Does Nitric Oxide Mediate the Vasodilator Activity of Nitroglycerin?

Abstract

Abstract-Nitroglycerin (glyceryl trinitrate, GTN) relaxes blood vessels primarily via activation of the soluble guanylyl cyclase (sGC)/cGMP/cGMP-dependent protein kinase (cGK-I) pathway. Although the precise mechanism of sGC activation by GTN in the vascular wall is unknown, the mediatory role of nitric oxide (NO) has been postulated. We tested the GTN/NO hypothesis in different types of isolated rat and rabbit blood vessels using two novel approaches:(1) EPR spin trapping using colloid Fe(DETC) 2 and (2) analysis of cGK-I-dependent phosphorylation of the vasodilator-stimulated phosphoprotein at Ser239 (P-VASP). For comparison, another organic nitrate, isosorbide dinitrate (ISDN), and endothelium-dependent vasodilator, calcium ionophore A23187, were tested. We found a marked discrepancy between GTN's strong vasoactivity (vasodilation and augmentation of P-VASP) and its poor NO donor properties. In aortas precontracted with phenylephrine, GTN, ISDN, and A23187 induced nearly full relaxations (Ͼ80%) and doubling of vascular P-VASP content at concentrations of 100 nmol/L, 100 mol/L, and 1 mol/L, respectively. GTN applied in vasorelaxant concentrations (10 to 1000 nmol/L) did not significantly increase the basal vascular NO production, in contrast to ISDN and A23187. The absence of GTN-derived NO was confirmed in rabbit vena cava and renal artery. A significant increase in vascular NO formation was observed only at suprapharmacological GTN concentrations (Ͼ10 mol/L). The concentration dependency of NO formation from GTN was comparable to that of ISDN, although the latter exhibits 100-folds lower vasorelaxant potency. We conclude that GTN activates the sGC/cGMP/cGK-I pathway and induces vasorelaxation without intermediacy of the free radical NO. 2 It is believed that the beneficial therapeutic effect of GTN is due to selective vasodilation of coronary arteries and venous capacitance vessels with minimal effect on arteriolar tone. 3 Several evidences indicate that the principal mechanism of GTN-induced smooth muscle relaxation is the activation of the intracellular enzyme, soluble guanylyl cyclase (sGC), and subsequent elevation of the cyclic guanosine-3Ј,-5Ј-monophosphate (cGMP) levels 4 -12 (see reviews 13,14 ). Among other effects, the elevated cGMP level leads to activation of cGMP-dependent protein kinase (cGK-I), which in turn mediates vasorelaxation via phosphorylation of several proteins regulating intracellular Ca 2ϩ mobilization. 15 However, the precise mechanism by which GTN activates vascular sGC is still controversially discussed. 16 -19 According to the hypothesis presently favored, GTN undergoes intracellular bioconversion into the putative metabolite nitric oxide (NO), which is known as a direct activator of sGC and as a smooth muscle relaxant. 5,8,20 -23 After the discovery of endothelium-derived NO, 24 -26 the idea of NO being the active principle of GTN has been widely accepted by the scientific community, and it has been often speculated that GTN can replace the compromised endothelial NO produ...

Evaluation of bioavailability of nitric oxide in coronary circulation by direct measurement of plasma nitric oxide concentration

“…that is characterized as the NO donor. Recently, different concentration-vasodilatory responses were reported for ISDN and GTN (58). GTN may apparently exert its vasodilatory activity independently of its NO-releasing properties, whereas ISDN exerts its activity by NO donation.…”

Effects of the nitric oxide donor, sodium nitroprusside, on resting leg glucose uptake in patients with type 2 diabetes

“…Many patients treated with nitrates frequently receive concomitant therapy with drugs such as beta-blockers or diuretics, which may adversely affect glucose metabolism and thus may mask any effect of nitrates on glucose uptake. Furthermore, there has been some controversy as to whether many organic nitrates are, in fact, true NO donors [36].…”

Vascular Bioactivation of Nitroglycerin Is Catalyzed by Cytosolic Aldehyde Dehydrogenase-2

“…40 This seemingly straightforward model is questioned by the failure of at least three independent laboratories to detect GTNderived NO in blood vessels. 11,41,42 Formation of S-nitrosoglutathione rather than NO has been proposed as possible explanation, 6 but activation of sGC by S-nitrosoglutathione requires release of NO, 43,44 leaving the problem unresolved. Although the negative findings might reflect a true lack of NO formation, it is conceivable that cytosolic ALDH2 interacts in a locally constrained manner with sGC, resulting in significant cGMP accumulation by low concentrations of free NO that are below the detection limit of C-labeled GTN as described in Methods, followed by quantification of reaction products by radio thin layer chromatography.…”