Cytotoxic effect and induction of sister chromatid exchange in exponentially growing rat 9L gliosarcoma cells after brief exposure to BrdU

The magnitude of DNA modulation in rat 9L gliosarcoma cells after a brief exposure to bromodeoxyuridine (BrdU) was studied by assaying colony-forming efficiency (CFE) and the number of sister chromatid exchanges (SCEs) per metaphase. The CFE assay showed that a 1-hr exposure to BrdU, at concentrations ranging from 10 to 1000 microM, produced a maximum cell kill of 5%. After a 2-hr exposure to 20 microM BrdU, the surviving fraction was 0.99, and even at a BrdU concentration of 1000 microM, 77% of the 9L cells survived. Compared with control cultures, the relative number of SCEs per metaphase in treated cultures was increased after a 1-hr exposure to BrdU at concentrations of 100 microM or more and after a 2-hr exposure to concentrations of 20 microM or more; no increase was observed in cells treated for 30 min with BrdU at concentrations up to 1000 microM. When the treated cells were allowed to grow in BrdU-free growth medium, the number of SCEs per metaphase returned to the control level within 24 hr, even after exposure to BrdU at concentrations as high as 1000 microM. These results demonstrate that exposure to BrdU at concentrations of up to 1000 microM for 30 min, 100 microM for 1 hr, and 20 microM for 2 hr causes little modulation of DNA.