Abstract
The biliary excretion of pravastatin in normal rats and Eisai hyperbiliruminemic rats (EHBRs) was examined in vivo andin vitro using bile canalicular membrane vesicles (CMVs).In vivo, the total body clearances at steady-state (CLtot) for both rat strains decreased as the infusion rate increased. At the lowest infusion rate,CLtot for normal rats was 1.6 times higher than that for EHBRs. Under this set of conditions, the biliary excretion clearance (CLbile), defined as the biliary excretion rate at steady-state divided by the concentration in the liver (Cliver), for normal rats was 3-fold higher than that for EHBRs. TheCLbile fell markedly with increasingCliver for normal rats and the Michaelis constant (KM ) forCliver was 180 μM; in contrast, the degree of saturation was slight if any in EHBRs. In vitro, the uptake of pravastatin by CMVs prepared from normal rats exhibited clear ATP-dependence, whereas only a minimal effect of ATP was observed on the uptake by CMVs from EHBRs. Transport kinetic studies were performed over a wide range of pravastatin concentration (0.2–10,000 μM) with a tracer tritium-labeled pravastatin. Saturation was observed both in the ATP-dependent (KM : 220 μM) and ATP-independent (KM : 480 μM) uptake by CMVs prepared from normal rats. ATP-dependent uptake of 2,4-dinitrophenyl glutathione, a typical substrate for the canalicular multispecific organic anion transporter (cMOAT), was inhibited by pravastatin in a concentration-dependent manner and the resultant inhibitory constant of pravastatin (170 μM) was comparable with theKM value of ATP-dependent pravastatin uptake itself. In conclusion, biliary excretion of pravastatin is mediated mainly by cMOAT in normal rats. This can explain the decrease in the biliary excretion of pravastatin in EHBRs.
Footnotes
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Send reprint requests to: Dr. Yuichi Sugiyama, Faculty of Pharmaceutical Sciences, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113, Japan.
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This study was supported in part by a Grant-in-Aid for Scientific Research provided by the Ministry of Education, Science and Culture of Japan.
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↵2 K. Ni’inuma, O. Takenaka, T. Horie, K. Kobayashi, Y. Kato, H. Suzuki and Y. Sugiyama, submitted toJ. Pharmacol. Exp. Ther.
- Abbreviations used are::
- HMG-CoA
- hydroxymethylglutaryl CoA, EHBRs
- Eisai hyperbilirubinemic rats
- cMOAT
- canalicular multispecific organic anion transporter
- CMVs
- bile canalicular membrane vesicles
- CLtot
- total body clearance
- I
- infusion rate
- Cp,ss
- plasma concentration of pravastatin at steady-state
- Vbile
- biliary excretion rate
- KM
- Michaelis constant
- Vmax
- maximum transport velocity
- Pdif
- nonspecific diffusion clearance
- AIC
- Akaike’s information criterion
- Ki
- inhibitory constant
- CLbile
- biliary excretion clearance
- Cliver
- pravastatin concentration in the liver
- DNP-SG
- 2
- 4-dinitrophenyl glutathione, GSH
- glutathione
- Tm
- maximal secretory rate.
- Received February 10, 1997.
- Accepted July 7, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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