Oxidative stress in Dahl salt-sensitive hypertension

Oxidative stress mediates the stimulation of sympathetic nerve activity in the phenol renal injury model of hypertension

Renal injury caused by the injection of phenol in the lower pole of one kidney increases blood pressure (BP), norepinephrine secretion from the posterior hypothalamic nuclei (PH), and renal sympathetic nerve activity in the rat. Renal denervation prevents these effects of phenol. We have also demonstrated that noradrenergic traffic in the brain is modulated by NO and interleukin-1beta. In this study, we tested the hypothesis that the increase in sympathetic nervous system (SNS) activity in the phenol renal injury model is because of activation of reactive oxygen species. To this end, first we examined the abundance of several components of reduced nicotinamide-adenine dinucleotide phosphate oxidase (identified as the major source of reactive oxygen species), including gp91phox/Nox2, p22phox, p47phox, and Nox3 using real-time PCR. Second, we evaluated the effects of 2 superoxide dismutase mimetic, tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl), and superoxide dismutase-polyethylene glycol on central and peripheral SNS activation caused by intrarenal phenol injection. Intrarenal injection of phenol raised BP, NE secretion from the PH, renal sympathetic nerve activity, and the abundance of reduced nicotinamide-adenine dinucleotide phosphate and reduced the abundance of interleukin-1beta and neural-NO synthase mRNA in the PH, paraventricular nuclei, and locus coeruleus compared with control rats. When tempol or superoxide dismutase-polyethylene glycol were infused in the lateral ventricle before phenol, the effects of phenol on BP and SNS activity were abolished. The studies suggest that central activation of the SNS in the phenol-renal injury model is mediated by increased reactive oxygen species in brain nuclei involved in the noradrenergic control of BP.

NADPH Oxidase in the Renal Medulla Causes Oxidative Stress and Contributes to Salt-Sensitive Hypertension in Dahl S Rats

Dahl salt-sensitive (SS) rats exhibit increased renal medullary oxidative stress and blood pressure salt-sensitivity compared with consomic, salt-resistant SS-13BN rats, despite highly similar genetic backgrounds. The present study examined potential sources of renal medullary superoxide in prehypertensive SS rats fed a 0.4% NaCl diet by assessing activity and protein levels of superoxide producing and scavenging enzymes. Superoxide production was nearly doubled in SS rats compared with SS-13BN rats as determined by urinary 8-isoprostane excretion and renal medullary oxy-ethidium microdialysate levels. Medullary superoxide production in tissue homogenates was greater in SS rats, and the NADPH oxidase inhibitor diphenylene iodonium preferentially reduced SS levels to those found in SS-13BN rats. Dinitrophenol, a mitochondrial uncoupler, eliminated the remaining superoxide production in both strains, whereas inhibition of xanthine oxidase, NO synthase, and cycloxygenase had no effect. L-arginine, NO synthase, superoxide dismutase, catalase, and glutathione peroxidase activities between SS and SS-13BN rats did not differ. Chronic blood pressure responses to a 4% NaCl diet were then determined in the presence or absence of the NADPH oxidase inhibitor apocynin (3.5 microg/kg per minute), chronically delivered directly into the renal medulla. Apocynin infusion reduced renal medullary interstitial superoxide from 1059+/-130 to 422+/-80 (oxyethidium fluorescence units) and mean arterial pressure from 175+/-4 to 157+/-6 mm Hg in SS rats, whereas no effects on either were observed in the SS-13(BN). We conclude that excess renal medullary superoxide production in SS rats contributes to salt-induced hypertension, and NADPH oxidase is the major source of the excess superoxide.

Renoprotective effect of long-term combined treatment with adrenomedullin and omapatrilat in hypertensive rats

BACKGROUND

Previous studies demonstrated that adrenomedullin (AM) is metabolized by neutral endopeptidases and that the renal effect of AM is augmented by the inhibition of neutral endopeptidases. We have recently shown that the long-term administration of AM has renoprotective effects.

OBJECT

This study assessed the chronic renoprotective effects of AM combined with a vasopeptidase inhibitor in hypertensive rats and attempted to elucidate the mechanism involved.

METHODS

We studied the following four groups: control Dahl salt-resistant (DR) rats, untreated Dahl salt-sensitive (DS) rats, omapatrilat (35 mg/kg per day)-treated DS rats; and human AM (500 ng/h) plus omapatrilat-treated DS rats. After 7 weeks' treatment, blood pressure, renal function, neurohumoral factors, gene expression levels, and histological findings were examined.

RESULTS

DS rats were characterized by increased blood pressure, decreased renal function, abnormal histological findings, and increased gene expression of collagen I and III, transforming growth factor beta (TGF-beta), and NADPH oxidase subunits (p40phox, p47phox, and gp91phox) in the renal cortex compared with DR rats. Compared with DS rats, omapatrilat significantly decreased systolic blood pressure (-26 mmHg), improved renal function, histological findings, and messenger RNA expression levels of collagen I, collagen III, and TGF-beta. Combined treatment with omapatrilat and AM further improved renal function, histological findings, and mRNA expression levels of collagen I, collagen III, and TGF-beta, without a further reduction in blood pressure. Only combined treatment decreased mRNA levels of p40phox, p47phox, and gp91phox. There were no differences in plasma AM or atrial natriuretic peptide levels among three DS groups.

CONCLUSION

Our results suggest that combined treatment with omapatrilat and AM provides additional renoprotective effects independent of blood pressure-lowering activity partly via inhibition of gene expressions of oxidative stress and extracellular matrix.

Molecular mechanisms and therapeutic strategies of chronic renal injury: physiological role of angiotensin II-induced oxidative stress in renal medulla

Renal medullary circulation has now been found to play a fundamental role in regulating long-term blood pressure control and fluid balance. Elevation of superoxide or reduction of nitric oxide (NO) in renal medulla decreases medullary blood flow and Na excretion, resulting in sustained hypertension. Angiotensin II (Ang II)-induced interaction of superoxide and NO was determined in thin tissue strips isolated from the renal outer medullary region of Sprague-Dawley rats using fluorescent microscopy techniques. Ang II can induce diffusion of NO, but not superoxide, from the medullary thick ascending limb (mTAL) to the surrounded vasa recta. However, when NO is reduced by the NO scavenger carboxy-PTIO, Ang II can induce superoxide diffusion from mTAL to vasa recta pericytes. Therefore, the physiological action of oxidative stress in renal medullary region is demonstrated as balance of superoxide and NO diffusion ("tubulo-vascular cross-talk"). These results explain how chronically hypoxic medulla can maintain blood flow. In other studies using chronically instrumented rats, we found that nearly 70% of Ang II-induced medullary renal injury was dependent on pressure determined by servo-control of renal perfusion pressure, whereas 30% of the injury was non-hemodynamic. We conclude that oxidative stress within the renal medulla can induce hypertension and also make the kidney functionally more vulnerable to the effects of Ang II.

Hydrogen peroxide causes uncoupling of dopamine D1-like receptors from G proteins via a mechanism involving protein kinase C and G-protein-coupled receptor kinase 2

Dopamine, via activation of D1-like receptors, inhibits Na,K-ATPase and Na,H-exchanger in renal proximal tubules and promotes sodium excretion. This effect of dopamine is not seen in conditions associated with oxidative stress such as hypertension, diabetes, and aging due to uncoupling of D1-like receptors from G proteins. To identify the role of oxidative stress in uncoupling of the D1-like receptors, we utilized primary cultures from rat renal proximal tubules. Hydrogen peroxide (H2O2), an oxidant, treatment to the cell cultures increased the level of malondialdehyde, a marker of oxidative damage. Further, H2O2 decreased membranous D1-like receptor numbers and proteins, D1-like agonist (SKF 38393)-mediated [35S]GTPgammaS binding and SKF 38393-mediated inhibition of Na,K-ATPase. Moreover, H2O2 treatment to the cultures caused membranous translocation of G-protein-coupled receptor kinase 2 (GRK 2) and increased serine phosphorylation of D1A receptors accompanied by an increase in protein kinase C (PKC) activity. Interestingly, PKC inhibitors blocked the H2O2-mediated stimulation of GRK 2 and serine phosphorylation of D1A receptors. Further, GRK 2 antisense but not scrambled oligonucleotides attenuated the effect of H2O2 on membranous expression of GRK 2. Moreover, direct activation of PKC with phorbol ester (PMA) resulted in reduction of SKF 38393-mediated [35S]GTPgammaS binding. We conclude that H2O2 stimulates PKC leading to the activation of GRK 2, which causes serine phopshorylation of D1A receptors and receptor G-protein uncoupling in these cells, resulting in impairment in D1-like receptor function.

Genetic variations of tubular sodium reabsorption leading to “primary” hypertension: from gene polymorphism to clinical symptoms

The definition of the most appropriate strategy to demonstrate causation of a given genetic-molecular mechanism in a complex multifactorial polygenic disease like hypertension is hampered by the underestimation of the complexity arising from the genetic and environmental interactions. To disentangle this complexity, we developed a strategy based on six steps: 1) isolation of a rodent model of hypertension (Milan hypertensive strain and Milan normotensive strain) that shares some pathophysiological abnormalities with human primary hypertension; 2) definition in the model of the sequence of events linking these abnormalities to a genetic molecular mechanism; 3) determination of the polymorphism of the three adducin genes discovered in the model both in rats and in humans; 4) comparison at biochemical and physiological levels between the rodent models and the hypertensive carriers of the “mutated” gene variants; 5) evaluation of the impact of the adducin genes in hypertension and its organ complications with association and linkage studies in humans, also considering the genetic and environmental interactions; and 6) development of a pharmacogenomic approach aimed at establishing the therapeutic benefit of a drug interfering with the sequence of events triggered by adducin and their effect's size. The bulk of data obtained demonstrates the importance of a multidisciplinary approach considering a variety of genetic and environmental interactions. Adducin functions within the cells as a heterodimer composed of a combination of three subunits. Each of these subunits is coded by genes mapping to different chromosomes. Therefore, the interaction among these genes, taken together with the interactions with other modulatory genes or with the environment, is indispensable to establish the adducin clinical impact. The hypothesis that adducin polymorphism favors the development of hypertension via an increased tubular sodium reabsorption is well supported by a series of consistent experimental and clinical data. Many mechanistic aspects, underlying the link between these genes and clinical symptoms, need to be clarified. The clinical effect size of adducin must be established also with the contribution of pharmacogenomics with a drug that selectively interferes with the sequence of events triggered by the mutated adducin.

NO-independent mechanism mediates tempol-induced renal vasodilation in SHR

We investigated whether tempol, a superoxide dismutase mimetic, affected renal hemodynamics and arterial pressure in spontaneously hypertensive rats (SHR) and Sprague-Dawley (SD) rats. We also examined whether tempol affected exaggerated renal vasoconstrictor responses to ANG II in SHR. To test whether the effects of tempol were due to a restored NO system, we used the NOS inhibitor Nw-nitro-l-arginine methyl ester (l-NAME). Renal blood flow (RBF) and mean arterial pressure (MAP) were measured in vivo by electromagnetic flowmetry and arterial catheterization in 10- to 12-wk-old anesthetized SHR and SD rats. Systolic arterial pressure (SAP) was measured in conscious rats using the tail cuff method. Tempol (1 mM) was given in the drinking water to SD (SD-T) and SHR (SHR-T) for 5–7 days for RBF measurements and for 15 days for SAP measurements. Age-matched SD (SD-C) and SHR (SHR-C) were used as controls. ANG II (1–4 ng) was administered as a bolus via a renal artery catheter. l-NAME was administered intravenously for 15–20 min. Renal vascular resistance (RVR) was elevated in SHR-C compared with SD-C. In SHR-T, baseline RVR was not different from SD-C and SD-T rats. Tempol had no effect on RVR in SD. l-NAME elevated RVR to the same extent in all four groups. Arterial pressure was not affected by tempol. The RVR responses to ANG II were higher in SHR-C than in the SD-C group. ANG II responses were not different between SHR-T and SD-T. Overall, tempol reduced the renovascular responses to ANG II in SHR. l-NAME elevated the effects of ANG II in SD-C rats but had no effect on the ANG II responses in the other groups. Thus l-NAME treatment did not influence tempol’s effects on baseline RVR or ANG II responses. We conclude that in SHR, tempol has a significant renal vasodilator effect and that it normalizes the increased renovascular ANG II sensitivity. As the effects of l-NAME are not greater in SHR-T rats, it is not likely that the elevated renal resistance and ANG II sensitivity in SHR are due to reactive oxygen species-induced quenching of nitric oxide.

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.

Activation of Vascular BK Channel by Tempol in DOCA-Salt Hypertensive Rats

Large-conductance Ca2+-activated potassium (BK) channels modulate vascular smooth muscle tone. Tempol, a superoxide dismutase (SOD) mimetic, lowers blood pressure and inhibits sympathetic nerve activity in normotensive and hypertensive rats. In the present study, we tested the hypotheses depressor responses caused by tempol are partly mediated by vasodilation. It was found that tempol, but not tiron (a superoxide scavenger), dose-dependently relaxed mesenteric arteries (MA) in anesthetized sham and deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Tempol also reduced perfusion pressure in isolated, norepinephrine (NE) preconstricted MA from sham and DOCA-salt hypertensive rats. Maximal responses in DOCA-salt rats were twice as large as those in sham rats. The vasodilation caused by tempol was blocked by iberiotoxin (IBTX, BK channel antagonist, 0.1 micromol/L) and tetraethylammonium chloride (TEA) (1 mmol/L). Tempol did not relax KCl preconstricted arteries in sham or DOCA-salt rats, and Nomega-nitro-L-arginine methyl ester (L-NAME), apamin, or glibenclamide did not alter tempol-induced vasodilation. IBTX constricted MA and this response was larger in DOCA-salt compared with sham rats. Western blots and immunohistochemical analysis revealed increased expression of BK channel alpha subunit protein in DOCA-salt arteries compared with sham arteries. Whole-cell patch clamp studies revealed that tempol enhanced BK channel currents in HEK-293 cells transiently transfected with mslo, the murine BK channel a subunit. These currents were blocked by IBTX. The data indicate that tempol activates BK channels and this effect contributes to depressor responses caused by tempol. Upregulation of the BK channel alpha subunit contributes to the enhanced depressor response caused by tempol in DOCA-salt hypertension.

Kidney immune cell infiltration and oxidative stress contribute to prenatally programmed hypertension

BACKGROUND

Prenatal environment has been shown to modify adult blood pressure profile, but the underlying mechanisms are not well understood. The role of renal immune cell infiltration, oxidative stress, and nitric oxide bioavailability in the pathogenesis was investigated.

METHODS

Adult hypertension in rat offspring was induced by maternal low protein diet. Oxidative stress was determined by quantitative immunoblotting for nitrotyrosine, and T-cell and macrophage content by immunostaining, in offspring kidneys before and after the onset of hypertension. Nitric oxide metabolites (NOx) were measured in 24-hour urines. A group of offspring was treated with the immunosuppressive drug mycophenolate mofetil (MMF) to reduce inflammation, or with the superoxide dismutase mimetic Tempol to reduce oxidative stress, for a 3-week period before the onset of hypertension.

RESULTS

During the prehypertensive stage, at 4 weeks of age, the low protein diet pups exhibited an increase in kidney nitrotyrosine content and in number of immune cells, both of which persisted in untreated animals after hypertension was established, at 8 weeks of age. Urine NOx was increased at 4 weeks and unchanged at 8 weeks of age. Both MMF and Tempol treatment prevented the immune cell infiltration, the increase in kidney nitrotyrosine abundance, and the development of hypertension. The effect on blood pressure persisted throughout the 4- to 10-week observation period after discontinuation of the treatments.

CONCLUSION

Renal oxidative stress and infiltrating immune cells may play a pathogenetic role in prenatally programmed hypertension. Nitric oxide bioavailability does not appear impaired.

Interactive effects of superoxide anion and nitric oxide on blood pressure and renal hemodynamics in transgenic rats with inducible malignant hypertension

Superoxide anion contributes to the pathogenesis of various forms of hypertension, but its role in the development of malignant hypertension remains unclear. The present study was performed to determine the influence of superoxide anion on blood pressure and renal hemodynamics in transgenic rats with inducible malignant hypertension [strain name: TGR(Cyp1a1Ren2)]. Malignant hypertension was induced in male Cyp1a1-Ren2 rats ( n = 6) through dietary administration of the aryl hydrocarbon, indole-3-carbinol (0.3%) for 7–9 days. Mean arterial pressure (MAP) and renal hemodynamics were measured in pentobarbital sodium-anesthetized Cyp1a1-Ren2 rats before and during intravenous infusion of the superoxide dismutase mimetic tempol (100 μmol/h). Basal MAP and renal vascular resistance (RVR) were elevated in rats induced with indole-3-carbinol compared with noninduced rats ( n = 5) (184 ± 4 vs. 127 ± 3 mmHg, P < 0.01, and 29 ± 2 vs. 21 ± 1 mmHg·ml−1·min·g, P < 0.01, respectively). Hypertensive rats had elevated excretion of urinary 8-isoprostane compared with normotensive rats (41 ± 4 vs. 13 ± 6 pg·min−1·g−1, P < 0.01). There were no differences in renal plasma flow and glomerular filtration rate between groups. Systemic administration of tempol decreased MAP (184 ± 4 to 151 ± 4 mmHg, P < 0.01) and RVR (29 ± 2 to 25 ± 2 mmHg·ml−1·min·g, P < 0.05) in hypertensive but not in normotensive Cyp1a1-Ren2 rats. In addition, tempol administration decreased urinary excretion of 8-isoprostane (41 ± 4 to 25 ± 4 pg·min−1·g−1, P < 0.05). Renal plasma flow and glomerular filtration rate remained unaltered during tempol administration in both groups. The administration of the nitric oxide synthase inhibitor nitro-l-arginine attenuated the decrease in MAP and RVR in response to tempol. These findings indicate that superoxide anion contributes to the elevated RVR and increased arterial blood pressure, by a mechanism that is at least in part nitric oxide dependent, in Cyp1a1-Ren2 rats with malignant hypertension.

Antihypertensive response to prolonged tempol in the spontaneously hypertensive rat

INTRODUCTION

Tempol is a permeant nitroxide superoxide dismutase (SOD) mimetic that lowers mean arterial pressure (MAP) in spontaneously hypertensive rats (SHRs). We investigated the hypothesis that the antihypertensive response entails a negative salt balance, blunting of plasma renin activity (PRA), endothelin-1 (ET-1), or catecholamines or correction of oxidative stress as indexed by 8-isoprostane prostaglandin F(2alpha) (PGF(2alpha)) (8-Iso).

METHODS

Groups (N= 6 to 8) of SHRs were infused for 2 weeks with vehicle or tempol (200 nmol/kg/min) or given tempol (2 mmol/L) in drinking water.

RESULTS

Tempol infusion reduced the MAP of anesthetized SHRs (150 +/- 5 vs. 126 +/- 6 mm Hg) (P < 0.005). Oral tempol did not change the heart rate but reduced the MAP of conscious SHRs (-23 +/- 6 mm Hg) (P < 0.01) but not Wistar-Kyoto (WKY) rats. Tempol infusion increased the PRA (2.2 +/- 0.2 vs. 5.0 +/- 0.9 ng/mL/hour) (P < 0.005), did not change excretion of nitric oxide (NO) [NO(2)+ NO(3) (NOx)], ET-1, or catecholamines but reduced excretion of 8-Iso (13.2 +/- 1.4 vs. 9.6 +/- 0.9 ng/24 hours; P < 0.01). Cumulative Na(+) balance and gain in body weight were unaltered by tempol infusion. Tempol prevented a rise in MAP with high salt intake.

CONCLUSION

Tempol corrects hypertension without a compensatory sympathoadrenal activation or salt retention. The response is independent of nitric oxide, endothelin, or catecholamines and occurs despite increased PRA. It is accompanied by a reduction in oxidative stress and is maintained during increased salt intake.

Antihypertensive and cognitive effects of grape polyphenols in estrogen-depleted, female, spontaneously hypertensive rats

Both endogenous and dietary estrogens reduce hypertension and enhance cognitive abilities in estrogen-depleted female spontaneously hypertensive rats (SHR). Many of the beneficial effects of estrogens/phytoestrogens also appear to be provided by other polyphenols (e.g., proanthocyanidins) in grape seed, which lack appreciable estrogenic receptor binding. The present study tested the hypothesis that similar to phytoestrogens, proanthrocyanidins in grape seed polyphenols reduce salt-sensitive hypertension in young, estrogen-depleted SHR. SHR were ovariectomized at 4 wk of age and placed on phytoestrogen-free diets with or without 0.5% grape seed extract added and with high (8.0%) or basal (0.6%) NaCl. After 10 wk on the diets, grape proanthrocyanidin supplementation significantly reduced arterial pressure in the rats fed the basal (10 mmHg) and high (26 mmHg)-NaCl diet, compared with the nonsupplemented controls. In vitro superoxide production was significantly reduced (23%) by the grape seed polyphenols. Spatial learning (8-arm-radial maze) in the SHR on the basal NaCl diets was improved by dietary grape seed polyphenols. These results indicate that grape seed polyphenols decrease arterial pressure in SHR, probably via an antioxidant mechanism.

Oxidative stress mediates angiotensin II-dependent stimulation of sympathetic nerve activity

Evidence indicates that angiotensin II (Ang II) enhances sympathetic nervous system (SNS) activity centrally and peripherally, but the exact mechanisms of this activation are not well established. We have previously shown that infusion of Ang II in the lateral cerebral ventricle raises blood pressure (BP), renal sympathetic nervous system activity (RSNA), and norepinephrine (NE) secretion from the posterior hypothalamic nuclei (PH), and reduces the abundance of interleukin-1beta (IL-1beta) and neuronal NO synthase (nNOS) mRNA in the PH. Pretreatment with an Ang II type 1 (AT1) receptor antagonist abolished these effects of Ang II. The data support the hypothesis that Ang II stimulates SNS through activation of AT1 receptors and downregulation of nNOS. In the current studies, we tested the hypothesis that the effects of Ang II on central SNS are mediated by reactive oxygen species. To this end, we evaluated the effects of Ang II alone or in combination with 2 superoxide dismutase (SOD) mimetics, tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl) and polyethylene glycol-SOD (PEG-SOD) on BP, NE secretion from the PH, RSNA, and abundance of IL-1beta and nNOS mRNA in the PH Ang II raised BP, NE secretion from the PH, and RSNA and reduced the abundance of IL-1beta and nNOS mRNA in the PH. Tempol and PEG-SOD completely abolished these actions of Ang II. In conclusion, these studies support the hypothesis that the effects of centrally administered Ang II on the SNS are mediated by increased oxidative stress in brain regions involved in the noradrenergic control of BP.

Effect of renal medullary H2O2 on salt-induced hypertension and renal injury

Dahl salt-sensitive (SS) and consomic, salt-resistant SS-13(BN) rats possess substantial differences in blood pressure salt-sensitivity even with highly similar genetic backgrounds. The present study examined whether increased oxidative stress, particularly H2O2, in the renal medulla of SS rats contributes to these differences. Blood pressure was measured using femoral arterial catheters in three groups of rats: 1) 12-wk-old SS and consomic SS-13(BN) rats fed a 0.4% NaCl diet, 2) SS rats fed a 4% NaCl diet and chronically infused with saline or catalase (6.9 microg x kg(-1) x min(-1)) directly into the renal medulla, and 3) SS-13(BN) fed high salt (4%) and infused with saline or H2O2 (347 nmol x kg(-1) x min(-1)) into the renal medullary interstitium. After chronic blood pressure measurements, renal medullary interstitial H2O2 concentration ([H2O2]) was collected by microdialysis and analyzed with Amplex red. Blood pressure and [H2O2] were both significantly higher in SS (126 +/- 3 mmHg and 145 +/- 17 nM, respectively) vs. SS-13(BN) rats (116 +/- 2 mmHg and 56 +/- 14 nM) fed a 0.4% diet. Renal interstitial catalase infusion significantly decreased [H2O2] (96 +/- 41 vs. 297 +/- 52 nM) and attenuated the hypertension (146 +/- 2 mmHg catalase vs. 163 +/- 4 mmHg saline) in SS rats after 5 days of high salt (4%). H2O2 infused into the renal medulla of consomic SS-13(BN) fed high salt (4%) for 7 days accentuated the salt sensitivity (145 +/- 2 mmHg H2O2 vs. 134 +/- 1 mmHg saline). [H2O2] was also increased in the treated group (83 +/- 1 nM H2O2 vs. 44 +/- 9 nM saline). These data show that medullary production of H2O2 may contribute to salt-induced hypertension in SS rats and that chromosome 13 of the Brown Norway contains gene(s) that protect against renal medullary oxidant stress.

Oxidative stress and antioxidant treatment in hypertension and the associated renal damage

Reactive oxygen species (ROS) are elevated in humans with hypertension many of which develop end-stage renal disease (ESRD), and antioxidant capacity is decreased. About one-half of essential hypertensives have a salt-sensitive type of hypertension, and the amount of renal damage that occurs in salt-sensitive hypertensives greatly exceeds that of non-salt-sensitive hypertensives. Antioxidant therapy can improve cardiovascular outcomes in humans but only if sufficient doses are used. Salt-sensitive hypertensive animal models, especially Dahl salt-sensitive rats, have been used to investigate the relationship between hypertension, ROS and end-stage renal damage. In experimental salt-sensitive hypertension, ROS increase and significant renal damage occur. In the Dahl salt-sensitive (S) rat on high Na for 3 weeks, renal damage is mild, renal levels of superoxide dismutase are decreased, and treatment with Tempol reduces arterial pressure. In the Dahl S rat on high Na for 5 weeks, renal damage is severe, GFR and renal plasma flow are decreased, and renal superoxide production is high. Treatment with vitamins C and E decreases renal superoxide production and renal damage and prevents the decrease in renal hemodynamics. Antioxidant treatment reduces arterial pressure, aortic superoxide production and renal inflammation in DOCA-salt rats, and decreases blood pressure and aortic superoxide release and increases bioactive nitric oxide in SHR stroke-prone rats. In conclusion, in both human and experimental salt-sensitive hypertension, superoxide production and renal damage are increased, antioxidant capacity is decreased, and antioxidant therapy can be helpful.

Sympatho-Inhibitory Action of Endogenous Adrenomedullin Through Inhibition of Oxidative Stress in the Brain

Central sympathetic activation is one of the possible mechanisms underlying hypertension, in which reactive oxygen species may play a role. Thus, we examined whether adrenomedullin, an antioxidant peptide, is involved in the central regulation of arterial pressure through sympatho-modulatory action. Adrenomedullin knockout mice were fed with high-salt diet for 4 weeks to stimulate adrenomedullin production. In the wild-type littermates, brain adrenomedullin content was significantly increased with salt loading, but not in the knockout mice. Intracerebroventricular hyperosmotic saline increased arterial pressure and sympathetic nerve activity in a dose-dependent fashion. With the normal salt diet, the hyperosmotic saline-induced response did not significantly differ between the knockout and wild-type mice; with the high-salt diet, however, the response was significantly greater in the knockout mice than in wild-type littermates (arterial pressure: 35.3±5.7% versus 20.1±2.1%, P <0.05; sympathetic nerve activity: 30.3±4.8% versus 15.9±1.5%, P <0.05; respectively). Moreover, pretreatment with 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol), a membrane-permeable superoxide dismutase mimetic, inhibited the augmented response to central hyperosmotic saline in salt-loaded knockout mice. Consistently, the hyperosmotic saline-induced production of reactive oxygen species, measured by the lucigenin chemiluminescence method, was significantly greater in the isolated hypothalamus of salt-loaded knockout mice than in that of salt-loaded wild-type ones. In conclusion, endogenous adrenomedullin in the brain may inhibit sympathetic activation through its antioxidant action.

Antioxidant Treatment Prevents Renal Damage and Dysfunction and Reduces Arterial Pressure in Salt-Sensitive Hypertension

The goal of this study was to test the hypothesis that oxidative stress in Dahl salt-sensitive (SS) rats on a high-sodium intake contributes to the progression of renal damage, the decreases in renal hemodynamics, and the development of hypertension. We specifically studied whether antioxidant therapy, using vitamins C and E, could help prevent renal damage and glomerular filtration rate (GFR) and renal plasma flow reductions and attenuate the increases in arterial pressure. Thirty-three 7- to 8-week old Dahl SS/Rapp strain rats were placed on either a high-sodium (8%) or a low-sodium (0.3%) diet with or without vitamin E (111 IU/d) in the food and 98 mg/d vitamin C in the drinking water for 5 weeks. Rats were equipped with indwelling arterial and venous catheters at day 21. By day 35 in the rats with high-sodium diet, vitamin C and E treatment significantly decreased renal cortical and medullary O2*- release, mean arterial pressure, urinary protein excretion, glomerular necrosis, and renal tubulointerstitial damage. At this time, GFR significantly decreased in the high-sodium diet group (1.6+/-0.2 mL/min) when compared with either the high-sodium plus vitamins C and E (2.9+/-0.2 mL/min) or the low-sodium diet group (2.9+/-0.3 mL/min). In SS rats on high-sodium diet, renal plasma flow decreased 40%, and this reduced flow was restored by vitamin treatment. In Dahl salt-sensitive hypertension, increased oxidative stress plays an important role in the renal damage, decreases in renal hemodynamics, and increases in arterial pressure that occur. Antioxidant treatment with vitamins C and E improves renal dysfunction, lessens renal injury, and decreases arterial pressure in Dahl salt-sensitive hypertension.

Roles of Nitric Oxide and Oxidative Stress in the Regulation of Blood Pressure and Renal Function in Prehypertensive Ren-2 Transgenic Rats

AIMS

The present study was performed to evaluate the role of nitric oxide (NO) and its interaction with superoxide anion (O2-) in the regulation of blood pressure (BP) and renal function during the developmental phase of hypertension in Ren-2 transgenic rats (TGR). The first aim was to compare BP and renal functional responses to acute NO synthase (NOS) inhibition achieved by intravenous (i.v.) infusion of Nomega-nitro-L-arginine-methyl ester (L-NAME) in prehypertensive heterozygous TGR and in transgene-negative Hannover Sprague-Dawley (HanSD) rats. The second aim was to evaluate whether scavenging of O2- by infusion of the superoxide dismutase mimetic tempol increases NO bioavailability which therefore should augment BP and renal functional responses to L-NAME.

METHODS

Rats were anesthetized, prepared for clearance experiments and BP and renal functional responses were evaluated in response to i.v. L-NAME administration (20 microg.100 g(-1).min(-1)) without or with tempol pretreatment (i.v., 300 microg.100 g(-1).min(-1)). In renal cortical tissue, nitrotyrosine protein expression was assessed by immunoblotting as marker of O2- production and urinary 8-epi-PGF(2alpha) excretion as marker of intrarenal oxidative stress was assessed by enzyme immunoassay.

RESULTS

BP, glomerular filtration rate (GFR), renal plasma flow (RPF) and sodium excretion were similar in TGR and HanSD. L-NAME infusion induced greater increases in BP in TGR than in HanSD (+42 +/- 4 vs. +25 +/- 3 mmHg, p < 0.05). In the absence of a significant change in GFR, L-NAME caused similar decreases in RPF (-32 +/- 6 and -25 +/- 4%, p < 0.05) in TGR and HanSD. Despite significantly higher renocortical expression of nitrotyrosine and urinary 8-epi-PGF2alpha excretion in TGR than in HanSD, pretreatment with tempol did not augment the rise in BP and the decrease in RPF induced by L-NAME.

CONCLUSIONS

The greater BP response to L-NAME in TGR suggests that prehypertensive TGR exhibit an enhanced NO activity in the systemic vasculature as compared with HanSD. Despite increased intrarenal oxidative stress in TGR, the dependency of the intrarenal vascular tone on NO appears to be similar in TGR and HanSD. The lack of a compensatory increase in renal NO activity may partially account for the enhanced renal vascular response to ANG II present in TGR.

Differential effects of salt on renal hemodynamics and potential pressure transmission in stroke-prone and stroke-resistant spontaneously hypertensive rats

Salt-supplemented stroke-prone spontaneously hypertensive rats (SHRsp) develop more severe hypertension-induced renal damage (HIRD) compared with their progenitor SHR. The present studies were performed to examine whether in addition to increasing the severity of hypertension salt also enhanced the transmission of such hypertension to the renal vascular bed in the SHRsp. "Step" and "dynamic" renal blood flow (RBF) autoregulation (AR) were examined in approximately 12-wk-old SHR and SHRsp after 3-5 days of an 8% NaCl diet. During step AR under anesthesia (n = 8-11), RBF was significantly higher in the SHRsp at all perfusion pressures (P < 0.01), but AR capacity was not different. Similarly, in separate conscious chronically instrumented rats (n = 8 each), both blood pressure (BP) and RBF were modestly but significantly higher at baseline before salt in the SHRsp (P < 0.05). However, transfer function analysis did not show significant differences in the admittance gain parameters. However, after 3-5 days of salt, although average BP was not significantly altered in either strain, RBF increased further in the SHRsp and there was a significantly greater transfer of BP into RBF power in the SHRsp. This was reflected in the significantly higher admittance gain parameters at most frequencies including the heartbeat frequency (P < 0.05 maximum). These differential hemodynamic effects of salt have the potential to enhance BP transmission to the renal vascular bed and also contribute to the more severe HIRD observed in the salt-supplemented SHRsp.

Salt-sensitive hypertension develops after transient induction of ANG II-dependent hypertension in Cyp1a1-Ren2 transgenic rats

Transient exposure to ANG II results in the development of salt-sensitive hypertension in rats. This study was performed to determine whether a transient hypertensive episode can induce salt-sensitive hypertension in transgenic rats with inducible expression of the mouse Ren2 renin gene [strain name TGR(Cyp1a1-Ren2)]. Systolic blood pressures were measured in conscious male Cyp1a1-Ren2 rats ( n = 6) during control conditions and during dietary administration of indole-3-carbinol (I3C; 0.15%, wt/wt), for 14 days. Systolic pressure increased from 135 ± 5 to 233 ± 7 mmHg by day 14. I3C administration was terminated and blood pressure returned to normal levels (137 ± 5 mmHg) within 10 days. Subsequently, the rats were placed on a high-salt diet (8% NaCl) for 10 days. Systolic pressure increased by 34 ± 2 mmHg throughout 10 days of the high-salt diet. Neither glomerular filtration rate nor renal plasma flow was altered in Cyp1a1-Ren2 rats with salt-sensitive hypertension. In a separate group of male Cyp1a1-Ren2 rats ( n = 6) transiently induced with 0.15% I3C for 14 days, administration of the superoxide dismutase mimetic tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl, 2 mM) attenuated the increase in systolic pressure induced by high salt. Systolic pressure increased by only 11 ± 1 mmHg throughout 8 days of a high-salt diet and tempol administration. Thus transient induction of ANG II-dependent hypertension via activation of the Cyp1a1-Ren2 transgene induces salt-sensitive hypertension in these transgenic rats. The attenuation by tempol of the high salt-induced blood pressure elevation indicates that ANG II-induced production of superoxide anion contributes to the development of salt-sensitive hypertension after transient induction of ANG II-dependent hypertension.

Effects of thiol antioxidant on reduced nicotinamide adenine dinucleotide phosphate oxidase in hypertensive Dahl salt-sensitive rats

Recent studies implicate of reactive oxygen species (ROS) in hypertension; however, whether reactive oxygen species promote hypertensive derangements is not fully clear. We thus investigated the effects of an antioxidant, N-acetyl-L-cysteine, on hypertensive Dahl salt-sensitive rats. High-salt intake for 4 weeks markedly elevated systolic arterial pressure, urinary excretion of protein, 8-isoprostane, and H(2)O(2), and the enzyme activity of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase along with the elevated expression of its subunits gp91phox and p47phox at the levels of mRNA and protein. Supplement with N-acetyl-L-cysteine reduced the increase in systolic arterial pressure and counteracted the elevation of urinary excretion of protein, 8-isoprostane, and H(2)O(2), and the increases in NADPH oxidase activity/expression in high-salt-loaded Dahl salt-sensitive rats. N-acetyl-L-cysteine supplement ameliorated plasma and urinary levels of thromboxane B(2) (an end metabolite of thromboxane A(2)), associated with improvement of both the abnormal contraction and the impaired nitric oxide-dependent relaxation in renal arteries. These results revealed that oxidative stress mediates hypertensive changes in Dahl salt-sensitive rats, because thiol antioxidant N-acetyl-L-cysteine attenuated the augmentation of local ROS production by diminishing the elevation of NADPH oxidase expression and ameliorated renal/vascular hypertensive changes.

Oxidant stress in kidneys of spontaneously hypertensive rats involves both oxidase overexpression and loss of extracellular superoxide dismutase

Oxidant stress is an important contributor to renal dysfunction and hypertension. We have previously demonstrated that regulation of renal oxygen consumption by nitric oxide (NO) is impaired in the kidney of spontaneously hypertensive rats (SHR) due to increased superoxide production. We further explored the mechanisms of enhanced oxidant stress in the kidney of SHR. Suppression of cortical oxygen consumption by bradykinin (BK) or enalaprilat (Enal), which act through stimulation of endogenous NO, was impaired in SHR (BK: -14.1 +/- 1.2%; Enal: -15.5 +/- 1.2%) and was restored by addition of apocynin, an inhibitor of assembly of the NAD(P)H oxidase complex (BK: -21.0 +/- 0.6%; Enal: -25.3 +/- 1.4%), suggesting this as the source of enhanced superoxide production. Addition of an angiotensin type 1 receptor blocker, losartan, also restored responsiveness to control levels (BK: -22.0 +/- 1.1%; Enal: -23.6 +/- 1.3%), suggesting that ANG II is responsible for enhanced oxidase activity. A similar defect in responsiveness to BK and Enal could be induced in Wistar-Kyoto kidneys by ANG II and was reversed by a superoxide scavenger (tempol), apocynin or losartan. Immunoblotting of cortical samples demonstrated enhanced expression of endothelial NO synthase (eNOS 1.9x) and NAD(P)H oxidase components (gp91(phox) 1.6x and Rac-1 4.5x). Expression of SOD-1 and -2 were unchanged, but SOD-3 was significantly decreased in SHR (0.5x). Thus NO bioavailability is impaired in SHR owing to an ANG II-mediated increase in superoxide production in association with enhanced expression of NAD(P)H oxidase components, despite increased expression of eNOS. Loss of SOD-3, an important superoxide scavenger, may also contribute to enhanced oxidant stress.

Tissue kallikrein attenuates salt-induced renal fibrosis by inhibition of oxidative stress

BACKGROUND

High salt intake induces hypertension, cardiac hypertrophy, and progressive renal damage. Progressive renal injury is the consequence of a process of destructive fibrosis. Using gene transfer approach, we have shown that the tissue kallikrein-kinin system (KKS) plays an important role in protection against renal injury in several hypertensive rat models. In this study, we further investigated the effect and potential mechanisms mediated by kallikrein on salt-induced renal fibrosis.

METHODS

Adenovirus harboring the human tissue kallikrein gene was delivered intravenously into Dahl salt-sensitive (DSS) rats on a high salt diet for 4 weeks. Two weeks after gene delivery, the effect of kallikrein on renal fibrosis was examined by biochemical and histologic analysis.

RESULTS

Kallikrein gene delivery resulted in reduced blood urea nitrogen (BUN), urinary protein and albumin levels in DSS rats on a high salt diet. Expression of recombinant human tissue kallikrein was detected in the sera and urine of rats injected with the kallikrein gene. Histologic investigation showed that kallikrein gene delivery significantly reduced glomerular and tubular fibrosis scores and collagen deposition, as well as renal cell proliferation, compared to rats on a high salt diet injected with control virus. Kallikrein gene transfer significantly increased nitric oxide and cyclic guanosine monophosphate (cGMP) levels in conjunction with reduced salt-induced nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide phosphate (NADH/NADPH) oxidase activity, superoxide production, transforming growth factor-beta1 (TGF-beta1) mRNA and protein levels, and TGF-beta1 immunostaining.

CONCLUSION

These results indicate that tissue kallikrein protects against renal fibrosis in hypertensive DSS rats through increased nitric oxide bioavailability and suppression of oxidative stress and TGF-beta expression.

Primed Polymorphonuclear Leukocytes, Oxidative Stress, and Inflammation Antecede Hypertension in the Sabra Rat

Hypertension is accompanied by systemic oxidative stress, inflammation, and priming of peripheral polymorphonuclear leukocytes (PMNLs), yet the involvement of these factors in the pathophysiology of hypertension is incompletely understood. We investigated the relationship between oxidative stress, primed PMNLs, and inflammation and the development of hypertension in the Sabra rat model of salt-sensitive hypertension. Sabra hypertension-resistant rats (SBN/y) (salt-resistant) and Sabra hypertension-prone rats (SBH/y) (salt-sensitive) were studied under normal conditions or during salt loading. Systolic blood pressure (BP) was measured by the tail-cuff method. The extent of oxidative stress was evaluated by the rate of superoxide release from PMNLs, plasma-reduced glutathione (GSH) levels, malondialdehyde (MDA) levels (estimated by thiobarbituric acid–reacting substances), and plasma-carbonylated fibrinogen (Western blotting). Plasma fibrinogen levels and the peripheral PMNL count served as indices of inflammation. In SBH/y and SBN/y provided regular chow without salt loading, BP did not rise above baseline values, yet superoxide release, plasma MDA, carbonylated fibrinogen, and PMNL count were higher in SBH/y than in SBN/y, whereas GSH levels were lower in SBH/y. Four weeks of salt loading resulted in a gradual increase in systolic BP in SBH/y to 205±3 mm Hg, whereas BP remained in SBN/y at baseline normotensive levels. All the parameters reflecting oxidative stress and inflammation were further aggravated with the development of hypertension in salt-loaded SBH/y. We conclude that primed PMNLs, oxidative stress, and inflammation antecede the development of hypertension in this experimental model of hypertension.

Arterial Pressure Response to the Antioxidant Tempol and ET B Receptor Blockade in Rats on a High-Salt Diet

We hypothesized that increased superoxide contributes to mean arterial pressure (MAP) regulation in male Sprague-Dawley rats fed a high-salt diet and/or during endothelin (ET B ) receptor blockade. Four groups on either a normal- or a high-salt diet were studied for 1 week: (1) control; (2) tempol, a superoxide dismutase mimetic, in their drinking water (1 mmol/L); (3) A-192621, an ET B antagonist, in their food (10 mg/kg daily); or (4) both tempol and A-192621. Without ET B blockade, tempol had no effect on MAP (telemetry) in rats on the normal-salt diet but significantly reduced MAP in rats on the high-salt diet (100±3 vs 112±2 mm Hg, P <0.05). On the normal-salt diet, A-192621 increased MAP with or without tempol. Under high-salt conditions, tempol attenuated the increase in MAP produced by A-192621, but only during the initial days of treatment. Plasma 8-isoprostanes were increased in all rats on the high-salt diet and were further increased after 3 days of A-192621 but not after 7 days; tempol inhibited the increase produced by A-192621 but had no influence on the increase produced by high salt. H 2 O 2 excretion was significantly higher in rats on a high-salt diet for the 7-day drug treatment compared with those on a normal-salt diet. Tempol further increased H 2 O 2 excretion in rats on a high-salt diet, an effect accelerated in A-192621–treated rats. These data suggest that blood pressure lowering by tempol in rats on a high-salt diet may be unrelated to reductions in superoxide and that renal H 2 O 2 may account for the limited ability of tempol to attenuate hypertension produced by ET B receptor blockade.

Impaired nitric oxide- and endothelium-derived hyperpolarizing factor-dependent dilation of renal afferent arteriole in Dahl salt-sensitive rats

BACKGROUND AND AIMS

We previously demonstrated that acetylcholine elicited nitric oxide-dependent sustained and endothelium-derived hyperpolarizing factor (EDHF)-dependent transient dilation of afferent arterioles. The present study examined whether free radicals interacted with nitric oxide-dependent and EDHF-dependent vasodilator mechanisms in renal microvessels of salt-sensitive hypertension, using the isolated perfused hydronephrotic kidney.

METHODS AND RESULTS

Following the pretreatment with indomethacin (100 micromol/L) with or without nitro- l-arginine methylester (100 micromol/L), the effect of acetylcholine on noradrenaline (0.3 micromol/L)-induced constriction was evaluated in kidneys from Dahl salt-sensitive and salt-resistant rats. Although acetylcholine (0.01-10 micromol/L) caused dose-dependent and sustained vasodilation of afferent arterioles, attenuated dilation was observed in Dahl salt-sensitive rats, compared with that in salt-resistant rats (58 +/- 4 vs 101 +/- 11% reversal at 10 micromol/L acetylcholine). In the presence of nitro- l-arginine methylester, acetylcholine elicited only transient dilation, with vasodilator response blunted in Dahl salt-sensitive rats (64 +/- 4 vs 100 +/- 9% reversal at 10 micromol/L acetylcholine). Furthermore, chronic (8-10 weeks) treatment with tempol caused partial restoration of acetylcholine (10 micromol/L)-induced sustained arteriolar dilation (71 +/- 3% reversal), but complete reversal of transient dilation (92 +/- 4% reversal). Finally, acute treatment with tempol not only improved the sustained component of the acetylcholine-induced dilation but also restored the impaired responsiveness of transient dilation in Dahl salt-sensitive rats.

CONCLUSION

Both sustained (nitric oxide-mediated) and transient (EDHF-mediated) components of acetylcholine-induced afferent arteriolar dilation were attenuated in Dahl salt-sensitive rats, which was attributed, in part, to enhanced free radical activity. A reversal of the sustained and transient vasodilation by the acute tempol treatment suggests possible interaction between free radicals and EDHF as well as increased bioavailability of nitric oxide.

Role of sodium and volume in the pathogenesis of hypertension in dialysis patients. Reflections on pathophysiological mechanisms

Volume and sodium excess is the predominant factor in the pathogenesis of hypertension in dialysis patients. However, except for anephric patients, the relationship between volume status, blood pressure, and hemodynamics is not straightforward, but may vary between subgroups of patients. In general, the sensitivity of blood pressure to changes in the volume/sodium status appears to be increased in patients with end-stage renal disease. The blood pressure response to changes in the sodium/volume status may be influenced by disturbances in the activity of and response to various neurohumoral mechanisms, such as the renin-angiotensin and sympathetic nervous systems, Na-K-ATP-ase inhibitors, and the nitric oxide system. Regarding these aspects, there might be a parallel with salt-sensitive (essential) hypertension. Preliminary data showed a beneficial effect of sodium removal beyond changes in the volume status. Also of interest is the fact that prolonging dialysis time may improve blood pressure control without clear changes in the fluid status. It is hypothesized that a reduction in exchangeable sodium, by increased diffusive transport of sodium, in combination with increased removal of vasopressor substances, might be partly responsible for the observed blood pressure changes during long dialysis times. In conclusion, sodium and volume overload and neurohumoral factors coincide in the pathogenesis of hypertension in dialysis patients. Nevertheless, their exact relationship has not yet been elucidated and deserves further study.

Reactive oxygen species stimulate central and peripheral sympathetic nervous system activity

Recent studies have implicated reactive oxygen species (ROS) in the pathogenesis of hypertension and activation of the sympathetic nervous system (SNS). Because nitric oxide (NO) exerts a tonic inhibition of central SNS activity, increased production of ROS could enhance inactivation of NO and result in activation of the SNS. To test the hypothesis that ROS may modulate SNS activity, we infused Tempol (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl), a superoxide dismutase mimetic, or vehicle either intravenously (250 microg x kg(-1) x min(-1)) or in the lateral ventricle (50 microg x kg body wt(-1) x min(-1)), and we determined the effects on blood pressure (BP), norepinephrine (NE) secretion from the posterior hypothalamus (PH) measured by the microdialysis technique, renal sympathetic nerve activity (RSNA) measured by direct microneurography, the abundance of neuronal NO synthase (nNOS)-mRNA in the PH, paraventricular nuclei (PVN), and locus coeruleus (LC) measured by RT-PCR, and the secretion of nitrate/nitrite (NO(x)) in the dialysate collected from the PH of Sprague-Dawley rats. Tempol reduced BP whether infused intravenously or intracerebroventricularly. Tempol reduced NE secretion from the PH and RSNA when infused intracerebroventricularly but raised NE secretion from the PH and RSNA when infused intravenously. The effects of intravenous Tempol on SNS activity were blunted or abolished by sinoaortic denervation. Tempol increased the abundance of nNOS in the PH, PVN, and LC when infused intracerebroventricularly, but it decreased the abundance of nNOS when infused intravenously. When given intracerebroventricularly, Tempol also reduced the concentration of NO(x) in the dialysate collected from the PH. Pretreatment with N(omega)-nitro-l-arginine methyl ester did not abolish the effects of intracerebral Tempol on BP, heart rate, NE secretion from the PH, and RSNA suggesting that the effects of Tempol on SNS activity may be in part dependent and in part independent of NO. In all, these studies support the notion that ROS may raise BP via activation of the SNS. This activation may be mediated in part by downregulation of nNOS and NO production, in part by mechanisms independent of NO. The discrepancy in results between intracerebroventricular and intravenous infusion of Tempol can be best explained by direct inhibitory actions on SNS activity when given intracerebral. By contrast, Tempol may exert direct vasodilation of the peripheral circulation and reflex activation of the SNS when given intravenously.

Redox-dependent signalling by angiotensin II and vascular remodelling in hypertension

1. Hypertension is associated with structural alterations of resistance arteries, a process known as remodelling (increased media-to-lumen ratio). 2. At the cellular level, vascular remodelling involves changes in vascular smooth muscle cell (VSMC) growth, cell migration, inflammation and fibrosis. These processes are mediated via multiple factors, of which angiotensin (Ang) II appears to be one of the most important in hypertension. 3. Angiotensin II signalling, via AT1 receptors, is upregulated in VSMC from resistance arteries of hypertensive patients and rats. This is associated with hyperactivation of vascular NADPH oxidase, leading to increased generation of reactive oxygen species (ROS), particularly O2- and H2O2. 4. Reactive oxygen species function as important intracellular second messengers to activate many downstream signalling molecules, such as mitogen-activated protein kinase, protein tyrosine phosphatases, protein tyrosine kinases and transcription factors. Activation of these signalling cascades leads to VSMC growth and migration, modulation of endothelial function, expression of pro-inflammatory mediators and modification of extracellular matrix. 5. Furthermore, ROS increase intracellular free Ca2+ concentration ([Ca2+]i), a major determinant of vascular reactivity. 6. All these processes play major roles in vascular injury associated with hypertension. Accordingly, ROS and the signalling pathways that they modulate provide new targets to regress vascular remodelling, reduce peripheral resistance and prevent hypertensive end-organ damage. 7. In the present review, we discuss the role of ROS as second messengers in AngII signalling and focus on the implications of these events in the processes underlying vascular remodelling in hypertension.

Inhibition of Na-K-ATPase in thick ascending limbs by NO depends on O2- and is diminished by a high-salt diet

A high-salt diet enhances nitric oxide (NO)-induced inhibition of transport in the thick ascending limb (THAL). Long exposures to NO inhibit Na-K-ATPase in cultured cells. We hypothesized that NO inhibits THAL Na-K-ATPase after long exposures and a high-salt diet would augment this effect. Rats drank either tap water or 1% NaCl for 7-10 days. Na-K-ATPase activity was assessed by measuring ouabain-sensitive ATP hydrolysis by THAL suspensions. After 2 h, spermine NONOate (SPM; 5 microM) reduced Na-K-ATPase activity from 0.44 +/- 0.03 to 0.30 +/- 0.04 nmol P(i).microg protein(-1).min(-1) in THALs from rats on a normal diet (P < 0.03). Nitroglycerin also reduced Na-K-ATPase activity (P < 0.04). After 20 min, SPM had no effect (change -0.07 +/- 0.05 nmol P(i).microg protein(-1).min(-1)). When rats were fed high salt, SPM did not inhibit Na-K-ATPase after 120 min. To investigate whether ONOO(-) formed by NO reacting with O(2)(-) was involved, we measured O(2)(-) production. THALs from rats on normal and high salt produced 35.8 +/- 0.3 and 23.7 +/- 0.8 nmol O(2)(-).min(-1).mg protein(-1), respectively (P < 0.01). Because O(2)(-) production differed, we studied the effects of the O(2)(-) scavenger tempol. In the presence of 50 microM tempol, SPM did not inhibit Na-K-ATPase after 120 min (0.50 +/- 0.05 vs. 0.52 +/- 0.07 nmol P(i).microg protein(-1).min(-1)). Propyl gallate, another O(2)(-) scavenger, also prevented SPM-induced inhibition of Na-K-ATPase activity. SPM inhibited pump activity in tubules from rats on high salt when O(2)(-) levels were increased with xanthine oxidase and hypoxanthine. We concluded that NO inhibits Na-K-ATPase after long exposures when rats are on a normal diet and this inhibition depends on O(2)(-). NO donors do not inhibit Na-K-ATPase in THALs from rats on high salt due to decreased O(2)(-) production.

Effects of tempol on renal angiotensinogen production in Dahl salt-sensitive rats

We have recently reported that Dahl salt-sensitive rats (DS) on high salt diet (HS) have an inappropriate augmentation of intrarenal angiotensinogen. Recent studies also reported that the augmented superoxide anion formation plays important roles in this animal model of hypertension. This study was performed to address the hypothesis that an inappropriate augmentation of intrarenal angiotensinogen by HS is caused by the augmented reactive oxygen species. Male DS (200-220 g) were maintained on low salt diet LS (N = 7) or HS (N = 27) for 4 weeks. The HS group was subdivided into three subgroups to receive null (N = 12), superoxide dismutase mimetic, tempol (3 mmol/l, N = 8), or vasodilator, hydralazine (0.5 mmol/l, N = 7) in drinking water during the period. Systolic BP was significantly increased in the DS+HS group compared to the DS+LS group (184+/-7 mmHg vs. 107+/-5 at 4-week). Tempol or hydralazine treatment equivalently attenuated the hypertension (128+/-3 and 127+/-5 at 4-week, respectively). Urinary excretion of thiobarbituric acid reactive substances at 4-week was significantly increased in the DS+HS group compared to the DS+LS group (0.66+/-0.05 micromol/day vs. 0.14+/-0.01). Tempol treatment prevented this effect (0.24+/-0.04) but hydralazine treatment only partially prevented the effect (0.40+/-0.03). Kidney angiotensinogen levels, measured by Western blot analysis, were significantly increased in the DS+HS group compared to the DS+LS group (32+/-5 densitometric units vs. 21+/-1). Tempol (14+/-3) but not hydralazine (32+/-5) treatment prevented the intrarenal angiotensinogen augmentation. The evidence suggests that the enhanced intrarenal angiotensinogen in DS challenged with HS is associated with the augmented reactive oxygen species.

Flavonoid-induced reduction of ENaC expression in the kidney of Dahl salt-sensitive hypertensive rat

Flavonoid, a plant extract, exhibits various biological actions. Dietary flavonoid intake is reported to reduce an elevated blood pressure, however the mechanism is unknown. The epithelial Na+ channel (ENaC) in the kidney plays a key role in the regulation of blood pressure by contributing to the Na+ reabsorption in renal tubules. Thus, we investigated the effect of quercetin, a flavonoid, on ENaC mRNA expression in the kidney of hypertensive Dahl salt-sensitive rats. Dahl salt-sensitive rats of 8 weeks were acclimated for 1 week in a metabolic cage and were subsequently kept for 4 weeks under four different conditions: (1) normal salt diet (0.3% NaCl), (2) normal salt diet with quercetin (10 mg/kg/day), (3) high-salt diet (8% NaCl), and (4) high-salt diet with quercetin. Quercetin diminished the alphaENaC mRNA expression in the kidney associated with reduction of the systolic blood pressure elevated by high-salt diet, suggesting that one of the mechanisms of the flavonoid's antihypertensive effect on salt-sensitive hypertension would be mediated through downregulation of ENaC expression in the kidney.

Possible contributions of reactive oxygen species and mitogen-activated protein kinase to renal injury in aldosterone/salt-induced hypertensive rats

Studies were performed to test the hypothesis that reactive oxygen species (ROS) and mitogen-activated protein kinase (MAPK) contribute to the pathogenesis of aldosterone/salt-induced renal injury. Rats were given 1% NaCl to drink and were treated with one of the following combinations for 6 weeks: vehicle (0.5% ethanol, SC, n=6); aldosterone (0.75 microg/H, SC, n=8); aldosterone plus a selective mineralocorticoid receptor antagonist; eplerenone (0.125% in chow, n=8); aldosterone plus an antioxidant; and tempol (3 mmol/L in drinking solution, n=8). The activities of MAPKs, including extracellular signal-regulated kinases (ERK)1/2, c-Jun-NH2-terminal kinases (JNK), p38MAPK, and big-MAPK-1 (BMK1) in renal cortical tissues were measured by Western blot analysis. Aldosterone-infused rats showed higher systolic blood pressure (165+/-5 mm Hg) and urinary excretion of protein (106+/-24 mg/d) than vehicle-infused rats (118+/-3 mm Hg and 10+/-3 mg/d). Renal cortical mRNA expression of p22phox, Nox-4, and gp91phox, measured by real-time polymerase chain reaction, was increased in aldosterone-infused rats by 2.3, 4.3, and 3.0-fold, respectively. Thiobarbituric acid-reactive substances (TBARS) content in renal cortex was also higher in aldosterone (0.23+/-0.02) than vehicle-infused rats (0.09+/-0.01 nmol/mg protein). ERK1/2, JNK, and BMK1 activities were significantly elevated in aldosterone-infused rats by 3.3, 2.3, and 3.0-fold, respectively, whereas p38MAPK activity was not changed. Concurrent administration of eplerenone or tempol to aldosterone-infused rats prevented the development of hypertension (127+/-2 and 125+/-5 mm Hg), and the elevations of urinary excretion of protein (10+/-2 and 9+/-2 mg/day) or TBARS contents (0.08+/-0.01 and 0.11+/-0.01 nmol/mg protein). Furthermore, eplerenone and tempol treatments normalized the activities of ERK1/2, JNK, and BMK1. These data suggest that ROS and MAPK play a role in the progression of renal injury induced by chronic elevations in aldosterone.

Tempol Lowers Blood Pressure and Sympathetic Nerve Activity But Not Vascular O 2 − in DOCA-Salt Rats

This study tested the hypothesis that depressor responses caused by tempol are not associated with reductions in vascular O 2 − levels in urethane-anesthetized deoxycorticosterone acetate (DOCA)-salt hypertensive rats. We compared the effects of intravenous (IV) administration of tempol, apocynin, superoxide dismutase-polyethylene glycol (PEG-SOD), and SOD on mean arterial blood pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA). In DOCA-salt rats, tempol (30 to 300 μmol/kg) dose-dependently decreased RSNA, MAP, and HR. Tempol (300 μmol/kg) decreased MAP from 140±5 to 83±4 mm Hg ( P <0.05). HR decreased from 435±15 to 390±12 bpm ( P <0.05). RSNA was reduced by 54%±6% from baseline. However, in the same rats, tempol did not reduce dihydroethidium-induced fluorescent signals in the aorta and vena cava. Apocynin (200 μmol/kg) did not lower MAP (142±5 mm Hg versus 140±6 mm Hg) or HR (428±15 bpm versus 420±13 bpm) and apocynin did not potentiate depressor responses caused by tempol. PEG-SOD (10 000 U/kg, bolus or 5000 U/kg bolus followed by a 30-minutes infusion of 500 U/kg/min) or SOD (25 000 U/kg, bolus or 10 000 U/kg bolus followed by a 30-minutes infusion of 1000 U/kg per minute) did not alter MAP or HR. It is concluded that depressor responses and decreases in HR and RSNA caused by acute tempol treatment are caused by direct sympathetic nerve activity inhibition that is not accompanied by SOD-mimetic action in the aorta or vena cava.

Renal oxidative stress in medullary thick ascending limbs produced by elevated NaCl and glucose

The effects of NaCl, glucose, and thyroid hormone on the production of superoxide (O2*-) within the renal medulla of Sprague-Dawley rats were examined. Responses of intracellular superoxide [O2*-]i in isolated medullary thick ascending limbs (mTALs) were studied using real-time fluorescent microscopy with measurement of the dehydroethidium (DHE) to ethidium (Eth) conversion ratio (Eth/DHE ratio unit). The results demonstrated that elevations of extracellular NaCl (from 152 to 252 mmol/L), D-glucose (from 5 to 25 mmol/L), and triiodo-thyronine (T3; 10 micromol/L) significantly increased [O2*-]i levels. Preincubation with superoxide scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid (1 mmol/L) significantly inhibited these responses. Stimulation with equamolar amounts of choline chloride or L-glucose failed to increase [O2*]i, indicating that these O2*- responses were not determined by changes in osmolality. The responses to NaCl, D-glucose, and T3 were abolished by pretreatment with the Na+/K+-ATPase pump inhibitor ouabain (4 mmol/L) and with Na+/H+ -exchanger inhibitor dimethylamiloride (100 micromol/L). We conclude that elevations of extracellular NaCl, D-glucose, or T3 levels can activate both the Na+/K+-ATPase pump and Na+/H+ exchanger in mTAL, which, in turn, is associated with increased intracellular concentrations of superoxide.

Renal and vascular oxidative stress and salt-sensitivity of arterial pressure

Oxidative stress occurs in a tissue or in the whole body when the total oxidant production exceeds the antioxidant capacity. Recent studies in human essential hypertension indicate that free radical production is increased and antioxidant levels are decreased, and more than one-half of these hypertensives have a salt-sensitive type of hypertension with progressive renal damage. Increased oxidative stress may also play a critical role in animal models of salt-sensitive hypertension. The stroke-prone spontaneously hypertensive rats (SHRSP) exhibits salt-sensitivity, vascular release of superoxide is increased, and total plasma antioxidant capacity is decreased. The superoxide release in the SHRSP rats inactivates nitric oxide, and superoxide dismutase (SOD) administration returns the bioactive nitric oxide levels to normal. The deoxycorticosterone acetate (DOCA)-salt hypertensive rat is salt-sensitive, aortic superoxide production is increased, and renal inflammation is significant. Treatment of the DOCA-salt rats with apocynin, an NADPH oxidase inhibitor, decreased aortic superoxide production and decreased arterial pressure. The Dahl salt-sensitive (S) rat has increased mesenteric microvascular and renal superoxide production and increased plasma levels of H2O2. The renal protein expression of SOD is decreased in the kidney of Dahl S rats, and long-term administration of Tempol, a superoxide mimetic, significantly decreased arterial pressure and renal damage. In conclusion, both human hypertension and experimental models of salt-sensitive hypertension have increased superoxide release, decreased antioxidant capacity and elevated renal damage.

Increased H(2)O(2) counteracts the vasodilator and natriuretic effects of superoxide dismutation by tempol in renal medulla

A membrane-permeable SOD mimetic, 4-hydroxytetramethyl-piperidine-1-oxyl (tempol), has been used as an antioxidant to prevent hypertension. We recently found that this SOD mimetic could not prevent development of hypertension induced by inhibition of renal medullary SOD with diethyldithiocarbamic acid. The present study tested a hypothesis that increased H2O2 counteracts the effects of tempol on renal medullary blood flow (MBF) and Na+ excretion (UNaV), thereby restraining the antihypertensive effect of this SOD mimetic. By in vivo microdialysis and Amplex red H2O2 microassay, it was found that interstitial H2O2 levels in the renal cortex and medulla in anesthetized rats averaged 55.91 +/- 3.66 and 102.18 +/- 5.16 nM, respectively. Renal medullary interstitial infusion of tempol (30 micromol x min-1x kg-1) significantly increased medullary H2O2 levels by 46%, and coinfusion of catalase (10 mg x min-1x kg-1) completely abolished this increase. Functionally, removal of H2O2 by catalase enhanced the tempol-induced increase in MBF, urine flow, and UNaV by 28, 41, and 30%, respectively. Direct delivery of H2O2 by renal medullary interstitial infusion (7.5-30 nmol x min-1x kg-1) significantly decreased renal MBF, urine flow, and UNaV, and catalase reversed the effects of H2O2. We conclude that tempol produces a renal medullary vasodilator effect and results in diuresis and natriuresis. However, this SOD mimetic increases the formation of H2O2, which constricts medullary vessels and, thereby, counteracts its vasodilator actions. This counteracting effect of H2O2 may limit the use of tempol as an antihypertensive agent under exaggerated oxidative stress in the kidney.

L-arginine reverses p47phox and gp91phox expression induced by high salt in Dahl rats

Derangements in the production and degradation of reactive oxygen species (ROS) as well as nitric oxide (NO) have been implicated in cardiovascular diseases. We explored how supplementation with l-arginine, an NO synthase substrate, restores such derangements of ROS/NO systems in Dahl salt-sensitive, hypertensive (DS) rats. We detected an increase of NADPH oxidase activity, a key enzyme that produces superoxide, in the membrane fraction of the renal cortex derived from DS rats loaded with high salt for 4 weeks; high salt loading also remarkably increased urinary H2O2, 8-isoprostane, and thromboxane B2 excretion and decreased plasma NO end products. These changes from high salt loading were counteracted by oral l-arginine supplementation. We further examined expression patterns of NADPH oxidase subunits in renal cortex derived from these animals. High salt loading increased gp91phox and p47phox but not p22phox or Rac1 or mRNA abundance, which were counteracted with L-arginine supplementation. Western blot analyses after subcellular fractionation revealed that l-arginine supplementation distinctly decreases membrane localization of p47phox protein, as it decreases total expression of Rac1 protein in DS rats with high salt loading. These results disclose that high salt loading causes a deficiency in available L-arginine amounts for NO synthases and induces NADPH oxidase activation in the renal cortex of DS rats, which l-arginine supplementation markedly restores. Since superoxide rapidly eliminates NO, which inhibits sodium reabsorption in the cortical collecting duct, superoxide production caused by upregulated NADPH oxidase activity in the renal cortex of high salt-loaded DS rats may accelerate sodium reabsorption and hypertension.