The labeling of lung phosphatidylcholine in premature rabbits

Lung phosphatidylcholine metabolism was studied in vivo in premature rabbits delivered by cesarean section as early in gestation as compatible with prolonged viability (28.9 days). The newborn rabbits initially required supplemental oxygen and had respiratory distress. The amount of phosphatidylcholine isolated from the lung parenchyma changed little over the first 3 days of life, while phosphatidylcholine in the alveolar wash increased in 3 days from 0.05--3.1 mumol/50 g animal. The phosphatidylcholine of the lungs of the premature rabbits was pulse labeled with isotopically labeled palmitic acid, choline, and phosphate given to the pregnant does 10 min before delivery of the newborns. After the initial incorporation period, the total amount of radioactive precursor palmitic acid, choline, or 32P) incorporated into lung phosphatidylcholine did not change for a period of 4 days. Labeled phosphatidylcholine was detected initially in alveolar wash 3 hr after administration of the three precursors and continued to accumulate for many hours. The biological half-life values for lung and alveolar phosphatidylcholine indicated that phosphatidylcholine was turning over very slowly. However, if the effect of dilution on the measured specific activity caused by phosphatidylcholine accumulation was considered, virtually no labeled alveolar or lung phosphatidylcholine disappeared during the 3--4 days of these observations. These results with premature newborn rabbits were similar to those for term newborn rabbits, but different from similar measurements made in the adult rabbit.