Marked depression of radiation-induced emesis in frogs following prior exposure to a brief dose of X-rays.

Acute emesis response to harmful doses of X-rays on frogs (Rana porosa porosa) was examined. Results showed that the number of radioemesis events following exposure to 0.85 Gy was slightly higher than in the sham control animals. The increase in emesis action became more pronounced when the total dose of radiation was raised to 2.5 Gy. Only 1 frog out of a total of 12 did not show vomiting following radiation, while no response was observed in sham control animals. Note that animals in which the low dose rate of radiation was applied to whole body did not display any changes in the emesis response relative to control animals. The present studies, and those by others, showed that a brief dose of X-rays prior to a second exposure to a sub-lethal dose might induce a tolerance to radiation. An additional experiment was conducted to examine whether a small conditioning dose could induce a depression of radioemesis (tolerance) following an exposure to high dose X-ray. With prior exposure to 0.3 Gy, only 1 frog out of a total of 5 frogs vomited as a result of radiation exposure. Suppression of the emetic response became significant when the pre-radiation dose was decreased to 0.1 Gy. On the contrary, increasing the small conditioning dose to 0.5 Gy resulted in a remarkable rise of radiation-induced emesis. This results indicate that exposure to the smaller dose of X-rays elicits a tolerance effect to toxic dose level of radiation.