Novel Advances in Our Understanding of Sex-Dependent Control of Blood Pressure

The vasculature critically contributes to blood pressure by regulating the size and elasticity of blood vessels, modulating vascular resistance, and producing factors that impact vascular tone. Sex differences have been well-documented in several vascular beds and are particularly influenced by sex hormones ( 33 – 36 ). One study reported that prepubescent females had stiffer large arteries and higher pulse pressure relative to age-matched males ( 37 ). However, this study noted that arterial stiffness decreased in females and increased in males post-puberty. This post-pubertal sex difference is maintained through mid-life, and though arterial stiffness increases with age in both men and women ( 38 ), menopausal women experience a more drastic increase in stiffening ( 39 , 40 ) ( Table 1 ). Intriguingly, loss of vascular compliance throughout the menopausal transition was linked to oxidative stress (see the section titled Oxidative Stress ) ( 40 ). One study utilized the four-core genotype (FCG) mouse model to assess the influence of sex hormones versus sex chromosome complement (the specific pair of sex chromosomes that determines an individual’s sex) on arterial stiffness. In the FCG model, sex chromosome complement is separated from the animal’s gonadal sex. The FCG mouse model was created by deleting the testes-determining Sry gene from the Y chromosome and subsequent insertion into an autosome, allowing it to be sorted and inherited separately from the Y chromosome. This allows for the generation of any of the four-core genotypes: XX and XY gonadal females and XX and XY gonadal males ( 41 ) ( Figure 1 ). This study demonstrated that, regardless of sex chromosome complement, castrated male mice exhibited greater arterial stiffness relative to their intact counterparts ( 42 ). Thus, androgens play a role in maintaining vascular tone, and this study provides insight into the increase in arterial stiffening that is observed in males with aging, a process that coincides with falling testosterone levels. Studies in both humans and animals imply that both conduit and resistance vessels from females exhibit less of a constrictive capability, relative to males, in response to vasoconstrictors and pressure ( 43 – 45 ). One possible reason for this lower myogenic tone in premenopausal females could be sex differences in calcium (Ca 2+ ) signaling. Ca 2+ signaling and the balance between cytosolic and intracellular Ca 2+ are essential for vascular smooth muscle–mediated hemodynamic changes to regulate blood pressure ( 46 ). A study demonstrated that cytosolic Ca 2+ levels in vascular smooth muscle cells were lower in female Wistar-Kyoto rats relative to males; this was also true in spontaneously hypertensive rats (SHRs) ( 47 ). Ovariectomy resulted in a significant increase in cytosolic Ca 2+ and contractility, which was reversed with estradiol treatment. There were no differences in contractility or Ca 2+ levels between intact and castrated males, indicating that female sex hormones regulate this difference in basal Ca 2+ concentration. Sex differences in cardiac structure and function may also impact blood pressure regulation. Not only is the heart 25% smaller in adult females than in males ( 48 , 49 ), but female hearts do not scale isometrically, displaying different proportions of wall thickness and chamber size from male hearts ( 50 ). Comparisons by lean body mass are able to reduce, but cannot fully eliminate, sex differences in cardiac geometry. Further, despite a faster heartbeat, females have a lower cardiac output and smaller stroke volume relative to males ( 50 ). Many of the structural sex differences in the cardiovascular system contribute to reduced vasoconstriction and lowered vascular resistance in females, ultimately contributing to lower normotensive blood pressure ( 50 , 51 ). The kidney is a key controller of blood pressure, acting as a primary controller of extracellular volume. Several lines of evidence indicate that sex differences in renal function are crucial in blood pressure control. First, the pressure-natriuresis relationship—a physiological process whereby increased renal perfusion pressure leads to increased sodium excretion ( 52 )—is left-shifted in female rats ( 53 ), and there are multiple indications that electrolytes are handled differently in male and female kidneys. A recent computational modeling study reported that the attenuation of the blood pressure response to hypertensive stimuli in women is likely dependent on the renal transporter abundance pattern ( 54 ). There are numerous sex differences in renal transporters, including altered phosphorylation and localization of Na + /H + exchanger 3 (NHE3) and altered expression of aquaporin-1 in the proximal tubule ( 55 ). Females also have increased expression of the cleaved forms of epithelial Na + channel (ENaC) α and γ as well as increased total and phosphorylated sodium chloride cotransporter (NCC) in the distal nephron ( 54 – 56 ). As a result of these sex differences in transporter expression in rats and mice, females depend less on proximal tubular reabsorption and rely more heavily on distal transporters ( 55 ); consequently, females excrete a saline load more rapidly than their male counterparts ( 55 ). Plasma potassium (K + ) is also lower in females, presumably due to alternations in renal K + handling ( 55 ). A computational modeling study examining blood pressure regulation in male and female rats found that differences in renal fluid and sodium handling in females were a likely contributor to the lowered salt sensitivity of females ( 54 ). Although sex differences in expression of renal transporters are important, as they can alter handling of electrolytes along the renal tubule, there are also sex differences in renal hemodynamics: A study in rats found that male kidneys were more vasodilated at baseline ( 57 ). Because male and female rats had the same number of glomeruli (despite females having lower kidney and body weights), the vascular resistance per glomerulus was significantly higher in females ( 57 ). Intriguingly, this study found that ovariectomy (only 7 days before the study!) was able to shift the baseline hemodynamics of females toward that of the male ( 57 ), implying an acute effect of gonadal hormones in mediating these hemodynamic differences. The renin-angiotensin-aldosterone system (RAAS) is a major pathway in the regulation of blood pressure, and there are known sex differences in this pathway; it is noteworthy here that the angiotensin type 2 receptor (AT2R) resides on the X chromosome ( 58 ). Estrogen increases angiotensinogen, while decreasing renin, angiotensin-converting enzyme (ACE), and AT1R ( 59 , 60 ); testosterone upregulates renin, ACE, and AT1R ( 61 ), and progesterone acts as an aldosterone antagonist ( 62 ). Additionally, estrogen reduces AT1R expression in adrenal glands in rats, leading to decreased aldosterone secretion ( 63 ). Studies in normotensive Hannover Sprague-Dawley rats with heterozygous transgenic mouse Ren 2 renin (TGR) expression found increased mean arterial pressure in both female and male 3-month-old TGR, with an absence of blood pressure difference. However, there was a marked sexual dimorphism: 6-month-old and 9-month-old TGR males are hypertensive, whereas females are normotensive ( 64 , 65 ). Importantly, sex chromosome complement has also been shown to influence angiotensin receptor expression ( 66 ). AT1R mediates the classical actions of angiotensin II (Ang II), including vasoconstriction, whereas AT2R activation leads to vasorelaxation, providing a regulatory and protective role ( Figure 2 ). Although the ratio of AT1R/AT2R is lower in females than in males in SHRs ( 67 ), it is unclear whether such differences persist in normotension. In early 2020, ACE2 (which catalyzes Ang II) ( 68 ) was identified as the receptor for SARS-CoV-2, and thus there has been a surge of research on ACE2 in recent years; in terms of sex differences, it is known that ACE2 activity is dependent on 17β-estradiol ( 69 ). In addition, women have higher ACE2 expression than men in resistance arteries ( 70 ). A recent intriguing study in dogs reported that ACE activity was higher in intact females compared to both spayed females and intact males ( 71 ). Normotensive women, regardless of premenopausal or postmenopausal status, are reported to have higher urine aldosterone levels compared to age-matched men ( 68 ). Women also exhibit greater salt-sensitive blood pressure, a more pronounced aldosterone and vascular response to Ang II, and higher upright aldosterone/plasma renin activity ( 68 ). Conversely, men on both restricted- and liberal-salt diets have higher basal levels of supine aldosterone, plasma renin activity, and other aldosterone secretagogues ( 68 ). Although there is some evidence that ACE inhibitors and angiotensin receptor blockers may be slightly more effective in men, the magnitude of the difference is small ( 72 – 74 ), and the existence of a sex difference is disputed given that several studies have found similar treatment effects in both sexes ( 62 ). The autonomic nervous system also plays an important role in blood pressure regulation. For example, the sympathetic nervous system (SNS) regulates arterial blood pressure in response to stressors and transient changes (such as shifting from a supine to an upright position) by influencing the baroreflex, cardiac output, stroke volume, and total peripheral resistance ( 75 ). Ganglionic blockade studies, which “shut off” the autonomic nervous system, have been used to assess its contribution to blood pressure regulation. One study reported that young women experienced only modest reductions in blood pressure compared to young men after ganglionic blockade, concluding that autonomic support of arterial blood pressure was lower in women relative to men ( 76 ). Intriguingly, another study compared autonomic support of blood pressure via ganglionic blockade in pre- and postmenopausal women and found that postmenopausal women exhibited significantly greater decreases in blood pressure relative to premenopausal women, suggesting that autonomic support of blood pressure in women becomes more relevant after menopause ( 77 ). Muscle sympathetic nerve activity (MSNA), used as a measure of SNS function, is not directly correlated to blood pressure in young men and women, though it has provided important insight into how the SNS offsets physiologic challenges to maintain a stable blood pressure. Despite significant interindividual differences in MSNA, several studies have shown that MSNA is positively correlated with total peripheral resistance and negatively correlated with cardiac output in young men, but not in young women ( 78 – 80 ). MSNA increases with age in both sexes and becomes more positively associated with blood pressure, though aging women exhibit a starker increase in MSNA ( 75 ). This difference appears to be explained, at least in part, by female sex hormones as resting MSNA fluctuates over the course of the menstrual cycle ( 81 ). Additionally, 8 weeks of transdermal estrogen replacement therapy in postmenopausal women caused a decrease in MSNA and a modest decrease in blood pressure ( 82 ). Another contributor to sex differences in sympathetic control of blood pressure in young individuals is the difference in adrenergic receptor expression and responsiveness. Young females exhibit greater β-adrenergic receptor responsiveness relative to age-matched males ( 83 ), which is the likely mechanism for how young females offset SNS-mediated vasoconstriction. Premenopausal females exhibit a greater increase in forearm blood flow relative to postmenopausal women in response to β-adrenergic stimulation ( 80 , 84 ). Female sex hormones likely contribute to this sex difference in β-adrenergic receptor responsiveness. For instance, one study demonstrated that ovariectomy in rats eliminated sex-specific differences in vasoconstriction and vasodilation in response to β-adrenergic stimulation, and also resulted in reduced vascular messenger RNA (mRNA) levels of β-adrenergic receptors ( 85 ). They further showed that estrogen supplementation to ovariectomized rats restored the sex-specific differences in vasoconstriction, vasodilation, and the vascular mRNA levels of β-adrenergic receptors. Neither orchiectomy nor testosterone treatment affected any of the abovementioned parameters. Normal levels of reactive oxygen species (ROS) are necessary for maintaining endothelial cell and vascular smooth muscle cell function, both of which are key in maintaining vascular tone, and hence, blood pressure. Both clinical and animal studies have demonstrated that males exhibit higher levels of ROS compared to females, which may be the result of both lower amounts and lower activity of NADPH oxidase in females ( 86 – 88 ). Sex hormones are linked to this phenomenon: Testosterone administration to both normotensive Wistar-Kyoto rats and SHRs elevated ROS production and increased protein expression of NADPH oxidase subunits in vascular smooth muscle cells ( 89 , 90 ). An examination of NADPH oxidase activity in cerebral arteries of Sprague-Dawley rats found that male arteries exhibited greater ROS production and greater NADPH oxidase subunit protein expression ( 91 ). The same study also reported that ovariectomy resulted in a threefold increase in superoxide production relative to intact females, and estradiol therapy in ovariectomized rats prevented this elevation in ROS production within cerebral arteries. Further evidence of a regulatory influence of sex hormones contributing to favorable redox balance in females includes studies comparing pre- and postmenopausal females. Multiple studies have demonstrated that postmenopausal women exhibit elevated ROS production, as well as a decline in antioxidants, relative to premenopausal women ( 92 , 93 ). Interestingly, hormone replacement therapy (in both naturally occurring postmenopausal women and in medically induced menopausal women) resulted in an increase of antioxidants relative to postmenopausal women not receiving hormone replacement therapy ( 94 – 96 ). ROS production is also closely linked to nitric oxide (NO) production, which is necessary for maintenance of blood pressure via its vasodilatory effect on vessels. Pathophysiologic conditions where ROS is elevated will lead to lower NO bioavailability (and hypertension) via direct interactions of ROS and NO ( 97 ). Basal sex differences exist with female Wistar-Kyoto rats exhibiting greater NO bioavailability relative to males ( 98 ). Intriguingly, ovariectomy in normotensive Sprague-Dawley rats reduced NO bioavailability and led to an increase in blood pressure; estrogen replacement in ovariectomized rats normalized blood pressure and improved endothelial function by increasing circulating levels of nitrates and nitrites ( 99 ). Testosterone has also been shown to be important in NO production, as castrated rats exhibited endothelial dysfunction and elevated oxidative stress via increased asymmetric dimethylarginine (AMDA), a potent NO synthase inhibitor. Testosterone supplementation in castrated rats prevented this elevation in AMDA ( 100 ). Basal sex differences in immune cells (both innate and adaptive compartments) exist in homeostatic physiology ( 101 ). For example, multiple clinical studies have demonstrated sex differences in immune cell composition, with females displaying a higher proportion of CD4 + T cells and circulating CD19 + B cells, while males exhibit greater proportions of monocytes, myeloid cells, and natural killer cells ( 102 ). However, because most studies of sex differences in immune cell responses and blood pressure have focused on hypertension, our discussion here centers on these data. A 2007 study demonstrated that mice lacking T and B cells (Rag1 −/− mice) exhibited dampened hypertension and did not develop vascular dysfunction in hypertension models [Ang II infusion and desoxycorticosterone acetate (DOCA)-salt ( 103 )]. Adoptive transfer of T cells, but not B cells, restored these pathologies. Further investigation demonstrated that adoptive transfer of splenic CD3 + T cells from wild-type (WT) male mice to Rag1 −/− male mice restored their sensitivity to hypertension with Ang II infusion while transfer of CD3 + T cells from female mice into male Rag1 −/− mice maintained a blunted hypertensive phenotype ( 104 ). Moreover, adoptive transfer of splenic CD3 + T cells from male mice into female Rag1 −/− recipient mice failed to restore sensitivity to hypertension with Ang II infusion ( 105 ). Further investigation of the immune cell milieu in the context of hypertension has revealed sex differences of specific T cell subtypes both in the circulation and within the renal compartment. For example, female SHRs ( 106 ) have more CD4 + T cells and proinflammatory T helper 17 (Th17) T cells within the circulation while male SHRs have more anti-inflammatory/immunosuppressive regulatory T cells (Tregs) ( 107 ). Interestingly, the inverse is true in the kidneys, with female SHRs demonstrating more Tregs within the kidney and male SHRs exhibiting more CD4 + and Th17 T cells. Female SHRs have lower blood pressure than males, and these findings suggest that the T cell milieu within the kidney plays an outsized role in determining blood pressure. Additional work has demonstrated similar findings in a different model of hypertension: Ang II infusion in Sprague-Dawley rats resulted in larger increases in CD4 + and Th17 T cells in male kidneys, while females displayed infiltration of immunosuppressive Tregs generating anti-inflammatory cytokine interleukin-10 ( 108 ). Sex hormones appear to be responsible for the protection offered by Tregs against hypertension in premenopausal females. A 2019 study found that menopause induction (via 4-vinylcyclohexene diepoxide) in CD3 + adoptively transferred Rag-1 −/− mice followed by Ang II infusion resulted in significantly greater increases in blood pressure relative to their premenopausal counterparts ( 109 ). Additionally, this study showed that Tregs in the spleen and kidney were significantly reduced in the menopausal mice relative to the premenopausal mice following Ang II infusion. Sex differences have been reported in the microbiome composition of both animal models and humans ( 110 , 111 ). Although resistance arteries from both male and female germ-free mice exhibit reduced vasoconstriction (as compared to mice with intact gut microbes), this difference is more pronounced in males than females ( 112 ). In addition, a distinct change in vascular remodeling is observed between male and female germ-free mice, with increased vascular stiffness in germ-free males versus conventional males, but similar stress-strain curves between germ-free and conventional females ( 112 ). Akkermansia muciniphila is reported to have sex differences, with some studies reporting higher Akkermansia in males, but other studies reporting a lower abundance in males ( 113 , 114 ). This variation may be influenced by factors such as animal strain, diet, and environmental conditions. Other studies have shown that Coprococcus comes is more abundant in male feces and has the ability to metabolize ACE inhibitors used for hypertension treatment, resulting in a reduction of their therapeutic efficacy ( 115 , 116 ). On the one hand, women, particularly at younger ages, display greater gut microbial diversity than men, with higher proportions of Firmicutes , Proteobacteria , Veillonella , and Blautia ( 111 , 117 , 118 ). On the other hand, men show a higher abundance of Bacteriodes - Prevotella than women, while postmenopausal woman display a similar microbiome composition to men ( 119 ). One way that gut microbes influence the host is via the generation of microbial metabolites. Some studies have reported sex differences in microbial metabolites that influence blood pressure, including short-chain fatty acids ( 120 , 121 ) and tryptophan derivatives ( 122 ). Trimethylamine N-oxide (TMAO) is reported to digest carnitine and choline, which are linked to cardiovascular disease ( 123 ). Women tend to have higher TMAO levels due to increased hepatic expression of flavin monooxygenase 3 ( 124 ). A recent study used machine learning models to predict blood pressure from metabolites and found that catecholamine derivatives were more important predictors for blood pressure in women, while sphingomyelins were more important in men ( 125 ). Most studies of the microbiome in blood pressure regulation have focused on hypertensive models, and thus we also briefly discuss this here. A study in mice indicated that Ang II has a sex-specific effect on microbially dependent plasma metabolites (although, the sex specificity was in the magnitude of the change rather than the direction) ( 126 ). A human subject study revealed that gut microbiota dysregulation was strongly related to 24-h ambulatory blood pressure in women but not in men ( 127 ). Similarly, sex-specific variation in β-diversity was seen in intestinal microbiota in both coronary heart disease–afflicted and noncardiovascular disease human subjects ( 128 ). However, a separate study reported that sex differences in β-diversity were associated with hypertension in skin—but not oral or gut microbiomes—in rats ( 129 ). Another recent study provided evidence for sex-specific roles of the gut microbiota and its metabolism in the development of salt-sensitive hypertension: Female rats transplanted with male cecal microbiota show higher salt-induced blood pressure increases compared to those with female microbiota ( 122 ). Intriguingly, a recent study evaluated Lactobacillus paracasei engineered to deliver human ACE2 (Lacto-hACE2) for blood pressure regulation in a hypertensive rat model lacking endogenous ACE2, and showed a sex-specific blood pressure reduction in female (Ace2 −/− ) rats, but not in males (Ace2 −/y ), after colonization with Lacto-hACE2 ( 130 ). Finally, we want to acknowledge that there are additional pathways and interactions that are not discussed above but that may make key contributions to sex differences in blood pressure. Indeed, blood pressure is a classic example of an output that is dynamically controlled by a large number of systems, as elegantly illustrated by Guyton’s circuit diagram ( 131 ). Modulators of blood pressure that are not already discussed above include additional hormonal influences, such as the actions of endothelin ( 132 ), which exerts its effects through two main receptor subtypes: endothelin receptor type A and B (ETA and ETB). ETA is predominantly expressed on vascular smooth muscle cells, where it promotes vasoconstriction. ETB receptors are located on both renal epithelial cells and endothelial cells, and ETB activation promotes natriuresis and vasodilation. Notably, males have greater ETA receptor activation, whereas females have greater ETB activation. There are also sex differences in the physiology of organ systems, such as the gastrointestinal and pulmonary systems, which also influence blood pressure. Adipose tissue distribution also displays sexually dimorphic patterns and has important implications for the development of hypertension. Premenopausal women tend to have more subcutaneous fat, whereas men tend to have more visceral fat, which is linked to higher blood pressure and cardiovascular disease risk ( 133 , 134 ).

In the following section, we highlight recent novel advances in the field with a focus on G protein–coupled estrogen receptor (GPER), Klotho, olfactory receptor 558 (OLFR558), and the FCG model. Recent studies have highlighted the role of GPER (GPR30) in contributing to sex differences in blood pressures. In humans, the GPER gene is found on chromosome 7p22, a region related to human arterial hypertension ( 135 , 136 ). It is broadly distributed in various tissues including breast, ovary, endometrium, testis, and prostate, as well as brain, lung, liver, pancreatic islets, adipose tissue, vasculature, and muscle ( 137 ). GPER can be activated by endogenous hormones including estrogen and aldosterone, synthetic estrogenic compounds, and natural origin phytoestrogenic compounds ( 137 ). GPER expression shows no sex differences in the heart in healthy rats, but female (not male) rats increase GPER expression (both mRNA and protein) with pressure overload ( 138 ). Additionally, GPER is also expressed in kidney tubules in rats, with significantly higher mRNA and protein levels in the outer medulla of females compared to males. This difference likely explains why intramedullary administration of the GPER agonist G-1 increases urinary flow and natriuresis in female but not male rats ( 139 ). WT males have higher blood pressure than females, and the sex difference in mean arterial pressure still exists in Gper knockout (KO) mice by tail-cuff, suggesting that Gper deletion does not affect mean arterial pressure. The sex difference in arterial stiffness is also detected in WT mice but is absent in Gper KO mice ( 140 ). Additionally, the systolic blood pressure sex difference is absent in Gper KO mice via tail-cuff reported by the same group, and it is related to endothelial dysfunction in females ( 141 ). Another study reported that Gper deletion did not influence blood pressure as measured by the more reliable telemetry method; however, in this study no sex difference in blood pressure was shown in Gper WT mice ( 142 ). Of note, a recent study demonstrated that activation of GPER protected against hypertension, which was negated by ETB antagonism in female rats ( 143 ). In both Gper WT and KO mice with Ang II-induced hypertension, sex differences in blood pressure are absent as measured by tail-cuff ( 140 ). In aldosterone-induced hypertensive WT mice, sex differences in blood pressure are amplified with elevated blood pressure in males, but these differences are absent when treated with GPER agonist or compared to ovariectomized females. Additionally, in aldosterone-induced hypertensive Gper KO mice, the sex differences in blood pressure are absent after 7 days of aldosterone treatment ( 144 ). GPER activation induces a relaxation response in mesenteric arteries from hypertensive rats of both sexes, but this GPER-induced relaxation occurs via differential endothelial mediators between males and females ( 145 ). Human studies have shown that GPER expression is lower in hypertension. Additionally, GPER expression is reduced after menopause in hypertensive women but is similar in normotensive women ( 146 ). Gender differences in hypertension may be associated with impaired GPER function, as the P16L allele, a lower-functioning variant of GPER, is nearly twice as prevalent in female hypertension patients as compared to male patients ( 147 ). Klotho is a single-pass transmembrane protein that is antiaging, helps to maintain cardiovascular health, and acts as a coreceptor for fibroblast growth factor 23 ( 148 , 149 ). As described below, there are sex differences in Klotho expression, and Klotho plays important roles in several key cardiovascular phenotypes including arterial stiffening and endothelial dysfunction. Klotho has three forms—transmembrane, shed, and secreted—and is highly expressed in the kidney ( 148 , 150 – 152 ). Secreted Klotho is most prevalent in humans ( 148 ). After birth there is a rapid increase in Klotho, which decreases with age ( 151 , 153 ). Of note, there are sex differences in Klotho regulation, and these differences could help to explain sex differences in cardiovascular risk and disease. One study of 2,475 participants found that after adjusting for sex, ethnicity, body mass index, and tobacco use, women had higher levels of Klotho, and these higher levels may be due to higher levels of estradiol in women ( 154 ). Klotho protects the kidney through various mechanisms, including inhibition of cell senescence and apoptosis, induction of autophagy, and antioxidation ( 149 ). Klotho has cardioprotective effects that are produced by inhibiting the phosphoinositide-3-kinase/protein kinase B/mechanistic target of rapamycin (PI3K/AKT/mTOR) signaling pathway and activating autophagy ( 81 ). It has also been shown that Klotho protects the cardiovascular system against arterial stiffness through endothelium-derived NO production ( 155 ), and low levels of Klotho are associated with increased arterial stiffness ( 156 ). While it is thought that Klotho may play a role in blood pressure regulation, more studies need to be conducted to examine sex differences in Klotho and normotension. One study found that a high-salt diet elevated blood pressure in aged and young Klotho-deficient mice, but when Klotho was supplemented (by intraperitoneal injection of transformed full-length mouse α-Klotho plasmid vector), the high-salt blood pressure elevation was mitigated, suggesting that decreased Klotho is associated with salt-induced hypertension ( 157 ). Two studies showed a relationship between triglyceride and Klotho levels in women and that Klotho could be associated with central obesity and hyperlipidemia in women ( 154 , 158 ). As previously mentioned, sex hormones impact Klotho expression ( 159 ); however, the literature is unclear on the exact mechanism. For example, testosterone increases Klotho and nuclear androgen receptor expression, and overexpression of the nuclear androgen receptor increases Klotho mRNA and protein ( 153 , 160 ). However, research also suggests that estrogen enhances Klotho gene expression, providing protective effects on the cardiovascular system in women ( 159 ). More research investigating hormones and their impact on Klotho secretion is important, especially when considering individuals receiving hormone therapy. Of note, Klotho is decreased in patients with essential hypertension or renovascular hypertension and is correlated with preeclampsia. In patients with arterial stiffness and hypertension, Klotho levels decreased by 45% relative to healthy controls ( 161 ). These data suggest that Klotho could be used as a potential biomarker for hypertension ( 148 , 162 , 163 ) or for postmenopausal women who are at risk for hypertension ( 164 ). Significantly decreased Klotho levels have also been observed in kidneys of patients with chronic kidney disease, suggesting that Klotho may play a role in chronic kidney disease development and progression ( 153 ). Studies have found that higher levels of soluble, or secreted Klotho, is associated with a lower incident of hypertension, lower blood pressure trajectories during follow-up, and a lower likelihood of cardiovascular disease, suggesting that higher circulating Klotho levels are protective ( 165 , 166 ). In summary, Klotho plays a vital role in better understanding blood pressure regulation, cardiovascular health, and kidney function and is a promising biomarker for the diagnosis of hypertension. Olfactory receptors are primarily known for their role in the olfactory epithelium, where they mediate the sense of smell. However, olfactory receptors are also expressed outside of the nose ( 167 ) where they serve as chemosensory G protein–coupled receptors. OLFR558 is an evolutionarily conserved olfactory receptor that is widely expressed in the body in both humans and mice, including kidney, heart, brown adipose tissue, skeletal muscle, bladder, thyroid, colon, and brain ( 168 – 174 ). Olfr558 mRNA expression is reported to be similar in males and females ( 170 ). In a recent study, we found that Olfr558 regulates blood pressure in a sex-specific manner ( 170 ). As mentioned above, WT male mice have higher blood pressure than females. However, this blood pressure difference is absent in Olfr558 whole-body KO mice. This is due to both an increase in blood pressure in female Olfr558 KOs, and a decrease in diastolic blood pressure in male Olfr558 KOs. Male Olfr558 KOs exhibit increases in renin (including renin mRNA level and plasma renin activity), but these parameters are unchanged in female Olfr558 KOs. Olfr558 KO mesenteric arteries from males have an exaggerated relaxation response to sodium nitroprusside that could also contribute to the lower blood pressure in male Olfr558 KOs. Olfr558 KO females have increased pulse wave velocity, suggesting elevated arterial stiffness, which is a likely contributor to the higher blood pressure in these mice. Of note, OR51E1 is the human ortholog of OLFR558, and a rare OR51E1 variant (missense mutation) has a significant sex-interaction effect on diastolic blood pressure: increasing diastolic blood pressure in women but decreasing it in men ( 170 ). While sex hormones have been implicated in blood pressure regulation for decades, their role in blood pressure regulation is complicated and cannot comprehensively explain all of the sex differences in blood pressure ( 5 , 22 ). For instance, hormone replacement therapy in postmenopausal women has been reported to lower or raise blood pressure or have no effect on it ( 5 ). This could be due to differences in formulation or duration, but it could also indicate that estrogen alone cannot rescue the rise in blood pressure associated with menopause. Testosterone supplementation in aging men also has conflicting impacts on blood pressure, with some studies indicating a lowering effect ( 175 ) and others indicating a hypertensive effect ( 176 ). Sex chromosomes are also implicated in blood pressure regulation, especially in the context of hypertension; for example, a Y chromosome–linked locus has been associated with hypertension in SHRs ( 177 , 178 ). The FCG mouse model represents a powerful tool to investigate the relative contributions of sex chromosome complement, sex hormones, and possible interactions between the two. In this model, sex chromosome complement is separated from the animal’s gonadal sex ( Figure 1 ). A study used the FCG model along with gonadectomies (GDX) to uncouple sex hormones from sex chromosomes and more clearly investigated the effect of sex chromosomes on blood pressure regulation ( 30 ). Following two weeks of Ang II infusion in gonadectomized FCG mice, they observed a strong sex chromosome effect on mean arterial pressure. Regardless of gonadal sex, GDX-XX mice (both gonadal males and gonadal females) exhibited 14 mmHg higher mean arterial pressure relative to GDX-XY mice following Ang II infusion. There were no differences in mean arterial pressure between FCG mice prior to Ang II infusion. This elevated hypertension in GDX-XX mice relative to GDX-XY mice is contrary to hypertensive responses observed in gonad-intact males and females, suggesting that sex chromosomes and sex hormones may have opposing effects on blood pressure regulation. Indeed, it leaves one to posit that the rapid increase in blood pressure observed in menopausal women is mediated by the decline in estrogen and its protective effect against hypertension, allowing for the unmasking of the X chromosome and its predisposition toward hypertension. Another study utilized GDX-FCG mice to investigate the impact of sex chromosomes on the baroreceptor reflex response to acute Ang II infusion ( 179 ). This study showed that sexually dimorphic Ang II bradycardiac baroreflex responses may result from sex chromosome complement. In response to Ang II infusion, the heart rate of both gonadal male and female GDX-XX mice lowered to a much greater extent than GDX-XY mice. This sex chromosome effect on the sexually dimorphic bradycardiac baroreflex response could have important implications for hypertension as reductions in baroreflex control of heart rate has been reported in hypertension ( 180 ). Although our article has focused on the current understanding in the field of normotensive sex differences, hypertension is a significant clinical problem that is estimated to contribute to 8.5 million deaths worldwide ( 106 ). This is also a growing problem: Between 1990 and 2019, the prevalence of hypertension in 30- to 79-year-olds doubled globally ( 106 ). Determining the optimal threshold for lowering blood pressure is important to help lower the risk of cardiovascular disease. However, clinical management of hypertension is identical in men and women, with blood pressure >130/80 defined as hypertension, and a systolic blood pressure over 120 noted as elevated. Given that women have a lower blood pressure at baseline, they must incur a greater increase in blood pressure from baseline before they are noted as hypertensive in the clinic. Furthermore, women are less likely to be prescribed antihypertensive medication than men with the same blood pressure ( 181 ). Looking ahead, it will be crucial to use data-driven approaches to determine whether blood pressure thresholds for hypertension should be sex-specific, and if so, what the optimal clinical guidelines should be for both sexes ( 62 ). Similarly, the mechanisms underlying menopausal hypertension ( 182 , 183 ) may be quite different from the mechanisms that typically drive “essential” hypertension. Thus, it will also be important to better understand mechanisms that drive blood pressure elevation in both cases so that we can identify clinical approaches that may be optimal for each “type” of hypertension. Efforts to reassess optimal blood pressures for each sex are also motivated by the fact that women incur increased risk for cardiovascular disease at lower blood pressures than men ( 10 ). Although men have a higher incidence (higher absolute risk) of cardiovascular diseases, women incur increases in their risk at much lower blood pressures. For example, when compared to the baseline risk (risk incurred by men with a systolic blood pressure <100 mmHg), men incur increased risk for cardiovascular disease once their systolic pressure is above 130 mmHg (hazard ratio 1.26 for 130–139 mmHg, aligning well with current guidelines for treatment of hypertension). On the other hand, women begin incurring increased risk with systolic pressures of 100–109 mmHg (when compared to the risk for women with systolic pressure <100 mmHg; hazard ratio 1.25). For myocardial infarction, this difference is even more stark: Women incur increased risk once their systolic pressure is above 110 mmHg (hazard ratio 1.64 for 110–119 mmHg), but men do not incur increased risk until their systolic pressure is 150–159 mmHg (hazard ratio 1.46). These data strongly imply that women not only have lower normotensive blood pressure, but their physiology is better optimized to function at lower blood pressures. It may be that women are better adapted for lower normotensive blood pressures in order to allow the cardiovascular system to better handle the drastic changes in cardiovascular function that occur in pregnancy (including a ~50% increase in blood volume) ( 184 ). Conversely, it is also possible that men’s physiology is optimized to operate at higher pressures. In either case, these data argue that sex-specific guidelines may be advantageous for the clinical management of blood pressure in both sexes. Understanding how sex differences impact lifestyle modifications will be key to providing better management of hypertension and blood pressure. In the DASH (Dietary Approaches to Stop Hypertension) trial, for example, only females had a reduction in blood pressure with sodium restriction ( 62 ). In a meta-analysis investigating the intake of sodium and potassium, it was found that women have a stronger relationship between the dietary sodium-potassium ratio and mortality (as well as a stronger relationship between dietary potassium and mortality) ( 185 ). Higher sodium intake (> 2,000 mg/day), higher sodium-potassium ratios, and lower potassium intake increased the risk of overall mortality and cardiovascular disease risk ( 185 ). In one study, women who consumed a diet high in fried foods had a greater risk for hypertension compared to women who rarely ate fried foods, but this risk was not seen in men ( 186 ). Another study found that adhering to a Mediterranean diet lowered cardiovascular risk in women but not men ( 186 ); similarly, it was reported that a diet high in legumes and whole grains reduced hypertension in women but not men ( 186 ). Overall, these studies reported that dietary modifications had a greater potential to reduce hypertension in women ( 186 ). Conversely, a meta-analysis showed that aerobic exercise reduced blood pressure more in males than females ( 62 ). Yet after exercise, women have lower vascular resistance and reduced vasoconstriction compared to males ( 51 ). A study investigating the mechanism of sex differences in blood pressure during exercise in normotensive adults found that the β-adrenergic receptor may mediate the decreased vasoconstrictor response during exercise in women ( 187 ). Thus, although diet and exercise can help with management of hypertension, the sex differences in their antihypertensive benefits should be noted, and uncovering the mechanisms underlying these differences may reveal unique insights. Pharmacological treatments also are subject to sex differences: For example, antihypertensive drugs can exhibit sex differences due to differences in enzyme metabolism and absorption ( 62 ). Adverse drug effects for management of blood pressure also show sex differences in women, and women have different side effect profiles than men for some hypertensive medications ( 62 ). A study comparing traditional cuff blood pressure to invasive aortic blood pressure found that blood pressure–related risks in women were underestimated by cuff blood pressure ( 188 ). Women are more aware of hypertension than men, but studies show that men have better blood pressure control during treatment than females ( 62 ). Thus, aside from sex differences in the underlying physiology, it will be relevant to consider biases in both clinician and patient behavior, as well as sex differences in pharmaceutical treatments. Of note, there are also aspects of blood pressure regulation that are specific to women: A subset of gynecological conditions can increase the risk of cardiovascular disease (e.g., polycystic ovary syndrome, endometriosis, uterine fibroids, and hysterectomy before 50 years of age) ( 189 ), and the incidence of hypertension increases in postmenopausal women ( 190 , 191 ). Thus, better understanding of how blood pressure is regulated in females both pre- and postmenopause would allow for better preventative risk assessments and hypertension treatment for women as they age. Finally, a related and emerging question is how to best manage blood pressure in transgender individuals. Although transgender individuals are at greater risk for adverse cardiovascular outcomes ( 192 – 195 ), little is known regarding how gender-affirming hormone therapy affects blood pressure ( 196 ). If sex-specific guidelines for clinical management of blood pressure are to be adopted, the impact of the varied and personalized nature of gender-affirming care would have to be considered.

Normotensive sex differences in blood pressure emerge at puberty. Females have lower blood pressure than males until menopause, when female blood pressure sharply increases. Sex differences have been documented within several systems that impact blood pressure regulation including the cardiovascular and renal systems, RAAS signaling, the autonomic nervous system, oxidative stress, immune cells, and the microbiome. Many of these sex differences are linked to sex hormones, as evidenced by gonadectomy studies and changes in or loss of sex differences in postmenopausal women. In this review, we have highlighted novel and recent advances in the field’s understanding of sexually dimorphic blood pressure regulation. Recent studies have demonstrated that GPER is important for sex differences in arterial stiffness as well as sex differences in hypertension, and that OLFR558 is necessary for sex differences in normotensive blood pressure. Emerging research on sex differences in Klotho will be important to our understanding of how Klotho may be targeted for blood pressure maintenance in hypertension. We also discussed the powerful FCG model as a way to investigate the contribution of sex chromosome complement to blood pressure regulation. Lastly, we highlight the real-world clinical impact of sex differences in blood pressure regulation, emphasizing the need to more clearly understand this phenomenon so that patients, regardless of their sex, can receive optimal treatment for hypertension and cardiovascular disease.

Normotensive blood pressure is differentially regulated in males and females ( 1 – 11 , 12 , 13 ). Females have lower blood pressure than men by ~10 mmHg starting around puberty ( 14 , 15 ), and this sex difference is maintained until menopause ( 9 ). The fact that this sex difference is seen from puberty to menopause implies a key role of gonadal hormones. In humans, sex differences in blood pressure have been documented in life course trajectory studies examining blood pressure over the lifespan in multiple cohorts ( 8 , 9 ), as well as in population studies ( 6 , 11 , 12 ). Females also have lower blood pressure than males in a number of other species including mice [both C57BL/6 and FCB/N ( 1 )], dogs ( 16 ), and chickens ( 17 , 18 ); notably, birds (including chickens) do not have the same sex chromosome inheritance as mammals (ZW, ZZ instead of XX, XY), again implicating a role for gonadal signals. Although blood pressure is generally lower in females during the reproductive years, blood pressure changes over the life course in both sexes. Both sexes experience increases in blood pressure with aging, but the slope of this increase is much steeper in females, such that the blood pressure of aged females can surpass that of aged males ( 9 ). This accelerated increase in blood pressure is likely driven in part by changes associated with menopause ( 19 ). Of note, blood pressure is also influenced in females by the physiological challenges associated with pregnancy: Despite a dramatic increase in blood volume during pregnancy, blood pressure is lowered in healthy pregnancy ( 20 ). In general, sex differences in physiology can be driven either by gonadal signals or by differences in sex chromosomes. In the case of sex differences in blood pressure, there is strong evidence that gonadal influences play a key role. However, the relationship between sex hormones and blood pressure is complicated ( 21 ). In animal studies, androgens have differential effects in males and females ( 22 ), the fall in androgens in aging men is thought to increase their risk for cardiovascular disease ( 23 – 25 ), and there is no clear effect of hormone replacement therapy on blood pressure in postmenopausal women [it has been reported to increase, decrease, or have no effect on blood pressure ( 26 – 29 )]. Furthermore, an influence of sex chromosomes cannot be ruled out, especially in a setting of hypertension ( 30 ). In addition to sex differences in blood pressure regulation during the healthy life course, there are also clear sex differences in hypertension in humans and in animal models. Sex differences in hypertension are generally better studied (versus sex differences in normotension), and several recent reviews have focused on this topic ( 18 , 22 , 31 , 32 ). Thus, in this review we focus on sex differences in blood pressure control in the healthy normotensive state. For some areas, normotensive sex differences are poorly studied, in which case we instead note reported sex differences in hypertension. We then focus our attention on several recent, novel advances that have shed important light on our current understanding of sex-dependent control of blood pressure.