Effect of Aqueous Glycerol Subphases on DPPC:POPG 7:3 Packing

Most model lung surfactant (LS) monolayer (DPPC:POPG 7:3) work is done at room temperature, far below the physiological 37C. The reason for this is often that in addition to the experimental difficulties of working at higher temperatures, also model LS monolayers are highly fluid and unstable at these temperatures. Fluid monolayers collapse by flowing into the subphase through vesicles or disks that ultimately separate from the surface and cause lipid depletion. It is clear that within the lungs, an effective surfactant replacement therapy requires the ability to remain at the surface through many compression cycles, which is only possible if fluid collapse does not occur. Furthermore, it is unlikely that nature would create such a fluid system where after each breathing cycle new material would need to be respread at the surface. We are exploring the possibility that in the lungs, the effective fluidity of the surfactant monolayer is decreased by subphase viscosity. Several observations point to the fact that the alveolar lining fluid (ALF), upon which the lung surfactant monolayer rests, does not have the dynamic viscosity of bulk water. First, the ALF is only 15 microns thick, such that the effective viscosity would be dominated not by fluid bulk properties but rather thin layer effects that increase the effective viscosity. Second, the chemical composition of ALF includes biopolymers that potentially increase its dynamic viscosity. And third, theoretical hydrodynamic calculations of pulmonary cleansing predict an ALF dynamic viscosity roughly an order of magnitude greater than bulk water [1].