Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells, and their underlying ionic mechanisms. It is therefore critical to further unravel the patho-physiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodelling) are discussed. The focus is human relevant findings obtained with clinical, experimental and computational studies, given that interspecies differences make the extrapolation from animal experiments to the human clinical settings difficult. Deepening the understanding of the diverse patholophysiology of human cellular electrophysiology will help developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions

Mechanisms of ventricular rate adaptation as a predictor of arrhythmic risk.

Background: Protracted QT interval (QTI) adaptation to abrupt heart rate (HR) changes has been identified as a clinical arrhythmic risk marker. This study investigates the ionic mechanisms of QTI rate adaptation and its relationship to arrhythmic risk. Methods and Results: Computer simulations and experimental recordings in human and canine ventricular tissue were used to investigate the ionic basis of QTI and action potential (AP) duration (APD) to abrupt changes in HR with a protocol commonly used in clinical studies. Time for 90% QTI adaptation is 3.5 min in simulations, in agreement with experimental and clinical data in human. APD adaptation follows similar dynamics, being faster in midmyocardial cells (2.5 min) than in endocardial/epicardial cells (3.5 min). Both QTI and APD adapt in two phases following an abrupt HR change: a fast initial phase with time constant 2 min driven by [Na(+)]i dynamics. Alterations in [Na(+)]i dynamics due to Na(+)/K(+) pump (INaK) inhibition result in protracted rate adaptation, and is associated with increased proarrhythmic risk, as indicated by AP triangulation and faster ICaL recovery from inactivation, leading to formation of early afterdepolarizations (EADs). Conclusions: This study suggests that protracted QTI adaptation could be an indicator of altered [Na(+)]i dynamics following INaK inhibition as it occurs in patients with ischemia or heart failure. Increased risk of cardiac arrhythmias in patients with protracted rate adaptation may be due to increased risk of EAD formation. Key words: action potentials, ventricles, ion channels, arrhythmia

Mechanisms of ventricular rate adaptation as a predictor of arrhythmic risk

Background: Protracted QT interval (QTI) adaptation to abrupt heart rate (HR) changes has been identified as a clinical arrhythmic risk marker. This study investigates the ionic mechanisms of QTI rate adaptation and its relationship to arrhythmic risk. Methods and Results: Computer simulations and experimental recordings in human and canine ventricular tissue were used to investigate the ionic basis of QTI and action potential (AP) duration (APD) to abrupt changes in HR with a protocol commonly used in clinical studies. Time for 90% QTI adaptation is 3.5 min in simulations, in agreement with experimental and clinical data in human. APD adaptation follows similar dynamics, being faster in midmyocardial cells (2.5 min) than in endocardial/epicardial cells (3.5 min). Both QTI and APD adapt in two phases following an abrupt HR change: a fast initial phase with time constant 2 min driven by [Na(+)]i dynamics. Alterations in [Na(+)]i dynamics due to Na(+)/K(+) pump (INaK) inhibition result in protracted rate adaptation, and is associated with increased proarrhythmic risk, as indicated by AP triangulation and faster ICaL recovery from inactivation, leading to formation of early afterdepolarizations (EADs). Conclusions: This study suggests that protracted QTI adaptation could be an indicator of altered [Na(+)]i dynamics following INaK inhibition as it occurs in patients with ischemia or heart failure. Increased risk of cardiac arrhythmias in patients with protracted rate adaptation may be due to increased risk of EAD formation. Key words: action potentials, ventricles, ion channels, arrhythmia

Nucleoside Diphosphate Kinase-C Suppresses cAMP Formation in Human Heart Failure

BACKGROUND: -Chronic heart failure (HF) is associated with altered signal transduction via beta-adrenoceptors and G proteins, and with reduced cAMP formation. Nucleoside diphosphate kinases (NDPKs) are enriched at the plasma membrane of end-stage HF patients, but the functional consequences of this are largely unknown, particularly for NDPK-C. Here, we investigated the potential role of NDPK-C in cardiac cAMP formation and contractility. METHODS: -Real-time PCR, (Far) Western blot, immunoprecipitation, and immunocytochemistry were used to study the expression, interaction with G proteins, and localization of NDPKs. cAMP levels were determined using immunoassays or fluorescent resonance energy transfer, and contractility was determined in cardiomyocytes (cell shortening) and in vivo (fractional shortening). RESULTS: -NDPK-C was essential for the formation of a NDPK-B/G proteins complex. Protein and mRNA levels of NDPK-C were up-regulated in end-stage human HF, in rats following chronic isoprenaline (ISO) stimulation through osmotic minipumps, and after incubation of rat neonatal cardiomyocytes with ISO. ISO also promoted translocation of NDPK-C to the plasma membrane. Overexpression of NDPK-C in cardiomyocytes increased cAMP levels and sensitized cardiomyocytes to ISO-induced augmentation of contractility, whereas NDPK-C knockdown decreased cAMP levels. In vivo, depletion of NDPK-C in zebrafish embryos caused cardiac edema and ventricular dysfunction. NDPK-B knockout mice had unaltered NDPK-C expression, but showed contractile dysfunction and exacerbated cardiac remodeling during chronic ISO stimulation. In human end-stage HF, the complex formation between NDPK-C and Galphai2 was increased, whereas NDPK-C/Galphas interaction was decreased, producing a switch that may contribute to an NDPK-C-dependent cAMP-reduction in HF. CONCLUSIONS: -Our findings identify NDPK-C as an essential requirement for both the interaction between NDPK isoforms and with G proteins. NDPK-C is a novel critical regulator of beta-adrenoceptor/cAMP signaling and cardiac contractility. By switching from Galphas to Galphai2 activation, NDPK-C may contribute to lower cAMP levels and the related contractile dysfunction in HF

Mapping genetic changes in the cAMP-signaling cascade in human atria

To obtain a quantitative expression profile of the main genes involved in the cAMP-signaling cascade in human control atria and in different cardiac pathologies.Expression of 48 target genes playing a relevant role in the cAMP-signaling cascade was assessed by RT-qPCR. 113 samples were obtained from right atrial appendages (RAA) of patients in sinus rhythm (SR) with or without atrium dilation, paroxysmal atrial fibrillation (AF), persistent AF or heart failure (HF); and left atrial appendages (LAA) from patients in SR or with AF. Our results show that right and left atrial appendages in donor hearts or from SR patients have similar expression values except for AC7 and PDE2A. Despite the enormous chamber-dependent variability in the gene-expression changes between pathologies, several distinguishable patterns could be identified. PDE8A, PKI3G and EPAC2 were upregulated in AF. Different phosphodiesterase (PDE) families showed specific pathology-dependent changes.By comparing mRNA-expression patterns of the cAMP-signaling cascade related genes in right and left atrial appendages of human hearts and across different pathologies, we show that 1) gene expression is not significantly affected by cardioplegic solution content, 2) it is appropriate to use SR atrial samples as controls, and 3) many genes in the cAMP-signaling cascade are affected in AF and HF but only few of them appear to be chamber (right or left) specific.AC, AKAP, PDE, Epac, PKA, CaMKII TRANSLATIONAL PERSPECTIVE: The cyclic AMP signaling pathway is important for atrial function. However, expression patterns of the genes involved in the atria of healthy and diseased hearts are still unclear. We give here a general overview of how different pathologies affect the expression of key genes in the cAMP signaling pathway in human right and left atria appendages. Our study may help identifying new genes of interest as potential therapeutic targets or clinical biomarkers for these pathologies and could serve as a guide in future gene therapy studies

Docosahexaenoic acid normalizes QT interval in long QT type 2 transgenic rabbit models in a genotype-specific fashion

AIM: Long QT syndrome (LQTS) is a cardiac channelopathy predisposing to ventricular arrhythmias and sudden cardiac death. Since current therapies often fail to prevent arrhythmic events in certain LQTS subtypes, new therapeutic strategies are needed. Docosahexaenoic acid (DHA) is a polyunsaturated fatty acid, which enhances the repolarizing I(Ks) current. METHODS AND RESULTS: We investigated the effects of DHA in wild type (WT) and transgenic long QT Type 1 (LQT1; loss of I(Ks)), LQT2 (loss of I(Kr)), LQT5 (reduction of I(Ks)), and LQT2–5 (loss of I(Kr) and reduction of I(Ks)) rabbits. In vivo ECGs were recorded at baseline and after 10 µM/kg DHA to assess changes in heart-rate corrected QT (QTc) and short-term variability of QT (STVQT). Ex vivo monophasic action potentials were recorded in Langendorff-perfused rabbit hearts, and action potential duration (APD(75)) and triangulation were assessed. Docosahexaenoic acid significantly shortened QTc in vivo only in WT and LQT2 rabbits, in which both α- and β-subunits of I(K)(s)-conducting channels are functionally intact. In LQT2, this led to a normalization of QTc and of its short-term variability. Docosahexaenoic acid had no effect on QTc in LQT1, LQT5, and LQT2–5. Similarly, ex vivo, DHA shortened APD(75) in WT and normalized it in LQT2, and additionally decreased AP triangulation in LQT2. CONCLUSIONS: Docosahexaenoic acid exerts a genotype-specific beneficial shortening/normalizing effect on QTc and APD(75) and reduces pro-arrhythmia markers STVQT and AP triangulation through activation of I(Ks) in LQT2 rabbits but has no effects if either α- or β-subunits to I(Ks) are functionally impaired. Docosahexaenoic acid could represent a new genotype-specific therapy in LQT2

Combined pharmacological block of IKr and IKs increases short-term QT interval variability and provokes torsades de pointes

Background and purpose: Assessing the proarrhythmic potential of compounds during drug development is essential. However, reliable prediction of drug-induced torsades de pointes arrhythmia (TdP) remains elusive. Along with QT interval prolongation, assessment of the short-term variability of the QT interval (STV(QT)) may be a good predictor of TdP. We investigated the relative importance of I-Ks and I-Kr block in development of TdP together with correlations between QTc interval, QT interval variability and incidence of TdP. Experimental approach: ECGs were recorded from conscious dogs and from anaesthetized rabbits given the IKr blocker dofetilide (DOF), the IKs blocker HMR-1556 (HMR) and their combination, intravenously. PQ, RR and QT intervals were measured and QTc and short-term variability of RR and QT intervals calculated. Key results: DOF increased QTc interval by 20% in dogs and 8% in rabbits. HMR increased QTc in dogs by 12 and 1.9% in rabbits. Combination of DOF+HMR prolonged QTc by 33% in dogs, by 16% in rabbits. DOF or HMR given alone in dogs or HMR given alone in rabbits induced no TdP. Incidence of TdP increased after DOF+HMR combinations in dogs (63%) and following HMR+DOF (82%) and DOF+HMR combinations (71%) in rabbits. STV(QT) markedly increased only after administration of DOF+HMR combinations in both dogs and rabbits. Conclusion and implications: STV(QT) was markedly increased by combined pharmacological block of IKr and IKs and may be a better predictor of subsequent TdP development than the measurement of QTc interval prolongation