Role of AT2 receptors in angiotensin II-stimulated contraction of small mesenteric arteries in young SHR

The Angiotensin AT2 Receptor: From a Binding Site to a Novel Therapeutic Target

Discovered more than 30 years ago, the angiotensin AT2 receptor (AT2R) has evolved from a binding site with unknown function to a firmly established major effector within the protective arm of the renin-angiotensin system (RAS) and a target for new drugs in development. The AT2R represents an endogenous protective mechanism that can be manipulated in the majority of preclinical models to alleviate lung, renal, cardiovascular, metabolic, cutaneous, and neural diseases as well as cancer. This article is a comprehensive review summarizing our current knowledge of the AT2R, from its discovery to its position within the RAS and its overall functions. This is followed by an in-depth look at the characteristics of the AT2R, including its structure, intracellular signaling, homo- and heterodimerization, and expression. AT2R-selective ligands, from endogenous peptides to synthetic peptides and nonpeptide molecules that are used as research tools, are discussed. Finally, we summarize the known physiological roles of the AT2R and its abundant protective effects in multiple experimental disease models and expound on AT2R ligands that are undergoing development for clinical use. The present review highlights the controversial aspects and gaps in our knowledge of this receptor and illuminates future perspectives for AT2R research. SIGNIFICANCE STATEMENT: The angiotensin AT2 receptor (AT2R) is now regarded as a fully functional and important component of the renin-angiotensin system, with the potential of exerting protective actions in a variety of diseases. This review provides an in-depth view of the AT2R, which has progressed from being an enigma to becoming a therapeutic target.

Estradiol Supplement or Induced Hypertension May Attenuate the Angiotensin II Type 1 Receptor Antagonist-Promoted Renal Blood Flow Response to Graded Angiotensin II Administration in Ovariectomized Rats

Backgrounds. Estrogen replacement therapy (ERT) and hypertension may influence females’ renin-angiotensin system (RAS) and its components. The angiotensin II (Ang II) type 1 receptor (AT1R) antagonist (losartan) may promote renal blood flow (RBF), and it is widely used in the clinic to control hypertension. The main objective of this study was the effects of estradiol or induced hypertension on RBF response to Ang II in losartan-treated ovariectomized (OVX) rats. Methods. Two groups of OVX rats were treated with placebo (group 1) and estradiol (group 2) for period of four weeks, and another group of OVX rats was subjected to induce hypertension by two-kidney one clip (2K1C) model (group 3). All the groups were subjected to the surgical procedure under anesthesia, and AT1R was blocked by losartan. RBF and renal vascular resistance (RVR) responses to Ang II administration were determined and compared. Results. Mean arterial (MAP) and renal perfusion (RPP) pressures in group 3 and uterus weight (UT) in group 2 were significantly more than other groups ( $P<0.05$ ). Ang II infusion resulted in dose-related percentage change increase in RBF and decrease in RVR. However, these responses in the OVX-estradiol and OVX-hypertensive rats were significantly lower than in the OVX-control group ( $P<0.05$ ). For instance, at the dose of 1000 ng/kg/min of Ang II administration, the percentage change of RBF was $45.1\pm 10.4\%$ , $17.9\pm 2.3\%$ , and $16.7\pm 4.7\%$ in the groups of 1 to 3, respectively. Conclusion. Losartan prescription in some conditions such as hypertension or ERT could worsen RBF and RVR responses to Ang II.

Anti-Apoptosis of Podocytes and Pro-Apoptosis of Mesangial Cells for Telmisartan in Alleviating Diabetic Kidney Injury

Podocytes damage and mesangial cells expansion are two important pathological manifestations of glomerular injury in early diabetes. Telmisartan, as an angiotensin type 1 (AT₁) receptor inhibitor, could improve advanced glycation end (AGE) products or angiotensin Ⅱ (Ang Ⅱ)-induced podocytes injury including detachment or apoptosis. In this current paper, we first confirmed the protective effect of telmisartan on early diabetic kidney injury in type 1 diabetic rats. Telmisartan reduced the loss of podocin and inhibited the expression of α-SMA, reflecting its protective effect on podocyte injury and mesangial proliferation, respectively. More interestingly we observed an opposite effect of telmisartan on the cell viability and apoptosis of podocytes and mesangial cells in a high-glucose environment in vitro. The anti-apoptotic effect of telmisartan on podocytes might be related to its inhibition of swiprosin-1 (a protein can mediate high glucose-induced podocyte apoptosis) expression. While telmisartan induced a high expression of PPARγ in mesangial cells, and GW9662 (a PPARγ antagonist) partially inhibited telmisartan-induced apoptosis and reduced viability of mesangial cells. In addition, high glucose-induced PKCβ1/TGFβ1 expression in mesangial cells could be blocked by telmisartan. These data provide a more precise cellular mechanism for revealing the protective effect of telmisartan in diabetic kidney injury.

Loss of Angiotensin II Type 2 Receptor Improves Blood Pressure in Elastin Insufficiency

There is ample evidence supporting a role for angiotensin II type 2 receptor (AT₂R) in counterbalancing the effects of angiotensin II (ang II) through the angiotensin II type 1 receptor by promoting vasodilation and having anti-inflammatory effects. Elastin insufficiency in both humans and mice results in large artery stiffness and systolic hypertension. Unexpectedly, mesenteric arteries from elastin insufficient (Eln^+/−) mice were shown to have significant vasoconstriction to AT₂R agonism in vitro suggesting that AT₂R may have vasoconstrictor effects in elastin insufficiency. Given the potential promise for the use of AT₂R agonists clinically, the goal of this study was to determine whether AT₂R has vasoconstrictive effects in elastin insufficiency in vivo. To avoid off-target effects of agonists and antagonists, mice lacking AT₂R (Agtr2^−/Y) were bred to Eln^+/− mice and cardiovascular parameters were assessed in wild-type (WT), Agtr2^−/Y, Eln^+/−, and Agtr2^−/Y;Eln^+/− littermates. As previously published, Agtr2^−/Y mice were normotensive at baseline and had no large artery stiffness, while Eln^+/− mice exhibited systolic hypertension and large artery stiffness. Loss of AT₂R in Eln^+/− mice did not affect large artery stiffness or arterial structure but resulted in significant reduction of both systolic and diastolic blood pressure. These data support a potential vasocontractile role for AT₂R in elastin insufficiency. Careful consideration and investigation are necessary to determine the patient population that might benefit from the use of AT₂R agonists.

Renin-Angiotensin System Induced Secondary Hypertension: The Alteration of Kidney Function and Structure

Long-term hypertension is known as a major risk factor for cardiovascular and chronic kidney disease (CKD). The Renin-angiotensin system (RAS) plays a key role in hypertension pathogenesis. Angiotensin II (Ang II) enhancement in Ang II-dependent hypertension leads to progressive CKD and kidney fibrosis. In the two-kidney one-clip model (2K1C), more renin is synthesized in the principal cells of the collecting duct than juxtaglomerular cells (JGCs). An increase of renal Ang I and Ang II levels and a decrease of renal cortical and medullary Ang 1–7 occur in both kidneys of the 2K1C hypertensive rat model. In addition, the activity of the angiotensin-converting enzyme (ACE) increases, while ACE2's activity decreases in the medullary region of both kidneys in the 2K1C hypertensive model. Also, the renal prolyl carboxypeptidase (PrCP) expression and its activity reduce in the clipped kidneys. The imbalance in the production of renal ACE, ACE2, and PrCP expression causes the progression of renal injury. Intrarenal angiotensinogen (AGT) expression and urine AGT (uAGT) excretion rates in the unclipped kidney are greater than the clipped kidney in the 2K1C hypertensive rat model. The enhancement of Ang II in the clipped kidney is related to renin secretion, while the elevation of intrarenal Ang II in the unclipped kidney is related to stimulation of AGT mRNA and protein in proximal tubule cells by a direct effect of systemic Ang II level. Ang II-dependent hypertension enhances macrophages and T-cell infiltration into the kidney which increases cytokines, and AGT synthesis in proximal tubules is stimulated via cytokines. Accumulation of inflammatory cells in the kidney aggravates hypertension and renal damage. Moreover, Ang II-dependent hypertension alters renal Ang II type 1 & 2 receptors (AT₁R & AT₂R) and Mas receptor (MasR) expression, and the renal interstitial fluid bradykinin, nitric oxide, and cGMP response to AT₁R, AT₂R, or BK B₂-receptor antagonists. Based on a variety of sources including PubMed, Google Scholar, Scopus, and Science-Direct, in the current review, we will discuss the role of RAS-induced secondary hypertension on the alteration of renal function.

Divergent impact of gestational diabetes mellitus between the thoracic and abdominal rat aorta: Influence of endothelium and angiotensin II receptors

Gestational diabetes mellitus (GDM) affects 5-10% of pregnancies and increases the risk of fetal and maternal adverse outcomes. Interestingly, the vascular response to AngII is decreased by pregnancy while the response is increased by diabetes. It remains unclear how GDM affects vascular tone and how angiotensin II receptors contribute to these changes. In this work, we sought to establish the vascular impact of a hypercaloric diet-induced GDM through changes in AT1 and AT2 receptor's expression. Female rats fed for 7 weeks with standard (SD) or hypercaloric (HD) diet were divided at week 4. Half of the rats of each group were mated to become pregnant and those fed with a HD developed GDM. AngII-induced vasoconstriction was measured in thoracic or abdominal aorta rings using a conventional isolated organ bath and AT1 and AT2 receptors were searched by immunohistochemistry. Experiments where conducted on the pregnant standard diet group (PSD) and the pregnant hypercaloric-gestational diabetes mellitus group (PHD-GDM). Vasoconstriction was reduced in the thoracic aorta (P < 0.05 vs PSD) but increased in the abdominal aorta of PHD-GDM rats (P < 0.05 vs PSD). Blockade of AT2 receptors using PD123319 decreased vasoconstriction, particularly in the abdominal aorta of PHD-GDM animals (P < 0.05 vs PSD). PHD-GDM increased AT1 receptors expression (P < 0.05 vs PSD). Also, PHD-GDM reverted physiologic hypoglycemia and hypotension of healthy pregnancy. Findings provide new insight into the hypercaloric diet induced damage on the vasculature during pregnancy.

Hyperdynamic circulatory syndrome in a mouse model transgenic for SerpinB3

INTRODUCTION AND OBJECTIVES

SerpinB3 is a cysteine protease inhibitor involved in several biological activities. It is progressively expressed in chronic liver disease, but not in normal liver. The role in vascular reactivity of this serpin, belonging to the same family of Angiotensin II, is still unknown. Our aim was to evaluate the in vivo and in vitro effects of SerpinB3 on systemic and splanchnic hemodynamics.

MATERIAL AND METHODS

Different hemodynamic parameters were evaluated by ultrasonography in two colonies of mice (transgenic for human SerpinB3 and C57BL/6J controls) at baseline and after chronic carbon tetrachloride (CCl₄) treatment. In vitro SerpinB3 effect on mesenteric microvessels of 5 Wistar-Kyoto rats was analyzed measuring its direct action on: (a) preconstricted arteries, (b) dose-response curves to phenylephrine, before and after inhibition of angiotensin II type 1 receptors with irbesartan. Hearts of SerpinB3 transgenic mice and of the corresponding controls were also analyzed by morphometric assessment.

RESULTS

In SerpinB3 transgenic mice, cardiac output (51.6±21.5 vs 30.1±10.8ml/min, p=0.003), hepatic artery pulsatility index (0.85±0.13 vs 0.65±0.11, p<0.001) and portal vein blood flow (5.3±3.2 vs 3.1±1.8ml/min, p=0.03) were significantly increased, compared to controls. In vitro, recombinant SerpinB3 had no direct hemodynamic effect on mesenteric arteries, but it increased their sensitivity to phenylephrine-mediated vasoconstriction (p<0.01). This effect was suppressed by inhibiting angiotensin II type-1 receptors.

CONCLUSIONS

In transgenic mice, SerpinB3 is associated with a hyperdynamic circulatory syndrome-like pattern, possibly mediated by angiotensin receptors.

Angiotensin-(1-9) prevents vascular remodeling by decreasing vascular smooth muscle cell dedifferentiation through a FoxO1-dependent mechanism

The renin-angiotensin system, one of the main regulators of vascular function, controls vasoconstriction, inflammation and vascular remodeling. Antagonistic actions of the counter-regulatory renin-angiotensin system, which include vasodilation, anti-proliferative, anti-inflammatory and anti-remodeling effects, have also been described. However, little is known about the direct effects of angiotensin-(1-9), a peptide of the counter-regulatory renin-angiotensin system, on vascular smooth muscle cells. Here, we studied the anti-vascular remodeling effects of angiotensin-(1-9), with special focus on the control of vascular smooth muscle cell phenotype. Angiotensin-(1-9) decreased blood pressure and aorta media thickness in spontaneously hypertensive rats. Reduction of media thickness was associated with decreased vascular smooth muscle cell proliferation. In the A7r5 VSMC cell line and in primary cultures of rat aorta smooth muscle cells, angiotensin-(1-9) did not modify basal proliferation. However, angiotensin-(1-9) inhibited proliferation, migration and contractile protein decrease induced by platelet derived growth factor-BB. Moreover, angiotensin-(1-9) reduced Akt and FoxO1 phosphorylation at 30 min, followed by an increase of total FoxO1 protein content. Angiotensin-(1-9) effects were blocked by the AT2R antagonist PD123319, Akt-Myr overexpression and FoxO1 siRNA. These data suggest that angiotensin-(1-9) inhibits vascular smooth muscle cell dedifferentiation by an AT2R/Akt/FoxO1-dependent mechanism.

Maternal high-fat diet increases vascular contractility in adult offspring in a sex-dependent manner

A maternal high-fat diet (HFD) is a risk factor for cardiovascular diseases in offspring. The aim of the study was to determine whether maternal HFD causes the epigenetic programming of vascular angiotensin II receptors (ATRs) and leads to heightened vascular contraction in adult male offspring in a sex-dependent manner. Pregnant rats were treated with HFD (60% kcal fat). Aortas were isolated from adult male and female offspring. Maternal HFD increased phenylephrine (PE)-and angiotensin II (Ang II)-induced contractions of the aorta in male but not female offspring. N^G-nitro-L-arginine (ʟ-NNA; 100 μM) abrogated the maternal HFD-induced increase in PE-mediated contraction. HFD caused a decrease in endothelium-dependent relaxations induced by acetylcholine in male but not female offspring. However, it had no effect on sodium nitroprusside-induced endothelium-independent relaxations of aortas regardless of sex. The AT₁ receptor (AT₁R) antagonist losartan (10 μM), but not the AT₂ receptor (AT₂R) antagonist PD123319 (10 μM), blocked Ang II-induced contractions in both control and HFD offspring in both sexes. Maternal HFD increased AT₁R but decreased AT₂R, leading to an increased ratio of AT₁R/AT₂R in HFD male offspring, which was associated with selective decreases in DNA methylation at the AT_1aR promoter and increases in DNA methylation at the AT₂R promoter. The vascular ratio of AT₁R/AT₂R was not significantly different in HFD female offspring compared with the control group. Our results indicated that maternal HFD caused a differential regulation of vascular AT₁R and AT₂R gene expression through a DNA methylation mechanism, which may be involved in HFD-induced vascular dysfunction and the development of a hypertensive phenotype in adulthood in a sex-dependent manner.

High-intensity aerobic training lowers blood pressure and modulates the renal renin-angiotensin system in spontaneously hypertensive rats

Background: This study aimed to verify the effects of high-intensity aerobic training (HIAT) on BP control and renin-angiotensin system (RAS) components in renal tissue of SHR. Ten SHRs received HIAT or control for 8-weeks. At the end of the training, the SBP showed a reduction of ~ 30mmHg (p < .01) in HIAT and increased by ~ 15 mmHg in the  control group. HIAT resulted in a higher release of nitrite, IL-6, ACE2 and ATR2. These results indicated an association between BP, NO and renal RAS.Abbreviations: JAA: writing, carried out all experimental procedures, performed statistical analysis, original draft and revised manuscript DMS: data interpretation, formal analysis, writing, editing and revised manuscript BAP: carried all experimental procedures, revised manuscritpt CPCG: carried all experimental procedures, revised manuscritpt MEN: experimental procedures, revised manuscript and data interpretation RWP: drafted and revised manuscript RCA: writing, experimental procedures, revised manuscript JP: writing, data interpretation and revised manuscript OLF: writing, original draft and revised manuscript.

Apocynin alters redox signaling in conductance and resistance vessels of spontaneously hypertensive rats

Chronic treatment with apocynin reduces blood pressure and prevents endothelial dysfunction development in spontaneously hypertensive rats (SHR). Mechanisms underlying apocynin effects on SHR remain unclear. Compared to diapocynin and other drugs, apocynin is a weak antioxidant, which suggests that its effects on SHR are associated with other mechanisms besides its antioxidant capacity. Angiotensin (Ang) II regulates NOX, the major reactive oxygen species (ROS) source in the cardiovascular system. We hypothesized that, by inhibiting NOX, apocynin could alter Ang II pressor and vasoconstrictor effects on SHR. We analyzed how Ang II affects blood pressure and vascular reactivity in aorta and mesenteric resistance arteries and evaluated plasma antioxidant capacity, NOX isoforms and subunits, NOS isoforms, AT₁ and AT₂ receptors expression, ROS production, and NOS activity in apocynin-treated SHR blood vessels (30 mg/Kg/day, p.o.). In SHR, apocynin reduced Ang II pressor effects, increased plasmatic antioxidant capacity, and blunted aortic and mesenteric NOX-dependent oxidants production and NOX2 and p47phox overexpression, which demonstrated that apocynin inhibits NOX in SHR blood vessels. Moreover, apocynin raised plasmatic and aortic nitrate/nitrite levels, maintained NOS activity and eNOS, p-eNOS, nNOS, iNOS, sGC-α, and sGC-β expression in mesenteric bed, diminished AT₁ expression in aorta and mesenteric bed, and elevated AT₂ expression in SHR aorta. Apocynin increased Ang II vasoconstriction endothelial modulation in SHR resistance arteries. All these results showed that in vivo treatment with apocynin alters several mechanisms that reduce Ang II pressor and vasoconstrictor effects on SHR. Such apocynin effects involve other mechanisms besides vascular ROS modulation, which improves NO availability in SHR vascular cells. These integrated data could help us to understand the promising apocynin activity as an antihypertensive drug that acts differently from the drugs that are currently being used in the clinical setting.

Relaxin treatment reduces angiotensin II-induced vasoconstriction in pregnancy and protects against endothelial dysfunction†

The peptide relaxin has gained considerable attention as a new vasoactive drug, largely through its beneficial therapeutic effects in cardiovascular disease. In this study, we tested the hypothesis that relaxin treatment alleviates systemic vascular dysfunction characteristic of hypertensive diseases of pregnancy. We investigated vascular effects and mechanisms of relaxin action in (i) pregnant relaxin-deficient (Rln-/-) mice with enhanced responses to angiotensin II (AngII) and (ii) arteries pre-incubated ex vivo in trophoblast conditioned media (TCM) to induce endothelial dysfunction. Pregnant Rln-/- mice received 0.5 μg/h recombinant human H2 relaxin (rhRLX: n = 5) or placebo (20 nM sodium acetate; n = 7) subcutaneously via osmotic minipumps for 5 days prior to gestational day 17.5. This treatment protocol significantly reduced AngII-mediated contraction of mesenteric arteries and increased plasma 6-keto prostaglandin F1α. These vascular effects were endothelium independent and likely involve smooth muscle-derived vasodilator prostanoids. In the second study, mesenteric arteries were incubated ex vivo for 24 h at 37°C in TCM, which contained high levels of soluble Flt-1 (>20 ng/ml) and soluble Eng (>1 ng/ml). TCM incubation caused significant reduction in endothelium-dependent relaxation and increased sensitivity to AngII. Co-incubation of arteries with rhRLX for 24 h (n = 6-16/treatment) prevented endothelial dysfunction but had no effect on AngII-mediated contraction. In conclusion, relaxin treatment prevents and/or reverses vascular dysfunction in mesenteric arteries, but acts through different vascular pathways depending on duration of relaxin treatment and type of vascular dysfunction. Overall, our data suggest that relaxin is a potential therapeutic to alleviate maternal systemic vascular dysfunction associated with hypertensive diseases in pregnant women.

Cellular distribution and interaction between extended renin-angiotensin-aldosterone system pathways in atheroma

The importance of the renin-angiotensin-aldosterone system (RAAS) in the development of atherosclerotic has been experimentally documented. In fact, RAAS components have been shown to be locally expressed in the arterial wall and to be differentially regulated during atherosclerotic lesion progression. RAAS transcripts and proteins were shown to be differentially expressed and to interact in the 3 main cells of atheroma: endothelial cells, vascular smooth muscle cells, and macrophages. This review describes the local expression and cellular distribution of extended RAAS components in the arterial wall and their differential regulation during atherosclerotic lesion development.

Testosterone downregulates angiotensin II type-2 receptor via androgen receptor-mediated ERK1/2 MAP kinase pathway in rat aorta

Introduction:

Blood pressure is lower in females than males. Angiotensin II type-2 receptor (AT₂R) induces vasodilation. This study determined whether sex differences in vascular AT₂R expression occur and if androgens exert control on AT₂R expression in the vasculature.

Methods:

AT₂Rs in the aorta of male and female Sprague-Dawley rats were examined following alteration in androgen levels by gonadectomy or hormone supplementation.

Results:

AT₂R mRNA and protein expression levels were lower in the aortas of males than females. In males, testosterone withdrawal by castration significantly elevated AT₂R mRNA and protein levels and testosterone replacement restored them. In females, increasing androgen levels decreased AT₂R mRNA and protein expression and this was attenuated by androgen receptor blocker flutamide. Ex vivo, dihydrotestosterone downregulated AT₂R in endothelium-intact but not endothelium-denuded aorta. Dihydrotestosterone-induced AT₂R downregulation in isolated aorta was blocked by an androgen receptor antagonist. Furthermore, blockade of ERK1/2 but not p38 MAP kinase or TGFβ signaling with specific inhibitors abolished dihydrotestosterone-induced AT₂R downregulation.

Conclusion:

Androgens downregulate AT₂R expression levels in aorta, in vivo and ex vivo. The androgen receptor-mediated ERK1/2 MAP kinase-signaling pathway may be a key mechanism by which testosterone downregulates AT₂R expression, implicating androgens’ contributing role to gender differences in vascular AT₂R expression.

High-Dose Estradiol-Replacement Therapy Enhances the Renal Vascular Response to Angiotensin II via an AT2-Receptor Dependent Mechanism

Physiological levels of estrogen appear to enhance angiotensin type 2 receptor- (AT₂R-) mediated vasodilatation. However, the effects of supraphysiological levels of estrogen, analogous to those achieved with high-dose estrogen replacement therapy in postmenopausal women, remain unknown. Therefore, we pretreated ovariectomized rats with a relatively high dose of estrogen (0.5 mg/kg/week) for two weeks. Subsequently, renal hemodynamic responses to intravenous angiotensin II (Ang II, 30–300 ng/kg/min) were tested under anesthesia, while renal perfusion pressure was held constant. The role of AT₂R was examined by pretreating groups of rats with PD123319 or its vehicle. Renal blood flow (RBF) decreased in a dose-related manner in response to Ang II. Responses to Ang II were enhanced by pretreatment with estradiol. For example, at 300 ng kg⁻¹ min⁻¹, Ang II reduced RBF by 45.7 ± 1.9% in estradiol-treated rats but only by 27.3 ± 5.1% in vehicle-treated rats. Pretreatment with PD123319 blunted the response of RBF to Ang II in estradiol-treated rats, so that reductions in RBF were similar to those in rats not treated with estradiol. We conclude that supraphysiological levels of estrogen promote AT₂R-mediated renal vasoconstriction. This mechanism could potentially contribute to the increased risk of cardiovascular disease associated with hormone replacement therapy using high-dose estrogen.

Chronic fetal exposure to caffeine altered resistance vessel functions via RyRs-BKCa down-regulation in rat offspring

Caffeine modifies vascular/cardiac contractility. Embryonic exposure to caffeine altered cardiac functions in offspring. This study determined chronic influence of prenatal caffeine on vessel functions in offspring. Pregnant Sprague-Dawley rats (5-month-old) were exposed to high dose of caffeine, their offspring (5-month-old) were tested for vascular functions in mesenteric arteries (MA) and ion channel activities in smooth muscle cells. Prenatal exposure to caffeine increased pressor responses and vasoconstrictions to phenylephrine, accompanied by enhanced membrane depolarization. Large conductance Ca2⁺-activated K⁺ (BK_Ca) channels in buffering phenylephrine-induced vasoconstrictions was decreased, whole cell BK_Ca currents and spontaneous transient outward currents (STOCs) were decreased. Single channel recordings revealed reduced voltage/Ca²⁺ sensitivity of BK_Ca channels. BK_Ca α-subunit expression was unchanged, BK_Ca β1-subunit and sensitivity of BK_Ca to tamoxifen were reduced in the caffeine offspring as altered biophysical properties of BK_Ca in the MA. Simultaneous [Ca²⁺]_i fluorescence and vasoconstriction testing showed reduced Ca²⁺, leading to diminished BK_Ca activation via ryanodine receptor Ca²⁺ release channels (RyRs), causing enhanced vascular tone. Reduced RyR1 was greater than that of RyR3. The results suggest that the altered STOCs activity in the caffeine offspring could attribute to down-regulation of RyRs-BK_Ca, providing new information for further understanding increased risks of hypertension in developmental origins.

Changes in protein and gene expression of angiotensin II receptors (AT1 and AT2) in aorta of diabetic and hypertensive rats

Diabetes and hypertension have been associated with cardiovascular diseases and stroke. Some reports have related the coexistence of hypertension and diabetes with increase in the risk of developing vascular complications. Recently some studies have shown results suggesting that in the early stages of diabetes and hypertension exist a reduced functional response to vasopressor agents like angiotensin II (Ang II), which plays an important role in blood pressure regulation mechanism through the activation of its AT1 and AT2 receptors. For that reason, the aim of this work was to study the gene and protein expression of AT1 and AT2 receptors in aorta of diabetic SHR and WKY rats. Diabetes was induced by the administration of streptozotocin (60 mg/kg i.p.). After 4 weeks of the onset of diabetes, the protein expression was obtained by western blot and the mRNA expression by RT-PCR. Our results showed that the hypertensive rats have a higher mRNA and protein expression of AT1 receptors than normotensive rats while the AT2 expression remained unchanged. On the other hand, the combination of diabetes and hypertension increased the mRNA and protein expression of AT1 and AT2 receptors significantly. In conclusion, our results suggest that diabetes with hypertension modifies the mRNA and protein expression of AT1 and AT2 receptors. However, the overexpression of AT2 could be associated with the reduction in the response to Ang II in the early stage of diabetes.

Metoprolol restores expression and vasodilatation function of AT2R in spontaneously hypertensive rats

Angiotensin II type 2 receptor (AT2R) is thought as an important regulatory target during antihypertensive treatment but its role in vasomotor regulation remains controversial. The interactional relationship between the sympathetic nervous systems and the renin-angiotensin-aldosterone system (RAS) has been revealed but poorly investigated. This work was designed to explore the effect of metoprolol (MET) treatment on the RAS, especially the expression and vasomotor function of AT2R, in spontaneously hypertensive rats (SHR). The results showed that upregulated renin activity and Ang II concentration of plasma in SHR were inhibited by MET treatment. In isolated superior mesenteric arteries from both Wistar-Kyoto rats and SHR, Ang II perfusion induced vasodilatation after AT1R inhibition by telmisartan, although the vasodilatation was harmed in SHR. Furthermore, AT2R inhibitor PD123319 arrested the vasodilatation induced by Ang II. SHR received MET exerted improved vasodilatation mediated by AT2R (47.29% ± 5.16% vs. 24.99% ± 4.93% for MET and SHR, respectively; P < 0.05). Western blot analysis showed that MET restored expression of AT2R in SHR, which may contribute to MET's antihypertensive effect. These results suggested an impact of β-adrenergic blocker on RAS and supported an important role of AT2R in antihypertensive treatment.

TRPC1, CaN and NFATC3 signaling pathway in the pathogenesis and progression of left ventricular hypertrophy in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) was used to study left ventricular hypertrophy (LVH) and its dynamic change after the interventions with Telmisartan and Amlodipine. The results showed that the expression of TRPC1, CaN and NFATC3 increased gradually with the pathogenesis and progression of LVH. Telmisartan reduced blood pressure and LVH, and down-regulated the expression of TRPC1, CaN and NFATC3 in left ventricle of SHR. Amlodipine reduced the blood pressure in SHR but had no impact on the hypertrophy and expression of above factors. Our data suggest that the pathogenesis and progression of LVH in SHR are related to upregulation of TRPC1, CaN and NFATC3 signaling pathway.

Upregulation of ERK1/2-eNOS via AT2 receptors decreases the contractile response to angiotensin II in resistance mesenteric arteries from obese rats

It has been clearly established that mitogen-activated protein kinases (MAPKS) are important mediators of angiotensin II (Ang II) signaling via AT1 receptors in the vasculature. However, evidence for a role of these kinases in changes of Ang II-induced vasoconstriction in obesity is still lacking. Here we sought to determine whether vascular MAPKs are differentially activated by Ang II in obese animals. The role of AT2 receptors was also evaluated. Male monosodium glutamate-induced obese (obese) and non-obese Wistar rats (control) were used. The circulating concentrations of Ang I and Ang II, determined by HPLC, were increased in obese rats. Ang II-induced isometric contraction was decreased in endothelium-intact resistance mesenteric arteries from obese compared with control rats and exhibited a retarded AT1 receptor antagonist response. Blocking of AT2 receptors and inhibition of either endothelial nitric oxide synthase (eNOS) or extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) restored Ang II-induced contraction in obese rats. Western blot analysis revealed increased protein expression of AT2 receptors in arteries from obese rats. Basal and Ang II-induced ERK1/2 phosphorylation was also increased in obese rats. Blockade of either AT1 or AT2 receptors corrected the increased ERK1/2 phosphorylation in arteries from obese rats to levels observed in control preparations. Phosphorylation of eNOS was increased in obese rats. Incubation with the ERK1/2 inhibitor before Ang II stimulation did not affect eNOS phosphorylation in control rats; however, it corrected the increased phosphorylation of eNOS in obese rats. These results clearly demonstrate that enhanced AT2 receptor and ERK1/2-induced, NO-mediated vasodilation reduces Ang II-induced contraction in an endothelium-dependent manner in obese rats.

Gestational exposure to elevated testosterone levels induces hypertension via heightened vascular angiotensin II type 1 receptor signaling in rats

Pre-eclampsia is a life-threatening pregnancy disorder whose pathogenesis remains unclear. Plasma testosterone levels are elevated in pregnant women with pre-eclampsia and polycystic ovary syndrome, who often develop gestational hypertension. We tested the hypothesis that increased gestational testosterone levels induce hypertension via heightened angiotensin II signaling. Pregnant Sprague-Dawley rats were injected with vehicle or testosterone propionate from Gestational Day 15 to 19 to induce a 2-fold increase in plasma testosterone levels, similar to levels observed in clinical conditions like pre-eclampsia. A subset of rats in these two groups was given losartan, an angiotensin II type 1 receptor antagonist by gavage during the course of testosterone exposure. Blood pressure levels were assessed through a carotid arterial catheter and endothelium-independent vascular reactivity through wire myography. Angiotensin II levels in plasma and angiotensin II type 1 receptor expression in mesenteric arteries were also examined. Blood pressure levels were significantly higher on Gestational Day 20 in testosterone-treated dams than in controls. Treatment with losartan during the course of testosterone exposure significantly attenuated testosterone-induced hypertension. Plasma angiotensin II levels were not significantly different between control and testosterone-treated rats; however, elevated testosterone levels significantly increased angiotensin II type 1 receptor protein levels in the mesenteric arteries. In testosterone-treated rats, mesenteric artery contractile responses to angiotensin II were significantly greater, whereas contractile responses to K(+) depolarization and phenylephrine were unaffected. The results demonstrate that elevated testosterone during gestation induces hypertension in pregnant rats via heightened angiotensin II type 1 receptor-mediated signaling, providing a molecular mechanism linking elevated maternal testosterone levels with gestational hypertension.

Functional closure of the ductus arteriosus at birth: evidence against an intermediary role of angiotensin II

The fetal ductus arteriosus (DA) closes postnatally first functionally and then structurally. Normal rise in blood oxygenation is regarded as a prime trigger, but closure may occur more slowly without this stimulus. Here, our aim was to assess the role of angiotensin II (Ang II) in functional closure of DA since its action may not be conditioned by oxygen. Experiments were performed with wild-type fetal and neonatal mice, using whole-body freezing technique to assess DA caliber in vivo. Transcripts for Ang II type 1 (AT1R) and type 2 (AT2R) receptors were also examined. We found that the AT1R antagonist olmesartan had no effect in the fetus, but delayed ductus closure in the neonate. However, this response was short-lived and disappeared upon concomitant treatment with the AT2R antagonist PD123319. Coincidentally, olmesartan promoted the Agtr2 transcript. We conclude that AT1R-based Ang II has no role in the functional closure of DA. Conversely, the compound may modulate this process through AT2R-mediated vasodilatation.

Attenuation of L-type Ca²⁺ channel expression and vasomotor response in the aorta with age in both Wistar-Kyoto and spontaneously hypertensive rats

Age-related vascular diseases are induced by vascular dysfunction, which involves changes in the vasomotor response. The voltage-dependent L-type calcium channel (VDCC) protein is involved in the regulation of vessel function (contraction/relaxation action). In the present study, we evaluated age-related vasomotor function and expression of the signal-related target proteins, including VDCC, using thoracic aorta from both 8- and 40-week old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). In contraction experiments using aortic rings, vasomotor responses of both phenylephrine-induced contraction and acetylcholine-induced relaxation were significantly attenuated with age in SHR, whereas WKY did not lose activity with age. Contraction induced by angiotensin II was impaired only for the 40-week old SHR among all the rat groups tested, although enhanced AT1R/reduced AT2R expression with age was observed for both WKY and SHR. In contrast, a vasomotor responsiveness to Bay K 8644 (a VDCC agonist) at the initial contraction phase was significantly attenuated in both 40-week WKY and SHR with significant reduction of VDCC protein expression. The reduced VDCC expression in 40-week old rats significantly lowered the relaxation activity of VDCC blockers, such as verapamil and Trp-His, but did not affect that of nifedipine. Taken together, we provided the first evidence that aging caused a reduction of VDCC expression in rat aorta, irrespective of the rat strain, along with diminishment of the therapeutic potential of VDCC blockers.

Opposite effects of angiotensins receptors type 2 and type 4 on streptozotocin induced diabetes vascular alterations in mice

Background

We examined the effect of chronic administration of angiotensin IV (AngIV) on the vascular alterations induced by type 1 diabetes in mice.

Methods

Diabetes was induced in adult Swiss mice with a single injection of streptozotocin (STZ). Mice were treated subcutaneously with AngIV (1.4 mg/kg/day) either immediately following diabetes induction (preventive treatment), or treated with AngIV (0.01 to 1.4 mg/kg), alone or with the AT4 receptor antagonist Divalinal or the AT2 receptor antagonist PD123319, for two weeks after 4 weeks of diabetes duration (rescue treatment). Acetylcholine-induced, endothelium-dependent relaxation (EDR) was measured in isolated aortic rings preparations. Histomorphometric measurements of the media thickness were obtained, and nitric oxide (NO) and superoxide anion production were measured by electron paramagnetic resonance in aorta and mesenteric arteries. The effect of diabetes on mesenteric vascular alterations was also examined in genetically modified mice lacking the AT2 receptor.

Results

Induction of diabetes with STZ was associated with a progressive decrease of EDR and an increase of the aortic and mesenteric media thickness already significant after 4 weeks and peaking at week 6. Immediate treatment with AngIV fully prevented the diabetes-induced endothelial dysfunction. Rescue treatment with AngIV implemented after 4 weeks of diabetes dose-dependently restored a normal endothelial function at week 6. AngIV blunted the thickening of the aortic and mesenteric media, and reversed the diabetes-induced changes in NO and O₂^•– production by the vessels. The protective effect of AngIV on endothelial function was completely blunted by cotreatment with Divalinal, but not with PD123319. In contrast, both the pharmacological blockade and genetic deletion of the AT2 receptor reversed the diabetes-induced morphologic and endothelial alteration caused by diabetes.

Conclusions

The results suggest an opposite contribution of AT2 and AT4 receptors to the vascular alterations caused by streptozotocin-induced diabetes in mice, since chronic stimulation of AT4 by AngIV and inhibition of AT2 similarly reverse diabetes-induced endothelial dysfunction and hypertrophic remodeling, and increase NO bioavailability.

Pathophysiology of vascular remodeling in hypertension

Vascular remodeling refers to alterations in the structure of resistance vessels contributing to elevated systemic vascular resistance in hypertension. We start with some historical aspects, underscoring the importance of Glagov's contribution. We then move to some basic concepts on the biomechanics of blood vessels and explain the definitions proposed by Mulvany for specific forms of remodeling, especially inward eutrophic and inward hypertrophic. The available evidence for the existence of remodeled resistance vessels in hypertension comes next, with relatively more weight given to human, in comparison with animal data. Mechanisms are discussed. The impact of antihypertensive drug treatment on remodeling is described, again with emphasis on human data. Some details are given on the three mechanisms to date which point to remodeling resistance arteries as an independent predictor of cardiovascular risk in hypertensive patients. We terminate by considering the potential role of remodeling in the pathogenesis of endorgan damage and in the perpetuation of hypertension.

Angiotensin II type 1 receptor blockade restores angiotensin-(1–7)-induced coronary vasodilation in hypertrophic rat hearts

The aim of the present study was to investigate the coronary effects of Ang-(1–7) [angiotensin-(1–7)] in hypertrophic rat hearts. Heart hypertrophy was induced by abdominal aorta CoA (coarctation). Ang-(1–7) and AVE 0991, a non-peptide Mas-receptor agonist, at picomolar concentration, induced a significant vasodilation in hearts from sham-operated rats. These effects were blocked by the Mas receptor antagonist A-779. Pre-treatment with L-NAME (NG-nitro-L-arginine methyl ester) or ODQ (1H-[1,2,4]oxadiazolo[4,3-a]quinozalin-1-one) [NOS (NO synthase) and soluble guanylate cyclase inhibitors respectively] also abolished the effect of Ang-(1–7) in control hearts. The coronary vasodilation produced by Ang-(1–7) and AVE 0991 was completely blunted in hypertrophic hearts. Chronic oral administration of losartan in CoA rats restored the coronary vasodilation effect of Ang-(1–7). This effect was blocked by A-779 and AT2 receptor (angiotensin II type 2 receptor) antagonist PD123319. Acute pre-incubation with losartan also restored the Ang-(1–7)-induced, but not BK (bradykinin)-induced, coronary vasodilation in hypertrophic hearts. This effect was inhibited by A-779, PD123319 and L-NAME. Chronic treatment with losartan did not change the protein expression of Mas and AT2 receptor and ACE (angiotensin-converting enzyme) and ACE2 in coronary arteries from CoA rats, but induced a slight increase in AT2 receptor in aorta of these animals. Ang-(1–7)-induced relaxation in aortas from sham-operated rats was absent in aortas from CoA rats. In vitro pre-treatment with losartan restored the Ang-(1–7)-induced relaxation in aortic rings of CoA rats, which was blocked by the Mas antagonist A-779 and L-NAME. These data demonstrate that Mas is strongly involved in coronary vasodilation and that AT1 receptor (angiotensin II type 1 receptor) blockade potentiates the vasodilatory effects of Ang-(1–7) in the coronary beds of pressure-overloaded rat hearts through NO-related AT2- and Mas-receptor-dependent mechanisms. These data suggest the association of Ang-(1–7) and AT1 receptor antagonists as a potential therapeutic avenue for coronary artery diseases.

Angiotensin II-mediated vascular changes in aged offspring rats exposed to perinatal nicotine

This study determined the long-term influence of prenatal nicotine exposure (PN) on blood pressure and vascular functions in the aged offspring rats. PN did not affect body weight and plasma adrenocorticotropic hormone level; however, it significantly reduced plasma angiotensin I and angiotensin II in both sexes. Systolic pressure in the male aged PN offspring was significantly higher. Angiotensin II-increased mean arterial pressure was higher in the aged PN offspring than that in the control regardless of sex. AT1 receptor blocker losartan, not AT2 receptor antagonist PD123319, reduced blood pressure in the aged PN rats more than that in the control. In the aged PN offspring, angiotensin II-increased vessel contraction and intracellular calcium level were higher in small mesenteric arteries. Acetylcholine-mediated vascular relaxation was weaker, and nitric oxide-related endothelial functions were damaged in aortic rings of PN offspring. Thickness of the wall of mesenteric arteries was increased in the male aged PN offspring. Ratio of AT1/AT2 receptors was significantly increased in the vessel of the PN group regardless of sex. These data provide new information on the very long term influence of PN on vascular structures and functions in the aged offspring, demonstrate that the aged PN female rats were not free of vascular risks after menopause, and suggest that multiple pathways may be involved in the detrimental alterations of the cardiovascular system of the PN rats.

Impact of the AT(2) receptor agonist C21 on blood pressure and beyond

It is now widely accepted that the angiotensin AT(2) receptor (AT(2)R) plays an important protective role during pathophysiologic conditions, acting as a repair system. The development of the first selective nonpeptide AT(2)R agonist C21 accelerated our understanding of AT(2)R-mediated protective signaling and actions. This article reviews the impact of C21 on blood pressure in normotensive and hypertensive animal models. Although C21 does not act as a classical antihypertensive drug, it could be useful in preventing hypertension-induced vascular and other end organ damages via anti-apoptotic, anti-fibrotic and anti-inflammatory actions. In particular, a strong body of evidence started to emerge around its anti-inflammatory feature. This property should be further investigated for potential clinical indications in cardiovascular diseases and beyond.

Sexual dimorphism in rodent models of hypertension and atherosclerosis

Approximately one third of all deaths are attributed to cardiovascular disease (CVD), making it the biggest killer worldwide. Despite a number of therapeutic options available, the burden of CVD morbidity continues to grow indicating the need for continued research to address this unmet need. In this respect, investigation of the mechanisms underlying the protection that premenopausal females enjoy from cardiovascular-related disease and mortality is of interest. In this review, we discuss the essential role that rodent animal models play in enabling this field of research. In particular, we focus our discussion on models of hypertension and atherosclerosis.

Central arterial aging and the epidemic of systolic hypertension and atherosclerosis

The structure and function of central arteries change throughout the lifetime of humans and animals. Since atherosclerosis and hypertension are prevalent in epidemic proportion among older persons, it is reasonable to hypothesize that specific mechanisms that underlie the arterial substrate that has been altered by an "aging process" are intimately linked to arterial diseases. Indeed, recent studies reveal a profile of arterial cell and matrix properties that emerges with advancing age within the grossly normal appearing aortic wall of both animals and humans. This profile is proinflammatory, and is manifested by intimal infiltration of fetal cells, increased production of angiotensin II (Ang II)-signaling pathway molecules, eg, matrix metalloproteases (MMPs), and monocyte chemoattractant protein (MCP-1), transforming growth factor B1 (TGF-beta1), enhanced activation of MMPs, TGF-beta, and NADPH oxidase, and reduced nitric oxide (NO) bioavailability. This profile is similar to that induced at younger ages in experimental animal models of hypertension or atherosclerosis. In humans, this proinflammatory state, which occurs in the absence of lipid deposition, appears to be attributable to aging, per se. Other well known human risk factors, eg, altered lipid metabolism, smoking, and lack of exercise, interact with this arterial substrate that is altered by aging and render the aging human artery fertile soil for facilitation of the initiation and progression of arterial diseases. Therapies to reduce or retard this age-associated proinflammatory state within the grossly appearing arterial wall central arteries, in addition to slowing arterial aging, per se, may have a substantial impact on the quintessential age-associated arterial diseases of our society.

HDAC4 mediates development of hypertension via vascular inflammation in spontaneous hypertensive rats

Histone deacetylases (HDACs) are transcriptional corepressors. Our recent study demonstrated that HDAC4 protein specifically increases in mesenteric artery from spontaneous hypertensive rats (SHR) compared with Wistar Kyoto rats (WKY). Vascular inflammation is important for pathogenesis of hypertension. We examined whether HDAC4 affects vascular inflammatory responses and promotes hypertension. In vivo, blood pressure, reactive oxygen species (ROS) production, and VCAM-1 expression in isolated mesenteric artery were elevated in young SHR (7 wk old) compared with age-matched WKY, which were prevented by long-term treatment of SHR with an HDACs inhibitor, trichostatin A (TSA; 500 μg·kg−1·day−1 for 3 wk). In isolated mesenteric artery, the increased angiotensin II-induced contraction in SHR was reversed by TSA. The endothelium-dependent relaxation induced by ACh in SHR was augmented by TSA. In cultured rat mesenteric arterial smooth muscle cells (SMCs), expression of HDAC4 mRNA and protein was increased by TNF-α (10 ng/ml). TSA (10 μM, pretreatment for 30 min) inhibited VCAM-1 expression and NF-κB phosphorylation induced by TNF (10 ng/ml, 24 h or 20 min) in SMCs. HDAC4 small interfering RNA inhibited TNF-induced monocyte adhesion, VCAM-1 expression, transcriptional activity of NF-κB, and ROS production in SMCs. The present results demonstrated that proinflammatory effects of HDACs may mediate the further development of hypertension in SHR. It is also suggested in cultured vascular SMCs that TNF-induced HDAC4 mediates vascular inflammation likely via VCAM-1 induction through ROS-dependent NF-κB activation.

Involvement of PPAR-γ in the neuroprotective and anti-inflammatory effects of angiotensin type 1 receptor inhibition: effects of the receptor antagonist telmisartan and receptor deletion in a mouse MPTP model of Parkinson's disease

BACKGROUND

Several recent studies have shown that angiotensin type 1 receptor (AT1) antagonists such as candesartan inhibit the microglial inflammatory response and dopaminergic cell loss in animal models of Parkinson's disease. However, the mechanisms involved in the neuroprotective and anti-inflammatory effects of AT1 blockers in the brain have not been clarified. A number of studies have reported that AT1 blockers activate peroxisome proliferator-activated receptor gamma (PPAR γ). PPAR-γ activation inhibits inflammation, and may be responsible for neuroprotective effects, independently of AT1 blocking actions.

METHODS

We have investigated whether oral treatment with telmisartan (the most potent PPAR-γ activator among AT1 blockers) provides neuroprotection against dopaminergic cell death and neuroinflammation, and the possible role of PPAR-γ activation in any such neuroprotection. We used a mouse model of parkinsonism induced by the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and co-administration of the PPAR-γ antagonist GW9662 to study the role of PPAR-γ activation. In addition, we used AT1a-null mice lesioned with MPTP to study whether deletion of AT1 in the absence of any pharmacological effect of AT1 blockers provides neuroprotection, and investigated whether PPAR-γ activation may also be involved in any such effect of AT1 deletion by co-administration of the PPAR-γ antagonist GW9662.

RESULTS

We observed that telmisartan protects mouse dopaminergic neurons and inhibits the microglial response induced by administration of MPTP. The protective effects of telmisartan on dopaminergic cell death and microglial activation were inhibited by co-administration of GW9662. Dopaminergic cell death and microglial activation were significantly lower in AT1a-null mice treated with MPTP than in mice not subjected to AT1a deletion. Interestingly, the protective effects of AT1 deletion were also inhibited by co-administration of GW9662.

CONCLUSION

The results suggest that telmisartan provides effective neuroprotection against dopaminergic cell death and that the neuroprotective effect is mediated by PPAR-γ activation. However, the results in AT1-deficient mice show that blockage of AT1, unrelated to the pharmacological properties of AT1 blockers, also protects against dopaminergic cell death and neuroinflammation. Furthermore, the results show that PPAR-γ activation is involved in the anti-inflammatory and neuroprotective effects of AT1 deletion.

Angiotensin II type 2 receptor agonists: where should they be applied?

INTRODUCTION

Angiotensin II, the active endproduct of the renin-angiotensin system (RAS), exerts its effects via angiotensin II type 1 and type 2 (AT(1), AT(2)) receptors. AT(1) receptors mediate all well-known effects of angiotensin II, ranging from vasoconstriction to tissue remodeling. Thus, to treat cardiovascular disease, RAS blockade aims at preventing angiotensin II-AT(1) receptor interaction. Yet RAS blockade is often accompanied by rises in angiotensin II, which may exert beneficial effects via AT(2) receptors.

AREAS COVERED

This review summarizes our current knowledge on AT(2) receptors, describing their location, function(s), endogenous agonist(s) and intracellular signaling cascades. It discusses the beneficial effects obtained with C21, a recently developed AT(2) receptor agonist. Important questions that are addressed are do these receptors truly antagonize AT(1) receptor-mediated effects? What about their role in the diseased state and their heterodimerization with other receptors?

EXPERT OPINION

The general view that AT(2) receptors exclusively exert beneficial effects has been challenged, and in pathological models, their function sometimes mimics that of AT(1) receptors, for example, inducing vasoconstriction and cardiac hypertrophy. Yet given its upregulation in various pathological conditions, the AT(2) receptor remains a promising target for treatment, allowing effects beyond blood pressure-lowering, for example, in stroke, aneurysm formation, inflammation and myocardial fibrosis.

New cardiovascular and pulmonary therapeutic strategies based on the Angiotensin-converting enzyme 2/angiotensin-(1-7)/mas receptor axis

Angiotensin (Ang)-(1–7) is now recognized as a biologically active component of the renin-angiotensin system (RAS). The discovery of the angiotensin-converting enzyme homologue ACE2 revealed important metabolic pathways involved in the Ang-(1–7) synthesis. This enzyme can form Ang-(1–7) from Ang II or less efficiently through hydrolysis of Ang I to Ang-(1–9) with subsequent Ang-(1–7) formation. Additionally, it is well established that the G protein-coupled receptor Mas is a functional ligand site for Ang-(1–7). The axis formed by ACE2/Ang-(1–7)/Mas represents an endogenous counter regulatory pathway within the RAS whose actions are opposite to the vasoconstrictor/proliferative arm of the RAS constituted by ACE/Ang II/AT₁ receptor. In this review we will discuss recent findings concerning the biological role of the ACE2/Ang-(1–7)/Mas arm in the cardiovascular and pulmonary system. Also, we will highlight the initiatives to develop potential therapeutic strategies based on this axis.

The roles of angiotensin II receptors in the portosystemic collaterals of portal hypertensive and cirrhotic rats

BACKGROUND/AIMS

In liver cirrhosis/portal hypertension, collaterals as varices may bleed and are influenced by vasoresponsiveness. An angiotensin blockade ameliorates portal hypertension but the influence on collaterals is unknown.

METHODS

Portal hypertension and cirrhosis were induced by portal vein (PVL) and common bile duct ligation (BDL). Hemodynamics, real-time PCR of angiotensin II receptors (AT(1)R, AT(2)R) in the left adrenal vein (LAV, sham) and splenorenal shunt derived from LAV (PVL, BDL) were performed. With an in situcollateral perfusion model, angiotensin II vasoresponsiveness with different preincubations was evaluated: (1) vehicle; (2) AT(1)R blocker losartan; (3) losartan plus nonselective nitric oxide synthase (NOS) inhibitor (N(ω)-nitro-L-arginine); (4) AT(2)R blocker PD123319; (5) PD123319 plus N(ω)-nitro-L-arginine; (6) N(ω)-nitro-L-arginine, and (7) losartan plus inducible NOS inhibitor aminoguanidine.

RESULTS

LAV AT(1)R and AT(2)R expression decreased in PVL and BDL rats. Losartan attenuated angiotensin II-elicited vasoconstriction but PD123319 had no effect. N(ω)-nitro-L-arginine but not aminoguanidine reversed the losartan effect.

CONCLUSIONS

Angiotensin receptors are downregulated in the collateral vessel of portal hypertensive and cirrhotic rats. The AT(1)R blockade attenuates the angiotensin II vasoconstrictive effect, suggesting AT(1)R mediates collateral vasoconstriction and the influence of AT(2)R is negligible. The lack of aminoguanidine influence indicates that endothelial NOS participates in the losartan effect.

Vascular mechanisms involved in angiotensin II-induced venoconstriction in hypertensive rats

To investigate the venoconstrictor effect of angiotensin II (Ang II) in spontaneously hypertensive rats (SHR), we used preparations of mesenteric venular beds and the circular muscle of the portal veins. Vessels were tested with Ang II in the presence or absence of losartan, PD 123319, HOE 140, L-NAME, indomethacin, or celecoxib. In the mesenteric venular bed of SHR, the effect of Ang II (0.1 nmol) was nearly abolished by losartan and enhanced by HOE 140, indomethacin, and celecoxib, while PD123319 and L-NAME had no effect. In portal vein preparations, cumulative-concentration response curves (CCRC) to Ang II (0.1-100 nmol/L) exhibited a lower maximal response (E(max)) in SHR compared to Wistar rats. AT(1) receptor expression was similar in the two strains, while AT(2) receptor levels were lower in SHR portal veins when compared to Wistar. In SHR portal veins, losartan shifted the CCRC to Ang II to the right, while indomethacin and HOE 140 increased the E(max) to Ang II. PD 123319, celecoxib, and L-NAME had no effect. Taken together, our results suggest that Ang II-induced venoconstriction in SHR is mediated by activation of AT(1) receptors and this effect may be counterbalanced by kinin B(2) receptor and COX metabolites. Furthermore, our data indicate that there are different cellular and molecular mechanisms involved in the regulation of venous tonus of normotensive and hypertensive rats. These differences probably reflect distinct factors that influence arterial and venous bed in hypertension.

Differential regulation of Nox1, Nox2 and Nox4 in vascular smooth muscle cells from WKY and SHR

The functional significance and regulation of NAD(P)H oxidase (Nox) isoforms by angiotensin II (Ang II) and endothelin-1 (ET-1) in vascular smooth muscle cells (VSMCs) from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) was studied. Expression of Nox1, Nox2, and Nox4 (gene and protein) and NAD(P)H oxidase activity were increased in SHR. Basal NAD(P)H oxidase activity was blocked by GKT136901 (Nox1/4 inhibitor) and by Nox1 siRNA in WKY cells and by siNOX1 and siNOX2 in SHR. Whereas Ang II increased expression of all Noxes in WKY, only Nox1 was influenced in SHR. Ang II-induced NAD(P)H activity was inhibited by siNOX1 in WKY and by siNOX1 and siNOX2 in SHR. ET-1 upregulated Nox expression only in WKY and increased NAD(P)H oxidase activity, an effect inhibited by siNOX1 and siNOX2. Nox1 co-localized with Nox2 but not with Nox4, implicating association between Nox1 and Nox2 but not between Nox1 and Nox4. These data highlight the complexity of Nox biology in VSMCs, emphasising that more than one Nox member, alone or in association, may be involved in NAD(P)H oxidase-mediated •O(2)(-) production. Nox1 regulation by Ang II, but not by ET-1, may be important in •O(2)(-) formation in VSMCs from SHR.

Casein-derived tripeptide Ile-Pro-Pro improves angiotensin-(1-7)- and bradykinin-induced rat mesenteric artery relaxation

AIMS

Milk casein-derived bioactive tripeptides isoleucine-proline-proline (Ile-Pro-Pro) and valine-proline-proline (Val-Pro-Pro) lower blood pressure in animal models of hypertension and humans. In some studies, their angiotensin-converting enzyme (ACE)-inhibitory effect has been demonstrated. Besides classical ACE-angiotensin II-AT(1)-receptor pathway (ACE-Ang II- AT(1)), the significance of ACE2-angiotensin-(1-7)-Mas-receptor (ACE2-Ang-(1-7)-Mas) axis in the blood pressure regulation has now been acknowledged. The present study was aimed to further evaluate the renin-angiotensin system (RAS)-related vascular effects of Ile-Pro-Pro in vitro using rat mesenteric arteries.

MAIN METHODS

Superior mesenteric arteries of spontaneously hypertensive rat (SHR) were isolated, cut into rings and mounted in standard organ bath chambers. Endothelium-intact arterial rings were incubated in Krebs solution either with Ile-Pro-Pro, proline-proline (Pro-Pro), isoleucine (Ile), proline (Pro) or captopril for 6h at +37°C and vascular reactivity was measured.

KEY FINDINGS

In the presence of AT(1)-antagonist valsartan, Ang II induced vasodilatation, which was more pronounced in the arteries incubated with Ile-Pro-Pro (P<0.05) compared to the other compounds. Ang-(1-7)-induced vasodilatation was augmented by Ile-Pro-Pro or Pro (P<0.001 vs. control). Mas-receptor antagonist A-779 did not alter the responses. Ile-Pro-Pro and Pro augmented also bradykinin-induced relaxations (P<0.001 vs. control). Control arteries and arteries incubated with captopril showed only slight relaxations at higher bradykinin concentrations.

SIGNIFICANCE

Casein-derived tripeptide Ile-Pro-Pro and amino acid Pro enhance the vasodilatory effect of Ang-(1-7) and bradykinin. The role of ACE2-Ang-(1-7)-Mas axis in the modulation of vascular tone by these compounds seems probable.

Aging-related changes in the nigral angiotensin system enhances proinflammatory and pro-oxidative markers and 6-OHDA-induced dopaminergic degeneration

An age-related proinflammatory, pro-oxidant state in the nigra may increase the vulnerability of dopaminergic neurons to additional damage. Angiotensin II, via type 1 (AT1) receptors, is one of the most important known inflammation and oxidative stress inducers. However, it is not known if there are age-related changes in the nigral angiotensin system. In aged rats, we observed increased activation of the nicotinamide adenine dinucleotide phosphate-oxidase (NADPH oxidase) complex and increased levels of the proinflammatory cytokines interleukin (IL)-1β and tumor necrosis factor (TNF)-α, which indicate pro-oxidative, proinflammatory state in the nigra. We also observed enhanced 6-hydroxydopamine (6-OHDA)-induced dopaminergic cell death in aged rats. This is associated with increased expression of AT1 receptors and decreased expression of AT2 receptors in aged rats, and is reduced by treatment with the AT1 antagonist candesartan. The present results indicate that brain angiotensin is involved in changes that may increase the risk of Parkinson's disease with aging. Furthermore, the results suggest that manipulation of the brain angiotensin system may constitute an effective neuroprotective strategy against aging-related risk of dopaminergic degeneration.

Endothelial progenitor cell senescence--is there a role for estrogen?

Recent studies have demonstrated that aging or senescence constitutes a potential limitation to the ability of endothelial progenitor cells (EPCs) to sustain ischemic tissue repair. Excess amount of reactive oxygen species (ROS) is involved in senescence, causing defective neovascularization. Conversely, estrogens have been shown to accelerate recovery of the endothelium after vascular injury. Estrogen reduces EPC senescence through augmentation of telomerase activity. In addition, the inhibition of EPC senescence by estrogen in vitro may improve the functional activity of EPCs in a way that is important for potential cell therapy. This review describes current understanding of EPC senescence and the role of estrogen in preventing EPC senescence.

The angiotensin II type 2 receptor in cardiovascular disease

Angiotensin II (Ang II) is considered the major final mediator of the renin-angiotensin system. The actions of Ang II have been implicated in many cardiovascular conditions, such as hypertension, atherosclerosis, coronary heart disease, restenosis, and heart failure. Ang II can act through two different receptors: Ang II type 1 (AT(1)) receptor and Ang II type 2 (AT(2)) receptor. The AT(1) receptor is ubiquitously expressed in the cardiovascular system and mediates most of the physiological and pathophysiological actions of Ang II. The AT(2) receptor is highly expressed in the developing foetus, but its expression is very low in the cardiovascular system of the normal adult. Expression of the AT(2) receptor can be modulated by pathological states associated with tissue remodelling or inflammation such as hypertension, atherosclerosis, and myocardial infarction. The precise role of the AT(2) receptor remains under debate. However, it appears that the AT(2) receptor plays a vasodilatory role, and may be enhanced as a countervailing mechanism in cardiac hypertrophy, and in presence of vascular injury in hypertension and atherosclerosis. Signalling pathways induced by the stimulation of the AT(2) receptor are poorly understood, but three main mechanisms have been described: (a) activation of protein phosphatases causing protein dephosphorylation; (b) activation of bradykinin/nitric oxide/cyclic guanosine 3',5'-monophosphate pathway; and (c) stimulation of phospholipase A(2) and release of arachidonic acid. Vasodilatory effects of the AT(2) receptor, probably the only well-established role of the AT(2) receptor, have been attributed to the second of these mechanisms. The participation of the AT(2) receptor in cardiovascular remodelling and inflammation is more controversial. In vitro, AT(2) receptor stimulation clearly inhibits cardiac and vascular smooth muscle growth and proliferation, and stimulates apoptosis. In vivo, the situation is less clear, and depending on the studies, the AT(2) receptor appears to be required for cardiac hypertrophic growth or contrariwise, the AT(2) receptor has demonstrated no effects on cardiac hypertrophy. Similar controversial findings have been reported in atherosclerosis. Here we discuss the role of the AT(2) receptor on cardiovascular structure and disease, and the signalling pathways induced by its activation.

Early effects of high glucose in retinal tissue cultures Renin-Angiotensin system-dependent and -independent signaling

The early effects of the diabetic milieu on retinal tissue and their relation to the Renin-Angiotensin system (RAS) activation are poorly known. Here we investigated RAS signaling in retinas explanted from adult rats exposed for 48 h to high glucose (HG), with or without the Angiotensin Converting Enzyme inhibitor enalaprilat, which blocks RAS. HG was observed to i) initiate a phosphotyrosine-dependent signaling cascade; ii) up-regulate Angiotensin(1) Receptor (AT(1)R); iii) activate src tyrosine kinase and increase phosphorylation of Pyk2, PLCgamma1 and ERK1/2; and iv) activate Akt and the transcription factor CREB. In the presence of enalaprilat, tyrosine phosphorylation signal and AT(1)R upregulation decreased and activation of PLCgamma1 and CREB reverted, showing their relation to RAS signaling. In line with Akt activation, no apoptosis or synapse degeneration was found. Müller glia was activated, but in a RAS-independent manner. Our results suggest that, in early phases of HG exposure, a pro-survival cell program may be induced in the retina.

Prenatal cocaine exposure differentially causes vascular dysfunction in adult offspring

Epidemiological studies have shown a clear association of adverse intrauterine environment and an increased risk of cardiovascular diseases and hypertension in adult life. The present study tested the hypothesis that prenatal cocaine exposure causes reprogramming of vascular reactivity, leading to an increased risk of hypertension in adult offspring. Pregnant rats received cocaine (30 mg kg(-1) day(-1)) or saline from days 15 to 21 of gestational age, and experiments were conducted in 3-month-old offspring. Cocaine had no effect on the baseline blood pressure but significantly increased norepinephrine-stimulated blood pressure and decreased the baroreflex sensitivity in male but not female offspring. The cocaine treatment significantly increased norepinephrine-induced contractions in pressurized resistance-sized mesenteric arteries but not in aortas, which was primarily because of a loss of endothelial NO synthase-mediated inhibition and an enhanced Ca(2+) sensitivity in mesenteric arteries. In addition, the cocaine treatment significantly attenuated the endothelium-dependent relaxation in mesenteric arteries in male but not female offspring. Endothelial NO synthase protein levels in aortas but not mesenteric arteries were significantly increased in the cocaine-treated animals. However, cocaine significantly decreased phosphorylation levels of endothelial NO synthase in both aortas and mesenteric arteries. The results suggest that prenatal cocaine exposure programs vascular contractility via changes in endothelial NO synthase-regulated Ca(2+) sensitivity of myofilaments in the sex- and tissue-dependent manners in resistance arteries leading to an increased risk of hypertension in male offspring.

Intrarenal angiotensin III infusion induces natriuresis and angiotensin type 2 receptor translocation in Wistar-Kyoto but not in spontaneously hypertensive rats

In Sprague-Dawley rats, renal angiotensin (Ang) type 2 receptors (AT(2)Rs) mediate natriuresis in response to renal interstitial (RI) D(1)-like receptor stimulation or RI Ang III infusion. After D(1)-like receptor activation, apical membrane (AM) but not total renal proximal tubule cell AT(2)R expression is increased, suggesting that AM AT(2)R translocation may be important for natriuresis. The onset of hypertension in spontaneously hypertensive rats (SHRs) is preceded by defects in renal sodium excretion. The present study examines AT(2)R-mediated natriuresis in response to RI Ang III infusion in Wistar-Kyoto rats (WKYs) and SHRs. WKYs and SHRs received RI Ang III infusion after 24 hours of systemic AT(1)R blockade with candesartan. In WKYs, urine sodium excretion rate increased from 0.043+/-0.01 to 0.191+/-0.06 micromol/min (P<0.05) in response to Ang III infusion, but identical conditions failed to increase the urine sodium excretion rate in SHRs. The increase in the urine sodium excretion rate was blocked by coinfusion of PD-123319, a selective AT(2)R antagonist. On confocal microscopy images, Ang III-infused WKYs demonstrated greater renal proximal tubule cell AM AT(2)R fluorescence intensity compared with SHRs (5385+/-725 versus 919+/-35; P<0.0001), and Western blot analysis demonstrated increased AM (0.050+/-0.003 versus 0.038+/-0.003; P<0.01) but not total cell AT(2)R expression in WKYs. In SHRs, AM AT(2)R expression remained unchanged in response to RI Ang III infusion. Thus, RI Ang III infusion elicits natriuresis and renal proximal tubule cell AT(2)R translocation in WKYs. Identical manipulations fail to induce natriuresis or AT(2)R translocation in SHRs, suggesting that defects in AT(2)R-mediated natriuresis and trafficking may be important to the development of hypertension in SHRs.

AT2 receptors: functional relevance in cardiovascular disease

The renin angiotensin system (RAS) is intricately involved in normal cardiovascular homeostasis. Excessive stimulation by the octapeptide angiotensin II contributes to a range of cardiovascular pathologies and diseases via angiotensin type 1 receptor (AT₁R) activation. On the other hand, tElsevier Inc.he angiotensin type 2 receptor (AT₂R) is thought to counter-regulate AT₁R function. In this review, we describe the enhanced expression and function of AT₂R in various cardiovascular disease settings. In addition, we illustrate that the RAS consists of a family of angiotensin peptides that exert cardiovascular effects that are often distinct from those of Ang II. During cardiovascular disease, there is likely to be an increased functional importance of AT₂R, stimulated by Ang II, or even shorter angiotensin peptide fragments, to limit AT₁R-mediated overactivity and cardiovascular pathologies.