Identification of system y+L as the high-affinity transporter for L-arginine in human platelets: up-regulation of L-arginine influx in uraemia

ADMA metabolism and clearance

The plasma concentration of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, is the resultant of many processes at cellular and organ levels. Post-translational methylation of arginine residues of pro teins plays a crucial role in the regulation of their functions, which include processes such as transcription, translation and RNA splicing. Because protein methylation is irreversible, the methylation signal can be turned off only by proteolysis of the entire protein. Consequently, most methylated proteins have high turnover rates. Free ADMA, which is formed during proteolysis, is actively degraded by the intracellular enzyme dimethylarginine dimethylaminohydrolase (DDAH). Some ADMA escapes degradation and leaves the cell via cationic amino acid transporters. These trans porters also mediate uptake of ADMA by neighboring cells or distant organs, thereby facilitating active interorgan transport. Clearance of ADMA from the plasma occurs in small part by urinary excretion, but the bulk of ADMA is degraded by intracellular DDAH, after uptake from the circulation. This review discusses the various processes involved in ADMA metabolism: protein methylation, proteolysis of methylated proteins, metabolism by DDAH, and interorgan transport. In addition, the role of the kidney and the liver in the clearance of ADMA is highlighted.

Hyperaggregability and impaired nitric oxide production in platelets from postmenopausal women

OBJECTIVE

Cardiovascular mortality increases after menopause in women. Nitric oxide is essential for proper platelet function inhibiting its aggregation and maintaining vascular haemostasis. Here, we investigated whether platelet function and intraplatelet l-arginine-nitric oxide pathway are impaired in postmenopausal women.

STUDY DESIGN

Cross-sectional.

MAIN OUTCOMES MEASURES

Blood was collected from 16 premenopausal and 12 postmenopausal women without any additional risk factor for cardiovascular disease. Platelet reactivity was measured by light transmission aggregometry. l-Arginine-nitric oxide pathway was assessed measuring transmembrane l-[(3)H]-arginine transport, nitric oxide synthase activity by the citrulline assay, and arginase activity by the conversion of l-[(14)C]arginine to l-[(14)C]-urea. The activity of antioxidant enzymes was measured by spectrophotometric assays. Protein expression was determined by Western blotting.

RESULTS

Platelet aggregation was increased in postmenopausal compared to premenopausal women. Postmenopausal women demonstrated reduced plasma levels of l-arginine, a lower nitric oxide synthase activity, similar endothelial and inducible nitric oxide synthase expression, and a compensatory increase in l-arginine transmembrane transport. Arginase expression and activity did not differ between groups. In regard to oxidative stress, no differences between groups were observed NAPDH oxidase subunits expression and protein carbonylation. However, the activity of the antioxidant enzyme superoxide dismutase and catalase protein levels in platelets were higher in postmenopausal women.

CONCLUSION

Postmenopausal women present increased platelet reactivity, which may be due to a reduction in intraplatelet nitric oxide synthesis. Platelet hyperaggregability is known to be associated with arterial and venous thromboembolic event; therefore, it may contribute to the heightened risk of cardiovascular adverse events in this population.

The role of exercise on L-arginine nitric oxide pathway in chronic heart failure

Chronic heart failure (CHF) is a pathological state with high morbidity and mortality and the full understanding of its genesis remain to be elucidated. In this syndrome, a cascade of neurohormonal and hemodynamic mechanisms, as well as inflammatory mediators, are activated to improve the impaired cardiac function. Clinical and experimental observations have shown that CHF is associated with a generalized disturbance in endothelium-dependent vasodilation, which may contribute to the progression of ventricular and vascular remodelling in this syndrome. There is also accumulating evidence that disturbances in nitric oxide (NO) availability is involved in the development of heart failure at the systemic and cardiac levels. NO is a ubiquitous signalling molecule which causes potent vasodilation, inhibits platelet activation and regulates the contractile properties of cardiac myocytes. It is generated from the amino acid L-arginine via constitutive and inducible isoforms of the enzyme NO synthase (NOS). There is evidence that exercise, a nonpharmacological tool, improves symptoms, fitness (VO_2peak), quality of life and NO bioavailability in CHF population. This review examines different aspects of the L-arginine-NO pathway and inflammation in the physiopathology of CHF and highlights the important beneficial effects of exercise in this disease.

Dengue fever activates the L-arginine-nitric oxide pathway: an explanation for reduced aggregation of human platelets

In patients with Dengue fever, a viral inflammatory syndrome, haemorrhage is a significant pathological feature, yet the underlying mechanisms remain unclear. Nitric oxide (NO) is an important regulator of platelet function, inhibiting aggregation, recruitment and adhesion to the vascular endothelium. We have investigated whether changes in the activity of the L-arginine-NO pathway in human platelets may account for increased bleeding in patients with Dengue fever. A total of 16 patients with Dengue fever and 18 age-matched healthy volunteers participated in the study. Collagen induced platelet aggregation in a dose-dependent manner in both Dengue patients and controls, but the degree of platelet aggregation was significantly reduced in the patient group. Elevated rates of L-arginine transport in Dengue fever patients were associated with enhanced NO synthase activity and elevated plasma fibrinogen levels. The present study provides the first evidence that Dengue fever is associated with increased L-arginine transport and NO generation and reduced platelet aggregation.

Nitric oxide deficiency in chronic kidney disease

The overall production of nitric oxide (NO) is decreased in chronic kidney disease (CKD) which contributes to cardiovascular events and further progression of kidney damage. There are many likely causes of NO deficiency in CKD and the areas surveyed in this review are: 1. Limitations on substrate (l-Arginine) availability, probably due to impaired renal l-Arginine biosynthesis, decreased transport of l-Arginine into endothelial cells and possible competition between NOS and competing metabolic pathways, such as arginase. 2. Increased circulating levels of endogenous NO synthase (NOS) inhibitors, in particular asymmetric dimethylarginine (ADMA). Increased methylation of proteins and their subsequent breakdown to release free ADMA may contribute but the major culprit is probably reduced ADMA catabolism by the enzymes dimethylarginine dimethylaminohydrolases. 3. Reduced renal cortex abundance of the neuronal NOS (nNOS)α protein correlates with injury while increasing nNOSβ abundance may provide a compensatory, protective response. Interventions that can restore NO production by targeting these various pathways are likely to reduce the cardiovascular complications of CKD as well as slowing the rate of progression.

Arginine, arginine analogs and nitric oxide production in chronic kidney disease

Nitric oxide (NO) production is reduced in renal disease, partially due to decreased endothelial NO production. Evidence indicates that NO deficiency contributes to cardiovascular events and progression of kidney damage. Two possible causes of NO deficiency are substrate (L-arginine) limitation and increased levels of circulating endogenous inhibitors of NO synthase (particularly asymmetric dimethylarginine [ADMA]). Decreased L-arginine availability in chronic kidney disease (CKD) is due to perturbed renal biosynthesis of this amino acid. In addition, inhibition of transport of L-arginine into endothelial cells and shunting of L-arginine into other metabolic pathways (e.g. those involving arginase) might also decrease availability. Elevated plasma and tissue levels of ADMA in CKD are functions of both reduced renal excretion and reduced catabolism by dimethylarginine dimethylaminohydrolase (DDAH). The latter might be associated with loss-of-function polymorphisms of a DDAH gene, functional inhibition of the enzyme by oxidative stress in CKD and end-stage renal disease, or both. These findings provide the rationale for novel therapies, including supplementation of dietary L-arginine or its precursor L-citrulline, inhibition of non-NO-producing pathways of L-arginine utilization, or both. Because an increase in ADMA has emerged as a major independent risk factor in end-stage renal disease (and probably also in CKD), lowering ADMA concentration is a major therapeutic goal; interventions that enhance the activity of the ADMA-hydrolyzing enzyme DDAH are under investigation.

ADMA metabolism and clearance

The plasma concentration of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, is the resultant of many processes at cellular and organ levels. Post-translational methylation of arginine residues of proteins plays a crucial role in the regulation of their functions, which include processes such as transcription, translation and RNA splicing. Because protein methylation is irreversible, the methylation signal can be turned off only by proteolysis of the entire protein. Consequently, most methylated proteins have high turnover rates. Free ADMA, which is formed during proteolysis, is actively degraded by the intracellular enzyme dimethylarginine dimethylaminohydrolase (DDAH). Some ADMA escapes degradation and leaves the cell via cationic amino acid transporters. These transporters also mediate uptake of ADMA by neighboring cells or distant organs, thereby facilitating active interorgan transport. Clearance of ADMA from the plasma occurs in small part by urinary excretion, but the bulk of ADMA is degraded by intracellular DDAH, after uptake from the circulation. This review discusses the various processes involved in ADMA metabolism: protein methylation, proteolysis of methylated proteins, metabolism by DDAH, and interorgan transport. In addition, the role of the kidney and the liver in the clearance of ADMA is highlighted.

Diminished L-arginine bioavailability in hypertension

L-Arginine is the precursor of NO (nitric oxide), a key endogenous mediator involved in endothelium-dependent vascular relaxation and platelet function. Although the concentration of intracellular L-arginine is well above the Km for NO synthesis, in many cells and pathological conditions the transport of L-arginine is essential for NO production (L-arginine paradox). The present study was designed to investigate the modulation of L-arginine/NO pathway in systemic arterial hypertension. Transport of L-arginine into RBCs (red blood cells) and platelets, NOS (NO synthase) activity and amino acid profiles in plasma were analysed in hypertensive patients and in an animal model of hypertension. Influx of L-arginine into RBCs was mediated by the cationic amino acid transport systems y+ and y+L, whereas, in platelets, influx was mediated only via system y+L. Chromatographic analyses revealed higher plasma levels of L-arginine in hypertensive patients (175+/-19 micromol/l) compared with control subjects (137+/-8 micromol/l). L-Arginine transport via system y+L, but not y+, was significantly reduced in RBCs from hypertensive patients (60+/-7 micromol.l(-1).cells(-1).h(-1); n=16) compared with controls (90+/-17 micromol.l(-1).cells(-1).h(-1); n=18). In human platelets, the Vmax for L-arginine transport via system y+L was 86+/-17 pmol.10(9) cells(-1).min(-1) in controls compared with 36+/-9 pmol.10(9) cells(-1).min(-1) in hypertensive patients (n=10; P<0.05). Basal NOS activity was decreased in platelets from hypertensive patients (0.12+/-0.02 pmol/10(8) cells; n=8) compared with controls (0.22+/-0.01 pmol/10(8) cells; n=8; P<0.05). Studies with spontaneously hypertensive rats demonstrated that transport of L-arginine via system y+L was also inhibited in RBCs. Our findings provide the first evidence that hypertension is associated with an inhibition of L-arginine transport via system y+L in both humans and animals, with reduced availability of L-arginine limiting NO synthesis in blood cells.

Uremic levels of urea inhibit L-arginine transport in cultured endothelial cells

Hypertension in end-stage renal disease (ESRD) may involve lack of endothelial nitric oxide (NO), as suggested by reduced total NO synthase (NOS) in dialysis patients. One reason might be due to substrate deficiency. To test the hypothesis that uremia is a state of intracellular L-arginine deficiency, uremic plasma was obtained from dialysis patients, and its effect was tested on arginine transport in cultured vascular endothelial cells. L-arginine transport (P < 0.01) was reduced in human dermal microvascular endothelial cells (HDMEC) incubated for 6 h with 20% uremic plasma from peritoneal dialysis and hemodialysis patients obtained immediately predialysis. Similar transport inhibition was seen with ESRD plasma in human glomerular capillary and bovine aortic endothelial cells. Hemodialysis partially reversed inhibition of L-arginine transport. HDMECs incubated for 6 h with synthetic media containing high (uremic) urea concentrations showed inhibition of L-arginine transport, but this was not competitive because acute exposure to urea had no impact on L-arginine transport. Over a 6-h period, urea-induced inhibition of L-arginine transport was not sufficient to inhibit NOS activity, but after 7 days NOS activity was reduced. These cellular findings suggest that substrate delivery may be lowered, thus reducing endothelial NOS activity and contributing to hypertension in ESRD patients.