Bystander effects in bullfrog tadpoles

Adaptive responses were observed using the micronucleus frequency in bullfrog tadpoles. In tanks in which control tadpoles were placed in contact with tadpoles that were previously housed in tritiated water (3.0× 104 Bq/L), the cells from all animals responded as though they were “adapted”. This suggests that direct exposure to 3.0× 104 Bq/L tritium contributes to an increased resistance to a high dose of radiation in liver cells. It also suggests that being in contact with tadpoles that were previously exposed to 3.0× 104 Bq/L tritium (bystander effect) contributes to an increased resistance to a high dose of radiation in liver cells. In vitro exposures were also conducted using primary cultures of liver cells obtained from an unexposednon-bystander tadpole. In these control cells, it was observed that exposure to 100mGy of 60Co gamma radiation (delivered at a dose rate of 5mGy/min) did not affect the micronucleus frequency whereas exposure to 4Gy (delivered at a dose rate of about 10.2Gy/min) increased the micronucleus frequency. Prior exposure to a low dose of 60Co gamma radiation (100mGy delivered at a dose rate of 5mGy/min) induced an adaptive response, protecting the cells from harm caused by exposure to subsequent high doses of 60Co gamma radiation (4 Gy delivered at a dose rate of about 10.2Gy/min). Using the adaptive response (determined using micronucleus assay) as a biomarker, the data obtained suggest that bystander effects do play a role in wild populations since bullfrog tadpoles that were not exposed to tritium responded like the tadpoles that were directly exposed to tritium after being placed in contact with them.