Role of pulmonary surfactant components in surface film formation and dynamics

Pulmonary surfactant is a mixture of lipids and proteins which is secreted by the epithelial type II cells into the alveolar space. Its main function is to reduce the surface tension at the air/liquid interface in the lung. T his is achieved by forming a surface film that consists of a monolayer whic h is highly enriched in dipalmitoylphosphatidylcholine and bilayer lipid/pr otein structures closely attached to it, The molecular mechanisms of film f ormation and of film adaptation to surface changes during breathing in orde r to remain a low surface tension at the interface, are unknown. The result s of several model systems give indications for the role of the surfactant proteins and lipids in these processes. In this review, we describe and com pare the model systems that are used for this purpose and the progress that has been made. Despite some conflicting results using different techniques , we conclude that surfactant protein B (SP-B) plays the major role in adso rption of new material into the interface during inspiration. SP-C's main f unctions are to exclude non-DPPC lipids from the interface during expiratio n and to attach the bilayer structures to the lipid monolayer. Surfactant p rotein A (SP-A) appears to promote most of SP-B's functions. We describe a model proposing that SP-A and SP-B create DPPC enriched domains which can r eadily be adsorbed to create a DPPC-rich monolayer at the interface. Furthe r enrichment in DPPC is achieved by selective desorption of non-DPPC lipids during repetitive breathing cycles. (C) 2000 Elsevier Science B.V. All rig hts reserved.