Reutilization of surfactant phosphatidylcholine in adult rabbits.

32P-saturated phosphatidylcholine was added to [3H]choline-labeled natural surfactant and the mixture was injected intratracheally into 87 adult rabbits. The rabbits were also given [14C]palmitate intravenously at the same time. Rabbits were killed in groups from 10 min to 72 h after injection. In each rabbit we measured the total recovered [3H]phosphatidylcholine (PC) in the alveolar wash, the ratio of [3H]PC to [32P]PC in the alveolar wash, and the specific activity of [14C]PC in the alveolar wash and lamellar bodies. Values were averaged for all rabbits killed at the same times and smooth curves were fit to the data by computer. From the intravenous [14C]palmitate data we calculated a turnover time for alveolar PC of 6.0 h. From the intratracheal labeling data, we calculated a turnover time for alveolar PC of 5.7 h and determined that alveolar PC was reutilized at an efficiency of only 23%. We also concluded that this reutilization occurred as intact molecules.