Vascular smooth muscle contains a depletable store of a vasodilator which is light-activated and restored by donors of nitric oxide.

Endothelium-denuded strips of rabbit thoracic aorta relax on exposure to light. This response is similar to endothelium-dependent relaxation as it is inhibited by hemoglobin and methylene blue, and is mediated by an increase in cyclic GMP. We now demonstrate that photorelaxation decreases on repeated exposure to light. The response can be restored by treating the depleted smooth muscle strips with acidified nitrite, but not nitrite alone, and with the nitric oxide donors, S-nitrosopenicillamine and glyceryl trinitrate, but not with hydralazine. These data indicate that photorelaxation is mediated in part by a "pool" of light-activated vasodilator(s) and suggest that this may act as a store of nitric oxide which could play a role in the regulation of vascular tone.