Effects of angiotensin II on cerebral blood vessels

Association of miR-155 and Angiotensin Receptor Type 1 Polymorphisms with the Risk of Ischemic Stroke in a Chinese Population

There is increasing evidence suggesting that dysregulation of miR-155 and its target angiotensin receptor type 1 (AT1R) are linked to the incidence of ischemic stroke (IS), but the underlying mechanisms remain to be clarified. In this study, we therefore sought to investigate how miR-155 and AT1R polymorphisms affect IS risk. We included 579 IS patients and 509 age-matched controls in the present analysis, genotyping individuals for the rs767649 polymorphism in miR-155, as well as for the rs1492099 and rs275653 polymorphisms in AT1R via iMLDR-TM genotyping technology. The allele and genotype frequencies for the assessed polymorphisms were comparable in IS patients and controls, without any detectable association between AT1R haplotype and IS risk. We conducted additional trial of ORG 10172 in acute stroke treatment-mediated stratification, which indicated that the AT1R rs1492099 T allele was linked to a decreased risk of large-artery atherosclerosis (LAA) stroke. We further found that those with the AT1R rs275653 AA genotype had a decreased risk of small-artery occlusion (SAO) strokes. We further confirmed elevated miR-155 expression in IS patients, but observed no link between the rs767649 polymorphism and expression of this microRNA. Similarly, rs1492099 and rs275653 polymorphisms did not impact AT1R expression levels. The miR-155 rs767649 polymorphism does not seem to be a key determinant of IS risk, whereas the AT1R rs1492099 polymorphism is linked to reduced LAA-stroke risk, and the rs275653 AA genotype is potentially protective against SAO strokes.

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.

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.

Cerebral vascular effects of angiotensin II: new insights from genetic models

Very little is known regarding the mechanisms of action of angiotensin II (Ang II) or the consequences of Ang II-dependent hypertension in the cerebral circulation. We tested the hypothesis that Ang II produces constriction of cerebral arteries that is mediated by activation of AT1A receptors and Rho-kinase. Basilar arteries (baseline diameter approximately 130 microm) from mice were isolated, cannulated and pressurized to measure the vessel diameter. Angiotensin II was a potent constrictor in arteries from male, but not female, mice. Vasoconstriction in response to Ang II was prevented by an inhibitor of Rho-kinase (Y-27632) in control mice, and was reduced by approximately 85% in mice deficient in expression of AT1A receptors. We also examined the chronic effects of Ang II using a model of Ang II-dependent hypertension, mice which overexpress human renin (R+) and angiotensinogen (A+). Responses to the endothelium-dependent agonist acetylcholine were markedly impaired in R+A+ mice (P<0.01) compared with controls, but were restored to normal by a superoxide scavenger (PEG-SOD). A-23187 (another endothelium-dependent agonist) produced vasodilation in control mice, but no response or vasoconstriction in R+A+ mice. In contrast, dilation of the basilar artery in response to a NO donor (NONOate) was similar in R+A+ mice and controls. Thus, Ang II produces potent constriction of cerebral arteries via activation of AT1A receptors and Rho-kinase. There are marked gender differences in cerebral vascular responses to Ang II. Endothelial function is greatly impaired in a genetic model of Ang II-dependent hypertension via a mechanism that involves superoxide.

Autocrine peptide mediators of cerebral endothelial cells and their role in the regulation of blood-brain barrier

A unique feature of cerebral endothelial cells (CECs) is the formation of the blood-brain barrier (BBB), which contributes to the stability of the brain microenvironment. CECs are capable of producing several substances mediating endothelium-dependent vasorelaxation or vasoconstriction, regulating BBB permeability, and participating in the regulation of cell-cell interactions during inflammatory and immunological processes. The chemical nature of these mediators produced by CECs ranges from gaseous anorganic molecules (e.g. nitric oxide) through lipid mediators (e.g. prostaglandins) to peptides. Peptide mediators are a large and diverse family of bioactive molecules which can elicit multiple effects on cerebral endothelial functions. In this review, we summarize current knowledge of peptide mediators produced by CECs, such as adrenomedullin, angiotensin, endothelin and several others and their role in the regulation of BBB functions.

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.

Cerebral ischemia enhances vascular angiotensin AT1 receptor-mediated contraction in rats

BACKGROUND AND PURPOSE

The aim of the study was to examine how focal cerebral ischemia affects the expression and function of vascular angiotensin II receptors.

MATERIALS AND METHODS

We used an intraluminal filament occlusion technique to occlude the right middle cerebral artery (MCA) of the rat. Myographs were used for functional studies of the MCA and real-time polymerase chain reaction, for determination of relative mRNA levels.

RESULTS

The contractile responses to angiotensin II were stronger in the right occluded MCA compared with the left MCA and the MCA from sham-operated rats 48 hours after MCA occlusion (P<0.05). The angiotensin II type 1 (AT1) receptor antagonists candesartan and losartan abolished the enhanced responses to angiotensin II (P<0.05), whereas the AT2 receptor antagonist PD123319 had no effect. The amount of AT1 receptor mRNA was lower in the occluded MCAs compared with nonoccluded MCAs 48 hours after occlusion (P<0.05), whereas the mRNA levels of angiotensin converting enzyme (ACE) were higher in the occluded arteries. The mRNA levels of the AT2 receptor and nuclear factor-kappaB were unchanged.

CONCLUSIONS

Focal cerebral ischemia in the rat upregulated the contractile responses to angiotensin II in the ipsilateral MCA, and this contraction was mediated by AT1 receptors. Real-time polymerase chain reaction revealed decreased AT1 receptor mRNA levels in the occluded MCA, whereas the amount of ACE mRNA was increased, suggesting locally enhanced angiotensin II production. These results support a role for AT1 receptors in cerebral ischemia, and we think that AT1 receptors might be a future therapeutic target in ischemic stroke.

Angiotensin II produces superoxide-mediated impairment of endothelial function in cerebral arterioles

BACKGROUND AND PURPOSE

Angiotensin II (Ang II) produces oxidative stress in vascular cells in culture and in extracranial conduit arteries. The goal of this study was to examine the hypothesis that Ang II produces superoxide-mediated impairment of endothelial function in cerebral microvessels.

METHODS

Diameter of cerebral arterioles (baseline diameter=104+/-3 microm) was measured with the use of a closed cranial window in anesthetized rabbits. Topical application of Ang II was used to avoid effects on arterial pressure.

RESULTS

Ang II (0.1 to 1 micromol/L for 2 hours) had no effect on baseline diameter (change in diameter of -3+/-2% in response to 1 micromol/L Ang II) but produced concentration-dependent inhibition of vasodilatation to the endothelium-dependent agonist bradykinin. For example, 1 micromol/L Ang II inhibited responses to 1 nmol/L bradykinin by almost 80%. These inhibitory effects of Ang II were prevented by the superoxide scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron; 10 mmol/L) or diphenylene iodonium (DPI; 3 micromol/L), an inhibitor of NAD(P)H oxidase. Ang II did not inhibit vasodilatation in response to nitroprusside, an endothelium-independent vasodilator.

CONCLUSIONS

These findings are the first evidence that local Ang II produces superoxide-mediated vascular dysfunction in cerebral microvessels. The results with DPI suggest that the source of superoxide may be an NAD(P)H oxidase.

Intracisternal administration of Angiotensin II AT1 receptor antisense oligodeoxynucleotides protects against cerebral ischemia in spontaneously hypertensive rats

Pharmacological blockade of peripheral and brain Angiotensin II (Ang II) AT(1) receptors protects against brain ischemia. To clarify the protective role of brain AT(1) receptors, we examined the effects of specific antisense oligodeoxynucleotides (AS-ODN) targeted to AT(1) receptor mRNA administered intracisternally to spontaneously hypertensive rats (SHRs), 4 and 7 days before middle cerebral artery (MCA) occlusion, and we determined the infarct size and tissue swelling 24 h after surgery. A single intracisternal injection of AT(1) mRNA receptor antisense oligodeoxynucleotides reduced systemic blood pressure for 5 days and AT(1) receptor binding for at least 4 days in the area postrema and the nucleus of the solitary tract. A similar injection of scrambled oligodeoxynucleotides (SC-ODN) was without effect. Both blood pressure and AT(1) receptor binding returned to normal 7 days after antisense receptor mRNA administration. Both the infarction size and the tissue swelling after middle cerebral artery occlusion were reduced when the antisense oligodeoxynucleotide was administered 7 days, but not 4 days, before the operation. We conclude that 4 to 5 days of decrease in brain AT(1) receptor binding by a single administration of an AT(1) receptor mRNA oligodeoxynucleotide are sufficient to significantly protect the brain against ischemia resulting from total occlusion of a major cerebral vessel.

Protection against ischemia and improvement of cerebral blood flow in genetically hypertensive rats by chronic pretreatment with an angiotensin II AT1 antagonist

BACKGROUND AND PURPOSE

Pretreatment with angiotensin II AT(1) receptor antagonists protects against cerebral ischemia. We studied whether modulation of cerebral blood flow (CBF) and morphometric changes in brain arteries participated in this protective mechanism.

METHODS

We pretreated adult spontaneously hypertensive rats with equally antihypertensive doses of candesartan (0.1 or 0.3 mg/kg per day), nicardipine (0.1 mg/kg per day), or captopril (3.0 mg/kg per day) for 3 or 28 days via subcutaneous osmotic minipumps followed by permanent left middle cerebral artery (MCA) occlusion distal to the origin of the lenticulostriate arteries. We measured CBF by autoradiography with 4-iodo-[N-methyl-(14)C]antipyrine 3 hours after operation and the areas of infarct and tissue swelling 24 hours after operation. Morphometric changes in the MCA were studied after antihypertensive treatment.

RESULTS

Twenty-eight days of candesartan pretreatment decreased the infarct area by 31%; reduced the CBF decrease at the peripheral area of ischemia and the cortical volume of severe ischemic lesion, where CBF was <0.50 mL/g per minute; increased the MCA external diameter by 16%; and reduced the media thickness of the MCA by 23%. Captopril pretreatment for 28 days decreased the infarct area by 25%. Pretreatment with candesartan for 3 days or nicardipine for 28 days was ineffective.

CONCLUSIONS

Angiotensin II system inhibition protects against neuronal injury more effectively than calcium channel blockade. Protection after AT(1) receptor blockade is not directly correlated with blood pressure reduction but with normalization of MCA media thickness, leading to increased arterial compliance and reduced CBF decrease during ischemia at the periphery of the lesion.

Dilatation of cerebral parenchymal vessels mediated by angiotensin type 1 receptor in cats

We report the effects of angiotensin II (ANG-II), as well as angiotensin II type 1 (AT1) and type 2 receptor antagonists (CV-11974 and PD-123319, respectively) on the cerebral parenchymal microvessels in cats using the photoelectric method. ANG-II continuously and dose-dependently increased the cerebral blood volume (CBV) for 15 min. Maximum CBV increases were +0.36+or-0.11 vol% for 0.01 nmol/kg (P<0.05), +0.51+or-0.24 vol% for 0.1 nmol/kg (P<0.05), +1.87+or-0.55 vol% for 1 nmol/kg (P<0.05), and +2.14+or-0.77 vol% for 10 nmol/kg (P<0.05). Systemic arterial blood pressure increased at only 1 min following ANG-II infusion (1 and 10 nmol/kg). CV-11974 and PD-123319 per se did not change the resting CBV. CV-11974 completely inhibited the vasodilatory action of ANG-II, however, PD-123319 did not block it. We conclude that ANG-II directly dilates the parenchymal vessels through the AT1 receptor without increasing systemic blood pressure, and that intrinsic ANG-II may not be associated with maintenance of resting vascular tone.

Impact of the renin-angiotensin system on cerebral perfusion following subarachnoid haemorrhage in the rat

1. This study investigated the effects of blocking the AT1 angiotensin receptors with irbesartan, either peripherally or centrally, on systemic blood pressure, intracranial pressure and cerebral perfusion pressure following experimental subarachnoid haemorrhage (SAH) in urethane-anaesthetized rats. Sympathetic nervous activation was determined by measuring plasma noradrenaline levels. 2. In untreated animals, SAH induced a sustained increased in intracranial pressure from 2.1 +/- 0.3 to 16 +/- 2 mmHg (3 h, P < 0.001). Cerebral perfusion pressure was reduced by 20 % (P < 0.001), this reduction being maintained for 3 h. Sympathetic activation was evident in the high level of plasma noradrenaline measured 3 h post-SAH (751 +/- 104 vs. 405 +/- 33 pg ml(-1), P < 0.05). 3. Acute peripheral pretreatment with irbesartan (3 mg kg(-1), I.V.) prevented the rise in plasma noradrenaline and further aggravated the decrease in cerebral perfusion pressure by producing transient systemic hypotension (blood pressure was 85 +/- 6 mmHg at 2 h post-SAH vs. 100 +/- 3 mmHg, P < 0.01). 4. Intracisternal pretreatment with irbesartan (0.035 mg) did not prevent the rise in plasma noradrenaline post-SAH but enhanced the rise in intracranial pressure by 75 % compared with untreated animals. 5. This study demonstrates that peripheral endogenous angiotensin II interacts with the sympathetic nervous system in order to maintain an adequate cerebral perfusion following SAH. Endogenous angiotensin II in the brain seems to exert a protective effect by counteracting the elevation in intracranial pressure that occurs following experimental SAH.

No effect of angiotensin II AT(2)-receptor antagonist PD 123319 on cerebral blood flow autoregulation

Blockade of the renin-angiotensin system with angiotensin-converting enzyme inhibitors (ACE-I) or angiotensin AT1-receptor antagonists shift the limits of autoregulation of cerebral blood flow (CBF) towards lower blood pressure (BP). The role of AT2-receptors in the regulation of the cerebral circulation is uncertain. Hence, the present study investigated the effect on CBF autoregulation of blocking of angiotensin AT2-receptors with PD 123319 in spontaneously hypertensive rats (SHR). Anaesthetised and ventilated SHR were given PD 123319, 0.36 mg/kg/min, intravenously, and compared with a control group. CBF was measured by the intracarotid 133xenon injection method and BP was raised by noradrenaline infusion and lowered by controlled haemorrhage in separate groups of rats. The limits of autoregulation were determined by computed least-sum-of-squares analysis. PD 123319 did not influence baseline CBF, but resulted in a minor BP decrease (10 control and 10 treated rats). The lower limit of CBF autoregulation (eight treated and eight control) as well as the upper limit of CBF autoregulation (eight treated and eight control) were not significantly different in PD 123319 and control animals (lower limit treated 102+/-4 mmHg and control 94+/-4; NS, and upper limit treated 171 +/- 10 mmHg and control 162+/-7; NS). These findings indicate that acute AT2-receptor blockade does not influence CBF autoregularion.

Effect of renin on brain arterioles and cerebral blood flow in rabbits

There is evidence of an intrinsic renin-angiotensin system in the brain. The goal of the study was to determine whether stimulation of endogenous angiotensin production by applying renin to the brain surface has an effect on pial arteriolar caliber and CBF. Pial vessel diameters were measured through a closed cranial window in anesthetized rabbits. Percent changes of blood flow in the cortical area under the cranial window were simultaneously measured by laser-Doppler flowmetry. Topical application of 0.01-0.1 U/ml renin induced maximum dilation of 18.9 +/- 4% (mean +/- SD) of pial arterioles within 2 min. Arteriolar calibers thereafter decreased slowly. Flow gradually increased to peak at 38 +/- 15% 50 min after renin application. Angiotensin I levels in jugular blood, as measured by radioimmunoassay, increased to a peak 40 min after topical renin application. Angiotensin II levels in jugular blood and both angiotensin I and II levels in blood samples from the femoral artery did not change. Diameter and flow changes were inhibited by intravenous pretreatment with the converting enzyme blocker captopril (10 mg/kg body wt i.v.). Captopril did not affect the vasodilation and flow increase in response to hypercapnia. Topically applied captopril (10(-5) M) blocked renin-induced arteriolar dilation. We conclude that renin increases pial arteriolar diameters and cortical blood flow in the rabbit brain. Stimulation of angiotensin production is likely to be a mediator of this response.

Angiotensin II contributes to cerebral vasodilatation during hypoxia in the rabbit

BACKGROUND

and Purpose Hypoxia increases cerebral blood flow (CBF). Hypoxia also exerts a major influence on the renin-angiotensin system. In addition to the circulating renin-angiotensin system, a local renin-angiotensin system appears to be present in the brain, and angiotensin II receptors have been identified in cerebral blood vessels. In this study we tested the hypothesis that endogenous angiotensin II attenuates dilatation of the cerebral vessels during hypoxia.

METHODS

Pentobarbital-anesthetized rabbits were prepared for measurement of blood flow (microspheres) and assigned to one of two groups: in group 1 (n = 11), rabbits were subjected to 30 minutes of stable hypoxia (PaO2 = 34 +/- 1 mm Hg, mean +/- SD) followed by 15 minutes of reoxygenation (PaO2 = 177 to 200 mm Hg). Blood flow was measured four times: under control conditions, after 15 and 30 minutes of hypoxia, and after 15 minutes of reoxygenation. This was a control group to characterize changes in CBF during hypoxia. In group 2 (n = 11), blood flow was measured as in the previous group except that an infusion of the angiotensin II receptor antagonist saralasin (1 microgram.kg-1.min-1 IV) was started with the onset of hypoxia and continued through reoxygenation to the end of the experiment. The goal of this group was to examine whether endogenous activation of receptors for angiotensin II influences increases in CBF during hypoxia. In a separate series of experiments we examined the influence of the angiotensin-converting enzyme (ACE) inhibitor captopril on the hypoxic response. Thus, in one group of rabbits we measured CBF in the same manner as in group 1 (n = 13). In another group of rabbits we also measured blood flow as in group 1 except that rabbits received 10 mg/kg of the ACE inhibitor captopril before the control measurement (n = 11). We tested for significant differences between groups using two-way ANOVA.

RESULTS

Under control conditions, CBF was similar in all groups and averaged 53 +/- 15 mL.min-1.100 g-1. During hypoxia, CBF increased to a greater extent in the absence versus the presence of saralasin (95 +/- 31 and 104 +/- 30 mL.min-1.100 g-1 versus 72 +/- 24 and 71 +/- 25 mL.min-1.100 g-1, respectively; P = .003). Increase in CBF during hypoxia was also significantly greater in the animals that did not receive captopril versus those that were treated with captopril (100 +/- 24 and 89 +/- 16 mL.min-1.100 g-1 versus 72 +/- 16 and 73 +/- 17 mL.min-1.100 g-1). To rule out the possibility that saralasin produced non-specific attenuation of cerebral vasodilatation, we tested the influence of hypercapnia on CBF in the absence and presence of saralasin. During normocapnia, CBF values were not significantly different in the absence and presence of saralasin (57 +/- 17 and 64 +/- 6 mL.min-1.100 g-1, respectively; P > .05). Hypercapnia increased CBF similarly in the absence and presence of saralasin (81 +/- 22 and 91 +/- 19 mL.min-1.100 g-1; PaCO2 = 61 +/- 2 and 60 +/- 2 mm Hg, respectively; P > .05).

CONCLUSIONS

Because the ACE inhibitor captopril and the angiotensin II receptor blocker saralasin attenuated increased in CBF during hypoxia, the findings suggest that endogenous release of angiotensin II contributes to the increase in CBF during hypoxia.

Cerebrovascular autoregulation in response to hypertension induced by NG-nitro-L-arginine methyl ester

Local neocortical blood flow and glucose utilization were measured in conscious rats using [14C]iodoantipyrine and [14C]2-deoxyglucose quantitative autoradiography, respectively, following intravenous injection of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (30 mg/kg). The dose of NG-nitro-L-arginine methyl ester was chosen so as to produce a level of hypertension equivalent to that produced in a parallel group of rats by the infusion of angiotensin-II (5 micrograms/ml at 0.5-2.0 ml/h). In those animals in which angiotensin-induced hypertension did not exceed 150 mmHg (mean arterial blood pressure), there were no significant effects upon cortical blood flow when compared to controls, but at higher pressures (157 +/- 1 mmHg), blood flow was significantly increased in circumscribed areas of cortex, most notably in parietal (from 204 +/- 10 to 780 +/- 44 ml/100 g per min) and occipital cortex (from 175 +/- 5 to 600 +/- 46 ml/100 g per min), whilst other cortical areas (e.g. temporal and frontal areas) were unchanged. Despite the fact that NG-nitro-L-arginine methyl ester increased blood pressure to levels (164 +/- 1 mmHg) which were in excess of the highest produced by angiotensin, there was no evidence of focal hyperaemia; indeed blood flow was significantly reduced in every cortical region except parietal area 1. No significant differences in glucose use were evident between any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral blood flow and cerebral blood flow velocity during angiotensin-induced arterial hypertension in dogs

Pressure-passive perfusion beyond the upper limit of cerebral blood flow (CBF) autoregulation may be deleterious in patients with intracranial pathology. Therefore, monitoring of changes in CBF would be of clinical relevance in situations where clinical evaluation of adequate cerebral perfusion is impossible. Noninvasive monitoring of cerebral blood flow velocity using transcranial Doppler sonography (TCD) may reflect relative changes in CBF. This study correlates the effects of angiotensin-induced arterial hypertension on CBF and cerebral blood flow velocity in dogs. Heart rate (HR) was recorded using standard ECG. Catheters were placed in both femoral arteries and veins for measurements of mean arterial blood pressure (MAP), blood sampling and drug administration. A left ventricular catheter was placed for injection of microspheres. Cerebral blood flow velocity was measured in the basilar artery through a cranial window using a pulsed 8 MHz transcranial Doppler ultrasound system. CBF was measured using colour-labelled microspheres. Intracranial pressure (ICP) was measured using an epidural probe. Arterial blood gases, arterial pH and body temperature were maintained constant over time. Two baseline measures of HR, MAP, CBF, cerebral blood flow velocity and ICP were made in all dogs (n = 10) using etomidate infusion (1.5 mg.kg-1 x hr-1) and 70% N2O in O2 as background anaesthesia. Following baseline measurements, a bolus of 1.25 mg angiotensin was injected i.v. and all variables were recorded five minutes after the injection. Mean arterial blood pressure was increased by 76%. Heart rate and ICP did not change. Changes in MAP were associated with increases in cortical CBF (78%), brainstem CBF (87%) and cerebellum CBF (64%).(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural localization of angiotensin II-like immunoreactivity (A II-LI) in the vegetative networks of the spinal cord of the guinea pig

The localization of angiotensin II-like immunoreactivity (A II-LI) has been studied at an ultrastructural level in the thoracic and sacral spinal cord vegetative regions of male and female guinea pigs. Most of the AII-LI structures are axon terminals and fibers localized in both vegetative networks which connect bilaterally and cranio-caudally and/or caudo-cranially the preganglionic vegetative nuclei in the thoracic and sacral spinal cord intermediate zone. The A II-LI product is localized predominantly in large granular and small vesicles of axon terminals and varicosites which form axo-somatic, axo-dendritic and axo-axonal synapses. Occasionally A II-LI is observed over cysterns of the endoplasmatic reticulum and throughout the cytoplasm of astrocytes which are neighbouring on blood vessels and are in contact with neuronal processi. Immunoreactive terminals are observed in direct contacts with endothelial cells too. These observations confirm recently described light-microscopic localization of Ang II-LI in the studied spinal cord areas, as well as its suggested role as transmitter and/or modulator of the neuronal function and regulatory role over the cardiovascular function and blood circulation.

Effects of angiotensin and atrial natriuretic peptide on the cerebral circulation

The purpose of the present study was to determine effects of angiotensin (ANG) II on the cerebral circulation. We measured the pial artery pressure (PAP) and CBF in anesthetized rabbits. ANG II (5 micrograms/min) was infused into each carotid artery, and systemic arterial pressure was maintained constant. During infusion of ANG II, there was a significant increase in CBF and fall of PAP, with no change in the large artery resistance (LAR) and a significant decrease in the small vessel resistance (SVR). To investigate whether prostaglandin modulated the ANG II-induced increase in CBF, indomethacin was administered (10 mg/kg i.v.) in another group of animals. Indomethacin itself reduced PAP and increased LAR significantly without changing CBF or SVR. Indomethacin did not attenuate the effects of ANG II on the cerebral circulation. The CMRO2 was assessed during ANG II intracarotid infusion in another group of rabbits. CMRO2 did not change during infusion of ANG II. We also investigated effects of alpha-atrial natriuretic peptide (ANP) on the cerebral circulation. Infusion of ANP (1 microgram/min) decreased LAR by 28% (p less than 0.05) without altering SVR. Administration of ANG II after ANP tended to reduce LAR (p greater than 0.05), with a significant decrease in SVR. The results of the present study suggest that high doses of ANG II can produce cerebral vasodilatation, particularly of small vessels. Blood-borne ANP dilated the large arteries of the cerebral circulation selectively and neither interfered with nor reversed the ANG II-induced increase in CBF.

Captopril improves neurologic outcome from incomplete cerebral ischemia in rats

We investigated the effects of the angiotensin-converting enzyme inhibitor captopril on neurologic outcome in a rat model of incomplete cerebral ischemia. Twenty male Sprague-Dawley rats were anesthetized with 70% nitrous oxide in oxygen and fentanyl (10 micrograms x kg-1 i.v. bolus, 25 micrograms x kg-1 x hr-1 i.v. continuous infusion). Animals in group 1 (n = 10) received no angiotensin-converting enzyme inhibitor while animals in group 2 (n = 10) were given 10 mg x kg-1 i.v. captopril 30 minutes prior to the ischemic period. Ischemia was produced by unilateral carotid artery ligation and hemorrhagic hypotension to 35 mm Hg for 30 minutes. Body temperature, arterial blood gases, and arterial pH were maintained constant. Neurologic outcome was evaluated every 24 hours for 3 days using a graded deficit score (0, normal; 18, stroke-related death). On the third day after ischemia, captopril significantly improved neurologic outcome (median deficit score = 4) compared with controls (median deficit score = 18) (p less than 0.05). These results suggest that reduced angiotensin II levels or increased tissue kinin concentrations may decrease ischemic brain injury.

Angiotensin II, vasopressin and GTP[gamma-S] inhibit inward-rectifying K+ channels in porcine cerebral capillary endothelial cells

Cerebral capillaries from porcine brain were isolated, and endothelial cells were grown in primary culture. The whole-cell tight seal patch-clamp method was applied to freshly isolated single endothelial cells, and cells which were held in culture up to one week. With high K+ solution in the patch pipette and in the bath we observed inward-rectifying K+ currents, showing a time-dependent decay in part of the experiments. Ba2+ (1-10 mM) in the bath blocked this current, whereas outside tetraethylammonium (10 mM) decreased the peak current but increased the steady-state current. Addition of 1 microM of angiotensin II or of arginine-vasopressin to the extracellular side caused a time-dependent inhibition of the inward-rectifying K+ current in part of the experiments. Addition of 100 microM GTP[gamma-S] to the patch pipette blocked the K+ inward rectifier. In cell-attached membrane patches two types of single inward-rectifying K+ channels were observed, with single channel conductances of 7 and 35 pS. Cell-attached patches were also obtained at the antiluminal membrane of intact isolated cerebral capillaries. Only one type of K+ channel with g = 30 pS was recorded. In conclusion, inwardly rectifying K+ channels, which can be inhibited by extracellular angiotensin II and arginine-vasopressin, are present in cerebral capillary endothelial cells. The inhibition of this K+ conductance by GTP[gamma-S] indicates that G-proteins are involved in channel regulation. It is suggested that angiotensin II and vasopressin regulate K+ transport across the blood-brain barrier, mediating their effects via G-proteins.

Regulation of large cerebral arteries and cerebral microvascular pressure

Resistance of large arteries appears to be greater in the cerebral circulation than in other vascular beds. Large arteries contribute importantly to total cerebral vascular resistance and are major determinants of local microvascular pressure. Recent studies have shown that resistance of large arteries and cerebral microvascular pressure are affected by several physiological stimuli, including changes in systemic blood pressure, increases in cerebral metabolism, activity of sympathetic nerves, and humoral stimuli such as circulating vasopressin and angiotensin. Stimuli such as sympathetic stimulation and vasopressin produce selective responses of large arteries and, thereby, regulate microvascular pressure without a significant change in cerebral blood flow. These findings lead to the new hypothesis that the brain may be sensitive to changes in cerebral microvascular pressure, resulting in activation of compensatory neurohumoral mechanisms. Important changes occur in large cerebral arteries under pathophysiological conditions. Chronic hypertension increases resistance of large cerebral arteries, which protects the microcirculation against hypertension. Atherosclerosis potentiates constrictor responses of large cerebral arteries to serotonin and thromboxane, which may contribute to vasospasm and transient ischemic attacks.

In vivo microscopy of the cerebral microcirculation using neonatal allografts in hamsters

Studies were performed to characterize the morphology and vascular reactivity of the allografted cerebral microcirculation. Cerebral cortical tissue was allografted into the cheek pouch of the hamster so that cerebral parenchymal vessels could be studied. The vascular morphology was characterized by a large number of looping vessels. The ultrastructural examination indicated viable cerebral tissue containing typical vessels, that is, "tight" junctions, not like those of the cheek pouch. Also, the microvasculature was impermeable to 150, 70, and 20 kDa fluorescein isothiocyanate dextrans. Angiotensin II and norepinephrine caused constriction of the cerebral vessels whereas adenosine caused dilation. Isoproterenol did not affect cerebral arterioles; however, it dilated cheek pouch arterioles. Thus, this preparation provides a satisfactory model for studying the living cerebral microcirculation.

Does angiotensin-II protect against strokes?

In the Medical Research Council trial for the treatment of mild hypertension, bendrofluazide showed an unexpected and sizeable benefit compared with propranolol in the reduction of stroke. It is suggested that this difference reflects the opposing actions of these drugs on the renin-angiotensin system. The hypothesis that angiotensin-II protects the distal smaller cerebral vessels, which are the usual site of vessel rupture in intracerebral haemorrhage, indicates that long-term benefit of angiotensin-converting-enzyme inhibitors in the treatment of hypertension cannot be assumed.

Angiotensin II decreases mortality rate in gerbils with unilateral carotid ligation

Evidence indicates that after vascular occlusion, infusion of angiotensin II restores blood supply to ischemic tissues by stimulating the development of collateral circulation through a mechanism independent of the mechanical effects of increased blood pressure. To test this effect in focal cerebral ischemia, angiotensin II was intravenously administered for four hours to gerbils immediately after unilateral carotid ligation. Three different pressor doses, 50, 250, and 500 ng/kg/min, were used, and mortality rate was evaluated at 1 and 2 days after vascular occlusion. Two additional groups similarly prepared were infused either with saline or with the pressor agent metaraminol. There was a significant inverse relationship between the infusion dose of angiotensin II and mortality: the greater the infusion dose of angiotensin II, the lower the mortality rate. Infusion of metaraminol, at the dose chosen to mimic the pressor effect of the highest angiotensin II dose, yielded a mortality rate which was statistically indistinguishable from that obtained with saline infusion. It is concluded that the mortality rate after unilateral carotid occlusion is significantly reduced by intravenous administration of angiotensin II through mechanisms unrelated to its hypertensive action. Evidence suggests that this may occur by the enhancement of the development of collateral circulation and therefore the reduction of the severity of brain ischemia.

Angiotensin II receptor binding sites in brain microvessels

We assessed the specific binding of 125I-labeled angiotensin II (125I-Ang II) to particulate fractions of the cerebral cortex and cerebellum and to microvessels obtained by bulk isolation from these two brain regions in the dog. 125I-Ang II binds to cerebral and cerebellar microvessels in a specific, saturable, and reversible manner and with high affinity (dissociation constant about 1 nM). Maximal binding of 125I-Ang II to brain microvessels was about 2-fold higher than the maximal binding to particulate fractions of the cerebellum and more than 15-fold higher than that of the cerebral cortex. No significant differences were noted between cerebral and cerebellar microvessels in their specific binding of Ang II. Furthermore, our finding that analogues of Ang II displace specific 125I-Ang II binding to brain microvessels in a rank order that correlates with their pharmacological activities confers biological relevance on the ligand-binding studies. These results strongly suggest that specific Ang II receptor binding sites are present in brain microvessels. Such Ang II receptors may have an important role in regulating the microcirculation of the brain.

Interaction between presynaptic facilitatory angiotensin II receptors and inhibitory muscarinic cholinoceptors on 3H-noradrenaline release in the rabbit heart

The existence of a functional interaction between presynaptic receptors modulating the release of noradrenaline was studied in the rabbit heart. Isolated right atria were prelabelled with 3H-noradrenaline and the overflow of tritium was induced by field stimulation (2 Hz, 0.1 ms duration, supramaximal voltage for a total of 180 pulses). In atria superfused with Krebs' solution containing 10 mumol/l cocaine and 30 mumol/l corticosterone, angiotensin II (10 nmol/l) increased the stimulation-evoked overflow of 3H-transmitter by 2.8-fold. The addition of atropine (0.3 mumol/l) to the perfusion medium, either in the presence or in the absence of uptake inhibitors, further enhanced the facilitatory effect of angiotensin II (3H-transmitter release increased by 3.5-fold). Exposure to 1 mumol/l carbachol decreased by 65% the stimulation-evoked release of 3H-transmitter while the facilitatory effect of angiotensin II determined in the presence of the muscarinic cholinoceptor agonist was enhanced (3H-transmitter release increased by 6.6-fold). Conversely, during sustained activation of presynaptic angiotensin receptors producing a 2.5-fold increase in the release of 3H-transmitter, the inhibitory effect of carbachol remained unchanged. These results suggest a functional interaction between presynaptic inhibitory muscarinic cholinoceptors and the presynaptic facilitatory angiotensin receptor which modulate the release of noradrenaline from cardiac noradrenergic nerves.

Cerebrospinal fluid pressure during cerebroventricular infusion of angiotensin and vasopressin

Rationale exists for suspecting that angiotensin (Ang) and arginine vasopressin (AVP) given by the intracerebroventricular (IVT) route can affect cerebrospinal fluid (CSF) pressure. This hypothesis was tested in conscious, unrestrained adult male Sprague-Dawley rats with IVT and left carotid arterial catheters. The rats were infused (IVT) for 30 min with artificial CSF followed by 30 additional minutes with CSF, Ang, (0.6 micrograms/h) AVP (5 or 50 ng/h), or AVP (5 or 50 ng/h) + Ang, (0.6 micrograms/h). Angiotensin evoked a central hypertensive effect (+ 16 mm Hg) and increased CSF pressure from 10 to 16 cm H2O (P less than 0.05). Neither dose of AVP affected blood or CSF pressures. The AVP (5 ng/h) prevented Ang-induced changes in blood and CSF pressures and AVP (50 ng/h) blocked only the Ang-induced rise in CSF pressure. These results raise the possibility that angiotensin and vasopressin participate in the regulation of CSF pressure.

Generalized cerebral vasoconstriction induced by intracarotid infusion of angiotensin II in the rabbit

This study investigated the influence of angiotensin II, perfused into one common carotid artery at a dose of 0.065 micrograms/kg/min, on the cerebrovascular resistance of the anesthetized rabbit by means of complementary in vivo methods. Heat clearance and mass spectrometry measurements indicated that in the homolateral caudate nucleus angiotensin induced a significant decrease in local blood flow (18.2 +/- 9%), a fall in pO2 (14.2 +/- 5.3%) and no significant change in pCO2. The [14C]ethanol tissue sampling technique revealed a significant decrease in flow in all 10 structures sampled in the brain. This decrease was similar in magnitude in both the ipsilateral and the contralateral hemisphere with regard to the site of injection. When expressed in terms of cerebrovascular resistance (CVR) and allowing for a slight increase in blood pressure (less than 10%), these results show that angiotensin II infusion induced an increase in CVR of 18-32%. We conclude that: A unilateral intracarotid infusion of a low dose of angiotensin II induces an increased vascular tone in all cerebral structures. This action, being bilateral, cannot readily be explained by a direct action of angiotensin II on the cerebral vessels in view of the very low recirculating concentration of angiotensin II (less than 10(-9) M). The hypothesis of a cerebral vasomotor influence of angiotensin II by action on a central structure is discussed.

Correlated electrical and mechanical responses of isolated rabbit pial arteries to some vasoactive drugs

Simultaneous measurements were made of spike activity and perfusion pressure (PA) in intact segments of rabbit middle cerebral artery in vitro. The segments were mounted on a Teflon tube designed so that the perfusing solution flowed in the annular space between the tube and the artery wall, thus magnifying the PA changes occurring when the artery constricted or dilated. A widened portion of the Teflon tube immobilized 1--2 mm of the artery segment for electrical recording with fine glass microelectrodes. Spontaneous spike activity (extra- and intracellular) was regularly observed. When a steady PA and spike discharge was obtained, tests were performed by substituting for the normal perfusion liquid, solutions containing 5 microgram/ml norepinephrine, 5 microgram/ml angiotensin II or 7.5 microgram/ml isoproterenol. Norepinephrine and angiotensin each increased spike frequency (+ 293 and + 126%) and PA (+ 6.6 and + 7.9 mm Hg) whereas isoproterenol decreased spike frequency (-89%) and PA (-22.9 mm Hg). These results a) confirm the presence of receptors to these agents in pial arteries, and b) demonstrate a high degree of correlation between membrane electrical events and mechanical activity of these spontaneously-active myovascular cells.