Preferential uptake of small-aggregate fraction of pulmonary surfactant in vitro

Homeostasis of pulmonary surfactant requires metabolic clearance of surfactant forms with decreased surface activity. Rabbit pulmonary surfactant was labeled in vivo with rhodamine-labeled dipalmitoylphosphatidylethanolamine (R-DPPE), isolated, and fractionated into large- and small-aggregate subfractions by differential centrifugation. Endocytosis of large (LA)- and small (SA)-aggregate surfactant by a mouse lung epithelial cell line (MLE-12) was evaluated in vitro by epifluorescence microscopy. More SA than LA surfactant was taken up by MLE-12 cells. Endocytosis of SA and LA surfactant was inhibited by preincubation of the subfractions with surfactant protein A and 3.3 mM Ca2+. The difference in uptake between SA and LA surfactant was lost for reconstituted organic extracts of the subfractions. Much of the difference in uptake of SA and LA surfactant may be attributed to the greater concentration of surfactant protein A in LA surfactant.