Impairment of neural nitric oxide-mediated relaxation after antigen exposure in guinea pig airways in vitro

Mini review: Neural mechanisms underlying airway hyperresponsiveness

Neural changes underly hyperresponsiveness in asthma and other airway diseases. Afferent sensory nerves, nerves within the brainstem, and efferent parasympathetic nerves all contribute to airway hyperresponsiveness. Inflammation plays a critical role in these nerve changes. Chronic inflammation and pre-natal exposures lead to increased airway innervation and structural changes. Acute inflammation leads to shifts in neurotransmitter expression of afferent nerves and dysfunction of M₂ muscarinic receptors on efferent nerve endings. Eosinophils and macrophages drive these changes through release of inflammatory mediators. Novel tools, including optogenetics, two photon microscopy, and optical clearing and whole mount microscopy, allow for improved studies of the structure and function of airway nerves and airway hyperresponsiveness.

Long noncoding RNA PAGBC contributes to nitric oxide (NO) production by sponging miR-511 in airway hyperresponsiveness upon intubation

BACKGROUND AND OBJECTIVES

In this study, we aimed to study the molecular mechanisms underlying the symptoms of hyperresponsiveness during intubation.

METHOD

The value of circulating long noncoding RNA (lncRNA)-prognosis-associated gallbladder cancer (PAGBC) in the prediction of hyperresponsiveness upon intubation during general anesthesia was evaluated via the receiver operating characteristic analyses of serum miR-511, serum PAGBC, and serum nitric oxide (NO). In addition, the possible association between lncRNA-PAGBC/NOS1 messenger RNA (mRNA) and miR-511 was further validated via real-time quantitative polymerase chain reaction, immunohistochemistry assay, computational analysis, and luciferase assay. Enzyme-linked immunosorbent assay and Western blot analysis were also conducted to establish the regulatory relationship among PAGBC, miR-511, and NO synthase 1 (NOS1).

RESULTS

Compared with circulating miR-511 and serum NO, circulating PAGBC was associated with a higher predictive value. In addition, a negative correlation was found between serum miR-511 and serum PAGBC (multicorrelation coefficient: -0.5) as well as between serum miR-511 and serum NO (multicorrelation coefficient: -0.6). In addition, both lncRNA-PAGBC and NO were decreased in patients with hyperresponsiveness, whereas the levels of miR-511 and NOS1 in these patients were similar to those in normal patients. Furthermore, our computational analyses and luciferase assays validated the direct binding between miR-511 and lncRNA-PAGBC, whereas NOS1 mRNA was identified as a virtual target gene of miR-511. Moreover, in the presence of lncRNA-PAGBC, we also observed an evident increase in the levels of NOS1 and NO accompanied by an obvious decrease of miR-511 expression.

CONCLUSION

LncRNA-PAGBC downregulated the expression of miR-511, which in turn upregulated the expression of NOS1 mRNA and led to the increase in NOS1 expression, thus leading to the inhibited responsiveness (normal-responsiveness rather than hyperresponsiveness) to intubation in patients.

A single nucleotide polymorphism in hsa‑miR‑146a is responsible for the development of bronchial hyperresponsiveness in response to intubation during general anesthesia

Bronchial hyperresponsiveness (BHR) is the most common clinical manifestation identified in asthmatic patients, and intubation is the major factor that stimulates the airway of patients receiving general anesthetic. In the present study, nitric oxide synthase 1 (NOS1) was identified as a target gene of micro (mi)R‑146a using in silico analysis and luciferase assay. Furthermore, the regulatory role of miR‑146a was demonstrated by the observation that the NOS1 expression level in pulmonary artery smooth muscle cells (PASMCs) transfected with miR‑146a mimics was significantly downregulated and the NOS1 expression level in PASMCs transfected with miR‑146a inhibitors was significantly upregulated. Additionally, it was identified that a polymorphism in pri‑miR‑146 interfered with mature processing and reduced the quantity of mature miRNA. To assess the association between the polymorphism and the development of BHR, 563 patients with basic pulmonary diseases, such as asthma, emphysema or bronchitis were enrolled in the present study. Each participant received a general anesthetic and the development of BHR was evaluated. The miR‑146a rs2910164 polymorphism CC genotype was identified to be significantly associated with a decreased risk of BHR in response to intubation when compared with the GG or GC genotype (odds ratio, 0.38; confidence interval, 0.18‑0.78). These findings indicate that the miR‑146a rs2910164 polymorphism is associated with a decrease risk of BHR, and the CC genotype increased the level of NOS1 expression, which was physiologically inhibited by wild‑type miR‑146a.

Nitrative stress in inflammatory lung diseases

Since the discovery of nitric oxide (NO), an intracellular signal transmitter, the role of NO has been investigated in various organs. In the respiratory system, NO derived from the constitutive type of NO synthase (cNOS, NOS1, NOS3) induces bronchodilation and pulmonary vasodilatation to maintain homeostasis. In contrast, the roles of excessive NO derived from the inducible type of NOS (iNOS, NOS2) in airway and lung inflammation in inflammatory lung diseases including bronchial asthma and chronic obstructive pulmonary disease (COPD) are controversial. In these inflammatory lung diseases, excessive nitrosative stress has also been observed. In asthma, some reports have shown that nitrosative stress causes airway inflammation, airway hyperresponsiveness, and airway remodeling, which are the features of asthma, whereas others have demonstrated the anti-inflammatory role of NO derived from NOS2. In the case of refractory asthma, more nitrosative stress has been reported to be observed in such airways compared with that in well-controlled asthmatics. In COPD, reactive nitrogen species (RNS), which are NO and NO-related molecules including nitrogen dioxide and peroxynitrite, cause lung inflammation, oxidative stress, activation of matrix metalloproteinase, and inactivation of antiprotease, which are involved in the pathophysiology of the disease. In the present paper, we review the physiological and pathophysiological effects of NO and NO-related molecules in the respiratory system and in inflammatory lung diseases.

The effects of NOS1 gene on asthma and total IgE levels in Taiwanese children, and the interactions with environmental factors

Asthma is a complex disorder, which is known to be affected by interactions between genetic and environmental factors. The aim of this study was to investigate the three microsatellite polymorphisms of GT repeats in intron 2, AAT repeats in intron 20, and CA repeats in exon 29 of the NOS1 gene in 155 asthmatic children and 301 control children, and the interaction with environmental factors in southern Taiwan. Total serum IgE, phadiatop test and genetic polymorphisms were measured. The genotype frequency of 14/14-AAT repeats of the NOS1 gene was significantly higher in the asthmatic group (p = 0.01). Total IgE concentrations were higher in asthmatic children (p = 0.015) carrying the NOS1 14/14-AAT genotype than in subjects with other polymorphisms. The gene and environmental interaction effects were 3.83-fold, 6.86-fold, and 8.04-fold (all corrected p-values <0.001) between subjects carrying at least one NOS1 14-AAT allele and exposure to cockroaches, high levels of total IgE, and positive response against the phadiatop test in asthmatic children. The findings of this study provide strong evidence that NOS1 gene with 14-AAT tandem repeats has a significant effect in asthmatic children. Environmental factors and atopic status will enhance the asthmatic risk for children who carry NOS1 susceptible allele.

Red wine polyphenolic compounds inhibit tracheal smooth muscle contraction during allergen-induced hyperreactivity of the airways

The aims of the study were to investigate the short and long-term effects of Provinol (red wine polyphenolic compounds) on tracheal smooth muscle reactivity using an in-vitro model of ovalbumin-induced airway inflammation in guinea-pig trachea, and to evaluate the role of nitric oxide (NO) in the bronchodilatory effect of Provinol.

The amplitude of tracheal smooth muscle contraction in response to mediators of bronchoconstriction —histamine (10 nM-1 mM), acetylcholine (10 nM-1 mM) and to allergen (ovalbumin 10−5-10−3 g mL−1) was used as a parameter of tracheal smooth muscle reactivity. To test the short-term effects of Provinol, isolated tracheal strips were pre-treated for 30 min with Provinol (10−4mg mL−1) alone or in combination with Nω-nitro-L-arginine methyl ester (L-NAME; 10−6mol L−1). To test the long-term effects of Provinol, isolated tracheal strips were prepared from guinea pigs that had been treated for 14 days with Provinol (20mg kg−1 per day) alone or in combination with L-NAME (40 mg kg−1 per day).

Incubation of tracheal smooth muscle with Provinol decreased the amplitude of contraction in response to ovalbumin, histamine and acetylcholine. The non-selective NO synthase inhibitor L-NAME partially abolished the effect of Provinol on acetylcholine and ovalbumin-induced but not histamine-induced bronchoconstriction. A similar profile was observed after 14 days' oral administration of Provinol.

In conclusion, Provinol inhibited the allergen- and spasmogen-induced contraction of tracheal smooth muscle in ovalbumin-sensitized guinea pigs via a mechanism that was mediated at least partially through the metabolism of NO.

Oxidative and nitrative stress in bronchial asthma

There has been a marked increase in the global prevalence, morbidity, and mortality of asthma, and its associated economic burden has also grown over the last 40 years. Approximately 300 million people worldwide currently have asthma, and its prevalence increases by 50% every decade. Airway inflammation is the most proximate cause of the recurrent episodes of airflow limitation in asthma. Recent research has revealed that numerous biologically active proinflammatory mediators are responsible for the pathogenesis of asthma. Among these mediators, there is increasing evidence that endogenous or exogenous reactive oxygen species (ROS) and reactive nitrogen species (RNS) are responsible for the airway inflammation of asthma. Many reports have shown that there is an excessive production of ROS and RNS in the airways of asthmatic individuals compared with healthy subjects. Excessively produced ROS and RNS have been reported to lead to airway inflammation, airway hyper-responsiveness, airway microvascular hyperpermeability, tissue injury, and remodeling in animal models and human studies. Although human lungs have a potent antioxidant system, excessive oxidative and nitrative stress leads to an imbalance of oxidants/antioxidants. This review describes the rapidly accruing data linking oxidative and nitrative events to the pathogenesis of bronchial asthma.

Pharmacology of airway smooth muscle proliferation

Airway smooth muscle thickening is a pathological feature that contributes significantly to airflow limitation and airway hyperresponsiveness in asthma. Ongoing research efforts aimed at identifying the mechanisms responsible for the increased airway smooth muscle mass have indicated that hyperplasia of airway smooth muscle, due in part to airway myocyte proliferation, is likely a major factor. Airway smooth muscle proliferation has been studied extensively in culture and in animal models of asthma, and these studies have revealed that a variety of receptors and mediators contributes to this response. This review aims to provide an overview of the receptors and mediators that control airway smooth muscle cell proliferation, with emphasis on the intracellular signalling mechanisms involved.

Extended NO analysis in asthma

The discovery of the flow dependence of exhaled NO made it possible to model NO production in the lung. The linear model provides information about the maximal flux of NO from the airways and the alveolar concentrations of NO. Nonlinear models give additional flow-independent parameters such as airway diffusing capacity and airway wall concentrations of NO. When these models are applied to patients with asthma, a clear-cut increase in NO flux is found, and this is caused by an increase in both airway diffusing capacity and airway wall concentrations of NO. There is no difference in alveolar concentrations of NO compared to healthy subjects, except in severe asthma where an increase has been found. Inhaled corticosteroids are able to reduce the airway wall concentrations but not diffusing capacity or alveolar concentrations. Oral prednisone affects the alveolar concentration, suggesting that in severe asthma there is a systemic component. Steroids distributed by any route do not affect the airway diffusing capacity. Therefore, the airway diffusing capacity should be in focus in testing new drugs or in combination treatment for asthma. Exhaled NO analysis is a promising tool in characterizing asthma in both adults and children. However, there is a strong need to agree on the models and to standardize the flow rates to be used for the modelling in order to perform a systematic and robust analysis of NO production in the lung.

Mechanism of relaxation induced by nicotine in normal and ovalbumin-sensitized guinea-pig trachea

Nicotine is an irritant molecule in the cigarette that contributes airway hyper-reactivity. The aim of this study was to investigate the mechanism of these effects and effects of nicotine on the isolated trachea preparations from control and ovalbumin-sensitized guinea-pigs. Nicotine (3x10(-5) to 3x10(-4) M) produced concentration-dependent relaxation on isolated trachea preparations precontracted by carbachol (10(-6) M) in both groups. We found that the relaxant effect of nicotine decreased in the presence of N(w)-nitro L-arginine methyl ester (L-NAME) (10(-6) M), and hexamethonium (10(-2) M) but not in the presence of alpha-bungarotoxin (10(-3) M), and tetrodotoxin (3.1x10(-6) M) in isolated trachea preparations in both groups. The relaxant effect of nicotine was less significant in isolated trachea preparations from ovalbumin-sensitized guinea-pigs than from control guinea-pigs (P<0.05). The contractions elicited by carbachol (10(-6) M) were not significantly different in the ovalbumin-sensitized group than in the control group. Nicotine (10(-4) M) significantly increased the cGMP levels in trachea preparations compared with the control preparations.(P<0.05). These results suggest that nicotine-induced relaxation response in normal and ovalbumin sensitized guinea-pigs trachea is at least in part mediated by nitric oxide (NO) since it was significantly reduced in the presence of L-NAME. The decreased relaxation response to nicotine in ovalbumin sensitized guinea-pigs trachea may be due to impaired production and/or liberation of NO.

The therapeutic potential of drugs targeting the arginase pathway in asthma

Arginine metabolism by arginases may be of importance in health and disease, either by competing with nitric oxide synthases for the common substrate or by the production of L-ornithine. L-ornithine serves as a precursor for L-proline and polyamines, which may be involved in tissue remodelling by promoting collagen synthesis and cell proliferation. Arginase activity potentiates airway reactivity by reducing the production of bronchodilatory nitric oxide. Increased arginase activity has been implicated in the development of allergen-induced airway hyper-responsiveness in experimental asthma. In addition, reduced L-arginine availability to inducible nitric oxide synthase by arginase may lead to an increased production of peroxynitrite, contributing to increased airway smooth muscle contractility, airway inflammation and cell damage in this disease. Recent studies demonstrate that the upregulation of arginase by T helper type 2 cytokines in lung tissue as well as in cultured airway fibroblasts indicates a possible role of the enzyme in airway re-modelling. These findings, in conjunction with human studies showing a role for arginase in acute asthma, open a new horizon for the therapeutic potential of drugs targeting the arginase pathway in asthma.

Reflex regulation of airway smooth muscle tone

Autonomic nerves in most mammalian species mediate both contractions and relaxations of airway smooth muscle. Cholinergic-parasympathetic nerves mediate contractions, whereas adrenergic-sympathetic and/or noncholinergic parasympathetic nerves mediate relaxations. Sympathetic-adrenergic innervation of human airway smooth muscle is sparse or nonexistent based on histological analyses and plays little or no role in regulating airway caliber. Rather, in humans and in many other species, postganglionic noncholinergic parasympathetic nerves provide the only relaxant innervation of airway smooth muscle. These noncholinergic nerves are anatomically and physiologically distinct from the postganglionic cholinergic parasympathetic nerves and differentially regulated by reflexes. Although bronchopulmonary vagal afferent nerves provide the primary afferent input regulating airway autonomic nerve activity, extrapulmonary afferent nerves, both vagal and nonvagal, can also reflexively regulate autonomic tone in airway smooth muscle. Reflexes result in either an enhanced activity in one or more of the autonomic efferent pathways, or a withdrawal of baseline cholinergic tone. These parallel excitatory and inhibitory afferent and efferent pathways add complexity to autonomic control of airway caliber. Dysfunction or dysregulation of these afferent and efferent nerves likely contributes to the pathogenesis of obstructive airways diseases and may account for the pulmonary symptoms associated with extrapulmonary disorders, including gastroesophageal reflux disease, cardiovascular disease, and rhinosinusitis.

Mechanisms underlying the relaxation induced by isokaempferide from Amburana cearensis in the guinea-pig isolated trachea

The present study examines possible mechanisms by which the flavonoid isokaempferide (IKPF; 5,7,4'-trihydroxy-3-methoxyflavone) from Amburana cearensis, a Brazilian medicinal plant popularly used as bronchodilator, induces relaxation of guinea-pig isolated trachea. In the trachea (with intact epithelium) contracted by carbachol, IKPF (1-1000 microM) caused a graded relaxation, and the epithelium removal increased the sensitivity of the airway smooth muscle to IKPF (EC50, in intact tissue: 77.4 [54.8-109.2] microM; in denuded epithelium: 15.0 [11.3-20.1] microM). The IKPF-induced relaxation was inhibited in 41% by the nitric oxide (NO) synthase inhibitor L-NAME (100 microM); in 31% and 50% by the soluble guanylate cyclase (sGC) inhibitor ODQ (3 and 33 microM); by propranolol (31%) and also by capsaicin (37%). In the trachea pre-contracted by 40 mM KCl the pre-incubation with glibenclamide (33 microM) or iberiotoxin (IbTX, 0.1 microM), selective K(+) channel inhibitors, inhibited the IKPF-induced relaxation by 39% and 38%, respectively. On the other hand, 4-aminopyridine (100 microM), a nonselective K(+) channel antagonist, did not significantly influence the effect of IKPF, while IbTX induced a rightward displacement of the IKPF concentration-response curve. However, in muscle pre-contracted with 120 mM KCl the relaxant effect of IKPF was significantly reduced and not affected by glibenclamide. In conclusion, these results indicate a direct and epithelium-independent relaxant effect of IKPF on smooth muscle fibers. Although this IKPF relaxant action seems to be multi-mediated, it occurs via both Ca(2+) and ATP-sensitive K(+) channels, but some other possible mechanisms unrelated to K(+) channels cannot be excluded.

Arginase strongly impairs neuronal nitric oxide-mediated airway smooth muscle relaxation in allergic asthma

BACKGROUND

Using guinea pig tracheal preparations, we have recently shown that endogenous arginase activity attenuates inhibitory nonadrenergic noncholinergic (iNANC) nerve-mediated airway smooth muscle relaxation by reducing nitric oxide (NO) production--due to competition with neuronal NO-synthase (nNOS) for the common substrate, L-arginine. Furthermore, in a guinea pig model of allergic asthma, airway arginase activity is markedly increased after the early asthmatic reaction (EAR), leading to deficiency of agonist-induced, epithelium-derived NO and subsequent airway hyperreactivity. In this study, we investigated whether increased arginase activity after the EAR affects iNANC nerve-derived NO production and airway smooth muscle relaxation.

METHODS

Electrical field stimulation (EFS; 150 mA, 4 ms, 4 s, 0.5-16 Hz)-induced relaxation was measured in tracheal open-ring preparations precontracted to 30% with histamine in the presence of 1 microM atropine and 3 microM indomethacin. The contribution of NO to EFS-induced relaxation was assessed by the nonselective NOS inhibitor Nomega-nitro-L-arginine (L-NNA, 100 microM), while the involvement of arginase activity in the regulation of EFS-induced NO production and relaxation was investigated by the effect of the specific arginase inhibitor Nomega-hydroxy-nor-L-arginine (nor-NOHA, 10 microM). Furthermore, the role of substrate availability to nNOS was measured in the presence of exogenous L-arginine (5.0 mM).

RESULTS

At 6 h after ovalbumin-challenge (after the EAR), EFS-induced relaxation (ranging from 3.2 +/- 1.1% at 0.5 Hz to 58.5 +/- 2.2% at 16 Hz) was significantly decreased compared to unchallenged controls (7.1 +/- 0.8% to 75.8 +/- 0.7%; P < 0.05 all). In contrast to unchallenged controls, the NOS inhibitor L-NNA did not affect EFS-induced relaxation after allergen challenge, indicating that NO deficiency underlies the impaired relaxation. Remarkably, the specific arginase inhibitor nor-NOHA normalized the impaired relaxation to unchallenged control (P < 0.05 all), which effect was inhibited by L-NNA (P < 0.01 all). Moreover, the effect of nor-NOHA was mimicked by exogenous L-arginine.

CONCLUSION

The results clearly demonstrate that increased arginase activity after the allergen-induced EAR contributes to a deficiency of iNANC nerve-derived NO and decreased airway smooth muscle relaxation, presumably via increased substrate competition with nNOS.

Genetic polymorphisms in arginase I and II and childhood asthma and atopy

BACKGROUND

A recent microarray study implicated arginase I (ARG1) and arginase II (ARG2) in mouse allergic asthma models and human asthma.

OBJECTIVES

To examine the association between genetic variation in ARG1 and ARG2 and childhood asthma and atopy risk.

METHODS

We enrolled 433 case-parent triads, consisting of patients with asthma 4 to 17 years old and their biologic parents, from the allergy clinic of a public hospital in Mexico City between 1998 and 2003. Atopy to 24 aeroallergens was determined by skin prick tests. We genotyped 4 single nucleotide polymorphisms (SNPs) of ARG1 and 4 SNPs of ARG2 with minor allele frequencies higher than 10% by using the TaqMan assay (Roche Molecular Systems, Pleasanton, Calif).

RESULTS

ARG1 SNPs and haplotypes were not associated with asthma, but all 4 ARG1 SNPs were associated with the number of positive skin tests (P = .007-.018). Carrying 2 copies of minor alleles for either of 2 highly associated ARG2 SNPs was associated with a statistically significant increased relative risk (RR) of asthma (1.5, 95% CI = 1.1-2.1 for arg2s1; RR = 1.6, 95% CI = 1.1-2.3 for arg2s2). The association was slightly stronger among children with a smoking parent (arg2s1 RR = 2.1, 95% CI = 1.2 - 3.9 with a smoking parent; RR = 1.2, 95% CI = 0.8-1.9 without; interaction P = .025). Haplotype analyses reduced the sample size but supported the single SNP results. One ARG2 SNP was related to the number of positive skin tests (P = .027).

CONCLUSION

Variation in arginase genes may contribute to asthma and atopy in children.

Arginase attenuates inhibitory nonadrenergic noncholinergic nerve-induced nitric oxide generation and airway smooth muscle relaxation

Background

Recent evidence suggests that endogenous arginase activity potentiates airway responsiveness to methacholine by attenuation of agonist-induced nitric oxide (NO) production, presumably by competition with epithelial constitutive NO synthase for the common substrate, L-arginine. Using guinea pig tracheal open-ring preparations, we now investigated the involvement of arginase in the modulation of neuronal nitric oxide synthase (nNOS)-mediated relaxation induced by inhibitory nonadrenergic noncholinergic (iNANC) nerve stimulation.

Methods

Electrical field stimulation (EFS; 150 mA, 4 ms, 4 s, 0.5 – 16 Hz)-induced relaxation was measured in tracheal preparations precontracted to 30% with histamine, in the presence of 1 μM atropine and 3 μM indomethacin. The contribution of NO to the EFS-induced relaxation was assessed by the nonselective NOS inhibitor L-NNA (0.1 mM), while the involvement of arginase activity in the regulation of EFS-induced NO production and relaxation was investigated by the effect of the specific arginase inhibitor nor-NOHA (10 μM). Furthermore, the role of substrate availability to nNOS in EFS-induced relaxation was measured in the presence of various concentrations of exogenous L-arginine.

Results

EFS induced a frequency-dependent relaxation, ranging from 6.6 ± 0.8% at 0.5 Hz to 74.6 ± 1.2% at 16 Hz, which was inhibited with the NOS inhibitor L-NNA by 78.0 ± 10.5% at 0.5 Hz to 26.7 ± 7.7% at 8 Hz (P < 0.01 all). In contrast, the arginase inhibitor nor-NOHA increased EFS-induced relaxation by 3.3 ± 1.2-fold at 0.5 Hz to 1.2 ± 0.1-fold at 4 Hz (P < 0.05 all), which was reversed by L-NNA to the level of control airways in the presence of L-NNA (P < 0.01 all). Similar to nor-NOHA, exogenous L-arginine increased EFS-induced airway relaxation (P < 0.05 all).

Conclusion

The results indicate that endogenous arginase activity attenuates iNANC nerve-mediated airway relaxation by inhibition of NO generation, presumably by limiting L-arginine availability to nNOS.

Nitric oxide in health and disease of the respiratory system

During the past decade a plethora of studies have unravelled the multiple roles of nitric oxide (NO) in airway physiology and pathophysiology. In the respiratory tract, NO is produced by a wide variety of cell types and is generated via oxidation of l-arginine that is catalyzed by the enzyme NO synthase (NOS). NOS exists in three distinct isoforms: neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS). NO derived from the constitutive isoforms of NOS (nNOS and eNOS) and other NO-adduct molecules (nitrosothiols) have been shown to be modulators of bronchomotor tone. On the other hand, NO derived from iNOS seems to be a proinflammatory mediator with immunomodulatory effects. The concentration of this molecule in exhaled air is abnormal in activated states of different inflammatory airway diseases, and its monitoring is potentially a major advance in the management of, e.g., asthma. Finally, the production of NO under oxidative stress conditions secondarily generates strong oxidizing agents (reactive nitrogen species) that may modulate the development of chronic inflammatory airway diseases and/or amplify the inflammatory response. The fundamental mechanisms driving the altered NO bioactivity under pathological conditions still need to be fully clarified, because their regulation provides a novel target in the prevention and treatment of chronic inflammatory diseases of the airways.

Nitric oxide synthase inhibition: therapeutic potential in asthma

Nitric oxide (NO) is synthesized from L-arginine in the human respiratory tract by enzymes of the NO synthase (NOS) family. Levels of NO in exhaled air are increased in asthma, and measurement of exhaled NO has been advocated as a noninvasive tool to monitor the underlying inflammatory process. However, the relation of NO to disease pathophysiology is uncertain, and in particular the fundamental question of whether it should be viewed primarily as beneficial or harmful remains unanswered. Exogenously administered NO has both bronchodilator and bronchoprotective properties. Although it is unlikely that NO is an important regulator of basal airway tone, there is good evidence that endogenous NO release exerts a protective effect against various bronchoconstrictor stimuli. This response is thought to involve one or both of the constitutive NOS isoforms, endothelial NOS (eNOS) and neuronal NOS (nNOS). Therefore, inhibition of these enzymes is unlikely to be therapeutically useful in asthma and indeed may worsen disease control. On the other hand, the high concentrations of NO in asthma, which are believed to reflect upregulation of inducible NOS (iNOS) by proinflammatory cytokines, may produce various deleterious effects. These include increased vascular permeability, damage to the airway epithelium, and promotion of inflammatory cell infiltration. However, the possible effects of iNOS inhibition on allergic inflammation in asthma have not yet been described and studies in animal models have yielded inconsistent findings. Thus, the evidence to suggest that inhibition of iNOS would be a useful therapeutic strategy in asthma is limited at present. More definitive information will require studies combining agents that potently and specifically target individual NOS isoforms with direct measurement of inflammatory markers.

Guanylyl cyclases, nitric oxide, natriuretic peptides, and airway smooth muscle function

Airway smooth muscle (ASM) plays an important role in asthma pathophysiology through its contractile and proliferative functions. The cyclic nucleotides adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) are second messengers capable of mediating the effects of a variety of drugs and hormones. There is a large body of evidence to support the hypothesis that cAMP is a mediator of the ASM's relaxant effects of drugs, such as beta2-adrenoceptor agonists, in human airways. Although most attention has been paid to this second messenger and the signal transduction pathways it activates, recent evidence suggests that cGMP is also an important second messenger in ASM with important relaxant and antiproliferative effects. Here, we review the regulation and function of cGMP in ASM and discuss the implications for asthma pathophysiology and therapeutics. Recent studies suggest that activators of soluble and particulate guanylyl cyclases, such as nitric oxide donors and natriuretic peptides, have both relaxant and antiproliferative effects that are mediated through cGMP-dependent and cGMP-independent pathways. Abnormalities in these pathways may contribute to asthma pathophysiology, and therapeutic manipulation may complement the effects of beta2-adrenoceptor agonists.

Arginase and asthma: novel insights into nitric oxide homeostasis and airway hyperresponsiveness

For many years it has been supposed that the production of an excess of nitric oxide (NO) by inducible NO synthase (iNOS) plays a major role in inflammatory diseases, including asthma. However, recent studies indicate that a deficiency of beneficial, bronchodilating constitutive NOS (cNOS)-derived NO is important in allergen-induced airway hyperresponsiveness. Although several mechanisms are proposed to explain the reduction of cNOS activity, reduced substrate availability, caused by a combination of increased arginase activity and decreased cellular uptake of L-arginine, appears to play a key role. Recent evidence also indicates that iNOS-induced pathophysiological effects involve substrate deficiency. Thus, at low concentrations of L-arginine iNOS produces both NO and superoxide anions, which results in the increased synthesis of the highly reactive, detrimental oxidant peroxynitrite. Based on these observations, we propose that a relative deficiency of NO caused by increased arginase activity and altered L-arginine homeostasis is a major factor in the pathology of asthma.

Nitric oxide in respiratory diseases

The formation and modulation of nitric oxide (NO) in the lungs is reviewed. Its beneficial and deleterious roles in airways diseases, including asthma, chronic obstructive pulmonary disease, and cystic fibrosis, and in animal models is discussed. The pharmacological effects of agents that modulate NO production or act as NO donors are described. The clinical pharmacology of these agents is described and the therapeutic potential for their use in airways disease is considered.

Multiple roles of nitric oxide in the airways

Nitric oxide is endogenously released in the airways by nitric oxide synthase. Functionally, two isoforms of this enzyme exist: constitutive and inducible. The former seems to protect airways from excessive bronchoconstriction while the latter has a modulatory role in inflammatory disorders of the airways such as asthma. This review explores the physiological and pathophysiological role of endogenous nitric oxide in the airways, and the clinical aspects of monitoring nitric oxide in exhaled air of patients with respiratory disease.

Increased arginase activity underlies allergen-induced deficiency of cNOS-derived nitric oxide and airway hyperresponsiveness

1. A deficiency of constitutive nitric oxide synthase (cNOS)-derived nitric oxide (NO), due to reduced availability of L-arginine, importantly contributes to allergen-induced airway hyperresponsiveness (AHR) after the early asthmatic reaction (EAR). Since cNOS and arginase use L-arginine as a common substrate, we hypothesized that increased arginase activity is involved in the allergen-induced NO deficiency and AHR. 2. Using a guinea-pig model of allergic asthma, we addressed this hypothesis by examining the effects of the specific arginase inhibitor N(omega)-hydroxy-nor-L-arginine (nor-NOHA) on the responsiveness to methacholine of isolated perfused tracheae from unchallenged control animals and from animals 6 h after ovalbumin challenge. Arginase activity in these preparations was investigated by measuring the conversion of L-[14C]arginine to [14C]urea. 3. Airways from allergen-challenged animals showed a 2 fold (P<0.001) increase in responsiveness to intraluminal (IL) administration of methacholine compared to controls. A similar hyperresponsiveness (1.8 fold, P<0.01) was observed in control airways incubated with the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM, IL), while L-NAME had no further effect on the airways from challenged animals. 4. Remarkably, 5 microM nor-NOHA (IL) normalized the hyperresponsiveness of challenged airways to basal control (P<0.001), and this effect was fully reversed again by 0.1 mM L-NAME (P<0.05). Moreover, arginase activity in homogenates of the hyperresponsive airways was 3.5 fold (P<0.001) enhanced compared to controls. 5. The results indicate that enhanced arginase activity contributes to allergen-induced deficiency of cNOS-derived NO and AHR after the EAR, presumably by competition with cNOS for the common substrate, L-arginine. This is the first demonstration that arginase is involved in the pathophysiology of asthma.

Reactive oxygen species as mediators in asthma

This review describes production and effects of reactive oxygen species (ROS) on airway function. ROS are important in many physiological processes but can also have detrimental effects on airway cells and tissues when produced in high quantities or during the absence of sufficient amounts of anti-oxidants. Therefore, these mediators play a prominent role in the pathogenesis of various inflammatory airway disorders, including asthma. Effects of ROS on airway function in asthma have been studied with isolated airway cells and tissues and with animal models and patients. With the use of inhibitors, transgenic animals and measurements of the release of ROS within the airways, it became clear that oxidative stress contributes to the initiation and worsening of inflammatory respiratory disorders.

Effects of nitrogen dioxide exposure and ascorbic acid supplementation on exhaled nitric oxide in healthy human subjects

BACKGROUND

Nitric oxide (NO) is detectable in the exhaled breath, is involved in airway defence and inflammation, and probably modulates bronchial smooth muscle tone. Given the sensitivity of nitrogen oxides to local redox conditions, we postulated that exposure to oxidant or antioxidant compounds could alter concentrations of NO in the exhaled breath (eNO). We assessed the effect of nitrogen dioxide (NO(2)) and ascorbic acid exposure on eNO in healthy human subjects.

METHODS

Ten healthy subjects were randomised to undergo a 20 minute single blind exposure to NO(2) (1.5 parts per million) or medical air in a crossover fashion. Exhaled NO and pulmonary function were measured before and for 3 hours after exposure. In a separate double blind crossover study 20 healthy subjects received ascorbic acid 500 mg twice daily or placebo for 2 weeks with a 6 week interim washout. Serum ascorbic acid levels and eNO were measured before and after each supplementation phase.

RESULTS

NO(2) induced a decrease of 0.62 (95% CI 0.32 to 0.92) ppb in the mean post-exposure eNO (p<0.01) with no change in forced expiratory volume in 1 second (FEV(1)). Oral supplementation with ascorbic acid increased the mean serum ascorbic acid concentration by 7.4 (95% CI 5.1 to 9.7) microg/ml (63%) but did not alter eNO.

CONCLUSIONS

NO(2) exposure causes a decrease in eNO, an effect which may be mediated through changes in epithelial lining fluid redox state or through a direct effect on epithelial cells. In contrast, ascorbic acid does not appear to play a significant role in the metabolism of NO in the epithelial lining fluid.

Airway nitric oxide diffusion in asthma: Role in pulmonary function and bronchial responsiveness

If the nitric oxide (NO) diffusing capacity of the airways (DNO) is the quantity of NO diffusing per unit time into exhaled gas (q) divided by the difference between the concentration of NO in the airway wall (Cw) and lumen, then DNO and C(w) can be estimated from the relationship between exhaled NO concentration and expiratory flow. In 10 normal subjects and 25 asthmatic patients before and after treatment with inhaled beclomethasone, DNO averaged 6.8 +/- 1.2, 25.5 +/- 3.8, and 22.3 +/- 2.7 nl/s/ppb x 10(-3), respectively; C(w) averaged 149 +/- 31.9, 255.3 +/- 46.4, and 108.3 +/- 14.3 ppb, respectively; and DNOC(w) (the maximal from diffusion) averaged 1,020 +/- 157.5, 6,512 +/- 866, and 2,416 +/- 208.5 nl/s x 10(-3), respectively. DNO and DNOC(w) in the asthmatic subjects before and after steroids were greater than in normal subjects (p < 0.0001), but C(w) was not different. Within asthmatic subjects, steroids caused C(w) and DNOC(w) to fall (p < 0.0001), but DNO was unchanged. DNOC(w) after steroids, presumably reflecting maximal diffusion of constitutive NO, was positively correlated with methacholine PC(20) and FEV(1)/FVC before or after steroids. The increased DNO measured in asthmatic patients may reflect upregulation of nonadrenergic, noncholinergic, NO-producing nerves in airways in compensation for decreased sensitivity of airway smooth muscle to the relaxant effects of endogenous NO.

Allergen-induced early and late asthmatic responses are not affected by inhibition of endogenous nitric oxide

Endogenous exhaled nitric oxide (NO) is increased during the late response to inhaled allergen in patients with asthma and may be bronchoprotective in asthma or have a deleterious effect when generated in excess under inflammatory conditions. To investigate this, we evaluated the effect of inhibiting endogenous NO production with nebulized NG-nitro-L-arginine methyl ester (L-NAME), a nonselective NO synthase (NOS) inhibitor, on early and late asthmatic responses to inhaled allergen in patients with mild allergic asthma. After a screening allergen challenge (AC), 22 male patients attended two visits conducted in a double-blind, randomized, placebo-controlled, crossover manner. Twelve patients demonstrating an early asthmatic response only (single responders) inhaled either L-NAME 170 mg or 0.9% saline 20 min before AC, with exhaled NO and FEV1 measured for 3 h. Ten patients demonstrating both early and late asthmatic responses (dual responders) were studied in a similar fashion but inhaled two further doses of L-NAME or placebo 3.5 and 7 h after the initial dose, with exhaled NO and FEV1 measured for 10 h. L-NAME reduced exhaled NO levels by 77 +/- 5% (p < 0.01) and 71 +/- 7% (p < 0.01) in single and dual responders, respectively, but had no significant effect on early or late asthmatic responses. Following AC in single responders, the mean (+/- SEM) maximum fall in FEV1 after L-NAME and saline was 21.2 +/- 2.9% and 23.8 +/- 3.0%, respectively, and in dual responders, 31.2 +/- 4.5% and 31.8 +/- 5. 8% during the early asthmatic responses, and 27.4 +/- 3.9% and 30.6 +/- 4.5% during the late asthmatic responses, respectively. Area under the curve (AUC) did not significantly differ. AUC0-2 h in single responders after L-NAME and saline was 20.2 +/- 3.9 and 24.9 +/- 4.4 Delta% FEV1/h, and in dual responders, 37.6 +/- 8.4 and 36.7 +/- 8.4 Delta% FEV1/h, respectively, and 106.2 +/- 18.9 and 117.1 +/- 22.4 Delta% FEV1/h, respectively, for the AUC4-10 h. This study suggests that in mild allergic asthma, endogenous NO neither protects against nor contributes to the processes underlying airway responses to inhaled allergen.

Role of endogenous nitric oxide in allergen-induced airway responses in guinea-pigs

1. Endogenous nitric oxide (NO) can be detected in exhaled air and accumulates in inflamed airways. However its physiological role has not been fully elucidated. In this study, we investigated a role for endogenous NO in allergen-induced airway responses. Sensitised guinea-pigs were treated with NG-nitro-L-arginine methyl ester L-NAME (2.0 mM) or aminoguanidine (AG) (2.0 mM) 30 min before the allergen challenge, and 3 and 4 h after the challenge. Alternatively, L-arginine (2.4 mM) treatment was performed 30 min before, and 2 and 3 h after the challenge. In all groups, ovalbumin (OVA) challenge (2 mg ml(-1) for 2 min) was performed, and airway responses, NO production, infiltration of inflammatory cells, plasma exudation and histological details were examined. 2. Allergen-challenged animals showed an immediate airway response (IAR) and a late airway response (LAR), which synchronised with an increase in exhaled NO. Treatment with L-NAME and AG did not affect IAR while they significantly blocked LAR (72% and 80% inhibition compared to vehicle) and production of NO (35% and 40% inhibition). On the other hand, treatment with L-arginine did not affect IAR but potentiated LAR (74% augmentation). 3. In bronchoalveolar lavage (BAL) fluid, allergen-induced increases in eosinophils were reduced by 48% for L-NAME treatment compared to vehicle, and increased by 56% for L-arginine treatment. 4. Treatment with L-NAME significantly decreased airway microvascular permeability to both Monastral blue (MB) and Evans blue (EB) dye (50.6% and 44% inhibition). 5. We conclude that allergen-induced LAR is closely associated with NO production, and that NO plays a critical role in inflammatory cell infiltration and plasma exudation in the allergic condition.