A Direct Test of the "Squeeze-Out" Hypothesis of Lung Surfactant Function. External Reflection FT-IR at the Air/Wave Interface

Abstract

The current theory of pulmonary surfactant function requires that very low surface tension be achieved and maintained in the alveolar surface film during compression (expiration). To effect this condition, it has been hypothesized that the unsaturated and/or fluid components of surfactant are selectively excluded or "squeezed out" from mixed monolayers containing both saturated and unsaturated phospholipids, leaving a surface film of essentially pure 1,2-dipalmitoylphosphatidylcholine (DPPC). External reflection Fourier transform infrared (FT-IR) spectroscopy has been employed to quantitatively test this hypothesis. Mixed monolayer films of acyl chain-perdeuterated 1,2-dipalmitoylphosphatidylcholine (DPPC-d62) with 1,2-dioleoylphosphatidylglycerol (DOPG), 1-palmitoyl, 2-oleoylPG (POPG), 1,2-dipalmitoylPG (DPPG) were examined in situ at the air/water interface as a function of surface pressure. The relative intensities of CD2 (CH2) stretching vibrations of the deuterated (proteated) components permitted quantitative determination of the relative concentrations of each in the film. For 7:1 (mol:mol) mixtures of DPPC-d62/DOPG, progressive, selective squeeze out of up to about 90% of the PG component is observed over a range of surface pressures from about 51 to 68 mN/m. The extent of maximal PG squeeze out was reduced to 61% for a 7:1 (mol:mol) mixture of DPPC-d62/POPG. This phenomenon, which is at least partially reversible, appears to require relatively high rates of film compression. Squeeze out was reduced (< 20%) for 7:1 (mol:mol) mixtures of DPPC-d62/DPPG or for 7:3 mixtures of DPPC-d62/POPG. Squeeze out requires that the lipid mixture achieve surface pressures greater than about 50-60 mN/m along with unsaturation (or at least conformational disorder) in the acyl chains of the non-DPPC component.(ABSTRACT TRUNCATED AT 250 WORDS)

The Pulmonary Surfactant System: Biochemical and Clinical Aspects

“…This process may occur either by selective insertion of DPPC during adsorption or by selective exclusion of other components of the surface film during reduction of the surface area. Evidence of the latter possibility, which results in the formation of different types of remnants, has been provided [245,276]. During the next inhalation, and expansion of the surface area of the alveoli, the hydrophobic surfactant proteins improve the respreading of lipids [234,303].…”

Reflection–absorption FT-IR spectroscopy of pentadecanoic acid at the air/water interface

“…RA bands may be positive or negative depending on the polarisation of the incident radiation, the proximity of the angle of incidence to the Brewster angle, the direction of the change in dipole moment during the normal mode of vibration [34], and the complex refractive index of the substrate [1,6,[35][36][37]. Negative absorbance bands have been observed by Dluhy and coworkers [1][2][3]5,10] from phospholipid monolayers at the A/W interface.…”

Molecular Structures and Interactions of Pulmonary Surfactant Components

“…This implies that more fluid, non-Pam,GroPCho, components which are unable to resist high surface pressures, are selectively squeezed out of the surface layer. It has been shown in vitro that unsaturated PtdGro species are actually squeezed out of binary mixtures with Pam,-GroPCho upon compression at an aidliquid interfacial layer (Pastrana-Rios et al, 1994).…”