Kinetic Basis for Compensated Hemopoiesis during Continuous Irradiation with Low Doses

The hemopoietic effects of low-level continuous ..gamma.. irradiation with a daily regime of 16.0, 2.5, 1.4, and 1.15 rad were studied in mice. Irradiation time varied from 3 to 38 days. During the irradiation in vivo TL dosimetry was applied. Between 43 and 50 rad the reduction in both erythropoiesis and the number of femoral colony-forming cells (CFU/sub 8/) were inversely related to irradiation time. If exposure time was extended to over 30 days, /sup 59/Fe uptake and stem cell level practically agreed with those of the control indicating a compensation within the hemopoietic system. At the same time, however, a significantly increased proliferation rate of CFU/sub 8/ was found. The threshold dose for a detectable increase in CFU/sub 8/ turnover rate was observed to be about 30 rad at 2.5 rad daily dose. The results suggest the following conclusions. (1) In spite of the virtually unaltered hemopoiesis during the low-level chronic irradiation, characteristic changes can be detected in the turnover rate of the CFU/sub 8/ population. Therefore, the number of CFU/sub 8/ in S phase is proposed to be a suitable tool for the investigation of the effects of low-level irradiation. (2) The significantly enhanced proliferation rate of CFU/submore » 8/ can be regarded as the basis for the maintenance of the normal stem cell level and hemopoiesis during low-level irradiation.« less