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.

The data suggest that a compromised mechanism of antioxidant defense and an increase in oxidative damage contribute to the development of hypertension and associated vascular dysfunction in 2K-1C rats, and that tempol and apocynin prevent these effects.

Nitric oxide (NO) produced by human platelets plays an important role in all stages of platelet activation. l-Arginine, the precursor for NO synthesis, modulates NO production by platelets. The l-arginine analogues asymmetric dimethylarginine (ADMA) and N(G)-monomethyl-l-arginine (l-NMMA) are endogenous inhibitors of nitric oxide synthase (NOS), involved in the physiopathology of arterial hypertension. The aim of the present study was to investigate the inhibitory effects of endogenous and exogenous l-arginine analogues on l-arginine influx in platelets from healthy controls and hypertensive patients. Twelve patients with uncomplicated essential hypertension (stage I) and 15 age-matched normotensive controls participated in the present study. Platelets were isolated and incubated with l-[(3)H]-arginine and increasing concentrations of l-arginine analogues (5-2000 micromol/L). The influx of l-arginine was inhibited in a concentration-dependent manner by l-NMMA in platelets from controls (K(i) = 42 +/- 6 micromol/L) and this inhibitory effect was markedly higher in hypertensive platelets (K(i) = 23 +/- 4 micromol/L). Similarly, the K(i) for ADMA inhibition of l-arginine transport was significantly more pronounced in platelets from hypertensive patients (K(i) = 16 +/- 1 micromol/L) compared with controls (K(i) = 27 +/- 2 micromol/L). In contrast, N(G)-nitro-l-arginine methyl ester (l-NAME) was found to be a weak inhibitor of l-arginine influx in platelets from controls (K(i) = 1917 +/- 319 micromol/L) and hypertensive patients (K(i) = 2279 +/- 578 micromol/L). Aminoguanidine, a selective inhibitor of inducible NOS, failed to inhibit l-arginine transport. Our findings provide the first evidence that ADMA and l-NMMA markedly inhibit l-arginine transport in human platelets, an effect that is more pronounced in hypertensive patients. It is possible that endogenous l-arginine analogues, by inhibiting NO synthesis, are involved in the platelet activation present in hypertension.

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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.