ADAPTIVE RESPONSE OF THE CHICKEN-EMBRYO TO LOW-DOSES OF X-IRRADIATION

Chicken embryos were x-irradiated in ovo with 5-30 cGy (=priming dose) at the 13th-15th day of development, After 3-48 h, brain- and liver-c ell suspensions were x-irradiated in vitro with (challenge) doses of 4 -32 Gy. Significantly less radiation damage was observed when the radi ation response was measured by scheduled DNA synthesis, nucleoid sedim entation and viscosity of alkaline cell lysates 12-36 h after the prin ting exposure. In vivo, pre-irradiation with 10 cGy enhanced regenerat ion as evidenced by the DNA content of chicken embryo brain and liver 24 h following a challenge dose of 4 Gy. From nucleoid sedimentation a nalyses in brain and liver cells immediately after irradiation with 16 Gy and after a 30-min repair period in the presence of aphidicolin, d ideoxythymidine and 3-aminobenzamide or in the absence of these DNA re pair inhibitors, it is concluded that a reduction of the initial radia tion damage is the dominant mechanism of the ''radio-adaptive'' respon se of the chicken embryo. Sedimentation of nucleoids from ethidium bro mide (EB) (0.75-400 mu g/ml)-treated cells suggests a higher tendency of ''radio-adapted'' cells to undergo positive DNA supercoiling in the presence of high EB concentrations.