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.

In mice fed fat-rich diets, the level of adipokines, the distribution of adipose tissue, and the metabolism of carbohydrates are more significantly influenced by the lipid content rather than the absolute amount of lipid.

Bleeding tendency in uraemic patients seems to be related to alterations in the activity of the L-arginine-nitric oxide (NO) signalling pathway in platelets. We have reported previously that L-arginine influx into human platelets is mediated by the high-affinity cationic amino acid transport system y(+)L. In the present study we examined the dependency of nitric oxide synthase (NOS) activity on L-arginine transport in platelets isolated from healthy controls and uraemic patients on haemodialysis. We investigated basal and ADP-stimulated NOS activity, as reflected by the conversion of L-[(3)H]arginine to L-[(3)H]citrulline, in platelets obtained from healthy controls and uraemic patients on haemodialysis. To determine whether NOS activity depended on L-arginine transport, we analysed the effects of competitive inhibitors of L-arginine transport via system y(+)L on NOS activity. Basal NOS activity was increased from 0.21+/-0.06 to 0.7+/-0.2 pmol/10(8) platelets ( n=9, P<0.05) in uraemic patients. Stimulation by ADP (10 micro M) significantly increased NOS activity (inhibitable by L-NAME) in control platelets (252%) but failed to increase further the elevated NOS activity in uraemic platelets. Homocysteine and L-leucine, competitive inhibitors of system y(+)L, markedly inhibited NOS activity in uraemic platelets. These observations indicate that platelets from uraemic patients on haemodialysis generate more NO than control platelets and that entry of L-arginine via system y(+)L is most likely rate-limiting for platelet NO production in chronic renal failure.

Kinetic studies of L-arginine transport in human platelets have identified a high-affinity, low-capacity transport system [Michaelis-Menten constant (K m ) about 10 µM] for cationic amino acids that also transports neutral amino acids with high affinity in the presence of Na + but not K + . These characteristics, together with our kinetic cis-inhibition studies, indicate that saturable L-arginine transport in human platelets is mediated via the system y + L and not the classic cationic transporter system y + . We present here the first evidence that L-arginine transport via system y + L is increased twofold in platelets from patients with chronic renal failure. System y + L has been described in human erythrocytes, peripheral blood mononuclear cells and placenta, and up-regulation of system y + L activity in human platelets could explain the paradox of increased nitric oxide (NO) production by uraemic platelets under conditions of decreased plasma L-arginine and elevated N G -monomethyl-L-arginine (L-NMMA) concentrations.

Pre-eclampsia (PE), a syndrome of pregnancy-induced hypertension, continues to be a leading cause of maternal and fetal morbidity and mortality. The aim of this study was to investigate whether changes in oxidative status are correlated with alterations in the L-arginine-nitric oxide pathway and platelet aggregation in PE. Plasma and platelets from women with PE (n=24) or normotensive pregnancy (NP, n=27) recruited in the third trimester of gestation were used to measure oxidative damage assessed by protein carbonyl content, antioxidant activities of superoxide dismutase (SOD), catalase (CAT) and nitrite levels. Transport of L-[(3)H]-arginine, as well as the activities of the nitric oxide (NO) synthase (eNOS and inducible NO synthase (iNOS)) and platelet aggregation, were also evaluated. Plasma nitrite levels and the activities of SOD and CAT were reduced in PE (5.2±2.7, 3.4±0.8, 0.3±0.4, respectively, P<0.05) compared with NP (8.7±2.3, 6.7±3.1, 1.0±0.5, respectively), whereas protein carbonyl content and L-arginine levels were not significantly different between PE and NP groups. In platelets, L-arginine transport was reduced in PE (19.2±10.5, P<0.05) compared with NP (62.0±31.1), whereas the NOS activity, eNOS and iNOS expression, nitrite levels and platelet aggregation were unaffected. Protein carbonyl content was increased, and CAT activity was reduced in platelets from PE (0.03±0.02, 0.55±0.30, respectively, P<0.05), compared with NP (0.005±0.005, 1.01±0.36, respectively). The data suggest that a systemic impairment of antioxidant defense mechanisms is associated with decreased plasma nitrite levels, which may contribute to hypertension in PE. Oxidative stress may contribute to the reduced influx of L-arginine in platelets. Compensatory mechanisms may contribute to the maintenance of NO production and its modulatory role on platelet function.

Nitric oxide (NO) inhibits platelet function and plays a key role in the regulation of cardiovascular homeostasis. Essential hypertension is characterized by an increased risk of thrombus formation, and by an inhibition of intraplatelet NO bioactivity. We have previously shown that membrane transport of L-arginine is a rate-limiting step for plateletderived NO synthesis. This study examined the effects of exercise on the platelet L-arginine-NO pathway and aggregation and systemic inflammation markers in 13 sedentary hypertensive patients subjected to 60 min of training activity (exercise group), predominantly aerobic, three times a week for a period of 12 weeks. Six sedentary hypertensive patients participated in the control group.After 12 weeks, L-arginine transport was significantly increased and associated with increased platelet NO synthase activity and cGMP levels and reduced platelet aggregation. Moreover, exercise training reduced plasma concentrations of fibrinogen and C-reactive protein and blood pressure. The control group did not change their previous intraplatelet L-arginine-NO results and systemic inflammatory markers levels. Thus, exercise training reduces inflammatory responses, restores NO synthesis in platelets and thereby contributes to the beneficial effects of exercise in hypertension. The present study adds exercise as a new tool to reduce morbidity and mortality associated with platelet activation in hypertension.

L-arginine is the physiological precursor for nitric oxide (NO) synthesis, and availability and transport of L-arginine modulate the rates of NO biosynthesis in circulating blood cells and the vasculature. NO is involved in many vascular functions such as vasodilation and inhibition of platelet aggregation and adhesion. We have established that reduced plasma L-arginine and NO production and increased tumour necrosis factor-alpha (TNF-alpha), fibrinogen, and C-reactive protein levels in malnourished uremic patients are associated with increased aggregability of platelets. Our findings may explain the increased cardiovascular mortality in patients with deficient nutritional status, leading to inflammation, oxidative stress, impaired L-arginine-NO signalling, and platelet activation. The aim of this review is to evaluate whether disturbances in the L-arginine-NO signalling pathway in chronic renal failure and atherosclerosis are affected by malnutrition and inflammation. We have included a brief overview of membrane transporters mediating influx of L-arginine and other cationic amino acids, as these transporters are involved in the potential benefits of L-arginine supplementation and platelet function in malnourished uremic patients.

Our results suggest that regular exercise training is a valuable adjunct to optimal medical management of HF, reducing platelet aggregation via antioxidant and anti-inflammatory effects, and, therefore, reducing the risk of future thrombotic events.