Na+ and pH dependence of proline and β-alanine absorption in rat small intestine

Aims: Early characterization of intestinal absorption of imino acids in mammals has demonstrated the existence of a Na+-dependent, Cl--independent transport system in rat small intestine, which is the only carrier for beta-alanine. Based on the substrate selectivity, it was proposed that the Proton Amino Acid Transporter 1 (PAT1) could be the same as this imino acid carrier. The present study characterizes the pH and Na+ dependence of proline and beta-alanine uptake in rat small intestine. Methods: Intestinal uptake of radiolabelled L-proline or beta-alanine was measured in brush border membrane vesicles and everted intestinal rings, in the presence and absence of Na+ and at different pH values. Results: The existence of an inwardly directed H+ gradient in the absence of Na+ enhanced the initial entry of proline and beta-alanine in brush border membrane vesicles, that reached a transient overshoot with maximal value around 30 s. In the absence of pH gradient, no overshoot was shown. In entire tissue, there was an increase of proline and beta-alanine uptake at acidic pH that was higher in the presence of Na+ than in its absence. This ion dependence and pH effect of the amino acids uptake also increased with the incubation period. Substrate inhibition studies confirmed that intestinal proline absorption in rat occurs mainly by system B and PAT1-like transporter. Conclusions: There is a Na+-independent, H+-dependent transporter of amino acids at the apical membrane of the rat enterocytes.