Complement complex C5b-8 induces PGI2 formation in cultured endothelial cells.

The effects of the terminal complement sequence on prostacyclin (PGI2) generation in antibody-sensitized pulmonary arterial endothelial cells were examined. Whereas C5b-7 complement complexes induced no PGI2 formation, addition of purified complement component C8 resulted in a time- and dose-dependent burst of PGI2 release 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. In contrast, W7 [N-(6-amino-hexyl)-5-chloro-1-naphthalene sulfonamide] and trifluoperazine, inhibitors of calmodulin activity, all reduced the C5b-8-dependent PGI2 generation. None of the inhibitors used impaired Ca2+ flux into the cells. One minute after addition of C8 to endothelial cells carrying C5b-7 complexes, a six- to seven-fold enhanced passive influx of 45Ca2+ into the cells was noted. An enhanced passive influx was also observed for 51Cr O4(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.