Terminal complement complex and endothelial cells

The effects of the terminal complement sequence on two endothelial cell functions (prostacyclin (PGI2) generation and permeability of an endothelial cell monolayer) were examined in antibody-sensitized pulmonary artery endothelial cells. Whereas C5b-7 complement complexes induced on PGI2 formation and no enhanced endothelial permeability, addition of purified complement component C8 resulted in a time- and dose-dependent burst of PGI2 release and in a substantially increased permeability of an endothelial cell monolayer in the absence of overt cell damage. Formation of the complete terminal complement complex C5b-9 enhanced PGI2 release but was accompanied by cytolysis. Extracellular Ca2+ was required for C5b-8 dependent PGI2 formation. Three different blockers of physiological calcium channels failed to suppress the observed stimulatory effect. One minute after addition of C8 to endothelial cells carrying C5b-7 complexes, a six to sevenfold enhanced passive influx of 45Ca2+ into the cells was noted. An enhanced passive influx was also observed for 51CrO4(2-), 3H-aminobutyric acid, and 3H-sucrose, but not for 3H-inulin and 3H-dextran. These data together suggest that complement C5b-8 complexes may serve as Ca2+-bypass gates in endothelial cells, the ensuing influx of Ca2+ leading to subsequent activation of the arachidonic acid pathway and the actin-myosin system.