The effect of hypoxia during irradiation on four-day death in mice given single and split doses of electrons

Anaesthetised mice were exposed while anoxic to single or split doses of electrons, and the values of four-day (intestinal) LD50 were compared with those for mice treated similarly while breathing oxygen. The values of LD50/4 for single exposures were 2,810 and 1,010 rads respectively, i.e. the oxygen enhancement ratio was 2·8. When fractionated doses were given in about that ratio, the time patterns of the split-dose recovery curves referring to anoxic and oxic mice were the same. Values of the LD50/4 were measured for 12-hour fractionation. The first doses were 1,600 or 600 rads to mice breathing nitrogen or oxygen. Dose-effect curves were the same for both groups, whether the mice were breathing nitrogen or oxygen during the second series of doses. Values of the LD50 were 3,980 and 1,410 rads when both doses were given under nitrogen and both under oxygen. Thus the differences in LD50 for single and split doses were 1,170 and 400 rads, i.e. the increments were in about the same ratio as the OER for single shots. The equal modification by anoxia of the effects of single and split doses contrasts with the markedly unequal extent of sparing by the fractionation of neutron and X-ray doses (Hornsey, et al., 1965). The two sets of results together strongly support the attribution of the sparing effect of fractionation on the gut syndrome to intracellular repair of sub-lethal damage.