Arginine and citrulline can yield antiatherosclerotic effects and …

Rapid Muscular Benefit
… by more efficiently raising arginine levels

By Will Block

Y ou may not know the benefits of chewing a full size aspirin (325 mg; not the baby kind) in the ambulance on the way to the hospital after you’ve had a heart attack. Learn about it! It could SAVE your life by interfering with ongoing platelet aggregation and increase your chances of both surviving and of preserving more of your heart cells known as myocytes.1 That’s the value of knowledge and rapid response!

Building Muscle Rapidly

Knowledge of supplementation with the right nutrients may also offer rapid response benefits. Perhaps not as dramatic as the aspirin example, but quite significantly when you consider the long run. The rapid generation of muscular development is such an example, as we shall learn.


With aging comes a gradual loss of
muscle mass and strength (this applies
to women as well as men).


Loss of Muscle with Age

With aging comes a gradual loss of muscle mass and strength (this applies to women as well as men). It’s called sarcopenia, and it may seem to be inevitable—although not necessarily to the degree that we commonly see in our society. As with every other aspect of our health, it all depends on how we have lived our lives thus far, in terms of diet, exercise, and other lifestyle choices—not to mention our genetic makeup. But quality knowledge goes a long way, and can overcome many of these barriers and slow or even reverse the long-term loss of muscle.

In the development of life on the planet, the amino acid arginine has proven to be invaluable. This is because its special chemistry and metabolism is crucial for the evolution of the metabolic and structural features of animals.2 Arginine and its derivative compounds act in muscle tissue as a major reserve for adenosine triphosphate (ATP)—the “molecular unit of currency” of intracellular energy transfer—and as a regulatory sink for phosphate. That’s because, after its addition and utilization, the removal of phosphate from cellular protein is a pivotal strategy in the regulation of metabolic processes.


Habitual supplementation with
arginine (hereafter arginine) and
citrulline (hereafter citrulline), or
each separately, has been shown to
exhibit antiatherosclerotic effects.


Arginine and Citrulline Increase for Nitric Oxide

Habitual supplementation with L-arginine (hereafter arginine) and L-citrulline (hereafter citrulline), or each separately, has been shown to exhibit antiatherosclerotic effects. That’s because nitric oxide (NO)—the discovery of which earned a Nobel Prize—is a widespread signaling molecule in the cardiovascular system, and both arginine and citrulline each helps to produce NO in the body. Moreover, NO protects against the initiation and progression of atherosclerosis in a variety of ways.

  • Vasodilation: NO produced from arginine by endothelial NO synthase (eNOS) plays an important role in regulating endothelium-dependent vasodilatation (widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls).

  • Antiplatelet Activity: Preventing the adhesion of blood cells and platelets along the endothelial cell layer of blood vessels, and inhibiting vascular smooth muscle cell proliferation.

  • Scavenging Activity: Oxygen radical species are disarmed, including the prevention of oxidation of LDL-cholesterol.

Thus, when NO bioavailability caused by endothelial impairment is reduced, atherosclerotic coronary artery disease may result.


With enough arginine, you can trigger
your own, internal biochemical
resources of mental and
physical power for enhanced vitality
and youthfulness.


Oral Supplementation with Arginine

To augment NO-dependent responses, oral treatment with arginine in animals and humans has been studied extensively, with the aim of suppressing the progression of atherosclerosis or its component processes by restoring physiological levels of NO. One of the principal explanations of the therapeutic function of arginine is the increased availability of substrate for eNOS (endothelial nitric oxide synthase), which helps generates NO in blood vessels and is involved with regulating vascular tone by inhibiting smooth muscle contraction and platelet aggregation.

The Role of Human Growth Hormone

If amino acids, as a class of molecules, are the building blocks of life, then arginine—the most versatile of them all—is the bedrock on which the foundation of life depends. Without its support, everything would come tumbling down. But with enough arginine, you can trigger your own, internal biochemical resources of mental and physical power for enhanced vitality and youthfulness.


In a recent clinical study, citrulline
supplementation dose-dependently
increased plasma arginine levels in
healthy human volunteers
more effectively than equivalent
doses of arginine itself.


Central to enhanced vitality and youthfulness, when used correctly arginine helps release human growth hormone (HGH) naturally, the rejuvenating benefits of which have been extensively reported in the scientific and popular media. Your body releases HGH easily throughout childhood and early adulthood, but this crucial process slows down as you age. For nearly 60 years, scientific researchers concerned with the natural release of HGH from the pituitary gland have selected arginine as a primary focus of their investigation, because it plays a key role in the regulatory mechanism that governs this process. HGH release is pulsatile, delivering the right quantities at the right times to serve the body’s changing needs.

Unfortunately, to produce adequate levels of HGH, especially in conjunction with a peak output exercise program, a lot of arginine is required—from 6–18 g/day to improve endothelial function in humans.

Citrulline as Precursor to Arginine

Citrulline is a water-soluble amino acid that is a potent endogenous precursor of arginine. Citrulline is not found in the diet, but it is synthesized in the body via a mechanism called the urea cycle. In this cycle, arginine is converted to urea and ornithine and ornithine is converted to citrulline, which is converted to argininosuccinate, which is converted to arginine, thus completing the cycle. Of note, once citrulline is produced by any mechanism, it has only one place to go, and that’s back to arginine.


The data show for the first time that
oral citrulline supplementation raises
plasma arginine concentration and
augments NO-dependent signaling in
a dose-dependent manner.


In a recent clinical study, citrulline supplementation dose-dependently increased plasma arginine levels in healthy human volunteers more effectively than equivalent doses of arginine itself.3 Oral arginine supplementation has been used in various studies to improve endothelium-dependent nitric oxide (NO)-mediated vasodilation. Yet arginine treatment is hampered by extensive presystemic elimination due to intestinal arginase activity. In contrast, citrulline is readily absorbed and at least in part converted to arginine. The aim of the study was to assess this metabolic conversion and its subsequent pharmacodynamic effects.


A combination of citrulline and
arginine along with antioxidants
has a better therapeutic effect on
high-cholesterol induced
atherosclerosis in rabbits.


In the study, which was double-blinded, randomized, and placebo-controlled with a cross-over feature, 20 healthy volunteers received six different dosing regimens of placebo, citrulline, and arginine. Tests were carried out to measure pharmacokinetic parameters, calculated after 1 week of oral supplementation. Other measures were taken to assess pharmacodynamic effects.

Citrulline dose-dependently increased plasma arginine concentration more effectively than arginine. The highest dose of citrulline (3 g/twice per day) increased minimum levels of arginine. Note that citrulline is not found in food, and that the study only lasted one week.


Citrulline plus arginine
supplementation caused a more rapid
increase in plasma arginine levels.


The data show for the first time that oral citrulline supplementation raises plasma arginine concentration and augments NO-dependent signaling in a dose-dependent manner. Also, the research demonstrates that citrulline supplementation improves arterial stiffness, decreases lipoprotein oxidation, reduces ankle blood pressure and carotid wave reflection, and causes a reduction in the heart rate-corrected QT interval as a marker of sudden cardiac death. Thus, citrulline supplementation exhibits several significant beneficial effects on the cardiovascular system.

New Study Emphasizes Rapid Increases in Arginine

The researcher’s previous study4 revealed that chronic administration of a combination of citrulline and arginine along with antioxidants has a better therapeutic effect on high-cholesterol induced atherosclerosis in rabbits.

Following this, the purpose of the human study3 was to evaluate the acute effects of simultaneous administration of citrulline plus arginine on plasma arginine, NO bioavailability and blood circulation associated with the NO-cGMP pathway.

Increasing Arginine Levels Rapidly

In another new study,5 researchers focused on a potential strategy for promoting the citrulline-to-arginine recycling pathway, which is the principal mechanism for sustaining localized arginine availability for eNOS-catalyzed NO production, through the simultaneous use of citrulline and arginine.

Rats or New Zealand white rabbits were treated orally with citrulline, or arginine, or a combination of each at half dosage. Following supplementation, plasma levels of arginine, NOx, cGMP and changes in blood circulation were determined sequentially. NOx is a generic term for the mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide).



The data showed for the first time
that a combination of oral citrulline
and arginine effectively and rapidly
augments NO-dependent responses at
the acute stage.


Citrulline plus arginine supplementation caused a more rapid increase in plasma arginine levels and marked enhancement of NO bioavailability, including plasma cGMP concentrations, than either amino acid by itself.

Figure 1 Blood flow levels in the central ear artery after administration. Rabbits were administered L-citrulline, L-arginine, or a combination of each at half dosage. Blood flow in the central ear artery was measured 40 min after administration. There was a significant difference from the control group.
LEM1501_VitD_Fig1_274.gif
(click on thumbnail for full sized image)

Blood flow in the central ear artery in rabbits was also significantly increased by citrulline plus arginine administration than for either arginine or citrulline alone or the control (see Fig. 1).

The data showed for the first time that a combination of oral citrulline and arginine effectively and rapidly augments NO-dependent responses at the acute stage. This combination approach may have clinical utility for the regulation of cardiovascular function in humans because, like the aspirin study, when your cardiovascular system is stressed, rapid relief can forego long term negative consequences. It may also make it easier and more efficient for those who seek to gain the benefits of HGH release in peak output exercise.

Short Term Action

The primary objective of the study was to determine the short-term action of oral citrulline plus arginine on plasma parameters for the NO-cGMP pathway. To the best of the researchers’ knowledge, this is the first study on the rapid kinetics of plasma arginine, NOx and cGMP levels after simultaneous administration in comparison with the single amino acids.

Rapid Increase in Arginine Levels

Figure 2 Schematic of the citrulline recycling pathway in endothelial cells.

LEM1501_VitD_Fig2_274.gif
(click on thumbnail for full sized image)

The major finding of the study is that a combination of oral citrulline and arginine effectively and rapidly increases plasma arginine and augments NO-dependent responses, particularly within 1 hour after supplementation. NO—the endothelium-derived vasoactive factor—is produced from arginine by eNOS via the citrulline/arginine recycling pathway (see Fig. 2).

To investigate the effects of arginine, the substrate for NO synthesis, many researchers have carried out studies on vascular function. They have reported that intervention with arginine can improve endothelial dysfunction in animal and humans models. Nonetheless, the recent literature has barely reported effects of chronic administration of arginine, because orally administered arginine is strongly trapped in the gastrointestinal tract and the hepatic tissue, where it is extensively catabolized by arginase (see Fig. 3), suggesting that it has limited oral bioavailability as a substrate for eNOS.

Alternatively, several studies have persuasively demonstrated that citrulline is an effective precursor of arginine, thus contributing to sustained arginine supply for eNOS activity.

Figure 3 A proposed mechanism for simultaneous administration with L-citrulline plus L-arginine leading to rapid raising of plasma L-arginine levels.
LEM1501_VitD_Fig3_274.gif
(click on thumbnail for full sized image)

Citrulline Suppresses Arginase Activity

Of interest, previous in vitro and in vivo research has demonstrated that citrulline suppresses arginase activity, acting as a strong allosteric inhibitor, and exerts an anti-hypertensive effect in animals via a mechanism involving arginase inhibition by citrulline. This could provide one possible mechanism for the rapid and significant increase in plasma arginine levels, suggesting that arginine could pass through the gastrointestinal tract and liver without being influenced by intestinal and hepatic-first pass effects, probably due to inhibition of arginase activity by citrulline (Fig. 3).


The major finding is that a
combination of oral citrulline and
arginine effectively and rapidly
increases plasma arginine and
augments NO-dependent responses,
particularly within 1 hour
after supplementation.


These findings suggest the role played by citrulline in upregulating arginine bioavailability, leading to enhancement of the NO-cGMP pathway. Upregulation of the citrulline-arginine recycling pathway and increased eNOS expression are accompanied by increased plasma NOx and cGMP. Here, the intracellular arginine concentration offers a sufficient level for arginine as a substrate for eNOS. However, a previous report suggests that intracellular endothelial arginine may not in fact be available for NO production. This is because cytosolic arginine availability for eNOS may be limited by its sequestration in caveolae in which the eNOS and the citrulline to arginine recycling pathway are localized.

Furthermore, recent reports suggest one possible mechanism in which the principal source of available arginine for NO production is uptake from extracellular, but not cytosolic arginine. This finding supports the notion that a combination of citrulline plus arginine effectively increases plasma arginine levels, leading to NO production. Other results of the study clearly indicate arginine-linked enhancement of plasma NOx and cGMP levels following simultaneous administration with citrulline and arginine.

It has been indicated that citrulline added to cultured endothelial cells sustains NOS activity and NO production under arginine-deficient conditions. The researchers speculated that direct uptake of citrulline into the endothelial cells may provide a substrate for the formation of arginine via citrulline/arginine recycling pathway and contribute to upregulation of eNOS activity. The time to reach peak concentration of plasma arginine after oral citrulline is approximately 2 hours in humans and 4 hours in animals (the researchers’ unpublished data). This suggests that the combination of citrulline plus arginine is useful for short-acting effects, as shown in the present study, while citrulline alone is useful for long-acting enhancement of arginine availability. Indeed, this suggestion is further supported by research findings that citrulline administration to rats significantly increases the area under the concentration–time curve to a greater extent than arginine itself (the data were not shown in the paper).

Long-Term Citrulline Plus Arginine Improves Vasorelaxation

The researcher’s previous study revealed that long-term citrulline plus arginine supplementation causes a significant improvement in endothelium-dependent vasorelaxation and dramatic regression in experimentally-induced atherosclerosis in rabbits. As one possible mechanism, they have also shown increased expression of eNOS protein by combination of these two amino acids. However, the short-term action of the combination of citrulline plus arginine on the NO-cGMP pathway remains to be explicated.


The combination of citrulline
plus arginine is useful for
short-acting effects, as shown in
the present study, while citrulline
alone is useful for long-acting
enhancement of
arginine availability.


Thus, this study proposes a potentially novel strategy for rapidly potentiating NO-cGMP dependent reactions in the short-term, which is capable of increasing plasma arginine and NOx levels as well as blood circulation rapidly and effectively, likely due to upregulation of the citrulline/arginine recycling pathway and eNOS expression.

Clinical Benefit of Arginine and Citrulline

Growing evidences suggest that endothelial dysfunction, which is strongly associated with reduced NO bioavailability, to be an initiating factor in cardiovascular diseases. The data suggests the clinical benefit on vascular function of citrulline plus arginine supplementation. However, of course, the researchers believe a further study in humans (only one?) is needed to be able to present a therapeutic option with clinical utility. Also, one of the benefits of high and sustained levels of arginine is growth hormone release, which can be extremely beneficial for amplifying the benefits of peak output exercise, so with citrulline onboard, it takes two to tango!

References

  1. Bøtker HE, Kharbanda R, Schmidt MR, et al. Remote ischaemic conditioning before hospital admission, as a complement to angioplasty, and effect on myocardial salvage in patients with acute myocardial infarction: a randomised trial. Lancet. 2010 Feb 27;375(9716):727-34.
  2. Hird FJR. The importance of arginine in evolution. Comp Biochem Physiol. 1986; 85B(2):285-8.
  3. Schwedhelm E, Maas R, Freese R, Jung D, Lukacs Z, Jambrecina A, Spickler W, Schulze F, Böger RH. Pharmacokinetic and pharmacodynamic properties of oral L-citrulline and L-arginine: impact on nitric oxide metabolism. Br J Clin Pharmacol. 2008 Jan;65(1):51-9.
  4. Hayashi T, Juliet PA, Matsui-Hirai H, Miyazaki A, Fukatsu A, Funami J, Iguchi A, Ignarro LJ. l-Citrulline and l-arginine supplementation retards the progression of high-cholesterol-diet-induced atherosclerosis in rabbits. Proc Natl Acad Sci U S A. 2005 Sep 20;102(38):13681-6.
  5. Morita M, Hayashi T, Ochiai M, Maeda M, Yamaguchi T, Ina K, Kuzuya M. Oral supplementation with a combination of l-citrulline and l-arginine rapidly increases plasma l-arginine concentration and enhances NO bioavailability. Biochem Biophys Res Commun. 2014 Oct 14;454(1):53-57.


Will Block is the publisher and editorial director of Life Enhancement magazine.

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