Development of a peripheral blood transcriptomic gene signature to predict bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is the most common lung disease of extreme prematurity, yet mechanisms that associate with or identify neonates with increased susceptibility for BPD are largely unknown. Combining artificial intelligence with gene expression data is a novel approach that may assist in better understanding mechanisms underpinning chronic lung disease and in stratifying patients at greater risk for BPD. The objective of this study is to develop an early peripheral blood transcriptomic signature that can predict preterm neonates at risk for developing BPD. Secondary analysis of whole blood microarray data from 97 very low birth weight neonates on day of life 5 was performed. BPD was defined as positive pressure ventilation or oxygen requirement at 28 days of age. Participants were randomly assigned to a training (70%) and testing cohort (30%). Four gene-centric machine learning models were built, and their discriminatory abilities were compared with gestational age or birth weight. This study adheres to the transparent reporting of a multivariable prediction model for individual prognosis or diagnosis (TRIPOD) statement. Neonates with BPD (n = 62 subjects) exhibited a lower median gestational age (26.0 wk vs. 30.0 wk, P < 0.01) and birth weight (800 g vs. 1,280 g, P < 0.01) compared with non-BPD neonates. From an initial pool (33,252 genes/patient), 4,523 genes exhibited a false discovery rate (FDR) <1%. The area under the receiver operating characteristic curve (AUC) for predicting BPD utilizing gestational age or birth weight was 87.8% and 87.2%, respectively. The machine learning models, using a combination of five genes, revealed AUCs ranging between 85.8% and 96.1%. Pathways integral to T cell development and differentiation were associated with BPD. A derived five-gene whole blood signature can accurately predict BPD in the first week of life.

Hepatic Insulin Resistance and Altered Gluconeogenic Pathway in Premature Baboons

Premature infants have altered glucose regulation early in life and increased risk for diabetes in adulthood. Although prematurity leads to an increased risk of diabetes and metabolic syndrome in adult life, the role of hepatic glucose regulation and adaptation to an early extrauterine environment in preterm infants remain unknown. The purpose of this study was to investigate developmental differences in glucose metabolism, hepatic protein content, and gene expression of key insulin-signaling/gluconeogenic molecules. Fetal baboons were delivered at 67%, 75%, and term gestational age and euthanized at birth. Neonatal baboons were delivered prematurely (67% gestation), survived for two weeks, and compared with similar postnatal term animals and underwent serial hyperinsulinemic-euglycemic clamp studies. Premature baboons had decreased endogenous glucose production (EGP) compared with term animals. Consistent with these results, the gluconeogenic molecule, phosphoenolpyruvate carboxykinase messenger RNA, was decreased in preterm baboons compared with terms. Hepatic insulin signaling was altered by preterm birth as evidenced by decreased insulin receptor-β, p85 subunit of phosphoinositide 3-kinase, phosphorylated insulin receptor substrate 1, and Akt-1 under insulin-stimulated conditions. Furthermore, preterm baboons failed to have the normal increase in glycogen synthase kinase-α from fetal to postnatal life. The blunted responses in hepatic insulin signaling may contribute to the hyperglycemia of prematurity, while impaired EGP leads to hypoglycemia of prematurity.

Neonatal intubation with direct laryngoscopy vs videolaryngoscopy: an extremely premature baboon model

OBJECTIVE

To compare the ability to successfully intubate extremely preterm baboons using conventional direct laryngoscopy (DL) vs videolaryngoscopy.

METHODS

A prospective randomized crossover study using experienced and inexperienced neonatal intubators. All participants were shown an educational video on intubation with each device, followed by attempt of the procedure. The time for successful intubation was the primary outcome.

RESULTS

Seven subjects comprised the experienced group, while 10 individuals were in the inexperienced group. The overall intubation success rate was comparable between both devices (53% vs 26%, P = 0.09); however, mean time to intubate with the conventional laryngoscope was faster (25.5 vs 39.4 s, P = 0.02). Although both groups intubated faster with DL, it only reached statistical significance in the inexperienced group (27.0 vs 48.7 s, P < 0.05).

CONCLUSION

Conventional DL and videolaryngoscopy are suitable modes for intubating extremely preterm baboons. Although experienced intubators prefer DL, intubation success rate and time to intubate with both devices were comparable. In inexperienced intubators, participants preferred and intubated faster with DL.

Effects of ibuprofen treatment on the developing preterm baboon kidney

Preterm neonates are commonly exposed postnatally to pharmacological treatments for a patent ductus arteriosus. Exposure of the developing kidney to nephrotoxic medications may adversely impact renal development. This study aimed to determine the effect of early postnatal ibuprofen treatment, both alone and in combination with a nitric oxide synthase inhibitor (NOSi), on renal development and morphology. Baboon neonates were delivered prematurely at 125-day (125d) gestation (term = 185d) and were euthanized at birth or postnatal day 6. Neonates were divided into four groups: 125d gestational controls (n = 8), Untreated (n = 8), Ibuprofen (n = 6), and ibuprofen (Ibu)+NOSi (n = 4). Animals in the Ibuprofen and Ibu+NOSi groups received five doses of ibuprofen, with the Ibuprofen+NOSi animals additionally administered a NOS inhibitor (N(G)-monomethyl-l-arginine). There was no difference among groups in body weight, kidney weight, or glomerular generation number. Nephrogenic zone width was significantly reduced in the Ibuprofen group (123.5 ± 7.4 μm) compared with the 125d gestational control (176.1 ± 6.9 μm) and Untreated animals (169.7 ± 78.8 μm). In the Ibu+NOSi group, nephrogenic zone width averaged 152.7 ± 3.9 μm, which was not significantly different from any other group. Morphologically abnormal glomeruli were present at a range of 0.0-22.9% in the Untreated group, 0.0-6.1% in the Ibuprofen group, and 0.0-1.4% in the Ibu+NOSi group. In conclusion, early postnatal ibuprofen exposure is associated with a reduced nephrogenic zone width, which may suggest the early cessation of nephrogenesis following treatment. Ultimately, this may impact the number of nephrons formed in the preterm kidney.

Effects of ibuprofen treatment on the developing preterm baboon kidney

Preterm neonates are commonly exposed postnatally to pharmacological treatments for a patent ductus arteriosus. Exposure of the developing kidney to nephrotoxic medications may adversely impact renal development. This study aimed to determine the effect of early postnatal ibuprofen treatment, both alone and in combination with a nitric oxide synthase inhibitor (NOSi), on renal development and morphology. Baboon neonates were delivered prematurely at 125-day (125d) gestation (term = 185d) and were euthanized at birth or postnatal day 6. Neonates were divided into four groups: 125d gestational controls (n = 8), Untreated (n = 8), Ibuprofen (n = 6), and ibuprofen (Ibu)+NOSi (n = 4). Animals in the Ibuprofen and Ibu+NOSi groups received five doses of ibuprofen, with the Ibuprofen+NOSi animals additionally administered a NOS inhibitor (N(G)-monomethyl-l-arginine). There was no difference among groups in body weight, kidney weight, or glomerular generation number. Nephrogenic zone width was significantly reduced in the Ibuprofen group (123.5 ± 7.4 μm) compared with the 125d gestational control (176.1 ± 6.9 μm) and Untreated animals (169.7 ± 78.8 μm). In the Ibu+NOSi group, nephrogenic zone width averaged 152.7 ± 3.9 μm, which was not significantly different from any other group. Morphologically abnormal glomeruli were present at a range of 0.0-22.9% in the Untreated group, 0.0-6.1% in the Ibuprofen group, and 0.0-1.4% in the Ibu+NOSi group. In conclusion, early postnatal ibuprofen exposure is associated with a reduced nephrogenic zone width, which may suggest the early cessation of nephrogenesis following treatment. Ultimately, this may impact the number of nephrons formed in the preterm kidney.

Anatomic closure of the premature patent ductus arteriosus: The role of CD14+/CD163+ mononuclear cells and VEGF in neointimal mound formation

Permanent closure of the newborn ductus arteriosus requires the development of neointimal mounds to completely occlude its lumen. VEGF is required for neointimal mound formation. The size of the neointimal mounds (composed of proliferating endothelial and migrating smooth muscle cells) is directly related to the number of VLA4 mononuclear cells that adhere to the ductus lumen after birth. We hypothesized that VEGF plays a crucial role in attracting CD14/CD163 mononuclear cells (expressing VLA4) to the ductus lumen and that CD14/CD163 cell adhesion to the ductus lumen is important for neointimal growth. We used neutralizing antibodies against VEGF and VLA-4 to determine their respective roles in remodeling the ductus of premature newborn baboons. Anti-VEGF treatment blocked CD14/CD163 cell adhesion to the ductus lumen and prevented neointimal growth. Anti-VLA-4 treatment blocked CD14/CD163 cell adhesion to the ductus lumen, decreased the expression of PDGF-B (which promotes smooth muscle migration), and blocked smooth muscle influx into the neointimal subendothelial space (despite the presence of increased VEGF in the ductus wall). We conclude that VEGF is necessary for CD14/CD163 cell adhesion to the ductus lumen and that CD14/CD163 cell adhesion is essential for VEGF-induced expansion of the neointimal subendothelial zone.

Relationship between circulating platelet counts and ductus arteriosus patency after indomethacin treatment

OBJECTIVE

To determine whether low platelet counts are related to the incidence of patent ductus arteriosus (PDA) after indomethacin treatment in preterm human infants.

STUDY DESIGN

Multivariable logistic regression modeling was used for a cohort of 497 infants, who received indomethacin (within 15 hours of birth).

RESULTS

Platelet counts were not related to the incidence of permanent closure after indomethacin constriction. There was a relationship between platelet counts and the initial degree of constriction; however, this relationship appeared to be primarily influenced by the high end of the platelet distribution curve. PDA incidence was similar in infants with platelet counts < 50 × 10⁹/L and those with platelet counts above this range. Only when platelet counts were consistently >230 ×10⁹/L was there a decrease in PDA incidence.

CONCLUSION

In contrast to the evidence in mice, low circulating platelet counts do not affect permanent ductus closure (or ductus reopening) in human preterm infants.

Patterns of gene expression in the ductus arteriosus are related to environmental and genetic risk factors for persistent ductus patency

Three independent risk factors (immature gestation, absence of antenatal glucocorticoid exposure, and presence of the rs2817399(A) allele of the gene TFAP2B) are associated with patent ductus arteriosus (PDAs) that fail to close during prostaglandin inhibition. We hypothesized that these three factors may affect a common set of genes that increase the risk of persistent PDA after birth. We studied baboon ductus from term, preterm, and glucocorticoid-treated preterm fetuses and found that both immature gestation and absence of antenatal glucocorticoid exposure decreased RNA expression of calcium- and potassium-channel genes involved in oxygen-induced constriction, and phosphodiesterase genes (that modulate cAMP/cGMP signaling). Ductus obtained from second trimester human pregnancies were genotyped for TFAP2B polymorphisms. When present, the rs2817399(A) allele also was associated with decreased expression of calcium- and potassium-channel genes. In contrast, alleles of two other TFAP2B polymorphisms, rs2817419(G) and rs2635727(T), which are not related to the incidence of PDA after birth, had no effect on RNA expression. In conclusion, three calcium- and potassium-channel genes (CACNA1G/ alpha1G, CACNB 2/CaL-beta2, and KCNA2/ Kv1.2) were similarly affected by each of the PDA risk factors. We speculate that these channels may play a significant role in closing the preterm ductus during prostaglandin inhibition and may be potential targets for future pharmacologic manipulations.

Oxygen-induced tension in the sheep ductus arteriosus: effects of gestation on potassium and calcium channel regulation

Compared with the full-term ductus arteriosus, the premature ductus is less likely to constrict when exposed to postnatal oxygen concentrations. We used isolated fetal sheep ductus arteriosus (pretreated with inhibitors of prostaglandin and nitric oxide production) to determine whether changes in K+ - and CaL-channel activity could account for the developmental differences in oxygen-induced tension. In the mature ductus, KV-channels appear to be the only K+-channels that oppose ductus tension. Oxygen concentrations between (2% and 15%) inhibit KV-channel activity, which increases the CaL-channel-mediated increase in tension. Low oxygen concentrations have a direct inhibitory effect on CaL-channel activity in the immature ductus; this is not the case in the mature ductus. In the immature ductus, three different K+-channel activities (KV, KCa, and KATP) oppose ductus tension and contribute to its decreased tone. Oxygen inhibits the activities of all three K+ -channels. The inhibitory effects of the three K+-channel activities decline with advancing gestation. The decline in K+ -channel activity is not due to decreased K+ -channel expression. Super-physiologic oxygen concentrations (>or=30% O2) constrict the ductus by using calcium-dependent pathways that are independent of K+- and CaL-channel activities. Super-physiologic oxygen concentrations eliminate the difference in tensions between the two age groups.

Ibuprofen-induced patent ductus arteriosus closure: physiologic, histologic, and biochemical effects on the premature lung

OBJECTIVE

The goal was to study the pulmonary, biochemical, and morphologic effects of a persistent patent ductus arteriosus in a preterm baboon model of bronchopulmonary dysplasia.

METHODS

Preterm baboons (treated prenatally with glucocorticoids) were delivered at 125 days of gestation (term: 185 days), given surfactant, and ventilated for 14 days. Twenty-four hours after birth, newborns were randomly assigned to receive either ibuprofen (to close the patent ductus arteriosus; n = 8) or no drug (control; n = 13).

RESULTS

After treatment was started, the ibuprofen group had significantly lower pulmonary/systemic flow ratio, higher systemic blood pressure, and lower left ventricular end diastolic diameter, compared with the control group. There were no differences in cardiac performance indices between the groups. Ventilation index and dynamic compliance were significantly improved with ibuprofen. The improved pulmonary mechanics in ibuprofen-treated newborns were not attributable to changes in levels of surfactant protein B, C, or D, saturated phosphatidylcholine, or surfactant inhibitory proteins. There were no differences in tracheal concentrations of cytokines commonly associated with the development of bronchopulmonary dysplasia. The groups had similar messenger RNA expression of genes that regulate inflammation and remodeling in the lung. Lungs from ibuprofen-treated newborns were significantly drier (lower wet/dry ratio) and expressed 2.5 times more epithelial sodium channel protein than did control lungs. By 14 days after delivery, control newborns had morphologic features of arrested alveolar development (decreased alveolar surface area and complexity), compared with age-matched fetuses. In contrast, there was no evidence of alveolar arrest in the ibuprofen-treated newborns.

CONCLUSIONS

Ibuprofen-induced patent ductus arteriosus closure improved pulmonary mechanics, decreased total lung water, increased epithelial sodium channel expression, and decreased the detrimental effects of preterm birth on alveolarization.

The role of monocyte-derived cells and inflammation in baboon ductus arteriosus remodeling

Inflammatory processes play a crucial role in the pathogenesis of atherosclerosis and other vascular disorders. We hypothesized that ischemia of the ductus arteriosus might initiate an active inflammatory response that could play a role in ductus remodeling and permanent closure. To test this hypothesis, we studied effects of postnatal ductus construction on inflammatory processes and remodeling in late-gestation fetal and newborn baboons, and preterm newborn baboons. After postnatal ductus constriction, the expression of several genes known to be essential for atherosclerotic remodeling [vascular cell adhesion molecule (VCAM)-1, E-selectin, IL-8, macrophage colony stimulating factor-1, CD154, interferon-gamma, IL-6, and tumor necrosis factor-alpha] was increased in the ductus wall. We were unable to detect intercellular adhesion molecule (ICAM)-1, ICAM-2, P-selectin, monocyte chemoattractant protein-1, or IL-1 by either real-time PCR or immunohistochemistry. VCAM-1, which is newly expressed by luminal cells of the closed ductus, is an important ligand for the mononuclear cell adhesion receptor VLA4. After postnatal constriction, VLA4+ monocytes/macrophages (CD68+ and CD14+) and, to a lesser extent, T-lymphocytes adhered to the ductus wall. Neutrophils and platelets were not observed. The extent of postnatal neointimal remodeling (both endothelial cell layering and subendothelial space thickening) was associated with the degree of mononuclear cell adhesion. Similarly, the extent of vasa vasorum ingrowth correlated with the invasion of CD68+ cells, from the adventitia into the muscle media. Based on these data, we conclude that the inflammatory response following postnatal ductus constriction may be as necessary for ductus remodeling as it is for atherosclerotic remodeling.

Early surfactant for neonates with mild to moderate respiratory distress syndrome: a multicenter, randomized trial

OBJECTIVE

We studied the efficacy and safety of electively providing surfactant to preterm infants with mild to moderate respiratory distress syndrome (RDS) not requiring mechanical ventilation.

STUDY DESIGN

A 5-center, randomized clinical trial was performed on 132 infants with RDS, birth weight >or=1250 grams, gestational age <or=36 weeks, postnatal age 4 to 24 hours, Fio(2) >or=40% for >or=1 hour, and no immediate need for intubation. Infants were randomly assigned to intubation, surfactant (Survanta, Ross Laboratories, Columbus, Ohio) administration, and expedited extubation (n=65) or expectant management (n=67) with subsequent intubation and surfactant treatment as clinically indicated. The primary outcome was duration of mechanical ventilation.

RESULTS

Infants in the surfactant group had a median duration of mechanical ventilation of 2.2 hours compared with 0.0 hours for control infants, since only 29 of 67 control infants required mechanical ventilation (P=.001). Surfactant-treated infants were less likely to require subsequent mechanical ventilation for worsening respiratory disease (26% vs 43%, relative risk=0.60; 95% CI, 0.37, 0.99). There were no differences in secondary outcomes (duration of nasal continuous positive airway pressure, oxygen therapy, hospital stay, or adverse outcomes).

CONCLUSIONS

Routine elective intubation for administration of surfactant to preterm infants >or=1250 grams with mild to moderate RDS is not recommended.

Effects of antenatal thyrotropin-releasing hormone, antenatal corticosteroids, and postnatal ventilation on surfactant mobilization in premature rabbits

OBJECTIVE

The effects of antenatal hormones on postnatal surfactant mobilization were evaluated in preterm rabbits.

STUDY DESIGN

Pregnant rabbits were treated with vehicle, betamethasone, or thyrotropin-releasing hormone for 2 days before cesarean section at 29 days' gestation (term 31 days). Newborns were mechanically ventilated or allowed to spontaneously breathe, and groups were compared by analysis of variance.

RESULTS

Neither antenatal corticosteroids nor thyrotropin-releasing hormone increased radiolabeled precursor incorporation, alveolar wash or total lung saturated phosphatidylcholine pools, lung clearance of radiolabeled rabbit surfactant, or estimated net secretion of saturated phosphatidylcholine. However, saturated phosphatidylcholine pools in alveolar wash increased 2.7-fold during the first 24 hours in spontaneously breathing rabbits versus 2.1-fold in mechanically ventilated thyrotropin-releasing hormone-treated and control rabbits (p less than 0.05). In addition, estimated net secretion of precursor-derived saturated phosphatidylcholine was 50% higher after 24 hours in spontaneously breathing rabbits.

CONCLUSION

Mechanical ventilation may have hindered the mobilization of surfactant saturated phosphatidylcholine pools to the alveolar space after birth in preterm rabbits, but maternal hormonal therapies did not appear to influence this adaptive process or change surfactant metabolism.

Effect of smoke inhalation on immediate changes in lung chemical mediators

We studied the effects of acute smoke exposure on lung and alveolar macrophage (AM) function in New Zealand white rabbits. Six rabbits were exposed to smoke (SE, N = 6) and a control group of rabbits (SS, N = 6) were exposed to sham smoke. The smoke exposure consisted of 60 tidal volume breaths of air and smoke which were aspirated by syringe from a sampling port of a smoke chamber. The smoke was generated by the combustion of 20 ml diesel fuel and 0.2 g polycarbonate plastic shavings. The smoke was administered in 8-9 min. The rabbits were then killed and the lungs were removed for lavage. Acute smoke exposure caused a significant (p = 0.037) increase in bronchoalveolar lavage fluid levels of leukotriene B4 in the SE rabbits; 643 (+/- 30, SEM) pg/ml compared to 539 (+/- 43, SEM) pg/ml for SS rabbits at 3-4 min post-exposure. Lung surfactant, measured as mumoles/kg phosphatidylcholine, was decreased (p = 0.039) in SE rabbits' bronchoalveolar lavage fluid, 1.07 (+/- 0.12, SEM) -vs- 1.45 (+/- 0.15, SEM) for SS. Furthermore, cultured SE alveolar macrophage superoxide secretion after stimulation with phorbol myristate acetate was significantly decreased versus SS alveolar macrophage superoxide values at 40 min in culture. We conclude that acute smoke exposure causes immediate increases in bronchoalveolar lavage fluid levels of LTB4, and decreases in alveolar macrophage superoxide production and lung surfactant. These changes in chemical mediators may contribute to the lung injury caused by the smoke insult.

Antenatal betamethasone dose effects in preterm rabbits studied at 27 days gestation

Pregnant rabbits received bethamethasone (0.05, 0.2, 0.4, or 0.5 mg.kg-1.day-1) or vehicle control for 2 days before delivery of fetuses at 27 days gestation to evaluate dose-related effects on surfactant pool sizes with and without ventilation, pressure-volume measurements, lung protein leaks, and precursor incorporation into lung saturated phosphatidylcholine (PC). Alveolar wash-saturated PC pool sizes in betamethasone-exposed fetuses were less than in controls (P less than 0.01). At higher doses, total lung saturated PC also decreased (P less than 0.01). Maximal lung volumes on pressure-volume loops were larger than controls only at the 0.4 mg.kg-1.day-1 dose (P less than 0.05). The larger maximal volumes, despite decreased saturated PC pools, indicated increased responsiveness of the steroid-treated lungs to the smaller saturated PC pool sizes. Vascular-to-alveolar iodinated albumin leak decreased with steroid treatment independently of dose (P less than 0.01). No consistent pattern of increased precursor incorporation into saturated PC by lung slices was seen. Our results indicate that, in preterm rabbits exposed to a range of maternal corticosteroid doses, the beneficial lung maturational effect of structural alterations with increased responses to endogenous saturated PC pools was maximal even at the lowest dose.

Surfactant metabolism in surfactant-treated preterm ventilated lambs

Preterm lambs were delivered at 132 days gestational age, treated with 100 mg/kg radiolabeled natural sheep surfactant or Surfactant TA, and ventilated for times up to 24 h. Compared with an untreated group that developed respiratory failure by 5 h, both surfactant-treated groups had stable respiratory function to 24 h. Although only approximately 13% of the labeled surfactant phosphatidylcholine was recovered by alveolar wash at 24 h, there was no significant loss of the labeled phosphatidylcholine from the lungs. Labeled palmitic acid intravascularly injected at 1 h of age comparably labeled lung phosphatidylcholine in the three groups of lambs at 5 h; however, only approximately 0.5% of the labeled phosphatidylcholine was secreted to the air spaces of surfactant-treated lambs at 24 h. Labeled lysophosphatidylcholine given with the natural sheep surfactant was taken up by the lungs, converted to phosphatidylcholine with 30-40% efficiency, and resecreted to the air spaces, demonstrating recycling of a phospholipid. The large surfactant aggregates recovered from alveolar washes by centrifugation were surface active and contained approximately 76% of the air-space phosphatidylcholine in both surfactant-treated groups. Although clinical status was comparable, alveolar washes and surfactant subfractions from Surfactant TA-treated lambs had better surface properties than did sheep surfactant-treated lambs. These studies identified no detrimental effects of surfactant treatments on endogenous surfactant metabolism and indicated that the surfactants used for treatments were recycled by the preterm ventilated lamb lung.

Effect of maternal hormone treatment on lung protein leakage and lung function of preterm newborn rabbits

We tested whether maternal administration of corticosteroids, thyrotropin-releasing hormone (TRH), triiodothyronine (T3) or their combinations, would improve lung function of ventilated preterm newborn rabbits. Maternal corticosteroids and T3 treatments did not improve lung compliance; TRH did. The major effect observed was a large improvement in lung compliance following maternal treatment with corticosteroids plus TRH in animals treated with surfactant. These agents made the lung "receptive" to the surfactant treatment. T3 did not improve lung function and no augmented response to surfactant was seen. None of the treated groups had surfactant pool sizes significantly different from controls. The mechanisms of action of corticosteroids and/or TRH seemed to be independent of changes in surfactant pool sizes.

Relationship between alveolar saturated phosphatidylcholine pool sizes and compliance of preterm rabbit lungs. The effect of maternal corticosteroid treatment

The relationship between alveolar saturated phosphatidylcholine pool size and compliance was evaluated in control and maternal corticosteroid-treated 27-, 28-, and 29-day gestational age preterm ventilated rabbits. Surfactant pool sizes below about 0.7 mumol saturated phosphatidylcholine/kg body weight did not alter compliance in either group. Compliance increased to values comparable to surfactant-treated or term newborn rabbits over a narrow range of saturated phosphatidylcholine pool sizes from about 0.7 to 1.2 mumol/kg for corticosteroid-treated rabbits and from about 0.9 to about 2.3 mumol/kg for control rabbits (p less than 0.01). When these results are compared with those from surfactant-treated rabbits, almost an order of magnitude less endogenous surfactant than exogenous surfactant is needed to alter compliance in preterm ventilated rabbits.

Vascular to alveolar leak of iron dextran (120 kD) in the immature ventilated rabbit lung

Rabbit fetuses were delivered by hysterotomy on day 27 or 28 of gestation. Immediately after birth, the animals were tracheotomized and received by intravenous injection 0.2 mu Ci radiolabeled albumin and 11 mg iron dextran in 0.2 ml saline. The newborn rabbits then were ventilated artificially with a tidal vol of 12 ml/kg for 5-20 min. One group of nonventilated animals served as controls. At the end of the experiment, one lung was lavaged via the airways and the other was fixed for histologic examination. The recovery of labeled albumin and iron dextran in the lavage fluid was quantified. Iron dextran complexes were easily identified in the lung sections by staining with Prussian blue. Iron dextran accumulated in the airspaces of animals delivered on day 27 (about 4% of the injected dose during 10-20 min of ventilation). The albumin leakage was slightly higher than that of the dextran, a result consistent with different mol wt of the markers. The vol density of leaking alveoli in histologic sections increased with time, from 0 at birth to a mean value of 0.36 after 20 min of ventilation. The leakage starts as a focal event, gradually involving more and more terminal airspaces. In the histologic sections, there was no indication of a significant leakage at the bronchiolar level, although the epithelium of terminal and preterminal airways was clearly injured in all ventilated animals.

Gestational effects of corticosteroids and surfactant in ventilated rabbits

Maternal corticosteroid treatments decreased lung protein leaks and increased the compliance responses to exogenous surfactant in 27-day preterm rabbits. We asked if maternal corticosteroid treatments at different gestational ages might alter these responses. Pregnant does were given 0.1 mg/kg betamethasone 48 and 24 h before study of the rabbits at 27, 28, and 29 days of gestational age and term newborns at 31 days of gestational age. Alternate rabbits at each gestation were treated with 50 mg/kg surfactant after delivery. Alveolar saturated phosphatidylcholine pool sizes increased with gestation similarly in control and corticosteroid-treated groups. Corticosteroids improved compliance relative to control values at 29 days of gestational age in animals not treated with surfactant and improved the compliance response to surfactant treatment at 27 and 28 days of gestational age. Corticosteroids decreased the leak of radiolabeled albumin to the lungs and alveolar washes at all preterm gestations with a maximum decrease to 16% of the control value at 29 days of gestation. Surfactant decreased this protein leak more effectively than did corticosteroids at the earlier gestations. There were potentially beneficial effects of corticosteroids either alone or together with surfactant at all preterm gestations studied. No protein leak or compliance effects of either treatment were noted in the term newborns.

Surfactant metabolism during hyperventilation of newborn lambs with atrial right to left shunts

We studied the effect on surfactant metabolism of 8 h of mechanical ventilation at tidal volumes of 13 +/- 0.3 ml/kg and very high tidal volumes of 28 +/- 1.5 ml/kg, with and without added CO2, in the presence of an atrial right to left shunt in 4- to 8-day-old lambs. Similarly aged, spontaneously breathing lambs were used as controls. Right to left atrial shunts were created by inflating a balloon in the right atrium after a Rashkind atrial septostomy, thus creating a stable, easily controlled atrial shunt. Radiolabeled surfactant phospholipid precursors were used to probe incorporation into and secretion of surfactant phosphatidylcholine, whereas intratracheally administered labeled natural surfactant was used to evaluate alveolar clearance. Protein leak from the vascular space to the lungs was measured using radioactive iodine-labeled albumins. At the end of the 8-h study period, tissue association of alveolar surfactant was significantly increased to 63% in the mechanically hyperventilated lambs as compared to 44% in those lambs mechanically ventilated but not hyperventilated (p less than 0.05) and to 39% in the spontaneously breathing control animals (p less than 0.05). No increased surfactant secretion or decreased compliance was detected with hyperventilation. However, the lambs had very large surfactant-saturated phosphatidylcholine pool sizes, and a large portion (50%) was already in the alveolar pool, even in the spontaneously breathing lambs. Precursor incorporation into saturated phosphatidylcholine was similar in all groups, and very low and comparable protein leaks were seen in the different groups of lambs.(ABSTRACT TRUNCATED AT 250 WORDS)

Clearance of surfactant phosphatidylcholine via the upper airways in rabbits

A possible route of clearance of surfactant phosphatidylcholine from the lungs is via the airways. To quantify surfactant loss via this pathway, latex bags were surgically placed into the abdomens of adult rabbits such that secretions cleared via the esophagus could be collected. The rabbits then were given treatment or trace doses of radiolabeled phosphatidylcholine-surfactant by tracheal injection and/or intravascular radiolabeled precursors of phosphatidylcholine. Labeled saturated phosphatidylcholine was measured in all fluids that were collected from the bags at 2-h intervals for 24 h and in alveolar washes and lung tissues at 24 h. No more than 7% of either treatment or trace doses of intratracheal surfactant-saturated phosphatidylcholine was lost via clearance up the airways over 24 h. Clearances of endogenously synthesized and secreted saturated phosphatidylcholine were estimated to be no more than 3% of the flux of labeled saturated phosphatidylcholine through the alveolar pool. These experiments demonstrate that surfactant phosphatidylcholine clearance via movement up the airways is not a major pathway leading to surfactant catabolism.

Pulmonary effects of acute prenatal asphyxia in ventilated premature lambs

The effect of profound repetitive prenatal asphyxial insults on the cardiopulmonary function of premature ventilated lambs was studied. Twenty-nine fetal lambs (approximately 138 days gestational age) were exteriorized. In 16 of these lambs, the umbilical cord was occluded for 4 min then released for 10 min. This asphyxial episode was repeated until the arterial pH was approximately 7.00, and the mean arterial blood pressure was less than 40 mmHg and falling. The 13 control lambs were simply exteriorized with the umbilical circulation intact. The lambs were then ventilated for 3-4 h. There were no differences between the control vs. asphyxiated lambs in pulmonary compliances (0.57 and 0.58 ml.cmH2O-1.kg-1) wet-to-dry weight ratios (8.18 and 7.55), cardiac outputs (177.8 and 141.8 ml.kg-1.min-1), surfactant-saturated phosphatidylcholine pool sizes, or atrial and/or ductal shunts. Asphyxia did not interfere with the redirection of blood away from atelectatic lung segments created by bronchial obstruction with balloon catheters. Also, although the bidirectional flux of protein into and out of the airways of these preterm lambs was large relative to term lambs, there was no effect of asphyxia on this protein leak. In this animal model, prenatal asphyxia did not impact negatively on the severity of the respiratory failure.

Corticosteroid potentiation of surfactant dose response in preterm rabbits

Fetal rabbits were treated with corticosteroids by maternal administration for 48 h before delivery at 27 days gestational age. Both corticosteroid-treated and control animals then received exogenous natural rabbit surfactant at birth at doses of 0-75 mg lipid/kg. After 10 min of ventilation at tidal volumes of 12-15 ml/kg, static pressure-volume measurements were made. At all surfactant doses there was a significantly higher maximal lung volume, higher dynamic compliance, and lower pressure requirement in the corticosteroid-treated than in the control rabbits (P less than 0.01). Control animals showed incremental improvements in dynamic compliances and maximal lung volumes up to a dose of 50 mg/kg, whereas corticosteroid treated animals improved to a maximum at the low dose of 15 mg/kg (P less than 0.01). However, surface tension as assessed by lung stability index improved with increasing surfactant dose but was not significantly different between corticosteroid-treated and control animals at a given dose. The results imply that maternal corticosteroid treatment potentiates surfactant replacement by a change in lung structure that is independent of surface tension effects.

Clearance of surfactant phosphatidylcholine from adult rabbit lungs

Rabbits were given various doses of rabbit surfactant and treatment doses of approximately 100 mg/kg body wt of calf surfactant and Surfactant TA by tracheal injection. The linear loss of radiolabeled phosphatidylcholine from the total lung (alveolar wash and lung tissue) was 3.1, 1.5, and 1.8%/h for rabbit surfactant, calf surfactant, and Surfactant TA, respectively. After 24 h only 6% rabbit, 19% calf, and 9.7% Surfactant TA phosphatidylcholine were recovered by alveolar wash, and alveolar macrophage fractions contained less than 1% of the injected labeled phosphatidylcholine. The loss of rabbit surfactant phosphatidylcholine 24 h after tracheal injection did not change for doses in the range of 0.5-70 mumol phosphatidylcholine per kilogram, indicating nonsaturable clearance pathways. Very little of the labeled rabbit surfactant phosphatidylcholine lost from the lungs could be recovered in other organs, and 90% of the recovered labeled phosphatidylcholine in the liver was unsaturated, implying de novo synthesis using precursors from degraded phosphatidylcholine. The surfactant did not change endogenous lung phosphatidylcholine synthesis or its secretion to the alveolus. There were no adverse effects of the surfactant treatments noted in healthy rabbits.

Clearance of treatment doses of surfactant. Effect of lipid extraction and aggregate sizes

Three-day-old rabbits were given intratracheal injections with a variety of surfactants at doses of about 100 mg lipid/kg, doses commonly used in clinical trials of surfactant for respiratory distress syndrome. Calf and sheep natural surfactants isolated by centrifugation of alveolar washes were compared with Surfactant-TA and two aggregate sizes of lipid solvent extracted sheep surfactant by measuring the percent recoveries of labeled phosphatidylcholine in alveolar washes and lung tissue at times to 48 h after surfactant injection. Surfactant-TA and the lipid extracted surfactants did not contain the 28 to 35-kdalton surfactant protein. All surfactants had similar linear clearance rates from the total lung (alveolar wash plus lung tissue), independent of species source, extraction with lipid solvents, or aggregate sizes of the phospholipids in suspension. There were no metabolic consequences to lipid extraction, the loss of 28-35 kdaltons of protein, or changes in aggregate sizes of surfactant lipids when injected in treatment doses into the airways of 3-day-old rabbits.

Corticosteroids and surfactant change lung function and protein leaks in the lungs of ventilated premature rabbits

Fetal rabbits were treated with corticosteroids by maternal administration for 48 h before delivery at 27 d gestational age. The treated and control rabbits were placed on ventilator-plethysmographs so that ventilation could be adjusted by regulation of tidal volumes to 10-13 ml/kg body wt. [125I]albumin was mixed with fetal lung fluid at birth, alternate rabbits from each litter were treated with Surfactant-TA, and [131I]albumin was injected intravascularly. The movement of the labeled albumins into and out of the alveolar wash and lung tissue was measured after 30 min of ventilation. Corticosteroid treatment (total dose, 0.2 mg/kg betamethasone) significantly decreased the protein leak across the endothelium (P less than 0.001) but increased the protein leak across the epithelium (P less than 0.001). Surfactant treatment decreased both the endothelial and epithelial leaks, and the combination of surfactant and corticosteroid treatments decreased endothelial leaks to 29% of control values and increased compliance more than either treatment alone. The 48-h corticosteroid treatment did not increase alveolar surfactant pool sizes. Corticosteroids significantly changed lung protein leaks independently of surfactant, and improved the response of the preterm lung to surfactant treatments.

Effects of distention of the preterm fetal lamb lung on lung function with ventilation

Constant distending pressure when applied to the immature fetal lamb lung was reported to improve compliance and prevent the subsequent development of the respiratory distress syndrome after delivery. These experiments were designed to repeat those observations and identify the mechanism(s) responsible for the effects of constant distending pressure on subsequent lung function. The compliance of the lungs of exteriorized fetal lambs at 130 to 133 days gestational age increased 2.5-fold after 1 h of 15 cm of distending pressure. However, subsequent ventilation of the lambs exposed to distending pressure and control lambs resulted in comparable sequential compliance and blood gas and pH measurements. Severity of lung disease as reflected by the peak inspiratory pressure needed to normalize Pco2 values decreased as surfactant-saturated phosphatidylcholine pool sizes increased (r values greater than 0.90) and minimum surface tensions of alveolar washes decreased, but the distention procedure did not change these relationships. Distention of the fetal lung did result in an apparent increase in pulmonary blood flow in the fetus and an increased leak of labeled albumin from the vascular space to the lung interstitium and airways during the 1-h period of ventilation after delivery. However, the leak of protein into the lungs exposed to the distending pressures was not increased during the period of exteriorization and distention, suggesting that distention sensitized the preterm lung to leak protein with subsequent mechanical ventilation. The leak of labeled albumin out of the airways was not changed by distention, and total lung water was not changed.(ABSTRACT TRUNCATED AT 250 WORDS)

Clearance of natural surfactant phosphatidylcholine from 3-day-old rabbit lungs: effects of dose and species

Surfactants were labeled in vivo with [3H] choline and the large aggregates of the surfactant were recovered by alveolar wash and centrifugation. The labeled natural surfactants from rabbit, sheep, cat, and pig were injected into the airways of 3-day-old rabbits, and the percent recoveries of the labeled surfactant-associated phosphatidylcholine were measured in alveolar washes, lung tissue after alveolar wash, and in the lungs (alveolar wash plus lung tissue). The rabbit surfactant-associated phosphatidylcholine was cleared from the lungs at a constant 15.6 +/- 1.8% per 24 h (mean +/- SE) of the injected doses of surfactant that contained from 0.41 to 10.2 mumol phosphatidylcholine. At all times following injection, approximately 50% of the labeled phosphatidylcholine remaining within the lungs was recovered in the alveolar wash and 50% with the lung tissue. The percent clearances for sheep, cat, and pig surfactant phosphatidylcholine in rabbits were 12.5, 16.6, and 16.3% per 24 h, respectively, values not different from that measured for rabbit surfactant. The results documented a slow clearance of exogenously administered surfactant phosphatidylcholine as a fixed percent of the injected dose that was unchanged by species source of the surfactant.