Effect of age on radiosensitivity of nuclear and mitochondrial DNA syntheses in the mouse liver

Abstract

Male ICR/JCL mice of various ages were irradiated with 400 R of X-rays, and changes in nuclear and mitochondrial DNA syntheses were studied by intraperitoneal injection of tritiated thymidine. In unirradiated control mouse liver, the nuclear DNA synthesis decreased very sharply with age up to 36 weeks, rising to a higher level at 50 and 80 weeks. Age-dependent increase in mean nuclear diameter of the hepatocytes, ranging from 5·11 ± 0·11 μm at 4 weeks to 13·94 ± 2·80 μm at 80 weeks, was observed in histological sections. The mitochondrial DNA synthesis did not show age-related change. X-irradiation inhibited the nuclear DNA synthesis in livers of 8- and 24-week-old mice during 24 hr after irradiation. In the liver of 36-week-old mouse, the nuclear DNA synthesis showed about 30 per cent increase over the control at 2 hr after irradiation, however, remained inhibited thereafter up to 24 hr. The nuclear DNA synthesis in 50-week-old mouse liver was depressed during 8 hr post-irradiation, recovering to about 70 per cent of the control at 24 hr. Forty-six per cent inhibition was observed in the nuclear DNA synthesis of 80-week-old mouse liver at 2 hr post-irradiation. The mitochondrial DNA syntheses in 8-, 24- and 50-week-old mouse livers were depressed during the early period after irradiation, but recovered to 49, 114 and 207 per cent of the control, respectively, at 24 hr after irradiation. In the liver of 36-week-old mouse, the mitochondrial DNA synthesis increased to about two-fold of the control at 2 hr after irradiation, then was inhibited up to 16 hr, and finally returned to the control level at 24 hr. The mitochondrial DNA synthesis in 80-week-old mouse liver was about 45 per cent of the control at 2 hr after irradiation.