STUDIES ON EXPERIMENTAL COLON TUMORIGENESIS IN RATS : 1. STRAIN DIFFERENCES IN INCIDENCE, LOCATION AND TUMOR TYPES OF 1, 2-DIMETHYLHYDRAZINE-INDUCED TUMORS

Susceptibility to 1, 2-dimethylhydrazine (DMH) was comparatively studied using two rats strains, Wistar-Furth substrain (WF/O) rats derived from the family with a high incidence of spontaneous colon tumor, and Long-Evans (LE) rats. Ten-week-old rats of both sexes were injected with DMH (20 mg/kg body weight) weekly for 20 weeks, and sacrificed at the 10th, 15th, 20th, and 25th week after the last injection. LE rats developed colon tumors more frequently in terms of crude incidence (21/21) (p<0.01) and the number of tumors per rat (6.1) (p<0.05) than those (17/25, 2.5) in WF/O rats. The predilection site of induced tumors was the descending colon in LE rats (59/128), in which adenomas were prevailing, where only a small number of tumors were found in WF/O rats (8/42). In both strains ascending colon was far more crowded with poorly differentiated adenocarcinoma than other sites of the colon (8/19, 14/34). It is likely that a resistant rat strain (WF/O) for DMH carcinogenesis has emerged from a parent rat strain prone to develop colonic tumors spontaneously.