Differences in the acute response of the various segments of rat intestine to treatment with the intestinal carcinogen, methylazoxymethanol acetate

Previous work has shown that single injections of methylazoxymethanol acetate in rats induce tumors predominantly in the colon, occasionally in the duodenum, and rarely in the jejunum and ileum. These studies describe the acute pathological and biochemical alterations induced by this carcinogen in the different segments of rat small intestine and colon. Karyorrhexis was found in crypts of duodenum, cecum, and all segments of colon at 6 hr after treatment. Much of the cellular debris was removed by 24 hr, although mitoses did not return to normal levels until the third day after treatment. No pathological alterations were found in jejunum or ileum, even as late as 24 hr after treatment. Studies of DNA synthesis at 24 hr after treatment indicated that jejunum and ileum were much less affected than were duodenum, cecum, or colon. In contrast, 5 fluorouracil and nitrogen mustard, agents that can inhibit proliferating cells but are not known to be intestinal carcinogens, affected all of the segments equally. The results indicate that a correlation exists between those segments of intestine acutely affected by methylazoxymethanol acetate and the sites of eventual tumor development. The level of deacetylase activity in the various intestinal segments did not correlate with sensitivity to methylazoxymethanol acetate induced inhibition of DNA synthesis. We also found that methylazoxymethanol acetate inhibited DNA synthesis in the duodenum and colon in rats with cannulated bile ducts. These data indicate that the carcinogen does not require biliary transport to the intestinal lumen to exert its biological effects. Mechanisms that might account for the observed selectivity in action of methylazoxymethanol acetate in the various rat intestinal segments are discussed.