Impact of surface tension on the conversion rate of large to small surfactant aggregates

The extracellular alveolar surfactant can be separated into the highly surface active large surfactant aggregates (LA) and the less active small surfactant aggregates (SA). Conversion of LA to SA is encountered upon cyclic surface area changes and demands the presence of enzymatic activity. In the present study we investigated the influence of surface tension on the conversion of LA to SA. Bronchoalveolar lavage fluid (BALF) obtained from healthy rabbits was cycled for various time periods in absence or presence of increasing amounts of serum proteins or oleic acid. LA were isolated, quantified and the minimum surface tension (gamma(min)) of uncycled or cycled LA was assessed in absence or presence of increasing amounts of serum proteins or oleic acid. In additional experiments, already cycled LA with a gamma(min) of approximately 20 mN/m were pooled, diluted to a similar PL concentration as original BALF and cycled a second time. Serum proteins and oleic acid dose-dependently: (i) increased the gamma(min) values of LA when added to the isolated LA; and (ii) decreased the LA to SA conversion when added to BALF. These events were directly correlated, suggesting inhibition of LA to SA conversion by increase in gamma(min) of the LA fraction. In line with this suggestion, already cycled LA displaying poor surface activity showed no further conversion in a second cycling maneuver, whereas LA being similarly prepared from uncycled BALF did. We conclude that LA to SA conversion is inversely correlated with the surface tension of the LA fraction. An increase in gamma(min) of the LA fraction during the cycling procedure blocks further LA to SA conversion and may thus represent a negative feedback mechanism.