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

Abstract

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.

Figure 1

The rats' lungs on days 3 (a–d) and 61 (e–h) after ⁵⁶MnO₂ exposure and the control (HE staining). There were no significant histological alternations in the lungs among Mn56x3 (a,b,e,f) and the control (c,d,g,h) groups. The structure of alveoli (alv) was normal with bronchiole (br) and blood vessels (b-v) in the Mn56x3 group. There were no signs of thickening of the alveolus wall even on day 61 postexposure (f) compared to the control (h). Original magnification × 40 (a,c,e,g), or × 100 (b,d,f,h).

Figure 2

The lung of rats on day 61 after ⁵⁶MnO₂ exposure (a,c) and the control (b,d). EVG staining indicated no significant changes in elastin (dark blue or black color) in the alveoli (alv) by ⁵⁶Mn exposure (a). Azan staining showed no signs of increased collagen deposition (blue color) in the alveolus wall (alv-w) in the ⁵⁶Mnx3 group (c). Original magnification, × 100 (a,b), or × 400 (c,d).

Figure 3

Relative mRNA expression levels of collagen-I, elastin, TGF-β, Smad2 and Smad7 genes in the lungs of rats on day 3 (left) and day 61 (right) after the exposure to ⁵⁶MnO₂ powder (Mn56x1, Mn56x2, Mn56x3), Cold MnO₂ powder (Cold Mn) or ⁶⁰Co-γ exposure (Co-60). *p < 0.05 vs. control; ^#p < 0.05 vs cold Mn.

Figure 4

Relative mRNA expression levels of AQP1, AQP4, and AQP5 genes in the lungs of rats on day 3 (left) and day 61 (right) after the exposure to ⁵⁶MnO₂ powder (Mn56x1, Mn56x2, Mn56x3), Cold MnO₂ powder (Cold Mn) or ⁶⁰Co-γ exposure (Co-60). *p < 0.05 vs. control; ^##p < 0.01 vs cold Mn.

Figure 5

Western blot analysis and densitometry for AQP5 in the lung on postexposure days 3 and 61. β-Actin was presented as an internal control. Lysates extracted from the lung were examined. *p < 0.05, **p < 0.01 vs. control; ^#p < 0.05, ^##p < 0.01 vs cold Mn.