Nitric oxide down-regulates endocytosis in rat liver endothelial cells

Hepatic sinusoidal endothelial cells (SEC) were challenged with inducers, blockers or donor of nitric oxide (NO) production in vitro to test the effect of this reactive nitrogen agent on endocytosis in SEC. NO was measured as its stable form nitrite (NO2-) in culture media and trace amounts of radioiodinated ligands were tested for endocytosis and binding after 6 and 24 h of incubation. Among several proinflammatory reagents tested, IL-1 beta induced most strongly NO synthesis after 24 h in culture. Although endocytosis was significantly enhanced in SEC that had been exposed to IL-1 beta for 6 h, prolonged exposure (24 h) to this proinflammatory cytokine, which triggered increased production of NO by the cells, yielded a decreased endocytic activity. The presence of aminoguanidine, an inhibitor of NO synthase, gave significant up-regulation of endocytosis compared with control cells. To fully verify the role of NO as an endocytosis modulator, SEC were preincubated with sodium nitroprusside, an exogenous NO donor. Again, increased concentration of NO2- in the medium was associated with decreased endocytosis of SEC. Binding studies at 4 degrees C revealed that the down-regulation of endocytosis in SEC after increased exposure to NO was due to a decreased number or affinity of receptors on the cell surface.