Discovery of a distinct binding site for angiotensin II (3-8), a putative angiotensin IV receptor

Regulation of K+ transport in the rat distal colon via angiotensin II subtype receptors and K+ -pathways

The role of angiotensin subtype-1 (AT1) and -2 (AT2) receptors in mediating the effects of angiotensin II (ANG II) on several K+ transporters was studied in rat distal colon using an Ussing chamber. Angiotensin II induced K+ secretion at two different doses. Secretion occurred at 10-(8) and 10-(4) M, as a result of an increase in serosal-to-mucosal flux (Js-m). The ANG II-induced stimulation of Js-m at a low dose (10-(8) M) was abolished by PD123319 while losartan did not alter the low-dose ANG II-dependent increase in Js-m. In contrast, the increase in Js-m induced by a high-dose of ANG II (10-(4) M) was blocked by losartan, whereas PD123319 partially reduced the stimulatory effect. In the presence of both blockers, high-dose ANG II induced an inhibition of basal Js-m. Low-dose ANG II activated the barium-sensitive K+ channels, whereas the Na+, K+, 2Cl- cotransporter and the Na+, K+ -ATPase pump were unchanged. At the high dose, ANG II activated the barium-sensitive K+ channels and the Na+, K+, 2Cl- cotransporter and inhibited the Na+, K+ -ATPase pump. These data indicate that ANG II stimulates serosal-to-mucosal K+ flux in the rat distal colon at high and low doses via different receptors and K+ transporters.

Ligands to the (IRAP)/AT4 receptor encompassing a 4-hydroxydiphenylmethane scaffold replacing Tyr2

Analogues of the hexapeptide angiotensin IV (Ang IV, Val(1)-Tyr(2)-Ile(3)-His(4)-Pro(5)-Phe(6)) encompassing a 4-hydroxydiphenylmethane scaffold replacing Tyr(2) and a phenylacetic or benzoic acid moiety replacing His(4)-Pro(5)-Phe(6) have been synthesized and evaluated in biological assays. The analogues inhibited the proteolytic activity of cystinyl aminopeptidase (CAP), frequently referred to as the insulin-regulated aminopeptidase (IRAP), and were found less efficient as inhibitors of aminopeptidase N (AP-N). The best Ang IV mimetics in the series were approximately 20 times less potent than Ang IV as IRAP inhibitors. Furthermore, it was found that the ligands at best exhibited a 140 times lower binding affinity to the membrane-bound IRAP/AT4 receptor than Ang IV. Although the best compounds still exert lower activities than Ang IV, it is notable that these compounds comprise only two amino acid residues and are considerably less peptidic in character than the majority of the Ang IV analogues previously reported as IRAP inhibitors in the literature.

Effects of angiotensin II and its metabolites in the rat coronary vascular bed: is angiotensin III the preferred ligand of the angiotensin AT2 receptor?

Aminopeptidases metabolize angiotensin II to angiotensin-(2-8) (=angiotensin III) and angiotensin-(3-8) (=angiotensin IV), and carboxypeptidases generate angiotensin-(1-7) from angiotensin I and II. Angiotensin-converting enzyme (ACE) inhibitors and/or angiotensin II type 1 (AT1) receptor blockers affect the concentrations of these metabolites, and they may thus contribute to the beneficial effects of these drugs, possibly through stimulation of non-classical angiotensin AT receptors. Here, we investigated the effects of angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) in the rat coronary vascular bed, with or without angiotensin AT1 - or angiotensin II type 2 (AT2) receptor blockade. Results were compared to those in rat iliac arteries and abdominal aortas. Angiotensin II, angiotensin III and angiotensin IV constricted coronary arteries via angiotensin AT1 receptor stimulation, angiotensin III and angiotensin IV being approximately 20- and approximately 8000-fold less potent than angiotensin II. The angiotensin AT2 receptor antagonist PD123319 greatly enhanced the constrictor effects of angiotensin III, starting at angiotensin III concentrations in the low nanomolar range. PD123319 enhanced the angiotensin II-induced constriction at submicromolar angiotensin II concentrations only. Angiotensin-(1-7) exerted no effects in the coronary circulation, although, at micromolar concentrations, it blocked angiotensin AT1 receptor-induced constriction. Angiotensin AT2 receptor-mediated relaxation did not occur in iliac arteries and abdominal aortas, and the constrictor effects of the angiotensin metabolites in these vessels were identical to those in the coronary vascular bed. In conclusion, angiotensin AT2 receptor activation in the rat coronary vascular bed results in vasodilation, and angiotensin III rather than angiotensin II is the preferred endogenous agonist of these receptors. Angiotensin II, angiotensin III, angiotensin IV and angiotensin-(1-7) do not exert effects through non-classical angiotensin AT receptors in the rat coronary vascular bed, iliac artery or aorta.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

Hypertension, RAS, and gender: what is the role of aminopeptidases?

Hypertension is the major risk factor for coronary heart disease, stroke, and renal disease. Also, it is probably the most important risk factor for peripheral vascular disease and vascular dementia. Although hypertension occurs in both men and women, gender differences have been observed. However, whether sex hormones are responsible for the observed gender-associated differences in arterial blood pressure, and which is their mechanism of action, remains unclear. Local and circulating renin-angiotensin systems (RAS) are examples of systems that may be involved in the pathogenesis of hypertension. Classically, angiotensin II (Ang II) has been considered as the effector peptide of the RAS, but Ang II is not the only active peptide. Several of its degradation products, including angiotensin III (Ang III) and angiotensin IV (Ang IV) also possess biological functions. These peptides are formed via the activity of several aminopeptidases. This review will briefly summarize what is known about gender differences in RAS-regulating aminopeptidase activities, their relationship with sex hormones, and their potential role in controlling blood pressure acting through local and circulating RAS.

Angiotensin, bradykinin and the endothelium

Angiotensins and kinins are endogenous peptides with diverse biological actions; as such, they represent current and future targets of therapeutic intervention. The field of angiotensin biology has changed significantly over the last 50 years. Our original understanding of the crucial role of angiotensin II in the regulation of vascular tone and electrolyte homeostasis has been expanded to include the discovery of new angiotensins, their important role in cardiovascular inflammation and the development of clinically useful synthesis inhibitors and receptor antagonists. While less applied progress has been achieved in the kinin field, there are continuous discoveries in bradykinin physiology and in the complexity of kinin interactions with other proteins. The present review focuses on mechanisms and interactions of angiotensins and kinins that deal specifically with vascular endothelium.

Enzymatic pathways of the brain renin-angiotensin system: unsolved problems and continuing challenges

The brain renin-angiotensin system continues to be enigmatic more than 40 years after the brain was first recognized to be a site of action of angiotensin II. This review focuses on the enzymatic pathways for the formation and degradation of the growing number of active angiotensins in the brain. A brief description and nomenclature of the peptidases involved in the processing of angiotensin peptides in the brain is given. Of primary interest is the array of enzymes that degrade radiolabeled angiotensins in receptor binding assays. This poses major challenges to studies of brain angiotensin receptors and it is debatable whether an accurate determination of brain angiotensin receptor binding kinetics has yet been made. The quandary facing the investigator of brain angiotensin receptors is the need to protect the radioligand from metabolic alteration while maintaining the characteristics of the receptors in situ. It is the tenet of this review that we have yet to fully understand the binding characteristics of brain angiotensin receptors and the extent of their distribution in the brain because of our inability to fully protect the angiotensins from metabolic alteration until equilibrium binding conditions can be attained.

Small potent ligands to the insulin-regulated aminopeptidase (IRAP)/AT4 receptor

Angiotensin IV analogs encompassing aromatic scaffolds replacing parts of the backbone of angiotensin IV have been synthesized and evaluated in biological assays. Several of the ligands displayed high affinities to the insulin-regulated aminopeptidase (IRAP)/AT(4) receptor. Displacement of the C-terminal of angiotensin IV with an o-substituted aryl acetic acid derivative delivered the ligand 4, which exhibited the highest binding affinity (K(i) = 1.9 nM). The high affinity of this ligand provides support to the hypothesis that angiotensin IV adopts a gamma-turn in the C-terminal of its bioactive conformation. Ligand (4) inhibits both human IRAP and aminopeptidase N-activity and induces proliferation of adult neural stem cells at low concentrations. Furthermore, ligand 4 is degraded considerably more slowly in membrane preparations than angiotensin IV. Hence, it might constitute a suitable research tool for biological studies of the (IRAP)/AT(4) receptor.

The effect of sodium and angiotensin-converting enzyme inhibition on the classic circulating renin-angiotensin system in autosomal-dominant polycystic kidney disease patients

BACKGROUND

It has been suggested that inappropriate stimulation of the renin-angiotensin system (RAS) is responsible for the increase in blood pressure that occurs in autosomal-dominant polycystic kidney disease (ADPKD) before the development of renal failure. However, the interpretation of previous studies in ADPKD patients is confounded by inadequate matching with control populations for blood pressure and renal function, or failure to control the sodium intake of participants.

METHODS

A double-blind, placebo-controlled study of two different sodium intakes (350 and 50 mmol/day for 5 days) in a group of 11 hypertensive ADPKD patients and eight matched control subjects with essential hypertension. In addition, blood pressure and hormonal responses were measured after the administration of the angiotensin-converting enzyme inhibitor enalapril for 3 days.

RESULTS

Blood pressure and hormonal responses of the RAS after a reduction in sodium intake and after the administration of enalapril were identical in ADPKD patients and controls.

CONCLUSIONS

Activation of the classic circulating RAS is no greater in hypertensive ADPKD patients than in individuals with essential hypertension.

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

Biphasic actions of angiotensin IV on renal blood flow in the rat

Our aim was to investigate the changes in renal blood flow during brief exposure of the renal vasculature to angiotensin IV (Ang IV). Total renal blood flow was measured by electromagnetic flowmetry in pentobarbital-anesthetized male Sprague Dawley rats. Intrarenal injection of Ang I, Ang II and Ang III produced a dose-dependent vasoconstriction. In contrast, Ang IV and Ang-(3-10) produced a dose-dependent rapid vasoconstriction (lasting seconds) followed by a transient vasodilatation (lasting minutes). The biphasic response to Ang IV was unchanged in rats pre-treated with captopril, whereas the Ang-(3-10) response was abolished implying that its vasoactive activity was due to the generation of Ang IV. The vasodilatory component of Ang IV was unaffected by indomethacin. The biphasic vasoactive response of Ang IV was unaffected by divalinal-Ang IV (AT(4) receptor antagonist) or PD 123319 (AT(2) receptor antagonist), but greatly reduced by losartan or L-158,809 (AT(1) receptor antagonists). These results suggest that Ang IV is distinct from other angiotensins in that it possesses non-prostaglandin mediated renal vasodilatory activity that is apparently linked to the renal vascular AT(1) receptor.

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.

Cyclic insulin-regulated aminopeptidase (IRAP)/AT4 receptor ligands

The angiotensin IV receptor (AT4 receptor) is the insulin-regulated aminopeptidase enzyme (IRAP, EC 3.4.11.3). This membrane-spanning enzyme belongs to the M1 family of zinc-dependent metallo-peptidases. It has been proposed that AT4 receptor ligands exert their physiological effects by binding to the active site of IRAP and thereby inhibiting the catalytic activity of the enzyme. The biological activity of a large series of linear angiotensin IV analogs was previously disclosed. Herein, the synthesis and biological evaluation of a series of angiotensin IV analogs, encompassing macrocyclic ring systems of different sizes, are presented. It is demonstrated that disulfide cyclizations of angiotensin IV can deliver ligands with high IRAP/AT4 receptor affinity. One ligand, with an 11-membered ring system (4), inhibited human IRAP and aminopeptidase N (AP-N) activity with similar potency as angiotensin IV but was considerably more stable than angiotensin IV toward enzymatic degradation. The compound provides a promising starting point for further optimization toward more drug-like derivatives. The cyclic constrained analogs allowed us to propose a tentative bioactive conformation of angiotensin IV and it seems that the peptide adopts an inverse gamma-turn at the C-terminal.

AT1 receptor-activated signaling mediates angiotensin IV-induced renal cortical vasoconstriction in rats

Angiotensin IV (ANG IV), an active ANG II fragment, has been shown to induce systemic and renal cortical effects by binding to ANG IV (AT(4)) receptors and activating unique signaling transductions unrelated to classical type 1 (AT(1)) or type 2 (AT(2)) receptors. We tested whether ANG IV exerts systemic and renal cortical effects on blood pressure, renal microvascular smooth muscle cells (VSMCs), and glomerular mesangial cells (MC) and, if so, whether AT(1) receptor-activated signaling is involved. In anesthetized rats, systemic infusion of ANG II, ANG III, or ANG IV (0.01, 0.1, and 1.0 nmol.kg(-1).min(-1) iv) caused dose-dependent increases in mean arterial pressure (MAP) and decreases in renal cortical blood flow (CBF; P < 0.01). ANG II also induced dose-dependent reductions in renal medullary blood flow (P < 0.01), whereas ANG IV did not. ANG IV-induced pressor and renal cortical vasoconstriction were completely abolished by AT(1) receptor blockade with losartan (5 mg/kg iv; P < 0.05). When ANG IV (1 nmol.kg(-1).min(-1)) was infused directly in the renal artery, CBF was reduced by >30%, and the response was also blocked by losartan (P < 0.01). In the renal cortex, unlabeled ANG IV displaced (125)I-labeled [Sar(1),Ile(8)]ANG II binding, whereas unlabeled ANG II (10 microM) inhibited (125)I-labeled Nle(1)-ANG IV (AT(4)) binding in a concentration-dependent manner (P < 0.01). In freshly isolated renal VSMCs, ANG IV (100 nM) increased intracellular Ca(2+) concentration, and the effect was blocked by losartan and U-73122, a selective inhibitor of phospholipase C/inositol trisphosphate/Ca(2+) signaling (1 microM). In cultured rat MCs, ANG IV (10 nM) induced mitogen-activated protein kinase extracellular/signal-regulated kinase 1/2 phosphorylation via AT(1) receptor- and phospholipase C-activated signaling. These results suggest that, at nanomolar concentrations, ANG IV can increase MAP and induce renal cortical effects by interacting with AT(1) receptor-activated signaling.

Angiotensin II and III upregulate body fluid volume of the clam worm Perinereis sp. via angiotensin II receptors in different manners

Angiotensin III (Ang III) as well as angiotensin II (Ang II) suppressed body weight loss of the clam worm Perinereis sp. under a hyper-osmotic condition, and enhanced body weight gain under a hypo-osmotic condition. Under a drying condition where the water inflow from outside the body was eliminated, Ang II suppressed body weight loss, but Ang III did not. Under these conditions, angiotensins I, IV, and (1-7) had no effect, and saralasin blocked the effects of Ang II and Ang III. It is concluded that Ang II and Ang III upregulate body fluid volume of the clam worm via Ang II receptors in different ways.

Vasoconstrictive effect of angiotensin IV in isolated rat basilar artery independent of AT1 and AT2 receptors

The effect of angiotensin IV (AngIV) was studied in freshly isolated rat basilar arteries (BAs) perfused at a constant rate. AngIV had no effect on basal BA perfusion pressure, but induced a marked concentration-dependent contraction in vessels precontracted by a 50-mM KCl solution (EC50=44.5+/-16 nM). This contraction was unaffected by the angiotensin AT1 receptor antagonist candesartan or the angiotensin AT2 receptor blocker PD123319, but was markedly inhibited by two different specific AT4 receptor antagonists, Nle1-Leu3 yen(CH2-NH2)3-4-AngIV and divalinal-AngIV. Removal of the endothelium abolished the contractile response to AngIV, and pretreatment of endothelium-intact arteries with the endothelin ETA/ETB receptors inhibitor PD142893 blocked the AngIV-induced contraction to the same extent. In BA pretreated with endothelin-1 (ET-1; 0.01 microM), AngIV-induced a concentration-dependent contraction, shifted to the left, compared with that observed with KCl precontraction, unaffected by candesartan but completely abolished by Nle1-Leu3 yen(CH2-NH2)3-4-AngIV. The contractile effect was not affected by endothelium removal in the presence of exogenous ET-1, in contrast to KCl pretreated BA, suggesting that endothelium was mandatory to unmask the effect of AngIV as a source of endogenous ET-1 release. Taken together, these results indicate that low (nanomolar) concentrations of AngIV exert a constrictive effect mediated by its specific binding site AT4 in the rat BA, and that this vasoactive effect is indirect and involves endogenous endothelin(s).

The antiatherogenic potential of blocking the renin-angiotensin system

Angiotensin converting enzyme (ACE) inhibitors have proved effective in preventing or ameliorating clinical manifestations of atherosclerosis, such as myocardial infarction (MI) and heart failure. Experimental evidence demonstrates their anti-atherogenic potential; ACE inhibitors do not only suppress the formation of proatherogenic angiotensin II (AII), but also enhance the formation and release of anti-atherogenic nitric oxide (NO) at local tissue sites; both mechanisms are implicated in the suppression of neointima formation in the balloon-injured vessel wall. A similar anti-atherogenic potential is provided by the blockade of the renin-angiotensin system (RAS) at the level of the angiotensin type-1 (AT1) receptor. AT1 receptor antagonists do not only block the proatherogenic actions of AII, but also induce an enhanced formation and release of anti-atherogenic NO at local tissue sites. AT1 receptor antagonists may therefore prove as effective as ACE inhibitors in patients with manifest atherosclerosis.

Endogenous cystinyl aminopeptidase in Chinese hamster ovary cells: characterization by [125I]Ang IV binding and catalytic activity

The angiotensin II C-terminal hexapeptide fragment angiotensin IV (Ang IV) exerts central and cardiovascular effects. Cystinyl aminopeptidase (EC 3.4.11.3), a membrane-associated zinc-dependent metallopeptidase of the M1 family, has recently been found to display high affinity for Ang IV and it was proposed to represent the AT4 receptor. We present evidence for the presence of endogenous cystinyl aminopeptidase in membranes from Chinese hamster ovary (CHO-K1) cells by binding studies with [125I]Ang IV and by measuring the cleavage of L-leucine-p-nitroanilide. The equilibrium dissociation constant of [125I]Ang IV in saturation binding studies (KD= 0.90 nM) was similar to the value (KD= 0.70 nM) calculated from the association and dissociation rates. Binding was displaced with high potency by the "AT4 receptor" ligands (Ang IV > divalinal1-Ang IV approximately LVV-hemorphin-7 approximately LVV-hemorphin-6 > Ang (3-7) > Ang III > Ang (4-8)) but not by AT1/AT2 receptor antagonists. Enzymatic activity in CHO-K1 cell membranes was competitively inhibited upto 94% by Ang IV and other "AT4 receptor" ligands (Ang IV > Ang III approximately divalinal1-Ang IV approximately Ang (3-7) approximately LVV-hemorphin-7 > Ang (4-8) approximately LVV-hemorphin-6). High affinity binding of [125I]Ang IV required the presence of metal chelators and the ligands such as Ang IV and LVV-hemorphin-7 displayed higher potency in the binding studies as in the enzyme assay. This difference in potency varied from one peptide to another. These pharmacological properties match those previously reported for the recombinantly-expressed human cystinyl aminopeptidase in embryonal kidney cells.

The brain angiotensin system and extracellular matrix molecules in neural plasticity, learning, and memory

The brain renin-angiotensin system (RAS) has long been known to regulate several classic physiologies including blood pressure, sodium and water balance, cyclicity of reproductive hormones and sexual behaviors, and pituitary gland hormones. These physiologies are thought to be under the control of the angiotensin II (AngII)/AT1 receptor subtype system. The AT2 receptor subtype is expressed during fetal development and is less abundant in the adult. This receptor appears to oppose growth responses facilitated by the AT1 receptor, as well as growth factor receptors. Recent evidence points to an important contribution by the brain RAS to non-classic physiologies mediated by the newly discovered angiotensin IV (AngIV)/AT4 receptor subtype system. These physiologies include the regulation of blood flow, modulation of exploratory behavior, and a facilitory role in learning and memory acquisition. This system appears to interact with brain matrix metalloproteinases in order to modify extracellular matrix molecules thus permitting the synaptic remodeling critical to the neural plasticity presumed to underlie memory consolidation, reconsolidation, and retrieval. There is support for an inhibitory influence by AngII activation of the AT1 subtype, and a facilitory role by AngIV activation of the AT4 subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning. The discovery of the AT4 receptor subtype, and its facilitory influence upon learning and memory, suggest an important role for the brain RAS in normal cognitive processing and perhaps in the treatment of dysfunctional memory disease states.

Quantification of hemorphins in Alzheimer's disease brains

The hemorphins are a family of opioid receptor-binding peptides originating from the beta-chain of hemoglobin and have been found at high levels within the central and peripheral nervous systems. In addition to opioid receptor binding, hemorphins have been shown to have a number of effects on the renin-angiotensin system, including inhibition of angiotensin-converting enzyme and angiotensin IV receptor binding. However, relatively few studies have examined the role of hemorphins in neurological diseases. Here we report the first study of hemorphins in Alzheimer's disease (AD) brains. Quantitative MALDI-TOF mass spectrometry was employed to assess levels of LVV and VV hemorphin-6 and -7 in 10 control and 10 AD brain tissue samples. LVV hemorphin-6 and total hemorphin levels were elevated in AD temporal neocortex but not in hippocampus, occipital lobe, or frontal lobe. The elevation of hemorphins is probably indicative of a vascular abnormality resulting from cerebral amyloid angiopathy associated with both neurodegenerative disease and aging.

Involvement of D1 dopamine receptors in the cognitive effects of angiotensin IV and des-Phe6 angiotensin IV

An important role for angiotensin IV (Ang IV) in the processes of learning and memory has now been well established. We have previously found that intracerebroventricular (ICV) administration of Ang IV as well as des-Phe6-Ang IV enhances learning of conditioned avoidance responses (CARs), facilitates recall of a passive avoidance (PA) task, and improves object recognition (OR) in rats. Since the dopaminergic system is crucial for the cognitive processes, in this study our aim was to determine the dopaminergic D1 mediation of these effects using SCH 23390 as a selective D1 receptor antagonist. Male Wistar rats (180-200 g), pretreated with SCH 23390 (R-[+]-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) 0.05 mg/kg intraperitoneally (IP), were given Ang IV or des-Phe6-Ang IV (1 nmol ICV) 1 h later and then tested in the above cognitive paradigms, as well as in the open field and an elevated 'plus' maze to control for the unspecific, respectively, motor and emotional, effects of our treatments. Both, Ang IV and des-Phe6-Ang IV effectively enhanced learning of CARs (P < 0.05), recall of PA (P < 0.001), and improved OR (P < 0.001). Pretreatment with SCH 23390 abolished the cognitive effects of both peptides. SCH 23390, Ang IV, and des-Phe6-Ang IV, given at the same doses and routes as in the cognitive tests, did not significantly influence crossings, rearings and bar approaches in the open field, nor the parameters measured in the elevated 'plus' maze, thus making a major contribution of the unspecific effects of our treatments to the results of the memory tests improbable. In conclusion, these results indicate that the functional dopaminergic D1 receptors are necessary for the Ang IV and des-Phe6-Ang IV cognitive effects to occur.

Norleucine1-Angiotensin IV alleviates mecamylamine-induced spatial memory deficits

The brain angiotensin AT4 receptor subtype has been implicated in cognitive processing. We initially established that intracerebroventricular administration of the nAChR-antagonist mecamylamine (mec) interfered with spatial memory performance in male Sprague-Dawley rats. Next we demonstrated that mec-induced deficits in spatial memory were overcome by the AT4 receptor-agonist Norleucine1-Angiotensin IV (Nle1-Ang IV). Nle1-Ang IV could not, however, compensate for spatial learning impairments precipitated by both mec and the mAChR-antagonist scopolamine. These findings support the importance of the AT4 receptor in cognitive processing and suggest that the ability of Nle1-Ang IV to improve spatial memory deficiencies may be dependant upon the brain cholinergic system.

AT4 receptor binding in the developing rabbit

The binding of the AT(4)-specific analog, divalinal-AngIV (Dival), was studied in rabbit fetuses of various gestational ages. Saturation isotherm and competition data from selected tissues indicate that fetal Dival binding sites are saturable and specific for AT(4) ligands. Autoradiographs revealed that binding was present in all the specimens examined. The peripheral nerves, kidneys, and heart were particularly heavily labeled. Labeling of some tissues, such as forming bones, was not constant as gestational age increased. Other tissues, including multilocular fat, sinus hairs, and enamel organs of nascent teeth, exhibited substantial binding as these tissues developed.

Effect of I.C.V. injection of AT4 receptor ligands, NLE1-angiotensin IV and LVV-hemorphin 7, on spatial learning in rats

Central administration of angiotensin IV (Ang IV) or its analogues enhance performance of rats in passive avoidance and spatial memory paradigms. The purpose of this study was to examine the effect of a single bolus injection of two distinct AT4 ligands, Nle1-Ang IV or LVV-haemorphin-7, on spatial learning in the Barnes circular maze. Mean number of days for rats treated with either Nle1-Ang IV or LVV-haemorphin-7 to achieve learner criterion is significantly reduced compared with controls (P < 0.001 and P < 0.05 respectively). This is due to enhanced ability of the peptide-treated rats to adopt a spatial strategy for finding the escape hatch. In all three measures of learning performance, (1) the number of errors made, (2) the distance travelled and (3) the latency in finding the escape hatch, rats treated with either 100 pmol or 1 nmol of Nle1-Ang IV or 100 pmol LVV-haemorphin-7 performed significantly better than the control groups. As early as the first day of testing, the rats treated with the lower dose of Nle1-Ang IV or LVV-haemorphin-7 made fewer errors (P < 0.01 and P < 0.05 respectively) and travelled shorter distances (P < 0.05 for both groups) than the control animals. The enhanced spatial learning induced by Nle1-Ang IV (100 pmol) was attenuated by the co-administration of the AT4 receptor antagonist, divalinal-Ang IV (10 nmol). Thus, administration of AT4 ligands results in an immediate potentiation of learning, which may be associated with facilitation of synaptic transmission and/or enhancement of acetylcholine release.

Effect of fimbria-fornix lesion on 125I-angiotensin IV (Ang IV) binding in the guinea pig hippocampus

Central administration of angiotensin IV (Ang IV) and its analogues facilitates memory retention and retrieval in normal animals and reverses amnesia induced by scopolamine or by bilateral perforant pathway lesions. Ang IV binds with high affinity and specificity to a novel binding site designated the AT(4) receptor. AT(4) receptors are abundant in the medial septum and hippocampus, a cholinergic pathway associated with memory processing. The aim of this study was to determine whether AT(4) receptors in the guinea pig hippocampus were associated with the neural input from the basal forebrain. The fimbria-fornix was lesioned by a unilateral-knife cut and the brain was processed for 125I-Ang IV binding, acetylcholinesterase, and cresyl violet staining. Unilateral lesions of the fimbria-fornix significantly reduced acetylcholinesterase staining in the ipsilateral hippocampus. The loss in cholinergic input to the hippocampus was associated with a small, but significant, reduction in 125I-Ang IV binding in the CA2 (-9%; P=0.001), and CA3 (-5%; P=0.003) of the rostral hippocampus. No other changes in 125I-Ang IV binding were observed. These results provide evidence that the majority of AT(4) receptor binding occurs in a post-synaptic locus in the guinea pig hippocampus.

AT1 and AT2 receptors in human glomerular endothelial cells at different passages

In human adult kidney angiotensin II (AngII) effects are mediated by the AT1 receptor, while the functions of AT2 receptors are mostly unknown. Since AngII regulates endothelial cell growth by AT1 and AT2 receptors, we analysed their functional aspects at different passages in human glomerular endothelial cells (GENC). Semiquantitative reverse transcription-polymerase chain reaction revealed the presence of AT1 and AT2 receptors between 2p and 15p cell passages with different levels of expression. In fact, binding studies of different families of displacement curves using AngII, DUP753 (AT1 antagonist), and PD123177 (AT2 antagonist) showed the presence of AT1a and AT2 receptors at 4p-9p while in GENC 2p only the presence of AT2. In terms of mitogenic activity, AngII was unable to stimulate GENC 2p growth. On the contrary, in GENC 4p-9p and 15p a significant thymidine incorporation was observed. This stimulatory effect seemed to be induced also by the concomitant release of PDGF-BB AT1a mediated. In conclusion, AT1a and AT2 receptors are represented in GENC with a different ratio depending upon the cell passage. AngII regulates the mitogenic effect through AT1a receptors (in later cell passages 4p-15p) involving the release of PDGF-BB, while AT2 (in early cell passage 2p) showed a predominant negative growth control.

Hormonal status modifies renin-angiotensin system-regulating aminopeptidases and vasopressin-degrading activity in the hypothalamus-pituitary-adrenal axis of male mice

Local renin-angiotensin systems (RAS) have been postulated in brain, pituitary and adrenal glands. These local RAS have been implicated, respectively, in the central regulation of the cardiovascular system and body water balance, the secretion of pituitary hormones and the secretion of aldosterone by adrenal glands. By other hand, it is known that the hypothalamus-pituitary-adrenal (HPA) axis is involved in blood pressure regulation, and is affected by sex hormones. The aim of the present work is to analyze the influence of testosterone on RAS-regulating aminopeptidase A, B and M activities and vasopressin-degrading activity in the HPA axis, measuring these activities in their soluble and membrane-bound forms in the hypothalamus, pituitary and adrenal glands of orchidectomized males and orchidectomized males treated subcutaneously with several doses of testosterone. The present data suggest that in male mice, testosterone influences the RAS- and vasopressin-degrading activities at all levels of the HPA axis.

The brain renin-angiotensin system: location and physiological roles

Angiotensinogen, the precursor molecule for angiotensins I, II and III, and the enzymes renin, angiotensin-converting enzyme (ACE), and aminopeptidases A and N may all be synthesised within the brain. Angiotensin (Ang) AT(1), AT(2) and AT(4) receptors are also plentiful in the brain. AT(1) receptors are found in several brain regions, such as the hypothalamic paraventricular and supraoptic nuclei, the lamina terminalis, lateral parabrachial nucleus, ventrolateral medulla and nucleus of the solitary tract (NTS), which are known to have roles in the regulation of the cardiovascular system and/or body fluid and electrolyte balance. Immunohistochemical and neuropharmacological studies suggest that angiotensinergic neural pathways utilise Ang II and/or Ang III as a neurotransmitter or neuromodulator in the aforementioned brain regions. Angiotensinogen is synthesised predominantly in astrocytes, but the processes by which Ang II is generated or incorporated in neurons for utilisation as a neurotransmitter is unknown. Centrally administered AT(1) receptor antagonists or angiotensinogen antisense oligonucleotides inhibit sympathetic activity and reduce arterial blood pressure in certain physiological or pathophysiological conditions, as well as disrupting water drinking and sodium appetite, vasopressin secretion, sodium excretion, renin release and thermoregulation. The AT(4) receptor is identical to insulin-regulated aminopeptidase (IRAP) and plays a role in memory mechanisms. In conclusion, angiotensinergic neural pathways and angiotensin peptides are important in neural function and may have important homeostatic roles, particularly related to cardiovascular function, osmoregulation and thermoregulation.

Minireview: overview of the renin-angiotensin system--an endocrine and paracrine system

Since the discovery of renin as a pressor substance in 1898, the renin-angiotensin (RAS) system has been extensively studied because it remains a prime candidate as a causative factor in the development and maintenance of hypertension. Indeed, some of the properties of the physiologically active component of the RAS, angiotensin II, include vasoconstriction, regulation of renal sodium and water absorption, and increasing thirst. Initially, its affect on blood pressure was thought to be mediated primarily through the classical endocrine pathway; that is, the generation of blood-borne angiotensin with actions in target tissues. More recently, however, it has become appreciated that a local autocrine or paracrine RAS may exist in a number of tissues, and that these may also play a significant role in regulating blood pressure. Some of the difficulties in studying tissue RAS stem from the limitations of pharmacology in not differentiating between RAS products made systemically from those synthesized locally. However, the development of transgenic animals with highly specific promoters to target the RAS to specific tissues provided important tools to dissect these systems. Thus, this minireview will discuss recent advances in understanding the relationship between endocrine and paracrine (tissue) RAS using transgenic models.

Angiotensin IV interacts with a juxtamembrane site on AT(4)/IRAP suggesting an allosteric mechanism of enzyme modulation

Angiotensin IV (Ang IV), the 3-8 fragment of angiotensin II, binds to a specific receptor (AT(4)) that has recently been identified as the transmembrane aminopeptidase insulin-regulated aminopeptidase (IRAP) based on the fact that the two proteins share several pharmacological and biochemical properties. Our binding studies indicated that bovine heart expresses relatively large amounts (1.2 pmol/mg protein) of high-affinity binding sites for Ang IV (K(d)=1.8 nM). A photoaffinity-labeling approach combined with mild trypsin digestion revealed that the AT(4) receptor of bovine heart is a single transmembrane domain protein (153 kDa) with a large extracellular fragment (143 kDa). After alkaline denaturation of the AT(4) receptor, trypsin digestion produced two small membrane-associated fragments (16.9 and 6.6 kDa). These results suggest that Ang IV interacts with a juxtamembrane domain of AT(4) receptor. The location of the juxtamembrane site of contact was different from that of the active site of IRAP, suggesting that Ang IV uses an allosteric mechanism to modulate the activity of the AT(4)/IRAP.

Potential of renin inhibition in cardiovascular disease

Since renin catalyses the first and rate-limiting step of the renin-angiotensin system (RAS) cascade, interruption of the generation of angiotensin II (Ang II) by renin inhibitors at this highly specific initial step of the cascade has long been a therapeutic goal. The early development of renin inhibitors was hampered by problems with bioavailability and high costs of synthesis. However, more recently a potent non-peptidic inhibitor of renin, aliskiren, with acceptable oral bioavailability, has been synthesised. Aliskiren effectively reduces Ang II levels in normal volunteers and has been shown to lower blood pressure (BP) in patients with mild-to-moderate hypertension. Renin inhibitors would be expected to have similar, but not identical effects to those of the established RAS antagonists. Due to the lack of effective alternative enzyme pathways, blockade of Ang II production may be more effective with renin inhibition than with angiotensin-converting enzyme (ACE) inhibition. Furthermore, because renin has high specificity for only one substrate, angiotensinogen, side-effects would be expected to be less frequent. It is currently unclear whether blockade of Ang II type 1 (AT1) receptors, leaving other Ang II receptors (AT2, AT3 and AT4) unblocked, is preferable to the reduction in plasma and tissue Ang II levels achieved with either ACE or renin inhibition. Pharmacological suppression of the RAS, through ACE inhibition, or blockade of AT1, beta-adrenoceptor or mineralocorticoid receptors, has been proven to reduce morbidity and mortality in patients with hypertension, diabetes mellitus, atherosclerosis, heart failure and nephropathy. While, to date, aliskiren has only been shown to reduce BP, it appears likely that orally-active renin inhibitors could prove useful in the management of a wide range of cardiovascular pathologies.

Possible involvement of the adipose tissue renin-angiotensin system in the pathophysiology of obesity and obesity-related disorders

Angiotensin II (Ang II), acting on the AT1 and AT2 receptors in mammalian cells, is the vasoactive component of the renin-angiotensin system (RAS). Several components of the RAS have been demonstrated in different tissues, including adipose tissue. Although the effects of Ang II on metabolism have not been studied widely, it is intriguing to assume that components of the RAS produced by adipocytes may play an autocrine, a paracrine and/or an endocrine role in the pathophysiology of obesity and provide a potential pathway through which obesity leads to hypertension and type 2 diabetes mellitus. In the first part of this review, we will describe the production of Ang II, the different receptors through which Ang II exerts its effects and summarize the concomitant intracellular signalling cascades. Thereafter, potential Ang II-induced mechanisms, which may be associated with obesity and obesity-related disorders, will be considered. Finally, we will focus on the different pharmaceutical agents that interfere with the RAS and highlight the possible implications of these drugs in the treatment of obesity-related disorders.

Therapeutic potential of renin inhibitors in the management of cardiovascular disorders

The renin-angiotensin system (RAS) is well recognized for its importance in regulation of BP, electrolyte balance and vascular growth. Pharmacological suppression of the RAS, through ACE inhibition and/or angiotensin receptor blockade, is a proven effective therapeutic approach to the treatment of a range of cardiovascular diseases. Renin is the enzyme that catalyzes the first and rate-limiting step of RAS, the cleavage of angiotensinogen to angiotensin I (A-I). A-I is then further converted by ACE to the biologically active vasoconstrictor, A-II. Interruption of the generation of A-II by renin inhibitors at this highly specific initial step of the cascade would be expected to have similar but not identical effects to those of the already well established RAS antagonists. Due to the lack of effective alternative enzyme pathways, blockade of A-II production may be more effective with renin inhibition than with ACE inhibition, and because of the high specificity of renin for only one substrate, namely angiotensinogen, adverse effects would be expected to be less frequent. It is currently unclear whether blockade of angiotensin II type 1 receptors (AT(1)), leaving other A-II receptors unblocked, is preferable to the reduction in plasma and tissue A-II levels achieved with either ACE or renin inhibition. The development of early peptidic and peptidomimetic renin inhibitors was hampered by problems with oral bioavailability and high costs of synthesis. However recent work has led to the synthesis of a potent non-peptidic inhibitor of renin, aliskiren, which has acceptable oral bioavailability. This renin inhibitor has been shown to effectively reduce A-II levels in normal volunteers and to lower BP in patients with mild to moderate hypertension. It appears likely that aliskiren is the first of a new class of agents that may prove useful in the management of patients with nephropathy, heart failure and atherosclerosis in addition to hypertension.

Cellular targets for angiotensin II fragments: pharmacological and molecular evidence

Although angiotensin II has long been considered to represent the end product of the renin-angiotensin system (RAS), there is accumulating evidence that it encompasses additional effector peptides with diverse functions. In this respect, angiotensin IV (Ang IV) formed by deletion of the two N terminal amino acids, has sparked great interest because of its wide range of physiological effects. Among those, its facilitatory role in memory acquisition and retrieval is of special therapeutic relevance. High affinity binding sites for this peptide have been denoted as AT(4)- receptors and, very recently, they have been proposed to correspond to the membrane-associated OTase/ IRAP aminopeptidase. This offers new opportunities for examining physiological roles of Ang IV in the fields of cognition, cardiovascular and renal metabolism and pathophysiological conditions like diabetes and hypertension. Still new recognition sites may be unveiled for this and other angiotensin fragments. Recognition sites for Ang-(1-7) (deletion of the C terminal amino acid) are still elusive and some of the actions of angiotensin III (deletion of the N terminal amino acid) in the CNS are hard to explain on the basis of their interaction with AT(1)-receptors only. A more thorough cross-talk between in vitro investigations on native and transfected cell lines and in vivo investigations on healthy, diseased and transgenic animals may prove to be essential to further unravel the molecular basis of the physiological actions of these small endogenous angiotensin fragments.

Angiotensins II and IV stimulate the activity of tyrosine kinases in estrogen-induced rat pituitary tumors

The effects of angiotensin II (AngII) and its fragment 3-8 (angiotensin IV, AngIV) on tyrosine kinase (TK) activity in estrogen-induced rat pituitary tumor homogenates were studied. It was found that both angiotensin peptides increase the TK activity. AngIV was effective in a lower concentration and its maximal effect was greater as compared to that of AngII. Moreover, the specific inhibitors of aminopeptidase A (EC33) and of aminopeptidase N (PC18) significantly attenuated the stimulatory effect of AngII. Because the action of both aminopeptidases is necessary to convert AngII into AngIV, this finding suggested that the effect of AngII on TK activity is (at least in part) exerted via AngIV.

Recent advances in angiotensin II signaling

Angiotensin II (Ang II) is a multifunctional hormone that influences the function of cardiovascular cells through a complex series of intracellular signaling events initiated by the interaction of Ang II with AT1 and AT2 receptors. AT1 receptor activation leads to cell growth, vascular contraction, inflammatory responses and salt and water retention, whereas AT2 receptors induce apoptosis, vasodilation and natriuresis. These effects are mediated via complex, interacting signaling pathways involving stimulation of PLC and Ca2+ mobilization; activation of PLD, PLA2, PKC, MAP kinases and NAD(P)H oxidase, and stimulation of gene transcription. In addition, Ang II activates many intracellular tyrosine kinases that play a role in growth signaling and inflammation, such as Src, Pyk2, p130Cas, FAK and JAK/STAT. These events may be direct or indirect via transactivation of tyrosine kinase receptors, including PDGFR, EGFR and IGFR. Ang II induces a multitude of actions in various tissues, and the signaling events following occupancy and activation of Ang receptors are tightly controlled and extremely complex. Alterations of these highly regulated signaling pathways may be pivotal in structural and functional abnormalities that underlie pathological processes in cardiovascular diseases such as cardiac hypertrophy, hypertension and atherosclerosis.

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.

Characterisation of a thymic renin-angiotensin system in the transgenic m(Ren-2)27 rat

We previously showed the rat thymus contains and secretes active renin. However, the cellular location of the thymic renin-angiotensin system (RAS) is unknown. To more easily study the thymic RAS we used the hypertensive transgenic (mRen-2)27 rat which overexpresses renin and angiotensin in extra-renal tissues. Comparisons were made with normotensive Sprague Dawley (SD) rats. All rats exhibited intense immunolabeling for renin protein and angiotensin in macrophages and thymic epithelial cells, however renin prosequence was not detected. In each rat strain, thymic renin was predominately active and highest in Ren-2 rats (Ren-2, 22.6+/-4.2, SD 0.8+/-0.1 mGoldblatt Units/g, mean+/-SEM). Renin mRNA was identified in Ren-2 and SD rat thymus by RT-PCR. Thymic angiotensin II concentrations/wet weight in Ren-2 (20.1+/-1.1 fmol/g) and SD (15.8+/-2.3 fmol/g) rats were similar to plasma. In conclusion, macrophages and epithelial cells are the source of active renin in the rat thymus. The thymic RAS may have actions systemically and may also influence local processes such as blood flow and cell growth.

Neural plasticity and the brain renin-angiotensin system

The brain renin-angiotensin system mediates several classic physiologies including body water balance, maintenance of blood pressure, cyclicity of reproductive hormones and sexual behaviors, and regulation of pituitary gland hormones. In addition, angiotensin peptides have been implicated in neural plasticity and memory. The present review initially describes the extracellular matrix (ECM) and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases, and tissue inhibitors of metalloproteinases in the maintenance and degradation of the ECM. It is the ECM that appears to permit synaptic remodeling and thus is critical to the plasticity that is presumed to underlie mechanisms of memory consolidation and retrieval. The interrelationship among long-term potentiation (LTP), CAMs, and synaptic strengthening is described, followed by the influence of angiotensins on LTP. There is strong support for an inhibitory influence by angiotensin II (AngII) and a facilitory role by angiotensin IV (AngIV), on LTP. Next, the influences of AngII and IV on associative and spatial memories are summarized. Finally, the impact of sleep deprivation on matrix metalloproteinases and memory function is described. Recent findings indicate that sleep deprivation-induced memory impairment is accompanied by a lack of appropriate changes in matrix metalloproteinases within the hippocampus and neocortex as compared with non-sleep deprived animals. These findings generally support an important contribution by angiotensin peptides to neural plasticity and memory consolidation.

Angiotensin IV is a potent agonist for constitutive active human AT1 receptors. Distinct roles of the N-and C-terminal residues of angiotensin II during AT1 receptor activation

The octapeptide hormone, angiotensin II (Ang II), exerts its major physiological effects by activating AT(1) receptors. In vivo Ang II is degraded to bioactive peptides, including Ang III (angiotensin-(2-8)) and Ang IV (angiotensin-(3-8)). These peptides stimulate inositol phosphate generation in human AT(1) receptor expressing CHO-K1 cells, but the potency of Ang IV is very low. Substitution of Asn(111) with glycine, which is known to cause constitutive receptor activation by disrupting its interaction with the seventh transmembrane helix (TM VII), selectively increased the potency of Ang IV (900-fold) and angiotensin-(4-8), and leads to partial agonism of angiotensin-(5-8). Consistent with the need for the interaction between Arg(2) of Ang II and Ang III with Asp(281), substitution of this residue with alanine (D281A) decreased the peptide's potency without affecting that of Ang IV. All effects of the D281A mutation were superseded by the N111G mutation. The increased affinity of Ang IV to the N111G mutant was also demonstrated by binding studies. A model is proposed in which the Arg(2)-Asp(281) interaction causes a conformational change in TM VII of the receptor, which, similar to the N111G mutation, eliminates the constraining intramolecular interaction between Asn(111) and TM VII. The receptor adopts a more relaxed conformation, allowing the binding of the C-terminal five residues of Ang II that switches this "preactivated" receptor into the fully active conformation.

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.

Comparison of effects of angiotensin peptides in the regulation of clitoral cavernosum smooth muscle tone

The isometric tension measurement and in vitro autoradiography were used in clitoral cavernosum smooth muscle (CSM). Angiotensin ANG III, ANG IV, ANG II and ANG I induced contractions in clitoral CSM strips. ANG III and ANG I- induced contraction was five times less active than ANG II, whereas ANG IV-induced contraction was 1181-fold less potent than ANG II. Contractile responses to ANG III, ANG IV, ANG II and ANG I were significantly inhibited by type 1 ANG II (AT 1) receptor antagonist Dup 753 but not by type 2 ANG II (AT2) receptor antagonist PD 123,319. Pre-treatment with Nomega-nitro-L-arginine methyl ester, nitric oxide (NO) synthase inhibitor accentuated force of contraction induced by ANG III, ANG IV and ANG II. Amastatin, an aminopeptidase inhibitor enhanced ANG III- and ANG IV-induced contractions. Specific binding sites for 125I-ANG II were found in the clitoral CSM. Specific binding of 125I-ANG II was displaced by unlabeled ANG peptides. This study suggests that the contractile responses to all four peptides of the ANG family are mediated via AT1 receptors but not AT2 receptors. Further, the rank order of potency of contraction was as follows, ANG II> ANG I>ANG III>ANG IV. It is also suggested that peptides of the ANG family have a cross-talk with the NO system and aminopeptidase is involved in the modulation of the tone of clitoral CSM by ANG III and ANG IV.

Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

Considerable evidence now suggests an interrelationship among long-term potentiation (LTP), extracellular matrix (ECM) reconfiguration, synaptogenesis, and memory consolidation within the mammalian central nervous system. Extracellular matrix molecules provide the scaffolding necessary to permit synaptic remodeling and contribute to the regulation of ionic and nutritional homeostasis of surrounding cells. These molecules also facilitate cellular proliferation, movement, differentiation, and apoptosis. The present review initially focuses on characterizing the ECM and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), in the maintenance and degradation of the ECM. The induction and maintenance of LTP is described. Debate continues over whether LTP results in some form of synaptic strengthening and in turn promotes memory consolidation. Next, the contribution of CAMs and TIMPs to the facilitation of LTP and memory consolidation is discussed. Finally, possible roles for angiotensins, MMPs, and tissue plasminogen activators in the facilitation of LTP and memory consolidation are described. These enzymatic pathways appear to be very important to an understanding of dysfunctional memory diseases such as Alzheimer's disease, multiple sclerosis, brain tumors, and infections.