Impaired relaxation of cerebral arteries in the absence of elevated salt intake in normotensive congenic rats carrying the Dahl salt-sensitive renin gene

The Role of Nitric Oxide in the Micro- and Macrovascular Response to a 7-Day High-Salt Diet in Healthy Individuals

This study aimed to investigate the specific role of nitric oxide (NO) in micro- and macrovascular response to a 7-day high-salt (HS) diet, specifically by measuring skin microvascular local thermal hyperemia and the flow-mediated dilation of the brachial artery, as well as serum NO and three NO synthase enzyme (NOS) isoform concentrations in healthy individuals. It also aimed to examine the concept of non-osmotic sodium storage in the skin following the HS diet by measuring body fluid status and systemic hemodynamic responses, as well as serum vascular endothelial growth factor C (VEGF-C) concentration. Forty-six young, healthy individuals completed a 7-day low-salt diet, followed by a 7-day HS diet protocol. The 7-day HS diet resulted in impaired NO-mediated endothelial vasodilation in peripheral microcirculation and conduit arteries, in increased eNOS, decreased nNOS, and unchanged iNOS concentration and NO serum level. The HS diet did not change the volume of interstitial fluid, the systemic vascular resistance or the VEGF-C serum level. These results indicate that the 7-day HS-diet induces systemic impairment of NO-mediated endothelial vasodilation, while dissociation in the eNOS and nNOS response indicates complex adaptation of main NO-generating enzyme isoforms to HS intake in healthy individuals. Our results failed to support the concept of non-osmotic sodium storage.

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.

Seven-Day Salt Loading Impairs Microvascular Endothelium-Dependent Vasodilation without Changes in Blood Pressure, Body Composition and Fluid Status in Healthy Young Humans

OBJECTIVES

We aimed to assess whether a 7-day high-salt (HS) diet affects endothelium-dependent and/or endothelium-independent microvascular function in the absence of changes in arterial blood pressure (BP), and to determine whether such microvascular changes are associated with changes in body composition and fluid status in healthy young humans.

MATERIALS AND METHODS

Fifty-three young healthy individuals (28 women and 25 men) were assigned to a 7-day low-salt diet (<3.5 g salt/day) followed by a 7-day HS diet (∼14 g salt/day). Skin microvascular blood flow in response to iontophoresis of acetylcholine (ACh) and sodium nitroprusside (SNP) was assessed by laser Doppler flowmetry, and BP, heart rate (HR), plasma renin activity (PRA), serum aldosterone, serum and 24 h-urine sodium, potassium, urea and creatinine levels, together with body composition and fluid status measurement with a 4-terminal portable impedance analyzer were measured before and after diet protocols.

RESULTS

BP, HR, body composition and fluid status were unchanged, and PRA and serum aldosterone level were significantly suppressed after HS diet. ACh-induced dilation (AChID) was significantly impaired, while SNP-induced dilation was not affected by HS diet. Impaired AChID and increased salt intake, as well as impaired AChID and suppressed renin-angiotensin system were significantly positively correlated. Changes in body composition and fluid status parameters were not associated with impaired AChID.

CONCLUSION

7-day HS diet impairs microvascular reactivity by affecting its endothelium-dependent vasodilation in young healthy individuals. Changes are independent of BP, body composition changes or fluid retention, but are the consequences of the unique effect of HS on endothelial function.

High salt diet impairs cerebral blood flow regulation via salt-induced angiotensin II suppression

OBJECTIVES

This study sought to determine whether salt-induced ANG II suppression contributes to impaired CBF autoregulation.

METHODS

Cerebral autoregulation was evaluated with LDF during graded reductions of blood pressure. Autoregulatory responses in rats fed HS (4% NaCl) diet vs LS (0.4% NaCl) diet were analyzed using linear regression analysis, model-free analysis, and a mechanistic theoretical model of blood flow through cerebral arterioles.

RESULTS

Autoregulation was intact in LS-fed animals as MAP was reduced via graded hemorrhage to approximately 50 mm Hg. Short-term (3 days) and chronic (4 weeks) HS diet impaired CBF autoregulation, as evidenced by progressive reductions of laser Doppler flux with arterial pressure reduction. Chronic low dose ANG II infusion (5 mg/kg/min, i.v.) restored CBF autoregulation between the pre-hemorrhage MAP and 50 mm Hg in rats fed short-term HS diet. Mechanistic-based model analysis showed a reduced myogenic response and reduced baseline VSM tone with short-term HS diet, which was restored by ANG II infusion.

CONCLUSIONS

Short-term and chronic HS diet lead to impaired autoregulation in the cerebral circulation, with salt-induced ANG II suppression as a major factor in the initiation of impaired CBF regulation.

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.

Decoding resistant hypertension signalling pathways

Resistant hypertension (RH) is a clinical condition in which the hypertensive patient has become resistant to drug therapy and is often associated with increased cardiovascular morbidity and mortality. Several signalling pathways have been studied and related to the development and progression of RH: modulation of sympathetic activity by leptin and aldosterone, primary aldosteronism, arterial stiffness, endothelial dysfunction and variations in the renin-angiotensin-aldosterone system (RAAS). miRNAs comprise a family of small non-coding RNAs that participate in the regulation of gene expression at post-transcriptional level. miRNAs are involved in the development of both cardiovascular damage and hypertension. Little is known of the molecular mechanisms that lead to development and progression of this condition. This review aims to cover the potential roles of miRNAs in the mechanisms associated with the development and consequences of RH, and explore the current state of the art of diagnostic and therapeutic tools based on miRNA approaches.

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.

Cerebral Cortical Microvascular Rarefaction in Metabolic Syndrome is Dependent on Insulin Resistance and Loss of Nitric Oxide Bioavailability

OBJECTIVE

Chronic presentation of the MS is associated with an increased likelihood for stroke and poor stroke outcomes following occlusive cerebrovascular events. However, the physiological mechanisms contributing to compromised outcomes remain unclear, and the degree of cerebral cortical MVD may represent a central determinant of stroke outcomes.

METHODS

This study used the OZR model of MS and clinically relevant, chronic interventions to determine the impact on cerebral cortical microvascular rarefaction via immunohistochemistry with a parallel determination of cerebrovascular function to identify putative mechanistic contributors.

RESULTS

OZR exhibited a progressive rarefaction (to ~80% control MVD) of the cortical microvascular networks vs. lean Zucker rats. Chronic treatment with antihypertensive agents (captopril/hydralazine) had limited effectiveness in blunting rarefaction, although treatments improving glycemic control (metformin/rosiglitazone) were superior, maintaining ~94% control MVD. Chronic treatment with the antioxidant TEMPOL severely blunted rarefaction in OZR, although this ameliorative effect was prevented by concurrent NOS inhibition.

CONCLUSIONS

Further analyses revealed that the maintenance of glycemic control and vascular NO bioavailability were stronger predictors of cerebral cortical MVD in OZR than was prevention of hypertension, and this may have implications for chronic treatment of CVD risk under stroke-prone conditions.

Enhancement of resting-state fcMRI networks by prior sensory stimulation

It is important to consider the effect of a previous experimental condition when analyzing resting-state functional connectivity magnetic resonance imaging (fcMRI) data. In this work, a simple sensory stimulation functional MRI (fMRI) experiment was conducted between two resting-state fcMRI acquisitions in anesthetized rats using a high-field small-animal MR scanner. Previous human studies have reported fcMRI network alteration by prior task/stimulus utilizing similar experimental paradigms. An anesthetized rat preparation was used to test whether brain regions with higher level functions are involved in post-task/stimulus fcMRI network alteration. We demonstrate significant fcMRI enhancement poststimulation in the sensory cortical, limbic, and insular brain regions in rats. These brain regions have been previously implicated in vigilance and anesthetic arousal networks. We tested their experimental paradigm in several inbred strains of rats with known phenotypic differences in anesthetic susceptibility and cerebral vascular function. Brown Norway (BN), Dahl Salt-Sensitive (SS), and consomic SSBN13 strains were tested. We have previously shown significant differences in blood oxygen level-dependent fMRI activity and fcMRI networks across these strains. Here we report statistically significant interstrain differences in regional fcMRI poststimulation enhancement. In the SS strain, poststimulation enhancement occurred in posterior sensory and limbic cortical brain regions. In the BN strain, poststimulation enhancement appeared in anterior cingulate and subcortical limbic brain regions. These results imply that a prior condition has a significant impact on fcMRI networks that depend on intersubject difference in genetics and physiology.

Vascular dysfunction precedes hypertension associated with a blood pressure locus on rat chromosome 12

We previously isolated a 6.1-Mb region of SS/Mcwi (Dahl salt-sensitive) rat chromosome 12 (13.4-19.5 Mb) that significantly elevated blood pressure (BP) (Δ+34 mmHg, P < 0.001) compared with the SS-12(BN) consomic control. In the present study, we examined the role of vascular dysfunction and remodeling in hypertension risk associated with the 6.1-Mb (13.4-19.5 Mb) locus on rat chromosome 12 by reducing dietary salt, which lowered BP levels so that there were no substantial differences in BP between strains. Consequently, any observed differences in the vasculature were considered BP-independent. We also reduced the candidate region from 6.1 Mb with 133 genes to 2 Mb with 23 genes by congenic mapping. Both the 2 Mb and 6.1 Mb congenic intervals were associated with hypercontractility and decreased elasticity of resistance vasculature prior to elevations of BP, suggesting that the vascular remodeling and dysfunction likely contribute to the pathogenesis of hypertension in these congenic models. Of the 23 genes within the narrowed congenic interval, 12 were differentially expressed between the resistance vasculature of the 2 Mb congenic and SS-12(BN) consomic strains. Among these, Grifin was consistently upregulated 2.7 ± 0.6-fold (P < 0.05) and 2.0 ± 0.3-fold (P < 0.01), and Chst12 was consistently downregulated -2.8 ± 0.3-fold (P < 0.01) and -4.4 ± 0.4-fold (P < 0.00001) in the 2 Mb congenic compared with SS-12(BN) consomic under normotensive and hypertensive conditions, respectively. A syntenic region on human chromosome 7 has also been associated with BP regulation, suggesting that identification of the genetic mechanism(s) underlying cardiovascular phenotypes in this congenic strain will likely be translated to a better understanding of human hypertension.

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.

AT1 receptors prevent salt-induced vascular dysfunction in isolated middle cerebral arteries of 2 kidney-1 clip hypertensive rats

BACKGROUND

Elevated blood pressure, elevated angiotensin II (ANG II), and ANG II suppression with high salt (HS) diet all contribute to vascular dysfunction. This study investigated the interplay of HS diet and vascular function in a high renin model of hypertension.

METHODS

Male Sprague-Dawley rats were subjected to 2 kidney-1 clip (2K1C) Goldblatt hypertension for 4 weeks and compared with sham-operated controls.

RESULTS

Middle cerebral arteries (MCA) of 2K1C rats and sham-operated controls fed normal salt (NS; 0.4% NaCl) diet dilated in response to acetylcholine (ACh) and reduced partial pressure of oxygen (PO2). Switching to HS (4% NaCl) diet for 3 days to reduce plasma renin activity (PRA) eliminated vasodilation to ACh and reduced PO2 in sham-operated controls, with no effect on vasodilation in 2K1C rats. AT1 receptor blockade (losartan, 20 mg/kg/day; 1 week) eliminated vasodilator responses to ACh and reduced PO2 in 2K1C rats fed NS or HS diet. ANG II infusion (5 ng/kg/min, intravenous) for 3 days to prevent salt-induced reductions in plasma ANG II restored vascular relaxation in MCA of sham-operated controls fed HS diet. Copper/zinc superoxide dismutase expression and total superoxide dismutase activity were significantly higher in arteries of 2K1C rats fed HS diet vs. sham-operated controls.

CONCLUSIONS

These results suggest that the sustained effects of elevated ANG II levels in 2K1C hypertension maintain endothelium-dependent vasodilatation via AT1 receptor-mediated preservation of antioxidant defense mechanisms despite significant elevations in blood pressure and salt-induced suppression of PRA.

Low-dose angiotensin II infusion restores vascular function in cerebral arteries of high salt-fed rats by increasing copper/zinc superoxide dimutase expression

BACKGROUND

This study examined the vasoprotective role of circulating angiotensin II (ANG II) levels in the cerebral circulation of high salt (HS)-fed (SS.BN-(D13hmgc41-13hmgc23)/Mcwi) (Ren1-BN) congenic rats, which carry a normally functioning renin allele from the Brown Norway (BN) rat on the Dahl salt-sensitive genetic background.

METHODS

Ren1-BN rats were placed on an HS (4.0% NaCl) diet for 3 days. The vasodilator response to acetylcholine (ACh; 10(-10) - 10(-6) mol/L) was assessed in isolated middle cerebral arteries (MCAs), and Western blots were performed to assess the expression of the antioxidant enzymes copper (Cu)/zinc (Zn) superoxide dismutase (SOD) and manganese (Mn) SOD in cerebral resistance vessels. A separate group of HS-fed animals were infused with either a subpressor dose of ANG II (100ng/kg/min) or saline vehicle via osmotic minipump for 3 days.

RESULTS

HS diet eliminated acetylcholine (ACh)-induced dilation in the MCAs of the congenic rats. Western blot analysis of antioxidant enzymes showed that Cu/Zn SOD and Mn SOD expression were significantly reduced in the cerebral resistance arteries of the HS-fed rats compared with control animals fed a normal salt diet. Infusion of ANG II restored the vasodilator response to ACh in the MCAs and increased Cu/Zn SOD (but not Mn SOD) expression compared with saline-infused animals.

CONCLUSIONS

These results indicate that prevention of salt-induced ANG II suppression prevents vascular dysfunction in the cerebral circulation by preventing the downregulation of Cu/Zn SOD and vascular oxidant stress that normally occurs with HS diet.

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.

Congenic mapping and sequence analysis of the Renin locus

Renin was the first blood pressure (BP) quantitative trait locus mapped by linkage analysis in the rat. Subsequent BP linkage and congenic studies capturing different portions of the renin region have returned conflicting results, suggesting that multiple interdependent BP loci may be residing in the chromosome 13 BP quantitative trait locus that includes Renin. We used SS-13(BN) congenic strains to map 2 BP loci in the Renin region (chr13: 45.2-49.0 Mb). We identified a 1.1-Mb protective Brown Norway region around Renin (chr13: 46.1-47.2 Mb) that significantly decreased BP by 32 mm Hg. The Renin protective BP locus was offset by an adjacent hypertensive locus (chr13: 47.2-49.0 Mb) that significantly increased BP by 29 mm Hg. Sequence analysis of the protective and hypertensive BP loci revealed 1433 and 2063 variants between Dahl salt-sensitive/Mcwi and Brown Norway rats, respectively. To further reduce the list of candidate variants, we regenotyped an overlapping SS-13(SR) congenic strain (S/renrr) with a previously reported BP phenotype. Sequence comparison among Dahl salt-sensitive, Dahl R, and Brown Norway reduced the number of candidate variants in the 2 BP loci by 42% for further study. Combined with previous studies, these data suggest that at least 4 BP loci reside within the 30-cM chromosome 13 BP quantitative trait locus that includes Renin.

Short-term hypercaloric diet induces blunted aortic vasoconstriction and enhanced vasorelaxation via increased nitric oxide synthase 3 activity and expression in Dahl salt-sensitive rats

AIM

To elucidate the role of the O(2)(-), H(2)O(2) or NO pathways in aortic angiotensin (Ang)II-induced vasoconstriction in Dahl salt-sensitive (SS) rats compared with control SS-13(BN) rats on a normal or hypercaloric diet.

METHODS

Aortic function was assessed using wire myography in 16-week-old rats maintained on a normal diet or a 4-week hypercaloric diet. Nitric oxide synthase (NOS) activity and expression was determined by the conversion of radio-labelled arginine to citrulline and Western blot analysis respectively.

RESULTS

On normal diet, AngII-induced vasoconstriction was greater in SS than SS-13(BN) rats. Polyethylene glycol superoxide dismutase (PEG-SOD) reduced the aortic AngII response similarly in both strains on normal diet. Catalase blunted, whereas N(ω)-Nitro-L-arginine methyl ester (L-NAME) did not affect the AngII response in SS rats. In SS-13(BN) rats, catalase had no effect and L-NAME enhanced AngII response. On hypercaloric diet, aortic AngII responsiveness was reduced in SS but unaltered in SS-13(BN) rats compared with their normal diet counterparts. PEG-SOD reduced the AngII response in both rats on hypercaloric diet. Catalase treatment did not alter aortic AngII response, while L-NAME increased the response in SS rats on hypercaloric diet. In SS-13(BN) rats on hypercaloric diet, catalase reduced and L-NAME did not alter the AngII response. Furthermore, aortic endothelial-dependent vasorelaxation was increased in SS rats on hypercaloric diet compared with normal diet and aortic NOS3 activity and expression was increased.

CONCLUSION

A short-term hypercaloric diet induces a blunted vasoconstrictive and enhanced vasodilatory phenotype in SS rats, but not in SS-13(BN) rats, via reduced H(2)O(2) and increased NOS3 function.

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.

Dahl salt-sensitive rats are protected against vascular defects related to diet-induced obesity

Obesity increases plasma renin activity and angiotensin II levels, leading to vascular damage, elevated blood pressure, diabetes mellitus, and renal damage. Because genetic deletion of crucial parts of the renin-angiotensin system protect against obesity-related cardiovascular defects, we hypothesized that Dahl salt-sensitive (SS) rats, a model of chronically low plasma renin activity and angiotensin II levels, would be protected against vascular defects during diet-induced obesity compared with SS.13(BN) consomic rats showing normal renin-angiotensin system regulation. We evaluated vascular function in middle cerebral arteries of SS or SS.13(BN) rats fed high-fat (45% kcal from fat) versus normal-fat diet for 15 to 20 weeks from weaning. Endothelium-dependent relaxation in response to acetylcholine (10(-8) to 10(-4) mol/L) was restored in middle cerebral arteries of high-fat SS rats versus normal-fat diet controls, whereas vasodilation to acetylcholine was dramatically reduced in high-fat SS 13(BN) rats versus normal-fat diet controls. These findings support the hypothesis that physiological levels of angiotensin II play an important role in maintaining normal vascular relaxation in cerebral arteries and suggest that the cerebral vasculature of the SS rat model is genetically protected against endothelial dysfunction in diet-induced obesity.

Introgression of the Brown Norway renin allele onto the Dahl salt-sensitive genetic background increases Cu/Zn SOD expression in cerebral arteries

BACKGROUND

Nitric oxide (NO)-dependent vasodilation is impaired in middle cerebral arteries (MCAs) from Dahl salt-sensitive (SS) rats that are fed normal salt (NS) diet, due to low plasma renin activity and chronic exposure to low plasma angiotensin II (ANG II) levels. NO-dependent vasodilator responses are rescued in MCAs from Ren1-BN congenic rats, which have a 2.0 Mbp portion of Brown Norway (BN) chromosome 13 containing the renin gene introgressed onto the Dahl SS genetic background.

METHODS

Vascular superoxide levels were measured with dihydroethidium (DHE) fluorescence in basilar arteries from 10- to 14-week-old, male Dahl SS and Ren1-BN congenic rats that fed NS diet. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and xanthine oxidase (XO) activity were also measured in cerebral artery tissue homogenates. Expression of the superoxide dismutase (SOD) enzymes was evaluated via western blotting in cerebral arteries from the two rat strains.

RESULTS

Superoxide levels were significantly higher in basilar arteries from Dahl SS rats compared to Ren1-BN congenic rats. NADPH oxidase and XO activity were similar between the two rat strains. Cu/Zn SOD expression was significantly higher in cerebral arteries from Ren1-BN congenic rats vs. those from Dahl SS rats. The expression of Mn-SOD was similar in cerebral arteries from both strains.

CONCLUSIONS

These findings suggest that introgressing the BN renin allele onto the Dahl SS genetic background to restore normal activity of the renin-angiotensin system (RAS) protects NO-dependent vascular relaxation in cerebral arteries by increasing the expression of Cu/Zn SOD and lowering vascular superoxide levels.