.-The aim of this study was to determine whether estrogen plays any role in the hepatic regeneration of nonligated lobe following portal branch ligation (PBL). Male rats were subjected to PBL on the left and middle lobes. Two and 7 days after PBL, the rats were killed and blood and liver samples were analyzed. Sham animals underwent only laparotomy. The serum estradiol levels were significantly elevated on day 2 following PBL and returned to normal levels on day 7. The expression of estrogen receptors (ER) in the liver evaluated by Western blotting did not show any change in the nonligated lobe compared with shams. Immunohistochemical study for ER showed a predominant ER expression in the hepatocyte nucleus in periportal area (zone 1), although there was no apparent difference in the amount and expression pattern between sham and PBL. However, chronic inhibition of ER by an ER antagonist (ICI 182,780) showed a significantly lower regeneration rate of the nonligated lobe compared with vehicle treatment. Liver regeneration-associated genes also were less activated in the ICI group. Moreover, portal venous flow, determined by fluorescent microsphere injection, was significantly lower in the ICI group compared with vehicle group. These changes correlated with the attenuated expression of endothelial nitric oxide synthase mRNA in both superior mesenteric arteries and veins. In conclusion, these results indicate that the estrogen's contribution on hepatic regeneration following PBL is at least partly mediated through maintaining mesenteric blood flow by mesenteric endothelial nitric oxide synthase upregulation rather than directly activating liver regeneration in the liver. portal vein embolization; nonligated lobe; estrogen receptor; endothelial nitric oxide synthase; mesenteric flow PREOPERATIVE PORTAL VEIN EMBOLIZATION (PVE) has been performed widely before extended hepatectomy for hepatoma (21), metastatic colorectal cancer (16), and biliary cancer (27). After PVE for the hepatic lobe to be resected, the nonembolized lobe enlarges and ensures the safety of major hepatectomy. Patients in the PVE group were better than the non-PVE group with respect to lower concentration of postoperative peak bilirubin, fewer occurrence of liver failure, and shorter hospital stay (5, 12). Interestingly, a report analyzing 84 consecutive cases of PVE demonstrated that female sex is preferable in the process of hepatic regeneration (14), indicating that gender or sex hormone milieu has some influence on the regeneration rate following PVE. A better hepatic regeneration rate in female subjects was also observed after major hepatectomy (31). However, these studies were retrospective analyses of patients and thus the precise mechanism with regard to the effect of gender or sex hormone in the process of hepatic regeneration following PVE is not known.Estrogen, a representative female sex hormone, has been shown to modulate hepatic regeneration in experimental hepatectomy models (4, 6 -8). Serum concentrations of estradiol were elevated f...