Endothelin B receptor-mediated protection against anoxia-reoxygenation injury in perfused rat liver: nitric oxide-dependent and -independent mechanisms.

This study aimed to investigate the roles of endothelin (ET) receptors in biliary dysfunction and cell injury in postischemic livers. Rat livers perfused with oxygenated Krebs-Henseleit solution were exposed to reoxygenation following 20-minute hypoxia. The anoxic perfusion decreased bile output and reduced cyclic guanosine monophosphate (cGMP) contents, an index of nitric oxide (NO) generation. Upon reoxygenation, the decreased bile was not fully recovered, and the resistance increased biphasically: an early transient spike accompanied by an elevated release of ET-1 and a rise accompanied by a cGMP elevation in the later period. The initial vasoconstriction appeared to be mediated by both ET(A) and ET(B) receptors, as judged by inhibitory effects of their antagonists, BQ-485 and BQ-788, respectively, while the late elevation of the resistance was not attenuated by these reagents, but rather enhanced by the ET(B) blockade. The BQ-788 treatment attenuated the reoxygenation-induced cGMP elevation and induced bile acid-dependent choleresis. However, such a change upon the ET(B) blockade coincided with dissociation of a recovery of phospholipids and aggravation of cell injury. The BQ-788-elicited deterioration of reoxygenation-elicited changes was attenuated by NO supplement with S-nitroso-N-acetyl penicillamine. N(omega)-Nitro-L-arginine methyl ester, an NO synthase inhibitor, mimicked biliary changes elicited by the ET(B) blockade but without causing notable cell injury. Under these circumstances, coadministration of clotrimazole, an inhibitor of cytochrome P450 mono-oxygenases, elicited the injury comparable with that observed under the ET(B) blockade. These results suggest that ET(B)-mediated signaling limits excessive bile acid excretion and plays a protective role against reoxygenation injury through mechanisms involving both NO-dependent and -independent processes.

Carbon monoxide from heme catabolism protects against hepatobiliary dysfunction in endotoxin-treated rat liver.

BACKGROUND AND AIMS: Liver is a major organ for heme detoxification under disease conditions, but its self-protective mechanisms against the toxicity are unknown. This study aimed to examine roles of carbon monoxide (CO), the gaseous product of heme oxygenase (HO), in ameliorating hepatobiliary dysfunction during catabolism of heme molecules in endotoxemic livers. METHODS: Vascular resistance and biliary flux of bilirubin-IXalpha, an index of HO-derived CO generation, were monitored in perfused livers of endotoxemic rats. Livers were perfused with HbO(2), which captures nitric oxide (NO) and CO, or metHb, a reagent trapping NO but not CO. RESULTS: In endotoxin-pretreated livers where inducible NO synthase and HO-1 overproduced NO and CO, HbO(2) caused marked vasoconstriction and cholestasis. These changes were not reproduced by the NO synthase inhibitor aminoguanidine alone, but by coadministration of zinc protoporphyrin-IX, an HO inhibitor. CO supplementation attenuated the events caused by aminoguanidine plus zinc protoporphyrin-IX, suggesting that simultaneous elimination of these vasorelaxing gases accounts for a mechanism for HbO(2)-induced changes. This concept was supported by observation that metHb did not cause any cholestasis; the reagent captures NO but triggers CO overproduction through rapid degradation of the heme by HO-1. CONCLUSIONS: These results suggest protective roles of CO against hepatobiliary dysfunction caused by heme overloading under stress conditions.