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 (Km) (70 and 6 μmol/L) and similar maximum velocities (Vmax) (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 Km values (12.6 ± 6.2 and 14.6 ± 1.9 μmol/L). These Km 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 Km and/or passive diffusion.
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