Studies on adaptation of V79 Chinese hamster cells to low doses of methylating agents

Pretreatment of V79 cells with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) or N-methyl-N-nitrosourea (MNU) significantly reduced the frequency of mutations (6-thioguanine resistance) induced by a challenge dose of these agents. Mutagenic adaptation can be observed after exposure to pretreatment doses which have no measurable toxic or cytogenetic effects, and which do not change DNA synthesis as measured at the moment of challenge. The influence of pretreatment on challenge dose-induced cell killing, chromosomal aberrations, and sister chromatid exchanges has also been studied. The effect of the challenge with respect to these end points was not in the same way dependent on the pretreatment dose as the challenge dose-induced mutation frequency. Therefore it is concluded that mutagenic adaptation and adaptation with respect to the other end points studied have no identical underlying causes.