Abstract
Abstract: :
Purpose: Nitric oxide (NO) is generally accepted to be the final product of the nitric oxide synthase (NOS) activity. However, recent evidences suggest that under certain conditions, NOS produces nitroxyl (HNO) rather than NO, that it may be converted to its oxidized form by endogenous oxidants. In this context, the aim of the present work was to examine comparatively the effect of NO and HNO on the hamster retinal nitridergic pathway. Methods: Angeli’s salt (AS) and sodium nitroprusside (SNP) were used as nitroxyl and NO donors, respectively. NOS activity was assessed through the conversion of 3H–L–arginine to 3H–L–citrulline, whereas 3H–L–arginine uptake was measured in a crude synaptosomal fraction. Retinal levels of cGMP were measured by radioimmunoassay. Reduced glutathione levels were assessed by a spectrophotometric assay, and retinal lipid peroxidation was determined by the quantification of the thiobarbituric acid reactive substances (TBARS) levels. Results: AS and SNP significantly inhibited hamster retinal NOS activity, and a non additive effect was observed when both compounds were added together. Only AS inhibited L–arginine uptake. AS and SNP significantly increased cGMP accumulation. In addition, reduced glutathione levels were significantly diminished in the presence of both donors, whereas only SNP increased retinal lipid peroxidation. Conclusions: The differential effect of HNO and NO on several parameters suggests that, at least in the hamster retina, the conversion of the former to its oxidized form (if it occurs) is not complete. Furthermore, the present results indicate that both compounds inhibit its own synthesis by decreasing NOS activity and in the case of HNO also by reducing L–arginine availability. Since cGMP is one of the most relevant molecules in the phototransduction mechanism, HNO and NO could participate in the processing of retinal light information.
Keywords: nitric oxide • second messengers: pharmacology/physiology • retina