Secretion of surfactant protein A and phosphatidylcholine from type II cells of human fetal lung

Surfactant protein A (SP-A) appears to have a role in lung immune defense as well as generation and metabolism of the alveolar surface-active film. Previous studies indicated that lamellar bodies isolated from lung tissue had a relatively low content of SP-A and that exogenous SP-A was needed for rapid formation of a surface-active film in vitro. We therefore tested the hypothesis that SP-A was secreted from type II cells primarily by a pathway separate from lamellar bodies. Cells were isolated from explants of human fetal lung that had been cultured with hormones to promote differentiation of type II cells, and secretion of surfactant lipid and SP-A were compared. Cultured cells secreted labeled phosphatidylcholine in a nearly linear fashion for 48 h. Basal secretion of SP-A, assayed by enzyme-linked immunosorbent assay, was linear for only 12 h after plating of cells; during this time, there was no accumulation of intracellular SP-A. Addition of secretagogues (phorbol ester, calcium ionophore, and beta-adrenergic agonist) stimulated phosphatidylcholine secretion approximately 4-fold. In contrast, the secretion rate of SP-A was not significantly affected by secretagogues. These findings indicate that a relatively small amount of secreted SP-A (approximately 10%) is released with lamellar bodies. Most SP-A is released by constitutive secretion and may be important for both surfactant- and nonsurfactant-related functions.