Stimulation of glycogenolysis and vasoconstriction in the perfused rat liver by the thromboxane A2 analogue U-46619.

Infusion of the thromboxane A2 analogue U-46619 into isolated perfused rat livers resulted in dose-dependent increases in glucose output and portal vein pressure, indicative of constriction of the hepatic vasculature. At low concentrations, e.g. less than or equal to 42 ng/ml, glucose output occurred only during agonist infusion; whereas at concentrations greater than or equal to 63 ng/ml, a peak of glucose output also was observed upon termination of agonist infusion coincident with relief of hepatic vasoconstriction. Effluent perfusate lactate/pyruvate and beta-hydroxybutyrate/acetoacetate ratios increased significantly in response to U-46619 infusion. Hepatic oxygen consumption increased at low U-46619 concentrations (less than or equal to 20 ng/ml) and became biphasic with a transient spike of increased consumption followed by a prolonged decrease in consumption at higher concentrations. Increased glucose output in response to 42 ng/ml U-46619 was associated with a rapid activation of glycogen phosphorylase, slight increases in tissue ADP levels, and no increase in cAMP. At 1000 ng/ml, U-46619 activation of glycogen phosphorylase was accompanied by significant increases in tissue levels of AMP and ADP, decreases in ATP, and slight increases in cAMP. In isolated hepatocytes, U-46619 did not stimulate glucose output or activate glycogen phosphorylase. Reducing the perfusate calcium concentration from 1.25 to 0.05 mM resulted in a marked reduction of the glycogenolytic response to U-46619 (42 ng/ml) with no efflux of calcium from the liver. U-46619-induced glucose output and vasoconstriction displayed a similar dose dependence upon the perfusate calcium concentration. Thus, U-46619 exerts a potent agonist effect on glycogenolysis and vasoconstriction in the perfused rat liver. The present findings support the concept that U-46619 stimulates hepatic glycogenolysis indirectly via vasoconstriction-induced hypoxia within the liver.