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 the rat colon, including both the induction and inhibition of electroneutral NaCl absorption in proximal and distal colon of rat, respectively. To establish the mechanism of the differential regulation of Na+ absorption by sodium depletion, this study utilized 1) HOE-694, a dose-dependent inhibitor of 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) Northern blot analyses of NHE isoform-specific mRNA abundance; and 3) Western blot analyses of NHE isoform-specific protein expression. HOE-694 inhibition studies establish that 25 mu M HOE-694-sensitive (NHE2) and 25 mu M HOE-694-insensitive (NHE3) Na+/H+ exchange activities are present in AMV of both proximal and distal colon of normal rats. In proximal colon, dietary sodium depletion enhanced both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance. In contrast, in distal colon both NHE2 and NHE3 isoform-specific Na+/H+ exchange activities, protein expression, and mRNA abundance were inhibited by sodium depletion. NHE 1 isoform-specific mRNA abundance in proximal or distal colon was not altered by sodium depletion. Differential effects by sodium depletion on Na+/H+ exchange in rat colon are tissue specific and isoform specific; sodium depletion both induces and inhibits apical Na+/H+ exchange at a pretranslational level.