IMPAIRED ACTIVITY OF BILE BILE CANALICULAR ORGANIC ANION TRANSPORTER (MRP2 CMOAT) IS NOT THE MAIN CAUSE OF ETHINYLESTRADIOL-INDUCED CHOLESTASIS IN THE RAT/

To test the hypothesis that impaired activity of the bile canalicular organic anion transporting system mrp2 (cmoat) is a key event in the e tiology of 17 alpha-ethinylestradiol (EE)-induced intrahepatic cholest asis in rats, EE (5 mg/kg subcutaneously daily) was administered to ma le normal Wistar (NW) and mrp2-deficient Groningen Yellow/Transport-de ficient Wistar (GY/TR-) rats. Elevated plasma bilirubin levels in GY/T R- rats increased upon EE-treatment from 65 +/- 8.4 mu mol/L to 183 +/ - 22.7 mu mol/L within 3 days, whereas bilirubin levels remained unaff ected in NW rats. Biliary bilirubin secretion was 1.5-fold increased i n NW rats but remained unaltered in GY/TR- rats. Plasma bile salt conc entrations remained unchanged in both strains, although hepatic levels of the sinusoidal Na+-taurocholate cotransporting protein (ntcp) were markedly reduced. Biliary secretion of endogenous bile salt was not a ffected in either strain. A clear reduction of mrp2 revels in liver pl asma membranes of NW rats was found after 3 days of treatment. The bil e salt-independent fraction of bile flow (BSIF) was reduced from 2.6 t o 2.0 mu L/min/100 g body weight in NW rats with a concomitant 62% red uction of biliary glutathione secretion. The absence of mrp2 and bilia ry glutathione in GY/TR- rats did not prevent induction of EE-cholesta sis; a similar absolute reduction of BSIF, i.e., from 1.1 to 0.6 mu L/ min/100 g body weight, was found in these animals. EE treatment caused a reduction of the maximal biliary secretory rate (S-RM) of the mrp2 substrate, dibromosulphthalein (DBSP), from 1,040 to 695 nmol/min/100 g body weight (-38%) in NW rats and from 615 to 327 nmol/min/100 g bod y weight (-46%) in GY/TR- rats. These results demonstrate that inhibit ion of mrp2 activity and/or biliary glutathione secretion is not the m ain cause of EE-induced cholestasis in rats. The data indicate that al ternative pathways exist for the biliary secretion of bilirubin and re lated organic anions that are also affected by EE.