Local nitric oxide levels reflect the degree of allergic airway inflammation after segmental allergen challenge in asthmatics.

Nitric oxide (NO) levels are increased in the exhaled air of asthmatics. As NO levels correlate with allergic airway inflammation, NO measurement has been suggested for disease monitoring. In patients with asthma, we previously demonstrated that intrabronchial treatment with a natural porcine surfactant enhanced airway inflammation after segmental allergen provocation. We studied whether local levels of NO reflect the degree of allergic airway inflammation following segmental allergen challenge with or without surfactant pretreatment. Segmental NO, as well as nitrite and nitrate in bronchoalveolar lavage (BAL) fluid, was measured before and after segmental challenge with either saline, saline plus allergen, or surfactant plus allergen in 16 patients with asthma and five healthy subjects. The data were compared with inflammatory BAL cells. Segmental NO levels were increased after instillation of saline (p < 0.05), or surfactant plus allergen in asthmatics (p < 0.05), and values were higher after surfactant plus allergen compared to saline challenge. Nitrate BAL levels were not altered after saline challenge but increased after allergen challenge (p < 0.05) and further raised by surfactant (p < 0.05), whereas nitrite levels were not altered by any treatment. Segmental NO and nitrate levels correlated with the degree of eosinophilic airway inflammation, and nitrate levels also correlated with neutrophil and lymphocyte numbers in BAL. In healthy subjects, NO, nitrite, and nitrate were unaffected. Thus, segmental NO and nitrate levels reflect the degree of allergic airway inflammation in patients with asthma. Measurement of both markers can be useful in studies using segmental allergen provocation, to assess local effects of potential immunomodulators.     

Nitric oxide-mediated vasodilation becomes independent of beta-adrenergic receptor activation with increased intensity of hypoxic exercise

Hypoxic vasodilation in skeletal muscle at rest is known to include β-adrenergic receptor-stimulated nitric oxide (NO) release. We previously reported that the augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes β-adrenergic mechanisms. However, it is unclear whether a β-adrenergic receptor-stimulated NO component exists during hypoxic exercise. We hypothesized that NO-mediated vasodilation becomes independent of β-adrenergic receptor activation with increased exercise intensity during hypoxic exercise. Ten subjects (7 men, 3 women; 23 ± 1 yr) breathed hypoxic gas to titrate arterial O(2) saturation to 80% while remaining normocapnic. Subjects performed two consecutive bouts of incremental rhythmic forearm exercise (10% and 20% of maximum) with local administration (via a brachial artery catheter) of propranolol (β-adrenergic receptor inhibition) alone and with the combination of propranolol and nitric oxide synthase inhibition [N(G)-monomethyl-l-arginine (l-NMMA)] under normoxic and hypoxic conditions. Forearm blood flow (FBF, ml/min; Doppler ultrasound) and blood pressure [mean arterial pressure (MAP), mmHg; brachial artery catheter] were assessed, and forearm vascular conductance (FVC, ml·min(-1)·100 mmHg(-1)) was calculated (FBF/MAP). During propranolol alone, the rise in FVC (Δ from normoxic baseline) due to hypoxic exercise was 217 ± 29 and 415 ± 41 ml·min(-1)·100 mmHg(-1) (10% and 20% of maximum, respectively). Combined propranolol-l-NMMA infusion during hypoxic exercise attenuated ΔFVC at 20% (352 ± 44 ml·min(-1)·100 mmHg(-1); P < 0.001) but not at 10% (202 ± 28 ml·min(-1)·100 mmHg(-1); P = 0.08) of maximum compared with propranolol alone. These data, when integrated with earlier findings, demonstrate that NO contributes to the compensatory vasodilation during mild and moderate hypoxic exercise; a β-adrenergic receptor-stimulated NO component exists during low-intensity hypoxic exercise. However, the source of the NO becomes less dependent on β-adrenergic mechanisms as exercise intensity increases.     

Effect of triiodothyronine-induced thyrotoxicosis on airway hyperresponsiveness

To determine whether thyrotoxicosis has an effect on the asthmatic state in subjects with mild asthma, airway responsiveness, lung function, and exercise capacity were measured in a randomized double-blind placebo-controlled trial before and after liothyronine (triiodothyronine, T3)-induced thyrotoxicosis. Baseline evaluation of 15 subjects with mild asthma included clinical evaluation, thyroid and routine pulmonary function tests, airway responsiveness assessment by methacholine inhalation challenge, and a symptom-limited maximal exercise test. For all subjects, the initial testing revealed that the dose of methacholine which provoked a 20% fall in forced expiratory volume in 1s (PD20) was in a range consistent with symptomatic asthma. There was no significant change in pulmonary function tests, airway reactivity (PD20), or exercise capacity in either the placebo or the T3-treated groups. Thyroid function tests confirmed mild sustained thyrotoxicosis in the T3-treated groups. We conclude that mild T3-induced thyrotoxicosis of 4-wk duration had no effect on lung function, airway responsiveness, or exercise capacity in subjects with mild asthma.     

Effects of rapid saline infusion on lung mechanics and airway responsiveness in humans.

Lung mechanics and airway responsiveness to methacholine (MCh) were studied in seven volunteers before and after a 20-min intravenous infusion of saline. Data were compared with those of a time point-matched control study. The following parameters were measured: 1-s forced expiratory volume, forced vital capacity, flows at 40% of control forced vital capacity on maximal (Vm(40)) and partial (Vp(40)) forced expiratory maneuvers, lung volumes, lung elastic recoil, lung resistance (Rl), dynamic elastance (Edyn), and within-breath resistance of respiratory system (Rrs). Rl and Edyn were measured during tidal breathing before and for 2 min after a deep inhalation and also at different lung volumes above and below functional residual capacity. Rrs was measured at functional residual capacity and at total lung capacity. Before MCh, saline infusion caused significant decrements of forced expiratory volume in 1 s, Vm(40), and Vp(40), but insignificantly affected lung volumes, elastic recoil, Rl, Edyn, and Rrs at any lung volume. Furthermore, saline infusion was associated with an increased response to MCh, which was not associated with significant changes in the ratio of Vm(40) to Vp(40). In conclusion, mild airflow obstruction and enhanced airway responsiveness were observed after saline, but this was not apparently due to altered elastic properties of the lung or inability of the airways to dilate with deep inhalation. It is speculated that it was likely the result of airway wall edema encroaching on the bronchial lumen.     

Inflammatory and mechanical factors of allergen-induced bronchoconstriction in mild asthma and rhinitis.

We studied whether different bronchial responses to allergen in asthma and rhinitis are associated with different bronchial inflammation and remodeling or airway mechanics. Nine subjects with mild asthma and eight with rhinitis alone underwent methacholine and allergen inhalation challenges. The latter was preceded and followed by bronchoalveolar lavage and bronchial biopsy. The response to methacholine was positive in all asthmatic but in only two rhinitic subjects. The response to allergen was positive in all asthmatic and most, i.e., five, rhinitic subjects. No significant differences between groups were found in airway inflammatory cells or basement membrane thickness either at baseline or after allergen. The ability of deep inhalation to dilate methacholine-constricted airways was greater in rhinitis than in asthma, but it was progressively reduced in rhinitis during allergen challenge. We conclude that 1) rhinitic subjects may develop similar airway inflammation and remodeling as the asthmatic subjects do and 2) the difference in bronchial response to allergen between asthma and rhinitis is associated with different airway mechanics.     

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.     

Role of exhaled nitric oxide in asthma

Nitric oxide (NO), an evanescent atmospheric gas, has recently been discovered to be an important biological mediator in animals and humans. Nitric oxide plays a key role within the lung in the modulation of a wide variety of functions including pulmonary vascular tone, nonadrenergic non-cholinergic (NANC) transmission and modification of the inflammatory response. Asthma is characterized by chronic airway inflammation and increased synthesis of NO and other highly reactive and toxic substances (reactive oxygen species). Pro- inflammatory cytokines such as TNFalpha and IL-1beta are secreted in asthma and result in inflammatory cell recruitment, but also induce calcium- and calmodulin-independent nitric oxide synthases (iNOS) and perpetuate the inflammatory response within the airways. Nitric oxide is released by several pulmonary cells including epithelial cells, eosinophils and macrophages, and NO has been shown to be increased in conditions associated with airway inflammation, such as asthma and viral infections. Nitric oxide can be measured in the expired air of several species, and exhaled NO can now be rapidly and easily measured by the use of chemiluminescence analysers in humans. Exhaled NO is increased in steroid-naive asthmatic subjects and during an asthma exacerbation, although it returns to baseline levels with appropriate anti-inflammatory treatment, and such measurements have been proposed as a simple non-invasive method of measuring airway inflammation in asthma. Here the chemical and biological properties of NO are briefly discussed, followed by a summary of the methodological considerations relevant to the measurement of exhaled NO and its role in lung diseases including asthma. The origin of exhaled NO is considered, and brief mention made of other potential markers of airway inflammation or oxidant stress in exhaled breath.     

How does airway inflammation modulate asthmatic airway constriction? An antigen challenge study.

During the late-phase (LP) response to inhaled allergen, mediators from neutrophils and eosinophils are released within the airways, resembling what occurs during an asthma attack. We compared the distribution of obstruction and degree of reversibility that follows a deep inspiration (DI) during early-phase (EP) and LP responses in nine asthmatic subjects challenged with allergen. Heterogeneity of constriction was assayed by determining frequency dependence of dynamic lung resistance and elastance, airway caliber by tracking airway resistance during a DI, and airway inflammation by measuring inflammatory cells in induced sputum postchallenge. Despite a paucity of eosinophils in the sputum at baseline (<1% of nonsquamous cells), asthmatic subjects showed a substantial EP response with highly heterogeneous constriction and reduced capacity to maximally dilate airways. The LP was associated with substantial airway inflammation in all subjects. However, five subjects showed only mild LP constriction, whereas four showed more marked LP constriction characterized by heterogeneous constriction similar to EP. Bronchoconstriction during LP was fully alleviated by administration of a bronchodilator. These findings, together with the impaired bronchodilatory response during a DI, indicate a physiological abnormality in asthma at the smooth muscle level and indicate that airway inflammation in asthma is associated with a highly nonuniform pattern of constriction. These data support the hypothesis that variability in responsiveness among asthmatic subjects derives from intrinsic differences in smooth muscle response to inflammation.     

Nitric oxide paradox in asthma

Asthma results from allergen-driven intrapulmonary Th2 response, and is characterized by intermittent airway obstruction, airway hyperreactivity (AHR), and airway inflammation. Accumulating evidence indicates that inflammatory diseases of the respiratory tract are commonly associated with elevated production of nitric oxide (NO). It has been shown that exhaled NO may be derived from constitutive NO synthase (NOS) such as endothelial (NOS 3) and neural (NOS 1) in normal airways, while increased levels of NO in asthma appear to be derived from inducible NOS2 expressed in the inflamed airways. Nevertheless, the functional role of NO and NOS isoforms in the regulation of AHR and airway inflammation in human or experimental models of asthma is still highly controversial. In the present commentary we will discuss the role of lipopolysaccharides contamination of allergens as key element in the controversy related to the regulation of NOS2 activity in experimental asthma.     

Frequency dependence of respiratory system mechanics during induced constriction in a murine model of asthma.

Airway dysfunction in asthma is characterized by hyperresponsiveness, heterogeneously narrowed airways, and closure of airways. To test the hypothesis that airway constriction in ovalbumin (OVA)-sensitized OVA-intranasally challenged (OVA/OVA) mice produces mechanical responses that are similar to those reported in asthmatic subjects, respiratory system resistance (Rrs) and elastance (Edyn,rs) spectra were obtained in OVA/OVA and control mice during intravenous methacholine (MCh) infusions. In control mice, MCh at 1,700 microg x kg(-1) x min(-1) produced 1) a 495 and 928% increase of Rrs at 0.5 Hz and 19.75 Hz, respectively, 2) a 33% rise in Edyn,rs at 0.5 Hz, and 3) a mild frequency (f)-dependent increase of Edyn,rs. The same MCh dose in OVA/OVA mice produced 1) elevations of Rrs at 0.5 Hz and 19.75 Hz of 1,792 and 774%, respectively, 2) a 390% rise in Edyn,rs at 0.5 Hz, and 3) marked f-dependent increases of Edyn,rs. During constriction, the f dependence of mechanics in control mice was consistent with homogeneous airway narrowing; however, in OVA/OVA mice, f dependence was characteristic of heterogeneously narrowed airways, closure of airways, and airway shunting. These mechanisms amplify the pulmonary mechanical responses to constrictor stimuli at physiological breathing rates and have important roles in the pathophysiology of human asthma.     

Airway nitric oxide release is reduced after PBS inhalation in asthma.

Exhaled nitric oxide (NO) is elevated in asthma, but the underlying mechanisms remain poorly understood. Recent results in subjects with asthma have reported a decrease in exhaled breath pH and ammonia, as well as altered expression and activity of glutaminase in both alveolar and airway epithelial cells. This suggests that pH-dependent nitrite conversion to NO may be a source of exhaled NO in the asthmatic airway epithelium. However, the anatomic location (i.e., airway or alveolar region) of this pH-dependent NO release has not been investigated and could impact potential therapeutic strategies. We quantified airway (proximal) and alveolar (peripheral) contributions to exhaled NO at baseline and then after PBS inhalation in stable (mild-intermittent to severe) asthmatic subjects (20-44 yr old; n = 9) and healthy controls (22-41 yr old; n = 6). The mean (SD) maximum airway wall flux (pl/s) and alveolar concentration (ppb) at baseline in asthma subjects and healthy controls was 2,530 (2,572) and 5.42 (7.31) and 1,703 (1,567) and 1.88 (1.29), respectively. Compared with baseline, there is a significant decrease in the airway wall flux of NO in asthma as early as 15 min and continuing for up to 60 min (maximum -28% at 45 min) after PBS inhalation without alteration of alveolar concentration. Healthy control subjects did not display any changes in exhaled NO. We conclude that elevated airway NO at baseline in asthma is reduced by inhaled PBS. Thus airway NO may be, in part, due to nitrite conversion to NO and is consistent with airway pH dysregulation in asthma.     

Impact of combined NO and PG blockade on rapid vasodilation in a forearm mild-to-moderate exercise transition in humans

We tested the hypothesis that nitric oxide (NO) and prostaglandins (PGs) contribute to the rapid vasodilation that accompanies a transition from mild to moderate exercise. Nine healthy volunteers (2 women and 7 men) lay supine with forearm at heart level. Subjects were instrumented for continuous brachial artery infusion of saline (control condition) or combined infusion of N(G)-nitro-L-arginine methyl ester (L-NAME) and ketorolac (drug condition) to inhibit NO synthase and cyclooxygenase, respectively. A step increase from 5 min of steady-state mild (5.4 kg) rhythmic, dynamic forearm handgrip exercise (1 s of contraction followed by 2 s of relaxation) to moderate (10.9 kg) exercise for 30 s was performed. Steady-state forearm blood flow (FBF; Doppler ultrasound) and forearm vascular conductance (FVC) were attenuated in drug compared with saline (control) treatment: FBF = 196.8 +/- 30.8 vs. 281.4 +/- 34.3 ml/min and FVC = 179.3 +/- 29.4 vs. 277.8 +/- 34.8 ml.min(-1).100 mmHg(-1) (both P < 0.01). FBF and FVC increased from steady state after release of the initial contraction at the higher workload in saline and drug conditions: DeltaFBF = 72.4 +/- 8.7 and 52.9 +/- 7.8 ml/min, respectively, and DeltaFVC = 66.3 +/- 7.3 and 44.1 +/- 7.0 ml.min(-1).100 mmHg(-1), respectively (all P < 0.05). The percent DeltaFBF and DeltaFVC were not different during saline infusion or combined inhibition of NO and PGs: DeltaFBF = 27.2 +/- 3.1 and 28.1 +/- 3.8%, respectively (P = 0.78) and DeltaFVC = 25.7 +/- 3.2 and 26.0 +/- 4.0%, respectively (P = 0.94). The data suggest that NO and vasodilatory PGs are not obligatory for rapid vasodilation at the onset of a step increase from mild- to moderate-intensity forearm exercise. Additional vasodilatory mechanisms not dependent on NO and PG release contribute to the immediate and early increase in blood flow in an exercise-to-exercise transition.     

Elevated levels of NO are localized to distal airways in asthma

The contribution of nitric oxide (NO) to the pathophysiology of asthma remains incompletely defined despite its established pro- and anti-inflammatory effects. Induction of the inducible nitric oxide synthase (iNOS), arginase, and superoxide pathways is correlated with increased airway hyperresponsiveness in asthmatic subjects. To determine the contributions of these pathways in proximal and distal airways, we compared bronchial wash (BW) to traditional bronchoalveolar lavage (BAL) for measurements of reactive nitrogen/oxygen species, arginase activation, and cytokine/chemokine levels in asthmatic and normal subjects. Levels of NO were preferentially elevated in the BAL, demonstrating higher level NOS activation in the distal airway compartment of asthmatic subjects. In contrast, DHE(+) cells, which have the potential to generate reactive oxygen species, were increased in both proximal and distal airway compartments of asthmatics compared to controls. Different patterns of cytokines and chemokines were observed, with a predominance of epithelial cell-associated mediators in the BW compared to macrophage/monocyte-derived mediators in the BAL of asthmatic subjects. Our study demonstrates differential production of reactive species and soluble mediators within the distal airways compared to the proximal airways in asthma. These results indicate that cellular mechanisms are activated in the distal airways of asthmatics and must be considered in the development of therapeutic strategies for this chronic inflammatory disorder.     

The overproduction of nitric oxide associated with neutrophilic predominance is relevant to airway mycotic infections in asthmatics undergoing prolonged glucocorticoid treatment

The complex relationship between the local inflammatory response and the spread of airway mycosis during prolonged glucocorticoid therapy in bronchial asthma patients remains unclear. We assessed the ability of airway leukocytes to produce nitric oxide (NO) in relation to differential inflammatory cell counts, levels of asthma severity, and coexisting airway mycotic infections. The study was carried out on leukocytes from the induced sputa (IS) of 14 patients with asthma complicated by mycotic airway infections undergoing prolonged glucocorticoid therapy (group FcA). Three groups of subjects without airway fungal infections were also studied: 18 glucocorticoid-treated asthmatics (group cA), 11 steroid-free asthmatics (group A), and 13 healthy control subjects (group H). In group FcA, both the level of spontaneous production of NO and the percentages of neutrophils in the IS were significantly higher than in all the remaining groups. Additionally, a significant positive correlation was noticed between the NO levels and both the percentages of neutrophils in the IS and the symptom intensity scores. The results suggest a possible predominant role of neutrophils in the overproduction of NO related to asthma severity and coexisting fungal infections in glucocorticoid-treated patients.     

Epithelium-dependent regulation of airways smooth muscle function. A histamine-nitric oxide pathway.

The airway epithelium is responsible for the production of a number of arachidonic acid and non-prostanoid inhibitory factors. Epithelium synthesises nitric oxide (NO) which may be important in regulating the function of airways smooth muscles. We studied in vitro the effect of histamine (100 nM-100 microM) which increases the NO release on rabbit airway smooth muscles induced by 80 mM KC1 in the presence or not of 10(-5) Methylene blue (MB) (inactivator of guanylate cyclase) or N(G)-monomethyl L-arginine (L-NMMA), a NOS inhibitor. All experiments were done in tracheal muscle strips from 28 rabbits with epithelium and after epithelium removal. The additional use of histamine (1 microM) on KC1 contraction induced a relaxation of 10% of the initial contraction. The additional use of L-NMMA decreased the relaxation to 5% of initial contraction. MB rather than L-NMMA increased the contraction significantly (p<0.01). Epithelium removal increased the contraction induced by KC1 (80 mM) and histamine (1 microM) by about 30% (p<0.001). NO release especially from epithelium regulates the airways smooth muscle functions. Damage to the epithelium may contribute to an increase in airways sensitivity, observed in asthma.     

Novel effects of PGF2 alpha on airway function in asthmatic subjects

The effects of inhaled prostaglandin F2 alpha (PGF2 alpha) have been examined in eight subjects with asthma. Incremental PGF2 alpha aerosol concentrations, ranging from 1 to 5,000 micrograms/ml, were administered at 15-min intervals. Plethysmographic specific airway conductance (sGaw), forced expiratory volume at 1 s (FEV1), and maximum expiratory flow at 50% vital capacity breathing air (Vmax50% air) and 80% He-20% O2 (Vmax50% He-O2) were measured after each dose and compared with saline control values. We observed unexpected triphasic dose-response characteristics, i.e., an initial decline in physiological variables at low concentrations (1-100 micrograms/ml), followed by improvement at intermediate concentrations (100-1,000 micrograms/ml) and a subsequent steep decline at high concentrations (1,000-5,000 micrograms/ml). Improvement in FEV1 and Vmax50% air between 100 and 1,000 micrograms/ml was associated with sGaw increases above control levels in six subjects and a significant fall in density-dependent index (Vmax50% He-O2/Vmax50% air) when compared with values before challenge and at low concentrations. Inhaled atropine (5 mg) improved prechallenge lung function but had no effect on PGF2 alpha dose-response characteristics. Intermediate PGF2 alpha concentrations given as a single dose consistently induced greater FEV1 reductions than the same concentration during graded dose challenges. Our findings are consistent with the demonstration of in vivo airway tachyphylaxis and indicate that airway effects of PGF2 alpha are far more complex than previously reported. Moreover, these novel effects suggest that, in addition to its well-known bronchoconstrictor effects, PGF2 alpha directly or indirectly causes airway relaxation, predominantly in large airways.     

Reduced hyperpnea-induced bronchospasm following repeated cold air challenge

This study assessed reduction in expiratory function in 12 asthmatic subjects both after 5 min of cold air provocation (CAP) with dry air conditioned to approximately 0 degrees C and after exercise (to 85% of predicted maximum heart rate) while breathing ambient room air (approximately 21 degrees C and 40% relative humidity). These assessments were done both before and after the following training protocol. Three 5-min periods of isocapnic cold air hyperpnea separated by 5-min rest periods were performed breathing 0 degrees to -10 degrees C air, for 36 sessions over 12 wk. As expected, pretraining expiratory function was significantly reduced (P less than 0.001) after both CAP and exercise. The posttraining reduction in expiratory function after CAP and exercise, however, was significantly less pronounced (largest P less than 0.05). These data support our hypothesis that repeated bouts of cold air challenge result in airway acclimatization to cold air and consequent decrease in exercise-induced bronchospasm. Acclimatization may result directly either by habituation of the airways or by vasodilation leading to increased bronchial blood flow and consequent reduced airway cooling. An unanticipated finding, though, is that repeated cold air challenge may also cause long-term inflammatory changes in the airways. A significant percentage of subjects experienced reduced base-line pulmonary function and overall exacerbation of asthma symptoms during the training period.     

Prostaglandins do not contribute to the nitric oxide-mediated compensatory vasodilation in hypoperfused exercising muscle

We tested the hypothesis that 1) prostaglandins (PGs) contribute to compensatory vasodilation in contracting human forearm subjected to acute hypoperfusion, and 2) the combined inhibition of PGs and nitric oxide would attenuate the compensatory vasodilation more than PG inhibition alone. In separate protocols, subjects performed forearm exercise (20% of maximum) during hypoperfusion evoked by intra-arterial balloon inflation. Each trial included baseline, exercise before inflation, exercise with inflation, and exercise after deflation. Forearm blood flow (FBF; ultrasound) and local (brachial artery) and systemic arterial pressure [mean arterial pressure (MAP); Finometer] were measured. In protocol 1 (n = 8), exercise was repeated during cyclooxygenase (COX) inhibition (Ketorolac) alone and during Ketorolac-NOS inhibition [N(G)-monomethyl-l-arginine (l-NMMA)]. In protocol 2 (n = 8), exercise was repeated during l-NMMA alone and during l-NMMA-Ketorolac. Forearm vascular conductance (FVC; ml·min(-1)·100 mmHg(-1)) was calculated from FBF (ml/min) and local MAP (mmHg). The percent recovery in FVC during inflation was calculated as (steady-state inflation + exercise value - nadir)/[steady-state exercise (control) value - nadir] × 100. In protocol 1, COX inhibition alone did not reduce the %FVC recovery compared with the control (no drug) trial (92 ± 11 vs. 100 ± 10%, P = 0.83). However, combined COX-nitric oxide synthase (NOS) inhibition caused a substantial reduction in %FVC recovery (54 ± 8%, P < 0.05 vs. Ketorolac alone). In protocol 2, the percent recovery in FVC was attenuated with NOS inhibition alone (69 ± 9 vs. 107 ± 10%, P < 0.01) but not attenuated further during combined NOS-COX inhibition (62 ± 10%, P = 0.74 vs. l-NMMA alone). Our data indicate that PGs are not obligatory to the compensatory dilation observed during forearm exercise with hypoperfusion.     

Inducible nitric oxide synthase (iNOS): role in asthma pathogenesis

Asthma is one of the most common chronic inflammatory disorder of the airways of the lungs, affecting more than 300 million people all over the world. Nitric oxide (NO) is endogenously produced in mammalian airways by nitric oxide synthase (NOS) and is known to regulate many aspects of human asthma, including the modulation of airway and vascular smooth muscle tone and the inflammation. Asthmatic patients show an increased expression of inducible nitric oxide synthase (iNOS) in airway epithelial cells and an increased level of NO in exhaled air. Using various NO inhibitors (non-specific or iNOS-specific) and gene knock-out experiments, controversial results have been obtained regarding iNOS's beneficial and deleterious effects in the disease. In the present review, we have attempted to summarize the results of these experiments and also the genetic studies being undertaken to understand the role of iNOS in asthma. It is argued that extensive biochemical, clinical and genetic studies will be required to assess the precise role of NO in the asthma. This may help in designing selective and more potent iNOS inhibitors and NO donors for developing novel therapeutics for the asthma patients.