The Na+/Ca²+ exchanger in cardiac ischemia/reperfusion injury

TRPV4 inhibition suppresses myocardial ischemia-reperfusion arrhythmia of mice by alleviating calcium handling abnormalities

BACKGROUND

Transient receptor potential vanilloid 4 (TRPV4), a calcium (Ca2+) permeable channel, is upregulated during myocardial ischemia-reperfusion (IR). Although TRPV4 inhibition has cardioprotective effects, its impact on arrhythmogenesis remains unclear.

OBJECTIVE

This study aimed to evaluate the antiarrhythmic effects of TRPV4 inhibition, using the TRPV4 antagonist GSK2193874 (GSK219) and TRPV4 knockout (TRPV4-/-) mice, after IR.

METHODS

Surface electrocardiogram and optical mapping recordings were performed during 15 minutes of global ischemia and 10 minutes of reperfusion in Langendorff perfused mouse hearts. Ca2+ sparks were detected by confocal microscopy, and protein expression was analyzed by Western blot.

RESULTS

GSK219 or TRPV4 deletion significantly decreased the incidence and duration of ventricular tachycardia during reperfusion. TRPV4 inhibition shortened Ca2+ transient (CaT) recovery, suppressed CaT alternans, and decreased Ca2+ leak without affecting IR-induced prolongation of action potential duration (APD) and APD alternations. Activation of TRPV4 by GSK101790A (GSK101) increased arrhythmia susceptibility and Ca2+ leak. Moreover, GSK101 prolonged CaT recovery and promoted CaT alternans, which were greatly avoided by pretreatment with Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor. Interestingly, IR or GSK101 markedly increased the phosphorylation of CaMKII, ryanodine receptors, and phospholamban, which was significantly blocked by TRPV4 inhibition.

CONCLUSION

TRPV4 inhibition exerts antiarrhythmic effects after IR by modulating CaMKII-dependent Ca2+ handling abnormalities, reducing CaT alternans and Ca2+ leak, without affecting APD.

Slack K+ channels confer protection against myocardial ischaemia/reperfusion injury

AIMS

Na+-activated Slack potassium (K+) channels are increasingly recognized as regulators of neuronal activity, yet little is known about their role in the cardiovascular system. Slack activity increases when intracellular Na+ concentration ([Na+]i) reaches pathophysiological levels. Elevated [Na+]i is a major determinant of the ischaemia and reperfusion (I/R)-induced myocardial injury; thus, we hypothesized that Slack plays a role under these conditions.

METHODS AND RESULTS

K+ currents in cardiomyocytes (CMs) obtained from wildtype but not from global Slack knockout mice were sensitive to electrical inactivation of voltage-sensitive Na+ channels. Live-cell imaging demonstrated that K+ fluxes across the sarcolemma rely on Slack, while the depolarized resting membrane potential in Slack-deficient CMs led to excessive cytosolic Ca2+ accumulation and finally to hypoxia/reoxygenation-induced cell death. Cardiac damage in an in vivo model of I/R was exacerbated in global and CM-specific conditional Slack mutants and largely insensitive to mechanical conditioning manoeuvres. Finally, the protection conferred by mitochondrial ATP-sensitive K+ (mitoKATP) channels required functional Slack in CMs.

CONCLUSION

Collectively, our study provides evidence for Slack's crucial involvement in the ion homeostasis of no or low O2-stressed CMs. Thereby, Slack activity opposes the I/R-induced fatal Ca2+-uptake to CMs supporting the cardioprotective signaling attributed to mitoKATP function.

A Comprehensive Review on Unveiling the Journey of Digoxin: Past, Present, and Future Perspectives

Digoxin, a cardiac glycoside derived from the foxglove plant (Digitalis spp.), has been utilized for centuries in managing various cardiac conditions due to its ability to increase myocardial contractility and regulate heart rate. This comprehensive review explores the historical context, pharmacological properties, clinical applications, efficacy, safety profile, challenges, and future perspectives of digoxin. Tracing its journey from traditional medicine to modern cardiovascular therapeutics, we delve into its mechanism of action, therapeutic indications, and clinical guidelines. While digoxin remains a cornerstone therapy for heart failure and atrial fibrillation, its narrow therapeutic index and individual variability in response pose challenges in clinical practice. Nevertheless, ongoing research efforts aim to elucidate its role in emerging therapeutic areas and technological advancements in drug delivery. Despite the advent of newer pharmacological agents, digoxin's enduring relevance lies in its established efficacy, affordability, and global accessibility. This review underscores the symbiotic relationship between tradition and progress in cardiovascular medicine, highlighting the timeless pursuit of medical innovation to optimize patient care.

Structural insight into the allosteric inhibition of human sodium-calcium exchanger NCX1 by XIP and SEA0400

Sodium-calcium exchanger proteins influence calcium homeostasis in many cell types and participate in a wide range of physiological and pathological processes. Here, we elucidate the cryo-EM structure of the human Na+/Ca2+ exchanger NCX1.3 in the presence of a specific inhibitor, SEA0400. Conserved ion-coordinating residues are exposed on the cytoplasmic face of NCX1.3, indicating that the observed structure is stabilized in an inward-facing conformation. We show how regulatory calcium-binding domains (CBDs) assemble with the ion-translocation transmembrane domain (TMD). The exchanger-inhibitory peptide (XIP) is trapped within a groove between the TMD and CBD2 and predicted to clash with gating helices TMs1/6 at the outward-facing state, thus hindering conformational transition and promoting inactivation of the transporter. A bound SEA0400 molecule stiffens helix TM2ab and affects conformational rearrangements of TM2ab that are associated with the ion-exchange reaction, thus allosterically attenuating Ca2+-uptake activity of NCX1.3.

The Effect of SARS-CoV-2 Spike Protein RBD-Epitope on Immunometabolic State and Functional Performance of Cultured Primary Cardiomyocytes Subjected to Hypoxia and Reoxygenation

Cardio complications such as arrhythmias and myocardial damage are common in COVID-19 patients. SARS-CoV-2 interacts with the cardiovascular system primarily via the ACE2 receptor. Cardiomyocyte damage in SARS-CoV-2 infection may stem from inflammation, hypoxia-reoxygenation injury, and direct toxicity; however, the precise mechanisms are unclear. In this study, we simulated hypoxia-reoxygenation conditions commonly seen in SARS-CoV-2-infected patients and studied the impact of the SARS-CoV-2 spike protein RBD-epitope on primary rat cardiomyocytes to gain insight into the potential mechanisms underlying COVID-19-related cardiac complications. Cell metabolic activity was evaluated with PrestoBlueTM. Gene expression of proinflammatory markers was measured by qRT-PCR and their secretion was quantified by Luminex assay. Cardiomyocyte contractility was analysed using the Myocyter plugin of ImageJ. Mitochondrial respiration was determined through Seahorse Mito Stress Test. In hypoxia-reoxygenation conditions, treatment of the SARS-CoV-2 spike RBD-epitope reduced the metabolic activity of primary cardiomyocytes, upregulated Il1β and Cxcl1 expression, and elevated GM-CSF and CCL2 cytokines secretion. Contraction time increased, while amplitude and beating frequency decreased. Acute treatment with a virus RBD-epitope inhibited mitochondrial respiration and lowered ATP production. Under ischaemia-reperfusion, the SARS-CoV-2 RBD-epitope induces cardiomyocyte injury linked to impaired mitochondrial activity.

Myocardial ischemia/reperfusion injury: Mechanisms of injury and implications for management (Review)

Myocardial infarction is one of the primary causes of mortality in patients with coronary heart disease worldwide. Early treatment of acute myocardial infarction restores blood supply of ischemic myocardium and decreases the mortality risk. However, when the interrupted myocardial blood supply is recovered within a certain period of time, it causes more serious damage to the original ischemic myocardium; this is known as myocardial ischemia/reperfusion injury (MIRI). The pathophysiological mechanisms leading to MIRI are associated with oxidative stress, intracellular calcium overload, energy metabolism disorder, apoptosis, endoplasmic reticulum stress, autophagy, pyroptosis, necroptosis and ferroptosis. These interplay with one another and directly or indirectly lead to aggravation of the effect. In the past, apoptosis and autophagy have attracted more attention but necroptosis and ferroptosis also serve key roles. However, the mechanism of MIRI has not been fully elucidated. The present study reviews the mechanisms underlying MIRI. Based on current understanding of the pathophysiological mechanisms of MIRI, the association between cell death-associated signaling pathways were elaborated, providing direction for investigation of novel targets in clinical treatment.

Modification of Ischemia/Reperfusion-Induced Alterations in Subcellular Organelles by Ischemic Preconditioning

It is now well established that ischemia/reperfusion (I/R) injury is associated with the compromised recovery of cardiac contractile function. Such an adverse effect of I/R injury in the heart is attributed to the development of oxidative stress and intracellular Ca²⁺-overload, which are known to induce remodeling of subcellular organelles such as sarcolemma, sarcoplasmic reticulum, mitochondria and myofibrils. However, repeated episodes of brief periods of ischemia followed by reperfusion or ischemic preconditioning (IP) have been shown to improve cardiac function and exert cardioprotective actions against the adverse effects of prolonged I/R injury. This protective action of IP in attenuating myocardial damage and subcellular remodeling is likely to be due to marked reductions in the occurrence of oxidative stress and intracellular Ca²⁺-overload in cardiomyocytes. In addition, the beneficial actions of IP have been attributed to the depression of proteolytic activities and inflammatory levels of cytokines as well as the activation of the nuclear factor erythroid factor 2-mediated signal transduction pathway. Accordingly, this review is intended to describe some of the changes in subcellular organelles, which are induced in cardiomyocytes by I/R for the occurrence of oxidative stress and intracellular Ca²⁺-overload and highlight some of the mechanisms for explaining the cardioprotective effects of IP.

Cardiac ischemia on-a-chip to investigate cellular and molecular response of myocardial tissue under hypoxia

Tissue engineering has enabled the development of advanced and physiologically relevant models of cardiovascular diseases, with advantages over conventional 2D in vitro assays. We have previously demonstrated development of a heart on-a-chip microfluidic model with mature 3D anisotropic tissue formation that incorporates both stem cell-derived cardiomyocytes and cardiac fibroblasts within a collagen-based hydrogel. Using this platform, we herein present a model of myocardial ischemia on-a-chip, that recapitulates ischemic insult through exposure of mature 3D cardiac tissues to hypoxic environments. We report extensive validation and molecular-level analyses of the model in its ability to recapitulate myocardial ischemia in response to hypoxia, demonstrating the 1) induction of tissue fibrosis through upregulation of contractile fibers, 2) dysregulation in tissue contraction through functional assessment, 3) upregulation of hypoxia-response genes and downregulation of contractile-specific genes through targeted qPCR, and 4) transcriptomic pathway regulation of hypoxic tissues. Further, we investigated the complex response of ischemic myocardial tissues to reperfusion, identifying 5) cell toxicity, 6) sustained contractile irregularities, as well as 7) re-establishment of lactate levels and 8) gene expression, in hypoxic tissues in response to ischemia reperfusion injury.

Considerations for the Bioengineering of Advanced Cardiac In Vitro Models of Myocardial Infarction

Despite the latest advances in cardiovascular biology and medicine, myocardial infarction (MI) remains one of the major causes of deaths worldwide. While reperfusion of the myocardium is critical to limit the ischemic damage typical of a MI event, it causes detrimental morphological and functional changes known as "reperfusion injury." This complex scenario is poorly represented in currently available models of ischemia/reperfusion injury, leading to a poor translation of findings from the bench to the bedside. However, more recent bioengineered in vitro models of the human heart represent more clinically relevant tools to prevent and treat MI in patients. These include 3D cultures of cardiac cells, the use of patient-derived stem cells, and 3D bioprinting technology. This review aims at highlighting the major features typical of a heart attack while comparing current in vitro, ex vivo, and in vivo models. This information has the potential to further guide in developing novel advanced in vitro cardiac models of ischemia/reperfusion injury. It may pave the way for the generation of advanced pathophysiological cardiac models with the potential to develop personalized therapies.

Acute hypobaric hypoxia and cardiac energetic response in prepubertal rats: Role of nitric oxide

NEW FINDINGS

What is the central question of this study? In adult rat hearts, exposure to hypobaric hypoxia increases tolerance to hypoxia-reoxygenation, termed endogenous cardioprotection. The mechanism involves the nitric oxide system and modulation of mitochondrial oxygen consumption. What is the cardiac energetic response in prepubertal rats exposed to hypobaric hypoxia? What is the main finding and its importance? Prepubertal rats, unlike adult rats, did not increase tolerance to hypoxia-reoxygenation in response acute exposure to hypobaric hypoxia, which impaired cardiac contractile economy. This finding could be related to a failure to increase nitric oxide synthase expression, hence modulation of mitochondrial oxygen consumption and ATP production.

ABSTRACT

Studies in our laboratory showed that exposure of rats to hypobaric hypoxia (HH) increased the tolerance of the heart to hypoxia-reoxygenation (H/R), involving mitochondrial and cytosolic nitric oxide synthase (NOS) systems. The objective of the present study was to evaluate how the degree of somatic maturation could alter this healthy response. Prepubertal male rats were exposed for 48 h to a simulated altitude of 4400 m in a hypobaric chamber. The mechanical energetic activity in perfused hearts and the contractile functional capacity of NOS in isolated left ventricular papillary muscles were evaluated during H/R. Cytosolic nitric oxide (NO), production of nitrites/nitrates (Nx), expression of NOS isoforms, mitochondrial O₂ consumption and ATP production were also evaluated. The left ventricular pressure during H/R was not improved by HH. However, the energetic activity was increased. Thus, the contractile economy (left ventricular pressure/energetic activity) decreased in HH. Nitric oxide did not modify papillary muscle contractility after H/R. Cytosolic p-eNOS-Ser1177 and inducible NOS expression were decreased by HH, but no changes were observed in NO production. Interestingly, HH increased Nx levels, but O₂  consumption and ATP production in mitochondria were not affected by HH. Prepubertal rats exposed to HH preserved cardiac contractile function, but with a high energetic cost, modifying contractile economy. Although this could be related to the decreased NOS expression detected, cytosolic NO production was preserved, maybe through the Nx metabolic pathway, without modification of mitochondrial ATP production and O₂  consumption. In this scenario, the treatment was unable to increase tolerance to H/R as observed in adult animals.

Lipidomics Provides New Insight into Pathogenesis and Therapeutic Targets of the Ischemia-Reperfusion Injury

Lipids play an essential role in both tissue protection and damage. Tissue ischemia creates anaerobic conditions in which enzyme inactivation occurs, and reperfusion can initiate oxidative stress that leads to harmful changes in membrane lipids, the formation of aldehydes, and chain damage until cell death. The critical event in such a series of harmful events in the cell is the unwanted accumulation of fatty acids that leads to lipotoxicity. Lipid analysis provides additional insight into the pathogenesis of ischemia/reperfusion (I/R) disorders and reveals new targets for drug action. The profile of changes in the composition of fatty acids in the cell, as well as the time course of these changes, indicate both the mechanism of damage and new therapeutic possibilities. A therapeutic approach to reperfusion lipotoxicity involves attenuation of fatty acids overload, i.e., their transport to adipose tissue and/or inhibition of the adverse effects of fatty acids on cell damage and death. The latter option involves using PPAR agonists and drugs that modulate the transport of fatty acids via carnitine into the interior of the mitochondria or the redirection of long-chain fatty acids to peroxisomes.

Sevoflurane Postconditioning Reduces Hypoxia-Reoxygenation Injury in H9C2 Embryonic Rat Cardiomyocytes and Targets the STRADA Gene by Upregulating microRNA-107

BACKGROUND Sevoflurane as a widely used inhalational general anesthetic that also has a cardioprotective role in hypoxia-reoxygenation (H/R) injury. This study aimed to investigate the effects of microRNA-107 (miR-107) on sevoflurane postconditioning (SpostC) in H9C2 embryonic rat cardiomyocytes and to use bioinformatics analysis to identify the molecular basis of cardioprotection from sevoflurane in human cardiac tissue. MATERIAL AND METHODS The STRADA gene was identified from the Gene Expression Omnibus (GEO) database. H9C2 embryonic rat cardiomyocytes were cultured with sevoflurane. Quantitative real-time polymerase chain reaction (qRT-PCR) and Western blot were used to measure the mRNA expression and protein expression of STRADA and miR-107 in H9C2 cells. TargetScanHuman version 7.2 was used to identify the target gene of miR-107 and to predict the STRADA 3'-UTR binding site of miR-107. The dual-luciferase reporter assay measured the relative luciferase activity. The cell proliferation rate and cell apoptosis were measured using the MTT assay and flow cytometry, respectively. RESULTS H/R injury in H9C2 cells following SpostC resulted in increased expression of miR-107 and reduced expression of STRADA. Specific binding of miR-107 was identified to STRADA 3'-UTR. Upregulation of the miR-107 in SpostC H/R injured H9C2 cells promoted cell proliferation, reduced cell apoptosis, and downregulating the protein expression of caspase-3. STRADA overexpression reduced the effects of a miR-107 mimic on SpostC. CONCLUSIONS SpostC reduced H/R injury in H9C2 embryonic rat cardiomyocytes by targeting the STRADA gene and by upregulating the expression of microRNA-107.

Palmitoylation: A Fatty Regulator of Myocardial Electrophysiology

Regulation of cardiac physiology is well known to occur through the action of kinases that reversibly phosphorylate ion channels, calcium handling machinery, and signaling effectors. However, it is becoming increasingly apparent that palmitoylation or S-acylation, the post-translational modification of cysteines with saturated fatty acids, plays instrumental roles in regulating the localization, activity, stability, sorting, and function of numerous proteins, including proteins known to have essential functions in cardiomyocytes. However, the impact of this modification on cardiac physiology requires further investigation. S-acylation is catalyzed by the zDHHC family of S-acyl transferases that localize to intracellular organelle membranes or the sarcolemma. Recent work has begun to uncover functions of S-acylation in the heart, particularly in the regulation of cardiac electrophysiology, including modification of the sodium-calcium exchanger, phospholemman and the cardiac sodium pump, as well as the voltage-gated sodium channel. Elucidating the regulatory functions of zDHHC enzymes in cardiomyocytes and determination of how S-acylation is altered in the diseased heart will shed light on how these modifications participate in cardiac pathogenesis and potentially identify novel targets for the treatment of cardiovascular disease. Indeed, proteins with critical signaling roles in the heart are also S-acylated, including receptors and G-proteins, yet the dynamics and functions of these modifications in myocardial physiology have not been interrogated. Here, we will review what is known about zDHHC enzymes and substrate S-acylation in myocardial physiology and highlight future areas of investigation that will uncover novel functions of S-acylation in cardiac homeostasis and pathophysiology.

NCX and EAAT transporters in ischemia: At the crossroad between glutamate metabolism and cell survival

Energy metabolism impairment is a central event in the pathophysiology of ischemia. The limited availability of glucose and oxygen strongly affects mitochondrial activity, thus leading to ATP depletion. In this setting, the switch to alternative energy sources could ameliorate cells survival by enhancing ATP production, thus representing an attractive strategy for ischemic treatment. In this regard, some studies have recently re-evaluated the metabolic role of glutamate and its potential to promote cell survival under pathological conditions. In the present review, we discuss the ability of glutamate to exert an "energizing role" in cardiac and neuronal models of hypoxia/reoxygenation (H/R) injury, focusing on the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺-dependent excitatory amino acid transporters (EAATs) as key players in this metabolic pathway.

NOX5 as a therapeutic target in cerebral ischemic injury

In this issue of the JCI, Casas et al. define a previously unknown role of the NADPH oxidase catalytic subunit NOX5 in cerebral infarction. Using a mouse expressing human NOX5 in the endothelium, the investigators show that NOX5 is activated and plays a deleterious role in promoting edema, infarction, and ultimately, worsened neurological function following cerebral ischemia. They provide evidence that this is due to the breakdown of the blood-brain barrier (BBB) and that a unique pharmacological inhibitor of NOX5, ML090, if given early, around the time of reoxygenation, can maintain BBB integrity. Future studies of NOX5 inhibition in humans, particularly in the setting of thrombolysis, are warranted.

Nerol Attenuates Ouabain-Induced Arrhythmias

Nerol (C₁₀H₁₈O) is a monoterpene found in many essential oils, such as lemon balm and hop. In this study, we explored the contractile and electrophysiological properties of nerol and demonstrated its antiarrhythmic effects in guinea pig heart preparation. Nerol effects were evaluated on atrial and ventricular tissue contractility, electrocardiogram (ECG), voltage-dependent L-type Ca²⁺ current (I_Ca,L), and ouabain-triggered arrhythmias. Overall our results revealed that by increasing concentrations of nerol (from 0.001 to 30 mM) there was a significant decrease in left atrium contractile force. This effect was completely and rapidly reversible after washing out (~ 2 min). Nerol (at 3 mM concentration) decreased the left atrium positive inotropic response evoked by adding up CaCl₂ in the extracellular medium. Interestingly, when using a lower concentration of nerol (30 μM), it was not possible to clearly observe any significant ECG signal alterations but a small reduction of ventricular contractility was observed. In addition, 300 μM nerol promoted a significant decrease on the cardiac rate and contractility. Important to note is the fact that in isolated cardiomyocytes, peak I_Ca,L was reduced by 58.9 ± 6.31% after perfusing 300 μM nerol (n=7, p<0.05). Nerol, at 30 and 300 μM, delayed the time of onset of ouabain-triggered arrhythmias and provoked a decrease in the diastolic tension induced by the presence of ouabain (50 μM). Furthermore, nerol preincubation significantly attenuated arrhythmia severity index without changes in the positive inotropism elicited by ouabain exposure. Taken all together, we may be able to conclude that nerol primarily by reducing Ca²⁺ influx through L-type Ca²⁺ channel blockade lessened the severity of ouabain-triggered arrhythmias in mammalian heart.

Effects of ticagrelor on the sodium/calcium exchanger 1 (NCX1) in cardiac derived H9c2 cells

Ticagrelor is a direct acting and reversibly binding P2Y₁₂ antagonist approved for the prevention of thromboembolic events. Clinical effects of ticagrelor cannot be simply accounted for by pure platelet inhibition, and off-target mechanisms can potentially play a role. In particular, recent evidence suggests that ticagrelor may also influence heart function and improve the evolution of myocardial ischemic injury by more direct effects on myocytes. The cardiac sodium/calcium exchanger 1 (NCX1) is a critical player in the generation and control of calcium (Ca²⁺) signals, which orchestrate multiple myocyte activities in health and disease. Altered expression and/or activity of NCX1 can have profound consequences for the function and fate of myocytes. Whether ticagrelor affects cardiac NCX1 has not been investigated yet. To explore this hypothesis, we analyzed the expression, localization and activity of NCX1 in the heart derived H9c2-NCX1 cells following ticagrelor exposure. We found that ticagrelor concentration- and time-dependently reduced the activity of the cardiac NCX1 in H9c2 cells. In particular, the inhibitory effect of ticagrelor on the Ca²⁺-influx mode of NCX1 was evident within 1 h and further developed after 24 h, when NCX1 activity was suppressed by about 55% in cells treated with 1 μM ticagrelor. Ticagrelor-induced inhibition of exchanger activity was reached at clinically relevant concentrations, without affecting the expression levels and subcellular distribution of NCX1. Collectively, these findings suggest that cardiac NCX1 is a new downstream target of ticagrelor, which may contribute to the therapeutic profile of ticagrelor in clinical practice.

CaMKII-dependent late Na+ current increases electrical dispersion and arrhythmia in ischemia-reperfusion

The mechanisms underlying Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-induced arrhythmias in ischemia-reperfusion (I/R) are not fully understood. We tested the hypothesis that CaMKII increases late Na⁺ current ( I_Na,L) via phosphorylation of Na_v1.5 at Ser⁵⁷¹ during I/R, thereby increasing arrhythmia susceptibility. To test our hypothesis, we studied isolated, Langendorff-perfused hearts from wild-type (WT) mice and mice expressing Na_v channel variants Na_v1.5-Ser571E (S571E) and Na_v1.5-Ser571A (S571A). WT hearts showed a significant increase in the levels of phosphorylated CaMKII and Na_v1.5 at Ser⁵⁷¹ [p-Na_v1.5(S571)] after 15 min of global ischemia (just before the onset of reperfusion). Optical mapping experiments revealed an increase in action potential duration (APD) and APD dispersion without changes in conduction velocity during I/R in WT and S571E compared with S571A hearts. At the same time, WT and S571E hearts showed an increase in spontaneous arrhythmia events (e.g., premature ventricular contractions) and an increase in the inducibility of reentrant arrhythmias during reperfusion. Pretreatment of WT hearts with the Na⁺ channel blocker mexiletine (10 μM) normalized APD dispersion and reduced arrhythmia susceptibility during I/R. We conclude that CaMKII-dependent phosphorylation of Na_v1.5 is a crucial driver for increased I_Na,L, arrhythmia triggers, and substrate during I/R. Selective targeting of this CaMKII-dependent pathway may have therapeutic potential for reducing arrhythmias in the setting of I/R. NEW & NOTEWORTHY Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of Na_v1.5 at Ser⁵⁷¹ leads to a prolongation of action potential duration (APD), increased APD dispersion, and increased arrhythmia susceptibility after ischemia-reperfusion in isolated mouse hearts. Genetic ablation of the CaMKII-dependent phosphorylation site Ser⁵⁷¹ on Na_v1.5 or low-dose mexiletine (to inhibit late Na⁺ current) reduced APD dispersion, arrhythmia triggers, and ventricular tachycardia inducibility.

Reverse mode of sodium/calcium exchanger subtype 1 contributes to detrusor overactivity in rats with partial bladder outflow obstruction

To investigate whether the reverse mode of sodium/calcium exchanger subtype 1 (NCX1) plays an important role in the excitability of detrusor cells in rats with partial bladder outflow obstruction (PBOO), PBOO was maintained for 6 weeks in forty female Wistar rats. Thirty of the animals exhibited non-voiding bladder contraction and comprised the DO group. An additional thirty sham-operated female Wistar rats were used as the control group. The expression levels of NCX1 were compared between the two groups by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), western blotting (WB), and double-label immunofluorescence. The contractions of detrusor strips in NCX reverse mode were measured in both groups using isometric tension. The role of NCX in the regulation of the intracellular Ca²⁺ concentration ([Ca²⁺]_i) of smooth muscle cells was observed in reverse mode using confocal microscopy, and the current was evaluated in the presence of the antagonist KB-R7943 (5 μM and 10 μM) using the whole-cell patch-clamp technique. The expression of NCX1 was significantly higher in the DO group than in the control group, as assessed by qRT-PCR, WB analysis and immunofluorescence. The volume and rate of Ca²⁺ ion flux through the NCX, as well as the NCX currents, were higher in the DO group than in the control group in both modes. Increased NCX1 levels may contribute to the establishment of DO after PBOO by elevating [Ca²⁺]_i in reverse mode under depolarization, potentially inducing cell excitability.

Role of glycogen synthase kinase following myocardial infarction and ischemia-reperfusion

Glycogen synthase kinase-3 beta (GSK3β) is principally is a glycogen synthase phosphorylating enzyme that is well known for its role in muscle metabolism. GSK3β is a serine/threonine protein Kinase, which is responsible for several essential roles in mammalian cells. This enzyme is implicated in the pathophysiology of many conditions involved in homeostasis and cellular immigration. GSK3β is involved in several pathways leading to neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. Increasing evidence has shown the potential importance of GSK3β in ischemic heart disease and ischemia-reperfusion pathologies. Reperfusion injury may occur in tissues after prolonged ischemia following reperfusion. Reperfusion injury can be life threatening. Reperfusion injury occurs due to a change in ionic homeostasis, excess free radical production, mitochondrial damage and cell death. There are however clear, cardiac-protective signals; although the molecular pathophysiology is not clearly understood. In normal physiology, GSK3β has a critical role in the cytoprotective pathway. However, it`s controversial role in ischemia and ischemia-reperfusion is a topic of current interest. In this review, we have opted to focus on GSK3β interactions with mitochondria in ischemic heart disease and expand on the therapeutic interventions.

Essential role of the Na+-Ca2+ exchanger (NCX) in glutamate-enhanced cell survival in cardiac cells exposed to hypoxia/reoxygenation

Myocardial ischemia culminates in ATP production impairment, ionic derangement and cell death. The provision of metabolic substrates during reperfusion significantly increases heart tolerance to ischemia by improving mitochondrial performance. Under normoxia, glutamate contributes to myocardial energy balance as substrate for anaplerotic reactions, and we demonstrated that the Na⁺/Ca²⁺ exchanger1 (NCX1) provides functional support for both glutamate uptake and use for ATP synthesis. Here we investigated the role of NCX1 in the potential of glutamate to improve energy metabolism and survival of cardiac cells subjected to hypoxia/reoxygenation (H/R). Specifically, in H9c2-NCX1 myoblasts, ATP levels, mitochondrial activities and cell survival were significantly compromised after H/R challenge. Glutamate supplementation at the onset of the reoxygenation phase significantly promoted viability, improved mitochondrial functions and normalized the H/R-induced increase of NCX1 reverse-mode activity. The benefits of glutamate were strikingly lost in H9c2-WT (lacking NCX1 expression), or in H9c2-NCX1 and rat cardiomyocytes treated with either NCX or Excitatory Amino Acid Transporters (EAATs) blockers, suggesting that a functional interplay between these transporters is critically required for glutamate-induced protection. Collectively, these results revealed for the first time the key role of NCX1 for the beneficial effects of glutamate against H/R-induced cell injury.

Late sodium current and intracellular ionic homeostasis in acute ischemia

Blockade of the late Na⁺ current (I _NaL) protects from ischemia/reperfusion damage; nevertheless, information on changes in I _NaL during acute ischemia and their effect on intracellular milieu is missing. I _NaL, cytosolic Na⁺ and Ca²⁺ activities (Na_cyt, Ca_cyt) were measured in isolated rat ventricular myocytes during 7 min of simulated ischemia (ISC); in all the conditions tested, effects consistently exerted by ranolazine (RAN) and tetrodotoxin (TTX) were interpreted as due to I _NaL blockade. The results indicate that I _NaL was enhanced during ISC in spite of changes in action potential (AP) contour; I _NaL significantly contributed to Na_cyt rise, but only marginally to Ca_cyt rise. The impact of I _NaL on Ca_cyt was markedly enhanced by blockade of the sarcolemmal(s) Na⁺/Ca²⁺ exchanger (NCX) and was due to the presence of (Na⁺-sensitive) Ca²⁺ efflux through mitochondrial NCX (mNCX). sNCX blockade increased Ca_cyt and decreased Na_cyt, thus indicating that, throughout ISC, sNCX operated in the forward mode, in spite of the substantial Na_cyt increment. Thus, a robust Ca²⁺ source, other than sNCX and including mitochondria, contributed to Ca_cyt during ISC. Most, but not all, of RAN effects were shared by TTX. (1) The paradigm that attributes Ca_cyt accumulation during acute ischemia to decrease/reversal of sNCX transport may not be of general applicability; (2) I _NaL is enhanced during ISC, when the effect of Na_cyt on mitochondrial Ca²⁺ transport may substantially contribute to I _NaL impact on Ca_cyt; (3) RAN may act mostly, but not exclusively, through I _NaL blockade during ISC.

ADP-Induced Ca2+ Signaling and Proliferation of Rat Ventricular Myofibroblasts Depend on Phospholipase C-Linked TRP Channels Activation Within Lipid Rafts

Nucleotides released during heart injury affect myocardium electrophysiology and remodeling through P2 purinoceptors activation in cardiac myofibroblasts. ATP and UTP endorse [Ca²⁺ ]_i accumulation and growth of DDR-2/α-SMA-expressing myofibroblasts from adult rat ventricles via P2Y₄ and P2Y₂ receptors activation, respectively. Ventricular myofibroblasts also express ADP-sensitive P2Y₁ , P2Y₁₂ , and P2Y₁₃ receptors as demonstrated by immunofluorescence confocal microscopy and western blot analysis, but little information exists on ADP effects in these cells. ADP (0.003-3 mM) and its stable analogue, ADPßS (100 μM), caused fast [Ca²⁺ ]_i transients originated from thapsigargin-sensitive internal stores, which partially declined to a plateau sustained by capacitative Ca²⁺ entry through transient receptor potential (TRP) channels inhibited by 2-APB (50 μM) and flufenamic acid (100 μM). Hydrophobic interactions between G_q/11 -coupled P2Y purinoceptors and TRP channels were suggested by prevention of the ADP-induced [Ca²⁺ ]_i plateau following PIP₂ depletion with LiCl (10 mM) and cholesterol removal from lipid rafts with methyl-ß-cyclodextrin (2 mM). ADP [Ca²⁺ ]_i transients were insensitive to P2Y₁ , P2Y₁₂ , and P2Y₁₃ receptor antagonists, MRS2179 (10μM), AR-C66096 (0.1 μM), and MRS2211 (10μM), respectively, but were attenuated by suramin and reactive blue-2 (100 μM) which also blocked P2Y₄ receptors activation by UTP. Cardiac myofibroblasts growth and type I collagen production were favored upon activation of MRS2179-sensitive P2Y₁ receptors with ADP or ADPßS (30 μM). In conclusion, ADP exerts a dual role on ventricular myofibroblasts: [Ca²⁺ ]_i transients are mediated by fast-desensitizing P2Y₄ receptors, whereas the pro-fibrotic effect of ADP involves the P2Y₁ receptor activation. Data also show that ADP-induced capacitative Ca²⁺ influx depends on phospholipase C-linked TRP channels opening in lipid raft microdomains. J. Cell. Physiol. 232: 1511-1526, 2017. © 2016 Wiley Periodicals, Inc.

Sodium-Calcium Exchanger in Pig Coronary Artery

This review focuses on the sodium-calcium exchangers (NCX) in the left anterior descending coronary artery smooth muscle. Bathing tissues in Na⁺-substituted solutions caused them to contract. In cultured smooth muscle cells, it increased the cytosolic Ca²⁺ concentration and extracellular entry of ⁴⁵Ca²⁺. All three activities were attributed to NCX since they were inhibited by NCX inhibitors. The tissues also expressed the sarco/endoplasmic reticulum (SER) Ca²⁺ pump SERCA2b whose activity was much greater than that of NCX. Inhibiting SERCA2b with thapsigargin decreased the NCX-mediated ⁴⁵Ca²⁺ accumulation by the cells. The decrease was not observed in cells loaded with the Ca²⁺-chelator BAPTA. The results are consistent with a limited diffusional space model with a proximity between NCX and SERCA2b. NCX molecules appear to be colocalized with the subsarcolemmal SERCA2b based on studies on membrane flotation experiments and microscopic fluorescence imaging of antibody-labeled cells. Thapsigargin inhibition of SERCA2b moved NCX even closer to SER. This provides a model for the NCX-mediated Ca²⁺ refilling of SER in the arterial smooth muscle. The model for the NCX-mediated refilling of the depleted SER proposed for smooth muscle did not apply to endothelium in which NCX levels were greater and SERCA levels were lower than in smooth muscle. The effect of thapsigargin on the NCX-mediated Ca²⁺ accumulation which was observed in smooth muscle was absent in the endothelium. We propose that the coupling between NCX and smooth muscle may be tissue dependent.

Cardioprotection in ischaemia-reperfusion injury: novel mechanisms and clinical translation

In recent decades, robust successes have been achieved in conquering the acutely lethal manifestations of heart disease. Nevertheless, the prevalence of heart disease, especially heart failure, continues to rise. Among the precipitating aetiologies, ischaemic disease is a leading cause of heart failure. In the context of ischaemia, the myocardium is deprived of oxygen and nutrients, which elicits a cascade of events that provokes cell death. This ischaemic insult is typically coupled with reperfusion, either spontaneous or therapeutically imposed, wherein blood supply is restored to the previously ischaemic tissue. While this intervention limits ischaemic injury, it triggers a new cascade of events that is also harmful, viz. reperfusion injury. In recent years, novel insights have emerged regarding mechanisms of ischaemia-reperfusion injury, and some hold promise as targets of therapeutic relevance. Here, we review a select number of these pathways, focusing on recent discoveries and highlighting prospects for therapeutic manipulation for clinical benefit.

Echinochrome A regulates phosphorylation of phospholamban Ser16 and Thr17 suppressing cardiac SERCA2A Ca²⁺ reuptake

Echinochrome A (Ech A), a marine bio-product isolated from sea urchin eggs, is known to have cardioprotective effects through its strong antioxidant and ATP-sparing capabilities. However, the effects of Ech A on cardiac excitation-contraction (E-C) are not known. In this study, we investigated the effects of Ech A on cardiac contractility and Ca(2+) handling in the rat heart. In ex vivo Langendorff hearts, Ech A (3 μM) decreased left ventricular developing pressure to 77.7 ± 6.5 % of basal level. In isolated ventricular myocytes, Ech A reduced the fractional cell shortening from 3.4 % at baseline to 2.1 %. Ech A increased both diastolic and peak systolic intracellular Ca(2+) ([Ca(2+)]i). However, the ratio of peak [Ca]i to resting [Ca]i was significantly decreased. Ech A did not affect the L-type Ca(2+) current. Inhibiting the Na(+)/Ca(2+) exchanger with either NiCl2 or SEA400 did not affect the Ech A-dependent changes in Ca(2+) handling. Our data demonstrate that treatment with Ech A results in a significant reduction in the phosphorylation of phospholamban at both serine 16 and threonine 17 leading to a significant inhibition of SR Ca(2+)-ATPase 2A (SERCA2A) and subsequent reduced Ca(2+) uptake into the intracellular Ca(2+) store. Taken together, our data show that Ech A negatively regulates cardiac contractility by inhibiting SERCA2A activity, which leads to a reduction in internal Ca(2+) stores.

Novel therapeutic strategies for ischemic heart disease

Despite significant advances in the physician's ability to initiate myocardial reperfusion and salvage heart tissue, ischemic heart disease remains one of the leading causes of death in the United States. Consequently, alternative therapeutic strategies have been intensively investigated, especially methods of enhancing the heart's resistance to ischemic cell death - so called "cardioprotective" interventions. However, although a great deal has been learned regarding the methods and mechanisms of cardioprotective interventions, an efficacious therapy has yet to be successfully implemented in the clinical arena. This review discusses the current understanding of cardioprotection in the context of ischemic heart disease pathophysiology, highlighting those elements of ischemic heart disease pathophysiology that have received less attention as potential targets of cardioprotective intervention.

Inhibition of the cardiac ATP-dependent potassium current by KB-R7943

KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea) was developed as a specific inhibitor of the sarcolemmal sodium-calcium exchanger (NCX) with potential experimental and therapeutic use. However, in cardiomyocytes KB-R7943 also effectively blocks several K(+) currents including the delayed rectifier, IKr, and background inward rectifier, IK1. In the present study we analyze the effects of KB-R7943 on the ATP-dependent potassium current (IKATP) recorded by whole-cell patch-clamp in ventricular cardiomyocytes from a mammal (mouse) and a fish (crucian carp). IKATP was induced by external application of a mitochondrial uncoupler CCCP (3×10(-7) M) and internal perfusion of the cell with ATP-free pipette solution. A weakly inwardly rectifying current with a large outward component, recorded in the presence of CCCP, was blocked with 10(-5) M glibenclamide by 56.1±4.6% and 56.9±3.6% in crucian carp and mouse ventricular myocytes, respectively. In fish cardiomyocytes IKATP was blocked by KB-R7943 with an IC50 value of 3.14×10(-7) M, while in mammalian cells IC50 was 2.8×10(-6) M (P<0.05). 10(-5) M KB-R7943 inhibited CCCP-induced IKATP by 99.9±0.13% and 97.5±1.2% in crucian carp and mouse ventricular myocytes, respectively. In crucian carp the IKATP is about an order of magnitude more sensitive to KB-R7943 than the background IK1, but in mammals IKATP and IK1 are almost equally sensitive to KB-R7943. Therefore, the ability of KB-R7943 to block IKATP should be taken into account together with INCX inhibition when investigating possible cardioprotective effects of this compound.

Intracellular Ca(2+) Transient Phase II Can be Assessed by Half-Logistic Function Model in Isolated Aequorin-Injected Mouse Left Ventricular Papillary Muscle

BACKGROUND

Myocardial contraction and relaxation are regulated by increases and decreases in intracellular cytoplasmic calcium (Ca(2+)) concentration ([Ca(2+)]i). In previous studies, we found that a half-logistic (h-L) function, which represents a half-curve of a symmetrical sigmoid logistic function with a boundary at the inflection point, curve-fits the first half of the ascending phase (CaTI) and the second half of the descending phase of the [Ca(2+)]i transient curve (CaTIV) better than a mono-exponential (m-E) function. In the present study, we investigated the potential application of an h-L function to the analysis of the second half of the ascending phase of the [Ca(2+)]i transient curve (CaTII).

METHODS

The [Ca(2+)]i transient was measured using the Ca(2+)-sensitive photoprotein aequorin, which was microinjected into 15 isolated left ventricular (LV) papillary muscles of mice. The observed CaTII data during the time duration from the point corresponding to the maximum of the first-order time derivative of Ca(2+) concentration (dCa/dtmax) to the point corresponding to the peak Ca(2+) concentration was curve-fitted by the least-squares method using the h-L and m-E function equations.

RESULTS

The mean correlation coefficient (r) values of the h-L and m-E curve-fits for CaTII were 0.9996 and 0.9984, respectively. The Z transformation of h-L r was larger than that of m-E r (p < 0.0001). H-L residual mean square (RMS) was smaller than m-E RMS (p < 0.001).

CONCLUSIONS

The h-L function tracks the magnitudes and time courses of CaTII more accurately than the m-E function in isolated aequorin-injected mouse LV papillary muscle. Compared with the m-E time constant, the h-L time constant of CaTII is a more reliable index for evaluating the time duration of the change in the increase in [Ca(2+)]i during the combination of the middle part of the contraction process and the early part of the relaxation process. CaTII can be assessed by the h-L function model in cardiac muscles. The h-L approach may provide a more useful model for studying each process in myocardial Ca(2+) handling.

KEY WORDS

Calcium handling; Calcium transient; Curve-fit; Half-Logistic function; Time constant.