Dynamic surface activity of films of lung surfactant phospholipids, hydrophobic proteins, and neutral lipids

Abstract

Surface pressure-area (pi-A) isotherms during dynamic cycling were measured for films of dipalmitoyl phosphatidylcholine (DPPC) and column-separated fractions of calf lung surfactant extract (CLSE). Emphasis was on defining the relative importance of lung surfactant phospholipids (PPL), neutral lipids (N), and hydrophobic proteins (SP) in facilitating dynamic respreading and surface tension lowering within the interfacial film itself. Solvent-spread films in a Wilhelmy balance were studied at 23 degrees and 37 degrees C over a range of cycling rates for initial concentrations giving both monomolecular and surface-excess films. A striking finding was that PPL films containing the complete mix of surfactant phospholipids had greatly improved dynamic respreading compared to DPPC, particularly in surface excess films (30 and 15 Angstrum 2/molecule). Hydrophobic SP gave an additional increase in dynamic respreading in SP&PL compared to PPL films for initial concentrations of 60, 30, and 15 Anstrum 2/molecule. Neutral lipids also improved respreading slightly in N&PL versus PPL films, but maximum surface pressures in N&PL films at 37 degrees C were consistently the lowest of any surfactant subfraction. Spread films of SP&PL at 60 and 30 Angstrum 2/molecule had lower maximum pressures than PPL, but maximum pressures were slightly larger for SP&PL films at high initial concentration (15 Anstrum 2/molecule). Supplementary oscillating bubble studies involving both adsorption and film dynamics at rapid cycling rate (20 cycles/min) showed that dispersions of CLSE and SP&PL lowered surface tension to < 1 mN/m, while PPL and N&PL had elevated minimums of 21 mN/m. These results show that secondary surfactant phospholipids in addition to DPPC are important in the film behavior of pulmonary surfactant, giving improved respreading and overall pi-A isotherms very different from disaturated phospholipids. Hydrophobic SP also increase respreading in the interfacial film, in addition to their known action in increasing surfactant adsorption. SP may also improve film stability at high interfacial concentrations of phospholipid, although they were destabilizing in more dilute films. Neutral lipids contributed minor increases in surfactant respreading, but were consistently detrimental to surface tension lowering.