RESPONSE OF V79 CELLS TO LOW-DOSES OF X-RAYS AND NEGATIVE PI-MESONS -CLONOGENIC SURVIVAL AND DNA STRAND BREAKS

Mammalian cells are hypersensitive to very low doses of X-rays (< 0.2 Gy), a response which is followed by increased radioresistance up to 1 Gy. Increased radioresistance is postulated to be a response to DNA d amage, possibly single-strand breaks, and it appears to be a character istic of low linear energy transfer (LET) radiation. Here we demonstra te a correspondence between the extent of the increased radioresistanc e and linear energy transfer of 250 kVp X-rays and plateau and Bragg p eak negative pi-mesons. The results support our hypothesis since the s ize of the increased radioresistant response appears to correspond to the number of radiation induced single-strand breaks. Furthermore, sin ce survival prior to the increased radioresistant response (< 0.2 Gy) was LET-independent, these data support the notion that the increased radioresistant response may dictate the overall survival response to h igher doses. However, while these data provide fur ther circumstantial evidence for the involvement of DNA strand breaks in the triggering o f increased radioresistance, more direct conclusions cannot be made. T he data are not accurate enough detect structure in the single-strand production of single-strand breaks being apparently linear with dose.