Backround: 

The apical Na⁺/H⁺ exchanger isoform 3 (NHE3) is believed to play an important role in intestinal H⁺/dipeptide uptake through its pH-regulatory function.

Aim: 

This study investigates the effect of PEPT1, NHE3 and Slc26a6 gene ablation on H⁺/dipeptide transport, and tries to understand the molecular function of NHE3 in H⁺-dipeptide transport in the native intestine.

Methods and Results: 

The luminal application of the dipeptide Gly-Sar resulted in a strong increase in murine small intestinal fluid absorption, a short circuit current (Isc) response in chambered jejunal mucosa, a time-dependent increase in ¹⁴C-Gly-Sar uptake in everted jejunal sacs in vitro, and a decrease in pH_i in BCECF-loaded enterocytes in isolated jejunal villi. Genetic ablation of PEPT1 abolished Gly-Sar uptake, Gly-Sar-induced fluid absorption, Isc response, and enterocyte acidification. Genetic ablation or pharmacological inhibition of NHE3 also abolished Gly-Sar-induced fluid absorption, diminished Isc response, but enhanced enterocyte acidification, and did not change PEPT1 abundancy in the brush border membrane. Genetic ablation of the apical Cl^-/HCO₃^- exchanger Slc26a6 also enhanced enterocyte acidification, but did not alter either Gly-Sar induced fluid absorption or Isc response. Contrary to expectation, the transmembrane proton gradient was irrelevant for sustained H⁺/dipeptide transport rates in native jejunum, but the luminal Na⁺ concentration, enterocyte Na⁺/K⁺ ATPase activity, and luminal membrane potential were essential determinants.

Conclusions: 

The data suggest that NHE3 as well as Slc26a6 regulate enterocyte pH_i during H⁺/dipeptide uptake. However, we speculate that NHE3 is the major Na⁺-uptake pathway into enterocytes during dipeptide-induced volume increase, and therefore essential to the maintenance of membrane potential negativity necessary to sustain H⁺/dipeptide transport rates in the native intestine.