Introgression of chromosome 13 in Dahl salt-sensitive genetic background restores cerebral vascular relaxation

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.

Triiodothyronine Reduces Vascular Dysfunction Associated with Hypertension by Attenuating Protein Kinase G/Vasodilator-Stimulated Phosphoprotein Signaling

Vascular dysfunction associated with hypertension comprises hypercontractility and impaired vasodilation. We have previously demonstrated that triiodothyronine (T3), the active form of thyroid hormone, has vasodilatory effects acting through rapid onset mechanisms. In the present study, we examined whether T3 mitigates vascular dysfunction associated with hypertension. To test the direct effects of T3 in hypertensive vessels, aortas from female Dahl salt-sensitive (Dahl SS) rats fed a high-salt diet (8% NaCl, HS group) and their age-matched controls fed a standard low-salt diet (0.3% NaCl, LS group) for 16 weeks were isolated and used in ex vivo vascular reactivity studies. We confirmed that the HS group exhibited a higher systolic blood pressure in comparison with the control LS group and displayed aortic remodeling. Aortas from both groups were pretreated with T3 (0.1 μM) for 30 minutes at 37°C in a 5% CO₂ incubator before functional vascular studies. T3 treatment significantly attenuated hypercontractility and improved impaired endothelium-dependent vasodilation in aortas from the HS group. These vascular improvements in response to T3 were accompanied by increased phosphorylation of vasodilator-stimulated phosphoprotein (VASP) at serine 239, a vasodilatory factor of the cGMP-dependent protein kinase (PKG)/VASP signaling pathway in vascular smooth muscle cells. Moreover, increased production of reactive oxygen species in aortas from the HS group were significantly reduced by T3, suggesting a potential antioxidant effect of T3 in the vasculature. These results demonstrate that T3 can mitigate hypertension-related vascular dysfunction through the VASP signaling pathway and by reducing vascular ROS production. SIGNIFICANCE STATEMENT: This study demonstrates that triiodothyronine (T3) directly acts on vascular tone and has a beneficial effect in hypertension-induced vascular dysfunction. T3 augmented vasodilation and diminished vasoconstriction in blood vessels from hypertensive rats in association with activation of the protein kinase G/vasodilator-stimulated phosphoprotein signaling pathway that activates vascular relaxation and exerted an antioxidant effect. Collectively, these results show that T3 is a potential vasoprotective agent with rapid action on hypertension-related vascular dysfunction.

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.

Haploinsufficiency of the Transcription Factor Ets-1 Is Renoprotective in Dahl Salt-Sensitive Rats

Studies using Dahl salt-sensitive (SS) rats identified specific quantitative trait loci that predispose animals to hypertension-associated albuminuria and kidney injury. We explored the hypothesis that kidney-specific expression of the transcription factor Ets-1, located within one of these loci on chromosome 8, mediates glomerular injury in SS hypertension. During the first week on a high-salt diet, SS rats and SS rats with only one functioning Ets-1 gene (ES rats) demonstrated similar increases in BP. However, serum creatinine concentration, albuminuria, and glomerular expression of ETS-1 and two ETS-1 targets, MCP-1 and MMP2, did not increase as substantially in ES rats as in SS rats. Mean BP subsequently increased further in SS rats and remained higher than that of ES rats for the rest of the study. After 4 weeks of high-salt intake, ES rats still showed a lower mean serum creatinine concentration and less albuminuria, as well as less histologic evidence of glomerular injury and kidney fibrosis, than SS rats did. To investigate the specific contribution of renal Ets-1, we transplanted kidneys from ES or SS rats into salt-resistant SS-Chr 13^BN/McwiCrl (SS-13BN) rats. Within 10 days on a high-salt diet, BP increased similarly in ES and SS allograft recipients, becoming significantly higher than the BP of control isograft recipients. However, mean serum creatinine concentration and albuminuria remained lower in ES allograft recipients than in SS allograft recipients at 2 weeks, and ES allografts showed less glomerular injury and interstitial fibrosis. In conclusion, reduced renal expression of ETS-1 prevented hypertension-associated kidney injury in SS rats.

Myocardial energetics is not compromised during compensated hypertrophy in the Dahl salt-sensitive rat model of hypertension

Salt-induced hypertension leads to development of left ventricular hypertrophy in the Dahl salt-sensitive (Dahl/SS) rat. Before progression to left ventricular failure, the heart initially undergoes a compensated hypertrophic response. We hypothesized that changes in myocardial energetics may be an early indicator of transition to failure. Dahl/SS rats and their salt-resistant consomic controls (SS-13(BN)) were placed on either a low- or high-salt diet to generate four cohorts: Dahl-SS rats on a low- (Dahl-LS) or high-salt diet (Dahl-HS), and SS-13(BN) rats on a low- (SSBN-LS) or high-salt diet (SSBN-HS). We isolated left ventricular trabeculae and characterized their mechanoenergetic performance. Our results show, at most, modest effects of salt-induced compensated hypertrophy on myocardial energetics. We found that the Dahl-HS cohort had a higher work-loop heat of activation (estimated from the intercept of the heat vs. relative afterload relationship generated from work-loop contractions) relative to the SSBN-HS cohort and a higher economy of contraction (inverse of the slope of the heat vs. active stress relation) relative to the Dahl-LS cohort. The maximum extent of shortening and maximum shortening velocity of the Dahl/SS groups were higher than those of the SS-13(BN) groups. Despite these differences, no significant effect of salt-induced hypertension was observed for either peak work output or peak mechanical efficiency during compensated hypertrophy.

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.

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.

Candidate genes for hypertension: insights from the Dahl S rat

Human genetic linkage and association studies have nominated many genes as possible contributors to disease. Mutating or deleting these genes in a relevant disease model can validate their association with disease and potentially uncover novel mechanisms of pathogenesis. Targeted genetic mutagenesis has only recently been developed in the rat, and this technique has been applied in the Dahl salt-sensitive (S) rat to investigate human candidate genes associated with hypertension. This mini-review communicates the findings of these studies and displays how targeted genetic mutagenesis may contribute to the discovery of novel therapies for patients.

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.

Reduced angiotensin II levels cause generalized vascular dysfunction via oxidant stress in hamster cheek pouch arterioles

OBJECTIVES

We investigated the effect of suppressing plasma angiotensin II (ANG II) levels on arteriolar relaxation in the hamster cheek pouch.

METHODS

Arteriolar diameters were measured via television microscopy during short-term (3-6days) high salt (HS; 4% NaCl) diet and angiotensin converting enzyme (ACE) inhibition with captopril (100mg/kg/day).

RESULTS

ACE inhibition and/or HS diet eliminated endothelium-dependent arteriolar dilation to acetylcholine, endothelium-independent dilation to the NO donor sodium nitroprusside, the prostacyclin analogs carbacyclin and iloprost, and the KATP channel opener cromakalim; and eliminated arteriolar constriction during KATP channel blockade with glibenclamide. Scavenging of superoxide radicals and low dose ANG II infusion (25ng/kg/min, subcutaneous) reduced oxidant stress and restored arteriolar dilation in arterioles of HS-fed hamsters. Vasoconstriction to topically-applied ANG II was unaffected by HS diet while arteriolar responses to elevation of superfusion solution PO2 were unaffected (5% O2, 10% O2) or reduced (21% O2) by HS diet.

CONCLUSIONS

These findings indicate that sustained exposure to low levels of circulating ANG II leads to widespread dysfunction in endothelium-dependent and independent vascular relaxation mechanisms in cheek pouch arterioles by increasing vascular oxidant stress, but does not potentiate O2- or ANG II-induced constriction of arterioles in the distal microcirculation of normotensive hamsters.

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

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.

Deficiency of renal cortical EGF increases ENaC activity and contributes to salt-sensitive hypertension

Various stimuli, including hormones and growth factors, modulate epithelial sodium channels (ENaCs), which fine-tune Na(+) absorption in the kidney. Members of the EGF family are important for maintaining transepithelial Na(+) transport, but whether EGF influences ENaC, perhaps mediating salt-sensitive hypertension, is not well understood. Here, the ENaC inhibitor benzamil attenuated the development of hypertension in Dahl salt-sensitive rats. Feeding these salt-sensitive rats a high-salt diet led to lower levels of EGF in the kidney cortex and enhanced the expression and activity of ENaC compared with feeding a low-salt diet. To directly evaluate the role of EGF in the development of hypertension and its effect on ENaC activity, we infused EGF intravenously while continuously monitoring BP of the salt-sensitive rats. Infusion of EGF decreased ENaC activity, prevented the development of hypertension, and attenuated glomerular and renal tubular damage. Taken together, these findings indicate that cortical EGF levels decrease with a high-salt diet in salt-sensitive rats, promoting ENaC-mediated Na(+) reabsorption in the collecting duct and the development of hypertension.

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.

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.

Pharmacogenomic strain differences in cardiovascular sensitivity to propofol

INTRODUCTION

A pharmacogenomic approach was used to further localize the genetic region responsible for previously observed enhanced cardiovascular sensitivity to propofol in Dahl Salt Sensitive (SS) versus control Brown Norway (BN) rats.

METHODS

Propofol infusion levels that decreased blood pressure by 50% were measured in BN.13(SS) rats (substitution of SS chromosome 13 into BN) and in five congenic (partial substitution) strains of SS.13(BN). The effect of superfused 2,6 diisopropylphenol on small mesenteric arterial vascular smooth muscle transmembrane potential was measured in congenic strains before and during superfusion with Rp-adenosine-3',5'-cyclic monophosphorothioate and 2.5 μM (Rp)-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, inhibitors of protein kinase A and G, respectively. The genetic locus and potential role of the renin gene in mediating vascular smooth muscle sensitivity to propofol were determined in three selected subcongenic SS.BN¹³ strains.

RESULTS

A 30-32% smaller propofol infusion rate reduced blood pressure by 50% in BN.13(SS) compared with BN and the SS.13(BN) congenic containing an 80 BN gene substitution. Compared with the 80 BN gene-containing SS.13(BN) congenic, SS exhibited greater protein kinase A dependent vascular smooth muscle hyperpolarization in response to propofol. Using subcongenics, the increased propofol-induced cardiovascular sensitivity and hyperpolarization was further localized to an eight-gene region (containing the BN renin gene). Blockade of angiotensin receptors with losartan in this subcongenic increased propofol-induced hyperpolarization by threefold to that observed in SS.

CONCLUSIONS

Enhanced cardiovascular sensitivity to propofol in SS (compared with BN) is caused by an altered renin gene. Through modified second messenger function, this differentially regulates vascular smooth muscle contractile state and reduces vascular tone, thereby exacerbating cardiovascular depression by propofol.

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.

High-salt diet and hypertension: focus on the renin-angiotensin system

A high-salt diet is one of the major risk factors in the development and maintenance of hypertension. Numerous experimental and observational studies have confirmed the association of sodium intake with blood pressure levels. The effects of a high-salt diet are related to the function of the renin-angiotensin system, which is normally suppressed by a high-salt diet. Endothelial dysfunction probably plays an important role in the influence of high sodium intake on blood pressure, although the exact mechanisms remain elusive. Genetic factors are known to be very important, and various consomic and congenic rat strains as animal models have proven to be very useful in bringing us a step closer to understanding the interaction between salt intake and hypertension. In this article, experimental data obtained in studies on animals and humans, as well as epidemiological data are reviewed.

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

This study evaluated endothelium-dependent vascular relaxation in response to acetylcholine (ACh) in isolated middle cerebral arteries (MCA) from Dahl salt-sensitive (Dahl SS) rats and three different congenic strains that contain a portion of Brown Norway (BN) chromosome 13 introgressed onto the Dahl SS genetic background through marker-assisted breeding. Two of the congenic strains carry a 3.5-Mbp portion and a 2.6-Mbp portion of chromosome 13 that lie on opposite sides of the renin locus, while the third contains a 2.0-Mbp overlapping region that includes the BN renin allele. While maintained on a normal salt (0.4% NaCl) diet, MCAs from Dahl SS rats and the congenic strains retaining the Dahl SS renin allele failed to dilate in response to ACh, whereas MCAs from the congenic strain carrying the BN renin allele exhibited normal vascular relaxation. In congenic rats receiving the BN renin allele, vasodilator responses to ACh were eliminated by nitric oxide synthase inhibition with N(G)-nitro-l-arginine methyl ester, angiotensin-converting enzyme inhibition with captopril, and AT(1) receptor blockade with losartan. N(G)-nitro-l-arginine methyl ester-sensitive vasodilation in response to ACh was restored in MCAs of Dahl SS rats that received either a 3-day infusion of a subpressor dose of angiotensin II (3 ng·kg(-1)·min(-1) iv), or chronic treatment with the superoxide dismutase mimetic tempol (15 mg·kg(-1)·day(-1)). These findings indicate that the presence of the Dahl SS renin allele plays a crucial role in endothelial dysfunction present in the cerebral circulation of the Dahl SS rat, even in the absence of elevated dietary salt intake, and that introgression of the BN renin allele rescues endothelium-dependent vasodilator responses by restoring normal activation of the renin-angiotensin system.

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.

Role of the renin angiotensin system on bone marrow-derived stem cell function and its impact on skeletal muscle angiogenesis

Autologous bone marrow cell (BMC) transplantation has been shown as a potential approach to treat various ischemic diseases. However, under many conditions BMC dysfunction has been reported, leading to poor cell engraftment and a failure of tissue revascularization. We have previously shown that skeletal muscle angiogenesis induced by electrical stimulation (ES) is impaired in the SS/Mcwi rats and that this effect is related to a dysregulation of the renin angiotensin system (RAS) that is normalized by the replacement of chromosome 13 derived from the Brown Norway rat (SS-13(BN)/Mcwi consomic rats). The present study explored bone marrow-derived endothelial cell (BM-EC) function in the SS/Mcwi rat and its impact on skeletal muscle angiogenesis induced by ES. SS/Mcwi rats were randomized to receive BMC from: SS/Mcwi; SS-13(BN)/Mcwi; SS/Mcwi rats infused with saline or ANG II (3 ng kg(-1) min(-1)). BMC were injected in the stimulated tibialis anterior muscle of SS/Mcwi rats. Vessel density was evaluated in unstimulated and stimulated muscles after 7 days of ES. BMC isolated from SS/Mcwi or SS/Mcwi rats infused with saline failed to restore angiogenesis induced by ES. However, BMC isolated from SS-13(BN)/Mcwi and SS/Mcwi rats infused with ANG II effectively restored the angiogenesis response in the SS/Mcwi recipient. Furthermore, ANG II infusion increased the capacity of BM-EC to induce endothelial cell tube formation in vitro and slightly increased VEGF protein expression. This study suggests that dysregulation of the RAS in the SS/Mcwi rat contributes to impaired BM-EC function and could impact the angiogenic therapeutic potential of BMC.

Restoration of cerebral vascular relaxation in renin congenic rats by introgression of the Dahl R renin gene

BACKGROUND

This study determined whether transfer of the renin gene from the Dahl salt-resistant (Dahl R) strain into the Dahl salt-sensitive (SS) genetic background restores the relaxation of middle cerebral arteries (MCAs) to different vasodilator stimuli in S/renRR renin congenic (SS.SR-(D13N1 and Syt2)/Mcwi) (RGRR) rats maintained on low-salt (0.4% NaCl) diet.

METHODS

Responses to vasodilator stimuli were evaluated in isolated MCA from SS (Dahl SS/Jr/Hsd/MCWi), RGRR rats, and Dahl R rats.

RESULTS

MCA from SS rats failed to dilate in response to acetylcholine (ACh; 10(-6) mol/l), hypoxia (PO2 reduction to 40-45 mm Hg), and iloprost (10(-11) g/ml). ACh- and hypoxia-induced dilations were present in Dahl R rats and restored in RGRR rats. MCA from RGRR and SS constricted in response to iloprost, whereas MCA from Dahl R rats dilated in response to iloprost. MCA from SS, RGRR, and Dahl R rats exhibited similar dilations in response to cholera toxin (10(-9) g/ml) and dialated in response to the nitric oxide (NO) donor DEA-NONOate (10(-5) mol/l).

CONCLUSIONS

(i) Restoration of normal regulation of the renin-angiotensin system restores dilations to ACh and hypoxia that are impaired in SS rats, (ii) prostacyclin signaling is impaired in SS and RGRR rats but intact in Dahl R rats, indicating that alleles other than the renin gene affect vascular relaxation in response to this agonist; and (iii) vascular smooth muscle sensitivity to NO is preserved in SS and RGRR and is not responsible for impaired arterial relaxation in response to ACh in SS rats.

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.

Consomic strategies to localize genomic regions related to vascular reactivity in the Dahl salt-sensitive rat

Chromosomal substitution strains afford the opportunity to discover regions of the rat genome that contain genes related to cardiovascular traits with the long-range goal of linking these genes to physiological function. PhysGen (Programs for Genomic Applications) created a consomic panel of rats derived from the introgression of a single chromosome (> or =95% of the BN chromosome, one at a time) of the Brown Norway (BN/NHsdMcwi) rat onto the homogeneous genetic background of the Dahl salt-sensitive rat (SS/JrHsdMcwi). For 3 wk before the experiment, the rats were maintained on a low-salt diet (0.4% NaCl). The dose response of aortic rings from each strain of rat to phenylephrine, acetylcholine, sodium nitroprusside, and three different levels of tissue bath hypoxia (10, 5, and 0% O2) was measured and compared with the parental SS rat. To maximize the possibility that differences among the strains would become apparent, each strain of rat including the parental SS and BN was also studied after being maintained on a high-salt diet (4.0% NaCl) for 3 wk. If the response of the aortic ring from a consomic strain to these vasoactive substances was different from that of the SS parental strain, it was concluded that the introgressed chromosome contained a gene or genes that contributed to that difference. Because the BN chromosome is removed from its native background and the SS rat loses a native chromosome, it is also necessary to consider the contribution of changes in gene-to-gene interaction.

Reduced Angiotensin II and Oxidative Stress Contribute to Impaired Vasodilation in Dahl Salt-Sensitive Rats on Low-Salt Diet

This study investigated the role of impaired angiotensin II (Ang II) modulation in contributing to reduced vascular relaxation in isolated middle cerebral arteries (MCA) (100 to 200 μm in diameter) of normotensive Dahl salt-sensitive (SS) rats maintained on low salt (LS) diet (0.4% NaCl) for 9 to 10 weeks. MCA from SS rats on LS diet (n=6 to 9) constricted in response to reduction of perfusate and superfusate PO 2 to 35 to 40 mm Hg or acetylcholine (ACh). Vasodilator responses to reduced PO 2 and ACh were restored in SS.13 BN consomic rats that are 98% genetically identical to SS rats, but exhibit normal regulation of their renin-angiotensin system (RAS). This restored dilation could be prevented by feeding SS.13 BN rats high-salt (HS) diet (4% NaCl) for 3 days to suppress Ang II. A continuous intravenous infusion of a subpressor dose (3 ng/kg per minute) of Ang II for 3 days restored vasodilator responses to ACh and reduced PO 2 in SS.13 BN rats on HS diet and in SS rats on LS diet. Superoxide scavenging with tempol (100 μmol/L) restored vasodilator responses to ACh and reduced PO 2 in MCA of SS rats on LS diet, but did not affect vasodilator responses in MCA of SS.13 BN rats on LS diet. These data indicate that exposure to chronically low Ang II levels leads to impaired vascular relaxation in SS rats, even when the animals are on LS diet and normotensive. This impaired relaxation appears to be mediated by increased levels of oxidative stress in the arteries.

Restoration of normal vascular relaxation mechanisms in cerebral arteries by chromosomal substitution in consomic SS.13BN rats

This study sought to identify the mechanisms of vascular relaxation that are rescued in middle cerebral arteries (MCA) of SS.13BN consomic rats by substituting chromosome 13 containing the renin gene from Brown Norway (BN) rats into the Dahl salt-sensitive (SS) genetic background. Isolated MCA from SS rats exhibited an indomethacin-sensitive constriction in response to acetylcholine (ACh) and hypoxia. ACh-induced dilation was NO dependent and hypoxic dilations were cyclooxygenase (COX) dependent in BN and SS.13BN rats. In SS rats, hypoxic dilation was restored by indomethacin and abolished by inhibiting cytochrome P-450 epoxygenases, suggesting a role for epoxyeicosatrienoic acids. MCA from SS and SS.13BN rats constricted and MCA from BN rats dilated in response to the stable prostacyclin analog iloprost. MCA from SS.13BN and BN rats (but not SS rats) dilated in response to the prostaglandin E2 receptor agonist butaprost. Hypoxia increased prostacyclin release in cerebral arteries from all the strains, whereas thromboxane A2 production was reduced in BN rat vessels only. These data suggest that SS rats may be less sensitive to vasodilator prostaglandins and that normalization of renin-angiotensin system regulation causes a switch from production of COX-derived vasoconstrictor metabolites (in SS rats) toward NO-dependent relaxation in response to ACh- and prostaglandin-dependent dilation in response to hypoxia in SS.13(BN) rats.