Acid-base effects on intestinal Na(+) absorption and vesicular trafficking

We examined for vesicular traffickingof the Na⁺/H⁺ exchanger (NHE) in pH-stimulatedileal and CO₂-stimulated colonic Na⁺absorption. Subapical vesicles in rat distal ileum were quantified bytransmission electron microscopy at ×27,500 magnification. Internalization of ileal apical membranes labeled withFITC-phytohemagglutinin was assessed using confocal microscopy, andpH-stimulated ileal Na⁺ absorption was measured afterexposure to wortmannin. Apical membrane protein biotinylation of ilealand colonic segments and Western blots of recovered proteins wereperformed. In ileal epithelial cells incubated inHCO/Ringer or HEPES/Ringer solution, the number ofsubapical vesicles, the relative quantity of apical membrane NHEisoforms 2 and 3 (NHE2 and NHE3, respectively), and apical membranefluorescence under the confocal microscope were not affected by pHvalues between 7.1 and 7.6. Wortmannin did not inhibit pH-stimulatedileal Na⁺ absorption. In colonic epithelial apicalmembranes, NHE3 protein content was greater at aPCO₂ value of 70 than 21 mmHg, was internalized when PCO₂ was reduced, and was exocytosed whenPCO₂ was increased. We conclude that vesicletrafficking plays no part in pH-stimulated ileal Na⁺absorption but is important in CO₂-stimulated colonicNa⁺ absorption.