['Low-flow hypoxia induces xanthine oxidase-dependent hydrogen peroxide production by hepatocytes in the midzone of blood-perfused rat livers and apoptosis in sinusoidal endothelial cells (SECs). As Bcl-2 is a potent inhibitor of apoptotic cell death and is localized mainly in the inner mitochondrial membrane and crista, the purpose of this study was to determine whether cell-specific changes in mitochondrial Bcl-2 levels could account for the hypoxia-induced apoptosis in SECs.', 'A low-flow hypoxia model was generated in isolated rat livers by reducing perfusate inflow pressure from 10 to 2.5 cmH2O for 2 h. Apoptosis was then evaluated using the TdT-mediated dUTP-digoxigenin nick end-labeling (TUNEL) method. Mitochondrial Bcl-2 protein levels were determined in hepatocytes and SECs using cryosectioning immunogold labeling electron microscopy.Results. TUNEL-positive nonparenchymal cells, identified as SECs, were observed predominantly in the midzone of low-flow hypoxic rat livers, whereas few parenchymal cells were stained. Mitochondrial Bcl-2 levels were higher in SECs than in hepatocytes under control conditions, but they declined significantly during hypoxia, though no morphological signs of apoptosis were apparent. In hepatocytes, by contrast, Bcl-2 levels were unaffected by hypoxia. Pretreatment with a specific xanthine oxidase inhibitor, sodium (-)-8-(3-methoxy-4-phenylsulfinylphenyl) pyrazolo [1,5-a]-1,3,5-triazine-4-olate monohydrate, which blocks production of hydrogen peroxide, also blocked both the hypoxia-induced apoptosis and the decline in mitochondrial Bcl-2 in SECs.'] Does hydrogen peroxide-dependent declines in Bcl-2 induce apoptosis in hypoxic liver?