109Cd uptake was studied using the highly differentiated TC7 clone of Caco-2 cells as a model of human enterocyte function. Intracellular accumulation of 0.3 μm 109Cd involved a rapid and a slow uptake phase, which resulted in complete equilibration (t ½= 17.3 ± 1.3 min) with an apparent in-to-out distribution ratio (α 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 α e values were observed at equilibrium. In both clones, the t ½ and α e values increased and decreased, respectively, upon addition of unlabeled Cd to the uptake media. In TC7 cells, 109Cd uptake at 1 min (U 1) was unaffected by Ca concentrations four order of magnitude in excess, but both U 0 and V demonstrated similar 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 μm). Cd efflux kinetics demonstrated partial reversibility in EDTA-containing solutions, suggesting that the taken up Cd might be both tightly and loosely bound to intracellular binding sites. However, the displacement of 109Cd measured at 65 min failed to reveal this heterogeneity: the data were found compatible with a model equation assuming the presence of one class of high-capacity high-affinity binding sites. We conclude that a slow-transport fast-intracellular binding mechanism of Cd uptake best accounts for these results and that Cd transport most likely involves a carrier-type of protein unrelated to Ca absorption.
The Journal of Membrane Biology – Springer Journals
Published: Jul 1, 1997
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