Biliary Secretion of α-Tocopherol and the Role of the mdr2 P-Glycoprotein in Rats and Mice

The mechanism by which α-tocopherol (α-T) is secreted into the bile is not known; however, we have previously demonstrated that treatment with piperonyl butoxide (PIP, 1 g/kg) results in increased biliary output of both α-T and phosphatidylcholine within 3 h of ip injection in rats and that the biliary output of both substances was prevented by chemicals that disrupt microtubules (Toxicol. Appl. Pharmacol.139, 411–417 (1996)). The P-glycoprotein (Pgp) encoded by the mdr2 gene has been shown to transport phosphatidylcholine into the bile; therefore, in the current study, we utilized the Pgp inhibitor verapamil to investigate the possible involvement of Pgps in the biliary secretion of α-T. When rats were iv injected with verapamil (4 mg/kg) 10 min prior to PIP treatment, verapamil prevented the PIP-induced increases in biliary α-T and phosphatidylcholine output and resulted in biliary α-T outputs that were significantly less than controls. Also, we determined that the biliary α-T levels in mdr2 knockout mice were 25% of those in wildtype mice; furthermore, mdr2 liver, lung, and kidney levels of α-T and glutathione differed from those of wildtype. To investigate the fate of biliary α-T, we injected14C-labeled α-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 α-T undergoes enterohepatic circulation and that the biliary secretion of α-T, basally and following chemical treatment, is dependent on the presence of a functioning mdr2 Pgp in rats and mice.