ATP-sensitive K+ channels mediate dilatation of cerebral arterioles during hypoxia

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).

Cerebral sympatholysis: experiments on in vivo cerebrovascular regulation and ex vivo cerebral vasomotor control

Whether cerebral sympathetic-mediated vasomotor control can be modulated by local brain activity remains unknown. This study tested the hypothesis that the application or removal of a cognitive task during a cold pressor test (CPT) would attenuate and restore decreases in cerebrovascular conductance (CVC), respectively. Middle cerebral artery blood velocity (transcranial Doppler) and mean arterial pressure (finger photoplethysmography) were examined in healthy adults (n = 16; 8 females and 8 males) who completed a control CPT, followed by a CPT coupled with a cognitive task administered either 1) 30 s after the onset of the CPT and for the duration of the CPT or 2) at the onset of the CPT and terminated 30 s before the end of the CPT (condition order was counterbalanced). The major finding was that the CPT decreased the index of CVC, and such decreases were abolished when a cognitive task was completed concurrently and restored when the cognitive task was removed. As a secondary experiment, vasomotor interactions between sympathetic transduction pathways (α1-adrenergic and Y1-peptidergic) and compounds implicated in cerebral blood flow control [adenosine, and adenosine triphosphate (ATP)] were explored in isolated porcine cerebral arteries (wire myography). The data reveal α1-receptor agonism potentiated vasorelaxation modestly in response to adenosine, and preexposure to ATP attenuated contractile responses to α1-agonism. Overall, the data suggest a cognitive task attenuates decreases in CVC during sympathoexcitation, possibly related to an interaction between purinergic and α1-adrenergic signaling pathways.NEW & NOTEWORTHY The present study demonstrates that the cerebrovascular conductance index decreases during sympathoexcitation and this response can be positively and negatively modulated by the application or withdrawal of a nonexercise cognitive task. Furthermore, isolated vessel experiments reveal that cerebral α1-adrenergic agonism potentiates adenosine-mediated vasorelaxation and ATP attenuates α1-adrenergic-mediated vasocontraction.

Vascular contraction of umbilical arteries of pregnant women with preeclampsia

Objective

Potassium channels have an important role in the vascular adaptation during pregnancy and a reduction in the expression of adenosine triphosphate-sensitive potassium channels (Katp) has been linked to preeclampsia. Activation of Katp induces vasodilation; however, no previous study has been conducted to evaluate the effects of the inhibition of these channels in the contractility of preeclamptic arteries. Glibenclamide is an oral antihyperglycemic agent that inhibits Katp and has been widely used in vascular studies.

Methods

To investigate the effects of the inhibition of Katp, umbilical arteries of preeclamptic women and women with healthy pregnancies were assessed by vascular contractility experiments, in the presence or absence of glibenclamide. The umbilical arteries were challenged with cumulative concentrations of potassium chloride (KCl) and serotonin.

Results

There were no differences between the groups concerning the maternal age and gestational age of the patients. The percentage of smokers, caucasians and primiparae per group was also similar. On the other hand, blood pressure parameters were elevated in the preeclamptic group. In addition, the preeclamptic group presented a significantly higher body mass index. The newborns of both groups presented similar APGAR scores and weights.

Conclusion

In the presence of glibenclamide, there was an increase in the KCl-induced contractions only in vessels from the PE group, showing a possible involvement of these channels in the disorder.

Evaluating the effect of post-traumatic hypoxia on the development of axonal injury following traumatic brain injury in sheep

Damage to the axonal white matter tracts within the brain is a key cause of neurological impairment and long-term disability following traumatic brain injury (TBI). Understanding how axonal injury develops following TBI requires gyrencephalic models that undergo shear strain and tissue deformation similar to the clinical situation and investigation of the effects of post-injury insults like hypoxia. The aim of this study was to determine the effect of post-traumatic hypoxia on axonal injury and inflammation in a sheep model of TBI. Fourteen male Merino sheep were allocated to receive a single TBI via a modified humane captive bolt animal stunner, or sham surgery, followed by either a 15 min period of hypoxia or maintenance of normoxia. Head kinematics were measured in injured animals. Brains were assessed for axonal damage, microglia and astrocyte accumulation and inflammatory cytokine expression at 4 hrs following injury. Early axonal injury was characterised by calpain activation, with significantly increased SNTF immunoreactivity, a proteolytic fragment of alpha-II spectrin, but not with impaired axonal transport, as measured by amyloid precursor protein (APP) immunoreactivity. Early axonal injury was associated with an increase in GFAP levels within the CSF, but not with increases in IBA1 or GFAP+ve cells, nor in levels of TNFα, IL1β or IL6 within the cerebrospinal fluid or white matter. No additive effect of post-injury hypoxia was noted on axonal injury or inflammation. This study provides further support that axonal injury post-TBI is driven by different pathophysiological mechanisms, and detection requires specific markers targeting multiple injury mechanisms. Treatment may also need to be tailored for injury severity and timing post-injury to target the correct injury pathway.

Recent insights into mechanisms of hypoxia-induced vasodilatation in the human brain

The cerebral vasculature manages oxygen delivery by adjusting arterial blood in-flow in the face of reductions in oxygen availability. Hypoxic cerebral vasodilatation, and the associated hypoxic cerebral blood flow reactivity, involve many vascular, erythrocytic and cerebral tissue mechanisms that mediate elevations in cerebral blood flow via micro- and macrovascular dilatation. This contemporary review focuses on in vivo human work - with reference to seminal preclinical work where necessary - on hypoxic cerebrovascular reactivity, particularly where recent advancements have been made. We provide updates with the following information: in humans, hypoxic cerebral vasodilatation is partially mediated via a - likely non-obligatory - combination of: (1) nitric oxide synthases, (2) deoxygenation-coupled S-nitrosothiols, (3) potassium channel-related vascular smooth muscle hyperpolarization, and (4) prostaglandin mechanisms with some contribution from an interrelationship with reactive oxygen species. And finally, we discuss the fact that, due to the engagement of deoxyhaemoglobin-related mechanisms, reductions in O2 content via haemoglobin per se seem to account for ∼50% of that seen with hypoxic cerebral vasodilatation during hypoxaemia. We further highlight the issue that methodological impediments challenge the complete elucidation of hypoxic cerebral reactivity mechanisms in vivo in healthy humans. Future research is needed to confirm recent advancements and to reconcile human and animal findings. Further investigations are also required to extend these findings to address questions of sex-, heredity-, age-, and disease-related differences. The final step is to then ultimately translate understanding of these mechanisms into actionable, targetable pathways for the prevention and treatment of cerebral vascular dysfunction and cerebral hypoxic brain injury.

Lower Oxygen Tension and Intracranial Hemorrhage in Veno-venous Extracorporeal Membrane Oxygenation

INTRODUCTION AND METHODS

We examined the relationship between 24-h pre- and post-cannulation arterial oxygen tension (PaO2) and arterial carbon dioxide tension (PaCO2) and subsequent acute brain injury (ABI) in patients receiving veno-venous extracorporeal membrane oxygenation (VV-ECMO) with granular arterial blood gas (ABG) data and institutional standardized neuromonitoring.

RESULTS

Eighty-nine patients underwent VV-ECMO (median age = 50, 63% male). Twenty (22%) patients experienced ABI; intracranial hemorrhage (ICH) was the most common diagnosis (n = 14, 16%). Lower post-cannulation PaO2 levels were significantly associated with ICH (66 vs. 81 mmHg, p = 0.007) and a post-cannulation PaO2 level < 70 mmHg was more frequent in these patients (71% vs. 33%, p = 0.007). PaCO2 parameters were not associated with ABI. By multivariable logistic regression, hypoxemia post-cannulation increased the odds of ICH (OR = 5.06, 95% CI:1.41-18.17; p = 0.01).

CONCLUSION

In summary, lower oxygen tension in the 24-h post-cannulation was associated with ICH development. The precise roles of peri-cannulation ABG changes deserve further investigation, as they may influence the management of VV-ECMO patients.

Prenatal exposure to alcohol impairs responses of cerebral arterioles to activation of potassium channels: Role of oxidative stress

BACKGROUND

Potassium channels play an important role in the basal tone and dilation of cerebral resistance arterioles in response to many stimuli. However, the effect of prenatal alcohol exposure (PAE) on specific potassium channel function remains unknown. The first goal of this study was to determine the influence of PAE on the reactivity of cerebral arterioles to activation of ATP-sensitive potassium (KATP ) and BK channels. Our second goal was to determine whether oxidative stress contributed to potassium channel dysfunction of cerebral arterioles following PAE.

METHODS

We fed Sprague-Dawley dams a liquid diet with or without alcohol (3% EtOH) for the duration of their pregnancy (21 to 23 days). We examined in vivo responses of cerebral arterioles in control and PAE male and female offspring (14 to 16 weeks after birth) to activators of potassium channels (Iloprost [BK channels] and pinacidil [KATP channels]), before and following inhibition of oxidative stress with apocynin.

RESULTS

We found that PAE impaired dilation of cerebral arterioles in response to activation of potassium channels with iloprost and pinacidil, and this impairment was similar in male and female rats. In addition, treatment with apocynin reversed the impaired vasodilation to iloprost and pinacidil in PAE rats to levels observed in control rats. This effect of apocynin also was similar in male and female rats.

CONCLUSIONS

PAE induces dysfunction in the ability of specific potassium channels to dilate cerebral arterioles which appears to be mediated by an increase in oxidative stress. We suggest that these alterations in potassium channel function may contribute to the pathogenesis of cerebral vascular abnormalities and/or behavioral/cognitive deficits observed in fetal alcohol spectrum disorders.

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.

Cerebral Oxygen Delivery and Consumption in Brain-Injured Patients

Organism survival depends on oxygen delivery and utilization to maintain the balance of energy and toxic oxidants production. This regulation is crucial to the brain, especially after acute injuries. Secondary insults after brain damage may include impaired cerebral metabolism, ischemia, intracranial hypertension and oxygen concentration disturbances such as hypoxia or hyperoxia. Recent data highlight the important role of clinical protocols in improving oxygen delivery and resulting in lower mortality in brain-injured patients. Clinical protocols guide the rules for oxygen supplementation based on physiological processes such as elevation of oxygen supply (by mean arterial pressure (MAP) and intracranial pressure (ICP) modulation, cerebral vasoreactivity, oxygen capacity) and reduction of oxygen demand (by pharmacological sedation and coma or hypothermia). The aim of this review is to discuss oxygen metabolism in the brain under different conditions.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Blood-brain barrier disruption in atrial fibrillation: a potential contributor to the increased risk of dementia and worsening of stroke outcomes?

Atrial fibrillation (AF) has become one of the most significant health problems worldwide, warranting urgent answers to currently pending questions on the effects of AF on brain function. Recent evidence has emerged to show an association between AF and an increased risk of developing dementia and worsening of stroke outcomes. A healthy brain is protected by the blood–brain barrier (BBB), which is formed by the endothelial cells that line cerebral capillaries. These endothelial cells are continuously exposed to shear stress (the frictional force generated by blood flow), which affects endothelial cell structure and function. Flow disturbances as experienced during AF can disrupt the BBB and leave the brain vulnerable to damage. Investigating the plausible mechanisms in detail, linking AF to cerebrovascular damage is difficult in humans, leading to paucity of available clinical data. Here, we discuss the available evidence for BBB disruption during AF due to altered cerebral blood flow, and how this may contribute to an increased risk of dementia and worsening of stroke outcomes.

KATP channels modulate cerebral blood flow and oxygen delivery during isocapnic hypoxia in humans

KEY POINTS

ATP-sensitive K⁺ (K_ATP ) channels mediate hypoxia-induced cerebral vasodilatation and hyperperfusion in animals. We tested whether K_ATP channels blockade affects the increase in human cerebral blood flow (CBF) and the maintenance of oxygen delivery (CDO₂ ) during hypoxia. Hypoxia-induced increases in the anterior circulation and total cerebral perfusion were attenuated under K_ATP channels blockade affecting the relative changes of brain oxygen delivery. Therefore, in humans, K_ATP channels activation modulates the vascular tone in the anterior circulation of the brain, contributing to CBF and CDO₂ responses to hypoxia.

ABSTRACT

ATP-sensitive K⁺ (K_ATP ) channels mediate hypoxia-induced cerebral vasodilatation and hyperperfusion in animals. We tested whether K_ATP channels blockade affects the increase in cerebral blood flow (CBF) and the maintenance of oxygen delivery (CDO₂ ) during hypoxia in humans. Nine healthy men were exposed to 5-min trials of normoxia and isocapnic hypoxia (IHX, 10% O₂ ) before (BGB) and 3 h after glibenclamide ingestion (AGB). Mean arterial pressure (MAP), arterial saturation ( S a O 2 ), partial pressure of oxygen ( P a O 2 ) and carbon dioxide ( P aC O 2 ), internal carotid artery blood flow (ICABF), vertebral artery blood flow (VABF), total (t)CBF (Doppler ultrasound) and CDO₂ were quantified during the trials. IHX provoked similar reductions in S a O 2 and P a O 2 , while MAP was not affected by oxygen desaturation or K_ATP blockade. A smaller increase in ICABF (ΔBGB: 36 ± 23 vs. ΔAGB 11 ± 18%, p = 0.019) but not in VABF (∆BGB 26 ± 21 vs. ∆AGB 27 ± 27%, p = 0.893) was observed during the hypoxic trial under K_ATP channels blockade. Thus, IHX-induced increases in tCBF (∆BGB 32 ± 19 vs. ∆AGB 14 ± 13%, p = 0.012) and CDO₂ relative changes (∆BGB 7 ± 13 vs. ∆AGB -6 ± 14%, p = 0.048) were attenuated during the AGB hypoxic trial. In a separate protocol, 6 healthy men (5 from protocol 1) underwent a 5-min exposure to normoxia and IHX before and 3 h after placebo (5 mg of cornstarch) ingestion. IHX reduced S a O 2 and P a O 2 , but placebo did not affect the ICABF, VABF, tCBF, or CDO₂ responses. Therefore, in humans, K_ATP channels activation modulates vascular tone in the anterior rather than the posterior circulation of the brain, contributing to tCBF and CDO₂ responses to hypoxia.

Inositol 1,4,5-Trisphosphate Receptors in Hypertension

Chronic hypertension remains a major cause of global mortality and morbidity. It is a complex disease that is the clinical manifestation of multiple genetic, environmental, nutritional, hormonal, and aging-related disorders. Evidence supports a role for vascular aging in the development of hypertension involving an impairment in endothelial function together with an alteration in vascular smooth muscle cells (VSMCs) calcium homeostasis leading to increased myogenic tone. Changes in free intracellular calcium levels ([Ca²⁺] _i ) are mediated either by the influx of Ca²⁺ from the extracellular space or release of Ca²⁺ from intracellular stores, mainly the sarcoplasmic reticulum (SR). The influx of extracellular Ca²⁺ occurs primarily through voltage-gated Ca²⁺ channels (VGCCs), store-operated Ca²⁺ channels (SOC), and Ca²⁺ release-activated channels (CRAC), whereas SR-Ca²⁺ release occurs through inositol trisphosphate receptor (IP₃R) and ryanodine receptors (RyRs). IP₃R-mediated SR-Ca²⁺ release, in the form of Ca²⁺ waves, not only contributes to VSMC contraction and regulates VGCC function but is also intimately involved in structural remodeling of resistance arteries in hypertension. This involves a phenotypic switch of VSMCs as well as an alteration of cytoplasmic Ca²⁺ signaling machinery, a phenomena tightly related to the aging process. Several lines of evidence implicate changes in expression/function levels of IP₃R isoforms in the development of hypertension, VSMC phenotypic switch, and vascular aging. The present review discusses the current knowledge of these mechanisms in an integrative approach and further suggests potential new targets for hypertension management and treatment.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Role of KATPChannels in the Regulation of Rat Dura and Pia Artery Diameter

The aim of the present study was to examine the effect of KATPchannel openers pinacidil and levcromakalim on rat dural and pial arteries as well as their inhibition by glibenclamide. We used an in-vivo genuine closed cranial window model and an in-vitro organ bath. Glibenclamide alone reduced the dural but not the pial artery diameter compared with controls. Intravenous pinacidil and levcro-makalim induced dural and pial artery dilation that was significantly attenuated by glibenclamide. In the organ bath pinacidil and levcromakalim induced dural and middle cerebral artery relaxation that was significantly attenuated by glibenclamide. In conclusion, KATPchannel openers induce increasing diameter/relaxation of dural and pial arteries after intravenous infusion in vivo and on isolated arteries in vitro. Furthermore, dural arteries were more sensitive to KATPchannel openers than pial arteries.

Contribution of epoxyeicosatrienoic acids to the cerebral blood flow response to hypoxemia

Adenosine A2A receptors and ATP-activated K(+) (KATP) channels contribute to part of the cerebral vasodilatory response to systemic hypoxia, but other mediators are likely involved. Epoxyeicosatrienoic acids (EETs) are cerebral vasodilators and are released from astrocytes exposed to hypoxia. Moreover, stimulation of metabotropic glutamate receptors (mGluR) produces vasodilation by an EET-dependent mechanism. Here, we tested the hypothesis that EET signaling and mGluR activation contribute to hypoxic vasodilation. Laser-Doppler flow was measured over cerebral cortex of anesthetized rats subjected to stepwise reductions in arterial oxygen saturation to 50-70%. Hypoxic reactivity was calculated as the slope of the change in laser-Doppler flow vs. the reciprocal of arterial oxygen content. Hypoxic reactivity significantly decreased from 9.2 ± 1.9 (±95% confidence interval) in controls with vehicle treatment to 2.6 ± 1.4 with the EET antagonist 14,15-epoxyeicosa-5(Z)-enoic acid, to 3.0 ± 1.5 with the EET synthesis inhibitor MS-PPOH, to 1.9 ± 2.3 with the combined mGluR subtype 1 and 5 antagonists 2-methyl-6-(phenylethynyl)pyridine and LY367385, to 5.6 ± 1.2 with the KATP channel inhibitor glibenclamide, and to 5.8 ± 2.3 with the A2A receptor antagonist SCH58261. However, reactivity was not significantly altered by the A2B receptor antagonist MRS1754 (6.7 ± 1.8; P = 0.28 Dunnett's test) or by the 20-hydroxyeicosatetraenoic acid synthesis inhibitor HET0016 (7.5 ± 2.3; P = 0.6). These data indicate that, in addition to the known contributions of A2A receptors and KATP channels to the increase in cerebral blood flow during hypoxia, EETs and mGluRs make a major contribution, possibly by mGluR stimulation and hypoxia-induced release of EETs. In contrast, A2B receptors do not make a major contribution, and 20-hydroxyeicosatetraenoic acid does not significantly limit hypoxic vasodilation.

Cellular signalling pathways mediating dilation of porcine pial arterioles to adenosine A₂A receptor activation

AIMS

Adenosine is a potent vasodilator contributing to cerebral blood flow regulation during metabolic stress. However, the distribution of adenosine receptor subtypes and underlying signalling mechanisms for dilation of pial arterioles remain unclear. The present study aimed at addressing these issues.

METHODS AND RESULTS

Isolated porcine pial arterioles were subjected to study of vasomotor function, localization of adenosine receptors, and production of nitric oxide (NO). Concentration-dependent vasodilation to adenosine was inhibited by A₂A receptor antagonist ZM241385 but not by A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine. A₂A receptors were detected in endothelium and smooth muscle of pial arterioles via immunohistochemistry. Adenosine significantly increased arteriolar production of NO, and the induced dilation was insensitive to KATP channel blocker glibenclamide but was attenuated by endothelial denudation, NO synthase inhibitor L-NAME, or guanylyl cyclase inhibitor ODQ in a similar manner. Both inward rectifier potassium (Kir) channel inhibitor barium and cAMP signalling inhibitor Rp-8-Br-cAMPS attenuated adenosine-induced dilation. In the presence of L-NAME or the absence of endothelium, addition of Rp-8-Br-cAMPS but not barium further reduced adenosine-induced responses. Barium diminished endothelium-independent vasodilation to NO donor sodium nitroprusside. Comparable to the adenosine-induced response, vasodilation to A₂A receptor agonist CGS21680 was attenuated by endothelial removal, ZM241385, L-NAME, barium, or Rp-8-Br-cAMPS, but not by glibenclamide.

CONCLUSION

Adenosine evokes dilation of porcine pial arterioles via parallel activation of endothelial and smooth muscle A₂A receptors. Stimulation of endothelial NO production activates smooth muscle guanylyl cyclase for vasodilation by opening Kir channels. Adenosine also activates smooth muscle cAMP signalling leading to vasodilation.

Vasodilation induced by oxygen/glucose deprivation is attenuated in cerebral arteries of SUR2 null mice

Physiological functions of arterial smooth muscle cell ATP-sensitive K(+) (K(ATP)) channels, which are composed of inwardly rectifying K(+) channel 6.1 and sulfonylurea receptor (SUR)-2 subunits, during metabolic inhibition are unresolved. In the present study, we used a genetic model to investigate the physiological functions of SUR2-containing K(ATP) channels in mediating vasodilation to hypoxia, oxygen and glucose deprivation (OGD) or metabolic inhibition, and functional recovery following these insults. Data indicate that SUR2B is the only SUR isoform expressed in murine cerebral artery smooth muscle cells. Pressurized SUR2 wild-type (SUR2(wt)) and SUR2 null (SUR2(nl)) mouse cerebral arteries developed similar levels of myogenic tone and dilated similarly to hypoxia (<10 mmHg Po(2)). In contrast, vasodilation induced by pinacidil, a K(ATP) channel opener, was ∼71% smaller in SUR2(nl) arteries. Human cerebral arteries also expressed SUR2B, developed myogenic tone, and dilated in response to hypoxia and pinacidil. OGD, oligomycin B (a mitochondrial ATP synthase blocker), and CCCP (a mitochondrial uncoupler) all induced vasodilations that were ∼39-61% smaller in SUR2(nl) than in SUR2(wt) arteries. The restoration of oxygen and glucose following OGD or removal of oligomycin B and CCCP resulted in partial recovery of tone in both SUR2(wt) and SUR2(nl) cerebral arteries. However, SUR(nl) arteries regained ∼60-82% more tone than did SUR2(wt) arteries. These data indicate that SUR2-containing K(ATP) channels are functional molecular targets for OGD, but not hypoxic, vasodilation in cerebral arteries. In addition, OGD activation of SUR2-containing K(ATP) channels may contribute to postischemic loss of myogenic tone.

Role of KATP and L-type Ca2+ channel activities in regulation of ovine uterine vascular contractility: effect of pregnancy and chronic hypoxia

OBJECTIVE

Our objective was to determine whether the pregnancy and high altitude long-term hypoxia-mediated changes in uterine artery contractility were regulated by K(ATP) and L-type Ca(2+) channel activities.

STUDY DESIGN

Uterine arteries were isolated from nonpregnant and near-term pregnant ewes that had been maintained at sea level (∼300 m) or exposed to high altitude (3801 m) for 110 days. Isometric tension was measured in a tissue bath.

RESULTS

Pregnancy increased diazoxide, but not verapamil-induced relaxations. Long-term hypoxia attenuated diazoxide-induced relaxations in near-term pregnant uterine arteries, but enhanced verapamil-induced relaxations in nonpregnant uterine arteries. Diazoxide decreased the maximal response (E(max)) of phenylephrine-induced contractions in near-term pregnant uterin arteries but not nonpregnant uterine arteries in normoxic sheep. In contrast, diazoxide had no effect on phenylephrine-induced E(max) in near-term pregnant uterine arteries but decreased it in nonpregnant uterine arteries in long-term hypoxia animals. Verapamil decreased the E(max) and pD(2) (-logEC(50)) of phenylephrine-induced contractions in both nonpregnant uterine arteries and near-term pregnant uterine arteries in normoxic and long-term hypoxia animals, except nonpregnant uterine arteries of normoxic animals in which verapamil showed no effect on the pD(2).

CONCLUSION

The results suggest that pregnancy selectively increases K(ATP), but not L-type Ca(2+) channel activity. Long-term hypoxia decreases the K(ATP) channel activity, which may contribute to the enhanced uterine vascular myogenic tone observed in pregnant sheep at high altitude hypoxia.

Muscle KATP channels: recent insights to energy sensing and myoprotection

ATP-sensitive potassium (K(ATP)) channels are present in the surface and internal membranes of cardiac, skeletal, and smooth muscle cells and provide a unique feedback between muscle cell metabolism and electrical activity. In so doing, they can play an important role in the control of contractility, particularly when cellular energetics are compromised, protecting the tissue against calcium overload and fiber damage, but the cost of this protection may be enhanced arrhythmic activity. Generated as complexes of Kir6.1 or Kir6.2 pore-forming subunits with regulatory sulfonylurea receptor subunits, SUR1 or SUR2, the differential assembly of K(ATP) channels in different tissues gives rise to tissue-specific physiological and pharmacological regulation, and hence to the tissue-specific pharmacological control of contractility. The last 10 years have provided insights into the regulation and role of muscle K(ATP) channels, in large part driven by studies of mice in which the protein determinants of channel activity have been deleted or modified. As yet, few human diseases have been correlated with altered muscle K(ATP) activity, but genetically modified animals give important insights to likely pathological roles of aberrant channel activity in different muscle types.

Role of 20-HETE in the hypoxia-induced activation of Ca2+-activated K+ channel currents in rat cerebral arterial muscle cells

The mechanism of sensing hypoxia and hypoxia-induced activation of cerebral arterial Ca(2+)-activated K(+) (K(Ca)) channel currents and vasodilation is not known. We investigated the roles of the cytochrome P-450 4A (CYP 4A) omega-hydroxylase metabolite of arachidonic acid, 20-hydroxyeicosatetraenoic acid (20-HETE), and generation of superoxide in the hypoxia-evoked activation of the K(Ca) channel current in rat cerebral arterial muscle cells (CAMCs) and cerebral vasodilation. Patch-clamp analysis of K(+) channel current identified a voltage- and Ca(2+)-dependent 238 +/- 21-pS unitary K(+) currents that are inhibitable by tetraethylammonium (TEA, 1 mM) or iberiotoxin (100 nM). Hypoxia (<2% O(2)) reversibly enhanced the open-state probability (NP(o)) of the 238-pS unitary K(Ca) current in cell-attached patches. This effect of hypoxia was not observed on unitary K(Ca) currents recorded from either excised inside-out or outside-out membrane patches. Inhibition of CYP 4A omega-hydroxylase activity increased the NP(o) of K(Ca) single-channel current. Hypoxia reduced the basal endogenous level of 20-HETE by 47 +/- 3% as well as catalytic formation of 20-HETE in cerebral arterial muscle homogenates as determined by liquid chromatography-mass spectrometry analysis. The concentration of authentic 20-HETE was reduced when incubated with the superoxide donor KO(2). Exogenous 20-HETE (100 nM) attenuated the hypoxia-induced activation of the K(Ca) current in CAMCs. Hypoxia did not augment the increase in NP(o) of K(Ca) channel current induced by suicide inhibition of endogenous CYP 4A omega-hydroxylase activity with 17-octadecynoic acid. In pressure (80 mmHg)-constricted cerebral arterial segments, hypoxia induced dilation that was partly attenuated by 20-HETE or by the K(Ca) channel blocker TEA. Exposure to hypoxia caused the generation of intracellular superoxide as evidenced by intense staining of arterial muscle with the fluorescent probe hydroethidine, by quantitation using fluorescent HPLC analysis, and by attenuation of the hypoxia-induced activation of the K(Ca) channel current by superoxide dismutation. These results suggest that the exposure of CAMCs to hypoxia results in the generation of superoxide and reduction in endogenous level of 20-HETE that may account for the hypoxia-induced activation of arterial K(Ca) channel currents and cerebral vasodilation.

Inhibitory effect of tramadol on vasorelaxation mediated by ATP-sensitive K+ channels in rat aorta

PURPOSE

Tramadol produces a conduction block similar to lidocaine by exerting a local anesthetic-like effect. The aims of this in vitro study were to determine the effects of tramadol on the vasorelaxant response induced by the adenosine triphosphate-sensitive K(+) (K(ATP)) channel opener, levcromakalim, in an endothelium-denuded rat aorta, and to determine whether this effect of tramadol is stereoselective.

METHODS

The effects of tramadol (racemic, R(-) and S(+): 10(-6), 10(-5), 5 x 10(-5) M), and glibenclamide on the levcromakalim dose-response curve were assessed in aortic rings that had been pre-contracted with phenylephrine. In the rings pretreated independently with naloxone, and glibenclamide, the levcromakalim dose-response curves were generated in the presence or absence of tramadol. The effect of tramadol on the dose-response curve of diltiazem was assessed.

RESULTS

Racemic, R(-) and S(+) tramadol (10(-5), 5 x 10(-5) M) attenuated (P < 0.0001) levcromakalim-induced relaxation in the ring with or without naloxone in a dose-dependent manner. The magnitude of the R(-)-tramadol-induced attenuation of vasorelaxant response induced by levcromakalim was greater (P < 0.05) than that induced by S(+)-tramadol. Glibenclamide almost abolished the levcromakalim-induced relaxation. Tramadol, 5 x 10(-5) M, did not significantly alter the diltiazem-induced relaxation.

CONCLUSION

These results suggest that a supraclinical dose (10(-5) M) of tramadol [racemic, R(-) and S(+)] attenuates the vasorelaxation mediated by the K(ATP) channels in the rat aorta. The R(-) tramadol-induced attenuation of vasorelaxation induced by levcromaklim was more potent than that induced by S(+) tramadol. This attenuation is independent of opioid receptor activation.

Augmented activity of adenosine triphosphate-sensitive K+ channels induced by droperidol in the rat aorta

Droperidol produces the inhibition of K+ channels in cardiac myocytes. However, the effects of droperidol on K+ channels have not been studied in blood vessels. Therefore, we designed the present study to determine whether droperidol modulates the activity of adenosine triphosphate (ATP)-sensitive K+ channels in vascular smooth muscle cells. Rat aortic rings without endothelium were suspended or used for isometric force and membrane potential recordings, respectively. Vasorelaxation and hyperpolarization induced by levcromakalim (10(-8) to 10(-5) M or 10(-5) M, respectively) were completely abolished by the ATP-sensitive K+ channel antagonist glibenclamide (10(-5) M). Droperidol (10(-7) M) and an alpha-adrenergic receptor antagonist phentolamine (3 x 10(-9) M) caused a similar vasodilator effect (approximately 20% of vasorelaxation compared with maximal vasorelaxation induced by papaverine [3 x 10(-4) M]), whereas glibenclamide did not alter vasorelaxation induced by droperidol. Droperidol (3 x 10(-8) M to 10(-7) M) augmented vasorelaxation and hyperpolarization produced by levcromakalim, whereas phentolamine (3 x 10(-9) M) did not alter this vasorelaxation. Glibenclamide (10(-5) M) abolished the vasodilating and hyperpolarizing effects of levcromakalim in the aorta treated with droperidol (10(-7) M). These results suggest that droperidol augments vasodilator activity via ATP-sensitive K+ channels. However, it is unlikely that this augmentation is mediated by the inhibition of alpha-adrenergic receptors in vascular smooth muscles.

Effects of hypoxia, anoxia, and metabolic inhibitors on KATP channels in rat femoral artery myocytes

Vascular ATP-sensitive potassium (KATP) channels have an important role in hypoxic vasodilation. Because KATP channel activity depends on intracellular nucleotide concentration, one hypothesis is that hypoxia activates channels by reducing cellular ATP production. However, this has not been rigorously tested. In this study we measured KATP current in response to hypoxia and modulators of cellular metabolism in single smooth muscle cells from the rat femoral artery by using the whole cell patch-clamp technique. KATP current was not activated by exposure of cells to hypoxic solutions (Po2 approximately 35 mmHg). In contrast, voltage-dependent calcium current and the depolarization-induced rise in intracellular calcium concentration ([Ca2+]i) was inhibited by hypoxia. Blocking mitochondrial ATP production by using the ATP synthase inhibitor oligomycin B (3 microM) did not activate current. Blocking glycolytic ATP production by using 2-deoxy-D-glucose (5 mM) also did not activate current. The protonophore carbonyl cyanide m-chlorophenylhydrazone (1 microM) depolarized the mitochondrial membrane potential and activated KATP current. This activation was reversed by oligomycin B, suggesting it occurred as a consequence of mitochondrial ATP consumption by ATP synthase working in reverse mode. Finally, anoxia induced by dithionite (0.5 mM) also depolarized the mitochondrial membrane potential and activated KATP current. Our data show that: 1) anoxia but not hypoxia activates KATP current in femoral artery myocytes; and 2) inhibition of cellular energy production is insufficient to activate KATP current and that energy consumption is required for current activation. These results suggest that vascular KATP channels are not activated during hypoxia via changes in cell metabolism. Furthermore, part of the relaxant effect of hypoxia on rat femoral artery may be mediated by changes in [Ca2+]i through modulation of calcium channel activity.

Role of 20-HETE in the pial arteriolar constrictor response to decreased hematocrit after exchange transfusion of cell-free polymeric hemoglobin

The cerebrovascular response to decreases in hematocrit and viscosity depends on accompanying changes in arterial O2 content. This study examines whether 1) the arteriolar dilation seen after exchange transfusion with a 5% albumin solution can be reduced by the K(ATP) channel antagonist glibenclamide (known to inhibit hypoxic dilation), and 2) the arteriolar constriction seen after exchange transfusion with a cell-free hemoglobin polymer to improve O2-carrying capacity can be blocked by inhibitors of the synthesis or vasoconstrictor actions of 20-HETE. In anesthetized rats, decreasing hematocrit by one-third with albumin exchange transfusion dilated pial arterioles (14 +/- 2%; SD), whereas superfusion of the surface of the brain with 10 muM glibenclamide blocked this response (-10 +/- 7%). Exchange transfusion with polymeric hemoglobin decreased the diameter of pial arterioles by 20 +/- 3% without altering arterial pressure. This constrictor response was attenuated by superfusing the surface of the brain with a 20-HETE antagonist, WIT-002 (10 microM; -5 +/- 1%), and was blocked by two chemically dissimilar selective inhibitors of the synthesis of 20-HETE, DDMS (50 microM; 0 +/- 4%) and HET-0016 (1 microM; +6 +/- 4%). The constrictor response to hemoglobin transfusion was not blocked by an inhibitor of nitric oxide (NO) synthase, and the inhibition of the constrictor response by DDMS was not altered by coadministration of the NO synthase inhibitor. We conclude 1) that activation of K(ATP) channels contributes to pial arteriolar dilation during anemia, whereas 2) constriction to polymeric hemoglobin transfusion at reduced hematocrit represents a regulatory response that limits increased O2 transport and that is mediated by increased formation of 20-HETE, rather than by NO scavenging.

Role of nitric oxide scavenging in vascular response to cell-free hemoglobin transfusion

Modified Hb solutions have been developed as O(2) carrier transfusion fluids, but of concern is the possibility that increased scavenging of nitric oxide (NO) within the plasma will alter vascular reactivity even if the Hb does not readily extravasate. The effect of decreasing hematocrit from approximately 30% to 18% by an exchange transfusion of a 6% sebacyl cross-linked tetrameric Hb solution on the diameter of pial arterioles possessing tight endothelial junctions was examined through a cranial window in anesthetized cats with and without a NO synthase (NOS) inhibitor. Superfusion of a NOS inhibitor decreased diameter, and subsequent Hb transfusion produced additional constriction that was not different from Hb transfusion alone but was different from the dilation observed by exchange transfusion of an albumin solution after NOS inhibition. In contrast, abluminal application of the cross-linked Hb produced constriction that was attenuated by the NOS inhibitor. Neither abluminal nor intraluminal cross-linked Hb interfered with pial arteriolar dilation to cromakalim, an activator of ATP-sensitive potassium channels. Pial vascular reactivity to hypocapnia and hypercapnia was unaffected by Hb transfusion. Microsphere-determined regional blood flow indicated selective decreases in perfusion after Hb transfusion in the kidney, small intestine, and neurohypophysis, which does not have tight endothelial junctions. Administration of a NOS inhibitor to reduce the basal level of NO available for scavenging before Hb transfusion prevented further decreases in blood flow to these regions compared with NOS inhibition alone. In contrast, blood flow to skeletal and left ventricular muscle increased, and cerebral blood flow was unchanged after Hb transfusion. This cross-linked Hb tetramer is known to appear in renal lymph but not in urine. We conclude that cell-free tetrameric Hb does not scavenge sufficient NO in the plasma space to significantly affect baseline tone in vascular beds with tight endothelial junctions but does produce substantial constriction in beds with porous endothelium. The data support increasing the molecular size of Hb by polymerization or conjugation to limit extravasation in all vascular beds to preserve normal vascular reactivity.

Lidocaine impairs vasodilation mediated by adenosine triphosphate-sensitive K+ channels but not by inward rectifier K+ channels in rat cerebral microvessels

Vasodilator effects of adenosine triphosphate (ATP)-sensitive, as well as inward rectifier, K+ channel openers have not been well demonstrated in cerebral microvessels. Although lidocaine impairs vasorelaxation via ATP-sensitive K+ channels in the rat aorta, the effects of this compound on K+ channels in the cerebral circulation have not been shown. We designed the present study to examine whether ATP-sensitive and inward rectifier K+ channels contribute to vasodilator responses in cerebral microvessels and whether the vasodilation mediated by these channels is inhibited by lidocaine. Rat brain slices were monitored using a computer-assisted videomicroscopy. Cerebral parenchymal arterioles (diameter, 5-10 microm) were contracted with prostaglandin F(2alpha), and thereafter potassium chloride (KCl), levcromakalim, or sodium nitroprusside was added to the perfusion chamber. Levcromakalim and KCl produced vasodilation of the cerebral parenchymal arterioles, which was abolished by an ATP-sensitive K+ channel antagonist, glibenclamide, or an inward rectifier K+ channel antagonist, barium chloride, respectively. Lidocaine (10(-5) to 3 x 10(-5) M) inhibited the dilation produced by levcromakalim but not by KCl or sodium nitroprusside. In parenchymal arterioles of the cerebral cortex, lidocaine seems to reduce vasodilation mediated by ATP-sensitive K+ channels but not by inward rectifier K+ channels.

Nitric oxide and endothelin in oxygen-dependent regulation of vascular tone of human umbilical vein

We investigated the possible contribution of nitric oxide (NO) and endothelin (ET) to oxygen-dependent regulation of human umbilical vein vascular tone by simultaneous registration of intracellular membrane potential and isometric tension of vessel strips with and without NO synthase inhibition [10-4 M N omega-nitro-L-arginine methyl ester (L-NAME)], ETA receptor blockade (10(-5) M BQ-123), or ETB receptor blockade (10(-7) M BQ-788) at Po2 values in the bath solution between 5 and 104 mmHg. Increasing PO2 above the physiological intrauterine range resulted in depolarization and an increase of isometric tension, whereas lowering PO2 resulted in hyperpolarization and a decrease in isometric tension. Removal of the endothelium reversed these effects. At PO2 values below 39 mmHg, intact preparations treated with either L-NAME, BQ-788, or BQ-123 were more depolarized than controls. In the case of treatment with L-NAME or BQ-123, this was accompanied by an increase in isometric tension. We conclude that it is NO that mediates the hypoxic hyperpolarization and vasodilatation of the human umbilical vein and that ET, via activation of ETB1 receptors on endothelial cells, contributes to this effect.

Angiogenic and neurotrophic effects of vascular endothelial growth factor (VEGF165): studies of grafted and cultured embryonic ventral mesencephalic cells

The present series of experiments investigated the effects of vascular endothelial growth factor (VEGF165) on adult rat striatal cerebrovasculature and embryonic dopamine (DA) neuron allografts in a rat model of Parkinson's disease (PD). We examined VEGF165's ability to (1) alter the vascular network of the adult rat striatum, (2) influence the vascular growth of solid embryonic day 14 (E14) ventral mesencephalic (VM) grafts when placed into a VEGF-pretreated host striatum, (3) alter the function and survival of E14 VM grafts when transplanted into an adult DA-deleted striatum, and (4) influence cell survival and neurite growth in cultures of E14 VM cells. We demonstrate here that a single bolus injection of VEGF165 into the adult rat striatum significantly increases the amount of vasculature in the vicinity of the injection site in a delayed and transient manner when compared to saline controls. Transplanting solid E14 VM grafts into the VEGF165-pretreated striatum resulted in a homogeneous distribution of small blood vessels throughout the graft, a pattern that closely resembles mature adult vasculature. In contrast, grafts in the control condition contained a patchy distribution of heavily dilated vessels. Behavioral measurements indicate that VEGF pretreatment of the intrastriatal graft site accelerates recovery of amphetamine-induced rotational asymmetry in unilateral 6-OHDA lesioned rats. Unexpectedly, however, VEGF pretreatments failed to increase survival of tyrosine hydroxylase-immunoreactive (THir) neurons in the grafts. In contrast to this finding in vivo, adding VEGF165 to glial-reduced E14 rat VM cultures produced a fourfold increase in THir cell survival and a doubling in the length of THir neurites. We conclude that with the proper method of delivery, VEGF165 may prove to be one of several strategies necessary to significantly improve the survival and function of fetal VM tissue grafts.

The role of K+ channels in vasorelaxation induced by hypoxia and the modulator effects of lidocaine in the rat carotid artery

Hypoxia induces vasodilation, partly via the activation of K(+) channels. Lidocaine impairs vasorelaxation mediated by a K(+) channel opener, suggesting that this antiarrhythmic drug may inhibit hypoxia-induced vasodilation mediated by K(+) channels. We designed the current study to determine whether, in the carotid artery, K(+) channels contribute to vasorelaxation in response to hypoxia and whether lidocaine modulates vasorelaxation induced by K(+) channels via pathophysiological and pharmacological stimuli. Rings of rat common carotid artery without endothelium were suspended for isometric force recording. During contraction to phenylephrine, hypoxia-induced vasorelaxation or concentration-response to an adenosine triphosphate-sensitive K(+) channel opener was obtained changing control gas to hypoxic gas and the cumulative addition of levcromakalim, respectively. Hypoxia-induced vasorelaxation was significantly reduced by glibenclamide (5 micro M) but not by iberiotoxin (0.1 micro M), apamin (0.1 micro M), BaCl(2) (10 micro M), or 4-aminopyridine (1 mM). Levcromakalim-induced vasorelaxation was completely abolished by glibenclamide. Lidocaine (10-100 micro M) concentration-dependently inhibited this vasodilation, whereas it did not affect hypoxia-induced vasodilation. These results suggest that adenosine triphosphate-sensitive K(+) channels play a role in hypoxia-induced vasodilation in the rat carotid artery and that lidocaine differentially modulates vasodilation via these channels activated by pathophysiological and pharmacological stimuli.

IMPLICATIONS

In rat carotid artery, levcromakalim produced vasorelaxation mediated by adenosine triphosphate (ATP)-sensitive K(+) channels, whereas hypoxia induced it partly via these channels. Lidocaine inhibited vasorelaxation induced by an ATP-sensitive K(+) channel opener but not by hypoxia, indicating the differential mechanisms of modulatory effects of this antiarrhythmic drug on vasodilation via ATP-sensitive K(+) channels activated by pathophysiological and pharmacological stimuli.

Vitamin K1 attenuates hypoxia-induced relaxation of rat carotid artery

Vascular responses to hypoxia are heterogeneous and involve the release of vasodilators substances such as nitric oxide (NO) and prostacyclin (PGI(2)). In vitro studies have shown that Vitamin K(1) modulates the release of arachidonic acid (AA) in vascular cells, and thus inhibits the capacity of blood vessels to synthesise vasodilator AA metabolites. The aim of our work was to investigate the effects of Vitamin K(1) on the hypoxia-induced vasorelaxation. Hypoxia was induced by changing the gas from 95% O(2)/5% CO(2) to a mixture containing 95% N(2)/5% CO(2). Rat carotid arteries were pre-contracted with phenylephrine (Phe, 10(-8)mol/l) and when the contraction reached a plateau, the bath was bubbled with 95% N(2)/5% CO(2) for 15 min. In intact rings, there was a total relaxation after 15 min of exposure to hypoxia. Removal of the endothelium strongly reduced hypoxia-induced relaxation. In intact rings, indomethacin and L-NAME reduced the hypoxic relaxation after 5 min of exposure but not after 10 or 15 min. Exposure of endothelium-intact rings to Vitamin K(1) (5 x 10(-6) and 5 x 10(-5)mol/l), L-NAME+indomethacin as well as the combination of L-NAME+indomethacin+Vitamin K(1) reduced the hypoxic relaxation after 5 and 10 min of exposure but not after 15 min. At 5 x 10(-7)mol/l Vitamin K(1) did not attenuate hypoxia-induced relaxation. It was also found that Vitamin K(1) (5 x 10(-6) and 5 x 10(-5)mol/l) inhibited ACh-induced relaxation in normoxic conditions. These results show that the effect of Vitamin K(1) on attenuating hypoxia-induced vasorelaxation is concentration-dependent and probably related to its action on endothelial cells.

Direct effects of acute hypoxia on the reactivity of peripheral arteries of the chicken embryo

In the chicken embryo, acute hypoxemia results in cardiovascular responses, including an increased peripheral resistance. We investigated whether local direct effects of decreased oxygen tension might participate in the arterial response to hypoxemia in the chicken embryo. Femoral arteries of chicken embryos were isolated at 0.9 of incubation time, and the effects of acute hypoxia on contraction and relaxation were determined in vitro. While hypoxia reduced contraction induced by high K(+) to a small extent (-21.8 +/- 5.7%), contractile responses to exogenous norepinephrine (NE) were markedly reduced (-51.1 +/- 3.2%) in 80% of the arterial segments. This effect of hypoxia was not altered by removal of the endothelium, inhibition of NO synthase or cyclooxygenase, or by depolarization plus Ca(2+) channel blockade. When arteries were simultaneously exposed to NE and ACh, hypoxia resulted in contraction (+49.8 +/- 9.3%). Also, relaxing responses to ACh were abolished during acute hypoxia, while the vessels became more sensitive to the relaxing effect of the NO donor sodium nitroprusside (pD(2): 5.81 +/- 0.21 vs. 5.31 +/- 0.27). Thus, in chicken embryo femoral arteries, acute hypoxia blunts agonist-induced contraction of the smooth muscle and inhibits stimulated endothelium-derived relaxation factor release. The consequences of this for in vivo fetal hemodynamics during acute hypoxemia depend on the balance between vasomotor influences of circulating catecholamines and those of the endothelium.

Fetal and adult cerebral artery K(ATP) and K(Ca) channel responses to long-term hypoxia

High-altitude long-term hypoxia (LTH) alters cerebral vascular contractile and relaxation responses in both fetus and adult. We tested the hypotheses that LTH-mediated vascular responses were secondary to altered K+ channel function and that in the fetus these responses differ from those of the adult. In middle cerebral arteries (MCA) from both nonpregnant adult and fetal (approximately 140 days gestation) sheep, which were either acclimatized to high altitude (3,820 m) or sea-level controls, we measured norepinephrine (NE)-induced contractions and intracellular Ca2+ concentration ([Ca2+]i) simultaneously, in the presence or absence of different K+ channel openers or blockers. In adult MCA, LTH was associated with approximately 20% decrease in NE-induced tension and [Ca2+]i, with a significant increase in Ca2+ sensitivity. In contrast, in fetal MCA, LTH failed to affect significantly NE-induced contraction or [Ca2+]i but significantly decreased the ATP-sensitive K+ (K(ATP)) channel and Ca2+-activated K+ (K(Ca)) channel-mediated relaxation. The significant effect of K(ATP) and K(Ca) channel activators on the relaxation responses and the fact that K+ channels play a key role in myogenic tone support the hypotheses that K+ channels play an important role in hypoxia-mediated responses. These results also support the hypothesis of significant developmental differences with maturation from fetus to adult.

Maturation alters the contribution of potassium channels to resting and 5HT-induced tone in small cerebral arteries of the sheep

To address the hypothesis that maturation alters the contribution of K-channels to resting and agonist-induced tone in small cerebral arteries, second branch middle cerebral arteries (approximately 200 microm) were taken from term fetal (139-141 days gestation) and adult sheep, denuded of endothelium, and mounted in myographs. After determination of length-tension relations, the arteries were stretched to 55, 100, and 145% of optimum length. At each level of stretch, contractile responses to 5 mM 4-aminopyridine (4-AP, voltage-sensitive K-channel blocker), 100 nM iberiotoxin (calcium-sensitive K-channel blocker), 10 microM glibenclamide (ATP-sensitive K-channel blocker), or 10 microM Ba(2+) (inward rectifier K-channel blocker) were recorded. In separate experiments, concentration--response relations were determined for 5-HT in the presence and absence of each of the four K-channel blockers at the same concentrations. Both 4-AP and iberiotoxin produced stretch-dependent contractions of greater magnitude in adult (37% for 4-AP and 43% for iberiotoxin at 100% optimum) than in fetal (5% for 4-AP and 7% for iberiotoxin at 100% optimum) arteries. 4-AP also enhanced the pD(2) for 5-HT in adult (from 7.15 to 7.49), but not in fetal, arteries. Conversely, glibenclamide attenuated the pD(2) for 5-HT in fetal (from 7.02 to 6.71), but not in adult, arteries. Iberiotoxin enhanced the pD(2) for 5-HT in both fetal (from 7.05 to 7.51) and adult (from 7.15 to 7.75) arteries. In addition, iberiotoxin enhanced maximum responses to 5-HT (from 59 to 82%) in adult but not fetal arteries. Finally, 4-AP enhanced the maximum responses to 5-HT in both fetal (from 67 to 85%) and adult (from 59 to 79%) arteries. These results indicate that maturation modulates the contribution of K(V), K(Ca), and K(ATP), but not K(IR) channels to basal and/or 5HT-induced cerebrovascular tone, and demonstrate that K(V) and K(Ca) channels are coupled to stretch-sensitive receptors, and that K(V) and K(Ca) limit contractile responses to 5-HT. To the extent that changes in pD(2) values reflect changes in agonist--ligand interactions, the data also suggest that K(V), K(Ca), and K(ATP) channels may possibly influence ligand--receptor binding for 5-HT.

Cibenzoline has an inhibitory effect on vasorelaxation mediated by adenosine triphosphate-sensitive K(+) channels in the rat carotid artery

Studies in cardiac myocytes have shown that cibenzoline reduces adenosine triphosphate (ATP)-sensitive K(+) currents, suggesting that this class Ia antiarrhythmic drug may modify the activity of ATP-sensitive K(+) channels in these preparations. The effects of class Ia antiarrhythmic drugs on vasodilation mediated by ion channels have not been studied. Therefore, we designed this study to examine whether cibenzoline may produce changes in vasorelaxation in response to a selective ATP-sensitive K(+) channel opener, levcromakalim, in the isolated rat carotid artery. Rings of rat carotid arteries without endothelium were suspended for isometric force recording. Concentration-response curves were obtained in a cumulative fashion. During submaximal contraction to phenylephrine (3 x 10(-7) M), vasorelaxation in response to levcromakalim (10(-8) to 10(-5) M) or 1-hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-1-triazene (NOC-7; 10(-10) to 10(-5) M) was obtained. During contraction to phenylephrine, levcromakalim induced concentration-dependent vasorelaxation. A selective ATP-sensitive K(+) channel antagonist, glibenclamide (5 x 10(-6) M), completely abolished vasorelaxation in response to levcromakalim, whereas a selective Ca(2+)-dependent K(+) channel antagonist, iberiotoxin (5 x 10(-8) M), did not affect the relaxation. Cibenzoline (10(-6) to 10(-5) M) significantly reduced vasorelaxation to levcromakalim in a concentration-dependent fashion. In contrast, cibenzoline (10(-5) M) did not alter vasorelaxation to a nitric oxide donor, NOC-7. These results suggest that from the clinically relevant concentrations, a novel class Ia antiarrhythmic drug, cibenzoline, impairs carotid vasodilation mediated by ATP-sensitive K(+) channels.

IMPLICATIONS

In isolated rat carotid artery, cibenzoline (10(-6) to 10(-5) M) reduced vasorelaxation to levcromakalim in a concentration-dependent fashion. These results suggest that from the clinically relevant concentrations, a novel class Ia antiarrhythmic drug, cibenzoline, impairs carotid vasodilation mediated by adenosine triphosphate-sensitive K(+) channels.

Evidence for a K(ATP) ion channel link in the inhibition of hypercapnic dilation of pial arterioles by 7-nitroindazole and tetrodotoxin

7-Nitroindazole, an inhibitor of neuronal nitric oxide synthase, reportedly inhibits hypercapnic dilation, but tetrodotoxin, an inhibitor of neuronal transmission, reportedly does not. Thus, evidence does not uniformly support the hypothesis of a neurogenic link to the hypercapnic response. Others suggest the hypercapnic response is mediated by a K(ATP) ion channel. In the following studies, we observed that topically administered tetrodotoxin inhibited dilations produced by hypercapnia. In addition, topical tetrodotoxin and either topical or intraperitoneal 7-nitroindazole, inhibited dilations produced by the K(ATP) channel openers, cromakalim and pinacidil. Inhibition of hypercapnic dilation and inhibition of dilation by the openers of the K(ATP) channel was immediately reversed by either L-lysine or L-arginine, amino acids previously shown to facilitate opening of the channel. The data strongly supports the previous conclusion that there is a K(ATP) ion channel link in the response of pial arterioles to hypercapnia. The location of the channel is not established by these data, nor is it known whether the action of tetrodotoxin on the channel was direct or indirect.

Structural and functional disruption of vascular smooth muscle cells in a transgenic mouse model of amyloid angiopathy

The deposition of amyloid Abeta peptide in the wall of cerebral vessels (cerebral amyloid angiopathy), can lead to weakness and rupture of the vessel wall, resulting in hemorrhagic stroke. The Tg2576 transgenic mouse line, overexpressing mutant amyloid precursor protein in an age-dependent manner, forms amyloid angiopathy morphologically similar to that seen in the human. We report here the structural and functional disruption of smooth muscle cells (SMCs) in the walls of pial vessels affected by amyloid deposition in the Tg2576 mouse. We demonstrate, using multiphoton imaging, that the arrangement of SMCs becomes disorganized before the onset of cell death, and that these disorganized SMCs are unable to respond appropriately to application of endothelial-dependent and endothelial-independent vasodilators in a closed-cranial window preparation.

Adenosine mediates nitric-oxide-independent renal vasodilation by activation of A2A receptors

OBJECTIVE

Adenosine dilates rabbit renal arteries by an endothelium-dependent, nitric oxide (NO)- and prostaglandin-independent mechanism. The aim was to identify the responsible P1-purinoceptor subtype and to investigate the involvement of K+-channels.

METHODS

Rabbit renal arteries were perfused with medium containing indomethacin (10 micromol/l). After preconstriction with noradrenaline (0.4 micromol/l), changes in vessel diameter by P1-purinoceptor agonists were measured with a photoelectric device. The P1-receptor subtype was characterised by selective antagonists.

RESULTS

Adenosine caused concentration-dependent dilation (EC50 approximately 7 micromol/l). The mRNA for A1, A2A and A3 receptors were demonstrated by reverse transcription-polymerase chain reaction from total RNA of renal arteries. The agonists CPCA (A2) and CGS21680 (A2A) dilated renal arteries (EC50 approximately 0.1 micromol/l), and CPA (A1) was ineffective. As demonstrated by experiments using two arteries in sequence, CPCA induced release of an endothelium-derived relaxing factor. NO synthase inhibition by NG-nitro-L-arginine methyl ester (L-NAME) had no effect on CPCA-induced dilation. The concentration-response curves of adenosine, CPCA and CGS21680 were shifted to the right by the A2A antagonist ZM241385 (1 micromol/l), but not by the A1 and A3 antagonists DPCPX (1 micromol/l) and MRS1220 (1 micromol/l). Iberiotoxin (0.1 micromol/l), a blocker of Ca2+-activated K+-channels, slightly shifted the dose- response curve of CPCA. Arteries preconstricted by KCl showed dilation to CPCA, but not to acetylcholine chloride (ACh).

CONCLUSION

Adenosine induces dilation of rabbit renal arteries through activation of A2A receptors. This effect depends on the release of an endothelium-derived relaxing factor, which is not NO. Dilation by ACh in the presence of L-NAME is likely to be mediated by K+ as an endothelium-derived relaxing factor. However, in the A2A-receptor-induced dilation of rabbit renal arteries, K+ does not play this role, suggesting the involvement of a further soluble factor in the receptor-induced dilatory function of the endothelium.

Mutually protective actions of kainic acid epileptic preconditioning and sublethal global ischemia on hippocampal neuronal death: involvement of adenosine A1 receptors and K(ATP) channels

Preconditioning with sublethal ischemia attenuates the detrimental effects of subsequent prolonged ischemic insults. This research elucidates potential in vivo cross-tolerance between different neuronal death-generating treatments such as kainate administration, which induces seizures and global ischemia. This study also investigates the effects of a mild epileptic insult on neuronal death in rat hippocampus after a subsequent, lethal epileptic stress using kainic acid (KA) as a model of epilepsy. Three preconditioning groups were as follows: group 1 was injected with 5 mg/kg KA before a 6-minute global ischemia; group 2 received a 3-minute global ischemia before 7.5 mg/kg KA; and group 3 was injected with a 5-mg/kg dose of KA before a 7.5-mg/kg KA injection. The interval between treatments was 3 days. Neuronal degeneration, revealed by the silver impregnation method and analysis of cresyl violet staining, was markedly reduced in rats preconditioned with a sublethal ischemia or a 5-mg/kg KA treatment. Labeling with terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'triphosphate-biotin nick-end labeling and DNA laddering confirmed the component of DNA fragmentation in the death of ischemic and epileptic neurons and its reduction in all preconditioned animals. The current study supports the existence of bidirectional cross-tolerance between KA excitotoxicity and global ischemia and suggests the involvement of adenosine A1 receptors and sulfonylurea- and ATP-sensitive K+ channels in this protective phenomenon.

Introduction of the closed cranial window technique in gerbils and verification by observation of the effects of specific drugs

The exact mechanisms of cerebral arterial hypoxia are not perfectly defined. Our purpose is to adapt and validate, with drugs well known in rats and rabbits, a closed cranial window technique in gerbils. The method was used with seventeen gerbils to measure diameter changes of the pial arterioles under normoxia (after the topical application of agonists and antagonists of ATP-sensitive and Ca2+-dependent potassium channels), as well as under hypoxia. In normoxia, aprikalim (10(-6) M), a direct activator of ATP-sensitive potassium channels, increases the diameter of pial arterioles by 10+/-2% (N = 17). This effect is inhibited by glibenclamide (10(-6) M), but not affected by iberiotoxin (10(-6) M), a specific inhibitor of Ca2+-dependent potassium channels. The adenosine-induced dilation by 19+/-5% (N = 17) is reduced by 59+/-16% with iberiotoxin, by 33+/-23% with glibenclamide and inhibited by theophylline (10(-5) M). In hypoxia (15% O2), pial arteriole diameters are increased by 24+/-5% (N = 17) and partially decreased by the application of glibenclamide and iberiotoxin to 59+/-11% and 54+/-5%, respectively. These data are similar to those obtained in other species and validate the closed cranial window technique on gerbils. They indicate that, as for rats and rabbits, both ATP-sensitive and Ca2+-dependent potassium channels are present in gerbil pial vessels and play a role in hypoxia.

Electrical and mechanical responses of rat middle cerebral arteries to reduced PO2 and prostacyclin

Isolated rat middle cerebral arteries were perfused and superfused with physiological salt solution equilibrated with a control (approximately 140 mmHg) or reduced (approximately 35-40 mmHg) PO2. In other experiments, cerebral arteries were isolated and prostacyclin release was determined by radioimmunoassay for 6-ketoprostaglandin F1alpha. Equilibration of the vessels with reduced PO2 (35 mmHg) solution caused a significant increase in prostacyclin release relative to control PO2 (140 mmHg) conditions. Exposure of middle cerebral arteries to reduced PO2 caused vascular smooth muscle (VSM) hyperpolarization and vessel relaxation, which could be blocked by 1 microM glibenclamide, an inhibitor of the ATP-sensitive K+ channel, but not by 1 mM tetraethylammonium (TEA), an inhibitor of the Ca2+-activated K+ channel. Glibenclamide also inhibited VSM hyperpolarization and vasodilation in response to the stable prostacyclin analog iloprost, but TEA did not affect iloprost-induced dilation of the vessel. Endothelial removal eliminated the electrical and mechanical responses of the arteries to reduced PO2, but vessel responses to iloprost were similar to those of intact vessels. The results of this study are consistent with the hypothesis that hypoxic dilation of rat middle cerebral arteries is due to VSM hyperpolarization mediated by prostacyclin-induced activation of glibenclamide-sensitive K+ channels.

Role of K+ channels in A2A adenosine receptor-mediated dilation of the pressurized renal arcuate artery

1. Adenosine A2A receptor-mediated renal vasodilation was investigated by measuring the lumenal diameter of pressurized renal arcuate arteries isolated from the rabbit. 2. The selective A2A receptor agonist CGS21680 dilated the arteries with an EC50 of 130 nM. The CGS21680-induced vasodilation was, on average, 34% less in endothelium-denuded arteries. 3. The maximum response and the EC50 for CGS21680-induced vasodilation in endothelium-intact arteries were not significantly affected by incubation with the K+ channel blockers apamin (100 nM), iberiotoxin (100 nM), 3,4-diaminopyridine (1 mM), glibenclamide (1 microM) or Ba2+ (10 microM). However, a cocktail mixture of these blockers did significantly inhibit the maximum response by almost 40%, and 1 mM Ba2+ alone or 1 mM Ba2+ in addition to the cocktail inhibited the maximum CGS21680-response by 58% and about 75% respectively. 4. CGS21680-induced vasodilation was strongly inhibited when the extracellular K+ level was raised to 20 mM even though the dilator response to 1 microM levcromakalim, a K(ATP) channel opener drug, was unaffected. 5. CGS21680-induced vasodilation was inhibited by 10 microM ouabain, an inhibitor of Na+/K(+)-ATPase, but ouabain had a similar inhibitory effect on vasodilation induced by 30 nM nicardipine (a dihydropyridine Ca2+ antagonist) or 1 microM levcromakalim. 6. The data suggest that K+ channel activation does play a role in A(2A) receptor-mediated renal vasodilation. The inhibitory effect of raised extracellular K+ levels on the A(2A) response may be due to K(+)-induced stimulation of Na+/K(+)-ATPase.