RECYCLING OF SURFACTANT LIPID AND APOPROTEIN-A STUDIED BY ELECTRON-MICROSCOPIC AUTORADIOGRAPHY

Recycling of lipid and apoprotein components by lung epithelium appear s to be a key part of the metabolism of surfactant, but many details o f this process are poorly understood, including its efficiency and whi ch intracellular pathways participate. We chose electron microscopic a utoradiography to track time-dependent changes in the intracellular lo cation of [methyl-H-3]choline dipalmitoyl phosphatidylcholine or I-125 -labeled apoprotein-A that were incorporated into liposomes and instil led intratracheally into rats or added to cultures of freshly isolated rat lung type II cells. At times from 2 to 120 min after the start of labeling, lungs or cells were fixed and processed for autoradiography . We found a time-dependent uptake of lipid and apoprotein by type II cells and by macrophages but not by other lung cell types. There was a time-dependent incorporation of lipid and apoprotein label into lamel lar bodies, but the labels appeared in other intracellular compartment s without a convincing time-dependent relationship to lamellar bodies. Although all type II cell organelles received some lipid-bound label, the apoprotein-bound label was not found within electron-dense multiv esicular bodies (d-MVB). Selective lack of apoprotein label in d-MVB i ndicated a segregation of lipid and apoprotein labels and suggested a unique role for the d-MVB, possibly as a degradation pathway. Our resu lts also provided conclusive morphological evidence of surfactant lipi d and apoprotein recycling by type II cells.