Abstract

In the present study, we compared the inhibitory effects of organic anions (including bile acids) on the uptake of taurocholate (TC) and estradiol 17β-d-glucuronide (E₂17βG), typical substrates for sodium taurocholate cotransporting polypeptide (Ntcp) and organic anion transporting polypeptide (oatp1), respectively, using primary cultured rat hepatocytes and Ntcp- or oatp1-transfected COS-7 cells. The Na⁺-dependent uptake of TC was inhibited by nine bile acids and five nonbile acid organic anions in a concentration-dependent manner, and their inhibitory effects were similar in both primary cultured rat hepatocytes and Ntcp-transfected COS-7 cells. BQ-123 (1 μM) and indomethacin (10 μM), both of which exhibit no Ntcp-mediated transport, significantly inhibited the Na⁺-dependent uptake of TC mediated by Ntcp. In addition, the Na⁺-independent uptake of E₂17βG was inhibited by 15 organic anions in a concentration-dependent manner, and their inhibitory effects were similar between primary cultured rat hepatocytes and oatp1-transfected COS-7 cells. BQ-123 (1 μM), pravastatin (1 μM), and indomethacin (10 μM), all of which do not undergo oatp1-mediated transport, significantly inhibited the Na⁺-independent uptake of E₂17βG mediated by oatp1. These results are consistent with the hypothesis that the hepatic uptake of TC and E₂17βG is predominantly mediated by Ntcp and oatp1, respectively. In addition, it was clearly demonstrated that we cannot refer to the substrate specificity of transporters based on inhibition studies.