ing a blood free, low-flow, reflow liver perfusion model. For A low-flow, reflow model of liver perfusion was used example, perfused livers from rats treated with a fish-oil diet in the rat to investigate the effects of L-arginine on repershowed a marked improvement in microcirculation and a sigfusion injury in the absence of blood elements. In connificant reduction in hepatic damage. 2 In addition, Gao et al. trast to in vivo studies, L-arginine cannot minimize hypfound that adenosine, a known vasodialator and an essential oxia by improving the microcirculation under these component in Carolina Rinse solution, improved survival folspecial conditions, but rather can only increase oxygen lowing liver transplantation. 3 delivery upon reflow. During reflow, lactate dehydroge-L-Arginine is the substrate for nitric oxide (NO) synthase, nase (LDH) release reached a new steady-state value of the enzyme responsible for the production of NO, a highly 35 { 3 U/g/h in livers perfused in the absence of L-argi-
unstable free radical gas that mediates smooth muscle relaxnine. L-Arginine (0.1 mmol/L) significantly reduced LDH ation in vascular tissue. 4 Endogenous nitric oxide production release during reflow to 14 { 1 U/g/h; higher concentracan also increase blood flow by reducing adherence of blood tions (1.0-3.0 mmol/L) were less effective and the arginine elements. 5 However, in higher doses, nitric oxide can cause analogue Nv-nitro L-arginine methyl ester (L-NAME, 0.3 cytotoxicity. One form of NO synthase is induced by cytokines mmol/L) reversed the protective effect completely. Infuto produce toxic levels of NO as a defense response. 4 sion of the biologically inactive isomer D-arginine (0.1
Previously, it has been shown that NO exhibits a protective mmol/L) had no effect on the observed reperfusion ineffect against endotoxin-induced hepatic injury in vivo. 6 It jury. Malondialdehyde (MDA) release during reflow avhas also been reported that treatment with L-arginine minieraged 92 { 10 nmol/g/h and was decreased significantly mizes hepatic reperfusion injury following a brief period of to 47 { 13 nmol/g/h with L-arginine (0.1 mmol/L). Oxygen cold storage. 7 While improving the microcirculation would uptake during reflow was not significantly altered by L- clearly minimize hypoxia, it is paradoxical that improving the arginine, although the time required to reach steadymicrocirculation leads to diminished rather than enhanced state values upon reflow was reduced significantly cellular injury because oxygen availability for free radical (about twofold), indicating improved microcirculation.
formation should be increased when the microcirculation is Trypan blue distribution time, which is used to index the improved. The purpose of this study, therefore, was to investihepatic microcirculation, was decreased significantly gate the effects of improvement of the microcirculation with from 330 { 17 to 227 { 31 seconds by L-arginine, an effect L-arginine on reperfusion injury in an attempt to understand also blocked by L-NAME. Additionally, L-arginine signifithis paradox. A low-flow, reflow model of liver perfusion was cantly increased both the rate of entry and exit of fluoreschosen for two important reasons. First, it is a convenient cein-dextran, a dye confined to the vascular space, by and economical means to investigate reperfusion injury in approximately 50%, also reflecting improved microcircuthe absence of the possible complications of blood elements.
lation. Collectively, these findings indicate that L-argi-
Second, arginine can be added specifically during the reoxynine protects against hypoxia/reoxygenation injury in a genation period to dissect the effect of hypoxia from reoxygenblood-free perfusion model specifically during the reoxyation. In this model, pericentral regions of the liver lobule genation period. It is likely that L-arginine rapidly reare made anoxic by reducing the flow rate for an initial period moves substrates for free radical generation by improvof the perfusion. Subsequently, normal flow rates are reing the microcirculation. (HEPATOLOGY 1996;24:163-168.) stored resulting in an oxygen dependent reperfusion injury in pericentral regions of the liver lobule. Preliminary ac-Disruption of the microvasculature is most likely a key counts of this work have appeared elsewhere. 8 factor in mechanisms of hepatic ischemia-reperfusion injury and graft failure following transplantation. 1 Furthermore, re-
Methods
cent studies demonstrated that reperfusion injury can be re-Chemicals. L-Arginine, D-arginine, fluorescein isothiocyanate dexduced by improving the hepatic microcirculation in vitro ustran, and L-arginine methyl ester (Nv-nitro L-arginine methyl ester [L-NAME]) were obtained from Sigma (St. Louis, MO); all other chemicals were from standard sources. Animals and Liver Perfusion. Male Sprague-Dawley rats weighing Abbreviations: NO, nitric oxide; L-NAME, Nv-nitro L-arginine methyl ester; LDH, lacbetween 150 and 200 g were fasted for 24 hours and subsequently tate dehydrogenase; MDA, malondialdehyde; FITC, fluorescein isothiocyanate.
anesthetized with phenobarbital (75 mg/kg) before surgery. All ani-From the