Localization of calmodulin in differentiating pulmonary type II epithelial cells.
Pulmonary surfactant is synthesized by alveolar type II pneumonocytes and stored in inclusions called lamellar bodies. In the present study we have investigated the role of the calcium-binding protein, calmodulin, in regulating surfactant secretion in differentiating rat fetal type II pneumonocytes. Lamellar body secretion is stimulated in differentiating type II cells in vitro by the calcium ionophore, A23187. A23187-induced secretion is blocked by the phenothiazine drugs, trifluoperazine and chlorpromazine, but is unaffected by the inactive analogs, trifluoperazine sulfoxide and chlorpromazine sulfoxide. Immunofluorescence studies on cultured type II pneumonocytes show that the percentage of Nomarski-dense intracellular granules, which stain positively with anticalmodulin antibody, increases when the cells are stimulated with the calcium ionophore, A23187. Since these Nomarski-dense granules are positively stained by phosphine-3R, these results indicate that increased amounts of immunoreactive calmodulin appear associated with lamellar body surfaces when the cells are stimulated for secretion. In addition, ultrastructural localization of calmodulin on isolated lamellar bodies using protein A-colloidal gold indicates that calmodulin is present on their outer surfaces. The endogenous calmodulin present on the isolated lamellar bodies is not removed by treatment of the lamellar bodies with ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid. Using the same localization technique, we found that isolated lamellar bodies are capable of binding exogenously added calmodulin in the presence of calcium. Taken together, these results implicate calmodulin in pulmonary surfactant secretion.