Atrioventricular Nodal Cells Research Articles

Pacemaker Cells of the Atrioventricular Node are CaV1.3 Dependent Oscillators

The atrioventricular node (AVN) can generate pacemaker activity in case of failure of the sino-atrial node (SAN). However, the mechanisms underlying pacemaking in AVN cells (AVNCs) are poorly understood. Voltage-dependent ion channels such as hyperpolarization-activated HCN channels, L-type Cav1.3 and T-type Cav3.1 channels are known to play a role in pacemaking of sino-atrial node cells (SANCs). Here, we investigate the role of these channels in AVNCs pacemaker activity using genetically modified mouse strains and show that they differentially impact pacemaking of AVNCs than of SANCs. Indeed, contrary to SANCs, Cav1.3 channels are necessary for pacemaking of AVNCs and accounted for the predominant fraction of ICa,L. Inactivation of Cav3.1 channels impaired automaticity in AVNCs by promoting sporadic block of automaticity and spontaneous cellular arrhythmia.Abolition of the cAMP sensitivity of HCN channels shifted the If activation to voltages negative to that spanning the diastolic depolarization and prevented AVNCs automaticity in basal conditions. However pacemaker activity could be restored to control levels by adrenergic receptor stimulation.Inactivation of both Cav1.3 and Cav3.1 results in abolishment of pacemaking. Inhibition of TTX-resistant (INar) Na+ current showed that this is a key contributor of the action potential (AP) threshold and upstroke velocity.Conclusion: 1. Spontaneous firing rate in AVNCs is strongly dependent from Cav1.3-mediated L type calcium current and from TTX resistant sodium current (INa,r). 2. In AVNCs the Cav1.3 isoform seems to be predominant compared to Cav1.2 isoform. 3. The fact that hyperpolarization of Cav1.3-/-AVNCs pacemaking can be observed suggests that the absence of pacemaker activity is not due to the impossibility to generate the upstroke phase of the AP but probably due to an imbalance between outward and inward currents during the diastolic depolarization.

Generation of Sodium-Permeable Cav1.3 Channel: Insights into the Molecular Basis for the Sustained Inward Current in Cardiac Pacemaker Cells

The sustained inward current (Ist) is a novel pacemaker current identified in spontaneously active sinoatrial and atrioventricular node cells of rabbits, guinea-pigs, rats and mice. Because Ist is activated and produces an inward current over the entire range of the slow diastolic depolarization, its contribution to the pacemaker activity has been suggested. However, due to the absence of specific blockers and unidentified molecular determinants, it is still difficult to directly investigate the significance of Ist in cardiac automaticity. Although Ist is a Na+ current, its pharmacological properties are qualitatively identical with those of L-type Ca2+ current (ICa,L). In the present study, we generated a Na+-permeable CaV1.3 channel by a substitution of key glutamate residue in the Ca2+-selective filter of the pore with lysine (E1160K). The CaV1.3-E1160K channel expressed in HEK cells evoked an inward current carried by Na+ even in the presence of extracellular Ca2+ (1.8 mM). The Na+ current was characterized by a slow inactivation kinetics, a low activation threshold (∼−60 mV) and sensitivity to ICa,L blockers such as nifedipine, diltiazem and Cd2+. These properties of the CaV1.3-E1160K current were very similar to those of Ist, suggesting that an ICa,L channel variant with altered ion selectivity may mediate Ist. Besides, application of the CaV1.3-E1160K channel to the biological pacemaker would be an intriguing approach to understand the impact of Ist on cardiac pacemaking.

Proteolytic Processing of HCN2 and Co-assembly with HCN4 in the Generation of Cardiac Pacemaker Channels

In sino-atrial and atrio-ventricular nodal cells, hyperpolarization-activated cyclic nucleotide-gated (HCN) inward current carrying cationic channels, I(f), are expressed that contribute importantly to the diastolic depolarization critical for cardiac pacemaker activity. Although previous studies have demonstrated myocardial expression of both the HCN2 and HCN4 subunits, the specific roles of these subunits in the generation of functional myocardial I(f) channels remain unclear. To explore the molecular compositions of functional cardiac I(f) channels, antibodies targeted against specific C- and N-terminal sequences in HCN2 and HCN4 were exploited to examine HCN2 and HCN4 subunit expression in adult (mouse) heart and to immunoprecipitate endogenous HCN-encoded cardiac I(f) channel complexes. Western blot experiments revealed that although the full-length HCN2 (105 kDa) and HCN4 (160 kDa) proteins are readily detected in transiently transfected HEK-293 cells and in adult (mouse) brain, the molecular mass of the HCN2 protein in the myocardium is approximately 60 kDa. In addition, the myocardial 60-kDa HCN2 protein lacks the C terminus, which contains the cAMP binding domain. In heterologous cells, the C-terminal-truncated HCN2 protein co-assembles with HCN4 to form functional heteromeric HCN channels, which activate faster than homomeric HCN2 or homomeric HCN4 channels, and display properties similar to endogenous myocardial I(f) channels Taken together, these results suggest that functional myocardial I(f) channels reflect the heteromeric assembly of HCN2 and HCN4 subunits and further that the HCN4 subunit underlies the cAMP-mediated regulation of cardiac I(f) channels.

Gene Expression Profiling of the Forming Atrioventricular Node Using a Novel Tbx3 -Based Node-Specific Transgenic Reporter

The atrioventricular (AV) node is a recurrent source of potentially life-threatening arrhythmias. Nevertheless, limited data are available on its developmental control or molecular phenotype. We used a novel AV nodal myocardium-specific reporter mouse to gain insight into the gene programs determining the formation and phenotype of the developing AV node. In this reporter, green fluorescent protein (GFP) expression was driven by a 160-kbp bacterial artificial chromosome with Tbx3 and flanking sequences. GFP was selectively active in the AV canal of embryos and AV node of adults, whereas the Tbx3-positive AV bundle and sinus node were devoid of GFP, demonstrating that distinct regulatory sequences and pathways control expression in the components of the conduction system. Fluorescent AV nodal and complementary Nppa-positive chamber myocardial cell populations of embryonic day 10.5 embryos and of embryonic day 17.5 fetuses were purified using fluorescence-activated cell sorting, and their expression profiles were assessed by genome-wide microarray analysis, providing valuable information concerning their molecular identities. We constructed a comprehensive list of sodium, calcium, and potassium channel genes specific for developing nodal or chamber myocardium. Furthermore, the data revealed that the AV node and the chamber (working) myocardium phenotypes diverge during development but that the functional gene classes characterizing both subtypes are maintained. One of the repertoires identified in the AV node-specific gene profiles consists of multiple neurotrophic factors and semaphorins, not yet appreciated to play a role in nodal development, revealing shared characteristics between nodal and nervous system development.

Four Dimensional (4-D) BioChemInfoPhysics Models of Cardiac Cellular and Sub-Cellular Vibrations (Oscillations)

Problem statement: Cardiovascular Diseases (CVD) continued to be the leading cause of death. Failure or abnormal cardiac cellular or sub-cellular vibrations (oscillations) could lead failure or abnormal heart beats that could cause CVD. Understanding the mechanisms of the vibrations (oscillations) could help to prevent or to treat the diseases. Scientists have studied the mechanisms for more than 100 years. To our knowledge, the mechanisms are still unclear today. In this investigation, based on published data or results, conservation laws of the momentum as well as the energy, in views of biology, biochemistry, informatics and physics (BioChemInfoPhysics), we proposed our models of cardiac cellular and sub-cellular vibrations (oscillations) of biological components, such as free ions in Biological Fluids (BF), Biological Membranes (BM), Ca++H+ (Ca++ and Na+K+) ATPases, Na+Ca++ exchangers (NCX), Ca++ carriers and myosin heads. Approach: Our models were described with 4-D (x, y, z, t or r, θ, z, t) momentum transfer equations in mathematical physics. Results: The momentum transfer equations were solved with free and forced, damped, un-damped and over-damped, vibrations (oscillations). The biological components could be modeled as resonators or vibrators (oscillators), such as liquid plasmas, membranes, active springs, passive springs and active swings. Conclusion: We systematically provided new insights of automation (ignition and maintain), transportation, propagation and orientation of the cardiac cellular and sub-cellular vibrations (oscillations) and resonances, with our BioChemInfoPhysics models of 4-D momentum transfer equations. Our modeling results implied: Auto-rhythmic cells (Sinoatrial Node Cells (SANC), Atrioventricular Node Cells (AVNC), Purkinje fibers), non-Auto-rhythmic ventricular myocytes and their Sarcoplasmic Reticulums (SR) work as Biological Liquid Plasma Resonators (BLPR). The resonators were biological clocks and mainly made of BF, BM and BM Transporters (BMT) that had mutually adapted and produced Biological Liquid Plasma Resonance Frequencies (BLPRF) for the resonators during their natural evolution. The resonators naturally vibrate (oscillate) near the SANC SR BLPRF that had the highest BLPRF among them.

Beating to time: calcium clocks, voltage clocks, and cardiac pacemaker activity

the heartbeat arises in the sinoatrial node (SAN) and then spreads throughout the heart. SAN function is therefore essential for normal cardiac physiology. The function of the SAN decreases with age, and this has been correlated with changes of ion channel expression ([12][1]). It is therefore of

Abstract 1512: Heteromeric Assembly of HCN4 and C-Terminal Truncated HCN2 Subunits in the Generation of Cardiac Pacemaker (If) Channels

In sino-atrial and atrio-ventricular nodal cells, hyperpolarization- activated cyclic nucleotide-gated (HCN) cationic channels are expressed that carry inward currents, I f , contributing importantly to the diastolic depolarization critical for cardiac pacemaker functioning. Although previous studies have demonstrated myocardial expression of HCN2 and HCN4 subunits, the roles of these subunits in the generation of mature, functional I f channels remains unclear. In preliminary experiments, completed on transiently transfected HEK293 cells expressing wild type and dominant negative mutant HCN2 and HCN4 subunits, we find that HCN2 and HCN4 preferentially co-assemble to form heteromeric channels. In further studies focused on defining the molecular compositions of functional cardiac I f channels, we developed and validated C-and N-terminal HCN2 and HCN4 subunit specific antibodies and exploited these to identify the HCN subunits expressed in adult (mouse) heart and to immunoprecipitate cardiac HCN-encoded I f channel complexes. These experiments revealed that the HCN2 and HCN4 proteins co-immunoprecipitate from heart, In addition, Western blot experiments revealed that, although the full length HCN2 (105Da) and HCN4 (160Da) proteins are expressed in transiently transfected HEK293 cells and in adult (mouse) brain, the molecular weight of the HCN2 protein in the heart is reduced to about 60 kDa. In addition, the mature, 60 kDa HCN2 protein lacks the C-terminal cAMP binding domain. These observations likely explain the results of previous studies demonstrating that the targeted deletion of HCN2 does not affect the cAMP sensitivity of I f . In addition, taken together, these results demonstrate that functional myocardial I f channels reflect the heteromeric assembly of HCN2 and HCN4 subunits and further that the HCN4 subunit regulates cAMP-mediated regulation of I f channels. This research has received full or partial funding support from the American Heart Association, AHA National Center.

Emerin and the Nuclear Lamina in Muscle and Cardiac Disease

The human genome is contained within the nucleus and is separated from the cytoplasm by the nuclear envelope. Mutations in the nuclear envelope proteins emerin and lamin A cause a number of diseases including premature aging syndromes, muscular dystrophy, and cardiomyopathy. Emerin and lamin A are implicated in regulating muscle- and heart-specific gene expression and nuclear architecture. For example, lamin A regulates the expression and localization of gap junction and intercalated disc components. Additionally, emerin and lamin A are also required to maintain nuclear envelope integrity. Demonstrating the importance of maintaining nuclear integrity in heart disease, atrioventricular node cells lacking lamin A exhibit increased nuclear deformation and apoptosis. This review highlights the present understanding of lamin A and emerin function in regulating nuclear architecture, gene expression, and cell signaling and discusses putative mechanisms for how specific mutations in lamin A and emerin cause cardiac- or muscle-specific disease.

Development of a biological cardiac pacemaker using gene therapy in a preclinical model

Objective: When cardiac atrioventricular nodal cells are damaged by disease, the implantation of a cardiac pacemaker becomes necessary. Several limitations and problems of artificial pacemakers have emerged during the past decades. In children and newborn babies with arrhythmia, initial size mismatch and their growth can pose a problem. Lead extraction and infections have additional risks. The aim of our study was the development of a biological cardiac pacemaker by in-vivo adenoviral transfection of ventricular cardiomyocytes in a large animal model with the gene encoding for Adenylate-Cyclase type VI (ACVI) in order to increase the intracellular concentration of cAMP and intrinsic rhythmic rate of the transfected cells, enabling ectopic pacing from the injection site.

Electrophysiological Effects of Carvedilol on Rabbit Heart Pacemaker Cells

The electrophysiological effects of carvedilol, a beta-blocking agent with vasodilating actions, have been studied on rabbit pacemaker cells using the whole-cell patch clamp technique. Nystatin-perforated patch recordings from the sinoatrial (SA) and atrioventricular (AV) nodes demonstrated that 1-3 microM of carvedilol caused a decrease in the spontaneous firing frequency, depolarization of the maximal diastolic potential, and prolongation of the action potential duration in both species. Voltage clamp experiments were performed using SA and AV node myocytes to identify and define the carvedilol-induced changes in the Ca(2+) current, I(Ca), delayed rectifier K(+) current, I(K), and hyperpolarization-activated inward current, I(f). In the SA node cells, 1 microM of carvedilol blocked I(K), I(Ca), and I(f) by 72%, 47%, and 22%, respectively. In the AV node cells, the corresponding reductions were 64% (I(K)) and 46% (I(Ca)), respectively. In both the SA and AV nodes the decrease in I(K) appeared to be mainly due to the rapidly activating component of the delayed rectifier, I(Kr), since the high dose of carvedilol blocked I(K) in the SA and AV nodes to a submaximal degree. In conclusion, effective doses of carvedilol have classical class III antiarrhythmic actions and a negative chronotropic effect resulting from the inhibition of I(K) and I(Ca). Both actions may be efficacious for treating supraventricular tachyarrhythmias.

Calmodulin kinase II activity is required for normal atrioventricular nodal conduction

Multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMKII) is abundant in myocardium. CaMKII activity is augmented by catecholamine stimulation, which enhances AV nodal conduction, suggesting the hypothesis that CaMKII also contributes to AV nodal conduction properties. The purpose of this study was to test the potential role of CaMKII in regulating AV nodal conduction in heart. We developed a novel mouse with genetic CaMKII inhibition by cardiac-specific expression of autocamtide 3 inhibitory peptide (AC3-I) mimicking a conserved sequence of the CaMKII regulatory domain. We also engineered a control transgenic mouse with cardiac expression of an inactive, scrambled version of AC3-I (autocamtide 3 control peptide [AC3-C]) and performed electrophysiologic measurements in vivo and in Langendorff-perfused isolated hearts. AC3-I and AC3-C were abundantly expressed in AV nodal cells. AC3-I mice with implanted ECG telemeters showed enhanced Wenckebach-type AV conduction block after isoproterenol (present in 9/9 mice) compared with AC3-C mice (present in 1/5 mice, P = .005). Intracardiac recordings showed significant PR and AH interval prolongation in AC3-I mice at baseline and after isoproterenol compared with AC3-C mice. HV durations were not different. Langendorff-perfused AC3-I hearts had significantly prolonged Wenckebach cycle lengths and AV nodal effective refractory periods compared with AC3-C hearts, whereas sinus node recovery time and left ventricular effective refractory times were similar between these genotypes. These studies define CaMKII as a critical determinant of normal and catecholamine-stimulated AV nodal conduction responses.

Foot formation in action potentials from atrioventricular nodal cells: Rosenblueth hypothesis or dual pathway electrophysiology?

Foot formation in action potentials from atrioventricular nodal cells: Rosenblueth hypothesis or dual pathway electrophysiology?

PROGRESS AND GAPS IN UNDERSTANDING THE ELECTROPHYSIOLOGICAL PROPERTIES OF MORPHOLOGICALLY NORMAL CELLS FROM THE CARDIAC ATRIOVENTRICULAR NODE

The atrioventricular node (AVN) is a small but critically important component of the cardiac electrical conduction system and is located at the junction between right atrium and right ventricle of the heart. It plays important roles in normal and abnormal impulse propagation. Mathematical models of the conduction properties of the AVN have been made, but detailed in silico reconstruction of AVN electrophysiology lags behind that of other cardiac regions. One important facet of detailed reconstruction of AVN electrical activity is the development of comprehensive, ionic conductance-based models of single cell electrophysiology. With a view to facilitating the construction of such models, this article reviews progress made regarding single AVN cell electrophysiological data during the last decade, predominantly focusing on that derived from morphologically normal AVN cells. Properties and potential roles of a range of currents are discussed: including L-type and T-type calcium currents (ICa,Land ICa,Trespectively), background current, hyperpolarization-activated current (If), delayed and transient outward potassium currents (IKrand Ito, respectively), sodium–calcium exchanger current (INaCa), the sustained inward current (Ist) and acetylcholine-activated potassium current (IKACh).

Long QT syndrome and anaesthesia

Long QT syndrome (LQTS) is an arrhythmogenic cardiovascular disorder resulting from mutations in cardiac ion channels. LQTS is characterized by prolonged ventricular repolarization and frequently manifests itself as QT interval prolongation on the electrocardiogram (ECG). The age at presentation varies from in utero to adulthood. The majority of symptomatic events are related to physical activity and emotional stress. Although LQTS is characterized by recurrent syncope, cardiac arrest, and seizure-like episodes, only about 60% of patients are symptomatic at the time of diagnosis. 3 The clinical features of LQTS result from a peculiar episodic ventricular tachyarrhythmia called ‘torsade de pointes’. ‘Twisting of the points’ describes the typical sinusoidal twisting of the QRS axis around the isoelectric line of the ECG. Usually torsade de pointes start with a premature ventricular depolarization, followed by a compensatory pause. The next sinus beat often has a markedly prolonged QT interval and abnormal T wave. This is followed by a ventricular tachycardia that is characterized by variation in the QRS morphology, and a constantly changing R-R interval (Fig. 1). The ‘short-long-short’ cycle length sequence heralding torsade de pointes is a hallmark of LQTS. Commonly, the episode of torsade de pointes is self-terminating, producing a syncopal episode or pseudoseizure, secondary to the abrupt decrease in cerebral blood flow. The majority of episodes of sudden death in LQTS result from ventricular fibrillation triggered by torsade de pointes, although the mechanism of this deterioration is unknown. Traditionally, LQTS has been classified as either familial (inherited) or acquired. However, it is likely that many patients with previously labelled acquired LQTS carry a silent mutation in one of the genes responsible for congenital LQTS. 22 119 The evidence for this hypothesis has been gradually emerging over the past few years. It is important for anaesthetists to be aware of this concept, as it means that a much higher proportion of the general population may be affected by asymptomatic mutations in genes encoding cardiac ion channels than was thought previously. The prevalence of LQTS in developed countries may be as high as 1 per 1100‐3000 of the population. 32 119 About 30% of congenital LQTS carriers have an apparently normal phenotype, and thus a normal QT interval, and remain undiagnosed until an initiating event. 105 Fatal arrhythmias associated with primary electrical disease of the heart such as the Brugada and LQTS, probably account for 19% of sudden deaths in children between 1 and 13 yr of age, and 30% of sudden deaths that occur between 14 and 21 yr of age. 10 Furthermore, there is a strong association between prolonged corrected QT interval (QTc) in the first week of life and risk of sudden infant death syndrome. 86

Characteristics of single cells isolated from the atrioventricular node of the adult guinea-pig heart.

To date, data regarding the cellular electrophysiology of the atrioventricular node (AVN) have derived from AVN cells isolated from the rabbit heart. The aim of this study was to characterise for the first time the electrophysiological properties of single cells isolated from the AVN of the guinea-pig heart. Cells were isolated from the AVN region by a combination of enzymatic and mechanical dispersion. Mean (+/-SEM) cell dimensions were 110.8+/-2.3 microm in length by 11.4+/-1.3 micro m in width (n=76 cells). Electrophysiological recordings were made using the whole-cell patch-clamp technique at 37 degrees C. Mean cell capacitance was 25.1+/-0.9 pF (n=43) and mean cell input resistance was 1,377+/-178 Muomega (n=21). Spontaneously active cells exhibited action potentials characteristic of cardiac pacemaker tissue. Under whole-cell voltage clamp, the mean 'zero current' potential was -39.7+/-4.1 mV (n=21). Voltage commands of 500 ms duration to a range of test potentials from a holding potential of -40 mV revealed a number of distinct current components. At potentials positive to -40 mV an early inward current was observed that exhibited a bell-shaped current voltage (I-V) relation, typical of L-type calcium current. A delayed outward current that increased progressively with test potential magnitude was also observed. Analysis of the outward current 'tails' on repolarisation to -40 mV showed this to be comprised of two components with half-maximal activation voltages of -17.2+/-1.8 mV, and +27.1+/-3.6 mV (n=13). 'Transient outward' current appeared absent from guinea-pig AVN cells. Hyperpolarising test pulses activated net inward current, for which three components could be observed: a barium-sensitive (100 microM Ba(2+)) inwardly rectifying current evident at the start of the voltage command and prominent at potentials negative to the diastolic potential range; a time-dependent, hyperpolarisation-activated current, and a residual background current. On repolarisation to -40 mV, a large inward current spike typical of cardiac Na current was observed in some cells. Notably, the following electrophysiological characteristics of guinea-pig AVN cells are distinct from those characteristics previously reported for the rabbit AVN: (1) an absence of transient outward current, (2) the presence of two components of delayed outward current, and (3) the presence of a Ba(2+)-sensitive inwardly rectifying current at negative voltages. The guinea-pig AVN isolated cell preparation may be valuable in providing additional insights into the cellular electrophysiology of this important region of the heart.

Electrophysiological effects of ibutilide on the delayed rectifier K + current in rabbit sinoatrial and atrioventricular node cells

Biophysical and pharmacological characteristics of the delayed rectifier K + current ( I K) of rabbit sinoatrial (SA) node and atrioventricular (AV) node cells have been studied using the whole-cell patch clamp technique together with a recently developed antiarrhythmic agent, ibutilide. Ibutilide is a potent blocker of the rapid delayed rectifier K + current, I Kr. Superfusion with ibutilide (10 −7 M) caused a decrease in the spontaneous firing frequency, depolarization of the maximal diastolic potential and prolongation of the action potential duration in both SA and AV node cells. In whole cell voltage clamp experiments done on myocytes from SA node, ibutilide (10 −7 M) blocked I K strongly (40%) and had smaller effects on Ca 2+ current (10%) and hyperpolarization-activated inward current, I f (11%). In AV node cells, the corresponding reductions were I K (68%), I Ca (13%) and I f (10%), respectively. A 10-fold increase in the concentration of ibutilide further decreased I K in SA node cells (67±8%), and blocked I K almost completely in AV node cells. These results are consistent with the hypothesis that the delayed rectifier K + current in SA node cell is generated by both I Kr and I Ks, whereas I Kr predominates in AV node cells. Knowledge of the differences in the distribution of I Kr, as well as the different sensitivity to blockers of I Kr in nodal cells, is important for understanding modifications of the automaticity, conduction velocity, and refractoriness by class III antiarrhythmic agents.

Rate-dependency of Action Potential Duration and Refractoriness in Isolated Myocytes from the Rabbit AV Node and Atrium

During atrial fibrillation, ventricular rate is determined by atrioventricular nodal (AVN) conduction, which in part is dependent upon the refractoriness of single AVN cells. The aims of this study were to investigate the rate-dependency of the action potential duration (APD) and effective refractory period (ERP) in single myocytes isolated from the AV node and atrium of rabbit hearts, using whole cell patch clamping, and to determine the contribution of the 4-aminopyridine (4-AP)-sensitive current, ITO1to these relationships in the two cell types. AVN cells had a more positive maximum diastolic potential (−60±1 v−71±2 mV), lower Vmax(8±2 v 144±17 V/s) and higher input resistance [420±46 v 65±7 MΩ (mean±s.eP<0.05n =9–33)], respectively, than atrial myocytes. Stepwise increases in rate from 75 beats/min caused activation failure and Wenckebach periodicity in AVN cells (at around 400 beats/min), but 1:1 activation in atrial cells (at up to 600 beats/min). Rate reduction from 300 to 75 beats/min shortened the ERP in both cell types (from 155±7 to 135±11 ms in AVN cells [P<0.05, n=6] and from 130±8 to 106±7 ms in atrial cells [P<0.05, n=10]). Rate increase from 300 to 480 and 600 beats/min shortened ERP in atrial cells, by 12±4% (n=8) and 26±7% (n=7), respectively (P<0.05). By contrast, AVN ERP did not shorten at rates >300 beats/min. In atrial cells, rate reduction to 75 beats/min caused marked shortening of APD50(from 51±6 to 29±6 ms, P<0.05). 4-AP (1 mm ) significantly prolonged atrial APD50at 75 beats/min (P<0.05, n=7), but not at 300 or 400 beats/min. In AVN cells, in contrast, there was less effect of rate change on APD, and 4-AP did not alter APD50at any rate. 4-AP also did not affect APD90or ERP in either cell type. In conclusion, a lack of ERP-shortening at high rates in rabbit single AVN cells may contribute to ventricular rate control. ITO1contributed to the APD50rate relation in atrial, but not AVN cells and did not contribute to the ERP rate relation in either cell type.

Sustained inward current during pacemaker depolarization in mammalian sinoatrial node cells.

Several time- and voltage-dependent ionic currents have been identified in cardiac pacemaker cells, including Na(+) current, L- and T-type Ca(2+) currents, hyperpolarization-activated cation current, and various types of delayed rectifier K(+) currents. Mathematical models have demonstrated that spontaneous action potentials can be reconstructed by incorporating these currents, but relative contributions of individual currents vary widely between different models. In 1995, the presence of a novel inward current that was activated by depolarization to the potential range of the slow diastolic depolarization in rabbit sinoatrial (SA) node cells was reported. Because the current showed little inactivation during depolarizing pulses, it was called the sustained inward current (I(st)). A similar current is also found in SA node cells of the guinea pig and rat and in subsidiary pacemaker atrioventricular node cells. Recently, single-channel analysis has revealed a nicardipine-sensitive, 13-pS Na(+) current, which is activated by depolarization to the diastolic potential range in guinea pig SA node cells. This channel differs from rapid voltage-gated Na(+) or L-type Ca(2+) channels both in unitary conductance and gating kinetics. Because I(st) was observed only in spontaneously beating SA node cells, ie, it was absent in quiescent cells dissociated from the same SA or atrioventricular node, an important role of I(st) for generation of intrinsic cardiac automaticity was suggested.

Surface potentials from the region of the atrioventricular node and their relation to dual pathway electrophysiology.

Clinical applications of the principles of dual atrioventricular nodal (AVN) electrophysiology in the treatment of AVN reentrant tachycardias rely on empirical findings, such as discontinued conduction curves or the presence of specific catheter-recorded signals. However, neither the shape of the conduction curve nor the surface electrograms have been validated as functionally related to the presence of slow or fast wavefronts. We performed in vitro studies using 10 rabbit atrial-AVN preparations. A bipolar roving electrode was used to explore the endocardial surface of the triangle of Koch during programmed electrical stimulation. Microelectrodes were impaled in AVN cells to correlate surface and intracellular responses. In 7 preparations, a specific area near the compact cell region produced surface electrograms that were dissociated in 2 distinct components, with progressive shortening of prematurity. Similar dissociation was demonstrated during Wenckebach periodicity and increased vagal tone. Cellular recordings supported the presence of early ("fast") and late ("slow") wavefronts, with different refractory properties. Although the fast-slow transition was a basis for discontinued propagation, the AVN conduction curves were smooth in the majority of cases. Exploration of the triangle of Koch during programmed pacing reveals the presence of dual-wavefront surface potentials. Clinical confirmation of these AVN potentials could provide a new, sensitive tool in defining dual AVN electrophysiology.

An investigation of the role played by the E-4031-sensitive (rapid delayed rectifier) potassium current in isolated rabbit atrioventricular nodal and ventricular myocytes.

The aim of this study was to measure and compare the profile of rapid delayed rectifier potassium current (IKr) elicited by action potential (AP) waveforms applied to isolated rabbit atrioventricular nodal (AVN) and ventricular myocytes. All measurements were made using whole-cell patch-clamp recordings at 37 degrees C. In AVN myocytes, IKr during voltage steps and slow ramp depolarisations showed "inward rectification" (characteristic for this channel) at positive potentials. The E-4031-sensitive current showed half-maximal activation at -10.8 +/- 0.86 mV, with a slope factor for the activation relation of 6.5 +/- 0.77 mV (n = 7). During AVN APs, IKr rapidly reached a peak after the AP upstroke and remained at similar amplitude until late in AP repolarisation. At the maximum diastolic potential following the AVN AP, a component of IKr remained which decayed during the pacemaker depolarisation, consistent with a role for the current in generating AVN pacemaker activity. In ventricular myocytes IKr was small at the beginning of the AP, and increased slowly during the AP plateau. Measurement of Ba-sensitive-inward rectifier K current (IK1) in ventricular myocytes revealed that IK1 rapidly increased during the final AP repolarisation phase, whilst IKr declined. It is concluded that IKr may participate in both AP repolarisation and the pacemaker depolarisation in AVN cells, whilst in ventricular myocytes, IKr and IK1 participate in controlling early and final AP repolarisation respectively.