GASTROENTEROLOGY 1997;113:1734 – 1740 Functional Expression of the Apical Na " -Dependent Bile Acid Transporter in Large But Not Small Rat Cholangiocytes GIANFRANCO ALPINI, SHANNON S. GLASER, REBECCA RODGERS, JO LYNNE PHINIZY, WILLIE E. ROBERTSON, JOHN LASATER, ALESSANDRA CALIGIURI, ZIGA TRETJAK, and GENE D. LESAGE Department of Internal Medicine, Scott & White Hospital and Texas A&M University Health Science Center College of Medicine, Temple, Texas Background & Aims: Bile acids interact with cholangio- respect to (1) secretin receptor (SR) gene expression; (2) cytes, resulting in cholangiocyte proliferation and in- secretin-induced opening of Cl 0 channels; and (3) secre- creases in ductal bile secretion in large but not small tin-induced cAMP levels. Because these functions were cholangiocytes. It was proposed that for bile acids to found in large cholangiocytes and large IBDUs (diameter exert these effects on cholangiocytes, a specific up- ú15 mm) and were absent in small cholangiocytes and take mechanism must be present in cholangiocytes. small IBDUs (diameter õ15 mm), we have proposed that The aim of this study was to show the expression of a hormone-responsive segments of the biliary tree are ú15- bile acid transporter in cholangiocytes. Methods: Small mm-diameter ducts. 9 and large cholangiocytes or intrahepatic bile duct units Multiple lines of evidence suggest that bile acids inter- (IBDUs) were isolated from normal rats, and gene ex- act with cholangiocytes. 10–12 For example, cholangiocytes pression for the apical Na / -dependent bile acid trans- in primary culture conjugate bile acids with glycine or porter (ABAT) and the 14-kilodalton ileal cytosolic taurine. 10 Other investigators have proposed a ‘‘cholehe- binding protein (IBABP) was assessed by ribonuclease- patic shunt mechanism’’ by which unconjugated bile protection assays. Tissue and subcellular distribution acids, protonated in the bile duct lumen, are reabsorbed of bile acid transporters was also studied. [ 14 C]- Taurocholate uptake into cholangiocytes was deter- by cholangiocytes and then returned to hepatocytes for mined. Results: Both ABAT and IBABP messenger resecretion via the peribiliary plexus. 11,12 A basolateral RNAs were detected in large but not small cholangio- transporter for conjugated bile acid has been localized in cytes. By immunohistochemistry, ABAT was present in IBDUs. 13 Moreover, we have shown that bile acids at 20 large but not small cholangiocytes. Immunofluores- mmol/L increase in vitro secretory activity and prolifera- cence showed ABAT to be present in the apical mem- tive capacity of large but not small cholangiocytes. 7 We brane of large IBDUs. A Na / -dependent saturable up- found these bile acid effects on large cholangiocytes to take of taurocholate was present in large but not small be completely dependent on the presence of sodium. 7 cholangiocytes. Conclusions: These proteins may me- Furthermore, in a bile acid – fed in vivo model 14 we found diate bile acid uptake from the duct lumen in large increased ductal secretion and cholangiocyte prolifera- ducts, resulting in modification of canalicular bile se- tion. We have proposed that a specific target for bile cretion and modulation of ductal bile secretion and acids exists on large cholangiocytes that signals the re- growth. sponses of cholangiocytes to bile acids. In the present studies, we examined for the presence of an Na / -depen- C dent transporter that may mediate our previously ob- holangiocytes participate in both basal and hor- mone-regulated ductal secretion. 1 Secretin stimu- served bile acid effects on large cholangiocytes. 7 We used lates ductal secretion by interaction with receptors on pure subpopulations of small and large cholangiocytes cholangiocytes 2 through increases in intracellular adeno- and small and large IBDUs. We found the presence of sine 3,5-cyclic monophosphate (cAMP) levels 3–5 and in- gene expression for both the apical Na / -dependent bile creases in the activity of a Cl 0 /HCO 3 -exchanger on chol- angiocytes. 3,6 We have shown that cholangiocytes are Abbreviations used in this paper: ABAT, apical Na / -dependent bile acid transporter; CK-19, cytokeratin 19; GAPDH, glyceraldehyde-3- functionally heterogeneous. 3,7–9 For example, we have phosphate dehydrogenase; IBABP, 14-kilodalton ileal cytosolic bind- shown 3,8,9 that subpopulations of cholangiocytes (sepa- ing protein; IBDU, intrahepatic bile duct unit. rated by size) or isolated intrahepatic bile duct units  1997 by the American Gastroenterological Association 0016-5085/97/$3.00 (IBDUs) of different diameters are heterogeneous with / 5e22$$0022 10-08-97 15:36:40 gasas WBS-Gastro