Oxidative stress in cells exposed to low levels of ionizing radiation

The ability of medium from gamma -irradiated cells to induce early events i n the apoptotic cascade, such as the mobilization of intracellular calcium, loss of mitochondrial membrane potential and increased levels of reactive oxygen species, in unirradiated cells was investigated. Medium from irradia ted human keratinocytes was harvested and transferred to unirradiated kerat inocytes. Intracellular calcium levels, mitochondrial membrane potential an d the level of reactive oxygen species were all monitored for a period of 2 4 h following medium transfer. Rapid calcium fluxes (within 30 s), loss of mitochondrial membrane potential and increases in reactive oxygen species ( from 6 h after medium transfer) were observed. There was no significant dif ference between the effects of medium generated by cells irradiated at 0.5 Gy or 5 Gy. The data suggest that a signal that leads to apoptosis is relea sed from cells undergoing radiation-induced oxidative stress.