EXPRESSION OF THE HEPATOCELLULAR CHLORIDE-DEPENDENT SULFOBROMOPHTHALEIN UPTAKE SYSTEM IN XENOPUS-LAEVIS OOCYTES

The expression of the basolateral chloride-activated organic anion uptake system of rat hepatocytes has been studied in Xenopus laevis oocytes. Injection of oocytes with rat liver poly(A)+RNA resulted in the functional expression of chloride-dependent sulfobromophthalein (BSP) uptake within 3-5 d. This expressed chloride-dependent BSP uptake system exhibited saturation kinetics (apparent K(m) approximately 6.2-mu-M) and efficiently extracted BSP from its binding sites on BSA. Furthermore, the chloride-activated portion of BSP uptake was inhibited by bilirubin (10-mu-M; inhibition 53%), 4,4'-diisothiocyano-2,2-disulfonic acid stilbene (DIDS, 100-mu-M; 80%), taurocholate (100-mu-M; 80%), and cholate (200-mu-M; 95%). In contrast to results with total rat liver mRNA, injection of mRNA derived from the Na+/bile acid cotransporter cDNA (Hagenbuch, B., B. Stieger, M. Foguet, H. Lubbert, and P. J. Meier. 1991. Proc. Natl. Acad. Sci. USA. In press.) had no effect on BSP uptake into oocytes. Size fractionation of total rat liver mRNA revealed that a 2.0- to 3.5-kb size-class mRNA was sufficient to express the hepatic chloride-dependent BSP uptake system. These data indicate that "expression cloning" in oocytes represents a promising approach to ultimately clone the cDNA coding for the hepatocyte high affinity, chloride-dependent organic anion uptake system. Furthermore, the results confirm that the Na+/bile acid cotransport system does not mediate BSP uptake.