Regional cerebral blood flow in conscious stroke-prone spontaneously hypertensive rats

Assessment of oxidative stress and antioxidant property using electron spin resonance (ESR) spectroscopy

The pathophysiology of hypertension or stroke is associated with an excess of ROS generation in the vascular system, and results in induction of various pathological cascades of cerebrovascular damage. We have demonstrated that electron spin resonance methods using a spin trap or spin probe will be useful for understanding redox status under conditions of oxidative stress in the spontaneously hypertensive rat or stroke-prone spontaneously hypertensive rat brain. We have used electron spin resonance imaging and noninvasive L-band electron spin resonance to characterize the higher degree of brain oxidative stress in the stroke-prone spontaneously hypertensive rat and spontaneously hypertensive rat than in the Wistar-Kyoto rat brain, and the lower extent of oxidative stress in the spontaneously hypertensive rat than in the stroke-prone spontaneously hypertensive rat brain. Indeed, we may be able to confirm propofol medium-chain triglyceride/long-chain triglyceride (MCT/LCT) as neuroprotective anesthesia and crocetin as antioxidant food factor against human stroke after screening for antioxidant properties in stroke models such as stroke-prone spontaneously hypertensive rat. Thus, our electron spin resonance biomedical application suggests that it could be used to assess antioxidant effects on oxidative stress in the brain using spontaneously hypertensive rat and stroke-prone spontaneously hypertensive rat. We hope that further advances in the instrumentation used for electron spin resonance imaging and the development of optimized nontoxic spin probes will make this technology even more promising for novel clinical prediction or noninvasive diagnosis of human stroke. After screening drugs or foods for antioxidant property using in vitro or in vivo electron spin resonance assessment, it will be possible to find and develop novel drugs or food factors with such properties for the prevention of stroke in the near future.

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.

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.

Structural alterations of tight junctions are associated with loss of polarity in stroke-prone spontaneously hypertensive rat blood-brain barrier endothelial cells

The mechanisms leading to stroke in stroke-prone spontaneously hypertensive rats (SHRSP) are not well understood. We tested the hypothesis that the endothelial tight junctions of the blood-brain barrier are altered in SHRSP prior to stroke. We investigated tight junctions in 13-week-old SHRSP, spontaneously hypertensive stroke-resistant rats (SHR) and age-matched Wistar-Kyoto rats (WKY) by electron microscopy and immunocytochemistry. Ultrathin sections showed no difference in junction structure of cerebral capillaries from SHRSP, SHR and WKY, respectively. However, using freeze-fracturing, we observed that the blood-brain barrier specific distribution of tight junction particles between P- and E-face in WKY (58.7+/-3.6%, P-face; 41.2+/-5.59%, E-face) and SHR (53.2+/-19. 3%, P-face; 55.6+/-13.25%, E-face) was changed to an 89.4+/-9.9% predominant E-face association in cerebral capillaries from SHRSP. However, the expression of the tight junction molecules ZO-1, occludin, claudin-1 and claudin-5 was not changed in capillaries of SHRSP. Permeability of brain capillaries from SHRSP was not different compared to SHR and WKY using lanthanum nitrate as a tracer. In contrast, analysis of endothelial cell polarity by distribution of the glucose-1 transporter (Glut-1) revealed that its abluminal:luminal ratio was reduced from 4:1 in SHR and WKY to 1:1 in endothelial cells of cerebral capillaries of SHRSP. In summary, we demonstrate that early changes exist in cerebral capillaries from a genetic model of hypertension-associated stroke. We suggest that a disturbed fence function of the tight junctions in SHRSP blood-brain barrier endothelial cells may lead to subtle changes in polarity. These changes may contribute to the pathogenesis of stroke.

Hemodynamics and metabolism in stroke-prone spontaneously hypertensive rats before manifestation of brain infarcts

Genomic screening of hybrids from stroke-prone (SHR-SP) and stroke-resistant spontaneously hypertensive rats (SHR) identified a STR1 locus on the rat chromosome 1, which correlates with the susceptibility to cerebral stroke but not with hypertension. The authors examined whether this genetic abnormality is associated with hemodynamic or metabolic alterations in the brain that can be detected before the manifestation of brain infarction. Starting at 6 weeks of age, SHR-SP were fed with a salt-rich diet to accelerate arterial hypertension. At the age of 12 weeks, animals developed functional symptoms and were age-matched with symptom-negative SHR-SP to differentiate between presymptomatic and postsymptomatic changes. Brains were investigated by multiparametric imaging comprising quantitative double-tracer autoradiography of CBF and cerebral protein synthesis (CPS); bioluminescence imaging of regional ATP, glucose, and lactate content; and umbelliferone fluoroscopic imaging of tissue pH. None of the animals exhibited focal hemodynamic or biochemical abnormalities. In symptom-negative SHR-SP, global CBF was 1.1+/-0.3 mL x g(-1) x min(-1), cortical CPS was 10.1+/-3.1 nmol x g(-1) x min(-1), and cortical ATP, glucose, lactate, and pH levels were in the normal range. In SHR-SP with functional symptoms, ATP, glucose, and lactate levels also were normal, but tissue pH exhibited periventricular alkalosis, CBF was significantly reduced to 0.7+/-0.2 mL x g(-1) x min(-1) (P < 0.001), and cortical CPS was significantly reduced to 6.7+/-2.1 nmol x g(-1) x min(-1) (P < 0.001). The decline in brain perfusion of SHR-SP correlated significantly with both the severity of functional deficits and the decline of protein synthesis. Our observations demonstrate that SHR-SP had already developed functional symptoms before the manifestation of overt brain infarcts and that the symptoms are initiated by a decline in global CBF and cortical CPS. Genetic abnormalities in SHR-SP are associated with a diffuse vascular process that results in global decompensation of blood flow well before the onset of focal brain infarction.

Effects of carbon dioxide inhalation on cerebral blood flow and oxygen tissue level in spontaneously hypertensive rabbits

BACKGROUND AND PURPOSE

Because previous studies have yielded conflicting results, this study was designed to investigate the efficiency of cerebrovascular reactivity to carbon dioxide in hypertension associated with moderate diffuse cerebral ischemic lesions.

METHODS

The effects of carbon dioxide inhalation on mean arterial blood pressure, heart and respiration rates, cerebral cortical blood flow, polarographically detected oxygen currents (oxygen availability), and cerebral electrical activity were compared in 14 spontaneously hypertensive and 16 normotensive rabbits anesthetized with urethane and alpha-chloralose. Blood flow was measured with the hydrogen clearance and thermal clearance methods.

RESULTS

In the resting state the frequency of electrical activity shifted to slower components, the levels of oxygen availability and cerebral blood flow were lower (p less than 0.01), and the ratio of the two latter parameters was greater (p less than 0.01) in hypertensive rabbits than in normotensive animals. Carbon dioxide inhalation induced more marked increases in cerebral blood flow, respiration rate, and oxygen availability in hypertensive (p less than 0.01) than in normotensive (p less than 0.05) rabbits. The ratio of oxygen availability to cerebral blood flow decreased (p less than 0.01) in the former and did not change significantly in the latter group. The carbon dioxide-induced rise in blood flow was also slower and more protracted in hypertensive rabbits (p less than 0.01). Histological investigation revealed groups of neurons with ischemic changes in the cortex of the hypertensive rabbits.

CONCLUSIONS

We suggest that in hypertensive rabbits the mild multiple ischemic lesions are the basis of functional disturbances, including reduced resting cerebral blood flow, greater oxygen tissue level, slower response to carbon dioxide, and greater vasodilatory capacity.

Effects of local reduction in pressure on distensibility and composition of cerebral arterioles

This study examined effects of local reductions in mean and pulse pressures on cerebral arterioles in normotensive Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). WKY and SHRSP underwent clipping of one carotid artery at 1 month of age. At 10-12 months of age, mechanics of pial arterioles were examined in vivo in anesthetized rats. Bilateral craniotomies were performed to expose pial arterioles in the sham and clipped cerebral hemispheres. Stress-strain relations were calculated from measurements of pial arteriolar pressure (servo null), diameter, and cross-sectional area of the arteriolar wall. Point counting stereology was used to quantitate individual components in the arteriolar wall. Before deactivation of smooth muscle with EDTA, mean (Pm) and pulse (Pp) pressures were significantly less (p less than 0.05) in clipped than in sham arterioles in WKY (Pm, 63 +/- 2 versus 73 +/- 2 mm Hg; Pp, 23 +/- 3 versus 30 +/- 3 mm Hg) and SHRSP (Pm, 94 +/- 4 versus 110 +/- 4 mm Hg; Pp, 27 +/- 2 versus 38 +/- 3 mm Hg). Cross-sectional area of the arteriolar wall was less (p less than 0.05) in clipped than in sham arterioles in both groups of rats (1,403 +/- 125 versus 1,683 +/- 125 microns2 in WKY; 1,436 +/- 72 versus 1,926 +/- 134 microns2 in SHRSP). There was a correlation between cross-sectional area of the vessel wall and pulse pressure (r2 = 0.66), but not mean pressure (r2 = 0.09). During maximal dilatation with EDTA, the stress-strain curve was shifted to the left in clipped arterioles of SHRSP, but not of WKY, which indicates that carotid clipping in SHRSP reduces passive distensibility of cerebral arterioles. The proportion of distensible components in the vessel wall (smooth muscle, elastin, and endothelium) was reduced in clipped arterioles in SHRSP, but not in WKY. These findings suggest that 1) vascular hypertrophy of cerebral arterioles is related more closely to pulse pressure than to mean pressure, and 2) reduction of pial arteriolar pressure completely prevents cerebral vascular hypertrophy and attenuates increases in passive distensibility of cerebral arterioles in SHRSP.

Post-ischaemic treatment with the prostacycline analogue TTC-909 reduces ischaemic brain injury

The effects of stable PGI analogue TTC-909 on CBF and glucose metabolism was studied in the chronic stage of cerebral ischaemia produced by occluding the distal MCA in SHRSP. Administration of TTC-909 (100 ng/kg/day during 7 days) prevented the development of ischaemic oedema and improved secondary metabolic derangement coupled to flow in postischaemic tissues, particularly in the ischaemic rim.

Regional cerebral blood flow in acute hypertension induced by adrenaline, noradrenaline and phenylephrine in the conscious rat

Hypertension was induced in conscious rats by intravenous infusion of phenylephrine (3, 6 or 12 micrograms kg-1 min-1), noradrenaline (3 micrograms min-1) or adrenaline (3 micrograms kg-1 min-1). Local cerebral blood flow was measured autoradiographically in 24 defined brain structures using [14C]iodoantipyrine as the diffusible tracer. The mean arterial pressure induced by adrenaline, noradrenaline and the two higher doses of phenylephrine was 158-168 mmHg with no significant differences between the groups. Only adrenaline significantly increased local cerebral blood flow in nine of the 24 structures studied. The smaller capacity for autoregulation after adrenaline compared with other drugs might be related to a beta-adrenoreceptor-stimulating effect.

The quantification of cerebral infarction following focal ischemia in the rat: influence of strain, arterial pressure, blood glucose concentration, and age

Focal cerebral ischemia was induced by occlusion of the middle cerebral artery in rats. The volumetric assessment of infarcted tissue, 2 days following occlusion, was calculated from the examination of eight preselected coronal sections. Five differing rat strains were examined. A small and variable infarcted volume was seen in Wistar-Kyoto rats; Sprague-Dawley rats had a relatively large, but still variable, infarcted volume. Of the normotensive rat strains, the most reproducible volume of infarcted tissue was seen in Fischer-344 rats; also the absolute value of the infarcted volume did not vary from one series to another in this strain. Chronic arterial hypertension, studied in both normal and stroke-prone spontaneously hypertensive rats, was associated with significantly larger infarction volumes. Age does not change the volume of necrosis: Fischer-344 rats were studied at 3, 9, and 20 months of age, and no significant differences were noted between these ages. Experimental diabetes was induced by the administration of streptozotocin 3 days prior to middle cerebral artery occlusion. Severe hyperglycemia (greater than 400 mg/dl) was associated with a considerably increased volume of infarction. The variability of the resultant lesion is high in the most commonly studied strains, but our results suggest that, for studies in normotensive rats, the use of the Fischer-344 strain produces a standardized and repeatable infarction that may be significantly modified by experimental interventions. Age is not a factor that affects the occlusion-induced infarction; in contrast, both chronic arterial hypertension and experimental diabetes aggravate the histological consequences of middle cerebral artery occlusion in the rat. We conclude that quantitative histological evaluation of infarct size allows a meaningful assessment of the gravity of focal cerebral ischemia.

Cerebral microangiopathy in stroke-prone spontaneously hypertensive rats. An immunohistochemical and ultrastructural study

The morphology of cerebral microvessels was studied immunohistochemically and ultrastructurally in 6- to 9-month-old normotensive Wistar-Kyoto rats (WKY), spontaneously hypertensive rats (SHR), and stroke-prone SHR (SHRSP) with a systolic blood pressure of 138 +/- 15 mm Hg, 189 +/- 9 mm Hg, and 258 +/- 30 mm Hg respectively. Regions with major opening of the blood-brain barrier (BBB) were revealed by an i.v. injection of Evans Blue. Multifocal BBB opening with massive leakage of plasma constituents rich in fibrinogen-fibrin-related antigen occurred in SHRSP with a blood pressure above 210-220 mm Hg. BBB-leakage sites were found in the cerebral cortex and the basal ganglia, most frequently in the arterial border zones. The perivascular tissue spaces were dilated within the BBB-leakage sites, in particular around arterioles. Damaged endothelial and smooth muscle cells were replaced by fibrin-like material, multiple layers of basement membranes and bundles of collagen fibrils surrounded by proliferated fibroblasts. The degenerative-infiltrative-proliferative disease process transformed short segments of single arterioles into severely thickened, tortuous and stenotic vessels. Fibrinoid degeneration, formation of microaneurysms and fibrin-rich vascular occlusions were observed. In contrast, only minor or no vascular alterations were seen in regions with preserved BBB in SHRSP and SHR. A severely increased intraluminal pressure load appears to be of major pathogenetic importance for breakdown of the BBB and initiation of the vascular disease process in SHRSP. However, since only short segments of a limited number of widely separated vessels are severely affected, and the number of affected vessels increase towards arterial end and border zones, additional predisposing and aggravating factors may play significant roles in the development of fibrinoid vascular lesions in arterial hypertension.

Pressure distribution in the pial arterial system of rats based on morphometric data and mathematical models

The objective of the present work was a theoretical evaluation of pial arterial pressures in normotensive rats and spontaneously hypertensive rats based on the geometry and topography of the pial arterial system as well as on various topological models of the vascular trees. Pial branches of the middle cerebral artery in the diameter range of 30-320 microns were selectively visualized by corrosion compound, and the diameter and length of vascular segments were measured. The vessels were classified into branching orders by the methods of Horsfield and Strahler. The steady-state pressure distribution in the pial arterial system was calculated assuming that the flow at the bifurcations was partitioned in proportion to a given power of the diameters of the daughter branches (diameter exponent). The maximum number of vascular segments along the longest branch varied between 16 and 33. The mean branching ratio was 4.14 +/- 0.23 (SD). The mean diameter of vessels classified into Strahler orders 1-5 were: 50 +/- 12, 71 +/- 19, 106 +/- 22, 168 +/- 22, and 191 +/- 7 microns, respectively. The calculated pressure drop in the pial trees of normotensive rats was approximately twice as large in proximal orders 3 and 4 than in distal orders 1 and 2. The mean pressure in arteries of order 1 ranged from 54.4 to 58.4 mm Hg in the normotensive rat (input pressure: 83 mm Hg), and from 77.2 to 89.0 mm Hg in the spontaneously hypertensive rat (input pressure: 110 mm Hg). The coefficient of variation of terminal pressures in vessels of order 1 increased linearly with the mean pressure drop in the system. The coefficient of variation in terminal pressure had a minimum as a function of the diameter exponent in case of each pial tree. At its minimum, it was higher in all spontaneously hypertensive rats (10.1-22.9%) than in any normotensive rats (6.0-8.5%). The corresponding diameter exponents were in most cases lower in the spontaneously hypertensive rat (1.3-2.5) than in the normotensive rat (2.5-3.0). Topologically consistent models of the pial arterial network predicted significantly less variation in intravascular pressures than was obtained by direct calculations. More idealized models suggested the dependence of coefficient of variation in terminal pressure on the total number of vascular segments contained by the tree. All models predicted the existence of the minimum of coefficient of variation in terminal pressure in function of the diameter exponent.(ABSTRACT TRUNCATED AT 400 WORDS)

A method for recording effects of anti-epileptic drugs on interictal discharge in the cat's cerebral cortex. Factors determining the distribution of external carotid artery infusions

The method utilizes infusion via the external carotid (ECA), the internal maxillary arteries and their anastomoses to the cerebral circulation. It takes into account the ipsilateral distribution of the carotid blood supply. A regular interictal epileptiform spiking from foci on both hemispheres was provided by local application to the cortical surface of small pieces of filter paper soaked in sodium benzylpenicillin, 100,000 IE ml-1. The infused drug affects the ipsilateral foci, and the contralateral one functions as a simultaneous untreated control. The stability of the interictal frequency and the effect of non-oxygen carrying solvents are described. The effect of changes in blood pressure, temperature and PCO2 are considered as well as the coupling between activity in ipsi- and contralateral foci. Experiments with infused radioactive microspheres were performed to determine the strictness of the ipsilateral distribution and the conditions under which it was upheld. With mean arterial blood pressures between 70 mm Hg and 170 mm Hg and infusion speeds between 1.0 ml min-1 and 6.3 ml min-1 the distribution to the contralateral cerebral hemisphere was 0.3% (SD 0.2, SEM 0.1). Infusions of [125I]albumin were used to determine the blood flow in ECA. The flow varied between 20 ml min-1 and 68 ml min-1. The higher values were seen when the extracerebral shunting was high. Conditions influencing the dilution of the infusion and its distribution within the brain were investigated. Important factors were carotid and cerebral blood flow, arterial blood pressure, speed and duration of the infusion, recirculation and cerebral temperature. Arterial PCO2, pH and PO2 should be carefully controlled. Computer-supported treatment of interictal spike frequency and amplitude, as well as of circulatory and respiratory parameters, was utilized. The method was tested in experiments with infusions of 5 alpha-pregnanolone. It was shown that infusions, shorter than the estimated circulation time, reduced the interictal spike frequency and amplitude recorded from the ipsilateral foci without effects on the contralateral ones.

Pentoxifylline: cerebral blood flow and glucose utilization in conscious spontaneously hypertensive rats

Pentoxifylline, 0.30 mg/kg/min, significantly reduced cerebral blood flow by 10-44% in 19 of 23 regions studied in conscious spontaneously hypertensive rats. Bilateral ligation of the common carotid arteries reduced cerebral blood flow to 24-46% of resting values in 20 structures; a further reduction to 10-27% of resting values was seen after pentoxifylline in 10 cortical or subcortical structures. Thus, in conscious hypertensive rats, there is no evidence that pentoxifylline redistributes blood flow from normal to low flow brain regions. Pentoxifylline did not reduce the metabolic rate of glucose.

The morphometry of consecutive segments in cerebral arteries of normotensive and spontaneously hypertensive rats

The media cross-sectional area, the media thickness, the internal radius and the ratio between media thickness and internal radius were determined in consecutive sections of extraparenchymal cerebral arteries of 7- and 12-month-old normotensive and spontaneously hypertensive rats. The study included intracranial pial and basal arteries as well as extracranial cervical arteries. In the chronically hypertensive rats the media to radius ratio was consistently higher than in normotensive rats over the entire calibre spectrum investigated (radius 5-400 micron). The increase of the ratio in the extracranial arteries of the hypertensive rats was exclusively due to a thicker media. In the basal intracranial arteries the increase of ratio was due to a thicker media and/or a smaller internal radius in 7- and 12-month-old rats with moderate hypertension (mean arterial pressure, MAP 171 +/- 8 and 177 +/- 7 mm Hg respectively). In 7-month-old rats with severe hypertension (MAP 204 +/- 11 mm Hg) the increase of ratio was mainly due to a smaller internal radius. The observed structural alterations are likely to be of hemodynamic importance.

Reaction of pial arteries and veins to hypercapnia in hypertensive and normotensive rats

The lumen diameters of the main cortical surface arteries were continuously monitored through a closed cranial window in spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY). The arterial diameter was significantly smaller in SHR (55 +/- 1 micron) than in WKY (87 +/- 1 micron) during resting conditions as well as during hypercapnic dilatation (87 +/- 2 micron compared to 117 +/- 5 micron). The per cent increase in diameter induced by hypercapnia was larger in SHR (54%) than in WKY (36%), presumably a consequence of the altered vascular wall to lumen ratio. Alpha-adrenoreceptor blockade with yohimbine and phenoxybenzamine had no significant effect on arterial diameter during hypercapnia. The diameters of the largest pial surface veins increased to the same extent in SHR and WKY during hypercapnia (about 10%).