Regulation of lung surfactant secretion.

Secretion of lung surfactant is the direct step in release of the lipoprotein-like product, synthesized in lung epithelial type II cells, onto the alveolar surface. Release of surfactant phosphatidylcholine (PC) proceeds via formation of surface pores during exocytosis of lamellar bodies. Surfactant secretion is regulated locally in the lung by changes in ventilation rate, possibly mediated by distension and altered intracellular pH. Secretion is also stimulated by various agents, including agonists for beta-adrenergic, purinoceptors, and vasopressin receptors and is associated with increased cytosolic Ca2+, cellular adenosine 3',5'-cyclic monophosphate, and activation of protein kinases. Limited studies suggest that secretion of surfactant protein A may be regulated by both cAMP-dependent and protein kinase C-dependent pathways. The integration of these various mechanisms for the in vivo regulation of surfactant secretion remains largely unexplored. Future research into the mechanisms involved in lamellar body fusion with the plasma membrane, role of protein phosphorylation, transient changes in cAMP and Ca2+, and coordination between the secretion of phospholipid and protein components of surfactant should enhance our understanding of secretion of surfactant "lipoprotein."