Opposing actions of angiotensin-(1-7) and angiotensin II in the brain of transgenic hypertensive rats

How Does SARS-CoV-2 Affect the Central Nervous System? A Working Hypothesis

Interstitial pneumonia was the first manifestation to be recognized as caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2); however, in just a few weeks, it became clear that the coronavirus disease-2019 (COVID-19) overrun tissues and more body organs than just the lungs, so much so that it could be considered a systemic pathology. Several studies reported the involvement of the conjunctiva, the gut, the heart and its pace, and vascular injuries such as thromboembolic complications and Kawasaki disease in children and toddlers were also described. More recently, it was reported that in a sample of 214 SARS-CoV-2 positive patients, 36.4% complained of neurological symptoms ranging from non-specific manifestations (dizziness, headache, and seizures), to more specific symptoms such hyposmia or hypogeusia, and stroke. Older individuals, especially males with comorbidities, appear to be at the highest risk of developing such severe complications related to the Central Nervous System (CNS) involvement. Neuropsychiatric manifestations in COVID-19 appear to develop in patients with and without pre-existing neurological disorders. Growing evidence suggests that SARS-CoV-2 binds to the human Angiotensin-Converting Enzyme 2 (ACE2) for the attachment and entrance inside host cells. By describing ACE2 and the whole Renin Angiotensin Aldosterone System (RAAS) we may better understand whether specific cell types may be affected by SARS-CoV-2 and whether their functioning can be disrupted in case of an infection. Since clear evidences of neurological interest have already been shown, by clarifying the topographical distribution and density of ACE2, we will be able to speculate how SARS-CoV-2 may affect the CNS and what is the pathogenetic mechanism by which it contributes to the specific clinical manifestations of the disease. Based on such evidences, we finally hypothesize the process of SARS-CoV-2 invasion of the CNS and provide a possible explanation for the onset or the exacerbation of some common neuropsychiatric disorders in the elderly including cognitive impairment and Alzheimer disease.

Vaccines against components of the renin-angiotensin system

Even though effective drugs for treating hypertension are available, a great percentage of patients have inadequate control of their blood pressure. Unwanted side effects and inappropriate oral drug adherence are important factors that contribute to the global problem of uncontrolled hypertension. Vaccination could provide a revolutionary therapy with long-lasting effects, increasing patient compliance and therefore better control of high blood pressure. Nowadays, current immunization approaches against hypertension target renin, angiotensin I, angiotensin II, and angiotensin II type 1 receptor, key elements of the renin–angiotensin system. This article reviews the different vaccination attempts with proteins and peptides against the different molecules of the renin–angiotensin system in the last two decades, safety issues, and other novel prospects biomarkers in hypertension, and summarizes the potential of this immunomodulatory approach in clinical practice.

Impact of COVID-19 on Alzheimer's Disease Risk: Viewpoint for Research Action

In the middle of the coronavirus disease 19 (COVID-19) outbreak, the main efforts of the scientific community are rightly all focused on identifying efficient pharmacological treatments to cure the acute severe symptoms and developing a reliable vaccine. On the other hand, we cannot exclude that, in Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) positive subjects, the virus infection could have long-term consequences, leading to chronic medical conditions such as dementia and neurodegenerative disease. Considering the age of SARS-CoV-2 infected subjects, the neuroinvasive potential might lead/contribute to the development of neurodegenerative diseases. Here, we analyzed a possible link between SARS-CoV-2 infection and Alzheimer's disease risk, hypothesizing possible mechanisms at the base of disease development. This reflection raises the need to start to experimentally investigating today the mechanistic link between Alzheimer's disease (AD) and COVID-19 to be ready tomorrow.

Role of the ACE2‑Ang‑(1‑7)‑Mas axis in blood pressure regulation and its potential as an antihypertensive in functional foods (Review)

The renin‑angiotensin system (RAS) serves a critical role in blood pressure regulation and prevention of cardiovascular diseases. Efforts to develop functional foods that enhance the RAS have focused on inhibition of angiotensin‑converting enzyme (ACE) activity in the ACE‑angiotensin II (Ang II)‑Ang II type 1 receptor axis. ACE2 and the Mas receptor are important components of this axis. ACE2 catalyzes Ang II into Ang‑(1‑7), which then binds to the G‑protein‑coupled receptor Mas. In addition, it induces nitric oxide release from endothelial cells and exerts antiproliferative, vasodilatory and antihypertensive effects. The present review examined recent findings regarding the physiological and biological roles of the ACE2‑Ang‑(1‑7)‑Mas axis in the cardiovascular system, discussed potential food‑derived ACE2‑activating agents, and highlighted initiatives, based on this axis, that aim to develop functional foods for the treatment of hypertension.

Beneficial versus harmful effects of Angiotensin (1-7) on impulse propagation and cardiac arrhythmias in the failing heart

Introduction. The presence of Angiotensin (1-7) (Ang 1-7) and ACE 2 in the ventricle of cardiomyopathic hamsters as well as the influence of Ang (1-7) on membrane potential, impulse propagation and cardiac excitability were investigated.

Methods. Histology and immunochemistry were used to demonstrate the presence of Ang (1-7) and ACE 2 in the ventricle of cardiomyopathic hamsters. Measurements of transmembrane potentials, conduction velocity and refractoriness were made using conventional intracellular microelectrodes. The influence of Ang (1-7) on sodium pump current was investigated in voltageclamped myocytes isolated from the ventricle.

Results. The results indicated the presence of Ang (1-7) and ACE 2 in myocytes of cardiomyopathic hamsters. Moreover, Ang (1-7) (10-8 M) hyperpolarised the heart cell, increased the conduction velocity, and I reduced transiently the action potential duration. The cardiac refractoriness was also increased by the heptapeptide, an effect in part reduced by an inhibitor of mas receptor. These findings indicate that Ang (1-7) has important antiarrhythmic properties. However, the beneficial effects of Ang (1-7) are dose-dependent because at higher concentration (10-7 M) the heptapeptide elicited an appreciable increase of action potential duration and early-after depolarisations. Since losartan (10-7 M) did not counteract this effect of the high dose of the heptapeptide, it is possible to conclude that activation of AT1-receptors is not involved in this effect of Ang (1-7).To investigate the mechanism of the hyperpolarising action of Ang (1-7) the influence of the heptapeptide on the sodium potassium pump current was studied in myocytes isolated from the ventricle of cardiomyopathic hamsters. The peak pump current density was measured under voltage clamp using the whole cell configuration. The results indicated that Ang (1-7) (10—8 M) enhanced the electrogenic sodium pump, an effect suppressed by ouabain (10—7 M).

Conclusions. Ang (1-7) has beneficial effects on the failing heart by activating the sodium pump, hyperpolarising the cell membrane and increasing the conduction velocity. These effects as well as the increment of refractoriness indicate that Ang (1-7) has antiarrhythmic properties. At higher concentrations (10—7 M), however, the heptapeptide induced early-after depolarisations which leads to the conclusion that an optimal generation of Ang (1-7) must be achieved to permit a protective role of Ang (1-7) on cardiac arrhythmias.

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.

Angiotensin converting enzyme 2: a new important player in the regulation of glycemia

In spite of the novel antidiabetic drugs available on the market, type 2 diabetes mellitus (T2DM) affects nearly 25 million people in the USA and causes about 5% of all deaths globally each year. Given the rate and proportion by which T2DM is affecting human beings, it is indispensable to identify new therapeutic targets that can control the disease. Recent preclinical and clinical studies suggest that attenuating the activity of the renin-angiotensin system (RAS) could improve glycemia in diabetic patients. Angiotensin-converting enzyme 2 (ACE2) counteracts RAS overactivity by degrading angiotensin-II (Ang-II), a vasoconstrictor, to Ang-(1-7) which is a vasodilator. A decrease in ACE2 and an increase in A disintegrin and metalloproteinase (ADAM17)-mediated shedding activity have been observed with the progression of T2DM, suggesting the importance of this mechanism in the disease. Indeed, restoration of ACE2 improves glycemia in db/db and Ang-II-infused mice. The beneficial effects of ACE2 can be attributed to reduced oxidative stress and ADAM17 expression in the islets of Langerhans in addition to the improvement of blood flow to the β-cells. The advantage of ACE2 over other RAS blockers is that ACE2 not only counteracts the negative effects of Ang-II but also increases Ang-(1-7)/Mas receptor (MasR) [a receptor through which Ang-(1-7) produces its actions] signaling in the cells. Increased Ang-(1-7)/MasR signaling has been reported to improve insulin sensitivity and glycemia in diabetic animals. Altogether, ACE2/Ang-(1-7)/MasR axis of the RAS appears to be protective in T2DM and strategies to restore ACE2 levels in the disease seem to be a promising therapy for Ang-II-mediated T2DM.

In vivo expression of angiotensin-(1-7) lowers blood pressure and improves baroreflex function in transgenic (mRen2)27 rats

Transgenic (mRen2)27 rats are hypertensive with impaired baroreflex sensitivity for control of heart rate compared with Hannover Sprague-Dawley rats. We assessed blood pressure and baroreflex function in male hemizygous (mRen2)27 rats (30-40 weeks of age) instrumented for arterial pressure recordings and receiving into the cisterna magna either an Ang-(1-7) fusion protein or a control fusion protein (CTL-FP). The maximum reduction in mean arterial pressure achieved was -38 ± 7 mm Hg on day 3, accompanied by a 55% enhancement in baroreflex sensitivity in Ang-(1-7) fusion protein-treated rats. Both the high-frequency alpha index (HF-α) and heart rate variability increased, suggesting increased parasympathetic tone for cardiac control. The mRNA levels of several components of the renin-angiotensin system in the dorsal medulla were markedly reduced including renin (-80%), neprilysin (-40%), and the AT1a receptor (-40%). However, there was a 2-fold to 3-fold increase in the mRNA levels of the phosphatases PTP-1b and dual-specificity phosphatase 1 in the medulla of Ang-(1-7) fusion protein-treated rats. Our finding that replacement of Ang-(1-7) in the brain of (mRen2)27 rats reverses in part the hypertension and baroreflex impairment is consistent with a functional deficit of Ang-(1-7) in this hypertensive strain. We conclude that the increased mRNA expression of phosphatases known to counteract the phosphoinositol 3 kinase and mitogen-activated protein kinases, and the reduction of renin and AT1a receptor mRNA levels may contribute to the reduction in arterial pressure and improvement in baroreflex sensitivity in response to Ang-(1-7).

Central angiotensin-(1-7) improves vagal function independent of blood pressure in hypertensive (mRen2)27 rats

Hypertensive transgenic (mRen2)27 rats with overexpression of the mRen2 gene have impaired baroreflex sensitivity for heart rate control and high nicotinamide adenine dinucleotide phosphate oxidase and kinase-to-phosphatase signaling activity in medullary tissue compared with normotensive Hannover Sprague-Dawley control rats. They also exhibit insulin resistance at a young age. To determine whether blocking angiotensin II actions, supplementing angiotensin-(1-7), or scavenging reactive oxygen species in brain differentially alters mean arterial pressure, baroreflex sensitivity, or metabolic function, while altering medullary signaling pathways in these animals, we compared intracerebroventricular infusions of the angiotensin II type 1 receptor antagonist candesartan (4 μg/5 μL/h), angiotensin-(1-7) (0.1 μg/5 μL/h), a reactive oxygen species scavenger tempol (25 μg/5 μL/h), or artificial cerebrospinal fluid (5 μL/h) for 2 weeks. Mean arterial pressure was reduced in candesartan-treated rats without significantly improving the vagal components of baroreflex function or heart rate variability. In contrast, angiotensin-(1-7) treatment significantly improved the vagal components of baroreflex function and heart rate variability at a dose that did not significantly lower mean arterial pressure. Tempol significantly reduced nicotinamide adenine dinucleotide phosphate oxidase activity in brain dorsal medullary tissue but had no effect on mean arterial pressure or autonomic function. Candesartan tended to reduce fat mass, but none of the treatments significantly altered indices of metabolic function or mitogen-activated protein kinase signaling pathways in dorsal medulla. Although additional dose response studies are necessary to determine the potential maximal effectiveness of each treatment, the current findings demonstrate that blood pressure and baroreflex function can be essentially normalized independently of medullary nicotinamide adenine dinucleotide phosphate oxidase or mitogen-activated protein kinase in hypertensive (mRen2)27 rats.

The functional role of PI3K in maintenance of blood pressure and baroreflex suppression in (mRen2)27 and mRen2.Lewis rat

The phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathway in brain of spontaneously hypertensive rats, but not Wistar-Kyoto (WKY) rats, contributes to elevated mean arterial pressure (MAP). The role of PI3K in the regulation of blood pressure or autonomic function in the nucleus tractus solitarii (NTS) is yet to be established in other Ang II-dependent models of hypertension. Thus, we microinjected PI3K inhibitors, wortmannin or LY294002, into the NTS, and measured MAP, baroreflex sensitivity (BRS) for heart rate (HR) control, and HR variability (HRV) in mRen2.Lewis congenic and (mRen2)27 transgenic rats. Bilateral NTS microinjections of wortmannin (100 nmol/L; 50 nL) reduced MAP in (mRen2)27 and mRen2.Lewis rats (33 ± 5 mm Hg, n = 7, and 32 ± 6 mm Hg, n = 9, respectively) for approximately 90 minutes. Spectral and sequence analysis showed improvements in spontaneous BRS and HRV (50%-100%) after treatment in both hypertensive strains. Injections of wortmannin into NTS of Hannover Sprague-Dawley or Lewis control rats failed to alter MAP, BRS, or HRV. In mRen2.Lewis, but not in control Lewis rats, LY294002 (50 μmole/L) reduced MAP and increased BRS and HRV similar to wortmannin. Thus, the pharmacologic blockade of the PI3K signaling pathway in NTS reveals an important contribution to resting MAP and BRS in rats with overexpression of the Ren2 gene.

Restoration of the blood pressure circadian rhythm by direct renin inhibition and blockade of angiotensin II receptors in mRen2.Lewis hypertensive rats

BACKGROUND

Alterations in the circadian arterial pressure rhythm predict cardiovascular mortality. We examined the circadian arterial pressure rhythm and the effect of renin-angiotensin system blockade in congenic mRen2.Lewis hypertensive rats, a renin-dependent model of hypertension derived from the backcross of transgenic hypertensive [mRen-2]27 rats with Lewis normotensive ones.

METHODS

Twenty-nine mRen2.Lewis hypertensive rats were randomly assigned to drink tap water (vehicle; n = 9), valsartan (30 mg/kg/day; n = 10), or valsartan (30 mg/kg/day) combined with aliskiren given subcutaneously (50 mg/kg/day; n = 10) for 2 weeks. Arterial pressure, heart rate, and locomotive activity were recorded with chronically implanted radiotelemetry probes. The awake/asleep ratio was calculated as [awake mean arterial pressure (MAP) mean - asleep MAP mean)] / (awake MAP mean) x 100. Plasma renin activity (PRA) and concentration (PRC), and plasma and kidney angiotensin II (Ang II) were measured by radioimmunoassay (RIAs).

RESULTS

Untreated hypertensive rats showed an inverse arterial pressure rhythm, higher at day and lower at night, accompanied by normal rhythms of heart rate and locomotive activity. Treatment with valsartan or aliskiren and valsartan normalized the elevated arterial pressure and the arterial pressure rhythm, with the combination therapy being more effective in reducing MAP and in restoring the awake/asleep ratio. While PRA and PRC increased with the treatments, the addition of aliskiren to valsartan partially reversed the increases in plasma Ang II levels. Valsartan and the aliskiren and valsartan combination markedly reduced the renal cortical content of Ang II.

CONCLUSION

The altered circadian arterial pressure rhythm in this renin-dependent hypertension model uncovers a significant role of Ang II in the desynchronization of the circadian rhythm of arterial pressure, heart rate, and locomotive activity.

Altered cardiovascular reflexes responses in conscious Angiotensin-(1-7) receptor Mas-knockout mice

This study evaluated the physiological importance of Angiotensin-(1-7) receptor Mas on reflex control of circulation. Experiments were performed in male Mas-knockout (Mas-KO) and Wild Type (WT) conscious mice (12-20 wk of age). Baroreceptor reflex was evaluated by the bradycardic response induced by phenylephrine (0.25 μg/5 μl, i.v.). Bezold-Jarisch reflex was evaluated by phenylbiguanide (0.5 μg/5 μl, i.v.) and chemoreflex by potassium cyanide (2.5 μg/5 μl, i.v.). Baseline mean arterial pressure was higher in Mas-KO (n=14) as compared with WT mice (n=18) (118±1 mmHg vs. 109±2 mmHg); however, heart rate was similar in both strains (615±30 bpm vs. 648±13 bpm). Baroreflex bradycardia was lower (0.78±0.44 ms/mmHg vs. 1.30±0.14 ms/mmHg) in Mas-KO compared with WT mice. The depressor (-17±5 mmHg vs. -45±6 mmHg) and bradycardic (-212±36 bpm vs. -391±29 bpm) components of the Bezold-Jarisch reflex were also lower in Mas-KO mice. In addition, chemoreflex pressor response (+20±3 mmHg vs. +12±0.8 mmHg) and bradycardic response (-250±74 bpm vs. -52±26 bpm) were significantly higher in Mas-KO. These results further advances previous studies by showing that the lack of Mas receptor induced important imbalance in the neural control of blood pressure, altering not only the baroreflex but also the chemo- and Bezold-Jarisch reflexes.

Advances in the renin angiotensin system focus on angiotensin-converting enzyme 2 and angiotensin-(1-7)

The contribution of the renin angiotensin system to physiology and pathology is undergoing a rapid reconsideration of its mechanisms from emerging new concepts implicating angiotensin-converting enzyme 2 and angiotensin-(1-7) as new elements negatively influencing the vasoconstrictor, trophic, and pro-inflammatory actions of angiotensin II. This component of the system acts to oppose the vasoconstrictor and proliferative effects on angiotensin II through signaling mechanisms mediated by the mas receptor. In addition, a reduced expression of the vasodepressor axis composed by angiotensin-converting enzyme 2 and angiotensin-(1-7) may contribute to the expression of essential hypertension, the remodeling of heart and renal function associated with this disease, and even the physiology of pregnancy and the development of eclampsia.

Preservation of intracellular renin expression is insufficient to compensate for genetic loss of secreted renin

The primary product of the renin gene is preprorenin. A signal peptide sorts renin to the secretory pathway in juxtaglomerular cells where it is released into the circulation to initiate the renin-angiotensin system cascade. In the brain, transcription of renin occurs from an alternative promoter encoding an mRNA starting with a new first exon (exon 1b). Exon 1b initiating transcripts skip over the classical first exon (exon 1a) containing the initiation codon for preprorenin. Exon 1b transcripts are predicted to use a highly conserved initiation codon within exon 2, producing renin, which should remain intracellular, because it lacks the signal peptide. To evaluate the roles of secreted and intracellular renin, we took advantage of the organization of the renin locus to generate a secreted renin (sRen)-specific knockout, which preserves intracellular renin expression. Expression of sRen mRNA was ablated in the brain and kidney, whereas intracellular renin mRNA expression was preserved in fetal and adult brains. We noted a developmental shift from the expression of sRen mRNA in the fetal brain to intracellular renin mRNA in the adult brain. Homozygous sRen knockout mice exhibited very poor survival at weaning. The survivors exhibited renal lesions, low hematocrit, an inability to generate a concentrated urine, decreased arterial pressure, and impaired aortic contraction. These results suggest that preservation of intracellular renin expression in the brain is not sufficient to compensate for a loss of sRen, and sRen plays a pivotal role in renal development and function, survival, and the regulation of arterial pressure.

The uterine placental bed Renin-Angiotensin system in normal and preeclamptic pregnancy

Previously, we demonstrated activation of the renin-angiotensin system in the fetal placental chorionic villi, but it is unknown whether the immediately adjacent area of the maternal uterine placental bed is regulated similarly. This study measured angiotensin peptides, renin-angiotensin system component mRNAs, and receptor binding in the fundus from nonpregnant subjects (n = 19) and in the uterine placental bed from normal (n = 20) and preeclamptic (n = 14) subjects. In the uterine placental bed from normal pregnant women, angiotensin II peptide levels and angiotensinogen, angiotensin-converting enzyme, angiotensin receptor type 1 (AT(1)), AT(2), and Mas mRNA expression were lower as compared with the nonpregnant subjects. In preeclamptic uterine placental bed, angiotensin II peptide levels and renin and angiotensin-converting enzyme mRNA expression were significantly higher than normal pregnant subjects. The AT(2) receptor was the predominant receptor subtype in the nonpregnant fundus, whereas all angiotensin receptor binding was undetectable in normal and preeclamptic pregnant uterine placental bed compared with nonpregnant fundus. These findings suggest that the maternal uterine placental bed may play an endocrine role by producing angiotensin II, which acts in the adjacent placenta to vasoconstrict fetal chorionic villi vessels where we have shown previously that AT(1) receptors predominate. This would lead to decreased maternal-fetal oxygen exchange and fetal nutrition, a known characteristic of preeclampsia.

Angiotensin modulation of rostral ventrolateral medulla (RVLM) in cardiovascular regulation

The rostral ventrolateral medulla (RVLM) and the presympathetic bulbospinal neurons in this region play a critical role in cardiovascular regulation. However, there is ambiguity regarding the precise anatomical coordinates of the RVLM and much still needs to be learned regarding the regulation and neurochemistry of this region. This brief review discusses some of these issues and focuses on the role of angiotensin-mediated signaling in the RVLM in blood pressure regulation.

Chronic immunoneutralization of brain angiotensin-(1-12) lowers blood pressure in transgenic (mRen2)27 hypertensive rats

Angiotensin-(1-12) [ANG-(1-12)] is a newly identified peptide detected in a variety of rat tissues, including the brain. To determine whether brain ANG-(1-12) participates in blood pressure regulation, we treated male adult (mRen2)27 hypertensive rats (24-28 wk of age) with Anti-ANG-(1-12) IgG or Preimmune IgG via an intracerebroventricular cannula for 14 days. Immunoneutralization of brain ANG-(1-12) lowered systolic blood pressure (-43 +/- 8 mmHg on day 3 and -26 +/- 7 mmHg on day 10 from baseline, P < 0.05). Water intake was lower on intracereroventricular day 6 in the Anti-ANG-(1-12) IgG group, accompanied by higher plasma osmolality on day 13, but there were no differences in urine volume, food intake, or body weight during the 2-wk treatment. In Preimmune IgG-treated animals, there were no significant changes in these variables over the 2-wk period. The antihypertensive effects produced by endogenous neutralization of brain ANG-(1-12) suggest that ANG-(1-12) is functionally active in brain pathways regulating blood pressure.

Effect of angiotensin receptor blockade on endothelial function: focus on olmesartan medoxomil

Endothelial dysfunction is the common link between cardiovascular disease risk factors and the earliest event in the cascade of incidents that results in target organ damage. Angiotensin II, the terminal pressor effector arm of the renin-angiotensin-aldosterone system, increases blood pressure (BP) by vasoconstriction and sodium and fluid retention, and has a pro-oxidative action that induces endothelial dysfunction and contributes to vascular remodeling. Angiotensin receptor blockers (ARBs) reduce BP and morbidity and mortality in patients with hypertension, ventricular hypertrophy, diabetes mellitus, and renal disease. Olmesartan medoxomil is a long-acting, well-tolerated, effective ARB that prevents or reverses endothelial dysfunction in animal models of atherosclerosis, hypertension, diabetes, nephropathy, and retinopathy. Olmesartan medoxomil, a prodrug of olmesartan approved for the treatment of hypertension, has been shown to ameliorate endothelial dysfunction in patients with hypertension or diabetes. In randomized studies, the drug reduces vascular inflammation and the volume of large atherosclerotic plaques, increases the number of regenerative endothelial progenitor cells in the peripheral circulation, improves endothelium-dependent relaxation, and restores the normal resistance vessel morphology. Importantly, the impact of olmesartan medoxomil on endothelial dysfunction is thought to be independent of BP lowering.

Angiotensin-converting enzyme 2 in the brain: properties and future directions

Angiotensin (Ang)-converting enzyme (ACE) 2 cleaves Ang-II into the vasodilator peptide Ang-(1-7), thus acting as a pivotal element in balancing the local effects of these peptides. ACE2 has been identified in various tissues and is supposed to be a modulator of cardiovascular function. Decreases in ACE2 expression and activity have been reported in models of hypertension, heart failure, atherosclerosis, diabetic nephropathy and others. In addition, the expression level and/or activity are affected by other renin-angiotensin system components (e.g., ACE and AT1 receptors). Local inhibition or global deletion of brain ACE2 induces a reduction in baroreflex sensitivity. Moreover, ACE2-null mice have been shown to exhibit either blood pressure or cardiac dysfunction phenotypes. On the other hand, over-expression of ACE2 exerts protective effects in local tissues, including the brain. In this review, we will first summarize the major findings linking ACE2 to cardiovascular function in the periphery then focus on recent discoveries related to ACE2 in the CNS. Finally, we will unveil new tools designed to address the importance of central ACE2 in various diseases, and discuss the potential for this carboxypeptidase as a new target in the treatment of hypertension and other cardiovascular diseases.

Angiotensin-(1-7) and baroreflex function in nucleus tractus solitarii of (mRen2)27 transgenic rats

BACKGROUND

Endogenous angiotensin (Ang)-(1-7) enhances, while Ang II attenuates, baroreceptor sensitivity (BRS) for reflex control of heart rate (HR) in Sprague-Dawley (SD) rats. In (mRen2)27 renin transgenic rats [(mRen2)], there is overexpression of the mouse Ren2 gene in brain, leading to elevated Ang II and reduced Ang-(1-7) in brain medullary, and associated with hypertension and impaired BRS.

METHODS

We therefore tested the contribution of endogenous Ang-(1-7) to BRS for control of HR and responses to cardiac vagal chemosensitive afferent fiber activation (CVA) with phenylbiguanide (PBG) in anesthetized SD and (mRen2) 27 rats before and after bilateral nucleus of the solitary tract (nTS) injection of the Ang-(1-7) receptor antagonist (D-Ala7)-Ang-(1-7).

RESULTS

(mRen2) 27 rats exhibited a approximately 50% impairment in BRS as compared with SD (P < 0.05). (D-Ala7)-Ang-(1-7) attenuated BRS by approximately 50% in SD rats, but was without effect in (mRen2) 27 rats. (D-Ala7)-Ang-(1-7) did not alter the responses to CVA by PBG (iv bolus) in either strain. There were no differences in the depressor effects of Ang-(1-7) injected into the nTS, nor were levels of mRNA different for angiotensin-converting enzyme, angiotensin-converting enzyme 2, neprilysin, or the mas receptor in medullary tissue from SD versus (mRen2)27 rats.

CONCLUSION

Endogenous Ang-(1-7) does not provide tonic input in the nTS to modulate BRS for control of HR in (mRen2)27 rats, which may contribute to impairment of BRS in these animals.

New angiotensins

Accumulation of a large body of evidence during the past two decades testifies to the complexity of the renin–angiotensin system (RAS). The incorporation of novel enzymatic pathways, resulting peptides, and their corresponding receptors into the biochemical cascade of the RAS provides a better understanding of its role in the regulation of cardiovascular and renal function. Hence, in recent years, it became apparent that the balance between the two opposing effector peptides, angiotensin II and angiotensin-(1-7), may have a pivotal role in determining different cardiovascular pathophysiologies. Furthermore, our recent studies provide evidence for the functional relevance of a newly discovered rat peptide, containing two additional amino acid residues compared to angiotensin I, first defined as proangiotensin-12 [angiotensin-(1-12)]. This review focuses on angiotensin-(1-7) and its important contribution to cardiovascular function and growth, while introducing angiotensin-(1-12) as a potential novel angiotensin precursor.

Regulation of c-fos, c-jun and c-myc Gene Expression by Angiotensin II in Primary Cultured Rat Astrocytes: Role of ERK1/2 MAP Kinases

We have previously shown that angiotensin II (Ang II) stimulates astrocyte growth through activation of ERK1/2 mitogen activated protein (MAP) kinases. In the current study, we determined whether Ang II stimulates the expression of c-fos, c-jun and c-myc in brainstem astrocyte cultures. Reverse transcriptase-PCR analysis showed c-fos, c-jun, and c-myc mRNAs were induced by Ang II. The EC50 values for Ang II stimulation of c-fos, c-jun and c-myc were 1.3, 1.68 and 1.4 nM, respectively. Ang II (100 nM) induced peak stimulation for all genes by 45 min followed by a gradual decline. Inhibition of ERK1/2 by PD98059 attenuated Ang II-induced c-fos and c-myc mRNA expression (by 75% and 100%, respectively) but was ineffective in preventing Ang II induction of c-jun. These studies show for the first time in brainstem astrocytes that Ang II induces the expression of c-fos, c-myc and c-jun, and showed that ERK1/2 mediate Ang II stimulation of c-fos and c-myc. These data implicate the ERK1/2 MAP kinase pathway as a divergent point in controlling Ang II stimulation of immediate early response genes in the central nervous system.

Involvement of angiotensin-(1-7) in the hypothalamic hypotensive effect of captopril in sinoaortic denervated rats

The role of anterior hypothalamic angiotensin-(1-7) (Ang-(1-7)) on blood pressure regulation was studied in sinoaortic denervated (SAD) rats. Since angiotensin-converting enzyme inhibitors increase endogenous levels of Ang-(1-7), we addressed the involvement of Ang-(1-7) in the hypotensive effect induced by captopril in SAD rats. Wistar rats 7 days after SAD or sham operation (SO) were anaesthetized and the carotid artery was cannulated for monitoring mean arterial pressure (MAP). A needle was inserted into the anterior hypothalamus for drug administration. Intrahypothalamic administration of Ang-(1-7) (5 pmol) was without effect in SO rats but reduced MAP in SAD rats by 15.5+/-3.2 mm Hg and this effect was blocked by 250 pmol [D-Ala(7)]-Ang-(1-7), a Mas receptor antagonist. Angiotensin II (Ang II) induced an increase in MAP in both groups being the effect greater in SAD rats (DeltaMAP=15.8+/-1.4 mm Hg) than in SO rats (DeltaMAP=9.6+/-1.0 mm Hg). Ang-(1-7) partially abolished the pressor response caused by Ang II in SAD rats. Whilst the captopril intrahypothalamic injection did not affect MAP in SO animals, it significantly reduced MAP in SAD rats (DeltaMAP=-13.3+/-1.9 mm Hg). Either [D-Ala(7)]-Ang-(1-7) or an anti-Ang-(1-7) polyclonal antibody partially blocked the MAP reduction caused by captopril. In conclusion, whilst Ang-(1-7) does not contribute to hypothalamic blood pressure regulation in SO normotensive animals, in SAD rats the heptapeptide induces a reduction of blood pressure mediated by Mas receptor activation. Although Ang-(1-7) is not formed in enough amount in the AHA of SAD animals to exert cardiovascular effects in normal conditions, our results suggest that enhancement of hypothalamic Ang-(1-7) levels by administration of captopril is partially involved in the hypotensive effect of the ACE inhibitor.

Regulation of Cardiovascular Control Mechanisms by Angiotensin-(1–7) and Angiotensin-Converting Enzyme 2

Among the molecular forms of angiotensin peptides generated by the action of renin on angiotensinogen (Aogen), both angiotensin II (Ang II) and the amino terminal heptapeptide angiotensin-(1–7) [Ang-(1–7)] are critically involved in the long-term control of tissue perfusion, cell-cell communication, development, and growth. Whereas an impressive body of literature continues to uncover pleiotropic effects of Ang II in the regulation of cell function, research on Ang-(1–7) has a shorter history as it was only 16 yr ago that a biological function for this heptapeptide was first demonstrated in the isolated rat neuro-hypophysial explant preparation (1). On the contrary, the synthesis of angiotonin/ hypertensin (now Ang II) was first obtained in 1957 (2), three decades ahead of the discovery of Ang-(1–7) biological properties.

Angiotensin-(1-7) inhibits angiotensin II-stimulated phosphorylation of MAP kinases in proximal tubular cells

Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE, which is not blocked by ACE inhibitors. High amounts of ACE2 are present in the proximal tubule, and ACE2 catalyzes generation of angiotensin 1-7 (Ang-(1-7)) by this segment. Ang-(1-7) binds to a receptor distinct from the AT1 or AT2 Ang II receptor, identified as the mas receptor. We studied the effects of Ang-(1-7) on Ang II-mediated cell signaling pathways in proximal tubule. In primary cultures of rat proximal tubular cells, activation of mitogen-activated protein kinases (MAPK) was detected by immunoblotting, in the presence or absence of agonists/antagonists. Transforming growth factor-beta1 (TGF-beta1) was measured by enzyme-linked immunosorbent assay. Ang II (5 min, 10(-7) M) stimulated phosphorylation of the three MAPK (p38, extracellular signal-related kinase (ERK 1/2), and c-Jun N-terminal kinase (JNK)). While incubation of proximal tubular cells with Ang-(1-7) alone did not significantly affect MAPK phosphorylation, Ang-(1-7) (10(-7) M) completely inhibited Ang II-stimulated phosphorylation of p38, ERK 1/2, and JNK. This inhibitory effect was reversed by the Ang-(1-7) receptor antagonist, D-Ala7-Ang-(1-7). Ang II significantly increased production of TGF-beta1 in proximal tubular cells, an effect that was partly inhibited by Ang-(1-7). Ang-(1-7) had no significant effect on cyclic 3',5'-adenosine monophosphate production in these cells. In summary, Ang-(1-7) inhibits Ang II-stimulated MAPK phosphorylation in proximal tubular cells. Generation of Ang-(1-7) by proximal tubular ACE2 could thereby serve a protective role by counteracting the effects of locally generated Ang II.

Advances in biochemical and functional roles of angiotensin-converting enzyme 2 and angiotensin-(1-7) in regulation of cardiovascular function

Angiotensin-converting enzyme 2 (ACE2) is the first human homologue of ACE to be described. ACE2 is a type I integral membrane protein that functions as a carboxypeptidase, cleaving a single hydrophobic/basic residue from the COOH-terminus of its substrates. Because ACE2 efficiently hydrolyzes the potent vasoconstrictor angiotensin II to angiotensin (1-7), this has changed our overall perspective about the classical view of the renin angiotensin system in the regulation of hypertension and heart and renal function, because it represents the first example of a feedforward mechanism directed toward mitigation of the actions of angiotensin II. This paper reviews the new data regarding the biochemistry of angiotensin-(1-7)-forming enzymes and discusses key findings such as the elucidation of the regulatory mechanisms participating in the expression of ACE2 and angiotensin-(1-7) in the control of the circulation.

Distinct roles for ANG II and ANG-(1-7) in the regulation of angiotensin-converting enzyme 2 in rat astrocytes

Angiotensin-converting enzyme 2 (ACE2) is a homolog of ACE that preferentially forms angiotensin-(1-7) [ANG-(1-7)] from angiotensin II (ANG II). Incubation of neonatal rat cerebellar or medullary astrocytes with ANG II reduced ACE2 mRNA by approximately 60%, suggesting transcriptional regulation of the enzyme. In contrast, ANG II had no effect on ACE mRNA in astrocytes isolated from either brain region, demonstrating a differential regulation of the two enzymes by ANG II. The ANG II-mediated reduction in ACE2 mRNA was blocked by the angiotensin type 1 (AT(1)) receptor antagonists losartan or valsartan; the angiotensin type 2 (AT(2)) antagonist PD123319 was ineffective. The reduction in ACE2 mRNA by ANG II also was associated with a 50% decrease in cerebellar and medullary ACE2 protein, which was blocked by losartan. Treatment of medullary astrocytes with ANG-(1-7), the product of ACE2 hydrolysis of ANG II, did not affect ACE2 mRNA; however, ANG-(1-7) prevented the ANG II-mediated reduction in ACE2 mRNA. The addition of [d-Ala(7)]-ANG-(1-7), a selective AT((1-7)) receptor antagonist, blocked the inhibitory actions of ANG-(1-7). These data are the first to demonstrate transcriptional regulation of ACE2 by ANG II and ANG-(1-7). Because ACE2 preferentially converts ANG II to ANG-(1-7), downregulation of the enzyme by ANG II constitutes a novel positive feed-forward system within the brain that may favor ANG II-mediated neural responses. Furthermore, the modulatory role of ANG-(1-7) in the transcriptional regulation of ACE2 by ANG II suggests a complex interplay between these peptides that is mediated by distinct receptor systems.

Distribution of angiotensin-(1-7) and ACE2 in human placentas of normal and pathological pregnancies

This work was designed to study the expression of the vasodilator peptide angiotensin-(1-7) [Ang-(1-7)] and its generating enzyme (ACE2) in the uteroplacental interface. Placentas were obtained from 11 early pregnancy failures (5 miscarriages and 6 ectopic pregnancies), 15 normotensive, and 10 preeclamptic gestations. In placental villi, the main sites of immunocytochemical expression of Ang-(1-7) and ACE2 were the syncytiotrophoblast, cytotrophoblast, endothelium and vascular smooth muscle of primary and secondary villi. Syncitial Ang-(1-7) expression in samples obtained from miscarriages and ectopic pregnancies was increased compared to normal term pregnancy [2.0 (2.0-2.25 for the 25 and 75% interquartile range) vs 1.3 (1.0-1.9), p<0.01]. In the maternal stroma, Ang-(1-7) and ACE2 were expressed in the invading and intravascular trophoblast and in decidual cells in all 3 groups. Ang-(1-7) and ACE2 staining was also found in arterial and venous endothelium and smooth muscle of the umbilical cord. The expression of Ang-(1-7) and ACE2 was similar in samples obtained from normal term or preeclamptic pregnancies, except for increased expression of ACE2 in umbilical arterial endothelium in preeclampsia [0.5 (0.5-0.8) vs 0.0 (0.0-0.0), p<0.01]. The uteroplacental location of Ang-(1-7) and ACE2 in pregnancy suggests an autocrine function of Ang-(1-7) in the vasoactive regulation that characterizes placentation and established pregnancy.

Cloning and characterization of a secreted form of angiotensin-converting enzyme 2

Angiotensin-converting enzyme 2 (ACE2) is a newly discovered, membrane-bound aminopeptidase responsible for the production of vasodilatory peptides such as angiotensin 1-7 (Ang 1-7). Thus, ACE2 is important in counteracting the adverse, vasoconstrictor effects of angiotensin II (Ang II). The objective of the present study was to clone and characterize a constitutively secreted form of ACE2 as a prelude to an investigation into its therapeutic potential in hypertension. A truncated form of ACE2 was cloned into a lentiviral vector behind the human elongation factor 1 alpha promoter (lenti-shACE2). Transfection experiments demonstrated that secreted human ACE2 (shACE2) was secreted constitutively into the medium. The kinetic properties of shACE2 were comparable to the human recombinant enzyme (rACE2). Transduction of human coronary artery endothelial cells and rat cardiomyocytes with lenti-shACE2 showed a significant secretion of the enzyme into the medium compared to its native, membrane-bound homolog (human ACE2 [hACE2]). In addition, systemic administration of lenti-shACE2 into neonatal rats resulted in a eightfold increase in ACE2 activity in the serum above control values. These observations establish that lenti-shACE2 can be used to transduce cardiovascularly relevant cells for the secretion of functional ACE2 enzyme both in vitro and in vivo. Collectively, these results set the stage for the use of these vectors to investigate the consequences of ACE2 over-expression in the pathogenesis of hypertension.

Enhanced expression of Ang-(1-7) during pregnancy

Pregnancy is a physiological condition characterized by a progressive increase of the different components of the renin-angiotensin system (RAS). The physiological consequences of the stimulated RAS in normal pregnancy are incompletely understood, and even less understood is the question of how this system may be altered and contribute to the hypertensive disorders of pregnancy. Findings from our group have provided novel insights into how the RAS may contribute to the physiological condition of pregnancy by showing that pregnancy increases the expression of both the vasodilator heptapeptide of the RAS, angiotensin-(1-7) [Ang-(1-7)], and of a newly cloned angiotensin converting enzyme (ACE) homolog, ACE2, that shows high catalytic efficiency for Ang II metabolism to Ang-(1-7). The discovery of ACE2 adds a new dimension to the complexity of the RAS by providing a new arm that may counter-regulate the activity of the vasoconstrictor component, while amplifying the vasodilator component. The studies reviewed in this article demonstrate that Ang-(1-7) increases in plasma and urine of normal pregnant women. In preeclamptic subjects we showed that plasma Ang-(1-7) was suppressed as compared to the levels found in normal pregnancy. In addition, kidney and urinary levels of Ang-(1-7) were increased in pregnant rats coinciding with the enhanced detection and expression of ACE2. These findings support the concept that in normal pregnancy enhanced ACE2 may counteract the elevation in tissue and circulating Ang II by increasing the rate of conversion to Ang-(1-7). These findings provide a basis for the physiological role of Ang-(1-7) and ACE2 during pregnancy.

Plasma and kidney angiotensin II levels and renal functional responses to AT1 receptor blockade in hypertensive Ren-2 transgenic rats

OBJECTIVE

The first aim of the present study was to assess plasma and kidney angiotensin II (ANG II) levels and renal cortical ANG II receptor subtype 1A (AT1A) mRNA expression in hypertensive Ren-2 transgenic rats (TGR) and in normotensive Hannover Sprague-Dawley (HanSD) rats. The second aim was to investigate potential differences between TGR and HanSD in blood pressure (BP) and renal functional responses to either intravenous (i.v.), i.e. systemic, or intrarenal (i.r.) AT1 receptor blockade with candesartan.

METHODS

Rats were anesthetized and prepared for clearance experiments. In series 1, ANG II concentrations were assayed by radioimmunoassay and renal cortical AT1A mRNA expression by semiquantitative reverse transcriptase-polyacrylamide gel electrophoresis. In series 2, BP and renal functional responses were evaluated after either i.v. or i.r. bolus administration of candesartan.

RESULTS

Plasma and kidney ANG II levels were significantly lower in TGR than in HanSD (39 +/- 5 versus 107 +/- 19 fmol/ml and 251 +/- 41 versus 571 +/- 95 fmol/g, respectively, P < 0.05). Renal AT1A mRNA expression was not different between TGR and HanSD. Intravenous candesartan caused comparable decreases in BP in TGR and HanSD and did not change renal plasma flow (RPF) or absolute and fractional sodium excretion in HanSD. In contrast, i.v. candesartan significantly increased RPF (+27 +/- 6%, P < 0.05) and absolute and fractional sodium excretion (+49 +/- 10 and + 42 +/- 9%, respectively P < 0.05) in TGR without changing glomerular filtration rate (GFR). Acute i.r. candesartan increased RPF by +36 +/- 6% (P < 0.05) in TGR but not in HanSD with a greater rise in absolute and fractional sodium excretion in TGR (+124 +/-8 and 97 +/- 9%, respectively) than in HanSD (+81 +/- 9 and +69 +/- 8%, respectively) (P < 0.05).

CONCLUSIONS

The enhanced responses of RPF and sodium excretion to AT1 receptor blockade in TGR suggest that renal hemodynamics and sodium excretion in TGR are under strong ANG II influence. The compromised ability of the kidney to respond to BP elevations by appropriate increases in sodium excretion may contribute to the maintenance of high BP in TGR. Thus, the present findings provide new insights into the pathophysiology of hypertension in this model.

Angiotensin-(1–7) inhibits the angiotensin II-enhanced norepinephrine release in coarcted hypertensive rats

Since it has been suggested that angiotensin (Ang) (1-7) functions as an antihypertensive peptide, we studied its effect on the Ang II-enhanced norepinephrine (NE) release evoked by K+ in hypothalami isolated from aortic coarcted hypertensive (CH) rats. The endogenous NE stores were labeled by incubation of the tissues with 3H-NE during 30 min, and after 90 min of washing, they were incubated in Krebs solution containing 25 mM KCl in the absence or presence of the peptides. Ang-(1-7) not only diminished the K+-evoked NE release from hypothalami of CH rats, but also blocked the Ang II-enhanced NE release induced by K+. Ang-(1-7) blocking action on the Ang II response was prevented by [D-Ala7]Ang-(1-7), an Ang-(1-7) specific antagonist, by PD 123319, an AT2-receptor antagonist, and by Hoe 140, a B2 receptor antagonist. Ang-(1-7) inhibitory effect on the Ang II facilitatory effect on K+-stimulated NE release disappeared in the presence of Nomega-nitro-L-arginine methylester and was restored by L-arginine. Our present results suggest that Ang-(1-7) may contribute to blood pressure regulation by blocking Ang II actions on NE release at the central level. This inhibitory effect is a nitric oxide-mediated mechanism involving AT2 receptors and/or Ang-(1-7) specific receptors and local bradykinin generation.

Functional genomics as an emerging strategy for the investigation of central mechanisms in experimental hypertension

Centrally mediated increases in sympathetic nerve activity and attenuated arterial baroreflexes contribute to the pathogenesis of hypertension. Despite the characterization of cellular and physiological mechanisms that regulate blood pressure and alterations that contribute to hypertension, the genetic and molecular basis of this pathophysiology remains poorly understood. Strategies to identify genes that contribute to central pathophysiologic mechanisms in hypertension include integrative biochemistry and physiology as well as functional genomics. This article summarizes recent progress in applying functional genomics to elucidate the genetic basis of altered central blood pressure regulatory mechanisms in hypertension. We describe approaches others and we have undertaken to investigate gene expression profiles in hypertensive models in order to identify genes that contribute to the pathogenesis of hypertension. Finally, we provide the readers a roadmap for negotiating the route from experimental findings of gene expression profiling to translating their therapeutic potential. The combination of gene expression profiling and the phenotypic characterization of in vitro and in vivo loss or gain of function experiments for candidate genes have the potential to identify genes involved in the pathogenesis of hypertension and may present novel targets for therapy.

Central depletion of angiotensinogen is associated with elevated AT1 receptors in the SFO and PVN

The brain renin-angiotensin system (RAS) is important in fluid balance and blood pressure regulation. In this study, we compared angiotensin (Ang) receptor density in the subfornical organ (SFO) and paraventricular nucleus (PVN) of a) brain angiotensinogen deficient rats (ASrAogen); b) those with high levels of brain Ang II [(mRen2)27]; c) Hannover Sprague Dawley (SD) rats at 48 and 68 wks of age. Since there was no difference between the two ages in any of the three strains, the data from the 48 and 68 wk time points were combined. There was a significantly higher level of AT1 receptors in the SFO and PVN of ASrAogen animals compared to both the SD and (mRen2)27 rats. This suggests that the brain RAS is important in regulating receptor density and that the differences may be explained by lower levels of the peptide locally. These higher levels of receptors suggest that the ASrAogen animals in adulthood and early aging would be more sensitive to either circulating or endogenous brain Ang II than the SD animals of similar age. In contrast, the similar receptor density in the (mRen2)27 and SD rats suggest that previous reports of reduced responses in the (mRen2)27 rats may result from differences in post receptor mechanisms such as intracellular signaling. Moreover, our data reveal that functional assessments are necessary in addition to receptor density levels to understand the consequences of long-term alterations in brain tissue peptides.

Vasopeptidase inhibition and Ang-(1-7) in the spontaneously hypertensive rat

BACKGROUND

Omapatrilat, a new vasopeptidase inhibitor, inhibits the activity of angiotensin-converting enzyme (ACE) and neutral endopeptidase 24.11 (NEP). Because these two enzymes participate in the degradation of the vasodilator and natriuretic peptide, angiotensin-(1-7) [Ang-(1-7)], we assessed whether omapatrilat treatment is associated with changes in the plasma and urinary excretion rates of the angiotensins.

METHODS

We investigated in spontaneously hypertensive rats (SHR) (0.24 kg body weight) the effect of omapatrilat on plasma and urinary concentrations of angiotensin (Ang) I, Ang II and Ang-(1-7) during 17 days of administration of either the drug (N = 15, 100 micromol/kg/day) or vehicle (N = 14) in the drinking water. Hemodynamic and renal excretory function studies were associated with histological examination of the expression of Ang-(1-7) in the kidneys of both vehicle and omapatrilat-treated SHRs.

RESULTS

Omapatrilat induced a sustained lowering of systolic blood pressure (-68 mm Hg) without changes in cardiac rate. The mild positive water balance produced by omapatrilat did not cause natriuresis or kaliuresis, although it was associated with a significant decrease in urine osmolality. Blood pressure normalization was accompanied by increases in plasma Ang I (2969%), Ang II (57%), and Ang-(1-7) (163%) levels, paralleling pronounced increases in urinary excretion rates of Ang I and Ang-(1-7) but not Ang II. Detection of Ang-(1-7) immunostaining in the kidneys of five other SHR exposed either to vehicle (N = 3) or omapatrilat (N = 2) ascertained the source of the Ang-(1-7) found in the urine. Intense Ang-(1-7) staining, more pronounced in omapatrilat-treated SHR, was found in renal proximal tubules throughout the outer and inner regions of the renal cortex and the thick ascending loop of Henle, whereas no Ang-(1-7)-positive immunostaining was found in glomeruli and distal tubules.

CONCLUSIONS

Omapatrilat antihypertensive effects caused significant activation of the renin-angiotensin system associated with increases in urinary excretion rates of Ang I and Ang-(1-7). Combined studies of Ang-(1-7) metabolism in urine and immunohistochemical studies in the kidney revealed the existence of an intrarenal source, which may account for the pronounced increase in the excretion rate of the vasodilator heptapeptide. These findings provide further evidence for a contribution of Ang-(1-7) to the regulation of renal function and blood pressure.

Formation of angiotensin-(1-7) from angiotensin II by the venom of Conus geographus

The binding of [3H]angiotensin II to AT(1) receptors on Chinese Hamster Ovary cells expressing the human AT(1) receptor (CHO-AT(1) cells) is potently inhibited by venoms of the marine snails Conus geographus and C. betulinus. On the other hand, the binding of the nonpeptide AT(1) receptor-selective antagonist [3H]candesartan is not affected but competition binding curves of angiotensin II and the peptide antagonist [Sar(1),Ile(8)]angiotensin II (sarile) are shifted to the right. These effects resulted from the breakdown of angiotensin II into smaller fragments that do not bind to the AT(1) receptor. In this context, angiotensin-(1-7) is the most prominent fragment and angiotensin-(1-4) and angiotensin-(1-5) are also formed but to a lesser extent. The molecular weight of the involved peptidases exceeds 50 kDa, as determined by gel chromatography and ultrafitration.

Angiotensin-(1-7) is involved in the endothelium-dependent modulation of phenylephrine-induced contraction in the aorta of mRen-2 transgenic rats

1. The contribution of the local vascular production of angiotensin-(1-7) [Ang-(1-7)] to the control of alpha-adrenergic-induced contractions in the aorta of Sprague-Dawley (SD) and TGR(mRen-2)27 [mRen-2] rats was studied. 2. In mRen-2 rats, contractile responses to phenylephrine were diminished as compared to control SD rats in endothelium containing but not in endothelium-denuded vessels. L-NAME increased contractile responses to phenylephrine in mRen-2 rats and, after nitric oxide synthase blockade, responses to phenylephrine became comparable in both strains. 3. Inhibition of angiotensin-converting enzyme (ACE) by captopril potentiated contractile responses in mRen-2 rats and diminished contractile responses in SD rats, both effects being dependent on the presence of a functional endothelium. The effect of captopril in mRen-2 rats was abolished in vessels pre-incubated with Ang-(1-7). 4. Blockade of Ang-(1-7) and bradykinin (BK) receptors by A-779 and HOE 140 respectively, increased phenylephrine-induced contraction in mRen-2, but not in SD rats. This effect was seen only in endothelium-containing vessels. 5. Angiotensin II AT(1) and AT(2) receptor blockade by CV 11974 and PD 123319 did not affect the contractile responses to phenylephrine in aortas of transgenic animals but diminished the response in SD rats. This effect was only seen in the presence of a functional endothelium. 6. It is concluded that the decreased contractile responses to phenylephrine in aortas of mRen-2 rats was dependent on an intact endothelium, the local release and action of Ang-(1-7) and bradykinin.

L-158,809 and (D-Ala(7))-angiotensin I/II (1-7) decrease PAI-1 release from human umbilical vein endothelial cells

The endothelium is a major source of plasminogen activator inhibitor-1 (PAI-1), which plays a critical role in the regulation of fibrinolysis. There are many reports on the increase in the expression of PAI-1 by angiotensin II (Ang II). In the present study, we investigated the effects of angiotensin-related substances on the release of PAI-1 from human umbilical vein endothelial cells (HUVECs). Ang II increased PAI-1 and tissue plasminogen activator (t-PA) release, while its metabolite angiotensin-(1-7) (Ang-(1-7)) amino acid fragment decreased them. Angiotensin Type 1 (AT1) receptor antagonist, L-158,809 (L-1), and Ang-(1-7) receptor antagonist, (D-Ala(7))-angiotensin I/II (1-7) (D-Ala), decreased PAI-1 and t-PA release; angiotensin Type 2 (AT2) antagonist, PD123,319 (PD), however, did not have any effects on the release of PAI-1 and t-PA. The addition of the equal concentration or 10-times-higher concentration of L-1 to Ang II did not change PAI-1 release compared to that by Ang II. Although Ang-(1-7) and L-1 decreased PAI-1 release, there were no additional effects on the decrease of the amounts of PAI-1 by the mixture of Ang-(1-7) and the equal concentration or 10-times-higher concentration of L-1 compared to those by Ang-(1-7). The equal concentration of D-Ala to Ang II did not change the amounts of PAI-1, but the addition of the 10-times-higher concentration of D-Ala to Ang II resulted in significant decrease of the amounts of PAI-1 compared to those by Ang II. The addition of equal concentration or 10-times-higher concentration of D-Ala to Ang-(1-7) showed the significant decrease of the amounts of PAI-1 compared to those by Ang-(1-7). In conclusion, L-158,809 and (D-Ala(7))-angiotensin I/III (1-7) may be used as profibrinolytic drugs.

The differential effect of angiotensin II and angiotensin 1-7 on norepinephrine, epinephrine, and dopamine concentrations in rat hypothalamus: the involvement of angiotensin receptors

Angiotensin 1-7 has been recently claimed the active member of the angiotensins' family. In the present study we compared the effect of angiotensin II and angiotensin 1-7 on the concentration of dopamine, serotonin, epinephrine, and norepinephrine and some of their metabolites in the rat hypothalamus, where the levels of angiotensins are particularly high. Intracerebroventricular injection of angiotensin II, but not angiotensin 1-7, time-dependently elevated the levels of both epinephrine (p < 0.05) and norepinephrine (p < 0.05) in the hypothalamus and both effects could be prevented by intracerebroventricular injection of either AT(1) (candesartan), AT(2) (PD123319) or AT(1-7) (A-779) receptor antagonist. Neither angiotensin II nor angiotensin 1-7 produced any changes in the level of dopamine, dihydroxyphenylacetic acid, homovanilic acid, serotonin, 5-hydroxyindoleacetic acid, or tryptophan at any time point in comparison with the control groups. However, AT(1) but not AT(2) receptor blockade, unmasked the stimulatory effect of angiotensin 1-7 on dopamine concentration in the hypothalamus. Thus, angiotensin II and its active metabolite angiotensin 1-7 regulate selectively, albeit differentially, adrenergic, noradrenergic and dopaminergic systems in the hypothalamus, the effects that involve AT(1), AT(2) and AT(1-7) angiotensin receptors.

Downregulation of the AT1A receptor by pharmacologic concentrations of Angiotensin-(1-7)

Angiotensin (Ang)-(1-7), the amino terminal heptapeptide fragment of Ang II, is an endogenous Ang peptide with vasodilatory and antiproliferative actions. Because Ang II causes vasoconstriction and promotes growth through activation of Ang type 1 (AT1) receptors, we investigated whether the actions of Ang-(1-7) are due to its regulation of these receptors. Studies were performed in CHO cells stably transfected with the AT1A receptor. Ang-(1-7) competed poorly with [125I]-Ang II for the AT1A binding site and was ineffective at shifting the IC50 for Ang II competition with [125I]-Ang II for binding to the AT1A receptor. However, if CHO-AT1A cells were pretreated with Ang-(1-7) and then treated with acidic glycine to remove surface-bound ligand, the heptapeptide caused a concentration-dependent reduction in Ang II binding, with a maximal inhibition to 67.8 +/- 4.6% of total (p < 0.05) at 1 microM Ang-(1-7) compared with a reduction to 24% of total by 10 nM Ang II. Ang-(1-7) pretreatment caused a small but significant decrease in the affinity of [125I]-Ang II for the AT1A receptor and a significant reduction in the total number of binding sites. The Ang-(1-7)-induced reduction in binding was rapid (occurring as early as 5 min after exposure to the peptide), was maintained for 30 min during continued exposure of the cells to Ang-(1-7), and rapidly recovered after removal of the heptapeptide. The AT1 receptor antagonist L-158,809 reduced the Ang-(1-7)-induced downregulation of the AT1A receptor, suggesting that interactions with AT1A receptors mediate the regulatory events. Pretreatment with 1 microM or 10 microM Ang-(1-7) significantly reduced inositol phosphate production in response to 10 nM Ang II. The decrease in binding and responsiveness of the AT1A receptor after exposure to micromolar concentrations of Ang-(1-7) suggests that the heptapeptide downregulates the AT1A receptor to reduce responses to Ang II. Because downregulation of the receptor only occurred at micromolar concentrations of the heptapeptide, our findings suggest that Ang-(1-7) is not a potent antagonist at the AT1A receptor. However, when the balance between Ang II and Ang-(1-7) is shifted in favor of Ang-(1-7), such as during inhibition of Ang-converting enzyme, some contribution of this mechanism may come into play.

Angiotensin-(1-7): an update

The renin-angiotensin system is a major physiological regulator of arterial pressure and hydro-electrolyte balance. Evidence has now been accumulated that in addition to angiotensin (Ang) II other Ang peptides [Ang III, Ang IV and Ang-(1-7)], formed in the limited proteolysis processing of angiotensinogen, are importantly involved in mediating several actions of the RAS. In this article we will review our knowledge of the biological actions of Ang-(1-7) with focus on the puzzling aspects of the mediation of its effects and the interaction Ang-(1-7)-kinins. In addition, we will attempt to summarize the evidence that Ang-(1-7) takes an important part of the mechanisms aimed to counteract the vasoconstrictor and proliferative effects of Ang II.

Angiotensin peptides acting at rostral ventrolateral medulla contribute to hypertension of TGR(mREN2)27 rats

We have previously demonstrated that microinjections of the selective angiotensin-(1-7) [ANG-(1-7)] antagonist, A-779, into the rostral ventrolateral medulla (RVLM) produces a significant fall in mean arterial pressure (MAP) and heart rate (HR) in both anesthetized and conscious rats. In contrast, microinjection of angiotensin II (ANG II) AT(1) receptor antagonists did not change MAP in anesthetized rats and produced dose-dependent increases in MAP when microinjected into the RVLM of conscious rats. In the present study, we evaluated whether endogenous ANG-(1-7) and ANG II acting at the RVLM contribute to the hypertension of transgenic rats harboring the mouse renin Ren-2 gene, TGR(mREN2)27. Unilateral microinjection of A-779 (0.1 nmol) produced a significant fall in MAP (-25 +/- 5 mmHg) and HR (-57 +/- 20 beats/min) of awake TGR rats. The hypotensive effect was greater than that observed in Sprague-Dawley (SD) rats (-9 +/- 2 mmHg). Microinjection of the AT(1) antagonist CV-11974 (0.2 nmol) produced a fall in MAP in TGR rats (-14 +/- 4 mmHg), contrasting with the pressor effect observed in SD rats (33 +/- 9 mmHg). These results indicate that endogenous ANG-(1-7) exerts a significant pressor action in the RVLM, contributing to the hypertension of TGR(mREN2)27 transgenic rats. The role of ANG II at the RVLM seems to be dependent on its endogenous level in this area.

Blood pressure reduction and diabetes insipidus in transgenic rats deficient in brain angiotensinogen

Angiotensin produced systemically or locally in tissues such as the brain plays an important role in the regulation of blood pressure and in the development of hypertension. We have established transgenic rats [TGR(ASrAOGEN)] expressing an antisense RNA against angiotensinogen mRNA specifically in the brain. In these animals, the brain angiotensinogen level is reduced by more than 90% and the drinking response to intracerebroventricular renin infusions is decreased markedly compared with control rats. Blood pressure of transgenic rats is lowered by 8 mmHg (1 mmHg = 133 Pa) compared with control rats. Crossbreeding of TGR(ASrAOGEN) with a hypertensive transgenic rat strain exhibiting elevated angiotensin II levels in tissues results in a marked attenuation of the hypertensive phenotype. Moreover, TGR(ASrAOGEN) exhibit a diabetes insipidus-like syndrome producing an increased amount of urine with decreased osmolarity. The observed reduction in plasma vasopressin by 35% may mediate these phenotypes of TGR(ASrAOGEN). This new animal model presenting long-term and tissue-specific down-regulation of angiotensinogen corroborates the functional significance of local angiotensin production in the brain for the central regulation of blood pressure and for the pathogenesis of hypertension.

Angiotensin-(1-7): a bioactive fragment of the renin-angiotensin system

Accumulating evidence suggests that angiotensin-(1-7) [Ang-(1-7)] is an important component of the renin-angiotensin system. As the most pleiotropic metabolite of angiotensin I (Ang I) it manifest actions which are most often the opposite of those described for angiotensin II (Ang II). Ang-(1-7) is produced from Ang I bypassing the prerequisite formation of Ang II. The generation of Ang-(1-7) is under the control of at least three enzymes, which include neprilysin, thimet oligopeptidase, and prolyl oligopeptidase depending on the tissue compartment. Both neprilysin and thimet oligopeptidase are also involved in the metabolism of bradykinin and the atrial natriuretic peptide. Moreover, recent studies suggest that in addition to Ang I and bradykinin, Ang-(1-7) is an endogenous substrate for angiotensin converting enzyme. This suggests that there is a complex relationship between the enzymatic pathways forming angiotensin II and other various vasodepressor peptides from either the renin-angiotensin system or other peptide systems. The antihypertensive actions of angiotensin-(1-7) are mediated by an angiotensin receptor that is distinct from the pharmacologically characterized AT1 or AT2 receptor subtypes. Ang-(1-7) mediates it antihypertensive effects by stimulating synthesis and release of vasodilator prostaglandins, and nitric oxide and potentiating the hypotensive effects of bradykinin.

Angiotensin-(1-7) immunoreactivity in the hypothalamus of the (mRen-2d)27 transgenic rat

The distribution of angiotensin-(1-7) immunoreactive neurons was compared to those of vasopressin-(VP) and oxytocin-(OT) immunoreactive (IR) neurons in the hypothalamus of adult (mRen-2d)27 transgenic hypertensive and Sprague-Dawley rats. In both strains, angiotensin (Ang)-(1-7)-IR cells were found in the supraoptic nucleus (SON), and in the anterior (ap-), medial (mp-), and lateral (lp-) parvocellular, and posterior magnocellular (pm-) subdivisions of the paraventricular (PVN) nucleus. Three-dimensional reconstructions showed that cells immunoreactive to Ang-(1-7) and VP were specifically co-distributed in the SON and in the pmPVN. Double-labeling neurons for both peptides revealed that both Ang-(1-7) and VP were colocalized in a subpopulation of neurons in the pmPVN and SON. In combination with previous studies, our results suggest that Ang-(1-7) and VP are colocalized, co-released and may have a combined action at a common target. In addition, the introduction of the mouse submandibular renin (mRen-2d) transgene into Sprague-Dawley rats does not appear to have altered the fundamental organization of hypothalamic peptide systems involved in fluid homeostasis.