Pulmonary NO synthase expression is attenuated in a fetal baboon model of chronic lung disease

Nitric oxide regulation of fetal and newborn lung development and function

In the developing lung, nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) signaling are essential in regulating lung formation and vascular tone. Animal studies have linked many anatomical and pathophysiological features of newborn lung disease to abnormalities in the NO/cGMP signaling system. They have demonstrated that driving this system with agonists and antagonists alleviates many of them. This research has spurred the rapid clinical development, testing, and application of several NO/cGMP-targeting therapies with the hope of treating and potentially preventing significant pediatric lung diseases. However, there are instances when the therapeutic effectiveness of these agents is limited. Studies indicate that injury-induced disruption of several critical components within the signaling system may hinder the promise of some of these therapies. Recent research has identified basic mechanisms that suppress NO/cGMP signaling in the injured newborn lung. They have also pinpointed biomarkers that offer insight into the activation of these pathogenic mechanisms and their influence on the NO/cGMP signaling system's integrity in vivo. Together, these will guide the development of new therapies to protect NO/cGMP signaling and safeguard newborn lung development and function. This review summarizes the important role of the NO/cGMP signaling system in regulating pulmonary development and function and our evolving understanding of how it is disrupted by newborn lung injury.

Pharmacotherapy in Bronchopulmonary Dysplasia: What Is the Evidence?

Bronchopulmonary Dysplasia (BPD) is a multifactorial disease affecting over 35% of extremely preterm infants born each year. Despite the advances made in understanding the pathogenesis of this disease over the last five decades, BPD remains one of the major causes of morbidity and mortality in this population, and the incidence of the disease increases with decreasing gestational age. As inflammation is one of the key drivers in the pathogenesis, it has been targeted by majority of pharmacological and non-pharmacological methods to prevent BPD. Most extremely premature infants receive a myriad of medications during their stay in the neonatal intensive care unit in an effort to prevent or manage BPD, with corticosteroids, caffeine, and diuretics being the most commonly used medications. However, there is no consensus regarding their use and benefits in this population. This review summarizes the available literature regarding these medications and aims to provide neonatologists and neonatal providers with evidence-based recommendations.

Perinatal Hyperoxia and Developmental Consequences on the Lung-Brain Axis

Approximately 11.1% of all newborns worldwide are born preterm. Improved neonatal intensive care significantly increased survival rates over the last decades but failed to reduce the risk for the development of chronic lung disease (i.e., bronchopulmonary dysplasia (BPD)) and impaired neurodevelopment (i.e., encephalopathy of prematurity (EoP)), two major long-term sequelae of prematurity. Premature infants are exposed to relative hyperoxia, when compared to physiological in-utero conditions and, if needed to additional therapeutic oxygen supplementation. Both are associated with an increased risk for impaired organ development. Since the detrimental effects of hyperoxia on the immature retina are known for many years, lung and brain have come into focus in the last decade. Hyperoxia-induced excessive production of reactive oxygen species leading to oxidative stress and inflammation contribute to pulmonary growth restriction and abnormal neurodevelopment, including myelination deficits. Despite a large body of studies, which unraveled important pathophysiological mechanisms for both organs at risk, the majority focused exclusively either on lung or on brain injury. However, considering that preterm infants suffering from BPD are at higher risk for poor neurodevelopmental outcome, an interaction between both organs seems plausible. This review summarizes recent findings regarding mechanisms of hyperoxia-induced neonatal lung and brain injury. We will discuss common pathophysiological pathways, which potentially link both injured organ systems. Furthermore, promises and needs of currently suggested therapies, including pharmacological and regenerative cell-based treatments for BPD and EoP, will be emphasized. Limited therapeutic approaches highlight the urgent need for a better understanding of the mechanisms underlying detrimental effects of hyperoxia on the lung-brain axis in order to pave the way for the development of novel multimodal therapies, ideally targeting both severe preterm birth-associated complications.

Evidence for the Management of Bronchopulmonary Dysplasia in Very Preterm Infants

Background: Very preterm birth often results in the development of bronchopulmonary dysplasia (BPD) with an inverse correlation of gestational age and birthweight. This very preterm population is especially exposed to interventions, which affect the development of BPD. Objective: The goal of our review is to summarize the evidence on these daily procedures and provide evidence-based recommendations for the management of BPD. Methods: We conducted a systematic literature research using MEDLINE/PubMed on antenatal corticosteroids, surfactant-replacement therapy, caffeine, ventilation strategies, postnatal corticosteroids, inhaled nitric oxide, inhaled bronchodilators, macrolides, patent ductus arteriosus, fluid management, vitamin A, treatment of pulmonary hypertension and stem cell therapy. Results: Evidence provided by meta-analyses, systematic reviews, randomized controlled trials (RCTs) and large observational studies are summarized as a narrative review. Discussion: There is strong evidence for the use of antenatal corticosteroids, surfactant-replacement therapy, especially in combination with noninvasive ventilation strategies, caffeine and lung-protective ventilation strategies. A more differentiated approach has to be applied to corticosteroid treatment, the management of patent ductus arteriosus (PDA), fluid-intake and vitamin A supplementation, as well as the treatment of BPD-associated pulmonary hypertension. There is no evidence for the routine use of inhaled bronchodilators and prophylactic inhaled nitric oxide. Stem cell therapy is promising, but should be used in RCTs only.

Inhaled nitric oxide use in neonates: Balancing what is evidence-based and what is physiologically sound

Inhaled nitric oxide is a powerful therapeutic used in neonatology. Its use is evidenced-based for term and near-term infants with persistent pulmonary hypertension; however, it is frequently used off-label both in term and preterm babies. This article reviews the off-label uses of iNO in infants. Rationale is discussed for a selective application of iNO based on physiologically guided principles, and new research avenues are considered.

Drugs for the Prevention and Treatment of Bronchopulmonary Dysplasia

Rates of bronchopulmonary dysplasia (BPD) are increasing. After preterm birth, there are important developmental periods in which neonates are more vulnerable to stressful events. These periods are opportunities for pharmacologic interventions. Many drugs remain inadequately tested and no new drugs have been approved in more than 25 years for BPD prevention or therapy. More progress is needed in defining appropriate end points based on the pathophysiology of BPD and postdischarge chronic pulmonary insufficiency of prematurity and to develop effective new drugs. In addition, much work is needed to better define perinatal factors, early postnatal findings, and physiologic phenotypes or endotypes.

Modulators of inflammation in Bronchopulmonary Dysplasia

Over 50 years after its first description, Bronchopulmonary Dysplasia (BPD) remains a devastating pulmonary complication in preterm infants with respiratory failure and develops in 30-50% of infants less than 1000-gram birth weight. It is thought to involve ventilator- and oxygen-induced damage to an immature lung that results in an inflammatory response and ends in aberrant lung development with dysregulated angiogenesis and alveolarization. Significant morbidity and mortality are associated with this most common chronic lung disease of childhood. Thus, any therapies that decrease the incidence or severity of this condition would have significant impact on morbidity, mortality, human costs, and healthcare expenditure. It is clear that an inflammatory response and the elaboration of growth factors and cytokines are associated with the development of BPD. Numerous approaches to control the inflammatory process leading to the development of BPD have been attempted. This review will examine the anti-inflammatory approaches that are established or hold promise for the prevention or treatment of BPD.

An update on the diagnosis and management of bronchopulmonary dysplasia (BPD)-associated pulmonary hypertension

The past decade of neonatal care has been highlighted by increased survival rates in smaller and more premature infants. Despite reduction in mortality associated with extreme prematurity, long term pulmonary morbidities remain a concern, with growing recognition of the clinical burden attributable to infants with bronchopulmonary dysplasia (BPD)-associated pulmonary hypertension (PH). Recent publications shed light on the critical contributions of maternal placental pathology and compromised intrauterine growth to fetal pulmonary vascular development. A body of literature has further clarified postnatal risk factors for PH, most notably the severity of BPD but surprisingly the additional presence of non-pulmonary morbidities including necrotizing enterocolitis (NEC). Limitations of current diagnostics persist with growing consideration of novel echocardiographic approaches as well as complementary non-invasive biomarkers to better identify infants at risk. In 2015, a joint report published by the American Heart Association and American Thoracic Society provided the first guidelines for the care of children with PH with limited content to address BPD-associated PH. These guidelines were expanded upon in an expert consensus report produced by the Pediatric Pulmonary Hypertension Network (PPHNet). These recommendations encouraged the use of standardized screening protocols and emphasized the importance of evaluation and treatment of comorbidities when PH is identified. Cardiac catheterization was recommended prior to initiation of therapy for more accurate quantification of pulmonary pressures, clarification of anatomy and guidance in the use of pharmacotherapy. Despite these guidelines, significant practice variation persists and gaps remain with respect to optimal evaluation and management of BPD-associated PH.

Update on Vitamin E and Its Potential Role in Preventing or Treating Bronchopulmonary Dysplasia

Vitamin E is obtained only through the diet and has a number of important biological activities, including functioning as an antioxidant. Evidence that free radicals may contribute to pathological processes such as bronchopulmonary dysplasia (BPD), a disease of prematurity associated with increased lung injury, inflammation and oxidative stress, led to trials of the antioxidant vitamin E (α-tocopherol) to prevent BPD with variable results. These trials were all conducted at supraphysiologic doses and 2 of these trials utilized a formulation containing a potentially harmful excipient. Since 1991, when the last of these trials was conducted, both neonatal management strategies for minimizing oxygen and ventilator-related lung injury and our understanding of vitamin E isoforms in respiratory health have advanced substantially. It is now known that there are differences between the effects of vitamin E isoforms α-tocopherol and γ-tocopherol on the development of respiratory morbidity and inflammation. What is not known is whether improvements in physiologic concentrations of individual or combinations of vitamin E isoforms during pregnancy or following preterm birth might prevent or reduce BPD development. The answers to these questions require adequately powered studies targeting pregnant women at risk of preterm birth or their premature infants immediately following birth, especially in certain subgroups that are at increased risk of vitamin E deficiency (e.g., smokers). The objective of this review is to compile, update, and interpret what is known about vitamin E isoforms and BPD since these first studies were conducted, and suggest future research directions.

Diagnostic Approach to Pulmonary Hypertension in Premature Neonates

Bronchopulmonary dysplasia (BPD) is a form of chronic lung disease in premature infants following respiratory distress at birth. With increasing survival of extremely low birth weight infants, alveolar simplification is the defining lung characteristic of infants with BPD, and along with pulmonary hypertension, increasingly contributes to both respiratory morbidity and mortality in these infants. Growth restricted infants, infants born to mothers with oligohydramnios or following prolonged preterm rupture of membranes are at particular risk for early onset pulmonary hypertension. Altered vascular and alveolar growth particularly in canalicular and early saccular stages of lung development following mechanical ventilation and oxygen therapy, results in developmental lung arrest leading to BPD with pulmonary hypertension (PH). Early recognition of PH in infants with risk factors is important for optimal management of these infants. Screening tools for early diagnosis of PH are evolving; however, echocardiography is the mainstay for non-invasive diagnosis of PH in infants. Cardiac computed tomography (CT) and magnetic resonance are being used as imaging modalities, however their role in improving outcomes in these patients is uncertain. Follow-up of infants at risk for PH will help not only in early diagnosis, but also in appropriate management of these infants. Aggressive management of lung disease, avoidance of hypoxemic episodes, and optimal nutrition determine the progression of PH, as epigenetic factors may have significant effects, particularly in growth-restricted infants. Infants with diagnosis of PH are managed with pulmonary vasodilators and those resistant to therapy need to be worked up for the presence of cardio-vascular anomalies. The management of infants and toddlers with PH, especially following premature birth is an emerging field. Nonetheless, combination therapies in a multi-disciplinary setting improves outcomes for these infants.

Role of the Nrf2/HO-1 axis in bronchopulmonary dysplasia and hyperoxic lung injuries

Bronchopulmonary dysplasia (BPD) is a chronic illness that usually originates in preterm newborns. Generally, BPD is a consequence of respiratory distress syndrome (RDS) which, in turn, comes from the early arrest of lung development and the lack of pulmonary surfactant. The need of oxygen therapy to overcome premature newborns' compromised respiratory function generates an increasing amount of reactive oxygen species (ROS), the onset of sustained oxidative stress (OS) status, and inflammation in the pulmonary alveoli deputies to respiratory exchanges. BPD is a severe and potentially life-threatening disorder that in the most serious cases, can open the way to neurodevelopmental delay. More importantly, there is no adequate intervention to hamper or treat BPD. This perspective article seeks to review the most recent and relevant literature describing the very early stages of BPD and hyperoxic lung injuries focussing on nuclear factor erythroid derived 2 (Nrf2)/heme oxygenase-1 (HO-1) axis. Indeed, Nrf2/HO1 activation in response to OS induced lung injury in preterm concurs to the induction of certain number of antioxidant, anti-inflammatory, and detoxification pathways that seem to be more powerful than the activation of one single antioxidant gene. These elicited protective effects are able to counteract/mitigate all multifaceted aspects of the disease and may support novel approaches for the management of BPD.

Aberrant Pulmonary Vascular Growth and Remodeling in Bronchopulmonary Dysplasia

In contrast to many other organs, a significant portion of lung development occurs after birth during alveolarization, thus rendering the lung highly susceptible to injuries that may disrupt this developmental process. Premature birth heightens this susceptibility, with many premature infants developing the chronic lung disease, bronchopulmonary dysplasia (BPD), a disease characterized by arrested alveolarization. Over the past decade, tremendous progress has been made in the elucidation of mechanisms that promote postnatal lung development, including extensive data suggesting that impaired pulmonary angiogenesis contributes to the pathogenesis of BPD. Moreover, in addition to impaired vascular growth, patients with BPD also frequently demonstrate alterations in pulmonary vascular remodeling and tone, increasing the risk for persistent hypoxemia and the development of pulmonary hypertension. In this review, an overview of normal lung development will be presented, and the pathologic features of arrested development observed in BPD will be described, with a specific emphasis on the pulmonary vascular abnormalities. Key pathways that promote normal pulmonary vascular development will be reviewed, and the experimental and clinical evidence demonstrating alterations of these essential pathways in BPD summarized.

Persistent Challenges in Pediatric Pulmonary Hypertension

Pulmonary hypertension and related pulmonary vascular diseases cause significant morbidities and high mortality and present many unique challenges toward improving outcomes in neonates, infants, and children. Differences between pediatric and adult disease are reflected in controversies regarding etiologies, classification, epidemiology, diagnostic evaluations, and therapeutic interventions. This brief review highlights several key topics reflecting recent advances in the field and identifies persistent gaps in our understanding of clinical pediatric pulmonary hypertension.

Therapeutic potential of soluble guanylate cyclase modulators in neonatal chronic lung disease

Supplemental oxygen after premature birth results in aberrant airway, alveolar, and pulmonary vascular development with an increased risk for bronchopulmonary dysplasia, and development of wheeze and asthma, pulmonary hypertension, and chronic obstructive pulmonary disease in survivors. Although stimulation of the nitric oxide (NO)-soluble guanylate cyclase (sGC)-cGMP signal transduction pathway has significant beneficial effects on disease development in animal models, so far this could not be translated to the clinic. Oxidative stress reduces the NO-sGC-cGMP pathway by oxidizing heme-bound sGC, resulting in inactivation or degradation of sGC. Reduced sGC activity and/or expression is associated with pathology due to premature birth, oxidative stress-induced lung injury, including impaired alveolar maturation, smooth muscle cell (SMC) proliferation and contraction, impaired airway relaxation and vasodilation, inflammation, pulmonary hypertension, right ventricular hypertrophy, and an aggravated response toward hyperoxia-induced neonatal lung injury. Recently, Britt et al. (10) demonstrated that histamine-induced Ca(2+) responses were significantly elevated in hyperoxia-exposed fetal human airway SMCs compared with normoxic controls and that this hyperoxia-induced increase in the response was strongly reduced by NO-independent stimulation and activation of sGC. These recent studies highlight the therapeutic potential of sGC modulators in the treatment of preterm infants for respiratory distress with supplemental oxygen. Such treatment is aimed at improving aberrant alveolar and vascular development of the neonatal lung and preventing the development of wheezing and asthma in survivors of premature birth. In addition, these studies highlight the suitability of fetal human airway SMCs as a translational model for pathological airway changes in the neonate.

Utility of large-animal models of BPD: chronically ventilated preterm lambs

This paper is focused on unique insights provided by the preterm lamb physiological model of bronchopulmonary dysplasia (BPD). Connections are also made to insights provided by the former preterm baboon model of BPD, as well as to rodent models of lung injury to the immature, postnatal lung. The preterm lamb and baboon models recapitulate the clinical setting of preterm birth and respiratory failure that require prolonged ventilation support for days or weeks with oxygen-rich gas. An advantage of the preterm lamb model is the large size of preterm lambs, which facilitates physiological studies for days or weeks during the evolution of neonatal chronic lung disease (CLD). To this advantage is linked an integrated array of morphological, biochemical, and molecular analyses that are identifying the role of individual genes in the pathogenesis of neonatal CLD. Results indicate that the mode of ventilation, invasive mechanical ventilation vs. less invasive high-frequency nasal ventilation, is related to outcomes. Our approach also includes pharmacological interventions that test causality of specific molecular players, such as vitamin A supplementation in the pathogenesis of neonatal CLD. The new insights that are being gained from our preterm lamb model may have important translational implications about the pathogenesis and treatment of BPD in preterm human infants.

Drug therapy for the prevention and treatment of bronchopulmonary dysplasia

INTRODUCTION

As more infants are surviving at younger gestational ages, bronchopulmonary dysplasia (BPD) remains as a frequent neonatal complication occurring after preterm birth. The multifactorial nature of the disease process makes BPD a challenging condition to treat. While multiple pharmacologic therapies have been investigated over the past two decades, there have been limited advances in the field. Often multiple therapies are used concurrently without clear evidence of efficacy, with potential for significant side effects from drug-drug interactions.

METHODS

Systematic literature review.

CONCLUSION

Although there is physiologic rationale for the use of many of these therapies, none of them has single-handedly altered the incidence, severity, or progression of BPD. Future research should focus on developing clinically significant end-points (short and long term respiratory assessments), investigating biomarkers that accurately predict risk and progression of disease, and creating appropriate stratification models of BPD severity. Applying a multi-modal approach to the study of new and existing drugs should be the most effective way of establishing the optimal prevention and treatment regimens for BPD.

Bronchopulmonary dysplasia early changes leading to long-term consequences

Neonatal chronic lung disease, i.e., bronchopulmonary dysplasia, is characterized by impaired pulmonary development resulting from the impact of different risk factors including infections, hyperoxia, and mechanical ventilation on the immature lung. Remodeling of the extracellular matrix, apoptosis as well as altered growth factor signaling characterize the disease. The immediate consequences of these early insults have been studied in different animal models supported by results from in vitro approaches leading to the successful application of some findings to the clinical setting in the past. Nonetheless, existing information about long-term consequences of the identified early and most likely sustained changes to the developing lung is limited. Interesting results point towards a tremendous impact of these early injuries on the pulmonary repair capacity as well as aging related processes in the adult lung.

Role of reactive oxygen species in neonatal pulmonary vascular disease

SIGNIFICANCE

Abnormal lung development in the perinatal period can result in severe neonatal complications, including persistent pulmonary hypertension (PH) of the newborn and bronchopulmonary dysplasia. Reactive oxygen species (ROS) play a substantive role in the development of PH associated with these diseases. ROS impair the normal pulmonary artery (PA) relaxation in response to vasodilators, and ROS are also implicated in pulmonary arterial remodeling, both of which can increase the severity of PH.

RECENT ADVANCES

PA ROS levels are elevated when endogenous ROS-generating enzymes are activated and/or when endogenous ROS scavengers are inactivated. Animal models have provided valuable insights into ROS generators and scavengers that are dysregulated in different forms of neonatal PH, thus identifying potential therapeutic targets.

CRITICAL ISSUES

General antioxidant therapy has proved ineffective in reversing PH, suggesting that it is necessary to target specific signaling pathways for successful therapy.

FUTURE DIRECTIONS

Development of novel selective pharmacologic inhibitors along with nonantioxidant therapies may improve the treatment outcomes of patients with PH, while further investigation of the underlying mechanisms may enable earlier detection of the disease.

Arginase I gene single-nucleotide polymorphism is associated with decreased risk of pulmonary hypertension in bronchopulmonary dysplasia

AIM

To test the hypothesis that there are single-nucleotide polymorphisms (SNPs) in genes of the l-arginine/nitric oxide pathway associated with pulmonary hypertension (PH) in neonates with bronchopulmonary dysplasia (BPD).

METHODS

Neonates with BPD were enrolled (n = 140) and clinical characteristics compared between case (BPD + PH) and control (BPD) groups. DNA was isolated from blood leucocytes and assayed for 17 SNPs in l-arginine/nitric oxide pathway genes by Sequenom massarray. Genes included carbamoyl-phosphate synthetase, ornithine transcarbamylase, argininosuccinate synthase, nitric oxide synthase and arginase. SNPs were selected from the National Center for Biotechnology Information database for their putative functionality. Calculated minor allele frequencies (MAF) of cases and controls were compared using χ2 and logistic regression.

RESULTS

Of the 140 patients with BPD, 26% had echocardiographic evidence of PH. Ventilation days were longer for cases than controls (mean 31 vs. 15 days, p < 0.05). Of the 17 SNPs, rs2781666 in arginase I gene was less common in cases (MAF = 0.23) than controls (MAF = 0.37, p = 0.04). The odds of PH decreased by 43% (p = 0.047) for each copy of the SNP minor allele in arginase I gene in patients with BPD.

CONCLUSION

Arginase I SNP (rs2781666) may be associated with protection against pulmonary hypertension in preterm neonates with BPD.

Exhaled nitric oxide in neonates with or without hypoxemic respiratory failure

BACKGROUND

Exhaled nitric oxide (eNO) is one of the airway condensate derived markers, reflecting mainly airway inflammation in asthma and other lung diseases. The changes of eNO levels as pathophysiology of neonatal hypoxemic respiratory failure (HRF) in early postnatal life have not been thoroughly studied. The present study was to establish a method for measuring eNO concentrations in neonates with or without HRF.

METHODS

Twenty-two newborn infants with HRF and 26 non-NRF controls were included within the first 24 hours of postnatal life. Their eNO levels were detected with a rapid-response chemiluminescence analyzer daily during the first week of their postnatal life, and lung mechanics and gas exchange efficiency were monitored at the same time, such as pulse oxygen saturation (SpO2), inspired fraction of oxygen (FiO2) and other parameters.

RESULTS

During the first two days of postnatal life, eNO values of HRF neonates were significantly higher than those of the control neonates (day 1, 7.9±3.2 vs. 5.8±1.8 parts per billion [ppb], P<0.05; day 2, 8.8±3.2 vs. 6.0±2.4 ppb, P<0.05), but there were no significant differences in the following days. With SpO2/FiO2 increasing, difference of eNO values between the HRF and non-HRF neonates became narrowed, but there was still a two-fold difference of eNO/[SpO2/(FiO2×100)] on days 5-7.

CONCLUSION

We established a method for measuring eNO and found difference in neonates with or without HRF, which diminished with prolonged postnatal days, reflecting pathophysiological characteristics of HRF.

Oxidative stress and bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the major cause of pulmonary disease in infants. The pathophysiology and management of BPD changed with the improvement of neonatal intensive care unit (NICU) management and with the increase of survival rates. Despite the improvements made, BPD is still a public health concern, resulting in frequent hospitalizations with high rates of mortality, impaired weight and height growth, and neurodevelopmental disorders. Lung injury in the neonatal period has multiple etiologic factors - genetic, hemodynamic, metabolic, nutritional, mechanical, and infectious mechanisms - act in a cumulative and synergic way. Free radical (FR) generation is largely recognized as the major cause of lung damage. Oxidative stress (OS) is the final common endpoint for a complex convergence of events, some genetically determined and some triggered by in utero stressors. Inflammatory placental disorders and chorioamnionitis also play an important role due to the coexistence of inflammatory and oxidative lesions. In addition, the contribution of airway inflammation has been extensively studied. The link between inflammation and OS injury involves the direct activation of inflammatory cells, especially granulocytes, which potentiates the inflammatory reaction. Individualized interventions to support ventilation, minimize oxygen exposure, minimize apnea, and encourage growth should decrease both the frequency and severity of BPD. Future perspectives suggest supplementation with enzymatic and/or non-enzymatic antioxidants. The use of antioxidants in preterm newborns particularly exposed to OS and at risk for BPD represents a logical strategy to ameliorate FRs injury, but further studies are needed to support this hypothesis.

Chronic lung disease in the preterm infant. Lessons learned from animal models

Neonatal chronic lung disease, also known as bronchopulmonary dysplasia (BPD), is the most common complication of premature birth, affecting up to 30% of very low birth weight infants. Improved medical care has allowed for the survival of the most premature infants and has significantly changed the pathology of BPD from a disease marked by severe lung injury to the "new" form characterized by alveolar hypoplasia and impaired vascular development. However, increased patient survival has led to a paucity of pathologic specimens available from infants with BPD. This, combined with the lack of a system to model alveolarization in vitro, has resulted in a great need for animal models that mimic key features of the disease. To this end, a number of animal models have been created by exposing the immature lung to injuries induced by hyperoxia, mechanical stretch, and inflammation and most recently by the genetic modification of mice. These animal studies have 1) allowed insight into the mechanisms that determine alveolar growth, 2) delineated factors central to the pathogenesis of neonatal chronic lung disease, and 3) informed the development of new therapies. In this review, we summarize the key findings and limitations of the most common animal models of BPD and discuss how knowledge obtained from these studies has informed clinical care. Future studies should aim to provide a more complete understanding of the pathways that preserve and repair alveolar growth during injury, which might be translated into novel strategies to treat lung diseases in infants and adults.

Diagnosis and treatment of pulmonary hypertension in infancy

Normal pulmonary vascular development in infancy requires maintenance of low pulmonary vascular resistance after birth, and is necessary for normal lung function and growth. The developing lung is subject to multiple genetic, pathological and/or environmental influences that can adversely affect lung adaptation, development, and growth, leading to pulmonary hypertension. New classifications of pulmonary hypertension are beginning to account for these diverse phenotypes, and or pulmonary hypertension in infants due to PPHN, congenital diaphragmatic hernia, and bronchopulmonary dysplasia (BPD). The most effective pharmacotherapeutic strategies for infants with PPHN are directed at selective reduction of PVR, and take advantage of a rapidly advancing understanding of the altered signaling pathways in the remodeled vasculature.

Soluble guanylate cyclase modulates alveolarization in the newborn lung

Nitric oxide (NO) regulates lung development through incompletely understood mechanisms. NO controls pulmonary vascular smooth muscle cell (SMC) differentiation largely through stimulating soluble guanylate cyclase (sGC) to produce cGMP and increase cGMP-mediated signaling. To examine the role of sGC in regulating pulmonary development, we tested whether decreased sGC activity reduces alveolarization in the normal and injured newborn lung. For these studies, mouse pups with gene-targeted sGC-α1 subunit truncation were used because we determined that they have decreased pulmonary sGC enzyme activity. sGC-α1 knockout (KO) mouse pups were observed to have decreased numbers of small airway structures and lung volume compared with wild-type (WT) mice although lung septation and body weights were not different. However, following mild lung injury caused by breathing 70% O2, the sGC-α1 KO mouse pups had pronounced inhibition of alveolarization, as evidenced by an increase in airway mean linear intercept, reduction in terminal airway units, and decrease in lung septation and alveolar openings, as well as reduced somatic growth. Because cGMP regulates SMC phenotype, we also tested whether decreased sGC activity reduces lung myofibroblast differentiation. Cellular markers revealed that vascular SMC differentiation decreased, whereas myofibroblast activation increased in the hyperoxic sGC-α1 KO pup lung. These results indicate that lung development, particularly during hyperoxic injury, is impaired in mouse pups with diminished sGC activity. These studies support the investigation of sGC-targeting agents as therapies directed at improving development in the newborn lung exposed to injury.

Progress in understanding the pathogenesis of BPD using the baboon and sheep models

Bronchopulmonary dysplasia (BPD) is among the most common chronic lung diseases in infants in the US. Improved survival of preterm infants who developed BPD is becoming increasingly important because of the high risk for persistent pulmonary morbidities such as poor respiratory gas exchange, pulmonary hypertension, and excess airway expiratory resistance later in life. This review focuses on unique insights provided by the two large-animal, physiological models of neonatal chronic lung disease: preterm baboons and preterm lambs. The models' are valuable because they contribute to better understanding of the underlying molecular pathogenic mechanisms. An epigenetic hypothesis is proposed as a pathogenic mechanism for BPD and its persistent pulmonary morbidities.

Pulmonary hypertension in bronchopulmonary dysplasia

Pulmonary hypertension (PH) is a common complication of neonatal respiratory diseases, including bronchopulmonary dysplasia (BPD), and recent studies have increased awareness that PH worsens the clinical course, morbidity and mortality of BPD. Recent evidence indicates that up to 18% of all extremely low-birth-weight infants will develop some degree of PH during their hospitalization, and the incidence rises to 25-40% of the infants with established BPD. Risk factors are not yet well understood, but new evidence shows that fetal growth restriction is a significant predictor of PH. Echocardiography remains the primary method for evaluation of BPD-associated PH, and the development of standardized screening timelines and techniques for identification of infants with BPD-associated PH remains an important ongoing topic of investigation. The use of pulmonary vasodilator medications, such as nitric oxide, sildenafil, and others, in the BPD population is steadily growing, but additional studies are needed regarding their long-term safety and efficacy.

Pharmacotherapy for pulmonary hypertension

Pulmonary arterial hypertension is a serious disease with significant morbidity and mortality. Although it can occur idiopathically, it is more commonly associated with other cardiac or lung diseases. While most of the available therapies have been tested in adult populations and most therapies in children remain off-label, new reports and randomized trials are emerging that inform the treatment of pediatric populations. This review discusses currently available therapies for pediatric pulmonary hypertension, their biological rationales, and evidence for their clinical effectiveness.

Pulmonary vasodilator therapy in the NICU: inhaled nitric oxide, sildenafil, and other pulmonary vasodilating agents

The perinatal transition from fetal to extrauterine life requires a dramatic change in the circulatory pattern as the organ of gas exchange switches from the placenta to the lungs. Pulmonary hypertension can occur during early newborn life, and present as early respiratory failure or as a complication of more chronic diseases, such as bronchopulmonary dysplasia. The most effective pharmacotherapeutic strategies for infants with persistent pulmonary hypertension of the newborn are directed at selective reduction of pulmonary vascular resistance. This article discusses currently available therapies for pulmonary hypertension, their biologic rationales, and evidence for their clinical effectiveness.

Interplay of endothelial and inducible nitric oxide synthases modulates the vascular response to ischaemia-reperfusion in the rabbit lung

AIM

Lung ischaemia-reperfusion induces nitric oxide synthesis and reactive nitrogen species, decreasing nitric oxide bioavailability. We hypothesized that in the ventilated lung, this process begins during ischaemia and intensifies with reperfusion, contributing to ischaemia-reperfusion-induced pulmonary vasoconstriction. The aim was to determine whether ischaemia-reperfusion alters inducible and endothelial nitric oxide synthase expression/activity, reactive nitrogen species generation, and nitric oxide bioavailability, potentially affecting pulmonary perfusion.

METHODS

Ischaemia-reperfusion was induced for various times in anesthetized rabbits with ventilated lungs by reversibly occluding the right pulmonary artery and initiating reperfusion. Nitric oxide synthase activity/expression and phosphorylation, reactive nitrogen species generation and total nitrate/nitrite were determined in lung tissue.

RESULTS

Inducible nitric oxide synthase expression and activity, and reactive nitrogen species formation coincided with increased pulmonary vascular resistance during reperfusion and increased with ischaemia duration, further increasing after 2-h reperfusion. Total nitrate/nitrite also increased with ischaemia but decreased after 2-h reperfusion. Pre-treatment with an inducible nitric oxide synthase inhibitor (1400W; Cayman Chemical Company, Ann Arbor, MI, USA) attenuated inducible nitric oxide synthase activity, reactive nitrogen species generation and pulmonary vascular resistance, but did not affect total nitrate/nitrite. Endothelial nitric oxide synthase expression was unchanged by ischaemia-reperfusion; however, its phosphorylation on serine 1177 and dephosphorylation on threonine 495 was uncoupled, suggesting decreased endothelial nitric oxide synthase activity. 1400W prevented uncoupling of endothelial nitric oxide synthase phosphorylation, maintaining its activity during reperfusion.

CONCLUSION

Ischaemia-reperfusion up-regulates inducible nitric oxide synthesis and/activity, which coincides with reduced endothelial nitric oxide synthase activity as suggested by its uncoupling and may contribute to ischaemia-reperfusion-induced pulmonary vasoconstriction.

Therapeutic approaches using nitric oxide in infants and children

Pulmonary hypertension contributes significantly to the morbidity and mortality associated with many pediatric pulmonary and cardiac diseases. Nitric oxide, a gas molecule, is a unique pharmaceutical agent that can be inhaled and thus delivered directly to the lung. Inhaled nitric oxide was approved by the FDA in 1999 as a therapy for infants with persistent pulmonary hypertension. Since then, the use of inhaled nitric oxide has expanded to other neonatal and pediatric conditions, and our knowledge of its properties and mechanisms of action has increased tremendously. This review discusses the physiology of nitric oxide signaling, the most common indications for its clinical use, and promising new investigations that may enhance endogenous production of nitric oxide and/or improve vascular response to it.

Nitric Oxide and Cellular Maturity Are Key Components of Pro-Inflammatory Cytokine-Induced Apoptosis of Human Fetal Lung Epithelial Cells

Inflammation is a major contributor to the pathogenesis of bronchopulmonary dysplasia (BPD). BPD is associated with prematurity of birth, sepsis, with increased production of both cytokines and nitric oxide, and with the shedding of bronchial epithelial cells. The pathological mechanisms involved in this disease remain unclear, in particular the role that epithelial maturity plays. The effects of pro-inflammatory cytokines upon immature and mature cells are examined within this study, using primary culture of human lung epithelial cells. Pro-inflammatory cytokines increase inducible nitric oxide synthase (iNOS) expression and raise NO production, irrespective of cellular maturity. Pre-incubation with 1400W, a specific iNOS inhibitor, abrogated pro-inflammatory cytokine-induced NO generation and apoptosis. However, immature fetal lung epithelial cells were uniquely sensitive to cellular injury in response to cytokine exposure. These observations suggest that pro-inflammatory cytokines, which are present within BPD, may cause apoptosis of lung epithelial cells via de novo generation of NO. Furthermore, the prematurity of lung epithelial cells may be a factor in free radical mediated pulmonary damage.

Impaired vascular endothelial growth factor signaling in the pathogenesis of neonatal pulmonary vascular disease

Abstract Of diverse growth factors that contribute to normal lung development, vascular endothelial growth factor (VEGF) plays an especially prominent role in the normal growth and development of the pulmonary circulation in the fetus and newborn. Strong experimental and clinical data support the role of impaired VEGF signaling in the pathogenesis of two major clinical disorders of the developing lung circulation: persistent pulmonary hypertension of the newborn (PPHN) and bronchopulmonary dysplasia (BPD). These disorders are each characterized by impaired vascular growth, structure and reactivity, which are at least partly due to endothelial cell dysfunction. This chapter will briefly discuss VEGF signaling during normal lung development and how disruption of VEGF signaling contribute to the pathogenesis of neonatal pulmonary vascular disease in these settings.

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.

Adrenomedullin promotes lung angiogenesis, alveolar development, and repair

Bronchopulmonary dysplasia (BPD) and emphysema are significant global health problems at the extreme stages of life. Both are characterized by alveolar simplification and abnormal distal airspace enlargement due to arrested development or loss of alveoli, respectively. Both lack effective treatments. Mechanisms that inhibit distal lung growth are poorly understood. Adrenomedullin (AM), a recently discovered potent vasodilator, promotes angiogenesis and has protective effects on the cardiovascular and respiratory system. Its role in the developing lung is unknown. We hypothesized that AM promotes lung angiogenesis and alveolar development. Accordingly, we report that lung mRNA expression of AM increases during normal alveolar development. In vivo, intranasal administration of the AM antagonist, AM22-52 decreases lung capillary density (12.4 +/- 1.5 versus 18 +/- 1.5 in control animals; P < 0.05) and impairs alveolar development (mean linear intercept, 52.3 +/- 1.5 versus 43.8 +/- 1.8 [P < 0.05] and septal counts 62.0 +/- 2.7 versus 90.4 +/- 3.5 [P < 0.05]) in neonatal rats, resulting in larger and fewer alveoli, reminiscent of BPD. This was associated with decreased lung endothelial nitric oxide synthase and vascular endothelial growth factor-A mRNA expression. In experimental oxygen-induced BPD, a model of arrested lung vascular and alveolar growth, AM attenuates arrested lung angiogenesis (vessel density, 6.9 +/- 1.1 versus 16.2 +/- 1.3, P < 0.05) and alveolar development (mean linear intercept, 51.9 +/- 3.2 versus 44.4 +/- 0.7, septal counts 47.6 +/- 3.4 versus 67.7 +/- 4.0, P < 0.05), an effect in part mediated by inhibition of apoptosis. AM also prevents pulmonary hypertension in this model, as assessed by decreased right ventricular hypertrophy and pulmonary artery medial wall thickness. Our findings suggest a role for AM during normal alveolar development. AM may have therapeutic potential in diseases associated with alveolar injury.

Oxygen toxicity and reactive oxygen species: the devil is in the details

Reactive oxygen species (ROS) serve as cell signaling molecules for normal biologic processes. However, the generation of ROS can also provoke damage to multiple cellular organelles and processes, which can ultimately disrupt normal physiology. An imbalance between the production of ROS and the antioxidant defenses that protect cells has been implicated in the pathogenesis of a variety of diseases, such as cancer, asthma, pulmonary hypertension, and retinopathy. The nature of the injury will ultimately depend on specific molecular interactions, cellular locations, and timing of the insult. This review will outline the origins of endogenous and exogenously generated ROS. The molecular, cellular, pathologic, and physiologic targets will then be discussed with a particular emphasis on aspects relevant to child development. Finally, antioxidant defenses that scavenge ROS and mitigate associated toxicities will be presented, with a discussion of potential therapeutic approaches for the prevention and/or treatment of human diseases using enzymatic and nonenzymatic antioxidants.

Inhaled nitric oxide for preterm neonates

The evidence for the benefits of inhaled nitric oxide (iNO) on gas exchange, cytokine-induced lung inflammation, and vascular dysfunction has been demonstrated by several animal and human studies. The use of iNO in extremely low birth weight neonates for the prevention of adverse outcomes like chronic lung disease and neurologic injury has been investigated, but the findings remain inconclusive. This review briefly outlines the biologic rationale for the use of iNO in preterm neonates and the results on the outcome measures of bronchopulmonary dysplasia and brain injury from the recent clinical trials. This article focuses on the potential toxicities, persistent controversies, and unanswered questions regarding the use of this treatment modality in this patient population at high risk for adverse outcomes.

Inhaled nitric oxide to prevent bronchopulmonary dysplasia in preterm neonates

Bronchopulmonary dysplasia is a chronic lung disease that affects premature infants and contributes to their morbidity and mortality. With the advent of prenatal steroids and postnatal exogenous surfactant and less aggressive respiratory support, premature infants can develop chronic oxygen dependency without even acute respiratory distress. This 'new bronchopulmonary dysplasia' could be the result of impaired postnatal growth. Several experimental studies have suggested a possible role of the vascular endothelial growth factor/nitric oxide (VEGF/NO) pathway in restoring pulmonary angiogenesis and enhancing distal lung growth. The results of the clinical studies are, however, inconclusive, and it is currently unclear which subsets of premature infants might benefit from inhaled nitric oxide. Besides, severe intracranial haemorrhage and/or cystic periventricular leucomalacia may affect the most immature babies, many of whom are spared from severe initial respiratory disease. Recently, inhaled nitric oxide was shown to significantly decrease the incidence of these neurological events, and to improve the long-term outcome in a few clinical trials. At times neuroprotective, at times neurotoxic, nitric oxide is capable of divergent effects depending upon the extent of cerebral damage, the redox state of the cell, and the experimental model used. Recently, our group found that inhaled nitric oxide had remote effects including angiogenesis and maturation on the developing brain in rodent pups. Thus, we await the results of the recently completed randomised clinical trial of inhaled nitric oxide to prevent bronchopulmonary dysplasia (the European Nitric Oxide or 'EUNO' trial) where, besides the primary endpoint of chronic oxygen dependency reduction at 36 weeks' postconceptional age, long-term lung and brain will be followed-up until 7 years of age.

Postnatal estradiol up-regulates lung nitric oxide synthases and improves lung function in bronchopulmonary dysplasia

RATIONALE

Nitric oxide (NO) plays an important role in lung development and perinatal lung function, and pulmonary NO synthases (NOS) are decreased in bronchopulmonary dysplasia (BPD) following preterm birth. Fetal estradiol levels increase during late gestation and estradiol up-regulates NOS, suggesting that after preterm birth estradiol deprivation causes attenuated lung NOS resulting in impaired pulmonary function.

OBJECTIVE

To test the effects of postnatal estradiol administration in a primate model of BPD over 14 days after delivery at 125 days of gestation (term = 185 d).

METHODS

Cardiopulmonary function was assessed by echocardiography and whole body plethysmography. Lung morphometric and histopathologic analyses were performed, and NOS enzymatic activity and abundance were measured.

MEASUREMENTS AND MAIN RESULTS

Estradiol caused an increase in blood pressure and ductus arteriosus closure. Expiratory resistance and lung compliance were also improved, and this occurred before spontaneous ductal closure. Furthermore, both oxygenation and ventilation indices were improved with estradiol, and the changes in lung function and ventilatory support requirements persisted throughout the study period. Whereas estradiol had negligible effect on indicators of lung inflammation and on lung structure assessed after the initial 14 days of ventilatory support, it caused an increase in lung neuronal and endothelial NOS enzymatic activity.

CONCLUSIONS

In a primate model of BPD, postnatal estradiol treatment had favorable cardiovascular impact, enhanced pulmonary function, and lowered requirements for ventilatory support in association with an up-regulation of lung NOS. Estradiol may be an efficacious postnatal therapy to improve lung function and outcome in preterm infants.

Bronchopulmonary dysplasia and early prophylactic inhaled nitric oxide in preterm infants: current concepts and future research strategies in animal models

We reviewed the literature on the use of inhaled nitric oxide and the influence of supplemental oxygen on bronchopulmonary dysplasia (BPD), and the role of endogenous nitric oxide-synthase, vascular endothelial growth factor, the interplay of nitric oxide and superoxide, protein nitration and the nuclear factor kappa B-pathway. BPD is a major cause of neonatal mortality and morbidity leading to arrested lung development in newborns. Several studies indicate that inhaled nitric oxide (iNO) improves pulmonary angiogenesis, lung alveolarization, distal lung growth and pulmonary function in preterm infants. Given the inconclusive results of clinical studies, however, it is unclear which subpopulations of infants might benefit. Moreover, data on iNO are conflicting whether exogenous nitric oxide is protective or damaging in the presence of hyperoxia. The toxicology of iNO is poorly understood and its potential interaction with oxygen has to be considered given that infants treated with iNO are also supplemented with oxygen. The underlying mechanisms of the effects of iNO in the newborn lung need further analysis. New data clarifying the role of endogenous nitric oxide-synthases, vascular endothelial growth factor (VEGF), the interplay of nitric oxide and superoxide, and protein nitration with concurrent iNO-therapy might answer some of these questions.

Expression of nitric oxide synthases and endogenous NO metabolism in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), a multifactorial disease of preterm neonates of complex etiology, is a significant problem within very low birth weight infants. Nitric oxide (NO) has been implicated in both the pathogenesis and as a potential therapeutic of this disease. At this time, there is little direct evidence of the changes in NO production and metabolism that occur within BPD in humans. Animal models have implied that reduced nitric oxide synthase (NOS) expression and NO production in the early stages of the disease may be critical factors. However, inflammation and hence iNOS expression, is also thought to play a role. In the present study we have utilized pathological samples to determine changes in the expression of NOS and NO metabolites within late stage BPD. It is our contention that within these samples iNOS expression is increased and associated with increased NO metabolite production. Mild immunostaining of all three nitric oxide synthase (NOS) enzymes (neuronal, inducible and endothelial) is observed in control lung with tight localization to the endothelium and epithelial airway. This tight localization was lost in samples from subjects with BPD. There was also a marked increase in iNOS expression throughout the lung tissue with strong coexistence with an epithelial cell marker cytokeratin. NO reaction products are altered with BPD as evidenced by increased S-nitrosothiol (SNO) and strong nitrotyrosine (NO(2)Y) imunoreactivity. This study demonstrates a strong correlation between products of NO reactivity and NOS localization in BPD.

Blunted hypoxic pulmonary vasoconstriction in experimental neonatal chronic lung disease

RATIONALE

Neonatal chronic lung disease (CLD), caused by prolonged mechanical ventilation (MV) with O(2)-rich gas, is the most common cause of long-term hospitalization and recurrent respiratory illness in extremely premature infants. Recurrent episodes of hypoxemia and associated ventilator adjustments often lead to worsening CLD. The mechanism that causes these hypoxemic episodes is unknown. Hypoxic pulmonary vasoconstriction (HPV), which is partially controlled by O(2)-sensitive voltage-gated potassium (K(v)) channels, is an important adaptive response to local hypoxia that helps to match perfusion and ventilation in the lung.

OBJECTIVES

To test the hypothesis that chronic lung injury (CLI) impairs HPV.

METHODS

We studied preterm lambs that had MV with O(2)-rich gas for 3 weeks and newborn rats that breathed 95%-O(2) for 2 weeks, both of which resulted in airspace enlargement and pulmonary vascular changes consistent with CLD.

MEASUREMENTS AND MAIN RESULTS

HPV was attenuated in preterm lambs with CLI after 2 weeks of MV and in newborn rats with CLI after 2 weeks of hyperoxia. HPV and constriction to the K(v)1.x-specific inhibitor, correolide, were preferentially blunted in excised distal pulmonary arteries (dPAs) from hyperoxic rats, whose dPAs exhibited decreased K(v)1.5 and K(v)2.1 mRNA and K(+) current. Intrapulmonary gene transfer of K(v)1.5, encoding the ion channel that is thought to trigger HPV, increased O(2)-sensitive K(+) current in cultured smooth muscle cells from rat dPAs, and restored HPV in hyperoxic rats.

CONCLUSIONS

Reduced expression/activity of O(2)-sensitive K(v) channels in dPAs contributes to blunted HPV observed in neonatal CLD.

Early inhaled nitric oxide treatment decreases apoptosis of endothelial cells in neonatal rat lungs after vascular endothelial growth factor inhibition

Vascular endothelial growth factor (VEGF) receptor blockade impairs lung growth and decreases nitric oxide (NO) production in neonatal rat lungs. Inhaled NO (iNO) treatment after VEGF inhibition preserves lung growth in infant rats by unknown mechanisms. We hypothesized that neonatal VEGF inhibition disrupts lung growth by causing apoptosis in endothelial cells, which is attenuated by early iNO treatment. Three-day-old rats received SU-5416, an inhibitor of VEGF receptor, or its vehicle and were raised in room air with or without iNO (10 ppm). SU-5416 reduced alveolar counts and lung vessel density by 28% ( P < 0.005) and 21% ( P < 0.05), respectively, as early as at 7 days of age. SU-5416 increased lung active caspase-3 protein by 60% at 5 days of age ( P < 0.05), which subsided by 7 days of age, suggesting a transient increase in lung apoptosis after VEGF blockade. Apoptosis primarily colocalized to lung vascular endothelial cells, and SU-5416 increased endothelial cell apoptotic index by eightfold at 5 days of age ( P <0.0001). iNO treatment after SU-5416 prevented the increases in lung active caspase-3 and in endothelial cell apoptotic index. There was no difference in alveolar type 2 cell number between control and SU-5416-treated rats. We conclude that neonatal VEGF receptor inhibition causes transient apoptosis in pulmonary endothelium, which is followed by persistently impaired lung growth. Early iNO treatment after VEGF inhibition reduces endothelial cell apoptosis in neonatal lungs. We speculate that enhancing endothelial cell survival after lung injury may preserve neonatal lung growth in bronchopulmonary dysplasia.

Maturational changes in the regulation of pulmonary vascular tone by nitric oxide in neonatal rats

Nitric oxide (NO) is an important regulator of vasomotor tone in the pulmonary circulation. We tested the hypothesis that the role NO plays in regulating vascular tone changes during early postnatal development. Isolated, perfused lungs from 7- and 14-day-old Sprague-Dawley rats were studied. Baseline total pulmonary vascular resistance (PVR) was not different between age groups. The addition of KCl to the perfusate caused a concentration-dependent increase in PVR that did not differ between age groups. However, the nitric oxide synthase (NOS) inhibitor N(omega)-nitro-L-arginine augmented the K(+)-induced increase in PVR in both groups, and the effect was greater in lungs from 14-day-old rats vs. 7-day-old rats. Lung levels of total endothelial, inducible, and neuronal NOS proteins were not different between groups; however, the production rate of exhaled NO was greater in lungs from 14-day-old rats compared with those of 7-day-old rats. Vasodilation to 0.1 microM of the NO donor spermine NONOate was greater in 14-day lungs than in 7-day lungs, and lung levels of both soluble guanylyl cyclase and cGMP were greater at 14 days than at 7 days. Vasodilation to 100 microM of the cGMP analog 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphate was greater in 7-day lungs than in 14-day lungs. Our results demonstrate that the pulmonary vascular bed depends more on NO production to modulate vascular tone at 14 days than at 7 days of age. The observed differences in NO sensitivity may be due to maturational increases in soluble guanylyl cyclase protein levels.

Disruption of NO-cGMP signaling by neonatal hyperoxia impairs relaxation of lung parenchyma

Exposure of immature lungs to hyperoxia for prolonged periods contributes to neonatal lung injury and airway hyperreactivity. We studied the role of disrupted nitric oxide-guanosine 3',5'-cyclic monophosphate (NO-cGMP) signaling in impairing the relaxant responses of lung tissue from hyperoxia-exposed rat pups. Pups were exposed to >/=95% O(2) or room air for 7 days starting from days 1, 5, or 14. The animals were killed, lungs were removed, and 1-mm-thick lung parenchymal strips were prepared. Lung parenchymal strips of room air or hyperoxic pups were preconstricted using bethanechol and then graded electrical field stimulation (EFS) was applied to induce relaxation. EFS-induced relaxation of lung parenchymal strips was greater at 7 and 12 days than at 21 days in room air-exposed rat pups. Hyperoxic exposure significantly reduced relaxation at 7 and 12 days but not 21 days compared with room air exposure. NO synthase blockade with N(omega)-nitro-l-arginine methyl ester diminished relaxant responses in room air but not in hyperoxic pups at 12 days. After incubation with supplemental l-arginine, the relaxation response of hyperoxic strips was restored. cGMP, a key mediator of the NO signaling pathway, also decreased in strips from hyperoxic vs. room air pups and cGMP levels were restored after incubation with supplemental l-arginine. In addition, arginase activity was significantly increased in hyperoxic lung parenchymal strips compared with room air lung parenchymal strips. These data demonstrate disruption of NO-cGMP signaling in neonatal rat pups exposed to hyperoxia and show that bioavailability of the substrate l-arginine is implicated in the predisposition of this model to airway hyperreactivity.