生体での作用

　これらの変動は生体レベルに反映される。発がんが最も関心の高い放射線の影響である。中程度から高線量では発がんリスクが増加することは良く知られている。しきい値なし直線仮説のように低線量でのリスクが外挿で決定できるのかは検討が必要である。動物実験では高線量と低線量で反対の作用が得られている。例えばC57BL/6Jマウスが1.75Gyの照射を週1回4週間受けると、6ヶ月で約半数に胸腺リンパ腫が発生する[12]。もしこれらの照射の6から24時間前に0.025、0.075、0.1Gyの照射を受けると発生の確率が1/2から1/3に低下する。著者らはこの作用を免疫の活性化によると考えている[13]。

まとめ

　線量-作用曲線は0.5Gy以上とそれ以下例えば0.025から0.2Gyではしばしば逆になることに気付くだろう。この点をここでは分子レベルで議論したが、これらを踏まえた仮説的な線量-応答曲線を図8に示す。

References

1. Liu S-Z. Radiation Hormesis with low-level exposures. Beijing: Science Publishers, 1996.
2. Li X-Y, Lu Z, Zhang YC, et al. Enhancing effect of low dose radiation on the proliferative activity of thymocytes in mice. J Radiat Res Radiat Proc 1994; 12, 124-126.
3. Liu S-Z, Bai O, Chen D, et al. Genes and protein molecules involved in the cellular activation induced by low dose radiation. J Radiat Res Radiat Proc 2000; 18: 175-186.
4. Liu S-Z. Biological defense and adaptation induced by low dose radiation. Human Ecol Risk Assess (HERA) 1998; 4:1217-1254.
5. Liu S-Z. Cellular and molecular basis of the stimulatory effect of low dose radiation on immunity. In Wei, L-X, Sugahars, T. and Tao, Z-F. (eds.), High Levels of Natural Radiation. Amsterdam: Elsevier Science BV, 1997; 341-353.
6. McKenna WG, Maity A, Muschel RJ. Radiation and the molecular biology of the cell cycle. Radiation Research 1895-1995, Proc 10th ICRR, Wurzburg, Germany, Aug. 27-Sept. 1, 1995; 629-634.
7. Ye F, Liu S-Z. Effects of whole body irradiation with X-rays on cell cycle progression of thymocytes in mice. Radiat Prot 2000; 20:189-192.
8. Liu S-Z, Xie F. Involvement of the Ca2+-protein kinase C and adenylate cyclase signal pathways in the activation of thymocytes in response to whole-body irradiation with low dose X-rays. Chin Med Sci J 2000; 15:1-7.
9. Liu S-Z, Su X, Zhang YC, et al. Signal transduction in lymphocytes after low dose radiation. Int J Occup Med Toxicol 1994; 3: 107-117.
10. Danet-Desnoyers G, Meyer MD, Gross TS, et al. Regulation of endometrial prostaglandin synthesis during early pregnancy in cattle: effects of phospholipases and calcium in vitro. Prostaglandins 1995; 50: 313-330.
11. Ghosh S, May MJ, Kopp EB. NF-kappaB and Rel proteins: evolutionarily conserved mediators of immune response. Annu Rev Immunol 1998; 16: 225-260.
12. Li X-Y?
13. Li X-J?
14. Zhang Y?
15. et al. Influence of low dose ionizing radiation on thymic lymphoma induced by carcinogenic dose radiation in C57BL/6J mice. Acad. Period Abst Chin 1998; 4: 1406-1407.
16. Li X-J, Yang Y, Li X-Y, et al. Immunologic mechanism of the suppressive effect of low dose radiation on thymic lymphoma induced by radiation. J Radiat Res Radiat Proc 1999; 17: 125-128.