Incorporation of biotinylated SP-A into rat lung surfactant layer, type IIcells, and Clara cells

The goal of this study was to compare the functions of Clara and type II ce lls during alveolar clearance and recycling of surfactant protein (SP) A, a secretory product of both cell types. We examined the incorporation of ins tilled biotinylated SP-A (bSP-A) into rat lung type II and Clara cells as a measure of clearance and recycling of the protein. Ultrastructural localiz ation of bSP-A was accomplished by an electron-microscopic immunogold techn ique at 7, 30, and 120 min after intratracheal instillation. Localization o f bSP-A was quantitatively evaluated within extracellular surfactant compon ents (lipid-rich forms: myelin figures, vesicles, and tubular myelin; and l ipid-poor hypophase) and in compartments of type II and Clara cells. bSP-A was incorporated into myelinic and vesicular forms of extracellular surfact ant, but tubular myelin and hypophase had little bSP-A. Lamellar bodies of type II cells demonstrated a significant time-dependent increase in their i ncorporation of bSP-A. There was a concentration of bSP-A in the secretory granules and mitochondria of Clara cells, but no Clara cell compartment sho wed a pattern of time-dependent change in immunolabeling. Our immunolabelin g data demonstrated a time-dependent movement of exogenous SP-A from extrac ellular components into type II cells and their secretory granules. Clara c ells did not demonstrate a time-dependent incorporation of bSP-A into their secretory granules during the period of this study. If Clara cells recycle SP-A, they must reach a steady state very quickly or very slowly.