The Role of
L-Arginine in Cardiovascular Health

U.S. Pharmacist Continuing Education
ACPE Program No. 430-000-99-015-H01
This program provides 1.0 hours of credit (0.1 CEU).
Lesson Expires: November 30, 2001

Subsequent to its discovery, the endogenous arterial vasodilator “endothelium-derived relaxing factor” (EDRF) was determined to be chemically identical to the free radical gas nitric oxide (NO). Known to be present in macrophages, NO is thought to mediate a number of macrophage cytotoxic actions. Endogenous NO is believed to mediate many diverse physiological processes including vasorelaxation, immune responses, adhesion of leukocytes and platelets, and neurotransmission. NO is not the same as nitrous oxide (N₂O), the general anesthetic known as “laughing gas.” The basic amino acid L-arginine is the precursor for the synthesis of NO in mammals.

As it can be synthesized endogenously from L-citrulline, L-arginine is classified as a non-essential amino acid in adults. However, in children and in conditions of accelerated growth as seen following trauma or infection, L-arginine synthesis may become inadequate. Thus, L-arginine may be considered “semi-essential” in certain situations. In addition to occurring in the liver, much of the endogenous synthesis of  L-arginine from L-citrulline takes place in the proximal tubule of the kidney during the formation of urea.

L-Arginine, which constitutes approximately 5% of the amino acid content of the typical adult diet, is absorbed in the lower 2/3 of the small intestine along with other basic amino acids. Absorption involves uptake by the gastrointestinal enterocytes, where about 60% of the L-arginine is metabolized, and only 40% reaches the systemic circulation intact. Although some protein-containing foods may have slightly higher L-arginine contents than others, nutritionally there is little difference between most proteins with respect to this amino acid. Thus, as described in this article, the only way to effectively deliver more L-arginine to the individual would be to supplement with the amino acid itself.

L-Arginine stimulates the secretion of a number of important hormones including hypothalamic corticotropin releasing factor (CRF), pituitary growth hormone and prolactin, pancreatic insulin, glucagon, pancreozymin and polypeptide, somatostatin, aldosterone, and adrenal catecholamines.¹ L-Arginine given parenterally (30 g) is used to determine the ability of the pituitary to release growth hormone.

Nitric Oxide and Nitric Oxide Synthase

Nitric oxide formed from L-arginine appears to be present in all cells in the body and is believed essential in a number of important homeostatic processes. In blood NO is rapidly inactivated by oxyhemoglobin to form methemoglobin. While NO normally has a very short half-life of only 3–5 seconds, some of the NO formed in vivo can survive 30–40 times longer if reacted with nitroso adducts on albumin. Most of the biological effects of NO are mediated via the activation of soluble guanylyl cyclase which increases cyclic-GMP in cells. However, some NO-mediated effects are guanylyl cyclase independent.

The production of NO from L-arginine occurs by way of NO synthase (NOS). Three isoforms of NOS exist: inducible (iNOS), endothelial (eNOS), and neuronal (nNOS). Normally levels of L-arginine are in the millimolar range. As the Km (half saturation concentration) for NOS is in the micromolar range, one would predict that NOS should be saturated with its substrate L-arginine, and that additional L-arginine should not affect NO production. However, under various pathologic conditions L-arginine has been shown to increase NO and influence physiological function, and thus this “arginine paradox” has been the subject of much investigation. One explanation that has gained the most interest is the finding that high levels (2–10 times normal) of the endogenous L-arginine analog asymmetric dimethyl-L-arginine (ADMA) are present during many of these pathologic conditions and can inhibit NOS.² The formation of ADMA is not from circulating free L-arginine, but appears to derive from the posttranslational methylation of peptide bound arginine in proteins (e.g., histones, heat shock proteins).³ Elevated ADMA, shown to be capable of inhibiting NOS, has been reported in renal failure, hypertension, preeclampsia, hypercholesterolemia, tobacco use, diabetes mellitus, and aging.⁴ Supplemental L-arginine is capable of competing with ADMA and overcoming this inhibitory effect.⁵ Elevations in cholesterol and associated atherogenic lipoproteins or glucose decrease the major catabolic enzyme involved in ADMA metabolism. This enzyme, dimethylarginine dimethylaminohydrolase (DDAH) is significantly decreased (40%–60%) in animals fed high-cholesterol diets or given streptozotocin which experimentally induces diabetes mellitus and hyperglycemia.⁶ The resulting disruption of ADMA metabolism by DDAH may explain the underlying dysregulation of NO synthesis in endothelial cells in various pathologic conditions.

An additional pathway involving the nonenzymatic synthesis of NO has been proposed, in which L-arginine combines with the highly reactive oxygen species hydrogen peroxide or superoxide anion to yield NO. This pathway may help to explain why L-arginine has been shown to be effective in some conditions characterized by oxidative stress.

Tissue injury and repair increases the demand for L-arginine.⁷ While initially there is a decrease in L-arginine and a corresponding increase in L-citrulline and NO in the injured tissue, during the repair period L-arginine continues to be depleted as L-ornithine production increases due to the action of arginase.

L-Arginine can undergo numerous metabolic fates. In addition to its role as a component of most proteins, this amino acid can be converted to urea, L-citrulline, L-ornithine, L-proline, L-glutamate, and polyamines such as putrescine. Creatine, the high-energy phosphate storage form found in skeletal muscles, is also formed from L-arginine. Recently the decarboxylation of L-arginine via L-arginine decarboxylase to form agmatine has been reported. Agmatine may act as an endogenous antihypertensive agent, similar in mechanism to that of clonidine. Thus L-arginine plays an important role in the body’s response to injury.

L-Arginine, the Endothelium and Cardiovascular Function

Within the cardiovascular system alterations in NO function have been linked to numerous diseases, many of which appear to originate in the vascular endothelial cells. A healthy vasculature is characterized by the presence of endothelial cell-produced, locally acting paracrine factors that favor vasodilation, blood fluidity and inhibition of cell proliferation. In contrast, numerous cardiovascular disease states are characterized by an abundance of endothelial factors causing vasoconstriction, inflammation, thrombolytic activity and cell proliferation. TABLE 1 lists some of the factors that typically predominate in healthy and pathological blood vessels. The delicate balance between these factors determines the overall health of the vasculature; thus, processes that can either augment or disrupt the synthesis, release, metabolism or actions of a particular paracrine factor may contribute to pathology or constitute an approach for a therapeutic intervention.

Atherosclerosis in experimental animals and in humans is associated with impaired vasodilation in response to normal physiological stimuli.⁸ Even patients at risk for the development of atherosclerosis, who have yet to demonstrate hypertension or other overt cardiovascular abnormalities, typically exhibit abnormal vasodilator responses when examined carefully.⁹ These abnormalities are thought to be partly due to an enhanced degradation of NO due to superoxide anion overproduction, reduced availability of the NOS cofactors, or an impaired synthesis of NO due to ADMA accumulation.¹⁰ While the degree of vasodilatory impairment is related to plasma low-density lipoprotein (LDL) levels and a number of other standard risk factors (e.g., smoking, hypertension, other hyperlipidemias), the ratio of L-arginine/ADMA in plasma may be the best correlated biochemical marker for predicting vascular dysfunction. Administration of L-arginine to experimental animals (hypercholesterolemic rabbits) and humans (hypercholesterolemic, hypertensive) has been shown to reverse the vasomotor dysfunction and restore vasodilatory responses to cholinergic agonists (e.g., methacholine, acetylcholine) that normally release EDRF.^11,12 Studies in cultured endothelial cells also support these findings and the role of ADMA.¹³

Exercise tolerance in experimental animals with compromised limb blood flow can be significantly improved by L-arginine administration. When administered a 6% L-arginine drinking solution, mice deficient in Apo-E, the apoprotein normally involved in the transport of cholesterol out of the circulation, showed a 61% increase in treadmill exercise performance.¹⁴ This beneficial effect of L-arginine was associated with an increase in overall NO synthesis, and could be blocked by a
NOS inhibitor. Patients with peripheral arterial disease also benefit from L-arginine supplementation (8 g, bid, 14 days) as evidenced by a 150% improvement in walking distance.¹⁵ Additionally, patients with angina associated with coronary artery disease given L-arginine (9 g/day orally for 6 months) demonstrated improved acetylcholine-induced coronary vascular relaxation (149% improvement) with a 70% decrease in anginal episodes.¹⁶ L-Arginine (6 g/day) improved exercise tolerance in anginal patients and increased the time to 1 mm ST-segment depression during exercise-stress testing.¹⁷

Platelet aggregation, leukocyte adhesion, proliferation of vascular smooth muscle and superoxide anion formation all enhance the formation of atherosclerotic plaques. NO inhibits these pro-atherogenic factors, and L-arginine administration reduces atherogenesis in a number of studies. Hypercholesterolemic rabbits and LDL receptor knockout mice both experience fewer intimal lesions when treated with L-arginine. On the other hand, administration of NOS inhibitors accelerates lesion formation. The potency of L-arginine in reducing lesion formation is similar to that observed with the HMG CoA reductase inhibitor lovastatin.¹⁸ Administration of L-arginine can lead to regression of preexisting intimal lesions in the hypercholesterolemic rabbit¹⁹ and can inhibit myointimal hyperplasia after balloon angioplasty.²⁰

Hypertension is characterized by endothelial dysfunction as evidenced by the finding that while vasodilator responses to endothelium-independent responses remain intact, such responses to endothelium-dependent processes (e.g., cholinergic agonist administration) are impaired, even in young patients just developing the disease.²¹ Administration of NOS inhibitors normally increases arterial blood pressure by 40% in experimental animals. While administration of NOS inhibitors to normotensive patients produces a similar hypertensive response, little response is observed in hypertensive patients following NOS inhibitor administration; this suggests the normally operating constant NO-mediated vasodilator tone is deficient in hypertension.²² Whether the primary defect in hypertension is endothelial function disruption or if the observed disruption is secondary to other pathologies is not known.

Several factors involved in the process of angiogenesis depend upon NO for their normal actions. Endothelial cells grown in culture and treated with proangiogenic factors (vascular endothelial growth factor, transforming growth factor beta, basic fibroblast growth factor) increase NO production, upregulate NOS, and are generally sensitive to inhibition by NOS inhibitors. Many effects of these angiogenic factors can be mimicked in vitro by administration of NO donors. In vivo administration of L-arginine (but not D-arginine) speeds healing and increases gastric blood flow in rats subjected to acid-induced ulcerations, a process known to require angiogenesis.²³

A number of disease states have been reported to respond beneficially to supplemental L-arginine. During sepsis and trauma, supplemental L-arginine improves nitrogen balance and reduces protein catabolism. In human breast cancer, supplementation with L-arginine increases the quantity and cytotoxic capability of lymphokine activated cells and natural-killer T-cells.²⁴ L-Arginine has also been shown to be effective in a number of renally involved diseases such as nephrosclerosis associated with diabetes mellitus and progressive renal failure.²⁵ Improvements in renal parameters such as glomerular filtration rate, renal blood flow and urinary protein excretion have been noted in experimental animal models for these disease states when L-arginine is administered.²⁶ Improvements in renal function may also help enhance the clearance of ADMA and thus relieve the inhibition of NOS observed in some renal disorders.

L-Arginine Safety

In the few studies to date in which L-arginine HCl has been administered parenterally, metabolic acidosis and alterations in some electrolytes (e.g., potassium, phosphorus) have been noted. While these appear to occur with very large parenteral doses of the hydrochloride salt, these effects would not be anticipated following oral administration of more modest doses. Of the limited studies of L-arginine administered orally in humans, few report any adverse effects following acute or chronic treatment. One report²⁷ in 1992 that caused much concern demonstrated the ability of L-arginine (30 g/day, QID for 3 days) to double the rate of tumor protein synthesis in patients with breast cancer. However, studies prior²⁸ and subsequent^29,30 failed to replicate these findings, and in fact demonstrated a reduction in tumor growth. However, since the effect of L-arginine in patients with active malignancy is not fully known, caution should be exercised if L-arginine use is being considered in such patients.

Generally doses up to 30 g/day are well tolerated, the most common adverse effects (nausea and diarrhea) being reported infrequently.³¹ No changes in liver function, blood glucose, or plasma electrolytes have been noted. One study with 9 g/day reported one patient with a recurrence of oral herpes lesions, which resolved upon discontinuance of L-arginine treatment. In a recent study of 24 hypercholesterolemic patients administered 14 g/day for 12 weeks, no changes in plasma insulin, growth hormone or other serum chemistries or hematological parameters were noted except for a slight but clinically nonsignificant increase in blood urea nitrogen.³² There were no reports of any adverse effects in this study. L-Arginine (9 g/day) administered for 6 months in patients with angina appeared to be well tolerated with no adverse effects noted.¹⁶ Another study administered a medical food containing L-arginine (6.6 g/day, divided dose) for one week to 43 hypercholesterolemic patients. Beneficial effects on exercise tolerance and vasodilator responses were noted. The multicomponent product also contained vitamins B6, B12, C, E, folate and niacin in a soy protein base. It appeared highly effective, with no reports of adverse effects.

Due to NO’s physiological role in the vasculature, a number of potential cautions and/or contraindications to the use of L-arginine exist. Because of the vasodilator role of NO associated with septic shock, patients with severe infections probably should not use L-arginine. Since NO is capable of promoting angiogenesis, patients with diabetic retinopathy should use L-arginine carefully, if at all. Finally, as described above, patients with active malignancies probably should avoid L-arginine supplementation.

Available L-Arginine Products

L-Arginine, readily available in the form of dietary supplements (100, 250, 500 mg capsules and tablets), is regulated by the provisions of the Dietary Supplements Health and Education Act (DSHEA) of 1994. A dietary supplement is “a product (other than tobacco) that is intended to supplement the diet that bears or contains one or more of the following dietary ingredients: a vitamin, a mineral, an herb or other botanical, an amino acid, a dietary substance for use by man to supplement the diet by increasing the total daily intake.”³³ However, the DSHEA regulations are extremely lax and there are essentially no requirements for content uniformity, stability, dissolution characteristics or bioavailability. Consumers may incorrectly conclude that since the product is sold in a pharmacy and shelved in the general vicinity of over-the-counter drugs like acetaminophen or aspirin (regulated as drugs), the product was produced with the same rigorous manufacturing controls required of all drugs. While there are no reports indicating problems with L-arginine-containing dietary supplements, the number of products that undergo pharmaceutical and quality assurance testing is unknown.

A product for L-arginine supplementation in the form of a medical food has recently been developed (HeartBar); it delivers L-arginine and a number of other antioxidants, vitamins and fiber. These other components have been shown to be of some benefit to patients with cardiovascular disease. A medical food is “formulated to be consumed or administered enterally under the supervision of a physician and is intended for the specific dietary management of a disease or condition for which distinctive nutritional requirements on the basis of recognized scientific principles are established by medical evaluation.”³³ Sold as a medical food, this product is intended to be used under a physician’s supervision and generally would be expected to be of a higher pharmaceutical quality than dietary supplements. Healthcare providers should be aware of the effects of medical food use, whereas there may be a lack of knowledge associated with use of dietary supplements.

Conclusion

Studies clearly demonstrate the potential of L-arginine supplementation to enhance NO-mediated cardiovascular health. The risk associated with L-arginine therapy appears minimal, except in a very small subset of patients (e.g., active malignancies, severe infection, diabetic retinopathy). Patients should consult their healthcare providers about using products containing L-arginine for cardiovascular benefits.

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