Inhibition of nitric oxide synthesis in ischemia/reperfusion of the rat liver is followed by impairment of hepatic microvascular blood flow

Impact of ischemia-reperfusion injury on dimensional changes of hepatic microvessels

Dimensional alteration of hepatic microvessels was demonstrated during reperfusion after normothermic hepatic ischemia. Using a specially designed cover glass, it was possible to relocate selected sites of observation and microvessels repeatedly throughout the whole reperfusion time. Twenty minutes of hepatic ischemia resulted in a decrease of sinusoidal diameter (mean +/- SEM; 10.0 +/- 0.3 microns at baseline, 8.2 +/- 0.2 microns after ischemia) and diameter of postsinusoidal venules (26.4 +/- 1.2 at baseline, 23.0 +/- 1.0 after ischemia). In the control group (no ischemia induced) no changes of these parameters were observed. Thus, the reduction of hepatic microvascular cross section was present during the early phase of reperfusion. Hepatic dysfunction was characterized by increased serum activity of liver enzymes and reduction of bile flow in the ischemia-exposed animals. It has been suggested that postischemic dimensional microvascular changes are involved in postischemic liver dysfunction.

Protective effect of ischemic preconditioning on liver preservation-reperfusion injury in rats

BACKGROUND

Ischemic-preconditioning is a process whereby a brief ischemic episode confers a state of protection against subsequent long-term ischemia-reperfusion injury. Ischemic preconditioning has been studied in heart and liver ischemia-reperfusion injury; however, few studies have been performed in the model of preservation-reperfusion injury in liver transplantation. The current study was designed to evaluate the ability of ischemic preconditioning to protect liver grafts from long-term preservation-reperfusion injury.

METHODS

Male Sprague Dawley rats were used as donors and recipients of orthotopic liver transplantation. Ischemic preconditioning was done by interruption of the portal vein and hepatic artery for 5, 10, and 20 min (5-10, 10-10, and 20-10 groups). Reflow was initiated by removal of the clamp for another 10 min in all groups. The liver was removed and placed in a bath with Euro-Collins solution for different preservation times. Tolerance of the transplanted liver to cold ischemia was determined by survival time and liver function tests. Rat tumor necrosis factor was analyzed by a bioassay. Nomega-Nitro-L-arginine methyl ester, L-arginine, or adenosine was administered to block or stimulate the synthesis of nitric oxide (NO) in the rats that received long-term-preserved liver grafts.

RESULTS

Twenty percent of syngeneic rats (n=10) that received a liver graft with a 16-hr cold ischemia time in Euro-Collins solution survived for more than 1 day and 10% survived for more than 5 days. In contrast, 87.5% of rats (n=8) that received a liver graft with ischemic preconditioning (10-10 group) and 16 hr of cold ischemia survived for more than 1 day and 75% for more than 5 days. Recipients of liver grafts with ischemic preconditioning had significantly reduced levels of serum aspartate transaminase and tumor necrosis factor-alpha, as well as increased bile flow, compared with recipients of liver grafts without ischemic preconditioning. Blockage of the NO pathway using Nomega-nitro-L-arginine methyl ester, a stereospecific competitive inhibitor of NO formation, attenuated the protective effect of ischemic preconditioning. Administration of one of two precursors of NO synthesis, L-arginine or adenosine, prolonged the survival of rats that received 16-hr-preserved liver grafts. In addition, L-arginine synergized with short-term ischemic pre conditioning (5-10 group) to increase the survival of rats that received a liver graft with a 16-hr cold ischemia time, and the survival rate was 83% after 5 days. Finally, prolonged ischemic preconditioning (> or = 20 min; 20-10 group) resulted in liver damage and loss of function.

CONCLUSION

The current results show that ischemic preconditioning protects the liver graft from subsequent long-term cold preservation-reperfusion injury in a rat liver transplantation model. The protective role of ischemic preconditioning may be mediated by the endogenous production of NO.