Mark R. Boyett

New Insights Into Pacemaker Activity Promoting Understanding of Sick Sinus Syndrome

Sick sinus syndrome is an abnormality involving the generation of the action potential by the sinus node and is characterized by an atrial rate inappropriate for physiological requirements. Manifestations include severe sinus bradycardia, sinus pauses or arrest, sinus node exit block, chronic atrial tachyarrhythmias, alternating periods of atrial bradyarrhythmias and tachyarrhythmias, and inappropriate responses of heart rate during exercise or stress.1 Although its incidence increases in an exponential-like manner with age,1 it can occur at all ages, including in the newborn.2 The mean age of patients with the syndrome is 68 years, with both genders being affected in approximately equal proportion.2 The syndrome occurs in 1 of every 600 cardiac patients older than 65 years and accounts for approximately half of implantations of pacemakers in the United States.2 The syndrome is a collection of conditions, with a variety of causes intrinsic or extrinsic relative to the node.2 Age-dependent degenerative fibrosis of the tissues of the node has been suggested to be a common cause,2 although this is disputed.1 Clinical aspects of the syndrome have been reviewed many times.1–3 In this review, coinciding with the centenary of the discovery of the node, we offer new insights into intrinsic causes that come from an understanding of the mechanisms underlying pacemaking. It is now exactly 100 years since Arthur Keith (Figure 1A) performed the work that introduced the world to the location of the sinus node.4,5 Keith had been intrigued by the actions of the musculature of the heart and was anxious to establish any muscular mechanisms involved in closure of the great caval veins as the right atrium contracted to deliver its load to the ventricles.5 He was aware of the work of James Mackenzie, then a medical practitioner working …

Molecular Architecture of the Human Sinus Node Insights Into the Function of the Cardiac Pacemaker

Background— Although we know much about the molecular makeup of the sinus node (SN) in small mammals, little is known about it in humans. The aims of the present study were to investigate the expression of ion channels in the human SN and to use the data to predict electrical activity. Methods and Results— Quantitative polymerase chain reaction, in situ hybridization, and immunofluorescence were used to analyze 6 human tissue samples. Messenger RNA (mRNA) for 120 ion channels (and some related proteins) was measured in the SN, a novel paranodal area, and the right atrium (RA). The results showed, for example, that in the SN compared with the RA, there was a lower expression of Nav1.5, Kv4.3, Kv1.5, ERG, Kir2.1, Kir6.2, RyR2, SERCA2a, Cx40, and Cx43 mRNAs but a higher expression of Cav1.3, Cav3.1, HCN1, and HCN4 mRNAs. The expression pattern of many ion channels in the paranodal area was intermediate between that of the SN and RA; however, compared with the SN and RA, the paranodal area showed greater expression of Kv4.2, Kir6.1, TASK1, SK2, and MiRP2. Expression of ion channel proteins was in agreement with expression of the corresponding mRNAs. The levels of mRNA in the SN, as a percentage of those in the RA, were used to estimate conductances of key ionic currents as a percentage of those in a mathematical model of human atrial action potential. The resulting SN model successfully produced pacemaking. Conclusions— Ion channels show a complex and heterogeneous pattern of expression in the SN, paranodal area, and RA in humans, and the expression pattern is appropriate to explain pacemaking.

New insights into Sick Sinus Syndrome

Sick sinus syndrome is an abnormality involving the generation of the action potential by the sinus node and is characterized by an atrial rate inappropriate for physiological requirements. Manifestations include severe sinus bradycardia, sinus pauses or arrest, sinus node exit block, chronic atrial tachyarrhythmias, alternating periods of atrial bradyarrhythmias and tachyarrhythmias, and inappropriate responses of heart rate during exercise or stress.1 Although its incidence increases in an exponential-like manner with age,1 it can occur at all ages, including in the newborn.2 The mean age of patients with the syndrome is 68 years, with both genders being affected in approximately equal proportion.2 The syndrome occurs in 1 of every 600 cardiac patients older than 65 years and accounts for approximately half of implantations of pacemakers in the United States.2 The syndrome is a collection of conditions, with a variety of causes intrinsic or extrinsic relative to the node.2 Age-dependent degenerative fibrosis of the tissues of the node has been suggested to be a common cause,2 although this is disputed.1 Clinical aspects of the syndrome have been reviewed many times.1–3 In this review, coinciding with the centenary of the discovery of the node, we offer new insights into intrinsic causes that come from an understanding of the mechanisms underlying pacemaking. It is now exactly 100 years since Arthur Keith (Figure 1A) performed the work that introduced the world to the location of the sinus node.4,5 Keith had been intrigued by the actions of the musculature of the heart and was anxious to establish any muscular mechanisms involved in closure of the great caval veins as the right atrium contracted to deliver its load to the ventricles.5 He was aware of the work of James Mackenzie, then a medical practitioner working …

Pacing-Induced Spontaneous Activity in Myocardial Sleeves of Pulmonary Veins After Treatment With Ryanodine

Background—Recent clinical electrophysiology studies and successful results of radiofrequency catheter ablation therapy suggest that high-frequency focal activity in the pulmonary veins (PVs) plays important roles in the initiation and perpetuation of atrial fibrillation, but the mechanisms underlying the focal arrhythmogenic activity are not understood. Methods and Results—Extracellular potential mapping of rabbit right atrial preparations showed that ryanodine (2 &mgr;mol/L) caused a shift of the leading pacemaker from the sinoatrial node to an ectopic focus near the right PV-atrium junction. The transmembrane potential recorded from the isolated myocardial sleeve of the right PV showed typical atrial-type action potentials with a stable resting potential under control conditions. Treatment with ryanodine (0.5 to 2 &mgr;mol/L) resulted in a depolarization of the resting potential and a development of pacemaker depolarization. These changes were enhanced transiently after an increase in the pacing rate: a self-terminating burst of spontaneous action potentials (duration, 33.6±5.0 s; n=32) was induced by a train of rapid stimuli (3.3 Hz) applied after a brief rest period. The pacing-induced activity was attenuated by either depletion of the sarcoplasmic reticulum of Ca2+ or blockade of the sarcolemmal Na+-Ca2+ exchanger or Cl− channels and potentiated by &bgr;-adrenergic stimulation. Conclusions—PV myocardial sleeves have the potential to generate spontaneous activity, and such arrhythmogenic activity is uncovered by modulation of intracellular Ca2+ dynamics.

Effect of Adrenergic Stimulation on Action Potential Duration Restitution in Humans

Background—Enhanced sympathetic activity facilitates complex ventricular arrhythmias and fibrillation. The restitution properties of action potential duration (APD) are important determinants of electrical stability in the myocardium. Steepening of the slope of APD restitution has been shown to promote wave break and ventricular fibrillation. The effect of adrenergic stimulation on APD restitution in humans is unknown. Methods and Results—Monophasic action potentials were recorded from the right ventricular septum in 18 patients. Standard APD restitution curves were constructed at 3 basic drive cycle lengths (CLs) of 600, 500, and 400 ms under resting conditions and during infusion of isoprenaline (15 patients) or adrenaline (3 patients). The maximum slope of the restitution curves was measured by piecewise linear regression segments of sequential 40-ms ranges of diastolic intervals in steps of 10 ms. Under control conditions, the maximum slope was steeper at longer basic CLs; eg, mean values for the maximum slope were 1.053±0.092 at CL 600 ms and 0.711±0.049 at CL 400 ms (±SEM). Isoprenaline increased the steepness of the maximum slope of APD restitution, eg, from a maximum slope of 0.923±0.058 to a maximum slope of 1.202±0.121 at CL 500 ms. The effect of isoprenaline was greater at the shorter basic CLs. A similar overall effect was observed with adrenaline. Conclusions—The adrenergic agonists isoprenaline and adrenaline increased the steepness of the slope of the APD restitution curve in humans over a wide range of diastolic intervals. These results may relate to the known effects of adrenergic stimulation in facilitating ventricular fibrillation.

Distribution and Functional Characterization of Equilibrative Nucleoside Transporter-4, a Novel Cardiac Adenosine Transporter Activated at Acidic pH

Adenosine plays multiple roles in the efficient functioning of the heart by regulating coronary blood flow, cardiac pacemaking, and contractility. Previous studies have implicated the equilibrative nucleoside transporter family member equilibrative nucleoside transporter-1 (ENT1) in the regulation of cardiac adenosine levels. We report here that a second member of this family, ENT4, is also abundant in the heart, in particular in the plasma membranes of ventricular myocytes and vascular endothelial cells but, unlike ENT1, is virtually absent from the sinoatrial and atrioventricular nodes. Originally described as a monoamine/organic cation transporter, we found that both human and mouse ENT4 exhibited a novel, pH-dependent adenosine transport activity optimal at acidic pH (apparent Km values 0.78 and 0.13 mmol/L, respectively, at pH 5.5) and absent at pH 7.4. In contrast, serotonin transport by ENT4 was relatively insensitive to pH. ENT4-mediated nucleoside transport was adenosine selective, sodium independent and only weakly inhibited by the classical inhibitors of equilibrative nucleoside transport, dipyridamole, dilazep, and nitrobenzylthioinosine. We hypothesize that ENT4, in addition to playing roles in cardiac serotonin transport, contributes to the regulation of extracellular adenosine concentrations, in particular under the acidotic conditions associated with ischemia.

Connexin45, a Major Connexin of the Rabbit Sinoatrial Node, Is Co-expressed with Connexin43 in a Restricted Zone at the Nodal-Crista Terminalis Border

The pacemaker of the heart, the sinoatrial (SA) node, is characterized by unique electrical coupling properties. To investigate the contribution of gap junction organization and composition to these properties, the spatial pattern of expression of three gap junctional proteins, connexin45 (Cx45), connexin40 (Cx40), and connexin43 (Cx43), was investigated by immunocytochemistry combined with confocal microscopy. The SA nodal regions of rabbits were dissected and rapidly frozen. Serial cryosections were double labeled for Cx45 and Cx43 and for Cx40 and Cx43, using pairs of antibody probes raised in different species. Dual-channel scanning confocal microscopy was applied to allow simultaneous visualization of the different connexins. Cx45 and Cx40, but not Cx43, were expressed in the central SA node. The major part of the SA nodal-crista terminalis border revealed a sharply demarcated boundary between Cx43-expressing myocytes of the crista terminalis and Cx45/Cx40-expressing myocytes of the node. On the endocardial side, however, a transitional zone between the crista terminalis and the periphery of the node was detected in which Cx43 and Cx45 expression merged. These distinct patterns of connexin compartmentation and merger identified suggest a morphological basis for minimization of contact between the tissues, thereby restricting the hyperpolarizing influence of the atrial muscle on the SA node while maintaining a communication route for directed exit of the impulse into the crista terminalis.

Computer Three-Dimensional Reconstruction of the Sinoatrial Node

Background—There is an effort to build an anatomically and biophysically detailed virtual heart, and, although there are models for the atria and ventricles, there is no model for the sinoatrial node (SAN). For the SAN to show pacemaking and drive atrial muscle, theoretically, there should be a gradient in electrical coupling from the center to the periphery of the SAN and an interdigitation of SAN and atrial cells at the periphery. Any model should include such features. Methods and Results—Staining of rabbit SAN preparations for histology, middle neurofilament, atrial natriuretic peptide, and connexin (Cx) 43 revealed multiple cell types within and around the SAN (SAN and atrial cells, fibroblasts, and adipocytes). In contrast to atrial cells, all SAN cells expressed middle neurofilament (but not atrial natriuretic peptide) mRNA and protein. However, 2 distinct SAN cell types were observed: cells in the center (leading pacemaker site) were small, were organized in a mesh, and did not express Cx43. In contrast, cells in the periphery (exit pathway from the SAN) were large, were arranged predominantly in parallel, often expressed Cx43, and were mixed with atrial cells. An ≈2.5-million-element array model of the SAN and surrounding atrium, incorporating all cell types, was constructed. Conclusions—For the first time, a 3D anatomically detailed mathematical model of the SAN has been constructed, and this shows the presence of a specialized interface between the SAN and atrial muscle.

Site of Origin and Molecular Substrate of Atrioventricular Junctional Rhythm in the Rabbit Heart

Abstract— During failure of the sinoatrial node, the heart can be driven by an atrioventricular (AV) junctional pacemaker. The position of the leading pacemaker site during AV junctional rhythm is debated. In this study, we present evidence from high-resolution fluorescent imaging of electrical activity in rabbit isolated atrioventricular node (AVN) preparations that, in the majority of cases (11 out of 14), the AV junctional rhythm originates in the region extending from the AVN toward the coronary sinus along the tricuspid valve (posterior nodal extension, PNE). Histological and immunohistochemical investigation showed that the PNE has the same morphology and unique pattern of expression of neurofilament160 (NF160) and connexins (Cx40, Cx43, and Cx45) as the AVN itself. Block of the pacemaker current, If, by 2 mmol/L Cs+ increased the AV junctional rhythm cycle length from 611±84 to 949±120 ms (mean±SD, n=6, P <0.001). Immunohistochemical investigation showed that the principal If channel protein, HCN4, is abundant in the PNE. As well as the AV junctional rhythm, the PNE described in this study may also be involved in the slow pathway of conduction into the AVN as well as AVN reentry, and the predominant lack of expression of Cx43 as well as the presence of Cx45 in the PNE shown could help explain its slow conduction.

3D virtual human atria: A computational platform for studying clinical atrial fibrillation

Despite a vast amount of experimental and clinical data on the underlying ionic, cellular and tissue substrates, the mechanisms of common atrial arrhythmias (such as atrial fibrillation, AF) arising from the functional interactions at the whole atria level remain unclear. Computational modelling provides a quantitative framework for integrating such multi-scale data and understanding the arrhythmogenic behaviour that emerges from the collective spatio-temporal dynamics in all parts of the heart. In this study, we have developed a multi-scale hierarchy of biophysically detailed computational models for the human atria--the 3D virtual human atria. Primarily, diffusion tensor MRI reconstruction of the tissue geometry and fibre orientation in the human sinoatrial node (SAN) and surrounding atrial muscle was integrated into the 3D model of the whole atria dissected from the Visible Human dataset. The anatomical models were combined with the heterogeneous atrial action potential (AP) models, and used to simulate the AP conduction in the human atria under various conditions: SAN pacemaking and atrial activation in the normal rhythm, break-down of regular AP wave-fronts during rapid atrial pacing, and the genesis of multiple re-entrant wavelets characteristic of AF. Contributions of different properties of the tissue to mechanisms of the normal rhythm and arrhythmogenesis were investigated. Primarily, the simulations showed that tissue heterogeneity caused the break-down of the normal AP wave-fronts at rapid pacing rates, which initiated a pair of re-entrant spiral waves; and tissue anisotropy resulted in a further break-down of the spiral waves into multiple meandering wavelets characteristic of AF. The 3D virtual atria model itself was incorporated into the torso model to simulate the body surface ECG patterns in the normal and arrhythmic conditions. Therefore, a state-of-the-art computational platform has been developed, which can be used for studying multi-scale electrical phenomena during atrial conduction and AF arrhythmogenesis. Results of such simulations can be directly compared with electrophysiological and endocardial mapping data, as well as clinical ECG recordings. The virtual human atria can provide in-depth insights into 3D excitation propagation processes within atrial walls of a whole heart in vivo, which is beyond the current technical capabilities of experimental or clinical set-ups.