Clearance of Large Amounts of Natural Surfactants and Liposomes of Dipalmitoylphosphatidylcholine from the Lungs of Rabbits

Three-day-old rabbits were given intratracheal injections of radiolabeled natural sheep surfactant, rabbit surfactant, or liposomes of dipalmitoylphosphatidylcholine that contained greater than three times the quantity of phosphatidylcholine present in the endogenous surfactant pool. The recoveries of radiolabeled phosphatidylcholine and total phosphatidylcholine in alveolar washes, lung tissue, and total lung (alveolar washes plus lung tissue) were measured for 72 h. Approximately half of the two natural surfactants rapidly became lung tissue associated, and phosphatidylcholine derived from the rabbit surfactant was cleared from the total lung twice as rapidly as was the phosphatidylcholine from sheep surfactant (20.7% versus 10.4% of the amount present at zero time/24 h, p less than .05). The alveolar surfactant pool size did not decrease despite the clearance of the exogenously administered material. The liposomes of dipalmitoylphosphatidylcholine were cleared from the total lung at the same rate as the rabbit surfactant phosphatidylcholine; however, compared to the natural surfactants, much less of this material became lung tissue associated. The administration of the natural rabbit surfactant did not decrease the amount of radiolabeled choline, palmitic acid, or 32P that was incorporated into lung phosphatidylcholine or the amount of labeled phosphatidylcholine that was secreted to the alveoli. However, sheep surfactant increased the percent of radiolabeled phosphatidylcholine recovered by alveolar wash. These experiments document differences in clearance rates, tissue and alveolar association patterns, and subtle effects on endogenous surfactant metabolism for two surfactants and liposomes of dipalmitoylphosphatidylcholine following intratracheal administration. Uniform metabolic responses should not be anticipated following treatments with different surfactant preparations.