Cerebrovascular effects of angiotensin converting enzyme inhibition involve large artery dilatation in rats

Patients with Prior Exposure to a Combination of Statins & Angiotensin-Converting Enzyme Inhibitors (ACE-Is)/Angiotensin Receptor Blockers (ARBs) Have Better Outcomes after Carotid Revascularization than Patients with Prior Exposure to Statins Alone: A MultiCenter Analysis

BACKGROUND

Statin use has been studied and confirmed to have a beneficial impact on perioperative carotid endarterectomy (CEA) and carotid artery stenting (CAS) outcomes. The benefits of Angiotensin-converting enzyme inhibitors (ACE-I) in hypertension, ischemic heart disease, heart failure, diabetes mellitus, and renal disease are well-known; however, the impact of continuing or withholding ACE-Is/angiotensin receptor blockers (ARBs) on CEA and CAS outcomes is not addressed well in the literature. This study aimed to evaluate the impact of preoperative statin use combined with ACE-Is/ARBs in patients undergoing CEA or CAS on mortality and morbidity using a multi-institutional database.

METHODS

Using the data of all patients who underwent carotid artery revascularization, including CEA, transcarotid artery revascularization, and transfemoral carotid artery stenting from 2016 to 2021 in the Vascular Quality Initiative data, we determined as our primary outcome 30-day mortality/stroke after carotid revascularization based on periop exposure to statins alone, or the combination of statins and ACE-Is/ARBs. Secondary outcomes were postop myocardial infarction and postop congestive heart failure. Poisson regression with robust variance was used to determine postop outcomes comparing the combination of statin and ACE-Is/ARBs group with statins alone group.

RESULTS

A total of 131,285 patients were included in the study, with 59,860 (46%) patients receiving statin only, and 71,425 (54%) receiving both statin and ACE-Is/ARBs preoperatively. Both patient groups differed significantly in preop clinical and demographic characteristics. After adjusting for potential confounders, the statins plus ACE-I/ARB group had a 12% lower risk of postop mortality/stroke (Incident Rate Ratio comparing Statin/ACE group to Statins Only group [IRR] 0.88, 95% confidence interval 0.81-0.95, P = 0.001), 18% lower risk of postop congestive heart failure (IRR 0.82, 95% CI 0.68-0.98, P = 0.029), and similar risk of postop myocardial infarction (IRR 1.05 95% confidence interval 0.91-1.20, P = 0.54) compared to the statin-only group.

CONCLUSION

Statins combined with ACE-Is/ARBs perioperatively offer better protection compared to statins alone in patients undergoing carotid revascularization surgery. We recommend the continuation of ACE-Is/ARBs use in patients undergoing carotid revascularization, especially if they have concurrent hypertension. Further prospective studies are needed to evaluate the benefit of adding ACE-Is/ARBs.

Role of Renin-Angiotensin-Aldosterone System Inhibition in Patients Undergoing Carotid Revascularization

Background

Previous data suggest that using renin‐angiotensin‐aldosterone system inhibitors (RAASIs) improves survival in patients with cardiovascular diseases. We sought to investigate the association of different patterns of use of RAASIs on perioperative and 1‐year outcomes following carotid revascularization.

Methods and Results

We investigated patients undergoing carotid revascularization, either with carotid endarterectomy or transfemoral carotid artery stenting, in the VQI (Vascular Quality Initiative) VISION (Vascular Implant Surveillance and Interventional Outcomes Network) data set between 2003 and 2018. We divided our cohort into 3 groups: (1) no history of RAASI intake, (2) preoperative intake only, and (3) continuous pre‐ and postoperative intake. The final cohort included 73 174 patients; 44.4% had no intake, 50% had continuous intake, and 5.6% had only preoperative intake. Compared with continuous intake, preoperative and no intake were associated with higher odds of postoperative stroke (odds ratio [OR], 1.7 [95% CI, 1.5–1.9]; P<0.001; OR, 1.1 [95% CI, 1.03–1.2]; P=0.010); death (OR, 4.8 [95% CI, 3.8–6.1]; P<0.001; OR, 1.9 [95% CI, 1.6–2.2]; P<0.001); and stroke/death (OR, 2.05 [95% CI, 1.8–2.3]; P<0.001; OR, 1.2 [95% CI, 1.1–1.3]; P<0.001), respectively. At 1 year, preoperative and no intake were associated with higher odds of stroke (hazard ratio [HR], 1.4 [95% CI, 1.3–1.6]; P<0.001; HR, 1.15, [95% CI, 1.08–1.2]; P<0.001); death (HR, 1.7 [95% CI, 1.5–1.9]; P<0.001; HR, 1.3 [95% CI, 1.2–1.4]; P<0.001); and stroke/death (HR, 1.5 [95% CI, 1.4–1.7]; P<0.001; HR, 1.2 [95% CI, 1.17–1.3]; P<0.001), respectively.

Conclusions

Compared with subjects discontinuing or never starting RAASIs, use of RAASIs before and after carotid revascularization was associated with a short‐term stroke and mortality benefit. Future clinical trials examining prescribing patterns of RAASIs should aim to clarify the timing and potential to maximize the protective effects of RAASIs in high‐risk vascular patients.

Bradykinin antagonist counteracts the acute effect of both angiotensin-converting enzyme inhibition and of angiotensin receptor blockade on the lower limit of autoregulation of cerebral blood flow

The lower limit of autoregulation of cerebral blood flow (CBF) can be modulated with both angiotensin-converting enzyme (ACE) inhibitors and angiotensin receptor blockers (ARB). The influence of bradykinin antagonism on ARB-induced changes was the subject of this study. CBF was measured in Sprague-Dawley rats with laser Doppler technique. The blood pressure was lowered by controlled bleeding. Six groups of rats were studied: a control group and five groups given drugs intravenously: an ACE inhibitor (enalaprilat), an ARB (candesartan), a bradykinin-2 receptor antagonist (Hoe 140), a combination of enalaprilat and Hoe 140, and a combination of candesartan and Hoe 140. In the control group, the lower limit of CBF autoregulation was 54±9 mm Hg (mean±s.d.), with enalaprilat it was 46±6, with candesartan 39±8, with Hoe 140 53±6, with enalaprilat/Hoe 140 52±6, and with candesartan/Hoe 140 50±7. Both enalaprilat and candesartan lowered the lower limit of autoregulation of CBF significantly. The bradykinin antagonist abolished not only the effect of the ACE inhibitor but surprisingly also the effect of the ARB on the lower limit of CBF autoregulation, the latter suggesting an effect on intravascular bradykinin.

Cerebral Autoregulation and Syncope

Whatever the pathogenesis of syncope is, the ultimate common cause leading to loss of consciousness is insufficient cerebral perfusion with a critical reduction of blood flow to the reticular activating system. Brain circulation has an autoregulation system that keeps cerebral blood flow constant over a wide range of systemic blood pressures. Normally, if blood pressure decreases, autoregulation reacts with a reduction in cerebral vascular resistance, in an attempt to prevent cerebral hypoperfusion. However, in some cases, particularly in neurally mediated syncope, it can also be harmful, being actively implicated in a paradox reflex that induces an increase in cerebrovascular resistance and contributes to the critical reduction of cerebral blood flow. This review outlines the anatomic structures involved in cerebral autoregulation, its mechanisms, in normal and pathologic conditions, and the noninvasive neuroimaging techniques used in the study of cerebral circulation and autoregulation. An emphasis is placed on the description of autoregulation pathophysiology in orthostatic and neurally mediated syncope.

Effect of nephrectomy and captopril on autoregulation of cerebral blood flow in rats

The present study investigated the effect of circulating versus locally present renin on cerebral blood flow (CBF) and its autoregulation in rats. CBF was measured repetitively with the intracarotid 133Xe injection method, whereas blood pressure was lowered to determine the lower limit of autoregulation. To remove renin from the blood, rats were bilaterally nephrectomized and kept alive with peritoneal dialysis for 48 h. Five groups of animals were studied: 1) nephrectomized dialyzed rats, 2) nephrectomized dialyzed rats given a single intravenous dose of the angiotensin-converting enzyme inhibitor captopril (10 mg/kg), 3) sham nephrectomized and dialyzed rats, 4) rats receiving drugs as dialyzed rats but no surgery, and 5) rats given the same diet as the other groups but no drugs and no surgery. Baseline blood pressure was significantly lower in nephrectomized rats compared with controls. Nephrectomy, captopril, sham operation, or dialysis did not influence baseline CBF. The lower limit of CBF autoregulation was significantly lower in nephrectomized (53 +/- 4 mmHg) and sham-operated (58 +/- 4 mmHg) rats compared with diet control rats (78 +/- 3 mmHg). Captopril significantly decreased the lower limit in nephrectomized rats (35 +/- 2 mmHg). Thus removal of circulating renin caused no change in the lower limit of autoregulation. By contrast, captopril lowered the lower limit even in the absence of circulating renin and hence appeared to exert its effect on components of the renin-angiotensin system in the cerebral resistance vessel walls.

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.

Pre-treatment with candesartan protects from cerebral ischaemia

Angiotensin II (Ang II) regulates cerebral blood flow by stimulating cerebral vasoconstriction via AT1-receptors. In adult spontaneously hypertensive rats (SHR), the cerebrovascular autoregulatory curve is shifted to the right, in the direction of higher blood pressures, an indication of excessive cerebrovascular vasoconstriction. A restricted capacity to dilate cerebral blood vessels may be responsible for the enhanced vulnerability to cerebrovascular ischaemia during hypertension. We found that chronic treatment with the AT1-receptor antagonist, candesartan, (0.5 mg/kg/day for 14 days, via osmotic minipumps implanted in the subcutaneous tissue) blocked Ang II binding to AT1-receptors in cerebral blood vessels and in brain areas involved in the regulation of cerebrovascular flow, and increased the ratio of lumen-wall area in the middle cerebral artery. Candesartan treatment normalised the lower part of the autoregulatory curve in SHR, and markedly decreased cerebral ischaemia as a consequence of middle cerebral artery occlusion with reperfusion. Protection from ischaemia is related to arterial remodelling, enhanced compensatory vasodilatation in the peripheral area of ischaemia, decreased reduction in cerebral blood flow following the occlusion of a major cerebral blood vessel, and protection from injury in the periphery of the lesion. Our results indicate that pre-treatment with AT1-antagonists such as candesartan could be of benefit in the prevention and treatment of brain ischaemia.

Effects of an angiotensin-converting enzyme inhibitor and a beta-blocker on cerebral arteriolar dilatation in hypertensive rats

We examined the effects of the angiotensin-converting enzyme inhibitor perindopril and the beta-blocker propranolol on dilator responses of cerebral arterioles in chronic hypertension. Dilator responses to acute hypotension were examined in untreated Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) that were untreated or treated for 3 months with a low (0.3 mg. kg(-1). day(-1)) or a high (2 mg. kg(-1). day(-1)) dose of perindopril or a dose of propranolol (250 mg. kg(-1). day(-1)) alone or in combination with the low dose of perindopril. Pressure (servo-null) and diameter were measured in cerebral arterioles during acute reductions in arterial pressure both before and during maximal dilatation (EDTA). The high dose of perindopril or the combination of propranolol and perindopril normalized cerebral arteriolar pressure (52+/-2 [mean+/-SEM], 49+/-2 mm Hg versus 50+/-2 mm Hg in WKY and 96+/-3 mm Hg in untreated SHRSP; P<0.05). In contrast, the low dose of perindopril or propranolol alone did not normalize arteriolar pressure (74+/-2 mm Hg and 58+/-3 mm Hg). Both the low and high doses of perindopril improved autoregulatory dilatation, maximal dilatation, and dilator reserve of cerebral arterioles in SHRSP, with the low dose of perindopril being almost as effective as the high dose of perindopril. Propranolol alone did not significantly improve dilator function of cerebral arterioles. Furthermore, dilator function of cerebral arterioles was not further improved by the addition of propranolol to the low dose of perindopril. These findings suggest that angiotensin-converting enzyme inhibitors, such as perindopril, may be more effective than propranolol in attenuating the impairment of cerebral autoregulatory vasodilatation, maximal dilatation, and dilator reserve during treatment of chronic hypertension.

Haemodynamics of arteriovenous malformations of the brain and consequences of resection: a review

The physiological manifestations of arteriovenous fistulae in humans have been studied since the 18th century. However, confusion regarding concepts of cerebral 'steal', 'normal perfusion breakthrough', and 'congestive hyperaemia' continue. Although the advent of more accurate monitoring of pressures and flows within the brain has provided useful information to help understand some of these proposed pathological hypotheses, disagreement still exists. The purpose of this review is to examine the current physiological data in attempt to explain the clinicopathological manifestations of arteriovenous malformations of the brain and the consequences of their removal.

Angiotensin II AT(1) blockade normalizes cerebrovascular autoregulation and reduces cerebral ischemia in spontaneously hypertensive rats

BACKGROUND AND PURPOSE

Angiotensin II, through stimulation of AT(1) receptors, not only controls blood pressure but also modulates cerebrovascular flow. We sought to determine whether selective AT(1) antagonists could be therapeutically advantageous in brain ischemia during chronic hypertension.

METHODS

We pretreated spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto controls with the AT(1) antagonist candesartan (CV-11974), 0.5 mg/kg per day, for 3 to 14 days, via subcutaneously implanted osmotic minipumps. We analyzed cerebral blood flow by laser-Doppler flowmetry, cerebral stroke in SHR after occlusion of the middle cerebral artery with reperfusion, and brain AT(1) receptors by quantitative autoradiography.

RESULTS

Candesartan treatment normalized blood pressure and the shift toward higher blood pressures at both the upper and lower limits of cerebrovascular autoregulation in SHR. Candesartan pretreatment of SHR for 14 days partially prevented the decrease in blood flow in the marginal zone of ischemia and significantly reduced the volume of total and cortical infarcts after either 1 or 2 hours of middle cerebral artery occlusion with reperfusion, relative to untreated SHR, respectively. This treatment also significantly reduced brain edema after 2 hours of middle cerebral artery occlusion with reperfusion. In SHR, candesartan markedly decreased AT(1) binding in areas inside (nucleus of the solitary tract) and outside (area postrema) the blood-brain barrier and in the middle cerebral artery.

CONCLUSIONS

Pretreatment with an AT(1) antagonist protected hypertensive rats from brain ischemia by normalizing the cerebral blood flow response, probably through AT(1) receptor blockade in cerebral vessels and in brain areas controlling cerebrovascular flow during stroke.

Topical carbonic anhydrase inhibition increases ocular pulse amplitude in high tension primary open angle glaucoma

BACKGROUND

Ocular pulse amplitude (OPA) is reduced in normal tension primary open angle glaucoma (NTP) patients when compared with healthy age matched controls (CTL) while increased OPA appears to protect ocular hypertensive patients from visual field loss. If NTP is accompanied by vasospasm, as in roughly half of the primary open angle glaucoma (POAG) population (independent of intraocular pressure, IOP), calcium channel blockers increase OPA and thus stabilise visual fields in these patients. Current glaucoma drugs reduce IOP but do not activate (compromised) ocular perfusion.

METHODS

The influence of dorzolamide, a topical carbonic anhydrase inhibitor in standard dosage (three times daily, one eye) on OPA, IOP, blood pressure, and heart rate was investigated in a randomised, prospective, masked clinical trial assessing the acute effects of dorzolamide v placebo before and 2 days after application in 33 cataract patients with (n = 14) and without (n = 19) high tension POAG (HTP) who provided informed consent.

RESULTS

Following application of dorzolamide (D) IOP (mm Hg, mean (SEM)) in HTPD (20.2 (0.5)/16.3 (0.5) and in CTLD (16.0 (0.5)/12.3 (0.5)) was highly significantly (p < 0.001) reduced and was significantly (p < 0.03) reduced in vehicle (V) treated eyes (HTPv: 20.3 (0.4)/19.0 (0.4)) and CTLv: 15.8 (0.4)/14.9 (0.3)) when compared with respective baseline measurements. OPA (mm Hg) in HTPD (2.1 (0.1)/2.5 (0.1)) and CTLD (2.2 (0.1)/2.6 (0.2)) eyes was significantly (p < 0.05) increased and unaffected in vehicle treated eyes when compared with respective baseline measurements. Systemic perfusion variables were also unchanged.

CONCLUSION

Dorzolamide increased OPA in HTP and CTL. Drugs stimulating OPA may improve prognosis of POAGs.

Effects of long-acting angiotensin-converting enzyme inhibitor, imidapril, on the lower limit of cerebral blood flow autoregulation in hypertensive rats

The objective of the present study was to examine the effects of a long-acting angiotensin converting enzyme inhibitor, imidapril ((4S)-1-methyl-3-¿(2S)-2-[N-(1S)-1-ethoxycarbonyl-3-phenylpropyl) amino] propionyl¿-2-oxoimidazolidine-4-carboxylic acid hydrochloride), for 7 days on the cerebral blood flow autoregulatory response to hypotension in hypertensive rats. We measured the cerebral blood flow at rest and during hemorrhagic hypotension, using laser-Doppler flowmetry. At the same time, the absolute baseline cerebral blood flow values in the parietal cortex were quantified with the hydrogen clearance method. After administration of imidapril at a dose of 5 mg/kg/day for 7 days, the resting value of mean arterial blood pressure was significantly decreased by 25 mm Hg (P < 0.001), cerebral vascular resistance was lowered by 14.4% (P < 0.05) and the lower limit of cerebral blood flow autoregulation was shifted to a lower level, 106+/-11 mm Hg (mean +/- S.D.), from 137+/-8 mm Hg in the control group (P < 0.001), while resting cerebral blood flow remained unchanged. The present results demonstrated that imidapril preserves cerebral blood flow and significantly shifts the lower limit of cerebral autoregulation towards lower blood pressure levels.

Enalaprilat controls postoperative hypertension while maintaining cardiac function and systemic oxygenation after neurosurgery

OBJECTIVE

The efficacy of intravenous enalaprilat in lowering postoperative hypertension.

DESIGN

Prospective, randomized, controlled, single blind trial.

SETTING

Surgical ICT in a university hospital (tertiary care center).

PATIENTS

18 neurosurgical patients subjected to the extirpation of a supratentorial intracerebral tumour were studied after detection of postoperative hypertension. This was defined as a constant elevation of systolic arterial pressure over 160 mmHg or diastolic arterial pressure over 95 mmHg.

INTERVENTIONS

Enalaprilat 0.015 mg kg-1 was injected within 5 min to 9 patients.

MEASUREMENTS AND RESULTS

Central haemodynamics and systemic oxygenation were assessed at baseline before enalaprilat injection, and repeatedly during four hours after the injection. The statistical analysis was performed with analysis of variance for repeated measurements. As compared to control patients, the blood pressure lowering effect of enalaprilat became evident within 15 min and lasted for over four hours (p = 0.008). It was mainly due to the reduced systemic vascular resistance. Enalaprilat also induced a small decline in myocardial perfusion pressure. Cardiac performance, preload, heart rate and systemic oxygenation were not affected by enalaprilat.

CONCLUSIONS

We found intravenous enalaprilat effective and safe in lowering postoperative hypertension following neurosurgery as assessed by it's effects on central haemodynamics and systemic oxygenation.

Role of angiotensin receptor subtypes in the response of rabbit brain arterioles to angiotensin

BACKGROUND AND PURPOSE

Angiotensin II has been reported to induce either constriction or dilation in the cerebral microcirculation. The goal of this study was to determine whether binding to different angiotensin II receptor subtypes may account for the divergent responses.

METHODS

Pial arterioles ranging a diameter from 28 to 136 microns were observed through a microscope in a closed cranial window preparation in anesthetized rabbits. Arteriolar responses to topical application of 10(-5) mol/L angiotensin II or the vasoactive angiotensin II degradation products L-arginine/angiotensin-(3-8) were measured by videometry. The effect of the subtype 1 receptor antagonist losartan and the subtype 2 antagonist PD 123319 on these responses was examined in separate groups of animals.

RESULTS

Topical coapplication of 10(-5) mol/L losartan or 10(-5) mol/L PD 123319 produced 55% and 62% inhibition of the dilator response to 10(-5) mol/L angiotensin II, respectively. Combined application of the antagonists caused 79% inhibition. Each of the antagonists almost completely blocked the response to L-arginine/angiotensin-(3-8). Acetylcholine-induced dilation of rabbit brain arterioles was unaffected by the antagonists.

CONCLUSIONS

Both of the known angiotensin II receptor subtypes appear to be involved in angiotensin II-induced dilation of rabbit cerebral arterioles. These results argue against the assumption that vasodilation is a specific function of one of these receptor subtypes, which might have explained the equivocal effects of angiotensin II by predominance of a certain receptor subtype in a given vascular bed.

Angiotensin II AT2 receptor stimulation extends the upper limit of cerebral blood flow autoregulation: agonist effects of CGP 42112 and PD 123319

The effects of the angiotensin II AT2 receptor ligands CGP 42112 and PD 123319, the AT1 antagonist losartan, and the nonselective angiotensin II antagonist Sar1,Ile8-angiotensin II on the upper limit of CBF autoregulation were studied in pentobarbital-anesthetized rats. Blood pressure was increased by intravenous phenylephrine infusion, while CBF was measured continuously from the parietal cortex by laser-Doppler flowmetry. Intravenous infusions of CGP 42112 (0.1 and 1 mg kg-1 min-1) and PD 123319 (0.36 and 1 mg kg-1 min-1) shifted the upper limit of CBF autoregulation toward higher blood pressures without affecting baseline CBF. Sar1,Ile8-angiotensin II (4 micrograms kg-1 min-1) had no effect on baseline CBF or CBF autoregulation but antagonized the effect of CGP 42112 and PD 123319. Losartan (10 mg/kg i.v. bolus) reduced baseline blood pressure and CBF and shifted the autoregulation curve toward higher blood pressures. Sar1,Ile8-angiotensin II blocked the effect of losartan on baseline CBF but not on CBF autoregulation. These results suggest that both CGP 42112 and PD 123319 exert their effects on CBF autoregulation through stimulation of angiotensin II AT2 receptors. The mechanism by which losartan affects CBF remains unclear.

Angiotensin converting enzyme inhibitors attenuate ischemic brain metabolism in hypertensive rats

BACKGROUND AND PURPOSE

Angiotensin converting enzyme (ACE) inhibitors are expected to modulate neuronal activities. The present study was designed to examine the beneficial effects of ACE inhibitors on microcirculation and metabolism in the ischemic brain.

METHODS

Cerebral ischemia was developed for 60 minutes in spontaneously hypertensive rats (SHR, n = 35) by bilateral carotid artery occlusion. ACE inhibitor (0.1 or 10 mg/kg SQ 29,852 or captopril) were intravenously injected 15 minutes before cerebral ischemia. Cerebral blood flow to the parietal cortex was measured with the H2 clearance technique. Lactate, pyruvate, and ATP in the brain were estimated by the enzymatic method.

RESULTS

Before cerebral ischemia, high doses of both SQ 29,852 and captopril significantly decreased mean arterial pressure by 15 to 25 mm Hg and reduced cerebral vascular resistance by 13% to 17% of the resting values. Cerebral blood flow and arterial pressure during ischemia were not altered by these ACE inhibitors. After 60 minutes of cerebral ischemia, tissue lactate in vehicle-treated SHR increased 6.6-fold and ATP decreased to 65% of the control values. Administration of SQ 29,852 or captopril significantly reduced the lactate levels to 1.6- to 3.1-fold and well preserved the ATP levels to 82% to 93% of the control.

CONCLUSIONS

These results suggest that inhibition of ACE activities may be protective for cerebral metabolism against ischemic insult.

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.