Effect of nitric oxide synthase inhibition on renal hemodynamics in humans: reversal by L-arginine

The restorative effect of apocynin and catalase in l-arginine induced hypotension on normotensive subjects – the role of oxidative stress

L-arginine is a substrate for nitric oxide synthase (NOS) responsible for the production of NO. This investigation studied the effect of apocynin, an NADPH oxidase inhibitor and catalase, an H2O2 scavenger on L-arginine induced oxidative stress and hypotension. Forty Wistar-Kyoto rats were treated for 14 days with vehicle, L-arginine (12.5mg/ml p.o.), L-arginine+apocynin (2.5mmol/L p.o.), L-arginine+catalase (10000U/kg/day i.p.) and L-arginine plus apocynin+catalase respectively. Weekly renal functional and hemodynamic parameters were measured and kidneys harvested at the end of the study for histopathological and renal NADPH oxidase 4 (Nox4) assessments. L-arginine administration in normotensive rats decreased systolic blood pressure (120±2 vs 91±2mmHg) and heart rate (298±21 vs 254±15b/min), enhanced urinary output (21.5±4.2 vs 32±1.9ml/24h , increased creatinine clearance (1.72±0.56 vs 2.62±0.40ml/min/kg), and fractional sodium excretion (0.88±0.16 vs 1.18±0.16 %), caused proteinuria (28.10±1.93 vs 35.26±1.69mg/kg/day) and a significant decrease in renal cortical blood perfusion (292±3 vs 258±5bpu) and pulse wave velocity (3.72±0.20 vs 2.84±0.13m/s) (all P<0.05). L-arginine increased plasma malondialdehyde (by ~206 % P<0.05) and NO (by ~51 %, P<0.05) but decreased superoxide dismutase (by ~31 %, P<0.05) and total antioxidant capacity (by ~35 %, P<0.05) compared to control. Renal Nox4 mRNA activity was approximately 2.1 fold higher (P<0.05) in the L-arginine treated rats but was normalized by apocynin and apocynin plus catalase treatment. Administration of apocynin and catalase, but not catalase alone to rats fed L-arginine, restored the deranged renal function and structure, prevented hypotension and enhanced the antioxidant capacity and suppressed Nox4 expression. These findings suggest that apocynin and catalase might be used prophylactically in states of oxidative stress.

Endothelin-1/nitric oxide balance and HOMA index in children with excess weight and hypertension: a pathophysiological model of hypertension

The aim of this study was to investigate the relationship between endothelin-1, nitric oxide, insulin resistance, and blood pressure in young subjects with a high prevalence of excess weight and/or elevated blood pressure. In a cohort of 238 children (mean age = 11.1 years), height, weight, waist circumference, and blood pressure were assessed. Body mass index, waist-to-height ratio, and blood pressure percentiles were calculated, and the children were classified as having excess weight and elevated blood pressure according to the International Obesity Task Force and the US blood pressure nomograms specific for gender, age and height, respectively. Endothelin-1 and nitric oxide production were assessed, and the homeostatic model assessment index was calculated. Forty-three percent of children were male, 71% had excess weight, and 37% had systolic and/or diastolic values above the ninetieth percentile. Plasma endothelin-1 and nitric oxide production were independently correlated (p < 0.05). In multivariate analyses, the HOMA index was associated with systolic and diastolic blood pressure (p = 0.01), and nitric oxide was independently related to diastolic blood pressure (p = 0.04), even after adjustment for measures of body composition. By using the waist-to-height ratio instead of BMI in the statistical model, the association between the homeostatic model assessment index and blood pressure was attenuated, while the results remained similar for nitric oxide. No correlation was found between endothelin-1 and blood pressure. In our study population, the correlation between nitric oxide and blood pressure and the lack of a relationship between endothelin-1 and blood pressure could be explained by an increase in the vasodilator effect of local and systemic nitric oxide, which counteracts the possible hypertensive effect of endothelin-1.

Poor glycemic control is related to increased nitric oxide activity within the renal circulation of patients with type 2 diabetes

OBJECTIVE

Experimental studies have shown that glucose releases endothelial nitric oxide (NO) and that NO contributes to renal hyperperfusion in models of diabetes. To examine whether this translates into the human condition, we studied the relationship between glycemic control and renal NO activity in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

A total of 113 patients with type 2 diabetes and a wide range of HbA1c concentrations were included. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were determined by constant infusion input clearance. Functional NO activity in the renal circulation was determined as change of RPF to infusion of the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA) (4.25 mg/kg). As additional markers, we measured urinary excretion of NO (UNOx) and L-arginine-to-asymmetrical dimethylarginine (ADMA) ratio in plasma.

RESULTS

Subjects within the highest tertile of HbA1c concentration had increased RPF (low, medium, and high tertiles 576 ± 17 vs. 585 ± 22 vs. 627 ± 33 mL/min/m(2), P = 0.05 by one-way ANOVA), while GFR was similar across tertiles. The response of RPF to NOS blockade was augmented in subjects with higher HbA1c levels (-55 ± 7 vs. -64 ± 8 vs. -86 ± 8 mL/min, P = 0.04 by one-way ANOVA). Further, L-arginine-to-ADMA ratio and UNOx were increased in subjects with higher HbA1c levels.

CONCLUSIONS

In line with experimental evidence, we could demonstrate in humans that poor glycemic control is related to higher NO activity and hyperperfusion of the kidney. The renal NO system may thus be a novel therapeutic target for improving renal hemodynamics in patients with diabetes.

Heme arginate improves reperfusion patterns after ischemia: a randomized, placebo-controlled trial in healthy male subjects

BACKGROUND

Heme arginate can induce heme oxygenase-1 to protect tissue against ischemia-reperfusion injury. Blood oxygen level dependent (BOLD) functional magnetic resonance imaging measures changes in tissue oxygenation with a high spatial and temporal resolution. BOLD imaging was applied to test the effect of heme arginate on experimental ischemia reperfusion injury in the calf muscles.

METHODS

A two period, controlled, observer blinded, crossover trial was performed in 12 healthy male subjects. Heme arginate (1 mg/kg body weight) or placebo were infused 24 h prior to a 20 min leg ischemia induced by a thigh cuff. 3 Tesla BOLD-imaging of the calf was performed and signal time courses from soleus, gastrocnemius and tibialis anterior muscle were available from 11 participants for technical reasons.

RESULTS

Peak reactive hyperemia signal of the musculature was significantly increased and occurred earlier after heme arginate compared to placebo (106.2 ± 0.6% at 175 ± 16s vs. 104.5 ± 0.6% at 221 ± 19s; p = 0.025 for peak reperfusion and p = 0.012 for time to peak).

CONCLUSIONS

A single high dose of heme arginate improves reperfusion patterns during ischemia reperfusion injury in humans. BOLD sensitive, functional MRI is applicable for the assessment of experimental ischemia reperfusion injury in skeletal muscle.

The interaction between endothelin-1 and nitric oxide in the vasculature: new perspectives

Nitric oxide (NO) and endothelin-1 (ET-1) are natural counterparts in vascular function, and it is becoming increasingly clear that an imbalance between these two mediators is a characteristic of endothelial dysfunction and is important in the progression of vascular disease. Here, we review classical and more recent data that suggest that ET-1 should be regarded as an essential component of NO signaling. In particular, we review evidence of the role of ET-1 in models of acute and chronic NO synthase blockade. Furthermore, we discuss the possible mechanisms by which NO modulates ET-1 activity. On the basis of these studies, we suggest that NO tonically inhibits ET-1 function, and in conditions of diminished NO bioavailability, the deleterious effects of unmitigated ET-1 actions result in vasoconstriction and eventually lead to vascular remodeling and dysfunction.

Effect of nitric oxide synthase inhibition on intrarenal oxygenation as evaluated by blood oxygenation level-dependent magnetic resonance imaging

OBJECTIVE

To investigate the feasibility of studying renal effects of nitric oxide synthase inhibition (NOSi) in humans by blood oxygenation level-dependent (BOLD) MRI. Nitric oxide (NO) is known to play a key role in the pathophysiology of hypertension and previous reports suggest reduced bioavailability of NO in the kidneys of hypertensive rats and hence show reduced response to NOSi using BOLD MRI. Ability to perform similar studies in humans could potentially lead to detection of early changes before development of symptoms, and to monitor novel interventions targeted toward improved NO bioavailability. The specific goals for this study were: (1) to examine whether lower doses and dose rate of administration of NOSi such as those previously used in humans can be detected by BOLD MRI in rat kidneys, (2) to compare changes in R2* to direct measures of renal medullary oxygen levels and blood flow using invasive probes (OxyLite/OxyFlo), and (3) to examine for the first time the effect of NOSi on intrarenal oxygenation in humans.

MATERIAL AND METHODS

In rat kidneys, acute changes in renal tissue oxygenation induced by different doses (2, 4, and 10 mg/kg) of N-nitro-L-arginine methyl ester were studied in 36 Sprague Dawley rats, which were equally divided into BOLD MRI and OxyLite/OxyFlo groups. Similarly in humans, acute changes in renal oxygenation were induced by 2 different NOS inhibitors NG-monomethyl-L-arginine (4.25 mg/kg) in 7 volunteers and N-nitro-L-arginine methyl ester (2 mg/kg and 4 mg/kg) in 6 healthy young volunteers. A multiple gradient echo sequence was used in both rats (TE = 4.4-57.8 milliseconds with 3.6 milliseconds interecho spacing) and humans (TE = 6.4-40.8 milliseconds with a 2.3 milliseconds interecho spacing) to acquire 16 T2*-weighted images. R2* maps were constructed by fitting a single exponential decay to the image data on pixel by pixel basis. R2* measurements in the cortex and medulla were performed by regions of interest analysis. Measurements were performed before and during infusion of NOSi.

RESULTS

In rats, NOSi decreased medullary pO2 and blood flow in a dose-dependent manner, and BOLD MRI showed an increase in medullary R2* consistent with the invasive pO2 measurements. In humans, BOLD MRI similarly showed an increase in medullary and cortical R2* after NOSi in a dose-dependent manner. In both rats and humans, the R2* values fell back toward baseline before the end of the infusion period.

CONCLUSION

Comparison of BOLD MRI measurements with those using invasive probes suggests that changes in blood flow are at least partly responsible for observed changes with BOLD MRI. Monitoring changes after NOSi by renal BOLD MRI in vivo in human kidneys are feasible, and preliminary findings are consistent with observations in rat kidneys. Future studies are warranted to fully understand the apparent reversal in R2* changes during the infusion of NOSi.

Resistance to renal damage by chronic nitric oxide synthase inhibition in the Wistar-Furth rat

Chronic nitric oxide synthase inhibition (NOSI) causes chronic kidney disease (CKD) in the Sprague Dawley (SD) rat. We previously showed that the Wistar-Furth (WF) rats are resistant to several models of CKD and maintain renal nitric oxide (NO) production compared with SD rats, whereas low-dose NOSI caused progression of CKD in WF rats. Here, we evaluate the impact of high-dose chronic NOSI in WF and SD rats, as well as intrarenal responses to an acute pressor dose of NOSI in the normal WF. Rats were given N(G)-nitro-l-arginine methyl ester (l-NAME) (150 and 300 mg/l for 6-10 wk) in the drinking water after an initial bolus tail vein injection. Both strains showed significant reductions in total NO production with chronic l-NAME. SD given 150 mg/l l-NAME for 6 wk developed proteinuria and renal injury, whereas WF rats receiving 150 mg/l l-NAME for 6-10 wk or 300 mg/l for 6 wk developed no proteinuria and minimal renal injury. Blood pressure was significantly elevated with chronic NOSI in both strains but was higher in the SD rat. There was little impact on renal nitric oxide synthase expression with l-NAME, except that cortical endothelial nitric oxide synthase abundance increased in WF after 6 wk (150 mg/l). Micropuncture experiments with acute pressor NOSI resulted in similar increases in systemic blood pressure in SD and WF rats, whereas WF rats showed a much smaller increment in glomerular blood pressure compared with SD rats. In conclusion, WF rats do not develop renal injury after chronic NOSI at, or above, a dose that causes significant injury in the SD rat. This protection may be associated with protection from glomerular hypertension.

Nitric oxide regulates retinal vascular tone in humans

The purpose of the present study was to investigate the contribution of basal nitric oxide (NO) on retinal vascular tone in humans. In addition, we set out to elucidate the role of NO in flicker-induced retinal vasodilation in humans. Twelve healthy young subjects were studied in a three-way crossover design. Subjects received an intravenous infusion of either placebo or NG-monomethyl-L-arginine (L-NMMA; 3 or 6 mg/kg over 5 min), an inhibitor of NO synthase. Thereafter, diffuse luminance flicker was consecutively performed for 16, 32, and 64 s at a frequency of 8 Hz. The effect of L-NMMA on retinal arterial and venous diameter was assessed under resting conditions and during the hyperemic flicker response. Retinal vessel diameter was measured with a Zeiss retinal vessel analyzer. L-NMMA significantly reduced arterial diameter (3 mg/kg: -2%; 6 mg/kg: -4%, P < 0.001) and venous diameter (3 mg/kg: -5%; 6 mg/kg: -8%, P < 0.001). After placebo infusion, flicker induced a significant increase in retinal vessel diameter (P < 0.001). At a flicker duration of 64 s, arterial diameter increased by 4% and venous diameter increased by 3%. L-NMMA did not abolish these hyperemic responses but blunted venous vasodilation (P = 0.017) and arterial vasodilation (P = 0.02) in response to flicker stimulation. Our data indicate that NO contributes to basal retinal vascular tone in humans. In addition, NO appears to play a role in flicker-induced vasodilation of the human retinal vasculature.

Vasodilator effects of L-arginine are stereospecific and augmented by insulin in humans

The amino acid l-arginine, the precursor of nitric oxide (NO) synthesis, induces vasodilation in vivo, but the mechanism behind this effect is unclear. There is, however, some evidence to assume that the l-arginine membrane transport capacity is dependent on insulin plasma levels. We hypothesized that vasodilator effects of l-arginine may be dependent on insulin plasma levels. Accordingly, we performed two randomized, double-blind crossover studies in healthy male subjects. In protocol 1 (n = 15), subjects received an infusion of insulin (6 mU x kg(-1) x min(-1) for 120 min) or placebo and, during the last 30 min, l-arginine or d-arginine (1 g/min for 30 min) x In protocol 2 (n = 8), subjects received l-arginine in stepwise increasing doses in the presence (1.5 mU x kg(-1) x min(-1)) or absence of insulin. Renal plasma flow and glomerular filtration rate were assessed by the para-aminohippurate and inulin plasma clearance methods, respectively. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation, and mean flow velocity in the ophthalmic artery was measured with Doppler sonography. l-arginine, but not d-arginine, significantly increased renal and ocular hemodynamic parameters. Coinfusion of l-arginine with insulin caused a dose-dependent leftward shift of the vasodilator effect of l-arginine. This stereospecific renal and ocular vasodilator potency of l-arginine is enhanced by insulin, which may result from facilitated l-arginine membrane transport, enhanced intracellular NO formation, or increased NO bioavailability.

Association of nitric oxide production and apoptosis in a model of experimental nephropathy

BACKGROUND

In recent studies increased amounts of nitric oxide (NO) and apoptosis have been implicated in various pathological conditions in the kidney. We have studied the role of NO and its association with apoptosis in an experimental model of nephrotic syndrome induced by a single injection of adriamycin (ADR).

METHODS

The alteration in the NO pathway was assessed by measuring nitrite levels in serum/urine and by evaluating the changes in vascular reactivity of the isolated perfused rat kidney (IPRK) system. Rats were stratified into control groups and ADR-induced nephropathy groups. These two groups were then divided into: group 1, animals receiving saline; and group 2, animals receiving aminoguanidine (AG) which is a specific inhibitor of inducible-NO synthase. On day 21, rats were sacrificed after obtaining material for biochemical analysis.

RESULTS

Histopathological examination of the kidneys of rats treated with ADR revealed focal areas of mesangial proliferation and mild tubulointerstitial inflammation. They also had significantly higher levels of proteinuria compared with control and treatment groups (P < 0.05). Urine nitrite levels were significantly increased in the ADR-nephropathy group (P < 0.05). In the IPRK phenylephrine and acetylcholine related responses were significantly impaired in the ADR-nephropathy group. Apoptosis was not detected in controls. However, in the ADR-nephropathy group, numerous apoptotic cells were identified in the tubulointerstitial areas. Double staining revealed numerous interstitial apoptotic cells to stain for ED1, a marker for monocytes/macrophages. Treatment with AG prevented the impairment of renal vascular bed responses and reduced both urine nitrite levels and apoptosis to control levels.

CONCLUSION

We suggest that interactions between NO and apoptosis are important in the pathogenesis of the ADR-induced nephrosis.

Pharmacokinetic-pharmacodynamic profile of systemic nitric oxide-synthase inhibition with L-NMMA in humans

AIMS

It has been demonstrated that inhibition of endothelium derived nitric oxide with NG-monomethyl-L-arginine (L-NMMA) results in a different cardiac and peripheral vascular response. The purpose of this study was to investigate the pharmacokinetic-pharmacodynamic profile of L-NMMA and pharmacokinetic interactions with L-arginine in healthy subjects.

METHODS

Plasma pharmacokinetics were analysed from two different studies: In study 1, 3 mg kg-1 L-NMMA was administered i.v. over 5 min and systemic haemodynamics, cardiac output (CO), fundus pulsation amplitude (FPA), and NO-exhalation (exhNO) were measured at baseline and 15, 65, 95, 155, and 305 min after start of drug administration (n=7). In study 2, 17 mg kg-1 min-1 of the physiologic substrate for nitric oxide synthase, L-arginine, was coinfused i.v. over 30 min with a primed constant infusion of 50 microg kg-1 min-1 L-NMMA (n=8).

RESULTS

Bolus infusion of L-NMMA resulted in a maximum plasma concentration of 12. 9+/-3.4 microg ml-1 (mean+/-s.d.) with elimination half-life of 63. 5+/-14.5 min and clearance of 12.2+/-3.5 ml min-1 kg-1 and caused a small hypertensive response, decreased CO by 13%, FPA by 26%, exhNO by 46% and increased systemic vascular resistance by 16% (P<0.05 each) 15 min after start of drug administration. Although only limited data points were available in the L-NMMA plasma concentration range between 0 and 4 microg ml-1, drug effects over time were in good agreement with an Emax model (r2>0.98 each), which also suggested that concentrations producing half-maximum effects were higher for FPA than for CO and exhNO. The coinfusion with L-arginine caused a nearly two-fold increase in plasma L-NMMA levels, indicating a pharmacokinetic interaction.

CONCLUSIONS

In the absence of a systemic hypertensive response, L-NMMA significantly decreased CO, exhNO, and FPA. The concentration calculated to produce a half maximal effect was equivalent for exhNO and CO, but markedly higher for FPA. Furthermore, measurement of FPA is susceptible to changes in L-NMMA levels at small plasma concentrations.

Renal preservation in cardiac surgery

There is no conclusive evidence that any pharmacological intervention is able to offer effective protection for the kidneys during cardiac surgery. More research is needed into the underlying mechanisms of postoperative renal failure, specifically with regard to the possible role played by endothelial factors and inflammatory response.

Exogenous L-arginine does not affect angiotensin II-induced renal vasoconstriction in man

AIMS

It has been suggested that provision of the substrate of nitric oxide (NO) synthesis, L-arginine, might influence the effects of renal vasoconstrictors. We have therefore studied the effects of pretreatment or concomitant administration of L-arginine on angiotensin II (ANG II)-increased renovascular resistance.

METHODS

The study was conducted in a double-blind, randomized, cross-over design. Eight healthy subjects were assigned to placebo or a continuous intravenous coinfusion of ANG II (5.0 ng kg[-1] min[-1], infusion period 75 min) with L-arginine (17 mg kg[-1] min[-1], infusion period 30 min). Nine further subjects received a continuous infusion of ANG II with or without pretreatment of L-arginine. Changes in renal plasma flow (RPF) were estimated by the steady state clearance of PAH.

RESULTS

L-arginine alone increased RPF to 110 +/- 10% over baseline (P < 0.003). The ANG II-induced decrease in RPF was not affected by pretreatment or coinfusion of L-arginine.

CONCLUSIONS

Our results demonstrate that a counterregulatory response of the renal vasculature to high levels of ANG II does not depend on exogenous L-arginine. In healthy subjects, this lack of functional antagonism at the renal vasculature is therefore not a result of NO substrate availability.

Exhaled NO during graded changes in inhaled oxygen in man

BACKGROUND

Nitric oxide (NO) is present in the exhaled air of animals and humans. In isolated animal lungs the amount of exhaled NO is decreased during hypoxia. A study was undertaken to determine whether changes in arterial oxygen tension affect levels of exhaled NO in humans.

METHODS

Sixteen healthy subjects were randomised to inhale different gas mixtures of oxygen and nitrogen in a double blind crossover study. Eight gas mixtures of oxygen and nitrogen (fractional inspired oxygen concentration (FiO2) 0.1 to 1.0) were administered. Exhaled NO was measured with a chemiluminescence detector from end expiratory single breath exhalation.

RESULTS

A dose-dependent change in exhaled NO during graded oxygen breathing was observed (p = 0.0012). The mean (SE) exhaled NO concentration was 31 (3) ppb at baseline, 39 (4) ppb at an FiO2 of 1.0, and 26 (3) ppb at an FiO2 of 0.1.

CONCLUSIONS

The NO concentration in exhaled air in healthy humans is dependent on oxygen tension. Hyperoxia increases the level of exhaled NO, which indicates increased NO production. The mechanism behind this phenomenon remains to be elucidated.