The Role of Nitroglycerin and Other Nitrogen Oxides in Cardiovascular Therapeutics

Abstract

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.

Keywords: angina; nitrate; nitrate-nitrite-NO pathway; nitric oxide; nitroglycerin; soluble guanylyl cyclase.

Copyright © 2017 American College of Cardiology Foundation. Published by Elsevier Inc. All rights reserved.

Figures

Figure 1. Sources of Nitric Oxide

Nitric oxide is a free radical that is synthesized by the family of NO synthases from L-arginine and oxygen, yielding L-citrulline as a co-product. In the blood vessel wall, NO is mainly produced by endothelial NOS (eNOS). There are two other isoforms of NOS that also produce NO from L-arginine: neuronal NOS (nNOS) and cytokine-inducible NOS (iNOS). Additionally, bacterial flora present in the mammalian oral cavity is rich in nitrate reductases and can convert dietary sources of nitrate to nitrite. In the extremely acidic environment (pH ≤ 3) of the gastric lumen, protonation of nitrite can, in turn, produce nitrous acid, which can spontaneously decompose to nitric oxide. This pathway of NO generation is referred to as the enterosalivary nitrate circulation (nitrate-nitrite-NO pathway). NOS, nitric oxide synthase; NO, nitric oxide; NO2−, inorganic nitrite; NO3−, inorganic nitrate.

Figure 2. Regulation of Vascular Tone by Nitric Oxide

Nitric oxide is a powerful vasodilator that induces formation of cGMP by activating soluble sGC in vascular smooth muscle cells. cGMP can bind to and enhance protein kinase G activity, cGMP-gated ion channels, and cGMP-sensitive phosphodiesterases. Protein kinase G promotes reuptake of cytosolic calcium into the sarcoplasmic reticulum, the movement of calcium from the intracellular to the extracellular environment, and the opening of calcium-activated potassium channels. These changes result in relaxation of vascular tone as the reduction in intracellular calcium impairs myosin light chain kinase’s ability to phosphorylate myosin, resulting in smooth muscle cell relaxation. eNOS, endothelial nitric oxide synthase; NO, nitric oxide; sGC, soluble guanylyl cyclase; GTP, guanosine triphosphate; cGMP, cyclic guanosine monophosphate; GMP, guanosine monophosphate; PDE, phosphodiesterase; Ca2+, calcium ion; cGMP-dep Kinase I, cyclic guanosine monophosphate-dependent protein kinase I; MLCK, myosin light-chain kinase; MLCP, myosin-light-chain phosphatase; MLC-Pi, phosphorylated myosin light chain; MLC, myosin light chain.

Figure 3

Chemical Structures of Key Nitrovasodilators.

Figure 4. Bioactivation of Organic Nitrates

Organic nitrates must undergo biotransformation to release a vasoactive molecule via one of two pathways: a high potency pathway mediated by ALDH-2, or a low potency pathway mediated by other enzymes or low-molecular-weight reductants. The high potency pathway is important at clinically relevant nitroglycerin concentrations (2−) and 1,2-glyceryl dinitrate from nitroglycerin. There are three proposed mechanisms for the release of the vasoactive molecule via this pathway: nitrogen oxide(s) is(are) formed via reduction of inorganic nitrite; nitric oxide is formed directly in response to interaction with ALDH-2; and inorganic nitrite released from mitochondria may be reduced by xanthine oxidase in the cytoplasm to form NO. The low potency pathway is important at suprapharmacological nitroglycerin concentrations (> 1 μM), is found in the smooth endoplasmic reticulum, and leads to formation of measurable amounts of NO in vascular tissues. In this pathway, nitroglycerin is biotransformed by proteins such as deoxyhemoglobin, deoxymyoglobin, cytrochrome P450, xanthine oxidase, glutathione-S-transferase, glyceraldehyde-3-phosphate dehydrogenase, or other ALDH isoforms; and by low-molecular-weight reductants such as cysteine, N-acetyl-cysteine, thiosalicylic acid, and ascorbate. GTN, glyceryl trinitrate (nitroglycerin); PETN, pentaerythrityl tetranitrate; ALDH2, aldehyde dehydrogenase-2; NO2−, inorganic nitrite; H+, hydrogen ion; Cyt Ox, cytochrome c oxidase; XO, xanthine oxidase; NOx, nitrogen oxides; ·NO, nitric oxide; PETriN, pentaerythrityl trinitrate; GDN, 1,2-glyceryl dinitrate; PEDN, pentaerythrityl dinitrate; sGC, soluble gyanylyl cyclase; cGMP, cyclic guanosine monophosphate; cGK-I, cyclic guanosine monophosphate-dependent protein kinase I; ISDN, isosorbide dinitrate; ISDM, isosorbide mononitrate; P450, cytochrome P450 enzyme(s); GMN, 1,2-glyceryl mononitrate; PEMN, pentaerythrityl mononitrate. Reproduced with permission from Munzel, T., Daiber, A. Pharmacology of Nitrovasodilators. In: Bryan, N., and Loscalzo, J., editors. Nitrite and Nitrate in Human Health and Disease. 2nd Ed., New York, Humana Press, 2017.

Figure 5. Novel Ways of Modulating the NOsGC-cGMP Pathway

Several novel modulators of the NO-sGC-cGMP pathway are undergoing active investigation, and the details are summarized in the main text. Methods of increasing endogenous cavnoxin production or introducing synthesized cavnoxin-like peptides could increase endogenous NO production. Transcriptional enhances of eNOS, the small molecules AVE3085 and AVE9488, have increased endogenous NO production, reversed eNOS uncoupling, and decreased eNOS production of superoxide anion in mice. The enterosalivary nitrate circulation (nitrate-nitrite-NO pathway) is a source of NO that is derived from dietary inorganic nitrate intake and production of NO from this pathway is enhanced by hypoxia and acidosis. Research dedicated to exploring therapeutic options, such as prebiotics, probiotics, and antimicrobial agents, that can modulate the microbiome and the nitrate-nitrite-NO pathway in heart failure, pulmonary hypertension, hypertension, obesity, and other cardiovascular disease states is ongoing. Increasing the bioavailability of endogenous NO is another potential approach to modulating the NO-sGC-cGMP pathway. The oxidized form of hemoglobin-α has a much lower affinity for NO than the reduced form, and, therefore, allows eNOS-generated NO to diffuse to underlying vascular smooth muscle cells. Since NADH-cytrochrome b5 reductase 3 reduces Fe3+-hemoglobin-α to Fe2+-hemoglobin-α, a potential way to increase NO bioavailability would be to inhibit NADH-cytochrome b5 reductase 3. Another possible strategy would be to disrupt the binding site of hemoglobin-α for eNOS, and a small peptide has been developed, hemoglobin-αX, as just such an inhibitor. Finally, modulating the NO-sGC-cGMP pathway in an NO-independent fashion is also being study. The ciguats modulate sGC activity to increase cGMP production independently of NO. The phosphodiesterases hydrolyze the phosphodiester bond of cGMP. Inhibition of these enzymes decreases cGMP breakdown. eNOS, endothelial nitric oxide synthase; NO, nitric oxide; NADH, nicotinamide adenine dinucleotide; Fe, iron; sGC, soluble gyanylyl cyclase; cGMP, cyclic guanosine monophosphate.

Central Illustration. Hemodynamic Effects of Nitroglycerin

The organic nitrates have several hemodynamic effects that are largely mediated through vasodilation of capacitance veins and conductance arteries. Seminal work in the 1970s, as detailed in the main text, showed that nitroglycerin acted primarily by reducing left ventricular oxygen requirements through a reduction in left ventricular volume. Nitric oxide-mediated dilation of capacitance veins decreases ventricular preload, which results in reduction in myocardial oxygen demand and left ventricular wall tension. This results in an increase in subendocardial myocardial blood flow. Nitrates also dilate large and medium sized coronary arteries and arterioles > 100 micrometers in diameter. This effect reduces left ventricular systolic wall tension via decreasing afterload and, therefore, also decreases myocardial oxygen demand. The hemodynamic effects of nitrates on the coronary vasculature also relieve angina. Nitrates dilate the epicardial coronary arteries, including stenotic segments, and also improve blood flow in coronary collaterals via decreasing resistance to collateral flow. Nitrates, particularly sodium nitroprusside, can also dilate the systemic arterial bed, and NO gas and sodium nitroprusside dilate the pulmonary vascular bed and inhibit hypoxia-induced pulmonary vasoconstriction. NO, nitric oxide; O2, oxygen.