Localization and thiol dependancy of endogenous nitro compounds-mediating urethral photo-relaxation

Using a nitric oxide (NO)-specific fluorescent probe, we have examined the location of NO generation in the urethra from sheep and rat when induced by either electrical field- or light-stimulation (EFS and LS, respectively). In addition, we studied the effect of specific glutathione (GSH) modifiers, acting upon different cellular GSH pools, on NO release and on urethral relaxation. Both EFS and LS led to fluorescence emission from a fiber network associated with neuronal NO synthase (nNOS) immunoreactive nerves. Both the relaxation and the fluorescence elicited by EFS were blocked by specific nNOS inhibitors, but these parameters were not significantly modified by endogenous GSH depletion. In contrast, the opposite was found for LS-induced responses. Moreover, when the mitochondrial pool was effectively reduced by incubation with ethacrynic acid, the responses to LS were further reduced until they disappeared after intensive LS. Our results confirm that while NO is released by nNOS activation, the photolytic breakdown of an endogenous nitro-compound, probably S-nitroso-glutathione, in nitrergic nerves (and in the vascular endothelium) is the only factor responsible for photo-relaxation. The possible role of this mechanism in NO inactivation and as a protective mechanism in NO-generating structures is further discussed.