Radiotoxicity of incorporated [3H]thymidine as studied by autoradiography and flow cytometry. Consequences for the interpretation of FLM data.

The radiotoxic effects of incorporated [3H]thymidine on proliferation kinetics of flash-labelled (30 min, 0.3 microCi/ml, 40 Ci/mM) L-929 cells in vitro were studied by means of autoradiography and flow cytometry. The flow cytometric results obtained by applying the BUdR-33258 Hoechst technique, using new evaluation procedures, showed that the labelled cells are delayed in their progression through the S and G2 + M phases, leading to mitotic delay. From autoradiographs, the fraction of labelled mitoses was determined and, in addition, the ratio of labelled and of unlabelled mitotic cells to all cells. The radiotoxic effects are not evident from the FLM curve, even if the ratio of labelled mitotic cells to all cells shows a highly distorted shape. A mathematical model has been developed that describes the perturbed cell kinetics due to radiotoxic effects of the incorporated [3H]thymidine. These findings have considerable consequences for the interpretation of autoradiographic data, especially of labelled mitoses curves.