The impact of hepatic steatosis on liver circulation, transplantation, ischaemia-reperfusion injury and its relation with nitrous oxide

Abstract

Obesity is now a major health threat in the developed world. To investigate the impact of obesity-induced steatosis on hepatic blood flow, portal venous and hepatic arterial blood flow were measured in the anaesthetized lean and obese Zucker rats. Hepatic tissue oxygenation status was assessed by analyzing the NAD(P)H fluorescence. The results show that although the steatotic liver has a remarkably reduced total and portal venous blood flow, the hepatic arterial blood flow is well maintained. However, the tissue in the steatotic liver is still in a relatively hypoxic state as compared with the lean Zucker rats.

The impact of systemic metabolic conditions on the development of liver steatosis was studied by transplanting normal non-steatotic liver grafts from lean Zucker rats to their obese littermates and following the changes in plasma lipid profile, body weight, graft perfusion, microvascular structure, and liver histology for 3 months. The results demonstrate that transplantation of a non-steatotic liver into an obese Zucker rat initially has a positive effect on lipid metabolism. However, 3 months after transplantation, the donor liver became steatotic with reduced perfusion. Hence, the recipient's metabolic status is pivotal in the maintenance of normal liver graft perfusion after transplantation.

To investigate the vulnerability of the steatotic liver to ischaemia-reperfusion (IR) injury, partial hepatic IR (45' /60') was induced in anaesthetized lean and obese Zucker rats. Mitochondrial membrane potential, hepatocyte nucleus density, and leukocyte adherence were quantitated using intravital fluorescent microscopy (IVFM) during the process. Plasma transaminases and liver histology were also studied. The results indicate that IR leads to significantly depressed mitochondrial membrane potential and greater injury in the steatotic than in the normal liver. In conclusion, the increased sensitivity of the steatotic liver to IR injury would appear to involve both alterations in the microcirculation and to cellular changes.

To elucidate the role of nitric oxide (NO) in the maintenance of hepatic haemodynamics under steatotic condition and during IR injury, hepatic haemodynamic parameters, hepatic microvascular features, and changes in mitochondrial membrane potential were measured during IR (60' /120') in the anaesthetized obese and lean Zucker rats with and without pretreatment with L-NAME (10mg·kg-1 i.a.). Plasma transaminase activities were also compared. The findings suggest that not only is NO vital in the control of hepatic haemodynamics and hepatic microcirculatory perfusion, it is also important in the maintenance of hepatic architecture and the alleviation of hepatic IR injury in both lean and obese Zucker rats. The results also suggest a protective role of NO in the maintenance of metabolic functions and cellular integrity during short-term IR in the steatotic liver.

In an attempt to address in vivo the function of specific nitric oxide synthase (NOS) isoform in hepatic haemodynamics and during hepatic IR, age-matched wildtype (WT),iNOS knockout (iNOS-/-), and eNOS knockout (eNOS-/-) mice were subjected to 60 min partial hepatic ischaemia and examined before and at 2h, 8h, and 24h after reperfusion. The results imply that eNOS is protective in the subacute phase of hepatic IR injury. Moreover, although a role of iNOS in reactive oxygen species (ROS) formation during IR is suggested, there is no evidence of significant protective effect in iNOS-/- mice toward IR injury. On the contrary, the iNOS-/- mutants exhibited delayed histologic recovery after IR insult, implying that iNOS-derived NO may be beneficial in the process.