Surface activity in vitro: role of surfactant proteins

dPattle, who provided some of the initial direct evidence for the presence of pulmonary surfactant in the lung, was also the first to show surfactant was susceptible to proteases such as trypsin. Pattle concluded surfactant w as a lipoprotein, Our group has investigated the roles of the surfactant pr oteins (SP-) SP-A, SP-B, and SP-C using a captive bubble tensiometer. These studies show that SP-C > SP-B > SP-A in enhancing surfactant lipid adsorpt ion (film formation) to the equilibrium surface tension of similar to 22-25 mN/m from the 70 mN/m of saline at 37 degreesC, In addition to enhancing a dsorption, surfactant proteins can stabilize surfactant films so that later al compression induced through surface area reduction results in the loweri ng of surface tension (gamma) from similar to 25 mN/m (equilibrium) to valu es near 0 mN/m. These low tensions, which are required to stabilize alveoli during expiration, are thought to arise through exclusion of fluid phospho lipids from the surface monolayer, resulting in an enrichment in the gel ph ase component dipalmitoylphosphatidylcholine (DPPC), The results are consis tent with DPPC enrichment occurring through two mechanisms, selective DPPC adsorption and preferential squeeze-out of fluid components such as unsatur ated phosphatidylcholine (PC) and phospharidylglycerol (PG) from the monola yer, Evidence for selective DPPC adsorption arises from experiments showing that the surface area reductions required to achieve gamma near 0 mN/m wit h DPPC/PG samples containing SP-B or SP-A plus SP-B films were less than th ose predicted for a pure squeeze-out mechanism. Surface activity improves d uring quasi-static or dynamic compression-expansion cycles, indicating the squeeze-out mechanism also occurs. Although SP-C was not as effective as SP -B in promoting selective DPPC adsorption, this protein is more effective i n promoting the reinsertion of lipids forced out of the surface monolayer f ollowing overcompression at low gamma values. Addition of SP-A to samples c ontaining SP-B but not SP-C limits the increase in gamma during expansion. It is concluded that the surfactant apoproteins possess distinct overlappin g functions. SP-B is effective in selective DPPC insertion during monolayer formation and in PG squeeze-out during monolayer compression. SP-A can pro mote adsorption during film formation, particularly in the presence of SP-B . SP-C appears to have a superior role to SP-B in formation of the surfacta nt reservoir and in reinsertion of collapse phase lipids. (C) 2001 Elsevier Science Inc, All rights reserved.