Effect of X-Radiation on DNA Metabolism in Various Tissues of the Rat: II. Recovery after Sublethal Doses of Irradiation

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Abstract
The effect of 100 r and 400 r of total-body X-irradiation on DNA (deoxyribonucleic acid) synthesis in thymus, spleen, and small intestine of the rat was studied over extended periods. The incorporation of thymidine-2-C14 into DNA during 1 hour was used as a relative measure of DNA synthesis. Following an initial inhibition the tissues recovered their ability to synthesize DNA. The recovery time depended on the tissue and radiation dose. In terms of specific activity of the isolated DNA, i.e., the relative rate of DNA synthesis, the maximum rate of recovery after 100 r for small intestine, spleen, and thymus was at 2,2, and 3 days, respectively. After 400 r the maximum specific activities were observed at 2, 6, and 4 days for the same tissues. Since there was an extensive initial loss of DNA from the irradiated organs, the recovery in terms of total activity of the DNA was less pronounced than that of the specific activity. In all cases the specific activity exceeded the control value at the time of maximum recovery, usually by a factor of approximately 2. For thymus after 400 r this "overshoot" was ten times as great as the control value, indicating an extremely active regeneration. It has been concluded that the effect of radiation on the DNA metabolism of the three tissues studied is qualitatively the same, differing only in the degree of acute cell death, in the duration of the delay of DNA synthesis in the surviving cells, and in the rate of recovery resulting from accelerated cell replication during regeneration.