/ ajplung.00478.2001.-The uptake of fluorescent-labeled liposomes (with a surfactant-like composition) by alveolar macrophages and alveolar type II cells was studied using flow cytometry, in vivo by instillation of the labeled liposomes in the trachea of ventilated rats followed by isolation of the alveolar cells and determination of the cell-associated fluorescence, and in vitro by incubation of isolated alveolar cells with the fluorescent liposomes. The results show that the uptake of liposomes by the alveolar cells is time and concentration dependent. In vivo alveolar macrophages internalize more than three times as many liposomes as alveolar type II cells, whereas in vitro, the amount of internalized liposomes by these cells is approximately the same. In vitro, practically all the cells (70-75%) internalize liposomes, whereas in vivo only 30% of the alveolar type II cells ingest liposomes vs. 70% of the alveolar macrophages. These results indicate that in vivo, only a small subpopulation of alveolar type II cells is able to internalize surfactant liposomes. pneumocyte; lung; liposome; fluorescence PULMONARY SURFACTANT lines the alveolar capillary membrane and plays an essential role in normal lung function. It is a complex of lipids and proteins synthesized by alveolar type II cells and is stored in lamellar bodies until it is secreted into the alveolar space (4). Within the alveolus, surfactant transforms to tubular myelin, which unfolds, and the surfactant lipids are rapidly inserted in the lipid monolayer present at the air-liquid interface (4). During a breathing cycle, lipids are squeezed out of the monolayer. To maintain the primary function of the monolayer, i.e., reduction of the surface tension, the loss of lipids from the monolayer has to be compensated by renewed insertion of lipids. Because the de novo synthesis of surfactant is insufficient to correct the natural loss (i.e., inactivation of surfactant), the alveolar type II cell not only produces newly synthesized surfactant but also reutilizes inactivated surfactant, derived from the alveolar space.The recycling of surfactant is a major pathway for surfactant in the alveolar space (14). Clearance by the mucociliary pathway (20) or degradation by alveolar type II cells and alveolar macrophages appears to be less important in the removal of surfactant lipids from the alveolar space.One important factor in the complex system of surfactant synthesis, secretion, recycling, clearing, and degradation is the uptake of surfactant lipids by alveolar type II cells and alveolar macrophages. Numerous in vitro studies have demonstrated that both cell types can internalize surfactant lipids, although their relative contribution in the uptake of surfactant lipids in the lung remains obscure. According to Miles et al. (11), based on in vitro studies, alveolar macrophages may be responsible for all the catabolism of surfactant lipids.In contrast, results of in vivo experiments suggest a primary role of alveolar type II cells in the uptake of surfactant lipids r...