Respiratory distress and surfactant inhibition following vagotomy in rabbits

We used the model of bilateral cervical vagotomy of adult rabbits to cause respiratory failure characterized by pulmonary edema, decreased lung compliance, and atelectasis. We documented an 18-fold increase in radiolabeled albumin leak from the vascular space into alveolar washes of vagotomy vs. sham-operated rabbits (P less than 0.01). Despite a twofold increase in percent of prelabeled saturated phosphatidylcholine secreted (P less than 0.01), the alveolar wash saturated phosphatidylcholine pool sizes were not different. The minimum surface tensions were 19.6 +/- 2.5 vs. 9.4 +/- 2.2 dyn/cm for alveolar washes from vagotomy and control rabbits, respectively (P less than 0.01). The soluble proteins from alveolar washes inhibited the surface tension lowering properties of natural surfactant, whereas those from the control rabbits did not (P less than 0.01). When vagotomy rabbits in respiratory failure were treated with 50 mg natural surfactant lipid per kilogram arterial blood gas values and compliances improved relative to control rabbits. Vagotomy results in alveolar pulmonary edema, and surfactant dysfunction despite normal surfactant pool sizes and respiratory failure. A surfactant treatment can improve the respiratory failure.     

The effect of betamethasone and fetal sex on the synthesis and maturation of lung surfactant phospholipids in rabbits

In the present study we investigated the maturation of the surfactant phospholipids and the role of fetal sex on the effect of betamethasone in male and female rabbit fetuses. Betamethasone was administered to the doe (0.2 mg/kg intramuscularly) 42 and 18 h prior to killing. The fetuses were studied at 27 and 28 days from conception. Results from the alveolar lavage show that male fetuses tended to have a lower disaturated phosphatidylcholine/sphingomyelin ratio and lower levels of phosphatidylinositol. Phosphatidylglycerol was detected in trace amounts. This was apparently due to the high extracellular levels of myo-inositol inhibiting the synthesis of surfactant phosphatidylglycerol while increasing the synthesis of surfactant phosphatidylinositol. Betamethasone increased the recovery of disaturated phosphatidylcholine and phosphatidylinositol from the lung lavage in both sexes. As studied in lung slices in vitro, the betamethasone treatment decreased the incorporation of glucose into phospholipids, including into the fatty acid moiety of disaturated phosphatidylcholine, although it had no significant effect on the incorporation of glucose into the glycerol moiety of disaturated phosphatidylcholine. However, the addition of palmitate increased the incorporation of glucose into the glycerol moiety of disaturated phosphatidylcholine. The betamethasone treatment did not increase the incorporation of [1-14C]pyruvate into disaturated phosphatidylcholine. Following betamethasone administration, the availability of fatty acids may become rate-limiting for the synthesis of surfactant phospholipids. Betamethasone increased the activities of phosphatidic acid phosphohydrolase and phosphatidate cytidyltransferase in a fraction of microsomal membranes. The present evidence suggests that the glucocorticoid-induced lung maturation and the maturation of the normal lung are associated with an increase in the activity of the enzymes which are involved in metabolizing phosphatidic acid to neutral and acidic surfactant secretion of the male fetus was not explained by possible sex-related differences in the biosynthesis of the phospholipids.     

Kinetics of the in vivo labeling of the acyl groups of rabbit lung phosphatidylcholine and disaturated phosphatidylcholine.

The kinetics of labeling of lung phosphatidylcholine and disaturated phosphatidylcholine were studied for periods from 0.75--120 min following intravenous injection of radiolabeled palmitic acid and choline into 3-day-old rabbits. The labeled palmitic acid was cleared rapidly from plasma, and rapidly appeared with identical incorporation kinetics in both phosphatidylcholine and disaturated phosphatidylcholine. The 2-acyl positions of both phosphatidylcholine and disaturated phosphatidylcholine were labeled preferentially soon after [14C]palmitic acid injection. The specific activities of palmitic acid in the 2-acyl positions of phosphatidylcholine and disaturated phosphatidylcholine 0.75 min after injection of labeled palmitic acid were 3.4 and 1.9 times, respectively, the specific activities of palmitic acid in the 1-acyl positions. By 120 min the label had randomized between the 1-acyl and 2-acyl positions, and the kinetics of that randomization were defined for both phosphatidylcholine and disaturated phosphatidylcholine. Choline did not pulse label lung phosphatidylcholine or disaturated phosphatidylcholine. The choline label appeared with equal specific activities in both phosphatidylcholine and disaturated phosphatidylcholine. Thus no analysis of the de novo synthesized product via the CDP-choline pathway was possible.     

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.     

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.     

Insulin receptors and insulin effects on type II alveolar epithelial cells

Type II alveolar epithelial cells (pneumocytes) were isolated to purity from adult rabbits and analyzed for the presence of cell surface insulin receptors and for effects of insulin on cells. Assays were performed on cells cultured for 24 h in Eagle's minimum essential medium. Insulin binding to cells in culture approached a steady-state level by 180 min at 15 degrees C and remained constant for at least 1 h. Competition experiments using native insulin, proinsulin and desoctapeptide supported specificity of binding. Scatchard analysis of binding revealed a class of high-affinity receptors with Kd = 1.5 X 10(-10) M and a low-affinity component with Kd = 4 X 10(-9) M. The number of receptors was estimated at 2000-4000/cell. Insulin added to cell cultures of type II pneumocytes in concentrations from 5 X 10(-11) to 5 X 10(-8) M resulted in a dose-related increase in uptake of 2-deoxyglucose by cells. Insulin also stimulated the incorporation of choline and glucose into phosphatidylcholine and disaturated phosphatidylcholine.     

Phospholipid biosynthesis and secretion by a cell line (A549) which resembles type II aleveolar epithelial cells.

The A549 cell line is a continuous cell line derived from a human adenocarcinoma of the lung. At low cell population density the cells contain relatively few lamellar bodies, but in mature cells in very confluent cultures lamellar bodies are abundant. The lamellar bodies from these cells are enriched for phosphatidylcholine and disaturated phosphatidylcholine. In mature cells, 45% of newly synthesized phosphatidylcholine is disaturated. Stimulation with the calcium ionophore A23187 produces exocytosis of phosphatidylcholine (46% disaturated). The A549 cell synthesizes, stores in lamellar bodies, and secretes phosphatidylcholine, and thus has many important biological properties of the alveolar epithelial type II cell.     

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%.     

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.     

Binding of adenovirus capsid to dipalmitoyl phosphatidylcholine provides a novel pathway for virus entry

Adenovirus (Ad) is an airborne, nonenveloped virus infecting respiratory epithelium. To study the mechanism of Ad entry, we used alveolar adenocarcinoma A549 cells, which have retained the ability of alveolar epithelial type II cells to synthesize the major component of pulmonary surfactant, disaturated phosphatidylcholine. Stimulation of phosphatidylcholine secretion by calcium ionophore or phorbol ester augmented the susceptibility of these cells to Ad. Both Ad infection and recombinant-Ad-mediated transfection increased in the presence of dipalmitoyl phosphatidylcholine (DPPC) liposomes in culture medium. Importantly, in the presence of DPPC liposomes, virus penetrates the cells independently of virus-specific protein receptors. DPPC vesicles bind Ad and are efficiently incorporated by A549 lung cells, serving as a virus vehicle during Ad penetration. To identify the viral protein(s) mediating Ad binding, a flotation of liposomes preincubated with structural viral proteins was employed, showing that the only Ad protein bound to DPPC vesicles was a hexon. The hexon preserved its phospholipid-binding properties upon purification, confirming its involvement in virus binding to the phospholipid. Given that disaturated phosphatidylcholine not only covers the inner surface of alveoli in the lungs but also reenters alveolar epithelium during lung surfactant turnover, Ad binding to this phospholipid may provide a pathway for virus entry into alveolar epithelium in vivo.     

The incorporation of choline into disaturated phosphatidylcholine in the microsomal, lamellar body, and surfactant fractions of hamster lung tissue

The specific activity of disaturated phosphatidylcholine in microsomes and lamellar bodies prepared from hamster lung tissue and in surfactant obtained by lung lavage was determined at various times following the intraperitoneal administration of [Me-3H]choline. The highest specific activity of disaturated phosphatidylcholine in the lung microsomes was attained 1 h after the administration of [3H]choline; thereafter, the specific activity declined. The specific activity of disaturated phosphatidylcholine in lamellar bodies increased steadily for 12 h after [3H]choline administration. The specific activity in lamellar bodies ater 12 h exceeded the maximum specific activity achieved in the microsomal fraction (p less than 0.005). The specific activity of the disaturated phosphatidylcholine in the alveolar lavage increased after an initial lag period of approximately 3 h, attaining the same specific activity as that of the lamellar bodies at the 12-h time point. The reported results are discussed in relation to the biosynthesis, storage, and secretion of the disaturated phosphatidylcholine associated with the lipoprotein, surfactant.     

Lysosomal phospholipase A2 is selectively expressed in alveolar macrophages

Lung surfactant is the surface-active agent comprised of phospholipids and proteins that lines pulmonary alveoli. Surfactant stabilizes the alveolar volume by reducing surface tension. Previously, we identified a lysosomal phospholipase A2, termed LPLA2, with specificity toward phosphatidylcholine and phosphatidylethanolamine. The phospholipase is localized to lysosomes, is calcium-independent, has an acidic pH optimum, and transacylates ceramide. Here, we demonstrate that LPLA2 is selectively expressed in alveolar macrophages but not in peritoneal macrophages, peripheral blood monocytes, or other tissues. Other macrophage-associated phospholipase A2s do not show a comparable distribution. LPLA2 is of high specific activity and recognizes disaturated phosphatidylcholine as a substrate. The lysosomal phospholipase A2 activity is six times lower in alveolar macrophages from mice with a targeted deletion of the granulocyte macrophage colony-stimulating factor (GM-CSF), a model of impaired surfactant catabolism, compared with those from wild-type mice. However, LPLA2 activity and protein levels are measured in GM-CSF null mice in which GM-CSF is expressed as a transgene under the control of the surfactant protein C promoter. Thus LPLA2 may be a major enzyme of pulmonary surfactant phospholipid degradation by alveolar macrophages and may be deficient in disorders of surfactant metabolism.     

Fetal lung disaturated phosphatidylcholine. Ostensible increase following exposure to dexamethasone

Fetal rat lung removed at 15 days gestation and placed in organ culture incorporates choline into phosphatidylcholine. Addition of 10(-9) M dexamethasone resulted in increased rates of choline incorporation per micrograms protein after both 6 and 12 days culture. This concentration of dexamethasone did not increase tissue phosphatidylcholine or disaturated phosphatidylcholine. Thus, at a culture time when dexamethasone had a significant effect on choline incorporation, there was no change in either the total phospholipid or disaturated phosphatidylcholine content of the lung tissue. The transplacental administration of dexamethasone decreased fetal lung DNA and phospholipid content. At the mid-range dosage tested (400 micrograms), dexamethasone depressed DNA (51%) appreciably more than total phosphatidylcholine (28%) and disaturated phosphatidylcholine (33%). These results show that the hormone does not increase the total amount of surfactant per lung. The increased disaturated phosphatidylcholine per mg DNA results in an ostensible beneficial effect of dexamethasone on surfactant and may reflect an increased proportion of Type II cells in fetal lung both in vitro and in vivo following hormone exposure. Disaturated phosphatidylcholine per Type II alveolar cell is no doubt increased but the trade-off is fewer total cells in the lung.     

A species comparison of alveolar size and surface forces

The independent roles of alveolar size and surface tension in relation to lung stability were investigated in 11 different mammalian species whose body weight ranged from 0.03 to 50 kg. This range in species provided a wide variation in subgross anatomy as well as a fourfold range in alveolar diameter. Alveolar diameter was estimated from the mean linear intercept (Lm) of fixed lungs. Quasi-static pressure-volume curves were determined in excised lungs and the percent volume remaining on deflation from total lung capacity at 30 cmH2O to 10 cmH2O (%V10) provided an index of deflation stability related to functional surfactant. Surface tension of lung extract was measured in the Wilhelmy balance, and the minimum surface tension measured provided an index of surface tension lowering capacity of surfactant. Relationships of %V10 with alveolar diameter and surface tension with alveolar diameter were examined for correlations. Our results indicated that despite a range in Lm between 31 and 133 micron (mouse to pig), %V10 did not change in proportion with Lm across species. Similarly, minimum surface tension was about the same (6.1 to 8.8 dyn/cm) across a threefold difference in alveolar diameter. These results suggest that a stable alveolar configuration is maintained by both surface and tissue forces in a complex manner yet to be analyzed.     

Alveolar pre-type II cells from the fetal rabbit lung: effect of confluence on the production of disaturated phosphatidylcholine

Pre-type II alveolar cells isolated from the fetal rabbit lung on the 24th gestational day have been maintained in vitro for 14 days in a chemically defined medium supplemented with hormone-stripped serum. These cells replicate in culture. Measurement of the incorporation of [14C]choline into cellular disaturated phospholipid indicated that those cells grown in vitro under standard conditions for 8 days (pre-confluent) incorporate the radioactive precursor at a similar rate to cells maintained for 14 days (post-confluent). Both dexamethasone and serum-free medium conditioned by monolayer cultures of fetal rabbit lung fibroblasts stimulated [14C]choline incorporation into disaturated phosphatidylcholine (PC) by the pre- and post-confluent cultures after 24 or 48 h of exposure: the conditioned medium was more effective than the steroid. These treatments had little effect on choline incorporation into disaturated phosphatidylcholine of preconfluent cells during the first 12 h. A marked response occurred by 24 h after which the labelling of disaturated phosphatidylcholine plateaued. In contrast, with post-confluent cells labelling of disaturated PC increased in a more linear fashion and only plateaued after 72 h. Determination of the ratio of incorporation of [14C]choline into disaturated versus unsaturated phospholipid indicated that serum-free medium conditioned by monolayer cultures of fetal lung fibroblasts specifically increased the level of radioactive precursor in the disaturated phospholipid in both the pre- and post-confluent cell monolayers.     

The synthesis of phosphatidylcholine by adult rat lung alveolar type II epithelial cells in primary culture

1. The formation of phosphatidylcholine from radioactive precursors was studied in adult rat lung alveolar type II epithelial cells in primary culture. 2. The incorporation of [Me-14C]choline into total lipids and phosphatidylcholine was stimulated by addition of palmitate, whereas the incorporation of [U-14C]glucose into phosphatidylcholine and disaturated phosphatidylcholine was stimulated by addition of choline. Addition of glucose decreased the absolute rate of incorporation of [1(3)-3H]glycerol into total lipids, phosphatidylcholine and disaturated phosphatidylcholine, decreased the percentage [1(3)-3H]glycerol recovered in phosphatidylcholine, but increased the percentage phosphatidylcholine label in the disaturated species. 3. At saturating substrate concentrations, the percentages of phosphatidylcholine radioactivity found in disaturated phosphatidylcholine after incubation with [1-(14)C]acetate (in the presence of glucose) [1-(14)C]palmitate (in the presence of glucose), [Me-14C]choline (in the presence of glucose and palmitate) and [U-14C]glucose (in the presence of choline and palmitate) were 78, 75, 74 and 90%, respectively. 4. Fatty acids stimulated the incorporation of [U-14C]glucose into the glycerol moiety of phosphatidylcholine. The degree of unsaturation of the added fatty acids was reflected in the distribution of [U-14C]glucose label among the different molecular species of phosphatidylcholine. It is suggested that the glucose concentration in the blood as related to the amount of available fatty acids and their degree of unsaturation may be factors governing the synthesis of surfactant lipids.     

Normal surface properties of phosphatidylglycerol-deficient surfactant from dog after acute lung injury

Lung surfactant was isolated from bronchoalveolar lavage of dogs during the late phase of recovery (15 days) from acute alveolar injury induced by subcutaneous injection of N-nitroso-N-methylurethane. This surfactant was compared with surfactant from control dogs in terms of in vitro surface properties, phospholipid composition and protein content, and those of its subfractions. Phospholipid composition and protein content were similar in the two groups, except that phosphatidylglycerol (PG) was markedly reduced and phosphatidylinositol (PI) was increased in the experimental group. In both, isopycnic densities of their subfractions in continuous sucrose density gradient were identical. The time course of surfactant adsorption was similar in both groups. Minimum surface tension (gamma min) was 4.1 +/- 1.5 dynes/cm in the experimental dogs and 3.8 +/- 1.3 dynes/cm in the controls. Surface compressibility (SC), stability index (SI), and dynamic respreadability (DR) of the surfactants from the two groups were nearly identical. When compared to an artificial surfactant composed of dipalmitoyl phosphatidylcholine (DPPC) and PG in 9:1 molar ratio a mixture of DPPC-PI 9:1 prepared identically showed similar gamma min, SC, SI, and DR, and a much higher surface adsorption rate. These results suggest that PG is not essential for normal in vitro surfactant function and that its role may be assumed by PI.     

Surfactant in the gas mantle of the snail Helix aspersa

Surfactant occurs in cyclically inflating and deflating, gas-holding structures of vertebrates to reduce the surface tension of the inner fluid lining, thereby preventing collapse and decreasing the work of inflation. Here we determined the presence of surfactant in material lavaged from the airspace in the gas mantle of the pulmonate snail Helix aspersa. Surfactant is characterized by the presence of disaturated phospholipid (DSP), especially disaturated phosphatidylcholine (PC), lavaged from the airspace, by the presence of lamellated osmiophilic bodies (LBs) in the airspaces and epithelial tissue, and by the ability of the lavage to reduce surface tension of fluid in a surface balance. Lavage had a DSP/phospholipid (PL) ratio of 0.085, compared to 0.011 in membranes, with the major PL being PC (45.3%). Cholesterol, the primary fluidizer for pulmonary surfactant, was similar in lavage and in lipids extracted from cell homogenates (cholesterol/PL: 0.04 and 0. 03, respectively). LBs were found in the tissues and airspaces. The surface activity of the lavage material is defined as the ability to reduce surface tension under compression to values much lower than that of water. In addition, surface-active lipids will vary surface tension, increasing it upon inspiration as the surface area expands. By these criteria, the surface activity of lavaged material was poor and most similar to that shown by pulmonary lavage of fish and toads. Snail surfactant displays structures, a biochemical PL profile, and biophysical properties similar to surfactant obtained from primitive fish, teleost swim bladders, the lung of the Dipnoan Neoceratodus forsteri, and the amphibian Bufo marinus. However, the cholesterol/PL and cholesterol/DSP ratios are more similar to the amphibian B. marinus than to the fish, and this similarity may indicate a crucial physicochemical relationship for these lipids.     

Pulmonary surfactant in birds: coping with surface tension in a tubular lung

As birds have tubular lungs that do not contain alveoli, avian surfactant predominantly functions to maintain airflow in tubes rather than to prevent alveolar collapse. Consequently, we have evaluated structural, biochemical, and functional parameters of avian surfactant as a model for airway surfactant in the mammalian lung. Surfactant was isolated from duck, chicken, and pig lung lavage fluid by differential centrifugation. Electron microscopy revealed a uniform surfactant layer within the air capillaries of the bird lungs, and there was no tubular myelin in purified avian surfactants. Phosphatidylcholine molecular species of the various surfactants were measured by HPLC. Compared with pig surfactant, both bird surfactants were enriched in dipalmitoylphosphatidylcholine, the principle surface tension-lowering agent in surfactant, and depleted in palmitoylmyristoylphosphatidylcholine, the other disaturated phosphatidylcholine of mammalian surfactant. Surfactant protein (SP)-A was determined by immunoblot analysis, and SP-B and SP-C were determined by gel-filtration HPLC. Neither SP-A nor SP-C was detectable in either bird surfactant, but both preparations of surfactant contained SP-B. Surface tension function was determined using both the pulsating bubble surfactometer (PBS) and capillary surfactometer (CS). Under dynamic cycling conditions, where pig surfactant readily reached minimal surface tension values below 5 mN/m, neither avian surfactant reached values below 15 mN/m within 10 pulsations. However, maximal surface tension of avian surfactant was lower than that of porcine surfactant, and all surfactants were equally efficient in the CS. We conclude that a surfactant composed primarily of dipalmitoylphosphatidylcholine and SP-B is adequate to maintain patency of the air capillaries of the bird lung.     

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.