Circadian Variation in te Induction of Intestinal Tumors by N-Methyl-N-nitrosourea in Male C57BL/6N Mice

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Abstract
Male C57BL/6N mice were administered a single ip injection of 30 mg of N- methyl-N-nitrosourea (MNU)/kg of body weight. Additional groups were treated similarly every 3 hours for the next 24 hours. Adenocarcinomas of the small intestine were the major treatment-related tumors, with the total incidence being 38% at 250 days after injection. There was a significant circadian variation for tumor induction; the maximum number of intestinal tumors (≈55%) tended to occur when the MNU was administered during the middle of the light period (6:00 to 18:00), while the tumor incidence was at a minimum (≈10%) when the MNU was given in the middle of the dark phase (18:00 to 6:00). These data are discussed in relation to DNA synthesis and repair and MNU-induced cellular toxicity.

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