Prevention and reversal of nitrate tolerance in patients with congestive heart failure

Comparison of the hemodynamic responses to molsidomine and isosorbide dinitrate in congestive heart failure

To evaluate the mechanisms involved in nitrate tolerance, we randomized 23 patients with congestive heart failure resulting from coronary artery disease to an isosorbide dinitrate or a molsidomine infusion. The drugs were titrated to decrease pulmonary capillary wedge pressure by > or = 30% or > or = 10 mm Hg. Then isosorbide dinitrate, molsidomine, or placebo was infused in a double-blind randomized manner for 24 hours. In all patients, treatment with enalapril was begun > or = 48 hours before the beginning of the protocol and was continued throughout the study to avoid renin-angiotensin activation. The pulmonary capillary wedge pressure remained significantly decreased at 24 hours during molsidomine infusion only. No significant increase in catecholamines occurred. Because molsidomine differs from organic nitrates by its property of directly stimulating guanylate cyclase without depending on thiol group availability, these results suggest that impaired biotransformation of nitrates is involved in tolerance induced by high doses of isosorbide dinitrate in congestive heart failure.

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.

Haemodynamic effects of glyceryl trinitrate during continuous 24 hour infusion in patients with heart failure

OBJECTIVE

To investigate whether the susceptibility to tolerance to glyceryl trinitrate is similar in different vascular beds in patients with chronic heart failure.

PATIENTS

Twenty patients with heart failure underwent a continuous infusion of glyceryl trinitrate over 24 hours followed by administration of N-acetylcysteine (5 g intravenously) in a bolus.

MAIN OUTCOME MEASURES

Haemodynamic measurements under control conditions, at peak titration of glyceryl trinitrate at 24 hours, and after N-acetylcysteine; plasma renin activity and packed cell volume.

RESULTS

After 24 hours of infusion the acute reduction in right atrial pressure had largely waned, while pulmonary vascular resistance remained improved and systemic resistance, which was not reduced at peak titration, significantly decreased with respect to control conditions. The effects of N-acetylcysteine and hormonal responses were different in patients who did and did not develop tolerance to glyceryl trinitrate.

CONCLUSIONS

The haemodynamic profile of glyceryl trinitrate changed substantially during the study from a predominantly venodilator action at peak titration to a predominantly arteriolar dilatation after 24 hours of infusion. The different effects of N-acetylcysteine and the different hormonal responses confirm the multifactorial pathogenesis of tolerance to glyceryl trinitrate.

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.

Nitrates in congestive heart failure

Nitrates are commonly used in the therapy of congestive heart failure (CHF). They exert beneficial hemodynamic effects by decreasing left ventricular filling pressure and systemic vascular resistance while modestly improving cardiac output. The improvement in left ventricular function caused by nitrates is the result of combined reduction in outflow resistance and mitral regurgitation, while decreased pericardial constraint and subendocardial ischemia may also contribute to the process. With continuous nitrate administration, complete arterial tolerance develops, while venous tolerance appears to be only partial. The major mechanism of tolerance is loss of vascular smooth muscle sensitivity to nitrates. An increase in total blood volume occurring during the first few hours of an acute administration may partly contribute to tolerance. The importance of reflex neurohumoral activation is controversial; although it may contribute to tolerance in CHF, its role does not appear to be major. Chronic continuous nitrate therapy in CHF improves submaximal and maximal exercise tolerance. In combination therapy with hydralazine, isosorbide dinitrate reduces mortality, although to a lesser extent than the angiotensin converting enzyme inhibitor enalapril. Intravenous or sublingual nitrates are first-line agents in the therapy of acute pulmonary edema. In severe CHF, refractory to standard medical therapy, a short course of intravenous nitroglycerin, with or without inotropic agents, can help break the vicious spiral of CHF. Because tolerance occurs without nitrate-free intervals and until an optimal schedule of administration is determined, it makes good sense to include a nightly nitrate-free interval when prescribing nitrates for CHF in order to maintain maximal benefit during the hours of activity.

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.

Pharmacodynamic models of nitroglycerin-induced hemodynamic tolerance in experimental heart failure

Pharmacodynamic tolerance during continuous nitroglycerin (NTG) infusion is a significant limitation of nitrate therapy. The mechanism of this phenomenon is not well-understood but may involve physiologic compensation which involves vasoconstriction. We have obtained pharmacodynamic data on NTG-induced hemodynamic tolerance in a rat model of congestive heart failure (CHF), which we have shown to mimic human behavior toward NTG in vivo. In this report, we developed two mechanism-based pharmacokinetic/pharmacodynamic models to describe the time-dependent effects of NTG infusion on left ventricular end-diastolic pressures (LVEDP) in CHF rats and compared their abilities to describe the observed hemodynamic data. Both mathematical models introduced a counter-regulatory vasoconstrictive effect as a result of NTG-induced vasodilation and assumed the magnitude of this effect to be driven by the extent of the initial hemodynamic effect produced by NTG. The decay of this counter-regulatory effect was described by a first-order process in both models. A model that assumed vasoconstriction to develop via two sequential first-order processes was statistically superior in describing the data, when compared to one that assumed a single first-order process and a lag phase. Both models provided similar estimates of the half-life for the disappearance of the vasoconstriction (t1/2 of vasoconstriction: 128min vs. 182min, respectively), and both predicted rebound elevation of LVEDP after abrupt NTG withdrawal. These results are consistent with a counter-regulatory mechanism of NTG-induced hemodynamic tolerance and suggest that such an approach may be useful for modeling other tolerance phenomena as well.

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.

Effect of prolonged nitrate therapy on left ventricular remodeling after canine acute myocardial infarction

BACKGROUND

Prolonged nitrate therapy during healing between 2 days and 6 weeks after anterior myocardial infarction has the potential for limiting further left ventricular remodeling (or changes in topography) and preserving function. Longterm therapy throughout healing over 6 weeks might be more beneficial than short-term therapy over the first 2 weeks after infarction.

METHODS AND RESULTS

The effect of prolonged nitrate therapy between 2 days and 6 weeks during healing after infarction on serial parameters of ventricular remodeling (scar expansion, scar thinning, ventricular dilation, and hypertrophy) and function (asynergy or akinesis plus dyskinesis and ejection fraction) by serial two-dimensional echocardiography, hemodynamics, postmortem topography (computerized planimetry, geometric maps, and radiographs), and collagen content (hydroxyproline) was studied in 64 instrumented dogs randomized 2 days after left anterior descending coronary artery ligation to various nitrate regimens (n = 32) over the first 2 weeks (subgroup 1: 2% transdermal nitroglycerin at 8 AM and 4 PM, n = 6; subgroup 2: 2% transdermal nitroglycerin plus 2.6 mg of sustained-release oral nitroglycerin at 8 AM, 3 PM, and 10 PM, n = 5; subgroup 3: oral isosorbide dinitrate, 30 mg at 8 AM and 4 PM, n = 11) or 6 weeks (subgroup 4: isosorbide dinitrate, n = 10) and in matching controls (n = 32). Nitrate therapy reduced left atrial pressure, mean arterial pressure, and the rate-pressure product compared with controls over the 6 weeks. Postmortem scar mass and hydroxyproline were similar in control and nitrate groups. However, scar stretching and thinning, cavity dilation, noninfarct wall hypertrophy, and apical bulging were less with nitrates, especially in the long-term subgroup 4. In vivo remodeling parameters between 2 days and 6 weeks after ligation showed that, compared with controls, nitrate therapy prevented further stretching of the asynergic segment, decreased the expansion index, decreased further scar thinning, prevented the increase in ventricular volumes, reduced the frequency of ventricular aneurysm, prevented the increase in ventricular mass, reduced the extent of asynergy, and improved ejection fraction. Although the beneficial effect on topography and function was seen in all nitrate subgroups, the overall benefit was greater with long-term therapy over 6 weeks (subgroup 4) than short-term therapy confined to the first 2 weeks (subgroups 1, 2, and 3).

CONCLUSIONS

Prolonged nitrate therapy, in various regimens during healing after infarction, effectively reduced left ventricular loading and prevented infarct thinning, further infarct expansion, progressive ventricular dilation, and the increase in mass. These effects were associated with decreased asynergy and improved ejection fraction. The beneficial effects were greater with long-term therapy over 6 weeks than short-term therapy over the first 2 weeks.

Absence of hemodynamic tolerance to nicorandil in patients with severe congestive heart failure

To evaluate whether hemodynamic tolerance develops to nicorandil, a nitrate and potassium channel opener, 14 patients with chronic heart failure (CHF) were treated with nicorandil and 11 were treated with nitroglycerin (GTN). Doses of GTN or nicorandil were titrated to achieve a > or = 20% reduction in pulmonary capillary wedge pressure (PCWP) within 1 hour, and the infusion was maintained at a constant rate for 24 hours. Both groups of patients had comparable hemodynamic parameters before drug infusions were started. The fall in PCWP was identical after 1 hour infusion of either GTN or nicorandil. In the GTN group, PCWP was not significantly different from the baseline value at 12 hours; however, in the nicorandil group, PCWP remained significantly lower than the preinfusion value for 24 hours. During the study period changes in plasma atrial natriuretic peptide (ANP) concentrations paralleled and correlated with changes in PCWP (r = 0.84, p < 0.001). These findings indicate that CHF patients develop hemodynamic tolerance to GTN within 12 hours of continuous infusion, but not to nicorandil, which remained hemodynamically effective during the 24-hour period of infusion. Furthermore, plasma ANP concentration may be a useful noninvasive index of hemodynamic tolerance during GTN or nicorandil therapy in patients with CHF.

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.

Enhancement of endothelium-dependent vasodilation by low-dose nitroglycerin in patients with congestive heart failure

BACKGROUND

Since organic nitroesters and endothelium-derived nitric oxide mediate vasodilation through a final common pathway, that is, by activation of soluble guanylate cyclase in vascular smooth muscle, nitroglycerin (NTG) could specifically enhance the endothelium-dependent vasodilatory response to acetylcholine (Ach) in patients with congestive heart failure (CHF) and endothelial cell dysfunction. Accordingly, the net effects of an intra-arterial infusion of NTG (10(-9) mol/L) on endothelium-dependent and endothelium-independent vasodilation were assessed in the forearm circulation of patients with CHF.

METHODS AND RESULTS

The forearm blood flow responses to intra-arterial administration of graded concentrations of Ach (10(-7) to 10(-5) mol/L) were determined by venous occlusion plethysmography (mL/min per 100 mL) in 18 patients with CHF and 5 age-matched normal subjects before and during intra-arterial infusion of NTG (10(-9) mol/L) for 20 minutes. In eight patients, the duration of the infusion of NTG (n = 5) or vehicle control solution (n = 3) was extended to 12 hours with measurement of the forearm blood flow responses to Ach at 20 minutes, 4 hours, and 12 hours. In five additional patients, forearm blood flow response to intra-arterial administration of two doses of phentolamine (0.05 and 0.5 mg) were determined before and during a 20-minute NTG infusion. Regional administration of NTG 10(-9) mol/L did not change resting forearm blood flow in either normal subjects or patients with CHF. Before administration of NTG 10(-9) mol/L, intra-arterial infusions of Ach 10(-7), 10(-5) and 10(-5) mol/L increased forearm blood flow to 14.7 +/- 6.2, 20.2 +/- 4.7, and 38.4 +/- 7.9 mL/min per 100 mL in normal subjects and to 4.1 +/- 0.8, 5.0 +/- 1.1, and 10.6 +/- 2.3 mL/min per 100 mL in patients with CHF. After administration of NTG 10(-9) mol/L for 20 minutes, the vasodilatory response to Ach significantly increased to 5.6 +/- 1.0, 6.9 +/- 1.6, and 17.7 +/- 3.4 mL/min per 100 mL in patients with CHF but did not change in normal subjects. The enhanced forearm blood flow responses to administration of Ach observed after 20 minutes of NTG administration in patients with CHF were sustained throughout a 12-hour NTG infusion. In contrast, regional administration of NTG did not change the vasodilatory responses to phentolamine.

CONCLUSIONS

NTG, when administered intra-arterially for 20 minutes at a dose that does not affect resting forearm blood flow, specifically increased the vasodilatory response to intra-arterial administration of Ach in patients with CHF but not in normal subjects. The vasodilatory response to Ach was consistently enhanced by low-dose NTG throughout a 12-hour period. The vasodilating effects of organic nitroesters on the peripheral vasculature of patients with CHF may result in part from an interaction with the vascular endothelium.

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.

Nitrate tolerance in vivo is not associated with depletion of arterial or venous thiol levels

Results from in vitro experiments suggest that development of nitrate tolerance is due to a depletion of vascular thiol compounds (ie, cysteine and glutathione [GSH]) necessary for the bioconversion of organic nitrates. However, it is unknown whether in vivo tolerance development is associated with changes in thiol levels. This study measures plasma and vessel tissue GSH and cysteine levels in nontolerant rats, nitrate-tolerant rats, and rats treated with the two characteristically different thiol donors N-acetyl-L-cysteine and L-2-oxothiazolidine-4-carboxylic acid (OXO). Chronically catheterized conscious rats received an intravenous infusion of either nitroglycerin (NTG, 0.2 mg/h) or matching placebo for 3 days. At day 3, the hypotensive effect of 2.5 mg NTG/kg was decreased by 74 +/- 6% (mean +/- SEM, P < .05) in the NTG-treated group (n = 7), indicating the development of tolerance. No change in the hypotensive effect of NTG was seen in the placebo group (n = 6, P > .05). Hemodynamic tolerance is not associated with changes in aorta cysteine or GSH levels as compared with the placebo group (cysteine, 77 +/- 14 versus 57 +/- 11 [mean + SEM] nmol/g; GSH, 414 +/- 62 versus 399 +/- 89 nmol/g; P > .05). However, the increase in vascular thiol levels seen after OXO treatment in nontolerant rats is completely absent in nitrate-tolerant animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of nitroglycerin for the treatment of acute myocardial infarction

After a decade of warnings against the use of nitrates in acute myocardial infarction (MI), they are becoming recognized for their potential to salvage left ventricular (LV) myocardium, geometry and function. Low-dose intravenous (IV) nitroglycerin (NTG) infusion for the first 48 h after acute MI, titrated to lower mean blood pressure by 10% to 30%, but not below 80 mmHg, has been shown to be safe, to improve hemodynamics, and to decrease infarct size, infarct expansion, complications, and deaths in a prospective, randomized, single-blind study of 310 patients. In addition, low-dose NTG infusion for the first 48 h, followed by prolonged buccal NTG given during healing after acute MI in an eccentric dose schedule to minimize tolerance, was found to limit further progressive remodeling and preserve LV function. Meta-analysis of nitrate studies in acute MI indicate that they improve survival. Preliminary and ongoing studies suggest that prolonged NTG therapy post MI can produce further benefit.

Altered peripheral vasodilator profile of nitroglycerin during long-term infusion of N-acetylcysteine

OBJECTIVES

The aim of this study was to compare the short- and long-term effects of intravenous nitroglycerin plus placebo and nitroglycerin plus N-acetylcysteine on peripheral arteries, veins and microcirculation in humans.

BACKGROUND

The thiol donor N-acetylcysteine may potentiate the hemodynamic response to nitrates in nitrate-tolerant and nontolerant patients. The vascular changes responsible for this effect are not clear.

METHODS

Eight male volunteers were treated with nitroglycerin (0.1 microgram/kg per min) combined with N-acetylcysteine (2 g intravenously, followed by 5 mg/kg per h) or placebo for 23 h in a double-blind, randomized, crossover study. Venous volume, the diameter of the radial and temporal arteries, calf blood flow and subcutaneous blood flow were measured at baseline and repeated after 1 and 23 h of infusion.

RESULTS

Prolonged coadministration of N-acetylcysteine and nitroglycerin potentiated the acute venodilator effect of nitroglycerin as estimated by changes in venous volume (nitroglycerin plus N-acetylcysteine, 4.45 +/- 0.36 ml/100 g; nitroglycerin plus placebo, 3.65 +/- 0.46 ml/100 g, mean +/- SEM, p < 0.05) and prevented development of tolerance as seen after 23 h of treatment with nitroglycerin plus placebo (4.35 +/- 0.25 vs. 3.47 +/- 0.41 ml/100 g, p < 0.05). N-acetylcysteine had no effect on nitroglycerin-induced changes in arterial diameters (p > 0.05) but significantly increased microcirculatory subcutaneous blood flow after 1 h (nitroglycerin plus N-acetylcysteine: 6.3 +/- 1.3 ml/100 g per min vs. nitroglycerin plus placebo: 3.5 +/- 0.3 ml/100 g per min, p < 0.05) and after 23 h (4.4 +/- 0.6 vs. 3.1 +/- 0.5 ml/100 g per min, p < 0.05).

CONCLUSIONS

The results suggest that coadministration of nitroglycerin and N-acetylcysteine in humans 1) potentiates and preserves nitroglycerin-induced venodilation and 2) augments the effect of nitroglycerin on small resistance vessels (regulating subcutaneous blood flow) without affecting the response to nitroglycerin in middle-sized arteries. Both the development of nitrate tolerance and the administration of N-acetylcysteine significantly change the normal vasodilator profile of nitroglycerin in humans.

Influence of endothelium and nitrovasodilators on free thiols and disulfides in porcine coronary smooth muscle

It is hypothesised that the well known development of tolerance to the vasodilating action of organic nitrates is contributed by intracellular depletion of free thiols occurring during repeated treatment with these drugs. Therefore, ring segments of porcine coronary arteries with and without endothelium were treated for 30 min with either vehicle or 100 microM of isosorbide-5-mononitrate, glyceryl trinitrate, S-nitroso-N-acetyl-D,L-penicillamine or N-(3-nitratopivaloyl)-1-cysteine-ethylester (SPM 3672), and the content of histochemically stained free thiols (-SH) and disulfides (S-S-) was measured densitometrically in single smooth muscle cells. In the presence of endothelium the content of -SH in smooth muscle cells of controls (n = 8) gave an extinction of 0.127 +/- 0.013 in the intima and 0.120 +/- 0.010 in the media. The corresponding values for S-S- were 0.684 +/- 0.084 and 0.535 +/- 0.120 (n = 8). Removal of endothelium reduced S-S- to 82.1 +/- 70% and increased -SH to 126.7 +/- 6.7%. Treatment with all nitrates reduced -SH in intact artery segments to a similar degree, ranging between 54.0 +/- 4.4 and 68.7 +/- 4.7% (n = 8-10). In contrast, S-S- content was less affected and reached values between 70.6 +/- 2.8 and 91.6 +/- 6.0% (n = 8-9). As evaluated by tension studies, tolerance developed for glycerol trinitrate and isosorbide-5-mononitrate but not for S-nitroso-N-acetyl-D,L-penicillamine. Induction of tolerance with glycerol trinitrate (0.1 mM) produced a significantly more pronounced attenuation in activity of isosorbide-5-mononitrate than tolerance induction with isosorbide-5-mononitrate (1 mM). In contrast, the potency of SPM 3672 was not reduced in glycerol trinitrate-tolerant arteries. We conclude that, in porcine coronary arteries, an intact endothelium modifies intracellular thiols and disulfides. In addition, nitrate tolerance is associated with, but probably not caused by, thiol depletion.

Nitrates and angina pectoris

This review discusses the mechanisms of action of the organic nitrates, nitrate tolerance, and the effects of nitrates in patients with stable angina pectoris. The nitrates are prodrugs that enter the vascular smooth muscle, where they are denitrated to form the active agent nitric oxide (NO). NO activates guanylate cyclase, which results in cyclic guanosine monophosphate (cGMP) production and vasodilation as a result of reuptake of calcium by the sarcoplasmic reticulum. NO is identical to endothelium-derived relaxing factor (EDRF), which induces vasodilation, inhibits platelet aggregation, reduces endothelium adhesion, and has anticoagulant and fibrinolytic effects. Thus, the nitrates may be more than vasodilators and, in addition to reducing ischemia, may affect the process of atherosclerosis. The vascular effects of nitrates are attenuated during sustained therapy. Although the basis for the phenomenon of nitrate tolerance is not completely understood, sulfhydryl depletion as well as neurohormonal activation and increased plasma volume may be involved. The administration of N-acetylcysteine, angiotensin-converting enzyme (ACE) inhibitors, or diuretics do not consistently prevent nitrate tolerance. At present, intermittent nitrate therapy is the only way to avoid nitrate tolerance. The intermittent administration of nitrates, however, cannot provide continuous therapeutic benefits, and thus monotherapy with nitrates is not suitable for many patients with stable angina pectoris.

Clinical pharmacology of organic nitrates

Although organic nitrates have been used in cardiovascular therapy for many years, various aspects of their pharmacology remain poorly understood. It is now known that organic nitrates produce nitric oxide (NO) in vascular smooth muscle cells, catalyzed by a membrane-bound enzyme that is not glutathione-S-transferase. Other nitrovasodilators, such as organic nitrites, sodium nitroprusside, and S-nitrosothiols, do not utilize the same enzyme for NO generation. The short-term hemodynamic action of various organic nitrates has been shown to be related to their pharmacokinetics, but their long-term therapeutic effects are limited by the development of pharmacologic tolerance. Nitrate sensitivity in patients can be restored daily after a nitrate-free period of 8-12 hours. Coadministration of nitrates with other vasodilators, such as captopril and hydralazine, may avoid the development of nitrate tolerance in patients with congestive heart failure.

Effect of apparent elimination half-life on nitroglycerin-induced hemodynamic rebound in experimental heart failure

Hemodynamic rebound after abrupt withdrawal may be an important consideration associated with nitroglycerin (NTG) monotherapy. This phenomenon may arise from unopposed neurohormonal vasoconstriction because of rapid elimination of NTG. The role of NTG pharmacokinetics in the development of hemodynamic rebound was examined using a rat model of congestive heart failure. NTG was infused for 90 min, then the dose was either abruptly stopped (n = 8) or gradually reduced by 20% every 20 min (n = 7). Abrupt withdrawal caused rebound elevations of left ventricular end-diastolic pressure (LVEDP) to about 25% above baseline values, at 30-60 min after drug termination (P < 0.01), but this was completely avoided by graded NTG withdrawal. A positive correlation was observed (P < 0.05) between the percentage reduction in LVEDP during infusion and the maximum percentage rebound in rats after abrupt withdrawal but not after graded withdrawal. These results suggest that NTG-induced hemodynamic rebound is related to its short biological half-life and that this phenomenon is consistent with a mechanism of neurohormonal compensation.

Nitrate tolerance and aging in isolated rat aorta

The purpose of this study was to determine whether there are age-related changes in the extent of in vitro-induced nitrate tolerance. Nitroglycerin pre-exposure (10 microM for 30 min) provoked a significant shift to the right of the dose-response curve to nitroglycerin in aortae isolated from rats of 8 weeks, 12 and 18 months. However, this shift to the right was significantly larger at 18 months, both when KCl or phenylephrine were used as contractile agents. On the contrary, nitroglycerin pre-exposure did not significantly alter the dose-dependent relaxation to Sin-1 (3-morpholinosydnonimine, the active metabolite of molsidomine) at 8 weeks, 12 and 18 months. These data indicate that the extent of the in vitro-induced nitrate tolerance is larger when aortae are isolated from senescent rats. This increase in tolerance does not appear to involve desensitization of guanylate cyclase.

Sulfhydryl group donors potentiate the hypotensive effect of acetylcholine in rats

Nitric oxide mediates the vasodilator and hypotensive responses of acetylcholine infusion. It has been reported that nitric oxide could be protected from free radical destruction by forming an S-nitrosothiol compound. Furthermore, sulfhydryl donors such as N-acetylcysteine or thiosalicylic acid enhance nitric oxide production from nitroglycerin. Consequently, the hypotensive effect of intravenous acetylcholine infusion might be potentiated during the simultaneous administration of sulfhydryl donors. The objective of the present study was to test in Okamoto spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats (1) whether the hypotensive effect of acetylcholine (10 micrograms/kg per minute) was affected by the simultaneous administration of N-acetylcysteine (10 micrograms/kg per minute) or thiosalicylic acid (10 micrograms/kg per minute), and (2) whether NG-nitro-L-arginine-methyl ester (100 micrograms/kg per minute) administration was able to reverse the changes induced by acetylcholine plus N-acetylcysteine or acetylcholine plus thiosalicylic acid. The administration of acetylcholine reduced (P < .05) mean arterial pressure in WKY rats (13 +/- 2%) and SHR (14 +/- 2%) without affecting urine flow rate, urinary sodium excretion, and glomerular filtration rate. In the presence of N-acetylcysteine, the acetylcholine-induced reduction in mean arterial pressure was potentiated (P < .05) in WKY rats (24 +/- 4%) and SHR (20 +/- 2%). These changes in mean arterial pressure were accompanied by significant reductions in urine flow rate and urinary sodium excretion in WKY rats, as well as in glomerular filtration rate in SHR.2

Double-blind, placebo-controlled study of the efficacy of flosequinan in patients with chronic heart failure. Principal Investigators of the REFLECT Study

OBJECTIVES

The aim of this study was to assess the efficacy of flosequinan in chronic heart failure.

BACKGROUND

Flosequinan is a new vasodilator drug that acts by interfering with the inositol-triphosphate/protein kinase C pathway, an important mechanism of vasoconstriction. The drug dilates both peripheral arteries and veins, is orally active and has a long duration of action that permits once-daily dosing. Previous studies have shown that flosequinan produces sustained hemodynamic benefits in heart failure, but large scale studies evaluating its clinical efficacy have not been reported.

METHODS

One hundred ninety-three patients with chronic heart failure (New York Heart Association functional class II or III and left ventricular ejection fraction < 40%) receiving digoxin and diuretic drugs were randomly assigned (double-blind) to the addition of flosequinan (100 mg once daily, n = 93) or placebo (n = 100) for 3 months. The clinical status and exercise tolerance of each patient was evaluated at the start of the study and every 2 to 4 weeks during the trial while background therapy remained constant.

RESULTS

After 12 weeks, maximal treadmill exercise time increased by 96 s in the flosequinan group but by only 47 s in the placebo group (p = 0.022 for the difference between groups). Maximal oxygen consumption increased by 1.7 ml/kg per min in the flosequinan group (n = 17) but by only 0.6 ml/kg per min in the placebo group (n = 23), p = 0.05 between the groups. Symptomatically, 55% of patients receiving flosequinan but only 36% of patients receiving placebo benefited from treatment (p = 0.018). In addition, fewer patients treated with flosequinan had sufficiently severe worsening of heart failure to require a change in medication or withdrawal from the study (p = 0.07). By intention to treat, seven patients in the flosequinan group and two patients in the placebo group died.

CONCLUSIONS

These findings indicate that flosequinan is an effective drug for patients with chronic heart failure who remain symptomatic despite treatment with digoxin and diuretic drugs. The effect of the drug on survival remains to be determined.

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

BACKGROUND

The present study was designed to investigate the intracellular production of cyclic GMP (cGMP) in platelets in response to nitroglycerin and to determine the potential clinical value of platelet cGMP as an indicator of the effects of nitroglycerin and nitrate tolerance.

METHODS AND RESULTS

Platelet cGMP levels and the diameters of the coronary arteries before and 2 minutes after intracoronary injection of 200 micrograms nitroglycerin were measured in 15 patients who had previously received nitrates (nitrates group) and in 16 who had not received any nitrates (no-nitrates group). Platelet cGMP levels increased significantly after nitroglycerin injection in the two groups, but plasma cGMP levels and plasma atrial natriuretic peptide levels did not change. The percent increase in platelet cGMP levels and the percent dilatation of the left anterior descending (LAD) and left circumflex (LCx) coronary arteries after nitroglycerin injection were higher in the no-nitrates group than in the nitrates group (platelet cGMP levels: artery, 74.2 +/- 18.3% versus 11.5 +/- 4.2%, P < .01; vein, 73.6 +/- 22.9% versus 9.0 +/- 3.1%, P < .01; coronary dilatation: LAD, 46.7 +/- 6.0% versus 9.9 +/- 2.5%, P < .01, LCx, 51.2 +/- 8.7% versus 6.1 +/- 3.0%, P < .01). The percent increase in platelet cGMP levels was significantly correlated with the percent dilatation of the coronary arteries (LAD: r = .90, P < .01; LCx: r = .92, P < .01) in the no-nitrates group and not in the nitrates group.

CONCLUSIONS

These results indicate that platelet cGMP can be used as an indicator for in situ evaluation of nitroglycerin effects and that patients who have received nitrates develop nitrate tolerance, which affects intracellular production of cGMP and vasodilation in the response to nitroglycerin.

Comparison of the effects of two different galenical preparations of glyceryl trinitrate on pulmonary artery pressure and on the finger pulse curve

The time course and the magnitude of the effect of glyceryl trinitrate (GTN) on central venous (pulmonary artery diastolic pressure-PAPd) and peripheral arterial (a/b-ratio of the finger pulse wave) haemodynamics were compared in a randomized double-blind cross-over study in 12 patients suffering from congestive heart failure (NYHA II-III) with elevated PAPd at rest (> or = 15 mmHg). The data were obtained in a bioavailability study of two sprays of glyceryl trinitrate, which differed in their galenical characteristics and in the dose of GTN (0.4 mg vs. 0.8 mg). Following sublingual administration of each spray, PAPd, a/b-ratio and the plasma concentrations of GTN and its metabolites were measured up to 30 min. The relative bioavailability of GTN of the test preparation was estimated to be 157%, 161% and 147%, when calculated from the plasma concentration-time data or the integrated effect of GTN on a/b-ratio or PAPd, respectively. The mean time courses of the decrease in PAPd and the increase in the a/b-ratio of the finger pulse curve were mirror images. Thus, there was a strong correlation between the mean values of PAPd and a/b-ratio following the administration of glyceryl trinitrate. Since the slope of the relationship differed considerably between the patients, the magnitude of effect of GTN on PAPd in the individual patient could not be predicted from the changes in a/b-ratio.

Release of nitric oxide from glyceryl trinitrate by captopril but not enalaprilat: in vitro and in vivo studies

1. The hypotensive effects of glyceryl trinitrate (GTN, 0.5 mg kg-1) but not of 3-morpholino-sydnonimine (SIN-1, 0.125 mg kg-1) in anaesthetized rats were attenuated following a seven day (using a q.i.d. dosing schedule) oral treatment with isosorbide-5-mononitrate (IS-5-MN; 5 mg kg-1) indicative of the induction of tolerance to GTN but not to SIN-1. The hypotensive effects of GTN did not decline when the sulphydryl (SH) containing angiotensin converting enzyme inhibitor (ACE-1), captopril (CPT, 5 mg kg-1) or the structurally unrelated SH-containing, N-acetylcysteine (NAC, 10 mg kg-1) but not the non-SH-containing ACE-I, enalaprilat (ENA, 5 mg kg-1) were given together with IS-5-MN for the seven days treatment. 2. The attenuated hypotensive effects of GTN (0.5 mg kg-1) in rats treated with IS-5-MN were also restored when CPT (1 mg kg-1) or NAC (2.5 mg kg-1) but not ENA (1 mg kg-1) was administered intraperitoneally (i.p.) 30 min before GTN. Furthermore, in control rats, CPT or NAC but not ENA given i.p. 30 min before GTN, potentiated its haemodynamic effects. These effects were blocked by methylene blue (10 mg kg-1). At the same doses, CPT or NAC did not affect the hypotensive effects of SIN-1. 3. The reduced ability of cultured tolerant smooth muscle cells (SMC, 24 x 103 cells) or endothelial cells(EC, 40 x 103 cells) to potentiate the anti-platelet effects of GTN (44 microM) was restored by CPT or NAC but not by ENA or glutathione (all at 0.5 mM). Potentiation of the anti-platelet effects of tolerant SMC or EC by CPT or NAC was abolished by co-incubation with oxyhaemoglobin (Oxy-Hb, 10 microM)indicative of nitric oxide (NO) formation.4. When GTN (150-2400 microM) was incubated with CPT, NAC or glutathione but not ENA (all at 0.1 mM) for 30 min in Krebs buffer at 37 degrees C a concentration-dependent increase in nitrite (NO2-)formation was observed. 5. The antiplatelet effects of GTN (5.5-352 microM) were potentiated by co-incubation with CPT or NAC but not with ENA or glutathione (all at 0.5 mM). The concentration of GTN required to inhibit platelet aggregation by 50% (IC50) was 110 +/- 2 microM for GTN alone, 14 +/- 2 microM for GTN in the presence of NAC and 30 +/- 2 microM for GTN in the presence of CPT. The potentiation of the effects of GTN by CPT or NAC was inhibited by co-incubation with Oxy-Hb (10 microM). By themselves, CPT or NAC did not inhibit platelet aggregation.6. The ability of CPT to restore (a) the haemodynamic effects of GTN in tolerant rats and (b) the reduced capacity of tolerant SMC or EC to potentiate the anti-platelet effects of GTN is not related to its ACE inhibitory activity.7. CPT also potentiated the hypotensive effects of GTN in non-tolerant rats, and in vitro CPT released NO from GTN in the absence of a GTN to NO converting cell, so that it is unlikely that reversal of tolerance by CPT is due to the replenishment of intracellular thiols. Rather it can be explained by the ability of CPT to release NO from GTN in the extracellular space. This extracellular formation of NO from GTN by CPT would then compensate for the impaired enzymic biotransformation of GTN to NO that develops during tolerance as was originally proposed for NAC.

Effect of therapy with an angiotensin-converting enzyme inhibitor on hemodynamic and counterregulatory responses during continuous therapy with nitroglycerin

OBJECTIVES

We sought to characterize the reflex counterregulatory responses throughout a 6-day period of continuous nitroglycerin therapy and to examine the effect of concurrent administration of a non-thiol angiotensin-converting enzyme inhibitor (benazepril) on the nature of those responses.

BACKGROUND

Therapy with nitroglycerin has been shown to be associated with reflex counterregulatory responses.

METHODS

Standing systolic blood pressure, hormonal responses, urinary sodium and hematocrit levels were monitored during 6 days of continuous transdermal nitroglycerin therapy in normal volunteers. Using a double-blind randomized parallel design, 11 subjects received placebo and 9 received benazepril. Hemodynamic responses to sublingual nitroglycerin administration were evaluated before and after sustained therapy with transdermal nitroglycerin.

RESULTS

Attenuation of the hypotensive response to transdermal nitroglycerin was rapid in the group receiving placebo and the group receiving benazepril. There were no significant hormonal responses to transdermal nitroglycerin in either group, and sodium retention was modest and transient. Hematocrit levels decreased after transdermal nitroglycerin therapy and remained depressed for the duration of nitroglycerin therapy, a finding that suggests plasma volume expansion. Blood pressure responses to sublingual nitroglycerin in both groups were similar before and after continuous transdermal nitroglycerin therapy.

CONCLUSIONS

These data suggest that plasma volume expansion plays a more important role than neurohormonal responses in the loss of nitrate effects during sustained therapy. That therapy with an angiotensin-converting enzyme inhibitor did not modify the hemodynamic responses to continuous nitroglycerin therapy supports this conclusion. Further investigation will be necessary to confirm whether therapy with an angiotensin-converting enzyme inhibitor has any role in the prevention of nitrate tolerance.

Newer concepts in the medical management of patients with congestive heart failure

Congestive heart failure (CHF) remains a major cause of morbidity and mortality in the United States, especially among the elderly. Although an underlying disturbance in cardiac function can be identified in most patients, manifestations of the disease are greatly influenced by other factors, particularly neurohumoral and peripheral adaptive responses which occur secondary to impaired cardiac function. The renin-angiotensin system (RAS) is integrally involved in the pathophysiology of CHF. Originally considered a humoral system, the RAS is now known to exist and operate within cardiac and vascular tissues. The importance of tissue-specific renin-angiotensin systems in CHF is presently under investigation. Most patients with symptomatic CHF benefit from the administration of an ACE inhibitor. Certain asymptomatic patients, such as those with severe left ventricular (LV) dysfunction and those who are at high risk for LV remodeling after anterior wall myocardial infarction, may also benefit from ACE inhibitor therapy. Diuretics and nitrates improve symptoms and often cardiac output in many patients with CHF. Although many new inotropic agents have been tested in CHF patients, none appear clinically superior to digitalis glycosides. The efficacy of digitalis glycosides in CHF may in part result from sympathoinhibitory properties such as the activation of baroreceptor mechanisms. Despite the fact that many CHF patients die from arrhythmias, treatment of asymptomatic ventricular arrhythmias in these patients is not recommended. Patients with symptomatic or sustained ventricular arrhythmias are best treated by a physician experienced in cardiac electrophysiology. Therapy with beta-blocking drugs for CHF patients is controversial. Anticoagulants are recommended for selected patients with CHF. Finally, exercise therapy may improve functional capacity in some patients with CHF through its effects on peripheral blood vessels and skeletal muscle tissues.

Paracetamol cardiotoxicity

A 29 year old man with a significant paracetamol overdose was found to have an abnormal electrocardiograph which, in the absence of hepatic encephalopathy, was considered due to a direct cardiotoxic effect of the drug. A functional coronary insufficiency resulting from inhibition of endothelium-derived relaxing factor secondary to depletion of sulphydryl groups is postulated, and it is suggested that in paracetamol poisoning evidence of cardiotoxicity alone may be sufficient justification for treatment with acetylcysteine.

Calcitonin gene-related peptide: a haemodynamic study of a novel vasodilator in patients with severe chronic heart failure

The haemodynamic and neurohumeral response to a novel vasodilator calcitonin gene-related peptide was assessed in 11 patients with severe chronic heart failure. To assess tolerance, a continuous 48-h infusion (n = 6) was compared with a regimen of two successive 8-h infusions (n = 5). Haemodynamic response profiles were similar for both regimens, though the continuous infusion was poorly tolerated. Reductions in afterload reflected by changes in systemic vascular resistance and systemic blood pressure led to increases in cardiac index of at least 24%. Increments in heart rate accounted for much of the increase in cardiac output, there being no significant change in stroke volume index. The response was maintained over the 48-h study period with no tachyphylaxis. Renin and angiotensin levels increased significantly after 24 h. Calcitonin gene-related peptide exerts a favourable haemodynamic response in patients with severe heart failure. The dose used in this study, however, caused troublesome side-effects, particularly when given by continuous infusion. Further studies are required to establish the therapeutic range of this new peptide.

Rationale for intermittent nitrate therapy

Tolerance to the pharmacologic and therapeutic effects of nitrate therapy is now well established. This phenomenon may be defined as either a decreased response to a given amount of nitrate or the need for an increased amount of nitrate to maintain a constant effect. Tolerance has been demonstrated with all forms of nitrate therapy that maintain continuous blood levels of the drug, including frequent oral dosing, constant intravenous infusion, and continuous transdermal delivery. It can develop rapidly after only a few doses of a nitrate preparation and tends to be partial rather than absolute. Strategies for the prevention of nitrate tolerance include the avoidance of maximum nitrate doses and the use of intermittent nitrate dosing regimens. Providing a relatively brief nitrate-free interval restores vascular responsiveness to nitrates, most likely due to a recovery of the metabolic mechanisms responsible for the therapeutic effect of these drugs. The duration of this period of nitrate abstinence varies, depending on the nitrate preparation used but is generally in the range of 8-12 hours. Such intermittent therapy not only reduces the risk of nitrate tolerance, but also provides a convenient approach to outpatient management.

Possible mechanisms of nitrate tolerance

Prolonged exposure to organic nitrates has been shown to lead to the rapid development of tolerance to the peripheral and coronary vasodilatory effects of these drugs. As a result of this phenomenon, the hemodynamic and anti-ischemic effects of nitrates may be rapidly attenuated in patients with ischemic heart disease, congestive heart failure, or both. This nitrate tolerance appears to be both dose- and time-dependent. Likely mechanisms proposed for its development are multifactorial and include depletion of sulfhydryl groups, a nitrate-mediated increase in blood volume, and neurohormonal stimulation with activation of vasoconstrictive mechanisms.

Mechanisms of interaction between the sulfhydryl precursor L-methionine and glyceryl trinitrate

BACKGROUND

L-Methionine potentiates systemic hemodynamic effects of intravenous glyceryl trinitrate (GTN) in tolerant and nontolerant patients to a similar extent as N-acetylcysteine (NAC). This potentiation of GTN action by L-methionine has been attributed to enhanced intracellular formation of nitrosothiols, known to be potent stimulators of soluble guanylyl cyclase. This study was performed to analyze directly the effects of L-methionine on GTN-induced dilation of large epicardial arteries and the venous capacitance system of the dog in the tolerant and nontolerant states. Cultured rat aortic vascular smooth muscle cells and purified guanylyl cyclase were used to study potential intracellular and extracellular mechanisms responsible for this interaction.

METHODS AND RESULTS

In awake nontolerant dogs, L-methionine (100 mg/kg) potentiated the tachycardic response to GTN (5.0 and 15 micrograms/kg/min) and enhanced the hypotensive action of GTN (1.5 and 5.0 micrograms/kg/min) in anesthetized, nonreflexic dogs. In nontolerant and tolerant dogs, however, L-methionine did not alter the dose-response of large epicardial artery dilation to intravenous GTN challenges and did not modify nitrate tolerance of the low pressure system of the dog. The infusion of L-methionine (100 mg/kg) significantly increased plasma methionine levels (from 52 +/- 12 to 1,141 +/- 239 microM), cystine levels (from 12 +/- 4 to 26 +/- 7 microM), but not homocystine levels. In vitro, the L-methionine conversion product L-cysteine (0.1-1.0 mM) but not homocysteine significantly enhanced the augmentation of purified guanylyl cyclase activity by GTN (100 microM). Incubation of cultured rat aortic smooth muscle cells with L-methionine (10 microM or 1 mM) did not result in a significant increase of free intracellular sulfhydryl group content.

CONCLUSIONS

The L-methionine conversion product L-cysteine mediates tolerance independent the potentiation of GTN action. This may result from an L-cysteine-induced formation of a vasoactive metabolite of GTN (nitric oxide) or nitrosothiol. This effect occurs primarily in the resistance vessel circulation, not in large epicardial arteries and veins. The lack of effect of L-methionine on sulfhydryl group content in large conductance vessels indicates that hepatic L-methionine metabolism constitutes the significant source of L-cysteine. These findings strongly suggest that administration of sulfhydryl-group precursor L-methionine does not represent a therapeutic alternative to a nitrate-free interval to restore nitrate sensitivity in tolerant large epicardial arteries and veins.

Biochemical mechanism of organic nitrate action

Increasing evidence suggests that organic nitrate action derives from their metabolic conversion to nitric oxide (NO) in the vascular smooth muscle cell. The primary catalytic activity of this process appears to reside at the cellular plasma membrane. There is no concrete evidence to indicate that NO formation is preceded by the production of inorganic nitrite ion or that the NO produced needs to form S-nitrosothiols before it can activate guanylate cyclase to produce cyclic guanosine 3',5'-monophosphate (cGMP). Although sulfhydryl donors can partially reverse nitroglycerin-induced tolerance in patients, this phenomenon (by itself) is not sufficient to implicate intracellular sulfhydryl depletion as an operating mechanism of clinical nitrate tolerance. This is because sulfhydryl donors can react with nitroglycerin extracellularly to form S-nitrosothiols, and nonsulfhydryl compounds, such as enalapril and hydralazine, can prevent the development of in vivo nitrate tolerance. In addition to the cellular biochemical reactions, organic nitrates also produce systemic biochemical effects through altering neurohormonal status. These systemic effects may contribute significantly to the development of nitrate tolerance in therapeutic situations.

Neurohormonal activation during nitrate therapy: a possible mechanism for tolerance

The hemodynamic and antianginal effects of the organic nitrates are greatly reduced during therapy designed to provide 24-hour protection throughout each day. Although the mechanisms of tolerance are not clearly understood, it is likely that tolerance is related to a combination of reduced sulfhydryl groups in vascular smooth muscle and neurohormonal activation. These changes would reduce nitrate-induced vasodilation and induce sodium and water retention with an increase in plasma volume. These reflex neurohormonal changes would reduce the hemodynamic effect of any given degree of nitrate-induced vasodilation. Although preliminary studies suggest that diuretic therapy or the use of converting enzyme inhibitors may reduce or abolish tolerance, further clinical studies are required to confirm these findings.

Nitrate resistance and tolerance: potential limitations in the treatment of congestive heart failure

Organic nitrates are commonly used in the treatment of chronic congestive heart failure (CHF). These drugs have been shown to improve exercise capacity and reduce symptoms and in combination with hydralazine to prolong life in patients with mild-to-moderate symptoms of CHF. Recent investigations, however, have indicated that nitrate-mediated benefit to patients with CHF may be limited by resistance to their hemodynamic effects seen in many patients and by early development of tolerance. The incidence, potential mechanisms, and possible methods for prevention of nitrate resistance and tolerance in patients with chronic CHF are reviewed.

Role of nitrates in unstable angina pectoris

Unstable angina pectoris is a clinically heterogeneous process with patient symptoms varying between reduced threshold for exertional angina and the occurrence of multiple episodes of rest pain. The major factors in the pathogenesis of unstable angina appear to be intracoronary platelet aggregation and thrombus formation secondary to fissuring or rupture of atheromatous plaques, with associated coronary vasoconstriction due to release of constrictor materials from aggregating platelets and deficiency of endothelium-related vasodilator activity. The latter factor is of particular interest in view of the similar biochemical mechanisms of action of nitroglycerin (NTG) and endothelium-derived relaxing factor (EDRF). The efficacy of NTG in limiting platelet aggregation is also of particular interest in this condition. Medical therapy in patients with unstable angina usually requires use of multiple agents. In the short term, there is a strong case for the use of intravenous heparin both to relieve pain and to reduce the risk of acute myocardial infarction. Aspirin is perhaps less effective in the short term, but very useful in long-term treatment of such patients. Despite their widespread clinical use, beta-adrenoceptor antagonists are probably only marginally beneficial, whereas dihydropyridine calcium antagonists such as nifedipine are potentially harmful as monotherapy and of questionable use in combination with other drugs. Other agents that are effective in relieving ischemic symptoms are the nondihydropyridine calcium antagonists verapamil and diltiazem and the oxygen-sparing agent perhexiline maleate. Despite a paucity of controlled trial data, nitrates are used in the vast majority of patients with unstable angina.

Different susceptibility to the development of nitroglycerin tolerance in the arterial and venous circulation in humans. Effects of N-acetylcysteine administration

BACKGROUND

Tolerance to the effects of organic nitrates develops rapidly during continuous exposure to these drugs; its main mechanism seems to be an intracellular sulfhydryl group depletion. However, the relative susceptibility to the development of nitroglycerin tolerance of the arterial or venous circulation in humans is still a matter of dispute.

METHODS AND RESULTS

Twenty patients with coronary artery disease underwent a continuous 24-hour nitroglycerin infusion followed by a bolus administration of N-acetylcysteine. Forearm blood flow (ml/100 ml/min) and venous volume (ml/100 ml) were measured by strain gauge plethysmography under control conditions, at the end of nitroglycerin titration, after 24-hour infusion, and after N-acetylcysteine; vascular resistance was calculated as mean cuff blood pressure divided by flow. After 24 hours of nitroglycerin infusion, the initial increase in venous volume was reduced 48% (p less than 0.01), whereas the acute effects on vascular resistance were not attenuated in the whole group. N-Acetylcysteine completely restored nitroglycerin venodilator effects in all 10 patients in whom attenuation of the venous effects was observed during the infusion period.

CONCLUSIONS

The data indicate that the susceptibility to the development of nitrate tolerance in humans is higher in the venous than in the arterial circulation, and that the sulfhydryl group donor N-acetylcysteine is extremely effective in reversing nitroglycerin tolerance in the venous circulation in humans.

Effects of diuretic therapy on the development of tolerance during continuous therapy with nitroglycerin

OBJECTIVES

This study examined the effects of concurrent diuretic therapy on the hemodynamic responses to short-term and sustained therapy with transdermal nitroglycerin.

BACKGROUND

Sodium retention and plasma volume expansion occur during therapy with nitroglycerin and may play a role in the loss of nitroglycerin effects during sustained therapy.

METHODS

Twenty-two normal male volunteers were treated for 1 week with either hydrochlorothiazide and amiloride (50 + 5 mg) (n = 11) or placebo (n = 11) in a randomized, double-blind fashion. All 22 subjects then received continuous transdermal nitroglycerin (19 +/- 1 mg/24 h) for 5 to 7 days.

RESULTS

On the first and last day of transdermal nitroglycerin therapy, standing heart rate, systolic blood pressure and hematocrit values were assessed at 8, 9 and 10 AM and 12 noon. Heart rate and blood pressure responses to sublingual nitroglycerin (0.6 mg) were also evaluated before and after sustained transdermal nitroglycerin therapy. A significant loss of the hemodynamic effects of transdermal and sublingual nitroglycerin occurred during sustained therapy in both the diuretic and placebo therapy groups. In both groups, transdermal nitroglycerin therapy was associated with a significant decrease in hematocrit that persisted for the entire treatment period.

CONCLUSIONS

These results suggest that diuretic therapy does not prevent plasma volume expansion or the loss of hemodynamic effects during sustained transdermal nitroglycerin therapy. The persistent decrease in hematocrit suggests that plasma volume expansion plays a role in the attenuation of nitrate effects. It also provides evidence of continued vascular activity of nitroglycerin despite loss of systemic hemodynamic effects.

Use of nitrates in ischemic heart disease

The organic nitrates have remarkably diverse actions that are or should be beneficial in patients with ischemic heart disease. These drugs are effective in all the important ischemic syndromes. Preliminary data in patients with acute infarction suggest that the drugs may be truly cardioprotective, resulting in improved mortality. This review has not discussed the role of nitrates in congestive heart failure or LV dysfunction, a subject of great importance. The nitrates are useful adjunctive agents in these syndromes, and the two VeHfT trials support the concept that long-term nitrate administration, in conjunction with hydralazine, may favorably alter the natural history of heart failure. This cardioprotective effect is similar to that suggested for the post-MI patient. The data are not strong enough for definitive conclusions at this time. The clinical benefits of nitrates in decreasing subjective (angina) and objective indices of ischemia in stable and unstable angina, as well as limited data in asymptomatic myocardial ischemia, are unequivocal and are as favorable as those for beta blockers or calcium antagonists. Tolerance is an important problem that unfavorably influences the potential benefits of nitrate therapy. I believe that this problem can be avoided with well-designed dosing regimens. Current research into endothelial biology in health and disease has further supported a physiologic role for the organic nitrates in patients with ischemic heart disease. The nitrate-platelet story, while controversial, is promising and offers another positive rationale for nitrate administration. The concept of nitrates replenishing disordered EDRF release or action is an exciting one. Physicians should feel fortunate to have such a remarkable group of drugs available for their patients.

In vitro and in vivo interactions of nitrovasodilators and zaprinast, a cGMP-selective phosphodiesterase inhibitor

We have examined the interaction of zaprinast, a selective inhibitor of cGMP phosphodiesterase, with guanylate cyclase activators on vascular smooth muscle relaxation in vitro and in vivo. Isolated porcine coronary arterial rings precontracted with prostaglandin F2 alpha (PGF2 alpha) were relaxed dose dependently by the guanylate cyclase activators nitroglycerin and nitroprusside, the cGMP phosphodiesterase inhibitor zaprinast and the endothelium-dependent agent bradykinin. A 1 h pretreatment with 0.5 mM nitroglycerin shifted the dose-response curve to nitroglycerin to the right by a factor of 90, reflecting the development of tolerance. The dose-response curve to sodium nitroprusside was also affected, albeit to a much lesser degree (9-fold increase in IC50). Both zaprinast and bradykinin remained unaffected by nitroglycerin pretreatment. A 30 min pretreatment of rings with zaprinast (1 microM) had no effect on nitroglycerin- or nitroprusside-induced relaxation in control rings, but enhanced vasorelaxation to both nitrovasodilators 7- and 2-fold, respectively, in tolerant rings. Similarly, a 30 min pretreatment of rings with 0.1 microM nitroprusside enhanced zaprinast-induced vasorelaxation 4- and 8-fold, respectively, in control and tolerant rings. Similar observations were made in vivo in anesthetized spontaneously hypertensive rats where zaprinast (0.1-3.0 mg/kg i.v.), caused dose-dependent reductions in mean arterial pressure. This effect was enhanced when rats had been pretreated with nitroprusside (1 micrograms/kg per min). In comparison, in zaprinast-pretreated rats the magnitude of depressor responses to nitroprusside (0.5-5.0 micrograms/kg) was not altered, but the duration of hypotensive response to the highest dose of nitroprusside was enhanced by zaprinast. These data demonstrate an enhanced vasodilatory response of nitrocompounds in combination with peak I-selective phosphodiesterase inhibitors.