Pre-treatment with candesartan protects from cerebral ischaemia

The Renin Angiotensin System as a Therapeutic Target in Traumatic Brain Injury

Traumatic brain injury (TBI) is a major public health problem, with limited pharmacological options available beyond symptomatic relief. The renin angiotensin system (RAS) is primarily known as a systemic endocrine regulatory system, with major roles controlling blood pressure and fluid homeostasis. Drugs that target the RAS are used to treat hypertension, heart failure and kidney disorders. They have now been used chronically by millions of people and have a favorable safety profile. In addition to the systemic RAS, it is now appreciated that many different organ systems, including the brain, have their own local RAS. The major ligand of the classic RAS, Angiotensin II (Ang II) acts predominantly through the Ang II Type 1 receptor (AT1R), leading to vasoconstriction, inflammation, and heightened oxidative stress. These processes can exacerbate brain injuries. Ang II receptor blockers (ARBs) are AT1R antagonists. They have been shown in several preclinical studies to enhance recovery from TBI in rodents through improvements in molecular, cellular and behavioral correlates of injury. ARBs are now under consideration for clinical trials in TBI. Several different RAS peptides that signal through receptors distinct from the AT1R, are also potential therapeutic targets for TBI. The counter regulatory RAS pathway has actions that oppose those stimulated by AT1R signaling. This alternative pathway has many beneficial effects on cells in the central nervous system, bringing about vasodilation, and having anti-inflammatory and anti-oxidative stress actions. Stimulation of this pathway also has potential therapeutic value for the treatment of TBI. This comprehensive review will provide an overview of the various components of the RAS, with a focus on their direct relevance to TBI pathology. It will explore different therapeutic agents that modulate this system and assess their potential efficacy in treating TBI patients.

ACE (Angiotensin-Converting Enzyme) Inhibition Reverses Vasoconstriction and Impaired Dilation of Pial Collaterals in Chronic Hypertension

Leptomeningeal anastomoses (LMAs) are pial collaterals that perfuse the penumbra and important for stroke outcome. We previously showed LMAs from SHRs (spontaneously hypertensive rats) were vasoconstricted compared with normotensive Wistar rats. Here, we investigated mechanisms by which hypertension causes LMA vasoconstriction. SHRs were treated with the ACE (angiotensin-converting enzyme) inhibitor captopril, an Ang II (angiotensin II)-independent antihypertensive agent hydralazine, or vehicle for 5 weeks in drinking water (n=8/group). A group of Wistar rats (n=8) had regular drinking water served as controls. Blood pressure was measured twice weekly by tail-cuff. LMAs were isolated and studied under pressurized conditions. Vasoreactivity of LMAs, including myogenic responses, reactivity to Rho-kinase inhibitor Y-27632, and nitric oxide were measured. Both captopril and hydralazine lowered blood pressure in SHRs similar to Wistar. However, only captopril normalized LMA increased tone compared with untreated SHRs (15±2% versus 50±3%; P<0.01) that was similar to Wistar (16±2%) but not hydralazine (38±6%; P>0.05). Vasodilatory response of LMAs to Y-27632 was impaired in SHRs compared with Wistar (28±3% versus 81±4%; P<0.01) that was restored by captopril (84±5%; P<0.01) and partially hydralazine (59±4%). LMAs from all groups constricted similarly to NOS (NO synthase) inhibition; however, the vasodilatory response of LMAs to the nitric oxide donor sodium nitroprusside was impaired in SHRs compared with Wistar rats (29±4% versus 80±2%; P<0.01) that was restored by captopril (84±4%; P<0.01), not hydralazine (38±8%; P>0.05). These results suggest that ACE inhibition during chronic hypertension reversed vascular dysfunction and hyperconstriction of LMAs that could improve stroke outcome by increasing collateral perfusion.

Effects of different classes of antihypertensive agents on the outcome of acute ischemic stroke

It is unclear whether antihypertensive treatment before stroke affects acute ischemic stroke severity and outcome. To evaluate this association, the authors studied 482 consecutive patients (age 78.8±6.7 years) admitted with acute ischemic stroke. Stroke severity was assessed at admission with the National Institutes of Health Stroke Scale (NIHSS). The outcome was assessed with rates of adverse outcome (modified Rankin scale at discharge ≥2). Independent predictors of severe stroke (NIHSS ≥16) were female sex and atrial fibrillation. Treatment with diuretics before stroke was associated with nonsevere stroke. At discharge, patients with adverse outcome were less likely to be treated before stroke with β-blockers or with diuretics. Independent predictors of adverse outcome were older age, higher NIHSS at admission, and history of ischemic stroke. Treatment with diuretics before stroke appears to be associated with less severe neurologic deficit in patients with acute ischemic stroke.

Transgenic Mice Overexpressing Human Angiotensin I Receptor Gene Are Susceptible to Stroke Injury

Hypertension is one of the co-morbid conditions for stroke and profoundly increases its incidence. Angiotensin II (AngII) is shown to be at the center stage in driving the renin angiotensin system via activation of angiotensin 1 receptor (AT1R). This makes the AT1R gene one of the candidates whose differential regulation leads to the predisposition to disorders associated with hypertension. A haplotype block of four SNPs is represented primarily by haplotype-I, or Hap-I (TTAA), and haplotype-II, or Hap-II (AGCG), in the promoter of human AT1R (hAT1R) gene. To better understand the physiological role of these haplotypes, transgenic (TG) mice containing Hap-I and Hap-II of the hAT1R gene in a 166-kb bacterial artificial chromosome (BAC) were generated. Mice received injection of endothelin-1 (1 mg/ml) directly in to the striatum and were evaluated for neurologic deficit scores and sacrificed for analysis of infarct volume and mRNA levels of various proteins. Mice containing Hap-I suffered from significantly higher neurological deficits and larger brain infarcts than Hap II. Similarly, the molecular analysis of oxidant and inflammatory markers in brains of mice showed a significant increase (p < 0.05) in NOX-1 (2.3-fold), CRP (4.3-fold), and IL6 (1.9-fold) and a corresponding reduced expression of antioxidants SOD (60%) and HO1 (55%) in Hap-I mice as compared to Hap-II mice. These results suggest that increased expression of hAT1R rendered Hap-I TG mice susceptible to stroke-related pathology, possibly due to increased level of brain inflammatory and oxidative stress markers and a suppressed antioxidant defense system.

Effects of angiotensin II type 1 receptor blocker and adiponectin on adipocyte dysfunction in stroke-prone spontaneously hypertensive rats

BACKGROUND

Hypoadiponectinemia in lipoatrophy may be related to worsening of hypertension in stroke-prone spontaneously hypertensive rats (SHRSP). One of the beneficial effects of candesartan (Angiotensin II Type 1 receptor blocker) for preventing hypertension may be increasing of adiponectin due to improvement of adipocyte dysfunction. In this study, we determined the effects of candesartan or adiponectin on pathophysiologic features and adipocyte dysfunction in SHRSP.

METHODS

Candesartan was administered to male SHRSP from 16 to 20 weeks of age (2 mg/kg/day). Adiponectin was cloned and intravenously administered to male SHRSP from 16 to 20 weeks of age. We examined biological parameters, as well as the expression and release of adipokines.

RESULTS

The SHRSP exhibited severe atrophy of visceral fat and progression of severe hypertension. The expression and release of leptin and adiponectin were impaired at 6 and 20 weeks of age. Candesartan suppressed the development of lipoatrophy and reduced the incidence of stroke at 20 weeks of age. Candesartan also enhanced the expression of adiponectin and leptin by inducing the overexpression of peroxisome proliferator activated receptor γ. Circulating level of leptin was significantly higher in candesartan group than in the control group, whereas adiponectin was similar in both groups. Intravenous administration of adiponectin resulted in enhancement of adiponectin expression in adipose tissue, but no remarkable effects were found in pathophysiology in SHRSP.

CONCLUSIONS

Our results indicate that candesartan protects against hypertension and adipocyte dysfunction in SHRSP. The induction of leptin expression appeared to be important factor in the inhibition of stroke lesions, whereas adiponectin was not a major regulator of blood pressure in SHRSP with genetic hypertension. Further studies are needed to elucidate the role of the renin-angiotensin system in adipose tissue dysfunction in relation to hypertensive end-organ damage.

Candesartan, an angiotensin II AT₁-receptor blocker and PPAR-γ agonist, reduces lesion volume and improves motor and memory function after traumatic brain injury in mice

Traumatic brain injury (TBI) results in complex pathological reactions, the initial lesion worsened by secondary inflammation and edema. Angiotensin II (Ang II) is produced in the brain and Ang II receptor type 1 (AT₁R) overstimulation produces vasoconstriction and inflammation. Ang II receptor blockers (ARBs) are neuroprotective in models of stroke but little is known of their effect when administered in TBI models. We therefore performed controlled cortical impact (CCI) injury on mice to investigate whether the ARB candesartan would mitigate any effects of TBI. We administered candesartan or vehicle to mice 5 h before CCI injury. Candesartan treatment reduced the lesion volume after CCI injury by approximately 50%, decreased the number of dying neurons, lessened the number of activated microglial cells, protected cerebral blood flow (CBF), and reduced the expression of the cytokine TGFβ1 while increasing expression of TGFβ3. Candesartan-treated mice also showed better motor skills on the rotarod 3 days after injury, and improved performance in the Morris water maze 4 weeks after injury. These results indicate that candesartan is neuroprotective, reducing neuronal injury, decreasing lesion volume and microglial activation, protecting CBF and improving functional behavior in a mouse model of TBI. Co-treatment with a peroxisome proliferator-activated receptor-gamma (PPARγ) antagonist significantly reduced some of the beneficial effects of candesartan after CCI, suggesting that PPARγ activation may contribute to part or to all of the neuroprotective effect of candesartan. Overall, our data suggest that ARBs with dual AT₁R-blocking and PPARγ activation properties may have therapeutic value in treating TBI.

A Comparative Study of Neuroprotective Effect of Single and Combined Blockade of AT1 Receptor and PARP-1 in Focal Cerebral Ischaemia in Rat

Background

Cerebral ischaemia results in enhanced expression of type 1 angiotensin receptor and oxidative stress. Free radicals due to oxidative stress lead to excessive DNA damage causing overactivation of poly (ADP-ribose) polymerase-1 resulting in neuronal death. Activation of both type 1 angiotensin receptors and poly (ADP-ribose) polymerase-1 following cerebral ischaemia takes place simultaneously, but until now, no study has explored the effect of combined blockade of both angiotensin type 1 angiotensin receptor and poly (ADP-ribose) polymerase-1 in cerebral ischaemia.

Aim

Our purpose was to compare the effect of single and combined treatment with angiotensin type 1 angiotensin receptor blocker, candesartan, and the poly (ADP-ribose) polymerase-1 inhibitor, 1, 5 isoquinolinediol, on brain damage and oxidative stress in transient focal cerebral ischaemia in rats.

Method

Transient focal cerebral ischaemia was induced in Sprague-Dawley rats by an intraluminal technique for two-hours following 48 h of reperfusion. Candesartan (0·05 mg/kg) was administered just after initiation of ischaemia followed by a repeat administration at 24 h while 1, 5 isoquinolinediol (0·1 mg/kg) was given one-hour after of ischaemia. After 24 h of reperfusion, neurological deficit was evaluated in the different treatment groups. After 48 h of reperfusion, the rats were sacrificed and the brain was isolated. Ischaemic brain damage by 2,3,5 triphenyl tetrazolium chloride staining, oxidative stress markers, and levels of reactive oxygen species were determined biochemically.

Result

Single treatment with candesartan and 1, 5 isoquinolinediol significantly reduced neurological deficit, infarct, and oedema volume as compared to ischaemic control and different vehicle groups for each of the drugs. However, treatment with candesartan + 1, 5 isoquinolinediol offered greater reduction in neurological deficit, cerebral infarct volume, and oedema as compared to single-drug treatments. Furthermore, treatment with candesartan + 1, 5 isoquinolinediol significantly decreased oxidative stress as compared to single treatments with each drug.

Conclusion

The study suggests that blockade of either type 1 angiotensin receptor or poly (ADP-ribose) polymerase-1 alone provides neuroprotection, but the better result was achieved when both type 1 angiotensin receptor and poly (ADP-ribose) polymerase-1 were blocked together by the combined use of their pharmacological inhibitor in transient cerebral ischaemia in rat.

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

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

Brain natriuretic peptide improves long-term functional recovery after acute CNS injury in mice

There is emerging evidence to suggest that brain natriuretic peptide (BNP) is elevated after acute brain injury, and that it may play an adaptive role in recovery through augmentation of cerebral blood flow (CBF). Through a series of experiments, we tested the hypothesis that the administration of BNP after different acute mechanisms of central nervous system (CNS) injury could improve functional recovery by improving CBF. C57 wild-type mice were exposed to either pneumatic-induced closed traumatic brain injury (TBI) or collagenase-induced intracerebral hemorrhage (ICH). After injury, either nesiritide (hBNP) (8 microg/kg) or normal saline were administered via tail vein injection at 30 min and 4 h. The mice then underwent functional neurological testing via rotorod latency over the following 5 days and neurocognitive testing via Morris water maze testing on days 24-28. Cerebral blood flow (CBF) was assessed by laser Doppler from 25 to 90 min after injury. After ICH, mRNA polymerase chain reaction (PCR) and histochemical staining were performed during the acute injury phase (<24 h) to determine the effects on inflammation. Following TBI and ICH, administration of hBNP was associated with improved functional performance as assessed by rotorod and Morris water maze latencies (p < 0.01). CBF was increased (p < 0.05), and inflammatory markers (TNF-alpha and IL-6; p < 0.05), activated microglial (F4/80; p < 0.05), and neuronal degeneration (Fluoro-Jade B; p < 0.05) were reduced in mice receiving hBNP. hBNP improves neurological function in murine models of TBI and ICH, and was associated with enhanced CBF and downregulation of neuroinflammatory responses. hBNP may represent a novel therapeutic strategy after acute CNS injury.

Short-term treatment of stroke-prone spontaneously hypertensive rats with an AT1 receptor blocker protects against hypertensive end-organ damage by prolonged inhibition of the renin-angiotensin system

The aim of the present study was to investigate the effects of short-term treatment with an AT(1) receptor blocker (ARB) on amelioration of hypertensive end-organ damage in stroke-prone spontaneously hypertensive rats (SHRSP). Male SHRSP were divided into two groups: (i) an ARB-treated group; and (ii) a control group. Candesartan (1 mg/kg per day) was administered orally from 6 to 11 weeks of age. At 20 weeks of age, plasma renin activity (PRA), angiotensin II concentrations, angiotensin-converting enzyme (ACE) activity and hydroperoxide content were measured. Expression of intercellular adhesion molecule (ICAM)-1, renin, AT(1) and AT(2) receptors was investigated by reverse transcription-polymerase chain reaction. Blood pressure in the ARB group was slightly lower at 7, 8, 11, 13-15 and 18 weeks of age, but no significant difference in blood pressure was found between the ARB and control groups at 20 weeks of age. All rats in the control group had cerebral oedema, whereas no lesions were found in the ARB group. In the ARB group, PRA, AII and hydroperoxide content were lower than in the control group. In the ARB-treated group, lower ICAM-1 expression was found in the cerebral cortex and slightly, albeit not significantly, lower expression of renin was found in the kidney. In contrast, AT(1) receptor expression in the cerebrum and kidney was higher in the ARB group compared with the control group. These results indicate that short-term treatment of SHRSP with ARB at a young age is effective in preventing cerebral oedema after maturation. Such beneficial effects of ARB may be due, in part, to decreased blood pressure and is likely mainly due to inhibition of total circulating and local renin-angiotensin systems.

Action on vascular risk factors: importance of blood pressure and lipid lowering in stroke secondary prevention

INTRODUCTION

Secondary stroke prevention comprises a broad spectrum of therapeutic actions that includes the appropriate management of risk factors and the action on blood pressure and serum lipids that are of great importance to decrease stroke recurrences.

METHODS

We conducted a review of the published studies analyzing the relevance of the treatment of blood pressure and serum lipids, with special attention to recent findings of clinical trials and current guidelines on stroke secondary prevention.

RESULTS

The relationship between blood pressure and stroke has been widely demonstrated; however, the role of serum lipids has been discussed for a long time. Recent results from epidemiological studies and clinical trials have demonstrated its role as modifiable risk factor for stroke. Blood pressure and lipid lowering are associated with significant reductions in recurrent strokes as well as in other vascular events in transient ischemic attack (TIA) or stroke patients. The PROGRESS and MOSES trials suggest that diuretics, angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers could confer additional benefits in stroke patients, and the SPARCL study did so for statins. These drugs are not only efficacious in the reduction of stroke recurrences, but also in other cardiovascular events.

CONCLUSIONS

Blood pressure and serum lipids are two important and modifiable vascular risk factors that should be taken into consideration when planning secondary stroke prevention measures. This approach should include hypotensive drugs (mainly the combination of diuretics and ACE inhibitors) with the objective to maintain normal blood pressure, avoiding levels >130/80 mm Hg in all stroke patients, and statins (atorvastatin 80 mg) in patients with noncardioembolic TIA or stroke.

Usefulness of ARBs and ACE Inhibitors in the Prevention of Vascular Dementia in the Elderly

Hypertension is a leading risk factor for vascular dementia. With the increasing burden of dementia, prevention and delay of cognitive decline are becoming a priority. Recent clinical trials have demonstrated that patients taking antihypertensive medications have a reduced incidence of dementia and cognitive impairment. Calcium channel blockers, angiotensin-converting enzyme inhibitors, and angiotensin receptor blockers appear to offer significant neuroprotection, even beyond blood pressure reduction. Evidence is emerging that the angiotensin receptor blockers offer superior neuroprotection. This finding has been attributed to the unique property of sustained blockade of the AT1 receptor, combined with simultaneous activation of the AT2 receptors. The use of angiotensin receptor blockers as first-line therapy for hypertension and cognitive protection in the elderly should be strongly considered.

Angiotensin II: multitasking in the brain

In addition to controlling systemic blood pressure, angiotensin II (Ang II) has several roles in the brain, including the regulation of cerebrovascular flow and the reaction to stress. In order to clarify the central effects of Ang II and its type 1 (AT1) receptors, we reviewed the literature reporting recent research on the effects of pretreatment with the AT1-receptor blocker, candesartan, on experimental ischemia, cerebrovascular remodeling, and inflammation in spontaneously hypertensive rats (SHRs), and the responses to stress induced by isolation and by cold-restraint. Angiotensin II regulates the brain circulation through stimulation of AT1-receptors located in the cerebrovascular endothelium and central pathways. SHRs express greater numbers of endothelial AT1-receptors and a central sympathetic overdrive, resulting in pathological cerebrovascular growth, inflammation, decreased cerebrovascular compliance, and enhanced vulnerability to brain ischemia. Sustained central AT1-receptor antagonism reverses these effects. Sustained reduction of AT1-receptor stimulation before stress prevents the hormonal and sympathoadrenal stress responses during isolation and prevents the gastric ulceration stress response to cold-restraint, indicating that increased AT1-receptor stimulation is essential to enhance the central sympathetic response and the formation and release of corticotropin-releasing factor (CRF) and arginine vasopressin that occur during stress. AT1-receptor blocking agents reverse the cortical alterations in CRF1 and benzodiazepine receptors characteristic of isolation stress, effects probably related to their anti-anxiety effect in rodents. Sustained reduction of Ang II tone by AT1-receptor antagonism could be considered as a preventive and therapeutic approach for brain ischemia and stress-related and mood disorders. Additional preclinical studies and controlled clinical trials are necessary to confirm the efficacy of this novel therapeutic approach.

AT1 receptor blockade regulates the local angiotensin II system in cerebral microvessels from spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Blockade of angiotensin II AT1 receptors in cerebral microvessels protects against brain ischemia and inflammation. In this study, we tried to clarify the presence and regulation of the local renin-angiotensin system (RAS) in brain microvessels in hypertension.

METHODS

Spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY) controls were treated with an AT1 receptor antagonist (candesartan, 0.3 mg/kg per day) via subcutaneous osmotic minipumps for 4 weeks. The expression and localization of RAS components and the effect of AT1 receptor blockade were assessed by Affymetrix microarray, qRT-PCR, Western blots, immunohistochemistry and immunofluorescence.

RESULTS

We found transcripts of most of RAS components in our microarray database, and confirmed their expression by qRT-PCR. Angiotensinogen (Aogen), angiotensin-converting enzyme (ACE) and AT1 receptors were localized to the endothelium. There was no evidence of AT2 receptor localization in the microvascular endothelium. In SHR, (pro)renin receptor mRNA and AT1 receptor mRNA and protein expression were higher, whereas Aogen, ACE mRNA and AT2 receptor mRNA and protein expression were lower than in WKY rats. Candesartan treatment increased Aogen, ACE and AT2 receptor in SHR, and increased ACE and decreased Aogen in WKY rats, without affecting the (pro)renin and AT1 receptors.

CONCLUSIONS

Increased (pro)renin and AT1 receptor expression in SHR substantiates the importance of the local RAS overdrive in the cerebrovascular pathophysiology in hypertension. AT1 receptor blockade and increased AT2 receptor stimulation after administration of candesartan may contribute to the protection against brain ischemia and inflammation.

CV-11974, angiotensin II type I receptor antagonist, protects against ischemia–reperfusion injury of the small intestine in rats

BACKGROUND

Angiotensin II has been implicated in the pathogenesis of vascular inflammation in various organs. The aim of the present study was to examine the effect of angiotensin II type I receptor antagonist, CV-11974, on reperfusion-induced small intestinal injury in rats.

METHODS

Intestinal damage was induced by clamping both the superior mesenteric artery and the celiac trunk for 30 min followed by reperfusion for 60 min in male Wistar rats. CV-11974 was given to the rats by intravenous injection 1 h before the vascular clamping. The intestinal mucosal injury and inflammation were evaluated by biochemical markers and histological findings. Thiobarbituric acid reactive substances and tissue-associated myeloperoxidase (MPO) activity were measured in the gastric mucosa as indices of lipid peroxidation and neutrophil infiltration. The expressions of pro-inflammatory cytokines (CINC-1) in intestinal mucosa were measured by enzyme-linked immunosorbent assay (ELISA) and reverse transcription-PCR (RT-PCR). In additional experiments with an in vitro flow system, human neutrophils were perfused on human umbilical vein endothelial cells (HUVEC) pretreated with anoxia-reoxygenation with or without CV-11974 and then the adhesive neutrophils were counted.

RESULTS

Reperfusion after ischemia resulted in an increase in luminal protein concentrations, hemoglobin concentrations, thiobarbituric acid reactive substances, and MPO activity. Pretreatment with CV-11974 significantly inhibited the increases in these parameters. CV-11974 also inhibited increases in intestinal CINC-1 protein and mRNA expression induced by ischemia-reperfusion. Moreover, in an in vitro study, CV-11974 significantly inhibited the adherence of neutrophils to HUVEC exposed to reoxygenation after anoxia.

CONCLUSIONS

These results suggest that the blockade of angiotensin II type I receptor by treatment with CV-11974 remarkably reduced the reperfusion-induced intestinal injury.

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.

Continuous inhibition of the renin-angiotensin system and protection from hypertensive end-organ damage by brief treatment with angiotensin II type 1 receptor blocker in stroke-prone spontaneously hypertensive rats

To investigate the short-term blockade of angiotensin II type 1 (AT1) receptor at the prehypertensive stage and its effects on hypertensive sequelae after maturation, we administered AT1 receptor blocker (ARB, 1 mg/kg/day) to male stroke-prone spontaneously hypertensive rats (SHRSP) from 5 to 10 weeks of age. Although blood pressure in the treated group was significantly lower than in the control group at 10 weeks of age, it gradually increased within 2-3 weeks and reached approximately 250 mm Hg at 17 weeks after cessation of the treatment (27 weeks of age), and reached parity with the control after 20 weeks of age. Nonetheless, hypertensive end-organ damage such as cerebral lesion, cardiac hypertrophy and nephrosclerosis were markedly suppressed in the treated group. Plasma renin activity (PRA), plasma angiotensin II (AII) levels and cerebral angiotensin-converting enzyme (ACE) activity were also significantly lower in the treated group than in the control group, indicating continuous suppression of the circulating and local renin-angiotensin system (RAS). In the brain, intercellular adhesion molecule-1 (ICAM-1) mRNA expression was significantly lower in the cerebral cortex of the treated group than in the control group, while AT1 receptor expression was similar. Such beneficial effects by ARB treatment were not found in the hydralazine-treated group, even though blood pressure changes were similar in both groups. These results demonstrated that early and transient treatment by ARB effective for the prevention of hypertensive end-organ damage. This may be due to the low concentration of plasma angiotensin II by continuous inhibition of RAS even after maturation.

Brain angiotensin II: new developments, unanswered questions and therapeutic opportunities

1. There are two Angiotensin II systems in the brain. The discovery of brain Angiotensin II receptors located in neurons inside the blood brain barrier confirmed the existence of an endogenous brain Angiotensin II system, responding to Angiotensin II generated in and/or transported into the brain. In addition, Angiotensin II receptors in circumventricular organs and in cerebrovascular endothelial cells respond to circulating Angiotensin II of peripheral origin. Thus, the brain responds to both circulating and tissue Angiotensin II, and the two systems are integrated. 2. The neuroanatomical location of Angiotensin II receptors and the regulation of the receptor number are most important to determine the level of activation of the brain Angiotensin II systems. 3. Classical, well-defined actions of Angiotensin II in the brain include the regulation of hormone formation and release, the control of the central and peripheral sympathoadrenal systems, and the regulation of water and sodium intake. As a consequence of changes in the hormone, sympathetic and electrolyte systems, feed back mechanisms in turn modulate the activity of the brain Angiotensin II systems. It is reasonable to hypothesize that brain Angiotensin II is involved in the regulation of multiple additional functions in the brain, including brain development, neuronal migration, process of sensory information, cognition, regulation of emotional responses, and cerebral blood flow. 4. Many of the classical and of the hypothetical functions of brain Angiotensin II are mediated by stimulation of Angiotensin II AT1 receptors. 5. Brain AT2 receptors are highly expressed during development. In the adult, AT2 receptors are restricted to areas predominantly involved in the process of sensory information. However, the role of AT2 receptors remains to be clarified. 6. Subcutaneous or oral administration of a selective and potent non-peptidic AT1 receptor antagonist with very low affinity for AT2 receptors and good bioavailability blocked AT1 receptors not only outside but also inside the blood brain barrier. The blockade of the complete brain Angiotensin II AT1 system allowed us to further clarify some of the central actions of the peptide and suggested some new potential therapeutic avenues for this class of compounds. 7. Pretreatment with peripherally administered AT1 antagonists completely prevented the hormonal and sympathoadrenal response to isolation stress. A similar pretreatment prevented the development of stress-induced gastric ulcers. These findings strongly suggest that blockade of brain AT1 receptors could be considered as a novel therapeutic approach in the treatment of stress-related disorders. 8. Peripheral administration of AT1 receptor antagonists strongly affected brain circulation and normalized some of the profound alterations in cerebrovascular structure and function characteristic of chronic genetic hypertension. AT1 receptor antagonists were capable of reversing the pathological cerebrovascular remodeling in hypertension and the shift to the right in the cerebral autoregulation, normalizing cerebrovascular compliance. In addition, AT1 receptor antagonists normalized the expression of cerebrovascular nitric oxide synthase isoenzymes and reversed the inflammatory reaction characteristic of cerebral vessels in hypertension. As a consequence of the normalization of cerebrovascular compliance and the prevention of inflammation, there was, in genetically hypertensive rats a decreased vulnerability to brain ischemia. After pretreatment with AT1 antagonists, there was a protection of cerebrovascular flow during experimental stroke, decreased neuronal death, and a substantial reduction in the size of infarct after occlusion of the middle cerebral artery. At least part of the protective effect of AT1 receptor antagonists was related to the inhibition of the Angiotensin II system, and not to the normalization of blood pressure. These results indicate that treatment with AT1 receptor antagonists appears to be a major therapeutic avenue for the prevention of ischemia and inflammatory diseases of the brain. 9. Thus, orally administered AT1 receptor antagonists may be considered as novel therapeutic compounds for the treatment of diseases of the central nervous system when stress, inflammation and ischemia play major roles. 10. Many questions remain. How is brain Angiotensin II formed, metabolized, and distributed? What is the role of brain AT2 receptors? What are the molecular mechanisms involved in the cerebrovascular remodeling and inflammation which are promoted by AT1 receptor stimulation? How does Angiotensin II regulate the stress response at higher brain centers? Does the degree of activity of the brain Angiotensin II system predict vulnerability to stress and brain ischemia? We look forward to further studies in this exiting and expanding field.

Angiogenic protection from focal ischemia with angiotensin II type 1 receptor blockade in the rat

Angiogenesis within an ischemic region of the brain may increase tissue viability and act to limit the extent of an infarct. The ANG II pathway can both stimulate and inhibit angiogenesis depending on the tissue and the activated receptors. Previous work showed that 2-wk losartan administration (ANG II type 1 receptor blockade) initiates a significant cerebral angiogenic response. We hypothesized that administration of losartan in the drinking water of rats for 2 wk before initiation of focal ischemia would decrease the extent of the resulting infarct. Adult male Sprague-Dawley rats were given losartan (50 mg/day) in drinking water for 2 wk before initiation of cerebral focal ischemia produced by cauterization of cortical surface vessels. Controls received normal drinking water. In control animals, three main vessels feeding the whisker barrel cortex were cauterized, resulting in cessation of blood flow. The same protocol was followed for losartan-treated animals but did not result in cessation of blood flow in the whisker barrel cortex. Another group of losartan-treated animals received between 8 and 14 cauterizations of surface vessels feeding the whisker barrel cortex, and cessation of blood flow was verified. Rats were killed 72 h after surgery. Morphological examination revealed angiogenesis, maintained vascular delivery, and significantly decreased infarct size in losartan-treated animals compared with controls. These results demonstrate that pretreatment with losartan reduces infarct size after cerebral focal ischemia and support the hypothesis that cerebral angiogenesis may be one of the mechanisms responsible.

Role of AT1 receptors and NAD(P)H oxidase in diabetes-aggravated ischemic brain injury

The objective of the present study was to examine the role of the angiotensin II type 1 receptor (AT1-R) in the diabetes-aggravated oxidative stress and brain injury observed in a rat model of combined diabetes and focal cerebral ischemia. Diabetes was induced by an injection of streptozotoxin (STZ; 55 mg/kg iv) at 8 wk of age. Two weeks after the induction of diabetes, some animals received continuous subcutaneous infusion of the AT1-R antagonist candesartan (0.5 mg·kg−1·day−1) for 14 days. Focal cerebral ischemia, induced by middle cerebral artery occlusion/reperfusion (MCAO), was conducted at 4 wk after STZ injection. Male Sprague-Dawley rats ( n = 189) were divided into five groups: normal control, diabetes, MCAO, diabetes + MCAO, and diabetes + MCAO + candesartan. The major observations were that 1) MCAO produced typical cerebral infarction and neurological deficits at 24 h that were accompanied by elevation of NAD(P)H oxidase gp91phox and p22phox mRNAs, and lipid hydroperoxide production in the ipsilateral hemisphere; 2) diabetes enhanced NAD(P)H oxidase gp91phox and p22phox mRNA expression, potentiated lipid peroxidation, aggravated neurological deficits, and enlarged cerebral infarction; and 3) candesartan reduced the expression of gp91phox and p22phox, decreased lipid peroxidation, lessened cerebral infarction, and improved the neurological outcome. We conclude that diabetes exaggerates the oxidative stress, NAD(P)H oxidase induction, and brain injury induced by focal cerebral ischemia. The diabetes-aggravated brain injury involves AT1-Rs. We have shown for the first time that candesartan reduces brain injury in a combined model of diabetes and cerebral ischemia.

Angiotensin II AT1 receptor blockade reverses pathological hypertrophy and inflammation in brain microvessels of spontaneously hypertensive rats

BACKGROUND AND PURPOSE

The spontaneously hypertensive rat (SHR) is vulnerable to brain ischemia and stress and exhibits a chronically stimulated brain angiotensin II system, cerebrovascular hypertrophy, and inflammation. Pretreatment with angiotensin II type 1 (AT1) receptor antagonists protects from brain ischemia and from stress and prevents the development of stress-induced gastric ulcers in part by reducing inflammation in the gastric mucosa. We studied whether AT1 receptor antagonists could exert antiinflammatory effects in the brain vasculature as a mechanism for their protective effects against ischemia.

METHODS

Ten-week-old SHR and normotensive Wistar-Kyoto male rats received the AT1 receptor antagonist candesartan (0.3 mg/kg per day) or vehicle for 28 days via osmotic minipumps. We studied AT1 receptors, intercellular adhesion molecule-1 (ICAM-1), endothelial nitric oxide synthase (eNOS), and number of macrophages by immunohistochemistry and Western blots.

RESULTS

We found increased endothelial AT1 receptor expression of brain microvessels and middle cerebral artery of SHR. Brain AT1 receptor inhibition reversed the pathological vascular hypertrophy, increased and normalized eNOS expression, and decreased ICAM-1 expression and the number of adherent and infiltrating macrophages in cerebral vessels of SHR.

CONCLUSIONS

The antiinflammatory effects of AT1 receptor antagonists may be an important mechanism in protecting against ischemia.