Oxidative stress in Dahl salt-sensitive hypertension

Role of renal medullary oxidative and/or carbonyl stress in salt-sensitive hypertension and diabetes

1. Salt-sensitive hypertension is commonly associated with diabetes, obesity and chronic kidney disease. The present review focuses on renal mechanisms involved in the development of this type of hypertension. 2. The renal medullary circulation plays an important role in the development of salt-sensitive hypertension. In vivo animal studies have demonstrated that the balance between nitric oxide (NO) and reactive oxygen species (ROS) in the renal medulla is an important element of salt-sensitive hypertension. The medullary thick ascending limb (mTAL) in the outer medulla is an important source of NO and ROS production and we have explored the mechanisms that stimulate their production, as well as the effects of NO superoxide and hydrogen peroxide on mTAL tubular sodium reabsorption and the regulation of medullary blood flow. 3. Angiotensin II-stimulated NO produced in the mTAL is able to diffuse from the renal mTAL to the surrounding vasa recta capillaries, providing a mechanism by which to increase medullary blood flow and counteract the direct vasoconstrictor effects of angiotensin II. Enhanced oxidative stress attenuates NO diffusion in this region. 4. Carbonyl stress, like oxidative stress, can also play an important role in the pathogenesis of chronic kidney disease, such as insulin resistance, salt-sensitive hypertension and renal vascular complications. 5. Despite the large number of studies undertaken in this area, there is as yet no drug available that directly targets renal ROS. Oxidative and/or carbonyl stress may be the next target of drug discovery to protect against salt-sensitive hypertension and associated end-organ damage.

Oxidative stress promotes hypertension and albuminuria during the autoimmune disease systemic lupus erythematosus

Several lines of evidence suggest that essential hypertension originates from an autoimmune-mediated mechanism. One consequence of chronic immune activation is the generation of oxygen-derived free radicals, resulting in oxidative stress. Renal oxidative stress has direct prohypertensive actions on renal microvascular and tubular function. Whether oxidative stress contributes to the prevalent hypertension associated with autoimmune disease is not clear. We showed previously that female NZBWF1 mice, an established model of the autoimmune disease systemic lupus erythematosus (SLE), develop hypertension associated with renal oxidative stress. In the present study we tested the hypothesis that oxidative stress contributes to autoimmune-mediated hypertension by treating SLE and control (NZW/LacJ) mice with tempol (2.0 mmol/L) and apocynin (1.5 mmol/L) in the drinking water for 4 weeks. Although the treatment did not alter SLE disease activity (assessed by plasma double-stranded DNA autoantibodies), blood pressure and renal injury (urinary albumin) were reduced in the treated SLE mice. Tempol plus apocynin-treated SLE mice had reduced expression of nitrosylated proteins in the renal cortex, as well as reduced urinary and renal cortical hydrogen peroxide, suggesting that treatment reduced renal markers of oxidative stress. These data suggest that renal oxidative stress plays an important mechanistic role in the development of autoimmune-mediated hypertension.

Association between circulating specific leukocyte types and incident chronic kidney disease: the Atherosclerosis Risk in Communities (ARIC) study

Progressive renal fibrosis is a characteristic of all the diseases that cause renal failure and is invariably accompanied by a prominent leukocyte infiltration in the kidney. The goal of this study was to determine the association between the circulating specific leukocyte types and incident chronic kidney disease (CKD). In a cohort of 10,056 middle-aged white and African American adults, levels of circulating neutrophils, lymphocytes, and monocytes were measured at baseline; blood pressure (BP) and serum creatinine were measured and estimated glomerular filtration rate (eGFR) was calculated at baseline and 3 and 9 years later; and surveillance for first hospitalization or death with CKD was carried out over a mean follow-up of 7.4 years (maximum, 11.9 years). Increased neutrophil levels and decreased lymphocyte levels were significantly associated with greater CKD incidence after adjustment for covariates. African Americans tended to have similar but stronger patterns of association between circulating leukocytes and CKD incidence than whites, although the differences between race groups were not statistically significant. We also found that eGFR and BP were higher at each visit in African Americans than whites between ages 45 and 65. These findings support a potential role for circulating specific leukocytes in the pathogenesis of kidney dysfunction, especially in African Americans, indicating the leukocyte-related renal mechanism of essential hypertension (HT).

Chronic antagonism of the mineralocorticoid receptor ameliorates hypertension and end organ damage in a rodent model of salt-sensitive hypertension

We investigated the effects of chronic mineralocorticoid receptor blockade with eplerenone on the development and progression of hypertension and end organ damage in Dahl salt-sensitive rats. Eplerenone significantly attenuated the progressive rise in systolic blood pressure (SBP) (204 ± 3 vs. 179±3 mmHg, p < 0.05), reduced proteinuria (605.5 ± 29.6 vs. 479.7 ± 26.1 mg/24h, p < 0.05), improved injury scores of glomeruli, tubules, renal interstitium, and vasculature in Dahl salt-sensitive rats fed a high-salt diet. These results demonstrate that mineralocorticoid receptor antagonism provides target organ protection and attenuates the development of elevated blood pressure (BP) in a model of salt-sensitive hypertension.

Cardiovascular-renal and metabolic characterization of a rat model of polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is the most common reproductive dysfunction in premenopausal women. PCOS is also associated with increased risk of cardiovascular disease when PCOS first occurs and later in life. Hypertension, a common finding in women with PCOS, is a leading risk factor for cardiovascular disease. The mechanisms responsible for hypertension in women with PCOS have not been elucidated.

OBJECTIVE

This study characterized the cardiovascular-renal consequences of hyperandrogenemia in a female rat model.

METHODS

Female Sprague-Dawley rats (aged 4-6 weeks) were implanted with dihydrotestosterone or placebo pellets lasting 90 days. After 10 to 12 weeks, blood pressure (by radiotelemetry), renal function (glomerular filtration rate, morphology, protein, and albumin excretion), metabolic parameters (plasma insulin, glucose, leptin, cholesterol, and oral glucose tolerance test), inflammation (plasma tumor necrosis factor-α), oxidative stress (mRNA expression of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits, p22(phox), p47(phox), gp91(phox), and NOX4), nitrate/nitrite excretion and mRNA expression of components of the renin-angiotensin system (angiotensinogen, angiotensin-I-converting enzyme [ACE], and AT1 receptor) were determined.

RESULTS

Plasma dihydrotestosterone increased 3-fold in hyperandrogenemic female (HAF) rats, whereas plasma estradiol levels did not differ compared with control females. HAF rats exhibited estrus cycle dysfunction. They also had increased food intake and body weight, increased visceral fat, glomerular filtration rate, renal injury, insulin resistance and metabolic dysfunction, oxidative stress, and increased expression of angiotensinogen and ACE and reduced AT1 receptor expression.

CONCLUSIONS

The HAF rat is a unique model that exhibits many of the characteristics of PCOS in women and is a useful model to study the mechanisms responsible for PCOS-mediated hypertension.

Association between circulating specific leukocyte types and blood pressure: the atherosclerosis risk in communities (ARIC) study

Although total white blood cell (WBC) count has been associated with hypertension, the association between specific WBC types and blood pressure (BP) levels has not been studied. In a cohort of 5746 middle-age African-American and white adults free of clinical cardiovascular disease and cancer and not taking hypertension or anti-inflammatory medications, BP was measured at baseline and 3, 6, and 9 years later. Levels of circulating neutrophils, lymphocytes, and monocytes were measured at baseline. In African-Americans, but much less so in whites, increased neutrophil levels and decreased lymphocyte levels were significantly associated with elevation of BP but did not influence the rate of change of BP over time. The mean BP difference between the highest and lowest quartiles of neutrophils was approximately 8 mm Hg for systolic BP (SBP), 4 mm Hg for mean arterial pressure (MAP), and 5 mm Hg for pulse pressure (PP). The mean BP difference between the lowest and highest quartiles of lymphocytes was approximately 6 mm Hg for SBP, 2 mm Hg for diastolic BP (DBP), 3 mm Hg for MAP, and 4 mm Hg for PP. Increased neutrophils and decreased lymphocytes are significantly correlated with the regulation of BP and the development of hypertension, especially in African-Americans.

Endothelin activation of reactive oxygen species mediates stress-induced pressor response in Dahl salt-sensitive prehypertensive rats

Experiments were designed to test the hypothesis that endothelin (ET) and/or reactive oxygen species contribute to the pressor response induced by acute air jet stress in normotensive Dahl salt-sensitive rats maintained on a normal salt diet (prehypertensive). Mean arterial pressure was chronically monitored by telemetry before and after 3-day treatment with the free radical scavenger 4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl (Tempol) or ET receptor antagonists ABT-627 (ET A antagonist) or A-182086 (ET A/B antagonist) supplied in the drinking water. Rats were restrained and subjected to pulsatile air jet stress (3 minutes). Plasma samples at baseline and during acute stress were analyzed for 8-isoprostane (measure of reactive oxygen species production) and ET. Neither Tempol nor ET receptor antagonist treatment had an effect on baseline mean arterial pressure or plasma 8-isoprostane. The pressor response to acute stress was accompanied by significant increases in plasma 8-isoprostane and ET. Tempol significantly reduced both the total pressor response (area under the curve) and the stress-mediated increase in plasma 8-isoprostane; conversely, Tempol had no effect on the stress-induced increase in plasma ET. Combined ET(A/B) antagonism, but not selective ET(A) receptor blockade, similarly suppressed the pressor response to stress and stress-mediated rise in 8-isoprostane. Together these results indicate that reactive oxygen species contribute to the pressor response to acute air jet stress. Furthermore, the increase in reactive oxygen species occurs downstream of ET(B) receptor activation.

Polyphenol-containing azuki bean (Vigna angularis) seed coats attenuate vascular oxidative stress and inflammation in spontaneously hypertensive rats

We investigated the effects of azuki bean (Vigna angularis) seed coats (ABSC), which contain polyphenols, on the vascular oxidative stress and inflammation associated with hypertension. Spontaneously hypertensive rats (SHR) and control normotensive Wistar-Kyoto (WKY) rats were divided into 2 groups each. One group was fed 0% ABSC; the other, a 1.0% ABSC-containing diet. Tail systolic blood pressure (SBP) was examined throughout ABSC treatment. At 8 weeks, vascular superoxide (O(2)(-)) production was measured by lucigenin-enhanced chemiluminescence. mRNA expressions of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits, macrophage chemoattractant protein-1 (MCP-1) and its receptor C-C chemokine receptor 2 (CCR2) in the aorta were analyzed by reverse transcriptase-polymerase chain reaction. Protein expressions of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were determined by western blotting. Polyphenol-containing ABSC suppressed the elevation of SBP throughout the treatment period. The NADPH-stimulated O(2)(-) level decreased significantly in the aorta of ABSC-treated SHR compared with the level of untreated SHR. The p47phox and Nox4 mRNA expression increased significantly in untreated SHR compared with that in WKY rats. Conversely, the level of p47phox mRNA was significantly lower in ABSC-treated SHR than in untreated SHR. The protein abundance of both iNOS and COX-2 was significantly decreased in the aorta of the ABSC-treated SHR compared with this abundance in untreated SHR. The MCP-1 and CCR2 mRNA expressions increased in untreated SHR, and these levels were significantly lower in ABSC-treated SHR. In conclusion, our results suggested that polyphenol-containing ABSC could attenuate vascular oxidative stress and inflammation during the progression of hypertension, and this may lead to an improvement in hypertension.

Flavonoids and age-related disease: risk, benefits and critical windows

Plant derived products are consumed by a large percentage of the population to prevent, delay and ameliorate disease burden; however, relatively little is known about the efficacy, safety and underlying mechanisms of these traditional health products, especially when taken in concert with pharmaceutical agents. The flavonoids are a group of plant metabolites that are common in the diet and appear to provide some health benefits. While flavonoids are primarily derived from soy, many are found in fruits, nuts and more exotic sources, e.g., kudzu. Perhaps the strongest evidence for the benefits of flavonoids in diseases of aging relates to their effect on components of the metabolic syndrome. Flavonoids from soy, grape seed, kudzu and other sources all lower arterial pressure in hypertensive animal models and in a limited number of tests in humans. They also decrease the plasma concentration of lipids and buffer plasma glucose. The underlying mechanisms appear to include antioxidant actions, central nervous system effects, gut transport alterations, fatty acid sequestration and processing, PPAR activation and increases in insulin sensitivity. In animal models of disease, dietary flavonoids also demonstrate a protective effect against cognitive decline, cancer and metabolic disease. However, research also indicates that the flavonoids can be detrimental in some settings and, therefore, are not universally safe. Thus, as the population ages, it is important to determine the impact of these agents on prevention/attenuation of disease, including optimal exposure (intake, timing/duration) and potential contraindications.

Tumor necrosis factor-alpha: a possible priming agent for the polymorphonuclear leukocyte-reduced nicotinamide-adenine dinucleotide phosphate oxidase in hypertension

In the Sabra rat, oxidative stress (OS) and inflammation precede the development of hypertension. Inhibition of the phagocytic NADPH oxidase attenuates the rise in blood pressure. The present study was set to identify possible priming agents for this enzyme and to test the hypothesis that the phagocytic NADPH oxidase contributes to OS and inflammation. Sabra salt-sensitive and Sabra salt-resistant rats were salt loaded or provided regular chow for 60 days with or without apocynin to inhibit NADPH oxidase. Levels of interleukin 6, tumor necrosis factor-alpha, and interferon-gamma served as indices of inflammation. Extracellular and intracellular levels of the polymorphonuclear leukocyte tumor necrosis factor-alpha receptors (p55 and p75) were assessed by flow cytometry in young and adult rats. NADPH oxidase activity and expression of p47phox were measured in polymorphonuclear leukocytes and aortic rings. Malondialdehyde and carbonylated fibrinogen served as indices of OS. Inflammatory and OS indices excluding interferon-gamma were higher in the hypertensive state and reduced by apocynin. Levels of malondialdehyde and tumor necrosis factor-alpha were elevated already in the prehypertensive state. No differences were found in the levels of p75. The extracellular expression of p55 was higher in adult Sabra salt-resistant compared with Sabra salt-sensitive rats (7.46+/-2.2% versus 2.1+/-0.5%; P<0.05), whereas levels of the intracellular p55 were higher in adult Sabra salt-sensitive rats (3.2+/-2% versus 1.1+/-0.5%; P<0.05). In young normotensive rats, the extracellular levels of p55 were higher in Sabra salt-sensitive compared with Sabra salt-resistant rats (10.6+/-5.2% versus 2.9+/-1.5%; P<0.01). Tumor necrosis factor-alpha plays a role in activation of the polymorphonuclear leukocyte NADPH oxidase, thereby contributing to systemic OS, inflammation, and the development of hypertension in this model.

PKC-alpha mediates flow-stimulated superoxide production in thick ascending limbs

We showed that luminal flow increases net superoxide (O(2)(-)) production via NADPH oxidase in thick ascending limbs. Protein kinase C (PKC) activates NADPH oxidase activity in phagocytes, cardiomyocytes, aortic endothelial cells, vascular smooth muscle cells, and renal mesangial cells. However, the flow-activated pathway that induces NADPH oxidase activity in thick ascending limbs is unclear. We hypothesized that PKC mediates flow-stimulated net O(2)(-) production by thick ascending limbs. Initiation of flow (20 nl/min) increased net O(2)(-) production from 4 +/- 1 to 61 +/- 12 AU/s (P < 0.007; n = 5). The NADPH oxidase inhibitor apocynin completely blocked the flow-induced increase in net O(2)(-) production (2 +/- 1 vs. 1 +/- 1 AU/s; P > 0.05; n = 5). Flow-stimulated O(2)(-) was also blocked in p47(phox)-deficient mice. We measured flow-stimulated PKC activity with a fluorescence resonance energy transfer (FRET)-based membrane-targeted PKC activity reporter and found that the FRET ratio increased from 0.87 +/- 0.02 to 0.96 +/- 0.04 AU (P < 0.05; n = 6). In the absence of flow, the PKC activator phorbol 12-myristate 13-acetate (200 nM) enhanced net O(2)(-) production from 5 +/- 2 to 92 +/- 6 AU/s (P < 0.001; n = 6). The PKC-alpha- and betaI-selective inhibitor Gö 6976 (100 nM) decreased flow-stimulated net O(2)(-) production from 54 +/- 15 to 2 +/- 1 AU/s (P < 0.04; n = 5). Flow-induced net O(2)(-) production was inhibited in thick ascending limbs transduced with dominant-negative (dn)PKC-alpha but not dnPKCbetaI or LacZ (Delta = 11 +/- 3 AU/s for dnPKCalpha, 55 +/- 7 AU/s for dnPKCbetaI, and 63 +/- 7 AU/s for LacZ; P < 0.001; n = 6). We concluded that flow stimulates net O(2)(-) production in thick ascending limbs via PKC-alpha-mediated activation of NADPH oxidase.

NOX2 is the primary source of angiotensin II-induced superoxide in the macula densa

Macula densa (MD)-mediated regulation of renal hemodynamics via tubuloglomerular feedback is regulated by interactions between factors such as superoxide (O(2)(-)) and angiotensin II (ANG II). We have reported that NaCl-induced O(2)(-) in the MD is produced by the NOX2 isoform of NADPH oxidase (NOX); however, the source of ANG II-induced O(2)(-) in MD is unknown. Thus we determined the pathways by which ANG II increased O(2)(-) in the MD by measuring O(2)(-) in ANG II-treated MMDD1 cells, a MD-like cell line. ANG II caused MMDD1 O(2)(-) levels to increase by more than twofold (P < 0.01). This increase was blocked by losartan (AT(1) receptor blocker) but not PD-123319 (AT(2) receptor antagonist). Apocynin (a NOX inhibitor) decreased O(2)(-) by 86% (P < 0.01), whereas oxypurinol (a xanthine oxidase inhibitor) and NS-398 (a cyclooxygenase-2 inhibitor) had no significant effect. The NOX-dependent increase in O(2)(-) was due to the NOX2 isoform; a short interfering (si)RNA against NOX2 blunted ANG II-induced increases in O(2)(-), whereas the NOX4/siRNA did not. Finally, we found that inhibiting the Rac1 subunit of NOX blunted ANG II-induced O(2)(-) production in NOX4/siRNA-treated cells but did not further decrease it in NOX2/siRNA-treated cells. Our results indicate that ANG II stimulates O(2)(-) production in the MD primarily via AT(1)-dependent activation of NOX2. Rac1 is required for the full activation of NOX2. This pathway may be an important component of ANG II enhancement of tubuloglomerular feedback.

Dynamic convergence and divergence of renal genomic and biological pathways in protection from Dahl salt-sensitive hypertension

Chromosome 13 consomic and congenic rat strains were analyzed to investigate the pattern of genomic pathway utilization involved in protection against salt-sensitive hypertension and renal injury. Introgression of the entire Brown-Norway chromosome 13 (consomic SS-13(BN)) or nonoverlapping segments of this chromosome (congenic strains, 16 Mbp in D13Rat151-D13Rat197 or 14 Mbp in D13Rat111-D13Got22) into the genome of the Dahl salt-sensitive rat attenuated salt-induced hypertension and proteinuria. mRNA abundance profiles in the renal cortex and the renal medulla from rats receiving 0.4% or 8% NaCl diets revealed two important features of pathway recruitment in these rat strains. First, the two congenic strains shared alterations in several pathways compared with Dahl salt-sensitive rats, despite the fact that the genomic segments introgressed in the two congenic strains did not overlap. Second, even though the genomic segment introgressed in each congenic strain was a part of the chromosome introgressed in the consomic strain, pathways altered in each congenic strain were not simply a subset of those altered in the consomic. Supporting the relevance of the mRNA data, differential expression of oxidative stress-related genes among the four strains of rats was associated with differences in urinary excretion of lipid peroxidation products. The findings suggest that different genetic alterations might converge to influence shared pathways in protection from hypertension, and that, depending on the genomic context, the same genetic alteration might diverge to affect different pathways.

Novel role of fumarate metabolism in dahl-salt sensitive hypertension

In a previous proteomic study, we found dramatic differences in fumarase in the kidney between Dahl salt-sensitive rats and salt-insensitive consomic SS-13(BN) rats. Fumarase catalyzes the conversion between fumarate and l-malate in the tricarboxylic acid cycle. Little is known about the pathophysiological significance of fumarate metabolism in cardiovascular and renal functions, including salt-induced hypertension. The fumarase gene is located on the chromosome substituted in the SS-13(BN) rat. Sequencing of fumarase cDNA indicated the presence of lysine at amino acid position 481 in Dahl salt-sensitive rats and glutamic acid in Brown Norway and SS-13(BN) rats. Total fumarase activity was significantly lower in the kidneys of Dahl salt-sensitive rats compared with SS-13(BN) rats, despite an apparent compensatory increase in fumarase abundance in Dahl salt-sensitive rats. Intravenous infusion of a fumarate precursor in SS-13(BN) rats resulted in a fumarate excess in the renal medulla comparable to that seen in Dahl salt-sensitive rats. The infusion significantly exacerbated salt-induced hypertension in SS-13(BN) rats (140+/-3 vs125+/-2 mm Hg in vehicle control at day 5 on a 4% NaCl diet; P<0.05). In addition, the fumarate infusion increased renal medullary tissue levels of H2O2. Treatment of cultured human renal epithelial cells with the fumarate precursor also increased cellular levels of H2O2. These data suggest a novel role for fumarate metabolism in salt-induced hypertension and renal medullary oxidative stress.

Effect of glutathione peroxidase mimic ebselen (PZ51) on endothelium and vascular structure of stroke‐prone spontaneously hypertensive rats

BACKGROUND

To investigate whether extrinsic antioxidant seleno-glutathione peroxidase mimic ebselen (PZ51) can protect endothelium and vascular structure of stroke-prone spontaneously hypertensive rats (SHRsp) during the chronic process of hypertension.

METHODS

Twenty-two 8-week-old SHRsp were randomized into a PZ51 group and a control group, and administered by gavage for 6 weeks. We examined the level of nitric oxide (NO) and malonaldehyde (MDA) in plasma. The intima-media thickness (IMT) of the common carotid artery (CCA) was measured by an image-analysis system. The endothelium of the CCA was observed by scanning electron microscopy. The eNOS protein of the major artery was assayed by immunohistochemistry and western blotting.

RESULTS

Compared with the control group, PZ51 decreased plasma MDA (7.88+/-1.06 vs 10.88+/-1.73 nmol/l, p<0.001) and increased plasma NO (40.02+/-9.74 vs 22.22+/-10.05 micromol/l, p<0.001), increased eNOS protein expression (8.25+/-2.36 vs 4.46+/-3.14, p=0.026), decreased IMT (69.85+/-5.47 vs 76.60+/-6.53 microm, p<0.05) significantly and alleviated the damage to the endothelium of the CCA.

CONCLUSION

Administration of PZ51 for 6 weeks can protect the endothelium and inhibit vascular remodeling, maybe due to its suppression of lipid peroxide formation and increase in eNOS protein expression.

Association between SNP heterozygosity and quantitative traits in the Framingham Heart Study

Associations between multilocus heterozygosity and fitness traits, also termed heterozygosity and fitness correlations (HFCs), have been reported in numerous organisms. These studies, in general, indicate a positive relationship between heterozygosity and fitness traits. We studied the association between genome-wide heterozygosity at 706 non-synonymous and synonymous SNPs and 19 quantitative traits, including morphological, biochemical and fitness traits in the Framingham Heart Study. Statistically significant association was found between heterozygosity and systolic and diastolic blood pressures as well as left ventricular diameter and wall thickness. These results suggest that heterozygosity may be associated with traits, such as blood pressure that closely track environmental variations. Balancing selection may be operating in the maintenance of heterozygosity and the major components of blood pressure and hypertension. Genome wide SNP heterozygosity may be used to understand the phenomenon of dominance as well as the evolutionary basis of many quantitative traits in humans.

Oxidative stress and hypertension

This review has summarized some of the data supporting a role of ROS and oxidant stress in the genesis of hypertension. There is evidence that hypertensive stimuli, such as high salt and angiotensin II, promote the production of ROS in the brain, the kidney, and the vasculature and that each of these sites contributes either to hypertension or to the untoward sequelae of this disease. Although the NADPH oxidase in these various organs is a predominant source, other enzymes likely contribute to ROS production and signaling in these tissues. A major clinical challenge is that the routinely used antioxidants are ineffective in preventing or treating cardiovascular disease and hypertension. This is likely because these drugs are either ineffective or act in a non-targeted fashion, such that they remove not only injurious ROS Fig. 5. Proposed role of T cells in the genesis of hypertension and the role of the NADPH oxidase in multiple cells/organs in modulating this effect. In this scenario, angiotensin II stimulates an NADPH oxidase in the CVOs of the brain, increasing sympathetic outflow. Sympathetic nerve terminals in lymph nodes activate T cells, and angiotensin II also directly activates T cells. These stimuli also activate expression of homing signals in the vessel and likely the kidney, which attract T cells to these organs. T cells release cytokines that stimulate the vessel and kidney NADPH oxidases, promoting vasoconstriction and sodium retention. SFO, subfornical organ. 630 Harrison & Gongora but also those involved in normal cell signaling. A potentially important and relatively new direction is the concept that inflammatory cells such as T cells contribute to hypertension. Future studies are needed to understand the interaction of T cells with the CNS, the kidney, and the vasculature and how this might be interrupted to provide therapeutic benefit.

Oxidative stress in the sympathetic premotor neurons contributes to sympathetic activation in renovascular hypertension

BACKGROUND

Based on previous data, we hypothesized that an increase of angiotensin II (Ang II)-via the Ang II type 1 (AT-1) receptor-in the rostral ventrolateral medulla (RVLM) and the paraventricular nucleus (PVN) of the hypothalamus could activate NAD(P)H oxidase that will produce superoxides resulting in increased sympathetic activity and hypertension.

METHODS

The mRNA expression of AT-1 receptors, NAD(P)H oxidase subunits (p47phox and gp91phox), and CuZnSOD were analyzed in the RVLM and PVN of male Wistar rats (Goldblatt hypertension model, 2K-1C). In addition, we administered Tempol 1 and 5 nmol into the RVLM, PVN, or systemically. The mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) were analyzed.

RESULTS

The AT-1 mRNA expression and NAD(P)H oxidase subunits was greater in the RVLM and PVN in 2K-1C compared to the control group. Furthermore, the CuZnSOD expression was similar in both groups. Tempol 1 nmol into the RVLM reduced MAP (15 +/- 1%) and RSNA (11 +/- 2%) only in 2K-1C rats. Tempol (5 nmol) in the same region decreased the MAP (12 +/- 4%) and RSNA (20 +/- 7%), respectively, only in 2K-1C. In the PVN, Tempol 5 nmol resulted in a significant fall in the MAP (24 +/- 1%) and in the RSNA (7.9 +/- 2%) only in the 2K-1C. Acute intravenous (IV) infusion of Tempol decreased MAP and RSNA in the 2K-1C but not in the control rats.

CONCLUSIONS

The data suggest that the hypertension and sympathoexcitation in 2K-1C rats were associated with an increase in oxidative stress within the RVLM, the PVN and systemically.

Time-course and mechanisms of restored vascular relaxation by reduced salt intake and angiotensin II infusion in rats fed a high-salt diet

OBJECTIVE

This study determined the mechanisms and time-course of recovery of vascular relaxation in middle cerebral arteries (MCAs) of salt-fed Sprague-Dawley rats returned to a low-salt (LS) diet (0.4% NaCl) or infused with low-dose angiotensin II (ANG II).

METHODS

Rats were fed a high-salt (HS) diet (4% NaCl) for 3 days or 4 weeks before returning to an LS diet for various periods. Other rats fed a HS diet (HS+ANG II) received a chronic (3 days) intravenous (i.v.) infusion of a low dose of ANG II (5 ng kg(-1) min(-1)) to prevent salt-induced ANG II suppression.

RESULTS

The HS diet eliminated the increase in cerebral blood flow in response to acetylcholine (ACh) infusion and the relaxation of MCA in response to ACh, iloprost, cholera toxin, and reduced PO2. Recovery of vascular relaxation was slow, requiring at least 2 weeks of the LS diet, regardless of the duration of exposure to a HS diet. Hypoxic dilation was mediated by cyclo-oxygenase metabolites and ACh-induced dilation was mediated via nitric oxide in LS rats and in HS rats returned to the LS diet or receiving ANG II infusion.

CONCLUSIONS

Returning to a LS diet for 2 weeks or chronic 3-day ANG II infusion restores the mechanisms that normally mediate cerebral vascular relaxation.

NADPH oxidase contributes to renal damage and dysfunction in Dahl salt-sensitive hypertension

The goal of this study was to test the hypothesis that NADPH oxidase contributes importantly to renal cortical oxidative stress and inflammation, as well as renal damage and dysfunction, and increases in arterial pressure. Fifty-four 7- to 8-wk-old Dahl salt-sensitive (S) or R/Rapp strain rats were maintained for 5 wk on a high sodium (8%) or high sodium + apocynin (1.5 mmol/l in drinking water). Arterial and venous catheters were implanted on day 21. By day 35 in the high-Na S rats, mRNA expression of renal cortical gp91phox, p22phox, p47phox, and p67phox NADPH subunits in S rats increased markedly, and treatment of high-Na S rats with the NADPH oxidase inhibitor apocynin resulted in significant decreases in mRNA expression of these NADPH oxidase subunits. At the same time, in apocynin-treated S rats 1) renal cortical GSH/GSSG ratio increased, 2) renal cortical O2(.-) release and NADPH oxidase activity decreased, and 3) renal glomerular and interstitial damage markedly fell. Apocynin also decreased renal cortical monocyte/macrophage infiltration, and apocynin, but not the xanthine oxidase inhibitor allopurinol, attenuated decreases in renal hemodynamics and lowered arterial pressure. These data suggest that NADPH oxidase plays an important role in causing renal cortical oxidative stress and inflammation, which lead to decreases in renal hemodynamics, renal cortical damage, and increases in arterial pressure.

Chemistry and antihypertensive effects of tempol and other nitroxides

Nitroxides can undergo one- or two-electron reduction reactions to hydroxylamines or oxammonium cations, respectively, which themselves are interconvertible, thereby providing redox metabolic actions. 4-Hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (tempol) is the most extensively studied nitroxide. It is a cell membrane-permeable amphilite that dismutates superoxide catalytically, facilitates hydrogen peroxide metabolism by catalase-like actions, and limits formation of toxic hydroxyl radicals produced by Fenton reactions. It is broadly effective in detoxifying these reactive oxygen species in cell and animal studies. When administered intravenously to hypertensive rodent models, tempol caused rapid and reversible dose-dependent reductions in blood pressure in 22 of 26 studies. This was accompanied by vasodilation, increased nitric oxide activity, reduced sympathetic nervous system activity at central and peripheral sites, and enhanced potassium channel conductance in blood vessels and neurons. When administered orally or by infusion over days or weeks to hypertensive rodent models, it reduced blood pressure in 59 of 68 studies. This was accompanied by correction of salt sensitivity and endothelial dysfunction and reduced agonist-evoked oxidative stress and contractility of blood vessels, reduced renal vascular resistance, and increased renal tissue oxygen tension. Thus, tempol is broadly effective in reducing blood pressure, whether given by acute intravenous injection or by prolonged administration, in a wide range of rodent models of hypertension.

Angiotensin II-dependent hypertension increases Na transport-related oxygen consumption by the thick ascending limb

Renal medullary superoxide (O(2)(-)) increases in angiotensin (Ang) II-dependent hypertension. O(2)(-) increases thick ascending limb Na transport, but the effect of Ang II-dependent hypertension on the thick ascending limb is unknown. We hypothesized that Ang II-dependent hypertension increases thick ascending limb NaCl transport because of enhanced O(2)(-) production and increased protein kinase C (PKC) alpha activity. We measured the effect of Ang II-dependent hypertension on furosemide-sensitive oxygen consumption (a measure of Na transport), O(2)(-) production, and PKCalpha translocation (a measure of PKCalpha activity) in thick ascending limb suspensions. Ang II-dependent hypertension increased furosemide-sensitive oxygen consumption (26.2+/-1.0% versus 36.6+/-1.2% of total oxygen consumption; P<0.01). O(2)(-) was also increased (1.1+/-0.2 versus 3.2+/-0.5 nmol of O(2)(-)/min per milligram of protein; P<0.03) in thick ascending limbs. Unilateral renal infusion of Tempol decreased O(2)(-) (2.4+/-0.4 versus 1.2+/-0.2 nmol of O(2)(-)/min per milligram of protein; P<0.04) and furosemide-sensitive oxygen consumption (32.8+/-1.3% versus 24.0+/-2.1% of total oxygen consumption; P<0.01) in hypertensive rats. Tempol did not affect O(2)(-) or furosemide-sensitive oxygen consumption in normotensive controls and did not alter systolic blood pressure. Ang II-dependent hypertension increased PKCalpha translocation (5.7+/-0.3 versus 13.8+/-1.4 AU per milligram of protein; P<0.01). Unilateral renal infusion of Tempol reduced PKCalpha translocation (5.0+/-0.9 versus 10.4+/-2.6 AU per milligram of protein; P<0.04) in hypertensive rats. Unilateral renal infusion of the PKCalpha inhibitor Gö6976 reduced furosemide-sensitive oxygen consumption (37.4+/-1.5% versus 25.1+/-1.0% of total oxygen consumption; P<0.01) in hypertensive rats. We conclude that Ang II-dependent hypertension enhances thick ascending limb Na transport-related oxygen consumption by increasing O(2)(-) and PKCalpha activity.

The antioxidant tempol attenuates pressure overload-induced cardiac hypertrophy and contractile dysfunction in mice fed a high-fructose diet

We have previously shown that high-sugar diets increase mortality and left ventricular (LV) dysfunction during pressure overload. The mechanisms behind these diet-induced alterations are unclear but may involve increased oxidative stress in the myocardium. The present study examined whether high-fructose feeding increased myocardial oxidative damage and exacerbated systolic dysfunction after transverse aortic constriction (TAC) and if this effect could be attenuated by treatment with the antioxidant tempol. Immediately after surgery, TAC and sham mice were assigned to a high-starch diet (58% of total energy intake as cornstarch and 10% fat) or high-fructose diet (61% fructose and 10% fat) with or without the addition of tempol [0.1% (wt/wt) in the chow] and maintained on the treatment for 8 wk. In response to TAC, fructose-fed mice had greater cardiac hypertrophy (55.1% increase in the heart weight-to-tibia length ratio) than starch-fed mice (22.3% increase in the heart weight-to-tibia length ratio). Treatment with tempol significantly attenuated cardiac hypertrophy in fructose-fed TAC mice (18.3% increase in the heart weight-to-tibia ratio). Similarly, fructose-fed TAC mice had a decreased LV area of fractional shortening (from 38+/-2% in sham to 22+/-4% in TAC), which was prevented by tempol treatment (33+/-3%). Markers of lipid peroxidation in fructose-fed TAC hearts were also blunted by tempol. In conclusion, tempol significantly blunted markers of cardiac hypertrophy, LV remodeling, contractile dysfunction, and oxidative stress in fructose-fed TAC mice.

Associations of hypertension and its complications with variations in the xanthine dehydrogenase gene

Hyperuricemia and oxidative stress participate in the pathophysiology of hypertension and its complications. Xanthine dehydrogenase (XDH) produces urate and, in its oxidase isoform, reactive oxygen species. Here we have studied whether or not the genetic variations in XDH could be implicated in hypertension and its complications. By sequencing the promoter region and all exons of XDH in 48 subjects, we identified three missense mutations (G172R, A932T, N1109T) in a heterozygous state in addition to 34 variations, including 15 common single nucleotide polymorphisms (SNPs). The three missense mutations and eight common SNPs (11488C>G, 37387A>G, 44408A>G, 46774G>A, 47686C>T, 49245A>T, 66292C>G, and 69901A>C) were genotyped in 953 hypertensive Japanese subjects and in 1,818 subjects from a general Japanese population. Four hypertensive patients with rare missense mutations (G172R or N1109T) in homozygous form had severe hypertension. Multivariate logistic regression analysis showed a significant association of three SNPs with hypertension in men: 47686C>T (exon 22, odds ratio [OR]: 1.52, p = 0.047) and 69901A>C (intron 31, OR: 3.14, p = 0.039) in the recessive model, and 67873A>C (N1109T) (exon 31, OR: 1.84, p = 0.018) in the dominant model. After full adjustment for all confounding factors, only one polymorphism (69901A>C) was found to be associated with carotid atherosclerosis in the dominant model (p = 0.028). Multiple logistic regression analysis showed that one SNP (66292C>G) was significantly associated with chronic kidney disease (CKD: estimated creatinine clearance < 60 ml/min) in the recessive model (p = 0.0006). Our results suggest that genetic variations in XDH contribute partly to hypertension and its complications, including atherosclerosis and CKD.

Aldosterone induces mesangial cell apoptosis both in vivo and in vitro

Both clinical and experimental reports indicate that aldosterone contributes to the progression of renal failure independent of its hemodynamic effects. In the present study, we evaluated effect of aldosterone on human mesangial cell (MC) growth. Aldosterone induced apoptotic and mitogenic effects on MCs. Aldosterone promoted MC apoptosis in a dose- and time-dependent manner. Spironolactone, a mineralocorticoid receptor antagonist, inhibited aldosterone-induced MC apoptosis. Similarly, antioxidants and free radical scavengers partially attenuated proapoaptotic effects of aldosterone. Aldosterone also enhanced dephosphorylation of phospho-Bad and accumulation of cytosolic cytochrome c in MCs. In in vivo studies, rats were randomly assigned to receive normal saline, aldosterone, or eplerenone + aldosterone for 28 days. Systolic blood pressure, urinary albumin excretion rate, serum creatinine, and aldosterone were measured. Aldosterone-infused rats developed elevated systolic blood pressure and albuminuria when compared with control rats. Aldosterone-treated rats also showed greater numbers of apoptosed MCs. This proapoptotic effect of aldosterone was inhibited by eplerenone, a selective aldosterone antagonist. These findings suggest that aldosterone, besides its hemodynamic effects, may also directly contribute to the occurrence of MC apoptosis.

Signaling pathways modulated by fish oil in salt-sensitive hypertension

Although many studies have indicated that fish oil (FO) improves cardiovascular risk factors and reduces histopathological manifestations of injury in experimental renal injury models, potential mechanisms underlying this protective effect have not been adequately defined. The objective of this study was to identify potential signaling pathways that confer protection in the Dahl rat model of salt-sensitive hypertension. Male Dahl salt-sensitive rats (n = 10/group) were provided with formulated diets containing 8% NaCl, 20% protein, and 25% FO or 25% corn oil (CO) for 28 days. FO reduced blood pressure (-11% at 4 wk; P < 0.05), urine protein excretion (-45% at 4 wk; P < 0.05), plasma cholesterol and triglyceride levels (-54%, P < 0.001; and -58%, P < 0.05), and histopathological manifestations of renal injury, including vascular hypertrophy, segmental and global glomerular sclerosis, interstitial fibrosis, and tubular atrophy. Interstitial inflammation was significantly reduced by FO (-32%; P < 0.001), as assessed by quantitative analysis of ED1-positive cells in sections of the renal cortex. FO reduced tubulointerstitial proliferative activity, as assessed by Western blot analysis of cortical homogenates for PCNA (-51%; P < 0.01) and quantitative analysis of Mib-1-stained sections of the renal cortex (-42%; P < 0.001). Decreased proliferative activity was associated with reduced phospho-ERK expression (-37%; P < 0.005) and NF-kappaB activation (-42%; P < 0.05). FO reduced cyclooxygenase (COX)-2 expression (-63%; P < 0.01) and membrane translocation of the NADPH oxidase subunits p47(phox) and p67(phox) (-26 and -34%; P < 0.05). We propose that FO ameliorates renal injury in Dahl salt-sensitive rats through the inhibition of ERK, decreased NF-kappaB activation, inhibition of COX-2 expression, and decreased NADPH oxidase activation.

Oxidative Stress Causes Renal Dopamine D1 Receptor Dysfunction and Salt-Sensitive Hypertension in Sprague-Dawley Rats

Renal dopamine plays an important role in maintaining sodium homeostasis and blood pressure (BP) during increased sodium intake. The present study was carried out to determine whether renal dopamine D1 receptor (D1R) dysfunction contributes to increase in salt sensitivity during oxidative stress. Male Sprague-Dawley rats, divided into various groups, received tap water (vehicle); 1% NaCl (high salt [HS]); l -buthionine sulfoximine (BSO), an oxidant; and HS plus BSO with or without Tempol, an antioxidant, for 12 days. Compared with vehicle, HS intake increased urinary dopamine production and decreased basal renal Na/K-ATPase activity but did not affect BP. BSO-treated rats exhibited oxidative stress and a mild increase in BP. In these rats, D1R expression and G protein coupling were reduced, and SKF38393, a D1R agonist, failed to inhibit Na/K-ATPase activity and promote sodium excretion. Concomitant administration of BSO and HS caused oxidative stress, D1R dysfunction, and a marked increase in BP. Although renal dopamine production was increased, it failed to reduce the basal Na/K-ATPase activity in these animals. Treatment of BSO plus HS rats with Tempol decreased oxidative stress and restored endogenous, as well as exogenous, D1R agonist-mediated Na/K-ATPase inhibition and normalized BP. In conclusion, during HS intake, the increased dopamine production via Na/K-ATPase inhibition prevents an increase in BP. During oxidative stress, D1R function is defective, and there is mild hypertension. However, in the presence of oxidative stress, HS intake causes marked elevation in BP, which results from a defective renal D1R function leading to the failure of dopamine to inhibit Na/K-ATPase and promote sodium excretion.

Cellular stretch increases superoxide production in the thick ascending limb

Superoxide (O(2)(-)) is an important regulator of kidney function. We have recently shown that luminal flow stimulates O(2)(-) production in the thick ascending limb (TAL), attributable in part to mechanical factors. Stretch, pressure and shear stress all change when flow increases in the TAL. We hypothesized that stretch rather than shear stress or pressure per se stimulates O(2)(-) production by TALs. We measured O(2)(-) production in isolated perfused rat TALs using fluorescence microscopy and dihydroethidium. Tubules were perfused with a Na-free solution to eliminate the confounding effect of Na transport. Flow induced an increase in O(2)(-) production from 29+/-4 to 90+/-8 AU/s (P<0.002; n=5). The response to flow is rapidly reversible. O(2)(-) production by TALs perfused at 10 nL/min decreased from 113+/-6 to 25+/-10 AU/s (P<0.003; n=4) 15 minutes after flow was stopped. Increasing pressure and stretch in the absence of shear stress caused a significant increase in O(2)(-) production (40+/-6 to 118+/-17 AU/s; P<0.02; n=5). In contrast, eliminating shear stress had no effect (107+/-9 versus 108+/-10 AU/s; n=5). Increasing stretch by 27+/-2% in the presence of flow while reducing pressure stimulated O(2)(-) production from 66+/-7 to 84+/-9 AU/s (29+/-8%; P<0.02; n=5). Tempol inhibited this increase (n=5). We conclude that increasing stretch rather than pressure or shear stress accounts for the mechanical aspect of flow-induced O(2)(-) production in the TAL. Stretch of the TAL during hypertension, diabetes, and salt loading may contribute to renal damage.

Interactions between oxidative stress and inflammation in salt-sensitive hypertension

The goal of this study was to test the hypothesis that increases in oxidative stress in Dahl S rats on a high-salt diet help to stimulate renal nuclear factor-kappaB (NF-kappaB), renal proinflammatory cytokines, and chemokines, thus contributing to hypertension, renal damage, and dysfunction. We specifically studied whether antioxidant treatment of Dahl S rats on high Na intake would decrease renal inflammation and thus attenuate the hypertensive and adverse renal responses. Sixty-four 7- to 8-wk-old Dahl S or R/Rapp strain rats were maintained for 5 wk on high Na (8%) or high Na + vitamins C (1 g/l in drinking water) and E (5,000 IU/kg in food). Arterial and venous catheters were implanted at day 21. By day 35 in the high-Na S rats, antioxidant treatment significantly increased the renal reduced-to-oxidized glutathione ratio and decreased renal cortical H(2)O(2) and O(2)(*-) release and renal NF-kappaB. Antioxidant treatment with vitamins C and E in high-Na S rats also decreased renal monocytes/macrophages in the glomeruli, cortex, and medulla, decreased tumor necrosis factor-alpha by 39%, and decreased monocyte chemoattractant protein-1 by 38%. Vitamin-treated, high-Na S rats also experienced decreases in arterial pressure, urinary protein excretion, renal tubulointerstitial damage, and glomerular necrosis and increases in glomerular filtration rate and renal plasma flow. In conclusion, antioxidant treatment of high-Na Dahl S rats decreased renal inflammatory cytokines and chemokines, renal immune cells, NF-kappaB, and arterial pressure and improved renal function and damage.

Enhanced Superoxide Production in Renal Outer Medulla of Dahl Salt-Sensitive Rats Reduces Nitric Oxide Tubular-Vascular Cross-Talk

Studies were conducted to determine whether the diffusion of NO from the renal medullary thick ascending limb (mTAL) to the contractile pericytes of surrounding vasa recta was reduced and, conversely, whether diffusion of oxygen free radicals was enhanced in the salt-sensitive Dahl S rat (SS/Mcwi). Angiotensin II ([Ang II] 1 μmol/L)–stimulated NO and superoxide (O 2 ·− ) production were imaged by fluorescence microscopy in thin tissue strips from the inner stripe of the outer medulla. In prehypertensive SS/Mcwi rats and a genetically designed salt-resistant control strain (consomic SS-13 BN ), Ang II failed to increase either NO or O 2 ·− in pericytes of isolated vasa recta. Ang II stimulation resulted in production of NO in epithelial cells of the mTAL that diffused to vasa recta pericytes of SS-13 BN rats but not in SS/Mcwi rats except when tissues were preincubated with the superoxide scavenger TIRON (1 mmol/L). Ang II resulted in a greater increase of O 2 ·− in the mTAL of SS/Mcwi compared with SS.13 BN mTAL. The O 2 ·− diffused to adjoining pericytes in tissue strips only in SS/Mcwi rats but not in control SS-13 BN rats. Diffusion of Ang II-stimulated O 2 ·− from mTAL to vasa recta pericytes was absent when tissue strips from SS/Mcwi rats were treated with the NO donor DETA-NONOate (20 μmol/L). We conclude that the SS/Mcwi rat exhibits increased production of O 2 ·− in mTAL that diffuses to surrounding vasa recta and attenuates NO cross-talk. Diffusion of O 2 ·− from mTAL to surrounding tissue could contribute to reduced bioavailability of NO, reductions of medullary blood flow, and interstitial fibrosis in the outer medulla of SS/Mcwi rats.

Distinct rapid and slow phases of salt-induced hypertension in Dahl salt-sensitive rats

OBJECTIVE

To test the hypothesis that Dahl salt-sensitive (Dahl-S) rats exhibit distinct and separable phases of salt sensitivity.

METHODS

Blood pressure (BP) telemetry was used to describe the detailed time course of salt-induced hypertension in Dahl-S rats and in hybrid rats derived from Dahl-S and Dahl salt-resistant strains.

RESULTS

Switching to a high salt (4% NaCl) diet led to a biphasic increase in BP. Phase-1 reached a plateau in 4 days whereas phase-2 progressed slowly over the subsequent 5 weeks. In hybrid rats, phase-1 was present in each rat whereas phase-2 was absent in many individuals. A correlation of the amplitude of the first and second phases was of borderline significance in Dahl-S rats (P = 0.053, R2 = 0.44, n = 9) but was clearly significant in hybrid rats (P < 0.0001, R2 = 0.78, n = 22). Increases in BP were reversible following 1 week of high salt but progressively less so after 4 and 7 weeks. Estimation of the chronic pressure-natriuresis relationship suggests that phase-1 is attributable to a reduced slope of this relationship. In contrast, phase-2 corresponds with a further reduction in slope and a progressive and irreversible resetting of the relationship to higher BP levels.

CONCLUSIONS

Two phases of salt sensitivity coexist and provide distinct contributions to salt-induced hypertension in Dahl-S rats. Our data also suggest that short-term measures of salt-sensitivity may be predictive of the effect of salt on the eventual progression of salt-induced hypertension.

Intrarenal renin-angiotensin system and counteracting protective mechanisms in angiotensin II-dependent hypertension

It is now well accepted that alterations in kidney function, due either to primary renal disease or to inappropriate hormonal influences on the kidney, are a cardinal characteristic in all forms of hypertension, and lead to a reduced ability of the kidneys to excrete sodium and the consequent development of elevated arterial pressures. However, it is also apparent that many extrarenal factors are important contributors to altered kidney function and hypertension. Central to many hypertensinogenic processes is the inappropriate activation of the renin-angiotensin system (RAS) and its downstream consequences by various pathophysiologic mechanisms. There may also be derangements in arachidonic acid metabolites, endothelium derived factors such as nitric oxide and carbon monoxide, and various paracrine and neural systems that normally interact with or provide a counteracting balance to the actions of the RAS. Thus, when the capacity of the kidneys to maintain sodium balance and extracellular fluid volume within appropriate ranges is compromised, increases in arterial pressure become necessary to re-establish normal balance.

Hydroxyl radical mediates the augmented angiotensin II responses in thoracic aorta of spontaneously hypertensive rats

AIM

To investigate the role of hydroxyl radical in augmented angiotensin II (Ang II) responses in the thoracic aorta of spontaneously hypertensive rats (SHR).

METHODS

To elucidate the role of hydroxyl radical, we used edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) as a tool for our study. The vascular responses to Ang II (10(-10) to 10(-6) mol/l), tert-butyl hydroperoxide (tBHP; 10(-6) to 10(-2) mol/l) and H(2)O(2) (10(-6) to 10(-2) mol/l) were constructed in aortic preparations obtained from control (WKY) and SHR in the absence and presence of edaravone.

RESULTS

The vascular responses to Ang II, tBHP and H(2)O(2) were found to be enhanced in aortic preparations from SHR as compared to control WKY rats. Edaravone selectively attenuated the augmented responses to Ang II but not to tBHP and H(2)O(2) suggesting that the .OH radical is involved in the augmented responses to Ang II. The elevated blood pressure in SHR was restored to a near normal value after 2 weeks of edaravone (10 mg kg(-1) i.p., b.i.d.) treatment.

CONCLUSION

From the results we infer that hydroxyl radical stress augments Ang II responses in the thoracic aorta of SHR and, by attenuating these enhanced vascular responses, edaravone could serve as an adjuvant antioxidant therapy for the vascular complications of hypertension.

Flow increases superoxide production by NADPH oxidase via activation of Na-K-2Cl cotransport and mechanical stress in thick ascending limbs

Superoxide (O2−) regulates renal function and is implicated in hypertension. O2−production increases in response to increased ion delivery in thick ascending limbs (TALs) and macula densa and mechanical strain in other cell types. Tubular flow in the kidney acutely varies causing changes in ion delivery and mechanical stress. We hypothesized that increasing luminal flow stimulates O2−production by NADPH oxidase in TALs via activation of Na-K-2Cl cotransport. We measured intracellular O2−in isolated rat TALs using dihydroethidium in the presence and absence of luminal flow and inhibitors of NADPH oxidase, Na-K-2Cl cotransport, and Na/H exchange. In the absence of flow, the rate of O2−production was 5.8 ± 1.4 AU/s. After flow was initiated, it increased to 29.7 ± 4.3 AU/s ( P < 0.001). O2−production was linearly related to flow. Tempol alone and apocynin alone blocked the flow-induced increase in O2−production (3.5 ± 1.7 vs. 4.5 ± 2.8 AU/s and 8.2 ± 2.1 vs. 10.6 ± 2.8 AU/s, respectively). Furosemide decreased flow-induced O2−production by 55% (37.3 ± 5.2 to 16.8 ± 2.8 AU/s; P < 0.002); however, dimethylamiloride had no effect. Finally, we examined whether changes in mechanical forces are involved in flow-induced O2−production by using a Na-free solution to perfuse TALs. In the absence of NaCl, luminal flow enhanced O2−production (1.5 ± 0.5 to 13.5 ± 1.1 AU/s; P < 0.001), ∼50% less stimulation than when flow was increased in the presence of luminal NaCl. We conclude that flow stimulates O2−production in TALs via activation of NADPH oxidase and that NaCl absorption due to Na-K-2Cl cotransport and flow-associated mechanical factors contribute equally to this process.

Insulin Resistance and Oxidant Stress: An Interrelation With Deleterious Renal Consequences?

Within the past years, several epidemiologic studies have shown that insulin resistance and hyperinsulinemia are associated with chronic kidney disease, and experimental data suggest that a number of background mechanisms could connect insulin resistance with renal injury. Moreover, the acute sodium-retaining action of insulin at the kidney level has been proposed to participate in the development of salt sensitivity in essential hypertension. Current knowledge suggests that oxidative stress can be involved in the development of renal injury and can also promote primary salt retention at the kidney level. Insulin resistance and hyperinsulinemia seem to be closely connected with oxidative stress in the form of a vicious circle. This article discusses the potential role of oxidative stress as a mediator of the renal effects of insulin resistance/hyperinsulinemia.

Immune suppression prevents renal damage and dysfunction and reduces arterial pressure in salt-sensitive hypertension

The goal of this study was to test the hypothesis that renal infiltration of immune cells in Dahl S rats on increased dietary sodium intake contributes to the progression of renal damage, decreases in renal hemodynamics, and development of hypertension. We specifically studied whether anti-immune therapy, using mycophenolate mofetil (MMF), could help prevent increases in renal NF-κB activation, renal infiltration of monocytes/macrophages, renal damage, decreases in glomerular filtration rate (GFR) and renal plasma flow, and increases in arterial pressure. Seventy-four 7-to 8-wk-old Dahl S, Rapp strain rats were maintained on an 8% Na, 8% Na + MMF (20 mg·kg−1·day−1), 0.3% Na, or 0.3% Na + MMF diet for 5 wk. Arterial and venous catheters were implanted at day 21. By day 35, renal NF-κB in 8% Na rats was 47% higher than in 0.3% Na rats and renal NF-κB was 41% lower in 8% Na + MMF rats compared with the 8% Na group. MMF treatment significantly decreased renal monocyte/macrophage infiltration and renal damage and increased GFR and renal plasma flow. In high-NA Dahl S rats mean arterial pressure increased to 182 ± 5 mmHg, and MMF reduced this arterial pressure to 124 ± 3 mmHg. In summary, in Dahl S rats on high sodium intake, treatment with MMF decreases renal NF-κB and renal monocyte/macrophage infiltration and improves renal function, lessens renal injury, and decreases arterial pressure. This suggests that renal infiltration of immune cells is associated with increased arterial pressure and renal damage and decreasing GFR and renal plasma flow in Dahl salt-sensitive hypertension.

The antinatriuretic effect of insulin: an unappreciated mechanism for hypertension associated with insulin resistance?

Insulin resistance is proposed to be causally related to the metabolic syndrome disorders, but a direct cause-and-effect relationship between insulin resistance and hypertension was not originally obvious. Previous data suggested that insulin promotes sodium retention from the kidney, and thus research efforts focused on this action among several other possible pathways connecting insulin resistance and hyperinsulinemia with hypertension. A review of numerous studies provides evidence that this antinatriuretic effect of insulin is preserved in states of metabolic insulin resistance, representing a major mechanism for blood pressure elevation. More recent experimental and clinical studies have added data about the exact tubular sites of this insulin action, its relation with the respective insulin action on potassium handling, its possible role in the development of salt sensitivity in essential hypertension, as well as the involvement of oxidant stress in these associations. This review summarizes the current state of knowledge in this area and attempts to highlight an important but rather overlooked pathway for hypertension development in the metabolic syndrome, the influence of high insulin levels leading to volume expansion.

Role of anemia in progression of chronic kidney disease

Anemia is a well-known consequence of chronic kidney disease (CKD), and its prevalence progressively increases when the estimated glomerular filtration rate decreases to less than 60 mL/min/1.73 m2. However, analyses of the consequences of anemia and of the mechanisms of progression of CKD suggest that anemia also could contribute to the deterioration of kidney function. This hypothesis is based mostly on experimental data that imply that hypoxia of tubular cells plays an important role in tubulointerstitial damage associated with CKD and, thus, in the progression of renal failure. It also is supported by the fact that red blood cells represent a major antioxidant component of blood and that oxidative stress appears to contribute to glomerulosclerosis and tubulointerstitial damage. In humans, post hoc analysis of the Reduction of End points in non insulin-dependent diabetes mellitus (NIDDM) with the Angiotensin II Antagonist Losartan study and analyses of smaller prospective cohorts of CKD patients have shown that anemia is an independent risk factor for progression of CKD. In addition, 3 small randomized studies have suggested that anemia correction could slow the progression of CKD. Thus, the existence of a relationship between anemia and progression of CKD is not only plausible biologically, but also is supported by observational studies and by small intervention studies. However, only a large, randomized, prospective trial will be able to establish if anemia correction can slow the progression of CKD effectively.

Double-Edged Action of SOD Mimetic in Diabetic Nephropathy

Oxidative stress plays an important role in the pathogenesis of diabetic complications, and we investigated the effect of superoxide dismutase (SOD) mimetic, tempol, in diabetic nephropathy. Streptozotocin-induced diabetic rats were treated with tempol from 2 weeks until 8 weeks. The expression of NADPH oxidase, catalase, and myeloperoxidase (MPO), superoxide dismutase activity, and production of peroxide and hypochlorite were evaluated. Tempol treatment prevented the increase in NADPH oxidase and peroxide production in the glomeruli of diabetic rat. Catalase was decreased without change in SOD activity, and MPO was enhanced in the kidney of diabetic rats. Tempol treatment stimulated SOD activity and increased the conversion of superoxide to hydrogen peroxide, and hydrogen peroxide on its hand was converted to hypochlorite by the increased MPO. The reduction of peroxide by tempol was followed by the decrease in TGF-beta and mesangial matrix expansion. However, tempol did not reduce hypochlorite or urinary protein excretion. In conclusion, tempol inhibited glomerular matrix expansion via suppression of peroxide production and TGF-beta, but it failed to reduce proteinuria, probably due to the increased hypochlorite production in diabetic nephropathy.

Isohumulones Derived from Hops Ameliorate Renal Injury via an Anti-Oxidative Effect in Dahl Salt-Sensitive Rats

Previous studies have reported that isohumulones, the bitter compounds in beer, improve insulin resistance and hyperlipidemia in several animal models. In this study, we examined whether isohumulones ameliorate renal injury. Dahl salt-sensitive hypertensive rats were fed a low-salt diet (LS), a high-salt diet (HS) or a high-salt diet containing 0.3% isohumulones (HS+IH) for 4 weeks. Urinary nitrite/nitrate (NOx) excretion was measured at 4 weeks along with blood pressure and urinary protein excretion. Renal injury was evaluated histologically and reactive oxygen species (ROS) and nitric oxide (NO) production in the renal cortex was visualized. Oxidative stress and NO synthase (NOS) expression were evaluated by immunohistochemical staining and Western blot analysis. Mean blood pressure was significantly decreased in the HS+IH group compared with the HS group at 4 weeks (158.1+/-8.7 vs. 177.5+/-3.7 mmHg; p<0.05). Isohumulones prevented the development of proteinuria in the HS+IH group compared with the HS group at 2 weeks (61.7+/-26.8 vs. 117.2+/-9.8 mg/day; p<0.05). Glomerulosclerosis and interstitial fibrosis scores were significantly decreased in the HS+IH group compared with the HS group (0.61+/-0.11 vs. 1.55+/-0.23, 23.7+/-6.8 vs. 36.1+/-3.5%; p<0.05 for both). In the HS group, increased ROS and decreased NO were observed in glomeruli in vivo. Isohumulones reduced the ROS production, leading to the restoration of bioavailable NO. Urinary NOx excretion was significantly increased in the HS+IH group compared with the HS group. Furthermore, renal nitrotyrosine was increased in the HS group compared with the LS group, and this effect was prevented by isohumulones. Renal NOS expression did not differ among the three groups. These results suggest that isohumulones may prevent the progression of renal injury caused by hypertension via an anti-oxidative effect.

Renal NF-κB activation and TNF-α upregulation correlate with salt-sensitive hypertension in Dahl salt-sensitive rats

Molecular mechanisms of salt-sensitive (SS) hypertension related to renal inflammation have not been defined. We seek to determine whether a high-salt (HS) diet induces renal activation of NF-κB and upregulation of TNF-α related to the development of hypertension in Dahl SS rats. Six 8-wk-old male Dahl SS rats received a HS diet (4%), and six Dahl SS rats received a low-sodium diet (LS, 0.3%) for 5 wk. In the end, mean arterial pressure was determined in conscious rats by continuous monitoring through a catheter placed in the carotid artery. Mean arterial pressure was significantly higher in the HS than the LS group (177.9 ± 3.7 vs. 109.4 ± 2.9 mmHg, P < 0.001). There was a significant increase in urinary albumin secretion in the HS group compared with the LS group (22.3 ± 2.6 vs. 6.1 ± 0.7 mg/day; P < 0.001). Electrophoretic mobility shift assay demonstrated that the binding activity of NF-κB p65 proteins in the kidneys of Dahl SS rats was significantly increased by 53% in the HS group compared with the LS group ( P = 0.007). ELISA indicated that renal protein levels of TNF-α, but not IL-6, interferon-γ, and CCL28, were significantly higher in the HS than the LS group (2.3 ± 0.8 vs. 0.7 ± 0.2 pg/mg; P = 0.036). We demonstrated that plasma levels of TNF-α were significantly increased by fivefold in Dahl SS rats on a HS diet compared with a LS diet. Also, we found that increased physiologically relevant sodium concentration (10 mmol/l) directly stimulated NF-κB activation in cultured human renal proximal tubular epithelial cells. These findings support the hypothesis that activation of NF-κB and upregulation of TNF-α are the important renal mechanisms linking proinflammatory response to SS hypertension.

NO synthase uncoupling in the kidney of Dahl S rats: role of dihydrobiopterin

NO synthase (NOS) can paradoxically contribute to the production of reactive oxygen species when l-arginine or the cofactor R-tetrahydrobiopterin (BH(4)) becomes limited. The present study examined whether NOS contributes to superoxide production in kidneys of hypertensive Dahl salt-sensitive (SS) rats compared with an inbred consomic control strain (SS-13(BN)) and tested the hypothesis that elevated dihydrobiopterin (BH(2)) levels are importantly involved in this process. This was assessed by determining the effects of l-nitroarginine methyl ester (l-NAME) inhibition of NOS on superoxide production and by comparing tissue concentrations of BH(4) and BH(2). A reverse-phase high-performance liquid chromatography method was applied for direct measurements of BH(4) and BH(2) using (S)-tetrahydrobiopterin as an internal standard. Superoxide concentrations were measured in vivo from medullary microdialysis fluid using dihydroethidine and in vitro using lucigenin. The results indicate the following: (1) that superoxide levels were elevated in the outer medulla of SS rats fed a 4% salt diet and could be inhibited by l-NAME. In contrast, l-NAME resulted in elevated superoxide production in consomic SS-13(BN) rats because of higher NOS activity; (2) SS rats showed a reduced ratio of BH(4)/BH(2) in the outer medulla that was driven by increased concentrations of BH(2); and (3) lower superoxide dismutase and catalase activities contributed to elevated reactive oxygen species in SS samples. Based on the shift of BH(4) to BH(2) and the observation of l-NAME inhibitable superoxide production, we conclude that NOS uncoupling occurs in the renal medulla of hypertensive SS rats fed a high-salt diet.

N-Acetylcysteine improves renal dysfunction, ameliorates kidney damage and decreases blood pressure in salt-sensitive hypertension

BACKGROUND

Salt-sensitive hypertension in humans and experimental animals causes progressive increases in renal damage and dysfunction. The Dahl salt-sensitive (S) rat closely mimics human salt-sensitive hypertension.

AIM

Our goal was to test the hypothesis that enhancing the glutathione system with dietary N-acetylcysteine administration in Dahl S rats on a high sodium intake for 5 weeks will attenuate the increases in arterial pressure, the decreases in renal hemodynamics and the increases in renal damage that normally occur in S rats on high sodium.

METHODS

Forty-four 7- to 8-week-old Dahl S/Rapp strain rats were maintained on a high sodium (8%), high sodium + N-acetylcysteine (4 g/kg per day), or low sodium (0.3%) diet for 5 weeks. Rats had arterial and venous catheters implanted at day 21.

RESULTS

By day 35 in the high-sodium rats, N-acetylcysteine treatment significantly increased the renal reduced-to-oxidized glutathione ratio, glomerular filtration rate, and renal plasma flow, and decreased renal cortical and medullary O2 release, urinary protein excretion, renal tubulointerstitial damage and glomerular necrosis. At this time, mean arterial pressure increased to 183 +/- 1 mmHg, and N-acetylcysteine reduced this arterial pressure to 121 +/- 4 mmHg. By day 35 in S high-sodium rats, N-acetylcysteine had caused a 91% decrease in glomerular necrosis and an 83% decrease in tubulointerstitial damage.

CONCLUSIONS

In Dahl S rats on high sodium intake, arterial pressure increases significantly and renal injury is pronounced. Treatment with N-acetylcysteine enhances the renal glutathione system, improves renal dysfunction and markedly decreases arterial pressure and renal injury in Dahl salt-sensitive hypertension.

Enhanced oxidative stress in kidneys of salt-sensitive hypertension: role of sensory nerves

To determine the mechanism(s) underlying enhanced oxidative stress in kidneys of salt-sensitive hypertension, neonatal Wistar rats were given vehicle or capsaicin (CAP, 50 mg/kg sc) on the first and second days of life. After being weaned, male rats were assigned into four groups and treated for 2 wk with the following: vehicle + a normal sodium diet (NS, 0.4%, CON-NS), vehicle + a high-sodium diet (HS, 4%, CON-HS), CAP + NS (CAP-NS), and CAP + HS (CAP-HS). Systolic blood pressure was significantly increased in CAP-HS but not CAP-NS or CON-HS rats. Plasma and urinary 8-iso-prostaglandin F(2alpha) levels increased by approximately 40% in CON-HS and CAP-HS rats compared with their respective controls fed a NS diet (P < 0.05), and these parameters were higher in CAP-HS compared with CON-HS rats. Superoxide (O(2)(-)*) levels in the renal cortex and medulla increased by approximately 45% in CAP-HS compared with CON-HS, CON-NS, and CAP-NS rats (P < 0.05). Enhanced O(2)(-)* levels in the cortex and medulla in CAP-HS rats were prevented by preincubation of renal tissues with apocynin, a selective NAD(P)H oxidase inhibitor. Protein expression of NAD(P)H oxidase subunits, including p47(phox) and gp91(phox) in the renal cortex and medulla, was significantly increased in CAP-HS compared with CON-HS, CON-NS, and CAP-NS rats. In contrast, protein expression and activities of Cu/Zn SOD and Mn SOD were significantly increased in the renal medulla in both CAP-HS and CON-HS but in the cortex in CAP-HS rats only. Creatinine clearance decreased by approximately 45% in CAP-HS rats compared with CON-HS, CON-NS, and CAP-NS rats (P < 0.05). O(2)(-)* levels in the renal cortex of CAP-HS rats negatively correlated with creatinine clearance (r = -0.76; P < 0.001). Therefore, regardless of enhanced SOD activity to suppress oxidative stress, increased oxidative stress in the kidney of CAP-treated rats fed a HS diet is likely the result of increased expression and activities of NAD(P)H oxidase, which may contribute to decreased renal function and increased blood pressure in these rats. Our results suggest that sensory nerves may play a compensatory role in attenuating renal oxidative stress during HS intake.

Upregulation of renal eNOS by high-sodium diet facilitates hypertension in doxorubicin-treated rats through enhanced oxidative stress

It is known that renal nitric oxide (NO) is an important controller of urinary sodium excretion. A defect in the kidney's NO system could cause salt-sensitive hypertension. Since it has been demonstrated that doxorubicin binds to the reductase domain of endothelial NO synthase (eNOS) and generates superoxide in vitro, we tested our hypothesis that a high-sodium diet would upregulate the expression of eNOS and enhance oxidative stress in the kidney of doxorubicin-treated rats, resulting in a facilitation of hypertension. At 4 weeks after treatment with doxorubicin in Sprague-Dawley rats, the systolic blood pressure significantly increased only in the high-sodium diet group. The expressions of eNOS protein in the renal cortex and medulla were significantly higher in high-sodium groups than in normal-sodium groups, regardless of doxorubicin treatment. In rats treated with doxorubicin, a biomarker of oxidative damage 8-hydroxy-2'-deoxyguanosine (8-OHdG) immunohistological staining of renal tissues showed strong staining of the proximal and distal tubules. In particular, rats with doxorubicin in the high-sodium diet group demonstrated a significant increase in urinary exertion of 8-OHdG as well as more prominently stained tubules against 8-OHdG antibody, but markedly lower urinary NO(x) excretion than in rats without doxorubicin, even than in the untreated, low-sodium group. In conclusion, these results indicate that the oxidative stress induced by doxorubicin impairs NO production in the kidney. As such, doxorubicin treatment appears to contribute to the development of salt-sensitive hypertension through reductive activation of upregulated eNOS by a high-sodium diet instead of NO production.

Hydronephrosis causes salt-sensitive hypertension in rats

BACKGROUND

Hypertension is a common disease in the Western world and approximately 5% of all cases are secondary to kidney malfunction. It is not clear whether unilateral hydronephrosis due to partial obstruction affects blood pressure.

AIM

The aim of this study was to determine whether hypertension develops and to investigate the effects of different salt diets on the blood pressure in hydronephrotic animals.

METHODS

Unilateral partial ureteral obstruction was created in 3-week-old Sprague-Dawley rats. A telemetric device was implanted 4-6 weeks later and blood pressure was measured on normal, low- and high-salt diets. Plasma samples were collected on all diets for renin analysis.

RESULTS

All hydronephrotic animals developed hypertension that correlated to the degree of hydronephrosis. The blood pressure increased slowly with time and was salt sensitive. In severe hydronephrosis, blood pressure increased from 118 +/- 5 mmHg on low salt to 140 +/- 6 mmHg on high salt intake, compared to control levels of 82 +/- 2 and 84 +/- 2 mmHg, respectively. Plasma renin concentration was increased in the hydronephrotic group of animals compared to controls on all diets, but the difference was only significant on a normal salt diet, 165 +/- 15 versus 86 +/- 12 microGU/ml respectively. In animals with severe hydronephrosis the plasma renin levels were lower, and the changes less, than in those with mild and moderate hydronephrosis.

CONCLUSION

This study demonstrates the presence of a salt-sensitive hypertension in hydronephrosis. A systemic effect of the renin-angiotensin system alone cannot be responsible for the hypertension.