Possible Inhibition of Focal Cerebral Ischemia by Angiotensin II Type 2 Receptor Stimulation

Candesartan augments ischemia-induced proangiogenic state and results in sustained improvement after stroke

BACKGROUND AND PURPOSE

We have shown that acute treatment with candesartan in an experimental model of stroke resulted in vascular protection and improved outcomes at 24 hours poststroke, but the mechanisms are unknown. We now examine effects of candesartan on proangiogenic factors and 7-day outcomes using the same treatment paradigm.

METHODS

Male Wistar rats underwent 3 hours of middle cerebral artery occlusion followed by reperfusion. A single dose of 1 mg/kg candesartan intravenously was given at reperfusion. Animals received neurobehavioral testing before middle cerebral artery occlusion, at 24 hours after middle cerebral artery occlusion, and at 7 days. Blood pressure was measured by telemetry. Animals euthanized at 24 hours had brain tissue and cerebrospinal fluid collected for matrix metalloproteinase activity, vascular endothelial growth factor expression, and tube formation assay. Neurobehavioral testing included elevated body swing test, Bederson, beam walk, and paw grasp. Cerebrovascular density was quantified using immunohistochemistry at 24 hours and 7 days.

RESULTS

Matrix metalloproteinase-2 activity and vascular endothelial growth factor expression were higher (P=0.035, P=0.042, respectively) and cerebrospinal fluid was significantly more proangiogenic (5x tube formation; P=0.002) in the candesartan group at 24 hours. Although no difference was seen in infarct size at 7 days, treatment improved Bederson scores (2.1 versus 2.9, P=0.0083), elevated body swing test (22.9 versus 39.4, P=0.021), and paw grasp (1.29 versus 2.88, P=0.0001) at 7 days. Candesartan treatment resulted in increased vascular density in the striatum at 7 days (P=0.037).

CONCLUSIONS

Candesartan after reperfusion augments ischemia-induced angiogenic state and provides long-term benefits. The beneficial effects may involve vascular protection and enhancement of early angiogenic remodeling.

Inhibition of the renin-angiotensin system and target organ protection

The renin-angiotensin system (RAS) is involved in the pathological mechanisms of target organ damage, as well as in the induction of hypertension. RAS inhibition by angiotensin converting enzyme (ACE) inhibitors and angiotensin (Ang) II receptor blockers can prevent tissue damage by inhibition of Ang II type 1 receptor signaling. A beneficial effect of RAS inhibition on the heart, vasculature and kidney in cardiovascular disease has been reported. However, RAS inhibition can also prevent fibroproliferative diseases and damage of other tissues, such as brain, adipose tissue and muscle, because local RAS has an important role in tissue damage compared with circulating RAS. Moreover, other players, such as Ang II type 2 receptor signaling, aldosterone and ACE2 have been highlighted. Furthermore, there has also been a focus on the emerging concept of regulation of RAS, such as receptor-interacting proteins and receptor modifications, in the new discovery of therapeutic agents for tissue protection. The RAS has a pivotal role in various target organ damage, with complicated mechanisms; therefore, blockade of RAS may be therapeutically effective in preventing organ damage, as well as in having an antihypertensive effect.

Protective effect of candesartan in experimental ischemic stroke in the rat mediated by AT2 and AT4 receptors

OBJECTIVES

The contribution of the AT2 and AT4 angiotensin receptors to the protective role of the AT1 receptor blocker candesartan in acute ischemic stroke was investigated.

METHODS

Embolic stroke was induced by injection of calibrated microspheres (50 microm) in the right internal carotid in Sprague-Dawley rats.

RESULTS

Inhibition of production of endogenous angiotensins by pretreatment for 24 h with lisinopril significantly increased mortality and infarct volume, whereas candesartan for 24 h reduced blood pressure to the same extent but had no deleterious effect. A more sustained pretreatment with candesartan for 5 days significantly decreased mortality, neurological deficit and infarct size. The AT2 receptor antagonist PD123319 and the AT4 receptor antagonist divalinal abolished the protective effect of 5 days' AT1 blockade. Combined blockade of AT2 and AT4 in candesartan pretreated rats resulted in an increased mortality, neurological deficit and infarct volume of similar magnitude to lisinopril pretreatment. Coadministration of lisinopril 24 h before surgery completely blunted the protective effect of candesartan pretreatment. Administration of exogenous angiotensin IV (1 nmol) reversed the deleterious effect of lisinopril pretreatment.

CONCLUSION

Protection against acute cerebral ischemia induced by AT1 blockade for 5 days is blood pressure independent and mediated by both AT2 and AT4 angiotensin receptors.

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.

Exaggeration of focal cerebral ischemia in transgenic mice carrying human Renin and human angiotensinogen genes

BACKGROUND AND PURPOSE

We examined the possibility that activation of the human brain renin-angiotensin system is involved in enhancement of ischemic brain damage using chimeric transgenic mice with human renin (hRN) and human angiotensinogen (hANG) genes.

METHODS

Chimeric (hRN/hANG-Tg) mice were generated by mating of hRN and hANG transgenic mice. Permanent occlusion of the middle cerebral artery (MCA) by an intraluminal filament technique induced focal ischemic brain lesions.

RESULTS

hRN/hANG-Tg mice showed higher angiotensin II levels in the plasma and brain. The ischemic brain area at 24 hours after MCA occlusion was significantly enlarged in hRN/hANG-Tg mice with an enhanced neurological deficit compared to that in wild-type, hRN-Tg and hANG-Tg mice. The reduction of cerebral blood flow in the periphery region of the MCA territory after MCA occlusion was markedly exaggerated in hRN/hANG-Tg mice. Superoxide anion production in the brain and arteries was also increased significantly in hRN/hANG-Tg mice even before MCA occlusion and was further enhanced after MCA occlusion. Treatment with an AT(1) receptor blocker, valsartan (3.0 mg/kg per day), for 2 weeks significantly reduced the ischemic brain area and improved the neurological deficit after MCA occlusion in hRN/hANG-Tg mice, similar to those in wild-type, hRN-Tg, and hANG-Tg mice, with restoration of cerebral blood flow in the peripheral region and decreases in superoxide anion production and blood pressure.

CONCLUSIONS

These results indicate that activation of the human renin-angiotensin system exaggerates ischemic brain damage mainly through stimulation of the AT(1) receptor and marked reduction of cerebral blood flow and enhanced oxidative stress.

Protective mechanisms of the angiotensin II type 1 receptor blocker candesartan against cerebral ischemia: in-vivo and in-vitro studies

BACKGROUND

Angiotensin II type 1 (AT1) receptor blockers decrease ischemia by mechanisms dependent on and independent of arterial blood pressure in hypertensive rats and AT1-R knockout mice, respectively. However, the detailed mechanisms underlying the effects of AT1 receptor blockers remain unclear.

AIMS

To elucidate the systemic and focal effects of AT1 receptor blockers against cerebral ischemia in in-vivo and in-vitro studies.

METHODS

Normotensive Wistar rats were treated for 2 weeks with 0.5 or 1 mg/kg candesartan cilexetil and then subjected to 2-h middle cerebral artery occlusion-reperfusion. Human umbilical endothelial cells were stimulated with the active form of candesartan and angiotensin II in the absence and presence of an angiotensin II type 2 (AT2) receptor antagonist.

RESULTS

In candesartan-pretreated hypotensive and nonhypotensive rats, blood pressure was moderately increased during middle cerebral artery occlusion and fell gradually to the baseline after the reperfusion; it remained elevated in the control even after the reperfusion occlusion. Candesartan treatment resulted in a decrease in the cortical infarct volume and oxidative damage, the hypoxic status was improved, and the expression of repair-associated and growth-associated proteins in the cortical penumbra was augmented. Candesartan also increased the eNOS mRNA level and the lumen size of the middle cerebral artery. In human umbilical endothelial cells, candesartan increased the eNOS protein level AT2-R dependently, inhibited the expression of nicotinamide adenine dinucleotide phosphate oxidase subunits and angiotensin II-induced intracellular reactive oxygen species and nitric oxide, and promoted the extracellular release of nitric oxide, suggesting that it augmented the bioavailability of nitric oxide.

CONCLUSION

Among the mechanisms candesartan exerts in its protection against cerebral ischemia, restoration of endothelial function may represent an attractive therapeutic goal to address cerebral ischemia.

The role of mineralocorticoid receptor expression in brain remodeling after cerebral ischemia

Mineralocorticoid receptors (MRs) are classically known to be expressed in the distal collecting duct of the kidney. Recently it was reported that MR is identified in the heart and vasculature. Although MR expression is also found in the brain, it is restricted to the hippocampus and cerebral cortex under normal condition, and the role played by MRs in brain remodeling after cerebral ischemia remains unclear. In the present study, we used the mouse 20-min middle cerebral artery occlusion model to examine the time course of MR expression and activity in the ischemic brain. We found that MR-positive cells remarkably increased in the ischemic striatum, in which MR expression is not observed under normal conditions, during the acute and, especially, subacute phases after stroke and that the majority of MR-expressing cells were astrocytes that migrated to the ischemic core. Treatment with the MR antagonist spironolactone markedly suppressed superoxide production within the infarct area during this period. Quantitative real-time RT-PCR revealed that spironolactone stimulated the expression of neuroprotective or angiogenic factors, such as basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF), whereas immunohistochemical analysis showed astrocytes to be cells expressing bFGF and VEGF. Thereby the incidence of apoptosis was reduced. The up-regulated bFGF and VEGF expression also appeared to promote endogenous angiogenesis and blood flow within the infarct area and to increase the number of neuroblasts migrating toward the ischemic striatum. By these beneficial effects, the infarct volume was significantly reduced in spironolactone-treated mice. Spironolactone may thus provide therapeutic neuroprotective effects in the ischemic brain after stroke.

The renin-angiotensin system contributes to renal fibrosis through regulation of fibrocytes

BACKGROUND

The renin-angiotensin system is a major pathway in the pathogenesis of cardiovascular and renal diseases. Bone marrow-derived fibrocytes, which are dual positive for CD45 and type I collagen, are now considered to contribute to the pathogenesis of various fibrotic diseases. We hypothesized that fibrocytes might contribute to renal fibrosis by an angiotensin II dependent pathway.

RESULTS

In murine models of renal fibrosis, angiotensin II type 2 receptor (AT2R)-deficient mice, when compared with wild-type mice, showed increased renal fibrosis and fibrocyte infiltration with a concomitant upregulation of renal transcripts of procollagen type I (alpha) (COL1A1). Fibrocyte numbers in the bone marrow also were increased in AT2R-deficient mice. By contrast, pharmacological inhibition of angiotensin II type 1 receptor (AT1R) with valsartan reduced the degree of renal fibrosis and the number of fibrocytes in both the kidney and the bone marrow. In isolated human fibrocytes, inhibition of AT2R signaling increased the angiotensin II-stimulated expression of type I collagen, whereas inhibition of AT1R decreased collagen synthesis. These results suggest that AT1R/AT2R signaling may contribute to the pathogenesis of renal fibrosis by at least two mechanisms: by regulating the number of fibrocytes in the bone marrow, and by activation of fibrocytes.

Temporary pretreatment with the angiotensin II type 1 receptor blocker, valsartan, prevents ischemic brain damage through an increase in capillary density

BACKGROUND AND PURPOSE

We investigated the effect of temporary treatment with a nonhypotensive dose of valsartan on ischemic brain damage in C57BL/6 mice.

METHODS

We separated the mice into 3 groups of valsartan treatment before middle cerebral artery (MCA) occlusion: (1) for 4 weeks: Val (2W, 2W); (2) for 2 weeks followed by its cessation for 2 weeks: Val (2W, -); and (3) no treatment for 4 weeks: Val (-, -).

RESULTS

Ischemic volume, DNA damage, superoxide production, and mRNA levels of monocyte chemoattractant protein-1 and tumor necrosis factor-alpha on the ipsilateral side after 24 hours of MCA occlusion were significantly reduced in both Val (2W, 2W) and Val (2W, -) mice compared with those in Val (-, -) mice, whereas these parameters were larger in Val (2W, -) mice than in Val (2W, 2W) mice. Moreover, mice in both the Val (2W, 2W) and Val (2W, -) groups exhibited an increase in cerebral blood flow in the peripheral territory of the MCA 1 hour after MCA occlusion, with increases in endothelial nitric oxide synthase activation and nitric oxide production. Before MCA occlusion, treatment with valsartan did not influence superoxide production or mRNA levels of monocyte chemoattractant protein-1 and tumor necrosis factor-alpha in the brain. However, the capillary density in the brain in both Val (2W, 2W) and Val (2W, -) mice was increased before MCA occlusion.

CONCLUSIONS

Our results suggest that temporary valsartan treatment could protect against ischemic brain damage even after its cessation, at least in part due to an increase in capillary density.

Neuroprotective role of angiotensin II type 2 receptor after transient focal ischemia in mice brain

This study assessed the time course of angiotensin (Ang) II type 1 and type 2 receptor expression after 60 min of ischemia/reperfusion in mice treated with a nonhypotensive dose of valsartan, an angiotensin II type 1 receptor antagonist. We also examined the potential neuroprotective mechanisms mediated by angiotensin II type 2 receptor. Mice were divided into two groups (n=64, each): valsartan-treated and control, vehicle groups. Infarct volume and neurological deficit scores were evaluated at several time points after ischemia, while immunohistochemical analyses were performed at serial time points after reperfusion. Valsartan significantly reduced the infarct volume and improved the neurological deficit scores (P<0.05). Both angiotensin II type 1 and type 2 receptors were upregulated at 24h and peaked at 72 h with type I receptors dominating in the ischemic penumbra of the vehicle group. Interestingly, angiotensin II type 2 receptor expression levels were significantly higher in the valsartan group than vehicle controls (P<0.001). Moreover, angiotensin II type 2 receptor upregulated phosphosignal transducer and activator of transcription-3, and B-cell lymphoma protein-2 (P<0.05). Our results indicated that angiotensin II type 2 receptor has antiapoptotic activity by activating the B-cell lymphoma protein-2 via the janus kinase/signal transducer and activator of transcription signaling pathway.

Targeting genes in the renin-angiotensin system

PURPOSE OF REVIEW

The renin-angiotensin system plays a key role in the regulation of blood pressure and fluid homeostasis. Owing to its critical contribution to blood pressure control, abnormalities of any component in this system can lead to hypertension and cardiovascular diseases. In this review, we will highlight studies using this approach to uncover new perspectives on the physiology of the renin-angiotensin system.

RECENT FINDINGS

Over the past decade, application of techniques for manipulating the genome of living animals, including gene targeting through homologous recombination in embryonic stem cells, has provided unique insights into the complex biology of the renin-angiotensin system. Along with advances in understanding functions of the classical components of the system, gene targeting has clarified the functions of newly discovered angiotensin-converting enzyme homologues.

SUMMARY

Since pharmacological antagonists of the renin-angiotensin system are widely used in clinical medicine, advances in the gene-targeting experiments of the system have helped to clarify the mechanisms of action of these agents and may provide clues for improved approaches for the treatment of hypertension and kidney diseases.

Inhibition of cognitive decline in mice fed a high-salt and cholesterol diet by the angiotensin receptor blocker, olmesartan

The metabolic syndrome is closely related to dietary habits and seems to be associated with impairment of cognitive function in humans. Angiotensin receptor blockers are widely used with the expectation of preventing cardiovascular events and stroke and potential amelioration of the metabolic syndrome. We examined the diet-induced changes of cognitive function in mice treated with a high-salt and high-cholesterol diet. C57BL/6J mice were fed a high-salt (2% NaCl in drinking water) and high-cholesterol (1.25% cholesterol, 10% coconut oil) diet (HSCD) or a normal diet (ND), and subjected to 20 trials of a passive avoidance task every week from 8weeks of age. An age-dependent decline of the avoidance rate starting from 10weeks of age was observed in HSCD mice, whereas the avoidance rate gradually increased in the ND group. Oral administration of an angiotensin receptor blocker, olmesartan, at a dose of 3mg/kg per day in drinking water from 8weeks of age prevents this decline of avoidance rate in HSCD mice (49% vs. 82% at 12weeks of age). Treatment with olmesartan significantly decreased serum glucose and cholesterol levels in HSCD mice, with a slight decrease in blood pressure. Administration of olmesartan in HSCD-fed mice showed a 1.6-fold increase in mRNA expression of a neuroprotective factor, MMS2, compared to HSCD-fed mice without olmesartan. Olmesartan attenuated the increase in superoxide anion production detected by dihydroethidium staining in the brain of HSCD mice. Our results suggest that olmesartan could be therapeutically effective in preventing the impairment of quality of life in persons on a high-fat and high-salt diet.

Essential hypertension

Essential hypertension can be defined as a rise in blood pressure of unknown cause that increases risk for cerebral, cardiac, and renal events. In industrialised countries, the risk of becoming hypertensive (blood pressure >140/90 mm Hg) during a lifetime exceeds 90%. Essential hypertension usually clusters with other cardiovascular risk factors such as ageing, being overweight, insulin resistance, diabetes, and hyperlipidaemia. Subtle target-organ damage such as left-ventricular hypertrophy, microalbuminuria, and cognitive dysfunction takes place early in the course of hypertensive cardiovascular disease, although catastrophic events such as stroke, heart attack, renal failure, and dementia usually happen after long periods of uncontrolled hypertension only. All antihypertensive drugs lower blood pressure (by definition) and this decline is the best determinant of cardiovascular risk reduction. However, differences between drugs exist with respect to reduction of target-organ disease and prevention of major cardiovascular events. Most hypertensive patients need two or more drugs for blood-pressure control and concomitant statin treatment for risk factor reduction. Despite the availability of effective and safe antihypertensive drugs, hypertension and its concomitant risk factors remain uncontrolled in most patients.

Does a change in angiotensin II formation caused by antihypertensive drugs affect the risk of stroke?

BACKGROUND

Stroke prevention by antihypertensive therapy is believed to be related to the fall in blood pressure (BP). Experimental data have documented that activation of non-AT1 receptors of angiotensin II may exert anti-ischaemic mechanisms in the brain. The present meta-analysis of various randomized clinical trials attempts to relate stroke risk to angiotensin II formation during antihypertensive therapy.

METHODS

Primary and secondary stroke prevention was examined in 26 prospective, randomized clinical trials including 206,632 patients without heart failure, in whom a total of 7,108 strokes occurred. The trials were selected because a difference in angiotensin II generation was expected between the two treatment arms on the basis of the drugs' pharmacodynamic effects, and allowed 36 evaluations of the relative risk of stroke.

FINDINGS

In placebo-controlled trials, stroke risk was significantly higher with angiotensin II-decreasing than increasing drugs, but systolic BP decreased less in the former. Compared with an active therapy having a neutral effect on angiotensin II formation, stroke risk was also higher with angiotensin-decreasing drugs than with angiotensin-increasing drugs, whereas BP decrease was comparable with both drug classes. When angiotensin II-decreasing drugs were directly compared with angiotensin II-increasing drugs in the same trials, stroke risk was significantly increased. On-treatment systolic BP was minimally and significantly higher with angiotensin II-decreasing drugs, but not large enough to explain the excess in stroke risk.

CONCLUSION

Within the limitations of the methodology, our meta-analysis supports the hypothesis that angiotensin II-decreasing drugs are less stroke protective than angiotensin II-increasing drugs, although this difference is not entirely explained by their smaller BP-lowering effect.

Inhibition of the renin-angiotensin system and the prevention of stroke

BP is the most important determinant of the risk of stroke. A small reduction in BP results in a substantial reduction of both ischemic and hemorrhagic stroke. Any of the commonly used antihypertensive drugs lower the incidence of stroke, with larger reductions in BP resulting in larger reductions in risk. Experimental evidence has linked the renin-angiotensin system (RAS) to the development and progression of cerebrovascular disease. Inhibition of the RAS has beneficial cerebrovascular effects and may reduce the risk of stroke in a manner possibly independent from the alterations of BP. Some clinical trials even suggest that ACE inhibitors and angiotensin II type 1 receptor antagonists (angiotensin receptor blockers [ARBs]) exert cerebroprotective effects beyond BP lowering, but the evidence is controversial. Studies on specific protective actions of antihypertensive drugs are generally hampered by the fact that any treatment-related difference in BP may play a dominant role in the prevention of stroke. There are also indications that the protective potency of ARBs might be superior to that of ACE inhibitors, due to their differential activation of angiotensin II type 2 receptors, but the clinical relevance of this mechanism is unclear. Some studies in primary prevention of stroke, acute stroke, and secondary prevention show advantages for ARBs beyond controlling BP alone. In primary prevention, the LIFE randomized trial showed a significant difference in stroke rate in favor of losartan compared with atenolol despite similar reductions in BP. In acute stroke, the role of hypertension and its treatment remains controversial. ACCESS, however, suggested that an ARB is safe in hypertensive acute stroke patients and may offer advantages independent from BP control. In secondary stroke prevention, there are very few antihypertensive trials. These trials show that BP lowering is at least as successful as in primary prevention, but the absolute stroke risk is much higher. An ACE inhibitor was effective compared with placebo in the PROGRESS trial. The MOSES study showed that eprosartan prevented vascular events more effectively than nitrendipine, despite similar BP-lowering effects. Hypertension is not only the most important risk factor for stroke, but is also closely correlated with cognitive decline and dementia. Therefore, prevention of cognitive decline or even improvement of slightly diminished brain function should be an important goal for antihypertensive treatment in the future. Some clinical data suggest advantages for ACE inhibitors, ARBs, and calcium channel antagonists. Currently, however, the existing data are not sufficient for clinical recommendations. Therefore, ongoing trials will further define the exact role of inhibitors of the RAS and are urgently needed in secondary prevention, in acute stroke, and in the prevention of cognitive decline.

Pretreatment with eplerenone reduces stroke volume in mouse middle cerebral artery occlusion model

Eplerenone, a mineralocorticoid receptor antagonist, is reported to be effective to prevent end-stage cardiovascular damage induced by aldosterone. However, the effect of eplerenone on brain damage is not fully understood. Here, we investigated whether pretreatment with eplerenone attenuates stroke size in mice subjected to middle cerebral artery occlusion. Middle cerebral artery occlusion with a microfilament technique induced focal ischemia, to approximately 25% of the total area in a coronal section of the brain. Treatment with eplerenone at a dose of 1.67 mg/g chow significantly reduced the ischemic area, ischemic volume, and neurological deficit, without a blood pressure-lowering effect. Laser-Doppler flowmetry analysis showed a decrease in surface cerebral blood flow in the peripheral region after 1 h of middle cerebral artery occlusion. This decrease was smaller in mice treated with eplerenone. Superoxide production evaluated by staining with dihydroethidium was attenuated in the ischemic area of the brain in eplerenone-treated mice. Taken together, our findings suggest that eplerenone has a protective effect on ischemic brain damage, at least partly due to improvement of cerebral blood flow in the penumbra and reduction of oxidative stress.

An update on non-peptide angiotensin receptor antagonists and related RAAS modulators

The renin-angiotensin-aldosterone-system (RAAS) is an important regulator of blood pressure and fluid-electrolyte homeostasis. RAAS has been implicated in pathogenesis of hypertension, congestive heart failure, and chronic renal failure. Aliskiren is the first non-peptide orally active renin inhibitor approved by FDA. Angiotensin Converting Enzyme (ACE) Inhibitors are associated with frequent side effects such as cough and angio-oedema. Recently, the role of ACE2 and neutral endopeptidase (NEP) in the formation of an important active metabolite/mediator of RAAS, ang 1-7, has initiated attempts towards development of ACE2 inhibitors and combined ACE/NEP inhibitors. Furukawa and colleagues developed a series of low molecular weight nonpeptide imidazole analogues that possess weak but selective, competitive AT1 receptor blocking property. Till date, many compounds have exhibited promising AT1 blocking activity which cause a more complete RAAS blockade than ACE inhibitors. Many have reached the market for alternative treatment of hypertension, heart failure and diabetic nephropathy in ACE inhibitor intolerant patients and still more are waiting in the queue. But, the hallmark of this area of drug research is marked by a progress in understanding molecular interaction of these blockers at the AT1 receptor and unraveling the enigmatic influence of AT2 receptors on growth/anti-growth, differentiation and the regeneration of neuronal tissue. Different modeling strategies are underway to develop tailor made molecules with the best of properties like Dual Action (Angiotensin And Endothelin) Receptor Antagonists (DARA), ACE/NEP inhibitors, triple inhibitors, AT2 agonists, AT1/TxA2 antagonists, balanced AT1/AT2 antagonists, and nonpeptide renin inhibitors. This abstract gives an overview of these various angiotensin receptor antagonists.

Angiotensin II mediates postischemic leukocyte-endothelial interactions: role of calcitonin gene-related peptide

Vascular inflammation and enhanced production of angiotensin II (ANG II) are involved in the pathogenesis of hypertension and diabetes, disease states that predispose the afflicted individuals to ischemic disorders. In light of these observations, we postulated that ANG II may play a role in promoting leukocyte rolling (LR) and adhesion (LA) in postcapillary venules after exposure of the small intestine to ischemia-reperfusion (I/R). Using an intravital microscopic approach in C57BL/6J mice, we showed that ANG II type I (AT1) or type II (AT2) receptor antagonism (with valsartan or PD-123319, respectively), inhibition of angiotensin-converting enzyme (ACE) with captopril, or calcitonin gene-related peptide (CGRP) receptor blockade (CGRP8-37) prevented postischemic LR but did not influence I/R-induced LA. However, both postischemic LR and LA were largely abolished by concomitant AT1 and AT2 receptor blockade or chymase inhibition (with Y-40079). Additionally, exogenously administered ANG II increased LR and LA, effects that were attenuated by pretreatment with a CGRP receptor antagonist or an NADPH oxidase inhibitor (apocynin). Our work suggests that ANG II, formed by the enzymatic activity of ACE and chymase, plays an important role in inducing postischemic LR and LA, effects that involve the engagement of both AT1 and AT2 receptors and may be mediated by CGRP and NADPH oxidase.

Blockade of AT1 Receptor Reduces Apoptosis, Inflammation, and Oxidative Stress in Normotensive Rats with Intracerebral Hemorrhage

Angiotensin II exerts its central nervous system effects primarily via its receptors AT1 and AT2, and it participates in the pathogenesis of ischemia via AT1. The selective AT1 receptor blocker (ARB) is used in the hypertension treatment, and it exerts a variety of pleiotropic effects, including antioxidative, antiapoptotic, and anti-inflammatory effects. In this study, we investigated the therapeutic effect of the ARB telmisartan in experimental intracerebral hemorrhage (ICH) in normotensive rats. ICH was induced via the collagenase infusion or autologous blood injection. Either telmisartan at 30 mg/kg/dose or phosphate-buffered saline was orally administered 2 h after ICH induction. We evaluated hemorrhage volume, brain water content, and functional recovery, and we performed the histological analysis for terminal deoxynucleotidyl transferase dUTP nick-end labeling, leukocyte infiltration, and microglia activation. A variety of intracellular signals, in terms of oxidative stress, apoptotic molecules, and inflammatory mediators, were also measured. Telmisartan reduced hemorrhage volume, brain edema, and inflammatory or apoptotic cells in the perihematomal area. Telmisartan was noted to induce the expression of endothelial nitric-oxide synthase and peroxisome proliferator-activated receptor gamma and decrease oxidative stress, apoptotic signal, tumor necrosis factor-alpha, and cyclooxygenase-2 expression. The telmisartan-treated rats exhibited less pronounced neurological deficits and recovered better. Thus, telmisartan seems to offer neural protection, including antiapoptosis, anti-inflammatory, and antioxidant benefits in the intracerebral hemorrhage rat model.

Angiotensin, bradykinin and the endothelium

Angiotensins and kinins are endogenous peptides with diverse biological actions; as such, they represent current and future targets of therapeutic intervention. The field of angiotensin biology has changed significantly over the last 50 years. Our original understanding of the crucial role of angiotensin II in the regulation of vascular tone and electrolyte homeostasis has been expanded to include the discovery of new angiotensins, their important role in cardiovascular inflammation and the development of clinically useful synthesis inhibitors and receptor antagonists. While less applied progress has been achieved in the kinin field, there are continuous discoveries in bradykinin physiology and in the complexity of kinin interactions with other proteins. The present review focuses on mechanisms and interactions of angiotensins and kinins that deal specifically with vascular endothelium.

The Pathophysiologic Role of the Brain Renin-Angiotensin System in Stroke Protection: Clinical Implications

The brain possesses the same renin-angiotensin system (RAS) as the systemic circulation. Recent experimental studies have shown that the brain RAS plays an important role in stroke and neuronal protection through its effector peptide angiotensin (Ang) II. Ang II exerts its stroke-protective effects through stimulation of Ang II type 2 (AT2) receptors. Angiotensin receptor blockers (ARBs) exert a dual influence, which is important in their stroke protective effects. They selectively block the Ang II type 1 (AT1) receptors, decreasing local vasoconstriction, and allow free Ang II to stimulate the unoccupied AT2 receptor and increase local vasodilation, resulting in the alleviation of local brain ischemia and limiting the volume and extent of brain loss. In contrast, angiotensin-converting enzyme (ACE) inhibitors, by decreasing the amount of Ang II production, may diminish the stroke-protective effects of Ang II. This perhaps could be a reason for the inferior stroke-protective effect of ACE inhibitors compared with ARBs, which has been demonstrated in several clinical trials. The evidence for this effect of ARBs compared with ACE inhibitors, however, is only indirect. Ongoing clinical trials with head-to-head comparisons of ARBs and ACE inhibitors will hopefully provide the needed information.

Blood pressure lowering after experimental cerebral ischemia provides neurovascular protection

BACKGROUND

There is evidence that acutely elevated blood pressure (BP) after stroke is associated with increased cerebral hemorrhage and edema. Previous experiments in our laboratory have shown that candesartan 1 mg/kg administered after reperfusion in a model of hypertension after experimental ischemic stroke reduces neurovascular damage and improves outcome. These results could be either mediated by BP lowering or a BP-independent cerebrovascular protective effect.

OBJECTIVES

To determine the contribution of BP lowering to the neurovascular protection previously reported with candesartan after stroke.

METHODS

Male Wistar rats (280-305 g) underwent 3 h of middle cerebral artery occlusion (MCAO). At reperfusion, either hydralazine 1 mg/kg (n = 8), enalapril 5 mg/kg (n = 7) or enalapril 10 mg/kg (n = 8) were administered intravenously. BP was measured by telemetry for 2 days before and 24 h after MCAO. After neurological function was assessed, brain tissue was processed for infarct size and hemoglobin content analyses.

RESULTS

Mean arterial pressure (MAP) increased from 92 to 124 mmHg immediately upon MCAO and decreased to 112 mmHg after reperfusion, remaining elevated for 24 h (P < 0.0001) in the saline group. Hydralazine reduced MAP (P = 0.048) and infarct size (53 versus 30%, P = 0.0083), and there was a trend towards decreased hemoglobin content. Enalapril 5 mg/kg did not significantly change MAP or other outcomes. Enalapril 10 mg/kg reduced MAP (P < 0.0001) and infarct size (53 versus 29%, P = 0.003). There was an intermediate effect on both hemoglobin content and neurological function, neither one was significant. The time course of BP lowering varied with each treatment.

CONCLUSION

Acute BP lowering after reperfusion in acute ischemic stroke is an effective strategy to achieve neurovascular protection. The rate, extent and mechanism of BP lowering may determine the magnitude of protection.

A third-generation, long-acting, dihydropyridine calcium antagonist, azelnidipine, attenuates stent-associated neointimal formation in non-human primates

BACKGROUND

Calcium antagonists have been shown to reduce atherogenesis and improve clinical outcomes in atherosclerotic vascular disease. No study has so far, however, addressed the effects of calcium antagonists on stent-associated neointimal formation. We therefore investigated whether a third-generation calcium antagonist, azelnidipine, attenuates in-stent neointimal formation in non-human primates.

METHOD

Male cynomolgus monkeys were fed a high cholesterol diet for 4 weeks, and were randomly assigned to three groups: a vehicle group and two other groups treated with azelnidipine at 3 and 10 mg/kg per day for an additional 24 weeks (n = 12 each). Multi-link stents were then implanted in the iliac artery.

RESULTS

Azelnidipine at the high dose reduced neointimal thickness (0.25 +/- 0.02 versus 0.19 +/- 0.02 mm; P < 0.05). Azelnidipine also reduced local oxidative stress and monocyte chemoattractant protein 1 (MCP-1) expression. No difference was found between the three groups in the degrees of injury score, inflammation score, plaque neovascularization, or plasma lipid levels. Azelnidipine also reduced MCP-1-induced proliferation/migration of vascular smooth muscle cells in vitro.

CONCLUSIONS

This study demonstrated for the first time that azelnidipine attenuates in-stent neointimal formation associated with the reduced expression of MCP-1 and smooth muscle proliferation/migration in the neointima. These data in non-human primates suggest potential clinical benefits of azelnidipine as a 'vasculoprotective calcium antagonist' in patients undergoing vascular interventions.

Inhibition of Matrix Metalloproteinase-9 Activity by Trandolapril after Middle Cerebral Artery Occlusion in Rats

We investigated whether an angiotensin-converting enzyme (ACE) inhibitor could inhibit matrix metalloproteinase (MMP) activities in cerebral infarct lesions after middle cerebral artery occlusion (MCAO) in rats. After placebo or trandolapril (5 mg/kg per day) was administered orally for 7 days, we permanently occluded the right middle cerebral artery. ACE activity in extracts from the infarct side of placebo-treated rats was significantly higher than that in extracts from the non-infarct side from 5 days after MCAO, though they did not differ at 1 day. ACE activities in extracts from both hemispheric segments in the trandolapril-treated group were significantly decreased compared with those in the placebo-treated group before MCAO, and this significant reduction persisted even at 7 days after MCAO. In the placebo-treated group, MMP-9 and MMP-2 activities in the infarct side were significantly increased at 12 h and at 1 day after MCAO, respectively. Trandolapril treatment significantly reduced MMP-9 and MMP-2 activities to 68.5% and 53.2%, respectively. Seven days after MCAO, the ratios of infarct areas to the hemispheric sectional areas in placebo- and trandolapril-treated rats were 55.4+/-2.1% and 30.9+/-2.9%, respectively, and this difference was significant. Neurological severity scores were significantly improved from 1 to 7 days after MCAO in trandolapril-treated rats. Cumulative survival in trandolapril-treated rats was significantly increased compared with that in placebo-treated rats. Thus, the inhibition of MMP-9 by trandolapril might be part of the mechanism that prevents cerebral damage after cerebral ischemia.

Synergistic protective effects of erythropoietin and olmesartan on ischemic stroke survival and post-stroke memory dysfunctions in the gerbil

OBJECTIVES

Treatment with erythropoietin and AT1 blockers is protective in experimental acute cerebral ischemia, with promising results in pilot clinical studies in human stroke. This paper examines the effects of using both agents as combination therapy in acute ischemic stroke.

METHODS

We used the single carotid ligation stroke model in the gerbil. Six groups of 50 gerbils were treated either with placebo, erythropoietin (intraperitoneally, 5000 IU/kg, 2 and 48 h after stroke), olmesartan (10 mg/kg per day in drinking water started 36 h after stroke), ramipril (2.5 mg/kg per day in drinking water started 36 h after stroke), erythropoietin + olmesartan, or erythropoietin + ramipril. Long-term (1 month) Kaplan-Meyer survival curves were obtained, and survivors were submitted at day 30 to immediate (object recognition test) and spatial (Morris water maze) memory function tests.

RESULTS

Erythropoietin alone and olmesartan alone, but not ramipril, significantly increased survival at day 30 compared with untreated controls (38, 30 and 6% versus 12%, respectively). Combined treatment with erythropoietin and olmesartan further increased the survival rate to 56%, whereas combined therapy with erythropoietin and ramipril decreased 30-day survival to 24% (P < 0.0001, erythropoietin + olmesartan versus erythropoietin + ramipril). Untreated stroke survivors had markedly altered performances in both the object recognition test (P = 0.0007) and the Morris water maze (P < 0.0001) tests at day 30 compared with normal gerbils. In erythropoietin-treated animals, ramipril therapy had no beneficial effect whereas olmesartan fully restored normal response to the memory tests.

CONCLUSION

Post-infarct treatment with olmesartan combined with early erythropoietin therapy has a protective effect on survival, and markedly improves long-term memory dysfunction in this experimental model.

The calcium-channel blocker, azelnidipine, enhances the inhibitory action of AT1 receptor blockade on ischemic brain damage

OBJECTIVE

The combined effects of a calcium-channel blocker (CCB) with an angiotensin (Ang) II type 1 (AT1) receptor blocker were investigated in focal brain ischemia induced by middle cerebral artery (MCA) occlusion.

METHODS AND RESULTS

In male C57BL/6J mice, permanent occlusion of the MCA-induced focal cerebral ischemia and neurological deficit after 24 h, accompanied by a reduction of cerebral blood flow and an increase in superoxide production in the ischemic area. Administration of azelnidipine, a CCB, at 1.0 mg/kg per day for 10 days significantly suppressed these changes after MCA without affecting systolic blood pressure. Such inhibitory effects of azelnidipine on brain ischemia could be observed in AT1a receptor-deficient mice. In addition, olmesartan, an AT1 receptor blocker, at 3.0 mg/kg per day also diminished the ischemic brain area and neurological score, as well as superoxide production and the reduction of cerebral surface blood flow in C57BL/6 mice. The combination of lower doses of azelnidipine (0.1 mg/kg per day) and olmesartan (0.5 mg/kg per day) significantly attenuated the ischemic brain area, neurological score, superoxide production and the reduction of cerebral surface blood flow after MCA occlusion in C57BL/6 mice, whereas either of these agents alone at these doses did not affect brain ischemia.

CONCLUSION

These results indicate that azelnidipine inhibited ischemic brain damage induced by MCA occlusion, at least in part, through suppression of blood flow change and oxidative stress via a signaling mechanism independent of AT1 receptor stimulation. Moreover, azelnidipine synergistically enhanced the inhibitory action of olmesartan on brain ischemia, suggesting beneficial combined effects of a CCB with an AT1 receptor blocker on ischemic brain damage.

Targeting angiotensin II type I receptors to reduce the risk of stroke in patients with hypertension

The incidence of stroke is rising among the elderly population. Hypertension is the single most important contributor to stroke, and hypertension treatment and control is the most important intervention to prevent it. Blood pressure reduction by any means will substantially reduce the risk of stroke. However, accumulating evidence indicates that angiotensin receptor blocker-based regimens may provide additional benefits in the prevention of stroke. This effect seems to be particularly prominent in high-risk patients and patients with isolated systolic hypertension. The pathophysiological mechanisms have not been clearly delineated, but favourable remodelling of the left atrium and reduction of the risk of atrial fibrillation has been suggested. In addition, blockade of the central AT1 receptors and stimulation of AT2 receptors and/or preferential reduction of central pressure by angiotensin receptor blockers may account for the beneficial protective effect.

Comparison of inhibitory action of candesartan and enalapril on brain ischemia through inhibition of oxidative stress

The effects of an angiotensin II type 1 (AT1) receptor blocker (ARB) on ischemic brain damage induced by middle cerebral artery (MCA) occlusion were compared with those of an angiotensin converting enzyme (ACE) inhibitor. Treatment of male C57BL/6J mice with an ARB, candesartan, reduced the brain ischemic area and neurological deficit after MCA occlusion at a non-hypotensive dose. In contrast, an ACE inhibitor, enalapril, did not reduce the brain ischemic area, and neurological deficit even at a hypotensive dose. Candesartan improved the reduction of brain surface blood flow after MCA occlusion, and inhibited the increase in superoxide production both in the cortex and brain arterial wall at non-hypotensive and hypotensive doses. However, enalapril did not affect the changes in blood flow and superoxide production in the brain after MCA occlusion. AT2 receptor expression in the ischemic area was increased at 3 h after MCA occlusion by pretreatment with candesartan, but not that with enalapril. AT1 receptor expression was neither affected by candesartan nor by enalapril. These results suggest that candesartan attenuated ischemic brain damage, at least partly, through inhibition of oxidative stress.

Susceptibility of brain microvascular endothelial cells to advanced glycation end products-induced tissue factor upregulation is associated with intracellular reactive oxygen species

There is accumulating evidence that advanced glycation end products (AGEs) are relevant to the formation of vascular complications in diabetes mellitus. The aim of this study was to investigate whether AGEs have a significant effect on tissue factor (TF) expression in brain microvascular endothelial cells compared with that in other arterial endothelial cells. Cultured bovine brain microvascular endothelial cells (BBMECs) and aortic endothelial cells (BAECs) were incubated in medium containing glyceraldehyde-derived AGE (glycer-AGE). TF mRNA expression, protein expression, and activity were measured at multiple time points after glycer-AGE incubation. Participation of reactive oxygen species (ROS) in the effect of glycer-AGE on TF expression was investigated by treatment with a free radical scavenger, edaravone, and intracellular ROS measurements with dihydroethidium (DHE). Basic TF mRNA expression was greater in BBMECs than in BAECs. Glycer-AGE significantly upregulated TF mRNA expression in both cells, and the upregulation was more prominent in BBMECs than in BAECs. TF protein expression and activity were also upregulated with a pattern of being greater in BBMECs than in BAECs. Edaravone significantly attenuated the AGE-induced upregulation of TF mRNA expression, protein expression, and activity. Intracellular ROS levels measured with DHE-stained fluorescent intensity were significantly upregulated by glycer-AGE with a pattern of being greater in BBMECs than in BAECs. AGE-induced ROS upregulation was attenuated by edaravone like AGE-induced TF upregulation. These results suggest that brain microvascular endothelial cells are more susceptible to AGE-induced TF upregulation than aortic endothelial cells, and that this susceptibility is associated with levels of intracellular ROS.

Angiotensin receptor blockers and cerebral protection in stroke

Stroke is a burden of modern civilization, causing death and disability. Nowadays it is universally accepted that inhibition of the renin-angiotensin system by angiotensin-converting enzyme inhibitors or angiotensin II type 1 (AT1) receptor blockers (ARBs) can effectively decrease the incidence of stroke in patients at risk. Here, we summarize current knowledge concerning the molecular mechanisms of the beneficial effects of inhibition of the renin-angiotensin system in stroke, with an emphasis on mechanisms beyond blood pressure reduction; in particular, neuroprotection. All major clinical studies comparing the effectiveness of ARBs with placebo or other blood pressure decreasing drugs in stroke are mentioned and commented on. These clinical data are complemented by data from a selection of animal experiments pivotal for the understanding of neuroprotective actions of ARBs. Clinical studies have shown that ARBs can be superior to other antihypertensive drugs in the prevention of stroke, even if there are no differences in blood pressures. Findings from animal experiments suggest that the underlying mechanisms include not just inhibition of the detrimental peripheral and central actions of angiotensin II mediated by AT1-receptors, but also stimulation of unopposed angiotensin II type 2 (AT2) receptors that are upregulated in the area of ischaemia. ARBs have been proven to be effective in the prevention of stroke via mechanisms that are both dependent on and independent of the antihypertensive abilities of the drugs.

Angiotensin II Type-2 Receptor Stimulation Prevents Neural Damage by Transcriptional Activation of Methyl Methanesulfonate Sensitive 2

The molecular mechanisms of the contribution of angiotensin II type-1 receptor blockers to neuronal protection are still unclear. Here, we investigated the effect of angiotensin II type-2 (AT2) receptor stimulation on neurons and cognitive function involving a new neuroprotective factor, methyl methanesulfonate sensitive 2 (MMS2). Angiotensin II treatment of neurospheres enhanced their differentiation and increased MMS2 expression. Knockdown of the MMS2 gene by small interference RNA (siRNA) significantly reduced the number of neurospheres, with loss of sphere formation. An angiotensin II type-1 receptor blocker, valsartan, enhanced such neurosphere differentiation and MMS2 induction, whereas an AT2 receptor antagonist, PD123319, inhibited them. After mice underwent permanent middle cerebral artery occlusion, AT2 receptor mRNA expression was significantly increased in the ischemic side of the brain. Passive avoidance rate to evaluate cognitive function was significantly impaired in AT2 receptor null (Agtr2-) mice compared with wild-type mice. Treatment with valsartan prevented the cognitive decline in wild-type mice, but this effect was weaker in Agtr2- mice. In ischemic brain regions, MMS2 was increased in wild-type mice, but not in Agtr2- mice. Valsartan also enhanced MMS2 expression to a greater degree in wild-type mice. Finally, intracerebroventricular administration of MMS2 siRNA showed more impaired avoidance rate after middle cerebral artery occlusion compared with that in control siRNA-transfected mice. These findings experimentally support the clinical evidence and indicate a unique mechanism of the AT2 receptor in brain protection.

Anti-fibrogenic function of angiotensin II type 2 receptor in CCl4-induced liver fibrosis

The renin-angiotensin system (RAS) contributes to fibrogenesis in a variety of organs. We recently showed that a lack of angiotensin (Ang) II type 1 (AT1) receptor activity reduces liver fibrosis. In this study, we investigated whether the Ang II type 2 (AT2) receptor is implicated in the development of liver fibrosis. A comparison was made between AT2-receptor knockout (AT2KO) and wild type (WT) mice after 4 weeks of treatment with carbon tetrachloride (CCl4). Fibrosis was assessed by Azan-Mallory staining and hepatic hydroxyproline (HP) content. The expression of fibrogenic mRNA was measured by real-time quantitative reverse-transcription polymerase chain reaction (PCR). Liver fibrosis evaluated by regular histological analyses and immunohistochemical alpha-SMA staining was observed in both groups of mice. The extent of fibrosis was greatest in the AT2KO mice. Fibrosis was associated with increases in hepatic HP content and mRNA expression for TGF-beta1 and alpha-SMA, as well as an increase in hepatic TBARS. These findings suggest that CCl4 induces oxidative stress which leads to activation of hepatic stellate cells (HSCs). These changes were considerably more pronounced in the AT2KO mice than the WT mice. Taken together, we conclude that AT2 signal has anti-fibrogenic and/or cytoprotective effects on oxidative stress-induced liver fibrosis. We therefore suggest that RAS-associated liver fibrogenesis may be determined by the balance between AT1 and AT2 signals.

Mechanisms of the Anti-Ischemic Effect of Angiotensin II AT( 1 ) Receptor Antagonists in the Brain

1. Circulating and locally formed Angiotensin II regulates the cerebral circulation through stimulation of AT(1) receptors located in cerebrovascular endothelial cells and in brain centers controlling cerebrovascular flow. 2. The cerebrovascular autoregulation is designed to maintain a constant blood flow to the brain, by vasodilatation when blood pressure decreases and vasoconstriction when blood pressure increases. 3. During hypertension, there is a shift in the cerebrovascular autoregulation to the right, in the direction of higher blood pressures, as a consequence of decreased cerebrovascular compliance resulting from vasoconstriction and pathological growth. In hypertension, when perfusion pressure decreases as a consequence of blockade of a cerebral artery, reduced cerebrovascular compliance results in more frequent and more severe strokes with a larger area of injured tissue. 4. There is a cerebrovascular angiotensinergic overdrive in genetically hypertensive rats, manifested as an increased expression of cerebrovascular AT(1) receptors and increased activity of the brain Angiotensin II system. Excess AT(1) receptor stimulation is a main factor in the cerebrovascular pathological growth and decreased compliance, the alteration of the cerebrovascular eNOS/iNOS ratio, and in the inflammatory reaction characteristic of cerebral blood vessels in genetic hypertension. All these factors increase vulnerability to brain ischemia and stroke. 5. Sustained blockade of AT(1) receptors with peripheral and centrally active AT(1) receptor antagonists (ARBs) reverses the cerebrovascular pathological growth and inflammation, increases cerebrovascular compliance, restores the eNOS/iNOS ratio and decreases cerebrovascular inflammation. These effects result in a reduction of the vulnerability to brain ischemia, revealed, when an experimental stroke is produced, in protection of the blood flow in the zone of penumbra and substantial reduction in neuronal injury. 6. The protection against ischemia resulting is related to inhibition of the Renin-Angiotensin System and not directly related to the decrease in blood pressure produced by these compounds. A similar decrease in blood pressure as a result of the administration of beta-adrenergic receptor and calcium channel blockers does not protect from brain ischemia. 7. In addition, sustained AT(1) receptor inhibition enhances AT(2) receptor expression, associated with increased eNOS activity and NO formation followed by enhanced vasodilatation. Direct AT(1) inhibition and indirect AT(2) receptor stimulation are associated factors normalizing cerebrovascular compliance, reducing cerebrovascular inflammation and decreasing the vulnerability to brain ischemia.8. These results strongly suggest that inhibition of AT(1) receptors should be considered as a preventive therapeutic measure to protect the brain from ischemia, and as a possible novel therapy of inflammatory conditions of the brain.

The pleiotropic effects of angiotensin receptor blockers

The angiotensin receptor blockers (ARBs) are very effective and safe antihypertensive drugs. They exert their antihypertensive effect through blockage of the angiotensin II, type 1 receptor and quite possibly through stimulation by angiotensin II of the unoccupied type 2 receptor. Besides hypertension, the ARBs have been found recently to be of value in the treatment of heart failure and diabetic nephropathy. In addition, ARBs have emerged lately as being very effective and perhaps superior to other antihypertensive drugs in the prevention of de novo or recurrent strokes. Other actions that may account for their stroke-protective effects include their antiatherogenic, antidiabetic, antiplatelet aggregating, hypouricemic, and atrial antifibrillatory actions. All these actions make the ARBs a true pleiotropic class of drugs. Each of the foregoing effects will be discussed briefly in this concise review.

Angiotensin II type 1 receptors in cerebral ischaemia-reperfusion: initiation of inflammation

Cerebral ischaemia-reperfusion injury is associated with an inflammatory response, with contributions from leucocytes and microglia. Formation of free radicals and nitric oxide contributes to the development of cerebral infarction and of the neurological deficit that follows transient focal ischaemia. The circulating and cerebral renin-angiotensin systems contribute, via stimulation of the angiotensin II (Ang II) types 1 (AT1) and 2 receptors, to the initiation or progression of inflammatory processes, and blockade of AT1-receptors prevents irreversible tissue injury and improves outcome from stroke in animal experiments. Such cerebral protection can be achieved even when treatment is initiated hours after established reperfusion. Blockade of AT1-receptors also reduces the incidence of stroke and cardiovascular mortality associated with stroke in patients; however, the mechanisms underlying the prevention of stroke by AT1-receptor blockade in patients remain to be elucidated. In this review we summarize the existing experimental and clinical data demonstrating that the renin-angiotensin system contributes to the inflammation and subsequent irreversible injury after cerebral ischaemia-reperfusion. We conclude that AT1-receptor blockade reduces cerebral ischaemia-reperfusion injury in part by attenuating inflammatory processes.

Hypertension after experimental cerebral ischemia: candesartan provides neurovascular protection

BACKGROUND

After ischemic stroke, hypertension increases the risk of recurrence, hemorrhage and fatal cerebral edema, but blood pressure (BP) lowering in the acute stroke period is controversial due to fears of infarct extension and worsened outcomes.

OBJECTIVE

To determine whether BP lowering with candesartan, initiated at reperfusion, can reduce neurovascular damage and improve outcome in a model of hypertension after experimental ischemic stroke.

METHODS

Male Wistar rats (280-305 g) underwent 3 h of middle cerebral artery occlusion (MCAO). At reperfusion, either saline (n = 18) or candesartan 1 mg/kg (n = 18) was administered intravenously. BP was measured by telemetry for 2 days before and 24 h after MCAO. Neurologic function was assessed and sacrifice occurred at 24 h after occlusion. Brain tissue was analyzed for infarct size, hemoglobin content and edema.

RESULTS

Mean BP increased from 96 to 124 mmHg immediately upon MCAO and decreased to 114 mmHg after reperfusion, remaining elevated for 24 h (P < 0.001) in the saline group. Candesartan reduced BP back to baseline and BP remained lower than in saline-treated animals until sacrifice (P < 0.001). Infarct size (54 versus 38%, P = 0.01) and hemoglobin content (23.4 versus 10.0 microg/g tissue; P = 0.03) and edema (17.97 versus 11.33%, P < 0.0001) were lower in the candesartan group. In addition, neurologic function at 24 h was improved (P = 0.0036) in the candesartan group.

CONCLUSIONS

Candesartan administered after reperfusion in acute ischemic stroke reduces neurovascular damage and improves outcome.

Cardioprotective mechanisms of eplerenone on cardiac performance and remodeling in failing rat hearts

Aldosterone may play a pivotal role in the pathophysiology of heart failure. To elucidate the beneficial cardioprotective mechanism of eplerenone, a novel selective aldosterone blocker, we hypothesized that eplerenone stimulates endothelial NO synthase (eNOS) through Akt and inhibits inducible NO synthase (iNOS) via nuclear factor kappaB (NF-kappaB) after the development of oxidative stress and activation of the lectin-like, oxidized, low-density lipoprotein receptor 1 (LOX-1) pathway in Dahl salt-sensitive rats with heart failure. Eplerenone (10, 30, and 100 mg/kg per day) was given from the age of the left ventricular hypertrophy stage (11 weeks) to the failing stage (18 weeks) for 7 weeks. The left ventricular end-systolic pressure-volume relationship was evaluated using a conductance catheter. Decreased percentage of fractional shortening by echocardiography and end-systolic pressure-volume relationship in failing rats was significantly ameliorated by eplerenone. Downregulated eNOS expression, eNOS and Akt phosphorylation, and NOS activity in failing rats were increased by eplerenone. Upregulated expression of the mineralocorticoid receptor aldosterone synthase (CYP11B2); NAD(P)H oxidase p22phox, p47phox, gp91phox, iNOS, and LOX-1; and activated p65 NF-kappaB, protein kinase CbetaII, c-Src, p44/p42 extracellular signal-regulated kinase, and p70S6 kinase phosphorylation were inhibited by eplerenone. Eplerenone administration resulted in significant improvement of cardiac function and remodeling and upregulation of sarcoplasmic reticulum Ca(2+)-ATPase expression. These findings suggest that eplerenone may have significant therapeutic potential for heart failure, and these cardioprotective mechanisms of eplerenone may be mediated in part by stimulating eNOS through Akt and inhibiting iNOS via NF-kappaB after activation of the oxidative stress-LOX-1 pathway and signal transduction pathway.