Angiotensin AT2 receptors directly stimulate renal nitric oxide in bradykinin B2-receptor-null mice

Sex-Specific Differences in Kidney Function and Blood Pressure Regulation

Premenopausal women generally exhibit lower blood pressure and a lower prevalence of hypertension than men of the same age, but these differences reverse postmenopause due to estrogen withdrawal. Sexual dimorphism has been described in different components of kidney physiology and pathophysiology, including the renin-angiotensin-aldosterone system, endothelin system, and tubular transporters. This review explores the sex-specific differences in kidney function and blood pressure regulation. Understanding these differences provides insights into potential therapeutic targets for managing hypertension and kidney diseases, considering the patient's sex and hormonal status.

The potential role of brain renin-angiotensin system in the neuropathology of Parkinson disease: Friend, foe or turncoat?

Parkinson disease (PD) is one of the most common neurodegenerative diseases of the brain. Of note, brain renin-angiotensin system (RAS) is intricate in the PD neuropathology through modulation of oxidative stress, mitochondrial dysfunction and neuroinflammation. Therefore, modulation of brain RAS by angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme inhibitors (ACEIs) may be effective in reducing the risk and PD neuropathology. It has been shown that all components including the peptides and enzymes of the RAS are present in the different brain areas. Brain RAS plays a critical role in the regulation of memory and cognitive function, and in the controlling of central blood pressure. However, exaggerated brain RAS is implicated in the pathogenesis of different neurodegenerative diseases including PD. Two well-known pathways of brain RAS are recognized including; the classical pathway which is mainly mediated by AngII/AT1R has detrimental effects. Conversely, the non-classical pathway which is mostly mediated by ACE2/Ang1-7/MASR and AngII/AT2R has beneficial effects against PD neuropathology. Exaggerated brain RAS affects the viability of dopaminergic neurons. However, the fundamental mechanism of brain RAS in PD neuropathology was not fully elucidated. Consequently, the purpose of this review is to disclose the mechanistic role of RAS in in the pathogenesis of PD. In addition, we try to revise how the ACEIs and ARBs can be developed for therapeutics in PD.

Regulation of vascular angiotensin II type 1 and type 2 receptor and angiotensin-(1-7)/MasR signaling in normal and hypertensive pregnancy

Normal pregnancy (Norm-Preg) is associated with a slight reduction in blood pressure (BP) and decreased BP response to vasoconstrictor stimuli such as angiotensin II (Ang II), although the renin-angiotensin-aldosterone system (RAAS) is upregulated. Preeclampsia (PE) is a complication of pregnancy manifested as hypertension-in-pregnancy (HTN-Preg), and dysregulation of angiotensin biosynthesis and signaling have been implicated. Ang II activates vascular Ang II type-1 receptor (AT1R) and Ang II type-2 receptor (AT2R), while angiotensin-(1-7) promotes Ang-(1-7)/MasR signaling. The role of AT1R in vasoconstriction and the activated cellular mechanisms are well-characterized. The sensitivity of vascular AT1R to Ang II and consequent activation of vasoconstrictor mechanisms decrease during Norm-Preg, but dramatically increase in HTN-Preg. Placental ischemia in late pregnancy could also initiate the release of AT1R agonistic autoantibodies (AT1AA) with significant impact on endothelial dysfunction and activation of contraction pathways in vascular smooth muscle including [Ca2+]c and protein kinase C. On the other hand, the role of AT2R and Ang-(1-7)/MasR in vascular relaxation, particularly during Norm-Preg and PE, is less clear. During Norm-Preg, increases in the expression/activity of vascular AT2R and Ang-(1-7)/MasR promote the production of endothelium-derived relaxing factors such as nitric oxide (NO), prostacyclin and endothelium-derived hyperpolarizing factor leading to generalized vasodilation. Aortic segments of Preg rats show prominent endothelial AT2R staining and increased relaxation and NO production in response to AT2R agonist CGP42112A, and treatment with AT2R antagonist PD123319 enhances phenylephrine-induced contraction. Decreased vascular AT2R and Ang-(1-7)/MasR expression and receptor-mediated mechanisms of vascular relaxation have been suggested in HTN-Preg animal models, but their role in human PE needs further testing. Changes in angiotensin-converting enzyme-2 (ACE2) have been observed in COVID-19 patients, and whether ACE2 influences the course of COVID-19 viral infection/immunity in Norm-Preg and PE is an intriguing area for research.

Advancement in Beneficial Effects of AVE 0991: A Brief Review

AVE 0991, a non-peptide analogue of Angiotensin-(1-7) [Ang-(1-7)], is orally active and physiologically well tolerated. Several studies have demonstrated that AVE 0991 improves glucose and lipid metabolism, and contains anti-inflammatory, anti-apoptotic, anti-fibrosis, and anti-oxidant effects. Numerous preclinical studies have also reported that AVE 0991 appears to have beneficial effects on a variety of systemic diseases, including cardiovascular, liver, kidney, cancer, diabetes, and nervous system diseases. This study searched multiple literature databases, including PubMed, Web of Science, EMBASE, Google Scholar, Cochrane Library, and the ClinicalTrials.gov website from the establishment to October 2022, using AVE 0991 as a keyword. This literature search revealed that AVE 0991 could play different roles via various signaling pathways. However, the potential mechanisms of these effects need further elucidation. This review summarizes the benefits of AVE 0991 in several medical problems, including the COVID-19 pandemic. The paper also describes the underlying mechanisms of AVE 0991, giving in-depth insights and perspectives on the pharmaceutical value of AVE 0991 in drug discovery and development.

Circulating renin-angiotensin systems mediated feedback controls over the mean-arterial pressure

The renin-angiotensin systems play pivotal role in cardiovascular physiology through its effects on regulating blood pressure and electrolyte homeostasis. Components of circulating RAS (cRAS) that include precursor angiotensinogen, two critical enzymes (renin and angiotensin-converting enzyme, ACE), their bioactive products, angiotensin- I, II together with its receptors (AT1R and AT2R) essentially determine this homeostasis. Most classical studies, however, showed the deleterious role of cRAS in elevating the blood pressure. Contemporary discovery of non-canonical components of the RAS has challenged this classic hypothesis that it can only exert deleterious effects on the cardiovascular systems. Using the classic cRAS model, we have designed in-silico experiments to test the hypothesis that AT2R-mediated feedback effects play pivotal role for maintaining the normal variation of the mean-arterial pressure (MAP).Beside the AT2R-mediation of downstream singling pathways consisting of several non-canonical RAS components, this study first time illustrated AT2R mediated feedback controls over the blood pressure regulation: one that impedes AT1R activity, and the other that downregulates renin. It has been shown that relatively stronger suppression of renin activity significantly contributes in maintaining the normal MAP and that tight AT2R-mediated regulation is relaxed in hyper-and hypotension. This control mechanism is noted to be robustly maintained with the MAP variations through an established linear steady-state relationship among renin, angiotensin I and angiotensin II. This examination suggests that AT2R-mediated downregulation of renin activities potentially counteracts the AT1R-mediated deleterious actions of Ang II. This study, therefore, provides a solid ground for considering different AT2 receptor adaptor protein and direct agonism at AT2R that can cause greater effects along with contemporary approaches of blocking AT1R mediation to attenuate hypertension or other cardiovascular disorders.

The AT1/AT2 Receptor Equilibrium Is a Cornerstone of the Regulation of the Renin Angiotensin System beyond the Cardiovascular System

The AT₁ receptor has mainly been associated with the pathological effects of the renin-angiotensin system (RAS) (e.g., hypertension, heart and kidney diseases), and constitutes a major therapeutic target. In contrast, the AT₂ receptor is presented as the protective arm of this RAS, and its targeting via specific agonists is mainly used to counteract the effects of the AT₁ receptor. The discovery of a local RAS has highlighted the importance of the balance between AT₁/AT₂ receptors at the tissue level. Disruption of this balance is suggested to be detrimental. The fine tuning of this balance is not limited to the regulation of the level of expression of these two receptors. Other mechanisms still largely unexplored, such as S-nitrosation of the AT₁ receptor, homo- and heterodimerization, and the use of AT₁ receptor-biased agonists, may significantly contribute to and/or interfere with the settings of this AT₁/AT₂ equilibrium. This review will detail, through several examples (the brain, wound healing, and the cellular cycle), the importance of the functional balance between AT₁ and AT₂ receptors, and how new molecular pharmacological approaches may act on its regulation to open up new therapeutic perspectives.

Signaling pathways in vascular function and hypertension: molecular mechanisms and therapeutic interventions

Hypertension is a global public health issue and the leading cause of premature death in humans. Despite more than a century of research, hypertension remains difficult to cure due to its complex mechanisms involving multiple interactive factors and our limited understanding of it. Hypertension is a condition that is named after its clinical features. Vascular function is a factor that affects blood pressure directly, and it is a main strategy for clinically controlling BP to regulate constriction/relaxation function of blood vessels. Vascular elasticity, caliber, and reactivity are all characteristic indicators reflecting vascular function. Blood vessels are composed of three distinct layers, out of which the endothelial cells in intima and the smooth muscle cells in media are the main performers of vascular function. The alterations in signaling pathways in these cells are the key molecular mechanisms underlying vascular dysfunction and hypertension development. In this manuscript, we will comprehensively review the signaling pathways involved in vascular function regulation and hypertension progression, including calcium pathway, NO-NOsGC-cGMP pathway, various vascular remodeling pathways and some important upstream pathways such as renin-angiotensin-aldosterone system, oxidative stress-related signaling pathway, immunity/inflammation pathway, etc. Meanwhile, we will also summarize the treatment methods of hypertension that targets vascular function regulation and discuss the possibility of these signaling pathways being applied to clinical work.

Angiotensin receptor blocker use is associated with upregulation of the memory-protective angiotensin type 4 receptor (AT4R) in the postmortem brains of individuals without cognitive impairment

The reported primary dementia-protective benefits of angiotensin II type 1 receptor (AT1R) blockers (ARB) are believed, at least in part, to arise from systemic effects on blood pressure. However, there is a specific and independently regulated brain renin-angiotensin system (RAS). Brain RAS acts mainly through three receptor subtypes; AT1R, AT2R, and AT4R. The AT1R promotes inflammation and mitochondrial reactive oxygen species generation. AT2R increases nitric oxide. AT4R is essential for dopamine and acetylcholine release. It is unknown whether ARB use is associated with changes in the brain RAS. Here, we compared the impact of treatment with ARB on not cognitively impaired individuals and individuals with Alzheimer's dementia using postmortem frontal-cortex samples of age- and sex-matched participants (70-90 years old, n = 30 in each group). We show that ARB use is associated with higher brain AT4R, lower oxidative stress, and amyloid-β burden in NCI participants. In AD, ARB use was associated with lower brain AT1R but had no impact on inflammation, oxidative stress, or amyloid-β burden. Our results may suggest a potential role for AT4R in the salutary effects for ARB on the brains of not cognitively impaired older adults.

Serum Concentrations of Losartan Metabolites Correlate With Improved Physical Function in a Pilot Study of Prefrail Older Adults

Losartan is an oral antihypertensive agent that is rapidly metabolized to EXP3174 (angiotensin-subtype-1-receptor blocker) and EXP3179 (peroxisome proliferator-activated receptor gamma [PPARγ] agonist), which was shown in animal studies to reduce inflammation, enhance mitochondrial energetics, and improve muscle repair and physical performance. We conducted an exploratory pilot study evaluating losartan treatment in prefrail older adults (age 70-90 years, N = 25). Participants were randomized to control (placebo) or treatment (daily oral losartan beginning at 25 mg per day and increasing every 8 weeks) for a total of 6 months. Fatigue, hyperkalemia, and hypotension were the most observed side effects of losartan treatment. Participants in the losartan group had an estimated 89% lower odds of frailty (95% confidence interval [CI]: 18% to 99% lower odds, p = .03), with a 0.3-point lower frailty score than the placebo group (95% CI: 0.01-0.5 lower odds, p = .04). Frailty score was also negatively associated with serum losartan and EXP3179 concentrations. For every one standard deviation increase in EXP3179 (ie, 0.0011 ng/μL, based on sample values above detection limit) and EXP3174 (ie, 0.27 ng/μL, based on sample values above detection limit), there was a 0.0035 N (95% CI: 0.0019-0.0051, p < .001) and a 0.0027 N (95% CI: 0.00054-0.0043, p = .007) increase in average knee strength, respectively.

CGP42112: the full AT2 receptor agonist and its role in the renin-angiotensin-aldosterone system: no longer misunderstood

For years, the AT2R-selective ligand CGP42112 has been erroneously characterized as a partial agonist, partly due to its ability to also interact with the AT1R at high concentrations. As late as 2009, it was still being characterized as an antagonist as well. In this perspective/opinion piece, we try to resolve the ambiguity that surrounds the efficacy of this compound by extensively reviewing the literature, tracing its beginnings to 1989, showing that CGP42112 has never been convincingly shown to be a partial agonist or an antagonist at the AT2R. While CGP42112 is now routinely characterized as an AT2R agonist, regrettably, there is a paucity of studies that can validate its efficacy as a full agonist at the AT2R, leaving the door open for continuing speculation regarding the extent of its efficacy. Hopefully, the information presented in this perspective/opinion piece will firmly establish CGP42112 as a full agonist at the AT2R such that it can once again be used as a tool to study the AT2R.

View of the Renin-Angiotensin System in Acute Kidney Injury Induced by Renal Ischemia-Reperfusion Injury

Renal ischemia-reperfusion injury (RIRI) is a sequence of complicated events that is defined as a reduction of the blood supply followed by reperfusion. RIRI is the leading cause of acute kidney injury (AKI). Among the diverse mediators that take part in RIRI-induced AKI, the renin-angiotensin system (RAS) plays an important role via conventional (angiotensinogen, renin, angiotensin-converting enzyme (ACE), angiotensin (Ang) II, and Ang II type 1 receptor (AT1R)) and nonconventional (ACE2, Ang 1-7, Ang 1-9, AT2 receptor (AT2R), and Mas receptor (MasR)) axes. RIRI alters the balance of both axes so that RAS can affect RIRI-induced AKI. In overall, the alteration of Ang II/AT1R and AKI by RIRI is important to consider. This review has looked for the effects and interactions of RAS activities during RIRI conditions.

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.

Angiotensin-(1-9) in hypertension

Angiotensin-(1-9) [Ang-(1-9)] is a peptide of the non-canonical renin-angiotensin system (RAS) synthesized from angiotensin I by the monopeptidase angiotensin-converting enzyme type 2 (ACE2). Using osmotic minipumps, infusion of Ang-(1-9) consistently reduces blood pressure in several rat hypertension models. In these animals, hypertension-induced end-organ damage is also decreased. Several pieces of evidence suggest that Ang-(1-9) is the endogenous ligand that binds and activates the type-2 angiotensin II receptor (AT2R). Activation of AT2R triggers different tissue-specific signaling pathways. This phenomenon could be explained by the ability of AT2R to form different heterodimers with other G protein-coupled receptors. Because of the antihypertensive and protective effects of AT2R activation by Ang-(1-9), associated with a short half-life of RAS peptides, several synthetic AT2R agonists have been synthesized and assayed. Some of them, particularly CGP42112, C21 and novokinin, have demonstrated antihypertensive properties. Only two synthetic AT2R agonists, C21 and LP2-3, have been tested in clinical trials, but none of them like an antihypertensive. Therefore, Ang-(1-9) is a promising antihypertensive drug that reduces hypertension-induced end-organ damage. However, further research is required to translate this finding successfully to the clinic.

Aging modifies receptor expression but not muscular contractile response to angiotensin II in rat jejunum

The involvement of renin-angiotensin system in the modulation of gut motility and age-related changes in mRNA expression of angiotensin (Ang II) receptors (ATR) are well accepted. We aimed to characterize, in vitro, the contractile responses induced by Ang II, in jejunum from young (3–6 weeks old) and old rats (≥ 1 year old), to evaluate possible functional differences associated to changes in receptor expression. Mechanical responses to Ang II were examined in vitro as changes in isometric tension. ATR expression was assessed by qRT-PCR. Ang II induced a contractile effect, antagonized by losartan, AT1R antagonist, and increased by PD123319, AT2R antagonist, as well by neural blocker ω-conotoxin and by nitric oxide (NO) synthase inhibitor. No difference in the response was observed between young and old groups. AT1 receptor-mediated contractile response was decreased by U-73122, phospholipase C (PLC) inhibitor; or 2-aminoethoxy-diphenylborate (2-APB), inositol triphosphate (IP₃) receptor inhibitor; or nifedipine, l-type calcium channel blocker. Age-related changes in the expression of both AT1 receptor subtypes, AT1a and AT1b, and of AT2 receptors were detected. In conclusion, Ang II modulates the spontaneous contractility of rat jejunum via postjunctional AT1 receptors, involving Ca²⁺ mobilization from intracellular stores, via PLC/IP₃ pathway, and Ca²⁺ influx from extracellular space, via l-type channels. Prejunctional AT2 receptors would counteract AT1 receptor effects, via NO synthesis. The observed age-related differences in the expression of all AT receptor subtypes are not reflected in the muscular contractile response to Ang II.

Novel Pharmacology Following Heteromerization of the Angiotensin II Type 2 Receptor and the Bradykinin Type 2 Receptor

The angiotensin type 2 (AT₂) receptor and the bradykinin type 2 (B₂) receptor are G protein-coupled receptors (GPCRs) that have major roles in the cardiovascular system. The two receptors are known to functionally interact at various levels, and there is some evidence that the observed crosstalk may occur as a result of heteromerization. We investigated evidence for heteromerization of the AT₂ receptor and the B₂ receptor in HEK293FT cells using various bioluminescence resonance energy transfer (BRET)-proximity based assays, including the Receptor Heteromer Investigation Technology (Receptor-HIT) and the NanoBRET ligand-binding assay. The Receptor-HIT assay showed that Gα_q, GRK2 and β-arrestin2 recruitment proximal to AT₂ receptors only occurred upon B₂ receptor coexpression and activation, all of which is indicative of AT₂-B₂ receptor heteromerization. Additionally, we also observed specific coupling of the B₂ receptor with the Gα_z protein, and this was found only in cells coexpressing both receptors and stimulated with bradykinin. The recruitment of Gα_z, Gα_q, GRK2 and β-arrestin2 was inhibited by B₂ receptor but not AT₂ receptor antagonism, indicating the importance of B₂ receptor activation within AT₂-B₂ heteromers. The close proximity between the AT₂ receptor and B₂ receptor at the cell surface was also demonstrated with the NanoBRET ligand-binding assay. Together, our data demonstrate functional interaction between the AT₂ receptor and B₂ receptor in HEK293FT cells, resulting in novel pharmacology for both receptors with regard to Gα_q/GRK2/β-arrestin2 recruitment (AT₂ receptor) and Gα_z protein coupling (B₂ receptor). Our study has revealed a new mechanism for the enigmatic and poorly characterized AT₂ receptor to be functionally active within cells, further illustrating the role of heteromerization in the diversity of GPCR pharmacology and signaling.

Angiotensin converting enzyme inhibitors and angiotensin II receptor blockers and outcomes in patients with acute decompensated heart failure: a systematic review and meta-analysis

BACKGROUND

Angiotensin-converting enzyme inhibitor (ACEi) and angiotensin-receptor blocker (ARB) are cornerstones in the treatment of heart failure with reduced ejection (HFrEF). However, there are limited data on their risk-benefit profile in patients with acute heart failure requiring hospitalizations.

METHODS

We did a meta-analysis pooling data from all studies examining the use of ACEi/ARB in patients hospitalized for heart failure compared to patients without ACEi/ARB use. We calculated pooled hazard ratios (HR) and their 95% confidence intervals (CI) using a random-effects model.

RESULTS

Twenty-five studies were included in the meta-analysis. Continued use of ACEi/ARBs in hospitalized patients with HFrEF was associated with lower 1-year mortality risk (pooled HR 0.68 [0.60-0.77] p < 0.001) and with lower 1-6-year mortality risk in those with heart failure preserved ejection fraction (HFpEF) (pooled HR 0.86 [0.78-0.94] p = 0.002). There were significant reductions in 1-year HF readmissions among hospitalized HFrEF patients (pooled HR 0.83 [0.73-0.95] p = 0.005).

CONCLUSION

Maintaining or initiating patients with HFrEF hospitalized for acute decompensated heart failure (ADHF) on ACEi/ARB is associated with a reduce risk of mortality and 1-year admissions, but the effect size is lower among those with HFpEF with more heterogeneous outcomes.

Crosstalk between the renin-angiotensin, complement and kallikrein-kinin systems in inflammation

During severe inflammatory and infectious diseases, various mediators modulate the equilibrium of vascular tone, inflammation, coagulation and thrombosis. This Review describes the interactive roles of the renin–angiotensin system, the complement system, and the closely linked kallikrein–kinin and contact systems in cell biological functions such as vascular tone and leakage, inflammation, chemotaxis, thrombosis and cell proliferation. Specific attention is given to the role of these systems in systemic inflammation in the vasculature and tissues during hereditary angioedema, cardiovascular and renal glomerular disease, vasculitides and COVID-19. Moreover, we discuss the therapeutic implications of these complex interactions, given that modulation of one system may affect the other systems, with beneficial or deleterious consequences.

The renin–angiotensin, complement and kallikrein–kinin systems comprise a multitude of mediators that modulate physiological responses during inflammatory and infectious diseases. This Review investigates the complex interactions between these systems and how these are dysregulated in various conditions, including cardiovascular diseases and COVID-19, as well as their therapeutic implications.

C21 preserves endothelial function in the thoracic aorta from DIO mice: role for AT2, Mas and B2 receptors

Compound 21 (C21), a selective agonist of angiotensin II type 2 receptor (AT2R), induces vasodilation through NO release. Since AT2R seems to be overexpressed in obesity, we hypothesize that C21 prevents the development of obesity-related vascular alterations. The main goal of the present study was to assess the effect of C21 on thoracic aorta endothelial function in a model of diet-induced obesity (DIO) and to elucidate the potential cross-talk among AT2R, Mas receptor (MasR) and/or bradykinin type 2 receptor (B2R) in this response. Five-week-old male C57BL6J mice were fed a standard (CHOW) or a high-fat diet (HF) for 6 weeks and treated daily with C21 (1 mg/kg p.o) or vehicle, generating four groups: CHOW-C, CHOW-C21, HF-C, HF-C21. Vascular reactivity experiments were performed in thoracic aorta rings. Human endothelial cells (HECs; EA.hy926) were used to elucidate the signaling pathways, both at receptor and intracellular levels. Arteries from HF mice exhibited increased contractions to Ang II than CHOW mice, effect that was prevented by C21. PD123177, A779 and HOE-140 (AT2R, Mas and B2R antagonists) significantly enhanced Ang II-induced contractions in CHOW but not in HF-C rings, suggesting a lack of functionality of those receptors in obesity. C21 prevented those alterations and favored the formation of AT2R/MasR and MasR/B2R heterodimers. HF mice also exhibited impaired relaxations to acetylcholine (ACh) due to a reduced NO availability. C21 preserved NO release through PKA/p-eNOS and AKT/p-eNOS signaling pathways. In conclusion, C21 favors the interaction among AT2R, MasR and B2R and prevents the development of obesity-induced endothelial dysfunction by stimulating NO release through PKA/p-eNOS and AKT/p-eNOS signaling pathways.

Role of Kinins in Hypertension and Heart Failure

The kallikrein-kinin system (KKS) is proposed to act as a counter regulatory system against the vasopressor hormonal systems such as the renin-angiotensin system (RAS), aldosterone, and catecholamines. Evidence exists that supports the idea that the KKS is not only critical to blood pressure but may also oppose target organ damage. Kinins are generated from kininogens by tissue and plasma kallikreins. The putative role of kinins in the pathogenesis of hypertension is discussed based on human mutation cases on the KKS or rats with spontaneous mutation in the kininogen gene sequence and mouse models in which the gene expressing only one of the components of the KKS has been deleted or over-expressed. Some of the effects of kinins are mediated via activation of the B2 and/or B1 receptor and downstream signaling such as eicosanoids, nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF) and/or tissue plasminogen activator (T-PA). The role of kinins in blood pressure regulation at normal or under hypertension conditions remains debatable due to contradictory reports from various laboratories. Nevertheless, published reports are consistent on the protective and mediating roles of kinins against ischemia and cardiac preconditioning; reports also demonstrate the roles of kinins in the cardiovascular protective effects of the angiotensin-converting enzyme (ACE) and angiotensin type 1 receptor blockers (ARBs).

Brain Renin-Angiotensin System at the Intersect of Physical and Cognitive Frailty

The renin–angiotensin system (RAS) was initially considered to be part of the endocrine system regulating water and electrolyte balance, systemic vascular resistance, blood pressure, and cardiovascular homeostasis. It was later discovered that intracrine and local forms of RAS exist in the brain apart from the endocrine RAS. This brain-specific RAS plays essential roles in brain homeostasis by acting mainly through four angiotensin receptor subtypes; AT₁R, AT₂R, MasR, and AT₄R. These receptors have opposing effects; AT₁R promotes vasoconstriction, proliferation, inflammation, and oxidative stress while AT₂R and MasR counteract the effects of AT₁R. AT₄R is critical for dopamine and acetylcholine release and mediates learning and memory consolidation. Consequently, aging-associated dysregulation of the angiotensin receptor subtypes may lead to adverse clinical outcomes such as Alzheimer’s disease and frailty via excessive oxidative stress, neuroinflammation, endothelial dysfunction, microglial polarization, and alterations in neurotransmitter secretion. In this article, we review the brain RAS from this standpoint. After discussing the functions of individual brain RAS components and their intracellular and intracranial locations, we focus on the relationships among brain RAS, aging, frailty, and specific neurodegenerative diseases, such as Alzheimer’s disease, Parkinson’s disease, and vascular cognitive impairment, through oxidative stress, neuroinflammation, and vascular dysfunction. Finally, we discuss the effects of RAS-modulating drugs on the brain RAS and their use in novel treatment approaches.

Recent Research Advances in Renin-Angiotensin-Aldosterone System Receptors

PURPOSE OF REVIEW

The renin-angiotensin-aldosterone system (RAAS) plays important roles in regulating blood pressure and body fluid, which contributes to the pathophysiology of hypertension and cardiovascular/renal diseases. However, accumulating evidence has further revealed the complexity of this signal transduction system, including direct interactions with other receptors and proteins. This review focuses on recent research advances in RAAS with an emphasis on its receptors.

RECENT FINDINGS

Both systemically and locally produced angiotensin II (Ang II) bind to Ang II type 1 receptor (AT1R) and elicit strong biological functions. Recent studies have shown that Ang II-induced activation of Ang II type 2 receptor (AT2R) elicits the opposite functions to those of AT1R. However, accumulating evidence has now expanded the components of RAAS, including (pro)renin receptor, angiotensin-converting enzyme 2, angiotensin 1-7, and Mas receptor. In addition, the signal transductions of AT1R and AT2R are regulated by not only Ang II but also its receptor-associated proteins such as AT1R-associated protein and AT2R-interacting protein. Recent studies have indicated that inappropriate activation of local mineralocorticoid receptor contributes to cardiovascular and renal tissue injuries through aldosterone-dependent and -independent mechanisms. Since the mechanisms of RAAS signal transduction still remain to be elucidated, further investigations are necessary to explore novel molecular mechanisms of the RAAS, which will provide alternative therapeutic agents other than existing RAAS blockers.

Estrogen Receptor-β Mediates Estradiol-Induced Pregnancy-Specific Uterine Artery Endothelial Cell Angiotensin Type-2 Receptor Expression

The pregnancy-augmented uterine vasodilation is linked to increased AT2R (angiotensin type-2 receptor) that mediates the vasodilatory effects of angiotensin II. However, the mechanisms controlling AT2R expression during pregnancy remain unclear. Estrogens are known to play a role in vascular adaptations during pregnancy. We hypothesized that estrogen stimulates uterine artery AT2R expression via ER (estrogen receptor)-β-dependent transcription in a pregnancy-specific endothelium-dependent manner. Plasma estradiol levels increased and peaked in late pregnancy and returned to prepregnant levels post-partum, correlating with uterine artery AT2R and ERβ upregulation. Estradiol stimulated AT2R mRNA expression in endothelium-intact but not endothelium-denuded late pregnant and nonpregnant rat uterine artery ex vivo. Consistently, estradiol stimulated AT2R mRNA expression in late pregnant but not nonpregnant primary human uterine artery endothelial cells in vitro, which was abolished by ER antagonist ICI 182,780. Higher ERα protein bound to ER-responsive elements in AT2R promoter in the nonpregnant arteries whereas higher ERβ bound in the pregnant state. ERα protein levels were similar but higher ERβ protein levels were expressed in pregnant versus nonpregnant human uterine artery endothelial cells. Estradiol stimulation recruited ERα to the AT2R promoter in the nonpregnant state and ERβ to the AT2R promoter in pregnancy; however, only ERβ recruitment mediated transactivation of the AT2R reporter gene in pregnant human uterine artery endothelial cells. Estradiol-induced AT2R expression was abolished by the specific ERβ (not ERα) antagonist 4-[2-Phenyl-5,7-bis(trifluoromethyl)pyrazolo[1,5-a]pyrimidin-3-yl]phenol (PHTPP) and mimicked by the specific ERβ (not ERα) agonist 2,3-bis(4-Hydroxyphenyl)-propionitrile (DPN) in pregnant human uterine artery endothelial cells in vitro. This study demonstrates a novel role of pregnancy-augmented ERβ in AT2R upregulation in the uterine artery and provides new insights into the mechanisms underlying uterine vascular adaptation to pregnancy.

Pregnancy upregulates angiotensin type 2 receptor expression and increases blood flow in uterine arteries of rats

Normal pregnancy is associated with decreased uterine vascular contraction and increased blood flow even though angiotensin II (AngII) levels are increased. AngII not only activates the angiotensin type 1 receptor (AT1R) to mediate vasoconstriction but also angiotensin type 2 receptor (AT2R) to cause vasodilation. We hypothesized that upregulation of AT2R expression and function accounts for increased uterine artery blood flow during pregnancy. Virgin, pregnant (at different days of gestation) and post-partum Sprague-Dawley rats were used to determine uterine artery hemodynamics using micro ultrasound and plasma angiotensin II levels by ELISA. Isolated uterine arteries were examined for AT1R and AT2R expression and isometric contraction/relaxation. Plasma AngII levels were steady up to mid-pregnancy, increased as pregnancy advanced, reaching a peak in late pregnancy, and then restored to pre-pregnant levels after delivery. The pattern of increase in AngII levels mirrored a parallel increase in uterine blood flow. AT1R expression did not change, but AT2R expression increased during pregnancy correlating with uterine blood flow increase. Treatment with the AT2R antagonist PD123319 reduced uterine arterial blood flow. Vasoconstriction to angiotensin II was blunted in pregnant rats. Treatment with PD123319 caused greater enhancement of AngII contraction in pregnant than virgin rats. Ex vivo exposure of estradiol to uterine arterial rings dose dependently upregulated AT2R expression, that was inhibited by estrogen receptor antagonist. These results demonstrate that elevated AngII levels during gestation induce an increase in uterine blood flow via heightened AT2R-mediated signaling. Estrogens appear to directly upregulate uterine vascular AT2R independent of any endogenous factors.

Antifibrotic Roles of RAAS Blockers: Update

The rennin-angiotensin-aldosterone system (RAAS) has been well documented in regulating blood pressure, fluid volume, and sodium balance. Overactivity of RAAS promotes both systemic and regional glomerular capillary hypertension, which could induce hemodynamic injury to the glomerulus, leading to kidney damage and renal fibrosis via profibrotic and proinflammatory pathway. Therefore, the use of RAAS inhibitors (i.e., ACEIs, ARBs, and MRAs) as the optional therapy has been demonstrated to prevent proteinuria, and kidney fibrosis and slow the decline of renal function effectively in the process of kidney disease during the last few decades. Recently, several new components of the RAAS have been discovered, including ACE2 and the corresponding ACE2/Ang (1-7)/Mas axis, which are also present in the kidney. Besides the classic RAAS inhibitors target the angiotensin-AT1-aldosterone axis, with the expanding knowledge about RAAS, a number of potential therapeutic targets in this system is emerging. Newer agents that are more specific are being developed. The present chapter outlines the insights of the RAAS agents (classic RAAS antagonists/the new RAAS drugs), and discusses its clinical application in the combat of renal fibrosis.

Apocynin alters redox signaling in conductance and resistance vessels of spontaneously hypertensive rats

Chronic treatment with apocynin reduces blood pressure and prevents endothelial dysfunction development in spontaneously hypertensive rats (SHR). Mechanisms underlying apocynin effects on SHR remain unclear. Compared to diapocynin and other drugs, apocynin is a weak antioxidant, which suggests that its effects on SHR are associated with other mechanisms besides its antioxidant capacity. Angiotensin (Ang) II regulates NOX, the major reactive oxygen species (ROS) source in the cardiovascular system. We hypothesized that, by inhibiting NOX, apocynin could alter Ang II pressor and vasoconstrictor effects on SHR. We analyzed how Ang II affects blood pressure and vascular reactivity in aorta and mesenteric resistance arteries and evaluated plasma antioxidant capacity, NOX isoforms and subunits, NOS isoforms, AT₁ and AT₂ receptors expression, ROS production, and NOS activity in apocynin-treated SHR blood vessels (30 mg/Kg/day, p.o.). In SHR, apocynin reduced Ang II pressor effects, increased plasmatic antioxidant capacity, and blunted aortic and mesenteric NOX-dependent oxidants production and NOX2 and p47phox overexpression, which demonstrated that apocynin inhibits NOX in SHR blood vessels. Moreover, apocynin raised plasmatic and aortic nitrate/nitrite levels, maintained NOS activity and eNOS, p-eNOS, nNOS, iNOS, sGC-α, and sGC-β expression in mesenteric bed, diminished AT₁ expression in aorta and mesenteric bed, and elevated AT₂ expression in SHR aorta. Apocynin increased Ang II vasoconstriction endothelial modulation in SHR resistance arteries. All these results showed that in vivo treatment with apocynin alters several mechanisms that reduce Ang II pressor and vasoconstrictor effects on SHR. Such apocynin effects involve other mechanisms besides vascular ROS modulation, which improves NO availability in SHR vascular cells. These integrated data could help us to understand the promising apocynin activity as an antihypertensive drug that acts differently from the drugs that are currently being used in the clinical setting.

Neprilysin Inhibitors and Bradykinin

Bradykinin has important physiological actions related to the regulation of blood vessel tone and renal function, and protection from ischemia reperfusion injury. However, bradykinin also contributes to pathological states such as angioedema and inflammation. Bradykinin is metabolized by many different peptidases that play a major role in the control of bradykinin levels. Peptidase inhibitor therapies such as angiotensin converting enzyme (ACE) and neprilysin inhibitors increase bradykinin levels, and the challenge for such therapies is to achieve the beneficial cardiovascular and renal effects without the adverse consequences such as angioedema that may result from increased bradykinin levels. Neprilysin also metabolizes natriuretic peptides. However, despite the potential therapeutic benefit of increased natriuretic peptide and bradykinin levels, neprilysin inhibitor therapy has only modest efficacy in essential hypertension and heart failure. Initial attempts to combine neprilysin inhibition with inhibition of the renin angiotensin system led to the development of omapatrilat, a drug that combines ACE and neprilysin inhibition. However, omapatrilat produced an unacceptably high incidence of angioedema in patients with hypertension (2.17%) in comparison with the ACE inhibitor enalapril (0.68%), although angioedema incidence was less in patients with heart failure with reduced ejection fraction (HFrEF) treated with omapatrilat (0.8%), and not different from that for enalapril therapy (0.5%). More recently, LCZ696, a drug that combines angiotensin receptor blockade and neprilysin inhibition, was approved for the treatment of HFrEF. The approval of LCZ696 therapy for HFrEF represents the first approval of long-term neprilysin inhibitor administration. While angioedema incidence was acceptably low in HFrEF patients receiving LCZ696 therapy (0.45%), it remains to be seen whether LCZ696 therapy for other conditions such as hypertension is also accompanied by an acceptable incidence of angioedema.

Angiotensin-(1-9) reduces cardiovascular and renal inflammation in experimental renin-independent hypertension

Hypertension-induced cardiovascular and renal damage can be mediated by activation of the renin-angiotensin-aldosterone system. There are different factors beyond renin-angiotensin-aldosterone system involved in hypertension and renal damage. Inflammation has emerged as an important mediator of hypertension and cardiovascular and kidney damage. Angiotensin-(1-9), a peptide of the renin-angiotensin system, counter-regulates both the physiological and pathological actions of angiotensin II. Recent data has shown that angiotensin-(1-9) protects the heart and blood vessels from adverse cardiovascular remodeling in experimental models of hypertension and/or heart failure and reduces cardiac fibrosis in stroke-prone, spontaneously hypertensive rats. These effects are mediated by the angiotensin II type 2 receptor (AT2R). However, it remains unknown whether angiotensin-(1-9) also has an anti-inflammatory effect. In the present study, we investigate whether angiotensin-(1-9) reduces inflammation and fibrosis in the heart, arteries, and kidney in a DOCA-salt hypertensive model and explore the mechanisms underlying the amelioration of end-organ damage. DOCA-salt hypertensive rats received: a) vehicle, b) angiotensin-(1-9), c) PD123319 (AT2R blocker), d) angiotensin-(1-9) plus A779 (a Mas receptor blocker) or e) angiotensin-(1-9) plus PD123319, and sham rats were used as a control. Our results showed that angiotensin-(1-9) decreased hypertension and increased vasodilation in DOCA-salt hypertensive rats. These actions were partially inhibited by PD123319. Moreover, angiotensin-(1-9) decreased diuresis, fibrosis, and inflammation. These beneficial effects were not mediated by Mas or AT2R blockers. We concluded that angiotensin-(1-9) protects against volume overload-induced hypertensive cardiovascular and kidney damage by decreasing inflammation in the heart, aortic wall, and kidney, through mechanisms independent of the Mas or AT2R.

Update on angiotensin AT2 receptors

PURPOSE OF REVIEW

This review updates major new findings and concepts introduced during the past year on the role of angiotensin II (Ang II) subtype 2 receptors (AT2Rs) in the control of blood pressure and renal function.

RECENT FINDINGS

AT2R activation prevents sodium (Na) retention and lowers blood pressure in the Ang II infusion model of experimental hypertension and prevents salt-sensitive hypertension in the obese Zucker rat model of obesity and the metabolic syndrome. Ang II metabolite, des-aspartyl-Ang II (Ang III) is the predominant AT2R agonist in the kidney and possibly also in the vasculature; a novel synthetic Ang III peptide, β-Pro-Ang III, is vasodepressor and lowers blood pressure in conscious spontaneously hypertensive rats in the presence of low-level Ang II type 1 receptor (AT1R) blockade. Because nitric oxide is a product of AT2R activation, a potential feed-forward loop, wherein nitric oxide increases AT2R transcription, may reinforce the beneficial actions of AT2R in the long term. AT2R activation also reduces proteinuria and oxidative stress in glomerulosclerotic kidneys of high-salt obese Zucker rats.

SUMMARY

Studies during the past year have helped to clarify the physiological and pathophysiological roles of AT2Rs and have enhanced the promise of AT2R agonists in cardiovascular and renal disease.

Angiotensin II type 2 receptor (AT2R) in renal and cardiovascular disease

Angiotensin II (Ang II) is well-considered to be the principal effector of the renin-angiotensin system (RAS), which binds with strong affinity to the angiotensin II type 1 (AT1R) and type 2 (AT2R) receptor subtype. However, activation of both receptors is likely to stimulate different signalling mechanisms/pathways and produce distinct biological responses. The haemodynamic and non-haemodynamic effects of Ang II, including its ability to regulate blood pressure, maintain water-electrolyte balance and promote vasoconstriction and cellular growth are well-documented to be mediated primarily by the AT1R. However, its biological and functional effects mediated through the AT2R subtype are still poorly understood. Recent studies have emphasized that activation of the AT2R regulates tissue and organ development and provides in certain context a potential counter-regulatory mechanism against AT1R-mediated actions. Thus, this review will focus on providing insights into the biological role of the AT2R, in particular its actions within the renal and cardiovascular system.

Vascular Kinin B1 and B2 Receptors Determine Endothelial Dysfunction through Neuronal Nitric Oxide Synthase

B₁- and B₂-kinin receptors are G protein-coupled receptors that play an important role in the vascular function. Therefore, the present study was designed to evaluate the participation of kinin receptors in the acetylcholine (ACh)-induced vascular relaxation, focusing on the protein-protein interaction involving kinin receptors with endothelial and neuronal nitric oxide synthases (eNOS and nNOS). Vascular reactivity, nitric oxide (NO·) and reactive oxygen species (ROS) generation, co-immunoprecipitation were assessed in thoracic aorta from male wild-type (WT), B₁- (B₁R^−/−), B₂- (B₂R^−/−) knockout mice. Some vascular reactivity experiments were also performed in a double kinin receptors knockout mice (B₁B₂R^−/−). For pharmacological studies, selective B₁- and B₂-kinin receptors antagonists, NOS inhibitors and superoxide dismutase (SOD) mimetic were used. First, we show that B₁- and B₂-kinin receptors form heteromers with nNOS and eNOS in thoracic aorta. To investigate the functionality of these protein-protein interactions, we took advantage of pharmacological tools and knockout mice. Importantly, our results show that kinin receptors regulate ACh-induced relaxation via nNOS signaling in thoracic aorta with no changes in NO· donor-induced relaxation. Interestingly, B₁B₂R^−/− presented similar level of vascular dysfunction as found in B₁R^−/− or B₂R^−/− mice. In accordance, aortic rings from B₁R^−/− or B₂R^−/− mice exhibit decreased NO· bioavailability and increased superoxide generation compared to WT mice, suggesting the involvement of excessive ROS generation in the endothelial dysfunction of B₁R^−/− and B₂R^−/− mice. Alongside, we show that impaired endothelial vasorelaxation induced by ACh in B₁R^−/− or B₂R^−/− mice was rescued by the SOD mimetic compound. Taken together, our findings show that B₁- and B₂-kinin receptors regulate the endothelium-dependent vasodilation of ACh through nNOS activity and indicate that molecular disturbance of short-range interaction between B₁- and B₂-kinin receptors with nNOS might be involved in the oxidative pathogenesis of endothelial dysfunction.

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.

Gender Difference in Renal Blood Flow Response to Angiotensin II Administration after Ischemia/Reperfusion in Rats: The Role of AT2 Receptor

Background. Renal ischemia/reperfusion (I/R) is one of the major causes of kidney failure, and it may interact with renin angiotensin system while angiotensin II (Ang II) type 2 receptor (AT2R) expression is gender dependent. We examined the role of AT2R blockade on vascular response to Ang II after I/R in rats. Methods. Male and female rats were subjected to 30 min renal ischemia followed by reperfusion. Two groups of rats received either vehicle or AT2R antagonist, PD123319. Mean arterial pressure (MAP), and renal blood flow (RBF) responses were assessed during graded Ang II (100, 300, and 1000 ng/kg/min, i.v.) infusion at controlled renal perfusion pressure (RPP). Results. Vehicle or antagonist did not alter MAP, RPP, and RBF levels significantly; however, 30 min after reperfusion, RBF decreased insignificantly in female treated with PD123319 (P = 0.07). Ang II reduced RBF and increased renal vascular resistance (RVR) in a dose-related fashion (P dose < 0.0001), and PD123319 intensified the reduction of RBF response in female (P group < 0.005), but not in male rats. Conclusion. The impact of the AT2R on vascular responses to Ang II in renal I/R injury appears to be sexually dimorphic. PD123319 infusion promotes these hemodynamic responses in female more than in male rats.

AT2 receptor activities and pathophysiological implications

Although angiotensin II subtype-2 receptor (AT2R) was discovered over 2 decades ago, its contribution to physiology and pathophysiology is not fully elucidated. Current knowledge suggests that under normal physiologic conditions, AT2R counterbalances the effects of angiotensin II subtype-1 receptor (AT1R). A major obstacle for AT2R investigations was the lack of specific agonists. Most of the earlier AT2R studies were performed using the peptidic agonist, CG42112A, or the nonpeptidic antagonist PD123319. CGP42112A is nonspecific for AT2R and in higher concentrations can bind to AT1R. Recently, the development of specific nonpeptidic AT2R agonists boosted the efforts in identifying the therapeutic potentials for AT2R stimulation. Unlike AT1R, AT2R is involved in vasodilation by the release of bradykinin and nitric oxide, anti-inflammation, and healing from injury. Interestingly, the vasodilatory effects of AT2R stimulation were not associated with significant reduction in blood pressure. In the kidney, AT2R stimulation produced natriuresis, increased renal blood flow, and reduced tissue inflammation. In animal studies, enhanced AT2R function led to reduction of cardiac inflammation and fibrosis, and reduced the size of the infarcted area. Similarly, AT2R stimulation demonstrated protective effects in vasculature and brain.

ACE2 and vasoactive peptides: novel players in cardiovascular/renal remodeling and hypertension

The renin-angiotensin system (RAS) is a key component of cardiovascular physiology and homeostasis due to its influence on the regulation of electrolyte balance, blood pressure, vascular tone and cardiovascular remodeling. Deregulation of this system contributes significantly to the pathophysiology of cardiovascular and renal diseases. Numerous studies have generated new perspectives about a noncanonical and protective RAS pathway that counteracts the proliferative and hypertensive effects of the classical angiotensin-converting enzyme (ACE)/angiotensin (Ang) II/angiotensin type 1 receptor (AT1R) axis. The key components of this pathway are ACE2 and its products, Ang-(1-7) and Ang-(1-9). These two vasoactive peptides act through the Mas receptor (MasR) and AT2R, respectively. The ACE2/Ang-(1-7)/MasR and ACE2/Ang-(1-9)/AT2R axes have opposite effects to those of the ACE/Ang II/AT1R axis, such as decreased proliferation and cardiovascular remodeling, increased production of nitric oxide and vasodilation. A novel peptide from the noncanonical pathway, alamandine, was recently identified in rats, mice and humans. This heptapeptide is generated by catalytic action of ACE2 on Ang A or through a decarboxylation reaction on Ang-(1-7). Alamandine produces the same effects as Ang-(1-7), such as vasodilation and prevention of fibrosis, by interacting with Mas-related GPCR, member D (MrgD). In this article, we review the key roles of ACE2 and the vasoactive peptides Ang-(1-7), Ang-(1-9) and alamandine as counter-regulators of the ACE-Ang II axis as well as the biological properties that allow them to regulate blood pressure and cardiovascular and renal remodeling.

Venthamarai chooranam, a polyherbal Siddha medicine, alleviates hypertension via AT₁R and eNOS signaling pathway in 2K1C hypertensive rats

The present study was aimed to scientifically demonstrate the anti-hypertensive action of Venthamarai chooranam (VMC) in renal hypertensive rats. Two Kidney One Clip (2K1C) Goldblatt model was adopted to induce hypertension in rats. Male Sprague Dawley rats (270-320 g) were randomized into sham (n = 6), vehicle-treated 2K1C (n = 9) and VMC-treated 2K1C (400 mg/kg, p.o; n = 8) and monitored for nine weeks. Systolic blood pressure (SBP), plasma nitrate/nitrite, carotid endothelial nitric oxide synthetase (eNOS), renal angiotensin type 1 receptor (AT₁R), angiotensin type 2 receptor (AT₂R), TNFα, IL-6, thioredoxin 1 (TRX1), and thioredoxin reductase 1 (TRXR1) mRNA expressions were studied. VMC upregulated eNOS expression which in turn improved plasma nitric oxide and decreased SBP in hypertensive rats. It down-regulated AT₁R and simultaneously upregulated AT₂R expression in comparison to vehicle-treated 2K1C rats. Further, renal TNFα and IL-6 expressions were down-regulated while TRX1 and TRXR1 were upregulated by VMC. VMC potentially interacts with renin-angiotensin components and endothelial functions, and thereby exerts its antihypertensive action. This is the first study to demonstrate the mechanism of anti-hypertensive action of VMC in an animal model of renovascular hypertension.

Angiotensin AT2 receptor agonist prevents salt-sensitive hypertension in obese Zucker rats

High-sodium intake is a risk factor for the pathogenesis of hypertension, especially in obesity. The present study is designed to investigate whether angiotensin type 2 receptor (AT2R) activation with selective agonist C21 prevents high-sodium diet (HSD)-induced hypertension in obese animals. Male obese rats were treated with AT2R agonist C21 (1 mg·kg(-1)·day(-1), oral) while maintained on either normal-sodium diet (NSD; 0.4%) or HSD (4%) for 2 wk. Radiotelemetric recording showed a time-dependent increase in systolic blood pressure in HSD-fed obese rats, being maximal increase (∼27 mmHg) at day 12 of the HSD regimen. C21 treatment completely prevented the increase in blood pressure of HSD-fed rats. Compared with NSD controls, HSD-fed obese rats had greater natriuresis/diuresis and urinary levels of nitrates, and these parameters were further increased by C21 treatment. Also, C21 treatment improved glomerular filtration rate in HSD-fed rats. HSD-fed rats expressed higher level of cortical ANG II, which was reduced to 50% by C21 treatment. HSD feeding and/or C21 treatment had no effects on cortical renin activity and the expression of angiotensin-converting enzyme (ACE) and chymase, which are ANG II-producing enzymes. However, ANG(1-7) concentration and ACE2 activity in the renal cortex were reduced by HSD feeding, and C21 treatment rescued both the parameters. Also, C21 treatment reduced the cortical expression of AT1R in HSD-fed rats, but had no effect of AT2R expression. We conclude that chronic treatment with the AT2R agonist C21 prevents salt-sensitive hypertension in obese rats, and a reduction in the renal ANG II/AT1R and enhanced ACE2/ANG(1-7) levels may play a potential role in this phenomenon.

Testosterone induces apoptosis in cardiomyocytes by increasing proapoptotic signaling involving tumor necrosis factor-α and renin angiotensin system

Anabolic androgenic steroids lead to cardiac complications and have been shown to exhibit proapoptotic effects in cardiac cells; however, the mechanism involved in those effects is unclear. The aim of this study was to assess whether apoptosis and the activation of caspase-3 (Casp-3) induced by testosterone in high concentrations involves increments in tumor necrosis factor-α (TNF-α) concentrations and angiotensin-converting enzyme (ACE) activity in cardiomyocytes (H9c2) cell cultures. Cardiomyocytes were treated with testosterone (5 × 10−6 mol/L), doxorubicin (9.2 × 10−6 mol/L), testosterone + etanercept (Eta; 6.67 × 10−5 mol/L), testosterone + losartan (Los; 10−7 mol/L), and testosterone + AC-DEVD-CHO (10−5 mol/L; Casp-3 inhibitor). Apoptosis was determined by flow cytometry and by the proteolytic activity of Casp-3. We demonstrated that incubation of H9c2 cells for 48 h with testosterone causes the apoptotic death of 60–70% of the cells and co-treatments with Eta, Los, or AC-DEVD-CHO reduced this effect. Testosterone also induces apoptosis (concentration dependent) and increases the proteolytic activity of Casp-3, which were reduced by co-treatments. TNF-α and ACE activities were elevated by testosterone treatment, while co-treatment with Los and Eta reduced these effects. We concluded that an interaction between testosterone, angiotensin II, and TNF-α induced apoptosis and Casp-3 activity in cultured cardiomyocytes, which contributed to the reduced viability of these cells induced by testosterone in toxic concentrations.

Angiotensin-(1-9) reverses experimental hypertension and cardiovascular damage by inhibition of the angiotensin converting enzyme/Ang II axis

BACKGROUND

Little is known about the biological effects of angiotensin-(1-9), but available evidence shows that angiotensin-(1-9) has beneficial effects in preventing/ameliorating cardiovascular remodeling.

OBJECTIVE

In this study, we evaluated whether angiotensin-(1-9) decreases hypertension and reverses experimental cardiovascular damage in the rat.

METHODS AND RESULTS

Angiotensin-(1-9) (600  ng/kg per min for 2 weeks) reduced already-established hypertension in rats with early high blood pressure induced by angiotensin II infusion or renal artery clipping. Angiotensin-(1-9) also improved cardiac (assessed by echocardiography) and endothelial function in small-diameter mesenteric arteries, cardiac and aortic wall hypertrophy, fibrosis, oxidative stress, collagen and transforming growth factor type β - 1 protein expression (assessed by western blot). The beneficial effect of angiotensin-(1-9) was blunted by coadministration of the angiotensin type 2(AT2) receptor blocker PD123319 (36  ng/kg per min) but not by coadministration of the Mas receptor blocker A779 (100  ng/kg per min). Angiotensin-(1-9) treatment also decreased circulating levels of Ang II, angiotensin-converting enzyme activity and oxidative stress in aorta and left ventricle. Whereas, Ang-(1-9) increased endothelial nitric oxide synthase mRNA levels in aorta as well as plasma nitrate levels.

CONCLUSION

Angiotensin-(1-9) reduces hypertension, ameliorates structural alterations (hypertrophy and fibrosis), oxidative stress in the heart and aorta and improves cardiac and endothelial function in hypertensive rats. These effects were mediated by the AT2 receptor but not by the angiotensin-(1-7)/Mas receptor axis.

Recent insights and therapeutic perspectives of angiotensin-(1-9) in the cardiovascular system

Chronic RAS (renin-angiotensin system) activation by both AngII (angiotensin II) and aldosterone leads to hypertension and perpetuates a cascade of pro-hypertrophic, pro-inflammatory, pro-thrombotic and atherogenic effects associated with cardiovascular damage. In 2000, a new pathway consisting of ACE2 (angiotensin-converting enzyme2), Ang-(1-9) [angiotensin-(1-9)], Ang-(1-7) [angiotensin-(1-7)] and the Mas receptor was discovered. Activation of this novel pathway stimulates vasodilation, anti-hypertrophy and anti-hyperplasia. For some time, studies have focused mainly on ACE2, Ang-(1-7) and the Mas receptor, and their biological properties that counterbalance the ACE/AngII/AT1R (angiotensin type 1 receptor) axis. No previous information about Ang-(1-9) suggested that this peptide had biological properties. However, recent data suggest that Ang-(1-9) protects the heart and blood vessels (and possibly the kidney) from adverse cardiovascular remodelling in patients with hypertension and/or heart failure. These beneficial effects are not modified by the Mas receptor antagonist A779 [an Ang-(1-7) receptor blocker], but they are abolished by the AT2R (angiotensin type 2 receptor) antagonist PD123319. Current information suggests that the beneficial effects of Ang-(1-9) are mediated via the AT2R. In the present review, we summarize the biological effects of the novel vasoactive peptide Ang-(1-9), providing new evidence of its cardiovascular-protective activity. We also discuss the potential mechanism by which this peptide prevents and ameliorates the cardiovascular damage induced by RAS activation.

AT₂ receptor activation induces natriuresis and lowers blood pressure

RATIONALE

Compound 21 (C-21) is a highly selective nonpeptide AT2 receptor (AT2R) agonist.

OBJECTIVE

To test the hypothesis that renal proximal tubule AT2Rs induce natriuresis and lower blood pressure in Sprague-Dawley rats and mice.

METHODS AND RESULTS

In rats, AT2R activation with intravenous C-21 increased urinary sodium excretion by 10-fold (P<0.0001); this natriuresis was abolished by direct renal interstitial infusion of specific AT2R antagonist PD-123319. C-21 increased fractional excretion of Na(+) (P<0.05) and lithium (P<0.01) without altering renal hemodynamic function. AT2R activation increased renal proximal tubule cell apical membrane AT2R protein (P<0.001) without changing total AT2R expression and internalized/inactivated Na(+)-H(+) exchanger-3 and Na(+)/K(+)ATPase. C-21-induced natriuresis was accompanied by an increase in renal interstitial cGMP (P<0.01); C-21-induced increases in urinary sodium excretion and renal interstitial cGMP were abolished by renal interstitial nitric oxide synthase inhibitor l-N(6)-nitroarginine methyl ester or bradykinin B2 receptor antagonist icatibant. Renal AT2R activation with C-21 prevented Na(+) retention and lowered blood pressure in the angiotensin II infusion model of experimental hypertension.

CONCLUSIONS

AT2R activation initiates its translocation to the renal proximal tubule cell apical membrane and the internalization of Na(+)-H(+) exchanger-3 and Na(+)/K(+)ATPase, inducing natriuresis in a bradykinin-nitric oxide-cGMP-dependent manner. Intrarenal AT2R activation prevents Na(+) retention and lowers blood pressure in angiotensin II-dependent hypertension. AT2R activation holds promise as a renal proximal tubule natriuretic/diuretic target for the treatment of fluid-retaining states and hypertension.

Role of angiotensin AT(2) receptors in natriuresis: Intrarenal mechanisms and therapeutic potential

The renin-angiotensin system is a coordinated hormonal cascade critical for the regulation of blood pressure (BP) and kidney function. Angiotensin (Ang) II, the major angiotensin effector peptide, binds to two major receptors, namely AT1 and AT2 receptors. The AT1 receptors engender antinatriuresis and raise BP, whereas AT2 receptors oppose these effects, inducing natriuresis and reducing BP. There is high AT2 receptor expression in the adult kidney, especially in the proximal tubule. In AT2 receptor-null mice, long-term AngII infusion results in pressor and antinatriuretic hypersensivivity compared with responses in wild-type mice. The major endogenous receptor ligand for AT2 receptor-mediated natriuretic responses appears to be des-aspartyl(1) -AngII (AngIII) instead of AngII. Recent studies have demonstrated that AngII requires metabolism to AngIII by aminopeptidase A to induce natriuresis and that inhibition of aminopeptidase N increases intrarenal AngIII and augments AngIII-induced natriuresis. The renal dopaminergic system is another important natriuretic pathway. Renal proximal tubule the D1 and D5 receptor subtypes (D1 -like receptors (D1LIKE R)) control approximately 50% of basal sodium excretion. Recently, we have found that natriuresis induced by proximal tubule D1LIKE R requires AT2 receptor activation and that D1LIKE R stimulation induces recruitment of AT2 receptors to the apical plasma membrane via a cAMP-dependent mechanism. Initial studies using the potent AT2 receptor non-peptide agonist Compound 21 demonstrate natriuresis in both the presence and absence of AT1 receptor blockade, indicating the therapeutic potential of this compound in fluid-retaining states and hypertension.

Vasopressor meets vasodepressor: The AT1-B2 receptor heterodimer

The AT1 receptor for the vasopressor angiotensin II is one of the most important drug targets for the treatment of cardiovascular diseases. Sensitization of the AT1 receptor system is a common feature contributing to the pathogenesis of many cardiovascular disorders but underlying mechanisms are not fully understood. More than a decade ago, evidence was provided for control of AT1R activation by heterodimerization with the B2 receptor for the vasodepressor peptide, bradykinin, a physiological counterpart of the vasoconstrictor angiotensin II. AT1-B2 receptor heterodimerization was shown to enhance AT1R-stimulated signaling under pathophysiological conditions such as experimental and human pregnancy hypertension. Notably, AT1R signal sensitization of patients with preeclampsia hypertension was attributed to AT1R-B2R heterodimerization. Vice versa, transgenic mice lacking the AT1-B2 receptor heterodimer due to targeted deletion of the B2R gene showed a significantly reduced AT1R-stimulated vasopressor response compared to transgenic mice with abundant AT1R-B2R heterodimerization. Biophysical methods such as BRET and FRET confirmed those data by demonstrating efficient AT1-B2 receptor heterodimerization in transfected cells and transgenic mice. Recently, a study on AT1R-specific biased agonism directed the focus to the AT1-B2 receptor heterodimer again. The β-arrestin-biased [Sar1,Ile4,Ile8]-angiotensin II promoted not only the recruitment of β-arrestin to the AT1R but also stimulated the down-regulation of the AT1R-associated B2 receptor by co-internalization. Thereby specific targeting of the AT1R-B2R heterodimer became feasible and could open the way to a new class of drugs, which specifically interfere with pathological angiotensin II-AT1 receptor system activation.

Enhanced angiotensin-converting enzyme activity and systemic reactivity to angiotensin II in normotensive rats exposed to a high-sodium diet

A high salt diet is associated with reduced activity of the renin-angiotensin-aldosterone system (RAAS). However, normotensive rats exposed to high sodium do not show changes in systemic arterial pressure. We hypothesized that, despite the reduced circulating amounts of angiotensin II induced by a high salt diet, the cardiovascular system's reactivity to angiotensin II is increased in vivo, contributing to maintain arterial pressure at normal levels. Male Wistar rats received chow containing 0.27% (control), 2%, 4%, or 8% NaCl for six weeks. The high-sodium diet did not lead to changes in arterial pressure, although plasma levels of angiotensin II and aldosterone were reduced in the 4% and 8% NaCl groups. The 4% and 8% NaCl groups showed enhanced pressor responses to angiotensin I and II, accompanied by unchanged and increased angiotensin-converting enzyme activity, respectively. The 4% NaCl group showed increased expression of angiotensin II type 1 receptors and reduced expression of angiotensin II type 2 receptors in the aorta. In addition, the hypotensive effect of losartan was reduced in both 4% and 8% NaCl groups. In conclusion these results explain, at least in part, why the systemic arterial pressure is maintained at normal levels in non-salt sensitive and healthy rats exposed to a high salt diet, when the functionality of RAAS appears to be blunted, as well as suggest that angiotensin II has a crucial role in the vascular dysfunction associated with high salt intake, even in the absence of hypertension.

The kallikrein-kinin system as a regulator of cardiovascular and renal function

Autocrine, paracrine, endocrine, and neuroendocrine hormonal systems help regulate cardio-vascular and renal function. Any change in the balance among these systems may result in hypertension and target organ damage, whether the cause is genetic, environmental or a combination of the two. Endocrine and neuroendocrine vasopressor hormones such as the renin-angiotensin system (RAS), aldosterone, and catecholamines are important for regulation of blood pressure and pathogenesis of hypertension and target organ damage. While the role of vasodepressor autacoids such as kinins is not as well defined, there is increasing evidence that they are not only critical to blood pressure and renal function but may also oppose remodeling of the cardiovascular system. Here we will primarily be concerned with kinins, which are oligopeptides containing the aminoacid sequence of bradykinin. They are generated from precursors known as kininogens by enzymes such as tissue (glandular) and plasma kallikrein. Some of the effects of kinins are mediated via autacoids such as eicosanoids, nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF), and/or tissue plasminogen activator (tPA). Kinins help protect against cardiac ischemia and play an important part in preconditioning as well as the cardiovascular and renal protective effects of angiotensin-converting enzyme (ACE) and angiotensin type 1 receptor blockers (ARB). But the role of kinins in the pathogenesis of hypertension remains controversial. A study of Utah families revealed that a dominant kallikrein gene expressed as high urinary kallikrein excretion was associated with a decreased risk of essential hypertension. Moreover, researchers have identified a restriction fragment length polymorphism (RFLP) that distinguishes the kallikrein gene family found in one strain of spontaneously hypertensive rats (SHR) from a homologous gene in normotensive Brown Norway rats, and in recombinant inbred substrains derived from these SHR and Brown Norway rats this RFLP cosegregated with an increase in blood pressure. However, humans, rats and mice with a deficiency in one or more components of the kallikrein-kinin-system (KKS) or chronic KKS blockade do not have hypertension. In the kidney, kinins are essential for proper regulation of papillary blood flow and water and sodium excretion. B2-KO mice appear to be more sensitive to the hypertensinogenic effect of salt. Kinins are involved in the acute antihypertensive effects of ACE inhibitors but not their chronic effects (save for mineralocorticoid-salt-induced hypertension). Kinins appear to play a role in the pathogenesis of inflammatory diseases such as arthritis and skin inflammation; they act on innate immunity as mediators of inflammation by promoting maturation of dendritic cells, which activate the body's adaptive immune system and thereby stimulate mechanisms that promote inflammation. On the other hand, kinins acting via NO contribute to the vascular protective effect of ACE inhibitors during neointima formation. In myocardial infarction produced by ischemia/reperfusion, kinins help reduce infarct size following preconditioning or treatment with ACE inhibitors. In heart failure secondary to infarction, the therapeutic effects of ACE inhibitors are partially mediated by kinins via release of NO, while drugs that activate the angiotensin type 2 receptor act in part via kinins and NO. Thus kinins play an important role in regulation of cardiovascular and renal function as well as many of the beneficial effects of ACE inhibitors and ARBs on target organ damage in hypertension.

Emerging role of angiotensin type 2 receptor (AT2R)/Akt/NO pathway in vascular smooth muscle cell in the hyperthyroidism

Hyperthyroidism is characterized by increased vascular relaxation and decreased vascular contraction and is associated with augmented levels of triiodothyronine (T3) that contribute to the diminished systemic vascular resistance found in this condition. T3 leads to augmented NO production via PI3K/Akt signaling pathway, which in turn causes vascular smooth muscle cell (VSMC) relaxation; however, the underlying mechanisms involved remain largely unknown. Evidence from human and animal studies demonstrates that the renin-angiotensin system (RAS) plays a crucial role in vascular function and also mediates some of cardiovascular effects found during hyperthyroidism. Thus, in this study, we hypothesized that type 2 angiotensin II receptor (AT2R), a key component of RAS vasodilatory actions, mediates T3 induced-decreased vascular contraction. Marked induction of AT2R expression was observed in aortas from T3-induced hyperthyroid rats (Hyper). These vessels showed decreased protein levels of the contractile apparatus: α-actin, calponin and phosphorylated myosin light chain (p-MLC). Vascular reactivity studies showed that denuded aortic rings from Hyper rats exhibited decreased maximal contractile response to angiotensin II (AngII), which was attenuated in aortic rings pre-incubated with an AT2R blocker. Further study showed that cultured VSMC stimulated with T3 (0.1 µmol/L) for 24 hours had increased AT2R gene and protein expression. Augmented NO levels and decreased p-MLC levels were found in VSMC stimulated with T3, both of which were reversed by a PI3K/Akt inhibitor and AT2R blocker. These findings indicate for the first time that the AT2R/Akt/NO pathway contributes to decreased contractile responses in rat aorta, promoted by T3, and this mechanism is independent from the endothelium.