The anatomical and histochemical alterations in rat kidneys after the parenteral administration of uranium nitrate [UO2(NO3)2·6H2O] have been studied. The histological effects produced by this agent over a wide range of dose levels were nearly identical in character and differed principally in their speed of evolution. The deposition of calcium always began at foci in the cytoplasm of the cells of the proximal convoluted tubules of the inner cortex. It remained intracellular until the cell boundaries were destroyed.

Deposits of calcium could be found before any other cellular damage could be demonstrated by histological examination. Later, when degeneration and necrosis were present, the foci of calcification were imperfectly related to them in location or degree. In contrast, the amount of calcification was correlated with the dose of uranium nitrate, being greatest in the kidneys of rats that received 20 mg./kg., next greatest in the 30 mg./kg. rats, less in the 10 mg./kg. rats, and slight in those that received 2 mg./kg.

Histochemical stains for ferric and ferrous iron, chondroitinsulfate, and polysaccharides gave results that were negative or unrelated to the deposits of calcium, thus making it unlikely that these substances held any appreciable amount of calcium in the tissue. Yet it is clear that some anion other than phosphate must be combined with part of the calcium; the results with the alizarin and von Kóssa stains confirmed an earlier result (1) in showing that the first deposits of calcium are formed without comparable accumulations of phosphate.

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