EVIDENCE FOR AN ATP-DEPENDENT BILE-ACID TRANSPORT PROTEIN OTHER THAN THE CANALICULAR LIVER ECTO-ATPASE IN RATS

Background & Aims: Canalicular secretion is rate limiting in overall b lood-to-bile transport of bile acids. Studies using transfected cells have implicated the canalicular ecto-adenosine triphosphatase (ecto-AT Pase) in adenosine triphosphate (ATP)-dependent bile acid transport. H owever, the structural features of this ecto-ATPase are not those anti cipated for an in-to-out ATP-dependent transporter. The aim of this st udy was to explore the possible existence of an ATP-dependent bile aci d transport mechanism distinct from ecto-ATPase. Methods: Bile acid tr ansport activity and ecto-AT-Pase expression were analyzed in primary rat hepatocytes, rat hepatoma HTC cells, and specially adapted HTC (HT C-R) cells using plasma membrane vesicles and Northern blot, slot blot , ribonuclease protection assay, and Western blot analyses. Results: P lasma membranes isolated from HTC-R cells exhibited ATP-dependent taur ocholate transport, which was manyfold greater than that in HTC cells. Hepatocytes showed the highest transport rates. Protein and RNA analy ses showed very low expression of ecto-ATPase in HTC and HTC-R cells c ompared with hepatocytes. There was no difference between the two cell types at both the RNA and protein level. Conclusions: These findings show the presence in HTC-R cells and, apparently in hepatocytes, of on e or more proteins other than the ecto-ATPase that mediate ATP-depende nt transport of bile acids.