Hyperoxia disrupts vascular and alveolar growth of the developing lung and contributes to the development of bronchopulmonary dysplasia (BPD). Endothelial progenitor cells (EPC) have been implicated in repair of the vasculature, but their role in lung vascular development is unknown. Since disruption of vascular growth impairs lung structure, we hypothesized that neonatal hyperoxia impairs EPC mobilization and homing to the lung, contributing to abnormalities in lung structure. Neonatal mice (1-day-old) were exposed to 80% O2at Denver's altitude (= 65% at sea level) or room air for 10 days. Adult mice were also exposed for comparison. Blood, lung, and bone marrow were harvested after hyperoxia. Hyperoxia decreased pulmonary vascular density by 72% in neonatal but not adult mice. In contrast to the adult, hyperoxia simplified distal lung structure neonatal mice. Moderate hyperoxia reduced EPCs (CD45−/Sca-1+/CD133+/VEGFR-2+) in the blood (55%; P < 0.03), bone marrow (48%; P < 0.01), and lungs (66%; P < 0.01) of neonatal mice. EPCs increased in bone marrow (2.5-fold; P < 0.01) and lungs (2-fold; P < 0.03) of hyperoxia-exposed adult mice. VEGF, nitric oxide (NO), and erythropoietin (Epo) contribute to mobilization and homing of EPCs. Lung VEGF, VEGF receptor-2, endothelial NO synthase, and Epo receptor expression were reduced by hyperoxia in neonatal but not adult mice. We conclude that moderate hyperoxia decreases vessel density, impairs lung structure, and reduces EPCs in the circulation, bone marrow, and lung of neonatal mice but increases EPCs in adults. This developmental difference may contribute to the increased susceptibility of the developing lung to hyperoxia and may contribute to impaired lung vascular and alveolar growth in BPD.

Exposure of newborn rats to hyperoxia impairs alveolarization and vessel growth, causing abnormal lung structure that persists during infancy. Recent studies have shown that impaired angiogenesis due to inhibition of vascular endothelial growth factor (VEGF) signaling decreases alveolar and vessel growth in the developing lung, and that nitric oxide (NO) mediates VEGFdependent angiogenesis. The purpose of this study was to determine whether hyperoxia causes sustained reduction of lung VEGF, VEGF receptor, or endothelial NO synthase (eNOS) expression during recovery, and whether inhaled NO improves lung structure in infant rats after neonatal exposure to hyperoxia. Newborn rat pups were randomized to hyperoxia [fraction of inspired oxygen (FiO 2 ), 1.00] or room air exposure for 6 d, and then placed in room air with or without inhaled NO (10 ppm) for 2 wk. Rats were then killed for studies, which included measurements of body weight, lung weight, right ventricular hypertrophy (RVH), morphometric analysis of alveolarization (by mean linear intercept (MLI), radial alveolar counts (RAC), and vascular volume (Vv), and immunostaining and Western blot analysis. In comparison with controls, neonatal hyperoxia reduced body weight, increased MLI, and reduced RAC in infant rats. Lung VEGF, VEGFR-2, and eNOS protein expression were reduced after hyperoxia. Inhaled NO treatment after hyperoxia increased body weight and improved distal lung growth, as demonstrated by increased RAC and Vv and decreased MLI. We conclude that neonatal hyperoxia reduced lung VEGF expression, which persisted during recovery in room air, and that inhaled NO restored distal lung growth in infant rats after neonatal hyperoxia. BPD is the chronic lung disease of infancy that follows ventilator and oxygen therapy for acute respiratory failure after premature birth (1). Traditionally, BPD has been defined by the presence of persistent respiratory signs and symptoms, the need for supplemental oxygen to treat hypoxemia, and an abnormal chest radiograph at 36 wk corrected age. Over the years, the clinical course of premature infants with BPD has changed, reflecting the effects of improved survival of babies who are less mature and have lower birth weights, as well as changes in therapeutic strategies, such as surfactant therapy and modes of mechanical ventilation (2,3). Infants with BPD now have less severe initial acute respiratory disease, and at autopsy, lung histology is predominantly characterized by arrested lung development, including impaired alveolar and vascular growth (4).Although mechanisms that impair lung growth in BPD are poorly understood, recent studies suggest that disruption of VEGF function plays a pivotal role in the pathogenesis of BPD. Bhatt et al. (5) have shown decreased lung VEGF mRNA and protein expression, as well as a reduction of the VEGF receptor, flt-1 (VEGFR-1), in the lungs of infants with fatal BPD. In addition, VEGF protein levels are reduced in the tracheal aspirates obtained from premature newborns with BPD, as well a...

VEGF plays a critical role during lung development and is decreased in human infants with bronchopulmonary dysplasia. Inhibition of VEGF receptors in the newborn rat decreases vascular growth and alveolarization and causes pulmonary hypertension (PH). Nitric oxide (NO) is a downstream mediator of VEGF, but whether the effects of impaired VEGF signaling are due to decreased NO production is unknown. Therefore, we sought to determine whether impaired VEGF signaling downregulates endothelial NO synthase (eNOS) expression in the developing lung and whether inhaled NO (iNO) decreases PH and improves lung growth after VEGF inhibition. Newborn rats received a single dose of SU-5416 (a VEGF receptor inhibitor) or vehicle by subcutaneous injection and were killed up to 3 wk of age for assessments of right ventricular hypertrophy (RVH), radial alveolar counts (RAC), lung eNOS protein, and NOx production in isolated perfused lungs (IPL). Neonatal treatment with SU-5416 increased RVH in infant rats and reduced RAC. Compared with controls, SU-5416 reduced lung eNOS protein expression by 89% at 5 days (P < 0.01). IPL studies from day 14 rats demonstrated increased baseline pulmonary artery pressure and lower perfusate NOx concentration after SU-5416 treatment. Importantly, iNO treatment prevented the increase in RVH and improved RAC after SU-5416 treatment. We conclude that treatment of neonatal rats with SU-5416 downregulates lung eNOS expression and that iNO therapy decreases PH and improves lung growth after SU-5416 treatment. We speculate that decreased NO production contributes to PH and decreases distal lung growth caused by impaired VEGF signaling.

In addition to its vasodilator properties, nitric oxide (NO) promotes angiogenesis in the systemic circulation and tumors. However, the role of NO in promoting normal lung vascular growth and its impact on alveolarization during development or in response to perinatal stress is unknown. We hypothesized that NO modulates lung vascular and alveolar growth and that decreased NO production impairs distal lung growth in response to mild hypoxia. Litters of 1-day-old mouse pups from parents that were heterozygous for endothelial nitric oxide synthase (eNOS) deficiency were placed in a hypobaric chamber at a simulated altitude of 12,300 ft (Fi(O(2)) = 0.16). After 10 days, the mice were killed, and lungs were fixed for morphometric and molecular analysis. Compared with wild-type controls, mean linear intercept (MLI), which is inversely proportional to alveolar surface area, was increased in the eNOS-deficient (eNOS -/-) mice [51 +/- 2 micro m (eNOS -/-) vs. 41 +/- 1 micro m (wild type); P < 0.01]. MLI was also increased in the eNOS heterozygote (+/-) mice (44 +/- 1 micro m; P < 0.03 vs. wild type). Vascular volume density was decreased in the eNOS -/- mice compared with wild-type controls (P < 0.03). Lung vascular endothelial growth factor (VEGF) protein and VEGF receptor-1 (VEGFR-1) protein content were not different between the study groups. In contrast, lung VEGFR-2 protein content was decreased from control values by 63 and 34% in the eNOS -/- and eNOS +/- mice, respectively (P < 0.03). We conclude that exposure to mild hypoxia during a critical period of lung development impairs alveolarization and reduces vessel density in the eNOS-deficient mouse. We speculate that NO preserves normal distal lung growth during hypoxic stress, perhaps through preservation of VEGFR-2 signaling.