Luminal Na+ homeostasis has an important role in intestinal peptide absorption in vivo

Intestinal cell line studies indicated luminal Na + homeostasis is essential for proton-coupled peptide absorption, because the driving force of PepT1 activity is supported by the apical Na + /H + exchanger NHE3. However, there is no direct evidence demonstrating the importance of in vivo luminal Na + for peptide absorption in animal experiments. To investigate the relationship between luminal Na + homeostasis and peptide absorption, we took advantage of claudin 15-deficient (cldn15 −/− ) mice, whereby Na + homeostasis is disrupted. We quantitatively assessed the intestinal segment responsible for peptide absorption using radiolabeled nonhydrolyzable dipeptide (glycylsarcosine, Gly-Sar) and nonabsorbable fluid phase marker polyethylene glycol (PEG) 4000 in vivo. In wild-type (WT) mice, the concentration ratio of Gly-Sar to PEG 4000 decreased in the upper jejunum, suggesting the upper jejunum is responsible for peptide absorption. Gly-Sar absorption was decreased in the jejunum of cldn15 −/− mice. To elucidate the mechanism underlining these impairments, a Gly-Sar-induced short-circuit ( I sc ) current was measured. In WT mice, increments of Gly-Sar-induced I sc were inhibited by the luminal application of a NHE3-specific inhibitor S3226 in a dose-dependent fashion. In contrast to in vivo experiments, robust Gly-Sar-induced I sc increments were observed in the jejunal mucosa of cldn15 −/− mice. Gly-Sar-induced I sc was inhibited by S3226 or a reduction of luminal Na + concentration, which mimics low luminal Na + concentrations in vivo . Our study demonstrates that luminal Na + homeostasis is important for peptide absorption in native epithelia and that there is a cooperative functional relationship between PepT1 and NHE3. NEW & NOTEWORTHY Our study is the first to demonstrate that luminal Na + homeostasis is important for proton-coupled peptide absorption in in vivo animal experiments.