Pretreatment of Chinese hamster v79 cells with MNU increases survival without affecting DNA repair or mutagenicity.

Exposure of Chinese hamster V79 cells to a non-toxic dose of N-methyl-N-nitrosourea, followed at intervals by exposure to toxic challenging doses of the same agent, resulted in increased survival of colony forming ability when these cells were compared with matched control cells that only received the challenging dose. The extent of the increase was dependent on the time interval between exposures, and rose to a maximum of about two-fold 5 days after the initial dose, declining slowly to control values on subsequent days. Whilst pretreatment enhanced survival, it altered neither the frequency of mutation to 6-thioguanine resistance, nor the formation or loss of 3-methyladenine, 7-methylguanine and O6-methylguanine. Modification of the conditions by which the initial dose was administered led to a reduction or abolition of the survival response. It is suggested that enhanced survival may result from alteration in the ability to recover from cellular damage rather than by improved DNA repair.