Toshiyuki Norimura

Changes with age in swimming performance of X-irradiated mice

Abstract The time required to swim 250 cm was determined once weekly for the entire life of fifteen pairs of male dd/K mice. The irradiated group was exposed to a single 224 rad of X-rays at 20 weeks of age. Median survival time (ST50) for the control was 88.9 weeks and that for the irradiated group was 77.4 weeks, and both regression lines relating to death rate and age were parallel. The swimming ability of control mice began to decrease when the mice were 40 weeks of age, after which there was a gradual reduction with age, at 0.00646/day. In the irradiated group, the swimming ability decreased from seven weeks after the irradiation. The time of 50% reduction of swimming speed (TRS50) for the control was 78.9 weeks and that for the irradiated group was 66.3 weeks, and the slopes of the regression lines relating reduction rate and age were similar. Differences between ST50 and TRS50 were 10 weeks in the control and 11 weeks in the irradiated group, respectively. These results indicate that there is no basic difference in the reduction in swimming ability between control and irradiated mice. The X-irradiation may simply mean that the reduction in the swimming ability is displaced to an earlier time, with no alteration in the rate of reduction, and that the earlier appearance in the irradiated group is related to premature aging as induced by irradiation.