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* From the Departments of Thoracic Medicine (Drs. Csoma, Balint, Donnelly, Barnes, and Kharitonov) and Pediatric Respiratory Care (Drs. Bush and Wilson), Imperial College School of Medicine, National Heart and Lung Institute, London, UK.
Correspondence to: Peter J. Barnes, DM, Department of Thoracic Medicine, Imperial College School of Medicine, National Heart and Lung Institute, Dovehouse St, London, SW3 6LY, United Kingdom; e-mail: p.j.barnes{at}ic.ac.uk
| Abstract |
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Design: Single-center observational study.
Patients: Fifteen children with PCD (seven boys; mean [± SEM] age, 10.3 ± 0.7 years; mean FEV1, 73 ± 2.1% predicted) were recruited along with 14 healthy age-matched subjects (seven boys; mean age, 11.5 ± 0.4 years; mean FEV1, 103 ± 5% predicted).
Interventions: We assessed the levels of nitrite (NO2-), NO2-/NO3- (NO2-/NO3-), and S-nitrosothiol in exhaled breath condensate, exhaled NO, and nasal NO from children with PCD compared to those in healthy children.
Measurements and results: The mean exhaled and nasal NO levels were markedly decreased in children with PCD compared to those without PCD (3.2 ± 0.2 vs 8.5 ± 0.9 parts per billion [ppb], respectively [p < 0.0001]; 59.6 ± 12.2 vs 505.5 ± 66.8 ppb, respectively [p < 0.001]). Despite the lower levels of exhaled NO in children with PCD, no differences were found in the mean levels of NO2- (2.9 ± 0.4 vs 3.5 ± 0.3 µM, respectively), NO2-/NO3- (35.2 ± 5.0 vs 34.3 ± 4.5 µM, respectively), or S-nitrosothiol (1.0 ± 0.2 vs 0.6 ± 0.1 µM, respectively) between children with PCD and healthy subjects.
Conclusion. These findings suggest that NO synthase activity may not be decreased as much as might be expected on the basis of low exhaled and nasal NO levels.
Key Words: breath condensate exhaled nitric oxide nitrite nitrite/nitrate primary ciliary dyskinesia S-nitrosothiols
| Introduction |
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The measurement of exhaled NO might be used as a screening procedure to detect PCD among patients with recurrent chest infections or male infertility due to immotile spermatozoa, and the diagnosis of PCD would then be confirmed by the saccharine test, nasal NO level, ciliary beat frequency (CBF), and electron microscopy.6 NO plays an important role in bactericidal activity in the lungs, sodium and chloride transport in the nasal epithelium, and CBF,7 so that a decrease in endogenous NO production might contribute to the characteristic recurrent chest infections in PCD patients. Low levels of exhaled and nasal NO in PCD patients are related to mucociliary dysfunction,1 8 and treatment with NO donor L-arginine increases nasal NO levels and also improves mucociliary transport in PCD patients.1 9 Considering the inflammatory nature of PCD, the markedly decreased exhaled and nasal NO levels are surprising, and the pathophysiologic basis of these observations are still not clear.
NO is a free radical that is rapidly oxidized, reduced, or complexed with other molecules depending on the microenvironment, leading to the formation of nitrite (NO2-), nitrate (NO3-), and powerful oxidant peroxynitrite (ONOO-).10 11 ONOO- formation may be decreased in patients with PCD due to the NO deficit. It may further impair host defense in patients with PCD, as ONOO- may also nitrate inflammatory proteins such as the chemokines and, therefore, reduce oxidative damage.12 In addition, ONOO- interacts directly with glutathione to form the nitrososthiol S-nitrosoglutathione, which may protect against further effects of ONOO-.13
NO metabolites, such as NO2-, NO3-, and S-nitrosothiol, previously have been detected in exhaled breath condensate.14 We speculated that the measurement of NO metabolites in exhaled breath condensate would provide further information about nitrogen-reactive species in the airways of patients with PCD. We hypothesized that, on the basis of the low levels of exhaled and nasal NO, exhaled NO metabolites would be low. The design of this study was to assess the levels of NO2-, NO3, and S-nitrosothiol in the exhaled breath condensate of children with PCD in comparison with healthy subjects.
| Materials and Methods |
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Pulmonary Function
FVC percent predicted and FEV1 percent predicted were measured with a dry spirometer (Vitalograph; Buckingham, UK), and the best value of three maneuvers was expressed as a percentage of the predicted value.
Exhaled NO Measurement
NO was measured by chemiluminescence analyzer (model LR2000; Logan Research Ltd; Rochester, UK) according to American Thoracic Society guidelines15
and European Respiratory Society guidelines16
on NO measurements, as described previously.2
17
Exhaled Breath Condensate
Exhaled breath condensate was collected by using a condenser, which allowed the noninvasive collection of nongaseous components of the expiratory air (EcoScreen; Jaeger; Würzburg, Germany), as described previously.14
Subjects breathed through a mouthpiece and a two-way nonrebreathing valve, which also served as a saliva trap. They were asked to breathe at a normal frequency and at tidal volume, wearing a nose clip, for a period of 8 min. The condensate, at least 700 µL, was collected on ice at -20°C and was stored at -70°C immediately.
NO2-, NO2- and NO3-, and S-Nitrosothiol Measurements
The quantification of NO2- was assessed by a fluorometric assay based on the reaction of NO2- with 2,3-diaminonaphthalene to form the fluorescent product 1-(H)-naphthotriazole.23
Briefly, the 100-µL sample (exhaled breath condensate) was mixed with 10 µL 0.05 mg/mL 2,3-diaminonaphthalene reagent in 0.625 M HCl. The reaction was allowed to proceed at room temperature in the dark and was terminated with the addition of 10 µL 1.4 M NaOH. The intensity of the fluorescent signal produced by the product was measured by a fluorometer (Biolite F1; Labtech International Ltd; Uckfield, UK) immediately. The incubation of samples with NO3- reductase allowed the NO3- present in the sample to be measured by this assay after being converted to NO2-.14
S-nitrosothiols were measured following the release of NO2- from S-nitrosothiols by 2 mM Hg2Cl using the above-mentioned procedure.14
To calculate the level of S-nitrosothiols, NO2- levels were subtracted.
Statistical Analysis
Data were expressed as the mean ± SEM. A Mann-Whitney test was used to compare groups. The correlation between the fractional concentration of exhaled NO and NO2-, NO2-/NO3-, and S-nitrosothiol, as well as NO metabolites and lung function (ie, FEV1) was determined by nonparametric Spearman correlation analysis. Significance was defined as a value of p < 0.05.
| Results |
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| Discussion |
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The normal levels of NO metabolites cast doubt on the hypothesis that reduced exhaled and nasal NO levels are the results of reduced NO synthase (NOS) activity. Nevertheless, reduced NOS activity cannot be completely excluded. Normal NOS activity is comparable with a decreased level of exhaled NO if increased NO metabolism or reduced NO diffusion into the airway lumen occurs. Elevated levels of NO2- and NO3-,18 19 and nitrotyrosine20 have been found in the exhaled condensate and sputum21 of patients with cystic fibrosis (CF) during clinical stability and during exacerbations. In children with CF and normal lung function, however, the NO2-/NO3- concentrations in BAL fluid are normal and concentrations of S-nitrosothiol are reduced.22 In contrast, elevated levels of NO2- and S-nitrosothiol are found in the exhaled breath condensate of adult patients with more severe CF.23 Therefore, it may be speculated that production/metabolism changes may take place in PCD patients, so that the total NO production may seem to be similar to those of control subjects.
Myeloperoxidase, which is a heme enzyme of neutrophils that uses hydrogen peroxide to oxidize chloride to hypochlorous acid, is capable of catalyzing the nitration of tyrosine, providing an alternative to ONOO- in the formation of 3-nitrotyrosine.24 At sites of neutrophilic inflammation, the presence of myeloperoxidase will lead to protein nitration because the cosubstrate tyrosine will be available to facilitate the reaction.25 Patients with stable CF have significantly higher levels of nitrotyrosine in exhaled breath condensate than do healthy subjects.20 This suggests that the nitration of proteins by myeloperoxidase may be an additional source of nitrotyrosine in patients with CF who have very low NO production. In fact, the level of myeloperoxidase is elevated in the sputum of CF patients and correlates with concentrations of nitrotyrosine,21 implying that the absence of an increase in exhaled NO does not exclude the possibility of NO participating in airway inflammation, including CF.
Free radicals released from neutrophils may increase NO metabolism by the conversion of NO to NO metabolites, such as NO2-, NO3-, and S-nitrosothiol,26 and may lead to chronic, recurrent neutrophil inflammation, as seen in patients with CF and PCD. In fact, a positive correlation between the NO2- levels in exhaled breath condensate and the number of circulating plasma neutrophils in CF patients has been demonstrated.18
Some bacteria have been shown to produce NO from NO2-.27 NO may play an important role in nonspecific host defense against bacterial, viral, and fungal infections. One of the general mechanisms of antimicrobial defenses involves the S-nitrosylation of NO by cysteine proteases. Therefore, reduced endogenous NO production, resulting in low exhaled and nasal NO levels, may contribute to recurrent chest infections in patients with PCD, CF, and Wegener granulomatosis.28
Patients with PCD are frequently or continuously treated with antibiotics, which can influence nasal and lower airway bacterial composition, which may influence NO production and NO metabolite levels in airway fluids. For example, it has been shown that benzoquinoid ansamycins were able to reduce NO2- accumulation, inducible NOS (iNOS) messenger RNA levels, and the cytokine-dependent activation of the iNOS promoter.29 If this is the case, decreased exhaled NO levels would be associated with elevated levels of NO metabolites in airway fluids. However, our results do not support this hypothesis since no differences were found in the levels of exhaled NO metabolites between patients with PCD and healthy subjects. Furthermore, there were no differences either in the levels of exhaled NO or in the levels of NO metabolites in exhaled breath condensate between patients with PCD who were and were not receiving continuous antibiotic treatment.
Airway hygiene depends largely on mucociliary clearance and the movement of viscoelastic mucus along the airway by the beating of the ciliary appendages of airway epithelial cells.30 The failure to keep the airways sterile by mucociliary clearance, resulting in chronic damage to the airway wall and up-regulation of mucus production, may be due to several of the following factors: (1) very low NO levels in PCD patients because of a deficiency of iNOS; (2) microbial toxin-induced dysfunction of the energy pathways required for ciliary beating (ie, secondary ciliary dyskinesia); and (3) abnormalities in the viscosity of mucus, including reduced salt content/osmolality, which results in it being unsuitable in quality for the cilia to move it. Therefore, methods of recognizing the prevalent mechanism behind the mucociliary clearance in PCD patients may be useful in disease management.
The effect of NO may be beneficial or deleterious, and both NOS inhibitors and substrates of NOS could have great therapeutic potential in the treatment of PCD patients. Currently, L-arginine supplementation has been studied in a variety of clinical situations in which the increase of NO production is desired. For example, digested L-arginine31 and inhaled L-arginine1 have been used in healthy subjects and patients with PCD to improve the bactericidal activity of the lungs, ciliary beating, and mucociliary beating.
Selective and more potent NOS inhibitors and NO donors, as well as noninvasive clinical methods with which to assess NO biochemistry will lead to a better understanding of its deleterious and beneficial effects, and to novel treatments for PCD patients.
In conclusion, our study has demonstrated that the levels of NO metabolites, such as NO2-, NO2-/NO3-, and S-nitrosothiol, in exhaled breath condensate are not different from normal in patients with PCD, despite the marked decrease in exhaled NO levels. This may suggest that NOS activity is not decreased to such an extent as we expected on the basis of the detection of exhaled NO.
| Footnotes |
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This study was supported by the European and Hungarian Respiratory Society, the Hungarian Immunology and Allergology Society (Hungary), and the National Heart and Lung Institute (UK).
Received for publication June 4, 2002. Accepted for publication December 11, 2002.
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