Role of the bile acid transport system in hepatocellular ouabain uptake
HeLa cells take up 3H-ouabain by internalization together with Na+/K+-ATPase (4). Similar endocytotic uptake was suggested for rat hepatocytes which excrete ouabain into bile (10). On isolated rat hepatocytes carrier mediated uptake of ouabain has been described (5) but any identification of the transport system is still lacking. By using rauwolfia alkaloide cevadine we have proved that ouabain in contrast to HeLa cells is not taken up in rat hepatocytes by Na+/K+-ATPase internalization (17). We speculate additional binding proteins in rat liver cell membrane with properties for glycoside translocation and suggest the bile acid transport system to be such a candidate. Ouabain uptake is inhibited by DIDS, probenecid, furosemide and cevadine which inhibit bile acid uptake concomitantly. Bile acids are inhibitors of ouabain uptake. Cells which lack bile acid uptake, e. g. AS-30 D ascites hepatoma cells and Ehrlich ascites tumor cells do not transport ouabain. Digitoxin, digoxin and cassaine as compared to ouabain are stronger inhibitors of 3H-ouabain uptake. They inhibit cholate uptake markedly.
In order to identify membrane proteins probably involved in ouabain uptake, rat hepatocytes were incubated with benzacidolysin-ouabain for photoaffinity labelling. The label was covalently attached to a 50 kDa protein which was not identified in transport deficient Ehrlich cells. With respect to recent results in the identification of the bile acid transporter from hepatocytes (14, 23) we propose ouabain as substrate for the hepatic bile acid uptake system.
KeywordsGlycoside Digoxin Ghost Photolysis Furosemide
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