EXPRESSION OF NHE-3 IN THE APICAL MEMBRANE OF RAT RENAL PROXIMAL TUBULE AND THICK ASCENDING LIMB

Apical membrane Na/H exchange is a principal mechanism of renal proxim al tubule Na absorption and H secretion, and thick ascending limb H se cretion. Based on current data on Na/H exchanger isoforms (NHE-1 to 5) , NHE-3 is the likeliest candidate for the apical membrane isoform. Th e present study localizes NHE-3 in rat kidney using polyclonal antiser a against cytoplasmic epitopes of rat NHE-3. These antisera recognized an similar to 87 kD protein in Na/H exchanger-deficient cells transfe cted with the rat NHE-3 gene but not in mock-transfected cells. All an tisera labeled an similar to 87 kD protein in plasma membranes from co rtex and outer medulla. Fractionation of cortical membranes showed lab eling in apical but not basolateral membranes. Cross linking studies s uggested existence of oligomeric forms of the transporter. Immunohisto chemistry showed strong staining of the apical membrane of S1 convolut ed, and S2 convoluted tubule with lesser staining of the S2 straight t ubule and absent staining of S3. Weak staining was observed in thin de scending limbs in the inner stripe and intense staining was seen in th e apical membrane of medullary and cortical thick ascending limbs. NHE -3 staining was absent in the remainder of the nephron. In summary, NH E-3 is the isoform responsible for NaCl and NaHCO3 absorption in the p roximal convoluted tubule, and NaHCO3 absorption in the thick ascendin g limb. In the S3 proximal tubule and the distal convoluted tubule, ap ical membrane Na/H exchange activity is likely mediated by other isofo rm(s) of the NHE family.