Polarized transport was observed not in hydrophilic compounds but in dextran in Caco-2 cell monolayers

We examined the efflux from the basal to apical side of fluorescein isothiocyanate-labeled dextran 4000 (FD-4) in Caco-2 cell monolayers using a diffusion chamber system. The permeation clearance (Pm) of FD-4 from the basal to apical side was 6 times greater than that from the apical to basal side at 37 degrees C. Such polarized efflux of FD-4 was evident at 37 degrees C, but not at 4 degrees C. Efflux of FD-4 was dose-dependently inhibited by colchicine, an endocytosis inhibitor. Adding an excess of unlabeled dextran (M.W. 10,000 Da) decreased the efflux of FD-4. On the other hand, transepithelial electrical resistance (TEER) did not change during the experiments under any conditions. These results suggest that the efflux of FD-4 across Caco-2 cell monolayers is apparently mediated by at least two polarized, energy requiring transport systems, one of which shows substrate specificity for dextran polysaccharides.