Evidence for an ATP-dependent bile acid transport protein other than the canalicular liver ecto-ATPase in rats.
Canalicular secretion is rate limiting in overall blood-to-bile transport of bile acids. Studies using transfected cells have implicated the canalicular ecto-adenosine triphosphatase (ecto-ATPase) in adenosine triphosphate (ATP)-dependent bile acid transport. However, the structural features of this ecto-ATPase are not those anticipated for an in-to-out ATP-dependent transporter. The aim of this study was to explore the possible existence of an ATP-dependent bile acid transport mechanism distinct from ecto-ATPase. Bile acid transport activity and ecto-ATPase expression were analyzed in primary rat hepatocytes, rat hepatoma HTC cells, and specially adapted HTC (HTC-R) cells using plasma membrane vesicles and Northern blot, slot blot, ribonuclease protection assay, and Western blot analyses. Plasma membranes isolated from HTC-R cells exhibited ATP-dependent taurocholate transport, which was many-fold greater than that in HTC cells. Hepatocytes showed the highest transport rates. Protein and RNA analyses showed very low expression of ecto-ATPase in HTC and HTC-R cells compared with hepatocytes. There was no difference between the two cell types at both the RNA and protein level. These findings show the presence in HTC-R cells and, apparently in hepatocytes, of one or more proteins other than the ecto-ATPase that mediate ATP-dependent transport of bile acids.