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 epit helium from injury. In isolated human duodenal enterocytes in vitro, multip le luminal membrane proteins are involved in acid/base transport. We postul ated that one or more isoforms of the Na+/H+ exchanger (NHE) family is loca ted on the apical surface of human duodenal mucosal epithelial cells and th ereby contributes to duodenal mucosal HCO3- transport. Duodenal biopsies we re obtained from human volunteers, and the presence of NHE2 and NHE3 was de termined by using previously characterized polyclonal antibodies (Ab 597 fo r NHE2 and Ab 1381 for NHE3). In addition, proximal duodenal mucosal HCO3- transport was measured in humans in vivo in response to luminal perfusion o f 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 locali zed principally to the brush border of duodenal villus cells. Sequential do ses of amiloride resulted in significant, step-wise increases in net duoden al HCO3- output. Inhibition of NHE2 with 10(-5) M and 10(-4) M amiloride si gnificantly increased net HCO3- output. Moreover, there was an additional, equivalent increase (P < 0.05) in duodenal HCO3- output with 10(-3) M amilo ride, 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 i ncreases in net HCO3- output; 3) NHE2 and NHE3 participate in human duodena l villus cell HCO3- transport; and 4) the contribution of NHE-related trans port events should be considered when studying duodenal HCO3- transport pro cesses.