Adrenal Epinephrine and the Regulation of Pulmonary Surfactant Release in Neonatal Rabbits

Adrenergic mechanisms influence surfactant metabolism; however the nature and importance of catechol-mediated regulation is unclear. We designed experiments to assess the role of endogenous adrenal epinephrine (E) synthesis in neonatal alveolar surfactant release. We administered the experimental adrenal epinephrine synthesis inhibitor SKF 29661 to pregnant rabbits to reduce fetal adrenal E content. Surfactant release was measured as total alveolar phosphatidylcholine (PC) content recovered by a thorough alveolar wash performed on animals sacrificed at birth and at 1 h of age. At a maternal dose of 100 mg/kg/day, SKF 29661 caused a 57% reduction in fetal adrenal E content (p less than .01), which was associated with a 40% reduction in alveolar PC at birth and a 23% reduction in alveolar PC at 1 h of age (p less than .01). Moreover, adrenal E correlated positively with total alveolar PC content (p less than .001). These results suggest that in the neonatal period, the ability of the fetal adrenal gland to synthesize E for secretion plays a role in the regulation of pulmonary surfactant release.

Immunocytochemical characterization of murine Hex, a homeobox-containing protein

A polyclonal antibody against a glutathione S:-transferase fusion protein containing the 76 COOH-terminal amino acids of Hex, a divergent homeobox gene, was raised in rabbits. Western blot and immunofluorescence reveal that Hex is a 35-37-kD soluble protein present both in the nucleus and cytoplasm of transfected and nontransfected cultured cells as well as in whole mouse embryo. Confocal microscopy of whole mount immunostained mouse embryos at E7. 5 and E8.5 demonstrates that Hex is differentially localized in the cytoplasm and nucleus of definitive endoderm, developing blood islands, and hepatic diverticulum. In particular, in the region of the foregut that gives rise to the liver, Hex expression is nuclear in the endodermal cells of the hepatic diverticulum, whereas expression is primarily cytoplasmic in cells lateral to the liver-forming region. This suggests that nuclear localization of Hex is involved in early hepatic specification and that compartmentalization of Hex protein plays an important role in its function during mouse development.

Hex expression suggests a role in the development and function of organs derived from foregut endoderm

Hex is a divergent homeobox gene expressed as early as E4.5 in the mouse and in a pattern that suggests a role in anterior-posterior patterning. Later in embryogenesis, Hex is expressed in the developing thyroid, lung, and liver. We now show Hex expression during thymus, gallbladder, and pancreas development and in the adult thyroid, lung, and liver. At E10.0, Hex is expressed in the 3rd pharyngeal pouch, from which the thymus originates, the endodermal cells of liver that are invading the septum transversum, the thyroid, the dorsal pancreatic bud, and gallbladder primoridum. At E13.5, expression is maintained at high levels in the thyroid, liver, epithelial cells lining the pancreatic and extrahepatic biliary ducts and is present in both the epithelial and mesenchymal cells of the lung. Expression in the thymus at this age is less than in the other organs. In the E16.5 embryo, expression persists in the thyroid, pancreatic, and bile duct epithelium, lung, and liver, with thymic expression dropping to barely detectable levels. By E18.5, expression in the thyroid and bile ducts remains high, whereas lung expression is markedly decreased. At this age, expression in the pancreas and thymus is no longer present. Finally, we show the cell types in the adult thyroid, lung, and liver that express Hex in the mature animal. Our results provide more detail on the potential role of Hex in the development of several organs derived from foregut endoderm and in the maintenance of function of several of these organs in the mature animal.

Divergent homeobox gene hex regulates promoter of the Na(+)-dependent bile acid cotransporter

The divergent homeobox gene Hex is expressed in both developing and mature liver. A putative Hex binding site was identified in the promoter region of the liver-specific Na(+)-bile acid cotransporter gene (ntcp), and we hypothesized that Hex regulates the ntcp promoter through this site. Successive 5'-deletions of the ntcp promoter in a luciferase reporter construct transfected into Hep G2 cells confirmed a Hex response element (HRE) within the ntcp promoter (nt -733/-714). Moreover, p-CMHex transactivated a heterologous promoter construct containing HRE multimers (p4xHRELUC), whereas a 5-bp mutation of the core HRE eliminated transactivation. A dominant negative form of Hex (p-Hex-DN) suppressed basal luciferase activity of p-4xHRELUC and inhibited activation of this construct by p-CMHex. Interestingly, p-CMHex transactivated the HRE in Hep G2 cells but not in fibroblast-derived COS cells, suggesting the possibility that Hex protein requires an additional liver cell-specific factor(s) for full activity. Electrophoretic mobility shift assays confirmed that liver and Hep G2 cells contain a specific nuclear protein that binds the native HRE. We have demonstrated that the liver-specific ntcp gene promoter is the first known target of Hex and is a useful tool for evaluating function of the Hex protein.

HNF3beta and GATA-4 transactivate the liver-enriched homeobox gene, Hex

The orphan homeobox gene, Hex, has a limited domain of expression which includes the developing and adult mouse liver. Hex is expressed in the developing liver coincident with the forkhead/winged helix transcription factor, Hepatocyte Nuclear Factor 3beta (HNF3beta). Although preliminary characterization of the mouse Hex promoter has recently been reported, the identity of the molecular regulators that drive liver expression is not known. We hypothesized that putative HNF3beta and GATA-4 elements within the Hex promoter would confer liver-enriched expression. A series of Hex promoter-driven luciferase reporter constructs were transfected in liver-derived HepG2 and fibroblast-like Cos cells+/-HNF3beta or GATA expression plasmids. The Hex promoter region from nt -235/+22 conferred basal activity in both HepG2 and Cos cells, with the region from -103/+22 conferring liver-enriched activity. HNF3beta and GATA-4 transactivated the promoter via response elements located within nt -103/+22, whereas Sp1 activated the -235/+22 construct. Mutation of the HNF3 element significantly reduced promoter activity in HepG2 cells, whereas this element in isolation conferred HNF3beta responsiveness to a heterologous promoter. Electrophoretic mobility shift assays were performed to confirm transcription factor:DNA binding. We conclude that HNF3beta and GATA-4 contribute to liver-enriched expression of Hex.

Fetal lung mRNA levels of Hox genes are differentially altered by maternal diabetes and butyrate in rats

Diabetes is known to be associated with delayed lung development in humans and in experimental animals. This includes delayed expression of surfactant apoproteins. An important component of the metabolic abnormalities in diabetes is elevated levels of analogs of butyric acid, and the effects of diabetes on surfactant apoproteins can be reproduced by exposure of fetal rat lung explants to butyrate. Dexamethasone has the opposite effects on lung development. In humans, antenatal exposure to dexamethasone results in a lower incidence of RDS, whereas in experimental animals, dexamethasone increases the expression of surfactant apoproteins. A subset of Hox genes are expressed in developing lung, and their level of expression decreases with advancing gestation. We hypothesized that: 1) lungs of fetuses of rats with streptozotocin-induced diabetes would have altered levels of expression of Hox genes, 2) the effect would be mediated in part through elevated levels of butyrate, and 3) dexamethasone would reverse the effect. We tested our hypotheses in vivo using fetuses from streptozotocin-treated rats and in vitro by treating lung explants from normal rats with sodium butyrate. Streptozotocin treatment increased expression of Hoxb-5 at 18 d of gestation, but did not affect Hoxa-5 expression. This was associated with a 20-fold increase in alpha-aminobutyrate levels. Dexamethasone tended to reverse this effect. In contrast, butyrate treatment of explants decreased the expression of Hoxa-5 and Hoxb-5. We conclude that diabetes alters expression of Hox genes, but that the effect of butyrate on lung development, and in particular on surfactant apoprotein expression, is independent of its effects on Hox genes.

Umbilical cord blood banking in The Netherlands

Cord blood banks are being developed in the United States and Europe. In The Netherlands, the EuroCord Nederland Foundation (ECN) has been established to coordinate the development of a national cord blood bank for unrelated transplants. The aim of ECN is to collect at least 5000 transplants for patients who lack an HLA-identical related or unrelated bone marrow donor. Four blood banks in Leiden, Groningen, Nijmegen and Amsterdam, Europdonor Foundation and the Department of Hematology of the Leiden University Medical Center participate in this organization. From March 1995 to November 1997, 720 units have been collected, processed, HLA typed, tested for transmissible diseases and cryopreserved in the Dutch cord blood bank.

Repeated doses of the perfluorocarbon FC-100 improve lung function of preterm lambs

Intratracheal administration of a single dose of the perfluorocarbon FC-100 improves lung function in surfactant-deficient animals. In this study we compared the response to repeated doses of FC-100 (3 mL/kg 3% solution, n = 5) with that observed after administration of Exosurf (5 mL/kg, n = 5) to mechanically ventilated preterm lambs of 125 d of gestation. The initial dose of FC-100 rapidly increased arterial PO2, decreased arterial PCO2, and improved arterial pH. Also dynamic lung compliance markedly improved with this agent. Administration of an additional dose of FC-100 resulted in relatively similar changes, albeit of lesser magnitude than those observed with the initial dose. In contrast, Exosurf did not improve these variables even after three doses. All lambs treated with FC-100 survived the 6-h study period, whereas one of the five Exosurf-treated lambs survived (p < 0.05). Mean arterial blood pressure and heart rate decreased in those lambs that received FC-100, but not in surviving lambs that received Exosurf. Our data demonstrate that repeated intratracheal administration of the perfluorocarbon FC-100 improves lung function and survival of surfactant-deficient lambs better than the synthetic surfactant Exosurf. We speculate that tensio-active agents with properties different from surfactant, such as FC-100, might improve lung function in preterm neonates with diseases due to surfactant deficiency.

Umbilical venous catheterization and the risk of portal vein thrombosis

Portal vein thrombosis has been associated with umbilical venous catheterization. We studied the incidence of portal vein thrombosis associated with umbilical venous catheterization with the catheter tip not in the portal venous system. Appropriate placement of an umbilical venous catheter in sick neonates is associated with a low risk of portal vein thrombosis (actual incidence, 1.3%).

Retinoic acid increases surfactant protein mRNA in fetal rat lung in culture

Retinoic acid has both early or immediate (within hours) and late (after days) effects on gene expression. We studied the early effects of retinoic acid on the surfactant protein (SP) genes. Exposure of fetal rat lung explants to all trans-retinoic acid for 4 h resulted in a significant dose-dependent increase in SP-A, -B, and -C mRNA with markedly different dose-response characteristics. The maximal (2.5x) increase in SP-A mRNA was observed with 10(-10) M retinoic acid, whereas treatment with 10(-5) M resulted in a tendency to decreased levels. In contrast, maximal stimulation of SP-C (6x) was noted at 10(-5) M retinoic acid and that of SP-B (2x) at 10(-7) to 10(-5) M retinoic acid. Similar differences in the dose-response characteristics of SP-A and SP-C were observed with 9-cis-retinoic acid. A retinoic acid response element consensus sequence was identified in the rat SP-A gene; we hypothesize that retinoic acid-receptor complexes act directly on the SP-A gene via this response element.

Expression of Hoxb genes in the developing mouse foregut and lung

Lung development in the mouse begins at embryonic day 9.5 (E9.5) when lung buds form in the foregut. Subsequently, there is extensive branching and cellular differentiation that depends upon specific epithelial-mesenchymal interactions. Homeobox genes are expressed in specific temporo-spatial patterns in the developing embryo and are known to be involved in axial patterning and specification of regional identity. Using whole mount in situ hybridization and immunohistochemistry, we studied the expression of Hoxb-1, b-2, b-3, b-4 and b-5 in the E9.5-E14.5 foreguts and lungs. Our results show that in E9.5 branchial arches and foregut, Hoxb genes are expressed in overlapping spatial domains and the anterior boundaries of these domains correspond to the position of a particular gene in the cluster-genes on the 3' end of the cluster are expressed more anteriorly in the branchial arches and foregut and those on the 5' end are expressed more posteriorly. Three of the genes, Hoxb-3, b-4, and b-5, are highly expressed in the foregut where the lung buds form. In contrast, in E10.5-E14.5 lung, there are two patterns of Hoxb gene expression. Hoxb-3 and b-4 are expressed in the mesenchyme of the trachea, mainstem bronchi, and distal lung, whereas Hoxb-2 and b-5 mRNA are present only in the mesenchyme of the distal lung buds. These results suggest that specific combinations of Hoxb gene expression are important in lung development and that Hoxb genes may be involved in specifying the differences between proximal (trachea and main bronchi) and distal (lung bud) mesenchyme.

Identification of Hox genes in newborn lung and effects of gestational age and retinoic acid on their expression

Hox genes are sequence-specific DNA transcription factors, which are important in embryonic development and are expressed in a number of fetal tissues, including the lung. Additionally, retinoic acid (RA) has been shown to modulate Hox gene expression in a number of cell types. The specific aims of this study were to 1) identify those Hox genes expressed in newborn mouse lung using reverse transcription-polymerase chain reaction (RT-PCR), 2) study the ontogeny of Hox gene expression in fetal mouse and rat lung by Northern analysis using cDNAs for mouse Hox genes, and 3) study the effects of RA on whole lung Hox mRNA levels in cultured fetal rat lung explants. Our data show that 16 different homeobox genes are expressed in newborn mouse lung. This includes seven Hox genes not previously identified in lung, as well as the divergent homeobox gene Hex. Steady-state mRNA levels of Hox A5 (Hox 1.3), B5 (Hox 2.1), B6 (Hox 2.2), and B8 (Hox 2.4) decrease with advancing gestational age in mouse lungs (E14 to adult). Similarly, Hox A5, B5, and B6 follow the same decreasing pattern of expression with advancing gestational age in rat lungs (E15 to adult). RA treatment of E17 rat lung explants in culture resulted in a significant dose- and time-dependent increase in Hox A5, B5, and B6 mRNA levels. The highest mRNA levels were seen in explants treated with 1 x 10(-5) M RA for 4-16 h. We conclude that there are many homeobox genes expressed in developing rodent lung and that their mRNA levels are affected by both gestational age and RA.

Heat shock does not induce tolerance to hyperoxia

Thermal stress is associated with the induction of a specific set of proteins called heat shock proteins and with the induction of thermal tolerance. Heat stress has been shown to be capable of inducing at least partial tolerance to other stresses, including some oxidant stresses. Furthermore, these oxidant stresses are reported to be inducers of heat shock proteins. We hypothesized that hyperoxic stress would induce heat shock proteins and that factors induced by thermal stress, including heat shock proteins, would offer at least partial protection from hyperoxic exposure. We were particularly interested in a level of protection that would be relevant to clinical situations. Lung fibroblasts and live animals were exposed to thermal stress and/or hyperoxic stress and examined for induction of HSP70 (the most conserved of the heat shock proteins) and for induced tolerance as determined by the ability of cells to metabolize 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and by comparison of lung wet to dry weight ratios in live animals. Each stress induced tolerance to itself, but there was no evidence of heat stress inducing tolerance to hyperoxic stress. Furthermore, there was only minimal induction of HSP70 mRNA by hyperoxic exposure. We conclude that some overlap of mechanisms of induced tolerance by hyperoxic and thermal stress exists, but that differences far outweigh similarities.

Effect of artificial surfactant on pulmonary function in preterm and full-term lambs

We hypothesized that agents very different from surfactant may still support lung function. To test this hypothesis, we instilled FC-100, a fluorocarbon, and Tween 20, a detergent, which have higher minimum surface tensions and less hysteresis than surfactant, into 15 full-term and 14 preterm lambs. FC-100 and Tween 20 were as efficient as natural surfactant in improving gas exchange and compliance in preterm lambs with respiratory failure. Dynamic compliance correlated with the equilibrium surface tension of the alveolar wash in both full-term (P less than 0.02) and preterm (P less than 0.008) lambs. Functional residual capacity in full-term and preterm lambs was lower after treatment with the two test agents than with surfactant, findings consistent with qualitative histology. Oxygenation in full-term lambs correlated with mean lung volumes (P less than 0.003), suggesting that the hysteresis and/or low minimum surface tension of surfactant may improve mean lung volume, and hence oxygenation, by maintaining functional residual capacity. The effects of the test agents suggest that agents with biophysical properties different from surfactant may still aid lung expansion.

Corticosteroids and intratracheal surfactant both alter the distribution between the airways and lung tissue of intratracheally administered radiolabeled phosphatidylcholine in the preterm rabbit

Developmental differences exist regarding quantitative aspects of surfactant phosphatidylcholine clearance from the alveolar space and its subsequent reutilization. We wished to further extend observations of this nature to prematurely delivered rabbits undergoing mechanical ventilation. In addition we tested the hypothesis that prenatal corticosteroid exposure and/or intratracheal surfactant at birth would produce alterations in the lung's clearance of phosphatidylcholine from the airways. Pregnant does were injected with either Ringer's lactate or betamethasone on days 25 and 26 of gestation. Fetuses were delivered at 27 days and given by intratracheal injection either surfactant or one-half strength Ringer's lactate, both of which were trace labeled with [3H]phosphatidylcholine. Fetuses then underwent mechanical ventilation for periods of time ranging from 10 to 120 min. Following ventilation, alveolar lavage and lung tissue were examined to determine the distribution of [3H]phosphatidylcholine between these two compartments. Antenatal corticosteroid exposure was associated with decreased recovery of the radiolabel from the alveolar space and increased recovery of the label from the lung tissue in comparison to control fetuses. Intratracheal surfactant was associated with persistence of the radiolabel within the alveolar space. Therapy with both of these modalities produced a radiolabel distribution that resembled that seen in fetuses receiving intratracheal surfactant alone.

Pulmonary function tests and fluid balance in neonates with chronic lung disease during dexamethasone treatment

Pulmonary function tests and fluid balance were measured serially during treatment with dexamethasone in seven ventilator-dependent, 14- to 27-day-old infants. The infants showed no improvement in respiratory status during the prior 5 days. Birth weights ranged from 540 to 900 g, with gestational ages of 24 to 26 weeks. The decision to treat the infants with dexamethasone was made by the clinical team. Pulmonary function tests were performed prior to the first dose and then every 12 hours until extubation. Significant differences were first seen after only 12 hours of treatment. Five infants were extubated within 48 hours of starting therapy. Before extubation at 48 hours, changes were found in dynamic compliance (74% increase), total pulmonary resistance by midvolume and regression methods (38% and 35% decreases, respectively), and expiratory time constant (49% increase), with P less than .01 in all cases. An increase in urine output was also observed in the first 12 hours. Improvements in chronic lung disease produced by dexamethasone are rapid and may result from dexamethasone-induced pulmonary fluid shifts.

Antenatal steroids, postnatal surfactant, and pulmonary function in premature rabbits

Antenatal corticosteroids reduce the incidence of the respiratory distress syndrome and improve pulmonary mechanics at least in part by mechanisms other than surfactant stimulation. We measured several aspects of pulmonary function in rabbits to better understand the mechanisms involved. Seven does were given intramuscular betamethasone and six were given vehicle on days 25 and 26 of gestation. Delivery was on day 27 (term = 31). Half of the fetuses from each litter were given rabbit surfactant before the first breath. All fetuses were then ventilated at a consistent tidal volume for 1 h. Pulmonary function tests included static and dynamic compliance, expiratory time constant, stress relaxation, total lung resistance, and total lung conductance. Steroid or surfactant treatment increased dynamic compliance, and the effects of both together were greater than either alone. Static compliance was affected more by surfactant than steroids, whereas lung resistance and conductance were affected more by steroids. The differences in action of the two therapies help account for the increased dynamic compliance seen with combination therapy.

Surface tension and pulmonary compliance in premature rabbits

In vitro surface properties of pulmonary surfactant thought to be essential to its ability to increase pulmonary compliance include minimum surface tension less than 10 dyn/cm and large surface tension variability and hysteresis. We tested four surface-active agents (Tween 20, a detergent; and FC-100, FC-430, and FC-431, industrial fluorocarbons), all lacking these properties, for their ability to increase pulmonary compliance in surfactant-deficient premature rabbits. Fetal rabbits were delivered by cesarean section at 27 days (full term = 31 days) and injected via tracheostomy with 50% lactated Ringer solution, adult rabbit surfactant, or one of the four experimental agents. Dynamic compliance was measured using 1 h of mechanical ventilation followed by alveolar lavage. Each experimental agent produced a dynamic compliance significantly higher than 50% lactated Ringer solution and statistically equal to or greater than natural surfactant. Equilibrium surface tension of the agents and minimum and equilibrium surface tension of the alveolar washes each correlated with compliance (P less than 0.05). This suggests that some surface properties of pulmonary surfactant believed to be essential are not, although surface tension does seem to play a role in pulmonary compliance.

Reutilization of surfactant phosphatidylglycerol and lysophosphatidylcholine by adult rabbits

Adult rabbits reutilize the phosphatidylcholine (PC) of surfactant much less efficiently than developing rabbits (22% vs. 95%). Comparisons of reutilization efficiency of other components of surfactant in adult rabbits have not been determined. We injected adult rabbits intratracheally with [3H]dipalmitoylphosphatidylcholine (DPPG) mixed with [14C]lysophosphatidylcholine (lysoPC) and natural surfactant or [14C]DPPC mixed with [3H]dipalmitoylphosphatidylglycerol (DPPG) and natural surfactant. Recovery in the alveolar wash and lamellar bodies of labelled DPPC, lysoPC and DPPG was determined at different times after injection. By plotting the ratio of [3H]DPPG to [14C]DPPC in the alveolar wash versus time after injection we found that phosphatidylglycerol was reutilized with an efficiency of only 0-7% which was much less than the reutilization of PC in these animals. At early times after injection, adult rabbits injected with [14C]lysoPC had a ratio of [14C]PC in their alveolar wash to lamellar bodies that was larger than 1.0. By comparison, 3-day old rabbits injected intratracheally with [14C]lysoPC had a ratio of [14C]PC in alveolar wash to lamellar bodies less than 1.0 at the earliest times measurable. Thus adult rabbits demonstrate a pathway for accumulation of PC in their alveolar space prior to its appearance in lamellar bodies. This was not detected in developing rabbits. As in developing rabbits, adult rabbits reutilize the phosphatidylglycerol of surfactant less efficiently than the PC of surfactant.

Betamethasone increases pulmonary compliance in part by surfactant-independent mechanisms in preterm rabbits

Antenatal exposure to corticosteroids is known to increase the pulmonary compliance of preterm animals. We wished to determine whether this was due solely to alteration in lung surfactant content. Rabbit does were injected with either vehicle alone or betamethasone on days 25 and 26 of gestation. Fetuses were delivered at 27 days and given either 50% lactated Ringer's or intratracheal natural surfactant prior to their first breath. Fetuses were mechanically ventilated at a tidal volume of 12 ml/kg for 60 min with periodic compliance measurements. Following ventilation an alveolar lavage was collected for phosphatidylcholine determination. Some fetuses did not undergo ventilation but had saline compliance studies instead. Fetuses given intratracheal surfactant had a higher dynamic compliance than fetuses exposed to antenatal corticosteroids (0.55 +/- 0.01 versus 0.48 +/- 0.02 ml/cm H2O/kg, respectively). Fetuses exposed to antenatal corticosteroids and given intratracheal surfactant had a dynamic compliance (0.66 +/- 0.02) that was greater than those exposed to either single therapy. This was found despite an alveolar surfactant content equal to that in fetuses receiving intratracheal surfactant alone. Saline compliance at birth was significantly greater for fetuses exposed antenatally to steroids. These data imply the existence of a nonsurfactant mechanism by which antenatal corticosteroids increase fetal pulmonary compliance.

Thresholds for physiological effects of plasma catecholamines in fetal sheep

To clarify the physiological role for the marked increases in circulating norepinephrine (NE) and epinephrine (E) that occur at birth, we performed graded infusions of NE and E in preterm (131 days) and full-term (142 days) fetal sheep. A variety of hemodynamic, metabolic, and endocrine responses to stepwise increases in plasma catecholamine levels were analyzed by computer-based graphical analysis of the dose-response curves. We determined the "threshold" value or minimum plasma concentration necessary to produce discernible effects. We observed increases in systolic blood pressure, diastolic blood pressure, and dP/dt beginning at plasma concentrations of 500-800 pg/ml of NE or E. In contrast, increases in plasma free fatty acid and glucose levels were observed at E concentrations as low as 50-100 pg/ml. Full-term animals had generally lower thresholds and higher peak responses than preterm animals. Because these thresholds for infused NE and E are well within the range of plasma catecholamine values seen at birth, these results underscore the importance of circulating catecholamines in the events of neonatal adaptation.

Normalization of arterial blood gases after treatment of surfactant-deficient lambs with Tween 20

Pulmonary surfactant (PS) is capable of lowering surface tension at an air-water interface to less than 10 dynes/cm. This property of PS is believed to be important for normal lung function. We tested the detergent Tween 20, which cannot lower surface tension at an air-water interface to less than 25 dynes/cm, for its ability to promote reasonable gas exchange in surfactant-deficient lambs delivered prematurely at 120 to 124 days gestation. Lambs were treated at birth with 75 mM NaCl (control), 5% Tween 20 in 75 mM NaCl, or PS in 75 mM NaCl and placed on infant ventilators. Control lambs had poor oxygenation and were in respiratory failure at 30 min of age (arterial PCO2 of 75.8 +/- 11 mmHg (mean +/- SE)) despite peak inspiratory pressures (PIP) of 32 +/- 0.7 cm H2O. Pulmonary surfactant treatment allowed adequate ventilation with a PIP of 23.8 +/- 1.2 cm H2O. Tween treatment also allowed adequate ventilation with a PIP of only 27 +/- 1.3 cm H2O. Minimal surface tensions in alveolar washes of control and Tween-treated lambs were 28.7 +/- 1.3 and 26.7 +/- 2.3 dynes/cm, respectively, compared with less than 10 dynes/cm for PS-treated lambs. Thus, Tween 20 did not alter the alveolar wash minimal surface tension when compared with that of the control animals but it did significantly improve gas exchange and lung compliance.

Reutilization of surfactant phosphatidylcholine in adult rabbits

32P-saturated phosphatidylcholine was added to [3H]choline-labeled natural surfactant and the mixture was injected intratracheally into 87 adult rabbits. The rabbits were also given [14C]palmitate intravenously at the same time. Rabbits were killed in groups from 10 min to 72 h after injection. In each rabbit we measured the total recovered [3H]phosphatidylcholine (PC) in the alveolar wash, the ratio of [3H]PC to [32P]PC in the alveolar wash, and the specific activity of [14C]PC in the alveolar wash and lamellar bodies. Values were averaged for all rabbits killed at the same times and smooth curves were fit to the data by computer. From the intravenous [14C]palmitate data we calculated a turnover time for alveolar PC of 6.0 h. From the intratracheal labeling data, we calculated a turnover time for alveolar PC of 5.7 h and determined that alveolar PC was reutilized at an efficiency of only 23%. We also concluded that this reutilization occurred as intact molecules.

Reutilization of phosphatidylglycerol and phosphatidylethanolamine by the pulmonary surfactant system in 3-day-old rabbits

Developing rabbits reutilize the phosphatidylcholine of surfactant with an efficiency of about 95%. The efficiency of reutilization of other components of surfactant have not been determined. 3-day-old rabbits were injected intratracheally with [3H]dipalmitoylphosphatidylcholine (DPPC) mixed with unlabeled natural surfactant and either disaturated [32P]phosphatidylglycerol (DSPG) or [14C]dipalmitoylphosphatidyl-ethanolamine (DPPE). The recovery of [3H]DPPC, [14C]DPPE, and [32P]DSPG in the alveolar wash was measured at different times after injection. By plotting the ratio of [32P]DSPG to [3H]DPPC or [14C]DPPE to [3H]DPPC counts/min in the alveolar wash vs. time after injection we showed that these two phospholipids are reutilized less efficiently than phosphatidylcholine. Based on other studies, several assumptions were made about the kinetics of surfactant phosphatidylethanolamine and phosphatidylglycerol. From the slopes of the semilog plots of total [14C]DPPE and total [32P]DSPG counts/min in the alveolar wash vs. time and these assumptions, we determined that these two phospholipids were reutilized at an efficiency of only 79%.

Surfactant pool sizes and severity of respiratory distress syndrome in prematurely delivered lambs

To quantify the relationship between surfactant pool size and severity of respiratory disease, 21 lambs were delivered at 134 to 136 days gestational age and ventilated by varying only peak inspiratory pressure to maintain tidal volume at 6.2 +/- 0.3 ml/kg (mean +/- SE) and thus to control PCO2. Compliance measurements were used to quantify the severity of lung disease. After alveolar wash, surfactant phosphatidylcholine, saturated phosphatidylcholine, and minimal surface tensions were estimated. Compliance correlated linearly with saturated phosphatidylcholine pool size (r = 0.755, p less than 0.001). The mean minimal surface tension of the alveolar washes was 17.4 +/- 1.7 dynes/cm, and alveolar washes from lambs with more compliant lungs had lower minimal surface tensions than did washes from lambs with poorly compliant lungs (p less than 0.001). Lung tissue of all lambs contained similar amounts of saturated and total phosphatidylcholine, and in vitro rates of incorporation of labeled choline and palmitate into phosphatidylcholine in lung slices were similar, independent of severity of lung disease. The pool size of surfactant within the alveoli is an important determinant of lung disease in premature lambs; however, surface tension, tissue maturity, and other factors may contribute to the severity of the disease.

Left ventricular performance and regional blood flows before and after ductus arteriosus occlusion in premature lambs treated with surfactant

The hemodynamic consequences of patent ductus arteriosus (PDA) were studied during the first few hours of life (1.9 +/- 0.7 hours) in 13 mechanically ventilated, surfactant-treated, preterm lambs (gestational age of 120-124 days). Cardiovascular pressures, left ventricular (LV) performance and regional blood flows were measured using cine left ventriculography and radionuclide-labeled microspheres before and after occlusion of the PDA with a catheter balloon. Before occlusion, the left-to-right shunt was 44 +/- 13%; after occlusion the shunt was negligible (2%). Heart rate (164 +/- 17 beats/min), LV end-diastolic pressure (5 +/- 3 mm Hg), ejection fraction (74 +/- 8%) and cardiac output (241 +/- 13 ml . min-1 . kg-1) were normal and did not change after ductus occlusion. However, "effective" systemic blood flow increased significantly after occlusion (130 +/- 53 vs 228 +/- 93 ml . min-1 . kg-1, p less than 0.001), as did blood flow to organs such as the brain, myocardium and gastrointestinal tract. The reduction of systemic blood flow in the presence of a left-to-right PDA shunt may be responsible for many early pathologic manifestations of the PDA "syndrome" of prematurity even in the absence of overt LV dysfunction.