Identification of Different Transport Systems for Bile Salts in Sinusoidal and Canalicular Membranes of Hepatocytes

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Abstract
The preservation of the functional polarity of hepatocytes in liver snips (1 .times. 2 .times. 4 mm) was demonstrated by fluorescent microscopic studies using the sodium salt of {N-[7-(4-nitrobenzo-2-oxa-1,3-diazol)]-3.beta.-amino-7.alpha.,12.alpha.-dihydroxy-5.beta.-cholan-24-oyl}-2-aminoethanesulfonic acid. This fluorescent bile salt derivative is not only taken up by hepatocytes of several cell layers at the surface of the snips but also secreted into bile canaliculi. The intact hepatobiliary transport of bile salts by hepatocytes of liver snips demonstrates that they are a useful system for the investigation of those transcellular transport processes which require the integrity of hepatic structure. Photoaffinity labelling of liver snips with the sodium salt of (7,7-azo-3.alpha.,12.alpha.-dihydroxy-5.beta.-[3.beta.-3H]cholan-24-oyl)-2-aminoethanesulfonic acid revealed that the bile-salt-binding membrane polypeptides with apparent Mr values of 54,000 and 48,000 are exclusively located in the sinusoidal membrane, whereas a single bile-salt-binding polypeptide with an apparent Mr of 100,000 is located in the bile-canalicular membrane. Photoaffinity labelling of liver snips at 4.degree. C, when transcellular bile-salt trnsport is insignificant, resulted in the labelling of the two sinusoidal membrane polypeptides and practically no labelling of the polypeptide with an apparent Mr of 100,000. This latter polypeptide was also not labelled when Ca2+ deprivation abolished bile secretion completely. These results indicate that the directed hepatobiliary transport of bile salts in hepatocytes is accomplished by transport systems which are different for sinusoidal uptake and canalicular secretion.