Renal and hepatic expression of the ammonium transporter proteins, RhB glycoprotein and RhC glycoprotein

A family of ammonium transporter proteins was recently identified. Members of this family, Rh B Glycoprotein (RhBG) and Rh C Glycoprotein (RhCG) are expressed in the kidney and the liver, important tissues for ammonium metabolism. Immunohistochemical studies demonstrate basolateral RhBG immunoreactivity in the connecting segment (CNT) and collecting ducts, but not in the proximal tubule or the loop of Henle. Colocalization with thiazide sensitive cotransporter and carbonic anhydrase II confirms expression in the CNT, initial collecting tubule (ICT), and throughout the collecting duct. Colocalization with AE1 and pendrin demonstrates expression is greatest in A-type intercalated cells in the cortical collecting duct (CCD), outer medullary collecting duct (OMCD) and inner medullary collecting duct (IMCD), present in the CCD principal cell, and not detectable in either pendrin-positive CCD intercalated cells or in non-intercalated cells in the OMCD and IMCD. RhCG inummoreactivity has a similar axial distribution as RhBG. However, RhCG immunoreactivity is apical, and is detectable in all CCD and outer stripe of OMCD cells. The liver, a second organ involved in ammonia metabolism, also expresses both RhBG and RhCG. Basolateral RhBG immunoreactivity is present in the perivenous hepatocyte, but is not present in either the periportal or midzonal hepatocyte. Hepatic RhCG mRNA is expressed at lower levels than RhBG, and RhCG protein is detected in bile duct epithelium. These findings indicate that RhBG and RhCG are involved in at least two organs that transport ammonia, and that they are located in sites where they are likely to mediate important roles in ammonia transport.