| Abstract: | The beta-adrenergic agonist isoproterenol inhibited the glycogenolytic response of platelet-activating factor (AGEPC, 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) in perfused livers derived from fed rats. AGEPC-stimulated hepatic vasoconstriction, measured by increases in portal vein pressure, also was inhibited by prior isoproterenol infusion. Isoproterenol-mediated inhibition of these hepatic responses to AGEPC was not apparent when isoproterenol (10 microM) was coinfused with the beta-receptor antagonist propranolol (75 microM) or when isoproterenol was replaced with the alpha-adrenergic agonist phenylephrine (10 microM). alpha-Agonist-induced glycogenolysis and vasoconstriction in the perfused liver was unaffected by isoproterenol infusion. Glucagon (2.3 nM) had no effect on the glycogenolytic or vasoconstrictive responses of the liver to AGEPC despite the fact that glucagon increased hepatic cAMP levels to a far greater extent than isoproterenol. Additionally, inhibition of the hepatic responses to AGEPC by isoproterenol occurred in perfused livers from mature rats (i.e. greater than 300 g) in which liver parenchymal cells lack functional beta-adrenergic receptors. The data presented in this study illustrate a specific inhibition of AGEPC-induced hepatic glycogenolysis and vasoconstriction by beta-adrenergic stimulation of the perfused liver. This inhibition appears to be mediated by interaction of isoproterenol with nonparenchymal cells within the liver. These findings are consistent with the concept that AGEPC stimulates hepatic glycogenolysis by an indirect mechanism involving hepatic vasoconstriction. |
