Multiple small exposures of filtered mid-UV radiation increase the resistance of Chinese hamster cells to far-UV, mid-UV and filtered mid-UV radiation.

The response of Chinese hamster cells (V79-MI2G) to multiple, low doses of filtered mid-UV radiation (wavelengths longer than 300 nm) were examined over an exposure period of 30 days. Cell survival and the induction of mutation at the hypoxanthine-guanine phosphoribosyl transferase (HGPRT) locus using resistance to 6-thioguanine (TG) were the endpoints in this study. With increasing total accumulated dose given at 500 J/m2/day as a single short exposure, an increased resistance to cell killing was observed. This increase in resistance to cell killing was accompanied by a gradual decrease in sensitivity to the induction of mutants resistant to 6-TG. Above total accumulated doses of 5000 J/m2 the frequency of 6-TG resistance did not increase. After multiple doses of filtered mid-UV radiation the cells became more resistant to subsequent challenges with acute doses of far-UV, mid-UV or filtered mid-UV. The increased resistance to the cell killing action and to the mutation induction by UV suggests that during exposure to low, multiple doses of filtered mid-UV radiation the cells become adapted to the damaging effects of filtered mid-UV radiation.