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

Angiotensin1-7 Protects Against Renal Ischemia-Reperfusion Injury via Regulating the Expression of NRF2 and microRNAs in Fisher 344 Rats

Ischemia/reperfusion (I/R) is considered the primary cause of acute kidney injury and is higher among older individuals. While ischemic episodes are hard to predict and prevent, detrimental ischemic effects could be mitigated by exogenous intervention. This study aims to identify the protective role of angiotensin (ANG)1-7 against I/R-induced renal injury in adult vs. aged rats. Adult and aged male Fisher 344 rats were subjected to 40-minute bilateral renal ischemia followed by 28-days reperfusion. ANG1-7 was administered intraperitoneally in ischemic rats for 28 days without or with Mas receptor antagonist A779. I/R increased blood pressure, plasma creatinine, urinary 8-isoprostane, and renal infiltration of pro and anti-inflammatory macrophages and reduced glomerular filtration rate in both adult and aged rats compared to shams. In addition to causing glomerular sclerosis and tubular damage, I/R increased the expression of pathogenic microRNAs (miRNAs): miR-20a-5p, miR-21-5p, miR-24-3p, and miR-194-5p in both the age groups. ANG1-7 treatment of ischemic rats mitigated oxidative stress and renal inflammation, restored renal structure and function, and reduced high blood pressure. Also, ANG1-7 suppressed the expression of pathogenic miRNAs. In addition, ANG1-7 treatment of I/R rats increased the expression of redox-sensitive transcription factor NRF2 and phase II antioxidant enzymes. The beneficial effects of ANG1-7 were sensitive to A779. Collectively, these data suggest that ANG1-7 associated with NRF2 activation could alleviate post-I/R-induced kidney injury and therefore serve as a potential therapeutic compound to protect against biochemical and morphological pathologies of I/R in both adults and aged populations.

Candidate Gene Polymorphisms Influence the Susceptibility to Salt Sensitivity of Blood Pressure in a Han Chinese Population: Risk Factors as Mediators

Genome-wide association studies suggest that there is a significant genetic susceptibility to salt sensitivity of blood pressure (SSBP), but it still needs to be verified in varied and large sample populations. We attempted to verify the associations between single-nucleotide polymorphisms (SNPs) in candidate genes and SSBP and to estimate their interaction with potential risk factors. A total of 29 candidate SNPs were genotyped in the 2,057 northern Han Chinese population from the Systems Epidemiology Study on Salt Sensitivity. A modified Sullivan’s acute oral saline load and diuresis shrinkage test (MSAOSL-DST) was used to identify SSBP. A generalized linear model was conducted to analyze the association between SNPs and SSBP, and Bonferroni correction was used for multiple testing. Mediation analysis was utilized to explore the mediation effect of risk factors. Eleven SNPs in eight genes (PRKG1, CYBA, BCAT1, SLC8A1, AGTR1, SELE, CYP4A11, and VSNL1) were identified to be significantly associated with one or more SSBP phenotypes (P < 0.05). Four SNPs (PRKG1/rs1904694 and rs7897633, CYP4A11/rs1126742, and CYBA/rs4673) were still significantly associated after Bonferroni correction (P < 0.0007) adjusted for age, sex, fasting blood glucose, total cholesterol, salt-eating habit, physical activity, and hypertension. Stratified analysis showed that CYBA/rs4673 was significantly associated with SSBP in hypertensive subjects (P < 0.0015) and CYP4A11/rs1126742 was significantly associated with SSBP in normotensive subjects (P < 0.0015). Subjects carrying both CYBA/rs4673-AA and AGTR1/rs2638360-GG alleles have a higher genetic predisposition to salt sensitivity due to the potential gene co-expression interaction. Expression quantitative trait loci analysis (eQTL) suggested that the above positive four SNPs showed cis-eQTL effects on the gene expression levels. Mediation analysis suggested that several risk factors were mediators of the relation between SNP and SSBP. This study suggests that the genetic variants in eight genes might contribute to the susceptibility to SSBP, and other risk factors may be the mediators.

Reinvestigation of the tonic natriuretic action of intrarenal dopamine: comparison of two variants of salt-dependent hypertension and spontaneously hypertensive rats

The intrarenal dopamine system has been thoroughly investigated at all levels, especially its role in salt-dependent and other forms of hypertension. However, the evidence regarding dopamine's tonic influence on renal tubular transport of sodium remains equivocal. We reinvestigated its tonic influence on sodium excretion and systemic and renal haemodynamics. Early effects of dopamine D1 receptor blockade using 90-min Schering 23390 (SCH) infusion were examined in anaesthetized rats on 7 days' high salt diet (HS), early uninephrectomized rats on 14 days' HS diet, drinking 1% saline (HS/UNX), and in spontaneously hypertensive rats (SHR). In the HS group (baseline BP ~133 mm Hg) renal intracortical SCH promptly decreased sodium, water and total solute excretion (UNa V, V, Uosm V), with significant difference from the solvent-infused group. BP and renal artery blood flow (RBF, Transonic probe) did not change. In HS/UNX model (baseline BP ~150 mm Hg), characterized by hypertrophy of the remaining kidney, the excretion parameters only tended to decrease whereas SCH induced an ~20% fall in RBF. In SHR (BP ~180 mm Hg), UNa V and V tended to increase in solvent-infused rats; this increasing tendency was abolished by SCH infusion. During experiments the renal vascular resistance increased significantly in SCH- and solvent-infused SHR. Despite some contradictory findings regarding the genuine tonic control of renal excretion by intrarenal dopamine, our results clearly support such role in rats on HS diet and in SHR, the model resembling human essential hypertension. The observations strengthen the experimental basis and the rationale for targeting the intrarenal dopamine system in attempts to combat arterial hypertension.

Renal Dopamine Receptors and Oxidative Stress: Role in Hypertension

Significance: The kidney plays an important role in the long-term control of blood pressure. Oxidative stress is one of the fundamental mechanisms responsible for the development of hypertension. Dopamine, via five subtypes of receptors, plays an important role in the control of blood pressure by various mechanisms, including the inhibition of oxidative stress. Recent Advances: Dopamine receptors exert their regulatory function to decrease the oxidative stress in the kidney and ultimately maintain normal sodium balance and blood pressure homeostasis. An aberration of this regulation may be involved in the pathogenesis of hypertension. Critical Issues: Our present article reviews the important role of oxidative stress and intrarenal dopaminergic system in the regulation of blood pressure, summarizes the current knowledge on renal dopamine receptor-mediated antioxidation, including decreasing reactive oxygen species production, inhibiting pro-oxidant enzyme nicotinamide-adenine dinucleotide phosphate (NADPH) oxidase, and stimulating antioxidative enzymes, and also discusses its underlying mechanisms, including the increased activity of G protein-coupled receptor kinase 4 (GRK4) and abnormal trafficking of renal dopamine receptors in hypertensive status. Future Directions: Identifying the mechanisms of renal dopamine receptors in the regulation of oxidative stress and their contribution to the pathogenesis of hypertension remains an important research focus. Increased understanding of the role of reciprocal regulation between renal dopamine receptors and oxidative stress in the regulation of blood pressure may give us novel insights into the pathogenesis of hypertension and provide a new treatment strategy for hypertension.

The Role of the Renal Dopaminergic System and Oxidative Stress in the Pathogenesis of Hypertension

The kidney is critical in the long-term regulation of blood pressure. Oxidative stress is one of the many factors that is accountable for the development of hypertension. The five dopamine receptor subtypes (D₁R–D₅R) have important roles in the regulation of blood pressure through several mechanisms, such as inhibition of oxidative stress. Dopamine receptors, including those expressed in the kidney, reduce oxidative stress by inhibiting the expression or action of receptors that increase oxidative stress. In addition, dopamine receptors stimulate the expression or action of receptors that decrease oxidative stress. This article examines the importance and relationship between the renal dopaminergic system and oxidative stress in the regulation of renal sodium handling and blood pressure. It discusses the current information on renal dopamine receptor-mediated antioxidative network, which includes the production of reactive oxygen species and abnormalities of renal dopamine receptors. Recognizing the mechanisms by which renal dopamine receptors regulate oxidative stress and their degree of influence on the pathogenesis of hypertension would further advance the understanding of the pathophysiology of hypertension.

Inactivation of Cys674 in SERCA2 increases BP by inducing endoplasmic reticulum stress and soluble epoxide hydrolase

BACKGROUND AND PURPOSE

The kidney is essential in regulating sodium homeostasis and BP. The irreversible oxidation of Cys⁶⁷⁴ (C674) in the sarcoplasmic/endoplasmic reticulum calcium ATPase 2 (SERCA2) is increased in the renal cortex of hypertensive mice. Whether inactivation of C674 promotes hypertension is unclear. Here we have investigated the effects on BP of the inactivation of C674, and its role in the kidney.

EXPERIMENTAL APPROACH

We used heterozygous SERCA2 C674S knock-in (SKI) mice, where half of C674 was substituted by serine, to represent partial irreversible oxidation of C674. The BP, urine volume, and urine composition of SKI mice and their littermate wild-type (WT) mice were measured. The kidneys were collected for cell culture, Na⁺ /K⁺ -ATPase activity, protein expression, and immunohistological analysis.

KEY RESULTS

Compared with WT mice, SKI mice had higher BP, lower urine volume and sodium excretion, up-regulated endoplasmic reticulum (ER) stress markers and soluble epoxide hydrolase (sEH), and down-regulated dopamine D₁ receptors in renal cortex and cells from renal proximal tubule. ER stress and sEH were mutually regulated, and both upstream of D₁ receptors. Inhibition of ER stress or sEH up-regulated expression of D₁ receptors, decreased the activity of Na⁺ /K⁺ -ATPase, increased sodium excretion, and lowered BP in SKI mice.

CONCLUSIONS AND IMPLICATIONS

The inactivation of SERCA2 C674 promotes the development of hypertension by inducing ER stress and sEH. Our study highlights the importance of C674 redox status in BP control and the contribution of SERCA2 to sodium homeostasis and BP in the kidney.

Renal Dopamine Oxidation and Inflammation in High Salt Fed Rats

Background

Oxidative stress and high salt intake could be independent or intertwined risk factors in the origin of hypertension. Kidneys are the major organ to regulate sodium homeostasis and blood pressure and the renal dopamine system plays a pivotal role in sodium regulation during sodium replete conditions. Oxidative stress has been implicated in renal dopamine dysfunction and development of hypertension, especially in salt‐sensitive animal models. Here we show the nexus between high salt intake and oxidative stress causing renal tubular dopamine oxidation, which leads to mitochondrial and lysosomal dysfunction and subsequently causes renal inflammation and hypertension.

Methods and Results

Male Sprague Dawley rats were divided into the following groups, vehicle (V)—tap water, high salt (HS)—1% NaCl, L‐buthionine‐sulfoximine (BSO), a prooxidant, and HS plus BSO without and with antioxidant resveratrol (R) for 6 weeks. Oxidative stress was significantly higher in BSO and HS+BSO–treated rat compared with vehicle; however, blood pressure was markedly higher in the HS+BSO group whereas an increase in blood pressure in the BSO group was modest. HS+BSO–treated rats had significant renal dopamine oxidation, lysosomal and mitochondrial dysfunction, and increased renal inflammation; however, HS alone had no impact on organelle function or inflammation. Resveratrol prevented oxidative stress, dopamine oxidation, organelle dysfunction, inflammation, and hypertension in BSO and HS+BSO rats.

Conclusions

These data suggest that dopamine oxidation, especially during increased sodium intake and oxidative milieu, leads to lysosomal and mitochondrial dysfunction and renal inflammation with subsequent increase in blood pressure. Resveratrol, while preventing oxidative stress, protects renal function and mitigates hypertension.

Kidney dopamine D1-like receptors and angiotensin 1-7 interaction inhibits renal Na+ transporters

The role of dopamine D₁-like receptors (DR) in the regulation of renal Na⁺ transporters, natriuresis, and blood pressure is well established. However, the involvement of the angiotensin 1-7 (ANG 1-7)-Mas receptor in the regulation of Na⁺ balance and blood pressure is not clear. The present study aimed to investigate the hypothesis that ANG 1-7 can regulate Na⁺ homeostasis by modulating the renal dopamine system. Sprague-Dawley rats were infused with saline alone (vehicle) or saline with ANG 1-7, ANG 1-7 antagonist A-779, DR agonist SKF38393, and antagonist SCH23390. Infusion of ANG 1-7 caused significant natriuresis and diuresis compared with saline alone. Both natriuresis and diuresis were blocked by A-779 and SCH23390. SKF38393 caused a significant, SCH23390-sensitive natriuresis and diuresis, and A-779 had no effect on the SKF38393 response. Concomitant infusion of ANG 1-7 and SKF38393 did not show a cumulative effect compared with either agonist alone. Treatment of renal proximal tubules with ANG 1-7 or SKF38393 caused a significant decrease in Na⁺-K⁺-ATPase and Na⁺/H⁺ exchanger isoform 3 activity. While SCH23390 blocked both ANG 1-7- and SKF38393-induced inhibition, the DR response was not sensitive to A-779. Additionally, ANG 1-7 activated PKG, enhanced tyrosine hydroxylase activity via Ser⁴⁰ phosphorylation, and increased renal dopamine production. These data suggest that ANG 1-7, via PKG, enhances tyrosine hydroxylase activity, which increases renal dopamine production and activation of DR and subsequent natriuresis. This study provides evidence for a unidirectional functional interaction between two G protein-coupled receptors to regulate renal Na⁺ transporters and induce natriuresis.

Primary Pediatric Hypertension: Current Understanding and Emerging Concepts

The rising prevalence of primary pediatric hypertension and its tracking into adult hypertension point to the importance of determining its pathogenesis to gain insights into its current and emerging management. Considering that the intricate control of BP is governed by a myriad of anatomical, molecular biological, biochemical, and physiological systems, multiple genes are likely to influence an individual's BP and susceptibility to develop hypertension. The long-term regulation of BP rests on renal and non-renal mechanisms. One renal mechanism relates to sodium transport. The impaired renal sodium handling in primary hypertension and salt sensitivity may be caused by aberrant counter-regulatory natriuretic and anti-natriuretic pathways. The sympathetic nervous and renin-angiotensin-aldosterone systems are examples of antinatriuretic pathways. An important counter-regulatory natriuretic pathway is afforded by the renal autocrine/paracrine dopamine system, aberrations of which are involved in the pathogenesis of hypertension, including that associated with obesity. We present updates on the complex interactions of these two systems with dietary salt intake in relation to obesity, insulin resistance, inflammation, and oxidative stress. We review how insults during pregnancy such as maternal and paternal malnutrition, glucocorticoid exposure, infection, placental insufficiency, and treatments during the neonatal period have long-lasting effects in the regulation of renal function and BP. Moreover, these effects have sex differences. There is a need for early diagnosis, frequent monitoring, and timely management due to increasing evidence of premature target organ damage. Large controlled studies are needed to evaluate the long-term consequences of the treatment of elevated BP during childhood, especially to establish the validity of the current definition and treatment of pediatric hypertension.

High salt diet impairs memory-related synaptic plasticity via increased oxidative stress and suppressed synaptic protein expression

Scope

A high salt (HS) diet is detrimental to cognitive function, in addition to having a role in cardiovascular disorders. However, the method by which an HS diet impairs cognitive functions such as learning and memory remains open.

Methods and results

In this study, we found that mice on a 7 week HS diet demonstrated disturbed short‐term memory in an object‐place recognition task, and both 4 week and 7 week HS treatments impaired long‐term memory, as evidenced in a fear conditioning test. Mechanistically, the HS diet inhibited memory‐related long‐term potentiation (LTP) in the hippocampus, while also increasing the levels of reactive oxygen species (ROS) in hippocampal cells and downregulating the expression of synapsin I, synaptophysin, and brain‐derived neurotrophic factor in specific encephalic region.

Conclusion

This suggests that oxidative stress or synaptic protein/neurotrophin deregulation was involved in the HS diet‐induced memory impairment. Thus, the present study provides novel insights into the mechanisms of memory impairment caused by excessive dietary salt, and underlined the importance of controlling to salt absorb quantity.

Oxidative Stress and Hypertensive Diseases

It has become clear that reactive oxygen species (ROS) contribute to the development of hypertension via myriad effects. ROS are essential for normal cell function; however, they mediate pathologic changes in the brain, the kidney, and blood vessels that contribute to the genesis of chronic hypertension. There is also emerging evidence that ROS contribute to immune activation in hypertension. This article discusses these events and how they coordinate to contribute to hypertension and its consequent end-organ damage.

Gut microbiota in hypertension

PURPOSE OF REVIEW

Hypertension, which is present in about one quarter of the world's population, is responsible for about 41% of the number one cause of death - cardiovascular disease. Not included in these statistics is the effect of sodium intake on blood pressure, even though an increase or a marked decrease in sodium intake can increase blood pressure. This review deals with the interaction of gut microbiota and the kidney with genetics and epigenetics in the regulation of blood pressure and salt sensitivity.

RECENT FINDINGS

The abundance of the gut microbes, Firmicutes and Bacteroidetes, is associated with increased blood pressure in several models of hypertension, including the spontaneously hypertensive and Dahl salt-sensitive rats. Decreasing gut microbiota by antibiotics can increase or decrease blood pressure that is influenced by genotype. The biological function of probiotics may also be a consequence of epigenetic modification, related, in part, to microRNA. Products of the fermentation of nutrients by gut microbiota can influence blood pressure by regulating expenditure of energy, intestinal metabolism of catecholamines, and gastrointestinal and renal ion transport, and thus, salt sensitivity.

SUMMARY

The beneficial or deleterious effect of gut microbiota on blood pressure is a consequence of several variables, including genetics, epigenetics, lifestyle, and intake of antibiotics. These variables may influence the ultimate level of blood pressure and control of hypertension.

Role of nuclear factor erythroid 2-related factor 2 in the oxidative stress-dependent hypertension associated with the depletion of DJ-1

Renal dopamine 2 receptor dysfunction is associated with oxidative stress and high blood pressure (BP). We have reported that DJ-1, an oxidative stress response protein, is positively regulated by dopamine 2 receptor in the kidney. The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) regulates the expression of several antioxidant genes. We tested the hypothesis that Nrf2 is involved in the renal DJ-1-mediated inhibition of reactive oxygen species production. We have reported that silencing dopamine 2 receptor in mouse renal proximal tubule cells decreases the expression of DJ-1. We now report that silencing DJ-1 or dopamine 2 receptor in mouse proximal tubule cells and mouse kidneys decreases Nrf2 expression and activity and increases reactive oxygen species production; BP is also increased in mice in which renal DJ-1 or dopamine 2 receptor is silenced. DJ-1(-/-) mice have decreased renal Nrf2 expression and activity and increased nitro-tyrosine levels and BP. Silencing Nrf2 in mouse proximal tubule cells does not alter the expression of DJ-1 or dopamine 2 receptor, indicating that Nrf2 is downstream of dopamine 2 receptor and DJ-1. An Nrf2 inducer, bardoxolone, normalizes the systolic BP and renal malondialdehyde levels in DJ-1(-/-) mice without affecting them in their wild-type littermates. Because Nrf2 ubiquitination is increased in DJ-1(-/-) mice, we conclude that the protective effect of DJ-1 on renal oxidative stress is mediated, in part, by preventing Nrf2 degradation. Moreover, renal dopamine 2 receptor and DJ-1 are necessary for normal Nrf2 activity to keep a normal redox balance and BP.

Nox and renal disease

Since the first demonstration of Nox enzyme expression in the kidney in the early 1990s and the subsequent identification of Nox4, or RENOX, a decade later, it has become apparent that the Nox family of reactive oxygen species (ROS) generating enzymes plays an integral role in the normal physiological function of the kidney. As our knowledge of Nox expression patterns and functions in various structures and specialized cell types within the kidney grows, so does the realization that Nox-derived oxidative stress contributes significantly to a wide variety of renal pathologies through their ability to modify lipids and proteins, damage DNA and activate transcriptional programmes. Diverse studies demonstrate key roles for Nox-derived ROS in kidney fibrosis, particularly in settings of chronic renal disease such as diabetic nephropathy. As the most abundant Nox family member in the kidney, much emphasis has been placed on the role of Nox4 in this setting. However, an ever growing body of work continues to uncover key roles for other Nox family members, not only in diabetic kidney disease, but in a diverse array of renal pathological conditions. The objective of the present review is to highlight the latest novel developments in renal Nox biology with an emphasis not only on diabetic nephropathy but many of the other renal disease contexts where oxidative stress is implicated.

Oxidative stress causes imbalance of renal renin angiotensin system (RAS) components and hypertension in obese Zucker rats

Background

Oxidative stress plays an important role in the pathogenesis of hypertension, especially in obesity‐related hypertension. The natriuretic and antinatriuretic components of the renal renin angiotensin system (RAS) maintain sodium homeostasis and blood pressure. Here, we test the hypothesis that increased oxidative stress leads to the imbalance of RAS components and hypertension in obese Zucker rats.

Methods and Results

Lean and obese rats received vehicle or tempol, a superoxide dismutase mimetic in the drinking water for 4 weeks. Compared with vehicle‐treated lean rats, vehicle‐treated obese rats exhibited higher blood pressure and increased renal oxidative stress, accompanied by increased diuretic and natriuretic responses to AT₁R antagonist (Candesartan) and AT₂R agonist (CGP‐42112A) and reduced diuretic and natriuretic response to MasR agonist (Ang‐[1 to 7]). Moreover, obese rats had higher ACE, AT₁R and AT₂R, lower ACE2 and MasR expressions in the kidney. All of the above‐mentioned abnormalities were reversed to some degree by tempol treatment. In primary cultures of renal proximal tubular (RPT) cells from lean and obese rats, tempol treatment also increased AT₂R, ACE2, and MasR expressions but decreased AT₁R and ACE expressions in obese rats.

Conclusions

Taken together, our study indicated that the imbalance of renal RAS components was associated with increased oxidative stress in obese rats. Furthermore, antioxidant treatment with tempol reversed the imbalance of renal RAS components and led to diuresis and natriuresis, which, at least in part, explains the blood pressure‐lowering effect of antioxidant supplementation in obesity‐related hypertension.

Atrial natriuretic peptide and renal dopaminergic system: a positive friendly relationship?

Sodium metabolism by the kidney is accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between antinatriuretic and natriuretic factors. Renal dopamine plays a central role in this interactive network. The natriuretic hormones, such as the atrial natriuretic peptide, mediate some of their effects by affecting the renal dopaminergic system. Renal dopaminergic tonus can be modulated at different steps of dopamine metabolism (synthesis, uptake, release, catabolism, and receptor sensitization) which can be regulated by the atrial natriuretic peptide. At tubular level, dopamine and atrial natriuretic peptide act together in a concerted manner to promote sodium excretion, especially through the overinhibition of Na+, K+-ATPase activity. In this way, different pathological scenarios where renal sodium excretion is dysregulated, as in nephrotic syndrome or hypertension, are associated with impaired action of renal dopamine and/or atrial natriuretic peptide, or as a result of impaired interaction between these two natriuretic systems. The aim of this review is to update and comment on the most recent evidences demonstrating how the renal dopaminergic system interacts with atrial natriuretic peptide to control renal physiology and blood pressure through different regulatory pathways.

Unique role of NADPH oxidase 5 in oxidative stress in human renal proximal tubule cells

NADPH oxidases are the major sources of reactive oxygen species in cardiovascular, neural, and kidney cells. The NADPH oxidase 5 (NOX5) gene is present in humans but not rodents. Because Nox isoforms in renal proximal tubules (RPTs) are involved in the pathogenesis of hypertension, we tested the hypothesis that NOX5 is differentially expressed in RPT cells from normotensive (NT) and hypertensive subjects (HT). We found that NOX5 mRNA, total NOX5 protein, and apical membrane NOX5 protein were 4.2±0.7-fold, 5.2±0.7-fold, and 2.8±0.5-fold greater in HT than NT. Basal total NADPH oxidase activity was 4.5±0.2-fold and basal NOX5 activity in NOX5 immunoprecipitates was 6.2±0.2-fold greater in HT than NT (P=<0.001, n=6-14/group). Ionomycin increased total NOX and NOX5 activities in RPT cells from HT (P<0.01, n=4, ANOVA), effects that were abrogated by pre-treatment of the RPT cells with diphenylene-iodonium or superoxide dismutase. Silencing NOX5 using NOX5-siRNA decreased NADPH oxidase activity (-45.1±3.2% vs. mock-siRNA, n=6-8) in HT. D1-like receptor stimulation decreased NADPH oxidase activity to a greater extent in NT (-32.5±1.8%) than HT (-14.8±1.8). In contrast to the marked increase in expression and activity of NOX5 in HT, NOX1 mRNA and protein were minimally increased in HT, relative to NT; total NOX2 and NOX4 proteins were not different between HT and NT, while the increase in apical RPT cell membrane NOX1, NOX2, and NOX4 proteins in HT, relative to NT, was much less than those observed with NOX5. Thus, we demonstrate, for the first time, that NOX5 is expressed in human RPT cells and to greater extent than the other Nox isoforms in HT than NT. We suggest that the increased expression of NOX5, which may be responsible for the increased oxidative stress in RPT cells in human essential hypertension, is caused, in part, by a defective renal dopaminergic system.

Crosstalk between dopamine receptors and the Na⁺/K⁺-ATPase (review)

Dopamine (DA) receptors, which belong to the G protein-coupled receptor family, are the target of ~50% of all modern medicinal drugs and constitute a large and diverse class of proteins whose primary function is to transduce extracellular stimuli into intracellular signals. Na+/K+-ATPase (NKA) is ubiquitous and crucial for the maintenance of intracellular ion homeostasis and excitability. Furthermore, it plays a critical role in diverse effects, including clinical cardiotonic and cardioprotective effects, ischemic preconditioning in the brain, natriuresis, lung edema clearance and other processes. NKA regulation is of physiological and pharmacological importance and has species- and tissue-specific variations. The activation of DA receptors regulates NKA expression/activity and trafficking in various tissues and cells, for example in the kidney, lung, intestine, brain, non-pigmented ciliary epithelium and the vascular bed. DA receptor-mediated regulation of NKA mediates a diverse range of cellular responses and includes endocytosis/exocytosis, phosphorylation/dephosphorylation of the α subunit of NKA and multiple signaling pathways, including phosphatidylinositol (PI)-phospholipase C/protein kinase (PK) C, cAMP/PKA, PI3K, adaptor protein 2, tyrosine phosphatase and mitogen-activated protein kinase/extracellular signal-regulated protein kinase. Furthermore, in brain and HEK293T cells, D1 and D2 receptors exist in a complex with NKA. Among D1 and D2 receptors and NKA, regulations are reciprocal, which leads to crosstalk between DA receptors and NKA. In the present study, the current understanding of signaling mechanisms responsible for the crosstalk between DA receptors and NKA, as well as with specific consequent functions, is reviewed.

Role of mitochondrial oxidative stress in hypertension

Based on mosaic theory, hypertension is a multifactorial disorder that develops because of genetic, environmental, anatomical, adaptive neural, endocrine, humoral, and hemodynamic factors. It has been recently proposed that oxidative stress may contribute to all of these factors and production of reactive oxygen species (ROS) play an important role in the development of hypertension. Previous studies focusing on the role of vascular NADPH oxidases provided strong support of this concept. Although mitochondria represent one of the most significant sources of cellular ROS generation, the regulation of mitochondrial ROS generation in the cardiovascular system and its pathophysiological role in hypertension are much less understood. In this review, the role of mitochondrial oxidative stress in the pathophysiology of hypertension and cross talk between angiotensin II signaling, pathways involved in mechanotransduction, NADPH oxidases, and mitochondria-derived ROS are considered. The possible benefits of therapeutic strategies that have the potential to attenuate mitochondrial oxidative stress for the prevention/treatment of hypertension are also discussed.

Effects of imperatorin, the active component from Radix Angelicae (Baizhi), on the blood pressure and oxidative stress in 2K,1C hypertensive rats

The 2-kidney, 1-clip (2K,1C) model of hypertension was used to investigate the potential antihypertensive and antioxidant effect of imperatorin extracted from the root of radix angelicae. After 10 weeks treatment of imperatorin, mean blood pressure (MBP) of 2K,1C hypertensive rats was obtained, and superoxide dismutase (SOD), nitric oxide (NO) and nitric oxide synthase (NOS) were measured. Malondialdehyde (MDA) and glutathione (GSH) levels, catalase (CATA), xanthine oxidase (XOD), angiotensinII (Ang II) and endothelin (ET) levels of kidney were evaluated with commercial kits. Nicotinamide adenine dinucleotidephosphate (NADPH) oxidase subunits of the renal cortial tissues were determined by RT-PCR and Western blot. 8-Iso-prostaglandin F2α (8-iso-PGF2α) of 24h urinary excretion was also measured by ELISA. MBP was significantly reduced by treatment with IMP (6.25, 12.5 and 25 mg/kg/day, i.g.) in 2K,1C hypertensive rats. Meanwhile, we found that renal CATA and XOD activities, GSH levels, plasma NO and NOS contents were significantly increased in IMP-treated groups. Plasma ET, renal Ang II levels, MDA and the 24h urinary excretion of 8-iso-PGF2α in the IMP treated group were lower than control SD group. After that, we found the mRNA expressions and protein levels of NADPH oxidase subunits in the clipped kidney were markedly reduced after IMP treated in 2K,1C hypertensive rats. IMP showed antihypertensive and antioxidant effects in the renal injury of renovascular hypertensive rats, suggesting that IMP could be of therapeutic use in preventing renal injury related hypertension.

A high-salt diet further impairs age-associated declines in cognitive, behavioral, and cardiovascular functions in male Fischer brown Norway rats

Aging-associated declines in cognitive, emotional, and cardiovascular function are well known. Environmental stress triggers critical changes in the brain, further compromising cardiovascular and behavioral health during aging. Excessive dietary salt intake is one such stressor. Here, we tested the effect of high salt (HS) on anxiety, learning-memory function, and blood pressure (BP) in male Fischer brown Norway (FBN) rats. Adult (A; 2 mo) and old (O; 20 mo) male rats were fed normal-salt (NS; 0.4% NaCl) or HS (8% NaCl) diets for 4 wk after being implanted with telemeter probes for conscious BP measurement. Thereafter, tests to assess anxiety-like behavior and learning-memory were conducted. The rats were then killed, and samples of plasma, urine, and brain tissue were collected. We found that systolic BP was higher in O-NS (117 ± 1.2 mm Hg) than in A-NS (105 ± 0.8 mm Hg) rats (P < 0.05). Furthermore, BP was higher in O-HS (124 ± 1.4 mm Hg) than in O-NS (117 ± 1.2 mm Hg) rats (P < 0.05). Moreover, anxiety-like behavior (light-dark and open-field tests) was not different between A-NS and O-NS rats but was greater in O-HS rats than in A-NS, O-NS, or A-HS rats (P < 0.05). Short-term memory (radial arm water maze test) was similar in A-NS and O-NS rats but was significantly impaired in O-HS rats compared with A-NS, O-NS, or A-HS rats (P < 0.05). Furthermore, oxidative stress variables (in plasma, urine, and brain) as well as corticosterone (plasma) were greater in O-HS rats when compared with A-NS, O-NS, or A-HS rats (P < 0.05). The antioxidant enzyme glyoxalase-1 expression was selectively reduced in the hippocampus and amygdala of O-HS rats compared with A-NS, O-NS, or A-HS rats (P < 0.05), whereas other antioxidant enzymes, glutathione reductase 1, manganese superoxide dismutase (SOD), and Cu/Zn SOD remained unchanged. We suggest that salt-sensitive hypertension and behavioral derangement are associated with a redox imbalance in the brain of aged FBN rats.

Winter cardiovascular diseases phenomenon

This paper review seasonal patterns across twelve cardiovascular diseases: Deep venous thrombosis, pulmonary embolism, aortic dissection and rupture, stroke, intracerebral hemorrhage, hypertension, heart failure, angina pectoris, myocardial infarction, sudden cardiac death, venricular arrythmia and atrial fibrillation, and discuss a possible cause of the occurrence of these diseases. There is a clear seasonal trend of cardiovascular diseases, with the highest incidence occurring during the colder winter months, which have been described in many countries. This phenomenon likely contributes to the numbers of deaths occurring in winter. The implications of this finding are important for testing the relative importance of the proposed mechanisms. Understanding the influence of season and other factors is essential when seeking to implement effective public health measures.

Winter Hypertension: Potential mechanisms

Hypertension exhibits a winter peak and summer trough in countries both north and south of the equator. A variety of explanations have been proposed to account for the seasonal nature of hypertension. It is likely that this reflects seasonal variations in risk factors. Seasonal variations have been demonstrated in a number of risk factors may play essential roles for seasonality of hypertension such as noradrenalin, catecholamine and vasopressin, vitamin D, and serum cholesterol. However, a number of studies have also suggested a direct effect of environmental temperature and physical activity on blood pressure. This paper was design to review the available evidence on seasonal variations in hypertension and possible explanations for them.

Grape powder supplementation prevents oxidative stress-induced anxiety-like behavior, memory impairment, and high blood pressure in rats

We examined whether or not grape powder treatment ameliorates oxidative stress-induced anxiety-like behavior, memory impairment, and hypertension in rats. Oxidative stress in Sprague-Dawley rats was produced by using L-buthionine-(S,R)-sulfoximine (BSO). Four groups of rats were used: 1) control (C; injected with vehicle and provided with tap water), 2) grape powder-treated (GP; injected with vehicle and provided for 3 wk with 15 g/L grape powder dissolved in tap water), 3) BSO-treated [injected with BSO (300 mg/kg body weight), i.p. for 7 d and provided with tap water], and 4) BSO plus grape powder-treated (GP+BSO; injected with BSO and provided with grape powder-treated tap water). Anxiety-like behavior was significantly greater in BSO rats compared with C or GP rats (P < 0.05). Grape powder attenuated BSO-induced anxiety-like behavior in GP+BSO rats. BSO rats made significantly more errors in both short- and long-term memory tests compared with C or GP rats (P < 0.05), which was prevented in GP+BSO rats. Systolic and diastolic blood pressure was significantly greater in BSO rats compared with C or GP rats (P < 0.05), whereas grape powder prevented high blood pressure in GP+BSO rats. Furthermore, brain extracellular signal-regulated kinase-1/2 (ERK-1/2) was activated (P < 0.05), whereas levels of glyoxalase-1 (GLO-1), glutathione reductase-1 (GSR-1), calcium/calmodulin-dependent protein kinase type IV (CAMK-IV), cAMP response element-binding protein (CREB), and brain-derived neurotrophic factor (BDNF) were significantly less (P < 0.05) in BSO but not in GP+BSO rats compared with C or GP rats. We suggest that by regulating brain ERK-1/2, GLO-1, GSR-1, CAMK-IV, CREB, and BDNF levels, grape powder prevents oxidative stress-induced anxiety, memory impairment, and hypertension in rats.

Cardiovascular and renal manifestations of glutathione depletion induced by buthionine sulfoximine

Oxidative stress contributes to the development of several cardiovascular diseases, including diabetes, renal insufficiency, and arterial hypertension. Animal studies have evidenced the association between higher blood pressure (BP) and increased oxidative stress, and treatment with antioxidants has been shown to reduce BP, while BP reduction due to antihypertensive drugs is associated with reduced oxidative stress. In 2000, it was first reported that oxidative stress and arterial hypertension were produced in normal Sprague-Dawley rats by oral administration of buthionine sulfoximine (BSO), which induces glutathione (GSH) depletion, indicating that oxidative stress may induce hypertension. The contribution of several potential pathogenic factors has been evaluated in the BSO rat model, the prototype of oxidative stress-induced hypertension, including vascular reactivity, endothelium-derived factors, renin-angiotensin system activity, TXA(2)-PGH(2) production, sodium sensitivity, renal dopamine-induced natriuresis, and sympathetic tone. This review summarizes the main factors implicated in the pathogenesis of BSO-induced hypertension and the alterations associated with GSH depletion that are related to renal function or BP control.

Potential dopamine-1 receptor stimulation in hypertension management

The role of dopamine receptors in blood pressure regulation is well established. Genetic ablation of both dopamine D1-like receptor subtypes (D1, D5) and D2-like receptor subtypes (D2, D3, D4) results in a hypertensive phenotype in mice. This review focuses on the dopamine D1-like receptor subtypes D1 and D5 (especially D1 receptors), as they play a major role in regulating sodium homeostasis and blood pressure. Studies mostly describing the role of renal dopamine D1-like receptors are included, as the kidneys play a pivotal role in the maintenance of sodium homeostasis and the long-term regulation of blood pressure. We also attempt to describe the interaction between D1-like receptors and other proteins, especially angiotensin II type 1 and type 2 receptors, which are involved in the maintenance of sodium homeostasis and blood pressure. Finally, we discuss a new concept of renal D1 receptor regulation in hypertension that involves oxidative stress mechanisms.

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Genetics of salt-sensitive hypertension

The assessment of salt sensitivity of blood pressure is difficult because of the lack of universal consensus on definition. Regardless of the variability in the definition of salt sensitivity, increased salt intake, independent of the actual level of blood pressure, is also a risk factor for cardiovascular morbidity and mortality and kidney disease. A modest reduction in salt intake results in an immediate decrease in blood pressure, with long-term beneficial consequences. However, some have suggested that dietary sodium restriction may not be beneficial to everyone. Thus, there is a need to distinguish salt-sensitive from salt-resistant individuals, but it has been difficult to do so with phenotypic studies. Therefore, there is a need to determine the genes that are involved in salt sensitivity. This review focuses on genes associated with salt sensitivity, with emphasis on the variants associated with salt sensitivity in humans that are not due to monogenic causes. Special emphasis is given to gene variants associated with salt sensitivity whose protein products interfere with cell function and increase blood pressure in transgenic mice.

Relationship of dietary cholesterol to blood pressure: the INTERMAP study

OBJECTIVE

A direct relationship of dietary cholesterol to blood pressure of men has been reported in a few observational studies from the USA. It is not clear whether this association prevails consistently, for example, in populations with varied dietary habits, across ethnic groups, and sexes. Cross-sectional data from the International Study of Macro/Micro-nutrients and Blood Pressure (INTERMAP) were used to assess relations of dietary cholesterol intake to blood pressure in men and women from four countries.

METHODS

Data include 83 nutrients from four multipass 24-h dietary recalls and two-timed 24-h urine collections; eight blood pressure readings, and questionnaire data, for 4680 participants ages 40-59 years from 17 population samples in Japan, People's Republic of China, UK, and USA.

RESULTS

With sequential models to control for multiple possible confounders (dietary, other), linear regression analyses showed that dietary cholesterol was directly related to SBP for all participants and for nonhypertensive individuals, but not to DBP. With adjustment for 12 variables, estimated SBP differences with 2SD for higher cholesterol intake (131.0 mg/1000 kcal) were 0.9 mmHg (P < 0.05) for all participants and 1.1 mmHg (P < 0.01) for nonhypertensive individuals, findings attenuated with addition of height and weight to the model.

CONCLUSION

INTERMAP found a low-order, positive relationship of dietary cholesterol intake to SBP with control for multiple possible confounders. Reduction of dietary cholesterol intake may contribute to prevention and control of adverse blood pressure levels in general populations.

Oxidative stress: a potential recipe for anxiety, hypertension and insulin resistance

We recently reported involvement of oxidative stress in anxiety-like behavior of rats. Others in separate studies have demonstrated a link between oxidative stress and hypertension as well as with type 2 diabetes/insulin resistance. In the present study, we have tested a putative role of oxidative stress in anxiety-like behavior, hypertension and insulin resistance using a rat model of oxidative stress. Oxidative stress in rats was produced by xanthine (0.1%; drinking water) and xanthine oxidase (5 U/kg; i.p.). X+XO-treated rats had increased plasma and urinary 8-isoprostane levels (a marker of oxidative stress) and increased malondialdehyde (MDA) levels in the hippocampus and amygdala as compared to control rats. Serum corticosterone (a systemic marker of stress and anxiety) levels also increased with X+XO treatment. Moreover, anxiety-like behavior measured via open-field and light-dark exploration behavior tests significantly increased in X+XO-treated rats. Mean arterial blood pressure measured in anesthetized rats increased in X+XO-treated compared to control rats. Furthermore, plasma insulin but not glucose levels together with homeostasis model assessment (HOMA), an index of insulin resistance, were higher in X+XO-treated rats. Our studies suggest that oxidative stress is a common factor that link anxiety-like behavior, hypertension and insulin resistance in rats.

Exercise reduces oxidative stress but does not alleviate hyperinsulinemia or renal dopamine D1 receptor dysfunction in obese rats

Impairment of renal dopamine D1 receptor (D1R)-mediated natriuresis is associated with hypertension in humans and animal models, including obese Zucker rats. We have previously reported that treatment of these rats with antioxidants or insulin sensitizers reduced insulin levels and oxidative stress, restored D1R-mediated natriuresis, and reduced blood pressure. Furthermore, the redox-sensitive transcription factor, nuclear factor-κB (NF-κB), has been implicated in impairment of D1R-mediated natriuresis during oxidative stress. In this study, we investigated the effect of exercise on insulin levels, oxidative stress, nuclear translocation of NF-κB, blood pressure, albuminuria, and D1R-mediated natriuresis. The exercise protocol involved treadmill exercise from 3 wk of age for 8 wk. Exercise reduced oxidative stress, nuclear translocation of NF-κB, and albuminuria. However, exercise did not reduce plasma insulin levels or blood pressure. Also, selective D1R agonist (SKF-38393)-mediated increases in sodium excretion and guanosine 5'-O-(3-thiotriphosphate) binding were impaired in obese rats compared with lean rats, and exercise did not restore this defect. We conclude that, while exercise is beneficial in reducing oxidative stress and renal injury, reducing insulin levels may be required to restore D1R-mediated natriuresis in this model of obesity and metabolic syndrome. Furthermore, this study supports previous observations that restoring D1R function contributes to blood pressure reduction in this model.

Dopamine-mediated inhibition of renal Na+/K+-ATPase in HK-2 cells is reduced by ouabain

1. Abnormal renal sodium handling is considered a major contributing factor in hypertension associated with chronic ouabain treatment. However, the molecular mechanisms involved in abnormal renal sodium handling have not been elucidated. Therefore, we investigated whether chronic ouabain treatment perturbs dopamine D(1) receptor function. 2. The expression and phosphorylation levels of the D(1) receptor in cells of the human proximal tubule cell line (HK-2) were determined using western blot analysis and reverse transcription polymerase chain reaction. The activity of the renal sodium/potassium pump (Na(+)/K(+)-ATPase) was measured using a colourimetric assay, and cyclic adenosine monophosphate accumulation was determined by performing a radioimmunoassay. 3. We showed that chronic ouabain treatment decreased the protein and mRNA expression levels of the D(1) receptor and increased the basal phosphorylation of the D(1) receptor in HK-2 cells. We also showed that in the presence of ouabain, HK-2 cells did not reveal the cyclic adenosine monophosphate accumulation and Na(+)/K(+)-ATPase inhibition induced by the D(1) receptor agonist fenoldopam. 4. We hypothesize that the ouabain-induced decrease in renal D(1) receptor function is responsible for the increase in renal sodium reabsorption, which eventually leads to ouabain-induced hypertension.

Chronic hyperleptinemia induces resistance to acute natriuretic and NO-mimetic effects of leptin

Apart from controlling energy balance, leptin, secreted by adipose tissue, is also involved in the regulation of cardiovascular function. Previous studies have demonstrated that acutely administered leptin stimulates natriuresis and vascular nitric oxide (NO) production and that these effects are impaired in obese animals. However, the mechanism of resistance to leptin is not clear. Because obesity is associated with chronically elevated leptin, we examined if long-term hyperleptinemia impairs acute effects of leptin on sodium excretion and NO production in the absence of obesity. Hyperleptinemia was induced in lean rats by administration of exogenous leptin at a dose of 0.5mg/kg/day for 7 days, and then acute effect of leptin (1mg/kg i.v.) was studied under general anesthesia. Leptin increased fractional sodium excretion and decreased Na(+),K(+)-ATPase activity in the renal medulla. In addition, leptin increased the level of NO metabolites and cyclic GMP in plasma and aortic wall. These acute effects of leptin were impaired in hyperleptinemic animals. In both control and hyperleptinemic groups the effect of leptin on Na(+) excretion and renal Na(+),K(+)-ATPase was abolished by phosphoinositide 3-kinase (PI3K) inhibitor, wortmannin, but not by protein kinase B/Akt inhibitor, triciribine,. In contrast, acute effect of leptin on NO metabolites and cGMP was abolished by triciribine but not by wortmannin. Leptin stimulated Akt phosphorylation at Ser(473) in aortic tissue but not in the kidney, and this effect was comparable in control and hyperleptinemic groups. These results suggest that hyperleptinemia may mediate "renal" and "vascular" leptin resistance observed in obesity.

Exercise activates redox-sensitive transcription factors and restores renal D1 receptor function in old rats

We have previously reported that age-associated oxidative stress via protein kinase C (PKC) increases D1 receptor (D1R) phosphorylation and causes D1R-G protein uncoupling in renal proximal tubules (RPTs) of old Fischer 344 rats. This results in reduced ability of D1R agonist SKF-38393 to inhibit Na+-K+-ATPase in RPTs of old rats. Here, we studied the effect of treadmill exercise on markers of oxidative stress, PKC, D1R phosphorylation, D1R-G protein coupling, and Na+-K+-ATPase activity in RPTs of adult and old rats. We found increased levels of malondialdehyde, a marker of oxidative stress, in RPTs of old rats, which decreased during exercise. Nuclear levels of nuclear erythroid-related factor (Nrf)-2 and nuclear factor (NF)-kappaB in RPTs, transcription factors involved in antioxidant enzyme gene transcription, increased in exercised old rats. This was accompanied by an increase in the activity and expression of antioxidant enzymes, superoxide dismutase and heme oxygenase-1. Age-related decrease in the levels of D1R mRNAs and proteins was attenuated during exercise. Furthermore, exercise in old rats decreased PKC activity and D1R phosphorylation and increased SKF-38393-mediated [35S]guanosine 5'-O-(3-thiotriphosphate) binding (an index of D1R-G protein coupling). SKF-38393 also caused inhibition of Na+-K+-ATPase in these animals. Also, exercise caused a decrease in proteinuria and increase in phosphaturia in old rats. These results suggest beneficial effects of exercise in terms of increasing antioxidant defenses, decreasing oxidative stress, and improving kidney function in general and D1R function in particular in aging. Both Nrf-2 and NF-kappaB seem to play key role in this phenomenon.

Inflammation compromises renal dopamine D1 receptor function in rats

We tested the effects of inflammation on renal dopamine D1 receptor signaling cascade, a key pathway that maintains sodium homeostasis and blood pressure during increased salt intake. Inflammation was produced by administering lipopolysaccharide (LPS; 4 mg/kg ip) to rats provided without (normal salt) and with 1% NaCl in drinking water for 2 wk (high salt). Control rats had saline injection and received tap water. We found that LPS increased the levels of inflammatory cytokines, interleukin-6, and tumor necrosis factor-alpha in the rats given either normal- or high-salt intake. Also, these rats had higher levels of oxidative stress markers, malondialdehyde and nitrotyrosine, and lower levels of antioxidant enzyme superoxide dismutase in the renal proximal tubules (RPTs). The nuclear levels of transcription factors NF-kappaB increased and Nrf2 decreased in the RPTs in response to LPS in rats given normal and high salt. Furthermore, D1 receptor numbers, D1 receptor proteins, and D1 receptor agonist (SKF38393)-mediated (35)S-GTPgammaS binding decreased in the RPTs in these rats. The basal activities of Na-K-ATPase in the RPTs were similar in control and LPS-treated rats given normal and high salt. SKF38393 caused inhibition of Na-K-ATPase activity in the primary cultures of RPTs treated with vehicle but not in the cultures treated with LPS. Furthermore, LPS caused an increase in blood pressure in the rats given high salt but not in the rats given normal salt. These results suggest that LPS differentially regulates NF-kappaB and Nrf2, produces inflammation, decreases antioxidant enzyme, increases oxidative stress, and causes D1 receptor dysfunction in the RPTs. The LPS-induced dysfunction of renal D1 receptors alters salt handling and causes hypertension in rats during salt overload.

Intrarenal dopamine attenuates deoxycorticosterone acetate/high salt-induced blood pressure elevation in part through activation of a medullary cyclooxygenase 2 pathway

Locally produced dopamine in the renal proximal tubule inhibits salt and fluid reabsorption, and a dysfunctional intrarenal dopaminergic system has been reported in essential hypertension and experimental hypertension models. Using catechol-O-methyl-transferase knockout (COMT(-/-)) mice, which have increased renal dopamine because of deletion of the major renal dopamine-metabolizing enzyme, we investigated the effect of intrarenal dopamine on the development of hypertension in the deoxycorticosterone acetate/high-salt (DOCA/HS) model. DOCA/HS led to significant increases in systolic blood pressure in wild-type mice (from 115+/-2 to 153+/-4 mm Hg), which was significantly attenuated in COMT(-/-) mice (from 114+/-2 to 135+/-3 mm Hg). In DOCA/HS COMT(-/-) mice, the D1-like receptor antagonist SCH-23390 increased systolic blood pressure (156+/-2 mm Hg). DOCA/HS COMT(-/-) mice also exhibited more urinary sodium excretion (COMT(-/-) versus wild-type: 3038+/-430 versus 659+/-102 micromol/L per 24 hours; P<0.01). Furthermore, DOCA/HS-induced renal oxidative stress was significantly attenuated in COMT(-/-) mice. COX-2-derived prostaglandins in the renal medulla promote sodium excretion, and dopamine stimulates medullary prostaglandin production. Renal medullary COX-2 expression and urinary prostaglandin E2 excretion were significantly higher in COMT(-/-) than in wild-type mice after DOCA/HS treatment. In DOCA/HS-treated COMT(-/-) mice, the COX-2 inhibitor SC-58236 reduced urinary sodium and prostaglandin E(2) excretion and increased systolic blood pressure (153+/-2 mm Hg). These studies indicate that an activated renal dopaminergic system attenuates the development of hypertension, at least in large part through activating medullary COX-2 expression/activity, and also decreases oxidative stress resulting from DOCA/HS.

Renal dopaminergic defect in C57Bl/6J mice

The C57Bl/6J mouse strain, the genetic background of many transgenic and gene knockout models, is salt sensitive and resistant to renal injury. We tested the hypothesis that renal dopaminergic function is defective in C57Bl/6J mice. On normal NaCl (0.8%, 1 wk) diet, anesthetized and conscious (telemetry) blood pressures were similar in C57Bl/6J and SJL/J mice. High NaCl (6%, 1 wk) increased blood pressure (approximately 30%) in C57Bl/6J but not in SJL/J mice and urinary dopamine to greater extent in SJL/J than in C57Bl/6J mice. Absolute and fractional sodium excretions were lower in SJL/J than in C57Bl/6J mice. The blood pressure-natriuresis plot was shifted to the right in C57Bl/6J mice. Renal expressions of D(1)-like (D(1)R and D(5)R) and angiotensin II AT(1) receptors were similar on normal salt, but high salt increased D(5)R only in C57Bl/6J. GRK4 expression was lower on normal but higher on high salt in C57Bl/6J than in SJL/J mice. Salt increased the excretion of microalbumin and 8-isoprostane (oxidative stress marker) and the degree of renal injury to a greater extent in SJL/J than in C57Bl/6J mice. A D(1)-like receptor agonist increased sodium excretion whereas a D(1)-like receptor antagonist decreased sodium excretion in SJL/J but not in C57Bl/6J mice. In contrast, parathyroid hormone had a similar natriuretic effect in both strains. These results show that defective D(1)-like receptor function is a major cause of salt sensitivity in C57Bl/6J mice, decreased renal dopamine production might also contribute. The relative resistance to renal injury of C57Bl/6J may be a consequence of decreased production of reactive oxygen species.

Renal functional, not morphological, abnormalities account for salt sensitivity in Dahl rats

BACKGROUND

The kidney's role in the pathogenesis of salt-induced hypertension remains unclear. However, it has been suggested that inherited morphological renal abnormalities may cause hypertension. We hypothesized that functional, not morphological, derangements in Dahl salt-sensitive rats' kidneys cause NaCl retention that leads to hypertension accompanied by renal pathologic changes and proteinuria.

METHOD

We studied hemodynamic, renal morphologic, and biochemical differences in Dahl salt-resistant and Dahl salt-sensitive rats fed low (0.05-0.23% NaCl) or elevated (1% NaCl) salt diets.

RESULTS

We found similar hemodynamics, equal numbers of glomeruli, normal renal medullary interstitial cells and their osmiophilic granules, and cortical morphology in normotensive Dahl salt-resistant and Dahl salt-sensitive rats fed low dietary salt. Furthermore, aldosterone secretion, caused by angiotensin II infusion in normotensive rats fed 0.23% NaCl, was significantly less in Dahl salt-sensitive than Dahl salt-resistant rats. Increasing NaCl to 1% caused renal vasoconstriction without changing cyclic GMP excretion in Dahl salt-sensitive rats; in Dahl salt-resistant rats, cyclic GMP increased markedly and renal vascular resistance remained unchanged. On 1% NaCl for 9 months, Dahl salt-sensitive rats developed marked hypertension, severe renal vasoconstriction, glomerulosclerosis, tubulointerstitial abnormalities, and marked proteinuria; hypertension resulted from increased total peripheral resistance, as occurs in essential hypertensive humans. No hemodynamic or renal pathologic changes occurred in Dahl salt-resistant rats, and proteinuria was minimal.

CONCLUSION

We conclude that renal functional, not morphological, abnormalities cause salt sensitivity in Dahl rats.

Reactive oxygen species and dopamine receptor function in essential hypertension

Essential hypertension is a major risk factor for stroke, myocardial infarction, and heart and kidney failure. Dopamine plays an important role in the pathogenesis of hypertension by regulating epithelial sodium transport and by interacting with vasoactive hormones and humoral factors. However, the mechanisms leading to impaired dopamine receptor function in hypertension states are not clear. Compelling experimental evidence indicates a role of reactive oxygen species (ROS) in hypertension, and there are increasing pieces of evidence showing that in conditions associated with oxidative stress, which is present in hypertensive states, dopamine receptor effects, such as natriuresis, diuresis, and vasodilation, are impaired. The goal of this review is to present experimental evidence that has led to the conclusion that decreased dopamine receptor function increases ROS activity and vice versa. Decreased dopamine receptor function and increased ROS production, working in concert or independent of each other, contribute to the pathogenesis of essential hypertension.

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.

Dopamine and angiotensin as renal counterregulatory systems controlling sodium balance

PURPOSE OF REVIEW

To review the recent evidence demonstrating how the renal dopaminergic and angiotensin systems control renal electrolyte balance through various receptor-mediated pathways with counterregulatory interactions.

RECENT FINDINGS

Stimulation of the renal rennin-angiotensin system results in increased sodium reabsorption, whereas the opposite is true for stimulation of the renal dopaminergic system. An underactive renal dopaminergic system has been associated with increased sodium reabsorption and hypertension. Recent findings indicate novel cell surface receptor-mediated mechanisms by which these two renal endocrine systems directly counterregulate each other. Each of the dopamine receptors (D1R through D5R) have been implicated in dopamine-mediated natriuresis, in addition to counterregulating the angiotensin type 1 R. Dopamine D1-like (D1R and D5R) stimulation has also been found to induce an AT2 receptor- dependent natriuresis. Recently, it has also been discovered that reactive oxygen species can play a role in inactivating the D1 receptor and activating the angiotensin type 1 R.

SUMMARY

Current therapeutic interventions for hypertension predominantly involve correction of an overactive rennin-angiotensin aldosterone system. Recent evidence suggests that stimulation of the renal dopaminergic system and possibly activation of AT2 receptors, as well as decreasing reactive oxygen species, may provide additional therapeutic approaches.

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.

Dopamine receptors and hypertension

Dopamine plays an important role in regulating renal function and blood pressure. Dopamine synthesis and dopamine receptor subtypes have been shown in the kidney. Dopamine acts via cell surface receptors coupled to G proteins; the receptors are classified via pharmacologic and molecular cloning studies into two families, D1-like and D2-like. Two D1-like receptors cloned in mammals, the D1 and D5 receptors (D1A and D1B in rodents), are linked to adenylyl cyclase stimulation. Three D2-like receptors (D2, D3, and D4) have been cloned and are linked mainly to adenylyl cyclase inhibition. Activation of D1-like receptors on the proximal tubules inhibits tubular sodium reabsorption by inhibiting Na/H-exchanger and Na/K-adenosine triphosphatase activity. Reports exist of defective renal dopamine production and/or dopamine receptor function in human primary hypertension and in genetic models of animal hypertension. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to hypertension. A primary defect in D1-like receptors and an altered signaling system in proximal tubules may reduce dopamine-mediated effects on renal sodium excretion. The molecular basis for dopamine receptor dysfunction in hypertension is being investigated, and may involve an abnormal posttranslational modification of the dopamine receptor.

Renal antioxidant enzymes and glutathione redox status in leptin-induced hypertension

Previously, we have demonstrated that leptin increases blood pressure (BP) in the rats through two oxidative stress-dependent mechanisms: stimulation of extracellular signal-regulated kinases (ERK) by H(2)O(2) and scavenging of nitric oxide (NO) by superoxide (O(2-.)). Herein, we examined if renal glutathione system and antioxidant enzymes determine the mechanism of prohypertensive effect of leptin. Leptin administered at 0.5 mg/kg/day for 4 or 8 days increased BP and renal Na(+),K(+)-ATPase activity and reduced fractional sodium excretion; these effects were prevented by NADPH oxidase inhibitor, apocynin. Superoxide scavenger, tempol, abolished the effect of leptin on BP and renal Na(+) pump in rats receiving leptin for 8 days, whereas ERK inhibitor, PD98059, was effective in animals treated with leptin for 4 days. Leptin administered for 4 days decreased glutathione (GSH) and increased glutathione disulfide (GSSG) in the kidney. In animals receiving leptin for 8 days GSH returned to normal level, which was accompanied by up-regulation of gamma-glutamylcysteine synthetase (gamma-GCS), a rate-limiting enzyme of the GSH biosynthetic pathway. In addition, superoxide dismutase (SOD) activity was decreased, whereas glutathione peroxidase (GPx) was increased in rats receiving leptin for 8 days. Cotreatment with gamma-GCS inhibitor, buthionine sulfoximine (BSO), accelerated, whereas GSH precursor, N-acetylcysteine (NAC), attenuated leptin-induced changes in gamma-GCS, SOD, and GPx. In addition, coadministration of BSO changed the mechanism of BP elevation from H(2)O(2)-ERK to (O(2-.))-NO dependent in animals receiving leptin for 4 days, whereas NAC had the opposite effect in rats treated with leptin for 8 days. These results suggest that initial change in GSH redox status induces decrease in SOD/GPx ratio, which results in greater amount of (O)2-.)) versus H(2)O(2) in later phase of leptin treatment, thus shifting the mechanism of BP elevation from H(2)O(2)-ERK to (O(2-.))-NO dependent.

Oxidative stress-induced renal angiotensin AT1 receptor upregulation causes increased stimulation of sodium transporters and hypertension

Reactive oxygen species have emerged as important molecules in cardiovascular dysfunction such as diabetes and hypertension. Recent work has shown that oxidative stress and angiotensin II signaling mutually regulate each other by multiple mechanisms and contribute to the development of hypertension. Most of the known biological actions of angiotensin II can be attributed to AT1 receptors. The present study was carried out to investigate the role of renal AT1 receptor signaling in oxidative stress-mediated hypertension. Male Sprague-Dawley rats received tap water (control) or 30 mM L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mM tempol (an antioxidant) for 2 wk. Compared with control rats, BSO-treated rats exhibited increased oxidative stress and reduced antioxidant levels and developed hypertension. BSO treatment also caused increased renal proximal tubular AT1 receptor protein abundance, message levels, and ligand binding. In these rats, angiotensin II caused significantly higher accumulation of inositol trisphosphate (IP3) and phospholipase C (PLC) activation which was sensitive to blockade by AT1 but not to AT2 antagonist. Also, angiotensin II-mediated, AT1-dependent MAP kinase, Na-K-ATPase, and Na/H exchanger 3 activation was higher in BSO-treated rats than in control rats. Tempol supplementation of BSO-treated rats restored redox status, normalized AT1 receptor expression, and decreased blood pressure. Tempol also normalized the angiotensin II-mediated, AT1-dependent IP3 accumulation and PLC, MAP kinase, Na-K-ATPase, and Na/H exchanger 3 stimulation. These data suggest that oxidative stress leads to AT1 receptor upregulation, which in turn causes overstimulation of sodium transporters and subsequently contributes to sodium retention and hypertension. Tempol, while reducing oxidative stress, normalizes AT1 receptor signaling and decreases blood pressure.