Distinct mechanisms of zinc uptake at the apical and basolateral membranes of caco-2 cells

Abstract

Zinc uptake mechanisms at the apical and basolateral membrane borders of caco-2 cells were examined. This human-derived cell line possesses many morphological and functional characteristics of absorptive small intestinal cells. By day 14, confluent and well-differentiated monolayers were formed when the cells were grown on porous polycarbonate filters. Labelled zinc was placed on the apical or basal side of the monolayer and its uptake by the cells, as well as its transport across the monolayer, were measured. Zinc uptake by the cells from the apical side was found to be a saturable process (Kt = 41 microM; Vmax = 0.3 nmols/cm2/10 min) with a diffusional term at higher concentrations (1.0 sec/cm). Apical uptake was not affected by metabolic inhibitors or potential zinc ligands. Zinc uptake from the basolateral side was concentration dependent (Kd = 1.3 sec/cm) and was partially inhibited (30%) by ouabain and vanadate, suggesting that the (Na-K)-ATPase on the basolateral membrane is involved in the serosal uptake of zinc by the cell. Transport of zinc across the monolayers from the apical or basolateral compartment was concentration dependent and was not affected by metabolic inhibitors. Zinc transport from the basolateral side was greater than 2-fold greater than apical transport. Hence, separate mechanisms can be distinguished with respect to zinc uptake at the apical and basolateral membranes of caco-2 cells.