LABELING AND BIOLOGICAL HALF-LIFE OF PHOSPHATIDYLCHOLINE IN SUBCELLULAR-FRACTIONS OF RABBIT LUNG

Rabbits were given [14C]palmitic acid, [3H]choline and ortho[32P] phosphate to label lung phosphatidylcholine. The rabbit lungs were lavaged to obtain an alveolar wash phosphatidylcholine sample, and subsequently samples of phosphatidylcholine were obtained from the lung parenchyma and from microscomal and lamellar body fractions. The specific activity of the phosphatidylcholine in each fraction was determined. The labeling of phosphatidylcholine and the biological half-life values for the labeled phosphatidylcholine in each lung fraction are presented. Phosphatidylcholine labeled with palmitic acid or choline reached a maximal specific activity rapidly in microsomal fractions. Maximal specific activity of lamellar body phosphatidylcholine and alveolar phosphatidylcholine was achieved by 2 and 6 h, respectively. The labeling of lecithin with phosphate was quite slow in all lung fractions relative to the other precursors. The decay curves for parenchymal and microsomal phosphatidylcholine had 2 exponential components; the disappearance of lamellar body and alveolar wash phosphatidylcholine was described by simple decay curves. The biological half-life values of palmitic acid-labeled and choline-labeled alveolar wash phosphatidylcholine were 16 and 30 h, respectively. Total phosphatidylcholine and desaturated phosphatidycholine decay rates in the parenchyma and alveolar wash were similar. The labeling of phosphatidylcholine in subcellular lung fractions indicated that the precursors did not cleanly pulse label the phosphatidylcholine of the lamella bodies and alveolar wash. The labeling patterns of the subcellular fractions were consistent with previous anatomical descriptions of surfactant synthesis and secretion.