Clearance of phosphatidylcholine and cholesterol from liposomes, liposomes loaded with metaproterenol, and rabbit surfactant from adult rabbit lungs.

Rabbits were given by tracheal instillation liposomes, liposomes carrying metaproterenol sulfate (MPS) and suspended in a MPS solution, rabbit surfactant, or rabbit surfactant suspended in a MPS solution. The lipid suspensions were labeled with [14C]cholesterol and [3H]phosphatidylcholine. The percent recoveries of the labels were measured over 24 h in alveolar wash, lung tissue after alveolar wash, and the total lungs. All clearance curves for both phosphatidylcholine and cholesterol from liposomes were the same in the presence or absence of MPS. Alveolar clearance curves for both labels from rabbit surfactant were the same; however, the surfactant-associated labels were cleared to the lung tissue more rapidly than were the liposome-derived labels. Despite different alveolar clearance curves, all clearance curves for cholesterol from the total lungs were similar at a rate of 20 to 30%/24 h of the injected labeled cholesterol. Phosphatidylcholine was cleared from the total lungs more rapidly, at rates from 35 to 56%/24 h. Although MPS did not change labeled liposomal lipid clearance, the beta-agonist increased surfactant lipid clearance. The different responses to beta-agonist and the different alveolar clearance curves indicated distinct alveolar-to-lung tissue metabolism for liposomal versus surfactant phosphatidylcholine and cholesterol, although overall lung clearance rates were similar.