Modification of Recovery from Potentially Lethal X-Ray Damage in Plateau Phase Chinese Hamster Cells

The survival of density inhibited Chinese hamster cells assayed immediately after 1500 rads (${\rm SF}_{0}$) was compared to that obtained from cells held for 6 hr at 37°C in their high density condition ( SF₆). The ratio of SF₆ to${\rm SF}_{0}$, designated the surviving fraction ratio (SFR) was determined for various post irradiation conditions. If the density inhibited cells were overlaid with full medium following irradiation, the SFR was approximately 4 while in Hanks' BSS the SFR was 15-20. The recovery from X-ray damage was inhibited by lowered temperature: the SFRs were 10 and 2 at 25° and 4°C, respectively, compared to 16 at 37°C. This inhibition was almost completely reversible following transfer of the cells from the lower temperatures to 37°C and further incubation. The SFR under full medium (postirradiation) conditions was not significantly modified by the presence of metabolic inhibitors at concentrations nontoxic to at least 70% of the cell population. When the irradiated cells were kept in suspension rather than left in monolayers during the incubation period, the SFR was reduced from 15-20 to 4-5.