Survival and mutation of Chinese hamster cells after ultraviolet irradiation and caffeine treatment.

UV-irradiated Chinese hamster cells on post-irradiation treatment with caffeine in growth medium for 24 h gave rise to biphasic UV-survival curves. At caffeine concentrations between 0.001 and 0.1 mM, control and caffeine-grown cells had similar survival curves initially from 0 to 30 J/m2. At fluences greater than 30 J/m2, there was effectively only little further killing of caffeine-grown cells beyond that observed at 30 J/m2. At concentrations of caffeine greater than 0.5 mM, there was a gradual sensitization in the early part of the survival curve with increasing caffeine concentrations; but at fluences greater than 30 J/m2, the slopes in the survival curves decreased very much. It has been proposed that the initial sensitization observed at low UV fluences is due to the suppression of post-replication repair by caffeine. After high fluences of UV exposures in these excision-deficient cells, in the presence of caffeine, the possibility of an induced 'SOS'-like repair process has been suggested. This suggestion was supported by the observation that caffeine increased the yield of the UV-induced 8-azaguanine-resistant mutants only for the cell population exposed to UV fluences greater than 30 J/m2.