Gender and the renin–angiotensin–aldosterone system

Angiotensinogen

Angiotensinogen is a glycoprotein synthesized mainly in the liver. Recent studies show that adipose tissue also secretes angiotensinogen. Angiotensinogen mRNA is expressed in the brain, heart, vascular system, kidney, and adrenal glands.

Previous studies have found that plasma angiotensinogen levels in postmenopausal women are slightly higher than [6] or not different from [7] those in men. Plasma angiotensinogen levels are increased with oestrogen replacement therapy, but this increase is observed only with oral administration [7–10]. This difference may be because of the first-pass effect in the liver, where angiotensinogen is synthesized. Thus, the effect of oestrogen on angiotensinogen levels seems to be dependent on the method of administration [11]. The angiotensinogen promoter is directly controlled by oestrogen [12,13].

Levels of angiotensinogen mRNA are higher in the kidneys and livers of male rats than in those of female rats [14–16]. Furthermore, angiotensinogen mRNA levels are decreased by castration and increased by administration of exogenous testosterone [14,15]. Subcutaneously administered estradiol also increases angiotensinogen mRNA levels in the liver and angiotensinogen levels in the plasma. In rat hepatocytes, oestrogen increases both angiotensinogen mRNA levels and angiotensinogen secretion, whereas dihydrotestosterone increases angiotensinogen mRNA levels without affecting angiotensinogen secretion [17].

Oral oestrogen replacement therapy significantly increases angiotensinogen levels, but its effects might be offset by decreases in renin concentrations and ACE activity. In other words, an excess of angiotensinogen might block the beneficial effects of oestrogen.

Renin

Renin is secreted by juxtaglomerular cells and converts angiotensinogen to angiotensin I. Renin mRNA is also expressed in the submandibular glands of mice.

The conflicting results of studies of gender differences in renin levels might be because of the method of measurement [11]. Plasma renin activity is defined as angiotensin I generation from an endogenous substrate. Therefore, plasma renin activity also depends on the concentration of substrate.

In postmenopausal women, plasma renin levels are lower than in men and are further decreased by oestrogen [6,7]. In contrast, Seely et al. [18] have shown in normotensive postmenopausal women that plasma renin levels are increased by transdermally administered oestrogen but are decreased by the combination of transdermally administered oestrogen and transvaginal micronized progesterone.

In hypertensive model rats, plasma renin levels are higher in males than in females [19]. In spontaneously hypertensive rats (SHRs), testosterone increases renal mRNA levels in females but neither castration nor testosterone alters renal renin levels in males [15]. Even in female hypertensive rats, the androgen antagonist flutamide decreases renal renin mRNA levels and renin concentrations [20]. In the submandibular gland, renin levels increase after puberty to become much higher in males than in females, but castration decreases renin levels [21].

Renin secretion is also stimulated by catecholamines. Thus, the inhibitory effect of oestrogen on renin is partly because of inhibition of catecholamines [22].

Although results have been inconsistent, most studies suggest that plasma renin is decreased by oestrogen and increased by androgens.

Angiotensin-converting enzyme (ACE)

As its name indicates, ACE converts angiotensin I to angiotensin II. Moreover, kininase II, which converts bradykinin to its inactive peptide, has been shown to be identical to ACE. Most ACE is in the lung, but ACE is also found in other tissues, such as the heart and vascular endothelium.

In children, ACE activity increases in boys and decreases in girls with age after puberty [23]. In healthy young adults, ACE activity is higher in men [24]. However, in premenopausal women [25] and postmenopausal women [6,7], plasma ACE activity is similar to that in men of the same age. With oestrogen replacement therapy, ACE activity has been found to decrease [26,27], to show no change [7], or to increase [27].

In monkeys, oral oestrogen alone or the combination of oral oestrogen and oral medroxyprogesterone significantly decreases ACE activity [28]. In rats, ovariectomy increases ACE activity in the kidney, lung, heart, abdominal aorta, and adrenal gland, but this increase is reversed by oestrogen [29,30]. In mouse ventricle, levels of ACE mRNA and ACE protein are higher in males than in females; this difference becomes apparent after puberty. Ovariectomy or castration decreases gender differences in ACE mRNA levels [31].

In hypertensive model rats, ACE activity is higher in males than in females [19], and oestrogen administration decreases ACE activity in the plasma, kidney, and aorta of ovariectomized females [32].

Although there are some variations, ACE activity is likely decreased by oestrogen and increased by androgens.

Angiotensin II

In normotensive premenopausal women, plasma angiotensin II levels are similar to those in men [25]. In normotensive postmenopausal women, plasma angiotensin II levels nearly double with oral administration of oestrogen and medroxyprogesterone but do not change with transdermal administration [27].

In monkeys, treatment with oral oestrogen alone or combined therapy with oestrogen and oral medroxyprogesterone increases angiotensin I levels. Oral oestrogen monotherapy or combination therapy with medroxyprogesterone appears to increase angiotensin II levels but the increase does not reach statistical significance. Both treatments significantly reduce the angiotensin II/angiotensin I ratio [28]. In female Sprague-Dawley (SD) rats, ovariectomy decreases plasma angiotensin II levels, but this decrease is reversed by oestrogen [33].

In mRen(2).Lewis hypertensive rats, plasma angiotensin II levels in the plasma and in the renal cortex and medulla are higher in males than in females, but cardiac angiotensin II levels do not differ between the sexes [19]. In ovariectomized female TGR(mREN2)27 rats, oestrogen decreases plasma angiotensin II levels [32]. In contrast, in SHRs, plasma angiotensin II levels are higher in females than in males, but renal cortical angiotensin II levels are similar in males and females [34].

There is no consistent difference in serum concentrations of angiotensin II between males and females or between untreated females and oestrogen-treated females. However, both circulating levels and tissue levels of angiotensin II are correlated with tissue injury. Thus, further observation is needed.

AT₁ receptor

The AT₁ receptor is a 7-transmembrane G-protein-coupled receptor [35], and stimulation of the AT₁ receptor induces Na reabsorption, vasoconstriction, cardiac hypertrophy, cardiac fibrosis, and other types of tissue damage. The AT₁ receptor stimulation also triggers aldosterone production in the adrenal gland.

In the adrenal gland, kidney, heart, and liver of ovariectomized premenopausal female beagles, subcutaneous administration of oestrogen decreases AT₁ receptor density [36].

In rat glomerulus, AT₁ receptor binding is lower in females than in males [37]. In rat heart, kidney, and aortic tissue, ovariectomy increases AT₁ receptor density, but this increase is reversed by oestrogen [30,38,39]. In rat adrenal gland, oestrogen decreases the number of AT₁ receptors without altering mRNA levels by post-transcriptional mechanisms [40]. Only in the uterus is oestrogen reported to increase AT₁ receptor density [41]. On the other hand, castration in male rats does not alter rates of AT₁ receptor binding [37].

In hypertensive model rats, AT₁ receptor protein expression is higher in the renal cortex of males than of females [34], but castration does not alter AT₁ receptor binding or AT₁ mRNA expression [42]. In the aorta and mesenteric microvessels of SHRs, AT₁ receptor mRNA levels are lower in females. Ovariectomy increases AT₁ mRNA to levels similar to those in males [43]. Oestrogen regulates AT₁ receptor expression by controlling the 5′ leader sequence of receptor mRNA binding proteins [41].

In isolated vascular smooth muscle cells from rats, AT₁ receptor mRNA levels and AT₁ receptor density are decreased by oestrogen but are increased by progesterone [44].

AT₁ receptor density is almost always decreased by oestrogen but does not appear to be affected by androgens. Progesterone might increase AT₁ receptor density.

AT₂ receptor

The AT₂ receptor is a 7-transmembrane G-protein-coupled receptor [35]. Although the precise function of AT₂ receptor has not been clarified, AT₂ receptor stimulation is believed to antagonize AT₁ receptor stimulation [45]. The expression of AT₂ receptor decreases after birth but is increased in certain pathophysiological conditions [45,46]. Because the AT₂ receptor gene is on the X chromosome [47], it is reasonable to assume that AT₂ receptor expression differs with gender.

Oestrogen increases AT₂ binding in the rat adrenal gland [39] and mouse kidney [48]. In hypertensive rats, AT₂ receptor mRNA levels in the kidney are higher in females than in males [43].

In the arteries of mice, AT₂ receptor mRNA expression is extremely low and does not differ between males and females. After injury, AT₂ receptor mRNA expression increases in males and to a much greater extent in females. Ovariectomy lessens this injury-dependent increase [49]. In mouse arteries, neointima formation, DNA synthesis, monocyte chemoattractant protein 1 expression, and levels of superoxide anion and nicotinamide adenine dinucleotide phosphate (NADPH) oxydase activity increase in response to artery injury but are decreased by valsartan. The protective effect of ARBs is greater in females than in males, but this difference is small in AT₂ receptor null mice [49].

Under pathophysiological conditions, AT₂ receptor expression is likely increased, especially by oestrogen, and ARBs might show a protective effect, at least in part, via an increase in AT₂ receptor activation. Moreover, treatment with the ARB losartan in SHRs increases AT₂ receptor mRNA expression of mesenteries only in females [50].

Vascular response to angiotensin II

In mice, the angiotensin II-induced increase in blood pressure is greater in males than in females [51,52].

The angiotensin II-induced increase in blood pressure is enhanced by both ovariectomy and castration in normal and hypertensive rats. Administration of oestrogen and testosterone, respectively, restore the effects of ovariectomy and castration [32,42,51,52]. When endogenous angiotensin II is blocked with an ACE inhibitor in SD rats, the angiotensin II-induced increase in blood pressure is greater in female with low salt levels and in males with high salt levels, indicating that female rats are less sensitive to salt [53].

In the aortic tissue of female rats, vasoconstriction in response to angiotensin II is increased by ovariectomy [38]. Aorta and mesenteric microvessels are hyporeactive to angiotensin II in female SHRs, and this response is increased after ovariectomy [43]. In the mesenteric arterioles of SHRs, NAD(P)H oxidase activity is higher in males than in females, and this difference is because of AT₁-dependent overexpression of the NAD(P)H oxidase compartment [54]. In cultured vascular smooth muscle cells, angiotensin II-dependent NAD(P)H oxidase activation is attenuated by oestrogen [55].

Thus, the vascular response to angiotensin II has been demonstrated to be decreased by oestrogen and increased by testosterone. In vascular tissue, oestrogen also inhibits NAD(P)H oxidase activation by angiotensin II.

Cardiac response to angiotensin II

In mice, angiotensin II increases NAD(P)H oxidase activity in males and ovariectomized females but not in nonovariectomized females [52].

In cardiac fibroblasts, oestrogen shows an inhibitory effect on angiotensin II-induced fibroblast-mediated remodeling of the extracellular matrix, gene expression, and signal transduction [56]. Oestrogen also inhibits angiotensin II-induced fibroblast proliferation and differentiation, nuclear factor κB activity, and p38 mitogen-activated protein kinase phosphorylation [57].

Thus, oestrogen appears to protect the heart against hypertrophy, remodeling, and fibrosis. Less is known about the effect of androgens on cardiac muscle.

Renal response to angiotensin II

When angiotensin II is infused into healthy young men and women, renal plasma flow decreases to a same extent in both sexes, but the glomerular filtration rate decreases only in women. This decreased response of the filtration fraction to angiotensin II in women would counteract the angiotensin II-induced increase in intraglomerular pressure [25].

In genetically hypertensive rats, castration decreases the response of renal vascular resistance to angiotensin II, but testosterone reverses the effect of castration [42]. When endogenous angiotensin II is blocked with an ACE inhibitor, the level of angiotensin II-induced albuminuria is higher in men than in women [53].

Less is known about gender differences in the renal response to angiotensin II. Therefore, additional studies are needed.

Aldosterone

Aldosterone is synthesized mainly in the adrenal gland in response to AT₁ receptor stimulation, ACTH, and hyperkalemia. Aldosterone binds to mineralocorticoid receptor (MR), a steroid hormone receptor. MR is present in kidney, heart, vascular tissue, and a variety of other tissues [58]. Aldosterone induces Na reabsorption, cardiac hypertrophy, cardiac fibrosis, proteinuria, vasoconstriction, and other types of tissue damage.

In premenopausal women, plasma aldosterone levels are lower than in men [25], but after menopause, the difference in plasma aldosterone levels between men and women disappear [6,7]. In postmenopausal women, effect of oestrogen replacement therapy on plasma aldosterone levels is not consistent [7,18], which might be because of presence or absence of progestins.

In ovariectomized premenopausal female dogs, oestrogen decreases aldosterone concentrations [36]. In the adrenal glands of ovariectomized rats, oestrogen decreases aldosterone content and angiotensin II-induced aldosterone secretion [39,40,59]. When ACTH is inhibited by dexamethasone, angiotensin II-induced aldosterone secretion is decreased by oestrogen. Oestrogen also inhibits ACTH secretion in response to angiotensin II [59]. In isolated rat zona glomerulosa cells, progesterone but not oestrogen increases aldosterone production [60].

In ovariectomized female Wistar rats, aldosterone with high salt induces hypertension, cardiac hypertrophy, vascular fibrosis, and increased osteopontin expression. Activation of either oestrogen receptor α or β protects against these pathologic changes [61].

Together, these findings suggest that oestrogen decreases aldosterone secretion in response to angiotensin II. Oestrogen might also inhibit ACTH, another aldosterone-stimulating factor. Oestrogen also attenuates aldosterone-induced tissue injury [62].

Progesterone binds MR, then antagonizes aldosterone [63,64]. However, small structural changes induce different effect among progestins. [65]. Almost all the progestins except for drospirenone lose anti-aldosterone effect [64,65]. Moreover, some progestins show androgen-like effect [66]. Drospirenone combined with oral oestrogen lowers blood pressure to a larger extent than oestrogen alone [67].

The Randomized Aldactone Evaluation Study for Congestive Heart Failure (RALES) showed that spironolactone improves the prognosis of patients with heart failure [68], and the Eplerenone Post-Acute Myocardial Infarction Heart Failure Efficacy and Survival Study (EPHESUS) showed that eplerenone improved the prognosis of patients with acute myocardial infarction complicated by heart failure [69]. However, risk reduction was higher in spironolactone than eplerenone although patient characteristics are different in two studies. Spironolactone, a nonselective MR antagonist, also blocks androgen receptors and stimulates progesterone receptors, whereas eplerenone is a selective MR antagonist. The beneficial effects of spironolactone might occur, in part, via the inhibition or activation of sex hormone receptors. However, in the subanalysis of RALES, the beneficial effects of spironolactone were identical in both men and women.

ACE2 and angiotensin (1-7) (Ang(1-7))

ACE2 is an ACE homologue that converts angiotensin II to angiotensin (1-7) (Ang(1-7)). The ACE2 gene is located on the X chromosome [70]; thus, it is reasonable that women show higher ACE2 activity. Ang(1-7) exerts an effect almost exactly opposite to that of angiotensin II [32,71,72].

In the renal cortex of female SD rats of the renal-wrap hypertension model, ACE2 activity and ACE2 protein expression are decreased by ovariectomy and restored by oestrogen. However, these changes are not observed in the heart [73]. In ovariectomized rats of the deoxycorticosterone acetate-salt hypertensive model, oestrogen increases cardiac ACE2 protein levels [74]. In contrast, in mRen(2).Lewis hypertensive rats, cortical and cardiac ACE2 activities are higher in males than in females [19]. In this model, however, plasma Ang(1-7) levels are higher in females, possibly because of the increased activity of neprilysin, which directly converts angiotensin I to Ang(1-7) [19]. In ovariectomized female TGR(mREN2)27 rats, oestrogen increases plasma Ang(1-7) levels [32].

In SHRs, long-term Ang(1-7) infusion lowers blood pressure. However, after 2 weeks, Ang(1-7)-induced attenuation is observed in females only. Plasma NO levels increase in response to Ang(1-7) in females only [75]. Oestrogen increases depressor responses to ANG(1-7) in ovariectomized female TGR(mREN2)27 rats but does not alter responses in nontransgenic rats [32]. In mesenteric arteries of ovariectomized female SD rats, Ang(1-7) induces significant dilation in the presence of oestrogen [76]. On the other hand, in the aortic rings of ovariectomized female SD rats, subcutaneously administered oestrogen decreases the relaxation response to ANG(1-7) [77].

Less is known about ACE2, but activation of ACE2 by oestrogen would have beneficial effects. The response to Ang(1-7) might also differ between the sexes.

Bradykinin

Bradykinin is a component of the kallikrein-kinin system, not the RAAS. However, the RAAS and the kallikrein-kinin system overlap clinically because ACE also converts bradykinin to its inactive peptide. Thus, ACE inhibitors would not only decrease angiotensin II but also increase bradykinin; one of the beneficial effects of ACE inhibitors is considered via bradykinin [78]. Cardiovascular effects of bradykinin are considered to be predominantly mediated via B₂ receptor [79], a 7-transmembrane G-protein-coupled receptor [35]. B₂ receptor stimulation induces NO production and vasorelaxation [79].

In normotensive postmenopausal women, plasma bradykinin levels are increased by orally administered oestrogen and medroxyprogesterone [27,80] but are decreased by transdermally administered oestrogen and orally administered medroxyprogesterone [27,81].

In female SHRs, the captopril-induced fall in blood pressure is blunted by ovariectomy but is restored by oestrogen. This effect of oestrogen is prevented by B₂ receptor antagonists or L-N(G)-nitro-arginine methyl ester (L-NAME) [82], suggesting that the bradykinin-NO pathway plays roles in ACE inhibitor treatment and its displayed gender difference.

Catecholamines

As described earlier, catecholamines, i.e., beta adrenoceptor stimulation, stimulate renin production. Thus, sex hormones might regulate the RAAS via the regulation of catecholamines. β-Adrenoceptor stimulation shows inotropic and chronotropic effects, whereas α-adrenoceptor stimulation induces vasoconstriction.

In normotensive postmenopausal women, transdermally administered oestrogen decreases the rate of sympathetic nerve discharge, which is an index of sympathetic activity [83]. In premenopausal women, low frequency percentage (LF%) and the low frequency/high frequency ratio (LF/HF), both of which are indexes of sympathetic activity in heart rate variability, are lower than in men. The LF% and LF/HF are increased by menopause and attenuated by orally administered oestrogen [84]. Catechol oestrogens inhibit the activity of tyrosine hydroxylase, which plays a key role in the synthesis of dopamine and norepinephrine [22].

These findings suggest that oestrogen inhibits the RAAS, in part by inhibiting sympathetic activity.

Natriuretic peptides

Natriuretic peptides are predominantly produced in the heart. Natriuretic peptides bind guanylate cyclase A receptor, which is a membranous guanylyl cyclase, and increase levels of cyclic guanine monophosphate (cGMP), which antagonizes several components of the RAAS [85]. Natriuretic peptides induce vasorelaxation and diuresis and also show cardioprotective effect [85].

Plasma levels of atrial natriuretic peptide (ANP) are higher in women than in men [6]. In postmenopausal women, oestrogen increases plasma levels of ANP [86] and brain natriuretic peptide (BNP) [86,87].

In female rats, ovariectomy produces a decrease in plasma ANP levels, which is reversed by oestrogen [33,88]. In rat atria, oestrogen increases basal ANP secretion but does not affect stretch-induced secretion. In contrast, testosterone does not alter basal secretion but decreases stretch-induced secretion [89]. In neonatal rat ventricular myocytes, oestrogen attenuates hypertrophy induced by phenylephrine and endothelin-1 by increasing ANP mRNA expression and cGMP activity [90].

Oestrogen increases level of natriuretic peptides, which are counterparts of the RAAS. Oestrogen exerts its protective effects, at least in part, by activating the ANP/BNP-cGMP pathway.

Hypertension

In general, the activity level of the RAAS is higher in men than in premenopausal women. In premenopausal women, lower RAAS activity is associated with lower blood pressure, which is lower than that in men [25]. The combination of transdermally administered oestrogen and orally administered medroxyprogesterone decreases mean blood pressure without changing angiotensin II levels, indicating that oestrogen modulates the AT₁ receptor or its downstream signaling [27]. In SHRs, blood pressures are lower in females than in males, and ovariectomy increases blood pressure; blood pressure is correlated with the AT₁/AT₂ mRNA expression ratio [43]. In female SD rats, ovariectomy produces an increase in blood pressure, which is reversed by subcutaneously administered oestrogen [33]. In the two-kidney, one-clip model, hypertension is induced more frequently in males than in females. Ovariectomy increases the rate of hypertension in females to a level similar to that in males [93].

The lower activity of the RAAS in women suggests that the efficacy of an RAAS inhibitor would be lower. Although many studies have found no gender difference in the effect of RAAS inhibition, several studies have. In patients of African descent, men (mean age, 46 years) show greater blood pressure responses to ACE inhibitors than do women (mean age, 52 years) [94]. In elderly patients with hypertension (mean age, 72 years), ACE inhibitors are superior to diuretics for decreasing all cardiovascular events and all causes of death in men, but both types of drugs have similar effects in women [95]. When an ACE inhibitor is administered to SHRs, the decrease in blood pressure is greater in males [96].

Left ventricular hypertrophy is a compensatory mechanism of pressure load, but hypertrophy itself exacerbates CHF. ACE inhibitors decrease hypertrophy independent of their blood pressure-lowering effects [97]. In premenopausal hypertensive women, left ventricular mass is lower than in men matched for blood pressure, age, and race. This difference disappears after menopause [98]. In postmenopausal women, transdermally administered oestrogen reduces left ventricular mass, as assessed with echocardiography, independent of blood pressure [99].

Chronic heart failure (CHF)

ACE inhibitors have been shown to improve outcomes in patients with CHF [100]; however, few studies have examined gender differences in their effects. Among women with CHF (median age, 80 years), survival is longer in those treated with an ARB than in those treated with an ACE inhibitor. Among men with CHF (median age, 76 years), survival is similar in those treated with an ARB and those treated with an ACE inhibitor. However, among men without hypertension, those treated with an ACE inhibitor show longer survival than do those treated with an ARB [101].

In female Dahl myocardial infarction model rats, ovariectomy decreases ejection fraction, and subcutaneously administered oestrogen or an ARB reverses the effect of ovariectomy [102]. In female TGR(mREN2)27 hypertensive model rats, the androgen antagonist flutamide decreases cardiac hypertrophy [20].

In females, oestrogen exerts cardioprotective effect by inhibiting the RAAS. That ARBs have greater efficacy than do ACE inhibitors indicates the important role of AT₂ receptors, especially in females. Oestrogen also inhibits the RAAS by activating the ANP/BNP-cGMP system.

Chronic kidney disease

Chronic kidney disease progresses to end-stage renal failure more slowly in women than in men [3,5].

In SHRs, microalbumin excretion is greater in males than in females; the gender difference is decreased by castration and tends to be decreased by ovariectomy. The ED-1-positive macrophage counts in the renal cortex are greater in males than in females, and this difference is decreased by ovariectomy or castration. Superoxide levels in the renal cortex are also higher in males than in females [34]. In female TGR(mREN2)27 hypertensive model rats, the androgen antagonist flutamide decreases blood pressure, urinary albumin secretion, kidney levels of collagen III mRNA and repairs histological damage in the kidney. This study suggests endogenous androgens induce organ damage even in females [20]. In mRen(2).Lewis hypertensive model rats, urinary protein excretion is higher in males than in females [19] and is increased by ovariectomy in the presence of high salt [103]. In female Dahl myocardial infarction model rats, proteinuria is increased by ovariectomy, decreased by subcutaneously administered oestrogen, and further decreased by the combination of oestrogen and an ARB [102]. In this model, ovariectomy exacerbates glomerulosclerosis. Oestrogen alone does not attenuate glomerulosclerosis but increases interstitial cell infiltration and thrombosis. The combination of oestrogen and an ARB decreases glomerulosclerosis, interstitial cell infiltration, and thrombosis. The combination of oestrogen and an ARB has a much greater effect than does an ARB alone [102].

In contrast to these studies, other studies have shown harmful effects of oestrogen. In female mRen.Lewis rats, high-salt-induced kidney injuries, assessed with urinary protein excretion, kidney injury molecule 1 mRNA, renal cortical insulin-like growth factor 1 mRNA, and protein, improve with ovariectomy [104]. In female rats receiving angiotensin II and the nitric oxide synthase inhibitor L-NAME, ovariectomy reduces proteinuria, glomerular injury, and fibrinoid necrosis of renal arterioles, and oestrogen replacement therapy restores the protective effects of ovariectomy. Under these conditions, AT₁ receptor expression is increased by oestrogen [105].