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Rabbit alveolar saturated phosphatidylcholine (SPC) can be separated by differential and density gradient centrifugation of lung lavage into three fractions. One fraction ('B'), which is analogous to conventionally prepared alveolar surfactant, contains 46.0 +/- 5.9% (SD) of lavage SPC, and is made of large multilamellar vesicles or sheet-like structures. A second fraction ('C') does not sediment after centrifugation at 80 000 g for 90 min, contains 29.8 +/- 14.0% of lavage SPC, and a uniform population of small vesicles. This fraction incorporates 3H-palmitate administered intravenously with a small delay with respect to fraction 'B'. A third fraction ('D') contains almost all the cells of lavage and less than 5% of lavage SPC. We conclude that alveolar SPC consists of more than one compartment. The possible significance of isolated fractions is discussed.

To clarify the steps following surfactant secretion, we injected adult rabbits with radioactive palmitate, lavaged the airways, removed the cells, separated by ultracentrifugation lavage components into two fractions (B and C), and followed the labeling of phospholipids of these fractions. The results were compatible with the view that total and saturated phosphatidylcholine are transferred from B to C. Furthermore, the fluxes of total and saturated phosphatidylcholine through fraction C (0.45 and 0.30 mumol . h-1 . g lung-1, respectively) were compatible with the actual estimates of surfactant recycling. The labeling of phosphatidylglycerol ruled out a simple precursor-product relationship between B and C but was compatible with a nonideal first-order relationship. The labeling of phosphatidylinositol, cardiolipin, and phosphatidylethanolamine was incompatible with the existence of a direct precursor-product relationship between B and C. The labeling of total and saturated phosphatidylcholine suggests that fraction B may be made by active surfactant, whereas fraction C may contain surfactant modified for reuptake or for reuptake and catabolism.

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