Effect of dietary sodium on estrogen regulation of blood pressure in 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.

Salt Sensitivity of Blood Pressure in Women

Several clinical and large population studies indicate that women are more salt-sensitive than men, yet the precise mechanisms by which the sexually dimorphic onset manifests remains incompletely understood. Here, we evaluate recent epidemiological data and highlight current knowledge from studies investigating sex-specific mechanisms of salt-sensitive blood pressure (SSBP). Emerging evidence indicates that women of all ethnicities are more salt-sensitive than men, at all ages both premenopausal and postmenopausal. However, menopause exacerbates severity and prevalence of SSBP, suggesting that female sex chromosomes predispose to and female sex hormones mitigate SSBP. Results from both human and rodent studies support the contribution of enhanced and inappropriate activation of the aldosterone-ECMR (endothelial cell mineralocorticoid receptor) axis promoting vascular dysfunction in females. Increases in adrenal response to angiotensin II, in association with higher ECMR expression and activation of endothelial ENaC (epithelial sodium channel) in females compared to males, are emerging as central players in the development of endothelial dysfunction and SSBP in females. Female sex increases the prevalence and susceptibility of SSBP and sex hormones and sex chromosome complement may exert antagonistic effects in the development of the female heightened SSBP.

New Mechanism for the Sex Differences in Salt-Sensitive Hypertension: The Role of Macula Densa NOS1β-Mediated Tubuloglomerular Feedback

Females are relatively resistant to salt-sensitive hypertension than males, but the mechanisms are not completely elucidated. We recently demonstrated a decisive role of macula densa neuronal NOS1β (nitric oxide synthase β)-mediated tubuloglomerular feedback (TGF) in the long-term control of glomerular filtration rate, sodium excretion, and blood pressure. In the present study, we hypothesized that the macula densa NOS1β-mediated TGF mechanism is different between male and female, thereby contributing to the sexual dimorphism of salt-sensitive hypertension. We used microperfusion, micropuncture, clearance of fluorescein isothiocyanate-inulin, and radio telemetry to examine the sex differences in the changes of macula densa NOS1β expression and activity, TGF response, natriuresis, and blood pressure after salt loading in wild-type and macula densa-specific NOS1 knockout mice. In wild-type mice, a high-salt diet induced greater increases in macula densa NOS1β expression and phosphorylation at Ser 1417, greater nitric oxide generation by the macula densa, and more inhibition in TGF response in vitro and in vivo in females than in males. Additionally, the increases of glomerular filtration rate, urine flow rate, and sodium excretion in response to an acute volume expansion were significantly greater in females than in males. The blood pressure responses to angiotensin II plus a high-salt diet were significantly less in females than in males. In contrast, these sex differences in TGF, natriuretic response, and blood pressure were largely diminished in knockout mice. In conclusion, macula densa NOS1β-mediated TGF is a novel and important mechanism for the sex differences in salt-sensitive hypertension.

The Angiotensin Type 1 Receptor Antagonist Losartan Prevents Ovariectomy-Induced Cognitive Dysfunction and Anxiety-Like Behavior in Long Evans Rats

Women who have bilateral oophorectomies prior to the age of natural menopause are at increased risk of developing mild cognitive decline, dementia, anxiety, and depressive type disorders. Clinical and animal studies indicate angiotensin type 1 receptor (AT₁R) blockers (ARBs) have blood pressure (BP)-independent neuroprotective effects. To investigate the potential use of ARBs in normotensive women at increased risk of developing neurocognitive problems, we studied a rat model of bilateral oophorectomy. Long Evans rats were sham-operated (Sham) or ovariectomized (Ovx) at 3 months of age and immediately treated continuously with vehicle (Veh) or the ARB losartan (Los) for the duration of the experiment. In contrast to many hypertensive rat models, ovariectomy did not increase mean arterial pressure (MAP) in these normotensive rats. Ovariectomized rats spent less time in the open arms of the elevated plus maze (EPM) [(% total time): Veh, 34.1 ± 5.1 vs. Ovx, 18.7 ± 4.4; p < 0.05] and in the center of the open field (OF) [(s): Veh, 11.1 ± 1.7 vs. Ovx, 6.64 ± 1.1; p < 0.05]. They also had worse performance in the novel object recognition (NOR) test as evidenced by a reduction in the recognition index [Veh, 0.62 ± 0.04 vs. Ovx, 0.45 ± 0.03; p < 0.05]. These adverse effects of ovariectomy were prevented by Los. Losartan also reduced plasma corticosterone in Ovx rats compared to Veh treatment [(ng/mL): Ovx–Veh, 238 ± 20 vs. Ovx–Los, 119 ± 42; p < 0.05]. Ovariectomy increased AT₁R mRNA expression in the CA3 region of the hippocampus (Hc) [(copies x 10⁶/µg RNA): Sham–Veh, 7.15 ± 0.87 vs. Ovx–Veh, 9.86 ± 1.7; p < 0.05]. These findings suggest the neuroprotective effects of this ARB in normotensive Ovx rats involve reduction of plasma corticosterone and blockade of increased AT₁R activity in the hippocampus. These data suggest ARBs have therapeutic potential for normotensive women at increased risk of developing cognitive and behavioral dysfunction due to bilateral oophorectomy prior to the natural age of menopause.

Renal T cell infiltration occurs despite attenuation of development of hypertension with hydralazine in Envigo's female Dahl rat maintained on a low-Na+ diet

Many studies have suggested that renal T cell infiltration contributes to the pathogenesis of salt-sensitive hypertension. To investigate this mechanism further, we determined T cell profiles in the kidney and lymphoid tissues as a function of blood pressure in the female Envigo Dahl salt-sensitive (SS) rat maintained on low-Na+ (LS) diet. Mean arterial pressure and heart rate were measured by telemetry in SS rats from 1 mo old (juvenile) to 4 mo old. Normotensive salt-resistant (SR) rats were included as controls. Frequencies of T helper (CD4+) cells were greater in the kidney, lymph nodes, and spleen in 4-mo-old hypertensive SS rats compared with normotensive SR animals and SS juvenile rats, suggesting that renal T cell infiltration contributes to hypertension in the SS rat on a LS diet. At 1.5 mo, half of the SS rats were treated with vehicle (Veh), and the rest received hydralazine (HDZ; 25 mg·kg-1·day-1) for 11 wk. HDZ impeded the development of hypertension compared with Veh-treated control rats [mean arterial pressure: 157 ± 4 mmHg in the Veh-treated group (n = 6) vs. 133 ± 3 mmHg in the HDZ-treated group (n = 7), P < 0.001] without impacting T helper cell frequencies in the tissues, suggesting that HDZ can overcome mechanisms of hypertension driven by renal T cell infiltration under the LS diet. Renal frequencies of CD4+CD25+ and CD4+CD25+FoxP3+ regulatory T cells were significantly higher in 4-mo-old hypertensive rats compared with normotensive SR rats and SS juvenile rats, suggesting that these T cell subpopulations play a compensatory role in the development of hypertension. Greater understanding of these T cell populations could lead to new therapeutic targets for treating inflammatory diseases associated with hypertension.

Salt-sensitive (Rapp) rats from Envigo spontaneously develop accelerated hypertension independent of ovariectomy on a low-sodium diet

Inbred salt-sensitive (SS) rats developed by John Rapp and distributed by Harlan (SS/JrHsd) were shown to model ovariectomy-induced hypertension because on a low-sodium (LS) diet, ovariectomized SS (SS-OVX) animals became hypertensive in contrast to their sham-operated (SS-SHAM) normotensive littermates. After Harlan merged with Envigo in 2015, inconsistencies in the LS normotensive phenotype were reported. To further investigate these inconsistencies, we studied the effects of ovariectomy on SS and salt-resistant (SR) rats purchased from Envigo (SS/JrHsd/Env) between 2015 and 2017. The mean arterial pressure (MAP) in SS rats on a LS diet exceeded 160 mmHg at 7 mo old. Ovariectomy at 3 mo had no detectable effect on MAP from 4 to 7 mo, nor did ovariectomy at 1.5 mo significantly affect MAP at 10 mo in either strain; only strain differences in MAP were observed [MAP: SR-SHAM ( n = 7 rats), 102 ± 3 mmHg; SR-OVX ( n = 6 rats), 114 ± 1 mmHg; SS-SHAM ( n = 7 rats), 177 ± 6 mmHg; SS-OVX ( n = 5 rats), 190 ± 12 mmHg; where P < 0.0001 vs. SR, same ovarian-status for SS-SHAM and SS-OVX, respectively]. Whole genome sequencing revealed more genomic variants of SS/JrHsd/Env, including single nucleotide and insertion deletion polymorphisms and higher heterozygous/homozygous ratios compared with the reference genome, than for SS/JrHsd/Mcwi and SS/Jr rats maintained in Milwaukee, WI and Toledo, OH, respectively, and which still exhibit normal blood pressure on a LS diet. These findings demonstrate that the female SS/JrHsd/Env rat has genetically diverged from the original phenotype, which was normotensive on a LS diet when the ovaries were intact but rapidly developed hypertension when the ovaries were removed. Nonetheless, the SS/JrHsd/Env rat could be a valuable model that complements other animal models of spontaneous hypertension used to investigate mechanisms of essential hypertension.

Orchidectomy attenuates high-salt diet-induced increases in blood pressure, renovascular resistance, and hind limb vascular dysfunction: role of testosterone

Sex hormone-dependent vascular reactivity is an underlying factor contributing to sex differences in salt-dependent hypertension. This study evaluated the role of androgens (testosterone) in high salt-induced increase in blood pressure (BP) and altered vascular reactivity in renal blood flow and perfused hind limb preparation. Weanling male rats (8 weeks old, 180-200 g) were bilaterally orchidectomised or sham operated with or without testosterone replacement (Sustanon 250, 10 mg/kg intramuscularly once in 3 weeks) and placed on a normal (0.3%) or high (4.0%) NaCl diet for 6 weeks. The high-salt diet (HSD) increased arterial BP, renal vascular resistance (RVR) and positive fluid balance (FB). These changes were accompanied by decreased plasma nitric oxide levels. The increased BP, RVR and FB observed in the rats fed a HSD were reversed by orchidectomy while testosterone replacement prevented the reversal. Phenylephrine (PE)-induced increased vascular resistance in the perfused hind limb vascular bed was enhanced by HSD, the enhanced vascular resistance was prevented by orchidectomy and testosterone replacement reversed orchidectomy effect. Vasorelaxation responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were impaired in HSD groups, orchidectomy attenuated the impairment, while testosterone replacement prevented the orchidectomy attenuation. These data suggested that eNOS-dependent and independently-mediated pathways were equally affected by HSD in vascular function impairment and this effect is testosterone-dependent in male Sprague-Dawley rats.

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.

Is immune system-related hypertension associated with ovarian hormone deficiency?

NEW FINDINGS

What is the topic of this review? This review summarizes recent data on the role of ovarian hormones and sex in inflammation-related hypertension. What advances does it highlight? The adaptive immune system has recently been implicated in the development of hypertension in males but not in females. The role of the immune system in the development of hypertension in women and its relationship to ovarian hormone production are highlighted. The immune system is known to contribute to the development of high blood pressure in males. However, the role of the immune system in the development of high blood pressure in females and the role of ovarian hormones has only recently begun to be studied. In animal studies, both the sex of the host and the T cell are critical biological determinants of susceptibility and resistance to hypertension induced by angiotensin II. In women, natural menopause is known to result in significant changes in the expression of genes regulating the immune system. Likewise, in animal models, ovariectomy results in hypertension and an upregulation in T-cell tumour necrosis factor-α-related genes. Oestrogen replacement results in decreases in inflammatory genes in the brain regions involved in blood pressure regulation. Together, these studies suggest that the response of the adaptive immune system to ovarian hormone deficiency is a significant contributor to hypertension in women.

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.

Evidence that the G protein-coupled membrane receptor GPR30 contributes to the cardiovascular actions of estrogen

Although female protection from cardiovascular diseases declines with the fall in circulating sex hormones experienced during menopause, clinical trials in older women fail to demonstrate beneficial effects for hormone replacement therapy. The recent discovery of GPR30, a membrane-bound estrogen receptor that is structurally and functionally unique from the steroid receptors ERα and ERβ, has unveiled additional signaling pathways by which estrogen may influence cardiovascular health. This review takes an organ-based approach to assess the expression and function of GPR30 in the cardiovascular system. We concluded that although the current literature does suggest a cardiovascular role for GPR30, additional exploration is necessary to fully elucidate the estrogenic actions mediated by this novel receptor.

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.

Orchidectomy attenuates impaired endothelial effects of a high-salt diet in Sprague–Dawley rats

The effect of sex hormones on vascular reactivity is considered one of the underlying factors contributing to gender differences in cardiovascular functions and diseases. Experiments were designed to investigate the role of androgens in salt-induced hypertension by assessing the relaxation response of isolated aortic rings to acetylcholine and sodium nitroprusside in the presence or absence of l-nitroarginine methyl ester in Sprague–Dawley rats. The rats were either orchidectomized or sham-operated, with or without testosterone replacement, and were placed on a normal or high-salt diet for 6 weeks. The results indicate a significant increase (p < 0.001) in the mean arterial blood pressure of rats on the high-salt diet, when compared with control or orchidectomized rats. Orchidectomy elicited a reduction in mean arterial blood pressure (p < 0.01), while testosterone replacement normalized mean arterial blood pressure to values seen in intact rats on the high-salt diet. The high-salt diet reduced the relaxation response to acetylcholine both in the presence and absence of inhibition of endothelial nitric oxide synthase with l-nitroarginine methyl ester. Bilateral orchidectomy attenuated the impaired endothelial function induced by the high-salt diet in rats, but this was reversed by concomitant administration of testosterone, suggesting a role for androgens in enhancing long-term vascular smooth muscle tone and hence maintenance of high blood pressure in salt-induced hypertension.

Time course of behavioral, physiological, and morphological changes after estradiol treatment of ovariectomized rats

Previous studies showed that treatment with 17-β-estradiol-3-benzoate (EB) reduces isoproterenol (ISOP) stimulated water intake by ovariectomized rats. This effect was observed 48h after the second of two EB injections, suggesting that the attenuation is attributable to classic EB actions to alter gene expression. However, in addition to classic, slowly-occurring, genomic effects, estrogens have more rapidly-occurring effects that may be nongenomic or 'nonclassical' genomic effects. Thus, it is possible that the EB attenuation of water intake stimulated by ISOP is genomic, nongenomic, or both. Accordingly, we measured ISOP-induced water intake by OVX rats at different times after EB injections, using time points likely to indicate classic genomic effects (48h or 24h) or nonclassical genomic or nongenomic effects (90min). We also examined EB effects on body weight, uterine weight, and plasma volume and Na(+) concentration in the same animals using the same time points and EB dose. EB treatment decreased water intake stimulated by ISOP in both the 24-h and 48-h groups; however, water intake in the 90-min group was not affected by EB. Uterine weight was unchanged 90min after EB, but was increased 24h after the first injection of EB. In contrast, body weight decreased after EB, but not until 48h after the second EB injection. Finally, EB did not alter plasma Na(+) concentration or hematocrit, though plasma protein concentration increased transiently 24h after EB treatment. Taken together, these findings suggest that the behavioral, morphological, and physiological effects of EB likely are attributable to slowly-occurring, classic genomic actions of estrogens. Moreover, the time course of the observed effects varied, suggesting tissue-specific differences in estrogen receptor density or subtype, or in co-activators or co-repressors that, ultimately, determine the timing and direction of EB effects.

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.