When the radionuclide125 I is localized in mouse testis as${}^{125}{\rm I}\text{-iododeoxyuridine}$ (an analogue of thymidine) and incorporated into the DNA of spermatogonial cells, the cytocidal effects are as severe as those due to densely ionizing α particles. In contrast,125 I confined to the cytoplasm of these cells is much less radiotoxic, the efficacy being the same as for selective irradiation of the testis with sparsely ionizing external X rays. The biological effects, in both cases, are strongly mitigated upon pretreatment of the testes with very small amounts (0.75 μg) of cysteamine, a radioprotector. These findings suggest an important role for such chemical agents in radiation protection and in understanding the mechanisms of radiation damage involving radionuclides incorporated in tissue.

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