N-acetylcysteine inhibits hypoxic pulmonary hypertension most effectively in the initial phase of chronic hypoxia

Mitochondrial Regulation of the Hypoxia-Inducible Factor in the Development of Pulmonary Hypertension

Pulmonary hypertension (PH) is a severe progressive lung disorder characterized by pulmonary vasoconstriction and vascular remodeling, culminating in right-sided heart failure and increased mortality. Data from animal models and human subjects demonstrated that hypoxia-inducible factor (HIF)-related signaling is essential in the progression of PH. This review summarizes the regulatory pathways and mechanisms of HIF-mediated signaling, emphasizing the role of mitochondria in HIF regulation and PH pathogenesis. We also try to determine the potential to therapeutically target the components of the HIF system for the management of PH.

Inhalants

Toxic inhalants include various xenobiotics. Irritants cause upper and lower respiratory tract injuries. Highly water-soluble agents injure the upper respiratory tract, while low water-soluble inhalants injure the lower track. Asphyxiants are divided into simple asphyxiants and chemical asphyxiants. Simple asphyxiants displace oxygen, causing hypoxia, while chemical asphyxiants also impair the body's ability to use oxygen. Cyanide is a classic chemical asphyxiant. Treatment includes hydroxocobalamin. Electronic cigarette or vaping use-associated lung injury (EVALI) is a relatively new illness. Patients present with respiratory symptoms and gastrointestinal distress. EVALI appears to be associated with vaping cannabinoids. Treatment is supportive and may include steroids.

Endothelial-to-Mesenchymal Transition in Pulmonary Arterial Hypertension

Endothelial-to-mesenchymal transition (EndMT) is a process that encompasses extensive transcriptional reprogramming of activated endothelial cells leading to a shift toward mesenchymal cellular phenotypes and functional responses. Initially observed in the context of embryonic development, in the last few decades EndMT is increasingly recognized as a process that contributes to a variety of pathologies in the adult organism. Within the settings of cardiovascular biology, EndMT plays a role in various diseases, including atherosclerosis, heart valvular disease, cardiac fibrosis, and myocardial infarction. EndMT is also being progressively implicated in development and progression of pulmonary hypertension (PH) and pulmonary arterial hypertension (PAH). This review covers the current knowledge about EndMT in PH and PAH, and provides comprehensive overview of seminal discoveries. Topics covered include evidence linking EndMT to factors associated with PAH development, including hypoxia responses, inflammation, dysregulation of bone-morphogenetic protein receptor 2 (BMPR2), and redox signaling. This review amalgamates these discoveries into potential insights for the identification of underlying mechanisms driving EndMT in PH and PAH, and discusses future directions for EndMT-based therapeutic strategies in disease management.

The effect of exposure to hypoxia on superoxide formation by alveolar macrophages is indirect

AIMS

To elucidate the role of alveolar macrophages (AM) in the pathogenesis of hypoxic pulmonary hypertension (HPH), we tested the effects of sustained hypoxia on AM polarization and on the formation of superoxide by AM in vivo and in vitro.

MAIN METHODS

Rat AM were obtained by bronchoalveolar lavage. 4-day exposure to hypoxia (10% O2) was carried out in vivo (rats in isobaric hypoxic chamber, controls kept in air) or in vitro (control AM in 21% O2 and 5% CO2). Superoxide production was measured by luminol-orthovanadate chemiluminescence, AM polarization was detected immunocytochemically. To ascertain the effect of substances contained in the alveolar environment, we cultivated cells also in the presence of non-cellular components of the bronchoalveolar lavage fluid (BALF) either from controls or from rats exposed to 4 days of hypoxia.

KEY FINDINGS

In vivo, but not in vitro, hypoxia increased AM superoxide production. Both types of hypoxia polarized AM into M2 (pro-proliferative) type. While the presence of control BALF attenuated superoxide production in AM cultivated in normoxia, BALF from the hypoxia-exposed rats had no effect. In AM cultivated in hypoxia, superoxide production was not altered by control BALF and elevated by BALF obtained from hypoxic rats.

SIGNIFICANCE

Hypoxia does not influence superoxide production by AM directly but rather by modulating their milieu and their sensitivity to external influences.

Impact of the mitochondria-targeted antioxidant MitoQ on hypoxia-induced pulmonary hypertension

Increased mitochondrial reactive oxygen species (ROS), particularly superoxide have been suggested to mediate hypoxic pulmonary vasoconstriction (HPV), chronic hypoxia-induced pulmonary hypertension (PH) and right ventricular (RV) remodelling.We determined ROS in acute, chronic hypoxia and investigated the effect of the mitochondria-targeted antioxidant MitoQ under these conditions.The effect of MitoQ or its inactive carrier substance, decyltriphenylphosphonium (TPP+), on acute HPV (1% O2 for 10 minutes) was investigated in isolated blood-free perfused mouse lungs. Mice exposed for 4 weeks to chronic hypoxia (10% O2) or after banding of the main pulmonary artery (PAB) were treated with MitoQ or TPP+ (50 mg/kg/day).Total cellular superoxide and mitochondrial ROS levels were increased in pulmonary artery smooth muscle cells (PASMC), but decreased in pulmonary fibroblasts in acute hypoxia. MitoQ significantly inhibited HPV and acute hypoxia-induced rise in superoxide concentration. ROS was decreased in PASMC, while it increased in the RV after chronic hypoxia. Correspondingly, MitoQ did not affect the development of chronic hypoxia-induced PH, but attenuated RV remodelling after chronic hypoxia as well as after PAB.Increased mitochondrial ROS of PASMC mediate acute HPV, but not chronic hypoxia-induced PH. MitoQ may be beneficial under conditions of exaggerated acute HPV.

Pregestational diabetes increases fetoplacental vascular resistance in rats

INTRODUCTION

Diabetes is a well-known risk factor in pregnancy. Because maternal diabetes involves oxidative stress that is also induced by chronic hypoxia and can alter vascular function, we sought to determine the effects of chronic maternal hyperglycemia on the fetoplacental vasculature in rats and to compare it with the effects of chronic hypoxia.

METHODS

Diabetes was induced in female rats by a streptozotocin injection at a neonatal age. When these animals reached adulthood, their hyperglycemia was confirmed and they were inseminated. Half of them were exposed to hypoxia (10% O₂) for the last week before the delivery. One day before the expected date of delivery, one of their placentae was isolated and perfused.

RESULTS

Fetoplacental vascular resistance was increased equally by experimental diabetes, chronic hypoxia, and their combination. Fetoplacental perfusion pressure-flow analysis suggested increased resistance in the small vessels in chronic hypoxia and in larger vessels in diabetes. Fetal plasma nitrotyrosine levels, measured as a marker of peroxynitrite (reaction product of superoxide and nitric oxide), mirrored the differences in fetoplacental resistance, suggesting a causative role. Fetoplacental vasoconstrictor reactivity to acute hypoxic stimuli was reduced similarly in all groups. Fasudil, a strong vasodilator agent, reduced fetoplacental vascular resistance similarly in all groups, suggesting that for the observed differences among the groups, the changes in vascular morphology were more important than variances in vascular tone.

DISCUSSION

Maternal diabetes increases fetoplacental vascular resistance to a similar extent as chronic hypoxia. These stimuli are not additive. Changes in vascular tone are not responsible for these effects.

Four-day pulse of sodium cromoglycate modulates pulmonary vessel wall remodeling during 21-day hypoxia in rats

AIM OF THE STUDY

Remodeling of pulmonary resistance arteries in rats due to 4-day hypoxia could be successfully suppressed by sodium cromoglycate. In this study, we tested the difference in the suppression between two distinct time patterns of cromoglycate administration during 21-day hypoxia. In the experiment, we focused on some details in both smooth muscle cells and extracellular matrix of pulmonary arterial walls.

METHODS

During 21-day hypoxia, rats were treated with sodium cromoglycate either in the first four days or in the last four days. The first four days were chosen to test efficiency of an initial pulse of cromoglycate to suppress pulmonary vascular remodeling. The last four-day administration tested possibility to block remodeling post hoc.

RESULTS

Initial pulse reduced and modified remodeling in all levels of pulmonary arteries, which comprises neomuscularization of prealveolar arteries, asymmetrical hypertrophy of tunica media in muscular pulmonary arteries and hypertrophy of tunica media and tunica adventitia in large conduit arteries. Terminal pulse had only negligible effect.

CONCLUSIONS

Only the initial cromoglycate therapy led to significant morphological suppression of remodeling. We therefore assume important role of initial remodeling influencing during long time hypoxia experiment.

Nitric oxide elevation in polytrauma is driven by oxygen radicals

A common problem in management of polytrauma - a simultaneous injury to more than one organ or organ system, at least one of them lethal without intervention - is a discrepancy between a relatively good initial state and a serious subsequent development. Since nitric oxide (NO) is produced in high quantities during tissue injury, we assumed that serum levels of NO (and its oxidation products, NOx) might serve as a prognostic marker of polytrauma severity. However, we found recently that NOx was increased in polytrauma, but not in the most severe cases. The present study was undertaken to test the hypothesis that serum NOx is reduced in severe polytrauma by concomitant overproduction of reactive oxygen species (ROS). Polytrauma was induced in rats under anesthesia by bilateral fracture of femurs and tibiae plus incision of the right liver lobe through laparotomy. Serum NOx was measured by chemiluminescence after hot acidic reduction. The role of ROS was assessed by treatment with an antioxidant, N-acetyl-L-cysteine (NAC). Experimental polytrauma elevated NOx from 11.0+/-0.7 to 23.8+/-4.5 ppb. This was completely prevented by NAC treatment (9.1+/-2.2 ppb). Serum NOx is elevated in severe polytrauma, and this is not reduced by ROS. On the contrary, ROS are necessary for the NOx elevation, probably because ROS produced by inflammatory cells activated by the polytrauma induce massive NO production.

Constitutive Reprogramming of Fibroblast Mitochondrial Metabolism in Pulmonary Hypertension

Remodeling of the distal pulmonary artery wall is a characteristic feature of pulmonary hypertension (PH). In hypoxic PH, the most substantial pathologic changes occur in the adventitia. Here, there is marked fibroblast proliferation and profound macrophage accumulation. These PH fibroblasts (PH-Fibs) maintain a hyperproliferative, apoptotic-resistant, and proinflammatory phenotype in ex vivo culture. Considering that a similar phenotype is observed in cancer cells, where it has been associated, at least in part, with specific alterations in mitochondrial metabolism, we sought to define the state of mitochondrial metabolism in PH-Fibs. In PH-Fibs, pyruvate dehydrogenase was markedly inhibited, resulting in metabolism of pyruvate to lactate, thus consistent with a Warburg-like phenotype. In addition, mitochondrial bioenergetics were suppressed and mitochondrial fragmentation was increased in PH-Fibs. Most importantly, complex I activity was substantially decreased, which was associated with down-regulation of the accessory subunit nicotinamide adenine dinucleotide reduced dehydrogenase (ubiquinone) Fe-S protein 4 (NDUFS4). Owing to less-efficient ATP synthesis, mitochondria were hyperpolarized and mitochondrial superoxide production was increased. This pro-oxidative status was further augmented by simultaneous induction of cytosolic nicotinamide adenine dinucleotide phosphate reduced oxidase 4. Although acute and chronic exposure to hypoxia of adventitial fibroblasts from healthy control vessels induced increased glycolysis, it did not induce complex I deficiency as observed in PH-Fibs. This suggests that hypoxia alone is insufficient to induce NDUFS4 down-regulation and constitutive abnormalities in complex I. In conclusion, our study provides evidence that, in the pathogenesis of vascular remodeling in PH, alterations in fibroblast mitochondrial metabolism drive distinct changes in cellular behavior, which potentially occur independently of hypoxia.

Contribution of reactive oxygen species to the pathogenesis of pulmonary arterial hypertension

Pulmonary arterial hypertension is associated with a decreased antioxidant capacity. However, neither the contribution of reactive oxygen species to pulmonary vasoconstrictor sensitivity, nor the therapeutic efficacy of antioxidant strategies in this setting are known. We hypothesized that reactive oxygen species play a central role in mediating both vasoconstrictor and arterial remodeling components of severe pulmonary arterial hypertension. We examined the effect of the chemical antioxidant, TEMPOL, on right ventricular systolic pressure, vascular remodeling, and enhanced vasoconstrictor reactivity in both chronic hypoxia and hypoxia/SU5416 rat models of pulmonary hypertension. SU5416 is a vascular endothelial growth factor receptor antagonist and the combination of chronic hypoxia/SU5416 produces a model of severe pulmonary arterial hypertension with vascular plexiform lesions/fibrosis that is not present with chronic hypoxia alone. The major findings from this study are: 1) compared to hypoxia alone, hypoxia/SU5416 exposure caused more severe pulmonary hypertension, right ventricular hypertrophy, adventitial lesion formation, and greater vasoconstrictor sensitivity through a superoxide and Rho kinase-dependent Ca2+ sensitization mechanism. 2) Chronic hypoxia increased medial muscularization and superoxide levels, however there was no effect of SU5416 to augment these responses. 3) Treatment with TEMPOL decreased right ventricular systolic pressure in both hypoxia and hypoxia/SU5416 groups. 4) This effect of TEMPOL was associated with normalization of vasoconstrictor responses, but not arterial remodeling. Rather, medial hypertrophy and adventitial fibrotic lesion formation were more pronounced following chronic TEMPOL treatment in hypoxia/SU5416 rats. Our findings support a major role for reactive oxygen species in mediating enhanced vasoconstrictor reactivity and pulmonary hypertension in both chronic hypoxia and hypoxia/SU5416 rat models, despite a paradoxical effect of antioxidant therapy to exacerbate arterial remodeling in animals with severe pulmonary arterial hypertension in the hypoxia/SU5416 model.

Thin Air Resulting in High Pressure: Mountain Sickness and Hypoxia-Induced Pulmonary Hypertension

With rising altitude the partial pressure of oxygen falls. This phenomenon leads to hypobaric hypoxia at high altitude. Since more than 140 million people permanently live at heights above 2500 m and more than 35 million travel to these heights each year, understanding the mechanisms resulting in acute or chronic maladaptation of the human body to these circumstances is crucial. This review summarizes current knowledge of the body's acute response to these circumstances, possible complications and their treatment, and health care issues resulting from long-term exposure to high altitude. It furthermore describes the characteristic mechanisms of adaptation to life in hypobaric hypoxia expressed by the three major ethnic groups permanently dwelling at high altitude. We additionally summarize current knowledge regarding possible treatment options for hypoxia-induced pulmonary hypertension by reviewing in vitro, rodent, and human studies in this area of research.

(1)H NMR-Based Analysis of Serum Metabolites in Monocrotaline-Induced Pulmonary Arterial Hypertensive Rats

AIMS

To study the changes of the metabolic profile during the pathogenesis in monocrotaline (MCT) induced pulmonary arterial hypertension (PAH).

METHODS

Forty male Sprague-Dawley (SD) rats were randomly divided into 5 groups (n = 8, each). PAH rats were induced by a single dose intraperitoneal injection of 60 mg/kg MCT, while 8 rats given intraperitoneal injection of 1 ml normal saline and scarified in the same day (W0) served as control. Mean pulmonary arterial pressure (mPAP) was measured through catherization. The degree of right ventricular hypertrophy and pulmonary hyperplasia were determined at the end of first to fourth weeks; nuclear magnetic resonance (NMR) spectra of sera were then acquired for the analysis of metabolites. Principal component analysis (PCA) and orthogonal partial least-squares discriminant analysis (OPLS-DA) were used to discriminate different metabolic profiles.

RESULTS

The prominent changes of metabolic profiles were seen during these four weeks. Twenty specific metabolites were identified, which were mainly involved in lipid metabolism, glycolysis, energy metabolism, ketogenesis, and methionine metabolism. Profiles of correlation between these metabolites in each stage changed markedly, especially in the fourth week. Highly activated methionine and betaine metabolism pathways were selected by the pathway enrichment analysis.

CONCLUSIONS

Metabolic dysfunction is involved in the development and progression of PAH.

Obesity-related pulmonary arterial hypertension in rats correlates with increased circulating inflammatory cytokines and lipids and with oxidant damage in the arterial wall but not with hypoxia

Obesity is causally linked to a number of comorbidities, including cardiovascular disease, diabetes, renal dysfunction, and cancer. Obesity has also been linked to pulmonary disorders, including pulmonary arterial hypertension (PAH). It was long believed that obesity-related PAH was the result of hypoventilation and hypoxia due to the increased mechanical load of excess body fat. However, in recent years it has been proposed that the metabolic and inflammatory disturbances of obesity may also play a role in the development of PAH. To determine whether PAH develops in obese rats in the absence of hypoxia, we assessed pulmonary hemodynamics and pulmonary artery (PA) structure in the diet-resistant/diet-induced obesity (DR/DIO) and Zucker lean/fatty rat models. We found that high-fat feeding (DR/DIO) or overfeeding (Zucker) elicited PA remodeling, neomuscularization of distal arterioles, and elevated PA pressure, accompanied by right ventricular (RV) hypertrophy. PA thickening and distal neomuscularization were also observed in DIO rats on a low-fat diet. No evidence of hypoventilation or chronic hypoxia was detected in either model, nor was there a correlation between blood glucose or insulin levels and PAH. However, circulating inflammatory cytokine levels were increased with high-fat feeding or calorie overload, and hyperlipidemia and oxidant damage in the PA wall correlated with PAH in the DR/DIO model. We conclude that hyperlipidemia and peripheral inflammation correlate with the development of PAH in obese subjects. Obesity-related inflammation may predispose to PAH even in the absence of hypoxia.

Reactive oxygen species and antioxidants in pulmonary hypertension

SIGNIFICANCE

Pulmonary hypertension is a devastating disorder without any available treatment strategies that satisfactorily promote the survival of patients. The identification of new therapeutic strategies to treat patients with pulmonary hypertension is warranted.

RECENT ADVANCES

Human studies have provided evidence that there is increased oxidative stress (lipid peroxidation, protein oxidation, DNA oxidation, and the depletion of small-molecule antioxidants) in patients with pulmonary hypertension. A variety of compounds with antioxidant properties have been shown to have beneficial therapeutic effects in animal models of pulmonary hypertension, possibly supporting the hypothesis that reactive oxygen species (ROS) are involved in the progression of pulmonary hypertension. Thus, understanding the molecular mechanisms of ROS actions could contribute to the development of optimal, antioxidant-based therapy for human pulmonary hypertension. One such mechanism includes action as a second messenger during cell-signaling events, leading to the growth of pulmonary vascular cells and right ventricular cells.

CRITICAL ISSUES

The molecular mechanisms behind promotion of cell signaling for pulmonary vascular cell growth and right ventricular hypertrophy by ROS are not well understood. Evidence suggests that iron-catalyzed protein carbonylation may be involved.

FUTURE DIRECTIONS

Understanding precise mechanisms of ROS actions should be useful for designing preclinical animal experiments and human clinical trials of the use of antioxidants and/or other redox compounds in the treatment of pulmonary hypertension.

Pyrrolidine dithiocarbamate attenuates the development of monocrotaline-induced pulmonary arterial hypertension

We aimed to demonstrate the potential protective effects of pyrrolidine dithiocarbamate (PDTC) on monocrotaline (MCT)-induced pulmonary arterial hypertension (PAH). Adult male rats were randomly assigned to 4 groups: control group, MCT-treated rats only, MCT-injected rats treated with PDTC, and PDTC-treated rats only. Blood and tissue samples were collected after the sacrifice. Levels of malondialdehyde (MDA) were measured by using the thiobarbituric acid method. Total antioxidant status (TAS) was determined using a commercially available ImAnOx kit. A histopathological evaluation was accomplished by scoring the degree of severity. Endothelial damage of the main pulmonary artery was evaluated by immunohistochemical labeling of endothelial cells using anti-rat endothelial cell antigen 1 (RECA-1) antibody. MCT-induced right ventricular hypertrophy (RVH) was reduced significantly in the MCT+PDTC-treated group. MDA levels were significantly lowered in the MCT+PDTC-treated group. TAS was significantly higher in the MCT+PDTC-treated group when compared with the rats with PAH. Histopathological examination demonstrated that PDTC treatment reduced the development of inflammation, hemorrhage and congestion, and collagen deposition. In conclusion, PDTC attenuated PAH and protected pulmonary endothelium in rats administered MCT. These findings suggest that PDTC treatment may provide a new effective therapeutic approach in the treatment of PAH.

Chronic hypoxia inhibits tetrahydrobiopterin-induced NO production in rat lungs

Tetrahydrobiopterin (BH4) is an essential cofactor for nitric oxide synthases (NOS). Oxidative stress oxidises BH4 to dihydrobioptein (BH2), resulting in the uncoupling of the two enzymatic domains of NOS and the production of superoxide rather than NO (NOS uncoupling). Oxidative stress is known to be increased in the early stage of chronic hypoxia. This study investigated the participation of NOS uncoupling in the early phase of hypoxia-induced pulmonary hypertension in rats. Rats were exposed to 10% O(2) for 4 days. We investigated the effect of BH4 in vitro on isolated rat lungs and isolated rat peripheral pulmonary blood vessels and in vivo on exhaled NO concentration in exhaled air. BH4 attenuated hypoxic pulmonary vasoconstriction in isolated lungs and its effect was reversed by l-NAME (NOS inhibitor). The main finding of the study is that the effect of BH4 was smaller in rats exposed to 4 days of hypoxia than in normoxic controls. The finding was similar in isolated pulmonary blood vessels. BH4 increased exhaled NO in both normoxic and hypoxic rats. This increase was blunted by l-NIL (specific iNOS inhibitor) and therefore attributable to iNOS. We conclude that BH4 increased NO production in both normoxic and hypoxic rats. The increase was, however, smaller in hypoxic lungs than in controls. We assume that the smaller increase in NO production in hypoxic lungs is due to the decreased BH4/BH2 ratio in chronic hypoxia and NOS uncoupling resulting from this condition.

HIV-1, reactive oxygen species, and vascular complications

Over 1 million people in the United States and 33 million individuals worldwide suffer from HIV/AIDS. Since its discovery, HIV/AIDS has been associated with an increased susceptibility to opportunistic infection due to immune dysfunction. Highly active antiretroviral therapies restore immune function and, as a result, people infected with HIV-1 are living longer. This improved survival of HIV-1 patients has revealed a previously unrecognized risk of developing vascular complications, such as atherosclerosis and pulmonary hypertension. The mechanisms underlying these HIV-associated vascular disorders are poorly understood. However, HIV-induced elevations in reactive oxygen species (ROS), including superoxide and hydrogen peroxide, may contribute to vascular disease development and progression by altering cell function and redox-sensitive signaling pathways. In this review, we summarize the clinical and experimental evidence demonstrating HIV- and HIV antiretroviral therapy-induced alterations in reactive oxygen species and how these effects are likely to contribute to vascular dysfunction and disease.

Reactive Oxygen Species Production in the Early and Later Stage of Chronic Ventilatory Hypoxia

Pulmonary hypertension resulting from chronic hypoxia is at least partly caused by the increased production of reactive oxygen species (ROS). The goal of the presented study was to investigate the dynamics and the site of production of ROS during chronic hypoxia. In our study Wistar rats were kept for 1, 4 and 21 days in an isobaric hypoxic chamber (FiO2=0.1), while controls stayed in normoxia. We compared NO production in expired air, plasma and perfusate drained from isolated rat lungs and measured superoxide concentration in the perfusate. We also detected the presence of superoxide products (hydrogen peroxide and peroxynitrite) and the level of ROS-induced damage expressed as the concentration of lipid peroxydation end products. We found that the production and release of ROS and NO during early phase of chronic hypoxia has specific timing and differs in various compartments, suggesting the crucial role of ROS interaction for development of hypoxic pulmonary hypertension.

Tissue localization of nanoparticles is altered due to hypoxia resulting in poor efficacy of curcumin nanoparticles in pulmonary hypertension

The present study is an attempt to leverage therapeutic benefits of curcumin in pulmonary hypertension by encapsulating it in biodegradable poly(lactide-co-glycolic) acid nanoparticles. Pulmonary hypertension is induced in experimental animals by subjecting them to chronic hypoxic conditions. The ability of curcumin encapsulated nanoparticles to manage pulmonary hypertension is measured by right ventricular hypertrophy, haematocrit, vascular remodelling and target tissue levels of curcumin. Further, single oral dose tissue distribution of the nanoparticulate curcumin was also assessed under normoxic and hypoxic conditions. Orally administered nanoparticulate curcumin failed to offer any protection against hypoxia induced pulmonary hypertension as indicated by insignificant changes in right ventricular hypertrophy and vascular remodelling that are similar to untreated groups. A significant difference in the target tissue levels was observed between normoxic vs. hypoxic rats. The study suggests that hypoxia has a major role in the particle localization in lungs probably due to the altered blood flow, increased barrier properties of the lung vasculature and decreased endocytosis. The target tissue levels of curcumin under hypoxia are much lower to that achieved in normoxic rats probably due to difference in particle dynamics, resulting in the failure of treatment.

Lung mast cells and hypoxic pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) is a syndrome characterized by the increase of pulmonary vascular tone and the structural remodeling of peripheral pulmonary arteries. Mast cells have an important role in many inflammatory diseases and they are also involved in tissue remodeling. Tissue hypoxia is associated with mast cell activation and the release of proteolytic enzymes, angiogenic and growth factors which mediate tissue destruction and remodeling in a variety of physiological and pathological conditions. Here we focused on the role of mast cells in the pathogenesis of hypoxic pulmonary hypertension from the past to the present.

Disodium cromoglycate attenuates hypoxia induced enlargement of end-expiratory lung volume in rats

Mechanism responsible for the enlargement of end-expiratory lung volume (EELV) induced by chronic hypoxia remains unclear. The fact that the increase in EELV persists after return to normoxia suggests involvement of morphological changes. Because hypoxia has been also shown to activate lung mast cells, we speculated that they could play in the mechanism increasing EELV similar role as in vessel remodeling in hypoxic pulmonary hypertension (HPH). We, therefore, tested an effect of mast cells degranulation blocker disodium cromoglycate (DSCG) on hypoxia induced EELV enlargement. Ventilatory parameters, EELV and right to left heart weight ratio (RV/LV+S) were measured in male Wistar rats. The experimental group (H+DSCG) was exposed to 3 weeks of normobaric hypoxia and treated with DSCG during the first four days of hypoxia, control group was exposed to hypoxia only (H), two others were kept in normoxia as non-treated (N) and treated (N+DSCG) groups. DSCG treatment significantly attenuated the EELV enlargement (H+DSCG = 6.1+/-0.8; H = 9.2+/-0.9; ml +/-SE) together with the increase in minute ventilation (H + DSCG = 190+/-8; H = 273 +/- 10; ml/min +/- SE) and RV/LV + S (H + DSCG = 0.39 +/- 0.03; H = 0.50 +/- 0.06).

Reduction of reactive oxygen species prevents hypoxia-induced CREB depletion in pulmonary artery smooth muscle cells

Hypoxia-induced pulmonary arterial hypertension (PAH) is a deadly disease characterized by progressive remodeling and persistent vasoconstriction of the pulmonary arterial system. Remodeling of the pulmonary artery (PA) involves smooth muscle cell (SMC) proliferation, hypertrophy, migration, and elevated extracellular matrix (ECM) production elicited by mitogens and oxidants produced in response to hypoxic insult. We previously reported that the transcription factor cAMP response element binding protein (CREB) is depleted in medial PA SMCs in remodeled, hypertensive vessels in rats or calves exposed to chronic hypoxia. In culture, CREB loss can be induced in PA SMCs by exogenous oxidants or platelet-derived growth factor. Forced depletion of CREB with small interfering RNA (siRNA) in PA SMCs is sufficient to induce their proliferation, hypertrophy, migration, dedifferentiation, and ECM production. This suggests that oxidant and/or mitogen-induced loss of CREB in medial SMCs is, in part, responsible for PA thickening. Here, we tested whether oxidant scavengers could prevent the loss of CREB in PA SMCs and inhibit SMC proliferation, migration, and ECM production using in vitro and in vivo models. Exposure of PA SMCs to hypoxia induced hydrogen peroxide (H2O2) production and loss of CREB. Treatment of SMCs with exogenous H2O2 or a second oxidant, Sin-1, elicited CREB depletion under normoxic conditions. Exogenous H2O2 also induced SMC proliferation, migration, and increased elastin levels as did forced depletion of CREB. In vivo, hypoxia-induced thickening of the PA wall was suppressed by the superoxide dismutase mimetic, Tempol, which also prevented the loss of CREB in medial SMCs. Tempol also reduced hypoxia-induced SMC proliferation and elastin deposition in the PA. The data indicate that CREB levels in the arterial wall are regulated in part by oxidants produced in response to hypoxia and that CREB plays a crucial role in regulating SMC phenotype and PA remodeling.

Gene expression profiling of sex differences in HIF1-dependent adaptive cardiac responses to chronic hypoxia

Although physiological responses to chronic hypoxia, including pulmonary hypertension and right ventricular hypertrophy, have been well described, the molecular mechanisms involved in cardiopulmonary adaptations are still not fully understood. We hypothesize that adaptive responses to chronic hypoxia are the result of altered transcriptional regulations in the right and left ventricles. Here we report results from the gene expression profiling of adaptive responses in a chronically hypoxic heart. Of 11 analyzed candidate genes, the expression of seven and four genes, respectively, was significantly altered in the right ventricle of hypoxic male and female mice. In the transcriptional profile of the left ventricle, we identified a single expression change in hypoxic males (Vegfa gene). To directly test the role of HIF1, we analyzed the expression profile in Hif1a partially deficient mice exposed to moderate hypoxia. Our data showed that Hif1a partial deficiency significantly altered transcriptional profiles of analyzed genes in hypoxic hearts. The expression changes were only detected in two genes in the right ventricle of Hif1a(+/-) males and in one gene in the right ventricle of Hif1a(+/-) females. First, our results suggest that hypoxia mainly affects adaptive expression profiles in the right ventricle and that each ventricle can respond independently. Second, our findings indicate that HIF1a plays an important role in adaptive cardiopulmonary responses and the dysfunction of HIF1 pathways considerably affects transcriptional regulation in the heart. Third, our data reveal significant differences between males and females in cardiac adaptive responses to hypoxia and indicate the necessity of optimizing diagnostic and therapeutic procedures in clinical practice, with respect to sex.

Direct effect of cigarette smoke on human pulmonary artery tension

The effect of chronic cigarette smoke on pulmonary artery (PA) tension has been studied extensively; nevertheless, the direct effect of cigarette smoke is poorly understood. We investigated the direct effect of cigarette smoke extract (CSE) on PA tension in non-smokers, smokers, and COPD patients in vitro. PA samples from 35 patients who underwent lung resection were examined by measuring isometric tension in response to increasing serotonin concentrations. CSE dose dependently inhibited the response to serotonin in smokers and COPD patients, and to a lesser extent in non-smokers. CSE-induced relaxation was similarly inhibited by the nonspecific nitric oxide synthase (NOS) inhibitor l-NOARG and the specific inducible NOS (iNOS) inhibitor l-NIL, mainly in non-smokers and smokers, and to a lesser extent in COPD patients. Immunostaining of iNOS in PA samples was greater for smokers and COPD patients compared with non-smokers, which explains the lesser effect of CSE on PA tension in non-smokers. Moreover, CSE induced the release of nitrite via iNOS in human PA smooth muscle cells. In conclusion, CSE inhibition of serotonin-induced PA contraction was mediated mainly by iNOS in non-smokers, smokers, and COPD patients, but in different ways, which may be explained by differential iNOS expression in the PA of these patients.

Distribution of kappa-opioid receptor in the pulmonary artery and its changes during hypoxia

The present study evaluated the distribution of kappa-opioid receptors (kappa-ORs) in pulmonary arteries (PAs) in rats and investigated whether kappa-ORs are altered in PAs during hypoxia. An animal model of hypobaric/hypoxic pulmonary hypertension and a pulmonary artery smooth muscle cell (PASMC) model of hypoxia were utilized. Distribution of kappa-ORs was determined by fluorescence immunohistochemistry and changes in kappa-ORs expression in PAs and PASMCs were determined by fluorescence immunohistochemistry or Western blot techniques. The kappa-ORs were primarily distributed in the smooth muscle layer of the PAs and in the nucleus of PASMCs. The expression of the kappa-ORs were increased in PAs of rats subjected to hypoxia for 1-4 week (P < 0.01). Accordingly, the expression of kappa-ORs in PASMCs were also increased when subjected to hypoxia for 12-36 hr (P < 0.05). The present study has provided evidence for the first time of the precise location of kappa-ORs in PAs and PASMCs of rats and that hypoxia upregulates expression of kappa-ORs.

N-acetylcysteine-induced vasodilation involves voltage-gated potassium channels in rat aorta

AIMS

N-acetylcysteine (NAC) has a protective effect against vascular dysfunction by decreasing the level of reactive oxygen species (ROS) in experimental and human hypertension. This study was designed to examine whether NAC would relax vascular rings in vitro via nitric oxide-cyclic guanosine monophosphate (NO-cGMP) pathway, extracellular Ca2+ and/or K+ channels.

MAIN METHODS

Rat aortic arteries were mounted in an organ bath, contracted with 0.1, 0.5 or 1 micromol/L phenylephrine to plateau, and the vasodilatory effect of NAC was examined in the absence or presence of ROS scavengers, inhibitors of NO-cGMP pathway or K+ channels. Vascular smooth muscle cells (VSMCs) were loaded with a calcium sensitive fluorescent dye fluo-3 AM, and [Ca2+](i) was determined with laser-scanning confocal microscopy.

KEY FINDINGS

NAC (0.1-4 mmol/L) dose-dependently relaxed rat aorta pre-contracted with phenylephrine. Endothelium removal, endothelial nitric oxide synthase inhibitor N(omega)-Nitro-l-arginine (L-NNA) (100 micromol/L) or soluble guanylyl cyclase (sGC) inhibitor (ODQ) (10 micromol/L) did not affect NAC-induced vasodilation. In contrast, NAC-induced vasodilation was blunted after extracellular calcium was removed and calcium imaging showed that 4 mmol/L NAC quickly decreased [Ca2+](i) in fluo-3 AM loaded VSMCs. NAC-induced vasodilation was significantly reduced in the presence of voltage-gated K+ channels (Kv) inhibitor 4-aminopyridine (4-AP).

SIGNIFICANCE

The vasodilatory effect of NAC may be explained at least partly by activation of voltage-gated K+ channels.

Resolution of established cardiac hypertrophy and fibrosis and prevention of systolic dysfunction in a transgenic rabbit model of human cardiomyopathy through thiol-sensitive mechanisms

BACKGROUND

Cardiac hypertrophy, the clinical hallmark of hypertrophic cardiomyopathy (HCM), is a major determinant of morbidity and mortality not only in HCM but also in a number of cardiovascular diseases. There is no effective therapy for HCM and generally for cardiac hypertrophy. Myocardial oxidative stress and thiol-sensitive signaling molecules are implicated in pathogenesis of hypertrophy and fibrosis. We posit that treatment with N-acetylcysteine, a precursor of glutathione, the largest intracellular thiol pool against oxidative stress, could reverse cardiac hypertrophy and fibrosis in HCM.

METHODS AND RESULTS

We treated 2-year-old beta-myosin heavy-chain Q403 transgenic rabbits with established cardiac hypertrophy and preserved systolic function with N-acetylcysteine or a placebo for 12 months (n=10 per group). Transgenic rabbits in the placebo group had cardiac hypertrophy, fibrosis, systolic dysfunction, increased oxidized to total glutathione ratio, higher levels of activated thiol-sensitive active protein kinase G, dephosphorylated nuclear factor of activated T cells (NFATc1) and phospho-p38, and reduced levels of glutathiolated cardiac alpha-actin. Treatment with N-acetylcysteine restored oxidized to total glutathione ratio, normalized levels of glutathiolated cardiac alpha-actin, reversed cardiac and myocyte hypertrophy and interstitial fibrosis, reduced the propensity for ventricular arrhythmias, prevented cardiac dysfunction, restored myocardial levels of active protein kinase G, and dephosphorylated NFATc1 and phospho-p38.

CONCLUSIONS

Treatment with N-acetylcysteine, a safe prodrug against oxidation, reversed established cardiac phenotype in a transgenic rabbit model of human HCM. Because there is no effective pharmacological therapy for HCM and given that hypertrophy, fibrosis, and cardiac dysfunction are common and major predictors of clinical outcomes, the findings could have implications in various cardiovascular disorders.

Hypoxic preconditioning protects rat hearts against ischaemia-reperfusion injury: role of erythropoietin on progenitor cell mobilization

Preconditioning, such as by brief hypoxic exposure, has been shown to protect hearts against severe ischaemia. Here we hypothesized that hypoxic preconditioning (HPC) protects injured hearts by mobilizing the circulating progenitor cells. Ischaemia-reperfusion (IR) injury was induced by left coronary ligation and release in rats kept in room air or preconditioned with 10% oxygen for 6 weeks. To study the role of erythropoietin (EPO), another HPC + IR group was given an EPO receptor (EPOR) antibody via a subcutaneous mini-osmotic pump 3 weeks before IR induction. HPC alone gradually increased haematocrit, cardiac and plasma EPO, and cardiac vascular endothelial growth factor (VEGF) only in the first two weeks. HPC improved heart contractility, reduced ischaemic injury, and maintained EPO and EPOR levels in the infarct tissues of IR hearts, but had no significant effect on VEGF. Interestingly, the number of CD34(+)CXCR4(+) cells in the peripheral blood and their expression in HPC-treated hearts was higher than in control. Preconditioning up-regulated cardiac expression of stromal derived factor-1 (SDF-1) and prevented its IR-induced reduction. The EPOR antibody abolished HPC-mediated functional recovery, and reduced SDF-1, CXCR4 and CD34 expression in IR hearts, as well as the number of CD34(+)CXCR4(+) cells in blood. The specificity of neutralizing antibody was confirmed in an H9c2 culture system. In conclusion, exposure of rats to moderate hypoxia leads to an increase in progenitor cells in the heart and circulation. This effect is dependent on EPO, which induces cell homing by increased SDF-1/CXCR4 and reduces the heart susceptibly to IR injury.

Lung EC-SOD overexpression attenuates hypoxic induction of Egr-1 and chronic hypoxic pulmonary vascular remodeling

Although production of reactive oxygen species (ROS) such as superoxide (O(2)(.-)) has been implicated in chronic hypoxia-induced pulmonary hypertension (PH) and pulmonary vascular remodeling, the transcription factors and gene targets through which ROS exert their effects have not been completely identified. We used mice overexpressing the extracellular antioxidant enzyme extracellular superoxide dismutase (EC-SOD TG) to test the hypothesis that O(2)(.-) generated in the extracellular compartment under hypoxic conditions contributes to PH through the induction of the transcription factor, early growth response-1 (Egr-1), and its downstream gene target, tissue factor (TF). We found that chronic hypoxia decreased lung EC-SOD activity and protein expression in wild-type mice, but that EC-SOD activity remained five to seven times higher in EC-SOD TG mice under hypoxic conditions. EC-SOD overexpression attenuated chronic hypoxic PH, and vascular remodeling, measured by right ventricular systolic pressures, proliferation of cells in the vessel wall, muscularization of small pulmonary vessels, and collagen deposition. EC-SOD overexpression also prevented the early hypoxia-dependent upregulation of the redox-sensitive transcription factor Egr-1 and the procoagulant protein TF. These data provide the first evidence that EC-SOD activity is disrupted in chronic hypoxia, and increased EC-SOD activity can attenuate chronic hypoxic PH by limiting the hypoxic upregulation of redox-sensitive genes.

Chronic hypoxia increases fetoplacental vascular resistance and vasoconstrictor reactivity in the rat

An increase in fetoplacental vascular resistance caused by hypoxia is considered one of the key factors of placental hypoperfusion and fetal undernutrition leading to intrauterine growth restriction (IUGR), one of the serious problems in current neonatology. However, although acute hypoxia has been shown to cause fetoplacental vasoconstriction, the effects of more sustained hypoxic exposure are unknown. This study was designed to test the hypothesis that chronic hypoxia elicits elevations in fetoplacental resistance, that this effect is not completely reversible by acute reoxygenation, and that it is accompanied by increased acute vasoconstrictor reactivity of the fetoplacental vasculature. We measured fetoplacental vascular resistance as well as acute vasoconstrictor reactivity in isolated perfused placentae from rats exposed to hypoxia (10% O2) during the last week of a 3-wk pregnancy. We found that chronic hypoxia shifted the relationship between perfusion pressure and flow rate toward higher pressure values (by ∼20%). This increased vascular resistance was refractory to a high dose of sodium nitroprusside, implying the involvement of other factors than increased vascular tone. Chronic hypoxia also increased vasoconstrictor responses to angiotensin II (by ∼75%) and to acute hypoxic challenges (by >150%). We conclude that chronic prenatal hypoxia causes a sustained elevation of fetoplacental vascular resistance and vasoconstrictor reactivity that are likely to produce placental hypoperfusion and fetal undernutrition in vivo.

Successful extracorporeal life support after potentially fatal pulmonary oedema caused by inhalation of nitric and hydrofluoric acid fumes

Two patients presented with potentially fatal pulmonary oedema after accidental exposure to nitric and hydrofluoric acid fumes during electroplating. Despite aggressive respiratory support, one succumbed to respiratory failure 3.5h after inhalation. The other patient also rapidly progressed to respiratory failure. Extracorporeal life support (ECLS) was started 5h after exposure at the ED. During ECLS, hypoxia improved, but pulmonary oedema shown by chest radiography became aggravated. N-Acetyl cysteine and calcium gluconate were given i.v. on the first day of admission and nebulised for 48 h after exposure. Pulmonary secretions were significantly reduced 24 h after the nebulising therapy began. Ultimately, the patient was discharged without serious pulmonary or neurological complications after 28 days of hospitalisation. In this case, early ECLS, nebulised antioxidant and antidote were available to treat potentially fatal pulmonary oedema after exposure to nitric and hydrofluoric acid fumes.

Role of reactive oxygen species in chronic hypoxia-induced pulmonary hypertension and vascular remodeling

Pulmonary hypertension is a life-threatening disease process that affects adults and children. Pediatric patients with lung diseases that can be complicated by alveolar hypoxia, such as bronchopulmonary dysplasia (BPD), are at risk for developing pulmonary hypertension, which leads to right heart failure and greatly increases morbidity and mortality. We review the evidence that reactive oxygen species (ROS) are generated by pulmonary vascular wall cells in response to a hypoxic exposure, and that this response contributes to chronic hypoxic pulmonary hypertension. We summarize the accumulating data implicating NADPH oxidase as a major source of O2 responsible for vascular remodeling and hypertension. We also consider the effects of chronic hypoxia on the clearance of O2 by superoxide dismutases, specifically extracellular superoxide dismutase, which is highly expressed in the pulmonary artery. We review the role of the activated vascular adventitial fibroblast in the generation of ROS and in the pathogenesis of vascular remodeling, and provide a rationale to consider the role of the activated fibroblast and ROS in hypoxic pulmonary hypertension using a clinically relevant bovine model of neonatal chronic hypoxic pulmonary hypertension.

Role of oxidative stress in PKC-δ upregulation and cardioprotection induced by chronic intermittent hypoxia

The aim was to determine whether increased oxidative stress during the adaptation to chronic intermittent hypoxia (CIH) plays a role in the induction of improved cardiac ischemic tolerance. Adult male Wistar rats were exposed to CIH in a hypobaric chamber (7,000 m, 8 h/day, 5 days/wk, 24–30 exposures). Half of the animals received antioxidant N-acetylcysteine (NAC; 100 mg/kg) daily before the exposure; the remaining rats received saline. Control rats were kept under normoxia and treated in a corresponding manner. One day after the last exposure (and/or NAC injection), anesthetized animals were subject to 20 min of coronary artery occlusion and 3 h of reperfusion for determination of infarct size. In parallel subgroups, biochemical analyses of the left ventricular myocardium were performed. Adaptation to CIH reduced infarct size from 56.7 ± 4.5% of the area at risk in the normoxic controls to 27.7 ± 4.9%. NAC treatment decreased the infarct size in the controls to 42.0 ± 3.4%, but it abolished the protection provided by CIH (to 41.1 ± 4.9%). CIH decreased the reduced-to-oxidized glutathione ratio and increased the relative amount of PKC isoform-δ in the particulate fraction; NAC prevented these effects. The expression of PKC-ε was decreased by CIH and not affected by NAC. Activities of superoxide dismutase, catalase, and glutathione peroxidase were affected by neither CIH nor NAC treatment. It is concluded that oxidative stress associated with CIH plays a role in the development of increased cardiac ischemic tolerance. The infarct size-limiting mechanism of CIH seems to involve the PKC-δ-dependent pathway but apparently not the increased capacity of major antioxidant enzymes.

Acute and chronic hypoxia as well as 7-day recovery from chronic hypoxia affects the distribution of pulmonary mast cells and their MMP-13 expression in rats

Chronic hypoxia results in pulmonary hypertension due to vasoconstriction and structural remodelling of peripheral lung blood vessels. We hypothesize that vascular remodelling is initiated in the walls of prealveolar pulmonary arteries by collagenolytic metalloproteinases (MMP) released from activated mast cells. Distribution of mast cells and their expression of interstitial collagenase, MMP-13, in lung conduit, small muscular, and prealveolar arteries was determined quantitatively in rats exposed for 4 and 20 days to hypoxia as well as after 7-day recovery from 20-day hypoxia (10% O2). Mast cells were identified using Toluidine Blue staining, and MMP-13 expression was detected using monoclonal antibody. After 4, but not after 20 days of hypoxia, a significant increase in the number of mast cells and their MMP-13 expression was found within walls of prealveolar arteries. In rats exposed for 20 days, MMP-13 positive mast cells accumulated within the walls of conduit arteries and subpleurally. In recovered rats, MMP-13 positive mast cells gathered at the prealveolar arterial level as well as in the walls of small muscular arteries; these mast cells stayed also in the conduit part of the pulmonary vasculature. These data support the hypothesis that perivascular pulmonary mast cells contribute to the vascular remodelling in hypoxic pulmonary hypertension in rats by releasing interstitial collagenase.

Therapeutic concepts for hypoxic pulmonary vasoconstriction involving ion regulation and the smooth muscle contractile apparatus

Hypoxic pulmonary vasoconstriction (HPV) and pulmonary hypertension present a common and formidable clinical problem for practicing intensivists, thoracic, transplant, and trauma surgeons. The Redox Theory for the mechanisms of HPV has provided researchers with a new understanding of the etiology behind HPV that has opened the door to many new avenues of therapy for the disease. Potassium channels have been proposed to be the main mediator contributing to HPV, and treatment concepts that attempt to manipulate the function and number of those channels have been explored. Additionally, attempts to transfer genes that express the formation of specific potassium channels directly into pulmonary hypertensive lungs have proven to be very promising. Finally, rho kinase (ROK) has been discovered to play a very central role in the formation of hypoxia-induced pulmonary hypertension, and the advent of very specific ROK inhibitors has shown positive clinical results. The purposes of this review are to: (1) briefly discuss some of the basic mechanisms that undergird HPV, including the Redox Theory for the mechanisms of HPV; (2) address current research involving treatments concepts related to ion channels; (3) report on research involving gene therapy to combat pulmonary hypertension; and (4) examine potential therapeutic avenues associated with inhibition of rho kinase.

Hypobaric-hypoxia-induced pulmonary damage in rats ameliorated by antioxidant erdosteine

Free radical-mediated injury to lung and pulmonary vasculature is an important mechanism in hypoxia-induced lung damage. In this study, we aimed to investigate the potential protective effects of erdosteine as an antioxidant agent on hypobaric hypoxia-induced pulmonary hypertension. Adult male rats were assigned randomly to three groups. The first group of rats was exposed to hypobaric-hypoxia and the second group was treated with erdosteine (20mg/kg, daily) for 2 weeks, during which time they were in a hypoxic chamber. These groups were compared with normoxic controls. All rats were sacrificed after 2 weeks. The hypoxia-induced increase in right ventricle to left ventricle plus septum weight ratio (from 0.20+/-0.01 to 0.26+/-0.01) was reduced significantly in the erdosteine-treated group (0.23+/-0.01). Malondialdehyde levels were elevated (from 0.33+/-0.11 to 0.59+/-0.02) and total antioxidant status was not changed significantly (from 1.77+/-0.42 to 2.61+/-0.23) by hypoxia. In contrast to the hypoxia-exposed group, malondialdehyde levels were significantly decreased in the erdosteine-treated group (0.37+/-0.02). Total antioxidant status (4.03+/-0.22) was significantly higher in erdosteine-treated rats when compared to non-treated rats. Histopathological examination demonstrated that erdosteine prevented inflammation and protected lung parenchyma and pulmonary endothelium of hypoxia-exposed rats.

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.