Stimulation of thromboxane release by extracellular UTP and ATP from perfused rat liver
Authors: HÄUSSINGER Dieter , BUSSHARDT Elisabeth , STEHLE Thomas , STOLL Barbara , WETTSTEIN Matthias , GEROK Wolfgang 
Published: March 2005

In isolated perfused rat liver, infusion of UTP (20 μM) led to a transient, about sevenfold stimulation of thromboxane release (determined as thromboxane B2), which did not parallel the time course of the UTP‐induced stimulation of glucose release. An increased thromboxane release was also observed after infusion of ATP (20 μM). Although the maximal increase of portal pressure following ATP was much smaller than with UTP (4.2 vs 11.5 cm H2O), the peak thromboxane release was similar with both nucleotides.
Indomethacin (10 μM) inhibited the UTP‐induced stimulation of thromboxane release and decreased the UTP‐induced maximal increase of glucose output and of portal pressure by about 30%. The thromboxane A2 receptor antagonist BM 13.177 (20 μM) completely blocked the pressure and glucose response to the thromboxane A2 analogue U‐46619 (200 nM) and decreased the ATP‐ and UTP‐induced stimulation of glucose output by about 25%, whereas the maximal increase of portal pressure was inhibited by about 50% and 30%, respectively. BM 13.177 and indomethacin inhibited the initial nucleotide‐induced overshoot of portal pressure increase, but had no effect on the steady‐state pressure increase which is obtained about 5 min after addition of ATP or UTP.
The leukotriene D4/E4 receptor antagonist LY 171883 (50 μM) inhibited not only the glucose and pressure response of perfused rat liver to leukotriene D4, but also to leukotriene C4 by about 90%. This suggests that leukotriene D4 (not C4) is the active metabolite in perfused liver and the effects of leukotriene C4 are probably due to its rapid conversion to leukotriene D4. LY 171883 also inhibited the response to the thromboxane A2 analogue U 46619 by 75–80%, whereas the response of perfused liver to leukotriene C4 was not affected by the thromboxane receptor antagonist BM 13.177 (20 μM). The glucose and pressure responses of the liver to extracellular UTP were inhibited by LY 171883 and by BM 13.177 by about 30%. This suggests that the inhibitory action of LY 171883 was due to a thromboxane receptor antagonistic side‐effect and that peptide leukotrienes do not play a major role in mediating the UTP response.
In isolated rat hepatocytes extracellular UTP (20 μM), ATP (20 μM), cyclic AMP (50 μM) and prostaglandin F2α(3 μM) increased glycogen phosphorylase a activity by more than 100%. A significant stimulation of glycogen phosphorylase was not observed with the thromboxane A2 analogue U‐46619 and leukotrienes C4 and D4, suggesting that the stimulation of glucose output by these compounds in the intact liver [Häussinger et al. (1988) Biol. Chem. Hoppe‐Seyler 369, 97–107] is not due to a direct effect on hepatic parenchymal cells.
The data show that extracellular nucleotides stimulate the formation of thromboxanes in perfused rat liver in addition to a recently demonstrated increase of prostaglandin D2 and E2 release [Tran‐Thi et al. (1988) Biol. Chem. Hoppe‐Seyler 369, 65–68] and that the glycogenolytic and pressure responses of the liver to extracellular nucleotides are partially mediated by thromboxanes. The data further indicate that UTP, ATP, prostaglandin F2α, but not the thromboxane A2 analogue U‐46619 and peptide leukotrienes can act directly on parenchymal liver cells to stimulate glycogenolysis. Although the metabolic and ion‐flux responses of perfused rat liver to extracellular UTP, U‐46619 and leukotriene C4/D4 are similar, different mechanisms seem to be involved. Peptide leukotrienes probably play a minor role in mediating the nucleotide response. Cyclooxygenase products do not explain the differences in response of perfused rat liver to extracellular ATP and UTP. The data substantiate the hypothesis of complex interactions between hepatic parenchymal and non‐parenchymal cells in mediating the response of the liver to extracellular nucleotides and icosanoids.