Differences in the Acute Response of the Various Segments of Rat Intestine to Treatment with the Intestinal Carcinogen, Methylazoxymethanol Acetate1

azoxymethane (see Ref. 15). One explanation to account for the difference between colon and small intestine is the fact that the intestinal flora (1, 5, 12) and the bile acids (1, 2, 8, 9) can influence the incidence of colonic tumors. Colonic tumors can be induced, however, in germ-free rats (1, 5, 12) and in segments of rat intestine distal to a colostomy (14). It appears,therefore, thatpartofthedifference inresponseof these 2 tissues to the tumorigenic effects of chemicals is due to factors inherent within the colonic epithelium itself. Experiments conducted by Gennaro et al. (3) support this suggestion. They transposed segments of mid-small intes tine to the colon and segments of colon to the mid-small intestine. Rats treated with azoxymethane developed tu mors of the colon, regardless of its location, while no tu mors appeared in the segments of small intestine exposed to the milieu of the colon. The above findings led us to investigate whether the different segments of small intestine and colon would also respond selectively to the acute effects of carcinogens. A carcinogen suitable forsuch a study is MAM acetate, which induces significant numbers of colon tumors in rats follow ing a single treatment, with only a few tumors appearing in the small intestine (15). We report herein that, in fact, there appears to exist a relationship between those segments of intestine affected acutely and the sites of tumor formation.