Nitric oxide activity in platelets of dengue haemorrhagic fever patients: the apparent paradoxical role of ADMA and l-NMMA.

Dengue haemorrhagic fever (DHF) is a prevalent acute disease that occurs in patients infected by an arbovirus in tropical and subtropical regions. We have previously shown increased intraplatelet nitric oxide (NO) production in patients with dengue fever associated with reduced platelet aggregation. In this study, l-arginine transport as well as expression and activity of nitric oxide synthase (NOS) isoforms in the presence or absence of l-arginine analogues were examined in 23 DHF patients. l-arginine transport and NOS activity in platelets were increased in patients with DHF compared with controls. However, platelet endothelial NOS (eNOS) and inducible (iNOS) protein levels did not differ between healthy controls and DHF patients. Endogenous or exogenous analogues did not inhibit platelet NOS activity from DHF patients. In contrast, endogenous l-arginine analogues [N(G)-monomethyl-l-arginine (l-NMMA) and asymmetric dimethylarginine (ADMA)] inhibited NOS activity in platelets from healthy subjects. These results show the first evidence that the intraplatelet l-arginine-NO pathway is activated in DHF patients. The lack of inhibition of NO formation in vitro by all l-arginine analogues tested in DHF platelets may suggest another mechanism by which NOS activity can be regulated.

Insulin resistance in obesity and metabolic syndrome: is there a connection with platelet l-arginine transport?

OBJECTIVE: Nitric oxide (NO) is a short-lived gaseous messenger with multiple physiological functions including regulation of blood flow, platelet adhesion and aggregation inhibition. NO synthases (NOS) catalyze the conversion of cationic amino acid L-arginine in L-citrulline and NO. Despite an increasing prevalence of obesity and metabolic syndrome (MetS) in the last decades, the exact mechanisms involved in the pathogenesis and cardiovascular complications are not fully understood. We have examined the effects of obesity and MetS on the L-arginine-NO-cGMP pathway in platelets from a population of adolescents. MATERIALS: A total of twenty six adolescent patients (13 with obesity and 13 with MetS) and healthy volunteers (n=14) participated in this study. Transport of L-arginine, NO synthase (NOS) activity and cGMP content in platelets were analyzed. Moreover, platelet function, plasma levels of L-arginine, metabolic and clinical markers were investigated in these patients and controls. RESULTS: L-arginine transport (pmol/10(9) cells/min) in platelets via system y(+)L was diminished in obese subjects (20.8±4.7, n=10) and MetS patients (18.4±3.8, n=10) compared to controls (52.3±14.8, n=10). The y(+)L transport system correlated negatively to insulin levels and Homeostasis Model Assessment of Insulin Resistance (HOMA IR) index. No differences in NOS activity and cGMP content were found among the groups. Moreover, plasma levels of L-arginine were not affected by obesity or MetS. DISCUSSION: Our study provides the first evidence that obesity and MetS lead to a dysfunction of L-arginine influx, which negatively correlates to insulin resistance. These findings could be a premature marker of future cardiovascular complications during adulthood.

Nitric oxide, malnutrition and chronic renal failure.

The conditionally essential amino acid L-arginine is the substrate for nitric oxide (NO) synthesis, a key second messenger involved in physiological functions including endothelium-dependent vascular relaxation and inhibition of platelet adhesion and aggregation. Extracellular L-arginine transport seems to be essential for the production of NO by the action of NO synthases (NOS), even when the intracellular levels of L-arginine are available in excess (L-arginine paradox). Chronic renal failure (CRF) is a complex clinical condition associated with accelerated atherosclerosis and thrombosis leading to cardiovascular events. Various studies document that markers of malnutrition and inflammation, such as low body mass index (BMI), C-reactive protein (CRP) and interleukin-6 (IL-6), are strong independent predictors of cardiovascular mortality in patients with end-stage renal disease (ESRD). There is considerable literature demonstrating that a disturbance in the nitric oxide control mechanism plays a role in mediating the haemodynamic and haemostatic disorders present in CRF. Endogenous analogues of L-arginine, ADMA and L-NMMA, which can inhibit NO synthesis and L-arginine transport, are increased whilst L-arginine is reduced in plasma from all stages of CRF patients. In this context, the uptake of L-arginine in blood cells is increased in undialysed CRF patients and in patients treated by CAPD and haemodialysis. In platelets obtained from haemodialysis patients, the activation of L-arginine transport and NO production was limited to well-nourished patients. Impairment in nitric oxide bioactivity, coupled with malnutrition and inflammation, may contribute to increased incidence of atherothrombotic events in CRF. This article summarizes the current knowledge of L-arginine-nitric oxide pathway and malnutrition in CRF and briefly describes possible therapeutic interventions.

Plasma amino acid profile and L-arginine uptake in red blood cells from malnourished uremic patients.

BACKGROUND: Patients with end-stage chronic renal failure (CRF) (uremia) have a high prevalence of inflammation, malnutrition, and oxidative stress. All of these features seem to be associated with the increased cardiovascular mortality observed in these patients. Nitric oxide (NO) is involved in the pathogenesis of CRF. The present study investigates the effects of nutritional status on L-arginine transport (NO precursor), plasma amino acid profile, and concentration of tumor necrosis factor (TNF)-alpha in uremic patients on hemodialysis (HD). METHODS: A total of 32 uremic patients on regular HD and 16 healthy controls were included in this study. Kinetic studies of L-arginine transport, mediated by cationic transport systems y(+) and y(+)L into red blood cells, plasma concentrations of amino acids (measured by high-performance liquid chromatography), and plasma TNF-alpha level (evaluated by enzyme-linked immunosorbent assay), were analyzed in malnourished and well-nourished patients (isolated by body mass index). RESULTS: L-arginine influx by system y(+) in red blood cells (micromol/L cells(-1)h(-1)) was increased in both malnourished (377 +/- 41) and well-nourished (461 +/- 63) patients with CRF compared with controls (287 +/- 28). Plasma levels of all cationic amino acids (L-arginine, L-ornithine, and L-lysine) were low in uremic patients compared with controls. Among the uremic population, the reduction in plasma cationic amino acids levels was greater in malnourished patients. L-cysteine and L-glutamate, precursors of glutathione, were dramatically increased in plasma from uremic patients, independently of nutritional status. In addition, TNF-alpha concentration in plasma was enhanced in malnourished uremic patients (3.4 +/- 0.7 pg/mL) compared with controls (1.2 +/- 0.1 pg/mL) and well-nourished patients (1.9 +/- 0.1 pg/mL). CONCLUSIONS: Our results suggest an increased catabolism of cationic amino acids, inflammatory markers, and oxidative stress in CRF, especially in malnourished patients. The reduced plasma concentration of plasma L-arginine is counterbalanced by enhanced rates of transport, resulting in an activation of NO synthesis in uremia.

Uremia, atherothrombosis and malnutrition: the role of L-arginine-nitric oxide pathway.

The uraemic syndrome is a complex condition that results from an accumulation of multiple waste compounds, combined with failure of the endocrine and homeostatic functions of the kidney in end-stage chronic renal failure (CRF) patients. Recently it has become clear that uraemia is a microinflammatory condition with a significant increase in inflammation markers. Malnutrition is a common pathological condition which exacerbates cardiovascular mortality in uraemic patients. Inadequate diet and a state of persistent catabolism play major roles in uraemic malnutrition, yet the underlying mechanisms have not been completely clarified. Malnourished patients present elevated levels of circulating cytokines, further aggravating the oxidative and inflammatory characteristics of uraemia. It has been suggested that abnormalities in nitric oxide bioactivity, coupled with malnutrition and inflammation, may contribute to increased incidence of atherothrombotic events in uraemia. Amongst the earliest indications of nutritional deficiency are low concentrations of plasma amino acids, including L-arginine, the precursor for nitric oxide (NO) synthesis. Atherosclerosis is an inflammatory disorder and NO is an important mediator of inflammation. There is a close association between thrombosis and platelet aggregation, and NO is involved in all stages of platelet activation. L-arginine inhibits platelet aggregation both in vitro and in vivo, while L-NMMA (NG-monomethyl-L-arginine), an endogenous L-arginine analogue and inhibitor of NO synthase (NOS), increases platelet activation and adhesion. The majority of studies in animal models and human patients indicate that the systemic production of NO is increased in uraemia. CRF patients show reduced plasma concentration of L-arginine, and the enhancement of L-arginine transport is essential to maintain increased NO synthesis in platelets taken from these patients. The present review provides an overview of recent advances in the understanding of the association among malnutrition, chronic inflammation and the L-arginine-nitric oxide pathway in uraemic patients, and related potential interventions that could improve clinical outcome in chronic renal failure.