Endothelin B Receptor-Mediated Protection Against Anoxia–Reoxygenation Injury in Perfused Rat Liver: Nitric Oxide-Dependent and -Independent Mechanisms

Abstract

Liver has been considered to be a major organ that greatly alters its functions through mechanisms involving endothelin (ET)-1, one of the most potent vasoconstrictors produced by vascular endothelial cells. 1-3 Two classes of ET receptors have been identified: ET A receptor is predominantly expressed on vascular smooth muscle cells and executes vasoconstriction 4,5 ; on the other hand, ET B receptor occurs in endothelial cells, Kupffer cells, and vascular smooth muscle cells, and its stimulation in endothelial cells induces nitric oxide (NO)-mediated vasorelaxation through activation of constitutive NO synthase. 4,[6][7][8] Recent investigation has also shown contribution of eicosanoids to ET-induced biological effects, suggesting that its signaling pathways involve both NO-dependent and -independent mechanisms. 9,10 The ET-1 administration has been shown to aggravate hepatic microvascular changes under varied disease models involving endotoxemia, D-galactosamine-induced liver damage, or anoxia-reoxygenation injury. 11-15 During anoxia-reoxygenation, serum ET-1 levels were elevated, and blockade of its effects by ET antagonists attenuated reperfusion injury, suggesting deteriorating effects of this peptide vasoconstrictor on hepatoportal hemodynamics and bile formation. 2,3 Mechanisms by which ET-1 aggravates the hepatic reperfusion injury have been thought to involve impairment of sinusoidal patency through its vasoconstrictive actions. 14,16 This notion was supported by previous studies showing that transportal administration of ET-1 induces a marked vasoconstriction at portal venules and a reduction of bile flow. [17][18][19] However, biological actions of ET-1 endogenously generated upon the oxygen paradox on biliary function and cell viability have not fully been investigated. Further receptor phenotypes of the ET-1 effects responsible for alterations in reoxygenation-induced hepatobiliary dysfunction are largely unknown.The purpose of this study was to examine effects of anoxia and reoxygenation on endogenous ET-1 release and its pathophysiologic links to maintenance of bile formation and cell viability in isolated perfused rat livers. The current results suggest that ET-1 is released during the early period of reoxygenation and stimulates ET B receptor-mediated signaling to trigger NO-dependent and -independent protective mechanisms against anoxia-reoxygenation injury.Abbreviations: ET, endothelin; L-NAME, N -nitro-L-arginine methyl ester; SNAP, S-nitroso-N-acetyl penicillamine; LDH, lactate dehydrogenase; cGMP, cyclic guanosine monophosphate; A/R, anoxia/reoxygenation; B/P, bile salt/phospholipid ratio.From the

Possible Protection of Sinusoidal Endothelial Cells by Endothelin B Receptor During Hepatic Warm Ischemia–Reperfusion

“…17,18 In the liver, ETB-R occurs in SECs, hepatic stellate cells, and Kupffer cells. 12,15 To reveal the role of ETB-R in hepatic microcirculation, various antagonists and agonists have been examined in previous studies; however, the results of such studies varied and the exact roles thus remain obscure. 12,14,15 Taniai et al 12 investigated the role of ETB-R in biliary dysfunction and cell viability in isolated perfused rat livers using ETB-R antagonist.…”

“…12,15 To reveal the role of ETB-R in hepatic microcirculation, various antagonists and agonists have been examined in previous studies; however, the results of such studies varied and the exact roles thus remain obscure. 12,14,15 Taniai et al 12 investigated the role of ETB-R in biliary dysfunction and cell viability in isolated perfused rat livers using ETB-R antagonist. These reports suggested that stimulated ETB-R-mediated signaling limited bile acid excretion and played a protective role against I/R injury through the mechanisms involving both the NO-dependent and independent processes.…”

“…[12][13][14] To determine whether an ETB-R defi ciency affects the NO release in I/R injury, we measured nitrite (NO 2 − ) and nitrate (NO 3 − ), the fi nal products of NO oxidation, using liquid chromatography. Liver NO 2 − and NO 3 − were extracted in the homogenates from the freeze-clamped livers.…”

“…9,10 The action of ET-1 in vascular tone is mediated by G protein coupled receptors, of which two classes of ET receptor have been identifi ed. 11,12 Endothelin A receptor (ETA-R) mainly exists in vascular smooth muscle cells and mediates vasoconstriction. 11,12 Two subtypes of Endothelin B receptors (ETB-Rs) have been found: one in endothelial cells, which has been shown to promote the synthesis of nitric oxide (NO), and the other in smooth muscle cells which functions similar to ETA-R. 11 ETB-R has both a pressor and depressor effect in vivo, and therefore the role of the ETB-R in vascular tone remains unclear.…”

“…In addition, some investigators have evaluated the roles of ETB-R in hepatic microcirculation using ETB-R agonists or antagonists. 12,14,15 However, the results of these studies were controversial, and the exact role of ETB-R in hepatic microcirculation remains to be elucidated. Until today, no report has yet evaluated the role of ETB-R without the administration of any agonistic or antagonistic agents.…”