Angiotensin II decreases mortality rate in gerbils with unilateral carotid ligation

Neuroprotection in Stroke-Focus on the Renin-Angiotensin System: A Systematic Review

Stroke is the primary cause of disability in the adult population. Hypertension represents the leading risk factor being present in almost half the patients. The renin-angiotensin system is involved in the physiopathology of stroke and has an essential impact on hypertension as a risk factor. This article targeted the role of the renin-angiotensin system in stroke neuroprotection by reviewing the current literature available. The mechanism of action of the renin-angiotensin system was observed through the effects on AT1, AT2, and Mas receptors. The neuroprotective properties ascertained by angiotensin in stroke seem to be independent of the blood pressure reduction mechanism, and include neuroregeneration, angiogenesis, and increased neuronal resistance to hypoxia. The future relationship of stroke and the renin-angiotensin system is full of possibilities, as new agonist molecules emerge as potential candidates to restrict the impairment caused by stroke.

Gynura procumbens Root Extract Ameliorates Ischemia-Induced Neuronal Damage in the Hippocampal CA1 Region by Reducing Neuroinflammation

Gynura procumbens has been used in Southeast Asia for the treatment of hypertension, hyperglycemia, and skin problems induced by ultraviolet irradiation. Although considerable studies have reported the biological properties of Gynura procumbens root extract (GPE-R), there are no studies on the effects of GPE-R in brain damages, for example following brain ischemia. In the present study, we screened the neuroprotective effects of GPE-R against ischemic damage and neuroinflammation in the hippocampus based on behavioral, morphological, and biological approaches. Gerbils received oral administration of GPE-R (30 and 300 mg/kg) every day for three weeks and 2 h after the last administration, ischemic surgery was done by occlusion of both common carotid arteries for 5 min. Administration of 300 mg/kg GPE-R significantly reduced ischemia-induced locomotor hyperactivity 1 day after ischemia. Significantly more NeuN-positive neurons were observed in the hippocampal CA1 regions of 300 mg/kg GPE-R-treated animals compared to those in the vehicle-treated group 4 days after ischemia. Administration of GPE-R significantly reduced levels of pro-inflammatory cytokines such as interleukin-1β, -6, and tumor necrosis factor-α 6 h after ischemia/reperfusion. In addition, activated microglia were significantly decreased in the 300 mg/kg GPE-R-treated group four days after ischemia/reperfusion compared to the vehicle-treated group. These results suggest that GPE-R may be one of the possible agents to protect neurons from ischemic damage by reducing inflammatory responses.

Pharmacological interventions into the renin-angiotensin system with ACE inhibitors and angiotensin II receptor antagonists: effects beyond blood pressure lowering

Hypertension is recognized as an important risk factor for cardiovascular morbidity and mortality. Lowering of blood pressure has been shown to minimize the risk of cardiovascular events, with the majority of antihypertensives demonstrating a similar ability to reduce coronary events and stroke for a given reduction in blood pressure. Agents that modify the activity of the renin-angiotensin system (RAS) have been proposed to exhibit additional effects that might go beyond simple blood pressure lowering. The RAS is a crucial system that regulates extracellular fluid volume and blood pressure. Proposed potential benefits of RAS blockade that go beyond blood pressure lowering include a reduction in platelet aggregation and thrombosis, blunting of cardiac and vascular remodeling, favorable metabolic effects and reno- and cerebro-protection. However, factors such as treatment adherence, duration of action of antihypertensive agents and differences in effects on central versus brachial blood pressure may also result in apparent differences in efficacy of different antihypertensives. The aim of this review article is to examine the available data from clinical studies of antihypertensive drugs for evidence of effects that might legitimately be claimed to go beyond simple blood pressure lowering.

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.

Angiotensin receptor subtype mediated physiologies and behaviors: new discoveries and clinical targets

The renin-angiotensin system (RAS) mediates several classic physiologies including body water and electrolyte homeostasis, blood pressure, cyclicity of reproductive hormones and sexual behaviors, and the regulation of pituitary gland hormones. These functions appear to be mediated by the angiotensin II (AngII)/AT(1) receptor subtype system. More recently, the angiotensin IV (AngIV)/AT(4) receptor subtype system has been implicated in cognitive processing, cerebroprotection, local blood flow, stress, anxiety and depression. There is accumulating evidence to suggest an inhibitory influence by AngII acting at the AT(1) subtype, and a facilitory role by AngIV acting at the AT(4) subtype, on neuronal firing rate, long-term potentiation, associative and spatial learning, and memory. This review initially describes the biochemical pathways that permit synthesis and degradation of active angiotensin peptides and three receptor subtypes (AT(1), AT(2) and AT(4)) thus far characterized. There is vigorous debate concerning the identity of the most recently discovered receptor subtype, AT(4). Descriptions of classic and novel physiologies and behaviors controlled by the RAS are presented. This review concludes with a consideration of the emerging therapeutic applications suggested by these newly discovered functions of the RAS.

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.

Vascular benefits of angiotensin receptor blockers

There is convincing evidence that angiotensin II, through activation of the angiotensin II type 1 (AT1) receptor, is involved in the atherosclerotic process. Similarly, angiotensin receptor blockers decrease vascular inflammation, hypertrophy and thrombosis, which are the key components of the progression of atherosclerosis. In addition, in several animal models, angiotensin receptor blockade was able to inhibit atherosclerosis. However, the effects of angiotensin receptor blockers on clinical outcome in cardiovascular patients remains to be established. Contradictory results have been found on the reduction of the risk on myocardial infarctions and in-stent restenosis, although there is solid evidence for cerebroprotective effects of these receptor blockers. These differences may be related to the role of the AT2 receptor. This review discusses the role of angiotensin II and angiotensin receptor blockers in the atherosclerotic process and its translation into clinical practice.

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.

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.

Angiotensin Receptor Type 1 Blockade in Astroglia Decreases Hypoxia-Induced Cell Damage and TNF Alpha Release

The present study investigated the role of angiotensin receptors (AT-R) in the survival and inflammatory response of astroglia upon hypoxic injury. Exposure of rat astroglial primary cultures (APC) to hypoxic conditions (HC) led to decreased viability of the cells and to a 3.5-fold increase in TNF-alpha release. AT-R type1 (AT1-R) antagonist losartan and its metabolite EXP3174 decrease the LDH release (by 36 +/- 9%; 45 +/- 6%) from APC under HC. Losartan diminished TNF-alpha release (by 40 +/- 15%) and the number of TUNEL-cells by 204 +/- 38% under HC, alone and together with angiotensin II (ATII), while EXP3174 was dependent on ATII for its effect on TNF-alpha. The AT2-R antagonist, PD123.319, did not influence the release of LDH and TNF-alpha under normoxic (NC) and HC. These data suggest that AT1-R may decrease the susceptibility of astrocytes to hypoxic injury and their propensity to release TNF-alpha. AT1-R antagonists may therefore be of therapeutic value during hypoxia-associated neurodegeneration.

Preoperative statin and diuretic use influence the presentation of patients undergoing carotid endarterectomy: Results of a large single-institution case-control study

OBJECTIVE

Patients who present for carotid endarterectomy (CEA) with symptoms of stroke or transient ischemic attack (TIA) have worse postoperative outcomes than patients with asymptomatic carotid disease. We undertook this study to see whether preoperative medication use or patient characteristics were associated with the presence of symptomatic cerebrovascular disease at the time of operation.

METHODS

A retrospective case-control study was performed among patients presenting for elective CEA at a single academic institution between 1994 and 2004. A total of 660 (42%) symptomatic patients were identified from an institutional database and compared with 901 (58%) control patients who were asymptomatic at the time of CEA. The independent association of cerebrovascular symptoms with patient variables was assessed by using multivariate logistic regression analysis after propensity score adjustment.

RESULTS

The mean age and sex distribution were similar between cases and controls, although symptomatic patients were more likely to have an ulcerative plaque (18% symptomatic vs 11% asymptomatic; P < .01). Compared with asymptomatic controls, patients presenting for CEA with symptoms of stroke or TIA were less likely to have hyperlipidemia (43% vs 55%; P < .01) or a history of coronary artery disease (43% vs 54%; P < .01) and were less likely to be receiving statins (35% vs 47%; P < .01), beta-blockers (34% vs 44%; P < .01), and diuretics (22% vs 31%; P < .01). After controlling for potential interaction and confounding by using propensity score adjustment and logistic regression analysis, preoperative use of statins (adjusted odds ratio, 0.72; 95% confidence interval, 0.56-0.92; P = .01) and diuretics (adjusted odds ratio, 0.74; 95% confidence interval, 0.58-0.95; P = .02) were independently associated with a lower likelihood of having cerebrovascular symptoms at the time of CEA.

CONCLUSIONS

We observed that patients receiving statins or diuretics were less likely to present for CEA with symptoms of stroke or TIA. These data raise the question of whether the preoperative use of these medications protects patients with carotid stenosis from developing symptomatic disease and contributes to improved outcomes among patients undergoing CEA. Further research is needed to assess the optimal medical management of patients before vascular surgery.

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.

Angiotensin II Receptor Expression Following Intestinal Transplantation in Mice

BACKGROUND

To further improve the success rate of intestinal transplantation there is a need to find early appearing indicators of rejection. The specific aim of this study was to compare Angiotensin (Ang) II type 1 receptor and Ang II type 2 receptor expression in relation to histological signs of rejection.

METHODS

Mice of the C57BL6 strain with syngeneic intestinal grafts were compared to mice subjected to allogeneic intestinal transplantation with BalbC strain as donors. Local expression of Ang II type 1 and 2 receptor was evaluated using rt-PCR and Western blot and compared to histological picture in grafts and native intestine.

RESULTS

The Ang II type 2 receptor protein expression was markedly up-regulated in the allogeneically transplanted graft from day 1 postoperatively. Histological signs of rejection were not seen until day 6.

CONCLUSION

Intestinal allograft transplantation in mice is associated with a marked up-regulation of the Ang II type 2 receptor. However, the detailed role of the renin-angiotensin system in the immune rejection following intestinal transplantation remains to be clarified.

Effect of Vasoactive Therapy on Cerebral Circulation

Many questions regarding blood pressure management after acute stroke remain unanswered, resulting in an ongoing debate about whether to treat hypertension acutely and how aggressively blood pressure should be lowered. This review discusses normal and altered cerebrophysiology and provides evidence supporting and opposing the active management of blood pressure within the first 24 hours after stroke. Commonly used intravenous antihypertensive agents and their cerebrovascular effects are reviewed, and therapeutic recommendations are given based on the available evidence.

Possible pathophysiologic mechanisms supporting the superior stroke protection of angiotensin receptor blockers compared to angiotensin-converting enzyme inhibitors: clinical and experimental evidence

Stroke is a major cause of death and disability and its incidence increases linearly with age and the level of systolic and diastolic blood pressure. Stroke, besides being a cause of long-term disability for the affected person, also imposes a significant burden on society and healthcare costs. Although good blood pressure control is very critical for stroke prevention, angiotensin receptor blockers (ARBs) may be superior to angiotensin-converting enzyme inhibitors (ACEIs) for the same degree of blood pressure control. This hypothesis has clinical and experimental support. ARBs prevent stroke incidence by blocking the angiotensin II (AII), AT1 receptors preventing brain ischaemia and allowing AII to stimulate the unoccupied AT2 receptors, which improve brain ischaemia. ACEIs, by reducing AII generation, are less effective in preventing stroke. This hypothesis provides evidence that AII plays an important role in the prevention of stroke. Certain ARBs like losartan, and telmisartan, irbesartan and candesartan possess additional properties which may play a role in stroke prevention, which is independent of AII. These include antiplatelet aggregating, hypouricemic, antidiabetic and atrial antifibrillatory effects. However, the most critical factor in stroke prevention is good blood pressure control irrespective of drug used.

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.

Angiotensin AT2 receptor protects against cerebral ischemia-induced neuronal injury

Several lines of clinical and experimental evidence suggest an important role of the renin-angiotensin system in ischemic brain injury although the cellular regulation of the angiotensin AT1 and AT2 receptors and their potential relevance in this condition have not yet been clearly defined. We first assessed the regulation of brain AT1 and AT2 receptors in response to transient unilateral medial cerebral artery occlusion in rats by real-time RT-PCR, Western blot, and immunofluorescence labeling. AT2 receptors in the peri-infarct zone were significantly upregulated 2 days after transient focal cerebral ischemia. Increased AT2 receptors, which were abundantly distributed in a large number of brain regions adjacent to the infarct area including cerebral frontal cortex, piriform cortex, striatum, and hippocampus, were exclusively expressed in neurons. By contrast, AT1 receptors, which remained unaltered, were mainly expressed in astrocytes. In neurons of ischemic striatum, increased AT2 receptors were associated with intense neurite outgrowth. Blockade of central AT2 receptors with PD123177 abolished the neuroprotective effects of central AT1 receptor blockade with irbesartan on infarct size and neurological outcome. In primary cortical neurons, stimulation of AT2 receptors supported neuronal survival and neurite outgrowth. Our data indicate that cerebral AT2 receptors exert neuroprotective actions in response to ischemia-induced neuronal injury, possibly by supporting neuronal survival and neurite outgrowth in peri-ischemic brain areas.

Neuroprotection with angiotensin receptor antagonists: a review of the evidence and potential mechanisms

The peptide hormone angiotensin (A)-II, the major effector peptide of the renin-angiotensin system (RAS), is well established to play a pivotal role in the systemic regulation of blood pressure, fluid, and electrolyte homeostasis. Recent biochemical and neurophysiologic studies have documented local intrinsic angiotensin-generating systems in organs and tissues such as the brain, retina, bone marrow, liver, and pancreas. The locally generated angiotensin peptides have multiple and novel actions including stimulating cell growth and anti-proliferative and/or antiapoptotic actions. In the mammalian brain, all components of the RAS are present including angiotensin receptor subtypes 1 (AT(1)) and 2 (AT(2)). A-II exerts most of its well defined physiologic and pathophysiologic actions, including those on the central and peripheral nervous system, through its AT(1) receptor subtype. While the AT(1) receptor is responsible for the classical effects of A-II, it has been found that the AT(2) receptor is linked to totally different signalling mechanisms and this has revealed hitherto unknown functions of A-II. AT(2) receptors are expressed at low density in many healthy adult tissues, but are upregulated in a variety of human diseases. This receptor not only contributes to stroke-related pathologic mechanisms (e.g. hypertension, atherothrombosis, and cardiac hypertrophy) but may also be involved in post-ischemic damage to the brain. It has been reported that the AT(2) receptor regulates several functions of nerve cells, e.g. ionic fluxes, cell differentiation, and neuronal tissue regeneration, and also modulates programmed cell death. In this article, we review the experimental evidence supporting the notion that blockade of brain AT(1) receptors can be beneficial with respect to stroke incidence and outcome. We further delineate how AT(2) receptors could be involved in neuronal regeneration following brain injury such as stroke or CNS trauma. The current review is focussed on some of the new functions arising from the locally formed A-II with particular attention to its emerging neuroprotective role in the brain.

Cerebroprotection mediated by angiotensin II: a hypothesis supported by recent randomized clinical trials

Based on the Medical Research Council study, Brown and Brown hypothesized in 1986 that angiotensin II could protect against strokes by causing vasoconstriction of the proximal cerebral arteries, thereby preventing Charcot-Bouchard aneurysms from rupturing. In light of this hypothesis, we evaluated the cerebroprotective effects of various drug classes in recent double-blinded, prospective, randomized trials, such as SHEP, PATS, CAPPP, HOPE, PROGRESS, INSIGHT, NORDIL, LIFE, SCOPE, ANBP2, and ALLHAT. Drugs that activate the AT2 receptors, such as diuretics, calcium antagonists, and angiotensin receptor blockers (ARBs), were consistently more beneficial for stroke reduction than drugs devoid of such activation, such as beta-blockers and angiotensin-converting enzyme (ACE) inhibitors, despite an equal fall in arterial pressure (at least in patients with a low incidence of cardiac complications). These clinical and epidemiologic observations are supported by experimental data documenting greater cerebroprotection with ARBs (which increase angiotensin II levels and stimulate the AT2 receptors) than with ACE inhibitors. Stroke is the most devastating consequence of hypertensive cardiovascular disease, and our hypothesis of cerebroprotection by AT2 receptor activation should be tested by a head-to-head comparison of an ARB with an ACE inhibitor.

Is the angiotensin ii type 2 receptor cerebroprotective?

Most of the deleterious effects of angiotensin II (Ang II) on blood pressure (BP), cardiovascular remodeling, and atherosclerosis are mediated by Ang II type 1 (AT1)-receptor activation. This explains why Ang-II-decreasing or blocking drugs have been successful in decreasing global cardiovascular morbimortality in patients with cardiac complications. However, in primary or secondary stroke prevention trials in patients with low cardiac risk, b-blockers and angiotensin-converting enzyme inhibitors (ACEIs), which decrease Ang II formation, seem to be less protective than thiazides and dihydropyridines, which increase Ang II. When compared with a beta-blocker, an Ang II-increasing AT1-receptor blocker better protects against stroke but not against cardiac events, whereas an ACEI gives the same protection against both cardiac and cerebral events. This dissociation between blood-pressure-independent cardiac and cerebral protection between b-blockers or ACEIs versus AT1-blockers in patients with low cardiac risk can be best explained if, besides the beneficial vascular effect of AT1-receptor blunting, there is evidence of a beneficial effect of non-AT1-receptor activation. In this review, we present experimental evidence for AT2- and AT4-receptor-mediated brain-anti-ischemic mechanisms and propose a direct comparison of AT1-blockers with ACEIs to prove the clinical effectiveness of non-AT1-mediated mechanisms in stroke prevention, particularly in patients with a higher risk for stroke than for cardiac complications.

Ischemic injury in experimental stroke depends on angiotensin II

Since pharmacological interactions of the renin-angiotensin system appear to alter the neurological outcome of stroke patients significantly, we examined the effect of elevated levels of angiotensin II and the role of its receptor subtype AT1 in brain infarction in transgenic mice after focal cerebral ischemia. Angiotensinogen-overexpressing and angiotensin receptor AT1 knockout mice underwent 1 h or 24 h permanent middle cerebral artery occlusion (MCAO). The current study revealed a much smaller penumbra size, i.e., brain tissue at risk, in angiotensinogen-overexpressing animals compared with their wild-type subgroup after 1 h MCAO, but an enlarged infarct size after 24 h. In contrast, a smaller lesion area of energy failure and a much larger penumbral area were found in AT1 knockout mice compared with wild-type littermates. Lower perfusion thresholds for ATP depletion and protein synthesis inhibition after MCAO in AT1-deficient mice and reduced cell damage in an in vitro model using embryonic neurons of AT1 knockout mice suggest injury mechanisms independent of arterial blood pressure. Our data, therefore, demonstrate a direct correlation between brain angiotensin II and the severity of ischemic injury in experimental stroke.

Protection against ischemia: a physiological function of the renin-angiotensin system

The renin-angiotensin system (RAS) is involved in a complex mechanism that serves to preserve the blood supply to organs so that they can maintain cellular function. Angiotensin II exerts this effect, independently of the blood pressure generated, through two time-related events: a fast opening of the reserve collateral circulation and a much slower response of new vessel formation or angiogenesis. This effect is observed in rats with ligation of the abdominal aorta and in gerbils with abrupt or progressive unilateral carotid artery ligation. Inhibition of the angiotensin-converting enzyme (ACE) or the angiotensin II receptor represses this effect, and it appears that it is mediated through a non-AT1 receptor site of angiotensin II. Many tumors, both benign and malignant, express renin and angiotensin. It seems that the stimulating action of angiotensin II on angiogenesis could also be involved in preserving the blood supply to tumor cells. Administration of converting enzyme inhibitors increases survival and decreases tumor size in tumor-bearing rats. These observations support the hypothesis that the RAS, directly or indirectly, is involved in situations in which the restoration of blood supply is critical for the viability of cells and that it is present not only in normal but also in pathological conditions such as tumors. In view of the ubiquitous presence of renins and angiotensins, it is also likely to be involved in other conditions, such as inflammation, arthritis, diabetic retinopathy, and retrolental fibroplasia, among others in which angiogenesis is prominent. In addition, angiotensin II could be involved, through the counterbalance of the AT1 and AT2 receptors, in the rarefaction of blood vessels as an etiologic component of essential hypertension.

Non-AT(1)-receptor-mediated protective effect of angiotensin against acute ischaemic stroke in the gerbil

Previous studies have shown that angiotensin II (Ang II), by mediating rapid recruitment of collateral circulation, has a protective effect in the setting of acute ischaemia. In an experimental model of acute cerebral ischaemia in the gerbil, Fernandez et al. have reported that the mechanism of the protective effect of Ang 11 is blood pressure (BP)-independent, and that the AT1-receptor antagonist, losartan, but not the ACE inhibitor (ACE-I),enalapril, decreases mortality following unilateral carotid artery ligation. The aim of this study was to examine there producibility of the respective effects of losartan and enalapril, and to verify that these differential effects are drug class-related. Acute cerebral ischaemia was induced in anaesthetised gerbils bv unilateral carotid ligation. The effect of pretreatment with two different ACE-I(enalapril and lisinopril), and two different AT1-receptor antagonists (losartan and candesartan), administered orally or intravenously, on mortality were compared. Kaplan-Meier survival curves at day three were analysed bv a log-rank test. Pretreatment with both enalapril and lisinopril significantly decreased survival at day three compared with controls, while the AT1-receptor antagonists losartan and candesartan, despite similarly lowering BP, did not increase mortality. Coadministration of losartan and enalapril increased mortality to the same extent as enalapril alone. This study confirms that Ang II contributes to protective mechanisms against acute cerebral ischaemia through non AT1-receptor-mediated, BP-independent effects.

Gerbil angiotensin II AT1 receptors are highly expressed in the hippocampus and cerebral cortex during postnatal development

Increasing evidence suggests that Angiotensin II, classically known from its many effects regulating salt and water homeostasis, is also involved in brain development and cognitive functions through activation of AT1 Angiotensin II receptors. The recently cloned gerbil AT1 receptor is expressed in brain areas controlling hydro-mineral homeostasis, and particularly highly expressed in limbic areas such as the hippocampal formation. We quantified the gerbil AT1 receptor messenger RNA expression and receptor binding by quantitative in situ hybridization and receptor autoradiography, respectively, in the hippocampal formation and cerebral cortex of gerbils during postnatal development. The receptor messenger RNA and binding were present from birth and showed a gradual and sustained increase through postnatal maturation in the CA1 and CA2 regions of the hippocampus and in the dentate gyrus. Conversely, in the CA3 region, no binding was detected while receptor messenger RNA peaked at 15 days after birth and disappeared in the adult. The highest receptor messenger RNA expression and binding were found in the septomedial portions of the CA1 region and at septal levels of the CA2 region. We detected the highest receptor messenger RNA expression at postnatal day one in the frontolateral pole of the cerebral hemispheres. In these areas, and in the frontoparietal and insular cortex, receptor messenger RNA dramatically decreased during postnatal life. Similarly, we found receptor messenger RNA expression in the cingulate, retrosplenial, perirhinal and infralimbic cortex with higher values during the first two weeks of development and decreased expression in the adult. However, receptor binding in the cerebral cortex, did not decrease during postnatal life. The differential profile of receptor messenger RNA expression and binding in the gerbil cortex and hippocampus during postnatal maturation suggest a role for AT1 receptors in the development and function of the corticohippocampal system.

Emerging features of brain angiotensin receptors

In mammalian brain, angiotensin II AT1 and AT2 receptor subtypes are apparently expressed only in neurons and not in glia. AT1 and AT2 receptor subtypes are sometimes closely associated, but apparently expressed in different neurons. Brain AT1/AT2 interactions may occur in selective cases as inter-neuron cross talk. There are two AT1 isoforms in rodents. AT1A, which predominates, and AT1B. There are also important inter-species differences in receptor expression. Relative lack of amino acid conservation in the gerbil gAT1A receptor substantially decreases affinity for the AT1 antagonists. AT1 receptors are expressed in brain areas regulating autonomic and hormonal responses. AT1A receptors are heterogeneously regulated in a number of experimental conditions. In specific areas, AT1A receptors are not normally expressed, but are induced under influence of reproductive hormones in dopaminergic neurons. There are AT1 and AT2 receptors also in areas related to limbic, sensory and motor functions and their expression is developmentally regulated. A picture is emerging of widespread, neuronally localized, heterogeneously regulated, closely associated brain angiotensin receptor subtypes, modulating multiple functions including neuroendocrine and autonomic responses, stress, cerebrovascular flow, and perhaps brain maturation, neuronal plasticity, memory and behavior.

Induced hypertension improves regional blood flow and protects against infarction during focal ischemia: time course of changes in blood flow measured by laser Doppler imaging

OBJECTIVE

To characterize changes in regional blood flow (rCBF) during and after a period of arterial occlusion and determine the effect on rCBF and on the extent of infarction when the mean arterial blood pressure is increased during the period of occlusion.

METHODS

rCBF in the middle cerebral artery (MCA) territory of rabbits was monitored using laser Doppler perfusion imaging before, during, and after a 1- or 2-hour period of MCA occlusion, and the size of the infarction was assessed by 2,3,5-triphenyltetrazolamine chloride staining after 2 hours of reperfusion. Test animals, the mean arterial blood pressure of which was increased by 65 mm Hg with intravenous phenylephrine during the ischemia, were compared with control animals that remained normotensive. The laser Doppler perfusion imager (Lisca Developments Co., Linköping, Sweden) scanned a 3-cm2 area of cortex with a resolution of 4 mm2 every 15 minutes.

RESULTS

MCA occlusion reduced rCBF to 71 +/- 2% of the control level (n = 24, P < 0.001). Hypertension (HTN) restored rCBF to 84 +/- 3% of the control level (n = 12, P < 0.01), but the HTN-induced improvement diminished with time, so that after 1 hour, there was no longer a significant difference between hypertensive and normotensive animals. HTN during the MCA occlusion caused a 97% reduction in infarct size (P < 0.05) in the animals subjected to 1 hour of occlusion but caused only a 45% reduction (P approximately 0.1) in the animals subjected to 2 hours of occlusion.

CONCLUSION

This study supports the use of HTN to minimize ischemic injury from short intervals of major intracranial vessel occlusion but fails to demonstrate protection when HTN is maintained during occlusions of more than 1 hour.

Induced hypertension during ischemia reduces infarct area after temporary middle cerebral artery occlusion in rats

BACKGROUND

Induced hypertension is one of the few interventions available to reverse acute ischemic neurologic deficit. The purpose of this study was to determine the safety and efficacy of hypertension during prolonged temporary focal ischemia.

METHODS

Anesthetized rats underwent 2 hours of endovascular middle cerebral artery occlusion. Angiotensin was given to increase mean arterial blood pressure to 40%-60% above baseline during ischemia. Neurologic deficit and infarct size in hypertensive rats were assessed at 72 hours.

RESULTS

Hypertensive rats had smaller infarcts than normotensive controls (mean, 12.40 +/- 3.71% versus 24.19 +/- 2.89; p < 0.05) without hemorrhage. Neuroscores were comparable.

CONCLUSION

Hypertension safely reduces infarction after 2 hours of focal ischemia in rats.

Inhibition of acetylcholinesterase modulates the autoregulation of cerebral blood flow and attenuates ischemic brain metabolism in hypertensive rats

We designed the present study to examine whether or not the inhibition of acetylcholinesterase modulates cerebral microcirculation in hypotension and improves brain metabolism in ischemia induced by bilateral carotid artery occlusion in hypertensive rats. Blood flow to the parietal cortex was determined by the H2 clearance method. Lactate, pyruvate, and ATP were estimated by enzymatic methods. Acetylcholinesterase inhibitor (AChEI, ENA-713), at 0.05, 0.1, or 0.5 mg/kg, was intravenously injected 10 min before either hemorrhagic hypotension or cerebral ischemia. The levels of acetylcholine in the control were 29.3 +/- 8.1 (mean +/- SD) and 39.5 +/- 8.1 pmol/mg in the cortex and hippocampus, respectively, and they were significantly decreased by 15-19% after 60 min of ischemia in the vehicle-treated rats. AChEI preserved the levels to 93-98% of the control (p < 0.05 versus vehicle). The lower limit of autoregulation was 74 +/- 9% of the resting values. The administration of AChEI helped preserve blood flow and lowered the limit to 64 +/- 6% (p < 0.05 versus control). After 60 min of ischemia, lactate increased 6.5-fold and ATP decreased to 64% of the control value. The administration of AChEI dose-dependently reduced the lactate level 1.9- to 3.9-fold and well preserved the ATP level to 94-97% of the control. The inhibition of acetylcholinesterase activity may preserve cerebral autoregulation during hypotension and protect cerebral metabolism against ischemic insult.

Brain angiotensin receptor subtypes in the control of physiological and behavioral responses

This review summarizes emerging evidence that supports the notion of a separate brain renin-angiotensin system (RAS) complete with the necessary precursors and enzymes for the formation and degradation of biologically active forms of angiotensins, and several binding subtypes that may mediate their diverse functions. Of these subtypes the most is known about the AT1 site which preferentially binds angiotensin II (AII) and angiotensin III (AIII). The AT1 site appears to mediate the classic angiotensin responses concerned with body water balance and the maintenance of blood pressure. Less is known about the AT2 site which also binds AII and AIII and may play a role in vascular growth. Recently, an AT3 site was discovered in cultured neoblastoma cells, and an AT4 site which preferentially binds AII(3-8), a fragment of AII now referred to as angiotensin IV (AIV). The AT4 site has been implicated in memory acquisition and retrieval, and the regulation of blood flow. In addition to the more well-studied functions of the brain RAS, we review additional less well investigated responses including regulation of cellular function, the modulation of sensory and motor systems, long term potentiation, and stress related mechanisms. Although the receptor subtypes responsible for mediating these physiologies and behaviors have not been definitively identified research efforts are ongoing. We also suggest potential contributions by the RAS to clinically relevant syndromes such as dysfunctions in the regulation of blood flow and ischemia, changes in cognitive affect and memory in clinical depressed and Alzheimer's patients, and angiotensin's contribution to alcohol consumption.

Nonpeptide angiotensin AT1 and AT2 receptor ligands modulate the upper limit of cerebral blood flow autoregulation in rats

We investigated the effect of angiotensin AT1 and AT2 receptor blockade on the upper limit of CBF autoregulation in pentobarbital-anesthetized rats. CBF was measured by laser-Doppler flowmetry from the parietal cortex and MABP was increased by intravenous phenylephrine infusion. Neither the AT1 antagonist losartan nor the AT2 ligand PD 123319 nor angiotensin II (ANG II) in the presence of losartan affected baseline CBF. When the blood pressure was increased in the control group, CBF remained fairly constant up to 145 mm Hg and increased steeply after 150 mm Hg. Both PD 123319 (7-10 mg/kg) and losartan (1-10 mg/kg) shifted the upper limit of CBF autoregulation toward higher pressures. Intravenous infusion of PD 123319 was more effective than bolus injection. The losartan effect was dose dependent. Selective stimulation of AT2 receptors with an intravenous ANG II infusion (0.54 micrograms/min) in the presence of losartan did not reverse the effect of losartan on CBF autoregulation, but, on the contrary, appeared to further shift the upper limit of autoregulation toward higher pressures. The results implicate a role for both AT1 and AT2 angiotensin receptors in the regulation of CBF.

Microvasculature of the nasal salt gland of the duckling, Anas platyrhynchos: quantitative responses to osmotic adaptation and deadaptation studied with vascular corrosion casting

The three-dimensional microvasculature of the nasal salt gland of the duckling was studied by vascular corrosion casting and scanning electron microscopy. Changes in the vascular volume of the gland in response to osmotic stress were also determined using cast weights and densities. The richly vascularized gland is supplied on its medial surface by large branches of the supraorbital and ethmoidal arteries. Numerous arterial branches enter the gland and distribute to lobes via the interlobar connective tissue. Lobar arterioles penetrate to the periductal areas of the lobes before dividing into capillaries supplying the ductal epithelium and secretory tubules. Capillaries envelope the secretory tubules and run radially from the ducts toward the lobe periphery, so that blood flows counter to the tubular secretion. Blood is collected via venous plexuses seen as distinct drainage units on the periphery of each lobe. Veins exhibit large numbers of bicuspid valves. Following 1 day and 4 days of osmotic loading (feeding 1% NaCl), vascular volume of the gland increased fivefold and ninefold, respectively, a response that precedes and exceeds that of the gland weight or Na,K-ATPase activity. When salt water-adapted ducklings were fed fresh water for only 24 hr (deadaptation), vascular volume fell to 2.8 times the control level. Changes in blood flow to the gland during osmotic adaptation and deadaptation are rapid and dramatic and may represent the initial steps in the control of gland secretion.

The angiotensin I converting enzyme inhibitors, captopril and Wy-44,655 attenuate the consequences of cerebral ischemia in renovascular hypertensive rats

Global cerebral ischemia (four vessel model) was induced in renovascular hypertensive rats (two kidney, one clip model) chronically treated with intraperitoneal administration of angiotensin I converting enzyme inhibitors, either captopril (100 mg/kg per day) or Wy-44,655 (10 mg/kg per day). Mortality following cerebral ischemia was higher in renovascular hypertensive rats than in normotensive controls. Reduction of blood pressure with captopril or Wy-44,655, lowered mortality. In surviving renovascular hypertensive and normotensive rats cerebral ischemia induced hyperactivity and lesions of the CA1 area of the hippocampus. Prolonged treatment with captopril--but not with Wy-44,655--reduced hyperactivity and the extent of the CA1 lesions. In conclusion, hypertension increases mortality following cerebral ischemia but does not affect the extent of brain injury in survivors. Prior treatment with converting enzyme inhibitors lowers mortality. Treatment with captopril attenuates brain injury in survivors.

Pial arterial pressure contribution to early ischemic brain edema

The effect of pial arterial pressure (PAP) on brain edema was examined in cats with middle cerebral artery (MCA) occlusion. Measurements of PAP and regional CBF (rCBF) were collected in the central core and the peripheral margin of the MCA territory over 180 min post MCA occlusion. Brain water content in each region was determined at the end of the experiment. MCA occlusion resulted in decreased PAP and rCBF in both the core (PAP = 13 mm Hg, rCBF = 9 ml/100 g/min) and the peripheral region (PAP = 15 mm Hg, rCBF = 18 ml/100 g/min). Brain edema developed in both the core and the peripheral region. Brain water content was correlated inversely with PAP in the core region and positively in the peripheral region. The results indicate that decreased blood flow contributes to cytotoxic edema in the core, and a hydrostatic pressure gradient preferentially enhances edema formation in the peripheral region. Maintenance of high perfusion pressure early after ischemia onset may suppress brain edema in the core region.

The combined effects of hypertension, hemodilution, and osmotherapy on the metabolic sequelae of acute experimental cerebral ischemia

The effects of induced hypertension, hemodilution, and osmotherapy (mannitol) have been assessed singly and in combination on the metabolic sequelae 2 hr after middle cerebral artery (MCA) occlusion in the anesthetized rat. All regimes that included hypertension and monotherapy with mannitol significantly reduced the rise in hemispheric lactate produced by vessel occlusion. No treatment caused increase cerebral edema, and mannitol produced a slight reduction in the hemisphere water content. The significance of the results is discussed.

Differences in mortality rate between abrupt and progressive carotid ligation in the gerbil: role of endogenous angiotensin II

Studies have shown that in comparison to rapid occlusion of a vessel, gradual occlusion produces less severe tissue ischemia due to a more effective development of collateral circulation. As other studies have shown that collateral circulation can be enhanced by stimulation of the endogenous renin-angiotensin II system, it was hypothesized that this system is involved in the mechanism of protection against ischemia that obtains during gradual vascular occlusion. To test this hypothesis, mortality rates were evaluated in gerbils subjected to gradual vascular occlusion by means of progressive carotid ligation while simultaneously infused with inhibitors of the renin-angiotensin II cascade--enalaprilat or saralasin. Groups of animals with either abrupt or progressive carotid ligation infused with saline served as controls. Results showed that (1) in saline-infused animals, there was a significant decrease in the mortality rate of progressive-ligated animals when compared to abrupt-ligated animals, and (2) administration of either enalaprilat or saralasin to progressive-ligated animals resulted in mortality rates that were indistinguishable from those of saline-infused abrupt-ligated animals. These results suggest that the endogenous renin-angiotensin system is indeed involved in an adaptive mechanism that occurs during progressive ligation of the carotid artery, and more specifically, that the relatively benign effect of progressive carotid ligation may be due to the action of angiotensin II to stimulate the development of collateral circulation and reduce the severity of focal brain ischemia.

The effect of increased blood pressure on hemispheric lactate and water content during acute cerebral ischaemia in the rat and gerbil

Infusions of metaraminol and angiotensin were used to test the effect of increased perfusion pressure on tissue metabolism and oedema after induction of regional cerebral ischaemia in the rat and the gerbil. An increase of mean arterial blood pressure of 30-40 mm Hg in the rat over the first 2 hours after diathermy of the middle cerebral artery prevented the 100% rise in hemisphere lactate seen in normotensive control animals. Angiotensin infusion also prevented early hemispheric oedema in this model. In the gerbil, 4 hours after placing a clip on one carotid artery, metaraminol-induced increases in blood pressure had no such protective effect on the metabolic changes or on oedema. When the clip was removed after 3 hours to permit 1 hour of reperfusion, lactate levels returned to normal but the degree of oedema was unchanged. Hypertension in this reperfusion model caused a slight but not statistically significant increase in oedema. The evidence suggests that moderate increases in blood pressure may be protective against the early metabolic sequelae of focal cerebral ischaemia, but there are potential problems with oedema formation. It is argued that a clinical trial should study the potentially beneficial effects of a brief early increase in blood pressure in the acute aftermath of ischaemic stroke.