Increase in peripheral blood flow due to extraocular direct irradiation of visible light in rats

We have conducted experiments to clarify the existence of extraretinal photosensitivity in mammals through the measurements of skin blood flow variation due to light irradiation. We found that blood flow shows a synchronized transient increase with a irradiation-nonirradiation sequence. The action spectrum of the phenomenon was found to show peaks at approximately 410-420 nm, 540-550 nm, and 570- 580 nm. These peaks coincide with the specific optical absorption peaks of B and Q (alpha,beta) bands in sixfold coordinated ferruos-heme complexes such as nitric oxide (NO)-Hb. The blood flow increase in the irradiated duration disappears when the rats are intraperitoneally injected with 1H-[1,2,3]oxydiazolo[4,3-a] quinoxalin-1-one (ODQ), which is an inhibitor of guanylate cyclase, and N(G)-monomethyl-L-arginine acetate and N(G)-nitro-L-arginine methyl ester, which are inhibitors of NO synthase. On the basis of the present results, we propose a photochemical model of the photosensitivity mechanism where optical absorption of the sixfold coordinated ferrous heme-NO complex plays a main role.