β-Estradiol 17β-d-glucuronide (E217G), an endogenous cholestatic metabolite of estradiol, has been identified as a substrate for both hepatic P-glycoprotein (P-gp) and the multispecific organic anion transporter (MOAT), the liver-specific homologue of the multidrug resistance protein. The aim of the present studies was to determine the role of hepatic P-gp and MOAT in E217G-mediated cholestasis and its biliary excretion using the isolated perfused rat liver. A bolus dose of E217G (2 µmol) alone decreased the bile flow maximally from 1.5 to 0.3 µl/min/g liver. In the presence of an infusion of 1.5 µm daunorubicin or 1.0 µm Taxol, P-gp substrates, E217G cholestasis was blocked such that 2 µmol E217G decreased the bile flow from 1.48 to 1.31 or from 1.70 to 1.31 µl/min/g liver, respectively. In the presence of 1 and 3 µm Taxol, the log dose-response curves for E217G cholestasis were shifted to the right 2-fold and 5-fold, respectively, in a parallel manner. Taxol (10 and 50 µm) inhibited the ATP-dependent transport of 10 µm E217G in canalicular plasma membrane vesicles by 46 and 81%, respectively. Daunorubicin (1.5 µm) also shifted the log dose-response curve for E217G cholestasis to the right about 4-fold. Neither Taxol nor daunorubicin decreased the biliary excretion of E217G. Infusion of cyclosporine (6 µm), an inhibitor of both P-gp and MOAT, significantly blocked both E217G cholestasis and biliary excretion, such that 16 µmol E217G decreased the bile flow only 15–20%. In contrast, bromosulfophthalein, a MOAT substrate, had no effect on either E217G-mediated cholestasis or its biliary excretion. These data indicate that P-gp plays an essential role in E217G-mediated cholestasis and suggest that MOAT functions to deliver high concentrations of E217G to P-gp.

1

This article is dedicated to Dr. Michael P. Gosland, November 22, 1962-February 26, 1996.

2

This work was supported by the National Institute of Child Health and Human Development Grant HD-13250, National Institute of Digestive Disease and Kidney Diseases Grant DK 46923, and Grant IRG 163E from the American Cancer Society.

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