IMMUNOHISTOCHEMICAL DETECTION OF CHLORIDE BICARBONATE ANION-EXCHANGERS IN HUMAN LIVER/

Sodium-independent Cl-/HCO3- exchange activity has been observed in is olated rat hepatocytes and intrahepatic bile duct epithelial cells, wh ere it is involved in intracellular pH regulation and, possibly, bilia ry bicarbonate secretion. Monoclonal antibodies to the membrane domain of human chloride/bicarbonate anion exchanger proteins, AE1 and AE2, were prepared so that we might determine by immunohistochemical method s the presence and location of these antiporters in the human liver. T o obtain the antibody against AE1, we immunized mice with injections o f washed human erythrocytes. The selected monoclonal antibody was foun d to be specific for the 17-kD proteolytic membrane fragment of AE1 pr otein. The antibody to AE2 was produced with a 14-mer synthetic peptid e, whose sequence corresponds specifically to amino acid residues 871 to 884 in the deduced primary structure of human kidney AE2 protein. W hen the monoclonal antibody to AE2 peptide was employed for the immuno histochemical study of liver specimens (by both immunofluorescence and immunoperoxidase), a clearly defined staining was present at the cana licular membrane of hepatocytes, as well as the luminal side of the me mbrane of bile duct epithelial cells from small and medium-sized bile ducts. No staining was observed in the liver parenchyma with the monoc lonal antibody to AE1, which instead strongly decorated the erythrocyt es in liver blood vessels. We conclude that AE2 immunoreactivity is pr esent in human liver, where it localizes very specifically to the memb rane regions, which appear most probably involved in the transport of bicarbonate to bile (i.e., the canalicular membrane of hepatocytes and the apical side of epithelial cells of small and medium bile ducts).