Expression of sodium-independent organic anion uptake systems of skate liver in Xenopus laevis oocytes

The expression of the basolateral sodium-independent organic onion uptake system of the little skate (Raja erinacea) has been studied in Xenopus laevis oocytes. Injection of oocytes with skate liver poly(A)⁺ RNA resulted in the functional expression of chloride-dependent sulfobromophthalein (BSP) uptake and sodium-independent taurocholate uptake within 3-5 days. The expressed chloride-dependent BSP uptake activity exhibited saturation kinetics [apparent Michaelis constant (K(m)) 1.8 μM] and efficiently extracted BSP from its binding sites on bovine serum albumin. The chloride-sensitive portion of BSP uptake was inhibited by bilirubin (10 μM; 27%), 4,4'- diisothiocyanostilbene-2,2'-disulfonic acid (100 μM; 57%), bumetanide (100 μM; 48%), taurocholate (200 μM; 515), and cholate (200 μM; 45%). Size fractionation of total skate liver mRNA revealed that a 1.8- to 2.9-kb size class mRNA was sufficient to express chloride-dependent BSP uptake and sodium-independent taurocholate uptake. In addition, a 1- to 1.7-kb size class mRNA expressed sodium-independent taurocholate uptake but had no effects on BSP uptake. This study confirms that an organic onion transport system for chloride-dependent BSP uptake, with characteristics similar to rat liver, is already expressed in the liver of lower vertebrates and thus represents a phylogenetically old system. Sodium-independent taurocholate uptake in skate liver may be mediated by two different transport systems.