Background: Surfactant components can be recycled (= reuptake→ resecretion) by alveolar type II cells. Surfactant protein A is involved in this process.

Question: Elucidation of the kinetics and mechanisms of this recycling process.

Methods: Surfactant resecretion assays in freshly isolated rat type II pneumocytes. Endocytosis and resecretion of surfactant-like liposomes labeled in the phospholipid surfactant protein A components.

Results: Surfactant components taken up by SP-A-mediated endocytosis are rapidly resecreted by type II cells (cycle time ≤ 15 min) in contrast to the much slower resecretion after liposome uptake in the absence of SP-A (p<0.05). It is energy dependent and inhibitable by extracellular SP-A but not by extracellular lipid. Surfactant lipids internalized by SP-A-mediated endocytosis are resecreted intact(96±0.2%) in contrast to lipids taken up in the absence of SP-A (≈ 50% degradation/resynthesis).

Conclusions: These studies will lead to a better understanding of how extracellular surfactant is incorporated into the endogenous surfactant pools. This may be useful to improve therapeutic strategies of surfactant substitution.