Contributions of nitric oxide synthase isozymes to exhaled nitric oxide and hypoxic pulmonary vasoconstriction in rabbit lungs

Effects of different nitric oxide synthases on pulmonary and systemic hemodynamics in hypoxic stress rat model

BACKGROUND

Under hypoxia, exaggerated compensatory responses may lead to acute mountain sickness. The excessive vasodilatory effect of nitric oxide (NO) can lower the hypoxic pulmonary vasoconstriction (HPV) and peripheral blood pressure. While NO is catalyzed by various nitric oxide synthase (NOS) isoforms, the regulatory roles of these types in the hemodynamics of pulmonary and systemic circulation in living hypoxic animals remain unclear. Therefore, this study aims to investigate the regulatory effects of different NOS isoforms on pulmonary and systemic circulation in hypoxic rats by employing selective NOS inhibitors and continuously monitoring hemodynamic parameters of both pulmonary and systemic circulation.

METHODS

Forty healthy male Sprague-Dawley (SD) rats were randomly divided into four groups: Control group (NG-nitro-D-arginine methyl ester, D-NAME), L-NAME group (non-selective NOS inhibitor, NG-nitro-L-arginine methyl ester), AG group (inducible NOS inhibitor group, aminoguanidine), and 7-NI group (neurological NOS inhibitor, 7-nitroindazole). Hemodynamic parameters of rats were monitored for 10 min after inhibitor administration and 5 min after induction of hypoxia [15% O2, 2200 m a. sl., 582 mmHg (76.5 kPa), Xining, China] using the real-time dynamic monitoring model for pulmonary and systemic circulation hemodynamics in vivo. Serum NO concentrations and blood gas analysis were measured.

RESULTS

Under normoxia, mean arterial pressure and total peripheral vascular resistance were increased, and ascending aortic blood flow and serum NO concentration were decreased in the L-NAME and AG groups. During hypoxia, pulmonary arterial pressure and pulmonary vascular resistance were significantly increased in the L-NAME and AG groups.

CONCLUSIONS

This compensatory mechanism activated by inducible NOS and endothelial NOS effectively counteracts the pulmonary hemodynamic changes induced by hypoxic stress. It plays a crucial role in alleviating hypoxia-induced pulmonary arterial hypertension.

Bacillus anthracis edema toxin inhibits hypoxic pulmonary vasoconstriction via edema factor and cAMP-mediated mechanisms in isolated perfused rat lungs

Bacillus anthracis edema toxin (ET) inhibited lethal toxin-stimulated pulmonary artery pressure (Ppa) and increased lung cAMP levels in our previous study. We therefore examined whether ET inhibits hypoxic pulmonary vasoconstriction (HPV). Following baseline hypoxic measures in isolated perfused lungs from healthy rats, compared with diluent, ET perfusion reduced maximal Ppa increases (mean ± SE percentage of maximal Ppa increase with baseline hypoxia) during 6-min hypoxic periods (FIO2 = 0%) at 120 min (16 ± 6% vs. 51 ± 6%, P = 0.004) and 180 min (11.4% vs. 55 ± 6%, P = 0.01). Protective antigen-mAb (PA-mAb) and adefovir inhibit host cell edema factor uptake and cAMP production, respectively. In lungs perfused with ET following baseline measures, compared with placebo, PA-mAb treatment increased Ppa during hypoxia at 120 and 180 min (56 ± 6% vs. 10 ± 4% and 72 ± 12% vs. 12 ± 3%, respectively, P ≤ 0.01) as did adefovir (84 ± 10% vs. 16.8% and 123 ± 21% vs. 26 ± 11%, respectively, P ≤ 0.01). Compared with diluent, lung perfusion with ET for 180 min reduced the slope of the relationships between Ppa and increasing concentrations of endothelin-1 (ET-1) (21.12 ± 2.96 vs. 3.00 ± 0.76 × 108 cmH2O/M, P < 0.0001) and U46619, a thromboxane A2 analogue (7.15 ± 1.01 vs. 3.74 ± 0.31 × 107 cmH2O/M, P = 0.05) added to perfusate. In lungs isolated from rats after 15 h of in vivo infusions with either diluent, ET alone, or ET with PA-mAb, compared with diluent, the maximal Ppa during hypoxia and the slope of the relationship between change in Ppa and ET-1 concentration added to the perfusate were reduced in lungs from animals challenged with ET alone (P ≤ 0.004) but not with ET and PA-mAb together (P ≥ 0.73). Inhibition of HPV by ET could aggravate hypoxia during anthrax pulmonary infection.NEW & NOTEWORTHY The most important findings here are edema toxin's potent adenyl cyclase activity can interfere with hypoxic pulmonary vasoconstriction, an action that could worsen hypoxemia during invasive anthrax infection with lung involvement. These findings, coupled with other studies showing that lethal toxin can disrupt pulmonary vascular integrity, indicate that both toxins can contribute to pulmonary pathophysiology during infection. In combination, these investigations provide a further basis for the use of antitoxin therapies in patients with worsening invasive anthrax disease.

Exhaled nitric oxide in ethnically diverse high-altitude native populations: A comparative study

OBJECTIVES

Andean and Tibetan high-altitude natives exhibit a high concentration of nitric oxide (NO) in the lungs, suggesting that NO plays an adaptive role in offsetting hypobaric hypoxia. We examined the exhaled NO concentration as well as partial pressure of several additional high-altitude native populations in order to examine the possibility that this putative adaptive trait, that is, high exhaled NO, is universal.

METHODS

We recruited two geographically diverse highland native populations, Tawang Monpa (TM), a Tibetan derived population in North-Eastern India (n = 95, sampled at an altitude of ~3,200 m), and Peruvian Quechua from the highland Andes (n = 412). The latter included three distinct subgroups defined as those residing at altitude (Q-HAR, n = 110, sampled at 4,338 m), those born and residing at sea-level (Q-BSL, n = 152), and those born at altitude but migrant to sea-level (Q-M, n = 150). In addition, we recruited a referent sample of lowland natives of European ancestry from Syracuse, New York. Fraction of exhaled NO concentrations were measured using a NIOX NIMO following the protocol of the manufacturer.

RESULTS

Partial pressure of exhaled nitric oxide (PENO) was significantly lower (p < .05) in both high-altitude resident groups (TM = 6.2 ± 0.5 nmHg and Q-HAR = 5.8 ± 0.5 nmHg), as compared to the groups measured at sea level (USA = 14.6 ± 0.7 nmHg, Q-BSL = 18.9 ± 1.6 nmHg, and Q-M = 19.2 ± 1.7 nmHg). PENO was not significantly different between TM and Q-HAR (p < .05).

CONCLUSION

In contrast to previous work, we found lower PENO in populations at altitude (compared to sea-level) and no difference in PENO between Tibetan and Andean highland native populations. These results do not support the hypothesis that high nitric oxide in human lungs is a universal adaptive mechanism of highland native populations to offset hypobaric hypoxia.

Carbonic anhydrase is not a relevant nitrite reductase or nitrous anhydrase in the lung

KEY POINTS

Carbonic anhydrase (CA) inhibitors such as acetazolamide inhibit hypoxic pulmonary vasoconstriction (HPV) in humans and other mammals, but the mechanism of this action remains unknown. It has been postulated that carbonic anhydrase may act as a nitrous anhydrase in vivo to generate nitric oxide (NO) from nitrite and that this formation is increased in the presence of acetazolamide. Acetazolamide reduces HPV in pigs without evidence of any NO generation, whereas nebulized sodium nitrite reduces HPV by NO formation; however; combined infusion of acetazolamide with sodium nitrite inhalation did not further increase exhaled NO concentration over inhaled nitrite alone in pigs exposed to alveolar hypoxia. We conclude that acetazolamide does not function as either a nitrous anhydrase or a nitrite reductase in the lungs of pigs, and probably other mammals, to explain its vasodilating actions in the pulmonary or systemic circulations.

ABSTRACT

The carbonic anhydrase (CA) inhibitors acetazolamide and its structurally similar analogue methazolamide prevent or reduce hypoxic pulmonary vasoconstriction (HPV) in dogs and humans in vivo, by a mechanism unrelated to CA inhibition. In rodent blood and isolated blood vessels, it has been reported that inhibition of CA leads to increased generation of nitric oxide (NO) from nitrite and vascular relaxation in vitro. We tested the physiological relevance of augmented NO generation by CA from nitrite with acetazolamide in anaesthetized pigs during alveolar hypoxia in vivo. We found that acetazolamide prevents HPV in anaesthetized pigs, as in other mammalian species. A single nebulization of sodium nitrite reduces HPV, but this action wanes in the succeeding 3 h of hypoxia as nitrite is metabolized and excreted. Pulmonary artery pressure reduction and NO formation as measured by exhaled gas concentration from inhaled sodium nitrite were not increased by acetazolamide during alveolar hypoxia. Thus, our data argue against a physiological role of carbonic anhydrase as a nitrous anhydrase or nitrite reductase as a mechanism for its inhibition of HPV in the lung and blood in vivo.

Circulating exosomes potentiate tumor malignant properties in a mouse model of chronic sleep fragmentation

Background

Chronic sleep fragmentation (SF) increases cancer aggressiveness in mice. Exosomes exhibit pleiotropic biological functions, including immune regulatory functions, antigen presentation, intracellular communication and inter-cellular transfer of RNA and proteins. We hypothesized that SF-induced alterations in biosynthesis and cargo of plasma exosomes may affect tumor cell properties.

Results

SF-derived exosomes increased tumor cell proliferation (~13%), migration (~2.3-fold) and extravasation (~10%) when compared to exosomes from SC-exposed mice. Similarly, Pre exosomes from OSA patients significantly enhanced proliferation and migration of human adenocarcinoma cells compared to Post. SF-exosomal cargo revealed 3 discrete differentially expressed miRNAs, and exploration of potential mRNA targets in TC1 tumor cells uncovered 132 differentially expressed genes that encode for multiple cancer-related pathways.

Methods

Plasma-derived exosomes from C57/B6 mice exposed to 6 wks of SF or sleep control (SC), and from adult human patients with obstructive sleep apnea (OSA) before (Pre) and after adherent treatment for 6 wks (Post) were co-cultured with mouse lung TC1 or human adenocarcinoma tumor cell lines, respectively. Proliferation, migration, invasion, endothelial barrier integrity and extravasation assays of tumor cells were performed. Plasma mouse exosomal miRNAs were profiled with arrays, and transcriptomic assessments of TC1 cells exposed to SF or SC exosomes were conducted to identify gene targets.

Conclusions

Chronic SF induces alterations in exosomal miRNA cargo that alter the biological properties of TC1 lung tumor cells to enhance their proliferative, migratory and extravasation properties, and similar findings occur in OSA patients, in whom SF is a constitutive component of their sleep disorder. Thus, exosomes could participate, at least in part, in the adverse cancer outcomes observed in OSA.

The endothelium in hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) in combination with hypercapnic pulmonary vasoconstriction redistributes pulmonary blood flow from poorly aerated to better ventilated lung regions by an active process of local vasoconstriction. Impairment of HPV results in ventilation-perfusion mismatch and is commonly associated with various lung diseases including pneumonia, sepsis, or cystic fibrosis. Although several regulatory pathways have been identified, considerable knowledge gaps persist, and a unifying concept of the signaling pathways that underlie HPV and their impairment in lung diseases has not yet emerged. In the past, conceptual models of HPV have focused on pulmonary arterial smooth muscle cells (PASMC) acting as sensor and effector of hypoxia in the pulmonary vasculature. In contrast, the endothelium was considered a modulating bystander in this scenario. For an ideal design, however, the oxygen sensor in HPV should be located in the region of gas exchange, i.e., in the alveolar capillary network. This concept requires the retrograde propagation of the hypoxic signal along the endothelial layer of the vascular wall and subsequent contraction of PASMC in upstream arterioles that is elicited via temporospatially tightly controlled endothelial-smooth muscle cell crosstalk. The present review summarizes recent work that provides proof-of-principle for the existence and functional relevance of such signaling pathway in HPV that involves important roles for connexin 40, epoxyeicosatrienoic acids, sphingolipids, and cystic fibrosis transmembrane conductance regulator. Of translational relevance, implication of these molecules provides for novel mechanistic explanations for impaired ventilation/perfusion matching in patients with pneumonia, sepsis, cystic fibrosis, and presumably various other lung diseases.

Genetic variance in nitric oxide synthase and endothelin genes among children with and without endothelial dysfunction

Background

The presence of endothelial dysfunction (ED) constitutes an early risk factor for cardiovascular disease (CVD) in children. Nitric oxide (NO) and endothelin (EDN) are generated in endothelial cells and are critical regulators of vascular function, with ED resulting from an imbalance between these two molecules. We hypothesized that genetic variants in NO synthase and EDN isoforms and its receptors (EDNRA and EDNRB) may account for a proportion of the risk for ED in developing children.

Methods

Consecutive children (ages 5–10 years) were prospectively recruited from the community. Time to peak post-occlusive reperfusion (Tmax) was considered as the indicator of either normal endothelial function (NEF; Tmax < 45 sec) or ED (Tmax ≥ 45 sec). Lipid profiles, high sensitivity C-reactive protein (hsCRP), fasting glucose and insulin were assayed using ELISA. Genomic DNA from peripheral blood was extracted and genotyped for NOS1 (209 SNPs), NOS2 (122 SNPs), NOS3 (50 SNPs), EDN1 (43 SNPs), EDN2 (48 SNPs), EDN3 (14 SNPs), EDNRA (27 SNPs), and EDNRB (23 SNPs) using a custom SNPs array. Linkage disequilibrium was analyzed using Haploview version 4.2 software.

Results

The relative frequencies of SNPs were evaluated in 122 children, 84 with NEF and 38 with ED. The frequencies of NOS1 (11 SNPs), and EDN1 (2 SNPs) were differentially distributed between NEF vs. ED, and no significant differences emerged for all other genes. Significant SNPs for NOS1 and EDN1 SNPs were further validated with RT-PCR.

Conclusions

Genetic variants in the NOS1 and EDN1 genes appear to account for important components of the variance in endothelial function, particularly when concurrent risk factors such as obesity exist. Thus, analysis of genotype-phenotype interactions in children at risk for ED will be critical for more accurate formulation of categorical CVD risk estimates.

Optimization of isolated perfused/ventilated mouse lung to study hypoxic pulmonary vasoconstriction

Hypoxic pulmonary vasoconstriction (HPV) is a compensatory physiological mechanism in the lung that optimizes the matching of ventilation to perfusion and thereby maximizes gas exchange. Historically, HPV has been primarily studied in isolated perfused/ventilated lungs; however, the results of these studies have varied greatly due to different experimental conditions and species. Therefore, in the present study, we utilized the mouse isolated perfused/ventilated lung model for investigation of the role of extracellular Ca²⁺ and caveolin-1 and endothelial nitric oxide synthase expression on HPV. We also compared HPV using different perfusate solutions: Physiological salt solution (PSS) with albumin, Ficoll, rat blood, fetal bovine serum (FBS), or Dulbecco's Modified Eagle Medium (DMEM). After stabilization of the pulmonary arterial pressure (PAP), hypoxic (1% O₂) and normoxic (21% O₂) gases were applied via a ventilator in five-minute intervals to measure HPV. The addition of albumin or Ficoll with PSS did not induce persistent and strong HPV with or without a pretone agent. DMEM with the inclusion of FBS in the perfusate induced strong HPV in the first hypoxic challenge, but the HPV was neither persistent nor repetitive. PSS with rat blood only induced a small increase in HPV amplitude. Persistent and repetitive HPV occurred with PSS with 20% FBS as perfusate. HPV was significantly decreased by the removal of extracellular Ca²⁺ along with addition of 1 mM EGTA to chelate residual Ca²⁺ and voltage-dependent Ca²⁺ channel blocker (nifedipine 1 μM). PAP was also reactive to contractile stimulation by high K⁺ depolarization and U46619 (a stable analogue of thromboxane A₂). In summary, optimal conditions for measuring HPV were established in the isolated perfused/ventilated mouse lung. Using this method, we further confirmed that HPV is dependent on Ca²⁺ influx.

Rat models of acute lung injury: exhaled nitric oxide as a sensitive, noninvasive real-time biomarker of prognosis and efficacy of intervention

Exhaled nitric oxide (eNO) has received increased attention in clinical settings because this technique is easy to use with instant readout. However, despite the simplicity of eNO in humans, this endpoint has not frequently been used in experimental rat models of septic (endotoxemia) or irritant acute lung injury (ALI). The focus of this study is to adapt this method to rats for studying ALI-related lung disease and whether it can serve as instant, non-invasive biomarker of ALI to study lung toxicity and pharmacological efficacy. Measurements were made in a dynamic flow of sheath air containing the exhaled breath from spontaneously breathing, conscious rats placed into a head-out volume plethysmograph. The quantity of eNO in exhaled breath was adjusted (normalized) to the physiological variables (breathing frequency, concentration of exhaled carbon dioxide) mirroring pulmonary perfusion and ventilation. eNO was examined on the instillation/inhalation exposure day and first post-exposure day in Wistar rats intratracheally instilled with lipopolysaccharide (LPS) or single inhalation exposure to chlorine or phosgene gas. eNO was also examined in a Brown Norway rat asthma model using the asthmagen toluene diisocyanate (TDI). The diagnostic sensitivity of adjusted eNO was superior to the measurements not accounting for the normalization of physiological variables. In all bioassays - whether septic, airway or alveolar irritant or allergic, the adjusted eNO was significantly increased when compared to the concurrent control. The maximum increase of the adjusted eNO occurred following exposure to the airway irritant chlorine. The specificity of adjustment was experimentally verified by decreased eNO following inhalation dosing of the non-selective nitric oxide synthase inhibitor amoniguanidine. In summary, the diagnostic sensitivity of eNO can readily be applied to spontaneously breathing, conscious rats without any intervention or anesthesia. Measurements are definitely improved by accounting for the disease-related changes in exhaled CO2 and breathing frequency. Accordingly, adjusted eNO appears to be a promising methodological improvement for utilizing eNO in inhalation toxicology and pharmacological disease models with fewer animals.

Hypoxic pulmonary vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Effects of ambient pressure on pulmonary nitric oxide

Airway nitric oxide (NO) has been proposed to play a role in the development of high-altitude pulmonary edema. We undertook a study of the effects of acute changes of ambient pressure on exhaled and alveolar NO in the range 0.5-4 atmospheres absolute (ATA, 379-3,040 mmHg) in eight healthy subjects breathing normoxic nitrogen-oxygen mixtures. On the basis of previous work with inhalation of low-density helium-oxygen gas, we expected facilitated backdiffusion and lowered exhaled NO at 0.5 ATA and the opposite at 4 ATA. Instead, the exhaled NO partial pressure (Pe(NO)) did not differ between pressures and averaged 1.21 ± 0.16 (SE) mPa across pressures. As a consequence, exhaled NO fractions varied inversely with pressure. Alveolar estimates of the NO partial pressure differed between pressures and averaged 88 (P = 0.04) and 176 (P = 0.009) percent of control (1 ATA) at 0.5 and 4 ATA, respectively. The airway contribution to exhaled NO was reduced to 79% of control (P = 0.009) at 4 ATA. Our finding of the same Pe(NO) at 0.5 and 1 ATA is at variance with previous findings of a reduced Pe(NO) with inhalation of low-density gas at normal pressure, and this discrepancy may be due to the much longer durations of low-density gas breathing in the present study compared with previous studies with helium-oxygen breathing. The present data are compatible with the notion of an enhanced convective backtransport of NO, compensating for attenuated backdiffusion of NO with increasing pressure. An alternative interpretation is a pressure-induced suppression of NO formation in the airways.

Smoking, COPD, and 3-nitrotyrosine levels of plasma proteins

BACKGROUND

Nitric oxide is a physiological regulator of endothelial function and hemodynamics. Oxidized products of nitric oxide can form nitrotyrosine, which is a marker of nitrative stress. Cigarette smoking decreases exhaled nitric oxide, and the underlying mechanism may be important in the cardiovascular toxicity of smoking. Even so, it is unclear if this effect results from decreased nitric oxide production or increased oxidative degradation of nitric oxide to reactive nitrating species. These two processes would be expected to have opposite effects on nitrotyrosine levels, a marker of nitrative stress.

OBJECTIVE

In this study, we evaluated associations of cigarette smoking and chronic obstructive pulmonary disease (COPD) with nitrotyrosine modifications of specific plasma proteins to gain insight into the processes regulating nitrotyrosine formation.

METHODS

A custom antibody microarray platform was developed to analyze the levels of 3-nitrotyrosine modifications on 24 proteins in plasma. In a cross-sectional study, plasma samples from 458 individuals were analyzed.

RESULTS

Average nitrotyrosine levels in plasma proteins were consistently lower in smokers and former smokers than in never smokers but increased in smokers with COPD compared with smokers who had normal lung-function tests.

CONCLUSIONS

Smoking is associated with a broad decrease in 3-nitrotyrosine levels of plasma proteins, consistent with an inhibitory effect of cigarette smoke on endothelial nitric oxide production. In contrast, we observed higher nitrotyrosine levels in smokers with COPD than in smokers without COPD. This finding is consistent with increased nitration associated with inflammatory processes. This study provides insight into a mechanism through which smoking could induce endothelial dysfunction and increase the risk of cardiovascular disease.

Enhancement of myofilament calcium sensitivity by acute hypoxia in rat distal pulmonary arteries

Hypoxic contraction of pulmonary arterial smooth muscle is thought to require increases in both intracellular Ca(2+) concentration ([Ca(2+)](i)) and myofilament Ca(2+) sensitivity, which may or may not be endothelium-dependent. To examine the effects of hypoxia and endothelium on Ca(2+) sensitivity in pulmonary arterial smooth muscle, we measured the relation between [Ca(2+)](i) and isometric force at 37°C during normoxia (21% O(2)-5% CO(2)) and after 30 min of hypoxia (1% O(2)-5% CO(2)) in endothelium-intact (E+) and -denuded (E-) rat distal intrapulmonary arteries (IPA) permeabilized with staphylococcal α-toxin. Endothelial denudation enhanced Ca(2+) sensitivity during normoxia but did not alter the effects of hypoxia, which shifted the [Ca(2+)](i)-force relation to higher force in E+ and E- IPA. Neither hypoxia nor endothelial denudation altered Ca(2+) sensitivity in mesenteric arteries. In E+ and E- IPA, hypoxic enhancement of Ca(2+) sensitivity was abolished by the nitric oxide synthase inhibitor N(ω)-nitro-l-arginine methyl ester (30 μM), which shifted normoxic [Ca(2+)](i)-force relations to higher force. In E- IPA, the Rho kinase antagonist Y-27632 (10 μM) shifted the normoxic [Ca(2+)](i)-force relation to lower force but did not alter the effects of hypoxia. These results suggest that acute hypoxia enhanced myofilament Ca(2+) sensitivity in rat IPA by decreasing nitric oxide production and/or activity in smooth muscle, thereby revealing a high basal level of Ca(2+) sensitivity, due in part to Rho kinase, which otherwise did not contribute to Ca(2+) sensitization by hypoxia.

Endothelial NOS-derived nitric oxide prevents injury resulting from reoxygenation in the hypoxic lung

To date, the role that NO derived from endothelial NO synthase (eNOS) plays in the development of the injuries occurring under hypoxia/reoxygenation (H/R) in the lung remains unknown and thus constitutes the subject of the present work. A follow-up study was conducted in Wistar rats submitted to H/R (hypoxia for 30 min; reoxygenation of 0 h, 48 h and 5 days), with or without prior treatment using the eNOS inhibitor L-NIO (20 mg/kg). Lipid peroxidation, apoptosis, protein nitration and NO production (NOx) were analysed. The results showed that L-NIO administration lowered NOx levels in all the experimental groups. Contrarily, the lipid peroxidation level and the percentage of apoptotic cells rose, implying that eNOS-derived NO may have a protective effect against the injuries occurring during H/R in the lung. These findings could open the possibility of future studies to design new therapies for this type of hypoxia based on NO-pharmacology.

High-Altitude Pulmonary Edema

High-altitude pulmonary edema (HAPE) is an uncommon form of pulmonary edema that occurs in healthy individuals within a few days of arrival at altitudes above 2,500–3,000 m. The crucial pathophysiology is an excessive hypoxia-mediated rise in pulmonary vascular resistance (PVR) or hypoxic pulmonary vasoconstriction (HPV) leading to increased microvascular hydrostatic pressures despite normal left atrial pressure. The resultant hydrostatic stress can cause both dynamic changes in the permeability of the alveolar capillary barrier and mechanical damage leading to leakage of large proteins and erythrocytes into the alveolar space in the absence of inflammation. Bronchoalveolar lavage (BAL) and pulmonary artery (PA) and microvascular pressure measurements in humans confirm that high capillary pressure induces a high-permeability non-inflammatory-type lung edema; a concept termed “capillary stress failure.” Measurements of endothelin and nitric oxide (NO) in exhaled air, NO metabolites in BAL fluid, and NO-dependent endothelial function in the systemic circulation all point to reduced NO availability and increased endothelin in hypoxia as a major cause of the excessive hypoxic PA pressure rise in HAPE-susceptible individuals. Other hypoxia-dependent differences in ventilatory control, sympathetic nervous system activation, endothelial function, and alveolar epithelial sodium and water reabsorption likely contribute additionally to the phenotype of HAPE susceptibility. Recent studies using magnetic resonance imaging in humans strongly suggest nonuniform regional hypoxic arteriolar vasoconstriction as an explanation for how HPV occurring predominantly at the arteriolar level can cause leakage. This compelling but not yet fully proven mechanism predicts that in areas of high blood flow due to lesser vasoconstriction edema will develop owing to pressures that exceed the structural and dynamic capacity of the alveolar capillary barrier to maintain normal alveolar fluid balance. Numerous strategies aimed at lowering HPV and possibly enhancing active alveolar fluid reabsorption are effective in preventing and treating HAPE. Much has been learned about HAPE in the past four decades such that what was once a mysterious alpine malady is now a well-characterized and preventable lung disease. This chapter will relate the history, pathophysiology, and treatment of HAPE, using it not only to illuminate the condition, but also for the broader lessons it offers in understanding pulmonary vascular regulation and lung fluid balance.

Inducible NOS inhibitor 1400W reduces hypoxia/re-oxygenation injury in rat lung

Nitric oxide (NO(*)) from inducible NO(*) synthase (iNOS) has been reported to either protect against, or contribute to, hypoxia/re-oxygenation lung injury. The present work aimed to clarify this double role in the hypoxic lung. With this objective, a follow-up study was made in Wistar rats submitted to hypoxia/re-oxygenation (hypoxia for 30 min; re-oxygenation of 0 h, 48 h, and 5 days), with or without prior treatment with the selective iNOS inhibitor 1400W (10 mg/kg). NO(*) levels (NOx), lipid peroxidation, apoptosis, and protein nitration were analysed. This is the first time-course study which investigates the effects of 1400W during hypoxia/re-oxygenation in the rat lung. The results showed that the administration of 1400W lowered NOx levels in all the experimental groups. In addition, lipid peroxidation, the percentage of apoptotic cells, and nitrated protein expression fell in the late post-hypoxia period (48 h and 5 days). Our results reveal that the inhibition of iNOS in the hypoxic lung reduced the damage observed before the treatment with 1400W, suggesting that iNOS-derived NO(*) may exert a negative effect on this organ during hypoxia/re-oxygenation. These findings are notable, since they indicate that any therapeutic strategy aimed at controlling excess generation of NO(*) from iNOS may be useful in alleviating NO(*)-mediated adverse effects in hypoxic lungs.

Endogenous nitric oxide can act as beneficial or deleterious in the hypoxic lung depending on the reoxygenation time

Nitric oxide (NO) has been implicated in many pathophysiological situations in the lung, including hypoxia/reoxygenation. This work seeks to clarify the current controversy concerning the double protective/toxic role of endogenous NO under hypoxia/reoxygenation situations in the lung by using a nitric oxide synthase (NOS) inhibitor, in a novel approach to address the problems raised from assaults under such circumstances. A follow-up study was conducted in Wistar rats submitted to hypoxia/reoxygenation (hypoxia for 30 min; reoxygenation of 0 h, 48 h, and 5 days), with or without prior treatment using the nonselective NOS inhibitor L-NAME (1.5 mM, in drinking water). Lipid peroxidation, apoptosis level, protein nitration, in situ NOS activity and NO production (NOx) were analyzed. This is the first work to focus on the time-course effects of L-NAME in the adult rat lung submitted to hypoxia/reoxygenation. The results showed that after L-NAME administration, in situ NOS activity was almost completely eliminated and consequently, NOx levels fell. Lipid peroxidation and the percentage of apoptotic cells rose at the earliest reoxygenation time (0 h), but decreased in the later period (48 h and 5 days). Also nitrated protein expression decreased at 48 h and 5 days posthypoxia. These results suggest that NOS-derived NO exerts two different effects on lung hypoxia/reoxygenation injury depending on the reoxygenation time: NO has a beneficial role just after the hypoxic stimulus and a deleterious effect in the later reoxygenation times. Moreover, we propose that this dual role of NO depends directly on the producer NOS isoform.

Influence of ventilatory settings on exhaled nitric oxide during high frequency oscillatory ventilation

Nitric oxide (NO), which is produced in the lower airways, diffuses from cells into the air space and can be measured in exhaled air. The influence of high frequency oscillatory ventilation on the production of exhaled NO (eNO) has not been thoroughly studied. The objectives of this study are to establish an animal model for evaluation of lower airway NO and to evaluate settings in terms of frequency, mean airway pressure (MAP), amplitude pressure (amplitude), and inspiratory time ratio (t(I)/t(E)) during high frequency oscillatory ventilation on the production of eNO. An observational animal study was performed on 12 female New Zealand White rabbits, which were anesthetized, tracheotomized and ventilated using a SensorMedics 3100A HFOV ventilator (SM3100A). The concentration of NO in exhaled gas was measured by chemiluminescence continuously from the nose and the side hole of the adaptor of endotracheal tube. The individual effects of the respiratory settings were evaluated. The results were analyzed by paired t-test or by the generalized estimating equation method. We found that the lower airway was the main source of the eNO, that amplitude, MAP, and t(I)/t(E) were positively correlated with the level of eNO and that frequency was negatively correlated with the level of eNO. These findings fit the stretch theory for the production of endogenous NO. Monitoring of eNO during HFOV may provide insights into lung mechanics and ventilation efficiency and be used in the future as a guide during clinical practice.

Pulmonary-artery pressure and exhaled nitric oxide in Bolivian and Caucasian high altitude dwellers

There is evidence that high altitude populations may be better protected from hypoxic pulmonary hypertension than low altitude natives, but the underlying mechanism is incompletely understood. In Tibetans, increased pulmonary respiratory NO synthesis attenuates hypoxic pulmonary hypertension. It has been speculated that this mechanism may represent a generalized high altitude adaptation pattern, but direct evidence for this speculation is lacking. We therefore measured systolic pulmonary-artery pressure (Doppler chocardiography) and exhaled nitric oxide (NO) in 34 healthy, middle-aged Bolivian high altitude natives and in 34 age- and sex-matched, well-acclimatized Caucasian low altitude natives living at high altitude (3600 m). The mean+/-SD systolic right ventricular to right atrial pressure gradient (24.3+/-5.9 vs. 24.7+/-4.9 mmHg) and exhaled NO (19.2+/-7.2 vs. 22.5+/-9.5 ppb) were similar in Bolivians and Caucasians. There was no relationship between pulmonary-artery pressure and respiratory NO in the two groups. These findings provide no evidence that Bolivian high altitude natives are better protected from hypoxic pulmonary hypertension than Caucasian low altitude natives and suggest that attenuation of pulmonary hypertension by increased respiratory NO synthesis may not represent a universal adaptation pattern in highaltitude populations.

Differential effects of nitric oxide synthesis on pulmonary vascular function during lung ischemia-reperfusion injury

Lung ischemia-reperfusion (IR) injury causes alveolar, epithelial and endothelial cell dysfunction which often results in decreased alveolar perfusion, characteristic of an acute respiratory distress syndrome. Nitric oxide (NO) from endothelium-derived NO synthase (eNOS) helps maintain a low pulmonary vascular resistance. Paradoxically, during acute lung injury, overproduction of NO via inducible NO synthase (iNOS) and oxidative stress lead to reactive oxygen and nitrogen species (ROS and RNS) formation and vascular dysfunction. RNS potentiate vascular and cellular injury by oxidation, by decreasing NO bioavailability, and by regulating NOS isoforms. RNS potentiate their own production by uncoupling NO production through eNOS by oxidation and disruption of Akt-mediated phosphorylation of eNOS. This review focuses on effects of NO which cause vascular dysfunction in the unique environment of the lung and presents a hypothesis for interplay between eNOS and iNOS activation with implications for development of new strategies to treat vascular dysfunction associated with IR.

Lessons in hypoxic adaptation from high-altitude populations

An increase in hemoglobin level is seen in virtually all lowlanders who move to or train at altitude; however, studies of high-altitude native populations illustrate that this response is not necessary for successful long-term residence. Indigenous populations living at the same altitude have differences not only in hemoglobin level but also in other traits like oxygen saturation. Support for a genetic causation for differences in features of oxygen transport, namely hemoglobin levels and oxygen saturation, is derived from kindred studies among the highlander populations. Indeed, evidence from Tibet suggests that inferred genes for high oxygen saturation are associated with higher offspring survival. It may be that signaling molecules like nitric oxide and transcription factors such as hypoxia-inducible factor could act as an upstream regulator for highlander traits. However, the preponderance of data suggests that it is unlikely that one process or even a common set of processes is responsible for successful biologic adaptation shown in all three resident high-altitude populations. Future studies will require the ability to identify combinations of genetic variants with outcomes including expression levels, appropriate phenotypes, and functional responses.

Capsaicin-sensitive nerves and neurokinins modulate non-neuronal nNOS expression in lung

We investigated the effects of substance P (SP) and neurokinin A (NKA) infusion and acute stimulation of capsaicin-sensitive sensory nerves fibers (CAP) on lung recruitment of neuronal nitric oxide synthase (nNOS)-positive inflammatory and respiratory epithelial (RE) cells in guinea-pigs. We evaluated if the effects of CAP stimulation were maintained until 14 days and had functional pulmonary repercussions. After 24h of CAP and 30 min after SP and NKA infusions there was an increase in nNOS-positive eosinophils and mononuclear cells compared to controls (P<0.05). SP group presented an increase in nNOS-positive RE (P<0.05). After 14 days of CAP stimulation, there was a reduction in resistance (R(rs)) and elastance (E(rs)) of respiratory system in capsaicin pre-treated animals. We noticed a correlation between nNOS-positive eosinophils (R=-0.644, P<0.05) and mononuclear cells (R=-0.88, P<0.001) and R(rs). Concluding, CAP and neurokinins increase nNOS expression by inflammatory and RE cells. The increase in nNOS expression induced by low and high doses stimulation of CAP is longstanding and correlated to pulmonary mechanical repercussions.

Resting tension affects eNOS activity in a calcium-dependent way in airways

The alteration of resting tension (RT) from 0.5 g to 2.5 g increased significantly airway smooth muscle contractions induced by acetylcholine (ACh) in rabbit trachea. The decrease in extracellular calcium concentration [Ca²⁺]_o from 2 mM to 0.2 mM reduced ACh-induced contractions only at 2.5 g RT with no effect at 0.5 g RT. The nonselective inhibitor of nitric oxide synthase (NOS), N^G-nitro-L-arginine methyl ester (L-NAME) increased ACh-induced contractions at 2.5 g RT. The inhibitor of inducible NOS, S-methylsothiourea or neuronal NOS, 7-nitroindazole had no effect. At 2.5 g RT, the reduction of [Ca²⁺]_o from 2 mM to 0.2 mM abolished the effect of L-NAME on ACh-induced contractions. The NO precursor L-arginine or the tyrosine kinase inhibitors erbstatin A and genistein had no effect on ACh-induced contractions obtained at 2.5 g RT. Our results suggest that in airways, RT affects ACh-induced contractions by modulating the activity of epithelial NOS in a calcium-dependent, tyrosine-phosphorylation-independent way.

Similar but not the same: normobaric and hyperbaric pulmonary oxygen toxicity, the role of nitric oxide

Pulmonary manifestations of oxygen toxicity were studied and quantified in rats breathing >98% O(2) at 1, 1.5, 2, 2.5, and 3 ATA to test our hypothesis that different patterns of pulmonary injury would emerge, reflecting a role for central nervous system (CNS) excitation by hyperbaric oxygen. At 1.5 atmosphere absolute (ATA) and below, the well-recognized pattern of diffuse pulmonary damage developed slowly with an extensive inflammatory response and destruction of the alveolar-capillary barrier leading to edema, impaired gas exchange, respiratory failure, and death; the severity of these effects increased with time over the 56-h period of observation. At higher inspired O(2) pressures, 2-3 ATA, pulmonary injury was greatly accelerated but less inflammatory in character, and events in the brain were a prelude to a distinct lung pathology. The CNS-mediated component of this lung injury could be attenuated by selective inhibition of neuronal nitric oxide synthase (nNOS) or by unilateral transection of the vagus nerve. We propose that extrapulmonary, neurogenic events predominate in the pathogenesis of acute pulmonary oxygen toxicity in hyperbaric oxygenation, as nNOS activity drives lung injury by modulating the output of central autonomic pathways.

Increase in exhaled nitric oxide and protective role of the nitric oxide system in experimental pulmonary embolism

BACKGROUND AND PURPOSE

Pulmonary embolism (PE) represents a real diagnostic challenge. PE is associated with pulmonary hypertension due to pulmonary vascular obstruction and vasoconstriction. We recently reported that pulmonary gas embolism transiently increases exhaled nitric oxide (FENO), but it is not known whether solid emboli may alter FENO, and whether an intact endogenous NO synthesis has a beneficial effect in experimental solid pulmonary embolism.

EXPERIMENTAL APPROACH

We used anaesthetised and ventilated rabbits in these experiments. To mimic PE, a single intravenous infusion of homogenized autologous skeletal muscle tissue (MPE) was given to rabbits with intact NO production (MPE of 60, 15, or 7.5 mg kg(-1); group 1) and to another group (group 2) with inhibited NO synthesis (L-NAME 30 mg kg(-1); MPE of 7.5, 15 or 30 mg kg(-1)).

KEY RESULTS

In group 1, after MPE, FENO increased rapidly and dose-dependently and FENO was still significantly elevated after 60 min with the two highest emboli doses. All these animals survived more than 60 min after embolization. In group 2, MPE of 7.5, 15 and 30 mg kg(-1), in combination with NO synthesis inhibition, resulted in 67%, 50% and 25% survival at 60 min respectively, representing a statistically significant decrease in survival. Cardiovascular and blood-gas changes after MPE were intensified by pre-treatment with NO synthesis inhibitor.

CONCLUSIONS AND IMPLICATIONS

We conclude that solid PE causes a sustained, dose-dependent increase in FENO, giving FENO a diagnostic potential in PE. Furthermore, intact NO production appears critical for tolerance to acute PE.

The interactive effects of hypoxia and nitric oxide on catecholamine secretion in rainbow trout (Oncorhynchus mykiss)

Experiments were performed to test the hypothesis that exposure of rainbow trout to repetitive hypoxia would result in a decreased capacity of chromaffin cells to secrete catecholamines owing to increased production of nitric oxide(NO), a potent inhibitor of catecholamine secretion. A partial sequence of trout neuronal nitric oxide synthase (nNOS) was cloned and its mRNA was found to be present in the posterior cardinal vein (PCV), the predominant site of chromaffin cells in trout. Using heterologous antibodies, nNOS and endothelial NOS (eNOS) were localized in close proximity to the chromaffin cells of the PCV.

Exposure of trout to acute hypoxia (5.33 kPa for 30 min) in vivoresulted in significant increases in plasma catecholamine and NO levels. However, after 4 days of twice-daily exposures to hypoxia, the elevation of plasma catecholamine levels during hypoxia was markedly reduced. Associated with the reduction in plasma catecholamine levels during acute hypoxia was a marked increase in basal and hypoxia-evoked circulating levels of NO that became apparent after 2-4 days of repetitive hypoxia. The capacity of the chromaffin cells of the hypoxia-exposed fish to secrete catecholamine was assessed by electrical stimulation of an in situ saline-perfused PCV preparation. Compared with control (normoxic) fish, the PCV preparations derived from fish exposed to repeated hypoxia displayed a significant reduction in electrically evoked catecholamine secretion that was concomitant with a marked increased in NO production. This additional rise in NO secretion in preparations derived from hypoxic fish was prevented after adding NOS inhibitors to the perfusate; concomitantly, the reduction in catecholamine secretion was prevented. The increased production of NO during hypoxia in vivo and during electrical stimulation in situ was consistent with significant elevations of nNOS mRNA and protein; eNOS protein was unaffected. These results suggest that the reduced capacity of trout chromaffin cells to secrete catecholamines after repeated hypoxia reflects an increase in the expression of nNOS and a subsequent increase in NO production during chromaffin-cell activation.

Modulation of nitric oxide pathways: therapeutic potential in asthma and chronic obstructive pulmonary disease

Nitric oxide (NO) is present in the exhaled breath of humans and other mammalian species. It is generated in the lower airways by enzymes of the nitric oxide synthase (NOS) family, although nonenzymatic synthesis and consumptive processes may also influence levels of NO in exhaled breath. The biological properties of NO in the airways are multiple, complex, and bidirectional. Under physiological conditions, NO appears to play a homeostatic bronchoprotective role. However, its proinflammatory properties could also potentially cause tissue injury and contribute to airway dysfunction in disease states such as asthma and chronic obstructive pulmonary disease (COPD). This article will review the physiological and pathophysiological roles of NO in the airways, discuss the rationale for the use of drugs that modulate NO pathways--nitric oxide synthase inhibitors and NO donors--to treat inflammatory airway diseases, and attempt to predict the likely therapeutic benefit of such agents.

Hypoxia Impairs Systemic Endothelial Function in Individuals Prone to High-Altitude Pulmonary Edema

RATIONALE

High-altitude pulmonary edema (HAPE) is characterized by excessive pulmonary vasoconstriction and is associated with decreased concentrations of nitric oxide (NO) in the lung.

OBJECTIVES

We hypothesized that individuals susceptible to HAPE (HAPE-S) would also have dysfunction of the vascular NO vasodilator pathway during hypoxia in the systemic vasculature.

METHODS

During normoxia (FI(O(2)) = 0.21) and 4 hours of normobaric hypoxia (FI(O(2)) = 0.12, corresponding to an altitude of 4,500 m above sea level) endothelium-dependent and endothelium-independent vasodilator responses to intraarterial infusion of acetylcholine (ACh) and sodium nitroprusside, respectively, were measured by forearm venous occlusion plethysmography in nine HAPE-S subjects and in nine HAPE-resistant control subjects.

MAIN RESULTS

Pulmonary artery systolic pressure increased from 22 +/- 3 to 33 +/- 6 mm Hg (p < 0.001) during hypoxia in control subjects, and from 25 +/- 4 to 50 +/- 9 mm Hg in HAPE-S subjects (p < 0.001). Despite similar responses during normoxia in both groups, ACh-induced changes in forearm blood flow markedly decreased during hypoxia in HAPE-S subjects (p = 0.01) but not in control subjects. The attenuated vascular response to ACh infusion during hypoxia inversely correlated with increased pulmonary artery systolic pressure (p = 0.04) and decreased plasma nitrite correlated with attenuated ACh-induced vasodilation in HAPE-S subjects (p = 0.02).

CONCLUSIONS

Hypoxia markedly impairs vascular endothelial function in the systemic circulation in HAPE-S subjects due to a decreased bioavailability of NO. Impairment of the NO pathway could contribute to the enhanced hypoxic pulmonary vasoconstriction that is central to the pathogenesis of HAPE.

Pulmonary vascular iNOS induction participates in the onset of chronic hypoxic pulmonary hypertension

Pathogenesis of hypoxic pulmonary hypertension is initiated by oxidative injury to the pulmonary vascular wall. Because nitric oxide (NO) can contribute to oxidative stress and because the inducible isoform of NO synthase (iNOS) is often upregulated in association with tissue injury, we hypothesized that iNOS-derived NO participates in the pulmonary vascular wall injury at the onset of hypoxic pulmonary hypertension. An effective and selective dose of an iNOS inhibitor, L-N6-(1-iminoethyl)lysine (L-NIL), for chronic peroral treatment was first determined (8 mg/l in drinking water) by measuring exhaled NO concentration and systemic arterial pressure after LPS injection under ketamine+xylazine anesthesia. A separate batch of rats was then exposed to hypoxia (10% O2) and given L-NIL or a nonselective inhibitor of all NO synthases, N(G)-nitro-L-arginine methyl ester (L-NAME, 500 mg/l), in drinking water. Both inhibitors, applied just before and during 1-wk hypoxia, equally reduced pulmonary arterial pressure (PAP) measured under ketamine+xylazine anesthesia. If hypoxia continued for 2 more wk after L-NIL treatment was discontinued, PAP was still lower than in untreated hypoxic controls. Immunostaining of lung vessels showed negligible iNOS presence in control rats, striking iNOS expression after 4 days of hypoxia, and return of iNOS immunostaining toward normally low levels after 20 days of hypoxia. Lung NO production, measured as NO concentration in exhaled air, was markedly elevated as early as on the first day of hypoxia. We conclude that transient iNOS induction in the pulmonary vascular wall at the beginning of chronic hypoxia participates in the pathogenesis of pulmonary hypertension.

Decreased exhaled nitric oxide in pulmonary arterial hypertension: response to bosentan therapy

RATIONALE

Decreased nitric oxide (NO) is considered an important pathogenetic mechanism in pulmonary arterial hypertension (PAH), but clear evidence is lacking.

OBJECTIVES

We used multiple techniques to assess endogenous NO in 10 patients with untreated PAH (8 idiopathic and 2 anorexigen-associated PAH) and 12 control subjects.

METHODS

After a nitrite/nitrate-restricted diet, NO metabolites (NOx) were assayed in 24-hour urine collections and exhaled NO (FE(NO)) determined at multiple expiratory flows. Analysis of the relation between FE(NO) and flow allowed derivation of three flow-independent parameters: airway wall concentration (C(W)), diffusing capacity (D(NO)), and alveolar concentration (C(A)). Seven patients underwent follow-up testing after 3 months of bosentan treatment.

RESULTS

At baseline, FE(NO) was markedly decreased at the two lowest expiratory flows in PAH: 21 +/- 4 versus 36 +/- 4 ppb at 18 ml/second and 11 +/- 2 versus 17 +/- 2 ppb at 50 ml/second, for subjects with PAH and control subjects, respectively (p < 0.05). C(W) was 33 +/- 11 ppb in subjects with PAH versus 104 +/- 34 in control subjects (p = 0.04). Urinary NOx was also reduced in PAH (42 +/- 6 microM NOx/mM creatinine versus 62 +/- 7 in control subjects; p = 0.04). After bosentan, FE(NO), C(W), and urine NOx increased to control values (p < 0.05). Exclusion of the two anorexigen cases did not alter these results.

CONCLUSIONS

FE(NO) at low expiratory flows was decreased in PAH due to reduced C(W). Bosentan reversed these abnormalities, suggesting that suppression of NO in PAH may have been caused by endothelin.

NOS3 deficiency augments hypoxic pulmonary vasoconstriction and enhances systemic oxygenation during one-lung ventilation in mice

Nitric oxide (NO), synthesized by NO synthases (NOS), plays a pivotal role in regulation of pulmonary vascular tone. To examine the role of endothelial NOS (NOS3) in hypoxic pulmonary vasoconstriction (HPV), we measured left lung pulmonary vascular resistance (LPVR), intrapulmonary shunting, and arterial Po2 (PaO2) before and during left mainstem bronchus occlusion (LMBO) in mice with and without a deletion of the gene encoding NOS3. The increase of LPVR induced by LMBO was greater in NOS3-deficient mice than in wild-type mice (151 ± 39% vs. 109 ± 36%, mean ± SD; P < 0.05). NOS3-deficient mice had a lower intrapulmonary shunt fraction than wild-type mice (17.1 ± 3.6% vs. 21.7 ± 2.4%, P < 0.05) during LMBO. Both real-time PaO2 monitoring with an intra-arterial probe and arterial blood-gas analysis during LMBO showed higher PaO2 in NOS3-deficient mice than in wild-type mice ( P < 0.05). Inhibition of all three NOS isoforms with Nω-nitro-l-arginine methyl ester (l-NAME) augmented the increase of LPVR induced by LMBO in wild-type mice (183 ± 67% in l-NAME treated vs. 109 ± 36% in saline treated, P < 0.01) but not in NOS3-deficient mice. Similarly, systemic oxygenation during one-lung ventilation was augmented by l-NAME in wild-type mice but not in NOS3-deficient mice. These findings indicate that NO derived from NOS3 modulates HPV in vivo and that inhibition of NOS3 improves systemic oxygenation during acute unilateral lung hypoxia.

L-lysine decreases nitric oxide production and increases vascular resistance in lungs isolated from lipopolysaccharide-treated neonatal pigs

Nitric oxide (NO) production may depend on the uptake of L-arginine (L-arg), the substrate for NO synthase in inflammatory lung diseases. The cellular transport of L-arg occurs via the cationic amino acid transporters (CAT), and L-lysine (L-lys) competitively inhibits CAT. Neonatal pigs were treated with lipopolysaccharide (LPS) or vehicle for 4 h. LPS increased exhaled NO (exNO; 0.026 +/- 0.003 to 0.046 +/- 0.003 nmol. kg(-1). min(-1); p < 0.005) and decreased mean systemic arterial blood pressure (89 +/- 4 to 67 +/- 4 mm Hg; p < 0.05), whereas vehicle did not affect exNO or mean systemic arterial blood pressure. The lungs were then isolated and perfused; exNO was greater in lungs from LPS-treated animals (0.08 +/- 0.01 nmol/kg/min) than in lungs from vehicle-treated animals (0.05 +/- 0.01 nmol. kg(-1). min(-1); p < 0.05). The addition of L-arg (0.3 mM) significantly (p < 0.05) increased exNO production in both groups of lungs (mean increase 0.04 +/- 0.01 nmol. kg(-1). min(-1) LPS-treated lungs, p < 0.05; mean increase 0.02 +/- 0.01 nmol. kg(-1). min(-1) vehicle-treated lungs); however, L-arg decreased pulmonary vascular resistance (PVR) only in LPS-treated lungs (mean decrease 0.03 +/- 0.01 mm Hg. ml(-1). kg(-1). min(-1), p < 0.05). L-lys caused a dose-dependent decrease in exNO production and a dose-dependent increase in PVR in LPS-treated lungs. L-lys decreased exNO only at 30 mM and had no effect on PVR in vehicle-treated lungs. In four lungs each from vehicle- and LPS-treated animals, reverse transcriptase-PCR demonstrated CAT-2 mRNA only in LPS-treated animals. These results suggest that the increased NO production in the lungs from LPS-treated animals depends on the uptake of vascular L-arg.

A bronchial epithelium-derived factor reduces pulmonary vascular tone in the newborn rat

The factors accounting for the maintenance of a low pulmonary vascular resistance postnatally are not completely understood. The aim of this study was to test the hypothesis that bronchial epithelium produces a factor capable of relaxing adjacent pulmonary arterial smooth muscle. We studied fourth-generation intralobar pulmonary arteries and bronchi of 4- to 8-day-old rats. Arteries were mounted on a wire myograph, alone or with the adjacent bronchus. The presence of the attached bronchus significantly reduced pulmonary artery force generation induced by the thromboxane analog (U-46619) or KCl whether the endothelium was present or absent ( P < 0.01). The converse was not true in that bronchial force generation was not affected when studied with the adjacent pulmonary artery. Mechanical removal of the bronchial epithelium or addition of the nitric oxide (NO) synthase (NOS) nonspecific ( NG-monomethyl-l-arginine) or the specific neuronal NOS (7-nitroindazole) inhibitors increased arterial force generation to levels comparable to the isolated artery preparation. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, significantly decreased ( P < 0.01) NO release of pulmonary arteries only when the adjacent bronchus was present. We conclude that bronchial epithelium in the newborn rat produces a factor capable of lowering pulmonary vascular muscle tone. This relaxant effect can be suppressed by NOS and phosphatidylinositol 3-kinase kinase inhibition, suggesting an action via NOS phosphorylation and NO release. We speculate that such a mechanism may be operative in vivo and plays an important role in control of pulmonary vascular resistance in the early postnatal period.

Nitric oxide scavenging by hemoglobin regulates hypoxic pulmonary vasoconstriction

Although the importance of red blood cells in augmenting hypoxic pulmonary vasoconstriction has been recognized for decades, only recently has it become clear that this occurs primarily because of the inactivation of nitric oxide (NO) by hemoglobin. This interaction between red blood cells, NO, and the pulmonary circulation is critical in understanding the effects of anemia and polycythemia on pulmonary blood flow distribution, gas exchange, and global O2 delivery and in understanding the development of hemoglobin-based oxygen carriers. This review will discuss the proposed mechanisms for initiation of hypoxic pulmonary vasoconstriction and regulation of hypoxic pulmonary vasoconstriction by red blood cells with an emphasis on hemoglobin-NO interactions. In addition, the review will discuss how biologic (S-nitrosation) or pharmacologic (cross-linking) modification of hemoglobin may affect pulmonary circulatory-hemoglobin interactions.

Dexamethasone suppresses iNOS yet induces GTPCH and CAT-2 mRNA expression in rat lungs

The in vivo mechanisms by which glucocorticoids inhibit nitric oxide expression await detailed investigation. In cell culture experiments, glucocorticoids have been shown to inhibit inducible nitric oxide synthase (iNOS) formation and activity. Glucocorticoids can inhibit iNOS activity in cultured cells by blocking arginine transport and inhibiting tetrahydrobiopterin biosynthesis. We recently reported that changes in intrapulmonary formation of nitric oxide in endotoxemic rats correspond with changes in transcription of the predominant arginine transporter cationic amino acid transporter (CAT)-2. Realizing that hemorrhagic shock induces nitric oxide overproduction in intact animals, we sought to explore whether glucocorticoids attenuate hemorrhagic shock-induced increases in intrapulmonary nitric oxide formation and whether they might do so by inhibiting the formation of tetrahydrobiopterin, iNOS protein, and CAT-2. We randomly assigned 10 male Sprague-Dawley rats to receive dexamethasone or normal saline. Bleeding the animals to a mean systemic blood pressure of between 40 and 45 mmHg created the hemorrhagic shock. Dexamethasone abrogated the increase in exhaled nitric oxide concentrations caused by hemorrhagic shock. At the end of the experiment, plasma nitrate/nitrite values were lower in the dexamethasone group than in the control group. The iNOS protein concentrations were also lower in the dexamethasone group than in the control group. Dexamethasone decreased the intrapulmonary iNOS mRNA concentrations yet increased both guanosine triphosphate cyclohydrolase I mRNA and CAT-2 mRNA. Our results support the idea that dexamethasone inhibits nitric oxide formation in a manner that is independent of tetrahydrobiopterin and arginine transport yet dependent on downregulation of iNOS mRNA expression.