Effect of heat on radiosensitivity at different developmental stages of embryos of the fish Oryzias latipes.

Embryos of the fish, Oryzias latipes, at various developmental stages were heated before or after gamma-irradiation. The lethal effects of heat and radiation treatments were determined by the hatchability of the irradiated embryos. The radiosensitivity suddenly decreased at 3-days after fertilization. Heat (41 degrees C), either preceding or following irradiation, decreased the shoulder region (Dq) of the dose-response curves of the embryos irradiated at the 1-3 day stages, and increased the slope (D0) only at the 3 day stage. The magnitude of enhancement of radiosensitivity by pre- and post-heating was found to depend on the time and temperature of the heat as well as the developmental stage of the embryos; in 1- and 2-day embryos the effectiveness of post-irradiation heating was larger than that of pre-irradiation heating whereas in 3-day embryos there was no difference between the effectiveness of pre- and post-treatment. The sequence of heat (41 degrees C) and radiation treatment, and the time interval between the two treatments, also influenced the effectiveness of heat on radiosensitivity, depending upon the developmental stage; recovery from heat effects was not observed within a time interval of up to 3 h before radiation exposure for either 1- or 3-day embryos, whereas recovery from the radiation lesion occurred, for only 1-day embryos, within 1 h of subsequent heat exposure.