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

We have conducted experiments to clarify the existence of extraretinal phot osensitivity in mammals through the measurements of skin blood flow variati on due to light irradiation. We found that blood flow shows a synchronized transient increase with a irradiation-nonirradiation sequence. The action s pectrum of the phenomenon was found to show peaks at similar to 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 fer ruos-heme complexes such as nitric oxide (NO)-Hb. The blood flow increase i n the irradiated duration disappears when the rats are intraperitoneally in jected 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 an d 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 th e photosensitivity mechanism where optical absorption of the sixfold coordi nated ferrous heme-NO complex plays a main role.