Membrane Cholesterol Regulates Smooth Muscle Phasic Contraction

Cholesterol and PIP2 Modulation of BKCa Channels

Ca2+/voltage-gated, large conductance K+ channels (BKCa) are formed by homotetrameric association of α (slo1) subunits. Their activity, however, is suited to tissue-specific physiology largely due to their association with regulatory subunits (β and γ types), chaperone proteins, localized signaling, and the channel's lipid microenvironment. PIP2 and cholesterol can modulate BKCa activity independently of downstream signaling, yet activating Ca2+i levels and regulatory subunits control ligand action. At physiological Ca2+i and voltages, cholesterol and PIP2 reduce and increase slo1 channel activity, respectively. Moreover, slo1 proteins provide sites that seem to recognize cholesterol and PIP2: seven CRAC motifs in the slo1 cytosolic tail and a string of positively charged residues (Arg329, Lys330, Lys331) immediately after S6, respectively. A model that could explain the modulation of BKCa activity by cholesterol and/or PIP2 is hypothesized. The roles of additional sites, whether in slo1 or BKCa regulatory subunits, for PIP2 and/or cholesterol to modulate BKCa function are also discussed.

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.

The Membrane Cholesterol Modulates the Interaction Between 17-βEstradiol and the BK Channel

BK channels are composed by the pore forming α subunit and, in some tissues, is associated with different accessory β subunits. These proteins modify the biophysical properties of the channel, amplifying the range of BK channel activation according to the physiological context. In the vascular cells, the pore forming BKα subunit is expressed with the β1 subunit, where they play an essential role in the modulation of arterial tone and blood pressure. In eukaryotes, cholesterol is a structural lipid of the cellular membrane. Changes in the ratio of cholesterol content in the plasma membrane (PM) regulates the BK channel activation altering its open probability, and hence, vascular contraction. It has been shown that the estrogen 17β-Estradiol (E2) causes a vasodilator effect in vascular cells, inducing a leftward shift in the V_0.5 of the GV curve. Here, we evaluate whether changes in the membrane cholesterol concentration modify the effect that E2 induces on the BKα/β1 channel activity. Using binding and electrophysiology assays after cholesterol depletion or enrichment, we show that the cholesterol enrichment significantly decreases the expression of the α subunit, while cholesterol depletion increased the expression of that α subunit. Additionally, we demonstrated that changes in the membrane cholesterol cause the loss of the modulatory effect of E2 on the BKα/β1 channel activity, without affecting the E2 binding to the complex. Our data suggest that changes in membrane cholesterol content could affect channel properties related to the E2 effect on BKα/β1 channel activity. Finally, the results suggest that an optimal membrane cholesterol content is essential for the activation of BK channels through the β1 subunit.

The Myometrium in Pregnant Women with Obesity

Obesity is a worldwide public health problem, affecting at least one-third of pregnant women. One of the main problems of obesity during pregnancy is the resulting high rate of cesarean section. The leading cause of this higher frequency of cesarean sections in obese women, compared with that in nonobese women, is an altered myometrial function that leads to lower frequency and potency of contractions. In this article, the disruptions of myometrial myocytes were reviewed in obese women during pregnancy that may explain the dysfunctional labor. The myometrium of obese women exhibited lower expression of connexin43, a lower function of the oxytocin receptor, and higher activity of the potassium channels. Adipokines, such as leptin, visfatin, and apelin, whose concentrations are higher in obese women, decreased myometrial contractility, perhaps by inhibiting the myometrial RhoA/ROCK pathway. The characteristically higher cholesterol levels of obese women alter myometrial myocyte cell membranes, especially the caveolae, inhibiting oxytocin receptor function, and increasing the K+ channel activity. All these changes in the myometrial cells or their environment decrease myometrial contractility, at least partially explaining the higher rate of cesarean of sections in obese women.

Cholesterol activates BK channels by increasing KCNMB1 protein levels in the plasmalemma

Calcium-/voltage-gated, large-conductance potassium channels (BKs) control critical physiological processes, including smooth muscle contraction. Numerous observations concur that elevated membrane cholesterol (CLR) inhibits the activity of homomeric BKs consisting of channel-forming alpha subunits. In mammalian smooth muscle, however, native BKs include accessory KCNMB1 (β₁) subunits, which enable BK activation at physiological intracellular calcium. Here, we studied the effect of CLR enrichment on BK currents from rat cerebral artery myocytes. Using inside-out patches from middle cerebral artery (MCA) myocytes at [Ca²⁺]_free=30 μM, we detected BK activation in response to in vivo and in vitro CLR enrichment of myocytes. While a significant increase in myocyte CLR was achieved within 5 min of CLR in vitro loading, this brief CLR enrichment of membrane patches decreased BK currents, indicating that BK activation by CLR requires a protracted cellular process. Indeed, blocking intracellular protein trafficking with brefeldin A (BFA) not only prevented BK activation but led to channel inhibition upon CLR enrichment. Surface protein biotinylation followed by Western blotting showed that BFA blocked the increase in plasmalemmal KCNMB1 levels achieved via CLR enrichment. Moreover, CLR enrichment of arteries with naturally high KCNMB1 levels, such as basilar and coronary arteries, failed to activate BK currents. Finally, CLR enrichment failed to activate BK channels in MCA myocytes from KCNMB1^-/- mouse while activation was detected in their wild-type (C57BL/6) counterparts. In conclusion, the switch in CLR regulation of BK from inhibition to activation is determined by a trafficking-dependent increase in membrane levels of KCNMB1 subunits.

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

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

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

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

Gestation changes sodium pump isoform expression, leading to changes in ouabain sensitivity, contractility, and intracellular calcium in rat uterus

Developmental and tissue‐specific differences in isoforms allow Na⁺, K⁺‐ATPase function to be tightly regulated, as they control sensitivity to ions and inhibitors. Uterine contraction relies on the activity of the Na⁺, K⁺ATPase, which creates ionic gradients that drive excitation‐contraction coupling. It is unknown whether Na⁺, K⁺ATPase isoforms are regulated throughout pregnancy or whether they have a direct role in modulating uterine contractility. We hypothesized that gestation‐dependent differential expression of isoforms would affect contractile responses to Na⁺, K⁺ATPase α subunit inhibition with ouabain. Our aims were therefore: (1) to determine the gestation‐dependent expression of mRNA transcripts, protein abundance and tissue distribution of Na⁺, K⁺ATPase isoforms in myometrium; (2) to investigate the functional effects of differential isoform expression via ouabain sensitivity; and (3) if changes in contractile responses can be explained by changes in intracellular [Ca²⁺]. Changes in abundance and distribution of the Na⁺, K⁺ATPase α, β and FXYD1 and 2 isoforms, were studied in rat uterus from nonpregnant, and early, mid‐, and term gestation. All α, β subunit isoforms (1,2,3) and FXYD1 were detected but FXYD2 was absent. The α1 and β1 isoforms were unchanged throughout pregnancy, whereas α2 and α3 significant decreased at term while β2 and FXYD1 significantly increased from mid‐term onwards. These changes in expression correlated with increased functional sensitivity to ouabain, and parallel changes in intracellular Ca²⁺, measured with Indo‐1. In conclusion, gestation induces specific regulatory changes in expression of Na⁺, K⁺ATPase isoforms in the uterus which influence contractility and may be related to the physiological requirements for successful pregnancy and delivery.

Regulation of Ca2+-Sensitive K+ Channels by Cholesterol and Bile Acids via Distinct Channel Subunits and Sites

Cholesterol (CLR) conversion into bile acids (BAs) in the liver constitutes the major pathway for CLR elimination from the body. Moreover, these steroids regulate each other's metabolism. While the roles of CLR and BAs in regulating metabolism and tissue function are well known, research of the last two decades revealed the existence of specific protein receptors for CLR or BAs in tissues with minor contribution to lipid metabolism, raising the possibility that these lipids serve as signaling molecules throughout the body. Among other lipids, CLR and BAs regulate ionic current mediated by the activity of voltage- and Ca²⁺-gated, K⁺ channels of large conductance (BK channels) and, thus, modulate cell physiology and participate in tissue pathophysiology. Initial work attributed modification of BK channel function by CLR or BAs to the capability of these steroids to directly interact with bilayer lipids and thus alter the physicochemical properties of the bilayer with eventual modification of BK channel function. Based on our own work and that of others, we now review evidence that supports direct interactions between CLR or BA and specific BK protein subunits, and the consequence of such interactions on channel activity and organ function, with a particular emphasis on arterial smooth muscle. For each steroid type, we will also briefly discuss several mechanisms that may underlie modification of channel steady-state activity. Finally, we will present novel computational data that provide a chemical basis for differential recognition of CLR vs lithocholic acid by distinct BK channel subunits and recognition sites.

Cholesterol depletion impairs contractile machinery in neonatal rat cardiomyocytes

Cholesterol regulates numerous cellular processes. Depleting its synthesis in skeletal myofibers induces vacuolization and contraction impairment. However, little is known about how cholesterol reduction affects cardiomyocyte behavior. Here, we deplete cholesterol by incubating neonatal cardiomyocytes with methyl-beta-cyclodextrin. Traction force microscopy shows that lowering cholesterol increases the rate of cell contraction and generates defects in cell relaxation. Cholesterol depletion also increases membrane tension, Ca2+ spikes frequency and intracellular Ca2+ concentration. These changes can be correlated with modifications in caveolin-3 and L-Type Ca2+ channel distributions across the sarcolemma. Channel regulation is also compromised since cAMP-dependent PKA activity is enhanced, increasing the probability of L-Type Ca2+ channel opening events. Immunofluorescence reveals that cholesterol depletion abrogates sarcomeric organization, changing spacing and alignment of α-actinin bands due to increase in proteolytic activity of calpain. We propose a mechanism in which cholesterol depletion triggers a signaling cascade, culminating with contraction impairment and myofibril disruption in cardiomyocytes.

Cholesterol influences potassium currents in inner hair cells isolated from guinea pig cochlea

OBJECTIVE

There is a correlation between serum hyperlipidemia and hearing loss. Cholesterol is an integral component of the cell membrane and regulates the activity of ion channels in the lipid bilayer. The aim of this study was to investigate the effects of cholesterol on the potassium currents in IHCs by using the cholesterol-depleting drug, MβCD, and water-soluble cholesterol.

METHODS

IHCs were acutely isolated from a mature guinea-pig cochlea and potassium currents were recorded. MβCD and water-soluble cholesterol were applied to IHCs under pressure puff pipettes.

RESULTS

IHCs showed outwardly rectifying currents (I_K,f and I_K,s) in response to depolarizing voltage pulses, with only a slight inward current (I_K,n) when hyperpolarized. In 10mM MβCD solutions, the amplitude of outward K currents reversely decreased; however, fast activation kinetics was preserved. In contrast, in solution of 1mM water-soluble cholesterol, the amplitude of outward K currents reversely increased. At the membrane potential of +110mV, relative conductances were 0.87±0.07 and 1.18±0.11 in MβCD solutions and cholesterol solutions, respectively.

CONCLUSION

The amplitude of K currents in isolated IHCs was reversely changed by cholesterol-depleting drug and water-soluble cholesterol. These results demonstrated the possibility of the involvement of IHC function in hyperlipidemia-induced inner ear disorders.

Impact of maternal obesity on the fetal electrocardiogram during labor

OBJECTIVE

Maternal obesity affects one in every five women giving birth worldwide. This condition is associated with adverse perinatal outcomes, as well as increased morbidity and mortality for mother and offspring.

METHODS

We carried out a prospective study at the University of Pecs Medical Center, Pecs, Hungary, between 1 January 2013 and 1 January 2014. We enrolled 60 obese (body mass index >30 kg/m(2)) low-risk pregnant women and 108 age-, ethnicity-, and parity-matched nonobese pregnant control subjects. The ST segment of the fetal electrocardiogram was assessed by STAN® monitoring. Neonatal outcomes and cord gas analysis of the umbilical vessels were evaluated after birth.

RESULTS

No infant with definitive metabolic acidosis was delivered in either group. We observed 32 and 106 ST events in the obese and control group, respectively, but this difference was not statistically significant. To date, none of the infants delivered as part of this study have demonstrated developmental insufficiency.

CONCLUSIONS

Obesity might not influence the fetal electrocardiogram during labor as an independent risk factor for adverse pregnancy outcomes. Studies with larger cohort sizes are needed to confirm our findings.

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.

Maternal age effects on myometrial expression of contractile proteins, uterine gene expression, and contractile activity during labor in the rat

Advanced maternal age of first time pregnant mothers is associated with prolonged and dysfunctional labor and significant risk of emergency cesarean section. We investigated the influence of maternal age on myometrial contractility, expression of contractile associated proteins (CAPs), and global gene expression in the parturient uterus. Female Wistar rats either 8 (YOUNG n = 10) or 24 (OLDER n = 10) weeks old were fed laboratory chow, mated, and killed during parturition. Myometrial strips were dissected to determine contractile activity, cholesterol (CHOL) and triglycerides (TAG) content, protein expression of connexin-43 (GJA1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caveolin 1 (CAV-1). Maternal plasma concentrations of prostaglandins PGE2, PGF2α, and progesterone were determined by RIA. Global gene expression in uterine samples was compared using Affymetrix Genechip Gene 2.0 ST arrays and Ingenuity Pathway analysis (IPA). Spontaneous contractility in myometrium exhibited by YOUNG rats was threefold greater than OLDER animals (P < 0.027) but maternal age had no significant effect on myometrial CAP expression, lipid profiles, or pregnancy-related hormones. OLDER myometrium increased contractile activity in response to PGF2α, phenylephrine, and carbachol, a response absent in YOUNG rats (all P < 0.002). Microarray analysis identified that maternal age affected expression of genes related to immune and inflammatory responses, lipid transport and metabolism, steroid metabolism, tissue remodeling, and smooth muscle contraction. In conclusion YOUNG laboring rat myometrium seems primed to contract maximally, whereas activity is blunted in OLDER animals and requires stimulation to meet contractile potential. Further work investigating maternal age effects on myometrial function is required with focus on lipid metabolism and inflammatory pathways.

Functional insights into modulation of BKCa channel activity to alter myometrial contractility

The large-conductance voltage- and Ca(2+)-activated K(+) channel (BKCa) is an important regulator of membrane excitability in a wide variety of cells and tissues. In myometrial smooth muscle, activation of BKCa plays essential roles in buffering contractility to maintain uterine quiescence during pregnancy and in the transition to a more contractile state at the onset of labor. Multiple mechanisms of modulation have been described to alter BKCa channel activity, expression, and cellular localization. In the myometrium, BKCa is regulated by alternative splicing, protein targeting to the plasma membrane, compartmentation in membrane microdomains, and posttranslational modifications. In addition, interaction with auxiliary proteins (i.e., β1- and β2-subunits), association with G-protein coupled receptor signaling pathways, such as those activated by adrenergic and oxytocin receptors, and hormonal regulation provide further mechanisms of variable modulation of BKCa channel function in myometrial smooth muscle. Here, we provide an overview of these mechanisms of BKCa channel modulation and provide a context for them in relation to myometrial function.

Functional assembly of Kv7.1/Kv7.5 channels with emerging properties on vascular muscle physiology

OBJECTIVE

Voltage-dependent K(+) (Kv) channels from the Kv7 family are expressed in blood vessels and contribute to cardiovascular physiology. Although Kv7 channel blockers trigger muscle contractions, Kv7 activators act as vasorelaxants. Kv7.1 and Kv7.5 are expressed in many vessels. Kv7.1 is under intense investigation because Kv7.1 blockers fail to modulate smooth muscle reactivity. In this study, we analyzed whether Kv7.1 and Kv7.5 may form functional heterotetrameric channels increasing the channel diversity in vascular smooth muscles.

APPROACH AND RESULTS

Kv7.1 and Kv7.5 currents elicited in arterial myocytes, oocyte, and mammalian expression systems suggest the formation of heterotetrameric complexes. Kv7.1/Kv7.5 heteromers, exhibiting different pharmacological characteristics, participate in the arterial tone. Kv7.1/Kv7.5 associations were confirmed by coimmunoprecipitation, fluorescence resonance energy transfer, and fluorescence recovery after photobleaching experiments. Kv7.1/Kv7.5 heterotetramers were highly retained at the endoplasmic reticulum. Studies in HEK-293 cells, heart, brain, and smooth and skeletal muscles demonstrated that the predominant presence of Kv7.5 stimulates release of Kv7.1/Kv7.5 oligomers out of lipid raft microdomains. Electrophysiological studies supported that KCNE1 and KCNE3 regulatory subunits further increased the channel diversity. Finally, the analysis of rat isolated myocytes and human blood vessels demonstrated that Kv7.1 and Kv7.5 exhibited a differential expression, which may lead to channel diversity.

CONCLUSIONS

Kv7.1 and Kv7.5 form heterotetrameric channels increasing the diversity of structures which fine-tune blood vessel reactivity. Because the lipid raft localization of ion channels is crucial for cardiovascular physiology, Kv7.1/Kv7.5 heteromers provide efficient spatial and temporal regulation of smooth muscle function. Our results shed light on the debate about the contribution of Kv7 channels to vasoconstriction and hypertension.

ROCK1 translocates from non-caveolar to caveolar regions upon KCl stimulation in airway smooth muscle

Airway smooth muscle (ASM) membrane depolarization through KCl opens L-type voltage dependent Ca2+ channels (Ca(v)1.2); its opening was considered the cause of KCl contraction. This substance is used to bypass intracellular second messenger pathways. It is now clear that KCl also activates RhoA/Rho kinase (ROCK) pathway. ROCK isoforms are characterized as ROCK1 and ROCK2. Because ROCK1 seems the most abundant isotype in lung, we studied its participation in KCl stimulated bovine ASM. With methyl-beta-cyclodextrin (MbetaCD) we disrupted caveolae, a membrane compartment considered as the RhoA/ROCK assembly site, and found that KCl contraction was reduced to the same extent (~26%) as Y-27632 (ROCK inhibitor) treated tissues. We confirmed that KCl induces ROCK activation and this effect was annulled by Y-27632 or MbetaCD. In isolated plasmalemma, ROCK1 was localized in non-caveolar membrane fractions in Western blots from control tissues, but it transferred to caveolae in samples from tissues stimulated with KCl. Ca(v)1.2 was found at the non-caveolar membrane fractions in control and MbetaCD treated tissues. In MbetaCD treated tissues stimulated with KCl, contraction was abolished by nifedipine; only the response to Ca(v)1.2 opening remained as the ROCK component disappeared. Our results show that, in ASM, the KCl contraction involves the translocation of ROCK1 from non-caveolar to caveolar regions and that the proper physiological response depends on this translocation.

Differential regulation of muscarinic M2 and M3 receptor signaling in gastrointestinal smooth muscle by caveolin-1

Caveolae act as scaffolding proteins for several G protein-coupled receptor signaling molecules to regulate their activity. Caveolin-1, the predominant isoform in smooth muscle, drives the formation of caveolae. The precise role of caveolin-1 and caveolae as scaffolds for G protein-coupled receptor signaling and contraction in gastrointestinal muscle is unclear. Thus the aim of this study was to examine the role of caveolin-1 in the regulation of Gq- and Gi-coupled receptor signaling. RT-PCR, Western blot, and radioligand-binding studies demonstrated the selective expression of M2 and M3 receptors in gastric smooth muscle cells. Carbachol (CCh) stimulated phosphatidylinositol (PI) hydrolysis, Rho kinase and zipper-interacting protein (ZIP) kinase activity, induced myosin phosphatase 1 (MYPT1) phosphorylation (at Thr(696)) and 20-kDa myosin light chain (MLC20) phosphorylation (at Ser(19)) and muscle contraction, and inhibited cAMP formation. Stimulation of PI hydrolysis, Rho kinase, and ZIP kinase activity, phosphorylation of MYPT1 and MLC20, and muscle contraction in response to CCh were attenuated by methyl β-cyclodextrin (MβCD) or caveolin-1 small interfering RNA (siRNA). Similar inhibition of PI hydrolysis, Rho kinase, and ZIP kinase activity and muscle contraction in response to CCh and gastric emptying in vivo was obtained in caveolin-1-knockout mice compared with wild-type mice. Agonist-induced internalization of M2, but not M3, receptors was blocked by MβCD or caveolin-1 siRNA. Stimulation of PI hydrolysis, Rho kinase, and ZIP kinase activities in response to other Gq-coupled receptor agonists such as histamine and substance P was also attenuated by MβCD or caveolin-1 siRNA. Taken together, these results suggest that caveolin-1 facilitates signaling by Gq-coupled receptors and contributes to enhanced smooth muscle function.

The adverse impact of maternal obesity on intrapartum and perinatal outcomes

Background. Confidential enquiries into maternal deaths in the UK have repeatedly highlighted increased maternal morbidity and mortality associated with maternal obesity. Objective. To determine the impact of increased body mass index (BMI) on intrapartum outcomes. Materials and Methods. A retrospective case-control analysis of intrapartum outcomes of the study group (100 women), with a BMI above 40 kg/m² (class III Obesity) at booking and a control group (100 women) with a booking BMI between 20 and 25 kg/m² was performed. Results. A statistically significant increase in delivery by caesarean section (OR 2.32, 95% CI 1.26–4.29), minor and major postpartum haemorrhage (OR 5.93, 95% CI 2.34–11.98, OR 16.11, 95% CI 2.08–125.09, resp.), perineal trauma (OR 2.59, 95% CI 1.44–4.69), and fetal macrosomia (OR 3.11, 95% CI 1.25–7.79) was noted in the study group. Babies also had an increased risk of having a lower APGAR scores in the study group as compared to the control group (OR 3.09, 95% CI 1.07–8.94). Conclusion. Women with a BMI > 40 kg/m² experience increased incidence of intrapartum complications and hence, input of skilled birth attendants during labour is essential to improve intrapartum outcomes.

The effects of watermelon (Citrullus lanatus) extracts and L-citrulline on rat uterine contractility

In uterine smooth muscle, the effects of watermelon and its citrulline content are unknown. The aims of this study were therefore, to determine the effects of watermelon extract and citrulline on the myometrium and to investigate their mechanisms of action. The effects of extracts of watermelon flesh and rind and L-citrulline (64 μmol/L) were evaluated on 3 types of contractile activity; spontaneous, those elicited by potassium chloride (KCl) depolarization, or oxytocin (10 nmol/L) application in isolated rat uterus. Inhibitors of nitric oxide (NO) and its mechanisms of action, N ω-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 100 μmol/L), LY83583 (1 μmol/L), and tetraethylamonium chloride (5 mmol/L), as well as Ca signaling pathways, were determined. Both flesh and rind extracts significantly decreased the force produced by all 3 mechanisms, in a dose-dependent manner. The extracts could also significantly decrease the force under conditions of sustained high Ca levels (depolarization and agonist) and when the force was produced only by sarcoplasmic reticulum (SR) Ca release. L-citrulline produced the same effects on force as watermelon extracts. With submaximal doses of extract, the additive effects of L-citrulline were found. The inhibitory effects of extracts and L-citrulline were reversed upon the addition of NO inhibitors, and pretreatment of tissues with these inhibitors prevented the actions of both extracts and L-citrulline. Thus, these data show that watermelon and citrulline are potent tocolytics, decreasing the force produced by calcium entry and SR release and arising by different pathways, including oxytocin stimulation. Their major mechanism is to stimulate the NO-cyclic guanosine monophosphate (cGMP) relaxant pathway.

Large conductance, calcium- and voltage-gated potassium (BK) channels: regulation by cholesterol

Cholesterol (CLR) is an essential component of eukaryotic plasma membranes. CLR regulates the membrane physical state, microdomain formation and the activity of membrane-spanning proteins, including ion channels. Large conductance, voltage- and Ca²⁺-gated K⁺ (BK) channels link membrane potential to cell Ca²⁺ homeostasis. Thus, they control many physiological processes and participate in pathophysiological mechanisms leading to human disease. Because plasmalemma BK channels cluster in CLR-rich membrane microdomains, a major driving force for studying BK channel-CLR interactions is determining how membrane CLR controls the BK current phenotype, including its pharmacology, channel sorting, distribution, and role in cell physiology. Since both BK channels and CLR tissue levels play a pathophysiological role in human disease, identifying functional and structural aspects of the CLR-BK channel interaction may open new avenues for therapeutic intervention. Here, we review the studies documenting membrane CLR-BK channel interactions, dissecting out the many factors that determine the final BK current response to changes in membrane CLR content. We also summarize work in reductionist systems where recombinant BK protein is studied in artificial lipid bilayers, which documents a direct inhibition of BK channel activity by CLR and builds a strong case for a direct interaction between CLR and the BK channel-forming protein. Bilayer lipid-mediated mechanisms in CLR action are also discussed. Finally, we review studies of BK channel function during hypercholesterolemia, and underscore the many consequences that the CLR-BK channel interaction brings to cell physiology and human disease.

Real-time dynamic movement of caveolin-1 during smooth muscle contraction of human colon and aged rat colon transfected with caveolin-1 cDNA

Caveolin-1 (cav-1) plays a key role in PKC-α and RhoA signaling pathways during acetylcholine (ACh)-induced contraction of colonic smooth muscle cells (CSMC). Aged rat CSMC showed sluggish contractility, concomitant with reduced expression of cav-1 with an associated reduction in activation of PKC-α and RhoA signaling pathway. Real-time monitoring of live human CSMC transfected with yellow fluorescent protein-tagged wild-type caveolin 1 cDNA (YFP-wt-cav-1) cDNA in the present study suggests that cav-1 cycles within and along the membrane in a synchronized, highly organized cytoskeletal path. These studies provide, for the first time, the advantages of real-time monitoring of the dynamic movement of caveolin in living cells. Rapid movement of cav-1 in response to ACh suggests its dynamic role in CSMC contraction. Human CSMC transfected with YFP-ΔTFT-cav-1 dominant negative cDNA show fluorescence in the cytosol of the CSMC and no movement of fluorescent cav-1 in response to ACh mimicking the response shown by aged rat CSMC. Transfection of CSMC from aged rat with YFP-wt-cav-1 cDNA restored the physiological contractile response to ACh as well as the dynamic movement of cav-1 along the organized cytoskeletal path observed in normal adult CSMC. To study the force generation by CSMC, three-dimensional colonic rings were bioengineered. Colonic bioengineered rings from aged CSMC showed reduced force generation compared with colonic bioengineered rings from adult CSMC. Colonic bioengineered rings from aged CSMC transfected with wt-cav-1 cDNA showed force generation similar to colonic bioengineered rings from adult rat CSMC. The data suggest that contraction in CSMC is dependent on cav-1 reorganization dynamics, which restores the physiological contractile response in aged CSMC. We hypothesize that dynamic movement of cav-1 is essential for physiological contractile response of colonic smooth muscle.

Biomechanical properties and innervation of the female caveolin-1-deficient detrusor

BACKGROUND AND PURPOSE

Caveolin-1-deficiency is associated with substantial urogenital alterations. Here, a mechanical, histological and biochemical characterization of female detrusors from wild-type and caveolin-1-deficient (KO) mice was made to increase the understanding of detrusor changes caused by lack of caveolae.

EXPERIMENTAL APPROACH

Length-tension relationships were generated, and we recorded responses to electrical field stimulation, the muscarinic receptor agonist carbachol and the purinoceptor agonist ATP. Tyrosine nitration and the contents of caveolin-1, cavin-1, muscarinic M₃ receptors, phospholipase C(β1), muscle-specific kinase (MuSK) and L-type Ca(2+) channels were determined by immunoblotting. Innervation was assessed by immunohistochemistry.

KEY RESULTS

Bladder to body weight ratio was not changed, nor was there any change in the optimum circumference for force development. Depolarization- and ATP-induced stress was reduced, as was carbachol-induced stress between 0.1 and 3 µM, but the supramaximal relative (% K(+)) response to carbachol was increased, as was M₃ expression. The scopolamine-sensitive component of the electrical field stimulation response was impaired, and yet bladder nerves contained little caveolin-1. The density of cholinergic nerves was unchanged, whereas CART- and CGRP-positive nerves were reduced. Immunoblotting revealed loss of MuSK.

CONCLUSIONS AND IMPLICATIONS

Ablation of caveolae in the female detrusor leads to generalized impairment of contractility, ruling out prostate hypertrophy as a contributing factor. Cholinergic neuroeffector transmission is impaired without conspicuous changes in the density of cholinergic nerves or morphology of their terminals, but correlating with reduced expression of MuSK.

The effects of a high-fat, high-cholesterol diet on markers of uterine contractility during parturition in the rat

Increasing levels of obesity within women of reproductive age is a major concern in the UK. Approximately, 13% of women aged <30 and 22% of 31- to 40-year-old women are obese. Obesity increases complications during pregnancy and the risk of caesarean section due to prolonged labour and poor uterine activity. The aim was to investigate whether a high-fat, high-cholesterol (HFHC) diet decreases markers of uterine contractility during parturition in the rat. Female Wistar rats were fed control (CON, n=10) or HFHC (n=10) diets for 6 weeks. Animals were mated and, once pregnant, maintained on their diet throughout gestation. On gestational day 19, rats were monitored continuously and killed at the onset of parturition. Body and fat depot weights were recorded. Myometrial tissue was analysed for cholesterol (CHOL), triglycerides (TAG), and expression of the contractile associated proteins gap junction protein alpha 1 (GJA1; also known as connexin-43, CX-43), prostaglandin-endoperoxide synthase 2 (PTGS2; also known as cyclo-oxygenase-2, COX-2) and caveolin-1 (CAV1) and maternal plasma for prostaglandin F(2)(α) (PGF(2)(α)) and progesterone. HFHC fed rats gained greater weight than CON (P<0.003) with significant increases in peri-renal fat (P<0.01). The HFHC diet increased plasma CHOL, TAG and progesterone, but decreased PGF(2)(α) versus CON (P<0.01, P<0.01, P=0.05 and P<0.02 respectively). Total CHOL and TAG levels of uterine tissue were similar. However, HFHC fed rats showed significant increases in PTGS2 (P<0.037), but decreases in GJA1 and CAV1 (P=0.059). In conclusion, a HFHC diet significantly increases body weight and alters lipid profiles that correlate with decreases in key markers of uterine contractility. Further work is required to ascertain whether these changes have adverse effects on uterine activity.

Cholesterol depletion alters amplitude and pharmacology of vascular calcium-activated chloride channels

AIMS

Calcium-activated chloride channels (CACCs) share common pharmacological properties with Kcnma1-encoded large conductance K(+) channels (BK(Ca) or K(Ca)1.1) and it has been suggested that they may co-exist in a macromolecular complex. As K(Ca)1.1 channels are known to localize to cholesterol and caveolin-rich lipid rafts (caveolae), the present study investigated whether Ca(2+)-sensitive Cl(-) currents in vascular myocytes were affected by the cholesterol depleting agent methyl-beta-cyclodextrin (M-betaCD).

METHODS AND RESULTS

Calcium-activated chloride and potassium currents were recorded from single murine portal vein myocytes in whole cell voltage clamp. Western blot was undertaken following sucrose gradient ultracentrifugation using protein lysates from whole portal veins. Ca(2+)-activated Cl(-) currents were augmented by 3 mg mL(-1) M-betaCD with a rapid time course (t(0.5) = 1.8 min). M-betaCD had no effect on the bi-modal response to niflumic acid or anthracene-9-carboxylate but completely removed the inhibitory effects of the K(Ca)1.1 blockers, paxilline and tamoxifen, as well as the stimulatory effect of the K(Ca)1.1 activator NS1619. Discontinuous sucrose density gradients followed by western blot analysis revealed that the position of lipid raft markers caveolin and flotillin-2 was altered by 15 min application of 3 mg mL(-1) M-betaCD. The position of K(Ca)1.1 and the newly identified candidate for CACCs, TMEM16A, was also affected by M-betaCD.

CONCLUSION

These data reveal that CACC properties are influenced by lipid raft integrity.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Cholesterol and ion channels

A variety of ion channels, including members of all major ion channel families, have been shown to be regulated by changes in the level of membrane cholesterol and partition into cholesterol-rich membrane domains. In general, several types of cholesterol effects have been described. The most common effect is suppression of channel activity by an increase in membrane cholesterol, an effect that was described for several types of inwardly-rectifying K(+) channels, voltage-gated K(+) channels, Ca(+2) sensitive K(+) channels, voltage-gated Na(+) channels, N-type voltage-gated Ca(+2) channels and volume-regulated anion channels. In contrast, several types of ion channels, such as epithelial amiloride-sensitive Na(+) channels and Transient Receptor Potential channels, as well as some of the types of inwardly-rectifying and voltage-gated K(+) channels were shown to be inhibited by cholesterol depletion. Cholesterol was also shown to alter the kinetic properties and current-voltage dependence of several voltage-gated channels. Finally, maintaining membrane cholesterol level is required for coupling ion channels to signalling cascades. In terms of the mechanisms, three general mechanisms have been proposed: (i) specific interactions between cholesterol and the channel protein, (ii) changes in the physical properties of the membrane bilayer and (iii) maintaining the scaffolds for protein-protein interactions. The goal of this review is to describe systematically the role of cholesterol in regulation of the major types of ion channels and to discuss these effects in the context of the models proposed.

Cholesterol depletion alters coronary artery myocyte Ca(2+) signalling in a stimulus-specific manner

Although there is evidence that caveolae and cholesterol play an important role in myocyte signalling processes, details of the mechanisms involved remain sparse. In this paper we have studied for the first time the clinically relevant intact coronary artery and measured in situ Ca(2+) signals in individual myocytes using confocal microscopy. We have examined the effect of the cholesterol-depleting agents, methyl-cyclodextrin (MCD) and cholesterol oxidase, on high K(+), caffeine and agonist-induced Ca(2+) signals. We find that cholesterol depletion produces a stimulus-specific alteration in Ca(2+) responses; with 5-HT (10microM) and endothelin-1 (10nM) responses being selectively decreased, the phenylephrine response (100microM) increased and the responses to high K(+) (60mM) and caffeine (10mM) unaffected. Agonist-induced Ca(2+) signals were restored when cholesterol was replenished using cholesterol-saturated MCD. In additional experiments, enzymatically isolated myocytes were patch clamped. We found that cholesterol depletion caused a selective modification of ion channel function, with whole cell inward Ca(2+) current being unaltered, whereas outward K(+) current was increased, due to BK(Ca) channel activation. There was also a significant decrease in cell capacitance. These data are discussed in terms of the involvement of caveolae in receptor localisation, Ca(2+) entry pathways and SR Ca(2+) release, and the role of these in agonist signalling.

Disruption of the maxi-K-caveolin-1 interaction alters current expression in human myometrial cells

BACKGROUND

One determinant of the total K+ myometrial smooth muscle cell (MSMC) current is the large conductance, calcium- and voltage-activated potassium channel (maxi-K channel). This channel provides a repolarizing current in response to excitatory stimuli, most notably in response to increases in the levels of intracellular Ca2+, and blocking the channel by pharmacological means induces the depolarization of MSMCs and also enhances contraction strength. In MSMCs, maxi-K channels can reside in the caveolae, where they associate with the scaffolding protein caveolin-1 (cav-1). The aim of this study was to investigate the consequences of this interaction - more specifically, how disruption of the association between the maxi-K channel and cav-1 may influence the current expression and excitability of myometrial cells - with the aim of better understanding the mechanisms that underlie the regulation of normal and aberrant uterine function.

METHODS

Myometrial biopsies were collected from women undergoing elective C-sections. From these samples, myometrial cells were isolated, cultured, infected with a virus containing either caveolin-1 (cav-1) siRNA or scrambled cav-1 siRNA, and finally subjected to patch-clamp analysis. Mutant caveolin-binding site maxi-K channel constructs were generated and transfected into mouse Ltk- fibroblasts. Channel activity, expression, association, and localization were examined by patch-clamping, Western blot, immunoprecipitation, and immunofluorescence, respectively.

RESULTS

The caveolin-1 siRNA suppressed the total K+ current in human myometrial smooth muscle cells (hMSMC), as evident from comparison to the currents generated by both non-infected cells and cells infected with scrambled siRNA controls. The interaction between the maxi-K channel and caveolin depends on a region in the channel's C-terminal caveolin-binding site. Mutations of aromatic residues in this site (mutant F1012A, mutant Y1007A, F1012A and mutant Y1007A, F1012A, Y1015A) resulted in a decrease in K+ current compared to that produced by wild-type channels transfected into mouse Ltk- fibroblasts. However, mutation of all three aromatic amino acids (mutant Y1007A, F1012A, Y1015A) was necessary to disrupt the association between caveolin and the maxi-K channel, as visualized by immunofluorescence and immunoprecipitation.

CONCLUSION

Our results suggest that disruption of the caveolin-binding site interferes with the cav-1/maxi-K channel interaction, and that lack of the cav-1/maxi-K channel interaction in MSMCs attenuates the total K+ channel current of the cell.

Calcium signalling in smooth muscle

Calcium signalling in smooth muscles is complex, but our understanding of it has increased markedly in recent years. Thus, progress has been made in relating global Ca2+ signals to changes in force in smooth muscles and understanding the biochemical and molecular mechanisms involved in Ca2+ sensitization, i.e. altering the relation between Ca2+ and force. Attention is now focussed more on the role of the internal Ca2+ store, the sarcoplasmic reticulum (SR), global Ca2+ signals and control of excitability. Modern imaging techniques have shown the elaborate SR network in smooth muscles, along with the expression of IP3 and ryanodine receptors. The role and cross-talk between these two Ca(2+) release mechanisms, as well as possible compartmentalization of the SR Ca2+ store are discussed. The close proximity between SR and surface membrane has long been known but the details of this special region to Ca2+ signalling and the role of local sub-membrane Ca2+ concentrations and membrane microdomains are only now emerging. The activation of K+ and Cl- channels by local Ca2+ signals, can have profound effects on excitability and hence contraction. We examine the evidence for both Ca2+ sparks and puffs in controlling ion channel activity, as well as a fundamental role for Ca2+ sparks in governing the period of inexcitability in smooth muscle, i.e. the refractory period. Finally, the relation between different Ca2+ signals, e.g. sparks, waves and transients, to smooth muscle activity in health and disease is becoming clearer and will be discussed.

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.

Insights into the uterus

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

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.

Poor uterine contractility in obese women

OBJECTIVE

The aim of the study was to elucidate the reason for the high rate of caesarean section in obese women. We examined the following hypotheses: (1) obese women have a high incidence of complications related to poor uterine contractility--caesarean section for dysfunctional labour and postpartum haemorrhage. 2) The myometrium from obese women has less ability to contract in vitro.

DESIGN

First, a clinical retrospective analysis of data from 3913 completed singleton pregnancies was performed. Secondly, in a prospective study the force, frequency and intracellular [Ca(2+)] flux of spontaneously contracting myometrium were related to the maternal body mass index.

SETTING

Liverpool Women's Hospital and University of Liverpool.

POPULATION

The clinical study involved all women who delivered in one hospital in 2002. The in vitro study myometrial biopsies were obtained from 73 women who had elective caesarean section at term.

RESULTS

Maternal obesity carried significant risk of caesarean section in labour that was highest for delay in the first stage of labour (OR 3.54). The increased risk of caesarean section in obese women largely occurred in women with normal- and not with high-birthweight infants. Obese women delivering vaginally had increased risk of prolonged first stage of labour and excessive blood loss. Myometrium from obese women contracted with less force and frequency and had less [Ca(2+)] flux than that from normal-weight women.

CONCLUSIONS

We suggest that these findings indicate that obesity may impair the ability of the uterus to contract in labour.

Oxysterol nuclear receptor LXRbeta regulates cholesterol homeostasis and contractile function in mouse uterus

The uterus is an organ where lipid distribution plays a critical role for its function. Here we show that nuclear receptor for oxysterols LXRbeta prevents accumulation of cholesteryl esters in mouse myometrium by controlling expression of genes involved in cholesterol efflux and storage (abca1 and abcg1). Upon treatment with an LXR agonist that mimics activation by oxysterols, expression of these target genes was increased in wild-type mice, whereas under basal conditions, lxralpha;beta(-/-) mice exhibited a marked decrease in abcg1 accumulation. This change resulted in a phenotype of cholesteryl ester accumulation. Besides, a defect of contractile activity induced by oxytocin or PGF2alpha was observed in mice lacking LXRbeta. These results imply that LXRbeta provides a safety valve to limit cholesteryl ester levels as a basal protective mechanism in the uterus against cholesterol accumulation and is necessary for a correct induction of contractions.

Increased Rho activation and PKC-mediated smooth muscle contractility in the absence of caveolin-1

Caveolae are omega-shaped membrane invaginations that are abundant in smooth muscle cells. Since many receptors and signaling proteins co-localize with caveolae, these have been proposed to integrate important signaling pathways. The aim of this study was to test whether RhoA/Rho-kinase and protein kinase C (PKC)-mediated Ca2+sensitization depends on caveolae using caveolin (Cav)-1-deficient (KO) and wild-type (WT) mice. In WT smooth muscle, caveolae were detected and Cav-1, -2 and -3 proteins were expressed. Relative mRNA expression levels were ∼15:1:1 for Cav-1, -2, and -3, respectively. Caveolae were absent in KO and reduced levels of Cav-2 and Cav-3 proteins were seen. In intact ileum longitudinal muscle, no differences in the responses to 5-HT or the muscarinic agonist carbachol were found, whereas contraction elicited by endothelin-1 was reduced. Rho activation by GTPγS was increased in KO compared with WT as shown using a pull-down assay. Following α-toxin permeabilization, no difference in Ca2+sensitivity or in Ca2+sensitization was detected. In KO femoral arteries, phorbol 12,13-dibutyrate (PDBu)-induced and PKC-mediated contraction was increased. This was associated with increased α1-adrenergic contraction. Following inhibition of PKC, α1-adrenergic contraction was normalized. PDBu-induced Ca2+sensitization was not increased in permeabilized femoral arteries. In conclusion, Rho activation, but not Ca2+sensitization, depends on caveolae in the ileum. Moreover, PKC driven arterial contraction is increased in the absence of caveolin-1. This depends on an intact plasma membrane and is not associated with altered Ca2+sensitivity.

Regulation of ecto-5'-nucleotidase activity via Ca2+-dependent, annexin 2-mediated membrane rearrangement?

The spatial segregation of the plasma membrane plays a prominent role in distinguishing and sorting a large number of signals a cell receives simultaneously. The plasma membrane comprises regions known as lipid rafts, which serve as signal-transduction hubs and platforms for sorting membrane-associated proteins. Ca(2+)-binding proteins of the annexin family have been ascribed a role in the regulation of raft dynamics. Glycosylphosphatidylinositol-anchored 5'-nucleotidase is an extracellular, raft-associated enzyme responsible for conversion of extracellular ATP into adenosine. Our results point to a regulation of ecto-5'-nucleotidase activity by Ca(2+)-dependent, annexin-mediated stabilization of membrane rafts.

Phenotypic conversion leads to structural and functional changes of smooth muscle sarcolemma

Continuous changes in the length of smooth muscles require a highly organized sarcolemmal structure. Yet, smooth muscle cells also adapt rapidly to altered environmental cues. Their sarcolemmal plasticity must lead to profound changes which affect transmembrane signal transduction as well as contractility. We have established porcine vascular and human visceral smooth muscle cultures of epithelioid and spindle-shaped morphology and determined their plasma membrane properties. Epithelioid cells from both sources contain a higher ratio of cholesterol to glycerophospholipids, and express a less diverse range of lipid-associated annexins. These findings point to a reduction in efficiency of membrane segregation in epithelioid cells. Moreover, compared to spindle-shaped cells, cholesterol is more readily extracted from epithelioid cells with methyl-beta-cyclodextrin and its synthesis is more susceptible to inhibition with lovastatin. The inability of epithelioid cells to process vasoactive metabolites, such as angiotensin or nucleotides further indicates that contractile properties are impaired. Phenotypic plasticity extends beyond the loss of smooth muscle cell marker genes. The plasma membrane has undergone profound functional changes which are incompatible with cyclic foreshortening, but might be important in the development of vascular disease.

Cellular cholesterol controls TRPC3 function: evidence from a novel dominant-negative knockdown strategy

TRPC3 (canonical transient receptor potential protein 3) has been suggested to be a component of cation channel complexes that are targeted to cholesterol-rich lipid membrane microdomains. In the present study, we investigated the potential role of membrane cholesterol as a regulator of cellular TRPC3 conductances. Functional experiments demonstrated that cholesterol loading activates a non-selective cation conductance and a Ca2+ entry pathway in TRPC3-overexpressing cells but not in wild-type HEK-293 (human embryonic kidney 293) cells. The cholesterol-induced membrane conductance exhibited a current-to-voltage relationship similar to that observed upon PLC (phospholipase C)-dependent activation of TRPC3 channels. Nonetheless, the cholesterol-activated conductance lacked negative modulation by extracellular Ca2+, a typical feature of agonist-activated TRPC3 currents. Involvement of TRPC3 in the cholesterol-dependent membrane conductance was further corroborated by a novel dominant-negative strategy for selective blockade of TRPC3 channel activity. Expression of a TRPC3 mutant, which contained a haemagglutinin epitope tag in the second extracellular loop, conferred antibody sensitivity to both the classical PLC-activated as well as the cholesterol-activated conductance in TRPC3-expressing cells. Moreover, cholesterol loading as well as PLC stimulation was found to increase surface expression of TRPC3. Promotion of TRPC3 membrane expression by cholesterol was persistent over 30 min, while PLC-mediated enhancement of plasma membrane expression of TRPC3 was transient in nature. We suggest the cholesterol content of the plasma membrane as a determinant of cellular TRPC3 activity and provide evidence for cholesterol dependence of TRPC3 surface expression.

Genetic ablation of caveolin-1 modifies Ca2+ spark coupling in murine arterial smooth muscle cells

L-type, voltage-dependent calcium (Ca(2+)) channels, ryanodine-sensitive Ca(2+) release (RyR) channels, and large-conductance Ca(2+)-activated potassium (K(Ca)) channels comprise a functional unit that regulates smooth muscle contractility. Here, we investigated whether genetic ablation of caveolin-1 (cav-1), a caveolae protein, alters Ca(2+) spark to K(Ca) channel coupling and Ca(2+) spark regulation by voltage-dependent Ca(2+) channels in murine cerebral artery smooth muscle cells. Caveolae were abundant in the sarcolemma of control (cav-1(+/+)) cells but were not observed in cav-1-deficient (cav-1(-/-)) cells. Ca(2+) spark and transient K(Ca) current frequency were approximately twofold higher in cav-1(-/-) than in cav-1(+/+) cells. Although voltage-dependent Ca(2+) current density was similar in cav-1(+/+) and cav-1(-/-) cells, diltiazem and Cd(2+), voltage-dependent Ca(2+) channel blockers, reduced transient K(Ca) current frequency to approximately 55% of control in cav-1(+/+) cells but did not alter transient K(Ca) current frequency in cav-1(-/-) cells. Furthermore, although K(Ca) channel density was elevated in cav-1(-/-) cells, transient K(Ca) current amplitude was similar to that in cav-1(+/+) cells. Higher Ca(2+) spark frequency in cav-1(-/-) cells was not due to elevated intracellular Ca(2+) concentration, sarcoplasmic reticulum Ca(2+) load, or nitric oxide synthase activity. Similarly, Ca(2+) spark amplitude and spread, the percentage of Ca(2+) sparks that activated a transient K(Ca) current, the amplitude relationship between sparks and transient K(Ca) currents, and K(Ca) channel conductance and apparent Ca(2+) sensitivity were similar in cav-1(+/+) and cav-1(-/-) cells. In summary, cav-1 ablation elevates Ca(2+) spark and transient K(Ca) current frequency, attenuates the coupling relationship between voltage-dependent Ca(2+) channels and RyR channels that generate Ca(2+) sparks, and elevates K(Ca) channel density but does not alter transient K(Ca) current activation by Ca(2+) sparks. These findings indicate that cav-1 is required for physiological Ca(2+) spark and transient K(Ca) current regulation in cerebral artery smooth muscle cells.

Maxi-K channels localize to caveolae in human myometrium: a role for an actin-channel-caveolin complex in the regulation of myometrial smooth muscle K+ current

Multiple cell-signaling pathways converge to modulate large-conductance, voltage- and Ca2+-sensitive K+ channel (maxi-K channel) activity and buffer cell excitability in human myometrial smooth muscle cells (hMSMCs). Recent evidence indicates that maxi-K channel proteins can target to membrane microdomains; however, their association with other proteins within these macromolecular complexes has not been elucidated. Biochemical isolation of detergent-resistant membrane fractions from human myometrium demonstrates the presence of maxi-K channels in lipid raft microdomains, which cofractionate with caveolins. In both nonpregnant and late-pregnant myometrium, maxi-K channels associate and colocalize with caveolar scaffolding proteins caveolin-1 and caveolin-2, but not caveolin-3. Disruption of cultured hMSMC caveolar complexes by cholesterol depletion with cyclodextrin increases an iberiotoxin-sensitive K+ current. Coimmunoprecipitations have indicated that the maxi-K channel also is associated with both α- and γ-actin. Immunocytochemical analysis indicates colocalization of maxi-K channels, actin, and caveolin-1 in primary cultures of hMSMCs. Further experiments using immunoelectron microscopy have shown the proximity of both actin and the maxi-K channel within the same cell surface caveolar structures. Functionally, disruption of the actin cytoskeleton in cultured hMSMCs by cytochalasin D and latrunculin A greatly increased the open-state probability of the channel, while stabilization of actin cytoskeleton with jasplakinolide abolished the effect of latrunculin A. These data indicate that the actin cytoskeleton is involved as part of a caveolar complex in the regulation of myometrial maxi-K channel function.

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.

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively “simple” mechanism involving activation of voltage-operated Ca2+channels that leads to increases in cytosolic free Ca2+([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+sensitization, the relationship between [Ca2+]iand force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]iand force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+desensitization. Such changes in Ca2+sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+sensitivity of smooth muscle contraction. KCl as a “simple” stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+sensitivity.