Angiotensin II type 2 receptor-mediated vasodilation. Focus on bradykinin, NO and endothelium-derived hyperpolarizing factor(s)

Molecular mechanisms of angiotensin type 2 receptor-mediated nitric oxide pathway in angiotensin II-induced vasorelaxation: Roles of potassium channels

A variety of biological functions is attributed to the renin-angiotensin system (RAS). One of them is regulating vascular tone through its final effector Angiotensin II (Ang II). Ang II action is mediated by the Angiotensin type 1 receptor (AT1-R) which plays a role in vasoconstriction, and Angiotensin type 2 receptor (AT2-R) which may result in vascular relaxation through the releasing of endothelium mediates relaxation factors such as Nitric Oxide (NO). Therefore, this study investigated the role of AT2-R in vasodilation after blocking the effect of AT1-R in the rat aorta. Furthermore, it is to determine whether or not Ang II through NO has a role in rat aorta dilation via using valsartan. For control isolated aortic rings were preincubated with Valsartan (AT1- R inhibitor) and then stimulated with angiotensin II dose-dependent. For treating aortic rings different blockers and inhibitors were used. Pd123177 (AT2- R inhibitor) (20 µM), an inhibitor of PKA H-89 (10 µM), eNOS inhibitor L-NAME (0.3 mM), with group of K channel blockers such as TEA (1 mM), 4-AP (1 mM), BaCl2 (1 mM), clotrimazole (0.03 mM) and GLIB (0.01 mM). Our analysis demonstrates vasodilation in aortic rings induced by Ang II after blocking ATI-R and this response was highly reliant on PKA/eNOS and cyclic guanosine monophosphate (cGMP). The data from this investigation provided evidence that Ca2 + activated K+ channels (KCa) and Voltage-dependent K channel (KV) mediated Ang II vasorelaxation. Finally, these results indicate that angiotensin II primarily induces dilatation AT2-R after inhibiting the angiotensin AT1 receptor through a cascade of signaling pathways involving many enzymes and plasma membrane protein channels.

Nitric oxide-cyclic GMP role in Ang II induced hyperpolarization in bovine aortic endothelium cell line (BAE-1)

Angiotensin II (Ang II), a mitogen-activated peptide, exerts numerous effects on the cardiovascular system including the regulation of blood pressure. The current study focused on the potential mechanisms that seem to be involved in Ang II vasodilation using bovine aortic endothelial cells (BAE-1) cell lines. Expression of the Ang II receptor (AT2) in BAE-1 was checked by western blots in the presence of valsartan (AT1 inhibitor). To check if Ang II's vasodilator impact was mediated by the nitric oxide (NO) pathway, the Griess reagent was used. Furthermore, cell-attached patch-clamp and fire-polished borosilicate electrodes with a resistance of 3-5 MΩ in the working solutions was used to record membrane currents from treated BAE-1. BEA-1 revealed 50 kDa immunoreactive bands that matched AT2. The concentration of AT2 was elevated in valsartan-treated cells in comparison to control cells. The biochemical experimental data indicated that the NO level increased in a concentration-dependent manner. Meanwhile, Ang II at a concentration of 1 µM, the level of NO increased more than at 100 µM. In patch-clamp experiments, K current and chord conductance were enhanced after incubation of Ang II with valsartan. When 100 µM Ang II was added, the current peaked rapidly and after 15 min of incubation, the maximum value was obtained, as opposed to 10 min and control (110.9 ± 13.3 pA control, 141.4 ± 30.4 pA after 10 min and 174.4 ± 49.3 pA after 15 min). Ang II type two receptor inhibitor (PD1231777) reduced the current and conductance induced by Ang II. The presented data revealed that Ang II released NO via the activation of AT2. K currents were stimulated by Ang II and evoked mainly a current consistent with the activation of K channels.

Novel Pharmacology Following Heteromerization of the Angiotensin II Type 2 Receptor and the Bradykinin Type 2 Receptor

The angiotensin type 2 (AT₂) receptor and the bradykinin type 2 (B₂) receptor are G protein-coupled receptors (GPCRs) that have major roles in the cardiovascular system. The two receptors are known to functionally interact at various levels, and there is some evidence that the observed crosstalk may occur as a result of heteromerization. We investigated evidence for heteromerization of the AT₂ receptor and the B₂ receptor in HEK293FT cells using various bioluminescence resonance energy transfer (BRET)-proximity based assays, including the Receptor Heteromer Investigation Technology (Receptor-HIT) and the NanoBRET ligand-binding assay. The Receptor-HIT assay showed that Gα_q, GRK2 and β-arrestin2 recruitment proximal to AT₂ receptors only occurred upon B₂ receptor coexpression and activation, all of which is indicative of AT₂-B₂ receptor heteromerization. Additionally, we also observed specific coupling of the B₂ receptor with the Gα_z protein, and this was found only in cells coexpressing both receptors and stimulated with bradykinin. The recruitment of Gα_z, Gα_q, GRK2 and β-arrestin2 was inhibited by B₂ receptor but not AT₂ receptor antagonism, indicating the importance of B₂ receptor activation within AT₂-B₂ heteromers. The close proximity between the AT₂ receptor and B₂ receptor at the cell surface was also demonstrated with the NanoBRET ligand-binding assay. Together, our data demonstrate functional interaction between the AT₂ receptor and B₂ receptor in HEK293FT cells, resulting in novel pharmacology for both receptors with regard to Gα_q/GRK2/β-arrestin2 recruitment (AT₂ receptor) and Gα_z protein coupling (B₂ receptor). Our study has revealed a new mechanism for the enigmatic and poorly characterized AT₂ receptor to be functionally active within cells, further illustrating the role of heteromerization in the diversity of GPCR pharmacology and signaling.

Angiotensin II type 1 receptor is involved in hypertension and vascular alterations caused by environmental toxicant hexachlorobenzene

Environmental hexachlorobenzene (HCB) increases blood pressure (BP) in female rats, causing alterations in arterial structure and function. Here we study the role of Angiotensin II receptor type 1 (AT1) in HCB-induced hypertension through the use of AT1 antagonist losartan. HCB-treated male rats showed a 22.7% increase in BP which was prevented by losartan. Losartan blocked HCB-induced changes in arterial morphology (decreased aorta cell number and increased wall thickness). Losartan also prevented HCB-induced alterations in artery relaxation by acetylcholine and nitroprusside but not the reduction in the maximum contraction by phenylephrine. Losartan rescued arterial molecular alterations caused by HCB (i.e. an increase in TGF-β1 and AT1 expression and a decrease in eNOS expression and nitrite levels) and reduced hydrogen sulfide plasma concentration. In conclusion: in this work we demonstrate that AT1 activity is involved in HCB effects on the vascular system leading to hypertension.

New perspectives on natural flavonoids on COVID-19-induced lung injuries

The SARS-CoV-2 virus, responsible for COVID-19, spread rapidly worldwide and became a pandemic in 2020. In some patients, the virus remains in the respiratory tract, causing pneumonia, respiratory failure, acute respiratory distress syndrome (ARDS), and sepsis, leading to death. Natural flavonoids (aglycone and glycosides) possess broad biological activities encompassing antiinflammatory, antiviral, antitumoral, antiallergic, antiplatelet, and antioxidant effects. While many studies have focused on the effects of natural flavonoids in experimental models, reports based on clinical trials are still insufficient. In this review, we highlight the effects of flavonoids in controlling pulmonary diseases, particularly the acute respiratory distress syndrome, a consequence of COVID-19, and their potential use in coronavirus-related diseases. Furthermore, we also focus on establishing a relationship between biological potential and chemical aspects of related flavonoids and discuss several possible mechanisms of action, pointing out some possible effects on COVID-19.

An overview of the classical and tissue-derived renin-angiotensin-aldosterone system and its genetic polymorphisms in essential hypertension

The renin-angiotensin-aldosterone system (RAAS) is a specific hormonal cascade implicated in the blood pressure control and sodium balance regulation. Several components of this pathway have been identified including renin, angiotensinogen, angiotensin-converting enzyme, angiotensins with a wide range of distinct subtypes and receptors, and aldosterone. The RAAS is not only confined to the systemic circulation but also exists locally in specific tissues such as the heart, brain, and blood vessels with a particular paracrine action. Alteration of RAAS function can contribute to the development of hypertension and the emergence of its associated end-organ damage. Genotypic variations of the different genes of RAAS cascade have been linked to the susceptibility to essential hypertension. Accordingly, to understand the pathogenesis of essential hypertension and its related complications, deep insight into the physiological and genetic aspects of RAAS with its different components and pathways is necessary. In this review, we aimed to illustrate the physiological and genetic aspects of RAAS and the underlying mechanisms which link this system to the predisposition to essential hypertension.

Exercise training attenuates angiotensin II-induced vasoconstriction in the aorta of normotensive but not hypertensive rats

NEW FINDINGS

What is the central question of this study? What are the effects of exercise on Ang II-induced vasoconstriction in aortas of normotensive rats and how do these effects occur in two-kidney-one-clip hypertensive animals? What is the main finding and its importance? In two-kidney rats, exercise training improves the Ang II-induced vasoconstriction by endothelium-derived NO released through AT₂ R activation. This effect of exercise training on the Ang II-induced vasoconstriction is blunted in two-kidney-one-clip hypertensive animals, possibly as a consequence of oxidative stress.

ABSTRACT

This study investigated the effects of both acute exercise and training on the Ang II-induced vasoconstriction in aorta of normotensive (two-kidney; 2K) and two-kidney-one-clip (2K1C) hypertensive rats, focusing on endothelial mechanisms related to nitric oxide (NO) and prostanoids. Aorta rings of 2K and 2K1C male Wistar rats, sedentary and trained, killed at rest and after acute exercise, were challenged with Ang II in either the absence or the presence of PD 123,319, a selective angiotensin receptor subtype 2 (AT₂ R) antagonist; N^ω -nitro-l-arginine methyl ester (l-NAME), a non-selective inhibitor of nitric oxide synthase; indomethacin, a non-selective inhibitor of cyclooxygenase; or Tiron, an analogue of superoxide dismutase. Aortas of sedentary and trained animals studied at rest were also submitted to histomorphometric analysis. Exercise training reduced the Ang II-induced vasoconstriction in aorta of 2K but not of 2K1C animals. This reduction of Ang II response in aortas of 2K animals was not found after endothelial removal or treatment with PD 123,319 or l-NAME. These results suggest that exercise training improves the modulation of Ang II-induced vasoconstriction in aorta of 2K animals, by endothelium-derived NO released due to the activation of AT₂ R. No exercise-induced change of Ang II response occurred in 2K1C animals, except in the presence of Tiron, which was evidence for reduction of such responses only in resting trained 2K1C animals. In 2K1C animals, NO modulation of Ang II-induced vasoconstriction might be suppressed by local oxidative stress. Moreover, exercise training slightly reduced the media layer thickness in the aortas of the 2K1C, but not 2K animals, which may indicate cardiovascular protection of these animals.

Connexin37-Dependent Mechanisms Selectively Contribute to Modulate Angiotensin II -Mediated Hypertension

Background Gap junction channels made of Connexin37 (Cx37) are expressed by aortic endothelial and smooth muscle cells of hypertensive mice, as well as by the renin-secreting cells of kidneys. Methods and Results To decipher whether Cx37 has any role in hypertension, angiotensin II (Ang II ) was infused in normotensive wild-type and Cx37-deficient mice (Cx37-/-). After 2 to 4 weeks, the resulting increase in blood pressure was lower in Cx37-/- than in wild-type mice, suggesting an alteration in the Ang II response. To investigate this possibility, mice were submitted to a 2-kidney, 1-clip procedure, a renin-dependent model of hypertension. Two weeks after this clipping, Cx37-/- mice were less hypertensive than wild-type mice and, 2 weeks later, their blood pressure had returned to control values, in spite of abnormally high plasma renin levels. In contrast, Cx37-/- and wild-type mice that received N-nitro-l-arginine-methyl-ester, a renin-independent model of hypertension, featured a similar and sustained increase in blood pressure. The data indicate that loss of Cx37 selectively altered the Ang II -dependent pathways. Consistent with this conclusion, aortas of Cx37-/- mice featured an increased basal expression of the Ang II type 2 receptors ( AT 2R), and increased transcripts levels of downstream signaling proteins, such as Cnksr1 and Ptpn6 ( SHP -1). Accordingly, the response of Cx37-/- mice aortas to an ex vivo Ang II exposure was altered, since phosphorylation levels of several proteins of the Ang II pathway ( MLC 2, ERK , and AKT ) remained unchanged. Conclusions These findings provide evidence that Cx37 selectively influences Ang II signaling, mostly via a modulation of the expression of the Ang II type 2 receptor.

Roles of angiotensin II type 2 receptor in mice with fetal growth restriction

Our previous report indicated that vascular injury enhances vascular remodeling in fetal growth restriction (FGR) mice. The angiotensin II type 2 receptor (AT₂R) is relatively highly expressed in fetal mice. Therefore, we investigated the roles of AT₂R in FGR-induced cardiovascular disease using AT₂R knockout (AT₂KO) mice. Dams (wild-type and AT₂KO mice) were fed an isocaloric diet containing 20% protein (NP) or 8% protein (LP) until delivery. Arterial blood pressure, body weight, and histological changes in organs were investigated in offspring. The birth weight of offspring from dams fed an LP diet (LPO) was significantly lower than that of offspring from dams fed an NP diet. The heart/body and kidney/body weight ratios in AT₂KO-LPO at 12 weeks of age were significantly higher than those in the other groups. Greater thickness of the left ventricular wall, larger cardiomyocyte size and enhancement of perivascular fibrosis were observed in AT₂KO-LPO. Interestingly, mRNA expression of collagen I and inflammatory cytokines was markedly higher in the AT₂KO-LPO heart at 6 weeks of age but not at 12 weeks of age. AT₂R signaling may be involved in cardiovascular disorders of adult offspring with FGR. Regulation of AT₂R could contribute to preventing future cardiovascular disease in FGR offspring.

Dietary restriction but not angiotensin II type 1 receptor blockade improves DNA damage-related vasodilator dysfunction in rapidly aging Ercc1Δ/− mice

DNA damage is an important contributor to endothelial dysfunction and age-related vascular disease. Recently, we demonstrated in a DNA repair-deficient, prematurely aging mouse model (Ercc1Δ/− mice) that dietary restriction (DR) strongly increases life- and health span, including ameliorating endothelial dysfunction, by preserving genomic integrity. In this mouse mutant displaying prominent accelerated, age-dependent endothelial dysfunction we investigated the signaling pathways involved in improved endothelium-mediated vasodilation by DR, and explore the potential role of the renin-angiotensin system (RAS). Ercc1Δ/− mice showed increased blood pressure and decreased aortic relaxations to acetylcholine (ACh) in organ bath experiments. Nitric oxide (NO) signaling and phospho-Ser1177-eNOS were compromised in Ercc1Δ/−. DR improved relaxations by increasing prostaglandin-mediated responses. Increase of cyclo-oxygenase 2 and decrease of phosphodiesterase 4B were identified as potential mechanisms. DR also prevented loss of NO signaling in vascular smooth muscle cells and normalized angiotensin II (Ang II) vasoconstrictions, which were increased in Ercc1Δ/− mice. Ercc1Δ/− mutants showed a loss of Ang II type 2 receptor-mediated counter-regulation of Ang II type 1 receptor-induced vasoconstrictions. Chronic losartan treatment effectively decreased blood pressure, but did not improve endothelium-dependent relaxations. This result might relate to the aging-associated loss of treatment efficacy of RAS blockade with respect to endothelial function improvement. In summary, DR effectively prevents endothelium-dependent vasodilator dysfunction by augmenting prostaglandin-mediated responses, whereas chronic Ang II type 1 receptor blockade is ineffective.

Exercise modulates the aortic renin-angiotensin system independently of estrogen therapy in ovariectomized hypertensive rats

The renin-angiotensin-system is an important component of cardiovascular control and is up-regulated under various conditions, including hypertension and menopause. The aim of this study was to evaluate the effects of swimming training and estrogen therapy (ET) on angiotensin-II (ANG II)-induced vasoconstriction and angiotensin-(1-7) [ANG-(1-7)]-induced vasorelaxation in aortic rings from ovariectomized spontaneously hypertensive rats. Animals were divided into Sham (SH), Ovariectomized (OVX), Ovariectomized treated with E2 (OE2), Ovariectomized plus swimming (OSW) and Ovariectomized treated with E2 plus swimming (OE2+SW) groups. ET entailed the administration of 5μg of 17β-Estradiol three times per week. Swimming was undertaken for sixty minutes each day, five times per week. Both, training and ET were initiated seven days following ovariectomy. Forty-eight hours after the last treatment or training session, the animals' systolic blood pressures were measured, and blood samples were collected to measure plasma ANG II and ANG-(1-7) levels via radioimmunoassay. In aortic rings, the vascular reactivity to ANG II and ANG-(1-7) was assessed. Expression of ANG-(1-7) in aortic wall was analyzed by immunohistochemistry. The results showed that both exercise and ET increased plasma ANG II levels despite attenuating systolic blood pressure. Ovariectomy increased constrictor responses to ANG II and decreased dilatory responses to ANG-(1-7), which were reversed by swimming independently of ET. Moreover, it was observed an apparent increase in ANG-(1-7) content in the aorta of the groups subjected to training and ET. Exercise training may play a cardioprotective role independently of ET and may be an alternative to ET in hypertensive postmenopausal women.

Hypotensive and antihypertensive effects of a hydroalcoholic extract from Senecio nutans Sch. Bip. (Compositae) in mice: Chronotropic and negative inotropic effect, a nifedipine-like action

ETHNOPHARMACOLOGICAL RELEVANCE

Senecio nutans Sch. Bip. (Compositae) is an endemic plant of South America used in the management of acute mountain sickness in the Andean communities. Currently, the direct effects of hydroalcoholic extract from S. nutans on the cardiovascular system are unknown. The aim of this study was to determine the effects and mechanism of action of S. nutans on cardiovascular function in normotensive and Angiotensin II (1μg/mL) hypertension mice models.

MATERIAL AND METHODS

Blood pressure and ECG measurements were simultaneously carried out on the mice and rats. The isolated right atrium, papillary muscle of the left ventricle and isolated heart of rat were used to study the cardiac functions and mechanisms.

RESULTS

S. nutans (40mg/Kg) induced a 30% and 12% significant (p<0.05) reduction of the mean arterial pressure (MAP) in normotensive and hypertensive mice respectively. This decrease was as a result of decrease in heart rate (HR) in normotensive (25%) and hypertensive model (31%). It also decreased the sinus rhythm in isolated right atrium of rat. Compared with Losartan, a known anti-hypertensive, S. nutans caused a dose-dependent negative inotropic effect (dP/dtmax) on Langendorff isolated heart system. While Losartan, decreased the MAP by 30% but had no effect on heart rate. The calcium blocker nifedipine had similar effects as S. nutans, decreasing the beat frequency of isolated right atrium and contractility of papillary muscle of the left ventricle of rat.

CONCLUSION

The results suggest an important clinical function in hypertension therapy, as S. nutans could decrease the blood pressure in hypertensive mice by decreasing the HR and contractility, leading to a reduction in myocardial oxygen demand.

Developmental acceleration of bradykinin-dependent relaxation by prenatal chronic hypoxia impedes normal development after birth

Bradykinin-induced activation of the pulmonary endothelium triggers nitric oxide production and other signals that cause vasorelaxation, including stimulation of large-conductance Ca(2+)-activated K(+) (BKCa) channels in myocytes that hyperpolarize the plasma membrane and decrease intracellular Ca(2+). Intrauterine chronic hypoxia (CH) may reduce vasorelaxation in the fetal-to-newborn transition and contribute to pulmonary hypertension of the newborn. Thus we examined the effects of maturation and CH on the role of BKCa channels during bradykinin-induced vasorelaxation by examining endothelial Ca(2+) signals, wire myography, and Western immunoblots on pulmonary arteries isolated from near-term fetal (∼ 140 days gestation) and newborn, 10- to 20-day-old, sheep that lived in normoxia at 700 m or in CH at high altitude (3,801 m) for >100 days. CH enhanced bradykinin-induced relaxation of fetal vessels but decreased relaxation in newborns. Endothelial Ca(2+) responses decreased with maturation but increased with CH. Bradykinin-dependent relaxation was sensitive to 100 μM nitro-L-arginine methyl ester or 10 μM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, supporting roles for endothelial nitric oxide synthase and soluble guanylate cyclase activation. Indomethacin blocked relaxation in CH vessels, suggesting upregulation of PLA2 pathways. BKCa channel inhibition with 1 mM tetraethylammonium reduced bradykinin-induced vasorelaxation in the normoxic newborn and fetal CH vessels. Maturation reduced whole cell BKCa channel α1-subunit expression but increased β1-subunit expression. These results suggest that CH amplifies the contribution of BKCa channels to bradykinin-induced vasorelaxation in fetal sheep but stunts further development of this vasodilatory pathway in newborns. This involves complex changes in multiple components of the bradykinin-signaling axes.

Angiotensin II type 2 receptor- and acetylcholine-mediated relaxation: essential contribution of female sex hormones and chromosomes

Angiotensin-induced vasodilation, involving type 2 receptor (AT2R)-induced generation of nitric oxide (NO; by endothelial NO synthase) and endothelium-derived hyperpolarizing factors, may be limited to women. To distinguish the contribution of female sex hormones and chromosomes to AT2R function and endothelium-derived hyperpolarizing factor-mediated vasodilation, we made use of the four-core genotype model, where the testis-determining Sry gene has been deleted (Y(-)) from the Y chromosome, allowing XY(-) mice to develop a female gonadal phenotype. Simultaneously, by incorporating the Sry gene onto an autosome, XY(-)Sry and XXSry transgenic mice develop into gonadal male mice. Four-core genotype mice underwent a sham or gonadectomy (GDX) operation, and after 8 weeks, iliac arteries were collected to assess vascular function. XY(-)Sry male mice responded more strongly to angiotensin than XX female mice, and the AT2R antagonist PD123319 revealed that this was because of a dilator AT2R-mediated effect occurring exclusively in XX female mice. The latter could not be demonstrated in XXSry male and XY(-) female mice nor in XX female mice after GDX, suggesting that it depends on both sex hormones and chromosomes. Indeed, treating C57bl/6 GDX male mice with estrogen could not restore angiotensin-mediated, AT2R-dependent relaxation. To block acetylcholine-induced relaxation of iliac arteries obtained from four-core genotype XX mice, both endothelial NO synthase and endothelium-derived hyperpolarizing factor inhibition were required, whereas in four-core genotype XY animals, endothelial NO synthase inhibition alone was sufficient. These findings were independent of gonadal sex and unaltered after GDX. In conclusion, AT2R-induced relaxation requires both estrogen and the XX chromosome sex complement, whereas only the latter is required for endothelium-derived hyperpolarizing factors.

Enhanced angiotensin-converting enzyme activity and systemic reactivity to angiotensin II in normotensive rats exposed to a high-sodium diet

A high salt diet is associated with reduced activity of the renin-angiotensin-aldosterone system (RAAS). However, normotensive rats exposed to high sodium do not show changes in systemic arterial pressure. We hypothesized that, despite the reduced circulating amounts of angiotensin II induced by a high salt diet, the cardiovascular system's reactivity to angiotensin II is increased in vivo, contributing to maintain arterial pressure at normal levels. Male Wistar rats received chow containing 0.27% (control), 2%, 4%, or 8% NaCl for six weeks. The high-sodium diet did not lead to changes in arterial pressure, although plasma levels of angiotensin II and aldosterone were reduced in the 4% and 8% NaCl groups. The 4% and 8% NaCl groups showed enhanced pressor responses to angiotensin I and II, accompanied by unchanged and increased angiotensin-converting enzyme activity, respectively. The 4% NaCl group showed increased expression of angiotensin II type 1 receptors and reduced expression of angiotensin II type 2 receptors in the aorta. In addition, the hypotensive effect of losartan was reduced in both 4% and 8% NaCl groups. In conclusion these results explain, at least in part, why the systemic arterial pressure is maintained at normal levels in non-salt sensitive and healthy rats exposed to a high salt diet, when the functionality of RAAS appears to be blunted, as well as suggest that angiotensin II has a crucial role in the vascular dysfunction associated with high salt intake, even in the absence of hypertension.

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.

Differential sensitivity to angiotensin II and norepinephrine in human uterine arteries

BACKGROUND

During pregnancy, uteroplacental responses to norepinephrine (NE) exceed systemic responses. In contrast, uteroplacental responses to angiotensin II (ANG II) are less than systemic. The explanation for these differences in uteroplacental sensitivity remain unclear but may reflect type 2 ANG II receptor (AT(2)R) predominance in uterine artery (UA) vascular smooth muscle (VSM).

OBJECTIVE

The objective of the study was to examine VSM sensitivity to KCl, NE, and ANG II in UA from nonpregnant (NP) and pregnant (P) women and determine VSM ANG II receptor subtype expression.

METHODS

Responses to KCl, NE, and ANG II were examined in endothelium-denuded UA rings from NP (n = 28) and P (n = 13; 34-40 wk gestation) women, and ANG II receptor subtype, α(1)-receptor and contractile proteins were measured.

RESULTS

KCl and NE dose dependently contracted UA (P < 0.001), P exceeding NP 2-fold or greater; but α(1)-receptor expression was unchanged. ANG II did not elicit dose effects in NP or P UA; however, P responses exceeded NP approximately 2-fold (P < 0.001) and were approximately 2.5-fold less than NE (P < 0.001). AT(2)R and AT(1)R expression were similar (P > 0.1) in VSM from NP and term P women. AT(1)R blockade abolished ANG II contractions (P < 0.001); AT(2)R blockade did not enhance ANG II sensitivity in UA with or without endothelium. Actin contents increased approximately 2-fold in term UA.

CONCLUSIONS

Sensitivity to α-stimulation exceeds ANG II in NP and P UA, explaining the differential uteroplacental sensitivity in pregnancy. Because AT(2)R predominate in UA VSM throughout reproduction, this contributes to the inherent refractoriness to ANG II in the uterine vasculature. The increase in UA contractile proteins at term P suggests remodeling, explaining the enhanced contractility seen.

Defining the differential sensitivity to norepinephrine and angiotensin II in the ovine uterine vasculature

The intact ovine uterine vascular bed (UVB) is sensitive to α-agonists and refractory to angiotensin II (ANG II) during pregnancy; the converse occurs in the systemic circulation. The mechanism(s) responsible for these differences in uterine sensitivity are unclear and may reflect predominance of nonconstricting AT(2) receptors (AT(2)R) in uterine vascular smooth muscle (UVSM). The contribution of the placental vasculature also is unclear. Third generation and precaruncular/placental arteries from nonpregnant (n = 16) and term pregnant (n = 23) sheep were used to study contraction responses to KCl, norepinephrine (NE), and ANG II (with/without ATR specific inhibitors) and determine UVSM ATR subtype expression and contractile protein content. KCl and NE increased third generation and precaruncular/placental UVSM contractions in a dose- and pregnancy-dependent manner (P ≤ 0.001). ANG II only elicited modest contractions in third generation pregnant UVSM (P = 0.04) and none in precaruncular/placental UVSM. Moreover, compared with KCl and NE, ANG II contractions were diminished ≥ 5-fold. Whereas KCl and ANG II contracted third generation>>precaruncular/placental UVSM, NE-induced contractions were similar throughout the UVB. However, each agonist increased third generation contractions ≥ 2-fold at term, paralleling increased actin/myosin and cellular protein content (P ≤ 0.01). UVSM AT(1)R and AT(2)R expression was similar throughout the UVB and unchanged during pregnancy (P > 0.1). AT(1)R inhibition blocked ANG II-mediated contractions; AT(2)R blockade, however, did not enhance contractions. AT(2)R predominate throughout the UVB of nonpregnant and pregnant sheep, contributing to an inherent refractoriness to ANG II. In contrast, NE elicits enhanced contractility throughout the ovine UVB that exceeds ANG II and increases further at term pregnancy.

Vasculoprotective effects of rosiglitazone through modulating renin-angiotensin system in vivo and vitro

Background

The peroxisome proliferator-activated receptor-γ (PPARγ) agonist rosiglitazone has been suggested to exert cardiovascular protection through the improvement of lipid metabolism, anti-inflammation, anti-proliferation etc. However, whether renin-angiotensin system (RAS) is involved in the vascular protective effects of PPARγ agonists is not fully understood. The present study aimed to investigate the effects of the renin-angiotensin system in vascular protection mediated by PPARγ agonists.

Objective

To investigate the actions of the renin-angiotensin system in vascular protection mediated by activation of PPARγ in vivo and in vitro.

Methods

Rats were fed a regular diet (n = 8), a cholesterol-rich diet plus methylthiouracil (80 mg/Kg/day, n = 10), a cholesterol-rich diet plus methylthiouracil and rosiglitazone (4 mg/kg/day, n = 10). The rosiglitazone treatment was started from one month after the start of cholesterol-rich diet plus methylthiouracil, and lasted five months. Cultured vascular smooth muscle cells (VSMCs) were pretreated with 1 μmol/L angiotensin II (ANG II) for 6 h and randomly divided into the control group; the ANG II group (1 μmol/L ANG II); the groups respectively treated with different concentration rosiglitazone (20, 30, 50) μmol/L for 12 h; the groups treated with 30 μmol/L rosiglitazone for (6, 12, 24) h. Morphology changes of the aortic tissues were observed by hematoxylin and eosin stain. The VSMC growth was detected by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric assay. Angiotensin II and expression of angiotensin receptors were determined by radioimmunoassay, reverse transcription polymerase chain reaction (RT-PCR), western blot, and immunohistochemistry.

Results

After 6 months, lipid deposition, VSMC proliferation and migration toward intima were observed in aortic tissues in the rats on a cholesterol-rich diet plus methylthiouracil, while these pathological changes induced by the cholesterol-rich diet were significantly suppressed by rosiglitazone. In addition, VSMC proliferation induced by ANG II was markedly inhibited by rosiglitazone. Rosiglitazone markedly down-regulated expression of angiotensin type 1 receptor (AT₁R) and up-regulated expression of angiotensin type 2 receptor (AT₂R) in the aortic tissues and ANG II-treated VSMCs.

Conclusions

The present study demonstrated that PPARγ agonist rosiglitazone suppressed ANG II-induced VSMC proliferation in vitro and early atherosclerotic formation evoked by cholesterol-rich diet in vivo. These vasculoprotective effects of rosiglitazone were mediated at least partially by reduction in local tissue ANG II concentration, down-regulation of AT₁R expression and up-regulation of AT₂R expression both at the mRNA and protein levels.

Genetic variance in CYP2C8 and increased risk of myocardial infarction

BACKGROUND

Epoxyeicosatrienoic acids (EETs) are important mediators in vasodilatation, acting as endothelium-derived hyperpolarizing factors. CYP2C enzymes catalyze the metabolism of arachidonic acid to EETs. Genetic variation within the genes encoding for these enzymes may result in differences in vascular response, among others in myocardial tissue, and may therefore increase the risk of myocardial infarction (MI). CYP2C8 and CYP2C9 are encoded by the genes of the same name. CYP2C9 polymorphisms have been associated with an increased risk of MI. As CYP2C8 is genetically linked to CYP2C9 and on account of its role in EET production, we hypothesized that CYP2C8 polymorphisms are associated with the risk of MI.

METHODS

This study was embedded within the Rotterdam study, a prospective population-based cohort study. The study population included all participants with successful genotyping and without prevalent MI (n=5199). Twenty-five tagging single nucleotide polymorphisms within and around the gene-coding areas of CYP2C8 and CYP2C9 were tested for an association with incident MI using survival analysis techniques with multivariable adjustment for potential confounders.

RESULTS

During follow-up, 290 persons developed an incident MI. One tag-SNP in the CYP2C8 gene was associated with incident MI after Bonferroni correction, rs1058932C>T (variant genotype hazard ratio 1.54; 95% CI: 1.22-1.95). There was a significant gene-sex interaction with a relative excess risk of 1.40 (95% CI: 0.33-2.47) for men.

CONCLUSION

SNP rs1058932C>T within the CYP2C8 gene is associated with an increased risk of MI, which is, possibly because of a vascular effect of sex steroids, highest in males.

Kininogen gene (KNG) variation has a consistent effect on aldosterone response to antihypertensive drug therapy: the GERA study

Recent experimental and clinical studies suggested that apart from playing an essential role in blood pressure homeostasis, aldosterone is involved in the pathophysiology of cardiovascular and renal diseases by inducing structural changes in the heart, kidney, and vessel wall. The interindividual variation of aldosterone response to antihypertensive treatment is considerable, and is at least partially explained by genetic variation. In this study, we investigated aldosterone response to two antihypertensive drugs-a thiazide diuretic and an angiotensin receptor blocker (ARB). Genetic variations in 50 candidate genes were tested for association with aldosterone response in four independent samples: African American (AA) responders to a diuretic (n = 289), AA responders to an ARB (n = 252), European American (EA) responders to a diuretic (n = 295) and EA responders to an ARB (n = 300). Linear regression was used to test the association with inclusion of age, sex, and body mass index as covariates. The results indicated the existence of one or more variants in the kininogen gene (KNG) that influence interindividual variation in aldosterone response. The significant association was replicated in three of four studied groups. The single nucleotide polymorphism rs4686799 was associated in AA and EA responders to the diuretic (P = 0.04 and P = 0.07, respectively), and rs5030062 and rs698078 were significantly associated in EA responders to the diuretic (P = 0.05 and P = 0.01) and EA responders to the ARB (P = 0.04 and P = 0.02). Although the clinical implication of KNG gene variation to antihypertensive drug response is yet to be determined, this novel candidate locus provides important new insights into drug response physiology.

Characterization of endothelial thromboxane receptors in rabbit aorta

An increased synthesis of thromboxane (TX) A(2) is associated with a number of cardiovascular diseases including atherosclerosis, unstable angina and hypertension. We previously identified a subgroup of NZW rabbits in which isolated arteries failed to contract to the TX agonists, U46619 or I-BOP. In vascular smooth muscle membranes, there was a significant decrease in TX receptors, termed TP. These rabbits are referred to as vTP- and those with the TP receptor are called vTP+. Because TP receptors are expressed in some types of endothelial cells, the present study was designed to determine whether functional TP receptors are present in endothelial cells cultured from aortas of vTP+ and vTP- rabbits. Radioligand binding studies were performed with (125)I-BOP. Aortic endothelial cells from vTP+ rabbits exhibited specific and saturable binding. In contrast, in endothelial preparations from vTP- rabbit aortas, no measurable binding to (125)I-BOP was detected. Using an anti-TP receptor antibody, we compared the amount of receptor expressed in endothelial cell lysates obtained from vTP+ and vTP- rabbits. Consistent with the results observed radioligand binding assays, the expression of TP receptor protein was decreased in vTP- compared to vTP+ endothelial cells. An in vitro wound healing assay was used on confluent monolayers of endothelial cells. In the untreated vTP+ cells, the area of the scratch was completely closed by 30 h. In the vTP+ cells treated with U46619 (3 microM), the rate of closure of the scratch area was reduced with approximately 12% of the scratch area remaining at 30 h. Pretreatment with the TP receptor antagonist, SQ 29548 (10 microM) prevented the inhibitory effect of U46619. The rate of closure of the scratch in the vTP- was not altered by U46619. In a separate study, U46619 (3 microM) increased the release of 6-keto PGF(1alpha), the stable metabolite of prostacyclin, in vTP+ but not vTP- endothelial cells. Pretreatment with SQ29548 (10 microM) or the cyclooxygenase inhibitor, indomethacin (10 microM) blocked the increase in vTP+ endothelial cells. In vascular reactivity studies in aortas from vTP+ rabbits, removal of the endothelium enhanced the vasoconstrictor response to U46619 indicating that activation of endothelial TP receptors may modulate vascular tone via the release of the vasodilator, prostacyclin. The results of this study suggest an important role for endothelial TP receptors in modulating vascular function.

Role of bone marrow renin-angiotensin system in the pathogenesis of atherosclerosis

The renin-angiotensin system (RAS) has been considered to be a circulating hormonal system that regulates blood pressure, blood flow, fluid volume and electrolyte balance. A growing body of evidence indicates local effects of an activated RAS, particularly in the cardiac, vascular, and renal systems. It is now well established that RAS, especially angiotensin II (Ang II) and Ang II type 1 receptor (AT1R) pathway, has significant pro-inflammatory actions on the vessel wall, leading to progression of atherosclerosis. Recent reports suggest that an activated RAS has local effects in bone marrow (BM), which contributes to the regulation of normal and malignant hematologic processes. We reported that AT1aR in BM cells participate in the pathogenesis of atherosclerosis by analyzing several BM chimeric mice whose BM cells were positive or negative for AT1aR. These results suggest that blockade of AT1R not only in vascular cells but also in BM could be an important strategy to prevent atherosclerosis. In this review, we overview recent findings on a role of RAS in the pathogenesis of atherosclerosis, and discuss functional contribution of a local RAS in BM to progression and destabilization of atherosclerotic plaque.

Angiotensin II receptors subtypes mediate diverse gene expression profile in adult hypertrophic cardiomyocytes

1. Although the systemic and cardiac renin-angiotensin systems are known to be activated in the setting of pressure overload, the actions and signaling mechanisms of angiotensin (Ang) II via AT(1) and AT(2) receptors in hypertrophic cardiomyocytes (CM) remain largely unclear. 2. Hypertrophic CM were prepared from rats with aortic banding for 8 weeks, cultured and then treated as follows: (i) 1 micromol/L AngII for 24 h; (ii) 10 micromol/L losartan (an AT(1) receptor antagonist) for 1 h followed by 1 micromol/L AngII for 24 h; and (iii) 10 micromol/L PD123319 (an AT(2) receptor antagonist) for 1 h followed by 1 micromol/L AngII for 24 h. Changes in the expression of genes following stimulation of AT(1) and AT(2) receptors specific to G-protein-coupled receptor (GPCR) signaling pathways were tested using GEArray (Superarray, Bethesda, MD, USA). The effects of AngII, acting via AT(1) and AT(2) receptors, on the expression of tumour necrosis factor (TNF)-alpha, interleukin (IL)-1beta and IL-6 were confirmed by reverse transcription-polymerase chain reaction and radioimmunoassay. 3. The genes regulated via stimulation of AT(1) receptors were mainly restricted to the signaling pathways including cAMP/protein kinase (PK) A, Ca(2+), PKC, protein tyrosine kinase, mitogen-activated protein kinases, phosphatidylinositol 3-kinase and nuclear factor-kappaB. In addition to these pathways related to activation of AT(1) receptors, four additional signaling pathways were found to be associated with stimulation of AT(2) receptors, including phospholipase C, nitric oxide/cGMP, Rho and Janus kinase/signal transducer and activator of transcription. Blockade of AT(2) receptors decreased the mRNA and protein expression of TNF-alpha and IL-1beta, whereas blockade of AT(1) receptors had no such effect. 4. In conclusion, in hypertrophic CM, AngII leads to distinct signaling responses mediated by AT(1) and AT(2) receptors. Stimulation of AT(2) receptors appears to have a greater influence on GPCR-signaling than stimulation of AT(1) receptors. Angiotensin II enhances the synthesis and secretion of TNF-alpha and IL-1beta in hypertrophic CM, which is mediated by AT(2), but not AT(1), receptors.

Epidemic meningitis, meningococcaemia, and Neisseria meningitidis

Meningococcus, an obligate human bacterial pathogen, remains a worldwide and devastating cause of epidemic meningitis and sepsis. However, advances have been made in our understanding of meningococcal biology and pathogenesis, global epidemiology, transmission and carriage, host susceptibility, pathophysiology, and clinical presentations. Approaches to diagnosis, treatment, and chemoprophylaxis are now in use on the basis of these advances. Importantly, the next generation of meningococcal conjugate vaccines for serogroups A, C, Y, W-135, and broadly effective serogroup B vaccines are on the horizon, which could eliminate the organism as a major threat to human health in industrialised countries in the next decade. The crucial challenge will be effective introduction of new meningococcal vaccines into developing countries, especially in sub-Saharan Africa, where they are urgently needed.

Angiotensin II type 2 receptor-dependent increases in nitric oxide synthase expression in the pulmonary endothelium is mediated via a Gαi3/Ras/Raf/MAPK pathway

We have previously reported that angiotensin II (ANG II) stimulated Src tyrosine kinase via a pertussis toxin-sensitive type 2 receptor, which, in turn, activates MAPK, resulting in an increase in nitric oxide synthase (NOS) expression in pulmonary artery endothelial cells (PAECs). The present study was designed to investigate the pathway by which ANG II activates Src leading to an increase in ERK1/ERK2 phosphorylation and an increase in NOS protein in PAECs. Transfection of PAECs with Gαi3dominant negative (DN) cDNA blocked the ANG II-dependent activation of Src, ERK1/ERK2 phosphorylation, and increase in NOS expression. ANG II stimulated an increase in tyrosine phosphorylation of sequence homology of collagen (Shc; 15 min) that was prevented when PAECs were pretreated with 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo-[3,4-d]pyrimidine (PP2), a Src inhibitor. ANG II induced a Src-dependent association between Shc and growth factor receptor-bound protein 2 (Grb2) and between Grb2 and son of sevenless (Sos), both of which were maximal at 15 min. The ANG II-dependent increase in Ras GTP binding was prevented when PAECs were pretreated with the AT2antagonist PD-123319 or with PP2 or were transfected with Src DN cDNA. ANG II-dependent activation of MAPK and the increase in endothelial NOS (eNOS) were prevented when PAECs were transfected with Ras DN cDNA or treated with FTI-277, a farnesyl transferase inhibitor. ANG II induction of Raf-1 phosphorylation was prevented when PAECs were pretreated with PD-123319 and PP2. Raf kinase inhibitor 1 prevented the ANG II-dependent increase in eNOS expression. Collectively, these data suggest that Gαi3, Shc, Grb2, Ras, and Raf-1 link Src to activation of MAPK and to the AT2-dependent increase in eNOS expression in PAECs.

Physiology of Local Renin-Angiotensin Systems

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

AT2 receptor-mediated vasodilation in the mouse heart depends on AT1A receptor activation

Angiotensin (Ang) II type 2 (AT(2)) receptors are believed to counteract Ang II type 1 (AT(1)) receptor-mediated effects. Here, we investigated AT(2) receptor-mediated effects on coronary and cardiac contractility in C57BL/6 mice. Hearts were perfused according to Langendorff. Baseline coronary flow (CF) and left ventricular systolic pressure (LVSP) were 2.7 +/- 0.1 ml min(-1) and 111 +/- 3 mmHg (n = 50), respectively. Ang II (n = 14) concentration dependently decreased CF and LVSP, by maximally 41 +/- 4 and 25 +/- 3%, respectively (pEC(50)s 7.41 +/- 0.12 and 7.65 +/- 0.12). The AT(1) receptor antagonist irbesartan (n = 4) abolished all Ang II-induced changes, whereas the AT(2) receptor antagonist PD123319 (n = 6) enhanced (P < 0.05) the effect of Ang II on CF (to 59 +/- 1%) and LVSP (to 44 +/- 2%), without altering its potency. A similar enhancement was observed in the presence of nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine methyl ester HCl (L-NAME; n = 4). On top of L-NAME, PD123319 no longer affected the response to Ang II (n = 4). The AT(2) receptor agonist CGP42112A (n = 4) did not affect CF or LVSP, nor did CGP42112A (n = 4) alter the constrictor response to the alpha(1)-adrenoceptor agonist phenylephrine. Furthermore, Ang II exerted no effects in hearts of AT(1A)(-/-) mice (n = 5), whereas its effects in hearts of AT(1A)(+/+) wild-type control mice (n = 7) were indistinguishable from those in hearts of C57BL/6 mice. In conclusion, Ang II exerts opposite effects on coronary and cardiac contractility in the mouse heart via activation of AT(1A) and AT(2) receptors. AT(2) receptor-mediated effects depend on NO and occur only in conjunction with AT(1A) receptor activation.

Anti-inflammatory effects of kinins via microglia in the central nervous system

Kinins are important biologically active peptides that are up-regulated after lesions in both the peripheral and central (CNS) nervous systems. Microglia are immune cells in the CNS and play an important role in the defense of the neuronal parenchyma. In cultured murine microglia, bradykinin (BK) induces mobilization of intracellular Ca2+, microglial migration, and increases the release of nitric oxide and prostaglandin E2. On the other hand, BK attenuates lipopolysaccharide-activated TNF-α and IL-1β release. These results suggest that BK functions as a signal in brain trauma and may have an anti-inflammatory role in the CNS.

Hypotensive shock and angio-oedema from angiotensin II receptor blocker: a class effect in spite of tripled tryptase values

In adverse reactions with shock, tripled tryptase values can support a diagnosis of anaphylaxis. A 51-year old physically fit woman experienced angio-oedema and hypotensive shock after irbesartan ingestion requiring noradrenaline infusion. Serum tryptase rose to three times the normal value. Total immunoglobulin E and skin prick tests were normal, however. As nonallergic increases in tryptase have been observed, e.g. during angio-oedema from angiotensin-converting enzyme inhibitors, and bradykinin itself can degranulate mast cells acutely, we interpret the reaction as a class effect. To our knowledge, our report is one of the first on shock and angio-oedema from irbesartan.