Adaptive protection against the induction of oxidative DNA damage after hyperbaric oxygen treatment

Hyperbaric oxygen (HBO) treatment (i.e. exposure to 100% oxygen at a pressure of 2.5 ATA for a total of three 20 min periods) of human subjects caused clear and reproducible DNA effects in the comet assay with leukocytes. Interestingly, DNA damage was detected only after the first treatment and not after further treatments under the same conditions, indicating an increase in antioxidant defences. We now demonstrate that blood taken 24 h after HBO treatment is well protected against the in vitro induction of DNA damage by hydrogen peroxide (H2O2). H2O2 treatment caused a significant induction of DNA effects in the comet assay and chromosome breakage in the micronucleus test in the blood of volunteers before HBO. The same treatment did not cause genotoxic effects 24 h after HBO. This protective effect lasted for at least 1 week. Experiments with isolated lymphocytes gave similar results, indicating that the adaptive response is a cellular effect. The cells were not comparably protected against the genotoxic effects of gamma-irradiation, suggesting increased scavenging of reactive oxygen species distant from nuclear DNA.