Expression and functional activity of P-glycoprotein in cultured hepatocytes from Oncorhynchus mykiss

P-glycoproteins encoded by multidrug resistance 1 (mdr1) genes are ATP-depe ndent transporters located in the plasma membrane that mediate the extrusio n of hydrophobic compounds from the cell. Using cultured isolated rainbow t rout hepatocytes, we characterized an mdr1-like transport mechanism of the teleost liver. Immunoblots with the monoclonal antibody C219, which recogni zes a conserved epitope of P-glycoproteins, revealed the presence of immuno reactive protein(s) of 165 kDa in trout liver and cultured hepatocytes. In trout liver sections, the immunohistochemistry with C219 stained bile canal icular structures. Compounds known to interfere with mdr1-dependent transpo rt (verapamil, vinblastine, doxorubicin, cyclosporin A, and vanadate) all i ncreased the accumulation of rhodamine 123 by hepatocytes. Verapamil, vinbl astine, and cyclosporin A decreased the efflux of rhodamine 123 from hepato cytes preloaded with rhodamine 123. By contrast, the substrate of the canal icular cation transporter tetraethylammonium and the inhibitor of the multi drug resistance-associated protein MK571 had no effect on rhodamine 123 tra nsport. The results demonstrate the presence of an mdr1-like transport syst em in the teleost liver and suggest its function in biliary excretion.