A captive bubble method reproduces the in situ behavior of lung surfactant monolayers

We tested a new captive bubble surface tensiometer with films adsorbed from aqueous suspensions of rabbit lung surfactant and a bovine lung surfactant lipid extract and with films of dipalmitoyl-sn-3-glycerophosphorylcholine (DPPC) spread from solvents. The lack of tubes penetrating the bubble surface eliminated potential leakage pathways for the surface film, which was compressed by increasing external pressure. Surface tensions and areas were calculated directly from bubble shapes without the need of pressure measurements. After only one to two compressions, the rabbit surfactant films exhibited the low surface tension, collapse rates, and compressibilities characteristic of the alveolar surface in situ and approached the behavior of spread DPPC films. The bubble "clicking" phenomenon described earlier by Pattle (Proc. R. Soc. Lond. B Biol. Sci. 148: 217-240, 1958) was also reproduced, but only with the bovine extract, which did not perform as well as the rabbit surfactant in surface tests. These findings suggest that surfactant apoprotein SP-A, which was probably present in the rabbit but not the bovine preparations, enhances both adsorption and stability of pulmonary surfactant monolayers.