TRANSEPITHELIAL GLYCYLSARCOSINE TRANSPORT IN INTESTINAL CACO-2 CELLS MEDIATED BY EXPRESSION OF H+-COUPLED CARRIERS AT BOTH APICAL AND BASAL MEMBRANES

Glycylsarcosine (Gly-Sar) transport in human intestinal epithelial (Caco-2) cells has been investigated. Gly-Sar transport, from apical to basal surfaces (J(a-b)), and intracellular accumulation are greatest when the apical medium is acidified (apical pH 6.0, basal pH 7.4). Both transport and accumulation are susceptible to saturation and competition. Similarly, Gly-Sar transport, from basal to apical surfaces (J(b-a)), is increased with acidification of the basal medium (basal pH 6.0, apical pH 7.4). Apical addition of 20 mM Gly-Sar (pH 6.0) to Caco-2 cell monolayers loaded with the pH indicator BCECF (2',7'-bis(2-carboxyethyl)-5-(6)-carboxyfluorescein) caused a marked cytosolic acidification. Basolateral application of 20 mM Gly-Sar (pH 6.0) also caused a fall in intracellular pH. These observations are consistent with the expression of H+-coupled dipeptide transporters at both membrane faces of the human intestinal epithelial Caco-2 cell line. We also provide direct evidence for dipeptide-stimulated H+-influx, across both apical and basolateral membranes, in this intact epithelial cell system.