Optimal absorptive transport of the dipeptide glycylsarcosine is dependent on functional Na+/H+ exchange activity

Optimal nutrient absorption across the intestinal epithelium is dependent on the co-ordinated activity of a number of membrane transporters. Di/tripeptide transport across the luminal membrane of the intestinal enterocyte is mediated by the H+-coupled di/tripeptide transporter hPepT1. hPepTI function is dependent on the existence of a pH gradient (maintained, in part, by the action of the Na+/H+ exchanger NHE3) across the apical membrane of the small intestinal epithelium. The physiological problem addressed here was to determine how two transporters (hPepTI and NHE3), involved in nutrient absorption and pH(i) homeostasis, function co-operatively to maximise dipeptide absorption when both operate suboptimally at typical mucosal surface pH values (pH 6.1-6.8). Functional hPepT1 activity in human intestinal epithelial (Caco-2) cell monolayers was determined by measurement of apical uptake and apical-to-basolateral transport of the dipeptide glycylsarcosine. The dependence of hPepTI on NHE3 activity was measured (either after Na+ removal or addition of the NHE3-selective inhibitor S 1611) using both Caco-2 cell monolayers and hPepT1-expressing Xenopus laevis oocytes. Apical glycylsarcosine uptake in Caco-2 cell monolayers was modulated by apical pH, extracellular Na+, incubation time and S1611. Uptake in hPepT1-expressing oocytes was independent of Na+ or S1611. We conclude that functional NHE3 activity is required to allow optimal absorption of dipeptides across the human intestinal epithelium.