BILIARY-SECRETION OF ALPHA-TOCOPHEROL AND THE ROLE OF THE MDR2 P-GLYCOPROTEIN IN RATS AND MICE

The mechanism by which alpha-tocopherol (alpha-T) is secreted into the bile is not known; however, we have previously demonstrated that trea tment with piperonyl butoxide (PIP, 1 g/kg) results in increased bilia ry output of both alpha-T and phosphatidylcholine within 3 h of ip inj ection in rats and that the biliary output of both substances was prev ented by chemicals that disrupt microtubules (Toxicol. Appl. Pharmacol . 139, 411-417 (1996)). The P-glycoprotein (Pgp) encoded by the mdr2 g ene has been shown to transport phosphatidylcholine into the bile; the refore, in the current study, we utilized the Pgp inhibitor verapamil to investigate the possible involvement of Pgps in the biliary secreti on of alpha-T. When rats were iv injected with verapamil (4 mg/kg) 10 min prior to PIP treatment, verapamil prevented the PIP-induced increa ses in biliary alpha-T and phosphatidylcholine output and resulted in biliary alpha-T outputs that were significantly less than controls. Al so, we determined that the biliary alpha-T levels in mdr2 knockout mic e were 25% of those in wildtype mice; furthermore, mdr2 liver, lung, a nd kidney levels of alpha-T and glutathione differed from these of wil dtype. To investigate the fate of biliary alpha-T, we injected C-14-la beled alpha-T into the bile duct cannulae of rats and determined that approximately 60% of the radioactivity was reabsorbed within 1 h. Our results indicate that alpha-T undergoes enterohepatic circulation and that the biliary secretion of alpha-T, basally and following chemical treatment, is dependent on the presence of a functioning mdr2 Pgp in r ats and mice. (C) 1998 Academic Press.