ENDOTHELIN STIMULATES PLATELET-ACTIVATING-FACTOR SYNTHESIS BY CULTURED RAT KUPFFER CELLS

Endothelins are potent peptide mediators that elicit glycogenolytic an d vasoconstrictor actions in the liver. Endothelins were found to stim ulate the synthesis and release of the lipid mediator platelet-activat ing factor in cultured rat Kupffer cells. Endothelin-mediated synthesi s of platelet-activating factor required extracellular calcium in that the calcium chelator, EGTA and nifedipine, a calcium ion channel bloc ker, inhibited platelet-activating factor synthesis. The phospholipase A(2) inhibitor 4-bromophenacyl bromide, strongly inhibited endothelin -induced platelet activating factor synthesis, Endothelin-stimulated p latelet activating factor synthesis was inhibited after treatment of K upffer cells with cholera toxin, whereas pertussis toxin inhibited onl y this response to endothelin-1. Agents that elevate intracellular cyc lic AMP levels were found to inhibit endothelin-induced platelet-activ ating factor synthesis in Kupffer cells. Staurosporine, a protein kina se C inhibitor minimized endothelin-induced platelet-activating factor synthesis but phorbol myristate acetate, an activator of protein kina se C, did not affect endothelin-induced platelet activating factor syn thesis. Thus, the current study demonstrates that activation of an end othelin receptor in cultured rat Kupffer cells results in the synthesi s and release of platelet-activating factor. The importance of endothe lin-mediated platelet-activating factor synthesis relates to the mecha nism of intercellular signaling occurring between endothelial cells (i .e., the site of endothelin synthesis) and Kupffer cells (i.e., the si te of formation of secondary mediators such as platelet-activating fac tor and eicosanoids) within the rat liver exposed to various types of pathophysiological episodes.