Synthesis and release of phosphatidylcholine by isolated porcine gastric mucous cells in primary culture

Phosphatidylcholine metabolism of rat trachea in relation to lung parenchyma and surfactant

Pulmonary surfactant prevents alveolar collapse and contributes to airway patency by reducing surface tension. Although alveolar surfactant, consisting mainly of phospholipids (PL) together with neutral lipids and surfactant-specific proteins, originates from type II pneumocytes, the contribution of airway epithelia to the PL fraction of conductive airway surfactant is still debated. We, therefore, analyzed the composition, synthesis, and release of phosphatidylcholine (PC) molecular species as the main surfactant PL of the rat trachea compared with the lung. Analyses of individual PC molecular species with HPLC and electrospray ionization mass spectrometry revealed that the rat trachea contained and synthesized much more palmitoyloleoyl-PC, palmitoyllinoleoyl-PC, and palmitoylarachidonoyl-PC, together with increased amounts of alkylacyl-PC, and less surfactant-specific species such as dipalmitoyl-PC than the lung. Organ cultures with [methyl-3H]choline as precursor of PC revealed that, in the trachea, synthesized PC was retained in the tissue, rather than secreted. [Methyl-3H]choline-labeled dipalmitoyl-PC was a negligible component in the trachea, and, in contrast to the lungs, palmitoyloleoyl-PC was enriched in tracheal secretions. We conclude that the surfactant fraction in the airways does not originate from the airways but is produced in the alveolar space and transported upward.

Pulmonary and gastric surfactants. A comparison of the effect of surface requirements on function and phospholipid composition

Surfactant is present in the alveoli and conductive airways of mammalian lungs. The presence of surface active agents was, moreover, demonstrated for avian tubular lungs and for the stomach and intestine. As the surface characteristics of these organs differ from each other, their surfactants possess distinct biochemical and functional characteristics. In the stomach so-called 'gastric surfactant' forms a hydrophobic barrier to protect the mucosa against acid back-diffusion. For this purpose gastric mucosal cells secrete unsaturated phosphatidylcholines (PC), but no dipalmitoyl-PC (PC16:0/16:0). By contrast, surfactant from conductive airways, lung alveoli and tubular avian lungs contain PC16:0/16:0 as their main component in similar concentrations. Hence, there is no biochemical relation between gastric and pulmonary surfactant. Alveolar surfactant, being designed for preventing alveolar collapse under the highly dynamic conditions of an oscillating alveolus, easily reaches values of <5 mN/m upon cyclic compression. Surfactants from tubular air-exposed structures, however, like the conductive airways of mammalian lungs and the exclusively tubular avian lung, display inferior compressibility as they only reach minimal surface tension values of approximately 20 mN/m. Hence, the highly dynamic properties of alveolar surfactant do not apply for surfactants designed for air-liquid interfaces of tubular lung structures.

Conductive airway surfactant: surface-tension function, biochemical composition, and possible alveolar origin

Alveolar surfactant is well known for its ability to reduce minimal surface tension at the alveolar air-liquid interface to values below 5 mN/m. In addition, it has been suggested that an analogous conductive airway surfactant is also present in the airways. To elucidate the composition, possible origin, and surface activity of conductive airway phospholipids (PL), we compared in adult porcine lungs the PL classes and phosphatidylcholine (PC) molecular species of nonpurified tracheal aspirate samples with those of bronchoalveolar lavage fluid (BAL), tracheobronchial epithelium, and lung parenchyma. We also analyzed PL and PC composition, protein content, and surface activity of surfactant isolated from tracheal aspirates (SurfTrachAsp), BAL (SurfBAL), and the 27,000 x g pellet of BAL (SurfP27000) by density-gradient centrifugation. Although PL composition revealed contributions of the airways to tracheal aspirates, the composition of PC molecular species of tracheal aspirates was similar to that of BAL and lung parenchyma, but differed considerably from that of airway epithelium. SurfTrachAsp had the same PL and PC composition as SurfBAL and SurfP27000, indicating that this fraction of tracheal aspirates may have originated from the alveoli. Nevertheless, minimal and maximal surface tensions were higher in SurfTrachAsp than in SurfBAL and SurfP27000. Analysis of surfactant proteins A, B, and C (SP-A, SP-B, and SP-C) revealed that SP-A was decreased and SP-B and SP-C were absent, whereas total protein was increased in SurfTrachAsp. We conclude that as compared with alveolar surfactant, PL of SurfTrachAsp show the same composition, but that surface-tension function is impaired and the concentration of surfactant proteins is decreased in SurfTrachAsp.

Rat gastric hydrophobic barrier: modulation of phosphatidylcholine molecular species by dietary lipids

Phospholipids protect the gastric mucosa by forming a proton-repellent hydrophobic layer on its luminal surface. We have recently shown that two molecular species of phosphatidylcholine (PC), PC16:0/18:1, and PC16:0/18:2, but not PC16:0/16:0, are predominantly released into gastric mucus. We investigated whether these molecular species in mucus are modified by dietary fat. Rats were fed (for three weeks) a diet supplemented with either 10% cod liver, palm, or sunflower oil, or 10% corn starch as a control. In tissue, cod liver oil decreased PC16:0/20:4 and PC18:0/20:4. Cod liver oil and palm oil increased PC16:0/18:1, whereas sunflower oil decreased PC16:0/18:1. Palm oil additionally decreased PC16:0/18:2, whereas the other diets had no effect on PC16:0/18:2. In mucus, however, PC16:0/18:1 and PC16:0/18:2 were not significantly altered by any diet. They were increased over tissue values and comprised 37.6 +/- 3.3 and 33.1 +/- 1.4 mol% in controls. PC16:0/16:0 was lower in mucus than in mucosa and even decreased by cod liver oil (1.2 +/- 0.2 vs. 2.7 +/- 0.3 mol%; P < 0.01). We conclude that PC16:0/18:1 and PC16:0/18:2 are modified by dietary fat in tissue. In gastric secretions, however, PC16:0/18:1 and PC16:0/18:2 are kept constant and together comprise 70 mol% of the released PC species, whereas PC16:0/16:0 does not play a role for the gastric hydrophobic barrier under any dietary treatment. Additionally, cod liver oil decreases the content of PC16:0/20:4 and PC18:0/20:4 in gastric mucosa, thereby possibly decreasing the formation of eicosanoids.