The effect of bilirubin on biliary lipid secretion: analysis by horseradish peroxidase associated intrahepatic vesicular transport system.

The mechanism of biliary lipid secretion is still controversial and there is no definite information regarding how bilirubin inhibits biliary phospholipid and cholesterol secretions without affecting bile salt secretion. In this study, the effects of bilirubin on intrahepatic vesicular transport and biliary lipid secretion were examined using bile-fistula rats. Horseradish peroxidase (HRP) was used as a tracer of intrahepatic vesicular transport. Bilirubin (5 mg/100 g BW) and/or HRP (5 mg/100 gBW) were injected through the mesenteric vein. Bile flow, biliary bile acid, biliary phospholipid and cholesterol outputs were examined in saline, HRP and HRP + bilirubin groups, respectively. Bile flow and biliary bile acid output were not affected by bilirubin administration. Biliary phospholipid and cholesterol as well as biliary HRP outputs were inhibited just after bilirubin administration, 42.8 +/- 6.1 SD% 47.7 +/- 5.1 SD%, and 33.4 +/- 3.8 SD%, respectively. These results suggested the participation of intrahepatic vesicular transport system in the inhibition of biliary lipid secretion by bilirubin and in its secretory mechanism.