The NO donor molsidomine reduces endothelin-1 gene expression in chronic hypoxic rat lungs

Therapeutic potential of carbon monoxide in hypertension-induced vascular smooth muscle cell damage revisited: from physiology and pharmacology

As a chronic and progressive disorder, hypertension remains to be a serious public health problem around the world. Among the different types of hypertension, pulmonary arterial hypertension (PAH) is a devastating disease associated with pulmonary arteriole remodeling, right ventricular failure and death. The contemporary management of systemic hypertension and PAH has substantially grown since more therapeutic targets and/or agents have been developed. Evolving treatment strategies targeting the vascular remodeling lead to improving outcomes in patients with hypertension, nevertheless, significant advancement opportunities for developing better antihypertensive drugs remain. Carbon monoxide (CO), an active endogenous gasotransmitter along with hydrogen sulfide (H₂S) and nitric oxide (NO), is primarily generated by heme oxygenase (HO). Cumulative evidence suggests that CO is considered as an important signaling molecule under both physiological and pathological conditions. Studies have shown that CO confers a number of biological and pharmacological properties, especially its involvement in the pathological process and treatment of hypertension-related vascular remodeling. This review will critically outline the roles of CO in hypertension-associated vascular remodeling and discuss the underlying mechanisms for the protective effects of CO against hypertension and vascular remodeling. In addition, we will propose the challenges and perspectives of CO in hypertensive vascular remodeling. It is expected that a comprehensive understanding of CO in the vasculature might be essential to translate CO to be a novel pharmacological agent for hypertension-induced vascular remodeling.

Nitric Oxide-cGMP Pathway Modulation in an Experimental Model of Hypoxic Pulmonary Hypertension

Manipulation of nitric oxide (NO) may enable control of progression and treatment of pulmonary hypertension (PH). Several approaches may modulate the NO-cGMP pathway in vivo. Here, we investigate the effectiveness of 3 modulatory sites: (i) the amount of l-arginine; (ii) the size of plasma NO stores that stimulate soluble guanylate cyclase; (iii) the conversion of cGMP into inactive 5′-GMP, with respect to hypoxia, to test the effectiveness of the treatments with respect to hypoxia-induced PH. Male rats (n = 80; 10/group) maintained in normoxic (21% O₂) or hypoxic chambers (10% O₂) for 14 days were subdivided in 4 sub-groups: placebo, l-arginine (20 mg/ml), the NO donor molsidomine (15 mg/kg in drinking water), and phoshodiesterase-5 inhibitor sildenafil (1.4 mg/kg in 0.3 ml saline, i.p.). Hypoxia depressed homeostasis and increased erythropoiesis, heart and right ventricle hypertrophy, myocardial fibrosis and apoptosis inducing pulmonary remodeling. Stimulating anyone of the 3 mechanisms that enhance the NO-cGMP pathway helped rescuing the functional and morphological changes in the cardiopulmonary system leading to improvement, sometimes normalization, of the pressures. None of the treatments affected the observed parameters in normoxia. Thus, the 3 modulatory sites are essentially similar in enhancing the NO-cGMP pathway, thereby attenuating the hypoxia-related effects that lead to pulmonary hypertension.

Chronic hypoxia changes gene expression profile of primary rat carotid body cells: consequences on the expression of NOS isoforms and ET-1 receptors

Sustained chronic hypoxia (CH) produces morphological and functional changes in the carotid body (CB). Nitric oxide (NO) and endothelin-1 (ET-1) play a major role as modulators of the CB oxygen chemosensory process. To characterize the effects of CH related to normoxia (Nx) on gene expression, particularly on ET-1 and NO pathways, primary cultures of rat CB cells were exposed to 7 days of CH. Total RNA was extracted, and cDNA-32P was synthesized and hybridized with 1,185 genes printed on a nylon membrane Atlas cDNA Expression Array. Out of 324 differentially expressed genes, 184 genes were upregulated, while 140 genes were downregulated. The cluster annotation and protein network analyses showed that both NO and ET-1 signaling pathways were significantly enriched and key elements of each pathway were differentially expressed. Thus, we assessed the effect of CH at the protein level of nitric oxide synthase (NOS) isoforms and ET-1 receptors. CH induced an increase in the expression of endothelial NOS, inducible NOS, and ETB. During CH, the administration of SNAP, a NO donor, upregulated ETB. Treatment with Tezosentan (ET-1 receptor blocker) during CH upregulated all three NOS isoforms, while the NOS blocker L-NAME induced upregulation of iNOS and ETB and downregulated the protein levels of ETA. These results show that CH for 7 days changed the cultured cell CB gene expression profile, the NO and ET-1 signaling pathways were highly enriched, and these two signaling pathways interfered with the protein expression of each other.

Superimposed Preeclampsia Exacerbates Postpartum Renal Injury Despite Lack of Long-Term Blood Pressure Difference in the Dahl Salt-Sensitive Rat

Preeclampsia results in increased susceptibility to hypertension and chronic kidney disease postpartum; however, the mechanisms responsible for disease progression in these women remain unknown. The purpose of this study was to test the hypothesis that 2 mechanisms contribute to the link between the maternal syndrome of preeclampsia and the increased postpartum risk of cardiovascular and renal disease: (1) increased T cells in the kidney and (2) a decreased NO:ET-1 (endothelin-1) ratio. Dahl S rats (a previously characterized model of preeclampsia superimposed on chronic hypertension) who experienced 2 pregnancies and virgin littermate controls were studied at 6 months of age. Mean arterial pressure was measured via telemetry, and renal injury was assessed through both histological analysis and measurement of urinary markers including nephrin, podocalyxin, and KIM-1 (kidney injury marker 1). Contributing mechanisms were assessed through flow cytometric analysis of renal T cells, quantification of plasma TNF-α (tumor necrosis factor-α) and IL-10 (interleukin-10), and quantification of urinary concentrations of NO metabolites and ET-1. Although prior pregnancy did not exacerbate the hypertension at 6 months, this group showed greater renal injury compared with virgin littermates. Flow cytometric analyses revealed an increase in renal T cells after pregnancy, and cytokine analysis revealed a systemic proinflammatory shift. Finally, the NO:ET-1 ratio was reduced. These results demonstrate that the link between the maternal syndrome of superimposed preeclampsia and postpartum risk of chronic kidney disease could involve both immune system activation and dysregulation of the NO:ET-1 balance.

Ponatinib attenuates experimental pulmonary arterial hypertension by modulating Wnt signaling and vasohibin-2/vasohibin-1

AIMS

An abnormal ratio of vasohibin-2/vasohibin-1 may be involved in the abnormal angiogenesis and vascular remodeling during pulmonary arterial hypertension (PAH).

MAIN METHODS

To evaluate the pharmacological actions of Ponatinib (AP) in experimental model of PAH, the effects of AP on TGF-β1-mediated endothelial-mesenchymal transition (EndoMT) in human pulmonary microvascular endothelial cells (HPMEC), and the hypoxic human pulmonary artery smooth muscle cells (HPASMC) proliferation and HPMEC in vitro, and on bleomycin (BLM)-induced PAH in vivo were investigated.

KEY FINDINGS

AP treatment resulted in a reduction of EndoMT in HPMECs with a decrease of vimentin, whereas an increase of VE-cadherin, reduction of fibroblast growth factor (FGF-2), vascular endothelial growth factor (VEGF) and vasohibin-2 (VASH-2), whereas an increase of vasohibin-1 (VASH-1) in the hypoxic HPMEC, a reduction of the HPASMC proliferation with decreases of wnt5a, β-catenin and cyclin D1 expression. AP ameliorated BLM-induced PAH in rats with reductions of FGF-2, VEGF, von Willebrand factor (vWF) and VASH-2 expression, whereas an increase of VASH-1 expression. AP ameliorated BLM-induced PAH in rats with reductions of the pathological score and the collagen deposition. In addition, AP ameliorated hemodynamics and right ventricular hypertrophy.

SIGNIFICANCES

Our results identified a therapeutic potential of AP in PAH therapy might be modulated VASH-2/VASH-1 and the Wnt signaling.

NOS1-dependent negative feedback regulation of the epithelial sodium channel in the collecting duct

With an increase in urine flow there is a significant increase in shear stress against the renal epithelium including the inner medullary collecting duct, resulting in an increase in nitric oxide (NO) production. The mechanisms of the shear stress-mediated increases in NO are undetermined. Previous studies found that shear stress increases epithelial sodium channel (ENaC) open probability and endothelin (ET)-1 production in an ENaC-dependent mechanism in the collecting duct (CD). Given that ET-1 stimulates NO production in the CD, we hypothesized that shear stress-induced NO production is downstream of shear stress-induced ENaC activation and ET-1 production in a negative feedback loop. We determined that nitric oxide synthase 1 (NOS1) and NOS3 contribute to shear stress-mediated NO production in the CD, that is attenuated by low doses of the ENaC inhibitors amiloride and benzamil. Moreover, ETB receptor blockade significantly blunted the shear stress-mediated NO production. We further elucidated whether mice lacking NOS1 in the collecting duct (CDNOS1KO) have an impaired renal ET-1 system in the CD. Although urinary ET-1 production and inner medullary ET receptor expression were similar between flox control and CDNOS1KO mice, acute ET-1 treatment significantly reduced ENaC open probability in CDs from flox mice but not CDNOS1KO mice compared with basal. Basal ENaC activity in CDs was similar between the genotypes. We conclude that during acute shear stress across the CD, ENaC acts in a negative feedback loop to stimulate NO production in an ETB/NOS1-dependent manner resulting in a decrease in ENaC open probability and promoting natriuresis.

[Endothelin: From discovery to pharmacotherapeutic innovations]

OBJECTIVES

Endothelin (ET) is a major therapeutic target in cardiopulmonary diseases. The purpose of this review is to present the main concepts concerning ET biology, its pathophysiological roles and the major pharmacological and medical advances recently developed around the concept of ET receptor blockade.

METHODS

Analysis of PubMed database (keywords: endothelin, endothelin receptor antagonists, pulmonary hypertension, etc.), and of abstract originating from recent international meetings.

RESULTS

ET is a peptide produced by vascular endothelial cells as well as by many other tissues. Both its production and its effects are activated in pathological situations associated with endothelial dysfunction. ET is characterized by a strong tropism toward tissues because of its polarized release, the strong tissue receptor density and high affinity of the receptors for the peptide. ET exerts several vascular effects, including vasoconstriction, proliferation and hypertrophy, as well as non-vascular effects, notably stimulation of cardiac hypertrophy, tissue fibrosis and inflammation. Both vascular and non-vascular effects depend on the stimulation of two receptor subtypes, ETA and ETB. ET receptor antagonists (ERA) demonstrated beneficial effects in many different pre-clinical models of cardiovascular and pulmonary diseases, and constitute a first-line treatment of patients with pulmonary arterial hypertension (PAH). Recently, the targeted search for a novel ERA led to the development of macitentan which, compared to existing ERA, show optimized tissue penetration, increased receptor affinity and in vivo pharmacological efficacy in pre-clinical models, associated with a favorable profile, in terms of hepatic safety and drug interactions. The clinical efficacy of macitentan in the treatment of PAH was recently demonstrated in the SERAPHIN trial, which contrasts with previous PAH trials because of its long duration, the high number of patients enrolled, and its primary endpoint evaluating morbidity/mortality. Results show a significant reduction of the primary composite morbidity/mortality endpoint (taking into account both progression of PAH and death) by 30 and 45% with macitentan 3 and 10mg, respectively, compared to placebo, and confirm on the large scale the favorable tolerance profile, especially at the hepatic level.

CONCLUSION

The extensive knowledge on the complexity of the ET system allowed the synthesis of a new antagonist optimized, in terms of pharmacological efficacy and safety, which also show promising therapeutic effects in PAH patients, with demonstrated results in a prospective study using a composite primary endpoint of morbidity-mortality.

Effects of simvastatin on pulmonary fibrosis, pulmonary hypertension and exercise capacity in bleomycin-treated rats

AIM

Pulmonary fibrosis is often complicated by pulmonary hypertension. Statins reduce fibroblast activity in vitro and pulmonary hypertension in vivo. We investigated whether Simvastatin exerts beneficial effects on pulmonary fibrosis and pulmonary hypertension in Bleomycin-treated rats in vivo.

METHODS

Rats were randomly assigned to controls, Bleomycin, Bleomycin plus Simvastatin from day 1 to 28 and Bleomycin plus Simvastatin from day 13 to 28. 28 days after Bleomycin instillation, right ventricular systolic pressure (RVSP), right ventricular mass (RV/(LV+S)), right ventricular and circulating brain natriuretic peptide (BNP) levels were determined to assess pulmonary hypertension. Pulmonary hydroxyproline content (HPC), pulmonary connective tissue growth factor (CTGF) transcription and lung compliance (LC) were analysed to characterize pulmonary fibrosis. Exercise capacity was determined by treadmill tests.

RESULTS

Compared with controls, Bleomycin increased RVSP, RV/(LV+S), BNP levels, HPC and CTGF transcription and decreased LC significantly. Simvastatin administered from day 1 to 28 normalized all these parameters. Simvastatin administered from day 13 to 28 had no effect on HPC and LC, but reduced RV/(LV+S) significantly and induced a strong trend to lower RVSP and BNP levels. Exercise capacity was reduced by Bleomycin. Simvastatin significantly improved exercise intolerance in both treatment groups.

CONCLUSIONS

Simvastatin prevents the development of pulmonary fibrosis, but fails to attenuate already established pulmonary fibrosis. In contrast, it ameliorates pulmonary hypertension and thereby exercise capacity in the prevention and the treatment group regardless of its effects on pulmonary fibrosis. Whether statins are a treatment option in humans with pulmonary fibrosis needs to be investigated by further study.

Nitric oxide decreases the expression of endothelin-converting enzyme-1 through mRNA destabilization

OBJECTIVE

Endothelial function depends on the equilibrium in the synthesis of vasoactive endothelial factors. It is well known that endothelin and nitric oxide (NO) exhibit reciprocal regulation. We assessed the ability of NO to regulate endothelin-converting enzyme-1 (ECE-1) expression in vascular endothelial cells.

METHODS AND RESULTS

Bovine aortic endothelial cells were incubated with 2 different NO donors as well as with a cyclic-GMP analog, dibutyryl-cGMP (dB-cGMP). ECE-1 protein content and mRNA expression were evaluated by Western blot and Northern blot, respectively, promoter activity by transfection experiments, ECE-1 activity by ELISA, and cGMP production by radioimmunoassay. Both NO donors decreased ECE-1 protein content, mRNA expression, and ECE-1 activity. ODQ, an inhibitor of soluble guanylyl cyclase, blocked those effects. NO donors raised cGMP levels, and dB-cGMP mimicked their effects on ECE-1 expression, which were blocked by KT5823, a nonspecific PKG inhibitor. The changes on ECE-1 expression were due to a destabilization on 3'-untranslated region (3'-UTR) of this mRNA, because the activity of a luciferase reporter construct containing the 3'-UTR of the ECE-1 gene was reduced by dB-cGMP in a PKG-dependent manner. The biological relevance of this regulation was confirmed in bovine aortic endothelial cells coincubated with macrophages in the presence of lipopolysaccharide, in eNOS-deficient mice, and in Wistar rats treated with NO donors. In every case, an inverse relationship was observed between NO and ECE-1 protein content.

CONCLUSION

Our results support that NO regulates ECE-1 expression through a cGMP/PKG-dependent regulatory mechanism at the post-transcriptional level via the 3'-UTR of the ECE-1 gene.

Improvement of bleomycin-induced pulmonary hypertension and pulmonary fibrosis by the endothelin receptor antagonist Bosentan

RATIONALE

There is evidence that endothelin plays a key role in the development of pulmonary hypertension (PH) in pulmonary fibrosis (PF). However, the functional consequence of the unselective endothelin receptor antagonist Bosentan in PH and PF has not yet been studied. Therefore, we investigated the effects of Bosentan on the development of PH in the model of Bleomycin-induced PF in rats.

METHODS

Adult male Wistar rats were randomly assigned to the following groups: untreated animals (controls), Bleomycin-induced PF (Bleomycin) and Bleomycin-induced PF treated with Bosentan (Bleomycin+Bosentan). Exercise capacity was evaluated by treadmill exercise testing. PH was assessed by right ventricular systolic pressure (RVSP) and right ventricular hypertrophy. For quantification of PF the hydroxyproline content in lung tissue (HPC) was measured.

RESULTS

Compared to controls, animals with Bleomycin-induced PF showed a significant reduction in exercise capacity (44% vs. 100%), significantly higher RVSP (65 mmHg vs. 23 mmHg), significantly more right ventricular hypertrophy (0.55 vs. 0.24) and significantly higher HPC (60.5 vs. 14.8). Bosentan treatment in animals with Bleomycin-induced PF resulted in significantly greater exercise capacity (98% vs. 44%) and a trend towards lower RVSP (52 mmHg vs. 65 mmHg), significantly less right ventricular hypertrophy (0.34 vs. 0.55) and significantly lower HPC (16.7 vs. 60.5) compared to untreated Bleomycin-induced PF.

CONCLUSION

Application of Bosentan in Bleomycin rats resulted in significantly higher exercise capacity as a result of improvements in PH and PF.

Endothelin-1 impairs nitric oxide signaling in endothelial cells through a protein kinase Cdelta-dependent activation of STAT3 and decreased endothelial nitric oxide synthase expression

In an ovine model of persistent pulmonary hypertension of the newborn (PPHN), endothelin-1 (ET-1) expression is increased, while endothelial nitric oxide synthase (eNOS) expression is decreased. However, the molecular mechanisms by which ET-1 attenuates eNOS expression in endothelial cells are not completely understood. Thus, the goal of this study was to determine if the overexpression of ET-1 decreases eNOS expression in pulmonary arterial endothelial cells isolated from fetal lambs. To increase the ET-1 expression, cells were transfected with a plasmid coding for Prepro-ET-1, a precursor of ET-1. After overexpression of Prepro-ET-1, ET-1 levels in the culture medium were significantly increased (control = 805.3 +/- 69.8; Prepro-ET-1 overexpression = 1351 +/- 127.9). eNOS promoter activity, protein levels, and NO generation were all significantly decreased by the overexpression of Prepro-ET-1. The decrease in transcription correlated with increased activity of protein kinase Cdelta (PKCdelta) and STAT3. Further, DNA binding activity of STAT3 was also increased by Prepro-ET-1 overexpression. The increase in STAT3 activity and decrease in eNOS promoter activity were inhibited by the overexpression of dominant negative mutants of PKCdelta or STAT3. Further, a 2 bp mutation in the STAT3 binding site in the eNOS promoter inhibited STAT3 binding and led to enhanced promoter activity in the presence of Prepro-ET-1 overexpression. In conclusion, ET-1 secretion is increased by Prepro-ET-1 overexpression. This results in activation of PKCdelta, which phosphorylates STAT3, increasing its binding to the eNOS promoter. This in turn decreases eNOS promoter activity, protein levels, and NO production. Thus, ET-1 can reduce eNOS expression and NO generation in fetal pulmonary artery endothelial cells through PKCdelta-mediated activation of STAT3.

Effects of olmesartan medoxomil as an angiotensin II-receptor blocker in chronic hypoxic rats

We established a rat chronic alveolar hypoxia in vivo model to evaluate the efficacy against hypoxic pulmonary hypertension of a new angiotensin II-receptor I blocker, olmesartan medoxomil. Three groups of rats were established: rats exposed for 2-6 weeks to 10% oxygen atmosphere in a normobaric chamber; hypoxic rats treated with olmesartan medoxomil oral administration (5 mg/day) every day; and control rats fed in a normoxic condition. After hypoxia treatment, the presence, etiology and severity of pulmonary hypertension, was echocardiographically evaluated, and expressions of brain natriuretic peptide (BNP), transforming growth factor (TGF-beta) and endothelin-1 genes measured by both immunohistochemical assay and real-time polymerase chain reaction. Olmesartan medoxomil significantly reduced the induction of hypoxic cor pulmonale not only on echocardiographical observations but also in BNP, TGF-beta and endothelin gene expressions in molecular studies. However, systolic blood pressure was independent of olmesartan medoxomil. The present study clearly indicates that the angiotensin II-type I-receptor blocker olmesartan medoxomil has significant efficacy for hypoxic cor pulmonale.

Nitric oxide decreases endothelin-1 secretion through the activation of soluble guanylate cyclase

The use of exogenous nitric oxide (NO) has been shown to alter the regulation of other endothelially derived mediators of vascular tone, such as endothelin-1 (ET-1). However, the interaction between NO and ET-1 appears to be complex and remains incompletely understood. One of the major actions of NO is the activation of soluble guanylate cyclase (sGC) with the subsequent generation of cGMP. Therefore, we undertook this study to test the hypothesis that NO regulates ET-1 production via the activation of the sGC/cGMP pathway. The results obtained indicated that the exposure of primary cultures of 4-wk-old ovine pulmonary arterial endothelial cells (4-wk PAECs) to the long-acting NO donor DETA NONOate induced both a dose- and time-dependent decrease in secreted ET-1. This decrease in ET-1 secretion occurred in the absence of changes in endothelin-converting enzyme-1 or sGC expression but in conjunction with a decrease in prepro-ET-1 mRNA. The changes in ET-1 release were inversely proportional to the cellular cGMP content. Furthermore, the NO-independent activator of sGC, YC-1, or treatment with a cGMP analog also produced significant decreases in ET-1 secretion. Conversely, pretreatment with the sGC inhibitor ODQ blocked the NO-induced decrease in ET-1. Therefore, we conclude that exposure of 4-wk PAECs to exogenous NO decreases secreted ET-1 resulting from the activation of sGC and increased cGMP generation.

The superoxide dismutase mimetic, tempol, blunts right ventricular hypertrophy in chronic hypoxic rats

1. The purpose of this study was to investigate whether a membrane-permeable superoxide dismutase mimetic, tempol, added either alone or in combination with the nitric oxide (NO) donor molsidomine, prevents the development of pulmonary hypertension (PH) in chronic hypoxic rats. 2. Chronic hypobaric hypoxia (10% oxygen) for 2 weeks increased the right ventricular systolic pressure (RVSP), right ventricle and lung wet weight. Relaxations evoked by acetylcholine (ACh) and the molsidomine metabolite SIN-1 were impaired in isolated proximal, but not distal pulmonary arteries, from chronic hypoxic rats. 3. Treatment with tempol (86 mg x kg(-1) day(-1) in drinking water) normalized RVSP and reduced right ventricular hypertrophy, while systemic blood pressure, lung and liver weights, and blunted ACh relaxation of pulmonary arteries were unchanged. 4. Treatment with molsidomine (15 mg x kg(-1) day(-1) in drinking water) had the same effects as tempol, except that liver weight was reduced, and potassium and U46619-evoked vasoconstrictions in pulmonary arteries were increased. Combining tempol and molsidomine did not have additional effects compared to tempol alone. ACh relaxation in pulmonary arteries was not normalized by these treatments. 5. The media to lumen diameter ratio of the pulmonary arteries was greater for the hypoxic rats compared to the normoxic rats, and was not reversed by treatment with tempol, molsidomine, or the combination of tempol and molsidomine. 6. We conclude that tempol, like molsidomine, is able to correct RVSP and reduce right ventricular weight in the rat hypoxic model. Functional and structural properties of pulmonary small arteries were little affected. The results support the possibility that superoxide dismutase mimetics may be a useful means for the treatment of PH.

The endothelin system in pulmonary hypertension

Pulmonary hypertension (PH) may result from numerous clinical entities affecting the pulmonary circulation primarily or secondarily. It is recognized that vascular endothelial dysfunction contributes to the development and perpetuation of PH by creating an imbalance between vasodilating and antiproliferative forces and between vasoconstrictive and proliferative forces. In that context, endothelin-1 (ET-1) overproduction was rapidly targeted as a plausible contributor to the pathogenesis of PH. The lung is recognized as the major site for ET production and clearance. In all animal models of PH studied, circulating plasma ET-1 levels are elevated, accompanied by an increase in lung tissue expression of the peptide. The use of selective ETA and dual ETA-ETB receptor antagonists in these models both in prevention and in therapeutic studies have confirmed the contribution of ET-1 to the rise in pulmonary vascular tone, pulmonary medial hypertrophy, and right ventricular hypertrophy. This is found consistently in models affecting the pulmonary circulation primarily or producing PH secondarily. Recent clinical trials in patients with pulmonary arterial hypertension have confirmed the therapeutic effectiveness of ET-receptor antagonists in humans. We offer a systematic review of the pathogenic role of the ET system in the development of PH as well as the rationale behind the preclinical and ongoing clinical trials with this new class of agents.

Effects of ET-A receptor blockade on eNOS gene expression in chronic hypoxic rat lungs

We tested the hypothesis that pulmonary endothelial nitric oxide synthase (eNOS) gene expression is primarily regulated by hemodynamic factors and is thus increased in rats with chronic hypoxic pulmonary hypertension. Furthermore, we examined the role of endothelin (ET)-1 in this regulatory process, since ET-1 is able to induce eNOS via activation of the ET-B receptor. Therefore, chronic hypoxic rats (10% O(2)) were treated with the selective ET-A receptor antagonist LU-135252 (50 mg x kg(-1) x day(-1)). Right ventricular systolic pressure and cross-sectional medial vascular wall area of pulmonary arteries rose significantly, and eNOS mRNA levels increased 1.8- and 2.6-fold after 2 and 4 wk of hypoxia, respectively (each P < 0.05). Pulmonary ET-1 mRNA and ET-1 plasma levels increased significantly after 4 wk of hypoxia (each P < 0.05). LU-135252 reduced right ventricular systolic pressure, vascular remodeling, and eNOS gene expression in chronic hypoxic rats (each P < 0.05), whereas ET-1 production was not altered. We conclude that eNOS expression in chronic hypoxic rat lungs is modified predominantly by hemodynamic factors, whereas the ET-B receptor-mediated pathway and hypoxia seem to be less important.

The NO − K+ Channel Axis in Pulmonary Arterial Hypertension

The prognosis of patients with pulmonary arterial hypertension (PAH) is poor. Available therapies (Ca(++)-channel blockers, epoprostenol, bosentan) have limited efficacy or are expensive and associated with significant complications. PAH is characterized by vasoconstriction, thrombosis in-situ and vascular remodeling. Endothelial-derived nitric oxide (NO) activity is decreased, promoting vasoconstriction and thrombosis. Voltage-gated K+ channels (Kv) are downregulated, causing depolarization, Ca(++)-overload and PA smooth muscle cell (PASMC) contraction and proliferation. Augmenting the NO and Kv pathways should cause pulmonary vasodilatation and regression of PA remodeling. Several inexpensive oral treatments may be able to enhance the NO axis and/or K+ channel expression/function and selectively decrease pulmonary vascular resistance (PVR). Oral L-Arginine, NOS' substrate, improves NO synthesis and functional capacity in humans with PAH. Most of NO's effects are mediated by cyclic guanosine-monophosphate (c-GMP). cGMP causes vasodilatation by activating K+ channels and lowering cytosolic Ca++. Sildenafil elevates c-GMP levels by inhibiting type-5 phosphodiesterase, thereby opening BK(Ca). channels and relaxing PAs. In PAH, sildenafil (50 mg-po) is as effective and selective a pulmonary vasodilator as inhaled NO. These benefits persist after months of therapy leading to improved functional capacity. 3) Oral Dichloroacetate (DCA), a metabolic modulator, increases expression/function of Kv2.1 channels and decreases remodeling and PVR in rats with chronic-hypoxic pulmonary hypertension, partially via a tyrosine-kinase-dependent mechanism. These drugs appear safe in humans and may be useful PAH therapies, alone or in combination.

The role of the NO axis and its therapeutic implications in pulmonary arterial hypertension

Pulmonary Arterial Hypertension (PAH) is a disease of the pulmonary vasculature leading to vasoconstriction and remodeling of the pulmonary arteries. The resulting increase in the right ventricular afterload leads to right ventricular failure and death. The treatment options are limited, expensive and associated with significant side effects. The nitric oxide (NO) pathway in the pulmonary circulation provides several targets for the development of new therapies for this disease. However, the NO pathway is modulated at multiple levels including transcription and expression of the NO synthase gene, regulation of the NO synthase activity, regulation of the production of cyclic guanomonophosphate (cGMP) by phosphodiesterases, postsynthetic oxidation of NO, etc. This makes the study of the role of the NO pathway very difficult, unless one uses multiple complementary techniques. Furthermore, there are significant differences between the pulmonary and the systemic circulation which make extrapolation of data from one circulation to the other very difficult. In addition, the role of NO in the development of pulmonary hypertension varies among different models of the disease. This paper reviews the role of the NO pathway in both the healthy and diseased pulmonary circulation and in several animal models and human forms of the disease. It focuses on the role of recent therapies that target the NO pathway, including L-Arginine, inhaled NO, the phosphodiesterase inhibitor sildenafil and gene therapy.

Hypoxia-induced pulmonary endothelin-1 expression is unaltered by nitric oxide

Nitric oxide (NO) attenuates hypoxia-induced endothelin (ET)-1 expression in cultured umbilical vein endothelial cells. We hypothesized that NO similarly attenuates hypoxia-induced increases in ET-1 expression in the lungs of intact animals and reasoned that potentially reduced ET-1 levels may contribute to the protective effects of NO against the development of pulmonary hypertension during chronic hypoxia. As expected, hypoxic exposure (24 h, 10% O(2)) increased rat lung ET-1 peptide and prepro-ET-1 mRNA levels. Contrary to our hypothesis, inhaled NO (iNO) did not attenuate hypoxia-induced increases in pulmonary ET-1 peptide or prepro-ET-1 mRNA levels. Because of this surprising finding, we also examined the effects of NO on hypoxia-induced increases in ET peptide levels in cultured cell experiments. Consistent with the results of iNO experiments, administration of the NO donor S-nitroso-N-acetyl-penicillamine to cultured bovine pulmonary endothelial cells did not attenuate increases in ET peptide levels resulting from hypoxic (24 h, 3% O(2)) exposure. In additional experiments, we examined the effects of NO on the activity of a cloned ET-1 promoter fragment containing a functional hypoxia inducible factor-1 binding site in reporter gene experiments. Whereas moderate hypoxia (24 h, 3% O(2)) had no effect on ET-1 promoter activity, activity was increased by severe hypoxic (24 h, 0.5% O(2)) exposure. ET-1 promoter activity after S-nitroso-N-acetyl-penicillamine administration during severe hypoxia was greater than that in normoxic controls, although activity was reduced compared with that in hypoxic controls. These findings suggest that hypoxia-induced pulmonary ET-1 expression is unaffected by NO.