While it is generally believed that hepatic clearance of lipopolysaccharide involves Kupffer cells, the mechanism involved has not been fully elucidated. This study assesses this phenomenon in terms of in vitro uptake and post-uptake modification experiments with an "' 1- labeled Salmonella minnesota lipopolysaccharide. lZ51-Lipopolysaccharide was added to Kupffer cells in suspension cultures under a variety of conditions. In vitro uptake of 1251-Lipopolysa~~haride was not saturable up to concentrations of 33.33 fig per ml. Kinetics experiments performed at 16.67 fig per ml demonstrated that Kupffer cells were unsaturable after 60 min of incubation. The kinetics of uptake could be inhibited, however, by incubation in the presence of a 10-fold excess of unlabeled lipopolysaccharide, indicating that a component of the uptake process may be limited. Energy dependence in this process was demonstrated by incubation in the presence of 1 m M 2-deoxyglucose which inhibited 1Z61-lipopolysaccharide uptake by approximately 30%. Pretreatment with 7.5 x 10+ M colchicine had no effect on kinetics, implying no role for the cell cytoskeleton in lipopolysaccharide uptake. These results are inconsistent with a receptor-mediated process as previously suggested. Modification of internalized label has been demonstrated by changes in buoyant density in CsCl isopyknic density gradients following overnight incubation with Kupffer cells. These results indicate that Kupffer cells clear bacterial endotoxin in vitro and post-uptake degradation occurs within 20 hr of incubation.
The Gram-negative flora of the terminal ileum and large intestine serves as a potential reservoir of endotoxin. Absorption of endotoxin from the gut has been demonstrated both in vitro by everted gut sac studies with radiolabeled endotoxin (1) and in uiuo by the Limulus assay to detect endotoxin in both portal and peripheral circulation (2-4). These studies reinforce the concept