Abstract-The cardiac electrical system is designed to ensure the appropriate rate and timing of contraction in all regions of the heart, which are essential for effective cardiac function. Well-controlled cardiac electrical activity depends on specialized properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Cardiac electrical specialization was first recognized in the mid 1800s, but over the past 15 years, an enormous amount has been learned about how specialization is achieved by differential expression of cardiac ion channels. More recently, many aspects of the molecular basis have been revealed. Although the field is potentially vast, an appreciation of key elements is essential for any clinician or researcher wishing to understand modern cardiac electrophysiology. This article reviews the major regionally determined features of cardiac electrical function, discusses underlying ionic bases, and summarizes present knowledge of ion channel subunit distribution in relation to functional specialization. Key Words: ion channels Ⅲ molecular biology Ⅲ conduction Ⅲ cardiac arrhythmias Ⅲ antiarrhythmic drugs C ardiac function depends on the appropriate timing of contraction in various regions, as well as on appropriate heart rate. To subserve these functions, electrical activity in each region is adapted to its specialized function. Regionally specialized cardiac electrical function was recognized in the mid 1800s, when Stannius 1 demonstrated that ligatures in the superior vena caval sinus region of the frog caused cardiac asystole, with the sinus continuing to beat. With the widespread application to cardiac ion channel study of patchclamp methodologies in the 1980s and molecular biology in the 1990s, many underlying mechanisms have been unraveled. The present article reviews the major regionally determined features of cardiac electrical function and the present knowledge regarding ionic and molecular bases. Overview of Regional Functional SpecificityFigure 1 illustrates typical regional action potential (AP) properties in the heart. The normal cardiac impulse originates in the sinoatrial node (SAN) and propagates through the atria to reach the atrioventricular node (AVN). From the AVN, electrical activity passes rapidly through the cable-like HisPurkinje system to reach the ventricles, triggering cardiac pumping action. Figure 2 shows the ionic currents involved in a schematic cardiac AP, provides standard abbreviations for currents and their corresponding subunits, and summarizes principal localization data discussed elsewhere in the present review. Ionic and Molecular Basis of Functional Specificity Sinoatrial Node Cellular Electrophysiology and FunctionThe SAN, located in the right atrial (RA) roof between the venae cavae, 2 is specialized for physiological pacemaker function. Heart rate control is achieved through autonomic regulation of SAN pacemaking. SAN APs have a relatively positive maximum diastolic potential (MDP...

Vernakalant (RSD1235) is an investigational drug recentlyshown to convert atrial fibrillation rapidly and safely in patients (J Am Coll Cardiol 44:2355-2361. Here, the molecular mechanisms of interaction of vernakalant with the inner pore of the Kv1.5 channel are compared with those of the class IC agent flecainide. Initial experiments showed that vernakalant blocks activated channels and vacates the inner vestibule as the channel closes, and thus mutations were made, targeting residues at the base of the selectivity filter and in S6, by drawing on studies of other Kv1.5-selective blocking agents. Block by vernakalant or flecainide of Kv1.5 wild type and mutants was assessed by whole-cell patch-clamp experiments in transiently transfected human embryonic kidney 293 cells. The mutational scan identified several highly conserved amino acids, Thr479, Thr480, Ile502, Val505, and Val508, as important residues for affecting block by both compounds. In general, mutations in S6 increased the IC 50 for block by vernakalant; I502A caused an extremely local 25-fold decrease in potency. Specific changes in the voltage-dependence of block with I502A supported the crucial role of this position. A homology model of the pore region of Kv1.5 predicted that, of these residues, only Thr479, Thr480, Val505, and Val508 are potentially accessible for direct interaction, and that mutation at additional sites studied may therefore affect block through allosteric mechanisms. For some of the mutations, the direction of changes in IC 50 were opposite for vernakalant and flecainide, highlighting differences in the forces that drive drug-channel interactions.

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Rate-dependent block of Na(+) channels represents the main antiarrhythmic mechanism of vernakalant in the fibrillating atrium. Open channel block of early transient outward currents and IK,ACh could also contribute.

Greater ERG and KvLQT1 abundance in pulmonary vein cardiomyocytes, lower abundance of Kir2.3 in pulmonary veins and differential pulmonary vein subcellular distribution of Kir2.3, ERG and KvLQT1 subunits may contribute to ionic current differences between pulmonary vein and left atrial cardiomyocytes.

Background Several clinical trials have shown that vernakalant is effective in terminating recent-onset atrial fibrillation (AF). The electrophysiological actions of vernakalant are not fully understood. Methods and Results Here we report the results of a blinded study comparing the in vitro canine atrial electrophysiological effects of vernakalant, ranolazine, and dl-sotalol. Action potential durations (APD50,75,90), effective refractory period (ERP), post-repolarization refractoriness (PRR), maximum rate of rise of the action potential (AP) upstroke (Vmax), diastolic threshold of excitation (DTE), conduction time (CT), and the shortest S1-S1 permitting 1:1 activation (S1-S1) were measured using standard stimulation and microelectrode recording techniques in isolated normal, non-remodeled canine arterially-perfused left atrial preparations. Vernakalant caused variable but slight prolongation of APD90 (p=n.s.), but significant prolongation of APD50 at 30 µM and rapid rates. In contrast, ranolazine and dl-sotalol produced consistent concentration- and reverse rate-dependent prolongation of APD90. Vernakalant and ranolazine caused rate-dependent, whereas dl-sotalol caused reverse rate-dependent, prolongation of ERP. Significant rate-dependent PRR developed with vernakalant and ranolazine, but not with dl-sotalol. Other INa-mediated parameters (i.e., Vmax, CT, DTE, and S1-S1) were also significantly depressed by vernakalant and ranolazine, but not by dl-sotalol. Only vernakalant elevated AP plateau voltage, consistent with blockade of IKur and Ito. Conclusions In isolated canine left atria, the effects of vernakalant and ranolazine were characterized by use-dependent inhibition of sodium channel-mediated parameters and those of dl-sotalol by reverse rate-dependent prolongation of APD90 and ERP. This suggests that during the rapid activation rates of AF, the INa blocking action of the mixed ion channel blocker vernakalant takes prominence. This mechanism may explain vernakalant’s anti AF efficacy.

Heteromeric channel assembly is a potential source of physiological variability. The potential significance of Kir2 subunit heterotetramerization has been controversial, but recent findings suggest that heteromultimerization of Kir2.1‐3 may be significant. This study was designed to investigate whether the recently described Kir2.4 subunit can form heterotetramers with the important subunit Kir2.1, and if so, to investigate whether the resulting heterotetrameric channels are functional. Co‐expression of either dominant negative Kir2.1 or Kir2.4 subunits in Xenopus oocytes with either wild‐type Kir2.1 or 2.4 strongly decreased resulting current amplitude. To examine physical association between Kir2.1 and Kir2.4, Cos‐7 cells were co‐transfected with a His6‐tagged Kir2.1 subunit (Kir2.1‐His6) and a FLAG‐tagged Kir2.4 subunit (Kir2.4‐FLAG). After pulldown with a His6‐binding resin, Kir2.4‐FLAG could be detected in the eluted cell lysate by Western blotting, indicating co‐assembly of Kir2.1‐His6 and Kir2.4‐FLAG. Expression of a tandem construct containing covalently linked Kir2.1 and 2.4 subunits led to robust current expression. Kir2.1‐Kir2.4 tandem subunit expression, as well as co‐injection of Kir2.1 and Kir2.4 cRNA into Xenopus oocytes, produced currents with barium sensitivity greater than that of Kir2.1 or Kir2.4 subunit expression alone. These results show that Kir2.4 subunits can co‐assemble with Kir2.1 subunits, and that co‐assembled channels are functional, with properties different from those of Kir2.4 or Kir2.1 alone. Since Kir2.1 and Kir2.4 mRNAs have been shown to co‐localize in the CNS, Kir2.1 and Kir2.4 heteromultimers might play a role in the heterogeneity of native inward rectifier currents.

Background-Nicotine is a main constituent of cigarette smoke and smokeless tobacco, known to increase the risk of sudden cardiac death. This study aimed at establishing ionic mechanisms underlying potential electrophysiological effects of nicotine. Methods and Results-Effects of nicotine on Kv4.3 and Kv4.2 channels expressed in Xenopus oocytes were studied at the whole-cell and single-channel levels. The effects of nicotine on the transient outward K ϩ current (I to ) were studied by use of whole-cell patch-clamp techniques in canine ventricular myocytes. Nicotine potently inhibited Kv4 current. The concentration for half-maximal inhibition (IC 50 ) was 40Ϯ4 nmol/L, and the current was abolished by 100 mol/L nicotine. The IC 50 for block of native I to was 270Ϯ43 nmol/L. The steady-state activation properties of Kv4.3 and I to were unaltered by nicotine, whereas positive shifts of the inactivation curves were observed. Of the total inhibition of Kv4.3 and I to by nicotine, 40% was due to tonic block and 60% was attributable to use-dependent block. Activation, inactivation, and reactivation kinetics were not significantly changed by nicotine. Nicotine reduced single-channel conductance, open probability, and open time but increased the closed time of Kv4.3. The effects of nicotine were not altered by antagonists to various neurotransmitter receptors, indicating direct effects on I to channels. Conclusions-Nicotine is a potent inhibitor of cardiac A-type K ϩ channels, with blockade probably due to block of closed and open channels. This action may contribute to the ability of nicotine to affect cardiac electrophysiology and induce arrhythmias.