Functional characterization of the basolateral rat liver organic anion transporting polypeptide

To characterize the transport functions of a recently cloned basolateral organic anion transporting polypeptide of rat hepatocytes we performed further kinetic transport and substrate cis‐inhibition studies in organic anion‐transporting polypeptide—cRNA injected Xenopus laevis oocytes. The studies demonstrate saturable Na⁺‐independent sulfobromophthalein (Michaelis‐Menten constant, 1.5 μmol/L) and taurocholate (Michaelis‐Menten constant, 50 μmol/L) uptake by organic anion‐transporting polypeptide. Sulfobromophthalein uptake was inhibited by the following organic anions: 0.01 mmol/L bilirubin (43%), 0.1 mmol/L indocyanine green (81%), 0.1 mmol/L 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS; 52%) and 1 mmol/L probenecid (74%). Competitive inhibition was shown for indocyanine green (inhibition constant about 1.3 μmol/L). Sulfobromophthalein and taurocholate uptakes were also inhibited by cholate, chenodeoxycholate, deoxycholate and ursodeoxycholate, as well as their glycine and taurine conjugates. Organic anion‐transporting polypeptide also mediated uptake of glycocholate, tauroursodeoxycholate and taurochenodeoxycholate. No cis‐inhibition of sulfobromophthalein uptake was seen in the presence of ATP, para‐aminohippuric acid, bumetanide, digitoxin, reduced glutathione, leukotriene C₄, nicotinic acid, ouabain, oxalate, rifampicin, succinate or sulfate. Furthermore, radioactively labeled paraaminohippuric acid, α‐ketoglutarate and reduced glutathione were not taken up by organic aniontransporting polypeptide in cRNA‐injected frog oocytes. These data confirm that organic aniontransporting polypeptide represents a novel hepatocellular organic anion uptake system that can mediate Na⁺‐independent transport of monovalent (e.g., bile acids) and divalent (e.g., sulfobromophthalein and indocyanine green) cholephilic organic anions. A variety of substrates previously shown to inhibit uptake of sulfobromophthalein and bile acids in perfused rat livers, isolated hepatocytes and basolateral membrane vesicles had no cis‐inhibitory effects on organic aniontransporting polypeptide—mediated organic anion transport in X. laevis oocytes. Thus additional Na⁺‐independent organic anion carriers must be present in the basolateral membranes of rat hepatocytes. (Hepatology 1994;20:411‐416.)