The permissive role of endothelial NO in CO-induced cerebrovascular dilation

Carbon monoxide poisoning is associated with increased risk of migraine in the long term: a nationwide population-based cohort study

Objective

Carbon monoxide poisoning can cause migraine-like attacks. However, the association between carbon monoxide poisoning and the risk of migraine has not been thoroughly studied. This study aimed to investigate the long-term risk of migraine in patients with carbon monoxide poisoning.

Methods

This nationwide, population-based cohort study was conducted using the administrative database of the National Health Insurance Service of Korea from 2002 to 2021. Patients with carbon monoxide poisoning with at least one visit documented according to the International Classification of Diseases, 10th Revision code T58 were included. Patients were only included if they had the same diagnostic code at two or more outpatient clinic visits. The primary outcome of this study was the incidence of migraine after carbon monoxide poisoning.

Results

The overall risk of migraine was higher in the carbon monoxide poisoning group regardless of age, sex, or use of hyperbaric oxygen therapy (adjusted hazard ratio, 1.37; 95% confidence interval, 1.28-1.48). The carbon monoxide poisoning group had a persistently higher cumulative incidence of migraine during the observation period than the control group.

Conclusion

Carbon monoxide poisoning was associated with an increased overall risk of developing migraine during long-term follow-up.

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.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Carbon monoxide inhalation induces headache but no migraine in patients with migraine without aura

INTRODUCTION

Carbon monoxide is an endogenously produced signaling gasotransmitter known to cause headache and vasodilation. We hypothesized that inhalation of carbon monoxide would induce migraine-like attacks in migraine without aura patients.

METHODS

In a randomized, double-blind, placebo-controlled crossover design, 12 migraine patients were allocated to inhalation of carbon monoxide (carboxyhemoglobin 22%) or placebo on two separate days. Headache and migraine characteristics were recorded during hospital (0-2 hours) and post-hospital (2-13 hours) phases.

RESULTS

Six patients (50%) developed migraine-like attacks after carbon monoxide compared to two after placebo (16.7%) ( p = 0.289). The median time to onset of migraine-like attacks after carbon monoxide inhalation was 7.5 h (range 3-12) compared to 11.5 h (range 11-12) after placebo. Nine out of 12 patients (75%) developed prolonged headache after carbon monoxide. The area under the curve for headache score (0-13 hours) was increased after carbon monoxide compared with placebo ( p = 0.033).

CONCLUSION

Carbon monoxide inhalation did not provoke more migraine-like attacks in migraine patients compared to placebo, but induced more headache in patients compared to placebo. These data suggest that non-toxic concentrations of carbon monoxide had low potency in migraine induction and that the carbon monoxide inhalation model is not suitable to study migraine.

Differential distribution and functional impact of BK channel beta1 subunits across mesenteric, coronary, and different cerebral arteries of the rat

Large conductance, Ca²⁺_i- and voltage-gated K⁺ (BK) channels regulate myogenic tone and, thus, arterial diameter. In smooth muscle (SM), BK channels include channel-forming α and auxiliary β1 subunits. BK β1 increases the channel's Ca²⁺ sensitivity, allowing BK channels to negatively feedback on depolarization-induced Ca²⁺ entry, oppose SM contraction and favor vasodilation. Thus, endothelial-independent vasodilation can be evoked though targeting of SM BK β1 by endogenous ligands, including lithocholate (LCA). Here, we investigated the expression of BK β1 across arteries of the cerebral and peripheral circulations, and the contribution of such expression to channel function and BK β1-mediated vasodilation. Data demonstrate that endothelium-independent, BK β1-mediated vasodilation by LCA is larger in coronary (CA) and basilar (BA) arteries than in anterior cerebral (ACA), middle cerebral (MCA), posterior cerebral (PCA), and mesenteric (MA) arteries, all arterial segments having a similar diameter. Thus, differential dilation occurs in extracranial arteries which are subjected to similar vascular pressure (CA vs. MA) and in arteries that irrigate different brain regions (BA vs. ACA, MCA, and PCA). SM BK channels from BA and CA displayed increased basal activity and LCA responses, indicating increased BK β1 functional presence. Indeed, in the absence of detectable changes in BK α, BA and CA myocytes showed an increased location of BK β1 in the plasmalemma/subplasmalemma. Moreover, these myocytes distinctly showed increased BK β1 messenger RNA (mRNA) levels. Supporting a major role of enhanced BK β1 transcripts in artery dilation, LCA-induced dilation of MCA transfected with BK β1 complementary DNA (cDNA) was as high as LCA-induced dilation of untransfected BA or CA.

Carbon monoxide may be an important molecule in migraine and other headaches

Introduction

Carbon monoxide was previously considered to just be a toxic gas. A wealth of recent information has, however, shown that it is also an important endogenously produced signalling molecule involved in multiple biological processes. Endogenously produced carbon monoxide may thus play an important role in nociceptive processing and in regulation of cerebral arterial tone.

Discussion

Carbon monoxide-induced headache shares many characteristics with migraine and other headaches. The mechanisms whereby carbon monoxide causes headache may include hypoxia, nitric oxide signalling and activation of cyclic guanosine monophosphate pathways. Here, we review the literature about carbon monoxide-induced headache and its possible mechanisms.

Conclusion

We suggest, for the first time, that carbon monoxide may play an important role in the mechanisms of migraine and other headaches.

Multifunctional role of astrocytes as gatekeepers of neuronal energy supply

Dynamic adjustments to neuronal energy supply in response to synaptic activity are critical for neuronal function. Glial cells known as astrocytes have processes that ensheath most central synapses and express G-protein-coupled neurotransmitter receptors and transporters that respond to neuronal activity. Astrocytes also release substrates for neuronal oxidative phosphorylation and have processes that terminate on the surface of brain arterioles and can influence vascular smooth muscle tone and local blood flow. Membrane receptor or transporter-mediated effects of glutamate represent a convergence point of astrocyte influence on neuronal bioenergetics. Astrocytic glutamate uptake drives glycolysis and subsequent shuttling of lactate from astrocytes to neurons for oxidative metabolism. Astrocytes also convert synaptically reclaimed glutamate to glutamine, which is returned to neurons for glutamate salvage or oxidation. Finally, astrocytes store brain energy currency in the form of glycogen, which can be mobilized to produce lactate for neuronal oxidative phosphorylation in response to glutamatergic neurotransmission. These mechanisms couple synaptically driven astrocytic responses to glutamate with release of energy substrates back to neurons to match demand with supply. In addition, astrocytes directly influence the tone of penetrating brain arterioles in response to glutamatergic neurotransmission, coordinating dynamic regulation of local blood flow. We will describe the role of astrocytes in neurometabolic and neurovascular coupling in detail and discuss, in turn, how astrocyte dysfunction may contribute to neuronal bioenergetic deficit and neurodegeneration. Understanding the role of astrocytes as a hub for neurometabolic and neurovascular coupling mechanisms is a critical underpinning for therapeutic development in a broad range of neurodegenerative disorders characterized by chronic generalized brain ischemia and brain microvascular dysfunction.

Cerebral hemodynamics: concepts of clinical importance

Cerebral hemodynamics and metabolism are frequently impaired in a wide range of neurological diseases, including traumatic brain injury and stroke, with several pathophysiological mechanisms of injury. The resultant uncoupling of cerebral blood flow and metabolism can trigger secondary brain lesions, particularly in early phases, consequently worsening the patient's outcome. Cerebral blood flow regulation is influenced by blood gas content, blood viscosity, body temperature, cardiac output, altitude, cerebrovascular autoregulation, and neurovascular coupling, mediated by chemical agents such as nitric oxide (NO), carbon monoxide (CO), eicosanoid products, oxygen-derived free radicals, endothelins, K+, H+, and adenosine. A better understanding of these factors is valuable for the management of neurocritical care patients. The assessment of both cerebral hemodynamics and metabolism in the acute phase of neurocritical care conditions may contribute to a more effective planning of therapeutic strategies for reducing secondary brain lesions. In this review, the authors have discussed concepts of cerebral hemodynamics, considering aspects of clinical importance.

Astrocyte inositol triphosphate receptor type 2 and cytosolic phospholipase A2 alpha regulate arteriole responses in mouse neocortical brain slices

Functional hyperemia of the cerebral vascular system matches regional blood flow to the metabolic demands of the brain. One current model of neurovascular control holds that glutamate released by neurons activates group I metabotropic glutamate receptors (mGluRs) on astrocytes, resulting in the production of diffusible messengers that act to regulate smooth muscle cells surrounding cerebral arterioles. The acute mouse brain slice is an experimental system in which changes in arteriole diameter can precisely measured with light microscopy. Stimulation of the brain slice triggers specific cellular responses that can be correlated to changes in arteriole diameter. Here we used inositol trisphosphate receptor type 2 (IP(3)R2) and cytosolic phospholipase A(2) alpha (cPLA(2)α) deficient mice to determine if astrocyte mGluR activation coupled to IP(3)R2-mediated Ca(2+) release and subsequent cPLA(2)α activation is required for arteriole regulation. We measured changes in astrocyte cytosolic free Ca(2+) and arteriole diameters in response to mGluR agonist or electrical field stimulation in acute neocortical mouse brain slices maintained in 95% or 20% O(2). Astrocyte Ca(2+) and arteriole responses to mGluR activation were absent in IP(3)R2(-/-) slices. Astrocyte Ca(2+) responses to mGluR activation were unchanged by deletion of cPLA(2)α but arteriole responses to either mGluR agonist or electrical stimulation were ablated. The valence of changes in arteriole diameter (dilation/constriction) was dependent upon both stimulus and O(2) concentration. Neuron-derived NO and activation of the group I mGluRs are required for responses to electrical stimulation. These findings indicate that an mGluR/IP(3)R2/cPLA(2)α signaling cascade in astrocytes is required to transduce neuronal glutamate release into arteriole responses.

Carbon monoxide as an endogenous vascular modulator

Carbon monoxide (CO) is produced by heme oxygenase (HO)-catalyzed heme degradation to CO, iron, and biliverdin. HO has two active isoforms, HO-1 (inducible) and HO-2 (constitutive). HO-2, but not HO-1, is highly expressed in endothelial and smooth muscle cells and in adjacent astrocytes in the brain. HO-1 is expressed basally only in the spleen and liver but can be induced to a varying extent in most tissues. Elevating heme, protein phosphorylation, Ca(2+) influx, and Ca(2+)/calmodulin-dependent processes increase HO-2 activity. CO dilates cerebral arterioles and may constrict or dilate skeletal muscle and renal arterioles. Selected vasodilatory stimuli, including seizures, glutamatergic stimulation, hypoxia, hypotension, and ADP, increase CO, and the inhibition of HO attenuates the dilation to these stimuli. Astrocytic HO-2-derived CO causes glutamatergic dilation of pial arterioles. CO dilates by activating smooth muscle cell large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. CO binds to BK(Ca) channel-bound heme, leading to an increase in Ca(2+) sparks-to-BK(Ca) channel coupling. Also, CO may bind directly to the BK(Ca) channel at several locations. Endothelial nitric oxide and prostacyclin interact with HO/CO in circulatory regulation. In cerebral arterioles in vivo, in contrast to dilation to acute CO, a prolonged exposure of cerebral arterioles to elevated CO produces progressive constriction by inhibiting nitric oxide synthase. The HO/CO system is highly protective to the vasculature. CO suppresses apoptosis and inhibits components of endogenous oxidant-generating pathways. Bilirubin is a potent reactive oxygen species scavenger. Still many questions remain about the physiology and biochemistry of HO/CO in the circulatory system and about the function and dysfunction of this gaseous mediator system.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

Time-dependent action of carbon monoxide on the newborn cerebrovascular circulation

Carbon monoxide (CO) causes cerebral arteriolar dilation in newborn pigs by the activation of large-conductance Ca(2+)-activated K(+) channels. In adult rat cerebral and skeletal muscle arterioles, CO has been reported to produce constriction caused by the inhibition of nitric oxide (NO) synthase (NOS). We hypothesized that, in contrast to dilation to acute CO, more prolonged exposure of newborn cerebral arterioles to elevated CO produces constriction by reducing NO. In piglets with closed cranial windows, pial arteriolar responses to isoproterenol (10(-6) M), sodium nitroprusside (SNP; 10(-7) and 3 x 10(-7) M), and L-arginine ethyl ester (L-Arg; 10(-5) and 10(-4) M) were determined before and after 2 h of treatment with CO. CO (10(-7) M) caused transient dilation and had no further effects. CO (2 x 10(-7) and 10(-6) M) initially caused vasodilation, but over the 2-h exposure, pial arterioles constricted and removal of the CO caused dilation. Exposure to elevated CO (2 h) did not alter dilation to SNP or isoproterenol. Conversely, the NOS substrate L-Arg caused dilation before CO that was progressively lost over 90 min of elevated CO. If NO was held constant, CO caused dilation that was sustained for 2 h. We conclude that in neonates, cerebral arteriole responses to CO are biphasic: dilation to acute elevation with subsequent constriction from NOS inhibition after more prolonged exposure. As a result, short episodic production of CO allows function as a dilator gasotransmitter, whereas prolonged elevation can reduce NO to elevate cerebrovascular tone. The interaction between heme oxygenase/CO and NOS/NO could form a negative feedback system in the control of cerebral vascular tone.

Glutamate regulates Ca2+ signals in smooth muscle cells of newborn piglet brain slice arterioles through astrocyte- and heme oxygenase-dependent mechanisms

Glutamate is the principal cerebral excitatory neurotransmitter and dilates cerebral arterioles to match blood flow to neural activity. Arterial contractility is regulated by local and global Ca(2+) signals that occur in smooth muscle cells, but modulation of these signals by glutamate is poorly understood. Here, using high-speed confocal imaging, we measured the Ca(2+) signals that occur in arteriole smooth muscle cells of newborn piglet tangential brain slices, studied signal regulation by glutamate, and investigated the physiological function of heme oxygenase (HO) and carbon monoxide (CO) in these responses. Glutamate elevated Ca(2+) spark frequency by approximately 188% and reduced global intracellular Ca(2+) concentration ([Ca(2+)](i)) to approximately 76% of control but did not alter Ca(2+) wave frequency in brain arteriole smooth muscle cells. Isolation of cerebral arterioles from brain slices abolished glutamate-induced Ca(2+) signal modulation. In slices treated with l-2-alpha-aminoadipic acid, a glial toxin, glutamate did not alter Ca(2+) sparks or global [Ca(2+)](i) but did activate Ca(2+) waves. This shift in Ca(2+) signal modulation by glutamate did not occur in slices treated with d-2-alpha-aminoadipic acid, an inactive isomer of l-2-alpha-aminoadipic acid. In the presence of chromium mesoporphyrin, a HO blocker, glutamate inhibited Ca(2+) sparks and Ca(2+) waves and did not alter global [Ca(2+)](i). In isolated arterioles, CORM-3 [tricarbonylchloro(glycinato)ruthenium(II)], a CO donor, activated Ca(2+) sparks and reduced global [Ca(2+)](i). These effects were blocked by 1H-(1,2,4)-oxadiazolo-(4,3-a)-quinoxalin-1-one, a soluble guanylyl cyclase inhibitor. Collectively, these data indicate that glutamate can modulate Ca(2+) sparks, Ca(2+) waves, and global [Ca(2+)](i) in arteriole smooth muscle cells via mechanisms that require astrocytes and HO. These data also indicate that soluble guanylyl cyclase is involved in CO activation of Ca(2+) sparks in arteriole smooth muscle cells.

Roles of glia limitans astrocytes and carbon monoxide in adenosine diphosphate-induced pial arteriolar dilation in newborn pigs

BACKGROUND AND PURPOSE

Astrocytes, neurons, and microvessels together form a neurovascular unit allowing blood flow to match neuronal activity. Adenosine diphosphate (ADP) is an important signaling molecule in the brain, and dilation in response to ADP is astrocyte-dependent in rats and newborn pigs. Carbon monoxide (CO), produced endogenously by catabolism of heme to CO, iron, and biliverdin via heme oxygenase, is an important cell-signaling molecule in the neonatal cerebral circulation. We hypothesize ADP stimulates CO production by glia limitans astrocytes and that this CO causes pial arteriolar dilation.

METHODS

Experiments were performed using anesthetized piglet with closed cranial windows, and freshly isolated piglet astrocytes and microvessels. Astrocyte injury was caused by topical application of L-2-alpha aminoadipic acid (2 mmol/L, 5 hours). Cerebrospinal fluid was collected from under the cranial windows for measurement of ADP-stimulated CO production. CO was measured by gas chromatography-mass spectroscopy analysis.

RESULTS

Before, but not after, astrocyte injury in vivo, topical ADP stimulated both CO production and dilation of pial arterioles. Astrocyte injury did not block dilation to isoproterenol or bradykinin. Chromium mesoporphyrin, an inhibitor of heme oxygenase, also prevented the ADP-induced increase in cerebrospinal fluid CO and pial arteriolar dilation caused by ADP, but not dilation to sodium nitroprusside. ADP also increased CO production by freshly isolated piglet astrocytes and cerebral microvessels, although the increase was smaller in the microvessels.

CONCLUSIONS

These data suggest that glia limitans astrocytes use CO as a gasotransmitter to cause pial arteriolar dilation in response to ADP.

Role of heme oxygenase-2 in pial arteriolar response to acetylcholine in mice with and without transfusion of cell-free hemoglobin polymers

Carbon monoxide derived from heme oxygenase (HO) may participate in cerebrovascular regulation under specific circumstances. Previous work has shown that HO contributes to feline pial arteriolar dilation to acetylcholine after transfusion of a cell-free polymeric hemoglobin oxygen carrier. The role of constitutive HO2 in the pial arteriolar dilatory response to acetylcholine was determined by using 1) HO2-null mice (HO2-/-), 2) the HO inhibitor tin protoporphyrin IX (SnPPIX), and 3) 4,5,6,7-tetrabromobenzotriazole (TBB), an inhibitor of casein kinase-2 (CK2)-dependent phosphorylation of HO2. In anesthetized mice, superfusion of a cranial window with SnPPIX decreased arteriolar dilation produced by 10 microM acetylcholine by 51%. After partial polymeric hemoglobin exchange transfusion, the acetylcholine response was normal but was reduced 72% by SnPPIX and 95% by TBB. In HO2-/- mice, the acetylcholine response was modestly reduced by 14% compared with control mice and was unaffected by SnPPIX. After hemoglobin transfusion in HO2-/- mice, acetylcholine responses were also unaffected by SnPPIX and TBB. In contrast, nitric oxide synthase inhibition completely blocked the acetylcholine responses in hemoglobin-transfused HO2-/- mice. We conclude 1) that HO2 activity partially contributes to acetylcholine-induced pial arteriolar dilation in mice, 2) that this contribution is augmented in the presence of a plasma-based hemoglobin polymer and appears to depend on a CK2 kinase mechanism, 3) that nitric oxide synthase activity rather than HO1 activity contributes to the acetylcholine reactivity in HO2-/- mice, and 4) that plasma-based polymeric hemoglobin does not scavenge all of the nitric oxide generated by cerebrovascular acetylcholine stimulation.

Carbon monoxide-releasing molecules (CO-RMs): vasodilatory, anti-ischaemic and anti-inflammatory activities

The well-known adverse effects of CO (carbon monoxide) intoxication are counterbalanced by its positive actions when small amounts are produced intracellularly by the cytoprotective enzyme HO-1 (haem oxygenase-1). As compelling scientific evidence accumulated to sustain that HO-1 plays a fundamental role in counteracting vascular and inflammatory disorders, we began to appreciate that a controlled delivery of CO to mammals may provide therapeutic benefits in a number of pathological states. This is the rationale for the recent development of CO-RMs (CO-releasing molecules), a group of compounds capable of carrying and liberating controlled quantities of CO in cellular systems, which offer a plausible tool for studying the pharmacological effects of this gas and identifying its mechanism(s) of action. The present review will highlight the encouraging results obtained so far on the vasodilatory, anti-ischaemic and anti-inflammatory effects elicited by CO-RMs in in vitro and in vivo models with an emphasis on the prospect of converting chemical CO carriers into CO-based pharmaceuticals.

Cerebroprotective functions of HO-2

The constitutive isoform of heme oxygenase, HO-2, is highly expressed in the brain and in cerebral vessels. HO-2 functions in the brain have been evaluated using pharmacological inhibitors of the enzyme and HO-2 gene deletion in in vivo animal models and in cultured cells (neurons, astrocytes, cerebral vascular endothelial cells). Rapid activation of HO-2 via post-translational modifications without upregulation of HO-2 expression or HO-1 induction coincides with the increase in cerebral blood flow aimed at maintaining brain homeostasis and neuronal survival during seizures, hypoxia, and hypotension. Pharmacological inhibition or gene deletion of brain HO-2 exacerbates oxidative stress induced by seizures, glutamate, and inflammatory cytokines, and causes cerebral vascular injury. Carbon monoxide (CO) and bilirubin, the end products of HO-catalyzed heme degradation, have distinct cytoprotective functions. CO, by binding to a heme prosthetic group, regulates the key components of cell signaling, including BK(Ca) channels, guanylyl cyclase, NADPH oxidase, and the mitochondria respiratory chain. Cerebral vasodilator effects of CO are mediated via activation of BK(Ca) channels and guanylyl cyclase. CO, by inhibiting the major components of endogenous oxidant-generating machinery, NADPH oxidase and the cytochrome C oxidase of the mitochondrial respiratory chain, blocks formation of reactive oxygen species. Bilirubin, via redox cycling with biliverdin, is a potent oxidant scavenger that removes preformed oxidants. Overall, HO-2 has dual housekeeping cerebroprotective functions by maintaining autoregulation of cerebral blood flow aimed at improving neuronal survival in a changing environment, and by providing an effective defense mechanism that blocks oxidant formation and prevents cell death caused by oxidative stress.

Age and species dependence of pial arteriolar responses to topical carbon monoxide in vivo

In newborn pigs, carbon monoxide (CO) contributes to regulation of cerebrovascular circulation. Results from isolated adult cerebral arteries suggest CO may have less dilatory potential in mature animals. However, few data are available on the direct effects of CO on cerebrovascular circulation in vivo except for those from newborn pigs. Therefore, we tested the hypothesis that i) rat cerebral arterioles dilate to CO in vivo and ii) CO-induced cerebrovascular dilatory responses are age dependent in pigs. Also, we examined whether the permissive role of nitric oxide in CO-induced dilation observed in piglets is present in older pigs and rats. Experiments used anesthetized newborn, 7-week-old, and juvenile (3- to 4-month-old) pigs and 3- to 4-month-old rats with closed cranial windows and topical applications of CO and sodium nitroprusside (SNP). Dilations to SNP were not different at different ages in pigs or between pigs and rats. CO produced pial arteriolar dilations in all groups. Dilation to 10(-5) M CO was reduced in juvenile pigs as compared to newborn and 7-week-old pigs, and tended to less at 10(-6) M CO. Dilations of rat pial arterioles to all concentrations were less than those of newborn and 7-week-old pigs, but not different from those of juvenile pig pial arterioles. In newborn and 7-week-old pigs, l-nitro-arginine (LNA) inhibited the dilation to CO, an effect reversed by a constant background of SNP. In contrast, LNA did not reduce dilation to CO in juvenile pigs or rats. In conclusion, rat pial arterioles like those in piglets dilate to CO in vivo, but there are age and species differences with regard to reactivity and interaction with NO.

Astrocyte-derived CO is a diffusible messenger that mediates glutamate-induced cerebral arteriolar dilation by activating smooth muscle Cell KCa channels

Astrocyte signals can modulate arteriolar tone, contributing to regulation of cerebral blood flow, but specific intercellular communication mechanisms are unclear. Here we used isolated cerebral arteriole myocytes, astrocytes, and brain slices to investigate whether carbon monoxide (CO) generated by the enzyme heme oxygenase (HO) acts as an astrocyte-to-myocyte gasotransmitter in the brain. Glutamate stimulated CO production by astrocytes with intact HO-2, but not those genetically deficient in HO-2. Glutamate activated transient K(Ca) currents and single K(Ca) channels in myocytes that were in contact with astrocytes, but did not affect K(Ca) channel activity in myocytes that were alone. Pretreatment of astrocytes with chromium mesoporphyrin (CrMP), a HO inhibitor, or genetic ablation of HO-2 prevented glutamate-induced activation of myocyte transient K(Ca) currents and K(Ca) channels. Glutamate decreased arteriole myocyte intracellular Ca2+ concentration and dilated brain slice arterioles and this decrease and dilation were blocked by CrMP. Brain slice arteriole dilation to glutamate was also blocked by L-2-alpha aminoadipic acid, a selective astrocyte toxin, and paxilline, a K(Ca) channel blocker. These data indicate that an astrocytic signal, notably HO-2-derived CO, is used by glutamate to stimulate arteriole myocyte K(Ca) channels and dilate cerebral arterioles. Our study explains the astrocyte and HO dependence of glutamatergic functional hyperemia observed in the newborn cerebrovascular circulation in vivo.

Cerebroprotective effects of the CO-releasing molecule CORM-A1 against seizure-induced neonatal vascular injury

Endogenous CO, a product of heme oxygenase activity, has vasodilator and cytoprotective effects in the cerebral circulation of newborn pigs. CO-releasing molecule (CORM)-A1 (sodium boranocarbonate) is a novel, water-soluble, CO-releasing compound. We addressed the hypotheses that CORM-A1 1) can deliver CO to the brain and exert effects of CO on the cerebral microvasculature and 2) is cerebroprotective. Acute and delayed effects of topically and systemically administered CORM-A1 on cerebrovascular and systemic circulatory parameters were determined in anesthetized newborn pigs with implanted closed cranial windows. Topical application of CORM-A1 (10(-7)-10(-5) M) to the brain produced concentration-dependent CO release and pial arteriolar dilation. Systemically administered CORM-A1 (2 mg/kg ip or iv) caused pial arteriolar dilation and increased cortical cerebrospinal fluid CO concentration. Systemic CORM-A1 did not have acute or delayed effects on blood pressure, heart rate, or blood gases. Potential cerebroprotective vascular effects of CORM-A1 (2 mg/kg ip, 30 min before seizures) were tested 2 days after bicuculline-induced epileptic seizures (late postictal period). In control piglets, seizures reduced postictal cerebrovascular responsiveness to selective physiologically relevant vasodilators (bradykinin, hemin, and isoproterenol) indicative of cerebrovascular injury. In contrast, in CORM-A1-pretreated animals, no loss of postictal cerebrovascular reactivity was observed. We conclude that systemically administered CORM-A1 delivers CO to the brain, elicits the vasodilator and cytoprotective effects of CO in the cerebral circulation, and protects the neonatal brain from cerebrovascular injury caused by epileptic seizures.

Mechanisms involved in the cerebrovascular dilator effects of N-methyl-d-aspartate in cerebral cortex

Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Interactions between NO, CO and an endothelium-derived hyperpolarizing factor (EDHF) in maintaining patency of the ductus arteriosus in the mouse

BACKGROUND AND PURPOSE

Prenatal patency of ductus arteriosus is maintained by prostaglandin (PG) E(2), possibly along with nitric oxide (NO) and carbon monoxide (CO), and cyclooxygenase (COX) deletion upregulates NO. Here, we have examined enzyme source and action of NO for ductus patency and whether NO and CO are upregulated by deletion of, respectively, heme oxygenase 2 (HO-2) and COX1 or COX2.

EXPERIMENTAL APPROACH

Experiments were performed in vitro and in vivo with wild-type and gene-deleted, near-term mouse fetuses.

KEY RESULTS

N(G)-nitro-L-arginine methyl ester (L-NAME) contracted the isolated ductus and its effect was reduced by eNOS, but not iNOS, deletion. L-NAME contraction was not modified by HO-2 deletion. Zinc protoporphyrin (ZnPP) also contracted the ductus, an action unaffected by deletion of either COX isoform. Bradykinin (BK) relaxed indomethacin-contracted ductus similarly in wild-type and eNOS-/- or iNOS-/-. BK relaxation was suppressed by either L-NAME or ZnPP. However, it reappeared with combined L-NAME and ZnPP to subside again with K(+) increase or K(+) channel inhibition. In vivo, the ductus was patent in wild-type and NOS-deleted fetuses. Likewise, no genotype-related difference was noted in postnatal closure.

CONCLUSIONS AND IMPLICATIONS

NO, formed mainly via eNOS, regulates ductal tone. NO and CO cooperatively mediate BK-induced relaxation in the absence of PGE(2). However, in the absence of PGE(2), NO and CO, BK induces a relaxant substance behaving as an endothelium-derived hyperpolarizing factor. Ductus patency is, therefore, sustained by a cohort of agents with PGE(2) and NO being preferentially coupled for reciprocal compensation.

Contributions of astrocytes and CO to pial arteriolar dilation to glutamate in newborn pigs

Astrocytes can act as intermediaries between neurons and cerebral arterioles to regulate vascular tone in response to neuronal activity. Release of glutamate from presynaptic neurons increases blood flow to match metabolic demands. CO is a gasotransmitter that can be related to neural function and blood flow regulation in the brain. The present study addresses the hypothesis that glutamatergic stimulation promotes perivascular astrocyte CO production and pial arteriolar dilation in the newborn brain. Experiments used anesthetized newborn pigs with closed cranial windows, piglet astrocytes, and cerebrovascular endothelial cells in primary culture and immunocytochemical visualization of astrocytic markers. Pial arterioles and arteries of newborn pigs are ensheathed by astrocytes visualized by glial fibrillary acidic protein staining. Treatment (2 h) of astrocytes in culture with L-2-alpha-aminoadipic acid (L-AAA), followed by 14 h in toxin free medium, dose-dependently increased cell detachment, suggesting injury. Conversely, 16 h of continuous exposure to L-AAA caused no decrease in endothelial cell attachment. In vivo, topical L-AAA (2 mM, 5 h) disrupted the cortical glia limitans histologically. Such treatment also eliminated pial arteriolar dilation to the astrocyte-dependent dilator ADP and to glutamate but not to isoproterenol or CO. Glutamate stimulated CO production by the brain surface that also was abolished following L-AAA. In contrast, tetrodotoxin blocked dilation to N-methyl-D-aspartate but not to glutamate, isoproterenol, or CO or the glutamate-induced increase in CO. The concurrent loss of CO production and pial arteriolar dilation to glutamate following astrocyte injury suggests astrocytes may employ CO as a gasotransmitter for glutamatergic cerebrovascular dilation.

Carbon monoxide and hydrogen sulfide: gaseous messengers in cerebrovascular circulation

This review focuses on two gaseous cellular messenger molecules, CO and H2S, that are involved in cerebrovascular flow regulation. CO is a dilatory mediator in active hyperemia, autoregulation, hypoxic dilation, and counteracting vasoconstriction. It is produced from heme by a constitutively expressed enzyme [heme oxygenase (HO)-2] expressed highly in the brain and by an inducible enzyme (HO-1). CO production is regulated by controlling substrate availability, HO-2 catalytic activity, and HO-1 expression. CO dilates arterioles by binding to heme that is bound to large-conductance Ca2+-activated K+ channels. This binding elevates channel Ca2+ sensitivity, that increases coupling of Ca2+ sparks to large-conductance Ca2+-activated K+ channel openings and, thereby, hyperpolarizes the vascular smooth muscle. In addition to dilating blood vessels, CO can either inhibit or accentuate vascular cell proliferation and apoptosis, depending on conditions. H2S may also function as a cerebrovascular dilator. It is produced in vascular smooth muscle cells by hydrolysis of l-cysteine catalyzed by cystathione gamma-lyase (CSE). H2S dilates arterioles at physiologically relevant concentrations via activation of ATP-sensitive K+ channels. In addition to dilating blood vessels, H2S promotes apoptosis of vascular smooth muscle cells and inhibits proliferation-associated vascular remodeling. Thus both CO and H2S modulate the function and the structure of circulatory system. Both the HO-CO and CSE-H2S systems have potential to interact with NO and prostanoids in the cerebral circulation. Much of the physiology and biochemistry of HO-CO and CSE-H2S in the cerebral circulation remains open for exploration.

Dependence of acetylcholine and ADP dilation of pial arterioles on heme oxygenase after transfusion of cell-free polymeric hemoglobin

Polymers of cell-free hemoglobin have been designed for clinical use as oxygen carriers, but limited information is available regarding their effects on vascular regulation. We tested the hypothesis that the contribution of heme oxygenase (HO) to acetylcholine-evoked dilation of pial arterioles is upregulated 2 days after polymeric hemoglobin transfusion. Dilator responses to acetylcholine measured by intravital microscopy in anesthetized cats were blocked by superfusion of the HO inhibitor tin protoporphyrin-IX (SnPPIX) in a group that had undergone exchange transfusion with hemoglobin 2 days earlier but not in surgical sham and albumin-transfused groups. However, immunoblots from cortical brain homogenates did not reveal changes in expression of the inducible isoform HO1 or the constitutive isoform HO2 in the hemoglobin-transfused group. To test whether the inhibitory effect of SnPPIX was present acutely after hemoglobin transfusion, responses were measured within an hour of completion of the exchange transfusion. In control and albumin-transfused groups, acetylcholine responses were unaffected by SnPPIX but were blocked by addition of the nitric oxide synthase inhibitor N(omega)-nitro-l-arginine (l-NNA) to the superfusate. In hemoglobin-transfused groups, the acetylcholine response was blocked by either SnPPIX or l-NNA alone. The effect of another HO inhibitor, chromium mesoporphyrin (CrMP), was tested on ADP, another endothelial-dependent dilator, in anesthetized rats. Pial arteriolar dilation to ADP was unaffected by CrMP in controls but was attenuated 62% by CrMP in rats transfused with hemoglobin. It is concluded that 1) polymeric hemoglobin transfusion acutely upregulates the contribution of HO to acetylcholine-induced dilation of pial arterioles in cats, 2) this upregulation persists 2 days after transfusion when 95% of the hemoglobin is cleared from the circulation, and 3) this acute upregulation of HO signaling is ubiquitous in that similar effects were observed with a different endothelial-dependent agonist (i.e., ADP) in a another species (rat).

Effects of carbon monoxide and heme oxygenase inhibitors in cerebral vessels of rats and mice

Carbon monoxide (CO) has been postulated to be a signaling molecule in many tissues, including the vasculature. We examined vasomotor responses of adult rat and mouse cerebral arteries to both exogenously applied and endogenously produced CO. The diameter of isolated, pressurized, and perfused rat middle cerebral arteries (MCAs) was not altered by authentic CO (10(-6) to 10(-4) M). Mouse MCAs, however, dilated by 21 +/- 10% at 10(-4) M CO. Authentic nitric oxide (NO., 10(-10) to 10(-7) M) dilated both rat and mouse MCAs. At 10(-8) M NO., rat vessels dilated by 84 +/- 4%, and at 10(-7) M NO., mouse vessels dilated by 59 +/- 9%. Stimulation of endogenous CO production through heme oxygenase (HO) with the heme precursor delta-aminolevulinic acid (10(-10) to 10(-4) M) did not dilate the MCAs of either species. The metalloporphyrin HO inhibitor chromium mesoporphyrin IX (CrMP) caused profound constriction of the rat MCA (44 +/- 2% at 3 x 10(-5) M). Importantly, this constriction was unaltered by exogenous CO (10(-4) M) or CO plus 10(-5) M biliverdine (both HO products). In contrast, exogenous CO (10(-4) M) reversed CrMP-induced constriction in rat gracilis arterioles. Control mouse MCAs constricted by only 3 +/- 1% in response to 10(-5) M CrMP. Magnesium protoporphyrin IX (10(-5) M), a weak HO inhibitor used to control for nonspecific effects of metalloporphyrins, also constricted the rat MCA to a similar extent as CrMP. We conclude that, at physiological concentrations, CO is not a dilator of adult rodent cerebral arteries and that metalloporphyrin HO inhibitors have nonspecific constrictor effects in rat cerebral arteries.

Carbon Monoxide: Endogenous Production, Physiological Functions, and Pharmacological Applications

Over the last decade, studies have unraveled many aspects of endogenous production and physiological functions of carbon monoxide (CO). The majority of endogenous CO is produced in a reaction catalyzed by the enzyme heme oxygenase (HO). Inducible HO (HO-1) and constitutive HO (HO-2) are mostly recognized for their roles in the oxidation of heme and production of CO and biliverdin, whereas the biological function of the third HO isoform, HO-3, is still unclear. The tissue type-specific distribution of these HO isoforms is largely linked to the specific biological actions of CO on different systems. CO functions as a signaling molecule in the neuronal system, involving the regulation of neurotransmitters and neuropeptide release, learning and memory, and odor response adaptation and many other neuronal activities. The vasorelaxant property and cardiac protection effect of CO have been documented. A plethora of studies have also shown the importance of the roles of CO in the immune, respiratory, reproductive, gastrointestinal, kidney, and liver systems. Our understanding of the cellular and molecular mechanisms that regulate the production and mediate the physiological actions of CO has greatly advanced. Many diseases, including neurodegenerations, hypertension, heart failure, and inflammation, have been linked to the abnormality in CO metabolism and function. Enhancement of endogenous CO production and direct delivery of exogenous CO have found their applications in many health research fields and clinical settings. Future studies will further clarify the gasotransmitter role of CO, provide insight into the pathogenic mechanisms of many CO abnormality-related diseases, and pave the way for innovative preventive and therapeutic strategies based on the physiologic effects of CO.

Permissive contributions of NO and prostacyclin in CO-induced cerebrovascular dilation in piglets

Endogenously produced CO is an important dilator in newborn cerebrovascular circulation. CO dilates cerebral arterioles by activating Ca2+-activated K+ channels, but modulatory actions of other effectors and second messenger inputs are unclear. Specifically, the mechanisms behind the obligatory permissive roles of prostacyclin and NO are uncertain. Therefore, the present study was performed using acutely implanted, closed cranial windows in newborn pigs to address the hypothesis that the permissive roles of NO and prostacyclin in cerebrovascular dilation in response to CO involve a common mechanism. The NO donor sodium nitroprusside restored dilation in response to CO after inhibition of that dilation with the prostaglandin cyclooxygenase inhibitor indomethacin. The stable prostacyclin analog iloprost restored CO-induced dilation blocked by the NO synthase inhibitor Nomega-nitro-L-arginine. Restoration of dilation in response to CO by the cGMP-dependent phosphodiesterase inhibitor zaprinast and blockade of CO dilation by the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole-[4,3-a]quinoxalin-1-one (ODQ) suggests involvement of the cGMP/PKG pathway. Iloprost or the cAMP-dependent dilator isoproterenol restored dilation in response to CO after ODQ administration. However, CO-induced dilation blocked by the cGMP-dependent PKG inhibitor Rp-8-[(4-chlorophenyl)thio]-cGMPS triethylamine could not be reversed by administration of sodium nitroprusside, iloprost, or isoproterenol. Conversely, PKA inhibition did not block dilation in response to CO. Overall, data indicate that activation of PKG is the predominant mechanism of the permissive actions of NO and prostacyclin for CO-induced pial arteriolar dilation.