Pulmonary Surfactant Reduces Surface Tension to Low Values near Zero through a Modified Squeeze-Out Mechanism

Abstract

Pulmonary surfactant stabilizes the lung by reducing surface tension (ST) to low values near zero at end expiration. Atomic force microscopy studies revealed that, as surface pressure increases, spread surfactant films initially form solid micro- and nanodomains and then generate 3-5 multilayers which apparently remain associated with the surface monolayer. Time-of-flight Secondary Ion Mass Spectroscopy analyses revealed selective squeeze-out must occur because the multilayers are enriched in unsaturated fluid phospholipids (PL) while the remaining monolayer becomes enriched in disaturated PL. Taken together, these results are consistent with a modified squeeze-out model where surfactant vesicles interact with the air-water interface through surfactant proteins B- and C-containing adsorption/fusion pores. PL migration onto the surface results in vesicle instability generating a monolayer at equilibrium surface pressure, which remains functionally associated with excess bilayer material. Initially, the monolayer and associated reservoir have similar composition, but film compression causes fluid PL to migrate through the fusion pores into the multilayers. Gel phase PL are restricted because they are sequestered in micro- or nanodomains. This process results in monolayers highly enriched in disaturated PL which reduce ST to near zero. Film expansion allows fluid PL to regain the surface through the fusion pores. This mechanism explains both the rapid reincorporation of surfactant PL into the monolayer during adsorption and during film expansion and the progressive improvement in surface activity during repeated compression.