Uranium(VI) complexation in cell culture medium:: influence of speciation on Normal Rat Kidney (NRK-52E) cell accumulation

Uranium bioavailability and toxicity are closely linked to the metal's speciation in solution. However in biological fluids or in media classically used for cell culture and subsequently for in vitro cell exposure -, uranium is rarely present as free-ion since these media contain non-negligible concentrations of potential ligands such as phosphate and bicarbonate but also co-ions such as calcium which can cause U(VI) complexes precipitation. The chemical form of uranium that is internalized in cells and interferes with biological processes is of major concern. Uranium toxicity and accumulation were evaluated in vitro on NRK-52(E) cells, model for rat renal proximal tubule. Uranium intracellular accumulation begins after 12 h exposure to 600 p,M U(VI); toxicity appears as soon as cells accumulated 25 to 30 mg U/g protein. Modification of uranium speciation in the exposure medium induces great changes in toxicity and cell accumulation. Comparison of toxicity and accumulation results to theoretical uranium speciation, calculated with the J-Chess computer program, shows that free-ion concentration can not explain the total uranium intracellular accumulation. Low molecular weight U(VI) complexes, such as UO2(CO3)(3)(4-) but also UO2PO4- could be implicated in U(VI) cellular accumulation and toxicity.