Copyright ©ERS Journals Ltd 2006 Effect of smoking on exhaled nitric oxide and flow-independent nitric oxide exchange parametersDepts of 1 Medical Cell Biology, Integrative Physiology, 3 Medical Sciences, Respiratory Medicine and Allergology, and, 4 Medical Sciences, Occupational and Environmental Medicine, and, 2 Asthma and Allergy Research Centre, Uppsala University, Uppsala, and, 5 Dept of Caring Sciences and Sociology, Section of Medical Science, University of Gävle, Gävle, Sweden, and 6 Dept of Engineering Physics and Mathematics, Helsinki University of Technology, Helsinki, Finland. CORRESPONDENCE: A. Malinovschi, Uppsala University, Dept of Medical Cell Biology, Integrative Physiology, Box 571, SE-751 23 Uppsala, Sweden. Fax: 46 184714938. E-mail: Andrei.Malinovschi{at}medcellbiol.uu.se Keywords: Exhaled nitric oxide, extended nitric oxide analysis, oral tobacco, smoking
Received: September 29, 2005
It is a well-known fact that smoking is associated with a reduction in exhaled nitric oxide (NO) levels. There is, however, limited knowledge relating to the smoking-induced changes in production or exchange of NO in different compartments of the airways. This study comprised 221 adult subjects from the European Community Respiratory Health Survey II, who were investigated in terms of their exhaled NO, lung function, immunoglobulin E sensitisation and smoking habits. The following parameters were determined using extended NO analysis: airway tissue nitric oxide concentration (Caw,NO), airway transfer factor (or diffusing capacity) for nitric oxide (Daw,NO), alveolar nitric oxide concentration (CA,NO) and fractional exhaled nitric oxide concentration at a flow rate of 50 mL·s-1 (FeNO,0.05). Maximum total airway nitric oxide flux (J'aw,NO) was calculated from Daw,NO(Caw,NOCA,NO). Current smokers (n = 35) exhibited lower (geometric mean) FeNO,0.05 (14.0 versus 22.8 ppb), Caw,NO (79.0 ;versus 126 ppb) and J'aw,NO (688 versus 1,153 pL·s-1) than never-smokers (n = 111). Ex-smokers (n = 75) were characterised by lower FeNO,0.05 (17.7 versus 22.8 ppb) and Jaw,NO (858 versus 1,153 pL·s-1) than never-smokers. These relationships were maintained after adjusting for potential confounders (sex, age, height, immunoglobulin E sensitisation and forced expiratory volume in one second), and, in this analysis, a negative association was found between current smoking and CA,NO. Snus (oral moist snuff) consumption (n = 21) in ex-smokers was associated with an increase in Daw,NO and a reduction in Caw,NO, after adjusting for potential confounders. Passive smoking was associated with a higher CA,NO. Using extended nitric oxide analysis, it was possible to attribute the reduction in exhaled nitric oxide levels seen in ex- and current smokers to a lower total airway nitric oxide flux in ex-smokers and reduced airway and alveolar nitric oxide concentrations in current smokers. The association between snus (oral tobacco) use and reduced nitric oxide concentrations in the airways and increased nitric oxide transfer from the airways warrants further studies. A reduction in exhaled nitric oxide (NO) levels was first observed in smokers in the early 1990s 1, 2, and this effect was found after both acute and chronic exposure to smoking 3. Passive smoking has also been found to reduce levels of exhaled NO in healthy subjects 4 and asthmatic children 5. Smoking cessation is accompanied by an increase in exhaled NO levels 6, and, in one report, NO levels normalised after smoking cessation 7. The possible mechanisms by which exhaled NO levels are reduced in smoking subjects are a potential negative feedback mechanism of the NO from the cigarette smoke, which could lead to downregulation of NO synthase (NOS) in the lungs 8, 9, an inadequate supply of cofactors necessary for NO production, such as tetrahydrobiopterin 10, and an increase in the breakdown of NO 11, 12. By modelling NO exchange dynamics, it is possible to obtain greater insight into the two NO-producing compartments, the airways and alveoli, which are characterised by two or three flow-independent NO exchange parameters, depending on the model that is used. A review on this topic has been published recently 13. The few studies investigating the effects of smoking on flow-independent NO exchange parameters indicate that smoking is related to a lower maximum total airway nitric oxide flux (J'aw,NO) 7, 14 and to a lower mean airway tissue nitric oxide concentration (Caw,NO) 15. There is, however, very limited information about the doseresponse relationship and the effect of past and passive smoking on exhaled NO levels and flow-independent NO exchange parameters. The aim of the present investigation was to study the effect of past, current and passive smoking on exhaled NO in a general population sample using flow-independent NO exchange parameters.
Population The subjects in the present study were participants in a follow-up of the European Community Respiratory Health Survey (ECRHS), which was performed in Uppsala during 19901991 16. Of the 622 subjects in the random sample of the ECRHS, 517 were reinvestigated 9 yrs later (19992000) in the ECRHS II 17. The majority of the subjects who were reinvestigated (n = 368) were seen at the hospital for a clinical examination, whereas the remaining 149 subjects participated only in a telephone survey, usually because they had moved outside the study area between the two surveys. Of the 368 subjects who attended for clinical examination, 225 (61%) were also willing to perform exhaled NO measurements. In four of the subjects, information was lacking about their current smoking status; the present investigation, therefore, included 221 subjects.
Questionnaires
Measurements of exhaled NO The system used for NO measurements was a computer-based single-breath NO system from Nitrograf (Hässelby, Sweden), which used a chemiluminescence analyser (Sievers NOA 280; Sievers, Boulder, CO, USA). The system was calibrated using a mixture of 460 ppb NO in nitrogen (AGA, Lidingö, Sweden) and the zero was set by feeding synthetic air (AGA) into a 2-L canister filled with Purafil II chemisorbent with purakol (Lindair, Ljusne, Sweden). The flow sensor was calibrated in the range 00.6 L·s-1 (Dry Cal DC-2 flow calibrator; BIOS International, Pompton Plains, NJ, USA). Checks of the calibration and flow rate of the sampling system were made on a daily basis and the zero was controlled before each measurement. The expiratory pressure for all subjects ranged 520 cmH2O in order to exclude a NO contribution from the nasal cavity. The mean value from three breaths (or two, if the NO concentrations were identical from the two breaths) was used for statistical analysis.
Application of the extended NO analysis
Lung function
Immunoglobulin E sensitisation
Tobacco use Never- and ex-smokers who answered in the affirmative to the question "Have you regularly (most days or nights) been exposed to tobacco smoke in the last 12 months?" were classified as passive smokers. Snus consumption was registered as a yes/no answer, without recording information about the amount consumed.
Statistical methods
Ethics
The study population comprised 115 males and 106 females. Their mean age was 43 yrs (range 2954 yrs); 35 (15.8%) were current smokers, 75 (33.9%) ex-smokers and 15 (6.8%) passive smokers. The subjects who underwent exhaled NO measurements did not differ from the other participants undergoing clinical examination in terms of sex, age, smoking history or passive smoking.
Current smoking
The association between current smoking and FeNO,0.05, Caw,NO and J'aw,NO, respectively, remained significant when adjusting for sex, age, height, IgE sensitisation and FEV1 (table 2
Past smoking Ex-smokers showed significantly lower FeNO,0.05 than never-smokers, whereas no significant differences were found for the other NO variables (table 1
In ex-smokers, the effects of smoking-related variables (amount previously smoked and latency respectively), snus consumption and lung function (assessed by FEV1) on exhaled NO levels were investigated. No associations were found between smoking-related variables and exhaled NO levels. Snus consumption in ex-smokers was associated with increased Daw,NO (p = 0.04) on univariate analysis. The association between snus consumption and Caw,NO was just above the level of significance (p = 0.06; fig. 1
Passive smoking Nonsmokers who were passive smokers exhibited significantly higher CA,NO than subjects who were not exposed, whereas there were no significant differences in terms of the other exhaled NO variables (table 4
The main finding of the present study is that current smoking is associated with a reduction in Caw,NO and CA,NO. It was also found that ex-smokers exhibited lower levels of exhaled NO than never-smokers, which was reflected in a lower J'aw,NO, and that passive smoking was associated with increased CA,NO. A surprising and novel finding was that, in ex-smokers, snus consumption was associated with a reduction in Caw,NO and an increase in Daw,NO. The model used in the present article to determine the NO flow-independent parameters has been validated 19 against the classical slopeintercept model 23. The choice of flow rates and method used to analyse the data affects the estimation of NO flow-independent parameters 13. Decreasing the highest flow rate increased the estimated CA,NO in a recent article that used a linear regression method and FeNO measurements at three flow rates ranging 100200 mL·s-1 24. The choice of lowest flow rate affects the estimation of Caw,NO and Daw,NO, and, theoretically, Caw,NO would be estimated more accurately by using as low as possible a flow rate, since the measured exhaled NO would be Caw,NO at a flow rate that tends towards 0 mL·s-1. The reduction in Caw,NO in current smokers is in accordance with a previous study 15. A recent study 25 was not able to demonstrate differences between smokers and nonsmokers in terms of the nonenzymatic production of NO and suggested that the lower levels of exhaled NO in smokers might be due to the downregulation of enzymatic NO production in the oropharyngeal and bronchial compartment. A negative feedback mechanism caused by the high levels of NO in cigarette smoke was postulated, in the early 1990s, as a possible mechanism 8, but this was only confirmed in 2003, in the case of inducible NOS in lung epithelial cells 9. Smoking is associated with reduced levels of tetrahydrobiopterin 10, which might reduce enzymatic NO production by uncoupling NOS, with resultant production of superoxide instead of NO 26. Superoxide can, in turn, react with NO to form peroxynitrite. The fact that NO consumption might be increased in smokers airways is also suggested by the increase in NO metabolites in exhaled breath condensate 12, 27. In the present study, current smoking was also associated with reduced CA,NO. This result is in accordance with a study of Delclaux et al. 14, who measured FeNO at six different flow rates, ranging 50300 mL·s-1, and used least-squares linear regression in order to obtain CA,NO and J'aw,NO. Delclaux et al. 14 found a trend towards higher values in healthy nonsmokers versus healthy smokers, but contradicted the results of two previous studies demonstrating either higher CA,NO in smokers 7, when linear regression was used on FeNO measurements performed at five flow rates, ranging 50320 mL·s-1, or no difference in CA,NO, in a study that used the same flow rates and model as the present study 15. One explanation for this apparent discrepancy might be that the results were adjusted for possible confounders, something that was not undertaken in the previous studies. Another possible methodological explanation for this difference might be the fact that the present study is based on a general population sample, whereas the previous investigations comprised healthy nonsmokers versus healthy smokers. Therriault et al. 28 reported that the N-nitrosamine 4-(N-methylnitrosamino)-1-(3-pyridyl)-1-butanone, a component of cigarette smoke, inhibited alveolar macrophages from producing NO, a finding that might explain the lower CA,NO in smokers. Another possible mechanism could be an increase in the permeability of the respiratory membrane for NO in chronic smokers 29. It was not possible to find a doseresponse relationship when studying the number of cigarettes smoked and levels of exhaled NO. This observation apparently contradicts a previous study of Kharitonov et al. 3, in which a strong correlation between the number of cigarettes smoked and FeNO was found, but this study was conducted in 1995 and used peak and not plateau exhaled NO concentrations, as in the current recommendations, which were followed in the present study. Takahashi et al. 30 looked at end-expiratory levels of NO and reported that levels of exhaled NO were not related to the number of cigarettes smoked. In the present study, ex-smokers exhibited lower FeNO,0.05 and J'aw,NO than never-smokers, and this difference remained after adjusting for sex, age, height, IgE sensitisation and FEV1. These results are in accordance with those of Robbins et al. 6, who reported an increase in FeNO but even lower levels of mean oral NO than in controls after 8 weeks of smoking cessation. The results are not in accordance with a previous study in which it was found that 4 weeks of smoking cessation resulted in an increase in FeNO in the ex-smoker group up to the same level as the healthy nonsmoking controls 7. This apparent discrepancy may be due to the fact that some of the subjects in the smoking cessation group in the previous study showed allergic symptoms and therefore higher baseline FeNO than the healthy controls, who were all nonallergic. The present results point towards a reduction in NO transfer through the apical membrane of the airway epithelial cells, which could be explained by the fact that smoking has been associated with the keratinisation of epithelial cells, as seen for oral mucosa 31 and tracheal epithelium 32, impeding NO diffusion. In the present study, no association was found between FeNO,0.05 and latency or amount of previous smoking in ex-smokers. However, somewhat surprisingly, it was found that snus consumption was associated with a reduction in Caw,NO and an increase in Daw,NO in ex-smokers. One possible reason for the reduction in Caw,NO in snus users may be an increase in the consumption of NO in the airways, possibly due to the transformation of NO to peroxynitrite. This suggestion is supported by the observations of Helen et al. 33, who found nicotine-induced peroxidative damage in the lungs, heart and liver of rats. A similar observation was made by Iho et al. 34, who looked at nicotine-stimulated neutrophils and noted that neutrophilic production of NO was reduced, suggesting that superoxide, produced by nicotine, generates peroxynitrite by reacting with preformed NO. The other observation, which was that Daw,NO was increased in snus consumers, might be explained by the higher oral production seen in snus users due to bacterial colonisation. The bacterial colonisation might be explained by the poorer oral hygiene reported in snus consumers, which would create a local environment in the oral mucosa conducive to bacterial growth and colonisation 35. There is evidence that the nicotine concentrations reported in the saliva of snus consumers might have a stimulatory effect on bacterial growth 36. An alternative or complementary explanation could be the high nitrate content of snus 37, which can be transformed in the oral cavity to nitrite by bacterial activity. Nitrite could subsequently be used as substrate to produce NO. The hypothesis that increased NO production in the oral cavity is reflected in the Daw,NO was indirectly supported by Törnberg et al. 38, who observed that, in tracheotomised subjects, the removal of the oropharyngeal compartment led to a significant reduction in Daw,NO, without affecting the other flow-independent NO exchange parameters. Törnberg et al. 38 measured exhaled NO at six different flow rates ranging 6300 mL·s-1 and used a nonlinear regression method to obtain Caw,NO, Daw,NO and CA,NO. It was not possible to demonstrate that reported passive smoking in nonsmokers was associated with reduced levels of exhaled NO. This finding is in agreement with some previous studies 3941 and also in partial agreement with the study of Warke et al. 5, which used questionnaire assessment of smoke exposure and was unable to show any effect of smoke exposure on FeNO in nonasthmatic children, although it was found in asthmatic children. It should be noted that the previous studies were conducted in a population of children and that the only available studies in adults have focused exclusively on the immediate effects of smoking 4, 42. It was found that passive smoking was associated with increased CA,NO, but, at the present time, the current authors have no clear idea as to the mechanism behind this observation. One possible explanation might be offered by the reduced permeability of the respiratory membrane seen in subjects exposed to sidestream smoke 43. The present investigation is one of the first studies to investigate flow-independent NO exchange parameters in a general population sample. The utilisation of objective markers of tobacco consumption and exposure would have been better, even though there are studies showing a reasonable association between self-reported and objectively measured tobacco use and exposure 4446. The use of flow-independent nitric oxide exchange parameters may aid understanding of the location of tobacco-induced changes in airway nitric oxide metabolism and exchange. In the present study, both current and past smoking were associated with reduced levels of exhaled nitric oxide. In current smokers, reduced nitric oxide levels were found in both the airways and alveoli, whereas, in ex-smokers, the maximum total airway nitric oxide flux was reduced. The association between snus (oral moist snuff) and reduced nitric oxide concentrations in the airways and increased nitric oxide transfer from the airways warrants further studies.
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