The interactions between pulmonary surfactant and meconium have been previously reported, however, no prior study has paid attention to the effect of meconium on the extracellular surfactant metabolism. We studied the effect of meconium on the rate of surfactant subtype conversion from surface-active large surfactant aggregates to inactive small surfactant aggregates using the in vitro surface area cycling method. Human meconium was added to a suspension of large surfactant aggregates isolated from swine lung lavage fluid and the mixture was cycled in a capped polystyrene tube at 37 degrees C. The conversion rate from large to small surfactant aggregates was dependent upon the amount of meconium added. By sufficiently increasing the surfactant concentration, meconium could not significantly affect the surfactant subtype conversion. The conversion rate was temperature-dependent and was slower at 4 degrees C. Heat treatment of meconium diminished the rate of conversion. The potent effect of meconium on surfactant subtype conversion appears to be related to an enzymatic protein containing a serine active site, since the conversion occurred at 37 degrees C and not at lower temperatures and the conversion was blocked in the presence of diisopropyl fluorophosphate.
Conclusion: Our study suggests that meconium can accelerate surfactant subtype conversion. We believe that the increased rate of conversion caused by meconium is a new mechanism for surfactant inactivation that could occur with lung injuries associated with meconium aspiration syndrome.