Vascular expression of angiotensin type 2 receptor in the adult rat: influence of angiotensin II infusion

Angiotensin AT2 receptors reduce inflammation and fibrosis in cardiovascular remodeling

The angiotensin AT2 receptor (AT2R), an important member of the "protective arm" of the renin-angiotensin system (RAS), has been recently defined as a therapeutic target in different pathological conditions. The AT2R activates complex signalling pathways linked to cellular proliferation, differentiation, anti-inflammation, antifibrosis, and induction or inhibition of apoptosis. The anti-inflammatory effect of AT2R activation is commonly associated with reduced fibrosis in different models. Current discoveries demonstrated a direct impact of AT2Rs on the regulation of cytokines, transforming growth factor beta1 (TGF-beta1), matrix metalloproteases (MMPs), and synthesis of the extracellular matrix components. This review article summarizes current knowledge on the AT2R in regard to immunity, inflammation and fibrosis in the heart and blood vessels. In particular, the differential influence of the AT2R on cardiovascular remodeling in preclinical models of myocardial infarction, heart failure and aneurysm formation are discussed. Overall, these studies demonstrate that AT2R stimulation represents a promising therapeutic approach to counteract myocardial and aortic damage in cardiovascular diseases.

The Other Angiotensin II Receptor: AT2R as a Therapeutic Target

The active hormone of the renin-angiotensin system (RAS), angiotensin II (Ang II), is involved in several human diseases, driving the development and clinical use of several therapeutic drugs, mostly angiotensin I converting enzyme (ACE) inhibitors and angiotensin receptor type I (AT₁R) antagonists. However, angiotensin peptides can also bind to receptors different from AT₁R, in particular, angiotensin receptor type II (AT₂R), resulting in biological and physiological effects different, and sometimes antagonistic, of their binding to AT₁R. In the present Perspective, the components of the RAS and the therapeutic tools developed to control it will be reviewed. In particular, the characteristics of AT₂R and tools to modulate its functions will be discussed. Agonists or antagonists to AT₂R are potential therapeutics in cardiovascular diseases, for agonists, and in the control of pain, for antagonists, respectively. However, controlling their binding properties and their targeting to the target tissues must be optimized.

Understanding the Two Faces of Low-Salt Intake

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

Antihypertensive nano-ceuticales based on chitosan biopolymer: Physico-chemical evaluation and release kinetics

Prime risk factor behind cardiovascular associated mortality and morbidity is hypertension. The main challenge with antihypertensive (AHT) drug therapy is their extreme hydrophobic nature and very low oral bio-availability; which result into higher dosage/frequency and associated side effects of drugs. The main objective of this study was to fabricate AHT nano-ceuticals in hydrophilic carriers of natural origin to improve drugs' solubility, protection and sustained release. AHT nano-carrier systems (NCS) encapsulating captopril, amlodipine and valsartan were fabricated using chitosan (CS) polymer by ionic gelation assisted ultra-sonication method. Drug encapsulation efficiencies of 92±1.6%, 91±0.9% and 87±0.5% were observed for captopril, valsartan and amlodipine respectively. Scanning electron microscopy (SEM) based analysis had revealed that captopril loaded polymeric NCS were regular, smooth and without any agglomeration. FTIR analyses of drug loaded and empty NCS demonstrated that drugs were molecularly dispersed inside the nanoparticles via week hydrogen bonding. Captopril and valsartan have demonstrated grafting reaction with N-H group of chitosan. Zeta sizer results had confirmed that average size of chitosan nanoparticles was below 100 nm. Encapsulation of captopril had reduced the surface charge value from +52.6±4.8 to +46.5±5.2 mV. Controlled release evaluation of highly encapsulated drug captopril had revealed a slow release in vitro from NCS in physiological buffer. Thus, here reported innovative AHT nano-ceuticals of polymeric origin can improve the oral administration of currently available hydrophobic drugs while providing the extended-release function.

Do intravenous and subcutaneous angiotensin II increase blood pressure by different mechanisms?

Angiotensin (Ang) II plays a key role in blood pressure regulation. Mechanisms of the pressor effect of chronic intravenous AngII administration include vasoconstriction, stimulation of the sympathetic nervous system and aldosterone production, as well as direct effects on renal excretion of sodium and water.  Chronic AngII administration by subcutaneous minipump at doses higher than required to increase blood pressure by the intravenous route has identified additional pressor mechanisms, including the immune system, cytokines and matrix metalloproteinases. However, pressor doses of subcutaneous AngII may exceed the angiotensinogen synthesis rate and produce inflammation, fibrosis and necrosis of skin overlying the minipump.  Evidence that chronic subcutaneous and intravenous AngII increase blood pressure by different mechanisms includes the prevention of the pressor effects of subcutaneous, but not intravenous, AngII by angiotensin-converting enzyme inhibition. Furthermore, low doses of subcutaneous AngII reduce blood pressure of female, but not male, rodents and higher doses are less pressor in females than in males, whereas intravenous AngII is equally pressor in males and females.  Pressor doses of chronic subcutaneous AngII produce greater weight loss, anorexia and reduced kidney weight and cause greater vascular, cardiac and renal pathology than equally pressor doses of chronic intravenous AngII.  The different effects of chronic intravenous and subcutaneous AngII suggest that these two models of hypertension give different information and may differ in their relevance to blood pressure regulation in physiological and pathological states such as hypertension in humans.

Modulation of haemodynamics, endogeneous antioxidant enzymes, and pathophysiological changes by selective inhibition of angiotensin II type 1 receptors in pressureoverload rats

Background

Constriction of the thoracic or abdominal aorta provides an experimental model of pressure-overload cardiac hypertrophy. Blockade of AT₁ receptors is beneficial in preventing target-organ damage in hypertension.

Objective

To examine the effect of angiotensin II receptor antagonists on blood pressure, endogenous antioxidant enzyme and histopathological changes in pressure-overload rats.

Methods

Pressure overload was produced by abdominal aortic banding (AAB) using a blunt 22-guage needle in male rats as a model of cardiac hypertrophy. After surgery, the AAB-induced hypertension (AABIH) rats were treated with losartan 40 mg/kg/day, candesartan 10 mg/kg/day, irbesartan 10 mg/kg/day per os for 16 weeks. At 16 weeks of surgery, the rats were observed for general characteristics and mortality, and we determined non-invasive blood pressure (NIBP), endogenous antioxidant enzyme catalase and superoxide dismutase (SOD) activities, and histology of the target organs.

Results

In the AABIH group, significant increase in systolic blood pressure was observed from weeks 3 to 16 compared with the control group, along with reduced serum catalase and SOD activities. The treated groups showed significant reduction in systolic BP and increase in serum SOD and catalase activities. The histological changes induced in the target organs, namely heart, liver, kidneys and thoracic aorta in the AABIH rats were attenuated in the treated rats.

Conclusion

Blockade of the AT₁ receptor caused an improvement in the myocardial antioxidant reserve and decreased oxidative stress in the hypertensive rats, which was evidenced by the protection observed in the treatment groups.

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.

Role of renin-angiotensin system and oxidative status on the maternal cardiovascular regulation in spontaneously hypertensive rats

BACKGROUND

The purpose of this study was to investigate the contribution of renin-angiotensin system (RAS) and oxidative status on the maternal cardiovascular regulation at the end of pregnancy in normotensive and spontaneously hypertensive rats (SHR).

METHODS

Blood pressure (BP), mesenteric arterial bed (MAB) reactivity, mesenteric oxidative damage, protein expression, and antioxidant activities were compared between four groups: SHR (SHR-P) and normotensive Wistar controls (W-P) in the 20th day of pregnancy or age-matched nonpregnant rats (SHR-NP and W-NP).

RESULTS

BP in W-P and SHR-P was reduced at the end of pregnancy. The vasodilator effects of angiotensin II (Ang II) and angiotensin 1-7 (Ang-(1-7)) were higher in SHR-P than in other groups. Endothelial nitric oxide synthase (eNOS) expression was increased in W-P and SHR-P compared to nonpregnant groups. Angiotensin-converting enzyme (ACE) and AT(1) receptor expressions were increased in SHR-NP compared to normotensive groups and pregnancy reduced their expressions in SHR. No difference was observed in AT(2) receptor expression among the groups. ACE2 expression was higher in hypertensive than normotensive groups. The levels of thiobarbituric acid-reactive substances (TBARS) were reduced in pregnant compared to nonpregnant groups. Superoxide dismutase (SOD) activity was reduced in SHR-P compared to SHR-NP. However, pregnancy increased catalase (CAT) and glutathione peroxidase (GPx) activities in normotensive rats and SHR, respectively.

CONCLUSIONS

The results suggest that the reduction of BP to normal values at the end of pregnancy in SHR may be related to an increased NO production and vasorelaxation to Ang II and Ang-(1-7) associated with decreased expression of vascular ACE and AT(1) receptors and oxidative status.

Prevention of salt-induced hypertension and fibrosis by AT1-receptor blockers in Dahl S rats

In Dahl S rats, high salt intake causes hypertension and cardiovascular hypertrophy and fibrosis, associated with an apparent increase in activity of tissue RAAS. In the current study, we assessed the effects of two AT1-receptor blockers (ARB) on AT1- and AT2-receptors and ACE densities and salt-induced cardiovascular changes. The hydrophilic ARB losartan (30 or 100 mg.kg.d) and the lipophilic ARB telmisartan (10 or 30 mg.kg.d) were administered once daily, and a high-salt diet was provided from 5 to 9 weeks. In Dahl S but not R rats, the high-salt diet caused marked hypertension, cardiac and kidney hypertrophy, and fibrosis. Both ARBs dose-dependently inhibited binding of Ang II to AT1-receptors and reversed the salt-induced increases in AT2-receptor densities in the CNS. Both ARBs at regular doses attenuated the salt-induced hypertension and, at high doses, prevented the increase in BP during the day but not during the night. Both ARBs similarly prevented high-salt-induced interstitial and perivascular fibrosis in the LV and RV as well as fibrosis in the aorta and renal tubules. RV hypertrophy was also prevented, but LV hypertrophy only partially, and kidney hypertrophy not at all. In Dahl S rats, AT1-receptor stimulation seems to play a critical role in salt-induced hypertension and fibrosis, but a lesser role in tissue hypertrophy.

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.

Type 1 angiotensin receptor (AT1-R)-mediated decrease in type 2 angiotensin receptor mRNA level is dependent on Gq and extracellular signal-regulated kinase 1//2 in AT1-R-transfected PC12 cells

Angiotensin II (Ang II) functions through two major Ang II receptor subtypes, type 1 (AT1-R) and type 2 (AT2-R), both of which are classified to be G protein-coupled receptors. AT2-R is highly expressed at the fetal stage, and in heart remodelling and brain ischaemia; therefore, it is important to clarify the regulatory mechanisms of AT2-R expression. Although AT1-R is generally believed to modulate AT2-R expression in some tissues or cells, the exact mechanism remains to be clarified. In the present study, we examined the effect of AT1-R stimulation on expression of endogenous rat AT2-R (rAT2-R) in AT1-R-transfected PC12 cells. rAT2-R mRNA and protein expression were decreased by Ang II in PC12 cells transfected with rAT1A-R in a time-dependent manner, mediated through a decline in mRNA stability. The C-terminus of G protein-coupled receptor (GPCR) is important for GPCR-mediated signal transduction. Therefore, we used C-terminus-deleted human AT1-R (hAT1-327STOP), which is thought to be a nondesensitised mutant of hAT1-R. As a result, Ang II decreased rAT2-R mRNA expression to a greater extent in cells transfected with hAT1-327STOP than with wild-type hAT1-R. The decrease was completely reversed by AT1-R antagonist candesartan, G(q) inhibitor YM254980, and mitogen-activated protein kinase (MAPK) kinase 1/2 inhibitor U0126, but not by pertussis toxin, which uncouples the receptor with G(i), or p38 MAPK inhibitor SB203580. We suggest, possibly for the first time, that the hAT1-R/G(q)/extracellular signal-regulated kinase 1/2 pathway is involved in the down-regulation of AT2-R using PC12 cells transfected with AT1-R.

Angiotensin II Signaling in Vascular Physiology and Pathophysiology

Initially recognized as a physiologic regulator of blood pressure and body fluid homeostasis, angiotensin (Ang) II has now been shown in innumerable experiments and clinical studies to contribute to the development and maintenance of cardiovascular disease. Dissection of its signaling mechanisms over the past decades has led to the discovery of several novel concepts, such as tissue-specific metabolism of Ang peptides. Identification and cloning of the various receptors through which Ang II acts on almost all tissues has led to the development of specific pharmacologic inhibitors with proven clinical benefit in patients with cardiovascular disorders. Work on the G-protein-coupled Ang II Type 1 receptor has demonstrated that different receptors interact through oligomerization, compartmentalization, and transactivation, and may explain how Ang II can activate G-protein-independent pathways. Unraveling the downstream effects of Ang II in specific cell types corroborates the importance of the cellular redox state on certain signaling pathways. Finally, the effects of Ang II on cell function and phenotype, such as the expression of inflammatory cytokines and receptors promoting the recruitment of inflammatory cells into vascular tissues, have indicated its role in local inflammation as a general pathogenetic basis of cardiovascular disease. The recognition of Ang II as a contributor to such fundamental pathophysiologic mechanisms, which are believed to be a common pathway for diverse cardiovascular risk factors like hypertension and diabetes, has greatly advanced our knowledge of pathologic signaling in vascular tissues and may help to eventually define novel targets for pharmacologic interventions.

Regulator of G protein signalling 2 ameliorates angiotensin II-induced hypertension in mice

Angiotensin II (Ang II) activates signalling pathways predominantly through the G-protein-coupled Ang II type 1 receptor (AT(1)R). The regulator of G protein signalling 2 (RGS2) is a negative G protein regulator. We hypothesized that RGS2 deletion changes blood pressure regulation by increasing the response to Ang II. To address this issue, we infused Ang II (0.5 mg kg(-1) day(-1)) chronically into conscious RGS2-deleted (RGS2(-/-)) and wild-type (RGS2(+/+)) mice, measured mean arterial blood pressure and heart rate (HR) with telemetry and assessed vasoreactivity and gene expression of AT(1A), AT(1B) and AT(2) receptors. Angiotensin II infusion increased blood pressure more in RGS2(-/-) than in RGS2(+/+) mice, while HR was not different between the groups, indicating a resetting of the baroreceptor reflex. Urinary catecholamine excretion was similar in Ang II-infused RGS2(-/-) and RGS2(+/+) mice, indicating a minor role of sympathetic tone for blood pressure differences. Myogenic tone and vasoreactivity in response to Ang II, endothelin-1 and phenylephrine were increased in isolated renal interlobar arterioles of RGS2(-/-) mice compared with RGS2(+/+) mice. The AT(1A), AT(1B) and AT(2) receptor gene expression was not different between RGS2(-/-) and RGS2(+/+) mice. Our findings suggest that RGS2 deletion promotes Ang II-dependent hypertension primarily through an increase of myogenic tone and vasoreactivity, probably by sensitization of AT(1) receptors.

Potentiation of the antihypertensive action of losartan by peripheral overexpression of the ANG II type 2 receptor

Our previous studies demonstrated that peripheral overexpression of angiotensin II (ANG II) type 2 receptors (AT(2)R) prevents hypertension-induced cardiac hypertrophy and remodeling without altering high blood pressure. This, coupled with the observations that AT(2)R play a role in the antihypertensive actions of ANG II type 1 receptor (AT(1)R) blockers (ARBs), led us to propose that peripheral overexpression of AT(2)R would improve the antihypertensive action of losartan (Los) in Sprague-Dawley (SD) rats made hypertensive via chronic infusion of ANG II. Here we utilized adenoviral vector-mediated AT(2)R gene transfer to test this hypothesis. A single intracardiac injection of adenoviral vector containing genomic AT(2)R (G-AT(2)R) DNA and enhanced green fluorescent protein (EGFP) gene controlled by cytomegalovirus (CMV) promoters (Ad-G-AT(2)R-EGFP; 5 x 10(9) infectious units) into adult SD rats produced robust AT(2)R overexpression in cardiovascular tissues (kidney, lung, heart, aorta, mesenteric artery, and renal artery) that persisted for 3-5 days postinjection. By 7 days post viral injection, the overexpressed AT(2)R are reduced toward basal values in certain tissues (lung, kidney, and heart) and are undetectable in others (kidney and blood vessels). In two separate protocols, we demonstrated that the hypotensive effect of Los (0.125, 0.5, and 1.0 mg/kg iv) was significantly greater in the AT(2)R-overexpressing animals (-40.7 +/- 4.3, -41.8 +/- 4.8, and -48.1 +/- 2.6 mmHg, respectively) compared with control vector (Ad-CMV-EGFP)-treated rats (-12.4 +/- 2.2, -20.2 +/- 3.4, and -27.3 +/- 3.4 mmHg, respectively). These results provide support for a depressor role of AT(2)R and the proposal that combined AT(2)R agonist and ARB treatment may be an improved therapeutic strategy for controlling hypertension.

Angiotensin II type 2 receptor expression after vascular injury: differing effects of angiotensin-converting enzyme inhibition and angiotensin receptor blockade

It has been suggested that the effects of angiotensin II type 1 receptor (AT1R) blockers are in part because of angiotensin II type 2 receptor (AT2R) signaling. Interactions between the AT2R and kinins modulate cardiovascular function. Because AT2R expression increases after vascular injury, we hypothesized that the effects on vascular remodeling of the AT1R blocker valsartan and the ACE inhibitor benazepril require AT2R signaling through the bradykinin 1 and 2 receptors (B1R and B2R). To test this hypothesis, Brown Norway rats were assigned to 8 treatments (n=16): valsartan, valsartan+PD123319 (AT2R inhibitor), valsartan+des-arg9-[Leu8]-bradykinin (B1R inhibitor), valsartan+HOE140 (B2R inhibitor), benazepril, benazepril+HOE140, amlodipine, and vehicle. After 1 week of treatment, carotid balloon injury was performed. Two weeks later, carotids were harvested for morphometry and analysis of receptor expression by immunohistochemistry and Western blotting. Valsartan and benazepril significantly reduced the intima:media ratio compared with vehicle. Blockade of AT2R, B1R, or B2R in the presence of valsartan prevented the reduction seen with valsartan alone. B2R blockade inhibited the effect of benazepril. Injury increased AT1R, AT2R, B1R, and B2R expression. Treatment with valsartan but not benazepril significantly increased intima AT2R expression 2-fold compared with vehicle, which was not reversed by inhibition of AT2R, B1R, and B2R. Functionally, valsartan increased intimal cGMP levels compared with vehicle, and this increase was inhibited by blocking the AT2R, B1R, and B2R. Results suggest that AT2R expression and increased cGMP represent a molecular mechanism that differentiates AT1R blockers, such as valsartan, from angiotensin-converting enzyme inhibitors like benazepril.

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.

Angiotensin II, type 2 receptor is not involved in the angiotensin II-mediated pro-atherogenic process in ApoE-/- mice

OBJECTIVE

Angiotensin II (Ang II) accelerates atherogenesis in ApoE mice via the angiotensin II, type 1 receptor (AT1) while the type 2 receptor (AT2) is suggested to counteract atherogenesis. To confirm and further explore this possibility, we studied the effect of AT2 receptor antagonism on Ang II-accelerated atherosclerosis.

METHODS

ApoE mice were fed a standard or high cholesterol diet (1.25%) for 4 weeks. Mice on each diet were treated with either Ang II (0.5 microg/kg per min) or Ang II in combination with PD123319 (3 mg/kg per day). Plaque distribution was assessed by en face quantification of the thoracic aorta and in cross-sections of the aortic root. Mean arterial pressure (MAP) was measured. AT1 and AT2 receptor expression were analysed using real-time polymerase chain reaction (PCR) and the localization of the AT2 receptor protein confirmed with immunohistochemistry.

RESULTS

Ang II infusion increased MAP only in mice on a standard diet (P < 0.001). Regardless of diet, Ang II-infused mice had 22-30 times increased plaque area in the thoracic aorta (P < 0.001 for both). Ang II had no effect on plaque in the aortic root. Plaque area was not affected by PD123319. AT2 receptor was heavily expressed in the plaques and increased six- to ninefold by a high cholesterol diet and Ang II infusion (P < 0.01).

CONCLUSION

Ang II increases the extent of atherosclerosis in ApoE mice. Despite up-regulation of the AT2 receptor, we found no support for an effect of the AT2 receptor on atherogenesis in this model.

Dynamic expression of the angiotensin II type 2 receptor and duodenal mucosal alkaline secretion in the Sprague-Dawley rat

Activation of angiotensin II type 2 receptors (AT2R) has been shown to stimulate duodenal mucosal alkaline secretion (DMAS) in Sprague-Dawley rats (S-D). This finding could not be confirmed in another line of S-D, and the present study investigates whether the level of AT2R expression determines the response to the AT2R agonist CGP42112A. DMAS was measured in anaesthetized rats using in situ pH-stat titration. Real-time PCR and Western blot were used to assess AT1R and AT2R RNA and protein expression, respectively. CGP42112A (0.1 microg kg(-1)min(-1) I.V.) elicited a 45% net increase in DMAS in the previous S-D line studied, whereas no change occurred in the new S-D line. Luminal administration of prostaglandin E2 (10(-5) M) increased DMAS similarly in both S-D lines. AT2R protein expression was significantly higher in tissue from the previous line compared to the new line. Individual AT1R to AT2R ratios (RNA and protein) were significantly higher in the new line compared to the previous S-D line. In the new S-D line intravenous infusion of angiotensin II (Ang II; 10 microg kg(-1) h(-1)) over 120 min significantly lowered the duodenal AT1aR to AT2R RNA ratio. Prolonged Ang II infusion over 240 min increased AT2R protein expression and evoked a 42% stimulatory response in DMAS to CGP42112A. The level of local AT2R expression determines the effect of the AT2R agonist CGP42112A on rat duodenal mucosal alkaline secretion. AT2R expression should be confirmed before interpreting the experimental effects of pharmacological interferences with this receptor.

Reversal of fructose-induced hypertension and insulin resistance by chronic losartan treatment is independent of AT2 receptor activation in rats

OBJECTIVES

To examine whether angiotensin II type 2 receptors (AT2R) are involved in the reversal of fructose-induced hypertension and insulin resistance after chronic angiotensin II type 1 receptor (AT1R) blockade.

METHODS

Sprague-Dawley rats on fructose-enriched or regular diets were pretreated with losartan, an AT1R antagonist, or vehicle for 2 weeks before two-step glucose and insulin clamp experiments with [3-3H]glucose infusion. The hepatic glucose production (HGP) and whole-body glucose uptake (WBGU) were calculated during basal, euglycemic and euglycemic hyperinsulinemic periods. Blood pressure was measured before and after acute losartan (10 mg/kg, i.v. bolus), alone or in combination of PD123319 (PD, 50 microg/kg per min), an AT2R antagonist, or CGP42112 (2 microg/kg per min), an AT2R agonist, during the clamp study.

RESULTS

In rats on a regular diet, acute infusion of losartan alone or in combination with PD, an AT2R blocker, did not alter blood pressure and glucose metabolism during experiments. Fructose feeding for 6 weeks significantly increased blood pressure and attenuated insulin-mediated suppression of HGP and stimulation of WBGU. Both acute and chronic administration of losartan suppressed fructose-induced hypertension. Concomitant treatment with PD and losartan blunted the acute but not chronic losartan-mediated depressor effect. Acute losartan treatment further reduced insulin-induced suppression of HGP, but simultaneously increased insulin-stimulated WBGU. These acute metabolic effects of losartan were eliminated when PD was co-administered with losartan. Conversely, chronic losartan pretreatment significantly enhanced suppression of HGP and increased stimulation of WBGU by insulin, which were not altered when PD or CGP 42112 was superimposed on losartan during the clamp experiments.

CONCLUSIONS

These results suggest that reversal of high fructose-induced hypertension and insulin resistance by chronic losartan treatment is not dependent on AT2R activation and that functional activation of AT1R plays a major role in the pathogenesis of high fructose-induced hypertension and insulin resistance.

Valsartan reduces interleukin-1beta secretion by peripheral blood mononuclear cells in patients with essential hypertension

BACKGROUND

Chronic low-grade inflammation response may contribute to the pathology of essential hypertension. Angiotensin II (Ang II) may be partly responsible for this process. Our early studies showed that individuals with essential hypertension had increased interleukin-1beta (IL-1beta) secretion by peripheral blood mononuclear cells (PBMCs). In this study, we investigated whether treatment with valsartan, an angiotensin receptor blocker, lowered IL-1beta secretion by PBMCs in patients with essential hypertension.

METHODS

Twenty-four patients with essential hypertension were randomized to treatment with valsartan (80 mg/day, group B) or matching routine therapy group (group A) for 2 weeks. PBMCs were isolated by gradient centrifugation. IL-1beta concentrations in supernatant from PBMCs were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Compared with routine therapy group, patients treated with valsartan had decreased secretion of IL-1beta in PBMCs after stimulated by lipopolysaccharide (2857+/-643 vs. 2146+/-508 pg/ml, P<0.05).

CONCLUSIONS

We suggest a direct anti-inflammatory effect of valsartan and a pro-inflammatory effect of Ang II in patients with essential hypertension.

High-Salt Diet Inhibits Expression of Angiotensin Type 2 Receptor in Resistance Arteries

Recent studies suggested that type 2 angiotensin receptor (AT2R) could contribute to regulation of blood pressure and/or vascular remodeling. A key question relates to the effects of potential modulators of vascular AT2R expression. In the present work, we evaluated if high salt intake (70 mmol/L NaCl in drinking water) could modulate rat mesenteric artery AT2R function and expression. Angiotensin II dose-response curves were studied in rat perfused pressurized small-diameter arteries in the presence of losartan (AT1R antagonist). Arteries were precontracted with phenylephrine, yielding ≈30% decrease in resting diameter. AT2R activation by angiotensin-induced dose-dependent relaxation of precontracted arteries (60.1±9.1% of phenylephrine-induced contraction, P <0.05). In contrast, AT2R-dependent relaxation was not observed in arteries obtained from rats on high-salt diet. Semi-quantitative reverse-transcription polymerase chain reaction experiments demonstrated reduced amount of AT2R mRNA in arteries of rats on high-salt diet (65.5±7.5% of control levels, P <0.05). Western blot studies demonstrated decreased AT2R in mesenteric artery protein fractions of high-salt diet rats (60.0±18.0 of control levels, P <0.05). In a second set of experiments, adrenalectomy (4 days) blunted AT2R-mediated vasorelaxation and decreased AT2R mRNA (72.0±11.0% of control levels, P <0.05). AT2R abundance in protein fractions of mesenteric arteries of ADX rats was also diminished (64.0±13% of control levels, P <0.05). Both, AT2R mRNA and protein downregulation were prevented by mineralocorticoid replacement therapy. Finally, physiological concentrations of aldosterone caused a dose-dependent increase in AT2R mRNA of small diameter mesenteric artery explants. The results are consistent with aldosterone-mediated upregulation AT2R.

ANG II in the paraventricular nucleus potentiates the cardiac sympathetic afferent reflex in rats with heart failure

Chronic heart failure (CHF) is characterized by sympathoexcitation, and the cardiac sympathetic afferent reflex (CSAR) is a sympathoexcitatory reflex. Our previous studies have shown that the CSAR was enhanced in CHF. In addition, central angiotensin II (ANG II) is an important modulator of this reflex. This study was performed to determine whether the CSAR evoked by stimulation of cardiac sympathetic afferent nerves (CSAN) in rats with coronary ligation-induced CHF is enhanced by ANG II in the paraventricular nucleus (PVN). Under alpha-chloralose and urethane anesthesia, renal sympathetic nerve activity (RSNA) was recorded. The RSNA responses to electrical stimulation (5, 10, 20, and 30 Hz) of the CSAN were evaluated. Bilateral microinjection of the AT1-receptor antagonist losartan (50 nmol) into the PVN had no significant effects in the sham group, but it abolished the enhanced RSNA response to stimulation in the CHF group. Unilateral microinjection of three doses of ANG II (0.03, 0.3, and 3 nmol) into the PVN resulted in dose-related increases in the RSNA responses to stimulation. Although ANG II also potentiated the RSNA response to electrical stimulation in sham rats, the RSNA responses to stimulation after ANG II into the PVN in rats with CHF were much greater than in sham rats. The effects of ANG II were prevented by pretreatment with losartan into the PVN in CHF rats. These results suggest that the central gain of the CSAR is enhanced in rats with coronary ligation-induced CHF and that ANG II in the PVN augments the CSAR evoked by CSAN, which is mediated by the central angiotensin AT1 receptors in rats with CHF.

Resistance to oxidative stress by chronic infusion of angiotensin II in mouse kidney is not mediated by the AT2 receptor

Wild-type mice are resistant to ANG II-induced renal injury and hence form an attractive model to study renal defense against ANG II. The present study tested whether ANG II induces expression of antioxidative genes via the AT2 receptor in renal cortex and thereby counteracts prooxidative forces. ANG II was infused in female C57BL/6J mice for 28 days and a subgroup received AT2 receptor antagonist (PD-123,319) for the last 3 days. ANG II induced hypertension and aortic hypertrophy; proteinuria and renal injury were absent. Urinary nitric oxide metabolites (NOx) were decreased, and lipid peroxide (TBARS) excretion remained unchanged. Expression of NADPH oxidase components was decreased in renal cortex but induced in aorta. Heme oxygenase-1 (HO-1) was induced in both renal cortex and aorta. In contrast, ANG II suggestively increased AT2 receptor expression in kidney but not in aorta. AT2 receptor blockade enhanced hypertension in ANG II-infused mice, reversed ANG II effects on NOx excretion, but did not affect TBARS. Despite its prohypertensive effect, expression of prooxidative genes in the renal cortex decreased rather than increased after short-term AT2 receptor blockade and renal HO-1 induction after ANG II was normalized. Thus chronic ANG II infusion in mice induces hypertension but not oxidative stress. In contrast to the response in aorta, gene expression of components of NADPH-oxidase was not enhanced in renal cortex. Although ANG II administration induced renal cortical AT2 receptor expression, blockade of that receptor did not unveil the AT2 receptor as intrarenal dampening factor of prooxidative forces.

High blood pressure reduction reverses angiotensin II type 2 receptor-mediated vasoconstriction into vasodilation in spontaneously hypertensive rats

BACKGROUND

We have previously shown that angiotensin II type 2 receptor (AT(2)R) stimulation causes endothelium-dependent vasodilation that does not desensitize after chronic angiotensin II type 1 receptor (AT1R) blockade, suggesting a role for AT2R in antihypertensive treatment.

METHODS AND RESULTS

We recorded mean arterial pressure (MAP) and investigated AT2R by Western blot analysis, immunohistochemistry, and function in isolated mesenteric resistance arteries (205 microm in diameter) from Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR) receiving the following for 4 weeks in drinking water: placebo, AT1R blockade (candesartan; 2 mg/kg per day), ACE inhibitor (perindopril; 3 mg/kg per day), nonselective vasodilator (hydralazine; 16 or 24 mg/kg per day), or candesartan plus hydralazine (16 mg/kg per day). In precontracted isolated arteries, AT2R stimulation (angiotensin II in the presence of candesartan) caused vasodilation in WKY rats (MAP=118 mm Hg) and vasoconstriction in SHR (MAP=183 mm Hg). In SHR treated with candesartan (MAP=146 mm Hg) or hydralazine (16 mg/kg per day; MAP=145 mm Hg), AT2R-induced contraction was reduced by 50%. In SHR treated with perindopril (MAP=125 mm Hg), AT2R stimulation induced vasodilation. In SHR treated with hydralazine (24 mg/kg per day; MAP=105 mm Hg) and in SHR treated with hydralazine (16 mg/kg per day) plus candesartan (MAP=102 mm Hg), an AT2R-mediated vasodilation was restored. Immunochemistry and Western blot analysis showed that AT2R expression, lower in SHR than in WKY rats, was restored to normal levels by treatments reducing arterial pressure in SHR.

CONCLUSIONS

Our results suggest that in resistance arteries of SHR, (1) AT2R is downregulated by hypertension, and (2) specific and nonspecific antihypertensive treatments restore AT(2)R expression and vasodilator functions.

Imatinib Attenuates Diabetic Nephropathy in Apolipoprotein E-Knockout Mice

In the diabetic kidney, clinical as well as experimental observations have shown an upregulation of growth factors such as PDGF. These studies, however, were not designed to address whether upregulation of PDGF is merely a manifestation of diabetic renal injury or whether PDGF plays an active role in the pathophysiology of diabetic nephropathy. The objectives of this study were first to assess whether PDGF-dependent pathways are involved in the development of diabetic nephropathy and second to determine the effects of PDGF receptor antagonism on this disorder and associated molecular and cellular processes. This study used the diabetic apolipoprotein E-knockout (apoE-KO) mouse, a recently described model of accelerated diabetic nephropathy. Diabetes was induced by injection of streptozotocin in 6-wk-old apoE-KO mice. Diabetic animals received treatment with a tyrosine kinase inhibitor that inhibits PDGF action, imatinib (STI-571, 10 mg/kg per d orally) or no treatment for 20 wk. Nondiabetic apoE-KO mice served as controls. This model of accelerated renal disease with albuminuria as well as glomerular and tubulointerstitial injury was associated with increased renal expression of PDGF-B, proliferating cells, and alpha-smooth muscle actin-positive cells. Furthermore, there was increased accumulation of type I and type IV collagen as well as macrophage infiltration. Imatinib treatment ameliorated both renal functional and structural parameters of diabetes as well as overexpression of a number of growth factors, collagens, proliferating cells, alpha-smooth muscle actin-positive cells, and macrophage infiltration within the kidney. Tyrosine kinase inhibition with imatinib seems to retard the development of experimental diabetic nephropathy.

AT1 receptor mRNA antisense normalizes enhanced cardiac sympathetic afferent reflex in rats with chronic heart failure

Previous studies showed that the cardiac sympathetic afferent reflex (CSAR) is enhanced in dogs and rats with chronic heart failure (CHF) and that central ANG II type 1 receptors (AT(1)R) are involved in this augmented reflex. The aim of this study was to determine whether intracerebroventricular administration and microinjection of antisense oligodeoxynucleotides targeted to AT(1)R mRNA would attenuate the enhanced CSAR and decrease resting renal sympathetic nerve activity (RSNA) in rats with coronary ligation-induced CHF. The CSAR was elicited by application of bradykinin to the epicardial surface of the left ventricle. Reflex responses to epicardial administration of bradykinin were enhanced in rats with CHF. The response to bradykinin was determined every 50 min after intracerebroventricular administration (lateral ventricle) or microinjection (into paraventricular nucleus) of antisense or scrambled oligonucleotides to AT(1)R mRNA. AT(1)R mRNA and protein levels in the paraventricular nucleus were significantly reduced 5 h after administration of antisense. Antisense significantly decreased resting RSNA and normalized the enhanced CSAR responses to bradykinin in rats with CHF. Scrambled oligonucleotides did not alter resting RSNA or the enhanced responses to bradykinin in rats with CHF. No significant effects were found in sham-operated rats after administration of either antisense or scrambled oligonucleotides. These results strongly suggest that central AT(1)R mRNA antisense reduces expression of AT(1)R protein and normalizes the augmentation of this excitatory sympathetic reflex and that genetic manipulation of protein expression can be used to normalize the sympathetic enhancement in CHF.

Increased renal vascular endothelial growth factor and angiopoietins by angiotensin II infusion is mediated by both AT1 and AT2 receptors

A link between angiotensin II and cell proliferation has previously been reported. However, there remains controversy as to the role of the individual angiotensin II receptor subtypes in mediating these effects and their link to angiogenic cytokines and their receptors. Male Sprague-Dawley rats were infused with either angiotensin II or vehicle for 14 d at a dose of 58.3 ng/min. Angiotensin II-infused rats received no treatment, an AT(1) receptor antagonist valsartan (30 mg/kg per d), or an AT(2) receptor antagonist PD123319 (830 ng/min). Gene expression of vascular endothelial growth factor (VEGF) and receptor VEGF-R2, as well as Tie-2 and its ligands angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) were assessed by reverse transcription-PCR. Protein expression was assessed by Western blotting and immunohistochemistry. Gene and protein expression of VEGF, Ang-1, and Ang-2 were increased by angiotensin II infusion. Valsartan and PD123319 attenuated angiotensin II-associated increases in VEGF gene and protein expression. Ang-1 and Ang-2 gene but not protein expression were reduced by both treatments. These changes occurred in the context of attenuation of angiotensin II-induced glomerular cell proliferation by both valsartan and PD123319. In situ hybridization and immunohistochemical studies localized VEGF, Ang-1, and Ang-2 expression to the epithelial cells of the glomerulus, and VEGF-R2 and Tie-2 receptors to the endothelial cells of the kidney. These findings extend the increasing evidence that the AT(2) receptor, in addition to the AT(1) receptor subtype, plays an important role in mediating the proliferative actions of angiotensin II in the kidney.

AT2 receptor-mediated vasodilatation is unmasked by AT1 receptor blockade in conscious SHR

In the present study, we investigated the role of the angiotensin II type 2 receptor (AT(2)) receptor in the regulation of regional haemodynamics in spontaneously hypertensive rats (SHR). We tested the hypothesis that AT(2) receptor activation directly causes vasodilatation. Mean arterial pressure (MAP), renal, mesenteric and hindquarters flows and conductances were measured in various groups of conscious rats that received the following drug combinations on separate days: the AT(1) receptor antagonist, candesartan (5 or 10 microg kg(-1) i.v.) alone, the AT(2) receptor agonist, CGP42112 (1 microg kg(-1) min(-1)) alone and candesartan plus CGP42112. Low-dose candesartan (5 microg kg(-1)) caused renal vasodilatation, while CGP 42112 alone caused minimal haemodynamic effects. In the presence of candesartan, CGP42112 caused a marked depressor effect together with generalised vasodilatation that was abolished by the coinfusion of the AT(2) receptor antagonist, PD123319 (50 microg kg(-1) min(-1)), with the candesartan and CGP42112 combination. PD123319, given alone, increased MAP and reduced renal and mesenteric conductances. We also confirmed that the enhanced vasodilatation evoked by candesartan plus CGP42112 was not due to additional AT(1) receptor blockade, since angiotensin II-mediated vasoconstriction was inhibited by a similar magnitude in the combination treatment compared with candesartan alone. Analogous experiments in Wistar-Kyoto rats did not demonstrate significantly enhanced effects due to candesartan plus CGP42112. Collectively, these data suggest that, in SHR, AT(2) receptors tonically modulate vascular tone and that direct AT(2) receptor-mediated vasodilatation was unmasked by AT(1) receptor blockade.

Imatinib Attenuates Diabetes-Associated Atherosclerosis

OBJECTIVE

Diabetes is associated with accelerated atherosclerosis, the major factor contributing to increased mortality and morbidity in the diabetic population. The molecular mechanisms by which diabetes promotes atherosclerosis are not fully understood. Platelet-derived growth factor has been shown to play a major role in the pathology of vascular diseases, but whether it plays a role in atherosclerosis associated with diabetes remains unknown. The aims of this study were to assess whether platelet-derived growth factor-dependent pathways are involved in the development of diabetes-induced atherosclerosis and to determine the effects of platelet-derived growth factor receptor antagonism on this disorder.

METHODS AND RESULTS

Diabetes was induced by injection of streptozotocin in 6-week-old apolipoprotein E knockout mice. Diabetic animals received treatment with a tyrosine kinase inhibitor that inhibits platelet-derived growth factor action, imatinib (STI-571, 10 mg/kg per day), or no treatment for 20 weeks. Nondiabetic apolipoprotein E knockout mice served as controls. Induction of diabetes was associated with a 5-fold increase in plaque area in association with an increase in aortic platelet-derived growth factor-B expression and platelet-derived growth factor-beta receptor phosphorylation as well as other prosclerotic and proinflammatory cytokines. Imatinib treatment prevented the development of atherosclerotic lesions and diabetes-induced inflammatory cytokine overexpression in the aorta.

CONCLUSIONS

Tyrosine kinase inhibition with imatinib appears to be a novel therapeutic option to retard the development of atherosclerosis, specifically in the context of diabetes.

Upregulation of angiotensin II type 2 receptor expression in estrogen-induced pituitary hyperplasia

Recent evidence shows that reexpression and upregulation of angiotensin II (ANG II) type 2 (AT2) receptor in adult tissues occur during pathological conditions such as tissue hyperplasia, inflammation, and remodeling. In particular, expression of functional AT2 receptors in the pituitary and their physiological significance and regulation have not been described. In this study, we demonstrate that chronic in vivo estrogen treatment, which induces pituitary hyperplasia, enhances local AT2 expression (measured by Western blot and RT-PCR) concomitantly with downregulation of ANG II type 1 (AT1) receptors. In vivo progesterone treatment of estrogen-induced pituitary hyperplasia did not modify either the ANG II receptor subtype expression pattern or octapeptide-induced and AT1-mediated calcium signaling. Nevertheless, an unexpected potentiation of the ANG II prolactin-releasing effect was observed in this group, and this response was sensitive to both AT1 and AT2 receptor antagonists. These data are the first to document that ANG II can act at the pituitary level through the AT2 receptor subtype and that estrogens display a differential regulation of AT1 and AT2 receptors at this level.

Angiotensin II–Induced Vascular Dysfunction Is Mediated by the AT1AReceptor in Mice

Many of the actions of angiotensin II (Ang II) are mediated by angiotensin type 1 receptors (AT1), of which there are 2 pharmacologically indistinguishable subtypes (AT1A and AT1B). The purpose of this study was to evaluate the effect of an AT1A homozygous deletion (AT1A-/-) on vascular reactivity. AT1A-/- mice and control littermates (AT1A+/+) were infused with vehicle (saline) or Ang II (1000 ng x kg(-1) x min(-1)) for 7 days by osmotic pumps. Systolic pressure was increased in AT1A+/+ mice (Delta45+/-8 mm Hg, P<0.0001) but unchanged in AT1A-/- mice (Delta5+/-3 mm Hg, P>0.13) on day 7. The carotid artery response to the vasodilators acetylcholine (ACh), nitroprusside, and papaverine and to the vasoconstrictors phenylephrine, U46619, 5-hydroxytryptamine (5-HT), and KCl were not different between vehicle-infused AT1A+/+ and AT1A-/- animals. Carotid relaxation to ACh was impaired and contraction to 5-HT was increased in Ang II-infused AT1A+/+ mice. Ang II did not affect carotid responses in AT1A-/- mice. Superoxide, measured by lucigenin (5 micromol/L), and hydroethidine staining were not different between AT1A+/+ and AT1A-/- mice after vehicle or Ang II infusion, suggesting that it was not contributing to the altered ACh and 5-HT responses. The Rho-kinase inhibitor Y-27632 (1 micromol/L) attenuated the 5-HT response in both vehicle- and Ang II-infused AT1A+/+ mice. Moreover, concentration-dependent relaxation to Y-27632 and RhoA protein expression were not different in vehicle- or Ang II-infused AT1A+/+. These data demonstrate that the AT1A receptor is required for Ang II-induced changes in carotid artery function.

Up-regulation of angiotensin II type 2 receptor in rat thoracic aorta by pressure-overload

We have examined whether expression of angiotensin II (Ang II) type 1 (AT(1)) and/or type 2 (AT(2)) receptors are changed in thoracic aorta under pressure-overload by abdominal aortic banding in rats and determined whether their changes are accompanied by alteration in contractile response of thoracic aorta to Ang II. AT(2) receptor mRNA levels determined by reverse transcription-polymerase chain reaction or quantitative real-time polymerase chain reaction were increased by about 300% in aortas 4, 7, 14, and 28 days after banding without changes in AT(1) receptor mRNA levels. Contractile response of aortic rings to Ang II was decreased in thoracic aortas 7 days after banding, and AT(2) receptor antagonist PD123319 (1-[[4-(dimethulamino)-3-methylphenyl]methyl]-5-(diphenylacetyl)-4,5,6,7-tetrahydro-1H-imidazo[4,5-c]pyridine-6-carboxylic acid ditrifluoroacetate) (10(-6) M) increased the Ang II responsiveness in pressure-loaded but not in sham rings. After removal of the endothelium or treatment with N(G)-nitro-L-arginine methyl ester (L-NAME), no differences were observed in Ang II responsiveness between sham and pressure-loaded rings. Either losartan (1 mg/kg/day i.p.) or candesartan (2 mg/kg/day p.o.) for 7 days after banding not only abolished the up-regulation of AT(2) receptor mRNA in aortas but also recovered their Ang II responsiveness. Basal cGMP levels were 2 times higher in pressure-loaded than in sham rings; both levels were not affected by Ang II (10(-7) M; 5 min), but greatly decreased by L-NAME (10(-4) M, 30 min). These results suggest that pressure-overload induces the up-regulation of AT(2) receptor expression in aortas via AT(1) receptor and thereby negatively modulates the vasoconstrictor sensitivity to Ang II, probably mediated by the mechanisms independent of the nitric oxide-cGMP system.

Angiotensin AT2 receptors: cardiovascular hope or hype?

British Journal of Pharmacology (2003) 140, 809–824. doi:10.1038/sj.bjp.0705448

Angiotensin II receptor blocker valsartan suppresses reactive oxygen species generation in leukocytes, nuclear factor-kappa B, in mononuclear cells of normal subjects: evidence of an antiinflammatory action

In view of the pro-oxidant and proinflammatory effects of angiotensin II, we have tested the hypothesis that valsartan, an angiotensin receptor blocker, may exert a suppressive action on reactive oxygen species (ROS) generation, nuclear factor kappa B (NF-kappa B) in mononuclear cells. Four groups of eight normal subjects were given 1) 160 mg daily of valsartan, 2) 80 mg daily of simvastatin, 3) 40 mg quinapril, or 4) no treatment. Fasting blood samples were obtained before treatment and at d 1, 8, and 14 (7 d after the cessation of the drug). After valsartan, ROS generation by polymorphonuclear cells and mononuclear cells fell significantly by more than 40% (P < 0.01). NF-kappa B binding activity and the expression of total cellular p65, a protein component of NF-kappa B, fell significantly (P < 0.01). The expression of inhibitor kappa B (I kappa B) increased significantly (P < 0.05). Plasma C-reactive protein (CRP) concentration fell significantly (P < 0.01). All indices, except I kappa B, reverted toward baseline, 7 d after the cessation of the drug. I kappa B persisted in an elevated state. Neither quinapril nor simvastatin given for 7 d produced a suppression of ROS generation, intranuclear NF-kappa B, p65, or CRP, and these two agents did not alter cellular I kappa B either. The untreated controls also did not demonstrate a change in their ROS generation or NF-kappa B binding activity or plasma CRP concentration. We conclude that valsartan at a modest dose exerts a profound and rapid ROS and inflammation-suppressive effect that may be relevant to its potential beneficial effects in atherosclerosis, diabetes, and congestive cardiac failure. In contrast, quinapril and simvastatin produced no similar effect over the period of 1 wk. Our observations may also have implications to clinical situations in which a rapid antiinflammatory effect is required.

A breaker of advanced glycation end products attenuates diabetes-induced myocardial structural changes

The formation of advanced glycation end products (AGEs) on extracellular matrix components leads to accelerated increases in collagen cross linking that contributes to myocardial stiffness in diabetes. This study determined the effect of the crosslink breaker, ALT-711 on diabetes-induced cardiac disease. Streptozotocin diabetes was induced in Sprague-Dawley rats for 32 weeks. Treatment with ALT-711 (10 mg/kg) was initiated at week 16. Diabetic hearts were characterized by increased left ventricular (LV) mass and brain natriuretic peptide (BNP) expression, decreased LV collagen solubility, and increased collagen III gene and protein expression. Diabetic hearts had significant increases in AGEs and increased expression of the AGE receptors, RAGE and AGE-R3, in association with increases in gene and protein expression of connective tissue growth factor (CTGF). ALT-711 treatment restored LV collagen solubility and cardiac BNP in association with reduced cardiac AGE levels and abrogated the increase in RAGE, AGE-R3, CTGF, and collagen III expression. The present study suggests that AGEs play a central role in many of the alterations observed in the diabetic heart and that cleavage of preformed AGE crosslinks with ALT-711 leads to attenuation of diabetes-associated cardiac abnormalities in rats. This provides a potential new therapeutic approach for cardiovascular disease in human diabetes.

Up-regulation of components of the renin-angiotensin system in the bile duct-ligated rat liver

BACKGROUND & AIMS

Angiotensin II (ANG II) has profibrotic actions in the heart and kidney, whereas blockade of the renin angiotensin system (RAS) attenuates injury. This study examines whether the RAS is present in the liver and examines its regulation in the bile duct-ligation model of hepatic fibrogenesis.

METHODS

Sham-operated and bile duct-ligated (BDL) Sprague-Dawley rats were studied. Gene and protein expression of hepatic renin, angiotensinogen, angiotensin-converting enzyme (ACE), and the angiotensin receptors AT1 and AT2 were assessed using real-time reverse-transcription polymerase chain reaction, in vitro autoradiography, and immunohistochemistry.

RESULTS

Angiotensinogen and renin messenger RNA were detected in sham liver but were not increased following BDL. Angiotensinogen protein was widely distributed in hepatocytes in both normal and injured livers, but in BDL livers, it was also expressed within areas of active fibrogenesis. Both ACE and AT1 receptor genes were up-regulated following BDL. The low level of ACE activity in sham animals was significantly increased in areas of active fibrogenesis in BDL livers. The AT1 receptor was present in both normal and diseased liver parenchyma, with increased AT1 receptor binding seen in fibrotic areas in the diseased liver. The AT2 receptor gene was not detected in normal or diseased liver.

CONCLUSIONS

Key elements of the RAS are present in normal liver tissue, and there is major up-regulation of the system in the bile duct-ligated liver. These findings are in keeping with recent experimental studies that have demonstrated antifibrotic effects of RAS blockade in the bile duct-ligated liver.

AT2 receptor-mediated relaxation is preserved after long-term AT1 receptor blockade

Angiotensin II type 2 receptor (AT2R) stimulation may cause vasodilation per se and may contribute to the antihypertensive effect produced by Angiotensin II type 1 receptor (AT1R) antagonists, given that AT1R blockade increases endogenous levels of Ang II, suggesting a physiological role for the unblocked AT2R. Thus, we first directly assessed whether or not there is desensitization to AT2R-mediated vasorelaxation because this is an important consideration, given the raised Ang II levels and the marked desensitization that is known to occur after AT1R stimulation. Second, we examined if AT2R-mediated vasorelaxation is preserved after long-term treatment with the AT1R antagonist candesartan cilexetil. Consecutive concentration-response curves to AT2R stimulation, with either Ang II (with AT1R blockade) or the selective agonist CGP42112, were studied in rat isolated mesenteric resistance arteries mounted in an arteriograph. AT2R stimulation with Ang II induced a concentration-dependent relaxation without desensitization. Similarly, CGP42112 evoked highly reproducible relaxation, which, like Ang II, was abolished by the AT2R antagonist PD123319. By contrast, AT1R-mediated contraction exhibited marked desensitization. In rats treated with candesartan cilexetil (2 mg/kg per day for 2 weeks), AT1R-mediated contraction was abolished, whereas AT2R-mediated relaxation evoked by either Ang II or CGP42112 was highly reproducible, PD123319-sensitive, and of a magnitude similar to that observed in naïve animals. Therefore, this study has provided unequivocal evidence for the reproducible nature of AT2R-mediated vasorelaxation during short-term and long-term AT1R blockade. Such preservation of AT2R function is a prerequisite for the consideration of physiological role(s) of AT2R during AT1R blockade.

Renal expression of angiotensin receptors in long-term diabetes and the effects of angiotensin type 1 receptor blockade

OBJECTIVE

The aims of this study were to assess the renal expression of angiotensin type 1 (AT1) and type 2 (AT2) receptors in diabetic spontaneously hypertensive rats (SHR) and the effect of AT1 receptor blockade on the expression of these receptors.

DESIGN

Diabetes was induced by injection of streptozotocin in SHRs. Irbesartan, an AT1 receptor antagonist, was given to diabetic SHRs for 32 weeks (15 mg/kg per day, n = 10). Diabetic (n = 10) and non-diabetic SHRs (n = 10) were studied concurrently. A separate group of control and diabetic Wistar-Kyoto (WKY) rats were also evaluated.

METHODS

Gene and protein expressions of the AT1 and AT2 receptor were assessed by reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry with specific antibodies andin vitro autoradiography with [125I]Sar(1), Ile(8) angiotensin II or [125I]CGP42112B.

RESULTS

Both AT1 and AT2 receptor mRNA levels in the kidney were reduced in diabetic SHRs compared to non-diabetic SHRs. Immunohistochemistry staining with specific antibodies showed a similar reduction in glomerular and tubulo-interstitial staining for both AT1 and AT2 receptors. Reduced binding for the AT1 and AT2 receptor was found in the kidney of diabetic SHRs. Diabetic SHRs developed albuminuria and had glomerular and tubulo-interstitial injury, which were prevented by treatment with irbesartan. Reduced expression of the AT1 receptor, but not the AT2 receptor, in diabetic SHRs was prevented by treatment with irbesartan. In diabetic WKY rats no such reduction in AT1 expression was observed, although there was a trend for reduced AT2 receptor expression.

CONCLUSIONS

These findings demonstrated that renal expression of both AT1 and AT2 receptor was reduced in long-term diabetic SHRs and that blockade of the AT1 receptor had disparate effects on expression of angiotensin II receptor subtypes.

Prevention of accelerated atherosclerosis by angiotensin-converting enzyme inhibition in diabetic apolipoprotein E-deficient mice

BACKGROUND

Atherosclerosis is a major complication of diabetes, but the mechanisms by which diabetes promotes macrovascular disease have not been fully delineated. Although several animal studies have demonstrated that inhibition of ACE results in a decrease in the development of atherosclerotic lesions, information about the potential benefits of these agents on complex and advanced atherosclerotic lesions as observed in long-term diabetes is lacking. The aim of this study was to evaluate whether treatment with the ACE inhibitor perindopril affects diabetes-induced plaque formation in the apolipoprotein E (apoE)-deficient mouse.

METHODS AND RESULTS

Diabetes was induced by injection of streptozotocin in 6-week-old apoE-deficient mice. Diabetic animals received treatment with perindopril (4 mg x kg(-1) x d(-1)) or no treatment for 20 weeks. Nondiabetic apoE-deficient mice were used as controls. Induction of diabetes was associated with a 4-fold increase in plaque area compared with nondiabetic animals. This accelerated atherosclerosis was associated with a significant increase in aortic ACE expression and activity and connective tissue growth factor and vascular cell adhesion molecule-1 expression. Perindopril treatment inhibited the development of atherosclerotic lesions and diabetes-induced ACE, connective tissue growth factor, and vascular cell adhesion molecule-1 overexpression in the aorta.

CONCLUSIONS

The activation of the local renin-angiotensin system in the diabetic aorta and the reduction in atherosclerosis with ACE inhibitor treatment provides further evidence that the renin-angiotensin system plays a pivotal role in the development and acceleration of atherosclerosis in diabetes.

Angiotensin type 2 receptor antagonism confers renal protection in a rat model of progressive renal injury

The role of the angiotensin type 2 (AT(2)) receptor in the pathogenesis of progressive renal injury has not been previously elucidated. The renal expression of the AT(1) and AT(2) receptors in subtotally nephrectomized rats (STNx) and the effects of AT(2) receptor blockade on renal injury were explored. Reduced renal expression of the AT(1) but not the AT(2) receptor was observed in STNx by reverse transcription-PCR, by in vitro autoradiography, and by immunohistochemical staining. The STNx rats were randomly assigned to AT(1) receptor antagonist valsartan, AT(2) receptor antagonist PD123319, or the combination of both for 4 wk. Increased proteinuria in STNx rats was reduced by PD123319 but to a lesser degree when compared with valsartan. Reduced gene and protein expression of the slit diaphragm protein nephrin was prevented by either valsartan or PD123319. Expression of osteopontin, proliferating cell nuclear antigen, and monocyte/macrophage infiltration was increased in STNx rats and was reduced by both AT(1) and AT(2) receptor antagonists. These effects of AT(2) receptor antagonism were observed in the presence of increased BP in STNx rats. These findings suggest that blockade of the AT(2) receptor alone confers a degree of renal protection; in particular, it seems that the combination of the AT(1) and AT(2) receptor antagonists may confer additive renal effects than either receptor antagonist as monotherapy.