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

Downregulation of Endogenous Hydrogen Sulfide Pathway Is Involved in Mitochondrion-Related Endothelial Cell Apoptosis Induced by High Salt

Background. The study aimed to investigate whether endogenous H₂S pathway was involved in high-salt-stimulated mitochondria-related vascular endothelial cell (VEC) apoptosis. Methods. Cultured human umbilical vein endothelial cells (HUVECs) were used in the study. H₂S content in the supernatant was detected. Western blot was used to detect expression of cystathionine gamma-lyase (CSE), cleaved-caspase-3, and mitochondrial and cytosolic cytochrome c (cytc). Fluorescent probes were used to quantitatively detect superoxide anion generation and measure the in situ superoxide anion generation in HUVEC. Mitochondrial membrane pore opening, mitochondrial membrane potential, and caspase-9 activities were measured. The cell apoptosis was detected by cell death ELISA and TdT-mediated dUTP nick end labeling (TUNEL) methods. Results. High-salt treatment downregulated the endogenous VEC H₂S/CSE pathway, in association with increased generation of oxygen free radicals, decreased mitochondrial membrane potential, enhanced the opening of mitochondrial membrane permeability transition pore and leakage of mitochondrial cytc, activated cytoplasmic caspase-9 and caspase-3 and subsequently induced VEC apoptosis. However, supplementation of H₂S donor markedly inhibited VEC oxidative stress and mitochondria-related VEC apoptosis induced by high salt. Conclusion. H₂S/CSE pathway is an important endogenous defensive system in endothelial cells antagonizing high-salt insult. The protective mechanisms for VEC damage might involve inhibiting oxidative stress and protecting mitochondrial injury.

High dietary sodium reduces brachial artery flow-mediated dilation in humans with salt-sensitive and salt-resistant blood pressure

Recent studies demonstrate that high dietary sodium (HS) impairs endothelial function in those with salt-resistant (SR) blood pressure (BP). The effect of HS on endothelial function in those with salt-sensitive (SS) BP is not currently known. We hypothesized that HS would impair brachial artery flow-mediated dilation (FMD) to a greater extent in SS compared with SR adults. Ten SR (age 42 ± 5 yr, 5 men, 5 women) and 10 SS (age 39 ± 5 yr, 5 men, 5 women) healthy, normotensive participants were enrolled in a controlled feeding study consisting of a run-in diet followed by a 7-day low dietary sodium (LS) (20 mmol/day) and a 7-day HS (300 mmol/day) diet in random order. Brachial artery FMD and 24-h BP were assessed on the last day of each diet. SS BP was individually assessed and defined as a change in 24-h mean arterial pressure (MAP) of >5 mmHg between the LS and HS diets (ΔMAP: SR -0.6 ± 1.2, SS 7.7 ± 0.4 mmHg). Brachial artery FMD was lower in both SS and SR individuals during the HS diet (P < 0.001), and did not differ between groups (P > 0.05) (FMD: SR LS 10.6 ± 1.3%, SR HS 7.2 ± 1.5%, SS LS 12.5 ± 1.7%, SS HS 7.8 ± 1.4%). These data indicate that an HS diet impairs brachial artery FMD to a similar extent in adults with SS BP and SR BP.

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.

Mutation of Plekha7 attenuates salt-sensitive hypertension in the rat

PLEKHA7 (pleckstrin homology domain containing family A member 7) has been found in multiple studies as a candidate gene for human hypertension, yet functional data supporting this association are lacking. We investigated the contribution of this gene to the pathogenesis of salt-sensitive hypertension by mutating Plekha7 in the Dahl salt-sensitive (SS/JrHsdMcwi) rat using zinc-finger nuclease technology. After four weeks on an 8% NaCl diet, homozygous mutant rats had lower mean arterial (149 ± 9 mmHg vs. 178 ± 7 mmHg; P < 0.05) and systolic (180 ± 7 mmHg vs. 213 ± 8 mmHg; P < 0.05) blood pressure compared with WT littermates. Albumin and protein excretion rates were also significantly lower in mutant rats, demonstrating a renoprotective effect of the mutation. Total peripheral resistance and perivascular fibrosis in the heart and kidney were significantly reduced in Plekha7 mutant animals, suggesting a potential role of the vasculature in the attenuation of hypertension. Indeed, both flow-mediated dilation and endothelium-dependent vasodilation in response to acetylcholine were improved in isolated mesenteric resistance arteries of Plekha7 mutant rats compared with WT. These vascular improvements were correlated with changes in intracellular calcium handling, resulting in increased nitric oxide bioavailability in mutant vessels. Collectively, these data provide the first functional evidence that Plekha7 may contribute to blood pressure regulation and cardiovascular function through its effects on the vasculature.

Transgenic overexpression of uncoupling protein 2 attenuates salt-induced vascular dysfunction by inhibition of oxidative stress

BACKGROUND

Ablation of uncoupling protein 2 (UCP2) has been involved in the enhancement of salt sensitivity associated with increased superoxide level and decreased nitric oxide (NO) bioavailability. However, the role of overexpression of UCP2 in salt-induced vascular dysfunction remains elusive.

METHODS

UCP2 transgenic (TG) and wild-type (WT) mice were placed on either a normal-salt (NS, 0.5%) or a high-salt (HS, 8%) diet for 12 weeks. Blood pressure (BP) and hypotensive responses were measured, and the vascular tone, superoxide level, and NO bioavailability in aortas were measured in each group.

RESULTS

The TG mice had increased expression and function of UCP2 in vascular smooth muscle cells. The acetylcholine (ACh)- and nitroglycerin (NTG)-induced hypotensive responses and aortic relaxations were significantly blunted in WT mice fed with an HS diet compared with an NS diet. These harmful effects were prevented in UCP2 TG mice. The impairments of ACh- and NTG-induced relaxation in aorta were inhibited by the endothelial NO synthase (eNOS) inhibitor L-NAME and mitochondrial antioxidant MitoQ, respectively. The HS intake led to a significant increase in superoxide production and a comparable decrease in NO bioavailability in aortas, and these effects were blunted in UCP2 TG mice. The expression of UCP2 was slightly increased in the HS group. However, the expression and phosphorylation of eNOS were not affected by an HS diet and overexpression of UCP2.

CONCLUSIONS

These findings suggest that overexpression of UCP2 can ameliorate salt-induced vascular dysfunction. This beneficial effect of UCP2 is mediated by decreased superoxide and reserved NO bioavailability.

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.

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.

High dietary sodium intake impairs endothelium-dependent dilation in healthy salt-resistant humans

BACKGROUND

Excess dietary sodium has been linked to the development of hypertension and other cardiovascular diseases. In humans, the effects of sodium consumption on endothelial function have not been separated from the effects on blood pressure. The present study was designed to determine if dietary sodium intake affected endothelium-dependent dilation (EDD) independently of changes in blood pressure.

METHOD

Fourteen healthy salt-resistant adults were studied (9M, 5F; age 33 ± 2.4 years) in a controlled feeding study. After a baseline run-in diet, participants were randomized to a 7-day high-sodium (300-350 mmol/day) and 7-day low-sodium (20 mmol/day) diet. Salt resistance, defined as a 5 mmHg or less change in a 24-h mean arterial pressure, was individually assessed while on the low-sodium and high-sodium diets and confirmed in the participants undergoing study (low-sodium: 85 ± 1 mmHg; high-sodium: 85 ± 2 mmHg). EDD was determined in each participant via brachial artery flow-mediated dilation on the last day of each diet.

RESULTS

Sodium excretion increased during the high-sodium diet (P < 0.01). EDD was reduced on the high-sodium diet (low: 10.3 ± 0.9%, high: 7.3 ± 0.7%; P < 0.05). The high-sodium diet significantly suppressed plasma renin activity (PRA), plasma angiotensin II, and aldosterone (P < 0.05).

CONCLUSION

These data demonstrate that excess salt intake in humans impairs endothelium-dependent dilation independently of changes in blood pressure.

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.

Ethers and esters derived from apocynin avoid the interaction between p47phox and p22phox subunits of NADPH oxidase: evaluation in vitro and in silico

NOX (NADPH oxidase) plays an important role during several pathologies because it produces the superoxide anion (O₂^•−), which reacts with NO (nitric oxide), diminishing its vasodilator effect. Although different isoforms of NOX are expressed in ECs (endothelial cells) of blood vessels, the NOX2 isoform has been considered the principal therapeutic target for vascular diseases because it can be up-regulated by inhibiting the interaction between its p47phox (cytosolic protein) and p22phox (transmembrane protein) subunits. In this research, two ethers, 4-(4-acetyl-2-methoxy-phenoxy)-acetic acid (1) and 4-(4-acetyl-2-methoxy-phenoxy)-butyric acid (2) and two esters, pentanedioic acid mono-(4-acetyl-2-methoxy-phenyl) ester (3) and heptanedioic acid mono-(4-acetyl-2-methoxy-phenyl) ester (4), which are apocynin derivatives were designed, synthesized and evaluated as NOX inhibitors by quantifying O₂^•− production using EPR (electron paramagnetic resonance) measurements. In addition, the antioxidant activity of apocynin and its derivatives were determined. A docking study was used to identify the interactions between the NOX2′s p47phox subunit and apocynin or its derivatives. The results showed that all of the compounds exhibit inhibitory activity on NOX, being 4 the best derivative. However, neither apocynin nor its derivatives were free radical scavengers. On the other hand, the in silico studies demonstrated that the apocynin and its derivatives were recognized by the polybasic SH3A and SH3B domains, which are regions of p47phox that interact with p22phox. Therefore this experimental and theoretical study suggests that compound 4 could prevent the formation of the complex between p47phox and p22phox without needing to be activated by MPO (myeloperoxidase), this being an advantage over apocynin.

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.

High salt intake does not produce additional impairment in the coronary artery relaxation of spontaneously hypertensive aged rats

The effect of a salt-based diet on the coronary responsiveness in aged hypertensive rats (SHR) still is unclear. We investigated the effects of high salt intake on the relaxation properties of coronary arteries of aged SHRs. Male SHR (32 week-old) received drinking water (SHR) or 1% NaCl solution (SHR-Salt) for 8 weeks. Isolated coronary segments were subjected to concentration-response curves to acetylcholine (ACh) in the presence or absence of L-NAME (100 μM), enalaprilate (10 μM), losartan (10 μM), and spironolactone (100 μM). Salt intake did not increase blood pressure in old SHRs, but caused ventricular hypertrophy. The endothelium-dependent relaxation in SHRs was lower than in Wistar rats. However, salt intake did not add further impairment. Both enalaprilate and losartan reduced the vasodilator response in coronary arteries from Wistar, but did not affect SHR-salt rats. Conversely, losartan attenuated the impaired ACh relaxation observed in SHR. Spironolactone reduced the relaxation induced by ACh in coronary arteries from Wistar rats but not in SHR. The renin-angiotensin-aldosterone system participates in the impaired coronary relaxation in aged SHR, but does not partake in this deleterious effect under increased salt intake, indicating that age could differentiate the effects of high sodium intake in coronary arteries of SHR.

Testosterone reduces vascular relaxation by altering cyclic adenosine monophosphate pathway and potassium channel activation in male Sprague Dawley rats fed a high-salt diet

OBJECTIVES

Male gender and high-salt diet are risk factors for hypertension. The effect of chronic exposure to testosterone is an increase in vascular tone but its influence upon responses induced by other vasoactive agents is not clear. We considered the possibility of interactions between testosterone and a high-salt diet in the mechanisms that are involved in the regulation of vascular tone. Therefore, we designed experiments to assess the involvement of the cyclic adenosine monophosphate (cAMP) pathway and potassium channel activation on vascular relaxation elicited by testosterone deficiency that was induced by orchidectomy in Sprague Dawley rats on a normal or high-salt diet.

METHOD

Weanling male rats were randomly divided into eight groups (n = 6 each) that were either orchidectomized or sham operated with or without testosterone replacement (10 mg/kg body weight of Sustanon 250 intramuscularly, Organon, Holland) and were placed on a normal or high-salt (0.3% or 8% NaCl) diet, respectively, for 6 weeks. Arterial blood pressure was determined before and weekly throughout the experiment using the tail-cuff method. Relaxation responses to forskolin and diazoxide were studied in noradrenaline (0.1 µM) precontracted aortic rings.

RESULTS

There was an increase in the systolic blood pressure of rats placed on a high-salt diet compared with control or orchidectomized rats. Orchidectomy elicited a reduction in the systolic blood pressure while testosterone replacement restored systolic blood pressure to values seen in intact rats. A high-salt diet reduced the relaxation response to forskolin and diazoxide but not in orchidectomized rats while testosterone replacement re-established the blunted relaxation response to forskolin and diazoxide.

CONCLUSION

Inhibition of potassium channel or adenylyl cyclase activation appears to contribute to the mechanisms by which a high-salt diet increases vascular tone. These effects were counteracted by orchidectomy in male Sprague Dawley rats.

High-salt diet increases hormonal sensitivity in skin pre-capillary resistance vessels

AIMS

Recent data indicate that the skin of rats on a high-salt diet is able to accumulate Na(+) without commensurate water. This extrarenal mechanism of Na(+) homoeostasis could affect skin vasoregulation. We hypothesized that the major resistance vessel of rat skin, the pre-capillary arterioles, has increased vasoreactivity within the physiological range of circulating ANG II, a hormone relevant to salt-sensitive hypertension.

METHODS AND RESULTS

Skin arterioles from skin and muscle were isolated using the agar-infusion technique. Vessels from rats fed high-salt and low-salt diet had similar lumen diameter and media area/lumen area ratio. Contractile sensitivity to ANG II was increased in skin vessels from high-salt vessels at all doses tested starting at 10(-10) m (P < 0.01). Pre-capillary arterioles from muscle displayed similar contractions to ANG II, independent of the diet. As ANG II and the renin-angiotensin system are strongly involved in salt conservation, we explored whether vasoreactivity for noradrenaline was increased as well, because this is a functionally unrelated hormone. At low doses, contractions were similar, but at 10(-5) and 10(-4) m, noradrenaline produced stronger contractions in skin vessels from high-salt compared with low-salt rats (P < 0.01).

CONCLUSIONS

Our data demonstrate significantly increased hormonal vasoreactivity of skin vessels from rats on a high-salt diet, which could increase peripheral resistance in many situations and contribute to higher pressure in salt-sensitive hypertension. As vessels from adjacent muscle were unaffected, we raise the interesting possibility that increased vasoreactivity in the skin could be linked to osmotically inactive Na(+) accumulation.

Differential modulation of uncoupling protein 2 in kidneys of stroke-prone spontaneously hypertensive rats under high-salt/low-potassium diet

The stroke-prone spontaneously hypertensive rat (SHRsp) represents an animal model of increased susceptibility to high-salt diet-induced cerebral and renal vascular injuries. High blood pressure and genetic factors are viewed as major contributing factors. In high-salt-loaded SHRsp and stroke-resistant SHR animals, we determined blood pressure levels, degree of kidney lesions, renal uncoupling protein 2 (UCP2) gene and protein expression levels along with rattus norvegicus (rno)-microRNA (miR) 24 and 34a gene expression, nuclear factor-κB protein levels, and oxidative stress. In vitro, UCP2 gene silencing was performed in renal mesangial cells. We found more severe degree of renal damage in SHRsp at the end of 4-week high-salt dietary treatment as compared with stroke-resistant SHR, despite comparable blood pressure levels, along with increased rate of inflammation and oxidative stress. Kidney UCP2 gene and protein expression levels were significantly downregulated under high-salt diet in SHRsp, but not in stroke-resistant SHR. Differential UCP2 regulation was paralleled by differential expression of kidney rno-miR 24 and 34a, known to target UCP2 gene, in the 2 strains. UCP2 gene silencing in renal mesangial cells led to increased rate of reactive oxygen species generation, increased inflammation and apoptosis, reduced cell vitality, and increased necrosis. In conclusion, high-salt diet downregulates the antioxidant UCP2-dependent mechanism in kidneys of SHRsp, but not of stroke-resistant SHR. A parallel differential kidney miR regulation under high-salt diet in the 2 strains may contribute to the differential UCP2 modulation. UCP2 is a critical protein to prevent oxidative stress damage in renal mesangial cells in vitro.

Age and exercise training alter signaling through reactive oxygen species in the endothelium of skeletal muscle arterioles

Exercise training ameliorates age-related impairments in endothelium-dependent vasodilation in skeletal muscle arterioles. Additionally, exercise training is associated with increased superoxide production. The purpose of this study was to determine the role of superoxide and superoxide-derived reactive oxygen species (ROS) signaling in mediating endothelium-dependent vasodilation of soleus muscle resistance arterioles from young and old, sedentary and exercise-trained rats. Young (3 mo) and old (22 mo) male rats were either exercise trained or remained sedentary for 10 wk. To determine the impact of ROS signaling on endothelium-dependent vasodilation, responses to acetylcholine were studied under control conditions and during the scavenging of superoxide and/or hydrogen peroxide. To determine the impact of NADPH oxidase-derived ROS, endothelium-dependent vasodilation was determined following NADPH oxidase inhibition. Reactivity to superoxide and hydrogen peroxide was also determined. Tempol, a scavenger of superoxide, and inhibitors of NADPH oxidase reduced endothelium-dependent vasodilation in all groups. Similarly, treatment with catalase and simultaneous treatment with tempol and catalase reduced endothelium-dependent vasodilation in all groups. Decomposition of peroxynitrite also reduced endothelium-dependent vasodilation. Aging had no effect on arteriolar protein content of SOD-1, catalase, or glutathione peroxidase-1; however, exercise training increased protein content of SOD-1 in young and old rats, catalase in young rats, and glutathione peroxidase-1 in old rats. These data indicate that ROS signaling is necessary for endothelium-dependent vasodilation in soleus muscle arterioles, and that exercise training-induced enhancement of endothelial function occurs, in part, through an increase in ROS signaling.

Relaxation response of abdominal aorta to androgens in orchidectomised Sprague-Dawley rats fed a high-salt diet

Previous studies have demonstrated the acute relaxant effects of androgens on normal arterial beds, but not on any with underlying or induced pathologies. This study investigated whether the status of the gonads affects the direct actions of androgens on isolated abdominal aorta from male Sprague-Dawley rats fed a high-salt diet. A high-salt diet reduced the relaxation response to exogenous testosterone, but not to dehydroepiandrosterone (DHEA). Orchidectomy reduced the relaxation response to both testosterone and DHEA, while testosterone replacement restored the acute vasorelaxant effect of testosterone and DHEA in both normal and high-salt diet fed rats. Gonadal status appears to be important in the acute vasorelaxant effect of androgens.

Interaction between maternal and offspring diet to impair vascular function and oxidative balance in high fat fed male mice

AIMS

To determine the impact of maternal and post-weaning consumption of a high fat diet on endothelium-dependent vasorelaxation and redox regulation in adult male mouse offspring.

METHODS

Female C57BL6J mice were fed an obesogenic high fat diet (HF, 45% kcal fat) or standard chow (C, 21% kcal fat) pre-conception and throughout pregnancy and lactation. Post-weaning, male offspring were continued on the same diet as their mothers or placed on the alternative diet to give 4 dietary groups (C/C, HF/C, C/HF and HF/HF) which were studied at 15 or 30 weeks of age.

RESULTS

There were significant effects of maternal diet on offspring body weight (p<0.004), systolic blood pressure (p = 0.026) and endothelium-dependent relaxation to ACh (p = 0.004) and NO production (p = 0.005) measured in the femoral artery. With control for maternal diet there was also an effect of offspring post-weaning dietary fat to increase systolic blood pressure (p<0.0001) and reduce endothelium-dependent relaxation (p = 0.022) and ACh-mediated NO production (p = 0.007). There was also a significant impact of age (p<0.005). Redox balance was perturbed, with altered regulation of vascular enzymes involved in ROS/NO signalling.

CONCLUSIONS

Maternal consumption of a HF diet is associated with changes in vascular function and oxidative balance in the offspring of similar magnitude to those seen with consumption of a high fat diet post-weaning. Further, this disadvantageous vascular phenotype is exacerbated by age to influence the risk of developing obesity, raised blood pressure and endothelial dysfunction in adult life.

Reduced hyperpolarization of endothelial cells following high dietary Na+: effects of enalapril and tempol

1. High dietary Na(+) is associated with impaired vascular endothelial function. However, the underlying mechanisms are not completely understood. In the present study, we investigated whether the endothelial hyperpolarization response to acetylcholine (ACh) exhibited any abnormalities in Wistar rats fed a high-salt diet (HSD) for 1 month and, if so, whether chronic treatment with the angiotensin-converting enzyme inhibitor enalapril or the anti-oxidant tempol could normalize the response. Membrane potential was recorded using the perforated patch-clamp technique on the endothelium of rat aorta. 2. Acetylcholine (2 μmol/L) produced a hyperpolarization sensitive to TRAM-34, a blocker of intermediate-conductance Ca(2+) -sensitive K(+) channels (IK(Ca)), but not to apamin, a blocker of small-conductance Ca(2+)-sensitive K(+) channels (SK(Ca)). NS309 (3 μmol/L), an activator of SK(Ca) and IK(Ca) channels, produced a hyperpolarization of similar magnitude as ACh. 3. In the HSD group, the ACh-evoked hyperpolarization was significantly attenuated compared with that in the control group, which was fed normal chow rather than an HSD. Similarly, the hyperpolarization produced by NS309 was weaker in tissues from HSD-fed rats. 4. Combination of HSD with chronic enalapril treatment (20 mg/kg per day for 1 month) normalized endothelial hyperpolarizing responses to ACh. Chronic tempol treatment (1 mmol/L in tap water for 1 month) prevented the reduced hyperpolarization to ACh. 5. The results of the present study indicate that excess in dietary Na(+) results in a failure of endothelial cells to generate normal IK(Ca) channel-mediated hyperpolarizing responses. Our observations implicate oxidative stress mediated by increased angiotensin II signalling as a mechanism underlying altered endothelial hyperpolarization during dietary salt loading.

Dietary sodium loading impairs microvascular function independent of blood pressure in humans: role of oxidative stress

Animal studies have reported dietary salt-induced reductions in vascular function independent of increases in blood pressure (BP). The purpose of this study was to determine if short-term dietary sodium loading impairs cutaneous microvascular function in normotensive adults with salt resistance. Following a control run-in diet, 12 normotensive adults (31 ± 2 years) were randomized to a 7 day low-sodium (LS; 20 mmol day(-1)) and 7 day high-sodium (HS; 350 mmol day(-1)) diet (controlled feeding study). Salt resistance, defined as a 5 mmHg change in 24 h mean BP determined while on the LS and HS diets, was confirmed in all subjects undergoing study (LS: 84 ± 1 mmHg vs. HS: 85 ± 2 mmHg; P > 0.05). On the last day of each diet, subjects were instrumented with two microdialysis fibres for the local delivery of Ringer solution and 20 mm ascorbic acid (AA). Laser Doppler flowmetry was used to measure red blood cell flux during local heating-induced vasodilatation (42°C). After the established plateau, 10 mm l-NAME was perfused to quantify NO-dependent vasodilatation. All data were expressed as a percentage of maximal cutaneous vascular conductance (CVC) at each site (28 mm sodium nitroprusside; 43°C). Sodium excretion increased during the HS diet (P < 0.05). The plateau % CVCmax was reduced during HS (LS: 93 ± 1 % CVCmax vs. HS: 80 ± 2 % CVCmax; P < 0.05). During the HS diet, AA improved the plateau % CVCmax (Ringer: 80 ± 2 % CVCmax vs. AA: 89 ± 3 % CVCmax; P < 0.05) and restored the NO contribution (Ringer: 44 ± 3 % CVCmax vs. AA: 59 ± 6 % CVCmax; P < 0.05). These data demonstrate that dietary sodium loading impairs cutaneous microvascular function independent of BP in normotensive adults and suggest a role for oxidative stress.

Establishment of a conditional transgenic mouse model expressing human uncoupling protein 2 in vascular smooth muscle cells

Increased oxidative stress is involved in the development of vascular dysfunction and remodeling. Uncoupling protein 2 (UCP2) regulates the production of reactive oxygen species in vascular smooth muscle cells (SMCs). To promote the study of the role of UCP2 in vascular diseases, a transgenic mouse model expressing human UCP2 (hUCP2) in vascular SMCs was established. We constructed a plasmid carrying the 2.3 kb rabbit smooth muscle myosin heavy chain promoter and the hUCP2 gene. We used this plasmid to produce transgenic mice by pro-nuclear microinjection. Six offspring were identified as founder mice that were used to establish a transgenic mouse lineage. The transgenic mice showed a significant increase in hUCP mRNA expression in the aorta. Moreover, hUCP2 overexpression inhibited the production of superoxide and increased the bioavailability of nitric oxide (NO). In this study, we established a hUCP2 transgenic mouse model, which will enable further studies on the role of UCP2 in vascular dysfunction and remodeling.

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.

High salt intake abolishes AT(2)-mediated vasodilation of pial arterioles in rats

BACKGROUND

Angiotensin II (Ang II) induces constriction (AT(1)) and dilation (AT(2) receptors) of cerebral arterioles. High sodium intake induces changes in receptors expression and loss of AT(2)-mediated vasodilation in extracerebral vessels. We investigated whether high salt modifies the AT(2)-mediated response of cerebral arterioles.

METHODS

Three-month-old male Wistar rats received drinking water supplemented or not with 1% NaCl. We measured at day 4 or 30 plasma aldosterone concentration, AT receptors expression (brain microvessels, western blot, RT-qPCR), internal diameter of pial arterioles (cranial window) following suffusion with Ang II (10(-6) mol/l, or 10(-8) mol/l + losartan 10(-5) mol/l), serotonin (5-HT, 10(-6) mol/l), sodium nitroprusside (10(-5) mol/l) and adenosine diphosphate (ADP, 10(-4) mol/l).

RESULTS

High salt did not modify arterial pressure, baseline arteriolar diameter, vasoconstriction to Ang II or 5-HT, nor vasodilation to SNP. High salt lowered plasma aldosterone concentration (d4 138 ± 71 not significant vs. control 338 ± 73; d30 150 ± 21 P < 0.05 vs. control 517 ± 79 μmol/l). AT receptors mRNA did not change while protein level of AT(2) receptors decreased at d4 (64 ± 9% of control, P < 0.05). AT(2)-mediated vasodilation (control d4; d30 8 ± 2; 5 ± 2%) was abolished at d4 (-2 ± 2%, P < 0.05) and reversed to vasoconstriction at d30 (-7 ± 2%, P < 0.05). ADP-induced vasodilation is abolished at d30 (2 ± 2, P < 0.05 vs. control 19 ± 4%).

CONCLUSION

High salt specifically abolishes AT(2)-mediated vasodilation, immediately, via decreased level of AT(2) receptor protein, and after 30 days, in association with abolition of endothelial vasodilation. Such loss of AT(2)-mediated vasodilation may be deleterious in case of stroke.

Acute and chronic angiotensin-(1-7) restores vasodilation and reduces oxidative stress in mesenteric arteries of salt-fed rats

This study determined the effect of ANG-(1-7) on salt-induced suppression of endothelium-dependent vasodilatation in the mesenteric arteries of male Sprague-Dawley rats. Chronic intravenous infusion of ANG-(1-7), oral administration of the nonpeptide mas receptor agonist AVE-0991, and acute preincubation of the arteries with ANG-(1-7) and AVE-0991 all restored vasodilator responses to both ACh and histamine that were absent in the arteries of rats fed a high-salt (4% NaCl) diet. The protective effects of ANG-(1-7) and AVE-0991 were inhibited by acute or chronic administration of the mas receptor antagonist A-779, the ANG II type 2 (AT(2)) receptor blocker PD-123319, or N-nitro-l-arginine methyl ester, but not the ANG II type 1 receptor antagonist losartan. Preincubation with the antioxidant tempol or the nitric oxide (NO) donor diethylenetriamine NONOate and acute and chronic administration of the AT(2) receptor agonist CGP-42112 mimicked the protective effect of ANG-(1-7) to restore vascular relaxation. Acute preincubation with ANG-(1-7) and chronic infusion of ANG-(1-7) ameliorated the elevated superoxide levels in rats fed a high-salt diet, but the expression of Cu/Zn SOD and Mn SOD enzyme proteins in the vessel wall was unaffected by ANG-(1-7) infusion. These results indicate that both acute and chronic systemic administration of ANG-(1-7) or AVE-0991 restore endothelium-dependent vascular relaxation in salt-fed Sprague-Dawley rats by reducing vascular oxidant stress and enhancing NO availability via mas and AT(2) receptors. These findings suggest a therapeutic potential for mas/AT(2) receptor activation in preventing the vascular oxidant stress and endothelial dysfunction associated with elevated dietary salt intake.

Salt sensitivity in blacks: evidence that the initial pressor effect of NaCl involves inhibition of vasodilatation by asymmetrical dimethylarginine

In healthy, mostly normotensive blacks, 19 salt-sensitive (SS) and 18 salt-resistant (SR), we tested the hypothesis that, in SS subjects, dietary NaCl loading induces its initial pressor effect by inducing a normal increase of cardiac output, while failing to induce a normal pressor-offsetting vasodilatation, consequent to its inhibition by asymmetrical dimethylarginine that is abnormally increased by NaCl. In SS and SR subjects, dietary NaCl loading, 250 from 30 mmol/d, over a 7-day period, induced similar, immediate increases in external Na(+) balance (by day 2, ≈360 mmol), plasma volume (+11%), and cardiac output (+8%). In SR subjects, from day 1, transient decreases occurred in both systemic vascular resistance (nadir: -13%, day 2) and mean arterial pressure (nadir: -5%, day 2). In SS subjects, systemic vascular resistance did not change over days 1 to 3, whereas mean arterial pressure increased progressively after day 1, ultimately by 10 mm Hg. Failure of systemic vascular resistance to normally decrease, while cardiac output normally increased, accounted for salt's initial pressor effect in the SS subjects. In SS subjects, baseline plasma levels of asymmetrical dimethylarginine (0.76 μmol/L) and symmetrical dimethylarginine (0.60 μmol/L), which does not affect vasodilatation, approximated those in SR subjects. In SS but not SR subjects, NaCl loading induced increases in asymmetrical dimethylarginine on both days 2 (+38%, median) and 7 (+14%, median). Symmetrical dimethylarginine changed in neither group. For all of the subjects combined, changes in asymmetrical dimethylarginine on day 2 predicted changes in systemic vascular resistance (R=0.751; P<0.001) and mean arterial pressure (R=0.527; P=0.006) on day 2 and similarly on day 7. These observations support the hypothesis tested.

Intradermal microdialysis of hypertonic saline attenuates cutaneous vasodilatation in response to local heating

We tested the hypothesis that microdialysis of hypertonic saline would attenuate the skin blood flow response to local heating. Seventeen healthy subjects (23 ± 1 years old) were studied. In one group (n = 9), four microdialysis fibres were placed in the forearm skin and infused with the following: (1) Ringer solution; (2) normal saline (0.9% NaCl); (3) hypertonic saline (3% NaCl); and (4) 10 mm l-NAME. A second group (n = 8) was infused with the following: (1) normal saline; (2) hypertonic saline; (3) normal saline + l-NAME; and (4) hypertonic saline + l-NAME. Red blood cell flux was measured via laser Doppler flowmetry during local heating to 42°C. Site-specific maximal vasodilatation was determined by infusing 28 mm sodium nitroprusside while the skin was heated to 43°C. Data were expressed as the percentage of maximal cutaneous vascular conductance (%CVC(max)). The local heating response at the Ringer solution and normal saline sites did not differ (n = 9; initial peak Ringer solution, 69 ± 6 versus normal saline, 66 ± 2%CVC(max); plateau Ringer solution, 89 ± 4 versus normal saline, 89 ± 5%CVC(max)). Hypertonic saline reduced the initial peak (n = 9; normal saline, 66 ± 2 versus hypertonic saline, 54 ± 4%CVC(max); P < 0.05) and plateau (normal saline, 89 ± 5 versus hypertonic saline, 78 ± 2%CVC(max); P < 0.05) compared with normal saline. Plateau %CVC(max) was attenuated to a similar value at the normal saline + l-NAME and hypertonic saline + l-NAME sites (n = 8; normal saline + l-NAME, 39 ± 6 and hypertonic saline + l-NAME, 39 ± 5%CVC(max)). The nitric oxide contribution (plateau %CVC(max) - l-NAME plateau %CVC(max)) was lower at the hypertonic saline site (normal saline, 55 ± 6 versus hypertonic saline, 35 ± 4; P < 0.01). These data suggest an effect of salt on the cutaneous response to local heating, which may be mediated through a decreased production and/or availability of nitric oxide.

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.

Orchidectomy attenuates impaired endothelial effects of a high-salt diet in Sprague–Dawley rats

The effect of sex hormones on vascular reactivity is considered one of the underlying factors contributing to gender differences in cardiovascular functions and diseases. Experiments were designed to investigate the role of androgens in salt-induced hypertension by assessing the relaxation response of isolated aortic rings to acetylcholine and sodium nitroprusside in the presence or absence of l-nitroarginine methyl ester in Sprague–Dawley rats. The rats were either orchidectomized or sham-operated, with or without testosterone replacement, and were placed on a normal or high-salt diet for 6 weeks. The results indicate a significant increase (p < 0.001) in the mean arterial blood pressure of rats on the high-salt diet, when compared with control or orchidectomized rats. Orchidectomy elicited a reduction in mean arterial blood pressure (p < 0.01), while testosterone replacement normalized mean arterial blood pressure to values seen in intact rats on the high-salt diet. The high-salt diet reduced the relaxation response to acetylcholine both in the presence and absence of inhibition of endothelial nitric oxide synthase with l-nitroarginine methyl ester. Bilateral orchidectomy attenuated the impaired endothelial function induced by the high-salt diet in rats, but this was reversed by concomitant administration of testosterone, suggesting a role for androgens in enhancing long-term vascular smooth muscle tone and hence maintenance of high blood pressure in salt-induced hypertension.

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.

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.

Uncoupling protein 2 ablation exacerbates high-salt intake-induced vascular dysfunction

BACKGROUND

Salt-induced vascular dysfunction in which underlying mechanisms involve reactive oxygen species (ROS)-mediated reduction of nitric oxide (NO) bioavailability has been well documented. Uncoupling protein 2 (UCP2) has been implicated in the vascular protection, specifically by decreasing ROS production. However, it is unclear how UCP2 affects vascular function in salt-loaded mice.

METHODS

UCP2-deficient (UCP2(-/-)) and wild-type (WT) mice were placed on either a normal-salt (NS, 0.5%) or a high-salt (HS, 8%) diet for 24 weeks. Blood pressure (BP), mesenteric arterial reactivity, superoxide production, and NO bioavailability in the intact vessels were measured in each group.

RESULTS

UCP2(-/-) mice on a HS diet had a higher BP than those on a NS diet (P < 0.01). However, BP in WT mice was not different between the NS and HS diet group. Phenylephrine (PE)-induced contraction was enhanced while acetylcholine (ACh)-elicited relaxation was impaired in mesenteric resistance arteries from the HS diet-fed WT mice. Importantly, the enhanced contraction and impaired relaxation were both further exacerbated in UCP2(-/-) mice. Similarly, the HS diet led to a moderate increase in superoxide production and a comparable decrease in NO availability in both aortas and mesenteric resistance vessels, and these effects were also remarkably enhanced in UCP2(-/-) mice.

CONCLUSIONS

These findings suggest that UCP2 plays an important role in preventing salt-sensitive hypertension, which may be achieved by suppressing superoxide production and reserving NO bioavailability in blood vessels.

Angiotensin-(1-7) and low-dose angiotensin II infusion reverse salt-induced endothelial dysfunction via different mechanisms in rat middle cerebral arteries

The goals of this study were to 1) determine the acute effect of ANG-(1-7) on vascular tone in isolated middle cerebral arteries (MCAs) from Sprague-Dawley rats fed a normal salt (NS; 0.4% NaCl) diet, 2) evaluate the ability of chronic intravenous infusion of ANG-(1-7) (4 ng·kg(-1)·min(-1)) for 3 days to restore endothelium-dependent dilation to acetylcholine (ACh) in rats fed a high-salt (HS; 4% NaCl) diet, and 3) determine whether the amelioration of endothelial dysfunction by ANG-(1-7) infusion in rats fed a HS diet is different from the protective effect of low-dose ANG II infusion in salt-fed rats. MCAs from rats fed a NS diet dilated in response to exogenous ANG-(1-7) (10(-10)-10(-5) M). Chronic ANG-(1-7) infusion significantly reduced vascular superoxide levels and restored the nitric oxide-dependent dilation to ACh (10(-10)-10(-5) M) that was lost in MCAs of rats fed a HS diet. Acute vasodilation to ANG-(1-7) and the restoration of ACh-induced dilation by chronic ANG-(1-7) infusion in rats fed a HS diet were blocked by the Mas receptor antagonist [D-ALA(7)]-ANG-(1-7) or the ANG II type 2 receptor antagonist PD-123319 and unaffected by ANG II type 1 receptor blockade with losartan. The restoration of ACh-induced dilation in MCAs of HS-fed rats by chronic intravenous infusion of ANG II (5 ng·kg(-1)·min(-1)) was blocked by losartan and unaffected by d-ALA. These findings demonstrate that circulating ANG-(1-7), working via the Mas receptor, restores endothelium-dependent vasodilation in cerebral resistance arteries of animals fed a HS diet via mechanisms distinct from those activated by low-dose ANG II infusion.

Hydroethidine- and MitoSOX-derived red fluorescence is not a reliable indicator of intracellular superoxide formation: another inconvenient truth

Hydroethidine (HE; or dihydroethidium) is the most popular fluorogenic probe used for detecting intracellular superoxide radical anion. The reaction between superoxide and HE generates a highly specific red fluorescent product, 2-hydroxyethidium (2-OH-E(+)). In biological systems, another red fluorescent product, ethidium, is also formed, usually at a much higher concentration than 2-OH-E(+). In this article, we review the methods to selectively detect the superoxide-specific product (2-OH-E(+)) and the factors affecting its levels in cellular and biological systems. The most important conclusion of this review is that it is nearly impossible to assess the intracellular levels of the superoxide-specific product, 2-OH-E(+), using confocal microscopy or other fluorescence-based microscopic assays and that it is essential to measure by HPLC the intracellular HE and other oxidation products of HE, in addition to 2-OH-E(+), to fully understand the origin of red fluorescence. The chemical reactivity of mitochondria-targeted hydroethidine (Mito-HE, MitoSOX red) with superoxide is similar to the reactivity of HE with superoxide, and therefore, all of the limitations attributed to the HE assay are applicable to Mito-HE (or MitoSOX) as well.

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.

Decreased arteriolar tetrahydrobiopterin is linked to superoxide generation from nitric oxide synthase in mice fed high salt

OBJECTIVE

Impaired endothelium-dependent arteriolar dilation in mice fed high salt (HS) is due to local oxidation of nitric oxide (NO) by superoxide anion (O(2) (-)). We explored the possibility that "uncoupled" endothelial nitric oxide synthase (eNOS) is the source of this O(2) (-).

METHODS

Levels of L-arginine (L-Arg), tetrahydrobiopterin (BH(4)), and O(2) (-) (hydroethidine oxidation) were measured in spinotrapezius muscle arterioles of mice fed normal salt (0.45%, NS) or (4%, HS) diets for 4 weeks, with or without dietary L-Arg supplementation. The contribution of NO to endothelium-dependent dilation was determined from the effect of N(omega)-nitro-L-arginine methyl ester (L-NAME) on responses to acetylcholine (ACh).

RESULTS

Arterioles in HS mice had lower [BH(4)] and higher O(2) (-) levels than those in NS mice. ACh further increased arteriolar O(2) (-) in HS mice only. L-Arg supplementation prevented the reduction in [BH(4)] in arterioles of HS mice, and O(2) (-) was not elevated in these vessels. Compared to NS mice, arteriolar ACh responses were diminished and insensitive to L-NAME in HS mice, but not in HS mice supplemented with L-Arg.

CONCLUSIONS

These findings suggest that eNOS uncoupling due to low [BH(4)] is responsible for O(2) (-) generation and reduced NO-dependent dilation in arterioles of mice fed a HS diet.

Angiotensin II, mechanotransduction, and pulsatile arterial hemodynamics in hypertension

The aortic blood pressure curve involves two components: a steady component, the mean arterial pressure (MAP), which is dependent on cardiac output and vascular resistance, and a pulsatile component pulse pressure (PP), which is dependent on arterial stiffness and pulse wave reflections. The transduction mechanisms of MAP and PP differ markedly, involving focal adhesion kinase for MAP and oxygen free radicals for PP. Angiotensin II (ANG II) and its blockade are associated with changed vascular resistance and MAP; however, their effects on PP (peripheral and mostly central PP) have been inadequately investigated. In hypertensive rats, when compared with their normotensive controls, ANG II blockade normalizes central PP (<50 mmHg) but not MAP when the same drug dosage is used for each. In hypertensive patients, ANG II blockade reduces arterial stiffness and pulse wave reflections, but with the same reduction in MAP, there is a greater reduction in central than peripheral PP, thereby increasing carotid-brachial PP amplification. With long-term ANG II blockade, the hypertensive arteriolar hypertrophy observed at baseline is corrected in association with reduced arteriolar reflection coefficients, reduced carotid arterial attachments linking alpha(5)-integrin to its ligand fibronectin, and decreased circulating C-reactive protein. When given a normal salt diet, each of these factors contributes separately in reducing arterial stiffness and wave reflections. These responses disappear with a high-salt diet, a condition that usually involves the activation of the local vascular renin-angiotensin-aldosterone system and can be prevented by its selective blockade. Thus ANG II inhibition seems to contribute independently in reducing central PP and aortic stiffness.

Renal dopaminergic defect in C57Bl/6J mice

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

Low dietary sodium intake is associated with enhanced vascular endothelial function in middle-aged and older adults with elevated systolic blood pressure

BACKGROUND

Age and increasing systolic blood pressure (BP) are associated with vascular endothelial dysfunction, but the factors involved are incompletely understood. We tested the hypothesis that vascular endothelial function is related to dietary sodium intake among middle-aged and older adults (MA and O) with elevated systolic BP.

METHODS

Data were analyzed on 25 otherwise healthy adults aged 48-73 years with high normal systolic BP or stage I systolic hypertension (130-159 mmHg). Self-reported sodium intake was <100 mmol/d in 12 (7 M) subjects (low sodium, 73+/-6 mmol/d) and between 100 and 200 mmol/d in 13 (9 M) subjects (normal sodium, 144+/-6 mmol/d).

RESULTS

Groups did not differ in other dietary factors, age, body weight and composition, BP, metabolic risk factors, physical activity and maximal aerobic capacity. Plasma concentrations of norepinephrine, endothelin-1, oxidized low-density lipoproteins (LDL), antioxidant status and inflammatory markers did not differ between groups. Brachial artery flow-mediated dilation (FMD) was 42% (mm Delta) to 52% (% Delta) higher in the low versus normal sodium group (p < 0.05). In all subjects, brachial artery FMD was inversely related to dietary sodium intake (FMD mm Delta r =-0.40, p < 0.05; %Delta r =-0.53, p < 0.01). Brachial artery FMD was not related to any other variable. In contrast, endothelium-independent dilation did not differ between groups (p >or= 0.24) and was not related to sodium intake in the overall group (p >or= 0.29).

CONCLUSIONS

Low sodium intake is associated with enhanced brachial artery FMD in MA and O with elevated systolic BP. These results suggest that dietary sodium restriction may be an effective intervention for improving vascular endothelial function in this high-risk group.

Angiotensin II maintains cerebral vascular relaxation via EGF receptor transactivation and ERK1/2

This study identified, on the integrative level, two components of the ANG II signaling pathway that lay downstream from the ANG II type 1 (AT(1)) receptor and are critically involved in maintaining vascular relaxation in cerebral resistance arteries. In these experiments, the relaxation of isolated middle cerebral arteries (MCA) in response to ACh (10(-9)-10(-5) M), iloprost (10(-16)-10(-11) g/ml), and reduced PO(2) was lost and the ratio of phospho-ERK/ERK1/2 was significantly reduced in aortas of male Sprague-Dawley rats fed a high-salt (HS; 4% NaCl) diet to suppress plasma ANG II levels. In salt-fed rats, relaxation of MCA in response to these vasodilator stimuli was restored by chronic (3 days) intravenous infusion of either ANG II (5 ngxkg(-1)xmin(-1)) or epidermal growth factor (EGF; 2 microg/h). The protective effect of ANG II infusion to restore vascular relaxation was eliminated by coinfusion of either the EGF receptor kinase inhibitor AG-1478 (20 microg/h), the ERK1/2 inhibitor PD-98059 (10 microg/h), or the protein synthesis inhibitor cycloheximide (5 microg/h). In rats fed a low-salt (0.4% NaCl) diet, MCA relaxation in response to ACh, reduced PO(2), and iloprost was eliminated by intravenous infusion of AG-1478, PD-98059, or cycloheximide. In ANG II-infused rats fed HS diet, and in rats fed LS diet, vasodilator responses to reduced PO(2) and iloprost were unaffected by the p38 MAP kinase inhibitor SB-203580 and the phosphatidylinositol 3-kinase inhibitor wortmannin. These findings indicate that maintenance of normal vascular relaxation mechanisms by ANG II in rat MCA requires activation of the EGF receptor kinase and ERK1/2.

Effects of ageing and exercise training on eNOS uncoupling in skeletal muscle resistance arterioles

Reduced availability of tetrahydrobiopterin (BH(4)) contributes to the age-related decline of nitric oxide (NO)-mediated vasodilatation of soleus muscle arterioles. Depending on availability of substrate and/or necessary co-factors, endothelial nitric oxide synthase (eNOS) can generate NO and/or superoxide (O(2)(-)). We evaluated the effects of age and chronic exercise on flow-induced vasodilatation and levels of NO and O(2)(-) in soleus muscle arterioles. Young (3 months) and old (22 months) male rats were exercise trained or remained sedentary (SED) for 10 weeks. Flow-stimulated NO and O(2)(-), as well as BH(4) and l-arginine content, were determined in soleus muscle arterioles. Flow-induced vasodilatation was assessed under control conditions and during the blockade of O(2)(-) and/or hydrogen peroxide. Exercise training enhanced flow-induced vasodilatation in arterioles from young and old rats. Old age reduced, and exercise training restored, BH(4) content and flow-stimulated NO availability. Flow-stimulated, eNOS-derived O(2)(-) levels were higher in arterioles from old SED compared to those from young SED rats. Exercise training increased flow-stimulated eNOS-derived O(2)(-) levels in arterioles from young but not old rats. O(2)(-) scavenging with Tempol reduced flow-induced vasodilatation from all groups except young SED rats. Addition of catalase to Tempol-treated arterioles eliminated flow-induced vasodilatation in arterioles from all groups. Catalase reduced flow-induced vasodilatation from all groups. In Tempol-treated arterioles, flow-induced vasodilatation was restored by deferoxamine, an iron chelator. These data indicate that uncoupling of eNOS contributes to the age-related decline in flow-induced vasodilatation; however, reactive oxygen species are required for flow-induced vasodilatation in soleus muscle arterioles from young and old rats.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Chemistry and antihypertensive effects of tempol and other nitroxides

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