Chronic intrauterine pulmonary hypertension impairs endothelial nitric oxide synthase in the ovine fetus

Antenatal mesenchymal stromal cell extracellular vesicle treatment preserves lung development in a model of bronchopulmonary dysplasia due to chorioamnionitis

Antenatal stressors such as chorioamnionitis (CA) increase the risk for bronchopulmonary dysplasia (BPD). Studies have shown that experimental BPD can be ameliorated by postnatal treatment with mesenchymal stromal cell-derived extracellular vesicles (MEx). However, the antenatal efficacy of MEx to prevent BPD is unknown. To determine whether antenatal MEx therapy attenuates intrauterine inflammation and preserves lung growth in a rat model of CA-induced BPD. At embryonic day (E)20, rat litters were treated with intra-amniotic injections of saline, endotoxin (ETX) to model chorioamnionitis, MEx, or ETX plus MEx followed by cesarean section delivery with placental harvest at E22. Placental and lung evaluations were conducted at day 0 and day 14, respectively. To assess the effects of ETX and MEx on lung growth in vitro, E15 lung explants were imaged for distal branching. Placental tissues from ETX-exposed pregnancies showed increased expression of inflammatory markers NLRP-3 and IL-1ß and altered spiral artery morphology. In addition, infant rats exposed to intrauterine ETX had reduced alveolarization and pulmonary vessel density (PVD), increased right ventricular hypertrophy (RVH), and decreased lung mechanics. Intrauterine MEx therapy of ETX-exposed pups reduced inflammatory cytokines, normalized spiral artery architecture, and preserved distal lung growth and mechanics. In vitro studies showed that MEx treatment enhanced distal lung branching and increased VEGF and SPC gene expression. Antenatal MEx treatment preserved distal lung growth and reduced intrauterine inflammation in a model of CA-induced BPD. We speculate that MEx may provide a novel therapeutic strategy to prevent BPD due to antenatal inflammation.

Repurposing of the PDE5 Inhibitor Sildenafil for the Treatment of Persistent Pulmonary Hypertension in Neonates

Nitric oxide (NO), an important endogenous signaling molecule released from vascular endothelial cells and nerves, activates the enzyme soluble guanylate cyclase to catalyze the production of cyclic guanosine monophosphate (cGMP) from guanosine triphosphate. cGMP, in turn, activates protein kinase G to phosphorylate a range of effector proteins in smooth muscle cells that reduce intracellular Ca²⁺ levels to inhibit both contractility and proliferation. The enzyme phosphodiesterase type 5 (PDE5) curtails the actions of cGMP by hydrolyzing it into inactive 5'-GMP. Small molecule PDE5 inhibitors (PDE5is), such as sildenafil, prolong the availability of cGMP and therefore, enhance NO-mediated signaling. PDE5is are the first-line treatment for erectile dysfunction but are also now approved for the treatment of pulmonary arterial hypertension (PAH) in adults. Persistent pulmonary hypertension in neonates (PPHN) is currently treated with inhaled NO, but this is an expensive option and around 1/3 of newborns are unresponsive, resulting in the need for alternative approaches. Here the development, chemistry and pharmacology of PDE5is, the use of sildenafil for erectile dysfunction and PAH, are summarized and then current evidence for the utility of further repurposing of sildenafil, as a treatment for PPHN, is critically reviewed.

Pulmonary Hypertension: The Hidden Danger for Newborns

Despite growing awareness of the clinical importance of pulmonary hypertension (PH) in preterm infants, uncertainty persists regarding the different clinical settings in which abnormalities of pulmonary vascular growth, function, and structure contribute to high morbidity and mortality, and potential interventions to improve outcomes are uncertain. A major gap for improving outcomes of preterm infants with PH has been the limited characterization of the distinct settings of PH and related disease-specific mechanisms in preterm infants that represent diverse pulmonary vascular phenotypes of prematurity. In comparison with term newborns, preterm infants have a higher risk for developing hypoxemia due to suprasystemic levels of PH in preterm infants shortly after birth or persistent pulmonary hypertension of the newborn (PPHN). Variable and milder levels of PH have also been demonstrated in preterm infants without evidence of severe hypoxemic respiratory failure, suggesting delayed vascular transition of the lung which is associated with higher risks of mortality and developing bronchopulmonary dysplasia (BPD). In addition, early echocardiographic signs of PH at day 7 are strongly associated with the subsequent diagnosis of BPD, late PH, and respiratory disease throughout early childhood. In infants with evolving or established BPD, PH that persists beyond the first few months of life in preterm infants is associated with high mortality. Recent data further show that PVD can persist and cause PH in prematurely born adults. Overall, more precise characterization and studies of diverse pulmonary vascular phenotypes in preterm infants will be likely to improve the development of therapeutic strategies to optimize care of preterm infants with PH.

Perinatal Hypoxia-Inducible Factor Stabilization Preserves Lung Alveolar and Vascular Growth in Experimental Bronchopulmonary Dysplasia

Rationale: Antenatal inflammation with placental dysfunction is strongly associated with high bronchopulmonary dysplasia (BPD) risk in preterm infants. Whether antenatal or postnatal HIF (hypoxia-inducible factor) augmentation can preserve lung structure and function and prevent pulmonary hypertension after intrauterine inflammation is controversial.Objectives: To determine whether antenatal or postnatal prolyl-hydroxylase inhibitor (PHi) therapy increases lung HIF expression, preserves lung growth and function, and prevents pulmonary hypertension in a rat model of chorioamnionitis-induced BPD caused by antenatal inflammation.Methods: Endotoxin (ETX) was administered to pregnant rats by intraamniotic injection at Embryonic Day 20, and pups were delivered by cesarean section at Embryonic Day 22. Selective PHi drugs, dimethyloxalylglycine or GSK360A, were administered into the amniotic space at Embryonic Day 20 or after birth by intraperitoneal injection for 2 weeks. Placentas and lung tissue were collected at birth for morphometric and Western blot measurements of HIF-1a, HIF-2a, VEGF (vascular endothelial growth factor), and eNOS (endothelial nitric oxide synthase) protein contents. At Day 14, lung function was assessed, and tissues were harvested to determine alveolarization by radial alveolar counts, pulmonary vessel density, and right ventricle hypertrophy (RVH).Measurements and Main Results: Antenatal PHi therapy preserves lung alveolar and vascular growth and lung function and prevents RVH after intrauterine ETX exposure. Antenatal administration of PHi markedly upregulates lung HIF-1a, HIF-2a, VEGF, and eNOS expression after ETX exposure.Conclusions: HIF augmentation improves lung structure and function, prevents RVH, and improves placental structure following antenatal ETX exposure. We speculate that antenatal or postnatal PHi therapy may provide novel strategies to prevent BPD due to antenatal inflammation.

Redox Signaling and Persistent Pulmonary Hypertension of the Newborn

Reactive oxygen species (ROS) are redox-signaling molecules that are critically involved in regulating endothelial cell functions, host defense, aging, and cellular adaptation. Mitochondria are the major sources of ROS and important sources of redox signaling in pulmonary circulation. It is becoming increasingly evident that increased mitochondrial oxidative stress and aberrant signaling through redox-sensitive pathways play a direct causative role in the pathogenesis of many cardiopulmonary disorders including persistent pulmonary hypertension of the newborn (PPHN). This chapter highlights redox signaling in endothelial cells, antioxidant defense mechanism, cell responses to oxidative stress, and their contributions to disease pathogenesis.

Molecular physiopathogenetic mechanisms and development of new potential therapeutic strategies in persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN) is a cyanogenic plurifactorial disorder characterized by failed postnatal drop of pulmonary vascular resistance and maintenance of right-to-left shunt across ductus arteriosus and foramen ovale typical of intrauterine life. The pathogenesis of PPHN is very complex and can result from functional (vasoconstriction) or structural (arteriolar remodeling, reduced pulmonary vessels density) anomalies of pulmonary circulation. Etiopathogenetic factors heterogeneity can strongly condition therapeutical results and prognosis of PPHN that is particularly severe in organic forms that are usually refractory to selective pulmonary vasodilator therapy with inhaled nitric oxide. This paper reports the more recent acquisitions on molecular physiopathogenetic mechanisms underlying functional and structural forms of PPHN and illustrates the bases for adoption of new potential treatment strategies for organic PPHN. These strategies aim to reverse pulmonary vascular remodeling in PPHN with arteriolar smooth muscle hypertrophy and stimulate pulmonary vascular and alveolar growth in PPHN associated with lung hypoplasia.In order to restore lung growth in this severe form of PPHN, attention is focused on the results of studies of mesenchymal stem cells and their therapeutical paracrine effects on bronchopulmonry dysplasia, a chronic neonatal lung disease characterized by arrested vascular and alveolar growth and development of pulmonary hypertension.

Chronic intrauterine pulmonary hypertension increases main pulmonary artery stiffness and adventitial remodeling in fetal sheep

Persistent pulmonary hypertension of the newborn (PPHN) is a clinical syndrome that is characterized by high pulmonary vascular resistance due to changes in lung vascular growth, structure, and tone. PPHN has been primarily considered as a disease of the small pulmonary arteries (PA), but proximal vascular stiffness has been shown to be an important predictor of morbidity and mortality in other diseases associated with pulmonary hypertension (PH). The objective of this study is to characterize main PA (MPA) stiffness in experimental PPHN and to determine the relationship of altered biomechanics of the MPA with changes in extracellular matrix (ECM) content and orientation of collagen and elastin fibers. MPAs were isolated from control and PPHN fetal sheep model and were tested by planar biaxial testing to measure stiffness in circumferential and axial vessel orientations. Test specimens were fixed for histological assessments of the vascular wall ECM constituents collagen and elastin. MPAs from PPHN sheep had increased mechanical stiffness (P < 0.05) and altered ECM remodeling compared with control MPA. A constitutive mathematical model and histology demonstrated that PPHN vessels have a smaller contribution of elastin and a greater role for collagen fiber engagement compared with the control arteries. We conclude that exposure to chronic hemodynamic stress in late-gestation fetal sheep increases proximal PA stiffness and alters ECM remodeling. We speculate that proximal PA stiffness further contributes to increased right ventricular impedance in experimental PPHN, which contributes to abnormal transition of the pulmonary circulation at birth.

The Robyn Barst Memorial Lecture: Differences between the fetal, newborn, and adult pulmonary circulations: relevance for age-specific therapies (2013 Grover Conference series)

Pulmonary arterial hypertension (PAH) contributes to poor outcomes in diverse diseases in newborns, infants, and children. Many aspects of pediatric PAH parallel the pathophysiology and disease courses observed in adult patients; however, critical maturational differences exist that contribute to distinct outcomes and therapeutic responses in children. In comparison with adult PAH, disruption of lung vascular growth and development, or angiogenesis, plays an especially prominent role in the pathobiology of pediatric PAH. In children, abnormalities of lung vascular development have consequences well beyond the adverse hemodynamic effects of PAH alone. The developing endothelium also plays critical roles in development of the distal airspace, establishing lung surface area for gas exchange and maintenance of lung structure throughout postnatal life through angiocrine signaling. Impaired functional and structural adaptations of the pulmonary circulation during the transition from fetal to postnatal life contribute significantly to poor outcomes in such disorders as persistent pulmonary hypertension of the newborn, congenital diaphragmatic hernia, bronchopulmonary dysplasia, Down syndrome, and forms of congenital heart disease. In addition, several studies support the hypothesis that early perinatal events that alter lung vascular growth or function may set the stage for increased susceptibility to PAH in adult patients ("fetal programming"). Thus, insights into basic mechanisms underlying unique features of the developing pulmonary circulation, especially as related to preservation of endothelial survival and function, may provide unique therapeutic windows and distinct strategies to improve short- and long-term outcomes of children with PAH.

Decreases in manganese superoxide dismutase expression and activity contribute to oxidative stress in persistent pulmonary hypertension of the newborn

A rapid increase in the synthesis and release of nitric oxide (NO) facilitates the pulmonary vasodilation that occurs during birth-related transition. Alteration of this transition in persistent pulmonary hypertension of the newborn (PPHN) is associated with impaired function of endothelial nitric oxide synthase (eNOS) and an increase in oxidative stress. We investigated the hypothesis that a decrease in expression and activity of mitochondrial localized manganese superoxide dismutase (MnSOD) in pulmonary artery endothelial cells (PAEC) increases oxidative stress and impairs eNOS function in PPHN. We isolated PAEC and pulmonary arteries from fetal lambs with PPHN induced by prenatal ductus arteriosus ligation or sham ligation (control). We investigated MnSOD expression and activity, tyrosine nitration of MnSOD, and mitochondrial O(2)(-) levels in PAEC from control and PPHN lambs. We introduced exogenous MnSOD via an adenoviral vector (ad-MnSOD) transduction into PAEC and pulmonary arteries of PPHN lambs. The effect of ad-MnSOD was investigated on: mitochondrial O(2)(-) levels, MnSOD and eNOS expression and activity, intracellular hydrogen peroxide (H(2)O(2)) levels, and catalase expression in PAEC. MnSOD mRNA and protein levels and activity were decreased and MnSOD tyrosine nitration was increased in PPHN-PAEC. ad-MnSOD transduction of PPHN-PAEC increased its activity two- to threefold, decreased mitochondrial O(2)(-) levels, and increased H(2)O(2) levels and catalase expression. ad-MnSOD transduction improved eNOS expression and function and the relaxation response of PPHN pulmonary arteries. Our observations suggest that decreased MnSOD expression and activity contribute to the endothelial dysfunction observed in PPHN.

Risk factors for persistent pulmonary hypertension of the newborn

In utero, pulmonary blood flow is closely circumscribed and oxygenation and ventilation occur via the placental circulation. Within the first few breaths of air-breathing life, the perinatal pulmonary circulation undergoes a dramatic transition as pulmonary blood flow increases 10-fold and the pulmonary arterial blood pressure decreases by 50% within 24 hours of birth. With the loss of the placental circulation, the increase in pulmonary flow enables oxygen to enter the bloodstream. The physiologic mechanisms that account for the remarkable transition of the pulmonary circulation include establishment of an air-liquid interface, rhythmic distention of the lung, an increase in shear stress and elaboration of nitric oxide from the pulmonary endothelium. If the perinatal pulmonary circulation does not dilate, blood is shunted away from the lungs at the level of the patent foramen ovale and the ductus arteriosus leading to the profound and unremitting hypoxemia that characterizes persistent pulmonary hypertension of the newborn (PPHN), a syndrome without either optimally effective preventative or treatment strategies. Despite significant advances in treatment, PPHN remains a major cause of morbidity and mortality in neonatal centers across the globe. While there is information surrounding factors that might increase the risk of PPHN, knowledge remains incomplete. Cesarean section delivery, high maternal body mass index, maternal use of aspirin, nonsteroidal anti-inflammatory agents and maternal diabetes mellitus are among the factors associated with an increased risk for PPHN. Recent data suggest that maternal use of serotonin reuptake inhibitors might represent another important risk factor for PPHN.

Voltage-dependent anion channel-2 interaction with nitric oxide synthase enhances pulmonary artery endothelial cell nitric oxide production

Increased pulmonary artery endothelial cell (PAEC) endothelium-dependent nitric oxide synthase (eNOS) activity mediates perinatal pulmonary vasodilation. Compromised eNOS activity is central to the pathogenesis of persistent pulmonary hypertension of the newborn (PPHN). Voltage-derived anion channel (VDAC)-1 was recently demonstrated to bind eNOS in the systemic circulation. We hypothesized that VDAC isoforms modulate eNOS activity in the pulmonary circulation, and that decreased VDAC expression contributes to PPHN. In PAECs derived from an ovine model of PPHN: (1) there is eNOS activity, but not expression; and (2) VDAC1 and -2 proteins are decreased. Immunocytochemistry, coimmunoprecipitation, and in situ proximity ligation assays in human PAECs (hPAECs) demonstrate binding between eNOS and both VDAC1 and -2, which increased upon stimulation with NO agonists. The ability of agonists to increase the eNOS/VDAC interaction was significantly blunted in hypertensive, compared with normotensive, ovine PAECs. Depletion of VDAC2, but not VDAC1, blocked the agonist-induced increase in eNOS activity in hPAECs. Overexpression of VDAC2 in hypertensive PAECs increased eNOS activity. Binding of VDAC2 enhances eNOS activity in the pulmonary circulation, and diminished VDAC2 constrains eNOS in PAECs derived from fetal lambs with chronic intrauterine pulmonary hypertension. We speculate that decreases in VDAC2 may contribute to the limited eNOS activity that characterizes pulmonary hypertension.

Cross talk between NADPH oxidase and autophagy in pulmonary artery endothelial cells with intrauterine persistent pulmonary hypertension

Autophagy is a process for cells to degrade proteins or entire organelles to maintain a balance in the synthesis, degradation, and subsequent recycling of cellular products. Increased reactive oxygen species formation is known to induce autophagy. We previously reported that increased NADPH oxidase (NOX) activity in pulmonary artery endothelial cells (PAEC) from fetal lambs with persistent pulmonary hypertension (PPHN) contributes to impaired angiogenesis in PPHN-PAEC compared with normal PAEC. We hypothesized that increased NOX activity in PPHN-PAEC is associated with increased autophagy, which, in turn, contributes to impaired angiogenesis in PPHN-PAEC. In the present study, we detected increased autophagy in PPHN-PAEC as shown by increased ratio of the microtubule-associated protein 1 light chain (LC3)-II to LC3-I and increased percentage of green fluorescent protein-LC3 punctate positive cells. Inhibiting autophagy by 3-methyladenine, chloroquine, and beclin-1 knockdown in PPHN-PAEC has led to decreased autophagy and increased in vitro angiogenesis. Inhibition of autophagy also decreased the association between gp91(phox) and p47(phox), NOX activity, and superoxide generation. A nonspecific antioxidant N-acetylcysteine and a NOX inhibitor apocynin decreased autophagy in PPHN-PAEC. In conclusion, autophagy may contribute to impaired angiogenesis in PPHN-PAEC through increasing NOX activity. Our results suggest that, in PPHN-PAEC, a positive feedback relationship between autophagy and NOX activity may regulate angiogenesis.

Sepiapterin improves angiogenesis of pulmonary artery endothelial cells with in utero pulmonary hypertension by recoupling endothelial nitric oxide synthase

Persistent pulmonary hypertension of the newborn (PPHN) is associated with decreased blood vessel density that contributes to increased pulmonary vascular resistance. Previous studies showed that uncoupled endothelial nitric oxide (NO) synthase (eNOS) activity and increased NADPH oxidase activity resulted in marked decreases in NO bioavailability and impaired angiogenesis in PPHN. In the present study, we hypothesize that loss of tetrahydrobiopterin (BH4), a critical cofactor for eNOS, induces uncoupled eNOS activity and impairs angiogenesis in PPHN. Pulmonary artery endothelial cells (PAEC) isolated from fetal lambs with PPHN (HTFL-PAEC) or control lambs (NFL-PAEC) were used to investigate the cellular mechanisms impairing angiogenesis in PPHN. Cellular mechanisms were examined with respect to BH4 levels, GTP-cyclohydrolase-1 (GCH-1) expression, eNOS dimer formation, and eNOS-heat shock protein 90 (hsp90) interactions under basal conditions and after sepiapterin (Sep) supplementation. Cellular levels of BH4, GCH-1 expression, and eNOS dimer formation were decreased in HTFL-PAEC compared with NFL-PAEC. Sep supplementation decreased apoptosis and increased in vitro angiogenesis in HTFL-PAEC and ex vivo pulmonary artery sprouting angiogenesis. Sep also increased cellular BH4 content, NO production, eNOS dimer formation, and eNOS-hsp90 association and decreased the superoxide formation in HTFL-PAEC. These data demonstrate that Sep improves NO production and angiogenic potential of HTFL-PAEC by recoupling eNOS activity. Increasing BH4 levels via Sep supplementation may be an important therapy for improving eNOS function and restoring angiogenesis in PPHN.

Cinaciguat, a soluble guanylate cyclase activator, augments cGMP after oxidative stress and causes pulmonary vasodilation in neonatal pulmonary hypertension

Although inhaled NO (iNO) therapy is often effective in treating infants with persistent pulmonary hypertension of the newborn (PPHN), up to 40% of patients fail to respond, which may be partly due to abnormal expression and function of soluble guanylate cyclase (sGC). To determine whether altered sGC expression or activity due to oxidized sGC contributes to high pulmonary vascular resistance (PVR) and poor NO responsiveness, we studied the effects of cinaciguat (BAY 58-2667), an sGC activator, on pulmonary artery smooth muscle cells (PASMC) from normal fetal sheep and sheep exposed to chronic intrauterine pulmonary hypertension (i.e., PPHN). We found increased sGC α(1)- and β(1)-subunit protein expression but lower basal cGMP levels in PPHN PASMC compared with normal PASMC. To determine the effects of cinaciguat and NO after sGC oxidation in vitro, we measured cGMP production by normal and PPHN PASMC treated with cinaciguat and the NO donor, sodium nitroprusside (SNP), before and after exposure to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ, an sGC oxidizer), hyperoxia (fraction of inspired oxygen 0.50), or hydrogen peroxide (H(2)O(2)). After treatment with ODQ, SNP-induced cGMP generation was markedly reduced but the effects of cinaciguat were increased by 14- and 64-fold in PPHN fetal PASMC, respectively (P < 0.01 vs. controls). Hyperoxia or H(2)O(2) enhanced cGMP production by cinaciguat but not SNP in PASMC. To determine the hemodynamic effects of cinaciguat in vivo, we compared serial responses to cinaciguat and ACh in fetal lambs after ductus arteriosus ligation. In contrast with the impaired vasodilator response to ACh, cinaciguat-induced pulmonary vasodilation was significantly increased. After birth, cinaciguat caused a significantly greater fall in PVR than either 100% oxygen, iNO, or ACh. We conclude that cinaciguat causes more potent pulmonary vasodilation than iNO in experimental PPHN. We speculate that increased NO-insensitive sGC may contribute to the pathogenesis of PPHN, and cinaciguat may provide a novel treatment of severe pulmonary hypertension.

Epigenetic regulation of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is diagnosed as a sustained elevation of pulmonary arterial pressure to more than 25 mm Hg at rest or to more than 30 mm Hg with exercise. PAH is an intrinsic disease of the pulmonary vascular smooth muscle and endothelial cells in association with plexiform lesions, medial thickening, concentric laminar intimal fibrosis and thrombotic lesions. Pulmonary vascular remodeling is the characteristic pathological change of PAH. The pathogenesis of PAH has been studied at the level of smooth muscle and endothelial cells. Existing research does not adequately explain susceptibility to the disease, and recent evidence reveals that epigenetic alterations may be involved in PAH. Epigenetics refers to all heritable changes in phenotype or in gene expression states, including chromatin remodeling, DNA methylation, histone modification and RNA interference, which are not involved in the DNA sequence itself. This review will focus on recent advances in epigenetics related to PAH, including epigenetic changes of superoxide dismutase, endothelial nitric oxide synthase and the bone morphogenetic protein signaling pathway. This will provide new insight for improved treatment and prevention of PAH. Future work aimed at specific epigenetic treatments may prove to be an effective therapy for patients with PAH.

Nitric oxide alterations following acute ductal constriction in the fetal lamb: a role for superoxide

Acute partial compression of the fetal ductus arteriosus (DA) results in an initial abrupt increase in pulmonary blood flow (PBF), which is followed by a significant reduction in PBF to baseline values over the ensuing 2-4 h. We have previously demonstrated that this potent vasoconstricting response is due, in part, to an endothelin-1 (ET-1)-mediated decrease in nitric oxide synthase (NOS) activity. In addition, in vitro data demonstrate that ET-1 increases superoxide levels in pulmonary arterial smooth muscle cells and that oxidative stress alters NOS activity. Therefore, the objectives of this study were to determine the potential role of superoxide in the alterations of hemodynamics and NOS activity following acute ductal constriction in the late-gestation fetal lamb. Eighteen anesthetized near-term fetal lambs were instrumented, and a lung biopsy was performed. After a 48-h recovery, acute constriction of the DA was performed by inflating a vascular occluder. Polyethylene glycol-superoxide dismutase (PEG-SOD; 1,000-1,500 units/kg, n = 7) or PEG-alone (vehicle control group, n = 5) was injected into the pulmonary artery before ductal constriction. Six animals had a sham operation. In PEG-alone-treated lambs, acute ductal constriction rapidly decreased pulmonary vascular resistance (PVR) by 88%. However, by 4 h, PVR returned to preconstriction baseline. This vasoconstriction was associated with an increase in lung superoxide levels (82%), a decrease in total NOS activity (50%), and an increase in P-eNOS-Thr495 (52%) (P < 0.05). PEG-SOD prevented the increase of superoxide after ductal constriction, attenuated the vasoconstriction, preserved NOS activity, and increased P-eNOS Ser1177 (307%, P < 0.05). Sham procedure induced no changes. These data suggest that an acute decrease in NOS activity that is mediated, in part, by increased superoxide levels, and alterations in the phosphorylation status of the endothelial NOS isoform, underlie the pulmonary vascular response to acute ductal constriction.

Neonatal pulmonary hypertension

When the normal cardiopulmonary transition fails to occur, the result is persistent pulmonary hypertension of the newborn. Severe persistent pulmonary hypertension of the newborn is estimated to occur in 2 per 1000 live-born term infants, and some degree of pulmonary hypertension complicates the course of >10% of all neonates with respiratory failure. This review article discusses the vascular abnormalities that are associated with neonatal pulmonary hypertension, including recognition of its role in severe bronchopulmonary dysplasia in preterm infants. A systematic review of the evidence for common therapies including inhaled nitric oxide, high-frequency ventilation, surfactant, and extracorporeal life support is included. Finally, this field is rapidly evolving, and the rationale for promising new treatment approaches is reviewed, including inhibition of phosphodiesterases and scavengers of reactive oxygen species.

Transforming growth factor-beta modulates the expression of nitric oxide signaling enzymes in the injured developing lung and in vascular smooth muscle cells

Nitric oxide signaling has an important role in regulating pulmonary development and function. Expression of soluble guanylate cyclase (sGC) and cGMP-dependent protein kinase I (PKGI), both critical mediators of nitric oxide (NO) signaling, is diminished in the injured newborn lung through unknown mechanisms. Recent studies suggest that excessive transforming growth factor-beta (TGF-beta) activity inhibits injured newborn lung development. To explore mechanisms that regulate pulmonary NO signaling, we tested whether TGF-beta decreases sGC and PKGI expression in the injured developing lung and pulmonary vascular smooth muscle cells (SMC). We found that chronic oxygen-induced lung injury decreased pulmonary sGCalpha(1) and PKGI immunoreactivity in mouse pups and that exposure to a TGF-beta-neutralizing antibody prevented this reduction of sGC and PKGI protein expression. In addition, TGF-beta(1) decreased expression of NO signaling enzymes in freshly isolated pulmonary microvascular SMC/myofibroblasts, suggesting that TGF-beta has a direct role in modulating NO signaling in the pup lung. Moreover, TGF-beta(1) decreased sGC and PKGI expression in pulmonary artery and aortic SMC from adult rats and mice, suggesting a general role for TGF-beta in modulating NO signaling in vascular SMC. Although other cytokines decrease sGC mRNA stability, TGF-beta did not modulate sGCalpha(1) or PKGIbeta mRNA turnover in vascular SMC. These studies indicate for the first time that TGF-beta decreases NO signaling enzyme expression in the injured developing lung and pulmonary vascular SMC. Moreover, they suggest that TGF-beta-neutralizing molecules might counteract the effects of injury on NO signaling in the newborn lung.

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.

Advances in the diagnosis and management of persistent pulmonary hypertension of the newborn

Rapid evaluation of a neonate who is cyanotic and in respiratory distress is essential for achieving a good outcome. Persistent pulmonary hypertension of the newborn (PPHN) can be a primary cause or a contributing factor to respiratory failure, particularly in neonates born at 34 weeks or more of gestation. PPHN represents a failure of normal postnatal adaptation that occurs at birth in the pulmonary circulation. Rapid advances in therapy in recent years have led to a remarkable decrease in mortality for the affected infants. Infants who survive PPHN are at significant risk for long-term hearing and neurodevelopmental impairments, however. This review focuses on the diagnosis, recent advances in management, and recommendations for the long-term follow-up of infants who have PPHN.

Chronic intrauterine pulmonary hypertension increases endothelial cell Rho kinase activity and impairs angiogenesis in vitro

Persistent pulmonary hypertension of the newborn (PPHN) is characterized by endothelial dysfunction and decreased vascular growth. The role of Rho kinase activity in modulating endothelial function and regulating angiogenesis during normal lung development and in PPHN is unknown. We hypothesized that PPHN increases Rho kinase activity in fetal pulmonary artery endothelial cells (PAECs) and impairs angiogenesis in vitro. Proximal PAECs were harvested from fetal sheep with partial ligation of the ductus arteriosus in utero (PPHN) and age-matched controls. Rho kinase activity was measured by RhoA, Rho GTP, and phosphorylated MYPT-1 protein content. The effects of Rho kinase activity on angiogenesis, endothelial nitric oxide (NO) synthase (eNOS) protein expression, and NO production were determined in normal and PPHN PAECs. Angiogenesis was assessed by tube formation in vitro with/without Y-27632 (a Rho kinase inhibitor) and calpeptin (a Rho kinase activator) in the presence/absence of N-nitro-l-arginine (l-NA, an NOS inhibitor). RhoA, Rho GTP, and phosphorylated MYPT-1 protein were increased in PPHN PAECs. Tube formation was reduced 29% in PPHN PAECs (P < 0.001) and increased with Y-27632 treatment in normal and PPHN PAECs, with PPHN PAECs achieving levels similar to those of normal PAECs. l-NA inhibited the Y-27632-induced increase in tube formation in normal, but not PPHN, PAECs. Calpeptin reduced tube formation in normal and PPHN PAECs. eNOS expression was reduced 42% in PPHN PAECs (P < 0.01). Y-27632 increased eNOS protein and NO production in normal and PPHN PAECs. Calpeptin decreased eNOS protein only in normal PAECs but reduced NO production in normal and PPHN PAECs. We conclude that Rho kinase activity is increased in PPHN PAECs and impairs angiogenesis and downregulates eNOS protein and NO production in vitro.

Muscarinic receptor M1 and phosphodiesterase 1 are key determinants in pulmonary vascular dysfunction following perinatal hypoxia in mice

Perinatal adverse events such as limitation of nutrients or oxygen supply are associated with the occurrence of diseases in adulthood, like cardiovascular diseases and diabetes. We investigated the long-term effects of perinatal hypoxia on the lung circulation, with particular attention to the nitric oxide (NO)/cGMP pathway. Mice were placed under hypoxia in utero 5 days before delivery and for 5 days after birth. Pups were then bred in normoxia until adulthood. Adults born in hypoxia displayed an altered regulation of pulmonary vascular tone with higher right ventricular pressure in normoxia and increased sensitivity to acute hypoxia compared with controls. Perinatal hypoxia dramatically decreased endothelium-dependent relaxation induced by ACh in adult pulmonary arteries (PAs) but did not influence NO-mediated endothelium-independent relaxation. The M(3) muscarinic receptor was implicated in the relaxing action of ACh and M(1) muscarinic receptor (M(1)AChR) in its vasoconstrictive effects. Pirenzepine or telenzepine, two preferential inhibitors of M(1)AChR, abolished the adverse effects of perinatal hypoxia on ACh-induced relaxation. M(1)AChR mRNA expression was increased in lungs and PAs of mice born in hypoxia. The phosphodiesterase 1 (PDE1) inhibitor vinpocetine also reversed the decrease in ACh-induced relaxation following perinatal hypoxia, suggesting that M(1)AChR-mediated alteration of ACh-induced relaxation is due to the activation of calcium-dependent PDE1. Therefore, perinatal hypoxia leads to an altered pulmonary circulation in adulthood with vascular dysfunction characterized by impaired endothelium-dependent relaxation and M(1)AChR plays a predominant role. This raises the possibility that muscarinic receptors could be key determinants in pulmonary vascular diseases in relation to "perinatal imprinting."

Intrauterine pulmonary hypertension impairs angiogenesis in vitro: role of vascular endothelial growth factor nitric oxide signaling

RATIONALE

Mechanisms that impair angiogenesis in neonatal persistent pulmonary hypertension (PPHN) are poorly understood.

OBJECTIVES

To determine if PPHN alters fetal pulmonary artery endothelial cell (PAEC) phenotype and impairs growth and angiogenesis in vitro, and if altered vascular endothelial growth factor-nitric oxide (VEGF-NO) signaling contributes to this abnormal phenotype.

METHODS

Proximal PAECs were harvested from fetal sheep that had undergone partial ligation of the ductus arteriosus in utero (PPHN) and age-matched control animals. Growth and tube formation +/- VEGF and NO stimulation and inhibition were studied in normal and PPHN PAECs. Western blot analysis was performed for VEGF, VEGF receptor-2 (VEGF-R2), and endothelial NO synthase (eNOS) protein content. NO production with VEGF administration was measured in normal and PPHN PAECs.

MEASUREMENTS AND MAIN RESULTS

PPHN PAECs demonstrate decreased growth and tube formation in vitro. VEGF and eNOS protein expression were decreased in PPHN PAECs, whereas VEGF-R2 protein expression was not different. VEGF and NO increased PPHN PAEC growth and tube formation to values achieved in normal PAECs. VEGF inhibition decreased growth and tube formation in normal and PPHN PAECs. NOS inhibition decreased growth in normal and PPHN PAECs, but tube formation was only reduced in normal PAECs. NO reversed the inhibitory effects of VEGF-R2 inhibition on tube formation in normal and PPHN PAECs. VEGF increased NO production in normal and PPHN PAECs.

CONCLUSIONS

PPHN in utero causes sustained impairment of PAEC phenotype in vitro, with reduced PAEC growth and tube formation and down-regulation of VEGF and eNOS protein. VEGF and NO enhanced growth and tube formation of PPHN PAECs.

Hypoxia-inducible factors HIF-1alpha and HIF-2alpha are decreased in an experimental model of severe respiratory distress syndrome in preterm lambs

Respiratory distress syndrome (RDS) secondary to preterm birth and surfactant deficiency is characterized by severe hypoxemia, lung injury, and impaired production of nitric oxide (NO) and vascular endothelial growth factor (VEGF). Since hypoxia-inducible factors (HIFs) mediate the effects of both NO and VEGF in part through regulation by prolyl-hydroxylase-containing domains (PHDs) in the presence of oxygen, we hypothesized that HIF-1alpha and -2alpha in the lung are decreased following severe RDS in preterm neonatal lambs. To test this hypothesis, fetal lambs were delivered at preterm gestation (115-day gestation, term = 145 days; n = 4) and mechanically ventilated for 4 h. Lambs developed respiratory failure characterized by severe hypoxemia despite treatment with mechanical ventilation with high inspired oxygen concentrations. Lung samples were compared with nonventilated control animals at preterm (115-day gestation; n = 3) and term gestation (142-day gestation; n = 3). We found that HIF-1alpha protein expression decreased (P < 0.05) and PHD-2 expression increased (P < 0.005) at birth in normal term animals before air breathing. Compared with age-matched controls, HIF-1alpha protein and HIF-2alpha protein expression decreased by 80% and 55%, respectively (P < 0.005 for each) in preterm lambs with RDS. Furthermore, VEGF mRNA was decreased by 40%, and PHD-2 protein expression doubled in RDS lambs. We conclude that pulmonary expression of HIF-1alpha, HIF-2alpha, and the downstream target of their regulation, VEGF mRNA, is impaired following RDS in neonatal lambs. We speculate that early disruption of HIF and VEGF expression after preterm birth and RDS may contribute to long-term abnormalities in lung growth, leading to bronchopulmonary dysplasia.

Oxidant stress from uncoupled nitric oxide synthase impairs vasodilation in fetal lambs with persistent pulmonary hypertension

Persistent pulmonary hypertension of newborn (PPHN) is associated with decreased NO release and impaired pulmonary vasodilation. We investigated the hypothesis that increased superoxide (O(2)(*-)) release by an uncoupled endothelial nitric oxide synthase (eNOS) contributes to impaired pulmonary vasodilation in PPHN. We investigated the response of isolated pulmonary arteries to the NOS agonist ATP and the NO donor S-nitroso-N-acetylpenicillamine (SNAP) in fetal lambs with PPHN induced by prenatal ligation of ductus arteriosus and in sham-ligated controls in the presence or absence of the NOS antagonist nitro-L-arginine methyl ester (L-NAME) or the O(2)(*-) scavenger 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron). ATP caused dose-dependent relaxation of pulmonary artery rings in control lambs but induced constriction of the rings in PPHN lambs. L-NAME, the NO precursor L-arginine, and Tiron restored the relaxation response of pulmonary artery rings to ATP in PPHN. Relaxation to NO was attenuated in arteries from PPHN lambs, and the response was improved by L-NAME and by Tiron. We also investigated the alteration in heat shock protein (HSP)90-eNOS interactions and release of NO and O(2)(*-) in response to ATP in the pulmonary artery endothelial cells (PAEC) from these lambs. Cultured PAEC and endothelium of freshly isolated pulmonary arteries from PPHN lambs released O(2)(*-) in response to ATP, and this was attenuated by the NOS antagonist L-NAME and superoxide dismutase (SOD). ATP stimulated HSP90-eNOS interactions in PAEC from control but not PPHN lambs. HSP90 immunoprecipitated from PPHN pulmonary arteries had increased nitrotyrosine signal. Oxidant stress from uncoupled eNOS contributes to impaired pulmonary vasodilation in PPHN induced by ductal ligation in fetal lambs.

Rho kinase activation maintains high pulmonary vascular resistance in the ovine fetal lung

Mechanisms that maintain high pulmonary vascular resistance (PVR) in the fetal lung are poorly understood. Activation of the Rho kinase signal transduction pathway, which promotes actin-myosin interaction in vascular smooth muscle cells, is increased in the pulmonary circulation of adult animals with experimental pulmonary hypertension. However, the role of Rho kinase has not been studied in the fetal lung. We hypothesized that activation of Rho kinase contributes to elevated PVR in the fetus. To address this hypothesis, we studied the pulmonary hemodynamic effects of brief (10 min) intrapulmonary infusions of two specific Rho kinase inhibitors, Y-27632 (15-500 microg) and HA-1077 (500 microg), in chronically prepared late-gestation fetal lambs (n = 9). Y-27632 caused potent, dose-dependent pulmonary vasodilation, lowering PVR from 0.67 +/- 0.18 to 0.16 +/- 0.02 mmHg x ml(-1) x min(-1) (P < 0.01) at the highest dose tested without lowering systemic arterial pressure. Despite brief infusions, Y-27632-induced pulmonary vasodilation was sustained for 50 min. HA-1077 caused a similar fall in PVR, from 0.39 +/- 0.03 to 0.19 +/- 0.03 (P < 0.05). To study nitric oxide (NO)-Rho kinase interactions in the fetal lung, we tested the effect of Rho kinase inhibition on pulmonary vasoconstriction caused by inhibition of endogenous NO production with nitro-L-arginine (L-NA; 15-30 mg), a selective NO synthase antagonist. L-NA increased PVR by 127 +/- 73% above baseline under control conditions, but this vasoconstrictor response was completely prevented by treatment with Y-27632 (P < 0.05). We conclude that the Rho kinase signal transduction pathway maintains high PVR in the normal fetal lung and that activation of the Rho kinase pathway mediates pulmonary vasoconstriction after NO synthase inhibition. We speculate that Rho kinase plays an essential role in the normal fetal pulmonary circulation and that Rho kinase inhibitors may provide novel therapy for neonatal pulmonary hypertension.

Chronic intrauterine pulmonary hypertension selectively modifies pulmonary artery smooth muscle cell gene expression

Pulmonary artery smooth muscle cell (PASMC) relaxation at birth results from an increase in cytosolic cGMP, cGMP-dependent and kinase-mediated activation of the Ca2+-sensitive K+channel (KCa), and closure of voltage-operated Ca2+channels (VOCC). How chronic intrauterine pulmonary hypertension compromises perinatal pulmonary vasodilation remains unknown. We tested the hypothesis that chronic intrauterine pulmonary hypertension selectively modifies gene expression to mitigate perinatal pulmonary vasodilation mediated by the cGMP kinase-KCa-VOCC pathway. PASMC were isolated from late-gestation fetal lambs that had undergone either ligation of the ductus arteriosus (hypertensive) or sham operation (control) at 127 days of gestation and were maintained under either hypoxic (∼25 Torr) or normoxic (∼120 Torr) conditions in primary culture. We studied mRNA levels for cGMP kinase Iα (PKG-1α), the α-chain of VOCC (Cav1.2), and the α-subunit of the KCachannel. Compared with control PASMC, hypertensive PASMC had decreased VOCC, KCa, and PKG-1α expression. In response to sustained normoxia, expression of VOCC and KCachannel decreased and expression of PKG-1α increased. In contrast, sustained normoxia had no effect on PKG-1α levels and an attenuated effect on VOCC and KCachannel expression in hypertensive PASMC. Protein expression of PKG-1α was consistent with the mRNA data. We conclude that chronic intrauterine pulmonary hypertension decreases PKG expression and mitigates the genetic effects of sustained normoxia on pulmonary vasodilation, because gene expression remains compromised even after sustained exposure to normoxia.

Pulmonary vascular effects of nitric oxide-cGMP augmentation in a model of chronic pulmonary hypertension in fetal and neonatal sheep

Persistent pulmonary hypertension of the newborn (PPHN) is partly due to impaired nitric oxide (NO)-cGMP signaling. BAY 41-2272 is a novel direct activator of soluble guanylate cyclase, but whether this drug may be an effective therapy for PPHN is unknown. We hypothesized that BAY 41-2272 would cause pulmonary vasodilation in a model of severe PPHN. To test this hypothesis, we compared the hemodynamic response of BAY 41-2272 to acetylcholine, an endothelium-dependent vasodilator, and sildenafil, a selective inhibitor of PDE5 in chronically instrumented fetal lambs at 1 and 5 days after partial ligation of the ductus arteriosus. After 9 days, we delivered the animals by cesarean section to measure their hemodynamic responses to inhaled NO (iNO), sildenafil, and BAY 41-2272 alone or combined with iNO. BAY 41-2272 caused marked pulmonary vasodilation, as characterized by a twofold increase in blood flow and a nearly 60% fall in PVR at day 1. Effectiveness of BAY 41-2272-induced pulmonary vasodilation increased during the development of pulmonary hypertension. Despite a similar effect at day 1, the pulmonary vasodilator response to BAY 41-2272 was greater than sildenafil at day 5. At birth, BAY 41-2272 dramatically reduced PVR and augmented the pulmonary vasodilation induced by iNO. We concluded that BAY 41-2272 causes potent pulmonary vasodilation in fetal and neonatal sheep with severe pulmonary hypertension. We speculate that BAY 41-2272 may provide a novel treatment for severe PPHN, especially in newborns with partial response to iNO therapy.

Pulmonary vascular effects of sildenafil on the development of chronic pulmonary hypertension in the ovine fetus

We investigated the pulmonary vascular effects of prophylactic use of sildenafil, a specific phosphodiesterase-5 inhibitor, in late-gestation fetal lambs with chronic pulmonary hypertension. Fetal lambs were operated on at 129 +/- 1 days gestation (term = 147 days). Ductus arteriosus (DA) was compressed for 8 days to cause chronic pulmonary hypertension. Fetuses were treated with sildenafil (24 mg/day) or saline. Pulmonary vascular responses to increase in shear stress and in fetal PaO2 were studied at, respectively, day 4 and 6. Percent wall thickness of small pulmonary arteries (%WT) and the right ventricle-to-left ventricle plus septum ratio (RVH) were measured after completion of the study. In the control group, DA compression increased PA pressure (48 +/- 5 to 72 +/- 8 mmHg, P < 0.01) and pulmonary vascular resistance (PVR) (0.62 +/- 0.08 to 1.15 +/- 0.11 mmHg x ml(-1) x min(-1), P < 0.05). Similar increase in PAP was observed in the sildenafil group, but PVR did not change significantly (0.54 +/- 0.06 to 0.64 +/- 0.09 mmHg x ml(-1) x min(-1)). Acute DA compression, after brief decompression, elevated PVR 25% in controls and decreased PVR 35% in the sildenafil group. Increased fetal PaO2 did not change PVR in controls but decreased PVR 60% in the sildenafil group. %WT and RVH were not different between groups. Prophylactic sildenafil treatment prevents the rise in pulmonary vascular tone and altered vasoreactivity caused by DA compression in fetal lambs. These results support the hypothesis that elevated PDE5 activity is involved in the consequences of chronic pulmonary hypertension in the perinatal lung.

Effects of BAY 41-2272, a soluble guanylate cyclase activator, on pulmonary vascular reactivity in the ovine fetus

Nitric oxide (NO)-cGMP signaling plays a critical role during the transition of the pulmonary circulation at birth. BAY 41-2272 is a novel NO-independent direct stimulator of soluble guanylate cyclase that causes vasodilation in systemic and local circulations. However, the hemodynamic effects of BAY 41-2272 have not been studied in the perinatal pulmonary circulation. We hypothesized that BAY 41-2272 causes potent and sustained fetal pulmonary vasodilation. We performed surgery on 14 fetal lambs (125-130 days gestation; term = 147 days) and placed catheters in the main pulmonary artery, aorta, and left atrium to measure pressures. An ultrasonic flow transducer was placed on the left pulmonary artery (LPA) to measure blood flow, and a catheter was placed in the LPA for drug infusion. Pulmonary vascular resistance (PVR) was calculated as pulmonary artery pressure minus left atrial pressure divided by LPA blood flow. BAY 41-2272 caused dose-related increases in pulmonary blood flow up to threefold above baseline and reduced PVR by 75% (P < 0.01). Prolonged infusion of BAY 41-2272 caused sustained pulmonary vasodilation throughout the 120-min infusion period. The pulmonary vasodilator effect of BAY 41-2272 was not attenuated by N(omega)-nitro-l-arginine, a NO synthase inhibitor. In addition, compared with sildenafil, a phosphodiesterase 5 inhibitor, the pulmonary vasodilator response to BAY 41-2272 was more prolonged. We conclude that BAY 41-2272 causes potent and sustained fetal pulmonary vasodilation independent of NO release. We speculate that BAY 41-2272 may have therapeutic potential for pulmonary hypertension associated with failure to circulatory adaptation at birth, especially in the setting of impaired NO production.

Soluble guanylyl cyclase during postnatal porcine pulmonary maturation

The nitric oxide (NO)/cGMP pathway plays a key role in the regulation of pulmonary vascular tone during the transition from the fetal to the neonatal circulation, and it is impaired in pathophysiological conditions such as pulmonary hypertension. In the present study, we have analyzed the changes in the function and expression of soluble guanylyl cyclase (sGC) in pulmonary arteries during early postnatal maturation in isolated third-branch pulmonary arteries from newborn (3-18 h of age) and 2-wk-old piglets. The expression of sGC beta(1)-subunit in pulmonary arteries increased with postnatal age both at the level of mRNA and protein. The catalytic region of porcine sGC beta(1) was sequenced, showing a 92% homology with the human sequence. This age-dependent increase in sGC expression correlated with increased vasorelaxant responses to the physiological sGC activator NO and to the exogenous sGC activator YC-1, but not to the membrane-permeable cGMP analog 8-bromoguanosine 3',5'-cyclic monophosphate. In conclusion, an increased expression of sGC in pulmonary conduit arteries from 2-wk-old compared with newborn piglets explains, at least partly, the age-dependent increase in the vasorelaxant response of NO and other activators of sGC.

New approaches for persistent pulmonary hypertension of newborn

The management of PPHN entered a new era with the development of inhaled NO therapy for the relief of pulmonary hypertension. The wider application of INO therapy and improved ventilation strategies led to a decrease in the need for invasive life-sustaining therapies such as ECMO. The remarkable advances in the understanding and treatment of PPHN were made possible by the extensive investigations in the laboratory using animal models. Further decreases in morbidity and mortality are possible with specific strategies targeted to correct the alterations in NO and prostacyclin biology and strategies to reduce lung injury. Further research is needed to understand the basis for the biologic susceptibility of some infants to environmental insults such as intra-uterine stressor exposure to NSAIDs in utero.

Chronic intrauterine pulmonary hypertension compromises fetal pulmonary artery smooth muscle cell O2 sensing

To test the hypothesis that chronic intrauterine pulmonary hypertension (PHTN) compromises pulmonary artery (PA) smooth muscle cell (SMC) O2 sensing, fluorescence microscopy was used to study the effect of an acute increase in Po2 on the cytosolic Ca2+ concentration ([Ca2+]i) of chronically hypoxic subconfluent monolayers of PA SMC in primary culture. PA SMCs were derived from fetal lambs with PHTN due to intrauterine ligation of the ductus arteriosus. Acute normoxia decreased [Ca2+]i in control but not PHTN PA SMC. In control PA SMC, [Ca2+]i increased after Ca2+-sensitive (KCa) and voltage-sensitive (Kv) K+ channel blockade and decreased after diltiazem treatment. In PHTN PA SMC, KCa blockade had no effect, whereas Kv blockade and diltiazem increased [Ca2+]i. Inhibition of sarcoplasmic reticulum Ca2+ ATPase activity caused a greater increase in [Ca2+]i in controls compared with PHTN PA SMC. Conversely, ryanodine caused a greater increase of [Ca2+]i in PHTN compared with control PA SMC. KCa channel mRNA is decreased and Kv channel mRNA is unchanged in PHTN PA SMC compared with controls. We conclude that PHTN compromises PA SMC O2 sensing, alters intracellular Ca2+ homeostasis, and changes the predominant ion channel that determines basal [Ca2+]i from KCa to Kv.

Endothelial nitric oxide synthase (NOS) is upregulated in rapid progressive pulmonary hypertension of the newborn

OBJECTIVE

To provide evidence for the upregulation of endothelial nitric oxide synthase (eNOS) or inducible nitric oxide synthase (iNOS) in the assumed imbalance in the pathophysiology of rapid progressive pulmonary hypertension of the newborn (RPPHN), which is characterized by abnormal hypertrophy of the pulmonary arterioles and arteries leading to increased pulmonary vascular resistance. Furthermore, to determine the cellular source and topographic distribution of eNOS and iNOS.

MATERIAL AND METHODS

Lung biopsies were taken from two term neonates with clinical and echocardiographic evidence of RPPH and of three controls. Biopsies were obtained at an early stage of the disease as well as at post mortem and examined immunohistochemically for the presence of eNOS, iNOS and nitrotyrosine.

RESULTS

The endothelial cells of pulmonary arterioles stained significantly for eNOS protein in RPPHN patients. This was not the case in the control infants. There were no differences for nitrotyrosine or iNOS between RPPHN patients and controls.

CONCLUSION

Rapid progressive pulmonary hypertension of the newborn leads to compensatory induction of eNOS synthesis specifically in endothelial cells of the pulmonary arterioles. This mechanism of compensation can lead to delayed presentation of RPPHN during the late neonatal period. Exogenous inhaled nitric oxide therapy does not lead to suppression of the endogenous synthesis of nitric oxide.

Systemic arteriovenous fistula leads to pulmonary artery remodeling and abnormal vasoreactivity in the fetal lamb

Several cases of systemic arteriovenous fistula diagnosed in the human fetus have been associated with the postnatal development of persistent pulmonary hypertension. The aim of this study was to determine the effects of a prenatally created systemic arteriovenous fistula on the structure and reactivity of the pulmonary circulation in the fetal lamb. A fistula between the jugular vein and carotid artery was created in fetal lambs at 119-124 days of gestation. At delivery (134-139 days), left pulmonary artery (LPA) pressure was increased in the fistula group (n = 12) compared with controls (n = 11, P < 0.01). The pulmonary vascular resistance was significantly higher in the fistula group (P < 0.05), whereas mean LPA blood flow was not statistically different between the two groups. Morphometric analysis of the pulmonary vascular bed revealed an increase in the number of peripheral muscular arteries, together with an increase in pulmonary arterial medial thickness in the fistula group. There was no difference in the relative number or size of intraacinar arteries. In vitro organ bath studies on pulmonary arterial rings showed impaired endothelium-dependent relaxation in the fistula group compared with controls. However, endothelial nitric oxide synthase protein expression was similar in both groups, whereas endothelium-independent relaxation to sodium nitroprusside was greater in the fistula group compared with controls. A systemic arteriovenous fistula leads to both structural and functional alteration of the pulmonary vasculature, which might lead to the development of persistent pulmonary hypertension after birth.

Decreased association of HSP90 impairs endothelial nitric oxide synthase in fetal lambs with persistent pulmonary hypertension

Persistent pulmonary hypertension of newborn (PPHN) is associated with decreased nitric oxide (NO) release and impaired pulmonary vasodilation. We investigated the hypothesis that decreased association of heat shock protein 90 (HSP90) with endothelial NO synthase (eNOS) impairs NO release and vasodilation in PPHN. The responses to the NOS agonist ATP were investigated in fetal lambs with PPHN induced by prenatal ligation of ductus arteriosus, and in sham ligation controls. ATP caused dose-dependent vasodilation in control pulmonary resistance arteries, and this response was attenuated in PPHN vessels. The response of control pulmonary arteries to ATP was attenuated by NG-nitro-l-arginine methyl ester (l-NAME), a NOS antagonist, and geldanamycin, an inhibitor of HSP90-eNOS interaction. The attenuated response to ATP observed in PPHN was improved by pretreatment of vessels with l-NAME or 4,5-dihydroxy-1,3-benzene-disulfonate, a superoxide scavenger. Pulmonary arteries from PPHN lambs had decreased basal levels of HSP90 in association with eNOS. Association of HSP90 with eNOS and NO release increased in response to ATP in control pulmonary artery endothelial cells, but not in cells from PPHN lambs. Decreased HSP90-eNOS interactions may contribute to the impaired NO release and vasodilation observed in the ductal ligation model of PPHN.

Role of platelet-derived growth factor in vascular remodeling during pulmonary hypertension in the ovine fetus

Platelet-derived growth factor (PDGF) is a potent smooth muscle cell mitogen that may contribute to smooth muscle hyperplasia during the development of chronic pulmonary hypertension (PH). We studied changes in PDGFalpha- and beta-receptor and ligand expression in lambs with chronic intrauterine PH induced by partial ligation of the ductus arteriosus (DA) at gestational age 124-128 days (term = 147 days). Western blot analysis performed on whole lung homogenates from PH animals after 8 days of DA ligation showed a twofold increase in PDGFalpha- and beta-receptor proteins compared with age-matched controls (P < 0.05). Lung PDGF-A and -B mRNA expression did not differ between PH and control animals. We treated PH animals with NX1975, an aptamer that selectively inhibits PDGF-B, by infusion into the left pulmonary artery for 7 days after DA ligation. NX1975 reduced the development of muscular thickening of small pulmonary arteries by 47% (P < 0.05) and right ventricular hypertrophy (RVH) by 66% (P < 0.02). Lung PDGFalpha- and beta-receptor expression is increased in perinatal PH, and NX1975 reduces the increase in wall thickness of small pulmonary arteries and RVH in this model. We speculate that PDGF signaling contributes to structural vascular remodeling in perinatal PH and that selective PDGF inhibition may provide a novel therapeutic strategy for the treatment of chronic PH.

Exhaled NO is reduced at an early stage of hypoxia-induced pulmonary hypertension in newborn piglets

Altered nitric oxide (NO) production could contribute to the pathogenesis of hypoxia-induced pulmonary hypertension. To determine whether parameters of lung NO are altered at an early stage of hypoxia-induced pulmonary hypertension, newborn piglets were exposed to room air (control, n = 21) or 10% O(2) (hypoxia, n = 19) for 3-4 days. Some lungs were isolated and perfused for measurement of exhaled NO output and the perfusate accumulation of nitrite and nitrate (NOx-), the stable metabolites of NO. Pulmonary arteries (20-600-microm diameter) and their accompanying airways were dissected from other lungs and incubated for NOx- determination. Abundances of the nitric oxide synthase (NOS) isoforms endothelial NOS and neural NOS were assessed in homogenates of PAs and airways. The perfusate NOx- accumulation was similar, whereas exhaled NO output was lower for isolated lungs of hypoxic, compared with control, piglets. The incubation solution NOx- did not differ between pulmonary arteries (PAs) of the two groups but was lower for airways of hypoxic, compared with control, piglets. Abundances of both eNOS and nNOS proteins were similar for PA homogenates from the two groups of piglets but were increased in airway homogenates of hypoxic compared with controls. The NO pathway is altered in airways, but not in PAs, at an early stage of hypoxia-induced pulmonary hypertension in newborn piglets.

Intrauterine hypertension decreases lung VEGF expression and VEGF inhibition causes pulmonary hypertension in the ovine fetus

Although vascular endothelial growth factor (VEGF) plays a vital role in lung vascular growth in the embryo, its role in maintaining endothelial function and modulating vascular structure during late fetal life has not been studied. We hypothesized that impaired lung VEGF signaling causes pulmonary hypertension, endothelial dysfunction, and structural remodeling before birth. To determine whether lung VEGF expression is decreased in an experimental model of persistent pulmonary hypertension of the newborn (PPHN), we measured lung VEGF and VEGF receptor protein content from fetal lambs 7-10 days after ductus arteriosus ligation (132-140 days gestation; term = 147 days). In contrast with the surge in lung VEGF expression during late gestation in controls, chronic intrauterine pulmonary hypertension reduced lung VEGF expression by 78%. To determine whether VEGF inhibition during late gestation causes pulmonary hypertension, we treated fetal lambs with EYE001, an aptamer that specifically inhibits VEGF(165). Compared with vehicle controls, EYE001 treatment elevated pulmonary artery pressure and pulmonary vascular resistance by 22 and 50%, respectively, caused right ventricular hypertrophy, and increased wall thickness of small pulmonary arteries. EYE001 treatment reduced lung endothelial nitric oxide synthase protein content by 50% and preferentially impaired the pulmonary vasodilator response to ACh, an endothelium-dependent agent. We conclude that chronic intrauterine pulmonary hypertension markedly decreases lung VEGF expression and that selective inhibition of VEGF(165) mimics the structural and physiological changes of experimental PPHN. We speculate that hypertension downregulates VEGF expression in the developing lung and that impaired VEGF signaling may contribute to the pathogenesis of PPHN.

Contribution of the K(Ca) channel to membrane potential and O2 sensitivity is decreased in an ovine PPHN model

Ca2+-sensitive K+ (K(Ca)) channels play an important role in mediating perinatal pulmonary vasodilation. We hypothesized that lung K(Ca) channel function may be decreased in persistent pulmonary hypertension of the newborn (PPHN). To test this hypothesis, pulmonary artery smooth muscle cells (PASMC) were isolated from fetal lambs with severe pulmonary hypertension induced by ligation of the ductus arteriosus in fetal lambs at 125-128 days gestation. Fetal lambs were killed after pulmonary hypertension had been maintained for at least 7 days. Age-matched, sham-operated animals were used as controls. PASMC K+ currents and membrane potentials were recorded using amphotericin B-perforated patch-clamp techniques. The increase in whole cell current normally seen in response to normoxia was decreased (333.9 +/- 63.6% in control vs. 133.1 +/- 16.0% in hypertensive fetuses). The contribution of the K(Ca) channel to the whole cell current was diminished in hypertensive, compared with control, fetal PASMC. In PASMC from hypertensive fetuses, a change from hypoxia to normoxia caused no change in membrane potential compared with a -14.6 +/- 2.8 mV decrease in membrane potential in PASMC from control animals. In PASMC from animals with pulmonary hypertension, 4-aminopyridine (4-AP) caused a larger depolarization than iberiotoxin, whereas in PASMC from control animals, iberiotoxin caused a larger depolarization than 4-AP. These data confirm the hypothesis that the contribution of the K(Ca) channel to membrane potential and O2 sensitivity is decreased in an ovine model of PPHN, and this may contribute to the abnormal perinatal pulmonary vasoreactivity associated with PPHN.

Oxygen-dependent signaling in pulmonary vascular smooth muscle

The pulmonary circulation constricts in response to acute hypoxia, which is reversible on reexposure to oxygen. On exposure to chronic hypoxia, in addition to vasoconstriction, the pulmonary vasculature undergoes remodeling, resulting in a sustained increase in pulmonary vascular resistance that is not immediately reversible. Hypoxic pulmonary vasoconstriction is physiological in the fetus, and there are many mechanisms by which the pulmonary vasculature relaxes at birth, principal among which is the acute increase in oxygen. Oxygen-induced signaling mechanisms, which result in pulmonary vascular relaxation at birth, and the mechanisms by which chronic hypoxia results in pulmonary vascular remodeling in the fetus and adult, are being investigated. Here, the roles of cGMP-dependent protein kinase in oxygen-mediated signaling in fetal pulmonary vascular smooth muscle and the effects of chronic hypoxia on ion channel activity and smooth muscle function such as contraction, growth, and gene expression were discussed.

Nitric oxide-endothelin-1 interactions after acute ductal constriction in fetal lambs

Acute partial compression of the fetal ductus arteriosus (DA) results in an initial increase in pulmonary blood flow (PBF) that is followed by acute vasoconstriction. The objective of the present study was to determine the role of nitric oxide (NO)-endothelin-1 (ET-1) interactions in the acute changes in pulmonary vascular tone after in utero partial constriction of the DA. Twelve late-gestation fetal lambs (132-140 days) were instrumented to measure vascular pressures and left PBF. After a 24-h recovery period, acute constriction of the DA was performed by partially inflating a vascular occluder, and the hemodynamic variables were observed for 4 h. In control lambs (n = 7), acute ductal constriction initially increased PBF by 627% (P < 0.05). However, this was followed by active vasoconstriction, such that PBF was restored to preconstriction values by 4 h. This was associated with a 43% decrease in total NO synthase (NOS) activity (P < 0.05) and a 106% increase in plasma ET-1 levels (P < 0.05). Western blot analysis demonstrated no changes in lung tissue endothelial NOS, preproET-1, endothelin-converting enzyme-1, or ET(B) receptor protein levels. The infusion of PD-156707 (an ET(A) receptor antagonist, n = 5) completely blocked the vasoconstriction and preserved NOS activity. These data suggest that the fetal pulmonary vasoconstriction after acute constriction of the DA is mediated by NO-ET-1 interactions. These include an increase in ET(A) receptor-mediated vasoconstriction and an ET(A) receptor-mediated decrease in NOS activity. The mechanisms of these NO-ET-1 interactions, and their role in mediating acute changes in PBF, warrant further studies.

Chronic hypertension impairs flow-induced vasodilation and augments the myogenic response in fetal lung

We hypothesized that altered vasoreactivity in perinatal pulmonary hypertension (PH) is characterized by abnormal responses to hemodynamic stress, including the loss of flow-induced vasodilation and an augmented myogenic response. Therefore, we studied the acute hemodynamic effects of brief compression of the ductus arteriosus (DA) in control fetal lambs and in lambs during exposure to chronic PH. In both groups, acute DA compression decreased pulmonary vascular resistance (PVR) by 20% at baseline (day 0). After 2 days of hypertension, acute DA compression paradoxically increased PVR by 50% in PH lambs, whereas PVR decreased by 25% in controls. During the 8-day study period, PVR increased during acute DA compression in PH lambs, whereas acute DA compression continued to cause vasodilation in controls. Brief treatment with the nitric oxide (NO) synthase inhibitor nitro-L-arginine (L-NA) increased basal PVR in control but not PH lambs, suggesting decreased NO production in PH lambs. Chronic hypertension increased the myogenic response after L-NA in PH lambs, whereas the myogenic response remained unchanged in controls. The myogenic response was inhibited by nifedipine in PH lambs, suggesting that the myogenic response is dependent upon the influx of extracellular calcium. We conclude that chronic PH impairs flow-induced vasodilation and increases the myogenic response in fetal lung. We speculate that decreased NO signaling and an augmented myogenic response contributes to abnormal vasoreactivity in PH.

eNOS and prostanoid enzymes in lungs of newborn piglets with chronic aortopulmonary shunts

Our purpose was to determine if abundance of proteins underlying nitric oxide (NO) and prostanoid production is altered in lungs of piglets with aortopulmonary shunts. We also evaluated whether shunted piglets exhibit abnormal pulmonary vascular responses to ACh, an endothelium-dependent agent that mediates dilation in part by NO and prostanoid release. At age 4-5 days, piglets underwent either a sham operation or placement of an aortopulmonary shunt. At age 5-6 wk, pulmonary arterial pressure (Ppa) and cardiac output by the thermodilution technique were measured in anesthetized piglets. Ppa responses to the endothelium-dependent agent, ACh, and to a non-endothelium-dependent agent, papaverine, were measured in perfused lungs. An immunoblot technique was applied to homogenates of whole lung tissue and two size groups of pulmonary arteries. In shunted piglets, Ppa and cardiac output were elevated, and Ppa responses to papaverine were reduced. ACh responses were not decreased when expressed relative to Ppa dilation with papaverine. Endothelial nitric oxide synthase (eNOS), cyclooxygenase-1, cyclooxygenase-2, prostacyclin synthase, and thromboxane synthase amounts were unaltered in all lung tissue homogenates. Altered abundance of eNOS and/or prostanoid enzymes does not contribute to the blunted dilation and the elevation in Ppa associated with aortopulmonary shunts in newborn piglets.

Chronic intrauterine pulmonary hypertension decreases calcium-sensitive potassium channel mRNA expression

Calcium-sensitive potassium (K(Ca)) channels play a critical role in mediating perinatal pulmonary vasodilation. Because infants with persistent pulmonary hypertension of the newborn (PPHN) have blunted vasodilator responses to birth-related stimuli, we hypothesized that lung K(Ca) channel gene expression is decreased in PPHN. To test this hypothesis, we measured K(Ca) channel gene expression in distal lung homogenates from both fetal lambs with severe pulmonary hypertension caused by prolonged compression of the ductus arteriosus and age-matched, sham-operated animals (controls). After at least 9 days of compression of the ductus arteriosus, fetal lambs were killed. To determine lung K(Ca) channel mRNA levels, primers were designed against the known sequence of the K(Ca) channel and used in semiquantitative RT-PCR, with lung 18S rRNA content as an internal control. Compared to that in control lambs, lung K(Ca) channel mRNA content in the PPHN group was reduced by 26 +/- 6% (P < 0.02), whereas lung voltage-gated K(+) 2.1 mRNA content was unchanged. We conclude that lung K(Ca) channel mRNA expression is decreased in an ovine model of PPHN. Decreased K(Ca) channel gene expression may contribute to the abnormal pulmonary vascular reactivity associated with PPHN.

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

Endothelin (ET)-1 contributes to regulation of pulmonary vascular tone and structure in the normal ovine fetus and in models of perinatal pulmonary hypertension. The hemodynamic effects of ET-1 are due to activation of its receptors. The ET(A) receptor mediates vasoconstriction and smooth muscle cell proliferation, whereas the ET(B) receptor mediates vasodilation. In a lamb model of chronic intrauterine pulmonary hypertension, ET(B) receptor activity and gene expression are decreased. To determine whether prolonged ET(B) receptor blockade causes pulmonary hypertension, we studied the hemodynamic effects of selective ET(B) receptor blockade with BQ-788. Animals were treated with an infusion of either BQ-788 or vehicle for 7 days. Prolonged BQ-788 treatment increased pulmonary arterial pressure and pulmonary vascular resistance (P < 0.05). The pulmonary vasodilator response to sarafotoxin 6c, a selective ET(B) receptor agonist, was attenuated after 7 days of BQ-788 treatment, demonstrating pharmacological blockade of the ET(B) receptor. Animals treated with BQ-788 had greater right ventricular hypertrophy and muscularization of small pulmonary arteries (P < 0. 05). Lung ET-1 levels were threefold higher in the animals treated with BQ-788 (P < 0.05). We conclude that prolonged selective ET(B) receptor blockade causes severe pulmonary hypertension and pulmonary vascular remodeling in the late-gestation ovine fetus.

Early abnormalities of pulmonary vascular development in the Fawn-Hooded rat raised at Denver's altitude

The Fawn-Hooded rat (FHR) is a genetic strain that has been extensively studied as a model of primary pulmonary hypertension in adult rats. Based on our recent observations that alveolar number and pulmonary arterial density are reduced in FHRs raised at Denver's altitude, we hypothesized that early abnormalities in pulmonary vascular development contribute to the progression of pulmonary hypertension in the FHR. We found that endothelial nitric oxide synthase (eNOS) protein content was lower in the lungs of fetal, 1- and 7-day-old, 3-week-old, and adult FHRs compared with that in the normal Sprague-Dawley (SDR) and Fischer rat strains, all raised at Denver's altitude. In contrast, lung expression of the endothelial proteins kinase insert domain-containing receptor/fetal liver kinase-1 (KDR/Flk-1) and platelet endothelial cell adhesion molecule-1 (CD31) was not different between strains. Barium arteriograms showed that pulmonary arterial density was reduced in 3-week-old FHRs compared with SDRs. Perinatal treatment of FHRs with mild hyperbaria to simulate sea-level alveolar PO(2) improved lung eNOS content and pulmonary vascular growth and reduced right ventricular hypertrophy. We conclude that the development of pulmonary hypertension in Denver-raised FHRs is characterized by reductions in lung eNOS expression and abnormal pulmonary vascular growth during the fetal, neonatal, and postnatal periods.

Developmental changes in endothelin expression and activity in the ovine fetal lung

Mechanisms that regulate endothelin (ET) in the perinatal lung are complex and poorly understood, especially with regard to the role of ET before and after birth. We hypothesized that the ET system is developmentally regulated and that the balance of ET(A) and ET(B) receptor activity favors vasoconstriction. To test this hypothesis, we performed a series of molecular and physiological studies in the fetal lamb, newborn lamb, and adult sheep. Lung preproET-1 mRNA levels, tissue ET peptide levels, and cellular localization of ET-1 expression were determined by Northern blot analysis, peptide assay, and immunohistochemistry in distal lung tissue from fetal lambs between 70 and 140 days (term = 145 days), newborn lambs, and ewes. Lung mRNA expression for the ET(A) and ET(B) receptors was also measured at these ages. We found that preproET-1 mRNA expression increased from 113 to 130 days gestation. Whole lung ET protein content was highest at 130 days gestation but decreased before birth in the fetal lamb lung. Immunolocalization of ET-1 protein showed expression of ET-1 in the vasculature and bronchial epithelium at all gestational ages. ET(A) receptor mRNA expression and ET(B) receptor mRNA increased from 90 to 125 and 135 days gestation. To determine changes in activity of the ET(A) and ET(B) receptors, we studied the effect of selective antagonists to the ET(A) or ET(B) receptors at 120, 130, and 140 days of fetal gestation. ET(A) receptor-mediated vasoconstriction increased from 120 to 140 days, whereas blockade of the ET(B) receptor did not change basal fetal pulmonary vascular tone at any age examined. We conclude that the ET system is developmentally regulated and that the increase in ET(A) receptor gene expression correlates with the onset of the vasodilator response to ET(A) receptor blockade. Although ET(B) receptor gene expression increases during late gestation, the balance of ET receptor activity favors vasoconstriction under basal conditions. We speculate that changes in ET receptor activity play important roles in regulation of pulmonary vascular tone in the ovine fetus.

Decreased synthesis and vasodilation to nitric oxide in piglets with hypoxia-induced pulmonary hypertension

Nitric oxide (NO) is thought to play an important role in the regulation of neonatal pulmonary vasculature. It has been suggested that neonates with pulmonary hypertension have a defective NO pathway. Therefore, we measured in 1-day-old piglets exposed to hypoxia (fraction of inspired O(2) = 0.10) for 3 or 14 days to induce pulmonary hypertension 1) the activity of NO synthase (NOS) via conversion of L-arginine to L-citrulline and the concentration of the NO precursor L-arginine in isolated pulmonary vessels, 2) the vasodilator response to the NO donor 3-morpholinosydnonimine-N-ethylcarbamide (SIN-1) and the cGMP analog 8-bromo-cGMP in isolated perfused lungs, and 3) the production of cGMP in response to SIN-1 in isolated perfused lungs. After 3 days of exposure to hypoxia, endothelial NOS (eNOS) activity was unaffected, whereas, after 14 days of hypoxia, eNOS activity was decreased in the cytosolic fraction of pulmonary artery (P < 0.05) but not of pulmonary vein homogenates. Inducible NOS activity was decreased in the cytosolic fraction of pulmonary artery homogenates after both 3 (P < 0.05) and 14 (P < 0.05) days of hypoxia but was unchanged in pulmonary veins. Pulmonary artery levels of L-arginine were unaffected by hypoxic exposure. After 3 days of exposure to hypoxia, the reduction in the dilator response to SIN-1 (P < 0.05) coincided with a decrease in cGMP production (P < 0.005), suggesting that soluble guanylate cyclase activity may be altered. When the exposure was prolonged to 14 days, dilation to SIN-1 remained decreased (P < 0.05) and, although cGMP production normalized, the dilator response to 8-bromo-cGMP decreased (P < 0.05), suggesting that, after prolonged exposure to hypoxia, cGMP-dependent mechanisms may also be impaired. In conclusion, neonatal hypoxia-induced pulmonary hypertension is associated with multiple disruptions in the NO pathway.