Abstract

Eisai hyperbilirubinemic rat (EHBR) is a mutant strain with a hereditary defect in canalicular multispecific organic anion transporter (cMOAT). We examined the uptake and mutual inhibition of S-(2,4-dinitrophenyl)-glutathione (DNP-SG), which is a typical substrate for cMOAT, and 6-hydroxy-5,7-dimethyl-2-methylamino-4-(3-pyridylmethyl) benzothiazole (E3040) glucuronide (E-glu) with canalicular membrane vesicles (CMV) prepared from Sprague-Dawley (SD) and EHBR rats to investigate the multiplicity of the organic anion transporter. The ATP-dependent uptake by CMV from SD rats had an apparent K_m of 17.6 μM for DNP-SG and 5.7 μM for E-glu, whereas the corresponding uptake by CMV from EHBR had an apparent K_m of 44.6 μM for E-glu. The effects of E-glu, 4-methylumbelliferone glucuronide (4 MUG), E3040 sulfate (E-sul) and 4-methylumbelliferone sulfate (4 MUS) on the uptake of [³H]DNP-SG were also examined. The uptake of [³H]DNP-SG was inhibited by glucuronides (E-glu and 4 MUG) in a concentration-dependent manner, although it was enhanced by the sulfate conjugates (E-sul and 4 MUS). This enhancement was shown to be caused by an increased DNP-SG affinity for the transporter. In CMV from SD rats, although ATP-dependent uptake of [³H]DNP-SG was almost completely inhibited by E-glu, that of [¹⁴C]E-glu was only reduced to about 30% of controls by DNP-SG. On the other hand, in CMV from EHBR, the ATP-dependent uptake of [¹⁴C]E-glu was not inhibited at all by DNP-SG. Kinetic analysis indicated that E-glu inhibited DNP-SG uptake competitively. In conclusion: 1) cMOAT recognizes both DNP-SG and E-glu, and another transporter present in SD rats is also involved in E-glu transport along with cMOAT; 2) the latter transporter is kinetically similar to the E-glu transporter present in EHBR; 3) E-sul enhances the uptake of DNP-SG by increasing the affinity of glucuronide for the transporter.