Estrogen depletion induces NaCl-sensitive hypertension in female spontaneously hypertensive rats

Hypertension after the Menopause: What Can We Learn from Experimental Studies?

Hypertension is the most prevalent cardiovascular disease of the adult population and is closely associated with serious cardiovascular events. The burden of hypertension with respect to vascular and other organ damage is greater in women. These sex differences are not fully understood. The unique feature in women is their transition to menopause accompanied by profound hormonal changes that affect the vasculature that are also associated with changes of blood pressure. Results from studies of hormone replacement therapy and its effects on the cardiovascular system are controversial, and the timing of treatment after menopause seems to be important. Therefore, revealing potential sex- and sex hormone-dependent pathophysiological mechanisms of hypertension in experimental studies could provide valuable information for better treatment of hypertension and vascular impairment, especially in postmenopausal women. The experimental rat models subjected to ovariectomy mimicking menopause could be useful tools for studying the mechanisms of blood pressure regulation after menopause and during subsequent therapy.

Estrogen depletion modulates aortic prothrombotic signaling in normotensive and spontaneously hypertensive female rats

AIM

In this study, we investigated how platelets and aorta contribute to the creation and maintenance of a prothrombotic state in an experimental model of postmenopausal hypertension in ovariectomized rats.

METHODS

Bilateral ovariectomy was performed in both 14-week-old female spontaneously hypertensive (SHR) and normotensive Wistar Kyoto (WKY) rats. The animals were kept in phytoestrogen free diet. Vascular parameters, platelet, coagulation and aortic prothrombotic functions and mechanisms were assessed.

RESULTS

Exacerbated platelet aggregation was observed in both SHR and WKY animals after ovariectomy. The mechanism was related to aortic COX2 downregulation and reduction in AMP, ADP, and ATP hydrolysis in serum and platelets. A procoagulant potential was observed in plasma from ovariectomized rats and this was confirmed by kallikrein and factor Xa generation in aortic rings. Aortic rings derived from ovariectomized SHR presented a greater thrombin generation capacity compared to equivalent rings from WKY animals. The mechanism involved tissue factor and PAR-1 upregulation as well as an increase in extrinsic coagulation and fibrinolysis markers in aorta and platelets. Aortic smooth muscle cells pre-treated with a plasma pool derived from estrogen-depleted animals developed a procoagulant profile with tissue factor upregulation. This procoagulant profile was dependent on inflammatory signalling, since NFκB inhibition attenuated the procoagulant activity and tissue factor expression.

CONCLUSIONS

A prothrombotic phenotype was observed in both WKY and SHR ovariectomized rats being associated with platelet hyperreactivity and tissue factor upregulation in aorta and platelets. The mechanism involves proinflammatory signalling that supports greater thrombin generation in aorta and vascular smooth muscle cells.

Ovariectomy uncovers purinergic receptor activation of endothelin-dependent natriuresis

We recently reported that natriuresis produced by renal medullary salt loading is dependent on endothelin (ET)-1 and purinergic (P2) receptors in male rats. Because sex differences in ET-1 and P2 signaling have been reported, we decided to test whether ovarian sex hormones regulate renal medullary ET-1 and P2-dependent natriuresis. The effect of medullary NaCl loading on Na⁺ excretion was determined in intact and ovariectomized (OVX) female Sprague-Dawley rats with and without ET-1 or P2 receptor antagonism. Isosmotic saline (284 mosmol/kgH₂O) was infused in the renal medullary interstitium of anesthetized rats during a baseline urine collection period, followed by isosmotic or hyperosmotic saline (1,800 mosmol/kgH₂O) infusion. Medullary NaCl loading significantly enhanced Na⁺ excretion in intact and OVX female rats. ET_A+B or P2 receptor blockade did not attenuate the natriuretic effect of medullary NaCl loading in intact females, whereas ET_A+B or P2 receptor blockade attenuated the natriuretic response to NaCl loading in OVX rats. Activation of medullary P2Y₂ and P2Y₄ receptors by UTP infusion had no significant effect in intact females but enhanced Na⁺ excretion in OVX rats. Combined ET_A+B receptor blockade significantly inhibited the natriuretic response to UTP observed in OVX rats. These data demonstrate that medullary NaCl loading induces ET-1 and P2-independent natriuresis in intact females. In OVX, activation of medullary P2 receptors promotes ET-dependent natriuresis, suggesting that ovarian hormones may regulate the interplay between the renal ET-1 and P2 signaling systems to facilitate Na⁺ excretion.

Hypotensive effects of genistein: From chemistry to medicine

Genistein (4', 5, 7-trihydroxyisoflavone), a naturally occurring flavonoid characteristic of Leguminoseae plants, is a phyto-oestrogen exerting oestrogenic activity as both an agonist and an antagonist substance. A large body of evidence suggests that genistein possesses many physiological and pharmacological properties that make this molecule a potential agent for the prevention and treatment of a number of chronic diseases. Growing evidence suggests that genistein could act as a vasodilating, anti-thrombotic, and anti-atherosclerotic agent, exerting these effects through different mechanisms of action. This paper aims to review data from the literature assessing the beneficial effects of genistein on hypertension, one of the most important cardiovascular disease risk factors along with hyperglycemia and hyperlidipemia. In addition, we discuss the chemistry, main sources and bioavailability of genistein. Scientific findings support genistein's potential as a promising anti-hypertensive agent in different experimental models. However, clinical trials are very limited and more research will be required before genistein intake can be recommended as part of therapies targeting raised blood pressure.

Salt-sensitive hypertension: mechanisms and effects of dietary and other lifestyle factors

Salt sensitivity, which is an increase in blood pressure in response to high dietary salt intake, is an independent risk factor for cardiovascular disease and mortality. It is associated with physiological, environmental, demographic, and genetic factors. This review focuses on the physiological mechanisms of salt sensitivity in populations at particular risk, along with the associated dietary factors. The interplay of mechanisms such as the renin-angiotensin aldosterone system, endothelial dysfunction, ion transport, and estrogen decrease in women contributes to development of salt sensitivity. Because of their effects on these mechanisms, higher dietary intakes of potassium, calcium, vitamin D, antioxidant vitamins, and proteins rich in L-arginine, as well as adherence to dietary patterns similar to the DASH (Dietary Approaches to Stop Hypertension) diet, can be beneficial to salt-sensitive populations. In contrast, diets similar to the typical Western diet, which is rich in saturated fats, sucrose, and fructose, together with excessive alcohol consumption, may exacerbate salt-sensitive changes in blood pressure. Identifying potential mechanisms of salt sensitivity in susceptible populations and linking them to protective or harmful dietary and lifestyle factors can lead to more specific guidelines for the prevention of hypertension and cardiovascular disease.

Tempereversed effects of ovariectomy on brain function in rats: Effects of age and type of soy product

BACKGROUD

The aims of this study were to analyze the differences in nutrient content and isoflavones in tempe flour and tofu flour, and to analyze the effect of tempe and tofu flour on cognitive function of female rats after ovariectomy.

METHOD

Seventy two (72) white female Sprague Dawley strain rats, aged 12 months were used for this study. Before the intervention 52 rats underwent ovariectomy (OVx) and they were grouped into 4 intervention groups: tempe flour (Tp), tofu flour (Tf), estradiol (E2), and casein as a control protein (Cs). The remaining 20 rats were classed as controls and had no ovariectomy (NO). Cognitive function was measured using a maze test. Oneway ANOVA with polynomial contrasts and post hoc LSD were used with a p-value<0.05 to indicate significance.

RESULTS

The content of nutrients (vitamin B6, vitamin B12, folic acid) and isoflavones (genistein) were all higher in tempe flour higher than in tofu flour. After 8 weeks, the Tp group had showed significantly increased cognitive function (P<0.05), while the Tf group, the E2 group and the Cs group also all had increased performance, but not significantly so (P>0.05). There was no change in scores in the NO group.

CONCLUSIONS

Intervention by tempe flour can increase cognitive function in female ovariectomized female rats. Further research should focus onother aspects of cognitive function and the content of amyloid plaques and neurotransmitter synthesis in the brain.

Effect of menopausal hormone therapy on components of the metabolic syndrome

The world population is aging, and women will spend an increasing share of their lives in a postmenopausal state that predisposes to metabolic dysfunction. Thus, the prevalence of metabolic syndrome (MetS) in women is likely to increase dramatically. This article summarizes the effects of menopause in predisposing to components of MetS including visceral obesity, dyslipidemia, type 2 diabetes (T2D) and hypertension (HTN). We also summarize the effects of menopausal hormone therapy (MHT) in reversing these metabolic alterations and discuss therapeutic advances of novel menopausal treatment on metabolic function.

Estrogen Regulates Angiotensin II Receptor Expression Patterns and Protects the Heart from Ischemic Injury in Female Rats

Previous studies have shown that female offspring are resistant to fetal stress-induced programming of ischemic-sensitive phenotype in the heart; however, the mechanisms responsible remain unclear. The present study tested the hypothesis that estrogen plays a role in protecting females in fetal programming of increased heart vulnerability. Pregnant rats were divided into normoxic and hypoxic (10.5% O2 from Day 15 to 21 of gestation) groups. Ovariectomy (OVX) and estrogen (E2) replacement were performed in 8-wk-old female offspring. Hearts of 4-mo-old females were subjected to ischemia and reperfusion injury in a Langendorff preparation. OVX significantly decreased postischemic recovery of left ventricular function and increased myocardial infarction, and no difference was observed between normoxic and hypoxic groups. The effect of OVX was rescued by E2 replacement. OVX decreased the binding of glucocorticoid receptor (GR) to glucocorticoid response elements at angiotensin II type 1 (Agtr1) and type 2 (Agtr2) receptor promoters, resulting in a decrease in Agtr1 and an increase in Agtr2 in the heart. Additionally, OVX decreased estrogen receptor (ER) expression in the heart and inhibited ER/GR interaction in binding to glucocorticoid response elements at the promoters. Consistent with the changes in Agtrs, OVX significantly decreased Prkce abundance in the heart. These OVX-induced changes were abrogated by E2 replacement. The results indicate that estrogen is not directly responsible for the sex dimorphism in fetal programming of heart ischemic vulnerability but suggest a novel mechanism of estrogen in regulating cardiac Agtr1/Agtr2 expression patterns and protecting female hearts against ischemia and reperfusion injury.

Female sex hormones protect against salt-sensitive hypertension but not essential hypertension

Initial studies found that female Dahl salt-sensitive (DS) rats exhibit greater blood pressure (BP) salt sensitivity than female spontaneously hypertensive rats (SHR). On the basis of the central role played by NO in sodium excretion and BP control, we further tested the hypothesis that blunted increases in BP in female SHR will be accompanied by greater increases in renal inner medullary nitric oxide synthase (NOS) activity and expression in response to a high-salt (HS) diet compared with DS rats. Gonad-intact and ovariectomized (OVX) female SHR and DS rats were placed on normal salt (NS; 0.4% salt) or HS (4% salt) diet for 2 wk. OVX did not alter BP in SHR, and HS diet produced a modest increase in BP. OVX significantly increased BP in DS rats on NS; HS further increased BP in all DS rats, although OVX had a greater increase in BP. Renal inner medullary NOS activity, total NOS3 protein, and NOS3 phosphorylated on serine residue 1177 were not altered by salt or OVX in either strain. NOS1 protein expression, however, significantly increased with HS only in SHR, and this corresponded to an increase in urinary nitrate/nitrite excretion. SHR also exhibit greater NOS1 and NOS3 protein expression than DS rats. These data indicate that female sex hormones offer protection against HS-mediated elevations in BP in DS rats but not SHR. We propose that the relative resistance to HS-mediated increases in BP in SHR is related to greater NOS expression and the ability to increase NOS1 protein expression compared with DS rats.

High sugar intake blunts arterial baroreflex via estrogen receptors in perinatal taurine supplemented rats

In adult rats, perinatal taurine depletion followed by high sugar intake alters neural and renal control of arterial pressure via the renin-angiotensin system. This study tests the hypothesis that perinatal taurine supplementation predisposes adult female rats to the adverse arterial pressure effect of high sugar intake via the renin-angiotensin system, rather than via estrogen. Female Sprague-Dawley rats were fed normal rat chow with 3% taurine (taurine supplementation, TS) or water alone (control, C) from conception to weaning. Their female offspring were fed normal rat chow with either 5% glucose in tap water (TSG, CG) or tap water alone (TSW, CW). At 7-8 weeks of age, the female offspring's renin-angiotensin system or estrogen receptors were inhibited by captopril or tamoxifen, respectively. Body weight, heart weight, kidney weight, mean arterial pressures (MAP), and heart rates were not significantly different among groups without captopril or tamoxifen. Captopril (but not tamoxifen) decreased MAP but not heart rates in all groups. In TSG compared to TSW, CW, and CG groups, baroreflex sensitivity of heart rate (BSHR) and renal nerve activity (BSRA) were significantly decreased. Neither captopril nor tamoxifen altered BSHR in TSG, but tamoxifen (but not captopril) restored TSG BSRA to CW or CG control levels. Perinatal taurine supplementation did not disturb sympathetic and parasympathetic nerve activity in the adult rats on high or basal sugar intake. Compared to its effect in CW and CG groups, tamoxifen increased sympathetic but decreased parasympathetic activity less in TSG and TSW groups. Inhibition of the renin-angiotensin system did not affect autonomic nerve activity in any group. These data suggest that in adult female rats that are perinatally supplemented with taurine, high sugar intake after weaning blunts arterial baroreflex via an estrogen (but not renin-angiotensin) mechanism.

Sex differences in primary hypertension

Men have higher blood pressure than women through much of life regardless of race and ethnicity. This is a robust and highly conserved sex difference that it is also observed across species including dogs, rats, mice and chickens and it is found in induced, genetic and transgenic animal models of hypertension. Not only do the differences between the ovarian and testicular hormonal milieu contribute to this sexual dimorphism in blood pressure, the sex chromosomes also play a role in and of themselves. This review primarily focuses on epidemiological studies of blood pressure in men and women and experimental models of hypertension in both sexes. Gaps in current knowledge regarding what underlie male-female differences in blood pressure control are discussed. Elucidating the mechanisms underlying sex differences in hypertension may lead to the development of anti-hypertensives tailored to one's sex and ultimately to improved therapeutic strategies for treating this disease and preventing its devastating consequences.

Regional differences in sexually dimorphic protein expression in the spontaneously hypertensive rat (SHR)

Hypertension is sexually dimorphic and modified by removal of endogenous sex steroids. This study tested the hypothesis that endogenous gonadal hormones exert differential effects on protein expression in the kidney and mesentery of SHR. At ~5 weeks of age male and female SHR underwent sham operation, orchidectomy, or ovariectomy (OVX). At 20-23 weeks of age, mean arterial pressure (MAP) was measured in conscious rats. The mesenteric arterial tree and kidneys were collected, processed for Western blots, and probed for Cu Zn superoxide dismutase (SOD1), soluble epoxide hydrolase (sEH), and Alpha 2A adrenergic receptor (A2AR) expression. MAP was unaffected by ovariectomy (Sham 164 ± 4: Ovariecttomy 159 ± 3 mm Hg). MAP was reduced by orchidectomy (Sham 189 ± 5:Orchidectomy 167 ± 2 mm Hg). In mesenteric artery, SOD1 expression was greater in male versus female SHR. Orchidectomy increased while ovariectomy decreased SOD1 expression. The kidney exhibited a different pattern of response. SOD1 expression was reduced in male compared to female SHR but gonadectomy had no effect. sEH expression was not significantly different among the groups in mesenteric artery. In kidney, sEH expression was greater in males compared to females. Ovariectomy but not orchidectomy increased sEH expression. A2AR expression was greater in female than male SHR in mesentery artery and kidney. Gonadectomy had no effect in either tissue. We conclude that sexually dimorphic hypertension is associated with regionally specific changes in expression of three key proteins involved in blood pressure control. These data suggest that broad spectrum inhibition or stimulation of these systems may not be the best approach for hypertension treatment. Instead regionally targeted manipulation of these systems should be investigated.

Blood pressure and mesenteric vascular reactivity in spontaneously hypertensive rats 7 months after gonadectomy

OBJECTIVE

Sexual dimorphism in the degree of high blood pressure (BP) has been observed in both animal and human hypertension. However, the mechanisms are still poorly understood. We tested the hypothesis that long-term loss of sex steroids promotes changes in mesenteric vascular reactivity that impact the maintenance of hypertension in the spontaneously hypertensive rats (SHR).

METHODS

Male SHR were sham operated (M-SHAM) or castrated (M-CX), and female SHR were sham-operated (F-SHAM) or ovariectomized (F-OVX) at 3 weeks of age. Seven months later, BP was measured in anesthetized rats, and vascular responsiveness was evaluated in the isolated perfused mesentery.

RESULTS

Mean arterial BP (mm Hg) was significantly greater in M-SHAM (186 ± 6) compared with F-SHAM (159 ± 5). Gonadectomy reduced BP in male SHR (M-CX: 160 ± 4) but had no significant effect in female SHR (F-OVX: 153 ± 7). Norepinephrine-induced constriction was similar in all groups. Gonadectomy attenuated serotonin-induced vasoconstriction in the mesentery. Acetylcholine (ACh)- and isoproterenol (ISO)-induced vasodilation was greater in female than male SHR. Ovariectomy of female SHR blunted ACh and ISO dilatory responses. ISO dose-response curves were shifted to the left in castrated male SHR.

CONCLUSIONS

Gonadectomy exerts long-term effects on mesenteric vascular reactivity and hypertension in the SHR.

Kinetic epidemiological model for elucidating sexual difference of hypertension (KCIS no.20)

BACKGROUND

Woman have lower rate of hypertension than man but it is still elusive how such gender difference can be explained by kinetic epidemiological curves.

OBJECTIVE

The aim of this paper was to develop a multi-state model for delineating the kinetic epidemiology of hypertension according to the Seventh Report of the Joint National Committee (JNC 7) classification criteria by gender, and to derive gender-specific kinetic curves.

METHODS

We used data from a population-based screening programme with 42,027 participants to fit a four-state Markov model corresponding to the classification of hypertension from the JNC 7.

RESULTS

The young man had higher progression rate but lower regression rate for the movement between normal and pre-hypertension than the young woman. Such gender difference disappeared after 50 years old. The mean sojourn time of pre- and stage 1 hypertension for man and stage 1 for woman was approximately 5 years. However, the corresponding figure for pre-hypertension for woman was 25 years at age 30, 10 years at age 40 and 5 years afterwards.

CONCLUSION

Elucidating the kinetic epidemiological curves of hypertension explains higher prevalence rate in young man than woman. These findings fit with the role of sex hormones regulating blood pressure demonstrated in the animal model.

Characterization of an animal model of postmenopausal cardiac hypertrophy and novel mechanisms responsible for cardiac decompensation using ovariectomized pressure-overloaded rats

OBJECTIVE

The development of animal models of cardiovascular disease are critical to define pathophysiological mechanisms and to advance diagnosis and therapy. The lack of a suitable animal model represents a failure to define the mechanisms responsible for postmenopausal myocardial hypertrophy in hypertension and adverse cardiac remodeling.

METHODS

In this review, we presented a rat model of postmenopausal myocardial hypertrophy, with particular focus on the similarities between the animal model and postmenopausal women regarding myocardial function as well as molecular and subcellular mechanisms. To elucidate the molecular mechanism of left ventricular (LV) hypertrophy and remodeling in postmenopausal women, we analyzed myocardial hypertrophy as well as cardiac function and hypertrophy-related protein expression in ovariectomized (OVX) and pressure overloaded (PO) rats.

RESULTS

The model is characterized by depletion of serum estrogen and increased heart-to-body weight and lung-to-body weight ratios. Moreover, the OVX-PO rats also show increased mean arterial blood pressure, LV end-diastolic pressure, LV developed pressure, and maximal rates of LV contraction and relaxation compared with the OVX group. Importantly, Akt activity was largely attenuated, and both endothelial nitric oxide synthase expression and activity were markedly reduced in the OVX-PO group. Finally, significant increased mortality was observed in the OVX-PO group after chronic isoproterenol administration.

CONCLUSIONS

Our results demonstrate that rats subject to OVX are unable to compensate for hypertrophy partly due to impaired Akt-endothelial nitric oxide synthase signaling along with deteriorated heart function and demonstrated increased mortality. In this review, we discussed the mechanisms of cardiac injury, which could play a critical role in postmenopausal hypertrophy, as well as the characteristics of the OVX-PO female rats as a model to test cardioprotective drugs in postmenopausal women.

Flavonoids and age-related disease: risk, benefits and critical windows

Plant derived products are consumed by a large percentage of the population to prevent, delay and ameliorate disease burden; however, relatively little is known about the efficacy, safety and underlying mechanisms of these traditional health products, especially when taken in concert with pharmaceutical agents. The flavonoids are a group of plant metabolites that are common in the diet and appear to provide some health benefits. While flavonoids are primarily derived from soy, many are found in fruits, nuts and more exotic sources, e.g., kudzu. Perhaps the strongest evidence for the benefits of flavonoids in diseases of aging relates to their effect on components of the metabolic syndrome. Flavonoids from soy, grape seed, kudzu and other sources all lower arterial pressure in hypertensive animal models and in a limited number of tests in humans. They also decrease the plasma concentration of lipids and buffer plasma glucose. The underlying mechanisms appear to include antioxidant actions, central nervous system effects, gut transport alterations, fatty acid sequestration and processing, PPAR activation and increases in insulin sensitivity. In animal models of disease, dietary flavonoids also demonstrate a protective effect against cognitive decline, cancer and metabolic disease. However, research also indicates that the flavonoids can be detrimental in some settings and, therefore, are not universally safe. Thus, as the population ages, it is important to determine the impact of these agents on prevention/attenuation of disease, including optimal exposure (intake, timing/duration) and potential contraindications.

Chronic dietary kudzu isoflavones improve components of metabolic syndrome in stroke-prone spontaneously hypertensive rats

The present study tested the long-term effects of dietary kudzu root extract supplementation on the regulation of arterial pressure, plasma glucose, and circulating cholesterol in stroke-prone spontaneously hypertensive rats (SP-SHR). Female SP-SHR were maintained for 2 months on a polyphenol-free diet, with or without the addition of 0.2% kudzu root extract. Half of the rats in each diet group were ovariectomized, whereas the other half remained intact. Following 2 months on the diets, the 0.2% kudzu root extract supplementation (compared to control diet) significantly lowered arterial pressure (11-15 mmHg), plasma cholesterol, fasting blood glucose (20-30%), and fasting plasma insulin in both the ovariectomized and intact SP-SHR. These results indicate that long-term dietary kudzu root extract supplementation can improve glucose, lipid, and blood pressure control in intact and ovariectomized SP-SHR.

Anti-hypertensive nutraceuticals and functional foods

Epidemiological studies have demonstrated that elevated blood pressure is one of the major risk factors for stroke and coronary heart disease (CHD). A close association between blood pressure and the incidence of cardiovascular diseases is well established if systolic/diastolic blood pressure is above 140/90 mmHg. In recent years, nutraceuticals and functional foods have attracted considerable interest as potential alternative therapies for treatment of hypertension, especially for prehypertensive patients, whose blood pressure is marginally or mildly high but not high enough to warrant the prescription of blood pressure-lowering medications. This review summarizes the findings of recent studies on the chemistry, production, application, efficacy, and mechanisms of popular blood pressure-lowering nutraceuticals and functional foods including the Dietary Approaches to Stop Hypertension (DASH) diet plan, L-arginine, chlorogenic acid, fermented milk, garlic, onion, tea, soybean, ginger, hawthorn, and fish oil.

Effects of botanical dietary supplements on cardiovascular, cognitive, and metabolic function in males and females

BACKGROUND

The onset of menopause marks a pivotal time in which the incidence of hypertension and of cardiovascular disease (CVD) begins to increase dramatically in women. Before menopause, the incidences of these diseases are significantly lower in women than in age-matched men. After menopause, the rates of these diseases in women eventually approximate those in men. The loss of endogenous estrogen at menopause has been traditionally believed to be the primary factor involved in these changes.

OBJECTIVE

This review summarizes recent findings regarding the effectiveness of botanicals in the treatment of some menopausal symptoms and other symptoms of aging (eg, rise in arterial pressure, cognitive decline, insulin resistance, and hyperlipidemia).

METHODS

Articles were selected for inclusion in this review based on the significance of the research and contribution to the current understanding of how each botanical elicits cardioprotective effects. To this end, PubMed and MEDLINE databases were searched, using terms that included the name of the specific botanical along with the relevant aspects of its action(s), such as blood pressure, glycemic control, and lipids. Most of the articles used were published within the past 5 years, although some older articles that were seminal in advancing the current understanding of botanicals were also included.

RESULTS

Soy has been found to lower plasma lipid concentrations and arterial pressure in postmenopausal women and age-matched men, and to have protective effects in heart disease and atherosclerosis of the carotid and coronary circulation. Soy was also found to lower fasting insulin concentrations and glycosylated hemoglobin concentrations. Grape seed extract, another frequently used botanical, contains polyphenols that have been found to reduce arterial pressure and salt-sensitive hypertension in estrogendepleted animal models.

CONCLUSION

Several botanical compounds have been found to have beneficial effects in the treatment of the symptoms of menopause and other symptoms of aging, including CVD, cognitive decline, and metabolic diseases.

Differential effects of sex steroids in young and aged female mRen2.Lewis rats: a model of estrogen and salt-sensitive hypertension

BACKGROUND

Male-female differences in the expression of hypertension and in end-organ damage are evident in both experimental models and human subjects, with males exhibiting a more rapid onset of cardiovascular disease and mortality than do females. The basis for these male-female differences is probably the balance of the complex effects of sex steroids (androgens, estrogen, progesterone) and their metabolites on the multiple regulatory systems that influence blood pressure (BP). A key target of estrogen and other steroids is likely to be the different components of the renin-angiotensin-aldosterone system (RAAS).

OBJECTIVE

The aim of this study was to review the current experimental evidence on the protective effects of estrogen in hypertensive models.

METHODS

The search terms estrogen , renin-aangiotensin-aldosterone system, renin receptor, salt-sensitivity, endorgan damage, hypertension, kidney, mRen2. Lewis, and injury markers were used to identify relevant publications in the PubMed database (restricted to the English language) from January 1990 to October 2007.

RESULTS

In a new congenic model that expresses the mouse renin 2 gene (mRen2. Lewis), estrogen depletion (via ovariectomy [OVX ]) in young rats was found to have a marked stimulatory effect on the progression of increased BP and cardiac dysfunction. Moreover, estrogen depletion exacerbated salt-sensitive hypertension and the extent of salt-induced cardiac and renal injury in young mRen2. Lewis rats, which probably reflected the inability to appropriately regulate various components of the RAAS. However, OVX in aged mRen2. Lewis rats conveyed renal protective effects from a high-salt diet compared with intact hypertensive littermates (64 weeks), and these effects were independent of changes in BP.

CONCLUSION

These studies in hypertensive mRen2. Lewis rats underscored the influence of ovarian hormones on BP and tissue injury, as well as the plasticity of this response, apparently due to age and salt status.

Estrogen protects against increased blood pressure in postpubertal female growth restricted offspring

Placental insufficiency in the rat results in intrauterine growth restriction and development of hypertension in prepubertal male and female growth-restricted offspring. However, after puberty, only male growth-restricted offspring remain hypertensive, whereas female growth-restricted offspring stabilize their blood pressure to levels comparable to adult female controls. Because female rats reach their maximum levels of estrogen at puberty, we hypothesize that estrogen may be a factor involved in the stabilization of blood pressure in adult female growth-restricted offspring. At 10 weeks of age, female control and growth-restricted offspring underwent ovariectomy or sham surgery and insertion of a telemetry probe. Mean arterial pressure was similar at 16 weeks of age between control (123+/-4 mm Hg) and growth-restricted offspring (122+/-2 mm Hg); however, ovariectomy led to a significant increase in blood pressure in growth-restricted offspring (140+/-2 mm Hg; P<0.05 versus intact counterpart) with no significant effect in controls (124+/-1 mm Hg). Estrogen replacement by subcutaneous minipellet initiated at 14 weeks of age in a subset of ovariectomized control and growth-restricted offspring reversed the effect of ovariectomy on blood pressure in growth-restricted offspring at 16 weeks of age (111+/-3 mm Hg; P<0.05 versus ovariectomized counterpart); renin angiotensin system blockade also abolished ovariectomy-induced hypertension in female growth-restricted offspring (106+/-2 mm Hg; P<0.05 versus ovariectomized counterpart). Therefore, sex differences are observed in this model of fetal programmed hypertension, and results from this study suggest that estrogen contributes to normalization of blood pressure in adult female growth-restricted offspring.

Effect of oestrogen on reactive oxygen species production in the aortas of ovariectomized Dahl salt-sensitive rats

OBJECTIVE

In the present study, we examined whether ovariectomy increases reactive oxygen species (ROS) and the expression of nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase and modulates the scavenger enzymes for ROS in the aortas of Dahl salt-sensitive (DSS) rats fed a high salt diet.

METHODS

DSS female rats were ovariectomized and fed a high salt diet (8% NaCl), or a high salt diet plus oestrogen supplement for 4 weeks. Urinary levels of hydrogen peroxide (H2O2) were measured by using 2',7'-dichlorofluorescein. The expression of an NADPH oxidase subunit p22phox, extracellular superoxide dismutase (ecSOD), glutathione peroxidase (GPx)1, GPx4 and monocyte chemoattractant protein 1 (MCP-1) messenger RNA was assessed by reverse transcription-polymerase chain reaction. The expression of MCP-1, and macrophage infiltration were examined by immunohistochemical analysis.

RESULTS

Ovariectomy increased superoxide production and the expression of NADPH oxidase subunit p22phox mRNA and protein in the aortas of DSS rats fed a high salt diet. In contrast, ovariectomy reduced the expression of ecSOD mRNA and protein and the expression of GPx1 and GPx4 mRNA in the aorta. Ovariectomy increased MCP-1 mRNA and protein expression and ED1-positive cells in the aorta.

CONCLUSIONS

Ovariectomy leads to an amplification of oxidative stress in DSS rats fed a high salt diet synergistically by an increase in the ROS-generating system and a decrease in the ROS-eliminating system, as shown in the increase in superoxide production and the urinary excretion of H2O2. Oestrogen supplementation counteracted these alterations, showing how oestrogen is antioxidative.

Cytochrome P450 enzymes: Central players in cardiovascular health and disease

Cardiovascular disease (CVD) is a human health crisis that remains the leading cause of death worldwide. The cytochrome P450 (CYP) class of enzymes are key metabolizers of both xenobiotics and endobiotics. Many CYP enzyme families have been identified in the heart, endothelium and smooth muscle of blood vessels. Furthermore, mounting evidence points to the role of endogenous CYP metabolites, such as epoxyeicosatrienoic acids (EETs), hydroxyeicosatetraenoic acids (HETEs), prostacyclin (PGI(2)), aldosterone, and sex hormones, in the maintenance of cardiovascular health. Emerging science and the development of genetic screening have provided us with information on the differences in CYP expression among populations and groups of individuals. With this information, a link between CYP expression and activity and CVD, such as hypertension, coronary artery disease (CAD), myocardial infarction, heart failure, stroke, and cardiomyopathy and arrhythmias, has been established. In fact many currently used therapeutic modalities in CVD owe their therapeutic efficacy to their effect on CYP metabolites. Thus, the evidence for the involvement of CYP in CVD is numerous. Concentrating on treatment modalities that target the CYP pathway makes ethical sense for the affected individuals and decreases the socioeconomic burden of this disease. However, more research is needed to allow the integration of this information into a clinical setting.

Estrogen and salt sensitivity in the female mRen(2).Lewis rat

The present study determined whether early loss of estrogen influences salt-sensitive changes in blood pressure, renal injury, and cardiac hypertrophy as well as the effects on the circulating renin-angiotensin-aldosterone system (RAAS) in the hypertensive female mRen( 2 ).Lewis strain. Ovariectomy (OVX) of heterozygous mRen( 2 ).Lewis rats on a normal salt (NS) diet (0.5% sodium) increased systolic blood pressure from 137 ± 3 to 177 ± 5 mmHg ( P < 0.01) by 15 wk but did not show any changes in cardiac-to-body weight index (CI), proteinuria, or creatinine clearance. Maintenance with a high-sodium (HS) diet (4%) increased blood pressure (203 ± 4 mmHg, P < 0.01), proteinuria (3.5 ± 0.3 vs. 6.4 ± 0.7 mg/day, P < 0.05), and CI (4.0 ± 0.1 vs. 5.2 ± 0.1 mg/kg, P < 0.01) but decreased creatinine clearance (0.89 ± 0.15 vs. 0.54 ± 0.06 ml/min, P < 0.05). OVX exacerbated the effects of salt on the degree of hypertension (230 ± 5 mmHg), CI (5.6 ± 0.2 mg/kg), and proteinuria (13 ± 3.0 mg/day). OVX increased the urinary excretion of aldosterone approximately twofold in animals on the NS diet (3.8 ± 0.5 vs. 6.6 ± 0.5 ng·mg creatinine−1·day−1, P < 0.05) and HS diet (1.4 ± 0.2 vs. 4.5 ± 1.0 ng·mg creatinine−1·day−1, P < 0.05). Circulating renin, angiotensin-converting enzyme, and angiotensin II were also significantly increased in the OVX group fed a HS diet. These results reveal that the protective effects of estrogen apart from the increase in blood pressure were only manifested in the setting of a chronic HS diet and suggest that the underlying sodium status may have an important influence on the overall effect of reduced estrogen.

Discoordinate regulation of renal nitric oxide synthase isoforms in ovariectomized mRen2. Lewis rats

Estrogen depletion markedly exacerbates hypertension in female congenic mRen2. Lewis rats, a model of tissue renin overexpression. Because estrogen influences nitric oxide synthase (NOS) and NO may exert differential effects on blood pressure, the present study investigated the functional expression of NOS isoforms in the kidney of ovariectomized (OVX) mRen2. Lewis rats. OVX-mRen2. Lewis exhibited an increase in systolic blood pressure (SBP) of 171 +/- 5 vs. 141 +/- 7 mmHg (P < 0.01) for intact littermates. Renal cortical mRNA and protein levels for endothelial NOS (eNOS) were reduced 50-60% (P < 0.05) and negatively correlated with blood pressure. In contrast, cortical neuronal NOS (nNOS) mRNA and protein levels increased 100 to 300% (P < 0.05). In the OVX kidney, nNOS immunostaining was more evident in the macula densa, cortical tubules, and the medullary collecting ducts compared with the intact group. To determine whether the increase in renal nNOS expression constitutes a compensatory response to the reduction in renal eNOS, we treated both intact and OVX mRen2. Lewis rats with the selective nNOS inhibitor L-VNIO from 11 to 15 wk of age. The nNOS inhibitor reduced blood pressure in the OVX group (185 +/- 3 vs. 151 +/- 8 mmHg, P < 0.05), but pressure was not altered in the intact group (146 +/- 4 vs. 151 +/- 4 mmHg). In summary, exacerbation of blood pressure in the OVX mRen2. Lewis rats was associated with the discoordinate regulation of renal NOS isoforms. Estrogen sensitivity in this congenic strain may involve the influence of NO through the regulation of both eNOS and nNOS.

Ovariectomy augments hypertension through rho-kinase activation in the brain stem in female spontaneously hypertensive rats

Estrogen protects against increases in arterial pressure (AP) by acting on blood vessels and on cardiovascular centers in the brain. The mechanisms underlying the effects of estrogen in the brain stem, however, are not clear. The aim of the present study was to determine whether ovariectomy affects AP via the Rho/Rho-kinase pathway in the brain stem. We performed bilateral ovariectomy in 12-week-old female spontaneously hypertensive rats. AP and heart rate (HR), measured using radiotelemetry in awake rats, were increased in ovariectomized rats compared with control rats (mean AP: 163+/-3 versus 144+/-4 mm Hg; HR: 455+/-4 versus 380+/-6 bpm). Continuous intracisternal infusion of Y-27632 significantly attenuated the ovariectomy-induced increase in AP and HR (mean AP: 137+/-6 versus 163+/-3 mm Hg; HR: 379+/-10 versus 455+/-4 bpm). In addition, we confirmed the increase of Rho-kinase activity in the brain stem in ovariectomized rats, and the increase was attenuated by intracisternal infusion of Y-27632 via the phosphorylated ezrin, radixin, and moesin (ERM) family, which are Rho-kinase target proteins. Furthermore, angiotensin II type 1 receptor expression in the brain stem was significantly greater in ovariectomized rats than in control rats, and the increase was partially reduced by intracisternal infusion of Y-27632. In a separate group of animals, we confirmed that the serum and cerebrospinal fluid 17beta-estradiol concentrations decreased in ovariectomized rats. These results suggest that depletion of endogenous estrogen by ovariectomy, at least in part, induces hypertension in female spontaneously hypertensive rats via activation of the renin-angiotensin system and the Rho/Rho-kinase pathway in the brain stem.

Extrarenal Na + Balance, Volume, and Blood Pressure Homeostasis in Intact and Ovariectomized Deoxycorticosterone-Acetate Salt Rats

Water-free Na+ storage may buffer extracellular volume and mean arterial pressure (MAP) in spite of Na+ retention. We studied the relationship among internal Na+, K+, water balance, and MAP in Sprague-Dawley rats, with or without deoxycorticosterone-acetate (DOCA) salt, with or without ovariectomy (OVX). The rats were fed a low-salt (0.1% NaCl) or high-salt (8% NaCl) diet for 5 weeks. DOCA salt increased MAP (161+/-14 versus 123+/-4 mm Hg; P<0.05), and DOCA-OVX salt increased MAP further (181+/-22 mm Hg; P<0.05). DOCA salt increased the total body Na+ by &40% to 45%; however, water-free Na+ retention by osmotically inactive Na+ storage and by osmotically neutral Na+/K+ exchange allowed the rats to maintain the extracellular volume close to normal. DOCA-OVX salt rats showed similar Na+ retention. However, their osmotically inactive Na+ storage capacity was greatly reduced and only partially compensated by neutral Na+/K+ exchange, resulting in greater volume retention despite similar Na+ retention. For every 1% wet weight total body water gain, MAP increased by 2.3+/-0.2 mm Hg in DOCA salt rats and 2.5+/-0.3 mm Hg in DOCA-OVX salt rats. Because water-free Na+ retention buffered total body water content by 8% to 11% wet weight, we conclude that this internal Na+ escape buffered MAP. Extrarenal Na+ and volume balance seem to play an important role in long-term volume and MAP control.

Salt sensitivity and hypertension after menopause: role of nitric oxide and angiotensin II

Hypertension is a major risk factor for cardiovascular disease and renal disease. After menopause, the incidence of hypertension increases in women to levels that equal or exceed that in men, suggesting a protective role of female sex hormones. Salt sensitivity of blood pressure is associated with an increased risk for development of hypertension and cardiovascular disease. We and others have demonstrated that after menopause, the prevalence of salt sensitivity increases, suggesting that female sex hormones influence renal sodium handling and blood pressure regulation. A homeostatic balance between the counteracting effects of nitric oxide (NO) and angiotensin (Ang) II on pressure natriuresis, renal hemodynamics, tubular sodium reabsorption, and oxidative stress plays an important role in modulating salt sensitivity as well as hypertensive end-organ injury. Estrogens modulate the activity and expression of NO and Ang II. We infer that after menopause, estrogen deficiency promotes an unbalance between NO and Ang II, resulting in disturbed renal sodium handling, oxidative stress, and hypertension, particularly in genetically prone women. A better understanding of the mechanisms underlying the development of postmenopausal hypertension and associated cardiovascular and renal diseases should provide insights into preventive and therapeutic strategies.

Antihypertensive and cognitive effects of grape polyphenols in estrogen-depleted, female, spontaneously hypertensive rats

Both endogenous and dietary estrogens reduce hypertension and enhance cognitive abilities in estrogen-depleted female spontaneously hypertensive rats (SHR). Many of the beneficial effects of estrogens/phytoestrogens also appear to be provided by other polyphenols (e.g., proanthocyanidins) in grape seed, which lack appreciable estrogenic receptor binding. The present study tested the hypothesis that similar to phytoestrogens, proanthrocyanidins in grape seed polyphenols reduce salt-sensitive hypertension in young, estrogen-depleted SHR. SHR were ovariectomized at 4 wk of age and placed on phytoestrogen-free diets with or without 0.5% grape seed extract added and with high (8.0%) or basal (0.6%) NaCl. After 10 wk on the diets, grape proanthrocyanidin supplementation significantly reduced arterial pressure in the rats fed the basal (10 mmHg) and high (26 mmHg)-NaCl diet, compared with the nonsupplemented controls. In vitro superoxide production was significantly reduced (23%) by the grape seed polyphenols. Spatial learning (8-arm-radial maze) in the SHR on the basal NaCl diets was improved by dietary grape seed polyphenols. These results indicate that grape seed polyphenols decrease arterial pressure in SHR, probably via an antioxidant mechanism.

Increases in CSF [Na+] precede the increases in blood pressure in Dahl S rats and SHR on a high-salt diet

In Dahl salt-sensitive (S) and salt-resistant (R) rats, and spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats, at 5-6 wk of age, a cannula was placed in the cisterna magna, and cerebrospinal fluid (CSF) was withdrawn continuously at 75 microl/12 h. CSF was collected as day- and nighttime samples from rats on a regular salt intake (0.6% Na+; R-Na) and then on a high salt intake (8% Na+; H-Na). In separate groups of rats, the abdominal aorta was cannulated and blood pressure (BP) and heart rate (HR) measured at 10 AM and 10 PM, with rats first on R-Na and then on H-Na. On H-Na, CSF [Na+] started to increase in the daytime of day 2 in Dahl S rats and of day 3 in SHR. BP and HR did not rise until day 3 in Dahl S rats and day 4 in SHR. In Dahl R and WKY rats, high salt did not change CSF [Na+], BP, or HR. In a third set of Dahl S rats, sampling of both CSF and BP was performed in each individual rat. Again, significant increases in CSF [Na+] were observed 1-2 days earlier than the increases in BP and HR. In a fourth set of Dahl S rats, BP and HR were recorded continuously by means of radiotelemetry for 5 days on R-Na and 8 days on H-Na. On H-Na, BP (but not HR) increased first in the nighttime of day 2. In another set of Dahl S rats, intracerebroventricular infusion of antibody Fab fragments binding ouabain-like compounds (OLC) with high affinity prevented the increase in BP and HR by H-Na but further increased CSF [Na+]. Finally, in Wistar rats on H-Na, intracerebroventricular infusion of ouabain increased BP and HR but decreased CSF [Na+]. Thus, in both Dahl S and SHR on H-Na, increases in CSF [Na+] preceded the increases in BP and HR, consistent with a primary role of increased CSF [Na+] in the salt-induced hypertension. An increase in brain OLC in response to the initial increase in CSF [Na+] appears to attenuate further increases in CSF [Na+] but at the "expense" of sympathoexcitation and hypertension.

Novel mechanisms responsible for postmenopausal hypertension

Blood pressure increases in many women after menopause. Hypertension is one of the major risk factors for cardiovascular disease. However, the mechanisms responsible for the postmenopausal increase in blood pressure are yet to be elucidated. Various humoral systems have been proposed to play a role in postmenopausal hypertension, such as changes in estrogen/androgen ratios, increases in endothelin and oxidative stress, and activation of the renin-angiotensin system (RAS). In addition, obesity, type II diabetes, and activation of the sympathetic nervous system are common in postmenopausal women and may also play important roles. However, progress in elucidating the mechanisms responsible for postmenopausal hypertension has been hampered by the lack of a suitable animal model. The aging female spontaneously hypertensive rat (SHR) exhibits many of the characteristics found in postmenopausal women. In this review, some of the possible mechanisms that could play a role in postmenopausal hypertension are discussed, as well as the characteristics of the aged female SHR as a model to study.

Postovariectomy Hypertension Is Linked to Increased Renal AT1Receptor and Salt Sensitivity

The functional balance between angiotensin II (Ang II) and nitric oxide (NO) plays a key role in modulating salt sensitivity. Estrogen has been shown to downregulate angiotensin type 1 (AT1) receptor expression and to increase the bioavailability of endothelium-derived NO, which decreases AT1 receptor expression. The present study tests the hypothesis that in the presence of genetic salt sensitivity, deficiency of endogenous estrogens after ovariectomy (OVX) fosters an upregulation of Ang II. Female Dahl salt-resistant (DR), Dahl salt-sensitive (DS), Wistar-Kyoto (WKY), and spontaneously hypertensive (SHR) rats underwent bilateral OVX or sham surgery (SHX) and were fed a normal salt diet (0.5% NaCl) for 14 weeks. Systolic blood pressures were measured every 2 weeks and were not significantly different between OVX and SHX for DR, WKY, and SHR groups. However, at the end of 14 weeks of normal salt diet, hypertension developed in DS OVX but not SHX rats (160+/-3 versus 136+/-3 mm Hg; P<0.05). Hypertension also developed in DS OVX rats pair-fed a normal salt diet (166+/-7 mm Hg). Development of hypertension in DS OVX rats was prevented by estrogen replacement (132+/-3 mm Hg), AT1 receptor blockade (119+/-3 mm Hg), or feeding a very low salt diet (0.1% NaCl; 129+/-4 mm Hg). Renal AT1 receptor protein expression was significantly elevated 2-fold in DS OVX relative to SHX rats and was prevented by estrogen replacement. These data strongly suggest that after OVX in salt-sensitive rats there is a lower threshold for the hypertensinogenic effect of salt that is linked to an activation of Ang II.

Osmotically inactive skin Na+ storage in rats

Compared with age-matched men, women are resistant to the hypertensive effects of dietary NaCl; however, after menopause, the incidence of salt-sensitive hypertension is similar in women and men. We recently suggested that osmotically inactive Na+ storage contributes to the development of salt-sensitive hypertension. The connective tissues, including those immediately below the skin that may serve as a reservoir for osmotically inactive Na+ storage, are affected by menopause. We tested the hypothesis that ovariectomy (OVX) might reduce osmotically inactive Na+ storage capacity in the body, particularly in the skin. Male, female-fertile, and female OVX Sprague-Dawley (SD) rats were fed a high (8%)- or low (0.1%)-NaCl diet. The groups received the diet for 4 or 8 wk. At the end of the experiment, subgroups received 0.9% saline infusion and urinary Na+ and K+ excretion was measured. Wet and dry weight (DW), water content in the body and skin, total body Na+ (rTBNa+) and skin Na+ (rSKNa+) content were measured relative to DW by desiccation and dry ashing. There were no gender differences in osmotically inactive Na+ storage in SD rats. All SD rats accumulated Na+ if fed 8% NaCl, but rTBNa+ was lower in OVX rats than in fertile rats on a low (P < 0.001)- and a high (P < 0.05)-salt diet. OVX decreased rSKNa+ (P < 0.01) in the rats. A high-salt diet led to Na+ accumulation (DeltaSKNa+) in the skin in all SD rats. Osmotically inactive skin Na+ accumulation was approximately 66% of DeltaSKNa+ in female and 82% in male-fertile rats, but there was no osmotically inactive Na+ accumulation in OVX rats fed 8% NaCl. We conclude that skin is an osmotically inactive Na+ reservoir that accumulates Na+ when dietary NaCl is excessive. OVX leads to an acquired reduction of osmotically inactive Na+ storage in SD rats that predisposes the rats to volume excess despite a reduced Na+ content relative to body weight.

Estrogen or the AT1 antagonist olmesartan reverses the development of profound hypertension in the congenic mRen2. Lewis rat

The influence of estrogen on the regulation of cardiovascular function remains a controversial and complex area of investigation. We assessed the effects of estrogen depletion in the congenic mRen(2). Lewis rat, established from the back-cross of the original (mRen2)-27 transgenic onto the Lewis inbred strain. Ovariectomy of heterozygous mRen(2). Lewis at 4 to 5 weeks resulted in a progressive increase in blood pressure compared with the sham surgery congenics at weeks 6 to 11. At 11 weeks, the ovariectomized mRen(2). Lewis (OVX) systolic blood pressure averaged 195+/-3.7 mm Hg versus 141+/-4.0 mm Hg for sham. Plasma Angiotensin (Ang) II, serum ACE activity, plasma renin concentration, as well as urinary excretion of Ang II, 8-isoprostane F2alpha, and endothelin-1 were elevated; however, renal mRNA levels of eNOS were suppressed after ovariectomy. Estrogen replacement reduced blood pressure below both the sham and OVX by 11 weeks (125+/-2.9 mm Hg, n=7, P<0.01 versus OVX and sham). Moreover, the AT1 receptor antagonist olmesartan (CS866; week 12 to 16) essentially normalized blood pressure to 113+/-5.4 mm Hg (n=6, P<0.01 versus OVX and sham). The attenuation of the hypertension was still evident 7 weeks after complete withdrawal of treatment (124+/-4.1 mm Hg at week 23). In summary, the OVX mRen.2. Lewis exhibited a rapid and sustained increase in blood pressure. Estrogen or olmesartan lowered pressure by a similar extent. We conclude that the ovary exerts considerable influence on the regulation of the blood pressure in the mRen2. Lewis strain, possibly by limiting activation of the renin-angiotensin system.

Effects of hormone replacement therapy on the sympathetic nervous system and blood pressure

Hypertension is a major health problem that significantly contributes to heart disease and stroke. While most studies of hypertension have focused on men, women also experience significant hypertension-related morbidity and mortality. However, the incidence of hypertension and cardiovascular disease is significantly lower in premenopausal women compared with men until the onset of menopause, at which time cardiovascular disease incidence increases dramatically in women and eventually approaches that in men. These observations indicate that the loss of estrogen contributes to menopause-related increases in blood pressure and cardiovascular disease, and suggest that the use of estrogen hormone replacement therapy could decrease the incidence of cardiovascular disease in postmenopausal women. However, new findings from the Women's Health Initiative study suggest that estrogen therapy has few positive benefits and some significant negative effects on the health of postmenopausal women, and these data have caused many to abandon long-term estrogen replacement therapy. Conversely, numerous clinical and basic research studies indicate that estrogen replacement therapy beneficially reduces blood pressure, thereby decreasing the incidence of hypertension and cardiovascular disease. Further, several of these studies suggest that one means by which estrogen lowers blood pressure is by decreasing sympathetic nervous system activity. This review examines the evidence supporting estrogen's ability to modulate sympathetic nervous system tone and thereby decrease arterial pressure.

Estrogen depletion increases blood pressure and hypothalamic norepinephrine in middle-aged spontaneously hypertensive rats

In male spontaneously hypertensive rats (SHR) a high NaCl diet increases arterial pressure via a reduction in anterior hypothalamic nucleus norepinephrine release. Young female SHR are relatively well protected from this NaCl-sensitive hypertension, but depletion of both endogenous and dietary estrogens greatly exacerbates NaCl-sensitive hypertension. This study tests the hypothesis that estrogen also protects late middle-aged female SHR from NaCl-sensitive hypertension and that this effect is mediated by an estrogen-related effect on hypothalamic norepinephrine release. Ten-month-old female SHR were ovariectomized and placed on a phytoestrogen-free diet containing either basal or high NaCl. Each rat was implanted with a silastic tube containing 17beta estradiol or vehicle. Three months later, arterial pressure and hypothalamic norepinephrine metabolite levels (MOPEG) were measured. On the basal NaCl diet, estrogen-depleted rats displayed increased arterial pressure (12 mm Hg) and decreased anterior hypothalamic nucleus MOPEG (20%). Both effects were reversed by estrogen treatment. In all groups, the high NaCl diet increased arterial pressure by over 35 mm Hg and reduced anterior hypothalamic nucleus MOPEG by >60%. Across all groups, there was a significant inverse correlation between arterial pressure and anterior hypothalamic nucleus MOPEG. These data suggest that both dietary NaCl excess and estrogen depletion raise arterial pressure in middle-aged female SHR by a decreasing hypothalamic norepinephrine.

Estrogen modulation of baroreflex function in conscious mice

It has been suggested that estrogen modulates baroreflex regulation of autonomic function. The present study evaluated the effects of estrogen on baroreflex regulation of heart rate in response to changes in blood pressure with phenylephrine (PE), ANG II, and sodium nitroprusside (SNP) in a conscious mouse model. Males and ovariectomized females with (OvxE+) and without (OvxE-) estradiol replacement chronically implanted with arterial and venous catheters were used in these studies. The slope of the baroreflex bradycardic responses to PE was significantly facilitated in OvxE+ females (-7.65 +/- 1.37) compared with OvxE- females (-4.5 +/- 0.4). Likewise, the slope of the baroreflex bradycardic responses to ANG II was significantly facilitated in OvxE+ females (-7.97 +/- 1.06) compared with OvxE- females (-4.8 +/- 1.6). Reflex tachycardic responses to SNP were comparable in all the groups. Finally, in male mice, the slope of ANG II-induced baroreflex bradycardia (-5.17 +/- 0.95) was significantly less than that induced by PE (-8.50 +/- 0.92), but this ANG II-mediated attenuation of reflex bradycardia was not observed in the female mice. These data support the hypothesis that estrogen facilitates baroreflex function in female mice and suggest that ANG II-mediated acute blunting of baroreflex regulation of heart rate may be sex dependent.