Irradiation with He–Ne Laser Can Influence the Cytotoxic Response of HeLa Cells to Ionizing Radiation

A monolayer of HeLa cells in plateau-phase of growth was exposed to He--Ne laser radiation (632.8 nm, 100 J/m2) either 5, 60 or 180 min before gamma-irradiation (0.2-10 Gy, 6.75 Gy/min). In the case of a 5 min interval between the two types of irradiations the survival curve was practically identical to the survival curve of the cells after gamma-irradiation only. In the case of He--Ne laser exposure 60 min before gamma-irradiation with doses > 5 Gy, a fraction of more resistant cells was revealed: their Do was twice as high as the Do of the main population (3.6 and 1.7 Gy, respectively). A preexposure of cells to He--Ne laser (100 J/m2) 60 or 180 min before gamma-irradiation (5 Gy) also changed the shape of the growth curves as compared with gamma-irradiated cells (stimulation in exponential phase of growth). It is proposed that the preexposure of cells to He--Ne laser radiation activates, in a subpopulation of cells, processes which speed up the repair of gamma-radiation damage. The possibility that the described phenomenon is an adaptive response of cells is discussed.