Effects of pulmonary surfactant proteins SP-B and SP-C and calcium ions on the surface properties of hydrophobic fractions of lung surfactant

This study focused on two hydrophobic fractions (HF-A and HF-B) isolated from porcine lung surfactant (LS) that had similar phospholipid composition, but HF-A consisted of the hydrophobic LS specific proteins (SP-B and SP-C), in contrast to HF-B. Monolayers spread in a Langmuir trough were formed at the air/water interface of both fractions and the rate of adsorption-desorption and the respreading potential of the LS constituents was studied during six consecutive compression/decompression cycles of the monolayers. By drawing a comparison between the behavior of HF-A and HF-B monolayers on the subphase of 150 mM NaCl, either with or without additional Ca2+, we estimated the role of hydrophobic LS proteins and Ca2+ ions for LS surface activity. The results demonstrated much higher ability of the HF-A sample, compared to HF-B, to maintain lower surface tension (gamma) during monolayer compression and its better respreading capacity during decompression. For instance, at a surface concentration corresponding to 80 Angstrom(2) per phospholipid molecule, the HF-A monolayers showed a much lower gamma(max) value (surface tension at 100% of the trough area), being ca. 31.0 mN/m, compared to the HF-B monolayers (gamma(max) congruent to 62.0 mN/m). The surface tension after compression to 20% of the initial area (gamma(min)) reached ca. 7.0 and 19.0 mN/m in the HF-A and HF-B monolayers, respectively. Better respreading of the HF-A monolayers compared to the HF-B monolayers was due to the faster adsorption and spreading of LS phospholipids during decompression, facilitated by the hydrophobic proteins. As the phospholipid composition of both fractions was similar, we showed that the hydrophobic surfactant proteins were responsible also for the prevention of the irreversible loss of material from the surface during monolayer compression/decompression. The effects observed demonstrated also that the hydrophobic surfactant proteins were the stronger determinant, compared with Ca2+ ions, for the surface tension decrease and respreading of the monolayers during film compression/decompression. For instance, when the HF-A monolayers were spread on a subphase with an additional 5 mM Ca2+ ion content, no significant changes were detected in the gamma(min) and gamma(max) values between the first and sixth cycle, compared to the monolayers spread on a subphase of 150 mM NaCl only. However, in the absence of positively charged SP-B and SP-C (HF-B sample) in highly compressed monolayers, Ca2+ ions were able to cause the effects shown by SP-B and SP-C, although to a less extent. The role of the electrostatic and hydrophobic interactions is discussed for the better respreading of LS components in the presence of LS proteins and Ca2+ ions.