Triggering of DNA strand breaks by 45 degrees C hyperthermia and its influence on the repair of gamma-radiation damage in human white blood cells

Human peripheral white blood cells, freshly isolated from normal individual donors, were exposed to hyperthermia. Heat-generated DNA strand break damage and white blood cell capacity to repair radiation-induced breaks were determined by a fluorometric alkaline unwinding assay. Strand breaks could be readily detected when white blood cells were incubated in a physiological salt solution at temperatures between 41 degrees and 46 degrees C, for times up to 90 min. The time course of strand break induction at 45 degrees C was characterized by a short initial lag, followed by a period of rapid break induction and subsequently a lower rate. Evidence is presented which suggests that the induction of DNA damage involved a triggering mechanism; a short treatment at 45 degrees C (10 to 20 min) initiated a cellular event which led to a rapid increase in the number of strand breaks during subsequent incubation of 37 degrees C. Continuous incubation at 45 degrees C produced less DNA damage than an initial period at 45 degrees C followed by incubation at 37 degrees C. This apparent triggering phenomenon was not due to a triggering of the respiratory burst in phagocytic cells, since no O2- could be detected; in fact, a 30-min treatment at 45 degrees C largely blocked the capacity of the cells to respond normally to a soluble stimulator of the respiratory burst. Unlike gamma-ray-induced breaks, 45 degrees C hyperthermia-induced breaks did not rejoin during subsequent incubation for up to 1 h at 37 degrees C