Biol. Pharm. Bull. 29(2) 348—353 (2006)

vide the first line of defense against environmental free radical attack caused by, for example, exposure to UV radiation, ionizing radiation, chemical oxidants, and aerobic microorganisms. Therefore, the skin has developed a complex antioxidant network that includes enzymatic and non-enzymatic components. Prolonged exposure of the skin to UV radiation results in inflammation including erythema and edema formation. Premature aging, immune suppression and skin cancer are its possible late effects. Erythema formation is the result of local increases of blood flow, in the regulation of which nitric oxide (NO) has been known to play an important role. In fact, UV radiation-mediated expression of inducible nitric oxide synthase (iNOS) was reported in the vessel endothelia of normal human skin and in cultured human dermal endothelial cells. iNOS has been implicated in the pathogeneses of various inflammatory syndromes, including asthma, transplant rejection, inflammatory bowel disease, rheumatoid arthritis, septic shock and parasitic infections. The expression of iNOS is also strongly implicated in psoriasis and other inflammatory skin conditions including UV irradiation as stated above. Reactive oxygen species (ROS) have been connected with the pathogenesis of both inflammation syndromes and UV-caused skin damage. Similarly, the predominant event for ionizing irradiation of cell and tissue is the generation of ROS, for example, hydroxyl radicals and superoxide anion. In addition, the involvement of iNOSderived NO in acute radiation syndrome is suggested by an increase of iNOS gene expression or iNOS enzyme activity in the liver, intestine, colon and ileum and brain. NO has been detected in liver and mammary tumorigenesis after X-ray irradiation. However, there is no report on the direct measurement of the generation of NO and the expression of related enzymes, nitric oxide synthase, in ionizing radiation induced skin inflammatory reactions. In the present study we tried to measure NO production and NOS expression in mouse skin after high-dose X-ray irradiation.