Fig 1. Alveolar structure and schematic design of isotope tracing.

³H-labeled methyl chloride choline is involved in surfactant phospholipid synthesis of alveolar type II cells. The level of ³H in phospholipids was monitored to detect the amount originating from endogenous phospholipid synthesis. The radioactivity of TPL and DSPC in BALF and LT indicates the total content of ³H-labeled choline chloride incorporated into TPL and DSPC, which reflects the newly synthesized TPL and DSPC and the body’s ability to synthesize PC. The secretory rate of TPL was expressed as the ratio of radioactivity in BALF and the whole lung (BALF+LT). The secretory rates of TPL and DSPC indicate the ability of alveolar type II epithelial cells to secrete PC into the alveolar space.