Arterial-venous nitroglycerin gradient during intravenous infusion in man

The Role of Nitroglycerin and Other Nitrogen Oxides in Cardiovascular Therapeutics

The use of nitroglycerin in the treatment of angina pectoris began not long after its original synthesis in 1847. Since then, the discovery of nitric oxide as a biological effector and better understanding of its roles in vasodilation, cell permeability, platelet function, inflammation, and other vascular processes have advanced our knowledge of the hemodynamic (mostly mediated through vasodilation of capacitance and conductance arteries) and nonhemodynamic effects of organic nitrate therapy, via both nitric oxide-dependent and -independent mechanisms. Nitrates are rapidly absorbed from mucous membranes, the gastrointestinal tract, and the skin; thus, nitroglycerin is available in a number of preparations for delivery via several routes: oral tablets, sublingual tablets, buccal tablets, sublingual spray, transdermal ointment, and transdermal patch, as well as intravenous formulations. Organic nitrates are commonly used in the treatment of cardiovascular disease, but clinical data limit their use mostly to the treatment of angina. They are also used in the treatment of subsets of patients with heart failure and pulmonary hypertension. One major limitation of the use of nitrates is the development of tolerance. Although several agents have been studied for use in the prevention of nitrate tolerance, none are currently recommended owing to a paucity of supportive clinical data. Only 1 method of preventing nitrate tolerance remains widely accepted: the use of a dosing strategy that provides an interval of no or low nitrate exposure during each 24-h period. Nitric oxide's important role in several cardiovascular disease mechanisms continues to drive research toward finding novel ways to affect both endogenous and exogenous sources of this key molecular mediator.

Efficacy of different forms of nitrates in angina pectoris

Nitroglycerin has maintained its position in the treatment of angina pectoris for more than a century. Efficacy of oral nitrates has been established and compares well with that of other anti-anginal drugs. New delivery systems are being developed for sustained systemic nitrate action. Beneficial action of nitrates in congestive heart failure and their crucial role in unstable angina and acute myocardial infarction has further widened their therapeutic use. A plausible hypothesis of the mechanism of nitrate-induced vasodilation has been presented, involving production of nitrosothiols and activation of guanylate cyclase in the vascular smooth muscle. Recent developments suggest that the rate degradation of nitrates and formation of nitrosothiols in the vascular smooth muscle are linked, offering an explanation to the relatively rapidly developing, but partial vascular tolerance during high-dose nitrate therapy.

Nitroglycerin attenuates human endothelial progenitor cell differentiation, function, and survival

Endothelial progenitor cells (EPCs) participate in angiogenesis and the response to chronic ischemia. Risk factors and cardiovascular disease attenuate EPC number, function, and survival. Continuous therapy with nitroglycerin (glyceryl trinitrate; GTN) is associated with increased vascular oxidative stress, leading to nitrate tolerance and endothelial dysfunction. Thus, GTN therapy may also affect EPCs. The purpose of this study was to determine whether continuous exposure to GTN in vivo or during ex vivo expansion affects the circulating number and functional characteristics of human EPCs. To determine the effects of continuous in vivo GTN exposure, EPCs isolated from 28 healthy males before and after receiving 0.6 mg/h GTN (n=17) or no treatment (n=11) for 1 week were expanded for 6 days and compared. To determine the effects of continuous ex vivo GTN exposure, EPCs isolated before randomization were expanded for 6 days in medium supplemented with 100 nM, 300 nM, or 1 microM GTN. EPCs expanded without GTN served as controls (n=10). In vivo, GTN exposure significantly increased the percentage of circulating cells expressing the EPC marker CD34 and increased the susceptibility of expanded EPCs to apoptosis but had no impact on the phenotypic differentiation or migration of EPCs. Ex vivo, GTN exposure increased apoptosis while decreasing phenotypic differentiation, migration, and mitochondrial dehydrogenase activity of EPCs, compared with EPCs expanded in the absence of GTN. Taken together, these results suggest that continuous GTN therapy might impair EPC-mediated processes, an effect that could be detrimental in the setting of ischemic cardiovascular disease.

Clinical pharmacokinetics and pharmacodynamics of glyceryl trinitrate and its metabolites

This review discusses the pharmacokinetics and pharmacodynamics of glyceryl trinitrate (nitroglycerin; GTN) pertinent to clinical medicine. The pharmacokinetics of GTN associated with various dose regimens are characterised by prominent intra- and inter-individual variability. It is, nevertheless, important to clearly understand the pharmacokinetics and characteristics of GTN to optimise its use in clinical practice and, in particular, to obviate the development of tolerance. Measurements of plasma concentrations of GTN and of 1,2-glyceryl dinitrate (1,2-GDN), 1,3-glyceryl dinitrate (1,3-GDN), 1-glyceryl mononitrate (1-GMN), and 2-glyceryl mononitrate (2-GMN), its four main metabolites, remain difficult and require meticulous techniques to obtain reliable results. Since GDNs have an effect on haemodynamic function, pharmacokinetic analyses that include the parent drug as well as the metabolites are important. Although the precise mechanisms of GTN metabolism have not been elucidated, two main pathways have been proposed for its biotransformation. The first is a mechanism-based biotransformation pathway that produces nitric oxide (NO) and contributes directly to vasodilation. The second is a clearance-based biotransformation or detoxification pathway that produces inorganic nitrite anions (NO(2) -). NO(2) - has no apparent cardiovascular effect and is not converted to NO in pharmacologically relevant concentrations in vivo. In addition, several non-enzymatic and enzymatic systems are capable of metabolising GTN. This complex metabolism complicates considerably the evaluation of the pharmacokinetics and pharmacodynamics of GTN. Regardless of the route of administration, concentrations of the metabolites exceed those of the parent compound by several orders of magnitude. During continuous steady-state delivery of GTN, for instance by a patch, concentrations of 1,2-GDN are consistently 2-7 times higher than those of 1,3-GDN, and concentrations of 2-GMN are 4-8 times higher than those of 1-GMN. Concentrations of GDNs are approximately 10 times higher, and of GMNs approximately 100 times higher, than those of GTN during sustained administration. The development of tolerance is closely related to the metabolism of GTN, and can be broadly categorised as haemodynamic tolerance versus vascular tolerance. Efforts are warranted to circumvent the development of tolerance and facilitate the use of GTN in clinical practice. Although this remains to be accomplished, it is likely that, in the near future, regimens will be developed based on a full understanding of the pharmacokinetics and pharmacodynamics of GTN and its metabolites.

Vasodilator responses of coronary conduit and resistance arteries to continuous nitroglycerin infusion in humans: a Doppler guide wire study

To examine the responses of coronary conduit and resistance arteries to the continuous i.v. administration of nitroglycerin in 15 patients with atypical chest pain, we measured coronary blood flow velocity in the left anterior descending coronary artery using a Doppler guide wire and the lumen diameter and cross-sectional area by quantitative coronary angiography. Systolic flow, diastolic flow, total coronary flow, and coronary vascular resistance were calculated. Stepwise increases in dose of nitroglycerin resulted in significant dose-dependent decrease in mean aortic pressure (p < 0.01) and increase in lumen diameter (p < 0.05). After nitroglycerin administration of 0.5 microg/kg/min, systolic flow decreased significantly by 89.9+/-15.7% (p < 0.01), and diastolic flow increased significantly by 74.2+/-37.1% (p < 0.05). Total coronary flow did not change significantly with the various doses of nitroglycerin. However, coronary vascular resistance decreased significantly at concentrations greater than 0.5 microg/kg/min nitroglycerin. Continuous nitroglycerin infusion did not reduce either diastolic or total coronary blood flow despite a significant reduction in coronary perfusion pressure. These results indicate that subendocardial blood flow might be maintained during continuous i.v. infusion of nitroglycerin within the clinical dose range.

Arteriovenous differences in plasma concentration of nicotine and catecholamines and related cardiovascular effects after smoking, nicotine nasal spray, and intravenous nicotine

BACKGROUND AND OBJECTIVES

Delivery of a high concentration bolus of nicotine through the arterial circulation is believed to be an important determinant of the addictive, behavioral, and physiologic effects of nicotine. To better understand the pharmacologic features of nicotine with different routes of administration, we measured arterial and venous plasma concentrations of nicotine, cotinine, epinephrine, and norepinephrine after tobacco smoking, intravenous nicotine infusion, and use of a nicotine nasal spray.

SUBJECTS AND METHODS

Arterial and venous blood samples were drawn simultaneously from 12 male smokers. Six subjects received a single dose of 1 mg nicotine nasal spray, and six subjects smoked cigarettes, one puff per minute for 10 minutes. All 12 subjects were administered nicotine as a 30-minute infusion beginning 70 minutes after administration of the nicotine nasal spray or commencement of smoking.

RESULTS

The mean peak arterial plasma concentrations of nicotine (Cmax) after smoking or administration of nicotine nasal spray, or intravenous nicotine averaged twofold those of venous plasma. For nicotine nasal spray, the time to Cmax was much faster for arterial than for venous plasma (median, 5 versus 18 minutes, p < 0.01). Intravenous nicotine produced the greatest increase in plasma epinephrine concentration, although smoking had a greater chronotropic effect. Acute tolerance to the chronotropic effects of nicotine was suggested at pharmacodynamic analysis with venous nicotine concentrations, whereas analysis of arterial concentrations found the opposite--a time lag between plasma concentration and effect.

CONCLUSION

Nicotine is rapidly absorbed from nicotine nasal spray. The Cmax of nicotine after smoking or administration of nicotine nasal spray, or intravenous nicotine is substantially higher in arterial than venous plasma. Acute tolerance to the chronotropic effects of nicotine is not apparent if arterial plasma concentrations are measured.

Specificity of different organic nitrates to elicit NO formation in rabbit vascular tissues and organs in vivo

1. In the present study we assessed the formation of nitric oxide (NO) from classical and thiol-containing organic nitrates in vascular tissues and organs of anaesthetized rabbits, and established a relationship between the relaxant response elicited by nitroglycerin (NTG) and NO formation in the rabbit isolated aorta. Furthermore, the effect of isolated cytochrome P450 on NO formation from organic nitrates was investigated. 2. Rabbits received diethyldithiocarbamate (DETC; 200 mg kg-1 initial bolus i.p. and 200 mg kg-1 during 20 min, i.v.) and either saline, or one of the following organic nitrates: nitroglycerin (NTG, 0.5 mg kg-1), isosorbide dinitrate (ISDN), N-(3-nitratopivaloyl)-L-cysteine ethylester (SPM 3672), S-carboxyethyl-N-(3-nitratopivaloyl)-L-cysteine ethylester (SPM 5185), at 10 mg kg-1 each. After 20 min the animals were killed, blood vessels and organs were removed, and subsequently analyzed for spin-trapped NO by cryogenic electron spin resonance (e.s.r.) spectroscopy. 3. In the saline-treated control group, NO remained below the detection limit in all vessels and organs. In contrast, all of the nitrates tested elicited measurable NO formation, which was higher in organs (liver, kidney, heart, lung, spleen) (up to 4.8 nmol g-1 20 min-1) than in blood vessels (vena cava, mesenteric bed, femoral artery, aorta) (up to 0.7 nmol g-1 20 min-1). Classical organic nitrates (NTG, ISDN) formed NO preferentially in the mesenteric bed and the vena cava, while the SPM compounds elicited comparable NO formation in veins and arteries. 4. Using a similar spin trapping technique, NO formation was assessed in vitro in phenylephrine-precontracted rabbit aortic rings. The maximal relaxation elicited by a first exposure (10 min) to NTG (0.3 to 10 microM) was positively correlated (r = 0.8) with the net increase (NTG minus basal) of NO spin-trapped during a second exposure to the same concentration of NTG in the presence of DETC. 5. Cytochrome P450 purified from rabbit liver enhanced NO formation in a NADPH-dependent fashion from NTG, but not from the other nitrates, as assessed by activation of purified soluble guanylyl cyclase. 6. We conclude that the vessel selective action of different organic nitrates in vivo reflects differences in vascular NO formation. Thus, efficient preload reduction by classical organic nitrates can be accounted for by higher NO formation in venous capacitance as compared to arterial conductance and resistance vessels. In contrast, NO is released from cysteine-containing nitrates (SPMs) to a similar extent in arteries and veins, presumably independently of an organic nitrate-specific biotransformation. Limited tissue bioavailability of NTG and ISDN might account for low NO formation in the aorta, while true differences in biotransformation seem to account for differences in NO formation in the other vascular tissues.

Use of intra-aortic nitroglycerin in the cardiac catheterization laboratory

The purpose of this study was to determine whether nitroglycerin (NTG) injected into the ascending aorta or left ventricle would safely and effectively lower blood pressure in hypertensive patients undergoing cardiac catheterization. Fifty bolus injections of 297 +/- 67 micrograms (mean +/- SD) NTG were given to patients with a systolic blood pressure (SBP) of > or = 140 mm Hg (mean SBP 188 +/- 20.1 mm Hg). An average drop in systolic blood pressure of 36 +/- 16 mmHg (P < 0.001), diastolic blood pressure of 19 +/- 7 mm Hg (P < 0.001), and left ventricular end-diastolic pressure of 4.7 +/- 4 mm Hg (P = 0.001) was well tolerated in each patient. The mean time to response was 11 +/- 3 sec. Intra-aortic injection of NTG is a safe and effective means to treat hypertensive patients in the cardiac catheterization laboratory.

Compartment model to describe peripheral arterial-venous drug concentration gradients with drug elimination from the venous sampling compartment

The central compartment of an n-compartment mammillary model was concatenated with a gradient compartment (i.e., venous sampling compartment) and with a conventional effect compartment to model arterial-venous drug concentration gradients and the arterial drug concentration vs effect relationship, respectively. To model drug metabolism during transit between the peripheral arterial and peripheral venous circulations, the gradient compartment included a first-order elimination path. Simulations of the model were employed to demonstrate the impact of sampling site (e.g., arterial vs peripheral venous) on analyses of the concentration vs effect relationship.

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.

Pharmacokinetics and pharmacodynamics of nitroglycerin and its dinitrate metabolites in conscious dogs: intravenous infusion studies

Intravenous infusions of nitroglycerin (GTN), 1,2-glyceryl dinitrate (1,2-GDN), and 1,3-glyceryl dinitrate (1,3-GDN) were given to four conscious dogs at 10 micrograms/min, 30 micrograms/min, 50 micrograms/min, and 70 micrograms/min of GTN and 20 micrograms/min and 100 micrograms/min of GDNs. The steady state plasma concentrations (Css) of GTN were reached after about 60 min whereas for 1,2-GDN and 1,3-GDN the Css were reached at about 150 min after the infusion began. Except for one dog, the Css of GTN were not proportional to infusion rate, however, all dogs together showed a good linear relationship between Css of GTN and infusion rates with an average correlation coefficient of 0.917 +/- 0.102. Large variability in GTN clearance after various infusion rates was observed in all dogs. The Css ratios of 1,2-GDN/GTN and 1,3-GDN/GTN yield overall averages of 31.5 +/- 17.2 and 5.47 +/- 3.19, respectively. Average Css ratios of metabolites 1,2-GDN/1,3-GDN were 5.78 +/- 1.23. This ratio is different from those obtained after iv bolus and oral dosing indicating that the biotransformation of GTN to 1,2-GDN and 1,3-GDN differs for each dosing route. The clearances for 1,2-GDN and 1,3-GDN were not changed over the dose range of 20 micrograms/min to 100 micrograms/min. Terminal half-lives of 1,2-GDN and 1,3-GDN postinfusion were similar to those values obtained after a single bolus dose (45 min). It appears that all the GTN dose at steady state can be accounted for by the formation of measurable 1,2-GDN and 1,3-GDN. Large intra- and interdog variations in systolic blood pressure decrease (SPD) following infusions of GTN were observed, however, all dogs showed a clear systolic blood pressure decrease when the highest infusion rate (70 micrograms/min) was given. No significant systolic blood pressure drop was detected following 20 micrograms/min infusions of 1,2-GDN or 1,3-GDN. It was clear that systolic blood pressure in all dogs decreased following 100 microgram/min infusions of 1,2-GDN or 1,3-GDN. When SPD values were plotted vs. log GTN concentrations following the infusion of 70 micrograms/min of GTN in all four dogs, a counterclockwise hysteresis was observed indicating the significant contribution of the active dinitrate metabolites to GTN pharmacodynamics.

The hepatic first-pass metabolism of problematic drugs

The first-pass hepatic metabolism of a number of important therapeutic agents is inconsistent with traditional models that assume that the hepatic extraction ratio of a drug is constant in each individual (independent of the concentration of drug in the hepatic sinusoidal blood and also independent of the history of exposure to the drug). In this review, the authors examine the first-pass metabolism of five "problematic drugs" (propranolol, lidocaine, propafenone, verapamil, and nitroglycerin). Each of these compounds has unique facets to its hepatic clearance and pharmacokinetics as well as striking similarities. Selected aspects of first-pass metabolism are reviewed, and a theory that may explain some of the unusual behavior of the four lipophilic bases (propranolol, lidocaine, propafenone, and verapamil) is presented. Finally, the unusual and variable clearance of nitroglycerin is discussed.

Nitroglycerin dinitrate metabolites do not affect the pharmacokinetics and pharmacodynamics of nitroglycerin in the dog: a preliminary report

Studies were carried out in conscious dogs to determine the effects of 1,2-glyceryl dinitrate (1,2-GDN) and 1,3-glyceryl dinitrate (1,3-GDN) on nitroglycerin (GTN) pharmacokinetics and pharmacodynamics. In the first set of experiments, steady state plasma levels (Css) of either 1,2-GDN or 1,3-GDN in three dogs were rapidly achieved by giving an iv bolus (77 micrograms/kg), followed immediately by an infusion (50 micrograms/min) of the same GDN. A single iv bolus dose of GTN (0.025 micrograms/kg) was given 50 min after beginning the GDN infusion and compared with plasma concentrations following a similar GTN dose in the absence of dosed GDNs. No significant differences in GTN AUC (p > 0.9) and CL(app) (p > 0.7) were found. In a second set of experiments, an infusion of nitroglycerin was begun in each of 4 dogs and continued for 160 min at an infusion rate of 100 micrograms/min. Steady state concentrations of GTN were achieved within 100 min, at which time the dog received, simultaneously, an iv bolus dose (5.14 mg) of one of the GDNs and an infusion dose (100 micrograms/min) of the same GDN. For both dinitrate metabolites no significant differences (p > 0.5) were found between control and interaction arterial and venous clearances, although venous GTN clearances tended to decrease in the presence of dosed GDNs. Steady state systolic blood pressure during GDN infusions could be further reduced when GTN doses were administered; however, the steady state systolic blood pressure decrease caused by GTN could not be further reduced by the GDN infusions. Results suggest that the GDNs do not inhibit nitroglycerin metabolism or hemodynamics at the dose levels studied here.

Intravenous nitroglycerin unloading in acute myocardial infarction

Low-dose intravenous nitroglycerin infusion can be safely administered during acute myocardial infarction to unload the left ventricle and salvage ischemic myocardium and left ventricular geometry and function. In an experimental conscious dog model, low-dose infusion titrated to decrease mean blood pressure by 10% over the first 6 hours after coronary artery ligation resulted in 51% decrease in infarct size, 54% decrease in preload, and more than 50% increase in collateral blood flow. The same benefits were seen when methoxamine was given to counteract that 10% decrease in blood pressure. Similar short-term nitroglycerin infusion also limited remodeling in the dog model. More important, no myocardial salvage was seen with excessive nitroglycerin-induced hypotension to levels less than 80 mm Hg. Clinically, prolonged low-dose nitroglycerin infusion was evaluated in a prospective, randomized, single-blinded, placebo-controlled study of 310 patients with acute infarction: 154 received nitroglycerin and 156 received placebo. Nitroglycerin was titrated to reduce mean blood pressure by 10% in normotensive patients and up to 30% in hypertensive (blood pressure greater than 140/90 mm Hg) patients, but not to less than 80 mm Hg. Nitroglycerin produced several benefits compared with placebo: (1) smaller creatine kinase infarct size; (2) less regional left ventricular dysfunction, better global ejection fraction, and less infarct expansion and thinning; (3) better clinical functional status and hemodynamics; (4) fewer inhospital complications such as acute left ventricular failure and dilation due to marked infarct expansion, left ventricular thrombus, cardiogenic shock, and infarct extension; and (5) fewer deaths up to 1 year in patients with anterior Q-wave infarction.

Assay of glyceryl trinitrate, isosorbide dinitrate, and their metabolites in plasma by large-bore capillary column gas-liquid chromatography

Two large-bore capillary columns, one with dimethyl polysiloxane (HP-1) as the stationary phase and the other with phenyl (50 per cent) methyl (50 per cent) polysiloxane (DB-17), were used to develop gas-liquid chromatographic (GLC) assays for measuring isosorbide dinitrate (ISDN), glyceryl trinitrate (GTN), and their metabolites. ISDN, isosorbide-2-mononitrate (2-ISMN), and isosorbide-5-mononitrate (5-ISMN) in plasma, ranging in concentration from 1 to 300 nM, and GTN, glyceryl-1,2-dinitrate (1,2-GDN), and glyceryl-1,3-dinitrate (1,3-GDN), ranging in concentration from 3 to 60 nM in plasma, were analysed on both columns. GLC analysis yielded baseline resolution of the analytes. The method using the dimethyl polysiloxane column gave a lower limit of detectability for GTN of 0.75 nM (signal/noise (s/n) = 2), and the procedure using the phenyl-methyl column provided a lower limit of detectability for ISDN of 81 pM (s/n = 2). The large-bore column GLC procedures exhibited shorter retention times for both ISDN and GTN than those previously reported for capillary-column assays. The chromatographic resolution of analytes and column efficiency of the large-bore capillary columns were comparable to the results previously found using capillary-column GC. The assays for ISDN and GTN have been shown to be appropriate for pharmacokinetic studies in volunteers and patients. We determined that the HP-1 column is appropriate for the analysis of GTN and metabolites, and the DB-17 column is suitable for analysis of ISDN and its metabolites. We conclude that the use of large-bore capillary columns provides rapid and reliable GLC assays for organic nitrates.

Age-related changes in the pharmacodynamics of intravenous glyceryl trinitrate

Comparable hemodynamic effects were obtained administering a much lower intravenous dose of glyceryl trinitrate (GTN) in elderly than in younger patients. The pharmacodynamics and kinetics of GTN were thus assessed in 2 groups of patients with acute myocardial infarction (group A: less than or equal to 65 years, 6 patients; group B: greater than or equal to 75 years, 6 patients). The arterial and venous dose-concentration relationship and the associated hemodynamic changes at end-point (EP: 10% reduction in mean systemic arterial pressure) were similar in the 2 groups. However, in older subjects EP was reached at a lower GTN infusion rate (0.11 +/- 0.04 vs 0.33 +/- 0.11 micrograms.kg-1.min-1, mean +/- S.D.; p less than 0.001), and with lower arterial and venous drug concentrations (arterial [GTN]: 1.2 +/- 0.1 vs 4.6 +/- 1.2 ng.ml-1; p less than 0.01; venous [GTN]; 0.09 +/- 0.05 vs 0.35 +/- 0.15 ng.ml-1; p less than 0.05), whereas overall GTN kinetics appeared to be substantially independent of age. Thus, the enhanced efficacy of GTN in advanced age seems to stem mainly from pharmacodynamic changes, which may be the consequence of dampened baroreceptor reflexes, as suggested by a lower heart rate increase per unitary fall in systolic arterial pressure observed in group B (0.12 +/- 0.07 vs 0.41 +/- 0.29 b.min-1.mmHg-1; p less than 0.05).

Source of pain and primitive dysfunction in migraine: an identical site?

Twenty common migraine patients received a one sided frontotemporal application of nitroglycerin (10 patients) or placebo ointment (10 patients) in a double blind study. Early onset migraine attacks were induced by nitroglycerin in seven out of 10 patients versus no patient in the placebo group. Subsequently 20 migraine patients, who developed an early onset attack with frontotemporal nitroglycerin, received the drug in a second induction test at other body areas. No early onset migraine was observed. Thus the migraine-inducing effect of nitroglycerin seems to depend on direct stimulation of the habitual site of pain, suggesting that the frontotemporal region is of crucial importance in the development of a migraine crisis. This is not consistent with a CNS origin of migraine attack.

Sustained high-dose nitroglycerin transcutaneous patch therapy in angina pectoris: evidence for attenuation of effect over time

The safety and efficacy of using continuous high-dose transcutaneous nitroglycerin in doses up to 100 mg/24 hours in chronic stable angina was assessed in 20 patients using serial treadmill testing. Patients had first to show a response to sublingual nitroglycerin with a 20% improvement in exercise time. All patients were then titrated with 20 mg (40 cm2), 60 mg (120 cm2), 80 mg (160 cm2) or 100 mg (200 cm2) patches, until intolerable headache in association with a 10 mmHg reduction in blood pressure and a ten-beat increment in heart rate. Drug was then discontinued for 2 days and patients underwent three repeat stress tests to reestablish a consistent drug-free baseline. Patients were then randomized in double-blind fashion to receive either active patch (N = 11) in previous titration dose or placebo patch (N = 9), with treadmill tests performed at 0 (1 hour after previous patch removal), 4, and 24 hours after patch application at baseline and at weeks 1 and 2. Venous blood was obtained for measurement of plasma nitroglycerin levels. After the first 24 hours of active patch therapy, there was a significant reduction in systolic blood pressure (P = .05), a significant increase in heart rate (P = .01), and a minor increase in exercise tolerance (P = .06) compared to placebo. At weeks 1 and 2, there was an attenuation of drug effect in all of these parameters. Plasma nitroglycerin levels demonstrated consistently high plasma levels over each 24-hour dosing interval, on day 1, week 1, and week 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Arterial-venous concentration gradient as a potential source of error in pharmacokinetic studies. Plasma concentration differences of 6-chloro-2-pyridylmethyl nitrate on constant infusion to rats

1. Plasma concentrations of 6-chloro-2-pyridylmethyl nitrate (CPMN) at different sampling sites in the circulation were determined during and after constant infusion in the rat. 2. An arterial-venous CPMN concentration gradient was found and characterized by the following trends. During CPMN infusion into the right atrium, plasma concentrations were higher in arterial (aortic arch) than venous (inferior vena cava) plasma. After cessation of infusion the venous plasma concentrations were significantly higher (P less than 0.05) than those in arterial samples. There was a low concentration gradient between the right atrium and the peripheral artery, but substantial difference between the peripheral artery and the vein. There was a 1.8-2.4 extraction ratio of CPMN across the arterial-venous bed.

Topical nitroglycerin: a potential treatment for impotence

The effect of 2 per cent nitroglycerin paste applied to the penile shaft of impotent subjects was evaluated in a placebo controlled double-blind study under laboratory conditions. After application of nitroglycerin paste or a placebo ointment base, penile tumescence was recorded through a strain gauge transducer while subjects viewed an erotic video presentation. Relative to the placebo paste the number of subjects demonstrating an increase in penile circumference after nitroglycerin (18 of 26) was significantly different than all other outcome possibilities (p less than 0.05). Noninvasive vascular assessment by ultrasonography demonstrated an increase in diameter and blood flow in the cavernous arteries after application of nitroglycerin paste. Nitroglycerin paste increases blood flow in the cavernous arteries and improves tumescence after erotic stimulation. This agent may represent a new therapy for impotence.

Comparative study of time-course of hemodynamic effect of isosorbide dinitrate spray and sublingual tablets in patients with pulmonary congestion

We compared the time-course of the hemodynamic effect of isosorbide dinitrate (ISDN), 5 mg, in the form of sublingual tablet and oral spray, in 15 patients with isolated chronic pulmonary congestion (pulmonary arterial end-diastolic pressure of 15 mmHg or more in the presence of normal or only slightly reduced cardiac index). Both formulations produced significant reductions in the pulmonary arterial end-diastolic pressure. The effect of ISDN tablet (sublingually) became evident at 10 minutes after administration and was maximal at 30 minutes. The effect of ISDN oral spray became evident at 3 minutes and reached a peak at 10 minutes. The magnitude of hemodynamic response was similar. These findings indicate that ISDN oral spray is superior to ISDN sublingual tablets for rapid relief of pulmonary congestion.

Simultaneous determination of nitroglycerin and its dinitrate metabolites by capillary gas chromatography with electron-capture detection

A sensitive gas chromatographic-electron-capture detection method for the simultaneous determination of the antianginal drug nitroglycerin (GTN) and its dinitrate metabolites (1,2-GDN and 1,3-GDN) was developed. Human plasma samples (1 ml) spiked with 2,6-dinitrotoluene as the internal standard were extracted once with 10 ml of a methylene chloride-pentane mixture (3:7, v/v). Using this solvent system, less contaminants are extracted into the organic phase from plasma, resulting in cleaner chromatograms and prolonged column life. A break point was observed on the standard curves of GTN and GDNs. The two linear regions for the detectable concentrations of GTN are 0.025-0.3 and 0.3-3 ng/ml and for 1,2-GDN and 1,3-GDN they are 0.1-1 and 1-10 ng/ml. The limits of detection by this method for GTN, 1,2-GDN and 1,3-GDN in plasma are 0.025, 0.1 and 0.1 ng/ml, respectively.

Nitrates

Nitroglycerin and the long-acting nitrates are playing an increasingly important role in cardiovascular medicine. These agents are recommended in all of the various anginal syndromes and are as effective as the beta-blockers and calcium channel antagonists. There is a definite place for nitrate therapy in treating the complications of acute myocardial infarction. These drugs are also highly effective as unloading therapy in congestive heart failure. The mechanisms of action of the nitrates are reviewed in this article. Information is provided regarding nitrate efficacy in all the major clinical syndromes in which these drugs are used. Finally, appropriate dosing strategies are suggested that should eliminate the potential problem of nitrate tolerance.

Pharmacokinetics and pharmacodynamics of organic nitrates

The pharmacokinetic and pharmacodynamic aspects of organic nitrates are discussed, with particular emphasis on the 3 major organic nitrates currently in use, nitroglycerin (NTG), isosorbide dinitrate and isosorbide-5-mononitrate. After intravenous administration, both NTG and isosorbide dinitrate exhibit large systemic clearances and both nitrates appear to be extensively distributed in vascular and other peripheral tissues. Two pharmacokinetic features appear particularly notable for NTG: there is a significant arteriovenous extraction of the drug, and its systemic clearance is related to cardiac output. Both of these features, plus other evidence, suggest that organic nitrates may be substantially removed from the systemic circulation by the vasculature itself. During nitrate tolerance, plasma drug concentrations remain elevated, but vascular activity is diminished. This apparent paradox might be explainable by a unifying hypothesis of reduced nitrate metabolism during vascular tolerance; thus, in the tolerant state, reduced systemic clearance of the intact drug brought about elevated plasma concentrations, whereas reduced cellular metabolism at the smooth muscle brought about a decrease in vascular activity. The complex relations among plasma kinetics, vascular metabolism and pharmacologic action of organic nitrates are still poorly understood.

Pharmacologic and pharmacokinetic comparison of antianginal agents

Calcium channel blockers, nitrates, and beta blockers are the primary agents used for the treatment of angina. Calcium has a central role in excitation-contraction, action potential generation, and ischemic cell death. The three currently available calcium antagonists are nifedipine, verapamil, and diltiazem. Second-generation agents are in development, and a classification system of calcium channel blockers is used to place the currently available agents and those on the horizon in perspective. Nitrate pharmacology and pharmacodynamics are possibly related to nitrate tolerance; however, this is a matter of some controversy. The beta blockers are all equally effective in the treatment of angina; therefore, drug selection is based on ancillary properties.

Glyceryl-1-nitrate pharmacokinetics in healthy volunteers

The plasma kinetics and urinary excretion of glyceryl-1-nitrate (G-1-N), a metabolite of glyceryl trinitrate with antianginal potential, were investigated in 10 healthy male volunteers, after intravenous infusion and oral administration of 20 mg G-1-N. The apparent volume of G-1-N distribution was 601 corresponding to 0.761 kg-1 body weight, on average. It is suggested that total body water is the principal biological correlate of the hydrophilic drug. Mean intravenous clearance was 283 ml min-1 or 3.61 ml min-1 kg-1. The average of elimination half-lives were 2.50 +/- 0.36 (s.d.) h after the intravenous and 2.54 +/- 0.40 (s.d.) h after the oral dose. Inter-subject variances of pharmacokinetic parameters were low compared to variances reported for glyceryl trinitrate. The coefficient of intra-subject variation of the elimination half-lives was 8.8%. 5.5% (i.v.) and 5.4% (p.o.) of the administered dose were excreted into urine up to 48 h after the administration. 1% (i.v.) and 1.5% (p.o.) were in the conjugated form. The oral dose was rapidly and almost completely absorbed. The oral bioavailability on the basis of areas under the curve amounted to 88.6% on the average. For clinical use, owing to its high oral bioavailability, long residence in the body, inactivation by metabolic conversion, and good predictability of kinetic parameters, G-1-N offers advantage over glyceryl trinitrate.

Pharmacokinetic-hemodynamic studies of transdermal nitroglycerin in congestive heart failure

Ten patients with chronic congestive heart failure were studied to assess the hemodynamic effects and bioavailability of a transdermal nitroglycerin delivery system. Nitrate sensitivity was defined by a prior 20 minute infusion of nitroglycerin, 21 micrograms/min. Five patients with a satisfactory hemodynamic response to intravenous nitroglycerin received two nitroglycerin patches of 10 cm2 each and five patients who did not achieve a satisfactory response to intravenous nitroglycerin (that is, greater than or equal to 25% reduction in left ventricular filling pressure) received larger doses of transdermal nitroglycerin. In the five nitrate-sensitive patients who received 20 cm2 of transdermal nitroglycerin, one study was terminated at 90 minutes, at which point there was no detectable hemodynamic response or arterial plasma nitroglycerin evident. Two patients had a minimal hemodynamic response and a peak plasma concentration of 1 ng/ml. A fourth patient had a short-lived hemodynamic response at 60 and 120 minutes and a plasma nitroglycerin concentration of 1 ng/ml at 60 minutes. A fifth patient had a hemodynamic response persisting for 24 hours and plasma concentrations between 0.6 and 1.1 ng/ml. The remaining five patients showed little or no hemodynamic response despite doses of transdermal nitroglycerin from 40 to 60 cm2. The highest plasma concentration achieved in these patients was 2 ng/ml and there was no relation between dose administered and plasma concentration achieved. In 4 of the 10 patients who subsequently received nitroglycerin ointment, there were a greater decrease in left- and right-sided filling pressures and greater increase in plasma nitroglycerin concentrations (from 1.6 to 4.8 ng/ml) than those that occurred with transdermal nitroglycerin.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical pharmacokinetics of glyceryl trinitrate following the use of systemic and topical preparations

Glyceryl trinitrate has been used for more than a century for the treatment of angina pectoris and, more recently, for the treatment of congestive heart failure. The introduction of transdermal delivery systems has renewed the controversy regarding the efficacy of the drug, mainly in the light of the development of tolerance. With concentrations of the order of 1 microgram/L or less, the measurement of glyceryl trinitrate in plasma is not easy: gas chromatography with electron capture detection has been used widely but recently gas chromatography-mass spectrometry has provided satisfactory results. Assay problems are most likely to be responsible for some of the unexpected results reported. Further factors which may confound the results of the study of plasma concentrations are the rapid metabolism of glyceryl trinitrate in blood in vitro, adsorption to containers and infusion sets, and the uptake and/or metabolism in vessel walls. From the intravenous infusion data, the large interindividual variability in plasma concentrations of glyceryl trinitrate is apparent. The plasma half-life is about 2 to 3 minutes; plasma clearance values reported vary from 216 to 3270 L/h, indicating extensive non-hepatic metabolism. With transdermal administration, mainly with the transdermal controlled delivery systems, plasma concentrations of glyceryl trinitrate appear to be maintained for up to 24 hours, with large interindividual variations. Despite the ability to maintain, for example with the transdermal delivery systems, relatively constant concentrations of glyceryl trinitrate, it has not been possible to find a relationship between plasma concentrations and pharmacological or clinical effects. This is in part due to the attenuation of the effects with time; from the available data it is clear that this attenuation occurs at a pharmacodynamic level (reflex adaptation and tolerance) and not at the pharmacokinetic level.

Influence of skin site on bioavailability of nitroglycerin ointment in congestive heart failure

Nitroglycerin ointment (12.5 to 50 mg) was administered in randomized fashion to three skin sites, arm, chest, or thigh, to compare the hemodynamic effects and bioavailability in nine patients with severe congestive heart failure. Hemodynamic parameters and arterial nitroglycerin concentrations were measured frequently for 12 hours after each application and for 90 minutes after removal of the ointment. During the study, left ventricular filling pressures decreased from control values of 25.0 +/- 8.6 mm Hg (arm), 25.7 +/- 10.9 mm Hg (chest), and 23.7 +/- 8.4 mm Hg (thigh) to 20.4 +/- 8.6 mm Hg, 20.4 +/- 8.5 mm Hg, and 20.0 +/- 7.5 mm Hg; p less than 0.05, less than 0.01, and difference not significant respectively. Peak nitroglycerin concentrations were 5.1 +/- 4.3 ng/ml (arm), 6.2 +/- 6.0 ng/ml (chest), and 4.1 +/- 6.3 ng/ml (thigh). No significant difference was observed in mean arterial pressure, left ventricular filling pressure, right atrial pressure, or nitroglycerin concentration among the sites. These data show that nitroglycerin ointment has similar bioavailability on the arm, chest, or thigh and therefore can be used interchangeably on these skin sites.

Cardiovascular alterations in severe pregnancy-induced hypertension: effects of intravenous nitroglycerin coupled with blood volume expansion

Control of blood pressure in severe pregnancy-induced hypertension has often relied on agents with an unpredictable onset and duration of action. Because intravenous nitroglycerin is a potent, rapidly acting agent with a hemodynamic half-life measured in minutes, we evaluated its cardiovascular effects with and without volume expansion in six patients with severe pregnancy-induced hypertension. Nitroglycerin alone reduced mean arterial pressure by 27.5% without any significant changes in heart rate, central venous pressure, or stroke volume. The pulmonary capillary wedge pressure fell from 9 +/- 3 to 4 +/- 2 mm Hg (p less than 0.05) while the cardiac index decreased from 3.51 +/- 0.67 to 2.87 +/- 0.76 L/min X m2. Oxygen delivery fell significantly (p less than 0.05), from 617 +/- 78 to 491 +/- 106 ml/min X m2. While volume expansion alone had no effect on mean arterial pressure, the combination of blood volume expansion and nitroglycerin resulted in a marked resistance to the hypotensive effect of nitroglycerin. Cardiac index, pulmonary capillary wedge pressure, and oxygen utilization were not significantly different from baseline values when volume expansion preceded nitroglycerin. We conclude that the ease with which nitroglycerin reduces blood pressure is dependent on the individual patient's volume status. Although volume expansion allows one to maintain cardiac index, pulmonary capillary wedge pressure, and oxygen utilization when used in combination with nitroglycerin, this benefit may be offset by a concomitant reduction in hypotensive capability.

PEEP reverses nitroglycerin-induced hypoxemia following coronary artery bypass surgery

Intravenous nitroglycerin is increasingly used during and after cardiac surgery to control blood pressure and improve subendocardial and peripheral circulation. A dramatic decrease in arterial oxygenation has, however, been reported in a number of poorly controlled clinical trials. In the present investigation 16 patients were studied 2-4 h after coronary artery bypass procedures. All were treated with a continuous infusion of nitroglycerin (1 microgram X kg-1 X min-1). Utilizing an on-off-on drug design, it was clearly established that nitroglycerin depresses arterial oxygenation by increasing the pulmonary venous admixture. Three possible underlying mechanisms are discussed, but at the present time no firm conclusion can be drawn as to the nature of the changes. Eight patients were ventilated with 1 kPa (10 cmH2O) positive end-expiratory pressure (PEEP) during the nitroglycerin infusion. PEEP-ventilation reversed nitroglycerin-induced changes in arterial oxygenation and pulmonary shunting without adversely affecting hemodynamic stability.

Influence of cardiopulmonary bypass on nitroglycerin clearance

The effect of cardiopulmonary bypass on the clearance of nitroglycerin (NTG) was studied in seven patients scheduled for coronary artery bypass graft surgery. Intravenous NTG was administered through nonadsorbing tubing at a starting dosage of 5-10 micrograms/min and was adjusted as needed. Blood samples were obtained from the radial artery and antecubital vein before bypass and from the arterial outlet of the oxygenator during bypass at least 30 minutes apart during a constant dosage or at least 30 minutes after a dosage change. Serum concentrations were analyzed for NTG by gas chromatography. Venous NTG concentrations were always lower than concurrent arterial concentrations, with an average arteriovenous extraction of 67.2%. Serum concentrations of NTG were generally within the range associated with a therapeutic response in congestive heart failure patients. Consistent with other reports, NTG concentrations varied widely among patients and considerable intrasubject fluctuations in drug concentrations were seen. The mean +/- SD apparent clearance of NTG before bypass of 0.044 +/- 0.02 L/kg/min increased 20% to 0.052 +/- 0.02 L/kg/min during bypass (P = .05). These results suggest that cardiopulmonary bypass increases the clearance of NTG; however, the magnitude appears to be small and only partially explains the reported increased dosage needed during cardiopulmonary bypass.

Pharmacokinetics of pentaerythritol tetranitrate following intra-arterial and oral dosing in the rat

The pharmacokinetics of pentaerythritol tetranitrate (2,2-bis(hydroxymethyl)-1,3-propanediol tetranitrate, 1) were studied in rats following a single intra-arterial or oral dose (2 mg/kg) of the 14C-labeled drug. Blood levels of the tetranitrate and its metabolites were determined using a thin-layer radiochromatographic procedure. The apparent systemic clearance of 1 was 0.61 +/- 0.16 L/min/kg (mean +/- SD, n = 6) which exceeded the value of normal cardiac output in rats. The steady-state volume of distribution was 4.2 +/- 1.1 L/kg (n = 6), and the elimination half-life was estimated at 5.8 +/- 0.6 min (n = 6). Blood levels of 1 were only detectable (higher than 4.0 ng/mL) in three of the six rats examined after the oral dose. The trinitrate derivative (2,2-bis(hydroxymethyl)-1,3-propanediol trinitrate, 2) the active metabolite of 1, was not detectable following oral dosing with the tetranitrate. The oral bioavailability of 1 was in the range of 0-8%. In spite of the low water solubility of 1 (i.e., 1 microgram/mL), a rather high fraction of the radioactive oral dose [25.7 +/- 10.3% (n = 4) versus 62.4 +/- 14.5% (n = 4) from the intra-arterial dose] was recovered in the urine. A significant portion of the intra-arterial dose (32.7 +/- 11.0%, n = 4) was eliminated in feces, indicating enterohepatic recycling of radioactivity. Analysis of the metabolite pattern in urine indicated extensive metabolism of 1, 2, and the dinitrate derivative 3 (2,2-bis(hydroxymethyl)-1,3-propanediol dinitrate). Less than 0.2% of the dose was recovered as unchanged drug and 2 following either route of administration.

Pharmacokinetics of oral glycerol-1-nitrate

The plasma kinetics and urinary excretion of glycerol-1-nitrate (G-1-N), a water soluble metabolite of glycerol trinitrate with anti-anginal potential, have been investigated in healthy human volunteers following oral doses of 10, 20 and 40 mg tablets and 20 mg as drops. In all volunteers G-1-N was rapidly absorbed. The mean concentration-time curves peaked 40 min after administration of tablets at 144 ng/ml (10 mg), 308 ng/ml (20 mg) and 573 ng/ml (40 mg). After the drops the peak of 324 ng/ml occurred at 1 h. The areas under the G-1-N concentration-time curve and the G-1-N peak heights were linear with dose. Tablets and drops can be regarded as bioequivalent with respect to area under the curve and elimination half-life. The bioavailability of the 20 mg tablet relative to the 20 mg drops was 98.6% in terms of area under the curve. The mean apparent half-life of G-1-N elimination from plasma was 2.69 +/- 0.67 h (n = 46). The mean residence time of G-1-N in the body was 4.65 h compared to 0.28 h for glycerol trinitrate after buccal administration. Female volunteers were found to have significantly lower areas under the curve than male volunteers. The difference was probably due to differences in body weight. Renal excretion does not play an important role in the elimination of oral G-1-N from the body. An overall average of 5.42% of the G-1-N dose was excreted in the urine; free drug accounted for 4.02% and conjugated drug for 1.40%.

Metabolite inhibition of nitroglycerin metabolism in sheep tissue homogenates

The metabolism of nitroglycerin in sheep tissue homogenates has been examined using tritiated nitroglycerin and a HPLC separation procedure. Nitroglycerin was metabolized by liver, lung, muscle, arterial and venous tissue to its dinitrometabolites and subsequently to mononitroglycerin. Addition of the dinitrometabolites substantially inhibited the degradation of nitroglycerin in all tissue homogenates.

Pharmacology of nitroglycerin and long-acting nitrates

Organic nitrates are available in a remarkably diverse variety of formulations, including sublingual, buccal and oral tablets, capsules, topical creams, ointments, patches, tapes, inhalable sprays and intravenous solutions. Although not all of these formulations are available in the United States, the array of drugs and dosages approved for use is extensive. It is only by weighing the pharmacologic properties of these agents against the patient's clinical status and needs that a concise and appropriate treatment regimen may be derived. Numerous recent studies have confirmed the protracted efficacy of the organic nitrates in the treatment of patients with angina pectoris and congestive heart failure (CHF) as evidenced by improvements in cardiac hemodynamics and desired clinical parameters. It is appropriate that the patient's dosage of nitrates be administered with a formulation most likely to be both clinically effective and well tolerated. The use of nitroglycerin and isosorbide dinitrate in the acute and chronic treatment of CHF will be discussed in the context of their unique pharmacologic and pharmacokinetic properties. A rationale for the most efficacious use of these agents will be presented. Tolerance phenomena and adverse effects (i.e., headache) will also be discussed from the perspective of their significance in chronic nitrate therapy.

Dose dependent pharmacokinetics of nitroglycerin after multiple intravenous infusions in healthy volunteers

Evaluation of the pharmacokinetics of nitroglycerin has been hindered in the past by the lack of specific and sensitive analytical procedures, and the unavailability of parenteral nitroglycerin and infusion sets which did not adsorb nitroglycerin. The purpose for this present study was to determine the pharmacokinetic parameters of nitroglycerin and the dinitrate metabolites after multiple intravenous infusions of nitroglycerin in healthy volunteers. Six volunteers received variable infusion rates of nitroglycerin. Generally, at 0, 40, 80, and 120 min, the infusion rates were adjusted to 10, 20, 40, and 10 micrograms/min, respectively. Plasma samples were drawn and analyzed for nitroglycerin and its 1,2- and 1,3-dinitrate metabolites using capillary GC. Steady-state nitroglycerin plasma concentrations attained at 10, 20, 40, and 10 micrograms/min were 0.44 +/- 0.31, 1.32 +/- 0.71, 4.23 +/- 1.50 and 1.04 +/- 0.43 ng/ml, respectively. As the infusion rate was increased, the steady-state concentrations increased disproportionately. When the dose was decreased from 40 to 10 micrograms/min, the steady-state nitroglycerin concentrations were always higher than those at the initial low infusion rate. Thus, in the majority of subjects, a hysteretic type of response was present. The hysteresis observed in the dose versus steady-state concentration curve may be explained by either end-product inhibition or saturable binding of nitroglycerin to blood vessels. The clearance values (5.5 to 711/min) were very high and far exceed the maximum possible hepatic clearance suggesting that nitroglycerin is metabolized by organs other than liver. Clearance was not directly related to plasma concentrations but was found to decrease to a constant value (approximately 11 +/- 6 l/min) as nitroglycerin concentrations initially increased.

Pharmacokinetic-hemodynamic interactions between vasopressin and nitroglycerin: comparison between intravenous and cutaneous routes of nitrate delivery

Addition of nitroglycerin (NTG) improves the hemodynamic response to vasopressin and may thus be useful in the treatment of gastrointestinal hemorrhage. We studied in the rat the influence of vasopressin on the disposition of a constant intravenous infusion of NTG and the cutaneous absorption of NTG ointment. The effect of NTG on the pharmacokinetics of vasopressin was also determined. Animals were divided into four groups: control, NTG, vasopressin and vasopressin + NTG. Infusions (or ointments) were maintained for 70 min; cardiac output and regional blood flows were determined with the microsphere technique. Both intravenous and cutaneous NTG resulted in similar hemodynamic responses. Vasopressin caused generalized vasoconstriction, while the addition of NTG reversed the deleterious systemic hemodynamic effects of vasopressin. Addition of vasopressin to NTG did not alter NTG systemic clearance nor did NTG affect vasopressin clearance. Of note, the systemic clearance of NTG was directly correlated with the cardiac output (r = 0.804), supporting a model of NTG distribution where blood vessels and/or extrahepatic tissues are the site of elimination of the drug. The marked reduction in skin blood flow by vasopressin did not decrease the steady-state plasma concentration of NTG nor the estimated cutaneous absorption rate of NTG ointment, indicating that cutaneous blood flow is not an important determinant in the absorption of NTG ointment. The skin is an appropriate route of delivery for NTG when combined with vasopressin.

Incomplete and delayed bioavailability of sublingual nitroglycerin

Eight healthy male volunteers received 16 doses of sublingual nitroglycerin tablets (0.4 mg). After 8 minutes, each subject rinsed out his mouth to halt the drug absorption process. The mouth rinses were assayed by high-performance liquid chromatography for residual nitroglycerin content. Each subject also received intravenous infusions of nitroglycerin so that the absolute bioavailability could be evaluated. Plasma nitroglycerin concentrations were determined using a specific and sensitive capillary gas chromatographic method capable of quantifying 25 pg/ml of nitroglycerin. The mean bioavailability (+/- standard deviation) of sublingual nitroglycerin, estimated from plasma concentrations, was 36.2 +/- 24.9% (range 2.6 to 113%). The amount of drug not absorbed after 8 minutes, as determined from the analysis of the mouth rinses, varied from 2.7 to 65.8% (mean 31.4 +/- 18.9%) of the administered sublingual dose. Mean nitroglycerin peak concentrations of 1.89 +/- 1.64 ng/ml were obtained at a mean peak time of 5.3 +/- 2.3 minutes. Thus, sublingual absorption is not instantaneous and can be relatively slow, with peak times of as long as 10 minutes. These data indicate that nitroglycerin pharmacokinetic values should not be estimated only from sublingual doses. Additionally, attempts to correlate pharmacodynamic measurements to sublingual doses must take into account the low and variable bioavailability and the potentially long peak times after sublingual nitroglycerin administration to patients.

Role of the liver in the disposition of intravenous nitroglycerin in the rat

Previous studies have indicated that the liver is the main site of nitroglycerin (NTG) elimination when the drug is systematically infused. To examine this hypothesis, we measured the apparent systemic clearance (Cls) of nitroglycerin in anesthesized rats receiving a constant intravenous infusion at a dose of 100 micrograms per kg per min. Animals were divided into shunt and sham groups; the former had undergone a portal vein ligation 10 days prior to the study, while the latter was subjected to a sham operation. On the study day, half of the animals of each group also received probenecid at 200 mg/kg, i.v., a drug previously reported to inhibit organic nitrate ester reductase (ONER) activity in rat liver. Arterial NTG samples were obtained at 41, 43 and 45 min of infusion in all four experimental groups; Cls was 439 +/- 32 ml per kg per min (mean +/- S.E.) in sham, 460 +/- 44 in sham and probenecid, 477 +/- 39 in shunt, and 461 +/- 34 in shunt and probenecid animals. During NTG infusion, hepatic blood flow (measured with a constant infusion of indocyanine green) was decreased markedly in shunted rats as was liver/body weight, indicating hepatic atrophy. The specific activity of hepatic ONER was similar in all four groups. In spite of marked differences in hepatic blood flow and hepatic mass, the Cls was similar in all four groups. The liver does not appear to be a major site for the elimination of systemic nitroglycerin as hitherto assumed.

Determination of picogram nitroglycerin plasma concentrations using capillary gas chromatography with on-column injection

A specific, sensitive, and precise capillary gas chromatographic (GC) assay capable of analyzing picogram concentrations of nitroglycerin in human plasma was developed. The analytical procedure involves a double extraction of 1 mL of plasma with pentane, after the addition of internal standard (1 ng of 2,6-dinitrotoluene), followed by evaporation and reconstitution in 50 microL of heptane. The extract (1 microL) was injected onto a capillary column using the on-column injection technique. The GC oven temperature was programmed from 120 degrees C to 180 degrees C at a rate of 5 degrees C/min. The oven temperature was then programmed to 250 degrees C and was maintained for 10 min. The nitroglycerin and internal standard retention times were 8.6 and 11.4 min, respectively. The position of the end of the capillary column inside the detector is a critical determinant of sensitivity: the column exit must be positioned such that nitroglycerin adsorption to the detector is minimized (i.e., sensitivity maximized). The assay limit of quantitation was 25 pg/mL (CV = 7.6%) using 1 mL of plasma. This GC assay, specific for nitroglycerin in the presence of its metabolites, isosorbide dinitrate, and several other drugs, may be used to quantitate plasma levels obtained after therapeutic nitroglycerin doses.

Pharmacokinetics of nitroglycerin after parenteral and oral dosing in the rat

The pharmacokinetics of nitroglycerin was characterized in detail using venous plasma after different intravenous bolus doses (0.15-2.48 mg/kg), intra-arterial infusion (8.2 micrograms/min over 5 h), and oral doses (7-100 mg/kg). Venous plasma clearance was found to be approximately 650 mL/kg and was independent of the intravenous or intra-arterial dose. This confirmed earlier reports that the venous plasma clearance of nitroglycerin in rats exceeded the value of normal cardiac output. A terminal half-life of approximately 15 min was observed after high intravenous bolus doses of nitroglycerin. This slow disappearance phase was likely rate limited by redistribution of drug back into the plasma. The bioavailability of oral nitroglycerin (F) showed an apparent Michaelis-Menten dependency on dose. F was less than 5% at doses less than 20 mg/kg, but increased to a plateau of approximately 20% from 50-100 mg/kg. First-pass metabolism of nitroglycerin is thus apparently controlled by at least two systems (sites or enzymes). Coadministration of mannitol hexanitrate, a potential competitive inhibitor of first-pass metabolism, did not increase F.

Specialized delivery systems for intravenous nitroglycerin. Are they necessary?

Nitroglycerin is absorbed in vitro into polyvinyl chloride tubing, and it has been recommended that nitroglycerin be administered intravenously through specialized polyethylene infusion sets. To determine if tubing type is essential to achieve physiologic effectiveness, we studied dose responses to intravenous nitroglycerin in 15 patients with heart failure using standard polyvinyl chloride tubing in seven (group 2) and special polyethylene infusion sets in seven (group 1) (one patient was excluded from analysis because of technical difficulties). We monitored heart rate, blood pressure, right atrial pressure, pulmonary artery pressure, pulmonary capillary wedge pressure, and cardiac output. Cardiac index, systemic and pulmonary vascular resistance, triple index, rate pressure product, stroke volume, stroke volume index, and stroke work index were calculated. Baseline and treatment measurements were obtained from five to 15 minutes after the infusion of 10, 20, 40, and 80 micrograms of nitroglycerin per minute. Over-all, systolic blood pressure decreased (p less than 0.05) about 8 percent and mean blood pressure approximately 12 percent, mean pulmonary artery pressure and mean pulmonary capillary wedge pressure decreased 30 to 40 percent, and the decline in mean right atrial pressure was 35 percent of baseline (all p less than 0.05). Heart rate and cardiac index did not change (p greater than 0.05). Pulmonary vascular resistance decreased slightly (p = 0.07) and systemic vascular resistance significantly (p less than 0.05). When the two groups were compared physiologic changes were virtually identical (p less than 0.05). Two-way analysis of variance for baseline corrected data proved no differences between tubing sets (p less than 0.05), but the infusion concentration rate was highly related to response (p = 0.0001). A significant (p less than 0.05) decrease in mean blood pressure and mean right atrial pressure was noted at lower dose rates (20 micrograms per minute and 40 micrograms per minute, respectively) in group 1. Beneficial hemodynamic effects in heart failure patients can, then, be predicted to occur at 80 micrograms per minute infusion rates; these responses seem independent of the type of infusion tubing system employed. Additionally, when patients given intravenous nitroglycerin for various reasons were followed for 48 hours, the majority receiving infusions via polyvinyl chloride tubing (group 2) did not require dosage adjustments. Also, at lower flow rates, more solution than calculated may be delivered when polyethylene tubing infusion sets are employed with volumetric infusion pumps.