Activity of germ-cell mutagens and nonmutagens in the rat spermatocyte UDS assay.

The ability of 13 chemicals of known germ-cell mutagenicity to induce unscheduled DNA synthesis (UDS) in rat spermatocytes was examined. At selected times following i.p. injection of test compounds, spermatocytes were isolated from Fischer 344 rats by enzymatic digestion of the seminiferous tubules and cultured for 24 h in the presence of [3H]thymidine. 7 compounds, methyl methanesulfonate, triethylenemelamine, cyclophosphamide, methylnitrosourea, ethylnitrosourea, procarbazine, and dibromochloropropane produced positive UDS responses in spermatocytes. These chemicals are also positive for specific locus mutations, heritable translocations, or dominant lethal mutations when administered to male rodents. Mitomycin C, which produces DNA interstrand crosslinks and induces heritable mutations and translocations in male germ cells, failed to stimulate UDS in rat spermatocytes. Germ-cell nonmutagens N-methyl-N'-nitro-N-nitrosoguanidine, dimethylnitrosamine, 4-nitroquinoline 1-oxide, and ethylene dibromide were negative in the rat spermatocyte UDS assay. Correlation of these results with those of other assays for heritable mutations in germ cells indicates that the in vivo/in vitro spermatocyte DNA repair assay is useful in predicting the mutagenic potential of chemicals in male germ cells.