PRECURSOR-PRODUCT RELATIONSHIP BETWEEN RABBIT TYPE-II CELL LAMELLAR BODIES AND ALVEOLAR SURFACE-ACTIVE MATERIAL - SURFACTANT TURNOVER TIME

To estimate the turnover time of alveolar surfactant in New Zealand white rabbits, [9,10-3H]palmitic acid and [2-14C]glycerol were injected i.v. From 1-48 h after injection the animals were killed, the lungs were lavaged for alveolar surfactant with saline and the lamellar bodies were isolated by homogenization and sucrose density gradient centrifugation. Lamellar bodies and alveolar surfactant had comparable phospholipid composition and surface activity. Lamellar bodies contained little DNA, no mitochondrial enzyme activity and < 5% contaminating phospholipids from microsomal and Golgi-enriched fractions. Radioactivity of phosphatidylcholine, saturated phosphatidylcholine and phosphatidylglycerol were measured for each isotope in lamellar bodies and surfactant at each time point. The plot of the integral with respect to time of the difference between lamellar body and surfactant specific activity vs. surfactant specific activity has a slope determined by the turnover time and a shape which tests the precision of the precursor-product relationship. This analysis does not assume a pulse label and allows for possible recycling of tracer from surfactant to lamellar bodies. Turnover times of 4-11 h were obtained. An imprecise precursor-product relationship was detected between lamellar bodies and alveolar surfactant, which is not due to experimental variability or to contamination of lamellar bodies by other subcellular fractions but may reflect imperfect mixing within surfactant compartments.