Injury accumulation and recovery in sheep during protracted gamma irradiation.

Although numerous studies have been conducted to describe injury accumulation and recovery in several mammalian species during and after exposure to ionizing radiation, the characterization of the time-course curve for injury and repair has never been fully resolved. It has been suggested by Blair, 2 that, after radiation injury, the recovery rate of animals increases exponentially with time. Extensive data by Sacher (1, 2) and by Sacher and Grahn (3) indicate that in the mouse radiation injury goes through a series of maxima at specific time intervals which correspond to intestinal and bone marrow death. Earlier investigations in our laboratory with multiple mammalian species after x-ray exposure have shown significant departures from the "hypothetical" exponential recovery pattern (4, 5). For example, a transient increased radiosensitivity has been detected for the hamster and the rabbit, whereas an increased radioresistance has been detected for the dog, swine, and sheep. Thus, any attempt to characterize the time course of injury accumulation or recovery by a single rate constant may be a gross oversimplification of a complex process involving various modalities of damage. Since the LDs5, the mean survival time, size, and body dimensions for sheep approximate those of man (LD50 and MST estimated), studies with sheep were extended for the purpose of evaluating hazards encountered following protracted irradiation exposure. The usual method for estimating mammalian recovery rates utilizes the split-dose technique as described by Hagen and Simmons (6). The technique consists in subjecting a group of animals to a single sublethal exposure to radiation and then determining the LD50 for these animals at subsequent time intervals. These measurements of the injury accumulation and the recovery from this injury during and after protracted exposure situations would provide a better under-