Carbon monoxide activates human intestinal smooth muscle L-type Ca2+ channels through a nitric oxide-dependent mechanism

Fabrication of CO-releasing surface to enhance the blood compatibility and endothelialization of TiO2 nanotubes on titanium surface

Although the construction of nanotube arrays with the micro-nano structures on the titanium surfaces has demonstrated a great promise in the field of blood-contacting materials and devices, the limited surface hemocompatibility and delayed endothelial healing should be further improved. Carbon monoxide (CO) gas signaling molecule within the physiological concentrations has excellent anticoagulation and the ability to promote endothelial growth, exhibiting the great potential for the blood-contact biomaterials, especially the cardiovascular devices. In this study, the regular titanium dioxide nanotube arrays were firstly prepared in situ on the titanium surface by anodic oxidation, followed by the immobilization of the complex of sodium alginate/carboxymethyl chitosan (SA/CS) on the self-assembled modified nanotube surface, the CO-releasing molecule (CORM-401) was finally grafted onto the surface to create a CO-releasing bioactive surface to enhance the biocompatibility. The results of scanning electron microscopy (SEM), X-ray energy dispersion spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) revealed that the CO-releasing molecules were successfully immobilized on the surface. The modified nanotube arrays not only exhibited excellent hydrophilicity but also could slowly release CO gas molecules, and the amount of CO release increased when cysteine was added. Furthermore, the nanotube array can promote albumin adsorption while inhibit fibrinogen adsorption to some extent, demonstrating its selective albumin adsorption; although this effect was somewhat reduced by the introduction of CORM-401, it can be significantly enhanced by the catalytic release of CO. The results of hemocompatibility and endothelial cell growth behaviors showed that, as compared with the CORM-401 modified sample, although the SA/CS-modified sample had better biocompatibility, in the case of cysteine-catalyzed CO release, the released CO could not only reduce the platelet adhesion and activation as well as hemolysis rate, but also promote endothelial cell adhesion and proliferation as well as vascular endothelial growth factor (VEGF) and nitric oxide (NO) expression. As a result, the research of the present study demonstrated that the releasing CO from TiO₂ nanotubes can simultaneously enhance the surface hemocompatibility and endothelialization, which could open a new route to enhance the biocompatibility of the blood-contacting materials and devices, such as the artificial heart valve and cardiovascular stents.

Carbon Monoxide Signaling: Examining Its Engagement with Various Molecular Targets in the Context of Binding Affinity, Concentration, and Biologic Response

Carbon monoxide (CO) has been firmly established as an endogenous signaling molecule with a variety of pathophysiological and pharmacological functions, including immunomodulation, organ protection, and circadian clock regulation, among many others. In terms of its molecular mechanism(s) of action, CO is known to bind to a large number of hemoproteins with at least 25 identified targets, including hemoglobin, myoglobin, neuroglobin, cytochrome c oxidase, cytochrome P450, soluble guanylyl cyclase, myeloperoxidase, and some ion channels with dissociation constant values spanning the range of sub-nM to high μM. Although CO's binding affinity with a large number of targets has been extensively studied and firmly established, there is a pressing need to incorporate such binding information into the analysis of CO's biologic response in the context of affinity and dosage. Especially important is to understand the reservoir role of hemoglobin in CO storage, transport, distribution, and transfer. We critically review the literature and inject a sense of quantitative assessment into our analyses of the various relationships among binding affinity, CO concentration, target occupancy level, and anticipated pharmacological actions. We hope that this review presents a picture of the overall landscape of CO's engagement with various targets, stimulates additional research, and helps to move the CO field in the direction of examining individual targets in the context of all of the targets and the concentration of available CO. We believe that such work will help the further understanding of the relationship of CO concentration and its pathophysiological functions and the eventual development of CO-based therapeutics. SIGNIFICANCE STATEMENT: The further development of carbon monoxide (CO) as a therapeutic agent will significantly rely on the understanding of CO's engagement with therapeutically relevant targets of varying affinity. This review critically examines the literature by quantitatively analyzing the intricate relationships among targets, target affinity for CO, CO level, and the affinity state of carboxyhemoglobin and provide a holistic approach to examining the molecular mechanism(s) of action for CO.

Beneficial and Detrimental Roles of Heme Oxygenase-1 in the Neurovascular System

Heme oxygenase (HO) has both beneficial and detrimental effects via its metabolites, including carbon monoxide (CO), biliverdin or bilirubin, and ferrous iron. HO-1 is an inducible form of HO that is upregulated by oxidative stress, nitric oxide, CO, and hypoxia, whereas HO-2 is a constitutive form that regulates vascular tone and homeostasis. In brains injured by trauma, ischemia-reperfusion, or Alzheimer’s disease (AD), the long-term expression of HO-1 can be detected, which can lead to cytotoxic ferroptosis via iron accumulation. In contrast, the transient induction of HO-1 in the peri-injured region may have regenerative potential (e.g., angiogenesis, neurogenesis, and mitochondrial biogenesis) and neurovascular protective effects through the CO-mediated signaling pathway, the antioxidant properties of bilirubin, and the iron-mediated ferritin synthesis. In this review, we discuss the dual roles of HO-1 and its metabolites in various neurovascular diseases, including age-related macular degeneration, ischemia-reperfusion injury, traumatic brain injury, Gilbert’s syndrome, and AD.

Carbon monoxide activation of delayed rectifier potassium currents of human cardiac fibroblasts through diverse pathways

To identify the effect and mechanism of carbon monoxide (CO) on delayed rectifier K⁺ currents (I_K) of human cardiac fibroblasts (HCFs), we used the wholecell mode patch-clamp technique. Application of CO delivered by carbon monoxidereleasing molecule-3 (CORM3) increased the amplitude of outward K⁺ currents, and diphenyl phosphine oxide-1 (a specific I_K blocker) inhibited the currents. CORM3- induced augmentation was blocked by pretreatment with nitric oxide synthase blockers (L-NG-monomethyl arginine citrate and L-NG-nitro arginine methyl ester). Pretreatment with KT5823 (a protein kinas G blocker), 1H-[1,-2,-4] oxadiazolo-[4,-3-a] quinoxalin-1-on (ODQ, a soluble guanylate cyclase blocker), KT5720 (a protein kinase A blocker), and SQ22536 (an adenylate cyclase blocker) blocked the CORM3 stimulating effect on I_K. In addition, pretreatment with SB239063 (a p38 mitogen-activated protein kinase [MAPK] blocker) and PD98059 (a p44/42 MAPK blocker) also blocked the CORM3's effect on the currents. When testing the involvement of S-nitrosylation, pretreatment of N-ethylmaleimide (a thiol-alkylating reagent) blocked CO-induced I_K activation and DL-dithiothreitol (a reducing agent) reversed this effect. Pretreatment with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)-21H,23H porphyrin manganese (III) pentachloride and manganese (III) tetrakis (4-benzoic acid) porphyrin chloride (superoxide dismutase mimetics), diphenyleneiodonium chloride (an NADPH oxidase blocker), or allopurinol (a xanthine oxidase blocker) also inhibited CO-induced I_K activation. These results suggest that CO enhances I_K in HCFs through the nitric oxide, phosphorylation by protein kinase G, protein kinase A, and MAPK, S-nitrosylation and reduction/oxidation (redox) signaling pathways.

Electrochemical Modulation of Carbon Monoxide-Mediated Cell Signaling

Despite the critical role played by carbon monoxide (CO) in physiological and pathological signaling events, current approaches to deliver this messenger molecule are often accompanied by off-target effects and offer limited control over release kinetics. To address these challenges, we develop an electrochemical approach that affords on-demand release of CO through reduction of carbon dioxide (CO2 ) dissolved in the extracellular space. Electrocatalytic generation of CO by cobalt phthalocyanine molecular catalysts modulates signaling pathways mediated by a CO receptor soluble guanylyl cyclase. Furthermore, by tuning the applied voltage during electrocatalysis, we explore the effect of the CO release kinetics on CO-dependent neuronal signaling. Finally, we integrate components of our electrochemical platform into microscale fibers to produce CO in a spatially-restricted manner and to activate signaling cascades in the targeted cells. By offering on-demand local synthesis of CO, our approach may facilitate the studies of physiological processes affected by this gaseous molecular messenger.

Secondhand Smoke Exposure Impairs Ion Channel Function and Contractility of Mesenteric Arteries

Cigarette smoke, including secondhand smoke (SHS), has significant detrimental vascular effects, but its effects on myogenic tone of small resistance arteries and the underlying mechanisms are understudied. Although it is apparent that SHS contributes to endothelial dysfunction, much less is known about how this toxicant alters arterial myocyte contraction, leading to alterations in myogenic tone. The study's goal is to determine the effects of SHS on mesenteric arterial myocyte contractility and excitability. C57BL/6J male mice were randomly assigned to either filtered air (FA) or SHS (6 h/d, 5 d/wk) exposed groups for a 4, 8, or 12-weeks period. Third and fourth-order mesenteric arteries and arterial myocytes were acutely isolated and evaluated with pressure myography and patch clamp electrophysiology, respectively. Myogenic tone was found to be elevated in mesenteric arteries from mice exposed to SHS for 12 wk but not for 4 or 8 wk. These results were correlated with an increase in L-type Ca2+ channel activity in mesenteric arterial myocytes after 12 wk of SHS exposure. Moreover, 12 wk SHS exposed arterial myocytes have reduced total potassium channel current density, which correlates with a depolarized membrane potential (Vm). These results suggest that SHS exposure induces alterations in key ionic conductances that modulate arterial myocyte contractility and myogenic tone. Thus, chronic exposure to an environmentally relevant concentration of SHS impairs mesenteric arterial myocyte electrophysiology and myogenic tone, which may contribute to increased blood pressure and risks of developing vascular complications due to passive exposure to cigarette smoke.

Carbon monoxide activates large-conductance calcium-activated potassium channels of human cardiac fibroblasts through various mechanisms

Carbon monoxide (CO) is a cardioprotectant and potential cardiovascular therapeutic agent. Human cardiac fibroblasts (HCFs) are important determinants of myocardial structure and function. Large-conductance Ca²⁺-activated K⁺ (BK) channel is a potential therapeutic target for cardiovascular disease. We investigated whether CO modulates BK channels and the signaling pathways in HCFs using whole-cell mode patch-clamp recordings. CO-releasing molecules (CORMs; CORM-2 and CORM-3) significantly increased the amplitudes of BK currents (IBK). The CO-induced stimulating effects on IBK were blocked by pre-treatment with specific nitric oxide synthase (NOS) blockers (L-N^G-monomethyl arginine citrate and L-N^G-nitroarginine methyl ester). 8-bromo-cyclic GMP increased IBK. KT5823 (inhibits PKG) or ODQ (inhibits soluble guanylate cyclase) blocked the CO-stimulating effect on IBK. Moreover, 8-bromo-cyclic AMP also increased IBK, and pre-treatment with KT5720 (inhibits PKA) or SQ22536 (inhibits adenylate cyclase) blocked the CO effect. Pre-treatment with N-ethylmaleimide (a thiol-alkylating reagent) also blocked the CO effect on IBK, and DL-dithiothreitol (a reducing agent) reversed the CO effect. These data suggest that CO activates IBK through NO via the NOS and through the PKG, PKA, and S-nitrosylation pathways.

Role of Carbon Monoxide in Host-Gut Microbiome Communication

Nature is full of examples of symbiotic relationships. The critical symbiotic relation between host and mutualistic bacteria is attracting increasing attention to the degree that the gut microbiome is proposed by some as a new organ system. The microbiome exerts its systemic effect through a diverse range of metabolites, which include gaseous molecules such as H₂, CO₂, NH₃, CH₄, NO, H₂S, and CO. In turn, the human host can influence the microbiome through these gaseous molecules as well in a reciprocal manner. Among these gaseous molecules, NO, H₂S, and CO occupy a special place because of their widely known physiological functions in the host and their overlap and similarity in both targets and functions. The roles that NO and H₂S play have been extensively examined by others. Herein, the roles of CO in host-gut microbiome communication are examined through a discussion of (1) host production and function of CO, (2) available CO donors as research tools, (3) CO production from diet and bacterial sources, (4) effect of CO on bacteria including CO sensing, and (5) gut microbiome production of CO. There is a large amount of literature suggesting the "messenger" role of CO in host-gut microbiome communication. However, much more work is needed to begin achieving a systematic understanding of this issue.

Participation of PKG and PKA-related pathways in the IFN-γ induced modulation of the BKCa channel activity in human cardiac fibroblasts

This study was devoted to elucidating the interferon (IFN)-γ-induced signaling pathway and the interaction between protein kinase G (PKG) and protein kinase A (PKA) through large-conductance Ca(2+)-activated K(+) channels in human cardiac fibroblasts. The I_K currents were recorded using a whole-cell patch clamp method. A large depolarization (+50 mV) and a high Ca²⁺ concentration (pCa 6.0) were used in the internal pipette solution to activate only the K_Ca channels. Iberiotoxin (Ibtx), which selectively inhibits BK_Ca channels at a concentration of 100 nmol/l, caused a significant reduction of basal I_K. Adding IFN-γ in the presence of Ibtx had no effect on I_K. Application of the IFN-γ caused a significant reduction in total K⁺ current amplitude, recorded with a 500 ms depolarizing pulse duration, to +50 mV from a holding potential of -80 mV. We tested the involvement of the sGC/cGMP/PKG signaling pathway by using specific PKG inhibitor KT 5823, potent sGC inhibitor NS 2028, and specific sGC agonist BAY 41-8543. The obtained data confirmed that only sGC participated in the IFN-γ-mediated BK_Ca channel modulation, which was mediated further by PKA. This study represents first evidence about the participation of the IFN-γ in the mechanisms responsible for BK_Ca modulation in HCFs. We also believe that this process occurs via negative crosstalk between the PKG- and PKA-associated pathways.

Electroacupuncture promotes the gastrointestinal motility of diabetic mice by CNP/NPR-B-cGMP and PDE3A-cGMP signaling

BACKGROUND

Electroacupuncture (EA) can promote gastrointestinal (GI) motility of diabetic mice, but the mechanism is not clearly elucidated. Natriuretic peptides (NPs) were related to the diabetes-induced gut dysfunction of mice, which may be associated with ICC (interstitial cells of cajal). Besides, EA could increase the ICC of diabetic mice. Our aim was to explore whether EA can promote the gut motility by CNP/NPR-B-cGMP and PDE3A-cGMP signaling in diabetic mice, and the relationship between NPs and ICC.

METHODS

Wild C57BL/6 male mice were divided into five groups: control group, diabetic mellitus (DM group), diabetic mellitus plus sham EA group (SEA), diabetic mellitus plus low-frequency EA group (LEA), and diabetic mellitus plus high-frequency group (HEA). Gastrointestinal motility was assessed by gastric emptying and GI transit test. Immunofluorescence staining was applied to assess the expression level of CNP, NPR-B, and c-Kit. Western blot, PCR, and ELISA were used to detect the level of CNP, NPR-B, PDE2A, PDE3A, c-Kit, mSCF, and cGMP content. The correlativity between NPR-B and mSCF was evaluated by Pearson's correlation and linear regression analyses.

KEY RESULTS

(a) EA could improve the GI dysfunction of diabetic mice. (b) CNP, NPR-B, and cGMP contents were decreased, but the level of PDE3A, c-Kit, and mSCF was increased in the EA groups. (c) There was a negative correlation between NPR-B and mSCF among the groups.

CONCLUSIONS AND INFERENCES

Electroacupuncture promotes the GI function by CNP/NPR-B-cGMP and PDE3A-cGMP signaling in diabetic mice; up-regulated mSCF/c-Kit signaling by EA may be mediated partially via down-regulation of CNP/NPR-B signaling.

CORM-401 induces calcium signalling, NO increase and activation of pentose phosphate pathway in endothelial cells

Carbon monoxide-releasing molecules (CO-RMs) induce nitric oxide (NO) release (which requires NADPH), and Ca²⁺ -dependent signalling; however, their contribution in mediating endothelial responses to CO-RMs is not clear. Here, we studied the effects of CO liberated from CORM-401 on NO production, calcium signalling and pentose phosphate pathway (PPP) activity in human endothelial cell line (EA.hy926). CORM-401 induced NO production and two types of calcium signalling: a peak-like calcium signal and a gradual increase in cytosolic calcium. CORM-401-induced peak-like calcium signal, originating from endoplasmic reticulum, was reduced by thapsigargin, a SERCA inhibitor, and by dantrolene, a ryanodine receptors (RyR) inhibitor. In contrast, the phospholipase C inhibitor U73122 did not significantly affect peak-like calcium signalling, but a slow and progressive CORM-401-induced increase in cytosolic calcium was dependent on store-operated calcium entrance. CORM-401 augmented coupling of endoplasmic reticulum and plasmalemmal store-operated calcium channels. Interestingly, in the presence of NO synthase inhibitor (l-NAME) CORM-401-induced increases in NO and cytosolic calcium were both abrogated. CORM-401-induced calcium signalling was also inhibited by superoxide dismutase (poly(ethylene glycol)-SOD). Furthermore, CORM-401 accelerated PPP, increased NADPH concentration and decreased the ratio of reduced to oxidized glutathione (GSH/GSSG). Importantly, CORM-401-induced NO increase was inhibited by the PPP inhibitor 6-aminonicotinamide (6-AN), but neither by dantrolene nor by an inhibitor of large-conductance calcium-regulated potassium ion channel (paxilline). The results identify the primary role of CO-induced NO increase in the regulation of endothelial calcium signalling, that may have important consequences in controlling endothelial function.

Carbon monoxide in lung cell physiology and disease

Carbon monoxide (CO) is an endogenously produced gas that has gained recognition as a biological signal transduction effector with properties similar, but not identical, to that of nitric oxide (NO). CO, which binds primarily to heme iron, may activate the hemoprotein guanylate cyclase, although with lower potency than NO. Furthermore, CO can modulate the activities of several cellular signaling molecules such as p38 MAPK, ERK1/2, JNK, Akt, NF-κB, and others. Emerging studies suggest that mitochondria, the energy-generating organelle of cells, represent a key target of CO action in eukaryotes. Dose-dependent modulation of mitochondrial function by CO can result in alteration of mitochondrial membrane potential, mitochondrial reactive oxygen species production, release of proapoptotic and proinflammatory mediators, as well as the inhibition of respiration at high concentration. CO, through modulation of signaling pathways, can impact key biological processes including autophagy, mitochondrial biogenesis, programmed cell death (apoptosis), cellular proliferation, inflammation, and innate immune responses. Inhaled CO is widely known as an inhalation hazard due to its rapid complexation with hemoglobin, resulting in impaired oxygen delivery to tissues and hypoxemia. Despite systemic and cellular toxicity at high concentrations, CO has demonstrated cyto- and tissue-protective effects at low concentration in animal models of organ injury and disease. These include models of acute lung injury (e.g., hyperoxia, hypoxia, ischemia-reperfusion, mechanical ventilation, bleomycin) and sepsis. The success of CO as a candidate therapeutic in preclinical models suggests potential clinical application in inflammatory and proliferative disorders, which is currently under evaluation in clinical trials.

Anti-apoptotic properties of carbon monoxide in porcine oocyte during in vitro aging

If fertilization of matured oocyte does not occur, unfertilized oocyte undergoes aging, resulting in a time-dependent reduction of the oocyte’s quality. The aging of porcine oocytes can lead to apoptosis. Carbon monoxide (CO), a signal molecule produced by the heme oxygenase (HO), possesses cytoprotective and anti-apoptotic effects that have been described in somatic cells. However, the effects of CO in oocytes have yet to be investigated. By immunocytochemistry method we detected that both isoforms of heme oxygenase (HO-1 and HO-2) are present in the porcine oocytes. Based on the morphological signs of oocyte aging, it was found that the inhibition of both HO isoforms by Zn-protoporphyrin IX (Zn-PP IX) leads to an increase in the number of apoptotic oocytes and decrease in the number of intact oocytes during aging. Contrarily, the presence of CO donors (CORM-2 or CORM-A1) significantly decrease the number of apoptotic oocytes while increasing the number of intact oocytes. We also determined that CO donors significantly decrease the caspase-3 (CAS-3) activity. Our results suggest that HO/CO contributes to the sustaining viability through regulation of apoptosis during in vitro aging of porcine oocytes.

Effects of nitric oxide on large-conductance Ca2+ -activated K+ currents in human cardiac fibroblasts through PKA and PKG-related pathways

The human cardiac fibroblast (HCF) is the most abundant cell type in the myocardium, and HCFs play critical roles in maintaining normal cardiac function. However, unlike cardiomyocytes, the electrophysiology of HCFs is not well established. In the cardiovascular system, Ca²⁺ -activated K⁺ (KCa) channels have distinct physiological and pathological functions, and nitric oxide (NO) plays a key role. In this study, we investigated the potential effects of NO on KCa channels in HCFs. We recorded strong oscillating, well-maintained outward K⁺ currents without marked inactivation throughout the test pulse period and detected outward rectification in the I-V curve; these are all characteristics that are typical of KCa currents. These currents were blocked with iberiotoxin (IBTX, a BKCa blocker) but not with TRAM-34 (an IKCa blocker). The amplitudes of the currents were increased with SNAP (an NO donor), and these increases were inhibited with IBTX. The SNAP-stimulating effect on the BKCa currents was blocked by pretreatment with KT5823 (a protein kinase G [PKG] inhibitor) or 1 H-[1,-2, -4] oxadiazolo-[4,-3-a] quinoxalin-1-one (ODQ; a soluble guanylate cyclase inhibitor). Additionally, 8-bromo-cyclic guanosine 3',5'-monophosphate (8-Br-cGMP) stimulated the BKCa currents, and pretreatment with KT5720 (a protein kinase A [PKA] inhibitor) and SQ22536 (an adenylyl cyclase inhibitor) blocked the NO-stimulating effect on the BKCa currents. Furthermore, 8-bromo-cyclic adenosine 3',5'-monophosphate (8-Br-cAMP) activated the BKCa currents. These data suggest that BKCa current is the main subtype of the KCa current in HCFs and that NO enhances these currents through the PKG and PKA pathways.

Carbon Monoxide Potentiation of L-Type Ca2+ Channel Activity Increases HIF-1α-Independent VEGF Expression via an AMPKα/SIRT1-Mediated PGC-1α/ERRα Axis

AIMS

The heme oxygenase-1 (HO-1)/carbon monoxide (CO) pathway induced in astrocytes after ischemic brain injury promotes vascular endothelial growth factor (VEGF) expression to maintain and repair neurovascular function. Although HO-1-derived CO has been shown to induce hypoxia-inducible factor-1α (HIF-1α)-dependent VEGF expression, the underlying mechanism independent of HIF-1α remains to be elucidated.

RESULTS

HO-1 and VEGF were coexpressed in astrocytes of ischemic mouse brain tissues. Experiments with specific siRNAs and pharmacological activators/inhibitors of various target genes demonstrated that astrocytes pre-exposed to the CO-releasing compound, CORM-2, or transfected with HO-1 increased HIF-1α-independent VEGF expression via sequential activation of the following signal cascades; Ca2+/calmodulin-dependent protein kinase kinase β-mediated AMP-activated protein kinase (AMPK)α activation, AMPKα-induced increases in nicotinamide phosphoribosyltransferase (NAMPT) expression and cellular NAD+ level, sirtuin 1 (SIRT1)-dependent peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) stabilization and activation, and PGC-1α/estrogen-related receptor (ERR)α-mediated VEGF expression. All of these sequential events were blocked by an L-type voltage-gated Ca2+ channel inhibitor and Ca2+ chelators, but not by other Ca2+ channel inhibitors.

INNOVATION

HO-1-derived CO elicits Ca2+ influx by activating L-type Ca2+ channels, which is a key player in HIF-1α-independent VEGF expression by activating the AMPKα-NAMPT-SIRT1-PGC-1α/ERRα pathway.

CONCLUSION

Our results provide new mechanistic insight into the possible role for L-type Ca2+ channels in HO-1/CO-induced angiogenesis. Antioxid. Redox Signal. 27, 21-36.

Mechanistic insight into the heme-independent interplay between iron and carbon monoxide in CFTR and Slo1 BKCa channels

Ion channels have been extensively reported as effectors of carbon monoxide (CO). However, the mechanisms of heme-independent CO action are still not known. Because most ion channels are heterologously expressed on human embryonic kidney cells that are cultured in Fe³⁺-containing media, CO may act as a small and strong iron chelator to disrupt a putative iron bridge in ion channels and thus to tune their activity. In this review CFTR and Slo1 BK_Ca channels are employed to discuss the possible heme-independent interplay between iron and CO. Our recent studies demonstrated a high-affinity Fe³⁺ site at the interface between the regulatory domain and intracellular loop 3 of CFTR. Because the binding of Fe³⁺ to CFTR prevents channel opening, the stimulatory effect of CO on the Cl^- and HCO₃^- currents across the apical membrane of rat distal colon may be due to the release of inhibitive Fe³⁺ by CO. In contrast, CO repeatedly stimulates the human Slo1 BK_Ca channel opening, possibly by binding to an unknown iron site, because cyanide prohibits this heme-independent CO stimulation. Here, in silico research on recent structural data of the slo1 BK_Ca channels indicates two putative binuclear Fe²⁺-binding motifs in the gating ring in which CO may compete with protein residues to bind to either Fe²⁺ bowl to disrupt the Fe²⁺ bridge but not to release Fe²⁺ from the channel. Thus, these two new regulation models of CO, with iron releasing from and retaining in the ion channel, may have significant and extensive implications for other metalloproteins.

Carbon monoxide contributes to the constipating effects of granisetron in rat colon

AIM

To investigate the mechanisms underlying the potential contribution of the heme oxygenase/carbon monoxide (HO/CO) pathway in the constipating effects of granisetron.

METHODS

For in vivo studies, gastrointestinal motility was evaluated in male rats acutely treated with granisetron [25, 50, 75 μg/kg/subcutaneous (sc)], zinc protoporphyrin IX [ZnPPIX, 50 μg/kg/intraperitoneal (ip)] and hemin (50 μmol/L/kg/ip), alone or in combination. For in vitro studies, the contractile neurogenic response to electrical field stimulation (EFS, 3, 5, 10 Hz, 14 V, 1 ms, pulse trains lasting 10 s), as well as the contractile myogenic response to acetylcholine (ACh, 0.1-100 μmol/L) were evaluated on colon specimens incubated with granisetron (3 μmol/L, 15 min), ZnPPIX (10 μmol/L, 60 min) or CO-releasing molecule-3 (CORM-3, 100, 200, 400 μmol/L) alone or in combination. These experiments were performed under co-treatment with or without atropine (3 μmol/L, a muscarinic receptor antagonist) or N^G-nitro-L-Arginine (L-NNA, 100 μmol/L, a nitric oxide synthase inhibitor).

RESULTS

Administration of granisetron (50, 75 μg/kg) in vivo significantly increased the time to first defecation (P = 0.045 vs vehicle-treated rats), clearly suggesting a constipating effect of this drug. Although administration of ZnPPIX or hemin alone had no effect on this gastrointestinal motility parameter, ZnPPIX co-administered with granisetron abolished the granisetron-induced constipation. On the other hand, co-administration of hemin and granisetron did not modify the increased constipation observed under granisetron alone. When administered in vitro, granisetron alone (3 μmol/L) did not significantly modify the colon’s contractile response to either EFS or ACh. Incubation with ZnPPIX alone (10 μmol/L) significantly reduced the colon’s contractile response to EFS (P = 0.016) but had no effect on contractile response to ACh. Co-administration of ZnPPIX and atropine (3 μmol/L) abolished the ZnPPIX-mediated decrease in contractile response to EFS. Conversely, incubation with CORM-3 (400 μmol/L) alone increased both the contractile response to EFS at 10 Hz (10 Hz: 71.02 ± 19.16 vs 116.25 ± 53.70, P = 0.01) and the contractile response to ACh (100 μmol/L) (P = 0.012). Co-administration of atropine abolished the CORM-3-mediated effects on the EFS-mediated response. When granisetron was co-incubated in vitro with ZnPPIX, the ZnPPIX-mediated decrease in colon contractile response to EFS was lost. On the other hand, co-incubation of granisetron and CORM-3 (400 μmol/L) further increased the colon’s contractile response to EFS (at 5 Hz: P = 0.007; at 10 Hz: P = 0.001) and to ACh (ACh 10 μmol/L: P = 0.001; ACh 100 μmol/L: P = 0.001) elicited by CORM-3 alone. L-NNA co-administered with granisetron and CORM-3 abolished the potentiating effect of CORM-3 on granisetron on both the EFS-induced and ACh-induced contractile response.

CONCLUSION

Taken together, findings from in vivo and in vitro studies suggest that the HO/CO pathway is involved in the constipating effects of granisetron.

A modern literature review of carbon monoxide poisoning theories, therapies, and potential targets for therapy advancement

The first descriptions of carbon monoxide (CO) and its toxic nature appeared in the literature over 100 years ago in separate publications by Drs. Douglas and Haldane. Both men ascribed the deleterious effects of this newly discovered gas to its strong interaction with hemoglobin. Since then the adverse sequelae of CO poisoning has been almost universally attributed to hypoxic injury secondary to CO occupation of oxygen binding sites on hemoglobin. Despite a mounting body of literature suggesting other mechanisms of injury, this pathophysiology and its associated oxygen centric therapies persists. This review attempts to elucidate the remarkably complex nature of CO as a gasotransmitter. While CO's affinity for hemoglobin remains undisputed, new research suggests that its role in nitric oxide release, reactive oxygen species formation, and its direct action on ion channels is much more significant. In the course of understanding the multifaceted character of this simple molecule it becomes apparent that current oxygen based therapies meant to displace CO from hemoglobin may be insufficient and possibly harmful. Approaching CO as a complex gasotransmitter will help guide understanding of the complex and poorly understood sequelae and illuminate potentials for new treatment modalities.

Diverse mechanisms underlying the regulation of ion channels by carbon monoxide

Carbon monoxide (CO) is firmly established as an important, physiological signalling molecule as well as a potent toxin. Through its ability to bind metal-containing proteins, it is known to interfere with a number of intracellular signalling pathways, and such actions can account for its physiological and pathological effects. In particular, CO can modulate the intracellular production of reactive oxygen species, NO and cGMP levels, as well as regulate MAPK signalling. In this review, we consider ion channels as more recently discovered effectors of CO signalling. CO is now known to regulate a growing number of different ion channel types, and detailed studies of the underlying mechanisms of action are revealing unexpected findings. For example, there are clear areas of contention surrounding its ability to increase the activity of high conductance, Ca(2+) -sensitive K(+) channels. More recent studies have revealed the ability of CO to inhibit T-type Ca(2+) channels and have unveiled a novel signalling pathway underlying tonic regulation of this channel. It is clear that the investigation of ion channels as effectors of CO signalling is in its infancy, and much more work is required to fully understand both the physiological and the toxic actions of this gas. Only then can its emerging use as a therapeutic tool be fully and safely exploited.

A novel role for carbon monoxide as a potent regulator of intracellular Ca2+and nitric oxide in rat pancreatic acinar cells

Carbon monoxide (CO) is known as an essential gaseous messenger that regulates a wide array of physiological and pathological processes, similar to nitric oxide (NO) and hydrogen sulfide. The aim of the present study was to elucidate the potential role of CO in Ca2+homeostasis and to explore the underlying mechanisms in pancreatic acinar cells. The exogenous application of a CO-releasing molecule dose-dependently increased intracellular Ca2+concentration ([Ca2+]i). A heme oxygenase (HO) inducer increased [Ca2+]iin a concentration-dependent manner, and the increase was diminished by an HO inhibitor. The CO-induced [Ca2+]iincrease persisted in the absence of extracellular Ca2+, indicating that Ca2+release is the initial source for the increase. The inhibition of G protein, phospholipase C (PLC), and inositol 1,4,5-trisphosphate (IP3) receptor diminished the CO-induced [Ca2+]iincrease. CO upregulated endothelial nitric oxide synthase (eNOS) expression and stimulated NO production, and NOS inhibitor, calmodulin inhibitor, or the absence of extracellular Ca2+eliminated the latter response. Blocking the phosphatidylinositol 3-kinase (PI3K)-Akt/protein kinase B (PKB) pathway abolished CO-induced NO production. Pretreatment with an NOS inhibitor, NO scavenger, or soluble guanylate cyclase inhibitor, did not affect the CO-induced [Ca2+]iincrease, indicating that NO, soluble guanylate cyclase, and cyclic guanosine 5′-monophosphate are not involved in the CO-induced [Ca2+]iincrease. CO inhibited the secretory responses to CCK-octapeptide or carbachol. We conclude that CO acts as a regulator not only for [Ca2+]ihomeostasis via a PLC-IP3-IP3receptor cascade but also for NO production via the calmodulin and PI3K-Akt/PKB pathway, and both CO and NO interact. Moreover, CO may provide potential therapy to ameliorate acute pancreatitis by inhibiting amylase secretion.

Inhibition of the cardiac Na⁺ channel Nav1.5 by carbon monoxide

Sublethal carbon monoxide (CO) exposure is frequently associated with myocardial arrhythmias, and our recent studies have demonstrated that these may be attributable to modulation of cardiac Na(+) channels, causing an increase in the late current and an inhibition of the peak current. Using a recombinant expression system, we demonstrate that CO inhibits peak human Nav1.5 current amplitude without activation of the late Na(+) current observed in native tissue. Inhibition was associated with a hyperpolarizing shift in the steady-state inactivation properties of the channels and was unaffected by modification of channel gating induced by anemone toxin (rATX-II). Systematic pharmacological assessment indicated that no recognized CO-sensitive intracellular signaling pathways appeared to mediate CO inhibition of Nav1.5. Inhibition was, however, markedly suppressed by inhibition of NO formation, but NO donors did not mimic or occlude channel inhibition by CO, indicating that NO alone did not account for the actions of CO. Exposure of cells to DTT immediately before CO exposure also dramatically reduced the magnitude of current inhibition. Similarly, l-cysteine and N-ethylmaleimide significantly attenuated the inhibition caused by CO. In the presence of DTT and the NO inhibitor N(ω)-nitro-L-arginine methyl ester hydrochloride, the ability of CO to inhibit Nav1.5 was almost fully prevented. Our data indicate that inhibition of peak Na(+) current (which can lead to Brugada syndrome-like arrhythmias) occurs via a mechanism distinct from induction of the late current, requires NO formation, and is dependent on channel redox state.

Review article: carbon monoxide in gastrointestinal physiology and its potential in therapeutics

BACKGROUND

While carbon monoxide (CO) is a known toxin, it is now recognised that CO is also an important signalling molecule involved in physiology and pathophysiology.

AIMS

To summarise our current understanding of the role of endogenous CO in the regulation of gastrointestinal physiology and pathophysiology, and to potential therapeutic applications of modulating CO.

METHODS

This review is based on a comprehensive search of the Ovid Medline comprehensive database and supplemented by our ongoing studies evaluating the role of CO in gastrointestinal physiology and pathophysiology.

RESULTS

Carbon monoxide derived from haem oxygenase (HO)-2 is predominantly involved in neuromodulation and in setting the smooth muscle membrane potential, while CO derived from HO-1 has anti-inflammatory and antioxidative properties, which protect gastrointestinal smooth muscle from damage caused by injury or inflammation. Exogenous CO is being explored as a therapeutic agent in a variety of gastrointestinal disorders, including diabetic gastroparesis, post-operative ileus, organ transplantation, inflammatory bowel disease and sepsis. However, identifying the appropriate mechanism for safely delivering CO in humans is a major challenge.

CONCLUSIONS

Carbon monoxide is an important regulator of gastrointestinal function and protects the gastrointestinal tract against noxious injury. CO is a promising therapeutic target in conditions associated with gastrointestinal injury and inflammation. Elucidating the mechanisms by which CO works and developing safe CO delivery mechanisms are necessary to refine therapeutic strategies.

Carbon monoxide induces cardiac arrhythmia via induction of the late Na+ current

RATIONALE

Clinical reports describe life-threatening cardiac arrhythmias after environmental exposure to carbon monoxide (CO) or accidental CO poisoning. Numerous case studies describe disruption of repolarization and prolongation of the QT interval, yet the mechanisms underlying CO-induced arrhythmias are unknown.

OBJECTIVES

To understand the cellular basis of CO-induced arrhythmias and to identify an effective therapeutic approach.

METHODS

Patch-clamp electrophysiology and confocal Ca(2+) and nitric oxide (NO) imaging in isolated ventricular myocytes was performed together with protein S-nitrosylation to investigate the effects of CO at the cellular and molecular levels, whereas telemetry was used to investigate effects of CO on electrocardiogram recordings in vivo.

MEASUREMENTS AND MAIN RESULTS

CO increased the sustained (late) component of the inward Na(+) current, resulting in prolongation of the action potential and the associated intracellular Ca(2+) transient. In more than 50% of myocytes these changes progressed to early after-depolarization-like arrhythmias. CO elevated NO levels in myocytes and caused S-nitrosylation of the Na(+) channel, Na(v)1.5. All proarrhythmic effects of CO were abolished by the NO synthase inhibitor l-NAME, and reversed by ranolazine, an inhibitor of the late Na(+) current. Ranolazine also corrected QT variability and arrhythmias induced by CO in vivo, as monitored by telemetry.

CONCLUSIONS

Our data indicate that the proarrhythmic effects of CO arise from activation of NO synthase, leading to NO-mediated nitrosylation of Na(V)1.5 and to induction of the late Na(+) current. We also show that the antianginal drug ranolazine can abolish CO-induced early after-depolarizations, highlighting a novel approach to the treatment of CO-induced arrhythmias.

Peroxynitrite mediates disruption of Ca2+ homeostasis by carbon monoxide via Ca2+ ATPase degradation

AIM

Sublethal carbon monoxide poisoning causes prolonged neurological damage involving oxidative stress. Given the central role of Ca(2+) homeostasis and its vulnerability to stress, we investigated whether CO disrupts neuronal Ca(2+) homeostasis.

RESULTS

Cytosolic Ca(2+) transients evoked by muscarine in SH-SY5Y cells were prolonged by CO (applied via the donor CORM-2), and capacitative Ca(2+) entry (CCE) was dramatically enhanced. Ca(2+) store mobilization by cyclopiazonic acid was similarly augmented, as was the subsequent CCE, and that evoked by thapsigargin. Ca(2+) rises evoked by depolarization were also enhanced by CO, and Ca(2+) levels often did not recover in its presence. CO increased intracellular nitric oxide (NO) and all effects of CO were prevented by inhibiting NO formation. However, NO donors did not mimic the effects of CO. The antioxidant ascorbic acid inhibited effects of CO on Ca(2+) signaling, as did the peroxynitrite scavenger, FeTPPS, and CO increased peroxynitrite formation. Finally, CO caused significant loss of plasma membrane Ca(2+)ATPase (PMCA) protein, detected by Western blot, and this was also observed in brain tissue of rats exposed to CO in vivo.

INNOVATION

The cellular basis of CO-induced neurotoxicity is currently unknown. Our findings provide the first data to suggest signaling pathways through which CO causes neurological damage, thereby opening up potential targets for therapeutic intervention.

CONCLUSION

CO stimulates formation of NO and reactive oxygen species which, via peroxynitrite formation, inhibit Ca(2+) extrusion via PMCA, leading to disruption of Ca(2+) signaling. We propose this contributes to the neurological damage associated with CO toxicity.

A quantitative model of human jejunal smooth muscle cell electrophysiology

Recently, a number of ion channel mutations have been identified in the smooth muscle cells of the human jejunum. Although these are potentially significant in understanding diseases that are currently of unknown etiology, no suitable computational cell model exists to evaluate the effects of such mutations. Here, therefore, a biophysically based single cell model of human jejunal smooth muscle electrophysiology is presented. The resulting cellular description is able to reproduce experimentally recorded slow wave activity and produces realistic responses to a number of perturbations, providing a solid platform on which the causes of intestinal myopathies can be investigated.

Therapeutic potential of heme oxygenase-1/carbon monoxide in lung disease

Heme oxygenase (HO), a catabolic enzyme, provides the rate-limiting step in the oxidative breakdown of heme, to generate carbon monoxide (CO), iron, and biliverdin-IXα. Induction of the inducible form, HO-1, in tissues is generally regarded as a protective mechanism. Over the last decade, considerable progress has been made in defining the therapeutic potential of HO-1 in a number of preclinical models of lung tissue injury and disease. Likewise, tissue-protective effects of CO, when applied at low concentration, have been observed in many of these models. Recent studies have expanded this concept to include chemical CO-releasing molecules (CORMs). Collectively, salutary effects of the HO-1/CO system have been demonstrated in lung inflammation/acute lung injury, lung and vascular transplantation, sepsis, and pulmonary hypertension models. The beneficial effects of HO-1/CO are conveyed in part through the inhibition or modulation of inflammatory, apoptotic, and proliferative processes. Recent advances, however, suggest that the regulation of autophagy and the preservation of mitochondrial homeostasis may serve as additional candidate mechanisms. Further preclinical and clinical trials are needed to ascertain the therapeutic potential of HO-1/CO in human clinical disease.

Carbon monoxide: a vital signalling molecule and potent toxin in the myocardium

Endogenous carbon monoxide (CO) is generated through the heme oxygenase-catalysed degradation of heme and is now established as an important, biologically active molecule capable of modulating a number of signalling pathways. Such pathways include those involving nitric oxide/guanylate cyclase, reactive oxygen species (ROS) and MAP kinases. In the heart, up-regulation of the inducible form of heme oxygenase (HO-1) following stresses such as ischemia/reperfusion provides cardioprotection, and much evidence indicates that CO accounts for many of these beneficial effects. One target of CO appears to be the L-type Ca(2+) channel; CO inhibits recombinant and native forms of this cardiac channel via mitochondria-derived ROS, which likely contributes to the protective effects of CO. In stark contrast, exposure to exogenous CO is toxic: chronic, low-level exposure can lead to myocardial injury and fibrosis, whereas acute exposure is associated with life-threatening arrhythmias. The molecular mechanisms accounting for such effects remain to be elucidated, but require future study before the potentially beneficial effects of CO therapy can be safely exploited. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Ion channels as target effectors for carbon monoxide

Our understanding of carbon monoxide (CO) as an endogenous signalling molecule has expanded enormously in recent years, to the point where it can now be safely exploited therapeutically. In more recent years, the ability of CO to modulate ion channel activity has added further diversity to its activity. Here, I summarize briefly the established ion channel targets of CO and describe in more detail its actions on two example ion channels (the cardiac L-type Ca(2+) channel and the neuronal delayed rectifier, Kv2.1). The physiological consequences of such modulation are also considered. It is clear that our knowledge of the cellular effects of this gas is incomplete, and further study is required to further understand and so more fully exploit its therapeutic potential.

Carbon monoxide: an emerging regulator of ion channels

Carbon monoxide is rapidly emerging as an important cellular messenger, regulating a wide range of physiological processes. Crucial to its role in both physiology and disease is its ability differentially to regulate several classes of ion channels, including examples from calcium-activated K(+) (BK(Ca)), voltage-activated K(+) (K(v)) and Ca(2+) channel (L-type) families, ligand-gated P2X receptors (P2X2 and P2X4), tandem P domain K(+) channels (TREK1) and the epithelial Na(+) channel (ENaC). The mechanisms by which CO regulates these ion channels are still unclear and remain somewhat controversial. However, available structure-function studies suggest that a limited range of amino acid residues confer CO sensitivity, either directly or indirectly, to particular ion channels and that cellular redox state appears to be important to the final integrated response. Whatever the molecular mechanism by which CO regulates ion channels, endogenous production of this gasotransmitter has physiologically important roles and is currently being explored as a potential therapeutic.

Nitric oxide synthase expression in the medullary respiratory related nuclei and its involvement in CO-mediated central respiratory effects in neonatal rats

The present study was conducted in order to observe the potential participation of the nitric oxide synthase-NO pathway in CO-mediated regulation of respiration of neonatal rats. An immunofluorescent histochemical technique was used to examine the existence of the neuronal nitric oxide synthase, a key enzyme of synthesizing NO, in medullary respiratory nuclei. The rhythmic respiratory-like discharges of hypoglossal rootlets of medullary slices were recorded to test the role of the nitric oxide synthase in CO-mediated respiratory effects. We observed neuronal nitric oxide synthase expressed in the medullary respiratory nuclei in conjunction with CO lengthened expiratory duration, decreased respiratory frequency, and increased inspiratory amplitude. These CO-mediated respiratory effects could be partially eliminated by prior treatment of the slices with Nω-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase. The results suggest that nitric oxide synthase-NO pathway might be involved in the CO-mediated central regulation of respiration at the level of medulla oblongata in neonatal rats.

Non-adrenergic non-cholinergic inhibition of gastrointestinal smooth muscle and its intracellular mechanism(s)

Relaxation of gastrointestinal smooth muscle caused by release of non-adrenergic non-cholinergic (NANC) transmitters from enteric nerves occurs in several physiologic digestive reflexes. Likely candidate NANC inhibitory agents include nitric oxide (NO), adenosine triphosphate (ATP), vasoactive intestinal peptide (VIP), pituitary adenylate cyclase-activating peptide (PACAP), carbon monoxide (CO), protease-activated receptors (PARs), hydrogen sulfide (H2S), neurotensin (NT) and beta-nicotinamide adenine dinucleotide (beta-NAD). Multiple NANC transmitters work in concert, are pharmacologically coupled and are closely coordinated. Individual contribution varies regionally in the gastrointestinal tract and between species. NANC inhibition of gastrointestinal smooth muscle involves several intracellular mechanisms, including increase of cyclic guanosine monophosphate (cGMP), increase of cyclic adenosine monophosphate (cAMP) and hyperpolarization of the cell membrane via direct or indirect activation of potassium ion (K+) channels.

Lysophosphatidyl choline modulates mechanosensitive L-type Ca2+ current in circular smooth muscle cells from human jejunum

The L-type Ca2+ channel expressed in gastrointestinal smooth muscle is mechanosensitive. Direct membrane stretch and shear stress result in increased Ca2+ entry into the cell. The mechanism for mechanosensitivity is not known, and mechanosensitivity is not dependent on an intact cytoskeleton. The aim of this study was to determine whether L-type Ca2+ channel mechanosensitivity is dependent on tension in the lipid bilayer in human jejunal circular layer myocytes. Whole cell currents were recorded in the amphotericin-perforated-patch configuration, and lysophosphatidyl choline (LPC), lysophosphatidic acid (LPA), and choline were used to alter differentially the tension in the lipid bilayer. Shear stress (perfusion at 10 ml/min) was used to mechanostimulate L-type Ca2+ channels. The increase in L-type Ca2+ current induced by shear stress was greater in the presence of LPC (large head-to-tail proportions), but not LPA or choline, than in the control perfusion. The increased peak Ca2+ current also did not return to baseline levels as in control conditions. Furthermore, steady-state inactivation kinetics were altered in the presence of LPC, leading to a change in window current. These findings suggest that changes in tension in the plasmalemmal membrane can be transmitted to the mechanosensitive L-type Ca2+ channel, leading to altered activity and Ca2+ entry in the human jejunal circular layer myocyte.

Role of the soluble guanylyl cyclase alpha1/alpha2 subunits in the relaxant effect of CO and CORM-2 in murine gastric fundus

Carbon monoxide (CO) has been shown to cause enteric smooth muscle relaxation by activating soluble guanylyl cyclase (sGC). In gastric fundus, the sGCalpha1beta1 heterodimer is believed to be the most important isoform. The aim of our study was to investigate the role of the sGCalpha1/alpha2 subunits in the relaxant effect of CO and CORM-2 in murine gastric fundus using wild-type (WT) and sGCalpha1 knock-out (KO) mice. In WT mice, CO (bolus)-induced relaxations were abolished by the sGC inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), while CORM-2- and CO (infusion)-induced relaxations were only partially inhibited by ODQ. In sGCalpha1 KO mice, relaxant responses to CO and CORM-2 were significantly reduced when compared with WT mice, but ODQ still had an inhibitory effect. The sGC sensitizer 1-benzyl-3-(5'-hydroxymethyl-2'-furyl-)-indazol (YC-1) was able to potentiate CO- and CORM-2-induced relaxations in WT mice but lost this potentiating effect in sGCalpha1 KO mice. Both in WT and sGCalpha1 KO mice, CO-evoked relaxations were associated with a significant cGMP increase; however, basal and CO-elicited cGMP levels were markedly lower in sGCalpha1 KO mice. These data indicate that besides the predominant sGCalpha1beta1 isoform, also the less abundantly expressed sGCalpha2beta1 isoform plays an important role in the relaxant effect of CO in murine gastric fundus; however, the sGC stimulator YC-1 loses its potentiating effect towards CO in sGCalpha1 KO mice. Prolonged administration of CO-either by the addition of CORM-2 or by continuous infusion of CO-mediates gastric fundus relaxation in both a sGC-dependent and sGC-independent manner.

Modulation of hTREK-1 by carbon monoxide

TREK-1 is a background K channel important in the regulation of neuronal excitability. Here, we demonstrate that recombinant human TREK-1 is activated by low concentrations of carbon monoxide (CO) and nitric oxide (NO), applied via their respective donor molecules. Related channels hTASK-1 and hTASK-3 were unaffected by CO. Effects of both CO and NO were prevented by preincubation of cells with the protein kinase G inhibitor, Rp-8-Br-PET-cGMPS. The effects of CO were independent of NO formation. At higher concentrations, both NO and CO were inhibitory. As both NO and CO are important neuronal gasotransmitters and TREK is crucial in regulating neuronal excitability, our results provide a novel means by which these gases may modulate neuronal activity.

Regulation of basal tone, relaxation and contraction of the lower oesophageal sphincter. Relevance to drug discovery for oesophageal disorders

The lower oesophageal sphincter (LOS) is a specialized region of the oesophageal circular smooth muscle that allows the passage of a swallowed bolus to the stomach and prevents the reflux of gastric contents into the oesophagus. The anatomical arrangement of the LOS includes semicircular clasp fibres adjacent to the lesser gastric curvature and sling fibres following the greater gastric curvature. Such anatomical arrangement together with an asymmetric intrinsic innervation and distinct proportion of neurotransmitters in both regions produces an asymmetric pressure profile. The LOS tone is myogenic in origin and depends on smooth muscle properties that lead to opening of L-type Ca(2+) channels; however it can be modulated by enteric motor neurons, the parasympathetic and sympathetic extrinsic nervous system and several neurohumoral substances. Nitric oxide synthesized by neuronal NOS is the main inhibitory neurotransmitter involved in LOS relaxation. Different putative neurotransmitters have been proposed to play a role together with NO. So far, only ATP or related purines have shown to be co-transmitters with NO. Acetylcholine and tachykinins are involved in the LOS contraction acting through acetylcholine M(3) and tachykinin NK(2) receptors. Nitric oxide can also be involved in the regulation of LOS contraction. The understanding of the mechanisms that originate and modulate LOS tone, relaxation and contraction and the characterization of neurotransmitters and receptors involved in LOS function are important to develop new pharmacological tools to treat primary oesophageal motor disorders and gastro-oesophageal reflux disease.

Mechanisms of relaxation by carbon monoxide-releasing molecule-2 in murine gastric fundus and jejunum

This study investigated the effects and mechanisms of action of carbon monoxide-releasing molecule-2 (CORM-2), compared to those of carbon monoxide (CO), in murine gastric fundus and jejunal circular smooth muscle. Functional in vitro experiments and cGMP measurements were conducted. In both tissues, CO and CORM-2 induced concentration-dependent relaxations. CO-induced relaxations were abolished by the soluble guanylyl cyclase (sGC) inhibitor ODQ, while CORM-2-evoked inhibitory responses were only partly prevented by ODQ. Relaxations elicited by CO (300 microM) were associated with a significant increase in cGMP levels, whereas for CORM-2 (300 microM) no significant increase in cGMP levels could be measured. The sGC sensitizer YC-1 was able to accelerate and potentiate both CO- and CORM-2-induced relaxations. Furthermore, the intermediate- and large-conductance Ca2+-activated K+ (IKCa-BKCa) channel blocker charybdotoxin significantly reduced CO- and CORM-2-induced relaxations in jejunal tissue; this same effect was observed with the BKCa channel blocker iberiotoxin. The combination of apamin plus charybdotoxin significantly reduced relaxations in gastric fundus and had synergistic inhibitory effects in jejunum. The NOS inhibitor L-NAME had no effect on the induced relaxations in gastric fundus, but significantly reduced CO- and CORM-2-evoked relaxations in jejunum. In conclusion, these results demonstrate that CO and CORM-2 produce relaxation in gastric fundus and jejunum via sGC and activation of KCa channels, and a nitric oxide (NO)-mediated amplification of CO signaling in jejunum is suggested.

Involvement of protein kinase A in nitric oxide stimulating effect on a BK(Ca) channel of human dermal fibroblasts

We reported previously that a large-conductance Ca2+-activated K+ (BK(Ca)) channel constitutes a significant fraction of the K+ current in human dermal fibroblasts, and that nitric oxide (NO) increases the open-channel probability (NPo) of BK(Ca) channels via a soluble guanylate cyclase (sGC)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) pathway. The purpose of this study was to investigate whether the adenylate cyclase (AC)/cAMP/protein kinase A (PKA) pathway may also be involved in NO action on BK(Ca) channels in human dermal fibroblasts. Electrophysiological single-channel recordings were performed on fifth-passage cells of human penile skin cultures. KT5720 (specific PKA inhibitor) blocked the stimulatory effect of sodium nitroprusside (NO donor) on BK(Ca) channels. By contrast, forskolin (AC activator) or 8-bromo-cAMP (cell-permeable cAMP analog) did not increase the NPo of the channel. The PKA catalytic subunit (PKAcs) alone did not increase the NPo of the channel in cell-attached and inside-out patches, however, PKAcs with cGMP increased the NPo. In contrast, PKAcs with cGMP did not increase the NPo of BK(Ca) channels with 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one pretreatment, and KT5720 pretreatment also blocked the stimulatory effect of 8-Br-cGMP. In conclusion, the present data suggest the involvement of PKA in the stimulatory effect of NO on the BK(Ca) channel in human dermal fibroblasts through cGMP.

Positive inotropic effects of carbon monoxide-releasing molecules (CO-RMs) in the isolated perfused rat heart

BACKGROUND AND PURPOSE

Carbon monoxide (CO) generated by the enzyme haeme oxygenase-1 (HO-1) during the breakdown of haeme is known to mediate a number of biological effects. Here, we investigated whether CO liberated from two water soluble carbon monoxide-releasing molecules (CO-RMs) exerts inotropic effects on the myocardium.

EXPERIMENTAL APPROACH

Rat isolated hearts perfused either at constant flow or constant pressure were used to test the effect of CO-RMs.

KEY RESULTS

CORM-3, a fast CO releaser, produced a direct positive inotropic effect when cumulative doses (3, 10 and 30 microg min(-1)) or a single dose (5 microM) were infused at either constant coronary pressure (CCP) or constant coronary flow (CCF). The inotropic effect mediated by CORM-3 was abolished by blockade of soluble guanylate cyclase or Na(+)/H(+) exchanger, but not by inhibitors of L-type Ca(2+) channels and protein kinase C. CORM-3 also caused a slight reduction in heart rate but did not alter coronary flow. In contrast, the slow CO releaser CORM-A1 produced significant coronary vasodilatation when given at the highest concentration (30 mug min(-1)) but exerted no effect on myocardial contractility or heart rate.

CONCLUSION AND IMPLICATIONS

A rapid CO release from CORM-3 exerts a direct positive inotropic effect on rat isolated perfused hearts, whereas CO slowly released by CORM-A1 had no effect on myocardial contractility but caused significant coronary vasodilatation. Both cGMP and Na(+)/H(+) exchange appear to be involved in this effect but further work is needed to determine the relative contribution of each pathway in CO-mediated inotropic effect.

Heme in intestinal epithelial cell turnover, differentiation, detoxification, inflammation, carcinogenesis, absorption and motility

The gastrointestinal tract is lined by a simple epithelium that undergoes constant renewal involving cell division, differentiation and cell death. In addition, the epithelial lining separates the hostile processes of digestion and absorption that occur in the intestinal lumen from the aseptic environment of the internal milieu by defensive mechanisms that protect the epithelium from being breached. Central to these defensive processes is the synthesis of heme and its catabolism by heme oxygenase (HO). Dietary heme is also an important source of iron for the body which is taken up intact by the enterocyte. This review describes the recent literature on the diverse properties of heme/HO in the intestine tract. The roles of heme/HO in the regulation of the cell cycle/apoptosis, detoxification of xenobiotics, oxidative stress, inflammation, development of colon cancer, heme-iron absorption and intestinal motility are specifically examined.

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Carbon monoxide rescues ischemic lungs by interrupting MAPK-driven expression of early growth response 1 gene and its downstream target genes

Carbon monoxide (CO), an endogenous cytoprotective product of heme oxygenase type-1 regulates target thrombotic and inflammatory genes in ischemic stress. Regulation of the gene encoding early growth response 1 (Egr-1), a potent transcriptional activator of deleterious thrombotic and inflammatory cascades, may govern CO-mediated ischemic lung protection. The exact signaling mechanisms underlying CO-mediated cytoprotection are not well understood. In this study we tested the hypothesis that inhibition of mitogen-activated protein kinase-dependent Egr-1 expression may be pivotal in CO-mediated ischemic protection. In an in vivo isogeneic rat lung ischemic injury model, inhaled CO not only diminished fibrin accumulation and leukostasis and improved gas exchange and survival but also suppressed extracellular signal-regulated kinase (ERK) activation, Egr-1 expression, and Erg DNA-binding activity in lung tissue. Additionally, CO-mediated inhibition of Egr-1 reduced expression of target genes, such as tissue factor, serpine-1, interleukin-1, and TNF-alpha. However, CO failed to inhibit serpine-1 expression after unilateral lung ischemia in mice null for the Egr-1 gene. In RAW macrophages in vitro, hypoxia-induced Egr-1 mRNA expression was ERK-dependent, and CO-mediated suppression of ERK activation resulted in Egr-1 inhibition. Furthermore, CO suppression of ERK phosphorylation was reversed by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one but was insensitive to cAMP-dependent protein kinase A inhibition with H89 and NO synthase inhibition with l-nitroarginine methyl ester. This finding indicates that CO suppresses ERK in a cGMP-dependent but cAMP/protein kinase A- and NO-independent manner. Together, these data identify a unifying molecular mechanism by which CO interrupts proinflammatory and prothrombotic mediators of ischemic injury.

Defects in cGMP-PKG pathway contribute to impaired NO-dependent responses in hepatic stellate cells upon activation

NO antagonizes hepatic stellate cell (HSC) contraction, although activated HSC in cirrhosis demonstrate impaired responses to NO. Decreased NO responses in activated HSC and mechanisms by which NO affects activated HSC remain incompletely understood. In normal rat HSC, the NO donor diethylamine NONOate (DEAN) significantly increased cGMP production and reduced serum-induced contraction by 25%. The guanylate cyclase (sGC) inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ) abolished 50% of DEAN effects, whereas the cGMP analog 8-bromoguanosine 3',5'-cyclic monophosphate (8-BrcGMP) reiterated half the observed DEAN response, suggesting both cGMP-dependent protein kinase G (PKG)-dependent and -independent mechanisms of NO-mediated antagonism of normal HSC contraction. However, NO donors did not increase cGMP production from in vivo activated HSC from bile duct-ligated rats and showed alterations in intracellular Ca(2+) accumulation suggesting defective cGMP-dependent effector pathways. The LX-2 cell line also demonstrated lack of cGMP generation in response to NO and a lack of effect of ODQ and 8-BrcGMP in modulating the NO response. However, cGMP-independent effects in response to NO were maintained in LX-2 and were associated with S-nitrosylation of proteins, an effect reiterated in primary HSC. Adenovirus-based overexpression of PKG significantly attenuated contraction of LX-2 by 25% in response to 8-BrcGMP. In summary, these studies demonstrate that NO affects HSC through cGMP-dependent and -independent pathways. The HSC activation process is associated with maintenance of cGMP-independent actions of NO but defects in cGMP-PKG-dependent NO signaling that are improved by PKG gene delivery in LX-2 cells. Activating targets downstream from NO-cGMP in activated HSC may represent a novel therapeutic target for portal hypertension.

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.

Nitric oxide increases carbon monoxide production by piglet cerebral microvessels

Carbon monoxide (CO) and nitric oxide (NO) can be involved in the regulation of cerebral circulation. Inhibition of production of either one of these gaseous intercellular messengers inhibits newborn pig cerebral arteriolar dilation to the excitatory amino acid glutamate. Glutamate can increase NO production. Therefore, the present study tests the hypothesis that NO, which is increased by glutamate, stimulates the production of CO by cerebral microvessels. Experiments used freshly isolated cerebral microvessels from piglets that express only heme oxygenase-2 (HO-2). CO production was measured by gas chromatography-mass spectrometry. Although inhibition of nitric oxide synthase (NOS) with N(omega)-nitro-l-arginine (l-NNA) did not alter basal HO-2 catalytic activity or CO production, l-NNA blocked glutamate stimulation of HO-2 activity and CO production. Furthermore, the NO donor sodium nitroprusside mimicked the actions of glutamate on HO-2 and CO production. The action of NO appears to be via cGMP because 8-bromo-cGMP mimics and 1H-[1,2,4]oxadiazole-[4,3-a]quinoxalin-1-one (ODQ) blocks glutamate stimulation of CO production and HO-2 catalytic activity. Inhibitors of neither casein kinase nor phosphotidylinositol 3-kinase altered HO-2 catalytic activity. Conversely, inhibition of calmodulin with calmidazolium chloride blocked glutamate stimulation of CO production and reduced HO-2 catalytic activity. These data suggest that glutamate may activate NOS producing NO that leads to CO synthesis via a cGMP-dependent elevation of HO-2 catalytic activity. These results are consistent with the findings in vivo that either HO or NOS inhibition blocks cerebrovascular dilation to glutamate in piglets.

Carbon monoxide protects cardiomyogenic cells against ischemic death through L-type Ca2+ channel inhibition

Carbon monoxide (CO) is known to protect myocardial and vascular cells against injuries due to ischemia-reperfusion or inflammation. We showed that a Ca(2+)-dependent protease calpain promotes necrotic cell death of cardiomyocyte-derived H9c2 cells due to hypoxia through alpha-fodrin proteolysis. Here, we show that ischemia induces necrotic cell death, which is inhibited by either CO, extracellular Ca(2+) deprivation or L-type Ca(2+) channel blockers. A whole cell patch-clamp experiment supports that CO inhibits L-type Ca(2+) channel mediated influx of Ca(2+) and the ischemic death of H9c2 cells.