Surfactant phospholipid catabolic rate is pool size dependent in mice

We increased surfactant pool size by surfactant treatment in mice to test if the catabolism of the major component of surfactant, saturated phosphatidylcholine (Sat PC), was rate limited. By intratracheal instillation, we gave mice trace doses, doses of 45 or 110 micromol/kg, or three doses of 110 micromol/kg of Sat PC in surfactant that contained radiolabeled dipalmitoylphosphatidylcholine (DPPC) and a radiolabeled phospholipase A-resistant ether analog of DPPC. Two strains of mice with 2-fold differences in alveolar and total Sat PC pool sizes were used; the mice with the higher pool sizes had a 2.3-fold higher steady-state catabolic rate. Acute increases in alveolar surfactant given by intratracheal instillation increased catabolic rates approximately 2-fold over the steady-state rates in both strains. There was minimal loss of the ether analog of DPPC from the lungs, and the alveolar macrophages did not accumulate more than 10% of the ether analog. In these two strains of mice, the catabolism of Sat PC was not rate limited because catabolic rate increased when alveolar pool sizes were increased.