Endothelin, a potent peptide agonist in the liver

Endothelin, a peptide mediator produced by vascular endothelial cells, caused sustained vasoconstriction of the portal vasculature in the perfused rat liver. The vasoactive effect of endothelin was accompanied by increased glycogenolysis and alterations in hepatic oxygen consumption. The endothelin-induced increase in the portal pressure was concentration-dependent with an EC50 of 1 nM. Endothelin-induced hepatic glycogenolysis was dose-dependent but exhibited a different EC50 than for the vasoconstrictive effects of endothelin. Hepatic vasoconstriction and glycogenolysis following endothelin infusion were inhibited when Ca2+ was removed from the perfusion medium. The endothelin-induced responses in the liver were not altered by prior infusion of phenylephrine (alpha-adrenergic agonist), isoproterenol (beta-adrenergic agonist), angiotensin II, glucagon, platelet-activating factor, or the platelet-activating factor antagonist, BN52021. However, repeated infusion of endothelin resulted in desensitization of the glycogenolytic response but was without a significant effect on hepatic vasoconstriction. Endothelin also stimulated metabolism of inositol phospholipids in isolated hepatocytes and Kupffer cells in primary culture. The present experiments demonstrate, for the first time, that endothelin is a very potent agonist in the liver eliciting both a sustained vasoconstriction of the hepatic vasculature and a significant increase in hepatic glucose output.