老年大鼠心肌磷脂和钙通道变化及其与心律失常的关系

目的探讨老年大鼠心肌磷脂和钙通道的变化及其与心律失常发生的关系。方法比较老年（２４～２６月龄）和青年（３～４月龄）大鼠极小量肾上腺素诱发心律失常的发生率、持续时间、病死率以及心室晚电位、心肌膜磷脂组分、钙通道二氢吡啶（ＤＨＰ）受体活性、超氧化物歧化酶（ＳＯＤ）活性的变化。结果老年大鼠心律失常诱发率高达１００％，病死率２３．５％，同时还存在膜卵磷脂降解；ＤＨＰ受体平均解离常数（ｋｄ值）增大４４％，最大结合率（Ｂｍａｘ）增高３２％；ＳＯＤ活性降低近５０％。结论老年大鼠易发心律失常，其机理可能与心肌膜稳定性降低及分子受体活性衰退有关。     

Changes in left ventricular repolarization and ion channel currents following a transient rate increase superimposed on bradycardia in anesthetized dogs

INTRODUCTION: We previously demonstrated in dogs that a transient rate increase superimposed on bradycardia causes prolongation of ventricular refractoriness that persists for hours after resumption of bradycardia. In this study, we examined changes in membrane currents that are associated with this phenomenon. METHODS AND RESULTS: The whole cell, patch clamp technique was used to record transmembrane voltages and currents, respectively, in single mid-myocardial left ventricular myocytes from dogs with 1 week of complete AV block; dogs either underwent 1 hour of left ventricular pacing at 120 beats/min or did not undergo pacing. Pacing significantly heightened mean phase 1 and peak plateau amplitudes by approximately 6 and approximately 3 mV, respectively (P < 0.02), and prolonged action potential duration at 90% repolarization from 235+/-8 msec to 278+/-8 msec (1 Hz; P = 0.02). Rapid pacing-induced changes in transmembrane ionic currents included (1) a more pronounced cumulative inactivation of the 4-aminopyridine-sensitive transient outward K+ current, Ito, over the range of physiologic frequencies, resulting from a approximately 30% decrease in the population of quickly reactivating channels; (2) increases in peak density of L-type Ca2+ currents, I(Ca.L), by 15% to 35 % between +10 and +60 mV; and (3) increases in peak density of the Ca2+-activated chloride current, I(Cl.Ca), by 30% to 120% between +30 and +50 mV. CONCLUSION: Frequency-dependent reduction in Ito combined with enhanced I(Ca.L) causes an increase in net inward current that may be responsible for the observed changes in ventricular repolarization. This augmentation of net cation influx is partially antagonized by an increase in outward I(Ca.Cl).     

Concentration-dependent effect of trapidil on slow action potentials in cardiac muscle

Trapidil, a coronary vasodilator and positive inotropic agent, was tested for its ability to affect the normal "fast" action potentials and the "slow" action potentials and contractions of isolated perfused chick hearts, and to affect the tissue cyclic AMP level. At 5 X 10(-3) M, trapidil completely blocked the fast Na+ channels in hearts perfused with normal Tyrode solution, since this dose abolished the action potential when verapamil (2 X 10(-6) M) was present to eliminate the inward slow current. To study effects on the slow channels, the fast Na+ channels were voltage-inactivated by partial depolarization to about -40 mV with an elevated (25 mM) K+-Tyrode solution, resulting in loss of excitability. At low concentrations (1 X 10(-4) - 1 X 10(-3) M), trapidil induced slow action potentials accompanied by contractions, even in the presence of a beta-adrenergic blocker. In contrast, at high concentrations (3 X 10(-3) - 1 X 10(-2) M), trapidil markedly depressed or blocked the isoproterenol-induced slow action potentials. Consistent with this dual action, in hearts perfused with normal Tyrode solution, trapidil exerted a small positive inotropic action at low doses and a considerable negative inotropic action at high doses, even though the intracellular cyclic AMP level was substantially elevated. That is, trapidil has actions similar to those of papaverine. It is concluded that trapidil blocks both fast Na+ channels and slow channels in cardiac muscle, the fast Na+ channels being more sensitive, and that low concentrations of trapidil induce slow channels by elevating the cyclic AMP level because of phosphodiesterase inhibition.     

Mathematical models of electrical activity of the pancreatic β-cell: A physiological review

Mathematical modeling of the electrical activity of the pancreatic β-cell has been extremely important for understanding the cellular mechanisms involved in glucose-stimulated insulin secretion. Several models have been proposed over the last 30 y, growing in complexity as experimental evidence of the cellular mechanisms involved has become available. Almost all the models have been developed based on experimental data from rodents. However, given the many important differences between species, models of human β-cells have recently been developed. This review summarizes how modeling of β-cells has evolved, highlighting the proposed physiological mechanisms underlying β-cell electrical activity.     

Effects of cytochalasin D on electrical restitution and the dynamics of ventricular fibrillation in isolated rabbit heart

Cytochalasin D in Rabbit Ventricle. INTRODUCTION: Cytochalasin D (cyto-D) has been used as an excitation-contraction uncoupler during optical mapping studies. However, its effects on action potential duration restitution (APDR) and dynamics during ventricular fibrillation (VF) are unclear. METHODS AND RESULTS: Langendorff-perfused rabbit hearts (N = 6) were immersed in a tissue chamber. Transmembrane potential was recorded using glass microelectrodes. APD measured to 90% repolarization (APD90) was used to construct the APDR curve. During regular pacing at 300-msec cycle length, increasing concentrations of cyto-D resulted in progressively prolonged APD90 (131 +/- 26 msec, 171 +/- 14 msec, and 177 +/- 14 msec) and steepened maximum slope of the APDR curve (1.1 +/- 0.2, 1.3 +/- 0.2, and 1.6 +/- 0.4 for control, 5 micromoles, and 10 micromoles, respectively; P < 0.01). Resting membrane potential, AP amplitude, and maximum dV/dt did not change. Cyto-D lengthened VF cycle length and APD90, and steepened the maximum slope of the APDR curve. However, cyto-D did not significantly change the diastolic interval. The dominant frequency of pseudoelectrocardiogram progressively decreased with increasing concentrations of cyto-D (15.2 +/- 0.6 Hz, 11.1 +/- 2.4 Hz, and 9.8 +/- 3.2 Hz for control, 5 micromoles, and 10 micromoles, respectively; P < 0.01). Sustained (>1 min) VF was repeatedly inducible at baseline and with 5 or 10 micromoles of cyto-D. CONCLUSION: Continuous perfusion of cyto-D at 5 or 10 micromoles prolonged APD90, steepened APDR slope, and reduced dominant frequency in rabbit ventricles. Cyto-D at these concentrations allowed induction of sustained VF.     

心钠素对豚鼠乳头肌动作电位和心室肌细胞钙通道电流的影响

应用标准玻璃微电极和全细胞膜片钳技术研究了心钠素（ＡＮＰ）对豚鼠乳头肌动作电位和心室肌细胞钙通道电流的影响及其对β受体兴奋的拮抗作用。结果表明：ＡＮＰ可明显缩短动作电位时程，尤其动作电位平台期缩短较为明显，２０ｎｍｏｌ／ＬＡＮＰ灌流心室乳头肌可明显抑制β受体兴奋所致动作电位平台期的延长。ＡＮＰ这种作用的机制与其抑制心室肌细胞钙通道电流有关。     

腺苷对心肌细胞的电生理作用及机制探讨

采用微电极技术及膜片钳全细胞记录方式，研究腺苷对豚鼠心肌细胞的电生理作用及其机制。结果表明：腺苷可明显缩短心房肌及房室结区细胞动作电位时程，降低房室结区细胞动作电位振幅、零相最大去极化速率，膜片钳上证明此为腺苷加强延迟整流性钾通道电流和抑制Ｌ型钙通道电流所致。对心室肌细胞无此明显的作用，但应用异丙肾上腺素后证明腺苷能拮抗β１受体的作用。腺苷的作用能被选择性腺苷Ａ１受体阻断剂８－环戊基－１，３－二丙基黄嘌呤消除，提示腺苷对心肌细胞的作用由Ａ１受体介导。本研究也探讨了腺苷对心房肌和心室肌作用区别的可能原因，以及心房肌和心室肌对异丙肾上腺素合用腺苷时反应不同的可能机制。     

Progress and prospects for optimum antiarrhythmic drug design

Summary Different class I drugs slow down to differing degrees the rate at which sodium channel availability, hence excitability, recovers after action potentials. Drugs that produce longer recovery half-times generally produce greater proarrhythmic side effects. Increased lipid solubility may improve a drug's potency for blocking channels yet with implications for adverse effects. Drug action may be potentiated in depolarized and acidotic tissue via modulation of the recovery process. A knowledge of molecular properties of antiarrhythmic drugs helps to define these modes of interraction with the sodium channels and, hence, will help in future drug design. Prospects for improving our understanding of ionic events involved in the repolarization phase of cardiac action potentials are also outlined. The development of successful strategies for controlling reentrant arrhythmias will probably require a thorough understanding of both class I and class III drug actions at the level of the membrane ion channel.Supported in part by NIH grants HL 24156, and a gift from the William Coldiron family.     

Increased Right Ventricular Repolarization Gradients Promote Arrhythmogenesis in a Murine Model of Brugada Syndrome

Repolarization Gradients in Brugada Syndrome. Introduction: Brugada syndrome (BrS) is associated with loss of Na⁺ channel function and increased risks of a ventricular tachycardia exacerbated by flecainide but reduced by quinidine. Previous studies in nongenetic models have implicated both altered conduction times and repolarization gradients in this arrhythmogenicity. We compared activation latencies and spatial differences in action potential recovery between different ventricular regions in a murine Scn5a+/? BrS model, and investigated the effect of flecainide and quinidine upon these. Methods and Results: Langendorff‐perfused wild‐type and Scn5a+/? hearts were subjected to regular pacing and a combination of programmed electrical stimulation techniques. Monophasic action potentials were recorded from the right (RV) and left ventricular (LV) epicardium and endocardium before and following flecainide (10 μM) or quinidine (5 μM) treatment, and activation latencies measured. Transmural repolarization gradients were then calculated from the difference between neighboring endocardial and epicardial action potential durations (APDs). Scn5a+/? hearts showed decreased RV epicardial APDs, accentuating RV, but not LV, transmural gradients. This correlated with increased arrhythmic tendencies compared with wild‐type. Flecainide increased RV transmural gradients, while quinidine decreased them, in line with their respective pro‐ and antiarrhythmic effects. In contrast, Scna5+/? hearts showed slowed conduction times in both RV and LV, exacerbated not only by flecainide but also by quinidine, in contrast to their differing effects on arrhythmogenesis. Conclusion: We use a murine genetic model of BrS to systematically analyze LV and RV action potential kinetics for the first time. This establishes a key role for accentuated transmural gradients, specifically in the RV, in its arrhythmogenicity. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1153‐1159)     

Dispersion and delay of electrical restitution in the globally ischaemic heart

Altemans of action potential duration (APD) has been shown tobe a precursor of ventricular fibrillation in ischaemic myocardium.We postulated that magnitude of alternans of APD during ischaemiadepends not only on the severity of ischaemia but also on disturbedbeat-to-beat restitution of APD. Monophasic action potentials were recorded simultaneously fromright (RV) and left ventricular (LV) epicardial sites of isolatedrabbit hearts. The inter-beat time courses of APD recovery weredetermined both during normal flow and ischaemia by interposingsingle cycle length changes ranging from 200 to 800 ms (=electricalrestitution) simultaneously at the three recording sites. During normal perfusion, electrical restitution curves showeda steep initial recovery of APD, attaining steady-state valuesat extrastimulus cycle lengths of only 298±12 ms, witha high degree of uniformity between the three recording sites(inter-site variability <2%). Ischaemia produced a markedslowing of electrical restitution which, on average, reacheda plateau at extrastimulus cycle lengths of 415±45 ms,650 ± 72 ms and >800 ms at 2 min, 5 min and 9 minof ischaemia, respectively (each P<0·001 vs control).In addition, iscliaemia resulted in a large inter-site variability,with RV and LV restitution curves deviating from each otherby as much as 28·5% (P<0·0001 vs baseline). We conclude that global ischaemia not only leads to a delayedbut also non-uniform electrical restitution. The delay in electricalrestitution may be causally related to the development of alternansof APD, whereas the dispersion of electrical restitution mayproduce electrical instability and set the stage for ischaemia-relatedventricular arrhythmias.     

Upregulation of microRNA-1 and microRNA-133 contributes to arsenic-induced cardiac electrical remodeling

Background

A large body of evidence showed that arsenic trioxide (As₂O₃), a front-line drug for the treatment of acute promyelocytic leukemia, induced abnormal cardiac QT prolongation, which hampers its clinical use. The molecular mechanisms for this cardiotoxicity remained unclear. This study aimed to elucidate whether microRNAs (miRs) participate in As₂O₃-induced QT prolongation.

Methods

A guinea pig model of As₂O₃-induced QT prolongation was established by intravenous injection with As₂O₃. Real-time PCR and Western blot were employed to determine the expression alterations of miRs and mRNAs, and their corresponding proteins.

Results

The QT interval and QRS complex were significantly prolonged in a dose-dependent fashion after 7-day administration of As₂O₃. As₂O₃ induced a significant upregulation of the muscle-specific miR-1 and miR-133, as well as their transactivator serum response factor. As₂O₃ depressed the protein levels of ether-a-go-go related gene (ERG) and Kir2.1, the K⁺ channel subunits responsible for delayed rectifier K⁺ current I_Kr and inward rectifier K⁺ current I_K1, respectively. In vivo transfer of miR-133 by direct intramuscular injection prolonged QTc interval and increased mortality rate, along with depression of ERG protein and I_Kr in guinea pig hearts. Similarly, forced expression of miR-1 widened QTc interval and QRS complex and increased mortality rate, accompanied by downregulation of Kir2.1 protein and I_K1. Application of antisense inhibitors to knockdown miR-1 and miR-133 abolished the cardiac electrical disorders caused by As₂O₃.

Conclusions

Deregulation of miR-133 and miR-1 underlies As₂O₃-induced cardiac electrical disorders and these miRs may serve as potential therapeutic targets for the handling of As₂O₃ cardiotoxicity.     

Mathematical models of electrical activity of the pancreatic β-cell: A physiological review

Mathematical modeling of the electrical activity of the pancreatic β-cell has beenextremely important for understanding the cellular mechanisms involved inglucose-stimulated insulin secretion. Several models have been proposed over the last 30y, growing in complexity as experimental evidence of the cellular mechanisms involved hasbecome available. Almost all the models have been developed based on experimental datafrom rodents. However, given the many important differences between species, models ofhuman β-cells have recently been developed. This review summarizes how modeling ofβ-cells has evolved, highlighting the proposed physiological mechanisms underlyingβ-cell electrical activity.     

Long QT syndrome: cellular basis and arrhythmia mechanism in LQT2

LQT2 is one form of the congenital long QT syndrome. It results from mutations in the human ether-a-go-go-related gene (HERG), and more than 80 mutations, usually causing single amino acid substitutions in the HERG protein, are known. HERG encodes the ion channel pore-forming subunit protein for the rapidly activating delayed rectifier K+ channel (I(Kr)) in the heart. This review summarizes current findings about mutations causing LQT2, the mechanisms by which mutations may cause the clinical phenotype of a reduction in I(Kr) and a prolonged QT interval, and how this may be involved in the generation of ventricular arrhythmias.     

牵拉刺激对豚鼠心室肌动作电位时程的影响及其机制

目的本实验观察了牵拉刺激对离体豚鼠左心室乳头肌动作电位的影响,为探讨心肌牵张激活离子通道电流的特点提供实验依据。方法实验采用标准微电极细胞内记录技术引导豚鼠心室乳头肌细胞动作电位,用微机化生理信号采集分析系统(NSA-Ⅲ)记录并处理信号。结果①牵拉心肌可加快整个复极过程,APD20、APD50和APD90均有缩短(P<0.05)。牵拉作用呈心肌长度依赖性。牵拉刺激对RP和APA无显著性影响(P>0.05)。②内向整流钾通道(IK1)阻断剂BaCl2(100μmol/L)可明显减弱因牵拉刺激导致的动作电位时程缩短。应用格列本脲、奎尼丁和氯化钆与未用药组比较无显著差异(P>0.05)。结论①在豚鼠心室肌存在牵拉刺激激活的外向电流,这种电流加快复极过程,使动作电位时程明显缩短。②该电流可能与内向整流钾通道(IK1)有关,但是与ATP敏感性钾通道和IKr无关。     

Endocannabinoids and cannabinoid analogues block human cardiac Kv4.3 channels in a receptor-independent manner

Endocannabinoids are amides and esters of long chain fatty acids that can modulate ion channels through both receptor-dependent and receptor-independent effects. Nowadays, their effects on cardiac K⁺ channels are unknown even when they can be synthesized within the heart. We have analyzed the direct effects of endocannabinoids, such as anandamide (AEA), 2-arachidonoylglycerol (2-AG), the endogenous lipid lysophosphatidylinositol, and cannabinoid analogues such as palmitoylethanolamide (PEA), and oleoylethanolamide, as well as the fatty acids from which they are endogenously synthesized, on human cardiac Kv4.3 channels, which generate the transient outward K⁺ current (I_to1). Currents were recorded in Chinese hamster ovary cells, which do not express cannabinoid receptors, by using the whole-cell patch-clamp. All these compounds inhibited I_Kv4.3 in a concentration-dependent manner, AEA and 2-AG being the most potent (IC₅₀ ∼ 0.3-0.4 µM), while PEA was the least potent. The potency of block increased as the complexity and the number of C atoms in the fatty acyl chain increased. The effects were not mediated by modifications in the lipid order and microviscosity of the membrane and were independent of the presence of MiRP2 or DPP6 subunits in the channel complex. Indeed, effects produced by AEA were reproduced in human atrial I_to1 recorded in isolated myocytes. Moreover, AEA effects were exclusively apparent when it was applied to the external surface of the cell membrane. These results indicate that at low micromolar concentrations the endocannabinoids AEA and 2-AG directly block human cardiac Kv4.3 channels, which represent a novel molecular target for these compounds.     

绝对不应期电刺激对心力衰竭豚鼠心室肌细胞动作电位时程和L型钙电流的影响

目的 探讨绝对不应期电刺激(ARPES)对健康和心力衰竭豚鼠心室肌细胞(分别简称NVM和FVM)动作电位(AP)时程(APD)和L型钙电流(ICa-L)的影响。方法 应用膜片钳技术中电流钳,首先记录基础刺激S1所诱发的AP(APS1),与S1延迟10 ms给予ARPES,记录ARPES给予后的AP(APARPES),比较APS1和APARPES的APD的值,以APD30、APD50和APD90代表动作电位复极30%、50％和90％时的APD值。分别以APS1和APARPES为测试电压,记录AP电压钳下的细胞膜ICa-L。结果(1)在NVM和FVM,ARPES应用后APD均明显延长,以APD30和APD50最为显著(P<0.01)。(2)在NVM,与APS1电压钳记录的ICa-L相比,APARPES电压钳记录的ICa-L电流强度有一过性的减弱和继发增强,但其单位膜电容下的电流强度的整合值略有降低(P<0.05)。在FVM,与APS1电压钳记录的ICa-L相比,APARPES电压钳记录的ICa-L电流强度的减弱程度明显减少,其单位膜电容下的电流强度的整合值是增加的(P<0.01)。结论 ARPES延长NVM和FVM的APD,对NVM和FVM的ICa-L具有不同的影响。     

Effects of dofetilide on electrical dispersion and arrhythmias in post-infarcted anesthetized dogs

An increase in dispersion of myocardial refractoriness has been shown to coincide with a greater risk of inducible ventricular arrhythmias. We compared the dispersion of electrophysiologic parameters and antiarrhythmic effects of dofetilide (0.03, 0.1, 0.3 and 1 mg/kg i.v.) in postinfarcted anesthetized dogs. Animals were tested for inducibility of arrhythmias using a programmed electrical stimulation (PES) protocol, and divided into inducible (1) and non-inducible (NI) groups. In addition, myocardial vulnerability was measured using ventricular fibrillation thresholds (VFT), as well as susceptibility to sudden cardiac death (SCD). Dofetilide significantly increased ventricular effective refractory periods (ERP) and monophasic action potential durations (APD) in a dose-dependent manner. The standard deviation of ERP, which was used as an index of dispersion of refractoriness, increased from sham (control value of 5.4±sd2.5 ms), non-inducible (control value of 11.0 ±5.5 and 8.0±3.7 ms for vehicle and dofetilide groups, respectively) and inducible states (control value of 17.3±6.2 and 21.6±7.1 ms for vehicle and dofetilide groups, respectively). However, dofetilide treatment did not alter dispersion of refractoriness over the dose range studied. Dofetilide did not significantly increase inducibility in the NI group (2 out of 8[25%] compared to 0 out of 9 [0 %] in vehicle treated animals). In the I group, dofetilide (0.3 mg/kg) treated animals converted 2 out of 7 (29 %) to NI, and 5 out of 7 (71 %; significant at p<0.05) to a NI or non sustained ventricular tachycardia. There were no significant changes in VFT following the last dose of dofetilide given. Dofetilide did not significantly affect SCD survival (88 % and 29 % in the NI and I group, resectively) relauve to vehicle (66 % and 50 % in the NI and I group, respectively). Although infarct sizes were significantly greater in the I groups, there was no difference between vehicle and dofetilide animals within these groups. In conclusion, dofetilide increased ERP and APD values, but did not affect dispersion of refractoriness. Thus, changes in dispersion of refractori ness may be used as a marker for inducibility in untreated animals, but it did not predict the antiarrhythmic effects observed with dofetilide.