Promotion by Sodium L‐Ascorbate in Rat Two‐stage Urinary Bladder Carcinogenesis is Dependent on the Interval of Administration

In our two‐stage model of rat urinary bladder carcinogenesis employing N‐butyl‐N‐(4‐hydroxybutyl)nitrosamine (BBN) as the initiator, sodium L‐ascorbate (Na‐AsA) exhibits dose‐dependent promotion. In the present study, in order to assess the possible reversibility of the promoting effects, we investigated how different administration periods of Na‐AsA influence its promoting activity. In experiment 1, rats were treated with 5% Na‐AsA for different administration periods with or without withdrawal and injected with 5‐bromo‐2′‐deoxyuridine (BrdU) to allow determination of the cell proliferation status. Replicative DNA synthesis in the urinary bladder epithelium was shown to return to normal after removal of the promoting stimulus. In experiment 2, rats were initially given BBN for 4 weeks and subsequently received 16 weeks of Na‐AsA, alternating with basal diet, at intervals of 4, 8 or 16 weeks, within total 32‐week period. The longer the continuous exposure to Na‐AsA, the greater the yield of papillomas and carcinomas in the urinary bladder. In experiment 3, Na‐AsA was given for 4 or 8 weeks after BBN initiation and the animals were killed at weeks 8 and 12. Both promotion of lesion development and increase of DNA synthesis in the urinary bladder epithelium were dependent on the length of exposure to Na‐AsA and the total period of exposure. The results indicate that the promoting effects of Na‐AsA in urinary bladder carcinogenesis are reversible to certain extent after its withdrawal, and the existence of cumulative exposure time threshold seems likely.