Acute Lethality Studies with the Rat: The LD 50 , Death Rate, and Recovery Rate

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Abstract
FAC(I)F1 rats, 96 to 212 days old, were irradiated once with 250-kvcp X-rays (HVL, 1.6 mm Cu) at a tissue-dose rate of 30 r/ minute. Males and females did not differ significantly in reaction, and their data were pooled. Of the deaths during postirradiation days 0-40, 98% occurred prior to day 29. For the pooled data of 6 experiments, the LD50 was 570 [plus or minus] 7 r and the slope function, S, of the dose-effect curve was 1.12 [plus or minus] 0.01. After the LD48, the mean killing time was 10 days; after higher doses it fell below 10 days. The rate of prompt recovery was determined with a method in which a nonlethal injuring dose (315 r) was given on day 0 and the residual injury was determined from the changes in the LD50(28) at various times up to day 21 thereafter. Recovery was an exponential function of time; for 50% recovery the time was 4.9 days. As recovery proceeded, both the mean and the standard deviation of the killing time tended to increase. Judged by mean killing time, at 21 days after the injuring exposure the rats were more variable in their response to X-rays and, on the average, slightly more resistant. The results of experiments using a comparable design for the study of prompt recovery after whole-body exposure were reviewed. In 7 strains of mice and 2 of rats, recovery on day 1 and thereafter could be treated as an exponential function of time, but the rates differed by as much as a factor of 5. In the hamster, the function was not exponential; in the monkey it was assumed to be exponential. It was noted that genetic and other factors, including the magnitude of the injuring dose, may influence the recovery process.