Loss of endothelium and receptor-mediated dilation in pial arterioles of rats fed a short-term high salt diet

Gamma-aminobutyric acid-salt attenuated high cholesterol/high salt diet induced hypertension in mice

Excessive salt intake induces hypertension, but several gamma-aminobutyric acid (GABA) supplements have been shown to reduce blood pressure. GABAsalt, a fermented salt by L. brevis BJ20 containing GABA was prepared through the post-fermentation with refined salt and the fermented GABA extract. We evaluated the effect of GABA-salt on hypertension in a high salt, high cholesterol diet induced mouse model. We analyzed type 1 macrophage (M1) polarization, the expression of M1 related cytokines, GABA receptor expression, endothelial cell (EC) dysfunction, vascular smooth muscle cell (VSMC) proliferation, and medial thicknesses in mice model. GABA-salt attenuated diet-induced blood pressure increases, M1 polarization, and TNF-α and inducible nitric oxide synthase (NOS) levels in mouse aortas, and in salt treated macrophages in vitro. Furthermore, GABA-salt induced higher GABAB receptor and endothelial NOS (eNOS) and eNOS phosphorylation levels than those observed in salt treated ECs. In addition, GABA-salt attenuated EC dysfunction by decreasing the levels of adhesion molecules (E-selectin, Intercellular Adhesion Molecule-1 [ICAM-1], vascular cell adhesion molecule-1 [VCAM-1]) and of von Willebrand Factor and reduced EC death. GABA-salt also reduced diet-induced reductions in the levels of eNOS, phosphorylated eNOS, VSMC proliferation and medial thickening in mouse aortic tissues, and attenuated Endothelin-1 levels in salt treated VSMCs. In summary, GABA-salt reduced high salt, high cholesterol diet induced hypertension in our mouse model by reducing M1 polarization, EC dysfunction, and VSMC proliferation.

Voluntary Wheel Running Attenuates Salt-Induced Vascular Stiffness Independent of Blood Pressure

BACKGROUND

Excess dietary salt can lead to the development of arterial stiffness and high blood pressure (BP). Regular physical activity can protect against arterial stiffening and lower BP. Less is known regarding the role of exercise on the vasculature independent of BP under high salt (HS) conditions. The aim of the study was to determine whether wheel running protects against the development of dietary salt-induced arterial stiffness independent of BP.

METHODS

Rats were maintained on either normal salt (NS; 0.49% NaCl) or HS (4.0% NaCl) diet for 6 weeks and further divided into a voluntary wheel running (NS-VWR, HS-VWR) or cage control group (NS, HS). Carotid-femoral pulse wave velocity (PWV) was measured using applanation tonometry at baseline (BSL) and 6 weeks.

RESULTS

BP was measured weekly and remained unchanged among groups throughout the 6 weeks (P > 0.05). PWV was elevated at 6 weeks in HS compared to baseline (HS-BSL, 3.27 ± 0.17 vs. HS-6 week, 4.13 ± 0.26 m/s; P < 0.05) and was lower at 6 weeks in both VWR groups (NS-VWR, 2.98 ± 0.29, HS-VWR, 3.11 ± 0.23 m/s) when compared to HS at 6 weeks (P < 0.05). This was supported by a significant increase in aortic collagen I in the HS group alone and transforming growth factor beta (TGF-β) was greater in the HS group compared to both NS groups (P < 0.05). Wheel running resulted in a greater aortic phosphorylated eNOS and SOD-2 in HS-WVR (P < 0.05) compared to HS.

CONCLUSIONS

These data suggest that VWR may protect against collagen accumulation through a TGF-β-mediated pathway by improving nitric oxide bioavailability and redox balance in rats.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Voluntary wheel running prevents salt-induced endothelial dysfunction: role of oxidative stress

Diets high in salt can lead to endothelial dysfunction, a nontraditional risk factor for cardiovascular disease (CVD). Exercise is known to reduce CVD risk; however, it remains unknown whether chronic physical activity can attenuate salt-induced endothelial dysfunction independent of blood pressure (BP) and whether these changes are due to an upregulation in endogenous antioxidants. Eight-week-old Sprague-Dawley rats were fed either a normal (NS; 0.49%)- or a high (HS; 4.0%)-salt diet and further divided into voluntary wheel running (NS-VWR, HS-VWR) and sedentary (NS, HS) groups for 6 wk. BP was measured weekly and remained unchanged within groups ( P = 0.373). Endothelium-dependent relaxation (EDR) was impaired in the femoral artery of HS compared with NS (38.6 ± 4.0% vs. 65.0 ± 3.6%; P = 0.013) animals, whereas it was not different between NS and HS-VWR (73.4 ± 6.4%; P = 0.273) animals. Incubation with the antioxidants TEMPOL ( P = 0.024) and apocynin ( P = 0.013) improved EDR in HS animals, indicating a role for reactive oxygen species (ROS). Wheel running upregulated the antioxidant superoxide dismutase-2 (SOD-2) ( P = 0.011) under HS conditions and lowered NOX4 and Gp91-phox, two subunits of NADPH oxidase. Wheel running elevated phosphorylated endothelial nitric oxide synthase (eNOS) ( P = 0.014) in HS-fed rats, demonstrating a role for physical activity and eNOS activity under HS conditions. Finally, there was a reduction in EDR ( P = 0.038) when femoral arteries from NS-VWR animals were incubated with TEMPOL or apocynin, suggesting there may be a critical level of ROS needed to maintain endothelial function. In summary, physical activity protected HS-fed rats from reductions in endothelial function, likely through increased SOD-2 levels and reduced oxidative stress. NEW & NOTEWORTHY Our data suggest that voluntary wheel running can prevent impairments in endothelium-dependent relaxation in the femoral artery of rats fed a high-salt diet. This appears to be independent of blood pressure and mediated through a decrease in expression of NADPH oxidases as a result of physical activity. These data suggest that increased chronic physical activity can protect the vasculature from a diet high in salt, likely through a reduction in oxidative stress.

Hypertension, dietary salt and cognitive impairment

Dementia is growing at an alarming rate worldwide. Although Alzheimer disease is the leading cause, over 50% of individuals diagnosed with Alzheimer disease have vascular lesions at autopsy. There has been an increasing appreciation of the pathogenic role of vascular risk factors in cognitive impairment caused by neurodegeneration. Midlife hypertension is a leading risk factor for late-life dementia. Hypertension alters cerebrovascular structure, impairs the major factors regulating the cerebral microcirculation, and promotes Alzheimer pathology. Experimental studies have identified brain perivascular macrophages as the major free radical source mediating neurovascular dysfunction of hypertension. Recent evidence indicates that high dietary salt may also induce cognitive impairment. Contrary to previous belief, the effect is not necessarily associated with hypertension and is mediated by a deficit in endothelial nitric oxide. Collectively, the evidence suggests a remarkable cellular diversity of the impact of vascular risk factors on the cerebral vasculature and cognition. Whereas long-term longitudinal epidemiological studies are needed to resolve the temporal relationships between vascular risk factors and cognitive dysfunction, single-cell molecular studies of the vasculature in animal models will provide a fuller mechanistic understanding. This knowledge is critical for developing new preventive, diagnostic, and therapeutic approaches for these devastating diseases of the mind.

Angiotensin-(1-7) Selectively Induces Relaxation and Modulates Endothelium-Dependent Dilation in Mesenteric Arteries of Salt-Fed Rats

This study investigated the acute effects of angiotensin-(1-7) and AVE0991 on active tone and vasodilator responses to bradykinin and acetylcholine in isolated mesenteric arteries from Sprague-Dawley rats fed a high-salt (HS; 4% NaCl) versus a normal salt (NS; 0.4% NaCl) diet. Angiotensin-(1-7) and AVE0991 elicited relaxation, and angiotensin-(1-7) unmasked vasodilator responses to bradykinin in arteries from HS-fed rats. These effects of angiotensin-(1-7) and AVE0991 were inhibited by endothelium removal, A779, PD123319, HOE140 and L-NAME. Angiotensin-(1-7) also restored the acetylcholine-induced relaxation that was suppressed by the HS diet. Vasodilator responses to bradykinin and acetylcholine in the presence of angiotensin-(1-7) were mimicked by captopril and the AT2 receptor agonist CGP42112 in arteries from HS-fed rats. Thus, in contrast to salt-induced impairment of vascular relaxation in response to vasodilator stimuli, angiotensin-(1-7) induces endothelium-dependent and NO-mediated relaxation, unmasks bradykinin responses via activation of mas and AT2 receptors, and restores acetylcholine-induced vasodilation in HS-fed rats. AT2 receptor activation and angiotensin-converting enzyme (ACE) inhibition shared the ability of angiotensin-(1-7) to enhance bradykinin and acetylcholine responses in HS-fed rats. These findings suggest a therapeutic potential for mas and/or AT2 receptor activation and ACE inhibition in restoring endothelial function impaired by elevated dietary salt intake or other pathological conditions.

Dairy cheese consumption ameliorates single-meal sodium-induced cutaneous microvascular dysfunction by reducing ascorbate-sensitive oxidants in healthy older adults

Chronic dairy product intake is associated with improved cardiovascular outcomes, whereas high dietary Na impairs endothelial function through increased oxidative stress and reduced nitric oxide (NO) bioavailability. The purpose of this study was to compare the effect of acute cheese consumption with consumption of Na from non-dairy sources on microvascular function. We hypothesised that dairy cheese ingestion would augment NO-dependent vasodilation compared with Na from non-dairy sources. On five visits, fourteen subjects (61 (sem 2) years, eight male/six female) consumed either 85 g dairy cheese (560 mg Na), 85 g soya cheese (560 mg Na), 65 g pretzels (560 mg Na), 170 g dairy cheese (1120 mg Na) or 130 g pretzels (1120 mg Na). Two intradermal microdialysis fibres were inserted in the ventral forearm for delivery of lactated Ringer's solution or 10 mm-ascorbate (antioxidant) during local skin heating (approximately 50 min). Erythrocyte flux was measured continuously by laser-Doppler flowmetry (LDF), and cutaneous vascular conductance (CVC=LDF/mean arterial pressure) was normalised as %CVCmax (28 mm-sodium nitroprusside). Following a plateau in CVC, 15 mm-N G -nitro-l-arginine-methyl-ester was perfused to quantify NO-dependent vasodilation (approximately 45 min). NO-dependent vasodilation was greater following consumption of dairy products (560 mg Na 57 (sem 3) %) (1120 mg Na 55 (sem 5) %) compared with soya (560 mg Na 42 (sem 3) %; P=0·002) or pretzels (560 mg Na 43 (sem 4) %; P=0·004) (1120 mg Na 46 (sem 3) %; P=0·04). Ascorbate augmented NO-dependent vasodilation following intake of soya (control: 42 (sem 3) v. ascorbate: 54 (sem 3) %; P=0·01) or pretzels (560 mg Na; control: 43 (sem 4) v. ascorbate: 56 (sem 3) %; P=0·006) (1120 mg Na; control: 46 (sem 5) v. ascorbate: 56 (sem 3) %; P=0·02), but not dairy products. Na ingestion via dairy products was associated with greater NO-dependent vasodilation compared with non-dairy products, a difference that was ameliorated with ascorbate perfusion. The antioxidant properties of dairy proteins may protect against Na-induced reductions in NO-dependent dilation.

Altered potassium ATP channel signaling in mesenteric arteries of old high salt-fed rats

[Purpose]

Both aging and the consumption of a high salt diet are associated with clear changes in the vascular system that can lead to the development of cardiovascular disease; however the mechanisms are not clearly understood. Therefore, we examined whether aging and the consumption of excess salt alters the function of potassium ATP-dependent channel signaling in mesenteric arteries

[Methods]

Young (7 months) and old (29 months) Fischer 344 x Brown Norway rats were fed a control or a high salt diet (8% NaCl) for 12 days and mesenteric arteries were utilized for vascular reactivity measurements.

[Results]

Acetylcholine-induced endothelium relaxation was significantly reduced in old arteries (81 ± 4%) when compared with young arteries (92 ± 2%). Pretreatment with the potassium-ATP channel blocker glibenclamide reduced relaxation to acetylcholine in young arteries but did not alter dilation in old arteries. On a high salt diet, endothelium dilation to acetylcholine was significantly reduced in old salt arteries (60 ± 3%) when compared with old control arteries (81 ± 4%). Glibenclamide reduced acetylcholine-induced dilation in young salt arteries but had no effect on old salt arteries. Dilation to cromakalim, a potassium-ATP channel opener, was reduced in old salt arteries when compared with old control arteries.

[Conclusion]

These findings demonstrate that aging impairs endothelium-dependent relaxation in mesenteric arteries. Furthermore, a high salt diet alters the function of potassium-ATP-dependent channel signaling in old isolated mesenteric arteries and affects the mediation of relaxation stimuli.

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.

The NRF2 knockout rat: a new animal model to study endothelial dysfunction, oxidant stress, and microvascular rarefaction

Nuclear factor (erythroid-derived 2)-like-2 (NRF2) is a master antioxidant and cell protective transcription factor that upregulates antioxidant defenses. In this study we developed a strain of Nrf2 null mutant rats to evaluate the role of reduced NRF2-regulated antioxidant defenses in contributing to endothelial dysfunction and impaired angiogenic responses during salt-induced ANG II suppression. Nrf2(-/-) mutant rats were developed using transcription activator-like effector nuclease technology in the Sprague-Dawley genetic background, and exhibited a 41-bp deletion that included the start codon for Nrf2 and an absence of immunohistochemically detectable NRF2 protein. Expression of mRNA for the NRF2-regulated indicator enzymes heme oxygenase-1, catalase, superoxide dismutase 1, superoxide dismutase 2, and glutathione reductase was significantly lower in livers of Nrf2(-/-) mutant rats fed high salt (HS; 4% NaCl) for 2 wk compared with wild-type controls. Endothelium-dependent dilation to acetylcholine was similar in isolated middle cerebral arteries (MCA) of Nrf2(-/-) mutant rats and wild-type littermates fed low-salt (0.4% NaCl) diet, and was eliminated by short-term (3 days) HS diet in both strains. Low-dose ANG II infusion (100 ng/kg sc) reversed salt-induced endothelial dysfunction in MCA and prevented microvessel rarefaction in wild-type rats fed HS diet, but not in Nrf2(-/-) mutant rats. The results of this study indicate that suppression of NRF2 antioxidant defenses plays an essential role in the development of salt-induced oxidant stress, endothelial dysfunction, and microvessel rarefaction in normotensive rats and emphasize the potential therapeutic benefits of directly upregulating NRF2-mediated antioxidant defenses to ameliorate vascular oxidant stress in humans.

Late gestational hypoxia and a postnatal high salt diet programs endothelial dysfunction and arterial stiffness in adult mouse offspring

Gestational hypoxia and high dietary salt intake have both been associated with impaired vascular function in adulthood. Using a mouse model of prenatal hypoxia, we examined whether a chronic high salt diet had an additive effect in promoting vascular dysfunction in offspring. Pregnant CD1 dams were placed in a hypoxic chamber (12% O2) or housed under normal conditions (21% O2) from embryonic day 14.5 until birth. Gestational hypoxia resulted in a reduced body weight for both male and female offspring at birth. This restriction in body weight persisted until weaning, after which the animals underwent catch-up growth. At 10 weeks of age, a subset of offspring was placed on a high salt diet (5% NaCl). Pressurized myography of mesenteric resistance arteries at 12 months of age showed that both male and female offspring exposed to maternal hypoxia had significantly impaired endothelial function, as demonstrated by impaired vasodilatation to ACh but not sodium nitroprusside. Endothelial dysfunction caused by prenatal hypoxia was not exacerbated by postnatal consumption of a high salt diet. Prenatal hypoxia increased microvascular stiffness in male offspring. The combination of prenatal hypoxia and a postnatal high salt diet caused a leftward shift in the stress-strain relationship in both sexes. Histopathological analysis of aortic sections revealed a loss of elastin integrity and increased collagen, consistent with increased vascular stiffness. These results demonstrate that prenatal hypoxia programs endothelial dysfunction in both sexes. A chronic high salt diet in postnatal life had an additive deleterious effect on vascular mechanics and structural characteristics in both sexes.

NADPH Oxidase Activity in Cerebral Arterioles Is a Key Mediator of Cerebral Small Vessel Disease-Implications for Prevention

Cerebral small vessel disease (SVD), a common feature of brain aging, is characterized by lacunar infarcts, microbleeds, leukoaraiosis, and a leaky blood-brain barrier. Functionally, it is associated with cognitive decline, dementia, depression, gait abnormalities, and increased risk for stroke. Cerebral arterioles in this syndrome tend to hypertrophy and lose their capacity for adaptive vasodilation. Rodent studies strongly suggest that activation of Nox2-dependent NADPH oxidase activity is a crucial driver of these structural and functional derangements of cerebral arterioles, in part owing to impairment of endothelial nitric oxide synthase (eNOS) activity. This oxidative stress may also contribute to the breakdown of the blood-brain barrier seen in SVD. Hypertension, aging, metabolic syndrome, smoking, hyperglycemia, and elevated homocysteine may promote activation of NADPH oxidase in cerebral arterioles. Inhibition of NADPH oxidase with phycocyanobilin from spirulina, as well as high-dose statin therapy, may have potential for prevention and control of SVD, and high-potassium diets merit study in this regard. Measures which support effective eNOS activity in other ways-exercise training, supplemental citrulline, certain dietary flavonoids (as in cocoa and green tea), and capsaicin, may also improve the function of cerebral arterioles. Asian epidemiology suggests that increased protein intakes may decrease risk for SVD; conceivably, arginine and/or cysteine-which boosts tissue glutathione synthesis, and can be administered as N-acetylcysteine-mediate this benefit. Ameliorating the risk factors for SVD-including hypertension, metabolic syndrome, hyperglycemia, smoking, and elevated homocysteine-also may help to prevent and control this syndrome, although few clinical trials have addressed this issue to date.

Excess salt increases infarct size produced by photothrombotic distal middle cerebral artery occlusion in spontaneously hypertensive rats

Cerebral circulation is known to be vulnerable to high salt loading. However, no study has investigated the effects of excess salt on focal ischemic brain injury. After 14 days of salt loading (0.9% saline) or water, spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) were subjected to photothrombotic middle cerebral artery occlusion (MCAO), and infarct volume was determined at 48 h after MCAO: albumin and hemoglobin contents in discrete brain regions were also determined in SHR. Salt loading did not affect blood pressure levels in SHR and WKY. After MCAO, regional cerebral blood flow (CBF), determined with two ways of laser-Doppler flowmetry (one-point measurement or manual scanning), was more steeply decreased in the salt-loaded group than in the control group. In SHR/Izm, infarct volume in the salt-loaded group was 112±27 mm3, which was significantly larger than 77±12 mm3 in the control group (p = 0.002), while the extents of blood-brain barrier disruption (brain albumin and hemoglobin levels) were not affected by excess salt. In WKY, salt loading did not significantly increase infarct size. These results show the detrimental effects of salt loading on intra-ischemic CBF and subsequent brain infarction produced by phototrhombotic MCAO in hypertensive rats.

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 sodium augments angiotensin II-induced vascular smooth muscle cell proliferation through the ERK 1/2-dependent pathway

Angiotensin II (Ang II)-induced vascular injury is exacerbated by high-salt diets. This study examined the effects of high-sodium level on Ang II-induced cell proliferation in rat vascular smooth muscle cells (VSMCs). The cells were cultured in a standard medium containing 137.5 mmol l(-1) of sodium. The high-sodium medium (140 mmol l(-1)) contained additional sodium chloride. Extracellular signal-regulated kinase (ERK) 1/2 phosphorylation was determined by western blot analysis. Cell proliferation was evaluated by [(3)H]-thymidine incorporation. Ang II (100 nmol l(-1)) significantly increased ERK 1/2 phosphorylation and cell proliferation in the both medium containing standard sodium and high sodium. High-sodium level augmented Ang II-induced ERK 1/2 phosphorylation and cell proliferation compared with standard sodium. Pre-treatment with candesartan (1 μmol l(-1), Ang II type 1 receptor blocker) or PD98095 (10 μmol l(-1), ERK kinase iinhibitor) abolished the proliferative effect induced by high sodium/Ang II. Pre-treatment with 5-N,N-hexamethylene amiloride (30 μmol l(-1), Na(+)/H(+) exchanger type 1 (NHE-1) inhibitor), but not SN-6 (10 μmol l(-1), Na(+)/Ca(2+) exchanger inhibitor) or ouabain (1 mmol l(-1), Na(+)/K(+)-ATPase inhibitor) attenuated ERK 1/2 phosphorylation or cell proliferation. Osmotic pressure or chloride had no effect on Ang II-induced proliferative changes. High-sodium level did not affect Ang II receptor expression. Ang II increased intracellular pH via NHE-1 activation, and high-sodium level augmented the pH increase induced by Ang II. These data suggest that high-sodium level directly augments Ang II-induced VSMC proliferation through NHE-1- and ERK 1/2-dependent pathways and may offer new insights into the mechanisms of vascular remodeling by high-sodium/Ang II.

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.

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.

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.

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.

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.

Central and baroreflex control of heart period during the wake-sleep cycle in consomic rats with different genetic susceptibility to hypertension

1. In spontaneously hypertensive rats (SHR), the contributions of the baroreflex and central autonomic commands to the control of heart period (HP) vary among wake-sleep states and are impaired during quiet wakefulness and rapid eye movement sleep (REMS), respectively. 2. Dahl salt-sensitive (SS) rats are genetically susceptible to salt-sensitive hypertension, the development of which depends on diet. Substitution of chromosome 13 of SS rats with that of Brown Norway rats confers salt-resistance to consomic SS-13BN rats. 3. In the present study, we tested whether differences in the central and baroreflex contributions to HP control occur among wake-sleep states in SS and SS-13BN rats and reflect genetic susceptibility to hypertension. Rats (n = 5 per group) were fed a prohypertensive diet late during development to minimize hypertension in SS rats and were instrumented with an arterial catheter and electrodes for discriminating wake-sleep states. 4. The cross-correlation function between HP and blood pressure indicated that, in SS and SS-13BN rats, the contributions of the baroreflex and central commands to the control of HP differed significantly among wake-sleep states, with central commands outweighing the baroreflex in REMS. However, these contributions did not differ significantly between SS and SS-13BN rats in any wake-sleep state. 5. The data suggest that differences in the central and baroreflex contributions to HP control among wake-sleep states, which have been demonstrated in SHR, can be generalized to other rat models used in hypertension research. Impairments in the baroreflex and central autonomic control of HP during quiet wakefulness and REMS, respectively, cannot be generalized as an index of genetic susceptibility to hypertension.

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.

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.

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.

Salt: its role in chronic kidney disease

Few controversies in medicine have such a long history as that of whether salt is identifiably dangerous or not dangerous. The most common reported association between excess dietary salt intake and clinical outcome has been in the field of hypertension, but dietary sodium intake mediates effects that go far beyond, and are independent of, extracellular fluid expansion and elevation in blood pressure. For nephrologists, clinical trials that demonstrate no negative outcome of a high salt diet in the general population are thus not particularly assuasive, because patients with chronic kidney disease (CKD) represent an entity that is by no means comparable to the general population. This review takes a look at the challenges associated with salt balance in CKD patients (particularly at K/DOQI stage 5), followed by a summary of current concepts believed to play a part in salt-mediated pathophysiology, and the conclusion, based on the present state of scientific knowledge, that it appears advisable to advocate low dietary salt intake in this patient population.

Role of superoxide and angiotensin II suppression in salt-induced changes in endothelial Ca2+ signaling and NO production in rat aorta

Male Sprague-Dawley rats were maintained on a low-salt (LS) diet (0.4% NaCl) or changed to a high-salt (HS) diet (4% NaCl) for 3 days. Increases in intracellular Ca2+ ([Ca2+]i) in response to methacholine (10 μM) and histamine (10 μM) were significantly attenuated in aortic endothelial cells from rats fed a HS diet, whereas thapsigargin (10 μM)-induced increases in [Ca2+]i were unaffected. Methacholine-induced nitric oxide (NO) production was eliminated in endothelial cells of aortas from rats fed a HS diet. Low-dose ANG II infusion (5 ng·kg−1·min−1 iv) for 3 days prevented impaired [Ca2+]i signaling response to methacholine and histamine and restored methacholine-induced NO production in aortas from rats on a HS diet. Adding Tempol (500 μM) to the tissue bath to scavenge superoxide anions increased NO release and caused Nω-nitro-l-arginine methyl ester-sensitive vascular relaxation in aortas from rats fed a HS diet but had no effect on methacholine-induced Ca2+ responses. Chronic treatment with Tempol (1 mM) in the drinking water restored NO release, augmented vessel relaxation, and increased methacholine-induced Ca2+ responses significantly in aortas from rats on a HS diet but not in aortas from rats on a LS diet. These findings suggest that 1) agonist-induced Ca2+ responses and NO levels are reduced in aortas of rats on a HS diet; 2) increased vascular superoxide levels contribute to NO destruction, and, eventually, to impaired Ca2+ signaling in the vascular endothelial cells; and 3) reduced circulating ANG II levels during elevated dietary salt lead to elevated superoxide levels, impaired endothelial Ca2+ signaling, and reduced NO production in the endothelium.

Chronic At1 Receptor Blockade Alters the Mechanisms Mediating Hypoxic Dilation in Middle Cerebral Arteries

The purpose of this study was to determine whether chronic blockade of the angiotensin II (ANG II) AT1 receptor under normal physiological conditions impairs vascular relaxation mechanisms in isolated middle cerebral arteries (MCA). Male Sprague-Dawley rats on a standard diet were given losartan (1 mg/mL) in the drinking water or normal water ad libitum for 7 days. Vessel diameters were measured by television microscopy before and during exposure to various vasodilator agonists and reductions in PO2 from 140 mm Hg to 35-45 mm Hg. Dilations to acetylcholine (1 microM), the stable prostacyclin analogue iloprost (10 pg/mL), and the Gs protein activator cholera toxin (1 ng/mL) were completely eliminated in vessels from losartan-treated animals. However, middle cerebral arteries from control and losartan-treated rats still demonstrated significant dilations in response to reduced PO2. Hypoxic dilation of middle cerebral arteries from control rats was eliminated by indomethacin (1 microM) and unaffected by the NOS inhibitor L-NAME (100 microM) whereas dilation in response to reduced PO2 in middle cerebral arteries from losartan-treated rats was eliminated by L-NAME and unaffected by indomethacin. Middle cerebral arteries from control and losartan-treated animals exhibited similar dilations in response to the NO-donor sodium nitroprusside (1 microM). These data suggest that AT1 receptor activation is important in maintaining normal vascular relaxation mechanisms in cerebral resistance arteries during normal physiological conditions, and that AT1 receptor blockade causes a shift in the mechanisms of hypoxic dilation of middle cerebral arteries from cyclooxygenase metabolites to NO.

Homocysteine causes cerebrovascular leakage in mice

Elevated plasma homocysteine (Hcy) is associated with cerebrovascular disease and activates matrix metalloproteinases (MMPs), which lead to vascular remodeling that could disrupt the blood-brain barrier. To determine whether Hcy administration can increase brain microvascular leakage secondary to activation of MMPs, we examined pial venules by intravital video microscopy through a craniotomy in anesthetized mice. Bovine serum albumin labeled with fluorescein isothiocyanate (BSA-FITC) was injected into a carotid artery to measure extravenular leakage. Hcy (30 microM/total blood volume) was injected 10 min after FITC-BSA injection. Four groups of mice were examined: 1) wild type (WT) given vehicle; 2) WT given Hcy (WT + Hcy); 3) MMP-9 gene knockout given Hcy (MMP-9-/- + Hcy); and 4) MMP-9-/- with topical application of histamine (10(-4) M) (MMP-9-/- + histamine). In the WT + Hcy mice, leakage of FITC-BSA from pial venules was significantly (P < 0.05) greater than in the other groups. There was no significant leakage of pial microvessels in MMP-9-/- + Hcy mice. Increased cerebrovascular leakage in the MMP-9-/- + histamine group showed that microvascular permeability could still increase by a mechanism independent of MMP-9. Treatment of cultured mouse microvascular endothelial cells with 30 microM Hcy resulted in significantly greater F-actin formation than in control cells without Hcy. Treatment with a broad-range MMP inhibitor (GM-6001; 1 microM) ameliorated Hcy-induced F-actin formation. These data suggest that Hcy increases microvascular permeability, in part, through MMP-9 activation.

Oxidative stress and antioxidant treatment in hypertension and the associated renal damage

Reactive oxygen species (ROS) are elevated in humans with hypertension many of which develop end-stage renal disease (ESRD), and antioxidant capacity is decreased. About one-half of essential hypertensives have a salt-sensitive type of hypertension, and the amount of renal damage that occurs in salt-sensitive hypertensives greatly exceeds that of non-salt-sensitive hypertensives. Antioxidant therapy can improve cardiovascular outcomes in humans but only if sufficient doses are used. Salt-sensitive hypertensive animal models, especially Dahl salt-sensitive rats, have been used to investigate the relationship between hypertension, ROS and end-stage renal damage. In experimental salt-sensitive hypertension, ROS increase and significant renal damage occur. In the Dahl salt-sensitive (S) rat on high Na for 3 weeks, renal damage is mild, renal levels of superoxide dismutase are decreased, and treatment with Tempol reduces arterial pressure. In the Dahl S rat on high Na for 5 weeks, renal damage is severe, GFR and renal plasma flow are decreased, and renal superoxide production is high. Treatment with vitamins C and E decreases renal superoxide production and renal damage and prevents the decrease in renal hemodynamics. Antioxidant treatment reduces arterial pressure, aortic superoxide production and renal inflammation in DOCA-salt rats, and decreases blood pressure and aortic superoxide release and increases bioactive nitric oxide in SHR stroke-prone rats. In conclusion, in both human and experimental salt-sensitive hypertension, superoxide production and renal damage are increased, antioxidant capacity is decreased, and antioxidant therapy can be helpful.

Salt-induced ANG II suppression impairs the response of cerebral artery smooth muscle cells to prostacyclin

Recent studies have demonstrated that cerebral arteries from rats fed a high-salt (HS) diet exhibit impaired vasodilation and altered electrophysiological response to reduction in Po2. The present study examined whether an increase in salt intake alters the response of vascular smooth muscle cells (VSMC) to prostacyclin, a crucial mediator of hypoxic dilation in cerebral arteries. VSMC were isolated from cerebral arteries of male Sprague-Dawley rats maintained on an HS (4% NaCl) or a low-salt diet (0.4% NaCl) for 3 days. The stable prostacyclin analog iloprost (10 ng/ml) inhibited serotonin (0.1–10 μM)-induced contractions and the increase in intracellular Ca2+concentration ([Ca2+]i) in VSMC isolated from arteries of animals fed the low-salt diet. In contrast, iloprost had no effect on serotonin-induced contractions and increases in [Ca2+]iin VSMC isolated from arteries of rats fed the HS diet. Preventing the fall in ANG in rats fed the HS diet by infusion of a low dose of ANG II (5 ng·kg−1·min−1iv) restored the inhibitory effect of iloprost on serotonin-induced contractions and increases in [Ca2+]iin VSMC from animals fed the HS diet. These effects were reversed by AT1receptor blockade with losartan. These results indicate that ANG II suppression secondary to elevated dietary salt intake impairs vascular relaxation and Ca2+regulation by prostacyclin.

Salt intake and progression of chronic kidney disease: An overlooked modifiable exposure? A commentary

The relationship between sodium chloride (salt) intake and blood pressure and cardiovascular disease has been debated for decades. Overlooked is whether there is a relationship between dietary electrolyte ingestion (both sodium and potassium) and risk for progression of kidney disease, particularly in patients who manifest early evidence of chronic kidney disease (CKD). Patients with CKD often are salt sensitive and respond to increased ingestion of sodium chloride with increased blood pressure. Of concern is the clinical evidence that salt-sensitive patients respond to increased salt intake, in the physiological range, with increased glomerular filtration fraction and proteinuria. Thus, these salt-induced changes in both systemic blood pressure and the renal microcirculation create a favorable theoretical scenario for progressive renal injury. Increased salt intake also attenuates the antihypertensive effects of most antihypertensive drugs. Consequently, salt intake must be considered a potential modifiable risk factor for the progression of kidney disease.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure; production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells; as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

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

This study investigated the role of changes in the expression of the cytochrome P-450 4A (CYP450-4A) enzymes that produce 20-hydroxyeicosatetraenoic acid (20-HETE) in modulating the responses of rat mesenteric resistance arteries to norepinephrine (NE) and reduced Po(2) after short-term (3-day) changes in dietary salt intake. The CYP450-4A2, -4A3, and -4A8 isoforms were all detected by RT-PCR in arteries obtained from rats fed a high-salt (HS, 4% NaCl) diet, whereas only the CYP450-4A3 isoform was detected in vessels from rats fed a low-salt (LS, 0.4% NaCl) diet. Expression of the 51-kDa CYP450-4A protein was significantly increased by a HS diet. Inhibiting 20-HETE synthesis with 30 muM N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) reduced the vasoconstrictor response to NE in arteries obtained from rats fed either a LS or HS diet, but NE sensitivity after DDMS treatment was significantly lower in vessels from rats on a HS diet. DDMS treatment also restored the vasodilator response to reduced Po(2) that was impaired in arteries from rats on a HS diet. These findings suggest that 1) a HS diet increases the expression of CYP450-4A enzymes in the mesenteric vasculature, 2) 20-HETE contributes to the vasoconstrictor response to NE in mesenteric resistance arteries, 3) the contribution of 20-HETE to the vasoconstrictor response to NE is greater in rats fed a HS diet than in rats fed a LS diet, and 4) upregulation of the production of 20-HETE contributes to the impaired dilation of mesenteric resistance arteries in response to hypoxia in rats fed a HS diet.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure;production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells;as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Heme oxygenase-mediated endothelial dysfunction in DOCA-salt, but not in spontaneously hypertensive, rat arterioles

Vascular heme oxygenase (HO) metabolizes heme to form carbon monoxide. Carbon monoxide inhibits nitric oxide synthase and promotes endothelium-dependent vasoconstriction. We reported HO-1-mediated endothelial dysfunction in Dahl salt-sensitive hypertension. Previous studies suggested that salt-sensitive hypertensive rats, but not spontaneously hypertensive rats (SHR), display endothelial dysfunction. This study examines the hypothesis that HO-1-mediated arteriolar endothelial dysfunction develops in deoxycorticosterone acetate (DOCA)-salt hypertensive (DOCA) rats, but not in SHR. Uninephrectomized (isoflurane anesthesia) male Sprague-Dawley rats received DOCA injections and saline drinking solution for 4 wk. Rats subjected to sham surgery received vehicle injections and tap water. Blood pressure was elevated in DOCA rats and SHR compared with sham and Wistar-Kyoto (WKY) groups. Aortic HO-1 expression and blood carboxyhemoglobin levels were elevated in the DOCA group, but not in SHR. In isolated gracilis muscle arterioles, ACh caused concentration-related vasodilation in all groups, with attenuated maximum responses in DOCA, but not in SHR, arterioles. Acute pretreatment with an inhibitor of HO, chromium mesoporphyrin, restored ACh-induced responses in DOCA arterioles to sham levels. ACh responses remained the same in SHR and WKY arterioles after chromium mesoporphyrin treatment. These data show that HO-1 levels and activity are increased and arteriolar responses to ACh are decreased in DOCA rats, but not in SHR. Furthermore, in DOCA arterioles, an inhibitor of HO restores ACh-induced vasodilation to sham levels. These results suggest that elevated HO-1 levels and activity, not resulting from hypertension per se, contribute to endothelial dysfunction in DOCA rats.

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

To evaluate the potential role of impaired renin-angiotensin system (RAS) function in contributing to reduced vascular relaxation in Dahl salt-sensitive (S) rats, responses to ACh (10(-6) mol/l) and hypoxia (Po(2) reduction to 40-45 mmHg) were determined in isolated middle cerebral arteries of Dahl S rats, Brown Norway (BN) rats, and consomic rats having chromosome 13 (containing the renin gene) or chromosome 16 of the BN rat substituted into the Dahl S genetic background (SS-13(BN) and SS-16(BN), respectively). Arteries of BN rats on a low-salt (LS) diet (0.4% NaCl) dilated in response to ACh and hypoxia, whereas dilation in response to these stimuli was absent in Dahl S rats on LS diet. Vasodilation to ACh and hypoxia was restored in SS-13(BN) rats on an LS diet but not in SS-16(BN) rats. High-salt diet (4% NaCl), to suppress ANG II, eliminated vasodilation to hypoxia and ACh in BN and in SS-13(BN) rats. Treatment of SS-13(BN) rats with the AT(1) receptor antagonist losartan also eliminated the restored vasodilation in response to ACh and hypoxia. These studies suggest that restoration of normal RAS regulation in SS-13(BN) consomic rats restores vascular relaxation mechanisms that are impaired in Dahl S rats.

NOS 3 subcellular localization in the regulation of nitric oxide production

Endothelium-derived nitric oxide (NO) is a key signalling molecule in the maintenance of cardiovascular health. Endothelial NO synthase (NOS 3), which catalyses the formation of NO, is targeted to the plasma membrane by dual acylation. In vitro studies suggest that membrane localization of NOS 3 is an important regulatory element of NO production. Dysfunction of the vascular endothelium and a decrease in NO bioavailability is associated with the development and progression of a number of cardiovascular diseases, including hypertension. Our laboratory has previously published that in salt-dependent hypertension there is an altered localization of NOS 3, with an increase in cytosolic expression. These data have led us to question whether the increased cytosolic NOS 3 expression is a form of compensation for endothelial dysfunction in hypertension, or an indicator and contributing factor to endothelial dysfunction. This review will outline the importance of subcellular localization in the regulation of NOS 3 in vitro, the role of NOS 3 in endothelial dysfunction associated with salt-dependent hypertension, and the potential physiological consequences of altered NOS 3 localization in vivo.

Renal and vascular oxidative stress and salt-sensitivity of arterial pressure

Oxidative stress occurs in a tissue or in the whole body when the total oxidant production exceeds the antioxidant capacity. Recent studies in human essential hypertension indicate that free radical production is increased and antioxidant levels are decreased, and more than one-half of these hypertensives have a salt-sensitive type of hypertension with progressive renal damage. Increased oxidative stress may also play a critical role in animal models of salt-sensitive hypertension. The stroke-prone spontaneously hypertensive rats (SHRSP) exhibits salt-sensitivity, vascular release of superoxide is increased, and total plasma antioxidant capacity is decreased. The superoxide release in the SHRSP rats inactivates nitric oxide, and superoxide dismutase (SOD) administration returns the bioactive nitric oxide levels to normal. The deoxycorticosterone acetate (DOCA)-salt hypertensive rat is salt-sensitive, aortic superoxide production is increased, and renal inflammation is significant. Treatment of the DOCA-salt rats with apocynin, an NADPH oxidase inhibitor, decreased aortic superoxide production and decreased arterial pressure. The Dahl salt-sensitive (S) rat has increased mesenteric microvascular and renal superoxide production and increased plasma levels of H2O2. The renal protein expression of SOD is decreased in the kidney of Dahl S rats, and long-term administration of Tempol, a superoxide mimetic, significantly decreased arterial pressure and renal damage. In conclusion, both human hypertension and experimental models of salt-sensitive hypertension have increased superoxide release, decreased antioxidant capacity and elevated renal damage.

p38 MAPK Inhibitors Ameliorate Target Organ Damage in Hypertension: Part 1. p38 MAPK-Dependent Endothelial Dysfunction and Hypertension

Numerous mediators, believed to play a role in endothelial dysfunction (e.g., neurohormones, cytokines, hypoxia, and stretch), have been shown to activate p38 mitogen-activated protein kinase (MAPK) in a variety of cell types. The purpose of the present study was to examine the regulation of p38 MAPK in endothelium and its role in endothelial dysfunction and salt sensitivity. In cultured human umbilical vein endothelial cells (HUVECs), tumor necrosis factor-alpha and lipopolysaccharide increased phosphorylation of p38 MAPK (P-p38 MAPK) and increased ICAM-1 expression. Preincubation with highly selective p38 MAPK inhibitors, 1-(1,3-dihydroxyprop-2-yl)-4-(4-fluorophenyl)-5-[2-phenoxypyrimidin-4-yl] imidazole (SB-239063AN) or SB-239063, dose dependently reduced intercellular adhesion molecule-1 expression in HUVECs. In spontaneously hypertensive-stroke prone rats (SHR-SP), P-p38 MAPK was localized by immunohistochemistry to the aortic endothelium and adventitia but was undetectable in aortae from normotensive rats. Introduction of a salt/fat diet (SFD) to the SHR-SP strain induced endothelial dysfunction (ex vivo vascular reactivity analysis), albuminuria, and an increase in blood pressure within 4 weeks. Chronic dietary dosing (approx. 100 mg/kg/day) with SB-239063AN inhibited the SFD diet-induced hypertension. In addition, delayed treatment also significantly improved survival and restored nitric oxide-mediated endothelium-dependent relaxation in SFD-SHR-SPs with established endothelial dysfunction. These results suggest an important role for p38 MAPK in endothelial inflammation and dysfunction as well as providing the first evidence for p38 MAPK-dependent hypertension.

High-salt diet impairs vascular relaxation mechanisms in rat middle cerebral arteries

Male Sprague-Dawley rats were maintained on a low-salt (LS) diet (0.4% NaCl) or a high-salt (HS) diet (4% NaCl) for 3 days or 4 wk. PO(2) reduction to 40-45 mmHg, the stable prostacyclin analog iloprost (10 pg/ml), and stimulatory G protein activation with cholera toxin (1 ng/ml) caused vascular smooth muscle (VSM) hyperpolarization, increased cAMP production, and dilation in cerebral arteries from rats on a LS diet. Arteries from rats on a HS diet exhibited VSM depolarization and constriction in response to hypoxia and iloprost, failed to dilate or hyperpolarize in response to cholera toxin, and cAMP production did not increase in response to hypoxia, iloprost, or cholera toxin. Low-dose angiotensin II infusion (5 ng x kg(-1) x min(-1) i.v.) restored normal responses to reduced PO(2) and iloprost in arteries from animals on a HS diet. These observations suggest that angiotensin II suppression with a HS diet leads to impaired relaxation of cerebral arteries in response to vasodilator stimuli acting at the cell membrane.

Early endothelial dysfunction following renal mass reduction in rats

BACKGROUND

Endothelial dysfunction has been previously described in severely hypertensive rats with renal mass reduction (RMR) receiving large dietary Na loads. Because hypertension and Na loading reduce endothelium-dependent vasodilation, the effect of renal failure per se is unclear.

METHODS

Responses to acetylcholine in noradrenaline-contracted isolated perfused mesenteric arteries were studied. Vessels were obtained from RMR rats kept on a normal diet, 3 and 10 days after surgery, and the results were compared with those from sham-operated rats (SN). The role of three putative mediators of endothelium-dependent vasodilation was assessed using: L-NAME (10(-4) mol L(-1)); indomethacin (INDO, 10(-5) mol L(-1)); and a mixture of charybdotoxin and apamin (C/A, both 10(-7) mol L(-1)), inhibitors of Ca-activated K-channels to mediate the effects of endothelium-derived hyperpolarizing factor (EDHF).

RESULTS

Response to acetylcholine but not that to nitroprusside (endothelium-independent) was decreased in RMR. L-NAME reduced further acetylcholine relaxations in SN but not in RMR. By contrary, INDO decreased acetylcholine vasodilation in RMR but had no effect in SN. C/A had similar effects in the SN and RMR rats. The levels of 6-keto prostaglandin F1alpha were elevated in the urine of the RMR rats and were perfusate from the RMR vessels.

CONCLUSION

Endothelial dysfunction occurs early after RMR, even when systolic blood pressure is only minimally elevated and Na intake is normal. This alteration may be because of decreased availability of nitric oxide, partially compensated by increased prostacyclin production.

Superoxide dismutase and oxidative stress in Dahl salt-sensitive and -resistant rats

The roles of oxidative stress and renal superoxide dismutase (SOD) levels and their association with renal damage were studied in Dahl salt-sensitive (S) and salt-resistant (R)/Rapp strain rats during changes in Na intake. After 3 wk of a high (8%)-Na diet in S rats, renal medullary Cu/Zn SOD was 56% lower and Mn SOD was 81% lower than in R high Na-fed rats. After 1, 2, and 3 wk of high Na, urinary excretion of F(2)-isoprostanes, an index of oxidative stress, was significantly greater in S rats compared with R rats. Plasma F(2)-isoprostane concentration increased in the 2-wk S high Na-fed group. After 3 wk, renal cortical and medullary superoxide production was significantly increased in Dahl S rats on high Na intake, and urinary protein excretion, an index of renal damage, was 273 +/- 32 mg/d in S high Na-fed rats and 35 +/- 4 mg/d in R high Na-fed rats (P < 0.05). In conclusion, salt-sensitive hypertension in the S rat is accompanied by marked decreases in renal medullary SOD and greater renal oxidative stress and renal damage than in R rats.

Change in endothelial function in mesenteric arteries of Sprague-Dawley rats fed a high salt diet

A high salt diet in some species results in elevated arterial blood pressure and alterations in vascular smooth muscle responses to agonists. Weanling male Sprague-Dawley rats were given either a high salt diet containing 8 % or a low salt diet of 0.4 % sodium chloride for a period of 4 weeks. At the end of the feeding period, tail systolic pressure was higher in the high salt than in low salt rats. The rats were then killed and the intestines removed. Vascular smooth muscle (VSM) responses were estimated from the changes in lumenal diameter of pressurised second order mesenteric resistance arteries. High salt diet resulted in enhanced VSM responses to noradrenaline. The vessels dilated in response both to acetylcholine and to sodium nitroprusside and the responses were similar in vessels from both high and low salt rats. However, vessels from high salt rats were resistant to the blocking of endothelium derived nitric oxide (EDNO) with L-NAME and the responses were instead abolished by blocking endothelium derived hyperpolarising factor (EDHF) with apamin and charybdotoxin. These results show that in Sprague-Dawley rats, a high salt diet enhances the vasoconstriction in response to noradrenaline. The vasodilatory responses to acetylcholine were not significantly changed. However, they appeared to be mediated mainly by EDHF rather than by EDNO as in the low salt animals.