Transport of the sulfated, amidated bile acid, sulfolithocholyltaurine, into rat hepatocytes is mediated by Oatp1 and Oatp2

The uptake of the sulfated bile acid sulfolithocholyltaurine (SLCT) was investigated in isolated rat hepatocytes and in HeLa cells transfected with complementary DNAs (cDNAs) of organic anion transporting polypeptides (Oatps) 1 and 2 cloned from rat liver. In hepatocytes, transport of SLCT was greatly reduced by bromosulfophthalein (BSP), estrone sulfate, the precursor bile acids cholyltaurine and lithocholyltaurine, and 4, 4′-diisothiocyanostilbene-2-2′-disulfonic acid (DIDS). However, SLCT transport was insensitive to 4-methylumbelliferyl sulfate, harmol sulfate, digoxin, fexofenadine, and lack of sodium ion. Because the estimation of kinetic constants was enhanced with use of inhibitors, BSP (1-50 μmol/L) was added to isolated rat hepatocytes to assess the various transport components for SLCT uptake. The resulting data showed a nonsaturable pathway and at least 2 pathways of different Michaelis-Menten constants (K_m) (70 and 6 μmol/L) and similar maximum velocities (V_max) (1.73 and 1.2 nmol/min/mg protein) and inhibition constants of 0.63 and 10.3 μmol/L for BSP. In expression systems, SLCT was taken up by Oatp1 and Oatp2 expressed in HeLa cells with similar K_m values (12.6 ± 6.2 and 14.6 ± 1.9 μmol/L). These K_m values were comparable to that observed for the high-affinity pathway in rat hepatocytes. In conclusion, the results suggest that transport of SLCT into rat liver is mediated in part by Oatp1 and Oatp2, high-affinity pathways, a lower-affinity pathway of unknown origin, and a nonsaturable pathway that is compatible with a transport system of high K_m and/or passive diffusion.