Prolonged endothelin B receptor blockade causes pulmonary hypertension in the ovine fetus

The role of teratogens in neural crest development

The neural crest (NC), discovered by Wilhelm His 150 years ago, gives rise to a multipotent migratory embryonic cell population that generates a remarkably diverse and important array of cell types during the development of the vertebrate embryo. These cells originate in the neural plate border (NPB), which is the ectoderm between the neural plate and the epidermis. They give rise to the neurons and glia of the peripheral nervous system, melanocytes, chondrocytes, smooth muscle cells, odontoblasts and neuroendocrine cells, among others. Neurocristopathies are a class of congenital diseases resulting from the abnormal induction, specification, migration, differentiation or death of NC cells (NCCs) during embryonic development and have an important medical and societal impact. In general, congenital defects affect an appreciable percentage of newborns worldwide. Some of these defects are caused by teratogens, which are agents that negatively impact the formation of tissues and organs during development. In this review, we will discuss the teratogens linked to the development of many birth defects, with a strong focus on those that specifically affect the development of the NC, thereby producing neurocristopathies. Although increasing attention is being paid to the effect of teratogens on embryonic development in general, there is a strong need to critically evaluate the specific role of these agents in NC development. Therefore, increased understanding of the role of these factors in NC development will contribute to the planning of strategies aimed at the prevention and treatment of human neurocristopathies, whose etiology was previously not considered.

Endothelin-1-Rho kinase interactions impair lung structure and cause pulmonary hypertension after bleomycin exposure in neonatal rat pups

Bronchopulmonary dysplasia (BPD) is the chronic lung disease associated with premature birth, characterized by impaired vascular and alveolar growth. In neonatal rats bleomycin decreases lung growth and causes pulmonary hypertension (PH), which is poorly responsive to nitric oxide. In the developing lung, through Rho kinase (ROCK) activation, ET-1 impairs endothelial cell function; however, whether ET-1-ROCK interactions contribute to impaired vascular and alveolar growth in experimental BPD is unknown. Neonatal rats were treated daily with intraperitoneal bleomycin with and without selective ETA (BQ123/BQ610) and ETB (BQ788) receptor blockers, nonselective ET receptor blocker (ETRB) (bosentan), or fasudil (ROCK inhibitor). At day 14, lungs were harvested for morphometrics, and measurements of Fulton's index (RV/LV+S), medial wall thickness (MWT), and vessel density. Lung ET-1 protein and ROCK activity (phospho-MYPT-1:total MYPT-1 ratio) were also measured by Western blot analysis. Bleomycin increased lung ET-1 protein expression by 65%, RV/LV+S by 60%, mean linear intercept (MLI) by 212%, and MWT by 140% and decreased radial alveolar count (RAC) and vessel density by 40 and 44%, respectively (P < 0.01 for each comparison). After bleomycin treatment, fasudil and bosentan partially restored RAC and vessel density and decreased MLI, RV/LV+S, and MWT to normal values. Bleomycin increased ROCK activity by 120%, which was restored to normal values by bosentan but not selective ETRB. We conclude that ET-1-ROCK interactions contribute to decreased alveolar and vascular growth and PH in experimental BPD. We speculate that nonselective ETRB and ROCK inhibitors may be effective in the treatment of infants with BPD and PH.

Hypoxic pulmonary hypertension of the newborn

Hypoxic pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance and pressure due to vascular remodeling and increased vessel tension secondary to chronic hypoxia during the fetal and newborn period. In comparison to the adult, the pulmonary vasculature of the fetus and the newborn undergoes tremendous developmental changes that increase susceptibility to a hypoxic insult. Substantial evidence indicates that chronic hypoxia alters the production and responsiveness of various vasoactive agents such as endothelium-derived nitric oxide, endothelin-1, prostanoids, platelet-activating factor, and reactive oxygen species, resulting in sustained vasoconstriction and vascular remodeling. These changes occur in most cell types within the vascular wall, particularly endothelial and smooth muscle cells. At the cellular level, suppressed nitric oxide-cGMP signaling and augmented RhoA-Rho kinase signaling appear to be critical to the development of hypoxic pulmonary hypertension of the newborn.

Vascular endothelin receptor type B: structure, function and dysregulation in vascular disease

Endothelin-1 (ET-1) is a major regulator of vascular function, acting via both endothelin receptor type A (ET(A)R) and type B (ET(B)R). Although the role of ET(A)R in vascular smooth muscle (VSM) contraction has been studied, little is known about ET(B)R. ET(B)R is a G-protein coupled receptor with a molecular mass of ~50 kDa and 442 amino acids arranged in seven transmembrane domains. Alternative splice variants of ET(B)R and heterodimerization and cross-talk with ET(A)R may affect the receptor function. ET(B)R has been identified in numerous blood vessels with substantial effects in the systemic, renal, pulmonary, coronary and cerebral circulation. ET(B)R in the endothelium mediates the release of relaxing factors such as nitric oxide, prostacyclin and endothelium-derived hyperpolarizing factor, and could also play a role in ET-1 clearance. ET(B)R in VSM mediates increases in [Ca(2+)](i), protein kinase C, mitogen-activated protein kinase and other pathways of VSM contraction and cell growth. ET-1/ET(A)R signaling has been associated with salt-sensitive hypertension (HTN) and pulmonary arterial hypertension (PAH), and ET(A)R antagonists have shown some benefits in these conditions. In search for other pathogenetic factors and more effective approaches, the role of alterations in endothelial ET(B)R and VSM ET(B)R in vascular dysfunction, and the potential benefits of modulators of ET(B)R in treatment of HTN and PAH are being examined. Combined ET(A)R/ET(B)R antagonists could be more efficacious in the management of conditions involving upregulation of ET(A)R and ET(B)R in VSM. Combined ET(A)R antagonist with ET(B)R agonist may need to be evaluated in conditions associated with decreased endothelial ET(B)R expression/activity.

Regulation of the Pulmonary Circulation in the Fetus and Newborn

During the development of the pulmonary vasculature in the fetus, many structural and functional changes occur to prepare the lung for the transition to air breathing. The development of the pulmonary circulation is genetically controlled by an array of mitogenic factors in a temporo-spatial order. With advancing gestation, pulmonary vessels acquire increased vasoreactivity. The fetal pulmonary vasculature is exposed to a low oxygen tension environment that promotes high intrinsic myogenic tone and high vasocontractility. At birth, a dramatic reduction in pulmonary arterial pressure and resistance occurs with an increase in oxygen tension and blood flow. The striking hemodynamic differences in the pulmonary circulation of the fetus and newborn are regulated by various factors and vasoactive agents. Among them, nitric oxide, endothelin-1, and prostaglandin I2 are mainly derived from endothelial cells and exert their effects via cGMP, cAMP, and Rho kinase signaling pathways. Alterations in these signaling pathways may lead to vascular remodeling, high vasocontractility, and persistent pulmonary hypertension of the newborn.

Endothelin receptor selectivity: evidence from in vitro and pre-clinical models of scleroderma

Scleroderma [systemic sclerosis (SSc)] is a spectrum of connective tissue diseases characterized by micro- and macro-vasculopathy, inflammation and autoimmunity and tissue remodelling that often leads to excessive scarring and fibrosis in both interstitial and vascular compartments. Pre-clinical investigations and gene association studies have led to improved understanding of the cell and molecular mechanisms underlying disease pathogenesis and to the identification of key molecular candidates that may represent potentially useful disease biomarkers and effective therapeutic targets. Studies on the endothelin (ET) system, pre-dominantly ET-1 and the cell surface receptors [type A (ET(A))] and type B (ET(B))], have provided evidence for an important role of this system in the vascular and fibrotic pathologies in SSc. To date, promising clinical results, utilizing dual/mixed ET receptor antagonism have been obtained in two of the vascular complications associated with SSc, ischaemic digital ulceration and pulmonary arterial hypertension. Evidence suggests that ET-1 is able to activate and re-program the functional phenotypes of vascular smooth muscle cells, microvascular pericytes and tissue fibroblasts into pro-fibrogenic cell populations with myofibroblasts-like properties. The impact of receptor-selective, over mixed-receptor, antagonism has also been studied in vitro with respect to cell differentiation and proliferation, extracellular matrix synthesis, production and deposition and in pathological cellular contraction. However, the complexity of the ET system, potential for receptor cross-talk, interactions with down-stream signal transduction cascades, as well as the potent inter-relationships with other important ligand-receptor pathways have made in vivo studies difficult to unravel. Moreover, little information is available on the role of the ET system and receptor selectivity in the recruitment and activation of mesenchymal progenitor cells in tissue remodelling and fibrosis or on the early inflammatory response. Here, we discuss the available pre-clinical evidence for the role of the ET system in tissue repair, scarring and fibrosis, using the connective tissue diseases SSc and model systems of fibrogenesis.

The endothelin system as a therapeutic target in cardiovascular disease: great expectations or bleak house?

There is considerable evidence that the potent vasoconstrictor endothelin-1 (ET-1) contributes to the pathogenesis of a variety of cardiovascular diseases. As such, pharmacological manipulation of the ET system might represent a promising therapeutic goal. Many clinical trials have assessed the potential of ET receptor antagonists in cardiovascular disease, the most positive of which have resulted in the licensing of the mixed ET receptor antagonist bosentan, and the selective ET(A) receptor antagonists, sitaxsentan and ambrisentan, for the treatment of pulmonary arterial hypertension (PAH). In contrast, despite encouraging data from in vitro and animal studies, outcomes in human heart failure have been disappointing, perhaps illustrating the risk of extrapolating preclinical work to man. Many further potential applications of these compounds, including resistant hypertension, chronic kidney disease, connective tissue disease and sub-arachnoid haemorrhage are currently being investigated in the clinic. Furthermore, experience from previous studies should enable improved trial design and scope remains for development of improved compounds and alternative therapeutic strategies. Although ET-converting enzyme inhibitors may represent one such alternative, there have been relatively few suitable compounds developed, and consequently, clinical experience with these agents remains extremely limited. Recent advances, together with an increased understanding of the biology of the ET system provided by improved experimental tools (including cell-specific transgenic deletion of ET receptors), should allow further targeting of clinical trials to diseases in which ET is involved and allow the therapeutic potential for targeting the ET system in cardiovascular disease to be fully realized.

Hypoxic pulmonary hypertension (HPH) and iptakalim, a novel ATP-sensitive potassium channel opener targeting smaller arteries in hypertension

Hypoxic pulmonary hypertension (HPH) is a serious and potentially devastating chronic disorder of the pulmonary circulation. Attempts to use drugs in the therapy of hypoxic pulmonary hypertension indicated the importance of prevention or reduction of vasoconstriction as well as of the reversal of remodeling within the cardiovascular system. Iptakalim (2,3-dimethyl-N-(1-methylethyl)-2-butylamine), a novel ATP-sensitive potassium channel opener, has the desired effects on hypoxic pulmonary arteries. Iptakalim decreases the elevated mean pressure in pulmonary arteries, and attenuates remodeling in the right ventricle, pulmonary arteries and airways. Moreover, iptakalim has selective antihypertensive effects: it significantly lowers arterial pressure in hypertensive animals, but has little if any effect in normotensive animals. In HPH iptakalim has selective effects on smaller arteries. Long-term iptakalim therapy decreases expression of sulfonylurea receptor 2 and of mRNA of inwardly rectifying potassium channel in smaller arteries of spontaneously hypertensive rats. Iptakalim inhibits the effects of endothelin-1, reduces the intracellular calcium concentration and inhibits the cell cycle in smooth muscle cells of pulmonary arteries. There is no evidence for the development of tolerance to the long-lasting antihypertensive action of iptakalim. At therapeutic doses iptakalim has no effects on the central nervous, respiratory, digestive, or endocrine systems. It has a broad therapeutic range, so that it can be safely used in the therapy of HPH.

Dual ET(A)/ET(B) vs. selective ET(A) endothelin receptor antagonism in patients with pulmonary hypertension

Since the identification of endothelin as a key mediator in the pathogenesis of several diseases, including pulmonary arterial hypertension (PAH), the pharmacologic control of the activated endothelin system with endothelin receptor antagonists (ETRA) has been a major therapeutic achievement for the treatment of patients with PAH. To date, dual ET(A)/ET(B) and selective ET(A) receptor antagonists have clinically been evaluated. To answer the question of whether selective or dual ETRA is preferable in patients with PAH, experimental and clinical data with relevance to the pulmonary circulation are reviewed in this article. Whereas experimental and clinical data provide unambiguous evidence that ET(A) receptors mediate the detrimental effects of ET-1, such as vasoconstriction and cell proliferation, the elucidation of the role of ET(B) receptors has been more complex. It has been shown that there is a subpopulation of ET(B) receptors on smooth muscle cells and fibroblasts mediating vasoconstriction and proliferation. On the contrary, there is clear evidence that endothelial ET(B) receptors continue to mediate vasodilation, vasoprotection and ET-1 clearance despite the pathology associated with pulmonary hypertension. More difficult to assess is the net effect of these mechanisms in patients to be treated with ETRA. When considering the available data from controlled clinical trials, nonselectivity does not appear to carry a relevant clinical benefit for the treatment of patients with PAH when compared with selective ET(A) receptor antagonism.

Increased hydrogen peroxide downregulates soluble guanylate cyclase in the lungs of lambs with persistent pulmonary hypertension of the newborn

Similar to infants born with persistent pulmonary hypertension of the newborn (PPHN), there is an increase in circulating endothelin-1 (ET-1) and decreased cGMP-mediated vasodilation in an ovine model of PPHN. These abnormalities lead to vasoconstriction and vascular remodeling. Our previous studies have demonstrated that reactive oxygen species (ROS) levels are increased in pulmonary arterial smooth muscle cells (PASMC) exposed to ET-1. Thus the initial objective of this study was to determine whether the development of pulmonary hypertension in utero is associated with elevated production of the ROS hydrogen peroxide (H(2)O(2)) and if this is associated with alterations in antioxidant capacity. Second we wished to determine whether chronic exposure of PASMC isolated from fetal lambs to H(2)O(2) would mimic the decrease in soluble guanylate cyclase expression observed in the ovine model of PPHN. Our results indicate that H(2)O(2) levels are significantly elevated in pulmonary arteries isolated from 136-day-old fetal PPHN lambs (P 0.05). In addition, we determined that catalase and glutathione peroxidase expression and activities remain unchanged. Also, we found that the overnight exposure of fetal PASMC to a H(2)O(2)-generating system resulted in significant decreases in soluble guanylate cyclase expression and nitric oxide (NO)-dependent cGMP generation (P 0.05). Finally, we demonstrated that the addition of the ROS scavenger catalase to isolated pulmonary arteries normalized the vasodilator responses to exogenous NO. As these scavengers had no effect on the vasodilator responses in pulmonary arteries isolated from age-matched control lambs this enhancement appears to be unique to PPHN. Overall our data suggest a role for H(2)O(2) in the abnormal vasodilation associated with the pulmonary arteries of PPHN lambs.

Development of occlusive neointimal lesions in distal pulmonary arteries of endothelin B receptor-deficient rats: a new model of severe pulmonary arterial hypertension

BACKGROUND

Human pulmonary arterial hypertension (PAH) is characterized by proliferation of vascular smooth muscle and, in its more severe form, by the development of occlusive neointimal lesions. However, few animal models of pulmonary neointimal proliferation exist, thereby limiting a complete understanding of the pathobiology of PAH. Recent studies of the endothelin (ET) system demonstrate that deficiency of the ET(B) receptor predisposes adult rats to acute and chronic hypoxic PAH, yet these animals fail to develop neointimal lesions. Herein, we determined and thereafter showed that exposure of ET(B) receptor-deficient rats to the endothelial toxin monocrotaline (MCT) leads to the development of neointimal lesions that share hallmarks of human PAH.

METHODS AND RESULTS

The pulmonary hemodynamic and morphometric effects of 60 mg/kg MCT in control (MCT(+/+)) and ET(B) receptor-deficient (MCT(sl/sl)) rats at 6 weeks of age were assessed. MCT(sl/sl) rats developed more severe PAH, characterized by elevated pulmonary artery pressure, diminished cardiac output, and right ventricular hypertrophy. In MCT(sl/sl) rats, morphometric evaluation revealed the presence of neointimal lesions within small distal pulmonary arteries, increased medial wall thickness, and decreased arterial-to-alveolar ratio. In keeping with this, barium angiography revealed diminished distal pulmonary vasculature of MCT(sl/sl) rat lungs. Cells within neointimal lesions expressed smooth muscle and endothelial cell markers. Moreover, cells within neointimal lesions exhibited increased levels of proliferation and were located in a tissue microenvironment enriched with vascular endothelial growth factor, tenascin-C, and activated matrix metalloproteinase-9, factors already implicated in human PAH. Finally, assessment of steady state mRNA showed that whereas expression of ET(B) receptors was decreased in MCT(sl/sl) rat lungs, ET(A) receptor expression increased.

CONCLUSIONS

Deficiency of the ET(B) receptor markedly accelerates the progression of PAH in rats treated with MCT and enhances the appearance of cellular and molecular markers associated with the pathobiology of PAH. Collectively, these results suggest an overall antiproliferative effect of the ET(B) receptor in pulmonary vascular homeostasis.

Developmental changes in endothelial vasoactive and angiogenic growth factors in the human perinatal lung

Little is known of the mechanisms underlying the marked fall in pulmonary vascular resistance that occurs at birth, but changes in the expression of endothelial vasoactive and angiogenic factors during lung development might play a key role. Nitric oxide, endothelin-1, and vascular endothelial growth factor have critical effects on vascular tone and cell growth. Here, we investigated the protein expression of endothelial nitric oxide synthase, endothelin-1 and its receptors, and vascular endothelial growth factor in pulmonary necropsy samples from 14 fetuses of different gestational ages and from 5 infants. Expression of endothelin-1 and its receptor endothelin-A was strong and stable. In contrast, expression of the endothelin-B receptor was weak in early gestation, then increased markedly in mid-gestation and remained high thereafter. The expression of endothelial nitric oxide synthase and vascular endothelial growth factor fell markedly after mid-gestation and remained low thereafter. These data point to a discrepancy between maturational and functional changes in human pulmonary vascular structures. The weak perinatal expression of endothelial nitric oxide could suggest that other potent vasodilatory mediators are responsible for the marked vasodilation observed at birth.

Endothelin-1 decreases endothelial NOS expression and activity through ETA receptor-mediated generation of hydrogen peroxide

Similar to infants born with persistent pulmonary hypertension of the newborn (PPHN), there is an increase in circulating endothelin-1 (ET-1) and decreased endothelial nitric oxide synthase (eNOS) gene expression in an ovine model of PPHN. These abnormalities lead to vasoconstriction and vascular remodeling. Our previous studies have demonstrated that reactive oxygen species (ROS) levels are elevated in the pulmonary arteries from PPHN lambs and that ET-1 increases ROS production in pulmonary arterial smooth muscle cells (PASMC) in culture. Thus the objective of this study was to determine whether there was a feedback mechanism between the ET-1-mediated increase in ROS in fetal PASMC (FPASMC) and a decrease in eNOS gene expression in fetal pulmonary arterial endothelial cells (FPAEC). Our results indicate that ET-1 increased H2O2 levels in FPASMC in an endothelin A receptor-dependent fashion. This was observed in both FPASMC monoculture and in cocultures of FPASMC and FPAEC. Conversely, ET-1 decreased H2O2 levels in FPAEC monoculture in an endothelin B receptor-dependent fashion. Furthermore, ET-1 decreased eNOS promoter activity by 40% in FPAEC in coculture with FPASMC. Promoter activity was restored in the presence of catalase. In FPAEC in monoculture treated with 0-100 microM H2O2, 12 microM had no effect on eNOS promoter activity, but it increased eNOS protein levels by 50%. However, at 100 microM, H2O2 decreased eNOS promoter activity and protein levels in FPAEC by 79 and 40%, respectively. These data suggest a role for smooth muscle cell-derived H2O2 in ET-1-mediated downregulation of eNOS expression in children born with PPHN.

Exaggerated hypoxic pulmonary hypertension in endothelin B receptor-deficient rats

Mechanisms by which endothelin (ET)-1 mediates chronic pulmonary hypertension remain incompletely understood. Although activation of the ET type A (ET(A)) receptor causes vasoconstriction, stimulation of ET type B (ET(B)) receptors can elicit vasodilation or vasoconstriction. We hypothesized that the ET(B) receptor attenuates the development of hypoxic pulmonary hypertension and studied a genetic rat model of ET(B) receptor deficiency (transgenic sl/sl). After 3 wk of severe hypoxia, the transgenic sl/sl pulmonary vasculature lacked expression of mRNA for the ET(B) receptor and developed exaggerated pulmonary hypertension that was characterized by elevated pulmonary arterial pressure, diminished cardiac output, and increased total pulmonary resistance. Plasma ET-1 was fivefold higher in transgenic sl/sl rats than in transgenic controls. Although mRNA for prepro-ET-1 was not different, mRNA for ET-converting enzyme-1 was higher in transgenic sl/sl than in transgenic control lungs. Hypertensive lungs of sl/sl rats also produced less nitric oxide metabolites and 6-ketoprostaglandin F(1alpha), a metabolite of prostacyclin, than transgenic controls. These findings suggest that the ET(B) receptor plays a protective role in the pulmonary hypertensive response to chronic hypoxia.

Natriuretic peptide gene expression in DOCA-salt hypertension after blockade of type B endothelin receptor

We investigated the effect of long-term in vivo blockade of the ET-1 receptor subtype B (ET(B)) with A-192621, a selective ET(B) antagonist, on atrial and ventricular natriuretic peptide (NP) gene expression in deoxycorticosterone acetate (DOCA)-salt hypertension. In this model, stimulation of the cardiac natriuretic peptide (NP) and the endothelin system and suppression of the renin-angiotensin system is observed. DOCA-salt induced significant hypertension, cardiac hypertrophy and increased NP plasma and left atrial and right and left ventricular NP gene expression. ET(B) blockade per se produced hypertension and left ventricular hypertrophy but induced little change on the levels of ventricular NP and only increased left atrial natriuretic factor (ANF) mRNA levels. Combined ET(B) blockade/DOCA-salt treatment worsened hypertension, increased left ventricular hypertrophy and induced right ventricular hypertrophy. All animals so treated had increased ventricular NP gene expression. Collagen III and beta-myosin heavy chain gene expression were enhanced in both the right and the left ventricle of DOCA-salt hypertensive rats. The results of this study suggest that the ET(B) receptor does not participate directly in the modulation of atrial or ventricular NP gene expression and that this receptor mediates a protective cardiovascular function. ET(B) blockade can induce significant ventricular hypertrophy without an increase in ANF or brain NP gene expression.

Modulation of pulmonary vasomotor tone in the fetus and neonate

The high pulmonary vascular resistance (PVR) of atelectatic, hypoxic, fetal lungs limits intrauterine pulmonary blood flow (PBF) to less than 10% of combined right and left ventricular output. At birth, PVR decreases precipitously to accommodate the entire cardiac output. The present review focuses on the role of endothelium-derived nitric oxide (NO), prostacyclin, and vascular smooth muscle potassium channels in mediating the decrease in PVR that occurs at birth, and in maintaining reduced pulmonary vasomotor tone during the neonatal period. The contribution of vasodilator and vasoconstrictor modulator activity to the pathophysiology of neonatal pulmonary hypertension is also addressed.

Effect of ABT-627 (A-147627), a potent selective ET(A) receptor antagonist, on the cardiopulmonary profile of newborn lambs with surgically-induced diaphragmatic hernia

1. Postnatal mortality in isolated congenital diaphragmatic hernia (CDH) is mainly related to the associated pulmonary hypertension (PH) and to right-to-left shunting. 2. Endothelins (ETs) are potent vasoconstrictors and pro-mitogenic peptides. Strong evidences support their participation in CDH and in the etiology of PH via the activation of ET(A) receptors (ET(A)-Rs). 3. Evaluation of the effect of ABT-627, a selective non-peptidic ET(A)-R antagonist, given from -15 to 210 min post-delivery (1 mg kg(-1) bolus +0.01 mg kg(-1) h(-1) infusion, i.v.), was conducted in the lamb model of CDH. 4. Severity of CDH was assessed in comparison to untreated controls (n=5). Untreated CDH lambs (n=7) had a higher mean pulmonary arterial pressure (MPAP; P<0.0001), lower mean blood pressure (MBP; P=0.0004), higher MPAP / MBP ratio (P<0.0001), lower arterial pH (P<0.0001), higher paCO(2) (P<0.0001), lower paO(2) (P<0.0001) and lower post-ductal pulsatile SaO(2) (P<0.0001) than untreated controls. 5. Treated controls (n=7) showed a higher MPAP, lower MBP, higher MPAP/MBP ratio, lower arterial pH, higher paCO(2), lower paO(2), lower post-ductal pulsatile SaO(2) and lower plasmatic ir-ET ratios compared to untreated controls (P<0.0001). 6. Treated CDH lambs (n=8) showed a higher MBP (P<0.0001), lower MPAP / MBP ratio (P<0.0001), higher arterial pH (P<0.0001), lower paCO(2) (P<0.0001), higher paO(2) (P=0.0228), higher post-ductal pulsatile SaO(2) (P=0.0016) and lower plasmatic ir-ET ratios (P=0.0247) when compared to untreated CDH lambs. 7. These observations revealed that, although acute perinatal treatment with a selective non-peptidic ET(A)-R antagonist had some adverse effects in controls, it attenuated the progressive cardiopulmonary deterioration that occurred after birth in CDH lambs.