Effects of Single and Continuous Irradiation of HeLa Cells at −196°C

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Abstract
HeLa cells have been irradiated when frozen to liquid nitrogen temperature, and the dose-response has then been assessed in terms of survival of the colony-forming ability of cells plated out after thawing. Survival curves have been obtained after acute irradiation under various conditions and computations indicate a common extrapolation number of 3·3 for these curves. Differences in the exponential slope indicate successive dose-modifying factors when (1) the cryoprotective agent dimethyl sulphoxide is added at room temperature, (2) when the temperature is lowered to −196°c, (3) when the cells are hypoxic. This oxygen effect has been confirmed by clone-size analysis. No evidence of recovery from sub-lethal damage was obtained from split-dose experiments in which the cells were maintained at liquid nitrogen temperature between acute doses, before being thawed and plated for survival. Cells continuously irradiated at −196°c showed the same survival values as those acutely irradiated, as well as showing a similar oxygen effect. Recovery from sub-lethal damage was not manifest immediately after the rapid thawing used in these experiments.