Time-dependent sensitivity of rat CFU-GM to total body irradiation

It has been shown that the light-darkness cycle affects the proliferative activity of the hemopoietic system, possibly acting on the distribution of the cells in the cell-cycle phases at different hours of the day. This could determine time-dependent modifications in ionizing radiation damage to hemopoietic progenitors.

In this study the influence of the irradiation time on the radiosensitivity of rat CFU-GM was assessed by in vitro clonogenic assay. Rats were exposed to 3 Gy gamma rays at four time points (00:00, 06:00, 12:00, 18:00 hours). Bone marrow CFU-GM cultures were performed at various intervals ranging between 12 hours and 45 days after irradiation and compared with unirradiated controls.

A marked decrease of femoral CFU-GM was observed in the five days following total body irradiation, regardless of the irradiation time point. From this interval on all the irradiated groups showed an increasing proliferation of CFU, which was particularly evident in the group irradiated during the day light period. At the latest intervals (45 days) the post-acute damage to hematopoietic progenitors lacked any evidence of time dependence.

With the experimental model used, time scheduling does not seem to affect markedly either the acute depletion of CFU-GM in the 5 days following total body irradiation or the late consequence to the CFU-GM compartment.