CACO-2 CELL-LINE USED AS AN IN-VITRO MODEL TO STUDY CADMIUM ACCUMULATION IN INTESTINAL EPITHELIAL-CELLS

Cd-109 uptake was studied using the highly differentiated TC7 clone of Caco-2 cells as a model of human enterocyte function. Intracellular a ccumulation of 0.3 mu M Cd-109 involved a rapid and a slow uptake phas e, which resulted in complete equilibration (t(1/2) = 17.3 +/- 1.3 min ) with an apparent in-to-out distribution ratio (alpha(e)) of 11.6 +/- 0.8. The amplitude of the rapid phase (U-0) and the rate of the slow phase (V) were similarly reduced in the less differentiated PF11 clone , but comparable alpha(e) values were observed at equilibrium. In both clones, the t(1/2), and alpha(e) values increased and decreased, resp ectively, upon addition of unlabeled Cd to the uptake media, In TC7 ce lls, Cd-109 uptake at 1 min (U-1) was unaffected by Ca concentrations four order of magnitude in excess, but both U-0 and V demonstrated sim ilar sensitivities to unlabeled Cd, Zn and sulfhydryl-reactive agents, Only U-0 disappeared when EDTA was present in the wash solutions. U-1 showed saturation kinetics and the data were found compatible with a model assuming rapid initial Cd binding and transport through a unique transport protein (K-m = 3.8 +/- 0.7 mu M). Cd efflux kinetics demons trated partial reversibility in EDTA-containing solutions, suggesting that the taken up Cd might be both tightly and loosely bound to intrac ellular binding sites, However, the displacement of Cd-109 measured at 65 min failed to reveal this heterogeneity: the data were found compa tible with a model equation assuming the presence of one class of high -capacity high-affinity binding sites. We conclude that a slow-transpo rt fast-intracellular binding mechanism of Cd uptake best accounts for these results and that Cd transport most likely involves a carrier-ty pe of protein unrelated to Ca absorption.