Chemical nature of nitric oxide storage forms in rat vascular tissue

Haldane, hot dogs, halitosis, and hypoxic vasodilation: the emerging biology of the nitrite anion

While it has long been known that the reduction of nitrite to nitric oxide (NO) forms iron-nitrosyl-myoglobin and is the basis of meat curing, a greater biological activity of the nitrite anion has only recently been appreciated. In the stomach, NO is formed from acidic reduction of nitrite and increases mucous barrier thickness and gastric blood flow (see the related study beginning on page 106). Nitrite levels in blood reflect NO production from endothelial NO synthase enzymes, and recent data suggest that nitrite contributes to blood flow regulation by reaction with deoxygenated hemoglobin and tissue heme proteins to form NO.

Circulating NO pool: assessment of nitrite and nitroso species in blood and tissues

The formation of nitric oxide (NO) has been linked to many regulatory functions in mammalian cells. With the appreciation that NO-mediated nitrosation reactions are involved in cell signaling and pathology there is a need to elucidate and better characterize the different biochemical pathways of NO in vivo. Despite significant methodological advances over the years one major obstacle in assessing the significance of nitrosated species and other NO-related metabolites remains: their reliable measurement in complex biological matrices. In this review we briefly discuss the major routes of NO metabolism and transport in the mammalian circulation, considering plasma, red blood cell, and tissue compartments separately. In addition, we attempt to give a recommendation as to the most appropriate analytical technique and sample processing procedures for the reliable quantification of either species.

Inhibition of arterial contraction by dinitrosyl-iron complexes: critical role of the thiol ligand in determining rate of nitric oxide (NO) release and formation of releasable NO stores by S-nitrosation

The inhibition of arterial tone produced by two nitric oxide (NO) derivatives of biological relevance, dinitrosyl-iron complexes with cysteine (DNIC-CYS) or with glutathione (DNIC-GSH), was compared. Both compounds induced vasorelaxation within the same concentration range (3-300 nM) in endothelium-denuded rat aortic rings. Consistent with a faster rate of NO release from DNIC-CYS than from DNIC-GSH, the relaxant effect of DNIC-CYS was rapid in onset and tended to recover with time, whereas the one of DNIC-GSH developed slowly and was sustained. In addition, DNIC-GSH (0.3 and 1 microM) but not DNIC-CYS (1 microM) induced, even after washout of the drug, a persistent hyporesponsiveness to vasoconstrictors and a relaxant effect of low molecular weight thiols like N-acetylcysteine (NAC, which can displace NO from preformed NO stores). Both effects of DNIC-GSH were associated with elevation of cyclic GMP content and were attenuated by NO scavengers or a cyclic GMP-dependent protein kinases inhibitor. In rings previously exposed to DNIC-GSH, addition of mercuric chloride (which can cleave the cysteine-NO bond of S-nitrosothiols) elicited relaxation, completely blunted the one of NAC and also abolished the persistent elevation of NO content. In conclusion, this study shows that whereas both DNIC-CYS and DNIC-GSH elicited a NO release-associated relaxant effect in isolated arteries, only DNIC-GSH induced an inhibition of contraction which persisted after drug removal. The persistent effect of DNIC-GSH was attributed to the formation of releasable NO stores in arterial tissue, most probably as S-nitrosothiols. Thus, the nature of the thiol ligand plays a critical role in determining the mechanisms and duration of the effect of LMW-DNIC in arteries.

Oxidative modification of low-density lipoprotein: lipid peroxidation by myeloperoxidase in the presence of nitrite

Oxidative modification of low-density lipoprotein (LDL) is a pivotal process in early atherogenesis and can be brought about by myeloperoxidase (MPO), which is capable of reacting with nitrite, a NO metabolite. We studied MPO-mediated formation of conjugated dienes in isolated human LDL in dependence on the concentrations of nitrite and chloride. This reaction was strongly stimulated by low concentrations (5-50 microM) of nitrite which corresponds to the reported concentration in the arterial vessel wall. Under these conditions no protein tyrosine nitration occurred; this reaction required much higher nitrite concentrations (100 microM-1 mM). Chloride neither supported lipid peroxidation alone nor was its presence mandatory for the effect of nitrite. We propose a prominent role of lipid peroxidation for the proatherogenic action of the MPO/nitrite system, whereas peroxynitrite may be competent for protein tyrosine nitration of LDL. Monomeric and oligomeric flavan-3-ols present in cocoa products effectively counteracted, at micromolar concentrations, the MPO/nitrite-mediated lipid peroxidation of LDL. Flavan-3-ols also suppressed protein tyrosine nitration induced by MPO/nitrite or peroxynitrite as well as Cu2+-mediated lipid peroxidation of LDL. This multi-site protection by (-)-epicatechin or other flavan-3-ols against proatherogenic modification of LDL may contribute to the purported beneficial effects of dietary flavan-3-ols for the cardiovascular system.

A liver-specific nitric oxide donor improves the intra-hepatic vascular response to both portal blood flow increase and methoxamine in cirrhotic rats

BACKGROUND/AIMS

A decreased intra-hepatic nitric oxide (NO) production participates on the pathogenesis of portal hypertension in cirrhosis. We tested the hemodynamic effects of a liver-specific NO donor (NCX-1000) derived from ursodeoxycholic acid in portal hypertensive cirrhotic rats.

METHODS

After a 14-day treatment with ursodeoxycholic acid or NCX-1000 by gavage, ascitic cirrhotic rats (CCl4-induced) were used in two studies: (1) in vivo mean arterial pressure (MAP), portal pressure (PP) and superior mesenteric artery (SMA) blood flow measurements before and during progressive blood volume expansion (blood infusion); and (2) in situ liver perfusion to obtain dose/response curves to methoxamine (alpha1-adrenergic agonist) and flow/pressure curves.

RESULTS

Basal heart rate, MAP, and PP were similar in both groups. During blood infusion, similar MAP and SMA flow increases were observed in both groups; however, PP increase observed in control rats was blunted in NCX-1000 treated rats (P=0.015). In liver perfusions, flow/pressure curves were similar in both groups; however, NCX-1000-treated livers showed a lower response to methoxamine (P=0.016). cGMP concentration in NCX-1000-treated livers was higher (P=0.015) than in controls.

CONCLUSIONS

Treatment with a liver-specific NO donor improves the portal system adaptability to portal blood flow increase and ameliorates the intra-hepatic response to methoxamine in cirrhotic rats.

Nitrite reduction to nitric oxide by deoxyhemoglobin vasodilates the human circulation

Nitrite anions comprise the largest vascular storage pool of nitric oxide (NO), provided that physiological mechanisms exist to reduce nitrite to NO. We evaluated the vasodilator properties and mechanisms for bioactivation of nitrite in the human forearm. Nitrite infusions of 36 and 0.36 micromol/min into the forearm brachial artery resulted in supra- and near-physiologic intravascular nitrite concentrations, respectively, and increased forearm blood flow before and during exercise, with or without NO synthase inhibition. Nitrite infusions were associated with rapid formation of erythrocyte iron-nitrosylated hemoglobin and, to a lesser extent, S-nitroso-hemoglobin. NO-modified hemoglobin formation was inversely proportional to oxyhemoglobin saturation. Vasodilation of rat aortic rings and formation of both NO gas and NO-modified hemoglobin resulted from the nitrite reductase activity of deoxyhemoglobin and deoxygenated erythrocytes. This finding links tissue hypoxia, hemoglobin allostery and nitrite bioactivation. These results suggest that nitrite represents a major bioavailable pool of NO, and describe a new physiological function for hemoglobin as a nitrite reductase, potentially contributing to hypoxic vasodilation.

Role of mitochondrial aldehyde dehydrogenase in nitrate tolerance

Glyceryl trinitrate (GTN) is used in the treatment of angina pectoris and cardiac failure, but the rapid onset of GTN tolerance limits its clinical utility. Research suggests that a principal cause of tolerance is inhibition of an enzyme responsible for the production of physiologically active concentrations of NO from GTN. This enzyme has not conclusively been identified. However, the mitochondrial aldehyde dehydrogenase (ALDH2) is inhibited in GTN-tolerant tissues and produces NO2- from GTN, which is proposed to be converted to NO within mitochondria. To investigate the role of this enzyme in GTN tolerance, cumulative GTN concentration-response curves were obtained for both GTN-tolerant and -nontolerant rat aortic rings treated with the ALDH inhibitor cyanamide or the ALDH substrate propionaldehyde. Tolerance to GTN was induced using both in vivo and in vitro protocols. The in vivo protocol resulted in almost complete inhibition of ALDH2 activity and GTN biotransformation in hepatic mitochondria, indicating that long-term GTN exposure results in inactivation of the enzyme. Treatment with cyanamide or propionaldehyde caused a dose-dependent increase in the EC50 value for GTN-induced relaxation of similar magnitude in both tolerant and nontolerant aorta, suggesting that although cyanamide and propionaldehyde inhibit GTN-induced vasodilation, these inhibitors do not affect the enzyme or system involved in tolerance development to GTN. Treatment with cyanamide or propionaldehyde did not significantly inhibit 1,1-diethyl-2-hydroxy-2-nitrosohydrazine-mediated vasodilation in tolerant or nontolerant aorta, indicating that these ALDH inhibitors do not affect the downstream effectors of NO-induced vasodilation. Immunoblot analysis indicated that the majority of vascular ALDH2 is present in the cytoplasm, suggesting that mitochondrial biotransformation of GTN by ALDH2 plays a minor role in the overall vascular biotransformation of GTN by this enzyme.

S-nitrosating nitric oxide donors induce long-lasting inhibition of contraction in isolated arteries

The ability of various nitric oxide (NO) donors to induce long-lasting inhibition of contraction in isolated arteries was compared. All the studied compounds elicited a relaxant effect in rat aortic rings precontracted with norepinephrine (NE). Almost maximal relaxation was obtained with 1 microM of each compound. The S-nitrosating agents S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine, S-nitroso-N-acetylcysteine, and sodium nitroprusside (1 microM) produced a decrease of the maximal effect of NE that persisted after removal of the drug. This hyporesponsiveness to NE was associated with a relaxant effect of N-acetylcysteine, a low-molecular weight thiol that can displace NO from cysteine-NO bonds. Such modifications of contraction were not observed in aortic rings previously exposed to 1 microM S-nitrosocysteine, glyceryl trinitrate, 3-morpholinosydnonimine, or 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA-NO). The same differential effects of GSNO and DEA-NO on contraction were also observed in porcine coronary arteries. Rat aortic rings previously exposed to 100 microM GSNO, but not to 100 microM DEA-NO, displayed a persistent increase in NO content (determined by NO spin trapping) and cysteine-NO residues (determined by immunostaining with an anti-cysteine-NO antiserum). The GSNO-induced increase in cysteine-NO residues in aortic tissue was prevented by the thiolmodifying agent p-hydroxymercuribenzoic acid. This study shows that in isolated arteries, the effects of S-nitrosating agents differed from those of other NO-donating agents. S- Nitrosating agents induced a persistent inhibition of contraction, which was attributed to the formation of releasable NO stores by S-nitrosation of tissue thiols. These differential effects of NO donors may be important for orientating their therapeutic indications.

Nitric oxide and the mechanism of rat vascular smooth muscle photorelaxation

Photorelaxation of vascular smooth muscle (VSM) was studied using segments of tail artery from normotensive rats (NTR) or spontaneously hypertensive rats (SHR). Isolated vessels with intact endothelium were perfused with Krebs solution containing phenylephrine. Perfusion pressures were recorded while arteries were irradiated with either visible (VIS; lambda=514.5 nm) or long wavelength ultra-violet (UVA; lambda=366 nm) light. VIS light produced a transient vasodilator response: a rapid decrease of pressure that recovered fully during the period (6 min) of illumination. An irradiated artery was refractory to a second period of illumination delivered immediately after the first, but its photosensitivity recovered slowly in the dark, a process called 'repriming'. Photorelaxations generated by UVA light were qualitatively different and consisted of two components: a phasic (or p-) component superimposed on a sustained (or s-) component. The p-component is similar to the VIS light-induced response in that both exhibit refractoriness and repriming depends upon endothelium-derived NO. In contrast, the s-component persists throughout the period of illumination and does not show refractoriness. We conclude that VIS light-induced photorelaxations and the p-component of UVA light-induced responses are mediated by the photochemical release of NO from a finite molecular 'store' that can be reconstituted afterwards in the dark. The s-component of the UVA light-induced response does not depend directly on endothelial NO and may result instead from a stimulatory effect of UVA light on soluble guanylate cyclase. NO-dependent photorelaxation is impaired in vessels from SHR while the s-component is enhanced.