Effect of in vivo nitrate tolerance on hypersensitivity to NO donors after NO-synthase blockade

Role of the lipid peroxidation product, 4-hydroxynonenal, in the development of nitrate tolerance

Tolerance to nitrates such as nitroglycerin (GTN) is associated with oxidative stress, inactivation of aldehyde dehydrogenase 2 (ALDH2), and decreased GTN-induced cGMP accumulation and vasodilation. We hypothesized that GTN-induced inactivation of ALDH2 results in increased 4-hydroxy-2-nonenal (HNE) adduct formation of key proteins involved in GTN bioactivation, and, consequently, an attenuated vasodilator response to GTN (i.e., tolerance). We used an in vivo GTN tolerance model, a cell culture model of nitrate action, and Aldh2(-/-) mice to assess whether GTN exposure resulted in HNE adduct formation, and whether exogenous HNE affected GTN-induced relaxation and cGMP accumulation. Immunoblot analysis indicated a marked increase in HNE adduct formation in GTN-tolerant porcine kidney epithelial cells (PK1) and in aortae from GTN-tolerant rats and untreated Aldh2(-/-) mice. Preincubation of PK1 cells with HNE resulted in a dose-dependent decrease in GTN-induced cGMP accumulation, and pretreatment of isolated rat aorta with HNE resulted in dose-dependent decreases in the vasodilator response to GTN, thus mimicking GTN-tolerance. Pretreatment of aortae from Aldh2(-/-) mice with 10 μM HNE resulted in a desensitized vasodilator response to GTN. In the in vivo rat tolerance model, changes in HNE adduct formation correlated well with the onset of GTN tolerance and tolerance reversal. Furthermore, coadministration of an HNE scavenger during the tolerance induction protocol completely prevented HNE adduct formation and GTN tolerance but did not prevent the inactivation of ALDH2. The data are consistent with a novel mechanism of GTN tolerance suggesting a primary role of HNE adduct formation in the development of GTN tolerance.

Changes in aldehyde dehydrogenase 2 expression in rat blood vessels during glyceryl trinitrate tolerance development and reversal

BACKGROUND AND PURPOSE

Recent studies have suggested an essential role for aldehyde dehydrogenase 2 (ALDH2) in the bioactivation of organic nitrates such as glyceryl trinitrate (GTN). In the present study, we utilized an in vivo GTN tolerance model to further investigate the role of ALDH2 in GTN bioactivation and tolerance.

EXPERIMENTAL APPROACH

We assessed changes in aortic ALDH activity, and in ALDH2 protein expression in various rat blood vessels (aorta, vena cava, femoral artery and femoral vein) during continuous GTN exposure (0.4 mg·h⁻¹ for 6, 12, 24 or 48 h) or after a 1-, 3- or 5-day drug-free period following a 48 h exposure to GTN, in relation to changes in vasodilator responses to GTN and in vascular GTN biotransformation.

KEY RESULTS

A decrease was observed in both ALDH2 protein expression (80% in tolerant veins and 30% in tolerant arteries after 48 h exposure to GTN) and aortic ALDH activity, concomitant with decreased vasodilator responses to GTN and decreased aortic GTN biotransformation. However, after a 24 h drug-free period following 48 h of GTN exposure, vasodilator responses to GTN and aortic GTN biotransformation activity had returned to control values, whereas vascular ALDH2 expression and aortic ALDH activity were still significantly depressed, and remained so for 3-5 days following cessation of GTN exposure.

CONCLUSIONS AND IMPLICATIONS

The dissociation of reduced ALDH activity and ALDH2 expression from the duration of the impaired vasodilator and biotransformation responses to GTN in nitrate-tolerant blood vessels, suggests that factors other than changes in ALDH2-mediated GTN bioactivation contribute to nitrate tolerance.

Impaired nitroglycerin biotransformation in patients with chronic heart failure

OBJECTIVE

Patients with chronic heart failure (CHF) often require higher doses of nitroglycerin (glyceryl trinitrate, GTN) than patients with normal cardiac function to achieve a given haemodynamic goal. Two pathways leading to biotransformation of GTN have been characterized; a high-affinity pathway operative in nanomolar concentration ranges yielding predominantly 1,2-glyceryl dinitrate (1,2-GDN), and a low-affinity pathway operative at higher, micromolar concentrations of GTN associated with a greater proportion of 1,3-GDN formation. We tested the hypothesis that, at a given GTN-induced blood pressure reduction, the CHF group would present with: (i) higher concentrations of GTN; and (ii) decreased ratios of 1,2-GDN/GTN and 1,2-GDN/1,3-GDN compared with healthy subjects (HS).

METHODS

Twelve patients with CHF (left ventricular ejection fraction 20 +/- 5%, NYHA III) and nine HS were investigated during a right cardiac catheterization. GTN was titrated intravenously until mean arterial blood pressure (MAP) was reduced by 15%.

RESULTS

At arterial GTN concentrations of 27.2 [10.0-57.8] nmol l(-1) in CHF and 2.8 [2.5-3.5] nmol l(-1) in HS [median (quartile range), P<0.05 between groups], MAP and mean capillary wedge pressures were reduced similarly in both groups (approx. 15% and 65%, respectively, P = NS between groups). The ratios of 1,2-GDN/GTN and 1,2-GDN/1,3-GDN were lower in CHF (0.86 [0.28-1.58] and 5.8 [5.6-6.3]) compared with HS [1.91 (1.54-2.23) and 7.6 (7.2-10.2), P<0.05], with a negative correlation between the 1,2-GDN/1,3-GDN ratio and the arterial GTN concentrations in the CHF patients (R = -0.8, P<0.05).

CONCLUSION

Patients with CHF have attenuated GTN responsiveness and decreased relative formation of 1,2-GDN in comparison with HS, indicating an altered biotransformation of GTN.

Biotransformation of glyceryl trinitrate by rat hepatic microsomal glutathione S-transferase 1

Although the biotransformation of organic nitrates by the cytosolic glutathione S-transferases (GSTs) is well known, the relative contribution of the microsomal GST (MGST1) to nitrate biotransformation has not been described. We therefore compared the denitration of glyceryl trinitrate (GTN) by purified rat liver MGST1 and cytosolic GSTs. Both MGST1 and cytosolic GSTs catalyzed the denitration of GTN, but the activity of MGST1 toward GTN was 2- to 3-fold higher. To mimic oxidative/nitrosative stress in vitro, we treated enzyme preparations with hydrogen peroxide, S-nitrosoglutathione, and peroxynitrite. Both oxidants and nitrating reagents increased the activity of MGST1 toward the GST substrate, 1-chloro-2,4-dinitrobenzene (CDNB) whereas these treatments inhibited GTN denitration by MGST1. Alkylation of the sole cysteine residue of MGST1 by N-ethylmaleimide markedly increased enzyme activity with CDNB as substrate but decreased the rate of GTN denitration. In aortic microsomes from GTN-tolerant animals, there was a decreased abundance of MGST1 dimers and trimers. In hepatic microsomes from GTN-tolerant animals, GTN biotransformation was unaltered whereas the rate of CDNB conjugation was doubled, suggesting that chronic GTN exposure causes structural modifications to the enzyme, resulting in increased activity to certain substrates. Collectively, these data indicate that MGST1 contributes significantly to the biotransformation of GTN and that chemical modification of the microsomal enzyme has differential effects on the catalytic activity toward different substrates.

The effect of intermittent administration of sustained release isosorbide dinitrate (sr-ISDN) in rats with volume overload heart

Recently, it has been reported that intermittent administration of nitrate, with a nitrate-free interval of 10 to 12 hr eliminated expression of tolerance, and maintained its hypotensive effect. In the present study, we evaluated whether nitrate tolerance developed or not with an intermittent administration of sr-ISDN (5 mg/kg/ once a day) in Wistar rats. The effect of this administration protocol for sr-ISDN on the volume overload heart model, aortovenous fistula, was also examined. Furthermore, blood pressure was monitored by radio telemetry during sr-ISDN (5 mg/kg/once a day) administration. Nitrate tolerance did not develop, and eccentric hypertrophy due to volume overload was moderated by sr-ISDN administration. Sr-ISDN administration maintained blood pressure lower level than the placebo group. In conclusion, prolonged intermittent administration of sr-ISDN maintained its hypotensive effect during the entire experiment period, without developing tolerance, and moderated efferent hypertrophy with attenuated volume overload.

Nitrates and NO release: contemporary aspects in biological and medicinal chemistry

Nitroglycerine has been used clinically in the treatment of angina for 130 years, yet important details on the mechanism of action, biotransformation, and the associated phenomenon of nitrate tolerance remain unanswered. The biological activity of organic nitrates can be said to be nitric oxide mimetic, leading to recent, exciting progress in realizing the therapeutic potential of nitrates. Unequivocally, nitroglycerine and most other organic nitrates, including NO-NSAIDs, do not behave as NO donors in the most fundamental action: in vitro activation of sGC to produce cGMP. The question as to whether the biological activity of nitrates results primarily or exclusively from NO donation will not be satisfactorily answered until the location, the apparatus, and the mechanism of reduction of nitrates to NO are defined. Similarly, the therapeutic potential of nitrates will not be unlocked until this knowledge is attained. Aspects of the therapeutic and biological activity of nitrates are reviewed in the context of the chemistry of nitrates and the elusive efficient 3e- reduction required to generate NO.

Effect of regular physical training on cutaneous microvascular reactivity

PURPOSE

To investigate whether regular intense physical training induces changes in microvascular reactivity of human glabrous and nonglabrous skin.

METHODS

Subjects were physically trained competitive cyclists (N = 19) and age-matched sedentary controls (N = 20). We measured cutaneous microvascular blood flow on the dorsum of the hand (nonglabrous skin) and on the finger pulp (glabrous skin) using the laser-Doppler (LD) method. Endothelium-dependent vasodilation was assessed by an iontophoretic application of acetylcholine (ACh) on the dorsum of the hand and by an induction of postocclusive reactive hyperemia (PRH) on the finger pulp. Endothelium-independent vasodilation was assessed on the dorsum of the hand by iontophoretically applied sodium-nitroprusside (SNP).

RESULTS

The ACh-evoked increase in LD flux (LDF) was significantly greater in the group of cyclists (7.5-fold +/- 0.5 increase) as compared with controls (5.9-fold +/- 0.5). We found no differences in the peak LDF during PRH, whereas the recovery time of PRH was significantly longer (241.5 +/- 21.6 s in trained vs 154.6 +/- 9.3 s in sedentary group) and the area under the PRH curve significantly larger in the group of trained subjects (19,066 +/- 2,653 PU x s in trained vs 12,168 +/- 864 PU x s in sedentary). In contrast, we found a significantly smaller response to SNP in the group of cyclists (6.2-fold +/- 0.5 increase) as compared with sedentary subjects (7.8-fold +/- 0.5 increase).

CONCLUSION

The results of our study point to a greater vasodilator capacity of endothelium in glabrous as well as in nonglabrous skin in the group of physically trained subjects. In addition, our results indicate that regular physical activity also modifies the reactivity of vascular smooth muscle cells.

Influence of the endothelium on ex vivo tolerance and metabolism of glyceryl trinitrate in rat aorta

The influence of the endothelium on glyceryl trinitrate metabolism and relaxation and the relationship to tolerance induced by transdermal glyceryl trinitrate was explored in rat aorta. Metabolism was assessed in artery segments incubated with glyceryl trinitrate (1.0 microM) for 2 min and the contents of 1,2- and 1,3-glyceryl dinitrate measured by gas chromatography. In non-tolerant arteries mean contents of glyceryl trinitrate, 1,2-glyceryl dinitrate and 1,3-glyceryl dinitrate were 3.2, 0.23 and 0.10 nmol/g, respectively; in tolerant arteries the content of 1,2-glyceryl dinitrate was reduced by approximately 60%. Endothelium removal or nitric oxide synthase (NOS) inhibition did not affect metabolite contents but increased the relaxant response to glyceryl trinitrate in the tolerant artery to an extent that tolerance was significantly attenuated. It is concluded that (i) tolerance is associated with depression of glyceryl trinitrate metabolism by an endothelium-independent mechanism and (ii) the endothelium contributes to tolerance by a mechanism which is independent of metabolism and may be linked with endothelial NOS.

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.