Endothelins are potent peptide mediators that elicit glycogenolytic and vasoconstrictor actions in the liver. Endothelins were found to stimulate the synthesis and release of the lipid mediator platelet-activating factor in cultured rat Kupffer cells. Endothelin-mediated synthesis of platelet-activating factor required extracellular calcium in that the calcium chelator, EGTA and nifedipine, a calcium ion channel blocker, inhibited platelet-activating factor synthesis. The phospholipase A2 inhibitor, 4-bromophenacyl bromide, strongly inhibited endothelin-induced platelet activating factor synthesis. Endothelin-stimulated platelet activating factor synthesis was inhibited after treatment of Kupffer cells with cholera toxin, whereas pertussis toxin inhibited only this response to endothelin-1. Agents that elevate intracellular cyclic AMP levels were found to inhibit endothelin-induced platelet-activating factor synthesis in Kupffer cells. Staurosporine, a protein kinase C inhibitor minimized endothelin-induced platelet-activating factor synthesis but phorbol myristate acetate, an activator of protein kinase C, did not affect endothelin-induced platelet activating factor synthesis. Thus, the current study demonstrates that activation of an endothelin receptor in cultured rat Kupffer cells results in the synthesis and release of platelet-activating factor. The importance of endothelin-mediated platelet-activating factor synthesis relates to the mechanism of intercellular signaling occurring between endothelial cells (i.e., the site of endothelin synthesis) and Kupffer cells (i.e., the site of formation of secondary mediators such as platelet-activating factor and eicosanoids) within the rat liver exposed to various types of pathophysiological episodes.
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