Effect of intravenous bilirubin infusion on biliary phospholipid secretion, hepatic P-glycoprotein expression, and biliary cytotoxicity in pigs

Background: Infusion of large intravenous bilirubin loads in bile acid-depl eted pigs reduces P-glycoprotein-dependent biliary phospholipid secretion a nd increases the cytotoxicity of bile. The reasons for the diminution of bi liary phospholipid secretion and the increase in biliary cytotoxicity are n ot known. This study was undertaken to determine whether the bilirubin-indu ced lowering of biliary phospholipid secretion is associated with alteratio ns in hepatic P-glycoprotein (P-gp) expression and to determine why bilirub in infusions increase biliary cytotoxicity. Methods: Hepatic bile was colle cted from bile acid-depleted pigs before and during intravenous bilirubin i nfusion. Hepatic P-gp expression was measured with protein blot analysis, u sing the P-gp-specific antibody C219. Biliary cytotoxicity was assayed agai nst erythrocytes. The biliary phospholipid fatty acid profile was determine d by means of gas chromatography. Results: Bilirubin infusions lowered bili ary phospholipid secretion by 69% without changing hepatic P-gp expression, suggesting that bilirubin infusions have an inhibitory effect on hepatic P -gp activity. Bilirubin infusions did not cause P-gp losses into bile. An u nequivocal, proportional relationship (r(2) = 0.80) pertained between cytot oxicity and the bile acid to phospholipid ratio in bile secreted before and during bilirubin infusion and in phosphatidylcholine-supplemented bile. Un conjugated bilirubin in bile did not contribute to biliary cytotoxicity. Bi liary phospholipids were always phosphatidylcholine much greater than phosp hatidylethanolamine, mainly of C-16:0,C- 18:2 and C-16:0,C- 18:1 fatty acid configuration. Conclusions: Intravenous bilirubin loads reduce biliary pho spholipid secretion without changing hepatic P-gp expression. Bilirubin inf usions increase biliary cytotoxicity by augmenting the biliary bile acid to phospholipid ratio.