The perfused isolated lung as a possible model for the study of lipid synthesis by type II cells in their natural environment

Effects of chronic exposure to ozone on pulmonary lipids in rats

Ozone is the most toxic component of photochemical oxidant air pollution. Exposure to high concentrations of ozone produces a variety of toxic effects in the lung, but it is not known to what extent prolonged exposure to low concentrations of ozone may contribute to the development of chronic lung disease. Phospholipids, important components of cellular membranes and surfactant, are necessary for the maintenance of normal lung structure and function. In order to test the effects of chronic exposure to environmentally relevant concentrations of ozone on phospholipid metabolism in the lung, rats were exposed to clean air or to 0.12, 0.25 or 0.50 ppm ozone for up to 18 months. The content and biosynthesis of phospholipids in both lung tissue and bronchopulmonary lavage fluid (surfactant) were measured. Incorporation of [14C]acetate into lung tissue phospholipids, an estimate of overall biosynthesis, decreased significantly at some time points in the study, while tissue phospholipid content tended to increase with both ozone concentration and with age. No changes were detected in phospholipid content of bronchopulmonary lavage fluid. These findings did not support the hypothesis that prolonged exposure of rats to environmentally relevant concentrations of ozone results in either qualitative or quantitative deficits in the pulmonary surfactant system.

Degradation of vasoactive intestinal peptide by isolated, ventilated, and perfused rat lungs

The degradation of vasoactive intestinal peptide (VIP) was studied using an isolated perfused rat lung model. 125iodine labelled VIP (125I-VIP) was used as a tracer. VIP was cleared from the perfusate by a single lung passage up to concentrations of 1 nmol l-1. The clearance rate was decreased at higher concentrations of VIP. VIP was taken up by the lung tissue and the cleavage products were re-extruded into the perfusate. The time delay of re-extrusion was increased at starting concentrations of VIP exceeding 1 nmol l-1 and in the presence of the lysosomal inhibitor chloroquine. After a bolus of 9 pmol or 40 nmol 125I-VIP into the pulmonary artery catheter 6.3 pmol or 2920 pmol, respectively, were bound by the lung. Most of the radioactive material was extruded within 25 min and consisted of low molecular weight 125I-labelled degradation products. We conclude that the receptors for VIP in the alveolar capillaries are of high affinity and capacity to extract VIP from the circulation and that lysosomes may be involved in the degradation. The degradation products are of low molecular weight.

Regulation of phospholipid synthesis by intracellular phospholipases in fetal rabbit type II pneumocytes

Exposure of fetal type II pneumocytes to phospholipase A2 inhibitors led to significantly reduced choline uptake and decreased synthesis of total and disaturated phosphatidylcholines from both [methyl-14C]choline and [9,10(n)-3H]palmitate precursors. The percentage of the total synthesized phosphatidylcholine recovered as disaturated phosphatidylcholine was increased when compared to that in control cultures, suggesting that unsaturated phosphatidylcholine synthesis was reduced to a greater extent than that of the disaturated species. Synthesis of sphingomyelin and phosphatidylethanolamine from labeled palmitate was also reduced, whereas that of phosphatidylinositol and phosphatidylglycerol was significantly increased. Addition of phospholipase C resulted in increased synthesis of phosphatidylcholine from both labeled precursors; no significant changes were found in synthesis of most of the other 3H-labeled lipids. Added phospholipase A2 did not lead to any changes in either choline or palmitate incorporation. However, when melittin (a phospholipase A2 activator) was added to the cultures, greater incorporation of both palmitate and choline was observed, along with a significant increase in the percentage of total cellular radioactivity in 14C-labeled lipids, indicating also stimulation of phosphatidylcholine synthesis. A marked increase in CTP: phosphorylcholine cytidylyltransferase activity was found after treatment of the cultures with phospholipase C. Exposure to quinacrine also increased the activity of this enzyme. Addition of phospholipase C and melittin to prelabeled pneumocyte cultures accelerated degradation of cell phospholipids and the release of free fatty acids as the main degradation products. These findings suggest that intracellular phospholipases are regulators of synthesis of surfactant phospholipids in fetal type II pneumocytes, and that activation or inhibition of these phospholipases could represent a mechanism through which hormones and pharmacological agents modify surfactant and other phospholipid synthesis.

Developmental changes in molecular species of phosphatidic acid in rat lung and liver during the perinatal stage

A new and sensitive method for the determination of the molecular species of phosphatidic acid was developed. The developmental profiles of the structural heterogeneity of phosphatidic acid in rat lung and liver are presented. The individual molecular species of phosphatidic acid at different stages of development were analyzed as [14C]dimethylphosphatidates. The content of phosphatidic acid gradually increased from 20 to 40 nmol/g wet tissue during development of both lung and liver. The relative distribution of the disaturated species of lung phosphatidic acid accounted for 10.1 +/- 2.4% at -2 days of gestation and slightly decreased at -1 day and 1 day after birth (8.4 +/- 1.1%), then increased significantly to the adult level of 14.6 +/- 3.2%. The developmental pattern of the disaturated species of phosphatidic acid was not similar to that of diacylglycerol or phosphatidylcholine. On the other hand, the relative distribution of the disaturated species of liver phosphatidic acid did not change (4.1-6.0%) during development. The level of the disaturated species of phosphatidic acid was significantly higher in lung than in liver at any stage of development. The overall results strongly suggest that disaturated phosphatidic acid could be effectively formed via the de novo pathway in fetal as well as in adult lung.

Synthesis of saturated phosphatidylcholine and phosphatidylglycerol by freshly isolated rat alveolar type II cells

Saturated phosphatidylcholine and phosphatidylglycerol are important components of pulmonary surface active material, but the relative contributions of different pathways for the synthesis of these two classes of phospholipids by alveolar type II cells are not established. We purified freshly isolated rat type II cells by centrifugal elutriation and incubated them with [1-14C]palmitate as the sole exogenous fatty acid in one series of experiments or with [9,10-3H]palmitate, mixed fatty acids (16:0, 18:1 and 18:2), and [U-14C]glucose in another series of experiments. Type II cells readily incorporated [1-14C]palmitate into saturated phosphatidic acid (55-59% of total phosphatidic acid), saturated diacylglycerol (82-87% of total diacylglycerol), saturated phosphatidylcholine (69-76% of total phosphatidylcholine), and saturated phosphatidylglycerol (55-59% of total phosphatidylglycerol). Saturated phosphatidic acid, diacylglycerol and phosphatidylglycerol were nearly equally labeled in the sn-1 and sn-2 positions, whereas saturated phosphatidylcholine was preferentially labeled in the sn-2 position. With [9,10-3H]palmitate and [U-14C]glucose, the labeling patterns of phosphatidic acid, diacylglycerol and phosphatidylglycerol were similar to each other but different from that of phosphatidylcholine. The glucose label was found predominantly in the unsaturated phosphatidylcholines at early times (3-10 min) and in the saturated phosphatidylcholines at later times (30-90 min). Similarly, the 3H/14C ratio was very high in saturated phosphatidylcholine and always above that in saturated diacylglycerol. We conclude that freshly isolated type II cells synthesize saturated phosphatidic acid, diacylglycerol, phosphatidylcholine and phosphatidylglycerol and that under our in vitro conditions the deacylation-reacylation pathway is important for the synthesis of saturated phosphatidylcholine but is less important for the synthesis of saturated phosphatidylglycerol. By the assumptions stated in the text during the pulse chase experiment de novo synthesis of saturated phosphatidylcholine from saturated diacylglycerol accounted for 25% of the total synthesis of saturated phosphatidylcholine.

Mechanisms involved in the synthesis of disaturated phosphatidylcholine by alveolar type II cells isolated from adult rat lung

1. Alveolar type II cells isolated from adult rat lung incorporated radioactively labelled palmitate predominantly into the 2-position of disaturated phosphatidylcholine. In disaturated diacylglycerol, however, the radioactivity was almost equally distributed between the 1- and 2-positions. 2. Exposure of isolated type II cells to the phospholipase A2 inhibitors 4-bromophenacylbromide or quinacrine dihydrochloride led to a decreased synthesis of total phosphatidylcholines from various labelled precursors. Interestingly, it also led to an increased degree of unsaturation of the phosphatidylcholines synthesized by the cells. 3. Incubation of type II cell sonicates with radioactively labelled CDPcholine resulted in the formation of labelled phosphatidylcholine; 56% of this phosphatidylcholine appeared to be disaturated. In similar experiments with homogenates from whole lung, 20% of the synthesized phosphatidylcholine was disaturated. 4. These results suggest that both direct synthesis de novo and remodeling of 1-saturated-2-unsaturated phosphatidylcholines contribute to the biosynthesis of disaturated phosphatidylcholine in isolated alveolar type II cells.

Properties of diacylglycerol kinase in adult and fetal rat lung

Diacylglycerol kinase activity is found in both adult and fetal lung. Approximately 27 and 52% of the total activity is found in microsomes and cytosol, respectively. The activity is maximal at pH 7.4. The apparent Km for ATP is 0.11 mM and 0.21 mM for cytosol and microsomes, respectively. The apparent Km for dioleoylglycerol is 0.05 mM for cytosol and 0.14 for microsomes. Maximal activity in cytosol and microsomes is obtained with 2.0 mM dexoycholate. Other detergents cannot substitute for deoxycholate. Phosphatidylglycerol stimulates activity in the absence and in the presence of deoxycholate. Phosphatidylserine also stimulates activity, whereas phosphatidylethanolamine was inactive and phosphatidylcholine inhibited the reaction. Linoleic acid produced inhibition. The general properties of the enzyme were similar for fetal and adult lung. Diacylglycerol kinase from microsomes and cytosol fraction from both fetal and adult lung was most active with dioleoylglycerol and diacylglycerol from egg phosphatidylcholine. Significantly lower activity was obtained with dipalmitoylglycerol. Phosphatidylglycerol did not alter the relative substrate preferences. The activity in microsomes increased with development from 19 days gestation to a maximal activity at 21 days gestation. Maximal activity was about 2-fold higher than the adult. The activity dropped rapidly reaching adult values prior to birth (22 days gestation). The activity in cytosol fractions increased gradually from 19 days gestation, reaching adult values by 22 days gestation.

Synthesis of disaturated phosphatidylcholine by cholinephosphotransferase in rat lung microsomes

1. Incubation of rat lung microsomes with cytidine diphospho[methyl-14C]choline resulted in synthesis of radioactively labeled phosphatidylcholine. 2. 10-15% of this phosphatidylcholine appeared to be disaturated species. In similar experiments with rat liver microsomes only 2-3% of the radioactivity was present in the disaturated species. 3. When de novo synthesis was blocked after 5 min by addition of Ca2+ no increase in the proportion of disaturated phosphatidylcholine was observed upon further incubation of lung microsomes. Under these conditions the enzymes involved in a remodeling mechanism, i.e. phospholipase A and acyl-CoA: lysophosphatidyl-choline acyltransferase, remain fully active. 4. Addition of diacylglycerols from egg phosphatidylcholine containing trace amounts of di[1-14C]palmitoyl glycerol resulted in direct incorporation of 14C label into phosphatidylcholine. The rate of phosphatidylcholine synthesis measured from incorporation of di[1-14C]palmitoyl glycerol equalled that observed with labeled CDP choline. 5. These results support the conclusion that disaturated phosphatidylcholine in lung can be formed by direct utilization of disaturated diacylglycerol and is not produced exclusively via remodelling of de novo synthesized unsaturated species.

Acylation of 1-palmitoyl-lysophosphatidylglycerol in a alveolar type II cells from rat lung

1. Alveolar type II cells from adult rat lung have the enzymic capability to acylate 1-palmitoyl-lysophosphatidyl-glycerol, using palmitoyl-CoA to form dipalmitoylphosphatidylglycerol. 2. On a protein basis the acylation of 1-palmitoyl-lysophosphatidylglycerol is at least 2-fold more active in sonicated type II cells than in whole lung homogenates. 3. Both type II cells and whole lung homogenates show higher activity towards palmitoyl-CoA than towards oleoyl-CoA for acylation of 1-palmitoyl-lysophosphatidylglycerol. 4. Both in type II cells and in whole lung homogenates the rates of acylation of 1-palmitoyl-lysophosphatidylglycerol and 1-palmitoyl-lysophosphatidylcholine with palmitate are of the same order of magnitude, while the rate of acylation of 1-palmitoyl-lysophosphatidylethanolamine is much lower.

De novo fatty acid synthesis in the perfused rat lung. Incorporation of palmitate into phospholipids

1. The incorporation of exogenously derived [14C]palmitate and endogenously synthesized [3H]palmitate (from 3H2O) was measured in the isolated perfused lung. 2. Over 40% of the fatty acid esterified into lung disaturated phosphatidylcholine was derived from de novo synthesis. 3. A major portion of the palmitate synthesized de novo was incorporated in the 2 position of disaturated phosphatidylcholine. 4. Streptozotocin-induced diabetes and the compound 5-(tetradecyloxy)-2-furoic acid markedly inhibited de novo fatty acid synthesis while the incorporation of exogenously supplied palmitate increased into disaturated phosphatidylcholine, primarily in the 2 position. 5. Treatment with insulin resulted in an increase in [14C]glucose incorporation into lung phospholipid, with the largest increase appearing in the glyceride-glycerol fraction of the phosphatidylcholine species. 6. Insulin neither stimulated de novo fatty acid synthesis nor increased exogenous palmitate incorporation. 7. These data show: (1) that de novo fatty acid synthesis in the perfused rat lung is involved in the remodeling reactions in the synthesis of phosphatidylcholine, and (2) that diabetes affects the relative contribution of de novo synthesized and exogenously supplied palmitate available for the esterification of lung phospholipid.

Pathways of palmitate metabolism in the isolated rat lung

Plasma fatty acids represent major precursors of lung lipids. In this study, the pathways of palmitate metabolism were measured in an isolated perfused rat lung. Lungs were ventilated with 5% CO2 in air and perfused with Krebs-Ringer bicarbonate containing 3% serum albumin and 0.25 mM [U-14C] and [9, 10(-3)H] palmitate. Fatty acid utilization was estimated by recovery of radiolabel in products of metabolism. Fourteen percent of a total 14C-fatty-acid utilization of 4.5 mumol fatty acid/100 min/g dry wt. was recovered as 14CO2. Degradation of fatty acid to acetyl CoA was indicated by a 3H2O production that was twice fatty acid oxidation to CO2. The majority of palmitate was recovered in lung phosphatidylcholines with a 14C to 3H ratio of 1.4 accounting for differences between 14C and 3H2O productions. Addition of glucose to the perfusate decreased fatty acid oxidation to CO2 by 32% but had no effect on 14C recovery in phospholipids. Perfusion with the uncoupler of oxidative phosphorylation 2,4-dinitrophenol stimulated fatty acid oxidation twofold but decreased 14C incorporation into lipids. These data together with estimates of fatty acid synthesis based on 3H2O incorporation into lipids, suggested that exogenous fatty acids and glucose both represent sources of carbon for de novo fatty acid synthesis and energy production.

The isolated perfused lung--a critical evaluation

Recent refinement and application of the technique of perfusing isolated intact lung preparations from suitable experimental animal has enhanced our knowledge of the non-respiratory functions of the lung. The technique consists of perfusion with whole blood or constituted media via pulmonary artery or artificially ventilated lungs. Such preparations have been known to be especially useful for studies of uptake, metabolism and disposition of exogenous and endogenous substances. Recent advances in unilateral, split-lung perfusion in which left and right lungs are unilaterally perfused simultaneously have enabled investigators to maintain paired controls during perfusion. Such techniques are useful in the study of the interactions of endogenous and exogenous chemicals in the lung tissue. Differences between subcellular or lung slice preparations and intact lung perfusion are to be expected on the basis of distorting natural vascular and extra-vascular barriers in the case of in vitro preparations. Areas in which perfused lungs have not been extensively used include uptake and disposition of gases, solvents and vapors, effects of toxic chemicals on respiratory and non-respiratory functions of the lung and alteration of pulmonary mechanisms and hemodynamics in the presence of interacting chemical or physical stimuli. With suitable modifications, application of isolated perfused lung preparations for these investigations should be technically feasible in the future.

Phospholipid-transfer activity in type II cells isolated from adult rat lung

The soluble fraction from adult rat lung type II cells stimulated the transfer of various phospholipids between either liposomes or rat lung microsomes and rat lung mitochondria. Compared to whole lung, type II cells are highly enriched in transfer activity suggesting that phospholipid-transfer proteins play a role in the transport of surfactant phospholipids. Sephadex chromatography of pH 5.1 supernatant from type II cells yielded only one fraction catalysing the transfer of phosphatidylcholine, phosphatidylglycerol and phosphatidylethanolamine, while chromatography of pH 5.1 supernatant from whole lung yielded in addition a specific phosphatidylcholine-transfer and a specific phosphatidylglycerol-transfer protein.