Abstract

Previous studies have demonstrated that phenobarbital treatment impairs the biliary excretion of acetaminophen glucuronide (AG), although the transport system(s) responsible for AG excretion into bile has not been identified. Initial studies in rat canalicular liver plasma membrane vesicles indicated that AG uptake was stimulated modestly by ATP, but not by membrane potential, HCO₃⁻, or pH gradients. To examine the role of the ATP-dependent canalicular transporter multidrug resistance-associated protein 2 (Mrp2)/canalicular multispecific organic anion transporter (cMOAT) in the biliary excretion of AG, the hepatobiliary disposition of acetaminophen, AG, and acetaminophen sulfate (AS) was examined in isolated perfused livers from control and TR⁻ (Mrp2-deficient) Wistar rats. Mean bile flow in TR⁻ livers was ∼0.3 μl/min/g of liver (∼4-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 (∼5-fold); however, concentrations were still measurable, suggesting that multiple mechanisms, including Mrp2-mediated active transport, may be involved in AS biliary excretion. AG and AS perfusate concentrations were significantly higher in livers from TR⁻ compared with control rats. Pharmacokinetic modeling of the data revealed that the rate constant for basolateral egress of AG increased significantly from 0.028 to 0.206 min⁻¹, consistent with up-regulation of a basolateral organic anion transporter in Mrp2-deficient rat livers. In conclusion, these data indicate that AG biliary excretion is mediated by Mrp2, and clearly demonstrate that substrate disposition may be influenced by alterations in complementary transport systems in transport-deficient animals.