A low-flow, reflow model of liver perfusion was used in the rat to investigate the effects of L-arginine on reperfusion injury in the absence of blood elements. In contrast to in vivo studies, L-arginine cannot minimize hypoxia by improving the microcirculation under these special conditions, but rather can only increase oxygen delivery upon reflow. During reflow, lactate dehydrogenase (LDH) release reached a new steady-state value of 35 +/- 3 U/g/h in livers perfused in the absence of L-arginine. L-Arginine (0.1 mmol/L) significantly reduced LDH release during reflow to 14 +/- 1 U/g/h; higher concentrations (1.0-3.0 mmol/L) were less effective and the arginine analogue Nomega-nitro L-arginine methyl ester (L-NAME, 0.3 mmol/L) reversed the protective effect completely. Infusion of the biologically inactive isomer D-arginine (0.1 mmol/L had no effect on the observed reperfusion injury. Malondialdehyde (MDA) release during reflow averaged 92 +/- 10 nmol/g/h and was decreased significantly to 47 +/- 13 nmol/g/h with L-arginine (0.1 mmol/L). Oxygen uptake during reflow was not significantly altered by L-arginine, although the time required to reach steady state values upon reflow was reduced significantly (about twofold), indicating improved microcirculation. Trypan blue distribution time, which is used to index the hepatic microcirculation, was decreased significantly from 330 +/- 17 to 227 +/- 31 seconds by L-arginine, an effect also blocked by L-NAME. Additionally, L-arginine, significantly increased both the rate of entry and exit of fluorescein-dextran, a dye confined to the vascular space, by approximately 50%, also reflecting improved microcirculation. Collectively , these findings indicate that L-arginine protects against hypoxia/reoxygenation injury in a blood-free perfusion model specifically during the reoxygenation period. It is likely that L-arginine rapidly removes substrates for free radical generation by improving the microcirculation.