Intracellular Ca2+, intercellular electrical coupling, and mechanical activity in ischemic rabbit papillary muscle. Effects of preconditioning and metabolic blockade

Calcium Role in Gap Junction Channel Gating: Direct Electrostatic or Calmodulin-Mediated?

The chemical gating of gap junction channels is mediated by cytosolic calcium (Ca2+i) at concentrations ([Ca2+]i) ranging from high nanomolar (nM) to low micromolar (µM) range. Since the proteins of gap junctions, connexins/innexins, lack high-affinity Ca2+-binding sites, most likely gating is mediated by a Ca2+-binding protein, calmodulin (CaM) being the best candidate. Indeed, the role of Ca2+-CaM in gating is well supported by studies that have tested CaM blockers, CaM expression inhibition, testing of CaM mutants, co-localization of CaM and connexins, existence of CaM-binding sites in connexins/innexins, and expression of connexins (Cx) mutants, among others. Based on these data, since 2000, we have published a Ca2+-CaM-cork gating model. Despite convincing evidence for the Ca2+-CaM role in gating, a recent study has proposed an alternative gating model that would involve a direct electrostatic Ca2+-connexin interaction. However, this study, which tested the effect of unphysiologically high [Ca2+]i on the structure of isolated junctions, reported that neither changes in the channel's pore diameter nor connexin conformational changes are present, in spite of exposure of isolated gap junctions to [Ca2+]i as high at the 20 mM. In conclusion, data generated in the past four decades by multiple experimental approaches have clearly demonstrated the direct role of Ca2+-CaM in gap junction channel gating.

Novel 2D/3D Hybrid Organoid System for High-Throughput Drug Screening in iPSC Cardiomyocytes

Human induced pluripotent stem cell cardiomyocytes (hiPSC-CMs) allow for high-throughput evaluation of cardiomyocyte (CM) physiology in health and disease. While multimodality testing provides a large breadth of information related to electrophysiology, contractility, and intracellular signaling in small populations of iPSC-CMs, current technologies for analyzing these parameters are expensive and resource-intensive. We sought to design a 2D/3D hybrid organoid system and harness optical imaging techniques to assess electromechanical properties, calcium dynamics, and signal propagation across CMs in a high-throughput manner. We validated our methods using a doxorubicin-based system, as the drug has well-characterized cardiotoxic, pro-arrhythmic effects. hiPSCs were differentiated into CMs, assembled into organoids, and thereafter treated with doxorubicin. The organoids were then replated to form a hybrid 2D/3D iPSC-CM construct where the 3D cardiac organoids acted as the source of electromechanical activity which propagated outwards into a 2D iPSC-CM sheet. The organoid recapitulated cardiac structure and connectivity, while 2D CMs facilitated analysis at an individual cellular level which recreated numerous doxorubicin-induced electrophysiologic and propagation abnormalities. Thus, we have developed a novel 2D/3D hybrid organoid model that employs an integrated optical analysis platform to provide a reliable high-throughput method for studying cardiotoxicity, providing valuable data on calcium, contractility, and signal propagation.

Gap Junction Channel Regulation: A Tale of Two Gates-Voltage Sensitivity of the Chemical Gate and Chemical Sensitivity of the Fast Voltage Gate

Gap junction channels are regulated by gates sensitive to cytosolic acidification and trans-junctional voltage (Vj). We propose that the chemical gate is a calmodulin (CaM) lobe. The fast-Vj gate is made primarily by the connexin's NH2-terminus domain (NT). The chemical gate closes the channel slowly and completely, while the fast-Vj gate closes the channel rapidly but incompletely. The chemical gate closes with increased cytosolic calcium concentration [Ca2+]i and with Vj gradients at Vj's negative side. In contrast, the fast-Vj gate closes at the positive or negative side of Vj depending on the connexin (Cx) type. Cxs with positively charged NT close at Vj's negative side, while those with negatively charged NT close at Vj's positive side. Cytosolic acidification alters in opposite ways the sensitivity of the fast-Vj gate: it increases the Vj sensitivity of negative gaters and decreases that of positive gaters. While the fast-Vj gate closes and opens instantaneously, the chemical gate often shows fluctuations, likely to reflect the shifting of the gate (CaM's N-lobe) in and out of the channel's pore.

The role of miR1 and miR133a in new-onset atrial fibrillation after acute myocardial infarction

BACKGROUND

The development of new-onset atrial fibrillation (NOAF) after acute myocardial infarction (AMI) is a clinical complication that requires a better understanding of the causative risk factors. This study aimed to explore the risk factors and the expression and function of miR-1 and miR-133a in new atrial fibrillation after AMI.

METHODS

We collected clinical data from 172 patients with AMI treated with emergency percutaneous coronary intervention (PCI) between October 2021 and October 2022. Independent predictors of NOAF were determined using binary logistic univariate and multivariate regression analyses. The predictive value of NOAF was assessed using the area under the receiver operating characteristic (ROC) curve for related risk factors. In total, 172 venous blood samples were collected preoperatively and on the first day postoperatively; the expression levels of miR-1 and miR-133a were determined using the polymerase chain reaction. The clinical significance of miR-1 and miR-133a expression levels was determined by Spearman correlation analysis.

RESULTS

The Glasgow prognostic score, left atrial diameter, and infarct area were significant independent risk factors for NOAF after AMI. We observed that the expression levels of miR-1 and miR-133a were significantly higher in the NOAF group than in the non-NOAF group. On postoperative day 1, strong associations were found between miR-133a expression levels and the neutrophil ratio and between miR-1 expression levels and an increased left atrial diameter.

CONCLUSIONS

Our findings indicate that the mechanism of NOAF after AMI may include an inflammatory response associated with an increased miR-1-related mechanism. Conversely, miR-133a could play a protective role in this clinical condition.

From evidence-based medicine to digital twin technology for predicting ventricular tachycardia in ischaemic cardiomyopathy

Survivors of myocardial infarction are at risk of life-threatening ventricular tachycardias (VTs) later in their lives. Current guidelines for implantable cardioverter defibrillators (ICDs) implantation to prevent VT-related sudden cardiac death is solely based on symptoms and left ventricular ejection fraction. Catheter ablation of scar-related VTs is performed following ICD therapy, reducing VTs, painful shocks, anxiety, depression and worsening heart failure. We postulate that better prediction of the occurrence and circuit of VT, will improve identification of patients at risk for VT and boost preventive ablation, reducing mortality and morbidity. For this purpose, multiple time-evolving aspects of the underlying pathophysiology, including the anatomical substrate, triggers and modulators, should be part of VT prediction models. We envision digital twins as a solution combining clinical expertise with three prediction approaches: evidence-based medicine (clinical practice), data-driven models (data science) and mechanistic models (biomedical engineering). This paper aims to create a mutual understanding between experts in the different fields by providing a comprehensive description of the clinical problem and the three approaches in an understandable manner, leveraging future collaborations and technological innovations for clinical decision support. Moreover, it defines open challenges and gains for digital twin solutions and discusses the potential of hybrid modelling.

Calmodulin-Connexin Partnership in Gap Junction Channel Regulation-Calmodulin-Cork Gating Model

In the past four decades numerous findings have indicated that gap junction channel gating is mediated by intracellular calcium concentrations ([Ca²⁺_i]) in the high nanomolar range via calmodulin (CaM). We have proposed a CaM-based gating model based on evidence for a direct CaM role in gating. This model is based on the following: CaM inhibitors and the inhibition of CaM expression to prevent chemical gating. A CaM mutant with higher Ca²⁺ sensitivity greatly increases gating sensitivity. CaM co-localizes with connexins. Connexins have high-affinity CaM-binding sites. Connexin mutants paired to wild type connexins have a higher gating sensitivity, which is eliminated by the inhibition of CaM expression. Repeated trans-junctional voltage (Vj) pulses progressively close channels by the chemical/slow gate (CaM’s N-lobe). At the single channel level, the gate closes and opens slowly with on-off fluctuations. Internally perfused crayfish axons lose gating competency but recover it by the addition of Ca-CaM to the internal perfusion solution. X-ray diffraction data demonstrate that isolated gap junctions are gated at the cytoplasmic end by a particle of the size of a CaM lobe. We have proposed two types of CaM-driven gating: “Ca-CaM-Cork” and “CaM-Cork”. In the first, the gating involves Ca²⁺-induced CaM activation. In the second, the gating occurs without a [Ca²⁺]_i rise.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

Prolonged ursodeoxycholic acid administration reduces acute ischaemia-induced arrhythmias in adult rat hearts

Acute myocardial ischaemia and reperfusion (I-R) are major causes of ventricular arrhythmias in patients with a history of coronary artery disease. Ursodeoxycholic acid (UDCA) has previously been shown to be antiarrhythmic in fetal hearts. This study was performed to investigate if UDCA protects against ischaemia-induced and reperfusion-induced arrhythmias in the adult myocardium, and compares the effect of acute (perfusion only) versus prolonged (2 weeks pre-treatment plus perfusion) UDCA administration. Langendorff-perfused adult Sprague-Dawley rat hearts were subjected to acute regional ischaemia by ligation of the left anterior descending artery (10 min), followed by reperfusion (2 min), and arrhythmia incidence quantified. Prolonged UDCA administration reduced the incidence of acute ischaemia-induced arrhythmias (p = 0.028), with a reduction in number of ventricular ectopic beats during the ischaemic phase compared with acute treatment (10 ± 3 vs 58 ± 15, p = 0.036). No antiarrhythmic effect was observed in the acute UDCA administration group. Neither acute nor prolonged UDCA treatment altered the incidence of reperfusion arrhythmias. The antiarrhythmic effect of UDCA may be partially mediated by an increase in cardiac wavelength, due to the attenuation of conduction velocity slowing (p = 0.03), and the preservation of Connexin43 phosphorylation during acute ischaemia (p = 0.0027). The potential antiarrhythmic effects of prolonged UDCA administration merit further investigation.

Calmodulin-Cork Model of Gap Junction Channel Gating-One Molecule, Two Mechanisms

The Calmodulin-Cork gating model is based on evidence for the direct role of calmodulin (CaM) in channel gating. Indeed, chemical gating of cell-to-cell channels is sensitive to nanomolar cytosolic calcium concentrations [Ca2+]i. Calmodulin inhibitors and inhibition of CaM expression prevent chemical gating. CaMCC, a CaM mutant with higher Ca2+-sensitivity greatly increases chemical gating sensitivity (in CaMCC the NH2-terminal EF-hand pair (res. 9-76) is replaced by the COOH-terminal pair (res. 82-148). Calmodulin colocalizes with connexins. Connexins have high-affinity CaM binding sites. Several connexin mutants paired to wild-type connexins have a high gating sensitivity that is eliminated by inhibition of CaM expression. Repeated transjunctional voltage (Vj) pulses slowly and progressively close a large number of channels by the chemical/slow gate (CaM lobe). At the single-channel level, the chemical/slow gate closes and opens slowly with on-off fluctuations. The model proposes two types of CaM-driven gating: "Ca-CaM-Cork" and "CaM-Cork". In the first, gating involves Ca2+-induced CaM-activation. In the second, gating takes place without [Ca2+]i rise. The Ca-CaM-Cork gating is only reversed by a return of [Ca2+]i to resting values, while the CaM-Cork gating is reversed by Vj positive at the gated side.

Tetrahydrobiopterin in energy metabolism and metabolic diseases

Tetrahydrobiopterin (BH4) is an endogenous cofactor for various enzymatic conversions of essential biomolecules including nitric oxide, tyrosine, dopamine, serotonin and phenylalanine. Depending on the physiological functions of these molecules, BH4 plays multiple roles in the cardiovascular, immune, nervous and endocrine systems. A deficiency of BH4 or an imbalance of the redox state of biopterin has been implicated in various cardiovascular and metabolic diseases. Therefore, supplementation with BH4 is considered as a therapeutic option for these diseases. In addition to the classical nitric oxide synthase (NOS)-dependent role of BH4, recent studies proposed novel NOS-independent roles of BH4 in health and disease conditions. This article reviews the updated role of BH4 in mitochondrial regulation, energy metabolism and cardiovascular and metabolic diseases.

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

Comparison of hemodynamics, cardiac electrophysiology, and ventricular arrhythmia in an open- and a closed-chest porcine model of acute myocardial infarction

Ventricular fibrillation (VF) during acute myocardial infarction (AMI) is an important contributor to sudden cardiac death. Large animal models are widely used to study AMI-induced arrhythmia, but the mode of AMI induction ranges from thoracotomy and surgical ligation of a coronary vessel (open chest) to minimally invasive techniques, including balloon occlusion (closed chest). How the choice of induction affects arrhythmia development is unclear. The aim of this study was to compare an open-chest and a closed-chest model with regard to hemodynamics, electrophysiology, and arrhythmia development. Forty-two female Danish Landrace pigs (20 open chest, 22 closed chest) were anesthetized, and occlusion of the mid-left anterior descending coronary artery was performed for 60 min. Opening the chest reduced blood pressure and cardiac output (Δ -22 mmHg, Δ -1.5 L/min from baseline, both P < 0.001 intragroup). Heart rate decreased with opening of the chest but increased with balloon placement (P < 0.001). AMI-induced ST elevation was lower in the open-chest group (P < 0.001). Premature ventricular contractions occurred in two distinct phases (0-15 and 15-40 min), the latter of which was delayed in the open-chest group (P = 0.005). VF occurred in 7 out of 20 and 12 out of 22 pigs in the open-chest and closed-chest groups, respectively (P = 0.337), with longer time-to-VF in the open-chest group (23.4 ± 1.2 min in open chest and 17.8 ± 1.4 min in closed chest; P = 0.007). In summary, opening the chest altered hemodynamic parameters and delayed the onset of ventricular arrhythmias. Hence, in the search for mechanisms and novel treatments of AMI-induced arrhythmia, caution should be taken when choosing between or comparing the results from these two models.NEW & NOTEWORTHY We demonstrated pronounced differences in hemodynamic parameters and time course of ventricular arrhythmias in regard to mode of infarct induction. Inducing myocardial infarction by thoracotomy and subsequent ligation decreased blood pressure and cardiac output and delayed the onset of ventricular arrhythmia, whereas balloon occlusion resulted in higher heart rates during infarct.

Single Bolus Rosuvastatin Accelerates Calcium Uptake and Attenuates Conduction Inhomogeneity in Failing Rabbit Hearts With Regional Ischemia-Reperfusion Injury

Acute statin therapy reduces myocardial ischemia/reperfusion (IR) injury-induced ventricular fibrillation (VF), but the underlying electrophysiological mechanisms remain unclear. This study sought to investigate the antiarrhythmic effects of a single bolus rosuvastatin injection in failing rabbit hearts with IR injury and to unveil the underlying molecular mechanisms. Rabbits were divided into rosuvastatin, rosuvastatin + L-NAME, control, and L-NAME groups. Intravenous bolus rosuvastatin (0.5 mg/kg) and/or L-NAME (10 mg/kg) injections were administered 1 hour and 15 minutes before surgery, respectively. Heart failure was induced using rapid ventricular pacing. Under general anesthesia with isoflurane, an IR model was created by coronary artery ligation for 30 minutes, followed by reperfusion for 15 minutes. Plasma NO end product levels were measured during IR. Then, hearts were excised and Langendorff-perfused for optical mapping studies. Cardiac tissues were sampled for Western blot analysis. Rosuvastatin increased plasma NO levels during IR, which was abrogated by L-NAME. Spontaneous VF during IR was suppressed by rosuvastatin (P < 0.001). Intracellular calcium (Cai) decay and conduction velocity were significantly slower in the IR zone. Rosuvastatin accelerated Cai decay, ameliorated conduction inhomogeneity, and reduced the inducibility of spatially discordant alternans and VF significantly. Western blots revealed significantly higher expression of enhancing endothelial NO-synthase and phosphorylated enhancing endothelial NO-synthase proteins in the Rosuvastatin group. Furthermore, SERCA2a, phosphorylated connexin43, and phosphorylated phospholamban were downregulated in the IR zone, which was attenuated or reversed by rosuvastatin. Acute rosuvastatin therapy before ischemia reduced IR-induced VF by improving SERCA2a function and ameliorating conduction disturbance in the IR zone.

Myocardial subcellular glycogen distribution and sarcoplasmic reticulum Ca2+ handling: effects of ischaemia, reperfusion and ischaemic preconditioning

Ischaemic preconditioning (IPC) protects against myocardial ischaemia-reperfusion injury. The metabolic and ionic effects of IPC remain to be clarified in detail. We aimed to investigate the effect of IPC (2 times 5 min ischaemia) on the subcellular distribution of glycogen and Ca²⁺-uptake and leakiness by the sarcoplasmic reticulum (SR) in response to ischaemia-reperfusion in cardiomyocytes of isolated perfused rat hearts (Wistar rats, 335 ± 25 g). As estimated by quantitative transmission electron microscopy, the pre-ischaemic contribution [%, mean (95% CI)] of three sub-fractions of glycogen relative to total glycogen was 50 (39:61) as subsarcolemmal, 41 (31:50) as intermyofibrillar, and 9 (5:13) as intramyofibrillar glycogen. After 25 min of ischaemia, the relative contribution (%) of subsarcolemmal glycogen decreased to 39 (32:47) in control hearts (Con) and to 38 (31:45) in IPC. After 15 min reperfusion the contribution of subsarcolemmal glycogen was restored to pre-ischaemic levels in IPC hearts, but not in Con hearts. IPC increased the left ventricular developed pressure following ischaemia-reperfusion compared with Con. In saponin-skinned cardiomyocyte bundles, ischaemia reduced the SR Ca²⁺-uptake rate, with no effect of IPC. However, IPC reduced a SR Ca²⁺-leakage at pre-ischaemia, after ischaemia and during reperfusion. In conclusion, subsarcolemmal glycogen was preferentially utilised during sustained myocardial ischaemia. IPC improved left ventricular function reflecting reduced ischaemia-reperfusion injury, mediated a re-distribution of glycogen towards a preferential storage within the subsarcolemmal space during reperfusion, and lowered SR Ca²⁺-leakage. Under the present conditions, we found no temporal associations between alterations in glycogen localisation and SR Ca²⁺ kinetics.

Reperfusion injury in ST-segment elevation myocardial infarction: the final frontier

ST-segment elevation myocardial infarction is a major cause of morbidity and mortality worldwide. Reperfusion injury (RI) following the opening of an occluded coronary artery mitigates the effect of reperfusion by further accentuating ischemic damage and increasing infarct size. Experimental studies have shown that nearly 50% of final infarct size is attributable to RI, an elusive phenomenon that remains resistant to treatment. This review proposes a hypothesis to explain the failure of strategies that have been used in an attempt to prevent RI. This hypothesis suggests that, after a certain duration of myocardial ischemia in the affected myocardium, three phases of myocardial damage occur: reversible ischemia, irreversible ischemia, and necrosis. In the reversible ischemia phase, cellular adaptive responses remain functional, and cellular repair and thus recovery of cellular functions is possible, whereas in the irreversible ischemia phase protective maneuvers fail to confer cytoprotection. Preventive therapies for RI fail because they cannot prevent cell death once cells have entered the irreversible ischemia phase, although they may succeed in postponing cell death. Failure to salvage myocardium with irreversible ischemia in addition to postponement and change in the mode of cell death (mainly from necrosis to apoptosis) by various RI preventive strategies may be the key to understanding the failure of these strategies in the clinical setting, despite their success in the reduction of infarct size in the experimental setting. Early reperfusion before large amounts of myocardium at risk reach the stage of irreversible ischemia is the best strategy for reduction of RI-related myocardial damage.

Connexins in Cardiovascular and Neurovascular Health and Disease: Pharmacological Implications

Connexins are ubiquitous channel forming proteins that assemble as plasma membrane hemichannels and as intercellular gap junction channels that directly connect cells. In the heart, gap junction channels electrically connect myocytes and specialized conductive tissues to coordinate the atrial and ventricular contraction/relaxation cycles and pump function. In blood vessels, these channels facilitate long-distance endothelial cell communication, synchronize smooth muscle cell contraction, and support endothelial-smooth muscle cell communication. In the central nervous system they form cellular syncytia and coordinate neural function. Gap junction channels are normally open and hemichannels are normally closed, but pathologic conditions may restrict gap junction communication and promote hemichannel opening, thereby disturbing a delicate cellular communication balance. Until recently, most connexin-targeting agents exhibited little specificity and several off-target effects. Recent work with peptide-based approaches has demonstrated improved specificity and opened avenues for a more rational approach toward independently modulating the function of gap junctions and hemichannels. We here review the role of connexins and their channels in cardiovascular and neurovascular health and disease, focusing on crucial regulatory aspects and identification of potential targets to modify their function. We conclude that peptide-based investigations have raised several new opportunities for interfering with connexins and their channels that may soon allow preservation of gap junction communication, inhibition of hemichannel opening, and mitigation of inflammatory signaling.

Role of astrocyte connexin hemichannels in cortical spreading depression

Cortical spreading depression (CSD) is an intriguing phenomenon consisting of massive slow brain depolarizations that affects neurons and glial cells. It has been recognized since 1944, but its pathogenesis has only been uncovered during the last decade. Acute brain injuries can be further complicated by CSD in >50% of severe cases. This phenomenon is repetitive and produces a metabolic overload that increments secondary damage. Propagation of CSD is known to be linked to excitotoxicity, but the mechanisms associated with its initiation remain less understood. It has been shown that CSD can be initiated by increases in extracellular [K⁺] ([K⁺]_e), and animal models use high [K⁺]_e to promote CSD. Connexin hemichannel activity increases due to high [K⁺]_e and low extracellular [Ca²⁺], conditions that occur after brain injury. Moreover, glial cell gap junction channels are fundamental in controlling extracellular medium composition, particularly in maintaining normal extracellular glutamate and K⁺ concentrations through "spatial buffering". However, the role of astrocytic gap junctions under tissue stress can change to damage spread in the acute damage zone whereas the reduced communication in adjacent zone would reduce cell dead propagation. Here, we review the main findings associated with CSD, and discuss the possible involvement of astrocytic connexin-based channels in secondary damage propagation. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Ischemic Preconditioning and Repeated Sprint Swimming: A Placebo and Nocebo Study

PURPOSE

Ischemic preconditioning (IPC) has been shown to improve performance of exercises lasting 10-90 s (anaerobic) and more than 90 s (aerobic). However, its effect on repeated sprint performance has been controversial, placebo effect has not been adequately controlled, and nocebo effect has not been avoided. Thus, the IPC effect on repeated sprint performance was investigated using a swimming task and controlling placebo/nocebo effects.

METHODS

Short-distance university swimmers were randomized to two groups. One group (n = 15, 24 ± 1 yr [mean ± SEM]) was exposed to IPC (ischemia cycles lasted 5 min) and control (CT) (no ischemia); another (n = 15, 24 ± 1 yr) to a placebo intervention (SHAM) (ischemia cycles lasted 1 min) and CT. Seven subjects crossed over groups. Subjects were informed IPC and SHAM would improve performance compared with CT and would be harmless despite circulatory occlusion sensations. The swimming task consisted of six 50-m all-out efforts repeated every 3 min.

RESULTS

IPC, in contrast with SHAM, reduced worst sprint time (IPC, 35.21 ± 0.73 vs CT, 36.53 ± 0.72 s; P = 0.04) and total sprints time (IPC, 203.7 ± 4.60 vs CT, 206.03 ± 4.57 s; P = 0.02), moreover augmented swimming velocity (IPC, 1.45 ± 0.03 vs CT, 1.44 ± 0.03 m·s; P = 0.049). Six of seven subjects who crossed over groups reduced total sprints time with IPC versus SHAM (delta = -3.95 ± 1.49 s, P = 0.09). Both IPC and SHAM did not change blood lactate concentration (P = 0.20) and perceived effort (P = 0.22).

CONCLUSION

IPC enhanced repeated sprint swimming performance in university swimmers, whereas a placebo intervention did not.

Beat-to-beat variability of cardiac action potential duration: underlying mechanism and clinical implications

Beat-to-beat variability of cardiac action potential duration (short-term variability, SV) is a common feature of various cardiac preparations, including the human heart. Although it is believed to be one of the best arrhythmia predictors, the underlying mechanisms are not fully understood at present. The magnitude of SV is basically determined by the intensity of cell-to-cell coupling in multicellular preparations and by the duration of the action potential (APD). To compensate for the APD-dependent nature of SV, the concept of relative SV (RSV) has been introduced by normalizing the changes of SV to the concomitant changes in APD. RSV is reduced by I_Ca, I_Kr, and I_Ks while increased by I_Na, suggesting that ion currents involved in the negative feedback regulation of APD tend to keep RSV at a low level. RSV is also influenced by intracellular calcium concentration and tissue redox potential. The clinical implications of APD variability is discussed in detail.

Mechanisms underpinning protection against eccentric exercise-induced muscle damage by ischemic preconditioning

Eccentric exercise training is effective for increasing muscle mass and strength, and improving insulin sensitivity and blood lipid profiles. However, potential muscle damage symptoms such as prolonged loss of muscle function and delayed onset of muscle soreness may restrict the use of eccentric exercise, especially in clinical populations. Therefore, strategies to reduce eccentric exercise-induced muscle damage (EIMD) are necessary, and an extensive number of scientific studies have tried to identify potential intervention modalities to perform eccentric exercises without adverse effects. The present paper is based on a narrative review of current literature, and provides a novel hypothesis by which an ischemic preconditioning (IPC) of the extremities may reduce EIMD. IPC consists of an intermittent application of short-time non-lethal ischemia to an extremity (e.g. using a tourniquet) followed by reperfusion and was discovered in clinical settings in an attempt to minimize inflammatory responses induced by ischemia and ischemia-reperfusion-injury (I/R-Injury) during surgery. The present hypothesis is based on morphological and biochemical similarities in the pathophysiology of skeletal muscle damage during clinical surgery and EIMD. Even though the primary origin of stress differs between I/R-Injury and EIMD, subsequent cellular alterations characterized by an intracellular accumulation of Ca²⁺, an increased production of reactive oxygen species or increased apoptotic signaling are essential elements for both. Moreover, the incipient immune response appears to be similar in I/R-Injury and EIMD, which is indicated by an infiltration of leukocytes into the damaged soft-tissue. Thus far, IPC is considered as a potential intervention strategy in the area of cardiovascular or orthopedic surgery and provides significant impact on soft-tissue protection and downregulation of undesired excessive inflammation induced by I/R-Injury. Based on the known major impact of IPC on skeletal muscle physiology and immunology, the present paper aims to illustrate the potential protective effects of IPC on EIMD by discussing possible underlying mechanisms.

Connexin 43 is an emerging therapeutic target in ischemia/reperfusion injury, cardioprotection and neuroprotection

Connexins are widely distributed proteins in the body that are crucially important for heart and brain functions. Six connexin subunits form a connexon or hemichannel in the plasma membrane. Interactions between two hemichannels in a head-to-head arrangement result in the formation of a gap junction channel. Gap junctions are necessary to coordinate cell function by passing electrical current flow between heart and nerve cells or by allowing exchange of chemical signals and energy substrates. Apart from its localization at the sarcolemma of cardiomyocytes and brain cells, connexins are also found in the mitochondria where they are involved in the regulation of mitochondrial matrix ion fluxes and respiration. Connexin expression is affected by age and gender as well as several pathophysiological alterations such as hypertension, hypertrophy, diabetes, hypercholesterolemia, ischemia, post-myocardial infarction remodeling or heart failure, and post-translationally connexins are modified by phosphorylation/de-phosphorylation and nitros(yl)ation which can modulate channel activity. Using knockout/knockin technology as well as pharmacological approaches, one of the connexins, namely connexin 43, has been identified to be important for cardiac and brain ischemia/reperfusion injuries as well as protection from it. Therefore, the current review will focus on the importance of connexin 43 for irreversible injury of heart and brain tissues following ischemia/reperfusion and will highlight the importance of connexin 43 as an emerging therapeutic target in cardio- and neuroprotection.

Myocardial electrotonic response to submaximal exercise in dogs with healed myocardial infarctions: evidence for β-adrenoceptor mediated enhanced coupling during exercise testing

INTRODUCTION

Autonomic neural activation during cardiac stress testing is an established risk-stratification tool in post-myocardial infarction (MI) patients. However, autonomic activation can also modulate myocardial electrotonic coupling, a known factor to contribute to the genesis of arrhythmias. The present study tested the hypothesis that exercise-induced autonomic neural activation modulates electrotonic coupling (as measured by myocardial electrical impedance, MEI) in post-MI animals shown to be susceptible or resistant to ventricular fibrillation (VF).

METHODS

Dogs (n = 25) with healed MI instrumented for MEI measurements were trained to run on a treadmill and classified based on their susceptibility to VF (12 susceptible, 9 resistant). MEI and ECGs were recorded during 6-stage exercise tests (18 min/test; peak: 6.4 km/h @ 16%) performed under control conditions, and following complete β-adrenoceptor (β-AR) blockade (propranolol); MEI was also measured at rest during escalating β-AR stimulation (isoproterenol) or overdrive-pacing.

RESULTS

Exercise progressively increased heart rate (HR) and reduced heart rate variability (HRV). In parallel, MEI decreased gradually (enhanced electrotonic coupling) with exercise; at peak exercise, MEI was reduced by 5.3 ± 0.4% (or -23 ± 1.8Ω, P < 0.001). Notably, exercise-mediated electrotonic changes were linearly predicted by the degree of autonomic activation, as indicated by changes in either HR or in HRV (P < 0.001). Indeed, β-AR blockade attenuated the MEI response to exercise while direct β-AR stimulation (at rest) triggered MEI decreases comparable to those observed during exercise; ventricular pacing had no significant effects on MEI. Finally, animals prone to VF had a significantly larger MEI response to exercise.

CONCLUSIONS

These data suggest that β-AR activation during exercise can acutely enhance electrotonic coupling in the myocardium, particularly in dogs susceptible to ischemia-induced VF.

Determinants of myocardial conduction velocity: implications for arrhythmogenesis

Slowed myocardial conduction velocity (θ) is associated with an increased risk of re-entrant excitation, predisposing to cardiac arrhythmia. θ is determined by the ion channel and physical properties of cardiac myocytes and by their interconnections. Thus, θ is closely related to the maximum rate of action potential (AP) depolarization [(dV/dt)_max], as determined by the fast Na⁺ current (I_Na); the axial resistance (r_a) to local circuit current flow between cells; their membrane capacitances (c_m); and to the geometrical relationship between successive myocytes within cardiac tissue. These determinants are altered by a wide range of pathophysiological conditions. Firstly, I_Na is reduced by the impaired Na⁺ channel function that arises clinically during heart failure, ischemia, tachycardia, and following treatment with class I antiarrhythmic drugs. Such reductions also arise as a consequence of mutations in SCN5A such as those occurring in Lenègre disease, Brugada syndrome (BrS), sick sinus syndrome, and atrial fibrillation (AF). Secondly, r_a, may be increased due to gap junction decoupling following ischemia, ventricular hypertrophy, and heart failure, or as a result of mutations in CJA5 found in idiopathic AF and atrial standstill. Finally, either r_a or c_m could potentially be altered by fibrotic change through the resultant decoupling of myocyte–myocyte connections and coupling of myocytes with fibroblasts. Such changes are observed in myocardial infarction and cardiomyopathy or following mutations in MHC403 and SCN5A resulting in hypertrophic cardiomyopathy (HCM) or Lenègre disease, respectively. This review defines and quantifies the determinants of θ and summarizes experimental evidence that links changes in these determinants with reduced myocardial θ and arrhythmogenesis. It thereby identifies the diverse pathophysiological conditions in which abnormal θ may contribute to arrhythmia.

The Effects of Heptanol on Electrical Coupling during Ischemia in the Perfused Isolated Rat Heart

The aims of the present study were to examine the effect of heptanol on electrical coupling during ischemia, and to assess whether changes in electrical coupling by heptanol is associated with its cardiac protection. Perfused isolated rat hearts were subjected to a 24 min infusion of heptanol (0.05, 0.1, 0.5 or 1.0 mmol/L) followed by 70 min of global no-flow ischemia or by 20 min of regional ischemia and 60 min of reperfusion. Heptanol markedly decreased arrhythmia scores during ischemia and reperfusion as well as reduced infarct size to a degree similar to that induced by ischemic preconditioning. In the prolonged ischemia model, heptanol delayed the onset of uncoupling, increased time to plateau, and decreased the maximal rate of uncoupling during ischemia. Ischemic preconditioning had similar effects on these parameters. These results demonstrate that treatment with the gap junction uncoupler heptanol confers cardioprotection against ischemia, and this effect is related to delayed electrical uncoupling during prolonged ischemia.

Desipramine prevents cardiac gap junction uncoupling

BACKGROUND AND PURPOSE

Uncoupling of cardiac gap junction channels is an important arrhythmogenic mechanism in ischemia/reperfusion. Antiarrhythmic peptide AAP10 (H-Gly-Ala-Gly-Hyp-Pro-Tyr-CONH(2)) has been shown to prevent acidosis-induced uncoupling and ischemia-related increase in dispersion. Previous structure-effect investigations and subsequent computer modeling studies indicated that the tricyclic antidepressant desipramine may exert similar effects as AAP10.

METHODS

We assessed the binding of (14)C-AAP10 to membranes of rabbit cardiac ventricles and its displacement with desipramine in a classical radioligand binding and competition study. Gap junction currents were measured between isolated pairs of human atrial cardiomyocytes under normal and acidotic (pH 6.3) conditions with or without 1 μmol/l desipramine using dual whole-cell voltage clamp. The effect of 1 μmol/l desipramine was assessed in isolated rabbit hearts (Langendorff technique) undergoing local ischemia by coronary occlusion with 256-channel electrophysiological mapping and subsequent analysis of connexin43 (Cx43) expression, phosphorylation (Western blot), and subcellular localization (immunohistology).

RESULTS

We found saturable (14)C-AAP10 binding to cardiac membranes (K (D), 0.29 ± 0.11 nmol/l; B (max), 42.5 ± 7.2 pmol/mg) which could be displaced by desipramine with a K (D.High) = 0.14 μmol/l and a K (D.Low) = 22 μmol/l. Acidosis reduced the gap junction conductance in human cardiomyocyte pairs from 24.1 ± 4.7 to 11.5 ± 2.5 nS, which could be significantly reversed by desipramine (26.6 ± 4.8 nS). In isolated hearts, ischemia resulted in significantly increased dispersion of activation-recovery intervals, loss of membrane Cx43, and dephosphorylation of Cx43, which all could be prevented by desipramine.

CONCLUSION

Desipramine seems to prevent the uncoupling of cardiac gap junctions and ischemia-related increase in dispersion.

Gating of connexin 43 gap junctions by a cytoplasmic loop calmodulin binding domain

Calmodulin (CaM) binding sites were recently identified on the cytoplasmic loop (CL) of at least three α-subfamily connexins (Cx43, Cx44, Cx50), while Cx40 does not have this putative CaM binding domain. The purpose of this study was to examine the functional relevance of the putative Cx43 CaM binding site on the Ca(2+)-dependent regulation of gap junction proteins formed by Cx43 and Cx40. Dual whole cell patch-clamp experiments were performed on stable murine Neuro-2a cells expressing Cx43 or Cx40. Addition of ionomycin to increase external Ca(2+) influx reduced Cx43 gap junction conductance (G(j)) by 95%, while increasing cytosolic Ca(2+) concentration threefold. By contrast, Cx40 G(j) declined by <20%. The Ca(2+)-induced decline in Cx43 G(j) was prevented by pretreatment with calmidazolium or reversed by the addition of 10 mM EGTA to Ca(2+)-free extracellular solution, if Ca(2+) chelation was commenced before complete uncoupling, after which g(j) was only 60% recoverable. The Cx43 CL(136-158) mimetic peptide, but not the scrambled control peptide, or Ca(2+)/CaM-dependent kinase II 290-309 inhibitory peptide also prevented the Ca(2+)/CaM-dependent decline of Cx43 G(j). Cx43 gap junction channel open probability decreased to zero without reductions in the current amplitudes during external Ca(2+)/ionomycin perfusion. We conclude that Cx43 gap junctions are gated closed by a Ca(2+)/CaM-dependent mechanism involving the carboxyl-terminal quarter of the connexin CL domain. This study provides the first evidence of intrinsic differences in the Ca(2+) regulatory properties of Cx43 and Cx40.

Calcium-mediated cell death during myocardial reperfusion

Reperfusion may induce additional cell death in patients with acute myocardial infarction receiving primary angioplasty or thrombolysis. Altered intracellular Ca(2+) handling was initially considered an essential mechanism of reperfusion-induced cardiomyocyte death. However, more recent studies have demonstrated the importance of Ca(2+)-independent mechanisms that converge on mitochondrial permeability transition (MPT) and are shared by cardiomyocytes and other cell types. This article analyses the importance of Ca(2+)-dependent cell death in light of these new observations. Altered Ca(2+) handling includes increased cytosolic Ca(2+) levels, leading to activation of calpain-mediated proteolysis and sarcoplasmic reticulum-driven oscillations; this can induce hypercontracture, but also MPT due to the privileged Ca(2+) transfer between sarcoplasmic reticulum and mitochondria through cytosolic Ca(2+) microdomains. In the opposite direction, permeability transition can worsen altered Ca(2+) handling and favour hypercontracture. Ca(2+) appears to play an important role in cell death during the initial minutes of reperfusion, particularly after brief periods of ischaemia. Developing effective and safe treatments to prevent Ca(2+)-mediated cardiomyocyte death in patients with transient ischaemia, by targeting Ca(2+) influx, intracellular Ca(2+) handling, or Ca(2+)-induced cell death effectors, is an unmet challenge with important therapeutic implications and large potential clinical impact.

Loss of αT-catenin alters the hybrid adhering junctions in the heart and leads to dilated cardiomyopathy and ventricular arrhythmia following acute ischemia

It is generally accepted that the intercalated disc (ICD) required for mechano-electrical coupling in the heart consists of three distinct junctional complexes: adherens junctions, desmosomes and gap junctions. However, recent morphological and molecular data indicate a mixing of adherens junctional and desmosomal components, resulting in a 'hybrid adhering junction' or 'area composita'. The α-catenin family member αT-catenin, part of the N-cadherin-catenin adhesion complex in the heart, is the only α-catenin that interacts with the desmosomal protein plakophilin-2 (PKP2). Thus, it has been postulated that αT-catenin might serve as a molecular integrator of the two adhesion complexes in the area composita. To investigate the role of αT-catenin in the heart, gene targeting technology was used to delete the Ctnna3 gene, encoding αT-catenin, in the mouse. The αT-catenin-null mice are viable and fertile; however, the animals exhibit progressive cardiomyopathy. Adherens junctional and desmosomal proteins were unaffected by loss of αT-catenin, with the exception of the desmosomal protein PKP2. Immunogold labeling at the ICD demonstrated in the αT-catenin-null heart a preferential reduction of PKP2 at the area composita compared with the desmosome. Furthermore, gap junction protein Cx43 was reduced at the ICD, including its colocalization with N-cadherin. Gap junction remodeling in αT-catenin-knockout hearts was associated with an increased incidence of ventricular arrhythmias after acute ischemia. This novel animal model demonstrates for the first time how perturbation in αT-catenin can affect both PKP2 and Cx43 and thereby highlights the importance of understanding the crosstalk between the junctional proteins of the ICD and its implications for arrhythmogenic cardiomyopathy.

Calcium and connexin-based intercellular communication, a deadly catch?

Ca(2+) is known as a universal messenger mediating a wide variety of cellular processes, including cell death. In fact, this ion has been proposed as the 'cell death master', not only at the intracellular but also at the intercellular level. The most direct form of intercellular spread of cell death is mediated by gap junction channels. These channels have been shown to propagate cell death as well as cell survival signals between the cytoplasm of neighbouring cells, reflecting the dual role of Ca(2+) signals, i.e. cell death versus survival. Its precursor, the unopposed hemichannel (half of a gap junction channel), has recently joined in as a toxic pore connecting the intracellular with the extracellular environment and allowing the passage of a range of substances. The biochemical nature of the so-called intercellular cell death molecule, transferred through gap junctions or released/taken up via hemichannels, remains elusive but several studies pinpoint Ca(2+) itself or its messenger inositol trisphosphate as the responsible masters in crime. Although direct evidence is still lacking, indirect data including Ca(2+) involvement in intercellular communication and cell death, and effects of intercellular communication on intracellular Ca(2+) homeostasis, support this hypothesis. In addition, hemichannels and their molecular building blocks, connexin or pannexin proteins, may exert their effects on Ca(2+)-dependent cell death at the intracellular level, independently from their channel functions. This review provides a cutting edge overview of the current knowledge and underscores the intimate connection between intercellular communication, Ca(2+) signalling and cell death.

Coronary artery disease affecting the atrial branches is an independent determinant of atrial fibrillation after myocardial infarction

BACKGROUND

During acute myocardial infarction (MI), the incidence of atrial fibrillation (AF) is 6% to 22%, and its occurrence in this setting is associated with increased short- and long-term morbidity and mortality.

OBJECTIVE

The purpose of this case control study was evaluate the characteristics associated with the development of new-onset AF.

METHODS

Of 2,460 consecutive patients with acute MI, 149 (6%) were identified as having AF within 7 days of MI. After excluding patients with prior AF, previously documented heart failure, reduced left ventricular (LV) ejection fraction, valvular heart disease, LV hypertrophy, AF after coronary artery bypass grafting, or pericarditis; we identified 42 AF patients in whom coronary anatomy was assessed by invasive angiography and cardiac structure and function was evaluated using transthoracic echocardiography. Another 42 patients from the same cohort with MI but no AF matched for age, gender, and LV ejection fraction were studied as controls.

RESULTS

AF patients were more likely to present with an inferior MI (P = .002) but less likely to present with ST-segment elevation MI (P = .02). Univariate associations with AF included indexed left atrial volume (P <.001), LV filling pressure (E/e'; P = .001), right atrial branch disease (P <.001), left atrial branch disease (P = .009), sinoatrial branch disease (P <.001), left main stem disease (P = .02), and time from onset of symptoms to coronary intervention (P = .002). In multivariable analysis, right and left coronary artery atrial branch disease (P = .02) were predictors of AF post-MI.

CONCLUSION

Coronary artery disease affecting the atrial branches is an independent predictor for the development of AF after MI.

Platelets and cardiac arrhythmia

Sudden cardiac death (SCD) remains one of the most prevalent modes of death in industrialized countries, and myocardial ischemia due to thrombotic coronary occlusion is its primary cause. The role of platelets in the occurrence of SCD extends beyond coronary flow impairment by clot formation. Here we review the substances released by platelets during clot formation and their arrhythmic properties. Platelet products are released from three types of platelet granules: dense core granules, alpha-granules, and platelet lysosomes. The physiologic properties of dense granule products are of special interest as a potential source of arrhythmic substances. They are released readily upon activation and contain high concentrations of serotonin, histamine, purines, pyrimidines, and ions such as calcium and magnesium. Potential arrhythmic mechanisms of these substances, e.g., serotonin and high energy phosphates, include induction of coronary constriction, calcium overloading, and induction of delayed after-depolarizations. Alpha-granules produce thromboxanes and other arachidonic-acid products with many potential arrhythmic effects mediated by interference with cardiac sodium, calcium, and potassium channels. Alpha-granules also contain hundreds of proteins that could potentially serve as ligands to receptors on cardiomyocytes. Lysosomal products probably do not have an important arrhythmic effect. Platelet products and ischemia can induce coronary permeability, thereby enhancing interaction with surrounding cardiomyocytes. Antiplatelet therapy is known to improve survival after myocardial infarction. Although an important part of this effect results from prevention of coronary clot formation, there is evidence to suggest that antiplatelet therapy also induces anti-arrhythmic effects during ischemia by preventing the release of platelet activation products.

Anisotropic conduction block and reentry in neonatal rat ventricular myocyte monolayers

Anisotropy can lead to unidirectional conduction block that initiates reentry. We analyzed the mechanisms in patterned anisotropic neonatal rat ventricular myocyte monolayers. Voltage and intracellular Ca (Ca(i)) were optically mapped under the following conditions: extrastimulus (S1S2) testing and/or tetrodotoxin (TTX) to suppress Na current availability; heptanol to reduce gap junction conductance; and incremental rapid pacing. In anisotropic monolayers paced at 2 Hz, conduction velocity (CV) was faster longitudinally than transversely, with an anisotropy ratio [AR = CV(L)/CV(T), where CV(L) and CV(T) are CV in the longitudinal and transverse directions, respectively], averaging 2.1 ± 0.8. Interventions decreasing Na current availability, such as S1S2 pacing and TTX, slowed CV(L) and CV(T) proportionately, without changing the AR. Conduction block preferentially occurred longitudinal to fiber direction, commonly initiating reentry. Interventions that decreased gap junction conductance, such as heptanol, decreased CV(T) more than CV(L), increasing the AR and causing preferential transverse conduction block and reentry. Rapid pacing resembled the latter, increasing the AR and promoting transverse conduction block and reentry, which was prevented by the Ca(i) chelator 1,2-bis oaminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA). In contrast to isotropic and uniformly anisotropic monolayers, in which reentrant rotors drifted and self-terminated, bidirectional anisotropy (i.e., an abrupt change in fiber direction exceeding 45°) caused reentry to anchor near the zone of fiber direction change in 77% of monolayers. In anisotropic monolayers, unidirectional conduction block initiating reentry can occur longitudinal or transverse to fiber direction, depending on whether the experimental intervention reduces Na current availability or decreases gap junction conductance, agreeing with theoretical predictions.

Myocardial Ca2+ handling and cell-to-cell coupling, key factors in prevention of sudden cardiac deathThis article is one of a selection of papers published in a special issue on Advances in Cardiovascular Research

Using whole-heart preparations, we tested our hypothesis that Ca2+ handling is closely related to cell-to-cell coupling at the gap junctions and that both are critical for the development and particularly the termination of ventricular fibrillation (VF) and hence the prevention of sudden arrhythmic death. Intracellular free calcium concentration ([Ca2+]i), ECG, and left ventricular pressure were continuously monitored in isolated guinea pig hearts before and during development of low K+-induced sustained VF and during its conversion into sinus rhythm facilitated by stobadine. We also examined myocardial ultrastructure to detect cell-to-cell coupling alterations. We demonstrated that VF occurrence was preceded by a 55.9% ± 6.2% increase in diastolic [Ca2+]i, which was associated with subcellular alterations indicating Ca2+ overload of the cardiomyocytes and disorders in coupling among the cells. Moreover, VF itself further increased [Ca2+]i by 58.2% ± 3.4% and deteriorated subcellular and cell-to-cell coupling abnormalities that were heterogeneously distributed throughout the myocardium. In contrast, termination of VF and its conversion into sinus rhythm was marked by restoration of basal [Ca2+]i, resulting in recovery of intercellular coupling linked with synchronous contraction. Furthermore, we have shown that hearts exhibiting lower SERCA2a (sarcoplasmic reticulum Ca2+-ATPase) activity and abnormal intercellular coupling (as in older guinea pigs) are more prone to develop Ca2+ overload associated with cell-to-cell uncoupling than hearts with higher SERCA2a activity (as in young guinea pigs). Consequently, young animals are better able to terminate VF spontaneously. These findings indicate the crucial role of Ca2+ handling in relation to cell-to-cell coupling in both the occurrence and termination of malignant arrhythmia.

Improving cardiac gap junction communication as a new antiarrhythmic mechanism: the action of antiarrhythmic peptides

Co-ordinated electrical activation of the heart is maintained by intercellular coupling of cardiomyocytes via gap junctional channels located in the intercalated disks. These channels consist of two hexameric hemichannels, docked to each other, provided by either of the adjacent cells. Thus, a complete gap junction channel is made from 12 protein subunits, the connexins. While 21 isoforms of connexins are presently known, cardiomyocytes typically are coupled by Cx43 (most abundant), Cx40 or Cx45. Some years ago, antiarrhythmic peptides were discovered and synthesised, which were shown to increase macroscopic gap junction conductance (electrical coupling) and enhance dye transfer (metabolic coupling). The lead substance of these peptides is AAP10 (H-Gly-Ala-Gly-Hyp-Pro-Tyr-CONH(2)), a peptide with a horseshoe-like spatial structure as became evident from two-dimensional nuclear magnetic resonance studies. A stable D: -amino-acid derivative of AAP10, rotigaptide, as well as a non-peptide analogue, gap-134, has been developed in recent years. Antiarrhythmic peptides act on Cx43 and Cx45 gap junctions but not on Cx40 channels. AAP10 has been shown to enhance intercellular communication in rat, rabbit and human cardiomyocytes. Antiarrhythmic peptides are effective against ventricular tachyarrhythmias, such as late ischaemic (type IB) ventricular fibrillation, CaCl(2) or aconitine-induced arrhythmia. Interestingly, the effect of antiarrhythmic peptides is higher in partially uncoupled cells and was shown to be related to maintained Cx43 phosphorylation, while arrhythmogenic conditions like ischaemia result in Cx43 dephosphorylation and intercellular decoupling. It is still a matter of debate whether these drugs also act against atrial fibrillation. The present review outlines the development of this group of peptides and derivatives, their mode of action and molecular mechanisms, and discusses their possible therapeutic potential.

Ischemic, genetic and pharmacological origins of cardiac arrhythmias: the contribution of the Quebec Heart Institute

Research in the field of basic electrophysiology at the Quebec Heart Institute (Laval Hospital, Quebec City, Quebec) has evolved since its beginning in the 1990s. Interests were focused on cardiac arrhythmias induced by drugs, allelic variants and metabolic factors produced during ischemia. The results have contributed to the creation of new standards in drug development, more specifically, testing all new drugs for their potential effects on cardiac potassium currents, which could produce life-threatening proarrhythmic effects. In a French-Canadian population, three heterozygous single nucleotide polymorphisms in hK(v)1.5, a gene encoding for a major atrial repolarizing current, were found. These variants affect the expression level of the hK(v)1.5 channel and change the inactivation process in the presence of its accessory beta subunit. Because these effects could shorten atrial action potential, their presence was tested in postcoronary bypass patients and a higher prevalence was found in patients with postoperative atrial fibrillation. Finally, three potentially proarrhythmic factors characteristic of ischemia were identified: pH decrease; oxygen free radicals, which both increase the flow of K(+) ions through human ether-a-go-go-related gene and hK(v)1.5, producing a reduction in action potential duration, frequently leading to cardiac arrhythmias; and lysophosphatidylcholine, a metabolite involved in the production of cardiac arrhythmias early during ischemia that was shown to be a major cause of electrical uncoupling. Over the past decade, the Quebec Heart Institute has provided a significant amount of original data in the field of basic cardiac electrophysiology, specifically concerning arrhythmias originating from pharmacological agents, genetic background and cardiac ischemia.

Ischemic preconditioning protects against arrhythmogenesis through maintenance of both active as well as passive electrical properties in ischemic canine hearts

BACKGROUND

The mechanisms for the antiarrhythmogenic effects of preconditioning in ischemic hearts, although well demonstrated, are not clear. We measured indices of activation and repolarization using data from a high-resolution epicardial sock electrode array in preconditioned (PC) and non-PC hearts in an attempt to gain further insight into protective mechanisms.

METHODS AND RESULTS

Five canine hearts were subjected to a coronary artery occlusion lasting at least 1 hour, and 5 were subjected to a similar occlusion preceded by a preconditioning protocol. Epicardial electrograms were recorded using a 490-electrode sock. Representative beats were selected at intervals of 1 minute for analysis. The mean ST elevation for the PC group both rose slowly after occlusion and also resolved more slowly than the non-PC group. Electrocardiographic markers for propagation such as Total Activation Time, the QRSRMS width, and magnitude of steepest downstroke of the QRS complex all showed that the PC group maintained conduction velocity initially and also varied less dramatically than the control group. The regression line slope computed on a scatter plot of QT width vs cycle length was 0.23 for the PC group and 0.58 for non-PC. During occlusion, the incidence of premature ventricular contractions (PVCs) peaked at approximately 17 minutes followed by a second peak at approximately 27 minutes in the non-PC group, the PC group showed similar peaks at approximately 24 and approximately 53 minutes respectively.

CONCLUSION

The slower rate of resolution of ST elevation in PC hearts suggests a delay in gap junction closure, thus maintaining intracellular resistivity and reducing the likelihood of arrhythmia. The speed of conduction is adequately maintained during the early stages of ischemia in PC hearts. The mQTi-mRR regression line, a surrogate measure of rate dependency of repolarization (restitution), has a lower slope in the PC case, thus suggesting a mechanism of reduced arrhythmogenesis. The conclusions are supported by a delay of peak PVCs in PC hearts.

Modulation of ischaemic contracture in mouse hearts: a 'supraphysiological' response to adenosine

While inhibition of ischaemic contracture was one of the first documented cardioprotective actions of exogenously applied adenosine, it is not known whether this is a normal function of endogenous adenosine generated during ischaemic stress. Additionally, the relevance of delayed contracture to postischaemic outcome is unclear. We tested the ability of endogenous versus exogenous adenosine to modify contracture (and postischaemic outcomes) in C57/Bl6 mouse hearts. During ischaemia, untreated hearts developed peak contracture (PC) of 85 +/- 5 mmHg at 8.9 +/- 0.8 min, with time to reach 20 mmHg (time to onset of contracture; TOC) of 4.4 +/- 0.3 min. Adenosine (50 microm) delayed TOC to 6.7 +/- 0.6 min, as did pretreatment with 10 microm 2-chloroadenosine (7.2 +/- 0.5 min) or 50 nm of A(1) adenosine receptor (AR) agonist N(6)-cyclohexyladenosine (CHA) (6.7 +/- 0.3 min), but not A(2A)AR or A(3)AR agonists (20 nm 2-[4-(2-carboxyethyl) phenethylamino]-5' N-methylcarboxamidoadenosine (CGS21680) or 150 nm 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA), respectively). Adenosinergic contracture inhibition was eliminated by A(1)AR gene knockout (KO), mimicked by A(1)AR overexpression, and was associated with preservation of myocardial [ATP]. This adenosine-mediated inhibition of contracture was, however, only evident after prolonged (10 or 15 min) and not brief (3 min) pretreatment. Ischaemic contracture was also insensitive to endogenously generated adenosine, since A(1)AR KO, and non-selective and A(1)AR-selective antagonists (50 microm 8-sulphophenyltheophylline and 150 nm 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), respectively), all failed to alter intrinsic contracture development. Finally, delayed contracture with A(1)AR agonism/overexpression or ischaemic 2,3-butanedione monoxime (BDM; 5 microm to target Ca(2+) cross-bridge formation) was linked to enhanced postischaemic outcomes. In summary, adenosinergic inhibition of contracture is solely A(1)AR mediated; the response is 'supraphysiological', evident only with significant periods of pre-ischaemic AR agonism (or increased A(1)AR density); and ischaemic contracture appears insensitive to locally generated adenosine, potentially owing to the rapidity of contracture development versus the finite time necessary for expression of AR-mediated cardioprotection.

Vacuum-assisted apical suction devices induce passive electrical changes consistent with myocardial ischemia during off-pump coronary artery bypass graft surgery

OBJECTIVE

Off-pump coronary artery bypass graft surgery is common therapy to completely revascularize diseased hearts. In order to graft posterior arteries in this procedure, the heart must be lifted from the chest cavity and manipulated to expose the surgical field using an apical suction device. This suction device may cause unwanted myocardial ischemia.

METHODS

In this observational study, we measured myocardial electrical impedance, a parameter that responds to myocardial ischemia, as well as ST-segment changes during off-pump coronary artery bypass graft surgery in 12 patients with two-vessel coronary artery disease undergoing revascularisation of the left anterior descending and the posterior descending coronary arteries. During the posterior descending artery revascularisation phase of the procedure the apical suction device was oriented over the electrodes used to measure myocardial electrical impedance, thus allowing us the opportunity to assess myocardial ischemia in this region of the heart.

RESULTS

In these 12 patients, myocardial electrical impedance progressively increased under the suction device during posterior coronary artery revascularisation, suggesting that myocardial ischemia developed in this region of the myocardium. ST-segment changes were negligible while the heart was vertically displaced (and the suction device attached), but increased immediately when the heart was returned to the neutral anatomical position.

CONCLUSION

Our data suggest that the apical suction device may cause ischemia while the heart is vertically displaced and electrically disconnected from the body. Under these conditions, ST-segment changes may not detect myocardial ischemia. Myocardial electrical impedance has the potential to reliably detect intraoperative myocardial ischemia under these circumstances.

Suppression of Calcium-Induced Repolarization Heterogeneity as a Mechanism of Nitroglycerin??s Antiarrhythmic Action

This study examined whether the antifibrillatory action of nitroglycerin (NTG) is attributable to reduction in calcium-induced heterogeneity of repolarization independent of autonomic and coronary vasodilatory influences. The effects of intrapericardial (IPC) NTG on coronary blood flow, contractility, repolarization, and arrhythmia susceptibility were measured in anesthetized pigs (N = 43). Autonomic influences were minimized by vagotomy and beta-adrenergic blockade (metoprolol, 1.25 mg/kg, intravenous). Electrophysiological parameters were tested at 30 min, a time when coronary hemodynamics had returned to baseline. Intracoronary calcium chloride (CaCl2, 50-mg bolus) injection augmented contractility (dP/dt(max), 1760 +/- 144 to 2769 +/- 274 mmHg/s, and following NTG, 1531 +/- 384 to 2138 +/- 242 mmHg/s, P < 0.0002), reflecting increased myocardial intracellular calcium. Calcium increased repolarization heterogeneity (interlead precordial T-wave heterogeneity, 95 +/- 15 to 264 +/- 33 microV, P < 0.006; T(peak)-T(end), an index of transmural dispersion of repolarization, 37 +/- 3 to 76 +/- 6 ms, P < 0.05) and lowered repetitive extrasystole threshold (RET; 24 +/- 2 to 13 +/- 1 mA, and following NTG, 32 +/- 4 to 18 +/- 1 mA, P < 0.0001). IPC NTG raised the RET from baseline by 33% and blunted calcium-induced contractility (dP/dt(max) by 23%, P < 0.05), repolarization changes (T-wave heterogeneity by 24%, P < 0.006; T(peak)-T(end) by 18%, P = 0.04), and arrhythmia vulnerability (RET by 39%, P < 0.003). Thus, the capacity of NTG to suppress calcium-induced repolarization heterogeneity is an important mechanism of its antiarrhythmic action, which is independent of autonomic and vasodilatory actions.

Rotigaptide (ZP123) prevents spontaneous ventricular arrhythmias and reduces infarct size during myocardial ischemia/reperfusion injury in open-chest dogs

The antiarrhythmic and cardioprotective effect of increasing gap junction intercellular communication during ischemia/reperfusion injury has not been studied. The antiarrhythmic peptide rotigaptide (previously ZP123), which maintains gap junction intercellular communication, was tested in dogs subjected to a 60-min coronary artery occlusion and 4 h of reperfusion. Rotigaptide was administered i.v. 10 min before reperfusion as a bolus + i.v. infusion at doses of 1 ng/kg bolus + 10 ng/kg/h infusion (n = 6), 10 ng/kg bolus + 100 ng/kg/h infusion (n = 5), 100 ng/kg bolus + 1000 ng/kg/h infusion (n = 8), 1000 ng/kg bolus + 10 mug/kg/h infusion (n = 6), and vehicle control (n = 5). Premature ventricular complexes (PVCs) were quantified during reperfusion. A series of four or more consecutive PVCs was defined as ventricular tachycardia (VT). The total incidence of VT was reduced significantly with the two highest doses of rotigaptide (20.3 +/- 10.9 and 4.3 +/- 4.1 events; p < 0.05) compared with controls (48.7 +/- 6.0). Total PVCs were reduced significantly from 25.1 +/- 4.2% in control animals to 11.0 +/- 4.4 and 1.7 +/- 1.3% after the two highest doses of rotigaptide. Infarct size, expressed as a percentage of the left ventricle, was reduced significantly from 13.2 +/- 1.9 in controls to 7.1 +/- 1.0 (p < 0.05) at the highest dose of rotigaptide. Ultrastructural evaluation revealed no differences in myocardial injury in the infarct area, area at risk, border zone, or normal zone in vehicle and rotigaptide-treated animals. However, rotigaptide did increase the presence of gap junctions in the area at risk (p = 0.022, Fisher's exact test). Rotigaptide had no effect on heart rate, blood pressure, heart rate-corrected QT interval, or left ventricular end-diastolic pressure. In conclusion, these results demonstrate that rotigaptide is a potent antiarrhythmic compound with cardioprotective effects and desirable safety.

DELAYED UNCOUPLING CONTRIBUTES TO THE PROTECTIVE EFFECT OF HEPTANOL AGAINST ISCHAEMIA IN THE RAT ISOLATED HEART

1. It is known that infusion of the gap junction uncoupler heptanol, before ischaemia or during reperfusion, limits myocardial infarct size. However, whether this cardiac effect is linked to the effect of heptanol on communication across gap junctions has not been elucidated. The aims of the present study were to examine the effect of heptanol on infarct size, arrhythmias and myocardial tissue resistance and to assess whether changes in electrical coupling correlate with cardiac protection. 2. Rat isolated, perfused hearts were subjected to a 24 min infusion of heptanol (0.05, 0.1, 0.5 or 1.0 mmol/L) followed by 20 min regional ischaemia and 60 min reperfusion, or by 70 min global no-flow ischaemia. The effective refractory period, action potential and conduction velocity were measured in papillary muscles from the right ventricle. Heptanol markedly decreased arrhythmia scores during ischaemia and reperfusion, as well as reducing infarct size to a degree similar to that induced by ischaemic preconditioning. In the prolonged ischaemia model, heptanol delayed the onset of uncoupling, increased time to plateau and decreased the maximal rate of uncoupling during ischaemia. Ischaemic preconditioning had similar effects on these parameters. In papillary muscle, heptanol reduced the conduction velocity of the action potential in a dose-dependent manner, but had no significant effect on resting potential, action potential amplitude, action potential duration, maximal upstroke of depolarization or effective refractory period. 3. These results demonstrate that treatment with the gap junction uncoupler heptanol confers cardioprotection against ischaemia and this effect is related to delayed electrical uncoupling during prolonged ischaemia.

Alterations of the bioenergetics systems of the cell in acute and chronic myocardial ischemia

The aim of the works presented here is to analyze the alterations induced by acute ischemia-reperfusion and chronic ischemia on mitochondrial function, in relation to alterations on heart function. Parameters of mitochondrial function were assessed on skinned fibers coming from isolated perfused rat hearts. The effects of chronic ischemia were studied on a rat model of left descending coronary artery stenosis. Two key events observed after acute ischemia-reperfusion and chronic ischemia are the decrease (or the loss) of the stimulatory effect of creatine and the alteration of outer mitochondrial permeability to cytochrome c and ADP. Taken together, these effects indicate the alteration of the intermembrane space architecture leading to the loss of intracellular adenine nucleotides compartmentation and possibly of functional coupling of mitochondrial creatine kinase and adenine nucleotide translocase. These alterations result in the impairment of intracellular energy transfer (channeling) from mitochondria to ATP-utilizing sites located in the cytosol. This may play a significant role in ischemic injury and alterations in heart function. We show that these effects were prevented by effective cardioprotective strategies like ischemic preconditioning or pharmacological preconditioning by perfusion of mitochondrial ATP-sensitive potassium channel openers. We hypothesize that an open mitochondrial ATP-sensitive potassium channel during ischemia maintains the tight structure of the intermembrane space that is required to preserve the normal low outer membrane permeability to ADP and ATP.

Effect of sarcolemmal rupture on myocardial electrical impedance during oxygen deprivation

Plasma membrane disruption is a characteristic feature of cell death induced by hypoxia or ischemia. Here, we investigated whether analysis of tissue electrical impedance allows detection of ongoing cell membrane rupture and necrotic cell death in hypoxic or ischemic myocardium. Twenty-eight isolated rat hearts were submitted to 5 h of ischemia (n = 8) or hypoxia (n = 20). Myocardial electrical impedance and lactate dehydrogenase (LDH) release were monitored. The time course of hypoxia-induced cell death was modified by altering pH (pH 7.4 or 6.4, 5 h) or by adding 3 or 10 mM glycine. Ischemia and hypoxia induced an increase in electrical impedance, followed by a plateau, and later a reduction. During hypoxia, LDH release started after a prolonged lapse of time (80.00 +/- 8.37 min at pH 7.4 and 122.50 +/- 11.82 min at pH 6.4). The onset of LDH release was followed by the onset of the late reduction in electrical impedance, and both were delayed by acidic pH (P < 0.05) and by glycine (P < 0.05). The times of onset of LDH release and of late electrical changes were significantly correlated (r = 0.752, P < 0.001). In separate experiments, induction of sarcolemmal rupture with Triton X-100 (n = 6) mimicked the late effects of ischemia or hypoxia on tissue impedance. The protective effects of glycine and acidosis on membrane disruption were confirmed (propidium iodide) in energy-deprived HL-1 cardiomyocytes. These results describe for the first time a late fall in electrical impedance in myocardium submitted to prolonged oxygen deprivation and demonstrate that this fall allows detection of ongoing cell necrosis.