Surfactant subtypes in experimental lung damage: radiation pneumonitis

Radiation pneumonitis, a chronic form of adult respiratory distress syndrome (ARDS), is known to be associated with physiological and biophysical abnormalities of the surface element of the lungs suggesting an impairment of the surfactant system. The alveolar surfactant of mice with radiation pneumonitis was fractionated into subtypes on continuous sucrose density gradients to examine their relative amounts, composition, ultrastructure, surface activity, and turnover kinetics. The total phospholipid and protein contents of the alveolar lavage were increased. The proportions of high buoyant density subtypes (normally surface active) were increased about twofold and that of the low buoyant density subtype (not surface active) was decreased or absent. The buoyant densities, ultrastructure, and phospholipid compositions of the major surfactant subtypes were not significantly altered. The surface activity of the normally surface-active subtypes, when purified free of extraneous material, was close to those of normal controls. Turnover studies of the kinetics of surfactant subtype phospholipids suggested increased secretion of surfactant but a delay in the conversion of the heavier subtypes into their low-density product. Most of the heavier material appeared not to enter the lighter pool, in contrast to findings in control mice. It is concluded that in this form of ARDS the surfactant subtypes are qualitatively normal but that their surface activity is impaired, presumably by extraneous material in the alveoli, and that proportions of surfactant subtypes are radically altered by a combination of increased synthesis and decreased metabolism of the heavier subtypes.