Effects of changes in excitability and intercellular coupling on synchronization in the rabbit sino-atrial node

The conduction velocity-potassium relationship in the heart is modulated by sodium and calcium

The relationship between cardiac conduction velocity (CV) and extracellular potassium (K⁺) is biphasic, with modest hyperkalemia increasing CV and severe hyperkalemia slowing CV. Recent studies from our group suggest that elevating extracellular sodium (Na⁺) and calcium (Ca²⁺) can enhance CV by an extracellular pathway parallel to gap junctional coupling (GJC) called ephaptic coupling that can occur in the gap junction adjacent perinexus. However, it remains unknown whether these same interventions modulate CV as a function of K⁺. We hypothesize that Na⁺, Ca²⁺, and GJC can attenuate conduction slowing consequent to severe hyperkalemia. Elevating Ca²⁺ from 1.25 to 2.00 mM significantly narrowed perinexal width measured by transmission electron microscopy. Optically mapped, Langendorff-perfused guinea pig hearts perfused with increasing K⁺ revealed the expected biphasic CV-K⁺ relationship during perfusion with different Na⁺ and Ca²⁺ concentrations. Neither elevating Na⁺ nor Ca²⁺ alone consistently modulated the positive slope of CV-K⁺ or conduction slowing at 10-mM K⁺; however, combined Na⁺ and Ca²⁺ elevation significantly mitigated conduction slowing at 10-mM K⁺. Pharmacologic GJC inhibition with 30-μM carbenoxolone slowed CV without changing the shape of CV-K⁺ curves. A computational model of CV predicted that elevating Na⁺ and narrowing clefts between myocytes, as occur with perinexal narrowing, reduces the positive and negative slopes of the CV-K⁺ relationship but do not support a primary role of GJC or sodium channel conductance. These data demonstrate that combinatorial effects of Na⁺ and Ca²⁺ differentially modulate conduction during hyperkalemia, and enhancing determinants of ephaptic coupling may attenuate conduction changes in a variety of physiologic conditions.

Resonant model-A new paradigm for modeling an action potential of biological cells

Organ level simulation of bioelectric behavior in the body benefits from flexible and efficient models of cellular membrane potential. These computational organ and cell models can be used to study the impact of pharmaceutical drugs, test hypotheses, assess risk and for closed-loop validation of medical devices. To move closer to the real-time requirements of this modeling a new flexible Fourier based general membrane potential model, called as a Resonant model, is developed that is computationally inexpensive. The new model accurately reproduces non-linear potential morphologies for a variety of cell types. Specifically, the method is used to model human and rabbit sinoatrial node, human ventricular myocyte and squid giant axon electrophysiology. The Resonant models are validated with experimental data and with other published models. Dynamic changes in biological conditions are modeled with changing model coefficients and this approach enables ionic channel alterations to be captured. The Resonant model is used to simulate entrainment between competing sinoatrial node cells. These models can be easily implemented in low-cost digital hardware and an alternative, resource-efficient implementations of sine and cosine functions are presented and it is shown that a Fourier term is produced with two additions and a binary shift.

Maximum heart rate in brown trout (Salmo trutta fario) is not limited by firing rate of pacemaker cells

Temperature-induced changes in cardiac output (Q̇) in fish are largely dependent on thermal modulation of heart rate ( fH), and at high temperatures Q̇ collapses due to heat-dependent depression of fH. This study tests the hypothesis that firing rate of sinoatrial pacemaker cells sets the upper thermal limit of fH in vivo. To this end, temperature dependence of action potential (AP) frequency of enzymatically isolated pacemaker cells (pacemaker rate, fPM), spontaneous beating rate of isolated sinoatrial preparations ( fSA), and in vivo fH of the cold-acclimated (4°C) brown trout ( Salmo trutta fario) were compared under acute thermal challenges. With rising temperature, fPM steadily increased because of the acceleration of diastolic depolarization and shortening of AP duration up to the break point temperature (TBP) of 24.0 ± 0.37°C, at which point the electrical activity abruptly ceased. The maximum fPM at TBP was much higher [193 ± 21.0 beats per minute (bpm)] than the peak fSA (94.3 ± 6.0 bpm at 24.1°C) or peak fH (76.7 ± 2.4 at 15.7 ± 0.82°C) ( P < 0.05). These findings strongly suggest that the frequency generator of the sinoatrial pacemaker cells does not limit fH at high temperatures in the brown trout in vivo.

Desmosomal junctions are necessary for adult sinus node function

AIMS

Current mechanisms driving cardiac pacemaker function have focused on ion channel and gap junction channel function, which are essential for action potential generation and propagation between pacemaker cells. However, pacemaker cells also harbour desmosomes that structurally anchor pacemaker cells to each other in tissue, but their role in pacemaker function remains unknown.

METHODS AND RESULTS

To determine the role of desmosomes in pacemaker function, we generated a novel mouse model harbouring cardiac conduction-specific ablation (csKO) of the central desmosomal protein, desmoplakin (DSP) using the Hcn4-Cre-ERT2 mouse line. Hcn4-Cre targets cells of the adult mouse sinoatrial node (SAN) and can ablate DSP expression in the adult DSP csKO SAN resulting in specific loss of desmosomal proteins and structures. Dysregulation of DSP via loss-of-function (adult DSP csKO mice) and mutation (clinical case of a patient harbouring a pathogenic DSP variant) in mice and man, respectively, revealed that desmosomal dysregulation is associated with a primary phenotype of increased sinus pauses/dysfunction in the absence of cardiomyopathy. Underlying defects in beat-to-beat regulation were also observed in DSP csKO mice in vivo and intact atria ex vivo. DSP csKO SAN exhibited migrating lead pacemaker sites associated with connexin 45 loss. In vitro studies exploiting ventricular cardiomyocytes that harbour DSP loss and concurrent early connexin loss phenocopied the loss of beat-to-beat regulation observed in DSP csKO mice and atria, extending the importance of DSP-associated mechanisms in driving beat-to-beat regulation of working cardiomyocytes.

CONCLUSION

We provide evidence of a mechanism that implicates an essential role for desmosomes in cardiac pacemaker function, which has broad implications in better understanding mechanisms underlying beat-to-beat regulation as well as sinus node disease and dysfunction.

Fibrosis: a structural modulator of sinoatrial node physiology and dysfunction

Heart rhythm is initialized and controlled by the Sinoatrial Node (SAN), the primary pacemaker of the heart. The SAN is a heterogeneous multi-compartment structure characterized by clusters of specialized cardiomyocytes enmeshed within strands of connective tissue or fibrosis. Intranodal fibrosis is emerging as an important modulator of structural and functional integrity of the SAN pacemaker complex. In adult human hearts, fatty tissue and fibrosis insulate the SAN from the hyperpolarizing effect of the surrounding atria while electrical communication between the SAN and right atrium is restricted to discrete SAN conduction pathways. The amount of fibrosis within the SAN is inversely correlated with heart rate, while age and heart size are positively correlated with fibrosis. Pathological upregulation of fibrosis within the SAN may lead to tachycardia-bradycardia arrhythmias and cardiac arrest, possibly due to SAN reentry and exit block, and is associated with atrial fibrillation, ventricular arrhythmias, heart failure and myocardial infarction. In this review, we will discuss current literature on the role of fibrosis in normal SAN structure and function, as well as the causes and consequences of SAN fibrosis upregulation in disease conditions.

Sinoatrial node reentry in a canine chronic left ventricular infarct model: role of intranodal fibrosis and heterogeneity of refractoriness

BACKGROUND

Reentrant arrhythmias involving the sinoatrial node (SAN), namely SAN reentry, remain one of the most intriguing enigmas of cardiac electrophysiology. The goal of the present study was to elucidate the mechanism of SAN micro-reentry in canine hearts with post-myocardial infarction (MI) structural remodeling.

METHODS AND RESULTS

In vivo, Holter monitoring revealed ventricular arrhythmias and SAN dysfunctions in post-left ventricular MI (6-15 weeks) dogs (n=5) compared with control dogs (n=4). In vitro, high-resolution near-infrared optical mapping of intramural SAN activation was performed in coronary perfused atrial preparations from MI (n=5) and controls (n=4). Both SAN macro- (slow-fast; 16-28 mm) and micro-reentry (1-3 mm) were observed in 60% of the MI preparations during moderate autonomic stimulation (acetylcholine [0.1 µmol/L] or isoproterenol [0.01-0.1 µmol/L]) after termination of atrial tachypacing (5-8 Hz), a finding not seen in controls. The autonomic stimulation induced heterogeneous changes in the SAN refractoriness; thus, competing atrial or SAN pacemaker waves could produce unidirectional blocks and initiate intranodal micro-reentry. The micro-reentry pivot waves were anchored to the longitudinal block region and produced both tachycardia and paradoxical bradycardia (due to exit block), despite an atrial ECG morphology identical to regular sinus rhythm. Intranodal longitudinal conduction blocks coincided with interstitial fibrosis strands that were exaggerated in the MI SAN pacemaker complex (fibrosis density: 37±7% MI versus 23±6% control; P<0.001).

CONCLUSIONS

Both tachy- and brady-arrhythmias can result from SAN micro-reentry. Postinfarction remodeling, including increased intranodal fibrosis and heterogeneity of refractoriness, provides substrates for SAN reentry.

Conduction barriers and pathways of the sinoatrial pacemaker complex: their role in normal rhythm and atrial arrhythmias

Since Keith and Flack's anatomical discovery of the sinoatrial node (SAN), the primary pacemaker of the heart, the question of how such a small SAN structure can pace the entire heart has remained for a large part unanswered. Recent advances in optical mapping technology have made it possible to unambiguously resolve the origin of excitation and conduction within the animal and human SAN. The combination of high-resolution optical mapping and histological structural analysis reveals that the canine and human SANs are functionally insulated from the surrounding atrial myocardium, except for several critical conduction pathways. Indeed, the SAN as a leading pacemaker requires anatomical (fibrosis, fat, and blood vessels) and/or functional barriers (paucity of connexins) to protect it from the hyperpolarizing influence of the surrounding atrium. The presence of conduction barriers and pathways may help explain how a small cluster of pacemaker cells in the SAN pacemaker complex manages to depolarize different, widely distributed areas of the right atria as evidenced functionally by exit points and breakthroughs. The autonomic nervous system and humoral factors can further regulate conduction through these pathways, affecting pacemaker automaticity and ultimately heart rate. Moreover, the conduction barriers and multiple pathways can form substrates for reentrant activity and thus lead to atrial flutter and fibrillation. This review aims to provide new insight into the function of the SAN pacemaker complex and the interaction between the atrial pacemakers and the surrounding atrial myocardium not only in animal models but also human hearts.

The relevance of non-excitable cells for cardiac pacemaker function

Age-dependent changes in the architecture of the sinus node comprise an increasing ratio between fibroblasts and cardiomyocytes. This change is discussed as a potential mechanism for sinus node disease. The goal of this study was to determine the mechanism through which non-excitable cells influence the spontaneous activity of multicellular cardiomyocyte preparations. Cardiomyocyte monolayers (HL-1 cells) or embryonic stem cell-derived cardiomyocytes were used as two- and three-dimensional cardiac pacemaker models. Spontaneous activity and conduction velocity (theta) were monitored by field potential measurements with microelectrode arrays (MEAs). The influence of fibroblasts (WT-fibs) was determined in heterocellular cultures of different cardiomyocyte and fibroblast ratios. The relevance of heterocellular gap junctional coupling was evaluated by the use of fibroblasts deficient for the expression of Cx43 (Cx43(-/-)-fibs). The beating frequency and of heterocellular cultures depended negatively on the fibroblast concentration. Interspersion of fibroblasts in cardiomyocyte monolayers increased the coefficient of the interbeat interval variability. Whereas Cx43(-/-)-fibs decreased theta significantly less than WT-fibs, their effect on the beating frequency and the beat-to-beat variability seemed largely independent of their ability to establish intercellular coupling. These results suggest that electrically integrated, non-excitable cells modulate the excitability of cardiac pacemaker preparations by two distinct mechanisms, one dependent and the other independent of the heterocellular coupling established. Whereas heterocellular coupling enables the fibroblast to depolarize the cardiomyocytes or to act as a current sink, the mere physical separation of the cardiomyocytes by fibroblasts induces bradycardia through a reduction in frequency entrainment.

Arrhythmia as a result of poor intercellular coupling in the sinus node: a simulation study

The effects of reduced intercellular coupling in the sinus node were investigated by means of simulations. Coupling was reduced both uniformly, and by introducing localized interaction blocks. In either case, model sinus node element networks typically splitted into frequency domains. These were defined as groups of neighbour elements which all attained the same mean firing frequency. In systems, simulating the vicinity of an impulse outlet to the atrium, the sinus node elements often splitted into two domains, one slowly firing just inside the outlet, and one normally firing large domain in the sinus node interior. This two-domain situation was analysed using a two-element system. Wenckebach conduction and advanced (m:1) exit blocks were seen, together with more odd block patterns and slow chaotic rhythms. The two-domain situation appeared also when two discrete outlets were considered. The slow domains around each outlet synchronized via the atrium. However, if there were some degree of exit block through one of the outlets only, brady-tachy like rhythms could be simulated due to a re-entrant circuit including both sinus node and atrial tissue. In conclusion, poor coupling in the sinus node seems to be sufficient to produce most arrhythmias in the sick sinus syndrome

A simulation of the SA node by a phase response curve-based model of a two-dimensional pacemaker cells array

This paper presents a simulation of the sino-atrial (SA) node by a two-dimensional pacemaker cells array model, based on phase response curve (PRC) interaction. This simple model of the cardiac pacemaker cells, involves only the most basic functional properties, which play a direct role in the determination of the SA node rhythm. The two most relevant functional properties of the pacemaker cells are: The intrinsic cycle length, an "internal" feature of each pacemaker cell, and the PRC, an "overall collective" function. The PRC contains the "information" about the type of interactions of each pacemaker cell with the outside world (i.e., interaction with neighboring cells, external stimulus, etc.), and "strength" of the interaction (strong, weak, etc.). We studied the spatial interaction among a large number of pacemaker cells (15 x 15), as a function of the regional variation of cells properties, the "electrical" coupling between cells (the PRC), and the appearance of regions with abnormal cycle lengths. We investigated the influence of those parameters on the mutual interaction between the pacemaker cells, on the activation pattern and conduction time of the array, and on a pseudo-electrocardioigram (ECG) signal. This study demonstrates that by representing the pacemaker cells in the SA node by only two fundamental features, and by applying a simple physical-mathematical model, we can create a global picture of the SA node system. This enables us to explore physiological phenomena related to the genesis and maintenance of the SA node activity, and to gain insight into the conditions which predispose the SA node instability, and conduction disturbances.

Intermittent sinus bigeminy as an expression of sinus parasystole: a case report

A case of sinus parasystole is reported. The diagnosis of sinus parasystole is relatively difficult because there is no difference between the basic sinus P wave and the parasystolic wave. Sinus parasystole is diagnosed according to the following electrocardiographic criteria: (1) premature P waves having contour identical to P waves of basic beats; (2) intervals between premature P waves mathematically related. In the case reported, the coupling intervals during long phases of intermittent sinus bigeminy were nearly fixed, because there was little variability in the returning cycles, making the diagnosis of sinus parasystole difficult.

Phase response curve based model of the SA node: simulation by two-dimensional array of pacemaker cells with randomly distributed cycle lengths

A simulation of the SA node is presented, based on a 2D array (15 x 15) model of randomly distributed pacemaker cells, interacting via a phase response curve (PRC). The model involves only the basic properties that play a direct role in the determination of the SA node rhythm: intrinsic cycle length and PRC. The PRC reflects the 'type' of interaction of each pacemaker cell with the outside world (neighbouring cells, external stimulus, etc.). A major outcome of this study is the demonstration that global dynamics and the degree of 'disorder' of the SA node are strongly affected by the cycle length distribution of the model, as well as spatial inhomogeneity in the cell-to-cell 'electrical' coupling (PRC). Those factors also determine the conduction velocity throughout the SA node and may therefore be responsible for anisotropic conduction. For example, lowering the PRC parameters (d and a) by 25% increases the array activation time from 46 to 126 ms. The model also responds appropriately to a perturbation such as a vagal pulse. This pulse produces a shift of the dominant pacemaker to another site in the array and a transient lengthening of the array cycle length, for example from 312 to 355 ms.

Differential expression of gap junction proteins in the canine sinus node

Electrical coupling of pacemaker cells at gap junctions appears to play an important role in sinus node function. Although the major cardiac gap junction protein, connexin43 (Cx43), is expressed abundantly in atrial and ventricular muscle, its expression in the sinus node has been a subject of controversy. The objectives of the present study were to determine whether Cx43 is expressed by sinus node myocytes, to characterize the spectrum of connexin expression phenotypes in sinus node pacemaker cells, and to define the spatial distribution of different connexin phenotypes in the intact sinus node. To fulfill these objectives, we performed high-resolution immunohistochemical analysis of disaggregated adult canine sinus node preparations. Using enhanced tissue preservation and antigen retrieval techniques, we also performed immunohistochemical studies on sections of intact canine sinus node tissue. Analysis of disaggregated sinus node preparations revealed three populations of pacemaker cells distinguished on the basis of connexin immunohistochemical phenotype: approximately 55% of cells expressed only connexin40 (Cx40); 30% to 35% of cells expressed Cx43, connexin45 (Cx45), and Cx40; and the remaining cells had no detectable connexin expression. In immunostained sections of intact sinus node, Cx43- and Cx45-positive cells were limited in their distribution and were observed in discrete bundles that appeared to abut atrial myocytes. In contrast, Cx40 immunoreactive signal was widely distributed in the sinus node region. These results indicate that subsets of pacemaker cells express distinct connexin phenotypes. Differential expression of connexins could create regions within the sinus node with different conduction properties, thereby contributing to the nonuniform conduction properties seen in this tissue.

Pacemaker synchronization of electrically coupled rabbit sinoatrial node cells

The effects of intercellular coupling conductance on the activity of two electrically coupled isolated rabbit sinoatrial nodal cells were investigated. A computer-controlled version of the "coupling clamp" technique was used in which isolated sinoatrial nodal cells, not physically in contact with each other, were electrically coupled at various values of ohmic coupling conductance, mimicking the effects of mutual interaction by electrical coupling through gap junctional channels. We demonstrate the existence of four types of electrical behavior of coupled spontaneously active cells. As the coupling conductance is progressively increased, the cells exhibit: (a) independent pacemaking at low coupling conductances, (b) complex dynamics of activity with mutual interactions, (c) entrainment of action potential frequency at a 1:1 ratio with different action potential waveforms, and (d) entrainment of action potentials at the same frequency of activation and virtually identical action potential waveforms. The critical value of coupling conductance required for 1:1 frequency entrainment was <0.5 nS in each of the five cell pairs studied. The common interbeat interval at a relatively high coupling conductance (10 nS), which is sufficient to produce entrainment of frequency and also identical action potential waveforms, is determined most by the intrinsically faster pacemaker cell and it can be predicted from the diastolic depolarization times of both cells. Evidence is provided that, at low coupling conductances, mutual pacemaker synchronization results mainly from the phase-resetting effects of the action potential of one cell on the depolarization phase of the other. At high coupling conductances, the tonic, diastolic interactions become more important.

Intramolecular interactions mediate pH regulation of connexin43 channels

We have previously proposed that acidification-induced regulation of the cardiac gap junction protein connexin43 (Cx43) may be modeled as a particle-receptor interaction between two separate domains of Cx43: the carboxyl terminal (acting as a particle), and a region including histidine 95 (acting as a receptor). Accordingly, intracellular acidification would lead to particle-receptor binding, thus closing the channel. A premise of the model is that the particle can bind its receptor, even if the particle is not covalently bound to the rest of the protein. The latter hypothesis was tested in antisense-injected Xenopus oocyte pairs coexpressing mRNA for a pH-insensitive Cx43 mutant truncated at amino acid 257 (i.e., M257) and mRNA coding for the carboxyl terminal region (residues 259-382). Intracellular pH (pHo) was recorded using the dextran form of the proton-sensitive dye seminaphthorhodafluor (SNARF). Junctional conductance (Gj) was measured with the dual voltage clamp technique. Wild-type Cx43 channels showed their characteristic pH sensitivity. M257 channels were not pH sensitive (pHo tested: 7.2 to 6.4). However, pH sensitivity was restored when the pH-insensitive channel (M257) was coexpressed with mRNA coding for the carboxyl terminal. Furthermore, coexpression of the carboxyl terminal of Cx43 enhanced the pH sensitivity of an otherwise less pH-sensitive connexin (Cx32). These data are consistent with a model of intramolecular interactions in which the carboxyl terminal acts as an independent domain that, under the appropriate conditions, binds to a separate region of the protein and closes the channel. These interactions may be direct (as in the ball-and-chain mechanism of voltage-dependent gating of potassium channels) or mediated through an intermediary molecule. The data further suggest that the region of Cx43 that acts as a receptor for the particle is conserved among connexins. A similar molecular mechanism may mediate chemical regulation of other channel proteins.

Effects of gap junction conductance on dynamics of sinoatrial node cells: two-cell and large-scale network models

A computational model of single rabbit sinoatrial (SA) node cells has been revised to fit data on regional variation of rabbit SA node cell oscillation properties. The revised model simulates differences in oscillation frequency, maximum diastolic potential, overshoot potential, and peak upstroke velocity observed in cells from different regions of the node. Dynamic properties of electrically coupled cells, each with different intrinsic oscillation frequency, are studied as a function of coupling conductance. Simulation results demonstrate at least four distinct regimes of behavior as coupling conductance is varied: a) independent oscillation (Gc < 1 pS); b) complex oscillation (1 < or = Gc < 220 pS); c) frequency, but not waveform entrainment (Gc > or = 220 pS); and d) frequency and waveform entrainment (Gc > or = 50 nS). The conductance of single cardiac myocyte gap junction channels is about 50 pS. These simulations therefore show that very few gap junction channels between each cell are required for frequency entrainment. Analyses of large-scale SA node network models implemented on the Connection Machine CM-200 supercomputer indicate that frequency entrainment of large networks is also supported by a small number of gap junction channels between neighboring cells.

Immunohistochemical localization of gap junction protein channels in hamster sinoatrial node in correlation with electrophysiologic mapping of the pacemaker region

INTRODUCTION

Gap junction proteins are thought to form the low resistance pathways that connect neighboring cells within the sinoatrial node, and to mediate pacemaker synchronization.

METHODS AND RESULTS

We have carried out microelectrode mapping experiments of the hamster sinoatrial region to localize the primary pacemaker area for subsequent light, electron, and immunofluorescence microscopic studies aimed at testing the hypothesis that the major cardiac gap junction protein (connexin43) is present in such an area. The site of earliest activation is unifocal and the pattern of activation, obtained by multiple sequential microelectrode recordings of the sinoatrial region, is qualitatively similar to that previously described for other species. However, quantitatively, the impulse transmission time from the primary pacemaker area to the crista (sulcus) terminalis in the hamster sinoatrial node is about 50% briefer than that of the guinea pig and five times faster than that of the rabbit. Immunolocalization studies in the hamster sinoatrial node using anti-connexin43 antisera demonstrated specific staining at the areas of cell-to-cell apposition and suggested that the apparently high degree of electrical coupling in this tissue is the result of abundant connexin43 expression. The immunofluorescence data were supported by light microscopic studies, which demonstrated the typical morphologic characteristics of sinus nodal cells in the pacemaker area. In addition, an electron microscopic study of the sinoatrial region revealed the presence of gap junctions in the junctional complex at areas of cell-to-cell contact.

CONCLUSION

Our results demonstrate that cells in the sinoatrial region of the hamster heart are electrically well coupled and strongly suggest that such coupling is mediated by gap junctional channels formed by connexin43.

Interpretation of epicardial mapping by means of computer simulations: applications to calcium, lidocaine and to BRL 34915

The aim of this work was to compare experimental investigations on effects of lidocaine, calcium and, BRL 34915 on reentries to simulated data obtained by use of a model of propagation based on the Huygens' construction method already described in previous works. Calcium and lidocaine effects are investigated on anisotropic conduction conditions. In both cases, reduction in conduction velocities are observed. In lidocaine case, a refractory area is located along the longitudinal axis. In agreement with experimental electrical mapping, the simulations show that the stabilization of reentrant excitation is mainly due to the existence of this refractory area around which the reentrant circuit can develop. The experimental study shows that BRL 34915 has both arrhythmogenic and antiarrhythmic effects. A detailed electrophysiological analysis has shown that drug infusion act on normal cardiac cells by decreasing the relative and absolute refractory period. BRL 34915 action is simulated by a decrease in the refractory period showing that the time frequency of the reentrant activity is increased and that the spatial size where the reentry is developing is becoming smaller. These two effects are arrhythmogenic, the simulated data being so in good agreement with the experimental ones.

Gap junctional channels in adult mammalian sinus nodal cells. Immunolocalization and electrophysiology

The subcellular mechanism of cell-to-cell communication in the natural pacemaker region of the mammalian heart was studied using electrophysiological and immunofluorescence techniques in isolated pairs of rabbit sinus nodal cells. By measuring whole-cell currents using a double patch-clamp approach, it was demonstrated that communication in the sinus node is mediated through gap junctional channels similar to those in other types of adult cardiac cell pairs. Macroscopic sinus nodal junctional resistance had a mean value of 387.9 +/- 97.1 M omega (mean +/- SEM, n = 10) and was greatly increased by superfusion with alkanols. Single-channel junctional conductance could be resolved in three cell pairs. Given their high membrane resistance (1.16 +/- 0.32 G omega, n = 12), the electrical coupling provided by as few as three gap junctional channels between nodal cells will allow for pacemaker synchronization. Further evidence for the presence of the channels was obtained from immunofluorescent double-labeling of desmin and the gap junction protein (connexin43) in sinus nodal tissue as well as in cultured sinus nodal cells. Using antisera against residues 243-257 of the connexin43 protein, a specific staining at the site of cell-to-cell apposition was demonstrated. These data provide direct evidence in favor of electronic coupling as the means for achieving pacemaker synchronization in the rabbit sinus node.

Immunolocalization and expression of functional and nonfunctional cell-to-cell channels from wild-type and mutant rat heart connexin43 cDNA

The carboxyl terminal cytoplasmic domain of distinct gap junction proteins may play an important role in assembly of functional channels as well as differential responsiveness to pH, voltage, and intracellular second messengers. Oligonucleotide-directed site-specific mutagenesis in a paired Xenopus laevis oocyte expression system was used to examine the expression of mRNAs encoding wild-type and carboxyl terminal mutant connexin43 (Cx43) proteins. Oocytes were stripped, injected with mRNA or distilled water (dH2O), preincubated for 16-20 hours, and then paired for 5-10 hours; this process was followed by electrophysiological recording using the dual voltage-clamp technique. Initial experiments compared the relative junctional conductances (Gjs) in oocyte pairs expressing Cx43 (382 amino acid residues) and two truncated mutants lacking most or a portion of the cytoplasmic carboxyl terminal. The shortest mutant (M241) contained 240 amino acid residues and was devoid of all phosphorylatable serine residues in the cytoplasmic tail; its length approximated the length of liver connexin26. The longest mutant (M257) tested contained 256 amino acid residues, including two serine residues. Oocyte pairs expressing M241 yielded a Gj similar to that of oocytes injected with dH2O, whereas M257 yielded a Gj similar to that of oocytes injected with Cx43. Immunoprecipitation studies showed that Cx43, M257, and M241 proteins were readily detectable in oocytes injected with their respective mRNAs, indicating that the lack of Gj observed with the M241 mRNA was not due to reduced translation. Immunocytochemical studies revealed that wild-type and both truncated mutants were localized to the area of cell-to-cell contact between the paired oocytes, indicating that protein targeting to the membrane was not inhibited in oocytes injected with M241 mRNA. Oocyte pairs expressing mutants in which serine residues were replaced with nonphosphorylatable amino acids (serine codon No. 255 AGC was converted to GCC, alanine, designated as M255S----A, and serine codon No. 244 AGC was converted to GGC, glycine, designated as M244S----G) showed Gjs similar to M257, indicating that these serine residues and, by inference, their phosphorylation state are not critical for expression of functional channels. The importance of the length of the carboxyl terminus was assessed by comparing the Gjs in a series of mutants that were intermediate in length between M257 and M241. Gradual shortening of the carboxyl terminus produced a gradual reduction of Gj relative to M257. However, simple deletion of amino acid residues 241-257 from the wild-type Cx43 did not affect Gj relative to M257.(ABSTRACT TRUNCATED AT 400 WORDS)

Experimental model of effects on normal tissue of injury current from ischemic region

An ischemic myocardial region contains cells with a depolarized resting membrane potential. This depolarization leads to an intercellular current flow between the ischemic region and the surrounding normal myocardial cells, which has been termed an "injury current." We have devised an experimental model system in which an isolated rabbit ventricular cell is electrically coupled to a model depolarized cell to evaluate the effects of this injury current on the electrical properties of a normal ventricular cell. We found that the action potential duration of the isolated cell could be reversibly altered by varying the coupling resistance such that the action potential duration was shortened by high values of coupling resistance but could be considerably prolonged by lower resistance coupling. We did not observe automaticity in the isolated cell as a consequence of coupling to the depolarized model. The changes in action potential duration were accompanied by alterations in the frequency at which the isolated cell could respond to repetitive stimuli. In addition, the depolarization of the isolated cell produced by the electrical coupling led to a significant increase in the cellular excitability. This last effect may be of particular importance in understanding the mechanisms for origination of arrhythmias in the border zone of myocardial ischemia.

Sinus parasystole

Sinus parasystole is the expression of a protected nondominant sinus pacemaker, which is totally independent of the dominant rhythm. Two forms of sinus parasystole are described: (1) an active form, where both the dominant and the parasystolic pacemakers are located within the sinus node and (2) a passive form, where the basic rhythm is ectopic and the sinus pacemaker is protected as a result of complete retrograde SA block. Three cases of sinus parasystole are analyzed. In the active form of the arrhythmia the parasystolic sinus P waves are identical to those of the basic sinus rhythm. The diagnosis is suggested by variably coupled premature sinus P waves occurring with mathematically related intervals. This relationship between the parasystolic intervals can not be precise whenever complicating factors such as modulation occur. The recognition of active sinus parasystole is difficult, since the parasystolic P waves do not differ from basic P waves, so that the pattern resembles that of sinus arrhythmia or sinus extrasystoles. The passive form of sinus parasystole is more easily recognized due to the clear-cut difference between the dominant ectopic atrial waves and the "parasystolic" sinus P waves, which manifest with variable coupling intervals and reflect mathematically related intervals in between.

Phase resetting and entrainment of pacemaker activity in single sinus nodal cells

The phase-resetting and entrainment properties of single pacemaker cells were studied using computer simulations in a model of the rabbit sinus nodal cell, as well as using the whole-cell patch-clamp (current-clamp) technique in isolated rabbit sinus nodal cells. Spontaneous electrical activity in the cell model was reconstructed using Hodgkin-Huxley-type equations describing time- and voltage-dependent membrane currents. In both simulations and experiments, single subthreshold current pulses (depolarizing or hyperpolarizing) were used to scan the spontaneous cycle of the cells. Such pulses perturbed the subsequent discharge, producing temporary phasic changes in pacemaker period, and enabled the construction of phase response curves. On the basis of these results, we studied entrainment characteristics of the cells. For example, application of repetitive pulses allowed for phasic changes in the spontaneous cycle and resulted in stable 1:1 entrainment at a range of basic cycle length around the spontaneous cycle, or a 2:1 pattern at basic cycle length values about half the spontaneous cycle length. Between the two entrainment zones, complex Wenckebach-like patterns (e.g., 5:4, 4:3, and 3:2) were observed. The experimental data from the isolated cell were further analyzed from a theoretical perspective, and the results showed that the topological characteristics of the phase-resetting behavior accounts for the experimentally observed patterns during repetitive stimulation of the cell. This first demonstration of phase resetting in single cells provides the basis for phenomena such as mutual entrainment between electrically coupled pacemaker cells, apparent intranodal conduction, and reflex vagal control of heart rate.

Electrotonic influences on action potentials from isolated ventricular cells

This work combines a theoretical study of electrical interactions between two excitable heart cells, using a variable coupling resistance, with experimental studies on isolated rabbit ventricular cells coupled with a variable coupling resistance to a passive resistance and capacitance circuit. The theoretical results show that the response of an isolated cell to an increased frequency of stimulation is strongly altered by the presence of a coupling resistance to another cell. As the coupling resistance gradually is decreased, the stimulated cell becomes able to respond successfully to more rapid stimulation, and then, at levels of coupling resistance that allow conduction between the two cells, the coupled pair of cells exhibits arrhythmic interactions not predicted by the intrinsic properties of either cell. The experimental results show that the isolated rabbit ventricular cell is extremely sensitive to even a very small electrical load, with shortening of the action potential by 50% with electrical coupling to a model cell (of similar input resistance and capacitance to the ventricular cell) as high as 1,000 M omega, even though the action potential amplitude and current threshold are very insensitive to the electrical load.

Bistabilities and annihilation phenomena in electrophysiological cardiac models

We have investigated the oscillatory behavior of cardiac cellular elements simulated by two electrophysiological models: the van Capelle and Durrer (VCD) model and the sinoatrial node cell model of Yanagihara, Noma, and Irisawa (YNI). The VCD model behavior was examined systematically by using continuation-bifurcation analysis. Bifurcation diagrams were constructed as a function of Qit1, an intrinsic parameter of the model, which sets both maximum diastolic potential and depolarization threshold of the cell. The existence of stable high amplitude oscillations was evidenced between two Hopf bifurcation points (HB). Near each HB, a zone of bistability was detected. Close to the HB that corresponded to high values of Qit1, a high amplitude periodic stable state coexisted with a stable steady state. Close to the other HB, in a narrow range of lower Qit1 values, a relatively high amplitude periodic stable state coexisted with a low amplitude periodic stable state. There was no stable steady state in the latter bistability zone. Through the use of phase-plane representations and the determination of separatrices between the different attractor basins, we could deduce the conditions of timing, polarity, and strength needed for a pulse perturbation to send the system from one state to another and vice versa. The YNI model was analyzed by numerical simulation, and the oscillatory behavior of the sinoatrial node cell was explored while applying a depolarizing bias current of various strengths. Results were similar to those obtained from the VCD model in that there were two bistability regions for two different ranges of applied bias current. Depending on current intensity, annihilation of pacemaker activity could be achieved in both zones. However, the coexistence of two oscillatory stable states was never observed in the YNI model. From the behavioral similarities of these different models, we can conclude that bistabilities and annihilation phenomena can be found in transitional zones between quiescence and rhythmic activity.

Structure-activity relations of the cardiac gap junction channel

Cardiac gap junction channels play the important roles of synchronizing pacemaker cells and allowing impulse propagation along the conduction system and throughout the ventricular myocardium. These channels, which support current flow in both longitudinal and tranverse directions, are permeable to anions and cations with radii less than approximately 0.5 nm and in rat heart have unitary conductances on the order of 50 pS. This unitary conductance is consistent with channel geometry described by a right cylindrical pore with diameter large enough for the brilliantly fluorescent dye molecule lucifer yellow to pass between cells. These channels, like others in biological systems, are opened and closed by various treatments, a process termed gating. Cytoplasmic acidification reduces junctional conductance (gj), an effect that is apparently potentiated by elevated myoplasmic Ca ions. Reduced gj also occurs in response to a variety of lipophilic molecules, including halothane, heptanol, and unsaturated fatty acids; the mechanism of action may involve disruption of the protein-lipid microenvironment of the gap junction channel. Arachidonic acid uncouples, and this effect is partially, but incompletely, blocked by an inhibitor of the lipoxygenase metabolic pathways. Cyclooxygenase inhibitors have no protective effects. Certain cyclic nucleotides can rapidly increase gj [adenosine 3',5'-cyclic monophosphate (cAMP)] or slightly decrease it [guanosine 3',5'-cyclic monophosphate (cGMP)], and agents that use these cyclic nucleotides as second messengers (isoproterenol and perhaps carbachol, respectively) produce consistent effects. Agents expected to cause protein kinase C activation (tumor-promoting phorbol esters and diacylglycerol) increase gj rapidly. The gap junction protein from rat heart has been cloned and sequenced. From the primary sequence for the protein, plausible sites of action within the putative cytoplasmic domains are proposed for each of these treatments. In response to gating stimuli that close the channel (halothane, CO2, heptanol), unitary channel conductance is unchanged, suggesting that these agents act by reducing open time probability. Together, these properties constitute the beginnings of our endeavor to define pharmacological agents that are potentially useful in therapeutic manipulation of synchronous discharge, conduction velocity, and isochronous wavefront propagation in cardiac tissue.

Chaotic activity in a mathematical model of the vagally driven sinoatrial node

Phase-locking behavior and irregular dynamics were studied in a mathematical model of the sinus node driven with repetitive vagal input. The central region of the sinus node was simulated as a 15 x 15 array of resistively coupled pacemakers with each cell randomly assigned one of 10 intrinsic cycle lengths (range 290-390 msec). Coupling of the pacemakers resulted in their mutual entrainment to a common frequency and the emergence of a dominant pacemaker region. Repetitive acetylcholine (ACh; vagal) pulses were applied to a randomly selected 60% of the cells. Over a wide range of stimulus intensities and basic cycle lengths, such perturbations resulted in a large variety of stimulus/response patterns, including phase locking (1:1, 3:2, 2:1, etc.) and irregular (i.e., chaotic) dynamics. At a low ACh concentration (1 microM), the patterns followed the typical Farey sequence of phase-locked behavior. At a higher concentration (5 microM), period doubling and aperiodic patterns were found. When a single pacemaker cell was perturbed with repetitive ACh pulses, qualitatively similar results were obtained. In both types of simulation, chaotic behavior was investigated using phase-plane (orbital) plots, Poincaré mapping, and return mapping. Period-doubling bifurcations (2:2, 4:4, and 8:8) were found temporally and spatially within the array. Under certain conditions of stimulation, the attractor in the return map during chaotic activity of the single cell resembled the Lorenz tent map. However, when electrical coupling between cells was allowed, the interactions with neighboring cells exhibiting chaotic dynamics resulted in characteristic alterations of the attractor geometry. Our results suggest that irregular dynamics obeying the rules derived from other chaotic systems are present during vagal stimulation of the sinus node. In addition, application of the same analytical tools to the analysis of simulation of reflex vagal control of sinus rate suggests that chaotic dynamics can be obtained in the physiologically relevant case of the baroreceptor reflex loop. These results may provide insight into the mechanisms of dynamic vagal control of heart rate and may help to provide insights into clinically relevant disturbances of cardiac rate and rhythm.

Mechanisms of sinoatrial pacemaker synchronization: a new hypothesis

A model of electrically coupled sinus node cells was used to investigate pacemaker coordination and conduction. Individual cells were simulated using differential equations describing transmembrane ionic currents. Intrinsic cycle lengths (periods) were adjusted by applying constant depolarizing or hyperpolarizing bias current, and cells were coupled through ohmic resistances to form two-dimensional arrays. Activation maps of 81-225 coupled cells showed an apparent wavefront conducting from a leading pacemaker region to the rest of the matrix even though the pattern actually resulted from mutual entrainment of all spontaneously beating cells. Apparent conduction time increased with increasing intercellular resistance. Appropriate selection of pacemaker cycle lengths and intercellular resistances permitted the accurate simulation of the activation sequence seen experimentally for the rabbit sinus node. Furthermore, a simulated acetylcholine pulse applied to a randomly selected 20% of the cells in this model produced a pacemaker shift that lasted several beats. These results support the hypothesis that sinus node synchronization occurs through a "democratic" process resulting from the phase-dependent interactions of thousands of pacemakers.

The influence of the atrial myocardium on impulse formation in the rabbit sinus node

In the isolated right atrium of the rabbit heart the influence of the atrial myocardium on impulse formation in the sinus node was investigated. Under normal conditions the pacemaker (earliest activation) was located in the center of the node where fibers with the highest rate of diastolic depolarization were found. After disconnection of the atrium from the sinus node spontaneous cycle length decreased from a mean of 348 ms to a mean of 288 ms (-18%) in all experiments (n = 15). This was accompanied by a shift of the pacemaker from the nodal center towards the border zone. By means of multiple microelectrode impalements changes in action potential configuration were studied. After disconnection of atrium and sinus node the rate of diastolic depolarization of fibers in the border zone was increased from a mean of 26 mV/s to a mean of 78 mV/s, whereas in the center of the sinus node no increase was found (mean: 52 mV/s). It was concluded that the fibers in the border zone of the sinus node are better pacemaker fibers than in the nodal center. However under normal conditions the intrinsic pacemaker properties of the border zone fibers are electronically depressed by the connected atrial myocardium.

Effects of increasing intercellular resistance on transverse and longitudinal propagation in sheep epicardial muscle

Propagation in cardiac muscle is faster in the longitudinal than in the transverse axis of the cells. Yet, as a result of the larger upstroke velocity of action potentials propagating transversely, it has been suggested that longitudinal propagation is more vulnerable to block. To study the relation between conduction velocity and maximal upstroke velocity (Vmax), as well as the time course of conduction delay and block in the transverse vs. longitudinal direction, thin square pieces of sheep epicardial muscle were superfused with the cellular uncoupler heptanol (1.5 mM). Action potentials were recorded with microelectrodes at opposite corners of the preparation while stimulating alternately in the longitudinal or transverse direction with bipolar electrodes located at contralateral corners. In all cases, block occurred more promptly for transverse than for longitudinal propagation. The decrease in conduction velocity was greater than expected for Vmax decay and, in some cases, Vmax increased while conduction velocity decreased. In the presence of high grade conduction impairment, foot potentials appeared and the upstrokes became "notched." We conclude that when intercellular coupling is impaired, transverse propagation is more vulnerable to block, and need not be dependent on changes in Vmax.

Properties of the inspiration-related activity of sympathetic preganglionic neurones of the cervical trunk in the cat

1. The experiments reported here have examined some temporal characteristics of the inspiration-related sympathetic discharge of the cat in control conditions and during forcing of the respiratory oscillator into marked deviations from its natural frequency. The purpose of these experiments was to establish whether or not the relation of sympathetic to phrenic nerve activity shows properties consistent with the hypothesis that the inspiration-related sympathetic discharge is driven by a neural oscillator, independent of, but coupled and stably entrained to, the brain-stem respiratory oscillator. 2. The electrical activity of the whole cervical sympathetic trunk (n = 26) or of small strands of the cervical trunk containing single units (n = 20) and of the phrenic nerve was recorded in pentobarbitone-anaesthetized, paralysed, artificially ventilated, sino-aortic denervated cats. Most of the cats were bilaterally vagotomized. 3. The onset of the inspiratory burst of the sympathetic preganglionic neurones had a fixed delay from the onset of the phrenic nerve burst. The level of activity within the burst, in whole cervical trunk recording, reached a maximum in early inspiration and then was maintained at approximately this level for the rest of inspiration (twenty-two out of twenty-six cats). In four cats the activity level increased throughout the burst. Individual sympathetic preganglionic neurones displaying inspiration-related burst firing were characteristically recruited in early inspiration and thereafter maintained an approximately constant firing frequency for the rest of inspiration. 4. Electrical stimulation of afferents in the superior laryngeal nerve during various phases of the respiratory cycle caused equivalent, phase-dependent, resetting patterns of both phrenic nerve and inspiration-related sympathetic discharge. 5. In cats with intact vagus nerves, entrainment of the brain-stem respiratory oscillator to the frequency of the respiratory pump was used to change the frequency of the former, within limits, by changing the frequency of the latter. Over the range of frequencies tested, the pump-to-phrenic delay varied as a function of frequency, while the delay between phrenic and sympathetic burst onset was essentially independent of frequency. 6. In hyperthermic, hypocapnic cats phrenic nerve burst frequency increased up to about 300 bursts/min from a value of 15 bursts/min in normothermia-normocapnia. At all frequencies within this range the sympathetic burst maintained a delay, with respect to the phrenic burst, which was essentially independent of frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulated parasystole originating in the sinoatrial node

A computer model of "modulated sinus parasystole" was devised in which two sinus pacemakers interacted electrotonically, entraining each other's periodicity according to their beat-to-beat phasic relationships. Depending on the preestablished rules, the model gave rise to various rhythm patterns that were similar to those recorded in patients with sinoatrial arrhythmias. The validity of the model in predicting clinically observed rhythm disturbances was tested in a case of sinoatrial extrasystolic activity. The sinoatrial origin of parasystolic discharges giving rise to various patterns of group beating in this case was diagnosed according to the following electrocardiographic criteria: premature P waves having contour identical to P waves of basic beats, variable coupling intervals, and absence of compensatory pauses (i.e., returning cycles having duration similar to that of the basic P-P interval). For the analysis, it was assumed that two distinct but closely apposed sinoatrial pacemaker centers were competing for activation of the heart. The model accurately simulated the arrhythmias in the electrocardiographic trace. The best fit was found when the two pacemakers interacted on the basis of "resetting" in one direction and electronic modulation in the other. In fact, under appropriate conditions, the model matched precisely all frequency-dependent patterns of extrasystolic activity observed in the trace. We conclude that the modulated parasystole hypothesis can readily explain the mechanism of sinus extrasystolic discharges whose returning cycle equals the basic P-P interval. Moreover, the model predicts that, when the rules for mutual entrainment between "dominant" and parasystolic sinus pacemaker are appropriate, the returning cycle can be shorter than the basic cycle.

Dynamic interactions and mutual synchronization of sinoatrial node pacemaker cells. A mathematical model

Dynamic interactions and mutual entrainment of coupled sinoatrial pacemaker cells with different intrinsic frequencies were investigated using a computerized mathematical model. Transmembrane potentials were simulated using equations of individual membrane currents based on voltage clamp data for the sinoatrial node. The intrinsic frequency of a given cell was altered by applying bias hyperpolarizing current, or by changing the amount of slow inward current. Cells were coupled through simple ohmic resistances to form linear arrays of two or more cells. Simulations closely reproduced previous experimental work showing that the mutual interactions between pacemakers are mediated electrotonically and show phase dependence. Results from the present simulations provide an explanation for the ionic basis of these phase-dependent interactions. In addition, it is demonstrated that the mutual entrainment of coupled pacemakers can lead to their coordinated behavior (synchronization). Two pacemaker cells can synchronize at simple harmonic (i.e., 1:1, 2:1, etc.) or more complex ratios (3:2, 5:3, etc.), depending on the differences in intrinsic frequencies and the degree of electrical coupling between cells. Simulations using larger numbers of linearly connected cells yielded various patterns of pacemaker activity including 2:1 sinoatrial block and complex dysrhythmic activity. The overall results may be used to predict higher order interactions of thousands of cells comprising the sinus node. Under such a scheme, synchronization occurs not by the conducted influence of a dominant pacemaker cell, but by the mutual "democratic" interaction of individual pacemaker cells.