ASSOCIATION OF SURFACTANT PROTEIN-C WITH ISOLATED ALVEOLAR TYPE-II CELLS

Surfactant protein C (SP-C) is a small. hydrophobic protein that is sy nthesized and secreted by alveolar type II cells. The mechanism of cle arance of SP-C from the alveolar airspace is not well understood, alth ough previous studies demonstrated that recombinant SP-C instilled int o the lungs of spontaneously breathing anaesthetized rats was taken up by type II cells and incorporated into lamellar bodies. The current i nvestigation was undertaken to characterize the interaction of a compl ex of SP-C and surfactant-like lipids with freshly isolated rat alveol ar type II cells under conditions in which the extracellular milieu ca n be regulated. SP-C was isolated from alveolar proteinosis lavage flu id and radiolabeled with I-125-Bolton-Hunter reagent. The radiolabeled protein retained its ability to facilitate adsorption of phospholipid s to an air/liquid interface. Labeled human SP-C associated with isola ted type II cells in a concentration-dependent manner that was also de pendent upon temperature and time. The association of labeled SP-C wit h isolated type II cells did not saturate up to 150 mu g/ml. SP-A sign ificantly enhanced the association of SP-C with isolated type II cells . Under the experimental conditions tested, SP-C was not degraded to T CA-soluble products. These results are consistent with the hypothesis that association or uptake of SP-C by type II, cells may be enhanced b y SP-A and that like SP-A, SP-C is recycled by type II cells.