Low dose effect of ionizing radiation on incorporation of iododeoxyuridine into bone marrow cells.

The reduction of the incorporation of IUdR in bone marrow cells depends on the time after irradiation and on te microenvironment of the cells. The strongest effect is observed 4 hours after irradiation. For absorbed doses above 40 rad, whole-body irradiated mice were more sensitive with respect to depression of IUdR incorporation in bone marrow cells, when the bone marrow cells were labelled in vivo, and less sensitive for in vitro labelling. The converse was observed for very small doses of 1 rad and below. Such small doses resulted in a significant depression of IUdR incorporation after in vivo irradiation and in vitro labelling and showed no effect at all after in vivo irradiation and in vivo labelling. The least effect of radiation was observed after both irradiation and labelling in vitro. Although the mechanisms are not fully understood, the biological results and microdosimetric considerations indicate that at the smallest doses the effect is due to functional changes of cellular organelles which control intracellular mechanisms. A working hypothesis is proposed for the reduction of IUdR incorporation at low doses as being due to functional changes of the cellular membranes.