Neonatal pulmonary oxygen toxicity in the rat and lung changes with aging

Neonatal hyperoxic lung injury favorably alters adult right ventricular remodeling response to chronic hypoxia exposure

The development of pulmonary hypertension (PH) requires multiple pulmonary vascular insults, yet the role of early oxygen therapy as an initial pulmonary vascular insult remains poorly defined. Here, we employ a two-hit model of PH, utilizing postnatal hyperoxia followed by adult hypoxia exposure, to evaluate the role of early hyperoxic lung injury in the development of later PH. Sprague-Dawley pups were exposed to 90% oxygen during postnatal days 0-4 or 0-10 or to room air. All pups were then allowed to mature in room air. At 10 wk of age, a subset of rats from each group was exposed to 2 wk of hypoxia (Patm = 362 mmHg). Physiological, structural, and biochemical endpoints were assessed at 12 wk. Prolonged (10 days) postnatal hyperoxia was independently associated with elevated right ventricular (RV) systolic pressure, which worsened after hypoxia exposure later in life. These findings were only partially explained by decreases in lung microvascular density. Surprisingly, postnatal hyperoxia resulted in robust RV hypertrophy and more preserved RV function and exercise capacity following adult hypoxia compared with nonhyperoxic rats. Biochemically, RVs from animals exposed to postnatal hyperoxia and adult hypoxia demonstrated increased capillarization and a switch to a fetal gene pattern, suggesting an RV more adept to handle adult hypoxia following postnatal hyperoxia exposure. We concluded that, despite negative impacts on pulmonary artery pressures, postnatal hyperoxia exposure may render a more adaptive RV phenotype to tolerate late pulmonary vascular insults.

Relationship of structural to functional impairment during alveolar-capillary membrane development

Bronchopulmonary dysplasia is a chronic lung disease of extreme preterm infants and results in impaired gas exchange. Although bronchopulmonary dysplasia is characterized histologically by alveolar-capillary simplification in animal models, it is clinically defined by impaired gas diffusion. With the use of a developmentally relevant model, we correlated alveolar-capillary structural simplification with reduced functional gas exchange as measured by the diffusing factor for carbon monoxide (DFCO). Neonatal mouse pups were exposed to >90% hyperoxia or room air during postnatal days 0 to 7, and then all pups were returned to room air from days 7 to 56. At day 56, DFCO was measured as the ratio of carbon monoxide uptake to neon dilution, and lungs were fixed for histologic assessment of alveolar-capillary development. Neonatal hyperoxia exposure inhibited alveolar-capillary septal development as evidenced by significantly increased mean linear intercept, increased airspace-to-septal ratio, decreased nodal density, and decreased pulmonary microvasculature. Importantly, alveolar-capillary structural deficits in hyperoxia-exposed pups were accompanied by a significant 28% decrease in DFCO (0.555 versus 0.400; P < 0.0001). In addition, DFCO was highly and significantly correlated with structural measures of reduced alveolar-capillary growth. Simplification of alveolar-capillary structure is highly correlated with impaired gas exchange function. Current mechanistic and therapeutic animal models of inhibited alveolar development may benefit from application of DFCO as an alternative physiologic indicator of alveolar-capillary development.

Assessment of inhibited alveolar-capillary membrane structural development and function in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of extreme prematurity and is defined clinically by dependence on supplemental oxygen due to impaired gas exchange. Optimal gas exchange is dependent on the development of a sufficient surface area for diffusion. In the mammalian lung, rapid acquisition of distal lung surface area is accomplished in neonatal and early adult life by means of vascularization and secondary septation of distal lung airspaces. Extreme preterm birth interrupts secondary septation and pulmonary capillary development and ultimately reduces the efficiency of the alveolar-capillary membrane. Although pulmonary health in BPD infants rapidly improves over the first few years, persistent alveolar-capillary membrane dysfunction continues into adolescence and adulthood. Preventative therapies have been largely ineffective, and therapies aimed at promoting normal development of the air-blood barrier in infants with established BPD remain largely unexplored. The purpose of this review will be: (1) to summarize the histological evidence of aberrant alveolar-capillary membrane development associated with extreme preterm birth and BPD, (2) to review the clinical evidence assessing the long-term impact of BPD on alveolar-capillary membrane function, and (3) to discuss the need to develop and incorporate direct measurements of functional gas exchange into clinically relevant animal models of inhibited alveolar development.

Knockdown of ERp57 increases BiP/GRP78 induction and protects against hyperoxia and tunicamycin-induced apoptosis

Supplemental oxygen therapy (hyperoxia) in preterm babies with respiratory stress is associated with lung injury and the development of bronchopulmonary dysplasia. Endoplasmic reticulum (ER) homeostasis plays critical roles in maintaining cellular functions such as protein synthesis, folding, and secretion. Interruption of ER homeostasis causes ER stress and triggers the unfolded protein response, which can lead to apoptosis in persistently stressed cells. ERp57 is an ER protein and is associated with calreticulin and calnexin in protein glycosylation. In this study, we found hyperoxia downregulated ERp57 in neonatal rat lungs and cultured human endothelial cells. Transient transfection of ERp57 small interfering RNA significantly knocked down ERp57 expression and reduced hyperoxia- or tunicamycin-induced apoptosis in human endothelial cells. Apoptosis was decreased from 26.8 to 9.9% in hyperoxia-exposed cells and from 37.8 to 5.0% in tunicamycin-treated cells. The activation of caspase-3 induced by hyperoxia or tunicamycin was diminished and immunoglobulin heavy chain-binding protein/glucose-regulated protein 78-kDa (BiP/GRP78) induction was increased in ERp57 knockdown cells. Overexpression of ERp57 exacerbated hyperoxia- or tunicamycin-induced apoptosis in human endothelial cells. Apoptosis was increased from 10.1 to 14.3% in hyperoxia-exposed cells and from 14.0 to 21.2% in tunicamycin-treated cells. Overexpression of ERp57 also augmented tunicamycin-induced caspase-3 activation and reduced BiP/GRP78 induction. Our results demonstrate that ERp57 can regulate apoptosis in human endothelial cells. It appears that knockdown of ERp57 confers cellular protection against hyperoxia- or tunicamycin-induced apoptosis by inhibition of caspase-3 activation and stimulation of BiP/GRP78 induction.

Heat shock protein 27 protects lung epithelial cells from hyperoxia-induced apoptotic cell death

Oxygen toxicity or hyperoxia is one of the major contributing factors in the development of bronchopulmonary dysplasia. Heat shock protein 27 (Hsp27) is an important chaperone protein in the postnatal lung development. However, the role of Hsp27 in lung epithelial cells during hyperoxia is unclear. Our studies by cDNA array and immunohistochemistry revealed that hyperoxia decreased Hsp27 expression in newborn rat lungs. Western blot showed that hyperoxic treatment significantly decreased Hsp27 protein expression in cultured human lung epithelial cells (A549). The expression of Hsp27 was decreased approximately twofold after 24-h and threefold after 48- and 72-h hyperoxic exposure compared with that of the A549 cells exposed to normoxia (p < 0.05, n = 3). Knockdown of Hsp27 expression by siRNA resulted in more apoptotic cell death in A549 cells. Overexpression of Hsp27 reduced hyperoxia-induced apoptotic cell death to 9.2% in Hsp27 overexpressing A549 cells from 12.6% in control A549 cells after 72-h hyperoxic exposure (p < 0.01, n = 8-9). Overexpression of Hsp27 also diminished hyperoxia-induced caspase-9 activation in A549 cells. Our results demonstrated that hyperoxia decreased Hsp27 expression in newborn rat lung and cultured human lung epithelial cells. Overexpression of Hsp27 could reduce hyperoxia-induced apoptosis in cultured human lung epithelial cells.

Surfactant Palmitoylmyristoylphosphatidylcholine Is a Marker for Alveolar Size during Disease

Two common lung-related complications in the neonate are respiratory distress syndrome, which is associated with a failure to generate low surface tension at the air-liquid interface because of pulmonary surfactant insufficiency, and bronchopulmonary dysplasia (BPD), a chronic lung injury with reduced alveolarization. Surfactant phosphatidylcholine (PC) molecular species composition during alveolarization has not been examined. Mass spectrometry analysis of bronchoalveolar lavage fluid of rodents and humans revealed significant changes in surfactant PC during alveolar development and BPD. In rats, total PC content rose during alveolarization, which was caused by an increase in palmitoylmyristoyl-PC (16:0/14:0PC) concentration. Furthermore, two animal models of BPD exhibited a specific reduction in 16:0/14:0PC content. In humans, 16:0/14:0PC content was specifically decreased in patients with BPD and emphysema compared with patients without alveolar pathology. Palmitoylmyristoyl-PC content increased with increasing intrinsic surfactant curvature, suggesting that it affects surfactant function in the septating lung. The changes in acyl composition of PC were attributed to type II cells producing an altered surfactant during alveolar development. These data are compatible with extracellular surfactant 16:0/14:0PC content being an indicator of alveolar architecture of the lung.

Effects of Glucocorticoids on Fetal and Neonatal Lung Development

Antenatal glucocorticoids have been used for 30 years to induce maturation of preterm fetal lungs. Stimulation of the pulmonary surfactant system has been regarded as the most important effect of antenatal glucocorticoids; however, as these drugs alter the expression of a large number of genes they affect the maturation of the lung in several other ways. Antioxidant enzyme production, lung fluid absorption and alveolar development are all affected by glucocorticoids administered in the perinatal period. There is evidence that glucocorticoids induce genes associated with the synthesis of surfactant proteins, fatty acid synthase, the epithelial sodium channel and the membrane protein sodium/potassium ATPase as well as several antioxidant enzymes including catalase, glutathione peroxidase and two superoxide dismutases. Glucocorticoids also increase the expression of vascular endothelial growth factor, which may inhibit alveolarization and lead to abnormally large alveoli. The use of both antenatal and postnatal glucocorticoids has increased in the past decade. However, as concerns about possible long-term effects have arisen, the mechanisms of how glucocorticoids alter the structure and function of the lungs needs to be determined to allow the development of more specific agents in the treatment of respiratory distress syndrome.

Chronic maternal nicotine exposure during gestation and lactation and the development of the lung parenchyma in the offspring

THE AIM OF THIS STUDY WAS TO INVESTIGATE THE EFFECT OF MATERNAL NICOTINE EXPOSURE DURING GESTATION AND LACTATION ON: (1) the development of the gas exchange area of the lungs of the offspring; and (2) to determine whether these effects are reversible. Pregnant rats received daily nicotine (subcutaneously 1mgkg(-1) body weight) during gestation and lactation. Nicotine administration started 1 day after mating and lasted until weaning on postnatal day 21. The offspring were exposed to nicotine via the placenta and mother's milk only. The lung tissue of the neonates was collected on postnatal days 14, 21, 35 and 42 and prepared for morphometry. The results obtained show that maternal nicotine exposure resulted in bigger alveolar volumes and suppressed alveolarisation in the lungs of the offspring. Flattening of the alveoli occurred as the animals aged and as a consequence the internal surface area available for gas exchange decreased; a condition that resembles panlobular emphysema. It is unlikely that these effects of maternal nicotine exposure during gestation and lactation on lung development in the offspring was due to a lower birth weight, or a reduction in the period of gestation, or a poor supply of nutrients to the offspring. The changes in the gas-exchange region of the nicotine-exposed rat pups appear to be irreversible.

Effects of hyperoxia on VEGF, its receptors, and HIF-2alpha in the newborn rat lung

Signaling through the hypoxia inducible factor (HIF)-VEGF-VEGF receptor system (VEGF signaling system) leads to angiogenesis and epithelial cell proliferation and is a key mechanism regulating alveolarization in lungs of newborn rats. Hyperoxia exposure (>95% O2 days 4-14) arrests lung alveolarization and may do so through suppression of the VEGF signaling system. Lung tissue mRNA levels of HIF-2alpha and VEGF increased from days 4-14 in normoxic animals, but hyperoxia suppressed these increases. Levels of HIF-2alpha and VEGF mRNA were correlated in the air but not the O2-treated group, suggesting that the low levels of HIF-2alpha observed at high O2 concentrations are not stimulating VEGF expression. VEGF164 protein levels increased with developmental age, and with hyperoxia to day 9, but continuing hyperoxia decreased levels by day 12. VEGFR1 and VEGFR2 mRNA expression also increased in air-exposed animals, and these, too, were significantly decreased by hyperoxia by day 9 and day 12, respectively. Receptor protein levels did not increase with development; however, O2 did decrease protein to less than air values. Hyperoxic suppression of VEGF signaling from days 9-14 may be one mechanism by which alveolarization is arrested.

Montelukast does not protect against hyperoxia-induced inhibition of alveolarization in newborn rats

Impaired lung development has been demonstrated in neonatal animals exposed to hyperoxia. High lung cys-leukotriene levels may be a contributing factor towards the increase in oxygen toxicity. We investigated the effect of cysteinyl-leukotriene inhibition using the receptor antagonist, montelukast (MK, Singulair), on hyperoxia-induced changes in lung parenchymal structure in neonatal rat pups. Rat pups were exposed to 21% O(2) (air) or 50% O(2) (moderate hyperoxia) from days 1-14 after birth, and were administered the cys-leukotriene receptor antagonist MK (1 mg/kg/day) or normal saline from days 4-14. Somatic growth and morphometric measurements were done on day 15. There was a significant increase in bronchoalveolar lavage fluid cysteinyl-leukotriene levels (+61.9%) when animals were exposed to hyperoxia. O(2) exposure significantly decreased the specific internal surface area by 13%. There was a nonsignificant 5.8% and 19.6% increase in mean chord length and mean alveolar diameter, respectively, as well as an 8.6% decrease in lung volume to body weight ratio. Inhibition of only one arm of the arachidonic-acid cascade by MK was not sufficient to prevent these oxygen-induced changes.

Neonatal exposure to 65% oxygen durably impairs lung architecture and breathing pattern in adult mice

STUDY OBJECTIVE

To test the hypothesis that exposure to hyperoxia during the postnatal period of rapid alveolar multiplication by septation would cause permanent impairments, even with moderate levels of hyperoxia.

DESIGN

We exposed mouse pups to 65% O(2) (hyperoxic mice) or normoxia (normoxic mice) during their first postnatal month, and we analyzed lung histology, pulmonary mechanics, blood gas, and breathing pattern during normoxia or in response to chemical stimuli in adulthood, when they reached 7 to 8 months of postnatal age.

RESULTS

Hyperoxic mice had fewer and larger alveoli than normoxic mice (number of alveoli per unit surface area of parenchyma, 266 +/- 62/mm(2) vs 578 +/- 77/mm(2), p < 0.0001) [mean +/- SD], the cause being impaired alveolarization (radial alveolar count, 5.8 +/- 0.2 in hyperoxic mice vs 10.5 +/- 0.5 in normoxic mice, p < 0.0001). Respiratory system compliance was higher in hyperoxic mice (0.098 +/- 0.006 mL/cm H(2)O) than in normoxic mice (0.064 +/- 0.006 mL/cm H(2)O, p < 0.016). Baseline tidal volume (VT) and breath duration (TTOT]) measured noninvasively by whole-body plethysmography were larger in hyperoxic mice than in normoxic mice (VT, + 15%, p < 0.01; TTOT, + 12%, p < 0.01). Despite these impairments, blood gas, baseline minute ventilation E, and E responses to hypoxia and hypercapnia were normal in hyperoxic mice, compared with normoxic mice.

CONCLUSION

Hyperoxic exposure during lung septation in mice may cause irreversible lung injury and breathing pattern abnormalities in adulthood at O(2) concentrations lower than previously thought. However, ventilatory function and body growth were preserved, and ventilatory function showed no major abnormalities, at least at rest, despite early oxygen-induced injuries.

Timing of hyperoxic exposure during alveolarization influences damage mediated by leukotrienes

Hyperoxic exposure of rat pups during alveolarization (postnatal days 4-14) severely retards alveolar development. Some aspects of this inhibition are mediated by leukotrienes (LTs) and may be time sensitive. We determined 1) the effects of exposure to hyperoxia (O(2)) during discrete periods before and during alveolarization on developing alveoli and 2) whether a relationship exists between O(2) and LTs in these periods. Pups were exposed to >95% O(2) from days 1 to 4, 4 to 9, 9 to 14, or 4 to 14 in the absence and presence of the LT synthesis inhibitor MK-0591. Both the level of in vitro lung tissue LT output on days 4, 9, and 14 and the degree of alveolarization on day 14 were determined. Pups exposed to O(2) from days 4 to 9 had a more profound inhibition of alveolarization on day 14 compared with those exposed to O(2) from days 1 to 4 or 9 to 14. Peptido-LT levels were significantly higher in pups exposed to O(2) on days 9 and 14 compared with pups in air and returned to normal once normoxia was restored. LT inhibition from days 4 to 14, 4 to 9, or 9 to 14 in pups exposed to O(2) from days 4 to 14 prevented the O(2)-induced inhibition of alveolarization. These data suggest that developing alveoli are sensitive to LTs shortly before and after day 9, significantly retarding certain parameters of alveolarization on day 14. We conclude that some of the effects of O(2) are not uniform throughout different stages of alveolarization and that this is likely related to the timing of LT exposure.

Magnetic resonance imaging of pulmonary damage in the term and premature rat neonate exposed to hyperoxia

Immaturity and oxygen toxicity have been implicated in the pathogenesis of the neonatal disease bronchopulmonary dysplasia. The present study aimed to investigate the use of magnetic resonance imaging (MRI) to assess hyperoxia-mediated lung injury in the term and premature neonate. Term (gestation, 22 d) and premature (21 d) rat pups were exposed to hyperoxia (>95%) or air for a 6-d period (n = 7) and assessed for lung damage by MRI. Pulmonary signal intensities of T1-weighted images were significantly increased in both hyperoxia-exposed term and premature neonates, relative to air-breathing controls (p < 0.01). T2-weighted MRI signal intensities were also greater in premature and term rat pups exposed to hyperoxia, but failed to reach significance (p > 0.05). Elevated MRI pulmonary signal intensities may have represented an increase in magnetic resonance-detectable free water, possibly indicating an increase in edema. Corresponding histologic evidence of lung injury was detected in both term and premature rat pups exposed to hyperoxia. Histologic samples indicated focal regions of alveolar hemorrhage, immune cell infiltration, edema, and collapse in both term and premature rat neonates exposed to hyperoxia. Alveolar air space was assessed (n = 5) by light microscopy within a 0.5 mm2 region of the superior left and inferior right pulmonary lobes of each treatment group. Alveolar area of the superior left lung lobe of the premature hyperoxia treatment group was significantly smaller than other treatment groups (p < 0.05). Reduced area for respiratory exchange was probably a result of observed focal areas of edema and collapse. MRI-detectable increases in lung signal intensity may have represented an increase in hyperoxia-induced pulmonary edema in the 6-d-old rat neonate. Increases in signal intensity correlated with the appearance of edema in pulmonary histologic samples. Premature delivery had a less defined effect on lung injury but possibly exacerbated hyperoxia-mediated pulmonary damage.

Lung volume, pulmonary vasculature, and factors affecting survival in congenital diaphragmatic hernia

OBJECTIVES

There is a wide variation in published mortality from congenital diaphragmatic hernia (CDH). The prevailing opinion is that this variation is related directly to the degree of pulmonary hypoplasia. Our aim was to test the hypothesis that other factors are important for outcome. The specific objectives of this study were: 1) to quantitate the degree of lung hypoplasia and pulmonary arterial wall thickness in infants eligible for, and treated with, extracorporeal membrane oxygenation (ECMO), using postmortem analysis of lung DNA, wet lung weight, lung volume, and vessel morphometrics; 2) to correlate the degree of lung hypoplasia and vascular changes with functional tests of oxygenation and estimated right ventricular systolic pressures (RVSP); 3) to determine the minimum lung volume necessary for survival; and 4) to determine contributory clinical factors as potential causes of death in ECMO-treated infants with CDH.

METHODOLOGY

We retrospectively analyzed all 90 infants with CDH admitted consecutively over a 9-year period to a children's hospital with an ECMO program. Infants were categorized as lived or died, with or without ECMO. Indication for ECMO was an evolving process; however, in general, it was the therapy of last resort for pulmonary insufficiency. Clinically, the single best oxygenation index before ECMO or CDH repair while on conventional ventilation, and serial echocardiograms before, during, and after ECMO, were obtained. Twelve of 14 infants dying with ECMO and 6 of 12 without ECMO had postmortem examinations. Lung volume, DNA content, wet weights, and arterial wall thickness at the level of alveolar ducts were measured in both lungs. Postmortem morphometric findings were correlated with in vivo tests of cardiopulmonary function and contributory clinical factors in mortality.

RESULTS

Sixty-three percent of all infants with CDH and 61% of ECMO-treated infants lived. All infants with CDH requiring ECMO had elevated RVSP/systolic systemic blood pressure ratios before ECMO (0.98 +/- 0.24). Eighty-eight percent of ECMO-treated infants with CDH decreased this ratio to < 0.5 within 14 days, regardless of lung size. However, infants dying with normal ratios still had increased arterial wall thickness and muscle in both lungs. In infants whose lung volume, DNA, and weight were > 45% of values predicted for age-matched controls, the oxygenation index ranged from 4 to 29, significantly less than that in infants with values < 45% of predicted values (range, 25 to 133). We speculate that eight infants with lung volumes > 45% of that for controls died from potentially preventable surgical and medical complications.

CONCLUSION

A minimum lung volume of 45% of the value predicted from age-matched controls is required for survival in ECMO-treated infants. The RVSP/systolic systemic blood pressure ratio can be reduced with ECMO to < 0.5 in the majority of infants, even with lung volumes inadequate for survival. We speculate that 9% of infants with adequate lung volume were potentially survivable, but died of medical and surgical complications.

Chronic modifications of lung and heart development in glucocorticoid-treated newborn rats exposed to hyperoxia or room air

We assessed the mechanics and morphology of the lung in 165 rats treated neonatally with either room air (RA), O2, RA + steroids, or O2 + steroids. Newborn Sprague-Dawley male rats were randomly assigned to these groups. O2-exposure (0.96-1.0 FiO2) lasted 5 days, and dexamethasone treatment consisted of eight daily S.C. injections of drug or buffer in successive doses of 0.5, 0.4, 0.3, 0.2, 0.1, 0.1, 0.1, and 0.1 mg/kg. At 58 days, right ventricular systolic pressure (RVP) was measured. At 60 days, all rats were sacrificed for obtaining lung weight and DNA, saline pressure-volume (P-V) curves, and morphometry. We weighted right ventricles (RV) and left ventricles + septa (LV). Hyperoxia alone did not, but steroid decreased survival rate to 79.4% (95.3% in RA rats, P < 0.02). Only 21 of 40 (52%) O2 + steroids rats survived, less than in both RA groups (P < 0.001). RV weight, RVP and muscularization of alveolar duct arteries were significantly increased in O2 vs. RA rats. In RA + steroids rats, weight of the LV was decreased but RV, RVP, and lung vasculature were not affected. These effects were additive in the O2 + steroid group. Wet lung weights and DNA were increased for RA + steroid rats over all others. O2 and steroids shifted the P-V curve to the left and O2 + steroids still further. Maximal lung volume increased significantly with RA + steroids and still further in O2 + steroids but not in O2 alone. O2 and steroids significantly increased the mean linear intercept and O2 + steroids even more so.(ABSTRACT TRUNCATED AT 250 WORDS)

Lung glutathione reductase induction in aging catalase-depleted frogs correlates with early survival throughout the life span

A comprehensive experimental study on free radical-related parameters was performed in the lung throughout the life span of 220 initially young or old frogs. No age related differences were found transversely or longitudinally for lung superoxide dismutase, catalase, Se-dependent and -independent glutathione peroxidases, glutathione reductase, GSH, GSSG, or GSSG/GSH ratio. Continuous catalase depletion with aminotriazole led to glutathione reductase induction in the lung after 14.5 months of experimentation. This was accompanied by a great increase in survival rate of treated animals in relation to controls (especially in the old group). After 26.5 months of experimentation, glutathione reductase induction was lost and GSSG/GSH values tended to increase. This was followed by a 3-month long period of acute decrease in survival rate of treated animals. It is suggested that a high antioxidant/prooxidant balance is of protective value against causes of early death and can possibly be used in the future (when appropriately controlled) to increase the number of healthy years of the normal life span.