Altered Hepatobiliary Disposition of Acetaminophen Glucuronide in Isolated Perfused Livers from Multidrug Resistance-Associated Protein 2-Deficient TR− Rats1
- 1Division of Drug Delivery and Disposition, School of Pharmacy (H.X., K.C.T., K.L.R.B.) and 2Curriculum in Toxicology, School of Medicine (E.S.W., K.L.R.B.), University of North Carolina at Chapel Hill, Chapel Hill, North Carolina; and 3Division of Gastroenterology and Hepatology, University Hospital, Gröningen, The Netherlands (P.L.M.J.)
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.
Footnotes
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Send reprint requests to: Kim L. R. Brouwer, Pharm.D., Ph.D., CB# 7360, 28 Beard Hall, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7360. E-mail:kbrouwer{at}unc.edu
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↵1 This work was supported by National Institutes of Health Grant GM41935 and National Institute of Environmental Health Sciences Grant T32 ES07126.
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↵2 Current address: Bristol-Myers Squibb, P.O. Box 4000, Princeton, NJ 08543.
- Abbreviations:
- AG
- acetaminophen glucuronide
- APAP
- acetaminophen
- Mrp2
- multidrug resistance-associated protein 2
- cMOAT
- canalicular multispecific organic anion transporter
- bLPM
- basolateral liver plasma membrane
- cLPM
- canalicular liver plasma membrane
- AS
- acetaminophen sulfate
- TC
- taurocholate
- KHCO3
- potassium bicarbonate
- KPAG
- rate constant for the basolateral egress of AG
- KOTHER
- first-order rate constant for all elimination pathways other than the formation of AG and AS
- KmAS
- Michaelis-Menten constant for AS formation
- KBAG
- rate constant for the canalicular egress of AG
- KBAS
- rate constant for the canalicular egress of AS
- VmaxAS
- maximum velocity for AS formation
- CLAS
- clearance of AS from perfusate to hepatocytes
- KAG
- first-order rate constant for AG formation
- V
- apparent volume of distribution of APAP
- KPAS
- rate constant for the basolateral egress AS
- VR
- volume of perfusate
- Mrp3
- multidrug resistance-associated protein 3
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- Received May 17, 2000.
- Accepted July 24, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
