SWI/SNF chromatin remodeling enzymes are associated with cardiac hypertrophy in a genetic rat model of hypertension.

Pathological cardiac hypertrophy is characterized by a sustained increase in cardiomyocyte size and re-activation of the fetal cardiac gene program. Previous studies implicated SWI/SNF chromatin remodeling enzymes as regulators of the fetal cardiac gene program in surgical models of cardiac hypertrophy. Although hypertension is a common risk factor for developing cardiac hypertrophy, there has not yet been any investigation into the role of SWI/SNF enzymes in cardiac hypertrophy using genetic models of hypertension. In this study, we tested the hypothesis that components of the SWI/SNF complex are activated and recruited to promoters that regulate the fetal cardiac gene program in hearts that become hypertrophic as a result of salt induced hypertension. Utilizing the Dahl salt-sensitive (S) rat model, we found that the protein levels of several SWI/SNF subunits required for heart development, Brg1, Baf180, and Baf60c, are elevated in hypertrophic hearts from S rats fed a high salt diet compared with normotensive hearts from Dahl salt-resistant (R) rats fed the same diet. Furthermore, we detected significantly higher levels of SWI/SNF subunit enrichment as well as evidence of more accessible chromatin structure on two fetal cardiac gene promoters in hearts from S rats compared with R rats. Our data implicate SWI/SNF chromatin remodeling enzymes as regulators of gene expression in cardiac hypertrophy resulting from salt induced hypertension. Thus we provide novel insights into the epigenetic mechanisms by which salt induced hypertension leads to cardiac hypertrophy.

Mitochondrial proteomic analysis reveals deficiencies in oxygen utilization in medullary thick ascending limb of Henle in the Dahl salt-sensitive rat.

The renal medullary thick ascending limb (mTAL) of the Dahl salt-sensitive (SS) rat is the site of enhanced NaCl reabsorption and excess superoxide production. In the present studies we isolated mitochondria from mTAL of SS and salt-resistant control strain SS.13(BN) rats on 0.4 and 8% salt diet for 7 days and performed a proteomic analysis. Purity of mTAL and mitochondria isolations exceeded 93.6 and 55%, respectively. Using LC/MS spectral analysis techniques we identified 96 mitochondrial proteins in four biological mTAL mitochondria samples, run in duplicate, as defined by proteins with a false discovery rate <5% and scan count ≥2. Seven of these 96 proteins, including IDH2, ACADM, SCOT, Hsp60, ATPA, EFTu, and VDAC2 were differentially expressed between the two rat strains. Oxygen consumption and high-resolution respirometry analyses showed that mTAL cells and the mitochondria in the outer medulla of SS rats fed high-salt diet exhibited lower rates of oxygen utilization compared with those from SS.13(BN) rats. These studies advance the conventional proteomic paradigm of focusing exclusively upon whole tissue homogenates to a focus upon a single cell type and specific subcellular organelle. The results reveal the importance of a largely unexplored role for deficiencies of mTAL mitochondrial metabolism and oxygen utilization in salt-induced hypertension and renal medullary oxidative stress.

Mechanisms of pressure-diuresis and pressure-natriuresis in Dahl salt-resistant and Dahl salt-sensitive rats.

BACKGROUND: Data on blood flow regulation, renal filtration, and urine output in salt-sensitive Dahl S rats fed on high-salt (hypertensive) and low-salt (prehypertensive) diets and salt-resistant Dahl R rats fed on high-salt diets were analyzed using a mathematical model of renal blood flow regulation, glomerular filtration, and solute transport in a nephron. RESULTS: The mechanism of pressure-diuresis and pressure-natriuresis that emerges from simulation of the integrated systems is that relatively small increases in glomerular filtration that follow from increases in renal arterial pressure cause relatively large increases in urine and sodium output. Furthermore, analysis reveals the minimal differences between the experimental cases necessary to explain the observed data. It is determined that differences in renal afferent and efferent arterial resistances are able to explain all of the qualitative differences in observed flows, filtration rates, and glomerular pressure as well as the differences in the pressure-natriuresis and pressure-diuresis relationships in the three groups. The model is able to satisfactorily explain data from all three groups without varying parameters associated with glomerular filtration or solute transport in the nephron component of the model. CONCLUSIONS: Thus the differences between the experimental groups are explained solely in terms of difference in blood flow regulation. This finding is consistent with the hypothesis that, if a shift in the pressure-natriuresis relationship is the primary cause of elevated arterial pressure in the Dahl S rat, then alternation in how renal afferent and efferent arterial resistances are regulated represents the primary cause of chronic hypertension in the Dahl S rat.

T lymphocytes mediate hypertension and kidney damage in Dahl salt-sensitive rats.

This study examined mechanisms by which immune cells participate in the development of hypertension and renal disease in Dahl salt-sensitive (SS) rats. Increasing dietary salt from 0.4% to 4.0% NaCl significantly increased renal infiltration of T lymphocytes from 8.8 +/- 1.2 x 10(5) to 14.4 +/- 2.0 x 10(5) cells/2 kidneys, increased arterial blood pressure from 131 +/- 2 to 165 +/- 6 mmHg, increased albumin excretion rate from 17 +/- 3 to 129 +/- 20 mg/day, and resulted in renal glomerular and tubular damage. Furthermore, renal tissue ANG II was not suppressed in the kidneys of SS rats fed 4.0% NaCl. Administration of the immunosuppressive agent mycophenolate mofetil (MMF; 20 mg.kg(-1).day(-1)) prevented the infiltration of T lymphocytes and attenuated Dahl SS hypertension and renal disease. In contrast to vehicle-treated rats, Dahl SS rats administered MMF demonstrated a suppression of renal tissue ANG II from 163 +/- 26 to 88 +/- 9 pg/g of tissue when fed high salt. Finally, it was demonstrated that the T lymphocytes isolated from the kidney possess renin and angiotensin-converting enzyme activity. These data indicate that infiltrating T cells are capable of participating in the production of ANG II and are associated with increased intrarenal ANG II, hypertension, and renal disease. The suppression of T-cell infiltration decreased intrarenal ANG II and prevented Dahl SS hypertension and kidney damage. As such, infiltrating cells are capable of participating in the established phase of Dahl SS hypertension.

Important genetic checkpoints for insulin resistance in salt-sensitive (S) Dahl rats.

Despite the marked advances in research on insulin resistance (IR) in humans and animal models of insulin resistance, the mechanisms underlying high salt-induced insulin resistance remain unclear. Insulin resistance is a multifactorial disease with both genetic and environmental factors (such as high salt) involved in its pathogenesis. High salt triggers insulin resistance in genetically susceptible patients and animal models of insulin resistance. One of the mechanisms by which high salt might precipitate insulin resistance is through its ability to enhance an oxidative stress-induced inflammatory response that disrupts the insulin signaling pathway. The aim of this hypothesis is to discuss two complementary approaches to find out how high salt might interact with genetic defects along the insulin signaling and inflammatory pathways to predispose to insulin resistance in a genetically susceptible model of insulin resistance. The first approach will consist of examining variations in genes involved in the insulin signaling pathway in the Dahl S rat (an animal model of insulin resistance and salt-sensitivity) and the Dahl R rat (an animal model of insulin sensitivity and salt-resistance), and the putative cellular mechanisms responsible for the development of insulin resistance. The second approach will consist of studying the over-expressed genes along the inflammatory pathway whose respective activation might be predictive of high salt-induced insulin resistance in Dahl S rats. Variations in genes encoding the insulin receptor substrates -1 and/or -2 (IRS-1, -2) and/or genes encoding the glucose transporter (GLUTs) proteins have been found in patients with insulin resistance. To better understand the combined contribution of excessive salt and genetic defects to the etiology of the disease, it is essential to investigate the following question:Question 1: Do variations in genes encoding the IRS -1 and -2 and/or genes encoding the GLUTs proteins predict high salt-induced insulin resistance in Dahl S rats?A significant amount of evidence suggested that salt-induced oxidative stress might predict an inflammatory response that upregulates mediators of inflammation such as the nuclear factor- kappa B (NF-kappa B), the tumor necrosis factor-alpha (TNF-alpha) and the c-Jun Terminal Kinase (JNK). These inflammatory mediators disrupt the insulin signaling pathway and predispose to insulin resistance. Therefore, the following question will be thoroughly investigated:Question 2: Do variations in genes encoding the NF-kappa B, the TNF-alpha and the JNK, independently or in synergy, predict an enhanced inflammatory response and subsequent insulin resistance in Dahl S rats in excessive salt environment?Finally, to better understand the combined role of these variations on glucose metabolism, the following question will be addressed:Question 3: What are the functional consequences of gene variations on the rate of glucose delivery, the rate of glucose transport and the rate of glucose phosphorylation in Dahl S rats?The general hypothesis is that "high-salt diet in combination with defects in candidate genes along the insulin signaling and inflammatory pathways predicts susceptibility to high salt-induced insulin resistance in Dahl S rats".

Preventive and therapeutic effect of brozopine on stroke in Dahl Salt-sensitive hypertensive rats.

Our aim was to explore the preventive and therapeutic effects of sodium (±)-5-bromo-2-(α-hydroxypentyl) benzoate (brand name: brozopine, BZP) on stroke in Dahl Salt-sensitive (Dahl-SS) hypertensive rats. Dahl-SS rats were fed a high-salt diet to observe the effect of BZP on blood pressure, and brain, heart, and kidney tissues. Additionally, the incidence of stroke was recorded according to the neurological score. The relative mechanisms investigated included anti-oxidative effects and anti-platelet aggregation. BZP reduced the incidence of stroke, neuronal necrosis in the brain, and cell swelling and inflammatory infiltration in the kidney. Its mechanisms were related to the increased activities of gluthatione peroxidase and catalase and the decreased level of plasma nitric oxide. BZP inhibited arachidonic acid (AA) - induced platelet aggregation (IC50: 12µM) rather than that of adenosine diphosphate (ADP) - and/or thrombin-induced platelet aggregation in vitro. Interestingly, BZP inhibited ADP-, thrombin-, or AA-induced platelet aggregation and elevated the level of AMP-activated protein kinase, cyclic guanosine monophosphate, and vasodilator-stimulated-phosphoprotein, and attenuated ATP contents and mitogen-activated protein kinase levels in platelet and inhibited thrombus formation in a carotid artery thrombosis model, dose-dependently, in Dahl-SS hypertensive-induced stroke rats. In conclusion, BZP can have therapeutic and preventive effects on stroke in Dahl-SS hypertensive rats, the mechanisms of which may be related to anti-oxidant, anti-platelet aggregation and anti-thrombus formation.

Altered structure and reduced distensibility of arteries in Dahl salt-sensitive rats.

OBJECTIVES: The role of salt sensitivity in arterial stiffening and the structural basis of reduced arterial distensibility were investigated in Dahl salt-sensitive (DS) rats. METHODS: Three-month-old male DS rats received a normal (0.7% NaCl) or a high-sodium (2% NaCl) diet for 3 months. Dahl salt-resistant (DR) rats were controls. Pressure-volume (distensibility) relationships were measured in in-vitro-perfused segments of right carotid and iliac arteries, in the presence and absence of extracellular calcium. The left carotid and iliac arteries were perfusion-fixed at 100 mmHg for morphometric measurements. RESULTS: The average monthly tail systolic blood pressure (SBP) of DS rats on normal and high-sodium diets were increased compared to that of DR rats. Compared to controls, carotid and iliac artery pressure-volume curves of DS rats on normal and high-sodium diets were shifted toward the pressure axis, without a change in elastic moduli. In DS rats, reduced distensibility of the carotid artery was accompanied by increased lumen diameter and increased thickness of media and elastic lamellae, the wall to lumen ratio being unchanged; wall thickness was increased and lumen diameter unchanged in the iliac artery. The high-sodium diet had no effect on either distensibility or dimensions of carotid and iliac arteries in DS or DR rats. CONCLUSION: Geometry (increased or unchanged lumen and increased wall thickness), rather than increased stiffness of wall components, appears to be the cause of reduced distensibility of arteries in DS rats. Structural and functional adaptation to salt sensitivity may occur on what is considered a 'normal' sodium diet.

Development of different phenotypes of hypertensive heart failure: systolic versus diastolic failure in Dahl salt-sensitive rats.

OBJECTIVE: There are two phenotypes of heart failure, systolic failure and isolated diastolic heart failure with preserved left ventricular systolic function. Although isolated diastolic heart failure frequently occurs, there are only models for diastolic dysfunction unassociated with heart failure and models with overt diastolic heart failure have not been established. We attempted to develop two different models, i.e. diastolic and systolic failure models, based on hypertension. MATERIALS AND METHODS: Dahl salt-sensitive rats were placed on 8% NaCl diet from 7 weeks old (7-week starting group) or 8 weeks old (8-week starting group). As an age-matched control, Dahl salt-sensitive rats were consistently placed on normal chow. In these rats, echocardiogram was serially recorded, followed by hemodynamic and histological studies. RESULTS: The 7-week starting rats showed a steep elevation in blood pressure and progressive left ventricular hypertrophy, and fell into overt heart failure at approximately 19 weeks. The development of heart failure was not associated with a decrease in left ventricular midwall fractional shortening or an increase in left ventricular end-diastolic dimension as compared with the age-matched control, which mimics the characteristics of clinically observed isolated diastolic heart failure. The 8-week starting rats showed a gradual rise in blood pressure and less progressive left ventricular hypertrophy, and fell into heart failure at approximately 26 weeks with a decrease in mid-wall fractional shortening and an increase in left ventricular end-diastolic dimension. Hemodynamic and histological studies at failing stage revealed comparable elevation of left ventricular end-diastolic pressure and comparable left ventricular fibrosis in both groups. CONCLUSION: These two different models of overt heart failure may be useful as models of isolated diastolic heart failure and systolic heart failure based on the same hypertensive heart disease, respectively, and may contribute to discrimination of the mechanisms of the development of the two different phenotypes of heart failure.

Differential nNOS gene expression in salt-sensitive and salt-resistant Dahl rats.

BACKGROUND: Several indications exist to suggest that an impaired production of nitric oxide might have a role in the development of salt-sensitive hypertension. OBJECTIVE: To examine whether the gene expression of the nitric oxide synthases (NOS) is altered in the salt-sensitive Dahl rat compared with that in the salt-resistant Dahl rat. DESIGN AND METHODS: The abundance of NOS mRNA was measured by RNase protection assay in different organs of salt-resistant and salt-sensitive Dahl rats. In addition, the zonal expression of NOS genes in the kidney under salt load and salt restriction was determined. RESULTS: The abundance of endothelial NOS mRNA was similar between the salt-resistant and salt-sensitive Dahl rat strains in all organs. Inducible NOS mRNA was not detectable by RNase protection assay in any organ. Neuronal NOS (nNOS) mRNA expression, however, was about 50% lower in brain and kidney of salt-sensitive Dahl rats than in salt-resistant Dahl rats. Within the kidney, nNOS mRNA levels were significantly decreased in salt-sensitive Dahl rats compared with those in salt-resistant Dahl rats, in cortex, outer and inner medulla (50, 40 and 30%, respectively) under all dietary conditions. A comparison of renal nNOS gene expression in Dahl rats with that in salt-insensitive Sprague- Dawley rats revealed that the abundance of renal nNOS was similar in salt-sensitive Dahl and Sprague-Dawley rats, but was increased in salt-resistant Dahl rats relative to that in Sprague-Dawley rats. CONCLUSION: These data suggest that nNOS gene expression is increased in salt-resistant Dahl rats compared with that in salt-sensitive Dahl rats. This increased nNOS expression of the salt-resistant Dahl strain might play a part in compensating for a defect of renal salt excretion in the Dahl strains.

Renal vascular morphology and haemodynamics in Dahl salt-sensitive rats on high salt-low potassium diet: neural and genetic influences.

OBJECTIVE: A dietary combination of high salt and low potassium (HS-LK) exacerbates hypertension in Dahl salt-sensitive (DS) rats and renders Dahl salt-resistant (DR) rats hypertensive. In both strains, the hypertension is accompanied by remodelling of the renal resistance vasculature, and is attenuated by peripheral chemical sympathectomy. In the current study, we sought to determine whether the sympathetic nervous system is causally involved in mediating the renal vascular and haemodynamic alterations associated with HS-LK feeding in Dahl rats. DESIGN: Two groups each of DS and DR rats were maintained on HS-LK diet (8% NaCl, 0.2% KCl) for 8 weeks. One group of DS (n = 9) and DR (n = 8) were treated with 6-hydroxydopamine (6-OHDA) in 0.001 N HCl vehicle to chemically ablate peripheral sympathetic nerve terminals. The two remaining groups (n = 8 each) received equivalent injections of vehicle. METHODS: At the end of the dietary regimen, arterial blood pressure (ABP), glomerular filtration rate (GFR) and renal blood flow (RBF) were measured, and the structure of intra-renal resistance vessels was examined by planar morphometric analysis of coronal sections prepared from perfusion-fixed kidneys. RESULTS: Both 6-OHDA-treated and untreated DS rats presented a greater degree of intra-renal vessel remodelling characterized by reduced lumen diameter in the absence (eutrophic) or presence (hypertrophic) of cross-sectional area expansion, higher renal vascular resistance (RVR) and lower GFR and RBF than DR rats. Chemical sympathectomy increased lumen diameters and reduced vascular wall expansion, resulting in a decrease in RVR and a concomitant increase in RBF and GFR in both strains; however, the effect was more prominent in the DS rats. CONCLUSIONS: We conclude that HS-LK-induced changes in intra-renal vessel structure and renal haemodynamic function in Dahl rats are, at least in part, dependent on the activity of the sympathetic nervous system.

Neuronal nitric oxide strongly suppresses sympathetic outflow in high-salt Dahl rats.

OBJECTIVE: To investigate the effects of a selective inhibitor of neuronal nitric oxide synthase (nNOS), 7-nitroindazole, on peripheral sympathetic outflow in Dahl rats. DESIGN AND METHODS: Dahl salt-sensitive and salt-resistant rats were fed either a regular-salt (0.4% NaCl) or a high-salt (8% NaCl) diet for 4 weeks. In chronically instrumented conscious rats, renal sympathetic nerve activity (RSNA) was measured in both baroreceptor-loaded and baroreceptor-unloaded states. The baroreceptor unload was performed by decreasing arterial pressure with occlusion of the inferior vena cava. RESULTS: 7-Nitroindazole (307 micromol/kg intraperitoneally) increased resting RSNA from 24 +/- 3% to 38 +/- 6% with an increase in mean arterial pressure of 15 +/- 3 mmHg, and increased baroreceptor-unloaded RSNA from 100% to 278 +/- 16% in salt-sensitive Dahl rats receiving a high-salt diet However, 7-nitroindazole did not increase resting RSNA, but did increase baroreceptor-unloaded RSNA from 100% to 179 +/- 15%, 177 +/- 15%, and 133 +/- 4% in salt-sensitive Dahl rats receiving a regular-salt diet, salt-resistant Dahl rats receiving a high-salt diet, and salt-resistant Dahl rats receiving a regular-salt diet, respectively. The high-salt diet significantly increased the baroreceptor-unloaded RSNA more than the regular-salt diet did, in both salt-sensitive and salt-resistant rats. After administration of the vehicle for 7-nitroindazole (peanut oil), L-arginine (100 micromol/kg per min for 10 min) decreased both resting and baroreceptor-unloaded RSNA, whereas after pretreatment with 7-nitroindazole, the L-arginine-induced suppression was reversed, in Dahl salt-sensitive rats receiving a high-salt diet. CONCLUSIONS: Neuronal nitric oxide may suppress the sympathetic discharge generated before baroreflex-mediated inhibition in all rats. This neuronal nitric oxide-mediated suppression was enhanced by the salt load in both salt-resistant and salt-sensitive Dahl rats. Finally, the neuronal nitric oxide-mediated suppression in tonic peripheral sympathetic outflow may be greatly enhanced in salt-sensitive hypertension.

Cerebrovascular alterations in pressure and protein kinase C-mediated constriction in Dahl salt-sensitive rats.

BACKGROUND: Dahl salt-sensitive (DSS) rats fed an 8.7% sodium chloride diet from weaning spontaneously developed hypertension and a 50% mortality rate by 5 weeks. Before death the rats exhibited behavioural signs of stroke and disruption of the blood-brain barrier. OBJECTIVES: To test the hypothesis that rats exhibiting stroke had middle cerebral arteries (MCAs) that had lost the ability to constrict in response to pressure, and to assess whether this defect was associated with abnormalities in protein kinase C (PKC)-mediated constriction. METHODS: MCAs were sampled from DSS rats before and after stroke and from Dahl salt-resistant (DSR) rats fed 8.7% NaCl. Constrictions in response to a 100 mmHg pressure step and to PKC activation by phorbol dibutyrate (PDB) (0.1 micromol/l) in the presence of nifedipine (3 micromol/l) were measured. RESULTS: MCAs from DSS rats after stroke constricted in response to vasopressin but were unable to constrict in response to pressure or PDB in the presence of nifedipine, whereas those from DSS rats before stroke and from DSR rats constricted in response to all the stimuli. The PKC inhibitors, chelerythrine (12 micromol/l) and bisindolylmaleimide (5 micromol/l) inhibited constrictions in response to pressure and to PDB in the presence of nifedipine. CONCLUSIONS: Constriction of the MCA in response to pressure is dependent on functional PKC signalling. Development of stroke in DSS rats fed a high-salt diet is associated with an inability of the MCAs to constrict in response to pressure, possibly because of the presence of an incompetent PKC system. The inability to constrict in response to pressure may cause blood flow abnormalities that contribute to disruption of the blood-brain barrier in these rats.

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.

Plasma 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol concentrations are decreased in hind limb unloaded Dahl salt-sensitive female rats.

Plasma 1,25-dihydroxyvitamin D (1,25-(OH)(2)D) concentration was shown to decrease during bed rest in several studies when baseline plasma 25-hydroxyvitamin D (25-OHD) concentration was sub-optimal. Dahl salt-sensitive female (S) rats, but not Dahl salt-resistant female (R) rats, demonstrated a 50% decrease in plasma 1,25-dihydroxycholecalciferol (1,25-(OH)(2)D(3)) concentration after 28 days of hind limb unloading (HU, disuse model) during low salt intake (0.3%). We tested the vitamin D endocrine system response of female S rats to hind limb unloading during high salt intake (2%, twice that of standard rat chow to mimic salt intake in the USA). Hind limb unloading resulted in lower plasma 25-OHD(3) concentrations in S-HU rats than in R-HU rats (P<0.05) and greater urinary loss of 25-OHD(3) by S-HU rats than by S rats (P<0.05). Plasma 1,25-(OH)(2)D(3) concentration of S-HU rats was half that of S rats, but was unchanged in R-HU rats. The association of low plasma 25-OHD concentration with decrease in plasma 1,25-(OH)(2)D concentration of hind limb unloaded rats and of bed rest participants (published studies) suggests that low vitamin D status might be a risk factor for decrease in plasma vitamin D hormone concentration during long-term immobilization or bed rest.

Failure to upregulate the adenosine2A receptor-epoxyeicosatrienoic acid pathway contributes to the development of hypertension in Dahl salt-sensitive rats.

Adenosine-activated renovascular dilatation in Sprague-Dawley (SD) rats is mediated by stimulating adenosine(2A) receptors (A(2A)R), which is linked to epoxyeicosatrienoic acid (EET) synthesis. The A(2A)R-EET pathway is upregulated by high salt (HS) intake in normotensive SD rats. Because this pathway is antipressor, we examined the role of the A(2A)R-EET pathway in Dahl salt-sensitive (SS) rats. Male Dahl salt-resistant (SR) and SS rats were fed either HS (8.0% NaCl) or normal salt (NS; 0.4% NaCl) diet for 7 days. On day 8, isolated kidneys were perfused with Krebs-Henseleit buffer containing indomethacin and N(G)-nitro-l-arginine methyl ester and preconstricted with phenylephrine. Bolus injections of the stable adenosine analog 2-chloroadenosine (2-CA; 0.1-20 microg) elicited dose-dependent dilation in both Dahl SR and SS rats. Dahl SR rats fed a HS diet demonstrated a greater renal vasodilator response to 10 microg of 2-CA, as measured by the reduction in renal perfusion pressure, than that of Dahl SR rats fed a NS diet (-104 +/- 6 vs. -77 +/- 7 mmHg, respectively; P < 0.05). In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed NS or HS diet (96 +/- 6 vs. 104 +/- 13 mmHg in NS- and HS-fed rats, respectively). In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A)R and the cytochrome P-450 2C23 and 2C11 epoxygenases, and elevated the renal efflux of EETs. Thus the Dahl SR rat is able to respond to HS intake by recruiting EET formation, whereas the Dahl SS rat appears to have exhausted its ability to increase EET synthesis above the levels observed on NS intake, and this inability of Dahl SS rats to upregulate the A(2A)R-EET pathway in response to salt loading may contribute to the development of salt-sensitive hypertension.

Reduced angiotensin II and oxidative stress contribute to impaired vasodilation in Dahl salt-sensitive rats on low-salt diet.

This study investigated the role of impaired angiotensin II (Ang II) modulation in contributing to reduced vascular relaxation in isolated middle cerebral arteries (MCA) (100 to 200 microm in diameter) of normotensive Dahl salt-sensitive (SS) rats maintained on low salt (LS) diet (0.4% NaCl) for 9 to 10 weeks. MCA from SS rats on LS diet (n=6 to 9) constricted in response to reduction of perfusate and superfusate PO2 to 35 to 40 mm Hg or acetylcholine (ACh). Vasodilator responses to reduced PO2 and ACh were restored in SS.13BN consomic rats that are 98% genetically identical to SS rats, but exhibit normal regulation of their renin-angiotensin system (RAS). This restored dilation could be prevented by feeding SS.13BN rats high-salt (HS) diet (4% NaCl) for 3 days to suppress Ang II. A continuous intravenous infusion of a subpressor dose (3 ng/kg per minute) of Ang II for 3 days restored vasodilator responses to ACh and reduced PO2 in SS.13BN rats on HS diet and in SS rats on LS diet. Superoxide scavenging with tempol (100 micromol/L) restored vasodilator responses to ACh and reduced PO2 in MCA of SS rats on LS diet, but did not affect vasodilator responses in MCA of SS.13BN rats on LS diet. These data indicate that exposure to chronically low Ang II levels leads to impaired vascular relaxation in SS rats, even when the animals are on LS diet and normotensive. This impaired relaxation appears to be mediated by increased levels of oxidative stress in the arteries.

Function of the isolated perfused kidney in young and adult spontaneously hypertensive and dahl salt-sensitive rats.

The aim of the study was to find a possible explanation for the development of hypertension in two genetically hypertensive rat strains. Isolated kidneys from spontaneously hypertensive rats (SHR) and Dahl salt-sensitive rats (Dahl-S) kept on a low-salt diet were perfused at normotensive pressure (110 mm Hg). In adult (12-week-old) rats, no significant change in either parameter of kidney function was found in the two strains. In contrast, substantial changes were observed in both strains at the age of 6 weeks. Glomerular filtration rate and filtration fraction were similarly reduced both in SHR and Dahl-S rats. In SHR, the amount of sodium excreted was reduced in parallel to the GFR decrease. Sodium excretion was lower in Dahl-S than in Dahl-R due to the former higher Na(+) tubular reabsorption. The same was true for water excretion and its tubular reabsorption in this strain. In conclusion, the decreased amount of sodium excreted found in young SHR and Dahl-S might contribute to the development of hypertension in these strains. However, the reasons for this reduction are different in these two strains. In SHR, it results from decreased GFR whereas, in Dahl-S it is mainly due to increased tubular reabsorption.

Body fluid expansion is not essential for salt-induced hypertension in SS/Jr rats.

To evaluate the importance of volume in the development of hypertension in inbred Dahl salt-sensitive rats (SS/Jr), we measured the changes in blood pressure (BP) that occurred with oral intake of food (salt) and water in rats whose body weight was permitted to increase versus those in which body weight was maintained constant with a servo-control system. We hypothesized that if volume expansion is essential in the development of hypertension, then BP would not increase if body weight was held constant. We found that oral presentation of chow containing 4% salt to SS/Jr rats caused BP to increase 32.2 +/- 2.9 mmHg over 4 days when body weight was controlled at its initial value. Plasma sodium increased from 142.0 to 145.2 meq/l during 4 days of high salt. Neither plasma volume, hematocrit, nor central venous pressure changed significantly on the high-salt diet. In contrast, the inbred Dahl salt-resistant rats (SR/Jr) did not increase their BP during body weight control when given 4% salt. This demonstrates that volume expansion is not an obligatory step in the pressure response to increased salt in SS/Jr rats. Our results obtained with oral presentation of salt, in contrast to intravenous, represent a physiological evaluation of the significance of volume changes in response to dietary salt because no potential regulatory reflexes have been bypassed.