[Inhibition of tumor induction by methylnitrosourea in resected urinary bladder].

In the present experiments the dependence of tumour induction upon the different phases of the cell cycle in the proliferating urinary bladder was examined. For stimulation of urothelial proliferation, a one-third resection of the bladder was performed in female Wistar rats. To synchronize the proliferating urothelial cells, hydroxyurea was given. The direct-acting urothelial carcinogen N-methyl-N-nitrosourea (MNU) was administered as a single intravesical dose during different cell cycle phases. The incidence of urothelial bladder tumors was 32.6% in the controls. By comparison, the tumour incidences were 18.9, 9.3, 21.7, 26.3, 25.0 and 30.0%, respectively, when MNU was instilled during the late Gi-, early and late S-, G2+M-, and early and late postmitotic phase. The results obtained from a total of 283 rats clearly document a cell cycle specific inhibition of tumour development in the proliferating urinary bladder particularly when the carcinogen was administered during the early S-phase. MNU has also been shown to produce mesenchymal tumours in the bladder (overall incidence: 4.9%) as well as urothelial tumours in the renal pelvis (3.2%) and ureters (1.4%).