Endothelium-Derived Hyperpolarizing Factor in the Brain

Cerebral Autoregulation: A Target for Improving Neurological Outcomes in Extracorporeal Life Support

Despite improvements in survival after illnesses requiring extracorporeal life support, cerebral injury continues to hinder successful outcomes. Cerebral autoregulation (CA) is an innate protective mechanism that maintains constant cerebral blood flow in the face of varying systemic blood pressure. However, it is impaired in certain disease states and, potentially, following initiation of extracorporeal circulatory support. In this review, we first discuss patient-related factors pertaining to venovenous and venoarterial extracorporeal membrane oxygenation (ECMO) and their potential role in CA impairment. Next, we examine factors intrinsic to ECMO that may affect CA, such as cannulation, changes in pulsatility, the inflammatory and adaptive immune response, intracranial hemorrhage, and ischemic stroke, in addition to ECMO management factors, such as oxygenation, ventilation, flow rates, and blood pressure management. We highlight potential mechanisms that lead to disruption of CA in both pediatric and adult populations, the challenges of measuring CA in these patients, and potential associations with neurological outcome. Altogether, we discuss individualized CA monitoring as a potential target for improving neurological outcomes in extracorporeal life support.

The conducted vascular response as a mediator of hypercapnic cerebrovascular reactivity: A modelling study

It is well established that the cerebral blood flow (CBF) shows exquisite sensitivity to changes in the arterial blood partial pressure of CO2 ( [Formula: see text] ), which is reflected by an index termed cerebrovascular reactivity. In response to elevations in [Formula: see text] (hypercapnia), the vessels of the cerebral microvasculature dilate, thereby decreasing the vascular resistance and increasing CBF. Due to the challenges of access, scale and complexity encountered when studying the microvasculature, however, the mechanisms behind cerebrovascular reactivity are not fully understood. Experiments have previously established that the cholinergic release of the Acetylcholine (ACh) neurotransmitter in the cortex is a prerequisite for the hypercapnic response. It is also known that ACh functions as an endothelial-dependent agonist, in which the local administration of ACh elicits local hyperpolarization in the vascular wall; this hyperpolarization signal is then propagated upstream the vascular network through the endothelial layer and is coupled to a vasodilatory response in the vascular smooth muscle (VSM) layer in what is known as the conducted vascular response (CVR). Finally, experimental data indicate that the hypercapnic response is more strongly correlated with the CO2 levels in the tissue than in the arterioles. Accordingly, we hypothesize that the CVR, evoked by increases in local tissue CO2 levels and a subsequent local release of ACh, is responsible for the CBF increase observed in response to elevations in [Formula: see text] . By constructing physiologically grounded dynamic models of CBF and control in the cerebral vasculature, ones that integrate the available knowledge and experimental data, we build a new model of the series of signalling events and pathways underpinning the hypercapnic response, and use the model to provide compelling evidence that corroborates the aforementioned hypothesis. If the CVR indeed acts as a mediator of the hypercapnic response, the proposed mechanism would provide an important addition to our understanding of the repertoire of metabolic feedback mechanisms possessed by the brain and would motivate further in-vivo investigation. We also model the interaction of the hypercapnic response with dynamic cerebral autoregulation (dCA), the collection of mechanisms that the brain possesses to maintain near constant CBF despite perturbations in pressure, and show how the dCA mechanisms, which otherwise tend to be overlooked when analysing experimental results of cerebrovascular reactivity, could play a significant role in shaping the CBF response to elevations in [Formula: see text] . Such in-silico models can be used in tandem with in-vivo experiments to expand our understanding of cerebrovascular diseases, which continue to be among the leading causes of morbidity and mortality in humans.

Experimental carotid baroreceptor stimulation reduces blood flow velocities in the anterior and middle cerebral arteries of healthy individuals

This study investigated effects of experimental baroreceptor stimulation on bilateral blood flow velocities in the anterior and middle cerebral arteries (ACA and MCA) using functional transcranial Doppler sonography. Carotid baroreceptors were stimulated by neck suction in 33 healthy participants. Therefore, negative pressure (- 50 mmHg) was applied; neck pressure (+ 10 mmHg) was used as a control condition. Heart rate (HR) and blood pressure (BP) were also continuously recorded. Neck suction led to reductions in bilateral ACA and MCA blood flow velocities, which accompanied the expected HR and BP decreases; HR and BP decreases correlated positively with the ACA flow velocity decline. The observations suggest reduction of blood flow in the perfusion territories of the ACA and MCA during baroreceptor stimulation. Baroreceptor-related HR and BP decreases may contribute to the cerebral blood flow decline. The findings underline the interaction between peripheral and cerebral hemodynamic regulation in autoregulatory control of cerebral perfusion.

Oxidative Stress, Vascular Endothelium, and the Pathology of Neurodegeneration in Retina

Oxidative stress (OS) is an imbalance between free radicals/ROS and antioxidants, which evokes a biological response and is an important risk factor for diseases, in both the cardiovascular system and central nervous system (CNS). The underlying mechanisms driving pathophysiological complications that arise from OS remain largely unclear. The vascular endothelium is emerging as a primary target of excessive glucocorticoid and catecholamine action. Endothelial dysfunction (ED) has been implicated to play a crucial role in the development of neurodegeneration in the CNS. The retina is known as an extension of the CNS. Stress and endothelium dysfunction are suspected to be interlinked and associated with neurodegenerative diseases in the retina as well. In this narrative review, we explore the role of OS-led ED in the retina by focusing on mechanistic links between OS and ED, ED in the pathophysiology of different retinal neurodegenerative conditions, and how a better understanding of the role of endothelial function could lead to new therapeutic approaches for neurodegenerative diseases in the retina.

Increased cerebral endothelium-dependent vasodilation in rats in the postcardiac arrest period

Cardiovascular lability is common after cardiac arrest. We investigated whether altered endothelial function is present in cerebral and mesenteric arteries 2 and 4 h after resuscitation. Male Sprague-Dawley rats were anesthetized, intubated, ventilated, and intravascularly catheterized whereupon rats were randomized into four groups. Following 7 min of asphyxial cardiac arrest and subsequent resuscitation, cardiac arrest and sham rats were observed for either 2 or 4 h. Neuron-specific enolase levels were measured in blood samples. Middle cerebral artery segments and small mesenteric arteries were isolated and examined in microvascular myographs. qPCR and immunofluorescence analysis were performed on cerebral arteries. In cerebral arteries, bradykinin-induced vasodilation was inhibited in the presence of either calcium-activated K⁺ channel blockers (UCL1684 and senicapoc) or the nitric oxide (NO) synthase inhibitor, N^ω-nitro-L-arginine methyl ester hydrochloride (l-NAME), whereas the combination abolished bradykinin-induced vasodilation across groups. Neuron-specific enolase levels were significantly increased in cardiac arrest rats. Cerebral vasodilation was comparable between the 2-h groups, but markedly enhanced in response to bradykinin, NS309 (an opener of small and intermediate calcium-activated K⁺ channels), and sodium nitroprusside 4 h after cardiac arrest. Endothelial NO synthase and guanylyl cyclase subunit α-1 mRNA expression was unaltered after 2 h, but significantly decreased 4 h after resuscitation. In mesenteric arteries, the endothelium-dependent vasodilation was comparable between corresponding groups at both 2 and 4 h. Our findings show enhanced cerebral endothelium-dependent vasodilation 4 h after cardiac arrest mediated by potentiated endothelial-derived hyperpolarization and NO pathways. Altered cerebral endothelium-dependent vasodilation may contribute to disturbed cerebral perfusion after cardiac arrest.NEW & NOTEWORTHY This is the first study, to our knowledge, to demonstrate enhanced endothelium-dependent vasodilation in middle cerebral arteries in a cardiac arrest rat model. The increased endothelium-dependent vasodilation was a result of potentiated endothelium-derived hyperpolarization and endothelial nitric oxide pathways. Immunofluorescence microscopy confirmed the presence of relevant receptors and eNOS in cerebral arteries, whereas qPCR showed altered expression of genes related to guanylyl cyclase and eNOS. Altered endothelium-dependent vasoregulation may contribute to disturbed cerebral blood flow in the postcardiac arrest period.

The Role of Neurovascular System in Neurodegenerative Diseases

The neurovascular system (NVS), which consisted of neurons, glia, and vascular cells, is a functional and structural unit of the brain. The NVS regulates blood-brain barrier (BBB) permeability and cerebral blood flow (CBF), thereby maintaining the brain's microenvironment for normal functioning, neuronal survival, and information processing. Recent studies have highlighted the role of vascular dysfunction in several neurodegenerative diseases. This is not unexpected since both nervous and vascular systems are functionally interdependent and show close anatomical apposition, as well as similar molecular pathways. However, despite extensive research, the precise mechanism by which neurovascular dysfunction contributes to neurodegeneration remains incomplete. Therefore, understanding the mechanisms of neurovascular dysfunction in disease conditions may allow us to develop potent and effective therapies for prevention and treatment of neurodegenerative diseases. This review article summarizes the current research in the context of neurovascular signaling associated with neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and Huntington's disease (HD). We also discuss the potential implication of neurovascular factor as a novel therapeutic target and prognostic marker in patients with neurodegenerative conditions. Graphical Abstract.

Impacts of Microgravity Analogs to Spaceflight on Cerebral Autoregulation

It is well known that exposure to microgravity in astronauts leads to a plethora physiological responses such as headward fluid shift, body unloading, and cardiovascular deconditioning. When astronauts return to Earth, some encounter problems related to orthostatic intolerance. An impaired cerebral autoregulation (CA), which could be compromised by the effects of microgravity, has been proposed as one of the mechanisms responsible for orthostatic intolerance. CA is a homeostatic mechanism that maintains cerebral blood flow for any variations in cerebral perfusion pressure by adapting the vascular tone and cerebral vessel diameter. The ground-based models of microgravity are useful tools for determining the gravitational impact of spaceflight on human body. The head-down tilt bed rest (HDTBR), where the subject remains in supine position at -6 degrees for periods ranging from few days to several weeks is the most commonly used ground-based model of microgravity for cardiovascular deconditioning. head-down bed rest (HDBR) is able to replicate cephalic fluid shift, immobilization, confinement, and inactivity. Dry immersion (DI) model is another approach where the subject remains immersed in thermoneutral water covered with an elastic waterproof fabric separating the subject from the water. Regarding DI, this analog imitates absence of any supporting structure for the body, centralization of body fluids, immobilization and hypokinesia observed during spaceflight. However, little is known about the impact of microgravity on CA. Here, we review the fundamental principles and the different mechanisms involved in CA. We also consider the different approaches in order to assess CA. Finally, we focus on the effects of short- and long-term spaceflight on CA and compare these findings with two specific analogs to microgravity: HDBR and DI.

Cerebral blood flow modulations during proactive control in chronic hypotension

In addition to complaints including fatigue, mood disturbance, dizziness or cold limbs, chronic low blood pressure (hypotension) is associated with reduced cognitive performance. Deficiencies in cerebral blood flow regulation may contribute to this impairment. This study investigated cerebral blood flow modulations during proactive control in hypotension. Proactive control refers to cognitive processes during anticipation of a behaviourally relevant event that allow optimization of readiness to react. Using functional transcranial Doppler sonography, bilateral blood flow velocities in the middle cerebral arteries were recorded in 40 hypotensive and 40 normotensive participants during a precued Stroop task. Hypotensive participants exhibited smaller bilateral blood flow increases during response preparation and longer response time. The group differences in blood flow and response time did not vary by executive function load, i.e. congruent vs. incongruent trials. Over the total sample, the flow increase correlated negatively with response time in trials with a higher executive function load. The findings indicate reduced cerebral blood flow adjustment during both the basic and more complex requirements of proactive control in hypotension. They also suggest a general deficit in attentional function and processing speed due to low blood pressure and cerebral hemodynamic dysregulations rather than particular impairments in executive functions.

Enhancement of bradykinin-induced relaxation by focal brain ischemia in the rat middle cerebral artery: Receptor expression upregulation and activation of multiple pathways

Focal brain ischemia markedly affects cerebrovascular reactivity. So far, these changes have mainly been related to alterations in the level of smooth muscle cell function while alterations of the endothelial lining have not yet been studied in detail. We have, therefore, investigated the effects of ischemia/reperfusion injury on bradykinin (BK)-induced relaxation since BK is an important mediator of tissue inflammation and affects vascular function in an endothelium-dependent manner. Focal brain ischemia was induced in rats by endovascular filament occlusion (2h) of the middle cerebral artery (MCA). After 22h reperfusion, both MCAs were harvested and the response to BK studied in organ bath experiments. Expression of the BK receptor subtypes 1 and 2 (B₁, B₂) was determined by real-time semi-quantitative RT-qPCR methodology, and whole mount immunofluorescence staining was performed to show the B₂ receptor protein expression. In control animals, BK did not induce significant vasomotor effects despite a functionally intact endothelium and robust expression of B₂ mRNA. After ischemia/reperfusion injury, BK induced a concentration-related sustained relaxation in all arteries studied, more pronounced in the ipsilateral than in the contralateral MCA. The B₂ mRNA was significantly upregulated and the B₁ mRNA displayed de novo expression, again more pronounced ipsi- than contralaterally. Endothelial cells displaying B₂ receptor immunofluorescence were observed scattered or clustered in previously occluded MCAs. Relaxation to BK was mediated by B₂ receptor activation, abolished after endothelium denudation, and largely diminished by blocking nitric oxide (NO) release or soluble guanylyl cyclase activity. Relaxation to BK was partially inhibited by charybdotoxin (ChTx), but not apamin or iberiotoxin suggesting activation of an endothelium-dependent hyperpolarization pathway. When the NO-cGMP pathway was blocked, BK induced a transient relaxation which was suppressed by ChTx. After ischemia/reperfusion injury BK elicits endothelium-dependent relaxation which was not detectable in control MCAs. This gain of function is mediated by B₂ receptor activation and involves the release of NO and activation of an endothelium-dependent hyperpolarization. It goes along with increased B₂ mRNA and protein expression, leaving the functional role of the de novo B₁ receptor expression still open.

Total Flavones of Rhododendron simsii Planch Flower Protect against Cerebral Ischemia-Reperfusion Injury via the Mechanism of Cystathionine-γ-Lyase-Produced H2S

Total flavones of Rhododendron simsii Planch flower (TFR) have a significant protective effect against cerebral ischemia-reperfusion injury. However, its mechanism is unclear. This study investigated the protection of TFR against cerebral ischemia-reperfusion injury via cystathionine-γ-lyase- (CSE-) produced H2S mechanism. CSE-/- mice and CSE-siRNA-transfected rat were used. Relaxation of cerebral basilar artery (CBA), H2S, and CSE mRNA were measured. TFR significantly inhibited cerebral ischemia-reperfusion-induced abnormal neurological symptom and cerebral infarct in the normal rats and the CSE+/+ mice, but not in the CSE-/- mice, and the inhibition was markedly attenuated in CSE-siRNA-transfected rat; TFR elicited a significant vasorelaxation in rat CBA, and the relaxation was markedly attenuated by removal of endothelium or CSE-siRNA transfection or coapplication of NO synthase inhibitor L-NAME and PGI2 synthase inhibitor Indo. CSE inhibitor PPG drastically inhibited TFR-evoked vasodilatation resistant to L-NAME and Indo in endothelium-intact rat CBA. TFR significantly increased CSE mRNA expression in rat CBA endothelial cells and H2S production in rat endothelium-intact CBA. The increase of H2S production resistant to L-NAME and Indo was abolished by PPG. Our data indicate that TFR has a protective effect against the cerebral ischemia-reperfusion injury via CSE-produced H2S and endothelial NO and/or PGI2 to relax the cerebral artery.

The Role of Endothelial Ca2+ Signaling in Neurovascular Coupling: A View from the Lumen

BACKGROUND

Neurovascular coupling (NVC) is the mechanism whereby an increase in neuronal activity (NA) leads to local elevation in cerebral blood flow (CBF) to match the metabolic requirements of firing neurons. Following synaptic activity, an increase in neuronal and/or astrocyte Ca2+ concentration leads to the synthesis of multiple vasoactive messengers. Curiously, the role of endothelial Ca2+ signaling in NVC has been rather neglected, although endothelial cells are known to control the vascular tone in a Ca2+-dependent manner throughout peripheral vasculature.

METHODS

We analyzed the literature in search of the most recent updates on the potential role of endothelial Ca2+ signaling in NVC.

RESULTS

We found that several neurotransmitters (i.e., glutamate and acetylcholine) and neuromodulators (e.g., ATP) can induce dilation of cerebral vessels by inducing an increase in endothelial Ca2+ concentration. This, in turn, results in nitric oxide or prostaglandin E2 release or activate intermediate and small-conductance Ca2+-activated K⁺ channels, which are responsible for endothelial-dependent hyperpolarization (EDH). In addition, brain endothelial cells express multiple transient receptor potential (TRP) channels (i.e., TRPC3, TRPV3, TRPV4, TRPA1), which induce vasodilation by activating EDH.

CONCLUSIONS

It is possible to conclude that endothelial Ca2+ signaling is an emerging pathway in the control of NVC.

DOCA-salt hypertension impairs artery function in rat middle cerebral artery and parenchymal arterioles

OBJECTIVE

Chronic hypertension induces detrimental changes in the structure and function of surface cerebral arteries. Very little is known about PAs, which perfuse distinct neuronal populations in the cortex and may play a role in cerebrovascular disorders. We investigated the effect of DOCA-salt induced hypertension on endothelial function and artery structure in PAs and MCAs.

METHODS

Uninephrectomized male Sprague-Dawley rats were implanted with a subcutaneous pellet containing DOCA (150 mg/kg b.w.) and drank salt water (1% NaCl and 0.2% KCl) for 4 weeks. Sham rats were uninephrectomized and drank tap water. Vasoreactivity and passive structure in the MCAs and the PAs were assessed by pressure myography.

RESULTS

Both MCAs and PAs from DOCA-salt rats exhibited impaired endothelium-dependent dilation (P<.05). In the PAs, addition of NO and COX inhibitors enhanced dilation in DOCA-salt rats (P<.05), suggesting that dysfunctional NO and COX-dependent signaling could contribute to impaired endothelium-mediated dilation. MCAs from DOCA-salt rats exhibited inward remodeling (P<.05).

CONCLUSIONS

Hypertension-induced MCA remodeling coupled with impaired endothelium-dependent dilation in both the MCAs and PAs may exacerbate the risk of cerebrovascular accidents and the associated morbidity and mortality.

Cerebral Autoregulation-oriented Therapy at the Bedside

This comprehensive review summarizes the evidence regarding use of cerebral autoregulation-directed therapy at the bedside and provides an evaluation of its impact on optimizing cerebral perfusion and associated functional outcomes. Multiple studies in adults and several in children have shown the feasibility of individualizing mean arterial blood pressure and cerebral perfusion pressure goals by using cerebral autoregulation monitoring to calculate optimal levels. Nine of these studies examined the association between cerebral perfusion pressure or mean arterial blood pressure being above or below their optimal levels and functional outcomes. Six of these nine studies (66%) showed that patients for whom median cerebral perfusion pressure or mean arterial blood pressure differed significantly from the optimum, defined by cerebral autoregulation monitoring, were more likely to have an unfavorable outcome. The evidence indicates that monitoring of continuous cerebral autoregulation at the bedside is feasible and has the potential to be used to direct blood pressure management in acutely ill patients.

Peroxynitrite formed during a transient episode of brain ischaemia increases endothelium-derived hyperpolarization-type dilations in thromboxane/prostaglandin receptor-stimulated rat cerebral arteries

AIM

Increased thromboxane A₂ and peroxynitrite are hallmarks of cerebral ischaemia/reperfusion (I/R). Stimulation of thromboxane/prostaglandin receptors (TP) attenuates endothelium-derived hyperpolarization (EDH). We investigated whether EDH-type middle cerebral artery (MCA) relaxations following TP stimulation are altered after I/R and the influence of peroxynitrite.

METHODS

Vascular function was determined by wire myography after TP stimulation with the thromboxane A₂ mimetic 9,11-dideoxy-9α, 11α -methano-epoxy prostaglandin F2α (U46619) in MCA of Sprague Dawley rats subjected to MCA occlusion (90 min)/reperfusion (24 h) or sham operation, and in non-operated (control) rats. Some rats were treated with saline or the peroxynitrite decomposition catalyst 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrinato iron (III) (20 mg kg^-1 ). Protein expression was evaluated in MCA and in human microvascular endothelial cells submitted to hypoxia (overnight)/reoxygenation (24 h) (H/R) using immunofluorescence and immunoblotting.

RESULTS

In U46619-pre-constricted MCA, EDH-type relaxation by the proteinase-activated receptor 2 agonist serine-leucine-isoleucine-glycine-arginine-leucine-NH₂ (SLIGRL) was greater in I/R than sham rats due to an increased contribution of small-conductance calcium-activated potassium channels (SK_Ca ), which was confirmed by the enlarged relaxation to the SK_Ca activator N-cyclohexyl-N-2-(3,5-dimethyl-pyrazol-1-yl)-6-methyl-4-pyrimidinamine. I/R and H/R induced endothelial protein tyrosine nitration and filamentous-actin disruption. In control MCA, either cytochalasin D or peroxynitrite disrupted endothelial filamentous-actin and augmented EDH-type relaxation. Furthermore, peroxynitrite decomposition during I/R prevented the increase in EDH-type responses.

CONCLUSION

Following TP stimulation in MCA, EDH-type relaxation to SLIGRL is greater after I/R due to endothelial filamentous-actin disruption by peroxynitrite, which prevents TP-induced block of SK_Ca input to EDH. These results reveal a novel mechanism whereby peroxynitrite could promote post-ischaemic brain injury.

Vascular TRP channels: performing under pressure and going with the flow

Endothelial cells and smooth muscle cells of resistance arteries mediate opposing responses to mechanical forces acting on the vasculature, promoting dilation in response to flow and constriction in response to pressure, respectively. In this review, we explore the role of TRP channels, particularly endothelial TRPV4 and smooth muscle TRPC6 and TRPM4 channels, in vascular mechanosensing circuits, placing their putative mechanosensitivity in context with other proposed upstream and downstream signaling pathways.

Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging

Cognitive performances are tightly associated with the maximal aerobic exercise capacity, both of which decline with age. The benefits on mental health of regular exercise, which slows the age-dependent decline in maximal aerobic exercise capacity, have been established for centuries. In addition, the maintenance of an optimal cerebrovascular endothelial function through regular exercise, part of a healthy lifestyle, emerges as one of the key and primary elements of successful brain aging. Physical exercise requires the activation of specific brain areas that trigger a local increase in cerebral blood flow to match neuronal metabolic needs. In this review, we propose three ways by which exercise could maintain the cerebrovascular endothelial function, a premise to a healthy cerebrovascular function and an optimal regulation of cerebral blood flow. First, exercise increases blood flow locally and increases shear stress temporarily, a known stimulus for endothelial cell maintenance of Akt-dependent expression of endothelial nitric oxide synthase, nitric oxide generation, and the expression of antioxidant defenses. Second, the rise in circulating catecholamines during exercise not only facilitates adequate blood and nutrient delivery by stimulating heart function and mobilizing energy supplies but also enhances endothelial repair mechanisms and angiogenesis. Third, in the long term, regular exercise sustains a low resting heart rate that reduces the mechanical stress imposed to the endothelium of cerebral arteries by the cardiac cycle. Any chronic variation from a healthy environment will perturb metabolism and thus hasten endothelial damage, favoring hypoperfusion and neuronal stress.

A new rodent model for obstructive sleep apnea: effects on ATP-mediated dilations in cerebral arteries

Obstructive sleep apnea (OSA), a condition in which the upper airway collapses during sleep, is strongly associated with metabolic and cardiovascular diseases. Little is known how OSA affects the cerebral circulation. The goals of this study were 1) to develop a rat model of chronic OSA that involved apnea and 2) to test the hypothesis that 4 wk of apneas during the sleep cycle alters endothelium-mediated dilations in middle cerebral arteries (MCAs). An obstruction device, which was chronically implanted into the trachea of rats, inflated to obstruct the airway 30 times/h for 8 h during the sleep cycle. After 4 wk of apneas, MCAs were isolated, pressurized, and exposed to luminally applied ATP, an endothelial P2Y2 receptor agonist that dilates through endothelial-derived nitric oxide (NO) and endothelial-dependent hyperpolarization (EDH). Dilations to ATP were attenuated ~30% in MCAs from rats undergoing apneas compared with those from a sham control group (P < 0.04 group effect; n = 7 and 10, respectively). When the NO component of the dilation was blocked to isolate the EDH component, the response to ATP in MCAs from the sham and apnea groups was similar. This finding suggests that the attenuated dilation to ATP must occur through reduced NO. In summary, we have successfully developed a novel rat model for chronic OSA that incorporates apnea during the sleep cycle. Using this model, we demonstrate that endothelial dysfunction occurred by 4 wk of apnea, likely increasing the vulnerability of the brain to cerebrovascular related accidents.

Regulation of cerebral blood flow

The control of cerebral blood flow is complex, and only beginning to be elucidated. Studies have identified three key regulatory paradigms. The first is cerebral pressure autoregulation, which maintains a constant flow in the face of changing cerebral perfusion pressure. Flow-metabolism coupling refers to the brains ability to vary blood flow to match metabolic activity. An extensive arborization of perivascular nerves also serves to modulate cerebral blood flow, so-called neurogenic regulation. Central to these three paradigms are two cell types: endothelium and astrocytes. The endothelium produces several vasoactive factors that are germane to the regulation of cerebral blood flow: nitric oxide, endothelium-dependent hyperpolarization factor, the eicosanoids, and the endothelins. Astrocytic foot processes directly abut the blood vessels, and play a key role in regulation of cerebral blood flow. Lastly, new research has been investigating cell-cell communication at the microvascular level. Several lines of evidence point to the ability of the larger proximal vessels to coordinate vasomotor responses downstream.

Cellular mechanisms underlying Hyperin-induced relaxation of rat basilar artery

BACKGROUND AND AIM

Hyperin, a flavonol compound extracted from the Chinese herb Abelmoschus manihot L. Medic, is reported to exert protective actions in cerebral ischemic injury. The specific aim of the present study was to study the relaxation of Hyperin in rat isolated basilar artery and identify the underlying cellular mechanisms.

METHODS

Rat isolated basilar artery segments were cannulated and perfused while being superfused with PSS solution. Vessel images were recorded by video microscopy and diameters measured. Membrane potential was recorded using glass microelectrodes to evaluate the basilar artery smooth muscle cell hyperpolarization.

RESULTS

Perfusion of Hyperin (1~100 μM) elicited a concentration-dependent relaxation of basilar artery segments preconstricted with 0.1 μM U46619. The response was significantly inhibited by the removal of the endothelium. Hyperin also elicited marked and concentration-dependent hyperpolarization of smooth muscle cells. 30 μM nitro-L-arginine (an inhibitor of nitric oxide synthase) and indomethacin (an inhibitor of cyclooxygenase), partially inhibited Hyperin-induced relaxation and hyperpolarization leaving an attenuated, but significant, endothelium-dependent relaxation and hyperpolarization. This remaining effect was almost completely blocked by 1mM tetraethylammonium (an inhibitor of Ca(2+)-activated K(+) channels), or by 100 μM DL-propargylglycine, an inhibitor of cystathionine-γ-lyase (a synthase of the endogenous H(2)S).

CONCLUSION

These findings show that Hyperin produces significant hyperpolarization in rat basilar artery smooth muscle cells and relaxation through both endothelium-dependent and endothelium-independent mechanisms. The underlying mechanisms appeared to be multi-factorial involving nitric oxide, prostacyclin, and endothelium-derived hyperpolarizing factor (EDHF). Our data further suggest that endogenous H(2)S is a component of the EDHF-mediated hyperpolarization and relaxation to Hyperin.

Soluble guanylate cyclase alpha1beta1 limits stroke size and attenuates neurological injury

BACKGROUND AND PURPOSE

Nitric oxide mediates endothelium-dependent vasodilation, modulates cerebral blood flow, and determines stroke outcome. Nitric oxide signals in part by stimulating soluble guanylate cyclase (sGC) to synthesize cGMP. To study the role of sGC in stroke injury, we compared the outcome of cerebral ischemia and reperfusion in mice deficient in the alpha(1) subunit of sGC (sGCalpha(1)(-/-)) with that in wild-type mice.

METHODS

Blood pressure, cerebrovascular anatomy, and vasoreactivity of pressurized carotid arteries were compared in both mouse genotypes. Cerebral blood flow was measured before and during middle cerebral artery occlusion and reperfusion. We then assessed neurological deficit and infarct volume after 1 hour of occlusion and 23 hours of reperfusion and after 24 hours of occlusion.

RESULTS

Blood pressure and cerebrovascular anatomy were similar between genotypes. We found that vasodilation of carotid arteries in response to acetylcholine or sodium nitroprusside was diminished in sGCalpha(1)(-/-) compared with wild-type mice. Cerebral blood flow deficits did not differ between the genotypes during occlusion, but during reperfusion, cerebral blood flow was 45% less in sGCalpha(1)(-/-) mice. Infarct volumes and neurological deficits were similar after 24 hours of occlusion in both genotypes. After 1 hour of ischemia and 23 hours of reperfusion, infarct volumes were 2-fold larger and neurological deficits were worse in sGCalpha(1)(-/-) than in the wild-type mice.

CONCLUSIONS

sGCalpha(1) deficiency impairs vascular reactivity to nitric oxide and is associated with incomplete reperfusion, larger infarct size, and worse neurological damage, suggesting that cGMP generated by sGCalpha(1)beta(1) is protective in ischemic stroke.

Integration of cerebrovascular CO2 reactivity and chemoreflex control of breathing: mechanisms of regulation, measurement, and interpretation

Cerebral blood flow (CBF) and its distribution are highly sensitive to changes in the partial pressure of arterial CO(2) (Pa(CO(2))). This physiological response, termed cerebrovascular CO(2) reactivity, is a vital homeostatic function that helps regulate and maintain central pH and, therefore, affects the respiratory central chemoreceptor stimulus. CBF increases with hypercapnia to wash out CO(2) from brain tissue, thereby attenuating the rise in central Pco(2), whereas hypocapnia causes cerebral vasoconstriction, which reduces CBF and attenuates the fall of brain tissue Pco(2). Cerebrovascular reactivity and ventilatory response to Pa(CO(2)) are therefore tightly linked, so that the regulation of CBF has an important role in stabilizing breathing during fluctuating levels of chemical stimuli. Indeed, recent reports indicate that cerebrovascular responsiveness to CO(2), primarily via its effects at the level of the central chemoreceptors, is an important determinant of eupneic and hypercapnic ventilatory responsiveness in otherwise healthy humans during wakefulness, sleep, and exercise and at high altitude. In particular, reductions in cerebrovascular responsiveness to CO(2) that provoke an increase in the gain of the chemoreflex control of breathing may underpin breathing instability during central sleep apnea in patients with congestive heart failure and on ascent to high altitude. In this review, we summarize the major factors that regulate CBF to emphasize the integrated mechanisms, in addition to Pa(CO(2)), that control CBF. We discuss in detail the assessment and interpretation of cerebrovascular reactivity to CO(2). Next, we provide a detailed update on the integration of the role of cerebrovascular CO(2) reactivity and CBF in regulation of chemoreflex control of breathing in health and disease. Finally, we describe the use of a newly developed steady-state modeling approach to examine the effects of changes in CBF on the chemoreflex control of breathing and suggest avenues for future research.

Effects of chronic in vivo administration of nitroglycerine on ACh-induced endothelium-dependent relaxation in rabbit cerebral arteries

BACKGROUND AND PURPOSE

In the setting of nitrate tolerance, endothelium-dependent relaxation is reduced in several types of peripheral vessels. However, it is unknown whether chronic in vivo administration of nitroglycerine modulates such relaxation in cerebral arteries.

EXPERIMENTAL APPROACH

Isometric force and smooth muscle cell membrane potential were measured in endothelium-intact strips from rabbit middle cerebral artery (MCA) and posterior cerebral artery (PCA).

KEY RESULTS

ACh (0.1-10 microM) concentration-dependently induced endothelium-dependent relaxation during the contraction induced by histamine in both MCA and PCA. Chronic (10 days) in vivo administration of nitroglycerine reduced the ACh-induced relaxation in PCA but not in MCA, in the presence of the cyclooxygenase inhibitor diclofenac (3 microM). In the presence of the NO-synthase inhibitor N (omega)-nitro-L-arginine (L-NNA, 0.1 mM) plus diclofenac, in MCA from both nitroglycerine-untreated control and -treated rabbits, ACh (0.1-10 microM) induced a smooth muscle cell hyperpolarization and relaxation, and these were blocked by the small-conductance Ca(2+)-activated K(+)-channel inhibitor apamin (0.1 microM), but not by the large- and intermediate-conductance Ca(2+)-activated K(+)-channel inhibitor charybdotoxin (0.1 microM). In contrast, in PCA, ACh (<3 microM) induced neither hyperpolarization nor relaxation under these conditions, suggesting that the endothelium-derived relaxing factor is NO in PCA, whereas endothelium-derived hyperpolarizing factor (EDHF) plays a significant role in MCA.

CONCLUSIONS AND IMPLICATIONS

It is suggested that in rabbit cerebral arteries, the function of the endothelium-derived relaxing factor NO and that of EDHF may be modulated differently by chronic in vivo administration of nitroglycerine.

Cerebral Autoregulation and Syncope

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

Myoendothelial gap junction frequency does not account for sex differences in EDHF responses in rat MCA

Previous findings from our laboratory have shown that dilations to endothelium-derived hyperpolarizing factor (EDHF) in rat middle cerebral artery (MCA) are less in females compared to males. Myoendothelial gap junctions (MEGJs) appear to mediate the transfer of hyperpolarization between endothelium and smooth muscle in males. In the present study, we hypothesized that MEGJs are the site along the EDHF pathway which is compromised in female rat MCA. Membrane potential in endothelium was measured using the voltage-sensitive dye di-8-ANEPPS and in smooth muscle using intracellular glass microelectrodes in the presence of l-NAME (3x10(-5 )M) and indomethacin (10(-5 )M). Electron microscopy was used to assess MEGJ characteristics. In endothelial cells, the di-8-ANEPPS fluorescence ratio change to 10(-5 )M UTP was similar in males (-2.9+/-0.5%) and females (-3.2+/-0.2%), indicating comparable degrees of endothelial cell hyperpolarization. However, smooth muscle cell hyperpolarization to 10(-5 )M UTP was significantly attenuated in females (0 mV hyperpolarization; -31+/-1.5 mV resting) compared to males (8 mV hyperpolarization; -28+/-1.7 mV resting). Ultrastructural evidence suggested that MEGJ frequency and area of contact were comparable between males and females. Taken together, our data suggest that in rat MCA, MEGJ frequency does not account for the reduced EDHF responses observed in females compared to males. We conclude that reduced myoendothelial coupling and/or homocellular coupling within the media may account for these differences.

Transient middle cerebral artery occlusion causes different structural, mechanical, and myogenic alterations in normotensive and hypertensive rats

Transient focal cerebral ischemia in the rat alters vessel properties, and spontaneously hypertensive rats (SHR) show a poorer outcome after ischemia. In the present study we examined the role of hypertension on vessel properties after ischemia-reperfusion. The right middle cerebral artery (MCA) was occluded (90 min) and reperfused (24 h) in SHR (n = 12) and Wistar-Kyoto rats (WKY; n = 11). Sham-operated rats (SHR, n = 10; WKY, n = 10) were used as controls. The structural, mechanical, and myogenic properties of the MCA were assessed by pressure myography. Nuclei distribution and elastin content and organization were analyzed by confocal microscopy. Infarct volume was larger in SHR than in WKY rats. Ischemia-reperfusion induced adventitial hypertrophy associated with an increase in the total number of adventitial cells. In addition, fenestrae area and arterial distensibility increased and myogenic tone decreased in the MCA of WKY rats after ischemia-reperfusion. Hypertension per se induced hypertrophic inward remodeling. Ischemia-reperfusion decreased the cross-sectional area of the MCA in SHR, without significant changes in distensibility, despite an increase in fenestrae area. In addition, MCA myogenic properties were not altered after ischemia-reperfusion in SHR. Our results indicate that in normotensive rats, MCA develops a compensatory mechanism (i.e., enhanced distensibility and decreased myogenic tone) that counteracts the effect of ischemia-reperfusion and ensures correct cerebral irrigation. These compensatory mechanisms are lost in hypertension, thereby explaining, at least in part, the greater infarct volume observed in SHR.

Endothelium-dependent vasodilation of cerebral arteries is altered with simulated microgravity through nitric oxide synthase and EDHF mechanisms

Cephalic elevations in arterial pressure associated with microgravity and prolonged bed rest alter cerebrovascular autoregulation in humans. Using the head-down tail-suspended (HDT) rat to chronically induce headward fluid shifts and elevate cerebral artery pressure, previous work has likewise shown cerebral perfusion to be diminished. The purpose of this study was to test the hypothesis that 2 wk of HDT reduces cerebral artery vasodilation. To test this hypothesis, dose-response relations for endothelium-dependent (2-methylthioadenosine triphosphate and bradykinin) and endothelium-independent (nitroprusside) vasodilation were determined in vitro in middle cerebral arteries (MCAs) from HDT and control rats. All in vitro measurements were done in the presence and absence of the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (10(-5) M) and cyclooxygenase inhibitor indomethacin (10(-5) M). MCA caveolin-1 protein content was measured by immunoblot analysis. Endothelium-dependent vasodilation to 2-methylthioadenosine triphosphate and bradykinin were both lower in MCAs from HDT rats. These lower vasodilator responses were abolished with N(G)-nitro-L-arginine methyl ester but were unaffected by indomethacin. In addition, HDT was associated with lower levels of MCA caveolin-1 protein. Endothelium-independent vasodilation was not altered by HDT. These results indicate that chronic cephalic fluid shifts diminish endothelium-dependent vasodilation through alterations in the endothelial nitric oxide synthase signaling mechanism. Such decrements in endothelium-dependent vasodilation of cerebral arteries could contribute to the elevations in cerebral vascular resistance and reductions in cerebral perfusion that occur after conditions of simulated microgravity in HDT rats.

Endothelial influences on cerebrovascular tone

The cerebrovascular endothelium exerts a profound influence on cerebral vessels and cerebral blood flow. This review summarizes current knowledge of various dilator and constrictor mechanisms intrinsic to the cerebrovascular endothelium. The endothelium contributes to the resting tone of cerebral arteries and arterioles by tonically releasing nitric oxide (NO•). Dilations can occur by stimulated release of NO•, endothelium-derived hyperpolarization factor, or prostanoids. During pathological conditions, the dilator influence of the endothelium can turn to that of constriction by a variety of mechanisms, including decreased NO• bioavailability and release of endothelin-1. The endothelium may participate in neurovascular coupling by conducting local dilations to upstream arteries. Further study of the cerebrovascular endothelium is critical for understanding the pathogenesis of a number of pathological conditions, including stroke, traumatic brain injury, and subarachnoid hemorrhage.

Arachidonic acid metabolites, hydrogen peroxide, and EDHF in cerebral arteries

We tested the hypotheses that EDHF in rat middle cerebral arteries (MCAs) involves 1) metabolism of arachidonic acid through the epoxygenase pathway, 2) metabolism of arachidonic acid through the lipoxygenase pathway, or 3) reactive oxygen species. EDHF-mediated dilations were elicited in isolated and pressurized rat MCAs by activation of endothelial P2Y2receptors with either UTP or ATP. All studies were conducted after the inhibition of nitric oxide synthase and cyclooxygenase with Nω-nitro-l-arginine methyl ester (10 μM) and indomethacin (10 μM), respectively. The inhibition of epoxygenase with miconazole (30 μM) did not alter EDHF dilations to UTP, whereas the structurally different epoxygenase inhibitor N-methylsulfonyl-6-(2-propargyloxyphenyl)hexanoic acid (20 or 40 μM) only modestly inhibited EDHF at the highest concentration of UTP. An antagonist of epoxyeicosatrienoic acids, 14,15-epoxyeicosa-5( Z)-enoic acid, had no effect on EDHF dilations to UTP. Chronic inhibition of epoxygenase in the rat with 1-aminobenzotriazol (50 mg/kg twice daily for 5 days) did not alter EDHF dilations. The inhibition of the lipoxygenase pathway with either 10 μM baicalein or 10 μM nordihydroguaiaretic acid produced no major inhibitory effects on EDHF dilations. The combination of superoxide dismutase (200 U/ml) and catalase (140 U/ml) had no effect on EDHF dilations. Neither tiron (10 mM), a cell-permeable scavenger of reactive oxygen species, nor deferoxamine (1 or 10 mM), an iron chelator that blocks the formation of hydroxyl radicals, altered EDHF dilations in rat MCAs. We conclude that EDHF dilations in the rat MCA do not involve the epoxygenase pathway, lipoxygenase pathway, or reactive oxygen species including H2O2.

A physiological model of cerebral blood flow control

The construction of a computational model of the human brain circulation is described. We combine an existing model of the biophysics of the circulatory system, a basic model of brain metabolic biochemistry, and a model of the functioning of vascular smooth muscle (VSM) into a single model. This represents a first attempt to understand how the numerous different feedback pathways by which cerebral blood flow is controlled interact with each other. The present work comprises the following: Descriptions of the physiology underlying the model; general comments on the processes by which this physiology is translated into mathematics; comments on parameter setting; and some simulation results. The simulations presented are preliminary, but show qualitative agreement between model behaviour and experimental results.

Possible Role for K + in Endothelium-Derived Hyperpolarizing Factor–Linked Dilatation in Rat Middle Cerebral Artery

Background and Purpose— Endothelium-derived hyperpolarizing factor (EDHF) and K + are vasodilators in the cerebral circulation. Recently, K + has been suggested to contribute to EDHF-mediated responses in peripheral vessels. The EDHF response to the protease-activated receptor 2 ligand SLIGRL was characterized in cerebral arteries and used to assess whether K + contributes as an EDHF.

Methods— Rat middle cerebral arteries were mounted in either a wire or pressure myograph. Concentration-response curves to SLIGRL and K + were constructed in the presence and absence of a variety of blocking agents. In some experiments, changes in tension and smooth muscle cell membrane potential were recorded simultaneously.

Results— SLIGRL (0.02 to 20 μmol/L) stimulated concentration and endothelium-dependent relaxation. In the presence of N G -nitro- l -arginine methyl ester, relaxation to SLIGRL was associated with hyperpolarization and sensitivity to a specific inhibitor of IK Ca , 1-[(2-chlorophenyl)diphenylmethyl]-1 H -pyrazole (1μmol/L), reflecting activation of EDHF. Combined inhibition of K IR with Ba 2+ (30μmol/L) and Na + /K + -ATPase with ouabain (1 μmol/L) markedly attenuated the relaxation to EDHF. Raising extracellular [K + ] to 15 mmol/L also stimulated smooth muscle relaxation and hyperpolarization, which was also attenuated by combined application of Ba 2+ and ouabain.

Conclusions— SLIGRL evokes EDHF-mediated relaxation in the rat middle cerebral artery, underpinned by hyperpolarization of the smooth muscle. The profile of blockade of EDHF-mediated hyperpolarization and relaxation supports a pivotal role for IK Ca channels. Furthermore, similar inhibition of responses to EDHF and exogenous K + with Ba 2+ and ouabain suggests that K + may contribute as an EDHF in the middle cerebral artery.

Responses of cerebral arterioles to ADP: eNOS-dependent and eNOS-independent mechanisms

ADP mediates platelet-induced relaxation of blood vessels and may function as an important intercellular signaling molecule in the brain. We used pharmacological and genetic approaches to examine mechanisms that mediate responses of cerebral arterioles to ADP, including the role of endothelial nitric oxide synthase (eNOS). We examined responses of cerebral arterioles (control diameter approximately 30 microm) in anesthetized wild-type (WT, eNOS+/+) and eNOS-deficient (eNOS-/-) mice using a cranial window. In WT mice, local application of ADP produced vasodilation that was not altered by indomethacin but was reduced by approximately 50% by NG-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (inhibitors of NOS and soluble guanylate cyclase, respectively). In eNOS-/- mice, responses to ADP were largely preserved, and a significant component of the response was resistant to L-NNA (a finding similar to that in WT mice treated with L-NNA). In the absence of L-NNA, responses to ADP were markedly reduced by charybdotoxin plus apamin [inhibitors of Ca2+-dependent K+ channels and responses mediated by endothelium-derived hyperpolarizing factor (EDHF)] in both WT and eNOS-/- mice. Thus pharmacological and genetic evidence suggests that a significant portion of the response to ADP in cerebral microvessels is mediated by a mechanism independent of eNOS. The eNOS-independent mechanism is functional in the absence of inhibited eNOS and most likely is mediated by an EDHF.

Cerebral artery reactivity changes during pregnancy and the postpartum period: a role in eclampsia?

Eclampsia is thought to be similar to hypertensive encephalopathy, whereby acute elevations in intravascular pressure cause forced dilatation (FD) of intrinsic myogenic tone of cerebral arteries and arterioles, decreased cerebrovascular resistance, and hyperperfusion. In the present study, we tested the hypothesis that pregnancy and/or the postpartum period predispose cerebral arteries to FD by diminishing pressure-induced myogenic activity. We compared the reactivity to pressure (myogenic activity) as well as factors that modulate the level of tone of third-order branches (<200 μm) of the posterior cerebral artery (PCA) that were isolated from nonpregnant (NP, n = 7), late-pregnant (LP, 19 days, n = 10), and postpartum (PP, 3 days, n = 8) Sprague-Dawley rats under pressurized conditions. PCAs from all groups of animals developed spontaneous tone within the myogenic pressure range (50–150 mmHg) and constricted arteries at 100 mmHg (NP, 30 ± 3; LP, 39 ± 4; and PP, 42 ± 7%; P > 0.05). This level of myogenic activity was maintained in the NP arteries at all pressures; however, both LP and PP arteries dilated at considerably lower pressures compared with NP, which lowered the pressure at which FD occurred from >175 for NP to 146 ± 6.5 mmHg for LP ( P < 0.01 vs. NP) and 162 ± 7.7 mmHg for PP ( P < 0.01 vs. NP). The amount of myogenic tone was also significantly diminished at 175 mmHg compared with NP: percent tone for NP, LP, and PP animals were 35 ± 2, 11 ± 3 ( P < 0.01 vs. NP), and 20 ± 7% ( P < 0.01 vs. NP), respectively. Inhibition of nitric oxide (NO) with 0.1 mM Nω-nitro-l-arginine (l-NNA) caused constriction of all vessel types that was significantly increased in the PP arteries, which demonstrates significant basal NO production. Reactivity to 5-hydroxytryptamine (serotonin) was assessed in the presence of l-NNA and indomethacin. There was a differential response to serotonin: PCAs from NP animals dilated, whereas LP and PP arteries constricted. These results suggest that both pregnancy and the postpartum period predispose the cerebral circulation to FD at lower pressures, a response that may lower cerebrovascular resistance and promote hyperperfusion when blood pressure is elevated, as occurs during eclampsia.

Testosterone suppresses endothelium-dependent dilation of rat middle cerebral arteries

Little is known about vascular effects of testosterone. We previously reported chronic testosterone treatment increases vascular tone in middle cerebral arteries (MCA; 300 μm diameter) of male rats. In the present study, we investigated the hypothesis that physiological levels of circulating testosterone affect endothelial factors that modulate cerebrovascular reactivity. Small branches of MCA (150 μm diameter) were isolated from orchiectomized (ORX) and testosterone-treated (ORX+T) rats. Intraluminal diameters were recorded after step changes in intraluminal pressure (20–100 Torr) in the absence or presence of NG-nitro-l-arginine-methyl ester (l-NAME), a nitric oxide synthase (NOS) inhibitor; indomethacin, a cyclooxygenase (COX) inhibitor; and/or apamin and charybdotoxin (CTX); and KCa channel blockers used to inhibit endothelium-derived hyperpolarizing factors (EDHF). At intraluminal pressures ≥60 Torr, arteries from ORX+T developed greater tone compared with ORX arteries. This difference was abolished by removal of the endothelium but remained after treatment of intact arteries with indomethacin or l-NAME. In addition, testosterone treatment had no effect on cerebrovascular production of endothelin-1 or prostacyclin nor did it alter protein levels of endothelial NOS or COX-1. Endothelium removal after l-NAME/indomethacin exposure caused an additional increase in tone. Interestingly, the latter effect was smaller in arteries from ORX+T, suggesting testosterone affects endothelial vasodilators that are independent of NOS and COX. Apamin/CTX, in the presence of l-NAME/indomethacin, abolished the difference in tone between ORX and ORX+T and resulted in vessel diameters similar to those of endothelium-denuded preparations. In conclusion, testosterone may modulate vascular tone in cerebral arteries by suppressing EDHF.

ATP-sensitive potassium channels in the cerebral circulation

BACKGROUND

In brain blood vessels, electrophysiological studies proving the existence of ATP-sensitive potassium channels (KATP) are scarce. However, numerous pharmacological studies establish the importance of KATP channels in these blood vessels. This review emphasizes the data supporting the importance of vascular KATP in the responses of brain blood vessels.

SUMMARY OF REVIEW

Electrophysiological data show the existence of KATP in smooth muscle and endothelium of brain vessels. A much larger number of studies in virtually all experimental species have shown that classic openers of KATP dilate brain arteries and arterioles. This response can by blocked by glibenclamide, a selective inhibitor of KATP opening. Several physiological or pathophysiological responses are also blocked by glibenclamide. KATP contains a multiplicity of potential sites of interaction with drugs of diverse, sometimes unrelated, structures. Drugs with imidazole or guanidinium groups are particularly likely to have effects on KATP. This complicates interpretation of the actions of such drugs when used as supposedly selective pharmacological probes for other putative targets. A pH-sensitive site on the internal surface of cloned channels may explain the glibenclamide-inhibitable dilation produced by intracellular acidosis and perhaps by CO2. In some situations KATP appears to be involved in either the synthesis/release or action of endothelium-derived mediators of cerebrovascular tone. The importance of KATP may be dependent on the portion of the cerebrovascular tree being studied and on diverse experimental conditions, age, species, and the presence of disease.

CONCLUSIONS

KATP have been shown to mediate a wide range of cerebrovascular response in physiologic or pathologic circumstances in a variety of experimental conditions. Their relevance to cerebrovascular responses in humans remains to be explored.

Altered calcium dynamics do not account for attenuation of endothelium-derived hyperpolarizing factor-mediated dilations in the female middle cerebral artery

BACKGROUND AND PURPOSE

The contribution of endothelium-derived hyperpolarizing factor (EDHF) to ATP-mediated dilations is significantly attenuated in the rat middle cerebral artery of intact and estrogen-treated ovariectomized (OVX) females compared with males and vehicle-treated OVX females. Since an increase in endothelial calcium appears to be a critical prerequisite in the EDHF response, we tested the hypothesis that endothelial cell intracellular calcium ([Ca(2+)](i)) fails to reach sufficient levels to elicit robust EDHF-mediated dilations in females and that this effect is mediated by estrogen.

METHODS

Vascular diameter and [Ca(2+)](i) were measured concomitantly in perfused middle cerebral artery segments with the use of videomicroscopy and fura 2 fluorescence, respectively.

RESULTS

In the presence of N(G)-nitro-L-arginine methyl ester and indomethacin, the dilation to 10(-5) mol/L ATP was significantly reduced (P<0.05) in intact females (42+/-8%; n=6) and estrogen-treated OVX females (25+/-6%; n=9) compared with intact males (89+/-5%; n=6) and vehicle-treated OVX females (92+/-2%; n=7). Contrary to our initial hypothesis, endothelial cell [Ca(2+)](i) increased to comparable levels in intact females (461+/-116 nmol/L), estrogen-treated OVX females (417+/-50 nmol/L), intact males (421+/-77 nmol/L), and vehicle-treated OVX females (530+/-92 nmol/L). In response to luminal ATP (10(-5) mol/L), smooth muscle cell [Ca(2+)](i) decreased to a greater degree in males (37+/-4%; n=8) compared with females (21+/-5%; n=7) and in vehicle-treated OVX females (18+/-7%; n=7) compared with estrogen-treated OVX females (3+/-5%; n=9).

CONCLUSIONS

Our data suggest that loss of a factor coupling EDHF to reduction of ionized smooth muscle cell [Ca(2+)](i) accounts for the attenuated EDHF-mediated dilations in the female middle cerebral artery.

Role of cytoplasmic phospholipase A2 in endothelium-derived hyperpolarizing factor dilations of rat middle cerebral arteries

Very little is known regarding the mechanism of action for the endothelium-derived hyperpolarizing factor (EDHF) response in cerebral vessels. The authors tested two hypotheses: (1) activation of the cytoplasmic form of phospholipase A (cPLA ) is involved with EDHF-mediated dilations in rat middle cerebral arteries; and (2) activation of the cPLA involves an increase in endothelial Ca through activation of phospholipase C. Middle cerebral arteries were isolated from the rat, pressurized to 85 mm Hg, and luminally perfused. The EDHF response was elicited by luminal application of uridine triphosphate (UTP) after NO synthase and cyclooxygenase inhibition (10 mol/L -nitro-l-arginine methyl ester and 10 mol/L indomethacin, respectively). AACOCF and PACOCF, inhibitors of cPLA (Ca -sensitive) and Ca -insensitive PLA (iPLA ), dose dependently attenuated the EDHF response. A selective inhibitor for iPLA2, haloenol lactone suicide substrate, had no effect on the EDHF response. The EDHF response elicited by UTP was accompanied by an increase in endothelial Ca (144 to 468 nmol/L), and the EDHF dilation was attenuated with U73122, a phospholipase C inhibitor. The authors conclude that the EDHF response elicited by luminal UTP in rat middle cerebral arteries involved activation of phospholipase C, an increase in endothelial Ca, and activation of cPLA.

Altered endothelial Ca2+ regulation after ischemia/reperfusion produces potentiated endothelium-derived hyperpolarizing factor-mediated dilations

BACKGROUND AND PURPOSE

Endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations are potentiated after several pathologies, including ischemia/reperfusion (I/R). However, no study to date has addressed the mechanism by which this potentiation occurs. This study tested the hypothesis that potentiated EDHF-mediated dilations are due to altered endothelial Ca2+ handling after I/R.

METHODS

Rat middle cerebral arteries (MCAs) were isolated after 2 hours of MCA occlusion and 24 hours of reperfusion (or sham surgery). This model has been previously demonstrated to produce potentiated EDHF-mediated dilations. MCAs were studied in a pressurized/perfused vessel chamber equipped for the simultaneous measurement of endothelial Ca2+ (with fura 2) and artery diameter. Measures were made after luminal administration of UTP (P2Y2 purinoceptor agonist), 2 MeS-ATP (P2Y1 purinoceptor agonist), and Br-A23187 (receptor-independent Ca2+ ionophore) for sham and I/R MCAs.

RESULTS

I/R resulted in significantly potentiated UTP-mediated dilations (through a P2Y2 purinoceptor) and endothelial Ca2+ responses in the presence of N(G)-nitro-L-arginine methyl ester (L-NAME) and indomethacin. Endothelial Ca2+ and diameter responses were also significantly potentiated with 2 MeS-ATP (through a P2Y1 purinoceptor) when L-NAME and indomethacin were absent. Br-A23187, a receptor-independent Ca2+ ionophore, produced significantly potentiated endothelial Ca2+ responses after I/R in the presence of L-NAME/indomethacin. Evaluation of artery diameter as a function of endothelial Ca2+ demonstrated no differences between sham and I/R groups.

CONCLUSIONS

These findings demonstrate that I/R results in augmented endothelial Ca2+ responses that appear to be downstream of the receptor level. Moreover, these data suggest that this augmented Ca2+ response contributes to the potentiated EDHF-mediated dilations after I/R.