Convertase activity in alveolar surfactant and lamellar bodies in fetal, newborn, and adult rabbits

Conversion of heavy-aggregate alveolar surfactant (H) to a light-aggregate, nonsurface active form (L) is believed to involve the activity of an enzym e, namely, convertase. This conversion can be reproduced in vitro by the su rface-area cycling technique. The purpose of the present study was to use t his technique to investigate the developmental aspects of convertase activi ty in fetal, newborn, and adult rabbits. H was isolated from alveolar lavag e from term [31-day gestation (31d)] fetal rabbit pups, 1-, 4-, and 7-day-o ld newborns, and adults, and the percent conversion to L was determined. To assess lamellar bodies (LB) as a potential source of activity in this spec ies, these structures were isolated from lung tissue of 27-day-gestation (2 7d) and 31d fetuses, 1-, 4-, and 7-day-old newborns, and adults and were cy cled the same as for H. LB contained considerable activity at each developm ental stage i.e., similar to 82% of a 27d LB preparation converted to L aft er 3 h of cycling. In the adult, this value was 78%. Very little conversion of H was obtained from fetal lung (i.e., <20% of the 31d fetal preparation converted to L), but, by postnatal day 4, this value was greatly increased (i.e., >80% conversion) and stayed elevated to adulthood. The activity for each H and LB fraction was temperature and concentration dependent and dim inished with storage at 4 degrees C. These data suggest the LB as the sourc e of convertase activity in the rabbit and demonstrate dramatic development al changes in this activity after release of the LB contents to the alveoli .