Dynamic Surface Properties of Pulmonary Surfactant Proteins SP-B and SP-C and Their Mixtures with Dipalmitoylphosphatidylcholine

Dynamic cyclic surface pressure (pi)-area measurements were performed on surfactant proteins SP-B and SP-C in pure and binary spread films with dipalmitoylphosphatidylcholine (DPPC). When the films of pure SP-B and SP-C were compressed beyond their collapse points (about 40 mN m-1), no appreciable irreversible loss of material occurred and the successive compression isotherms were reproducible. A similar reversible collapse for the proteins was observed when their binary films with DPPC were compressed up to high surface pressures (pi approximately 65 mN m-1), which did not surpass the collapse for DPPC (about 72 mN m-1). In this case, SP-B, squeezed out at 50 mN m-1 during compression of the SP-B/DPPC monolayers that contained > or = 10 weight % protein, reinserted in the films during their subsequent expansion. Likewise, SP-C-DPPC complexes were reversibly excluded at pi approximately 55 mN m-1 from the SP-C/DPPC films that contained > or = 5 weight % protein. Dynamic compression of the mixed protein-lipid films beyond the collapse pressure of DPPC showed that SP-B and SP-C improved the respreading of DPPC in a concentration dependent manner. SP-B was more effective in promoting the respreading of DPPC than was SP-C, as indicated by the collapse plateau length ratio criterion. The results from this study suggest a possible interfacial role for SP-B and SP-C in lipid replenishment at the alveolar-air interface, through enhancement of the respreading of DPPC collapse phases (SP-B and SP-C) or through reversible removal of phospholipid (SP-C) during dynamic cyclic compression-expansion of the alveolar surface.