Oestrogen attenuates the increases in blood pressure and platelet aggregation in ovariectomized and salt-loaded Dahl salt-sensitive 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.

Cardiac adaptation to hypertension in adult female Dahl salt-sensitive rats is dependent on ovarian function, but loss of ovarian function does not predict early maladaptation

Aim of study was to examine experimentally the adult female hypertensive heart in order to determine the role of ovary function in the response of the heart to salt-dependent hypertension. Dahl salt-sensitive rats, age 12 weeks, with/without ovariectomy were fed a standard (0.3% NaCl) or high-salt diet (8%) for 16 weeks. Mean arterial blood pressure monitored noninvasively in conscious state increased significantly by high salt. Echocardiography was performed at baseline and endpoint. Heart function and molecular changes were evaluated at endpoint by left ventricle catheterization, by sirius red staining for collagen and by gene expression using quantitative RT-PCR for selected genes. At endpoint, significant concentric hypertrophy was present with high salt. Increase in relative wall thickening with high salt compared to normal diet was more pronounced with intact ovaries (0.33 ± 0.02 and 0.57 ± 0.04 vs. 0.29 ± 0.00 and 0.46 ± 0.03) as was the reduction in midwall fractional shortening (20 ± 0.6 and 14 ± 2 vs. 19 ± 0.9 and 18 ± 1). Ovariectomy increased stroke volume and decreased the ratio of mitral peak velocity of early filling (E) to early diastolic mitral annular velocity (E') (E/E' ratio) when compared to hearts from intact rats. High salt increased expression of collagen I and III genes and perivascular collagen in the heart slightly, but % interstitial collagen by sirius red staining remained unchanged in intact rats and decreased significantly by ovariectomy. Added volume load but not deterioration of function or structure characterized the nonfailing hypertensive heart of salt-sensitive females ovariectomized at mature age when compared to corresponding intact females.

Recent advances in central cardiovascular control: sex, ROS, gas and inflammation

The central nervous system (CNS) in concert with the heart and vasculature is essential to maintaining cardiovascular (CV) homeostasis. In recent years, our understanding of CNS control of blood pressure regulation (and dysregulation leading to hypertension) has evolved substantially to include (i) the actions of signaling molecules that are not classically viewed as CV signaling molecules, some of which exert effects at CNS targets in a non-traditional manner, and (ii) CNS locations not traditionally viewed as central autonomic cardiovascular centers. This review summarizes recent work implicating immune signals and reproductive hormones, as well as gasotransmitters and reactive oxygen species in the pathogenesis of hypertension at traditional CV control centers. Additionally, recent work implicating non-conventional CNS structures in CV regulation is discussed.

Hemodynamic effect of angiotensin II receptor blockade in postmenopausal women on a high-sodium diet: A double-blind, randomized, placebo-controlled study

BACKGROUND

Hypertension becomes increasingly prevalent after menopause. Postmenopausal women are more responsive to salt than premenopausal women, and they have been reported to develop marked renal vasoconstriction on a high-sodium diet.

OBJECTIVE

The aim of this study was to assess whether angiotensin II receptor blockade can restore a normal pattern of renal response to salt in postmenopausal women on a high-sodium diet. We also assessed segmental renal sodium handling in that population.

METHODS

Normotensive and hypertensive postmenopausal women not receiving hormone replacement therapy were enrolled in this prospective, double-blind, placebo-controlled, crossover study. They were assigned to receive irbesartan 150 mg or placebo for 6 weeks; the sequence in which they received irbesartan or placebo was randomized. During the last week of treatment, they received a high-sodium diet (250 mmol/d). Ambulatory blood pressure (ABP), glomerular filtration rate (GFR), and effective renal plasma flow (ERPF) were measured using sinistrin and para-amino-hippurate clearances. Renal sodium handling was assessed by measuring endogenous lithium clearance on day 7 of the high-salt diet.

RESULTS

Nineteen women (mean age, 54.7 years; range, 43-72 years; 7 normotensive subjects [mean age, 53.4 years; range, 47-61 years] and 12 hypertensive subjects [mean age, 55.4 years; range, 43-72 years]) were included in the study. When the data for all 19 subjects were pooled, ABP was significantly lower with irbesartan than placebo both during the day (120 [3]/79 [2] vs 127 [3]/85 [2] mm Hg; both, P < 0.01) and at night (systolic BP, 107 [4] vs 111 [4] mm Hg [P < 0.01] and diastolic BP, 71 [2] vs 75 [2] mm Hg [P < 0.05]). Compared with placebo, irbesartan was not associated with a significant change in GFR in either the normotensive or the hypertensive women. When the data for all 19 subjects were pooled, irbesartan was associated with a significant increase in ERPF compared with placebo (372 [21] vs324 [18] mL/min · 1.73 m(2); P < 0.05). When the hypertensive and normotensive women were considered separately, the effect was more pronounced in the hypertensive women than in the normotensive women, but the changes did not reach statistical significance. When the data for all subjects were pooled, irbesartan was associated with a significant increase in daytime urinary sodium excretion compared with placebo (135 [13] vs 106 [13] μmol/min; P < 0.05) and a significant decrease at night (109 [13] vs 136 [19] μmol/min; P < 0.05). Fractional excretion of lithium (FELi), an inverse marker of proximal sodium reabsorption, increased significantly during the daytime with irbesartan compared with placebo (47% [6.5%] vs 35% [4.7%]; P < 0.05). At nighttime, FELi was significantly higher in the hypertensive subjects receiving irbesartan compared with placebo (43% [7.2%] vs 29% [6.5%]; P < 0.05). The fractional distal reabsorption of sodium did not change significantly with irbesartan compared with placebo.

CONCLUSIONS

The results from this study suggest that angiotensin II receptor blockade had a favorable impact on BP, renal hemodynamics, and renal sodium handling in these salt-replete postmenopausal women. Blockade of the renin-angiotensin system restored the normal pattern of renal response to high sodium intake in these women.

Estrogen therapy, independent of timing, improves cardiac structure and function in oophorectomized mRen2.Lewis rats

OBJECTIVE

mRen2.Lewis rats exhibit exacerbated increases in blood pressure, left ventricular (LV) remodeling, and diastolic impairment after the loss of estrogens. In this same model, depletion of estrogens has marked effects on the cardiac biopterin profile concomitant with suppressed nitric oxide release. With respect to the establishment of overt systolic hypertension after oophorectomy (OVX), we assessed the effects of timing long-term 17β-estradiol (E2) therapy on myocardial function, myocardial structure, and the cardiac nitric oxide system.

METHODS

OVX (n = 24) or sham operation (Sham; n = 13) was performed in 4-week-old female mRen2.Lewis rats. After randomization, OVX rats received E2 immediately (OVX + E2-early; n = 7), E2 at 11 weeks of age (OVX + E2-late; n = 8), or no E2 at all (OVX; n = 9).

RESULTS

E2-early was associated with lower body weight, less hypertension-related cardiac remodeling, and decreased LV filling pressure compared with OVX rats without E2 supplementation. E2-late similarly attenuated the adverse effects of ovarian hormone loss on tissue Doppler-derived LV filling pressures and perivascular fibrosis, and significantly improved myocardial relaxation or mitral annular velocity (e'). Early and late exposures to E2 decreased dihydrobiopterin, but only E2-late yielded significant increases in cardiac nitrite concentrations.

CONCLUSIONS

Although there are some similarities between E2-early and E2-late treatments in relation to preservation of diastolic function and cardiac structure after OVX, the lusitropic potential of E2 is most consistent with late supplementation. The cardioprotective effects of E2-late are independent of blood pressure and may have occurred through regulation of cardiac biopterins and nitric oxide production.

Migraine: possible role of shear-induced platelet aggregation with serotonin release

BACKGROUND

Migraine patients are at an increased risk for stroke, as well as other thromboembolic events. This warrants further study of the role of platelets in a proportion of migraine patients.

OBJECTIVE

To extend the "platelet hypothesis" using literature data and observations made in a rat model of shear stress-induced platelet aggregation. Such aggregation causes release of serotonin, leading to vasoconstriction during sufficiently strong aggregation and to long-lasting vasodilation when aggregation diminishes. This vasodilation also depends on nitric oxide and prostaglandin formation.

RESULTS

A role for platelet aggregation in a number of migraineurs is indicated by reports of an increased platelet activity during attacks and favorable effects of antiplatelet medication. We hypothesize that in those patients, a migraine attack with or without aura may both be caused by a rise in platelet-released plasma serotonin, albeit at different concentration. At high concentrations, serotonin may cause vasoconstriction and, consequently, the neuronal signs of aura, whereas at low concentrations, it may already stimulate perivascular pain fibers and cause vasodilation via local formation of nitric oxide, prostaglandins, and neuropeptides. Platelet aggregation may be unilaterally evoked by elevated shear stress in a stenotic cervico-cranial artery, by reversible vasoconstriction or by other cardiovascular abnormality, eg, a symptomatic patent foramen ovale. This most likely occurs when a migraine trigger has further enhanced platelet aggregability; literature shows that many triggers either stimulate platelets directly or reduce endogenous platelet antagonists like prostacyclin.

CONCLUSION

New strategies for migraine medication and risk reduction of stroke are suggested.

Prenatal testosterone exposure leads to hypertension that is gonadal hormone-dependent in adult rat male and female offspring

Prenatal testosterone exposure impacts postnatal reproductive and endocrine function, leading to alterations in sex steroid levels. Because gonadal steroids are key regulators of cardiovascular function, it is possible that alteration in sex steroid hormones may contribute to development of hypertension in prenatally testosterone-exposed adults. The objectives of this study were to evaluate whether prenatal testosterone exposure leads to development of hypertension in adult males and females and to assess the influence of gonadal hormones on arterial pressure in these animals. Offspring of pregnant rats treated with testosterone propionate or its vehicle (controls) were examined. Subsets of male and female offspring were gonadectomized at 7 wk of age, and some offspring from age 7 to 24 wk received hormone replacement, while others did not. Testosterone exposure during prenatal life significantly increased arterial pressure in both male and female adult offspring; however, the effect was greater in males. Prenatal androgen-exposed males and females had more circulating testosterone during adult life, with no change in estradiol levels. Gonadectomy prevented hyperandrogenism and also reversed hypertension in these rats. Testosterone replacement in orchiectomized males restored hypertension, while estradiol replacement in ovariectomized females was without effect. Steroidal changes were associated with defective expression of gonadal steroidogenic genes, with Star, Sf1, and Hsd17b1 upregulation in testes. In ovaries, Star and Cyp11a1 genes were upregulated, while Cyp19 was downregulated. This study showed that prenatal testosterone exposure led to development of gonad-dependent hypertension during adult life. Defective steroidogenesis may contribute in part to the observed steroidal changes.

Effects of estrogen on cardiovascular injury in ovariectomized female DahlS.Z-Lepr(fa)/Lepr(fa) rats as a new animal model of metabolic syndrome

Although recent clinical trials have found an increased incidence of cardiovascular disease in women on estrogen replacement therapy, the underlying mechanism remains unclear. We have recently characterized DahlS.Z-Lepr(fa)/Lepr(fa) (DS/obese) rats, derived from a cross between Dahl salt-sensitive and Zucker rats, as a new animal model of metabolic syndrome. We have now examined the effects of estrogen replacement on cardiac pathophysiology in ovariectomized female DS/obese (Ovx-DS/obese) rats. Animals subjected to ovariectomy at 7 weeks of age were implanted subcutaneously with a 60-day release pellet containing 0.5 mg of 17β-estradiol (E(2)) or placebo at 8 weeks. Age-matched female homozygous lean littermates (DahlS.Z-Lepr(+)/Lepr(+) or DS/lean rats) of DS/obese rats served as controls. Animals were maintained on a normal diet and were subjected to echocardiography followed by various pathological analyses at 13 weeks of age. Ovx-DS/obese rats manifested hypertension at 7 weeks of age and thereafter and showed left ventricular (LV) fibrosis and diastolic dysfunction at 13 weeks. Treatment with E(2) attenuated hypertension in Ovx-DS/obese rats but had no effect on blood pressure in ovariectomized female DS/lean (Ovx-DS/lean) rats. E(2) treatment exacerbated LV fibrosis and diastolic dysfunction, as well as further increased cardiac oxidative stress and inflammation in Ovx-DS/obese rats, and it elicited similar effects in Ovx-DS/lean rats. E(2) reduced food intake, body weight, and visceral fat content in both Ovx-DS/obese and Ovx-DS/lean rats. E(2) treatment attenuated hypertension and obesity but exacerbated LV fibrosis and diastolic dysfunction in Ovx-DS/obese rats, with these latter effects being associated with increased cardiac oxidative stress and inflammation.

Changes in oestrogen receptor alpha expression in the nucleus of the solitary tract of the rat over the oestrous cycle and following ovariectomy

Oestrogen is capable of modulating autonomic outflow and baroreflex function via actions on groups of neurones in the brainstem. We investigated the presence of oestrogen receptor (ER) alpha in a part of the nucleus of the solitary tract (NTS) associated with central cardiovascular control, aiming to determine whether ERalpha mRNA and protein expression is correlated with levels of circulating oestrogen during the oestrous cycle. Polymerase chain reaction (PCR) detected ERalpha mRNA in the NTS at each stage of the oestrous cycle, from ovariectomised, sham-operated and male rats. Real-time PCR showed variations in ERalpha mRNA expression during the oestrous cycle, with the highest levels seen in oestrus, and lowest levels in metoestrus (P < 0.05 versus oestrus) and proestrus (P < 0.05 versus oestrus). Expression in males was lower than in dioestrus and oestrus females (P < 0.05). After ovariectomy, ERalpha mRNA levels were decreased compared to sham-operated animals (P < 0.01). Confocal fluorescence immunohistochemistry with stereological analysis showed that numbers of ERalpha immunoreactive cell nuclei per mm(3) of tissue in the caudal NTS were significantly greater in proestrus than in other groups of rats (P < 0.05). There were also differences among the groups in the extent of colocalisation of ERalpha in neurones immunoreactive for tyrosine hydroxylase and nitric oxide synthase. These results imply a complex pattern of region-specific oestrogen signalling in the NTS and suggest that ERalpha expression in this important autonomic nucleus may be related to circulating oestrogen levels. This may have consequences for the regulation of autonomic tone and baroreflex sensitivity when oestrogen levels decline, for example following menopause.

Chronic treatment with the G protein-coupled receptor 30 agonist G-1 decreases blood pressure in ovariectomized mRen2.Lewis rats

The mRen2.Lewis congenic strain is an estrogen-sensitive model of hypertension whereby estrogen depletion produces a significant and sustained increase in blood pressure. The recent identification of G protein-coupled receptor 30 (GPR30) as a third estrogen receptor isotype prompted us to test the hypothesis that this novel receptor exhibits beneficial cardiovascular actions in the hypertensive female mRen2.Lewis rat. Intact female, ovariectomized female (OVX), and male mRen2.Lewis rats were treated with the selective GPR30 agonist G-1 or vehicle via osmotic minipump for 2 wk. G-1 significantly reduced systolic blood pressure in OVX (178 +/- 7 to 142 +/- 10 mm Hg, P < 0.001, n = 8) but not intact female (144 +/- 3 to 143 +/- 5 mm Hg, P > 0.05, n = 5) or male mRen2.Lewis rats (207 +/- 7 to 192 +/- 5 mm Hg, P > 0.05, n = 7). G-1 did not alter uterine or body weight in OVX, suggesting activation of a receptor distinct from estrogen receptor-alpha and -beta. In isolated aortic rings from OVX, G-1 reduced constriction in response to angiotensin II. Vascular angiotensin-converting enzyme and angiotensin type 1 receptor mRNA were also lower, whereas angiotensin-converting enzyme-2 mRNA was increased. G-1 treatment in OVX was not associated with alterations in either endothelial nitric oxide synthase expression or acetylcholine-induced relaxation. Immunohistochemical staining for GPR30 was evident in both the intima and media of the aorta. We conclude that the novel estrogen receptor GPR30 may contribute to the beneficial cardiovascular actions of estrogen in female mRen2.Lewis rats through regulation of vascular components of the renin-angiotensin system.

Neuropeptidergic mediators of spontaneous physical activity and non-exercise activity thermogenesis

Lean individuals have high levels of spontaneous physical activity (SPA) and the energy expenditure derived from that activity, termed non-exercise activity thermogenesis or NEAT, appears to protect them from obesity. Conversely, obesity in different human populations is characterized by low levels of SPA and NEAT. Like in humans, elevated SPA in rats appears to protect against obesity: obesity-resistant rats have significantly greater SPA and NEAT than obesity-prone rats. We review the literature on brain mechanisms important in mediating SPA and NEAT. The focus is on neuropeptides, including cholecystokinin, corticotropin-releasing hormone (also known as corticotropin-releasing factor), neuromedin U, neuropeptide Y, leptin, agouti-related protein, orexin-A (also known as hypocretin-1), and ghrelin. We also review information regarding interactions between these neuropeptides and dopamine, a neurotransmitter important in mediating motor function. Finally, we present evidence that elevated signaling of pathways mediating SPA and NEAT may protect against weight gain and obesity.

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.

The endocrine system in chronic nitric oxide deficiency

The experimental model of chronic inhibition of nitric oxide (NO) production has proven to be a useful tool to study cardiovascular and renal lesions produced by this type of hypertension, which are similar to those found in human hypertension. It also offers a unique opportunity to study the interaction of NO with the humoral systems, known to have a role in the normal physiology of vascular tone and renal function. This review provides a thorough and updated analysis of the interactions of NO with the endocrine system. There is special focus on the main vasoactive factors, including the renin-angiotensin-aldosterone system, catecholamines, vasopressin, and endothelin among others. Recent discoveries of crosstalk between the endocrine system and NO are also reported. Study of these humoral interactions indicates that NO is a molecule with ubiquitous function and that its inhibition alters virtually to all other known regulatory systems. Thus, hypothyroidism attenuates the pressor effect of NO inhibitor N-nitro-L-arginine methyl ester, whereas hyperthyroidism aggravates the effects of NO synthesis inhibition; the sex hormone environment determines the blood pressure response to NO blockade; NO may play a homeostatic role against the prohypertensive effects of mineralocorticoids, thyroid hormones and insulin; and finally, NO deficiency affects not only blood pressure but also glucose and lipid homeostasis, mimicking the human metabolic syndrome X, suggesting that NO deficiency may be a link between metabolic and cardiovascular disease.

Chronic estrogen treatment reduces vaso-constrictor responses in insulin resistant rats

Previous experiments have shown that chronic estrogen treatment via subcutaneous implants prevented insulin-induced blood pressure elevation and increased insulin sensitivity in ovariectomized female rats. In vitro vascular studies were performed using isolated mesenteric arteries to determine the effect of chronic estrogen and insulin treatments on vascular responses to vasoconstrictor agents. Female Wistar rats were assigned to the following groups: sham-operated, sham-operated plus insulin, sham-operated plus insulin plus estrogen, ovariectomized, ovariectomized plus insulin, and ovariectomized plus insulin plus estrogen. Chronic insulin and estrogen treatments were initiated with subcutaneous placement of insulin implants (2 U/d) and 17β-estradiol implants (0.5 mg/pellet, 60 day release) at the back of the neck. After 8 weeks of treatment, mesenteric arteries were isolated for assessment of constrictor responses to norepinephrine and the thromboxane A2 analogue U46619 in the presence or absence of the endothelium. The results show that chronic estrogen treatment attenuated the vascular constrictor responses to norepinephrine and U46619 only in endothelium intact vessels. Incubation with insulin did not significantly affect norepinephrine-induced vascular smooth muscle contraction. The study provides evidence that the mechanism by which estrogen prevents insulin-induced blood pressure elevation in insulin-treated ovariectomized rats is by influencing endothelium-derived vasoactive factors such as thromboxane A2.

Estrogen modulates the mechanical homeostasis of mouse arterial vessels through nitric oxide

We have recently shown that estrogen causes vessel dilation through receptor-mediated stimulation of nitric oxide (NO) production. Here, we hypothesize that estrogen modulates the mechanical homeostasis in the blood vessel wall through NO production. The mechanical properties of female ovariectomized (ovx) mice, female mice lacking the gene for endothelial NO synthase (eNOS(-/-)), and control female and male mice were studied to test the hypothesis. The femoral and carotid arteries and aorta were cannulated in situ and mechanically distended. The stress, strain, elastic modulus, and wall thickness of vessels in ovx and eNOS(-/-) mice, as well as intact female and male mice, were determined. Western blot and immunohistochemistry were used to assess eNOS protein expression in the aorta. Moreover, NO by-products of the femoral and carotid artery were determined by measuring the levels of nitrite and nitrate. Our results show that ovariectomy and eNOS(-/-) significantly decrease the strain in all arteries. Furthermore, the eNOS protein was significantly reduced in ovx mice. Finally, the NO metabolites were significantly decreased both in ovx and eNOS(-/-) mice. We found statistically significant correlations between the structural (wall thickness), mechanical (stress, strain, and elastic modulus), and biochemical parameters (NO by-products). These novel results connect NO to the structural and mechanical properties of the vessel wall. Hence, the effect of endogenous estrogen on the arterial mechanical properties is mediated by the regulation of NO derived from eNOS.

17Beta-estradiol attenuates PDGF signaling in vascular smooth muscle cells at the postreceptor level

Estrogens are known to display significant vasoprotective effects in premenopausal women. PDGF is an important mediator of vascular smooth muscle cell (VSMC) migration and proliferation, and thus atherogenesis. We analyzed the effects of 17beta-estradiol (E2) on beta-PDGF receptor (beta-PDGFR) expression/activation and PDGF-dependent VSMC proliferation, migration, and downstream signaling events. Pretreatment of VSMCs with E2 (0.3 microM-0.1 mM) for 24 h concentration-dependently inhibited PDGF-induced proliferation and migration up to 85.5 +/- 15.8% and 79.4 +/- 9.8%, respectively (both P < 0.05). These effects were prevented by coincubation with the ER antagonist ICI-182780. E2 did not alter beta-PDGFR expression, nor did it impair the ligand-induced tyrosine phosphorylation of the beta-PDGFR and consecutive binding of the receptor-associated signaling molecules Src homology region 2-containing phosphatase-2, PLC-gamma, phosphatidylinositol 3-kinase, and RasGAP. Thus estrogens inhibited PDGF-induced cellular responses at the postreceptor level. Although stimulation of VSMCs with PDGF-BB led to a transient increase of rac-1 activity, pretreatment with E2 for 24 h concentration-dependently inhibited PDGF-induced rac-1 activation. Furthermore, inhibition of rac-1 by Clostridium sordellii lethal toxin or overexpression of dominant-negative rac-1 (rac-N17) significantly inhibited PDGF-induced VSMC migration, indicating that rac-1 activity is essential for PDGF-dependent cellular responses. E2 did not further reduce PDGF-induced migration in rac-N17-overexpressing cells, suggesting that it diminishes VSMC migration by altering rac-1 activity. We conclude that E2 attenuates PDGF-dependent cellular functions of VSMCs downstream of the beta-PDGFR via inhibition of rac-1. These observations offer a molecular explanation for the vasoprotective effects of estrogens.

Impact of Nitric Oxide Synthase Inhibitor and Chloride Channel Antagonist on Mesenteric Vascular Conductance in Anesthetized Dahl Normotensive and Hypertensive Rats

The effects of nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) and chloride channel antagonist niflumic acid on vascular responsiveness to the effect of alpha1-adrenoceptor stimulation in the mesenteric bed of Dahl salt-resistant normotensive (SRN) and salt-sensitive hypertensive (SSH) rats were examined. Dahl salt-resistant and salt-sensitive rats were fed a high-salt diet (4% NaCl) for 7 weeks, and blood pressure, heart rate, and mesenteric blood flow were measured before and after treatment with L-NAME (0.3 mg/kg, IV) and/or niflumic acid (10 mg/kg, IV). Morphometry of the primary mesenteric blood vessel was also assessed. Administration of alpha1-adrenoceptor agonist cirazoline produced a dose-dependent increase in blood pressure, decrease in heart rate, mesenteric blood flow, and mesenteric vascular conductance in SRN and SSH rats. L-NAME significantly increased basal blood pressure and decreased basal mesenteric blood flow and vascular conductance in SRN but not in SSH rats. Niflumic acid attenuation of cirazoline-mediated decreases in mesenteric blood flow and vascular conductance was more pronounced in the SRN than SSH rats. This difference in the inhibitory actions of niflumic acid was absent following its concomitant administration with L-NAME. It seems that tonic release of nitric oxide modulates niflumic acid-sensitive chloride channels in vascular muscle. Blood vessels from SSH rats had significantly larger smooth muscle thickness and lumen diameter, but the ratio of the 2 were not different between the SRN and SSH. Our findings support the view that alterations in receptor-mediated signal transduction, rather than just changes in blood vessel architecture, are responsible for differences in behavior of blood vessels in salt-induced hypertensive rats.

Gender difference in functional properties of Na,K-ATPase in the heart of spontaneously hypertensive rats

The aim of present study was the investigation of functional properties of the cardiac Na,K-ATPase in 16 weeks old male and female spontaneously hypertensive rats (SHR). The Na,K-ATPase activity in the presence of increasing concentrations of ATP, as well as Na(+) was lower in SHR of both genders, as compared to respective normotensive controls. Evaluation of kinetic parameters revealed a significant decrease of the maximum velocity (V(max)) in males (30% for ATP-activation, 40% for Na(+)-activation), as well as in females (24% for ATP, 29% for Na(+)), indicating a hypertension-induced diminution of the number of active enzyme molecules in cardiac sarcolemma. Insignificant changes were observed in the value of Michaelis-Menten constant (K(m)) in both cases. The concentration of sodium that gives half-maximal reaction velocity (K(Na)), increased by 38% in male and by 70% in female SHR. This impairment in the affinity of the Na(+)-binding site together with decreased number of active Na,K-ATPase molecules are probably responsible for the deteriorated enzyme-function in hearts of SHR. Direct comparison of SHR of both genders showed, that the enzyme from female hearts seems to be adapted better to hypertension as documented by its increased activity as a consequence of improved ability to bind and utilize ATP, as suggested by 32% decrease of K(m) value in females. In addition, the enzyme from female hearts is able to increase its activity (by 41%) in the presence of increasing sodium concentration even in the range where the enzyme from male hearts is already saturated.

Role of sex, gonadectomy and sex hormones in the development of nitric oxide inhibition-induced hypertension

In this study we have evaluated the influence of sex, gonadectomy and sex hormones on the development of L-NAME-induced hypertension in the rat, focusing our investigation on blood pressure (BP), plasma renin activity (PRA), cardiac hypertrophy and proteinuria. Three experiments were performed to investigate: (i) the influence of sex on the development of L-NAME-induced hypertension; (ii) the effects of gonadectomy on the dimorphism of L-NAME-induced hypertension; and (iii) the effects of testosterone in ovariectomized female and of 17beta-oestradiol in orchidectomized male rats. Male L-NAME-treated rats had higher BP values than females. Orchidectomy of L-NAME-treated rats reduced BP to the levels of females and ovariectomy did not affect hypertension in females. Oestrogenized and orchidectomized males had a BP level similar to intact female L-NAME-treated rats. However, androgenization and ovariectomy did not change BP in female L-NAME-treated rats. PRA was greater in intact male L-NAME hypertensive rats than in female rats, and gonadectomy protected against the increase in PRA such that PRA was similar among all the groups. Intact female hypertensive rats showed significantly greater ventricular hypertrophy compared with male hypertensive rats. Male L-NAME hypertensive rats had increased proteinuria that was not present in female rats. Moreover, testosterone increased proteinuria in males and females regardless of the BP level. Male L-NAME-treated rats developed higher BP, PRA and proteinuria than female rats, but were more resistant to the development of cardiac hypertrophy. The higher PRA of male L-NAME-treated rats might be involved in the sex-dependent dimorphism of this model of hypertension.

Involvement of atrial natriuretic peptide in blood pressure reduction induced by estradiol in spontaneously hypertensive rats

The aim of the present study was to determine the involvement of atrial natriuretic peptide (ANP) in blood pressure (BP) alterations induced by estradiol treatment. Spontaneously hypertensive rats (SHR) and Wistar rats (WR) were ovariectomized and, after 3 weeks, were injected daily for 4 days with estradiol benzoate (E2; 5 microg/100 g/day) or a vehicle. One day after the last injection, the animals were decapitated, blood was collected, and both right and left atrial appendages were quickly removed for determination of ANP by radioimmunoassay (RIA), or used for ANP mRNA determination. Estradiol treatment induced a significant reduction of blood pressure in SHR, but not in WR. This reduction was correlated with the increase of plasma ANP levels that were significantly increased in E2-treated, compared with vehicle-treated, SHR. E2-treated SHR showed significant increases in ANP concentration in the right and left atria compared to the vehicle-treated animals. These observations were confirmed by ANP mRNA. In summary, the present study shows that short-term estradiol treatment reduces the blood pressure of ovariectomized SHR, but not of WR. This reduction was highly correlated with increased plasma estradiol and ANP levels. These results suggest that ANP is involved in mediating the effect of estradiol on blood pressure reduction.

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.

Gender differences in the regulation of vascular tone

1. The greater incidence of hypertension and coronary artery disease in men and post-menopausal women compared with premenopausal women has suggested vascular protective effects of the female sex hormone oestrogen. However, vascular effects of the female sex hormone progesterone and the male sex hormone testosterone have also been suggested. 2. Oestrogen, progesterone and testosterone receptors have been identified in the plasmalemma, cytosol and nuclear compartments of vascular cells. The interaction of sex hormones with their specific receptors triggers not only long-term genomic vascular effects, but also acute non-genomic vascular responses. 3. Sex hormones may activate endothelium-dependent vascular relaxation pathways, including the nitric oxide-cGMP and prostacyclin-cAMP pathways and a hyperpolarizing factor pathway. 4. Sex hormones may also inhibit the mechanisms of vascular smooth muscle contraction, such as [Ca2+]i, protein kinase C and other protein kinases. 5. The sex hormone-induced stimulation of endothelium-dependent vascular relaxation and inhibition of vascular smooth muscle contraction may contribute to the gender differences in vascular tone and may represent potential beneficial vascular effects of hormone-replacement therapy during natural and surgically induced deficiencies of gonadal hormones.

Interactions between endothelial nitric oxide synthase and sex hormones in vascular protection in mice

The vasculoprotective effects of sex hormones, particularly estrogens, have been attributed to their ability to increase the bioavailability of nitric oxide through activation of endothelial nitric oxide synthase (eNOS). To dissect the relative contribution in vivo of eNOS, sex hormones, and their interaction in two complex vascular phenotypes, hypertension and atherosclerosis, we used mice doubly deficient in eNOS and apoE (nnee) or lacking only apoE (NNee). Females and males were gonadectomized at 1 month of age and implanted either with control pellets or pellets releasing 17beta-estradiol (E2). Hormonally intact nnee mice have elevated blood pressure (BP) and increased atherosclerosis compared with NNee mice, but on removal of gonads, BP and atherosclerosis decreased significantly in nnee mice but not in NNee mice. Three months of treatment with exogenous E2 dramatically reduced atherosclerosis and significantly lowered BP in both NNee and nnee mice compared with animals treated with control pellets. Thus exogenous E2 has strong BP-lowering and atheroprotective effects in apoE-deficient mice, but eNOS is not essential for either effect. Endogenous sex hormones, on the other hand, cause significant damage to the vasculature in the absence of eNOS, but these effects are overridden by interactions between eNOS and sex hormones.