Pathways of angiotensin formation and function in the brain

Ang-(1-7)/MasR axis promotes functional recovery after spinal cord injury by regulating microglia/macrophage polarization

BACKGROUND

Inflammatory response is an essential part of secondary injury after spinal cord injury (SCI). During this period, the injury may be exacerbated through the release of a large number of inflammatory factors and the polarization of infiltrating macrophages and microglia towards M1. Ang-(1-7), mainly generated by Ang II via angiotensin-converting enzyme 2 (ACE2), can specifically bind to the G protein-coupled receptor Mas (MasR) and plays an important role in regulating inflammation and alleviating oxidative stress.

METHODS

We aimed to investigate whether activating the Ang-(1-7)/MasR axis in rats after SCI can regulate local neuroinflammation to achieve functional recovery and obtain its potential mechanism. MasR expression of bone marrow-derived macrophages was determined by Western blot. Immunofluorescence, Western blot, Flow cytometry, and RT-qPCR were applied to evaluate the polarization of Ang-(1-7) on macrophages and the regulation of inflammatory cytokines. Previous evaluation of the spinal cord and bladder after SCI was conducted by hematoxylin-eosin staining, Basso, Beattie, and Bresnahan (BBB) score, inclined plate test, electrophysiology, and catwalk were used to evaluate the functional recovery of rats.

RESULTS

MasR expression increased in macrophages under inflammatory conditions and further elevated after Ang-(1-7) treatment. Both in vivo and in vitro results confirmed that Ang-(1-7) could regulate the expression of inflammatory cytokines by down-regulating proinflammatory cytokines and up-regulating anti-inflammatory cytokines, and bias the polarization direction of microglia/macrophages to M2 phenotypic. After SCI, Ang-(1-7) administration in situ led to better histological and functional recovery in rats, and this recovery at least partly involved the TLR4/NF-κB signaling pathway.

CONCLUSION

As shown in our data, activating Ang-(1-7)/MasR axis can effectively improve the inflammatory microenvironment after spinal cord injury, promote the polarization of microglia/macrophages towards the M2 phenotype, and finally support the recovery of motor function. Therefore, we suggest using Ang-(1-7) as a feasible treatment strategy for spinal cord injury to minimize the negative consequences of the inflammatory microenvironment after spinal cord injury.

Angiotensin II and the Cardiac Parasympathetic Nervous System in Hypertension

The renin-angiotensin-aldosterone system (RAAS) impacts cardiovascular homeostasis via direct actions on peripheral blood vessels and via modulation of the autonomic nervous system. To date, research has primarily focused on the actions of the RAAS on the sympathetic nervous system. Here, we review the critical role of the RAAS on parasympathetic nerve function during normal physiology and its role in cardiovascular disease, focusing on hypertension. Angiotensin (Ang) II receptors are present throughout the parasympathetic nerves and can modulate vagal activity via actions at the level of the nerve endings as well as via the circumventricular organs and as a neuromodulator acting within brain regions. There is tonic inhibition of cardiac vagal tone by endogenous Ang II. We review the actions of Ang II via peripheral nerve endings as well as via central actions on brain regions. We review the evidence that Ang II modulates arterial baroreflex function and examine the pathways via which Ang II can modulate baroreflex control of cardiac vagal drive. Although there is evidence that Ang II can modulate parasympathetic activity and has the potential to contribute to impaired baseline levels and impaired baroreflex control during hypertension, the exact central regions where Ang II acts need further investigation. The beneficial actions of angiotensin receptor blockers in hypertension may be mediated in part via actions on the parasympathetic nervous system. We highlight important unknown questions about the interaction between the RAAS and the parasympathetic nervous system and conclude that this remains an important area where future research is needed.

Renin-angiotensin system blockade in the COVID-19 pandemic

In the early months of the coronavirus disease 2019 (COVID-19) pandemic, a hypothesis emerged suggesting that pharmacologic inhibitors of the renin–angiotensin system (RAS) may increase COVID-19 severity. This hypothesis was based on the role of angiotensin-converting enzyme 2 (ACE2), a counterregulatory component of the RAS, as the binding site for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), allowing viral entry into host cells. Extrapolations from prior evidence led to speculation that upregulation of ACE2 by RAS blockade may increase the risk of adverse outcomes from COVID-19. However, counterarguments pointed to evidence of potential protective effects of ACE2 and RAS blockade with regard to acute lung injury, as well as substantial risks from discontinuing these commonly used and important medications. Here we provide an overview of classic RAS physiology and the crucial role of ACE2 in systemic pathways affected by COVID-19. Additionally, we critically review the physiologic and epidemiologic evidence surrounding the interactions between RAS blockade and COVID-19. We review recently published trial evidence and propose important future directions to improve upon our understanding of these relationships.

Renin-angiotensin system in vertebrates: phylogenetic view of structure and function

Renin substrate, biological renin activity, and/or renin-secreting cells in kidneys evolved at an early stage of vertebrate phylogeny. Angiotensin (Ang) I and II molecules have been identified biochemically in representative species of all vertebrate classes, although variation occurs in amino acids at positions 1, 5, and 9 of Ang I. Variations have also evolved in amino acid positions 3 and 4 in some cartilaginous fish. Angiotensin receptors, AT₁ and AT₂ homologues, have been identified molecularly or characterized pharmacologically in nonmammalian vertebrates. Also, various forms of angiotensins that bypass the traditional renin-angiotensin system (RAS) cascades or those from large peptide substrates, particularly in tissues, are present. Nonetheless, the phylogenetically important functions of RAS are to maintain blood pressure/blood volume homeostasis and ion-fluid balance via the kidney and central mechanisms. Stimulation of cell growth and vascularization, possibly via paracrine action of angiotensins, and the molecular biology of RAS and its receptors have been intensive research foci. This review provides an overview of: (1) the phylogenetic appearance, structure, and biochemistry of the RAS cascade; (2) the properties of angiotensin receptors from comparative viewpoints; and (3) the functions and regulation of the RAS in nonmammalian vertebrates. Discussions focus on the most fundamental functions of the RAS that have been conserved throughout phylogenetic advancement, as well as on their physiological implications and significance. Examining the biological history of RAS will help us analyze the complex RAS systems of mammals. Furthermore, suitable models for answering specific questions are often found in more primitive animals.

A new strategy for treating hypertension by blocking the activity of the brain renin-angiotensin system with aminopeptidase A inhibitors

Hypertension affects one-third of the adult population and is a growing problem due to the increasing incidence of obesity and diabetes. Brain RAS (renin-angiotensin system) hyperactivity has been implicated in the development and maintenance of hypertension in several types of experimental and genetic hypertension animal models. We have identified in the brain RAS that APA (aminopeptidase A) and APN (aminopeptidase N), two membrane-bound zinc metalloproteases, are involved in the metabolism of AngII (angiotensin II) and AngIII (angiotensin III) respectively. The present review summarizes the main findings suggesting that AngIII plays a predominant role in the brain RAS in the control of BP (blood pressure). We first explored the organization of the APA active site by site-directed mutagenesis and molecular modelling. The development and the use in vivo of specific and selective APA and APN inhibitors EC33 and PC18 respectively, has allowed the demonstration that brain AngIII generated by APA is one of the main effector peptides of the brain RAS, exerting a tonic stimulatory control over BP in conscious hypertensive rats. This identified brain APA as a potential therapeutic target for the treatment of hypertension, which has led to the development of potent orally active APA inhibitors, such as RB150. RB150 administered orally in hypertensive DOCA (deoxycorticosteroneacetate)-salt rats or SHRs (spontaneously hypertensive rats) crosses the intestinal, hepatic and blood-brain barriers, enters the brain, generates two active molecules of EC33 which inhibit brain APA activity, block the formation of brain AngIII and normalize BP for several hours. The decrease in BP involves two different mechanisms: a decrease in vasopressin release into the bloodstream, which in turn increases diuresis resulting in a blood volume reduction that participates in the decrease in BP and/or a decrease in sympathetic tone, decreasing vascular resistance. RB150 constitutes the prototype of a new class of centrally acting antihypertensive agents and is currently being evaluated in a Phase Ib clinical trial.

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.

Angiotensin-(1-7), Angiotensin-Converting Enzyme 2, and New Components of the Renin Angiotensin System

The discovery of angiotensin-(1-7) [Ang-(1-7)] in 1988 represented the first deviation from the traditional biochemical cascade of forming bioactive angiotensin peptides. Prior to that time, the biological actions of angiotensin II (Ang II) were being investigated as it relates to cardiovascular function, including hypertension, cardiac hypertrophy and failure, as well as biological actions in the brain and kidney. We now know that Ang II elicits a whole host of actions both within and outside of the cardiovascular system. Furthermore, the discovery of Ang-(1-7) by our laboratory was also the first indication of a biologically active angiotensin peptide that further studies revealed served to counter-balance the actions of Ang II. This chapter reviews the data demonstrating the role of the vasodepressor axis of the renin angiotensin system in the regulation of cardiovascular function and the new data that shows the existence of angiotensin-(1-12) as a novel alternate substrate for the production of angiotensin peptides. The ultimate role of this discovery, as well as the continuing elucidation of mechanisms pertaining to RAS physiology, will likely be clarified in the coming years, in hopes of improving the treatment of cardiovascular disease.

Influence of gender and genetic variability on plasma angiotensin peptides

INTRODUCTION

We analysed the influence of three polymorphisms of the renin-angiotensin system (RAS) (I/D from angiotensin-converting enzyme [ACE], M235T from angiotensinogen gene [ATG] and A1166C from AT1 receptors) on plasma levels of angiotensin I (Ang I), angiotensin II (Ang II) and angiotensin-(1-7) [Ang-(1-7)].

MATERIALS AND METHODS

The study population consisted of a homogeneous group of 93 healthy subjects (43 men and 50 women, mean age: 20.67+/-2.75 years). The mean blood pressure (BP) was 126+/-7/76+/-5 (SD) mmHg and the mean body mass index (BMI) was 22.4+/-2.5 kg/m2. Angiotensin peptides were separated by high performance liquid chromatography (HPLC) and quantified by radio immuno assay (RIA). Genotypes were determined by polymerase chain reaction (PCR) and restriction enzyme analysis.

RESULTS

Mean peptide levels were 92.48+/-102.12 pg/ml for Ang I, 22.35+/-10 pg/ml for Ang II, and 31.65+/-27.46 pg/ml for Ang-(1-7). Men had significantly higher levels of Ang-(1-7) (37.76+/-36.47 pg/ml) than women (26.04+/-13.98 pg/ml) (p<0.05). Among genotypes of each polymorphism, men with the T allele showed higher Ang- (1-7) levels compared with those with the MM genotype (p<0.05). Genotype analysis in women showed that higher Ang I levels were related with the DD genotype. When both genders were compared according to genotype, higher values of Ang-(1-7) levels and its molar ratios were found in men, and there was significantly greater Ang I levels in DD genotypes in women than men (136.72+/-112.43 vs . 65.36+/-46.83 pg/mL).

CONCLUSIONS

Significant correlations were found between Ang I and Ang II as well as between Ang II and Ang-(1-7) in the different study group distributions. No correlation was found between levels of Ang I and Ang-(1-7). Certain genotypes exert an influence on angiotensin peptide plasma levels which can only be seen when the population is divided according to gender.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Analysis of the mechanisms underlying the vasorelaxant action of angiotensin II in the isolated rat carotid

It has been suggested that low concentrations of angiotensin II cause vasoconstriction whereas high concentrations evoke vasodilation. Thus, this work aimed to functionally characterize the mechanisms underlying the relaxation induced by angiotensin II at high concentrations in isolated rat carotid rings. Experiments using standard muscle bath procedures showed that angiotensin II (0.01-3 microM) concentration dependently induces relaxation of phenylephrine-pre-contracted rings. No differences between intact or denuded endothelium were found. The angiotensin II-induced relaxation was strongly inhibited by saralasin, the non-selective antagonist of angiotensin II receptors but not by the selective antagonists of AT1 and AT2 receptors, losartan and PD123319, respectively. However, A-779, a selective angiotensin-(1-7) receptor antagonist, reduced the relaxation induced by angiotensin II. Administration of exogenous angiotensin-(1-7) on pre-contracted tissues produced concentration-dependent relaxation, which was also inhibited by A-779. HOE-140, the selective antagonist of the bradykinin in B2 receptor did not produce any significant effect on angiotensin II-induced relaxation. Pre-incubation of denuded-rings with N G-nitro-L-arginine methyl ester (L-NAME) or 1H-[1,2,4] Oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) reduced angiotensin II-induced relaxation. On the other hand, neither indomethacin nor tetraethylammonium (TEA) produced any significant effect. The major new finding of this work is that high concentrations of angiotensin II induce relaxation of the rat carotid via activation of the NO-cGMP pathway through a mechanism that seems to be partially dependent on activation of angiotensin-(1-7) receptors.

The role of bradykinin, AT2 and angiotensin 1-7 receptors in the EDRF-dependent vasodilator effect of angiotensin II on the isolated mesenteric vascular bed of the rat

1. The mechanisms involved in the vasodilator actions of angiotensin II (Ang II) have not yet been completely elucidated. We investigated the potential mechanisms that seem to be involved in the Ang II vasodilator effect using rat isolated mesenteric vascular bed (MVB). 2. Under basal conditions, Ang II does not affect the perfusion pressure of MVB. However, in vessels precontracted with norepinephrine, Ang II induces vasodilation followed by vasoconstriction. Vasoconstrictor, but not the vasodilation of Ang II, is inhibited by AT(1) antagonist (losartan). The vasodilator effect of Ang II was not inhibited by AT(2), angiotensin IV and angiotensin 1-7 receptor antagonists alone (PD 123319, divalinal, A 779, respectively). 3. The vasodilator effect of Ang II is significantly reduced by endothelial removal (deoxycholic acid), but not by indomethacin. Inhibition of NO-synthase by N(G)-nitro-l-arginine methyl ester (l-NAME) and guanylyl cyclase by 1H-[1,2,3] oxadiazolo [4,4-a] quinoxalin-1-one (ODQ) reduces the vasodilator effect of Ang II. This effect is also reduced by tetraethylammonium (TEA) or l-NAME, and a combination of l-NAME plus TEA increases the inhibitory effect of the antagonists alone. However, indomethacin does not change the residual vasodilator effect observed in vessels pretreated with l-NAME plus TEA. 4. In vessels precontracted with norepinephrine and depolarized with KCl 25 mm or treated with Ca(2+)-dependent K(+) channel blockers (charybdotoxin plus apamin), the effect of Ang II was significantly reduced. However, this effect is not affected by ATP and voltage-dependent K(+) channel blockers (glybenclamide and 4-aminopyridine). 5. Inhibition of kininase II with captopril significantly potentiates the vasodilator effect of bradykinin (BK) and Ang II in the rat MVB. The inhibitory effect of the B(2) receptor antagonist HOE 140 on the vasodilator effect of Ang II is further enhanced by PD 123319 and/or A 779. 6. The present findings suggest that BK plays an important role in the endothelium-dependent vasodilator effect of Ang II. Probably, the link between Ang II and BK release is modulated by receptors that bind PD 123319 and A 779.

A method to evaluate the renin-angiotensin system in rat renal cortex using a microdialysis technique combined with HPLC-fluorescence detection

A microdialysis (MD) technique, combined with HPLC-fluorescence (FL) detection, was developed for the evaluation of the tissue-specific renin-angiotensin system (RAS) in the rat renal cortex. An MD probe constructed with a hydrophilic hollow fiber dialysis tubing, AN69, showed high recovery (more than 50%) in vitro for all four angiotensins: angiotensin I (Ang I), Ang II, Ang III, and Ang (1-7). Angiotensins, successfully derivatized with m-BS-ABD-F, a water-soluble fluorogenic reagent that has a 2,1,3-benzoxadiazole (benzofurazan) structure, could be simultaneously determined by coupled-column HPLC. The detection limit for Ang I, Ang II, Ang III, and Ang (1-7) were 94, 44, 47, and 83 fmol, respectively. All these peptides were determined with good linearity (0.0125-3.1 microM, equivalent to 0.25-62 pmol, correlation coefficient >0.99) and good precision (recovery >91%). In the MD studies, generation of Ang (1-7) and Ang II was observed when Ang I was perfused, and Ang (1-7) was the major biologically active angiotensin found in the dialysate samples. The concentration of Ang (1-7) and Ang II in the dialysate samples showed good correlation to that of Ang I in a MD perfusate (20-100 microM). Cleavage of Ang I to Ang (1-7) was drastically suppressed by the co-perfusion of phoshoramidon (0.5-5 mM), an inhibitor of neprilysin, which generates Ang (1-7) from Ang I. These results are consistent with the previously reported characteristics of tissue-specific renal RAS, suggesting that our MD/HPLC-FL system may have the potential to be employed to evaluate tissue-specific RAS in the rat renal cortex.

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.

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.

Angiotensin-(1--7) in the ovine fetus

In the adult animal, ANG-(1-7) may counterbalance some effects of ANG II. Its effects in the fetus are unknown. Basal ANG-(1-7), ANG I, ANG II, and renin concentrations were measured in plasma from ovine fetuses and their mothers (n = 10) at 111 days of gestation. In the fetus, concentrations of ANG I, ANG-(1-7), and ANG II were 86 +/- 21, 13 +/- 2, and 14 +/- 2 fmol/ml, respectively. In the ewe, concentrations of ANG I were significantly lower (20 +/- 4 fmol/ml, P < 0.05) as were concentrations of ANG-(1-7) (2.9 +/- 0.6 fmol/ml), whereas ANG II concentrations were not different (10 +/- 1 fmol/ml). Plasma renin concentrations were higher in the fetus (4.8 +/- 1.1 pmol ANG I x ml(-1) x h(-1)) than in the ewe (0.9 +/- 0.2 pmol x ml(-1) x h(-1), P < 0.05). Infusion of ANG-(1-7) (approximately 9 microg/h) for a 3-day period caused a significant increase in plasma concentrations of ANG-(1-7) reaching a maximum of 448 +/- 146 fmol/ml on day 3 of infusion. Plasma levels of ANG I and II as well as renin were unchanged by the infusion. Urine flow rate, glomerular filtration rate, and fetal arterial blood pressure did not change and were not different than values in fetuses receiving a saline infusion for 3 days (n = 5). However, the osmolality of amniotic and allantoic fluid was significantly higher in fetuses that received ANG-(1-7). Also, compared with the saline-infused animals, mRNA expression levels of renin, the AT(1) receptor, and AT(2) receptor were elevated in kidneys of fetuses that received infusions of ANG-(1-7). Infusion of an ANG-(1-7) antagonist ([D-Ala(7)]-ANG-(1-7), 20 microg/h) for 3 days had no effect on fetal blood pressure or renal function. In conclusion, although infusion of ANG-(1-7) did not affect fetal urine flow rate, glomerular filtration rate, or blood pressure, changes in fetal fluids and gene expression indicate that ANG-(1-7) may play a role in the fetal kidney.

Angiotensin receptors--evolutionary overview and perspectives

The structure of the angiotensin molecule has been well preserved throughout the vertebrate scale with some amino acid variations. Specific angiotensin receptors (AT receptors) that mediate important physiological functions have been noted in a variety of tissues and species. Physiological and pharmacological characterization of AT receptors and, more recently, molecular cloning studies have elucidated the presence of AT receptor subtypes. Comparative studies suggest that an AT receptor subtype homologous to the mammalian type 1 receptor subtype (AT(1)), though pharmacologically distinct, is present in amphibians and birds, whereas AT receptors cloned from teleosts show low homology to both AT(1) and AT(2) receptor subtypes. Furthermore, receptors differing from both the AT(1)-homologue receptor and AT(2) receptor exist in some non-mammalian species. This may suggest that the prototype AT receptor evolved in primitive vertebrates and diverged to more than one type of AT receptor subtype during phylogeny. Furthermore, phenotypic modulation of AT receptors appears to occur during individual development/maturation.

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.

Effect of selective angiotensin antagonists on the antidiuresis produced by angiotensin-(1-7) in water-loaded rats

In the present study we evaluated the nature of angiotensin receptors involved in the antidiuretic effect of angiotensin-(1-7) (Ang-(1-7)) in water-loaded rats. Water diuresis was induced in male Wistar rats weighing 280 to 320 g by water load (5 ml/100 g body weight by gavage). Immediately after water load the rats were treated subcutaneously with (doses are per 100 g body weight): 1) vehicle (0.05 ml 0.9% NaCl); 2) graded doses of 20, 40 or 80 pmol Ang-(1-7); 3) 200 nmol Losartan; 4) 200 nmol Losartan combined with 40 pmol Ang-(1-7); 5) 1.1 or 4.4 nmol A-779; 6) 1.1 nmol A-779 combined with graded doses of 20, 40 or 80 pmol Ang-(1-7); 7) 4.4 nmol A-779 combined with graded doses of 20, 40 or 80 pmol Ang-(1-7); 8) 95 nmol CGP 42112A, or 9) 95 nmol CGP 42112A combined with 40 pmol Ang-(1-7). The antidiuretic effect of Ang-(1-7) was associated with an increase in urinary Na+ concentration, an increase in urinary osmolality and a reduction in creatinine clearance (CCr: 0.65 +/- 0.04 ml/min vs 1.45 +/- 0.18 ml/min in vehicle-treated rats, P < 0.05). A-779 and Losartan completely blocked the effect of Ang-(1-7) on water diuresis (2.93 +/- 0.34 ml/60 min and 3.39 +/- 0.58 ml/60 min, respectively). CGP 42112A, at the dose used, did not modify the antidiuretic effect of Ang-(1-7). The blockade produced by Losartan was associated with an increase in CCr and with an increase in sodium and water excretion as compared with Ang-(1-7)-treated rats. When Ang-(1-7) was combined with A-779 there was an increase in CCr and natriuresis and a reduction in urine osmolality compared with rats treated with Ang-(1-7) alone. The observation that both A-779, which does not bind to AT1 receptors, and Losartan blocked the effect of Ang-(1-7) suggests that the kidney effects of Ang-(1-7) are mediated by a non-AT1 angiotensin receptor that is recognized by Losartan.

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.

Diuresis and natriuresis produced by long term administration of a selective Angiotensin-(1-7) antagonist in normotensive and hypertensive rats

In this study we evaluated the renal effects of chronic administration of the selective Angiotensin-(1-7)[Ang-(1-7)] antagonist, A-779, in normotensive and spontaneously hypertensive rats (SHR). Male adult SHR and Wistar rats were housed in metabolic cages with tap water and standard chow, for three-five days before starting infusion (Alzet osmotic mini-pumps) of A-779 (Wistar: 1 microg/h, n = 9; 2.5 microg/h, n = 6; SHR:2.5 microg/h, n = 6) or vehicle (0.9% NaCl - 1 microl/h, n = 7 and n = 10 for SHR and Wistar rats, respectively). Urine volume, water and food intake and urinary Na+ were measured daily. On the last day of infusions mean arterial pressure (MAP) was recorded and urine and blood samples were collected to determine renal function parameters. Chronic infusion of A-779 produced a sustained increase in diuresis in normotensive rats [seventh day values: 0.75+/-0.08 ml/h (1 microg/h) and 0.94+/-0.13 ml/h (2.5 microg/h) vs. 0.42 + 0.03 ml/h for the control group, P<0.05] associated to a dose-dependent increase in the creatinine clearance. In SHR, diuresis increased significantly after chronic infusion of A-779 (fifth day values: 0.44 + 0.06 ml/h vs. 0.25+/-0.04 ml/h for the control group, P<0.05), without changes in creatinine clearance. Infusion of A-779 in normotensive rats produced a decrease in water reabsorption. A-779 infusion also produced a dose-dependent increase in urinary Na+ excretion (1.49 + 0.14 mEq, 1 microg/h vs. 2.37+/-0.22 mEq, 2.5 microg/h, P<0.05), in Wistar rats, without modifying the fractional excretion of Na+. In SHR, urinary Na+ excretion was also increased by A-779 (2.21+/-0.46 mEq vs. 0.94+/-0.22 mEq for the control group, P<0.05). No significant changes in blood pressure were observed. These findings suggest that endogenous Ang-(l-7) participates in the control of hydroelectrolyte balance by modulating water excretion, acting at tubular and glomerular sites.

N-domain-specific substrate and C-domain inhibitors of angiotensin-converting enzyme: angiotensin-(1-7) and keto-ACE

We used the isolated N- and C-domains of the angiotensin 1-converting enzyme (N-ACE and C-ACE; ACE; kininase II) to investigate the hydrolysis of the active 1-7 derivative of angiotensin (Ang) II and inhibition by 5-S-5-benzamido-4-oxo-6-phenylhexanoyl-L-proline (keto-ACE). Ang-(1-7) is both a substrate and an inhibitor; it is cleaved by N-ACE at approximately one half the rate of bradykinin but negligibly by C-ACE. It inhibits C-ACE, however, at an order of magnitude lower concentration than N-ACE; the IC50 of C-ACE with 100 micromol/L Ang I substrate was 1.2 micromol/L and the Ki was 0.13. While searching for a specific inhibitor of a single active site of ACE, we found that keto-ACE inhibited bradykinin and Ang I hydrolysis by C-ACE in approximately a 38- to 47-times lower concentration than by N-ACE; IC50 values with C-ACE were 0.5 and 0.04 micromol/L. Furthermore, we investigated how Ang-(1-7) acts via bradykinin and the involvement of its B2 receptor. Ang-(1-7) was ineffective directly on the human bradykinin B2 receptor transfected and expressed in Chinese hamster ovary cells. However, Ang-(1-7) potentiated arachidonic acid release by an ACE-resistant bradykinin analogue (1 micromol/L), acting on the B2 receptor when the cells were cotransfected with cDNAs of both B2 receptor and ACE and the proteins were expressed on the plasma membrane of Chinese hamster ovary cells. Thus like other ACE inhibitors, Ang-(1-7) can potentiate the actions of a ligand of the B2 receptor indirectly by binding to the active site of ACE and independent of blocking ligand hydrolysis. This potentiation of kinins at the receptor level can explain some of the well-documented kininlike actions of Ang-(1-7).

Angiotensin II has depressor effects in pregnant and nonpregnant women

Studies in anesthetized animals suggest that angiotensin II evokes a depressor as well as a pressor effect, which becomes evident on cessation of infusion. We have studied 18 nonpregnant and 8, 23, and 22 women in the first, second, and third trimesters of pregnancy to determine whether such an effect is present in conscious women, whether it is dose dependent, and whether it is influenced by pregnancy. Angiotensin II was infused intravenously in doubling concentrations at 10-minute intervals until a pressor effect of approximately 20 mm Hg was observed. The infusion was stopped, and blood pressure was monitored at 2-minute intervals for 30 minutes. There was a significant diastolic depressor effect after stopping angiotensin II in the nonpregnant women and those in the second and third trimesters of pregnancy. Individual women required differing doses of angiotensin II to evoke the standardized pressor response. It was thus possible to examine the depressor response in each group in relation to infused doses of angiotensin II. In nonpregnant women and in those in the second and third trimesters of pregnancy, the depressor response was dose dependent (P<.001). At any given dose, the depressor response deepened as pregnancy progressed (P<.001). Basal plasma prostacyclin concentrations rise in pregnancy, and angiotensin II can stimulate prostacyclin synthesis. This might mediate the depressor effect. In conclusion, the diminished pressor response to angiotensin II in normal pregnancy may be partly due to an increasing depressor effect of the hormone.

Counterregulatory actions of angiotensin-(1-7)

Angiotensin (Ang)-(1-7) is a bioactive component of the renin-angiotensin system that is formed endogenously from either Ang I or Ang II. The first actions described for Ang-(1-7) indicated that the peptide mimicked some of the effects of Ang II, including the release of prostanoids and vasopressin. However, Ang-(1-7) is devoid of vasoconstrictor, central pressor, or thirst-stimulating actions. In fact, new findings reveal depressor, vasodilator, and antihypertensive actions that may be more apparent in hypertensive animals or humans. Thus, the accumulating evidence suggests that Ang-(1-7) may oppose the actions of Ang II either directly or by stimulation of prostaglandins and nitric oxide. These observations are significant because they may explain the effective antihypertensive action of converting enzyme inhibitors in a variety of non-renin-dependent models of experimental and genetic hypertension as well as most forms of human hypertension. In this context, studies in humans and animals showed that the antihypertensive action of converting enzyme inhibitors correlated with increases in plasma levels of Ang-(1-7). In this review, we summarize our knowledge of the mechanisms accounting for the counterregulatory actions of Ang-(1-7) and elaborate on the emerging concept that Ang-(1-7) functions as an antihypertensive peptide within the cascade of the renin-angiotensin system.

Role of angiotensin-(1-7) in the modulation of the baroreflex in renovascular hypertensive rats

In this study, we evaluated the effect produced by lateral ventricle (intracerebroventricular, I.C.V.) infusion of the selective angiotensin (Ang)-(1-7) antagonist, D-Ala7-Ang-(1-7) (A-779), in the modulation of the baroreflex control of heart rate in two-kidney, one clip renovascular hypertensive rats (2K1C) treated with the angiotensin-converting enzyme (ACE) inhibitor enalapril. Twenty days after the surgery to produce renovascular hypertension, I.C.V. cannulas were implanted in the rats with blood pressure (BP) greater than 145 mm Hg (n=33) and in sham-operated rats (n=32). Five days later, the rats were treated with enalapril (10 mg x kg(-1) x d(-1); 6 days, in the drinking water) or vehicle (tap water). On the sixth day of treatment, direct continuous BP recording and measurement of reflex changes in heart rate elicited by phenylephrine were made in conscious rats before and at 1 hour of I.C.V. infusion of saline (8 microL/h) or A-779 (4 microg/h). To evaluate the degree of ACE blockade produced by enalapril treatment, the pressor effect of Ang I (50 ng, I.V., and 100 ng, I.C.V.) and plasma ACE activity was determined. As expected, enalapril treatment in 2K1C produced a significant fall in BP, significant attenuation in the pressor response of Ang I (I.V.), and a reduction in plasma ACE activity. In addition, enalapril treatment increased the baroreflex sensitivity (0.76+/-0.04 versus 0.43+/-0.04 ms/mm Hg in 2K1C untreated rats). I.C.V. infusion of A-779 reverted the improvement in baroreflex sensitivity produced by enalapril treatment in 2K1C (from 0.80+/-0.07 to 0.42+/-0.08 ms/mm Hg) and also attenuated the baroreflex sensitivity in untreated 2K1C (0.36+/-0.05 versus 0.48+/-0.06 ms/mm Hg) and untreated sham-operated rats (1.21+/-0.05 versus 0.78+/-0.17 ms/mm Hg). These results suggest that central endogenous Ang-(1-7) is involved at least in part in the improvement of baroreflex sensitivity observed in 2K1C after peripheral chronic ACE inhibition.

Effect of angiotensin-(1-7) on reperfusion arrhythmias in isolated rat hearts

There is increasing evidence that angiotensin-(1-7)(Ang-(1-7)) is an endogenous biologically active component of the renin-angiotensin system(RAS). In the present study, we investigated the effects of Ang-(1-7) on reperfusion arrhythmias in isolated rat hearts. Isolated rat hearts were perfused with two different media, i.e., Krebs-Ringer (2.52 mM CaCl2) and low-Ca2+ Krebs-Ringer (1.12 mM CaCl2). In hearts perfused with Krebs-Ringer, Ang-(1-7) produced a concentration-dependent (27-210 nM) reduction in coronary flow (25% reduction at highest concentration), while only slight and variable changes in contraction force and heart rate were observed. Under the same conditions, angiotensin II (Ang II; 27 and 70 nM) produced a significant reduction in coronary flow (39% and 48%, respectively) associated with a significant increase in force. A decrease in heart rate was also observed. In low-Ca2+ Krebs-Ringer solution, perfusion with Ang-(1-7) or Ang II at 27 nM concentration produced similar changes in coronary flow, contraction force and heart rate. In isolated hearts perfused with normal Krebs-Ringer, Ang-(1-7) produced a significant enhancement of reperfusion arrhythmias revealed by an increase in the incidence and duration of ventricular tachycardia and ventricular fibrillation (more than 30-min duration). The facilitation of reperfusion arrhythmias by Ang-(1-7) was associated with an increase in the magnitude of the decreased force usually observed during the postischemic period. The effects of Ang-(1-7) were abolished in isolated rat hearts perfused with low-Ca2+ Krebs-Ringer. The effect of Ang II (27 nM) was similar but less pronounced than that of Ang-(1-7) at the same concentration. These results indicate that the heart is a site of action for Ang-(1-7) and suggest that this heptapeptide may be involved in the mediation of the cardiac effects of the RAS.

Renal actions of angiotensin-(1-7)

The heptapeptide angiotensin-(1-7) is considered to be a biologically active endproduct of the renin-angiotensin system. This angiotensin, which is devoid of the most known actions of angiotensin II such as induction of drinking behavior and vasoconstriction, has several selective effects in the brain and periphery. In the present article we briefly review recent evidence for a physiological role of angiotensin-(1-7) in the control of hydroelectrolyte balance.

Pathologic consequences of increased angiotensin II activity

Advances in molecular medicine and pharmacology have allowed clinicians to critically reassess the renin-angiotensin system. Angiotensin II (AII) participates in the control of cardiovascular function and electrolyte balance, and plays a part in the regulation of cellular oncogenes and the expression of growth factors. The expression of the proteins of the renin-angiotensin system in organs other than the kidneys suggests that these diverse actions are associated with the peptide in the local environment. Tissue renin-angiotensin activity has prompted the investigation of alternate pathways for the production of AII and characterization of novel forms of angiotensin peptides that counteract the vasoconstrictor and proliferative actions of AII. The heptapeptide angiotensin-(1-7) appears to be critically involved in regulating the angiotensinogen activity of AII through stimulation of vasodilator prostaglandins and release of nitric oxide. Study in this area has been accelerated by the identification of receptors that convey the actions of angiotensin peptides at the cellular level and the pharmacologic characterization of agents that inhibit the ability of AII to bind to target receptors. The introduction of a new class of orally active AII-receptor blockers has provided a specific test of the role of AII in the development of essential hypertension and the potential for improved therapy for hypertension and cardiac and vascular sequelae.

Changes in the baroreflex control of heart rate produced by central infusion of selective angiotensin antagonists in hypertensive rats

We have recently shown that an angiotensin-(1-7) [Ang-(I-7)] analogue, D-Ala7-Ang-(1-7) (A-779), is a selective Ang-(1-7) antagonist with no significant action on angiotensin type 1 or type 2 receptors. The availability of selective angiotensin antagonists prompted us to evaluate the role of Ang-(1-7) and Ang II on central modulation of the baroreflex control of heart rate in normotensive Wistar rats and spontaneously hypertensive rats (SHR). Blood pressure recording and reflex changes in heart rate elicited by intravenous bolus injections of phenylephrine were made before and within 1 and 3 hours of intracerebroventricular (ICV, lateral ventricle) infusion of saline (8 microL/h), A-779 (4 microg/h), DuP 753 (100 microg/h), or CGP 42112A (50 mu g/h) in conscious rats. The slope of the relationship between changes in pulse interval versus changes in mean arterial pressure was used as an index of the baroreflex control of heart rate. ICV infusion of saline or any of the antagonists did not significantly change basal levels of mean arterial pressure and heart rate in SHR (170 +/- 6 mm Hg nd 360 +/- 9 beats per minute, respectively; n = 29) or Wistar rats (108 +/- 2 mm Hg and 377 +/- 6 beats per minute, respectively; n=29). Three hours of ICV infusion of A-779 markedly decreased baroreflex sensitivity in Wistar rats (from a basal slope of 1.09 +/- O.3). In contrast, A-779 did not significantly alter the depressed baroreflex sensitivity of SHR (0.61 +/- O.l). ICV infusion of DuP 753 produced a significant increase (60 percent) in baroreflex control of heart rate in both Wistar rats and SHR. Saline or CGP 42112A infusions did not significantly alter baroreflex control of heart rate. These results suggest that endogenous Ang II and Ang-(1-7) are differentially affecting central baroreflex modulation, acting probably through distinct receptor subtypes. Although the central Ang II inhibitory effect is mediated by the type 1 receptor subtype, the facilitatory effect of Ang-(1-7) might be mediated by a different, unidentified receptor.

Evidence for a physiological role of angiotensin-(1-7) in the control of hydroelectrolyte balance

In this study we evaluated the possibility that angiotensin-(1-7) [Ang-(1-7)] acts as an endogenous osmoregulatory peptide by determining the effect of acute administration of its selective antagonist [D-Ala7]Ang-(1-7) (A-779) on renal function parameters in rats. In addition, we investigated the physiological mechanisms involved in the antidiuretic effect of Ang-(1-7). The antidiuretic effect of Ang-(1-7) (40 pmol/0.05 mL per 100 g BW) in water-loaded rats was completely blocked by A-779 (vehicle-treated, 3.34 +/- 0.43 mL/h; Ang-(1-7), 1.48 +/- 0.23; A-779, 2.72 +/- 0.35; Ang-(1-7) plus A-779, 3.26 +/- 0.49). In contrast, the antidiuretic effect of Ang-(1-7) was not significantly changed by a vasopressin V2 receptor antagonist in a dose that completely blocked the antidiuresis produced by an equipotent dose of vasopressin. In addition, Ang-(1-7) administration did not significantly change vasopressin plasma levels in water-loaded rats. The antidiuretic effect of Ang-(1-7) in water-loaded rats was associated with a reduction of creatinine clearance (0.68 +/- 0.04 versus 1.38 +/- 0.32 mL/min in vehicle-treated rats, P <.05) and an increase in urine osmolality (266.8 +/- 32.7 versus 182.8 +/- 14 mOsm/kg in vehicle-treated rats, P <.05). An effect of Ang-(1-7) in tubular water transport was demonstrated in vitro by a fourfold increase in the hydraulic conductivity of inner medullary collecting ducts in the presence of 1 nmol/L Ang-(1-7). Subcutaneous administration of A-779 (2.3 to 9.2 nmol/100 g) produced a significant increase in urine volume (4.6 nmol/100 g, 0.45 +/- 0.12 mL/h; vehicle-treated rats, 0.16 +/- 0.03 mL/h; P <.05) comparable to that of acute administration of a vasopressin V2 receptor antagonist. The diuretic effect of A-779 was associated with an increase in creatinine clearance and decrease in urine osmolality. In contrast, no significant effects on urine volume were observed after systemic administration of angiotensin subtype 1 or 2 receptor antagonists (DuP 753 and CGP 42112A, respectively). These findings suggest that endogenous Ang-(1-7), acting on specific receptors, participates in the control of hydroelectrolyte balance by influencing especially water excretion.

Coronary kinin generation mediates nitric oxide release after angiotensin receptor stimulation

Our goal was to determine whether angiotensin II (Ang II) and its metabolic fragments release nitric oxide and the mechanisms by which this occurs in blood vessels from the canine heart. We incubated 20 mg of microvessels or large coronary arteries in phosphate-buffered saline for 20 minutes and measured nitrite release. Nitrite release increased from 27 +/- 2 up to 103 +/- 5, 145 +/- 17, 84 +/- 4, 107 +/- 16, and 54 +/- 4 pmol/mg (P < .05) in response to 10(-5) mol/L of Ang I, II, III, IV, and Ang-(1-7), respectively. The effects of all angiotensins were blocked by N omega-nitro-L-arginine methyl ester (100 mumol/L), indicating that nitrite was a product of nitric oxide metabolism, and by Hoe 140 (10 mumol/L), a specific bradykinin B2 receptor antagonist, indicating a potential role for local kinin formation. The protease inhibitors aprotinin (10 mumol/L) and soybean trypsin inhibitor, which block local kinin formation, inhibited nitrite release by all of the angiotensins. Angiotensin nonselective (saralasin), type 1-specific (losartan), and type 2-specific (PD 123319) receptor antagonists abolished the nitrite released in response to all the fragments. Angiotensin type 1 and type 2 and receptors mediate nitrite release after Ang I, II, III, and Ang-(1-7), whereas only type 2 receptors mediate nitrite release after Ang IV. Similar results were obtained in large coronary arteries. In summary, formation of nitrite from coronary microvessels and large arteries in the normal dog heart in response to angiotensin peptides is due to the activation of local kinin production in the coronary vessel wall.

Angiotensin-(1-7) binding at angiotensin II receptors in the rat brain

Angiotensin-(1-7) (Ang-(1-7)) is reported to be equipotent with angiotensin II (AII) in producing some central biological effects but the receptors responsible for these actions have not been defined. Three classes of receptor have been proposed: AT1, AT2, and a putative Ang-(1-7) selective receptor. This study specifically evaluates Ang-(1-7) competition at AII binding sites (AT1 and AT2) in the rat brain. 125I Sar1 Ile8 AII (269-312 pM) was used to conduct receptor autoradiographic binding assays in brain sections. Competition with Ile5 AII and Val5 AII was similar at nuclei in which either AT1 or AT2 receptor subtypes predominate (Ki = 11-18 nM). Ang-(1-7) competed 150-fold less effectively than native AII at AT1 predominant brain nuclei (Ki = 2.4 microM). At brain regions where AT2 receptors predominate, Ang-(1-7) showed a very low affinity (Ki = 104 microM) for the majority of the 125I Sar1 Ile8 AII binding sites (AT2). A small proportion of 125I Sar1 Ile8 AII binding sites showed an affinity of 2.0 microM, presumably AT1 receptors present in those brain regions. For biological responses where Ang-(1-7) is reported to be equipotent with AII, it is unlikely that these actions are mediated by the widely distributed AT1 or AT2 receptor subtypes which recognize 125I Sar1 Ile8 AII.

The brain renin-angiotensin system: molecular mechanisms of cell to cell interactions

The components of the Renin-Angiotensin System (RAS) have been found to be expressed in the brain. Angiotensinogen, the high molecular weight precursor of the system, is widely distributed and expressed in areas not related to control of blood pressure and body fluid homeostasis as well. It has been shown that it is regulated by steroid hormones independently from the liver and that it is also regulated in a different manner in several brain areas. Angiotensin II, the effector peptide of the system, may be generated in the brain via the classical pathway, using renin and angiotensin converting enzyme or directly from angiotensinogen by cathepsin G or tonin. N-terminal peptides of angiotensin II have been found in several brain areas with ANG (1-7) involved in vasopressin release however without influence on blood pressure and with ANG III acting as potent as ANG II. Transgenic animals may be used to study the pathophysiology of an activated brain RAS.

Evidence that angiotensin-(1-7) plays a role in the central control of blood pressure at the ventro-lateral medulla acting through specific receptors

In this study we determined which angiotensin receptors may mediate the cardiovascular effects elicited by angiotensin-(1-7) [Ang-(1-7)] in the rostral ventrolateral medulla (RVLM) and caudal pressor area (CPA) of the ventrolateral medulla (VLM) of anesthetized rats. Furthermore the role of endogenous angiotensins in these areas was also investigated. The pressor effect produced by unilateral microinjection of Ang-(1-7) into the RVLM or CPA was not modified by either the AT1 receptor antagonist, DuP 753 or by the AT2 receptor antagonist, CGP 42112A, but was completely blocked by the Ang-(1-7) selective antagonist, A-779. In contrast, the pressor effect produced by microinjection of angiotensin II (Ang II) was completely blocked by DuP 753 but was not changed by CGP 42112A or A-779. Bilateral microinjection of A-779 into the RVLM or CPA produced a significant fall in mean arterial pressure and heart rate. Microinjection of DuP 753 produced a pressor effect comparable to bilateral injection of vehicle. These results indicate that, although Ang II acts in the VLM through an AT1 receptor subtype, the cardiovascular effects produced by microinjection of Ang-(1-7) into the RVLM and CPA are mediated by a specific angiotensin receptor (AT5?). Furthermore, our data provide evidence that endogenous Ang-(1-7) participates at the VLM in the neural control of arterial blood pressure.

Effects of angiotensin II and angiotensin-(1-7) on the release of [3H]norepinephrine from rat atria

We examined the effects of angiotensin II (Ang II) and Ang-(1-7) on the release of [3H]norepinephrine elicited by nerve stimulation (2 Hz, 0.5 millisecond, for 2 minutes) in rat atria isolated with their cardioaccelerans nerves. The stimulation-induced release of [3H]norepinephrine was increased 50% by 3 x 10(-8) mol/L of either peptide. No further increase in [3H]norepinephrine release was observed with peptide concentrations up to 3 x 10(-7) mol/L. This effect was completely blocked by the nonselective angiotensin receptor antagonist saralasin (1 x 10(-7) mol/L). The type 1 angiotensin receptor antagonist DuP 753 (1 x 10(-6) mol/L) entirely prevented the increases in [3H]norepinephrine caused by Ang II and Ang-(1-7). On the other hand, the type 2 angiotensin receptor antagonist PD 123319 (1 x 10(-6) mol/L) prevented the increase in [3H]norepinephrine release elicited by Ang-(1-7) but not by Ang II. These results suggest that Ang-(1-7), like Ang II, could have a neuromodulatory function in rat atria via activation of specific angiotensin receptor subtypes, which could be the subtype 1 angiotensin receptor for Ang II and subtypes 1 and 2 for Ang-(1-7).

Formation of des-Asp-angiotensin I in the hypothalamic extract of normo- and hypertensive rats

Exogenous angiotensin I (ANG I) was degraded to mainly des-Asp-ANG I instead of ANG II in the hypothalamic homogenate of the Sprague Dawley (SD), Wistar Kyoto (WKY), left renal artery stenosed hypertensive SD (LRAS), deoxycorticosterone acetate/salt-induced hypertensive SD (DOCA-salt) and spontaneously hypertensive rats (SHR). In the same homogenate, ANG II was degraded to ANG III and ANG III remained unchanged during the first 10 min of incubation. However, all the homogenates were able to catalyse hippuryl-L-histidyl-L-leucine to hippuric acid and the catalysis was completely inhibited by 3 microM captorpil. The data show that the angiotensin converting enzyme present in the hypothalamus when extracted by the normal laboratory procedures is not able to hydrolyse ANG I to ANG II. In addition, the aminopeptidase that degraded ANG I to des-Asp-ANG I was not inhibited by amastatin, bestatin and EDTA, indicating that it is not aminopeptidase A or B. The formation of hippuric acid was significantly higher in the homogenate of the LRAS whilst the SHR and DOCA-salt showed significant higher rate of des-Asp-ANG I formation than in the normotensive control rats.

Differential regulation of brain angiotensin II in genetically hypertensive and normotensive rats after nephrectomy

To investigate the role of tissue angiotensin II (Ang II) in the maintenance of hypertension after nephrectomy in spontaneously hypertensive rats (SHR), Ang II levels were measured in various tissues of both 12-week-old SHR and normotensive control, Wistar-Kyoto rats (WKY), 48 h after nephrectomy or sham operation. Ang II was determined by radioimmunoassay coupled with high performance liquid chromatography. Nephrectomy caused a decrease of plasma renin activity and plasma Ang II concentration in both SHR and WKY. Aortic Ang II levels were significantly lowered by nephrectomy only in WKY, and not in SHR. Ang II levels in hypothalamic block, brainstem and cerebellum of SHR increased after nephrectomy, whereas those of WKY were unchanged. Intracerebroventricular administration of ceronapril, an angiotensin converting enzyme inhibitor, significantly decreased sustained high blood pressure in SHR 48 h after nephrectomy compared with vehicle administration, whereas intravenous administration had no effect. These results suggest that in spite of the important role of the renal renin-angiotensin system in maintenance of high blood pressure in SHR, control mechanisms may switch to other systems after nephrectomy, and that the increased brain Ang II levels after nephrectomy may be related to these mechanisms.

Effects of central angiotensin II and angiotensin III on baroreflex regulation

In the present study the cardiovascular effects of intracerebroventricularly (i.c.v.) applied angiotensin II (AN II) and angiotensin III (AN III) were analysed in conscious Wistar rats. The baroreceptor heart reflex (BHR) was elicited by intravenous bolus injection of both phenylephrine (1 microgram) and sodium nitroprusside (5 micrograms) before and after i.c.v. administration (1.5 and 15 min) of the peptides. Administration of 20 ng and 200 ng AN II produced a short increase in inter-beat interval (IBI) and a long-lasting increase in mean blood pressure (MBP), inclusive of a drinking response. Only after the high dose of 200 ng AN II we found a continuous impairment in the BHR for reflex bradycardia. Inversely, the small doses of both 100 pg AN II and 100 pg AN III were without effects on IBI and MBP; they induced an enhancement in BHR for the reflex bradycardia and after 100 pg AN II it was also found for the reflex tachycardia. Pretreatment with 20 nmol amastatin (AM), a specified aminopeptidase A inhibitor, followed by 100 pg An II suppressed the enhancement in BHR. AM alone was without effects in this respect. These findings suggest that: 1) the influence of central angiotensin on the BHR could be dose-dependent in the opposite way and 2) AN III seems to be the active form and involved in the central blood pressure regulatory mechanism.

Characterization of a new angiotensin antagonist selective for angiotensin-(1-7): evidence that the actions of angiotensin-(1-7) are mediated by specific angiotensin receptors

In this study we describe a new angiotensin antagonist [Asp1-Arg2-Val3-Tyr4-Ile5-His6-D-Ala7, (A-779)] selective for the heptapeptide angiotensin-(1-7) [Ang-(1-7)]. A-779 blocked the antidiuretic effect of Ang-(1-7) in water-loaded rats and the changes in blood pressure produced by Ang-(1-7) microinjection into the dorsal-medial and ventrolateral medulla. In contrast, A-779 did not change the dipsogenic, pressor, or myotropic effects of angiotensin II (Ang II). Also, A-779 did not affect the antidiuretic effect of vasopressin or the contractile effects of angiotensin III, bradykinin, or substance P on the rat ileum. In the rostral ventrolateral medulla, the pressor effect produced by Ang-(1-7) microinjection was completely blocked by A-779 but not by AT1 or AT2 receptor antagonists (DUP 753 and CGP 42112A, respectively). Conversely, the pressor effect produced by Ang II was not changed by A-779 but was completely blocked by DUP 753. Binding studies substantiated these observations: A-779 did not compete significantly for 125I-Ang II binding to adrenocortical membranes at up to a 1 microM concentration. Low affinity binding was also observed in adrenomedullary membranes with an IC50 greater than 10 microM. Our results show that A-779 is a potent and selective antagonist for Ang-(1-7). More importantly, our data indicate that specific angiotensin receptors mediate the central and peripheral actions of Ang-(1-7).

Plasma angiotensin(1-7) immunoreactivity is increased by salt load, water deprivation, and hemorrhage

In this study we investigated the effects of dehydration and hemorrhage on circulating levels of the heptapeptide, angiotensin(1-7). In water-deprived rats, a twofold increase in plasma angiotensin(1-7) was associated with similar increases in plasma renin activity, and angiotensin I and angiotensin II levels. In salt-loaded rats, plasma angiotensin(1-7) levels increased fourfold; however, other components of the renin-angiotensin system were suppressed or unchanged. In salt-loaded rats, increases in plasma angiotensin II levels in response to hemorrhage in normal rats were severely blunted, whereas angiotensin(1-7) plasma levels increased proportionately to the loss of blood volume. These results suggest that angiotensin(1-7) plasma concentration can be selectively regulated during dehydration and hemorrhage.

Comparative effects of angiotensin-(1-7) and angiotensin II on piglet pial arterioles

BACKGROUND AND PURPOSE

Recent investigations indicated that degradation fragments of angiotensins could be involved in the regulation of the cerebral circulation and that their effects might be mediated by prostaglandins. The present study was designed to examine the effect of angiotensin-(1-7), a major endogenous heptapeptide fragment, on cerebral arteriolar diameter and compare it with the octapeptide angiotensin II, and further to determine whether prostaglandins mediate their effects.

METHODS

Newborn, anesthetized pigs were equipped with a closed cranial window, and the diameter of one pial arteriole was measured using intravital microscopy.

RESULTS

Topical application of angiotensin-(1-7) (n = 9) increased the diameter by 6.8 +/- 5.3% (mean +/- SEM), 10.4 +/- 5.2%, 14.3 +/- 5.9%, and 17.5 +/- 7.7% (P < .05) at 10(-7), 10(-6), 10(-5), and 10(-4) mol/L, respectively (baseline, 94 +/- 3 microns). Topical application of angiotensin II (n = 8) increased the diameter by 9.6 +/- 7.0%, 9.6 +/- 7.6%, 11.3 +/- 8.4% (P < .05), and 5.5 +/- 7.9% at 10(-7), 10(-6), 10(-5), and 10(-4) mol/L, respectively (baseline, 94 +/- 5 microns). After administration of indomethacin (5 mg/kg IV), which did not significantly change the baseline arteriolar diameter, neither angiotensin-(1-7) at 10(-4) mol/L nor angiotensin II at 10(-5) mol/L caused significant vasodilation.

CONCLUSIONS

The results indicate that angiotensin-(1-7) is a modest dilator in the cerebral circulation, as is angiotensin II, and that prostaglandins may mediate responses.

Modulation of phospholipase A2 activity and sodium transport by angiotensin-(1-7)

Angiotensin II (Ang II) receptors are coupled to a variety of signal transduction mechanisms. In the kidney, Ang II at nanomolar concentration binds to proximal tubular cells and stimulates phospholipase A2 (PLA2), which in turn catalyzes the hydrolysis of phosphatidylcholine into lysophosphatidylcholine (LPC) and fatty acid. This signal transduction pathway has been shown to be an important modulator of sodium transport. The kidney cortex possesses the enzyme necessary to convert angiotensin I (Ang I) directly to Ang-(1-7) bypassing Ang II as an intermediate. The present investigation was undertaken to determine whether Ang-(1-7) influences epithelial cell function by comparing this heptapeptide with Ang II as a modulator of PLA2 activity and sodium transport. Proximal tubular cells were labeled in tissue culture with 3H-choline and PLA2 activity was measured by quantitation of LPC. We found that Ang II (10(-9) M to 10(-6) M) significantly increased PLA2 activity (154 +/- 36% to 209 +/- 94%). Similar results were obtained with Ang-(1-7) (240 +/- 130% to 353 +/- 40%). The bioactivity of the peptides was assayed by its ability to regulate transcellular 22Na flux. Ang II (10(-9) M) inhibited 22Na flux by 12 +/- 2% while Ang-(1-7) (10(-9) M) inhibited 22Na flux by 20 +/- 5%. These results suggest that one potential role of Ang-(1-7) in the regulation of kidney epithelial electrolyte transport may involve activation of PLA2.

Cardiovascular effects produced by micro-injection of angiotensin-(1-7) on vasopressor and vasodepressor sites of the ventrolateral medulla

In this study, we determined the cardiovascular effects produced by micro-injection of the heptapeptide Angiotensin-(1-7) [Ang-(1-7)] into the rat ventrolateral medulla (VLM). Micro-injection of Ang-(1-7) into the rostral VLM and the caudal pressor area of the VLM produced significant increases in arterial pressure, comparable to that observed with micro-injection of Ang II. The changes in arterial pressure were associated with more variable changes in heart rate (HR) (usually tachycardia). On the other hand, micro-injection of Ang-(1-7) into the caudal depressor area induced decreases in arterial pressure and HR. The results suggest that, besides Ang II, Ang-(1-7) is involved in the mediation of the cardiovascular actions of the renin-angiotensin system in the VLM.

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.

Elevated angiotensin-converting enzyme (kininase II) in the cerebrospinal fluid of neuroleptic-treated schizophrenic patients

Disturbances of water homeostasis have frequently been reported in schizophrenia. Water homeostasis is regulated by arginine vasopressin (AVP), the renin-angiotensin system and natriuretic hormones. The aim of this study was to determine the activity of the central renin-angiotensin system in schizophrenia by measuring levels of angiotensin-converting enzyme (ACE) in the cerebrospinal fluid (CSF) and blood in 14 in-patients with schizophrenia on neuroleptic medication and in 9 healthy volunteers. The levels of CSF ACE were significantly higher in the schizophrenia group. There were no correlations between CSF ACE and gender, age, age at first episode, duration of illness, term of hospitalization or neuroleptic dosage. No correlations between CSF ACE and serum ACE were found in either group. The authors suggest an activated central renin-angiotensin system in schizophrenia at least during antipsychotic drug treatment, which may cause 'psychogenic' polydipsia in some patients. ACE and the brain renin-angiotensin system may also play a role in the regulation of neuron growth and differentiation in schizophrenia.