Carcinogenicity of N-Methyl-N′-nitro-N-nitrosoguanidine to the Livers and Kidneys of Rainbow Trout (Salmo gairdneri) Exposed as Embryos234

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Abstract
Rainbow trout (Salmo gairdneri) embryos 21 days post fertilization were exposed to aqueous solutions of N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) at concentrations of 0, 10, 30, and 100 ppm for 1 hour. The resulting fry were fed a casein control diet for 1 year and sampled at 9 and 12 months for determination of tumor development. At 9 months the incidences of fish bearing hepatocellular carcinomas/total number of fish were 0/40, 0/40, 13/40, and 21/40 for the control, 10-ppm exposure group, 30-ppm exposure group, and 100-ppm exposure group, respectively. These incidence figures increased to 0/60, 4/60, 36/60, and 51/60, respectively, at 12 months. Lower but still significant incidences of nephroblastomas were also present. The incidences of fish bearing nephroblastomas/total number of fish were 0/40, 0/40, 1/40, and 3/40 at 9 months and 0/60, 1/60, 3/60, and 5/60 at 12 months in the control group, 10-ppm exposure group, 30-ppm exposure group, and 100-ppm exposure group, respectively. A marked sex reversal (female to male) trend was also noted in the group exposed to 100 ppm MNNG. This trend was manifested by a pronounced rile in the male/female ratio and by histologically demonstrated hermaphroditism. This experiment demonstrated that a single short exposure of 21-day trout embryos to MNNG in the ambient water was sufficient to induce in a dose-dependent fashion high incidences of hepatocellular carcinoma and lower incidences of nephroblastomas within 9–12 months.