Characterization of phospholipids accumulated in pulmonary-surfactant compartments of rats intratracheally exposed to silica

Abstract

Phospholipids in the lung fractions, i.e. alveolar free cells, extracellular pulmonary surfactant, intracellular pulmonary surfactant (lamellar bodies) and microsomal fractions, of rats were examined 28 days after intratracheal injection of silica (40 mg/kg). Significant accumulations of phospholipids were observed in the extracellular- and intracellular-surfactant fractions of rats exposed to silica. The prominent phospholipid accumulated was phosphatidylcholine (PC), consisting mainly of the dipalmitoyl species. However, a compositional change in acidic phospholipids of surfactant fractions was produced by the silica treatment. The percentage of phosphatidylglycerol (PG) was significantly decreased; in contrast, that of phosphatidylinositol (PI) was increased. Thus the ratio PG/PI in the surfactant fractions was markedly decreased in response to silica. This compositional change in both acidic phospholipids occurred even in the early stages, i.e. before appreciable accumulations of alveolar phospholipids were noticed. The molecular-species profiles of both acidic phospholipids in the surfactant fractions were distinctly different from each other. The dipalmitoyl species accounted for more than 30% of PG and less than 6% of PI, respectively. These species patterns of PG and PI were similar in control and silica-treated rats. These findings suggest two possibilities that (1) PG and PI destined for pulmonary surfactant are synthesized from each specific CDP-diacylglycerol (DG) pool having different molecular species in the lung, or (2) individual enzymes responsible for synthesis of surfactant PG and PI have substrate specificities for molecular species of CDP-DG, thereby producing PG and PI having different molecular species in surfactant compartments.