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

Platelets in Renal Disease

Kidney disease is a major global health concern, affecting millions of people. Nephrologists have shown interest in platelets because of coagulation disorders caused by renal diseases. With a better understanding of platelets, it has been found that these anucleate and abundant blood cells not only play a role in hemostasis, but also have important functions in inflammation and immunity. Platelets are not only affected by kidney disease, but may also contribute to kidney disease progression by mediating inflammation and immune effects. This review summarizes the current evidence regarding platelet abnormalities in renal disease, and the multiple effects of platelets on kidney disease progression. The relationship between platelets and kidney disease is still being explored, and further research can provide mechanistic insights into the relationship between thrombosis, bleeding, and inflammation related to kidney disease, and elucidate targeted therapies for patients with kidney disease.

Changes in the Plasma and Platelet Nitric Oxide Biotransformation Metabolites during Ischemic Stroke-A Dynamic Human LC/MS Metabolomic Study

Despite improvement in the management of modifiable cardiovascular risk factors, ischemic stroke remains the leading cause of morbidity and mortality in the adult population. The aim of this study was to analyze the time-dependent dynamic differences in expression of the nitric oxide (NO) metabolic pathway in the platelet and plasma compartment between subjects with and without ischemic stroke. Additionally, the interplay between these parameters and platelet aggregation was investigated. A total of 418 patients in acute phase of non-cardioembolic stroke were investigated. Following the inclusion and exclusion criteria, finally 40 subjects with stroke and 39 demographically matched healthy participants were enrolled. Neurological physical examination, followed by assessment of the platelet and plasma levels of the nitric oxide synthase (NOS) inhibitors, including asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA), as well as NOS substrate-L-Arginine were performed dynamically three times within the first 24-h, then on the 3rd and 7th day after the stroke onset, which was compared with the healthy control. The platelet L-Arginine concentration was significantly higher on the 1st and 3rd day of stroke, while the plasma levels were significantly lower on exact days in comparison to the control. The competitive NOS-inhibitors in platelets were stably elevated in stroke subjects, whereas no significant differences in plasma compartment were noted. The arachidonic-acid-induced platelet aggregation was negatively associated with the platelet NOS substrate bioavailability, as assessed by the L−Arginine ADMA-ratio on the 3rd and 7th day. Subjects with non-cardioembolic ischemic stroke are characterized by elevated platelet levels of NOS inhibitors. Management of stroke results in increasing the platelet L-Arginine concentration and subsequent NO bioavailability in the platelet compartment.

Comparison of the Surgical Resection and Infarct 5/6 Nephrectomy Rat Models of Chronic Kidney Disease

The 5/6 nephrectomy rat remnant kidney model is commonly employed to study chronic kidney disease (CKD). This model requires removal of one whole kidney and two-thirds of the other. The two most common ways of producing the remnant kidney are surgical resection of poles, known as the polectomy (Pol) model, or ligation of upper and lower renal arterial branches, resulting in pole infarction (Inf). These models have much in common, but also major phenotypic differences, and thus respectively model unique aspects of human CKD. The purpose of this review is to summarize phenotypic similarities and differences between these two models and their relation to human CKD, while emphasizing their vascular phenotype. In this article we review studies that have evaluated arterial blood pressure, the renin-angiotensin-aldosterone-system (RAAS), autoregulation, nitric oxide, single nephron physiology, angiogenic and anti-angiogenic factors, and capillary rarefaction in these two models. Phenotypic similarities: both models spontaneously develop hallmarks of human CKD including uremia, fibrosis, capillary rarefaction, and progressive renal function decline. They both undergo whole-organ hypertrophy, hyperfiltration of functional nephrons, reduced renal expression of angiogenic factor VEGF, increased renal expression of the anti-angiogenic thrombospondin-1, impaired renal autoregulation, and abnormal vascular nitric oxide physiology. Key phenotypic differences: the Inf model develops rapid-onset, moderate-to-severe systemic hypertension, and the Pol model early normotension followed by mild-to-moderate hypertension. The Inf rat has a markedly more active renin-angiotensin-aldosterone-system. Comparison of these two models facilitates understanding of how they can be utilized for studying CKD pathophysiology (e.g., RAAS dependent or independent pathology).

Cardiorenal Syndrome: New Pathways and Novel Biomarkers

Cardiorenal syndrome (CRS) is a multi-organ disease characterized by the complex interaction between heart and kidney during acute or chronic injury. The pathogenesis of CRS involves metabolic, hemodynamic, neurohormonal, and inflammatory mechanisms, and atherosclerotic degeneration. In the process of better understanding the bi-directional pathophysiological aspects of CRS, the need to find precise and easy-to-use markers has also evolved. Based on the new pathophysiological standpoints and an overall vision of the CRS, the literature on renal, cardiac, metabolic, oxidative, and vascular circulating biomarkers was evaluated. Though the effectiveness of different extensively applied biomarkers remains controversial, evidence for several indicators, particularly when combined, has increased in recent years. From new aspects of classic biomarkers to microRNAs, this review aimed at a 360-degree analysis of the pathways that balance the kidney and the heart physiologies. In this delicate system, different markers and their combination can shed light on the diagnosis, risk, and prognosis of CRS.

Understanding Erectile Dysfunction in Hypertensive Patients: The Need for Good Patient Management

Erectile dysfunction (ED) is defined as a man’s consistent or recurrent inability to attain and/or maintain penile erection enough for successful vaginal intercourse. ED affects a large part of the population, increasing its incidence with age and comorbidities. It is estimated by the year 2025, 322 million men will suffer from ED. Incidence of ED has been related not only to chronic diseases such as diabetes mellitus, metabolic syndrome, hyperlipidemia, psychiatric diseases or urinary tract diseases, but also to hypertension and especially to antihypertensive treatments. This review summarizes current knowledge about the management of ED in hypertensive men and its role as cardiovascular disease predictor.

Threshold levels of extracellular l-arginine that trigger NOS-mediated ROS/RNS production in cardiac ventricular myocytes

l-Arginine (L-Arg) is the substrate for nitric oxide synthase (NOS) to produce nitric oxide (NO), a signaling molecule that is key in cardiovascular physiology and pathology. In cardiac myocytes, L-Arg is incorporated from the circulation through the functioning of system-y⁺ cationic amino acid transporters. Depletion of L-Arg leads to NOS uncoupling, with O₂ rather than L-Arg as the terminal electron acceptor, resulting in superoxide formation. The reactive oxygen species (ROS) superoxide (O₂˙^-), combined with NO, may lead to the production of the reactive nitrogen species (RNS) peroxynitrite (ONOO^-), which is recognized as a major contributor to myocardial depression. In this study we aimed to determine the levels of external L-Arg that trigger ROS/RNS production in cardiac myocytes. To this goal, we used a two-step experimental design in which acutely isolated cardiomyocytes were loaded with the dye coelenterazine that greatly increases its fluorescence quantum yield in the presence of ONOO^- and O₂˙^- Cells were then exposed to different concentrations of extracellular L-Arg and changes in fluorescence were followed spectrofluorometrically. It was found that below a threshold value of ~100 µM, decreasing concentrations of L-Arg progressively increased ONOO^-/ O₂˙^--induced fluorescence, an effect that was not mimicked by d-arginine or l-lysine and was fully blocked by the NOS inhibitor l-NAME. These results can be explained by NOS aberrant enzymatic activity and provide an estimate for the levels of circulating L-Arg below which ROS/RNS-mediated harmful effects arise in cardiac muscle.

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.

Nitric oxide synthetic pathway and cGMP levels are altered in red blood cells from end-stage renal disease patients

Red blood cells (RBCs) enzymatically produce nitric oxide (NO) by a functional RBC-nitric oxide synthase (RBC-NOS). NO is a vascular key regulatory molecule. In RBCs its generation is complex and influenced by several factors, including insulin, acetylcholine, and calcium. NO availability is reduced in end-stage renal disease (ESRD) and associated with endothelial dysfunction. We previously demonstrated that, through increased phosphatidylserine membrane exposure, ESRD-RBCs augmented their adhesion to human cultured endothelium, in which NO bioavailability decreased. Since RBC-NOS-dependent NO production in ESRD is unknown, this study aimed to investigate RBC-NOS levels/activation, NO production/bioavailability in RBCs from healthy control subjects (C, N = 18) and ESRD patients (N = 27). Although RBC-NOS expression was lower in ESRD-RBCs, NO, cyclic guanosine monophosphate (cGMP), RBC-NOS Serine1177 phosphorylation level and eNOS/Calmodulin (CaM)/Heat Shock Protein-90 (HSP90) interaction levels were higher in ESRD-RBCs, indicating increased enzyme activation. Conversely, following RBCs stimulation with insulin or ionomycin, NO and cGMP levels were significantly lower in ESRD- than in C-RBCs, suggesting that uremia might reduce the RBC-NOS response to further stimuli. Additionally, the activity of multidrug-resistance-associated protein-4 (MRP4; cGMP-membrane transporter) was significantly lower in ESRD-RBCs, suggesting a possible compromised efflux of cGMP across the ESRD-RBCs membrane. This study for the first time showed highest basal RBC-NOS activation in ESRD-RBCs, possibly to reduce the negative impact of decreased NOS expression. It is further conceivable that high NO production only partially affects cell function of ESRD-RBCs maybe because in vivo they are unable to respond to physiologic stimuli, such as calcium and/or insulin.

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.

L-arginine catabolism is driven mainly towards nitric oxide synthesis in the erythrocytes of patients with type 2 diabetes at first clinical onset

BACKGROUND

We investigated the l-arginine (l-Arg)-nitric oxide (NO) metabolic pathway in the erythrocytes (RBCs) and plasma of subjects with type 2 diabetes at first clinical onset.

METHODS

RBCs and plasma were collected from 26 patients with type 2 diabetes at first clinical onset and 19 age-matched non-diabetes subjects as controls. l-Arg content was assayed by capillary electrophoresis. We measured arginase activity and nitrate/nitrite concentrations by spectrophotometry, and glycosylated haemoglobin (HbA1c) by standardized immunoturbidimetry.

RESULTS

We found that, when compared with controls, l-Arg content was similar in RBCs while decreased in the plasma of patients with type 2 diabetes. Interestingly, arginase activity was lower in RBCs and increased in plasma of patients with diabetes. NO production was higher in RBCs in patients with type 2 diabetes, while no difference was found in the plasma of our subjects.

CONCLUSIONS

l-Arg catabolism is driven mainly towards NO synthesis in RBCs of patients with type 2 diabetes at first clinical onset. The decreased RBC arginase activity could be considered a potential mechanism of increased RBC NO production in early diabetes. Therefore, the RBC pool would represent a potentially compensatory intravascular compartment for endothelial dysfunction in diabetes.

Nitric oxide transport in blood: a third gas in the respiratory cycle

The trapping, processing, and delivery of nitric oxide (NO) bioactivity by red blood cells (RBCs) have emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency. We present here an expanded paradigm for the human respiratory cycle based on the coordinated transport of three gases: NO, O₂, and CO₂. By linking O₂ and NO flux, RBCs couple vessel caliber (and thus blood flow) to O₂ availability in the lung and to O₂ need in the periphery. The elements required for regulated O₂-based signal transduction via controlled NO processing within RBCs are presented herein, including S-nitrosothiol (SNO) synthesis by hemoglobin and O₂-regulated delivery of NO bioactivity (capture, activation, and delivery of NO groups at sites remote from NO synthesis by NO synthase). The role of NO transport in the respiratory cycle at molecular, microcirculatory, and system levels is reviewed. We elucidate the mechanism through which regulated NO transport in blood supports O₂ homeostasis, not only through adaptive regulation of regional systemic blood flow but also by optimizing ventilation-perfusion matching in the lung. Furthermore, we discuss the role of NO transport in the central control of breathing and in baroreceptor control of blood pressure, which subserve O₂ supply to tissue. Additionally, malfunctions of this transport and signaling system that are implicated in a wide array of human pathophysiologies are described. Understanding the (dys)function of NO processing in blood is a prerequisite for the development of novel therapies that target the vasoactive capacities of RBCs.

Plasma concentrations of arginine and asymmetric dimethylarginine do not reflect their intracellular concentrations in peripheral blood mononuclear cells

OBJECTIVE

Production of nitric oxide (NO) from arginine is inhibited by endogenously produced monomethylarginine (MMA) and asymmetric dimethylarginine (ADMA). Elevated levels of ADMA, by limiting NO production, may lead to endothelial dysfunction and cardiovascular disease. Symmetric dimethylarginine (SDMA) and the arginine homolog homoarginine have also been associated with cardiovascular disease. Although NO synthesis, as well as generation of MMA, ADMA, SDMA and homoarginine, occurs intracellularly, these biomarkers are usually measured in plasma. Despite extensive transmembrane transport, it is not clear whether plasma levels of these biomarkers are a valid proxy for their intracellular levels in the cardiovascular system. Since it is difficult to obtain vascular tissue from healthy humans, we explored the relations between concentrations of these biomarkers in plasma and intracellular concentrations in peripheral blood mononuclear cells (PBMC).

METHODS

In PBMC and plasma of 27 healthy subjects, concentrations of arginine, MMA, ADMA, SDMA, and homoarginine were determined using stable isotope dilution liquid chromatography tandem mass spectrometry.

RESULTS

In PBMC, significant positive correlations were observed among arginine and its methylated forms (ρ = 0.43 to 0.81) and these correlations were slightly less pronounced in plasma. Homoarginine was not significantly correlated with (methylated) arginine in either PBMC or plasma. Plasma concentrations of arginine and its methylated forms showed non-significant inverse associations with their respective intracellular concentrations in PBMC and only for homoarginine was a weak positive association observed (ρ = 0.37).

CONCLUSION

In healthy individuals, plasma levels of arginine, MMA, ADMA, and SDMA poorly reflect their intracellular levels in PBMC.

Role of cellular L-arginine uptake and nitric oxide production on renal blood flow and arterial pressure regulation

PURPOSE OF REVIEW

L-Arginine (L-Arg) is the substrate for nitric oxide (NO) formation. Reduced NO bioavailability, particularly within the renal circulation, has been identified as a key factor in the pathogenesis of hypertension. This review focuses on the pathogenic role of abnormal L-Arg transport, particularly within the kidney, in hypertension.

RECENT FINDINGS

Most recent studies have attempted to restore NO bioavailability in cardiovascular diseases with the use of antioxidants to reduce NO inactivation, but this approach has failed to provide beneficial effects in the clinical setting. We argue that this may be due to reduced NO formation in hypertension, which has largely been overlooked as a means of restoring NO bioavailability in cardiovascular diseases. Recent data indicate that renal L-Arg transport plays an important role in regulating both renal perfusion and function and the long-term set point of arterial pressure in health. Perturbations in the renal L-Arg transport system can give rise to abnormal renal perfusion and function, initiating hypertension and related renal damage.

SUMMARY

Accordingly, we propose that L-Arg transporters are a new treatment target in hypertension and in disease states where renal NO bioavailability is disturbed.

Cardiorenal syndrome: pathophysiology and potential targets for clinical management

Combined dysfunction of the heart and the kidneys, which can be associated with haemodynamic impairment, is classically referred to as cardiorenal syndrome (CRS). Cardiac pump failure with resulting volume retention by the kidneys, once thought to be the major pathophysiologic mechanism of CRS, is now considered to be only a part of a much more complicated phenomenon. Multiple body systems may contribute to the development of this pathologic constellation in an interconnected network of events. These events include heart failure (systolic or diastolic), atherosclerosis and endothelial cell dysfunction, uraemia and kidney failure, neurohormonal dysregulation, anaemia and iron disorders, mineral metabolic derangements including fibroblast growth factor 23, phosphorus and vitamin D disorders, and inflammatory pathways that may lead to malnutrition-inflammation-cachexia complex and protein-energy wasting. Hence, a pathophysiologically and clinically relevant classification of CRS based on the above components would be prudent. With the existing medical knowledge, it is almost impossible to identify where the process has started in any given patient. Rather, the events involved are closely interrelated, so that once the process starts at a particular point, other pathways of the network are potentially activated. Current therapies for CRS as well as ongoing studies are mostly focused on haemodynamic adjustments. The timely targeting of different components of this complex network, which may eventually lead to haemodynamic and vascular compromise and cause refractoriness to conventional treatments, seems necessary. Future studies should focus on interventions targeting these components.

The glomerular proteome in a model of chronic kidney disease

Adequate kidney function is crucial in sustaining vertebrate homeostasis. Certain diseases can diminish renal function and lead to end-stage renal disease. Diabetes mellitus and hypertension are the main causes of glomerulosclerosis and albuminuria in adults. The molecular mechanisms that trigger these maladaptive changes are still unsatisfyingly described. We previously introduced 2-D DIGE in combination with focused tissue isolation methods to analyze protein expression in glomeruli. Glomeruli, the crucial compartments in albuminuric renal diseases, were extracted using magnetic particles from subtotally nephrectomized FVB mice (n = 6); this 5/6 nephrectomy in FVB mice is a model of chronic kidney disease. Analysis of protein expression levels from glomerular protein lysates was performed using 2-D DIGE and compared with glomerular protein lysates from mice that underwent sham surgery. The comparison of about 2100 detectable spots between both groups revealed 48 protein spots that showed significant differential expression. Of those, 33 proteins could be identified using nanoLC-ESI MS. The metalloproteinase meprin 1 alpha, the beta galactoside-binding-lectin galectin-1 and dimethylarginine dimethylaminohydrolase 1, a key enzyme in NO metabolism, were found to be differentially regulated, thus implying a role in the pathogenesis and pathophysiology of progressive kidney disease. In conclusion, 2-D DIGE protein analysis of smallest sample sizes from specific organ compartments provides focused protein expression results, which help in gaining an understanding of the molecular mechanisms of chronic kidney disease.

Role of the human erythrocyte in generation and storage of asymmetric dimethylarginine

Proteolytic activity in whole blood may lead to release of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA). We investigated the role of the human erythrocyte in storage and generation of ADMA in healthy controls (n = 36) and critically ill patients (n = 38). Both free and total (sum of free and protein-incorporated) ADMA were measured. Upon incubation of intact erythrocytes with extracellular ADMA (0 to 40 μmol/l), equilibrium between intra- and extracellular ADMA was reached within 3 h. Compared with controls, patients had significantly higher basal concentrations of ADMA in plasma (0.88 ± 0.75 vs. 0.41 ± 0.07 μmol/l) and erythrocytes (1.28 ± 0.55 vs. 0.57 ± 0.14 μmol/l). Intracellular and plasma ADMA were significantly correlated in the patient group only (r = 0.834). Upon lysis, followed by incubation at 37°C for 2 h, free ADMA increased sevenfold (to 8.60 ± 3.61 μmol/l in patients and 3.90 ± 0.78 μmol/l in controls). In lysates of controls, free ADMA increased further to 9.85 ± 1.35 μmol/l after 18 h. Total ADMA was 15.43 ± 2.44 μmol/l and did not change during incubation. The increase of free ADMA during incubation corresponded to substantial release of ADMA from the erythrocytic protein-incorporated pool (21.9 ± 4.6% at 2 h and 60.8 ± 7.6% at 18 h). ADMA was released from proteins other than hemoglobin, which only occurred after complete lysis and was blocked by combined inhibition of proteasomal and protease activity. Neither intact nor lysed erythrocytes mediated degradation of free ADMA. We conclude that intact erythrocytes play an important role in storage of ADMA, whereas upon erythrocyte lysis large amounts of free ADMA are generated by proteolysis of methylated proteins, which may affect plasma levels in hemolysis-associated diseases.

Platelet activation, oxidative stress and overexpression of inducible nitric oxide synthase in moderate heart failure

1. Chronic heart failure (CHF) is a common disabling disorder associated with thromboembolic events, the genesis of which is not yet fully understood. Nitric oxide (NO), derived from the vascular endothelium and platelets, has an important role in the physiological regulation of blood flow. It is generated from the amino acid L-arginine via NO synthase (NOS). 2. The main objective of the present study was to investigate NO production and its relationship with platelet aggregation, oxidative stress, inflammation and related amino acids in patients with moderate CHF. The expression and activity of NOS isoforms were analysed by western blotting and conversion of L-[(3)H]-arginine to L-[(3)H]-citrulline, respectively, in CHF patients (n = 12) and healthy controls (n = 15). Collagen- and ADP-induced platelet aggregation, oxidative stress (thiobarbituric acid-reactive substances (TBARS) formation and superoxide dismutase (SOD) activity) and plasma levels of amino acids and inflammatory markers (fibrinogen and C-reactive protein (CRP)) were also determined. 3. Both collagen- and ADP-induced platelet aggregation were increased in CHF patients compared with controls. Platelets from CHF patients did not show any changes in NOS activity in the presence of overexpression of inducible NOS. Systemic and intraplatelet TBARS production was elevated, whereas SOD activity was decreased in CHF patients. l-arginine plasma concentrations were lower in CHF patients than in controls. Systemic levels of CRP and fibrinogen were increased in CHF patients. 4. The results show that, in patients with moderate CHF, there is platelet activation and reduced intraplatelet NO bioavailability due to oxidative stress, which suggests a role for platelets in the prothrombotic state.

Increased nitric oxide production in platelets from severe chronic renal failure patients

Nitric oxide (NO) production occurs through oxidation of the amino acid L-arginine by NO synthase (NOS). NO inhibits platelet activation by increasing the levels of cyclic guanosine monophosphate (cGMP), thus maintaining vascular homeostasis. Our group previously demonstrated (da Silva et al. 2005) an enhancement of the L-arginine-NO-cGMP pathway in platelets taken from chronic renal failure (CRF) patients on haemodialysis associated with reduced platelet aggregation. We investigate the platelet L-arginine-NO-cGMP pathway, platelet function, and inflammation from patients in CRF on conservative treatment. A total of 42 CRF patients and 42 controls (creatinine clearance = 27 ± 3 vs. 93 ± 1 mL per min per 1.73 m2, respectively) participated in this study. NOS activity and expression and cGMP concentration were measured in platelets. Platelet aggregation induced by collagen or ADP was evaluated and plasma levels of fibrinogen were determined by the Clauss method. A marked increase in basal NOS activity was seen in undialysed CRF patients compared with controls, accompanied by an elevation of fibrinogen plasma levels. There were no differences in expression of NOS and in cGMP levels. In this context, platelet aggregation was not affected. We provide the first evidence of increased intraplatelet NO biosynthesis in undialysed CRF patients, which can be an early marker of future haemostatic abnormalities during dialysis treatment.

Short-term effects of L-citrulline supplementation on arterial stiffness in middle-aged men

BACKGROUND

Nitric oxide (NO) plays a key role in the maintenance of vascular tone, contributing to the functional regulation of arterial stiffness. Although oral L-citrulline could become the effective precursor of L-arginine (substrate for endothelial NO synthase) via the L-citrulline/ L-arginine pathway, little is known about the efficacy of L-citrulline application on arterial stiffness.

OBJECTIVE

We examined the short-term effects of L-citrulline supplementation on arterial stiffness in humans.

METHODS

In a double-blind, randomized, placebo-controlled parallel-group trial, 15 healthy male subjects (age: 58.3 ± 4.4 years) with brachial-ankle pulse wave velocity (baPWV; index of arterial stiffness >1400 cm/sec) were given 5.6g/day of L-citrulline (n=8) or placebo (n=7) for 7 days. baPWV and various clinical parameters were measured before (baseline) and after oral supplementation of L-citrulline or placebo.

RESULTS

Compared with the placebo group, baPWV was significantly reduced in the L-citrulline group (p<0.01). No significant differences in blood pressure (BP) were found between the two groups, and no correlation was observed between BP and baPWV. The serum nitrogen oxide (NOx, the sum of nitrite plus nitrate) and NO metabolic products were significantly increased only in the L-citrulline group (p<0.05). Plasma citrulline, arginine and the ratio of arginine/asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase (arginine/ADMA ratio) were significantly increased in the L-citrulline group compared with the placebo group (p<0.05, p<0.01, p<0.05, respectively). Moreover, there was a correlation between the increase of plasma arginine and the reduction of baPWV (r=-0.553, p<0.05).

CONCLUSION

These findings suggest that short-term L-citrulline supplementation may functionally improve arterial stiffness, independent of blood pressure, in humans.

The role of asymmetric dimethylarginine and arginine in the failing heart and its vasculature

Nitric oxide (NO) is formed from arginine by the enzyme nitric oxide synthase (NOS). Asymmetric dimethylarginine (ADMA) can inhibit NO production by competing with arginine for NOS binding. Therefore, the net amount of NO might be indicated by the arginine/ADMA ratio. In turn, arginine can be metabolized by the enzyme arginase, and ADMA by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). While ADMA has been implicated as a cardiovascular risk factor, arginine supplementation has been indicated as a treatment in cardiac diseases. This review discusses the roles of ADMA and arginine in the failing heart and its vasculature. Furthermore, it proposes nutritional therapies to improve NO availability.

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.

Cellular ADMA: regulation and action

Asymmetric (N(G),N(G)) dimethylarginine (ADMA) is present in plasma and cells. It can inhibit nitric oxide synthase (NOS) that generates nitric oxide (NO) and cationic amino acid transporters (CATs) that supply intracellular NOS with its substrate, l-arginine, from the plasma. Therefore, ADMA and its transport mechanisms are strategically placed to regulate endothelial function. This could have considerable clinical impact since endothelial dysfunction has been detected at the origin of hypertension and chronic kidney disease (CKD) in human subjects and may be a harbinger of large vessel disease and cardiovascular disease (CVD). Indeed, plasma levels of ADMA are increased in many studies of patients at risk for, or with overt CKD or CVD. However, the levels of ADMA measured in plasma of about 0.5micromol.l(-1) may be below those required to inhibit NOS whose substrate, l-arginine, is present in concentrations many fold above the Km for NOS. However, NOS activity may be partially inhibited by cellular ADMA. Therefore, the cellular production of ADMA by protein arginine methyltransferase (PRMT) and protein hydrolysis, its degradation by N(G),N(G)-dimethylarginine dimethylaminohydrolase (DDAH) and its transmembrane transport by CAT that determines intracellular levels of ADMA may also determine the state of activation of NOS. This is the focus of the review. It is concluded that cellular levels of ADMA can be 5- to 20-fold above those in plasma and in a range that could tonically inhibit NOS. The relative importance of PRMT, DDAH and CAT for determining the intracellular NOS substrate:inhibitor ratio (l-arginine:ADMA) may vary according to the pathophysiologic circumstance. An understanding of this important balance requires knowledge of these three processes that regulate the intracellular levels of ADMA and arginine.

Characteristics and function of cardiac mitochondrial nitric oxide synthase

We used laser scanning confocal microscopy in combination with the nitric oxide (NO)-sensitive fluorescent dye DAF-2 and the reactive oxygen species (ROS)-sensitive dyes CM-H(2)DCF and MitoSOX Red to characterize NO and ROS production by mitochondrial NO synthase (mtNOS) in permeabilized cat ventricular myocytes. Stimulation of mitochondrial Ca(2+) uptake by exposure to different cytoplasmic Ca(2+) concentrations ([Ca(2+)](i) = 1, 2 and 5 microm) resulted in a dose-dependent increase of NO production by mitochondria when L-arginine, a substrate for mtNOS, was present. Collapsing the mitochondrial membrane potential with the protonophore FCCP or blocking the mitochondrial Ca(2+) uniporter with Ru360 as well as blocking the respiratory chain with rotenone or antimycin A in combination with oligomycin inhibited mitochondrial NO production. In the absence of L-arginine, mitochondrial NO production during stimulation of Ca(2+) uptake was significantly decreased, but accompanied by increase in mitochondrial ROS production. Inhibition of mitochondrial arginase to limit L-arginine availability resulted in 50% inhibition of Ca(2+)-induced ROS production. Both mitochondrial NO and ROS production were blocked by the nNOS inhibitor (4S)-N-(4-amino-5[aminoethyl]aminopentyl)-N'-nitroguanidine and the calmodulin antagonist W-7, while the eNOS inhibitor L-N(5)-(1-iminoethyl)ornithine (L-NIO) or iNOS inhibitor N-(3-aminomethyl)benzylacetamidine, 2HCl (1400W) had no effect. The superoxide dismutase mimetic and peroxynitrite scavenger MnTBAP abolished Ca(2+)-induced ROS generation and increased NO production threefold, suggesting that in the absence of MnTBAP either formation of superoxide radicals suppressed NO production or part of the formed NO was transformed quickly to peroxynitrite. In the absence of L-arginine, mitochondrial Ca(2+) uptake induced opening of the mitochondrial permeability transition pore (PTP), which was blocked by the PTP inhibitor cyclosporin A and MnTBAP, and reversed by L-arginine supplementation. In the presence of the mtNOS cofactor (6R)-5,6,7,8,-tetrahydrobiopterin (BH(4); 100 microm) mitochondrial ROS generation and PTP opening decreased while mitochondrial NO generation slightly increased. These data demonstrate that mitochondrial Ca(2+) uptake activates mtNOS and leads to NO-mediated protection against opening of the mitochondrial PTP, provided sufficient availability of l-arginine and BH(4). In conclusion, our data show the importance of L-arginine and BH(4) for cardioprotection via regulation of mitochondrial oxidative stress and modulation of PTP opening by mtNOS.

Blood content of asymmetric dimethylarginine: new insights into its dysregulation in renal disease

BACKGROUND

Plasma asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, is significantly elevated in patients with kidney disease and is a potential risk factor for cardiovascular disease. Here, we tested whether human whole blood (WB), as in rodent blood, can accumulate free ADMA and whether this accumulation is a function of disease burden.

METHODS

In 16 healthy control subjects (CO), 18 patients with ESRD and 18 matched hypertensive patients with normal renal function (HTN), we compared using high-pressure liquid chromatography baseline plasma and WB supernatant (WBSUP) ADMA and symmetrical dimethylarginine (SDMA) concentrations and accumulation during a 5-h incubation. We measured protein turnover in incubated WBSUP to determine if proteolytic processes drive ADMA accumulation.

RESULTS

Elevated plasma ADMA was confirmed in ESRD and HTN populations while basal WBSUP ADMA was significantly higher in ESRD subjects than controls (P = 0.05 versus CO; P = 0.02 versus HTN). Plasma SDMA followed a similar pattern. Incubation of WBSUP resulted in ADMA release from protein-incorporated stores while SDMA was unaffected. ADMA accumulation in ESRD samples was significantly greater than that in HTN (P = 0.03). CO and HTN men showed significantly greater ADMA accumulation than women (P = 0.01 and P = 0.003, respectively) but no gender difference was observed in the ESRD group (P = 0.26). ADMA accumulation correlated with ex vivo protein turnover (R = 0.76, P < 0.0001).

CONCLUSIONS

Human blood is capable of releasing physiologically significant quantities of ADMA via proteolytic pathways. Dysregulated ADMA release from WB reservoirs may contribute to the distinctly high plasma ADMA levels in ESRD populations.

The arginine-creatine pathway is disturbed in children and adolescents with renal transplants

Cardiovascular disease is an important cause of morbidity in recipients of renal transplants. The aim of the present study was to analyze the status of the arginine-creatine pathway in such patients, given the relationship between the arginine metabolism and both renal function and the methionine-homocysteine cycle. Twenty-nine children and adolescents (median age 13, range 6-18 years), who had received a renal allograft 14.5-82.0 months before, were recruited for the study. On immunosuppressive therapy, all patients evidenced an adequate level of renal function. Plasma concentrations of homocysteine and glycine were significantly higher, whereas urinary excretions of guanidinoacetate and creatine were significantly lower than controls. Urinary excretions of guanidinoacetate and creatine correlated positively with creatinine clearance. Urinary excretion of creatine was negatively correlated with plasma concentration of homocysteine. The demonstration of disturbances in the arginine-creatine pathway in patients with well-functioning renal transplants and in absence of chronic renal failure represents a novel finding. We speculate that the low urinary excretion of guanidinoacetate and creatine is probably related to the nephrotoxic effect of immunosuppressive therapy and to defective methylation associated with the presence of hyperhomocysteinemia.

l-Arginine transporters in cardiovascular disease: A novel therapeutic target

The amino acid l-arginine participates in a variety of key biochemical and physiological activities, including its well-recognized role as the key substrate for nitric oxide (NO) biosynthesis. The current review describes the cellular influences on arginine metabolism with particular focus on the transport of l-arginine in the endothelium. It details the processes by which intracellular and extracellular levels of l-arginine may influence nitric oxide production and further documents the imbalance that is evident in various cardiovascular disease states. In man, impairment of l-arginine transport has been observed in hypertension, heart failure, and renal disease, and it may thus be a relevant therapeutic target for rectification of nitric oxide pathogenesis in these conditions.

L-Arginine currents in rat cardiac ventricular myocytes

L-Arginine (L-Arg) is a basic amino acid that plays a central role in the biosynthesis of nitric oxide, creatine, agmantine, polyamines, proline and glutamate. Most tissues, including myocardium, must import L-Arg from the circulation to ensure adequate intracellular levels of this amino acid. This study reports novel L-Arg-activated inward currents in whole-cell voltage-clamped rat ventricular cardiomyocytes. Ion-substitution experiments identified extracellular L-Arg as the charge-carrying cationic species responsible for these currents, which, thus, represent L-Arg import into cardiac myocytes. This result was independently confirmed by an increase in myocyte nitric oxide production upon extracellular application of L-Arg. The inward movement of Arg molecules was found to be passive and independent of Na(2+), K(2+), Ca(2+) and Mg(2+). The process displayed saturation and membrane potential (V(m))-dependent kinetics, with a K(0.5) for l-Arg that increased from 5 mm at hyperpolarizing V(m) to 20 mm at +40 mV. L-Lysine and L-ornithine but not D-Arg produced currents with characteristics similar to that activated by L-Arg indicating that the transport process is stereospecific for cationic L-amino acids. L-Arg current was fully blocked after brief incubation with 0.2 mm N-ethylmaleimide. These features suggest that the activity of the low-affinity, high-capacity CAT-2A member of the y(2+) family of transporters is responsible for L-Arg currents in acutely isolated cardiomyocytes. Regardless of the mechanism, we hypothesize that a low-affinity arginine transport process in heart, by ensuring substrate availability for sustained NO production, might play a cardio-protective role during catabolic states known to increase Arg plasma levels severalfold.

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.

Increased L-arginine transport via system b0,+ in human proximal tubular cells exposed to albumin

Albumin has complex effects on PTECs (proximal tubular epithelial cells) and is able to stimulate growth or injury depending on its bound moieties. Albumin itself is a mitogen, inducing proliferation through a number of pathways. In PTEC exposed to purified albumin, polyamines are required for entry into the cell cycle and are critical for proliferation. Polyamines are synthesized from L-ornithine (itself derived by the action of arginase on L-arginine), and the transport and availability of L-arginine may thus be important for subsequent polyamine-dependent proliferation. In the present study we investigated radiolabelled cationic amino-acid transport in cultured PTEC exposed to 20 mg/ml ultrapure recombinant human albumin, describing the specific kinetic characteristics of transport and the expression of transporters. L-[3H]Arginine transport capacity in human PTEC is increased after exposure for 24 h to human albumin, mediated by the broad-scope high-affinity system b0,+ and, to a lesser extent, system y+L (but not system y+) transport. Increased transport is associated with increased b0,+-associated transporter expression. Inhibition of phosphoinositide 3-kinase, a key regulator of albumin endocytosis and signalling, inhibited proliferation, but had no effect on the observed increase in transport. PTEC proliferated in response to albumin. L-Lysine, a competitive inhibitor of L-arginine transport, had no effect on albumin-induced proliferation; however, arginine deprivation effectively reversed the albumin-induced proliferation observed. In conclusion, in PTEC exposed to albumin, increased L-arginine transport is mediated by increased transcription and activity of the apical b0,+ transport system. This may make L-arginine available as a substrate for the downstream synthesis of polyamines, but is not critical for cell proliferation.

The role of obesity and its bioclinical correlates in the progression of chronic kidney disease

In spite of a progressive fall in the incidence of traditional risk factors of cardiovascular morbidity (cigarette smoking, high blood pressure, and hyperlipidemia), there is an upward trend in the prevalence of obesity and chronic kidney disease (CKD). Furthermore, there is a strong correlation between body mass indices and the relative risk of progression of CKD. The close biophysiological interaction between obesity and CKD is evident by a similar occurrence of comorbidities including insulin resistance, hyperlipidermia, endothelial dysfunction, and sleep disorders. Truncal obesity is a primary component of metabolic syndrome; unlike peripheral fat, the visceral adipocytes are more resistant to insulin. In addition, lipolysis results in a release of free fatty acid and TG, whereas hypertriglycedemia is potentiated by uremic activation of fatty acid synthase. Hypertriglycedemia and low HDL cholesterol increase the relative risk of progression of CKD. Furthermore, endothelial inflammation and premature atherosclerosis are promoted by hyperhomocysteinemia and oxidation of LDL, both of which are commonly observed in CKD and obesity. Predominance of oxidative stress in both obesity and azotemia stimulate synthesis of angiotensin II, which in turn increases TGF-B and plasminogen activator inhibitor-1, thereby propagating glomerular fibrosis. Furthermore, local synthesis of angiotensinogen by adipocytes, leptin activation of sympathetic nervous system, and hyperinsulinemia contribute to the development of hypertension in obesity and CKD. In addition, increased renal tubular expression of Na-K-ATPase and a blunted response to natiuretic hormones in obesity promote salt and water retention. Glomerular hyperfiltration from systemic volume load and hypertension results in mesangial cellular proliferation and progressive renal fibrosis. In addition, maternal nutritional deprivation increases the incidence of obesity, hypertension, and diabetes in adulthood. Reduced fetal protein synthesis contributes to oxidative glomerular injury and impairment of renal morphogenesis. Thus, kidneys are poorly equipped to handle physiologic stress that may result from the rapid body growth and programmed metabolic dysfunction later in life. Finally, in order to minimize morbidity of obesity-related kidney disease, preventive strategy must include optimal maternal health care, promotion of healthy nutrition and routine physical exercise, and early detection of CKD.

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.

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.

Effects of nutritional status on the L-arginine-nitric oxide pathway in platelets from hemodialysis patients

BACKGROUND

Malnutrition is a common feature in chronic renal failure and adversely affects patient morbidity and mortality. We here investigate the effects of nutritional status on the L-arginine-nitric oxide signaling pathway and platelet function in chronic renal failure patients on regular hemodialysis.

METHODS

Platelet aggregation was correlated with plasma amino acid profiles, L-arginine transport, and nitric oxide synthase (NOS) activity determined by conversion of L-[(3)H]-arginine to L-[(3)H]-citrulline and accumulation of intracellular cyclic guanosine monophospate (cGMP) in platelets from malnourished and well-nourished chronic renal failure patients on regular hemodialysis (N = 78).

RESULTS

Transport of L-arginine (pmol/10(9)cells/min) via y(+) L system was increased in well-nourished (104 +/- 15) compared to controls (57 +/- 11) or malnourished chronic renal failure patients (55 +/- 13). Basal NOS activity (pmol/10(8)cells) was enhanced in well-nourished chronic renal failure patients (0.51 +/- 0.01) compared to controls (0.18 +/- 0.01) or malnourished chronic renal failure patients (0.08 +/- 0.03). In addition, basal cGMP levels are elevated in platelets from well-nourished chronic renal failure compared to malnourished uremic patients. Platelet aggregation induced by collagen is impaired in well-nourished chronic renal failure patients compared to malnourished patients and controls. Plasma L-arginine levels are reduced in chronic renal failure patients and even lower in malnourished patients.

CONCLUSION

Our findings provide the first evidence that L-arginine transport via the high affinity system y(+) L and nitric oxide synthesis are only stimulated in platelets from well-nourished chronic renal failure patients, leading to impaired platelet aggregation. The absence of this adaptive response in the l-arginine-nitric oxide pathway in platelets from malnourished chronic renal failure patients may account for the enhanced occurrence of thrombotic events in these patients.

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 micromol/L per cell per h) and CAPD patients (539 +/- 61 micromol/L per cell per h) compared with controls (251 +/- 39 micromol/L per cell per h). High-pressure liquid chromatography measurements demonstrated low levels of plasma L-arginine in uraemic patients both on CAPD (54 +/- 3 micromol/L) and not yet on dialysis (80 +/- 6 micromol/L) compared with control subjects (146 +/- 14 micromol/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.

Emerging drugs for renal failure

Renal failure involves a significant impairment of the essential functions of the kidney, which can be either acute with sudden and rapid onset (acute renal failure [ARF]) or chronic with gradual onset (chronic renal failure [CRF]). ARF, if detected early, may be halted or reversed, whereas CRF is generally irreversible. Without treatment or intervention, both forms of renal failure lead to end stage renal failure (ESRF) or end stage renal disease (ESRD), requiring renal replacement therapy (RRT) in the form of dialysis or renal transplantation for survival. However, provision of RRT requires expert teams working in specialised units, making therapy of patients with renal failure expensive; furthermore, RRT is complex, with its own complications. Although pharmacological interventions have shown promise in experimental models, these have not been as successful in the clinical setting (e.g., administration of atrial natriuretic peptide, low-dose dopamine). At present, drugs are administered during CRF to either reduce one of the many risk factors of CRF (e.g., angiotensin-converting enzyme inhibitors, statins) or to deal with the consequences of CRF (e.g., erythropoietin, calcitriol). Recent evidence suggests that some of these interventions may provide further direct beneficial effects via reduction of renal inflammation. Although these interventions have greatly improved the prospects for patients suffering ESRF, the development of novel drugs and therapies with which to reduce the consequences of renal failure and ESRD remain topics of great interest. This article reviews the therapies available for the prevention and management of renal failure in adults and describes, in detail, emerging drugs and novel interventions that may soon become available for the treatment or prevention of ESRF.

Pathophysiology of Erectile Dysfunction

INTRODUCTION

Multiple regulatory systems are involved in normal erectile function. Disruption of psychological, neurological, hormonal, vascular, and cavernosal factors, individually, or in combination, can induced erectile dysfunction (ED). The contribution of neurogenic, vascular, and cavernosal factors was thoroughly reviewed by our committee, while psychological and hormonal factors contributing to ED were evaluated by other committees.

AIM

To provide state of the art knowledge on the physiology of ED.

METHODS

An international consultation in collaboration with the major urology and sexual medicine associations assembled over 200 multidisciplinary experts from 60 countries into 17 committees. Committee members established specific objectives and scopes for various male and female sexual medicine topics. The recommendations concerning state-of-the-art knowledge in the respective sexual medicine topic represent the opinion of experts from five different continents developed in a process over a 2-year period. Concerning the pathophysiology of ED committee, there were seven experts from five different countries.

MAIN OUTCOME MEASURE

Expert opinion was based on the grading of evidence-based medical literature, widespread internal committee discussion, public presentation, and debate.

RESULTS

The epidemiology and classification of neurogenic ED was reviewed. The evidence for the association between vascular ED and atherosclerosis/hypercholesterolemia, hypertension and diabetes was evaluated. In addition, the pathophysiological mechanisms implicated in vascular ED were defined, including: arterial remodeling, increased vasoconstriction, impaired neurogenic vasodilatation, and impaired endothelium-dependent vasodilatation. The possible mechanisms underlying the association between chronic renal failure and ED were also evaluated as well as the evidence supporting the association of ED with various classes of medications.

CONCLUSIONS

A better understanding of how diseases interfere with the physiological mechanisms that regulate penile erection has been achieved over the last few years, which helps establish a strategy for the prevention and treatment of ED.

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.

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

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.

Plasma membrane transporters for arginine

The supply of arginine may become rate limiting for enzymatic reactions that use this semiessential amino acid as a substrate (e.g., nitric oxide, agmatine, creatine, and urea synthesis), particularly under conditions of high demand such as growth, sepsis, or wound healing. In addition, arginine acts as a signaling molecule that regulates essential cellular functions such as protein synthesis, apoptosis, and growth. In the past decade, a number of carrier proteins for amino acids have been identified on the molecular level. They belong to different gene families, exhibit overlapping but distinctive substrate specificities, and can further be distinguished by their requirement for the cotransport or countertransport of inorganic ions. A number of these transporters function as exchangers rather than uniporters. Uptake of amino acids by these transporters therefore depends largely on the intracellular substrate composition. Hence, there is a complex crosstalk between transporters for cationic and neutral amino acids as well as for peptides. This article briefly reviews current knowledge regarding mammalian plasma membrane transporters that accept arginine as a substrate.

Low water-soluble uremic toxins

The uremic syndrome is the result of the retention of solutes, which under normal conditions are cleared by the healthy kidneys. Uremic retention products are arbitrarily subdivided according to their molecular weight. Low-molecular-weight molecules are characterized by a molecular weight below 500 D. The purpose of the present publication is to review the main water soluble, nonprotein bound uremic retention solutes, together with their main toxic effects. We will consecutively discuss creatinine, glomerulopressin, the guanidines, the methylamines, myo-inositol, oxalate, phenylacetyl-glutamine, phosphate, the polyamines, pseudouridine, the purines, the trihalomethanes, and urea per se.

Ceramide-induced Intracellular Oxidant Formation, Iron Signaling, and Apoptosis in Endothelial Cells

Sphingolipid ceramide (N-acetylsphingosine), a bioactive second messenger lipid, was shown to activate reactive oxygen species (ROS), mitochondrial oxidative damage, and apoptosis in neuronal and vascular cells. The proapoptotic effects of tumor necrosis factor-alpha, hypoxia, and chemotherapeutic drugs were attributed to increased ceramide formation. Here we investigated the protective role of nitric oxide (.NO) during hydrogen peroxide (H(2)O(2))-mediated transferrin receptor (TfR)-dependent iron signaling and apoptosis in C(2)-ceramide (C(2)-cer)-treated bovine aortic endothelial cells (BAECs). Addition of C(2)-cer (5-20 microm) to BAECs enhanced .NO generation. However, at higher concentrations of C(2)-cer (> or =20 microm), .NO generation did not increase proportionately. C(2)-cer (20-50 microm) also resulted in H(2)O(2)-mediated dichlorodihydrofluorescein oxidation, reduced glutathione depletion, aconitase inactivation, TfR overexpression, TfR-dependent uptake of (55)Fe, release of cytochrome c from mitochondria into cytosol, caspase-3 activation, and DNA fragmentation. N(w)-Nitro-l-arginine methyl ester (l-NAME), a nonspecific inhibitor of nitricoxide synthases, augmented these effects in BAECs at much lower (i.e. nonapoptotic) concentrations of C(2)-cer. The 26 S proteasomal activity in BAECs was slightly elevated at lower concentrations of C(2)-cer (< or =10 microm) but was greatly suppressed at higher concentrations (>10 microm). Intracellular scavengers of H(2)O(2), cell-permeable iron chelators, anti-TfR receptor antibody, or mitochondria-targeted antioxidant greatly abrogated C(2)-cer- and/or l-NAME-induced oxidative damage, iron signaling, and apoptosis. We conclude that C(2)-cer-induced H(2)O(2) and TfR-dependent iron signaling are responsible for its prooxidant and proapoptotic effects and that .NO exerts an antioxidative and cytoprotective role.

Myocardial Protection after Systemic Application of L-Arginine during Reperfusion

The L-arginine-nitric oxide (NO) pathway plays an important role in ischemia-reperfusion injury. In the present study we investigated the role of NO-precursor L-arginine on cardiac and pulmonary function after reversible hypothermic ischemia. Twelve anesthetized dogs underwent cardiopulmonary bypass. After 60 minutes of hypothermic cardiac arrest, reperfusion was started with application of either saline vehicle (control, n = 6) or L-arginine (40 mg/kg i.v. bolus then 3 mg/kg i.v. infusion during the first 20 minutes of reperfusion, n = 6). The vasodilative response to acetylcholine was significantly higher in the L-arginine group (P < 0.05). The preload recruitable stroke work of the left ventricle decreased significantly after reperfusion, however remained unchanged in the L-arginine group. Arterial blood gas analysis did not show any difference between the two groups. Plasma L-arginine concentration reached peak level at 20 minutes of administration (675.0 +/- 66.6 versus 207.7 +/- 14.5 in the L-arginine group, P < 0.05) and returned to baseline at 40 minutes, while in the control group remained unchanged during ischemia and reperfusion (276.2 +/- 71.6 versus 283.8 +/- 38.5, P < 0.05). Plasma nitrite concentration followed L-arginine changes parallel, however nitrate levels increased slower. Supplementation with L-arginine during reperfusion prevents myocardial and endothelial dysfunction, however does not have any overriding effect on pulmonary function. Considerably rapid elimination of plasma L-arginine was demonstrated during early reperfusion.

L-Carnitine Ameliorates Oxidative Damage due to Chronic Renal Failure in Rats

Chronic renal failure (CRF) is associated with oxidative stress that promotes production of reactive oxygen species. L-Carnitine is a cofactor required for transport of long-chain fatty acids into the mitochondrial matrix. Recent research has shown that some clinical conditions (i.e., anorexia, chronic fatigue, coronary heart disease, diphtheria, hypoglycemia, and male infertility) benefit from exogenous supplementation of L-carnitine. The aim of this study was to examine the role of L-carnitine in protecting the aorta, heart, corpus cavernosum, and kidney tissues against oxidative damage in a rat model of CRF. Male Wistar albino rats were randomly assigned to either the CRF group or the sham-operated control group, which had received saline or L-carnitine (500 mg/kg, i.p.) for 4 weeks. CRF was evaluated by BUN and serum creatinine measurements. Aorta and corporeal tissues were used for contractility studies or stored along with heart and kidney tissues for the measurement of malondialdehyde (MDA) and glutathione (GSH) levels. Plasma MDA, GSH levels and erythrocyte superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) activities were also studied. In the CRF group, the contraction and the relaxation of aorta and corpus cavernosum samples decreased significantly compared with controls and were partially reversed by L-carnitine treatment. In the CRF group, there were significant increases in tissue MDA with marked reductions in GSH levels in all tissues and plasma compared with controls. In the plasma SOD, CAT and GSH-Px activities were also reduced. All these effects were reversed by L-carnitine as well. The increase in MDA level and the concomitant decrease in GSH level of tissues and plasma and also suppression of the antioxidant enzyme activities in plasma demonstrate that oxidative mechanisms are involved in CRF-induced tissue damage. L-carnitine, possibly via its free radical scavenging and antioxidant properties, ameliorates oxidative organ injury and CRF-induced dysfunction of the aorta and corpus cavernosum. These results suggest that L-carnitine supplementation may have some benefit in CRF patients.