State-of-the-Art lecture. Antiproliferative actions of angiotensin-(1-7) in vascular smooth muscle

Angiotensin-(1-7) Inhibits Growth of Human Lung Adenocarcinoma Xenografts in Nude Mice through a Reduction in Cyclooxygenase-2

Angiotensin-(1-7) [Ang-(1-7)] is an endogenous peptide of the renin-angiotensin system with vasodilator and antiproliferative properties. Our previous studies showed that Ang-(1-7) reduced serum-stimulated growth of human lung cancer cells in vitro through activation of a unique AT((1-7)) receptor. The current study investigates the effect of Ang-(1-7) on lung tumor growth in vivo, using a human lung tumor xenograft model. Athymic mice with tumors resulting from injection of A549 human lung cancer cells were treated for 28 days with either i.v. saline or Ang-(1-7), delivered by implanted osmotic mini-pumps. Treatment with Ang-(1-7) reduced tumor volume by 30% compared with the size before treatment; in contrast, tumor size in the saline-treated animals increased 2.5-fold. These results correlate with a reduction in the proliferation marker Ki67 in the Ang-(1-7)-infused tumors when compared with the saline-infused tumor tissues. Treatment with Ang-(1-7) significantly reduced cyclooxygenase-2 (COX-2) mRNA and protein in tumors of Ang-(1-7)-infused mice when compared with mice treated with saline as well as in the parent A549 human lung cancer cells in tissue culture. These results suggest that Ang-(1-7) may decrease COX-2 activity and proinflammatory prostaglandins to inhibit lung tumor growth. In contrast, the heptapeptide had no effect on COX-1 mRNA in xenograft tumors or A549 cells. Because Ang-(1-7), a peptide with antithrombotic properties, reduces growth through activation of a selective AT((1-7)) receptor, our results suggest that the heptapeptide represents a novel treatment for lung cancer by reducing COX-2.

Evidence for a new angiotensin-(1-7) receptor subtype in the aorta of Sprague-Dawley rats

We have recently described, in the mouse aorta, the vasodilator effect of angiotensin-(1-7) (Ang-(1-7)) was mediated by activation of the Mas Ang-(1-7) receptor and that A-779 and D-Pro7-Ang-(1-7) act as Mas receptor antagonists. In this work we show pharmacological evidence for the existence of a different Ang-(1-7) receptor subtype mediating the vasodilator effect of Ang-(1-7) in the aorta from Sprague-Dawley (SD) rats. Ang-(1-7) induced an endothelium-dependent vasodilator effect in aortic rings from SD rats which was inhibited by removal of the endothelium and by L-NAME (100 microM) but not by indomethacin (10 microM). The Ang-(1-7) receptor antagonist D-Pro7-Ang-(1-7) (0.1 microM) abolished the vasodilator effect of the peptide. However, the other specific Ang-(1-7) receptor antagonist, A-779 in concentrations up to 10 microM, did not affect vasodilation induced by Ang-(1-7). The Ang II AT1 and AT2 receptors antagonists CV11974 (0.01 microM) and PD123319 (1 microM), respectively, the bradykinin B2 receptor antagonist HOE 140 (1 microM) and the inhibitor of ACE captopril (10 microM) did not change the effect of Ang-(1-7). Our results show that in the aorta of SD rats, the vasodilator effect of Ang-(1-7) is dependent on endothelium-derived nitric oxide. This effect is mediated by the activation of Ang-(1-7) receptors sensitive to D-Pro7-Ang-(1-7), but not to A-779, which suggests the existence of a different Ang-(1-7) receptor subtype.

Localized accumulation of angiotensin II and production of angiotensin-(1-7) in rat luteal cells and effects on steroidogenesis

These studies aim to investigate subcellular distribution of angiotensin II (ANG II) in rat luteal cells, identify other bioactive angiotensin peptides, and investigate a role for angiotensin peptides in luteal steroidogenesis. Confocal microscopy showed ANG II distributed within the cytoplasm and nuclei of luteal cells. HPLC analysis showed peaks that eluted with the same retention times as ANG-(1-7), ANG II, and ANG III. Their relative concentrations were ANG II >or= ANG-(1-7) > ANG III, and accumulation was modulated by quinapril, an inhibitor of angiotensin-converting enzyme (ACE), Z-proprolinal (ZPP), an inhibitor of prolyl endopeptidase (PEP), and parachloromercurylsulfonic acid (PCMS), an inhibitor of sulfhydryl protease. Phenylmethylsulfonyl fluoride (PMSF), a serine protease inhibitor, did not affect peptide accumulation. Quinapril, ZPP, PCMS, and PMSF, as well as losartan and PD-123319, the angiotensin receptor type 1 (AT1) and type 2 (AT2) receptor antagonists, were used in progesterone production studies. ZPP significantly reduced luteinizing hormone (LH)-dependent progesterone production (P < 0.05). Quinapril plus ZPP had a greater inhibitory effect on LH-stimulated progesterone than either inhibitor alone, but this was not reversed by exogenous ANG II or ANG-(1-7). Both PCMS and PMSF acutely blocked LH-stimulated progesterone, and PCMS blocked LH-sensitive cAMP accumulation. Losartan inhibited progesterone production in permeabilized but not intact luteal cells and was reversed by ANG II. PD-123319 had no significant effect on luteal progesterone production in either intact or permeabilized cells. These data suggest that steroidogenesis may be modulated by angiotensin peptides that act in part through intracellular AT1 receptors.

The effect of angiotensin 1-7 on tyrosine kinases activity in rat anterior pituitary

Angiotensin 1-7 (Ang 1-7) is a peptide originated from Ang II. It is known that in vessels Ang 1-7 shows opposite effects to Ang II. Ang 1-7 can modify processes of proliferation. However, Ang 1-7 action in pituitary gland cells was never studied. Moreover, the specific binding sites for Ang 1-7 are still unknown. The aim of this study was to examine the effects of Ang 1-7 on tyrosine kinases (PTKs) activity in the anterior pituitary. The reaction of phosphorylation was carrying out in presence of different concentration of Ang 1-7 and losartan (antagonist of AT1 receptor) and PD123319 (antagonist of AT2). Our results show that Ang 1-7 inhibited activity of PTK to 60% of basic activity. Losartan did not change the Ang 1-7-induced changes in PTKs activity. The presence of PD123319 together with Ang 1-7 caused stronger inhibition PTKs activity than Ang 1-7 alone. These observations suggest that Ang 1-7 binds to the novel, unknown, specific for this peptide receptor.

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

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

Are multiple angiotensin receptor types involved in angiotensin (1-7) actions on isolated rat portal vein

Angiotensin (1-7) [Ang (1-7)] is a bioactive component of the renin angiotensin system. Ang (1-7) may interact with angiotensin type 1 (AT1) or type 2 (AT2) receptors and with Ang (1-7) - specific receptors. We examined the interactions between different doses of Ang (1-7) (1 nM-1 microM) and angiotensin II (Ang II) (10 and 100 nM) on isolated rat portal vein. In endothelium-denuded portal vein rings, Ang (1-7) inhibited contractile effects induced by Ang II. The effects of Ang (1-7) were modified by indomethacin, N(G)-nitro-L-arginine methyl ester (L-NAME), (D-Ala7)-Angiotensin (1-7) (H-2888) and losartan. Our results suggest that on rat isolated portal vein rings without endothelium, Ang (1-7) reduces Ang II-induced contractions by acting mostly on Ang (1-7) specific receptors, and this effect is mediated by vasodilatatory prostaglandins. At high concentrations, Ang (1-7) effects are mediated by AT1-receptors, though to a lesser extent than by Ang (1-7) specific receptors.

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

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

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.

In vivo characterization of the angiotensin-(1-7)-induced dopamine and gamma-aminobutyric acid release in the striatum of the rat

The effect of angiotensin (Ang)-1-7 on dopamine, gamma-aminobutyric acid (GABA) and glutamate release in the striatum of the rat was examined using in vivo microdialysis. Ang-(1-7) was administered locally in the striatum through the microdialysis probe. At a concentration of 100 microm, Ang-(1-7) caused a significant increase in extracellular dopamine and GABA but had no effect on glutamate release. The Ang-(1-7)-induced dopamine release was blocked by EC33, an inhibitor of aminopeptidase A, an enzyme which converts Ang-(1-7) into Ang-(3-7), suggesting that this effect occurs after metabolism into Ang-(3-7). Indeed, administration of Ang-(3-7) (10-100 microm) into the striatum caused a more potent increase in the striatal dopamine release than Ang-(1-7). Because Ang-(3-7) is an inhibitor of insulin-regulated aminopeptidase (IRAP) and because Ang IV, another IRAP inhibitor, also causes a concentration-dependent increase in dopamine in the rat striatum, IRAP may be involved in this effect. In contrast, EC33 had no effect on the Ang-(1-7)-induced GABA increase but the GABA release was blocked by the putative AT(1-7) receptor antagonist A779 (0.1 microm) and by the nitric oxide synthase inhibitor L-NAME (1 mm). These drugs could not block the effect of Ang-(1-7) on the striatal dopamine release suggesting that only the observed effects on GABA release are mediated by the AT(1-7) receptor and/or are associated with a release of nitric oxide.

Angiotensin-(1-7) Enhances Anti-Aggregatory Effects of the Nitric Oxide Donor Sodium Nitroprusside

Patients with ischemic heart disease have platelets that are resistant to the anti-aggregatory effects of nitric oxide (NO) donors. This NO resistance is associated with increased whole blood superoxide radical (O2-) content. Angiotensin II (Ang II) has been shown to augment O2- formation. Recent studies have demonstrated that angiotensin-(1-7) [Ang-(1-7)] has opposite actions to those of Ang II in the vasculature. This study compares the effects of Ang-(1-7) and Ang II on platelet aggregation and platelet responsiveness to the NO donor sodium nitroprusside (SNP). Platelet aggregation was induced by the thromboxane A2 mimetic U46619 (1-5 micromol/L), and the inhibitory effects of SNP (10 micromol/L) on the rate and extent of aggregation were quantified. Ang II did not induce aggregation, but 10-100 nmol/L Ang II potentiated U46619-induced aggregation by 21+/-6% in the absence and by 26+/-9% in the presence of SNP (P<0.01 for both), in blood samples from 8 normal subjects. By contrast, Ang-(1-7) alone did not affect platelet aggregation, but 10-100 nmol/L Ang-(1-7) potentiated the anti-aggregatory effects of SNP in blood samples from both normal subjects (n=17) and patients with acute coronary syndromes (n=17). This effect of Ang-(1-7) was bimodal, and at higher concentrations of Ang-(1-7), potentiation was abolished. The maximum incremental effects of Ang-(1-7) on inhibition of aggregation were 25+/-4% and 28+/-5%, for rate and extent of aggregation respectively (P<0.01 for both), corresponding to a 2.3-fold potentiation of the anti-aggregatory effect of SNP. Platelets from patients were resistant to the anti-aggregatory effect of SNP, but potentiation of SNP effects by Ang-(1-7) was similar for patients and normal subjects. Thus, Ang-(1-7) potentiates the anti-aggregatory effects of NO donor, and may therefore counteract platelet NO resistance that accompanies cardiovascular disease.

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

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

Angiotensin-(1-7) binds to specific receptors on cardiac fibroblasts to initiate antifibrotic and antitrophic effects

ANG-(1-7) improves the function of the remodeling heart. Although this peptide is generated directly within the myocardium, the effects of ANG-(1-7) on cardiac fibroblasts that play a critical role in cardiac remodeling are largely unknown. We tested the hypothesis that specific binding of ANG-(1-7) to cardiac fibroblasts regulates cellular functions that are involved in cardiac remodeling. 125I-labeled ANG-(1-7) binding assays identified specific binding sites of ANG-(1-7) on adult rat cardiac fibroblasts (ARCFs) with an affinity of 11.3 nM and a density of 131 fmol/mg protein. At nanomolar concentrations, ANG-(1-7) interacted with specific sites that were distinct from ANG II type 1 and type 2 receptors without increasing cytosolic Ca2+ concentration. At these concentrations, ANG-(1-7) had inhibitory effects on collagen synthesis as assessed by [3H]proline incorporation and decreased mRNA expression of growth factors in ARCFs. These effects of ANG-(1-7) contrasted with effects of ANG II. Pretreatment of ARCFs with ANG-(1-7) inhibited ANG II-induced increases in collagen synthesis and in mRNA expression of growth factors, including endothelin-1 and leukemia inhibitory factor. ANG-(1-7) pretreatment also inhibited the stimulatory effects of conditioned medium from ANG II-treated ARCFs on [3H]leucine incorporation and atrial natriuretic factor mRNA expression, markers of hypertrophy, in cardiomyocytes. Thus ANG-(1-7) interacted with specific receptors on ARCFs to exert potential antifibrotic and antitrophic effects that could reverse ANG II effects. These results suggest that ANG-(1-7) may play an important role in the heart in regulating cardiac remodeling.

ACE2: A novel therapeutic target for cardiovascular diseases

Hypertension afflicts over 65 million Americans and poses an increased risk for cardiovascular morbidity such as stroke, myocardial infarction and end-stage renal disease resulting in significant mortality. Overactivity of the renin-angiotensin system (RAS) has been identified as an important determinant that is implicated in the etiology of these diseases and therefore represents a major target for therapy. In spite of the successes of drugs inhibiting various elements of the RAS, the incidence of hypertension and cardiovascular diseases remain steadily on the rise. This has lead many investigators to seek novel and innovative approaches, taking advantage of new pathways and technologies, for the control and possibly the cure of hypertension and related pathologies. The main objective of this review is to forward the concept that gene therapy and the genetic targeting of the RAS is the future avenue for the successful control and treatment of hypertension and cardiovascular diseases. We will present argument that genetic targeting of angiotensin-converting enzyme 2 (ACE2), a newly discovered member of the RAS, is ideally poised for this purpose. This will be accomplished by discussion of the following: (i) summary of our current understanding of the RAS with a focus on the systemic versus tissue counterparts as they relate to hypertension and other cardiovascular pathologies; (ii) the newly discovered ACE2 enzyme with its physiological and pathophysiological implications; (iii) summary of the current antihypertensive pharmacotherapy and its limitations; (iv) the discovery and design of ACE inhibitors; (v) the emerging concepts for ACE2 drug design; (vi) the current status of genetic targeting of the RAS; (vii) the potential of ACE2 as a therapeutic target for hypertension and cardiovascular disease treatment; and (viii) future perspectives for the treatment of cardiovascular diseases.

Angiotensin-(1-7) inhibits growth of cardiac myocytes through activation of the mas receptor

Peptide hormones such as ANG II and endothelin contribute to cardiac remodeling after myocardial infarction by stimulating myocyte hypertrophy and myofibroblast proliferation. In contrast, angiotensin-(1-7) [ANG-(1-7)] infusion after myocardial infarction reduced myocyte size and attenuated ventricular dysfunction and remodeling. We measured the effect of ANG-(1-7) on protein and DNA synthesis in cultured neonatal rat myocytes to assess the role of the heptapeptide in cell growth. ANG-(1-7) significantly attenuated either fetal bovine serum- or endothelin-1-stimulated [(3)H]leucine incorporation into myocytes with no effect on [(3)H]thymidine incorporation. [d-Ala(7)]-ANG-(1-7), the selective ANG type 1-7 (AT(1-7)) receptor antagonist, blocked the ANG-(1-7)-mediated reduction in protein synthesis in cardiac myocytes, whereas the AT(1) and AT(2) angiotensin peptide receptors were ineffective. Serum-stimulated ERK1/ERK2 mitogen-activated protein kinase activity was significantly decreased by ANG-(1-7) in myocytes, a response that was also blocked by [d-Ala(7)]-ANG-(1-7). Both rat heart and cardiac myocytes express the mRNA for the mas receptor, and a 59-kDa immunoreactive protein was identified in both extracts of rat heart and cultured myocytes by Western blot hybridization with the use of an antibody to mas, an ANG-(1-7) receptor. Transfection of cultured myocytes with an antisense oligonucleotide to the mas receptor blocked the ANG-(1-7)-mediated inhibition of serum-stimulated MAPK activation, whereas a sense oligonucleotide was ineffective. These results suggest that ANG-(1-7) reduces the growth of cardiomyocytes through activation of the mas receptor. Because ANG-(1-7) is elevated after treatment with angiotensin-converting enzyme inhibitors or AT(1) receptor blockers, ANG-(1-7) may contribute to their beneficial effects on cardiac dysfunction and ventricular remodeling after myocardial infarction.

Angiotensin‐converting enzyme 2 as a novel target for gene therapy for hypertension

Less than one-third of patients with hypertension have their blood pressures (BP) controlled with current traditional therapeutic approaches for the treatment and control of hypertension. Pharmacological approaches may have reached a plateau in their effectiveness and thus newer innovative strategies need to be studied not only to increase the number of patients that can achieve BP control, but also to find a way to cure, not just manage, the disease. Continuous advances in gene delivery systems coupled with the completion of the Human Genome Project, now make it possible to investigate genetic means for the treatment and possible cure for hypertension. The renin-angiotensin system (RAS) has long been known to regulate BP, and salt and water metabolism. This system is unique in having both a peripheral circulating system and a tissue-based system. Each of these components have been ascribed a variety of physiological effects that have been associated with not only an increase in BP, but also in a variety of the pathophysiological manifestations associated with hypertension, such as cardiac hypertrophy and kidney dysfunction. We and others have used an antisense gene therapy approach, targeting the classical components of the RAS, to effectively attenuate the development of hypertension and related cardiovascular pathophysiologies in numerous experimental models of hypertension. Recently other components of the RAS have been elucidated and some of these components may be potential targets in a gene therapy approach. This article will focus on angiotensin-converting enzyme 2 (ACE2) as a new, potential target of gene therapy for hypertensive disorders.

The renin-angiotensin-aldosterone system, glucose metabolism and diabetes

In diabetes mellitus (DM), the circulating renin-angiotensin system (RAS) is suppressed, but the renal tissue RAS is activated. Hyperglycemia increases tissue angiotensin II (Ang II), which induces oxidative stress, endothelial damage and disease pathology including vasoconstriction, thrombosis, inflammation and vascular remodeling. In early DM, the type 1 Ang II (AT(1)) receptor is upregulated but the type 2 Ang II (AT(2)) receptor is downregulated. This imbalance can predispose the individual to tissue damage. Hyperglycemia also increases the production of aldosterone, which has an unknown contribution to tissue damage. The insulin resistance state is associated with upregulation of the AT(1) receptor and an increase in oxygen free radicals in endothelial tissue caused by activation of NAD(P)H oxidase. Treatment with an AT(1) receptor blocker normalizes oxidase activity and improves endothelial function. An understanding of the tissue renin-angiotensin-aldosterone system, which is a crucial factor in the progression of tissue damage in DM, is imperative for protection against tissue damage in this chronic disease.

Low sodium diet inhibits the local counter-regulator effect of angiotensin-(1-7) on angiotensin II

OBJECTIVE

The heptapeptide angiotensin-(1-7) [Ang-(1-7)] has been identified as a versatile, endogenous inhibitor of the renin-angiotensin system (RAS). As the therapeutic response to exogenous RAS inhibitors, such as AT1 receptor antagonists, is altered by changes in salt intake, we investigated the effect of a low, normal and high sodium diet on the antagonism of Ang II by Ang-(1-7). The role of angiotensin receptor subtypes and the endothelium was assessed.

METHODS

Male Wistar rats received a normal sodium (0.3% NaCl), high sodium (2.0% NaCl) or low sodium (0.05% NaCl) diet for 10 days. Vascular responses were assessed ex vivo in thoracic aortic rings in the presence of the nitric oxide (NO) inhibitor N-monomethyl-l-arginine (l-NMMA) to avoid aspecific vasodilator effects of Ang-(1-7).

RESULTS

After a normal or high salt diet, Ang-(1-7) significantly decreased maximal Ang II-induced vascular constrictions by 40-50%. After a low salt diet this non-competitive antagonism disappeared. The AT2 receptor antagonist PD 123319 and the Ang-(1-7) receptor antagonist A779 attenuated the effect of Ang-(1-7) found in rats fed with a normal or high sodium diet. Further, removal of endothelium and pretreatment with the prostaglandin synthesis inhibitor indomethacin (10 mol/l) abolished the non-competitive antagonism by Ang-(1-7).

CONCLUSION

Ang-(1-7) elicits a specific, endothelium-dependent and non-competitive antagonism of Ang II, which involves AT2 and Ang-(1-7) receptors but is independent of NO production. This non-competitive antagonism of Ang-(1-7) is abolished by a low sodium intake in normotensive rats, suggesting that it serves as a negative feedback towards Ang II in response to an altered sodium intake.

Histological evaluation of the effects of angiotensin peptides on wound repair in diabetic mice

Recent studies have shown that angiotensin peptides accelerate dermal repair. Histological observation of samples taken at the termination of studies showed that the wounds treated with peptides were mature and organized by day 25 after full thickness excision in diabetic mice. However, the mechanisms by which this acceleration occurs has not been determined. In the experiments described here, the effect of angiotensin peptides (AII, A(1-7) and NorLeu (3)-A(1-7) on the quality of the healing wound was evaluated histologically. Administration of the peptides accelerated collagen deposition, re-epithelialization and new blood vessels formation. By day 4, the percentage of the wound with collagen increased two- to six-fold depending upon the peptide. The increase by angiotensin peptides continued throughout the experimental period. On days 4 and 7 9 (only) after injury, exposure to angiotensin peptides increased the number of blood vessels at wound site two-to three-fold. Finally, the percentage of the wound site covered with new epithelium increased after administration of angiotensin peptides. Re-epithelialization was observed as early as day 4 in wounds treated ith angiotensin peptides. The increase was greater at later time points (up to 8-fold ar day 14 with NorLeu(3)-A(1-7) had an increase in neutrophils and macrophages on day 4 after wounding. Overall, administration of these peptides resulted in a healing site that was more mature, including reorganization of the collagen into a basket-weave appearance. Further, these studies confirm the superiority of NorLeu(3)-A(1-7) to AII and A(1-7) in wound healing evaluated at a microscopic level.

The intrarenal renin-angiotensin system and diabetic nephropathy

The renin-angiotensin system (RAS) is a coordinated cascade of proteins and peptide hormones, the principal effector of which is angiotensin II (ANG II). Evidence now indicates that the kidney regulates its function via a self-contained RAS in a paracrine fashion. In diabetic nephropathy, the intrarenal generation of ANG II is increased, in spite of suppression of the systemic RAS. This increase can contribute to the progression of diabetic nephropathy via several hemodynamic, tubular and growth-promoting actions. ANG II induces insulin resistance. ANG II type-1 (AT(1)) and type-2 (AT(2)) receptors are downregulated in chronic diabetes, but decreased AT(2) receptor expression might contribute to early diabetic nephropathy by reducing AT(2) receptor-mediated beneficial actions that are counter-regulatory to those of the AT(1) receptor. AT(2) receptor stimulation might account for part of the renal protection seen with AT(1) receptor blockade. A rat model of accelerated diabetic nephropathy is the (mREN-2) 27 renin transgenic rat treated with streptozotocin in which both the intrarenal and extrarenal RAS is activated.

Role of 3-hydroxy-3-methylglutaryl coenzyme a reductase inhibitors, angiotensin-converting enzyme inhibitors, cyclooxygenase-2 inhibitors, and aspirin in anti-inflammatory and immunomodulatory treatment of cardiovascular diseases

The immunologic response in atherosclerosis involves not only intrinsic cells of the artery wall, but also circulating leukocytes, lymphocytes, and macrophages. Interaction of various arms of the immune response modulates plaque development and stability, and it is conceivable that immunologic effects of some cardiovascular therapies may contribute to their mechanism of benefit. The preponderance of data has accrued with the 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors (statins). Statin effects, such as inhibition of T cell activation, tissue factor expression, or reduction of platelet hyperreactivity, may elicit beneficial effects in vitro and in vivo in patients with coronary artery disease. Moreover, aspirin may limit oxidation of lipoproteins and fibrinogen, and it may inhibit cytokine-induced nitric oxide synthase II expression. The hypothesis that selective inhibition of cyclooxygenase-2 (COX-2) may increase risk of myocardial infarction is controversial and may also be of questionable clinical significance. Finally, angiotensin-converting enzyme (ACE) inhibitors not only reduce proinflammatory mediators, such as interleukin-6, but also enhance the concentration of anti-inflammatory cytokines, such as interleukin-10. Because ACE is expressed at the shoulder region of atherosclerotic plaques, and ACE activity is enhanced in unstable plaques, ACE inhibition may also contribute to plaque stability. This article reviews the potential immunomodulatory potencies of aspirin, COX-2 inhibitors, statins, and ACE inhibitors as established pharmacotherapy in patients with coronary artery disease.

A local pancreatic renin-angiotensin system: endocrine and exocrine roles

The renin-angiotensin system (RAS) is classically characterized as a circulating hormonal system primarily through the production of the physiologically active product angiotensin II (Ang II) that plays a crucial role in the regulation of blood pressure, fluid and electrolyte homeostasis. In addition to this circulating RAS, numerous tissues and organs have been recently demonstrated to exhibit their own RAS products and activities. Such an intrinsic RAS can modulate the specific local functions of their respective tissues and organs, frequently in a paracrine and autocrine manner. Recent findings from our laboratories and others have made a significant contribution on the expression, localization, regulation, and potential role of a local RAS in the pancreas. Although, it is quite intriguing that components of the local pancreatic RAS are responsive to various physiological and pathophysiological conditions, the crucial role of this system in regulating the exocrine and endocrine functions and ultimately the clinical relevance to pancreatic disease is still largely equivocal. Of particular interest in this context are the actions of pancreatic RAS on the growth, anti-proliferation and free radical generation in the pancreas. The aims of the current article focus on the emerging data on the local pancreatic RAS; its involvement in exocrine acinar and endocrine islet aspects, and the clinical significance in the pancreas are particularly addressed. The target for the local pancreatic RAS may provide a new insight into future management of various clinical conditions including islet transplants, diabetes mellitus, pancreatic cancer, pancreatitis and cystic fibrosis.

Angiotensin-(1-7) reduces renal angiotensin II receptors through a cyclooxygenase-dependent mechanism

In the kidney, angiotensin-(1-7) [Ang-(1-7)] exhibits diuretic and natriuretic properties associated with an increase in prostaglandin production. The prohypertensive effects of Ang II are attenuated in rats infused with Ang-(1-7), consistent with recent work showing that Ang-(1-7) downregulates AT1 receptors in Chinese hamster ovary-AT1A or vascular smooth muscle cells. To determine whether exposure to Ang-(1-7) reduces AT1 receptors in the kidney through an increase in prostaglandin production, kidney slices from Sprague-Dawley rats were incubated with 10 n -1 microM Ang-(1-7) in the presence or absence of 5 microM meclofenamate, a cyclooxygenase inhibitor. Following these treatments, the kidney slices were retrieved, frozen, and sectioned for determination of [125I]-Ang II binding using in vitro receptor autoradiography. Greater than 90% of the specific binding was competed for by losartan, indicating that the majority of binding was to the AT1 receptor. Incubation of kidney slices with 1 microM Ang-(1-7) caused a 20% reduction in [125I]-Ang II binding (n = 8) in the cortical tubulointerstitium, which was prevented when Ang-(1-7)-treated slices were incubated in the presence of 5 microM meclofenamate (1 +/- 2% increase, n = 8; p < 0.05). Incubation with 5 microM meclofenamate alone had no effect on [125I]-Ang II binding (-3 +/- 3%). The decrease in [125I]-Ang II binding with Ang-(1-7) was also blocked by the Ang-(1-7) antagonist [d-Ala7]-Ang-(1-7). Treatment with 1 microM [d-Ala7]-Ang-(1-7) alone had no effect on [125I]-Ang II binding (-3 +/- 6% of control). Pretreatment with 1 microM Ang II caused a similar reduction in [125I]-Ang II binding in the cortical tubulointerstitium. Neither Ang-(1-7) nor Ang II had any effect on [125I]-Ang II binding in the glomeruli and the area of the vasa recta of the kidney. These original findings suggest that prior exposure to Ang-(1-7) or Ang II causes a modest decrease in the number of AT1 receptors in the cortical tubulointerstitial area of the kidney. The reduction in Ang II binding by Ang-(1-7) was blocked by meclofenamate and [d-Ala7]-Ang-(1-7), suggesting that cyclooxygenase products released through activation of a novel receptor participate in this effect.

Angiotensin receptors contribute to blood pressure homeostasis in salt-depleted SHR

This study evaluated the contribution of angiotensin peptides acting at various receptor subtypes to the arterial pressure and heart rate of adult 9-wk-old male conscious salt-depleted spontaneously hypertensive rats (SHR). Plasma ANG II and ANG I in salt-depleted SHR were elevated sevenfold compared with peptide levels measured in sodium-replete SHR, whereas plasma ANG-(1-7) was twofold greater in salt-depleted SHR compared with salt-replete SHR. Losartan (32.5 micromol/kg), PD-123319 (0.12 micromol. kg(-1). min(-1)), [d-Ala(7)]ANG-(1-7) (10 and 100 pmol/min), and a polyclonal ANG II antibody (0.08 mg/min) were infused intravenously alone or in combination. Combined blockade of AT(2) and AT((1-7)) receptors significantly increased the blood pressure of losartan-treated SHR (+15 +/- 1 mmHg; P < 0.01); this change did not differ from the blood pressure elevation produced by the sole blockade of AT((1-7)) receptors (15 +/- 4 mmHg). On the other hand, sole blockade of AT(2) receptors in losartan-treated SHR increased mean arterial pressure by 8 +/- 1 mmHg (P < 0.05 vs. 5% dextrose in water as vehicle), and this increase was less than the pressor response produced by blockade of AT((1-7)) receptors alone or combined blockade of AT((1-7)) and AT(2) receptors. The ANG II antibody increased blood pressure to the greatest extent in salt-depleted SHR pretreated with only losartan (+11 +/- 2 mmHg) and to the least extent in salt-depleted SHR previously treated with the combination of losartan, PD-123319, and [d-Ala(7)]ANG-(1-7) (+7 +/- 1 mmHg; P < 0.01). Losartan significantly increased heart rate, whereas other combinations of receptor antagonists or the ANG II antibody did not alter heart rate. Our results demonstrate that ANG II and ANG-(1-7) act through non-AT(1) receptors to oppose the vasoconstrictor actions of ANG II in salt-depleted SHR. Combined blockade of AT(2) and AT((1-7)) receptors and ANG II neutralization by the ANG II antibody reversed as much as 67% of the blood pressure-lowering effect of losartan.

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.

Angiotensin-(1-7) inhibits angiotensin II-induced signal transduction

The inhibitory effects of angiotensin-(1-7) on angiotensin II-induced vasoconstriction, growth of vascular smooth muscle cells, stimulation of protein kinase C, extracellular signal-regulated kinases (ERK), and angiotensin subtype 1 receptor (AT1) and subtype 2 receptor (AT2) mRNA expression were investigated. The hemodynamic effects of angiotensin-(1-7) were measured in Wistar rats. Vasoconstriction was measured using aortic rings. DNA synthesis or protein synthesis was measured in cultured vascular smooth muscle cells using [3H] thymidine or [3H] leucine incorporation, respectively. Angiotensin II stimulated protein kinase C and ERK1/2 were measured by Western blot analysis using phosphospecific protein kinase C and ERK1/2 antibodies. AT1 and AT2 receptor mRNA expression was measured using reverse-transcription polymerase chain reaction. Infusion of angiotensin II significantly increased whereas infusion of angiotensin-(1-7) had no effects on mean arterial blood pressure in Wistar rats. Angiotensin-(1-7) dose-dependently showed partial antagonism on angiotensin II-induced contraction of aortic rings. Angiotensin-(1-7) showed partial antagonism on angiotensin II-induced DNA synthesis and protein synthesis. Angiotensin-(1-7) showed partial antagonism on angiotensin II-induced activation of protein kinase C and ERK1/2. The administration of angiotensin-(1-7) showed partial antagonism on angiotensin II-induced downregulation of AT1 receptor mRNA expression, whereas AT2 receptor mRNA expression was unchanged. Angiotensin-(1-7) showed partial antagonism on angiotensin II-induced intracellular signal transduction and may play a crucial role in the adaptation process of AT1 receptors to sustained stimulation of angiotensin II.

Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers in atherosclerosis

Angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARBs) effectively interfere with the renin-angiotensin system and exert various beneficial actions on cardiac and vascular structure and function, beyond their blood pressure-lowering effects. Randomized, controlled clinical trials have shown that ACE inhibitors improve endothelial function, cardiac and vascular remodeling, retard the anatomic progression of atherosclerosis, and reduce the risk of myocardial infarction, stroke, and cardiovascular death. Therefore, these agents are recommended in the treatment of a wide range of patients at risk for adverse cardiovascular outcomes, including those with coronary disease, prior stroke, peripheral arterial disease, high-risk diabetes, hypertension, and heart failure. ARBs are effective blood pressure- lowering and renoprotective agents and can be used in heart failure in patients who do not tolerate ACE inhibitors. The role of ARBs in the prevention of atherosclerosis and its sequelae is currently under investigation. The use of combined ACE inhibitor plus ARB therapy offers theoretical advantages over the use of each of these agents alone and is also under investigation in large, randomized clinical trials.

Multiple angiotensin receptor subtypes in normal and tumor astrocytes in vitro

A role for neuropeptide receptors in glial tumorigenesis has recently been proposed. Although angiotensin receptors are known to mediate proliferative effects in many cell types, including brain astrocytes, the possible participation of these receptors in glial tumorigenesis remains unknown. In the present study, we have examined the expression of the molecularly defined angiotensin receptor subtypes AT(1a), AT(1b), and AT(2) in normal perinatal rat astrocytes and in a panel of tumor adult astrocytoma cells, using the reverse transcriptase-polymerase chain reaction (RT-PCR). Subsequently, we compared the mitogenic effect of the angiotensins A(1-8), A(2-8), A(3-8) and the heptapeptide "metabolite" A(1-7), on both normal and tumor astrocytes, measured in terms of the incorporation of tritiated thymidine. Our results indicate that AT(1a), AT(1b), and AT(2) angiotensin receptor mRNA is commonly expressed by many of these cells. Of notable exception is the astrocytoma U373 which was not found to express AT(1) or AT(2) mRNA. Chronic (24-h) incubation of cells with A(1-8) and A(1-7) lead to the induction of mitogenesis, even in the AT(1) and AT(2) mRNA negative astrocytoma cell line U373. Moreover, pharmacological analysis indicated that the observed mitogenic effects are not mediated by the AT(1) or AT(2) type receptors, but rather by a novel, specific A((1-7)) angiotensin receptor, since mitogenesis was shown to be partially blocked by the A(1-7) analogue D-Ala(7)A(1-7) and by the protease inhibitor orthophenanthroline (100 microM). Using Fura-2 spectrophotometry, we found that activation of this receptor does not alter intracellular calcium levels; however, preincubation with the protein kinase kinase inhibitor U0126 (10 microM) was found to inhibit these mitogenic effects partially. Overall, these results which demonstrate that normal and tumor astrocytes express a greater variety of angiotensin receptor subtypes than previously thought, support the idea that A(1-7) and its receptor signaling system may play an important role in shaping the astrocyte population during development. Moreover, the untimely expression of this A((1-7)) receptor may represent an important etiological component in the development of brain astrocytomas.

Renal interstitial fluid angiotensin I and angiotensin II concentrations during local angiotensin-converting enzyme inhibition

It was recently demonstrated that angiotensin II (AngII) concentrations in the renal interstitial fluid (RIF) of anesthetized rats were in the nanomolar range and were not reduced by intra-arterial infusion of an angiotensin-converting enzyme (ACE) inhibitor (enalaprilat). This study was performed to determine changes in RIF AngI and AngII concentrations during interstitial administration of ACE inhibitors (enalaprilat and perindoprilat). Studies were also performed to determine the effects of enalaprilat on the de novo formation of RIF AngII elicited by interstitial infusion of AngI. Microdialysis probes (cut-off point, 30,000 D) were implanted in the renal cortex of anesthetized rats and were perfused at 2 micro l/min. The effluent dialysate concentrations of AngI and AngII were measured by RIA, and reported values were corrected for the equilibrium rates at this perfusion rate. Basal RIF AngI (0.74 +/- 0.05 nM) and AngII (3.30 +/- 0.17 nM) concentrations were much higher than plasma AngI and AngII concentrations (0.15 +/- 0.01 and 0.14 +/- 0.01 nM, respectively; n = 27). Interstitial infusion of enalaprilat through the microdialysis probe (1 or 10 mM in the perfusate; n = 5 and 8, respectively) significantly increased RIF AngI concentrations but did not significantly alter AngII concentrations. However, perindoprilat (10 mM in the perfusate, n = 7) significantly decreased RIF AngII concentrations by 22 +/- 4% and increased RIF AngI concentrations. Interstitial infusion of AngI (100 nM in the perfusate, n = 7) significantly increased the RIF AngII concentration to 8.26 +/- 0.75 nM, whereas plasma AngI and AngII levels were not affected (0.15 +/- 0.02 and 0.14 +/- 0.02 nM, respectively). Addition of enalaprilat to the perfusate (10 mM) prevented the conversion of exogenously added AngI. These results indicate that addition of AngI in the interstitial compartment leads to low but significant conversion to AngII via ACE activity (blocked by enalaprilat). However, the addition of ACE inhibitors directly into the renal interstitium, via the microdialysis probe, either did not reduce RIF AngII levels or reduced levels by a small fraction of the total basal level, suggesting that much of the RIF AngII is formed at sites not readily accessible to ACE inhibition or is formed via non-ACE-dependent pathways.

Effects of angiotensins on day-night fluctuations and stress-induced changes in blood pressure

In this study we evaluated by telemetry the effects of ANG II and ANG-(1-7) infusion on the circadian rhythms of blood pressure (BP) and heart rate (HR) and on the cardiovascular adjustment resulting from restraint stress in rats. ANG II or ANG-(1-7) or vehicle were infused subcutaneously for 7 days. Restraint stress was carried out before, during, and after infusion at 7-day intervals. Parallel with an increase in MAP, ANG II infusion produced an inversion of MAP circadian rhythm with a significant MAP acrophase inversion. It also produced bradycardia during the first 3 days of infusion. Thereafter, HR progressively increased, reaching values similar to or above those of the control period at the end of the infusion period. HR circadian variation was not changed by ANG II infusion. Strikingly, ANG II significantly attenuated the increase in MAP induced by restraint stress without altering the HR response. ANG-(1-7) infusion produced a slight but significant decrease in MAP restricted to the daytime period. No significant changes in the MAP acrophase were observed. In addition, ANG-(1-7) infusion produced a small but significant sustained bradycardia. ANG-(1-7) did not change cardiovascular responses to restraint stress. These data indicate that ANG II can influence the activity of brain areas involved in the determination of stress-induced or circadian-dependent variations of blood pressure without changing HR fluctuations. A significant modulatory influence of ANG-(1-7) on basal MAP and HR is also suggested.

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.

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.

ACEH/ACE2 is a novel mammalian metallocarboxypeptidase and a homologue of angiotensin-converting enzyme insensitive to ACE inhibitors

A human zinc metalloprotease (termed ACEH or ACE2) with considerable homology to angiotensin-converting enzyme (ACE) (EC 3.4.15.1) has been identified and subsequently cloned and functionally expressed. The translated protein contains an N-terminal signal sequence, a single catalytic domain with zinc-binding motif (HEMGH), a transmembrane region, and a small C-terminal cytosolic domain. Unlike somatic ACE, ACEH functions as a carboxypeptidase when acting on angiotensin I and angiotensin II or other peptide substrates. ACEH may function in conjunction with ACE and neprilysin in novel pathways of angiotensin metabolism of physiological significance. In contrast with ACE, ACEH does not hydrolyse bradykinin and is not inhibited by typical ACE inhibitors. ACEH is unique among mammalian carboxypeptidases in containing an HEXXH zinc motif but, in this respect, resembles a bacterial enzyme, Thermus aquaticus (Taq) carboxypeptidase (EC 3.4.17.19). Collectrin, a developmentally regulated renal protein, is homologous with the C-terminal region of ACEH but has no similarity with ACE and no catalytic domain. Thus, the ACEH protein may have evolved as a chimera of a single ACE-like domain and a collectrin domain. The collectrin domain may regulate tissue response to injury whereas the catalytic domain is involved in peptide processing events.

Angiotensin-(1-7) reverts the stimulatory effect of angiotensin II on the proximal tubule Na(+)-ATPase activity via a A779-sensitive receptor

Recently, we demonstrated that the stimulatory effect of Ang II on the Na(+)-ATPase activity in proximal tubules is reversed, in a dose-dependent manner, by Ang-(1-7) [Biochim. Biophys. Acta 1467 (2000) 189]. In the present paper, we characterized the receptor involved in this phenomenon. The preincubation of the Na(+)-ATPase with 10(-8) M Ang II increases the enzyme activity from 7.50+/-0.02 (control) to 12.40+/-1.50 nmol Pi mg(-1) min(-1) (p<0.05). Addition of 10(-9) M Ang-(1-7) completely reverts this effect returning the ATPase activity to the control level. This effect seems to be specific to Ang-(1-7) since Ang III (10(-12)-10(-8) M) does not modify the stimulation of the renal proximal tubule Na(+)-ATPase activity by Ang II. Saralasin abolishes the Ang-(1-7) effect in a dose-dependent manner being the maximal effect obtained at 10(-11) M. The increase in A779 concentration (from 10(-12) to 10(-7) M), a specific Ang-(1-7) antagonist, also abolishes the Ang-(1-7) effect. On the other hand, PD123319 (10(-8)-10(-6) M), an AT(2) antagonist receptor, and losartan (10(-12)-10(-7) M), an AT(1) antagonist receptor, does not modify the effect of Ang-(1-7). Taken together, these data indicate that Ang-(1-7) reverts the stimulatory effect of Ang II on the Na(+)-ATPase activity in proximal tubule through a A779-sensitive receptor.

Prevention of radiation-induced nephropathy and fibrosis in a model of bone marrow transplant by an angiotensin II receptor blocker

Nephropathy, interstitial pneumopathy, and renal and lung fibrosis are major complications of bone marrow transplantation (BMT). This study evaluated the antifibrotic property of an angiotensin II (A2) type-1 receptor blocker (L-159,809) and compared it with those of Captopril and Enalapril, two angiotensin-converting enzyme (ACE) inhibitors, in a rat model of BMT. Male WAG/Rij/MCW rats received a preparative regimen of 60 mg/kg body wt of cytoxan (i.p., Days 9 and 8) and 18.5 Gy of total body irradiation (TBI) in six twice daily fractions (Days 2, 1, and 0) followed immediately (Day 0) by BMT. Modifiers were given in drinking water from Day 10 until autopsy, 8 weeks after BMT. Rats treated with TBI plus cytoxan alone developed severe nephropathy. Trichrome staining showed marked collagen deposition in glomeruli, renal interstitium, and renal arteries and arterioles (especially in their adventitia). Collagen deposition and renal damage were markedly reduced by the three modifiers. Of the three, L-158,809-treated rats had slightly thinner vessels and slightly less collagen than nonirradiated normal controls. The study shows the effectiveness of these drugs in the protection of the renal parenchyma from the development of radiation-induced fibrosis. It also indicates a role for angiotensin II in the modulation of collagen synthesis.

Autoradiographic analysis and regulation of angiotensin receptor subtypes AT(4), AT(1), and AT((1-7)) in the kidney

Receptor autoradiography revealed that angiotensin AT(4) receptors were abundantly expressed in normal mammalian (mouse, rat, gerbil, guinea pig, rabbit) and avian (sparrow, chicken, turkey) kidneys and were more extensively distributed than previously reported (including proximal and distal segments of the nephron, interstitium, renal artery, vein, and ureter). Angiotensin AT(4) receptors were generally found to be more abundant than angiotensin AT(1) receptors in mammalian kidneys, whereas angiotensin AT((1-7)) receptors were not detected in either mammalian or avian kidneys. Rats subjected to various chronic treatments were found to preferentially decrease kidney AT(4) receptor density (furosemide, puromycin aminonucleoside, nitro-L-arginine methyl ester), decrease kidney AT(1) receptor density (bilateral ureteral obstruction), or increase kidney AT(1) receptor distribution in the inner medulla (water diuresis). These results indicate that the AT(4) receptor can be expressed in numerous cell types within the normal kidney of several species. Furthermore, several models of renal dysfunction and injury have been identified that selectively alter kidney AT(4) density and may potentially aid in elucidating the role of this novel angiotensin receptor system in renal function.

Beyond the usual strategies for blood pressure reduction: therapeutic considerations and combination therapies

Rapidly accumulating clinical data have repeatedly demonstrated not only the critical importance of even small increases in blood pressure as a pathophysiologic factor in the development of cardiovascular disease, particularly in individuals with diabetes mellitus, but also the therapeutic necessity of more aggressive blood pressure reduction and the achievement of progressively lower blood pressure targets in reducing cardiovascular event rates. JNC VI has defined optimal blood pressure as <or=120/80 mm Hg, and Stage 1 hypertension as >or=140/80 mm Hg. Target blood pressures are now <or=130/80 mm Hg in patients with diabetes and <125/75 mm Hg for patients with hypertensive renal disease with proteinuria of >1 gm/24 hours. Achieving such target pressures is increasingly difficult, particularly in diabetic patients with chronic renal disease, who require complex multidrug antihypertensive regimens. This review attempts to provide some suggestions for constructing such antihypertensive regimens, and provides considerations for the appropriate use of diuretics and the most effective drug combinations. Factors potentially contributing to drug resistant hypertension include such problems as failure to maximize drug dosing, suboptimal diuretic use, noncompliance, and possible confounding effects of such concomitant medications as nonsteroidal and anti-inflammatory drugs or decongestants. The issues underlying drug-resistant hypertension are listed, together with strategies for overcoming this problem.

Protection against ischemia: a physiological function of the renin-angiotensin system

The renin-angiotensin system (RAS) is involved in a complex mechanism that serves to preserve the blood supply to organs so that they can maintain cellular function. Angiotensin II exerts this effect, independently of the blood pressure generated, through two time-related events: a fast opening of the reserve collateral circulation and a much slower response of new vessel formation or angiogenesis. This effect is observed in rats with ligation of the abdominal aorta and in gerbils with abrupt or progressive unilateral carotid artery ligation. Inhibition of the angiotensin-converting enzyme (ACE) or the angiotensin II receptor represses this effect, and it appears that it is mediated through a non-AT1 receptor site of angiotensin II. Many tumors, both benign and malignant, express renin and angiotensin. It seems that the stimulating action of angiotensin II on angiogenesis could also be involved in preserving the blood supply to tumor cells. Administration of converting enzyme inhibitors increases survival and decreases tumor size in tumor-bearing rats. These observations support the hypothesis that the RAS, directly or indirectly, is involved in situations in which the restoration of blood supply is critical for the viability of cells and that it is present not only in normal but also in pathological conditions such as tumors. In view of the ubiquitous presence of renins and angiotensins, it is also likely to be involved in other conditions, such as inflammation, arthritis, diabetic retinopathy, and retrolental fibroplasia, among others in which angiogenesis is prominent. In addition, angiotensin II could be involved, through the counterbalance of the AT1 and AT2 receptors, in the rarefaction of blood vessels as an etiologic component of essential hypertension.

Renoprotective and anti-hypertensive effects of combined valsartan and perindopril in progressive diabetic nephropathy in the transgenic (mRen-2)27 rat

BACKGROUND

We have previously reported that severe glomerulosclerosis progressively develops in the streptozotocin (STZ) diabetic transgenic (mRen-2)27 rat. In this diabetic model, monotherapy with either angiotensin converting enzyme inhibition (ACEI) or angiotensin type 1 (AT(1)) receptor blockade is largely renoprotective. The objective of the present study was to determine if a combination therapy at lower doses than monotherapy would confer greater renoprotection.

METHODS

At 6 weeks of age, non-diabetic control and STZ diabetic female heterozygous Ren-2 rats were randomized to receive vehicle, the AT(1) receptor blocker valsartan (V, 20 mg/kg/day), the ACEI perindopril (P, 6 mg/kg/day), or a combination of low-dose V+P (V, 3 mg/kg/day plus P, 0.5 mg/kg/day) for 12 weeks.

RESULTS

Systolic blood pressure was lowered with all treatments, but the greatest reductions were observed with V monotherapy and combination V+P therapy. All treatments reduced albuminuria, the decline in glomerular filtration rate, and cortical collagen staining, to the same extent. The glomerulosclerotic index was increased with diabetes and reduced with V and P monotherapy. However, the low-dose combination therapy was more effective than single therapy and reduced severe glomerulosclerosis to levels observed in non-diabetic controls.

CONCLUSIONS

Monotherapy with either V or P reduced blood pressure and retarded the decline in renal function and glomerulosclerosis in the diabetic Ren-2 rat. Combination therapy has the additional benefit of requiring only low doses of AT(1) receptor blockade and ACEI to achieve superior renoprotective effects in this diabetic nephropathy model.

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

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

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

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

Effects of angiotensin IV and angiotensin-(1-7) on basal and angiotensin II-stimulated cytosolic Ca2+ in mesangial cells

This study analyzed the influence of two main metabolites of angiotensin II, angiotensin IV and angiotensin-(1-7), on basal and angiotensin II-dependent [Ca2+](i) in rat mesangial cells. Angiotensin IV behaved as a weak agonist. Its effects were abolished by angiotensin AT(1) receptor antagonists. Treatment with angiotensin II abolished the effect of a subsequent treatment with angiotensin IV whereas two successive angiotensin IV-dependent [Ca2+](i) peaks were obtained. Angiotensin II increased [Ca2+](i) in a Ca2+-free medium whereas angiotensin IV was inactive. Leucine-valine-valine-hemorphin 7, a hemorphin specific for the angiotensin AT(4) receptor, was devoid of any agonistic or antagonistic effect. In contrast, angiotensin-(1-7), if without influence on basal [Ca2+](i), inhibited angiotensin II- and angiotensin IV-dependent [Ca2+](i) increases. Total inhibition of the angiotensin IV effect was obtained whereas association of angiotensin-(1-7) to 8-(NN-diethylamino)-octyl-3,4,5-trimethoxybenzoate, an inhibitor of inositol phosphate-mediated Ca2+ release, was necessary to suppress the effect of angiotensin II. These results provide evidence that angiotensin II metabolites may participate in the control of [Ca2+](i) in mesangial cells at the initial stage of binding to the angiotensin AT(1) receptors.

Optimal strategies for preventing progression of renal disease: should angiotensin converting enzyme inhibitors and angiotensin receptor blockers be used together?

Interruption of the renin-angiotensin system (RAS) with angiotensin converting enzyme (ACE) inhibitors or angiotensin AT(1) receptor blockers has been shown to delay progression in a variety of renal diseases, suggesting that the RAS, and its major effector molecule, angiotensin II, are important players in renal pathophysiology. Both antagonists combine inhibition of deleterious effects of angiotensin II with activation of potentially beneficial pathways mediated by nitric oxide and prostaglandins. Some concerns have been raised about the completeness of the RAS blockade achieved by these agents. ACE-independent pathways can generate angiotensin II, whereas increases in angiotensin II levels may compete with the AT(1) receptor blocker at the receptor site. It has been suggested that an ACE inhibitor/AT(1) receptor blocker combination offers a better therapeutic effect than treatment with either agent alone. In this review, we focus on mechanisms of actions of ACE inhibitors and AT(1) receptor blockers, implicate them in the rationale for the use of an ACE inhibitor/AT(1) receptor blocker combination, and discuss evidence evaluating the renal effects of the combination as compared to the effects of a single agent. There is a surprising lack of information about the nephroprotective potential of the combination, allowing no consistent conclusions about the superiority of the combination over the single agent. Several experimental and clinical reports suggest that in some conditions, the combination may be beneficial. Rather than providing unequivocal evidence for the use of combination treatment in the renal disease, these studies should be considered as stimuli for more detailed exploration of this issue.

Regulated expression of pancreatic renin-angiotensin system in experimental pancreatitis

Our previous studies have provided evidence for the existence of an intrinsic renin-angiotensin system (RAS) in the rat pancreas, which may play a role in the regulation of pancreatic microcirculation and ductal secretion. Such a pancreatic RAS has recently shown to be activated by chronic hypoxia. The activation of a local RAS in the pancreas by chronic hypoxia and its significance of changes may be important for the physiological and pathophysiological aspects of the pancreas. In the present study, the regulation of experimentally induced acute pancreatitis on the expression of local RAS in the pancreas was investigated using Western blot, semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) and immunohistochemical approaches. Results from Western blot demonstrated that experimentally induced pancreatitis caused significantly increased expression of the pancreatic RAS component proteins. In keeping with the protein level, RT-PCR analysis also revealed the enhanced expression of pancreatic RAS genes, notably the angiotensinogen in experimental pancreatitis. Immunohistochemical results further demonstrated that increased immunoreactivity for RAS in experimental pancreatitis was predominantly localized to the endothelia and epithelia of pancreatic vasculature and ductal system respectively. The data indicate that experimental pancreatitis may elicit activation of a local RAS in the pancreas. Such an activation of pancreatic RAS and its significance of differential changes in individual RAS components could play a role in the pathophysiology of acute pancreatitis

Angiotensin-(1-7): an update

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