In vivo relevance of Mrp2-mediated biliary excretion of the Amanita mushroom toxin demethylphalloin

To determine which efflux carriers are involved in hepatic phalloidin elimination, hepatobiliary ³H]-demethylphalloin (DMP) excretion was studied in normal Wistar rats and in Mrp2 deficient TR(−) Wistar rats as well as in normal wild-type FVB mice, Mdr1a,b(−/−) knockout mice, and Bcrp1(−/−) knockout mice by in situ bile duct/gallbladder cannulation. A subtoxic dose of 0.03 mg DMP/kg b.w. was used, which did not induce cholestasis in any tested animal. Excretion of DMP into bile was not altered in Mdr1a,b(−/−) mice or in Bcrp1(−/−) mice compared with wild-type FVB mice. Whereas 17.6% of the applied dose was excreted into bile of normal Wistar rats, hepatobiliary excretion decreased to 7.9% in TR(−) rats within 2 h after intravenous application. This decrease was not due to reduced cellular DMP uptake, as shown by normal expression of Oatp1b2 in livers of TR(−) rats and functional DMP uptake into isolated TR(−) rat hepatocytes. Tissue concentrations of phalloidin were also not altered in any of the transgenic mice. Interestingly, the decrease of biliary DMP excretion in the TR(−) rats was not followed by any increase of phalloidin accumulation in the liver but yielded a compensatory excretion of the toxin into urine, indicating that hepatocytes of TR(−) rats expelled phalloidin back into blood circulation.