[Roles of surfactant protein A (SP-A) and phospholipids on regulation of surfactant secretion from alveolar type II epithelial cells].

Surfactant protein A (SP-A) is known to inhibit surfactant secretion from pulmonary alveolar type II epithelial cells (type II cells). It is unknown whether SP-A acts as an inhibitor in the presence of lipids since SP-A mostly exists complexed with surfactant lipids. In this paper we focused on this issue using purified surfactant, purified SP-A and synthesized lipid liposomes. Surfactant secretion from type II cells was evaluated as TPA-stimulated secretion of radiolabeled lipids. The results demonstrated that 1) the inhibition of lipiD secretion by purified surfactant depended on its lipid components, not on the SP-A component, 2) the inhibitory effect of SP-A purified from surfactant was reversed in the presence of lipid liposomes, and 3) SP-A free from surfactant was unable to inhibit lipid secretion. We conclude from these results that SP-A can potentially inhibit surfactant secretion, but surfactant secretion is mainly regulated by surfactant lipids.