The rate of DNA synthesis in HeLa cells during combined long-term and acute exposures to lead.

Asynchronous populations of HeLa cells growing as monolayers were incubated in a nutritive medium supplemented with a low concentration of 10(-5) M lead chloride for several months. At various time intervals the [3H]thymidine uptake was measured in the DNA of the cells by a scintillation counting technique. During the first 2 weeks of incubation, the rate of DNA synthesis was slightly reduced. After 3 weeks the synthesis returned to normal and later it was a little enhanced. The pretreated cells tolerated acute intoxication by a high lead concentration of 2.5 x 10(-4) M without a change in the rate of DNA synthesis. The cells also proliferated 1 year in a culture medium supplemented with a progressively increasing lead concentration. However, the survival of cells at the highest lead concentration lasted only a few days. After return to a metal-free medium the cells again became very sensitive to lead as shown by the reduced rate of DNA synthesis. The transitory character of adaptation indicates that there was no direct genetic involvement.