Integration of genetics into a systems model of electrocardiographic traits using humanCVD BeadChip

<p>Background—Electrocardiographic traits are important, substantially heritable determinants of risk of arrhythmias and sudden cardiac death.</p> <p>Methods and Results—In this study, 3 population-based cohorts (n=10 526) genotyped with the Illumina HumanCVD Beadchip and 4 quantitative electrocardiographic traits (PR interval, QRS axis, QRS duration, and QTc interval) were evaluated for single-nucleotide polymorphism associations. Six gene regions contained single nucleotide polymorphisms associated with these traits at P<10−6, including SCN5A (PR interval and QRS duration), CAV1-CAV2 locus (PR interval), CDKN1A (QRS duration), NOS1AP, KCNH2, and KCNQ1 (QTc interval). Expression quantitative trait loci analyses of top associated single-nucleotide polymorphisms were undertaken in human heart and aortic tissues. NOS1AP, SCN5A, IGFBP3, CYP2C9, and CAV1 showed evidence of differential allelic expression. We modeled the effects of ion channel activity on electrocardiographic parameters, estimating the change in gene expression that would account for our observed associations, thus relating epidemiological observations and expression quantitative trait loci data to a systems model of the ECG.</p> <p>Conclusions—These association results replicate and refine the mapping of previous genome-wide association study findings for electrocardiographic traits, while the expression analysis and modeling approaches offer supporting evidence for a functional role of some of these loci in cardiac excitation/conduction.</p&gt

One-Dimensional Mathematical Model of the Atrioventricular Node Including Atrio-Nodal, Nodal, and Nodal-His Cells

AbstractMathematical models are a repository of knowledge as well as research and teaching tools. Although action potential models have been developed for most regions of the heart, there is no model for the atrioventricular node (AVN). We have developed action potential models for single atrio-nodal, nodal, and nodal-His cells. The models have the same action potential shapes and refractoriness as observed in experiments. Using these models, together with models for the sinoatrial node (SAN) and atrial muscle, we have developed a one-dimensional (1D) multicellular model including the SAN and AVN. The multicellular model has slow and fast pathways into the AVN and using it we have analyzed the rich behavior of the AVN. Under normal conditions, action potentials were initiated in the SAN center and then propagated through the atrium and AVN. The relationship between the AVN conduction time and the timing of a premature stimulus (conduction curve) is consistent with experimental data. After premature stimulation, atrioventricular nodal reentry could occur. After slow pathway ablation or block of the L-type Ca2+ current, atrioventricular nodal reentry was abolished. During atrial fibrillation, the AVN limited the number of action potentials transmitted to the ventricle. In the absence of SAN pacemaking, the inferior nodal extension acted as the pacemaker. In conclusion, we have developed what we believe is the first detailed mathematical model of the AVN and it shows the typical physiological and pathophysiological characteristics of the tissue. The model can be used as a tool to analyze the complex structure and behavior of the AVN

Nickel inhibits β-1 adrenoceptor mediated activation of cardiac CFTR chloride channels

Cardiac ventricular myocytes exhibit a protein kinase A-dependent Cl(−) current (I(Cl.PKA)) mediated by the cystic fibrosis transmembrane conductance regulator (CFTR). There is conflicting evidence regarding the ability of the divalent cation nickel (Ni(2+)), which has been used widely in vitro in the study of other cardiac ionic conductances, to inhibit I(Cl.PKA). Here the action of Ni(2+) on I(Cl.PKA) activated by β-adrenergic stimulation has been elucidated. Whole-cell patch-clamp recordings were made from rabbit isolated ventricular myocytes. Externally applied Ni(2+) blocked I(Cl.PKA) activated by 1 μM isoprenaline with a log IC(50) (M) of −4.107 ± 0.075 (IC(50) = 78.1 μM) at +100 mV and −4.322 ± 0.107 (IC(50) = 47.6 μM) at −100 mV. Thus, the block of I(Cl.PKA) by Ni(2+) was not strongly voltage dependent. Ni(2+) applied internally via the patch-pipette was ineffective at inhibiting isoprenaline-activated I(Cl,PKA), but in the same experiments the current was suppressed by external Ni(2+) application, indicative of an external site of Ni(2+) action. In the presence of 1 μM atenolol isoprenaline was ineffective at activating I(Cl.PKA), but in the presence of the β2-adrenoceptor inhibitor ICI 118,551 isoprenaline still activated Ni(2+)-sensitive I(Cl.PKA). Collectively, these data demonstrate that Ni(2+) ions produce marked inhibition of β1-adrenoceptor activated ventricular I(Cl.PKA) at submillimolar [Ni(2+)]: an action that is likely to involve an interaction between Ni(2+) and β1-adrenoceptors. The concentration-dependence for I(Cl.PKA) inhibition seen here indicates the potential for confounding effects on I(Cl,PKA) to occur even at comparatively low Ni(2+) concentrations, when Ni(2+) is used to study other cardiac ionic currents under conditions of β-adrenergic agonism

Inhibition of spontaneous activity of rabbit atrioventricular node cells by KB-R7943 and inhibitors of sarcoplasmic reticulum Ca²⁺ ATPase

The atrioventricular node (AVN) can act as a subsidiary cardiac pacemaker if the sinoatrial node fails. In this study, we investigated the effects of the Na–Ca exchange (NCX) inhibitor KB-R7943, and inhibition of the sarcoplasmic reticulum calcium ATPase (SERCA), using thapsigargin or cyclopiazonic acid (CPA), on spontaneous action potentials (APs) and [Ca(2+)](i) transients from cells isolated from the rabbit AVN. Spontaneous [Ca(2+)](i) transients were monitored from undialysed AVN cells at 37 °C using Fluo-4. In separate experiments, spontaneous APs and ionic currents were recorded using the whole-cell patch clamp technique. Rapid application of 5 μM KB-R7943 slowed or stopped spontaneous APs and [Ca(2+)](i) transients. However, in voltage clamp experiments in addition to blocking NCX current (I(NCX)) KB-R7943 partially inhibited L-type calcium current (I(Ca,L)). Rapid reduction of external [Na(+)] also abolished spontaneous activity. Inhibition of SERCA (using 2.5 μM thapsigargin or 30 μM CPA) also slowed or stopped spontaneous APs and [Ca(2+)](i) transients. Our findings are consistent with the hypothesis that sarcoplasmic reticulum (SR) Ca(2+) release influences spontaneous activity in AVN cells, and that this occurs via [Ca(2+)](i)-activated I(NCX); however, the inhibitory action of KB-R7943 on I(Ca,L) means that care is required in the interpretation of data obtained using this compound

Modulation by endothelin-1 of spontaneous activity and membrane currents of atrioventricular node myocytes from the rabbit heart

BACKGROUND: The atrioventricular node (AVN) is a key component of the cardiac pacemaker-conduction system. Although it is known that receptors for the peptide hormone endothelin-1 (ET-1) are expressed in the AVN, there is very little information available on the modulatory effects of ET-1 on AVN electrophysiology. This study characterises for the first time acute modulatory effects of ET-1 on AVN cellular electrophysiology. METHODS: Electrophysiological experiments were conducted in which recordings were made from rabbit isolated AVN cells at 35–37°C using the whole-cell patch clamp recording technique. RESULTS: Application of ET-1 (10 nM) to spontaneously active AVN cells led rapidly (within ∼13 s) to membrane potential hyperpolarisation and cessation of spontaneous action potentials (APs). This effect was prevented by pre-application of the ET(A) receptor inhibitor BQ-123 (1 µM) and was not mimicked by the ET(B) receptor agonist IRL-1620 (300 nM). In whole-cell voltage-clamp experiments, ET-1 partially inhibited L-type calcium current (I(Ca,L)) and rapid delayed rectifier K(+) current (I(Kr)), whilst it transiently activated the hyperpolarisation-activated current (I(f)) at voltages negative to the pacemaking range, and activated an inwardly rectifying current that was inhibited by both tertiapin-Q (300 nM) and Ba(2+) ions (2 mM); each of these effects was sensitive to ET(A) receptor inhibition. In cells exposed to tertiapin-Q, ET-1 application did not produce membrane potential hyperpolarisation or immediate cessation of spontaneous activity; instead, there was a progressive decline in AP amplitude and depolarisation of maximum diastolic potential. CONCLUSIONS: Acutely applied ET-1 exerts a direct modulatory effect on AVN cell electrophysiology. The dominant effect of ET-1 in this study was activation of a tertiapin-Q sensitive inwardly rectifying K(+) current via ET(A) receptors, which led rapidly to cell quiescence

Characterization of recombinant hERG K+ channel inhibition by the active metabolite of amiodarone desethyl-amiodarone

UNLABELLED: The aim of this study was to determine the effects of desethyl-amiodarone (DEA), the major metabolite of the class III antiarrhythmic drug amiodarone, on human ether-à-go-go-related gene (hERG) encoded potassium channel current. MATERIALS AND METHODS: Whole-cell patch clamp recordings were made at 37 degrees C of ionic current (I(hERG)) carried by recombinant hERG channels expressed in HEK-293 cells. RESULTS: Desethyl-amiodarone inhibited I(hERG) with a half-maximal inhibitory concentration of approximately 158 nmol/L, compared with approximately 47 nmol/L for amiodarone. The inhibitory action of DEA on I(hERG) was contingent on channel gating, showing significant time and voltage dependence. Desethyl-amiodarone also produced an approximately -9 mV shift in the voltage dependence of activation of I(hERG); however, there was no significant preference for activated over inactivated channels. CONCLUSIONS: Because hERG underlies native cardiac "I(Kr)" channels, hERG/I(Kr) inhibition by DEA as well as amiodarone may contribute to the overall effects of amiodarone administration on cardiac repolarization

Inhibition of sarcoplasmic reticulum Ca²⁺-ATPase decreases atrioventricular node-paced heart rate in rabbits

Recent data indicate that Ca2+ cycling in isolated atrioventricular node (AVN) cells contributes to setting spontaneous rate. The aim of the present study was to extend this observation to the intact AVN in situ, by evaluating the effects of inhibiting sarcoplasmic reticulum Ca2+ uptake with cyclopiazonic acid (CPA) on intact AVN spontaneous activity and its response to isoprenaline. A model of the AVN-paced heart was produced to investigate intact AVN automaticity, by surgical ablation of the sino-atrial node (SAN) in the rabbit Langendorff-perfused heart. Electrograms were recorded from a site close to the AVN (triangle of Koch), an atrial site above the AVN, the left atrium and right ventricle, enabling AVN pacing of the preparation to be confirmed. Before SAN ablation, the heart rate was 166.8 ± 5.4 beats min−1. Ablation of the SAN was clearly indicated by a sudden and significant decrease of heart rate to 108.6 ± 9.6 beats min−1 (P < 0.01, n = 10). Isoprenaline (100 nM) increased AVN rate to 187.8 ± 12.0 beats min−1 after 1 min of application (P < 0.01, n = 10). Cyclopiazonic acid (10 and 30 μM) decreased AVN rate to 81.6 ± 4.8 (n = 9) and 77.4 ± 6.0 beats min−1 (n = 7), respectively [P < 0.05, 10 or 30 μM CPA versus control (n = 10)] and also reduced the AVN rate increase in response to isoprenaline from 78.8 ± 10.0 to 46.8 ± 6.8 and 26.7 ± 5.3%, respectively (P < 0.01). These inhibitory effects of CPA on the intact AVN rate and its response to isoprenaline indicate that Ca2+ cycling is important to the intact AVN spontaneous activity and its acceleration during sympathetic stimulation