Role of cholesterol in pulmonary surfactant at ambient and physiological temperatures.

Abstract

Pulmonary surfactant is the mixture of lipids and proteins that lowers surface tension in the lungs. The material forms a thin film on the aqueous layer that lines the alveolar air-sacks. When compressed by the shrinking alveolar surface area during exhalation, the surfactant film achieves very low surface tensions. Our studies used monolayers of DPPC with variable amounts of cholesterol as a simple model of the alveolar film. A widely held supposition contends that, relative to the multilamellar vesicles secreted by the type II pneumocytes, the alveolar film is greatly enriched in DPPC. We have recently published that structural changes by which cholesterol causes faster collapse of DPPC monolayers and determined the structure of calf lung surfactant extract (CLSE) and DPPC/cholesterol mixed films with Xchol = 0.0-0.4. This work is an extension of this effort, but at physiological temperature of 37⁰C. Here, we compare the structure of CLSE and DPPC/cholesterol mixtures with varying Xchol at ambient and physiological temperatures. We have used grazing incidence X-ray off-specular scattering (GIXOS) and grazing incidence X-ray diffraction (GIXD) to obtain the detailed structural data of the interfacial films. GIXOS measurements showed that CLSE remains a single monolayer at surface pressures from 15mN/m to 45 mN/m, contrary to popular predictions that pulmonary surfactant must form a multilayer at low surface pressures. Both CLSE and DPPC/cholesterol films at 37C produced a single diffraction peak, indicating little change in the lattice-parameters of the ordered film. Predictably, with heating both CLSE and DPPC/cholesterol films showed the significant broadening of the GIXD peak and the dramatic decrease in its maximum scattered intensity, indicating a radical loss of the long range order among the hydrocarbon chains and the extensive conversion of ordered to disordered structures.