Abstract:
The effects of residual radiation from atomic bombs have been considered to be minimal
because of its low levels of external radioactivity. However, studies involving atomic bomb survivors
exposed to only residual radiation in Hiroshima and Nagasaki have indicated possible adverse
health effects. Thus, we investigated the biological effects of radioactive dust of manganese dioxide
56 (56MnO2
), a major radioisotope formed in soil by neutron beams from a bomb. Previously,
we investigated C57BL mice exposed to 56MnO2 and found pulmonary gene expression changes
despite low radiation doses. In this study, we examined the effects in a radiation-sensitive strain
of mice, BALB/c, and compared them with those in C57BL mice. The animals were exposed to
56MnO2 particles at two radioactivity levels and examined 3 and 65 days after exposure. The mRNA
expression of pulmonary pathophysiology markers, including Aqp1, Aqp5, and Smad7, and radiationsensitive genes, including Bax, Phlda3, and Faim3, was determined in the lungs. The radiation doses
absorbed in the lungs ranged from 110 to 380 mGy; no significant difference was observed between
the two strains. No exposure-related pathological changes were observed in the lungs of any group.
However, the mRNA expression of Aqp1 was significantly elevated in C57BL mice but not in BALB/c
mice 65 days after exposure, whereas no changes were observed in external γ-rays (2 Gy) in either
strain. In contrast, Faim3, a radiation-dependently downregulated gene, was reduced by 56MnO2
exposure in BALB/c mice but not in C57BL mice. These data demonstrate that inhalation exposure to
56MnO2 affected the expression of pulmonary genes at doses <380 mGy, which is comparable to 2 Gy
of external γ-irradiation, whereas the responses differed between the two mouse strains.
