Effects of sublethal irradiation on changes in crypts of the mouse colon during treatment with 1,2-dimethylhydrazine

The effects of low-dose X-irradiation on the progression of early neoplastic changes induced in the colon by 1,2-dimethylhydrazine (DMH) were assessed. Outbred female Swiss-Webster mice were treated with DMH only, DMH-irradiation, irradiation only, or 0.001 M EDTA. Animals were pulsed with [3H]thymidine for 1 hour, and the distal colon was radioautographed. The number of labeled cells per crypt in animals tested with DMH-irradiation was greater (P less than 0.01) than the number in "DMH-only" animals, but a corresponding increase in the length of the crypts did not occur. After 16 weeks of treatment, the percentage of animals with focal atypia was similar in the DMH-irradiation group (87%), and the DMH-only group (100%), but there were more focal atypias per animal in the DMH-only group. After 20 weeks, 14% of the DMH-irradiation group had visible tumors compared to 63% of the DMH-only group. The results suggest that DMH increased the number of cells at risk for irradiation; thus the surge in proliferation subsequent to cell depletion was amplified. However, the development of precancerous lesions was not promoted.