Evidence for an intracellular barrier to cadmium transport through caco-2 cell monolayers

Abstract

¹⁰⁹Cd transport was studied in the highly differentiated TC7 clone of the enterocytic‐like Caco‐2 cells grown on filters. Accumulation curves for 0.3 μM ¹⁰⁹Cd over 12 h from the apical (AP) or the basal (BL) sides revealed a three‐step mechanism involving: 1) a zero‐time accumulation A₀; 2) a fast process A_f (t_1/2 ≤ 10 min); and 3) a slow process of uptake A_S (5 h ≤ t_1/2 ≤ 10 h) responsible for the major cellular levels of ¹⁰⁹Cd. The relative contribution of adsorption to total accumulation is greater for short exposure times (≤35%), but is no longer significant after the exposure times needed to reach equilibrium. Transepithelial transport was less than 4% of the cellular level at 12 h. A negligible but specific binding onto the BL surface of the filters was characterized. Saturable systems of accumulation with comparable affinities (K_m = 2.5 ± 0.5 and 5.4 ± 0.4 μM) but distinct capacities (V_max = 8.9 ± 1.2 and 312 ± 22 pmol/min/mg protein) were identified at the AP and BL cell membranes, respectively. Efflux studies revealed that Cd accumulation is only partially reversible, with an exclusive metal release at the same side. A 2‐h exposure on both sides simultaneously failed to demonstrate any competition for cellular accumulation: uptake was additive relative to AP and BL uptake values. These data suggest that A_f leads to an accumulation of loosely bound Cd, whereas A_S represents irreversible intracellular binding processes. We conclude that Cd transport occurs exclusively by a transcellular route and that saturation of the intracellular high‐capacity binding sites is the rate‐limiting step in Cd absorption. J. Cell. Physiol. 180:285–297, 1999.