High-resolution mapping of the blood pressure QTL on chromosome 7 using Dahl rat congenic strains

It was previously shown using Dahl salt-sensitive (S) and salt-resistant (R) rats that a blood pressure quantitative trait locus (QTL) was present on rat chromosome 7. In the present work, this QTL was localized to a region less than 0.54 cM in size on the linkage map using a series of congenic strains. This region was contained in a single yeast artificial chromosome that was 220 kb long. This small segment still contained the primary candidate locus Cyp11b1 (11beta-hydroxylase), but the adjacent candidate genes Cyp11b2 (aldosterone synthase) and Cyp11b3 were ruled out. It is concluded that 11beta-hydroxylase, through its known genetic variants altering the production of 18-hydroxy-11-deoxy corticosterone, is very likely to account for the blood pressure QTL on chromosome 7 in the Dahl rat model of hypertension. This QTL accounts for about 23 mm Hg under the condition of 2% NaCl diet for 24 days.     

Alpha1 Na,K-ATPase and Na,K,2Cl-cotransporte/D3mit3 loci interact to increase susceptibility to salt-sensitive hypertension in Dahl S(HSD) rats

BACKGROUND: Essential (multigenic) hypertension is a complex multifactorial disease whose genetic etiology has not been unraveled on a major locus-effect investigative paradigm. As with other complex genetic diseases, applying an interacting loci paradigm could be critical in the elucidation of genetic determinants. Having defined the alpha1 Na,K-ATPase (alpha1NK) as a hypertension susceptibility gene in Dahl salt-sensitive (Dahl S) rats, we determined whether alphaINK interacts with another renal epithelial Na transporter to increase susceptibility to salt-sensitive hypertension. We focused on alpha1NK and Na,K,2Cl-cotransporter (NKC) as an a priori candidate interacting gene pair because they comprise a functionally linked Na transport system in renal thick ascending limb of Henle (TALH) epithelial cells and exhibit altered function in prehypertensive Dahl S rats in contrast to Dahl salt-resistant normotensive (Dahl R) rats. MATERIAL AND METHOD: Cosegregation analysis of alphaNK and NKC loci was done in a (Dahl S x Dahl R) F2 cohort characterized for blood pressure by radiotelemetry using the D2mghII microsatellite marker in the alpha1NK gene and the D3mit3 microsatellite marker close to the NKC gene (NKC/D3mit3 locus). Single locus and digenic analyses were performed to establish the individual and interactive genetic contribution to salt-sensitive hypertension. Molecular analysis was then done to support the NKC gene as the likely candidate gene interacting with alpha1NK in Dahl salt-sensitive hypertension pathogenesis. RESULTS: Compared with respective single locus analysis, digenic analysis of 96 F2 (Dahl S x Dahl R) hybrid male rats revealed cosegregation of alpha1NK and NKC/D3mit3 loci as interacting pair with salt-sensitive hypertension with markedly increased significance for systolic (one-way ANOVA p = 10(-6)), diastolic (p = 10(-5)), and mean arterial (p = 10(-6)) blood pressures. Concordantly, two-way ANOVA detected interaction between alpha1NK and NKC loci in determining the levels of systolic (p = 0.004), diastolic (p = 0.008), and mean arterial (p = 0.006) pressures. To unravel potential NKC molecular dysfunction(s) involved in hypertension pathogenesis, we investigated putative differences between Dahl S and Dahl R rats in nucleotide sequence and isoform gene expression of the renal-specific Na,K,2Cl-cotransporter. Molecular analysis revealed an inversion of alternatively spliced NKC-isoform ratios (4B:4A:4F) between Dahl S and Dahl R prehypertensive kidneys supported by four mutations in intron-3 immediately upstream to alternatively spliced exons 4B, 4A, and 4F. No nucleotide changes were detected within the aminoacid encoding exons of NKC. CONCLUSIONS: Altogether, these current data and previous characterization of the role of the Q276L alpha1NK molecular variant in Dahl S hypertension provide cumulative compelling evidence that alpha1NK and NKC/D3mit3 loci interact to increase susceptibility to hypertension in Dahl S rats and that NKC is the likely candidate gene that interacts with alpha 1NK. More importantly, the data substantiate gene interaction as an operative mechanism in multigenic hypertension.     

Elevated atrial natriuretic polypeptide in plasma of hypertensive Dahl salt-sensitive rats

The relationship between circulating atrial natriuretic polypeptide (ANP) and blood pressure was studied in inbred Dahl salt-sensitive (S) and inbred Dahl salt-resistant (R) rats. Two month old S and R rats raised on normal rat chow had only small differences in blood pressure and no difference in plasma ANP levels. In contrast, when 6-month-old rats also raised on normal chow were studied, S had markedly elevated blood pressure and a 4 fold increase in plasma ANP compared to R. Similar strain differences in blood pressure and plasma ANP could be induced in young rats by feeding them diets high in salt. In six week old S and R rats which had been fed high salt diet for 3 weeks the S rats showed higher blood pressure and plasma ANP than R rats. The high plasma ANP levels seen in the hypertensive S rats were interpreted to be a response to hypertension and not a cause of hypertension. There was no qualitative strain difference in the plasma ANP molecule as assessed by reverse phase high pressure liquid chromatography.     

Differential genetic basis for pre-menopausal and post-menopausal salt-sensitive hypertension

Essential hypertension affects 75% of post-menopausal women in the United States causing greater cardiovascular complications compared with age-matched men and pre-menopausal women. Hormone replacement and current anti-hypertensive therapies do not correct this post-menopausal increased risk suggesting a distinct pathogenic framework. We investigated the hypothesis that distinct genetic determinants might underlie susceptibility to salt sensitive hypertension in pre-menopausal and post-menopausal states. To determine whether distinct genetic loci contribute to post-menopausal salt-sensitive hypertension, we performed a genome-wide scan for quantitative trait loci (QTLs) affecting blood pressure (BP) in 16-month old post-menopausal F2 (Dahl S×R)-intercross female rats characterized for blood pressure by radiotelemetry. Given identical environments and high salt challenge, post-menopausal BP levels were significantly higher than observed in pre-menopausal (post-menopausal versus pre-menopausal SBP, P<0.0001) and ovariectomized (post-menopausal versus ovariectomized SBP, P<0.001) F2-intercross female rats. We detected four significant to highly significant BP-QTLs (BP-pm1 on chromosome 13, LOD 3.78; BP-pm2 on chromosome 11, LOD 2.76; BP-pm3 on chromosome 2, LOD 2.61; BP-pm4 on chromosome 4, LOD 2.50) and two suggestive BP-QTLs (BP-pm5 on chromosome 15, LOD 2.37; BP-f1 on chromosome 5, LOD 1.65), four of which (BP-pm2, BP-pm3, BP-pm4, BP-pm5) were unique to this post-menopausal cohort. These data demonstrate distinct polygenic susceptibility underlying post-menopausal salt-sensitive hypertension providing a pathway towards the identification of mechanism-based therapy for post-menopausal hypertension and ensuing target-organ complications.     

Localization of a blood pressure QTL to a 2.4-cM interval on rat chromosome 9 using congenic strains

A blood pressure (BP) quantitative trait locus (QTL) was previously found on rat chromosome 9 using Dahl salt-sensitive (S) and Dahl salt-resistant (R) rats. A congenic strain, S.R(chr9), constructed by introgressing an R chromosomal segment into the S background, previously proved the existence of a BP QTL in a large 34.2-cM segment of chromosome 9. In the current work congenic substrains were constructed from the progenitor congenic strain, S.R(chr9). BP and heart weight comparisons between these congenic substrains and their S control localized the BP QTL to a 4.6-cM interval. Two solute carrier (Na(+)/H(+) exchanger) genes, Nhe2 and Nhe4, were excluded as candidates based on their map locations. A second iteration of congenic substrains was used to localize the QTL further to a 2.4-cM interval. Another solute carrier (Cl(-)/HCO3- exchanger) gene, Ae3, is in this reduced interval and was sequenced for both S and R strains, but no coding sequence variations were found. Ae3 mRNA was not differentially expressed in the kidney of congenic compared to S rats. Although the identity of the QTL remains unknown its map location has been reduced from an interval of 34.2 to 2.4 cM.     

Defining the blood pressure QTL on Chromosome 7 in Dahl rats by a 177-kb congenic segment containing Cyp11b1

Previously we reported that there is a blood pressure quantitative trait locus (QTL) on rat Chromosome (Chr) 7 seen when comparing Dahl salt-sensitive (S) rats and Dahl salt-resistant (R) rats. Evidence was also presented that this QTL was due to genetic variants in the adrenal steroidogenic enzyme 11beta-hydroxylase ( Cyp11b1). A series of congenic strains supported this contention. In the present work we have constructed a final congenic substrain that retains a blood pressure effect and that has an introgressed congenic segment which includes Cyp11b1 and is < 177 kb in size. None of the other genes in the congenic region (eight known genes) have known biological functions for influencing blood pressure. We believe that we have reached the limit of resolution for congenic analysis of a QTL in a rodent animal model, and we conclude that Cyp11b1 causes the observed QTL on rat Chr 7 in Dahl rats.     

Renal medullary endothelin-1 is decreased in Dahl salt-sensitive rats

Although it is well established that the renal endothelin (ET-1) system plays an important role in regulating sodium excretion and blood pressure through activation of renal medullary ET(B) receptors, the role of this system in Dahl salt-sensitive (DS) hypertension is unclear. The purpose of this study was to determine whether the DS rat has abnormalities in the renal medullary endothelin system when maintained on a high sodium intake. The data indicate that Dahl salt-resistant rats (DR) on a high-salt diet had a six-fold higher urinary endothelin excretion than in the DR rats with low Na(+) intake (17.8 ± 4 pg/day vs. 112 ± 44 pg/day). In sharp contrast, urinary endothelin levels increased only twofold in DS rats in response to a high Na(+) intake (13 ± 2 pg/day vs. 29.8 ± 5.5 pg/day). Medullary endothelin concentration in DS rats on a high-Na(+) diet was also significantly lower than DR rats on a high-Na(+) diet (31 ± 2.8 pg/mg vs. 70.9 ± 5 pg/mg). Furthermore, DS rats had a significant reduction in medullary ET(B) receptor expression compared with DR rats while on a high-Na(+) diet. Finally, chronic infusion of ET-1 directly into the renal medulla blunted Dahl salt-sensitive hypertension. These data indicate that a decrease in medullary production of ET-1 in the DS rat could play an important role in the development of salt-sensitive hypertension observed in the DS rat.     

Mapping a blood pressure quantitative trait locus to a 5.7-cM region in Dahl salt-sensitive rats

A region on rat Chromosome (Chr) 2 of the Dahl salt-sensitive rat (S) was shown previously to contain a quantitative trait locus (QTL) for blood pressure (BP). This was achieved first by linkage, followed by the use of congenic strains. A congenic strain, designated S.MNS-D2Mit6/Adh, contained a segment of Chr 2 from the Milan Normotensive (MNS) rat in the S genetic background. Since the region containing the QTL was roughly 80 cM in size, a further reduction was needed towards the positional or candidate gene cloning. Currently, two congenic substrains were made from the original strain S.MNS-D2Mit6/Adh. One of these two substrains showed a BP-lowering effect, whereas the other substrain did not. Deducing the segment not shared in the two substrains, the BP QTL has to be present in a chromosome region of roughly 5.7 cM between the marker D2Rat303 and the locus for the neutroendopeptidase gene (Nep). Nep is not included within the segment. This region does not seem to contain any candidate genes well known for the BP control. Thus, the final identification of the QTL will most likely lead to the discovery of a brand new gene for the BP regulation. Received: 14 December 2000 / Accepted: 18 January 2001     

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

ABSTRACT: 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 resistance 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.     

The response of Dahl salt-sensitive and salt-resistant female rats to a space flight model

Vitamin D metabolism in the Dahl salt-sensitive (S) rat, a model of salt-induced hypertension, differs from that in the Dahl salt-resistant (R) rat. We have tested the hypothesis that differences in vitamin D metabolism would render the Dahl S rat more susceptible than the Dahl R rat to the effects of a space flight model. Dahl female rats were tail suspended (hind limb unloaded) for 28 days, while fed a low salt (3 g/kg sodium chloride) diet. Plasma 25-OHD concentrations of S rats were significantly lower than that of R rats. Plasma 1,25-(OH)2D concentration was 50% lower in unloaded than in loaded S rats, but was unaffected in unloaded R rats. The left soleus muscle weight and breaking strength of the left femur (torsion test) were 50% and 25% lower in unloaded than in loaded S and R rats. The mineral content of the left femur, however, was significantly lower (by 11%) only in unloaded S rats. We conclude that female S rats are more vulnerable than female R rats to decreases in plasma 1,25-(OH)2D concentration and femur mineral content during hind limb unloading, but equally vulnerable to muscle atrophy and reduced breaking strength of the femur.     

Mechanisms of salt-sensitive hypertension: role of inducible nitric oxide synthase

The goal of this study was to determine the role of inducible nitric oxide synthase (iNOS) in the arterial pressure, renal hemodynamic, renal excretory, and hormonal changes that occur in Dahl/Rapp salt-resistant (R) and salt-sensitive (S) rats during changes in Na intake. Thirty-two R and S rats, equipped with indwelling arterial and venous catheters, were subjected to low (0.87 mmol/day) or high (20.6 mmol/day) Na intake, and selective iNOS inhibition was achieved with intravenous aminoguanidine (AG, 12.3 mg. kg(-1). h(-1)). After 5 days of AG, mean arterial pressure increased to 121 +/- 3% control in the R-high Na AG rats compared with 98 +/- 1% control (P < 0.05) in the R-high Na alone rats, and S-high Na rats increased their arterial pressure to 123 +/- 3% control compared with 110 +/- 2% control (P < 0.05) in S-high Na alone rats. AG caused no significant changes in renal hemodynamics, urinary Na or H(2)O excretion, plasma renin activity, or cerebellar Ca-dependent NOS activity. The data suggest that nitric oxide produced by iNOS normally helps to prevent salt-sensitive hypertension in the Dahl R rat and decreases salt sensitivity in the Dahl S rat.     

Interval mapping and congenic strains for a blood pressure QTL on rat Chromosome 13

The renin locus (Ren) on rat Chromosome (Chr) 13 had previously been shown to cosegregate with blood pressure in crosses involving Dahl salt-sensitive (S) and Dahl salt-resistant (R) rats. In the present work, interval mapping of blood pressure on Chr 13 with a large F₂ (S × R), n = 233, population yielded a maximum LOD = 4.2 for linkage to blood pressure, but the quantitative trait locus (QTL) was only poorly localized to a large 35-centiMorgan (cM) segment of Chr 13. In the linkage analysis, the S-rat QTL allele (S) was associated with higher, and the R-rat QTL allele (R) with lower blood pressure, the difference between homozygotes being about 20 mm Hg. A congenic strain was made by introgressing the R-rat Ren allele into the recipient S strain. This congenic strain showed a 24 mm Hg reduction (P = 0.004) in blood pressure compared with S rats for rats fed 2% NaCl diet for 24 days; this difference was confirmed by two other independent tests. Two congenic substrains were derived from the first congenic strain with shorter R Chr 13 segments on the S background. Comparisons among these congenic strains showed that a blood pressure QTL was in the 24-cM chromosomal segment between Syt2 and D13M1Mit108. This segment does not include the renin locus, which is thus excluded from being the gene on rat Chr 13 responsible for genetic differences in blood pressure detected by linkage analysis. Received: 20 December 1996 / Accepted: 7 April 1997     

Flavonoid-induced reduction of ENaC expression in the kidney of Dahl salt-sensitive hypertensive rat

Flavonoid, a plant extract, exhibits various biological actions. Dietary flavonoid intake is reported to reduce an elevated blood pressure, however the mechanism is unknown. The epithelial Na+ channel (ENaC) in the kidney plays a key role in the regulation of blood pressure by contributing to the Na+ reabsorption in renal tubules. Thus, we investigated the effect of quercetin, a flavonoid, on ENaC mRNA expression in the kidney of hypertensive Dahl salt-sensitive rats. Dahl salt-sensitive rats of 8 weeks were acclimated for 1 week in a metabolic cage and were subsequently kept for 4 weeks under four different conditions: (1) normal salt diet (0.3% NaCl), (2) normal salt diet with quercetin (10 mg/kg/day), (3) high-salt diet (8% NaCl), and (4) high-salt diet with quercetin. Quercetin diminished the alphaENaC mRNA expression in the kidney associated with reduction of the systolic blood pressure elevated by high-salt diet, suggesting that one of the mechanisms of the flavonoid's antihypertensive effect on salt-sensitive hypertension would be mediated through downregulation of ENaC expression in the kidney.     

Enhanced sympathoexcitatory and pressor responses to central Na+ in Dahl salt-sensitive vs. -resistant rats

An enhanced responsiveness to increases in cerebrospinal fluid (CSF) Na+ by high salt intake may contribute to salt-sensitive hypertension in Dahl salt-sensitive (S) rats. To test this hypothesis, sympathetic and pressor responses to acute and chronic increases in CSF Na+ were evaluated. In conscious young (5-6 wk old) and adult (10-11 wk old) Dahl S and salt-resistant (R) rats as well as weight-matched Wistar rats, hemodynamic [blood pressure (BP) and heart rate (HR)] and sympathetic [renal sympathetic nerve activity (RSNA)] responses to 10-min intracerebroventricular infusions of artificial CSF (aCSF) and Na+-rich aCSF (containing 0.2-0.45 M Na+) were evaluated. Intracerebroventricular Na+-rich aCSF increased BP, RSNA, and HR in a dose-related manner. The extent of these increases was significantly larger in Dahl S versus Dahl R or Wistar rats and young versus adult Dahl S rats. In a second set of experiments, young Dahl S and R rats received a chronic intracerebroventricular infusion of aCSF or Na+-rich (0.8 M) aCSF (5 microl/h) for 14 days, with the use of osmotic minipumps. On day 14 in conscious rats, CSF was sampled and BP, HR, and RSNA were recorded at rest and in response to air stress, intracerebroventricular alpha2-adrenoceptor agonist guanabenz, intracerebroventricular ouabain, and intravenous phenylephrine and nitroprusside to estimate baroreflex function. The infusion of Na+-rich aCSF versus aCSF increased CSF Na+ concentration to the same extent but caused severe versus mild hypertension in Dahl S and Dahl R rats, respectively. After central Na+ loading, hypothalamus "ouabain" significantly increased in Dahl S and only tended to increase in Dahl R rats. Moreover, sympathoexcitatory and pressor responses to intracerebroventricular exogenous ouabain were attenuated by Na+-rich aCSF to a greater extent in Dahl S versus Dahl R rats. Responses to air-jet stress or intracerebroventricular guanabenz were enhanced by Na+-rich aCSF in both strains, but the extent of enhancement was significantly larger in Dahl S versus Dahl R. Na+-rich aCSF impaired arterial baroreflex control of RSNA more markedly in Dahl S versus R rats. These findings indicate that genetic control of mechanisms linking CSF Na+ with brain "ouabain" is altered in Dahl S rats toward sympathetic hyperactivity and hypertension.     

Complete genome searches for quantitative trait loci controlling blood pressure and related traits in four segregating populations derived from Dahl hypertensive rats

The Dahl salt-sensitive rat is one of the principal animal models of hereditary hypertension. Genome-wide searches were undertaken to detect quantitative trait loci (QTLs) that influence blood pressure, cardiac mass, and body weight in four F₂ populations derived from Dahl salt-sensitive rats and different inbred normotensive control strains of rat. We detected three QTLs associated with one or more of the phenotypes, using a stringent statistical criterion for linkage (p < 0.00003). These included a novel QTL linked to blood pressure on rat Chromosome (Chr) 12, and another QTL on rat Chr 3 linked to body weight. A QTL on rat Chr 10 for which linkage to blood pressure has been described in other crosses was found to be a principal determinant of blood pressure and cardiac mass in some but not all of the crosses examined here. Three other regions showed evidence of linkage to these phenotypes with a less stringent statistical criterion of linkage at QTLs previously reported in other studies. As part of our study, microsatellite markers have been developed for three candidate genes for investigation in hypertension, and the genes have been localized by linkage mapping. These are: the rat Gs alpha subunit (Gnas) gene, the alpha-1B adrenergic receptor (Adra1b), and the Na⁺, K⁺-ATPase beta2 subunit (Atp1b2) gene. Received: 29 June 1998 / Accepted: 30 October 1998     

Aldosterone-induced abnormal regulation of ENaC and SGK1 in Dahl salt-sensitive rat

Aldosterone plays a crucial role in controlling mineral balance in our body. The mechanism of aldosterone has been reported to elevate renal Na+ reabsorption by stimulating expression of epithelial Na+ channel (ENaC) and also activate an ENaC-regulating protein kinase, serum and glucocorticoid-regulated kinase 1 (SGK1). However, it is unknown whether aldosterone shows its stimulatory action on ENaC and SGK1 under an abnormal, salt-sensitive hypertensive condition. To clarify this point, we studied how aldosterone regulates expression of ENaC and SGK1 in Dahl salt-sensitive (DS) rat that shows hypertension with high salt diet. RNA and protein were extracted from the kidney 6 h after application of aldosterone (1.5 mg/kg body weight) subcutaneously injected into adrenalectomized DS and Dahl salt-resistant (DR) rats. Aldosterone decreased mRNA expression of beta- and gamma-ENaC in DS rat unlike DR rat, while aldosterone increased alpha-ENaC mRNA expression in DS rat similar to DR rat. Further, we found that aldosterone elevated SGK1 expression in DR rat, but not in DS rat. These observations indicate that ENaC and SGK1 are abnormally regulated by aldosterone in salt-sensitive hypertensive rats, suggesting that disturbance of the aldosterone regulation would be one of factors causing salt-sensitive hypertension.     

Effect of 3beta-hydroxysteroid dehydrogenase inhibition by trilostane on blood pressure in the Dahl salt-sensitive rat

The brains of rats and humans express the enzymes required for the synthesis of aldosterone from cholesterol, including the 3beta-steroid dehydrogenase that catalyzes the conversion of pregnenolone to progesterone in the pathway of adrenal steroid synthesis. Salt-induced hypertension in the Dahl inbred salt-sensitive (SS/jr) rat is associated with normal to low levels of circulating aldosterone, yet it is abrogated by the central infusion of mineralocorticoid receptor antagonists. To test the hypothesis that de novo synthesis of aldosterone in the brain has a pathophysiological role in the salt-induced hypertension of the SS rat, the 3beta-steroid dehydrogenase antagonist trilostane was infused continuously intracerebroventricularly or subcutaneously in two different cohorts of Dahl SS/jr rats, one female, the other male, during and after the development of salt-induced hypertension. The doses of trilostane used had no effect on blood pressure when infused subcutaneously. Animals receiving vehicle intracerebroventricularly experienced a 30- to 45-mmHg increase in systolic blood pressure measured by tail cuff. The intracerebroventricular, but not subcutaneous, infusion of 0.3 microg/h trilostane effectively blocked the increase in systolic blood pressure and reversed the hypertension produced by drinking 0.9% saline. Trilostane was equally effective in female and male rats. Weight gain, serum aldosterone and corticosterone concentrations, and behavior assessed subjectively and by elevated plus maze were unchanged by the trilostane treatment. These studies suggest that the synthesis in the brain of a mineralocorticoid receptor agonist, probably aldosterone, is responsible in part for the salt-induced hypertension of the inbred Dahl SS/jr rat.     

Central infusion of aliskiren prevents sympathetic hyperactivity and hypertension in Dahl salt-sensitive rats on high salt intake

Central infusion of an angiotensin type 1 (AT(1)) receptor blocker prevents sympathetic hyperactivity and hypertension in Dahl salt-sensitive (S) rats on high salt. In the present study, we examined whether central infusion of a direct renin inhibitor exerts similar effects. Intracerebroventricular infusion of aliskiren at the rate of 0.05 mg/day markedly inhibited the increase in ANG II levels in the cerebrospinal fluid and in blood pressure (BP) caused by intracerebroventricular infusion of rat renin. In Dahl S rats on high salt, intracerebroventricular infusion of aliskiren at 0.05 and 0.25 mg/day for 2 wk similarly decreased resting BP in Dahl S rats on high salt. In other groups of Dahl S rats, high salt intake for 2 wk increased resting BP by ～25 mmHg, enhanced pressor and sympathoexcitatory responses to air-stress, and desensitized arterial baroreflex function. All of these effects were largely prevented by intracerebroventricular infusion of aliskiren at 0.05 mg/day. Aliskiren had no effects in rats on regular salt. Neither high salt nor aliskiren affected hypothalamic ANG II content. These results indicate that intracerebroventricular infusions of aliskiren and an AT(1) receptor blocker are similarly effective in preventing salt-induced sympathetic hyperactivity and hypertension in Dahl S rats, suggesting that renin in the brain plays an essential role in the salt-induced hypertension. The absence of an obvious increase in hypothalamic ANG II by high salt, or decrease in ANG II by aliskiren, suggests that tissue levels do not reflect renin-dependent ANG II production in sympathoexcitatory angiotensinergic neurons.     

Role of the adenosine(2A) receptor-epoxyeicosatrienoic acid pathway in the development of salt-sensitive hypertension

Activation of rat adenosine(2A) receptors (A(2A) R) dilates preglomerular microvessels, an effect mediated by epoxyeicosatrienoic acids (EETs). High salt (HS) intake increases epoxygenase activity and adenosine levels. A greater vasodilator response to a stable adenosine analog, 2-chloroadenosine (2-CA), was seen in kidneys obtained from HS-fed rats which was mediated by increased EET release. Because this pathway is antipressor, we examined the role of the A(2A) R-EET pathway in a genetic model of salt-sensitive hypertension, the Dahl salt-sensitive (SS) rats. Dahl salt resistant (SR) rats fed a HS diet demonstrated a greater renal vasodilator response to 2-CA. In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed normal salt (NS) or HS diet. In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A) R and cytochrome P450 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. In vivo inhibition of the A(2A) R-EET pathway in Dahl SR rats fed a HS diet results in reduced renal EETs levels, diminished natriuretic capacity and hypertension, thus supporting a role for the A(2A) R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading. An 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.