Lung surfactant apoprotein SP-A (26-36 kDa) binds with high affinity to isolated alveolar type II cells.

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Abstract
Pulmonary surfactant is synthesized and secreted by alveolar type II cells. These cells recycle surfactant lipids by an internalization process that is enhanced in vitro by the surfactant proteins with molecular masses of 26-36 kDa (SP-A). SP-A also inhibits the secretion of lipid by type II cells. These results suggest that SP-A may play a role in feedback regulation of surfactant pool size and are consistent with the hypothesis that the type II cell surface has receptors for SP-A. The goal of this study is to characterize the binding of radioiodinated SP-A to isolated rat type II cells. Binding of SP-A to type II cells at 4.degree.C has a K1/2 of approximately 5 .times. 10-10 M, is saturable, and is inhibited by excess unlabeled SP-A. Binding is dependent on calcium and is reduced by heat treatment of SP-A. The binding of a proteolytic fragment of SP-A that is produced by collagenase treatment is reduced by excess unlabled SP-A. The binding of the fragment to macrophages and lung fibroblasts is not inhibited by excess unlabeled SP-A. Trypsinization of the type II cell surface reduces the binding of both intact SP-A and the collagenase-resistant fragment. These results show that SP-A binds to type II cells with high affinity and suggest that these cells have receptors that recognize the carboxyl-terminal domain of SP-A.