Effects of Gamma Rays in the 0.5-50-cGy Range on the Conformation of Chromatin in Mammalian Cells

The effects of¹³⁷ Cs γ-ray quanta in mammalian cells were studied using the method of anomalous viscosity time dependence (AVTD). Several different cell types were exposed: VH-10 human fibroblasts, BALB/c mouse splenocytes and Sprague-Dawley rat thymocytes. The cells were irradiated with doses of 0.1-50 cGy and then lysed for viscosity measurements. It was established for all types of cells that exposure to a dose of 0.5 cGy resulted in a statistically significant reduction in viscosity peaks. This reduction reached a maximum value approximately 40-60 min after irradiation. The reduction of viscosity was revealed at doses up to 4 cGy for human fibroblasts with the maximum effect observed at about 2 cGy. The opposite response, an increase in viscosity, was observed after exposing the cells to 10-50 cGy. From the linear approximation of this dose dependence, the increase in viscosity started at doses above 4 cGy. The effect of increased viscosity disappeared with time after irradiation, with kinetics similar to that of DNA repair. Repair of this effect of AVTD depended strongly on temperature in the 0-37°C range. In contrast, the kinetics of the effect of 0.5 cGy did not depend on temperature. Thus two different responses of chromatin were observed in mammalian cells after low (<4 cGy) and high (>4 cGy) doses of radiation. The specific inhibitor of DNA topoisomerase II, etoposide, was shown to increase the peaks of AVTD significantly. These data provide further evidence that the effects of AVTD correlate with changes in chromatin conformation.