LATERAL PHASE-SEPARATION IN INTERFACIAL FILMS OF PULMONARY SURFACTANT

To determine if lateral phase separation occurs in films of pulmonary surfactant, we used epifluorescence microscopy and Brewster angle micr oscopy (BAM) to study spread films of calf lung surfactant extract (CL SE). Both microscopic methods demonstrated that compression produced d omains of liquid-condensed lipids surrounded by a liquid-expanded film , The temperature dependence of the pressure at which domains first em erged for CLSE paralleled the behavior of its most prevalent component , dipalmitoyl phosphatidylcholine (DPPC), although the domains appeare d at pressures 8-10 mN/m higher than for DPPC over the range of 20-37 degrees C. The total area occupied by the domains at room temperature increased to a maximum value at 35 mN/m during compression. The area o f domains reached 25 +/- 5% of the interface, which corresponds to the predicted area of DPPC in the monolayer, At pressures above 35 mN/m, however, both epifluorescence and BAM showed that the area of the doma ins decreased dramatically. These studies therefore demonstrate a pres sure-dependent gap in the miscibility of surfactant constituents. The monolayers separate into two phases during compression but remain larg ely miscible at higher and lower surface pressures.