Human duodenal mucosal brush border Na⁺/H⁺ exchangers NHE2 and NHE3 alter net bicarbonate movement

The proximal duodenal mucosa secretes HCO₃^- that serves to protect the epithelium from injury. In isolated human duodenal enterocytes in vitro, multiple luminal membrane proteins are involved in acid/base transport. We postulated that one or more isoforms of the Na⁺/H⁺ exchanger (NHE) family is located on the apical surface of human duodenal mucosal epithelial cells and thereby contributes to duodenal mucosal HCO₃^- transport. Duodenal biopsies were obtained from human volunteers, and the presence of NHE2 and NHE3 was determined by using previously characterized polyclonal antibodies (Ab 597 for NHE2 and Ab 1381 for NHE3). In addition, proximal duodenal mucosal HCO₃^- transport was measured in humans in vivo in response to luminal perfusion of graded doses of amiloride; 10^-5-10^-4 M amiloride was used to inhibit NHE2 and 10^-3 M amiloride to inhibit NHE3. Both NHE2 and NHE3 were localized principally to the brush border of duodenal villus cells. Sequential doses of amiloride resulted in significant, step-wise increases in net duodenal HCO^-₃ output. Inhibition of NHE2 with 10^-5 M and 10^-4 M amiloride significantly increased net HCO₃^- output. Moreover, there was an additional, equivalent increase (P < 0.05) in duodenal HCO₃^- output with 10^-3 M amiloride, which inhibited NHE3. We conclude that 1) NHE2 and NHE3 are localized principally to the brush border of human duodenal villus epithelial cells; 2) sequential inhibition of NHE2 and NHE3 isoforms resulted in step-wise increases in net HCO₃^- output; 3) NHE2 and NHE3 participate in human duodenal villus cell HCO₃^- transport; and 4) the contribution of NHE-related transport events should be considered when studying duodenal HCO₃^- transport processes.