Intra- and extracellular compartmentalization of the surface-active fraction in dog lung

Adult mongrel dogs were killed at various times after injection of (3)H-labeled palmitate. The lungs were removed and subjected to an extensive saline lavage. The surface-active fraction was isolated from the lavage and from homogenized residual lung by a procedure based upon differential centrifugation in sucrose solutions. The material isolated from the lavage was designated extracellular surfactant; material from the residual lung was designated intracellular surfactant. Both had similar chemical composition and surface activity. The results of the isotopic labeling studies demonstrate that the two fractions have distinctly different specific activity curves. Label was incorporated into the intracellular surfactant rapidly and reached a peak at 1 hr. No radioactivity was found in the extracellular surfactant for the first 15 min, and the specific activity increased much more slowly than in the intracellular surfactant. These results demonstrate at least two anatomically distinct metabolic "pools" of pulmonary surfactant in the lung. While our data are not conclusive, one possible interpretation is that the biosynthesis of pulmonary surfactant takes place intracellularly with a subsequent secretion onto the alveolar surface.