Altered hepatobiliary disposition of acetaminophen glucuronide in isolatedperfused livers from multidrug resistance-associated protein 2-deficient TR- rats

Previous studies have demonstrated that phenobarbital treatment impairs the biliary excretion of acetaminophen glucuronide (AG), although the transpor t system(s) responsible for AG excretion into bile has not been identified. Initial studies in rat canalicular liver plasma membrane vesicles indicate d that AG uptake was stimulated modestly by ATP, but not by membrane potent ial, HCO3-, or pH gradients. To examine the role of the ATP-dependent canal icular transporter multidrug resistance-associated protein 2 (Mrp2)/canalic ular multispecific organic anion transporter (cMOAT) in the biliary excreti on of AG, the hepatobiliary disposition of acetaminophen, AG, and acetamino phen sulfate (AS) was examined in isolated perfused livers from control and TR- (Mrp2-deficient) Wistar rats. Mean bile flow in TR- livers was similar to0.3 mul/min/g of liver (similar to4-fold lower than control). AG biliary excretion was decreased (>300-fold) to negligible levels in TR- rat livers , indicating that AG is an Mrp2 substrate. Similarly, AS biliary excretion in TR- livers was decreased (similar to5-fold); however, concentrations wer e still measurable, suggesting that multiple mechanisms, including Mrp2-med iated active transport, may be involved in AS biliary excretion. AG and AS perfusate concentrations were significantly higher in livers from TR- compa red with control rats. Pharmacokinetic modeling of the data revealed that t he rate constant for basolateral egress of AG increased significantly from 0.028 to 0.206 min(-1), consistent with up-regulation of a basolateral orga nic anion transporter in Mrp2-deficient rat livers. In conclusion, these da ta indicate that AG biliary excretion is mediated by Mrp2, and clearly demo nstrate that substrate disposition may be influenced by alterations in comp lementary transport systems in transport-deficient animals.