IbTX; 100 nM) were 9.8+/-0.8 (counts/h) and 8.0+/-1.3 (counts/h), respectively. Apamin (1 microM ), a blocker of SK(Ca), induced no contraction in guinea pig trachea and did not affect ChTX-induced oscillatory contractions. In Ca2+ free solution, no ChTX-induced contraction was observed. Nifedipine (100 nM), a blocker of voltage-dependent Ca2+ channels, and SK&F 96365 (10 microM), a blocker of capacitative Ca2+ entry, completely abolished ChTX-induced oscillatory contractions. Ryanodine (1 microM) decreased the amplitude, but increased the frequency of the oscillatory contractions. Thapsigargin (1 microM) changed contractions from the oscillatory type to the sustained type. Moreover, the protein kinase C (PKC) inhibitor, bisindolylamaleimide I (1 microM), decreased the amplitude and frequency, but PKC activator, phorbol 12-myristate 13-acetate (1 microM), increased the frequency of oscillatory contractions. These results suggest that K(Ca) inhibitors-induced oscillatory contractions are initiated by Ca2+ influx through L-type voltage-dependent Ca2+ channels. The ryanodine-sensitive calcium release channels in the sarcoplasmic reticulum may play an important role in maintaining the oscillatory contractions. Moreover, PKC activity modulates these oscillatory contractions.

Invited review: regulation of myosin phosphorylation in smooth muscle

Phosphorylation of the regulatory light chains of myosin II (rMLC) by the Ca(2+)/calmodulin-dependent myosin light-chain kinase (MLCK) and dephosphorylation by a type 1 phosphatase (MLCP), which is targeted to myosin by a regulatory subunit (MYPT1), are the predominant mechanisms of regulation of smooth muscle tone. The activities of both enzymes are modulated by several protein kinases. MLCK is inhibited by the Ca(2+)/calmodulin-dependent protein kinase II, whereas the activity of MLCP is increased by cGMP and perhaps also cAMP-dependent protein kinases. In either case, this results in a decrease in the Ca(2+) sensitivity of rMLC phosphorylation and force production. The activity of MLCP is inhibited by Rho-associated kinase, one of the effectors of the monomeric GTPase Rho, and protein kinase C, leading to an increase in Ca(2+) sensitivity. Hence, smooth muscle tone appears to be regulated by a network of activating and inactivating intracellular signaling cascades.

Divergence in murine myometrium spontaneous and oxytocin-stimulated contractile responses to serine/threonine protein phosphatase-1 inhibition

Reversible phosphorylation is essential in regulating uterine contractions. Identification, characterization, and functional understanding of myometrium protein phosphatase(s) are lacking. Okadaic acid (OA), which inhibits protein phosphatase-1 (PP1) and PP2A, has been shown to alter uterine contractions. Experiments were conducted to determine the 1) identity of the myometrial OA-sensitive PP, 2) influence of OA on spontaneous and oxytocin (OT)-stimulated myometrial contractions, and 3) expression of uterine PPs during sexual development. Western blot analysis indicated the presence of PP1(alpha) and PP2A in immature and mature mice. As determined by immunohistochemistry, gonadotropin-stimulated adult mouse uteri contain PP1(alpha) in longitudinal and circular myometrial layers and endometrial epithelium. Conversely, PP2A was localized to the endometrial stroma. Cumulative addition of OA (n = 9; 10, 100, 250, 500, 1000 nM) did not significantly alter spontaneous contractions of mouse uterine horns in comparison to vehicle-treated controls (n = 9). By the end of the test period OA- and vehicle-treated uteri displayed a comparable decline in uterine contractions to 79.2% and 63.7%, respectively, of basal contractile activity. Pretreatment of uterine tissue with OA (1 microM; n = 7) significantly reduced contractile response to increasing concentrations of OT (8, 16, 32, 64 nM) in comparison to vehicle pretreatment (dimethyl sulfoxide; n = 7). At the end of the OT-administration period, contractile activity was 160.4% and 67.3% of basal contractile activity for vehicle (no OA) and OA-pretreated groups, respectively. During the early prepubertal period PP1(alpha) was expressed in longitudinal myometrium and absent in circular myometrium; whereas, during the transition to sexual maturity PP1(alpha) was observed in both the longitudinal and circular myometrium. In summary, these studies have indicated 1) that PP1 is the primary myometrial OA-sensitive PP; 2) that inhibition of PP1 had no effect on spontaneous contractions, whereas it markedly inhibited OT-stimulated uterine contractions; and 3) that PP1 is differentially expressed in the circular and longitudinal myometrium in relation to sexual development.

Preterm labour

Spontaneous preterm labour remains a major obstetric problem because of the high incidence of neonatal mortality or long-term handicap associated with it. The drugs available for the management of preterm labour are poorly effective and have potentially serious side-effects for the mother or fetus. In recent years, there has been a remarkable increase in the knowledge of the biochemical mechanism underlying uterine quiescence and contractility. Many of the G protein-coupled receptors that participate in the regulation of myometrial activity have been cloned and characterized, and their intracellular signalling pathways have been elucidated. The role of G protein receptor kinases in uterine tachyphylaxis is better understood. New developments in our understanding of the cellular mechanisms involved in uterine contractions in idiopathic and infection-associated preterm labour are expected, which will lead to better, more selective therapy for this problem. However, much research remains to be done before the mechanism of human parturition is fully understood.

Pharmacomechanical coupling: the role of calcium, G-proteins, kinases and phosphatases

The concept of pharmacomechanical coupling, introduced 30 years ago to account for physiological mechanisms that can regulate contraction of smooth muscle independently of the membrane potential, has since been transformed from a definition into what we now recognize as a complex of well-defined, molecular mechanisms. The release of Ca2+ from the SR by a chemical messenger, InsP3, is well known to be initiated not by depolarization, but by agonist-receptor interaction. Furthermore, this G-protein-coupled phosphatidylinositol cascade, one of many processes covered by the umbrella of pharmacomechanical coupling, is part of complex and general signal transduction mechanisms also operating in many non-muscle cells of diverse organisms. It is also clear that, although the major contractile regulatory mechanism of smooth muscle, phosphorylation/dephosphorylation of MLC20, is [Ca2+]-dependent, the activity of both the kinase and the phosphatase can also be modulated independently of [Ca2+]i. Sensitization to Ca2+ is attributed to inhibition of SMPP-1M, a process most likely dominated by activation of the monomeric GTP-binding protein RhoA that, in turn, activates Rho-kinase that phosphorylates the regulatory subunit of SMPP-1M and inhibits its myosin phosphatase activity. It is likely that the tonic phase of contraction activated by a variety of excitatory agonists is, at least in part, mediated by this Ca(2+)-sensitizing mechanism. Desensitization to Ca2+ can occur either through inhibitory phosphorylation of MLCK by other kinases or autophosphorylation and by activation of SMPP-1M by cyclic nucleotide-activated kinases, probably involving phosphorylation of a phosphatase activator. Based on our current understanding of the complexity of the many cross-talking signal transduction mechanisms that operate in cells, it is likely that, in the future, our current concepts will be refined, additional mechanisms of pharmacomechanical coupling will be recognized, and those contributing to the pathologenesis diseases, such as hypertension and asthma, will be identified.

Calcium transients and the effect of a photolytically released calcium chelator during electrically induced contractions in rabbit rectococcygeus smooth muscle

Intracellular Ca2+ was determined with the fura-2 technique during electrically induced contractions in the rabbit rectococcygeus smooth muscle at 22 degreesC. The muscles were electrically activated to give short, reproducible contractions. Intracellular [Ca2+] increased during activation; the increase in [Ca2+] preceded force development by approximately 2 s. After cessation of stimulation Ca2+ fell, preceding the fall in force by approximately 4 s. The fluorescence properties of fura-2 were determined with time-resolved spectroscopy using synchrotron light at the MAX-storage ring, Lund, Sweden. The fluorescence decay of free fura-2 was best described by two exponential decays (time constants approximately 0.5 and 1.5 ns) at low Ca2+ (pCa 9). At high Ca2+ (pCa 4.5), fluorescence decay became slower and could be fitted by one exponential decay (1.9 ns). Time-resolved anisotropy of free fura-2 was characteristic of free rotational motion (correlation time 0.3 ns). Motion of fura-2 could be markedly inhibited by high concentrations of creatine kinase. Time-resolved spectroscopy measurements of muscle fibers loaded with fura-2 showed that the fluorescence lifetime of the probe was longer, suggesting an influence of the chemical environment. Anisotropy measurements revealed, however, that the probe was mobile in the cells. The Ca2+-dependence of contraction and relaxation was studied using a photolabile calcium chelator, diazo-2, which could be loaded into the muscle cells in a similar manner as fura-2. Photolysis of diazo-2 leads to an increase in its Ca2+-affinity and a fall in free Ca2+. When muscles that had been loaded with diazo-2 were illuminated with UV light flashes during the rising phase of contraction, the rate of contraction became slower, suggesting a close relation between intracellular Ca2+ and the cross-bridge interaction. In contrast, photolysis during relaxation did not influence the rate of force decay, suggesting that relaxation of these contractions is not determined by the rate of Ca2+ removal or due to an increased Ca2+ sensitivity, but instead is limited by other processes such as deactivation by dephosphorylation or detachment of tension-bearing cross-bridges, possibly regulated by thin filament systems.

Clostridium difficile toxin B inhibits carbachol-induced force and myosin light chain phosphorylation in guinea-pig smooth muscle: role of Rho proteins

1. Clostridium difficile toxin B glucosylates the Ras-related low molecular mass GTPases of the Rho subfamily thereby inactivating them. In the present report, toxin B was applied as a tool to test whether Rho proteins participate in the carbachol-induced increase in the Ca2+ sensitivity of force and myosin light chain (MLC) phosphorylation in intact intestinal smooth muscle. 2. Small strips of the longitudinal muscle of guinea-pig small intestine were incubated in toxin B (40 ng ml-1) overnight. Carbachol-induced force and intracellular [Ca2+], and, in a separate series, force and MLC phosphorylation, were determined. 3. Carbachol induced a biphasic contraction: an initial rapid increase in force (peak 1) followed by a partial relaxation and a second delayed increase in force (peak 2). The peak of the Ca2+ signal measured with fura-2 preceded peak 1 of force and then declined to a lower suprabasal steady-state level. Peak 2 was not associated with a significant increase in [Ca2+]. Toxin B nearly completely inhibited peak 2 while peak 1 was not significantly inhibited. Toxin B had no effect on the Ca2+ transient. 4. In control strips, MLC phosphorylation at peak 2 was 27.7% which was significantly higher than the resting value (18.6%). The inhibition of the second, delayed, rise in force induced by toxin B was associated with complete inhibition of the increase in MLC phosphorylation. The resting MLC phosphorylation was not significantly different from that of the control strips. 5. The initial increase in MLC phosphorylation determined 3 s after exposure to carbachol was 54% in the control strips. Toxin B also inhibited this initial phosphorylation peak despite the fact that the Ca2+ transient and the initial increase in force were not inhibited by toxin B. This suggests that Rho proteins play an important role in setting the balance between MLC phosphorylation and dephosphorylation reactions even at high levels of intracellular Ca2+. 6. These findings are consistent with the hypothesis that the delayed rise in force elicited by carbachol is due to an increase in the Ca2+ sensitivity of MLC phosphorylation mediated by Rho proteins.

Modulation of the calcium sensitivity of the smooth muscle contractile apparatus: molecular mechanisms, pharmacological and pathophysiological implications

Smooth muscle contraction is the basis of the physiological reactivity of several systems (vascular, respiratory, gastrointestinal, urogenital ...). Hyperresponsiveness of smooth muscle may also contribute to a variety of problems such as arterial hypertension, asthma and spontaneous abortion. An increase in cytoplasmic calcium concentration ([Ca2+]i) is the key event in excitation-contraction coupling in smooth muscle and the relationship linking the [Ca2+]i value to the force of contraction represents the calcium sensitivity of the contractile apparatus (CaSCA). Recently, it has become evident that CaSCA can be modified upon the action of agonists or drugs as well as in some pathophysiological situations. Such modifications induce, at a fixed [Ca2+]i value, either an increase (referred to as sensitization) or a decrease (desensitization) of the contraction force. The molecular mechanisms underlying this modulation are not yet fully elucidated. Nevertheless, recent studies have identified sites of regulation of the actomyosin interaction in smooth muscle. Sensitization primarily results from the inhibition of myosin light chain phosphatase (MLCP) by intracellular messengers such as arachidonic acid or protein kinase C. In addition, phosphorylation of thin filament-associated proteins, caldesmon and calponin, increases CaSCA. Activation of small (monomeric) G-proteins such as rho or ras is also involved. Desensitization occurs as a consequence of phosphorylation of myosin light chain kinase (MLCK) by the calcium-calmodulin activated protein kinase II, or stimulation of MLCP by cyclic GMP-activated protein kinase. In the present review, examples of physiological modulation of CaCSA as well as pharmacological and pathophysiological implications are illustrated for some smooth muscles.

Stimulus-dependent modulation of smooth muscle intracellular calcium and force by altered intracellular pH

Measurements of simultaneous force and intracellular Ca2+ concentration ([Ca2+]i) in rat uterine smooth muscle have been made to elucidate the mechanisms involved when force produced spontaneously, by high-K+ depolarization or carbachol is altered by a change of intracellular pH (pHi). Rises in force and [Ca2+]i were closely correlated for all forms of contraction, with the Ca2+ transient peaking before force. In spontaneously active preparations, alkalinization significantly increased, and acidification decreased, force and [Ca2+]i. Inhibition of the sarcoplasmic reticulum ATPase (cyclopiazonic acid) did not affect these changes, whereas removal of external Ca2+ abolished both responses, suggesting that the effect of pHi is on Ca2+ entry. Alkalinization caused a prolongation of the action potential complex, associated with a potentiation of contractile activity. Acidification produced hyperpolarization and abolition of action potentials and spontaneous activity, but did not prevent brief applications of carbachol or high-K+ from producing depolarization and increasing force, suggesting no impairment of the mechanism of generation of the action potential. For depolarized preparations, acidification increased tonic force and [Ca2+]i; the increase in the calcium signal persisted in zero-external calcium. In the presence of carbachol, acidification transiently increased force and [Ca2+]i, followed by a reduction in both. It is concluded that changes in pHi act at more than one step in excitation-contraction coupling and that changes in [Ca2+]i can account for most of the changes in uterine force.

Myosin phosphorylation and the control of myometrial contraction/relaxation

In summary, phosphorylation of the regulatory light chain of myosin by Ca2+/CaM-dependent MLCK plays an important role in smooth muscle contraction. Although there have been major advances in our understanding of the regulation and physiological functions of contractile proteins in smooth muscle in recent years, very little information exists on the functional status of these proteins in human myometrium during pregnancy. The simple view that contractile force in smooth muscle is proportionate to cytoplasmic Ca2+ concentrations (Ca2+i) and myosin light chain phosphorylation is now more complex as more experiments provide insights into mechanisms of regulation of the contractile elements. MLCK can be phosphorylated, which desensitizes its activation by Ca2+/CaM, and protein phosphatase activity toward myosin may also be regulated. Examples in smooth muscle tissue are sparse, and the different mechanisms by which these processes may be adapted in uterine smooth muscle during pregnancy are not well-defined. Much research is needed to define further the cellular, biochemical, and molecular basis for these physiological processes involved in the regulation of uterine smooth muscle contraction and relaxation.