Ionic mechanisms and Ca2+ handling in airway smooth muscle

Association of serum total IgE and allergen-specific IgE with insulin resistance in adolescents: an analysis of the NHANES database

BACKGROUND

Recent studies have found that total immunoglobulin E (IgE) and allergen-specific IgE were associated with some metabolic diseases. However, the role of IgE in metabolism among adolescents is still unclear. Herein, this study aims to investigate the associations of serum total IgE and allergen-specific IgE with insulin resistance (IR) in adolescents, in order to provide some reference for the prevention and treatment of metabolic diseases in a young age.

METHODS

Data of 870 adolescents were extracted from the National Health and Nutrition Examination Survey (NHANES) database in 2005-2006 in this cross-sectional study. Weighted univariate and multivariate logistic regression analyses were utilized to screen covariates and explore the relationships of serum total IgE and allergen-specific IgE with IR. The evaluation indexes were odds ratios (ORs) and 95% confidence intervals (CIs). In addition, these relationships were also assessed in subgroups of allergy history, asthma history, and number of allergens.

RESULTS

Among eligible adolescents, 168 had IR. No significant association between serum total IgE level and IR was found. However, adolescents with higher level of allergen-specific IgE to rye grass [OR = 0.47, 95%CI: (0.25-0.91)], white oak [OR = 0.57, 95%CI: (0.37-0.88)], or peanut [OR = 0.38, 95%CI: (0.15-0.97)] seemed to have lower odds of IR, whereas those had higher level of shrimp-specific IgE [OR = 2.65, 95%CI: (1.21-5.84)] have increased odds of IR. In addition, these associations between allergen-specific IgE and IR were also discovered in adolescents who had allergy history or asthma history, or had different numbers of allergens.

CONCLUSION

Paying attention to different allergens in adolescents may be important in the early identification of IR among this high-risk population. The study results relatively provided some reference for further exploration on IR prevention.

The elusive Nav1.7: From pain to cancer

Voltage-gated sodium channels (Nav) are protein complexes that play fundamental roles in the transmission of signals in the nervous system, at the neuromuscular junction and in the heart. They are mainly present in excitable cells where they are responsible for triggering action potentials. Dysfunctions in Nav ion conduction give rise to a wide range of conditions, including neurological disorders, hypertension, arrhythmia, pain and cancer. Nav family 1 is composed of nine members, named numerically from 1 to 9. A Nax family also exists and is involved in body-fluid homeostasis. Of particular interest is Nav1.7 which is highly expressed in the sensory neurons of the dorsal root ganglions, where it is involved in the propagation of pain sensation. Gain-of-function mutations in Nav1.7 cause pathologies associated with increased pain sensitivity, while loss-of-function mutations cause reduced sensitivity to pain. The last decade has seen considerable effort in developing highly specific Nav1.7 blockers as pain medications, nonetheless, sufficient efficacy has yet to be achieved. Evidence is now conclusively showing that Navs are also present in many types of cancer cells, where they are involved in cell migration and invasiveness. Nav1.7 is anomalously expressed in endometrial, ovarian and lung cancers. Nav1.7 is also involved in Chemotherapy Induced Peripheral Neuropathy (CIPN). We propose that the knowledge and tools developed to study the role of Nav1.7 in pain can be exploited to develop novel cancer therapies. In this chapter, we illustrate the various aspects of Nav1.7 function in pain, cancer and CIPN, and outline therapeutic approaches.

Cellular mechanism underlying the facilitation of contractile response induced by IL-25 in mouse tracheal smooth muscle

Asthma is a common heterogeneous respiratory disease characterized by airway inflammation and airway hyperresponsiveness (AHR) which is associated with abnormality in smooth muscle contractility. The epithelial cell-derived cytokine IL-25 is implicated in type 2 immune pathology including asthma, whereas the underlying mechanisms have not been fully elucidated. This study aims to investigate the effects of IL-25 on mouse tracheal smooth muscle contractility and elucidate the cellular mechanisms. Incubation with IL-25 augmented the contraction of mouse tracheal smooth muscles, which could be suppressed by the L-type voltage-dependent Ca²⁺ channel (L-VDCC) blocker nifedipine. Furthermore, IL-25 enhanced the cytosolic Ca²⁺ signals and triggered up-regulation of α1C L-VDCC (Ca_V1.2) in primary cultured mouse tracheal smooth muscle cells. Knocking down IL-17RA/IL-17RB receptors or inhibiting the transforming growth factor-β-activated kinase 1 (TAK1)-tumor progression locus 2 (TPL2)-MAPK kinase 1/2 (MEK1/2)-ERK1/2-activating protein-1 (AP-1) signaling pathways suppressed the IL-25-elicited up-regulation of Ca_V1.2 and hyperreactivity in tracheal smooth muscles. Moreover, inhibition of TPL2, ERK1/2 or L-VDCC alleviated the AHR symptom induced by IL-25 in a murine model. This study revealed that IL-25 potentiated the contraction of tracheal smooth muscle and evoked AHR via activation of TPL2-ERK1/2-Ca_V1.2 signaling, providing novel targets for the treatment of asthma with a high-IL-25 phenotype.

In vitro contractile studies within isolated tissue baths: Translational research from Visible Heart® Laboratories

The isolated tissue bath research methodology was first developed in 1904. Since then, it has been recognized as an important tool in pharmacology and physiology research, including investigations into neuromuscular disorders. The tissue bath is still used routinely as the instrument for performing the "gold standard" test for clinical diagnosis of malignant hyperthermia susceptibility - the caffeine-halothane contracture test. Our research group has utilized this tool for several decades for a range of research studies, and we are currently one of four North American diagnostic centers for determining susceptibility for malignant hyperthermia. This review provides a brief summary of some of the historical uses of the tissue bath. Important experimental considerations for the operation of the tissue bath are further described. Finally, we discuss the different studies our group has performed using isolated tissue baths to highlight the broad potential applications.

Topical therapy with negative allosteric modulators of the calcium-sensing receptor (calcilytics) for the management of asthma: the beginning of a new era?

In this review article we present the evidence to date supporting the role of the calcium-sensing receptor (CaSR) as a key, pluripotential molecular trigger for asthma and speculate on the likely benefits of topical therapy of asthma with negative allosteric modulators of the CaSR: calcilytics.

Autocrine regulation of airway smooth muscle contraction by diacylglycerol kinase

Diacylglycerol kinase (DGK), a lipid kinase, catalyzes the conversion of diacylglycerol (DAG) to phosphatidic acid, thereby terminating DAG-mediated signaling by Gq-coupled receptors that regulate contraction of airway smooth muscle (ASM). A previous study from our laboratory demonstrated that DGK inhibition or genetic ablation leads to reduced ASM contraction and provides protection for allergen-induced airway hyperresponsiveness. However, the mechanism by which DGK regulates contractile signaling in ASM is not well established. Herein, we investigated the role of prorelaxant cAMP-protein kinase A (PKA) signaling in DGK-mediated regulation of ASM contraction. Pretreatment of human ASM cells with DGK inhibitor I activated PKA as demonstrated by the phosphorylation of PKA substrates, VASP, Hsp20, and CREB, which was abrogated when PKA was inhibited pharmacologically or molecularly using overexpression of the PKA inhibitor peptide, PKI. Furthermore, inhibition of DGK resulted in induction of cyclooxygenase (COX) and generation of prostaglandin E₂ (PGE₂ ) with concomitant activation of Gs-cAMP-PKA signaling in ASM cells in an autocrine/paracrine fashion. Inhibition of protein kinase C (PKC) or extracellular-signal-regulated kinase (ERK) attenuated DGK-mediated production of PGE₂ and activation of cAMP-PKA signaling in human ASM cells, suggesting that inhibition of DGK activates the COX-PGE₂ pathway in a PKC-ERK-dependent manner. Finally, DGK inhibition-mediated attenuation of contractile agonist-induced phosphorylation of myosin light chain 20 (MLC-20), a marker of ASM contraction, involves COX-mediated cAMP production and PKA activation in ASM cells. Collectively these findings establish a novel mechanism by which DGK regulates ASM contraction and further advances DGK as a potential therapeutic target to provide effective bronchoprotection in asthma.

Cellular Mechanism Underlying the Facilitation of Contractile Response Induced by Tumor Necrosis Factor-α in Mouse Tracheal Smooth Muscle

The proinflammatory cytokine tumor necrosis factor-α (TNF-α) augments intracellular Ca²⁺ signaling and contractile responses of airway smooth muscles, leading to airway hyperresponsiveness. However, the underlying mechanism has not been fully elucidated. This study aimed to investigate the cellular mechanism of the potentiated contraction of mouse tracheal smooth muscle induced by TNF-α. The results showed that TNF-α triggered facilitation of mouse tracheal smooth muscle contraction in an epithelium-independent manner. The TNF-α-induced hypercontractility could be suppressed by the protein kinase C inhibitor GF109203X, the tyrosine kinase inhibitor genistein, the Src inhibitor PP2, or the L-type voltage-dependent Ca²⁺ channel blocker nifedipine. After TNF-α incubation, the α_1C L-type Ca²⁺ channel (Ca_V1.2) was up-regulated in primary cultured mouse tracheal smooth muscle cells. Pronounced phosphotyrosine levels also were observed in mouse tracheas. In conclusion, this study showed that TNF-α enhanced airway smooth muscle contraction via protein kinase C-Src-Ca_V1.2 pathways, which provides novel insights into the pathologic role of proinflammatory cytokines in mediating airway hyperresponsiveness.

Investigation of the relaxing effect of a camphor nanoemulsion on rat isolated trachea

Asthma is a chronic inflammatory disease that targeting lower airways, being characterized by bronchial smooth muscle hyper responsiveness and mucus hypersecretion. Asthma is considered the most common respiratory disease in the world, affecting approximately 235 million individuals. The main therapy sometimes fails to establish clinical improvement in patients, which leads to a constant search for new alternatives. Camphor is a transparent solid monoterpene with a strong aroma, which due to its high lipophilicity is insoluble in water. Nanostructured carrier systems have shown promise as a delivery system for lipophilic compounds such as monoterpenes. Therefore, the objective of this work was to evaluate the relaxant effect of nanoemulsified camphor (NEC), as well as the mechanism of action of that monoterpene, in isolated rat trachea. The results obtained demonstrated that NEC promote relaxation of the isolated rat trachea when smooth muscle contraction was induced by both carbachol (CCh) and KCl, presenting a pCE50 of 2.25 ± 0.27 and 3.30 ± 0.07, respectively. In the presence of dexamethasone (DEXA), tetraethylammonium (TEA), glibenclamide (GLIB), 1H-[1,2,4]-oxadiazole-[4,3,-a]-quinoxaline-1-one (ODQ) and ruthenium red (RR) there was a significant difference in at least one of the evaluated pharmacological parameters, such as concentration-response curves shape, Emax or pCE50. As conclusion, NEC may be involved with β-adrenergic receptors, channels for K+ sensitive to ATP (KATP) or Channels for K+ opened by Ca2+ (KCa), increase in prostanoids and with receptor channel with transient potential (TRPv). In conclusion, β-adrenergic receptors, prostanoids, nitric oxide (NO), ATP-sensitive K+ channels (KATP), Ca2+-opened K+ channels (KCa), and transient receptor potential cation channel subfamily V (TRPV) are involved in the relaxing effect of NEC. In addition, the mechanism of action of NEC may be involved with the signal transduction pathway Nitric Oxide/soluble guanylyl cyclase/cGMP/cGMP-activated protein kinase. NEC, therefore, demonstrates spasmolytic activity when presenting tracheal relaxation compared to CCh and KCl contracturants.

Cannabis roots: Pharmacological and toxicological studies in mice

ETHNOPHARMACOLOGICAL RELEVANCE

There are many studies and therapeutic properties attributed to the flowers and leaves of the Cannabis species, but even with few pharmacological studies, Cannabis sativa L. (Cannabaceae) roots presents several therapeutic indications in folk medicine.

AIM OF THE STUDY

This study aimed to evaluate the anti-inflammatory and spasmolytic effects as well as the toxicological profile of the aqueous extract of Cannabis sativa roots (CsAqEx) in mice.

MATERIALS AND METHODS

We assessed the anti-inflammatory effect with carrageenan-induced leukocyte migration assay, and carrageenan and histamine-induced paw edema methods; The spasmolytic effect was assessed through in vitro assays with isolated mice trachea. To assess motor coordination and mobility, mice went through the rotarod and open field tests, respectively. For the single-dose toxicity study, we administered CsAqEx at the dose of 1000 mg/kg by gavage. In a repeated dose toxicity study, animals received CsAqEx at doses of 25 mg or 100 mg/kg for 28 days.

RESULTS

The CsAqEx inhibited the migration of leukocytes at the doses of 25, 50, and 100 mg/kg. The CsAqEx showed anti-inflammatory activity after the intraplantar injection of carrageenan, presenting a reduction in edema formation at all tested doses (12.5, 25, 50 and 100 mg/kg). The dose of 12.5 mg/kg of CsAqEx prevented edema formation after intraplantar injection of histamine. In an organ bath, 729 μg/mL of CsAqEx did not promote spasmolytic effect on isolated mice tracheal rings contracted by carbachol (CCh) or potassium chloride (KCl). We did not observe clinical signs of toxicity in the animals after acute treatment with CsAqEx, which suggested that the median lethal dose (LD₅₀) is greater than 1000 mg/kg. Repeated dose exposure to the CsAqEx did not produce significant changes in hematological, biochemical, or organ histology parameters.

CONCLUSIONS

The results suggest that the anti-inflammatory effect of CsAqEx is related to the reduction of vascular extravasation and migration of inflammatory cells, without effects on the central nervous system. Moreover, there was no spasmolytic effect on airway smooth muscle and no toxicity was observed on mice.

Relaxation effects of Eriobotrya japonica toward tracheal smooth muscle via action mechanism on histamine-1 receptor and phosphodiesterase-5 enzyme

The Eriobotrya japonica leaves have the activity to relax the smooth muscle in the respiratory tract. However, the mechanism of action due to that activity has never been carried out. This study aims to determine the relaxation effects of E. japonica leaves extract in the isolated trachea of the guinea pigs through the inhibition of the histamine-1 (H-1) receptor and the phosphodiesterase-5 (PDE-5) enzyme. The determination of the relaxation effects was carried out by using histamine to contract smooth muscle within the tracheal tract, followed by adding cumulative concentrations of extract. Michaelis–Menten kinetics equation was used to determine the antagonist type of extract toward H-1 receptor. The understanding of mechanism of action of the extract toward PDE-5 enzyme was performed by incubating the smooth muscle using sildenafil. The percentage value of responses, originated from the relaxation effect of the extract toward the trachea was analyzed by using the t-independent test. The result showed that the extract was able to relax the smooth muscle, which was contracted by histamine, and there was a positive correlation between concentration and relaxation effect (P < 0.05; r = 0.973). The extract also antagonized the histamine as a noncompetitive antagonist. The incubation within the trachea with sildenafil demonstrated equal relaxation effect, produced by the extract. It can be concluded that E. japonica extract had relaxation effect within the isolated trachea as antagonist noncompetitive toward H-1 receptor and inhibitor of the PDE-5 enzyme.

The role of diacylglycerol kinases in allergic airway disease

Asthma is an obstructive inflammatory airway disease. Airway obstruction is mediated by hyperresponsive airway smooth muscle cell contraction, which is induced and compounded by inflammation caused by T lymphocytes. One important signal transduction pathway that is involved in the activation of these cell types involves the generation of a lipid second messenger known as diacylglycerol (DAG). DAG levels are controlled in cells by a negative regulator known as DAG kinase (DGK). In this review, we discuss how the DAG signaling pathway attenuates the pathological function of immune cells and airway smooth muscle cells in allergic airway disease and asthma. Furthermore, we discuss how the enhancement of the DAG signaling pathway through the inhibition of DGK may represent a novel therapeutic strategy for these diseases.

Research progress in the mechanism of calcium ion on contraction and relaxation of airway smooth muscle cells

lntracellular calcium ion is the key secondary messenger system of the cellular processes in airway smooth muscle cells(ASMc). The treatment and regulation of Ca²⁺ in airway smooth muscle (ASM) is, in part, to associated with many airway diseases such as asthma, COPD and pulmonary fibrosis. The mechanism of contraction and relaxation of ASM is a concerned aspect in airway diseases. This review emphasizes established and recent discoveries whice show the research progress of Ca²⁺ on cell contraction and relaxation in ASM in recent years, to provide theoretical support and new targets for clinical prevention and treatment of perioperative bronchospasm and variousrespiratory related diseases.

Canonical Transient Potential Receptor-3 Channels in Normal and Diseased Airway Smooth Muscle Cells

All seven canonical transient potential receptor (TRPC1-7) channel members are expressed in mammalian airway smooth muscle cells (ASMCs). Among this family, TRPC3 channel plays an important role in the control of the resting [Ca²⁺]_i and agonist-induced increase in [Ca²⁺]_i. This channel is significantly upregulated in molecular expression and functional activity in airway diseases. The upregulated channel significantly augments the resting [Ca²⁺]_i and agonist-induced increase in [Ca²⁺]_i, thereby exerting a direct and essential effect in airway hyperresponsiveness. The increased TRPC3 channel-mediated Ca²⁺ signaling also results in the transcription factor nuclear factor-κB (NF-κB) activation via protein kinase C-α (PKCα)-dependent inhibitor of NFκB-α (IκBα) and calcineurin-dependent IκBβ signaling pathways, which upregulates cyclin-D1 expression and causes cell proliferation, leading to airway remodeling. TRPC3 channel may further interact with intracellular release Ca²⁺ channels, Orai channels and Ca²⁺-sensing stromal interaction molecules, mediating important cellular responses in ASMCs and the development of airway diseases.

Estrogen receptors differentially regulate intracellular calcium handling in human nonasthmatic and asthmatic airway smooth muscle cells

Asthma is defined as chronic inflammation of the airways and is characterized by airway remodeling, hyperresponsiveness, and acute bronchoconstriction of airway smooth muscle (ASM) cells. Clinical findings suggest a higher incidence and severity of asthma in adult women, indicating a concrete role of sex steroids in modulating the airway tone. Estrogen, a major female sex steroid mediates its role through estrogen receptors (ER) ERα and ERβ, which are shown to be expressed in human ASM, and their expression is upregulated in lung inflammation and asthma. Previous studies suggested rapid, nongenomic signaling of estrogen via ERs reduces intracellular calcium ([Ca²⁺]_i), thereby promoting relaxation of ASM. However, long-term ER activation on [Ca²⁺]_i regulation in human ASM during inflammation or in asthma is still not known. In Fura-2-loaded nonasthmatic and asthmatic human ASM cells, we found that prolonged (24 h) exposure to ERα agonist (PPT) increased [Ca²⁺]_i response to histamine, whereas ERβ activation (WAY) led to decreased [Ca²⁺] compared with vehicle. This was further confirmed by ER overexpression and knockdown studies using various bronchoconstrictor agents. Interestingly, ERβ activation was more effective than 17β-estradiol in reducing [Ca²⁺]_i responses in the presence of TNF-α or IL-13, while no observable changes were noticed with PPT in the presence of either cytokine. The [Ca²⁺]_i-reducing effects of ERβ were mediated partially via L-type calcium channel inhibition and increased Ca²⁺ sequestration by sarcoplasmic reticulum_. Overall, these data highlight the differential signaling of ERα and ERβ in ASM during inflammation. Specific ERβ activation reduces [Ca²⁺]_i in the inflamed ASM cells and is likely to play a crucial role in regulating ASM contractility, thereby relaxing airways.

Naringin and Naringenin Relax Rat Tracheal Smooth by Regulating BKCa Activation

Naringin and its aglycone, naringenin, occur naturally in our regular diet and traditional Chinese medicines. This study aimed to detect an effective therapeutic approach for cough variant asthma (CVA) through evaluating the relaxant effect of these two bioactive herbal monomers as antitussive and antiasthmatic on rat tracheal smooth muscle. The relaxant effect was determined by measuring muscular tension with a mechanical recording system in rat tracheal rings. Cytosolic Ca²⁺ concentration was measured using a confocal imaging system in primary cultured tracheal smooth muscle cells. In rat tracheal rings, addition of both naringin and naringenin could concentration dependently relax carbachol (CCh)-evoked tonic contraction. This epithelium-independent relaxation could be suppressed by BaCl₂, tetraethylammonium, and iberiotoxin (IbTX), but not by glibenclamide. After stimulating primary cultured tracheal smooth muscle cells by CCh or high KCl, the intracellular Ca²⁺ increase could be inhibited by both naringin and naringenin, respectively. This reaction was also suppressed by IbTX. These results demonstrate that both naringin and naringenin can relax tracheal smooth muscle through opening big conductance Ca²⁺-activated K⁺ channel, which mediates plasma membrane hyperpolarization and reduces Ca²⁺ influx. Our data indicate a potentially effective therapeutic approach of naringin and naringenin for CVA.

Inflammatory mediators mediate airway smooth muscle contraction through a G protein-coupled receptor-transmembrane protein 16A-voltage-dependent Ca2+ channel axis and contribute to bronchial hyperresponsiveness in asthma

BACKGROUND

Allergic inflammation has long been implicated in asthmatic hyperresponsiveness of airway smooth muscle (ASM), but its underlying mechanism remains incompletely understood. Serving as G protein-coupled receptor agonists, several inflammatory mediators can induce membrane depolarization, contract ASM, and augment cholinergic contractile response. We hypothesized that the signal cascade integrating on membrane depolarization by the mediators might involve asthmatic hyperresponsiveness.

OBJECTIVE

We sought to investigate the signaling transduction of inflammatory mediators in ASM contraction and assess its contribution in the genesis of hyperresponsiveness.

METHODS

We assessed the capacity of inflammatory mediators to induce depolarization currents by electrophysiological analysis. We analyzed the phenotypes of transmembrane protein 16A (TMEM16A) knockout mice, applied pharmacological reagents, and measured the Ca²⁺ signal during ASM contraction. To study the role of the depolarization signaling in asthmatic hyperresponsiveness, we measured the synergistic contraction by methacholine and inflammatory mediators both ex vivo and in an ovalbumin-induced mouse model.

RESULTS

Inflammatory mediators, such as 5-hydroxytryptamin, histamine, U46619, and leukotriene D₄, are capable of inducing Ca²⁺-activated Cl^- currents in ASM cells, and these currents are mediated by TMEM16A. A combination of multiple analysis revealed that a G protein-coupled receptor-TMEM16A-voltage-dependent Ca²⁺ channel signaling axis was required for ASM contraction induced by inflammatory mediators. Block of TMEM16A activity may significantly inhibit the synergistic contraction of acetylcholine and the mediators and hence reduces hypersensitivity.

CONCLUSIONS

A G protein-coupled receptor-TMEM16A-voltage-dependent Ca²⁺ channel axis contributes to inflammatory mediator-induced ASM contraction and synergistically activated TMEM16A by allergic inflammatory mediators with cholinergic stimuli.

Regulation of RhoA/ROCK and sustained arterial contraction by low cytosolic Ca2+ levels during prolonged depolarization of arterial smooth muscle

The role of L-type Ca²⁺ channels (LTCCs) and RhoA/Rho kinase (ROCK) on depolarization-induced sustained arterial contraction lasting several minutes is already known. However, in vivo, vascular smooth muscle cells can be depolarized for longer periods, inducing substantial inactivation of LTCCs and markedly reducing Ca²⁺ influx into the myocytes. We have examined, in femoral arterial rings, the role of LTCCs and RhoA/ROCK during long-lasting depolarization. Our results reveal a new vasoreactive response after 20-30min of depolarization in 2.5mM external Ca²⁺ that has not been identified previously with shorter stimuli. Prolonged depolarization-induced arterial contraction was permanently abolished when arterial rings were treated with 100nM external Ca²⁺ or 20nM nifedipine. However, when Ca²⁺ influx was restricted, applying ~7μM external Ca²⁺ solution or 3nM nifedipine, vasorelaxation was transient, and isometric force slowly increased after 30min and maintained its level until the end of the stimulus. Under these conditions, arterial contraction showed the same temporal course of RhoA activity and was sensitive to fasudil, nifedipine and cyclopiazonic acid. Ca²⁺-response curve in β-escin permeabilized arteries was also sensitive to ROCK inhibitors. Thus, although long-lasting depolarization inactivates LTCCs, the reduced Ca²⁺ entry can induce a detectable arterial contraction via RhoA/ROCK activation.

Emerging concepts in smooth muscle contributions to airway structure and function: implications for health and disease

Airway structure and function are key aspects of normal lung development, growth, and aging, as well as of lung responses to the environment and the pathophysiology of important diseases such as asthma, chronic obstructive pulmonary disease, and fibrosis. In this regard, the contributions of airway smooth muscle (ASM) are both functional, in the context of airway contractility and relaxation, as well as synthetic, involving production and modulation of extracellular components, modulation of the local immune environment, cellular contribution to airway structure, and, finally, interactions with other airway cell types such as epithelium, fibroblasts, and nerves. These ASM contributions are now found to be critical in airway hyperresponsiveness and remodeling that occur in lung diseases. This review emphasizes established and recent discoveries that underline the central role of ASM and sets the stage for future research toward understanding how ASM plays a central role by being both upstream and downstream in the many interactive processes that determine airway structure and function in health and disease.

Targeting the γ-Aminobutyric Acid A Receptor α4 Subunit in Airway Smooth Muscle to Alleviate Bronchoconstriction

We previously demonstrated that airway smooth muscle (ASM) cells express γ-aminobutyric acid A receptors (GABA(A)Rs), and that GABA(A)R agonists acutely relax ASM. Among the GABA(A)R α subunits, human ASM cells express only α4 and α5, providing the opportunity for selective pharmacologic targeting. Novel GABA(A)R-positive allosteric modulators designed for enhanced α4/α6 subunit selectivity were synthesized using iterative computational analyses (CMD-45 and XHe-III-74). Studies using oocyte heterologous expression systems confirmed that CMD-45 and XHe-III-74 led to significantly greater augmentation of currents induced by a 3% maximal effective concentration (EC3) of GABA [EC3]-induced currents in oocytes expressing α4 or α6 subunits (along with β3 and γ2) compared with other α subunits. CMD-45 and XHe-III-74 also led to greater ex vivo relaxation of contracted wild-type mouse tracheal rings compared with tracheal rings from GABA(A)R α4 subunit (Gabra4) knockout mice. Furthermore, CMD-45 and XHe-III-74 significantly relaxed precontracted human ASM ex vivo, and, at a low concentration, both ligands led to a significant leftward shift in albuterol-mediated ASM relaxation. In vivo, inhaled XHe-III-74 reduced respiratory system resistance in an asthmatic mouse model. Pretreatment of human ASM cells with CMD-45 and XHe-III-74 inhibited histamine-induced increases in intracellular calcium concentrations in vitro, an effect that was lost when calcium was omitted from the extracellular buffer, suggesting that inhibition of calcium influx due to alterations in plasma membrane potential may play a role in the mechanism of ASM relaxation. Selective targeting of the GABA(A)R α4 subunit with inhaled ligands may be a novel therapeutic pathway to treat bronchoconstriction, while avoiding sedative central nervous system effects, which are largely mediated by α1-3 subunit-containing GABA(A)Rs in the brain.

Cystic Fibrosis Transmembrane Conductance Regulator in Sarcoplasmic Reticulum of Airway Smooth Muscle. Implications for Airway Contractility

RATIONALE

An asthma-like airway phenotype has been described in people with cystic fibrosis (CF). Whether these findings are directly caused by loss of CF transmembrane conductance regulator (CFTR) function or secondary to chronic airway infection and/or inflammation has been difficult to determine.

OBJECTIVES

Airway contractility is primarily determined by airway smooth muscle. We tested the hypothesis that CFTR is expressed in airway smooth muscle and directly affects airway smooth muscle contractility.

METHODS

Newborn pigs, both wild type and with CF (before the onset of airway infection and inflammation), were used in this study. High-resolution immunofluorescence was used to identify the subcellular localization of CFTR in airway smooth muscle. Airway smooth muscle function was determined with tissue myography, intracellular calcium measurements, and regulatory myosin light chain phosphorylation status. Precision-cut lung slices were used to investigate the therapeutic potential of CFTR modulation on airway reactivity.

MEASUREMENTS AND MAIN RESULTS

We found that CFTR localizes to the sarcoplasmic reticulum compartment of airway smooth muscle and regulates airway smooth muscle tone. Loss of CFTR function led to delayed calcium reuptake following cholinergic stimulation and increased myosin light chain phosphorylation. CFTR potentiation with ivacaftor decreased airway reactivity in precision-cut lung slices following cholinergic stimulation.

CONCLUSIONS

Loss of CFTR alters porcine airway smooth muscle function and may contribute to the airflow obstruction phenotype observed in human CF. Airway smooth muscle CFTR may represent a therapeutic target in CF and other diseases of airway narrowing.

Menthol inhibiting parasympathetic function of tracheal smooth muscle

Menthol is used as a constituent of food and drink, tobacco and cosmetics nowadays. This cold receptor agonist has been used as a nasal inhalation solution in the daily life. The effect of menthol on nasal mucosa in vivo is well known; however, the effect of the drug on tracheal smooth muscle has been rarely explored. Therefore, during administration of the drug for nasal symptoms, it might also affect the trachea via oral intake or inhalation. We used our preparation to test the effectiveness of menthol on isolated rat tracheal smooth muscle. A 5 mm long portion of rat trachea was submersed in 30 ml Krebs solution in a muscle bath at 37ºC. Changes in tracheal contractility in response to the application of a parasympathetic mimetic agent were measured using a transducer connected to a Pentium III computer equipped with polygraph software. The following assessments of menthol were performed: (1) effect on tracheal smooth muscle resting tension; (2) effect on contraction caused by 10^-6 M methacholine as a parasympathetic mimetic; (3) effect of the drug on electrically induced tracheal smooth muscle contractions. Results indicated that addition of a parasympathetic mimetic to the incubation medium caused the trachea to contract in a dose-dependent manner. Addition of menthol at doses of 10^-5 M or above elicited a relaxation response to 10^-6 M methacholine-induced contraction. Menthol could also inhibit electrical field stimulation (EFS) induced spike contraction. However, it alone had a minimal effect on the basal tension of trachea as the concentration increased. We concluded that the degree of drug-induced tracheal contraction or relaxation was dose-dependent. In addition, this study indicated that high concentrations of menthol might actually inhibit parasympathetic function of the trachea.

Matrix stiffness-modulated proliferation and secretory function of the airway smooth muscle cells

Multiple pulmonary conditions are characterized by an abnormal misbalance between various tissue components, for example, an increase in the fibrous connective tissue and loss/increase in extracellular matrix proteins (ECM). Such tissue remodeling may adversely impact physiological function of airway smooth muscle cells (ASMCs) responsible for contraction of airways and release of a variety of bioactive molecules. However, few efforts have been made to understand the potentially significant impact of tissue remodeling on ASMCs. Therefore, this study reports how ASMCs respond to a change in mechanical stiffness of a matrix, to which ASMCs adhere because mechanical stiffness of the remodeled airways is often different from the physiological stiffness. Accordingly, using atomic force microscopy (AFM) measurements, we found that the elastic modulus of the mouse bronchus has an arithmetic mean of 23.1 ± 14 kPa (SD) (median 18.6 kPa). By culturing ASMCs on collagen-conjugated polyacrylamide hydrogels with controlled elastic moduli, we found that gels designed to be softer than average airway tissue significantly increased cellular secretion of vascular endothelial growth factor (VEGF). Conversely, gels stiffer than average airways stimulated cell proliferation, while reducing VEGF secretion and agonist-induced calcium responses of ASMCs. These dependencies of cellular activities on elastic modulus of the gel were correlated with changes in the expression of integrin-β1 and integrin-linked kinase (ILK). Overall, the results of this study demonstrate that changes in matrix mechanics alter cell proliferation, calcium signaling, and proangiogenic functions in ASMCs.

No evidence for altered intracellular calcium-handling in airway smooth muscle cells from human subjects with asthma

Background

Asthma is characterized by airway hyper-responsiveness and variable airflow obstruction, in part as a consequence of hyper-contractile airway smooth muscle, which persists in primary cell culture. One potential mechanism for this hyper-contractility is abnormal intracellular Ca²⁺ handling.

Methods

We sought to compare intracellular Ca²⁺ handling in airway smooth muscle cells from subjects with asthma compared to non-asthmatic controls by measuring: i) bradykinin-stimulated changes in inositol 1,4,5-trisphosphate (IP₃) accumulation and intracellular Ca²⁺ concentration, ii) sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) expression, iii) mechanisms of cytoplasmic Ca²⁺ clearance assessed following instantaneous flash photolytic release of Ca²⁺ into the cytoplasm.

Results

We found no differences in airway smooth muscle cell basal intracellular Ca²⁺ concentrations, bradykinin-stimulated IP₃ accumulation or intracellular Ca²⁺ responses. Quantification of SERCA2 mRNA or protein expression levels revealed no differences in ASM cells obtained from subjects with asthma compared to non-asthmatic controls. We did not identify differences in intracellular calcium kinetics assessed by flash photolysis and calcium uncaging independent of agonist-activation with or without SERCA inhibition. However, we did observe some correlations in subjects with asthma between lung function and the different cellular measurements of intracellular Ca²⁺ handling, with poorer lung function related to increased rate of recovery following flash photolytic elevation of cytoplasmic Ca²⁺ concentration.

Conclusions

Taken together, the experimental results reported in this study do not demonstrate major fundamental differences in Ca²⁺ handling between airway smooth muscle cells from non-asthmatic and asthmatic subjects. Therefore, increased contraction of airway smooth muscle cells derived from asthmatic subjects cannot be fully explained by altered Ca²⁺ homeostasis.

Selective targeting of the α5-subunit of GABAA receptors relaxes airway smooth muscle and inhibits cellular calcium handling

The clinical need for novel bronchodilators for the treatment of bronchoconstrictive diseases remains a major medical issue. Modulation of airway smooth muscle (ASM) chloride via GABAA receptor activation to achieve relaxation of precontracted ASM represents a potentially beneficial therapeutic option. Since human ASM GABAA receptors express only the α4- and α5-subunits, there is an opportunity to selectively target ASM GABAA receptors to improve drug efficacy and minimize side effects. Recently, a novel compound (R)-ethyl8-ethynyl-6-(2-fluorophenyl)-4-methyl-4H-benzo[f]imidazo[1,5-a][1,4] diazepine-3-carboxylate (SH-053-2'F-R-CH3) with allosteric selectivity for α5-subunit containing GABAA receptors has become available. We questioned whether this novel GABAA α5-selective ligand relaxes ASM and affects intracellular calcium concentration ([Ca(2+)]i) regulation. Immunohistochemical staining localized the GABAA α5-subunit to human ASM. The selective GABAA α5 ligand SH-053-2'F-R-CH3 relaxes precontracted intact ASM; increases GABA-activated chloride currents in human ASM cells in voltage-clamp electrophysiology studies; and attenuates bradykinin-induced increases in [Ca(2+)]i, store-operated Ca(2+) entry, and methacholine-induced Ca(2+) oscillations in peripheral murine lung slices. In conclusion, selective subunit targeting of endogenous α5-subunit containing GABAA receptors on ASM may represent a novel therapeutic option to treat severe bronchospasm.

Revisiting the usefulness of thromboxane-A2 modulation in the treatment of bronchoconstriction in asthma

Airway smooth muscle (ASM) is the effector cell in the bronchoconstrictory pathway. It is believed that the bronchoconstriction present in asthma is associated with changes in the airway milieu that affect ASM excitation-contraction coupling and Ca(2+)-handling. Asthmatics also react differently to ventilatory mechanical strain. Deep inspiration (DI), which produces bronchodilation in healthy individuals, is less effective in asthmatics, and even enhances bronchoconstriction in moderate to severely affected patients. Our laboratory has previously studied the mechanotransductory pathway of airway stretch-activated contractions (Rstretch) leading to DI-induced bronchoconstriction. We demonstrated the ability of agonists acting through thromboxane A2 (TxA2) receptors to amplify airway Rstretch responses. Despite the involvement of excitatory prostanoids in bronchoconstriction, clinical trials on treatments targeting TxA2-synthase inhibition and TP-receptor antagonism have produced mixed results. Studies in Western populations produced mostly negative results, whereas studies performed in Asian populations showed mostly positive outcomes. In this review, we discuss the role of TxA2-synthase inhibition and TP-receptor antagonism in the treatment of asthmatics. We present information regarding variations in study designs and the possible role of TP-receptor gene polymorphisms in previous study outcome discrepancies. Perhaps future studies should focus on asthmatic patients with DI-induced bronchoconstriction in particular, planting the seed for the individualized treatments for asthmatics.

Cigarette smoke enhances proliferation and extracellular matrix deposition by human fetal airway smooth muscle

Cigarette smoke is a common environmental insult associated with increased risk of developing airway diseases such as wheezing and asthma in neonates and children. In adults, asthma involves airway remodeling characterized by increased airway smooth muscle (ASM) cell proliferation and increased extracellular matrix (ECM) deposition, as well as airway hyperreactivity. The effects of cigarette smoke on remodeling and contractility in the developing airway are not well-elucidated. In this study, we used canalicular-stage (18-20 wk gestational age) human fetal airway smooth muscle (fASM) cells as an in vitro model of the immature airway. fASM cells were exposed to cigarette smoke extract (CSE; 0.5-1.5% for 24-72 h), and cell proliferation, ECM deposition, and intracellular calcium ([Ca(2+)]i) responses to agonist (histamine 10 μM) were used to evaluate effects on remodeling and hyperreactivity. CSE significantly increased cell proliferation and deposition of ECM molecules collagen I, collagen III, and fibronectin. In contrast, [Ca(2+)]i responses were not significantly affected by CSE. Analysis of key signaling pathways demonstrated significant increase in extracellular signal-related kinase (ERK) and p38 activation with CSE. Inhibition of ERK or p38 signaling prevented CSE-mediated changes in proliferation, whereas only ERK inhibition attenuated the CSE-mediated increase in ECM deposition. Overall, these results demonstrate that cigarette smoke may enhance remodeling in developing human ASM through hyperplasia and ECM production, thus contributing to development of neonatal and pediatric airway disease.

Arachidonate-regulated Ca(2+) influx in human airway smooth muscle

Plasma membrane Ca(2+) influx, especially store-operated Ca(2+) entry triggered by sarcoplasmic reticulum (SR) Ca(2+) release, is a key component of intracellular calcium concentration ([Ca(2+)]i) regulation in airway smooth muscle (ASM). Agonist-induced Ca(2+) oscillations in ASM that involve both influx and SR mechanisms have been previously demonstrated. In nonexcitable cells, [Ca(2+)]i oscillations involve Ca(2+) influx via arachidonic acid (AA) -stimulated channels, which show similarities to store-operated Ca(2+) entry, although their molecular identity remains undetermined. Little is known about AA-regulated Ca(2+) channels or their regulation in ASM. In enzymatically dissociated human ASM cells loaded with the Ca(2+) indicator, fura-2, AA (1-10 μM) triggered [Ca(2+)]i oscillations that were inhibited by removal of extracellular Ca(2+). Other fatty acids, such as the diacylglycerol analog, 1-oleoyl-2-acetyl-SN-glycerol, oleic acid, and palmitic acid (10 μM each), failed to elicit similar [Ca(2+)]i responses. Preincubation with LaCl3 (1 μM or 1 mM) inhibited AA-induced oscillations. Inhibition of receptor-operated channels (SKF96,365 [10 μM]), lipoxygenase (zileuton [10 μM]), or cyclooxygenase (indomethacin [10 μM]) did not affect oscillation parameters. Inhibition of SR Ca(2+) release (ryanodine [10 μM] or inositol 1,4,5-trisphosphate receptor inhibitor, xestospongin C [1 μM]) decreased [Ca(2+)]i oscillation frequency and amplitude. Small interfering RNA against caveolin-1, stromal interaction molecule 1, or Orai3 (20 nM each) reduced the frequency and amplitude of AA-induced [Ca(2+)]i oscillations. In ASM cells derived from individuals with asthma, AA increased oscillation amplitude, but not frequency. These results are highly suggestive of a novel AA-mediated Ca(2+)-regulatory mechanism in human ASM, reminiscent of agonist-induced oscillations. Given the role of AA in ASM intracellular signaling, especially with inflammation, AA-regulated Ca(2+) channels could potentially contribute to increased [Ca(2+)]i in diseases such asthma.

Ca2+ handling and sensitivity in airway smooth muscle: emerging concepts for mechanistic understanding and therapeutic targeting

Free calcium ions within the cytosol serve as a key secondary messenger system for a diverse range of cellular processes. Dysregulation of cytosolic Ca(2+) handling in airway smooth muscle (ASM) has been implicated in asthma, and it has been hypothesised that this leads, at least in part, to associated changes in both the architecture and function of the lung. Significant research is therefore directed towards furthering our understanding of the mechanisms which control ASM cytosolic calcium, in addition to those regulating the sensitivity of its downstream effector targets to calcium. Key aspects of the recent developments in this field were discussed at the 8th Young Investigators' Symposium on Smooth Muscle (2013, Groningen, The Netherlands), and are outlined in this review.

KCNQ (Kv7) potassium channel activators as bronchodilators: combination with a β2-adrenergic agonist enhances relaxation of rat airways

KCNQ (Kv7 family) potassium (K(+)) channels were recently found in airway smooth muscle cells (ASMCs) from rodent and human bronchioles. In the present study, we evaluated expression of KCNQ channels and their role in constriction/relaxation of rat airways. Real-time RT-PCR analysis revealed expression of KCNQ4 > KCNQ5 > KCNQ1 > KCNQ2 > KCNQ3, and patch-clamp electrophysiology detected KCNQ currents in rat ASMCs. In precision-cut lung slices, the KCNQ channel activator retigabine induced a concentration-dependent relaxation of small bronchioles preconstricted with methacholine (MeCh; EC50 = 3.6 ± 0.3 μM). Bronchoconstriction was also attenuated in the presence of two other structurally unrelated KCNQ channel activators: zinc pyrithione (ZnPyr; 1 μM; 22 ± 7%) and 2,5-dimethylcelecoxib (10 μM; 24 ± 8%). The same three KCNQ channel activators increased KCNQ currents in ASMCs by two- to threefold. The bronchorelaxant effects of retigabine and ZnPyr were prevented by inclusion of the KCNQ channel blocker XE991. A long-acting β2-adrenergic receptor agonist, formoterol (10 nM), did not increase KCNQ current amplitude in ASMCs, but formoterol (1-1,000 nM) did induce a time- and concentration-dependent relaxation of rat airways, with a notable desensitization during a 30-min treatment or with repetitive treatments. Coadministration of retigabine (10 μM) with formoterol produced a greater peak and sustained reduction of MeCh-induced bronchoconstriction and reduced the apparent desensitization observed with formoterol alone. Our findings support a role for KCNQ K(+) channels in the regulation of airway diameter. A combination of a β2-adrenergic receptor agonist with a KCNQ channel activator may improve bronchodilator therapy.

Arctigenin exhibits relaxation effect on bronchus by affecting transmembrane flow of calcium

Arctigenin, a lignan extract from Arctium lappa (L.), exhibits anti-inflammation, antioxidation, vasodilator effects, etc. However, the effects of arctigenin on bronchus relaxation are not well investigated. This study aimed to investigate how arctigenin regulates bronchus tone and calcium ion (Ca(2+)) flow. Trachea strips of guinea pigs were prepared for testing the relaxation effect of arctigenin to acetylcholine, histamine, KCl, and CaCl2, respectively. Furthermore, L-type calcium channel currents were detected by patch-clamp, and intracellular Ca(2+) concentration was detected by confocal microscopy. The results showed that arctigenin exhibited relaxation effect on tracheae to different constrictors, and this was related to decreasing cytoplasmic Ca(2+) concentration by inhibiting Ca(2+) influx partly through L-type calcium channel as well as promoting Ca(2+) efflux. In summary, this study provides new insight into the mechanisms by which arctigenin exhibits relaxation effect on bronchus and suggests its potential use for airway disease therapy.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.

Chloride channel blockers promote relaxation of TEA-induced contraction in airway smooth muscle

Enhanced airway smooth muscle (ASM) contraction is an important component in the pathophysiology of asthma. We have shown that ligand gated chloride channels modulate ASM contractile tone during the maintenance phase of an induced contraction, however the role of chloride flux in depolarization-induced contraction remains incompletely understood. To better understand the role of chloride flux under these conditions, muscle force (human ASM, guinea pig ASM), peripheral small airway luminal area (rat ASM) and airway smooth muscle plasma membrane electrical potentials (human cultured ASM) were measured. We found ex vivo guinea pig airway rings, human ASM strips and small peripheral airways in rat lungs slices relaxed in response to niflumic acid following depolarization-induced contraction induced by K(+) channel blockade with tetraethylammonium chloride (TEA). In isolated human airway smooth muscle cells TEA induce depolarization as measured by a fluorescent indicator or whole cell patch clamp and this depolarization was reversed by niflumic acid. These findings demonstrate that ASM depolarization induced contraction is dependent on chloride channel activity. Targeting of chloride channels may be a novel approach to relax hypercontractile airway smooth muscle in bronchoconstrictive disorders.

Emerging airway smooth muscle targets to treat asthma

Asthma is characterized in part by variable airflow obstruction and non-specific hyperresponsiveness to a variety of bronchoconstrictors, both of which are mediated by the airway smooth muscle (ASM). The ASM is also involved in the airway inflammation and airway wall remodeling observed in asthma. For all these reasons, the ASM provides an important target for the treatment of asthma. Several classes of drugs were developed decades ago which targeted the ASM - including β-agonists, anti-cholinergics, anti-histamines and anti-leukotrienes - but no substantially new class of drug has appeared recently. In this review, we summarize the on-going work of several laboratories aimed at producing novel targets and/or tools for the treatment of asthma. These range from receptors and ion channels on the ASM plasmalemma, to intracellular effectors (particularly those related to cyclic nucleotide signaling, calcium-homeostasis and phosphorylation cascades), to anti-IgE therapy and outright destruction of the ASM itself.

Oxygen dose responsiveness of human fetal airway smooth muscle cells

Maintenance of blood oxygen saturation dictates supplemental oxygen administration to premature infants, but hyperoxia predisposes survivors to respiratory diseases such as asthma. Although much research has focused on oxygen effects on alveoli in the setting of bronchopulmonary dysplasia, the mechanisms by which oxygen affects airway structure or function relevant to asthma are still under investigation. We used isolated human fetal airway smooth muscle (fASM) cells from 18-20 postconceptual age lungs (canalicular stage) to examine oxygen effects on intracellular Ca(2+) ([Ca(2+)](i)) and cellular proliferation. fASM cells expressed substantial smooth muscle actin and myosin and several Ca(2+) regulatory proteins but not fibroblast or epithelial markers, profiles qualitatively comparable to adult human ASM. Fluorescence Ca(2+) imaging showed robust [Ca(2+)](i) responses to 1 μM acetylcholine (ACh) and 10 μM histamine (albeit smaller and slower than adult ASM), partly sensitive to zero extracellular Ca(2+). Compared with adult, fASM showed greater baseline proliferation. Based on this validation, we assessed fASM responses to 10% hypoxia through 90% hyperoxia and found enhanced proliferation at <60% oxygen but increased apoptosis at >60%, effects accompanied by appropriate changes in proliferative vs. apoptotic markers and enhanced mitochondrial fission at >60% oxygen. [Ca(2+)](i) responses to ACh were enhanced for <60% but blunted at >60% oxygen. These results suggest that hyperoxia has dose-dependent effects on structure and function of developing ASM, which could have consequences for airway diseases of childhood. Thus detrimental effects on ASM should be an additional consideration in assessing risks of supplemental oxygen in prematurity.

Caffeine relaxes smooth muscle through actin depolymerization

Caffeine is sometimes used in cell physiological studies to release internally stored Ca(2+). We obtained evidence that caffeine may also act through a different mechanism that has not been previously described and sought to examine this in greater detail. We ruled out a role for phosphodiesterase (PDE) inhibition, since the effect was 1) not reversed by inhibiting PKA or adenylate cyclase; 2) not exacerbated by inhibiting PDE4; and 3) not mimicked by submillimolar caffeine nor theophylline, both of which are sufficient to inhibit PDE. Although caffeine is an agonist of bitter taste receptors, which in turn mediate bronchodilation, its relaxant effect was not mimicked by quinine. After permeabilizing the membrane using β-escin and depleting the internal Ca(2+) store using A23187, we found that 10 mM caffeine reversed tone evoked by direct application of Ca(2+), suggesting it functionally antagonizes the contractile apparatus. Using a variety of molecular techniques, we found that caffeine did not affect phosphorylation of myosin light chain (MLC) by MLC kinase, actin-filament motility catalyzed by MLC kinase, phosphorylation of CPI-17 by either protein kinase C or RhoA kinase, nor the activity of MLC-phosphatase. However, we did obtain evidence that caffeine decreased actin filament binding to phosphorylated myosin heads and increased the ratio of globular to filamentous actin in precontracted tissues. We conclude that, in addition to its other non-RyR targets, caffeine also interferes with actin function (decreased binding by myosin, possibly with depolymerization), an effect that should be borne in mind in studies using caffeine to probe excitation-contraction coupling in smooth muscle.

Reverse-mode NCX current in mouse airway smooth muscle: Na(+) and voltage dependence, contributions to Ca(2+) influx and contraction, and altered expression in a model of allergen-induced hyperresponsiveness

AIM

We examined the electrophysiological properties of reverse-mode Na(+) /Ca(2+) exchange (NCX) in mouse airway smooth muscle (ASM), assessing its contributions to regulation of [Ca(2+) ], and its expression in acute and chronic airway hyperresponsiveness (AHR).

METHODS

Membrane currents were studied in single murine ASM cells under voltage clamp at -60 mV using ramp depolarizing commands to +80 mV. Confocal fluorimetric and RT-PCR techniques were used to monitor changes in cytosolic [Ca(2+) ] and NCX expression, respectively.

RESULTS

With standard KCl-containing electrode, 30 μm KB-R7943 (an inhibitor of reverse-mode NCX activity) exhibited variable effects on membrane current, indicating modulation of more than one conductance. KB-R7943 activated outwardly rectifying current that was inhibited by 100 μm iberiotoxin (blocker of large-conductance Ca(2+) -dependent K(+) channels), indicating a direct enhancing effect of KB-R7943 on those K(+) channels. After obviating K(+) currents, we found that a current sensitive to 4-4'-diisothiocyanostilbene-2,2'-disulfonic acid (blocker of Ca(2+) -dependent Cl- channels) was markedly increased by elevating [Na(+) ] in the electrode solution to 13, 15.5 and 18 mm and suppressed by KB-R7943, indicating Ca(2+) influx via reverse-mode NCX activity. With conditions preventing Ca(2+) influx through voltage-dependent Ca(2+) channels but promoting that through NCX, we found that introduction of Ca(2+) led to marked but transient KB-R7943-sensitive elevation of [Ca(2+) ]. Additionally, KB-R7943 suppressed cholinergically evoked Ca(2+) waves. Finally, NCX1 expression was not significantly changed in allergen-induced AHR acute model but increased approx. 2.5-fold in a chronic model.

CONCLUSION

Reverse-mode NCX activity leads to a physiologically relevant increase in [Ca(2+) ] even under control conditions, and this may be exaggerated in allergen-induced AHR and asthma.

Pharmacology and therapeutics of bronchodilators

Bronchodilators are central in the treatment of of airways disorders. They are the mainstay of the current management of chronic obstructive pulmonary disease (COPD) and are critical in the symptomatic management of asthma, although controversies around the use of these drugs remain. Bronchodilators work through their direct relaxation effect on airway smooth muscle cells. at present, three major classes of bronchodilators, β(2)-adrenoceptor (AR) agonists, muscarinic receptor antagonists, and xanthines are available and can be used individually or in combination. The use of the inhaled route is currently preferred to minimize systemic effects. Fast- and short-acting agents are best used for rescue of symptoms, whereas long-acting agents are best used for maintenance therapy. It has proven difficult to discover novel classes of bronchodilator drugs, although potential new targets are emerging. Consequently, the logical approach has been to improve the existing bronchodilators, although several novel broncholytic classes are under development. An important step in simplifying asthma and COPD management and improving adherence with prescribed therapy is to reduce the dose frequency to the minimum necessary to maintain disease control. Therefore, the incorporation of once-daily dose administration is an important strategy to improve adherence. Several once-daily β(2)-AR agonists or ultra-long-acting β(2)-AR-agonists (LABAs), such as indacaterol, olodaterol, and vilanterol, are already in the market or under development for the treatment of COPD and asthma, but current recommendations suggest the use of LABAs only in combination with an inhaled corticosteroid. In addition, some new potentially long-acting antimuscarinic agents, such as glycopyrronium bromide (NVA-237), aclidinium bromide, and umeclidinium bromide (GSK573719), are under development, as well as combinations of several classes of long-acting bronchodilator drugs, in an attempt to simplify treatment regimens as much as possible. This review will describe the pharmacology and therapeutics of old, new, and emerging classes of bronchodilator.

Airway smooth muscle electrophysiology in a state of flux?

Activation of chloride currents and release of internally sequestered Ca2+ in airway smooth muscle have long been associated with excitation and contraction. Surprisingly, however, two recent publications (Deshpande DA, Wang WC, McIlmoyle EL, Robinett KS, Schillinger RM, An SS, Sham JS, Liggett SB. Nat Med 16: 1299–1304, 2010; Gallos G, Yim P, Chang S, Zhang Y, Xu D, Cook JM, Gerthoffer WT, Emala CW Sr. Am J Physiol Lung Cell Mol Physiol 302: L248–L256, 2012) have linked both events to relaxation. This begs a closer look at our understanding of airway smooth muscle electrophysiology and its contribution to excitation-contraction coupling. This Editorial Focus highlights those two aforementioned studies and several other equally paradoxical findings and proposes some possible reinterpretations of the data and/or new directions of research in which the answers might be found.

Chloride in airway smooth muscle: the ignored anion no longer?

This Perspectives accompanies an Editorial Focus that summarizes new developments concerning the role of chloride in airway smooth muscle physiology. We provide several observations and mechanistic insights to reconcile recent experimental evidence with existing paradigms concerning chloride channel-mediated effects on airway smooth muscle tone. In addition, we highlight the potentially complex and dynamic nature that chloride currents and membrane potential have on calcium handling and airway smooth muscle contractility.

Caveolin-1 regulation of store-operated Ca(2+) influx in human airway smooth muscle

Caveolae, plasma membrane invaginations with constitutive caveolin proteins, harbour proteins involved in intracellular calcium ([Ca(2+)](i)) regulation. In human airway smooth muscle (ASM), store-operated Ca(2+) entry (SOCE) is a key component of [Ca(2+)](i) regulation, and contributes to increased [Ca(2+)](i) in inflammation. SOCE involves proteins Orai1 and stromal interaction molecule (STIM)1. We investigated the link between caveolae, SOCE and inflammation in ASM. [Ca(2+)](i) was measured in human ASM cells using fura-2. Small interference RNA (siRNA) or overexpression vectors were used to alter expression of caveolin-1 (Cav-1), Orai1 or STIM1. Tumour necrosis factor (TNF)-α was used as a representative pro-inflammatory cytokine. TNF-α increased SOCE following sarcoplasmic reticulum Ca(2+) depletion, and increased whole-cell and caveolar Orai1 (but only intracellular STIM1). Cav-1 siRNA decreased caveolar and whole-cell Orai1 (but not STIM1) expression, and blunted SOCE, even in the presence of TNF-α. STIM1 overexpression substantially enhanced SOCE: an effect only partially reversed by Cav-1 siRNA. In contrast, Orai1 siRNA substantially blunted SOCE even in the presence of TNF-α. Cav-1 overexpression significantly increased Orai1 expression and SOCE, especially in the presence of TNF-α. These results demonstrate that caveolar expression and regulation of proteins such as Orai1 are important for [Ca(2+)](i) regulation in human ASM cells and its modulation during inflammation.

Alteration of Ryanodine-receptors in Cultured Rat Aortic Smooth Muscle Cells

Vascular smooth muscle cells can obtain a proliferative function in environments such as atherosclerosis in vivo or primary culture in vitro. Proliferation of vascular smooth muscle cells is accompanied by changes in ryanodine receptors (RyRs). In several studies, the cytosolic Ca²⁺ response to caffeine is decreased during smooth muscle cell culture. Although caffeine is commonly used to investigate RyR function because it is difficult to measure Ca²⁺ release from the sarcoplasmic reticulum (SR) directly, caffeine has additional off-target effects, including blocking inositol trisphosphate receptors and store-operated Ca²⁺ entry. Using freshly dissociated rat aortic smooth muscle cells (RASMCs) and cultured RASMCs, we sought to provide direct evidence for the operation of RyRs through the Ca²⁺- induced Ca²⁺-release pathway by directly measuring Ca²⁺ release from SR in permeabilized cells. An additional goal was to elucidate alterations of RyRs that occurred during culture. Perfusion of permeabilized, freshly dissociated RASMCs with Ca²⁺ stimulated Ca²⁺ release from the SR. Caffeine and ryanodine also induced Ca²⁺ release from the SR in dissociated RASMCs. In contrast, ryanodine, caffeine and Ca²⁺ failed to trigger Ca²⁺ release in cultured RASMCs. These results are consistent with results obtained by immunocytochemistry, which showed that RyRs were expressed in dissociated RASMCs, but not in cultured RASMCs. This study is the first to demonstrate Ca²⁺ release from the SR by cytosolic Ca²⁺ elevation in vascular smooth muscle cells, and also supports previous studies on the alterations of RyRs in vascular smooth muscle cells associated with culture.

Kv7 potassium channels in airway smooth muscle cells: signal transduction intermediates and pharmacological targets for bronchodilator therapy

Expression and function of Kv7 (KCNQ) voltage-activated potassium channels in guinea pig and human airway smooth muscle cells (ASMCs) were investigated by quantitative reverse transcriptase polymerase chain reaction (qRT-PCR), patch-clamp electrophysiology, and precision-cut lung slices. qRT-PCR revealed expression of multiple KCNQ genes in both guinea pig and human ASMCs. Currents with electrophysiological and pharmacological characteristics of Kv7 currents were measured in freshly isolated guinea pig and human ASMCs. In guinea pig ASMCs, Kv7 currents were significantly suppressed by application of the bronchoconstrictor agonists methacholine (100 nM) or histamine (30 μM), but current amplitudes were restored by addition of a Kv7 channel activator, flupirtine (10 μM). Kv7 currents in guinea pig ASMCs were also significantly enhanced by another Kv7.2-7.5 channel activator, retigabine, and by celecoxib and 2,5-dimethyl celecoxib. In precision-cut human lung slices, constriction of airways by histamine was significantly reduced in the presence of flupirtine. Kv7 currents in both guinea pig and human ASMCs were inhibited by the Kv7 channel blocker XE991. In human lung slices, XE991 induced robust airway constriction, which was completely reversed by addition of the calcium channel blocker verapamil. These findings suggest that Kv7 channels in ASMCs play an essential role in the regulation of airway diameter and may be targeted pharmacologically to relieve airway hyperconstriction induced by elevated concentrations of bronchoconstrictor agonists.

Uridine adenosine tetraphosphate induces contraction of airway smooth muscle

Contraction of airway smooth muscle (ASM) plays an important role in the regulation of air flow and is potentially involved in the pathophysiology of certain respiratory diseases. Extracellular nucleotides regulate ASM contraction via purinergic receptors, but the signaling mechanisms involved are not fully understood. Uridine adenosine tetraphosphate (Up(4)A) contains both pyrimidine and purine moieties, which are known to potentially activate P2X and P2Y receptors. Both P2X and P2Y receptors have been identified in the lung, including airway epithelial cells and ASM. We report here a study of purinergic signaling in the respiratory system, with a focus on the effect of Up(4)A on ASM contraction. Up(4)A induced contraction of rat isolated trachea and extrapulmonary bronchi as well as human intrapulmonary bronchioles. Up(4)A-induced contraction was blocked by di-inosine pentaphosphate, a P2X antagonist, but not by suramin, a nonselective P2 antagonist. Up(4)A-induced contraction was also attenuated by α,β-methylene-ATP-mediated P2X receptor desensitization. Several P2X receptors were detected at the mRNA level: P2X1, P2X4, P2X6, and P2X7, and to a lesser extent P2X3. Furthermore, the Up(4)A response was inhibited by removal of extracellular Ca(2+) and by the presence of the L-type Ca(2+) channel blocker, nifedipine, or the Rho-associated kinase inhibitor, H1152. We conclude that Up(4)A stimulates ASM contraction, and the underlying signaling mechanism appears to involve P2X (most likely P2X1) receptors, extracellular Ca(2+) entry via L-type Ca(2+) channels, and Ca(2+) sensitization through the RhoA/Rho-associated kinase pathway. This study will add to our understanding of the pathophysiological roles of extracellular nucleotides in the lung.

Caveolin-1 in cytokine-induced enhancement of intracellular Ca(2+) in human airway smooth muscle

Diseases such as asthma are characterized by airway hyperresponsiveness. Enhanced airway smooth muscle (ASM) intracellular Ca(2+) ([Ca(2+)](i)) response to agonist stimulation leading to increased airway constriction has been suggested to contribute to airway hyperresponsiveness. Caveolae are flask-shaped plasma membrane invaginations that express the scaffolding protein caveolin and contain multiple proteins important in [Ca(2+)](i) signaling (e.g., agonist receptors, ion channels). We recently demonstrated that caveolae and caveolin-1 are important in [Ca(2+)](i) regulation in human ASM. Proinflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-13 modulate [Ca(2+)](i) in ASM. We hypothesized that cytokine upregulation of caveolar signaling in ASM contributes to enhanced agonist-induced [Ca(2+)](i) in inflammation. Enzymatically dissociated human ASM cells were exposed to medium (control), 20 ng/ml TNF-α, or 50 ng/ml IL-13 for 24 h. Caveolae-enriched membrane fractions displayed substantial increase in caveolin-1 and -2 expressions by TNF-α and IL-13. Transfection with caveolin-1-mRed DNA substantially accelerated and increased plasma membrane caveolin-1 expression by TNF-α and to a lesser extent by IL-13. Caveolin-1 enhancement was inhibited by nuclear factor-κB and mitogen-activated protein kinase inhibitors. In fura 2-loaded ASM cells, [Ca(2+)](i) responses to 1 μM ACh, 10 μM histamine, or 10 nM bradykinin were all exaggerated by TNF-α as well as IL-13 exposure. However, disruption of caveolae using caveolin-1 suppression via small-interfering RNA resulted in significant blunting of agonist-induced [Ca(2+)](i) responses of vehicle and TNF-α-exposed cells. These functional data were correlated to the presence of TNFR(1) receptor (but not the IL-4/IL-13 receptor) within caveolae. Overall, these results indicate that caveolin-1 plays an important role in airway inflammation by modulating the effect of specific cytokines on [Ca(2+)](i).

Molecular expression and functional role of canonical transient receptor potential channels in airway smooth muscle cells

Multiple canonical or classic transient receptor potential (TRPC) molecules are expressed in animal and human airway smooth muscle cells (SMCs). TRPC3, but not TRPC1, is a major molecular component of native non-selective cation channels (NSCCs) to contribute to the resting [Ca(2+)](i) and muscarinic increase in [Ca(2+)](i) in freshly isolated airway SMCs. TRPC3-encoded NSCCs are significantly increased in expression and activity in airway SMCs from ovalbumin-sensitized/challenged "asthmatic" mice, whereas TRPC1-encoded channel activity, but not its expression, is largely augmented. The upregulated TRPC3- and TRPC1-encoded NSCC activity both mediate "asthmatic" membrane depolarization in airway SMCs. Supportively, tumor necrosis factor-α (TNFα), an important asthma mediator, increases TRPC3 expression, and TRPC3 gene silencing inhibits TNFα-mediated augmentation of acetylcholine-evoked increase in [Ca(2+)](i) in passaged airway SMCs. In contrast, TRPC6 gene silencing has no effect on 1-oleoyl-2-acetyl-sn-glycerol (OAG)-evoked increase in [Ca(2+)](i) in primary isolated cells. These findings provide compelling information indicating that TRPC3-encoded NSCCs are important for physiological and pathological cellular responses in airway SMCs. However, continual studies are necessary to further determine whether, which, and how TRPC-encoded channels are involved in cellular responses in normal and diseased (e.g., asthmatic) airway SMCs.

Neurokinin-neurotrophin interactions in airway smooth muscle

Neurally derived tachykinins such as substance P (SP) play a key role in modulating airway contractility (especially with inflammation). Separately, the neurotrophin brain-derived neurotrophic factor (BDNF; potentially derived from nerves as well as airway smooth muscle; ASM) and its tropomyosin-related kinase receptor, TrkB, are involved in enhanced airway contractility. In this study, we hypothesized that neurokinins and neurotrophins are linked in enhancing intracellular Ca(2+) concentration ([Ca(2+)](i)) regulation in ASM. In rat ASM cells, 24 h exposure to 10 nM SP significantly increased BDNF and TrkB expression (P < 0.05). Furthermore, [Ca(2+)](i) responses to 1 μM ACh as well as BDNF (30 min) effects on [Ca(2+)](i) regulation were enhanced by prior SP exposure, largely via increased Ca(2+) influx (P < 0.05). The enhancing effect of SP on BDNF signaling was blunted by the neurokinin-2 receptor antagonist MEN-10376 (1 μM, P < 0.05) to a greater extent than the neurokinin-1 receptor antagonist RP-67580 (5 nM). Chelation of extracellular BDNF (chimeric TrkB-F(c); 1 μg/ml), as well as tyrosine kinase inhibition (100 nM K252a), substantially blunted SP effects (P < 0.05). Overnight (24 h) exposure of ASM cells to 50% oxygen increased BDNF and TrkB expression and potentiated both SP- and BDNF-induced enhancement of [Ca(2+)](i) (P < 0.05). These results suggest a novel interaction between SP and BDNF in regulating agonist-induced [Ca(2+)](i) regulation in ASM. The autocrine mechanism we present here represents a new area in the development of bronchoconstrictive reflex response and airway hyperreactive disorders.

Serum 25-hydroxy-vitamin D and ionised calcium in relation to lung function and allergen skin tests

BACKGROUND

Evidence suggests that higher levels of vitamin D and calcium are associated with greater lung function and that vitamin D is inversely associated with atopic sensitisation. It is unknown whether the associations of vitamin D and calcium with lung function are independent of each other or mediated by atopic sensitisation.

OBJECTIVE

To study the associations of 25-hydroxyvitamin D (25(OH)D) and ionised calcium levels with lung function and specific allergen sensitisation in adolescents (12-19 years) and adults (20-59 years) and to assess whether the associations with lung function are due to altered atopic sensitisation.

METHODS

This is a cross-sectional analysis of the data from the third National Health and Nutrition Examination Survey.

RESULTS

The 25(OH)D levels were positively associated with forced vital capacity in adolescents (0.035 (95% confidence interval (CI): 0.007-0.064) s.d.; s.d. in model adjusted for multiple confounders). This association and the previously reported association between higher serum levels of 25(OH)D and better lung function in adults were independent of serum calcium levels, which were not associated with lung function. In adults, calcium was associated with sensitisation to grass allergens (odds ratio per s.d., 1.17 (95% CI: 1.03-1.32), 1.15 (95% CI: 1.01-1.31) and 1.18 (95% CI: 1.06-1.32) for white oak, Bermuda grass and short ragweed, respectively) and peanut odds ratio 1.21 (95% CI: 1.02-1.43) after adjusting for age, gender and race/ethnicity, but these associations attenuated towards the null after adjusting for additional confounders. The associations were independent of 25(OH)D levels, which were not associated with allergen sensitisation.

CONCLUSIONS

Circulating levels of 25(OH)D are positively associated with lung function and this does not seem to be driven by allergen sensitisation or influenced by calcium levels.

Contribution of Rho kinase to the early phase of the calcium-contraction coupling in airway smooth muscle

We investigated theoretically and experimentally the role of Rho kinase (RhoK) in Ca(2+)-contraction coupling in rat airways. Isometric contraction was measured on tracheal, extrapulmonary and intrapulmonary bronchial rings. Intracellular [Ca(2+)] was recorded in freshly isolated tracheal myocytes. Stimulation by carbachol (0.3 and 10 μm) and 50 mm external KCl induced a short-time, Hill-shaped contraction obtained within 90 s, followed by a sustained or an additional delayed contraction. Responses of [Ca(2+)](i) to acetylcholine consisted in a fast peak followed by a plateau and, in 42% of the cells, superimposed Ca(2+) oscillations. The RhoK inhibitor Y27632 (10 μm) did not alter the [Ca(2+)](i) response. Whatever the agonist, Y27632 did not modify the basal tension but decreased the amplitude of the short-duration response, without altering the additional delayed contraction. The Myosin Light Chain Phosphatase (MLCP) inhibitor calyculin A increased the basal tension and abolished the effect of RhoK. KN93 (Ca(2+)-calmodulin-dependent protein kinase II inhibitor) and DIDS (inhibitor of Ca(2+)-activated Cl(-) channels) had no influence on the RhoK effect. We built a theoretical model of Ca(2+)-dependent active/inactive RhoK ratio and subsequent RhoK-dependent MLCP inactivation, which was further coupled with a four-state model of the contractile apparatus and Ca(2+)-dependent MLCK activation. The model explains the time course of the short-duration contraction and the role of RhoK by Ca(2+)-dependent activation of MLCK and RhoK, which inactivates MLCP. Oscillatory and non-oscillatory [Ca(2+)](i) responses result in a non-oscillatory contraction, the amplitude of which is encoded by the plateau value and oscillation frequency. In conclusion, Ca(2+)-dependent but CaMK II-independent RhoK activation contributes to the early phase of the contractile response via MLCP inhibition.

The 1.3 isoform of Na+-Ca 2+ exchanger expressed in guinea pig tracheal smooth muscle is less sensitive to KB-R7943

The sodium-calcium exchanger (NCX) plays a major role in the regulation of cytosolic Ca(2+) in muscle cells. In this work, we performed force experiments to explore the role of NCX during contraction and relaxation of Cch-stimulated guinea pig tracheal smooth muscle strips. This tissue showed low sensitivity to NCX inhibitor KB-R7943 (IC50, 57 +/- 2 microM), although a complete relaxation was obtained by NCX inhibition at 100 microM. Interestingly, relaxation after washing the agonist was prolonged in the absence of external Na(+), whereas washing without Na(+) and in the presence of KB-R7943 resembled control conditions with physiological solution. Altogether, this suggests the reversal of NCX to a Ca(2+) influx mode by the manipulation on the Na(+) gradient, which can be inhibited by KB-R7943. In order to understand the low sensitivity to KB-R7943, we studied the molecular aspects of the NCX expressed in this tissue and found that the isoform of NCX expressed is 1.3, similar to that described in human tracheal smooth muscle. Sequencing revealed that amino acid 19 in exon B is phenylalanine, whereas in its human counterpart is leucine, and that the first amino acid after exon D is aspartate instead of glutamate in humans. Results herein presented are discussed in term of their possible functional implications in the exchanger activity and thus in airway physiology.