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.     

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.     

Lysophosphatidylcholine uptake and metabolism in the adult rabbit lung

Tracer quantities of 3H-labeled lysoPC and 32P-labeled natural rabbit surfactant were given intratracheally via a bronchoscope and [14C]palmitate was given intravenously to 25 rabbits with labeled PC and lysoPC measured in the alveolar wash, lung homogenate, lamellar bodies and microsomes at five times from 10 min to 6 h after tracheal injection. Surprisingly, only 31% of the administered lysoPC remained in its original form in the total lungs (alveolar wash + lung homogenate) by 10 min, of which 77% was in the alveolar wash. Meanwhile, by 10 min an additional 37% was already converted to PC, of which more than 98% was in the lung homogenate. LysoPC continued to be rapidly and efficiently converted to PC, with 62% conversion measured at 3 h. The converted lysoPC initially appeared with high specific activity in microsomes, then in lamellar bodies, and finally in the alveolar wash. The intravascular palmitate labeled lung PC had similar specific activity-time profiles in the subcellular fractions, while intratracheally administered natural rabbit surfactant had a constantly low specific activity in microsomes and much higher specific activities in lamellar bodies and alveolar wash. Another 25 rabbits received intratracheal lysoPC labeled in both the choline and palmitate moieties and then were studied from 1 to 24 h after tracheal injection. The ratio of the palmitate to choline labels indicated uptake and conversion to PC primarily by direct acylation rather than transacylation and by intact reuptake and conversion rather than breakdown and resynthesis. LysoPC is an attractive 'metabolic probe' of surfactant metabolism which undergoes very rapid and efficient intracellular conversion to PC via a subcellular pathway that parallels the remodeling and de novo synthetic pathways.     

Liposomes of dipalmitoylphosphatidylcholine associate with natural surfactant

Unilamellar liposomes of an average diameter of 0.05 micron formed by sonication of dipalmitoylphosphatidylcholine associate in vitro with the large aggregate forms of natural surfactant. The liposomal-surfactant aggregates are stable and previously associated liposomes are not released from the aggregates by the addition of more liposomes. Radiolabeled liposomes, surfactant, and preformed liposomal-surfactant aggregates were injected at a dose of 8-10 mg lipid (about 2-times the endogenous surfactant pool size) into the airways of 3-day-old rabbits. Following airway injection, labeled phosphatidylcholine from the liposomal-surfactant aggregates were recovered in approximately equal amounts by alveolar wash and in the residual lung tissue fractions. This recovery pattern and the clearance kinetics were equivalent for 48 h after airway injection to those measured with radiolabeled surfactant alone. In contrast, following the injection of liposomes alone, labeled phosphatidylcholine from the liposomes was recovered primarily by alveolar wash at 3 and 24 h. The overall clearance of the liposomal-derived phosphatidylcholine from the lung was more rapid than was the clearance of the phosphatidylcholine from the surfactant or liposome-surfactant complexes. Liposomes can interact with surfactant in vitro, and the liposomes associated with the surfactant aggregate have a metabolic fate in vivo similar to surfactant and different from liposomes alone.     

Exogenous surfactant changes the phenotype of alveolar macrophages in mice

Alveolar macrophages are essential for the maintenance of surfactant homeostasis. We asked whether surfactant treatment would change alveolar macrophage number and whether the alveolar macrophage phenotype would become activated or apoptotic when challenged in vivo with exogenous surfactant. Surfactant pool size in mice was increased by repetitive surfactant treatments containing 120 mg/kg (110 micromol/kg) saturated phosphatidylcholine. The number of alveolar macrophages recovered by alveolar lavage decreased after the first dose by 49% and slightly increased after the second and third doses. Up to 28.5% of the macrophages became large and foamy, and their appearance normalized within 12 h. Surfactant treatment did not increase the percent of apoptotic or necrotic cells. The alveolar macrophages were not activated as indicated by no change in expression of CD14, CD16, CD54, CD95, and scavenger receptor class A types I and II after surfactant treatment. Surfactant treatment in healthy mice transiently changed the phenotype of alveolar macrophages to large and foamy without indications of changes in the surface markers characteristic of activation.     

The significance of reutilization of surfactant phosphatidylcholine

To assess the magnitude of reutilization of surfactant phosphatidylcholine, 68 3-day-old rabbits were injected intratracheally with a trace dose of [3H]choline-labeled surfactant mixed with [14C]palmitate-labeled synthetic dipalmitoylphosphatidylcholine. After timed kills we measured the total phosphatidylcholine associated counts/min in whole lung and alveolar wash and the specific activities of phosphatidylcholine in the alveolar wash, lamellar bodies, and microsomes isolated from the lung of each rabbit. Using a modification of the compartment analysis of Skinner et al. (Skinner, S. M., Clark, R. E., Baker, N., and Shipley, R. A. (1959) Am. J. Physiol. 196, 238-244), we found that surfactant phosphatidylcholine was reutilized with greater than 90% efficiency. The turnover time of the alveolar wash phosphatidylcholine was estimated to be 10.1 h and 9.3 h as measured by the 3H and 14C labels, respectively. From the ratios of alveolar wash-associated natural to synthetic phosphatidylcholine specific activities and from similar ratios obtained in 30 additional rabbits using [14C]choline-labeled natural surfactant and [3H]choline-labeled dipalmitoylphosphatidylcholine, we showed that phosphatidylcholine was reutilized intact rather than as component parts. Within 6 h of injection, the synthetic dipalmitoylphosphatidylcholine functioned metabolically as that administered in the form of natural surfactant.     

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.     

Altered surfactant function and metabolism in rabbits with acute lung injury

Lung injury was induced in rabbits with N-nitroso-N-methylurethane (NNNMU), and saturated phosphatidylcholine (Sat PC) pool sizes and phospholipid compositions were measured in alveolar wash subfractions isolated by differential centrifugation (large and small surfactant aggregates). Surfactant metabolism also was studied using intravascular and intratracheal radiolabels. Protein permeability, gas exchange, and compliance were significantly abnormal as lung injury progressed. At peak injury, there was a decrease in the large aggregate Sat PC pool size in alveolar wash accompanied by increased uptake of Sat PC from the air space and increased specific activity of both intravascular and intratracheal radiolabels in lamellar bodies. This was followed by a marked rise in the small aggregate pool size in the alveolar wash and increased secretion of Sat PC into the air spaces. Phospholipid compositions, total phospholipid-to-protein ratios, and in vivo functional studies using a preterm ventilated rabbit model were abnormal for both large and small aggregate surfactant fractions from the lung-injured rabbits. These studies characterize quantitative, qualitative, and functional changes of alveolar wash surfactant subfractions in NNNMU-injured lungs.     

Metabolism of intratracheally administered unsaturated phosphatidylcholines in adult rabbits

Twenty-five adult rabbits were each injected intratracheally with a solution containing 1-palmitoyl-2-[3H]palmitoyl phosphatidylcholine (DPPC) and 1-palmitoyl-2-[14C]oleoyl-PC that had been associated with with 32P-labeled natural rabbit surfactant. The animals were killed in groups of 5 at 1, 4, 8, 15 and 24 h after isotope injection. Isotope recovery and PC specific activities were measured in alveolar washes, lung homogenates, lamellar bodies and microsomes. The percent clearance per h of PC was very similar for the three labels and were; 3.56, 3.44 and 3.00%, respectively, for the 3H-, 14C- and 32P-labeled PC in the total lung (alveolar wash plus lung homogenate) and 3.84, 3.79 and 3.70%, respectively, for alveolar wash alone. The intracellular pathways of the three labels were assessed by comparing the specific activities in the lamellar bodies over 24 h as well as comparing the ratios of lamellar body to microsome specific activities over this period. These ratios were very similar for the monoenoic and saturated PC labels over time, indicating comparable recycling. In a separate experiment, three other unsaturated species; 1,2-[14C]dioleoyl-PC, 1-palmitoyl-2-[14C]linoleoyl-PC, and 1-palmitoyl-2-[14C]arachidonyl-PC were compared to 1-palmitoyl-2-[14C]oleoyl-PC. Recovery in the alveolar wash and total lung were similar at 16 h for all four labeled phospholipids. The intracellular pathways were also similar, except for the arachidonyl compound. More relative to the lamellar bodies as compared to the other. Thus, the catabolic pathways were similar for the saturated and unsaturated PC species initially present in the airspaces. The only metabolic difference between the compounds appears to be in the intracellular handling of the arachidonic species.     

Intrapulmonary catabolism of surfactant-saturated phosphatidylcholine in rabbits

Intrapulmonary surfactant catabolism was investigated by use of a phospholipase A1- and A2-resistant analogue of dipalmitoylphosphatidylcholine (DPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPC ether). [14C]DPC ether, made into liposomes with [3H]DPC and associated with 32P-labeled rabbit surfactant, was given intratracheally to 1-kg rabbits, which were killed at preset times to 48 h. Recoveries of radiolabel as saturated phosphatidylcholine (Sat PC) isolated from alveolar wash (AW), postlavage lung homogenate (LH), and alveolar macrophages were measured. All groups had similar AW and LH Sat PC pool sizes, indicating no perturbation of endogenous Sat PC pools. Despite a nearly fivefold accumulation of [14C]DPC ether in the lung by 48 h (P less than 0.01), the three probes had similar alveolar clearance curves. Furthermore, the Sat PC reutilization efficiency (41.6%) and turnover time (5.9 h) calculated for DPC ether were not different from values for the DPC and rabbit surfactant. Of the DPC ether (0.7%) and DPC (9%) labels recovered as PC in organs outside the lung, greater than 85% was unsaturated, indicating de novo synthesis using precursors from degraded PC. DPC ether was a useful probe of intrapulmonary DPC catabolism, and after alveolar uptake there was no direct reentry of intact DPC from the catabolic compartment(s) into the secretory pathway.     

Surface activity following natural surfactant treatment in premature lambs

Premature lambs with respiratory failure [CO2 partial pressure (PCO2) greater than 70 Torr] were treated with 50 mg/kg 3H-labeled natural surfactant by tracheal instillation. Minimum surface tensions of sequential samples suctioned from the airways fell from 25 +/- 3 dyn/cm before treatment to 8 +/- 5 dyn/cm after treatment and again rose to 32 +/- 2 dyn/cm at death. Minimum surface tensions of alveolar wash samples taken at death were 27 +/- 4 dyn/cm, whereas surfactant fractions reisolated from the alveolar washes lowered surface tension to under 10 dyn/cm. The alveolar washes, surfactant reisolated from the alveolar washes, and natural surfactant had similar phospholipid compositions; however, the alveolar washes contained about 40 times more protein per micromole phosphatidylcholine. The natural surfactant used for treatment apparently was inactivated by an inhibitor of surfactant function. After intravenous injections of [14C]palmitic acid, labeled saturated phosphatidylcholine appeared on the airways, indicating endogenous synthesis and secretion. However, the specific activity of the 3H-labeled saturated phosphatidylcholine in the natural surfactant used for treatment decreased by only 30 +/- 4% in the alveolar wash; thus the treatment dose was not diluted to a large extent by endogenous pools.     

Macrophage and type II cell catabolism of SP-A and saturated phosphatidylcholine in mouse lungs

Type II cells and macrophages are the major cells involved in the alveolar clearance and catabolism of surfactant. We measured type II cell and macrophage contributions to the catabolism of saturated phosphatidylcholine and surfactant protein A (SP-A) in mice. We used intratracheally administered SP-A labeled with residualizing (125)I-dilactitol-tyramine, radiolabeled dipalmitoylphosphatidylcholine ([(3)H]DPPC), and its degradation-resistant analog [(14)C]DPPC-ether. At 15 min and 7, 19, 29, and 48 h after intratracheal injection, the mice were killed; alveolar lavage was then performed to recover macrophages and surfactant. Type II cells and macrophages not recovered by the lavage were subsequently isolated by enzymatic digestion of the lung. Radioactivity was measured in total lung, lavage fluid macrophages, alveolar washes, type II cells, and lung digest macrophages. Approximately equal amounts of (125)I-dilactitol-tyramine-SP-A and [(14)C]DPPC-ether associated with the macrophages (lavage fluid plus lung digest) and type II cells when corrected for the efficiency of type II cell isolation. Eighty percent of the macrophage-associated radiolabel was recovered from lung digest macrophages. We conclude that macrophages and type II cells contribute equally to saturated phosphatidylcholine and SP-A catabolism in mice.     

Phospholipid content, composition and biosynthesis during fetal lung development in the rabbit.

The phospholipid content and composition of lung wash and lung tissue as well as the activities of the enzymes involved in the synthesis of phosphatidylcholine and phosphatidylglycerol (the major surface active components of pulmonary surfactant) were studied in the rabbit during fetal lung development. In lung wash the amount of phospholipid increased four-fold during the period 27-31 day's gestation. There was a further ten-fold increase following the onset breathing. During the same period the amount of phosphatidylcholine in lung wash increased from 29% of the total phospholipid to 80% while the amount of sphingomyelin decreased from 38% to 2%. The amount of phosphatidylcholine in lung tissue also increased during development but to a much lesser extent. During fetal lung development the activities of choline kinase and cholinephosphate cytidyltransferase changed little, cholinephosphotranserase decreased while lysophosphatidic acid acyltransferase and lysolecithin acyltransferase increased. There was a postnatal increase in the activities of cholinephosphate cytidyltransferase, cholinephosphotransferase and both acyltransferases. The amount of phosphatidylglycerol, as a percentage of the total phospholipid, in lung wash and lung tissue as well as the activity of pulmonary glycerolphosphate phosphatidyltransferase did not change appreciably during development.     

Characterization of rabbit lung lysosomes and their role in surfactant dipalmitoylphosphatidylcholine catabolism

Although alveolar surfactant is rapidly catabolized in adult rabbit lungs, the pathways have not been characterized. Pathways of surfactant secretion and recycling involve lamellar bodies and multivesicular bodies, organelles shown to be related to lysosomes by cytochemistry and autoradiography. Since lysosomes are central to intracellular catabolic events, it is possible that lysosomes are involved in intrapulmonary surfactant catabolism. Lysosomes relatively free of contaminating organelles (as determined morphologically and by marker enzymes for mitochondria, endoplasmic reticulum, peroxisomes, and plasma membranes) were obtained from post-lavage lung homogenates of 1-kg rabbits by differential centrifugation in buffered sucrose and gradient separation in percoll (density, 1.075-1.165). The role of lung lysosomes in catabolism of dipalmitoylphosphatidylcholine (DPC) was then studied in rabbits killed 4, 12, and 24 h following intratracheal injection of [3H]DPC and [14C] dihexadecyl phosphatidylcholine (DPC-ether). While equal amounts of label were in the lamellar body containing fractions at 4 h, nearly 6-fold more DPC-ether label than DPC label was recovered in the lysosomal fractions. By 24 h, there was 15-fold more DPC-ether in the lysosomes. This is the first report of successful isolation of lysosomes relatively free of other organelles from rabbit lungs. The tracer studies indicate DPC and DPC-ether follow similar intracellular processing after alveolar uptake. The subsequent accumulation of the ether analog in the lysosomal fractions supports a role for these organelles in surfactant DPC catabolism.     

Lack of correlation of severity of lung disease with the phosphatidylcholine concentration in fetal lung fluid from premature lambs at 133-136 days gestational age

Fetal lung fluid was collected following tracheotomy at the time of delivery of 40 premature lambs at 133-136 days gestational age. The concentration of phosphatidylcholine and saturated photophatidylcholine in fetal lung fluid was compared with the severity of lung disease of the lambs as assessed after 3 to 10 h of controlled mechanical ventilation with only peak inspiratory pressures varied to control the PCO2 values. Phosphatidylcholine concentration in fetal lung fluid did not correlate with the peak inspiratory pressures needed to ventilate the lambs, total lung compliance values, or the surfactant phosphatidylcholine pool sizes measured by alveolar wash after sacrifice. The ratio of saturated to total phosphatidylcholine was constant (0.55 +/- 0.02) and independent of concentration of phosphatidylcholine in the fetal lung fluid. The fetal lung fluid contained only about 0.7% of the final surfactant phosphatidylcholine pool released by the lambs to the alveoli after birth. Within a narrow gestational age range characterized by lung disease of widely varying severity, the phosphatidylcholine concentrations in fetal lung fluid were not predictive of the severity of lung disease.     

Mitigation of pulmonary hyperoxic injury by administration of exogenous surfactant

We studied the effects of surfactant supplementation on the progression of lung injury in rabbits exposed to 100% O2 for 64 h and returned to room air for 24 h. At this time, rabbits not treated with surfactant exhibit a severe lung injury with hypoxemia, increased alveolar premeability to solute, decreased total lung capacity (TLC) and lung edema. For surfactant treatment, 125 mg of calf lung surfactant extract (CLSE), suspended in 6-8 ml of normal saline, were instilled intratracheally at 0 and 12 h posthyperoxic exposure. At 24 h postexposure, these CLSE-treated rabbits compared with saline controls had significantly higher amounts of lung phospolipids (34 +/- 4 vs. 4.5 +/- 0.6 mumol/kg body wt) and increased TLC (42 +/- 2 vs. 27 +/- 1 ml/kg), with significantly lower amounts of alveolar protein (36 +/- 3 vs. 56 +/- 3 mg/kg) and decreased lung wet weight-to-dry weight ratios (5.6 +/- 0.1 vs. 6.3 +/- 0.3). Surfactant supplementation also decreased the degree of lung atelectasis as reflected by the increase in arterial O2 partial pressure (PaO2) after breathing 100% O2 for 20 min (PaO2 = 460 +/- 31 vs. 197 +/- 52 Torr). These findings indicate that instillation of exogenous surfactant mitigates the progression of hyperoxic lung injury in rabbits.     

Surfactant replacement attenuates the increase in alveolar permeability in hyperoxia

Rabbits exposed to hyperoxia develop surfactant deficiency, abnormal lung mechanics, and increased permeability to solute. We investigated whether replenishment of depleted alveolar surfactant by the intratracheal instillation of calf lung surfactant extract (CLSE) would mitigate the increase in alveolar permeability to solute. Twenty-eight rabbits were exposed to 100% O2 for 72 h and received intratracheal instillations of 125 mg CLSE (approximately 170 mumol dipalmitoyl phosphatidylcholine) at 24 and 48 h. The interlobar and intralobar distribution of CLSE was quantified by adding [14C]dipalmitoyl phosphatidylcholine liposes into the instillate and measuring the levels of activity in lung tissue. CLSE was nonuniformly distributed in the different lung lobes, the right lower lobe receiving more CLSE than the rest. Alveolar epithelial permeability to solute was assessed by instilling 10 ml isotonic saline, which contained a trace amount of [57Co]cyanocobalamin, in the right lower lobe and measuring the disappearance of the tracer from the alveolar saline and its appearance in the arterial blood during a 1-h period. CLSE treatment was associated with significantly increased 72-h survival in hyperoxia compared with saline-treated controls (number of survivors: 16/17 vs. 5/11, P less than 0.01). CLSE treatment significantly reduced the rate constant for the movement of cyanocobalamin out of the alveolar space (24 +/- 5 vs. 42 +/- 6 min-1 x 10(-3), P less than 0.01) and tracer appearance in the blood at the end of the study (7 +/- 1 vs. 34 +/- 13%, P less than 0.01) when compared with values in saline controls.(ABSTRACT TRUNCATED AT 250 WORDS)     

Surfactant treatments alter endogenous surfactant metabolism in rabbit lungs

The effect of exogenous surfactant on endogenous surfactant metabolism was evaluated using a single-lobe treatment strategy to compare effects of treated with untreated lung within the same rabbit. Natural rabbit surfactant, Survanta, or 0.45% NaCl was injected into the left main stem bronchus by use of a Swan-Ganz catheter. Radio-labeled palmitic acid was then given by intravascular injection at two times after surfactant treatment, and the ratios of label incorporation and secretion in the left lower lobe to label incorporation and secretion in the right lung were compared. The treatment procedure resulted in a reasonably uniform surfactant distribution and did not disrupt lobar pulmonary blood flow. Natural rabbit surfactant increased incorporation of palmitate into saturated phosphatidylcholine (Sat PC) approximately 2-fold (P less than 0.01), and secretion of labeled Sat PC increased approximately 2.5-fold in the surfactant-treated left lower lobe relative to the right lung (P less than 0.01). Although Survanta did not alter incorporation, it did increase secretion but not to the same extent as rabbit surfactant (P less than 0.01). Alteration of endogenous surfactant Sat PC metabolism in vivo by surfactant treatments was different from that which would have been predicted by previous in vitro studies.     

Injury,inflammation, and remodeling in fetal sheep lung after intra-amniotic endotoxin

Chorioamnionitis is frequent in preterm labor and increases the risk of bronchopulmonary dysplasia. We hypothesized that intra-amniotic endotoxin injures the lung in utero, causing a sequence of inflammation and tissue injury similar to that which occurs in the injured adult lung. Preterm lamb lungs at 125 days gestational age were evaluated for indicators of inflammation, injury, and repair 5 h, 24 h, 72 h, and 7 days after 4 mg of intra-amniotic endotoxin injection. At 5 h, the epithelial cells in large airways expressed heat shock protein 70, and alveolar interleukin-8 was increased. Surfactant protein B (SP-B) decreased in alveolar type II cells at 5 h, and SP-B in lung tissue and alveolar lavage fluid increased by 72 h. By 24 h, neutrophils were recruited into the large airways, and cell death was the highest. Alveolar type II cells decreased by 25% at 24 h, and proliferation was highest at 72 h, consistent with tissue remodeling. Intra-amniotic endotoxin caused surfactant secretion, inflammation, cell death, and remodeling as indications of lung injury. The recovery phase was accompanied by maturational changes in the fetal lung.     

Role of lamellar inclusions in surfactant production: studies on phospholipid composition and biosynthesis in rat and rabbit lung subcellular fractions.

Lamellar inclusion bodies in the type II alveolar epithelial cell are believed to be involved in pulmonary surfactant production. However, it is not clear whether their role is that of synthesis, storage, or secretion. We have examined the phospholipid composition and fatty acid content of rabbit lung wash, lamellar bodies, mitochondria, and microsomes. Phosphatidylcholine and phosphatidylglycerol, the surface-active components of pulmonary surfactant, accounted for over 80% of the total phospholipid in lung wash and lamellar bodies but for only about 50% in mitochondria and microsomes. Phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, and sphingomyelin accounted for over 40% of the total in mitochondria and microsomes but for only 6% in lung wash and 15% in lamellar bodies. The fatty acid composition of lamellar body phosphatidylcholine was similar to that of lung wash, but different from that of mitochondria and microsomes, in containing palmitic acid as a major component with little stearic acid and few fatty acids of chain length greater than 18 carbon atoms. The biosynthesis of phosphatidylcholine and phosphatidylglycerol was examined in the mitochondrial, microsomal, and lamellar body fractions from rat lung. Cholinephosphotransferase was largely microsomal. The activity in the lamellar body fraction could be attributed to microsomal contamination. The activity of glycerolphosphate phosphatidyltransferase, however, was high in the lamellar body fraction, although it was highest in the mitochondria and was also active in the microsomes. These data suggest that the lamellar bodies are involved both in the storage of the lipid components of surfactant and in the synthesis of at least one of those components, phosphatidylglycerol.