DOSE AND DOSE-RATE DEPENDENCE OF THE FREQUENCY OF HPRT DEFICIENT T-LYMPHOCYTES IN THE SPLEEN OF THE CS-137 GAMMA-IRRADIATED MOUSE

The frequency of hypoxanthine phosphoribosyl transferase (HPRT) defici ent splenic T lymphocytes was measured in the Cs-137 gamma-irradiated mouse by the T cell cloning method. Doses from 0.3 to 6 Gy were applie d at the dose-rates 0.5 Gy/min, 1 Gy/day and 1 Gy/week. Mutants were d etermined 8-10 and 30-40 weeks after the end of exposure. Radiation-in duced mutant frequency (MF(i)) was calculated by subtracting the age c orrected spontaneous mutant frequency (MF(sp)) from total mutant frequ ency (MF) found in irradiated animals. Data were fitted to linear and lineal-quadratic dose-response models. MF(i) depended markedly on dose , dose-rate and time after exposure. When mutants were determined 8-10 weeks after acute irradiation (0.5 Gy/min) the dose-effect curve fitt ed the linear-quadratic equation MF(i) = 6.9 X 10(-6) Gy + 1.2 X 10(-6 ) Gy(2), whereas in low dose-rate experiments (1 Gy/day, 1 Gy/week) th e dose-effect curves were linear. The slope of the linear regression w as about 3 X 10(-6). When low dose-rate-irradiated animals were killed 30-40 weeks after irradiation, MF(i) was about one-third of that obse rved after 8 weeks. The dose dose-rate effectiveness factor (DDREF) fo r radiation mutagenicity was calculated in animals that had been expos ed 8-10 weeks previously. For doses < 2 Gy the reduction in effectiven ess was about 1.5 when the irradiation dose-rate was less than or equa l to 1 Gy/day. For higher doses DDREF was 3-5.