Developmental changes of sheep surfactant: in vivo function and in vitro subtype conversion.

Developmental differences in the intrinsic characteristics of surfactant have not been evaluated. Therefore, heavy-subtype surfactant was recovered from alveolar washes of 132-, 139-, and 148-day preterm lambs, 2- to 3-day-old newborn ventilated lambs, and adult sheep. The density of heavy-subtype surfactant and surfactant protein-A-to-saturated phosphatidylcholine ratios increased significantly with developmental age. In contrast, percent conversion from heavy to light surfactant forms was more rapid for surfactant from preterm animals than for surfactant from mature or adult animals. The function of the heavy-subtype surfactant was tested by treating ventilated 27-day gestational age preterm rabbits. The surfactant from the most immature animals was less effective at improving compliance or maintaining lung volumes on deflation than was surfactant from newborn or adult animals. These results demonstrate intrinsic and functional differences in surfactant from developing compared with mature sheep that correlated with the surfactant protein-A-content. The pattern of changes indicates that the preterm animal is at a disadvantage, because the surfactant is intrinsically abnormal relative to that of the adult.