Conversion of newly released, heavy-aggregate alveolar surfactant (H) to a light-aggregate, non-surface active form (L) in vivo is believed to involve activity of an enzyme “convertase”. This conversion can be reproduced in vitro by the surface area cycling technique. The purpose of the present study was to determine the developmental aspects of this activity in fetal and newborn rabbits.

H was isolated as a pellet from alveolar lavage returns (ALR) by centrifugation at 10,000 × g for 30 min from term (31 d) fetal rabbit pups, 4 d and 7 d newborns and adults, cycled for 180 min at 37°C and the percent conversion to L determined. To assess lamellar bodies as a potential source of this activity, these structures were isolated from the post-lavaged lung tissue of the 31 d fetal, 4 d and 7 d newborns, adults and cycled as for H. Lamellar bodies from unlavaged 27d fetal pups were included in this portion of the study.

At each developmental stage studied, lamellar bodies contained considerable activity i.e. approximately 82% conversion in the 27 d fetal preparation and 78% in the adult. There was very little conversion of H obtained from fetal lung i.e. less than 20% conversion in the 31d fetal preparation but by the 4th postnatal day this activity was greatly increased (i.e. > 80% conversion) and stayed elevated to adulthood. The activity for each fraction was temperature and concentration dependent and diminished with storage at 4°C.

These data suggest that lamellar bodies may be the source of“convertase” activity in the rabbit and that there are developmental changes in this activity following release of the lamellar body contents to the alveoli. The low rate of conversion of H in term fetal rabbits may prolong the availability of this metabolically active form of alveolar surfactant and thus contribute to the superior response often observed in preterm newborn rabbit pups treated with this surfactant preparation.(Supported by Grace Maternity Hospital Foundation.)