H+-COUPLED DIPEPTIDE (GLYCYLSARCOSINE) TRANSPORT ACROSS APICAL AND BASAL BORDERS OF HUMAN INTESTINAL CACO-2 CELL MONOLAYERS DISPLAY DISTINCTIVE CHARACTERISTICS

Transepithelial transport and intracellular accumulation of the dipeptide glycylsarcosine (Gly-Sar) were studied using intact monolayers of the human intestinal epithelial cell line, Caco-2. Gly-Sar transport was demonstrated in both absorptive (apical-to-basal) and secretory (basal-to-apical) directions. In both directions, transport and accumulation were enhanced in the presence of a pH gradient (pH(o) < pH(i)). Under conditions similar to those found at the intestinal membrane in vivo (apical pH 6.0, basolateral pH 7.4), net absorption (145.2 pmol/cm2 per h) was observed, although experimental conditions could also be manipulated (apical pH 7.4, basolateral pH 6.0) so that net secretion was observed. Transport and accumulation (in both directions) were inhibited in the presence of either 20 mM (unlabelled) Gly-Sar or 20 mM cephalexin (an aminocephalosporin antibiotic). When added to either the apical or basolateral surface of BCECF (2',7',-bis(2-carboxyethyl)-5(6)-carboxyfluorescein)-loaded Caco-2 cell monolayers Gly-Sar (20 mM), at pH 6.0, caused a marked intracellular acidification, demonstrating that dipeptide absorption is accompanied by H+-flow into the cells. Cephalexin (20 mM) had similar effects (as Gly-Sar) when presented at the apical surface but also caused a marked intracellular acidification when perfused into the basolateral chamber at pH 7.4. In contrast, addition of Gly-Sar (20 mM) to the basolateral chamber (at pH 7.4) had no effect. Transepithelial absorption of dipeptides (Gly-Sar) and beta-lactam antibiotics (cephalexin) at low concentrations is predominately via a transcellular route mediated by carrier mechanisms located at both apical and basolateral membranes. Interestingly, Gly-Sar and cephalexin transport across the basolateral membrane (and, therefore, exit from the cell) display both common and distinct characteristics suggesting that more than one mechanism may be responsible for exit into the basolateral space.