Cell Cycle Progression in Irradiated Endothelial Cells Cultured from Bovine Aorta

Logarithmically growing endothelial cells from bovine aortas were exposed to single doses of 0-10 Gy of 60 Co γ rays, and cell cycle phase distribution and progression were examined by flow cytometry and autoradiography. In some experiments, cells were synchronized in the cell cycle with hydroxyurea (1 mM). Cell number in sham-irradiated control cultures doubled in approximately 24 h. Estimated cycle stage times for control cells were 14.4 h for G1 phase, 7.2 h for S phase, and 2.4 h for ${\rm G}_{2}+{\rm M}$ phase. Irradiated cells demonstrated a reduced distribution at the G1/ S-phase border at 4 h, and an increased distribution in ${\rm G}_{2}+{\rm M}$ phase at 24 h postirradiation. Autoradiographs of irradiated cells after continuous [3 H]thymidine labeling indicated a block in G1 phase or at the G1/ S-phase border. The duration of the block was dose dependent (2-3 min/cGy). Progression of the endothelial cells through S phase after removal of the hydroxyurea block also was retarded by irradiation, as demonstrated by increased distribution in early S phase and decreased distribution in late S phase. These results indicate that progression of asynchronous cultured bovine aortic endothelial cells through the DNA synthetic cycle is susceptible to radiation inhibition at specific sites in the cycle, resulting in redistribution and partial synchronization of the population. Thus aortic endothelial cells, diploid cells from a normal tissue, resemble many immortal cell types that have been examined in this regard in vitro.