Alterations in prostaglandin production in spontaneously hypertensive rat smooth muscle cells

Protective effects of perindopril against indomethacin-induced gastric mucosal damage through modulation of DDAH-1/ADMA and ACE-2/ANG-(1-7) signaling pathways

Indomethacin is a widely used nonsteroidal anti-inflammatory drug; however, its clinical utility is accompanied by serious adverse reactions including peptic ulcers. The current study aims to investigate the protective potential of perindopril against indomethacin-induced gastric injury in rats. Perindopril (4 mg/kg) was administered orally for 7 days and indomethacin (60 mg/kg, single oral dose) was administered on the 7^th day, 1 h after perindopril administration. Pantoprazole was used as a standard agent. Ulcer index (UI), preventive index ratio (PI), histopathological examination, oxidative stress, and inflammatory biomarkers were investigated. Perindopril significantly decreased UI while increased PI and counteracted histopathological aberrations induced by indomethacin. It alleviated indomethacin-induced oxidative stress by lowering NO while increasing GSH content and superoxide dismutase activity. Perindopril significantly downregulated TNF-α and asymmetric dimethylarginine (ADMA), while significantly upregulated COX-2, PGE-2, dimethylarginine dimethylaminohydrolase-1 (DDAH-1), ANG-(1-7), and ACE-2 expression. Together, these findings suggest the gastroprotective effects of perindopril through modulation of DDAH-1/ADMA and ACE-2/ANG-(1-7) signaling.HIGHLIGHTSPerindopril attenuated gastric histopathological damage.It increased GSH content and SOD activity while decreased NO content.It modulated gastric ADMA and DDAH-1 activity.It reduced TNF-α, while increased COX-2 and PGE-2 expression.It upregulated ACE-2 activity and ANG-(1-7) protein expression.

Angiotensin II induces cyclooxygenase 2 expression in rat astrocytes via the angiotensin type 1 receptor

We previously showed that Angiotensin (Ang) II stimulated pro-inflammatory and mitogenic actions in astrocytes suggesting that astrocytes are emerging as key players in neuroinflammation. Evidence suggests that neuroinflammation may contribute to central sympathetic overactivity and elevated blood pressure. Further, cyclooxygenase (Cox)-derived prostanoids were implicated in Ang II-dependent hypertension. Cox2 is one of two Cox isoenzymes that is responsible for the formation of prostanoids from arachidonic acid. Constitutively expressed Cox2 has a protective and homeostatic role in the cardiovascular and renal systems. Inducible Cox2 has been associated with pathogenic stimuli resulting in inflammatory conditions and cancers. In this study, we investigated the effect of Ang II on Cox2 protein and mRNA expression in brainstem and cerebellum astrocytes, and determined whether any differences in Cox2 expression exist in spontaneously hypertensive rat (SHR) astrocytes compared to their normotensive control Wistar rats. We demonstrated that Ang II increased Cox2 protein and mRNA levels relative to untreated controls in a time-dependent manner, in Wistar and SHR brainstem and cerebellum astrocytes. Increases in Cox2 protein expression were evident within 4 h, with subsequent sustained elevation for several hours followed by a decline at 48 h. Ang II-induced Cox2 protein levels were higher in Wistar compared to SHRs in both brainstem and cerebellum astrocytes for the majority of time points examined. The Ang II-induced Cox2 mRNA levels increased within 8 h followed by a rapid decline to almost basal levels at later time points. At the earlier time points, Cox2 mRNA elevation were higher in SHR compared to Wistar rat astrocytes. These Ang II actions were mediated by the Ang type I receptor. Our results corroborate previous reports of Ang II's ability to stimulate neuroinflammatory mediators in astrocytes. Cox2-derived prostaglandins might play a role in brain-renin angiotensin system associated hypertension, and astrocytes could be significant players.

Renal Blood Flow Response to Angiotensin 1-7 versus Hypertonic Sodium Chloride 7.5% Administration after Acute Hemorrhagic Shock in Rats

Background. Angiotensin 1-7 (Ang1-7) plays an important role in renal circulation. Hemorrhagic shock (HS) may cause kidney circulation disturbance, and this study was designed to investigate the renal blood flow (RBF) response to Ang1-7 after HS. Methods. 27 male Wistar rats were subjected to blood withdrawal to reduce mean arterial pressure (MAP) to 45 mmHg for 45 min. The animals were treated with saline (group 1), Ang1-7 (300 ng·kg(-1) min(-1)), Ang1-7 in hypertonic sodium chloride 7.5% (group 3), and hypertonic solution alone (group 4). Results. MAP was increased in a time-related fashion (P time < 0.0001) in all groups; however, there was a tendency for the increase in MAP in response to hypertonic solution (P = 0.09). Ang1-7, hypertonic solution, or combination of both increased RBF in groups 2-4, and these were significantly different from saline group (P = 0.05); that is, Ang1-7 leads to a significant increase in RBF to 1.35 ± 0.25 mL/min compared with 0.55 ± 0.12 mL/min in saline group (P < 0.05). Conclusion. Although Ang1-7 administration unlike hypertonic solution could not elevate MAP after HS, it potentially could increase RBF similar to hypertonic solution. This suggested that Ang1-7 recovers RBF after HS when therapeutic opportunities of hypertonic solution are limited.

Inhibition of the Prostaglandin Transporter PGT Lowers Blood Pressure in Hypertensive Rats and Mice

Inhibiting the synthesis of endogenous prostaglandins with nonsteroidal anti-inflammatory drugs exacerbates arterial hypertension. We hypothesized that the converse, i.e., raising the level of endogenous prostaglandins, might have anti-hypertensive effects. To accomplish this, we focused on inhibiting the prostaglandin transporter PGT (SLCO2A1), which is the obligatory first step in the inactivation of several common PGs. We first examined the role of PGT in controlling arterial blood pressure blood pressure using anesthetized rats. The high-affinity PGT inhibitor T26A sensitized the ability of exogenous PGE₂ to lower blood pressure, confirming both inhibition of PGT by T26A and the vasodepressor action of PGE₂ T26A administered alone to anesthetized rats dose-dependently lowered blood pressure, and did so to a greater degree in spontaneously hypertensive rats than in Wistar-Kyoto control rats. In mice, T26A added chronically to the drinking water increased the urinary excretion and plasma concentration of PGE₂ over several days, confirming that T26A is orally active in antagonizing PGT. T26A given orally to hypertensive mice normalized blood pressure. T26A increased urinary sodium excretion in mice and, when added to the medium bathing isolated mouse aortas, T26A increased the net release of PGE₂ induced by arachidonic acid, inhibited serotonin-induced vasoconstriction, and potentiated vasodilation induced by exogenous PGE₂. We conclude that pharmacologically inhibiting PGT-mediated prostaglandin metabolism lowers blood pressure, probably by prostaglandin-induced natriuresis and vasodilation. PGT is a novel therapeutic target for treating hypertension.

Role of renin-angiotensin system and metabolites of angiotensin in the mechanism of gastric mucosal protection

Angiotensin II, the main effector of the renin-angiotensin system (RAS), is generated from the precursor angiotensinogen by the actions of renin, angiotensin-converting enzyme, chymase and various peptidases. RAS is essential in the control of blood pressure and body fluid homeostasis but their involvement in the mechanism of the protection of gastric mucosa has not been extensively studied. On the other hand, angiotensin-(1-7) which acts on the Mas receptor, exhibits a potent vasodilatory activity and attenuates the gastric lesions induced by various ulcerogens. In this review, the mechanism of RAS, the antagonists of angiotensin AT1 and AT2 receptors and angiotensin-(1-7) in formation of gastric damage is discussed with possible translating relevance to treatment modalities in the protection against gastric mucosal injury.

Role of angiotensin-(1-7) in gastroprotection against stress-induced ulcerogenesis. The involvement of mas receptor, nitric oxide, prostaglandins, and sensory neuropeptides

Angiotensin-(1-7) [Ang-(1-7)] is a major vasoactive metabolite of angiotensin I (Ang I), both being important components of the renin-angiotensin system (RAS). Ang-(1-7) acting via Mas receptor was documented in kidneys, heart, brain, and gastrointestinal (GI)-tract. We studied the gastroprotective activity of exogenous Ang-(1-7) in rats exposed to water immersion and restraint stress (WRS) without or with A-779 [d-Ala7-Ang-(1-7), an antagonist of Ang-(1-7) Mas receptors], AVE 0991 (5-formyl-4-methoxy-2-phenyl-1[[4-[2-(ethylaminocarbonylsulfonamido)-5-isobutyl-3-thienyl]-phenyl]-methyl]-imidazole), the agonist of Ang-(1-7) receptor, as well as the inhibition of nitric-oxide (NO) synthase, the suppression of cyclo-oxygenase (COX)-1 (indomethacin, SC-560 [5-(4-chloro-phenyl)-1-(4-methoxyphenyl)-3-trifluoromethyl-pyrazole]), the activity COX-2 (rofecoxib), and denervation with capsaicin. The mRNA expression of constitutively expressed nitric-oxide synthase (cNOS), inducible nitric-oxide synthase (iNOS), interleukin (IL)-1β, and tumor necrosis factor (TNF)-α was analyzed by reverse transcription polymerase chain reaction. The WRS lesions were dose-dependently reduced by pretreatment with Ang-(1-7), which also caused an increase in gastric blood flow (GBF) and luminal content of NO. COX-1 and COX-2 inhibitors or L-NNA (N5-[imino(nitroamino)methyl]-L-ornithine) reversed the reduction in lesion number and the rise in GBF evoked by Ang-(1-7). Ang II augmented the WRS lesions, decreased GBF and increased the plasma IL-1β and TNF-α levels. Capsaicin denervation attenuated the reduction of Ang-(1-7)-induced gastric lesions and the rise in GBF; these effects were restored by supplementation with calcitonin gene-related peptide (CGRP). The cNOS mRNA was upregulated while iNOS, IL-1β and TNF-α mRNAs were downregulated in Ang-(1-7)-pretreated rats. We conclude that Ang-(1-7), in contrast to Ang II, which worsened WRS ulcerogenesis, affords potent gastroprotection against WRS ulcerogenesis via an increase in GBF mediated by NO, endogenous prostaglandins, sensory neuropeptides, and anti-inflammatory action involving the inhibition of proinflammatory markers iNOS, IL-1β, and TNF-α.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Newer Insights into the Biochemical Physiology of the Renin–Angiotensin System: Role of Angiotensin-(1-7), Angiotensin Converting Enzyme 2, and Angiotensin-(1-12)

Knowledge of the mechanisms by which the rennin–angiotensin system contributes to cardiovascular pathology continues to advance at a rapid pace as newer methods and therapies uncover the nature of this complex system and its fundamental role in the regulation of blood pressure and tissue function. The characterization of the biochemical pathways and functions mediated by angiotensin-(1-7) [Ang-(1-7)], angiotensin converting enzyme 2 (ACE2), and the mas receptor has revealed a vasodepressor and antiproliferative axis that within the rennin–angiotensin system opposes the biological actions of angiotensin II (Ang II). In addition, new research expands on this knowledge by demonstrating additional mechanisms for the formation of Ang II and Ang-(1-7) through the existence of an alternate form of the angiotensinogen substrate [angiotensin-(1-12)] which generates Ang II and even Ang-(1-7) through a non-renin dependent action. Altogether, this research paves the way for a better understanding of the intracellular mechanisms involved in the synthesis of angiotensin peptides and its consequences in terms of cell function in both physiology and pathology.

Reduced circulating levels of angiotensin-(1--7) in systemic sclerosis: a new pathway in the dysregulation of endothelial-dependent vascular tone control

OBJECTIVE

Systemic sclerosis (SSc) impairs endothelium-dependent vasodilatation. Among angiotensin I (Ang I)-derived compounds, vasoconstrictor angiotensin II (Ang II) and vasodilator angiotensin-(1-7) (Ang-(1-7)), cleaved from ACE and neutral endopeptidase (NEP) 24.11, respectively, play an important role in vascular tone regulation. Ang-(1-7) may act independently or by activating other vasodilating molecules, such as nitric oxide (NO) or prostaglandin I2 (PGI2). Our aim was to assess, in patients with SSc, circulating levels of Ang I, Ang II and Ang-(1-7), with their metabolising enzymes ACE and NEP, and levels of NO and PGI2, and to correlate them to the main characteristics of SSc.

METHODS

Levels of Ang I, Ang II, Ang-(1-7), NEP, ACE, NO and PGI2 were measured in 32 patients with SSc, who were also assessed for humoral and clinical characteristics, and 55 controls.

RESULTS

Plasma Ang I, Ang II and Ang-(1-7) levels were lower in patients with SSc than in controls (p<0.001in all cases). When Ang II and Ang-(1-7) levels were expressed as a function of the available Ang I, lower Ang-(1-7) levels in patients with SSc than in controls were confirmed (p<0.001), while no difference was found for Ang II levels. In patients with SSc, the Ang II/Ang-(1-7) ratio indicated a prevalence of Ang II over Ang-(1-7), while in controls Ang-(1-7) was prevalent (p<0.001). Levels of ACE, NEP, NO and PGI2 were lower in patients with SSc than in controls (p<0.05 in all cases).

CONCLUSION

In patients with SSc, prevalence of the vasoconstricting Ang II over the vasodilator Ang-(1-7) suggests a dysfunction of the angiotensin-derived cascade that may contribute to dysregulation of vascular tone.

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

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

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.

New insight into abnormal muscle vasodilatory responses in aged hypertensive rats by in vivo nuclear magnetic resonance imaging of perfusion

OBJECTIVE

Increased peripheral arterial resistances and decreased maximum vasodilation are characteristic features of chronic hypertension. However, little data are available in the literature regarding the possible alterations in the temporal patterns of vasodilatory responses elicited by various stimuli.

DESIGN

This question was addressed by measuring skeletal muscle perfusion using nuclear magnetic resonance imaging combined with arterial spin labeling.

METHODS

Ninety-week-old male spontaneously hypertensive (SHR; n = 7) and normotensive Wistar Kyoto (WKY; n = 8) rats were studied, and calf muscle perfusion was measured at rest and during reactive hyperemia following total ischemia of 5 and 30 min duration.

RESULTS

Reactive hyperemia profiles differed according to duration of ischemia. In WKY rats, 5 min of ischemia induced a short peak of hyperemia lasting no more than 63 s, while 30 min of ischemia were followed by a prolonged hyperemic response of 261 s. In SHRs, after 5 min of ischemia, peak muscle arterial conductance was decreased to 0.5 +/- 0.3 versus 0.9 +/- 0.3 ml.min(-1).100 g(-1).mm Hg(-1) in the WKY rats (p < 0.05), as expected. After 30 min of ischemia, there was, in addition, a shortening of the hyperemic response duration. Time to post-ischemic half normalization of arterial conductance was 38 +/- 24 s in the SHRs versus 149 +/- 58 s in the WKY rats (p < 0.001).

CONCLUSION

In vivo perfusion measurement not only confirmed the existence of a reduced maximum peripheral vasodilation in chronically hypertensive rats, it revealed a blunted hyperemic response after prolonged ischemia in the SHRs, which might be an important contributing factor to the increased sensitivity to ischemia in hypertension.

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.

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.

Endogenous prostacyclin increases neuronal nitric oxide release in mesenteric artery from spontaneously hypertensive rats

The aim of this study was to analyse the possible influence of endogenous prostacyclin on neuronal nitric oxide (NO) release induced by electrical field stimulation in mesenteric arteries from spontaneously hypertensive rats (SHR). Preincubation with the prostacyclin synthesis inhibitor tranylcypromine decreased NO release induced by electrical field stimulation, which was reversed by exogenous prostacyclin. Preincubation with tranylcypromine increased basal and electrical field stimulation-induced [3H]noradrenaline release. The nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl esther (L-NAME) increased the vasoconstrictor response induced by electrical field stimulation. In the presence of tranylcypromine, L-NAME did not modify the vasoconstrictor response induced by electrical field stimulation. In the presence of tranylcypromine and prostacyclin, LNAME increased the vasoconstrictor response to electrical field stimulation. These results indicate that endogenous prostacyclin positively modulates the neuronal NO release induced by electrical field stimulation and that this neuronal NO participates in the regulation of the vasomotor response.

Molecular mechanisms of inhibition of vascular growth by angiotensin-(1-7)

Angiotensin (Ang) peptides play a critical role in regulating vascular reactivity and structure. We showed that Ang-(1-7) reduced smooth muscle growth after vascular injury and attenuated the proliferation of vascular smooth muscle cells (VSMCs). This study investigated the molecular mechanisms of the antiproliferative effects of Ang-(1-7) in cultured rat aortic VSMCs. Ang-(1-7) caused a dose-dependent release of prostacyclin from VSMCs, with a maximal release of 277.9+/-25.2% of basal values (P<0.05) by 100 nmol/L Ang-(1-7). The cyclooxygenase inhibitor indomethacin significantly attenuated growth inhibition by Ang-(1-7). In contrast, neither a lipoxygenase inhibitor nor a cytochrome p450 epoxygenase inhibitor prevented the antiproliferative effects of Ang-(1-7). These results suggest that Ang-(1-7) inhibits vascular growth by releasing prostacyclin. Ang-(1-7) caused a dose-dependent release of cAMP, which might result from prostacyclin-mediated activation of adenylate cyclase. The cAMP-dependent protein kinase inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate attenuated the Ang-(1-7)-mediated inhibition of serum-stimulated thymidine incorporation. Finally, Ang-(1-7) inhibited Ang II stimulation of mitogen-activated protein kinase activities (ERK1/2). Incubation of VSMCs with concentrations of Ang-(1-7) up to 1 micromol/L had no effect on ERK1/2 activation. However, preincubation with increasing concentrations of Ang-(1-7) caused a dose-dependent reduction in Ang II-stimulated ERK1/2 activities. Ang-(1-7) (1 micromol/L) reduced 100 nmol/L Ang II-stimulated ERK1 and ERK2 activation by 42.3+/-6.2% and 41.2+/-4.2%, respectively (P<0.01). These results suggest that Ang-(1-7) inhibits vascular growth through the release of prostacyclin, through the prostacyclin-mediated production of cAMP and activation of cAMP-dependent protein kinase, and by attenuation of mitogen-activated protein kinase activation.

Newly recognized components of the renin-angiotensin system: potential roles in cardiovascular and renal regulation

The renin-angiotensin system (RAS) is a coordinated hormonal cascade in the control of cardiovascular, renal, and adrenal function that governs body fluid and electrolyte balance, as well as arterial pressure. The classical RAS consists of a circulating endocrine system in which the principal effector hormone is angiotensin (ANG) II. ANG is produced by the action of renin on angiotensinogen to form ANG I and its subsequent conversion to the biologically active octapeptide by ANG-converting enzyme. ANG II actions are mediated via the ANG type 1 receptor. Here, we discuss recent advances in our understanding of the components and actions of the RAS, including local tissue RASs, a renin receptor, ANG-converting enzyme-2, ANG (1-7), the function of the ANG type 2 receptor, and ANG receptor heterodimerization. The role of the RAS in the regulation of cardiovascular and renal function is reviewed and discussed in light of these newly recognized components.

Extracellular acidosis results in higher intracellular acidosis and greater contraction in spontaneously hypertensive rat aorta

Acidic pH induces a contraction in aorta from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. The contractile response to acidic pH in SHR aorta is greater than that in WKY aorta. The purpose of this study was to investigate the correlation among extracellular pH (pH(o)), intracellular pH (pH(i)) and contraction in order to understand the exaggerated contractile response to acidic pH in SHR aorta. pH(i) measurement showed that at pH(o) 6.5, intracellular acidification was greater in SHR aorta than in WKY aorta. Decreasing pH(o) further to 6.2 in WKY aorta produced intracellular acidification close to that achieved at pH(o) 6.5 in SHR aorta, and at this level, the difference in contractile response between the two strains was also abolished. These results suggest that acidic pH(i), but not pH(o), is closely correlated with the contractile response and that the exaggerated contractile response in SHR aorta is due to a greater fall in pH(i).

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.

Functional role of Cl- channels in acidic pH-induced contraction of the aorta of spontaneously hypertensive and Wistar Kyoto rats

pH regulates various cellular functions. Previously, we have described that acidic pH produces depolarization and contraction in isolated aorta from spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats [Br. J. Pharmacol. 118 (1996) 485]. The aim of the present study was to investigate the involvement of Cl- channels in acidic pH-induced contraction. Changing the pH of the bathing solution from 7.4 to 6.5 induced a contraction in both SHR and WKY aorta, which was 127.50+/-13.32% and 79.27+/-0.94% of the 64.8 mM KCl-induced contraction, respectively. The acidic pH-induced contraction was partially inhibited by the voltage-dependent Ca2+ channel (VDCC) blockers, verapamil (1 microM) and nifedipine (0.1 microM). The Cl- channel inhibitors, diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) (0.5 mM), 9-anthracene chloride (0.5 mM), indanyloxyacetic acid (30 microM) and niflumic acid (3 microM) also inhibited the acidic pH-induced contraction and the degree of attenuation was comparable to that of VDCC blockers. DIDS, 9-anthracene chloride and niflumic acid at concentrations used to inhibit the acidic pH-induced contraction also inhibited the 10 microM phenylephrine-induced contraction partially, without affecting the 64.8 mM KCl-induced contraction, whereas both the contractions were inhibited by indanyloxyacetic acid with equal efficacy. Indanyloxyacetic acid but not DIDS, 9-anthracene chloride or niflumic acid inhibited the 24.8 mM KCl-induced contraction. Simultaneous measurement of cytosolic Ca2+ and tension showed that niflumic acid reversed the increase in intracellular Ca2+ level and inhibited the contraction caused by acidic pH. Similarly, acidic pH depolarized the cultured vascular smooth muscle cells from SHR and the depolarization was completely reversible after the administration of niflumic acid. All these results suggest that the activation of Cl- channels is an important mechanism underlying the depolarization and contraction induced by acidic pH in SHR and WKY aortas.

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.

Prostanoids, but not nitric oxide, counterregulate angiotensin II mediated vasoconstriction in vivo

To evaluate the modulating effects of nitric oxide and prostanoids during angiotensin II-mediated vasoconstriction, male Wistar rats (n=25) were infused with increasing doses of angiotensin II following pretreatment with the cyclooxygenase inhibitor indomethacin, the nitric oxide-synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) plus sodium nitroprusside to restore mean arterial blood pressure, or saline. Hemodynamics were studied with the radioactive microsphere method. Indomethacin did not alter systemic or regional hemodynamics. L-NAME+sodium nitroprusside reduced cardiac output, as well as systemic and renal vascular conductance. Angiotensin II increased mean arterial blood pressure and heart rate, and decreased systemic vascular conductance as well as vascular conductance in gastrointestinal tract, kidney, skeletal muscle, skin, mesentery+pancreas, spleen and adrenal. Indomethacin enhanced the angiotensin II-mediated effects in all vascular beds, whereas L-NAME+sodium nitroprusside enhanced its effect in mesentery+pancreas only. In conclusion, vasodilator prostanoids, but not nitric oxide, counterregulate angiotensin II-mediated vasoconstriction in vivo.

Effects of angiotensin-(1-7) on forearm circulation in normotensive subjects and patients with essential hypertension

Previous animal studies have shown that angiotensin (Ang)-(1-7) is a biologically active component of the renin-angiotensin system, acting as a vasoactive agent, and may play a role in the blood pressure regulation. There is little information, however, on the effect of Ang-(1-7) on human circulation or the mechanism of its action. To investigate the effect of Ang-(1-7) on forearm circulation and to determine whether this effect is altered in patients with essential hypertension, we measured change in forearm blood flow using venous occlusion plethysmography in response to intra-arterial infusion of Ang-(1-7) (10(-10), 10(-9), and 10(-8) mol/min; for 5 minutes) in normotensive control subjects (n=8) and patients with essential hypertension (n=8). Infusion of Ang-(1-7) significantly increased the forearm blood flow response in a dose-dependent manner in both normotensive control subjects (28.7+/-9.7%, at 10(-8) mol/min; P<0.05) and hypertensive patients (31.8+/-15.2%, at 10(-8) mol/min; P<0.05). The vasodilatory effect of Ang-(1-7) was similar in the two groups. Intra-arterial infusion of N(G)-monomethyl-L-arginine, a nitric oxide synthesis inhibitor, did not alter the forearm blood flow response to Ang-(1-7) in either group. These findings suggest that Ang-(1-7) causes vasodilation in forearm circulation of normotensive subjects and patients with essential hypertension through a pathway that is independent of nitric oxide synthesis.

Angiotensin-(1-7) downregulates the angiotensin II type 1 receptor in vascular smooth muscle cells

Angiotensin (Ang)-(1-7) is a biologically active peptide of the renin-angiotensin system that has both vasodilatory and antiproliferative activities that are opposite the constrictive and proliferative effects of angiotensin II (Ang II). We studied the actions of Ang-(1-7) on the Ang II type 1 (AT(1)) receptor in cultured rat aortic vascular smooth muscle cells to determine whether the effects of Ang-(1-7) are due to its regulation of the AT(1) receptor. Ang-(1-7) competed poorly for [(125)I]Ang II binding to the AT(1) receptor on vascular smooth muscle cells, with an IC(50) of 2.0 micromol/L compared with 1.9 nmol/L for Ang II. The pretreatment of vascular smooth muscle cells with Ang-(1-7) followed by treatment with acidic glycine to remove surface-bound peptide resulted in a significant decrease in [(125)I]Ang II binding; however, reduced Ang II binding was observed only at micromolar concentrations of Ang-(1-7). Scatchard analysis of vascular smooth muscle cells pretreated with 1 micromol/L Ang-(1-7) showed that the reduction in Ang II binding resulted from a loss of the total number of binding sites [B(max) 437.7+/-261.5 fmol/mg protein in Ang-(1-7)-pretreated cells compared with 607.5+/-301.2 fmol/mg protein in untreated cells, n=5, P<0.05] with no significant effect on the affinity of Ang II for the AT(1) receptor. Pretreatment with the AT(1) receptor antagonist L-158,809 blocked the reduction in [(125)I]Ang II binding by Ang-(1-7) or Ang II. Pretreatment of vascular smooth muscle cells with increasing concentrations of Ang-(1-7) reduced Ang II-stimulated phospholipase C activity; however, the decrease was significant (81.2+/-6.4%, P<0.01, n=5) only at 1 micromol/L Ang-(1-7). These results demonstrate that pharmacological concentrations of Ang-(1-7) in the micromolar range cause a modest downregulation of the AT(1) receptor on vascular cells and a reduction in Ang II-stimulated phospholipase C activity. Because the antiproliferative and vasodilatory effects of Ang-(1-7) are observed at nanomolar concentrations of the heptapeptide, these responses to Ang-(1-7) cannot be explained by competition of Ang-(1-7) at the AT(1) receptor or Ang-(1-7)-mediated downregulation of the vascular AT(1) receptor.

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.

Contribution of angiotensin-(1-7) to blood pressure regulation in salt-depleted hypertensive rats

We exposed 63 adult spontaneously hypertensive rats (SHR) and 10 (mRen-2)27 transgenic hypertensive rats to a 12-day regimen of either a normal diet (0.5%) or a low-salt diet (0.05%) to evaluate the hypothesis that the vasodepressor heptapeptide, angiotensin-(1-7) [Ang-(1-7)], buffers the pressor effects of angiotensin II during endogenous stimulation of the renin-angiotensin system. Catheters were inserted into a carotid artery and jugular vein under light anesthesia the day before the experiment. Separate groups of conscious instrumented SHR were given short-term infusions of an affinity-purified monoclonal Ang-(1-7) antibody or the neprilysin inhibitor SCH 39370. In addition, SHR and (mRen-2)27 rats were given the Ang-(1-7) receptor antagonist [D-Ala(7)]Ang-(1-7). Exposure to the low-salt diet increased plasma renin activity and elevated plasma levels of angiotensin I and angiotensin II in SHR by 81% and 68%, respectively, above values determined in SHR fed a normal salt diet. Concentrations of angiotensin I and angiotensin II were also higher in the kidney of salt-depleted SHR, whereas plasma and renal tissue levels of Ang-(1-7) were unchanged. Infusion of the Ang-(1-7) antibody produced dose-dependent pressor and tachycardic responses in salt-depleted SHR but no effect in SHR maintained on a normal-salt diet. A comparable cardiovascular response was produced in salt-depleted SHR given either SCH 39370 or [D-Ala(7)]Ang-(1-7). These agents had negligible effects on SHR fed a normal-salt diet. Blockade of Ang-(1-7) receptors produced a similar cardiovascular response in (mRen-2)27 transgenic hypertensive rats fed a low-salt diet. Injections of the heat-inactivated antibody or the subsequent infusion of the antibody to rats given [D-Ala(7)]Ang-(1-7) produced no additional effects. The data support the hypothesis that the hemodynamic effects of neurohormonal activation after salt restriction stimulate a tonic depressor action of Ang-(1-7).

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.

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

Hemodynamic factors, circulating hormones, paracrine factors, and intracrine factors influence vascular smooth muscle growth and plasticity. The well-characterized role of angiotensin II in the modulation of vascular tone and cell function may be critically involved in the mechanisms by which vascular smooth muscle responds to signals associated with vascular endothelial dysfunction and increases in oxidative stress. Studies from this laboratory suggest that the trophic actions of angiotensin II may be intrinsically regulated by angiotensin-(1-7), a separate product of the angiotensin system derived from the common substrate, angiotensin I. Exposure of cultured vascular smooth muscle cells to angiotensin-(1-7) inhibited the trophic actions of angiotensin II and reduced the expression of the mitogenic effects of both normal serum and platelet-derived growth factor. The growth-inhibitory actions of angiotensin-(1-7) were blocked by the selective D-alanine(7)-angiotensin-(1-7) antagonist and the nonselective angiotensin receptor blocker sarcosine(1)-threonine(8)-angiotensin II. In contrast, subtype-selective antagonists for the AT(1) and AT(2) receptors had no effect on the inhibitory actions of angiotensin-(1-7), a finding that is consistent with the pharmacological characterization of a high-affinity (125)I-labeled angiotensin-(1-7) binding site in the vasculature by use of selective and nonselective angiotensin II receptor antagonists. The relevance of these findings to the proliferative response of vascular smooth muscle cells after endothelial injury was confirmed by assessment of the effect of a 12-day infusion of angiotensin-(1-7) on neointimal formation. In these experiments, the proliferative response produced by injuring the carotid artery was inhibited by angiotensin-(1-7) through a mechanism that could not be explained by changes in arterial pressure. Because plasma angiotensin-(1-7) increased to levels comparable to those found in animals and human subjects given therapeutic doses of angiotensin-converting enzyme inhibitors, angiotensin-(1-7) may be one factor participating in the reversal of vascular proliferation during inhibition of angiotensin II formation or activity.

Angiotensin-(1-7) reduces smooth muscle growth after vascular injury

Regulation of vascular smooth muscle cell growth is critical to the maintenance of normal blood flow and vessel patency. Angiotensin-(1-7) [Ang-(1-7)] inhibits proliferation of vascular smooth muscle cells in vitro and opposes the mitogenic effects of angiotensin II. The present study investigated whether Ang-(1-7) inhibits vascular smooth muscle cell growth in vivo by determining its effect on neointimal formation and medial remodeling in balloon-injured carotid arteries. The carotid arteries of adult male Sprague-Dawley rats were injured with a balloon embolectomy catheter. Ang-(1-7) in saline (24 microg/kg per hour) or saline alone was infused intravenously for 12 days after injury. Pumps containing bromodeoxyuridine were implanted at the same time to determine DNA synthesis. Intravenous infusion increased plasma Ang-(1-7) to 166. 0+/-41.2 fmol/mL (n=6) compared with 46.9+/-4.2 fmol/mL (n=8) in saline-infused rats. Plasma concentrations of Ang II were not changed by Ang-(1-7) infusion. Elevation in circulating Ang-(1-7) had no effect on either blood pressure or heart rate compared with saline controls. Histomorphometric analysis of carotid arteries indicated that Ang-(1-7) infusion significantly reduced neointimal area compared with rats infused with saline (0.063+/-0.011 versus 0. 100+/-0.009 mm2; P<0.05). In contrast, Ang-(1-7) infusion had no effect on medial area of the injured or the contralateral uninjured artery compared with saline controls. Ang-(1-7) infusion also reduced the rate of DNA synthesis in both the neointima and the media of the injured vessels. Therefore, exogenous Ang-(1-7) inhibited vascular smooth muscle cell proliferation associated with balloon-catheter injury. Similar increases in endogenous plasma Ang-(1-7) and inhibition of neointimal growth were observed in rats after angiotensin-converting enzyme inhibitor or angiotensin type 1 receptor antagonist administration, suggesting that Ang-(1-7) may contribute to the in vivo antiproliferative effects of these agents on vascular smooth muscle.

Synergistic effect of angiotensin-(1-7) on bradykinin arteriolar dilation in vivo

The interaction between angiotensin [Ang-(1-7)] and bradykinin (BK) was determined in the mesentery of anesthetized Wistar rats using intravital microscopy. Topical application of BK and Ang-(1-7) induced vasodilation that was abolished by the BK B2 receptor antagonist HOE-140 and the Ang-(1-7) antagonist A-779, respectively. BK (1 pmol)-induced vasodilation, but not SNP and ACh responses, was potentiated by Ang-(1-7) 10 pmol and 100 pmols. The effect of 100 pmol of Ang-(1-7) on BK-induced vasodilation was abolished by A-779, indomethacin, and L-nitroarginine methyl esther, whereas losartan was without effect. Enalaprilat treatment enhanced the BK- and Ang-(1-7)-induced vasodilation and the potentiating effect of Ang-(1-7) on BK vasodilation. The potentiation of BK-induced vasodilation by Ang-(1-7) is a receptor-mediated phenomenon dependent on cyclooxygenase-related products and NO release.

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

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

Antihypertensive effects of angiotensin-(1-7)

Accumulating evidence suggests that angiotensin-(1-7)(Ang-(1-7)) is an important component of the renin-angiotensin system and that the actions of the peptide may either contribute to or oppose those of Ang II. Ang-(1-7) can be converted directly from Ang I bypassing prerequisite formation of Ang II. Formation of Ang-(1-7) is under the control of at least three endopeptidases depending on the tissue compartment and include neprilysin, thimet oligopeptidase and prolyl oligopeptidase. Both neprilysin and thimet oligopeptidase are also involved in the metabolism of bradykinin and the atrial natriuretic peptide. Moreover, recent studies suggest that in addition to Ang I and bradykinin, Ang-(1-7) is an endogenous substrate for angiotensin converting enzyme. These enzymatic pathways may contribute to a complex relationship between the hypertensive actions of Ang II and various vasodepressor peptides from either the renin-angiotensin system or other peptide systems. Ang-(1-7) is devoid of the vasoconstrictor, central pressor, or thirst-stimulating actions associated with Ang II. In fact, new findings reveal depressor, vasodilator, and antihypertensive actions that may be more apparent in hypertensive animals or humans. Thus, Ang-(1-7) may oppose the actions of Ang II directly or as a result of increasing prostaglandins or nitric oxide. In this review, we examine the mechanisms by which Ang-(1-7) may contribute to cardiovascular regulation.

Downregulation of angiotensin II type 1 receptor gene transcription by nitric oxide

Nitric oxide (NO) plays an important role not only in the regulation of blood vessel tone, but also in the growth of vascular smooth muscle cells (VSMC). The precise mechanism involved in the inhibition of VSMC growth by NO is not known. To further explore the effect of NO on VSMC growth, we examined the effect of NO on the expression of angiotensin II type 1 receptor (AT1-R) that is important for hypertrophy and hyperplasia of VSMC. S-nitroso acetyl DL-penicillamine (SNAP; 200 micromol/L), a potent NO donor, suppressed expression level of AT1-R mRNA by 90% and AT1-R number by 60% after 24 hours of stimulation. The suppressive effect was dose-dependent. Actinomycin D, which is an inhibitor of gene transcription, did not affect the decrease of AT1-R mRNA by NO. Cyclic guanosine monophosphate (cGMP) analogue, 8 bromo-cGMP, did not affect AT1-R mRNA level. Deletion mutants of the promoter region of rat AT1a-R gene were fused to luciferase reporter gene and introduced to VSMC. Transfected cells were stimulated with SNAP, and luciferase activity was measured. Inhibitory effect of NO was still observed in the shortest deletion mutant that contained 61 bp upstream from transcription start site. In this DNA segment, two DNA binding protein were observed by gel mobility shift assay, and one of these binding proteins was decreased on stimulation by NO. NO downregulates AT1-R gene expression independently of cGMP. A DNA binding protein that binds to the proximal promoter region of AT1-R gene may be responsible for this inhibitory effect. The inhibition of AT1-R gene expression may be implicated in the anti-atherogenic property of NO.

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

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

Prostacyclin release by rat cardiac fibroblasts: inhibition of collagen expression

Cardiac fibroblasts, as the source of extracellular matrix for the left ventricle, subserve important functions to cardiac remodeling and fibrotic development following myocardial infarction or with pressure-overload cardiac hypertrophy. The fibroblast may be the target cell for angiotensin-converting enzyme inhibitors (ACEI) that are cardioprotective and reverse collagen deposition and remodeling but whose mechanisms of action remain controversial. Because we previously documented phenotypic differences between cardiac fibroblasts from the spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) left ventricle, the present study evaluated whether phenotypic differences also exist in the release of endogenous arachidonic acid metabolites or in the activation of phospholipase D, and the importance of observed differences to the formation of collagen and the mechanism of action of ACEI. The experimental design compared endogenous sources of arachidonic acid with exogenous prelabeling of cells. Angiotensin II stimulated greater arachidonic acid release than bradykinin, and WKY cells were more responsive than SHR. The major prostanoid formed by cardiac fibroblasts was prostaglandin I2 (PGI2), with more prostacyclin production by WKY cells than SHR cells both under nonstimulated conditions and in response to angiotensin II or bradykinin. Beraprost, a PGI2 analogue, was shown to decrease growth rate and DNA synthesis of fibroblasts and to inhibit mRNA expression for collagen types I and III, with SHR cells being less responsive to beraprost than WKY cells. These results potentially implicate eicosanoid metabolism, particularly PGI2, in collagen formation, fibrotic development, and cardiac remodeling, and they imply that the SHR genetic hypertension model may be predisposed to excess cardiac fibrosis.

Counterregulatory actions of angiotensin-(1-7)

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

Bovine aortic endothelial cells contain an angiotensin-(1-7) receptor

Angiotensin-(1-7) is a novel peptide of the renin-angiotensin system that counteracts the pressor and proliferative responses to angiotensin II. We now report that cultured bovine aortic endothelial cells contain a saturable, high-affinity [125I]angiotensin-(1-7) binding site with an affinity of 19.3 +/- 10.7 nmol/L and a density of 1351 +/- 710 fmol/mg protein. Angiotensin-(1-7) competed at a second lower-affinity site, with an IC50 of 2.9 mumol/L. The high-affinity angiotensin II receptor antagonist sarcosine1-isoleucine8-angiotensin II blocked [125I]angiotensin-(1-7) binding to bovine aortic endothelial cells at both a high- (IC50 = 1.3 nmol/L) and a low-affinity (IC50 = 6.2 mumol/L) binding site. In contrast, D-alanine7-angiotensin-(1-7) completely blocked [125I]angiotensin-(1-7) binding, with an IC50 of 19.8 nmol/L, suggesting that D-alanine7-angiotensin-(1-7) may selectively block responses to angiotensin-(1-7) in endothelial cells. Neither the AT1 antagonist losartan nor the AT2 antagonist PD 123319 exhibited significant competition for [125I]angiotensin-(1-7) binding to endothelial cells isolated from bovine aorta, in agreement with the absence of detectable mRNAs encoding typical angiotensin receptor subtypes 1 or 2 (AT1 or AT2). Angiotensin II also competed for [125I]angiotensin-(1-7) binding to bovine aortic endothelial cells; however, the relative affinity was 13-fold lower than angiotensin-(1-7), suggesting a preference for angiotensin-(1-7) over angiotensin II. These results demonstrate that bovine aortic endothelial cells contain a unique non-AT1, non-AT2 angiotensin receptor that preferentially binds angiotensin-(1-7).

Effect of an endothelin antagonist on hemodynamic responses to angiotensin II

We determined changes in blood pressure, cardiac output, and total peripheral conductance evoked by intravenous infusions of angiotensin II (Ang II) in conscious, unrestrained normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) before and after pretreatment with bosentan, a nonselective endothelin antagonist. Blood pressure was recorded by radiotelemetry and cardiac output by ultrasonic transit-time flow probes. Bosentan per se failed to affect basal blood pressure and evoked only small changes in cardiac output and total peripheral conductance in both strains. The pressor effects of Ang II were exaggerated in SHR compared with WKY. Strikingly, bosentan pretreatment blunted the increases in blood pressure, the fall in cardiac output, and the decreases in conductance evoked by lower doses of Ang II but not higher doses of the peptide. This effect was observed in both rat strains but was more pronounced in SHR. These data suggest that endothelin contributes to the hemodynamic effects of Ang II in both SHR and WKY and that endothelin may contribute to the exaggerated pressor responsiveness of SHR to Ang II.

Angiotensin-(1-7) inhibits vascular smooth muscle cell growth

Although angiotensin II (Ang II) and the heptapeptide Ang-(1-7) differ by only one amino acid, the two peptides produce different responses in vascular smooth muscle cells. We previously showed that Ang II stimulated phosphoinositide hydrolysis, whereas Ang II and Ang-(1-7) released prostaglandins. We now report that Ang II and Ang-(1-7) differentially modulate rat aortic vascular smooth muscle cell growth. Ang-(1-7) inhibited [3H]thymidine incorporation in response to stimulation by fetal bovine serum, platelet-derived growth factor, or Ang II. The reduction in serum-stimulated thymidine incorporation by Ang-(1-7) depended on the concentration of the heptapeptide over the range of 1 nmol/L to 1 mumol/L, with a maximal inhibition of 60% by 1 mumol/L Ang-(1-7). Ang-(1-7) also inhibited the serum-stimulated increase in cell number to a maximum of 77% by 1 mumol/L Ang-(1-7). The attenuation of serum-stimulated thymidine incorporation by Ang-(1-7) was unaffected by antagonists selective for angiotensin type 1 (AT1) or type 2 (AT2) receptors; however, [Sar1,Ile1]Ang II and [Sar1,Thr2]Ang II were effective antagonists, indicating that growth inhibition by Ang-(1-7) was a result of angiotensin receptor activation. In contrast, Ang II stimulated [3H]thymidine incorporation in cultured vascular smooth muscle cells over the same concentration range, with a maximal stimulation of 314% at 1 mumol/L Ang II. Ang II also increased the total number of cells (to 145% of control), suggesting that enhanced thymidine incorporation was associated with vascular smooth muscle cell proliferation. The AT1 antagonist losartan or L-158,809 but not AT2 antagonists blocked [3H]thymidine incorporation by Ang II. These results suggest that Ang-(1-7) and Ang II exhibit opposite effects on the regulation of vascular smooth muscle cell growth. The inhibition of proliferation by Ang-(1-7) appears to be mediated by a novel angiotensin receptor that is not inhibited by AT1 or AT2 receptor antagonists.

The mechanism of acidic pH-induced contraction in aortae from SHR and WKY rats enhanced by increasing blood pressure

1. Effect of pH on vascular smooth muscle contraction was analyzed by use of biochemical and pharmacological techniques. 2. In the aorta isolated from spontaneously hypertensive rats (SHR) decreasing extracellular pH (pH0) caused a rapid acidification of intracellular pH accompanied by a pH0-dependent increase in tension. The contraction of the SHR aorta was remarkable compared with that of the Wistar Kyoto rat (WKY) aorta. 3. Removal of NH4Cl caused a transient decrease in intracellular pH followed by a marked increase in tension. 4. Both contraction and intracellular Ca2+ mobilization induced by acidic pH0 were markedly inhibited by removal of extracellular Ca2+, verapamil and adenosine, whereas these were not affected by tetrodotoxin or Gd3+, a stretch-activated cation channel blocker. Furthermore, cromakalim (a K+ channel opener) inhibited acidic pH0-induced contraction (APIC). 5. Acidic pH0 induced a depolarization of cultured smooth muscle cells from SHR aorta. 6. Blood pressure elevated with increasing age of WKY and SHR accompanied by an increase in APIC. Feeding WKY with NG-nitro-L-arginine, an inhibitor of nitric oxide synthases caused a marked elevation of their blood pressure followed by an increase in APIC. 7. These results suggest that APIC is caused by Ca2+ influx mediated through the activation of voltage-sensitive Ca2+ channels mainly due to acidic pH0-induced depolarization of the plasma membrane of smooth muscle cells. It is also suggested that APIC is strengthened by the elevation of blood pressure.

Role of calcium in angiotensin II-induced prostaglandin release and DNA synthesis in rat vascular smooth muscle cells

Cellular calcium modulates enzyme activity, cell proliferation, and differentiation. In vascular smooth muscle cells (VSMC), calcium may contribute to increased vascular contractility and structural alterations in both hypertension and atherosclerosis. We investigated the role of calcium in angiotensin II (AII)-induced prostaglandin release and DNA synthesis in VSMC. Prostaglandin levels were determined by radioimmunoassay, and DNA synthesis was determined by the incorporation of [3H]thymidine. AII dose-dependently stimulated the release of prostaglandin E2 and prostaglandin I2, and this effect was synergistically enhanced by the Ca2+ ionophore A23187. Conversely, the AII response was inhibited by EGTA, a chelator of Ca2+ ions and by verapamil and nifedipine, two Ca2+ channel blockers or by incubation of the cells without exogenous Ca2+. TMB-8, an inhibitor of calcium mobilization, also strongly reduced angiotensin response. Similar results were obtained for angiotensin III (AIII) and vasopressin, two other agonists of prostaglandin production. AII- or serum-stimulated DNA synthesis was almost abolished by EGTA, whereas TMB-8, verapamil, and nifedipine had little or no effect. The production of prostaglandins triggered by angiotensins and vasopressin in VSMC is dependent on both intracellular and extracellular calcium, with calcium entering through L-type Ca2+ channels. Extracellular calcium is important for AII and serum mitogenic activity, but L-type Ca2+ channels do not appear to be implicated.

Pressor and reflex sensitivity is altered in spontaneously hypertensive rats treated with angiotensin-(1-7)

We have suggested that angiotensin-(1-7) [Ang-(1-7)] may oppose the pressor activity of angiotensin II (Ang II). This hypothesis was supported by the fact that long-term intravenous infusion of Ang-(1-7) transiently lowers blood pressure in spontaneously hypertensive rats (SHR). We now investigated whether the pressor sensitivity to bolus injections of either phenylephrine (PE) or Ang II was altered on day 12 of an Ang-(1-7) infusion when blood pressure in the SHR had returned to hypertensive levels. SHR (n = 10) and WKY rats (n = 8) were given Ang-(1-7) intravenously via osmotic minipumps at a dose of 24 micrograms/kg per hour for 2 weeks. On day 12 of the infusion, mean arterial pressure and heart rate in halothane-anesthetized rats were similar in Ang-(1-7)-treated SHR (142 +/- 6 mm Hg; 388 +/- 9 beats per minute) and those infused with vehicle (146 +/- 5 mm Hg; 392 +/- 13 beats per minute). Pressor responsiveness to PE in Ang-(1-7)-treated SHR was 22% less at a dose of 10 micrograms, while pressor responses to Ang II decreased by 20% and 25% at doses of 0.05 and 0.1 micrograms, respectively, compared with the vehicle-treated SHR (P < .05). There were no effects of the Ang-(1-7) infusion on pressor responses to Ang II or PE in WKY rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II-induced responses in vascular smooth muscle cells: inhibition by non-peptide receptor antagonists

The present study investigates the effect of angiotensin II and LR-B/081 (-methyl 2-[[4-butyl-2-methyl-6-oxo-5-[[2'-(1H-tetra-zol-5-yl) [1,1'-biphenyl]-4-yl] methyl]-1(6H)-pyrimidinyl] methyl]-3-thiophenecarboxylate), a novel non-peptide angiotensin II receptor antagonist, on both early and late responses in rat vascular smooth muscle cells. Angiotensin II induced a rapid and transient elevation of inositol trisphosphate intracellular levels, triggered the release of both prostaglandin E2 and prostaglandin I2 (EC50 = 21 +/- 3 and 16 +/- 2 nM, respectively), and, in long-term studies, increased leucine and thymidine incorporation. All angiotensin II effects were antagonized by LR-B/081 and losartan, the reference non-peptide angiotensin AT1-selective receptor antagonist, whereas they were unaffected by PD123177 (1-(4-amino-3-methylphenyl)methyl-5-diphenylacetyl-4,5,6,7-tetr ahy dro-1H- imidazo[4,5-c]pyridine carboxylic acid), a non-peptide angiotensin AT2-selective receptor antagonist. LR-B/081 displayed a much higher potency than losartan in inhibiting angiotensin II-induced prostaglandin E2 (IC50 = 0.15 +/- 0.02 and 39 +/- 9 nM, respectively) and prostaglandin I2 release (IC50 = 0.18 +/- 0.04 and 134 +/- 40 nM, respectively) and was also more potent in blocking the increase in protein synthesis (IC50 = 242 +/- 119 nM and 1221 +/- 687 nM, respectively). Moreover, LR-B/081 and losartan blocked the response to angiotensin III but failed to inhibit the prostaglandin release stimulated by vasopressin or the mitogenic effect of serum. LR-B/081 and losartan were devoid of intrinsic properties in the experimental conditions employed.(ABSTRACT TRUNCATED AT 250 WORDS)

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

Lack of specific antagonists to the amino-terminal heptapeptide angiotensin-(1-7) [Ang-(1-7)] prompted us to evaluate the central effects of delivering a specific affinity-purified Ang-(1-7) antibody on the blood pressure and heart rate of 12-week-old conscious homozygous female rats (n = 12) expressing the mouse submandibular Ren-2d gene [(mRen-2d)27] in their genome. Another group of transgenic hypertensive and strain-matched Sprague-Dawley controls were injected with a specific Ang II monoclonal antibody (KAA8). Cerebroventricular administration of the affinity-purified Ang-(1-7) antibody in conscious transgenic hypertensive rats caused significant dose-related elevations in blood pressure associated with tachycardia. The hypertensive response was augmented in transgenic rats studied 7 to 10 days after cessation of lisinopril therapy. Neutralization of Ang II with the Ang II antibody caused a hemodynamic response opposite to that obtained with the Ang-(1-7) antibody. All doses of the Ang II antibody produced hypotension and bradycardia. The magnitude of the depressor response was significantly augmented in transgenic rats weaned off lisinopril therapy. In contrast, central administration of either the Ang-(1-7) or Ang II antibodies had no effect on normotensive rats. Central injections of an affinity-purified IgG fraction were ineffective in both control and transgene-positive rats. These data suggest that in the brain of transgenic hypertensive rats, Ang-(1-7) opposes the action of Ang II on the central mechanism or mechanisms that contribute to the maintenance of this model of hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin-(1-7) and nitric oxide interaction in renovascular hypertension

New studies suggest that vasodilator systems may play an important role in restraining the rise in peripheral vascular resistance associated with the evolution of arterial hypertension. We characterized in conscious dogs the hemodynamic and hormonal effects of 4 weeks of feeding either the nitric oxide synthase inhibitor N omega-nitro-L-arginine (3 mg.kg-1.d-1) or the nitric oxide precursor L-arginine (0.3 mg.kg-1.d-1) during the evolution of two-kidney, one clip hypertension. Inhibition of nitric oxide production elicited a form of hypertension more severe than that produced in placebo-fed two-kidney, one clip dogs. The higher levels of blood pressure were accompanied by lower levels of plasma renin activity and lower angiotensin II concentrations. During the chronic phase of renovascular hypertension, the fall in blood pressure produced by acute systemic injections of lisinopril or losartan was significantly reduced in dogs given the nitric oxide inhibitor. In contrast, chronic administration of L-arginine had no effect on the magnitude of hypertension or on the increases in renin activity and hyperangiotensinemia associated with the evolution of renal hypertension. Likewise, the fall in blood pressure produced by pharmacological blockade of angiotensin II was not different from that recorded in untreated renal hypertensive dogs. The vasodilator component of the blood pressure response due to intravenous injections of angiotensin-(1-7) (1 to 100 nmol/kg) was augmented in both untreated and L-arginine-treated two-kidney, one clip hypertensive dogs, but was significantly attenuated in hypertensive dogs fed the nitric oxide synthase inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)