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Hepatobiliary excretions of drugs from the blood to the bile include two essential transmembrane processes: uptake into hepatocytes and secretion from hepatocytes. The purpose of this study was to clarify the transport mechanisms underlying these processes for a new non-peptide endothelin antagonist, (+)-(5S,6R,7R)-2-butyl-7-[2((2S)-2-carboxypropyl)-4-methoxyphenyl]-5-(3,4-methylenedioxy-phenyl)cyclopentenol[1,2-b]pyridine-6-carboxylic acid (J-104132).Biliary excretion of J-104132 was assessed in rats after intravenous injection. To evaluate the hepatic uptake process, J-104132 was incubated with freshly isolated rat hepatocytes and the uptake of J-104132 was calculated. To evaluate the biliary secretion process, the uptake of J-104132 into rat canalicular membrane vesicles that were isolated from normal Sprague-Dawley rats or Eisai hyperbilirubinemic rats was measured.After intravenous injection, J-104132 was recovered from the bile quantitatively (99.7 +/- 1.3%) as its intact form. J-104132 was taken up by isolated rat hepatocytes in a time- and temperature-dependent manner. The uptake was saturable with Km and Vmax of 5.7 microM and 564 pmol/min/10(6) cells, respectively. The uptake was Na+ independent and was reduced in the presence of ATP depleters (rotenone and carbonyl cyanide-p-(trifluoromethoxy)-phenylhydrazone), organic anions (dibromosulfophthalein, indocyanine green, BQ-123, and pravastatin), and bile acids (taurecholate and cholate). In Sprague-Dawley rats, J-104132 was taken up by canalicular membrane vesicle ATP-dependently with Km and Vmax values of 6.1 microM and 552 pmol/min/mg protein, respectively. However, ATP-dependent uptake disappeared in Eisai hyperbilirubinemic rats.These data suggest that energy-dependent and carrier-mediated transport systems play important roles in hepatobiliary excretion of J-104132 (both uptake and secretion processes), which is the main excretion route in rats. As for the secretion process of J-104132, an involvement of mrp2 was demonstrated. |
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