Effects of caffeine on L-cells exposed to mitomycin C.

Mouse L-cells were grown in suspension culture and exposed to varying concentrations, from 0.2 to 4 ¿ig/ml,of mitomycin C for periods ranging from 0.5 to 3.5 hr. A plot of cell colony-forming ability versus concentration of mitomycin C for different times of exposure yielded a series of exponential survival curves which were related by the fact that the product of the time of exposure and the con centration of mitomycin C required to reduce survival to 10% (CÃ-o) were a constant. However, the value of C10 was dependent on the pH of the cell culture at the time of exposure to mitomycin C. If, immediately after exposure to mitomycin C, the survival of the cells was measured in medium containing 2 mM caffeine, the C10 was 0.5 times that observed in the absence of caffeine. When the treated cells were incubated in the absence of caffeine for one generation time and their survival was measured in its presence, no difference in C\ o over that observed in normal medium was seen. With a synchronous population of L-cells, evidence was obtained that cells exposed to mitomycin C which passed through their first DNA-synthetic period in the absence of caffeine had no additional loss of viability when plated in it at later times. These results indicate that the same system shown previously to exist in L-cells for the repair or alteration of ultraviolet light damage is also func tional against damage produced by mitomycin C.