Mechanisms of impaired endothelium-dependent cerebral vasodilatation in response to bradykinin in hypertensive rats

Endothelial Cells and the Cerebral Circulation

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

Retinal Vascular Pathology in a Rat Model of Cerebral Small Vessel Disease

Introduction: The initial disease stages of hypertensive arteriopathy (HA) and cerebral amyloid angiopathy (CAA), the two main forms of sporadic human cerebral small vessel diseases (CSVD), are too subtle to be detectable on clinical routine imaging. Small vessel disease (SVD) is a systemic condition, affecting not only the brain, but also other organs. The retina appears as an ideal marker for the early detection of incipient CSVD. We therefore investigated the retinal microvasculature of the spontaneously hypertensive stroke-prone rat (SHRSP), an animal model of sporadic CSVD.

Materials and Methods: The brains and retinas of 26 male SHRSP (18–44 weeks) were examined histologically and immunohistochemically for the presence of HA phenomena (erythrocyte thrombi, small perivascular bleeds) and amyloid angiopathy (AA).

Results: CAA and AA in the retina showed a significant correlation with age (CAA: rho = 0.55, p = 0.005; AA: rho = 0.89, p < 0.001). The number of erythrocyte thrombi in the brain correlated with the severity of retinal erythrocyte thrombi (rho = 0.46, p = 0.023), while the occurrence of CAA correlated with the appearance of AA in the retina (rho = 0.51, p = 0.012). Retinal SVD markers predicted CSVD markers with good sensitivity.

Conclusions: These results indicate that SVD also occurs in the retinal microvasculature of SHRSP and the prediction of cerebral erythrocyte thrombi and CAA might be possible using retinal biomarkers. This underlines the important role of the investigation of the retina in the early diagnosis of CSVD.

The importance of comorbidities in ischemic stroke: Impact of hypertension on the cerebral circulation

Comorbidities are a hallmark of stroke that both increase the incidence of stroke and worsen outcome. Hypertension is prevalent in the stroke population and the most important modifiable risk factor for stroke. Hypertensive disorders promote stroke through increased shear stress, endothelial dysfunction, and large artery stiffness that transmits pulsatile flow to the cerebral microcirculation. Hypertension also promotes cerebral small vessel disease through several mechanisms, including hypoperfusion, diminished autoregulatory capacity and localized increase in blood-brain barrier permeability. Preeclampsia, a hypertensive disorder of pregnancy, also increases the risk of stroke 4-5-fold compared to normal pregnancy that predisposes women to early-onset cognitive impairment. In this review, we highlight how comorbidities and concomitant disorders are not only risk factors for ischemic stroke, but alter the response to acute ischemia. We focus on hypertension as a comorbidity and its effects on the cerebral circulation that alters the pathophysiology of ischemic stroke and should be considered in guiding future therapeutic strategies.

Blood pressure and cognition among older adults: a meta-analysis

Hypertension has adverse effects on cognition, can alter cerebral vasculature integrity, and is associated with the pathogenesis of dementia. Using meta-analysis, we correlated blood pressure to multiple cognitive domains among older adults free of clinical stroke and dementia. We identified 230 studies indexed in PubMed and PsycINFO relating blood pressure and cognition. After applying exclusion criteria, we selected n = 12 articles with n = 4,076 participants (age range 43-91 years). Meta-analysis yielded an association between blood pressure and episodic memory (r = -.18, p < .001) and between blood pressure and global cognition (r = -.07, p < .001). When limiting analyses to studies adjusting for vascular covariates (n = 8, n = 2,141), blood pressure was modestly related to global cognition (r = -.11, p < .001), attention (r = .14, p = .002), and episodic memory (r = -.20, p < .001) with a trend for language (r = -.22, p = .07). Findings underscore the need to manage blood pressure as a key prevention method in minimizing abnormal cognitive aging prior to the onset of clinical dementia.

Cerebral microcirculatory responses of insulin-resistant rats are preserved to physiological and pharmacological stimuli

OBJECTIVE

Previously, we have shown that IR impairs the vascular reactivity of the major cerebral arteries of ZO rats prior to the occurrence of Type-II diabetes mellitus. However, the functional state of the microcirculation in the cerebral cortex is still being explored.

METHODS

We tested the local CoBF responses of 11-13-week-old ZO (n = 31) and control ZL (n = 32) rats to several stimuli measured by LDF using a closed cranial window setup.

RESULTS

The topical application of 1-100 μm bradykinin elicited the same degree of CoBF elevation in both ZL and ZO groups. There was no significant difference in the incidence, latency, and amplitude of the NMDA-induced CSD-related hyperemia between the ZO and ZL groups. Hypercapnic CoBF response to 5% carbon-dioxide ventilation did not significantly change in the ZO compared with the ZL. Topical bicuculline-induced cortical seizure was accompanied by the same increase of CoBF in both the ZO and ZL at all bicuculline doses.

CONCLUSIONS

CoBF responses of the microcirculation are preserved in the early period of the metabolic syndrome, which creates an opportunity for intervention to prevent and restore the function of the major cerebral vascular beds.

NAC changes the course of cerebral small vessel disease in SHRSP and reveals new insights for the meaning of stases - a randomized controlled study

Background

N-Acetylcystein (NAC) reduces the reperfusion injury and infarct size in experimental macroangiopathic stroke. Here we now investigate the impact of NAC on the development of the histopathology of microangiopathic cerebrovascular disease including initial intravasal erythrocyte accumulations, blood–brain-barrier (BBB)-disturbances, microbleeds and infarcts.

Methods

Spontaneously Hypertensive Stroke-Prone Rats (SHRSP) were treated with NAC (12 mg/kg body weight, daily oral application for three to 30 weeks) and compared to untreated SHRSP. In all rats the number of microbleeds, thromboses, infarcts and stases were quantified by HE-staining. Exemplary brains were stained against von Willebrand factor (vWF), IgG, Glutathione and GFAP.

Results

NAC animals exhibited significant more microbleeds, a greater number of vessels with BBB-disturbances, but also an elevation of Glutathione-levels in astrocytes surrounding small vessels. NAC-treatment reduced the numbers of thromboses, infarcts and arteriolar stases.

Conclusions

NAC reduces the frequency of thromboses and infarcts to the expense of an increase of small microbleeds in a rat model of microangiopathic cerebrovascular disease. We suppose that NAC acts via an at least partial inactivation of vWF resulting in an insufficient sealing of initial endothelial injury leading to more small microbleeds. By elevating Glutathione-levels NAC most likely exerts a radical scavenger function and protects small vessels against extended ruptures and subsequent infarcts. Finally, it reveals that stases are mainly caused by endothelial injuries and restricted thromboses.

The pathologic cascade of cerebrovascular lesions in SHRSP: is erythrocyte accumulation an early phase?

Cerebral small vessel disease (CSVD) is associated with vessel wall changes, microbleeds, blood-brain barrier (BBB) disturbances, and reduced cerebral blood flow (CBF). As spontaneously hypertensive stroke-prone rats (SHRSP) may be a valid model of some aspects of human CSVD, we aimed to identify whether those changes occur in definite temporal stages and whether there is an initial phenomenon beyond those common vascular alterations. Groups of 51 SHRSP were examined simultaneously by histologic (Hematoxylin-Eosin, IgG-Immunohistochemistry, vessel diameter measurement) and imaging methods (Magnetic Resonance Imaging, 201-Thallium-Diethyldithiocarbamate/99m-Technetium-HMPAO Single Photon Emission Computed Tomography conducted as pilot study) at different stages of age. Vascular pathology in SHRSP proceeds in definite stages, whereas an age-dependent accumulation of erythrocytes in capillaries and arterioles represents the homogeneous initial step of the disease. Erythrocyte accumulations are followed by BBB disturbances and microbleeds, both also increasing with age. Microthromboses, tissue infarctions with CBF reduction, and disturbed potassium uptake represent the final stage of vascular pathology in SHRSP. Erythrocyte accumulations--we parsimoniously interpreted as stases--without cerebral tissue damage represent the first step of vascular pathology in SHRSP. If that initial phenomenon could be identified in patients, these erythrocyte accumulations might be a promising target for implementing prophylactic and therapeutic strategies in human CSVD.

Novel isoforms of NADPH-oxidase in cerebral vascular control

Reactive oxygen species (ROS) are thought to play an important role in the initiation and progression of a variety of vascular diseases. Furthermore, accumulating evidence indicates that ROS may also serve as important cell signalling molecules for the regulation of normal vascular function. Recently, a novel family of proteins (Nox1, 2 and 4) that act as the catalytic subunit of the superoxide (O2-) producing enzyme NADPH-oxidase has been discovered in vascular cells. There is now preliminary evidence suggesting that NADPH-oxidase-derived ROS may serve as a physiological vasodilator mechanism in the cerebral circulation. Moreover, the activity of NADPH-oxidase is profoundly greater in cerebral versus systemic arteries. Studies have shown that Nox1, Nox2 (also known as gp91phox) and Nox4 are all expressed in cerebral arteries, suggesting that multiple isoforms of NADPH-oxidase may be important for ROS production by cerebral arteries. Enhanced NADPH-oxidase activity is associated with several vascular-related diseases, including hypertension, stroke, subarachnoid haemorrhage and Alzheimer's dementia; however, the consequences of this for cerebral vascular function are controversial. For example, there is some evidence suggesting that NADPH-oxidase-derived O2- may play a role in endothelial dysfunction of cerebral arteries and a subsequent rise in cerebral vascular tone, associated with hypertension. However, activation of NADPH-oxidase elicits cerebral vasodilatation in vivo, and this mechanism is enhanced in chronic hypertension. While further supportive evidence is needed, it is an intriguing possibility that NADPH-oxidase-derived ROS may play a protective role in regulating cerebral vascular tone during disease.

Reactive oxygen species: influence on cerebral vascular tone

Reactive oxygen species have multiple effects on vascular cells. Defining the sources and the impact of the various reactive oxygen species within the vessel wall has emerged as a major area of study in vascular biology. This review will focus on recent findings related to effects of reactive oxygen species on cerebral vascular tone. Effects of superoxide radical, hydrogen peroxide, and the reactive nitrogen species peroxynitrite are summarized. Although higher concentrations may be important for cerebral vascular biology in disease, relatively low concentrations of reactive oxygen species may function as signaling molecules involved with normal regulation of cerebral vascular tone. The mechanisms by which reactive oxygen species affect vascular tone may be quite complex, and our understanding of these processes is increasing. Additionally, the role of reactive oxygen species as mediators of endothelium-dependent relaxation is addressed. Finally, the consequences of the molecular interactions of superoxide with nitric oxide and arachidonic acid are discussed.

Endothelium-derived reactive oxygen species: their relationship to endothelium-dependent hyperpolarization and vascular tone

Opinions on the role of reactive oxygen species (ROS) in the vasculature have shifted in recent years, such that they are no longer merely regarded as indicators of cellular damage or byproducts of metabolism--they may also be putative mediators of physiological functions. Hydrogen peroxide (H2O2), in particular, can initiate vascular myocyte proliferation (and, incongruously, apoptosis), hyperplasia, cell adhesion, migration, and the regulation of smooth muscle tone. Endothelial cells express enzymes that produce ROS in response to various stimuli, and H2O2 is a potent relaxant of vascular smooth muscle. H2O2 itself can mediate endothelium-dependent relaxations in some vascular beds. Although nitric oxide (NO) is well recognized as an endothelium-derived dilator, it is also well established, particularly in the microvasculature, that another factor, endothelium-derived hyperpolarizing factor (EDHF), is a significant determinant of vasodilatory tone. This review primarily focuses on the hypothesis that H2O2 is an EDHF in resistance arteries. Putative endothelial sources of H2O2 and the effects of H2O2 on potassium channels, calcium homeostasis, and vascular smooth muscle tone are discussed. Furthermore, given the perception that ROS can more likely elicit cytotoxic effects than perform signalling functions, the arguments for and against H2O2 being an endogenous vasodilator are assessed.

Biphasic effect of hydrogen peroxide on skeletal muscle arteriolar tone via activation of endothelial and smooth muscle signaling pathways

We hypothesized that hydrogen peroxide (H2O2) has a role in the local regulation of skeletal muscle blood flow, thus significantly affecting the myogenic tone of arterioles. In our study, we investigated the effects of exogenous H2O2 on the diameter of isolated, pressurized (at 80 mmHg) rat gracilis skeletal muscle arterioles (diameter of approximately 150 microm). Lower concentrations of H2O2 (10(-6)-3 x 10(-5) M) elicited constrictions, whereas higher concentrations of H2O2 (6 x 10(-5)-3 x 10(-4) M), after initial constrictions, caused dilations of arterioles (at 10(-4) M H2O2, -19 +/- 1% constriction and 66 +/- 4% dilation). Endothelium removal reduced both constrictions (to -10 +/- 1%) and dilations (to 33 +/- 3%) due to H2O2. Constrictions due to H2O2 were completely abolished by indomethacin and the prostaglandin H2/thromboxane A2 (PGH2/TxA2) receptor antagonist SQ-29548. Dilations due to H2O2 were significantly reduced by inhibition of nitric oxide synthase (to 38 +/- 7%) but were unaffected by clotrimazole or sulfaphenazole (inhibitors of cytochrome P-450 enzymes), indomethacin, or SQ-29548. In endothelium-denuded arterioles, clotrimazole had no effect, whereas H2O2-induced dilations were significantly reduced by charybdotoxin plus apamin, inhibitors of Ca(2+)-activated K+ channels (to 24 +/- 3%), the selective blocker of ATP-sensitive K+ channels glybenclamide (to 14 +/- 2%), and the nonselective K(+)-channel inhibitor tetrabutylammonium (to -1 +/- 1%). Thus exogenous administration of H2O2 elicits 1) release of PGH2/TxA2 from both endothelium and smooth muscle, 2) release of nitric oxide from the endothelium, and 3) activation of K+ channels, such as Ca(2+)-activated and ATP-sensitive K+ channels in the smooth muscle resulting in biphasic changes of arteriolar diameter. Because H2O2 at low micromolar concentrations activates several intrinsic mechanisms, we suggest that H2O2 contributes to the local regulation of skeletal muscle blood flow in various physiological and pathophysiological conditions.

Catalase has negligible inhibitory effects on endothelium-dependent relaxations in mouse isolated aorta and small mesenteric artery

1. The current study examined the hypothesis that endothelial production of hydrogen peroxide (H2O2) mediates relaxations to acetylcholine (ACh) in aorta and small mesenteric arteries (SMA) from mice. 2. Relaxations to ACh (0.01-10 microM) and H2O2 (0.1-1000 microM) were produced in aorta and SMA isolated from wild-type C57BL/6 mice and type II diabetic mice (db/db). In SMA, relaxations to ACh were produced in the presence of N omega-nitro-L-arginine methyl ester (100 microM) and indomethacin (Indo, 10 microM). 3. 1-H[1,2,4]oxadiazolo[4,3-]quinoxalin-1-one (10 microM) significantly reduced ACh-induced relaxations in SMA, abolished responses in aorta, but had no effect on relaxations induced by H2O2. Catalase (2500 U ml-1) abolished responses to H2O2, but did not alter relaxations to ACh in the SMA and only caused a small rightward shift in responses to ACh in the aorta. 4. ACh-, but not H2O2-, mediated relaxations were significantly reduced by tetraethylammonium (10 mM), the combination of apamin (1 microM) and charybdotoxin (100 nM), and 25 mm potassium chloride (KCl). Higher KCl (60 mM) abolished relaxations to both ACh and H2O2. Polyethylene glycolated superoxide dismutase (100 U ml-1), the catalase inhibitor 3-amino-1,2,4-triazole (3-AT, 50 mM) and treatment with the copper chelator diethyldithiolcarbamate (3 mM) did not affect relaxations to ACh. 5. H2O2-induced relaxations were endothelium-independent and were not affected by ethylene diamine tetraacetic acid (EDTA 0.067 mM), 4-aminopyridine (1 mM), ouabain (100 microM) and barium (30 microM), 3-AT or Indo. 6. Although the data from this study show that H2O2 dilates vessels, they do not support the notion that H2O2 mediates endothelium-dependent relaxations to ACh in either aorta or SMA from mice.

Cerebral vascular effects of reactive oxygen species: Recent evidence for a role of NADPH-oxidase

1. Reactive oxygen species (ROS) are a diverse family of molecules that are produced throughout the vascular wall. Many ROS, such as the superoxide anion (*O2-) and hydrogen peroxide (H2O2), are now known to act as cellular signalling molecules within blood vessels. In particular, these molecules can exert powerful effects on vascular tone. 2. Cerebral arteries are relatively unusual in their responsiveness to ROS. Unlike in many systemic vessels, both *O2- and H2O2 can cause vasodilatation in the cerebral microcirculation. 3. Reactive oxygen species can be produced in the vasculature via a variety of mechanisms; however, it appears that the primary source of *O2- within blood vessels is the enzyme NADPH-oxidase. 4. In cerebral vessels, activation of NADPH-oxidase causes both *O2- production and vasodilatation, indicating that NADPH-oxidase-derived ROS may have a functional role in the regulation of cerebral vascular tone. 5. Elevated levels of NADPH-oxidase activity and expression occur in cardiovascular disease states such as hypertension, atherosclerosis and subarachnoid haemorrhage. 6. Thus, ROS may contribute to the regulation of cerebral vascular tone during both physiological and pathological conditions.

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.

The Endothelium in Health and Disease-A Target for Therapeutic Intervention

In this review we discuss the contribution of NO, prostacyclin and endothelium-derived relaxing factor--endothelium-derived hyperpolarizing factor, or EDHF, to vascular function. We also explore the hypotheses (1): that tissues can store NO as nitrosothiols (RSNOs) and (2) that such RSNO stores can be modulated by physiological and pathophysiological processes. Notably in the microcirculation, EDHF appears to play an important role in the regulation of vascular tone. Leading candidates for EDHF include extracellular potassium (K+), an epoxygenase product, hydrogen peroxide and/or a contribution from myoendothelial gap junctions. Data from our laboratory indicate that in mouse vessels, different endothelium-dependent vasodilators, such as acetylcholine and protease-activated receptor (PAR) agonists, release different endothelium-derived relaxing factors. The combination of two K-channel toxins, apamin and charybdotoxin, inhibits EDHF activity in most protocols. Endothelial dysfunction is considered as the major risk factor and a very early indicator of cardiovascular disease including the cardiovascular complications of type I & types II diabetes. Impaired endothelium-dependent vasodilatation results primarily from a decreased synthesis of endothelium-derived nitric oxide (NO) and/or an increase in the production of reactive oxygen species such as superoxide. We have shown that the administration of tetrahydrobiopterin, an important co-factor for nitric oxide synthase (NOS) partially restores endothelial function (1) in leptin-deficient mice (db/db) with spontaneous type II diabetes, as well as (2) in human vascular tissue harvested for coronary artery bypass grafting (CABG). These data suggest that a deficiency in the availability of tetrahydrobiopterin plays an important role in vascular dysfunction associated with Type II diabetes. In addition, changes in the contribution of EDHF occur in vascular tissue from the db/db mice suggesting a compensatory increase in EDHF production; whether this alteration in EDHF production is physiological or pathophysiological remains controversial.

Hydrogen peroxide acts as an EDHF in the piglet pial vasculature in response to bradykinin

We investigated the mechanism of EDHF-mediated dilation to bradykinin (BK) in piglet pial arteries. Topically applied BK (3 micromol/l) induced vasodilation (62 +/- 12%) after the administration of N(omega)-nitro-L-arginine methyl ester (L-NAME) and indomethacin, which was inhibited by endothelial impairment or by the BK(2) receptor antagonist HOE-140 (0.3 micromol/l). Western blotting showed the presence of BK(2) receptors in brain cortex and pial vascular tissue samples. The cytochrome P-450 antagonist miconazole (20 micromol/l) and the lipoxygenase inhibitors baicalein (10 micromol/l) and cinnamyl-3,4-dyhydroxy-alpha-cyanocinnamate (1 micromol/l) failed to reduce the BK-induced dilation. However, the H(2)O(2) scavenger catalase (400 U/ml) abolished the response (from 54 +/- 11 to 0 +/- 2 microm; P < 0.01). The ATP-dependent K(+) (K(ATP)) channel inhibitor glibenclamide (10 micromol/l) had a similar effect as well (from 54 +/- 11 to 16 +/- 5 microm; P < 0.05). Coapplication of the Ca(2+)-dependent K(+) channel inhibitors charybdotoxin (0.1 micromol/l) and apamin (0.5 micromol/l) failed to reduce the response. We conclude that H(2)O(2) mediates the non-nitric oxide-, non-prostanoid-dependent vasorelaxation to BK in the piglet pial vasculature. The response is mediated via BK(2) receptors and the opening of K(ATP) channels.

Inhibition of copper/zinc superoxide dismutase impairs NO.-mediated endothelium-dependent relaxations

The superoxide anion (O-2.) appears to be an important modulator of nitric oxide (NO.) bioavailability. The present study was designed to characterize the role of copper/zinc superoxide dismutase (Cu/Zn SOD) in endothelium-dependent relaxations. Cu/Zn SOD was inhibited with the Cu2+ chelator diethyldithiocarbamic acid (DETCA). In isolated canine basilar arteries, DETCA (7.6 x 10(-3) M) inhibited total vascular SOD activity by 46% (P < 0.0001, n = 6-8 dogs). DETCA (7.6 x 10(-3) M) significantly reduced relaxations to bradykinin and A-23187 (P < 0.05, n = 7-11). The inhibitory effect of DETCA was abolished by the O-2. scavenger 4,5-dihydroxy-1,3-benzenedisulfonic acid (Tiron; 9.4 x 10(-3) M; P < 0.05, n = 6-13). Tiron significantly potentiated the relaxations to bradykinin in control rings (P < 0.05, n = 13), and the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (L-NAME; 3 x 10(-4) M) abolished these relaxations (P < 0.0001, n = 6). DETCA and Tiron had no effect on the relaxations to diethylamine-NONOate or forskolin (P > 0.05, n = 6). Our results demonstrate that endothelium-dependent relaxations mediated by NO. are impaired after the inhibition of Cu/Zn SOD. Relaxations to bradykinin (but not A-23187) were significantly augmented by Tiron. Pharmacological scavenging of O-2. reverses the effect of Cu/Zn SOD inhibition.

Potassium channels mediate dilatation of cerebral arterioles in response to arachidonate

We tested the hypothesis that cerebral vasodilatation in response to arachidonate is dependent on activation of cyclooxygenase and cytochrome P-450 pathways and formation of endogenous reactive oxygen species and is mediated by activation of potassium channels. The diameter of cerebral arterioles was measured using cranial windows in anesthetized rats. Under control conditions [baseline diameter = 45 +/- 1 micrometer (mean +/- SE)], arachidonate (1-100 microM) and papaverine (10-50 microM) produced concentration-dependent vasodilatation. Cerebral vasodilator responses to arachidonate, but not papaverine, were abolished during topical application of indomethacin (10 microM, an inhibitor of cyclooxygenase) or catalase (100 U/ml, which inactivates hydrogen peroxide). In contrast, clotrimazole (10 microM) and 17-ODYA (20 microM), inhibitors of cytochrome P-450 activity, had no effect on dilator responses of cerebral arterioles to arachidonate. Superoxide dismutase (SOD, 100 U/ml) had no effect on vasodilator responses to papaverine or lower concentrations of arachidonate, whereas dilator responses to 100 microM arachidonate were inhibited modestly (by 22%) by SOD. Similarly, deferoxamine (1 mM) partly inhibited dilator responses to 10 and 100 microM arachidonate (by approximately 30% at each concentration). Tetraethylammonium ion (1 mM) or iberiotoxin (50 nM), inhibitors of calcium-activated potassium channels, markedly inhibited vasodilatation in response to arachidonate (by 70-90%) but not papaverine. These findings suggest that dilatation of cerebral arterioles in response to arachidonate is mediated largely by endogenously formed reactive oxygen species, which are generated from cyclooxygenase activity, and activation of calcium-activated potassium channels. Thus activation of potassium channels appears to be a major mechanism of cerebral vasodilatation in response to reactive oxygen species produced endogenously.

Endothelium-dependent relaxation to hydrogen peroxide in canine basilar artery: a potential new cerebral dilator mechanism

In prostglandin F2alpha(PGF2alpha)-precontracted isolated canine basilar arterial rings, hydrogen peroxide (H2O2) produced endothelium-dependent relaxations at concentrations of from 4.4 x 10(-7) - approximately 4.4 x 10(-5) M. Removal of extracellular Ca2+ ([Ca2+]0) attenuated the relaxant effects of H2O2. Complete inhibition of H2O2 relaxant action was obtained after buffering intracellular Ca2+ ([Ca2+]i), in the endothelial cells, with 10 microM 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM). The H2O2-induced relaxations could be abolished completely by 1200 u/ml catalase and was suppressed significantly by 0.5 microM atropine, 150 microM NG-monomethyl-arginine (L-NMMA), 50 microM NG-nitro-L-arginine methyl ester (L-NAME), 1 microM Fe2+, or 5 microM methylene blue. These inhibitory effects of L-NMMA, L-NAME, or atropine could be reversed partly by 50 microM L-arginine. The Fe2+ inhibition of H2O2-stimulated relaxation was reduced significantly by either 1 mM deferoxamine (a Fe2+ chelator) or 100 microM dimethyl sulfoxide (DMSO, a *OH scavenger). Such relaxant effects of H2O2 were enhanced, significantly, by an acetylcholinesterase antagonist, neostigmine. A variety of pharmacological antagonists (of diverse vasodilator agents) could not inhibit the relaxant action of H2O2. Our observations suggest that at suitable pathophysiological concentrations, H2O2 could induce release of an endothelium-derived relaxing factor (EDRF), probably nitric oxide (NO), from endothelial cells of the canine cerebral artery. The H2O2 relaxant effects are clearly Ca2+-dependent, require formation of cyclic guanosine monophosphate (cGMP), and may be associated with release of endogenous acetylcholine (ACh).

Hypercapnic cerebral blood flow in spontaneously hypertensive rats

OBJECTIVE

Hypercapnic cerebral vasodilation appears to be endothelium-dependent, as it involves nitric oxide and prostaglandins. Since chronic hypertension has been associated with impaired endothelial function, we designed a study to find out whether hypercapnic cerebral blood flow and its nitric oxide- and prostaglandin-sensitive component is reduced in spontaneously hypertensive rats (SHR) compared with normotensive controls.

METHODS

Cerebral blood flow was measured in enflurane-anesthetized SHR (n=53), Wistar-Kyoto (WKY, n=20) and Sprague-Dawley (n=50) rats using the hydrogen clearance method. Cerebral blood flow was measured during eucapnia and hypercapnia; it was also assessed after administering either nonisoform-selective or isoform-selective neuronal nitric oxide synthase inhibitors and during inhibition of prostaglandin production.

RESULTS

Hypercapnic cerebral blood flow did not differ among the strains. Nitric oxide synthase inhibition with intracortical N(G)-monomethyl-L-arginine reduced hypercapnic cerebral blood flow in SHR by 23+/-4% and in Sprague-Dawley rats by 23+/-7% without affecting eucapnic flow. Intraperitoneal administration of the inhibitor of neuronal nitric oxide synthase, 7-nitroindazole, reduced eucapnic flow by 18+/-5% in SHR and 27+/-5% in WKY rats, and hypercapnic flow by 48+/-3 and by 51+/-6%, respectively. Indomethacin produced a similar decrease in hypercapnic flow in Sprague-Dawley rats and SHR (49+/-5 and 62+/-4%, respectively).

CONCLUSION

Hypercapnic cerebral blood flow was not impaired in SHR. The contribution of nitric oxide- and prostaglandin-dependent vasodilation appeared to be intact Our results are consistent with the hypothesis that neuronal rather than endothelial production of nitric oxide may be responsible for maintaining hypercapnic cerebral vasodilation in SHR.

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

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

Superoxide dismutase partially restores impaired dilatation of the basilar artery during diabetes mellitus

The goal of this study was to test the hypothesis that administration of superoxide dismutase restores nitric oxide synthase-mediated dilatation of the basilar artery during diabetes mellitus. We measured the diameter of the basilar artery in vivo in nondiabetic and diabetic rats (streptozotocin; 50-60 mg/kg i.p.) in response to nitric oxide synthase-dependent agonists (acetylcholine and bradykinin) and a nitric oxide synthase-independent agonist (nitroglycerin) before and during application of superoxide dismutase. Topical application of acetylcholine (1.0 and 10 microM) and bradykinin (1.0 and 10 microM) produced dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. However, the magnitude of vasodilation produced by acetylcholine and bradykinin was significantly less in diabetic rats. Topical application of nitroglycerin (0.1 and 1.0 microM) produced similar dose-related dilatation of the basilar artery in nondiabetic and diabetic rats. Treatment with superoxide dismutase (150 U/ml) did not alter baseline diameter of the basilar artery in nondiabetic and diabetic rats. However, topical application of superoxide dismutase partially restored nitric oxide synthase-dependent dilatation of the basilar artery in diabetic rats towards that observed in nondiabetic rats. Superoxide dismutase did not alter dilatation of the basilar artery in nondiabetic rats. These findings suggest that impaired nitric oxide synthase-dependent dilatation of the basilar artery during diabetes mellitus may be related, in part, to enhanced release of oxygen-derived free radicals.

Cerebrovascular effects of nitric oxide manipulation in spontaneously hypertensive rats

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

Potassium channels and the cerebral circulation

1. Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. 2. In cerebral blood vessels, two potassium channels have received considerable study recently: ATP-sensitive and calcium-dependent potassium channels. Activation of these potassium channels appears to play a major role in the relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, reactive oxygen species and hypoxia. 3. The functional influence of ATP-sensitive and calcium-dependent potassium channels may be altered in disease states, including hypertension, diabetes and subarachnoid haemorrhage.

Vasomotor and permeability effects of bradykinin in the cerebral microcirculation

All components of an intracerebral kallikrein-kinin system have been described. Thus, bradykinin (BK) acting from the parenchymal site as well as from the blood site may influence cerebral microcirculation. BK is a potent dilator of extra- and intraparenchymal cerebral arteries when acting from the perivascular site. The vasomotor effect of BK is mediated by B2 receptors which appear to be located at the abluminal membrane of the endothelial cell. The effect of BK is mediated by NO. prostanoids, free radicals, H2O2 or leukotrienes depending on the animal species and on the location of the artery. Selective opening of the blood-brain barrier for small tracers (Na(+)-fluorescein; MW, 376) has been found in cats during cortical superfusion or intraarterial application of BK. This leakage is mediated by B2 receptors located at the luminal and abluminal membrane of the endothelial cells. Formation of brain edema has been found after ventriculo-cisternal perfusion or interstitial infusion of BK. This can be explained by increase of vascular permeability and cerebral blood flow due to arterial dilation thus enhancing driving forces for the extravasation. An increase of the BK concentration in the interstitial space of the brain up to concentrations which induce extravasation, dilatation and oedema formation has been found under several pathological conditions. Thus, BK may be involved in oedema formation after cold lesion, concussive brain injury, traumatic spinal cord and ischemic brain injury. The mediator role of BK in brain edema is further supported by therapeutic results. Brain swelling due to cold lesion or ischemia could be diminished by treatment with kallikrein-inhibitors. Similarly, dilatation of cerebral arterioles after concussive brain injury was reduced by blockade of B2 receptors. Thus, all criteria favour BK as one mediator of vasogenic oedema.

Bradykinin dilates rat middle cerebral artery and its large branches via endothelial B2 receptors and release of nitric oxide

Ring segments of rat middle cerebral artery (MCA) were prepared for measurement of isometric force and precontracted with 10(-4) M uridine triphosphate (UTP). Concentration-effect curves (CEC) were constructed for bradykinin (BK, 10(-8)(-10)(-1) M) in segments with functionally intact (E+) or denuded (E-) endothelium. E- segments did not dilate to BK. The BK receptor was characterized by application of specific B1 or B2 antagonists [des-Arg4-Leu8] BK (10(-5) M) and [D-Arg4-Hyp3-Thi5-D-Tic7-Oic8] BK (HOE140, 3 x 10(-7) M), respectively, or B2 agonist [des-Arg9] BK (10(-8)-10(-4) M). Involvement of nitric oxide (NO) was tested with NG-nitro-L-arginine (LNNA, 10(-4) M). BK induced concentration-dependent relaxation with a maximal effect (Emax) of 40.86 +/- 1.50% at 10(-4) M and a pD2 (-log10 EC50) of 6.818 +/- 0.044. This relaxation could be prevented with HOE140 or LNNA, but was not influenced by [des-Arg(9)-Leu] BK. [des-Arg9] BK did not induce any effect. These results demonstrate that BK induced relaxation via endothelial B2 receptors and release of NO in isolated rat MCA.

Beneficial vascular and metabolic effects of cobalt-ATP in spontaneously hypertensive rabbits with diffuse chronic cerebral ischaemia

The in vivo effects of adenosine triphosphate (ATP) have not been investigated in cerebrovascular diseases. The use of the long-acting cobalt-ATP complex (Co-ATP) permits us to observe the effects of ATP without the influence of its metabolites. This study was designed to compare the effects of intravenous Co-ATP on the cerebral blood flow (CBF), polarographically detected oxygen currents (O2a), mean arterial blood pressure (MABP), heart rate, respiration rate, cerebral electrical activity, arterial blood gases, pH, and glucose in 13 normotensive (NT) rabbits to those in 14 stroke-prone spontaneously hypertensive (HT) animals. CBF was measured by the hydrogen and heat clearance methods. In response to Co-ATP, MABP decreased and CBF increased significantly in both groups. The decrease in MABP was more marked in HT rabbits, while CBF response was 25% smaller than in NT animals. The ratio of O2a to CBF diminished moderately and simultaneously with the CBF increase in NT rabbits. In HT rabbits, the decrease in O2a/CBF was larger and began when CBF response reached its maximum. We suggest that despite the restricted CBF response, long-acting ATP should still be taken into consideration as a supplementary treatment of hypertensive encephalopathy because of its beneficial effects on cerebral metabolism and hypertension.

Involvement of calcitonin gene-related peptide (CGRP) and nitric oxide (NO) in the pial artery dilatation elicited by cortical spreading depression

The aim of the present study was to examine whether the initial transient arterial dilatation during cortical spreading depression (CSD) was mediated by the release of calcitonin gene-related peptide (CGRP) and/or nitric oxide (NO). This question is of interest as the initial phase of CSD appears to be a model of events occurring during functional hyperemia and during the first period of classic migraine. Using an open cranial window technique, pial arterial diameter in the parietal cortex of cats was recorded with an image splitting method. Employing micropuncture technique, perivascularly applied CGRP8-37 did not alter the resting diameter of pial arteries but antagonized concentration dependently (5 x 10(-9)-10(-6) M) the dilatation (35%) due to 5 x 10(-8) M CGRP. NG-Nitro-L-Arginine (NOLAG, 10(-4) M) also had no effect on resting diameter of pial arteries, indicating that their resting tone is neither mediated by a continuous release of CGRP nor of NO. CSD was triggered by a remote intracortical injection of KCl (150 mM) and recorded by a microelectrode placed adjacent to the artery under investigation. CSD elicited a transient negative DC shift which was accompanied by a peak dilatation of 44 +/- 5.2% (S.E.M.). This dilatation was reduced by approximately 50% during topical application of 10(-7) M CGRP8-37 and 10(-4) M NOLAG each. A 75% inhibition of the CSD-induced dilatation was found during simultaneous application of both compounds. These data indicate that the initial dilatation during CSD is mediated, at least in part, by a release of CGRP and NO.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of diabetes mellitus on response of the basilar artery to activation of ATP-sensitive potassium channels

Our goal was to determine whether responses of the basilar artery to activation of ATP-sensitive potassium channels are altered during diabetes mellitus. We measured changes in diameter of the basilar artery in vivo in non-diabetic and diabetic rats (streptozotocin; 50-60 mg/kg i.p.) in response to activation of ATP-sensitive potassium channels using aprikalim (RP 52891) and levcromakalim (BRL 38227). Aprikalim (1.0 microM) dilated the basilar artery in non-diabetic rats by 27 +/- 6%, but by only 11 +/- 3% in diabetic rats (means +/- S.E.; P < 0.05). Levcromakalim (1.0 microM) dilated the basilar artery in non-diabetic rats by 45 +/- 11%, but by only 20 +/- 5% in diabetic rats (P < 0.05). Nitroglycerin (1.0 microM) dilated the basilar artery by 20 +/- 5% in non-diabetic rats and 17 +/- 2% in diabetic rats (P > 0.05). Thus, impaired dilatation of pial arterioles in diabetic rats in response to aprikalim and levcromakalim is not related to a non-specific effect of diabetes mellitus on vasodilatation. The findings of the present study suggest that ATP-sensitive potassium channels are functional in the rat basilar artery in vivo and are altered during diabetes mellitus.

The complex role of nitric oxide in the pathophysiology of focal cerebral ischemia

Nitrogen monoxide (NO) has recently emerged as an important mediator of cellular and molecular events which impacts the pathophysiology of cerebral ischemia. Although tempting to ask whether NO is "good or bad" for cerebral ischemia, the question underestimates the complexities of NO chemistry and physiology as well as oversimplifies the pathophysiology of focal cerebral ischemia. Important vascular and neuronal actions of NO have been defined which both enhance tissue survival and mediate cellular injury and death, and these will be reviewed. Strategies which modify NO synthesis and/or metabolism may someday assume therapeutic importance, but not until the tissue compartments generating NO, the activities of the enzymes that are inducibly and constitutively expressed, and the redox state of NO during the stages of ischemic injury, are defined with greater precision. Our knowledge of these processes is rudimentary. This review will summarize the evidence from animal models which supports an emerging role for NO in ischemic pathophysiology. Important aspects of NO synthesis and inhibitors of this process will also be discussed.

Endothelium-derived vasoactive factors and regulation of the cerebral circulation

Vasoactive factors produced and released by endothelium exert a powerful influence on vascular tone in the cerebral circulation. Endothelium-derived relaxing factor (EDRF), which has been identified as nitric oxide (NO) or an NO-containing compound, is produced under basal conditions in cerebral blood vessels. EDRF mediates endothelium-dependent relaxation in response to a number of stimuli in the cerebral circulation. The influence of NO on the cerebral circulation appears to be particularly important and complex because both neurons and glia, in addition to endothelium, produce NO in response to some stimuli. Neuronally derived NO may mediate local vasodilation in response to increased neuronal activity. In addition to EDRF, cerebral endothelium may produce other relaxing factors, including prostacyclin, endothelium-derived hyperpolarizing factor, and oxygen-derived free radicals. Several pathophysiological conditions are associated with impaired endothelium-dependent responses that may involve the decreased production of EDRF and release of endothelium-derived contracting factors, such as the cyclooxygenase products of arachidonic acid and endothelin. The release of endothelin, an extremely potent and long-lasting vasoconstrictor peptide, may contribute to vasospasm after subarachnoid hemorrhage.

Cerebral circulation during diabetes mellitus

Diabetes mellitus is characterized by hyperglycemia, a decrease in circulating insulin and the development of macro- and microvascular pathology. Hyperglycemia appears to be a primary determinant for the structural, biochemical and functional changes that occur in large and small blood vessels during diabetes mellitus. While much research has focused on the effects of diabetes mellitus on the peripheral circulation, it is clear that diabetes mellitus also has profound effects on the cerebral circulation. Thus, the focus of this review is to discuss morphological and functional alterations in the cerebral circulation during diabetes mellitus.

Regulation of the cerebral circulation by endothelium

Endothelium exerts an important influence on cerebral vascular tone through the production and release of a diverse group of vasoactive factors. Relaxing factors produced by endothelium include nitric oxide (or a nitric oxide-containing compound), a hyperpolarizing factor, and prostacyclin. Endothelium-derived contracting factors include cyclooxygenase products of arachidonic acid and endothelins. Several pathophysiological conditions are associated with increased formation of endothelium-derived contracting factors. Such endothelial dysfunction in the cerebral circulation may shift the balance of vascular tone toward constriction and may potentially contribute to the onset or maintainance of cerebral ischemia and stroke.