Interval therapy to avoid nitrate tolerance: paradise regained?

Possible usefulness of apocynin, an NADPH oxidase inhibitor, for nitrate tolerance: prevention of NO donor-induced endothelial cell abnormalities

The long-term benefits of nitroglycerin therapy are limited by tolerance development. Understanding the precise nature of mechanisms underlying nitroglycerin-induced endothelial cell dysfunction may provide new strategies to prevent tolerance development. In this line, we tested interventions to prevent endothelial dysfunction in the setting of nitrate tolerance. When bovine aortic endothelial cells (BAECs) were continuously treated with nitric oxide (NO) donors, including nitroglycerin, over 2-3 days, basal production of nitrite and nitrate (NO(x)) was diminished. The diminished basal NO(x) levels were mitigated by intermittent treatment allowing an 8-h daily nitrate-free interval during the 2- to 3-day treatment period. Addition of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitor apocynin restored the basal levels of NO(x) that were decreased by continuous nitroglycerin treatment of BAECs. Apocynin caused significant improvement of increased mRNA and protein levels of endothelial nitric oxide synthase (eNOS) in BAECs given nitroglycerin continuously over the treatment period. Apocynin also reduced endothelial production of reactive oxygen species (ROS) after continuous nitroglycerin treatment. These results showed an essential similarity to the effects of a nitrate-free interval. Application of the NOS inhibitor N(omega)-nitro- l-arginine methyl ester caused a recovery effect on basal NO(x) and eNOS expression but was without effect on ROS levels in continuously NO donor-treated BAECs. In conclusion, the present study characterized abnormal features and functions of endothelial cells following continuous NO donor application. We suggest that inhibition of NADPH oxidase, by preventing NO donor-induced endothelial dysfunction, may represent a potential therapeutic strategy that confers protection from nitrate tolerance development.

Effects of a nitrate-free interval on tolerance, vasoconstrictor sensitivity and vascular superoxide production

OBJECTIVES

In the present study, we tested whether a nitrate-free interval is able to prevent increases in vascular superoxide (O2*-) and the development of hypersensitivity to vasoconstrictors and whether this may result in restoration of vascular nitroglycerin (NTG) sensitivity.

BACKGROUND

Intermittent NTG-patch treatment (12 h patch on/patch-off) has been shown to increase ischemic periods in patients with stable coronary arteries, suggesting a rebound-like situation during the patch-off period. Recently, we demonstrated that long-term treatment with NTG induces tolerance, which was in part related to increases in vascular O2*- and increased vasoconstrictor sensitivity.

METHODS

New Zealand white rabbits received a continuous application of NTG patches (0.4 mg/h) or an intermittent application of NTG patches (12 h patch on, 12 h patch off) for three days. Isometric tension studies were performed with aortic rings, and vascular O2*- was estimated using lucigenin-derived chemiluminescence (5 micromol/liter). Expression of the copper/zinc (Cu/Zn) superoxide dismutase (SOD) was assessed by Western blotting, and SOD activity was measured by autooxidation of 6-hydroxydopamine.

RESULTS

Continuous treatment with NTG caused tolerance to NTG, cross-tolerance to the endothelium-dependent vasodilator acetylcholine, increased vascular O2*-, reduced Cu/Zn SOD expression and increased sensitivity to vasoconstrictors such as phenylephrine, serotonin and angiotensin II. On/off treatment with NTG improved tolerance, corrected endothelial dysfunction and decreased vascular O2*-. In addition the reduction in SOD expression was less pronounced, whereas increases in the sensitivity to vasoconstrictors such as phenylephrine and serotonin remained nearly unchanged.

CONCLUSIONS

Enhanced vasoconstrictor sensitivity may explain, at least in part, the rebound phenomena observed in patients during a 12-h NTG patch-off period.

Attenuation of anti-ischemic efficacy during chronic therapy with nicorandil in patients with stable angina pectoris

After 2 weeks of nicorandil therapy, time to ischemia on stress testing was significantly less than on day 1 and not different from placebo. These data are consistent with attenuation of the anti-ischemic effects of this drug and suggest that the potassium channel-opening properties do not compensate for development of attenuation to the nitrate component of nicorandil.

Impaired modulation of sympathetic excitability by nitric oxide after long-term administration of organic nitrates in pigs

BACKGROUND

Endogenous nitric oxide (NO) reduces sympathetic vasoconstriction by attenuating neuronal excitability in the brain stem and inhibition of postganglionic neurotransmission. We studied whether this modulation of sympathetic circulatory control by NO may be altered during chronic administration of NO donor drugs in pigs.

METHODS AND RESULTS

Nitrate tolerance was induced by oral administration of isosorbide dinitrate (ISDN, 4 mg/kg per day for 4 weeks) in eight pigs. Four of them were chronically instrumented for the measurement of mean arterial blood pressure and cardiac output in the conscious state. ISDN treatment caused hemodynamic tolerance to NO donors and significantly increased the hypotensive responses to pharmacologic ganglionic blockade in conscious pigs. In general anesthesia, ISDN-treated animals and age-matched controls (n=5) had similar baseline renal sympathetic nerve activity and in both groups neither inhibition of NO synthases (NOS) nor administration of NO donors to the brain stem by intracerebroventricular (i.c.v.) infusions caused significant changes in baseline renal sympathetic nerve activity. However, whereas sympathoexcitatory responses to glutamate (0.5 mL, 0.1 mol/L, i.c.v.) or electrical stimulation of somatic nerve afferents were significantly potentiated by central NOS inhibition and attenuated by NO donors in controls, these treatments no longer had significant effects in ISDN-treated pigs. Furthermore, reflex sympathetic activation in response to intravenous NO donor treatment was more pronounced in nitrate tolerant animals, which suggests loss of central sympathoinhibitory effects of NO. Subsequent histology on brain stem slices with NADPH-diaphorase as NOS marker revealed significant reduction of NOS density in ISDN-treated pigs.

CONCLUSIONS

Long-term administration of organic nitrates reduces the number of NO-producing neurons in the brain stem and causes loss of inhibitory effects of NO on sympathetic excitability. This component of tolerance to organic nitrates may be important in patients confronted frequently with sympathetic activation caused by mental and/or physical stressors.

Effect of pre-treatment with transdermal glyceryl trinitrate on myocardial ischaemia during coronary angioplasty

OBJECTIVE

In the light of the reported inconsistent anti-ischaemic and antianginal effects of transdermal glyceryl trinitrate, its efficacy and influence on the effects of intracoronary glyceryl trinitrate were examined during coronary angioplasty, which provides a model of controlled, reversible ischaemia.

DESIGN

Double blind, randomised study of the effect of transdermal and intracoronary glyceryl trinitrate on ischaemia during coronary angioplasty.

PATIENTS

40 patients with isolated severe stenosis of the left anterior descending coronary artery.

INTERVENTIONS

Patients were randomised (double blind) to transdermal glyceryl trinitrate (10 mg per day) and placebo, starting four to six hours before angioplasty. After 4 one-minute balloon inflations intracoronary glyceryl trinitrate was injected (0.2 mg) and then 4 further one-minute inflations were performed.

MAIN OUTCOME MEASURES

The time to angina and the time to > 0.2 mV ST shift on surface electrocardiogram (ECG) or intracoronary ECG during the individual inflations.

RESULTS

These times did not significantly differ during initial inflations between transdermal glyceryl trinitrate (27 (11), 25 (9), and 19 (9) s, respectively) and placebo (34 (11), 30 (8), and 21 (7) s. After intracoronary glyceryl trinitrate, they were significantly prolonged compared with the initial values, without differences between patients with transdermal glyceryl trinitrate (37 (10), 30 (8), and 23 (8) s, respectively) or placebo (39 (15), 36 (11), and 28 (12) s). Ischaemic preconditioning was not seen.

CONCLUSIONS

Transdermal glyceryl trinitrate (10 mg per day), unlike intracoronary glyceryl trinitrate, did not alleviate the myocardial ischaemia produced by balloon inflation during coronary angioplasty.

Magnitude and nature of noncompliance with treatment using isosorbide dinitrate in patients with ischemic heart disease

Isosorbide dinitrate is one of the most commonly prescribed medications for the treatment of ischemic heart disease. It has been demonstrated to be ineffective relative to placebo when taken inappropriately. This study objectively documents the magnitude and nature of compliance in 68 ambulatory patients who were prescribed isosorbide dinitrate three times a day. Each patient received a 9-week supply of medication in a vial equipped with a computerized monitor (MEMS-4 cap; APREX Corporation, Fremont, CA) and were informed as to the purpose of the study. Of the patients, 74% were classified into the low and 16% into the high compliance category, defined as compliant < 70% and > or = 85% of study days, respectively. The mean (+/- SD) percent days in which isosorbide dinitrate was taken three times was 66% (+/- 29), and the mean number of days in which it was taken three times with a 10-hour "nitrate free period" was 53% (+/- 31). It is concluded that the magnitude of noncompliance in patients prescribed isosorbide dinitrate three times daily is substantial (74%). The nature of this noncompliance is often due to failure to observe a 10- to 12-hour nitrate free period. Given the need to take isosorbide dinitrate appropriately and given the demonstrated magnitude of noncompliance, physicians should take these factors into consideration when selecting this agent for the management of coronary artery disease relative to other pharmacologic options.

Intermittent transdermal nitrates do not improve ischemia in patients taking beta-blockers or calcium antagonists: potential role of rebound ischemia during the nitrate-free period

OBJECTIVES

This study was conducted to determine whether rebound ischemia occurs during nitrate-free periods with intermittent cutaneous nitroglycerin therapy in patients with angina pectoris who are receiving background antianginal therapy.

BACKGROUND

Rebound angina has been suggested to be a complication of the nitrate-free period with long-term cutaneous nitroglycerin therapy given intermittently to prevent tolerance.

METHODS

Fifty-two patients with stable effort angina taking either a beta-adrenergic blocking agent (n = 25) or diltiazem (n = 22) or their combination (n = 5) completed a randomized, double-blind, placebo-controlled crossover study of cutaneous nitroglycerin patches (50 mg). Active or placebo patches were worn for 1 week, applied at 8 AM and removed at 10 PM to provide a 10-h daily nitrate-free (or placebo-free) period. During the last 48 h of each study phase, a Holter monitor was used to detect ischemia.

RESULTS

Only 31 patients experienced ischemia during either phase of the study (23 during the patch-off period). A total of 463 ischemic episodes were recorded: 246 during placebo and 217 during nitroglycerin (p = 0.8, for per patient comparison). The majority (88%) of ischemic episodes were silent. Mean (+/- SEM) duration of ischemia during the total 48-h period was similar during active and placebo phases (35.5 +/- 15.0 min/24 h for active therapy vs. 29.7 +/- 9.8 for placebo, p = 0.8). This was due to an increase in duration of ischemia with active therapy during the patch-off period (46.9 +/- 17.9 min/24 h for active therapy vs. 22.5 +/- 9.2 for placebo, p = 0.07) and a decrease during the patch-on period (27.5 +/- 14.0 min/24 h for active therapy vs. 34.5 +/- 11.0 min/24 h for placebo, p = 0.16). The pattern of diurnal distribution of ischemic episodes differed between active and placebo phases. During placebo there was a nadir in the incidence of ischemia in the overnight patch-off period, with a significantly lower incidence between midnight and 6 AM (25 episodes) compared with the mean number of episodes during the three other 6-h periods (73 episodes, p < 0.001). During the nitroglycerin patch-off period, there was a loss of this overnight nadir, with the same incidence of ischemia between midnight and 6 AM (53 episodes) as the mean number of episodes for the three other 6-h periods (54 episodes).

CONCLUSIONS

The majority of patients taking background antianginal therapy experienced no ischemia during the patch-off period. In the 44% of patients with ischemia during this period, there was a nonsignificant increase in the duration of ischemia with active therapy. Although this result was statistically inconclusive, the change in the distribution of diurnal ischemia offers suggestive evidence that rebound ischemia may be a problem with regard to intermittent cutaneous nitroglycerin.

Which drug to choose for stable angina pectoris: a comparative study between bisoprolol and nitrates

The choice between beta-blockade or nitrates as first line treatment for stable angina pectoris is based upon the different mechanisms of action and patient characteristics. We performed a clinical trial comparing the efficacy of the longacting beta-blocker bisoprolol once daily and the short acting nitrate, isosorbide dinitrate, three times daily in the reduction of anginal complaints in daily life and under stress. Thirty patients were enrolled in a double-blind randomised cross-over study. Both bisoprolol and isosorbide dinitrate were effective in reducing anginal attacks and nitroglycerin consumption significantly, but bisoprolol was significantly more effective than isosorbide dinitrate. Bisoprolol improved the workload during bicycle exercise testing significantly, but the improvement with isosorbide dinitrate was not significant. Despite the reduction in maximal rate pressure product, bisoprolol was significantly (P < 0.05) more effective at improving total workload and reducing the time to onset of angina than isosorbide dinitrate. The rate pressure product did not change significantly with isosorbide dinitrate. In this study, bisoprolol 10 mg once daily was more effective and caused less side effects than isosorbide dinitrate 20 mg three times a day. It seems questionable if monotherapy of isosorbide dinitrate 20 mg t.i.d is an adequate drug regime for stable angina pectoris.

Risk of rebound phenomenon during nitrate withdrawal

Organic nitrates are first-line drugs in the therapy and prevention of angina. These compounds, are acutely effective yet some formulations demonstrate a rapid decline in effect with chronic use. In this review the mechanisms of development of nitrate tolerance and the different strategies to prevent it are considered. If frequent dosing, high dosages and long acting preparations giving constant 24 h plasma GTN levels are more likely to cause tolerance, nitrate-low periods seem to be effective in restoring the drug's efficacy. Intermittent therapy with GTN patches, an effective way to prevent tolerance, raises the problem of the rebound phenomenon during the removal period. Considerable variations in its occurrence have been reported and in this review the factors that may influence the incidence of the rebound are discussed. The dangers of rebound can be lessened by concomitant anti-anginal drugs or avoiding any abrupt decline in blood nitrate concentrations. The use of beta-blockers or calcium channel blockers during intermittent therapy with GTN patches and oral preparations of isosorbide dinitrate or isosorbide 5-mononitrate seem to be effective for this purpose.

Efficacy of intermittent (eight hours off) transdermal nitrate therapy in stable angina

The efficacy of intermittent (16 h on/8 h off) transdermal nitrate therapy (0.4 mg/h) was assessed in 46 patients with chronic stable angina, all but one of whom were on concomitant beta-blocker and/or calcium antagonist therapy. The study was a randomised, double-blind placebo-controlled crossover with two consecutive 7-day treatment periods, conducted at two centres. Prior to entry into the study patients were screened for exercise test reproducibility and nitrate response. Patch efficacy was determined by maximal symptom limited treadmill stress testing (Bruce protocol) and subjective diary card data. Analysis of variance showed that active treatment significantly improved time to onset of angina (P < 0.001), time to 1 mm ST-segment depression (P < 0.001) and total exercise duration (P < 0.001) compared with placebo from 385.7, 310.8 and 458.1 s to 455.7, 385.2 and 497.1 s, respectively. Nitrate patch therapy significantly reduced diastolic and systolic blood pressures without significantly increasing resting heart rate. There was a marginal reduction in the incidence of angina attacks and sublingual nitroglycerine consumption during active treatment. We conclude that intermittent transdermal nitrate therapy significantly improves exercise tolerance in patients with chronic angina pectoris and that this improvement occurs in addition to conventional anti-anginal therapy.

Placebo effect of nitrate monotherapy for myocardial ischemia

Nitrate monotherapy was assessed by treadmill exercise stress testing in 18 patients with significant but relatively asymptomatic myocardial ischemia who were receiving no other antianginal therapy. In addition, prolonged ambulatory electrocardiographic monitoring was performed in 7 patients with demonstrable ischemia during baseline monitoring. After baseline assessment, 5 treatment periods were used in a random order (each of 1 week duration), incorporating 2 dose levels of transdermal nitrate (10 and 20 mg/24 hours) and isosorbide dinitrate (ISDN) (30 and 60 mg/day in divided doses) with a 10-hour nitrate-free interval every 24 hours, as well as a placebo period using a double-blind technique. All treatment periods (including placebo) showed a significant (p < 0.01) 45 to 69% prolongation in the time to 1 mm ST depression during exercise. Paired baseline times of 231 +/- 28 and 233 +/- 30 seconds increased to 367 +/- 37 seconds with 30 mg/day of ISDN, 393 +/- 37 seconds with 60 mg/day of ISDN, 381 +/- 31 seconds with 10 mg/day of transdermal nitrate, and 372 +/- 33 seconds with 20 mg/day of transdermal nitrate. The value for placebo was 342 +/- 29 seconds, which was not significantly different from active treatment (p > 0.1). Some patients appeared to individually respond to > or = 1 nitrate preparation significantly more than to placebo, but this appeared to be unpredictable and largely independent of dosage level and route of administration. There was a qualitatively similar but statistically insignificant reduction in total ischemic time during ambulatory monitoring.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of 8:00 a.m. and 2:00 p.m. doses of isosorbide-5-mononitrate during twice-daily therapy in stable angina pectoris

"Trough" plasma concentrations of isosorbide-5-mononitrate (IS-5-MN), an active metabolite of isosorbide dinitrate, of less than 95 ng/ml are considered necessary to prevent development of tolerance to isosorbide dinitrate and IS-5-MN. In a double-blind, crossover, placebo-controlled study, the effects of IS-5-MN during twice daily eccentric therapy were evaluated in 18 patients with reproducible exercise-induced angina who were nitrate responders. In a random order, patients received either placebo or IS-5-MN (20 mg) at 8 a.m. and 2 p.m. for 1 week each. Average trough plasma IS-5-MN concentrations before the 8 a.m. and 2 p.m. doses were 67 and 226 ng/ml, respectively, and increased to 382 and 488 ng/ml 2 hours after the 8 a.m. and 2 p.m. doses, respectively. Despite a more than threefold higher trough plasma IS-5-MN concentration before the 2 p.m. dose than before the 8 a.m. dose, the increase in exercise duration 2 hours after the doses was similar (1.21 minutes [21%] after 8 a.m. dose, and 1.08 minutes [19%] after 2 p.m. dose). These increases in exercise duration after IS-5-MN were significantly (p less than 0.01) greater than those observed after placebo (0.17 minutes [3%] after 8 a.m. dose, and -0.05 minute [-0.5%] after 2 p.m. dose). Reduction in standing systolic blood pressure at 2 hours after the doses was also nearly identical after the 8 a.m. and 2 p.m. doses of IS-5-MN (21 [15%] and 19 [14%] mm Hg, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and long-term effects on myocardial ischemia of intermittent and continuous transdermal nitrate therapy in stable angina

The aim of this study was to compare the efficacy and safety of continuous and intermittent transdermal nitrate therapy using ambulatory electrocardiographic (Holter) monitoring. Eighty-five patients with stable angina pectoris and positive exercise test results participated during their concomitant antiischemic medication in a randomized open trial lasting 12 weeks. After a 3-week run-in period with continuous therapy (10 mg/24 hours), patients were randomized to either continuous- or intermittent-therapy groups. In the intermittent-therapy group the patients removed their patch at night (the mean patch-off period was 10 hours). Forty-eight-hour Holter monitoring was performed in each patient after randomization, and again after 2 and 12 weeks. Eighteen patients withdrew, 9 in each group. A total of 11,194 hours of electrocardiography were recorded and 607 ischemic episodes were detected, of which 79% were asymptomatic and 95% appeared during daytime. The number of ischemic episodes per 48 hours with intermittent therapy was 3.1 +/- 0.7 (mean +/- SEM) after randomization, 1.8 +/- 0.4 at 2 weeks and 2.0 +/- 0.6 at 12 weeks. With continuous therapy the respective numbers were 3.8 +/- 1.1, 3.5 +/- 0.9 and 4.2 +/- 1.2. The differences were not statistically significant because a large number of patients (30%) had no ischemic episodes on Holter recording. However, when examining 47 patients with episodes during the study, the number of episodes was significantly reduced in the intermittent-therapy group (p less than 0.05 at 12 weeks). The changes in asymptomatic and symptomatic episodes were concordant. No changes and differences between the treatment groups were seen in nighttime episodes.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic stable angina pectoris. Risk stratification and treatment

Workup of stable angina patients begins with careful history taking and evaluation of various risk factors, physical examination, and a resting electrocardiogram (ECG). A noninvasive exercise stress test is valuable for risk stratification. Abnormalities on a resting ECG or equivocal results on a stress test warrant an exercise test combined with thallium scintigraphy, which is more sensitive and specific. Cardiac catheterization is advisable for patients with chest discomfort and multiple risk factors, even if results of thallium testing are negative. Patients with severe or progressive angina or congestive heart failure should also have cardiac catheterization. Nitrates, beta-adrenergic blockers, and calcium channel blockers are cornerstones of medical therapy. Revascularization with coronary artery bypass graft is recommended for patients with left main coronary artery disease, left ventricular dysfunction, or severe proximal three-vessel coronary artery disease. Percutaneous transluminal coronary angioplasty (PTCA) is a good alternative for one- or two-vessel disease. Three-vessel PTCA can be accomplished, but its real role still remains to be established.

Enhancement of the efficacy of isosorbide dinitrate by captopril in stable angina pectoris

This study was designed to assess whether the angiotensin-converting enzyme inhibitor captopril could potentiate the efficacy of a single dose of oral isosorbide dinitrate (ISDN) in patients with coronary artery disease. Fourteen men (mean age 53 years) with stable angina pectoris were studied. In each patient the efficacy of placebo, captopril (50 to 100 mg), ISDN (10 mg), and a combination of captopril (50 to 100 mg) and ISDN (10 mg) was assessed by repeated exercise treadmill tests performed before and 1, 2, 3 and 6 hours after administration of a single dose. A single-blind, randomized technique was applied. According to the mean data in the whole group of 14 patients, captopril alone produced no improvement in exercise duration to the onset of angina and to angina of moderate severity compared with placebo. The magnitude of ST-segment depression did not significantly change after captopril administration. ISDN alone significantly increased exercise duration to onset of angina and to angina of moderate severity (antianginal effect) and decreased the magnitude of ST-segment depression (antiischemic effect) 1 to 3 hours after administration. Combined administration of ISDN and captopril resulted in more expressed antianginal and antiischemic effects; at 2, 3 and 6 hours these effects with ISDN plus captopril were significantly more pronounced than those with ISDN alone. According to individual data, the most marked potentiation of ISDN efficacy was observed in patients who had poor response to ISDN alone. In all 6 patients in whom ISDN alone was ineffective, after the addition of captopril the desired antianginal effect was obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between organic nitrates and thiol groups

Nitroglycerin and the organic nitrates (RONO2) can be considered prodrugs that require conversion to an active intracellular moiety that initiates vascular smooth muscle relaxation. Vasodilation of veins and arteries occurs when the enzyme guanylate cyclase (GC) is activated, initiating the conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP); this is the final pathway for vascular dilation caused by the nitrovasodilators (organic nitrates, sodium nitroprusside, and molsidomine) as well as endothelium-derived relaxing factor (EDRF). The common denominator appears to be the intracellular production of nitric oxide (NO), which is the activated product of organic nitrate denitration. Nitrate tolerance has been associated with a relative depletion or unavailability of thiol groups that are involved in the initial step of denitration of RONO2. Sulfhydryl groups (SH) are oxidized during this process; with continuous nitrate exposure, decreased nitrate metabolism within the vascular smooth muscle cell occurs as a direct result of the depletion of reduced SH groups. Thus, less NO is formed and cGMP production is diminished, with a subsequent decrease or absence of vasodilation. In addition, SH groups or thiols are required for the production of S-nitrosothiols (RSNO). These short-lived compounds have been identified as an end product of organic nitrate metabolism and as possibly obligatory for the induction of GC. It is unclear, however, as to whether S-nitrosothiols are a necessary by-product of NO production from organic nitrates. It appears that RSNO can be formed outside the cell membrane and may be able to induce vasorelaxation after penetrating the cell and initiating GC activation. Exogenous SH donors, particularly N-acetylcysteine (NAC), have been employed to provide intracellular thiols in efforts to prevent or reverse nitrate tolerance. Nitrate physiologic actions are accentuated following NAC administration in the absence of tolerance. Although controversial, the concept that NAC or other thiols might be able to prevent the development of nitrate tolerance is being actively studied in laboratories around the world. Methionine has also been utilized as an SH donor with some success. Not all data are consistent, however, and the ultimate role of thiol donors for the prevention or reversal of nitrate tolerance remains uncertain. Finally, there has been considerable interest in supplying thiols by use of the SH-containing angiotensin converting enzyme inhibitors, such as captopril. This approach does not seem promising, probably because insufficient thiol can be supplied by therapeutic dosages of these drugs.

Recovery of vascular smooth muscle relaxation from nitroglycerin-induced tolerance following a drug-free interval. A time-course in vitro study

Hemodynamic tolerance occurs upon continuous exposure of vascular tissues to nitroglycerin (NTG). This phenomenon is believed to be due to the depletion of the tissue sulfhydryl (SH) group, which is essential for NTG-induced increase in tissue cyclic GMP and vasorelaxation. To determine the effect of an NTG-free interval on recovery of tissue cyclic GMP accumulation and vasorelaxation following development of NTG tolerance, isolated rat aortic rings were kept in Krebs physiologic buffer at 37 degrees, precontracted with epinephrine, and exposed to NTG. The mean concentration of NTG, which relaxed the rings by 50% (EC50) upon first exposure, was 1.1 x 10(-7) M (N = 20), and vascular cyclic GMP levels after NTG increased from 21 to 46 fmol/mg (P less than 0.02). A second exposure to NTG 15 min later increased the EC50 to 1.3 x 10(-4) M and cyclic GMP levels did not change (P less than 0.001 vs first NTG exposure), indicating tolerance to NTG. However, acetylcholine-mediated relaxation of aortic rings was preserved even in NTG-tolerant rings. A second exposure of tissues to NTG separated by 30, 60, and 120 min from the first exposure progressively decreased the EC50, such that at 120 min the EC50 of NTG was 0.4 x 10(-7) M (P = NS vs first NTG exposure). Tissue cyclic GMP levels increased from 14 to 71 fmol/mg (P = NS vs first NTG exposure). These data confirm development of tolerance to the vasorelaxant effects of NTG following initial exposure. An interval of 2 hr between multiple exposures of tissues to NTG results in preservation of the smooth muscle relaxation and an increase in tissue cyclic GMP in response to NTG.