NA+-INDEPENDENT, H+-COUPLED TRANSEPITHELIAL BETA-ALANINE ABSORPTION BY HUMAN INTESTINAL CACO-2 CELL MONOLAYERS

Beta-alanine transport across intact human intestinal epithelial (Caco-2) cell layers has been investigated. In Na+-containing solutions, net absorptive flux of beta-alanine from apical-to-basal surfaces is small or absent, despite Na+-dependent intracellular beta-alanine accumulation across both apical and basal surfaces. Upon apical acidification (apical pH 6.0, basal pH 7.5), beta-alanine absorptive flux and accumulation across the apical surface are increased. In Na+-free conditions, a significant absorptive flux of beta-alanine is observed, which is markedly stimulated upon apical acidification (pH 6.0). Cellular accumulation of beta-alanine across the apical but not basal surface is observed in Na+-free conditions, and this is increased by acidic (pH 6.0) solutions. Absorptive beta-alanine flux in Na+-free conditions with acidic apical solutions displays saturation kinetics and competitive inhibition by alanine and glycine, but not valine or serine. Addition of 20 mM beta-alanine to the apical solution of epithelial monolayers loaded with the pH indicator 2',7'-bis(2-carboxyethyl-5(6)-carboxyfluorescein) causes a marked decrement in intracellular pH. Beta-alanine transport is also electrogenic, a concentration-dependent increase in an inward short circuit current being observed in voltage-clamped epithelial monolayers. We conclude that a proton-dependent, but Na+-independent, amino acid transporter is expressed at the apical membrane of human intestinal Caco-2 cells, and we provide direct evidence for amino acid-stimulated proton influx across the apical membrane in this intact epithelial cell system.