Nitric oxide production and absorption in trachea, bronchi, bronchioles, and respiratory bronchioles of humans

Airway gas exchange and exhaled biomarkers

During inspiration and expiration, gases traverse the conducting airways as they are transported between the environment and the alveolar region of the lungs. The term "conducting" airways is used broadly as the airway tree is thought largely to provide a conduit for the respiratory gases, oxygen and carbon dioxide. However, despite a significantly smaller surface area, and thicker barrier separating the gas phase from the blood when compared to the alveolar region, the airway tree can participate in gas exchange under special conditions such as high water solubility, high chemical reactivity, or production of the gas within the airway wall tissue. While these conditions do not apply to the respiratory gases, other gases demonstrate substantial exchange of the airways and are of particular importance to the inflammatory response of the lungs, the medical-legal field, occupational health, metabolic disorders, or protection of the delicate alveolar membrane. Given the significant structural differences between the airways and the alveolar region, the physical determinants that control airway gas exchange are unique and require different models (both experimental and mathematical) to explore. Our improved physiological understanding of airway gas exchange combined with improved analytical methods to detect trace compounds in the exhaled breath provides future opportunities to develop new exhaled biomarkers that are characteristic of pulmonary and systemic conditions.

Relationships of online exhaled, offline exhaled, and ambient nitric oxide in an epidemiologic survey of schoolchildren

Field measurements of exhaled nitric oxide (FeNO) and ambient nitric oxide (NO) are useful to assess both respiratory health and short-term air pollution exposure. Online real-time measurement maximizes data quality and comparability with clinical studies, but offline delayed measurement may be more practical for large epidemiological studies. To facilitate cross-comparison in larger studies, we measured FeNO and concurrent ambient NO both online and offline in 362 children at 14 schools in 8 Southern California communities. Offline breath samples were collected in bags at 100 ml/s expiratory flow with deadspace discard; online FeNO was measured at 50 ml/s. Scrubbing of ambient NO from inhaled air appeared to be nearly 100% effective online, but 50-75% effective offline. Offline samples were stored at 2-8 degrees C and analyzed 2-26 h later at a central laboratory. Offline and online FeNO showed a nearly (but not completely) linear relationship (R(2)=0.90); unadjusted means (ranges) were 10 (4-94) and 15 (3-181) p.p.b., respectively. Ambient NO concentration range was 0-212 p.p.b. Offline FeNO was positively related to ambient NO (r=0.30, P<0.0001), unlike online FeNO (r=0.09, P=0.08), indicating that ambient NO artifactually influenced offline measurements. Offline FeNO differed between schools (P<0.001); online FeNO did not (P=0.26), suggesting artifacts related to offline bag storage and transport. Artifact effects were small in comparison with between-subject variance of FeNO. An empirical statistical model predicting individual online FeNO from offline FeNO, ambient NO, and lag time before offline analysis gave R(2)=0.94. Analyses of school or age differences yielded similar results from measured or model-predicted online FeNO.

CONCLUSIONS

Either online or offline measurement of exhaled NO and concurrent ambient NO can be useful in field epidemiology. Influence of ambient NO on exhaled NO should be examined carefully, particularly for offline measurements.

Axial distribution heterogeneity of nitric oxide airway production in healthy adults

Model simulations of nitric oxide (NO) transport considering molecular diffusion showed that the total bronchial NO production needed to reproduce a given exhaled value is deeply influenced by its axial distribution. Experimental data obtained by fibroscopy were available about proximal airway contribution (Silkoff PE, McClean PA, Caramori M, Slutsky AS. Zamel N. Respir Physiol 113: 33-38, 1998), and recent experiments using heliox instead of air gave insight on the peripheral airway production (Shin HW, Condorelli P, Rose-Gottron CM, Cooper DM, George SC. J Appl Physiol 97: 874-882, 2004; Kerckx Y, Michils A, Van Muylem A. J Appl Physiol 104: 918-924, 2008). This theoretical work aimed at obtaining a realistic distribution of NO production in healthy adults by meeting both proximal and peripheral experimental constraints. To achieve this, a model considering axial diffusion with geometrical boundaries derived from Weibel's morphometrical data was divided into serial compartments, each characterized by its axial boundaries and its part of bronchial NO production. A four-compartment model was able to meet both criteria. Two compartments were found to share all the NO production: one proximal (generations 0 and 1; 15-25% of the NO production) and one inside the acinus (proximal limit, generations 14-16; distal limit, generations 16 and 17; 75-85% of the NO production). Remarkably, this finding implies a quasi nil production in the main part of the conducting airways and in the acinar airways distal to generation 17. Given the chosen experimental outcomes and reliant on their accuracy, this very inhomogeneous distribution is likely the more realistic one that may be achieved with a "one-trumpet"-shaped model. Refinement should come from a more realistic description of the acinus structure.

Airway nitric oxide output is reduced in bronchiectasis

BACKGROUND

Increased concentrations of exhaled nitric oxide (NO) have been detected in inflammatory lung diseases including asthma and have been attributed to increased expression and activity of inducible nitric oxide synthase (iNOS) within the airways. However, previous studies of exhaled NO in patients with bronchiectasis have yielded conflicting results, with reports of both increased and normal NO values. Recent evidence from animal models suggests that chronic airway infection reduces NO production within the lung, despite causing increased iNOS expression. We tested the hypothesis that, in human subjects with bronchiectasis, chronic airway infection reduces NO output from the conducting airways.

METHODS

Using a recently described two-compartment model, we measured separately the contributions of the conducting airways and the alveoli to exhaled NO in nine patients with stable bronchiectasis and eight control subjects before and after inhaled glucocorticoid therapy.

RESULTS

We found that airway NO output was significantly lower in bronchiectasis than in normal airways whereas NO output from the alveoli was similar to that of control subjects. High-dose inhaled glucocorticoid therapy did not alter airway or alveolar NO production.

CONCLUSIONS

These findings demonstrate that, in patients with bronchiectasis, airway NO output is reduced and that iNOS does not contribute significantly to airway NO production.

Exhaled nasal nitric oxide output is reduced in humans at night during the sleep period

The physiologic function of nasal nitric oxide (NO) release is unknown. In prior experiments, topical NG-nitro-L-arginine methyl ester (L-NAME) on nasal mucosa reduced exhaled nasal NO output and caused daytime sleepiness. We hypothesized that nasal NO output is reduced at night during the sleep period. We measured exhaled nasal NO concentration and minute ventilation and calculated nasal NO output in humans over 24 h. Daytime awake NO output was greater than NO output at night during sleep or transient wakefulness. Exhaled NO concentration decreased during sleep along with minute ventilation. A daytime voluntary reduction in minute ventilation also decreased nasal NO output but exhaled NO concentration increased. Nasal NO output was not changed by body position. We conclude that exhaled nasal NO output is decreased at night due to decreased mass flow of NO into nasal air in addition to decreased minute ventilation. Our findings suggest a role of nasal NO in sleep or in the physiologic processes accompanying sleep.

Exercise-induced bronchoconstriction alters airway nitric oxide exchange in a pattern distinct from spirometry

Exhaled nitric oxide (NO) is altered in asthmatic subjects with exercise-induced bronchoconstriction (EIB). However, the physiological interpretation of exhaled NO is limited because of its dependence on exhalation flow and the inability to distinguish completely proximal (large airway) from peripheral (small airway and alveolar) contributions. We estimated flow-independent NO exchange parameters that partition exhaled NO into proximal and peripheral contributions at baseline, postexercise challenge, and postbronchodilator administration in steroid-naive mild-intermittent asthmatic subjects with EIB (24-43 yr old, n = 9) and healthy controls (20-31 yr old, n = 9). The mean +/- SD maximum airway wall flux and airway diffusing capacity were elevated and forced expiratory flow, midexpiratory phase (FEF(25-75)), forced expiratory volume in 1 s (FEV(1)), and FEV(1)/forced vital capacity (FVC) were reduced at baseline in subjects with EIB compared with healthy controls, whereas the steady-state alveolar concentration of NO and FVC were not different. Compared with the response of healthy controls, exercise challenge significantly reduced FEV(1) (-23 +/- 15%), FEF(25-75) (-37 +/- 18%), FVC (-12 +/- 12%), FEV(1)/FVC (-13 +/- 8%), and maximum airway wall flux (-35 +/- 11%) relative to baseline in subjects with EIB, whereas bronchodilator administration only increased FEV(1) (+20 +/- 21%), FEF(25-75) (+56 +/- 41%), and FEV(1)/FVC (+13 +/- 9%). We conclude that mild-intermittent steroid-naive asthmatic subjects with EIB have altered airway NO exchange dynamics at baseline and after exercise challenge but that these changes occur by distinct mechanisms and are not correlated with alterations in spirometry.

The effects of hyperpnea on exhaled nitric oxide synthesis in normal subjects

STUDY OBJECTIVES

To determine if the concentration of nitric oxide (NO) in the lungs increases with hyperpnea by contrasting calculated production (ie, the product of the fractional expired NO concentration [FeNO] and minute ventilation [Ve]) [Vno] with the amount of NO in equilibrium with the conducting airways (eNOair) and the amount of NO diffusing from the alveoli (eNOalv).

DESIGN

Observational study.

SETTING

University teaching hospital.

PARTICIPANTS

Normal subjects.

INTERVENTIONS

Measurements were made in 16 healthy people during and after 4 min of tidal breathing (10 L/min) and isocapnic hyperventilation of 60 L/min.

MEASUREMENTS AND RESULTS

FeNO was measured by collecting the exhaled air during the last minute of each trial and passing it through a chemiluminescence analyzer. The expired NO levels in the plateau phases of slow (30 mL/s) and fast (200 mL/s) single-breath exhalations were also obtained before and after hyperventilation. The Vno (mean +/- SEM) increased from 89.8 +/- 12.3 to 329.1 +/- 36.2 nL/min as Ve rose (p < 0.001). However, neither the quantities of eNOair nor eNOalv changed with hyperventilation (eNOair range before to after, 34.9 +/- 7.7 to 30.9 +/- 6.4 parts per billion [ppb], p = 0.96; eNOalv range before to after, 7.3 +/- 1.5 to 6.5 +/- 1.1 ppb, p = 0.97).

CONCLUSIONS

These data demonstrate that the amount of NO in equilibrium with the airway walls and alveoli are not altered by hyperpnea. Rather, the apparent augmentation in Vno in such circumstances appears to be an arithmetic artifact.

A new and more accurate technique to characterize airway nitric oxide using different breath-hold times

Exhaled nitric oxide (NO) arises from both airway and alveolar regions of the lungs, which provides an opportunity to characterize region-specific inflammation. Current methodologies rely on vital capacity breathing maneuvers and controlled exhalation flow rates, which can be difficult to perform, especially for young children and individuals with compromised lung function. In addition, recent theoretical and experimental studies demonstrate that gas-phase axial diffusion of NO has a significant impact on the exhaled NO signal. We have developed a new technique to characterize airway NO, which requires a series of progressively increasing breath-hold times followed by exhalation of only the airway compartment. Using our new technique, we determined values (means +/- SE) in healthy adults (20-38 yr, n = 8) for the airway diffusing capacity [4.5 +/- 1.6 pl.s(-1).parts per billion (ppb)(-1)], the airway wall concentration (1,340 +/- 213 ppb), and the maximum airway wall flux (4,350 +/- 811 pl/s). The new technique is simple to perform, and application of this data to simpler models with cylindrical airways and no axial diffusion yields parameters consistent with previous methods. Inclusion of axial diffusion as well as an anatomically correct trumpet-shaped airway geometry results in significant loss of NO from the airways to the alveolar region, profoundly impacting airway NO characterization. In particular, the airway wall concentration is more than an order of magnitude larger than previous estimates in healthy adults and may approach concentrations (approximately 5 nM) that can influence physiological processes such as smooth muscle tone in disease states such as asthma.

Cyclic adenosine monophosphate inhibits nitric oxide-induced apoptosis of cardiac muscle cells in a c-Jun N-terminal kinase-dependent manner

Cyclic adenosine monophosphate (cAMP) modulates various agent-induced apoptosis. In this study, we observed that cAMP had a significantly protective effect on nitric oxide (NO)-induced cytotoxicity in H9c2 cardiac muscle cells. Pretreatment with DBcAMP (cAMP analogue) or forskolin (adenylyl cyclase activator) also significantly prevented the SNP-induced apoptosis in H9c2 cells. In contrast, H-89 or KT5720 (PKA inhibitor) reversed the protective effects of DBcAMP. In this study, DBcAMP or forskolin reduced SNP-induced JNK/SAPK activation to the basal level, but KT5720 reversed the inhibitory effects of these two agents. In contrast to JNK/SAPK activation, DBcAMP and forskolin significantly enhanced SNP-activated p38 MAPK phosphorylation and did not affect SNP-mediated ERK activation. KT5720 reversed the effects of DBcAMP and forskolin on p38 MAPK phosphorylation. The inhibition of the JNK pathway by transfection of a dominant negative mutant of JNK/SAPK markedly reduced the extent of SNP-induced cell death. Taken together, we suggest that JNK/SAPK is related to cAMP-protective effect in SNP-induced apoptosis. In addition, c-AMP relating agents protected SNP-induced cell death in neonatal rat ventricular cardiomyocytes. The cAMP-relating agent-induced protective effect is not restricted in H9c2 cardiac muscle cells.

Probing the impact of axial diffusion on nitric oxide exchange dynamics with heliox

Exhaled nitric oxide (NO) is a potential noninvasive index of lung inflammation and is thought to arise from the alveolar and airway regions of the lungs. A two-compartment model has been used to describe NO exchange; however, the model neglects axial diffusion of NO in the gas phase, and recent theoretical studies suggest that this may introduce significant error. We used heliox (80% helium, 20% oxygen) as the insufflating gas to probe the impact of axial diffusion (molecular diffusivity of NO is increased 2.3-fold relative to air) in healthy adults (21–38 yr old, n = 9). Heliox decreased the plateau concentration of exhaled NO by 45% (exhalation flow rate of 50 ml/s). In addition, the total mass of NO exhaled in phase I and II after a 20-s breath hold was reduced by 36%. A single-path trumpet model that considers axial diffusion predicts a 50% increase in the maximum airway flux of NO and a near-zero alveolar concentration (CaNO) and source. Furthermore, when NO elimination is plotted vs. constant exhalation flow rate (range 50–500 ml/s), the slope has been previously interpreted as a nonzero CaNO (range 1–5 ppb); however, the trumpet model predicts a positive slope of 0.4–2.1 ppb despite a zero CaNO because of a diminishing impact of axial diffusion as flow rate increases. We conclude that axial diffusion leads to a significant backdiffusion of NO from the airways to the alveolar region that significantly impacts the partitioning of airway and alveolar contributions to exhaled NO.

Modeling pulmonary nitric oxide exchange

Nitric oxide (NO) was first detected in the exhaled breath more than a decade ago and has since been investigated as a noninvasive means of assessing lung inflammation. Exhaled NO arises from the airway and alveolar compartments, and new analytical methods have been developed to characterize these sources. A simple two-compartment model can adequately represent many of the observed experimental observations of exhaled concentration, including the marked dependence on exhalation flow rate. The model characterizes NO exchange by using three flow-independent exchange parameters. Two of the parameters describe the airway compartment (airway NO diffusing capacity and either the maximum airway wall NO flux or the airway wall NO concentration), and the third parameter describes the alveolar region (steady-state alveolar NO concentration). A potential advantage of the two-compartment model is the ability to partition exhaled NO into an airway and alveolar source and thus improve the specificity of detecting altered NO exchange dynamics that differentially impact these regions of the lungs. Several analytical techniques have been developed to estimate the flow-independent parameters in both health and disease. Future studies will focus on improving our fundamental understanding of NO exchange dynamics, the analytical techniques used to characterize NO exchange dynamics, as well as the physiological interpretation and the clinical relevance of the flow-independent parameters.

Characterizing airway and alveolar nitric oxide exchange during tidal breathing using a three-compartment model

Exhaled nitric oxide (NO) may be a useful marker of lung inflammation, but the concentration is highly dependent on exhalation flow rate due to a significant airway source. Current methods for partitioning pulmonary NO gas exchange into airway and alveolar regions utilize multiple exhalation flow rates or a single-breath maneuver with a preexpiratory breath hold, which is cumbersome for children and individuals with compromised lung function. Analysis of tidal breathing data has the potential to overcome these limitations, while still identifying region-specific parameters. In six healthy adults, we utilized a three-compartment model (two airway compartments and one alveolar compartment) to identify two potential flow-independent parameters that represent the average volumetric airway flux (pl/s) and the time-averaged alveolar concentration (parts/billion). Significant background noise and distortion of the signal from the sampling system were compensated for by using a Gaussian wavelet filter and a series of convolution integrals. Mean values for average volumetric airway flux and time-averaged alveolar concentration were 2,500 +/- 2,700 pl/s and 3.2 +/- 3.4 parts/billion, respectively, and were strongly correlated with analogous parameters determined from vital capacity breathing maneuvers. Analysis of multiple tidal breaths significantly reduced the standard error of the parameter estimates relative to the single-breath technique. Our initial assessment demonstrates the potential of utilizing tidal breathing for noninvasive characterization of pulmonary NO exchange dynamics.

Airway diffusing capacity of nitric oxide and steroid therapy in asthma

Exhaled nitric oxide (NO) concentration is a noninvasive index for monitoring lung inflammation in diseases such as asthma. The plateau concentration at constant flow is highly dependent on the exhalation flow rate and the use of corticosteroids and cannot distinguish airway and alveolar sources. In subjects with steroid-naive asthma (n = 8) or steroid-treated asthma (n = 12) and in healthy controls (n = 24), we measured flow-independent NO exchange parameters that partition exhaled NO into airway and alveolar regions and correlated these with symptoms and lung function. The mean (+/-SD) maximum airway flux (pl/s) and airway tissue concentration [parts/billion (ppb)] of NO were lower in steroid-treated asthmatic subjects compared with steroid-naive asthmatic subjects (1,195 +/- 836 pl/s and 143 +/- 66 ppb compared with 2,693 +/- 1,687 pl/s and 438 +/- 312 ppb, respectively). In contrast, the airway diffusing capacity for NO (pl.s-1.ppb-1) was elevated in both asthmatic groups compared with healthy controls, independent of steroid therapy (11.8 +/- 11.7, 8.71 +/- 5.74, and 3.13 +/- 1.57 pl.s-1.ppb-1 for steroid treated, steroid naive, and healthy controls, respectively). In addition, the airway diffusing capacity was inversely correlated with both forced expired volume in 1 s and forced vital capacity (%predicted), whereas the airway tissue concentration was positively correlated with forced vital capacity. Consistent with previously reported results from Silkoff et al. (Silkoff PE, Sylvester JT, Zamel N, and Permutt S, Am J Respir Crit Med 161: 1218-1228, 2000) that used an alternate technique, we conclude that the airway diffusing capacity for NO is elevated in asthma independent of steroid therapy and may reflect clinically relevant changes in airways.

Impact of high-intensity exercise on nitric oxide exchange in healthy adults

PURPOSE

After exercise, exhaled NO concentration has been reported to decrease, remain unchanged, or increase. A more mechanistic understanding of NO exchange dynamics after exercise is needed to understand the relationship between exercise and NO exchange.

METHODS

We measured several flow-independent NO exchange parameters characteristic of airway and alveolar regions using a single breath maneuver and a two-compartment model (maximum flux of NO from the airways, J'(awNO), pL x s-1; diffusing capacity of NO in the airways, D(awNO), pL x s-1 x ppb-1; steady state alveolar concentration, C(alv,ss), ppb; mean airway tissue NO concentration, C(awNO), ppb), as well as serum IL-6 at baseline, 3, 30, and 120 min after a high-intensity exercise challenge in 10 healthy adults (21-37 yr old).

RESULTS

D(awNO) (mean +/- SD) increased (37.1 +/- 44.4%), whereas J'(awNO) and C(awNO) decreased (-7.27 +/- 11.1%, -26.1 +/- 24.6%, respectively) 3 min postexercise. IL-6 increased steadily after exercise to 481% +/- 562% above baseline 120 min postexercise.

CONCLUSION

High-intensity exercise acutely enhances the ability of NO to diffuse between the airway tissue and the gas phase, and exhaled NO might be used to probe both the metabolic and physical properties of the airways.

Modeling of impact of gas molecular diffusion on nitric oxide expired profile

Present descriptions of nitric oxide (NO) transport in the lungs use two compartment models: airway compartment without mixing and alveolar compartment with perfect mixing. These models neglect NO molecular diffusion in the airways. To assess the impact of axial diffusion on expired NO profile, we solved a transport equation that incorporated diffusion, convection, and NO sources in the symmetrical Weibel model of the lung. When NO parameters computed from experimental data with the two compartment models are used in our model as NO sources, simulated end-expired NO is 29-45 and 64-78% of experimental values at expiratory flows of 50 and 2,000 ml/s, respectively. These lower values are because of NO axial diffusion: During expiration, NO back diffusion (opposed to convection) prevents some NO from being expired, so a two- to fivefold increase of airway NO excretion is necessary to simulate end-expired NO consistent with experimental data. We conclude that, insofar as a significant amount of NO is produced in small airways, models neglecting NO axial diffusion underestimate excretion in the airways.

Impact of axial diffusion on nitric oxide exchange in the lungs

Nitric oxide (NO) appears in the exhaled breath and is a potentially important clinical marker. The accepted model of NO gas exchange includes two compartments, representing the airway and alveolar region of the lungs, but neglects axial diffusion. We incorporated axial diffusion into a one-dimensional trumpet model of the lungs to assess the impact on NO exchange dynamics, particularly the impact on the estimation of flow-independent NO exchange parameters such as the airway diffusing capacity and the maximum flux of NO in the airways. Axial diffusion reduces exhaled NO concentrations because of diffusion of NO from the airways to the alveolar region of the lungs. The magnitude is inversely related to exhalation flow rate. To simulate experimental data from two different breathing maneuvers, NO airway diffusing capacity and maximum flux of NO in the airways needed to be increased approximately fourfold. These results depend strongly on the assumption of a significant production of NO in the small airways. We conclude that axial diffusion may decrease exhaled NO levels; however, more advanced knowledge of the longitudinal distribution of NO production and diffusion is needed to develop a complete understanding of the impact of axial diffusion.

Role of airway endogenous nitric oxide on lung function during and after exercise in mild asthma

We hypothesized that nitric oxide (NO), a known mild bronchodilator that can be released by several cell types within pulmonary airways, might protect airways during exercise in asthmatic subjects. We studied 17 individuals with documented exercise-induced asthma (screening exercise evaluation) on 2 study days: after treatment with inhaled NO synthase inhibitor N(G)-monomethyl-l-arginine (l-NMMA; 2 ml of 25 mg/ml mist) and after treatment with saline vehicle. Pulmonary resistance (Rl, esophageal manometry) rose and forced expiratory volume in 1 s fell more after l-NMMA compared with saline treatment, suggesting a bronchoprotective role for NO at baseline. The rise in Rl seen after l-NMMA treatment was nearly completely reversed early in exercise, suggesting a non-NO-mediated bronchodilation. A slow rise in Rl during constant-load exercise and dramatic increase in Rl after exercise were the same on the 2 treatment days, indicating little role for NO in regulating airway function during and after exercise. We conclude that endogenous NO plays little role in regulating airway function during and after exercise in subjects with mild asthma.

Analysis of exhaled nitric oxide by the helium bolus method

STUDY OBJECTIVES

The precise anatomic sites contributing to exhaled nitric oxide (eNO) are still unknown. The present study was designed to analyze profiles of eNO by referring to the He exhalation curve and examining the effects of breath-holding and expiratory flow rates on eNO.

PARTICIPANTS

Healthy volunteers and patients with stable asthma. MEASUREMENTS AND RESULTS We used the He bolus method of the closing volume, and simultaneously analyzed the concentrations of exhaled He and nitric oxide (NO). By referring to the He exhalation curve, the expired gas was divided into three parts: airway dead space (phase 1), a mixture of airway and alveolar gas (phase 2), and alveolar gas (phase 3 and phase 4). The eNO profiles showed a peak in phase 2 (peak eNO) and decreased gradually to a plateau in the latter half of phase 3 (plateau eNO). The levels of peak eNO were higher than those of plateau eNO in both normal subjects and asthmatic patients. Breath-holding increased levels of peak eNO 2.5-fold in both normal subjects and asthmatic patients, but it did not affect the levels of plateau eNO. The levels of peak eNO increased as the expiratory flow rate decreased, and the levels of plateau eNO showed a similar flow dependency.

CONCLUSION

A peak value of eNO concentration profiles may directly express the production of NO in the airway.

Pulmonary membrane diffusing capacity and capillary blood volume measured during exercise from nitric oxide uptake

STUDY OBJECTIVES

To validate lung diffusing capacity for nitric oxide (DLNO) as an index of conductance of the alveolar-capillary membrane during exercise, we compared DLNO to lung diffusing capacity for carbon monoxide (DLCO) and pulmonary membrane diffusing capacity for carbon monoxide (DMCO), and compared pulmonary capillary blood volume (Vc) calculated by two methods.

SETTING AND PARTICIPANTS

The study was performed at a university medical center involving 12 nonsmoking healthy volunteers (age range, 23 to 79 years). DLCO, DLNO, cardiac output (c), and lung volume were measured simultaneously at rest and during graded ergometer exercise by a rebreathing technique. Pulmonary membrane diffusing capacity and Vc were compared by (1) the classic technique of Roughton and Forster from DLCO measured at two alveolar oxygen tension (PAO(2)) levels, and (2) from DLNO and DLCO assuming negligible erythrocyte resistance to nitric oxide (NO) uptake, ie, DLNO approximately equal to pulmonary membrane diffusing capacity for nitric oxide.

RESULTS

In all subjects, DLNO increased linearly from rest to exercise; age, c, and lung volume were the major determinants of DLNO by stepwise regression analysis. The DLNO/DLCO ratio averaged 3.98 +/- 0.38 (+/- SD) and the DLNO/DMCO ratio averaged 2.49 +/- 0.28 irrespective of exercise intensity. Changing PAO(2) did not alter DLNO. Brief exposure to 40 ppm of inhaled NO during 16 s of rebreathing did not alter either DLCO or c. Estimates of pulmonary membrane diffusing capacity and Vc by the two methods showed a strong correlation.

CONCLUSION

Results support DLNO as a direct measure of pulmonary membrane diffusing capacity, allowing the estimation of Vc in a single rebreathing maneuver during exercise. The DLNO-DLCO rebreathing technique can be applied clinically in the investigation of pulmonary microvascular regulation.

Flow-independent nitric oxide exchange parameters in healthy adults

Currently accepted techniques utilize the plateau concentration of nitric oxide (NO) at a constant exhalation flow rate to characterize NO exchange, which cannot sufficiently distinguish airway and alveolar sources. Using nonlinear least squares regression and a two-compartment model, we recently described a new technique (Tsoukias et al. J Appl Physiol 91: 477-487, 2001), which utilizes a preexpiratory breath hold followed by a decreasing flow rate maneuver, to estimate three flow-independent NO parameters: maximum flux of NO from the airways (J(NO,max), pl/s), diffusing capacity of NO in the airways (D(NO,air), pl x s(-1) x ppb(-1)), and steady-state alveolar concentration (C(alv,ss), ppb). In healthy adults (n = 10), the optimal breath-hold time was 20 s, and the mean (95% intramaneuver, intrasubject, and intrapopulation confidence interval) J(NO,max), D(NO,air), and C(alv,ss) are 640 (26, 20, and 15%) pl/s, 4.2 (168, 87, and 37%) pl x s(-1) x ppb(-1), and 2.5 (81, 59, and 21%) ppb, respectively. J(NO,max) can be estimated with the greatest certainty, and the variability of all the parameters within the population of healthy adults is significant. There is no correlation between the flow-independent NO parameters and forced vital capacity or the ratio of forced expiratory volume in 1 s to forced vital capacity. With the use of these parameters, the two-compartment model can accurately predict experimentally measured plateau NO concentrations at a constant flow rate. We conclude that this new technique is simple to perform and can simultaneously characterize airway and alveolar NO exchange in healthy adults with the use of a single breathing maneuver.

Sodium nitroprusside induces apoptosis of H9C2 cardiac muscle cells in a c-Jun N-terminal kinase-dependent manner

Sodium nitroprusside (SNP) induces apoptosis in H9C2 cardiac muscle cells. Treatment with an exogenous NO donor SNP (2 mM) to H9C2 cells resulted in apoptotic morphological changes; a bright blue-fluorescent condensed nuclei and chromatin fragmentation by fluorescence microscope of Hoechst 33258-staining. The activity of caspase-3 like protease was increased during SNP-induced cell death. However, the activity of caspase-1 like protease was not affected by SNP. Pretreatment with Z-VAD-FMK (a pan-caspase inhibitor) or Ac-DEVD-CHO (a specific caspase-3 inhibitor) abrogated the SNP-induced cell death. SNP markedly activated three MAP kinases (JNK/SAPK, ERK and p38 MAP kinase) in the cardiac muscle cells. In this study, selective inhibition of the ERK or p38 MAPK pathway (by PD98059 or SB203580, respectively) had no effect on the extent of SNP-induced apoptosis in cardiac muscle cells. In contrast, inhibition of the JNK pathway by transfection of a dominant negative mutant of JNK markedly reduced the extent of SNP-induced cell death. Taken together, we suggest that JNK/SAPK will be related to SNP-induced apoptosis of H9C2 cardiac muscle cells.

The lung diffusing capacity for nitric oxide in rats is increased during endotoxemia

Rats, when injected with endotoxin, begin to exhale nitric oxide (NO) within 1 h. This study measured the diffusing capacity for NO in the lungs of rats (DL(NO)) under both control and endotoxemic conditions, and it also estimated the rate at which endogenous NO (VP(NO)) enters the distal compartment of the lung, both in control rats and during endotoxemia. DL(NO) increased from 0.68 +/- 0.12 (SE) ml. min(-1). mmHg(-1) in control rats to 1.17 +/- 0.25 ml. min(-1). mmHg(-1) in endotoxemic rats. VP(NO) was 2.6 +/- 0.5 nl/min in control rats and attained a value of 218.6 +/- 50.1 nl/min at the height of NO exhalation 3 h after the endotoxin. We suggest that increased DL(NO) reflects an increase in pulmonary membrane diffusing capacity, caused by a pulmonary hypertension that is due to neutrophil aggregation in the lung capillaries. DL(NO) may also be increased by an enlarged pulmonary capillary volume because of the vasodilatory effects of the endogenous NO that is produced by the lung in response to the endotoxin. NO production by the lungs in response to endotoxin is unique in that it is the only situation reported to date in which pathologically induced increases in NO exhalation originate from the alveolar compartment of the lung, as opposed to the small conducting airways.

Microscopic modeling of NO and S-nitrosoglutathione kinetics and transport in human airways

Nitric oxide (NO) appears in the exhaled breath and is elevated in inflammatory diseases. We developed a steady-state mathematical model of the bronchial mucosa for normal small and large airways to understand NO and S-nitrosoglutathione (GSNO) kinetics and transport using data from the existing literature. Our model predicts that mean steady-state NO and GSNO concentrations for large airways (generation 1) are 2.68 nM and 113 pM, respectively, in the epithelial cells and 0.11 nM (approximately 66 ppb) and 507 nM in the mucus. For small airways (generation 15), the mean concentrations of NO and GSNO, respectively, are 0.26 nM and 21 pM in the epithelial cells and 0.02 nM (approximately 12 ppb) and 132 nM in the mucus. The concentrations in the mucus compare favorably to experimentally measured values. We conclude that 1) the majority of free NO in the mucus, and thus exhaled NO, is due to diffusion of free NO from the epithelial cell and 2) the heterogeneous airway contribution to exhaled NO is due to heterogeneous airway geometries, such as epithelium and mucus thickness.

Role of nitric oxide in the airway response to exercise in healthy and asthmatic subjects

A role of nitric oxide (NO) has been suggested in the airway response to exercise. However, it is unclear whether NO may act as a protective or a stimulatory factor. Therefore, we examined the role of NO in the airway response to exercise by using N-monomethyl-L-arginine (L-NMMA, an NO synthase inhibitor), L-arginine (the NO synthase substrate), or placebo as pretreatment to exercise challenge in 12 healthy nonsmoking, nonatopic subjects and 12 nonsmoking, atopic asthmatic patients in a double-blind, crossover study. Fifteen minutes after inhalation of L-NMMA (10 mg), L-arginine (375 mg), or placebo, standardized bicycle ergometry was performed for 6 min using dry air, while ventilation was kept constant. The forced expiratory volume in 1-s response was expressed as area under the time-response curve (AUC) over 30 min. In healthy subjects, there was no significant change in AUC between L-NMMA and placebo treatment [28.6 +/- 17.0 and 1.3 +/- 20.4 (SE) for placebo and L-NMMA, respectively, P = 0.2]. In the asthmatic group, L-NMMA and L-arginine induced significant changes in exhaled NO (P < 0.01) but had no significant effect on AUC compared with placebo (geometric mean +/- SE: -204.3 +/- 1.5, -186.9 +/- 1.4, and -318.1 +/- 1.2%. h for placebo, L-NMMA, and L-arginine, respectively, P > 0.2). However, there was a borderline significant difference in AUC between L-NMMA and L-arginine treatment (P = 0.052). We conclude that modulation of NO synthesis has no effect on the airway response to exercise in healthy subjects but that NO synthesis inhibition slightly attenuates exercise-induced bronchoconstriction compared with NO synthase substrate supplementation in asthma. These data suggest that the net effect of endogenous NO is not inhibitory during exercise-induced bronchoconstriction in asthma.

Simultaneous measurement of nitric oxide production by conducting and alveolar airways of humans

Human airways produce nitric oxide (NO), and exhaled NO increases as expiratory flow rates fall. We show that mixing during exhalation between the NO produced by the lower, alveolar airways (VL(NO)) and the upper conducting airways (VU(NO)) explains this phenomenon and permits measurement of VL(NO), VU(NO), and the NO diffusing capacity of the conducting airways (DU(NO)). After breath holding for 10-15 s the partial pressure of alveolar NO (PA) becomes constant, and during a subsequent exhalation at a constant expiratory flow rate the alveoli will deliver a stable amount of NO to the conducting airways. The conducting airways secrete NO into the lumen (VU(NO)), which mixes with PA during exhalation, resulting in the observed expiratory concentration of NO (PE). At fast exhalations, PA makes a large contribution to PE, and, at slow exhalations, NO from the conducting airways predominates. Simple equations describing this mixing, combined with measurements of PE at several different expiratory flow rates, permit calculation of PA, VU(NO), and DU(NO). VL(NO) is the product of PA and the alveolar airway diffusion capacity for NO. In seven normal subjects, PA = 1.6 +/- 0.7 x 10(-6) (SD) Torr, VL(NO) = 0.19 +/- 0.07 microl/min, VU(NO) = 0.08 +/- 0.05 microl/min, and DU(NO) = 0.4 +/- 0.4 ml. min(-1). Torr(-1). These quantitative measurements of VL(NO) and VU(NO) are suitable for exploring alterations in NO production at these sites by diseases and physiological stresses.