Cerebral circulation in chronic arterial hypertension

From hypertension to stroke: mechanisms and potential prevention strategies

Stroke is a major cause of disability and death worldwide. Prevention aimed at risk factors of stroke is the most effective strategy to curb the stroke pandemic. Hypertension is one of the most important risk factors for stroke. Despite the substantial evidence of the benefits of lowering blood pressure, conventional treatment does not normalize the burden of major cardiovascular events in patients with hypertension. Fully understanding the factors involved in the hypertension-induced stroke helps to develop new strategies for stroke prevention. Antihypertensive therapies selected should have positive blood pressure-independent effects on stroke risk. This review summarizes the factors involved in the hypertension-induced stroke, such as oxidative stress, inflammation, and arterial baroreflex dysfunction, and potential strategies for its prevention, therefore, provides clues for clinicians.

Placental ischemia impairs middle cerebral artery myogenic responses in the pregnant rat

One potential mechanism contributing to the increased risk for encephalopathies in women with preeclampsia is altered cerebral vascular autoregulation resulting from impaired myogenic tone. Whether placental ischemia, a commonly proposed initiator of preeclampsia, alters cerebral vascular function is unknown. This study tested the hypothesis that placental ischemia in pregnant rats (caused by reduced uterine perfusion pressure [RUPP]) leads to impaired myogenic responses in middle cerebral arteries. Mean arterial pressure was increased by RUPP (135±3 mm Hg) compared with normal pregnant rats (103±2 mm Hg) and nonpregnant controls (116±1 mm Hg). Middle cerebral arteries from rats euthanized on gestation day 19 were assessed in a pressure arteriograph under active (+Ca(2+)) and passive (0 Ca(2+)) conditions, whereas luminal pressure was varied between 25 and 150 mm Hg. The slope of the relationship between tone and pressure in the middle cerebral artery was 0.08±0.01 in control rats and was similar in normal pregnant rats (0.05±0.01). In the RUPP model of placental ischemia, this relationship was markedly reduced (slope=0.01±0.00; P<0.05). Endothelial dependent and independent dilation was not different between groups, nor was there evidence of vascular remodeling assessed by the wall:lumen ratio and calculated wall stress. The impaired myogenic response was associated with brain edema measured by percentage of water content (RUPP P<0.05 versus control and normal pregnant rats). This study demonstrates that placental ischemia in pregnant rats leads to impaired myogenic tone in the middle cerebral arteries and that the RUPP model is a potentially important tool to examine mechanisms leading to encephalopathy during preeclamptic pregnancies.

Hypertension impairs vascular reactivity in the pediatric brain

BACKGROUND AND PURPOSE

Chronic hypertension impairs cerebrovascular regulation in adults, but its effects on the pediatric population are unknown. The objective of this study was to investigate cerebrovascular abnormalities in hypertensive children and adolescents.

METHODS

Sixty-four children and adolescents aged 7 to 20 years underwent transcranial Doppler examinations of the middle cerebral artery at the time of rebreathing CO2. Time-averaged maximum mean cerebral blood flow velocity and end-tidal CO2 were used to quantify cerebrovascular reactivity during hypercapnia. Patients were clinically categorized as hypertensive, prehypertensive, or white coat hypertensive based on 24-hour ambulatory blood pressure measurements. Their reactivities were compared with 9 normotensive control subjects and evaluated against baseline mean blood pressure z-scores and loads.

RESULTS

Untreated hypertensive children had significantly lower hypercapnic reactivity than normotensive children (2.556 +/- 1.832 cm/s x mm Hg versus 4.256 +/- 1.334 cm/s x mm Hg, P < 0.05). Baseline mean diastolic blood pressure z-scores (r = -0.331, P = 0.037) and diastolic blood pressure loads (r = -0.351, P = 0.026) were inversely related to reactivity.

CONCLUSIONS

Untreated hypertensive children and adolescents have blunted reactivity to hypercapnia, indicating deranged vasodilatory reactivity. The inverse relationship between diastolic blood pressure indices and reactivity suggests that diastolic blood pressure may be a better predictor of cerebral end organ damage than systolic blood pressure.

Differential effects of age and history of hypertension on regional brain volumes and iron

Aging affects various structural and metabolic properties of the brain. However, associations among various aspects of brain aging are unclear. Moreover, those properties and associations among them may be modified by age-associated increase in vascular risk. In this study, we measured volume of brain regions that vary in their vulnerability to aging and estimated local iron content via T2* relaxometry. In 113 healthy adults (19-83 years old), we examined prefrontal cortex (PFC), primary visual cortex (VC), hippocampus (HC), entorhinal cortex (EC), caudate nucleus (Cd), and putamen (Pt). In some regions (PFC, VC, Cd, and Pt) age-related differences in iron and volume followed similar patterns. However, in the medial-temporal structures, volume and iron content exhibited different age trajectories. Whereas age-related volume reduction was mild in HC and absent in EC, iron content evidenced significant age-related declines. In hypertensive participants significantly greater iron content was noted in all examined regions. Thus, iron content as measured by T2* may be a sensitive index of regional brain aging and may reveal declines that are more prominent than gross anatomical shrinkage.

Intersection between metabolic dysfunction, high fat diet consumption, and brain aging

Deleterious neurochemical, structural, and behavioral alterations are a seemingly unavoidable aspect of brain aging. However, the basis for these alterations, as well as the basis for the tremendous variability in regards to the degree to which these aspects are altered in aging individuals, remains to be elucidated. An increasing number of individuals regularly consume a diet high in fat, with high-fat diet consumption known to be sufficient to promote metabolic dysfunction, although the links between high-fat diet consumption and aging are only now beginning to be elucidated. In this review we discuss the potential role for age-related metabolic disturbances serving as an important basis for deleterious perturbations in the aging brain. These data not only have important implications for understanding the basis of brain aging, but also may be important to the development of therapeutic interventions which promote successful brain aging.

PPAR{gamma} activation prevents hypertensive remodeling of cerebral arteries and improves vascular function in female rats

BACKGROUND AND PURPOSE

Previous studies have shown that peroxisome proliferator-activated receptor gamma (PPARgamma), a ligand-activated transcription factor expressed in vascular cells, is protective of the vasculature. We hypothesized that activation of PPARgamma could prevent hypertensive remodeling of cerebral arteries and improve vascular function.

METHODS

Ten female Sprague-Dawley rats were treated with the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME) for 5 weeks, 8 were treated with l-NAME plus the PPARgamma activator rosiglitazone, and 8 received no treatment and served as controls. Blood pressure, myogenic activity, passive diameters and wall thickness of cerebral arteries, and brain capillary density were compared between the groups.

RESULTS

Treatment with l-NAME caused an increase in arterial blood pressure that was sustained with rosiglitazone treatment. l-NAME also caused inward hypertrophic remodeling and enhanced myogenic reactivity of cerebral arteries that was reversed by rosiglitazone. In addition, l-NAME hypertension caused rarefaction of brain capillaries by approximately 12%, whereas treatment with rosiglitazone increased capillary density by approximately 20%.

CONCLUSIONS

PPARgamma activation may be an effective and clinically relevant way to prevent hypertensive remodeling of cerebral arteries and capillary rarefaction as well as improving vascular function without affecting blood pressure.

Participation of oxidative stress on rat middle cerebral artery changes induced by focal cerebral ischemia: beneficial effects of 3,4-dihydro-6-hydroxy-7-methoxy-2,2-dimethyl-1(2H)-benzopyran (CR-6)

Cerebral ischemia followed by reperfusion alters vessel properties of brain arteries in rats, inducing an inflammatory response and excessive generation of reactive oxygen species. This study investigated the participation of oxidative stress on vessel properties after ischemia/reperfusion and the beneficial effects of 3,4-dihydro-6-hydroxy-7-methoxy-2,2-dimethyl-1(2H)-benzopyran (CR-6). The right middle cerebral artery was occluded (90 min) and reperfused (24 h). Sham-operated animals were used as controls. Ischemic rats were treated either with CR-6 (100 mg/kg in 1 ml olive oil) or vehicle (1 ml olive oil) administered orally at 2 and 8 h after the onset of ischemia. The structural, mechanical, and myogenic properties of the middle cerebral artery (MCA) were assessed by pressure myography. Superoxide anion ( ) production was evaluated by ethidium fluorescence, and protein tyrosine nitrosylation was determined by immunofluorescence. Infarct volume was smaller in rats treated with CR-6. In MCA, ischemia/reperfusion increased wall thickness, cross-sectional area, wall/lumen, and decreased wall stress. CR-6 treatment prevented all of these changes induced by ischemia/reperfusion. However, impaired myogenic response and larger lumen diameter in active conditions observed after ischemia/reperfusion were not modified by CR-6. Treatment with CR-6 prevented the increase in production and partially prevented the enhanced protein tyrosine nitrosylation that occurred in response to ischemia/reperfusion. Our findings suggest that oxidative stress is involved in the alterations of MCA properties observed after ischemia/reperfusion and that CR-6 induces protection.

Pressure-induced constriction is inhibited in a mouse model of reduced betaENaC

Recent studies suggest certain epithelial Na(+) channel (ENaC) proteins may be components of mechanosensitive ion channel complexes in vascular smooth muscle cells that contribute to pressure-induced constriction in middle cerebral arteries (MCA). However, the role of a specific ENaC protein, betaENaC, in pressure-induced constriction of MCAs has not been determined. The goal of this study was to determine whether pressure-induced constriction in the MCA is altered in a mouse model with reduced levels of betaENaC. Using quantitative immunofluorescence, we found whole cell betaENaC labeling in cerebral vascular smooth muscle cells (VSMCs) was suppressed 46% in betaENaC homozygous mutant (m/m) mice compared with wild-type littermates (+/+). MCAs from betaENaC +/+ and m/m mice were isolated and placed in a vessel chamber for myographic analysis. Arteries from betaENaC+/+ mice constricted to stepwise increases in perfusion pressure and developed maximal tone of 10 +/- 2% at 90 mmHg (n = 5). In contrast, MCAs from betaENaC m/m mice developed significantly less tone (4 +/- 1% at 90 mmHg, n = 5). Vasoconstrictor responses to KCl (4-80 mM) were identical between genotypes and responses to phenylephrine (10(-7)-10(-4) M) were marginally altered, suggesting that reduced levels of VSMC betaENaC specifically inhibit pressure-induced constriction. Our findings indicate betaENaC is required for normal pressure-induced constriction in the MCA and provide further support for the hypothesis that betaENaC proteins are components of a mechanosensor in VSMCs.

Cerebral vasomotor reactivity before and after blood pressure reduction in hypertensive patients

BACKGROUND

Hypertension is associated with cerebrovascular remodeling and endothelial dysfunction, which may reduce cerebral vasomotor reactivity to CO2. Treatment combining blood pressure (BP) reduction with inhibition of vascular effects of angiotensin II may reverse these changes. However, the reduction in BP at the onset of treatment can compromise cerebral perfusion and exhaust vasomotor reserve, leading to impaired CO2 reactivity.

METHODS

Eleven patients (nine men, two women) with newly diagnosed, untreated mild-to-moderate hypertension aged (mean (s.d.)) 52 (9) years, and eight controls (seven men, one woman) aged 46 (10) years were studied. Patients received losartan/hydrochlorothiazide (50/12.5 or 100/25 mg) to reduce BP to <140/<90 mm Hg within 1-2 weeks. BP (Finapres), heart rate (HR), CBFV (cerebral blood flow velocity, transcranial Doppler), cerebrovascular resistance, and CO2 reactivity were measured at baseline, after the rapid BP reduction, and after long-term treatment (3-4 months).

RESULTS

At baseline, hypertension was not associated with reduced CO2 reactivity. Treatment effectively lowered BP from 148 (12)/89 (7) to 130 (15)/80 (9) after 1-2 weeks and 125 (10)/77 (7) mm Hg after 3-4 months (P = 0.003). CO2 reactivity was not affected by the reduction in BP within 2 weeks, and long-term treatment did not augment reactivity.

CONCLUSIONS

In hypertension without diabetes or advanced cerebrovascular disease, CO2 reactivity is not reduced, and rapid normalization (within 2 weeks) of BP does not exhaust vasomotor reserve. CO2 reactivity did not change between 2 and 12 weeks of treatment, which argues against a direct vascular effect of angiotensin II inhibition within this period.

Impaired hemodynamics and endothelial vasomotor function via endoperoxide-mediated vasoconstriction in the carotid artery of spontaneously hypertensive rats

The fact that endothelium removal increases diameter and compliance in the common carotid artery (CCA) of spontaneously hypertensive rats (SHR) and that improving CCA endothelium-dependent vasorelaxation has been shown to normalize a reduced systolic blood flow through the SHR CCA compared with normotensive Wistar-Kyoto rats (WKY) suggests that endothelial vasomotor dysfunction may be linked to altered large artery hemodynamics in hypertension. The experiments herein were designed to further investigate WKY and SHR CCA hemodynamics and endothelium-dependent vasomotor functions. It was hypothesized that CCA blood flow and conductance would be reduced throughout the cardiac cycle in SHR and that endothelium-dependent contractile activity would impair SHR CCA vasorelaxation. We report that mean, maximal systolic, and diastolic blood flow was reduced in SHR vs. WKY CCA, as was vascular conductance. Pressure was augmented in SHR CCA and accompanied by late systolic flow augmentation so that total flow during systole was indeed no different between strains, possibly explained by earlier lower body wave reflection. While ACh stimulation in isolated precontracted WKY CCA caused a robust nitric oxide (NO)-mediated vasorelaxation, endothelium-dependent, cyclooxygenase (COX)-mediated contractile activity stimulated by high ACh concentration impaired NO- and non-NO/non-COX-mediated vasorelaxation in precontracted SHR CCA. In quiescent CCA, this endothelium-dependent contractile response was COX-1 and thromboxane-prostanoid receptor mediated and modulated by the availability of NO. These data collectively suggest that endothelium-dependent, COX-mediated endoperoxide signaling in the CCA of SHR may elicit vasoconstriction, which could shift the mechanical properties of this conduit artery and contribute to reduced CCA blood flow in vivo.

Obesity increases blood pressure, cerebral vascular remodeling, and severity of stroke in the Zucker rat

Obesity is a risk factor for stroke, but the mechanisms by which obesity increases stroke risk are unknown. Because microvascular architecture contributes to the outcome of stroke, we hypothesized that middle cerebral arteries (MCAs) from obese Zucker rats (OZRs) undergo inward remodeling and develop increased myogenic tone compared with those in lean Zucker rats (LZRs). We further hypothesized that OZRs have an increased infarct after cerebral ischemia and that changes in vascular structure and function correlate with the development of hypertension in OZRs. Blood pressure was measured by telemetry in LZRs and OZRs from 6 to 17 weeks of age. Vessel structure and function were assessed in isolated MCAs. Stroke damage was assessed after ischemia was induced for 60 minutes followed by 24 hours of reperfusion. Although mean arterial pressure was similar between young rats (6 to 8 weeks old), mean arterial pressure was higher in adult (14 to 17 weeks old) OZRs than in LZRs. MCAs from OZRs had a smaller lumen diameter and increased myogenic vasoconstriction compared with those from LZRs. After ischemia, infarction was 58% larger in OZRs than in LZRs. Before the development of hypertension, MCA myogenic reactivity and lumen diameter, as well as infarct size, were similar between young LZRs and OZRs. Our results indicate that the MCAs of OZRs undergo structural remodeling and that these rats have greater cerebral injury after cerebral ischemia. These cerebrovascular changes correlate with the development of hypertension and suggest that the increased blood pressure may be the major determinant for stroke risk in obese individuals.

Hypertension and cerebrovascular dysfunction

Essential hypertension has devastating effects on the brain, being the major cause of stroke and a leading cause of dementia. Hypertension alters the structure of cerebral blood vessels and disrupts intricate vasoregulatory mechanisms that assure an adequate blood supply to the brain. These alterations threaten the cerebral blood supply and increase the susceptibility of the brain to ischemic injury as well as Alzheimer's disease. This review focuses on the mechanisms by which hypertension disrupts cerebral blood vessels, highlighting recent advances and outstanding issues.

Optimizing selective cerebral perfusion: deleterious effects of high perfusion pressures

OBJECTIVE

Selective cerebral perfusion is a proven adjunct to hypothermia for neuroprotection in complex aortic surgery. The ideal conditions for the provision of selective cerebral perfusion, however, including optimal perfusion pressure, remain unknown. We investigated the effects of various perfusion pressures during selective cerebral perfusion on cerebral physiology and outcome in a long-term porcine model.

METHODS

Thirty piglets (26.3 +/- 1.4 kg), cooled to 20 degrees C on cardiopulmonary bypass with alpha-stat pH management (mean hematocrit 23.6%), were randomly assigned to 90 minutes of selective cerebral perfusion at a pressure of 50 (group A), 70 (group B), or 90 (group C) mm Hg. With fluorescent microspheres and sagittal sinus sampling, cerebral blood flow and cerebral oxygen metabolism were assessed at baseline, after cooling, at two points during selective cerebral perfusion, and for 2 hours after cardiopulmonary bypass. Visual evoked potentials were monitored during recovery. Neurobehavioral scores were assessed blindly from standardized videotaped sessions for 7 postoperative days.

RESULTS

Cerebral blood flow during selective cerebral perfusion was significantly increased by higher-pressure perfusion (P = .04), although all groups sustained similar levels of cerebral oxygen metabolism during selective cerebral perfusion (P = .88). After the end of cardiopulmonary bypass, the cerebral oxygen metabolism increased to above baseline in all groups, with the highest levels seen in group C (P = .06). Intracranial pressure was significantly higher during selective cerebral perfusion in group C (P = .0002); visual evoked potentials did not differ among groups. Neurobehavioral scores were significantly better in group A (P = .0002).

CONCLUSION

Selective cerebral perfusion at 50 mm Hg provides neuroprotection superior to that at higher pressures. The increased cerebral blood flow with higher-pressure selective cerebral perfusion is associated with cerebral injury, reflected by high post-cardiopulmonary bypass cerebral oxygen metabolism and poorer neurobehavioral recovery.

Sustained pharmacological inhibition of deltaPKC protects against hypertensive encephalopathy through prevention of blood-brain barrier breakdown in rats

Hypertensive encephalopathy is a potentially fatal condition associated with cerebral edema and the breakdown of the blood-brain barrier (BBB). The molecular pathways leading to this condition, however, are unknown. We determined the role of deltaPKC, which is thought to regulate microvascular permeability, in the development of hypertensive encephalopathy using deltaV1-1 - a selective peptide inhibitor of deltaPKC. As a model of hypertensive encephalopathy, Dahl salt-sensitive rats were fed an 8% high-salt diet from 6 weeks of age and then were infused s.c. with saline, control TAT peptide, or deltaV1-1 using osmotic minipumps. The mortality rate and the behavioral symptoms of hypertensive encephalopathy decreased significantly in the deltaV1-1-treated group relative to the control-treated group, and BBB permeability was reduced by more than 60%. Treatment with deltaV1-1 was also associated with decreased deltaPKC accumulation in capillary endothelial cells and in the endfeet of capillary astrocytes, which suggests decreased microvasculature disruption. Treatment with deltaV1-1 prevented hypertension-induced tight junction disruption associated with BBB breakdown, which suggests that deltaPKC may specifically act to dysregulate tight junction components. Together, these results suggest that deltaPKC plays a role in the development of hypertension-induced encephalopathy and may be a therapeutic target for the prevention of BBB disruption.

HAEMODYNAMIC CONTRIBUTIONS TO THE PATHOGENESIS OF PREECLAMPSIA AND ECLAMPSIA

Current hypotheses regarding the origins of preeclampsia have focused on the “Two stage model”. This model suggests that the primary steps in the pathophysiologic sequence of preeclampsia are initiated by abnormal placentation including the classic finding of abnormal trophoblast invasion of maternal decidual spiral arteries. The second stage of the sequence includes the elaboration of a single or multiple substances from these disordered placentas which contribute to the generalized maternal systemic illness, eventually manifesting as endothelial injury, hypertension and proteinuria. Recent studies have focused on the role of pro and anti-angiogenic peptides as potential placentally derived aetiologic agents in this pathophysiologic sequence, although other placental products have been highlighted in recent research. Despite the fact that this modeling of preeclampsia has widespread support significant limitations to this hypothesis can be identified.

Increased Pain Sensitivity in Low Blood Pressure

There is broad evidence for a functional interaction between the cardiovascular and pain regulatory systems. One result of this interaction is the reduced sensitivity to acute pain in individuals with elevated blood pressure, which has been established in numerous studies. In contrast to this, possible alterations in pain perception related to the lower range of blood pressure have not yet been investigated. In the present study pain sensitivity was assessed in 30 hypotensive women (mean blood pressure 95/56 mmHg) and 30 normotensive control persons (mean blood pressure 119/77 mmHg) based on a cold pressor test. Possible effects on pain perception of hypotension-related impairment of subjective state were controlled for by including a mood-scale. The hypotensive as compared to the normotensive group displayed lower pain threshold and pain tolerance levels, as well as increased sensory and affective experiences of pain. Moreover, a slight negative correlation was found, both in hypotensive and control persons, between pain sensitivity and the degree of blood pressure increase during the execution of the cold pressor test. In accordance with the previous findings on hypertension-related hypoalgesia, the present results suggest an inverse relationship between blood pressure and pain sensitivity across the total blood pressure spectrum. Different degrees of pain attenuation through afferent input from the arterial baroreceptor system are discussed as a physiological mechanism mediating this relationship.

The neurovascular dysfunction induced by angiotensin II in the mouse neocortex is sexually dimorphic

Women are less susceptible to the cerebrovascular complications of hypertension, such as a stroke and vascular dementia. The mechanism of such protection may be related to a reduced vulnerability of women to the cerebrovascular actions of hypertension. To test this hypothesis, we used a model of hypertension based on infusion of angiotensin II (ANG II), an octapeptide that plays a key role in hypertension and produces cerebrovascular dysregulation. Cerebral blood flow (CBF) was monitored by laser-Doppler flowmetry in anesthetized (urethane-chloralose) C57BL/6J male and female mice equipped with a cranial window. ANG II administration (0.25 mug.kg(-1).min(-1) iv x 30-45 min) elevated arterial pressure equally in both sexes but attenuated the CBF increase induced by whisker stimulation or by the endothelium-dependent vasodilator acetylcholine (ACh) in male but not in female mice. The administration of ANG II for 7 days (2.74 mg.kg(-1).day(-1)), using osmotic minipumps, also attenuated these cerebrovascular responses in male, but not female, mice. The reduced susceptibility to the effect of ANG II in female mice was abolished by ovariectomy and reinstated by estrogen administration to ovariectomized mice. Administration of estrogen to male mice abolished the ANG II-induced attenuation of CBF responses. We conclude that female mice are less susceptible to the cerebrovascular dysregulation induced by ANG II, an effect related to estrogen. Such protection from the deleterious cerebrovascular effects of hypertension may play a role in the reduced vulnerability to the cerebrovascular complications of hypertension observed in women.

Effects of benidipine, a long-lasting dihydropyridine-Ca2+ channel blocker, on cerebral blood flow autoregulation in spontaneously hypertensive rats

Chronic hypertension shifts cerebral blood flow (CBF) autoregulation towards higher blood pressure. We examined whether or not benidipine, a long-lasting dihydropyridine calcium channel blocker (CCB), improves the CBF autoregulation in spontaneously hypertensive rats (SHRs). CBF was analyzed by laser-Doppler flowmetry during stepwise hypotension by controlled bleeding. The lower limit of CBF autoregulation was calculated as the mean arterial blood pressure at which CBF decreased by 10% of the baseline. Mean arterial blood pressure and cerebral vascular resistance in SHRs were higher than those in normotensive Wistar rats. Oral administration of benidipine (3 mg/kg) for 8 d lowered the mean arterial blood pressure and cerebral vascular resistance, which were equivalent to the effects of amlodipine (3 mg/kg), another CCB, or candesartan (1 mg/kg), an Angiotensin II type-1 receptor blocker. The lower limit of CBF autoregulation in SHRs (142+/-4 mmHg) was significantly shifted to a higher-pressure level compared with Wistar rats (59+/-2 mmHg). The lower limit of CBF autoregulation was significantly lower in the benidipine-treated group (91+/-4 mmHg) than that in the control SHRs, and similar to that of the amlodipine group (97+/-6 mmHg). Benidipine reduced the lower limit of CBF autoregulation more effectively than candesartan (109+/-4 mmHg). In conclusion, benidipine shifted the limit of CBF autoregulation towards lower blood pressure in SHRs under hypotensive conditions by hemorrhage. These results suggest that benidipine may be useful for the treatment of hypertensive patients with the elderly or cerebrovascular disorders, in whom autoregulation of CBF is impaired.

The challenge of blood pressure management in neurologic emergencies

Hypertensive crises are commonly seen in the emergency department, and acute stroke is often the inciting etiology of a hypertensive crisis. Cerebral autoregulation is disrupted in acute stroke, and efforts to lower blood pressure may reduce cerebral perfusion and worsen outcomes. Although most patients with stroke have elevated blood pressure, evidence from clinical trials to guide therapy are scarce. Current national guidelines recommend lowering blood pressure after stroke only if end-organ damage is present or if systolic/diastolic blood pressures exceed 220/120 or 185/110 mm Hg in patients ineligible and in those eligible to receive thrombolytic drug therapy, respectively. Recommended pharmacologic interventions for elevated blood pressure after acute ischemic stroke include labetalol, nicardipine, or nitroprusside, depending on the severity of the elevation. Similar recommendations have been made for intracerebral hemorrhage. Subarachnoid hemorrhage is managed with nimodipine and other calcium channel blockers to prevent vasospasm and improve clinical outcomes. Data from ongoing clinical trials may improve guidance about the management of elevated blood pressure after acute stroke.

Middle cerebral artery structure and distensibility during developing and established phases of hypertension in the spontaneously hypertensive rat

OBJECTIVE

The aims of the current study were to examine the structural properties of middle cerebral arteries (MCA) from young (5-7 weeks) and adult (20-24 weeks) spontaneously hypertensive rats (SHR), compared with age-matched Wistar-Kyoto (WKY) control rats.

DESIGN

MCA segments (8-10 per group) were secured onto glass pipettes in a small vessel chamber and studied using a pressure arteriograph system. Vessels were perfused in Ca2+-free physiological salt solution to ensure the absence of tone. The wall thickness and lumen diameter were recorded at intraluminal pressures ranging from 3 to 180 mmHg using a video dimension analyser.

RESULTS

There was a borderline increase in systolic pressure of the young SHR, compared with WKY controls, but the systolic pressure of the older SHR was significantly raised. The MCA lumen diameter from young SHR was reduced across the entire pressure range and arterial distensibility was not reduced, compared with WKY vessels. The MCA lumen diameter from adult SHR was reduced at high pressure, but converged with the lumen diameter of the WKY vessels at 3 mmHg, and the stress-strain relation was shifted to the left, compared with the WKY vessels; nevertheless, the slope of the tangential elastic modulus-stress relation was not significantly increased. The pressure-wall cross-sectional area relationship did not differ between strains at either time point.

CONCLUSIONS

These data demonstrate eutrophic inward remodelling of the MCA from young SHR, compared with WKY controls. In the adult SHR the structural changes are probably a consequence of a reduced arterial distensibility.

Linkage analysis of neointimal hyperplasia and vascular wall transformation after balloon angioplasty

Neointimal hyperplasia (NIH), a result of vascular injury, is due to the migration and proliferation of smooth muscle cells through the media and internal elastic lamina leading to vascular occlusion. We used a rat model to find the genetic regions controlling NIH after endothelial denudation in two divergent inbred strains of rats. The Brown Norway (BN) and spontaneously hypertensive rat (SHR) strains have a 2.5-fold difference in injury-induced NIH. A population of 301 F2(SHR × BN) rats underwent a standard vascular injury followed by phenotyping 8 wk after injury to identify quantitative trait loci (QTL) responsible for this strain difference. Interval mapping identified two %NIH QTL on rat chromosomes 3 and 6 [logarithm of odds (LOD) scores 2.5, 2.2] and QTL for other injured vascular wall changes on rat chromosomes 3, 4, and 15 (LOD scores 2.0–4.6). Also, QTL for control vessel media width (MW) and media area (MA) were found on chromosome 6 with LOD scores of 2.3 and 2.5, suggesting that linkage exists between these control vessel parameters and NIH production. These results represent the first genetic analysis for the identification of NIH QTL and QTL associated with the vascular injury response.

Pregnancy prevents hypertensive remodeling of cerebral arteries: a potential role in the development of eclampsia

We investigated how hypertension during pregnancy affected passive structural (wall:lumen, wall stress) and active (myogenic activity) responses of the cerebral circulation. Female nonpregnant (NP; n=8) Sprague Dawley rats were compared with late-pregnant (LP; day 19 to 20, n=6) rats. Some animals were treated with the NO synthase inhibitor nitro-L-arginine in their drinking water to raise blood pressure. LP rats (n=6) were treated for the last 7 days of pregnancy (last trimester) to mimic preeclampsia and compared with NP rats treated for the same duration (n=8). Active and passive responses were determined on isolated and pressurized third-order posterior cerebral arteries. Nitro-L-arginine treatment significantly raised blood pressure in both groups of animals that was associated with increased wall thickness and wall:lumen ratio in the NP hypertensive animals versus controls (P<0.05). In contrast, this response to pressure was absent in LP animals, which had similar wall measurements. In addition, arteries from NP hypertensive animals had increased myogenic tone and pressure of forced dilatation compared with NP control animals (P<0.01). Again, this response was lacking in the LP hypertensive animals that had similar tone and pressure of forced dilatation as normotensive controls. The increased tone and wall thickness decreased wall stress in the NP hypertensive animals, a response that did not occur in LP hypertensive animals. Because medial hypertrophy is considered a protective response to elevated blood pressure, these results suggest that hypertension in pregnancy may predispose the cerebral circulation to autoregulatory breakthrough and blood-brain-barrier disruption when blood pressure is elevated, as during eclampsia.

Arterial hypertension and brain damage--evidence from animal models (review)

Hypertension is an important risk factor for cerebrovascular disease including stroke and has also a role in the development of vascular cognitive impairment (VCI) and vascular dementia (VaD). Research on pathophysiology and treatment of hypertensive brain damage may benefit from the availability of animal models. This paper has reviewed the main animal models of hypertension in which brain damage is documented. Spontaneously hypertensive rats (SHR) represent the animal model more largely used. In these rats cerebrovascular changes, brain atrophy, loss of nerve cells in cerebrocortical areas, and glial reaction were documented. Several changes observed in SHR are similar to those found by in vivo imaging studies in essential hypertensives. It is documented that brain gets benefit from lowering abnormally elevated blood pressure and that reduction of hypertension protects brain from stroke and probably reduces the incidence of VaD. The influence of anti-hypertensive treatment on brain structure and function in animal models of hypertension is reviewed. Among classes of drugs investigated, dihydropyridine-type Ca2+ antagonists were those with a most documented protective effect on hypertensive brain damage. Limits and perspectives in the use of animal models for assessing brain damage caused by hypertension and protection from it are discussed.

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.

Persistent hypertension does not alter the cerebral blood flow and glucose utilization in young-adult Dahl salt-sensitive rats

The Dahl- Iwai salt-sensitive (DS) rat develops hypertension due to a high-salt diet without any structural alterations of the brain arteries and arterioles. We investigated the effect of persistent hypertension on the regional cerebral blood flow (rCBF) and regional cerebral glucose utilization (rCGU) in the DS rats. The rats were fed either a high-salt diet (HSD; 8% NaCl, n = 5) or a low-salt diet (LSD; 0.3% NaCl, n = 6) from 8 to 16 weeks of age, and the HSD group developed hypertension lasting for 1 month. At 16 weeks of age, the rCBF was measured in the sensorimotor and visual cortices using the hydrogen clearance method, and the rCGU was measured in 26 different brain structures using the [(14)C]deoxyglucose method. The mean arterial pressure was significantly higher in the HSD group (168+/-7 mm Hg) than in the LSD group (139+/-3 mm Hg) (P < 0.01). The mean rCBF and the rCGU values tended to be lower in the HSD group than in the LSD group; however, there were no statistically significant differences except for the reduced rCGU value in the nucleus accumbens. These results suggest that hypertension itself does not alter either the rCBF or the rCGU in young-adult DS rats. This indicates that the functional / structural changes of the cerebral arteries and arterioles that are associated with hypertension appear to be responsible for altered rCBF and rCGU in other animal models of hypertension.

Structural factors increase blood pressure through the interaction of resistance vessel geometry with neurohumoral and local factors: estimates in rabbits with renal cellophane-wrap hypertension with intact effectors and during neurohumoral blockade

BACKGROUND

The structural changes in hypertension include narrowing of the lumen of the large resistance vessels and an increase in their wall thickness : lumen ratio. Their haemodynamic role has been controversial.

OBJECTIVE

To examine resting haemodynamics and the responses to graded drug-induced changes in tone in renal cellophane-wrap (wrap) and sham-operated (sham) rabbits, when their neurohumoral effectors were intact and during high-level blockade.

METHODS

Each rabbit was implanted with a flow probe for measuring cardiac output, had catheters inserted for drug infusions, and underwent mean arterial pressure (MAP) measurements. Resting values were determined and we infused graded doses of dilator and constrictor drugs: acetylcholine or adenosine; angiotensin II or methoxamine. The dilator and constrictor dose-response curves were combined into a single relationship for MAP, cardiac output, total peripheral conductance (TPC) and heart rate; total peripheral resistance (TPR) was estimated as 1/TPC.

RESULTS

Throughout the range of vascular tone and with intact effector function, MAP was greater and TPC lower in wrap than in sham rabbits, and cardiac output was the same in both groups. The ratios of wrap : sham slopes of the log dose-response regression lines were 0.47 for TPC, 2.04 for TPR and 1.89 for MAP. Thus MAP and TPR responses were enhanced to the same degree in wrap rabbits. During neurohumoral block, baroreflex-mediated heart rate responses were abolished. In addition, resting vascular tone was lower than with intact effectors in both wrap and sham rabbits; however, the ratios of wrap : sham slopes for TPC and TPR were similar to those with intact effectors, whereas the ratio for the slope for MAP was slightly smaller, although still enhanced.

CONCLUSION

In wrap hypertension, the enhanced MAP and TPR responses are in accord with an interaction between vascular geometry and the sum of altered neurohumoral + local activity, plus a rarefaction component.

High-intensity focused ultrasound selectively disrupts the blood-brain barrier in vivo

High-intensity focused ultrasound (HIFU) has been shown to generate lesions that destroy brain tissue while disrupting the blood-brain barrier (BBB) in the periphery of the lesion. BBB opening, however, has not been shown without damage, and the mechanisms by which HIFU induces BBB disruption remain unknown. We show that HIFU is capable of reversible, nondestructive, BBB disruption in a targeted region-of-interest (ROI) (29 of 55 applications; 26 of 55 applications showed no effect); this opening reverses after 72 h. Light microscopy demonstrates that HIFU either entirely preserves brain architecture while opening the BBB (18 of 29 applications), or generates tissue damage in a small volume within the region of BBB opening (11 of 29 applications). Electron microscopy supports these observations and suggests that HIFU disrupts the BBB by opening capillary endothelial cell tight junctions, an isolated ultrastructural effect that is different from the mechanisms through which other (untargeted) modalities, such as hyperosmotic solutions, hyperthermia and percussive injury disrupt the BBB.

Effect of cerebral embolization on regional autoregulation during cardiopulmonary bypass in dogs

BACKGROUND

Embolization during cardiopulmonary bypass probably alters cerebral autoregulation. Therefore, using laser Doppler flowmetry we investigated the cerebral blood flow velocity changes in response to changes in arterial pressure, before and after embolization in a canine bypass model.

METHODS

After Institutional Animal Care and Use Committee approval, 8 anesthetized dogs had a laser Doppler flow probe positioned over the temporoparietal dura. During 37 degrees C cardiopulmonary bypass, the cerebral blood flow velocity response to changing mean arterial pressure (40 to 85 mm Hg in random order) was assessed before and after systemic embolization of 100 mg of 97-microm latex microspheres.

RESULTS

Before embolization, cerebral blood flow velocity increased 39% as mean arterial pressure increased from 40 to 85 mm Hg. Following embolization, a 94% increase in cerebral blood flow velocity was demonstrated over the same mean arterial pressure range. The slopes of the curves relating cerebral blood flow velocity to mean arterial pressure were 0.21+/-0.74 and 1.31+/-0.87, before and after embolization (p = 0.016) respectively.

CONCLUSIONS

Regional cerebral blood flow autoregulation may be impaired by microembolization known to occur during cardiopulmonary bypass, increasing the dependence of cerebral blood flow on mean arterial pressure.

Should antihypertensive therapies be given to patients with acute ischemic stroke?

Hypertension is a major risk factor for stroke and many patients with acute stroke have elevated blood pressures. The management of hypertension in the setting of acute ischaemic stroke remains a source of confusion and controversy. Lowering blood pressure in this setting may be hazardous because of impaired cerebral autoregulation. Treatment may be considered in patients who are otherwise candidates for thrombolytic therapy, patients who have severe hypertension or patients who have specific concomitant medical conditions including acute myocardial infarction, aortic dissection, hypertensive encephalopathy, or severe left ventricular failure. In choosing an agent for acute treatment, drugs that can produce a precipitous decline in blood pressure (e.g. sublingual calcium antagonists) should be avoided. Drugs with the capacity to dilate cerebral vessels should be used with caution as they have the potential to increase intracranial pressure. Long term management of hypertension in poststroke patients is often required. The potential for certain classes of drugs (e.g. alpha2-adrenergic receptor agonists and alpha1-adrenergic receptor antagonists) to impair the recovery process should be considered when choosing an antihypertensive for treatment of these patients.

Effects of treatment with nilvadipine on cerebral ischemia in rats

The protective effects of a Ca2+ antagonist, nilvadipine, on focal cerebral ischemia were studied in male spontaneously hypertensive rats. The animals received either nilvadipine (3mg x kg(-1) x day(-1)) or a vehicle subcutaneously. Group 1 (n=11) was treated for 7 days, and Group 2 (n=11) for 14 days. The middle cerebral artery was occluded on the 6th (Group 1) or 13th (Group 2) day of the treatment, and neuropathological outcomes were quantified 24 hours later. The mean arterial blood pressure was significantly reduced with nilvadipine to normal levels. The % infarct volumes of Groups 1 (37+/-2) and 2 (34+/-3) were significantly less than those of their controls (39+/-3 [n=11] and 40+/-4 [n=12], respectively), although the difference between Groups 1 and 2 was not significant. When infarct areas were compared in each of 8 coronal sections, the infarct size had decreased in the 5 posterior sections in Group 2, but only in 2 sections of Group 1. A significant decrease in the edema volumes was observed in Group 2, but not in Group 1. Thus, nilvadipine provided protective effects against cerebral ischemia in rats having chronic hypertension, and the effects were dependent on the duration of treatment.

Effect of generalised sympathetic activation by cold pressor test on cerebral haemodynamics in healthy humans

There is no general agreement regarding several aspects of the role of the sympathetic system on cerebral haemodynamics such as extent of effectiveness, operational range and site of action. This study was planned to identify the effect of a generalised sympathetic activation on the cerebral haemodynamics in healthy humans before it is masked by secondary corrections, metabolic or myogenic in nature. A total of 35 healthy volunteers aged 20-35 underwent a 5 min lasting cold pressor test (CPT) performed on their left hand. The cerebral blood flow (CBF) velocity in the middle cerebral arteries and arterial blood pressure were recorded with transcranial Doppler sonography and with a non-invasive finger-cuff method, respectively. The ratio of arterial blood pressure to mean blood velocity (ABP/Vm) and Pulsatility Index (PI) were calculated throughout each trial. CPT induced an increase in mean ABP (range 2-54 mmHg depending on the subject) and only a slight, though significant, increase in blood velocity in the middle cerebral artery (+2.4 and +4.4% on ipsi- and contralateral side, respectively). During CPT, the ratio ABP/Vm increased and PI decreased in all subjects on both sides. These changes began simultaneously with the increase in blood pressure. The increase in ABP/Vm ratio is attributed to an increase in the cerebrovascular resistance, while the concomitant reduction in PI is interpreted as due to the reduction in the compliance of the middle cerebral artery. The results suggest that generalised increases in the sympathetic discharge, causing increases in ABP, can prevent concomitant increases in CBF by acting on both small resistance and large compliant vessels. This effect is also present when a slight increase in blood pressure occurs, which suggests a moderate increase in the sympathetic discharge, i.e. when ABP remains far below the upper limit of CBF autoregulation.

Cerebral hemodynamics in young hypertensive subjects and effects of atenolol treatment

The aim of this study was to evaluate changes in cerebral hemodynamics in young patients with uncomplicated hypertension before and after effective antihypertensive treatment with a beta-blocker drug. Changes in mean flow velocity in the middle cerebral artery from normal condition to hypercapnia were evaluated by means of a transcranial Doppler in 42 hypertensive patients and 21 healthy subjects comparable for age and sex distribution. We obtained hypercapnia with breath-holding and evaluated cerebrovascular reactivity with the breath-holding index (BHI). After a baseline evaluation (time 0), patients were randomly assigned to a placebo (group 1) or atenolol (group 2) therapy. The evaluation was repeated after 30 (time 1) and 60 (time 2) days of treatment. Before treatment, hypertensive patients had significantly lower BHI values (0.96 +/- 0.1 group 1 and 0.85 +/- 0.3 group 2) than controls (1.69 +/- 0.4) (P < 0.0001). During treatment, mean blood pressure significantly decreased in group 2 patients. In the same group, BHI values significantly increased with respect to the pre-treatment evaluation: 1.39 +/- 0.2 at time 1 and 1.44 +/- 0.2 at time 2 (P < 0.0001). On the contrary, mean blood pressure and BHI values remained unchanged in the placebo group. Furthermore, BHI values were significantly higher in group 2 than in group 1 patients at times 1 (P < 0.001) and 2 (P < 0.0001). These findings suggest that hypertension causes reduced capability of cerebral vessels to adapt to functional changes. This condition, which is reversible after treatment, could be implicated in the increased susceptibility to ischemic stroke in hypertension.

Critical cerebral perfusion pressure during tepid heart operations in dogs

BACKGROUND

The management of blood pressure during cardiopulmonary bypass varies widely. This may be particularly relevant with the trend to warmer bypass temperatures and an older patient population. Therefore, we examined the minimal perfusion pressure that maintains cerebral oxygen delivery during cardiopulmonary bypass at 33 degrees C.

METHODS

Ten dogs were placed on bypass and body temperature was reduced to 33 degrees C (alpha-stat pH management). At six randomly ordered mean arterial blood pressures (35, 40, 45, 50, 60, and 70 mm Hg), cerebral blood flow, oxygen delivery, and metabolic rate were determined.

RESULTS

Cerebral oxygen delivery was stable if the mean arterial pressure was greater than or equal to 60 mm Hg. If mean arterial pressure was less than or equal to 50 mm Hg, cerebral oxygen delivery decreased, and at less than 45 mm Hg cerebral ischemia was seen.

CONCLUSIONS

In a dog without vascular disease, the brain becomes perfusion pressure-dependent at a mean arterial pressure of approximately 50 mm Hg. There is no leftward shift of the cerebral autoregulatory curve during bypass at 33 degrees C. Greater support of mean arterial pressure during "tepid" cardiopulmonary bypass is indicated in the current adult surgical population that is older and has vascular comorbidity.

Comparative reactivity and mechanical properties of human isolated internal mammary and radial arteries

OBJECTIVE

The aim of this study was to analyse the arterial wall mechanics and the vasoreactive properties of the radial artery in comparison with those of the internal mammary artery and to discuss their implications for coronary bypass grafts.

METHODS

Measurements of pressure and diameter were obtained from cylindrical segments, whereas measurements of reactivity were obtained from ring segments from the same arteries. We used an echo-tracking technique of high resolution enabling to investigate, in vitro, the diameter and the wall thickness of arterial cylindrical segments. Furthermore, the compliance, distensibility and incremental elastic modulus of the radial and of the mammary arteries were determined for a wide range of transmural pressure (0-200 mmHg) in presence and absence of norepinephrine (NE).

RESULTS

Our results show that NE caused vasoconstriction of the two arteries. Strain was found significantly higher for the radial artery than for the internal mammary artery at any given value of stress both in the presence and in the absence of NE. In presence of NE, compliance for radial artery, in the overall transmural pressure range, did not change, whereas, distensibility was significantly increased and the elastic modulus was significantly decreased. Under the same conditions, the distensibility of the mammary artery tended to decrease and its elastic modulus to increase. In parallel, the vasoreactive properties of the two arteries confirmed the previous results showing that radial artery developed a significant higher tension to vasoconstricting agents (KCl, NE and phenylephrine (PHE)) and higher relaxation to isradipine than internal mammary artery. Moreover, radial artery displayed a lesser sensitivity to sodium nitroprusside than internal mammary artery. Furthermore, sensitivity to NE was found to be 7-fold higher for radial artery than for internal mammary artery.

CONCLUSION

Taken together, data on the mechanical and reactive properties of radial and internal mammary arteries show why the radial artery displayed a higher potential for spasm than the internal mammary artery and why the use of Ca2+ channel blocker can decrease the incidence of occlusion and spasm.

Relaxant effects of vasodilator peptides on isolated basilar arteries from stroke-prone spontaneously hypertensive rats

1. The relaxant of vasodilator peptides were examined in ring preparations of basilar arteries from stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto (WKY) rats. 2. Vasoactive intestinal peptide and peptide histidine isoleucine produced similar endothelium-independent relaxations in basilar arteries from WKY rats and SHRSP. Calcitonin gene-related peptide (CGRP) elicited endothelium-independent relaxations in both groups and the CGRP-induced relaxation was greater in SHRSP than in WKY rats. Substance P and neurokinin A did nor relax basilar arteries from either group. 3. Both WKY rat and SHRSP basilar arteries were relatively insensitive to atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide. 4. Bradykinin (BK) potently relaxed basilar arteries with endothelium, but BK produced contractions in endothelium-rubbed arteries in both WKY rats and in SHRSP. There was no significant difference in the relaxant response to BK between WKY rat and SHRSP arteries. 5. Adrenomedullin (AM) produced endothelium-independent relaxations in both groups and the relaxant response to AM was significantly greater in SHRSP than in WKY rats. 6. Human CGRP(8-37;mumol/L), a CGRP receptor antagonist, significantly inhibited both relaxant responses induced by CGRP and AM in WKY rats and in SHRSP arteries. 7. Among various peptides tested, the responses to CGRP and AM were higher in basilar arteries from SHRSP than in those from WKY rats. The relaxation produced by AM may be via CGRP receptors in WKY rat and SHRSP basilar arteries.

Decreased CGRP level with increased sensitivity to CGRP in the pial arteries of spontaneously hypertensive rats

It was aimed to investigate the importance of calcitonin gene-related peptide (CGRP) in maintenance of normal cerebral microcirculation. We examined both the functional (in vivo) and biochemical effects (in vitro) of CGRP on the pial arteries of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). When mock cerebrospinal fluid containing capsaicin (3 x 10(-7) M) was suffused over the cortical surface, the diameter of pial arteries of SHR was transiently increased and rapidly returned to the baseline level, while the capsaicin-induced increase in pial arterial diameters of WKY was large and sustained for a longer duration (> 10 min). Capsaicin-induced vasodilation was significantly attenuated by pretreatment with CGRP8-37, a CGRP1, receptor antagonist, in both WKY and SHR. On the other hand, cortical suffusion with CGRP (10(-9) approximately 10(-6) M) exerted a larger enhancement in the vasodilation of pial artery of SHR than WKY. The CGRP-induced vasodilation was significantly antagonized by CGRP8-37 in both WKY and SHR. The released level of CGRP-like immunoreactivity (CGRP-LI) from the pial artery was significantly lower in SHR (12.3 +/- 1.2 fmol/mm2/hr) than that in WKY (24.5 +/- 3.9 fmol/mm2/hr). CGRP (10(-6) M)-induced stimulation of cyclic AMP formation was rather larger in the pial arteries from SHR (50.2 +/- 5.8 fmol/mm2/30 min, p < 0.05) than those from WKY (34.5 +/- 3.8 fmol/mm2/30 min). These data suggest that, in the pial arteries of SHR, the transient vasodilation to capsaicin and enhanced vasodilation to CGRP are related to the decreased CGRP level in the cerebral microvascular beds, consequently leading to increased sensitivity of the CGRP receptors to CGRP.

Arterial wall remodelling and stiffness in hypertension: heterogeneous aspects

1. Hypertension is associated with hypertrophy and decreased operating distensibility of the large artery wall. Because similar pathological and functional changes are observed with ageing, hypertension is often looked upon as an accelerated form of aging. 2. Considering the decrease in arterial distensibility observed with both ageing and hypertension, whether the change is due to age, an increase in distending pressure or to hypertension-induced changes in large artery structural properties may be much debated. 3. The purpose of the present review is to study the effects of aging and hypertension on structural (lumen diameter and arterial wall thickness) and functional (distensibility) properties of large central and medium-sized arteries in humans. 4. From clinical studies in subjects with hypertension with or without advanced renal disease, it is suggested that age- and hypertension-induced structural arterial changes are quite heterogeneous, depending on the topography of the vessel and on the severity of the underlying disease.

Effects of ramipril and enalapril on cerebral blood flow in elderly patients with asymptomatic carotid artery occlusive disease

We wished to determine in asymptomatic hypertensive patients with > or = 70% stenosis of an internal carotid artery the change in regional cerebral blood flow (rCBF) produced by ramipril and enalapril and to evaluate the influence of age on drug-induced changes in rCBF. In a prospective, randomized, single-blind, placebo-controlled investigation, using the 133Xenon inhalation technique, we assessed baseline rCBF in 15 patients (9 men and 6 women aged 60-79 years) after a 24-h antihypertensive drug-free period. All patients then received a single 5-mg oral dose of ramipril, enalapril, or placebo. rCBF was reassessed 2 h postdose. There was no significant change in the median rCBF in any of the three treatment groups. Neither did we observe any lateralization of BF to any specific cerebral region in any of the three groups. There were no observed or patient-reported adverse events (AE). Single 5-mg oral doses of either ramipril or enalapril did not decrease CBF significantly in asymptomatic hypertensive patients with > or = 70% stenosis of an internal carotid artery and are probably safe starting doses in such patients. In addition, this effect was not modified by age.

Sympathetically-induced changes in microvascular cerebral blood flow and in the morphology of its low-frequency waves

The effect of bilateral cervical sympathetic nerve stimulation on microvascular cerebral blood flow, recorded at various depths in the parietal lobe and in ponto-mesencephalic areas, was investigated by laser-Doppler flowmetry in normotensive rabbits. These areas were chosen as representative of the vascular beds supplied by the carotid and vertebro-basilar systems, which exhibit different degrees of sympathetic innervation, the former being richer than the latter. Sympathetic stimulation at 30 imp/s affects cerebral blood flow in 77% of the parietal lobe and in 43% of the ponto-mesencephalic tested areas. In both cases the predominant effect was a reduction in blood flow (14.7 +/- 5.1% and 4.1 +/- 2.4%, respectively). The extent of the reduction in both areas was less if the stimulation frequency was decreased. Sometimes mean cerebral blood flow showed a small and transient increase, mainly in response to low-frequency stimulation. The morphology was analysed of low-frequency spontaneous oscillations in cerebral blood flow, attributed to vasomotion. Present in 41% of the tested areas (frequency 4-12 cycles/min, peak-to-peak amplitude 10-40% of mean value), these waves decreased in amplitude and increased in frequency during sympathetic stimulation, irrespective of changes in mean flow. The possibility has been proposed that the sympathetic action on low-frequency spontaneous oscillations may contribute to the protective influence that this system is known to exert on the blood-brain barrier in hypertension.

Long-term time course of regional changes in cholinergic indices following transient ischemia in the spontaneously hypertensive rat brain

Using an animal model of forebrain ischemia in spontaneously hypertensive rats (SHR) by 3-h bilateral carotid occlusion, and various indices of the cerebral cholinergic system were assessed for periods up to 24 weeks. The lesions observed histologically in the hippocampus of SHR 2 weeks after ischemia were less severe than those in the frontal cortex. Marked elevation of acetylcholine concentration was transiently observed in the frontal cortex, hippocampus and thalamus + midbrain at 2 weeks, and in the striatum at 1-4 weeks after ischemia. Choline acetyltransferase activity remained unchanged in all regions throughout the experimental period except for a minimal decrease in the frontal cortex at 4 weeks. Choline esterase (ChE) activity was slightly decreased in the frontal cortex at 2-4 weeks after ischemia but recovered by 8 weeks. A decrease in the hippocampus was seen at 8 weeks. The B(max) for the M1-receptor was significantly reduced by 2 weeks in the frontal cortex and by 4 weeks in the hippocampus. Low B(max) values in both regions persisted through week 24. These delayed hippocampal changes in the ChE activity and M1-receptor in SHR were similar to those of the very much delayed changes in M1-receptor previously reported in the gerbil model for transient ischemia. In contrast, Wistar-Kyoto rats (WKY), used as normotensive controls, exhibited no histological or biochemical changes for up to 24 weeks. The difference between SHR and WKY may depend on the more severe cerebral blood flow depletion during carotid ligation in the former. The chronic state of SHR after the transient ischemia may be a useful pathophysiological model for human cerebral infarctions with hypertension.

Cerebral microregional oxygen balance during chronic versus acute hypertension in middle cerebral artery occluded rats

This study was performed to compare microregional 0(2) supply and consumption balance in spontaneously hypertensive rats (SHR), normotensive Wistar Kyoto rats (WKY), and in phenylephrine-induced acutely hypertensive WKY (WKY + ph) rats. Under isoflurane anesthesia, a middle cerebral artery (MCA) of SHR (n = 7) and WKY (n = 14) rats was occluded. Seven of the WKY rats were infused with phenylephrine (WKY + ph) to keep the mean arterial pressure (MAP) at the same level as that of the SHR. In all animals, 1 h after MCA occlusion, regional cerebral blood flow (rCBF) was determined using an autoradiographic technique, and microregional arterial and venous 02 saturations were determined using microspectrophotometry. MAP was 76 +/- 4 (SD), 136 +/- 15, and 132 +/- 12 mm Hg for the WKY, WKY + ph, and SHR groups, respectively. All variables describing regional O2 balance and rCBF were similar between the SHR and the WKY groups in the ischemic cortex as well as in the contralateral cortex. With phenylephrine infusion, rCBF of both the ischemic cortex and the contralateral cortex were increased in the WKY group. The average 02 supply-to-consumption ratio in the ischemic cortex was higher in the WKY + ph than in the WKY or SHR group. In the ischemic cortex, heterogeneity of venous 02 saturation (SvO2), expressed as a coefficient of variation (CV = 100 X SD/mean), was significantly lower in the WKY + ph (18.3 +/- 2.4) group than in the SHR (30.5 +/- 11.8) or in the WKY (31.3 +/- 9.0) group. The number of veins with low 02 saturation (SvO2 < 40%) in the ischemic cortex was significantly lower in the WKY + ph than in the SHR or in the WKY group. Our data suggest that in chronically hypertensive animals, cerebrovascular adaptations enable the microregional 02 balance in focal ischemia to be maintained at a level similar to that of normotensive animals. However, in normotensive animals with focal cerebral ischemia, an acute increase of MAP improves microregional O2 balance.