The aim of this study was to elucidate whether bumetanide, which is a competitive inhibitor of carrier mediated bile acid uptake in liver cells, is transported by bile acid carriers. The expression of hepatocellular transport proteins for bile acid uptake and the uptake of the loop diuretic bumetanide was therefore studied in Xenopus laevis oocytes by injection of rat liver poly(A)(+)-RNA. Three hours after injection, a 70% increase in [3H]taurocholate uptake versus noninjected oocytes was accompanied by an increase in only 24% in the uptake of [3H]bumetanide. Size fractionation of the poly(A)(+)-RNA yielded 33 mRNA fractions of which fraction 21 accounted for an 800% increase of taurocholate transport with only a slight increase in bumetanide uptake. Bumetanide transport was coded by mRNA-fraction 18, which stimulated uptake by 160-200% with a concomitant small increase in taurocholate uptake. Uptake of cholate was induced by both mRNA fractions with almost 2.5 fold greater expression by the bumetanide fraction. Oocyte transport of taurocholate (expressed by fraction 21) and bumetanide transport (expressed by fraction 18) were characterized in terms of Na+ dependency, inhibition by 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid (DIDS) and mutual competition. The results indicate that the bumetanide transporter mRNA is clearly different from the mRNA for the taurocholate transport protein. The mRNA fraction 18 was used for the construction of a cDNA library.