Effects of high-salt diet on CYP450-4A omega-hydroxylase expression and active tone in mesenteric resistance arteries

Review Article on Molecular Mechanism of Regulation of Hypertension by Macro-elements (Na, K, Ca and Mg), Micro-elements/Trace Metals (Zn and Cu) and Toxic Elements (Pb and As)

Hypertension (HT) is a medical condition arising due to increase in blood pressure (BP) prevalent worldwide. The balanced dietary intakes of macro-elements and micro-elements including Na, K, Ca, Mg, Zn, and Cu have been described to maintain BP in humans by regulating the osmolarity of blood, cells/tissues, prevention of generation of oxidative and nitrosative stress (OANS), and endothelial damage through their functioning as important components of renin-angiotensin-aldosterone system (RAAS), antioxidant enzyme defense system, and maintenance of blood vascular-endothelial and vascular smooth muscle cell (VSMC) functions. However, inadequate/excess dietary intakes of Na/K, Ca/Mg, and Zn/Cu along with higher Pb and As exposures recognized to induce HT through common mechanisms including the followings: endothelial dysfunctions due to impairment of vasodilatation, increased vasoconstriction and arterial stiffness, blood clotting, inflammation, modification of sympathetic activity and higher catecholamine release, increased peripheral vascular resistance, and cardiac output; increased OANS due to reduced and elevated activities of extracellular superoxide dismutase and NAD(P)H oxidase, less nitric oxide bioavailability, decrease in cGMP and guanylate cyclase activity, increase in intracellular Ca2+ ions in VSMCs, and higher pro-inflammatory cytokines; higher parathyroid and calcitriol hormones; activation/suppression of RAAS resulting imbalance in blood Na+, K+, and water regulated by renin, angiotensin II, and aldosterone through affecting natriuresis/kaliuresis/diuresis; elevation in serum cholesterol and LDL cholesterol, decrease in HDL cholesterol due to defect in lipoprotein metabolism. The present study recommends the need to review simple dietary mineral intervention studies/supplementation trials before keeping their individual dietary excess intakes/exposures in consideration because their interactions lead to elevation and fall of their concentrations in body affecting onset of HT.

Salt Sensitivity of Blood Pressure in Black People: The Need to Sort Out Ancestry Versus Epigenetic Versus Social Determinants of Its Causation

Race is a social construct, but self-identified Black people are known to have higher prevalence and worse outcomes of hypertension than White people. This may be partly due to the disproportionate incidence of salt sensitivity of blood pressure in Black people, a cardiovascular risk factor that is independent of blood pressure and has no proven therapy. We review the multiple physiological systems involved in regulation of blood pressure, discuss what, if anything is known about the differences between Black and White people in these systems and how they affect salt sensitivity of blood pressure. The contributions of genetics, epigenetics, environment, and social determinants of health are briefly touched on, with the hope of stimulating further work in the field.

Effect of Chronically Suppressed Plasma Angiotensin II on Regulation of the CYP4A/20-HETE Pathway in the Dahl Salt-Sensitive Rat

In Dahl salt-sensitive (SS) rats, impaired vascular relaxation can be restored by: (1) minipump infusion of a low (sub-pressor) dose of angiotensin II (ANG II) to restore physiological levels of plasma ANG II, (2) inhibition of 20-HETE production, and (3) introgression of a normally functioning renin allele from the Brown Norway rat (SS-13^BN consomic rat). Unlike SS rats, SS-13^BN rats have normal levels of ANG II on a normal-salt diet and suppressed ANG II on a high-salt (HS) diet. This study tested whether chronically low ANG II levels in SS rats upregulate cytochrome P450-4A (CYP4A) increasing the production of the vasoconstrictor 20-HETE. Although salt-induced suppression of ANG II levels increased reactive oxygen species (ROS) in basilar arteries from SS-13^BN rats in previous studies, this study showed no change in vascular 20-HETE levels in response to ANGII suppression. CYP4A inhibition significantly reduced vascular ROS levels and restored endothelium-dependent relaxation in response to acetylcholine in the middle cerebral artery (MCA) of SS rats and HS-fed SS-13^BN rats. These data demonstrate that both the renin-angiotensin system and the CYP4A/20-HETE pathway play a direct role in the vascular dysfunction of the Dahl SS rat but are independent of each other, even though they may both contribute to vascular dysfunction through ROS production.

Sodium Intake as a Cardiovascular Risk Factor: A Narrative Review

While sodium is essential for human homeostasis, current salt consumption far exceeds physiological needs. Strong evidence suggests a direct causal relationship between sodium intake and blood pressure (BP) and a modest reduction in salt consumption is associated with a meaningful reduction in BP in hypertensive as well as normotensive individuals. Moreover, while long-term randomized controlled trials are still lacking, it is reasonable to assume a direct relationship between sodium intake and cardiovascular outcomes. However, a consensus has yet to be reached on the effectiveness, safety and feasibility of sodium intake reduction on an individual level. Beyond indirect BP-mediated effects, detrimental consequences of high sodium intake are manifold and pathways involving vascular damage, oxidative stress, hormonal alterations, the immune system and the gut microbiome have been described. Globally, while individual response to salt intake is variable, sodium should be perceived as a cardiovascular risk factor when consumed in excess. Reduction of sodium intake on a population level thus presents a potential strategy to reduce the burden of cardiovascular disease worldwide. In this review, we provide an update on the consequences of salt intake on human health, focusing on BP and cardiovascular outcomes as well as underlying pathophysiological hypotheses.

Is too much salt harmful? Yes

The contribution of high sodium intake to hypertension and to the severity of immune-mediated diseases is still being heatedly debated in medical literature and in the lay media. This review aims to demonstrate two conflicting views on the topic, with the first part citing the detrimental effects of excessive salt consumption. Sodium plays a central role in volume and blood pressure homeostasis, and the positive correlation between sodium intake and blood pressure has been extensively researched. Despite the fact that the average of global daily salt consumption exceeds recommendations of international associations, health damage from excessive salt intake is still controversial. Individual differences in salt sensitivity are in great part attributed to this contradiction. Patients suffering from certain diseases as well as other vulnerable groups-either minors or individuals of full age-exhibit more pronounced blood pressure reduction when consuming a low-sodium diet. Furthermore, findings from the last two decades give insight into the concept of extrarenal sodium storage; however, the long-term consequences of this phenomenon are lesser known. Evidence of the relationship between sodium and autoimmune diseases are cited in the review, too. Nevertheless, further clinical trials are needed to clarify their interplay. In conclusion, for salt-sensitive risk groups in the population, even stricter limits of sodium consumption should be set than for young, healthy individuals. Therefore, the question raised in the title should be rephrased as follows: "how much salt is harmful" and "for whom is elevated salt intake harmful?"

Sodium Intake and Hypertension

The close relationship between hypertension and dietary sodium intake is widely recognized and supported by several studies. A reduction in dietary sodium not only decreases the blood pressure and the incidence of hypertension, but is also associated with a reduction in morbidity and mortality from cardiovascular diseases. Prolonged modest reduction in salt intake induces a relevant fall in blood pressure in both hypertensive and normotensive individuals, irrespective of sex and ethnic group, with larger falls in systolic blood pressure for larger reductions in dietary salt. The high sodium intake and the increase in blood pressure levels are related to water retention, increase in systemic peripheral resistance, alterations in the endothelial function, changes in the structure and function of large elastic arteries, modification in sympathetic activity, and in the autonomic neuronal modulation of the cardiovascular system. In this review, we have focused on the effects of sodium intake on vascular hemodynamics and their implication in the pathogenesis of hypertension.

Salt-induced effects on microvascular function: A critical factor in hypertension mediated organ damage

Salt has been linked very closely to the occurrence and complications of arterial hypertension. A large percentage of patients with essential hypertension are salt-sensitive; that is, their blood pressure increases with increased salt intake and decreases with its reduction. For this reason, emphasis is placed on reducing salt intake to better regulate blood pressure. In day-to-day clinical practice this is viewed as mandatory for hypertensive patients who are judged to be salt-sensitive. Previous studies have highlighted the negative effect of high-salt diets on macrovascular function, which also affects blood pressure levels by increasing peripheral resistances. More recent studies provide a better overview of the pathophysiology of microvascular disorders and show that they are largely due to the overconsumption of salt. Microvascular lesions, which have a major impact on the functioning of vital organs, are often not well recognized in clinical practice and are not paid sufficient attention. In general, the damage caused by hypertension to the microvascular network is likely to be overlooked, while reversion of the damage is only rarely considered as a therapeutic target by the treating physician. The purpose of this review is to summarize the impact and the harmful consequences of increased salt consumption in the microvascular network, their significance and pathophysiology, and at the same time to place some emphasis on their treatment and reversion, mainly through diet.

Role of vascular reactive oxygen species in regulating cytochrome P450-4A enzyme expression in Dahl salt-sensitive rats

OBJECTIVE

The potential contribution of CYP4A enzymes to endothelial dysfunction in Dahl salt-sensitive rats was determined by comparison to SS-5^BN consomic rats having chromosome 5 carrying CYP4A alleles from the BN rat introgressed into the SS genetic background.

METHODS

The following experiments were performed in cerebral arteries from HS-fed SS and SS-5^BN rats ± the SOD inhibitor DETC and/or the superoxide scavenger Tempol: (i) endothelial function was determined via video microscopy ± acute addition of the CYP4A inhibitor DDMS or Tempol; (ii) vascular oxidative stress was assessed with DHE fluorescence ± acute addition of DDMS, l-NAME, or PEG-SOD; and (iii) CYP4A protein levels were compared by western blotting.

RESULTS

In DETC-treated SS-5^BN and HS-fed SS rats, (i) DDMS or Tempol ameliorated vascular dysfunction, (ii) DDMS reduced vascular oxidative stress to control levels, (iii) chronic Tempol treatment reduced vascular CYP4A protein expression, and (iv) combined treatment with Tempol and l-NAME prevented the reduction in CYP4A protein expression in MCA of HS-fed SS rats.

CONCLUSION

The CYP4A pathway plays a role in vascular dysfunction in SS rats and there appears to be a direct role of reduced NO availability due to salt-induced oxidant stress in upregulating CYP4A enzyme expression.

Evaluation of Vascular Control Mechanisms Utilizing Video Microscopy of Isolated Resistance Arteries of Rats

This protocol describes the use of in vitro television microscopy to evaluate vascular function in isolated cerebral resistance arteries (and other vessels), and describes techniques for evaluating tissue perfusion using Laser Doppler Flowmetry (LDF) and microvessel density utilizing fluorescently labeled Griffonia simplicifolia (GS1) lectin. Current methods for studying isolated resistance arteries at transmural pressures encountered in vivo and in the absence of parenchymal cell influences provide a critical link between in vivo studies and information gained from molecular reductionist approaches that provide limited insight into integrative responses at the whole animal level. LDF and techniques to selectively identify arterioles and capillaries with fluorescently-labeled GS1 lectin provide practical solutions to enable investigators to extend the knowledge gained from studies of isolated resistance arteries. This paper describes the application of these techniques to gain fundamental knowledge of vascular physiology and pathology in the rat as a general experimental model, and in a variety of specialized genetically engineered "designer" rat strains that can provide important insight into the influence of specific genes on important vascular phenotypes. Utilizing these valuable experimental approaches in rat strains developed by selective breeding strategies and new technologies for producing gene knockout models in the rat, will expand the rigor of scientific premises developed in knockout mouse models and extend that knowledge to a more relevant animal model, with a well understood physiological background and suitability for physiological studies because of its larger size.

The role of 20-HETE in cardiovascular diseases and its risk factors

Arachidonic acid (AA) is metabolized in mammals by enzymes of the CYP4A and 4F families to 20-hydroxyeicosatetraeonic acid (20-HETE) which plays an important role in the regulation of renal function, vascular tone and arterial pressure. In the vasculature, 20-HETE is a potent vasoconstrictor, the up-regulation of which contributes to inflammation, oxidative stress, endothelial dysfunction and an increase in peripheral vascular resistance in models of obesity, diabetes, ischemia/reperfusion, and vascular oxidative stress. Recent studies have established a role for 20-HETE in normal and pathological angiogenic conditions. We discuss in this review the synthesis of 20-HETE and how it and various autacoids, especially the renin-angiotensin system, interact to promote hypertension, vasoconstriction, and vascular dysfunction. In addition, we examine the molecular mechanisms through which 20-HETE induces these actions and the clinical implication of inhibiting 20-HETE production and activity.

High salt intake as a multifaceted cardiovascular disease: new support from cellular and molecular evidence

Scientists worldwide have disseminated the idea that increased dietary salt increases blood pressure. Currently, salt intake in the general population is ten times higher than that consumed in the past and at least two times higher than the current recommendation. Indeed, a salt-rich diet increases cardiovascular morbidity and mortality. For a long time, however, the deleterious effects associated with high salt consumption were only related to the effect of salt on blood pressure. Currently, several other effects have been reported. In some cases, the deleterious effects of high salt consumption are independently associated with other common risk factors. In this article, we gather data on the effects of increased salt intake on the cardiovascular system, from infancy to adulthood, to describe the route by which increased salt intake leads to cardiovascular diseases. We have reviewed the cellular and molecular mechanisms through which a high intake of salt acts on the cardiovascular system to lead to the progressive failure of a healthy heart.

Salt, Angiotensin II, Superoxide, and Endothelial Function

Proper function of the vascular endothelium is essential for cardiovascular health, in large part due to its antiproliferative, antihypertrophic, and anti-inflammatory properties. Crucial to the protective role of the endothelium is the production and liberation of nitric oxide (NO), which not only acts as a potent vasodilator, but also reduces levels of reactive oxygen species, including superoxide anion (O2•-). Superoxide anion is highly injurious to the vasculature because it not only scavenges NO molecules, but has other damaging effects, including direct oxidative disruption of normal signaling mechanisms in the endothelium and vascular smooth muscle cells. The renin-angiotensin system plays a crucial role in the maintenance of normal blood pressure. This function is mediated via the peptide hormone angiotensin II (ANG II), which maintains normal blood volume by regulating Na+ excretion. However, elevation of ANG II above normal levels increases O2•- production, promotes oxidative stress and endothelial dysfunction, and plays a major role in multiple disease conditions. Elevated dietary salt intake also leads to oxidant stress and endothelial dysfunction, but these occur in the face of salt-induced ANG II suppression and reduced levels of circulating ANG II. While the effects of abnormally high levels of ANG II have been extensively studied, far less is known regarding the mechanisms of oxidant stress and endothelial dysfunction occurring in response to chronic exposure to abnormally low levels of ANG II. The current article focuses on the mechanisms and consequences of this less well understood relationship among salt, superoxide, and endothelial function.

High salt intake increases blood pressure in normal rats: putative role of 20-HETE and no evidence on changes in renal vascular reactivity

Background/Aims

High salt (HS) intake may elevate blood pressure (BP), also in animals without genetic salt sensitivity. The development of salt-dependent hypertension could be mediated by endogenous vasoactive agents; here we examined the role of vasodilator epoxyeicosatrienoic acids (EETs) and vasoconstrictor 20-hydroxyeicosatetraenoic acid (20-HETE).

Methods

In conscious Wistar rats on HS diet systolic BP (SBP) was examined after chronic elevation of EETs using 4-[4-(3-adamantan-1-yl-ureido)-cyclohexyloxy]-benzoic acid (c-AUCB), a blocker of soluble epoxide hydrolase, or after inhibition of 20-HETE with 1-aminobenzotriazole (ABT). Thereafter, in acute experiments the responses of renal artery blood flow (Transonic probe) and renal regional perfusion (laser-Doppler) to intrarenal acetylcholine (ACh) or norepinephrine were determined.

Results

HS diet increased urinary 20-HETE excretion. The SBP increase was not reduced by c-AUCB but prevented by ABT until day 5 of HS exposure. Renal vasomotor responses to ACh or norepinephrine were similar on standard and HS diet. ABT but not c-AUCB abolished the responses to ACh.

Conclusions

20-HETE seems to mediate the early-phase HS diet-induced BP increase while EETs are not engaged in the process. Since HS exposure did not alter renal vasodilator responses to Ach, endothelial dysfunction is not a critical factor in the mechanism of salt-induced blood pressure elevation.

20-HETE and blood pressure regulation: clinical implications

20-Hydroxy-5, 8, 11, 14-eicosatetraenoic acid (20-HETE) is a cytochrome P450 (CYP)-derived omega-hydroxylation metabolite of arachidonic acid. 20-HETE has been shown to play a complex role in blood pressure regulation. In the kidney tubules, 20-HETE inhibits sodium reabsorption and promotes natriuresis, thus, contributing to antihypertensive mechanisms. In contrast, in the microvasculature, 20-HETE has been shown to play a pressor role by sensitizing smooth muscle cells to constrictor stimuli and increasing myogenic tone, and by acting on the endothelium to further promote endothelial dysfunction and endothelial activation. In addition, 20-HETE induces endothelial angiotensin-converting enzyme, thus, setting forth a potential feed forward prohypertensive mechanism by stimulating the renin-angiotensin-aldosterone system. With the advancement of gene sequencing technology, numerous polymorphisms in the regulatory coding and noncoding regions of 20-HETE-producing enzymes, CYP4A11 and CYP4F2, have been associated with hypertension. This in-depth review article discusses the biosynthesis and function of 20-HETE in the cardiovascular system, the pharmacological agents that affect 20-HETE action, and polymorphisms of CYP enzymes that produce 20-HETE and are associated with systemic hypertension in humans.

Amelioration of salt-induced vascular dysfunction in mesenteric arteries of Dahl salt-sensitive rats by missense mutation of extracellular superoxide dismutase

Superoxide dismutase (SOD) enzymes, including extracellular SOD (ecSOD), are important for scavenging superoxide radicals (O2(·-)) in the vasculature. This study investigated vascular control in rats [SS-Sod(3m1Mcwi) (ecSOD(E124D))] with a missense mutation that alters a single amino acid (E124D) of ecSOD that produces a malfunctioning protein in the salt-sensitive (Dahl SS) genetic background. We hypothesized that this mutation would exacerbate endothelial dysfunction due to elevated vascular O2(·-) levels in SS, even under normal salt (NS; 0.4% NaCl) conditions. Aortas of ecSOD(E124D) rats fed standard rodent chow showed enhanced sensitivity to phenylephrine and reduced relaxation to acetylcholine (ACh) vs. SS rats. Endothelium-dependent dilation to ACh was unaffected by the mutation in small mesenteric arteries of ecSOD(E124D) rats fed NS diet, and mesenteric arteries of ecSOD(E124D) rats were protected from endothelial dysfunction during short-term (3-5 days) high-salt (HS; 4% NaCl) diet. ACh-induced dilation of mesenteric arteries of ecSOD(E124D) rats and SS rats fed NS diet was inhibited by N(G)-nitro-l-arginine methyl ester and/or by H2O2 scavenging with polyethylene glycol-catalase at higher concentrations of ACh. Total SOD activity was significantly higher in ecSOD(E124D) rats vs. SS controls fed HS diet, most likely reflecting a compensatory response to loss of a functional ecSOD isoform. These findings indicate that, contrary to its effect in the aorta, this missense mutation of ecSOD in the SS rat genome has no negative effect on vascular function in small resistance arteries, but instead protects against salt-induced endothelial dysfunction, most likely via compensatory mechanisms involving an increase in total SOD activity.

The effect of high salt intake on endothelial function: reduced vascular nitric oxide in the absence of hypertension

Within the last 25 years, it has become increasingly clear that high dietary salt intake represents a risk factor for the development of cardiovascular disease that is independent of its well-known ability to increase arterial pressure in some individuals. Studies in normotensive experimental animals and human subjects have revealed that a key feature of this pressure-independent effect of dietary salt is a profound reduction in vascular nitric oxide (NO) bioavailability that limits endothelium-dependent dilation. This reduction in NO is strongly associated with increased levels of reactive oxygen species (ROS) generated by NAD(P)H oxidase, xanthine oxidase or uncoupled endothelial NO synthase within the vascular wall, leading not only to scavenging of NO but also to disruption of some signaling pathways that mediate its production. The mechanistic link between high salt intake and elevated levels of enzymatically generated ROS in the peripheral vasculature is not clear, but a reduction in circulating angiotensin II may play a key role in this regard. Additional studies are needed to further elucidate the mechanisms, both at the systemic level and within the vascular wall, that trigger these salt-induced deficits in endothelial function, and to further clarify how the attendant loss of NO may disrupt tissue blood flow regulation and ultimately lead to adverse cardiovascular events.

Role of the CYP4A/20-HETE pathway in vascular dysfunction of the Dahl salt-sensitive rat

20-HETE (20-hydroxyeicosatetraenoic acid), a vasoconstrictor metabolite of arachidonic acid formed through the action of CYP4A (cytochrome P450-4A) in vascular smooth muscle cells, has been implicated in the development of hypertension and vascular dysfunction. There have been a number of reports in human subjects demonstrating an association between elevated urinary excretion of 20-HETE and hypertension, as well as increased 20-HETE production and vascular dysfunction. The Dahl SS (salt-sensitive) rat is a genetic model of salt-sensitive hypertension that exhibits vascular dysfunction, even when maintained on a normal-salt diet and before the development of hypertension. This mini-review highlights our current research on the role of CYP4A and 20-HETE in the vascular dysfunction of the Dahl SS rat. In our studies, the SS rat is compared with the consomic SS-5BN rat, having chromosome 5 from the salt-resistant Brown Norway rat (carrying all CYP4A genes) introgressed on to the SS genetic background. Our laboratory has demonstrated restoration of normal vascular function in the SS rat with inhibition of the CYP4A/20-HETE pathway, suggesting a direct role for this pathway in the vascular dysfunction in this animal model. Our studies have also shown that the SS rat has an up-regulated CYP4A/20-HETE pathway within their cerebral vasculature compared with the SS-5BN consomic rat, which causes endothelial dysfunction through the production of ROS (reactive oxygen species). Our data shows that ROS influences the expression of the CYP4A/20-HETE pathway in the SS rat in a feed-forward mechanism whereby elevated ROS stimulates production of 20-HETE. The presence of this vicious cycle offers a possible explanation for the spiralling effects of elevated 20-HETE on the development of vascular dysfunction in this animal model.

Introgression of Brown Norway CYP4A genes on to the Dahl salt-sensitive background restores vascular function in SS-5(BN) consomic rats

The present study tested the hypothesis that the Dahl SS (salt-sensitive) rat has vascular dysfunction due, in part, to the up-regulation of the CYP4A/20-HETE (cytochrome P450 ω-hydroxylase 4A)/20-hydroxyeicosatetraenoic acid) system. To assess the role of vascular 20-HETE, SS rats were compared with SS-5(BN) consomic rats, carrying CYP4A alleles on chromosome 5 from the normotensive BN (Brown Norway) introgressed on to the SS genetic background. Cerebral arteries from SS-5(BN) rats had less CYP4A protein than arteries from SS rats fed either NS (normal-salt, 0.4% NaCl) or HS (high-salt, 4.0% NaCl) diet. ACh (acetylcholine)-induced dilation of MCAs (middle cerebral arteries) from SS and SS-5(BN) rats was present in SS-5(BN) rats fed on either an NS or HS diet, but absent in SS rats. In SS rats fed on either diet, ACh-induced dilation was restored by acute treatment with the CYP4A inhibitor DDMS (N-methyl-sulfonyl-12,12-dibromododec-11-enamide) or the 20-HETE antagonist 20-HEDE [20-hydroxyeicosa-6(Z),15(Z)-dienoic acid]. The restored response to ACh in DDMS-treated SS rats was inhibited by L-NAME (N(G)nitro-L-arginine methyl ester) and unaffected by indomethacin or MS-PPOH [N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide]. Vascular relaxation responses to the NO donor C(5)FeN(6)Na(2)O were intact in both SS and SS-5(BN) rats and unaffected by the acute addition of DDMS, indicating that the vascular dysfunction of the SS rat is due to a reduced bioavailability of NO instead of failure of the VSMCs (vascular smooth muscle cells) to respond to the vasodilator. Superoxide levels in cerebral arteries of SS-5(BN) rats [evaluated semi-quantitatively by DHE (dihydroethidium) fluorescence] were lower than those in the arteries of SS rats. These findings indicate that SS rats have an up-regulation of the CYP4A/20-HETE pathway resulting in elevated ROS (reactive oxygen species) and reduced NO bioavailability causing vascular dysfunction.

Flutamide increases aldosterone levels in gonadectomized male but not female Wistar rats

BACKGROUND

Sex-specific differences in blood pressure (BP) suggest an important modulating role of testosterone in the kidney. However, little is known about the interaction between androgens and the mineralocorticoid aldosterone. Our objective was to determine the effects of testosterone in gonadectomized male and female rats on a low-salt diet, and to determine the effect of androgen receptor (AR) blockade by flutamide on BP and on aldosterone levels.

METHODS

Normotensive male and female Wistar rats were gonadectomized and put on a low-salt diet. They were treated for 16 days with testosterone or placebo. In addition, the animals received the AR antagonist flutamide or placebo, respectively. BP was measured by tail-cuff method, 24-h urine samples were collected in metabolic cages and blood was collected for hormonal measurements.

RESULTS

Testosterone increased BP in males and females, and this effect could be blocked by flutamide. Flutamide treatment itself significantly increased aldosterone levels in male but not in female rats. These elevated aldosterone levels could be lowered by testosterone treatment during AR blockade. Accordingly to aldosterone levels, flutamide increased in males the serum sodium/potassium to urinary sodium/potassium ratio, an in vivo indicator of renal aldosterone action.

CONCLUSIONS

Testosterone regulates BP in male and female gonadectomized rats via the AR. Flutamide by itself exerts influence over aldosterone in the absence of gonadal steroid replacement suggesting AR involvement in renal sodium handling. These flutamide effects were sex-specific and not seen in female rats.

CYP4A11 T8590C polymorphism, salt-sensitive hypertension, and renal blood flow

OBJECTIVE AND METHODS

A loss-of-function cytosine (C) for thymidine (T) transition at nucleotide 8590 of CYP4A11 has been associated with increased blood pressure in humans. We tested the hypothesis that CYP4A11 T8590C genotype is associated with salt sensitivity in the International Hypertensive Pathotype cohort.

RESULTS

CYP4A11 T8590C genotype was associated with hypertension in whites. Among normotensive individuals, CYP4A11 T8590C genotype was associated with mean arterial pressure (MAP) during both high and low salt diets, such that there was no relationship between genotype and salt sensitivity of blood pressure. Among hypertensive individuals, CYP4A11 T8590C genotype did not associate with MAP during high salt intake, whereas MAP decreased with increasing number of C alleles during salt restriction. Consequently, among hypertensive individuals, change in MAP with salt restriction was greatest in individuals homozygous for the C allele (-10.9 ± 9.9, -11.1 ± 12.3, and -18.8 ± 12.0 mmHg in TT, CT, and CC groups, respectively, P = 0.02). In both normotensive and hypertensive individuals, individuals homozygous for the C allele exhibited an attenuated increase in renal blood flow during high salt. CYP4A11 genotype did not affect pressor responses to Angiotensin II in normotensive or hypertensive individuals.

CONCLUSION

The loss-of-function CYP4A11 8590C allele is associated with a diagnosis of hypertension and, in normotensive individuals, with higher blood pressure regardless of salt intake. Among hypertensive individuals, the C allele is associated with salt-sensitive blood pressure. Impaired renal vasodilation during high salt intake may contribute to salt sensitivity. Studies are needed to determine whether CYP4A11 T8590C genotype predicts responses to medications that affect sodium homeostasis in hypertensive individuals.

Interaction of nitric oxide and the cytochrome P-450 system on blood pressure and renal function in the rat: dependence on sodium intake

AIM

Interaction was examined of nitric oxide (NO) and cytochrome P-450 (CYP-450)-dependent arachidonic acid derivatives, 20-HETE and EETs, in control of arterial pressure (MABP) and renal function. Modification of this interaction by changing sodium intake was also studied.

METHODS

On low, standard or high Na diet (LS, STD and HS rats respectively) effects of sequential blockade of NO synthases (NOS) and CYP-450 enzyme activity on MABP, renal blood flow (RBF, Transonic probe), renal medullary perfusion (MBF, laser-Doppler technique), medullary tissue NO (selective electrode) and renal excretion were examined in anaesthetized rats. All NOS were blocked with N(ϖ) -nitro-l-arginine methyl ester (l-NAME), the neuronal NOS with S-methyl-l-thiocitrulline (SMTC), and CYP-450 with 1-aminobenzotriazole (ABT).

RESULTS

In each diet group the baseline MABP was highest in rats pre-treated with l-NAME. CYP-450 inhibition significantly decreased MABP only in LS (-9%) and HS rats (-22%) pre-treated with l-NAME. This MABP decrease correlated directly with the dietary sodium content (r = 0.644, P < 0.001). CYP-450 inhibition decreased RBF in LS and HS rats (not in HS pre-treated with l-NAME). Acute exclusion of CYP-450 significantly increased MBF only in STD, SMTC pre-treated rats; in HS group it significantly increased medullary tissue NO by about 1.0 nA. The post-ABT changes in renal excretion occurred in LS and HS rats, irrespective of the status of NO synthesis.

CONCLUSIONS

Both NO- and CYP-450-dependent agents contribute to blood pressure and kidney function control, however, the role of 20-HETE and EETs becomes crucial only under conditions of high sodium intake or after NOS inhibition.

Modulation by cytochrome P450-4A ω-hydroxylase enzymes of adrenergic vasoconstriction and response to reduced PO₂ in mesenteric resistance arteries of Dahl salt-sensitive rats

OBJECTIVE

This study evaluated the contribution of the 20-HETE/cytochrome P450-4A ω-hydroxylase (CYP4A) system to the early development of salt-induced vascular changes in Dahl salt-sensitive (SS) rats.

METHODS

CYP4A expression and 20-HETE production were evaluated and responses to norepinephrine, endothelin, and reduced PO₂ were determined by video microscopy in isolated mesenteric resistance arteries from SS rats fed high salt (HS; 4% NaCl) diet for three days vs. low salt (LS; 0.4% NaCl) controls.

RESULTS

CYP4A enzyme inhibition with dibromododecenyl methylsulfimide (DDMS) selectively reduced norepinephrine sensitivity and restored impaired vasodilation in response to reduced PO₂ in SS rats fed HS diet. In the presence of DDMS, vasodilatation to reduced PO₂ was eliminated by indomethacin and unaffected by l-NAME in rats fed LS diet, and eliminated by l-NAME and unaffected by indomethacin in rats fed HS diet. The 20-HETE agonist WIT003 restored norepinephrine sensitivity in DDMS-treated arteries of HS-fed rats. HS diet increased vascular 20-HETE production and CYP4A protein levels by ∼24% and ∼31%, respectively, although these differences were not significant.

CONCLUSIONS

These findings support the hypothesis that the 20-HETE/CYP4A system modulates vessel responses to norepinephrine and vascular relaxation to reduced PO₂ in mesenteric resistance arteries of SS rats fed HS diet.

Vascular responses in aortic rings of a consomic rat panel derived from the Fawn Hooded Hypertensive strain

The present experiments, utilizing the high-throughput vascular protocol of PhysGen (Program for Genomic Applications) characterized the responses of aortic rings to vasoconstrictor (phenylephrine) and vasodilator (acetylcholine, sodium nitroprusside, and reduced tissue bath Po(2)) stimuli in consomic rat strains derived from a cross between the Fawn Hooded Hypertensive rat (FHH/EurMcwi) and the Brown Norway normotensive (BN/NHsdMcwi) rat. The effects of substituting individual BN chromosomes into the FHH genetic background were determined in animals that were maintained on a low-salt (0.4% NaCl) diet or switched to a high-salt (4% NaCl) diet for 3 wk. Sex-specific differences were evaluated in male and female consomic rats on similar dietary salt intake. Multiple chromosomes affected various vascular reactivity phenotypes in the FHH × BN consomic panel, and substantial salt-dependent changes in vascular reactivity and sex-specific differences in aortic reactivity were observed in individual consomic strains. However, compared with earlier studies of consomic rats derived from a cross between the BN rat and the Dahl salt-sensitive (SS) rat, only 3-7% of the vascular phenotypes were affected in a similar manner by substituting specific BN chromosomeschromosomes into the FHH genetic background versus the SS genetic background. The findings of the present study stress the potential value of consomic rat panels in gaining insight into genetic factors influencing vascular reactivity and suggest that the chromosomes that appear to be involved in the determination of aortic ring reactivity in different rodent models of hypertension are highly strain- and sex specific.

Modulation by salt intake of the vascular response mediated through adenosine A(2A) receptor: role of CYP epoxygenase and soluble epoxide hydrolase

High-salt intake can change the effect of adenosine on arterial tone in mice. The aim of this study was to clarify the mechanism by which this occurs. Using aortas from mice fed a 4% NaCl (HS) or 0.45% NaCl (NS) diet for 4-5 wks, concentration-response curves for ACh, 5'-N-ethylcarboxamidoadenosine (NECA; adenosine analog) and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate [CGS-21680; A(2A) adenosine receptor (A(2A) AR) agonist] were obtained with N(omega)-nitro-L-arginine methyl ester (L-NAME; nitric oxide inhibitor, 10(-4) M), methylsulfonyl-propargyloxyphenylhexanamide [MS-PPOH; a CYP (cytochrome P-450) epoxygenase blocker, 10(-5) M including CYP2J2], 12-(3-adamantan-1-yl-ureido)dodecanoic acid [AUDA; soluble epoxide hydrolase (sEH) blocker, 10(-5) M], dibromo-dodecenyl-methylsulfimide [DDMS; CYP omega-hydroxylase (CYP4A blocker), 10(-5) M], glibenclamide (K(ATP) channel blocker; 10(-5) M) and 5-hydroxydecanoate (5-HD; mitochondrial-K(ATP) channel blocker, 10(-4) M). HS dose response to ACh (10(-7) - 10(-5) M) was not different from NS (P > 0.05). Relaxation to 10(-6) M NECA was greater in the HS group (28.4 +/- 3.9%) than in the NS group (4.1 +/- 2.3%). Relaxation to 10(-6) M CGS-21680 was also greater in HS (27.9 +/- 4.5%) than in NS (4.9 +/- 2.2%). L-NAME was able to block the dose response of ACh (10(-7) - 10(-5) M) equally in both HS and NS (P > 0.05), whereas L-NAME did not block CGS-21680-induced response in HS. In HS the CGS-21680 response was greatly reduced by MS-PPOH (to 4.7 +/- 2.0%) and 5-HD (to 8.9 +/- 2.2%), and also abolished by glibenclamide (-1.0 +/- 5.9%). In NS, the CGS-21680 response was increased by AUDA (to 26.3 +/- 3.4%) and DDMS (to 27.2 +/- 3.0%). Compared with NS, HS vessels showed increased CYP2J2 and A(2A) AR expression (46 and 74% higher, respectively) but decreased sEH, CYP4A, and A(1) AR expression (75, 30, and 55% lower, respectively). These data suggest that in mice fed NS-containing diet, upregulation of arterial A(1) receptor causes vasoconstriction via increased sEH and CYP4A proteins. However, in mice fed HS-containing diet, upregulation of A(2A) receptor protein triggers vascular relaxation through ATP-sensitive (K(+)) channels via upregulation of CYP2J2 enzyme.

Modulation of vascular O2 responses by cytochrome 450-4A omega-hydroxylase metabolites in Dahl salt-sensitive rats

OBJECTIVE

This study evaluated the role of the 20-HETE/cytochrome P450-4A omega-hydroxylase (CYP450-4A) system in microvascular regulation in the skeletal muscle circulation following short-term (three-day) exposure to a high-salt (HS) diet in Dahl salt-sensitive (SS) rats.

METHODS

The effects of inhibiting CYP450-4A on resting diameter, O(2)-induced constriction, and vasodilator responses to acetylcholine (ACh) and the nitric oxide (NO) donor, sodium nitroprusside (SNP), were evaluated in cremasteric arterioles of SS rats fed a low- (LS; 0.4% NaCl) or high-salt (HS; 4% NaCl) diet for three days.

RESULTS

The HS diet upregulated CYP450-4A mRNA expression and led to an enhanced constriction of arterioles in response to elevated PO(2) in SS rats, which could be blocked by inhibiting CYP450-4A enzymes with dibromododecenyl methylsulfimide (DDMS). DDMS also inhibited resting tone significantly in SS rats fed the HS, but not the LS, diet, despite similar resting diameters and active tone in the two groups. Arteriolar dilations in response to ACh and SNP were similar in SS rats fed the LS vs. the HS diet and were unaffected by DDMS.

CONCLUSIONS

These findings suggest that CYP450-4A enzymes contribute to resting tone and to an enhanced response to elevated PO(2) in arterioles of Dahl-SS rats fed the HS diet.

Sex-specific differences in chromosome-dependent regulation of vascular reactivity in female consomic rat strains from a SSxBN cross

High-throughput studies in the Medical College of Wisconsin Program for Genomic Applications (Physgen) were designed to link chromosomes with physiological function in consomic strains derived from a cross between Dahl salt-sensitive SS/JrHsdMcwi (SS) and Brown Norway normotensive BN/NHsdMcwi (BN) rats. The specific goal of the vascular protocol was to characterize the responses of aortic rings from these strains to vasoconstrictor and vasodilator stimuli (phenylephrine, acetylcholine, sodium nitroprusside, and bath hypoxia) to identify chromosomes that either increase or decrease vascular reactivity to these vasoactive stimuli. Because previous studies demonstrated sex-specific quantitative trait loci (QTLs) related to regulation of cardiovascular phenotypes in an F2 cross between the parental strains, males and females of each consomic strain were included in all experiments. As there were significant sex-specific differences in aortic sensitivity to vasoconstrictor and vasodilator stimuli compared with the parental SS strain, we report the results of the females separately from the males. There were also sex-specific differences in aortic ring sensitivity to these vasoactive stimuli in consomic strains that were fed a high-salt diet (4% NaCl) for 3 wk to evaluate salt-induced changes in vascular reactivity. Differences in genetic architecture could contribute to sex-specific differences in the development and expression of cardiovascular diseases via differential regulation and expression of genes. Our findings are the first to link physiological traits with specific chromosomes in female SS rats and support the idea that sex is an important environmental variable that plays a role in the expression and regulation of genes.

Dietary salt enhances benzamil-sensitive component of myogenic constriction in mesenteric arteries

Recent work from our laboratory indicates that epithelial Na(+) channel (ENaC) function plays an important role in modulating myogenic vascular reactivity. Increases in dietary sodium are known to affect vascular reactivity. Although previous studies have demonstrated that dietary salt intake regulates ENaC expression and activity in epithelial tissue, the importance of dietary salt on ENaC expression in vascular smooth muscle cells (VSMCs) and its role in myogenic constriction is unknown. Therefore, the goal of the present study was to determine whether dietary salt modulates ENaC expression and function in myogenic vasoconstriction. To accomplish this goal, we examined ENaC expression in freshly dispersed VSMCs and pressure-induced vasoconstrictor responses in isolated mesenteric resistance arteries from normotensive Sprague-Dawley rats fed a normal-salt (NS; 0.4% NaCl) or high-salt (HS; 8% NaCl for 2 wk) diet. VSMCs from the mesenteric arteries of NS-fed animals express alpha-, beta-, and gamma-ENaC. The HS diet reduced whole cell alpha- and gamma-ENaC and induced a pronounced translocation of beta-ENaC from intracellular regions toward the VSMC membrane (approximately 336 nm). Associated with this change in expression was a change in the importance of ENaC in pressure-induced constriction. Pressure-induced constriction in NS-fed animals was insensitive to ENaC inhibition with 1 microM benzamil, suggesting that ENaC proteins do not contribute to myogenic constriction in mesenteric arteries under NS intake. In contrast, ENaC inhibition blocked pressure-induced constriction in HS-fed animals. These data suggest that dietary sodium regulates ENaC expression and the quantitative importance of the vascular ENaC signaling pathway contributing to myogenic constriction.

Effect of High-Salt Diet on Vascular Relaxation and Oxidative Stress in Mesenteric Resistance Arteries

This study tested the hypothesis that superoxide levels are elevated in isolated mesenteric resistance arteries (100-300 microm) from rats fed a short-term high-salt (HS) diet (4% NaCl for 3 days) compared to controls fed a low-salt (LS) diet (0.4% NaCl). Vascular relaxation induced by the superoxide dismutase mimetic tempol (4-hydroxytetramethylpiperidine-1-oxyl), the NADPH oxidase inhibitor apocynin and the xanthine/xanthine oxidase inhibitor oxypurinol was significantly larger in mesenteric arteries from animals fed HS diet compared to arteries from animals fed LS diet. Basal superoxide levels assessed via dihydroethidine (DHE) fluorescence were significantly elevated in arteries from rats fed HS diet, and were reduced by tempol, apocynin and oxypurinol, but not by L-NAME. Basal and methacholine-induced NO production (assessed by DAF-2T fluorescence) was significantly reduced in arteries from rats fed HS diet versus arteries from rats on LS diet. Impaired methacholine-induced NO release and vascular relaxation were restored by tempol and apocynin, but not by oxypurinol. These data suggest that the elevated production of superoxide by NADPH oxidase and xanthine/xanthine oxidase contribute to elevated basal superoxide levels, reduced NO release and impaired vascular relaxation in mesenteric resistance arteries of rats fed HS diet.

High dietary salt reduces the contribution of 20-HETE to arteriolar oxygen responsiveness in skeletal muscle

The coupling of tissue blood flow to cellular metabolic demand involves oxygen-dependent adjustments in arteriolar tone, and arteriolar responses to oxygen can be mediated, in part, by changes in local production of 20-HETE. In this study, we examined the long-term effect of dietary salt on arteriolar oxygen responsiveness in the exteriorized, superfused rat spinotrapezius muscle and the role of 20-HETE in this responsiveness. Rats were fed either a normal-salt (NS, 0.45%) or high-salt (HS, 4%) diet for 4–5 wk. There was no difference in steady-state tissue Po2 between NS and HS rats, and elevation of superfusate oxygen content from 0% to 10% caused tissue Po2 to increase by the same amount in both groups. However, the resulting reductions in arteriolar diameter and blood flow were less in HS rats than NS rats. Inhibition of 20-HETE formation with N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or 17-octadecynoic acid (17-ODYA) attenuated oxygen-induced constriction in NS rats but not HS rats. Exogenous 20-HETE elicited arteriolar constriction that was greatly reduced by the large-conductance Ca2+-activated potassium (KCa) channel inhibitors tetraethylammonium chloride (TEA) and iberiotoxin (IbTx) in NS rats and a smaller constriction that was less sensitive to TEA or IbTx in HS rats. Arteriolar responses to exogenous angiotensin II were similar in both groups but more sensitive to inhibition with DDMS in NS rats. Norepinephrine-induced arteriolar constriction was similar and insensitive to DDMS in both groups. We conclude that 20-HETE contributes to oxygen-induced constriction of skeletal muscle arterioles via inhibition of KCa channels and that a high-salt diet impairs arteriolar responses to increased oxygen availability due to a reduction in vascular smooth muscle responsiveness to 20-HETE.

Pathogenesis of salt sensitivity of blood pressure

The effect of sodium in human blood pressure has been a contentious subject of considerable debate for decades. Nonetheless, it is generally conceded that there is heterogeneity to alterations in sodium and extracellular fluid volume in the blood pressure responses of normal and hypertensive humans. Although there are many forms of experimental and clinical hypertension that are clearly related to abnormalities of sodium handling and metabolism, it has been only the advent of the genetic revolution that has provided critical new insight into the mechanisms involved in many of these conditions. In this review, the clinical manifestations of salt sensitivity and several clinical syndromes associated with abnormal sodium metabolism are discussed, and factors that appear to be involved in many of the clinical abnormalities as well as relevant new insights derived from basic research are elucidated.