Vasoactivity of nitrite in the iliac artery of the toad, Rhinella marina

Nitrite (NO₂^-) causes vasodilation in mammals due to the formation of (nitric oxide) NO by endogenous NO₂^- reduction in the vascular wall. In this study, we determined if a similar mechanism operates in amphibians. Dual-wire myography of the iliac artery from Rhinella marina showed that applied NO₂^- caused a concentration-dependent vasodilation in normoxia (21% O₂; EC₅₀ 438 µM). Hypoxia (0.63% O₂) significantly increased the maximal dilation to NO₂^- by 5 % (P = 0.0398). The addition of oxyhemoglobin significantly increased the EC₅₀ (P = 0.0144; EC₅₀ 2236 µM), but did not affect the maximal vasodilation. In contrast, partially deoxygenated hemoglobin (90% desaturation) did not affect the EC₅₀ (P = 0.1189) but significantly (P = 0.0012) increased the maximal dilation to NO₂^- by 11%. The soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) completely abolished the response to NO₂- (P < 0.0001), and of the nitric oxide synthase inhibitors, only vinyl-L-NIO (P = 0.0028) significantly reduced the NO₂^- vasodilation. The xanthine oxidoreductase inhibitor, allopurinol (P = 0.927), the NO-scavenger C-PTIO (P = 0.478), and disruption of the endothelium (P = 0.094) did not affect the NO₂^- vasodilation. Incubation of iliac arteries with 1 mM NO₂^- did not a cause a change in cGMP concentration (P = 0.407). Plasma NO₂^- was found to be 0.86 ± 0.20 µmol.L^-1, while nitrate (NO₃^-) was 19.55 ± 2.55 µmol.L^-1. Both cygb and ngb mRNAs were expressed in the iliac artery and it is possible that these globins facilitate NO₂^- reduction in hypoxia. In addition, NO₂^- intracellular disproportionation processes could be important in the generation of NO from NO₂^-.

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