Elsevier

Microvascular Research

Volume 70, Issue 3, November 2005, Pages 129-136
Microvascular Research

The role of nitric oxide in the modulation of hepatic microcirculation and tissue oxygenation in an experimental model of hepatic steatosis

https://doi.org/10.1016/j.mvr.2005.08.001Get rights and content

Abstract

Background:

Impairment of hepatic microcirculation in fatty liver has been assumed to reduce tolerance of the liver against ischemia–reperfusion injury. The present study was aimed to investigate the role of nitric oxide (NO) in the regulation of hepatic microcirculation and tissue oxygenation in hepatic steatosis.

Methods:

Sprague–Dawley rats (200–250 g) were fed a 2% cholesterol diet (n = 12) to induce hepatic steatosis or normal diet (n = 12) served as controls for 12 weeks. Hepatic blood flow, microcirculation, tissue oxyhemoglobin (HbO2) and cytochrome c oxidase radox status (Cyt Ox) in response to intravenous bolus administrations of l-arginine (300 mg/kg) or l-NAME (20 mg/kg) were assessed.

Results:

Animals which developed moderate hepatic steatosis showed significant increase in tissue level of total lipids. Portal blood flow and hepatic microcirculation were significantly reduced as compared to controls (5.7 ± 0.9 vs. 9.7 ± 0.9 ml/min, P = 0.003 and 114.5 ± 9.5 vs. 167.3 ± 10.0 flux unit, P = 0.003). l-Arginine improved hepatic arterial and portal blood flows as well as microcirculation in fatty livers (P < 0.05), while l-NAME significantly worsened these parameters (P < 0.05). Hepatic tissue HbO2 and Cyt Ox were improved both in fatty and control livers following l-arginine, while l-NAME resulted in decreased HbO2 and Cyt Ox although a transit increase in tissue oxygenation was observed in fatty livers.

Conclusions:

NO is involved in the modulation of hepatic microcirculatory perfusion and oxygenation in cholesterol-induced hepatic steatosis. NO metabolisms may be regulated as a potential therapeutic strategy for impaired microcirculation in hepatic steatosis.

Introduction

Hepatic steatosis which is the result of the abnormal accumulation of triacylglycerol within the cytoplasm of hepatocytes, attributed to the effects of alcohol excess, obesity, diabetes or drugs is a common histological finding in human liver biopsies (Ijaz et al., 2003). Fatty liver affects about 25% of the donors for liver transplantation since severe steatosis is associated with a high risk of primary non-function following liver transplantation. This poor initial function of fatty livers has been shown to be associated with an impairment of the hepatic microcirculation (HM) (Aiba et al., 2001, McCuskey et al., 2004). A significant reduction in HM in liver donors with steatosis was found as compared to normal liver donors (Seifalian et al., 1997, Seifalian et al., 1998, Astarcioglu et al., 2004). Experimental studies in animal models with fatty liver showed that fatty infiltration, classified as mild (<30%), moderate (30–60%) or severe (>60%), reduced total hepatic blood flow and hepatic parenchymal microcirculation and there was an inverse correlation between the degree of fat infiltration and both total hepatic blood flow and flow in the microcirculation (Seifalian et al., 1999). The compromised HM has been suggested to be the result of sinusoidal compression due to the steatosis and hepatocyte enlargement (Akamatsu et al., 1993, Ijaz et al., 2003). However, the precise mechanisms behind the decreased microcirculation in fatty liver have yet to be clarified.

Nevertheless, it is generally accepted that blood flow in the HM is normally maintained by a balance of vasodilators, such as nitric oxide (NO) and vasoconstrictors, like endothelin 1. The role of NO in the regulation of systemic and hepatic hemodynamics involved in liver disease has been extensively studied (Clemens and Zhang, 1999, Rockey, 2001, Wiest and Groszmann, 1999, Yang et al., 2003, Koti et al., 2005). Infusion of an NOS inhibitor enhanced ethanol-induced vasoconstriction in the portal vein whereas the simultaneous infusion of an NO precursor reversed it. These results have suggested that endogenous NO acts as a vasodilator to reduce ethanol-induced vasoconstriction, thus improving the HM (Oshita et al., 1994). An l-arginine infusion was found to selectively augment the portal blood flow in cirrhotic liver (Kakumitsu et al., 1998). NO administration and arginine supplementation in sepsis also demonstrated beneficial effects on hemodynamics and microcirculation (Luiking et al., 2004). However, it is not known if endogenous NO is involved in the hepatic hemodynamic changes resulted from steatosis. There is a general consensus that fatty liver compromises HM regardless of the reason why fatty liver develops in the first place. This may be one of the fundamental reasons why fatty livers tend to be more susceptible to the injurious effects of ischemia and reperfusion. However, there is no published data to date showing the role of NO in the regulation of HM in fatty liver. Therefore, this study investigated the effects of NO stimulation and inhibition on HM and tissue oxygenation in an experimental model of moderate hepatic steatosis induced by rich cholesterol diet.

Section snippets

Animal model and surgical preparation

The study was conducted under a license granted by the Home Office in accordance with the Animals (Scientific Procedures) Act 1986. Male Sprague–Dawley rats weighing between 250 and 300 g were used in this study. Hepatic steatosis was induced by feeding animals with commercial high (2%) cholesterol diet for 12 weeks. The control animals were fed ad libitum. The development of the hepatic steatosis was examined macroscopically and confirmed by histology and tissue lipid analysis.

Prior to the

Animal model of hepatic steatosis

All animals tolerated the high cholesterol diet with no mortality. At laparotomy, liver showed moderate fatty change and, histologically, macrovesicular steatosis was seen under light microscopy. Total lipids in liver tissue of the fatty animals were significantly increased as compared with controls (Fig. 1A). However, there were no significant differences in plasma levels of cholesterol and triglyceride between control and fatty animals (Fig. 1B). There was no significant hepatocellular injury

Discussion

Disrupted HM observed in fatty liver (Hakamada et al., 1997, Teramoto et al., 1993) has been suggested to be the result of sinusoidal compression due to hepatocyte enlargement (Akamatsu et al., 1993) and decreased erythrocyte deformability (Shiraishi et al., 1993). In the present study, HM in the fatty liver induced by high cholesterol diet was significantly reduced that was consistent with significant reductions in PVF while hepatic arterial flow remained unchanged. These results have

Acknowledgments

This study was supported by RMO grant and internal departmental grant.

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