Previously, we examined the formation of reactive oxygen species (ROS) in T lymphocytes following 5 Gy of irradiation. Using a CCD camera system, we monitored fluorescence in T lymphocytes loaded with the succinimidyl ester of dichlorodihydrofluorescein diacetate (H2DCFDA), which is non-fluorescent until oxidized by ROS. We found that ROS formation occurred immediately after irradiation, continued for several hours, and resulted in oxidative DNA damage. Therefore, the origin of the hyper-radiosensitivity of T lymphocytes seemed to be the high production of ROS in the mitochondrial DNA following irradiation. In this study, we examined radiation-induced ROS formation in adult articular chondrocytes, which were demonstrated to be highly resistant to apoptosis in our previous study. We found that ROS formation was actually scarcely seen after irradiation of up to 20 Gy in these cells. Therefore, the origin of the great difference of radiosensitivity between T lymphocytes and adult articular chondrocytes is considered to lie in the degree of ROS formation following irradiation, with this difference possibly resulting from the scavenging acuity of these two kinds of normal tissue cells for free radicals including hydroxyl radicals.

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