In vivo induction and reversal of nitroglycerin tolerance in human coronary arteries

Oral N-acetylcysteine as an adjunct to standard medical therapy improved heart function in cases with stable class II and III systolic heart failure

BACKGROUND

This research attempted to assess whether N-acetylcysteine (NAC) as adjunctive therapy can be useful in the treatment of patients with heart failure (HF).

METHODS

Fifty-five cases with diagnosed systolic HF and stable symptomatic New York Heart Association (NYHA) functional class II and III and on optimal medical treatment of HF for at least 3 months were assigned for receiving oral NAC (600 mg twice daily) or placebo for 12 weeks. The outcomes were changes in the echocardiographic hemodynamic indices as well as the patients' functional capacity assessed by NYHA classification over a 12-week treatment.

RESULTS

Compared to placebo, NAC more significantly improved the systolic left ventricular (LV) function expressed as the ejection fraction and Tei index. These changes are accompanied by more improvement in other LV echocardiographic indices including LV end-diastolic volume index and LV global longitudinal strain in the patients receiving NAC in comparison with those receiving placebo. In parallel with the improvement of LV function, right ventricular (RV) function expressed as RV fractional area change and RV Tei-index also got more improvement in those receiving NAC than those receiving placebo. However, the change in RV global longitudinal strain did not show a significant difference between study groups. Additionally, at week 12, the distribution of the NYHA functional class also shifted toward a better outcome in the NAC group in comparison with the placebo group; however, it was not significant.

CONCLUSIONS

These preliminary data support experimental findings showing that NAC supplementation is able to improve heart function.

TRIAL REGISTRATION

The registration of the trial was done at the Iranian Registry of Clinical Trials ( www.irct.ir ). Identifier code: IRCT20120215009014N333. Registration date: 2020-01-11.

Pathobiology and evolving therapies of coronary artery vasospasm

Coronary artery vasospasm is a known cause of chest pain and requires a high level of clinical suspicion for diagnosis. It also remains in the differential diagnosis for patients presenting with type 2 myocardial infarction. There are few randomized controlled trials for guideline-based prevention and treatment for coronary artery vasospasm. In this article, we review updated concepts in coronary artery vasospasm. Specifically, our aim is to provide current evidence of pathophysiology, identify the risk factors, propose a diagnostic algorithm, review available evidence of evolving therapies, and identify patients who would benefit from automatic implantable cardioverter defibrillators.

Protection from Reperfusion Injury with Intracoronary N-Acetylcysteine in Patients with STEMI Undergoing Primary Percutaneous Coronary Intervention in a Cardiac Tertiary Center

BACKGROUND

Evidence suggests that oxidative stress plays a principal role in myocardial damage following ischemia/reperfusion events. Recent studies have shown that the antioxidant properties of N-acetylcysteine (NAC) may have cardioprotective effects in high doses, but-to the best of our knowledge-few studies have assessed this.

OBJECTIVES

Our objective was to investigate the impact of high-dose NAC on ischemia/reperfusion injury.

METHODS

We conducted a randomized double-blind placebo-controlled trial in which 100 consecutive patients with ST-elevation myocardial infarction undergoing percutaneous coronary intervention (PCI) were randomly assigned to the case group (high-dose NAC 100 mg/kg bolus followed by intracoronary NAC 480 mg during PCI then intravenous NAC 10 mg/kg for 12 h) or the control group (5% dextrose). We measured differences in peak creatine kinase-myocardial band (CK-MB) concentration, highly sensitive troponin T (hs-TnT), thrombolysis in myocardial infarction (TIMI) flow, myocardial blush grade (MBG), and corrected thrombolysis in myocardial infarction frame count (cTFC).

RESULTS

The peak CK-MB level was comparable between the two groups (P = 0.327), but patients receiving high-dose NAC demonstrated a significantly larger reduction in hs-TnT (P = 0.02). In total, 94% of the NAC group achieved TIMI flow grade 3 versus 80% of the control group (P = 0.03). No significant differences were observed between the two groups in terms of changes in the cTFC and MBG.

CONCLUSIONS

In this study, NAC improved myocardial reperfusion markers and coronary blood flow, as revealed by differences in peak hs-TnT and TIMI flow grade 3 levels, respectively. Further studies with large samples are warranted to elucidate the role of NAC in this population. ClinicalTrials.gov identifier: NCT01741207, and the Iranian Registry of Clinical Trials (IRCT; http://irct.ir ) registration number: IRCT201301048698N8.

Early Use of N-acetylcysteine With Nitrate Therapy in Patients Undergoing Primary Percutaneous Coronary Intervention for ST-Segment-Elevation Myocardial Infarction Reduces Myocardial Infarct Size (the NACIAM Trial [N-acetylcysteine in Acute Myocardial Infarction])

BACKGROUND

Contemporary ST-segment-elevation myocardial infarction management involves primary percutaneous coronary intervention, with ongoing studies focusing on infarct size reduction using ancillary therapies. N-acetylcysteine (NAC) is an antioxidant with reactive oxygen species scavenging properties that also potentiates the effects of nitroglycerin and thus represents a potentially beneficial ancillary therapy in primary percutaneous coronary intervention. The NACIAM trial (N-acetylcysteine in Acute Myocardial Infarction) examined the effects of NAC on infarct size in patients with ST-segment-elevation myocardial infarction undergoing percutaneous coronary intervention.

METHODS

This randomized, double-blind, placebo-controlled, multicenter study evaluated the effects of intravenous high-dose NAC (29 g over 2 days) with background low-dose nitroglycerin (7.2 mg over 2 days) on early cardiac magnetic resonance imaging-assessed infarct size. Secondary end points included cardiac magnetic resonance-determined myocardial salvage and creatine kinase kinetics.

RESULTS

Of 112 randomized patients with ST-segment-elevation myocardial infarction, 75 (37 in NAC group, 38 in placebo group) underwent early cardiac magnetic resonance imaging. Median duration of ischemia pretreatment was 2.4 hours. With background nitroglycerin infusion administered to all patients, those randomized to NAC exhibited an absolute 5.5% reduction in cardiac magnetic resonance-assessed infarct size relative to placebo (median, 11.0%; [interquartile range 4.1, 16.3] versus 16.5%; [interquartile range 10.7, 24.2]; P=0.02). Myocardial salvage was approximately doubled in the NAC group (60%; interquartile range, 37-79) compared with placebo (27%; interquartile range, 14-42; P<0.01) and median creatine kinase areas under the curve were 22 000 and 38 000 IU·h in the NAC and placebo groups, respectively (P=0.08).

CONCLUSIONS

High-dose intravenous NAC administered with low-dose intravenous nitroglycerin is associated with reduced infarct size in patients with ST-segment-elevation myocardial infarction undergoing percutaneous coronary intervention. A larger study is required to assess the impact of this therapy on clinical cardiac outcomes.

CLINICAL TRIAL REGISTRATION

Australian New Zealand Clinical Trials Registry. URL: http://www.anzctr.org.au/. Unique identifier: 12610000280000.

Postpartum coronary vasospasm with literature review

Acute myocardial infarction during pregnancy or the postpartum period is rare. We report a case of a 39-year-old postpartum woman who developed non-ST-elevation myocardial infarction due to severe diffuse coronary vasospasm. To our knowledge, this is the first case of angiographically evidenced coronary vasospasm, in a postpartum woman, with resistance to intracoronary nitroglycerin.

Hemodynamic action of captopril in coronary patients with heart failure tolerant to nitroglycerin

BACKGROUND

At present there is little dispute that clinical tolerance of organic nitrates occurs during long-term treatment of patients with stable angina pectoris and congestive heart failure.

HYPOTHESIS

Captopril exerts a favorable hemodynamic effect in coronary patients with heart failure who are clinically tolerant to nitroglycerin.

METHODS

Development of nitrate tolerance was observed during intravenous nitroglycerin treatment (10 mg/h) in 16 of 19 patients (7 women, 12 men; mean age 56 +/- 8 years) with coronary heart disease [stenosis > or = 75%, New York Heart Association (NYHA) classes II-III). The criterion applied was a loss of efficacy of at least 50% with regard to mean pulmonary capillary wedge pressure compared with the maximum effect of nitrate. The effect of captopril (50 mg p.o.) was determined in a blank test. Captopril (50 mg p.o.) was administered again at the stage of clinically manifest nitrate tolerance.

RESULTS

Compared with the effect of captopril alone, significantly more pronounced reductions in mean pulmonary capillary wedge pressure (33% compared with 27%) and in mean pulmonary arterial pressure (36% compared with 17%) and significantly greater increases in cardiac index (14% compared with 7%) and stroke work index (34% compared with 18%) (p < 0.05 in each case; Wilcoxon test for linked random samples) were measured. Maintaining nitroglycerin infusion, the effect of captopril (at least 90% of the maximum effect) lasted for 123 +/- 24 min. The baseline values (at least 75% decline in the effect of captopril) were only reached after 369 +/- 34 min.

CONCLUSION

The results document a favorable hemodynamic effect of captopril in nitrate tolerance which is significantly better than that of captopril alone.

Role of mitochondrial aldehyde dehydrogenase in nitroglycerin-induced vasodilation of coronary and systemic vessels: an intact canine model

BACKGROUND

It has recently been shown that mitochondrial aldehyde dehydrogenase 2 (mtALDH) catalyzes the formation of 1,2-glyceryl dinitrate and nitrite from nitroglycerin (glyceryl trinitrate [GTN]) within mitochondria, leading to production of cGMP and vasorelaxation. However, whether this mechanism operates in the systemic and coronary beds that subserve the antianginal action of GTN is not known. In this study, we address this question in an intact canine model.

METHODS AND RESULTS

Fourteen healthy mongrel dogs (weight, 20 to 25 kg) were studied. Coronary blood flow and hemodynamics were continuously monitored by a pulse Doppler flow probe implanted around the left circumflex coronary artery and with catheters in left ventricle and aorta, respectively. Each dog was given a 1-mL bolus injection of GTN, sodium nitroprusside (SNP), or adenosine through a catheter in the left atrium before and 30 minutes after infusion of cyanamide (17 mg/kg), an inhibitor of mtALDH. Cyanamide significantly inhibited both the classic dehydrogenase and GTN reductase activities of mtALDH in situ and attenuated the coronary blood flow increase and declines in blood pressure and left ventricular end-diastolic pressure produced by GTN in vivo. In contrast, mtALDH inhibition had no effect on the coronary and systemic effects of SNP and adenosine.

CONCLUSIONS

Our data suggest that mtALDH contributes to GTN biotransformation in vivo and thus at least partly underlies the antianginal mechanism of drug action. Our findings also highlight the differences in biometabolism of clinically relevant nitrosovasodilators.

Mechanisms of nitrate tolerance: potential roles of folate

More than 100 years since their introduction in cardiovascular therapy, nitrates continue to be widely used in ischaemic heart disease despite incomplete knowledge of their intimate mechanism of action. Particularly, the development of a progressive attenuation of their efficacy over prolonged use (tolerance) continues to be the subject of current investigation. Newer findings point to the role of increased intracellular oxidative stress as a mechanism for tolerance and to folic acid derivatives as pharmacologic means to attenuate its development. This paper reviews nitrate mechanism of action, the history of nitrate tolerance and newer findings related to the use of folate to prevent this phenomenon.

Wei Lun Visiting Professorial Lecture: Nitric oxide in the regulation of vascular function: an historical overview

The field of nitric oxide (NO) research has developed in explosive proportions since the discovery of endogenous NO in 1986. The biological importance of NO was first shown by the findings that nitroglycerin causes vasodilation by liberating NO in the smooth muscle, and activating guanylate cyclase to raise smooth muscle levels of cyclic GMP. NO also inhibits platelet aggregation by cyclic GMP mechanisms. NO activates guanylate cyclase by heme dependent mechanisms involving the formation of a nitrosyl-heme complex. The high pharmacological potency of NO was finally understood when NO was shown to be formed endogenously, and to be the same as EDRF. Based on these properties of NO, new drugs can be developed as vasodilators and antiplatelet agents for the treatment of a variety of vascular disorders including impotency. NO elicits many other actions in mammalian systems including inhibition of cell proliferation, airway bronchodilation, antimicrobial effects, other host defense effects, and also modulates learning and memory as well as other central functions. This allows for an extensive opportunity to develop novel drugs for the diagnosis, prevention, and treatment of a number of different diseases, many of which are vascular in origin.

Tolerance to nitroglycerin induced by isosorbide-5-mononitrate infusion in vivo

We examined the development of tolerance to nitroglycerin (glyceryl trinitrate; GTN) in the rat when isosorbide-5-mononitrate (ISMN) or GTN was continuously infused. Under pentobarbital anesthesia (60 mg/kg, i.p.), mean arterial blood pressure was measured via the left common carotid artery. Bolus injection of ISMN (0.25-250 mg/kg) and GTN (0.25 microg/kg-2.5 mg/kg) was made into the right external jugular vein. ISMN (2.5 mg/h/rat for 7 d), GTN (1.3 microg/h/rat for 7 d), or GTN (0.2 mg/h/rat for 3 d) was infused continuously using an osmotic pump embedded subcutaneously. Bolus injection of ISMN and GTN decreased arterial blood pressure in a dose-dependent manner. The hypotensive effect of ISMN was 2000 times less potent than that of GTN. The GTN-induced hypotensive effect was not affected after continuous infusion of ISMN, whereas it was attenuated after continuous infusion of GTN at either dose. Chronic treatment with ISMN does not induce GTN tolerance as easily as treatment with GTN.

Effects of N-acetylcysteine on nitroglycerin-induced relaxation and protein phosphorylation of porcine coronary arteries

We investigated the effects of the sulfhydryl-donor, N-acetylcysteine (NAC), on nitroglycerin (NTG)-induced relaxation of the vascular smooth muscle. Addition of histamine to isolated porcine coronary arteries induced an initial rapid contraction followed by a gradual decrease in tonic contraction. NTG applied to the coronary artery strips before histamine caused relaxation of the histamine-induced rapid (3 min) and tonic (48 min) contraction. The inhibition of the tonic contraction by NTG was less at 48 min than at 3 min. Application of NAC (NTG-NAC) enhanced the relaxing effects of NTG on the histamine-induced tonic contraction rather than the acute contraction. In phosphorylation studies, changes in the phosphorylation of an intermediate filament, desmin, were parallel with changes in contraction in NTG-treated and NTG-NAC samples at 48 min. These phosphorylation changes of desmin at 48 min, which might be responsible for tonic phase contraction, were more extensive than those of myosin light chain (MLC) phosphorylation at 3 min, which might be responsible for acute contraction. These results suggest that treatment with the sulfhydryl donor, NAC, inhibited the phosphorylation of desmin associated with the enhancement of NTG-induced relaxation, which might be related to the mechanisms of recovery from NTG tolerance by sulfhydryl groups.

Nitric oxide as a signaling molecule in the vascular system: an overview

In retrospect, basic research in the fields of nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) during the past two decades appears to have followed a logical course, beginning with the findings that NO and cGMP are vascular smooth muscle relaxants, that nitroglycerin relaxes smooth muscle by metabolism to NO, progressing to the discovery that mammalian cells synthesize NO, and finally the revelation that NO is a neurotransmitter mediating vasodilation in specialized vascular beds. A great deal of basic and clinical research on the physiologic and pathophysiologic roles of NO in cardiovascular function has been conducted since the discovery that endothelium-derived relaxing factor (EDRF) is NO. The new knowledge on NO should enable investigators in this field to develop novel and more effective therapeutic strategies for the prevention, diagnosis, and treatment of numerous cardiovascular disorders. The goal of this review was to highlight the early research that led to our current understanding of the pathophysiologic role of NO in cardiovascular medicine. Furthermore, we discussed the possible mechanism of some drugs interfering with NO signaling cascade.

Effect of nitroglycerin and nicorandil on regional poststenotic quantitative coronary blood flow in coronary artery disease: a combined digital quantitative angiographic and intracoronary doppler study

Little information is available concerning the effects of nitrates and potassium channel openers on local poststenotic blood flow in coronary artery disease (CAD). Combined quantitative digital angiography (QCA) and intracoronary Doppler (IVADO) velocity measurements were used to determine changes in absolute poststenotic blood flow after intracoronary injection of 0.2 mg nitroglycerin and 0.5 mg nicorandil. Quantitative blood flow (QBF) was calculated from average peak-flow velocity (APV) and angiographic cross-sectional area (CSA): QBF (ml/min) = CSA x APV x 0.5. In group I (n = 9), 0.5 mg nicorandil i.c. was identified as optimal to achieve maximal vasodilatation. In patients with CAD (group II, n = 12), i.c. injection of 0.5 mg nicorandil induced a significant increase in poststenotic CSA (+38%) and QBF (+50%). In contrast, 0.2 mg nitroglycerin (group III, n = 12) increases poststenotic CSA (+38%) without a significant change in QBF (+23%). Additional application of nicorandil in these patients induced further significant increases in CSA (+55%) and QBF (+48%) compared with baseline. There were no significant changes in stenosis area. Poststenotic blood flow can be increased by nicorandil after application of nitroglycerin. This effect is most likely mediated by the potassium channel-opening effect of nicorandil. Combined use of QCA and IVADO is a unique approach to measure local poststenotic QBF in patients with CAD.

Nitrate therapy is an alternative to furosemide/morphine therapy in the management of acute cardiogenic pulmonary edema

BACKGROUND

Nitrates are superior to furosemide in the management of acute pulmonary edema associated with myocardial infarction; however, their role in the absence of infarction is unclear.

METHODS AND RESULTS

A randomized comparison was undertaken of the relative effectiveness of primary therapy with either intravenous morphine/furosemide (men/women; n = 32) or nitroglycerin/N-acetylcysteine (NTG/NAC; n = 37) in consecutive patients with acute pulmonary edema. The primary end point was change in PaO2/FIO2 over the first 60 minutes of therapy. Secondary end points were needed for mechanical respiratory assistance (ie, continuous positive airway pressure via mask or intubation and ventilation) and changes in other gas exchange parameters. Both treatment groups showed improvement in oxygenation after 60 minutes of therapy; however, this reached statistical significance only with NTG/NAC therapy. There was no significant difference between groups in the assessed parameters (95% CI for differences in Pao2/FIO2: furosemide/morphine -12 to 23 and NTG/NAC 4 to 44), a finding also confirmed in 32 patients presenting with respiratory failure. Only 11% of the study group required mechanical ventilatory assistance (continuous positive airway pressure in 4 patients and intubation and ventilation in 3 patients).

CONCLUSIONS

NTG/NAC therapy is as effective as furosemide/morphine in the initial management of acute pulmonary edema, regardless of the presence or absence of respiratory failure. The necessity for mechanical ventilatory assistance is infrequent in these patients, regardless of the initial medical treatment regimen.

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.

The Utility of Nitric Oxide in the Postoperative Period

Inhaled nitric oxide (iNO) is a pulmonary-selective vaso dilator with minimal bronchodilator activity in humans. NO also inhibits platelet and neutrophil activation and adhesion and inhibits ischemia-reperfusion injury. The pulmonary vasodilatory property of iNO causes a reduc tion in pulmonary vascular resistance and improvement in arterial oxygenation in a wide spectrum of diseases characterized by pulmonary hypertension and hypox emia. Promising examples of diseases for which NO may provide beneficial physiologic effects are primary and secondary pulmonary hypertension, right ventricu lar failure, cardiac transplantation, pulmonary embo lism, protamine reactions, acute respiratory distress syndrome, lung transplantation and, perhaps, chronic obstructive airways disease. The usefulness of iNO may be improved by concomitant therapy with pulmonary- selective intravenous vasoconstrictors (eg, Almitrine; Vectarian, Neuilly, France) and cGMP phosphodiester ase V inhibitors (eg, Zaprinast; Research Biochemicals International, Natick, MA). Almitrine improves oxygen ation, synergistically with iNO, and may be useful in disease states characterized primarily by hypoxemia. Zaprinast may be useful for weaning iNO and avoidance of rebound pulmonary hypertension.

Long-term nitroglycerin treatment is associated with supersensitivity to vasoconstrictors in men with stable coronary artery disease: prevention by concomitant treatment with captopril

OBJECTIVES

We examined whether long-term nitroglycerin (NTG) treatment leads to an increase in sensitivity to vasoconstrictors. To assess a potential role of the renin-angiotensin system in mediating this phenomenon, we treated patients concomitantly with the angiotensin-converting enzyme (ACE) inhibitor captopril.

BACKGROUND

The anti-ischemic efficacy of organic nitrates is rapidly blunted by the development of nitrate tolerance. The underlying mechanisms are most likely multifactorial and may involve increased vasoconstrictor responsiveness.

METHODS

Forearm blood flow and vascular resistance were determined by using strain gauge plethysmography. The short-term responses to intraarterial angiotensin II (1, 3, 9 and 27 ng/min) and phenylephrine (an alpha-adrenergic agonist drug, 0.03, 0.1, 0.3 and 1 microg/min) were studied in 40 male patients with stable coronary artery disease. These patients were randomized into four groups receiving 48 h of treatment with NTG (0.5 microg/kg body weight per min) or placebo with or without the ACE inhibitor captopril (25 mg three times daily).

RESULTS

In patients treated with NTG alone, the maximal reductions in forearm blood flow in response to angiotensin II and phenylephrine were markedly greater (-64 +/- 3% and -53 +/- 4%, respectively) than those in patients receiving placebo (-41 +/- 2% and -42 +/- 2%, respectively). Captopril treatment completely prevented the NTG-induced hypersensitivity to angiotensin II and phenylephrine (-33 +/- 3% and -35 +/- 3%, respectively) but had no significant effect on blood flow responses in patients without NTG treatment (-34 +/- 2% and -37 +/- 3%, respectively).

CONCLUSIONS

We conclude that continuous administration of NTG is associated with an increased sensitivity to phenylephrine and angiotensin II that is prevented by concomitant treatment with captopril. The prevention of NTG-induced hypersensitivity to vasoconstrictors by ACE inhibition indicates an involvement of the renin-angiotensin system in mediating this phenomenon.

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.

Effect of transdermal nitroglycerin or N-acetylcysteine, or both, in the long-term treatment of unstable angina pectoris

OBJECTIVES

This study was designed to evaluate whether the addition of transdermal nitroglycerin or oral N-acetylcysteine, or both, to conventional medical therapy improves the natural history of unstable angina pectoris.

BACKGROUND

Transdermal nitroglycerin is widely used to treat angina pectoris, but the development of tolerance is a major problem that may reduce its clinical efficacy. It has been suggested that the addition of N-acetylcysteine to nitroglycerin reverses the development of tolerance, potentiates the hemodynamic response to nitroglycerin and may improve in-hospital prognosis in unstable angina.

METHODS

We assessed the efficacy of adding transdermal nitroglycerin or oral N-acetylcysteine, or both, to conventional medical therapy in a randomized, double-blind, placebo-controlled trial involving 200 patients with unstable angina who were followed up for 4 months.

RESULTS

Outcome events--death, myocardial infarction or refractory angina requiring revascularization--occurred in 31% of patients receiving nitroglycerin, 42% of those receiving N-acetylcysteine, 13% of those receiving nitroglycerin plus N-acetylcysteine and 39% of those receiving placebo (p = 0.0052). Kaplan-Meier curves showed a higher probability (p < 0.01) of no failure of medical treatment in the group receiving both nitroglycerin and N-acetylcysteine than in those receiving placebo, N-acetylcysteine or nitroglycerin alone. The combination of nitroglycerin and N-acetylcysteine was associated with a high incidence of side effects (35%), mainly intolerable headache, which was almost twice as frequent as in patients receiving nitroglycerin alone.

CONCLUSIONS

The combination of nitroglycerin and N-acetylcysteine, associated with conventional medical therapy in the long-term treatment of patients with unstable angina, reduces the occurrence of outcome events. However, the high incidence of side effects limits the clinical applicability of this therapeutic strategy at least at the dosage used in the present study.

Persistent inhibition of platelets during continuous nitroglycerin therapy despite hemodynamic tolerance

BACKGROUND

Nitroglycerin has been shown to possess antiplatelet properties in both animals and humans. Tolerance to the hemodynamic effects of nitroglycerin develops with continuous therapy, but it is unclear whether there is tolerance to its antiplatelet effect.

METHODS AND RESULTS

Tolerance to nitroglycerin was studied by exposing porcine aortic media to flowing arterial blood from control pigs (n = 9) or pigs treated with continuous nitroglycerin patches (Nitro-dur, 0.8 mg/h; n = 11) at a shear rate of 3397 s-1 for 3 minutes. Relative to baseline, mean arterial pressure fell by approximately 10% at 3 and 24 hours (P < .05) but returned to baseline at 48 hours of continuous nitroglycerin treatment, whereas no significant changes were observed in control animals. Autologous 51Cr-labeled platelet deposition (x 10(6)/cm2) on the aortic media at baseline and 3, 24, and 48 hours remained stable in control animals, with mean values of 94.8 +/- 5.9, 89.4 +/- 8.3, 89.3 +/- 8.8, and 84.3 +/- 5.7, respectively. However, in pigs treated continuously with nitroglycerin for 48 hours, platelet deposition was reduced significantly at 3 (65.9 +/- 4.8), 24 (63.8 +/- 6.4), and 48 hours (56.5 +/- 7.3) of nitroglycerin treatment compared with baseline (93.1 +/- 3.6). Platelet aggregation induced by thrombin also decreased at 3 (12.4 +/- 1.3), 24 (12.6 +/- 1.7), and 48 hours (10.8 +/- 1.6) of nitroglycerin treatment compared with baseline (16.3 +/- 1.4) but remained unchanged in the control group. Also, nitroglycerin treatment increased intraplatelet cGMP at 3, 24, and 48 hours compared with baseline.

CONCLUSIONS

This study demonstrates the persistent inhibition of platelet function and platelet deposition on an injured arterial wall by continuous nitroglycerin therapy despite hemodynamic tolerance.

N-acetylcysteine attenuates nitroglycerin tolerance in patients with angina pectoris and normal left ventricular function

The aim of this study was to assess whether N-acetylcysteine (NAC) is able to prevent tolerance to a 48-hour infusion of nitroglycerin (NTG) in the setting of normal left ventricular function. In 16 patients, the hemodynamic response to 0.8 mg sublingual (s.l.) NTG was assessed by measuring mean arterial, pulmonary artery, pulmonary capillary wedge and right atrial pressures, cardiac output, and calculation of the systemic and pulmonary vascular resistances. The parameters were obtained at baseline and 1 to 10 minutes after the s.l. NTG application (day 1). NTG was started at 1.5 microg/kg/min; concomitantly, a bolus of 2,000 mg of NAC was administered, followed by an infusion of 5 mg/kg/hour. Both infusions were continued for 48 hours, and the hemodynamic study was repeated (day 3). The same measurements were obtained in a matched control group of 15 patients with NTG infusion alone. Plasma renin activity, aldosterone, and norepinephrine were measured before and after the infusion period. The first s.l. NTG infusion (day 1) caused a significant decrease in mean arterial (p <0.01), pulmonary artery (p <0.001), and right atrial pressures (p <0.001), and in systemic (p <0.01) and pulmonary vascular resistances (p <0.001) in both groups. After the 48-hour infusion (day 3), there was a total loss of nitrate-mediated vasodilation (pressure values and vascular resistances day 3 > day 1) in 5 of 16 patients (NAC nonresponders), whereas in the other 11 of 16 patients (NAC responders), there was significant vasodilation throughout the infusion period. Tolerance had developed in 14 of 15 patients with NTG infusion alone. The same difference (responder vs nonresponder vs NTG alone) held true regarding the response to the second s.l. NTG infusion after 48 hours. The neurohormonal counter-regulation and intravascular volume expansion (increase in plasma renin activity, p <0.001, and norepinephrine, p <0.05; decrease in aldosterone, p <0.01) did not differ between responders and nonresponders. We conclude that NAC attenuates tolerance development to a continuous NTG infusion in a specific patient subgroup and that this occurs despite the same amount of neurohormonal counter-regulation and intravascular volume expansion compared with patients with tolerance development.

Effects of angiotensin-converting enzyme inhibitor alacepril in patients with stable effort angina during chronic isosorbide dinitrate treatment

Nitrate tolerance has been reported to be reversed by certain types of angiotensin-converting enzyme (ACE) inhibitors. We examined whether alacepril, a new long-acting oral ACE inhibitor, has beneficial effects against exercise-induced angina in patients with stable effort angina after substantial isosorbide dinitrate (ISDN) treatment. Thirteen men with stable effort angina were treated with oral ISDN (80 mg/d) for >3 weeks. After this period, efficacy of single oral administration of either alacepril (50 mg) or its placebo on exercise-induced angina and electrocardiographic changes was examined by treadmill exercise test in a double-blind crossover design. Alacepril significantly improved the exercise duration by 9.1% (p=0.03), the time to 1 mm ST-segment depression by 19% (p<0.01), and the maximal ST-segment depression by 33% (p=0.015) compared with placebo. Alacepril did not significantly alter the rate-pressure product, a marker of myocardial oxygen demand, during exercise test compared with placebo. Plasma renin activity was significantly increased (p<0.05) after administration of alacepril, indicating that alacepril significantly blocked ACE activity in our patients. In conclusion, a single oral administration of the ACE inhibitor alacepril (50mg) elicited beneficial effects against exercise-induced myocardial ischemia in patients with stable effort angina during chronic nitrate treatment. These effects may be mediated by increased coronary blood flow.

Nitrates in the treatment of congestive heart failure

Nitrates have been widely used for the treatment of patients with chronic congestive heart failure. Although the use of these drugs has not been evaluated by large-scale studies traditionally used for evaluation of new therapy, multiple studies over the years have demonstrated their favorable effects. Organic nitrates have been shown to have a beneficial effect on ischemia, hemodynamic profile, magnitude of mitral regurgitation, endothelial function, and cardiac remodeling. These drugs alone or in combination with hydralazine have improved exercise capacity, maximal oxygen consumption, cardiac function, and survival. The use of nitrates in patients with heart failure has been limited by reduced responsiveness (resistance) and early development of tolerance. Nitrate resistance is due to reduced vascular response and results in the need to use a larger dose of any nitrate preparation when used for the treatment of patients with heart failure compared to patients without heart failure. Recent information suggests that nitrate tolerance is caused by increased levels of superoxide at the vascular wall, which leads to reduced nitric oxide level and to increased sensitivity to vasoconstrictive mechanisms, such as endothelin and angiotensin II. Intermittent dosing of nitrates allowing a 12-hour nitrate-free interval is effective in preventing nitrate tolerance and is, therefore, recommended. Recent information suggests that augmentation of nitrate dose by the use of an escalating dose regimen and a concomitant use of hydralazine can prevent or overcome the effect of nitrate tolerance in patients with heart failure.

Nitric oxide and nitrovasodilators: similarities, differences, and interactions

The endothelium functions as a semipermeable membrane separating the blood from the body and allowing the transport of macromolecules from the blood to the interstitial space. The endothelium secretes a number of diffusible substances. These include endothelium-derived relaxing factor (EDRF), endothelium-derived hyperpolarizing factor (EDHF), and prostacyclin, in addition to vasoconstrictors including endothelin, angiotensin, and endothelium-derived contracting factor. EDRF is now known to be nitric oxide, or a closely related molecule, which affects signaling by stimulation of soluble guanylate cyclase, causing increased intracellular levels of cyclic guanosine monophosphate (cGMP), in turn leading to relaxation of vascular smooth muscle as well as a variety of additional effects that include altered function of platelets and cardiac myocytes. Nitric oxide can be made available to cellular elements in two ways: by endogenous synthesis via one or more of the three nitric oxide synthases now known to exist in mammalian species; or by exogenous administration of pharmacologic sources of nitric oxide, usually as organic nitrate vasodilators that can be metabolically converted to biologically activated nitric oxide. This process appears to require free sulfydryl groups. The metabolic machinery necessary to convert organic nitrates to a biologically active form exists mainly in the vasculature and not in the myocardium. Numerous studies have demonstrated that the presence of coronary artery disease is associated with interruption of the endogenous production of nitric oxide. Under these circumstances, exogenous nitrates still produce coronary vasodilation as well as relaxation of vascular smooth muscle in the periphery. Other articles in this supplement will focus on the vascular effects of nitric oxide and nitrovasodilators; this article will conclude with a brief discussion of the role of the nitric oxide pathway in the control of cardiac autonomic responsiveness and the potential role of cytokines and the nitric oxide pathway to impair the ability of the myocardium to respond to catecholamines or other stimuli with a normal increase in contractile function.

Tolerance to nitroglycerin in rabbit aorta. Investigating the involvement of the mu isozyme of glutathione S-transferases

We have proposed that glutathione S-transferases (GSTs), especially the mu isozyme, play a critical role in the metabolism of nitroglycerin (glyceryl trinitrate, GTN), leading to pharmacologic effects. Here we study this enzyme(s) during tolerance development in male New Zealand white rabbits. Each aorta was divided into two segments designated as GTN pretreated and buffer control. Tolerance was induced in rabbit aortic strips so assigned by incubation with GTN (0.22 mM). The activity of the mu isozyme and of total GSTs was determined in portions f each segment. In each rabbit aorta, the response to GTN (0.5 microM) was determined in GTN-pretreated and buffer-pretreated strips by measuring cyclic GMP levels (N = 7 pairs) and percent relaxation (N = 4 pairs). In GTN-pretreated strips, a significant decrease was observed in the activity of the mu isozyme of GST, while the total GST activity was unchanged as compared with control strips. The decrease in isozyme activity correlated very well with the decrease in response to GTN. Two rabbit aortae did not become tolerant, and the activity of the mu isozyme was also not affected. The levels of thiols were not affected by GTN pretreatment and aortae tolerant to GTN did not develop tolerance to S-nitroso acetylpenicillamine (SNAP), indicating that thiol depletion and guanylate cyclase desensitization probably play a minor role in tolerance development to GTN in our model. These studies suggest that tolerance to GTN in rabbit aorta in vitro is associated with a decrease in GST mu activity, which correlates well with the decrease in GTN response.

Infarct Size Limitation: acute N-acetylcysteine defense (ISLAND trial): preliminary analysis and report after the first 30 patients

Our previous experimental research and initial clinical observations regarding the use of N-acetylcysteine in the treatment of ischemic and reperfusion injury in acute myocardial infarction gave rise to a study entitled the Infarct Size Limitation: Acute N-acetylcysteine Defense (ISLAND) trial. Today, this randomized, echocardiographically and angiographically controlled study includes the first 30 patients with a first anterior wall myocardial infarction: Group A (n = 10) consisting of patients with successful recanalization of the infarct-related left anterior descending artery by streptokinase without any further treatment, Group B (n = 10) consisting of patients with failed infarct-related artery recanalization, and Group C (n = 10) comprising patients who had successful streptokinase-induced recanalization of the left anterior descending artery plus N-acetylcysteine administration at a dose of 100 mg/kg body weight. The parameters monitored in our study include changes in global and regional left ventricular ejection fraction of the infarct-related segment using echocardiography and, using electrocardiograms and the Wagner QRS scoring system, the amounts of acutely jeopardized and finally infarcted myocardium. In Group A, global left ventricular ejection fraction rose nonsignificantly within 2 weeks from 37.5 +/- 9.6% to 38.5 +/- 13.8%; it declined significantly in Group B from 36.2 +/- 6.1% to 30.1 +/- 6.7% (p < 0.05), while it considerably improved in Group C from 41.7 +/- 4.1% to 59.6 +/- 8.1% (p < 0.001). Regional left ventricular ejection fraction changed significantly only in Group C: from -4.5 +/- 27.3 to 45.6 +/- 16.3 (p < 0.001). In Group A, in which the amount of acutely jeopardized myocardium was 21.7 +/- 7.2, infarction actually occurred in 20.4 +/- 9.7% (practically no myocardial salvage). In Group B, risk area was 18.1 +/- 4.3%, but infarct size rose to a resulting 29.1 +/- 6.0%. Significant myocardial salvage was accomplished only in Group C: of 26.2 +/- 8.1% of jeopardized myocardium, infarct size was reduced to 10.8 +/- 7.1% (salvage by 58.8%). Also, basic division of patients by therapy showed that, although those with nonidentical findings on their coronary arteries were included into the same groups, patients treated with streptokinase plus N-acetylcysteine had significantly more favorable values of the monitored parameters than those treated with streptokinase alone. We conclude our interim analysis suggests that N-acetylcysteine has a beneficial effect, reducing the functional and structural impacts of myocardial infarction.

Influence of captopril on nitroglycerin-mediated vasodilation and development of nitrate tolerance in arterial and venous circulation

We investigated whether captopril is able to potentiate vasodilation and prevent tolerance to a 48-hour infusion of nitroglycerin (NTG). Twenty-six patients were randomly assigned to a 7-day regimen of captopril (50 mg/day) or placebo. The hemodynamic response to a 0.8 mg sublingual NTG dose was assessed by measuring mean arterial pressure (MAP), mean pulmonary artery pressure (PAP), pulmonary capillary wedge pressure (PCWP), right atrial pressure (RAP), and cardiac output (CO), and calculating systemic (SVR) and pulmonary vascular resistances (PVR). The parameters were obtained serially at baseline and 1 to 10 minutes after the sublingual NTG application (day 1). Then intravenous NTG was started and maintained for 48 hours (1.5 micrograms/kg/min), and the hemodynamic study was repeated (day 3). There was no difference between the captopril and the placebo groups at day 1 (baseline values and response to sublingual NTG). After the 48-hour infusion, there was a complete loss of the NTG effects in the placebo group (day 1 vs day 3: PAP, 20 +/- 5 mm Hg vs 21 +/- 8 mm Hg; MAP, 86 +/- 11 mm Hg vs 90 +/- 9 mm Hg; SVR, 1295 +/- 330 mm Hg vs 1380 +/- 465 dyne.sec.cm-5) whereas there was still evidence of a persistent vasodilation in the captopril group (day 1 vs day 3: PAP, 19 +/- 4 mm Hg vs 13 +/- 4 mm Hg; MAP, 84 +/- 9 mm Hg vs 74 +/- 10 mm Hg; SVR, 1265 +/- 280 mm Hg vs 1140 +/- 425 dyne.sec.cm-5). The response to sublingual NTG on day 3 was markedly attenuated in the placebo group only. We conclude that captopril does not increase the vasodilatory response to nitroglycerin but is able to prevent developing nitrate tolerance in arterial and venous circulation.

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.

N-acetylcysteine in combination with nitroglycerin and streptokinase for the treatment of evolving acute myocardial infarction. Safety and biochemical effects

BACKGROUND

N-acetylcysteine (NAC) has been shown to potentiate the effects of nitroglycerin (NTG) and to have antioxidant activity. This is the first study to assess the safety and effect of NAC in the treatment of evolving acute myocardial infarction (AMI).

METHODS AND RESULTS

Patients with AMI received either 15 g NAC infused over 24 hours (n = 20) or no NAC (n = 7), combined with intravenous NTG and streptokinase. Peripheral venous plasma malondialdehyde (MDA), reduced (GSH) and oxidized (GSSG) glutathione concentrations, and rate of reperfusion (using continuous ST-segment analysis) were measured. Cardiac catheterization was performed between days 2 and 5. No significant adverse events occurred. Less oxidative stress occurred in patients treated with NAC than in patients not receiving NAC (GSH to GSSG ratio 44 +/- 25 versus 19 +/- 13 at 4 hours, P < .05). NAC concentration (mean 172 +/- 79 mumol/L at 4 hours) was correlated to GSH concentration (P = .006). MDA concentrations were lower (P = .001) over the first 8 hours of treatment with NAC. There was a trend toward more rapid reperfusion (median 58 minutes, 95% confidence interval [CI] 48 to 98 minutes versus median 95 minutes, 95% CI 59 to 106 minutes; P = .17) and better preservation of left ventricular function (cardiac index 3.4 +/- 0.8 versus 2.6 +/- 0.27 L.min.m2, P = .009) with NAC treatment.

CONCLUSIONS

NAC in combination with NTG and streptokinase appeared to be safe for the treatment of evolving AMI and was associated with significantly less oxidative stress, a trend toward more rapid reperfusion, and better preservation of left ventricular function.

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.

Evidence for enhanced vascular superoxide anion production in nitrate tolerance. A novel mechanism underlying tolerance and cross-tolerance

We sought to examine mechanisms underlying nitroglycerin (NTG) tolerance and "cross-tolerance" to other nitrovasodilators. Rabbits were treated for 3 d with NTG patches (0.4 mg/h) and their aortic segments studied in organ chambers. Relaxations were examined after preconstriction with phenylephrine. In NTG tolerant rabbit aorta, relaxations to cGMP-dependent vasodilators such as NTG (45 +/- 6%), SIN-1 (69 +/- 7%), and acetylcholine (ACh, 64 +/- 5%) were attenuated vs. controls, (90 +/- 2, 94 +/- 3, and 89 +/- 2% respectively, P < 0.05 for all), while responses to the cAMP-dependent vasodilator forskolin remained unchanged. In tolerant aorta, endothelial removal markedly enhanced relaxations to NTG and SIN-1 (82 +/- 4 and 95 +/- 3%, respectively). Other studies were performed to determine how the endothelium enhances tolerance. Vascular steady state .-O2 levels (assessed by lucigenin chemiluminescence) was increased twofold in tolerant vs. control vessels with endothelium (0.31 +/- 0.01 vs. 0.61 +/- 0.01 nmol/mg per minute). This difference was less in vessels after denudation of the endothelium. Diphenylene iodonium, an inhibitor of flavoprotein containing oxidases, and Tiron a direct .-O2 scavenger normalized .-O2 levels. In contrast, oxypurinol (1 mM) an inhibitor of xanthine oxidase, rotenone (50 microM) an inhibitor of mitochondrial electron transport and NG-nitro-L-arginine (100 microM) an inhibitor of nitric oxide synthase did not affect the chemiluminescence signals from NTG-tolerant aortas. Pretreatment of tolerant aorta with liposome-entrapped, pH sensitive superoxide dismutase (600 U/ml) significantly enhanced maximal relaxation in response to NTG, SIN-1, and ACh, and effectively reduced chemiluminescence signals. These studies show that continuous NTG treatment is associated with increased vascular .-O2-production and consequent inhibition of NO. mediated vasorelaxation produced by both exogenous and endogenous nitrovasodilators.

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.

Effects of continuous intravenous infusion of isosorbide dinitrate on development of tolerance to vasodilating action in human epicardial coronary arteries

This study was performed to determine the effects of long-term intravenous infusion on the coronary vasodilating actions of continuous intravenous and bolus intracoronary administration of isosorbide dinitrate (ISDN). With quantitative coronary angiography, the coronary diameter and the vasodilating response to intracoronary ISDN (1 mg) at angiographically normal segments were studied before and after intravenous administration of ISDN, 10 to 60 micrograms/min for 1 hour, 2 days, or 5 days. The vasodilating effects of intravenous ISDN were 72% +/- 13%, 65% +/- 21%, and 6% +/- 11% of the response to intracoronary ISDN in the baseline study in each group. Irrespective of the duration of intravenous infusion, subsequent intracoronary ISDN dilated coronary arteries to extent similar to that observed in each baseline study. In conclusion, significant coronary vasodilating effects of intravenous ISDN were observed after a 2-day infusion, whereas tolerance to the vasodilating effects apparently developed within 5 days of infusion. The vasodilating response to bolus intracoronary ISDN was preserved even when the vasodilating effects of intravenous ISDN were no longer present.

Short and long-acting oral nitrates for stable angina pectoris

Nitroglycerin (NTG) spray and sublingual tablets rapidly relieve an established attack of angina, and their infrequent use is not associated with the development of tolerance. Although, following a suitable nitrate-free interval, the first dose of oral, long-acting nitrates produces significant hemodynamic effects, increases angina free walking, and decreases exercise-induced ischemia, during continued long-term therapy tolerance limits their usefulness. Appropriate dosing regimens of controlled-release formulations of isosorbide dinitrate (ISDN) and controlled-release NTG during long-term therapy have not been established. Use of immediate-release formulation of 15-120 mg of ISDN in a qid regimen lead to a marked reduction in the size and duration of antianginal effects compared to the initial dose. Asymmetric tid therapy with 30 mg of ISDN (7 a.m., 1 p.m., and 6 p.m.) is also associated with the development of partial tolerance and appears to provide antianginal prophylaxis for only a period of 6 hours each day. Asymmetric bid therapy with ISDN at 7 a.m. and noon may give sustained effect but is supported by only a single, small study that did not examine effectiveness after the noon dose in long-term use. Isosorbide-5-mononitrate (IS-5-MN) has been the subject of more recent studies than other nitrates because of attempts to bring a number of products into the U.S. market. IS-5-MN in qid, tid, and standard bid (8 a.m. and 8 p.m.) dosing regimens produce tolerance. Asymmetric regimens of immediate-release IS-5-MN (10 and 20 mg) given bid (once in the morning and again 7 hours later) decrease the development of tolerance compared to symmetric regimens and produce an increased exercise duration after each dose of the day; the 20 mg bid dosing is more effective. Similarly, once-daily 120 and 240 mg controlled-release IS-5-MN does not produce tolerance and gives a sustained increase in daytime exercise duration. Both asymmetric bid immediate-release and once-daily controlled-release IS-5-MN preparations do not produce deterioration in exercise performance prior to the administration of the medication in the morning (i.e., no zero-hour effect). Further studies are needed to establish useful dosing regimens for ISDN, for controlled-release ISDN, and for controlled-release nitroglycerin. None of the dosing regimens of any oral, long-acting nitrate (including IS-5-MN) provide 24 hour antianginal and antiischemic effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Phenomenon of nitrate tolerance

The phenomenon of nitrate tolerance has now been appreciated for almost a century, and our understanding of this process has greatly improved during the past 20 years. Therapeutic nitrates are now recognized as exogenous sources of nitric oxide (or nitrosothiols), which appears to be a primary mediator of natural vasodilatation. Nitrates have been clearly shown to have vasodilatory and antiplatelet effects, both of which diminish during continuous exposure. Nitrate tolerance has been documented with most nitrate preparations when the patient is given continuous nitrate therapy. Tolerance to nitrates may occur in any patient, regardless of underlying illness, medication dose, or serum concentration of NTG. The cause of this phenomenon is multifactorial; there appear to be both cellular and systemic processes involved. To date, no adjuvant pharmacologic intervention has conclusively demonstrated benefit in preventing, abating, or reversing nitrate tolerance. Interruption of nitrate exposure for as little as 8 to 12 hours does appear to be the best means of preventing or reversing tolerance. Nevertheless, some patients with objective tolerance continue to experience relief of symptoms. In addition, despite laboratory-documented cross-tolerance, patients receiving continuous nitrate therapy at usual clinical doses may continue to benefit from the hemodynamic and antianginal effects of SL NTG. Hence, nitrate tolerance is a real entity, but the clinical importance of this phenomenon remains controversial. Finally, further investigation will need to address quality-of-life issues and perhaps assess relief of ischemia by other means.

Potentiation of isosorbide dinitrate effects with N-acetylcysteine in patients with chronic heart failure

BACKGROUND

Supply of sulfhydryl groups with the administration of N-acetylcysteine (NAC) has been reported to reverse tolerance to nitroglycerin but not to isosorbide dinitrate (ISDN). Lack of interaction between NAC and ISDN was suggested as an explanation for these findings. The present study was therefore designed to further evaluate this hypothesis. For this purpose, we compared the hemodynamic and hormonal effects of ISDN when given alone and in combination with NAC.

METHODS AND RESULTS

We performed a randomized, cross-over design evaluation of the hemodynamic and hormonal effects of ISDN and ISDN + NAC in 14 patients with chronic congestive heart failure due to left ventricular systolic dysfunction. The findings of this study demonstrated a substantial NAC-mediated potentiation of ISDN effect on mean right atrial pressure (-11 +/- 21% versus -38 +/- 27%, -17 +/- 20% versus -34 +/- 27%, and -7 +/- 20% versus -25 +/- 26% at 2, 3, and 4 hours, respectively; all P < .05), mean pulmonary artery wedge pressure (-18 +/- 16% versus -33 +/- 14%, -15 +/- 25% versus -33 +/- 19%, -14 +/- 22% versus -25 +/- 22%, and -16 +/- 16% versus -26 +/- 16% at 2, 3, 4, and 5 hours, respectively; all P < .05), mean pulmonary artery pressure (-8 +/- 11% versus -20 +/- 15% at 3 hours, P < .05), and cardiac output (an increase of 2 +/- 16% versus 25 +/- 20% at 4 hours, P < .05). Although there were no significant changes in serum catecholamine levels and plasma renin concentration with both regimens, ISDN + NAC resulted in a greater fall in plasma levels of atrial natriuretic peptide (296 +/- 251 pg/mL after ISDN versus 202 +/- 118 pg/mL after ISDN + NAC, P < .05).

CONCLUSIONS

The results of this study provide strong evidence for the existence of an interaction between thiols and ISDN and further support the role of sulfhydryl groups in the activation and therapeutic action of organic nitrates. The discrepancy between the results of this study demonstrating NAC-induced potentiation of ISDN effects and a previous study showing failure to reverse ISDN tolerance with NAC may suggest that ISDN-NAC interaction requires normal intracellular levels of sulfhydryl groups and does not occur after intracellular sulfhydryl group depletion.

Mechanisms of nitrate tolerance

There is now little dispute that clinical tolerance of organic nitrates occurs, particularly when these drugs are used by themselves to treat patients with stable angina pectoris and congestive heart failure. Classical hypotheses of nitrate tolerance suggest the phenomenon to result from vascular depletion of critical sulfhydryl groups, which are necessary to bring about vasorelaxation from nitrates. While this mechanism of nitrate tolerance probably operates when isolated blood vessels are exposed to high concentrations of nitrate in vitro, there is little evidence to suggest that it contributes to clinical nitrate tolerance. Instead, emerging data suggest that nitrates can cause significant shifts in fluid distribution and secretion of neurohormonal factors that can modulate their vasorelaxant effects. Use of angiotensin converting enzyme inhibitors and diuretics in conjunction with nitrates may alleviate the development of tolerance, but the experience has not been universally favorable. Other receptor-effector systems that affect cardiovascular function, such as the adrenergic system, may also be affected by nitrate tolerance. The mechanisms of nitrate tolerance are therefore likely to be multifactorial, involving vascular biochemical changes, physiologic compensation, and possibly receptor regulation.

Nitroglycerin inhibits experimental thrombosis and reocclusion after thrombolysis

Nitroglycerin inhibits platelet aggregation in vitro, but its effect on thrombosis and platelet function in vivo is controversial. This study assessed the effect of nitroglycerin on primary thrombus formation in response to vessel wall injury and secondary thrombus formation, or rethrombosis, after lysis of an existing thrombus. In the first protocol the right carotid artery was instrumented with a flow probe, stenosis, an anodal electrode, and a proximal infusion line. A 300 microA anodal current was used to induce endothelial injury and subsequent thrombotic occlusion of the vessel. Anisoylated plasminogen streptokinase activator complex (APSAC; 0.05 U/kg intraarterially) was injected proximal to the thrombus 30 minutes after occlusion. After APSAC, nitroglycerin (1 microgram/kg/min intraarterially, n = 7) or vehicle (n = 6) was infused proximal to the thrombus for 3 hours. Reocclusion occurred in two of seven nitroglycerin-treated dogs and six of six vehicle-treated dogs (p < 0.05). In the second protocol both carotid arteries were instrumented as described previously. Anodal current (300 microA, 180 minutes) was applied to the right carotid (n = 12) artery to determine control times to occlusion. The left carotid artery served as the test vessel, receiving either nitroglycerin (1 microgram/kg/min intraarterially, n = 6) or trimethaphan (0.05 mg/kg/hr intraarterially, n = 6). Trimethaphan was used to produce controlled hypotension to match the approximately 10% decrease in mean arterial blood pressure that was observed during nitroglycerin infusion. Control arteries and those treated with trimethaphan formed occlusive thrombi in all instances. Nitroglycerin infusion resulted in a lower incidence of occlusion (1 of 6; p < 0.05 vs control value) and inhibited ex vivo platelet aggregation to adenosine diphosphate and arachidonic acid (p < 0.05). Local infusion of nitroglycerin reduced the formation of primary thrombi, independent of the hypotensive effect of the drug, and exerted systemic effects on platelet aggregation. Furthermore, platelet inhibition with nitroglycerin reduced the incidence of secondary thrombus formation (rethrombosis) after thrombolysis. The results suggest that a potential benefit of nitroglycerin therapy may be derived from its ability to inhibit thrombotic events in patients with unstable angina or myocardial infarction.

S-nitrosothiol modulation of tolerance to glyceryl trinitrate in bovine isolated coronary artery

The aim of the study was to ascertain whether induction of tolerance to glyceryl trinitrate (GTN) was modified by nitrosothiols. Vasodilator responses to GTN and to two nitrosothiols, namely S-nitroso-N-acetylpenicillamine (SNAP) and S-nitroso-N-acetylcaptopril (SNOCAP) were measured in bovine coronary artery rings constricted with the thromboxane mimetic agent U46619. Pre-exposure of vessels to SNAP (10 microM) for 1 h resulted in cross-tolerance to GTN comparable with tolerance induced by GTN (1.0 microM). Tolerance was increased after pre-exposure to both agents. SNAP alone in lower concentrations (0.01, 0.1 and 1.0 microM) did not produce cross tolerance to GTN, but in combination with GTN (1.0 microM) caused a small but significant decrease in tolerance to GTN. This attenuating effect of SNAP on GTN tolerance was reproduced by SNOCAP (0.01 microM). We conclude that the nitrosothiols exert a biphasic effect on induction of tolerance to GTN.