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Despite impressive medical advances1 that have led to diminished cardiovascular death rates in some countries over the past 20 years, cardiovascular disease remains the leading cause of death in developed countries such as the United States. This promises to worsen as a result of aging populations; the incipient obesity and type II diabetes epidemic; sedentary lifestyle; and continued abuse of tobacco, alcohol, and other substances. Cardiovascular disease is clearly multifactorial, and the approach to its prevention necessarily likewise. Candidates for prevention include cyclic guanosine 3′,5′-cyclic monophosphate (cGMP)–dependent signaling networks initiated by natriuretic peptides (NPs) and nitric oxide (NO), which demonstrate characteristics deemed worthy of diagnostic and therapeutic exploitation. cGMP signaling contributes to the function and interaction of several vascular cell types, and its dysfunction could be involved in major destructive processes such as atherosclerosis, hypertension, diabetic complications, (re)stenosis, and tissue infarction, as well as the undermining of clinical therapy like in the case of nitrate tolerance. This review takes a focused look at key elements of the cGMP signaling cascade in vascular tissue, particularly recent advances in our knowledge of cGMP-dependent protein kinase (cGK, also known as PKG) function. Finally, we discuss the potential of clinical monitoring of cGK activity for assessing the functional status of cGMP signaling and for guiding the design of therapeutic strategies to improve vascular function. One of the 2 major synthetic pathways for cGMP generation from guanosine 5′-triphosphate (GTP) is directed by NPs (Figure 1), consisting of atrial (ANP), B-type (BNP), and C-type (CNP) natriuretic peptides, which act via the membrane receptor guanylate cyclases GC-A (highest affinity for ANP, BNP) and GC-B (highest affinity for CNP). ANP and BNP, released from the heart by mechanical stretch in response to increased atrial pressure/volume, and CNP, released from the endothelium, can all cause smooth muscle (SM) …

BACKGROUND L-Methionine potentiates systemic hemodynamic effects of intravenous glyceryl trinitrate (GTN) in tolerant and nontolerant patients to a similar extent as N-acetylcysteine (NAC). This potentiation of GTN action by L-methionine has been attributed to enhanced intracellular formation of nitrosothiols, known to be potent stimulators of soluble guanylyl cyclase. This study was performed to analyze directly the effects of L-methionine on GTN-induced dilation of large epicardial arteries and the venous capacitance system of the dog in the tolerant and nontolerant states. Cultured rat aortic vascular smooth muscle cells and purified guanylyl cyclase were used to study potential intracellular and extracellular mechanisms responsible for this interaction. METHODS AND RESULTS In awake nontolerant dogs, L-methionine (100 mg/kg) potentiated the tachycardic response to GTN (5.0 and 15 micrograms/kg/min) and enhanced the hypotensive action of GTN (1.5 and 5.0 micrograms/kg/min) in anesthetized, nonreflexic dogs. In nontolerant and tolerant dogs, however, L-methionine did not alter the dose-response of large epicardial artery dilation to intravenous GTN challenges and did not modify nitrate tolerance of the low pressure system of the dog. The infusion of L-methionine (100 mg/kg) significantly increased plasma methionine levels (from 52 +/- 12 to 1,141 +/- 239 microM), cystine levels (from 12 +/- 4 to 26 +/- 7 microM), but not homocystine levels. In vitro, the L-methionine conversion product L-cysteine (0.1-1.0 mM) but not homocysteine significantly enhanced the augmentation of purified guanylyl cyclase activity by GTN (100 microM). Incubation of cultured rat aortic smooth muscle cells with L-methionine (10 microM or 1 mM) did not result in a significant increase of free intracellular sulfhydryl group content. CONCLUSIONS The L-methionine conversion product L-cysteine mediates tolerance independent the potentiation of GTN action. This may result from an L-cysteine-induced formation of a vasoactive metabolite of GTN (nitric oxide) or nitrosothiol. This effect occurs primarily in the resistance vessel circulation, not in large epicardial arteries and veins. The lack of effect of L-methionine on sulfhydryl group content in large conductance vessels indicates that hepatic L-methionine metabolism constitutes the significant source of L-cysteine. These findings strongly suggest that administration of sulfhydryl-group precursor L-methionine does not represent a therapeutic alternative to a nitrate-free interval to restore nitrate sensitivity in tolerant large epicardial arteries and veins.

Nitroglycerin (NTG) has been the foremost anti-ischemic agent used in clinical medicine for more than a century. NTG is a potent vasodilator of veins, arterial conductance vessels, and collaterals that has minimal effects on arteriolar tone. At the cellular level, NTG is biotransformed by a still unknown enzymatic process in endothelial cells, smooth muscle, and to some extent platelets, causing it to release the vasodilator and anti-aggregatory principle nitric oxide. The 2 major drawbacks of nitrate therapy that have been shown to be important are the rapid development of nitrate tolerance1 and endothelial dysfunction2,3⇓ within several days of prolonged NTG treatment. There is a growing body of evidence that both NTG-induced side effects may be at least in part secondary to NTG-stimulated production of oxygen-derived free radicals within vascular tissue.4–6⇓⇓ Increased oxidative stress correlates positively with increased cardiovascular event rates,7 which may, at least in part, explain why NTG-therapy either failed to improve or worsened the prognosis in patients with acute myocardial infarction and chronic ischemic heart disease, respectively. See p 208 For years, investigators have been debating whether NTG has significant antiplatelet activity. Several studies demonstrated that inhibition of platelet aggregation by NTG occurs in suprapharmacological dosis, denying any clinically relevant antiplatelet activity. Whether or not NTG inhibits platelet aggregation clearly depends on the chosen experimental conditions. For example, in washed platelets, the lowest effective concentrations of NTG that inhibit platelet aggregation are consistently in the μmol/L range and are therefore in concentrations that are 1000 times higher than those achieved with in vivo NTG therapy (nmol/L range), a finding which has raised doubt about the physiological significance of platelet inhibition by NTG. As mentioned above, NTG has to be biotransformed in order to release NO. The weak antiplatelet activity of NTG, …

Background —Chronic in vivo treatment with nitroglycerin (NTG) induces tolerance to nitrates and cross-tolerance to nitrovasodilators and endothelium-derived nitric oxide (NO). We previously identified increased vascular superoxide formation and reduced NO bioavailability as one causal mechanism. It is still controversial whether intracellular downstream signaling to nitrovasodilator-derived NO is affected as well. Methods and Results —We therefore studied the effects of 3-day NTG treatment of rats and rabbits on activity and expression of the immediate NO target soluble guanylyl cyclase (sGC) and on the cGMP-activated protein kinase I (cGK-I). Tolerance was induced either by chronic NTG infusion via osmotic minipumps (rats) or by NTG patches (rabbits). Western blot analysis, semiquantitative reverse transcription–polymerase chain reaction, and Northern blot analysis revealed significant and comparable increases in the expression of sGC α 1 and β 1 subunit protein and mRNA. Studies with the oxidative fluorescent dye hydroethidine revealed an increase in superoxide in the endothelium and smooth muscle. Stimulation with NADH increased superoxide signals in both layers. Although cGK-I expression in response to low-dose NTG was not changed, a strong reduction in vasodilator-stimulated phosphoprotein (VASP) serine239 phosphorylation (specific substrate of cGK-I) was observed in tolerant tissue from rats and rabbits. Concomitant in vivo and in vitro treatment with vitamin C improved tolerance, reduced oxidative stress, and improved P-VASP. Conclusions —We therefore conclude that increased expression of sGC in the setting of tolerance reflects a chronic inhibition rather than an induction of the sGC–cGK-I pathway and may be mediated at least in part by increased vascular superoxide.

Background In vitro data suggest that reduced bioconversion of nitroglycerin (NTG) to nitric oxide (NO) contributes to the development of vascular and hemodynamic tolerance to NTG. We examined the in vivo validity of this hypothesis by measuring NTG-derived NO formation by in vivo spin-trapping of NO in vascular tissues from nitrate-tolerant and -nontolerant rats. Methods and Results Five groups (n=6 to 8 each) of conscious chronically catheterized rats received NTG (0.2 or 1 mg/h IV) for 72 hours (nitrate-tolerant groups). Four other groups received either NTG vehicle (placebo, for 72 hours) or were left untreated (control). Nitrate tolerance was substantiated by a reduced (55% to 85%) hypotensive response to NTG in vivo and a reduced relaxation to NTG in isolated aortic rings. NTG-derived NO formation in aorta, vena cava, heart, and liver was measured as NOFe(DETC) 2 and NO-heme complexes formed in vivo during 35 minutes combined with ex vivo cryogenic electron spin resonance spectroscopy. NO formation was significantly ( P Conclusions The results suggest that vascular and hemodynamic NTG tolerance occurs despite high and similar rates of NO formation by NTG in tolerant and nontolerant target tissues. This finding is compatible with the assumption that reduced biological activity of NO, rather than reduced bioconversion of NTG to NO, contributes to in vivo development of nitrate tolerance.

Background The objectives of this study were (1) to assess glyceryl trinitrate (GTN)–derived nitric oxide (NO) formation in vascular tissues and organs of anesthetized rabbits in vivo, (2) to establish a correlation between tissue NO levels and a biological response, and (3) to verify biotransformation of GTN to NO by cytochrome P-450. Methods and Results NO was trapped in tissues in vivo as a stable paramagnetic mononitrosyl-iron-diethyldithiocarbamate complex [NOFe(DETC) 2 ]. After removal of the tissues, NO was determined by cryogenic electron spin resonance spectroscopy. NO formation in vitro was assessed by spin trapping and by activation of soluble guanylyl cyclase. The GTN-elicited decrease in coronary perfusion pressure was monitored in isolated, constant-flow perfused rabbit hearts. NO was not detected in control tissues. In GTN-treated rabbits, NO formation was higher in organs than in vascular tissues and higher in venous than in arterial vessels. In isolated hearts, ventricular NO levels and decreases in coronary perfusion pressure achieved by GTN were closely correlated. Purified cytochrome P-450 catalyzed NO formation from GTN in a P-450–NADPH reductase– and NADPH–dependent fashion. Conclusions Since GTN-derived NO formation in myocardial tissue correlates to the GTN-elicited vasodilator response, we conclude that GTN-derived NO detected in vivo correlates with the systemic effects of GTN. Therefore, the higher rate of NO formation detected in veins compared with arteries explains the preferential venodilator activity of GTN. High NO formation in cytochrome P-450–rich organs in vivo and efficient NO formation from GTN by cytochrome P-450 in vitro highlights the importance of this pathway for NO formation from GTN in the intact organism.

N-acetylcysteine is assumed to reverse nitrate tolerance by replenishing depleted intracellular sulfhydryl groups, but data on interactions of N-acetylcysteine and nitrates in patients with stable angina are controversial and disappointing. Therefore, we studied the effect of N-acetylcysteine on nitrate responsiveness of epicardial arteries and of the venous system (assessed as changes in effective vascular compliance) in dogs (n = 12) during long-term nitroglycerin treatment (1.5 micrograms/kg/min i.v. for 5-6 days). In dogs with nitroglycerin-specific tolerance (shift of venous or epicardial artery dilation to 15-17-fold higher dosages), N-acetylcysteine (100 mg/kg i.v.) had no dilator effect and did not alter the dose-response relations of nitroglycerin. Yet, in nontolerant dogs (n = 17), N-acetylcysteine augmented (1.5-2.0-fold) the dilation of epicardial arteries and the reduction of peripheral vascular resistance induced by 0.5-1.5 micrograms/kg/min nitroglycerin. In vitro, the augmentation of purified guanylate cyclase activity by nitroglycerin (10-100 microM) was potentiated by N-acetylcysteine (0.01-1.0 mM) in saline or in canine plasma, but N-acetylcysteine alone was ineffective. We conclude that 1) N-acetylcysteine does not restore nitroglycerin responsiveness in tolerant epicardial arteries or veins in vivo, 2) a small, tolerance-independent augmentation of nitroglycerin-induced dilation may result from N-acetylcysteine-induced extracellular formation of a stimulant of guanylate cyclase from nitroglycerin.