The significance of reutilization of surfactant phosphatidylcholine.

To assess the magnitude of reutilization of surfactant phosphatidylcholine, 68 3-day-old rabbits were injected intratracheally with a trace dose of [3H]choline-labeled surfactant mixed with [14C]palmitate-labeled synthetic dipalmitoylphosphatidylcholine. After timed kills we measured the total phosphatidylcholine associated counts/min in whole lung and alveolar wash and the specific activities of phosphatidylcholine in the alveolar wash, lamellar bodies, and microsomes isolated from the lung of each rabbit. Using a modification of the compartment analysis of Skinner et al. (Skinner, S. M., Clark, R. E., Baker, N., and Shipley, R. A. (1959) Am. J. Physiol. 196, 238-244), we found that surfactant phosphatidylcholine was reutilized with greater than 90% efficiency. The turnover time of the alveolar wash phosphatidylcholine was estimated to be 10.1 h and 9.3 h as measured by the 3H and 14C labels, respectively. From the ratios of alveolar wash-associated natural to synthetic phosphatidylcholine specific activities and from similar ratios obtained in 30 additional rabbits using [14C]choline-labeled natural surfactant and [3H]choline-labeled dipalmitoylphosphatidylcholine, we showed that phosphatidylcholine was reutilized intact rather than as component parts. Within 6 h of injection, the synthetic dipalmitoylphosphatidylcholine functioned metabolically as that administered in the form of natural surfactant.