T. M. C. Brunini

Abnormalities in l-arginine transport and nitric oxide biosynthesis in chronic renal and heart failure

Patients with chronic renal and heart failure present with hypertension and widespread vasoconstriction, respectively. Although systemic release of nitric oxide (NO) may be elevated in both pathological syndromes, enhanced production of NO fails to overcome endothelial dysfunction. Plasma concentrations of L-arginine, a cationic amino acid precursor for NO synthesis, are reduced whilst levels of the endogenous L-arginine analogues, asymmetric and symmetric dimethyl arginine and N(G)-monomethyl-L-arginine, seem to be elevated. We have reported that transport of L-arginine via the cationic amino acid transporters y(+)/CAT and/or y(+)L are up-regulated in erythrocytes, peripheral blood mononuclear cells and platelets from both patients with either chronic renal or heart failure. A possible explanation why NO serves as a failing counter-regulatory mechanism in both these pathologies is that availability of L-arginine for NO production is reduced despite the observed increase in membrane transport. This review examines the mechanisms underlying alterations in NO production in chronic renal and heart failure, and the possible role of L-arginine transport in vascular and platelet dysfunction observed in both syndromes.

Inhibition of L-arginine transport in platelets by asymmetric dimethylarginine and NG-monomethyl-L-arginine: Effects of arterial hypertension

1. 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 NG‐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.

Chronic exercise reduces platelet activation in hypertension: upregulation of the L-arginine-nitric oxide pathway

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 platelet‐derived 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.

Chronic exercise leads to antiaggregant, antioxidant and anti-inflammatory effects in heart failure patients

Background Heart failure (HF) patients are at an increased risk of thrombotic events. Here, we investigated the effects of exercise training on platelet function and factors involved in its modulation in HF. Design and methods Thirty HF patients were randomized to 6 months of supervised exercise training or to a control group that remained sedentary. Exercise training consisted of 30 min of moderate-intensity treadmill exercise, followed by resistance and stretching exercises, performed three times a week. Blood was collected before and after the intervention for platelet and plasma obtainment. Results Peak VO2 increased after exercise training (18.0 ± 2.2 vs. 23.8 ± 0.5 mlO2/kg/min; p < 0.05). Exercise training reduced platelet aggregation induced by both collagen and ADP (approximately –6%; p < 0.05), as well as platelet nitric oxide synthase activity (0.318 ± 0.030 vs. 0.250 ± 0.016 pmol/108 cells; p < 0.05). No difference in the above-mentioned variables were observed in the control group. No significant difference was observed in intraplatelet cyclic guanosine monophosphate levels among groups. There was a significant increase in the activity of the antioxidant enzymes superoxide dismutase and catalase in plasma and platelets, resulting in a decrease in both lipid and protein oxidative damage. Systemic levels of the inflammatory markers C-reactive protein, fibrinogen, and tumour necrosis factor α were also reduced in HF after training. Conclusions 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.

ACTIVATION OF l‐ARGININE TRANSPORT IN UNDIALYSED CHRONIC RENAL FAILURE AND CONTINUOUS AMBULATORY PERITONEAL DIALYSIS PATIENTS

1 Treatment with haemodialysis and continuous ambulatory peritoneal dialysis (CAPD) presents different pathophysiological profiles and it has been suggested that clinical outcome in chronic renal failure may depend on the mode of dialysis. The transport of l‐arginine, a precursor of nitric oxide, into blood cells is increased in uraemic patients on haemodialysis. The present study was designed to investigate l‐arginine transport into red blood cells (RBC) in uraemic patients not yet on dialysis and on CAPD therapy. 2 Eleven uraemic patients not yet on dialysis and 17 on CAPD were included in the study. l‐Arginine transport into RBC and plasma and RBC amino acid profiles were analysed in these sets of patients. 3 l‐Arginine transport via system y+, but not y+L, into RBC, was significantly increased in undialysed uraemic patients (459 40 mmol/L per cell per h) and CAPD patients (539 61 mmol/L per cell per h) compared with controls (251 39 mmol/L per cell per h). Hihg‐pressure liquid chromatography measurements demonstrated low levels of plasma l‐arginine in uraemic patients both on CAPD (54 3 mmol/L) and not yet on dialysis (80 6 mmol/L) compared with control subjects (146 14 mmol/L). 4 Our findings provide the first evidence that uraemic patients not yet on dialysis and on CAPD present with an activation of l‐arginine transport via system y+ into RBC associated with reduced plasma levels of l‐arginine.

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 (VO2peak), 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.

Characterization of cationic amino acid transport systems in rat erythrocytes: Lack of effect of uraemia on L-arginine influx

1 Chronic renal failure (CRF) is associated with the abnormal regulation of nitric oxide (NO) synthesis at the systemic level. The transport of l‐arginine, upregulated in blood cells from uraemic patients, modulates NO synthesis in this pathological condition. The model of partial nephrectomy in rats is widely accepted as a valid model of uraemia. Because there are no reports of l‐arginine transport in blood cells from uraemic rats, the aim of the present study was to investigate l‐arginine transport in red blood cells (RBCs) from these rats. 2 The kinetics of l‐arginine transport in RBC and plasma and the amino acid profiles of RBC were investigated in control, sham‐operated and subtotally nephrectomized rats. 3 l‐Arginine transport was mediated via the cationic amino acid transport system y+ and a transport system with kinetics resembling the human system y+L. In control RBC, the apparent Ki for l‐leucine inhibition of l‐arginine transport via system y+L was 0.16 ± 0.02 and 4.8 ± 2 mmol/L in the presence of Li+ and Na+, respectively. 4 The Vmax values for l‐arginine transport via system y+L and system y+ were similar in RBC from control sham‐operated and uraemic rats. Moreover, l‐arginine concentrations in plasma and RBC were not affected by uraemia. 5 The findings of the present study provide the first evidence that l‐arginine transport in rat erythrocytes is mediated by two distinct cationic transport systems with characteristics of systems y+ and y+L, which accept neutral amino acids only in the presence of Li+. In contrast with previous studies in uraemic patients, plasma levels and maximal transport rates of l‐arginine were not altered in this rat model of CRF.

Platelet hyperaggregability in obesity: is there a role for nitric oxide impairment and oxidative stress?

Epidemiological evidence has shown that platelet activation markers are consistently elevated in obesity, contributing to its prothrombotic state. In order to improve the understanding of the regulation of platelet function in obesity, the aim of this study was to investigate the l‐arginine‐nitric oxide (NO) pathway in obese adults without other cardiovascular risk factor. Seventeen obese (body mass index [BMI] 35.9±1.0 kg/m2) and eighteen age‐matched normal weight subjects (BMI 22.0±0.6 kg/m2) were included in this study. l‐arginine influx was measured with incubation of l‐[3H]‐arginine. NO synthase (NOS) and arginase activities were determined by the citrulline assay and the conversion of l‐[14C]‐arginine to [14C]‐urea, respectively. Cyclic guanosine monophosphate (cGMP) content was evaluated by enzyme‐linked immunosorbent assay. In addition, the study analyzed: platelet aggregation; intraplatelet antioxidant enzymes, via superoxide dismutase (SOD) and catalase activities; and systemic levels of l‐arginine, fibrinogen, and C‐reactive protein (CRP). Obese patients presented a significant decrease of platelet l‐arginine influx, NOS activity, and cGMP levels, along with platelet hyperaggregability. On the presence of NO donor, platelet aggregation was similar between the groups. The fibrinogen and CRP systemic levels were significantly higher and SOD activity was reduced in obesity. No significant differences were observed in plasma levels of l‐arginine and intraplatelet arginase and catalase activities between groups. The diminished NO bioavailability associated with inflammatory status and impaired enzymatic antioxidant defence may contribute to future cardiovascular complications in obesity.