Functional characterization of the basolateral rat liver organic anion transporting polypeptide

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Abstract
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.)