Nitrate-free Period Research Articles

Indirect sulfur reduction via polysulfide contributes to serious odor problem in a sewer receiving nitrate dosage

Nitrate dosing is commonly used to control hydrogen sulfide production in sewer systems. However, quick rebound of the sulfide concentration after nitrate depletion has been observed and results in more serious odor and corrosion problem. To investigate the mechanism of sulfide regeneration in the nitrate-free period, a laboratory-scale sewer reactor was run for 30 days to simulate sulfide production and oxidation with intermittent nitrate addition. The results show that nitrate addition substantially reduced the sulfide concentration, but the produced elemental sulfur was then quickly reduced back to sulfide in nitrate-free periods. This induced more and more sulfide production in the sewer reactor. Elemental sulfur and polysulfide reductions were found in the sewage in nitrate-free periods, showing their contributions to the sulfide regeneration. Through batch tests, polysulfide was confirmed as the key intermediate for accelerating sulfur reduction during the nitrate-free period in the sewer. Sulfide production rates significantly increased by 65% and 59% in the presences of tetrasulfide and sulfur with sulfide, respectively, at the beginning of the test. While polysulfide formation was prevented by the ferrous chloride addition, the sulfur reduction rate remarkably decreased from 12.8 mgS/L-h to 1.8 mgS/L-h. This indicates that direct sulfur reduction was significantly slower than the indirect sulfur reduction via polysulfide; the latter process could be the cause for the quick rebound of the sulfide concentration in the sewer with intermittent nitrate dosing. Thus, the pathways of sulfur transformations in a sewer, both in the presence and absence of nitrate, were proposed. Microbial community analysis results reveal that some common sulfate-reducing bacteria (SRB) genera in sewer sediment were possible sulfur reducers. According to this finding, the effect and strategy of nitrate dosing for hydrogen sulfide control in sewers should be re-evaluated and re-considered.

Effect of calcitonin gene-related peptide antagonist on the cardiovascular events, mortality, and prostaglandin E2 production by nitrate-induced tolerant rats with acute myocardial infarction.

BackgroundAnti-ischemic effects of NO releasing by nitroglycerin (NTG) and the release of calcitonin gene-related peptide (CGRP) are involved in the decrease of vascular remodeling in different cardiovascular diseases. Using a nitrate-free period is still generally required to prevent nitrate tolerance and should be used as the first-line option to maintain adequate symptom control and on an individual basis. Personalized anti-ischemic concerns require the urgent change of paradigm from interventional measures to predictive, preventive, and personalized treatment with organic nitrates and its combination with drugs that may improve prognosis and drugs that can be added for patients who remain symptomatic despite therapy with the other classes of agents. The purpose of this study was to evaluate the influence of human calcitonin gene-related peptide antagonist (CGRP8-37) on cardiohemodynamic events, prostaglandin E2 (PGE2) plasma concentration, the severity of ventricular arrhythmias, and mortality occurring during acute myocardial infarction (AMI) in NTG-tolerant and nontolerant rats.MethodsIn the pilot study of efficacy of calcitonin gene-related peptide antagonist (CGRP8-37), 58 male Wistar rats were included. All procedures were performed according to protocols approved by the General Animal Care and Use Committee. Adult male rats underwent surgery to induce AMI by ligating the left anterior descending coronary artery or SHAM. ECG was used to confirm myocardial ischemia. In each experiment, a rat was maintained under anesthesia for the duration of the experiment. At the end of the experiment, the rat was killed by an overdose of pentobarbital. All animals in accordance with the received pharmacological agent were randomized into three groups: I—received only NTG, 50 mg/kg daily, s.c. injections b.i.d. 3 days prior to AMI; II—received NTG by the same dose, route, and frequency of administration + CGRP antagonist (CGRP8-37), 10 μg/kg two times daily by a similar period of administration; and III—served as control (C) group without preliminary tolerance to NTG.ResultsSubcutaneous injections of NTG (50 mg/kg) 30 min prior to AMI in NTG-tolerant animals (group I) and in NTG-tolerant rats + CGRP antagonist (group II) caused minor changes in blood pressure and heart period that was accompanied before NTG s.c. administration with blunted baroreflex sensitivity in response to i.v. administration of sodium nitroprusside in these groups of rats (0.66 ± 0.05 and 0.56 ± 0.04 ms/mmHg, P < 0.05, respectively) in comparison to C (group III) animals (0.9 ± 0.1 ms/mmHg). AMI 1 h duration was associated with a high incidence of ventricular arrhythmia and significant mortality in group I (70 %) and especially in group II (90 %) animals at 72 h after reperfusion as compared with group III rats (56 %), that correlated to a decrease of PGE2 plasma content in group II (2.2 ± 0.4 ng/ml, P < 0.001) and group I (3.6 ± 0.2 ng/ml, P < 0.01) vs. control group of rats (4.8 ± 0.3 ng/ml).ConclusionsCGRP could be involved in the mechanism of nitrate tolerance via the inhibition of release of the potent vasodilator CGRP leading to exacerbation of acute myocardial ischemia. The influence of CGRP antagonist could enhance this condition.

Rationale for the Use of High Dose Sustained-Release Isosorbide-5-Mononitrate in Ischemic Heart Disease and Chronic Heart Failure

Isosorbide-5-mononitrate is one of the two pharmacologically active metabolites of isosorbide dinitrate. At variance from its parent drug, it has a longer elimination half-life, no metabolic first-pass, and greater bioavailability, allowing once-daily administration as standard or as sustained-release formulations. Several trials have shown that isosorbide-5-mononitrate, in the form of sustained-released capsules administered once daily at doses ranging between 50 and 100 mg, is an effective symptomatic drug for the treatment of stable angina and chronic heart failure (CHF), and is now indicated for these conditions by American and European guide- lines. In particular, at 80 mg once-daily sustained-release isosorbide-5-mononitrate has been shown to have trough plasma levels below the minimum therapeutic concentration (100 ng/mL), ensuring a nitrate-free period as sufficient as to avoid nitrate tolerance. This 80 mg dosage is the only high dose sustained-release formulation of isosorbide-5-mononitrate currently marketed in Italy. Isosorbide-5-mononitrate also exerts positive hemodynamic effects (reduction in filling pressure and systemic vascular resistance, with increase in cardiac output) in heart failure in as- sociation with standard medical therapy and hydralazine, with a positive impact on patient prog- nosis. (Heart International 2007; 3: 98-111)

Optimal nitrate therapy with a once-daily sustained-release formulation of isosorbide mononitrate.

During long-term prophylaxis of angina with oral nitrates, sustained high plasma nitrate concentrations produce partial or complete tolerance to both the haemodynamic and the clinical effects of the drug. There is substantial evidence that this can be prevented by an adequate nitrate-free or nitrate-low period during each 24 h dosing interval. However, a nitrate-free interval carries the risk of a rebound increase in myocardial ischaemia. Once-daily formulations of isosorbide mononitrate deliver high plasma nitrate concentrations that improve exercise tolerance in patients with angina for at least 12 h after dosing. During the remainder of the dosage interval, plasma nitrate concentrations fall but are sufficient to protect against coronary artery spasm overnight. Myocardial ischaemia has a marked circadian rhythm. All ischaemic events (total ischaemic burden, myocardial infarction and sudden cardiac death) are most frequent in the hours immediately after waking. Oral anti-ischaemic prophylaxis should ideally provide protection during this critical period, in order to minimize symptoms, maximize exercise capacity and perhaps also to reduce the risk of clinical events. The ideal long-acting nitrate formulation should therefore provide a rapid rise in plasma nitrate concentration as well as maintaining prolonged efficacy throughout the dosing interval. Elantan LA is a sustained-release capsule formulation of isosorbide mononitrate for once-daily dosing. This capsule contains pellets which release 30% of the dose immediately, while 70% is released slowly to maintain the therapeutic response. The pharmacokinetic profile of this formulation prevents the development of tolerance, while also conferring long-term anti-anginal efficacy. Patients reported an improvement in both severity of angina and quality of life indices when their therapy was changed from multiple daily dosing with isosorbide dinitrate to once-daily dosing with Elantan LA (50 mg). The anti-anginal effect of Elantan LA is attained rapidly after dosing. Within 30 min of ingestion, there are clinically significant improvements in exercise tolerance, comparable with the speed of onset after an immediate-release formulation of isosorbide mononitrate. Elantan LA is an effective once-daily prophylaxis for angina which also produces a rapid onset of therapeutic effect. The release profile of this formulation maximizes protection against the morning surge in myocardial ischaemia.

The effect of continuous versus intermittent treatment with transdermal nitroglycerin on pacing-induced preconditioning in conscious rabbits.

1. Tolerance to the hypotensive effect of nitroglycerin (NG) blocks preconditioning induced by rapid ventricular pacing (RVP) in rabbits. In the present work the effect of continuous versus intermittent treatment with transdermal nitroglycerin on the pacing-induced preconditioning phenomenon was studied in conscious rabbits. 2. RVP (500 beats min-1 over 5 min) increased left ventricular end-diastolic pressure (LVEDP) from baseline 4.1 +/- 0.9 to postpacing 13.8 +/- 2.9 mmHg (P < 0.001) with a right intraventricular ST-segment elevation of 1.25 +/- 0.13 mV, two indicators of myocardial ischaemia. These changes were significantly attenuated when the RVP period was preceded by a preconditioning pacing of the same rate and duration with an interpacing interval of 5 min. 3. Protection by preconditioning was abolished when the animals had been made tolerant to the vasodilator effect of 30 micrograms kg-1 NG by the application of transdermal NG (approx. 0.07 mg kg-1 h-1) over 7 days. Furthermore, transdermal NG per se attenuated both RVP-induced ST-segment elevation and LVEDP-increase over the 7 day period. 4. With intermittent transdermal NG treatment (12 h 'patch on' vs 'patch off'), neither development of vascular tolerance nor attenuation of the NG- or preconditioning-induced anti-ischaemic effects were observed. However, the severity of pacing-induced myocardial ischaemia was significantly increased during the 'patch off' periods. 5. In a second set of experiments, postpacing changes in cardiac cyclic GMP and cyclic AMP levels were determined by means of radioimmunoassay in chronically instrumented anaesthetized open-chest rabbits with the same NG-treatment protocols. Preconditioning reduced postpacing increase in cyclic AMP with an increase in cyclic GMP concentrations in hearts of the untreated animals and in those given patches intermittently during both 'patch on' and 'patch off' periods. However, the preconditioning effect on either cyclic nucleotide was blocked in the tolerant animals. 6. Transdermal NG increased resting levels of both cardiac cyclic nucleotides in the non-tolerant but not in the tolerant state. The postpacing increase in cyclic AMP content was inhibited by transdermal NG, independent of vascular tolerance development, whereas an cyclic GMP content was exclusively seen in the non-tolerant animals. 7. We conclude that the anti-ischaemic effect of NG is independent of the cyclic GMP mechanism in the tolerant state. While intermittent NG therapy prevents development of vascular tolerance and preserves preconditioning, the nitrate-free periods yield an increased susceptibility of the heart to ischaemic challenges.

Nitrate tolerance, rebound, and their clinical relevance in stable angina pectoris, unstable angina, and heart failure

Vascular tolerance develops rapidly in isolated vascular strips exposed to millimolar concentrations of nitroglycerin. Several mechanisms, including depletion of sulfhydryl groups, reduced biotransformation of nitrates to NO or nitrosothiols, oxygen free radical injury, and downregulation of a membrane-bound enzyme or a nitrate receptor, have been proposed, but the exact mechanism responsible for in-vitro tolerance remains unknown. In-vivo tolerance of the beneficial effects of nitrates on hemodynamics, myocardial ischemia, and exercise performance develops rapidly. It has been suggested, but remains to be proven, that development of venous tolerance and not arterial tolerance is responsible for the attenuation of nitrate effects during long-term nitrate therapy. Several mechanisms, including neurohormonal activation, depletion of sulfhdryl groups, and the shift of fluid from the extravascular to intravascular compartment have been implicated. However, the use of agents to counteract these mechanisms (ACE inhibitors, sulfhydryl donors, diuretics) has produced conflicting results. Thus, at present the mechanism responsible for in vivo tolerance to nitrates remains unknown. Both in vitro and in vivo vascular tolerance to nitrates can be prevented or minimized by providing nitrate-free or low-nitrate intervals. However, during nitrate-free periods, rebound phenomena (rest angina in patients with ischemic heart disease or a deterioration in exercise performance prior to the renewal of the morning dose in patients with stable angina) remain a clinical concern. When treating patients with stable angina pectoris, it must be recognized that none of the nitrate preparations or formulations can provide round-the-clock antianginal or antiischemic prophylaxis. In these patients, beneficial antianginal and antiischemic effects of nitrates for 10-14 hours during the daytime can be maintained by using formulations and dosing regimens that avoid or minimize the development of tolerance (standard formulation of isosorbide-5-mononitrate, 20 mg in the morning and 7 hours later; slow-release formulation of isosorbide-5-mononitrate, 120-240 mg once a day; or nitroglycerin patch delivering 0.6 nitroglycerin per hour for 10-12 hours each day). Only the patch on and off treatment is associated with nitrate rebound. Although intermittent nitrate therapy is not associated with the development of tolerance, this strategy cannot be recommended for treating unstable angina because rebound angina during nitrate-free periods complicates clinical decision making. In the acute phase of unstable angina, continuous treatment with intravenous nitroglycerin is recommended because it permits rapid up- or down-titration. Tolerance towards antianginal and antiischemic effects does develop in a substantial number of patients with 24 hours, but this can be overridden by dose escalation and restoration of the therapeutic effectiveness of nitroglycerin. Tolerance towards the beneficial effects of nitrates on hemodynamics and on exercise performance also develops rapidly during continuous or long-term nitrate therapy, and for these reasons nitrates are not used as first-line therapy to treat chronic heart failure. In combination with hydralazine, high-dose isosorbide dinitrate (30-40 mg four times a day) improves survival, but this combination therapy is inferior to ACE inhibitors.

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 blockign 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 amand removed at 10 pmto 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 ovenight 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 anti-anginal 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.

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.

Mechanisms Involved in the Development of Tolerance to Nitrovasodilators

The identification of endothelium-derived relaxing factor (EDRF) as nitric oxide (NO) has highlighted the clinical importance of NO-containing drugs (nitrovasodilators and molsidomine). The development of tolerance and cross-tolerance is a well-known problem with the classic nitrovasodilators. Although counterregulatory reflex mechanisms might be involved, desensitization of the vascular smooth muscle cells is the best documented mechanism of tolerance. Impaired metabolization of organic nitrates due to deficiency of reduced thiol groups seems to be the major mechanism responsible for this vascular desensitization. Tolerance preventing strategies (nitrate-free periods, administration of N-acetylcysteine, and use of drugs that do not require thiolgroups for activation of guanylate cyclase) are based on this mechanism.

Circadian Influence in Cardiovascular Disease (Part 1)

This department will offer stylized clinical presentations of current cardiovascular issues. Questions raised as a case history evolves highlight key elements that are elaborated upon in the discussions which follow.

Effects of dosing intervals on the development of tolerance to high dose transdermal nitroglycerin

To investigate the antiischemic efficacy and development of tolerance to transdermal nitroglycerin, 14 patients with chronic, stable angina pectoris were studied using continuous ambulatory electrocardiographic monitoring. Patients demonstrated initial hemodynamic responsiveness to sublingual nitroglycerin and were titrated to a maximally tolerated dose of 30 to 60 mg/24 hours (52 ± 5 mg). Two crossover phases were used in a randomized, double-blind, placebo-controlled manner: continuous nitroglycerin therapy (patches containing active drug worn for 24 hours) and intermittent nitroglycerin therapy (12-hour active drug followed by a 12-hour nitrate-free period). There were no differences in frequency or duration of ischemic episodes between the placebo days of each phase. A significant effect in frequency of episodes was observed between placebo and treatment days of continuous therapy (p < 0.05). Nonsignificant reductions in frequency and duration of ischemic episodes also occurred during intermittent therapy. The major antiischemic effect of transdermal nitroglycerin therapy occurred during the first day of treatment but was lost by 48 hours. Reductions in frequency and duration of ischemic episodes (p < 0.05) were present on day 1 of continuous therapy but ischemic episodes returned to placebo levels by day 2, suggesting the development of tolerance. Intermittent therapy did not prevent the development of tolerance on day 2 of treatment. The results demonstrate that the use of high doses of transdermal nitroglycerin in patients with chronic, stable coronary artery disease produced a beneficial reduction in the frequency and duration of ischemia. However, the antiischemic benefit was lost between 24 and 48 hours after the onset of continuous and intermittent therapy, presumably due to tolerance. Alternate doses and nitrate-free periods require further study to determine if the beneficial effects on ischemia are sustained.

Nitrate tolerance in angina pectoris

Tolerance to the hemodynamic and antianginal effects of the organic nitrates develops rapidly during therapy. This has been documented with a variety of nitrate preparations and with different routes of administration. Dosing strategies designed to provide therapeutic plasma nitrate concentrations throughout the 24 hours of the day are regularly associated with the development of tolerance. Recent information indicates that dosing schedules providing a nitrate-free period will permit continued efficacy of the organic nitrates without tolerance development. This can be accomplished in several ways. With oral preparations of isosorbide dinitrate, medication can be given 3 times daily, omitting the evening dose. Buccal nitroglycerin is usually given 3 times daily after meals, and this has been shown not to be associated with tolerance. Likewise, preliminary studies suggest that removal of the nitroglycerin patches for a period of several hours each day will prevent the development of tolerance. Much needs to be learned about the nitrate-free period. With oral isosorbide dinitrate, it appears that 12 hours is required, but this could be substantially less with nitroglycerin preparations. It is also possible that several short periods with low nitrate levels each day would suffice. Thus, simply increasing the interval between dosing of oral preparations may provide an adequate, low, nitrate-free period.

Nitrate tolerance

The organic nitrates are the most widely used agents in the management of patients with angina pectoris. When initially administered by the oral route, the nitrates produce profound changes in systemic hemodynamics and significant and prolonged improvement in exercise duration. It has been shown that during short periods of regular oral nitrate administration, the hemodynamic, antiischemic and antianginal effects of the nitrates are greatly reduced. Thus, when initially administered, oral isosorbide dinitrate prolongs exercise duration for a period of several hours, but during sustained 4-times-daily therapy, exercise tolerance is improved for only 2 hours after administration. Studies with transdermal preparations of isosorbide dinitrate and nitroglycerin also show improvement during short-term administration for up to 8 hours, but after several days of once-daily therapy, the effects of these agents are simiar to placebo. It is apparent that nitrate tolerance is a clinically relevant problem. Although tolerance develops rapidly during nitrate therapy, it is reversed promptly during nitratefree periods. Oral nitrates maintain their antianginal effects when given 2 or 3 times daily with provision of a nitrate-free period. Studies are currently underway to investigate the effects of intermittent administration schedules with transdermal nitrate preparations.