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 post-ischemic 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^W-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.