title: Different transport mechanisms for cadmium and mercury in Caco-2 cells: inhibition of Cd uptake by Hg without evidence for reciprocal effects.
abstract: Cadmium/Hg interactions have been studied in the TC7 clone of the enterocytic-like Caco-2 cells to test the hypothesis that these metals may compete for intestinal transport. Comparison of the kinetic parameter values for 203Hg(II) and 109Cd(II) uptake in a serum-free medium revealed that Hg is accumulated much more rapidly and to higher concentrations. The very rapid uptake/binding step and the initial uptake rate of 109Cd were both significantly inhibited by an excess of unlabeled Cd or Hg (apparent K(i) for Hg of 9.3 +/- 1.2 microM) without reciprocal effects. 109cadmium uptake was highly sensitive to temperature and a significant fraction of accumulation (12%) was EDTA extractable. 203Hg uptake remained insensitive to temperature or the EDTA washing procedure. However, the uptake of both tracers was half-decreased when an excess of the respective unlabeled metal was added in the stop solution, suggesting an exchange mechanism for adsorption. Cell pretreatment with N-ethylmaleimide (NEM) led to a 30% decrease or a 73% increase in the 3-min specific transport of 109Cd when NEM was still present in or removed from the uptake medium, respectively. NEM had no effect on 203Hg uptake. Overall our results suggest the involvement of a saturable specific mechanism for Cd, which is highly sensitive to inhibition by Hg and NEM under some conditions, and a nonspecific passive diffusion for Hg. The Hg- or NEM-induced inhibition of Cd uptake likely involves a thiol-mediated reaction, but our results suggest that NEM pretreatment may activate other cellular mechanisms leading to a stimulatory effect.
authors.affiliation: Département de Chimie, Centre TOXEN, Université du Québec à Montréal, C.P. 8888, Succ. Centre-ville, H3C 3P8, Montréal, Québec, Canada.
authors.forename: Ismaël
authors.initials: I
authors.lastname: Aduayom
authors.forename: Peter G C
authors.initials: PG
authors.lastname: Campbell
authors.forename: Francine
authors.initials: F
authors.lastname: Denizeau
authors.forename: Catherine
authors.initials: C
authors.lastname: Jumarie
mesh_terms: D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D018938:Caco-2 Cells / Q000187:drug effects / Q000378:metabolism; D002104:Cadmium / Q000037:antagonists & inhibitors* / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D006801:Humans; D008628:Mercury / Q000494:pharmacology*; D013696:Temperature
chemical_list: D002104:Cadmium; D008628:Mercury