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 b iliary dysfunction and cell injury in postischemic livers. Rat livers perfu sed 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 full y 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 ETA and ETB receptors, as judged by inhibitory effe cts of their antagonists, BQ-485 and BQ-788, respectively, while the late e levation of the resistance was not attenuated by these reagents, but rather enhanced by the ETB blockade. The BQ-788 treatment attenuated the reoxygen ation-induced cGMP elevation and induced bile acid-dependent choleresis. Ho wever, such a change upon the ETB blockade coincided with dissociation of a recovery of phospholipids and aggravation of cell injury. The BQ-788-elici ted deterioration of reoxygenation-elicited changes was attenuated by NO su pplement with S-nitroso-N-acetyl penicillamine, N-w-Nitro-L-arginine methyl ester, an NO synthase inhibitor, mimicked biliary changes elicited by the ETB blockade but without causing notable cell injury. Under these circumsta nces, coadministration of clotrimazole, an inhibitor of cytochrome P450 mon o-oxygenases, elicited the injury comparable with that observed under the E TB blockade. These results suggest that ETB-mediated signaling limits exces sive bile acid excretion and plays a protective role against reoxygenation injury through mechanisms involving both NO-dependent an processes.