Sodium nitrite causes relaxation of the isolated rat aorta: By stimulating both endothelial NO synthase and activating soluble guanylyl cyclase in vascular smooth muscle

Ingestion of dietary nitrites lowers arterial blood pressure in experimental animals and in humans. However, the exact mechanism underlying the hypotensive effect of nitrite remains unclear. The present study compared nitrite-induced responses in rings (with or without endothelium) of aortae of 18-20weeks old Wistar-Kyoto Rats (WKY) and spontaneously hypertensive (SHR) rats and investigated the underlying mechanism. Relaxations of aortae from WKY and SHR to increasing concentrations (1nM-100μM) of sodium nitrite (NaNO2) were determined during sustained contractions to phenylephrine, in the absence and presence of pharmacological agents. The nitrite-induced relaxations were concentration-dependent and larger in SHR than in WKY aortic rings. Inhibition of endothelial nitric oxide synthase (eNOS) and the absence of endothelium decreased nitrite-induced relaxations in both WKY and SHR aortae, indicating the role of endothelium-derived nitric oxide (NO) in the response. The involvement of eNOS was further confirmed by increases in phosphorylation of eNOS at ser1177 in HUVEC cells following treatment with sodium nitrite. The presence of NO scavengers decreased the relaxation to nitrite in both WKY and SHR preparations while inhibition of soluble guanylyl cyclase (sGC) abolished the response, indicating that besides producing NO, nitrite also induces relaxation by directly activating the enzyme. Thus, the present study demonstrates that the sensitivity to exogenous nitrite is increased in the aorta of the SHR compared to that of the WKY. The endothelium-dependent component of the relaxation to nitrite involves activation of eNOS with production of endothelium-derived NO, while the endothelium-independent component is due to stimulation of sGC.