Biological activities of angiotensin II-(1-6)-hexapeptide and angiotensin II-(1-7)-heptapeptide in man

Angiotensin-(1-7) infusion in COVID-19 patients admitted to the ICU: a seamless phase 1-2 randomized clinical trial

BACKGROUND

The coronavirus-related disease (COVID-19) is mainly characterized by a respiratory involvement. The renin-angiotensin system (RAS) has a relevant role in the pathogenesis of COVID-19, as the virus enters host's cells via the angiotensin-converting enzyme 2 (ACE2).

METHODS

This investigator-initiated, seamless phase 1-2 randomized clinical trial was conceived to test the safety and efficacy of continuous short-term (up to 7 days) intravenous administration of Angiotensin-(1-7) in COVID-19 patients admitted to two intensive care units (ICU). In addition to standard of care, intravenous administration of Angiotensin-(1-7) was started at 5 mcg/Kg day and increased to 10 mcg/Kg day after 24 h (Phase 1; open label trial) or given at 10 mcg/Kg day and continued for a maximum of 7 days or until ICU discharge (Phase 2; double-blind randomized controlled trial). The rate of serious adverse events (SAEs) served as the primary outcome of the study for Phase 1, and the number of oxygen free days (OFDs) by day 28 for Phase 2.

RESULTS

Between August 2020 and July 2021, when the study was prematurely stopped due to low recruitment rate, 28 patients were included in Phase 1 and 79 patients in Phase 2. Of those, 78 were included in the intention to treat analysis, and the primary outcome was available for 77 patients. During Phase 1, one SAE (i.e., bradycardia) was considered possibly related to the infusion, justifying its discontinuation. In Phase 2, OFDs did not differ between groups (median 19 [0-21] vs. 14 [0-18] days; p = 0.15). When patients from both phases were analyzed in a pooled intention to treat approach (Phase 1-2 trial), OFDs were significantly higher in treated patients, when compared to controls (19 [0-21] vs. 14 [0-18] days; absolute difference -5 days, 95% CI [0-7] p = 0.04).

CONCLUSIONS

The main findings of our study indicate that continuous intravenous infusion of Angiotensin-(1-7) at 10 mcg/Kg day in COVID-19 patients admitted to the ICU with severe pneumonia is safe. In Phase II intention to treat analysis, there was no significant difference in OFD between groups. Trial Registration ClinicalTrials.gov Identifier: NCT04633772-Registro Brasileiro de Ensaios Clínicos, UTN number: U1111-1255-7167.

Counter-regulatory renin-angiotensin system in hypertension: Review and update in the era of COVID-19 pandemic

Cardiovascular disease is the major cause of mortality and disability, with hypertension being the most prevalent risk factor. Excessive activation of the renin-angiotensin system (RAS) under pathological conditions, leading to vascular remodeling and inflammation, is closely related to cardiovascular dysfunction. The counter-regulatory axis of the RAS consists of angiotensin-converting enzyme 2 (ACE2), angiotensin (1-7), angiotensin (1-9), alamandine, proto-oncogene Mas receptor, angiotensin II type-2 receptor and Mas-related G protein-coupled receptor member D. Each of these components has been shown to counteract the effects of the overactivated RAS. In this review, we summarize the latest insights into the complexity and interplay of the counter-regulatory RAS axis in hypertension, highlight the pathophysiological functions of ACE2, a multifunctional molecule linking hypertension and COVID-19, and discuss the function and therapeutic potential of targeting this counter-regulatory RAS axis to prevent and treat hypertension in the context of the current COVID-19 pandemic.

ACE2 in the renin-angiotensin system

In 2020 we are celebrating the 20th anniversary of the angiotensin-converting enzyme 2 (ACE2) discovery. This event was a landmark that shaped the way that we see the renin-angiotensin system (RAS) today. ACE2 is an important molecular hub that connects the RAS classical arm, formed mainly by the octapeptide angiotensin II (Ang II) and its receptor AT1, with the RAS alternative or protective arm, formed mainly by the heptapeptides Ang-(1-7) and alamandine, and their receptors, Mas and MrgD, respectively. In this work we reviewed classical and modern literature to describe how ACE2 is a critical component of the protective arm, particularly in the context of the cardiac function, coagulation homeostasis and immune system. We also review recent literature to present a critical view of the role of ACE2 and RAS in the SARS-CoV-2 pandemic.

ACE2, angiotensin 1-7 and skeletal muscle: review in the era of COVID-19

Angiotensin converting enzyme-2 (ACE2) is a multifunctional transmembrane protein recently recognised as the entry receptor of the virus causing COVID-19. In the renin–angiotensin system (RAS), ACE2 cleaves angiotensin II (Ang II) into angiotensin 1-7 (Ang 1-7), which is considered to exert cellular responses to counteract the activation of the RAS primarily through a receptor, Mas, in multiple organs including skeletal muscle. Previous studies have provided abundant evidence suggesting that Ang 1-7 modulates multiple signalling pathways leading to protection from pathological muscle remodelling and muscle insulin resistance. In contrast, there is relatively little evidence to support the protective role of ACE2 in skeletal muscle. The potential contribution of endogenous ACE2 to the regulation of Ang 1-7-mediated protection of these muscle pathologies is discussed in this review. Recent studies have suggested that ACE2 protects against ageing-associated muscle wasting (sarcopenia) through its function to modulate molecules outside of the RAS. Thus, the potential association of sarcopenia with ACE2 and the associated molecules outside of RAS is also presented herein. Further, we introduce the transcriptional regulation of muscle ACE2 by drugs or exercise, and briefly discuss the potential role of ACE2 in the development of COVID-19.

Similarity and dissimilarity between angiotensin A and angiotensin II in cardiovascular functions in a rat model

Angiotensin (Ang) A differs from Ang II in a single N-terminal alanine residue. The aim of this study was to investigate whether the effects of Ang A on postischemic cardiac injury and hemodynamics differ from Ang II. After euthanizing Sprague-Dawley rats, hearts were perfused with Krebs-Henseleit buffer for a 20 min preischemic period with or without Ang A or Ang II, followed by 20 min global ischemia and 50 min reperfusion. The blood pressure was measured in anesthetized rats. Ang A (0.1, 1.0, 10 μg/kg) deteriorated the postischemic left ventricular hemodynamics in a dose-dependent manner, which was similar to that by Ang II. Ang A (10 μg/kg) increased the infarct size and the lactate dehydrogenase level, and decreased the coronary flow, which were attenuated by the pretreatment with Ang type 1 receptor (AT1R) antagonist (losartan) but not by AT2R antagonist (PD123319). Ang A increased the expression of apoptotic proteins and decreased the expression of antioxidative proteins. Interestingly, Ang A increased the atrial natriuretic peptide (ANP) level in coronary effluent and in atrial perfusate but Ang II did not increase it. Ang A increased mean arterial blood pressure, which was less potent than Ang II. These results suggest that Ang A has a similar effect on postischemic injury via AT1R and less potent vasopressor effect but opposite effect on ANP secretion as compared to Ang II.

Angiotensin-(1-7): Translational Avenues in Cardiovascular Control

Despite decades of research and numerous treatment approaches, hypertension and cardiovascular disease remain leading global public health problems. A major contributor to regulation of blood pressure, and the development of hypertension, is the renin-angiotensin system. Of particular concern, uncontrolled activation of angiotensin II contributes to hypertension and associated cardiovascular risk, with antihypertensive therapies currently available to block the formation and deleterious actions of this hormone. More recently, angiotensin-(1-7) has emerged as a biologically active intermediate of the vasodilatory arm of the renin-angiotensin system. This hormone antagonizes angiotensin II actions as well as offers antihypertensive, antihypertrophic, antiatherogenic, antiarrhythmogenic, antifibrotic and antithrombotic properties. Angiotensin-(1-7) elicits beneficial cardiovascular actions through mas G protein-coupled receptors, which are found in numerous tissues pivotal to control of blood pressure including the brain, heart, kidneys, and vasculature. Despite accumulating evidence for favorable effects of angiotensin-(1-7) in animal models, there is a paucity of clinical studies and pharmacokinetic limitations, thus limiting the development of therapeutic agents to better understand cardiovascular actions of this vasodilatory peptide hormone in humans. This review highlights current knowledge on the role of angiotensin-(1-7) in cardiovascular control, with an emphasis on significant animal, human, and therapeutic research efforts.

Efficacy of lanthionine-stabilized angiotensin-(1-7) in type I and type II diabetes mouse models

Native angiotensin-(1-7) exerts many therapeutic effects. However, it is rapidly degraded by ACE and other peptidases. This drawback is largely eliminated for lanthionine-stabilized angiotensin-(1-7), termed cAng-(1-7), which is fully resistant to ACE and has strongly increased resistance to other peptidases. Goal of the present study was to test whether cAng-(1-7) has therapeutic activity in diabetes mouse models: in a multiple low dose streptozotocin-induced model of type I diabetes and / or in a db/db model of type II diabetes. In the type I diabetes model cAng-(1-7) caused in an increase in the insulin level of 133% in week 4 (p < 0.001) compared to vehicle, and in the type II diabetes model an increase of 55% of the insulin level in week 8 (p < 0.05) compared to vehicle. cAng-(1-7) reduced blood glucose levels in the type I model by 37% at day 22 (p < 0.001) and in the type II diabetes model by 17% at day 63 of treatment (p < 0.001) and in an oral glucose tolerance test in a type II diabetes model, by 17% at week 4 (p < 0.01). cAng-(1-7) also caused a reduction of glycated hemoglobin levels in the type II diabetes model of 21% in week 6 (p < 0,001). These data are consistent with therapeutic potential of cAng-(1-7) in type I and II diabetes.

Comparative effects of angiotensin II and angiotensin-(4-8) on blood pressure and ANP secretion in rats

Angiotensin II (Ang II) is metabolized from N-terminal by aminopeptidases and from C-terminal by Ang converting enzyme (ACE) to generate several truncated angiotensin peptides (Angs). The truncated Angs have different biological effects but it remains unknown whether Ang-(4-8) is an active peptide. The present study was to investigate the effects of Ang-(4-8) on hemodynamics and atrial natriuretic peptide (ANP) secretion using isolated beating rat atria. Atrial stretch caused increases in atrial contractility by 60% and in ANP secretion by 70%. Ang-(4-8) (0.01, 0.1, and 1 µM) suppressed high stretch-induced ANP secretion in a dose-dependent manner. Ang-(4-8) (0.1 µM)-induced suppression of ANP secretion was attenuated by the pretreatment with an antagonist of Ang type 1 receptor (AT₁R) but not by an antagonist of AT₂R or AT₄R. Ang-(4-8)-induced suppression of ANP secretion was attenuated by the pretreatment with inhibitor of phospholipase (PLC), inositol triphosphate (IP₃) receptor, or nonspecific protein kinase C (PKC). The potency of Ang-(4-8) to inhibit ANP secretion was similar to Ang II. However, Ang-(4-8) 10 µM caused an increased mean arterial pressure which was similar to that by 1 nM Ang II. Therefore, we suggest that Ang-(4-8) suppresses high stretch-induced ANP secretion through the AT₁R and PLC/IP₃/PKC pathway. Ang-(4-8) is a biologically active peptide which functions as an inhibition mechanism of ANP secretion and an increment of blood pressure.

Angiotensin-(1-7) and Mas

Angiotensin-(1-7) is a vasoactive peptide of the renin–angiotensin system (RAS), which is generated mainly by angiotensin-converting enzyme 2 (ACE2) and exerts its actions via activation of its receptor Mas. The Ang-(1-7)/ACE2/Mas axis is nowadays considered to be a main mechanism, which counterbalances the vasoconstrictive actions of classical RAS, which includes renin, ACE, ANG II, and its receptors AT₁ and AT₂. Whereas the classical RAS has been known for more than 100 years, the protective arm of the RAS was relatively recently discovered. Both Mas and Ang-(1-7) were first described almost 30 years ago; however, it took an additional 15 years until the interaction of these components was revealed. Here, we will describe the story of Mas and Ang-(1-7), which was full of errors and uncertainty at the beginning, until the interrelationship between the two was unveiled in 2003.

A systematic review to investigate whether Angiotensin-(1-7) is a promising therapeutic target in human heart failure

Context. Heart failure (HF) is a common condition causing much morbidity and mortality despite major advances in pharmacological and device therapies. Preclinical data suggest a cardioprotective role of Angiotensin-(1-7) in animal models of HF. Objective. Perform a systematic review on the effects of Angiotensin-(1-7) on humans, focusing on HF. Results. 39 studies were included in the review (4 in human HF and (35) in non-HF patients). There is only one intervention study on 8 patients with human HF, using Angiotensin-(1-7), with forearm blood flow (FBF) as the endpoint. Angiotensin-(1-7) caused no significant effect on FBF in this HF study but caused vasodilation in 3 out of 4 non-HF studies. In one other non-HF study, Angiotensin-(1-7) infusion led to a significant increase in blood pressure in normal men; however, effects were <0.03% that of angiotensin II. Cardioprotective effects seen in non-HF studies include for instance beneficial actions against atherosclerosis and myocardial fibrosis. Conclusions. The main finding of our systematic review is that Angiotensin-(1-7) plays an important cardioprotective role in HF in animals and in patients without heart failure. More research is required to test the hypothesis that Angiotensin-(1-7) benefits patients with heart failure.

Lentivirus-mediated overexpression of angiotensin-(1-7) attenuated ischaemia-induced cardiac pathophysiology

Myocardial infarction (MI) results in cell death, development of interstitial fibrosis, ventricular wall thinning and ultimately, heart failure. Angiotensin-(1-7) [Ang-(1-7)] has been shown to provide cardioprotective effects. We hypothesize that lentivirus-mediated overexpression of Ang-(1-7) would protect the myocardium from ischaemic injury. A single bolus of 3.5 × 10(8) transducing units of lenti-Ang-(1-7) was injected into the left ventricle of 5-day-old male Sprague-Dawley rats. At 6 weeks of age, MI was induced by ligation of the left anterior descending coronary artery. Four weeks after the MI, echocardiography and haemodynamic parameters were measured to assess cardiac function. Postmyocardial infarction, rats showed significant decreases in fractional shortening and dP/dt (rate of rise of left ventricular pressure), increases in left ventricular end-diastolic pressure, and ventricular hypertrophy. Also, considerable upregulation of cardiac angiotensin-converting enzyme (ACE) mRNA was observed in these rats. Lentivirus-mediated cardiac overexpression of Ang-(1-7) not only prevented all these MI-induced impairments but also resulted in decreased myocardial wall thinning and an increased cardiac gene expression of ACE2 and bradykinin B2 receptor (BKR2). Furthermore, in vitro experiments using rat neonatal cardiac myocytes demonstrated protective effects of Ang-(1-7) against hypoxia-induced cell death. This beneficial effect was associated with decreased expression of inflammatory cytokines (tumour necrosis factor-α and interleukin-6) and increased gene expression of ACE2, BKR2 and interleukin-10. Our findings indicate that overexpression of Ang-(1-7) improves cardiac function and attenuates left ventricular remodelling post-MI. The protective effects of Ang-(1-7) appear to be mediated, at least in part, through modulation of the cardiac renin-angiotensin system and cytokine production.

The role of the renin-angiotensin system in liver fibrosis

Hepatic fibrosis, which is characterized by progressive inflammation and deposition of extracellular matrix components, is a common response to chronic liver disease. Hepatic fibrogenesis is a dynamic process that involves several liver cell types including hepatic stellate cells and Kupffer cells. In addition, recent evidence indicates that bile duct epithelial cells (i.e. cholangiocytes) also participate in the progression of biliary fibrosis that is observed during chronic cholestatic liver diseases, such as primary sclerosing cholangitis. To date, there are no effective treatments for hepatic fibrosis. Several recent studies have demonstrated that the renin-angiotensin system (RAS) plays a key role in hepatic fibrosis. Therapies targeting the RAS may represent a promising paradigm for the prevention and treatment of hepatic fibrosis in the setting of chronic liver disease. In this review, we provide a comprehensive update on the role of RAS in the pathogenesis of hepatic fibrosis in both animal models and human studies. We will discuss the profibrotic mechanisms activated by the RAS and the cell types involved. Studies that have utilized angiotensin receptor blockers (ARBs) and angiotensin-converting enzyme (ACE) inhibitors to modulate the RAS in order to ameliorate hepatic fibrosis will also be discussed. Although the cumulative evidence supports the potential for the use of ARBs and ACE inhibitors as treatment for hepatic fibrosis, extensive studies of the effectiveness of RAS therapeutics are necessary in patients with chronic liver disease.

Blood pressure and renal hemodynamic effects of angiotensin fragments

Angiotensin (Ang) II, the main effector peptide of the renin-Ang system, increases arterial blood pressure through Ang II type 1A (AT(1a)) receptor-dependent arterial vasoconstriction and by decreasing renal salt and water excretion through extrarenal and intrarenal mechanisms. AT(2) receptors are assumed to oppose these responses mediated by AT(1) receptors, thereby attenuating the pressor effects of Ang II. Nevertheless, a possible role of AT(2) receptors in the regulation of renal hemodynamics and sodium homeostasis remains to be unclear. Several other Ang fragments such as Ang III, Ang IV, Ang-(1-7) and Ang A have also been shown to display biological activity. In this review, we focus on the effects of these Ang on blood pressure, renal hemodynamics and sodium water handling, and discuss the receptors involved in these actions.

Phosphoinositide hydrolysis increase by angiotensin-(1–7) in neonatal rat brain

Angiotensin (Ang)-(1-7) is an endogenous peptide hormone of the renin-angiotensin system which exerts diverse biological actions, some of them counterregulate Ang II effects. In the present study potential effect of Ang-(1-7) on phosphoinositide (PI) turnover was evaluated in neonatal rat brain. Cerebral cortex prisms of seven-day-old rats were preloaded with [(3)H]myoinositol, incubated with additions during 30 min and later [(3)H]inositol-phosphates (IPs) accumulation quantified. It was observed that PI hydrolysis enhanced 30% to 60% in the presence of 0.01 nM to 100 nM Ang-(1-7). Neither 10 nM [D-Ala(7)]Ang-(1-7), an Ang-(1-7) specific antagonist, nor 10 nM losartan, an angiotensin II type 1 (AT(1)) receptor antagonist, blocked the effect of 0.1 nM Ang-(1-7) on PI metabolism. The effect of 0.1 nM Ang-(1-7) on PI hydrolysis was not reduced but it was even significantly increased in the simultaneous presence of [D-Ala(7)]Ang-(1-7) or losartan. PI turnover enhancement achieved with 0.1 nM Ang-(1-7) decreased roughly 30% in the presence of 10 nM PD 123319, an angiotensin II type 2 (AT(2)) receptor antagonist. The antagonists alone also enhanced PI turnover. Present findings showing an increase in PI turnover by Ang-(1-7) represent a novel action for this peptide and suggest that it exerts a function in this signaling system in neonatal rat brain, an effect involving, at least partially, angiotensin AT(2) receptors.

Biological properties of the angiotensin peptides other than angiotensin II: implications for hypertension and cardiovascular diseases

Several peptides of the RAS other than angiotensin (1-8) have been identified. They are generally referred as 'angiotensin fragments': Ang (2-8), Ang (3-8) and Ang (1-7) and have been detected in human tissues. There is evidence that they may play a functional role in humans by acting in concert with angiotensin (1-8) and aldosterone. Available knowledge on the pathways leading to synthesis and degradation of angiotensin fragments, as well as on their interactions with receptors and on their possible role in cardiovascular homeostasis and disease are reviewed.

Is "somatic" angiotensin I-converting enzyme a mechanosensor?

"Somatic" angiotensin I-converting enzyme (ACE) appears to be one of the evolutionary advances that made a closed circulation possible, and may have contributed to the Cambrian "explosion" of species approximately 540 million years ago. It also appears to be at the origin of a large number of common human diseases. A model is proposed in which the duplicated form of ACE ("somatic" ACE) functions as a mechanotransducer, defending downstream vessels and tissues from an increase in pressure. In the model, ACE senses shear stress (blood velocity) in regions of turbulent blood flow. An increase in shear stress strips an autoinhibitor tripeptide, FQP, from the N-terminal active site, thereby activating it. The C-terminal domain is constitutively activated by chloride. This model explains the clinical superiority of hydrophobic ACE inhibitors relative to hydrophilic ones.

Angiotensin(1-7) potentiates bradykinin-induced vasodilatation in man

BACKGROUND

It has been clearly demonstrated that angiotensin(1-7) potentiates the vasodilating effect of bradykinin in isolated vessels of animals.

OBJECTIVE

To investigate the interaction between angiotensin(1-7) Ang(1-7) and bradykinin in human forearm resistant vessels of normotensive healthy men in vivo, by the measurement of forearm blood flow using venous occlusion, strain-gauge plethysmography with intra-arterial infusions of peptides in a placebo-controlled, double-blind, cross-over design.

METHODS

In eight men, bradykinin was infused intra-arterially twice; placebo, Ang(1-7), or angiotensin II was co-infused with the second infusion. The effect of inhibition of nitric oxide synthase on the interaction between Ang(1-7) and bradykinin was also tested in eight other individuals. The effects of Ang(1-7) were analyzed by analysis of variance (ANOVA) and by the ratios of individually derived areas under the dose-response curves (AUC) of bradykinin, adjusted for changes in the AUCs by repeated infusions of bradykinin with placebo.

RESULTS

Ang(1-7) (1000 pmol/min) significantly potentiated the vasodilating effect of bradykinin compared with the effect of saline (P = 0.0471, ANOVA) and in a dose-dependent manner (adjusted AUC ratio [95% confidence interval (CI)] 2.75 (1.72 to 3.78) with 1000 pmol/min, 1.62 (1.31 to 1.93) with 100 pmol/min, and 0.98 (0.80, to 1.09) with 10 pmol/min). This effect was completely abolished by co-infusion of NG-monomethyl-l-arginine [AUC ratio 0.98 (0.90 to 1.04)]. Ang(1-7) did not affect the vasodilating effects of either acetylcholine or sodium nitroprusside.

CONCLUSIONS

Ang(1-7) potentiates the vasodilating effect of bradykinin, possibly through a mechanism(s) involving nitric oxide release, in human forearm resistance vessels.

Gender differences in the attenuation of salt-induced hypertension by angiotensin (1-7)

Chronic infusion of angiotensin (1-7) [Ang-(1-7)] lowers blood pressure in spontaneously hypertensive rats (SHR). To assess the role of Ang-(1-7) in salt-induced hypertension, Ang-(1-7) (24 microg/kg/hr) or saline was administered chronically via osmotic minipump into the jugular vein of 5-6 wk-old male (M) and female (F) Dahl salt-sensitive rats placed on a high-salt (8% NaCl) diet for 2 weeks. Blood pressure (BP) and heart rate were measured prior to the start of the diet and weekly thereafter. Ang-(1-7) significantly attenuated the BP increase after 1 wk on the diet in both M and F rats, but after 2 weeks only in F rats. Enhanced release of prostacyclin, (6-keto PGF1 alpha), following Ang-(1-7) treatment was observed in both M and F rats. In addition, significant increases in aortic blood flow and plasma levels of nitric oxide were observed in the F rats following Ang-(1-7) treatment. These findings demonstrate that the reduction in BP is due to both prostacyclin and NO and that there is a gender difference in the attenuation of salt-induced hypertension by Ang-(1-7).

Angiotensin-(1-7) does not affect vasodilator or TPA responses to bradykinin in human forearm

Studies in isolated vessels and rat models of hypertension suggest that angiotensin (Ang)-(1-7) potentiates the vasodilator effect of bradykinin, possibly through ACE inhibition. We therefore tested the hypothesis that Ang-(1-7) potentiates the vasodilator or tissue plasminogen activator (TPA) response to bradykinin in the human forearm vasculature. Graded doses of Ang-(1-7) (10, 100, and 300 pmol/min), bradykinin (47, 94, and 189 pmol/min), and Ang I (1, 10, and 30 pmol/min) were administered through the brachial artery to 8 normotensive subjects in random order. Thirty minutes after initiation of a constant infusion of Ang-(1-7) (100 pmol/min), bradykinin and Ang I infusions were repeated. There were no systemic hemodynamic effects of the agonists. Bradykinin significantly increased forearm blood flow (P<0.001, from 3.8+/-0.5 to 13.9+/-3.1 mL/min per 100 mL at 189 pmol/min) and net TPA release (P=0.007, from 1.1+/-1.0 to 23.6+/-6.2 ng/min per 100 mL at 189 pmol/min), whereas Ang I caused vasoconstriction (P=0.003, from 3.3+/-0.4 to 2.5+/-0.3 mL/min per 100 mL at 30-pmol/min dose). There was no effect of Ang-(1-7) on either forearm blood flow (P=0.62, 3.3+/-0.4 to 3.5+/-0.4 mL/min per 100 mL at 300 pmol/min) or TPA release (P=0.52, from 0.7+/-0.8 to 1.0+/-0.7 ng/min/100 mL at 300 pmol/min). Moreover, there was no effect of 100 pmol/min Ang-(1-7) on the vasodilator [P=0.46 for Ang-(1-7) effect] or TPA [P=0.82 for Ang-(1-7) effect] response to bradykinin or the vasoconstrictor response to Ang I [P=0.62 for Ang-(1-7) effect]. These data do not support a role of Ang-(1-7), given at supraphysiological doses, in the regulation of human peripheral vascular resistance or fibrinolysis.

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-(1-7) reduces norepinephrine release through a nitric oxide mechanism in rat hypothalamus

Angiotensin (Ang)-(1-7) elicits a facilitatory presynaptic effect on peripheral noradrenergic neurotransmission, and because biological responses to the heptapeptide on occasion are tissue specific, the present investigation was undertaken to study its action on noradrenergic neurotransmission at the central level. In rat hypothalamus labeled with [(3)H]-norepinephrine, 100 to 600 nmol/L Ang-(1-7) diminished norepinephrine released by 25 mmol/L KCl. This effect was blocked by the selective angiotensin type 2 receptor antagonist PD 123319 (1 micromol/L) and by the specific Ang-(1-7) receptor antagonist ([D-Ala(7)]Ang-(1-7) (1 micromol/L) but not by losartan (10 nmol/L to 1 micromol/L), a selective angiotensin type 1 receptor antagonist. The inhibitory effect on noradrenergic neurotransmission caused by Ang-(1-7) was prevented by 10 micromol/L N(omega)-nitro-L-arginine methylester, an inhibitor of nitric oxide synthase activity, and was restored by 100 micromol/L L-arginine, precursor of nitric oxide synthesis. Methylene blue (10 micromol/L), an inhibitor of guanylate cyclase considered as the target of nitric oxide action, as well as Hoe 140 (10 micromol/L), a bradykinin B(2)-receptor antagonist, prevented the inhibitory effect of the heptapeptide on neuronal norepinephrine release, whereas no modification was observed in the presence of 0.1 to 10 micromol/L indomethacin, a cyclooxygenase inhibitor. Our results indicate that Ang-(1-7) has a tissue-specific neuromodulatory effect on noradrenergic neurotransmission, being inhibitory at the central nervous system by a nitric oxide-dependent mechanism that involves angiotensin type 2 receptors and local bradykinin production.

Angiotensin-(1-7) attenuates vasoconstriction evoked by angiotensin II but not by noradrenaline in man

Angiotensin-(1-7) has been suggested to be a novel vasodilating peptide. We investigated the direct vascular effect of angiotensin-(1-7) in human forearm resistant vessels, particularly with regard to the interaction with angiotensin II, in healthy normotensive men by strain-gauge venous occlusion plethysmography with intra-arterial infusions of peptides. Intra-arterial infusion of angiotensin-(1-7) at 0.1 to 2000 pmol/min did not cause vasodilatation but rather reduced forearm blood flow by approximately 10% at the highest dose. A placebo-controlled study showed that angiotensin-(1-7) at 0.5 to 40 nmol/min caused weak but significant vasoconstriction (P=0.0016 by ANOVA). Angiotensin-(1-7) at 100 pmol/min, but not at 10 pmol/min, significantly shifted the angiotensin II dose-response curve toward the right (mean+/-SD of percent changes in forearm blood flow: -19+/-17%, -33+/-22%, -55+/-12%, -63+/-10%, and -68+/-5% at 5, 10, 25, 50, and 100 pmol/min of angiotensin II, respectively, with saline; 5+/-13%, 0. 9+/-18%, -40+/-16%, -54+/-9%, and -61+/-6% with angiotensin-(1-7), P=0.0021 by ANOVA). Angiotensin-(1-7) did not affect the dose-response curve of noradrenaline [3+/-12%, 5+/-16%, -20+/-22%, -31+/-18%, and -40+/-12% at 25, 50, 100, 300, and 600 pmol/min of noradrenaline, respectively, with saline; -4+/-15%, -2+/-23%, -29+/-22%, -34+/-16%, and -42+/-9% with angiotensin-(1-7)]. Our results suggest that angiotensin-(1-7) antagonizes vasoconstriction by angiotensin II in human resistant vessels and might act as an endogenous angiotensin II antagonist.

Angiotensin-(1-7) binds at the type 1 angiotensin II receptors in rat renal cortex

Significant angiotensin (Ang) (1-7) production occurs in kidney and effects on renal function have been observed. The present study was undertaken to investigate binding characteristics of the heptapeptide to Ang II receptors present in rat renal cortex. [125I]-Ang II binding to rat glomeruli membranes was analyzed in the presence of increasing concentrations of Ang II, Ang-(1-7), DUP 753 and PD 123319. Linearity of the Scatchard plot of the [125I]-Ang II specific binding to rat glomeruli membranes indicated a single population of receptors, with a Kd value of 0.7 +/- 0.1 nM and a Bmax of 198 +/- 0.04 fmol/mg protein. DUP 753, an specific AT1 receptor antagonist, totally displaced the specific binding of [125I]-radiolabelled hormone with a Ki of 15.8 +/- 0.9 nM, while no changes were observed in the presence of the selective AT2 receptor antagonist, PD 123319. The specific [125I]-Ang II binding to rat glomerular membranes was displaced by Ang-(1-7) with high affinity (Ki = 8.0 +/- 3.2 nM). We conclude that radioligand binding assays in the presence of selective Ang II antagonists DUP 753 and PD 123319 suggest the unique presence of AT1, receptors in rat glomeruli and a possible role in the control of the biological renal effects of Ang-(1-7).

Effect of angiotensin-(1-7) and bradykinin in patients with heart failure treated with an ACE inhibitor

Angiotensin-(1-7) is a product of angiotensin processing that has been proposed to have vasodepressor effects, both on its own and in combination with bradykinin, which may be pathophysiologically and therapeutically important. Despite this, there has been very little examination of its effects in humans and none in heart failure patients or in other patients treated with ACE inhibitors. We therefore sought to determine the effects of angiotensin-(1-7) in patients with heart failure treated with an ACE inhibitor, as well as any interaction with the effects of bradykinin. A locally active dose of angiotensin-(1-7), alone and in combination with bradykinin, was infused into the nondominant brachial artery while forearm blood flow was measured by venous occlusion plethysmography in 8 patients with heart failure treated with ACE inhibitors. Although bradykinin on its own caused profound vasodilation, there was no effect of angiotensin-(1 to 7) on its own or any effect of angiotensin-(1-7) on the response to bradykinin. We conclude that angiotensin-(1-7) is biologically inactive in the forearm circulation of patients with heart failure treated with an ACE inhibitor. The contrast between these findings and previously reported preclinical findings calls into question the relevance of angiotensin-(1-7) to the hemodynamic effects of ACE inhibitors.

Angiotensin-(1-7) is a modulator of the human renin-angiotensin system

The renin-angiotensin system is important for cardiovascular homeostasis. Currently, therapies for different cardiovascular diseases are based on inhibition of angiotensin-converting enzyme (ACE) or angiotensin II receptor blockade. Inhibition of ACE blocks metabolism of angiotensin-(1-7) to angiotensin-(1-5) and can lead to elevation of angiotensin-(1-7) levels in plasma and tissue. In animal models, angiotensin-(1-7) itself causes or enhances vasodilation and inhibits vascular contractions to angiotensin II. The function of angiotensin-(1-5) is unknown. We investigated whether angiotensin-(1-7) and angiotensin-(1-5) inhibit ACE or antagonize angiotensin-induced vasoconstrictions in humans. ACE activity in plasma and atrial tissue was inhibited by angiotensin-(1-7) up to 100%, with an IC(50) of 3.0 and 4.0 micromol/L, respectively. In human internal mammary arteries, contractions induced by angiotensin I and II and the non-ACE-specific substrate [Pro(11),D-Ala(12)]-angiotensin I were antagonized by angiotensin-(1-7) (10(-5) mol/L) in a noncompetitive way, with a 60% inhibition of the maximal response to angiotensin II. Contractions to ACE-specific substrate [Pro(10)]-angiotensin I were also inhibited, an effect only partly accounted for by antagonism of angiotensin II. Angiotensin-(1-5) inhibited plasma ACE activity with a potency equal to that of angiotensin I but had no effect on arterial contractions. In conclusion, angiotensin-(1-7) blocks angiotensin II-induced vasoconstriction and inhibits ACE in human cardiovascular tissues. Angiotensin-(1-5) only inhibits ACE. These results show that angiotensin-(1-7) may be an important modulator of the human renin-angiotensin system.

Potentiation of the hypotensive effect of bradykinin by angiotensin-(1-7)-related peptides

In this study, we evaluated the bradykinin potentiating activity and ACE inhibitory activity of several Ang-(1-7)-related peptides: Ang-(2-7), Ang-(3-7), Ang-(4-7), Ang-(1-6), Ang-(1-5) and the selective antagonist of Ang-(1-7): D-[Ala7]Ang-(1-7) (A-779). In vivo experiments were performed in freely moving Wistar rats. ACE activity was evaluated by a fluorometric assay in rat plasma using Hip-His-Leu as a substrate. Intravenous injections of Ang-(1-7) (2.2 nmol) transformed the effect of a single dose of bradykinin (1 nmol) into the effect produced by a double dose. A similar bradykinin potentiating activity was demonstrated for Ang-(2-7) and Ang-(3-7). On the other hand, Ang-(1-5), Ang-(1-6), Ang-(4-7) and A-779 did not change the hypotensive effect of bradykinin in doses ranging from 8 up to 25 nmols. The hypotensive effect of bradykinin was increased by intravenous infusion (0.3 ng/min) of Ang-(1-7) > Ang-(2-7) > Ang-(3-7). Conversely, Ang-(1-5), Ang-(1-6), Ang-(4-7) or A-779 did not change the hypotensive effect of bradykinin. ACE inhibition with Ang-(1-7) related peptides occurred in the order: Ang-(2-7) > or = Ang-(3-7) > Ang-(1-7) [>>] Ang-(1-5) > Ang-(4-7) > or = Ang-(1-6) > or = A-779. A-779 in concentrations up to 10(-5) M did not change the ACE inhibitory activity of Ang-(1-7). These results suggest that Ang-(1-7), Ang-(2-7) and Ang-(3-7) can modulate bradykinin actions in vivo. More important, our data pointed out that alternative mechanisms besides interaction with ACE are required to explain the bradykinin potentiating activity of Ang-(1-7).

Interaction of bradykinin and angiotensin-(1-7) in the central modulation of the baroreflex control of the heart rate

OBJECTIVE

Previous studies have shown that angiotensin-(1-7) potentiates the vascular actions of bradykinin. In the present study, we evaluated the interaction of bradykinin and angiotensin-(1-7) in the central modulation of baroreflex control of the heart rate.

MATERIALS AND METHODS

Blood pressure and reflex bradycardia, elicited by intravenous injection of phenylephrine, were evaluated in conscious male Wistar rats before and at the end of 1 h of an intracerebroventricular infusion of angiotensin-(1-7) at 0.5 or 1.0 microg/h combined with bradykinin at 2.5 microg/h; or angiotensin-(1-7) at 2.0 microg/h combined with bradykinin at 4.0 microg/h; or angiotensin-(1-7) alone at 2.0 or 4.0 microg/h; or bradykinin alone at 4.0 or 8.0 microg/h; or saline at 8 microl/h. In addition, baroreflex bradycardia was evaluated before and at the end of 1 and 2 h of intracerebroventricular infusion of angiotensin-(1-7) at 4 microg/h for 2 h; or saline at 8 microl/h in the first hour followed by HOE 140 at 90 ng/h in the second hour; or angiotensin-(1-7) at 4 microg/h in the first hour followed by angiotensin-(1-7) at 4 microg combined with HOE 140 at 90 ng/h in the second hour; or HOE 140 at 90 ng/h in the first hour followed by HOE 140 at 90th ng/h combined with angiotensin-(1-7) at 4 microg/h in the second hour; or saline at 8 microl/h for 2 h.

RESULTS

The intracerebroventricular infusion of angiotensin-(1-7) or bradykinin alone required a dose of 4.0 and 8.0 microg/h, respectively, to facilitate baroreflex control of the heart. However, a simultaneous infusion of these peptides at subeffective rates was able to produce a significant increase in baroreflex sensitivity. In addition, the facilitation of the baroreflex control of the heart rate induced by angiotensin-(1-7) at 4.0 microg/h was inhibited by HOE 140.

CONCLUSIONS

These results suggest that centrally, bradykinin and angiotensin-(1-7) can interact in order to modulate baroreflex control of the heart rate. In addition, our data indicate that the central modulatory effect of angiotensin-(1-7) on the baroreflex is mediated, at least in part, by the release of kinins.

Angiotensin-(1-7) and the rat aorta: modulation by the endothelium

Peptide metabolites of angiotensin I and II are active components of the renin-angiotensin system. One such peptide is angiotensin-(1-7), which has been shown to be present in various tissues and has properties distinct from those of angiotensin II. We examined the effects of angiotensin-(1-7) on endothelium-intact and denuded rat aorta. Second, we evaluated whether an interaction occurred between angiotensin-(1-7) and angiotensin peptides, as well as noradrenaline. Finally, we addressed whether the responses to angiotensin-(1-7) were mediated by an AT1 receptor. Angiotensin-(1-7) produced concentration-dependent relaxations of the rat aorta that were significantly greater in endothelium-intact preparations (81.1 +/- 18.9% and 29.6 +/- 2.9% for intact and denuded, respectively). Angiotensin-(1-7) inhibited responses generated to angiotensin I, II, III, and noradrenaline. In endothelium-denuded preparations, angiotensin-(1-7) produced a rightward shift of the concentration-effect curves to angiotensin II and noradrenaline. In addition, the inhibition against angiotensin I and II was significantly greater in endothelium-intact preparations [mean median inhibitory concentration (IC50) values for endothelium-intact preparations, 1.25 x 10(-9) M and 1.57 x 10(-9) M for angiotensin I and II, respectively; and for endothelium-denuded preparations, 1.77 x 10(-8) M and 1.17 x 10(-8) M for angiotensin I and II, respectively). Losartan did not affect relaxations in endothelium-intact preparations but caused a significant potentiation of the relaxation by angiotensin-(1-7) in denuded preparations. We conclude that angiotensin-(1-7) is a component of the renin-angiotensin system that acts to modulate the pressor effects of angiotensin II and noradrenaline.

Potentiation of the hypotensive effect of bradykinin by short-term infusion of angiotensin-(1-7) in normotensive and hypertensive rats

In this study we evaluated the effect of angiotensin-(1-7) on the hypotensive action of bradykinin (BK) in normotensive rats, renal hypertensive rats (RHR), and spontaneously hypertensive rats (SHR). In addition, we evaluated the effect of angiotensin-converting enzyme (ACE) inhibition with enalaprilat treatment (10 mg/kg I.V.) on the BK-potentiating activity of Ang-(1-7). Renal hypertension was produced by aorta coarctation between the origin of renal arteries. Ang-(1-7) (0.3 pmol/min) or saline (0.9% NaCl, 5 microL/min) was infused intravenously in conscious male Wistar rats, adult SHR, or RHR. Intravenous bolus injections of BK (0.1 to 1.6 nmol in RHR and SHR; 0.625 to 5 nmol in Wistar rats) were made before and within 30 and 60 minutes of Ang-(1-7) infusion. Ang-(1-7) infusion did not change mean arterial pressure (MAP) of Wistar rats (MAP=97+/-3 mm Hg), RHR (MAP=173+/-3 mm Hg), or SHR (MAP=177+/-5 mm Hg). In Wistar rats, Ang-(1-7) increased the BK hypotensive effect by 24+/-6% within 60 minutes of infusion. No significant changes were observed at 30 minutes of infusion. In additional groups of rats, Ang-(1-7) (5 pmol/min, n=5) was infused alone or combined with its selective antagonist D-Ala7-Ang-(1-7) (A-779) (5 pmol/min, n=6). The bradykinin-potentiating activity of Ang-(1-7) was completely abolished by A-779. In SHR and RHR, Ang-(1-7) significantly increased the hypotensive effect of BK by 59+/-8% and 57+/-9.8%, respectively, within 60 minutes of infusion. No significant changes were observed with saline infusion. In Wistar rats, enalaprilat treatment increased the BK-potentiating activity of Ang-(1-7) transforming the effect of 0.3 pmol/min into that observed with a rate 16-fold higher (5 pmol/min). On the other hand, in SHR enalaprilat did not change the Ang-(1-7) effect, while it abolished the BK potentiation in RHR. Our data show that the BK-potentiating activity of Ang-(1-7) is preserved and even augmented in hypertensive rats. The finding that the BK-potentiating activity of Ang-(1-7) could be demonstrated at a very low infusion rate suggests that this angiotensin can act as an endogenous modulator of the vascular actions of kinins. ACE inhibition can influence differently the BK-potentiating activity of Ang-(1-7) in normotensive and hypertensive rats.

Possible participation of nitric oxide in the increase of norepinephrine release caused by angiotensin peptides in rat atria

In rat atria isolated with their cardioaccelerans nerves and labeled with [3H]norepinephrine, exposure to 1 x 10(-7) mol/L angiotensin II (Ang II) and 1 x 10(-7) mol/L Ang-(1-7) increased the release of radioactivity elicited by nerve stimulation (0.5-millisecond-long square-wave pulses at 2 Hz during 2 minutes) by 90% and 60%, respectively. The facilitatory effect on noradrenergic neurotransmission caused by both peptides was stereospecifically prevented by N omega-nitro-L-arginine methyl ester (1 x 10(-4) mol/L), an inhibitor of nitric oxide synthase that catalyzes the conversion of L-arginine to nitric oxide, as well as by 1 x 10(-5) mol/L methylene blue, a substance that inhibits the guanylate cyclase considered as the final target of nitric oxide action. On the other hand, the precursor of nitric oxide synthesis. L-arginine (1 x 10(-3) mol/L), reversed the prevention produced by N omega-nitro-L-arginine methyl ester on the increased release of norepinephrine caused by Ang II and Ang-(1-7). The present results suggest that nitric oxide could be involved in the neuromodulatory function elicited by both Ang II and Ang-(1-7) in rat atria. The physiological role of this observation is still under study.

Angiotensin-(1-7) potentiates the hypotensive effect of bradykinin in conscious rats

Treatment with angiotensin-converting enzyme inhibitors increases the angiotensin-(1-7) [Ang-(1-7)] and bradykinin concentrations in plasma and tissue. In this study we evaluated the interaction between these peptides by determining the effect of Ang-(1-7) on the hypotensive action of bradykinin in conscious rats. Administration of Ang-(1-7) (5 nmol) did not change mean arterial pressure or heart rate. However, the hypotensive effect of bradykinin, produced by an intravenous or intra-arterial route, was potentiated by Ang-(1-7) in a dose-dependent manner. The Ang-(1-7) doses necessary to transform the effect of a single dose of bradykinin into that produced by a double dose (potentiating unit) were 2 nmol i.v. and 5 nmol IA. The Ang-(1-7) dose used did not change either the pressor effect of Ang II or the hypotensive effect of sodium nitroprusside. The bradykinin-potentiating Ang-(1-7) activity was significantly attenuated by pretreatment with indomethacin (5 mg/kg IM, n = 4). In an additional group the bradykinin-potentiating activity of Ang-(1-7) was evaluated 30 minutes after treatment with the angiotensin-converting enzyme inhibitor enalaprilat (10 mg/kg i.v., n = 9). Under this condition the bradykinin-potentiating activity of Ang-(1-7) was substantially increased, resulting in a potentiating unit of approximately 0.2 nmol IV. Pretreatment with indomethacin (5 mg/kg IM, n = 7) also attenuated the bradykinin-potentiating activity of Ang-(1-7) in enalaprilat-treated rats. These results show that Ang-(1-7) is a bradykinin-potentiating peptide in vivo. Furthermore, the data obtained with indomethacin suggest that prostaglandins participate in the mechanism of the bradykinin potentiation by Ang-(1-7). More importantly, these data suggest that the interaction between Ang-(1-7) and bradykinin can contribute to the pharmacological effects of angiotensin-converting enzyme inhibitors.

Cardiovascular actions of angiotensin(1-7)

Angiotensin(1-7) had a compound effect on blood pressure of pithed Sprague-Dawley rats. The initial phase of the response consisted of an increase in MAP of short duration and independent of injected dose, followed by a decline of arterial pressure to values below baseline. Both the magnitude (range: -4 +/- 1 to -13 +/- 1 mmHg) and the duration (range: 83 +/- 13 to 255 +/- 17 s) of the depressor response correlated with the dose of peptide. Indomethacin (5 mg/kg) eliminated the depressor component. Only [Sar1,Thr8]Ang II inhibited the effect of Ang(1-7) completely. We conclude that angiotensin(1-7) possesses myotonic actions that are in part related to release of vasodilator prostaglandins through an angiotensin receptor other than AT1 or AT2.

Differential responses to angiotensin-(1-7) in the feline mesenteric and hindquarters vascular beds

Regional vascular responses to angiotensin (Ang)-(1-7), a heptapeptide derivative of Ang II were investigated in the feline hindquarters and mesenteric vascular beds under conditions of controlled flow. In the mesenteric vascular bed, injections of Ang-(1-7) in doses of 1, 3 and 10 micrograms produced dose-dependent decreases in mesenteric perfusion pressure whereas at doses of 30 and 100 micrograms, increases were observed. In contrast, in the hindquarters circulation, low doses produced increases while high doses produced decreases in perfusion pressure. In both vascular beds the degree of vasoconstriction was weak, being less than 1% of that elicited by Ang II. The vasoconstrictor effect of Ang-(1-7) in both the mesenteric and hindquarters vascular bed was blocked by DuP 753 (1 mg/kg i.v.), an Ang receptor subtype 1 (AT1) antagonist. The vasodilator responses in both vascular beds were partially blocked by the nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (100 mg/kg i.v.) but were unaffected by the cyclooxygenase inhibitor, meclofenamate (2.5 mg/kg i.v.). The present results show that in the peripheral vascular bed of the cat, Ang-(1-7) causes vasodilation or modest vasoconstriction, depending on the dose and the regional vascular bed studied. The present data also suggest that the vasodilator effect of the peptide may be mediated in part by the release of endothelium-derived relaxing factor and the vasoconstrictor effect by activation of the AT1 receptor subtype.

Electrophysiological responses of angiotensin peptides on the rat isolated nodose ganglion

Previous autoradiographic studies have identified angiotensin II (AII) binding sites over the nodose ganglion and along the vagal afferent neurons. In the present study, we examined whether these binding sites are functional receptors by measuring d.c. potential changes by extracellular recording techniques in the rat isolated nodose ganglion preparation in response to superfusion of angiotensin peptides. It was found that AII, as well as AI and AIII elicited concentration-dependent depolarisation of the nodose ganglion. However, the amino terminal angiotensin heptapeptide, A(1-7), failed to evoke any significant response. The AII receptor antagonist, saralasin had no intrinsic activity, but caused a concentration-dependent blockade of AII-induced depolarisation. This study provides evidence for direct neuronal effects of angiotensin peptides on rat vagal afferent neurons. Moreover, this preparation is a relatively convenient one in which to study functional neuronal AII receptor mechanisms on central or peripheral terminals of vagal sensory neurons.

Neurophysiological responses to angiotensin-(1-7)

The aim of this study was to investigate the action of the heptapeptide angiotensin-(1-7) on the spontaneous activity of paraventricular neurons using microiontophoresis. Recent immunocytochemical investigations have shown that this product of angiotensin I is predominantly located in cells and fibers of the forebrain and brain stem. Our results show that most neurons in the paraventricular nucleus are excited by angiotensin-(1-7) at a dose of 50-80 nA. In comparison with angiotensin II or angiotensin III, the onset of response and the occurrence of the maximal effect were significantly delayed. With higher doses of angiotensin-(1-7), there was a decrease in latency and a dose-dependent increase in firing frequency. Of all the angiotensin compounds tested, angiotensin III was the most potent. Preliminary results obtained with an angiotensin antagonist show that the action of angiotensin II, angiotensin III, and angiotensin-(1-7) is blocked by the angiotensin receptor subtype 2 antagonist CGP 42112A. Because the angiotensin-(1-7) system in the brain is associated with central vasopressinergic pathways, vasopressin was tested in a similar way. Neurons in the paraventricular nucleus that were excited by iontophoretically applied angiotensins showed a weak response to vasopressin. Occasionally, a small excitatory action was observed. Our results support the hypothesis that the heptapeptide angiotensin-(1-7) is a biologically active neuropeptide. The data also suggest that amino terminal fragments of angiotensin II are not inactive degradation products.

Prostaglandin production in response to angiotensin-(1-7) in rabbit isolated vasa deferentia

Angiotensin-(1-7) is a predominant metabolite of angiotensin I in brain tissue. Its neuromodulatory and prostaglandin (PG) synthesizing capabilities were investigated in the rabbit isolated vas deferens. This metabolite had no significant effect as a neuromodulator, however it increased PGE synthesis in the vasa deferentia with a potency equivalent to that of angiotensin II. The angiotensin-(1-7) has a unique spectrum of activity among the angiotensin peptides to selectively increase PG synthesis. It could be useful in defining the relevance of angiotensin-induced PG synthesis in various systems, particularly in neuronal tissue. Angiotensin-(1-7) potentially could be useful in defining angiotensin receptor subtypes, as well.

Biological activity of angiotensin-(1-7) heptapeptide in the hamster heart

Angiotensin II has been reported to have both a positive inotropic effect and a coronary constrictor action in the hamster heart. To study the contribution to these responses of phenylalanine in position 8, we assessed the direct cardiac effects of angiotensin-(1-7), which lacks phenylalanine in position 8. Syrian hamsters were used to determine the effects of angiotensin-(1-7) on cardiac performance in the diseased and normal hearts. We used the isolated isovolumic heart preparation perfused either at a constant pressure of 50 mm Hg or at a constant coronary (myocardial) flow rate of 7 ml/min (seven cardiomyopathic hamsters [CMH] and seven normal hamsters [NH] in each subgroup). At constant perfusion pressure, coronary (myocardial) flow rate decreased (p less than 0.01) in both CMH and NH (-31 +/- 8% vs. -39 +/- 4% of baseline, respectively); but the percent decrease in left ventricular pressure and the first derivative of left ventricular pressure over time (LV + dP/dt) was significant only in NH (-8 +/- 1% and -9 +/- 4%) but not in CMH (-14 +/- 5% and -21 +/- 8%). On the other hand, at a constant coronary (myocardial) flow rate, left ventricular pressure and LV + dP/dt tended to increase in both CMH and NH (+10 +/- 3% and +6 +/- 2% of baseline vs. +7 +/- 7% and +7 +/- 5%, respectively) but these changes were not significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for extracellular, but not intracellular, generation of angiotensin II in the rat adrenal zona glomerulosa

Based on the observation that high levels of renin and angiotensin II (Ang II) are found in the adrenal zona glomerulosa (ZG), it has been postulated that Ang II is formed intracellularly by the renin-converting enzyme cascade in this tissue. To test this hypothesis, we examined renin-angiotensin system components in subcellular fractions of the rat adrenal ZG. Renin activity and immunoreactive-Ang II (IR-Ang II) were observed in vesicular fractions but were not colocalized. In addition, angiotensinogen, angiotensin I, and converting enzyme were not observed in the renin or IR-Ang II-containing vesicular fractions. These data do not support the hypothesis that Ang II is formed intracellularly within the renin-containing vesicles of the ZG. Rather, since modulatable renin release from adrenal ZG slices was observed and renin activity was found in dense vesicular fractions (33-39% sucrose), it is likely that Ang II formation in the ZG is extracellular and initiated by the release of vesicular renin. Receptor-mediated endocytosis and subsequent degradation of Ang II in ZG lysosomes have been shown by others. The presence of IR-Ang II in light vesicular fractions (15% sucrose) and the finding of a high correlation between ZG IR-Ang II and Ang II receptor levels suggest that the primary occurrence of this peptide in the ZG is by receptor-mediated endocytosis. In ZG lysosomal fractions 125I-labeled Ang II was degraded to 125I-labeled des-[Phe8]Ang II. Since Ang II antibodies do not recognize des-[Phe8]Ang II, these findings explain why IR-Ang II in the ZG is due predominantly to Ang II and not to its C-terminal immunoreactive fragments.

Release of vasopressin from the rat hypothalamo-neurohypophysial system by angiotensin-(1-7) heptapeptide

We have recently shown that hydrolysis of labeled angiotensin I in canine brainstem homogenate causes a rapid accumulation of the heptapeptide angiotensin-(1-7) [Ang-(1-7)]. Although this angiotensin fragment has no vasopressor activity, its consistent generation in brain homogenate led us to study its potential neurosecretory effects in the rat hypothalamo-neurohypophysial system (HNS) in vitro. Ang-(1-7) or angiotensin II (Ang II) was added to HNS perifusate in concentrations of 0.04, 0.4, and 4 microM, and release of arginine vasopressin (AVP) during each treatment was quantified as a percentage of the AVP release detected in the preceding collection period. Base-line release of AVP averaged 281 +/- 47 pg per 15 min (mean +/- SEM) in HNS explants (five experiments, five explants per chamber) perifused in Krebs solution at 37 degrees C, after a 1-hr equilibration period. At 0.04 microM, Ang II or Ang-(1-7) did not stimulate AVP release. Ang II increased AVP release over the control value by 172% +/- 44% and 268% +/- 66% at 0.4 and 4 microM, respectively; the same concentrations of Ang-(1-7) increased AVP release by 134% +/- 12% and 216% +/- 45%. The responses to Ang II and Ang-(1-7) at the highest concentration were both significant (P less than 0.05), and comparison by two-way analysis of variance indicated that Ang II and Ang-(1-7) were equipotent in stimulating AVP release over the range of concentrations studied. In the presence of the competitive Ang II antagonist [Sar1,Thr8]Ang II (20 microM), the release of AVP increased approximately equal to 2-fold. Neither Ang II nor Ang-(1-7) (4 microM) caused a further enhancement of AVP release in the presence of [Sar1,Thr8]Ang II. These data suggest that a hydrophobic residue in position 8 of the angiotensin peptide is not essential for activation of angiotensin receptors in the rat HNS. Moreover, the equipotence of Ang II and Ang-(1-7) indicates that Ang-(1-7) may participate in the control of AVP release.

Characterization by high performance liquid chromatography of angiotensin peptides in the plasma and cerebrospinal fluid of the dog

A high performance liquid chromatography (HPLC) method is described for the separation of angiotensin (Ang) peptides and their subsequent quantification by radioimmunoassay in plasma and cerebrospinal fluid (CSF). The use of the ion-pair solvent heptafluorobutyric acid in gradient HPLC achieves baseline resolution of Ang I, Ang II, and the C-terminal fragments des-[Asp1]-Ang I, des-[Asp1]-Ang II, des-[Asp1,Arg2]-Ang II and des-[Asp1,Arg2,Val3]-Ang II in approximately 25 min. Recovery of synthetic Ang standards after phenylsilica extraction and HPLC separation was greater than 70% for each peptide in both plasma and CSF. Ang I and Ang II were shown to be the major immunoreactive Ang components in plasma, and Ang II, des-[Asp1,Arg2]-Ang II and des-[Asp1,Arg2,Val3]-Ang II in CSF.