Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

Effects of high-fat diet on sympathetic neurotransmission in mesenteric arteries from Dahl salt-sensitive rat

Obesity hypertension is driven by sympathetic neurotransmission to the heart and blood vessels. We tested the hypothesis that high-fat diet (HFD)-induced hypertension is driven by sympathetic neurotransmission to mesenteric arteries (MA) in male but not female Dahl salt-sensitive (Dahl ss) rat. Rats were fed a control diet (CD; 10 kcal% from fat) or HFD (60 kcal% from fat) beginning at 3 weeks (wk) of age; measurements were made at 10-, 17- and 24-wk. Body weight increased with HFD, age and sex. Mean arterial pressure (MAP) was higher in HFD versus CD rats from both sexes at 17- and 24-wk. MA constriction measured using pressure myography, and electrical field stimulation (EFS, 0.2-30 Hz) was greater in HFD versus CD in males at 17-wk; this was not due to changes in α2 autoreceptor or norepinephrine transporter (NET) function. Prazosin (α1-AR antagonist) and suramin (P2 receptor antagonist) inhibited neurogenic MA constriction equally in all groups. Arterial reactivity to exogenous norepinephrine (NE; 10^-8 - 10^-5 M) was lower in HFD versus CD at 10-wk in males. Female MA reactivity to exogenous ATP was lower at 24-weeks compared to earlier time points. HFD did not affect tyrosine hydroxylase (TH) or the vesicular nucleotide transporter (VNUT) nerve density in MA from both sexes. NE content was lower in MA but higher in plasma at 24-wk compared to 10- and 17-wk in both sexes. In conclusion, HFD-induced hypertension is not driven by increased sympathetic neurotransmission to MA in male and female Dahl ss rats.

Effects of High Salt-Low Potassium Diet on Blood Pressure and Vascular Reactivity in Male Sprague Dawley Rats

Sodium (Na) intake increases vascular reactivity. Whether low potassium (K) intake affects vascular reactivity-associated blood pressure (BP) changes is uncertain. This study aimed to determine whether Na-induced increases in BP and vascular reactivity are altered by low K intake. Male Sprague Dawley rats were assigned to 3 dietary groups for 6 weeks: a standard Na-K diet (control, n = 12), a high Na-normal K diet (HS-NormK, n = 12), and a high Na-low K diet (HS-LowK, n = 12). BP was measured at baseline and after the dietary intervention. Na and K excretions and vascular reactivity were measured after the dietary intervention. The Na/K ratio was significantly higher in the HS-LowK compared with the other groups. Systolic and diastolic BPs increased significantly in the HS-NormK and HS-LowK groups. In mesenteric arteries, the dose-response curves for phenylephrine-induced contractions shifted to the left and the EC50 (mean ± SD) was significantly lower in the HS-NormK (0.51 ± 0.17 μM, P = 0.003) and HS-LowK (0.69 ± 0.14 μM, P = 0.005) groups compared with the control (3.24 ± 0.79 μM). Systolic (r = -0.58 P = 0.002) and diastolic (r = -0.61 P = 0.001) BPs were associated with the EC50 of phenylephrine-induced contraction in mesenteric arteries. High Na intake induces increased alpha-1 receptor responsiveness in mesenteric arteries, which may be responsible for the increase in BP and is not affected by low dietary K intake.

Increased activity of the orexin system in the paraventricular nucleus contributes to salt-sensitive hypertension

The orexin system is involved in arginine vasopressin (AVP) regulation, and its overactivation has been implicated in hypertension. However, its role in salt-sensitive hypertension (SSHTN) is unknown. Here, we tested the hypothesis that hyperactivity of the orexin system in the paraventricular nucleus (PVN) contributes to SSHTN via enhancing AVP signaling. Eight-week-old male Dahl salt-sensitive (Dahl S) and age- and sex-matched Sprague-Dawley (SD) rats were placed on a high-salt (HS; 8% NaCl) or normal-salt (NS; 0.4% NaCl) diet for 4 wk. HS intake did not alter mean arterial pressure (MAP), PVN mRNA levels of orexin receptor 1 (OX1R), or OX2R but slightly increased PVN AVP mRNA expression in SD rats. HS diet induced significant increases in MAP and PVN mRNA levels of OX1R, OX2R, and AVP in Dahl S rats. Intracerebroventricular infusion of orexin A (0.2 nmol) dramatically increased AVP mRNA levels and immunoreactivity in the PVN of SD rats. Incubation of cultured hypothalamus neurons from newborn SD rats with orexin A increased AVP mRNA expression, which was attenuated by OX1R blockade. In addition, increased cerebrospinal fluid Na⁺ concentration through intracerebroventricular infusion of NaCl solution (4 µmol) increased PVN OX1R and AVP mRNA levels and immunoreactivity in SD rats. Furthermore, bilateral PVN microinjection of the OX1R antagonist SB-408124 resulted in a greater reduction in MAP in HS intake (-16 ± 5 mmHg) compared with NS-fed (-4 ± 4 mmHg) anesthetized Dahl S rats. These results suggest that elevated PVN OX1R activation may contribute to SSHTN by enhancing AVP signaling.NEW & NOTEWORTHY To our best knowledge, this study is the first to investigate the involvement of the orexin system in salt-sensitive hypertension. Our results suggest that the orexin system may contribute to the Dahl model of salt-sensitive hypertension by enhancing vasopressin signaling in the hypothalamic paraventricular nucleus.

Brain Gαi 2 -subunit proteins and the prevention of salt sensitive hypertension

To counter the development of salt-sensitive hypertension, multiple brain G-protein-coupled receptor (GPCR) systems are activated to facilitate sympathoinhibition, sodium homeostasis, and normotension. Currently there is a paucity of knowledge regarding the role of down-stream GPCR-activated Gα-subunit proteins in these critically important physiological regulatory responses required for long-term blood pressure regulation. We have determined that brain Gαi₂-proteins mediate natriuretic and sympathoinhibitory responses produced by acute pharmacological (exogenous central nociceptin/orphanin FQ receptor (NOP) and α₂-adrenoceptor activation) and physiological challenges to sodium homeostasis (intravenous volume expansion and 1 M sodium load) in conscious Sprague–Dawley rats. We have demonstrated that in salt-resistant rat phenotypes, high dietary salt intake evokes site-specific up-regulation of hypothalamic paraventricular nucleus (PVN) Gαi₂-proteins. Further, we established that PVN Gαi₂ protein up-regulation prevents the development of renal nerve-dependent sympathetically mediated salt-sensitive hypertension in Sprague–Dawley and Dahl salt-resistant rats. Additionally, failure to up-regulate PVN Gαi₂ proteins during high salt-intake contributes to the pathophysiology of Dahl salt-sensitive (DSS) hypertension. Collectively, our data demonstrate that brain, and likely PVN specific, Gαi₂ protein pathways represent a central molecular pathway mediating sympathoinhibitory renal-nerve dependent responses evoked to maintain sodium homeostasis and a salt-resistant phenotype. Further, impairment of this endogenous “anti-hypertensive” mechanism contributes to the pathophysiology of salt-sensitive hypertension.

Brain heterotrimeric Gαi₂-subunit protein-gated pathways mediate central sympathoinhibition to maintain fluid and electrolyte homeostasis during stress

Fluid and electrolyte homeostasis is integral to blood pressure regulation. However, the central molecular mechanisms regulating the neural control of sodium excretion remain unclear. We have demonstrated that brain Gαi(2)-subunit protein pathways mediate the natriuretic response to α(2)-adrenoreceptor activation in vivo. Consequently, we examined the role of brain Gαi(2) proteins in the neural mechanisms facilitating fluid and electrolyte homeostasis in response to acute [i.v. volume expansion (VE)] or chronic stressful stimuli (dietary sodium restriction vs. supplementation) in conscious Sprague-Dawley rats. Selective oligodeoxynucleotide (ODN)-mediated down-regulation of brain Gαi(2) proteins, but not a scrambled ODN, abolished the renal sympathoinhibitory response and attenuated the natriuresis to VE. In scrambled ODN-treated rats, chronic changes in dietary sodium intake evoked an endogenous, hypothalamic paraventricular nucleus (PVN)-specific, decrease (sodium deficiency) or increase (sodium excess) in PVN Gαi(2) proteins; plasma norepinephrine levels were inversely related to dietary sodium content. Finally, in rats treated with an ODN to prevent high salt-induced up-regulation of brain Gαi(2) proteins, animals exhibited sodium retention, global sympathoexcitation, and elevated blood pressure. Collectively, these data demonstrate that PVN Gαi(2) protein pathways play an endogenous role in maintaining fluid and electrolyte balance by controlling the influence the sympathetic nervous system has on the renal handling of sodium.

Hypothalamic paraventricular nucleus G alpha q subunit protein pathways mediate vasopressin dysregulation and fluid retention in salt-sensitive rats

Central Gαz and Gαq protein-gated pathways play a pivotal role in modulating (inhibiting vs. stimulating, respectively) vasopressin release and urine output; these studies examined the role of brain Gαz/Gαq proteins in the regulation of vasopressin secretion during high-salt challenge. We examined the effects of 21-d normal or high salt intake on plasma vasopressin levels, daily sodium and water balance, and brain Gαz and Gαq protein levels in male Sprague-Dawley (SD), Dahl salt-resistant (DSR), and Dahl salt-sensitive (DSS) rats. Additionally, the effect of central Gαq protein down-regulation on these parameters and the diuretic response evoked by pharmacological [nociceptin/orphanin FQ; 5.5 nmol intracerebroventricularly (icv)] and physiological stimuli (isotonic-saline volume expansion, 5% bodyweight, iv) was examined. After 21 d of high salt intake, DSS, but not SD or DSR rats, exhibited vasopressin dysregulation, as evidenced by elevated plasma vasopressin levels (P < 0.05), marked positive water (and sodium) balance (P < 0.05), and an impaired diuretic response to pharmacological and physiological stimuli (P < 0.05). Chronic high salt intake (21 d) evoked down-regulation of Gαq (P < 0.05), but not Gαz, proteins in the hypothalamic paraventricular nucleus of SD and DSR, but not DSS rats. In salt-challenged (21 d) DSS rats, acute oligodeoxynucleotide-mediated down-regulation of central Gαq proteins returned plasma vasopressin to control levels (P < 0.05), decreased salt-induced water retention (P < 0.05), and restored the profound diuretic responses to pharmacological and physiological stimuli (P < 0.05). Therefore, the down-regulation of PVN Gαq proteins plays a critical counter-regulatory role in preventing vasopressin hypersecretion in salt-resistant phenotypes and may represent a new therapeutic target in pathophysiological states featuring vasopressin dysregulation.

Inhibition of the adenosine2A receptor-epoxyeicosatrienoic acid pathway renders Dahl salt-resistant rats hypertensive

Adenosine-induced renovasodilation in Dahl rats is mediated via activation of adenosine(2A) receptors (A(2A)Rs) and stimulation of epoxyeicosatrienoic acid (EET) synthesis. Unlike Dahl salt-resistant rats, salt-sensitive rats exhibit an inability to upregulate the A(2A)R-EET pathway with salt loading; therefore, we examined the effect of in vivo inhibition of the A(2A)R-EET pathway on blood pressure and the natriuretic response to salt-loading in Dahl salt-resistant rats. N-Methylsulfonyl-6-(2-propargyloxyphenyl)hexanamide (MS-PPOH; 20 mg/kg per day), an epoxygenase inhibitor, or ZM241385 (ZM; 5 mg/kg per day), an A(2A)R antagonist, was given daily as an IV bolus dose for 3 days before and after placing rats on high salt intake (2% saline). After 3 days of high salt, systolic blood pressure per 24 hours increased from 108+/-2 mm Hg to 136+/-5 mm Hg and 140+/-4 mm Hg when treated with MS-PPOH or ZM, respectively (P<0.001). Plasma levels of EETs and dihydroxyeicosatrienoic acids during salt loading and MS-PPOH (29.3+/-1.8 ng/mL) or ZM treatment (9.8+/-0.5 ng/mL) did not increase to the same extent as in vehicle-treated rats (59.4+/-1.7 ng/mL; P<0.001), and renal levels of EETs+dihydroxyeicosatrienoic acids were 2-fold lower with MS-PPOH or ZM treatment. On day 3 of the high salt intake, MS-PPOH- and ZM-treated rats exhibited a positive Na(+) balance, and plasma Na(+) levels were significantly increased (163.3+/-1.2 and 158.1+/-4.5 mEq/L, respectively) compared with vehicle-treated rats (142.1+/-1 mEq/L), reflecting a diminished natriuretic capacity. These data support a role for the A(2A)R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading.

Role of central nervous system aldosterone synthase and mineralocorticoid receptors in salt-induced hypertension in Dahl salt-sensitive rats

In Dahl salt-sensitive (S) rats, high salt intake increases cerebrospinal fluid (CSF) Na+ concentration ([Na+]) and blood pressure (BP). Intracerebroventricular (ICV) infusion of a mineralocorticoid receptor (MR) blocker prevents the hypertension. To assess the role of aldosterone locally produced in the brain, we evaluated the effects of chronic central blockade with the aldosterone synthase inhibitor FAD286 and the MR blocker spironolactone on changes in aldosterone and corticosterone content in the hypothalamus and the increase in CSF [Na+] and hypertension induced by high salt intake in Dahl S rats. After 4 wk of high salt intake, plasma aldosterone and corticosterone were not changed, but hypothalamic aldosterone increased by ∼35% and corticosterone tended to increase in Dahl S rats, whereas both steroids decreased by ∼65% in Dahl salt-resistant rats. In Dahl S rats fed the high-salt diet, ICV infusion of FAD286 or spironolactone did not affect the increase in CSF [Na+]. ICV infusion of FAD286 prevented the increase in hypothalamic aldosterone and 30 mmHg of the 50-mmHg BP increase induced by high salt intake. ICV infusion of spironolactone fully prevented the salt-induced hypertension. These results suggest that, in Dahl S rats, high salt intake increases aldosterone synthesis in the hypothalamus and aldosterone acts as the main MR agonist activating central pathways contributing to salt-induced hypertension.

Effects of enalapril, tempol, and eplerenone on salt-induced hypertension in dahl salt-sensitive rats

The renin-angiotensin-aldosterone system (RAAS) has been implicated in the pathophysiology of salt-induced hypertension. Angiotensin converting enzyme inhibitors, angiotensin II-type 1 receptor blockers, and aldosterone receptor blockers are used to treat hypertension and congestive heart disease. In addition to their blood pressure lowering effects, they appear to protect against myocardial, renal, and vascular damage. In various models of hypertension, generation of reactive oxygen species is increased in the vasculature and that treatment with antioxidants or superoxide dismutase mimetics (e.g., tempol) improves vascular function and structure and reduces blood pressure. The purpose of this study was to examine the effects of enalapril, an angiotensin II converting enzyme inhibitor; eplerenone, a selective aldosterone receptor antagonist; and tempol, a superoxide dismutase mimetic, on salt-induced hypertension in Dahl Salt-Sensitive rats. The rats were placed on a high salt (HS; 8%) diet for 3 weeks prior to switching to a normal salt (0.3%) diet for an additional 3 weeks. While on the normal salt (NS) diet, rats were treated with enalapril (30 mg/kg/day in the drinking water), eplerenone (100 mg/kg/day by gavage), tempol (1 mM/day in the drinking water), eplerenone + enalapril, eplerenone + enalapril + tempol, or without drug treatment (control). After 3 weeks on HS diet, systolic blood pressure rose from 127 +/- 7 to 206 +/- 11 mm Hg and remained elevated when switched to NS diet. Subsequently, treatment with eplerenone alone or in combination with enalapril and tempol produced a stepwise reduction in systolic blood pressure reaching -80 mm Hg; however, enalapril and tempol alone produced more modest pressure reduction (approximately -35 mmHg). Plasma levels of prostacyclin and nitric oxide were elevated in rats treated with enalapril and eplerenone alone or in combination. Enalapril and eplerenone alone and in combination reduced heart and kidney levels of angiotensin II and aldosterone when compared with control. Renal and heart levels of reduced glutathione were diminished by eplerenone alone; however, enalapril tended to attenuate the effect of eplerenone on reduced glutathione levels in the heart. The findings from this study suggest that eplerenone reduces salt-induced hypertension by increasing endothelium-derived relaxing factors, inhibiting RAAS components and oxidative stress. (353words).

Alterations in aldosterone and angiotensin II levels in salt-induced hypertension

Several studies have demonstrated that plasma renin-angiotensin activity is reduced in rats administered a high salt diet. We evaluated changes in plasma and tissue levels of aldosterone (ALDO) and angiotensin II (A-II), as well as the reduced-to-oxidized glutathione ratio. Male Dahl salt-sensitive (SS) rats were placed on either a high-salt (8% NaCl; HS) or a normal-salt (0.3% NaCl; NS) diet for 3 weeks. Prior to and weekly on the diets, systolic blood pressure was measured by tail cuff plethysmography. Levels of A-II and ALDO in plasma, heart, and kidney were analyzed by enzyme immunoassay. Reduced and oxidized gluthatione were simultaneously measured by HPLC fluorescence detection. Heart and kidney tissues were prepared for histological analysis. Systolic blood pressure in animals on a HS diet was significantly elevated above that of those on a NS diet. High salt caused a reduction in both plasma A-II and ALDO levels; while their levels in the heart and kidney were increased. Exposure to a high-salt diet led to the enlargement of both heart and kidney. The reduced-to-oxidized glutathione ratio in plasma, heart and kidney was lowered by exposure to a HS diet. Kidneys from animals on a high-salt diet showed fibroid necrosis associated with wrinkling and thickening of the glomerular capillary wall, while hearts were hypertrophic. Taken together, high dietary salt induces inappropriate activation of the local renin-angiotensin-aldosterone systems. Tissue levels of angiotensin II and aldosterone may be more reflective of the severity of vascular maladaptations than are plasma levels, and may play a greater role in the maintenance of hypertension.

Effects of dietary fat, NaCl, and fructose on renal sodium and water transporter abundances and systemic blood pressure

Dietary fructose, NaCl, and/or saturated fat have been correlated with mean arterial pressure (MAP) rises in sensitive strains of rats. Dysregulation of sodium and/or water reabsorption by the kidney may contribute. Using radiotelemetry and parallel semiquantitative immunoblotting, we examined the effects of various diets on MAP and the regulation of abundance of the major renal sodium and water transport proteins in male Sprague-Dawley rats. In study 1, rats (∼275 g) were fed one of four diets for 4 wk ( n = 6/group): 1) control, 2) 65% fructose, 3) control + added NaCl (2.59%), or 4) fructose + NaCl. In study 2, 5% butter (fat) was added to the above four diets. Both fat and NaCl, but not fructose, caused modest rises in MAP (5–10 mmHg) and increased the day-to-night ratio in diastolic blood pressure. NaCl or fructose increased kidney size. Creatinine clearance was increased by salt or fat, and fractional excretion of sodium was decreased by fat. In study 1, high NaCl markedly reduced plasma renin and aldosterone and its regulated proteins in whole kidney, i.e., the thiazide-sensitive Na-Cl cotransporter and the α- and γ (70-kDa band)-subunits of the epithelial sodium channel. These effects were blunted by fat. Fructose increased the abundance of the sodium phosphate cotransporter, whereas it decreased the bumetanide-sensitive Na-K-2Cl cotransporter and aquaporin-2. Overall, doubling of dietary fat appeared to impair dietary sodium adaptation, i.e., blunt the downregulation of aldosterone-mediated effects, thus allowing blood pressure to rise at an accelerated rate.

Altered cardiovascular regulation in arginine vasopressin-overexpressing transgenic rat

Although arginine vasopressin (AVP), an antidiuretic hormone, has been widely acknowledged to play an important role in cardiovascular regulation via V1a receptors (V1aR), its precise significance remains unclear. In this study, we investigated the effects of long-standing high plasma AVP status on cardiovascular regulation in the AVP-overexpressing transgenic (Tg) rat. Adult male homozygous Tg rats were compared with age-matched normal Sprague-Dawley rats as controls. There were no significant differences in mean arterial blood pressure (BP; MABP) or heart rate between Tg and control rats in the basal state. Subcutaneous injection of AVP significantly increased MABP in controls but did not cause any apparent increase in MABP in Tg rats. BP recovery from hemorrhage-induced hypotension was significantly delayed in Tg compared with control rats. Pretreatment with a selective V1aR antagonist, OPC-21268, which is thought to restore the downregulation of V1aR, markedly improved both of these impaired responses. Northern blot analysis confirmed that decreased expression of V1aR mRNA and pretreatment with V1aR antagonist significantly restored the downregulation of V1aR mRNA. These results suggest that the Tg rat has decreased sensitivity to the hypertensive effect of AVP due to downregulation of V1aR, which may function as an adaptive mechanism to maintain normal BP against chronic hypervasopressinemia. In addition, impaired restoration of BP after hemorrhage-induced hypotension in Tg rats supports a physiological role of AVP in cardiovascular regulation.

Participation of renal and circulating endothelin in salt-sensitive essential hypertension

Salt sensitivity of blood pressure is a cardiovascular risk factor, independent of and in addition to hypertension. In essential hypertension, a conglomerate of clinical and biochemical characteristics defines a salt-sensitive phenotype. Despite extensive research on multiple natriuretic and antinatriuretic systems, there is no definitive answer yet about the major causes of salt-sensitivity, probably reflecting the complexity of salt-balance regulation. The endothelins, ubiquitous peptides first described as potent vasoconstrictors, also have vasodilator, natriuretic and antinatriuretic actions, depending on their site of generation and binding to different receptors. We review the available data on endothelin in salt-sensitive essential hypertension and conclude that abnormalities of renal endothelin may play a primary role. More importantly, the salt-sensitive patient may have blood pressure-dependency on endothelin in all states of salt balance, thus predicting that endothelin receptor blockers will have a major therapeutic role in salt-sensitive essential hypertension.

1A-779 attenuates angiotensin-(1-7) depressor response in salt-induced hypertensive rats

Chronic infusion of angiotensin-(1-7) [Ang-(1-7)] lowers blood pressure in salt-induced and spontaneously hypertensive (SHR) rats. In the present study, we have examined the acute effect of Ang-(1-7) in salt-induced hypertension using Dahl salt-sensitive rats placed on low (0.3%) or high (8.0% NaCl) salt diets for 2 weeks. Rats fed a high salt diet showed a greater rise in BP than those fed a low salt diet. Ang-(1-7) (24 microg/kg) reduced mean arterial pressure (MAP), enhanced the release of prostacyclin and nitric oxide, and suppressed thromboxane A(2) levels. A-779 (48 microg/kg, i.v), a selective Ang-(1-7) antagonist, partially blocked these effects of Ang-(1-7). The Ang-(1-7)-induced depressor response observed in these animals was related to an increase in vasodilatory prostanoids, a decrease in the constrictor prostanoid thromboxane A(2), and an increase in nitric oxide levels in both plasma and isolated aortic rings.

Exaggerated natriuresis after selective AT1 receptor blockade in Dahl salt-sensitive rats

Salt-sensitive individuals are susceptible to develop hypertension when exposed to high salt-diet. Such a phenomenon is considered to be due to a genetic impairment in the renal excretion of sodium. In the present studies extent of endogenous angiotensin-II (Ang-II) mediated antinatriuresis was comparatively evaluated in Dahl salt-sensitive (SS) and salt-resistant (SR) rats, using a selective AT1 receptor antagonist, candesartan. In addition, differences in plasma renin activity and characteristics of Ang-II receptors in the renal cortical tubular membranes were also examined. Under INACTIN anesthesia AT1 receptor blockade resulted in significant increases in renal sodium excretion, which was several-fold greater in SS rats than that observed in SR rats. These observations suggest that antinatriuretic function of endogenous angiotensin-II is exaggerated in SS rats. This functional overexpression appears to be related to an increase in the affinity of Ang-II receptors in renal cortical tubular membranes but not to receptor density or plasma renin activity. It is proposed that salt-dependent hypertension in Dahl salt-sensitive rats may be due to enhanced Ang-II mediated sodium retention.

Gender differences in the attenuation of salt-induced hypertension by angiotensin (1-7)

Chronic infusion of angiotensin (1-7) [Ang-(1-7)] lowers blood pressure in spontaneously hypertensive rats (SHR). To assess the role of Ang-(1-7) in salt-induced hypertension, Ang-(1-7) (24 microg/kg/hr) or saline was administered chronically via osmotic minipump into the jugular vein of 5-6 wk-old male (M) and female (F) Dahl salt-sensitive rats placed on a high-salt (8% NaCl) diet for 2 weeks. Blood pressure (BP) and heart rate were measured prior to the start of the diet and weekly thereafter. Ang-(1-7) significantly attenuated the BP increase after 1 wk on the diet in both M and F rats, but after 2 weeks only in F rats. Enhanced release of prostacyclin, (6-keto PGF1 alpha), following Ang-(1-7) treatment was observed in both M and F rats. In addition, significant increases in aortic blood flow and plasma levels of nitric oxide were observed in the F rats following Ang-(1-7) treatment. These findings demonstrate that the reduction in BP is due to both prostacyclin and NO and that there is a gender difference in the attenuation of salt-induced hypertension by Ang-(1-7).

The role of gender in salt-induced hypertension

To evaluate gender differences in salt-induced hypertension, female and male Dahl salt-sensitive rats were fed high (8.0% NaCl, HS) and low (0.3% NaCl, LS) salt diets. During a 3-week treatment period, blood pressure was significantly elevated in both female and male HS groups compared to their respective LS groups. The blood pressure and 4 week mortality rate of the female HS group, however, were significantly lower than those of the male HS group. Renal and aortic blood flows were reduced in male rats on HS diet compared to the LS group, while, in females, renal blood flow was elevated and aortic flow was maintained while on HS diet. Plasma prostaglandin E2 and prostacyclin levels were higher in females than males and unaffected by diet. In contrast, plasma nitric oxide levels were reduced by HS, regardless of gender. In isolated aortic rings, HS diet caused a smaller elevation in the stimulated norepinephrine release ratio in female rats than in males. Thus, salt-induced hypertension is associated with a reduction in levels of nitric oxide regardless of gender. Plasma prostaglandin E2 and prostacyclin levels were higher in females. Taken together, the higher plasma prostaglandin levels and reduced sympathetic activity in females may be contributing factors in their lower blood pressure and reduced mortality.

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.