Chromosomal responses to ionizing radiation reminiscent of an adaptive response in cultured Chinese hamster cells.

When Chinese hamster V79 cells were internally exposed to low level chronic beta-rays from incorporated tritiated thymidine (3H-dThd), they showed an "adaptive" response to the induction of chromosomal damage by subsequent higher acute doses of gamma-rays. The yield of sister-chromatid exchanges (SCEs) in the 3H-dThd pretreated cells was less than the yield induced by gamma-rays alone (protective effects), and the micronucleus frequency was less than the sum of the induced frequencies by 3H-dThd and gamma-rays separately (below-additivity effects). No adaptation to the micronucleus induction by gamma-rays was observed after the 3H-adapted cells had divided once and when 3-aminobenzamide (3AB) was given before the challenge doses. The cross-resistance study revealed that the 3H-adapted cells were resistant to SCE induction but not to the micronucleus inductions by the challenge doses of reactor radiations. The results suggest that the SCE adaptation and the micronucleus adaptation or clastogenic adaptation are probably caused by different, inducible adaptive repair pathways.