5-bromodeoxyuridine substitution and differential radiosensitivity of uni- and bi-filar chromatids

Abstract

Radiosensitivity of 5-bromodeoxyuridine(BrdU)-substituted chromatids has been studied in CHO cells (Wolff and Bodycote, 1975; Ikushima, 1977; Wolff and Fijtman, 1981) and in muntjac lymphocytes (Jacob, 1979). Equal sensitivity of uniand bi-filarly BrdU-substituted chromatids was observed by Wolff and Bodycote (1975) when the cells were X-irradiated in (32 after growing them for 2 replication cycles in BrdU medium. However, Ikushima (1977) reported that the light chromatid showed 2 times more sensitivity than the dark one when subjected to visible light flashes. In view of these contradictory results in CHO cells, Jacob (1979) studied the sensitivity of uniand bi-filarly BrdU-substituted chromatids in muntjac lymphocytes and reported that the bifilar chromatid is 3 times more sensitive than the unifilar one. Recently, Wolff and Fijtman (1981) using different concentrations of BrdU studied the sensitivity of light and dark chromatids and reported that when high BrdU concentrations were used, both the chromatids had equal sensitivity. However, with lower concentrations of BrdU, the light chromatid was found to be more sensitive. They have explained the latter result to be due to the dark and the light chromatids remaining unsubstituted and unifilarly substituted respectively, as a result of depletion of BrdU in the culture medium during the second cycle. On this basis, Wolff and Fijtman (1981) have interpreted the differential sensitivity obtained in my earlier study as due to depletion of BrdU during the second cycle leading to the formation of TT-TB chromatids. In this communication, additional data are provided to confirm that the dark and the light chromatids in my earlier study were actually uniand bi-filarly substituted and not as interpreted by Wolff and Fijtman. The most simple reason to believe that the second cycle chromatids in my study were uniand bi-filarly substituted was the fact that quite a few third-cycle metaphases were obtained in those cultures. A total of 542 metaphases was scored from 4 repeated cultures and the percentages of I, II and III cycle metaphases were 29, 60 and 11, respectively. The presence of third cycle metaphases with their