Functional role of angiotensin II AT2 receptor in modulation of AT1 receptor-mediated contraction in rat uterine artery: involvement of bradykinin and nitric oxide

Affinin, Isolated from Heliopsis longipes, Induces an Antihypertensive Effect That Involves CB1 Cannabinoid Receptors and TRPA1 and TRPV1 Channel Activation

In previous studies, we demonstrated that the ethanolic extract of Heliopsis longipes roots and its main alkamide, affinin, elicit a vasorelaxant effect through a mechanism involving activation of the gasotransmitter pathways and stimulation of cannabinoid type 1 receptors and transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1 channels. However, it has not yet been demonstrated whether the EEH and affinin are capable of lowering high blood pressure. Therefore, the aim of the present study was to determine the effect of the oral administration of the EEH and affinin on the systolic blood pressure of NG-nitro-L-arginine methyl ester-induced hypertensive rats and to explore the participation of cannabinoid receptors and transient receptor potential channels in the mechanism of action of this alkamide. Our results showed that the ethanolic extract of H. longipes and affinin significantly lowered systolic blood pressure and induced an improvement in endothelial function, which is associated with increased serum nitric oxide levels. Inhibition of cannabinoid type 1 receptors by rimonabant (3 mg/kg), transient receptor potential ankyrin 1 channels by HC-030031 (8 mg/kg), and transient receptor potential vanilloid 1 channels by capsazepine (5 mg/kg) significantly decreased the antihypertensive effect induced by affinin, suggesting that the blood pressure-lowering effect of this alkamide involves activation of cannabinoid type 1 receptors and transient receptor potential ankyrin 1 and transient receptor potential vanilloid 1 channels.

The antihypertensive action of C-phycocyanin is related to the prevention of angiotensin II-caused vascular dysfunction in chronic kidney disease

C-phycocyanin (CPC) is a photosynthetic protein found in Arthrospira maxima with a nephroprotective and antihypertensive activity that can prevent the development of hemodynamic alterations caused by chronic kidney disease (CKD). However, the complete nutraceutical activities are still unknown. This study aims to determine if the antihypertensive effect of CPC is associated with preventing the impairment of hemodynamic variables through delaying vascular dysfunction. Twenty-four normotensive male Wistar rats were divided into four groups: (1) sham + 4 mL/kg/d vehicle (100 mM of phosphate buffer, PBS) administered by oral gavage (og), (2) sham + 100 mg/kg/d og of CPC, (3) CKD induced by 5/6 nephrectomy (CKD) + vehicle, (4) CKD + CPC. One week after surgery, the CPC treatment began and was administrated daily for four weeks. At the end treatment, animals were euthanized, and their thoracic aorta was used to determine the vascular function and expression of AT1, AT2, and Mas receptors. CKD-induced systemic arterial hypertension (SAH) and vascular dysfunction by reducing the vasorelaxant response of angiotensin 1-7 and increasing the contractile response to angiotensin II. Also, CKD increased the expression of the AT1 and AT2 receptors and reduced the Mas receptor expression. Remarkably, the treatment with CPC prevented SAH, renal function impairment, and vascular dysfunction in the angiotensin system. In conclusion, the antihypertensive activity of CPC is associated with avoiding changes in the expression of AT1, AT2, and Mas receptors, preventing vascular dysfunction development and SAH in rats with CKD.

Female mice are protected from impaired parenchymal arteriolar TRPV4 function and impaired cognition in hypertension

Hypertension is a leading modifiable risk factor for cerebral small vessel disease. Our laboratory has shown that endothelium-dependent dilation in cerebral parenchymal arterioles (PAs) is dependent on transient receptor potential vanilloid 4 (TRPV4) activation, and this pathway is impaired in hypertension. This impaired dilation is associated with cognitive deficits and neuroinflammation. Epidemiological evidence suggests that women with midlife hypertension have an increased dementia risk that does not exist in age-matched men, though the mechanisms responsible for this are unclear. This study aimed to determine the sex differences in young, hypertensive mice to serve as a foundation for future determination of sex differences at midlife. We tested the hypothesis that young hypertensive female mice would be protected from the impaired TRPV4-mediated PA dilation and cognitive dysfunction observed in male mice. Angiotensin II (ANG II)-filled osmotic minipumps (800 ng/kg/min, 4 wk) were implanted in 16- to 19-wk-old male C56BL/6 mice. Age-matched female mice received either 800 ng/kg/min or 1,200 ng/kg/min ANG II. Sham-operated mice served as controls. Systolic blood pressure was elevated in ANG II-treated male mice and in 1,200 ng ANG II-treated female mice versus sex-matched shams. PA dilation in response to the TRPV4 agonist GSK1016790A (10-9-10-5 M) was impaired in hypertensive male mice, which was associated with cognitive dysfunction and neuroinflammation, reproducing our previous findings. Hypertensive female mice exhibited normal TRPV4-mediated PA dilation and were cognitively intact. Female mice also showed fewer signs of neuroinflammation than male mice. Determining the sex differences in cerebrovascular health in hypertension is critical for developing effective therapeutic strategies for women.NEW & NOTEWORTHY Vascular dementia is a significant public health concern, and the effect of biological sex on dementia development is not well understood. TRPV4 channels are essential regulators of cerebral parenchymal arteriolar function and cognition. Hypertension impairs TRPV4-mediated dilation and memory in male rodents. Data presented here suggest female sex protects against impaired TRPV4 dilation and cognitive dysfunction during hypertension. These data advance our understanding of the influence of biological sex on cerebrovascular health in hypertension.

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.

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.

Role of the central renin‑angiotensin system in hypertension (Review)

Present in more than one billion adults, hypertension is the most significant modifiable risk factor for mortality resulting from cardiovascular disease. Although its pathogenesis is not yet fully understood, the disruption of the renin-angiotensin system (RAS), consisting of the systemic and brain RAS, has been recognized as one of the primary reasons for several types of hypertension. Therefore, acquiring sound knowledge of the basic science of RAS and the under- lying mechanisms of the signaling pathways associated with RAS may facilitate the discovery of novel therapeutic targets with which to promote the management of patients with cardiovascular and kidney disease. In total, 4 types of angiotensin II receptors have been identified (AT1R-AT4R), of which AT1R plays the most important role in vasoconstriction and has been most extensively studied. It has been found in several regions of the brain, and its distribution is highly associated with that of angiotensin-like immunoreactivity in nerve terminals. The effect of AT1R involves the activation of multiple media and signaling pathways, among which the most important signaling pathways are considered to be AT1R/JAK/STAT and Ras/Raf/MAPK pathways. In addition, the regulation of the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and cyclic AMP response element-binding (CREB) pathways is also closely related to the effect of ATR1. Their mechanisms of action are related to pro-inflammatory and sympathetic excitatory effects. Central AT1R is involved in almost all types of hypertension, including spontaneous hypertension, salt-sensitive hypertension, obesity-induced hypertension, renovascular hypertension, diabetic hypertension, L-NAME-induced hypertension, stress-induced hypertension, angiotensin II-induced hyper- tension and aldosterone-induced hypertension. There are 2 types of central AT1R blockade, acute blockade and chronic blockade. The latter can be achieved by chemical blockade or genetic engineering. The present review article aimed to high- light the prevalence, functions, interactions and modulation means of central AT-1R in an effort to assist in the treatment of several pathological conditions. The identification of angiotensin-derived peptides and the development of AT-2R agonists may provide a wider perspective on RAS, as well as novel therapeutic strategies.

Activation of angiotensin type 2 receptor attenuates testosterone-induced hypertension and uterine vascular resistance in pregnant rats†

Preeclampsia is a pregnancy-related hypertensive disorder with unclear mechanisms. While hypersensitivity to angiotensin II via vasoconstrictive angiotensin type-1 receptor (AT1R) is observed in preeclampsia, the importance of vasodilatory angiotensin type-2 receptor (AT2R) in the control of vascular dysfunction is less clear. We assessed whether AT1R, AT2R, and endothelial nitric oxide synthase (eNOS) expression are altered in placental vessels of preeclamptic women and tested if ex vivo incubation with AT2R agonist Compound 21 (C21; 1 μM) could restore AT1R, AT2R, and eNOS balance. Further, using a rat model of gestational hypertension induced by elevated testosterone, we examined whether C21 (1 μg/kg/day, oral) could preserve AT1R and AT2R balance and improve blood pressure, uterine artery blood flow, and vascular function. Western blots revealed that AT1R protein level was higher while AT2R and eNOS protein were reduced in preeclamptic placental vessels, and AT2R agonist C21 decreased AT1R and increased AT2R and eNOS protein levels in preeclamptic vessels. In testosterone dams, blood pressure was higher, and uterine artery blood flow was reduced, and C21 treatment reversed these levels similar to those in controls dams. C21 attenuated the exaggerated Ang II contraction and improved endothelium-dependent vasorelaxation in uterine arteries of testosterone dams. These C21-mediated vascular effects were associated with decreased AT1R and increased AT2R and eNOS protein levels. C21 also increased serum nitrate/nitrite and bradykinin production in testosterone dams and attenuated the fetoplacental growth restriction. Thus, AT1R upregulation and AT2R downregulation are observed in preeclampsia and testosterone model, and increasing AT2R activity could help restore AT1R and AT2R balance and improve gestational vascular function.

Anti-Hypertensive Potential and Epigenetics of Angiotensin II type 2 Receptor (AT2R)

BACKGROUND

Renin angiotensin system (RAS) is a critical pathway involved in blood pressure regulation. Octapeptide, angiotensin II (Ang II), is a biologically active compound of RAS pathway which mediates its action by binding to either angiotensin II type 1 receptor (AT1R) or angiotensin II type 2 receptor (AT2R). Binding of Ang II to AT1R facilitates blood pressure regulation, whereas AT2R is primarily involved in wound healing and tissue remodeling.

OBJECTIVES

Recent studies have highlighted the additional role of AT2R to counterbalance the detrimental effects of AT1R. Activation of angiotensin II type 2 receptor using AT2R agonist has shown the effect on natriuresis and release of nitric oxide. Additionally, AT2R activation has been found to inhibit angiotensin converting enzyme (ACE) and enhance angiotensin receptor blocker (ARB) activity. These findings highlight the potential of AT2R as a novel therapeutic target against hypertension.

CONCLUSION

The potential role of AT2R highlights the importance of exploring additional mechanisms that might be crucial for AT2R expression. Epigenetic mechanisms, including DNA methylation and histone modification, have been explored vastly with relation to cancer, but the role of such mechanisms in the expression of AT2R has recently gained interest.

How ACE inhibitors transformed the renin-angiotensin system

The renin-angiotensin system (RAS) now underlies the successful treatment of almost 50% of the patients in cardiovascular medicine, with serious possibilities of extension to diabetes, Alzheimer's disease and cancer. This clinical transformation started just over 50 years ago, with the unexpected identification of a bradykinin-potentiating peptide from snake venom, as a potent inhibitor of ACE which led to the development of the first synthetic inhibitor, captopril, followed by the angiotensin receptor blockers. This article analyses the transformation of the RAS into its different stages, from academic experiments to clinical use and back to the laboratory, identifying the critical events involved, both clinical and scientific. The analysis also assesses the contributions of chance, coincidence, and conviction that were crucial in this transformation. Although questions remain, the transformation of the RAS over the past five decades provides a success story for medicine, for pharmacology, and, most significantly, for patients.

Pregnancy upregulates angiotensin type 2 receptor expression and increases blood flow in uterine arteries of rats

Normal pregnancy is associated with decreased uterine vascular contraction and increased blood flow even though angiotensin II (AngII) levels are increased. AngII not only activates the angiotensin type 1 receptor (AT1R) to mediate vasoconstriction but also angiotensin type 2 receptor (AT2R) to cause vasodilation. We hypothesized that upregulation of AT2R expression and function accounts for increased uterine artery blood flow during pregnancy. Virgin, pregnant (at different days of gestation) and post-partum Sprague-Dawley rats were used to determine uterine artery hemodynamics using micro ultrasound and plasma angiotensin II levels by ELISA. Isolated uterine arteries were examined for AT1R and AT2R expression and isometric contraction/relaxation. Plasma AngII levels were steady up to mid-pregnancy, increased as pregnancy advanced, reaching a peak in late pregnancy, and then restored to pre-pregnant levels after delivery. The pattern of increase in AngII levels mirrored a parallel increase in uterine blood flow. AT1R expression did not change, but AT2R expression increased during pregnancy correlating with uterine blood flow increase. Treatment with the AT2R antagonist PD123319 reduced uterine arterial blood flow. Vasoconstriction to angiotensin II was blunted in pregnant rats. Treatment with PD123319 caused greater enhancement of AngII contraction in pregnant than virgin rats. Ex vivo exposure of estradiol to uterine arterial rings dose dependently upregulated AT2R expression, that was inhibited by estrogen receptor antagonist. These results demonstrate that elevated AngII levels during gestation induce an increase in uterine blood flow via heightened AT2R-mediated signaling. Estrogens appear to directly upregulate uterine vascular AT2R independent of any endogenous factors.

Perivascular adipose tissue (PVAT) in atherosclerosis: a double-edged sword

Perivascular adipose tissue (PVAT), the adipose tissue that surrounds most of the vasculature, has emerged as an active component of the blood vessel wall regulating vascular homeostasis and affecting the pathogenesis of atherosclerosis. Although PVAT characteristics resemble both brown and white adipose tissues, recent evidence suggests that PVAT develops from its own distinct precursors implying a closer link between PVAT and vascular system. Under physiological conditions, PVAT has potent anti-atherogenic properties mediated by its ability to secrete various biologically active factors that induce non-shivering thermogenesis and metabolize fatty acids. In contrast, under pathological conditions (mainly obesity), PVAT becomes dysfunctional, loses its thermogenic capacity and secretes pro-inflammatory adipokines that induce endothelial dysfunction and infiltration of inflammatory cells, promoting atherosclerosis development. Since PVAT plays crucial roles in regulating key steps of atherosclerosis development, it may constitute a novel therapeutic target for the prevention and treatment of atherosclerosis. Here, we review the current literature regarding the roles of PVAT in the pathogenesis of atherosclerosis.

Direct Activation of Angiotensin II Type 2 Receptors Enhances Muscle Microvascular Perfusion, Oxygenation, and Insulin Delivery in Male Rats

Angiotensin II receptors regulate muscle microvascular recruitment and the delivery of nutrients, oxygen, and insulin to muscle. Although angiotensin type 1 receptor antagonism increases muscle microvascular perfusion and insulin action, angiotensin type 2 receptor blockade markedly restricts muscle microvascular blood volume and decreases muscle delivery of insulin. To examine the effects of direct type 2 receptor stimulation using Compound 21 (C21) on microvascular perfusion, insulin delivery and action, and tissue oxygenation in muscle, overnight-fasted adult male rats were infused with C21 systemically. C21 potently increased microvascular blood volume without altering microvascular flow velocity or blood pressure, resulting in a net increase in microvascular blood flow in muscle. This was associated with a substantial increase in muscle interstitial oxygen saturation and insulin delivery into the skeletal and cardiac muscle. These effects were neutralized by coinfusion of the type 2 receptor antagonist or nitric oxide synthase inhibitor. Superimposing C21 infusion on insulin infusion increased insulin-mediated whole body glucose disposal by 50%. C21 significantly relaxed the preconstricted distal saphenous artery ex vivo. We have concluded that direct type 2 receptor stimulation markedly increases muscle microvascular perfusion through nitric oxide biosynthesis and enhances insulin delivery and action in muscle. These findings provide a physiologic mechanistic insight into type 2 receptor modulation of insulin action and suggest that type 2 receptor agonists might have therapeutic potential in the management of diabetes and its associated complications.

Prenatal Hypoxia Induced Dysfunction in Cerebral Arteries of Offspring Rats

BACKGROUND

Hypoxia during pregnancy could cause abnormal development and lead to increased risks of vascular diseases in adults. This study determined angiotensin II (AII)-mediated vascular dysfunction in offspring middle cerebral arteries (MCA).

METHODS AND RESULTS

Pregnant rats were subjected to hypoxia. Vascular tension in offspring MCA by AII with or without inhibitors, calcium channel activities, and endoplasmic reticulum calcium stores were tested. Whole-cell patch clamping was used to investigate voltage-dependent calcium channel currents. mRNA expression was tested using quantitative real-time polymerase chain reaction. AII-mediated MCA constriction was greater in male offspring exposed to prenatal hypoxia. AT1 and AT2 receptors were involved in the altered AII-mediated vasoconstriction. Prenatal hypoxia increased baseline activities of L-type calcium channel currents in MCA smooth muscle cells. However, calcium currents stimulated by AII were not significantly changed, whereas nifedipine inhibited AII-mediated vasoconstrictions in the MCA. Activities of IP3/ryanodine receptor-operated calcium channels, endoplasmic reticulum calcium stores, and sarcoendoplasmic reticulum membrane Ca2+-ATPase were increased. Prenatal hypoxia also caused dysfunction of vasodilatation via the endothelium NO synthase. The mRNA expressions of AT1A, AT1B, AT2R, Cav1.2α1C, Cav3.2α1H, and ryanodine receptor RyR2 were increased in the prenatal-hypoxia group.

CONCLUSIONS

Hypoxia in pregnancy could induce dysfunction in both contraction and dilation in the offspring MCA. AII-increased constriction in the prenatal-hypoxia group was not mainly dependent on the L-type and T-type calcium channels; it might predominantly rely on the AII receptors, IP3/ryanodine receptors, and the endoplasmic reticulum calcium store as well as calcium ATPase.

Epochs in the depressor/pressor balance of the renin-angiotensin system

The renin-angiotensin system (RAS) plays a commanding role in the regulation of extracellular fluid homoeostasis. Tigerstadt and Bergman first identified the RAS more than two centuries ago. By the 1980s a voyage of research and discovery into the mechanisms and actions of this system led to the development of drugs that block the RAS, which have become the mainstay for the treatment of cardiovascular and renal disease. In the last 25 years new components of the RAS have come to light, including the angiotensin type 2 receptor (AT2R) and the angiotensin-converting enzyme 2 (ACE2)/angiotensin-(1-7) [Ang(1-7)]/Mas receptor (MasR) axis. These have been shown to counter the classical actions of angiotensin II (AngII) at the predominant angiotensin type 1 receptor (AT1R). Our studies, and those of others, have demonstrated that targeting these depressor RAS pathways may be therapeutically beneficial. It is apparent that the evolution of both the pressor and depressor RAS pathways is distinct throughout life and that the depressor/pressor balance of the RAS vary between the sexes. These temporal patterns of expression suggest that therapies targeting the RAS could be optimized for discrete epochs in life.

AT2 Receptors: Potential Therapeutic Targets for Hypertension

The renin-angiotensin system (RAS) is arguably the most important and best studied hormonal system in the control of blood pressure (BP) and the pathogenesis of hypertension. The RAS features its main effector angiotensin II (Ang II) acting via its 2 major receptors, angiotensin type-1(AT1R) and type-2 (AT2R). In general, AT2Rs oppose the detrimental actions of Ang II via AT1Rs. AT2R activation induces vasodilation and natriuresis, but its effects to lower BP in hypertension have not been as clear as anticipated. Recent studies, however, have demonstrated that acute and chronic AT2R stimulation can induce natriuresis and lower BP in the Ang II infusion model of experimental hypertension. AT2R activation induces receptor recruitment from intracellular sites to the apical plasma membranes of renal proximal tubule cells via a bradykinin, nitric oxide, and cyclic guanosine 3',5' monophosphate signaling pathway that results in internalization and inactivation of sodium (Na+) transporters Na+-H+ exchanger-3 and Na+/K+ATPase. These responses do not require the presence of concurrent AT1R blockade and are effective both in the prevention and reversal of hypertension. This review will address the role of AT2Rs in the control of BP and Na+ excretion and the case for these receptors as potential therapeutic targets for hypertension in humans.

A Low-Protein Diet Enhances Angiotensin II Production in the Lung of Pregnant Rats but not Nonpregnant Rats

Pulmonary angiotensin II production is enhanced in pregnant rats fed a low-protein (LP) diet. Here we assessed if LP diet induces elevations in angiotensin II production in nonpregnant rats and whether Ace expression and ACE activity in lungs are increased. Nonpregnant rats were fed a normal (CT) or LP diet for 8, 12, or 17 days and timed pregnant rats fed for 17 days from Day 3 of pregnancy. Plasma angiotensin II, expressions of Ace and Ace2, and activities of these proteins in lungs, kidneys, and plasma were measured. These parameters were compared among nonpregnant rats or between nonpregnant and pregnant rats fed different diets. Major findings are as follows: (1) plasma angiotensin II levels were slightly higher in the LP than CT group on Days 8 and 12 in nonpregnant rats; (2) expression of Ace and Ace2 and abundance and activities of ACE and ACE2 in lungs, kidneys, and plasma of nonpregnant rats were unchanged by LP diet except for minor changes; (3) the abundance and activities of ACE in lungs of pregnant rats fed LP diet were greater than nonpregnant rats, while those of ACE2 were decreased. These results indicate that LP diet-induced increase in pulmonary angiotensin II production depends on pregnancy.

Attenuation of angiotensin type 2 receptor function in the rostral ventrolateral medullary pressor area of the spontaneously hypertensive rat

We hypothesized that blockade of angiotensin II type 2 receptors (AT2Rs) in the rostral ventrolateral medullary pressor area (RVLM) may elicit sympathoexcitatory responses which are smaller in hypertensive rats compared to normotensive rats. This hypothesis was tested in urethane-anesthetized, artificially ventilated male 14-week-old spontaneously hypertensive rats (SHR). Age-matched male Wistar-Kyoto rats (WKY) and Wistar rats were used as controls. PD123319 (AT2R antagonist) was microinjected into the RVLM and mean arterial pressure (MAP), heart rate (HR) and greater splanchnic nerve activity (GSNA) were recorded. Increases in MAP, HR and GSNA elicited by unilateral microinjections of PD123319 into the RVLM were significantly smaller in SHR when compared with those in WKY and Wistar rats. Unilateral microinjections of l-glutamate (l-Glu) into the RVLM elicited greater increases in MAP and GSNA in SHR compared to those in WKY. AT2R immunoreactivity was demonstrated in the RVLM neurons which were retrogradely labeled from the intermediolateral cell column (IML) of the spinal cord. These results indicate that AT2Rs are present on the RVLM neurons projecting to the IML and their blockade results in sympathoexcitatory responses. Activation of AT2Rs has an inhibitory influence in the RVLM and these receptors are tonically active. Attenuation of the function of AT2Rs in the RVLM may play a role in genesis and/or maintenance of hypertension in SHR.

Enhanced mesenteric arterial responsiveness to angiotensin II is androgen receptor-dependent in prenatally protein-restricted adult female rat offspring

Gestational protein restriction results in intrauterine growth restriction and hypertension in adult female growth-restricted rats. Enhanced vascular responsiveness to angiotensin II is observed, and blockade of the renin-angiotensin system abolishes hypertension in adult growth-restricted rats, suggesting that the renin-angiotensin system contributes to intrauterine growth restriction-induced hypertension. Moreover, growth-restricted adult rats have higher plasma testosterone levels, and antiandrogen treatment abolishes hypertension, indicating an important role for testosterone. We hypothesized that androgens may play a pivotal role in the enhanced responsiveness to Ang II and hypertension. Female offspring of pregnant rats fed 20% protein (control) or 6% protein diet (protein restricted), at 6 mo of age, were studied. Plasma testosterone and mean arterial pressure in protein-restricted offspring were significantly higher compared to controls. Flutamide treatment (10 mg/kg/day subcutaneously for 10 days) reduced mean arterial pressure in protein-restricted offspring but was without significant effect in controls. Vascular Agtr1/Agtr2 ratio was significantly higher in protein-restricted offspring, an effect that was reversed by flutamide. Flutamide treatment did not have any effect on Agtr1/Agtr2 ratio in controls. Enhanced contractile response to angiotensin II in mesenteric arteries was observed in protein-restricted offspring compared with control. Flutamide treatment reversed the enhanced contractile response to angiotensin II in protein-restricted offspring without significant effect in controls. Vascular reactivity to phenylephrine was similar between the control and protein-restricted offspring with and without flutamide treatment, suggesting that enhanced contractile response and flutamide's reversal effect is specific to angiotensin II. These results suggest that prenatally protein-restricted rats exhibit an enhanced responsiveness to angiotensin II that is testosterone-dependent.

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.

Perivascular fat, AMP-activated protein kinase and vascular diseases

Perivascular adipose tissue (PVAT) is an active endocrine and paracrine organ that modulates vascular function, with implications for the pathophysiology of cardiovascular disease (CVD). Adipocytes and stromal cells contained within PVAT produce mediators (adipokines, cytokines, reactive oxygen species and gaseous compounds) with a range of paracrine effects modulating vascular smooth muscle cell contraction, proliferation and migration. However, the modulatory effect of PVAT on the vascular system in diseases, such as obesity, hypertension and atherosclerosis, remains poorly characterized. AMP-activated protein kinase (AMPK) regulates adipocyte metabolism, adipose biology and vascular function, and hence may be a potential therapeutic target for metabolic disorders such as type 2 diabetes mellitus (T2DM) and the vascular complications associated with obesity and T2DM. The role of AMPK in PVAT or the actions of PVAT have yet to be established, however. Activation of AMPK by pharmacological agents, such as metformin and thiazolidinediones, may modulate the activity of PVAT surrounding blood vessels and thereby contribute to their beneficial effect in cardiometabolic diseases. This review will provide a current perspective on how PVAT may influence vascular function via AMPK. We will also attempt to demonstrate how modulating AMPK activity using pharmacological agents could be exploited therapeutically to treat cardiometabolic diseases.

Angiotensin-(1-7) and angiotensin-(1-9): function in cardiac and vascular remodelling

The RAS (renin-angiotensin system) is integral to cardiovascular physiology; however, dysregulation of this system largely contributes to the pathophysiology of CVD (cardiovascular disease). It is well established that AngII (angiotensin II), the main effector of the RAS, engages the AT1R (angiotensin type 1 receptor) and promotes cell growth, proliferation, migration and oxidative stress, all processes which contribute to remodelling of the heart and vasculature, ultimately leading to the development and progression of various CVDs, including heart failure and atherosclerosis. The counter-regulatory axis of the RAS, which is centred on the actions of ACE2 (angiotensin-converting enzyme 2) and the resultant production of Ang-(1-7) [angiotensin-(1-7)] from AngII, antagonizes the actions of AngII via the receptor Mas, thereby providing a protective role in CVD. More recently, another ACE2 metabolite, Ang-(1-9) [angiotensin-(1-9)], has been reported to be a biologically active peptide within the counter-regulatory axis of the RAS. The present review will discuss the role of the counter-regulatory RAS peptides Ang-(1-7) and Ang-(1-9) in the cardiovascular system, with a focus on their effects in remodelling of the heart and vasculature.

Vascular Tissue Contractility Changes Following Late Gestational Exposure to Multi-Walled Carbon Nanotubes or their Dispersing Vehicle in Sprague Dawley Rats

Multi-walled carbon nanotubes (MWCNTs) are increasingly used in industry and in nanomedicine raising safety concerns, especially during unique life-stages such as pregnancy. We hypothesized that MWCNT exposure during pregnancy will increase vascular tissue contractile responses by increasing Rho kinase signaling. Pregnant (17-19 gestational days) and non-pregnant Sprague Dawley rats were exposed to 100 μg/kg of MWCNTs by intratracheal instillation or intravenous administration. Vasoactive responses of uterine, mesenteric, aortic and umbilical vessels were studied 24 hours post-exposure by wire myography. The contractile responses of the vessel segments were different between the pregnant and non-pregnant rats, following MWCNT exposure. Maximum stress generation in the uterine artery segments from the pregnant rats following pulmonary MWCNT exposure was increased in response to angiotensin II by 4.9 mN/mm² (+118%), as compared to the naïve response and by 2.6 mN/mm² (+40.7%) as compared to the vehicle exposed group. Following MWCNT exposure, serotonin induced approximately 4 mN/mm² increase in stress generation of the mesenteric artery from both pregnant and non-pregnant rats as compared to the vehicle response. A significant contribution of the dispersion medium was identified as inducing changes in the contractile properties following both pulmonary and intravenous exposure to MWCNTs. Wire myographic studies in the presence of a Rho kinase inhibitor and RhoA and Rho kinase mRNA/protein expression of rat aortic endothelial cells were unaltered following exposure to MWCNTs, suggesting absent/minimal contribution of Rho kinase to the enhanced contractile responses following MWCNT exposure. The reactivity of the umbilical vein was not changed; however, mean fetal weight gain was reduced with dispersion media and MWCNT exposure by both routes. These results suggest a susceptibility of the vasculature during gestation to MWCNT and their dispersion media-induced vasoconstriction, predisposing reduced fetal growth during pregnancy.

Aliskiren reduces myocardial ischemia-reperfusion injury by a bradykinin B2 receptor- and angiotensin AT2 receptor-mediated mechanism

Angiotensin-converting enzyme inhibitors and angiotensin AT1 receptor blockers reduce myocardial ischemia-reperfusion injury via bradykinin B2 receptor- and angiotensin AT2 receptor-mediated mechanisms. The renin inhibitor aliskiren increases cardiac tissue kallikrein and bradykinin levels. In the present study, we investigated the effect of aliskiren on myocardial ischemia-reperfusion injury and the roles of B2 and AT2 receptors in this effect. Female Sprague-Dawley rats were treated with aliskiren (10 mg/kg per day) and valsartan (30 mg/kg per day), alone or in combination, together with the B2 receptor antagonist icatibant (0.5 mg/kg per day) or the AT2 receptor antagonist PD123319 (30 mg/kg per day), for 4 weeks before myocardial ischemia-reperfusion injury. Aliskiren increased cardiac bradykinin levels and attenuated valsartan-induced increases in plasma angiotensin II levels. In vehicle-treated rats, myocardial infarct size (% area at risk, mean±SEM, n=7-13) was 43±3%. This was reduced to a similar extent by aliskiren, valsartan, and their combination to 24±3%, 25±3%, and 22±2%, respectively. Icatibant reversed the cardioprotective effects of aliskiren and the combination of aliskiren plus valsartan, but not valsartan alone, indicating that valsartan-induced cardioprotection was not mediated by the B2 receptor. PD123319 reversed the cardioprotective effects of aliskiren, valsartan, and the combination of aliskiren plus valsartan. Aliskiren protects the heart from myocardial ischemia-reperfusion injury via a B2 receptor- and AT2 receptor-mediated mechanism, whereas cardioprotection by valsartan is mediated via the AT2 receptor. In addition, aliskiren attenuates valsartan-induced increases in angiotensin II levels, thus preventing AT2 receptor-mediated cardioprotection by valsartan.

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.

Emerging role of angiotensin type 2 receptor (AT2R)/Akt/NO pathway in vascular smooth muscle cell in the hyperthyroidism

Hyperthyroidism is characterized by increased vascular relaxation and decreased vascular contraction and is associated with augmented levels of triiodothyronine (T3) that contribute to the diminished systemic vascular resistance found in this condition. T3 leads to augmented NO production via PI3K/Akt signaling pathway, which in turn causes vascular smooth muscle cell (VSMC) relaxation; however, the underlying mechanisms involved remain largely unknown. Evidence from human and animal studies demonstrates that the renin-angiotensin system (RAS) plays a crucial role in vascular function and also mediates some of cardiovascular effects found during hyperthyroidism. Thus, in this study, we hypothesized that type 2 angiotensin II receptor (AT2R), a key component of RAS vasodilatory actions, mediates T3 induced-decreased vascular contraction. Marked induction of AT2R expression was observed in aortas from T3-induced hyperthyroid rats (Hyper). These vessels showed decreased protein levels of the contractile apparatus: α-actin, calponin and phosphorylated myosin light chain (p-MLC). Vascular reactivity studies showed that denuded aortic rings from Hyper rats exhibited decreased maximal contractile response to angiotensin II (AngII), which was attenuated in aortic rings pre-incubated with an AT2R blocker. Further study showed that cultured VSMC stimulated with T3 (0.1 µmol/L) for 24 hours had increased AT2R gene and protein expression. Augmented NO levels and decreased p-MLC levels were found in VSMC stimulated with T3, both of which were reversed by a PI3K/Akt inhibitor and AT2R blocker. These findings indicate for the first time that the AT2R/Akt/NO pathway contributes to decreased contractile responses in rat aorta, promoted by T3, and this mechanism is independent from the endothelium.

Losartan increases muscle insulin delivery and rescues insulin's metabolic action during lipid infusion via microvascular recruitment

Insulin delivery and transendothelial insulin transport are two discrete steps that limit muscle insulin action. Angiotensin II type 1 receptor (AT1R) blockade recruits microvasculature and increases glucose use in muscle. Increased muscle microvascular perfusion is associated with increased muscle delivery and action of insulin. To examine the effect of acute AT1R blockade on muscle insulin uptake and action, rats were studied after an overnight fast to examine the effects of losartan on muscle insulin uptake (protocol 1), microvascular perfusion (protocol 2), and insulin's microvascular and metabolic actions in the state of insulin resistance (protocol 3). Endothelial cell insulin uptake was assessed, using (125)I-insulin as tracer. Systemic lipid infusion was used to induce insulin resistance. Losartan significantly increased muscle insulin uptake (∼60%, P < 0.03), which was associated with a two- to threefold increase in muscle microvascular blood volume (MBV; P = 0.002) and flow (MBF; P = 0.002). Losartan ± angiotensin II had no effect on insulin internalization in cultured endothelial cells. Lipid infusion abolished insulin-mediated increases in muscle MBV and MBF and lowered insulin-stimulated whole body glucose disposal (P = 0.0001), which were reversed by losartan administration. Inhibition of nitric oxide synthase abolished losartan-induced muscle insulin uptake and reversal of lipid-induced metabolic insulin resistance. We conclude that AT1R blockade increases muscle insulin uptake mainly via microvascular recruitment and rescues insulin's metabolic action in the insulin-resistant state. This may contribute to the clinical findings of decreased cardiovascular events and new onset of diabetes in patients receiving AT1R blockers.

AT2 receptors: beneficial counter-regulatory role in cardiovascular and renal function

The renin-angiotensin system (RAS) is a coordinated hormonal cascade intimately involved in cardiovascular and renal control and blood pressure regulation. Angiotensin II (Ang II), the major RAS effector peptide, binds two distinct receptors, the angiotensin type-1 receptor (AT(1)R) and the angiotensin type-2 (AT(2)R) receptor. The vast majority of the physiological actions of Ang II, almost all of them detrimental, are mediated by AT(1)Rs. In contrast, AT(2)Rs negatively modulate the actions of AT(1)Rs under the majority of circumstances and generally possess beneficial effects. AT(2)Rs induce vasodilation in both resistance and capacitance vessels, mediating natriuresis directly and via interactions with dopamine D1 receptors in the renal proximal tubule. AT(2)Rs inhibit renin biosynthesis and secretion and protect the kidneys from inflammation and ischemic injury. Our understanding of the exact role of AT(2)Rs in physiology and pathophysiology continues to expand; the purpose of this review is to provide an up-to-date summary of the functional role of AT(2)Rs at the organ, tissue, cellular, and subcellular levels with emphasis on the vascular and renal actions that bear on blood pressure regulation and hypertension.

Protein restriction to pregnant rats increases the plasma levels of angiotensin II and expression of angiotensin II receptors in uterine arteries

Whether gestational protein restriction affects the renin-angiotensin system (RAS) in uterine artery remains unknown. In this study, we hypothesized that gestational protein restriction alters the expression of RAS components in uterine artery. In study one, time-scheduled pregnant Sprague Dawley rats were fed a normal or low-protein (LP) diet from Day 3 of pregnancy until they were killed at Days 19 and 22. The uterine arteries were collected and used for gene expression of Ace, Ace2, Agtr1a, Agtr1b, Agtr2, Esr1, and Esr2 by quantitative real-time PCR and/or Western blotting. LP increased plasma levels of angiotensin II in pregnant rats. In the uterine artery, the expressions of Agtr1a, Agtr1b, and Esr1 were increased by LP at Days 19 and 22 of pregnancy, whereas the abundance of AGTR1 and AGTR2 was increased by LP at Day 19 of pregnancy. The expression of Ace2 was not detectable in rat uterine artery. In study two, virgin female rats were ovariectomized and implanted with either 17beta-estradiol (E2), progesterone (P4), both E2 and P4, or placebo pellets until they were killed 7 days later. In rat uterine artery, E2 and P4 reduced the expression of Agtr1a, and E2 increased the expression of Agtr1b and Agtr2, but neither E2 nor P4 regulated the expression of Ace. These results indicate that gestational protein restriction induces an increase in Agtr1 expression in uterine artery, and thus may exacerbate the vasoconstriction to elevated angiotensin II present in maternal circulation, and that female sex hormones also play a role in this process.

Differential mechanisms of ang (1-7)-mediated vasodepressor effect in adult and aged candesartan-treated rats

Angiotensin (1-7) (Ang (1-7)) causes vasodilator effects in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) via angiotensin type 2 receptors (AT₂R). However, the role of vascular AT₂R in aging is not known. Therefore, we examined the effect of aging on Ang (1-7)-mediated vasodepressor effects and vascular angiotensin receptor localization in aging. Blood pressure was measured in conscious adult (~17 weeks) and aged (~19 months) normotensive rats that received drug combinations in a randomised fashion over a 4-day protocol: (i) Ang (1-7) alone, (ii) AT₁R antagonist, candesartan, alone, (iii) Ang (1-7) and candesartan, or (iv) Ang-(1-7), candesartan, and the AT₂R antagonist, PD123319. In a separate group of animals, the specific MasR antagonist, A779, was administered in place of PD123319. Receptor localisation was also assessed in aortic sections from adult and aged WKY rats by immunofluorescence. Ang (1-7) reduced blood pressure (~15 mmHg) in adult normotensive rats although this effect was dependant on the background dose of candesartan. This depressor effect was reversed by AT₂R blockade. In aged rats, the depressor effect of Ang (1-7) was evident but was now inhibited by either AT₂R blockade or MasR blockade. At the same time, AT₂R, MasR, and ACE2 immunoreactivity was markedly elevated in aortic sections from aged animals. These results indicate that the Ang (1-7)-mediated depressor effect was preserved in aged animals. Whereas Ang (1-7) effects were mediated exclusively via stimulation of AT₂R in adult WKY, with aging the vasodepressor effect of Ang (1-7) involved both AT₂R and MasR.

Angiotensin II receptors modulate muscle microvascular and metabolic responses to insulin in vivo

OBJECTIVE

Angiotensin (ANG) II interacts with insulin-signaling pathways to regulate insulin sensitivity. The type 1 (AT(1)R) and type 2 (AT(2)R) receptors reciprocally regulate basal perfusion of muscle microvasculature. Unopposed AT(2)R activity increases muscle microvascular blood volume (MBV) and glucose extraction, whereas unopposed AT(1)R activity decreases both. The current study examined whether ANG II receptors modulate muscle insulin delivery and sensitivity.

RESEARCH DESIGN AND METHODS

Overnight-fasted rats were studied. In protocol 1, rats received a 2-h infusion of saline, insulin (3 mU/kg/min), insulin plus PD123319 (AT(2)R blocker), or insulin plus losartan (AT(1)R blocker, intravenously). Muscle MBV, microvascular flow velocity, and microvascular blood flow (MBF) were determined. In protocol 2, rats received (125)I-insulin with or without PD123319, and muscle insulin uptake was determined.

RESULTS

Insulin significantly increased muscle MBV and MBF. AT(2)R blockade abolished insulin-mediated increases in muscle MBV and MBF and decreased insulin-stimulated glucose disposal by ~30%. In contrast, losartan plus insulin increased muscle MBV by two- to threefold without further increasing insulin-stimulated glucose disposal. Plasma nitric oxide increased by >50% with insulin and insulin plus losartan but not with insulin plus PD123319. PD123319 markedly decreased muscle insulin uptake and insulin-stimulated Akt phosphorylation.

CONCLUSIONS

We conclude that both AT(1)Rs and AT(2)Rs regulate insulin's microvascular and metabolic action in muscle. Although AT(1)R activity restrains muscle metabolic responses to insulin via decreased microvascular recruitment and insulin delivery, AT(2)R activity is required for normal microvascular responses to insulin. Thus, pharmacologic manipulation aimed at increasing the AT(2)R-to-AT(1)R activity ratio may afford the potential to improve muscle insulin sensitivity and glucose metabolism.

LOX-1 and angiotensin receptors, and their interplay

The renin-angiotensin system (RAS) plays an important role in regulating blood pressure, water-salt balance and the pathogenesis of cardiovascular diseases. Angiotensin II (Ang II) is the physiologically active mediator and mediates the main pathophysiological actions in RAS. Ang II exerts the effects by activating its receptors, primarily type 1 (AT1R) and type 2 (AT2R). Most of the known pathophysiological effects of Ang II are mediated by AT1R activation. The precise physiological function of AT2R is still not clear. Generally, AT2R is considered to oppose the effects of AT1R. Lectin-like oxidized low-density lipoprotein scavenger receptor-1 (LOX-1) is one of the major receptors responsible for binding, internalizing and degrading ox-LDL. The activation of LOX-1 has been known to be related to many pathophysiological events, including endothelial dysfunction and injury, fibroblast growth, and vascular smooth muscle cell hypertrophy. Many of these alterations are present in atherosclerosis, hypertension, and myocardial ischemia and remodeling. A growing body of evidence suggests the existence of a cross-talk between LOX-1 and Ang II receptors. Their interplays are embodied in the reciprocal regulation of their expression and activity. Their interplays are involved in a series of signals. Recent studies suggests that reactive oxygen species (ROS), nitric oxide (NO), protein kinase C (PKC) and mitogen activated protein kinases (MAPKs) are important signals responsible for their cross-talk. This paper reviews these aspects of dyslipidemia and RAS activation.

Lack of heterologous receptor desensitization induced by angiotensin II type 1 receptor activation in isolated normal rat thoracic aorta

We tested whether heterologous receptor desensitization induced by activation of AT1 receptors may explain the purported relaxation produced by angiotensin II in normal rat aorta. Also, the role for AT2 receptors in the promotion of vasodilation was studied. In endothelium-intact and endothelium-denuded aortic rings, angiotensin II elicited biphasic contractions, which were significantly depressed when repeated in each tissue. Angiotensin II produced biphasic responses on phenylephrine preconstricted endothelium-intact and endothelium-denuded tissues, without reducing precontractile tone. These responses were abolished in the presence of the AT1 receptor antagonist losartan, but no relaxing responses to angiotensin II were uncovered. PD123319 did not influence angiotensin II responses in endothelium-intact tissues precontracted with phenylephrine; thus, under AT2 receptors blockade the contractile effects of angiotensin II were not overexposed. In conclusion, angiotensin II-induced biphasic responses can be attributed to AT1 receptors activation and rapid desensitization with time. Desensitization proved to be homologous in nature, since precontractile tone induced by phenylephrine was not depressed by angiotensin II (i.e., angiotensin II did not induce heterologous α1-adrenergic receptors desensitization). We found no functional evidence of the participation of AT2 receptors in angiotensin II elicited biphasic contractions. Angiotensin II does not exert relaxant effects in normal rat aorta.

The potential role of the angiotensin subtype 2 receptor in cardiovascular protection

Advances in our knowledge of the renin-angiotensin system have led to better understanding of the mechanisms contributing to the development of cardiovascular, renal, and metabolic disorders. Similarly, the discovery of new components of this system offers opportunities to develop new therapeutic tools to manage these diseases. Angiotensin subtype 2 (AT2) receptor represents one of those components with the potential for improving cardiovascular protection. Current knowledge suggests that the AT2 receptor antagonizes the effects of the angiotensin subtype 1 receptor. AT2 receptor activation is linked to vasodilation, nitric oxide production, and antiproliferative and anti-inflammatory effects. The role of the AT2 receptor in inducing natriuresis, inhibiting renin release, limiting atherosclerosis, and promoting remodeling after myocardial infarction is reviewed in this article.

The impact of maternal protein restriction during rat pregnancy upon renal expression of angiotensin receptors and vasopressin-related aquaporins

BACKGROUND

Maternal protein restriction during rat pregnancy is known to impact upon fetal development, growth and risk of disease in later life. It is of interest to understand how protein undernutrition influences the normal maternal adaptation to pregnancy. Here we investigated the mechanisms regulating renal haemodynamics and plasma volume during pregnancy, in the context of both normal and reduced plasma volume expansion. The study focused on expression of renal angiotensin receptors (ATR) and vasopressin-related aquaporins (AQP), hypothesising that an alteration in the balance of these proteins would be associated with pregnancy per se and with compromised plasma volume expansion in rats fed a low-protein diet.

METHODS

Female Wistar rats were mated and fed a control (18% casein) or low-protein (9% casein) diet during pregnancy. Animals were anaesthetised on days 5, 10, 15 and 20 of gestation (n = 8/group/time-point) for determination of plasma volume using Evans Blue dye, prior to euthanasia and collection of tissues. Expression of the ATR subtypes and AQP2, 3 and 4 were assessed in maternal kidneys by PCR and western blotting. 24 non-pregnant Wistar rats underwent the same procedure at defined points of the oestrous cycle.

RESULTS

As expected, pregnancy was associated with an increase in blood volume and haemodilution impacted upon red blood cell counts and haemoglobin concentrations. Expression of angiotensin II receptors and aquaporins 2, 3 and 4 was stable across all stages of the oestrus cycle. Interesting patterns of intra-renal protein expression were observed in response to pregnancy, including a significant down-regulation of AQP2. In contrast to previous literature and despite an apparent delay in blood volume expansion in low-protein fed rats, blood volume did not differ significantly between groups of pregnant animals. However, a significant down-regulation of AT2R protein expression was observed in low-protein fed animals alongside a decrease in creatinine clearance.

CONCLUSION

Regulatory systems involved in the pregnancy-induced plasma volume expansion are susceptible to the effects of maternal protein restriction.

Angiotensin II type 1 and type 2 receptors regulate basal skeletal muscle microvascular volume and glucose use

Angiotensin II causes vasoconstriction via the type 1 receptor (AT(1)R) and vasodilatation through the type 2 receptor (AT(2)R). Both are expressed in muscle microvasculature, where substrate exchanges occur. Whether they modulate basal muscle microvascular perfusion and substrate metabolism is not known. We measured microvascular blood volume (MBV), a measure of microvascular surface area and perfusion, in rats during systemic infusion of angiotensin II at either 1 or 100 ng/kg per minute. Each caused a significant increase in muscle MBV. Likewise, administration of the AT(1)R blocker losartan increased muscle MBV by >3-fold (P<0.001). Hindleg glucose extraction and muscle interstitial oxygen saturation simultaneously increased by 2- to 3-fold. By contrast, infusing AT(2)R antagonist PD123319 significantly decreased muscle MBV by >or=80% (P<0.001). This was associated with a significant decrease in hindleg glucose extraction and muscle oxygen saturation. AT(2)R antagonism and inhibition of NO synthase each blocked the losartan-induced increase in muscle MBV and glucose uptake. In conclusion, angiotensin II acts on both AT(1)R and AT(2)R to regulate basal muscle microvascular perfusion. Basal AT(1)R tone restricts muscle MBV and glucose extraction, whereas basal AT(2)R activity increases muscle MBV and glucose uptake. Pharmacological manipulation of the balance of AT(1)R and AT(2)R activity affords the potential to improve glucose metabolism.

Endothelium-dependent relaxation factor released by perivascular adipose tissue

OBJECTIVE

Recent studies have demonstrated that perivascular adipose tissue (PVAT) releases vascular relaxation factor(s), but the identity of this relaxation factor remains unknown. Here, we examined if angiotensin 1-7 [Ang-(1-7)] is one of the relaxation factors released by PVAT.

METHOD

Morphological and functional methods were used to study aorta from adult Wistar rats.

RESULTS

Immunohistochemical staining showed abundant presence of Ang-(1-7) in aortic PVAT. In vessels with PVAT removed but intact endothelium (PVAT - E+), contraction induced by phenylephrine was attenuated by preincubation with Ang-(1-7). PVAT - E+ vessels precontracted with phenylephrine showed a concentration-dependent relaxation response to Ang-(1-7), and this response was abolished by the removal of endothelium. Relaxation response induced by Ang-(1-7) was also prevented by Ang-(1-7) receptor (Mas) antagonist (A779), nitric oxide synthase inhibitor, and nitric oxide scavenger. Ang-(1-7) did not cause a relaxation response in aorta precontracted with KCl, and the relaxation response to Ang-(1-7) was also blocked by calcium-dependent potassium (K(Ca)) channel blockers. Incubation of PVAT + E+ vessels with A779 or angiotensin-converting enzyme 2 inhibitor DX600 or angiotensin-converting enzyme inhibitor enalaprilat increased the contraction induced by phenylephrine. Transfer of donor solution incubated with PVAT + E+ vessel to recipient PVAT - E+ vessel caused a relaxation response. This relaxation response was abolished when donor vessels were incubated with DX600 or enalaprilat or when recipient vessels were incubated with A779.

CONCLUSION

Ang-(1-7) released by PVAT acts on the endothelium to cause the release of nitric oxide, and nitric oxide acts as a hyperpolarizing factor through K(Ca) channels to cause relaxation of the blood vessel.

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.

Candesartan decreases carotid intima-media thickness by enhancing nitric oxide and decreasing oxidative stress in patients with hypertension

Candesartan has been reported to produce nitric oxide (NO) and to decrease oxidative stress in animal studies. We investigated candesartan's effect on the production of NO and oxidative stress as well as on carotid intima-media thickness (IMT) in hypertensive patients. One-hundred age-matched hypertensive patients were enrolled into an angiotensin II receptor blocker (ARB) group (n=50) or a non-ARB group (n=50). The ARB group was treated with candesartan 8 mg and, when needed, Ca channel blockers, angiotesin-converting enzyme (ACE) inhibitors, and/or beta-blockers. The non-ARB group was treated with drugs other than ARB. Carotid IMT was assessed by echocardiography before and 12 and 24 months after treatment. The urine levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG), an indicator of oxidative stress, and the serum levels of NOx, an indicator of NO, were measured. Blood pressure decreased to below 140/90 mmHg to the same extent in both groups. Carotid IMT decreased significantly in the ARB group, but not in the non-ARB group, at 12 and 24 months after treatment. The urine levels of 8-OHdG decreased significantly at 6 and 12 months after treatment in the ARB group but did not decrease in the non-ARB group. The serum levels of NOx increased significantly at 6 and 12 months after treatment in the ARB group but not in the non-ARB group. In conclusion, candesartan decreases carotid IMT by enhancing NO production and decreasing oxidative stress in patients with hypertension.

Upregulation of AT2 receptor and iNOS impairs angiotensin II-induced contraction without endothelium influence in young normotensive diabetic rats

Diabetes and insulin resistance are associated with an increased risk of hypertension and cardiovascular disease. Recent evidence demonstrates that AT2 receptors (AT2R) play an important role in the hemodynamic control of hypertension by vasodilation. The quantitative significance of AT2R in the establishment of diabetic vascular dysfunction, however, is not well defined and needs further investigation. Goto-Kakizaki (GK) rats, a polygenic model of spontaneous normotensive type 2 diabetes, were used to examine any abnormalities in cardiovascular function associated with AT2R at the early stage of the disease without endothelium influence. Using a myograph to measure the isometric force, we observed that ANG II-induced contraction was impaired in denuded GK aorta compared with control Wistar-Kyoto (WKY) aorta and exhibited a retarded AT1R antagonist response and enhanced Rho kinase signaling. When AT1R were blocked, ANG II induced a significant vasodilation of precontracted GK aorta via AT2R. The protein and mRNA of AT2R were increased in diabetic GK denuded aorta. Blocking AT2R restored the ANG II-induced contraction in the GK vasculature to control levels, demonstrating a counteractive role for AT2R in AT1R-induced contraction. Inhibition of inducible nitric oxide synthase (iNOS) by NG-monomethyl-L-arginine mimicked AT2R inhibition in denuded GK aorta, suggesting that AT2R-induced vasodilation was dependent on iNOS/NO generation. The protein and mRNA of iNOS were also increased in GK aorta. In conclusion, these results clearly demonstrate that enhanced AT2R and iNOS-induced, NO-mediated vasodilation impair ANG II-induced contraction in an endothelium-independent manner at the early stage of type 2 diabetes.

Angiotensin AT2 receptors: control of renal sodium excretion and blood pressure

The renin-angiotensin system is a coordinated hormonal cascade of crucial importance in cardiovascular and renal function. The primary effector peptide angiotensin II functions at two major receptors, the AT1 and AT2 receptors. AT2 receptors mediate vasodilation and natriuresis. Regarding vasodilator actions, AT2 receptors oppose the AT1 receptor-mediated vasoconstrictor action of angiotensin II. Regarding the natriuretic actions of AT2 receptors, des-aspartyl 1-angiotensin II, rather than angiotensin II, is the preferred agonist. Regarding both the vasodilator and natriuretic properties of AT2 receptors, the beneficial blood pressure reduction and natriuretic responses to AT1 receptor blockade are mediated, at least in part, by AT2 receptor activation. In addition, AT2 receptor activation suppresses renin biosynthesis and release at renal juxtaglomerular cells. Therefore, AT2 receptors are potential therapeutic targets in hypertension.

Arterial stiffness and haemodynamic response to vasoactive medication in subjects with insulin-resistance syndrome

INSR (insulin-resistance syndrome) affects 25% of the Australian population and is associated with increased cardiovascular risk. In the present study, we postulated that early cardiovascular changes in these individuals may be associated with an activated RAS (renin–angiotensin system). We studied 26 subjects: 13 with INSR [waist circumference, 99±6 cm; HOMA (homoeostasis model assessment) score, 2.5±0.3] and 13 NCs (normals controls; waist circumference, 77±2 cm; HOMA score, 1.4±0.2). All received intravenous GTN (glyceryl trinitrate; 10, 20 and 40 μg/min), L-NMMA (NG-monomethyl-L-arginine; 3 mg/kg of body weight), AngII (angiotensin II; 8 and 16 ng/min), the selective AT2R (AngII type 2 receptor) inhibitor PD123319 (10 and 20 μg/min) and AngII (16 ng/min)+PD123319 (20 μg/min). At the end of each infusion, arterial stiffness indices [SI (stiffness index) and RI (reflection index)] and haemodynamic parameters were measured. There was a significantly higher RI response to AngII (P=0.0004 for both 8 and 16 ng/min doses) and to PD123319 (P=0.004 and P=0.03 for 10 and 20 μg/min doses respectively) in subjects with INSR compared with NCs. Co-infusion of AngII and PD123319 did not lead to additive changes in RI. RI responses to L-NMMA and GTN were not significantly different in both groups. No significant differences in SI and haemodynamic responses were detected. In conclusion, AT1R (AngII type 1 receptor) and AT2R activity produce arterial stiffness changes in subjects with INSR. Evidence of increased AT1R- and AT2R-mediated responses in small-to-medium-sized arteries in INSR was found, and may play an early role in the pathogenesis of vascular changes in INSR before haemodynamic changes become apparent.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.