Effects of cilazapril on cerebral vasodilatation in hypertensive rats

Methyl Palmitate Modulated NMDA-Induced Cerebral Hyperemia in Hypertensive Rats

N-methyl-D-aspartate (NMDA) receptors were found to be dysfunctional in hypertensive rats. Methyl palmitate (MP) has been shown to diminish the nicotine-induced increase in blood flow in the brainstem. The aim of this study was to determine how MP modulated NMDA-induced increased regional cerebral blood flow (rCBF) in normotensive (WKY), spontaneously hypertensive (SHR), and renovascular hypertensive (RHR) rats. The increase in rCBF after the topical application of experimental drugs was measured using laser Doppler flowmetry. Topical NMDA application induced an MK-801-sensitive increase in rCBF in anesthetized WKY rats, which was inhibited by MP pretreatments. This inhibition was prevented by pretreatment with chelerythrine (a PKC inhibitor). The NMDA-induced increase in rCBF was also inhibited by the PKC activator in a concentration-dependent manner. Neither MP nor MK-801 affected the increase in rCBF induced by the topical application of acetylcholine or sodium nitroprusside. Topical application of MP to the parietal cortex of SHRs, on the other hand, increased basal rCBF slightly but significantly. MP enhanced the NMDA-induced increase in rCBF in SHRs and RHRs. These results suggested that MP had a dual effect on the modulation of rCBF. MP appears to play a significant physiological role in CBF regulation.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Hypertension, dietary salt and cognitive impairment

Dementia is growing at an alarming rate worldwide. Although Alzheimer disease is the leading cause, over 50% of individuals diagnosed with Alzheimer disease have vascular lesions at autopsy. There has been an increasing appreciation of the pathogenic role of vascular risk factors in cognitive impairment caused by neurodegeneration. Midlife hypertension is a leading risk factor for late-life dementia. Hypertension alters cerebrovascular structure, impairs the major factors regulating the cerebral microcirculation, and promotes Alzheimer pathology. Experimental studies have identified brain perivascular macrophages as the major free radical source mediating neurovascular dysfunction of hypertension. Recent evidence indicates that high dietary salt may also induce cognitive impairment. Contrary to previous belief, the effect is not necessarily associated with hypertension and is mediated by a deficit in endothelial nitric oxide. Collectively, the evidence suggests a remarkable cellular diversity of the impact of vascular risk factors on the cerebral vasculature and cognition. Whereas long-term longitudinal epidemiological studies are needed to resolve the temporal relationships between vascular risk factors and cognitive dysfunction, single-cell molecular studies of the vasculature in animal models will provide a fuller mechanistic understanding. This knowledge is critical for developing new preventive, diagnostic, and therapeutic approaches for these devastating diseases of the mind.

The Influence of Stroke Risk Factors and Comorbidities on Assessment of Stroke Therapies in Humans and Animals

The main driving force behind the assessment of novel pharmacological agents in animal models of stroke is to deliver new drugs to treat the human disease rather than to increase knowledge of stroke pathophysiology. There are numerous animal models of the ischaemic process and it appears that the same processes operate in humans. Yet, despite these similarities, the drugs that appear effective in animal models have not worked in clinical trials. To date, tissue plasminogen activator is the only drug that has been successfully used at the bedside in hyperacute stroke management. Several reasons have been put forth to explain this, but the failure to consider comorbidities and risk factors common in older people is an important one. In this article, we review the impact of the risk factors most studied in animal models of acute stroke and highlight the parallels with human stroke, and, where possible, their influence on evaluation of therapeutic strategies.

Protecting against vascular disease in brain

Endothelial cells exert an enormous influence on blood vessels throughout the circulation, but their impact is particularly pronounced in the brain. New concepts have emerged recently regarding the role of this cell type and mechanisms that contribute to endothelial dysfunction and vascular disease. Activation of the renin-angiotensin system plays a prominent role in producing these abnormalities. Both oxidative stress and local inflammation are key mechanisms that underlie vascular disease of diverse etiology. Endogenous mechanisms of vascular protection are also present, including antioxidants, anti-inflammatory molecules, and peroxisome proliferator-activated receptor-γ. Despite their clear importance, studies of mechanisms that underlie cerebrovascular disease continue to lag behind studies of vascular biology in general. Identification of endogenous molecules and pathways that protect the vasculature may result in targeted approaches to prevent or slow the progression of vascular disease that causes stroke and contributes to the vascular component of dementia and Alzheimer's disease.

Do in vivo experimental models reflect human cerebral small vessel disease? A systematic review

Cerebral small vessel disease (SVD) is a major cause of stroke and dementia. Pathologically, three lesions are seen: small vessel arteriopathy, lacunar infarction, and diffuse white matter injury (leukoaraiosis). Appropriate experimental models would aid in understanding these pathologic states and also in preclinical testing of therapies. The objective was to perform a systematic review of animal models of SVD and determine whether these resemble four key clinicopathologic features: (1) small, discrete infarcts; (2) small vessel arteriopathy; (3) diffuse white matter damage; (4) cognitive impairment. Fifteen different models were included, under four categories: (1) embolic injuries (injected blood clot, photochemical, detergent-evoked); (2) hypoperfusion/ischaemic injury (bilateral common carotid occlusion/stenosis, striatal endothelin-1 injection, striatal mitotoxin 3-NPA); (3) hypertension-based injuries (surgical narrowing of the aorta, or genetic mutations, usually in the renin-angiotensin system); (4) blood vessel damage (injected proteases, endothelium-targeting viral infection, or genetic mutations affecting vessel walls). Chronic hypertensive models resembled most key features of SVD, and shared the major risk factors of hypertension and age with human SVD. The most-used model was the stroke-prone spontaneously hypertensive rat (SHR-SP). No model described all features of the human disease. The optimal choice of model depends on the aspect of pathophysiology being studied.

Cerebrovascular support for cognitive processing in hypertensive patients is altered by blood pressure treatment

Hypertension is associated with mild decrements in cognition. In addition, regional cerebral blood flow responses during memory processing are blunted in parietal and thalamic areas among untreated hypertensive adults, who, compared with normotensive subjects, manifest greater correlation in blood flow response across task-related brain regions. Here, we test whether pharmacological treatment of hypertension normalizes regional cerebral blood flow responses and whether it does so differentially according to drug class. Treatment with lisinopril, an angiotensin-converting enzyme blocker, known to enhance vasodilative responsivity, was compared with treatment with atenolol, a beta-blocker. Untreated hypertensive volunteers (n=28) were randomly assigned and treated for 1 year. Whole brain and regional cerebral flow responses to memory processing and acutely administered acetazolamide, a vasodilator, were assessed pretreatment and posttreatment. Peripheral brachial artery dilation during reactive hyperemia was also measured. Quantitative blood flow measures showed no difference in the magnitude of regional cerebral blood flow responses pretreatment and posttreatment to either memory tasks or acetazolamide injection. Brachial artery flow-mediated dilation increased with treatment. No differences between medications were observed. In brain regions active in memory processing, however, regional cerebral blood flow responses were more highly correlated after treatment. Specificity of cerebral blood flow to different regions appears to decline with treatment of hypertension. This greater correlation among active brain regions, which is present as well in untreated hypertensive relative to normotensive volunteers, may represent compensation in the face of less region-specific responsivity in individuals with hypertension.

Hemodynamic Influences of Losartan on the Brain in Hypertensive Patients

The effects of angiotensin II receptor blockers on cerebral hemodynamics in humans have not been well elucidated. The present study evaluated the effects of losartan on cerebral hemodynamics in hypertensive patients using positron emission tomography. Ten patients with essential hypertension (mean age, 60.8 years) were examined. In each patient, regional cerebral blood flow was measured by [O-15] labeled water positron emission tomography before and after the oral administration of losartan for 8 to 23 weeks. In 8 patients, the baseline regional cerebral blood flow measurement was followed by 1,000 mg of acetazolamide challenge to measure the cerebral perfusion reserve. Systemic blood pressures before and after treatment were 153.8 +/- 10.8/96.0 +/- 6.5 mmHg (systolic mean +/- SD/diastolic mean +/- SD) and 133.4 +/- 11.2/83.6 +/- 6.5 mmHg, respectively; this difference was significant. The baseline global cerebral blood flow values before and after treatment were 38.4 +/- 6.9 ml/min/100 g and 38.2 +/- 8.2 ml/min/100 g, respectively; this difference was not significant. The results of the global cerebral blood flow response to the acetazolamide challenges were not statistically different before and after treatment. A regional analysis showed no statistical difference in regional cerebral blood flow or cerebral perfusion reserve throughout the brain before and after treatment. Losartan's effect on reducing the blood pressure did not affect either the baseline regional cerebral blood flow or the cerebral perfusion reserve in patients with mild to moderate hypertension. The inclusion of losartan in anti-hypertensive regimens could be advantageous for cerebral circulation in patients with essential hypertension.

The stroke-prone spontaneously hypertensive rat: how good is it as a model for cerebrovascular diseases?

1. Cerebrovascular diseases (CVDs) in humans are a mixture of diseases with different etiologies. 2. Although the stroke-prone spontaneously hypertensive rat (SHRSP) cannot represent all types of CVDs, it is probably a good genetic model for particular types such as lacunar infarction and intracerebral hemorrhage. 3. Genetic studies suggested that SHRSP has genetic susceptibility to stroke independent of its severe hypertension. Studies on SHRSP may provide useful information with which to dissect genetic susceptibility to particular types of CVDs.

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.

Autoregulation of blood pressure and thought: preliminary results of an application of brain imaging to psychosomatic medicine

OBJECTIVE

This presentation seeks to demonstrate the use of brain imaging techniques for understanding the interaction between hypertension and psychosocial function.

METHODS

The historical background for the study of brain function among hypertensive patients is reviewed. An initial and a current project examining rCBF with 15O water radiotracer and PET in unmedicated hypertensives and normotensives are described. The rCBF response is assessed during the performance of spatial and verbal working memory tasks of increasing memory load. The assessment also addresses the influence on rCBF and performance of white matter hyperintensities and the presence of carotid artery thickening.

RESULTS

Initial results suggest that hypertensives relative to normotensives show less CBF and less posterior parietal rCBF in response to increases in memory load. Hypertensives, however, increase lateral prefrontal (Broca's area)/insula and amygdala/hippocampal rCBF more than normotensives.

CONCLUSION

Initial results are sufficient to show that hypertension induces changes in rCBF. A tentative hypothesis is that a relatively general decrease in rCBF responsivity induces specific compensatory cognitive strategies as well as subcortical activation. The rCBF changes appear to have implications for information processing and, as such, hold promise for understanding prior reports relating hypertension to affective regulation and cardiovascular reactivity. Imaging techniques provide a powerful tool for psychosomatic research.

Long-term effects of benidipine on cerebral vasoreactivity in hypertensive rats

We tested the hypothesis that long-term application of a Ca2+ channel blocker would ameliorate the functional and morphological deterioration of the cerebral arteries during hypertension. Male spontaneously hypertensive rats (SHR) were fed a standard rat chow, containing a low (3 mg/kg/day) or high dose (6 mg/kg/day) of benidipine, a Ca2+ channel blocker, for 2 months. Using a cranial window, we examined responses of the basilar artery to acetylcholine, sodium nitroprusside, (-)-(3S,4R)-4-(N-acetyl-N-hydroxyamino)-6-cyano-3,4-dihydro-2,2-dimethyl-2H-1-benzopyran-3-ol (Y-26763; an opener of ATP-sensitive K+ channels), and (R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632; an inhibitor of Rho-associated kinase). Mean arterial pressure of the control group was 193+/-5 mm Hg (mean+/-S.E.M.), while that of the low-dose benidipine group was 183+/-5 mm Hg and that of the high-dose group was 159+/-4 mm Hg. Dilator responses of the basilar artery to acetylcholine and Y-26763 were impaired in SHR compared with those of normotensive Wistar-Kyoto (WKY) rats and treatment with benidipine enhanced the vasodilator responses to acetylcholine and Y-26763 in SHR. Y-27632-induced dilatation of the basilar artery was enhanced in SHR compared to that in WKY rats and the vasodilatation was reduced by benidipine in SHR. Sodium nitroprusside caused similar dilatation of the basilar artery, in both WKY rats and the SHR control group, and benidipine did not affect nitroprusside-induced dilatation of the artery in SHR. The wall of the basilar artery was significantly thicker in SHR than in WKY rats and benidipine treatment reduced the wall thickness of the artery in SHR. These findings suggest that chronic treatment with a Ca2+ channel blocker may enhance the dilator capacity and reduce contractility of the basilar artery during hypertension. Benidipine may also ameliorate the morphological changes of the basilar artery in hypertension.

Why are strokes related to hypertension? Classic studies and hypotheses revisited

Although it seems obvious that excessive intravascular pressure is the cause of spontaneous intracerebral haemorrhage, the available evidence instead suggests that haemorrhage arises from previous ischaemic damage to the walls of small blood vessels. This interpretation unifies the aetiology of cerebral infarction and intracerebral haemorrhage. It is supported by much pathological evidence and also fits with observations on spontaneous stroke-prone hypertensive rats, which have smaller cerebral arteries than Wistar-Kyoto rats. Ischaemic damage to the brain probably occurs during spontaneous dips in aortic pressure in the presence of atheromatous arterial lesions and arteriolar narrowing by lipohyaline deposits. It may also follow long-lasting arterial spasm provoked by sudden pressure elevations. Local factors, especially unevenness of cerebral perfusion, probably determine the site of an infarct and whether it becomes haemorrhagic or not. In the long term, hypotensive drugs will lessen atheroma deposition. In the short term, they may act by reducing or preventing damaging arteriolar spasm.

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.

Bradykinin mediates the acute effect of an angiotensin-converting enzyme inhibitor on cerebral autoregulation in rats

BACKGROUND AND PURPOSE

In patients with stroke and long-standing hypertension, the autoregulation curve of cerebral blood flow (CBF) shifts toward higher blood pressure levels. Angiotensin-converting enzyme (ACE) inhibitors reduce blood pressure and shift the autoregulation curve back to normal in hypertensive patients. ACE inhibitors have 2 major pharmacological properties: they inhibit both the production of angiotensin II and the breakdown of kinins. Hence, we investigated whether the effect of an ACE inhibitor on the lower limit of CBF autoregulation is mediated by the potentiation of bradykinin-mediated vasodilatation.

METHODS

In 28 male Sprague-Dawley rats, CBF was measured by laser-Doppler flowmetry during stepwise controlled hypotension. The lower limit of CBF autoregulation was defined as the mean arterial pressure at which CBF decreased by 20% of the baseline value. The rats were treated with an ACE inhibitor, captopril, in the captopril group; a bradykinin BK2-receptor antagonist, Hoe140, in the Hoe140 group; and both agents in the captopril+Hoe140 group. Other rats served as a control group. The lower limits of CBF autoregulation were compared among the 4 groups.

RESULTS

In the captopril group, the lower limit of CBF autoregulation was 43+/-8 mm Hg (mean+/-SD), which was significantly lower than that in the control group (57+/-14 mm Hg). Inhibition of bradykinin abolished the effect of captopril on the lower limit of CBF autoregulation. Hoe140 alone had no significant effect on the lower limit of CBF autoregulation.

CONCLUSIONS

These results suggest that the shift of the lower limit of CBF autoregulation by captopril is mediated, at least in part, by bradykinin.

Vasopeptidase inhibition has potent effects on blood pressure and resistance arteries in stroke-prone spontaneously hypertensive rats

The antihypertensive agent omapatrilat represents a novel approach to antihypertensive therapy, namely vasopeptidase inhibition. Omapatrilat (BMS-186716) concomitantly inhibits neutral endopeptidase and angiotensin-converting enzyme, leading to protection from degradation of natriuretic and other hypotensive peptides in addition to interruption of the renin-angiotensin system. Although the potency of omapatrilat on reduction of blood pressure has been reported, its effects on resistance artery structure and function were unknown. We tested omapatrilat in stroke-prone spontaneously hypertensive rats (SHRSP), a malignant model of hypertension, with the hypothesis that it would improve the structure and endothelial function of mesenteric resistance arteries. Ten-week-old SHRSP were treated orally for 10 weeks with omapatrilat (40 mg/kg per day). Mesenteric arteries (lumen <300 microm) were studied on a pressurized myograph. After 10 weeks, untreated SHRSP had a systolic blood pressure of 230+/-2 mm Hg that was significantly reduced (P<0.05) by omapatrilat (145+/-3 mm Hg). Omapatrilat treatment improved endothelium-dependent relaxation of resistance arteries as elicited by acetylcholine (10(-5) mol/L) but had no significant effect on endothelium-independent relaxation produced by a nitric oxide donor (sodium nitroprusside). This suggested that there existed endothelial dysfunction in SHRSP that was corrected by vasopeptidase inhibition, probably in part caused by the potent blood pressure-lowering effect of omapatrilat. Media width and media/lumen ratio were significantly decreased (P<0.05) by omapatrilat, and a trend (P=0.07) to increase lumen diameter was observed. Vascular stiffness (slope of the elastic modulus versus stress curve) was unaltered by omapatrilat. In conclusion, omapatrilat, acting as a potent antihypertensive agent, may improve structure and endothelial function of resistance arteries in SHRSP, a severe form of genetic hypertension.

Effect of chronic treatment with perindopril on endothelium-dependent relaxation of aorta and carotid artery in SHRSP

Endothelium-dependent relaxation of aorta and carotid artery from stroke-prone spontaneously hypertensive rats (SHRSP) and the effect of chronic treatment of SHRSP with perindopril, an angiotensin converting enzyme inhibitor, on endothelium-dependent relaxation were studied. Endothelium-dependent relaxation was induced by acetylcholine (ACh) in preparations of SHRSP and normotensive Wistar Kyoto rats (WKY) precontracted with noradrenaline. The ACh-induced relaxation in both preparations was abolished by L-nitroarginine. The ACh-induced relaxation was impaired in preparations from SHRSP and contraction was observed at high concentrations of ACh. In the presence of indomethacin, impairment of endothelium-dependent relaxation in SHRSP was minimized and the contraction was inhibited. The relaxation with sodium nitroprusside did not differ between the preparations from WKY and SHRSP. Treatment of SHRSP with perindopril (2 mg/kg/day) for 6 weeks decreased systolic blood pressure and improved the ACh-induced relaxation of aorta and carotid artery. The treatment inhibited the contraction by higher concentrations of ACh in the presence of L-nitroarginine. These results indicate that the impairment of endothelium-dependent relaxation in aorta and carotid artery of SHRSP may be caused by the reduced availability of nitric oxide. The perindopril-treatment may prevent these changes in SHRSP.

Larger anastomoses in angiotensinogen-knockout mice attenuate early metabolic disturbances after middle cerebral artery occlusion

Abnormalities in the homeostasis of the renin-angiotensin system have been implicated in the pathogenesis of vascular disorders, including stroke. The authors investigated whether angiotensinogen (AGN) knockout mice exhibit differences in brain susceptibility to focal ischemia, and whether such differences can be related to special features of the collateral circulation. Wild-type and AGN-knockout mice were submitted to permanent suture occlusion of the middle cerebral artery (MCA). The collateral vascular system was visualized by systemic latex infusion, and the ischemic lesions were identified by cresyl-violet staining. The core and penumbra of the evolving infarct were differentiated by bioluminescence and autoradiographic imaging of ATP and protein biosynthesis, respectively. In wild-type mice, mean arterial blood pressure was 95.0 +/- 8.6 mm Hg, and the diameter of fully relaxed anastomotic vessels between the peripheral branches of the anterior and middle cerebral arteries 26.6 +/- 4.0 microm. In AGN knockouts, mean arterial blood pressure was significantly lower, 71.5 +/- 8.5 mm Hg (P < .01), and the anastomotic vessels were significantly larger, 29.4 +/- 4.6 microm (P < .01). One hour after MCA occlusion, AGN-knockout mice exhibited a smaller ischemic core (defined as the region of ATP depletion) but a larger penumbra (the area of disturbed protein synthesis with preserved ATP). At 24 hours after MCA occlusion, this difference disappeared, and histologically visible lesions were of similar size in both strains. The observations show that in AGN-knockout mice the more efficient collateral blood supply delays ischemic injury despite the lower blood pressure. Pharmacologic suppression of angiotensin formation may prolong the therapeutic window for treatment of infarcts.

Attenuation and recovery of brain stem autoregulation in spontaneously hypertensive rats

Cerebral large arteries dilate actively around the lower limits of CBF autoregulation, mediated at least partly by nitric oxide, and maintain CBF during severe hypotension. We tested the hypothesis that this autoregulatory response of large arteries, as well as the response of arterioles, is altered in spontaneously hypertensive rats (SHR) and that the altered response reverts to normal during long-term antihypertensive treatment with cilazapril, an angiotensin-converting enzyme inhibitor. In anesthetized 6- to 7-month-old normotensive Wistar-Kyoto rats (WKY), 4- and 6- to 7-month-old SHR without antihypertensive treatment, and 6- to 7-month-old SHR treated with cilazapril for 10 weeks, local CBF to the brain stem was determined with laser-Doppler flowmetry and diameters of the basilar artery and its branches were measured through a cranial window during stepwise hemorrhagic hypotension. The lower limit of CBF autoregulation shifted upward in untreated SHR to 90 to 105 mm Hg from 30 to 45 mm Hg in WKY, and it reverted to 30 to 45 mm Hg in treated SHR. In response to severe hypotension, the basilar artery dilated by 21 +/- 6% (mean +/- SD) of the baseline internal diameter in WKY. The vasodilation was impaired in untreated SHR (10 +/- 8% in 4-mo-old SHR and 4 +/- 5% in 6- to 7-month-old SHR), and was restored to 22 +/- 10% by treatment with cilazapril (P < 0.005). Dilator responses of branch arterioles to hypotension showed similar attenuation and recovery as that of the basilar artery. The data indicate that chronic hypertension impairs the autoregulatory dilation of the basilar artery as well as branch arterioles and that antihypertensive treatment with cilazapril restores the diminished dilation toward normal.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Arterial function in mineralocorticoid-NaCl hypertension: influence of angiotensin-converting enzyme inhibition

Angiotensin-converting enzyme inhibitors have been suggested to improve the function of arterial endothelium and smooth muscle not only through inhibition of angiotensin II formation and reduction of blood pressure, but also via additional pathways, e.g. potentiation of endogenous kinins and enhancement of endothelial autacoid formation. Therefore, we investigated whether 10-week-long quinapril therapy (10 mg kg-1 day-1) could beneficially influence the function of mesenteric arterial rings in vitro in deoxycorticosterone-NaCl-treated Wistar-Kyoto rats, a model of hypertension which is known to be resistant to angiotensin-converting enzyme inhibition. The quinapril treatment had no long-term blood pressure-lowering effect nor did it reduce the associated cardiac hypertrophy in deoxycorticosterone-NaCl hypertension. In noradrenaline-precontracted arterial rings the endothelium-dependent relaxations to acetylcholine and adenosine 5'-diphosphate as well as the endothelium-independent relaxations to nitroprusside and isoprenaline were clearly attenuated in the deoxycorticosterone-NaCl-treated rats. However, the quinapril therapy was without significant effect on any of these dilatory responses. In the presence of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester, the relaxations to acetylcholine in untreated and quinapril-treated hypertensive animals were practically absent, whereas in normotensive rats distinct relaxations to higher concentrations of acetylcholine were still present. Interestingly, when endothelium-dependent hyperpolarization was prevented by precontracting the preparations with potassium chloride, no differences were found in relaxations to acetylcholine and adenosine 5'-diphosphate between the study groups. Exogenous bradykinin induced small comparable contractions in endothelium-intact mesenteric arterial rings from all study groups. In conclusion, the 10-week-long quinapril therapy did not have any significant effects on arterial function in deoxycorticosterone-NaCl hypertensive rats. Therefore, the present results stress the roles of reduced blood pressure and diminished angiotensin II formation in the beneficial vascular effects of long-term angiotensin-converting enzyme inhibition in the present model of hypertension. Furthermore, since the relaxations to acetylcholine and adenosine 5'-diphosphate in the deoxycorticosterone-NaCl-treated rats were attenuated in the absence and presence of nitric oxide synthase inhibition but not under conditions which prevented hyperpolarization, impaired endothelium-dependent relaxation to agonists can be attributed to diminished endothelium-dependent hyperpolarization in this model of hypertension.

Cerebrovascular effects of nitric oxide manipulation in spontaneously hypertensive rats

1. Evidence that nitric oxide (NO) bioactivity is altered in chronic hypertension is conflicting, possibly as a result of heterogeneity in both the nature of the dysfunction and in the disease process itself. The brain is particularly vulnerable to the vascular complications of chronic hypertension, and the aim of this study was to assess whether differences in the cerebrovascular responsiveness to the NO synthase (NOS) inhibitors, NG-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI), and to the NO donor 3-morpholinosydnonimine (SIN-1) might indicate one possible source of these complications. 2. Conscious spontaneously hypertensive (SHR) and WKY rats, were treated with L-NAME (30 mg kg-1, i.v.), 7-NI (25 mg kg-1, i.p.), (0.54 or 1.8 mg kg-1 h-1, continuous i.v. infusion) or saline (i.v.), 20 min before the measurement of local cerebral blood flow (LCBF) by the fully quantitative [14C]-iodoantipyrine autoradiographic technique. 3. With the exception of mean arterial blood pressure (MABP), there were no significant differences in physiological parameters between SHR and WKY rats within any of the treatment groups, or between treatment groups. L-NAME treatment increased MABP by 27% in WKY and 18% in SHR groups, whilst 7-NI had no significant effect in either group. Following the lower dose of SIN-1 infusion, MABP was decreased to a similar extent in both groups (around -20%). There was no significant difference in MABP between groups following the higher dose of SIN-1, but this represented a decrease of -41% in SHR and -21% in WKY rats. 4. With the exception of one brain region (nucleus accumbens), there were no significant differences in basal LCBF between WKY and SHR. L-NAME produced similar decreases in LCBF in both groups, ranging between -10 and -40%. The effect of 7-NI upon LCBF was more pronounced in the SHR (ranging from -34 to -57%) compared with the WKY (ranging from -14 to -43%), and in seven out of the thirteen brain areas examined there were significant differences in LCBF. 5. Following the lower dose of SIN-1, in the WKY 8 out of the 13 brain areas examined showed significant increases in blood flow compared to the saline treated animals. In contrast, only 2 brain areas showed significant increases in flow in the SHR. In the rest of the brain areas examined the effects of SIN-1 upon LCBF were less marked than in the WKY. 6. Infusion of the higher dose of SIN-1 resulted in further significant increases in LCBF in the WKY group (ranging between +30% and +74% compared to saline-treated animals), but no significant effects upon LCBF were found in the SHR. As a result, there were significant differences in LCBF between SIN-1-treated WKY and SHR in six brain areas. In most brain areas examined, cerebral blood flow in SHR following the higher dose of SIN-1 was less than that measured with the lower dose of SIN-1. 7. Despite comparable reductions in MABP (approximately 20%) in both groups, calculated cerebrovascular resistance (CVR) confirmed that the vasodilator effects of the lower dose of SIN-1 were significantly more pronounced throughout the brain in the WKY (ranging between -3% and -50%; median = -38%) when compared to the SHR (ranging between -10% and -36%; median = -26%). In the animals treated with the higher dose of SIN-1, CVR changes were broadly similar in both groups (median = -45% in WKY and -42% in SHR), but with the reduction in MABP in SHR being twice that found in WKY, this is in keeping with an attenuated blood flow response to SIN-1 in the SHR. 8. The results of this study indicate that NO-dependent vasodilator capacity is reduced in the cerebrovasculature of SHR. In addition, the equal responsiveness to a non-specific NOS inhibitor but an enhanced effectiveness of a specific neuronal NO inhibitor upon LCBF in the SHR could be consistent with an upregulation of the neuronal NO system.

Altered cerebrovascular response to a potassium channel opener in hypertensive rats

We examined whether the effect of Y-26763, an ATP-sensitive potassium channel opener, on cerebral blood flow is altered in stroke-prone spontaneously hypertensive rats (SHRSP) and, if altered, whether long-term antihypertensive treatment with cilazapril, an angiotensin-converting enzyme inhibitor, is capable of preventing the change. Cerebral blood flow during intracarotid infusion of Y-26763 was measured in anesthetized SHRSP and normotensive Wistar-Kyoto rats (WKY) as control. Y-26763 increased cerebral blood flow in a dose-dependent manner in WKY, and glibenclamide, a selective inhibitor of ATP-sensitive potassium channels, inhibited the Y-26763-induced increase in cerebral blood flow. In contrast, the response to Y-26763 in SHRSP was significantly impaired compared with that in WKY. Antihypertensive treatment with cilazapril lowered blood pressure toward normal and prevented the impaired response in cerebral blood flow to Y-26763 in SHRSP. These findings suggest that (1) ATP-sensitive potassium channels contribute to the regulation of cerebral blood flow in rats, (2) the response to an ATP-sensitive potassium channel opener is markedly diminished in hypertensive rats, and (3) the altered response to an ATP-sensitive potassium channel opener during chronic hypertension can be prevented by long-term antihypertensive treatment.

Antihypertensive therapy and arterial function in experimental hypertension

1. Alterations in the function of the endothelium and arterial smooth muscle may be important in the establishment of hypertension. Thus, the possible favorable influences of blood pressure-lowering agents on vascular responsiveness may be important in the chronic antihypertensive actions of these compounds. 2. A number of reports have suggested that ACE inhibitors can improve arterial function in hypertension, whereas the knowledge about the vascular effects of other antihypertensive drugs, like beta-blockers, calcium channel blockers, and diuretics remains rather limited. 3. In this article, the effects of antihypertensive therapy on arterial function in human and experimental hypertension are reviewed.

Nitric oxide release from kidneys of hypertensive rats treated with imidapril

To examine whether endothelial dysfunction in hypertension is reversible or not, we studied the effects of imidapril, an angiotensin-converting enzyme inhibitor, on nitric oxide release in stroke-prone spontaneously hypertensive rats (SHR) and deoxycorticosterone acetate (DOCA)-salt hypertensive rats. After a 4-week treatment with imidapril (1 or 10 mg/d SC) or vehicle, acetylcholine-induced vasodilation and nitric oxide release in the isolated kidneys were determined. Nitric oxide release was measured by a chemiluminescense assay. Imidapril lowered blood pressure in stroke-prone SHR in a dose-dependent manner. Untreated stroke-prone SHR exhibited significantly attenuated responses to acetylcholine (10(-8) mol/L) of both renal perfusion pressure (stroke-prone SHE 42 +/- 4% versus Wistar-Kyoto rats [WKY] 58 +/- 4% [mean +/- SE], P < .01) and nitric oxide release (stroke-prone SHR +7.6 +/- 2.1 versus WKY +29.7 +/- 9.7 fmol/min per gram of kidney wt, P < .01). Imidapril at 10 mg/d significantly increased acetylcholine-induced renal vasodilation and nitric oxide release in stroke-prone SHR (renal perfusion pressure, 56 +/- 3%; nitric oxide release, +27.1 +/- 6.4 fmol/min per gram of kidney wt; both P < .01 versus stroke-prone SHR treated with vehicle). On the other hand, imidapril neither decreased blood pressure nor changed nitric oxide release induced by acetylcholine in DOCA-salt hypertensive rats. Staining for endothelial nitric oxide synthase and brain nitric oxide synthase was clearly detected in the kidneys of both stroke-prone SHR and WKY, whereas staining intensity was weaker in DOCA-salt hypertensive rats. Inducible nitric oxide synthase immunoreactivity was barely noticeable in any type of rat. Thus, imidapril restored endothelial damage by pressure-dependent mechanisms. Most of the nitric oxide detected in the perfusate seemed to be derived from constitutive nitric oxide synthase.

Sick vessel syndrome. Recovery of atherosclerotic and hypertensive vessels

This review describes vascular changes in atherosclerotic and hypertensive vessels as well as effects of treatment. Changes in vascular structure in both atherosclerosis and hypertension are characterized by thickening of the vessel wall and vascular "remodeling." Remodeling tends to preserve the size of the lumen in atherosclerotic vessels and results in a smaller lumen in hypertensive vessels. Changes in vascular function are characterized by preservation of smooth muscle relaxation, with the exception of activity of ATP-sensitive potassium channels, and dysfunction of endothelium. Regression of atherosclerosis, by treatment of hyperlipidemia, results in quite rapid removal of lipid from the vessel wall but with inconsistent improvement in maximal vasodilator capacity. In contrast, endothelial function improves during regression of atherosclerosis, and hyperresponsiveness to serotonin subsides rapidly. Effective treatment of hypertension produces regression of vascular hypertrophy, and some approaches (especially angiotensin-converting enzyme inhibitors) are effective in correcting vascular remodeling. Endothelium-dependent relaxation generally improves during antihypertensive treatment. Reduction in pulse pressure may be more important than reduction in mean arterial pressure in reversing the structural and functional abnormalities of hypertensive vessels.

Differential cerebrovascular responsiveness in spontaneously hypertensive rats following antihypertensive treatment with clonidine and verapamil

Numerous studies have been reported examining the effects of antihypertensive treatment on peripheral vascular responsiveness in spontaneously hypertensive rats (SHR). This study was conducted to determine the effects of chronic treatment with 2 antihypertensive agents on cerebrovascular responsiveness in male SHR and Wistar-Kyoto (WKY) rats. SHR and WKY (3-4 weeks old) received either placebo, clonidine (CLON, 10 mg pellet) or verapamil (VER, 5 mg pellet). Vascular reactivity studies on the basilar artery, using standard smooth muscle bath techniques, were conducted following 6 weeks of treatment. Both CLON and VER significantly attenuated the rise in blood pressure in SHR. Basilar artery responsiveness to KCl, serotonin (5-HT), and calcium were significantly increased whereas responses to acetylcholine (ACH), isoproterenol (ISO) and sodium nitroprusside (SNP) were significantly reduced in SHR compared to WKY. CLON had no effect on basilar artery responsiveness to either the contractile or relaxation agents in SHR. However, although responses to KCl, 5-HT and calcium were not affected by VER in SHR, VER significantly increased the responses to ACH, ISO and SNP. Neither CLON nor VER treatment affected basilar artery responsiveness to any of the agents in WKY. These data demonstrate that, even though CLON and VER have similar antihypertensive effects, differential effects of the 2 agents on cerebrovascular responsiveness in the SHR are apparent. This would suggest that the vascular effects of VER and CLON are dependent upon the mechanism of action of the agents and not simply due to prevention of the elevation in blood pressure.