The relationship between the population size and the postirradiation mortality of mice with respect to individual differences in stress reactivity.
Differences in radiosensitivity, expressed by the death rate up to the 30th day after irradiation by single whole-body exposures in the region of LD50, were followed in mice caged in groups by five, 15 and 25 individuals. No marked differences in the whole=group mortality of differently sized collectives were detected. Preirradiation as well as postirradiation leucocyte counts (after a sublethal irradiation) exhibit, however, differences, which correspond to an assumed different degree of the adrenocortical activity in differently sized collectives. Also the follow-up of the stress reactivity of the animals by measuring the Na/K ratio in 24-hour-samples of urine prior to irradiation demonstrated marked differences between the groups with low number and high number of animals in a cage. By correlating values of this indirect indicator of the adrenocortical activity with manifestations of the individual radiosensitivity it was found that the extremely hyporeactive animals (belonging to groups with five animals per cage) and the extremely hyperreactive ones (belonging to groups with 25 animals per cage) exhibit a markedly higher mortality. The absence of differences in the whole-group mortality of differently sized collectives can be explained by the existence of nonlinear "U"-shaped relations between the adrenocortical activity and the individuals' radiosensitivity. The results support the conception on the unfavorable prognostic significance of unbalanced reactions of homeostatic functions of system regulation under conditions of radiation syndrome.