Differential regulation of NHE isoforms by sodium depletion in proximal and distal segments of rat colon

Dietary sodium depletion has multiple diverse effects on ion transport in t he rat colon, including both the induction and inhibition of electroneutral NaCl absorption in proximal and distal colon of rat, respectively. To esta blish the mechanism of the differential regulation of Na+ absorption by sod ium depletion, this study utilized 1) HOE-694, a dose-dependent inhibitor o f Na+/H+ exchanger (NHE) isoforms, in studies of proton gradient-driven Na- 22 uptake (i.e., Na+/H+ exchange) by apical membrane vesicles (AMV); 2) Nor thern blot analyses of NHE isoform-specific mRNA abundance; and 3) Western blot analyses of NHE isoform-specific protein expression. HOE-694 inhibitio n studies establish that 25 mu M HOE-694-sensitive (NHE2) and 25 mu M HOE-6 94-insensitive (NHE3) Na+/H+ exchange activities are present in AMV of both proximal and distal colon of normal rats. In proximal colon, dietary sodiu m depletion enhanced both NHE2 and NHE3 isoform-specific Na+/H+ exchange ac tivities, protein expression, and mRNA abundance. In contrast, in distal co lon both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance were inhibited by sodium depletion. NHE 1 i soform-specific mRNA abundance in proximal or distal colon was not altered by sodium depletion. Differential effects by sodium depletion on Na+/H+ exc hange in rat colon are tissue specific and isoform specific; sodium depleti on both induces and inhibits apical Na+/H+ exchange at a pretranslational l evel.