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L-Arginine, Nitric Oxide and Atherosclerosis. Rhobert W. Evans, PhD. . University of Pittsburgh. Content :. •L-Arginine metabolism •Nitric oxide •Synthases •Dimethylarginines •Effects

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slide1

L-Arginine, Nitric Oxide and Atherosclerosis

Rhobert W. Evans, PhD.

University of Pittsburgh

content
Content:

•L-Arginine metabolism

•Nitric oxide

•Synthases

•Dimethylarginines

•Effects

•Arginine Supplementation

•Infusion/ Oral

•Endothelial Function

•Epidemiology

source of arginine
Source of Arginine
  • L-Arginine is considered a semi-essential amino acid: it becomes essential in growing children, during pregnancy or after injury.
  • A Western diet provides about 4-6 g/day of which 40-50% is absorbed.
slide5
•The liver produces considerable amounts of arginine during the urea cycle, but little is available for synthesis.

• The intestines produce citrulline which is converted by other tissues (kidney, 80%) into L-arginine which is then made available to other tissues.

slide6

NH2

NH2

NADPH+ H+

C=O

C=NH2+

NADP+

NADP+

NH

NH

CH2

CH2

CH2

CH2

CH2

CH2

2O2

2O2

NO + H2O

Nitric Oxide

H C NH3+

H C NH3+

COO-

COO-

L-Arginine

L-Citrulline

The synthesis of nitric oxide (NO) is catalyzed by nitric oxide synthase (NOS). The reaction is more complicated than indicated in the figure.

nitric oxide synthase
Nitric Oxide Synthase
  • Three Isoforms
      • Neuronal (constitutive, calcium dependent)
      • Endothelial (constitutive, calcium dependent)
      • Macrophages (inducible, calcium independent). Can lead to high levels of NO being formed.
nitric oxide effects via formation of cgmp
Nitric Oxide Effects (via formation of cGMP)
  • Relaxes smooth muscle
  • Inhibits platelet aggregation and activation
  • Neurotransmitter
  • Tumoricidal and bactericidal agent from macrophages (excess can damage healthy tissue)
slide9
• Many studies involve infusion or dietary supplements of L-arginine in both animals and humans.

• The physiological effects elicited were unexpected as the km of NOS for L-arginine is about 2 μM whereas the circulating levels of L-arginine are about 100 μM.

slide10
• The explanation may involve the presence of naturally occurring inhibitors of NOS (ADMA and NMA). These two analogues of L-arginine plus SDMA are also competitors for the y+ transport system that delivers L-arginine to NOS.
structure of l arginine and circulatory analogues
Structure of L-arginine and Circulatory Analogues

CH3

CH3

CH3

CH3

CH3

NH

NH2

NH

NH

NH

N

NH

N

C

C

C

C

NH

NH

NH

NH

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH

CH

CH

CH

NH2

COOH

NH2

COOH

NH2

COOH

NH2

COOH

L-ARGININE

L-NMA

ADMA

SDMA

as shown in the metabolic pathway slide 14
As shown in the metabolic pathway (slide 14):

•L-arginine is methylated while a component of proteins by:

•PRMT (type I): occurs in nucleus, many substrates forms ADMA and NMA

•PRMT (type II) : specific for myelin basic protein, forms SDMA and NMA

•The methylated analogues are released by hydrolysis during normal protein turnover

slide13
• The methylated analogues are removed by renal excretion or catabolism

•DDAH type I associated with neural NOS

•DDAH type II associated with endothelial NOS

•Neither DDAH is active on SDMA

•DPT (a minor pathway) acts on all three analogues

•The enzymes are particularly active in kidney

slide14

Protein

PRMT (types I and II)

Modified Protein Containing ADMA+ SDMA+ NMA

Hydrolysis

ADMA +SDMA +NMA

DDAH (types I and II)

DPT

Acetylated Products

Renal

Excretion

α-keto acid products

Citrulline + Methylamines

PRMT: Protein arginine methyltransferase

ADMA: Asymmetrical dimethylarginine

SDMA: Symmetrical dimethylarginine

NMA: N-monomethylarginine

DDAH: Dimethylaminohydrolase

DPT: Dimethylarginine pyruvate transferase

slide16

In slides 18 and 19, results from Cooke et al (1992) are shown. The investigators fed male rabbits either (a) normal chow (control) or (b) 1% cholesterol diet; or (c) 1% cholesterol diet supplemented with drinking water containing 2.25% L-arginine HCl. After 10 weeks of dietary intervention, analyses indicated:

slide17
Endothelium dependent relaxation of the thoracic aortae elicited by acetylcholine was reduced in cholesterol-fed animals and the response was significantly ameliorated by L-arginine.
  • L-arginine also significantly reduced the lesion surface area in the descending thoracic aorta elicited by cholesterol diets (intima thickness also reduced)
slide20
•Candipan et al (1996) fed rabbits either normal chow (controls) or 0.5% cholesterol chow for 10 weeks and then the cholesterol group received either vehicle or L-arginine (2.25% in water) (arginine group) for an additional 13 weeks.
slide21
•Histomorphic measurements indicated a gradual deterioration in the cholesterol group (intima-media thickness) and this was ameliorated by L-arginine at 18 weeks but not at 23 weeks (slide 22).

•This may indicate that the effects of L-arginine are not sustained.

slide22

Intima, mm2

Candipan, RC, et al. Arteriosclerosis, Thrombosis, and Vascular Biology (1996) 16(1): 44-50.

slide23
In human studies, Drexler et al (1994) infused 18 cardiac transplant recipients with acetylcholine (10-6, 10-5, 10-4 mol/L) before and after intravenous with L-arginine (10 mg/ kg. min. for 20 minutes). (slide 24)
  • Acetylcholine elicited a dose-dependent constriction of the coronary artery that was attenuated by L-arginine (p <0.01 at 10-4 acetylcholine)
slide24

Before L-Arginine

After L-Arginine

Drexler, H, et al. Circulation (1994) 89(4):1615

slide25
• Böger et al (1998) reported that infusion of L-arginine ameliorated the clinical symptoms of intermittent claudication in patients with peripheral arterial occlusive disease.

• 13 patients received two intravenous infusions of L-arginine (8 g each) for 3 weeks.

slide26
•13 patients received no infusions (control group)

•Both groups maintained normal walking exercises.

•Results indicated that L-arginine improved pain-free walking distance (slide 27) by 230± 63% (p < 0.05). Absolute walking distance also improved by 155 ±48% (p < 0.05).

slide27

Pain-free Walking Distance (m)

Böger, RH, et al. J Am Coll Cardiol (1998) 32(5): 1336-44.

slide28
•Physiological effects have also been elicited by infusion of NOS inhibitors.

•Vallance et al (1992) infused ADMA(8 μmol/min for 5 min into 5 volunteers) and observed a decrease in forearm blood flow (slide 30)

slide29
•McVeigh et al (2001) infused L-NAME (NG- nitro-L-arginine methyl ester) into 15 healthy men and observed an increase in systemic vascular resistance (slide 31) and a decrease in small artery compliance (slide 32)

•The effects were ameliorated by infusion of L-arginine but not by D-arginine.

slide31

† p < 0.01 versus control

‡ p < 0.01 D-arginine versus L-arginine

McVeigh, GE, et al. Clinical Science. (2001)100: 387-393.

slide32

* p < 0.05 versus controls

† p < 0.01 versus controls

‡ p < 0.01 D-arginine versus L-arginine

McVeigh, GE et al. Clinical Science. (2001) 100: 387-393.

slide34

•Epidemiological studies have observed associations between ADMA concentrations and subclinical and clinical measures of atherosclerosis.

•Miyazaki et al (1999) studied 116 subjects with no symptoms of coronary or peripheral artery disease and not taking medications. Results indicated:

slide35
•Plasma ADMA levels were significantly correlated with intima-media thickness (slide 36)

•Stepwise multiple regression analysis indicated plasma ADMA was a significant determinant of the intima-media thickness (slide 37)

slide38
•Zoccalli et al (2001) studied 225 haemodialysis patients with end-stage renal disease.

•Plasma ADMA was significantly and independently correlated with all-cause mortality and fatal and non-fatal cardiovascular events. No significant associations were observed for plasma SDMA or L-arginine (slides 40 and 41).

slide39
• Valkonen et al (2001), in a prospective case-control study analyzed the association between ADMA and the risk of acute coronary events.

•Among non-smoking men, ADMA was a significant risk factor for acute coronary events. The conclusions were dependent on presence or absence of a history of coronary heart disease (CHD): not significant in the absence of a history of CHD; significant in the presence of a history of CHD (slide 42).

slide40

All-cause mortality

Hazard ratio*

p

Fully adjusted

p

Unit of increase

(95%Cl)

hazard ratio*

(95% Cl)

ADMA

1 μmol/L

1.28(1.16-1.41)

<0.0001

1.26(1.11-1.41)

0.0001

SDMA

1 μmol/L

1.02(0.93-1.11)

0.73

1.06(0.94-1.18)

0.34

L-arginine

10 mmol/L

1.01(0.89-1.14)

0.92

0.92(0.80-1.05)

0.22

Zoccalli, C, et al. Lancet (2001) 358: 2113-2117.

slide41

Fatal and non-fatal cardiovascular events

Zoccalli, C, et al. Lancet (2001) 358: 2113-2117.

slide43

Future Directions:

  • Are the effects of L-arginine supplementation sustained?
  • Will L-arginine supplementation be of clinical benefit?
  • How general will any benefit be?
slide44

Ackknowledgments

  • I would like to thank Ms. Meghan Dabkowski for her assistance in the preparation of this presentation.