The anatomy and physiology of the sinoatrial node--a contemporary review

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

The in vivo regulation of heart rate in the murine sinoatrial node by stimulatory and inhibitory heterotrimeric G proteins

Reciprocal physiological modulation of heart rate is controlled by the sympathetic and parasympathetic systems acting on the sinoatrial (SA) node. However, there is little direct in vivo work examining the role of stimulatory and inhibitory G protein signaling in the SA node. Thus, we designed a study to examine the role of the stimulatory (Gαs) and inhibitory G protein (Gαi2) in in vivo heart rate regulation in the SA node in the mouse. We studied mice with conditional deletion of Gαs and Gαi2 in the conduction system using cre-loxP technology. We crossed mice in which cre recombinase expression was driven by a tamoxifen-inducible conduction system-specific construct with “Gαs floxed” and “Gαi2 floxed” mice. We studied the heart rate responses of adult mice compared with littermate controls by using radiotelemetry before and after administration of tamoxifen. The mice with conditional deletion of Gαs and Gαi2 had a loss of diurnal variation and were bradycardic or tachycardic, respectively, in the daytime. In mice with conditional deletion of Gαs, there was a selective loss of low-frequency power, while with deletion of Gαi2, there was a loss of high-frequency power in power spectral analysis of heart rate variability. There was no evidence of pathological arrhythmia. Pharmacological modulation of heart rate by isoprenaline was impaired in the Gαs mice, but a muscarinic agonist was still able to slow the heart rate in Gαi2 mice. We conclude that Gαs- and Gαi2-mediated signaling in the sinoatrial node is important in the reciprocal regulation of heart rate through the autonomic nervous system.

Ca²⁺-dependent phosphorylation of Ca²⁺ cycling proteins generates robust rhythmic local Ca²⁺ releases in cardiac pacemaker cells

The spontaneous beating of the heart is governed by spontaneous firing of sinoatrial node cells, which generate action potentials due to spontaneous depolarization of the membrane potential, or diastolic depolarization. The spontaneous diastolic depolarization rate is determined by spontaneous local submembrane Ca²⁺ releases through ryanodine receptors (RyRs). We sought to identify specific mechanisms of intrinsic Ca²⁺ cycling by which sinoatrial node cells, but not ventricular myocytes, generate robust, rhythmic local Ca²⁺ releases. At similar physiological intracellular Ca²⁺ concentrations, local Ca²⁺ releases were large and rhythmic in permeabilized sinoatrial node cells but small and random in permeabilized ventricular myocytes. Furthermore, sinoatrial node cells spontaneously released more Ca²⁺ from the sarcoplasmic reticulum than did ventricular myocytes, despite comparable sarcoplasmic reticulum Ca²⁺ content in both cell types. This ability of sinoatrial node cells to generate larger and rhythmic local Ca²⁺ releases was associated with increased abundance of sarcoplasmic reticulum Ca²⁺-ATPase (SERCA), reduced abundance of the SERCA inhibitor phospholamban, and increased Ca²⁺-dependent phosphorylation of phospholamban and RyR. The increased phosphorylation of RyR in sinoatrial node cells may facilitate Ca²⁺ release from the sarcoplasmic reticulum, whereas Ca²⁺-dependent increase in phosphorylation of phospholamban relieves its inhibition of SERCA, augmenting the pumping rate of Ca²⁺ required to support robust, rhythmic local Ca²⁺ releases. The differences in Ca²⁺ cycling between sinoatrial node cells and ventricular myocytes provide insights into the regulation of intracellular Ca²⁺ cycling that drives the automaticity of sinoatrial node cells.

Identification and detection of the isolated sinus venosus from the Asian toad

The pacemaker activity of mammalian sinoatrial node (SAN) of the heart plays a fundamental role in the integration of vital functions. Studying factors such as drugs that influence pacemaker activity of SAN has its significance. In this study, we isolated sinus venosus, SAN from toads (Bufo gargarizans), and analysed its electronic signal, histological characteristics and the influence of acetylcholine (ACh) and ivabradine on its pacemaker activity using PowerLab® and Chart® 5.0 software. We found that when isolated sinus venosus was treated with ACh, its histological distribution was disorganized and inter-beat (RR) interval was also broadened. The high frequency normalized unit (HFnu) and Poincaré plot of heart rate variability (HRV) of the isolated sinus venosus was also altered upon ACh treatment in a time-dependent and dose-dependent manner. When treated with ivabradine, these parameters of HRV such as square root of the mean of the squared differences between adjacent NN intervals (RMSSD) and HFnu were in the upward tendency, but low frequency normalized unit and low frequency/high frequency were in the opposite tendency. Taken together, we have developed a new model for studying the influences of drugs on autorhythmicity using isolated sinus venosus of the toad. With this model, we showed that ACh and ivabradine may affect the pacemaker activity by stimulating muscarinic receptor or inhibiting If current, respectively.

P-wave morphology: underlying mechanisms and clinical implications

Increasing awareness of atrial fibrillation (AF) and its impact on public health revives interest in identification of noninvasive markers of predisposition to AF and ECG-based risk stratification. P-wave duration is generally accepted as the most reliable noninvasive marker of atrial conduction, and its prolongation has been associated with history of AF. However, patients with paroxysmal AF without structural heart disease may not have any impressive P-wave prolongation, thus suggesting that global conduction slowing is not an obligatory requirement for development of AF. P-wave morphology is therefore drawing increasing attention as it reflects the three-dimensional course of atrial depolarization propagation and detects local conduction disturbances. The factors that determine P-wave appearance include (1) the origin of the sinus rhythm that defines right atrial depolarization vector, (2) localization of left atrial breakthrough that defines left atrial depolarization vector, and (3) the shape and size of atrial chambers. However, it is often difficult to distinguish whether P-wave abnormalities are caused by atrial enlargement or interatrial conduction delay. Recent advances in endocardial mapping technologies have linked certain P-wave morphologies with interatrial conduction patterns and the function of major interatrial conduction routes. The value of P-wave morphology extends beyond cardiac arrhythmias associated with atrial conduction delay and can be used for prediction of clinical outcome of a wide range of cardiovascular disorders, including ischemic heart disease and congestive heart failure.

Cell and gene therapy for arrhythmias: Repair of cardiac conduction damage

Action potentials generated in the sinoatrial node (SAN) dominate the rhythm and rate of a healthy human heart. Subsequently, these action potentials propagate to the whole heart via its conduction system. Abnormalities of impulse generation and/or propagation in a heart can cause arrhythmias. For example, SAN dysfunction or conduction block of the atrioventricular node can lead to serious bradycardia which is currently treated with an implanted electronic pacemaker. On the other hand, conduction damage may cause reentrant tachyarrhythmias which are primarily treated pharmacologically or by medical device-based therapies, including defibrillation and tissue ablation. However, drug therapies sometimes may not be effective or are associated with serious side effects. Device-based therapies for cardiac arrhythmias, even with well developed technology, still face inadequacies, limitations, hardware complications, and other challenges. Therefore, scientists are actively seeking other alternatives for antiarrhythmic therapy. In particular, cells and genes used for repairing cardiac conduction damage/defect have been investigated in various studies both in vitro and in vivo. Despite the complexities of the excitation and conduction systems of the heart, cell and gene-based strategies provide novel alternatives for treatment or cure of cardiac arrhythmias. This review summarizes some highlights of recent research progress in this field.

Use of antiarrhythmic drugs in elderly patients

Human aging is a global issue with important implications for current and future incidence and prevalence of health conditions and disability. Cardiac arrhythmias, including atrial fibrillation, sudden cardiac death, and bradycardia requiring pacemaker placement, all increase exponentially after the age of 60. It is important to distinguish between the normal, physiological consequences of aging on cardiac electrophysiology and the abnormal, pathological alterations. The age-related cardiac changes include ventricular hypertrophy, senile amyloidosis, cardiac valvular degenerative changes and annular calcification, fibrous infiltration of the conduction system, and loss of natural pacemaker cells and these changes could have a profound effect on the development of arrhythmias. The age-related cardiac electrophysiological changes include up- and down-regulation of specific ion channel expression and intracellular Ca(2+) overload which promote the development of cardiac arrhythmias. As ion channels are the substrates of antiarrhythmic drugs, it follows that the pharmacokinetics and pharmacodynamics of these drugs will also change with age. Aging alters the absorption, distribution, metabolism, and elimination of antiarrhythmic drugs, so liver and kidney function must be monitored to avoid potential adverse drug effects, and antiarrhythmic dosing may need to be adjusted for age. Elderly patients are also more susceptible to the side effects of many antiarrhythmics, including bradycardia, orthostatic hypotension, urinary retention, and falls. Moreover, the choice of antiarrhythmic drugs in the elderly patient is frequently complicated by the presence of co-morbid conditions and by polypharmacy, and the astute physician must pay careful attention to potential drug-drug interactions. Finally, it is important to remember that the use of antiarrhythmic drugs in elderly patients must be individualized and tailored to each patient's physiology, disease processes, and medication regimen.

Bradycardia following retinoic acid differentiation syndrome in a patient with acute promyelocytic leukaemia

The authors describe a 28-year-old woman with newly diagnosed acute promyelocytic leukaemia (APL), who developed junctional bradycardia after receiving the molecular-targeted therapy all-trans retinoic acid (ATRA) and the anthracycline-based chemotherapeutic agent idarubicin following sepsis and the APL differentiation syndrome. The patient was asymptomatic of the bradycardia. Electrolytes and cardiac imaging were unremarkable. No other cases have been reported in this context and the mechanisms of the sinus node dysfunction are unclear. The patient achieved normal sinus rhythm after ATRA was withheld. The patient recovered and went on to achieve complete remission after re-starting ATRA and idarubicin.

Quantification of fiber orientation in the canine atrial pacemaker complex using optical coherence tomography

The atrial pacemaker complex is responsible for the initiation and early propagation of cardiac impulses. Optical coherence tomography (OCT), a nondestructive imaging modality with spatial resolutions of ∼1 to 15 μm, can be used to identify unique fiber orientation patterns in this region of the heart. Functionally characterized canine sinoatrial nodes (SAN) (n=7) were imaged using OCT up to ∼1  mm below the endocardial tissue surface. OCT images were directly compared to their corresponding histological sections. Fiber orientation patterns unique to the crista terminalis (CT), SAN, and surrounding atrial myocardium were identified with dominant average fiber angles of 89 ± 12 deg, 110 ± 16 deg, and 95 ± 35 deg, respectively. Both the CT and surrounding atrial myocardium displayed predominantly unidirectionally based fiber orientation patterns within each specimen, whereas the SAN displayed an increased amount of fiber disarray manifested quantitatively as a significantly greater standard deviation in fiber angle distribution within specimens [33 ± 7 deg versus 23 ± 5 deg, atrium (p=0.02); 18 ± 3 deg, CT (p=0.0003)]. We also identified unique, local patterns of fiber orientation specific to the functionally characterized block zone. We demonstrate the ability of OCT in detecting components of the atrial pacemaker complex which are intimately involved in both normal and abnormal cardiac conduction.

RGS Proteins in Heart: Brakes on the Vagus

It has been nearly a century since Otto Loewi discovered that acetylcholine (ACh) release from the vagus produces bradycardia and reduced cardiac contractility. It is now known that parasympathetic control of the heart is mediated by ACh stimulation of G(i/o)-coupled muscarinic M2 receptors, which directly activate G protein-coupled inwardly rectifying potassium (GIRK) channels via Gβγ resulting in membrane hyperpolarization and inhibition of action potential (AP) firing. However, expression of M2R-GIRK signaling components in heterologous systems failed to recapitulate native channel gating kinetics. The missing link was identified with the discovery of regulator of G protein signaling (RGS) proteins, which act as GTPase-activating proteins to accelerate the intrinsic GTPase activity of Gα resulting in termination of Gα- and Gβγ-mediated signaling to downstream effectors. Studies in mice expressing an RGS-insensitive Gα(i2) mutant (G184S) implicated endogenous RGS proteins as key regulators of parasympathetic signaling in heart. Recently, two RGS proteins have been identified as critical regulators of M2R signaling in heart. RGS6 exhibits a uniquely robust expression in heart, especially in sinoatrial (SAN) and atrioventricular nodal regions. Mice lacking RGS6 exhibit increased bradycardia and inhibition of SAN AP firing in response to CCh as well as a loss of rapid activation and deactivation kinetics and current desensitization for ACh-induced GIRK current (I(KACh)). Similar findings were observed in mice lacking RGS4. Thus, dysregulation in RGS protein expression or function may contribute to pathologies involving aberrant electrical activity in cardiac pacemaker cells. Moreover, RGS6 expression was found to be up-regulated in heart under certain pathological conditions, including doxorubicin treatment, which is known to cause life-threatening cardiotoxicity and atrial fibrillation in cancer patients. On the other hand, increased vagal tone may be cardioprotective in heart failure where acetylcholinesterase inhibitors and vagal stimulation have been proposed as potential therapeutics. Together, these studies identify RGS proteins, especially RGS6, as new therapeutic targets for diseases such as sick sinus syndrome or other maladies involving abnormal autonomic control of the heart.

The voltage-gated channel accessory protein KCNE2: multiple ion channel partners, multiple ways to long QT syndrome

The single-pass transmembrane protein KCNE2 or MIRP1 was once thought to be the missing accessory protein that combined with hERG to fully recapitulate the cardiac repolarising current IKr. As a result of this role, it was an easy next step to associate mutations in KCNE2 to long QT syndrome, in which there is delayed repolarisation of the heart. Since that time however, KCNE2 has been shown to modify the behaviour of several other channels and currents, and its role in the heart and in the aetiology of long QT syndrome has become less clear. In this article, we review the known interactions of the KCNE2 protein and the resulting functional effects, and the effects of mutations in KCNE2 and their clinical role.