REGULATORY MECHANISMS OF SURFACTANT SECRETION

Surfactant secretion is a critical regulated process in the metabolism of pulmonary surfactant. Presumably, because this process is vital to the survival of the organism, there are several independent pathways for stimulating secretion which work through different cell surface re ceptors and signaling mechanisms. In addition, there is apparent homeo static regulation in that two components of surfactant, namely SP-A an d dipalmitoylphosphatidylcholine, can inhibit secretion. Although secr etion of surfactant has been studied for over two decades, there remai ns some important issues to be resolved. In vivo secretion can be stim ulated by hyperventilation or even a single large breath. However, we do not know the biochemical mechanism for this physiologically importa nt form of stimulation. In vitro, we know many of the proximal events in signaling, but we do not know how the lamellar bodies move within a cell or the docking mechanism at the plasma membrane. Many investigat ors have demonstrated that SP-A will inhibit secretion in vitro, but t he mechanism is not known. Finally, there is a route of secretion of S P-A independent of lamellar bodies, but we do not know details of this pathway nor its regulation. (C) 1998 Elsevier Science B.V. All rights reserved.