Possible modifier of surfactant function

Lamellar bodies and alveolar lavage from adult mammalian lung contain unusually high concentrations of phosphatidylglycerol that could serve as a sensitive indicator of surfactant. Phosphatidylglycerol was absent and phosphatidylinositol was correspondingly prominent in surfactant from the preterm rabbit fetus. Phosphatidylglycerol rapidly appeared and phosphatidylinositol decreased following the delivery. Surfactant isolated from the prematurely born rabbit or from humans with respiratory distress syndrome never contained phosphatidylglycerol. Comparison between lamellar bodies from fetal and postnatal rabbits revealed remarkably similar composition except for the acidic phospholipids; however, the physicochemical properties were different. The compressibility of the surface film (i.e. the ratio of the fractional decrease in surface area and the corresponding decrease in surface tension) at low surface tensions was higher with fetal than with postnatal surfactant, whereas the difference in minimum surface tensions was small. These data suggest that phosphatidylglycerol is not an essential component required for the formation of the complex, but it improves the properties of surfactant in stabilizing the alveoli. Supplementary key words lecithin * lung mechanics lamellar inclusion body . dipalmitoyl Lung surfactant lines the air-liquid interface of the terminal airways. By reducing the surface tension on expiration, it contributes to the stability of these tiny air spaces (1, 2). Surfactant is synthesized in type I1 alveolar cells and is stored in intracellular lamellar inclusion bodies prior to the release into alveoli. Surfactant lining of interfaces is deficient in some life-threatening conditions. Perhaps the most significant of these is the respiratory distress syndrome (RDS) of the newborn, a developmental disease characterized by progressive atelectasis of lungs (3, 4). This condition can be predicted by phospholipid analysis in the amniotic fluid prior to birth (5). Disaturated lecithin is the major component of surfactant. In addition, proteins and certain other lipid components are present (see ref. 6 for review). Phosphatidylglycerol is the second most abundant of the surfactant lipids (710) and like disaturated lecithin, is present in uniquely high concentration. We have speculated that its possible role is as ( I ) an intermediate or an end product of some other surfactant component, (2) a structural component of lamellar bodies necessary for their assembly, or (3) a component contributing to surfactant function (9). Studies on biosynthesis of phospholipids in lung fractions has yielded no evidence in favor of the first alternative (1 1). CDP-diglyceride serves as a common precursor for both phosphatidylinositol and phosphatidylglycerol, and the latter compound seems to be an end product rather than a precursor. The present approach to investigate surfactant phosphatidylglycerol-its ontogeny in mammals and distribution in some vertebrate species-enabled us to further evaluate its function.