Platelet cyclic GMP. A potentially useful indicator to evaluate the effects of nitroglycerin and nitrate tolerance

Metabolomics-Driven Elucidation of Cellular Nitrate Tolerance Reveals Ascorbic Acid Prevents Nitroglycerin-Induced Inactivation of Xanthine Oxidase

Glyceryl trinitrate (GTN) has found widespread use for the treatment of angina pectoris, a pathological condition manifested by chest pain resulting from insufficient blood supply to the heart. Metabolic conversion of GTN, a nitric oxide (NO) pro-drug, into NO induces vasodilation and improves blood flow. Patients develop tolerance to GTN after several weeks of continuous use, limiting the potential for long-term therapy. The mechanistic cause of nitrate tolerance is relatively unknown. We developed a cell culture model of nitrate tolerance that utilizes stable isotopes to measure metabolism of ¹⁵N₃-GTN into ¹⁵N-nitrite. We performed global metabolomics to identify the mechanism of GTN-induced nitrate tolerance and to elucidate the protective role of vitamin C (ascorbic acid). Metabolomics analyses revealed that GTN impaired purine metabolism and depleted intracellular ATP and GTP. GTN inactivated xanthine oxidase (XO), an enzyme that is critical for the metabolic bioactivation of GTN into NO. Ascorbic acid prevented inactivation of XO, resulting in increased NO production from GTN. Our studies suggest that ascorbic acid has the ability to prevent nitrate tolerance by protecting XO, but not aldehyde dehydrogenase (another GTN bioactivating enzyme), from GTN-induced inactivation. Our findings provide a mechanistic explanation for the previously observed beneficial effects of ascorbic acid in nitrate therapy.

Pharmacodynamic interaction between intravenous nitroglycerin and oral sacubitril/valsartan (LCZ696) in healthy subjects

PURPOSE

Sacubitril/valsartan (LCZ696) and nitroglycerin share the second messenger cGMP and lower blood pressure. Given the potential for co-administration of both drugs in patients with heart failure, this study was designed to investigate the potential for a pharmacodynamic drug interaction affecting blood pressure.

METHODS

In this double-blind, placebo-controlled, randomised, crossover study, 40 healthy subjects received sacubitril/valsartan 200 mg bid (97/103 mg bid) or placebo for 5 days. Two hours after the morning dose of sacubitril/valsartan or placebo on day 5, subjects received intravenous nitroglycerin infusion at increasing doses up to 40 μg/min or placebo. Serial measurements of blood pressure (BP), heart rate, biomarkers and sacubitril/valsartan pharmacokinetics were conducted.

RESULTS

Administration of nitroglycerin alone led to a dose- and time-dependent decrease in supine systolic BP (SBP) and diastolic BP (DBP) which was similar when nitroglycerin was co-administered with sacubitril/valsartan. At the highest dose of nitroglycerin, the mean (95% CI) decrease from baseline of SBP/DBP was 19.54 (- 21.99, - 17.09)/12.38 (- 13.85, - 10.92) mmHg for nitroglycerin alone compared to 22.63 (- 25.06, - 20.21)/12.94 (- 14.38, - 11.49) mmHg when co-administered with sacubitril/valsartan. Co-administration of sacubitril/valsartan and nitroglycerin did not result in further plasma cGMP increase compared to sacubitril/valsartan alone. The co-administration of nitroglycerin and sacubitril/valsartan was safe and well tolerated and did not impact the pharmacokinetics of sacubitril/valsartan.

CONCLUSIONS

The results from this study demonstrate no pharmacodynamic drug interaction between nitroglycerin and sacubitril/valsartan in healthy subjects, suggesting that no change of dose selection and escalation recommendations or clinical monitoring during nitroglycerin administration is required.

Effects of the NO/soluble guanylate cyclase/cGMP system on the functions of human platelets

Platelets are circulating sentinels of vascular integrity and are activated, inhibited, or modulated by multiple hormones, vasoactive substances or drugs. Endothelium- or drug-derived NO strongly inhibits platelet activation via activation of the soluble guanylate cyclase (sGC) and cGMP elevation, often in synergy with cAMP-elevation by prostacyclin. However, the molecular mechanisms and diversity of cGMP effects in platelets are poorly understood and sometimes controversial. Recently, we established the quantitative human platelet proteome, the iloprost/prostacyclin/cAMP/protein kinase A (PKA)-regulated phosphoproteome, and the interactions of the ADP- and iloprost/prostacyclin-affected phosphoproteome. We also showed that the sGC stimulator riociguat is in vitro a highly specific inhibitor, via cGMP, of various functions of human platelets. Here, we review the regulatory role of the cGMP/protein kinase G (PKG) system in human platelet function, and our current approaches to establish and analyze the phosphoproteome after selective stimulation of the sGC/cGMP pathway by NO donors and riociguat. Present data indicate an extensive and diverse NO/riociguat/cGMP phosphoproteome, which has to be compared with the cAMP phosphoproteome. In particular, sGC/cGMP-regulated phosphorylation of many membrane proteins, G-proteins and their regulators, signaling molecules, protein kinases, and proteins involved in Ca²⁺ regulation, suggests that the sGC/cGMP system targets multiple signaling networks rather than a limited number of PKG substrate proteins.

Paradoxical normoxia-dependent selective actions of inorganic nitrite in human muscular conduit arteries and related selective actions on central blood pressures

BACKGROUND

Inorganic nitrite dilates small resistance arterioles via hypoxia-facilitated reduction to vasodilating nitric oxide. The effects of nitrite in human conduit arteries have not been investigated. In contrast to nitrite, organic nitrates are established selective dilators of conduit arteries.

METHODS AND RESULTS

We examined the effects of local and systemic administration of sodium nitrite on the radial artery (a muscular conduit artery), forearm resistance vessels (forearm blood flow), and systemic hemodynamics in healthy male volunteers (n=43). Intrabrachial sodium nitrite (8.7 μmol/min) increased radial artery diameter by a median of 28.0% (25th and 75th percentiles, 25.7% and 40.1%; P<0.001). Nitrite (0.087-87 μmol/min) displayed conduit artery selectivity similar to that of glyceryl trinitrate (0.013-4.4 nmol/min) over resistance arterioles. Nitrite dose-dependently increased local cGMP production at the dose of 2.6 μmol/min by 1.1 pmol·min(-1)·100 mL(-1) tissue (95% confidence interval, 0.5-1.8). Nitrite-induced radial artery dilation was enhanced by administration of acetazolamide (oral or intra-arterial) and oral raloxifene (P=0.0248, P<0.0001, and P=0.0006, respectively) but was inhibited under hypoxia (P<0.0001) and hyperoxia (P=0.0006) compared with normoxia. Systemic intravenous administration of sodium nitrite (8.7 μmol/min) dilated the radial artery by 10.7% (95% confidence interval, 6.8-14.7) and reduced central systolic blood pressure by 11.6 mm Hg (95% confidence interval, 2.4-20.7), augmentation index, and pulse wave velocity without changing peripheral blood pressure.

CONCLUSIONS

Nitrite selectively dilates conduit arteries at supraphysiological and near-physiological concentrations via a normoxia-dependent mechanism that is associated with cGMP production and is enhanced by acetazolamide and raloxifene. The selective central blood pressure-lowering effects of nitrite have therapeutic potential to reduce cardiovascular events.

Clinical and functional correlates of platelet cyclic GMP in essential hypertensives

BACKGROUND

Platelets play a central role in atherothrombosis, which is responsible of major cardiovascular complications in human hypertension. Nitric oxide (NO) inhibits platelet aggregation via the second messenger cyclic guanosine monophosphate (cGMP). In essential hypertensives (EHs), we examined the relationship between platelet cGMP and clinical, hemodynamic, humoral variables as well as the responses to aggregating agents.

METHODS

In untreated EHs (male/female 106/43, age 44.4 +/- 1.1 years, smokers yes/no 38/111), blood pressure (BP), heart rate (HR), and stroke volume (SV) (impedance cardiography) were assessed after supine rest and venous blood was sampled for platelet cGMP (radioimmunoassay on acid extracts of washed platelets), plasma cGMP, atrial natriuretic peptide (ANP), renin activity, aldosterone and platelet aggregation to epinephrine (EPI, 5 micromol/l), and adenosine diphosphate (ADP) (4 micromol/l) (optical aggregometry on platelet-rich plasma (PRP)).

RESULTS

Platelet cGMP (7.0 +/- 0.3 pmol/10(9) cells, mean +/- s.e.m.) was lower in males and smokers than in their counterparts (P < 0.01 for both). Among the variables tested, platelet cGMP was related to number of cigarettes (-0.21), high-density lipoprotein cholesterol (HDLc) (r = 0.32), aldosterone (r = -0.21), and hemoglobin (-0.16); in a multivariate analysis that also included sex, HDLc was the best predictor of platelet cGMP. The aggregating response to EPI (r = -0.28), but not to ADP (r = -0.07, ns), was inversely related to platelet cGMP levels.

CONCLUSIONS

cGMP in resting platelets of EHs is positively predicted by HDLc and is inversely related to the aggregating response to EPI. It is suggested that a defect of the platelet NO/cGMP system could identify uncomplicated EHs at higher risk of thrombotic events during surges of sympathetic activity.

Attenuated severity of new acute ischemic events in patients with previous coronary heart disease receiving long-acting nitrates

BACKGROUND

Platelet aggregation and secondary vasoconstriction are key events in the genesis of acute coronary syndromes.

HYPOTHESIS

Since nitrates have vasodilatory and antiaggregant effects, treatment with long-acting nitrates at the time of onset of acute coronary syndromes could be associated with attenuation of their severity.

METHODS

A consecutive series of 533 patients with acute coronary syndrome and past history of coronary artery disease admitted to the Cardiology Service of a general hospital was studied. A specific questionnaire assessed the use of nitrates and other relevant drugs, as well as other clinical variables. The diagnosis of unstable angina or acute myocardial infarction (MI) was established according to clinical, electrocardiographic, and enzymatic criteria.

RESULTS

In the whole cohort, 169 patients had MI and 364 had unstable angina. Previous use of long-acting nitrates was significantly more common in patients with unstable angina (56%) than in those with MI (37%) (p < 0.0001). Multivariate analysis identified being a nonsmoker [odds ratio: 95%, confidence limits (CL) 0.37, 0.23-0.59], previous unstable angina (CL 0.62, 0.41-0.92), use of aspirin (CL 0.58, 0.41-0.92), and use of long-acting nitrates (CL 0.61, 0.40-0.93) as the independent predictors of the development of unstable angina rather than MI; of these the combination of nitrates and aspirin was the strongest predictor.

CONCLUSIONS

Long-acting nitrates as well as aspirin are suggested to have a protective or modifying effect on the development of acute coronary syndromes, favoring unstable angina rather than acute MI.

Lack of alteration of endogenous nitric oxide pathway during prolonged nitric oxide inhalation in intensive care unit patients

OBJECTIVE

To compare hemodynamic and gasometric variables and the plasma concentrations of nitric oxide metabolites (cyclic guanosine monophosphate and nitrate and nitrite), endothelin-1, and renin-angiotensin metabolites before and after the start of nitric oxide inhalation, after prolonged nitric oxide inhalation, and before and after nitric oxide withdrawal.

DESIGN

Prospective study.

SETTING

Surgical intensive care unit, university hospital.

SUBJECTS

Patients with acute lung injury and right ventricular failure.

INTERVENTIONS

Nitric oxide inhalation (10-12 ppm) during a median of 2.9 days (12 hrs to 6.5 days).

MEASUREMENTS AND MAIN RESULTS

The pulmonary vasodilator effects of inhaled nitric oxide improved arterial oxygenation in patients with acute lung injury (p < .05) and reduced right atrial pressure in patients with right ventricular dysfunction (p < .01). These beneficial effects lasted the whole period of prolonged inhaled nitric oxide therapy up to 6.5 days. However, when inhaled nitric oxide was withdrawn, pulmonary vasodilator effects rapidly disappeared, and Pao2/Fio2 ratio markedly deteriorated in all studied patients to return to pre-inhaled nitric oxide levels. Changes in plasma cyclic guanosine monophosphate and nitrate and nitrite paralleled those of pulmonary vasodilatory effects. An immediate increase in plasma cyclic guanosine monophosphate with a slightly delayed increase in plasma nitrate and nitrite was observed at inhaled nitric oxide start with no attenuation during the prolonged inhaled nitric oxide therapy. A marked decrease toward pre-inhaled nitric oxide levels was seen within hours of inhaled nitric oxide withdrawal. In addition, no alteration of plasma endothelin-1 or renin-angiotensin mediators was observed during or after inhaled nitric oxide therapy.

CONCLUSIONS

Our study showed a lack of attenuation in the beneficial effects of inhaled nitric oxide and a lack of alteration of endogenous nitric oxide, endothelin-1, and renin-angiotensin pathways during prolonged nitric oxide inhalation.

Reduced flow-mediated vasodilation is not due to a decrease in production of nitric oxide in preeclampsia

OBJECTIVE

Our aim was to determine the reduced function of endothelial nitric oxide in preeclampsia by use of noninvasive techniques in vivo.

STUDY DESIGN

With the use of a high-resolution ultrasound transducer, diameters of brachial artery were measured after reactive hyperemia in 20 nonpregnant women, 20 normotensive pregnant women, and 15 women with preeclampsia. The concentrations of cyclic guanosine monophosphate were measured in samples of platelets from all groups.

RESULTS

Flow-mediated vasodilation at 1 minute after deflation was higher in the normotensive pregnant women (115.1% +/- 6.5%) than in the nonpregnant women (108.7% +/- 3.9%); flow-mediated vasodilation was lower in women with preeclampsia (106.8% +/- 2.7%) than in the normotensive pregnant women. The concentration of platelet cyclic guanosine monophosphate was higher in the normotensive pregnant women than in the nonpregnant women (2.21 +/- 1.10 pmol/mL/10(8) cells vs 0.746 +/- 0.381 pmol/mL/10(8) cells). There was no difference between the normotensive pregnant and the preeclamptic group (2.81 +/- 1.82 pmol/mL/10(8) cells). Furthermore, the increase in cyclic guanosine monophosphate by sodium nitroprusside in platelet samples that were obtained from the normotensive pregnant women was larger than the samples from the nonpregnant women (6.20 +/- 4.2 pmol/mL/10(8) cells vs 1.62 +/- 0.81 pmol/mL/10(8) cells). The increase in cyclic guanosine monophosphate from the women with preeclampsia did not differ from that in the normotensive pregnant women (5.84 +/- 3.73 pmol/ml/10(8) cells).

CONCLUSION

These results indicate that reduced endothelial nitric oxide activity might be due to a reduction of nitric oxide-cyclic guanosine monophosphate activity rather than its production in preeclampsia.

Induction of heme oxygenase-1 expression inhibits platelet-dependent thrombosis

Heme oxygenase-1 (HO-1) plays a key role in protecting tissue from oxidative stress. Although some studies implicate HO-1 in modulating thrombosis after vascular injury, the impact of HO-1 on the rate of clot formation in vivo is poorly defined. This study examined the potential function of HO-1 in regulating platelet-dependent arterial thrombosis. Platelet-rich thrombi were induced in C57BL/6J mice by applying 10% ferric chloride to the exposed carotid artery. Mean occlusion time of wild-type mice (n = 10) was 14.6 +/- 1.0 min versus 12.9 +/- 0.6 min for HO-1-/- mice (n = 11, p = 0.17). However, after challenge with hemin, mean occlusion time was significantly longer in wild-type mice (16.3 +/- 1.2 min, n = 15) than HO-1-/- mice (12.0 +/- 1.0 min, n = 9; p = 0.021). Hemin administration induced an approximately twofold increase in oxidative stress, measured as plasma thiobarbituric acid reactive substances. Immunohistochemical analysis revealed that hemin induced a robust increase in HO-1 expression within the carotid arterial wall. Ex vivo blood clotting within a collagen-coated perfusion chamber was studied to determine whether the accelerated thrombosis observed in HO-1-/- mice was contributed to by effects on the blood itself. Under basal conditions, mean clot formation during perfusion of blood over collagen did not differ between wild-type mice and HO-1-/- mice. However, after hemin challenge, mean clot formation was significantly increased in HO-1-/- mice compared with wild-type controls. These results suggest that, under basal conditions, HO-1 does not exert a significant effect on platelet-dependent clot formation in vivo. However, under conditions that stimulate HO-1 production, platelet-dependent thrombus formation is inhibited by HO-1. Enhanced HO-1 expression in response to oxidative stress may represent an adaptive response mechanism to down-regulate platelet activation under prothrombotic conditions.

Effect of transdermal nitroglycerin on inflammatory mediators in patients with peripheral atherosclerotic vascular disease

OBJECTIVE

To demonstrate that nitroglycerin improves biological markers of arterial inflammation in patients with peripheral vascular disease.

BACKGROUND

Atherosclerosis is an inflammatory disease in which there is an increase in active inflammation markers such as C-reactive protein and other factors released by endothelial cells. Nitroglycerin acts by a chemical liberation of nitric oxide. We have previously published the results from several controlled clinical trials confirming an anti-inflammatory action of nitroglycerin.

METHODS

Forty patients with peripheral vascular disease entered a randomized, double-blind, placebo-controlled pilot study for 6 weeks. Twenty-one patients were treated with continuous application of a transdermal nitroglycerin patch (15 mg/24 hours) on the anterior face of the thigh. Venous blood samples were obtained before treatment and 2 and 6 weeks after. We measured plasma levels of C-reactive protein, cGMP (also intraplatelet cGMP), E-selectin, ICAM, VCAM-1, IL-6, and nitrites/nitrates.

RESULTS

No biological parameter was modified in the placebo group. On the contrary, nitroglycerin significantly reduced plasma levels of C-reactive protein and sE-selectin and increased the levels of intraplatelet cGMP.

CONCLUSIONS

The results of this preliminary study show that nitroglycerin has an anti-inflammatory action in patients with peripheral vascular disease. This may provide a new therapeutic approach to understanding the efficacy of nitrovasodilators in the improvement of atherosclerotic syndromes.

Inhaled NO inhibits platelet aggregation and elevates plasma but not intraplatelet cGMP in healthy human volunteers

Effects of inhaled nitric oxide (NO) on human platelet function are controversial. It is uncertain whether intraplatelet cGMP mediates the effect of inhaled NO on platelet function. We investigated the effect of 30 ppm inhaled NO on platelet aggregation and plasma and intraplatelet cGMP in 12 subjects. We performed platelet aggregation studies by using a photooptical aggregometer and five agonists (ADP, collagen, epinephrine, arachidonic acid, and ristocetin). During inhalation, the maximal extent of platelet aggregation decreased by 75% with epinephrine (P < 0.005), 56% with collagen (P < 0.005), and 20% with arachidonic acid (P < 0.05). Responses to ADP (8% P > 0.05) and ristocetin (5% P > 0.05) were unaffected. Platelet aggregation velocity decreased by 64% with collagen (P < 0.005), 60% with epinephrine (P < 0.05), 33% with arachidonic acid (P < 0.05), and 14% with ADP (P > 0.05). Plasma cGMP levels increased from 2.58 +/- 0.43 to 9.99 +/- 5.57 pmol/ml (P < 0.005), intraplatelet cGMP levels were unchanged (means +/- SD: 1.96 +/- 0.58 vs. 2.71 +/- 1.67 pmol/109 platelets; P > 0.05). Inhaled NO inhibits platelet aggregation via a cGMP independent mechanism.

Platelet resistance to the antiaggregating effect of N-acetyl-L-cysteine in obese, insulin-resistant subjects

INTRODUCTION

We investigated whether the platelets from obese subjects are sensitive as those from controls to the antiaggregating effects of N-acetyl-L-cysteine (NAC)-an antioxidant thiol that increases availability of endogenous nitric oxide (NO)-and of superoxide dismutase (SOD) and amifostine which act as scavengers of superoxide anion.

MATERIALS AND METHODS

In platelets from obese subjects (n=20, body mass index [BMI]=34.2+/-1.9 kg/m(2), homeostasis model assessment [HOMA] index=5.5+/-1.1) and controls (n=20, BMI=21.4+/-0.6 kg/m(2), HOMA index=1.4+/-0.2), we investigated the effects of NAC on aggregation and on 3',5'-cyclic guanosine monophosphate (cGMP) synthesis and the interplay between NAC and the organic nitrates glyceryl trinitrate (GTN) and sodium nitroprusside (SNP). Similar experiments were carried out with SOD and amifostine.

RESULTS

We found that a 3-min platelet exposure to NAC decreased aggregation and increased cGMP in controls, but not in obese subjects. Only more prolonged incubations exerted a small effect also in obese subjects. GTN and SNP increased platelet cGMP in both groups, but their effect was much lower in obese subjects. NAC (3 mmol/l), SOD (150 U/ml), and amifostine (50 micromol/l) enhanced the increase of cGMP elicited by NO donors, but again, the effect was much lower in obese subjects.

CONCLUSIONS

Since antioxidants do not restore the effects of NO in platelets from obese subjects, we hypothesize that oxidative stress is not the unique cause of platelet resistance to NO in obesity and suggest that a resistance to the NO action at the guanylate cyclase level could play a role in this phenomenon, potentially involved in the increased atherothrombotic risk linked to obesity.

Platelet and leukocyte activation during aortoiliac angiography and angioplasty

OBJECTIVE

to evaluate platelet and leukocyte activation during aortoiliac angiography and percutaneous transluminal angioplasty (PTA).

METHODS

an observational study of 14 patients with aortoiliac atherosclerotic disease, nine of whom underwent PTA. The proportion of fibrinogen-, and P-selectin positive platelets, P-selectin expression on platelets, intraplatelet cGMP and cAMP, CD18 positive granulocytes, CD18 expression on granulocytes, plasma (p)-neopterin, p-TNF alpha and p- interleukin-6 were repeatedly measured in arterial blood during angiography and in venous blood before and after.

RESULTS

compared to a previous venous sample, arterial intraplatelet cAMP was increased proximal to the atherosclerotic lesion before contrast infusion and PTA (median 18 [range: 14-22] vs 16 [15-21] pmol/10(9) platelets p<0.05), and intraplatelet cGMP was increased proximal to the lesion after contrast infusion and PTA (1.2 [0.8-3.9] vs 0.9 [0.6-2.5] pmol/10(9) platelets p<0.05). Four hours after angiography, both the proportion of P-selectin positive platelets (28[11-55]%) and platelet P-selectin expression (9[6-40]) had decreased (p<0.05), from arterial values distal to the lesion before contrast infusion and PTA (57 [24-78]% and 26 [10-83]). Granulocyte CD18 expression was lower during angiography than in a previous venous sample.

CONCLUSIONS

the results are compatible with platelet but not leukocyte activation during peripheral angiography.

Mechanisms involved in agonist-induced hyperaggregability of platelets from normal pregnancy

There is substantial evidence of increased platelet reactivity in vivo and in vitro during pregnancy. Platelet activation occurs in pregnancy with a risk of the development of preeclampsia. In this study, platelet behavior was studied during 28-40 weeks of gestation in a group of women who remained normotensive and a group of nonpregnant female controls. Platelet aggregation and ATP release stimulated by agonists (i.e. collagen and adenosine 5'-diphosphate) were markedly enhanced in washed platelets from pregnant subjects. Furthermore, the collagen-evoked increase in intracellular Ca(2+) ([Ca(2+)](i)) mobilization in fura-2-AM-loaded platelets was also enhanced in pregnant subjects. Moreover, the binding activity of fluorescein isothiocyanate-triflavin toward the platelet glycoprotein IIb/IIIa complex did not significantly differ between the nonpregnant and pregnant groups. In addition, the amount of thromboxane A(2) (TxA(2)) formation from pregnant subjects was significantly greater than that from nonpregnant subjects in both resting and collagen-activated platelets. On the other hand, prostaglandin E(2) formation in the presence of imidazole in either resting or arachidonic acid (100 microM)-treated platelets did not significantly differ between these two groups. The levels of cyclic AMP formation in both resting and prostaglandin E(1) (10 microM)-treated platelets from pregnant subjects were significantly lower than those in nonpregnant subjects. Nitric oxide production was measured by a chemiluminescence detection method in this study. The extent of nitrate production in either resting or collagen-stimulated platelets from pregnant subjects did not significantly differ from that of platelets from the nonpregnant group. We conclude that the agonist-induced hyperaggregability of platelets from normal pregnancy may be due, at least partly, to an increase in TxA(2) formation and a lowering of the level of cyclic AMP formation, which leads to increased [Ca(2+)](i) mobilization and finally to enhanced platelet aggregation and ATP release.

Increased plasma endothelin-1 and intraplatelet cyclic guanosine monophosphate in men with disturbed glucose metabolism

Plasma endothelin-1, the nitric oxide (NO) mediator intraplatelet cyclic guanosine monophosphate (cGMP), the prostacyclin mediator cyclic adenosine monophosphate (cAMP) and the macrophage derived inflammatory mediator plasma neopterin were measured in men with Type 2 diabetes mellitus (n=91), impaired glucose tolerance (IGT; n=51), previously abnormal glucose tolerance (PAGT; n=20), and 34 healthy control men. Plasma endothelin-1was higher in men with Type 2 diabetes mellitus than in controls [4.1 (1.0-14.3) vs. 2.1 (0.2-8. 7) ng/l; P<0.001). Intraplatelet cGMP was higher in men with PAGT [0. 84 (0.57-2.76) pmol/10(9) platelets; P<0.05], IGT [0.85 (0.48-3.53); P<0.001] and Type 2 diabetes mellitus [0.90 (0.47-3.86); P<0.001] than in controls [0.70 (0.42-1.70]. No differences existed between groups concerning intraplatelet cAMP or plasma neopterin. Plasma endothelin-1 correlated with fasting plasma glucose (r=0.33; P<0.001) and HbA1(c) (r=0.29; P<0.001). In conclusion, elevated plasma endothelin-1 in Type 2 diabetes mellitus and its relationship to glucose and HbA1(c) suggest a putative role for endothelin-1 in diabetic endothelial cell damage. Increased cGMP indicating enhanced production/activity of NO suggests that factors other than reduced NO activity contribute to enhanced platelet aggregation in diabetes.

Factors determining clockwise and counterclockwise conduction patterns in atrial reentrant tachycardias: a rabbit model of atrial flutter

INTRODUCTION

In the development of atrial flutter due to reentry, the crista terminalis is supposed to pose a conduction barrier, but the role of its longitudinal conduction in determining the propagation pattern of the reentrant impulse is not known. In rabbit right atrial preparations, we induced reentrant atrial tachycardias and examined the effects of transverse section of the crista terminalis on the development and conduction patterns of arrhythmias.

METHODS AND RESULTS

Right atrial preparations from 12 albino rabbits were placed endocardial surface down in a chamber with an array of 48 bipolar electrodes to draw activation maps. A single premature stimulus was delivered to induce tachycardias at the free wall. In the control, five instances of tachycardia per preparation were induced and another five were induced after cutting the crista terminalis. In the control, the mean duration of tachycardia was 127.1+/-25.2 seconds. The tachycardia was counterclockwise in 39 of 60 instances, clockwise in 12, and undetermined in 4 defined as "atypical." After transverse section of the crista terminalis, the duration was prolonged to 372.6+/-30.4 seconds, but the conduction patterns were not changed. In the free wall, counterclockwise reentry had a broader wavefront and faster conduction than clockwise reentry.

CONCLUSION

Longitudinal conduction block at the crista terminalis contributed to maintenance of reentrant atrial tachycardias, but had no influence on their propagation patterns. Clockwise and counterclockwise rotation of impulses in reentrant tachycardias had different paths and velocities of the wavefront in the free wall of the right atrium.

A new approach for extracellular spin trapping of nitroglycerin-induced superoxide radicals both in vitro and in vivo

Anti-ischemic therapy with nitrates is complicated by the induction of tolerance that potentially results from an unwanted coproduction of superoxide radicals. Therefore, we analyzed the localization of in vitro and in vivo, glyceryl trinitrate (GTN)-induced formation of superoxide radicals and the effect of the antioxidant vitamin C and of superoxide dismutase (SOD). Sterically hindered hydroxylamines 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CP-H) and 1-hydroxy-4-phosphonooxy-2,2,6,6-tetramethylpiperidin (PP-H) can be used for in vitro and in vivo quantification of superoxide radical formation. The penetration/incorporation of CP-H or PP-H and of their corresponding nitroxyl radicals was examined by fractionation of the blood and blood cells during a 1-h incubation. For monitoring in vivo, GTN-induced (130 microg/kg) O2*- formation CP-H or PP-H were continuously infused (actual concentration, 800 microM) for 90 to 120 min into rabbits. Formation of superoxide was determined by SOD- or vitamin C-inhibited contents of nitroxide radicals in the blood from A. carotis. The incubation of whole blood with CP-H, PP-H, or corresponding nitroxyl radicals clearly shows that during a 1-h incubation, as much as 8.3% of CP-H but only 0.9% of PP-H is incorporated in cytoplasm. Acute GTN treatment of whole blood and in vivo bolus infusion significantly increased superoxide radical formation as much as 4-fold. Pretreatment with 20 mg/kg vitamin C or 15,000 U/kg superoxide dismutase prevented GTN-induced nitroxide formation. The decrease of trapped radicals after treatment with extracellularly added superoxide dismutase or vitamin C leads to the conclusion that GTN increases the amount of extracellular superoxide radicals both in vitro and in vivo.

The natural course of atherosclerosis. Part I: incidence and progression

The natural course of early atherogenesis is not well established. The current prospective survey was designed to monitor 5-year changes in carotid atherosclerosis in a large, stratified random sample of the general population using high-resolution duplex ultrasound (Bruneck Study). Incidence rates of carotid atherosclerosis ranged from near zero to 184 per 1000 person-years. Most atherosclerotic lesions developed at sites with enhanced wall thickness. Incidence of atherosclerosis in premenopausal women was less than half of that observed in men of equal age. The sex difference disappeared within 5 years after menopause and may possibly be attributed to sex variations in body iron stores. Preexisting atherosclerotic lesions may experience 1 of 2 different types of disease progression. 1) The first main type of plaque growth causing nonstenotic or diffuse dilative atherosclerosis was characterized by slow and continuous plaque extension, which usually affected several lesions simultaneously and did not primarily focus on the carotid bifurcation. This step-by-step process relied on a cumulative exposure to well-known risk factors such as hyper-lipidemia. Compensatory enlargement of the artery at the site of active atherosclerosis effectively preserved a (near) normal lumen. 2) The second main type of plaque growth was characterized by occasional prominent increases in lesion size. This process primarily occurred in the internal carotid artery and was mediated by procoagulant risk factors in a way that peak levels were relevant rather than cumulative exposure. As the main underlying pathomechanism, atherothrombosis may be hypothesized. Marked increases in plaque size and insufficient vascular remodeling acted synergistically in producing a significant compromise of the lumen. The current study provides novel insights into the natural course of early carotid atherosclerosis, thereby focusing on disease incidence and various types of spontaneous disease progression. Nonstenotic or diffuse dilating atherosclerosis and focal stenotic disease were found to constitute epidemiologically and etiologically distinct disease entities that develop and proceed independently of each other.

Transcardiac 5-hydroxytryptamine release and impaired coronary endothelial function in patients with vasospastic angina

1. The present study was designed to test the hypotheses whether platelet degranulation across the coronary bed is detectable during non-ischaemic periods in patients with vasospastic angina (VSA) and whether the exogenous nitric oxide (NO) donor nitroglycerin (GTN) is able to modify platelet degranulation, reflecting an impaired endothelial production of NO. 2. We studied 13 patients with VSA and 10 controls. The time course of coronary sinus (CS) plasma 5-hydroxytryptamine (5-HT) levels was evaluated every 4 h before and after intravenous infusion of GTN over a period of 40 h. Coronary sinus plasma 5-HT levels were significantly higher at any measured time point in patients with VSA compared with control and were significantly decreased in patients with VSA following treatment with GTN, but not in controls. Femoral artery plasma 5-HT levels remained almost constant throughout the study. The ratio of CS:aorta 6-keto-prostaglandin F1 alpha was significantly and inversely correlated with the transcardiac plasma 5-HT difference only in patients with VSA (r = -0.68; P < 0.02; n = 13). 3. The time course of CS 5-HT levels confirmed significant platelet degranulation across the coronary bed supplied by the spasming artery in patients with VSA and this was modified by GTN. The present data suggest that platelet degranulation occurs during non-ischaemic periods in patients with VSA and that prostacyclin biosynthesis may be a compensatory response to an impaired endothelial release of NO, limiting the degree of the effects of platelet degranulation.

Randomized, double-blind, placebo-controlled study of carvedilol on the prevention of nitrate tolerance in patients with chronic heart failure

OBJECTIVES

This study was designed to evaluate the effect of carvedilol on nitrate tolerance in patients with chronic heart failure.

BACKGROUND

The attenuation of cyclic guanosine 5'-monophosphate (cGMP) production due to inactivation of guanylate cyclase by increased superoxide has been reported as a mechanism of nitrate tolerance. Carvedilol has been known to combine alpha/beta-blockade with antioxidant properties.

METHODS

To evaluate the effect of carvedilol on nitrate tolerance, 40 patients with chronic heart failure were randomized to four groups that received either carvedilol (2.5 mg once a day [carvedilol group, n=10]), metoprolol (30 mg once a day [metoprolol group, n=10]), doxazosin (0.5 mg once a day [doxazosin group, n=10]) or placebo (placebo group, n=10). Vasodilatory response to nitroglycerin (NTG) was assessed with forearm plethysmography by measuring the change in forearm blood flow (FBF) before and 5 min after sublingual administration of 0.3 mg NTG, and at the same time blood samples were taken from veins on the opposite side to measure platelet cGMP. Plethysmography and blood sampling were obtained serially at baseline (day 0); 3 days after carvedilol, metoprolol, doxazosin or placebo administration (day 3); and 3 days after application of a 10-mg/24-h NTG tape concomitantly with carvedilol, metoprolol, doxazosin or placebo (day 6).

RESULTS

There was no significant difference in the response of FBF (%FBF) and cGMP (%cGMP) to sublingual NTG on day 0 and day 3 among the four groups. On day 6, %FBF and %cGMP were significantly lower in the metoprolol, doxazosin and placebo groups than on day 0 and day 3, but these parameters in the carvedilol group were maintained.

CONCLUSIONS

These results indicated that carvedilol may prevent nitrate tolerance in patients with chronic heart failure during continuous therapy with NTG.

Preventive effects of carvedilol on nitrate tolerance--a randomized, double-blind, placebo-controlled comparative study between carvedilol and arotinolol

OBJECTIVES

This study was designed to compare the preventive efect of nitrate tolerance between carvedilol with antioxidant properties and arotinolol without antioxidant properties.

BACKGROUND

The attenuation of cyclic guanosine monophosphate (cGMP) production due to inactivation of guanylate cyclase by increased superoxide has been reported as a mechanism of nitrate tolerance. Carvedilol has been known to combine alpha- and beta-blockade with antioxidant properties.

METHODS

To evaluate the preventive effect of nitrate tolerance, 24 patients with untreated hypertension were randomized to receive either carvedilol (10 mg twice a day [carvedilol group, n=8]), arotinolol (10 mg twice a day [arotinolol group, n=8]), or placebo (placebo group, n=8). Vasodilatory response to nitroglycerin (NTG) was assessed with forearm plethysmography by measuring the change in forearm blood flow (FBF) before and 5 min after sublingual administration of 0.3 mg NTG, and at the same time blood samples were taken from veins on the opposite side to measure platelet cGMP. Plethysmography and blood sampling were obtained serially at baseline (day 0), 3 days after carvedilol, arotinolol or placebo administration (day 3) and 3 days after application of a 20 mg/24 h NTG tape concomitantly with carvedilol, arotinolol or placebo (day 6).

RESULTS

There was no significant difference in the response of FBF (%FBF) and cGMP (%cGMP) to sublingual administration of NTG on days 0 and 3 among the three groups. On day 6, %FBF and %cGMP were significantly lower in the arotinolol group and the placebo group than days 0 and 3, but these parameters in the carvedilol group were maintained.

CONCLUSIONS

The results indicated that carvedilol with antioxidant properties may prevent the development of nitrate tolerance during continuous therapy with NTG compared with arotinolol without antioxidant properties.

Efficacy and rebound phenomenon related to intermittent nitroglycerin therapy for the prevention of nitrate tolerance

Intermittent transdermal therapy of nitroglycerin (NTG) has been recommended for the prevention of nitrate tolerance, but a rebound phenomenon has been reported to occur following removal of the NTG tape. The present study investigated the effects of intermittent NTG therapy on vasodilatory response and the intracellular production of cyclic GMP (cGMP). The study group comprised 12 healthy adults and measurements were taken of the platelet cGMP level, the venous volume (VV) (by forearm plethysmography) and the plasma levels of neurohormonal factors before and 5 min after administration of 0.3 mg of sublingual nitroglycerin (NTG) during the following 4 phases: (i) the control phase (8.00 h); (ii) the continuous phase (8.00 h; 7 days after continuous application of a 10 mg/24 h NTG tape); (iii) the intermittent application phase (8.00 h; 7 days after intermittent application of NTG tape, applied at 21.00 h and removed at 9.00 h); and (iv) the intermittent removal phase (13.00 h; 4 h after removal of the NTG tape in the intermittent phase). The percentage increase in cGMP (%cGMP) and venous volume (%VV) were significantly lower in the continuous phase than the control phase, but there was no difference between the control and the intermittent application phases. However, in the intermittent removal phase, the cGMP level before sublingual NTG, the %cGMP and the %VV were unchanged, but the VV before sublingual NTG was significantly lower than in the control phase. Plasma renin activity and the plasma level of angiotensin II were significantly increased in the continuous phase, the intermittent application phase, and the intermittent removal phase. In conclusion, intermittent transdermal NTG therapy prevented nitrate tolerance in the production of cGMP and vasodilation, but induced a rebound phenomenon after removal of the NTG tape. The rebound phenomenon following the tape removal may be related to some other mechanism, such as activation of neurohormonal factors.

Preventive effects of angiotensin-converting enzyme inhibitors on nitrate tolerance during continuous transdermal application of nitroglycerin in patients with chronic heart failure

This study was designed to investigate the effect of angiotensin-converting enzyme (ACE) inhibitors with and without a sulfhydryl group on intracellular production of cGMP, forearm blood flow, and neurohormonal factors during continuous transdermal application of nitroglycerin in patients with chronic heart failure. Platelet cGMP level and forearm blood flow were measured before and 5 min after sublingual administration of nitroglycerin (NTG) in 20 patients with chronic heart failure during the following 4 phases: (1) baseline phase; (2) NTG phase (1 week after NTG tape 10 mg/day); (3) CPT phase (1 week after both captopril 37.5 mg/day and NTG tape 10 mg/day); and (4) ENL phase (1 week after both enalapril 5 mg/day and NTG tape 10 mg/day). The platelet GMP level before sublingual NTG and forearm blood flow were significantly higher during the 3 phases with NTG tape than during the control phase. The percent increases in platelet cGMP level and forearm blood flow after sublingual NTG were significantly lower during the NTG phase than during the baseline phase. In contrast, concomitant application of ACE inhibitors maintained the percent increase in platelet cGMP level and forearm blood flow. These results indicate that concomitant therapy with ACE inhibitors may be helpful in preventing the attenuation of intracellular cGMP production in patients with chronic heart failure during continuous transdermal application of NTG.

Randomized, double-blind, placebo-controlled study of the preventive effect of supplemental oral vitamin C on attenuation of development of nitrate tolerance

OBJECTIVES

This study sought to evaluate the preventive effect of vitamin C, an antioxidant, on the development of nitrate tolerance.

BACKGROUND

Decreased intracellular production of cyclic guanosine monophosphate (cGMP) is a mechanism of nitrate tolerance, and increased superoxide levels and reduced activation of guanylate cyclase have been observed in vitro.

METHODS

In this double-blind, placebo-controlled study, 24 normal volunteers and 24 patients with ischemic heart disease (IHD) were randomized to receive either vitamin C (2 g three times daily [vitamin C group, n=12]) or placebo (placebo group, n=12). The vasodilator response to nitroglycerin was assessed with forearm plethysmography by measuring the change in FBF before and 5 min after sublingual administration of 0.3 mg of nitroglycerin. Blood samples were simultaneously obtained to measure platelet cGMP levels. FBF was measured, and blood sampling was performed serially at baseline (day 0), 3 days after administration of vitamin C or placebo (day 3) and 3 days after application of a 10-mg/24-h nitroglycerin tape concomitantly with oral vitamin C or placebo (day 6).

RESULTS

There were no differences between the vitamin C and placebo groups in percent increases in FBF (%FBF) or platelet cGMP levels (%cGMP) after administration of sublingual nitroglycerin on day O (%FBF: normal volunteers 31+/-8 vs. 32+/-10; patients with IHD 32+/-9 vs. 32+/-8; %cGMP: normal volunteers 37+/-9 vs. 39+/-10; patients with IHD 38+/-10 vs. 39+/-10 [vitamin C group vs. placebo group]) or day 3 (%FBF: normal volunteers 32+/-9 vs. 33+/-9; patients with IHD 31+/-10 vs. 31+/-10; %cGMP: normal volunteers 36+/-8 vs. 37+/-9; patients with IHD 39+/-11 vs. 38+/-10 [vitamin C group vs. placebo group]). The %FBF and %cGMP in the placebo group were significantly lower on day 6 than in the vitamin C group (%FBF: normal volunteers 30+/-8 vs. 19 4, p < 0.01; patients with IHD 29+/-9 vs. 17+/-6, p < 0.01; %cGMP: normal volunteers 36 10 vs. 17+/-6, p < 0.01; patients with IHD 37+/-11 vs. 15+/-5, p < 0.01 [vitamin C group vs. placebo group]).

CONCLUSIONS

These results indicate that combination therapy with vitamin C is potentially useful for preventing the development of nitrate tolerance.

Unexpected, tolerance-devoid vasomotor and platelet actions of pentaerythrityl tetranitrate

Efficacy of nitrate therapy is limited by tolerance. A surprising upregulation of ex vivo platelet activity, a decrease in platelet thiol levels, and an enhanced release of vasoconstrictors from platelets is associated with enhanced superoxide-mediated oxidant stress leading to vascular tolerance to nitrates. We tested the NO-donor pentaerythrityl tetranitrate (PETN), which to date had not been precisely tested either with regard to the induction of tolerance or to a potential development of changes in platelet activity in comparison with glycerol trinitrate (GTN). Long-term instrumented dogs nonintermittently received: 1.5 microg/kg/min GTN, i.v., with or without vitamin C (55 microg/kg/min, i.v.) or PETN 4 x 60 mg/day orally for 5 days. Tested daily were (a) the dilation of the epicardial arteries, (b) thrombin-induced (0.5 U/ml) increases of the intracellular Ca2+ concentration and aggregability of platelets, (c) concentrations of reduced low-molecular-weight thiols (LMTs) in plasma and platelets, and (d) formation of reactive oxygen species (ROSs). During nonintermittent PETN and during GTN with additional vitamin C, a 9.8 +/- 0.4% coronary artery dilation was observed in contrast to that with GTN alone, which resulted in complete tolerance at day 4. This vascular tolerance was associated with enhanced platelet activity and formation of ROSs (incubated platelets) and a 38 +/- 3% reduction in LMT. These unfavorable changes were absent in the presence of PETN or with additional vitamin C as an antioxidant. Vascular tolerance associated with platelet upregulation is avoided either by nonintermittent nitroglycerin (5 days) when vitamin C is coadministered or by pentaerythrityl tetranitrate without the coadministration of vitamin C.

Smooth muscle cell expression of type I cyclic GMP-dependent protein kinase is suppressed by continuous exposure to nitrovasodilators, theophylline, cyclic GMP, and cyclic AMP

A key component of the nitric oxide-cyclic guanosine monophosphate (cGMP) pathway in smooth muscle cells (SMC) is the type I GMP-dependent protein kinase (PK-G I). Activation of PK-G I mediates the reduction of cytoplasmic calcium concentrations and vasorelaxation. In this manuscript, we demonstrate that continuous exposure of SMC in culture to the nitrovasodilators S-nitroso-N-acetylpenicillamine (SNAP) or sodium nitroprusside (SNP) results in approximately 75% suppression of PK-G I mRNA by 48 h. PK-G I mRNA and protein were also suppressed by continuous exposure to cGMP analogues 8-bromo- and 8-(4-chlorophenylthio) guanosine-3,5-monophosphate or the cAMP analogue dibutyryl cAMP. These results suggest that activation of one or both of the cyclic nucleotide-dependent protein kinases mediates PK-G I mRNA suppression. Using isoform-specific cDNA probes, only the PK-G I alpha was detected in SMC, either at baseline or after suppression, while PK-G I beta was not detected, indicating that isoform switch was not contributing to the gene regulation. Using the transcription inhibitor actinomycin D, the PK-G I mRNA half-life in bovine SMC was observed to be 5 h. The half-life was not affected by the addition of SNAP to actinomycin D, indicating no effect on PK-G I mRNA stability. Nuclear runoff studies indicated a suppression of PK-G I gene transcription by SNAP. PK-G I suppression was also observed in vivo in rats given isosorbide dinitrate in the drinking water, with a dose-dependent suppression of PK-G I protein in the aorta. PK-G I antigen in whole rat lung extract was also suppressed by administration of isosorbide or theophylline in the drinking water. These data may contribute to our understanding of nitrovasodilator resistance, a phenomenon resulting from continuous exposure to nitroglycerin or other nitrovasodilators.

Effects of enalapril during continuous nitrate therapy: analysis of diameter of coronary arteries and platelet cyclic guanosine monophosphate

To investigate the effects of enalapril, an angiotensin-converting enzyme inhibitor, on nitrate tolerance during continuous nitrate therapy, coronary artery diameters and platelet cyclic guanosine monophosphate (cGMP) levels were measured before and 2 minutes after intracoronary injection of nitroglycerin 200 microg in 60 patients with coronary artery disease and were compared among 20 patients treated with nitrates (nitrate group), 20 patients treated with both nitrates and enalapril (enalapril group), and 20 untreated patients (control group). The percent increase in platelet cGMP and coronary dilatation in the nitrate group was significantly less than in the control group, but the percent increase in the enalapril group was significantly greater than that in the nitrate group. These results indicate that enalapril may be helpful as concomitant therapy to maintain the effect of nitrates during continuous nitrate therapy.

Use of intracardiac echocardiography in interventional electrophysiology

Intracardiac echocardiography is emerging as a potentially useful tool during RF ablation procedures. There are a number of potential benefits of direct endocardial visualization during RF ablation including: (1) precise anatomical localization of the ablation catheter tip in relation to important endocardial structures, which cannot be visualized with fluoroscopy; (2) reduction in fluoroscopy time; (3) evaluation of catheter tip tissue contact; (4) confirmation of lesion formation and identification of lesion size and continuity; (5) immediate identification of complications; and (6) as a research tool to help in understanding the critical role played by specific endocardial structures in arrhythmogenesis. This article will review existing data and speculate as to possible future roles for intracardiac echocardiography in interventional electrophysiology.

Nitroglycerin tolerance at the platelet level in patients with angina pectoris

Suppression of platelet aggregation may be an important component of the therapeutic effect of nitroglycerin (NTG). Because of the phenomenon of hemodynamic tolerance to NTG, we tested the hypothesis that the anti-platelet effects of NTG in humans are also subject to tolerance induction. In patients with stable angina who had not received nitrates for at least 24 hours before study, sublingual administration of NTG (300 microg; n = 17) attenuated the reversal of adenosine diphosphate-induced platelet aggregation by NTG applied in vitro. Three minutes after in vivo NTG administration, concentration of NTG producing 50% reversal of aggregation (C50) increased from 7.9 +/- 1.9 x 10(-5) to 5.5 +/- 0.3 x 10(-4) M (p <0.01); this change persisted for at least 60 minutes. There was no concomitant change in C50 values for sodium nitroprusside applied in vitro. Basal activity of platelet guanylate cyclase and its response to sodium nitroprusside were not affected after administration of NTG. Brief intravenous infusion of NTG (10 microg/min for 10 minutes) produced no significant changes in platelet responses to NTG in vitro. However, prolonged infusion of NTG (5 microg/min for 24 hours, patients with unstable angina pectoris, n = 11) caused suppression of in vitro platelet response to NTG. Platelets from patients receiving prophylactic nitrates (n = 19) were less responsive to the antiaggregatory effects of NTG in vitro than those from patients who had not received nitrates in the previous 24 hours (n = 21). Thus, clinical exposure to NTG, even in very low doses, induces tolerance to antiaggregatory effects of NTG. This phenomenon is not associated either with cross tolerance to sodium nitroprusside or with down-regulation of platelet guanylate cyclase.

Antiplatelet and antiproliferative effects of SCH 51866, a novel type 1 and type 5 phosphodiesterase inhibitor

SCH 51866 is a potent and selective PDE1 and PDE5 inhibitor. The antiplatelet, antiproliferative, and hemodynamic effects of SCH 51866 were compared with those of E4021, a highly selective PDE5 inhibitor. SCH 51866 inhibited PDE1 and PDE5 isozymes with a 50% inhibitory concentration (IC50) of 70 and 60 nM, respectively. SCH 51866 and E4021 inhibited washed human platelet aggregation induced by collagen with an IC50 of 10 and 4 microM, respectively, and attenuated (p < 0.05) the adhesion of 111indium-labeled platelets to the nylon filament-injured rat aorta. The doses of SCH 51866 and E4021 that inhibited platelet adhesion caused significant increases in platelet cyclic guanosine monophosphate (cGMP; p < 0.05). SCH 51866 (1-10 mg/kg, p.o. twice daily) but not E4021 (3-30 mg/kg, p.o twice daily) inhibited neointima formation in the carotid arteries of spontaneously hypertensive rats (SHRs) subjected to balloon angioplasty. Moreover, SCH 51866 (0.3-10 mg/kg, p.o.) elicited dose-dependent reduction in blood pressure in SHRs, whereas E4021 (3-30 mg/kg, p.o.) did not affect blood pressure in SHRs. In conclusion, the data suggest that inhibition of PDE1 and PDE5 isozymes by SCH 51866 exerts antiplatelet and vascular protective effects. In comparison, inhibition of PDE5 alone by E4021 exhibited antiplatelet effects without affecting neointima formation.

Sustained antiplatelet properties of nitroglycerin during hemodynamic tolerance in rats

Organic nitrates possess important antiplatelet actions that are useful in the treatment of unstable angina and myocardial infarction, but the susceptibility of platelets to nitrate tolerance has not been extensively studied. In normal conscious rats, we showed that continuous infusion of nitroglycerin (NTG) at 10 micrograms/min had no significant effect on mean arterial pressure (MAP) as compared with control, but hemodynamic tolerance could be demonstrated by MAP response to a bolus intravenous (i.v.) NTG challenge. By this criterion, continuous 8-h NTG infusion produced hemodynamic tolerance (a decrease in MAP response of 45.7 +/- 19.9%, p < 0.05), whereas D5W control and S-nitroso-N-acetylpenicillamine (SNAP) infusions did not. During NTG infusion, platelet-rich plasma (PRP) cyclic GMP was increased by 41.4 +/- 13.6% as compared with control and remained increased throughout the infusion (p < 0.05). Bleeding time during a 2-h infusion of NTG was 8.9 +/- 1.2 min as compared to 3.8 +/- 0.4 min in controls (p < 0.05). After 8-h of NTG infusion, the bleeding time was 10.2 +/- 1.4 min versus 4.4 +/- 0.4 min in controls (p < 0.05). NTG also decreased the PRP platelet concentration by 30% in 8 h, whereas D5W had no effect. In vitro experiments showed that platelets in themselves do not produce significant amounts of cyclic GMP. These data indicate that the biochemical and antiaggregation effects of NTG on platelets are not diminished during hemodynamic tolerance and that these effects may be dependent on extraplatelet production of nitric oxide (NO).

Atrial flutter mapping and ablation. I. Studying atrial flutter mechanisms by mapping and entrainment

Endocardial mapping has led to a detailed knowledge of reentry mechanisms in atrial flutter. Multipolar and deflecting tip catheters allow recording local electrograms from multiple areas of the right atrium, and from the coronary sinus. In common flutter, with the typical "sawtooth" pattern, there is circular activation of the right atrium in a "counterclockwise" direction, descending in the anterior and lateral walls, and ascending in the septum and posterior wall. Superior and inferior vena cava, linked by a "line" of functional block in the posterolateral wall, make the central obstacle for circular activation. The cranial and caudal turning points are the atrial "roof," and the isthmus between the inferior vena cava and the tricuspid valve. Complex conduction patterns, probably including slow conduction are detectable in the low septal area, around the coronary sinus. Atypical flutter, without the sharp negative deflections of common flutter, sometimes shows circular activation in the right atrium, rotating in the opposite direction of common flutter (clockwise). Other atypical flutters show no circular right atrial activation, and only partial data from coronary sinus activation, combined with the response to atrial stimulation (entrainment) allow the diagnosis of left atrial reentry, without a precise delimitation of the circuits. In patients having undergone cardiac surgery, atypical flutter may be based on reentry around surgical scars. To our knowledge, the mechanism of type II flutter has not been disclosed in humans.

Tolerance to nitrates and simultaneous upregulation of platelet activity prevented by enhancing antioxidant state

We analysed the induction of tolerance to nitrates both in the vasculature (in vivo) and platelets (ex vivo). Simultaneously, we tested mechanisms underlying the induction of tolerance and interventions to prevent or overcome this phenomenon. For this purpose nitroglycerin (GTN 1.5 micrograms/kg per min i.v.), alone or in combination with ascorbate (55 micrograms/kg per min i.v.) as antioxidant, was infused continuously for a period of 5 days into chronically instrumented dogs. Along with haemodynamic parameters, ex vivo platelet function was continuously monitored. Following the start of GTN infusions there was a maximal coronary dilator response (245 +/- 15 microm) and, as an index of venodilation, a fall of left ventricular end-diastolic pressure (by 2.3 +/- 0.4 mmHg). Both responses declined progressively and disappeared during the infusion period. However, in combination with ascorbate as antioxidant the dilator responses were maintained fully throughout the infusion period. With GTN alone there was a progressive, unexpected upregulation of platelet activity demonstrated by enhanced thrombin-stimulated intracellular Ca2+ levels and increases in the microviscosity of platelet membranes (indicating enhanced receptor expression) associated with a progressive impairment in basal, unstimulated cGMP levels. These changes could also be prevented completely by i.v. co-administration of ascorbate. From these results it is concluded that vascular tolerance is closely reflected by simultaneous changes in platelet function and further, that both can be prevented completely by appropriate antioxidants such as ascorbate.

Tolerance to nitroglycerin in vascular smooth muscle cells is not affected by the level of intracellular glutathione or L-cysteine

A major hypothesis for the mechanism of tolerance to nitroglycerin (NTG) is that continued use causes a decrease in thiol donors within the vascular smooth muscle cell that are essential for the effect of NTG. We tested this idea directly in the target cell. NTG tolerance, measured as reduced formation of intracellular cyclic guanosine monophosphate (cGMP), was induced in pig coronary smooth muscle cells. The consequence of altering intracellular levels of the thiol donors, glutathione (GSH) and L-cysteine (L-cys), was determined. Incubating cells with 100 microM NTG for 1 h caused an 83% reduction in cGMP formation in response to acute readministration of 200 microM NTG for 2 min but was not associated with a reduction in intracellular GSH or L-cys. This result was not altered when intracellular GSH levels were increased three-fold by including 1 mM GSH in the incubation buffer. Also, recovery from tolerance was not affected by supplementation with GSH. Further, the response of cGMP to NTG was not altered by inhibiting the synthesis of GSH and lowering intracellular levels of GSH by 77%. Similar findings were made with supplemental L-cys or N-acetyl-L-cysteine. These results do not support the hypothesis that tolerance to NTG is the result of a reduction of the thiol donors GSH and L-cys within vascular smooth muscle cells.

Inhibitory effect of alcohol on cyclic GMP accumulation in human platelets

The effects of ethanol on cyclic GMP (cGMP) in washed human platelets were studied in the presence and absence of sodium nitroprusside (SNP), nitric oxide donor which stimulates guanylate cyclase. SNP stimulated cGMP accumulation in a dose-dependent fashion. After 1 min exposure to 100 microM SNP, the level of cGMP was approximately four-fold that in vehicle-treated platelets. Alcohol had no effect on basal cGMP, but inhibited SNP-induced cGMP accumulation at 17, 85 and 170 mM. In further experiments, platelets were incubated for 0, 0.5, 1 2 or 5 min with 10 microM SNP, with or without 100 microM zaprinast, a selective cGMP-phosphodiesterase (PDE) inhibitor and 85 mM ethanol. In the presence of zaprinast but not alcohol, cGMP levels rose continuously, to 10-fold the basal level at 5 min. Without zaprinast, cGMP levels were lower and reached a plateau by 2 min. Accumulation of cGMP was attenuated by alcohol 2 and 5 min after SNP addition, both in zaprinast-treated platelets and those without zaprinast. Thus, alcohol inhibits platelet cGMP accumulation stimulated by nitric oxide donor. Its mechanism probably does not involve a major effect on PDE, because the inhibition was observed in the presence or absence of zaprinast. We hypothesize that alcohol inhibits guanylate cyclase, contributing to its complex functional effects in platelets.

Expression of constitutive endothelial nitric oxide synthase in human blood platelets

Nitric oxide (NO) activates the soluble isoform of guanylate cyclase in platelets and inhibits platelet function. Several studies suggest the existence of a pathway for NO synthesis in platelets as a form of feedback inhibition, but the identity of the NO synthase (NOS) isoform present within platelets is unknown. We isolated human platelets, and synthesized cDNA from platelet RNA for analysis by PCR. Primers for human neuronal or inducible NOS failed to yield a PCR signal. However, primers specific for endothelial NOS (ecNOS) amplified a DNA band of the expected size. Analysis of nucleotide sequence revealed that the amplified DNA is ecNOS. NOS enzyme activity was detected in the platelet particulate subcellular fraction, as previously demonstrated for ecNOS in other cells. Thus, ecNOS is present in human platelets, and may play a role in the regulation of platelet function by an endogenous NO pathway.

Escalating nitrate dose overcomes early attenuation of hemodynamic effect caused by nitrate tolerance in patients with heart failure

This study was performed to examine the hypothesis that an early attenuation of nitrate effect caused by nitrate tolerance can be overcome by dose increase. We compared hemodynamic effects of constant dose (40 to 120 mg) of oral isosorbide dinitrate (ISDN) given every 6 hours for three doses followed by 12 hours of nitrate washout interval with those of escalating dose (40 mg, 80 mg, and 120 mg) in two similar groups of patients with chronic congestive heart failure. Escalating ISDN dose resulted in a progressive increase in ISDN blood level and overcame early attenuation of effect occurring with a constant dose. At peak effect of the third ISDN dose, a significantly greater reduction was seen in mean right atrial pressure (-59% +/- 27% vs -20% +/- 22%, p < 0.01), mean pulmonary artery pressure (-29% +/- 11% vs -11% +/- 15%, p < 0.01) and mean pulmonary artery wedge pressure (-39% +/- 19% vs -19% +/- 23%, p < 0.05) with the escalating dose. These findings demonstrate that early attenuation of hemodynamic effect caused by nitrate tolerance can be overcome by dose escalation.

Pathophysiology of congenital diaphragmatic hernia. X: Localization of nitric oxide synthase in the intima of pulmonary artery trunks of lambs with surgically created congenital diaphragmatic hernia

The pathophysiology of congenital diaphragmatic hernia (CDH) results from a combination of pulmonary hypoplasia, pulmonary hypertension, and surfactant deficiency. Previously we demonstrated that inhaled nitric oxide (NO), a known vasodilator, only improves oxygenation and decreases pulmonary artery pressures when the lamb model of CDH is pretreated with exogenous surfactant. Nitric oxide synthase (NOS) in endothelial cells is responsible for the production of NO, a mediator of smooth muscle cell relaxation. Pulmonary hypertension in CDH may result from a defect in the endogenous production of NO. Our aim was to determine whether the main pulmonary artery trunks in CDH lambs have NOS immunoreactivity. Cryostat sections of paraformaldehyde-fixed specimens of pulmonary artery and aortic rings from 10 CDH lambs and five control lambs were processed for NADPH-diaphorase activity. Immunolocalization of NOS was studied in paraformaldehyde-fixed sections and compared with serially cut specimens from identical rings that were tested for NADPH-diaphorase activity. Intense NADPH-diaphorase staining was present in the intimal layer (endothelial lining) of the pulmonary artery and aortic rings of both the CDH and control lambs. This activity colocalized with NOS immunoreactivity in all specimens. Both NOS immunoreactivity and NADPH-diaphorase staining were lacking in cartilage, which were used as negative controls. NOS is present in the main pulmonary artery trunks of CDH lambs. To our knowledge, this is the first report of NOS immunoreactivity in CDH. We can only speculate whether this activity is preserved in other areas of the vascular tree in CDH, ie, pulmonary capillaries and veins. Perhaps the pulmonary hypertension in CDH is not caused by an NOS deficiency.

Portal and systemic haemodynamic response to acute and chronic administration of low and high dose isosorbide-5-mononitrate in patients with cirrhosis

Oral isosorbide-5-mononitrate (Is-5-Mn) was given in doses of 10 and 40 mg acutely and chronically (twice daily for four weeks), allowing a nitrate free interval to 25 patients with cirrhosis. Both 10 mg and 40 mg Is-5-Mn reduced the hepatic venous pressure gradient acutely and chronically, without evidence of tolerance. This was achieved by a reduction in the wedged hepatic venous pressure. The effect on mean azygos blood flow was variable with no significant mean change seen acutely or after chronic use with either dose. The variability was dependent not on the dose used but on the initial azygos flow; the flow in patients with initially low values increased and those with high azygos flows decreased after nitrate challenge. The development of the porto-collateral flow seems an important parameter in predicting haemodynamic response to Is-5-Mn.

Parallel tolerance between platelet cyclic GMP and preload effects of nitroglycerin in anaesthetized mini-pigs

The effects of acute intravenous nitroglycerin (NTG) administration on platelet cyclic GMP in relation to changes in indices of preload (end-diastolic volume) and afterload (effective arterial elastance) were evaluated in the anaesthetized mini-pig, using pressure-volume analysis. NTG (1-30 micrograms kg-1 min-1, i.v.) elicited a dose-dependent fall in preload and afterload, and an increase in arterial blood platelet cyclic GMP. Repeated doses of NTG (30 micrograms kg-1 min-1) resulted in tolerance to the preload but not afterload effects. The increases in platelet cyclic GMP were also attenuated, being highly correlated with the preload changes. Therefore, platelet cyclic GMP appears to reflect NTG-induced venous tolerance, rather than arterial responsiveness. The measurement of platelet cyclic GMP may represent a simple approach for monitoring the degree of venous tolerance to NTG in animals or patients, facilitating further mechanistic investigations.

Exogenous nitric oxide prevents endotoxin-induced glomerular thrombosis in rats

Nitric oxide (NO) synthesized from L-arginine is an endogenous vasodilator and inhibitor of platelet adhesion and aggregation. Gram-negative lipopolysaccharide (LPS) can induce NO synthesis, which may mediate the pathophysiologic effects of endotoxemia. In addition, our previous studies suggested that LPS-induced NO may protect against thrombosis in rats. In the present study, male Sprague-Dawley rats given LPS (0.1 mg/kg) i.p. increased their urinary excretion of NO2 + NO3 (stable end-products of NO) by 4.3-fold. Rats given 10 micrograms/kg/hr i.v. of nitroglycerin (GTN), an exogenous NO donor, showed a similar increase. L-NAME, an inhibitor of NO synthesis, abrogated the increase in urinary NO2 + NO3 in LPS-treated rats but not in rats given GTN. Glomerular thrombosis developed in rats given LPS + L-NAME (thrombosis score = 3.02 +/- 0.4), while those given LPS + L-NAME + GTN were largely protected (thrombosis score = 1.37 +/- 0.5, P < 0.05). Atrial natriuretic peptide (ANP), an NO-independent vasodilator, neither increased urinary NO2 + NO3 nor prevented glomerular thrombosis (thrombosis score = 2.68 +/- 0.5, NS). Hydralazine, another vasodilator without effects on NO or platelets, also failed to prevent glomerular thrombosis in rats given LPS + L-NAME. We conclude that in endotoxemia, the antithrombogenic properties of endogenously synthesized NO are important in preventing alomerular thrombosis. The exogenously NO donor, GTN, can substitute for the antithrombogenic effect of endogenous NO. Clinically, administration of NO synthesis inhibitors to treat endotoxic shock may need to be combined with concomitant administration of exogenous NO donors to prevent microvascular thrombosis.

Coronary vasomotor responses: role of endothelium and nitrovasodilators

The endogenous nitrovasodilator endothelium-derived nitric oxide (EDNO) is continuously synthetized enzymatically by NO synthase from L-arginine and is released from endothelial cells. Enhanced, superimposed EDNO release can be stimulated by various local and circulating factors, such as bradykinin, ATP, etc., but also most importantly by viscous drag-induced shear stress of the bloodstream acting on the endothelial lining. Thus luminal release suppresses leukocyte adhesion (expression of adhesion molecules), platelet activation, platelet adhesion, and platelet aggregation, and abluminal release counteracts myogenic and neurogenic coronary constrictor tone, thereby increasing myocardial perfusion and dilating large coronary artery calibers. Thus endothelial impairment and denudation (hypercholesterolemia, atheromatosis, balloon catheter interventions) favor excessive constrictor tone and myocardial ischemia. Under these conditions EDNO can be supplemented by compounds (e.g., nitroglycerin, isosorbide dinitrate) converted by biological systems into NO. In addition, it can be supplemented by compounds that even spontaneously release NO (e.g., sydnonimines such as SIN-1 and sodium nitroprusside). EDNO and exogenously supplemented NO stimulate soluble guanylyl cyclase, increase cGMP levels, and bring about vascular relaxation, particularly in those still compliant sections in which EDNO production is impaired and cGMP levels are thus diminished. Exogenous nitrovasodilators are preferentially converted (in the presence of cysteine) enzymatically in large coronary arteries, improving coronary conductance, and in the venous bed (preload reduction), resulting in an improved O2 supply/demand ratio. During chronic, continuous application, neurohormonal counterregulation and diminished enzymatic biotransformation into NO may reduce their effectiveness, resulting in tolerance, particularly in the most sensitive vascular sections, such as veins and coronary arteries. This drawback can be overcome by applying spontaneously NO-releasing compounds, intermittent therapy, or intermittent interposition of other vasodilator principles.

Plasma arteriovenous cGMP difference as a useful indicator of nitrate tolerance in patients with heart failure

BACKGROUND

The present study was performed to evaluate the effects of nitroglycerin (GTN) on plasma arteriovenous cGMP production and to compare its hemodynamic effects in patients with congestive heart failure (CHF). We also estimated the potential clinical value of plasma arteriovenous cGMP production as an indicator of nitrate tolerance.

METHODS AND RESULTS

Plasma arterial and venous cGMP levels, atrial natriuretic peptide level, and hemodynamic parameters were measured before and after GTN infusion in 14 patients with CHF. Although the plasma levels of arterial cGMP and atrial natriuretic peptide decreased immediately after GTN, the plasma level of venous cGMP did not change. GTN infusion caused a dose-dependent increase in plasma arteriovenous cGMP production, and there was a positive correlation between the decrease of pulmonary capillary wedge pressure and the increase of plasma arteriovenous cGMP production immediately after GTN. Hemodynamic tolerance was observed after both 12 and 24 hours, when plasma arteriovenous GMP production was also attenuated.

CONCLUSIONS

These findings indicate that the plasma arteriovenous cGMP difference is a clinical indicator of vasodilatory action of GTN and a useful indicator of nitrate tolerance in patients with CHF.

Nitric oxide and nitrovasodilators: similarities, differences and potential interactions

Many similarities exist between the exogenous nitrates and endothelium-derived relaxing factor, which is nitric oxide or a thiol derivative. Both act by way of guanylate cyclase, which increases intracellular concentrations of cyclic guanosine monophosphate, resulting in smooth muscle cell relaxation and antiplatelet effects. Thiols may be important in the biotransformation of exogenous nitrates and other intracellular processes involving nitric oxide. As such, important interactions might be expected between nitrates and endothelium-dependent processes that involve nitric oxide. This review explores the mechanisms of action, biologic effects and potential interactions between nitrates and endothelium-derived relaxing factor.