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

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Abstract
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 γ-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 γ-irradiation only. In the case of He–Ne laser exposure 60 min before γ-irradiation with doses > 5 Gy, a fraction of more resistant cells was revealed: their D0 was twice as high as the D0 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 γ-irradiation (5 Gy) also changed the shape of the growth curves as compared with γ-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 γ-radiation damage. The possibility that the described phenomenon is an adaptive response of cells is discussed.