ALVEOLAR SURFACTANT AGGREGATE CONVERSION IN VENTILATED NORMAL AND INJURED RABBITS

Alveolar surfactant can be separated into two subtypes; large aggregat es and small aggregates. Large aggregates represent the surface active form of surfactant and are the metabolic precursors of small aggregat es. Previous studies examined the mechanism by which large aggregates are converted into small aggregates in vitro. We used intratracheal in jection of radiolabeled large aggregates in rabbits to probe the aggre gate conversion in vivo. After this injection, animals were mechanical ly ventilated for 60 min. After the animals were billed, the lungs wer e lavaged, and the percentage of radiolabel present in the small aggre gate fraction was determined. Our results showed that ventilation resu lted in aggregate conversion and that increases in tidal volume, but n ot in respiratory rate, correlated with increased conversion. Aggregat e conversion in rabbits with acute lung injury correlated significantl y with severity of injury. We conclude that a change in surface area ( i.e., respiration) is necessary for aggregate conversion in vivo and t hat the ventilation strategy can affect this conversion. Furthermore, increased aggregate conversion in injured lungs might contribute to in creased small-to-large aggregate ratios in these lungs compared with n ormal lungs.