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

Abstract

Conversion of heavy-aggregate alveolar surfactant (H) to a light-aggregate, nonsurface active form (L) is believed to involve the activity of an enzyme, namely, convertase. This conversion can be reproduced in vitro by the surface-area cycling technique. The purpose of the present study was to use this technique to investigate the developmental aspects of convertase activity in fetal, newborn, and adult rabbits. H was isolated from alveolar lavage from term [31-day gestation (31d)] fetal rabbit pups, 1-, 4-, and 7-day-old newborns, and adults, and the percent conversion to L was determined. To assess lamellar bodies (LB) as a potential source of activity in this species, these structures were isolated from lung tissue of 27-day-gestation (27d) and 31d fetuses, 1-, 4-, and 7-day-old newborns, and adults and were cycled the same as for H. LB contained considerable activity at each developmental stage i.e., approximately 82% of a 27d LB preparation converted to L after 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 diminished with storage at 4 degreesC. These data suggest the LB as the source of convertase activity in the rabbit and demonstrate dramatic developmental changes in this activity after release of the LB contents to the alveoli.

EXOCYTOSIS OF LUNG SURFACTANT: From the Secretory Vesicle to the Air-Liquid Interface

“…At first glance, this hypothesis seems appealing because it provides a new level of regulation for extracellular surfactant metabolism and plausible because it was known previously that LBs contain cytochemically demonstrable enzymes. Indeed, the convertase could be a product of the type II cells, co-secreted with lipids into the ALF (161,162). However, as Williams pointed out earlier (141), a co-secretion of enzymes with their substrates would necessitate that their activity be blocked within the vesicles and reactivated once the material is released.…”

Alveolar epithelial type II cell: defender of the alveolus revisited

“…This process is thought to involve both SP-B [87] and SP-C (for review, see [71,86]). The final step of conversion, from surface-active surfactant into inactive vesicular forms, appears to depend on an AE2-cell-derived enzyme termed convertase [88,89]. …”

Alveolar metabolism of natural vs. synthetic surfactants in preterm newborn rabbits

“…The natural surfactants were isolated from adult and fetal rabbit lungs using differential and density gradient procedures, which have been described in detail in previous reports (21,22,23). We have previously reported that the surfactant preparation isolated by this procedure consists entirely of characteristic lamellar body (LB)-like structures, as evidenced by electron microscopic analysis (22), and is virtually free from contamination by other nonsurfactant membrane components, as determined by marker enzyme assays (22).…”

“…We recently used this model to examine the effects of the intratracheal administration of different surfactant preparations on lung water clearance (16,24) in preterm rabbits ventilated for periods of up to 6 h. A surfactant preparation obtained from term fetal rabbits (FRS) was associated with longer survival and improved lung water clearance compared with an adult rabbit surfactant preparation (ARS) and two commercially available preparations, Exosurf and Survanta. In vitro studies on FRS in our laboratory (23) have shown that, after 3 h of surface area cycling (7), FRS converted from the biologically active heavyaggregate form (H) to the inactive light-aggregate (L) form at a significantly slower rate than ARS (i.e., Ͻ20% vs. ϳ60%). If FRS functions in a similar manner when instilled into lungs, the bioavailability of its active (H) form would be expected to be longer compared with ARS and other surfactants.…”