The kidney is the critical target of uranium exposure because uranium accumulates in the proximal tubules and causes tubular damage, but the chemical nature of uranium in kidney, such as its chemical status in the toxic target site, is poorly understood. Micro-X-ray absorption fine-structure (µXAFS) analysis was used to examine renal thin sections of rats exposed to uranyl acetate. The UL_III-edge X-ray absorption near-edge structure spectra of bulk renal specimens obtained at various toxicological phases were similar to that of uranyl acetate: their edge position did not shift compared with that of uranyl acetate (17.175 keV) although the peak widths for some kidney specimens were slightly narrowed. µXAFS measurements of spots of concentrated uranium in the micro-regions of the proximal tubules showed that the edge jump slightly shifted to lower energy. The results suggest that most uranium accumulated in kidney was uranium (VI) but a portion might have been biotransformed in rats exposed to uranyl acetate.

Although the kidneys comprise a critical target of uranium exposure, the dynamics of renal uranium distribution have remained obscure. Uranium is considered to function physiologically in the form of uranyl ions that have high affinity for phosphate groups. The present study applied microbeam-based elemental analysis to precisely determine the distribution of phosphorus and uranium in the kidneys of male Wistar rats exposed to uranium. One day after a single subcutaneous injection of uranyl acetate (2 mg/kg), areas of concentrated phosphorus were scattered in the S3 segments of the proximal tubule of the kidneys, whereas the S3 segments in control rats and in rats given a lower dose of uranium (0.5 mg/kg) contained phosphorus without concentrated phosphorus. Areas with concentrated phosphorus contained uranium 4- to 14-fold more than the mean uranium concentration (126–472 vs. 33.1 ± 4.6 μg/g). The chemical form of uranium in the concentrated phosphorus examined by XAFS was uranium (VI), suggesting that the interaction of uranyl ions with the phosphate groups of biomolecules could be involved in the formation of uranium concentration in the proximal tubules of kidneys in rats exposed to uranium.

Nephrotoxicity is the primary health effect of uranium exposure. However, the renal transport and intracellular behavior of uranium remains to be clearly elucidated. In the present study, the intracellular uranium distribution was examined with the cell lines derived from the S3 segment of mouse renal proximal tubules, which is a toxic target site of uranium, using microbeam-based elemental analysis. Uranium exposure at 100 μM for 24 h (non-toxic phase) was performed in S3 cells. Two types of measurement specimens, including those that are adhesive cell specimens and cryosection specimens, were examined for the positional relationship of the intracellular localization of uranium. Based on the combined results of single-cell imaging from the two types of cell specimens, uranium was distributed inside the cell and localized in the cytoplasm near the cell nucleus. In some cells, uranium was colocalized with phosphorus and potassium. The amount of uranium accumulated in S3 cells was estimated using thin section-standards. The mean uranium content of three adhesive cells was hundreds of femtogram per cell. Thus, we believe that single-cell imaging would be useful for studies on renal uranium transportation and cellular behavior.