Endothelin Receptor A Blockade Ameliorates Hypothermic Ischemia– Reperfusion-Related Microhemodynamic Disturbances during Liver Transplantation in the Rat Xing-yi Zhang, M.D., Richard J. B. Francis, Cheuk-kwan Sun, M.D., and Antony M. Wheatley, Ph.D. Microcirculation Research Laboratory, Department of Physiology, Otago School of Medical Sciences, University of Otago, P.O. Box 913, Dunedin, New Zealand Submitted for publication February 19, 2001; published online November 27, 2001 Background. The objective of this study was to in- vestigate the effect of graft treatment with specific endothelin receptor antagonists (ET A and ET B ) on the microhemodynamic disturbances which occur follow- ing ischemia/reperfusion injury during orthotopic liver transplantation (OLT) in the rat. Materials and methods. OLT was performed in male Sprague–Dawley rats. An ET A receptor antagonist (BQ-610; 0.3 mg/kg) or ET B receptor antagonist IRL- 1038 (20 nmol/kg) was administered intraportally into liver grafts in vitro at the beginning of 2- and 6-h cold storage (4°C) using physiological saline. Sham-operated animals served as controls (Cont). Seven groups were studied: Cont; vehicle—2 h (saline treated); ET B antagonist—2 h; ET A antagonist—2 h; vehicle— 6 h; ET A antagonist— 6 h; and ET B antagonist— 6 h. At 1 h after graft implantation, the liver microcirculation was inves- tigated by intravital fluorescence microscopy. Results. In vehicle-treated livers, the hepatic micro- circulation was markedly impaired compared with the Cont as manifested by a reduced lobular perfusion index, increased incidence of sinusoidal nonperfusion, elevated leukocyte adhesion in sinusoids and terminal hepatic venules, and increased hepatic venous resis- tance (23-fold; 6-h group). In addition, plasma liver enzymes were significantly elevated in the vehicle treated groups. Alterations to all these parameters were markedly reduced in the ET A receptor antagonist-treated liver grafts although there was still evidence of hepatic injury. The ET B receptor antago- nist had little effect on the I/R-induced changes to the hepatic microcirculation. Conclusions. Our results indicate that the ET A an- tagonism ameliorates hypothermic I/R-related micro- hemodynamic disturbances during OLT in the rat, suggesting that application of an ET A antagonist to liver grafts may have therapeutic potential in human liver transplantation. © 2001 Elsevier Science Key Words: endothelin; intravital microscopy; isch- emia/reperfusion; liver; rat; transplantation. INTRODUCTION Liver graft dysfunction related to hypothermic isch- emia and reperfusion (I/R) injury, during the organ transplantation operation, remains a major contribu- tor to morbidity and mortality in clinical liver trans- plantation [1]. The precise mechanism of hepatic I/R injury is still obscure. Current thinking, however, sug- gests that disruption to the hepatic microcirculation is a pivotal determinant of this injury [2] with (i) leukocyte– endothelium interaction and (ii) an imbal- ance in the production of vasoconstrictor and vasodila- tor substances within the liver. In addition, other fac- tors including reactive oxygen species, inflammatory cytokines, leukotrienes, platelet-activating factor, and proteases are all considered to contribute in part to the injuries (see [1] for an overview). Endothelins (ET) are synthesized in endothelial cells and released into the blood stream where they primar- ily exert their action via two ET receptors, the ET A , and the ET B receptors. ET-1 has been shown to exert a powerful and prolonged vasoconstriction giving rise to an increase in total peripheral resistance and a de- crease in cardiac output [3, 4]. Only in the recent past has the effect of ET on hepatic hemodynamics been investigated. In the anesthetized rat, the application of exogenous ET has been shown to reduce hepatic per- fusion [5], increase portal pressure [6], and cause a decrease in sinusoidal width [6] and cholestasis [7]. Both ET A and ET B receptors are found on vascular Journal of Surgical Research 102, 63–70 (2002) doi:10.1006/jsre.2001.6246, available online at http://www.idealibrary.com on 63 0022-4804/01 $35.00 © 2001 Elsevier Science All rights reserved.