Carbon monoxide modulates electrical activity of murine myocardium via cGMP-dependent mechanisms

The role and mechanism of heme oxygenase-1 in arrhythmias

The global incidence and prevalence of arrhythmias are continuously increasing. However, the precise mechanisms of underlying arrhythmogenesis and the optimal measures for effective treatment remain incompletely understood. The inducible form of heme oxygenase, known as heme oxygenase-1 (HO-1), is recognized as a potent antioxidant molecule capable of exerting anti-inflammatory and anti-apoptotic effects. Recent research indicates that HO-1 plays a role in preventing arrhythmias by mitigating cardiac remodeling, including electrical remodeling, ion remodeling, and structural remodeling. This review aimed to consolidate current knowledge regarding the involvement of HO-1 in arrhythmias and elucidate its underlying mechanisms of action.

Carbon monoxide signaling and soluble guanylyl cyclase: Facts, myths, and intriguing possibilities

The endogenous signaling roles of carbon monoxide (CO) have been firmly established at the pathway level. For CO's molecular mechanism(s) of actions, hemoproteins are generally considered as possible targets. Importantly, soluble guanylyl cyclase (sGC) is among the most widely referenced molecular targets. However, the affinity of CO for sGC (K_d: 240 μM) is much lower than for other highly abundant hemoproteins in the body, such as myoglobin (K_d: 29 nM) and hemoglobin (K_d: 0.7 nM-4.5 μM), which serve as CO reservoirs. Further, most of the mechanistic studies involving sGC activation by CO were based on in-vitro or ex-vivo studies using CO concentrations not readily attenable in vivo and in the absence of hemoglobin as a competitor in binding. As such, whether such in-vitro/ex-vivo results can be directly extrapolated to in-vivo studies is not clear because of the need for CO to be transferred from a high-affinity binder (e.g., hemoglobin) to a low-affinity target if sGC is to be activated in vivo. In this review, we discuss literature findings of sGC activation by CO and the experimental conditions; examine the myths in the disconnect between the low affinity of sGC for CO and the reported activation of sGC by CO; and finally present several possibilities that may lead to additional studies to improve our understanding of this direct CO-sGC axis, which is yet to be convincingly established as playing generally critical roles in CO signaling in vivo.

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.

An Overview of the Potential Therapeutic Applications of CO-Releasing Molecules

Carbon monoxide (CO) has long been known as the “silent killer” owing to its ability to form carboxyhemoglobin—the main cause of CO poisoning in humans. Its role as an endogenous neurotransmitter, however, was suggested in the early 1990s. Since then, the biological activity of CO has been widely examined via both the direct administration of CO and in the form of so-called “carbon monoxide releasing molecules (CORMs).” This overview will explore the general physiological effects and potential therapeutic applications of CO when delivered in the form of CORMs.

Phosphodiesterases 3 and 4 Differentially Regulate the Funny Current, If, in Mouse Sinoatrial Node Myocytes

Cardiac pacemaking, at rest and during the sympathetic fight-or-flight response, depends on cAMP (3',5'-cyclic adenosine monophosphate) signaling in sinoatrial node myocytes (SAMs). The cardiac "funny current" (I_f) is among the cAMP-sensitive effectors that drive pacemaking in SAMs. I_f is produced by hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels. Voltage-dependent gating of HCN channels is potentiated by cAMP, which acts either by binding directly to the channels or by activating the cAMP-dependent protein kinase (PKA), which phosphorylates them. PKA activity is required for signaling between β adrenergic receptors (βARs) and HCN channels in SAMs but the mechanism that constrains cAMP signaling to a PKA-dependent pathway is unknown. Phosphodiesterases (PDEs) hydrolyze cAMP and form cAMP signaling domains in other types of cardiomyocytes. Here we examine the role of PDEs in regulation of I_f in SAMs. I_f was recorded in whole-cell voltage-clamp experiments from acutely-isolated mouse SAMs in the absence or presence of PDE and PKA inhibitors, and before and after βAR stimulation. General PDE inhibition caused a PKA-independent depolarizing shift in the midpoint activation voltage (V_1/2) of I_f at rest and removed the requirement for PKA in βAR-to-HCN signaling. PDE4 inhibition produced a similar PKA-independent depolarizing shift in the V_1/2 of I_f at rest, but did not remove the requirement for PKA in βAR-to-HCN signaling. PDE3 inhibition produced PKA-dependent changes in I_f both at rest and in response to βAR stimulation. Our results suggest that PDE3 and PDE4 isoforms create distinct cAMP signaling domains that differentially constrain access of cAMP to HCN channels and establish the requirement for PKA in signaling between βARs and HCN channels in SAMs.

Effects of exogenous nicotinamide adenine dinucleotide (NAD+) in the rat heart are mediated by P2 purine receptors

Background

Recently, NAD+ has been considered as an essential factor, participating in nerve control of physiological functions and intercellular communication. NAD+ also has been supposed as endogenous activator of P1 and P2 purinoreceptors. Effects of extracellular NAD+ remain poorly investigated in cardiac tissue. This study aims to investigate the effects of extracellular NAD+ in different types of supraventricular and ventricular working myocardium from rat and their potential mechanisms.

Methods

The standard technique of sharp microelectrode action potential recording in cardiac multicellular preparations was used to study the effects of NAD+.

Results

Extracellular NAD+ induced significant changes in bioelectrical activity of left auricle (LA), right auricle (RA), pulmonary veins (PV) and right ventricular wall (RV) myocardial preparations. 10–100 μM NAD+ produced two opposite effects in LA and RA – quickly developing and transient prolongation of action potentials (AP) and delayed sustained AP shortening, which follows the initial positive effect. In PV and RV only AP shortening was observed in response to NAD+ application. In PV preparations AP shortening induced by NAD+ may be considered as a potential proarrhythmic effect. Revealed cardiotropic effects of NAD+ are likely to be mediated by P2 purine receptors, since P1 blocker DPCPX failed to affect them and P2 antagonist suramin abolished NAD + −induced alterations of electrical activity. P2X receptors may be responsible for NAD + −induced short-lasting AP prolongation, while P2Y receptors mediate persistent AP shortening. The latter effect is partially removed by PLC inhibitor U73122 showing the potential involvement of phosphoinositide signaling pathway in mediation of NAD+ cardiotropic effects.

Conclusions

Extracellular NAD+ is supposed to be a novel regulator of cardiac electrical activity. P2 receptors represent the main target of NAD+ at least in the rat heart.

Evidence for a role of heme oxygenase-1 in the control of cardiac function in zebrafish (Danio rerio) larvae exposed to hypoxia

Carbon monoxide (CO) is a gaseous neurotransmitter produced from the breakdown of heme via heme oxygenase-1 (HO-1; hypoxia-inducible isoform) and heme oxygenase-2 (HO-2; constitutively expressed isoform). In mammals, CO is involved in modulating cardiac function. The role of the HO-1/CO system in the control of heart function in fish, however, is unknown and investigating its physiological function in lower vertebrates will provide a better understanding of the evolution of this regulatory mechanism. We explored the role of the HO-1/CO system in larval zebrafish (Danio rerio) in vivo by investigating the impact of translational gene knockdown of HO-1 on cardiac function. Immunohistochemistry revealed the presence of HO-1 in the pacemaker cells of the heart at 4 days post-fertilization and thus the potential for CO production at these sites. Sham-treated zebrafish larvae (experiencing normal levels of HO-1) significantly increased heart rate (fH) when exposed to hypoxia (PwO2 =30 mmHg). Zebrafish larvae lacking HO-1 expression after morpholino knockdown (morphants) exhibited significantly higher fH under normoxic (but not hypoxic) conditions when compared with sham-treaded fish. The increased fH in HO-1 morphants was rescued (fH was restored to control levels) after treatment of larvae with a CO-releasing molecule (40 µmol l(-1) CORM). The HO-1-deficient larvae developed significantly larger ventricles and when exposed to hypoxia they displayed higher cardiac output ([Formula: see text]) and stroke volume (SV). These results suggest that under hypoxic conditions, HO-1 regulates [Formula: see text] and SV presumably via the production of CO. Overall, this study provides a better understanding of the role of the HO-1/CO system in controlling heart function in lower vertebrates. We demonstrate for the first time the ability for CO to be produced in presumptive pacemaker cells of the heart where it plays an inhibitory role in setting the resting cardiac frequency.