Surfactant displaces particles toward the epithelium in airways and alveoli.

This study was designed to investigate the early stages of particle deposition on airway and alveolar surfaces. To do this we used morphometric studies of aerosol deposition, in situ measurements of surface tension, and in vitro assays of particle displacement and mathematical modelling. We observed that latex particles, equal or less than 6 microns in diameter deposited in hamster lungs were submerged in the subphase of the alveolar lining layer and became completely coated with an osmiophilic film. Similar results were obtained for particles deposited in the conductive airways which were also covered with a surface active film, having a surface tension of 32 +/- 2 dyn.cm-1. In vitro experiments showed that pulmonary surfactant promotes the displacement of particles from air to the aqueous phase and that the extent of particle immersion depends on the surface tension of the surface active film. The lower the surface tension the greater is the immersion of the particles into the aqueous subphase. Mathematical analysis of the forces acting on a particle deposited on an air-fluid interface show that for small particles (less than 100 microns) the surface tension force is several orders of magnitude greater than forces related to gravity. Thus, even at the relatively high surface tension obtained in the airways (32 +/- 2 dyn.cm-1) particles will still be displaced into the aqueous subphase. Particles in peripheral airways and alveoli likely are below the surfactant film and submerged in the subphase. This may promote clearance by macrophages. In addition, particle displacement into the subphase is likely to increase the contact between the epithelial cell and particle. Toxic or allergenic particles would be available to interact with epithelial cells and this may be important in the pathophysiology of airway disease.