An examination of the different variables affecting surfactant aggregate conversion in vitro

Pulmonary surfactant exists in 2 major subtypes, the freshly secreted, surface-active large surfactant aggregates (LA) and their metabolic product, the less surface active, small aggregates (SA). Conversion of LA into SA can be studied using an in vitro technique, surface area cycling, which involves the rotation of a suspension of LA end-over-end so that the surface area of the liquid changes twice each cycle. In order to further elucidate the mechanisms involved in aggregate conversion, we have examined the effects of time, temperature, change in surface area, cycling speed, surfactant concentration, and albumin on aggregate conversion in vitro. Three different surfactant preparations were used; rabbit LA, sheep LA, and an exogenous surfactant preparation, bovine lipid extract surfactant (BLES). Based on our results that showed that these variables affected aggregate conversion, we concluded that the adsorption of surfactant at the changing air-liquid interface was an important step in aggregate conversion. However, the results also indicated that aggregate conversion was not solely due to the adsorption characteristics of the surfactant. Other surfactant properties, such as the activity of serine protease or film stability at the air-liquid interface, may also be important in aggregate conversion.