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ToxSci Advance Access originally published online on June 2, 2005
Toxicological Sciences 2005 87(1):15-26; doi:10.1093/toxsci/kfi216
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© The Author 2005. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org

The Chemical Species of Aluminum Influences Its Paracellular Flux across and Uptake into Caco-2 Cells, a Model of Gastrointestinal Absorption

Yuzhao Zhou* and Robert A. Yokel*,{dagger},1

* Graduate Center for Toxicology, University of Kentucky Medical Center, Lexington, Kentucky 40536–0305; {dagger} College of Pharmacy, University of Kentucky Medical Center, Lexington, Kentucky 40536–0082

Received January 25, 2005; accepted May 31, 2005

Aluminum (Al) can cause neurotoxicity, a low-turnover osteomalacia, and microcytic anemia. To test the null hypothesis that the chemical form (species) of Al does not influence its mechanism or rate of absorption from the gastrointestinal tract, Al flux across and uptake into Caco-2 cells was investigated. Caco-2 cells were grown on porous membranes mounted in vertical diffusion chambers or in 35-mm-diameter plastic cell culture dishes. When 8 mM 27Al was introduced as the ion, citrate, maltolate, fluoride, or hydroxide, the apical to basolateral apparent permeability (Papp) of Al correlated highly with the Papp of lucifer yellow (LY), a paracellular marker, except when introduced as Al hydroxide. The uptake rate of Al when introduced as the fluoride was > when introduced as the ion > maltolate > citrate > hydroxide. The activation energy of Al introduced as the ion, citrate, maltolate, and fluoride, determined from Årrhenius plots, was 13–22 KJ/mol, suggesting diffusion-mediated uptake. With exposure to 2 µM Al (containing 26Al as a tracer) introduced as the ion, hydroxide, citrate, and fluoride, Al and LY Papp were consistent with results obtained with 8 mM Al, but were not Al species dependent. Approximately 0.015% of the 26Al fluxed across the cell monolayer; 0.75% was associated with cells. Lumogallion staining imaged by confocal laser microscopy showed Al co-localized with DAPI in the nucleus. The results suggest that (1) soluble Al species predominantly diffuse through the paracellular pathway, (2) the ligand-dependent flux rate of Al is due to an effect on the tight junctions, (3) Caco-2 cell uptake of Al is a diffusion process, and (4) the ligand can influence the rate of cellular Al uptake.

Key Words: aluminum absorption; Caco-2 cells; chemical species; flux; paracellular pathway and uptake.


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