Colocalization of demethylating enzymes and NOS and functional effects of methylarginines in rat kidney

Symmetric dimethylarginine (SDMA) as endogenous marker of renal function--a meta-analysis

BACKGROUND

Dosing of most drugs must be adapted in renal insufficiency, making accurate assessment of renal function essential in clinical medicine. Furthermore, even modest impairment of renal function has been recognized as a cardiovascular risk factor. The purpose of this analysis was to identify the role of symmetric dimethylarginine (SDMA), the structural isomer of the cardiovascular risk marker asymmetric dimethylarginine, as an endogenous marker of renal function.

METHODS

Comprehensive searches of Medline and the Cochrane Library from 1970 to February 2006 were performed to identify studies that evaluated the correlation between SDMA and renal function. The search was augmented by scanning references of identified articles and reviews. The correlation coefficients (R) were recorded from each study for the values of 1/SDMA and clearance estimates and for SDMA and creatinine levels. The summary correlation coefficients with 95% confidence intervals (CIs) were pooled using the random-effects method.

RESULTS

In 18 studies involving 2136 patients systemic SDMA concentrations correlated highly with inulin clearance [R = 0.85 (CI 0.76-0.91, P < 0.0001)], as well as with various clearance estimates combined [R = 0.77 (CI 0.65-0.85, P < 0.0001)] and serum creatinine [R = 0.75 (CI 0.46-089, P < 0.0001)].

CONCLUSIONS

SDMA exhibits some properties of a reliable marker of renal function. Future studies have to clarify whether SDMA is indeed suited to improve diagnosis and eventually optimize care of patients.

Asymmetric dimethylarginine (ADMA): a cardiovascular and renal risk factor on the move

The endogenous inhibitor of the nitric oxide synthase, asymmetric dimethylarginine (ADMA), by reducing nitric oxide (NO) availability, may trigger pro-atherogenic effects. A high plasma concentration of this substance has been associated to intima-media thickening, left ventricular hypertrophy and all-cause and cardiovascular mortality in patients with end-stage renal disease, and to coronary events in males in the general population. Recent studies show that ADMA predicts renal disease progression and death in patients with moderate to severe renal insufficiency. ADMA may be at the crossroad of the atherosclerosis process and may represent an important factor in the high risk associated with renal insufficiency.

Symmetrical dimethylarginine: a new combined parameter for renal function and extent of coronary artery disease

Symmetrical dimethylarginine (SDMA) is the structural isomer of the endogenous nitric oxide synthase (NOS) inhibitor asymmetric dimethylarginine. Whereas the major route of asymmetric dimethylarginine elimination is the hydrolytic degradation by dimethylarginine dimethylaminohydrolase, SDMA is eliminated by renal excretion. SDMA does not directly inhibit NOS but is a competitor of arginine transport. This study showed for the first time that measurement of SDMA can be a marker of estimated GFR and extent of coronary artery disease (CAD). In 97 patients with CAD, SDMA was a marker of estimated GFR. On multiple regression analysis of the CAD parameter stenosis score, SDMA was the only parameter retained. In addition, endothelial cells from the third passage were cultured in medium that contained 70 micromol/L arginine and was incubated for 24 h in the presence of various concentration of SDMA (0, 2, 5, 10, and 100 micromol/L). The levels of nitrate and nitrite in conditioned media, the protein expression of NOS, and the content of reactive oxygen species in endothelial cells were determined. SDMA inhibited dose dependently the NO synthesis in intact endothelial cells, whereas it had no effect on protein expression of NOS. This effect was associated with an increase in reactive oxygen species. Co-incubation with L-arginine but not D-arginine reversed the effect of SDMA on NOS pathway. Our data suggest that SDMA reduced the endothelial NO synthesis, probably by limiting L-arginine supply to NOS. It is concluded that SDMA might be a useful parameter for detecting patients in very early stages of chronic kidney disease and for determining their risk for developing cardiovascular disease.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

Asymmetric dimethylarginine: a cardiovascular risk factor and a uremic toxin coming of age?

The idea that asymmetric dimethylarginine (ADMA) accumulation may be a cardiovascular risk factor in patients with end-stage renal disease was advanced by Vallance in 1992. During the last decade, the relationship between ADMA and adverse cardiovascular events, including death, in dialysis patients has been investigated thoroughly. Several studies have shown that, independently of other risk factors, ADMA is strongly associated with intima-media thickness of the carotid artery and left ventricular mass, particularly concentric left ventricular hypertrophy. Furthermore, cohort studies in both the general population and the dialysis population showed a strong and independent link between ADMA, all-cause mortality, and cardiovascular events. Circumstantial evidence indicates that norepinephrine and ADMA may be in the same causal pathway leading to cardiovascular complications in patients with end-stage renal disease. Several lines of evidence show that high ADMA levels may exert toxic effects in various cell types. High ADMA levels have been associated with alterations in the regulation of cerebral blood flow and neural function, with insulin resistance, thyroid dysfunction, and alterations in bone homeostasis, fertility, and erectile function. The clinical significance of decreasing plasma ADMA concentrations, if any, is unknown. Well-designed and carefully conducted studies are needed to further clarify the role of ADMA in the pathophysiological states of renal disease and explore possible treatment options to improve the prognosis of patients with elevated ADMA levels. ADMA may enable us to predict risk and follow up the course of renal diseases.

Asymmetrical dimethylarginine plasma clearance persists after acute total nephrectomy in rats

Elevated plasma concentrations of symmetrical dimethylarginine (SDMA) and asymmetrical dimethylarginine (ADMA) are repeatedly associated with kidney failure. Both ADMA and SDMA can be excreted in urine. We tested whether renal excretion is necessary for acute, short-term maintenance of plasma ADMA and SDMA. Sprague-Dawley rats underwent sham operation, bilateral nephrectomy (NPX), ureteral ligation, or ureteral section under isoflurane anesthesia. Tail-snip blood samples (250 microl) were taken before and at 6- or 12-h intervals for 72 h after operation. Plasma clearance was assessed in intact and NPX rats. High-performance liquid chromatography determined SDMA and ADMA concentrations. Sodium, potassium, creatinine, blood urea nitrogen (BUN), and body weight were also measured. Forty-eight hours after NPX, SDMA increased 25 times (0.23 +/- 0.03 to 5.68 +/- 0.30 microM), whereas ADMA decreased (1.17 +/- 0.08 to 0.73 +/- 0.08 microM) by 38%. Creatinine and BUN increased, paralleling SDMA. Sham-operated animals showed no significant changes. Increased SDMA confirms continuous systemic production of SDMA and its obligatory renal excretion, much like creatinine. In contrast, decreased plasma ADMA suggests that acute total NPX either reduced systemic ADMA formation and/or systemic hydrolysis of ADMA increased 48-h post-NPX. However, plasma clearance of ADMA appeared unchanged 48 h after NPX. We conclude that renal excretory function is needed for SDMA elimination but not needed for acute, short-term ADMA elimination in that systemic hydrolysis is fully capable of clearing plasma ADMA.

Asymmetric dimethylarginine and progression of chronic kidney disease: the mild to moderate kidney disease study

Reduced bioavailability of nitric oxide (NO) is thought to play an important role in progression of renal damage. The hypothesis that the endogenous NO synthase inhibitor asymmetric dimethylarginine (ADMA) is involved in progression of kidney disease was tested. Plasma ADMA concentrations and other putative progression factors were assessed in 227 relatively young patients (45.7 +/- 12.6 yr) with nondiabetic kidney diseases and mild to moderate renal failure. Progression assessed as doubling of serum creatinine and/or renal replacement therapy was evaluated prospectively. Baseline plasma ADMA concentrations in renal patients correlated significantly with serum creatinine (r = 0.595), GFR (r = -0.591), age (r = 0.281), and proteinuria (r = 0.184; all P < 0.01). Patients who reached an end point during follow-up were significantly older (P < 0.05) and had significantly higher creatinine, ADMA, and parathyroid hormone blood concentrations and protein excretion rates at baseline, whereas GFR and hemoglobin were significantly lower (all P < 0.01). Cox regression analysis revealed baseline serum creatinine (odds ratio 2.00; 95% confidence interval [CI] 1.61 to 2.49; P < 0.001) and ADMA (odds ratio 1.47; 95% CI 1.12 to 1.93 for an increment of 0.1 mumol/L; P < 0.006) as independent predictors of disease progression. In patients with ADMA levels above median, progression was significantly faster (P < 0.0001), and their mean follow-up time to a progression end point was 52.8 mo (95% CI 46.9 to 58.8) as compared with 71.6 mo (95% CI 66.2 to 76.9) in patients with ADMA levels below the median. The endogenous NO synthase inhibitor ADMA is significantly associated with progression of nondiabetic kidney diseases. Lowering plasma ADMA concentrations may be a novel therapeutic target to prevent progressive renal impairment.

Protective Effect of L-Arginine Intake on the Impaired Renal Vascular Responses in the Gentamicin-Treated Rats

The purpose of this study was to investigate the effect of gentamicin (100 mg/kg/day, i.p.) treatment on endothelium-dependent and -independent vasodilation in isolated perfused rat kidney, and the effect of amino acid L-arginine (in the drinking water, 2.25 g/l) on renal dysfunction induced by gentamicin. When gentamicin-treated groups were compared with the control group, it was observed that BUN and creatinine levels increased significantly. Also, the relaxant responses induced by acetylcholine, sodium nitroprusside and pinacidil decreased. Histopathological examination indicated acute tubular necrosis in this group. In animals treated with gentamicin together with L-arginine, there was a significant amelioration in the BUN and creatinine levels. The vasodilator responses were similar to those of the control group. Histopathological examination indicated only hydropic degeneration in tubular epithelium of kidney. Co-administration of L-NG-nitroarginine methyl ester (L-NAME) (112.5 mg/l), an inhibitor of nitric oxide synthase, and L-arginine to rats treated with gentamicin did not change the protective effect of L-arginine. In rats receiving L-NAME alone, the level of BUN and creatinine and vasodilation to acetylcholine were not significantly different when compared to those of the control group, while relaxant responses to sodium nitroprusside and pinacidil were increased. These results suggest that gentamicin leads to an impairment in vascular smooth muscle relaxation in addition to acute tubular necrosis in the rat kidney. Supplementation of L-arginine has an important protective effect on gentamicin-induced nephropathy.

Elevation of asymmetric dimethylarginine (ADMA) in patients developing hepatic failure after major hepatectomy

BACKGROUND

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of the arginine-nitric oxide pathway. It is conceivable that its concentration is tightly regulated by urinary excretion and degradation by the enzyme dimethylarginine dimethylaminohydrolase, which is highly expressed in the liver. In rats, we showed a high net hepatic uptake of ADMA. Therefore, we aimed to confirm the role of the liver in humans and hypothesized elevated ADMA levels after major liver resection by a reduction of functional liver mass and injury to the remnant liver.

METHODS

Patients undergoing a major hepatic resection (HEP, n = 17) or major abdominal surgery (MAS, n = 12) were included and followed in time. In addition, ADMA levels were measured in 4 patients having severe hepatic failure after a liver resection. Plasma ADMA concentration was measured by high-performance liquid chromatography.

RESULTS

Preoperatively and on days 1, 3, and 5, plasma levels of ADMA were higher in HEP patients when compared with MAS patients. In HEP patients with prolonged (>7 days) hepatic injury, ADMA levels were especially elevated. On the first postoperative day, ADMA significantly correlated to bilirubin concentration (r = .528, p < .05) as a marker of postoperative hepatic function. Besides, in patients with severe hepatic failure, ADMA levels were highly elevated.

CONCLUSIONS

In the present study, evidence was found for the role of the liver in the elimination of ADMA in humans. Increased levels of ADMA occur in the postoperative course after a major hepatic resection, especially when liver function is severely impaired. Further studies need to assess the role of ADMA in the development of complications after liver surgery.

Differential regulation of glomerular and interstitial endothelial nitric oxide synthase expression in the kidney of hibernating ground squirrel

Hibernating animals transiently reduce renal function during their hypothermic periods (torpor), while completely restoring it during their periodical rewarming to euthermia (arousal). Moreover, structural integrity of the kidney is preserved throughout the hibernation. Nitric oxide (NO) generated by endothelial nitric oxide synthase (eNOS) is a crucial vasodilatory mediator and a protective factor in the kidney. We investigated renal NOS expression in hibernating European ground squirrels after 1 day and 7 days of torpor (torpor short, TS, and torpor long, TL, respectively), at 1.5 and at 10 h of rewarming (arousal short, AS, and arousal long, AL, respectively), and in continuously euthermic animals after hibernation (EU). For that purpose, we performed NOS activity assay, immunohistochemistry and real-time PCR analysis. Immunohistochemistry revealed a decreased glomerular eNOS expression in hibernating animals (TS, TL, AS, and AL) compared to non-hibernating animals (EU, p < 0.05), whereas no difference was found in the expression of interstitial eNOS. Expression of iNOS and nNOS did not differ between all groups. The reduced glomerular eNOS was associated with a significantly lower eNOS mRNA levels and NOS activity of whole kidney during torpor and arousal (TS, TL, AS, and AL) compared to EU. In all methods used, torpid and aroused squirrels did not differ. These results demonstrate differential regulation of eNOS in glomeruli and interstitium of hibernating animals, which is unaffected during arousal. The differential regulation of eNOS may serve to reduce ultrafiltration without jeopardizing tubular structures during hibernation.

Expression of endothelial nitric oxide synthase in developing rat kidney

Endothelium-derived nitric oxide (NO) is synthesized within the developing kidney and may play a crucial role in the regulation of renal hemodynamics. The purpose of this study was to establish the expression and intrarenal localization of the NO-synthesizing enzyme endothelial NO synthase (eNOS) during kidney development. Rat kidneys from 14 ( E14)-, 16 ( E16)-, 18 ( E18)-, and 20-day-old ( E20) fetuses and 1 ( P1)-, 3 ( P3)-, 7 ( P7)-, 14 ( P14)-, and 21-day-old ( P21) pups were processed for immunocytochemical and immunoblot analysis. In fetal kidneys, expression of eNOS was first observed in the endothelial cells of the undifferentiated intrarenal capillary network at E14. At E16, strong eNOS immunoreactivity was observed in the endothelial cells of renal vesicles, S-shaped bodies (stage II glomeruli), and stage III glomeruli at the corticomedullary junction. At E18- 20, early-stage developing glomeruli located in the subcapsular region showed less strong eNOS immunoreactivity than those of E16. The eNOS-positive immature glomeruli were observed in the nephrogenic zone until 7 days after birth. In fetal kidneys, eNOS was also expressed in the medulla in the endothelial cells of the capillaries surrounding medullary collecting ducts. After birth, eNOS immunostaining gradually increased in the developing vascular bundles and peritubular capillaries in the medulla and was highest at P21. Surprisingly, eNOS was also expressed in proximal tubules, in the endocytic vacuolar apparatus, only at P1. The strong expression of eNOS in the early stages of developing glomeruli and vasculature suggests that eNOS may play a role in regulating renal hemodynamics of the immature kidney.

Asymmetric dimethylarginine (ADMA): the silent transition from an 'uraemic toxin' to a global cardiovascular risk molecule

Endothelial dysfunction as a result of reduced bioavailability of nitric oxide (NO) plays a central role in the process of atherosclerotic vascular disease. In endothelial cells NO is synthesized from the amino acid l-arginine by the action of the NO synthase (NOS), which can be blocked by endogenous inhibitors such as asymmetric dimethylarginine (ADMA). Acute systemic administration of ADMA to healthy subjects significantly reduces NO generation, and causes an increase in systemic vascular resistance and blood pressure. Increased plasma ADMA levels as a result of reduced renal excretion have been associated with atherosclerotic complications in patients with terminal renal failure. However, a significant relationship between ADMA and traditional cardiovascular risk factors such as advanced age, high blood pressure and serum LDL-cholesterol, has been documented even in individuals without manifest renal dysfunction. As a consequence, the metabolism of ADMA by the enzyme dimethylarginine dimethylaminohydrolase (DDAH) has come into the focus of cardiovascular research. It has been proposed that dysregulation of DDAH with consecutive increase in plasma ADMA concentration and chronic NOS inhibition is a common pathophysiological pathway in numerous clinical conditions. Thus, ADMA has emerged as a potential mediator of atherosclerotic complications in patients with coronary heart disease, peripheral vascular disease, stroke, etc., being the culprit and not only an innocent biochemical marker of the atherosclerotic disease process.

Expression profile of a human inducible nitric oxide synthase promoter reporter in transgenic mice during endotoxemia

Inducible nitric oxide synthase (iNOS) is involved in many physiological and pathophysiological processes, including septic shock and acute kidney failure. Little is known about transcriptional regulation of the human iNOS gene in vivo under basal conditions or in sepsis. Accordingly, we developed transgenic mice carrying an insertional human iNOS promoter-reporter gene construct. In these mice, the proximal 8.3 kb of the human iNOS 5′-flanking region controls expression of the reporter gene of enhanced green fluorescent protein (EGFP). Patterns of human iNOS promoter/EGFP transgene expression in tissues were examined by fluorescence microscopy and immunoblotting. Endogenous murine iNOS was basally undetectable in kidney, intestine, spleen, heart, lung, liver, stomach, or brain. In contrast, EGFP from the transgene was basally expressed in kidney, brain, and spleen, but not the other tissues of the transgenic mice. Bacterial lipopolysaccharide induced endogenous iNOS expression in kidney, intestine, spleen, lung, liver, stomach, and heart, but not brain. In contrast, human iNOS promoter/EGFP transgene expression was induced above basal levels only in intestine, spleen, brain, stomach, and lung. Within kidney, human iNOS promoter/EGFP fluorescence was detected most prominently in proximal tubules of the outer cortex and collecting ducts and colocalized with endogenous mouse iNOS. Within the collecting duct, both endogenous iNOS and the human iNOS promoter/EGFP transgene were expressed in cells lacking aquaporin-2 immunoreactivity, consistent with expression in intercalated cells. Although it remains possible that essential regulatory elements reside in remote locations of the gene, our data concerning this 8.3-kb region provide the first in vivo evidence suggesting differential transcriptional control of the human iNOS gene in these organs and marked differences in transcriptional regulatory regions between the murine and human genes.

Elimination of asymmetric dimethylarginine by the kidney and the liver: a link to the development of multiple organ failure?

Asymmetric dimethylarginine (ADMA) is a recently recognized endogenous inhibitor of nitric oxide production. Its role in cardiovascular disease is emerging, and ADMA appears to be an important causal factor in dysfunction of the vascular system. Several studies show that ADMA accumulates during renal failure, and ADMA has been identified as causing the cardiovascular complications accompanying renal failure. In addition to the kidney, we recently suggested an important role for the liver as an ADMA-eliminating organ. In a population of critically ill patients, hepatic failure was the most prominent determinant of ADMA concentration, and, notably, high ADMA concentration proved to be a strong and independent risk factor for intensive care unit mortality in these patients. We here summarize the role of both the kidney and the liver in the regulation of ADMA levels. In addition, the potential central role of ADMA as a causative factor in the development of multiple organ failure is discussed.

Gut and liver handling of asymmetric and symmetric dimethylarginine in the rat under basal conditions and during endotoxemia

INTRODUCTION/AIM

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase enzymes, whereas symmetric dimethylarginine (SDMA) competes with arginine transport. Although both dimethylarginines may be important regulators of the arginine-NO pathway, their metabolism is largely unknown. In previous studies, evidence was found for the liver in the metabolism of dimethylarginines. We aimed to investigate dimethylarginine handling of the gut and the liver in detail under basal conditions and during endotoxemia.

METHODS

Twenty-one male Wistar rats were used for this study. Endotoxemia was induced by lipopolysaccharide (LPS) infusion (8 mg/kg). Blood flow was measured using radiolabeled microspheres according to the reference sample method. Concentration of dimethylarginines were measured by high-performance liquid chromatography. The combination of arteriovenous concentration difference and organ blood flow allowed calculation of net organ fluxes and fractional extraction (FE) rates.

RESULTS

Arterial plasma concentration of ADMA was lower in LPS rats, in contrast to a higher SDMA concentration. For the gut, net release of ADMA was found, which was higher in LPS rats. In contrast, for the gut, net uptake of SDMA was found, which was lower in LPS rats. For the liver, a high net uptake of ADMA was found in both groups, while FE was significantly increased in LPS rats. Hepatic handling of SDMA was negligible.

CONCLUSION

The liver plays an important role in eliminating ADMA from the circulation and endotoxemia stimulates this capacity. In contrast to the liver, the gut releases ADMA. Endotoxemia results in a reduced systemic ADMA concentration.

The DDAH/ADMA/NOS pathway

An increasing number of reports in the literature indicate that endogenously produced inhibitors of nitric oxide synthase (NOS), particularly asymmetric dimethylarginine (ADMA) regulate nitric oxide generation in numerous disease states. Two dimethylarginine dimethylaminohydrolase (DDAH) enzymes metabolise ADMA. We and others have postulated that activity of DDAH is a key determinant of ADMA levels in vivo. This review summarises recent advances in the regulation and function of DDAH enzymes and its role in the regulation of nitric oxide generation.

Elevated plasma asymmetric dimethylarginine as a marker of cardiovascular morbidity in early diabetic nephropathy in type 1 diabetes

OBJECTIVE

Increased plasma concentration of asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase, has been associated with endothelial dysfunction, insulin resistance, and atherosclerosis in nondiabetic populations. In end-stage renal failure, circulating ADMA is elevated and a strong predictor of cardiovascular outcome. This study investigated the relation between ADMA and diabetic micro- and macrovascular complications in a large cohort of type 1 diabetic patients with and without early diabetic nephropathy.

RESEARCH DESIGN AND METHODS

ADMA concentrations in plasma were determined by a high-performance liquid chromatography method in 408 type 1 diabetic patients with overt diabetic nephropathy (252 men; mean age 42.7 years [SD 11.0], mean duration of diabetes 28 years [SD 9], median serum creatinine level 102 micromol/l [range 52-684]). A group of 192 patients with longstanding type 1 diabetes and persistent normoalbuminuria served as control subjects (118 men; mean age 42.6 years [SD 10.2], mean duration of diabetes 27 years [SD 9]).

RESULTS

In patients with diabetic nephropathy, mean +/- SD plasma ADMA concentration was elevated 0.46 +/- 0.08 vs. 0.40 +/- 0.08 micromol/l in normoalbuminuric patients (P<0.001). An increase in plasma ADMA of 0.1 micromol/l increased the odds ratio of nephropathy to 2.77 (95% CI 1.89-4.05) (P<0.001). Circulating ADMA increased in nephropathy patients with declining kidney function, as indicated by elevated values in the lower quartiles of glomerular filtration rate (<76 ml.min(-1).1.73 m(-2)) (P<0.001 ANOVA). Mean ADMA levels were similar in patients with or without diabetic retinopathy (P>0.2). However, in 44 patients with nephropathy and history of myocardial infarction and/or stroke, ADMA was significantly elevated at 0.48 +/- 0.08 micromol/l compared with 0.46 +/- 0.08 micromol/l in patients without major cardiovascular events (P=0.05).

CONCLUSIONS

Elevated circulating ADMA may contribute to the excess cardiovascular morbidity and mortality in early diabetic nephropathy.

Cardiovascular effects of systemic nitric oxide synthase inhibition with asymmetrical dimethylarginine in humans

BACKGROUND

Increased blood concentrations of the endogenous nitric oxide synthase (NOS) inhibitor asymmetrical dimethylarginine (ADMA) have been linked to excess cardiovascular morbidity and mortality and to progression of renal disease. We evaluated systemic cardiovascular effects of ADMA infusion in healthy subjects using invasive techniques, ie, right heart catheter and inulin/para-aminohippurate clearance.

METHODS AND RESULTS

Plasma ADMA concentrations encountered in patients with cardiovascular diseases, ie, between 2 and 10 micromol/L, caused a significant (P<0.05) decrease in concentrations of plasma cGMP, the main second messenger of NO. In addition, cardiac output was significantly lower (5.3+/-0.4 versus 5.8+/-0.6 L/min; P<0.05 versus baseline), and systemic vascular resistance was significantly higher (1403+/-123 versus 1221+/-100 dyn x s x cm(-5); P<0.05 versus baseline). The infusion of 0.25 mg ADMA x kg(-1) x min(-1) or 3 microg N(G)-nitro-L-arginine methyl ester x kg(-1) x min(-1), a potent synthetic NOS inhibitor with long action, resulted in a comparable decrease in effective renal plasma flow (from 670+/-40 to 596+/-29 mL x min(-1); P<0.05) and an increase in renovascular resistance (from 79+/-5 to 90+/-7 mm Hg x mL(-1) x min(-1); P<0.05). Moreover, administration of ADMA caused significant sodium retention and blood pressure increase (both P<0.05). The observed effects of ADMA in the systemic circulation were sustained corresponding to a mean plasma half-life of 23.5+/-6.8 minutes, calculated from plasma ADMA decay curves in healthy subjects.

CONCLUSIONS

Systemic ADMA infusion is responsible for a short-term, modest decrease in cardiac output with comparable decrease in effective renal plasma flow while increasing systemic vascular resistance and blood pressure in a dose-related manner.

Handling of asymmetrical dimethylarginine and symmetrical dimethylarginine by the rat kidney under basal conditions and during endotoxaemia

BACKGROUND

Asymmetrical dimethylarginine (ADMA) is capable of inhibiting nitric oxide synthase enzymes, whereas symmetrical dimethylarginine (SDMA) competes with arginine transport. The potential role of inflammation in the metabolism of ADMA has been elucidated in an in vitro model using tumour necrosis factor-alpha, resulting in a decreased activity of the ADMA-degrading enzyme dimethylarginine dimethylaminohydrolase (DDAH). The kidney probably plays a crucial role in the metabolism of ADMA by both urinary excretion and degradation by DDAH. We aimed to further elucidate the role of the kidney in a rat model under basal conditions and during endotoxaemia.

METHODS

Twenty-five male Wistar rats weighing 275-300 g were used for this study. The combination of arteriovenous concentration differences and kidney blood flow allowed calculation of net organ fluxes. Blood flow was measured using radiolabelled microspheres according to the reference sample method. Concentrations of ADMA, SDMA and arginine were measured by high-performance liquid chromatography.

RESULTS

The kidney showed net uptake of both ADMA and SDMA and fractional extraction rates were 35% and 31%, respectively. Endotoxaemia resulted in a lower systemic ADMA concentration (P = 0.01), which was not explained by an increased net renal uptake. Systemic SDMA concentrations increased during endotoxaemia (P = 0.007), which was accompanied by increased creatinine concentrations.

CONCLUSIONS

The rat kidney plays a crucial role in the regulation of concentrations of dimethylarginines, as both ADMA and SDMA were eliminated from the systemic circulation in substantial amounts. Furthermore, evidence for the role of endotoxaemia in the metabolism of dimethylarginines was obtained as plasma levels of ADMA were significantly lower in endotoxaemic rats.

Asymmetric dimethylarginine: a cardiovascular risk factor in renal disease?

Endothelial dysfunction due to reduced availability of nitric oxide (NO) is an early step in the course of atherosclerotic vascular disease. NO is synthesized from the amino acid L-arginine by the action of the NO synthase (NOS), which can be blocked by endogenous inhibitors such as asymmetric dimethylarginine (ADMA). In laboratory animals, administration of ADMA significantly reduces NO generation, and causes an increase of blood pressure and renal vascular resistance. In clinical studies, a strong correlation between increased ADMA blood levels and impaired endothelial-dependent vasodilatation, and cardiovascular morbidity and mortality has been documented in different populations, including in patients with renal disease. Thus, ADMA seems to be the culprit, and not just an innocent biochemical bystander, of the atherosclerotic disease process. Moreover, reduced NO availability is involved in the progression of renal disease, and increased ADMA blood levels may contribute to this process. Interventions that lower ADMA blood levels in renal patients could, therefore, modulate their atherogenic profile and interfere with progression of renal failure.

Asymmetric dimethylarginine, blood pressure, and renal perfusion in elderly subjects

BACKGROUND

Reduced availability of nitric oxide (NO) is thought to contribute to the age-associated increase of renovascular tone and blood pressure. We assessed blood concentrations of the endogenous NO synthase inhibitor asymmetric dimethylarginine (ADMA) as well as renal hemodynamics, comparing young (n=24, 13 men, 25+/-1 years) and elderly (n=24, 13 men, 69+/-2 years) healthy subjects and elderly subjects with essential hypertension (n=24, 13 men, 70+/-2 years).

METHODS AND RESULTS

Plasma ADMA concentration and renovascular resistance (RVR) were significantly higher (P<0.05) and effective renal plasma flow (ERPF) significantly lower (P<0.05) in elderly (2.77+/-0.20 micromol/L, 125+/-10 mm Hg/mL per minute, 487+/-26 mL/min per 1.73 m2) than in young healthy subjects (1.30+/-0.11, 77+/-3, 654+/-18). Both ADMA levels and RVR were higher and ERPF lower in the hypertensive elderly subjects (3.53+/-0.23, 163+/-11, 427+/-19; P<0.05 versus both groups). In contrast, plasma concentrations of the biologically inactive stereoisomer symmetric dimethylarginine, l-arginine, and homocysteine were similar in the 3 groups studied. In the logistic regression analysis only ADMA was an independent determinant of both ERPF (P<0.001; r2=0.80) and RVR (P<0.002; r2=0.86). In addition, ADMA (P<0.002) and serum glucose (P<0.036) were independently related (r2=0.67) to the level of blood pressure.

CONCLUSIONS

These results are compatible with the notion that accumulation of the endogenous NO synthase inhibitor ADMA in senescent individuals is involved in the decrease of renal perfusion and increase of blood pressure.

Male gender increases sensitivity to renal injury in response to cholesterol loading

Males are at greater risk for renal injury than females. This may relate to nitric oxide (NO) availability, because female rats have higher renal endothelial NO synthase (NOS) levels. Previously, our laboratory found susceptibility to proteinuria induced by NOS inhibition in male compared with female rats. Dyslipidemia and hypercholesterolemia dose dependently decreased renal NOS activity and caused renal injury in female rats. We hypothesized that exposure of male rats to hypercholesterolemia would lead to more renal injury in male than in female rats due to an a priori lower renal NO system. Female and male rats were fed no, low-dose, or high-dose cholesterol for 24 wk. Cholesterol feeding dose dependently increased proteinuria in both female and male rats, but male rats developed more proteinuria at similar plasma cholesterol (P < 0.001). Control males had lower renal NOS activity than control females (4.44 +/- 0.18 vs. 7.46 +/- 0.37 pmol. min(-1). mg protein(-1); P < 0.05), and cholesterol feeding decreased renal NOS activity in males and in females (P < 0.05). Cholesterol-fed males developed significantly more vascular, glomerular, and tubulointerstitial monocyte/macrophage influx and injury than females. Thus under baseline conditions, male rats have lower renal NOS activity than female rats. This may explain why male rats are more sensitive to renal injury by factors that decrease NO availability, such as hypercholesterolemia.

Cardiovascular and renal control in NOS-deficient mouse models

Nitric oxide (NO) plays an essential role in the maintenance of cardiovascular and renal homeostasis. Endogenous NO is produced by three different NO synthase (NOS) isoforms: endothelial NOS (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS). To investigate which NOS is responsible for NO production in different tissues, NOS knockout (-/-) mice have been generated for the three isoforms. This review focuses on the regulation of cardiovascular and renal function in relation to blood pressure homeostasis in the different NOS-/- mice. Although regulation of vascular tone and cardiac function in eNOS-/- has been extensively studied, far less is known about renal function in these mice. eNOS-/- mice are hypertensive, but the mechanism responsible for their high blood pressure is still not clear. Less is known about cardiovascular and renal control in nNOS-/- mice, probably because their blood pressure is normal. Recent data suggest that nNOS plays important roles in cardiac function, renal homeostasis, and regulation of vascular tone under certain conditions, but these are only now beginning to be studied. Inasmuch as iNOS is absent from the cardiovascular system under physiological conditions, it may become important to blood pressure regulation only during pathological conditions related to inflammatory processes. However, iNOS is constitutively expressed in the kidney, where its function is largely unknown. Overall, the study of NOS knockout mice has been very useful and produced many answers, but it has also raised new questions. The appearance of compensatory mechanisms suggests the importance of the different isoforms to specific processes, but it also complicates interpretation of the data. In addition, deletion of a single gene may have physiologically significant effects in addition to those being studied. Thus the presence or absence of a specific phenotype may not reflect the most important physiological function of the absent gene.

Adrenomedullin overexpression to inhibit cuff-induced arterial intimal formation

Adrenomedullin (AM) inhibits vascular smooth muscle cell proliferation stimulated by fetal calf serum and platelet-derived growth factor in vitro. In this study, an adenovirus expressing AM (AxCAAM) was created to examine the in vivo action of AM. Femoral arteries of Wistar rats were wrapped with a silicone cuff and treated with adenovirus expressing Escherichia coli beta-galactosidase (AxCALacZ) or AxCAAM. Immunoreactivity for endothelial nitric oxide synthase (eNOS) was reduced in the endothelium of cuff-injured arteries and was associated with increased local DNA synthesis. Consequently, the intimal formation measured by the intimal-to-medial ratio was significantly increased at 14 and 28 days after the cuff placement. AxCAAM-infected arteries increased the expression of eNOS in the endothelium and inducible NOS in the media and the adventitia. AxCAAM significantly decreased the intimal-to-medial ratio by 40% at 14 days and 51% at 28 days, whereas AxCALacZ showed no changes compared with cuff-injured control arteries. AM overexpression effectively limits intimal hyperplasia by reducing cell proliferation through a nitric oxide-dependent pathway of eNOS. Our findings suggest the possibility of a therapeutic use of the AM gene for the prevention of vascular proliferative disorders.

Accumulated endogenous NOS inhibitors, decreased NOS activity, and impaired cavernosal relaxation with ischemia

We examined whether endogenous inhibitors of nitric oxide (NO) synthesis are involved in the impaired cavernosal relaxation with ischemia in rabbits. Two weeks after cavernosal ischemia caused by partial vessel occlusion, endothelium-dependent and electrical field stimulation (EFS)-induced neurogenic NO-mediated relaxations, but not sodium nitroprusside (SNP)-induced relaxation, were significantly impaired in the isolated corpus cavernosum. The Ca(2+)-dependent NO synthase (NOS) activity and the basal and stimulated cGMP productions with carbachol or EFS were significantly decreased after ischemia. Supplementation of excess L-arginine partially recovered both of the impaired relaxations. The contents of N(G)-monomethyl-L-arginine (L-NMMA) and asymmetric N(G), N(G)-dimethyl-L-arginine (ADMA) but not L-arginine and symmetric N(G),N'(G)-dimethyl-L-arginine (SDMA) were increased in the cavernosal tissues after ischemia. Authentic L-NMMA and ADMA but not SDMA concentration dependently inhibited both relaxations without affecting the relaxation produced by SNP in the control. Excess L-arginine abolished the inhibition with L-NMMA and ADMA. These results suggest that the impaired NO-mediated cavernosal relaxations after ischemia are closely related to the decreased NOS activity and the increased accumulation of L-NMMA and ADMA.

[Biological and pathophysiological roles of endogenous methylarginines as inhibitors of nitric oxide synthase]

Protein arginine N-methyltransferases (PRMTs) catalyse the methylation of guanidinonitrogen(s) of arginine to produce NG-monomethyl-L-arginine (L-NMMA), asymmetric NG,NG-dimethyl-L-arginine (ADMA) and symmetric NG,NG-dimethyl-L-arginine (SDMA), which are subsequently released into the cytoplasm following proteolysis. Free intracellular L-NMMA and ADMA, but not SDMA, are inhibitors of all three isoforms of nitric oxide synthases (nNOS, eNOS and iNOS). L-NMMA and ADMA, but not SDMA, are actively metabolized by dimethylarginine dimethylaminohydrolase (DDAH) to L-citrulline and methylamine (and dimethylamine). Free methylarginines are detectable in cell cytosol, plasma and tissues. Elevated ADMA has been detected in the plasma of patients or experimental animals with hypercholesterolemia, renal failure, atherosclerosis, hypertension, thrombotic microangiopathy, peripheral arterial occlusive disease and in the regenerated endothelial cells after angioplasty. Moreover, in the non-cardiovascular field, ADMA was increased in the urethral tissue following ischemia and in the plasma of patients with schizophrenia and multiple sclerosis. Altered biosynthesis of NO has been implicated in the pathogenesis of these diseases, and it is possible to consider that the accumulation of endogenous L-NMMA and ADMA underlies the impaired NO generation and increased O2- production. We described herein the biosynthesis, transmembrane transport, metabolic pathway and possible pathophysiological roles of endogenous methylarginines.

Marked increase of asymmetric dimethylarginine in patients with incipient primary chronic renal disease

In patients with uremia, increased blood concentrations of the endogenous nitric oxide synthase inhibitor asymmetric dimethylarginine (ADMA) have been linked to the severity of atherosclerosis and to excess cardiovascular mortality. The ADMA levels and several traditional cardiovascular risk factors were assessed in 44 untreated nonsmoking patients with confirmed primary chronic renal disease at different stages of renal disease. True GFR was assessed by means of the inulin-clearance technique. For comparison, nonsmoking subjects matched with respect to age, gender, and body-mass index were examined. Mean plasma ADMA concentration was markedly higher (P < 0.0001) in all patients combined (4.2 +/- 0.9 micromol/L) than in control subjects (n = 16; age 45 +/- 10 yr; serum creatinine 1.0 +/- 0.1 mg/dl; ADMA 1.4 +/- 0.7 micromol/L). However, mean ADMA levels were similar in patients with normal renal function (n = 16; age 41 +/- 9 yr; serum creatinine 1.1 +/- 0.1 mg/dl; GFR 120 +/- 14 ml x min(-1) x 1.73 m2; ADMA 4.0 +/- 0.7 micromol/L), in patients with moderate renal failure (n = 15; 47 +/- 7 yr; 1.8 +/- 0.3 mg/dl; 65 +/- 10 ml x min(-1) x 1.73 m2; 3.8 +/- 0.6 micromol/L) and in patients with advanced renal failure (n = 13; 46 +/- 9 yr; 4.2 +/- 0.9 mg/dl; 25 +/- 4 ml x min(-1) x 1.73 m2; 4.7 +/- 1.2 micromol/L). Furthermore, ADMA levels were increased to the same extent in normotensive (n = 17; 4.0 +/- 0.8 micromol/L) and in hypertensive (n = 27; 4.2 +/- 0.9 micromol/L) patients. In contrast to ADMA, mean total plasma homocysteine concentration were similar in control subjects (10.6 +/- 2.9 micromol/L) and in patients with normal GFR (11.0 +/- 2.9 micromol/L), but were significantly higher in patients with moderate renal failure (17.7 +/- 4.1 micromol/L) and particularly in patients with advanced renal failure (28.2 +/- 10.6 micromol/L). Finally, mean total serum cholesterol concentrations were comparable in the control group and in the three groups of patients with renal disease. In contrast to several traditional cardiovascular risk factors, markedly increased blood concentrations of ADMA, a putative biochemical marker of atherosclerosis, are present even in nonsmoking patients without diabetes with incipient primary renal disease. Thus, the early increase of ADMA levels may be of relevance for the excess cardiovascular morbidity and mortality due to arterio- and atherosclerotic complications in patients with renal disease.

Antiproliferative effect of nitric oxide on rat glomerular mesangial cells via inhibition of mitogen-activated protein kinase

The effect of nitric oxide (NO) donors and lipopolysaccharide (LPS) on the proliferation of rat glomerular mesangial cells was characterized. Exogenous application of a NO donor inhibited serum-induced proliferation in a time- and dose-dependent manner. S-Nitrosoglutathione (GSNO) also increased cGMP generation and arachidonic acid release, but it did not cause any measurable increase in the cytosolic Ca2+ concentration. Chelation of cytosolic Ca2+ or inhibition of mitogen-activated protein kinase (MAPK) kinase had an inhibitory effect on proliferation, but neither enhanced the antiproliferative effect of GSNO. In contrast, inhibition of guanylate cyclase or phospholipase A2 had no effect on proliferation, but partially reversed GSNO-induced antiproliferation by approximately 98 and 65%, respectively. GSNO did not cause cell death. Incubation of cells with LPS induced endogenous NO generation and had an antiproliferative effect. LPS-induced antiproliferation was reversed completely by inhibition of nitric oxide synthase and partially by inhibition of guanylate cyclase or phospholipase A2. GSNO or LPS inhibited serum-induced MAPK activation, and both effects were partially reversed by inhibition of guanylate cyclase or phospholipase A2. Inclusion of 8-bromo-cGMP or arachidonic acid in the growth medium resulted in a similar antiproliferative effect. In conclusion, in rat glomerular mesangial cells, MAPK inhibition and an antiproliferative effect could be induced by either an increase in the cellular concentration of NO or exposure of the cells to LPS. Part of the effect of NO was attributable to the increased cellular cGMP generation and arachidonic acid release.

Structural and functional characterization of the Zn(II) site in dimethylargininase-1 (DDAH-1) from bovine brain. Zn(II) release activates DDAH-1

L-N(omega),N(omega)-dimethylarginine dimethylaminohydrolase-1 (DDAH-1) is a Zn(II)-containing enzyme that, through hydrolysis of side-chain methylated l-arginines, regulates the activity of nitric-oxide synthase. Herein we report the structural and functional properties of the Zn(II)-binding site in DDAH-1 from bovine brain. Activity measurements of the native and metal-free enzyme have revealed that the endogenously bound Zn(II) inhibits the enzyme. Native DDAH-1 could be fully or partially activated using various concentrations of phosphate, imidazole, histidine, and histamine, a process that is paralleled by the release of Zn(II). The slow activation of the enzyme by the bulky complexing agents EDTA and 1,10-phenantroline suggests that the Zn(II)-binding site is partially buried in the protein structure. The apparent Zn(II)-dissociation constant of 4.2 nm, determined by 19F NMR using the chelator 5F-BAPTA (1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid), lies in the range of intracellular free Zn(II) concentrations. These results suggest a regulatory role for the Zn(II)-binding site. The coordination environment of the Zn(II) in DDAH-1 has been examined by Zn K-edge x-ray absorption spectroscopy. The extended x-ray absorption fine structure observed is consistent with Zn(II) being coordinated by 2 S and 2 N (or O) atoms. The biological implications of these findings are discussed.

Plasmin-dependent and -independent effects of plasminogen activators and inhibitor-1 on ex vivo angiogenesis

Plasminogen activator (PA) inhibitor-1 (PAI-1) has been recognized as a surrogate marker of endothelial dysfunction in diseases associated with impaired angiogenesis, including atherosclerosis, diabetic vasculopathy, and nephropathy. To establish the necessary and sufficient components of the PA system [PAI-1, urokinase-type PA (uPA), or tissue-type PA (tPA), and plasminogen (Plg)] for angiogenesis, we examined angiogenic competence of vascular explant cultures obtained from mice deficient in PAI-1, tPA, uPA, and Plg. To gain insight into the requirement for different matrix-degrading systems during endothelial cell migration across plasmin-degradable basement membranes compared with profibrotic areas containing plasmin-nondegradable collagen, we contrasted vascular sprouting in collagen with Matrigel lattices. PAI-1(-/-) vessels showed an increased capillary sprouting in both collagen and Matrigel. Deficiency of uPA significantly reduced the rate of sprouting, whereas tPA(-/-) vessels showed a profound inhibition of capillary sprouting. The Plg(-/-) vessels failed to sprout, a defect that was restored not only by exogenous Plg, but also by the addition of PAs; a nonproteolytic effect of tPA was observed in Matrigel. Zymography revealed no differences in the activity of metalloproteinase (MMP)-2 and -9 in wild-type and PAI-1(-/-) vessels, but demonstrated reduced MMP-9 activity in all angiogenesis-deficient vessels. In summary, 1) PAI-1 by itself is a modest inhibitor of endothelial sprouting, 2) tPA and Plg are indispensable for angiogenesis in this model, 3) Plg is not the only substrate for PAs, and 4) the activity of MMP-9 is undetectable in explant cultures from tPA and Plg knockout mice.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.

Phosphorylation of neurofibromin by cAMP-dependent protein kinase is regulated via a cellular association of N(G),N(G)-dimethylarginine dimethylaminohydrolase

The neurofibromatosis type 1 (NF1) tumor suppressor (neurofibromin) is thought to play crucial roles in cellular Ras- and cAMP-dependent kinase (PKA)-associated signals. In this study, we identified a cellular neurofibromin-associating protein, N(G),N(G)-dimethylarginine dimethylaminohydrolase (DDAH) that is known as a cellular NO/NOS regulator. The interaction of DDAH was mainly directed to the C-terminal domain (CTD) and to the cysteine/serine-rich domain (CSRD) of neurofibromin, coinciding with the regions containing specific PKA phosphorylation sites. DDAH increased PKA phosphorylation of native neurofibromin in a dose-dependent manner, especially affecting the phosphorylation of CSRD. These findings suggest that the PKA accessibility of neurofibromin was regulated via DDAH interaction, and this regulation may modulate the cellular function of neurofibromin that is implicated in NF1-related pathogenesis.

Circulating endothelial nitric oxide synthase inhibitory factor in some patients with chronic renal disease

BACKGROUND

Chronic renal disease (CRD) is associated with hypertension and reduced synthesis of nitric oxide (NO). Here, we investigated whether there is a circulating endothelial NO synthase (eNOS) inhibitory factor(s) in some patients with CRD that might directly influence endothelial NOS.

METHODS

Human dermal microvascular endothelial cells (HDMECs) were incubated for six hours with 20% plasma from subjects with normal renal function (PCr = 0.8 +/- 0.2 mg%), and patients with moderate renal insufficiency of various causes (PCr = 4.0 +/- 1.5 mg%) and impact on NOS activity, transport of L-arginine, and abundance of eNOS protein were measured. Plasma concentrations of asymmetric and symmetric dimethyl L-arginine (ADMA and SDMA) were also measured.

RESULTS

There was no effect of any human plasma on L-arginine transport. The NOS activity was variable in CRD patients and fell into two subgroups: CRD I, individual values similar to control, and CRD II, individual values lower than control mean. The effect of CRD plasma on NOS activity in cultured cells was not related to the primary disease, but was predicted by plasma ADMA levels since plasma ADMA was elevated in CRD II versus both control and CRD I. Blood urea nitrogen and creatinine levels were uniformly elevated in CRD plasma. The abundance of eNOS protein was unaffected by plasma.

CONCLUSION

High plasma levels of ADMA in CRD patients are independent of reduced renal clearance, suggesting an alteration in ADMA synthesis and/or degradation. High ADMA is a marker and is partly responsible for the inhibition of eNOS activity in cultured cells and may also result in reduced eNOS activity in vivo, with consequent hypertension.

Serum concentrations of asymmetric (ADMA) and symmetric (SDMA) dimethylarginine in renal failure patients

BACKGROUND

Nitric oxide (NO) synthesis is inhibited by the ADMA that accumulates in the plasma of patients with renal failure; however, the concentration of SDMA also is enhanced. Therefore, it has been hypothesized that ADMA and SDMA may contribute to hypertension in these patients.

METHODS

We measured the concentrations of ADMA, SDMA and 21 endogenous amino acids in 257 persons by high pressure liquid chromatography (HPLC).

RESULTS

The plasma concentrations of both ADMA and SDMA were significantly elevated in patients with chronic renal failure (CRF). The increase was more pronounced for SDMA (2.05 +/- 0.1 micromol/L vs. 0.5 +/- 0.04 micromol/L), whereas it was only moderate for ADMA (0.85 +/- 0.03 micromol/L vs. 0.73 +/- 0.06 micromol/L). In dialysis patients, the concentrations were further increased (ADMA, 1.05 +/- 0.04 micromol/L; SDMA, 2.68 +/- 0.13 micromol/L). After kidney transplantation, the concentration of SDMA returned to the baseline value (1.15 +/- 0.11 micromol/L), but that of ADMA remained enhanced (0.99 +/- 0.06 micromol/L).

CONCLUSIONS

In CRF, especially the concentration of SDMA is significantly increased. Not only ADMA, but also SDMA are likely to be responsible for hypertension. Competition for reabsorption between SDMA and arginine within the kidney has to be considered for the interpretation of changes in the ratio between dimethylarginines and arginine in renal failure. Hemodialysis is not suitable for a long-lasting removal of methylarginines. Whether the administration of arginine could have promising effects on hypertension and complications of CRF needs to be studied in prospective trials.

Inhibition of nitric oxide synthase activity by early and advanced glycation end products in cultured rabbit proximal tubular epithelial cells

Nitric oxide (NO) is important in the regulation of renal tubular function. We have investigated whether glycated proteins could impair the NO production by examining the effects of Amadori products (AP-BSA) and advanced glycation end products (AGE-BSA) on primary cultures of rabbit proximal tubular epithelial (PTE) cells. Nitric oxide synthase activity was assessed by measurement of the conversion of L-arginine to L-citrulline and by production of NO, after short-term (30 min) or long-term (1 or 3 days) incubation. Short incubations of PTE cells with either 200 microg/ml AP-BSA or 40 microg/ml AGE-BSA significantly decreased NO production. AP-BSA (3000 microg/ml) inhibited the Ca(2+)-dependent NOS activity even though above 50 microg/ml it increased Ca(2+)-independent NOS activity. In contrast, 40 microg/ml AGE-BSA inhibited both isoforms of NOS. Longer incubations with 200 microg/ml AP-BSA or 250 microg/ml AGE-BSA decreased NO release and inhibited Ca(2+)-dependent and -independent NOS activities. APs did not affect NO release by S-nitroso-N-acetyl-penicillamine (SNAP), while 250 microg/ml AGEs decreased it. After 3 days incubation, glycation products had no effect on the NOS cell content. Cell viability and proliferation were not modified under these experimental conditions, suggesting that the fall in NO production was not due to there being fewer cells. These data indicate that APs and AGEs directly inhibit NOS activity, and additionally that AGEs quench released NO. Thus, both types of glycated proteins alter the production of NO by PTE cells and could participate in the renal tubule dysfunction associated with aging and diabetes.

Renal expression of constitutive NOS and DDAH: separate effects of salt intake and angiotensin

BACKGROUND

Nitric oxide (NO) is generated from NO synthase (NOS) isoforms. These enzymes can be inhibited by asymmetric dimethylarginine, which is inactivated by N(G)-N(G)-dimethylarginine dimethylaminohydrolase (DDAH). The neuroneal (nNOS) type I and endothelial (eNOS) type III constitutive NOS isoforms are expressed predominantly in the macula densa and microvascular endothelium of the renal cortex, respectively. DDAH is expressed at sites of NOS expression. Since NO may coordinate the renal responses to angiotensin II (Ang II) and changes in salt intake, we tested the hypothesis that salt intake regulates the expression of nNOS, eNOS and DDAH by Ang II acting on type 1 (AT(1)) receptors.

METHODS

Groups (N = 6) of rats were adapted to low-salt (LS) or high-salt (HS) intakes for 10 days. Other groups of LS and HS rats received the AT(1) receptor antagonist losartan for six days (to test the effects of salt independent of AT(1) receptors). A further group of HS rats received an infusion of Ang II for six days (to test the effect of Ang II independent of salt intake).

RESULTS

Compared with HS rats, there was a significant (P < 0.05) increase in LS rats of nNOS protein in kidney and immunohistochemical expression in the macula densa, and of eNOS protein expression and immunohistochemical expression in the microvascular endothelium, and of DDAH protein expression. Losartan prevented these effects of salt on the expression of eNOS or DDAH, both of which were also increased by Ang II infusions in HS rats. In contrast, losartan did not prevent the effects of salt on nNOS expression, which was unresponsive to Ang II infusion. The generation of NO(2)(-) released by slices of renal cortex, in the presence of saturating concentrations of L-arginine, was increased by LS, compared to HS, independent of losartan and by Ang II during HS.

CONCLUSION

The expressions of eNOS in cortical microvascular endothelium and DDAH in kidney are enhanced by Ang II acting on AT(1) receptors. The expression of nNOS in the macula densa is enhanced by salt restriction independent of Ang II or AT(1) receptors.

Localization of inward rectifier potassium channel Kir7.1 in the basolateral membrane of distal nephron and collecting duct

Inward rectifier potassium channels (Kir) play an important role in the K(+) secretion from the kidney. Recently, a new subfamily of Kir, Kir7.1, has been cloned and shown to be present in the kidney as well as in the brain, choroid plexus, thyroid, and intestine. Its cellular and subcellular localization was examined along the renal tubule. Western blot from the kidney cortex showed a single band for Kir7.1 at 52 kD, which was also observed in microdissected segments from the thick ascending limb of Henle, distal convoluted tubule (DCT), connecting tubule, and cortical and medullary collecting ducts. Kir7.1 immunoreactivity was detected predominantly in the DCT, connecting tubule, and cortical collecting duct, with lesser expression in the thick ascending limb of Henle and in the medullary collecting duct. Kir7.1 was detected by electron microscopic immunocytochemistry on the basolateral membrane of the DCT and the principal cells of cortical collecting duct, but neither type A nor type B intercalated cells were stained. The message levels and immunoreactivity were decreased under low-K diet and reversed by low-K diet supplemented with 4% KCl. By the double-labeling immunogold method, both Kir7.1 and Na(+), K(+)-ATPase were independently located on the basolateral membrane. In conclusion, the novel Kir7.1 potassium channel is located predominantly in the basolateral membrane of the distal nephron and collecting duct where it could function together with Na(+), K(+)-ATPase and contribute to cell ion homeostasis and tubular K(+) secretion.

Defective fluid and HCO(3)(-) absorption in proximal tubule of neuronal nitric oxide synthase-knockout mice

Using renal clearance techniques and in situ microperfusion of proximal tubules, we examined the effects of N(G)-monomethyl-L-arginine methyl ester (L-NAME) on fluid and HCO(3)(-) transport in wild-type mice and also investigated proximal tubule transport in neuronal nitric oxide synthase (nNOS)-knockout mice. In wild-type mice, administration of L-NAME (3 mg/kg bolus iv) significantly increased mean blood pressure, urine volume, and urinary Na(+) excretion. L-NAME, given by intravenous bolus and added to the luminal perfusion solution, decreased absorption of fluid (60%) and HCO(3)(-) (49%) in the proximal tubule. In nNOS-knockout mice, the urinary excretion of HCO(3)(-) was significantly higher than in the wild-type mice (3.12 +/- 0.52 vs. 1. 40 +/- 0.33 mM) and the rates of HCO(3)(-) and fluid absorption were 62 and 72% lower, respectively. Both arterial blood HCO(3)(-) concentration (20.7 vs. 25.7 mM) and blood pH (7.27 vs. 7.34) were lower, indicating a significant metabolic acidosis in nNOS-knockout mice. Blood pressure was lower in nNOS-knockout mice (76.2 +/- 4.6 mmHg) than in wild-type control animals (102.9 +/- 8.4 mmHg); however, it increased in response to L-NAME (125.5 +/- 5.07 mmHg). Plasma Na(+) and K(+) were not significantly different from control values. Our data show that a large component of HCO(3)(-) and fluid absorption in the proximal tubule is controlled by nNOS. Mice without this isozyme are defective in absorption of fluid and HCO(3)(-) in the proximal tubule and develop metabolic acidosis, suggesting that nNOS plays an important role in the regulation of acid-base balance.

Assembly of multiple dystrobrevin-containing complexes in the kidney

Dystrophin is the key component in the assembly and maintenance of the dystrophin-associated protein complex (DPC) in skeletal muscle. In kidney, dystroglycan, an integral component of the DPC, is involved in kidney epithelial morphogenesis, suggesting that the DPC is important in linking the extracellular matrix to the internal cytoskeleton of kidney epithelia. Here, we have investigated the molecular architecture of dystrophin-like protein complexes in kidneys from normal and dystrophin-deficient mice. Using isoform-specific antibodies, we show that the different cell types that make up the kidney maintain different dystrophin-like complexes. These complexes can be broadly grouped according to their dystrobrevin content: beta-dystrobrevin containing complexes are present at the basal region of renal epithelial cells, whilst alpha-dystrobrevin-1 containing complexes are found in endothelial and smooth muscle cells. Furthermore, these complexes are maintained even in the absence of all dystrophin isoforms. Thus our data suggest that the functions and assembly of the dystrophin-like complexes in kidney differ from those in skeletal muscle and implicate a protein other than dystrophin as the primary molecule in the assembly and maintenance of kidney complexes. Our findings also provide a possible explanation for the lack of kidney pathology in Duchenne muscular dystrophy patients and mice lacking all dystrophin isoforms.

Constitutive nitric oxide synthesis in the kidney--functions at the juxtaglomerular apparatus

Tubulo-vascular information transfer at the renal juxtaglomerular apparatus (JGA) serves to adjust the biosynthesis and release of renin, the key enzyme of the renin angiotensin system, and to regulate glomerular arteriolar muscle tone. The macula densa serves as a sensor of tubular NaCl. Concentration-dependent salt uptake through the Na-K-2Cl cotransporter located in the apical membrane of macula densa cells triggers a signal transduction cascade that involves the synthesis of nitric oxide (NO) through a type 1 NO synthase (NOS1) which is described with respect to its complex mRNA structure and regulatory aspects. The anatomical and functional targets of the NO-soluble guanylyl cyclase-cGMP pathway at the JGA are reviewed.

Nitric oxide dysfunction in the pathophysiology of preeclampsia

Researchers disagree as to the importance of nitric oxide (NO) in preeclampsia. Many researchers have alluded to NO's possible primary or secondary role in the development of preeclampsia, but few have correlated the dysfunction of nitric oxide production with the other metabolic derangements seen in this condition. This paper will review the evidence that the primary dysfunction in preeclampsia is a relative deficiency of available NO (secondary to oxidative degradation) and an excess of peroxynitrite (ONOO(-)). The combination of a deficiency of NO and an increase in ONOO(-) can directly or indirectly initiate the vast majority of physiological and serological changes associated with preeclampsia, such as blood pressure, increased glomerular filtration rate, proteinuria, platelet dysfunction, increased thromboxane and endothelin, and a decrease in prostacyclin. Understanding the complex role of nitric oxide in this condition may explain why previous interventions have been unsuccessful and suggest possible strategies for prevention and treatment in the future.

Nitric oxide synthase in the JGA of the SHR: expression and role in tubuloglomerular feedback

The spontaneously hypertensive rat (SHR) has an enhanced tubuloglomerular feedback (TGF) and a diminished buffering by juxtaglomerular apparatus (JGA)-derived NO. We examined the hypothesis that these effects are due to decreases in nitric oxide synthase (NOS) expression or limited availability of L-arginine or tetrahydrobiopterin (BH(4)). SHR had significantly (P < 0.05) greater mRNA abundance (by RT-PCR) or protein (by Western analysis) for neuronal NOS (nNOS, or type I) and endothelial cell NOS (ecNOS, or type III) in renal cortex or isolated glomeruli, respectively. There was prominent expression of ecNOS in glomerular endothelium and nNOS in macula densa. Maximal TGF responses, assessed from changes in proximal stop-flow pressure during orthograde loop of Henle (LH) perfusion, were greater in SHR [Wistar-Kyoto (WKY), 8.1 +/- 0.3 (n = 46) vs. SHR, 10.3 +/- 0.3 mmHg (n = 57); P < 0.001]. Unlike WKY, TGF responses of SHR were unresponsive to microperfusion of the nNOS inhibitor, 7-nitroindazole (7-NI, 10(-4) M) [WKY, 9.5 +/- 0.5 to 13.2 +/- 0.7 (n = 13, P < 0.001) vs. SHR, 11.8 +/- 0.7 to 12.5 +/- 0.6 mmHg (n = 19, not significant)], or to L-arginine (10(-3) M) [WKY, 7.7 +/- 0.8 to 6.3 +/- 0.4 (n = 10, P < 0.05) vs. SHR, 10.4 +/- 0.7 to 10.6 +/- 0.7 mmHg (n = 10, not significant)]. Neither BH(4) (10(-4) M) nor sepiapterin (10(-4) M), its stable precursor, modified TGF responses in WKY or in SHR, nor did they restore a response to microperfusion of 7-NI in SHR. In conclusion, there is a diminished role for NO from nNOS in blunting of TGF in SHR which cannot be ascribed to limited NOS expression or availability of substrate or BH(4).