Nitric Oxide and Asthma

Archives of Disease in Childhood 1995; 72: 259-262 259

CURRENT TOPIC Arch Dis Child: first published as 10.1136/adc.72.3.259 on 1 March 1995. Downloaded from

Nitric oxide and asthma

Bozhena Zoritch

Since nitric oxide (NO) was first described as Role ofNO as vasodilator endothelium derived relaxing factor (EDRF),I NO dependent vasodilator tone is entirely there has been compelling evidence that NO is locally regulated and as such is probably one of involved in very many biological processes.2-9 the simplest and most fundamental of adaptive It has at least four roles in the lungs.3 These are mechanism in the cardiovascular system. as a pulmonary vasodilator, bronchodilator, Endothelium dependent relaxation was non-adrenergic non-cholinergic transmitter, demonstrated in many vascular preparations, and inflammatory mediator. There are three including veins, arteries, and microvessels.'5 types of NO synthase, the enzyme catalysing However, while the arteries and arterioles are the formation of NO - endothelial, neuronal in a continuous state ofNO mediated vasodila- (both known as constitutive or cNOs), and tion, basal release of NO does not control the macrophage or inducible (iNOs) forms (fig resting tone of peripheral veins, although it 1).8 Agonists activate cNOs by an increase in may have an effect in central veins. Endothelial calcium ion (Ca2+) concentration resulting in NO synthesis influences the adhesion and release of NO in seconds.10 iNOs is part of the aggregation of platelets and leucocytes. immune system and is induced by cytokines, Endothelial dysfunction, either genetic or endotoxin, and lipopolysaccharide resulting in acquired, can lead to cardiovascular disease formation of larger amounts of NO.11 including atherosclerosis, essential and preg- However, this reaction involves gene transcrip- nancy induced hypertension.6 In the lung, tion, and is therefore much slower, resulting in pulmonary vessels also synthesise NO continu- increased production ofNO over several hours ously and this seems to be important for or even days. It can be blocked by glucocorti- maintaining blood flow within the lungs and costeroids. Once formed, NO activates soluble matching ventilation to perfusion.'6 Reduced guanylyl cyclase (sGG) after binding to its NO release may be the mechanism under- http://adc.bmj.com/ haem moiety to initiate a three dimensional lying hypoxic pulmonary vasoconstriction. change in the shape of the enzyme which Inhalation of NO can have a therapeutic effect increases its activity and consequently the pro- in diseases associated with pulmonary vaso- duction of cyclic guanosine 3'5'monophos- constriction.'7-'9 NO was first used in adult University of phate (cGMP).12 13 The rise in cGMP results respiratory distress syndrome as a pulmonary Deparment of Child in relaxation of smooth muscle but the mech- vasodilator.20 In 1992 two groups in North

Health, Southampton anism by which this happens is unknown. America used NO in treating persistent fetal on September 25, 2021 by guest. Protected copyright. General Hospital, When produced in large quantities, NO is Tremona Road, circulation successfully without causing Southampton involved in tissue damagecnannand cell death. hypotension or methaemoglobinaemia.21 22 It S016 6YD Inactivation of enzymes containing transition is already recognised that NO is an alternative Correspondence to: metals, including mitochondrial enzymes, is to extracorporeal membrane oxygenation in Dr Zoritch. one of the proposed toxic effects. 4 selected cases. NO in higher concentrations can have adverse influences on capillary permeability, Vasodilator Cytokines Steroids causing oedema and plasma leak. In inflammatory lung conditions, such as asthma, induc- A Receptor B receptor lInhibitors tion of NOs in endothelium as well as epithelium may contribute to hyperaemia of L-arginine ^, + , L-arginine Induction mucosa, airway narrowing, and broncho- cNsN Ca2NO constriction. L-citrulline L-Citrullif' Endothelial cell NO Endothelial cell Epithelial cell NO as a bronchodilator Glyceryl trinitrate and sodium nitroprusside, whose vasodilator activity is partly mediated sGC) by NO production, relax airway smooth mus- cGMP GTP cle in vitro, resulting in an increase in sGC Relaxation Smooth muscle cell activity and increase in cGMP.23 It has been proposed that the epithelial lining produces an Figure 1 (A) cNOs is activated by a rise in Ca2+ in response to activation to a surface epithelial derived relaxing factor, which is receptor. (B) iNOs is induced by cytokines or lipopolysaccharide resulting in larger similar to EDRF, but is not certain to be amounts ofNOformation (from Barnes and BelvisP). GTP=guanosine triphosphate. NO.24 There is some evidence that relaxation 260 Zoritch

of precontracted guinea pig trachea is induced hydroxyl radicals.32 High concentration of NO by NO.2526 In asthma, the high concentration may have effects on DNA and be genotoxic and ofNO produced by iNOs in epithelial cells may cytotoxic.33 Arch Dis Child: first published as 10.1136/adc.72.3.259 on 1 March 1995. Downloaded from suppress the activity of cNOs, and cause enhancement in intracellular calcium concentration and airway constriction. The histamine Role ofNO as neurotransmitter and carbachol induced increase in cGMP pro- There is increasing evidence that NO may duction in the respiratory system has been function as a neurotransmitter of inhibitory shown to be an L-arginine dependent process.26 non-adrenergic non-cholinergic (NANC) It could be speculated that the epithelial layer, nerves or purinergic pathways as they were by releasing NO, acts as a negative feedback sys- previously described34 (fig 2). NO appears to tem to histamine induced contractions. In account for the bronchodilator NANC response asthma, high concentrations of NO produced in vitro in human central and peripheral by inducible NO synthases in inflammatory or airways. Nerves containing NOs appear to epithelial cells may downregulate the activity of supply bronchial vessels, airway smooth muscle, cNOs. This may disturb the breaking mech- and submucosal glands. Parasympathetic, anism that prevents enhanced airway constric- sympathetic, and sensory ganglia supplying the tions. Damage or removal of epithelium airways also contain NOs. NOs inhibitors increases smooth muscle responsiveness.27 In potentiate the cholinergic neural response asthma, epithelium is often damaged which may by acting as a functional antagonist to also affect production of NO by cNOs which acetylcholine in airway smooth muscle. would contribute to bronchospasm.28 Airway There is evidence that NO modulates reflex responsiveness to histamine is increased bronchoconstriction.35 Bronchodilator NANC after intratracheal inoculation of parainfluenza nerves are the only neural bronchodilator type 3 virus and can be blocked by inhaling pathway in the human airway. L-arginine.29 Therefore, it is likely that the The question for research is whether there is deficiency in endogenous NO production any defect in those nerves in diseased airways. after a viral infection is due to a dysfunction of There is some evidence for this. Although the cNOs. A challenging hypothesis is that NANC responses in tissues from patients with bronchial hyperresponsiveness may be sup- mild asthma were not altered, responses were pressed by blocking the activity of the inducible significantly reduced from patients with cystic form of NOs by using glucocorticosteroids. fibrosis.36 Further information is needed on This would prevent downregulation of cNOs by the NANC neural bronchodilator pathway to iNOs. The effect of NO as a bronchodilator in unravel whether it has a part to play in the humans has been disappointing. High concen- pathophysiology of early asthma in children. trations of inhaled NO (>20 000 parts per Persson et al demonstrated that NO in billion) caused an increase in airway resistance exhaled air increased with exercise in healthy in healthy subjects.30 An alternative might be to volunteers.37 It would be of value to study the http://adc.bmj.com/ manipulate release of neuronal NO which change of NO concentration in exercise should give selective bronchodilatation. This induced asthma to establish if there is any might be particularly effective in wheezy infants abnormality in the modulation of broncho- who respond poorly to P agonists or in chronic constriction in response to exercise in obstructive pulmonary disease. However, there asthmatics. Perhaps the bronchoconstriction are potential dangers ofinhaling NO in the pres- produced during exercise in subjects with

ence of oxygen because of the formation of asthma and cystic fibrosis is due to the on September 25, 2021 by guest. Protected copyright. nitrate and nitrous and nitric acid which may increased NO release. increase airway responsiveness and in high con- Active smoking decreases the amount of centration may cause pulmonary oedema.31 NO in exhaled air in healthy people.38 The Peroxynitrite may generate tissue damaging regulation of NO synthesis by neuronal cNOs may be impaired in children whose mothers smoke in pregnancy and who were exposed Airway smooth to passive smoking in infancy which may muscle cell predispose them to wheezing illnesses. Peripheral nerve NO-cysteine complex A L-Citrulline Role of NO as an inflammatory mediator Macrophages express iNOs after exposure to * L-arginine / NO --w NO0 cytokines, in particular tumour necrosis factor- 1 ct, interferon y, and interleukin- 1 l as well as NOs endotoxins. Thus NO contributes to their cytotoxic effect."1 The cytotoxic activity of macrophages as well as nitrite and nitrate formation are blocked by inhibitors of iNOs, such Depolarisation Ca2+ L-NMMA as glucocorticoids.39 L-NAME Epithelial cells are also capable ofproducing NO after induction of iNOs by cytokines. Recent evidence elegantly demonstrated how iNOs can be induced by tumour necrosis Figure 2 Inhibitowy NANC nerves in airways utilise NO as a neurotransmitter (from Barnes and Belvisz). GTP=guanosine triphosphate; L-NMMA =NG-monomethyl-L- factor-cx in cell lines from human bronchial arginine; L-NAME=NG-nitro-L-arginine methylester. mucosa.40 The same paper described using Nitric oxide and asthma 261

specific antisera to stain for iNOs on lung cantly increased in asthmatics if measured biopsy specimens from a group of people with nasally but became 2-3 fold higher if measured asthma. There was increased iNOs expression orally compared to controls.47 This suggests Arch Dis Child: first published as 10.1136/adc.72.3.259 on 1 March 1995. Downloaded from on the samples from asthmatics who were not that the major source of NO in normal airway on inhaled steroids compared with samples is bronchial epithelium but when this is shed, from patients with lung cancer. In asthma as in inflammation, it is fibroblasts and there is upregulation ofmany cytokines includ- macrophages that produce NO. ing tumour necrosis factor-ox, interferon -y, and The majority of studies measuring NO in interleukin- 1 B.41 It is, therefore, easy to under- exhaled air involve adult patients. However, stand why iNOs is upregulated. Lundberg et al have used a direct nasal NO has an immunomodulatory role in sampling technique to measure NO in exhaled asthma and most likely in other inflammatory air from children.48 It would be of particular conditions via effects on T lymphocytes,42 interest to look at NO production in infants neutrophils,43 and macrophages.II Further- and young children in order to unravel the role more, mast cells and fibroblasts are also of inflammation in early asthma. This could capable of producing NO44 and mast cell then lead to development of early intervention degranulation causing release of many strategies. Whether inhibiting of the NO bronchoconstriction mediators is a feature inflammatory pathway will have any direct of asthma. It is likely that in the early therapeutic effect in asthma remains to be stages of asthma NO acts as an endogenous seen. Considering the information already bronchodilator to counteract the broncho- available regarding the site of production of constriction caused by mast cell degranulation. NO in asthma and atopic rhinitis, and the However, when greater quantities of NO are technical difficulties associated with collecting produced in established asthma, its production samples in children, careful subject selection is a double edged sword. It causes plasma leak will be needed for future studies. as a result ofvasodilatation and has a cytotoxic effect contributing to epithelial shedding. Finally the presence of NO in car exhaust, Thus NO is involved in chronic lung injury gas cookers and cigarette smoke, may in part in asthma and probably also in cystic fibrosis. explain the increased incidence of asthma over It mediates the effect of ozone in damaging the the last few decades. epithelium,45 which may be significant in view of the increased incidence of asthma in recent I am grateful to Professor J 0 Warner for his helpful comments times. Further evidence of its importance in and advice. asthma comes from a study by Kharitonov et al 1 Palmer RMJ, Ferridge AG, Moncada S. Nitric oxide release looking at NO concentration in expired air accounts for the biological activity ofendothelium-derived from asthmatic patients.46 Measured by relaxing factor. Nature 1987; 327: 524-6. 2 Moncada S, Palmer RMJ, Higgs EA. Nitric oxide: physiol- chemiluminescence, it was significantly raised ogy, pathophysiology and pharmacology. Pharmacol Rev in subjects with asthma who were not on 1991; 43: 109-41. http://adc.bmj.com/ 3 Barnes PJ, Belvisi MG. Nitric oxide and lung disease. inhaled or oral steroids, compared with con- Thorax 1993; 48: 1034-43. trols and subjects on steroids. The concentra- 4 Barnes PJ. Nitric oxide and airways. Eur Respir J 1993; 6: 163-5. tion of NO in exhaled air was reduced by 5 Moncada S, Higgs A. The L-arginine-nitric oxide pathway. inhaling an arginine analogue, L-NMMA N EnglJ Med 1993; 329: 2002-12. 6 Anggard E. Nitric oxide: mediator, murderer and medicine. (NG-monomethyl-L-arginine), which inhibits Lancet 1994; 343: 1199-206. NO synthesis. The increased level of NO in 7 Gaston B, Drazen JM, Loscalzo J, Stamler JS. The biology of nitrogen oxides in the airway. American Journal of asthmatics has been confirmed by other Respiratory Critical Care Medicine 1994; 149: 538-51. on September 25, 2021 by guest. Protected copyright. authors.374748 Further development of this 8 Vallance P, Collier J. Biology and clinical relevance of nitric oxide. BM3r 1994; 309: 454-7. technique may make it possible to monitor 9 Knowles RG, Moncada S. Nitric oxide synthases in mam- lung inflammation by measuring NO in mals. BiochemJ' 1994; 298: 249-58. 10 Bredt DS, Snyder SH. Isolation ofnitric oxide synthetase, a exhaled air and this may be a valuable tool in calmodulin-requiring enzyme. Proc Natl Acad Sci USA directing anti-inflammatory treatment.49 1990; 87: 682-5. 11 Hibbs JB, Taintor RR, Vaurin Z. Macrophage cytotoxicity: Asthma is an obvious clinical situation where it role of L-arginine deimunase activity and imino nitrogen can be used, although it might be of value in oxidation to nitrite. Science 1987; 235: 473-6. 12 Waldman SA, Murad F. Biochemical mechanisms underly- children with cystic fibrosis and broncho- ing vascular smooth muscle relaxation: the guanylate pulmonary dysplasia. cyclase-cyclic GMP system. Y Cardiovasc Pharmacol 1988; 12 (suppl 5): S115-8. There is some evidence that in healthy 13 Liu SF, Crawley DE, Rohde JAL, Evans TW, Barnes PJ. people the major part of NO in exhaled air Role of nitric oxide and guanosine 3'5' cyclic monophos- phate in mediating non-adrenergic, non-cholinergic originates in the nasal airways with only a relaxation in guinea-pig pulmonary arteries. Br J minor contribution from lower airways.4748 Pharmacol 1992; 107: 861-6. 14 Hibbs JB, Vavrin Z, Taintor RR. L-arginine is required post This relatively large production should be expression of the activated macrophage effector mech- taken into account when measuring NO as an anism causing selective metabolic inhibition in target cells. J Immunol 1987; 138: 550-65. indicator of inflammation in lower airways. 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