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

As birds have tubular lungs that do not contain alveoli, avian surfactant p redominantly 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 surfac tant 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. Phosp hatidylcholine molecular species of the various surfactants were measured b y HPLC. Compared with pig surfactant, both bird surfactants were enriched i n dipalmitoylphosphatidylcholine, the principle surface tension-lowering ag ent in surfactant, and depleted in palmitoylmyristoylphosphatidylcholine, t he other disaturated phosphatidylcholine of mammalian surfactant. Surfactan t protein (SP)-A was determined by immunoblot analysis, and SP-B and SP-C w ere determined by gel-filtration HPLC. Neither SP-A nor SP-C was detectable in either bird surfactant, but both preparations of surfactant contained S P-B. Surface tension function was determined using both the pulsating bubbl e surfactometer (PBS) and capillary surfactometer (CS). Under dynamic cycli ng 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 equall y 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.