Sex- and age-related differences in the chronic pressure-natriuresis relationship: role of the angiotensin type 2 receptor

Modulation of blood pressure by dietary potassium and sodium: sex differences and modeling analysis

High Na+ intake has been linked to elevations in blood pressure, whereas K+ has the opposite effect. The underlying mechanisms involve complex interactions among renal function, fluid volume, fluid-regulatory hormones, vasculature, cardiac function, and the autonomic nervous system. These mechanisms are likely modulated by sex, given the known sex differences in blood pressure regulation and the higher prevalence of hypertension in men. The source of these observed sex differences may be traced to organ and tissue levels, given that kidney function, intrarenal renin-angiotensin system components, renal sympathetic nervous activity, and nitric oxide bioavailability all exhibit sex differences. To assess the functional impact of each of these sex differences, we developed sex-specific computational models to simulate whole-body Na+, K+, and fluid homeostasis, and the effects on blood pressure. The models describe the interactions among the renal system, cardiovascular system, gastrointestinal system, renal sympathetic nervous system, and renin-angiotensin-aldosterone system. Model simulations suggest that women's attenuated blood pressure response to hypertensive stimuli, including high Na+ intake, may be largely attributable to the female renal transporter abundance pattern. In addition, we investigated the causal link between high K+ intake and blood pressure reduction. The models simulate renal response to high K+ intake, including the immediate gastrointestinal feedforward signals to the kidneys to increase K+ excretion, and the longer-term response to decrease proximal fractional Na+ reabsorption and distal K+ reabsorption. With these assumptions, simulations of high K+ intake yielded kaliuresis, natriuresis, and a substantial reduction in blood pressure, even when combined with high Na+ intake.NEW & NOTEWORTHY Excessive dietary Na+ raises blood pressure, whereas a high K+ diet has the opposite effect. The underlying mechanisms are moderated by sex and involve multiple organs and tissues. How do high K+-induced alternations in kidney function lower blood pressure, and how do those mechanisms differ between men and women? To answer these questions, we conducted computer simulations to simulate whole-body fluid and electrolyte homeostasis, and the effects of Na+ and K+ intake on blood pressure.

Salt sensitivity of blood pressure: mechanisms and sex-specific differences

Salt sensitivity of blood pressure (SSBP) is an independent risk factor for cardiovascular disease in individuals with or without hypertension. However, the mechanisms and management of SSBP remain unclear, mainly because the diagnosis of this condition relies on salt loading-depletion protocols that are not feasible in the clinic. The prevalence of hypertension is lower in premenopausal women than in men, but this sex-specific difference is reversed after menopause. Whether excessive SSBP in women at any age contributes to this reversal is unknown, but many clinical studies that have rigorously assessed for SSBP using salt loading-depletion protocols have confirmed that SSBP is more prevalent in women than in men, including during premenopausal age. In this Review, we discuss sex-specific mechanisms of SSBP. We describe sex-related differences in renal transporters, hypertensive pregnancy, SSBP in autoimmune disorders and mitogen-activated protein kinase signalling pathways, and highlight limitations and lessons learned from Dahl salt-sensitive rat models.

Sex-Specific Differences in the Pathophysiology of Hypertension

Hypertension is one of the most common comorbidities in cardiometabolic diseases, affecting nearly one third of adults. As a result, its pathophysiological mechanisms have been studied extensively and are focused around pressure natriuresis, the renin-angiotensin system (RAS), the sympathetic nervous system, oxidative stress, and endothelial dysfunction. Additionally, hypertension secondary to other underlying etiologies also exists. While clinical evidence has clearly shown differences in hypertension development in males and females, relatively little is known about the pathophysiological mechanisms behind these differences. Sex hormones likely play a key role, as they modulate many factors related to hypertension development. In this review, we postulate the potential role for sexually dimorphic fat metabolism in the physiology of hypertension. In brief, estrogen promotes subcutaneous fat deposition over visceral fat and increases in mass via adaptive hyperplasia rather than pathogenic hypertrophy. This adipose tissue subsequently produces anti-inflammatory effects and inhibits metabolic dysfunction-associated fatty liver disease (MAFLD) and RAS activation, ultimately leading to decreased levels of hypertension in pre-menopausal females. On the other hand, androgens and the lack of estrogens promote visceral and ectopic fat deposition, including in the liver, and lead to increased circulating pro-inflammatory cytokines and potentially subsequent RAS activation and hypertension development in males and post-menopausal females. Understanding the sex-specific differences in fat metabolism may provide deeper insights into the patho-mechanisms associated with hypertension and lead to more comprehensive sex-specific care.

Epigenetic mechanisms differentially regulate blood pressure and renal dysfunction in male and female Npr1 haplotype mice

We determined the epigenetic mechanisms regulating mean arterial pressure (MAP) and renal dysfunction in guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) gene-targeted mice. The Npr1 (encoding NPRA) gene-targeted mice were treated with class 1 specific histone deacetylase inhibitor (HDACi) mocetinostat (MGCD) to determine the epigenetic changes in a sex-specific manner. Adult male and female Npr1 haplotype (1-copy; Npr1+/-), wild-type (2-copy; Npr1+/+), and gene-duplicated heterozygous (3-copy; Npr1++/+) mice were intraperitoneally injected with MGCD (2 mg/kg) for 14 days. BP, renal function, histopathology, and epigenetic changes were measured. One-copy male mice showed significantly increased MAP, renal dysfunction, and fibrosis than 2-copy and 3-copy mice. Furthermore, HDAC1/2, collagen1alpha-2 (Col1α-2), and alpha smooth muscle actin (α-SMA) were significantly increased in 1-copy mice compared with 2-copy controls. The expression of antifibrotic microRNA-133a was attenuated in 1-copy mice but to a greater extent in males than females. NF-κB was localized at significantly lower levels in cytoplasm than in the nucleus with stronger DNA binding activity in 1-copy mice. MGCD significantly lowered BP, improved creatinine clearance, and repaired renal histopathology. The inhibition of class I HDACs led to a sex-dependent distinctive stimulation of acetylated positive histone marks and inhibition of methylated repressive histone marks in Npr1 1-copy mice; however, it epigenetically lowered MAP, repaired renal fibrosis, and proteinuria and suppressed NF-kB differentially in males versus females. Our results suggest a role for epigenetic targets affecting hypertension and renal dysfunction in a sex-specific manner.

Microvascular angiotensin II type 2 receptor function is enhanced in young females and declines in a model of murine aging

Introduction

Angiotensin II (AngII) affects cardiovascular health, mediating impacts through AngII type 1 (AT1R) and type 2 (AT2R) receptors. The present study investigated sex and aging-related differences in microvascular AngII receptor function in mice and humans.

Methods

Mesenteric resistance arteries (MRA) were isolated from 3-, 12-, and 18-month-old female and male C57/Bl6 mice. Wire myography was used to measure vasoconstriction to AngII and vasodilation to an AT2R agonist (compound 21, C21). Seven healthy adults (3 premenopausal women and 4 age-matched men) were recruited to participate in a study measuring cutaneous microvascular vasoconstriction to AngII in the presence and absence of 10 μM PD123319, an AT2R antagonist.

Results

In murine MRA, AngII-induced constriction increases by 18 months in females and by 12 months in males. AT2R-mediated vasodilation was reduced with age in females only, which corresponds with a female-specific decrease in mesenteric AT2R mRNA expression. AT2R inhibition enhances AngII-induced constriction in young female, but not male, mice. Clinical data support that premenopausal women have attenuated AngII constriction vs. men, which is abrogated by AT2R inhibition. AT2R expression is greater in primary aortic smooth muscle cells, but not endothelial cells, from young women compared with men.

Conclusions

These data demonstrate enhanced microvascular AT2R function in young female mice and young women. There is a female-specific loss of AT2R function with age in mice, concomitant with declining AT2R expression. These findings implicate AT2R as a sex-specific target for microvascular dysfunction and aging-associated cardiovascular disease.

Sex differences in blood pressure regulation and hypertension: renal, hemodynamic, and hormonal mechanisms

The teleology of sex differences has been argued since at least as early as Aristotle's controversial Generation of Animals more than 300 years BC, which reflects the sex bias of the time to contemporary readers. Although the question "why are the sexes different" remains a topic of debate in the present day in metaphysics, the recent emphasis on sex comparison in research studies has led to the question "how are the sexes different" being addressed in health science through numerous observational studies in both health and disease susceptibility, including blood pressure regulation and hypertension. These efforts have resulted in better understanding of differences in males and females at the molecular level that partially explain their differences in vascular function and renal sodium handling and hence blood pressure and the consequential cardiovascular and kidney disease risks in hypertension. This review focuses on clinical studies comparing differences between men and women in blood pressure over the life span and response to dietary sodium and highlights experimental models investigating sexual dimorphism in the renin-angiotensin-aldosterone, vascular, sympathetic nervous, and immune systems, endothelin, the major renal sodium transporters/exchangers/channels, and the impact of sex hormones on these systems in blood pressure homeostasis. Understanding the mechanisms governing sex differences in blood pressure regulation could guide novel therapeutic approaches in a sex-specific manner to lower cardiovascular risks in hypertension and advance personalized medicine.

Genetically engineered Lactobacillus paracasei rescues colonic angiotensin converting enzyme 2 (ACE2) and attenuates hypertension in female Ace2 knock out rats

Engineered gut microbiota represents a new frontier in medicine, in part serving as a vehicle for the delivery of therapeutic biologics to treat a range of host conditions. The gut microbiota plays a significant role in blood pressure regulation; thus, manipulation of gut microbiota is a promising avenue for hypertension treatment. In this study, we tested the potential of Lactobacillus paracasei, genetically engineered to produce and deliver human angiotensin converting enzyme 2 (Lacto-hACE2), to regulate blood pressure in a rat model of hypertension with genetic ablation of endogenous Ace2 (Ace2-/- and Ace2-/y). Our findings reveal a sex-specific reduction in blood pressure in female (Ace2-/-) but not male (Ace2-/y) rats following colonization with the Lacto-hACE2. This beneficial effect of lowering blood pressure was aligned with a specific reduction in colonic angiotensin II, but not renal angiotensin II, suggesting the importance of colonic Ace2 in the regulation of blood pressure. We conclude that this approach of targeting the colon with engineered bacteria for delivery of ACE2 represents a promising new paradigm in the development of antihypertensive therapeutics.

Determinants of Renal Micro-Perfusion as Assessed with Contrast-Enhanced Ultrasound in Healthy Males and Females

(1) Background: The renal microcirculation is essential to maintain the renal function, but its determinants in humans have been poorly studied. Contrast-enhanced ultrasound (CEUS) allows the non-invasive quantification of the cortical micro-perfusion at the bedside using the perfusion index (PI). The aims of this study were to assess whether differences exist in PI between healthy males and females and to identify clinical determinants associated with cortical micro-perfusion. (2) Methods: Healthy, normotensive volunteers (eGFR > 60 mL/min/1.73 m², no albuminuria) underwent CEUS under standardized conditions with the destruction-reperfusion (DR) technique. The mean PI of four DR sequences was reported as the primary outcome measure (3) Results: A total of 115 subjects (77 females and 38 males) completed the study; the mean ± SD age was, respectively, 37.1 ± 12.2 and 37.1 ± 12.7 years in females and males, and the mean eGFR was 105.9 ± 15.1 and 91.0 ± 17.4 mL/min/1.73 m². The PI (median) was higher in females than in males, i.e., 2705 (IQR 1641-3777) vs. 1965 (IQR 1294-3346) arbitrary units (a.u), p = 0.02). A correlation analysis showed positive associations between PI and eGFR, female sex, heart rate, plasma renin activity (PRA) and plasma aldosterone concentrations (PAC), negative associations with potassium, bicarbonate and systolic blood pressure, and no associations with age, body mass index and renal resistive index (RRI). In a multivariate linear regression analysis, only PRA remained significantly associated with PI. (4) Conclusions: Although the PI was higher among females, this association was no longer significant after adjustment for covariates. There was no difference in females tested during the follicular or the luteal phases. In conclusion, the PI was only weakly influenced by classic clinical variables, but was positively associated with PRA, suggesting that the renin-angiotensin system plays a role in the regulation of the cortical micro-perfusion in humans. Identifying which other factors contribute to the large variations in micro-perfusion across individuals needs further study.

Female mice are protected from impaired parenchymal arteriolar TRPV4 function and impaired cognition in hypertension

Hypertension is a leading modifiable risk factor for cerebral small vessel disease. Our laboratory has shown that endothelium-dependent dilation in cerebral parenchymal arterioles (PAs) is dependent on transient receptor potential vanilloid 4 (TRPV4) activation, and this pathway is impaired in hypertension. This impaired dilation is associated with cognitive deficits and neuroinflammation. Epidemiological evidence suggests that women with midlife hypertension have an increased dementia risk that does not exist in age-matched men, though the mechanisms responsible for this are unclear. This study aimed to determine the sex differences in young, hypertensive mice to serve as a foundation for future determination of sex differences at midlife. We tested the hypothesis that young hypertensive female mice would be protected from the impaired TRPV4-mediated PA dilation and cognitive dysfunction observed in male mice. Angiotensin II (ANG II)-filled osmotic minipumps (800 ng/kg/min, 4 wk) were implanted in 16- to 19-wk-old male C56BL/6 mice. Age-matched female mice received either 800 ng/kg/min or 1,200 ng/kg/min ANG II. Sham-operated mice served as controls. Systolic blood pressure was elevated in ANG II-treated male mice and in 1,200 ng ANG II-treated female mice versus sex-matched shams. PA dilation in response to the TRPV4 agonist GSK1016790A (10-9-10-5 M) was impaired in hypertensive male mice, which was associated with cognitive dysfunction and neuroinflammation, reproducing our previous findings. Hypertensive female mice exhibited normal TRPV4-mediated PA dilation and were cognitively intact. Female mice also showed fewer signs of neuroinflammation than male mice. Determining the sex differences in cerebrovascular health in hypertension is critical for developing effective therapeutic strategies for women.NEW & NOTEWORTHY Vascular dementia is a significant public health concern, and the effect of biological sex on dementia development is not well understood. TRPV4 channels are essential regulators of cerebral parenchymal arteriolar function and cognition. Hypertension impairs TRPV4-mediated dilation and memory in male rodents. Data presented here suggest female sex protects against impaired TRPV4 dilation and cognitive dysfunction during hypertension. These data advance our understanding of the influence of biological sex on cerebrovascular health in hypertension.

Smooth Muscle Mineralocorticoid Receptor Promotes Hypertension After Preeclampsia

BACKGROUND

Preeclampsia is a syndrome of high blood pressure (BP) with end organ damage in late pregnancy that is associated with high circulating soluble VEGF receptor (sFlt1 [soluble Fms-like tyrosine kinase 1]). Women exposed to preeclampsia have a substantially increased risk of hypertension after pregnancy, but the mechanism remains unknown, leaving a missed interventional opportunity. After preeclampsia, women have enhanced sensitivity to hypertensive stress. Since smooth muscle cell mineralocorticoid receptors (SMC-MR) are activated by hypertensive stimuli, we hypothesized that high sFlt1 exposure in pregnancy induces a postpartum state of enhanced SMC-MR responsiveness.

METHODS

Postpartum BP response to high salt intake was studied in women with prior preeclampsia. MR transcriptional activity was assessed in vitro in sFlt1-treated SMC by reporter assays and PCR. Preeclampsia was modeled by transient sFlt1 expression in pregnant mice. Two months post-partum, mice were exposed to high salt and then to AngII (angiotensin II) and BP and vasoconstriction were measured.

RESULTS

Women exposed to preeclampsia had significantly enhanced salt sensitivity of BP verses those with a normotensive pregnancy. sFlt1 overexpression during pregnancy in mice induced elevated BP and glomerular endotheliosis, which resolved post-partum. The sFlt1 exposed post-partum mice had significantly increased BP response to 4% salt diet and to AngII infusion. In vitro, SMC-MR transcriptional activity in response to aldosterone or AngII was significantly increased after transient exposure to sFlt1 as was aldosterone-induced expression of AngII type 1 receptor. Post-partum, SMC-MR-KO mice were protected from the enhanced response to hypertensive stimuli after preeclampsia. Mechanistically, preeclampsia mice exposed to postpartum hypertensive stimuli develop enhanced aortic stiffness, microvascular myogenic tone, AngII constriction, and AngII type 1 receptor expression, all of which were prevented in SMC-MR-KO littermates.

CONCLUSIONS

These data support that sFlt1-induced vascular injury during preeclampsia produces a persistent state of enhanced sensitivity of SMC-MR to activation. This contributes to postpartum hypertension in response to common stresses and supports testing of MR antagonism to mitigate the increased cardiovascular risk in women after PE.

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.

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Kidney Angiotensin in Cardiovascular Disease: Formation and Drug Targeting

The concept of local formation of angiotensin II in the kidney has changed over the last 10-15 years. Local synthesis of angiotensinogen in the proximal tubule has been proposed, combined with prorenin synthesis in the collecting duct. Binding of prorenin via the so-called (pro)renin receptor has been introduced, as well as megalin-mediated uptake of filtered plasma-derived renin-angiotensin system (RAS) components. Moreover, angiotensin metabolites other than angiotensin II [notably angiotensin-(1-7)] exist, and angiotensins exert their effects via three different receptors, of which angiotensin II type 2 and Mas receptors are considered renoprotective, possibly in a sex-specific manner, whereas angiotensin II type 1 (AT1) receptors are believed to be deleterious. Additionally, internalized angiotensin II may stimulate intracellular receptors. Angiotensin-converting enzyme 2 (ACE2) not only generates angiotensin-(1-7) but also acts as coronavirus receptor. Multiple, if not all, cardiovascular diseases involve the kidney RAS, with renal AT1 receptors often being claimed to exert a crucial role. Urinary RAS component levels, depending on filtration, reabsorption, and local release, are believed to reflect renal RAS activity. Finally, both existing drugs (RAS inhibitors, cyclooxygenase inhibitors) and novel drugs (angiotensin receptor/neprilysin inhibitors, sodium-glucose cotransporter-2 inhibitors, soluble ACE2) affect renal angiotensin formation, thereby displaying cardiovascular efficacy. Particular in the case of the latter three, an important question is to what degree they induce renoprotection (e.g., in a renal RAS-dependent manner). This review provides a unifying view, explaining not only how kidney angiotensin formation occurs and how it is affected by drugs but also why drugs are renoprotective when altering the renal RAS. SIGNIFICANCE STATEMENT: Angiotensin formation in the kidney is widely accepted but little understood, and multiple, often contrasting concepts have been put forward over the last two decades. This paper offers a unifying view, simultaneously explaining how existing and novel drugs exert renoprotection by interfering with kidney angiotensin formation.

The Na+/H+ Exchanger 3 in the Intestines and the Proximal Tubule of the Kidney: Localization, Physiological Function, and Key Roles in Angiotensin II-Induced Hypertension

The sodium (Na⁺)/hydrogen (H⁺) exchanger 3 (NHE3) is one of the most important Na⁺/H⁺ antiporters in the small intestines of the gastrointestinal tract and the proximal tubules of the kidney. The roles of NHE3 in the regulation of intracellular pH and acid-base balance have been well established in cellular physiology using in vitro techniques. Localized primarily on the apical membranes in small intestines and proximal tubules, the key action of NHE3 is to facilitate the entry of luminal Na⁺ and the extrusion of intracellular H⁺ from intestinal and proximal tubule tubular epithelial cells. NHE3 is, directly and indirectly, responsible for absorbing the majority of ingested Na⁺ from small and large intestines and reabsorbing >50% of filtered Na⁺ in the proximal tubules of the kidney. However, the roles of NHE3 in the regulation of proximal tubular Na⁺ transport in the integrative physiological settings and its contributions to the basal blood pressure regulation and angiotensin II (Ang II)-induced hypertension have not been well studied previously due to the lack of suitable animal models. Recently, novel genetically modified mouse models with whole-body, kidney-specific, or proximal tubule-specific deletion of NHE3 have been generated by us and others to determine the critical roles and underlying mechanisms of NHE3 in maintaining basal body salt and fluid balance, blood pressure homeostasis, and the development of Ang II-induced hypertension at the whole-body, kidney, or proximal tubule levels. The objective of this invited article is to review, update, and discuss recent findings on the critical roles of intestinal and proximal tubule NHE3 in maintaining basal blood pressure homeostasis and their potential therapeutic implications in the development of angiotensin II (Ang II)-dependent hypertension.

Pressure-Natriuresis Response Is Diminished in Old Age

BACKGROUND

Age-related alterations in renal sodium handling affect blood pressure (BP). We aimed to clarify whether the pressure-natriuresis response changes with age, leading to BP elevation.

METHODS

A total of 4,859 participants with normal renal function from the Korean Genome and Epidemiology Study (KoGES) and 235 patients with non-diabetic chronic kidney disease (CKD) from the ESPECIAL trial were included and divided into the younger and older groups. In ESPECIAL, participants took olmesartan from weeks 0 to 16 and were educated about a low-salt diet (LSD) from weeks 8 to 16.

RESULTS

In both studies, older participants showed lower estimated glomerular filtration rate (eGFR) and urine concentration index and higher albuminuria. In KoGES, BP was higher and urine sodium was lower in older participants. In ESPECIAL, diastolic BP at 0 week was lower in older participants. Olmesartan reduced BP in both groups, whereas LSD decreased systolic BP only in older participants. Urine sodium increased in younger participants but decreased in older participants after olmesartan use. In KoGES, urine sodium was correlated with BP in both groups after adjusting for age, sex, and eGFR; however, the correlation coefficient was lower in older participants. In ESPECIAL, only younger participants showed a significant positive association between systolic BP and urine sodium in multiple regression analysis.

CONCLUSIONS

The pressure-natriuresis response was diminished in older participants with or without CKD.

Relaxin as an anti-fibrotic treatment: Perspectives, challenges and future directions

Fibrosis refers to the scarring and hardening of tissues, which results from a failed immune system-coordinated wound healing response to chronic organ injury and which manifests from the aberrant accumulation of various extracellular matrix components (ECM), primarily collagen. Despite being a hallmark of prolonged tissue damage and related dysfunction, and commonly associated with high morbidity and mortality, there are currently no effective cures for its regression. An emerging therapy that meets several criteria of an effective anti-fibrotic treatment, is the recombinant drug-based form of the human hormone, relaxin (also referred to as serelaxin, which is bioactive in several other species). This review outlines the broad anti-fibrotic and related organ-protective roles of relaxin, mainly from studies conducted in preclinical models of ageing and fibrotic disease, including its ability to ameliorate several aspects of fibrosis progression and maturation, from immune cell infiltration, pro-inflammatory and pro-fibrotic cytokine secretion, oxidative stress, organ hypertrophy, cell apoptosis, myofibroblast differentiation and ECM production, to its ability to facilitate established ECM degradation. Studies that have compared and/or combined these therapeutic effects of relaxin with current standard of care medication have also been discussed, along with the main challenges that have hindered the translation of the anti-fibrotic efficacy of relaxin to the clinic. The review then outlines the future directions as to where scientists and several pharmaceutical companies that have recognized the therapeutic potential of relaxin are working towards, to progress its development as a treatment for human patients suffering from various fibrotic diseases.

Sex-specific adaptations to high-salt diet preserve electrolyte homeostasis with distinct sodium transporter profiles

Kidneys continuously filter an enormous amount of sodium and adapt kidney Na+ reabsorption to match Na+ intake to maintain circulatory volume and electrolyte homeostasis. Males (M) respond to high-salt (HS) diet by translocating proximal tubule Na+/H+ exchanger isoform 3 (NHE3) to the base of the microvilli, reducing activated forms of the distal NaCl cotransporter (NCC) and epithelial Na+ channel (ENaC). Males (M) and females (F) on normal-salt (NS) diet present sex-specific profiles of "transporters" (cotransporters, channels, pumps, and claudins) along the nephron, e.g., F exhibit 40% lower NHE3 and 200% higher NCC abundance than M. We tested the hypothesis that adaptations to HS diet along the nephron will, likewise, exhibit sexual dimorphisms. C57BL/6J mice were fed for 15 days with 4% NaCl diet (HS) versus 0.26% NaCl diet (NS). On HS, M and F exhibited normal plasma [Na+] and [K+], similar urine volume, Na+, K+, and osmolal excretion rates normalized to body weight. In F, like M, HS lowered abundance of distal NCC, phosphorylated NCC, and cleaved (activated) forms of ENaC. The adaptations associated with achieving electrolyte homeostasis exhibit sex-dependent and independent mechanisms. Sex differences in baseline "transporters" abundance persist during HS diet, yet the fold changes during HS diet (normalized to NS) are similar along the distal nephron and collecting duct. Sex-dependent differences observed along the proximal tubule during HS show that female kidneys adapt differently from patterns reported in males, yet achieve and maintain fluid and electrolyte homeostasis.

Relaxin Attenuates Organ Fibrosis via an Angiotensin Type 2 Receptor Mechanism in Aged Hypertensive Female Rats

Background

The antifibrotic effects of recombinant human relaxin (RLX) in the kidney are dependent on an interaction between its cognate receptor (RXFP1) and the angiotensin type 2 receptor (AT₂R) in male models of disease. Whether RLX has therapeutic effects, which are also mediated via AT₂R, in hypertensive adult and aged/reproductively senescent females is unknown. Thus, we determined whether treatment with RLX provides cardiorenal protection via an AT₂R-dependent mechanism in adult and aged female stroke-prone spontaneously hypertensive rats (SHRSPs).

Methods

In 6-month-old (6MO) and 15-month-old ([15MO]; reproductively senescent) female SHRSP, systolic BP (SBP), GFR, and proteinuria were measured before and after 4 weeks of treatment with vehicle (Veh), RLX (0.5 mg/kg per day s.c.), or RLX+PD123319 (AT₂R antagonist; 3 mg/kg per day s.c.). Aortic endothelium-dependent relaxation and fibrosis of the kidney, heart, and aorta were assessed.

Results

In 6MO SHRSP, RLX significantly enhanced GFR by approximately 25% (P=0.001) and reduced cardiac fibrosis (P=0.01) as compared with vehicle-treated counterparts. These effects were abolished or blunted by PD123319 coadministration. In 15MO females, RLX reduced interstitial renal (P=0.02) and aortic (P=0.003) fibrosis and lowered SBP (13±3 mm Hg; P=0.04) relative to controls. These effects were also blocked by PD123319 cotreatment (all P=0.05 versus RLX treatment alone). RLX also markedly improved vascular function by approximately 40% (P<0.001) in 15MO SHRSP, but this was not modulated by PD123319 cotreatment.

Conclusions

The antifibrotic and organ-protective effects of RLX, when administered to a severe model of hypertension, conferred cardiorenal protection in adult and reproductively senescent female rats to a great extent via an AT₂R-mediated mechanism.

AT2R deficiency in mice accelerates podocyte dysfunction in diabetic progeny in a sex-dependent manner

AIMS/HYPOTHESIS

The angiotensin II receptor type 2 (AT₂R) may be a potential therapeutic target for the treatment of hypertension and chronic kidney disease (CKD). The expression and function of AT₂R in the vasculature and kidney appear sexually dimorphic. We hypothesised that Agtr2 knockout dams (AT₂RKO) with gestational diabetes would program their offspring for subsequent hypertension and CKD in a sex-dependent manner.

METHODS

Age- and sex-matched offspring of non-diabetic and diabetic dams of wild-type (WT) and AT₂RKO mice were followed from 4 to 20 weeks of age and were monitored for development of hypertension and nephropathy; a mouse podocyte cell line (mPOD) was also studied.

RESULTS

Body weight was progressively lower in female compared with male offspring throughout the lifespan. Female but not male offspring from diabetic AT₂RKO dams developed insulin resistance. Compared with the offspring of non-diabetic dams, the progeny of diabetic dams had developed more hypertension and nephropathy (apparent glomerulosclerosis with podocyte loss) at 20 weeks of age; this programming was more pronounced in the offspring of AT₂RKO diabetic dams, particularly female AT₂RKO progeny. Female AT₂RKO offspring had lower basal ACE2 glomerular expression, resulting in podocyte loss. The aberrant ACE2/ACE ratio was far more diminished in glomeruli of female progeny of diabetic AT₂RKO dams than in male progeny. Knock-down of Agtr2 in mPODs confirmed the in vivo data.

CONCLUSIONS/INTERPRETATION

AT₂R deficiency accelerated kidney programming in female progeny of diabetic dams, possibly due to loss of protective effects of ACE2 expression in the kidney.

Considering Sex as a Biological Variable in Basic and Clinical Studies: An Endocrine Society Scientific Statement

In May 2014, the National Institutes of Health (NIH) stated its intent to "require applicants to consider sex as a biological variable (SABV) in the design and analysis of NIH-funded research involving animals and cells." Since then, proposed research plans that include animals routinely state that both sexes/genders will be used; however, in many instances, researchers and reviewers are at a loss about the issue of sex differences. Moreover, the terms sex and gender are used interchangeably by many researchers, further complicating the issue. In addition, the sex or gender of the researcher might influence study outcomes, especially those concerning behavioral studies, in both animals and humans. The act of observation may change the outcome (the "observer effect") and any experimental manipulation, no matter how well-controlled, is subject to it. This is nowhere more applicable than in physiology and behavior. The sex of established cultured cell lines is another issue, in addition to aneuploidy; chromosomal numbers can change as cells are passaged. Additionally, culture medium contains steroids, growth hormone, and insulin that might influence expression of various genes. These issues often are not taken into account, determined, or even considered. Issues pertaining to the "sex" of cultured cells are beyond the scope of this Statement. However, we will discuss the factors that influence sex and gender in both basic research (that using animal models) and clinical research (that involving human subjects), as well as in some areas of science where sex differences are routinely studied. Sex differences in baseline physiology and associated mechanisms form the foundation for understanding sex differences in diseases pathology, treatments, and outcomes. The purpose of this Statement is to highlight lessons learned, caveats, and what to consider when evaluating data pertaining to sex differences, using 3 areas of research as examples; it is not intended to serve as a guideline for research design.

In Aged Females, the Enhanced Pressor Response to Angiotensin II Is Attenuated By Estrogen Replacement via an Angiotensin Type 2 Receptor-Mediated Mechanism

[Figure: see text].

Angiotensin II Type 2 Receptor: A Target for Protection Against Hypertension, Metabolic Dysfunction, and Organ Remodeling

The renin-angiotensin system is of vital significance not only in the maintenance of blood pressure but also because of its role in the pathophysiology of different organ systems in the body. Of the 2 Ang II (angiotensin II) receptors, the AT₁R (Ang II type 1 receptor) has been extensively studied for its role in mediating the classical functions of Ang II, including vasoconstriction, stimulation of renal tubular sodium reabsorption, hormonal secretion, cell proliferation, inflammation, and oxidative stress. The other receptor, AT₂R (Ang II type 2 receptor), is abundantly expressed in both immune and nonimmune cells in fetal tissue. However, its expression is increased under pathological conditions in adult tissues. The role of AT₂R in counteracting AT₁R function has been discussed in the past 2 decades. However, with the discovery of the nonpeptide agonist C21, the significance of AT₂R in various pathologies such as obesity, hypertension, and kidney diseases have been examined. This review focuses on the most recent findings on the beneficial effects of AT₂R by summarizing both gene knockout studies as well as pharmacological studies, specifically highlighting its importance in blood pressure regulation, obesity/metabolism, organ protection, and relevance in the treatment of coronavirus disease 2019 (COVID-19).

Salt-sensitive hypertension in chronic kidney disease: distal tubular mechanisms

Chronic kidney disease (CKD) causes salt-sensitive hypertension that is often resistant to treatment and contributes to the progression of kidney injury and cardiovascular disease. A better understanding of the mechanisms contributing to salt-sensitive hypertension in CKD is essential to improve these outcomes. This review critically explores these mechanisms by focusing on how CKD affects distal nephron Na+ reabsorption. CKD causes glomerulotubular imbalance with reduced proximal Na+ reabsorption and increased distal Na+ delivery and reabsorption. Aldosterone secretion further contributes to distal Na+ reabsorption in CKD and is not only mediated by renin and K+ but also by metabolic acidosis, endothelin-1, and vasopressin. CKD also activates the intrarenal renin-angiotensin system, generating intratubular angiotensin II to promote distal Na+ reabsorption. High dietary Na+ intake in CKD contributes to Na+ retention by aldosterone-independent activation of the mineralocorticoid receptor mediated through Rac1. High dietary Na+ also produces an inflammatory response mediated by T helper 17 cells and cytokines increasing distal Na+ transport. CKD is often accompanied by proteinuria, which contains plasmin capable of activating the epithelial Na+ channel. Thus, CKD causes both local and systemic changes that together promote distal nephron Na+ reabsorption and salt-sensitive hypertension. Future studies should address remaining knowledge gaps, including the relative contribution of each mechanism, the influence of sex, differences between stages and etiologies of CKD, and the clinical relevance of experimentally identified mechanisms. Several pathways offer opportunities for intervention, including with dietary Na+ reduction, distal diuretics, renin-angiotensin system inhibitors, mineralocorticoid receptor antagonists, and K+ or H+ binders.

Defective Renal Angiotensin III and AT2 Receptor Signaling in Prehypertensive Spontaneously Hypertensive Rats

Background

Previous studies demonstrated that angiotensin (Ang) III, not Ang II, is the predominant endogenous agonist for Ang type‐2 receptor (AT₂R)‐induced natriuresis in normal rats, and that hypertensive 12‐week‐old spontaneously hypertensive rats (SHR) lack natriuretic responses to Ang III. This study tested whether prehypertensive SHR already have defective Ang III‐induced natriuresis and determined possible mechanisms.

Methods and Results

Female and male normotensive 4‐week‐old SHR and Wistar Kyoto rats were studied after 24‐hour systemic AT₁R blockade. Left kidneys received 30 minute renal interstitial infusions of vehicle followed by Ang III (3.5, 7.0, 14, and 28 nmol/kg per min; each dose for 30 minutes). Right kidneys received vehicle infusions. In 4‐week‐old Wistar Kyoto rats, renal interstitial Ang III increased urine sodium (Na⁺) excretion but failed to induce natriuresis in 4‐week‐old SHR. Renal Ang III levels were similar between Wistar Kyoto rats and SHR, making increased Ang III degradation as a possible cause for defective natriuresis in SHR unlikely. In Wistar Kyoto rats, renal interstitial Ang III induced translocation of AT₂Rs to apical plasma membranes of renal proximal tubule cells. Simultaneously, Ang III induced retraction of the major Na⁺ transporter Na⁺‐H⁺ exchanger‐3 (NHE‐3) from apical membranes and internalization of Na⁺/K⁺ATPase (NKA) from basolateral membranes of renal proximal tubule cells. Consistent with NHE‐3 and NKA retraction, Ang III increased pSer⁵⁵²‐NHE‐3 and decreased pSer²³‐NKA. In contrast, in SHR, intrarenal Ang III failed to induce AT₂R translocation, NHE‐3 or NKA retraction, pSer⁵⁵²‐NHE‐3 phosphorylation, or pSer²³‐NKA dephosphorylation.

Conclusions

These results indicate impaired Ang III/AT₂R signaling as a possible primary defect in prehypertensive SHR.

See Editorial Zhuo and Li

Sex differences in solute transport along the nephrons: effects of Na+ transport inhibition

Each day, ~1.7 kg of NaCl and 180 liters of water are reabsorbed by nephron segments in humans, with urinary excretion fine tuned to meet homeostatic requirements. These tasks are coordinated by a spectrum of renal Na⁺ transporters and channels. The goal of the present study was to investigate the extent to which inhibitors of transepithelial Na⁺ transport (T_Na) along the nephron alter urinary solute excretion and how those effects may vary between male and female subjects. To accomplish that goal, we developed sex-specific multinephron models that represent detailed transcellular and paracellular transport processes along the nephrons of male and female rat kidneys. We simulated inhibition of Na⁺/H⁺ exchanger 3 (NHE3), bumetanide-sensitive Na⁺-K⁺-2Cl^- cotransporter (NKCC2), Na⁺-Cl^- cotransporter (NCC), and amiloride-sensitive epithelial Na⁺ channel (ENaC). NHE3 inhibition simulations predicted a substantially reduced proximal tubule T_Na, and NKCC2 inhibition substantially reduced thick ascending limb T_Na. Both gave rise to diuresis, natriuresis, and kaliuresis, with those effects stronger in female rats. While NCC inhibition was predicted to have only minor impact on renal T_Na, it nonetheless had a notable effect of enhancing excretion of Na⁺, K⁺, and Cl^-, particularly in female rats. Inhibition of ENaC was predicted to have opposite effects on the excretion of Na⁺ (increased) and K⁺ (decreased) and to have only a minor impact on whole kidney T_Na. Unlike inhibition of other transporters, ENaC inhibition induced stronger natriuresis and diuresis in male rats than female rats. Overall, model predictions agreed well with measured changes in Na⁺ and K⁺ excretion in response to diuretics and Na⁺ transporter mutations.

Electrolyte and transporter responses to angiotensin II induced hypertension in female and male rats and mice

AIM

Sexual dimorphisms are evident along the nephron: Females (F) exhibit higher ratios of renal distal to proximal Na⁺ transporters' abundance, greater lithium clearance (C_Li ) more rapid natriuresis in response to saline infusion and lower plasma [K⁺ ] vs. males (M). During angiotensin II infusion hypertension (AngII-HTN) M exhibit distal Na⁺ transporter activation, lower proximal and medullary loop transporters, blunted natriuresis in response to saline load, and reduced plasma [K⁺ ]. This study aimed to determine whether responses of F to AngII-HTN mimicked those in M or were impacted by sexual dimorphisms evident at baseline.

METHODS

Sprague Dawley rats and C57BL/6 mice were AngII infused via osmotic minipumps 2 and 3 weeks, respectively, and assessed by metabolic cage collections, tail-cuff sphygmomanometer, semi-quantitative immunoblotting of kidney and patch-clamp electrophysiology.

RESULTS

In F rats, AngII-infusion increased BP to 190 mm Hg, increased phosphorylation of cortical NKCC2, NCC and cleavage of ENaC two to threefold, increased ENaC channel activity threefold and aldosterone 10-fold. K⁺ excretion increased and plasma [K⁺ ] decreased. Evidence of natriuresis in F included increased urine Na⁺ excretion and C_Li , and decreased medullary NHE3, NKCC2 and Na,K-ATPase abundance. In C57BL/6 mice, AngII-HTN increased abundance of distal Na⁺ transporters, suppressed proximal-medullary transporters and reduced plasma [K⁺ ] in both F and M.

CONCLUSION

Despite baseline sexual dimorphisms, AngII-HTN provokes similar increases in BP, aldosterone, distal transporters, ENaC channel activation and K⁺ loss accompanied by similar suppression of proximal and loop Na⁺ transporters, natriuresis and diuresis in females and males.

The anti-fibrotic actions of relaxin are mediated through AT2 R-associated protein phosphatases via RXFP1-AT2 R functional crosstalk in human cardiac myofibroblasts

Fibrosis is a hallmark of several cardiovascular diseases. The relaxin family peptide receptor 1 (RXFP1) agonist, relaxin, has rapidly occurring anti-fibrotic actions which are mediated through RXFP1 and angiotensin II receptor crosstalk on renal and cardiac myofibroblasts. Here, we investigated whether this would allow relaxin to indirectly activate angiotensin II type 2 receptor (AT₂ R)-specific signal transduction in primary human cardiac myofibroblasts (HCMFs). The anti-fibrotic effects of recombinant human relaxin (RLX; 16.8 nM) or the AT₂ R-agonist, Compound 21 (C21; 1 μM), were evaluated in TGF-β1-stimulated HCMFs, in the absence or presence of an RXFP1 antagonist (1 μM) or AT₂ R antagonist (0.1 μM) to confirm RXFP1-AT₂ R crosstalk. Competition binding for RXFP1 was determined. Western blotting was performed to determine which AT₂ R-specific protein phosphatases were expressed by HCMFs; then, the anti-fibrotic effects of RLX and/or C21 were evaluated in the absence or presence of pharmacological inhibition (NSC95397 (1 μM) for MKP-1; okadaic acid (10 nM) for PP2A) or siRNA-knockdown of these phosphatases after 72 hours. The RLX- or C21-induced increase in ERK1/2 and nNOS phosphorylation, and decrease in α-SMA (myofibroblast differentiation) and collagen-I expression by HCMFs was abrogated by pharmacological blockade of RXFP1 or the AT₂ R, confirming RXFP1-AT₂ R crosstalk in these cells. HCMFs were found to express AT₂ R-dependent MKP-1 and PP2A phosphatases, while pharmacological blockade or siRNA-knockdown of either phosphatase also abolished RLX and/or C21 signal transduction in HCMFs (all P < .05 vs RLX or C21 alone). These findings demonstrated that RLX can indirectly activate AT₂ R-dependent phosphatase activity in HCMFs by signaling through RXFP1-AT₂ R crosstalk, which have important therapeutic implications for its anti-fibrotic actions.

The Renin-Angiotensin System in Hypertension, a Constantly Renewing Classic: Focus on the Angiotensin AT2-Receptor

It is common knowledge that the renin-angiotensin system (RAS), in particular angiotensin II acting through the angiotensin AT₁-receptor (AT₁R), is pivotal for the regulation of blood pressure (BP) and extracellular volume. More recent findings have revealed that the RAS is far more complex than initially thought and that it harbours additional mediators and receptors, which are able to counteract and thereby fine-tune AT₁R-mediated actions. This review will focus on the angiotensin AT₂-receptor (AT₂R), which is one of the "counter-regulatory" receptors within the RAS. It will review and discuss data related to the role of the AT₂R in regulation of BP and focus on the following 3 questions: Do peripheral AT₂R have an impact on BP regulation, and, if so, does this effect become apparent only under certain conditions? Are central nervous system AT₂R involved in regulation of BP, and, if so, which brain areas are involved and what are the mechanisms? Does dysfunction of AT₂R contribute to the pathogenesis of hypertension in preeclampsia?

Maternal hypoxia developmentally programs low podocyte endowment in male, but not female offspring

Fetal hypoxia is a common complication of pregnancy. We have previously reported that maternal hypoxia in late gestation in mice gives rise to male offspring with reduced nephron number, while females have normal nephron number. Male offspring later develop proteinuria and renal pathology, including glomerular pathology, whereas female offspring are unaffected. Given the central role of podocyte depletion in glomerular and renal pathology, we examined whether maternal hypoxia resulted in low podocyte endowment in offspring. Pregnant CD1 mice were allocated at embryonic day 14.5 to normoxic (21% oxygen) or hypoxic (12% oxygen) conditions. At postnatal day 21, kidneys from mice were immersion fixed, and one mid-hilar slice per kidney was immunostained with antibodies directed against p57 and synaptopodin for podocyte identification. Slices were cleared and imaged with a multiphoton microscope for podometric analysis. Male hypoxic offspring had significantly lower birth weight, nephron number, and podocyte endowment than normoxic male offspring (podocyte number; normoxic 62.86 ± 2.26 podocytes per glomerulus, hypoxic 53.38 ± 2.25; p < .01, mean ± SEM). In contrast, hypoxic female offspring had low birth weight but their nephron and podocyte endowment was the same as normoxic female offspring (podocyte number; normoxic 62.38 ± 1.86 podocytes per glomerulus, hypoxic 61.81 ± 1.80; p = .88). To the best of our knowledge, this is the first report of developmentally programmed low podocyte endowment. Given the well-known association between podocyte depletion in adulthood and glomerular pathology, we postulate that podocyte endowment may place offspring at risk of renal disease in adulthood, and explain the greater vulnerability of male offspring.

Serelaxin and the AT2 Receptor Agonist CGP42112 Evoked a Similar, Nonadditive, Cardiac Antifibrotic Effect in High Salt-Fed Mice That Were Refractory to Candesartan Cilexetil

Fibrosis is involved in the majority of cardiovascular diseases and is a key contributor to end-organ dysfunction. In the current study, the antifibrotic effects of recombinant human relaxin-2 (serelaxin; RLX) and/or the AT₂R agonist CGP42112 (CGP) were compared with those of the established AT₁R antagonist, candesartan cilexetil (CAND), in a high salt-induced cardiac fibrosis model. High salt (HS; 5%) for 8 weeks did not increase systolic blood pressure in male FVB/N mice, but CAND treatment alone significantly reduced systolic blood pressure from HS-induced levels. HS significantly increased cardiac interstitial fibrosis, which was reduced by either RLX and/or CGP, which were not additive under the current experimental conditions, while CAND failed to reduce HS-induced cardiac fibrosis. The antifibrotic effects induced by RLX and/or CGP were associated with reduced myofibroblast differentiation. Additionally, all treatments inhibited the HS-induced elevation in tissue inhibitor of matrix metalloproteinases-1, together with trends for increased MMP-13 expression, that collectively would favor collagen degradation. Furthermore, these antifibrotic effects were associated with reduced cardiac inflammation. Collectively, these results highlight that either RXFP1 or AT₂R stimulation represents novel therapeutic strategies to target fibrotic conditions, particularly in HS states that may be refractory to AT₁R blockade.

FGF-21 ameliorates essential hypertension of SHR via baroreflex afferent function

Hypertension is a common complication of metabolic abnormalities associated with cardiovascular system and characterized by sexual dimorphism in mammals. Fibroblast growth factor-21 (FGF-21) plays a critical role in metabolic-disorder related hypertension through the afferent loop of baroreflex. However, the gender difference in FGF-21-mediated blood pressure (BP) regulation via sexual dimorphic expression of FGFRs in the nodose (NG) and nucleus tractus solitarius (NTS) were not elucidated in physiological and genomic form of hypertension. The gene and protein expression of FGFRs were tested by qRT-PCR, immunoblotting and immunostaining; the serum level of FGF21 was tested using ELISA; The BP was monitored while FGF21 was nodose microinjected. The results showed that more potent BP reduction was confirmed in female vs. male rats by nodose microinjection of rhFGF-21 along with higher expression of FGFR2 and FGFR4 in the nodose compared with age-match male and ovariectomized (OVX) rats, rather than other receptor subtypes, which is consistent well with immunohistochemical analysis. Additionally, serum FGF-21 was significantly higher in female-WKY, and this level of FGF-21 was dramatically declined in spontaneous hypertensive rats (SHR) with significant down-regulation of FGFR1/R4 for male-SHR and FGFR2/FGFR4 for female-SHR, respectively. Apparently, high BP of SHR of either sex could be reduced by rhFGF-21 nodose microinjection. These data extends our current understanding that sexual-specific distribution/expression of FGF-21/FGFRs is likely to contribute at least partially to sexual dimorphism of baroreflex afferent function on BP regulation in rats. FGF-21-mdiated BP reduction sheds new light on clinical management of primary/genomic form of hypertension.

AT1R-AT2R-RXFP1 Functional Crosstalk in Myofibroblasts: Impact on the Therapeutic Targeting of Renal and Cardiac Fibrosis

BACKGROUND

Recombinant human relaxin-2 (serelaxin), which has organ-protective actions mediated via its cognate G protein-coupled receptor relaxin family peptide receptor 1 (RXFP1), has emerged as a potential agent to treat fibrosis. Studies have shown that serelaxin requires the angiotensin II (AngII) type 2 receptor (AT₂R) to ameliorate renal fibrogenesis in vitro and in vivo. Whether its antifibrotic actions are affected by modulation of the AngII type 1 receptor (AT₁R), which is expressed on myofibroblasts along with RXFP1 and AT₂R, is unknown.

METHODS

We examined the signal transduction mechanisms of serelaxin when applied to primary rat renal and human cardiac myofibroblasts in vitro, and in three models of renal- or cardiomyopathy-induced fibrosis in vivo.

RESULTS

The AT₁R blockers irbesartan and candesartan abrogated antifibrotic signal transduction of serelaxin via RXFP1 in vitro and in vivo. Candesartan also ameliorated serelaxin's antifibrotic actions in the left ventricle of mice with cardiomyopathy, indicating that candesartan's inhibitory effects were not confined to the kidney. We also demonstrated in a transfected cell system that serelaxin did not directly bind to AT₁Rs but that constitutive AT₁R-RXFP1 interactions could form. To potentially explain these findings, we also demonstrated that renal and cardiac myofibroblasts expressed all three receptors and that antagonists acting at each receptor directly or allosterically blocked the antifibrotic effects of either serelaxin or an AT₂R agonist (compound 21).

CONCLUSIONS

These findings have significant implications for the concomitant use of RXFP1 or AT₂R agonists with AT₁R blockers, and suggest that functional interactions between the three receptors on myofibroblasts may represent new targets for controlling fibrosis progression.

Sex Differences in Blood Pressure Hemodynamics in Middle-Aged Adults With Overweight and Obesity

High blood pressure (BP) is the strongest modifiable risk factor for cardiovascular disease. Overweight/obesity is a major risk factor of high BP. Multiple sex differences exist in mechanistic pathways that increase BP in overweight/obesity. They may result in a sex-specific pattern of BP hemodynamics—males and females may vary in the relative contributions of stroke volume, total peripheral resistance (TPR), and heart rate to higher BP. We investigated this possibility in a population-based sample of middle-aged adults (36–65 years). The total sample (n=618) included 289 males and 329 females; 79% of males and 66% of females were overweight. In all, we measured BP, stroke volume, TPR, and heart rate beat-by-beat during a 52-minute protocol that included changes in posture and mental stress. We assessed the relative contributions of stroke volume, TPR, and heart rate to BP at each minute of the protocol. We observed marked sex differences in BP hemodynamics in overweight/obese individuals: the main determinant of higher BP was TPR in males (49% versus only 35% in females, P =0.008), whereas it was stroke volume in females (51% versus only 35% in males, P =0.006). These sex differences were most apparent when standing or sitting at rest. No such differences were seen in normal-weight individuals in whom the main determinant of higher BP was TPR in both sexes. Our study suggests that, in middle-aged adults, marked sex differences exist in BP hemodynamics, contributing to high BP in overweight/obese but not normal-weight individuals. As such, this study may contribute to precision medicine in hypertension.

Proximal Tubule-Specific Deletion of the NHE3 (Na+/H+ Exchanger 3) in the Kidney Attenuates Ang II (Angiotensin II)-Induced Hypertension in Mice

The present study directly tested the hypothesis that the NHE3 (Na⁺/H⁺ exchanger 3) in the proximal tubules of the kidney is required for the development of Ang II (angiotensin II)-induced hypertension using PT-Nhe3^-/- (proximal tubule-specific NHE3 knockout) mice. Specifically, PT-Nhe3^-/- mice were generated using the SGLT2-Cre/Nhe3^loxlox approach, whereas Ang II-induced hypertension was studied in 12 groups (n=5-12 per group) of adult male and female wild-type (WT) and PT-Nhe3^-/- mice. Under basal conditions, systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure were significantly lower in male and female PT-Nhe3^-/- than WT mice (P<0.01). A high pressor, 1.5 mg/kg per day, intraperitoneal or a slow pressor dose of Ang II, 0.5 mg/kg per day, intraperitoneal for 2 weeks significantly increased systolic blood pressure, diastolic blood pressure, and mean arterial blood pressure in male and female WT mice (P<0.01), but the hypertensive response to Ang II was markedly attenuated in male and female PT-Nhe3^-/- mice (P<0.01). Ang II impaired the pressure-natriuresis response in WT mice, whereas proximal tubule-specific deletion of NHE3 improved the pressure-natriuresis response in Ang II-infused PT-Nhe3^-/- mice (P<0.01). AT₁ receptor blocker losartan completely blocked Ang II-induced hypertension in both WT and PT-Nhe3^-/- mice (P<0.01). However, inhibition of nitric oxide synthase with L-N^G-Nitroarginine methyl ester had no effect on Ang II-induced hypertension in WT or PT-Nhe3^-/- mice (not significant). Furthermore, Ang II-induced hypertension was significantly attenuated by an orally absorbable NHE3 inhibitor AVE0657. In conclusion, NHE3 in the proximal tubules of the kidney may be a therapeutical target in hypertension induced by Ang II or with increased NHE3 expression in the proximal tubules.

Sex Differences in Renal Inflammation and Injury in High-Fat Diet-Fed Dahl Salt-Sensitive Rats

We examined the impact of sex on high-fat diet (HFD)-induced renal alterations in Dahl salt-sensitive and Sprague Dawley rats. In Dahl rats, HFD (60% kcal from fat for 24-26 weeks starting at weaning) significantly and equally increased blood pressure in males and females when compared with rats fed a control diet (10% kcal from fat). Male Dahl rats on HFD exhibited progressive renal histological injury and moderately increased renal macrophage infiltration at 10 and 24 weeks of feeding when compared with males on control diet. Female Dahl rats had lower grade renal injury and less macrophage infiltration (except at 17 weeks) than males regardless of diet. Male Dahl rats on both diets showed progressively increasing numbers of renal T-cells, a pattern not observed in females. HFD per se did not significantly affect renal T-cell number. Male Dahl rats had lower renal regulatory T-cells cell ratio than females at 24 weeks. Renal macrophage and T-cell infiltrations were highly correlated to final mean arterial pressure levels in males but not in females. Sprague Dawley rats fed HFD were normotensive without significant renal injury/inflammation after 24 weeks of feeding. In summary, HFD feeding fails to increase arterial blood pressure in Sprague Dawley rats but strongly promotes hypertension in both male and female Dahl salt-sensitive rats. Only Dahl males, however, exhibited blood pressure-associated renal inflammation and injury. Maintenance of regulatory T-cells ratio may protect against hypertension-associated renal injury/inflammation but not HFD-induced hypertension.

Sex differences in mechanisms of arterial stiffness

Arterial stiffness progressively increases with aging and is an independent predictor of cardiovascular disease (CVD) risk. Evidence supports that there are sex differences in the time course of aging-related arterial stiffness and the associated CVD risk, which increases disproportionately in postmenopausal women. The association between arterial stiffness and mortality is almost twofold higher in women versus men. The differential clinical characteristics of the development of arterial stiffness between men and women indicate the involvement of sex-specific mechanisms. This review summarizes the current literature on sex differences in vascular stiffness induced by aging, obesity, hypertension, and sex-specific risk factors as well as the impact of hormonal status, diet, and exercise on vascular stiffness in males and females. An understanding of the mechanisms driving sex differences in vascular stiffness has the potential to identify novel sex-specific therapies to lessen CVD risk, the leading cause of death in males and females. LINKED ARTICLES: This article is part of a themed section on The Importance of Sex Differences in Pharmacology Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.21/issuetoc.

The potential role of myosin motor proteins in mediating the subcellular distribution of NHE3 in the renal proximal tubule

Isoform 3 of the Na⁺/H⁺ exchanger (NHE3) is responsible for the majority of the reabsorption of NaCl, NaHCO₃, and, consequently, water in the renal proximal tubule. As such, this transporter plays an essential role in acid-base balance and extracellular fluid volume homeostasis and determining systemic arterial blood pressure levels. NHE3 activity is modulated by a number of mechanisms, including the redistribution of the transporter between the body of the microvilli (where NHE3 is active) and the base of the microvilli (where NHE3 is less active). Although the physiological, pathophysiological, and pharmacological importance of the subcellular distribution of NHE3 has been well established, the exact mechanism whereby NHE3 is translocated along microvilli microdomains of the proximal tubule apical membrane is unknown. Nonmuscle myosin IIA and unconventional myosin VI move cargoes in anterograde and retrograde directions, respectively, and are known to redistribute along with NHE3 in the proximal tubule in response to a variety of natriuretic and antinatriuretic stimuli, including stimulation or inhibition of the renin-angiotensin system, high dietary Na⁺ intake, and high blood pressure. Therefore, this review aims to discuss the current evidence that suggests a potential role of myosin IIA and myosin VI in mediating the subcellular distribution of NHE3 along the kidney proximal tubule microvilli and their possible contribution in modifying NHE3-mediated Na⁺ reabsorption under both physiological and pathophysiological conditions.

Lack of Suppression of Aldosterone Production Leads to Salt-Sensitive Hypertension in Female but Not Male Balb/C Mice

Clinical studies indicate that salt-sensitive hypertension is more prevalent in women than in men. However, animal models of salt sensitivity have primarily focused on the mechanisms of salt sensitivity in male animals; therefore, elucidation of these mechanisms in female animal models is needed. We have previously shown that female Balb/C mice have higher aldosterone synthase expression and aldosterone production than males. We hypothesized that female Balb/C mice develop salt-sensitive increases in blood pressure. Seven-day feeding of a 4% NaCl high-salt (HS) diet increased blood pressure in female mice without altering blood pressure in males. Females on an HS diet displayed no apparent increases in sodium retention as assessed by 24-hour urine collection, sodium balance measure, and saline loading excretion analysis. Females on an HS diet exhibited lower renin-angiotensin system activity (plasma Ang II [angiotensin II], plasma renin activity, and ACE [angiotensin-converting enzyme] activity) compared with males but developed a salt-induced elevation in adrenal aldosterone synthase expression and retained higher aldosterone levels than males on HS. This resulted in a higher aldosterone/plasma renin activity ratio in females compared with males on HS feeding. Adrenal mRNA expression of angiotensinogen and leptin receptor was increased in female mice on an HS diet. HS impaired endothelium-dependent relaxation in female mice only. MR (mineralocorticoid receptor) inhibition (eplerenone) restored blood pressure and endothelial function in females on an HS diet. Collectively, these data indicate that Balb/C mice develop sex-discrepant salt-sensitive hypertension likely via aldosterone-MR-mediated mechanisms involving impaired endothelium-dependent relaxation in females only. This study presents the first model of spontaneous sex-specific salt sensitivity, which mimics the human pathology.

The angiotensin type 2 receptor and the kidney

PURPOSE OF REVIEW

Angiotensin II is a main regulator of kidney function. Renal actions mediated by the angiotensin AT1 receptor have been well known for many years. In contrast, several details of angiotensin AT2 receptor actions in kidney physiology and pathophysiology were only described very recently. These findings are reviewed in this article.

RECENT FINDINGS

Regarding the role of the angiotensin AT2 receptor in kidney physiology, a major recent finding was that the AT2 receptor-mediated inhibition of Na-H exchanger-3 and Na/K-ATPase in the renal proximal tubules is caused by internalisation of these transporters, thus reducing reabsorption and increasing natriuresis/diuresis. Regarding renal pathology, several studies demonstrated an attenuation of renal injury caused by diabetes or by obesity with or without high-salt diet through anti-inflammatory, antifibrotic, and antioxidative mechanisms. Generally, AT2 receptor expression seems increased and AT2 receptor-mediated effects stronger in female and obese animals.

SUMMARY

The recent findings about the role of the angiotensin AT2 receptor in renal health and disease strongly suggest that pharmacological targeting of this receptor with selective agonists is a promising therapeutic strategy for inducing diuresis/natriuresis (also additive to established diuretics) and for the treatment of diabetic nephropathy or kidney disease of other pathogenesis.

Mechanisms of altered renal sodium handling in age-related hypertension

The prevalence of hypertension rises with age to approximately two out of three adults over the age of 60 in the United States. Although the mechanisms underlying age-related hypertension are incompletely understood, sodium homeostasis is critical to the long-term regulation of blood pressure and there is strong evidence that aging is associated with alterations in renal sodium handling. This minireview focuses on recent advancements in our understanding of the vascular, neurohumoral, and renal mechanisms that influence sodium homeostasis and promote age-related hypertension.

Sex-specific differences in hypertension and associated cardiovascular disease

Although intrinsic mechanisms that regulate arterial blood pressure (BP) are similar in men and women, marked variations exist at the molecular, cellular and tissue levels. These physiological disparities between the sexes likely contribute to differences in disease onset, susceptibility, prevalence and treatment responses. Key systems that are important in the development of hypertension and cardiovascular disease (CVD), including the sympathetic nervous system, the renin-angiotensin-aldosterone system and the immune system, are differentially activated in males and females. Biological age also contributes to sexual dimorphism, as premenopausal women experience a higher degree of cardioprotection than men of similar age. Furthermore, sex hormones such as oestrogen and testosterone as well as sex chromosome complement likely contribute to sex differences in BP and CVD. At the cellular level, differences in cell senescence pathways may contribute to increased longevity in women and may also limit organ damage caused by hypertension. In addition, many lifestyle and environmental factors - such as smoking, alcohol consumption and diet - may influence BP and CVD in a sex-specific manner. Evidence suggests that cardioprotection in women is lost under conditions of obesity and type 2 diabetes mellitus. Treatment strategies for hypertension and CVD that are tailored according to sex could lead to improved outcomes for affected patients.

Sexual Dimorphic Pattern of Renal Transporters and Electrolyte Homeostasis

Compared with males, females have lower BP before age 60, blunted hypertensive response to angiotensin II, and a leftward shift in pressure natriuresis. This study tested the concept that this female advantage associates with a distinct sexual dimorphic pattern of transporters along the nephron. We applied quantitative immunoblotting to generate profiles of transporters, channels, claudins, and selected regulators in both sexes and assessed the physiologic consequences of the differences. In rats, females excreted a saline load more rapidly than males did. Compared with the proximal tubule of males, the proximal tubule of females had greater phosphorylation of Na+/H+ exchanger isoform 3 (NHE3), distribution of NHE3 at the base of the microvilli, and less abundant expression of Na+/Pi cotransporter 2, claudin-2, and aquaporin 1. These changes associated with less bicarbonate reabsorption and higher lithium clearance in females. The distal nephrons of females had a higher abundance of total and phosphorylated Na+/Cl- cotransporter (NCC), claudin-7, and cleaved forms of epithelial Na+ channel (ENaC) α and γ subunits, which associated with a lower baseline plasma K+ concentration. A K+-rich meal increased the urinary K+ concentration and decreased the level of renal phosphorylated NCC in females. Notably, we observed similar abundance profiles in female versus male C57BL/6 mice. These results define sexual dimorphic phenotypes along the nephron and suggest that lower proximal reabsorption in female rats expedites excretion of a saline load and enhances NCC and ENaC abundance and activation, which may facilitate K+ secretion and set plasma K+ at a lower level.

Sex and gender differences in hypertensive kidney injury

Hypertension is a complex, multifaceted disorder, affecting ~1 in 3 adults in the United States. Although hypertension occurs in both men and women, there are distinct sex differences in the way in which they develop hypertension, with women having a lower incidence of hypertension until the sixth decade of life. Despite observed sex differences in hypertension, little is known about the molecular mechanisms underlying the development of hypertension in females, primarily because of their underrepresentation in both clinical and experimental animal studies. The first goal of this review is to provide a concise overview of the participation of women in clinical trials, including a discussion of the importance of including females in basic science research, as recently mandated by the National Institutes of Health. The remaining portion of the review is dedicated to identifying clinical and experimental animal studies that concentrate on gender and sex differences in hypertensive kidney disease, ending with a proposed role for T cells in mediating sex differences in blood pressure.

Infusion of Valproic Acid Into the Renal Medulla Activates Stem Cell Population and Attenuates Salt-Sensitive Hypertension in Dahl S Rats

BACKGROUND

Our previous study has detected a stem cell deficiency in the renal medulla in Dahl salt-sensitive (S) rats. This study determined whether infusion of valproic acid (VA), an agent known to stimulate the stem cell function, attenuated salt-sensitive hypertension in Dahl S rats.

METHODS

Uninephrectomized Dahl S rats were infused with vehicle or VA (50mg/kg/d) into the renal medulla and fed with a low (LS) or high salt diet (HS). Stem cell marker and number were analyzed by immunohistochemistry, Real-time RT-PCR and Western blot. Sodium excretion and blood pressure were measured.

RESULTS

VA significantly increased the mRNA and protein levels of FGF2, a stem cell niche factor, and CD133, a stem cell marker. The number of CD133+ cells was significantly increased in the renal medulla in VA-treated rats. Meanwhile, high salt-induced increases in the mRNA level of proinflammatory factors interleukin-1β and interleukin-6 were blocked in VA-treated rats. Functionally, sodium excretion in response to the blood pressure increase and acute sodium loading was significantly enhanced, sodium retention attenuated, high salt-induced increase of blood pressure reduced in VA-treated rats.

CONCLUSION

Activation of stem cell function by VA inhibits the activation of proinflammatory factors and attenuates salt-sensitive hypertension in Dahl S rats.

Anti-fibrotic actions of relaxin

Fibrosis refers to the hardening or scarring of tissues that usually results from aberrant wound healing in response to organ injury, and its manifestations in various organs have collectively been estimated to contribute to around 45-50% of deaths in the Western world. Despite this, there is currently no effective cure for the tissue structural and functional damage induced by fibrosis-related disorders. Relaxin meets several criteria of an effective anti-fibrotic based on its specific ability to inhibit pro-fibrotic cytokine and/or growth factor-mediated, but not normal/unstimulated, fibroblast proliferation, differentiation and matrix production. Furthermore, relaxin augments matrix degradation through its ability to up-regulate the release and activation of various matrix-degrading matrix metalloproteinases and/or being able to down-regulate tissue inhibitor of metalloproteinase activity. Relaxin can also indirectly suppress fibrosis through its other well-known (anti-inflammatory, antioxidant, anti-hypertrophic, anti-apoptotic, angiogenic, wound healing and vasodilator) properties. This review will outline the organ-specific and general anti-fibrotic significance of exogenously administered relaxin and its mechanisms of action that have been documented in various non-reproductive organs such as the cardiovascular system, kidney, lung, liver, skin and tendons. In addition, it will outline the influence of sex on relaxin's anti-fibrotic actions, highlighting its potential as an emerging anti-fibrotic therapeutic.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Epochs in the depressor/pressor balance of the renin-angiotensin system

The renin-angiotensin system (RAS) plays a commanding role in the regulation of extracellular fluid homoeostasis. Tigerstadt and Bergman first identified the RAS more than two centuries ago. By the 1980s a voyage of research and discovery into the mechanisms and actions of this system led to the development of drugs that block the RAS, which have become the mainstay for the treatment of cardiovascular and renal disease. In the last 25 years new components of the RAS have come to light, including the angiotensin type 2 receptor (AT2R) and the angiotensin-converting enzyme 2 (ACE2)/angiotensin-(1-7) [Ang(1-7)]/Mas receptor (MasR) axis. These have been shown to counter the classical actions of angiotensin II (AngII) at the predominant angiotensin type 1 receptor (AT1R). Our studies, and those of others, have demonstrated that targeting these depressor RAS pathways may be therapeutically beneficial. It is apparent that the evolution of both the pressor and depressor RAS pathways is distinct throughout life and that the depressor/pressor balance of the RAS vary between the sexes. These temporal patterns of expression suggest that therapies targeting the RAS could be optimized for discrete epochs in life.

AT2 Receptors: Potential Therapeutic Targets for Hypertension

The renin-angiotensin system (RAS) is arguably the most important and best studied hormonal system in the control of blood pressure (BP) and the pathogenesis of hypertension. The RAS features its main effector angiotensin II (Ang II) acting via its 2 major receptors, angiotensin type-1(AT1R) and type-2 (AT2R). In general, AT2Rs oppose the detrimental actions of Ang II via AT1Rs. AT2R activation induces vasodilation and natriuresis, but its effects to lower BP in hypertension have not been as clear as anticipated. Recent studies, however, have demonstrated that acute and chronic AT2R stimulation can induce natriuresis and lower BP in the Ang II infusion model of experimental hypertension. AT2R activation induces receptor recruitment from intracellular sites to the apical plasma membranes of renal proximal tubule cells via a bradykinin, nitric oxide, and cyclic guanosine 3',5' monophosphate signaling pathway that results in internalization and inactivation of sodium (Na+) transporters Na+-H+ exchanger-3 and Na+/K+ATPase. These responses do not require the presence of concurrent AT1R blockade and are effective both in the prevention and reversal of hypertension. This review will address the role of AT2Rs in the control of BP and Na+ excretion and the case for these receptors as potential therapeutic targets for hypertension in humans.

Sex- and age-related differences in arterial pressure and albuminuria in mice

Background

Animal models have become valuable experimental tools for understanding the pathophysiology and therapeutic interventions in cardiovascular disease. Yet to date, few studies document the age- and sex-related differences in arterial pressure, circadian rhythm, and renal function in normotensive mice under basal conditions, across the life span. We hypothesized that mice display similar sex- and age-related differences in arterial pressure and renal function to humans.

Methods

Mean arterial pressure (MAP) and circadian rhythm of arterial pressure were measured over 3 days via radiotelemetry, in 3- and 5-month-old (adult) and 14- and 18-month-old (aged) FVB/N and in 5-month-old (adult) C57BL/6 male and female normotensive mice. In FVB/N mice, albuminuria from 24-h urine samples as well as body, heart, and kidney weights were measured at each age.

Results

Twenty-four-hour MAP was greater in males than females at 3, 5, and 14 months of age. A similar sex difference in arterial pressure was observed in C57BL/6 mice at 5 months of age. In FVB/N mice, 24-h MAP increased with age, with females displaying a greater increase between 3 and 18 months of age than males, such that MAP was no longer different between the sexes at 18 months of age. A circadian pattern was observed in arterial pressure, heart rate, and locomotor activity, with values for each greater during the active (night/dark) than the inactive (day/light) period. The night-day dip in MAP was greater in males and increased with age in both sexes. Albuminuria was greater in males than females, increased with age in both sexes, and rose to a greater level in males than females at 18 months of age.

Conclusions

Arterial pressure and albuminuria increase in an age- and sex-specific manner in mice, similar to patterns observed in humans. Thus, mice represent a useful model for studying age and sex differences in the regulation of arterial pressure and renal disease. Understanding the mechanisms that underlie the pathophysiology of cardiovascular disease may lead to new and better-tailored therapies for men and women.