Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally-determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

The molecular clock mechanism underlies circadian rhythms and is defined by a transcription-translation feedback loop. Bmal1 encodes a core molecular clock transcription factor. Germline Bmal1 knockout mice show a loss of circadian variation in heart rate and blood pressure, and they develop dilated cardiomyopathy. We tested the role of the molecular clock in adult cardiomyocytes by generating mice that allow for the inducible cardiomyocyte-specific deletion of Bmal1 (iCS⌬Bmal1). ECG telemetry showed that cardiomyocyte-specific deletion of Bmal1 (iCS⌬Bmal1 Ϫ/Ϫ ) in adult mice slowed heart rate, prolonged RR and QRS intervals, and increased episodes of arrhythmia. Moreover, isolated iCS⌬Bmal1 Ϫ/Ϫ hearts were more susceptible to arrhythmia during electromechanical stimulation. Examination of candidate cardiac ion channel genes showed that Scn5a, which encodes the principle cardiac voltage-gated Na ϩ channel (NaV1.5), was circadianly expressed in control mouse and rat hearts but not in iCS⌬Bmal1 Ϫ/Ϫ hearts. In vitro studies confirmed circadian expression of a human Scn5a promoter-luciferase reporter construct and determined that overexpression of clock factors transactivated the Scn5a promoter. Loss of Scn5a circadian expression in iCS⌬Bmal1 Ϫ/Ϫ hearts was associated with decreased levels of NaV1.5 and Na ϩ current in ventricular myocytes. We conclude that disruption of the molecular clock in the adult heart slows heart rate, increases arrhythmias, and decreases the functional expression of Scn5a. These findings suggest a potential link between environmental factors that alter the cardiomyocyte molecular clock and factors that influence arrhythmia susceptibility in humans. cardiac excitability; circadian; heart; ion channels; Scn5a; Na ϩ current CIRCADIAN RHYTHMS are approximate 24-h cycles in biology. These rhythms are present at the systems level, the tissue level, the single cell and molecular levels (16,33,34,38). There are several examples of circadian rhythms in the cardiovascular system, with heart rate, blood pressure, and substrate metabolism exhibiting distinct oscillations over time of day (12,13,40,41). The mechanism that underlies circadian function is the molecular clock. The molecular clock is defined, in a simple way, by a transcription-translation feedback mechanism that is composed of the core clock genes Clock, Bmal1, Per1, Per2, Cry1, and Cry2. CLOCK and BMAL1 are transcription factors that heterodimerize and activate transcription of Per1, Per2, Cry1, and Cry2. PER1, PER2, CRY1, and CRY2 form multimers in the cytoplasm of the cell, translocate to the nucleus, and act to inhibit CLOCK:BMAL1 function. This cycle takes ϳ24 h and is the fundamental mechanism underlying circadian rhythms. Components of the core clock have also been shown to regulate the expression of genes outside the clock mechanism, and these genes are designated as clock-controlled genes (CCGs). CCGs often encode transcription factors or proteins that control rate-limiting steps in cell physiology (42).In the last 8 years, resear...

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Background Sudden Cardiac Death (SCD) follows a diurnal variation. Data suggest the timing of SCD is influenced by circadian (~24 hour) changes in neurohumoral and cardiomyocyte-specific regulation of the heart’s electrical properties. Objective The basic helix-loop-helix transcription factors BMAL1 and CLOCK coordinate the circadian expression of select genes. We tested whether Bmal1 expression in cardiomyocytes contributes to K+ channel expression and diurnal changes in ventricular repolarization. Methods We utilized transgenic mice that allow for the inducible cardiomyocyte-specific deletion of Bmal1 (iCSΔBmal1−/−). We used quantitative PCR, voltage-clamping, promoter-reporter bioluminescence assays, and electrocardiographic (ECG) telemetry. Results Although several K+ channel gene transcripts were downregulated in iCSΔBmal1−/− mouse hearts, only Kcnh2 exhibited a robust circadian pattern of expression that was disrupted in iCSΔBmal1−/− hearts. Kcnh2 underlies the rapidly activating delayed-rectifier K+ current (IKr), and IKr recorded from iCSΔBmal1−/− ventricular cardiomyocytes was ~50% compared to control myocytes. Promoter-reporter assays demonstrated that the human Kcnh2 promoter is transactivated by the co-expression of BMAL1 and CLOCK. ECG analysis showed iCSΔBmal1−/− mice developed a prolongation in the heart rate corrected QT (QTc) interval during the light (resting)-phase. This was secondary to an augmented circadian rhythm in the uncorrected QT interval without a corresponding change in the RR interval. Conclusion The molecular clock in the heart regulates the circadian expression of Kcnh2, modifies K+ channel gene expression and is important for normal ventricular repolarization. Disruption of the cardiomyocyte circadian clock mechanism likely unmasks diurnal changes in ventricular repolarization that could contribute to an increased risk of cardiac arrhythmias/SCD.

This paper is the outcome of the fourth UC Davis Systems Approach to Understanding Cardiac Excitation-Contraction Coupling and Arrhythmias Symposium, a biannual event that aims to bring together leading experts in subfields of cardiovascular biomedicine to focus on topics of importance to the field. The theme of the 2016 symposium was 'K Channels and Regulation'. Experts in the field contributed their experimental and mathematical modelling perspectives and discussed emerging questions, controversies and challenges on the topic of cardiac K channels. This paper summarizes the topics of formal presentations and informal discussions from the symposium on the structural basis of voltage-gated K channel function, as well as the mechanisms involved in regulation of K channel gating, expression and membrane localization. Given the critical role for K channels in determining the rate of cardiac repolarization, it is hardly surprising that essentially every aspect of K channel function is exquisitely regulated in cardiac myocytes. This regulation is complex and highly interrelated to other aspects of myocardial function. K channel regulatory mechanisms alter, and are altered by, physiological challenges, pathophysiological conditions, and pharmacological agents. An accompanying paper focuses on the integrative role of K channels in cardiac electrophysiology, i.e. how K currents shape the cardiac action potential, and how their dysfunction can lead to arrhythmias, and discusses K channel-based therapeutics. A fundamental understanding of K channel regulatory mechanisms and disease processes is fundamental to reveal new targets for human therapy.

BACKGROUND Loss-of-function mutations in the gene KCNQ1 encoding the Kv7.1 K+ channel cause long QT syndrome type 1 (LQT1), whereas gain-of-function mutations are associated with short QT syndrome as well as familial atrial fibrillation (FAF). However, KCNQ1 mutation pleiotropy, which is capable of expressing both LQT1 and FAF, has not been demonstrated for a discrete KCNQ1 mutation. The genotype–phenotype relationship for a family with FAF suggests a possible association with the LQT1 p.Arg231Cys-KCNQ1 (R231C-Q1) mutation. OBJECTIVE The purpose of this study was to determine whether R231C-Q1 also can be linked to FAF. METHODS The R231C-Q1 proband with AF underwent genetic testing for possible mutations in 10 other AF-linked genes plus KCNH2 and SCN5A. Sixteen members from five other R231C-positive LQT1 families were genetically tested for 21 single nucleotide polymorphisms (SNPs) to determine if the FAF family had discriminatory SNPs associated with AF. R231C-Q1 was expressed with KCNE1 (E1) in HEK293 cells, and Q1E1 currents (IQ1E1) were analyzed using the whole-cell patch-clamp technique. RESULTS Genetic analyses revealed no additional mutations or discriminatory SNPs. Cells expressing WT-Q1 and R231C-Q1 exhibited some constitutively active IQ1E1 and smaller maximal IQ1E1 compared to cells expressing WT-Q1. CONCLUSION Constitutively active IQ1E1 and a smaller peak IQ1E1 are common features of FAF-associated and LQT1-associated mutations, respectively. These data suggest that the mixed functional properties of R231C-Q1 may predispose some families to LQT1 or FAF. We conclude that R231C is a pleiotropic missense mutation capable of LQT1 expression, AF expression, or both.

The slowly activating delayed rectifier K current (I ) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca ([Ca ] ) and β-adrenergic receptor (β-AR) stimulation modulate I amplitude and kinetics, but details of these important I regulators and their interaction are limited. We assessed the [Ca ] dependence of I in steady-state conditions and with dynamically changing membrane potential and [Ca ] during an AP. I was recorded from freshly isolated rabbit ventricular myocytes using whole-cell patch clamp. With intracellular pipette solutions that controlled free [Ca ] , we found that raising [Ca ] from 100 to 600 nm produced similar increases in I as did β-AR activation, and the effects appeared additive. Both β-AR activation and high [Ca ] increased maximally activated tail I , negatively shifted the voltage dependence of activation, and slowed deactivation kinetics. These data informed changes in our well-established mathematical model of the rabbit myocyte. In both AP-clamp experiments and simulations, I recorded during a normal physiological Ca transient was similar to I measured with [Ca ] clamped at 500-600 nm. Thus, our study provides novel quantitative data as to how physiological [Ca ] regulates I amplitude and kinetics during the normal rabbit AP. Our results suggest that micromolar [Ca ] , in the submembrane or junctional cleft space, is not required to maximize [Ca ] -dependent I activation during normal Ca transients.

We illustrate the work necessary to reverse course after identification of a KCNQ1 variant interpreted erroneously as causing long QT syndrome (LQTS) and to identify the true cause of a case of sudden death in the young. Surrogate genetic testing of a decedent's living brother identified a rare KCNQ1-V133I variant, which prompted an implantable cardioverter defibrillator and subsequent diagnosis of LQTS in other family members. Subsequently, this presumed LQT1 family came to our institution for further clinical evaluation and research-based investigations, including KCNQ1-V133I variant-specific analysis of the decedent, heterologous expression studies of KCNQ1-V133I, and a whole-exome molecular autopsy along with genomic triangulation using his unaffected parents' DNA. After evaluating several V133I-positive family members, clinical doubt was cast on the veracity of the previously levied diagnosis of LQT1, resulting in a re-opening of the case and an intense pursuit of the lethal substrate. Furthermore, the decedent tested negative for V133I, and heterologous expression studies demonstrated a normal cellular phenotype for V133I-containing Kv7.1 channels. Instead, after whole-exome molecular autopsy, a de novo pathogenic variant (p.R454W) in DES-encoded desmin was identified. As detailed herein, the forensic evaluation of sudden death in the young requires meticulous focus on the decedent followed by a careful and deliberate assessment of the decedent's relatives. Surrogate genetic testing can have disastrous consequences and should be avoided. Genetic test results require careful scrutiny to avoid unintended and potentially devastating repercussions. Although the root cause of the decedent's tragic death would have remained a mystery, the unintended consequences for the living relatives described herein might have been avoided based on clinical grounds alone. All family members had electrocardiograms with normal QT intervals, making the diagnosis of familial LQTS unlikely. As such, if the clinicians caring for these patients had focused solely on clinical data from the survivors, there might have been no reason to embark on a path of inappropriate treatment based on genetic testing.