Dipalmitoylphosphatidylcholine and cholesterol in monolayers spread from adsorbed films of pulmonary surfactant

Pulmonary surfactant forms a surface film that consists of a monolayer and a monolayer-associated reservoir. The extent to which surfactant components including the main component, dipalmitoylphosphatidylcholine (DPPC), are a dsorbed into the monolayer, and how surfactant protein SP-A affects their a dsorptions, is not clear. Transport of cholesterol to the surface region fr om dispersions of bovine lipid extract surfactant [BLES(chol)] with or with out SP-A at 37 degreesC was studied by measuring surface radioactivities of [4-C-14] cholesterol-labeled BLES(chol), and the Wilhelmy plate technique was used to monitor adsorption of monolayers. Results showed that transport of cholesterol was lipid concentration dependent. SP-A accelerated lipid a dsorption but suppressed the final level of cholesterol in the surface. Sur factant adsorbed from a dispersion with or without SP-A was transferred via a wet filter paper to a clean surface, where the surface radioactivity and surface tension were recorded simultaneously. It was observed that 1) surf ace radioactivity was constant over a range of dispersion concentrations; 2 ) cholesterol and DPPC were transferred simultaneously; and 3) SP-A limited transfer of cholesterol. These results indicate that non-DPPC components o f pulmonary surfactant can be adsorbed into the monolayer. Studies in the t ransfer of [1-C-14]DPPC-labeled BLES(chol) to an equal or larger clean surf ace area revealed that SP-A did not increase selective adsorption of DPPC i nto the monolayer. Evaluation of transferred surfactant with a surface bala nce indicated that it equilibrated as a monolayer. Furthermore, examination of transferred surfactants from dispersions with and without prespread BLE S(chol) monolayers revealed a functional contiguous association between ads orbed monolayers and reservoirs.