PHASE-TRANSITIONS IN FILMS OF LUNG SURFACTANT AT THE AIR-WATER-INTERFACE

Pulmonary surfactant maintains a putative surface-active film at the a ir-alveolar fluid interface and prevents lung collapse at low volumes. Porcine lung surfactant extracts (LSE) were studied in spread and ads orbed films at 23 +/- 1 degrees C using epifluorescence microscopy com bined with surface balance techniques. By incorporating small amounts of fluorescent probe 1-palmitoyl-2-nitrobenzoxadiazole dodecanoyl phos phatidylcholine (NBD-PC) in LSE films the expanded (fluid) to condense d (gel-like) phase transition was studied under different compression rates and ionic conditions, Firms spread from solvent and adsorbed fro m vesicles both showed condensed (probe-excluding) domains dispersed i n a background of expanded (probe-including) phase, and the appearance of the films was similar at similar surface pressure. In quasistatica lly compressed LSE films the appearance of condensed domains occurred at a surface pressure (pi) of 13 mN/m. Such domains increased in size and amounts as pi was increased to 35 mN/m, and their amounts appeared to decrease to 4% upon further compression to 45 mN/m. Above pi of 45 mN/m the LSE films had the appearance of filamentous materials of fin ery divided dark and light regions, and such features persisted up to a pi near 68 mN/m. Some of the condensed domains had typical kidney be an shapes, and their distribution was similar to those seen previously in films of dipalmitoylphosphatidylcholine (DPPC), the major componen t of surfactant. Rapid cyclic compression and expansion of LSE films r esulted in features that indicated a possible small (5%) loss of fluid components from such films or an increase in condensation efficiency over 10 cycles. Calcium (5 mM) in the subphase of LSE films altered th e domain distribution, decreasing the size and increasing the number a nd total amount of condensed phase domains. Calcium also caused an inc rease in the value of pi at which the maximum amount of independent co ndensed phase domains were observed to 45 mN/m. It also induced format ion of large amounts of novel, nearly circular domains containing prob e above pi of 50 mN/m, these domains being different in appearance tha n any seen at lower pressures with calcium or higher pressures in the absence of calcium. Surfactant protein-A (SP-A) adsorbed from the subp hase onto solvent-spread LSE films, and aggregated condensed domains i n presence of calcium. This study indicates that spread or adsorbed lu ng surfactant films can undergo expanded to condensed, and possibly ot her, phase transitions at the air-water interface as lateral packing d ensity increases. These phase transitions are affected by divalent cat ions and SP-A in the subphase, and possibly by loss of material from t he surface upon cyclic compression and expansion.