Ca²⁺ and endonuclease activation in radiation-induced lymphoid cell death

Exposure of lymphoid cells to X-irradiation caused an increase in cytosolic Ca²⁺ ([Ca²⁺]_i) in rat thymocytes but not in rat splenocytes. The Ca²⁺ elevation in rat thymocytes was transitory and by 2 h postirradiation, when the chromatin degradation began to be appreciable, the [Ca²⁺]_i had returned to control level. Inhibitors of RNA and protein syntheses prevented the radiation-induced [Ca²⁺]_i rise in thymocytes. Pretreatment of both cell types with Ca²⁺ chelators also prevented DNA fragmentation, chromatin condensation, and cell death caused by X-irradiation. These data suggest that Ca²⁺ plays an important role in the perpetuation of apoptosis in both thymocytes and splenocytes although a Ca²⁺ elevation, which may serve as a signal in thymocytes, does not appear to be required to initiate radiation-induced DNA fragmentation in splenocytes.