Platelet-activating factor may act as a second messenger in the release of icosanoids and superoxide anions from leukocytes and endothelial cells.

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Abstract
Platelet-activating factor (PAF) is generated by endothelial cells, polymorphonuclear leukocytes, and macrophages after activation by appropriate receptor agonists, but much of the PAF remains intracellular. We have investigated whether PAF formation is important for the subsequent generation of icosanoids and superoxide anions by these cells. The generation of prostacyclin and leukotriene B4 were measured by radioimmunoassay, superoxide anion was measured by reduction of cytochrome c, and PAF was measured by bioassay. In each cell type, PAF formation preceded or accompanied icosanoid generation. Bradykinin-induced prostacyclin generation in endothelial cells was markedly reduced by the PAF receptor antagonists WEB 2086 or CV 6209. In guinea pig adherent macrophages in vitro, basal prostacyclin generation and that induced by endotoxin and fMet-Leu-Phe were inhibited by either WEB 2086 (1-100 .mu.M) or CV 6209 (0.1-10 .mu.M). In isolated rabbit polymorphonuclear leukocytes, fMet-Leu-Phe stimulated the generation of both leukotriene B4 and superoxide anion. WEB 2086 and CV 6209 caused concentration-dependent inhibition of both these markers of leukocyte activation. These observations lead us to suggest that PAF may be a second messenger in leukocytes and endothelial cells.

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