Some years ago we performed two experiments aimed at studying the influence of the background radiation on living matter by exploiting the low radiation background environment in the underground Gran Sasso Laboratory of the INFN. Their results were consistent with the hypothesis that the "normal" background radiation determines an adaptive response, although they cannot be considered conclusive. PULEX-3 (the third experiment of the series) is aimed at comparing the effects of different background radiation environments on metabolism of cultured mammalian cells, with substantial improvements with respect to the preceding ones. The experiment was designed to minimize variabilities, by maintaining two cultures of Chinese hamster V79 cells in exponential growth for up to ten months in the underground Gran Sasso Laboratory (LNGS), while two other cultures were maintained in parallel in a biological laboratory installed at the LNGS outside the tunnel. Exposure due to 7-rays was reduced by a factor of about 10 in the underground laboratory while the Rn concentration was small in both cases. After ten months the cells grown in the underground laboratory, compared to those grown in the external one, exhibited: i) a significantly lower capacity to scavenge reactive oxygen species (ROS), and ii) an increased sensitivity to the mutagenic effect of rays. Since the probability that this finding is due to casual induction of radiosensitive mutants is extremely low, it corroborates the hypothesis that cells grown in a "normal" background radiation environment exhibit an adaptive response when challenged with genotoxic agents, which is lost after many generations in a low background radiation environment.

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