Surface Dilatational Behavior of Pulmonary Surfactant Components Spread on the Surface of a Pendant Drop. 1. Dipalmitoyl Phosphatidylcholine and Surfactant Protein C

Surface rheological characteristics of the components that are part of the film floating on the alveolar lining fluid in mammalian lungs are not well established. We measured the surface dilatational elasticity and viscosity of two of these pulmonary surfactant components, dipalmitoyl phosphatidylcholine (DPPC) and pulmonary surfactant protein C (SP-C). Pure DPPC or DPPC with 2 mol % SP-C was spread on the surface of a pendant phosphate buffered saline drop. Harmonic drop oscillation experiments were carried out below the surface pressure of protein squeeze-out in a frequency range of 0.006−0.25 Hz and at 20, 30, and 40 °C. We found that the dilatational elasticity of DPPC and DPPC/SP-C films increased up to a surface pressure of 40−45 mN/m. The dilatational viscosity of DPPC and DPPC/SP-C films dropped in the frequency range of human respiration (0.4−0.2 Hz). Dilatational elasticity and viscosity decrease with temperature. We conclude that films containing the pulmonary surfactant components DPPC and SP-C are in general viscoelastic but at breathing frequencies spontaneously elastic. SP-C stabilizes DPPC films, forming an alveolar lining layer of low compressibility at surface pressures below the pressure of protein squeeze-out and of high compressibility in the squeeze-out region.