EPAC1 and 2 inhibit K currents via PLC/PKC and NOS/PKG pathways in rat ventricular cardiomyocytes.

The exchange protein directly activated by cAMP (EPAC) has been implicated in cardiac proarrhythmic signaling pathways including spontaneous diastolic Ca leak from sarcoplasmic reticulum and increased action potential duration (APD) in isolated ventricular cardiomyocytes. The action potential (AP) lengthening following acute EPAC activation is mainly due to a decrease of repolarizing steady-state K current (IK) but the mechanisms involved remain unknown. This study aimed to assess the role of EPAC1 and EPAC2 in the decrease of IK and to investigate the underlying signaling pathways.

Inhibition of EPAC1 signaling pathway alters atrial electrophysiology and prevents atrial fibrillation.

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and is associated with increased mortality and morbidity. The Exchange Protein directly Activated by cAMP (EPAC), has been implicated in pro-arrhythmic signaling pathways in the atria, but the underlying mechanisms remain unknown. In this study, we investigated the involvement of EPAC1 and EPAC2 isoforms in the genesis of AF in wild type (WT) mice and knockout (KO) mice for EPAC1 or EPAC2.

The antimalarial MMV688533 provides potential for single-dose cures with a high barrier to parasite resistance.

The emergence and spread of resistance to first-line antimalarials creates an imperative to identify and develop potent preclinical candidates with distinct modes of action. Here, we report the identification of MMV688533, an acylguanidine that was developed following a whole-cell screen with compounds known to hit high-value targets in human cells. MMV688533 displays fast parasite clearance in vitro and is not cross-resistant with known antimalarials.

TRPM4 Participates in Aldosterone-Salt-Induced Electrical Atrial Remodeling in Mice.

Aldosterone plays a major role in atrial structural and electrical remodeling, in particular through Ca-transient perturbations and shortening of the action potential. The Ca-activated non-selective cation channel Transient Receptor Potential Melastatin 4 (TRPM4) participates in atrial action potential. The aim of our study was to elucidate the interactions between aldosterone and TRPM4 in atrial remodeling and arrhythmias susceptibility.

Transient receptor potential channels in cardiac health and disease.

Transient receptor potential (TRP) channels are nonselective cationic channels that are generally Ca permeable and have a heterogeneous expression in the heart. In the myocardium, TRP channels participate in several physiological functions, such as modulation of action potential waveform, pacemaking, conduction, inotropy, lusitropy, Ca and Mg handling, store-operated Ca entry, embryonic development, mitochondrial function and adaptive remodelling. Moreover, TRP channels are also involved in various pathological mechanisms, such as arrhythmias, ischaemia-reperfusion injuries, Ca-handling defects, fibrosis, maladaptive remodelling, inherited cardiopathies and cell death.

Argon Exposure Induces Postconditioning in Myocardial Ischemia-Reperfusion.

Background And Purpose: Cardioprotection against ischemia-reperfusion (I/R) damages remains a major concern during prehospital management of acute myocardial infarction. Noble gases have shown beneficial effects in preconditioning studies. Because emergency proceedings in the context of myocardial infarction require postconditioning strategies, we evaluated the effects of argon in such protocols on mammalian cardiac tissue.

TRPM4 non-selective cation channel variants in long QT syndrome.

Background: Long QT syndrome (LQTS) is an inherited arrhythmic disorder characterized by prolongation of the QT interval, a risk of syncope, and sudden death. There are already a number of causal genes in LQTS, but not all LQTS patients have an identified mutation, which suggests LQTS unknown genes.

Methods: A cohort of 178 LQTS patients, with no mutations in the 3 major LQTS genes (KCNQ1, KCNH2, and SCN5A), was screened for mutations in the transient potential melastatin 4 gene (TRPM4).

TRPM4 non-selective cation channels influence action potentials in rabbit Purkinje fibres.

Key Points: The transient receptor potential melastatin 4 (TRPM4) inhibitor 9-phenanthrol reduces action potential duration in rabbit Purkinje fibres but not in ventricle. TRPM4-like single channel activity is observed in isolated rabbit Purkinje cells but not in ventricular cells. The TRPM4-like current develops during the notch and early repolarization phases of the action potential in Purkinje cells.

TRPM4 in cardiac electrical activity.

TRPM4 forms a non-selective cation channel activated by internal Ca(2+). Its functional expression was demonstrated in cardiomyocytes of several mammalian species including humans, but the channel is also present in many other tissues. The recent characterization of the TRPM4 inhibitor 9-phenanthrol, and the availability of transgenic mice have helped to clarify the role of TRPM4 in cardiac electrical activity, including diastolic depolarization from the sino-atrial node cells in mouse, rat, and rabbit, as well as action potential duration in mouse cardiomyocytes.

Rapid and MR-Independent IK1 Activation by Aldosterone during Ischemia-Reperfusion.

In ST elevation myocardial infarction (STEMI) context, clinical studies have shown the deleterious effect of high aldosterone levels on ventricular arrhythmia occurrence and cardiac mortality. Previous in vitro reports showed that during ischemia-reperfusion, aldosterone modulates K+ currents involved in the holding of the resting membrane potential (RMP). The aim of this study was to assess the electrophysiological impact of aldosterone on IK1 current during myocardial ischemia-reperfusion.

The calcium stored in the sarcoplasmic reticulum acts as a safety mechanism in rainbow trout heart.

Cardiomyocyte contraction depends on rapid changes in intracellular Ca(2+). In mammals, Ca(2+) influx as L-type Ca(2+) current (ICa) triggers the release of Ca(2+) from sarcoplasmic reticulum (SR) and Ca(2+)-induced Ca(2+) release (CICR) is critical for excitation-contraction coupling. In fish, the relative contribution of external and internal Ca(2+) is unclear.

Current recordings at the single channel level in adult mammalian isolated cardiomyocytes.

This chapter describes appropriate methods to investigate mammalian cardiac channels properties at the single channel level. Cell isolation is performed from new born or adult heart by enzymatic digestion on minced tissue or using the Langendorff apparatus. Isolation proceeding is suitable for rabbit, rat, and mouse hearts.

Proarrhythmic effects of aldosterone during myocardial ischemia-reperfusion: implication of the sarcolemmal-KATP channels.

Objective: To assess the electrophysiological impact of aldosterone during myocardial ischemia-reperfusion.

Methods: We used an in vitro model of "border zone" using rabbit right ventricle and standard microelectrodes.

Results: Aldosterone (10 and 100 nmol/L) shortened ischemic action potential [action potential duration at 90% of repolarization (APD90), from 55 ± 3 to 39 ± 1 ms and 36 ± 3 ms, respectively, P < 0.

Implication of the TRPM4 nonselective cation channel in mammalian sinus rhythm.

Background: The transient receptor potential melastatin 4 (TRPM4) channel is expressed in the sinoatrial node, but its physiologic roles in this tissue with cardiac pacemaker properties remain unknown. This Ca(2+)-activated nonselective cation channel (NSCCa) induces cell depolarization at negative potentials. It is implicated in burst generation in neurons and participates in induction of ectopic beating in cardiac ventricular preparations submitted to hypoxia/reoxygenation.

Molecular genetics and functional anomalies in a series of 248 Brugada cases with 11 mutations in the TRPM4 channel.

Brugada syndrome (BrS) is a condition defined by ST-segment alteration in right precordial leads and a risk of sudden death. Because BrS is often associated with right bundle branch block and the TRPM4 gene is involved in conduction blocks, we screened TRPM4 for anomalies in BrS cases. The DNA of 248 BrS cases with no SCN5A mutations were screened for TRPM4 mutations.

Epac activator critically regulates action potential duration by decreasing potassium current in rat adult ventricle.

Sympathetic stimulation is an important modulator of cardiac function via the classic cAMP-dependent signaling pathway, PKA. Recently, this paradigm has been challenged by the discovery of a family of guanine nucleotide exchange proteins directly activated by cAMP (Epac), acting in parallel to the classic signaling pathway. In cardiac myocytes, Epac activation is known to modulate Ca(2+) cycling yet their actions on cardiac ionic currents remain poorly characterized.

Nifedipine blocks ondansetron electrophysiological effects in rabbit purkinje fibers and decreases early after depolarization incidence.

We hypothesized that a high concentration of nifedipine (1 μM), known to inhibit at least 75%of L-type Ca++ current, might counteract proarrhythmic dose-dependent effects of ondansetron (0.1 to 10 μM) in rabbit Purkinje fibers. Ondansetron is a 5-HT3 receptor antagonist commonly prescribed to prevent nausea and vomiting caused by cancer chemotherapy, radiation therapy, and surgery but may increase the risk of developing prolongation of the QT interval of the electrocardiogram, which can lead to an abnormal and potentially fatal heart rhythm and recently raised FDA concerns and warnings.

Transient receptor potential melastatin 4 inhibitor 9-phenanthrol abolishes arrhythmias induced by hypoxia and re-oxygenation in mouse ventricle.

Background And Purpose: Hypoxia and subsequent re-oxygenation are associated with cardiac arrhythmias such as early afterdepolarizations (EADs), which may be partly explained by perturbations in cytosolic calcium concentration. Transient receptor potential melastatin 4 (TRPM4), a calcium-activated non-selective cation channel, is functionally expressed in the heart. Based on its biophysical properties, it is likely to participate in EADs.

I(Ks) blockade in border zone arrhythmias from guinea-pig ventricular myocardium submitted to simulated ischemia and reperfusion.

I(Ks) blockade might be a promising way to treat tachyarrhythmia because of the accumulation of activated potassium channels. However, I(Ks) blockade during ischemia/reperfusion has not been investigated. Thus, the electrophysiological effects of two I(Ks) blockers, chromanol 293B (10 μm) and HMR 1556 (1 μm), were assessed in an in vitro model of border zone between normal and ischemic/reperfused right ventricular myocardium from guinea-pigs, and classic electrophysiological parameters and the incidence of arrhythmias were studied.

The non-selective monovalent cationic channels TRPM4 and TRPM5.

Transient Receptor Potential (TRP) proteins are non-selective cationic channels with a consistent Ca(2+)-permeability, except for TRPM4 and TRPM5 that are not permeable to this ion. However, Ca(2+) is a major regulator of their activity since both channels are activated by a rise in internal Ca(2+). Thus TRPM4 and TRPM5 are responsible for most of the Ca(2+)-activated non-selective cationic currents (NSC(Ca)) recorded in a large variety of tissues.

I(Kr) vs. I(Ks) blockade and arrhythmogenicity in normoxic rabbit Purkinje fibers: does it really make a difference?

The electrophysiological (standard intracellular microelectrode technique) and pro-arrhythmic (occurrence of early after-depolarization) effects of five class III agents acting on delayed rectifier current (I(K)), rapid (I(Kr)), and/or slow (I(Ks)) components have been studied in rabbit Purkinje fibers taken near the septum and submitted in vitro to reduced stimulation rate (from 1 to 0.5 Hz) in the absence or presence of epinephrine (10 nm) during normoxic conditions. There were two I(Kr) blockers (d-sotalol and dofetilide), two I(Ks) blockers (chromanol 293B and HMR 1556), and a non-selective I(K) blocker (azimilide).

The electrophysiological effects of racemic ketamine and etomidate in an in vitro model of "border zone" between normal and ischemic/reperfused guinea pig myocardium.

Background: Etomidate and ketamine are used during induction of anesthesia in high-risk patients. However, their effects on action potential (AP) variables and ischemia/reperfusion-induced arrhythmias and conduction blocks are unknown.

Methods: Guinea pig right ventricular muscle strips were mounted in a 5-mL double chamber bath with the strips separated into two zones by an impermeable latex membrane.

Mechanisms underlying adaptation of action potential duration by pacing rate in rat myocytes.

Heart rate is an essential determinant of cardiac performance. In rat ventricular myocytes, a sudden increase in rate yields to a prolongation of the action potential duration (APD). The mechanism underlying this prolongation is controversial: it has been proposed that the longer APD is due to either: (1) a decrease in K+ currents only or (2) an increase in Ca2+ current only.

Ca2+ currents in cardiac myocytes: Old story, new insights.

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling.