Phosphorus Localization and Its Involvement in the Formation of Concentrated Uranium in the Renal Proximal Tubules of 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.

Uranium XAFS analysis of kidney from rats exposed to uranium.

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 U LIII-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.

Cellular localization of uranium in the renal proximal tubules during acute renal uranium toxicity.

Renal toxicity is a hallmark of uranium exposure, with uranium accumulating specifically in the S3 segment of the proximal tubules causing tubular damage. As the distribution, concentration and dynamics of accumulated uranium at the cellular level is not well understood, here, we report on high-resolution quantitative in situ measurements by high-energy synchrotron radiation X-ray fluorescence analysis in renal sections from a rat model of uranium-induced acute renal toxicity. One day after subcutaneous administration of uranium acetate to male Wistar rats at a dose of 0.5 mg uranium kg(-1) body weight, uranium concentration in the S3 segment of the proximal tubules was 64.9 ± 18.2 µg g(-1) , sevenfold higher than the mean renal uranium concentration (9.7 ± 2.4 µg g(-1) ). Uranium distributed into the epithelium of the S3 segment of the proximal tubules and highly concentrated uranium (50-fold above mean renal concentration) in micro-regions was found near the nuclei. These uranium levels were maintained up to 8 days post-administration, despite more rapid reductions in mean renal concentration. Two weeks after uranium administration, damaged areas were filled with regenerating tubules and morphological signs of tissue recovery, but areas of high uranium concentration (100-fold above mean renal concentration) were still found in the epithelium of regenerating tubules. These data indicate that site-specific accumulation of uranium in micro-regions of the S3 segment of the proximal tubules and retention of uranium in concentrated areas during recovery are characteristics of uranium behavior in the kidney.

Uranium dynamics and developmental sensitivity in rat kidney.

Renal toxicity is the principal health concern after uranium exposure. Children are particularly vulnerable to uranium exposure; with contact with depleted uranium in war zones or groundwater contamination the most likely exposure scenarios. To investigate renal sensitivity to uranium exposure during development, we examined uranium distribution and uranium-induced apoptosis in the kidneys of neonate (7-day-old), prepubertal (25-day-old) and adult (70-day-old) male Wistar rats. Mean renal uranium concentrations increased with both age-at-exposure and exposure level after subcutaneous administration of uranium acetate (UA) (0.1-2 mg kg(-1) body weight). Although less of the injected uranium was deposited in the kidneys of the two younger rat groups, the proportion of the peak uranium content remaining in the kidneys after 2 weeks declined with age-at-exposure, suggesting reduced clearance in younger animals. In situ high-energy synchrotron radiation X-ray fluorescence analysis revealed site-specific accumulation of uranium in the S3 segment of the proximal tubules, distributed in the inner cortex and outer stripe of the outer medulla. Apoptosis and cell loss in the proximal tubules increased with age-at-exposure to 0.5 mg kg(-1) UA. Surprisingly, prepubertal rats were uniquely sensitive to uranium-induced lethality from the higher exposure levels. Observations of increased apoptosis in generating/re-generating tubules particularly in prepubertal rats could help to explain their high mortality rate. Together, our findings suggest that age-at-exposure and exposure level are important parameters for uranium toxicity; uranium tends to persist in developing kidneys after low-level exposures, although renal toxicity is more pronounced in adults.