ATP-sensitive potassium channels may participate in the coupling of neuronal activity and cerebrovascular tone

KATP channels in cerebral hemodynamics: a systematic review of preclinical and clinical studies

Cumulative evidence suggests that ATP-sensitive potassium (KATP) channels act as a key regulator of cerebral blood flow (CBF). This implication seems to be complicated, since KATP channels are expressed in several vascular-related structures such as smooth muscle cells, endothelial cells and pericytes. In this systematic review, we searched PubMed and EMBASE for preclinical and clinical studies addressing the involvement of KATP channels in CBF regulation. A total of 216 studies were screened by title and abstract. Of these, 45 preclinical and 6 clinical studies were included. Preclinical data showed that KATP channel openers (KCOs) caused dilation of several cerebral arteries including pial arteries, the middle cerebral artery and basilar artery, and KATP channel inhibitor (KCI) glibenclamide, reversed the dilation. Glibenclamide affected neither the baseline CBF nor the baseline vascular tone. Endothelium removal from cerebral arterioles resulted in an impaired response to KCO/KCI. Clinical studies showed that KCOs dilated cerebral arteries and increased CBF, however, glibenclamide failed to attenuate these vascular changes. Endothelial KATP channels played a major role in CBF regulation. More studies investigating the role of KATP channels in CBF-related structures are needed to further elucidate their actual role in cerebral hemodynamics in humans. Systematic review registration: Prospero: CRD42023339278 (preclinical data) and CRD42022339152 (clinical data).

Brain capillary pericytes are metabolic sentinels that control blood flow through a KATP channel-dependent energy switch

Despite the abundance of capillary thin-strand pericytes and their proximity to neurons and glia, little is known of the contributions of these cells to the control of brain hemodynamics. We demonstrate that the pharmacological activation of thin-strand pericyte KATP channels profoundly hyperpolarizes these cells, dilates upstream penetrating arterioles and arteriole-proximate capillaries, and increases capillary blood flow. Focal stimulation of pericytes with a KATP channel agonist is sufficient to evoke this response, mediated via KIR2.1 channel-dependent retrograde propagation of hyperpolarizing signals, whereas genetic inactivation of pericyte KATP channels eliminates these effects. Critically, we show that decreasing extracellular glucose to less than 1 mM or inhibiting glucose uptake by blocking GLUT1 transporters in vivo flips a mechanistic energy switch driving rapid KATP-mediated pericyte hyperpolarization to increase local blood flow. Together, our findings recast capillary pericytes as metabolic sentinels that respond to local energy deficits by increasing blood flow to neurons to prevent energetic shortfalls.

Brain Bioenergetics in Chronic Hypertension: Risk Factor for Acute Ischemic Stroke

Chronic hypertension is one of the key modifiable risk factors for acute ischemic stroke, also contributing to determine greater neurological deficits and worse functional outcome when an acute cerebrovascular event would occur. A tight relationship exists between cerebrovascular autoregulation, neuronal activity and brain bioenergetics. In chronic hypertension, progressive adaptations of these processes occur as an attempt to cope with the demanding necessity of brain functions, creating a new steady-state homeostatic condition. However, these adaptive modifications are insufficient to grant an adequate response to possible pathological perturbations of the established fragile hemodynamic and metabolic homeostasis. In this narrative review, we will discuss the main mechanisms by which alterations in brain bioenergetics and mitochondrial function in chronic hypertension could lead to increased risk of acute ischemic stroke, stressing the interconnections between hemodynamic factors (i.e. cerebral autoregulation and neurovascular coupling) and metabolic processes. Both experimental and clinical pieces of evidence will be discussed. Moreover, the potential role of mitochondrial dysfunction in determining, or at least sustaining, the pathogenesis and progression of chronic neurogenic hypertension will be considered. In the perspective of novel therapeutic strategies aiming at improving brain bioenergetics, we propose some determinant factors to consider in future studies focused on the cause-effect relationships between chronic hypertension and brain bioenergetic abnormalities (and vice versa), so to help translational research in this so-far unfilled gap.

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.

Compromised regulation of the rat brain parenchymal arterioles in vasopressin-associated acute hyponatremia

OBJECTIVE

In this study, we examined the effect of acute hyponatremia associated with vasopressin (AVP) on the responses of the isolated rat's MCAs and PAs to acidosis, nitric oxide donor (SNAP) and to endothelium-dependent vasodilator ATP.

METHODS

The studies were performed on isolated, perfused and pressurized MCAs and PAs in control conditions and during AVP-associated hyponatremia. Hyponatremia was induced in vitro by lowering Na⁺ concentration from 144 to 121 mmol/L in intra- and extravascular fluid in the presence of AVP.

RESULTS

Parenchymal arterioles showed greater response to an increase in H⁺ and K⁺ ions concentration and to ATP in comparison with MCAs in control normonatremic conditions. Both PAs and MCAs constricted in response to acute hyponatremia associated with AVP. Interestingly, disordered regulation of vascular tone was observed in PAs but not in MCAs. The abnormalities in the regulation comprised a significant reduction of PA response to acidosis and the absence of the response to the administration of SNAP or ATP.

CONCLUSIONS

Arginine vasopressin-associated hyponatremia leads to constriction and dysregulation of PAs which may impair neurovascular coupling.

Cerebrospinal fluid dynamics and intracranial pressure elevation in neurological diseases

The fine balance between the secretion, composition, volume and turnover of cerebrospinal fluid (CSF) is strictly regulated. However, during certain neurological diseases, this balance can be disrupted. A significant disruption to the normal CSF circulation can be life threatening, leading to increased intracranial pressure (ICP), and is implicated in hydrocephalus, idiopathic intracranial hypertension, brain trauma, brain tumours and stroke. Yet, the exact cellular, molecular and physiological mechanisms that contribute to altered hydrodynamic pathways in these diseases are poorly defined or hotly debated. The traditional views and concepts of CSF secretion, flow and drainage have been challenged, also due to recent findings suggesting more complex mechanisms of brain fluid dynamics than previously proposed. This review evaluates and summarises current hypotheses of CSF dynamics and presents evidence for the role of impaired CSF dynamics in elevated ICP, alongside discussion of the proteins that are potentially involved in altered CSF physiology during neurological disease. Undoubtedly CSF secretion, absorption and drainage are important aspects of brain fluid homeostasis in maintaining a stable ICP. Traditionally, pharmacological interventions or CSF drainage have been used to reduce ICP elevation due to over production of CSF. However, these drugs are used only as a temporary solution due to their undesirable side effects. Emerging evidence suggests that pharmacological targeting of aquaporins, transient receptor potential vanilloid type 4 (TRPV4), and the Na+-K+-2Cl- cotransporter (NKCC1) merit further investigation as potential targets in neurological diseases involving impaired brain fluid dynamics and elevated ICP.

Are you in or out? Leukocyte, ion, and neurotransmitter permeability across the epileptic blood-brain barrier

The credo that epileptic seizures can be initiated only by "epileptic" neurons has been recently challenged. The recognition of key astrocytic-neuronal communication, and the close interaction and crosstalk between astrocytes and brain endothelial cells, has shifted attention to the blood-brain barrier (BBB) and the "neurovascular unit." Therefore, the pursuit of mechanisms of seizure generation and epileptogenesis now includes investigations of cerebral blood flow and permeability of cerebral microvessels. For example, leukocyte adhesion molecules at the BBB have been proposed to play a role as an initiating factor for pilocarpine-induced status epilepticus, and a viral infection model with a strong BBB etiology has been used to study epileptogenesis. Finally, the fact that in nonepileptic subjects seizures can be triggered by BBB disruption, together with the antiseizure effects obtained by administration of potent antiinflammatory "BBB repair" drugs, has increased the interest in neuroinflammation; both circulating leukocytes and resident microglia have been studied in this context. The dual scope of this review is the following: (1) outline the proposed role of BBB damage and immune cell activation in seizure disorders; and (2) explain how increased cerebrovascular permeability causes neuronal misfiring. The temporal sequence linking seizures to peripheral inflammation and BBB dysfunction remains to be clarified. For example, it is still debated whether seizures cause systemic inflammation or vice versa. The topographic localization of fundamental triggers of epileptic seizures also remains controversial: Are immunologic mechanisms required for seizure generation brain-specific or is systemic activation of immunity sufficient to alter neuronal excitability? Finally, the causative role of "BBB leakage" remains a largely unresolved issue.

Mechanism of alkalosis-induced constriction of rat cerebral penetrating arterioles

Cerebral arterioles are in close contact with the supplied tissue and are strong regulators of cerebrovascular tone. Transient ischemia can cause brain intracellular alkalosis producing vasoconstriction. However, the mechanisms of alkalosis-induced cerebral arteriolar constriction are poorly understood. Here, we determined the vascular responses to alkalosis under different conditions by monitoring the internal diameter of pressurized penetrating arterioles isolated from the rat cerebrum with an operating microscope. The roles of Na+/H+ exchanger (NHE), Na+/Ca²+ exchanger (NCX), Na+/K+-adenosine triphosphatase (NKA), and potassium (K+) channels during alkalosis were examined using specific inhibitors. Our results indicated that the extent of constriction of the penetrating arterioles was dependent on alkaline pH. Moreover, the alkalosis-induced vasoconstriction was significantly attenuated by inhibitors of NHE, NCX, and NKA, but not K+ channel inhibitors. Therefore, we concluded that NHE, NKA, and NCX are important regulators involved in alkalosis-induced vasoconstriction of rat cerebral penetrating arterioles.

Impaired rapid modulation of cerebral hemodynamics during a planning task in schizophrenia

OBJECTIVE

Patients with schizophrenia show deficits in planning, and the Stockings of Cambridge (SOC) is a task that assesses planning performance. This study was undertaken to investigate rapid changes of cerebral hemodynamics during separate phases of SOC in schizophrenia and normals by means of functional transcranial Doppler sonography (fTCD).

METHODS

We included 21 patients with chronic schizophrenia and a control group of 20 healthy subjects in the study. They underwent fTCD of the middle (MCA) and anterior cerebral arteries (ACA) during performance of SOC.

RESULTS

The main finding was that healthy subjects significantly modulated the early cerebral hemodynamic response along distinct conditions of SOC, whereas we observed no significant differences in patients. Normally, there was an up-regulation of cerebral hemodynamics during mental planning, and about zero values were observed during movement execution. Patients showed lower development of the early cerebral hemodynamic response during planning of SOC.

CONCLUSIONS

The findings of this study suggest a uniform pattern of cerebral hemodynamic regulation during a planning task in schizophrenia, whereas healthy subjects modulated such a response along a planning-movement execution sequence.

SIGNIFICANCE

We provide novel evidence that modulation of cerebral hemodynamics is compromised in schizophrenia, and that fTCD constitutes a proper method to measure these alterations.

Brain water and ion fluxes: a hard-to-die hypothesis to explain seizures

Increased Seizure Duration and Slowed Potassium Kinetics in Mice Lacking Aquaporin-4 Water Channels. Binder DK, Yao X, Zador Z, Sick TJ, Verkman AS, Manley GT. Glia 2006;53:631–636. The glial water channel aquaporin-4 (AQP4) has been hypothesized to modulate water and potassium fluxes associated with neuronal activity. In this study, we examined the seizure phenotype of AQP4 -/- mice using in vivo electrical stimulation and electroencephalographic (EEG) recording. AQP4 -/- mice were found to have dramatically prolonged stimulation-evoked seizures after hippocampal stimulation compared to wild-type controls (33 μ 2 s vs. 13 μ 2 s). In addition, AQP4 -/- mice were found to have a higher seizure threshold (167 μ 17 A vs. 114 μ 10 A). To assess a potential effect of AQP4 on potassium kinetics, we used in vivo recording with potassium-sensitive microelectrodes after direct cortical stimulation. Although there was no significant difference in baseline or peak [K+]o, the rise time to peak [K+]o ( t1/2, 2.3 μ 0.5 s) as well as the recovery to baseline [K+]o ( t1/2, 15.6 μ 1.5 s) were slowed in AQP4 -/- mice compared to WT mice ( t1/2, 0.5 μ 0.1 and 6.6 μ 0.7 s, respectively). These results implicate AQP4 in the expression and termination of seizure activity and support the hypothesis that AQP4 is coupled to potassium homeostasis in vivo.

Ketogenic diet metabolites reduce firing in central neurons by opening K(ATP) channels

A low-carbohydrate ketogenic diet remains one of the most effective (but mysterious) treatments for severe pharmacoresistant epilepsy. We have tested for an acute effect of physiological ketone bodies on neuronal firing rates and excitability, to discover possible therapeutic mechanisms of the ketogenic diet. Physiological concentrations of ketone bodies (beta-hydroxybutyrate or acetoacetate) reduced the spontaneous firing rate of neurons in slices from rat or mouse substantia nigra pars reticulata. This region is thought to act as a "seizure gate," controlling seizure generalization. Consistent with an anticonvulsant role, the ketone body effect is larger for cells that fire more rapidly. The effect of ketone bodies was abolished by eliminating the metabolically sensitive K(ATP) channels pharmacologically or by gene knock-out. We propose that ketone bodies or glycolytic restriction treat epilepsy by augmenting a natural activity-limiting function served by K(ATP) channels in neurons.

Neurovascular coupling in the normal brain and in hypertension, stroke, and Alzheimer disease

The brain is critically dependent on a continuous supply of blood to function. Therefore, the cerebral vasculature is endowed with neurovascular control mechanisms that assure that the blood supply of the brain is commensurate to the energy needs of its cellular constituents. The regulation of cerebral blood flow (CBF) during brain activity involves the coordinated interaction of neurons, glia, and vascular cells. Thus, whereas neurons and glia generate the signals initiating the vasodilation, endothelial cells, pericytes, and smooth muscle cells act in concert to transduce these signals into carefully orchestrated vascular changes that lead to CBF increases focused to the activated area and temporally linked to the period of activation. Neurovascular coupling is disrupted in pathological conditions, such as hypertension, Alzheimer disease, and ischemic stroke. Consequently, CBF is no longer matched to the metabolic requirements of the tissue. This cerebrovascular dysregulation is mediated in large part by the deleterious action of reactive oxygen species on cerebral blood vessels. A major source of cerebral vascular radicals in models of hypertension and Alzheimer disease is the enzyme NADPH oxidase. These findings, collectively, highlight the importance of neurovascular coupling to the health of the normal brain and suggest a therapeutic target for improving brain function in pathologies associated with cerebrovascular dysfunction.

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.

KATP-channel-induced vasodilation is modulated by the Na,K-pump activity in rabbit coronary small arteries

The purpose of the study was to evaluate the importance of the Na,K-pump in relaxations induced by K(ATP)-channel openers in rabbit coronary small arteries. Arterial segments were mounted in myographs for recording of isometric tension. Whole-cell patch clamp was used to assess K(ATP)-channel currents in isolated smooth muscle cells from the arteries. In arteries preconstricted with the thromboxane A(2) analogue U46619 pinacidil and cromakalim induced concentration-dependent relaxations. In arteries preconstricted with potassium (124 mM) only high concentrations of pinacidil had a small relaxant effect. In arteries preconstricted with U46619 pinacidil-induced relaxations were unaffected by pretreatment with N(omega)-nitro-L-arginine (L-NNA) and only slightly reduced after mechanical removal of the endothelium. Pinacidil induced relaxations were not significantly affected by 1 microM glibenclamide. However, the relaxations were partly inhibited in potassium-free media and by 1 microM ouabain. In contrast, the concentration-dependent relaxation to cromakalim was partly blocked by 1 microM glibenclamide and partly by 1 microM ouabain and when both drugs were present the inhibition increased. Ouabain (1 microM) and glibenclamide (1 microM) each partly inhibited an ATP-sensitive current induced by pinacidil and cromakalim. In the presence of both inhibitors a greater inhibition was seen. When the solution in the patch pipette was sodium-free the current was reduced and ouabain had no effect. The study suggests that the relaxation to cromakalim and most likely pinacidil is mediated through opening of K(ATP) channels. Inhibition of the Na,K-pump, however, may change the local environment for the K(ATP) channels (i.e. increases the ATP/ADPratio and/or decreases the transmembrane potassium gradient), which partly prevents the activation of the K(ATP)-channel current.

Protection against hypoxic–ischemic injury in transgenic mice overexpressing Kir6.2 channel pore in forebrain

The role of the K-ATP channel pore-forming subunit Kir6.2 on protection from cerebral hypoxic-ischemic injury was assessed in transgenic mice overexpressing normal Kir6.2 or a dominant negative form (AFA) of this subunit in the forebrain. The resulting mice overexpress either the Kir6.2 or the AFA transgene mainly in the cerebral cortex and hippocampus. The Kir6.2 transgenic mice are resistant to hypoxic-ischemic injury showing a decreased region of cortical damage as compared to the dominant negative AFA and the wild-type mice. Moreover, the overexpression of Kir6.2 allowed an important silencing of the neurons present in forebrain regions thus protecting them from ischemic injury. Interestingly, the phenotype observed in Kir6.2 transgenic mice was observed without increased sulfonylurea binding. Taken together, these results indicate that the transgenic overexpression of Kir6.2 in forebrain significantly protects mice from hypoxic-ischemic injury and neuronal damage seen in stroke.

Hypercapnic acidosis activates KATP channels in vascular smooth muscles

ATP-sensitive K+ channels (KATP) couple intermediary metabolism to cellular activity, and may play a role in the autoregulation of vascular tones. Such a regulation requires cellular mechanisms for sensing O2, CO2, and pH. Our recent studies have shown that the pancreatic KATP isoform (Kir6.2/SUR1) is regulated by CO2/pH. To identify the vascular KATP isoform(s) and elucidate its response to hypercapnic acidosis, we performed these studies on vascular smooth myocytes (VSMs). Whole-cell and single-channel currents were studied on VSMs acutely dissociated from mesenteric arteries and HEK293 cells expressing Kir6.1/SUR2B. Hypercapnic acidosis activated an inward rectifier current that was K+-selective and sensitive to levcromakalim and glibenclamide with unitary conductance of approximately 35pS. The maximal activation occurred at pH 6.5 to 6.8, and the current was inhibited at pH 6.2 to 5.9. The cloned Kir6.1/SUR2B channel responded to hypercapnia and intracellular acidification in an almost identical pattern to the VSM current. In situ hybridization histochemistry revealed expression of Kir6.1/SUR2B mRNAs in mesenteric arteries. Hypercapnia produced vasodilation of the isolated and perfused mesenteric arteries. Pharmacological interference of the KATP channels greatly eliminated the hypercapnic vasodilation. These results thus indicate that the Kir6.1/SUR2B channel is a critical player in the regulation of vascular tones during hypercapnic acidosis.

Mechanism of extracellular K+-induced local and conducted responses in cerebral penetrating arterioles

BACKGROUND AND PURPOSE

Extracellular concentration of potassium ion ([K+]o) may have a significant influence on the cerebral circulation in health and disease. Mechanisms of [K+]o-induced conducted vasomotor responses in cerebral arterioles, possibly linking microvascular regulation to neuronal activity, have not been examined.

METHODS

We analyzed vascular responses to small increases of [K+]o (up to 5 mmol/L) in isolated, cannulated, and pressurized rat cerebral arterioles (36.5+/-1.4 micro m). [K+]o was elevated globally through extraluminal application or locally through micropipette, while arteriolar diameter was measured online.

RESULTS

Elevation of [K+]o (5 mmol/L) produced dilation that was inhibited by ouabain but not BaCl2. Locally applied [K+]o (3 to 5 mmol/L) produced a biphasic response (initial constriction followed by dilation), both of which were conducted to the remote site (distance 1142+/-68 microm). Endothelial impairment inhibited conducted but not local biphasic responses. Extraluminal ouabain attenuated local and conducted secondary dilation but not initial constriction. The local biphasic response was unaffected by extraluminal or intraluminal BaCl2. Extraluminal but not intraluminal BaCl2 impaired both conducted constriction and dilation.

CONCLUSIONS

In rat penetrating arteriole, (1) [K+]o (3 to 5 mmol/L) strongly regulates arteriolar tone and causes conducted vasomotor responses; (2) local responses to elevated [K+]o are endothelium independent but conducted responses are dependent on an intact endothelium; (3) smooth muscle Na+-K+-ATPase activation is the generator of conducted dilation; and (4) smooth muscle inward rectifier potassium channels sustain conduction. Our findings suggest that potassium-induced conducted vasomotor responses may link local neuronal activity to microvascular regulation, which may be attenuated in pathological conditions.

Neuronal NO mediates cerebral vasodilator responses to K+ in hypertensive rats

Potassium ion (K+) normally causes cerebral vasodilatation by activating inwardly rectifying K+ (K(IR)) channels. We tested whether chronic hypertension affects the magnitude and/or mechanism of K+-induced cerebral vasodilatation in vivo. Basilar artery responses were examined in anesthetized Wistar-Kyoto (WKY; mean arterial pressure, 114+/-4 mm Hg) and spontaneously hypertensive (SHR; 176+/-3 mm Hg) rats. In WKY, elevating cerebrospinal fluid K+ concentration from 3 mmol/L to 5 and 10 mmol/L caused vasodilatation (percent maximum, 12+/-1 and 48+/-7, respectively). The response to 5 mmol/L K+ was greater in SHR (percent maximum, 17+/-2 [P<0.05 versus WKY] and 49+/-4). The K(IR) channel inhibitor, barium ion (Ba2+, 100 micromol/L) selectively inhibited dilator responses to 5 and 10 mmol/L K+ by approximately 75% in WKY. In SHR, Ba2+ had no effect on the response to 5 mmol/L K+, and only partially inhibited (by approximately 40%) the response to 10 mmol/L K+. The nonselective NO synthase (NOS) inhibitor N(omega)-nitro-L-arginine methyl ester, the neuronal NOS (nNOS) inhibitor 1-(2-trifluromethyl-phenyl)imidazole, and the N-type calcium channel inhibitor omega-conotoxin GVIA, were all without effect in WKY, but markedly inhibited the response to 5 mmol/L K+ in SHR. When applied together with Ba2+, each of these inhibitors also profoundly reduced responses to 10 mmol/L K+ in SHR. Immunostaining of basilar arteries revealed that the perivascular nNOS-containing nerve plexus was denser in SHR. Thus, K+ dilates the normotensive basilar artery predominantly via K(IR) channel activation. During chronic hypertension, small physiological elevations in K+ dilate the basilar artery by an nNOS-dependent mechanism that appears to be upregulated in a compensatory manner.

Role of endothelial nitric oxide and smooth muscle potassium channels in cerebral arteriolar dilation in response to acidosis

BACKGROUND AND PURPOSE

Potassium channels or nitric oxide or both are major mediators of acidosis-induced dilation in the cerebral circulation. However, these contributions depend on a variety of factors such as species and vessel location. The present study was designed to clarify whether potassium channels and endothelial nitric oxide are involved in acidosis-induced dilation of isolated rat cerebral arterioles.

METHODS

Cerebral arterioles were cannulated and monitored with an inverted microscope. Acidosis (pH 6.8 to 7.4) produced by adding hydrogen ions mediated dilation of the cerebral arterioles in a concentration-dependent manner. The role of nitric oxide and potassium channels in response to acidosis was examined with several specific inhibitors and endothelial damage.

RESULTS

The dilation was significantly inhibited by potassium chloride (30 mmol/L) and glibenclamide (3 micromol/L; ATP-sensitive potassium channel inhibitor). We found that 30 micromol/L BaCl2 (concentration-dependent potassium channel inhibitor) also affected the dilation; however, an additional treatment of 3 micromol/L glibenclamide did not produce further inhibition. Tetraethylammonium ion (1 mmol/L; calcium-activated potassium channel inhibitor) and 4-aminopyridine (100 micromol/L; voltage-dependent potassium channel inhibitor) as well as ouabain (10 micromol/L; Na-K ATPase inhibitor) and N-methylsulphonyl-6-(2-proparglyloxyphenyl) hexanamide (1 micromol/L; cytochrome P450 epoxygenase inhibitor) did not alter acidotic dilation. N(omega)-Monomethyl-L-arginine (10 micromol/L) and N(omega)-nitro-L-arginine (10 micromol/L) as nitric oxide synthase inhibitor blunted the dilation. Furthermore, the dilation was significantly attenuated after the endothelial impairment. Additional treatment with glibenclamide (3 micromol/L) further reduced the dilation in response to acidosis.

CONCLUSIONS

Endothelial nitric oxide and smooth muscle ATP-sensitive potassium channels contribute to acidosis-induced dilation of rat cerebral arterioles. Endothelial damage caused by pathological conditions such as subarachnoid hemorrhage or traumatic brain injury may contribute to reduced blood flow despite injury-induced cerebral acidosis.

Cyclooxygenase-1 participates in selected vasodilator responses of the cerebral circulation

Cyclooxygenase (COX) is a prostanoid-synthesizing enzyme present in 2 isoforms: COX-1 and COX-2. Although it has long been hypothesized that prostanoids participate in cerebrovascular regulation, the lack of adequate pharmacological tools has led to conflicting results and has not permitted investigators to define the relative contribution of COX-1 and COX-2. We used the COX-1 inhibitor SC-560 and COX-1-null (COX-1(-/-)) mice to investigate whether COX-1 plays a role in cerebrovascular regulation. Mice were anesthetized (urethane and chloralose) and equipped with a cranial window. Cerebral blood flow (CBF) was measured by laser Doppler flowmetry or by the (14)C-iodoantipyrine technique with quantitative autoradiography. In wild-type mice, SC-560 (25 micromol/L) reduced resting CBF by 21+/-4% and attenuated the CBF increase produced by topical application of bradykinin (-59%) or calcium ionophore A23187 (-49%) and by systemic hypercapnia (-58%) (P<0.05 to 0.01). However, SC-560 did not reduce responses to acetylcholine or the increase in somatosensory cortex blood flow produced by vibrissal stimulation. In COX-1(-/-) mice, resting CBF assessed by (14)C-iodoantipyrine was reduced (-13% to -20%) in cerebral cortex and other telencephalic regions (P<0.05). The CBF increase produced by bradykinin, A23187, and hypercapnia, but not acetylcholine or vibrissal stimulation, were attenuated (P<0.05 to 0.01). The free radical scavenger superoxide dismutase attenuated responses to bradykinin and A23187 in wild-type mice but not in COX-1(-/-) mice, suggesting that COX-1 is the source of the reactive oxygen species known to mediate these responses. The data provide evidence for a critical role of COX-1 in maintaining resting vascular tone and in selected vasodilator responses of the cerebral microcirculation.

Role of potassium channels in regulation of brain arteriolar tone: comparison of cerebrum versus brain stem

BACKGROUND AND PURPOSE

Potassium channels are important regulators of resting tone in large cerebral arteries, but their activity and distribution may vary according to vessel location and species studied. In the cerebral microcirculation in vivo, however, these channels appear to be silent at rest. Our goal was to determine the activity of potassium channels of brain arterioles from 2 origins under basal conditions in vitro.

METHODS

Penetrating cerebral (40. 9+/-2.2 microm control diameter) and brain stem (36.2+/-1.2 microm) arterioles of rats were prepared from middle cerebral and basilar arteries, respectively. The internal diameter of cannulated and pressurized vessel was monitored with the inverted microscope before and after administration of potassium channel inhibitors. In addition, we studied the effect of nitric oxide synthase inhibition on potassium channel activity.

RESULTS

Cerebral and brain stem arterioles were significantly constricted by 4-aminopyridine and low concentration of BaCl(2) but not by glibenclamide. The addition of N:(omega)-nitro-L-arginine to 4-aminopyridine further decreased diameters of both arterioles. Tetraethylammonium ion caused a significant constriction of brain stem but not cerebral arteriole. The brain stem arteriole was further constricted by additional N:(omega)-nitro-L-arginine.

CONCLUSIONS

Voltage-dependent and inward-rectifier, but not ATP-sensitive, potassium channels are active under basal conditions of rat cerebral and brain stem arterioles. There is a regional difference in the activity of calcium-activated potassium channels, which, at rest, are open in brain stem but silent in cerebral arterioles. In addition, basal endogenous nitric oxide may not contribute to the activation of voltage-dependent and calcium-activated potassium channels.