Phase Transitions in Films of Lung Surfactant at the Air-Water Interface

Abstract

Pulmonary surfactant maintains a putative surface-active film at the air-alveolar fluid interface and prevents lung collapse at low volumes. Porcine lung surfactant extracts (LSE) were studied in spread and adsorbed films at 23 +/- 1 degrees C using epifluorescence microscopy combined with surface balance techniques. By incorporating small amounts of fluorescent probe 1-palmitoyl-2-nitrobenzoxadiazole dodecanoyl phosphatidylcholine (NBD-PC) in LSE films the expanded (fluid) to condensed (gel-like) phase transition was studied under different compression rates and ionic conditions. Films spread from solvent and adsorbed from vesicles both showed condensed (probe-excluding) domains dispersed in a background of expanded (probe-including) phase, and the appearance of the films was similar at similar surface pressure. In quasistatically 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 finely 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 bean shapes, and their distribution was similar to those seen previously in films of dipalmitoylphosphatidylcholine (DPPC), the major component of surfactant. Rapid cyclic compression and expansion of LSE films resulted 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 the domain distribution, decreasing the size and increasing the number and total amount of condensed phase domains. Calcium also caused an increase in the value of pi at which the maximum amount of independent condensed phase domains were observed to 45 mN/m. It also induced formation of large amounts of novel, nearly circular domains containing probe above pi of 50 mN/m, these domains being different in appearance than any seen at lower pressures with calcium or higher pressures in the absence of calcium. Surfactant protein-A (SP-A) adsorbed from the subphase onto solvent-spread LSE films, and aggregated condensed domains in presence of calcium. This study indicates that spread or adsorbed lung surfactant films can undergo expanded to condensed, and possibly other, phase transitions at the air-water interface as lateral packing density increases. These phase transitions are affected by divalent cations and SP-A in the subphase, and possibly by loss of material from the surface upon cyclic compression and expansion.

Anionic Pulmonary Surfactant Phospholipids Inhibit Inflammatory Responses from Alveolar Macrophages and U937 Cells by Binding the Lipopolysaccharide-interacting Proteins CD14 and MD-2

“…It is not yet known whether POPG can exist in segregated domains within the surfactant monolayer or within the alveolar hypophase present in the lung. However, biophysical studies provide good evidence that DPPC can exist in distinct domains in the surfactant layer, and thus, it is plausible that POPG could also be present in segregated domains (48). In our studies, the highest levels of POPG or PI used are only 10% of the levels found in the alveolar compartment of the lung.…”

Lipid compositional analysis of pulmonary surfactant monolayers and monolayer-associated reservoirs

“…It is recognized that spread and adsorbed films containing gel phase phospholipids can spontaneously form liquid condensed domains enriched in DPPC (54,(62)(63)(64) [as reviewed in ref. (19)].…”

Synthetic Pulmonary Surfactant Preparations: New Developments and Future Trends

“…However, cholesterol is well recognized as a component that provides mechanical stability to cell membranes [48] and an important structural element to modulate their lateral structure, including segregation of specialized domains such as lipid rafts [49,50]. Recent results have shown that physiological proportions of cholesterol modulate the lateral organization and distribution of proteins and lipids in pulmonary surfactant membranes [51] and films [52,53], as well as the dynamical behaviour of surfactant films subjected to compression-expansion cycling [54]. Presence of cholesterol could be particularly important to permit surfactant films to reach and sustain the lowest surface tensions at 100% humidity [55], a condition thought to exist in the alveolar spaces but poorly reproduced in in vitro models.…”