Induction of the renal pelvic and ureteral carcinomas by N-butyl-N-(4-hydroxybutyl)nitrosamine in SD/cShi rats with spontaneous hydronephrosis

Differences in susceptibility to N-butyl-N-(4-hydroxybutyl)nitrosamine-induced urinary bladder carcinogenesis between SD/gShi rats with spontaneous hypospermatogenesis and SD/cShi rats with spontaneous hydronephrosis

Differences in susceptibility to N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN)-induced urinary bladder carcinogenesis between two substrains of male Sprague-Dawley rats were examined. One substrain was SD/gShi, which has spontaneous hypospermatogenesis, and the other was SD/cShi, which is a sister strain of SD/gShi, and has normal testis but spontaneous hydronephrosis. SD/gShi rats had a lower incidence of urinary bladder tumors and had lower 5-bromo-2'-deoxyuridine labeling indices in the urinary bladder epithelium than SD/cShi rats when BBN was given. SD/gShi rats had significantly lower urinary concentrations of N-butyl-N-(3-carboxypropyl)nitrosamine (BCPN), which is a metabolite and proximate carcinogen of BBN. In vitro analysis also showed significantly less BCPN formation, using an S9 mix derived from the liver and kidney, in SD/gShi rats than in SD/cShi rats. BCPN formation in vitro was markedly inhibited by non-selective cytochrome P450 (CYP) inhibitors, but not alcohol dehydrogenase inhibitor. However, analysis of CYP proteins including hepatic CYP1A1/2, 2B1/2, 2E1, and 3A2 and renal CYP2E1 and 3A2 revealed no significant variation in levels in either tissue in the groups. There were also no significant intergroup differences in the mutagenicity of carcinogens, including heterocyclic amines and N-nitrosamines, activated by CYP1A1/2 and CYP2E1 and/or CYP2B1/2, respectively. These results suggest that SD/gShi rats are less susceptible to BBN, possibly because less BCPN is produced by CYP isoforms other than those investigated. A contribution of CYP4B1 to the strain difference is also possible.

Hepatocarcinogenic activity of N-butyl-N-(4-hydroxybutyl)nitrosamine in rats is not modified by sodium L-ascorbate

Male F344 and Wistar Shionogi (WS) rats were treated with 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) for 20 weeks and then killed at week 36 (experiment 1). Although reduction of body weight increase was found, no effects on liver weights were noted. Formalin-fixed and paraffin-embedded liver tissues from rats killed terminally were cut and stained for glutathione S-transferase placental form (GST-P) immunohistochemically. Marked elevation of quantitative values of small GST-P positive (GST-P+) foci were apparent in both strains of rat administered BBN. In experiment 2, both sexes of F344 rats were given 0.05% BBN in the drinking water for 4 weeks and then fed diet containing 0 or 5.0% sodium L-ascorbate (SA) for 32 weeks. No body and liver weight changes were evident in any group. Quantitative values for small GST-P+ foci were increased in both sexes of rats exposed to BBN but were not modified by additional SA treatment. Thus, it was confirmed that the selective bladder carcinogen BBN also acts as a liver carcinogen. These results, from the quantitative analysis of small GST-P+ foci as end point marker lesions, indicate that the liver tumor modifying potential of test chemicals can be evaluated in rats by using an initiation/promotion protocol for urinary bladder carcinogenesis.

Strain as a determinant factor in the differential responsiveness of rats to chemicals

The beneficial effects derived from the use of chemicals in agriculture, energy production, transportation, pharmaceuticals, and other products that improve the quality of life are clearly established. However, continued exposure to these chemicals is only advantageous in conditions where the benefit far outweighs toxic manifestations. By law, determination of risk of toxicity necessitates the use of laboratory animals to establish whether chemical exposure is safe for humans. To simulate the human condition, it is incumbent upon investigators to choose a species in which pharmacokinetic and toxicokinetic principles are established and resemble those of humans. Some of the advantages to the use of rat in chemical toxicity testing include (a) similarities in metabolism, anatomy, and physiological parameters to humans; (b) the short life span, especially for carcinogenesis study; (c) the availability, ease of breeding, and maintenance at a relatively low cost; and (d) the existence of a large database to enable comparison of present to reported literature findings. However, the choice of rat can be complicated by several factors such as sex, age, and nutrition, but especially strain, where currently there are over 200 different strains of rat known to exist. The aim of this review is to demonstrate that there are differences in the responsiveness of rat strains to chemicals and that the susceptibility observed is dependent on the tissue examined. It is evident that the genotype differs among strains, and this may be responsible for differences in sensitivities to chemicals. Awareness of strain as a factor in susceptibility to toxicant action needs to be taken into account in interpretation of relevance of risk of toxicity for humans.