The recovery of the survival curve shoulder after protracted hypoxia

Abstract

Experimental findings are reported on post-hypoxic recovery. HeLa cells which had been maintained at 37°C in 95 per cent nitrogen, 5 per cent CO2 for 22 hours were moved to an atmosphere of 95 per cent air, 5 per cent CO2. At varying times thereafter the cells were given a dose of 420 rads of 100 kV X rays and it was found that the survival increased with time. Survival curves determined ten minutes and five hours after removal to aerobic conditions showed that the increase in survival was due to an increase in the shoulder. The shapes of these post-hypoxic recovery curves at both 37°C and room temperature were very similar to split X-ray dose recovery curves at the same temperatures, the 37°C curve exhibiting a dip at two hours. The possible significance of these results to radiotherapy in hyperbaric oxygen is discussed. While oxygen may sensitize hypoxic cells through its dose modifying effect it may also produce a protective effect by the increase in the shoulder described above, and this protective effect will increase with the duration of the exposure to hyperbaric conditions. The relevance to the mechanisms of recovery of the similarities between post-hypoxic and split X-ray dose recovery curves is also discussed.