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 (Surf(TrachAsp)), BAL (Surf(BAL)), and the 27,000 x g pellet of BAL (Surf(P27000)) 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. Surf(TrachAsp) had the same PL and PC composition as Surf(BAL) and Surf(P27000), indicating that this fraction of tracheal aspirates may have originated from the alveoli. Nevertheless, minimal and maximal surface tensions were higher in Surf(TrachAsp) than in Surf(BAL) and Surf(P27000). 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 Surf(TrachAsp). We conclude that as compared with alveolar surfactant, PL of Surf(TrachAsp) show the same composition, but that surface-tension function is impaired and the concentration of surfactant proteins is decreased in Surf(TrachAsp).