Nitric oxide and airway disease

Nitric oxide (NO) may play an important role in regulating airway function and in the pathophysiology of inflammatory airway diseases. Endothelium-derived NO may be important in regulating airway blood flow and, indirectly, plasma exudation. NO is the neurotransmitter of bronchodilator nerves in human airways and counteracts the bronchoconstriction due to cholinergic neural mechanisms. Inducible NO synthase (iNOS) is expressed in human epithelial cells in response to pro-inflammatory cytokines and oxidants, probably via activation of the transcription factor nuclear factor kappa B (NF-kappa B). There is increased expression of iNOS in the epithelium of asthmatic patients and in lung macrophages in bronchiectasis. This may account for the increased concentration of NO in the exhaled air of patients with inflammatory airways disease. Increased NO production in the airways may result in hyperaemia, plasma exudation, mucus secretion and indirectly increased proliferation of Th2 lymphocytes responsible for eosinophilic inflammation. Glucocorticoids inhibit the induction of iNOS in epithelial cells and reduce the elevated exhaled NO to normal values. Selective inhibitors of iNOS may be useful in the treatment of inflammatory airway diseases, such as asthma, in the future.

Immunomodulation by theophylline in asthma. Demonstration by withdrawal of therapy

Theophylline is the most widely used anti-asthma drug worldwide and is classified as a bronchodilator, although there is increasing evidence that it may have immunomodulatory effects. We have investigated the effects of theophylline withdrawal under placebo control in 27 asthmatic patients (25 to 70 yr) treated with long-term theophylline who were also treated with high dose inhaled corticosteroids. We measured asthma symptoms (diary card), lung function (spirometry and home records of peak expiratory flow), and peripheral leukocyte populations using dual color flow cytometry. In eight of these patients, we examined fiberoptic bronchial biopsies by immunocytochemistry. We also studied peripheral blood lymphocytes in eight asthmatic patients who have never received theophylline. Mean steady state plasma theophylline concentrations during theophylline therapy were 8.6 +/- 0.9 mg/L. Theophylline withdrawal was associated with a significant increase in asthma symptoms, particularly at night, and a fall in spirometry and morning peak flow. This was accompanied by a significant fall in peripheral blood monocytes (CD14+, activated CD4+ T-lymphocytes (CD4+/CD25+) and activated CD8+ T-cells (CD8+/HLA-DR+) in patients with a plasma theophylline > 5 mg/L. The lymphocyte populations in theophylline-naive patients were similar to those found after theophylline withdrawal. Bronchial biopsies showed a mirror image of the peripheral blood with an increase in CD4+ and CD8+ lymphocytes in the airway. Chronic treatment with theophylline, even at low plasma concentrations, controls asthma symptoms and has effects on T-lymphocyte populations in the peripheral blood which are the inverse of those observed in the airways.(ABSTRACT TRUNCATED AT 250 WORDS)

Bradykinin receptor subtypes in rat lung: effect of interleukin-1 beta

We have characterized bradykinin (BK) receptors in the rat lung and studied the effect of recombinant human interleukin-1 beta (IL-1 beta) on BK receptors in vitro and in vivo. In lung membranes, saturation studies with [3]BK revealed a single class of specific and saturable binding sites. The BK B1 antagonist des-Arg9[Leu8]-BK was less effective in displacing [3H]BK binding sites from lung membranes. In contrast, the selective BK B2 antagonists, Hoe 140 (D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]-BK) and NPC 567 (D-Arg-[Hyp3,D-Phe7]-BK) fully inhibited the binding of [3H]BK to lung membranes with Ki values of 96.7 +/- 17.8 pM and 9.0 +/- 2.5 nM, respectively. Intratracheal administration of 500 U of IL-1 beta induced airway hyper-responsiveness to inhaled BK and neutrophilia in bronchoalveolar lavage fluid 18 to 24 hr later. Compared to naive or saline-treated animals, IL-1 beta had no effect on [3H]BK binding characteristics at 4, 12 or 24 hr after IL-1 beta administration. Twenty-four hours after IL-1 beta instillation, there was no change in the affinity of the selective BK B1 or B2 antagonists when compared to control animals. In vivo, the selective BK B2 receptor antagonists, NPC 567 (3 mumol kg-1 i.v.) and Hoe 140 (100 nmol kg-1 i.v.), inhibited BK-induced increase in lung resistance, whereas the selective BK B1 antagonist, des-Arg9[Leu8]-BK (10 mumol kg-1 i.v.), was without effect. These data suggest that the action of BK in the rat lung is dependent mainly on the activation of the BK B2 receptor subtype.(ABSTRACT TRUNCATED AT 250 WORDS)

Allergen-induced late asthmatic reactions are associated with elevation of exhaled nitric oxide

The concentration of nitric oxide (NO) is increased in the exhaled air of asthmatic patients and may reflect cytokine-mediated inflammation in the airways. We investigated whether allergen-induced inflammation causes an elevation in the level of exhaled NO. Of 25 patients who underwent allergen challenge, 16 developed dual early and late responses, whereas eight had a single early response. In the patients with a dual response, the maximal fall in FEV1 during the late response was 26.8 +/- 4.2% at 9 h and there was a significant increase in the level of exhaled NO (maximal increase of 59.4 +/- 9.8%) 10 h after challenge. There was a significant relationship between the size of the late response and the increase in exhaled NO (r = 0.75, p < 0.01). In patients who have a single early response, there was no significant increase in exhaled NO, with the exception of a single time point at 21 h. In five patients given a control challenge with methacholine there was no change in exhaled NO. There was no increase in exhaled NO after inhaled histamine in any of the patient groups. We conclude that the late asthmatic response to allergen is associated with elevated exhaled NO concentrations and that this provides further evidence that exhaled NO may reflect allergic inflammation in asthmatic airways, and may be a useful marker in monitoring asthma and its response to anti-inflammatory treatments. Whether endogenously produced NO plays a pathophysiologic role in the late response remains to be determined.

Effect of a novel potent platelet-activating factor antagonist, modipafant, in clinical asthma

Platelet-activating factor (PAF), proposed as an important inflammatory mediator in asthma, reproduces several of the features of asthma, such as microvascular leakage, mucus secretion, bronchoconstriction, and possibly increased airway responsiveness. Modipafant (UK-80,067) is the (+)-enantiomer of UK-74,505, a potent and specific PAF antagonist. We have assessed the effect of modipafant over 28 d in adult subjects with moderately severe asthma in a placebo-controlled parallel group study. A total of 218 patients with asthma were enrolled into the single-blind run-in, of whom 120 (93 males and 27 females, mean age 41.0 yr) entered the double-blind treatment phase after demonstrating symptomatic asthma in the final week of the single-blind run-in phase. Patients could take up to 1600 micrograms inhaled corticosteroid and an inhaled beta 2 agonist as rescue medication. A total of 59 patients with asthma took modipafant (one 50 mg capsule twice daily), and 61 took matched placebo. There was no significant difference between placebo and modipafant in diurnal variation in PEF, morning and evening PEF, clinic FEV1, rescue bronchodilator usage, symptom score, or airway responsiveness. We previously showed that the racemate UK-74,505 had no effect on antigen challenge, and this study shows that the active (+)-enantiomer modipafant has no effect in chronic asthma. This suggests that PAF is not an important mediator in asthma.

Glucocorticoid inhibition of RANTES expression in human lung epithelial cells

An influx of eosinophils into the lungs occurs in several pulmonary disorders. However, the mechanisms involved remain unknown. Lung epithelial cell release of eosinophil chemotactic factors such as RANTES or macrophage inflammatory protein-1 alpha (MIP-1 alpha) could account for the influx of eosinophils into the lungs. In order to demonstrate the potential role for lung epithelial cells to release RANTES and/or MIP-1 alpha, we investigated the mRNA expression and protein release in cultured A549 cells. Tumor necrosis factor-alpha (TNF alpha) and interleukin-1 beta (IL-1 beta) induced a time- and dose-dependent increase in RANTES mRNA expression and protein release. In contrast, MIP-alpha protein release was not detectable in these cells. As corticosteroids decrease the influx of eosinophils into the lungs in vivo, we also investigated the capacity of dexamethasone to decrease the TNF alpha-induced RANTES release and mRNA expression; both were decreased in a time- and concentration-dependent manner. Dexamethasone did not affect the TNF alpha-induced RANTES mRNA half-life and did not require protein synthesis to manifest an inhibitory effect. Supernatant from cells stimulated with TNF alpha and IL-1 beta increased eosinophil chemotaxis and this was also inhibited by dexamethasone. These findings suggest a role for RANTES release by lung epithelial cells in the recruitment of eosinophils into the lungs in pulmonary disorders such as interstitial lung diseases, idiopathic pulmonary fibrosis, or asthma and suggest that one beneficial effect of corticosteroids may be inhibition of lung epithelial cell RANTES mRNA expression and protein release.

Beta-adrenoceptor agonists interfere with glucocorticoid receptor DNA binding in rat lung

Inhaled beta 2-adrenoceptor agonists are the most effective bronchodilator treatment in asthma, yet paradoxically high doses may be associated with increased asthma morbidity and mortality. Steroids are the most effective therapy in controlling asthmatic inflammation and act by binding to specific sequences of DNA (GRE), thus modulating gene transcription. We report that in rat lung, the beta 2-adrenoceptor agonists, salbutamol and fenoterol, decrease the binding of glucocorticoid receptors to GRE, by 46 +/- 4% although it has no effect on the affinity or number of glucocorticoid receptors. The inhibition of GRE binding by salbutamol is concentration-dependent, can be blocked by propranolol and is seen following forskolin treatment. This effect appears to be due to an interaction between the glucocorticoid receptor and the transcription factor, cAMP response element binding protein (CREB), which is activated by high concentrations of beta 2-adrenoceptor agonists. We suggest that by this mechanism high doses of inhaled beta 2-adrenoceptor agonists may inhibit the anti-inflammatory effects of endogenous glucocorticoids and exogenous corticosteroids used for asthma therapy.

Dexamethasone inhibits ozone-induced gene expression of macrophage inflammatory protein-2 in rat lung

To address the potential role of the chemokine macrophage inflammatory protein-2 (MIP-2) in airway inflammation, we examined whether MIP-2 may play a role in ozone-induced neutrophilic inflammation of airways and its modulation by dexamethasone in rat lung. Following ozone exposure, MIP-2 mRNA expression in the lung peaked at 2 h after exposure and slowly declined thereafter. Dexamethasone suppressed ozone-induced MIP-2 mRNA expression and neutrophil accumulation in the lung. We suggest that the MIP-2 mRNA induction may switch on the neutrophilic influx observed in this model of lung inflammation. Furthermore, the MIP-2 expression is regulated by dexamethasone which may represent one of the mechanisms by which glucocorticoids exert their potent anti-inflammatory properties.

Use of a fixed combination beta 2-agonist and steroid dry powder inhaler in asthma

In many countries the majority of patients with asthma are now treated with inhaled beta 2-agonists as required and regular inhaled steroids. Compliance with asthma medication is poor, particularly for inhaled steroids, which do not give immediate relief of symptoms. An inhaler combining a beta 2-agonist and a steroid may improve compliance and give better control of asthma. We have studied the effect of a fixed combination dry powder inhaler (Turbuhaler) containing terbutaline (250 micrograms) with budesonide (200 micrograms) compared with each drug given alone in a double-blind, crossover study in 74 patients with mild to moderate asthma. This was a multicenter study performed in general practice. After a 1-wk run-in period with placebo inhaler, each treatment was given twice daily for 4 wk in randomized order. Patients measured symptom scores and rescue inhaled beta 2-agonist use and recorded peak expiratory flow (PEF) twice daily. In nine patients recruited to a hospital center, airway responsiveness was measured by the concentration of methacholine required to produce a fall in FEV1 of 20% (PC20) at the end of each treatment period. Of 74 patients randomized, 68 completed all three treatment periods. Their mean age was 37 (18-60) yr and PEF 80 (43-116) % predicted. Combination treatment gave a significant improvement in morning and evening PEF budesonide or terbutaline treatment periods (p < 0.01), and a significant reduction in asthma symptom scores and rescue beta 2-agonist use (p < 0.05). There was a significant patient preference for the combination inhaler.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in binding of glucocorticoid receptor to DNA in steroid-resistant asthma

Although glucocorticosteroids are a very effective treatment for asthma and other chronic inflammatory diseases, a small proportion of patients are resistant to their therapeutic effects. The molecular mechanism for this steroid resistance is unclear. Steroid resistance cannot be explained by pharmacokinetic mechanisms, by a defect in the binding of steroids to glucocorticoid receptors, nor by defective nuclear translocation of this receptor, thereby suggesting that the molecular abnormality lies distal to nuclear translocation. We examined the ability of nuclear translocated glucocorticoid receptors to bind to their DNA binding sites (GRE) using electrophoretic mobility shift assays in PBMC from patients with steroid-sensitive and steroid-resistant asthma. The binding of the glucocorticoid receptor to DNA in these patients was also studied using Scatchard analysis. Dexamethasone induced a significant rapid and sustained twofold increase in GRE binding in PBMCs from steroid-sensitive asthmatic patients and nonasthmatic individuals, but this was markedly reduced in steroid-resistant asthmatic patients. Scatchard analysis of glucocorticoid receptor-GRE binding showed no change in binding affinity but did show a reduced number of receptors available for DNA binding in the steroid-resistant patients. These results suggest that the ability of the glucocorticoid receptor to bind to GRE is impaired in steroid-resistant patients because of a reduced number of receptors available for binding to DNA.

Differences in binding of glucocorticoid receptor to DNA in steroid-resistant asthma

Although glucocorticosteroids are a very effective treatment for asthma and other chronic inflammatory diseases, a small proportion of patients are resistant to their therapeutic effects. The molecular mechanism for this steroid resistance is unclear. Steroid resistance cannot be explained by pharmacokinetic mechanisms, by a defect in the binding of steroids to glucocorticoid receptors, nor by defective nuclear translocation of this receptor, thereby suggesting that the molecular abnormality lies distal to nuclear translocation. We examined the ability of nuclear translocated glucocorticoid receptors to bind to their DNA binding sites (GRE) using electrophoretic mobility shift assays in PBMC from patients with steroid-sensitive and steroid-resistant asthma. The binding of the glucocorticoid receptor to DNA in these patients was also studied using Scatchard analysis. Dexamethasone induced a significant rapid and sustained twofold increase in GRE binding in PBMCs from steroid-sensitive asthmatic patients and nonasthmatic individuals, but this was markedly reduced in steroid-resistant asthmatic patients. Scatchard analysis of glucocorticoid receptor-GRE binding showed no change in binding affinity but did show a reduced number of receptors available for DNA binding in the steroid-resistant patients. These results suggest that the ability of the glucocorticoid receptor to bind to GRE is impaired in steroid-resistant patients because of a reduced number of receptors available for binding to DNA.

Transcriptional down-regulation of m2 muscarinic receptor gene expression in human embryonic lung (HEL 299) cells by protein kinase C

m2 muscarinic receptor gene expression was investigated following stimulation of protein kinase C (PKC) with the phorbol ester 4 beta-phorbol dibutyrate (PDBu) in HEL 299 cells. PDBu (100 nM) caused a time-dependent decrease in the steady-state levels of m2 receptor mRNA and in specific [3H]N-methyl-scopolamine binding. Preincubation with the PKC inhibitor GF-109203X inhibited the reduction in M2 receptor and mRNA levels induced by PDBu, confirming the involvement of PKC. Chronic PDBu treatment also caused desensitization of the receptor as forskolin-stimulated cAMP accumulation, inhibited by carbachol in control cells, was lost upon treatment with PDBu for 24 h. Co-incubation with PDBu and the protein synthesis inhibitor cycloheximide, inhibited PDBu-mediated reduction of m2 receptor mRNA, indicating new protein synthesis is required for down-regulation. Half-life studies using the transcriptional inhibitor actinomycin D suggested that the stability of the m2 receptor mRNA was not altered by PDBu treatment (t1/2 = 2 h). Nuclear run-on assays showed a 50% reduction in the rate of m2 receptor gene transcription after treatment with PDBu for 12 h. In conclusion we have provided evidence for heterologous regulation of m2 receptor gene expression through changes in gene transcription resulting in uncoupling of M2 receptors. Furthermore, the synthesis of an unidentified factor is required for the down-regulation process.

Cyclic nucleotides and phosphodiesterases and airway function

The cyclic nucleotides, cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) play an important role in the regulation of airway smooth muscle tone and activation of inflammatory cells. Intracellular concentrations of these nucleotides are tightly regulated by phosphodiesterases (PDEs). Several families of PDEs are now recognized and, within each isoenzyme family, molecular cloning has revealed multiple members. PDE III and IV are important in the breakdown of cAMP; PDE III is involved in the regulation of airway smooth muscle tone, whereas PDE IV is more important in inflammatory cells, including mast cells, macrophages, eosinophils, T-lymphocytes and epithelial cells. PDE V is involved in the breakdown of cGMP in airway and vascular smooth muscle. Regulation of PDE expression in health and disease is now under investigation. Several selective PDE inhibitors have recently been developed, and experimental studies indicate that PDE IV inhibitors may be useful as anti-inflammatory treatments in asthma. Clinical trials in asthma are now in progress.

Nitric oxide and asthmatic inflammation

Asthmatic patients show an increased expression of inducible nitric oxide synthase (iNOS) in airway epithelial cells and an increased level of nitric oxide (NO) in exhaled air. The NO derived from airway epithelial cells may be a mechanism for amplifying and perpetuating asthmatic inflammation, through inhibition of T helper 1 (Th1) cells and their production of interferon gamma (IFN-gamma). This would result in an increase in the number of Th2 cells and the cytokines interleukin 4 (IL-4) (which is important for IgE expression) and IL-5 (which plays a critical role in the recruitment of eosinophils into the airways). Although this mechanism may be part of our nonspecific airway defence against parasite invasion, it appears to have been activated inappropriately in asthma. Here, Peter Barnes and Eddy Liew argue that the development of specific iNOS inhibitors may represent a novel therapeutic approach for asthma and other allergic diseases.

Evidence for the involvement of cGMP in neural bronchodilator responses in humal trachea

1. We have investigated the correlation between relaxation and changes in cyclic nucleotide content of human tracheal smooth muscle (HTSM) in vitro following inhibitory non-adrenergic non-cholinergic (i-NANC) neural bronchodilator responses evoked by electrical field stimulation (EFS), and compared these with changes seen with sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1) and vasoactive intestinal peptide (VIP). The effects of N omega-nitro-L-arginine methyl ester (L-NAME), Methylene Blue and alpha-chymotrypsin (alpha-CT) were studied. 2. EFS (10 Hz, 1 ms, 40 V for 30 s) evoked a time-dependent relaxation accompanied by a concurrent rise in cGMP, both of which were maximal at 30 s and unaffected by epithelium removal. Levels of cAMP were more variable than those of cGMP and were not significantly changed at any time point. 3. SIN-1 (1 mM) and SNP (100 microM) also produced time-dependent relaxations which were maximal between 2 and 8 min, accompanied by concomitant rises in cGMP; however, these changes were larger than those associated with i-NANC relaxations. cAMP levels were unchanged at all time points. 4. EFS-evoked i-NANC relaxations and cGMP increases (time, t = 30 s) were inhibited by L-NAME. The effects were partially reversed by L-arginine (1 mM), but not by D-arginine. D-NAME and alpha-CT (2 u ml-1) had no effect on either relaxation or cGMP accumulation. Tetrodotoxin (TTX, 3 microM) inhibited both relaxation and cGMP accumulation. 5. VIP (1 microM) also produced a time-dependent relaxation associated with a concurrent rise in cAMP levels with no change in cGMP levels. 6. Methylene Blue (10 microM) partially inhibited EFS (10 Hz)-evoked i-NANC relaxation and cGMP accumulation, and almost completely inhibited both relaxation and cGMP accumulation evoked by SIN-1 (1 mM). Methylene Blue had no significant effect on relaxation or cGMP accumulation evoked by SNP (100 microM). 7. Neural i-NANC relaxations in HTSM are associated with a concurrent selective accumulation of cGMP which is unaffected by epithelium removal. This is inhibited in a stereoselective manner by L-NAME and mimicked by SNP and SIN-1; however, cGMP accumulation was greatly increased with SNP and SIN-1 suggesting compartmentalized changes in cGMP content. VIP also caused relaxation associated with an increase of cAMP; however, no evidence was found for VIP being involved in i-NANC relaxation. Hence nitric oxide (NO), or a NO-containing complex, appears to mediate i-NANC responses in human trachea in vitro.

Ozone-induced airway hyperresponsiveness: role of superoxide anions, NEP, and BK receptors

We investigated the role of reactive oxygen species in ozone-induced airway hyperresponsiveness (AHR) in Brown Norway rats. Airway responsiveness to inhaled acetylcholine (ACh) and bradykinin (BK) and inflammatory cell recruitment in bronchoalveolar lavage fluid (BALF) were measured in vivo. Neutral endopeptidase (NEP) activity assay and measurement of BK-receptor binding sites in Brown Norway rat lungs were carried out in vitro. Apocynin (5 mg/kg), an inhibitor of superoxide anion-generating NADPH oxidase, was administered perorally 30 min before a 3- or 6-h exposure to 3 ppm of ozone, and the animals were studied 18-24 h postexposure. Ozone induced increases in airway responsiveness to ACh and BK and in neutrophil counts in BALF. Apocynin inhibited the increase in airway responsiveness to BK but not to ACh without affecting the neutrophil counts in BALF. The antioxidants allopurinol and deferoxamine prevented ozone-induced AHR to both ACh and BK but did not reduce neutrophil counts. To further examine the mechanisms of ozone-induced AHR to BK, we measured NEP activity and the density of BK receptors in vitro after ozone exposure. Ozone exposure had no significant effect either on NEP activity or on the affinity and the number of BK receptors in lungs from rats treated with or without apocynin. We conclude that superoxide anions released from inflammatory cells in the airway may be involved in ozone-induced AHR. Inactivation of NEP or upregulation of BK receptors do not appear to be involved, but the possibility of localized changes cannot be excluded.

Role of neutral endopeptidase in bronchial hyperresponsiveness to bradykinin induced by IL-1 beta

Interleukin-1 beta (IL-1 beta) induces bronchial hyperresponsiveness (BHR) to bradykinin but not to acetylcholine. We examined whether this was mediated through the inhibition of neutral endopeptidase (NEP) activity and/or through the enhancement of airway microvascular leakage (AML) by IL-1 beta. We administered human recombinant IL-1 beta (500 U) or saline intratracheally and 24 h later measured the airway responses to bradykinin (1 mM; 45 breaths). IL-1 beta-treated rats showed a decrease of 18.5 and 21.1% of NEP activity in the lungs and tracheobronchial tree, respectively (P < 0.05), associated with an augmented response in total lung resistance to bradykinin but with no increase in Evans blue dye extravasation used as a marker of AML. Phosphoramidon (0.1 and 1 mM; 90 breaths), an NEP inhibitor, induced a dose-dependent increase in lung resistance to bradykinin without further enhancing BHR induced by IL-1 beta. Bradykinin-induced AML was not enhanced by phosphoramidon in either saline- or IL-1 beta-treated rats. Similarly, after captopril (1 mM; 90 breaths), an inhibitor of angiotensin-converting enzyme, there was no further enhancement of BHR to bradykinin induced by IL-1 beta. BHR to bradykinin induced by IL-1 beta may result from an inhibition of peptidase activity, such as NEP and angiotensin-converting enzyme, and is not associated with an enhancement of AML.

L-arginine increases exhaled nitric oxide in normal human subjects

1. Endogenous nitric oxide plays an important physiological role and is synthesized by several isoforms of nitric oxide synthase from the semiessential amino acid L-arginine. Nitric oxide is detectable in the exhaled air of normal individuals and may be used to monitor the formation of nitric oxide in the respiratory tract. 2. We have investigated the effect of orally administered L-arginine (0.05, 0.1, 0.2 g/kg) compared with matched placebo on the concentration of nitric oxide in the exhaled air in 23 normal individuals. 3. L-Arginine caused significant increases in the concentration of nitric oxide in exhaled air at doses of 0.1 and 0.2 mg/kg, which was maximal 2 h after administration. This was associated with an increase in the concentration of L-arginine and nitrate in plasma. There were no significant changes in heart rate, blood pressure or forced expiratory volume in 1 s. 4. These results suggest that an increase in the amount of substrate for nitric oxide synthase can increase the formation of endogenous nitric oxide. This may have therapeutic relevance in diseases in which there is defective production of nitric oxide.

Increased nitric oxide in exhaled air of normal human subjects with upper respiratory tract infections

Viral infection may induce the expression of nitric oxide (NO) synthase, resulting in increased NO formation that has an antiviral effect. NO may be produced by various cells of the upper and lower respiratory tract, and may be detected in the exhaled air. We have studied the levels of exhaled NO in 18 normal subjects during symptomatic upper respiratory tract infections and during recovery 3 weeks later. Exhaled NO was measured using a modified chemiluminescence analyser. At the time of symptoms of upper respiratory tract infection, the peak exhaled NO values were 315 +/- 57 ppb (mean +/- SEM) and decreased to 87 +/- 9 ppb during recovery. Recovery values of exhaled NO were similar to those reported in age-matched normal control subjects (88 +/- 3 ppb, n = 72). These findings suggest that symptomatic upper respiratory tract infections markedly increase the concentration of NO in exhaled air. This may reflect the induction of nitric oxide synthase (NOS) in upper and lower respiratory tract, and may be relevant to viral exacerbations of asthma.