Biological impacts on the lungs in rats internally exposed to radioactive ⁵⁶MnO₂ particle

To understand the radiation effects of the atomic bombing of Hiroshima and Nagasaki among the survivors, radiation from neutron-induced radioisotopes in soil should be considered in addition to the initial radiation directly received from the bombs. ⁵⁶Mn, which emits both β particles and γ-rays, is one of the dominant radioisotopes created in soil by neutrons from the bomb. Thus we investigated the biological effects of internal exposure to ⁵⁶MnO₂ particle in the lung of male Wistar rats comparing to the effects of external ⁶⁰Co-γ irradiation. Absorbed doses of internal irradiation of lungs were between 25 and 65 mGy in ⁵⁶MnO₂-exposed animals, while the whole body doses were between 41 and 100 mGy. Animals were examined on days 3 and 61 after the exposure. There were no remarkable pathological changes related to ⁵⁶MnO₂ particle exposure. However, mRNA and protein expressions of aquaporin 5 increased significantly in the lung tissue on day 3 postexposure in ⁵⁶MnO₂ groups (by 1.6 and 2.9 times, respectively, in the highest dose group). Smad7 mRNA expression was also significantly elevated by 30% in the highest dose group of ⁵⁶MnO₂. Our data demonstrated that internal exposure to ⁵⁶MnO₂ induced significant biological responses including gene expression changes in the lungs, while external ⁶⁰Co-γ irradiation of 2 Gy did not show any changes.