Synchronizatin of pulsation rates in isolated cardiac myocytes

Mechanical activities of self-beating cardiomyocyte aggregates under mechanical compression

Since the discovery of synchronous pulsations in cardiomyocytes (CMs), electrical communication between CMs has been emphasized; however, recent studies suggest the possibility of mechanical communication. Here, we demonstrate that spherical self-beating CM aggregates, termed cardiac spheroids (CSs), produce enhanced mechanical energy under mechanical compression and work cooperatively via mechanical communication. For single CSs between parallel plates, compression increased both beating frequency and beating energy. Contact mechanics revealed a scaling law on the beating energy, indicating that the most intensively stressed cells in the compressed CSs predominantly contributed to the performance of mechanical work against mechanical compression. For pairs of CSs between parallel plates, compression immediately caused synchronous beating with mechanical coupling. Compression tended to strengthen and stabilize the synchronous beating, although some irregularity and temporary arrest were observed. These results suggest that mechanical compression is an indispensable control parameter when evaluating the activities of CMs and their aggregates.

Coordinating an Ensemble of Chemical Micromotors via Spontaneous Synchronization

Spatiotemporal coordination of a nanorobot ensemble is critical for their operation in complex environments, such as tissue removal or drug delivery. Current strategies of achieving this task, however, rely heavily on sophisticated, external manipulation. We here present an alternative, biomimetic strategy by which oscillating Ag Janus micromotors spontaneously synchronize their dynamics as chemically coupled oscillators. By quantitatively tracking the kinetics at both an individual and cluster level, we find that synchronization emerges as the oscillating entities are increasingly coupled as they approach each other. In addition, the synchronized beating of a cluster of these oscillating colloids was found to be dominated by substrate electroosmosis, revealed with the help of an acoustic trapping technique. This quantitative, systematic study of synchronizing micromotors could facilitate the design of biomimetic nanorobots that spontaneously communicate and organize at micro- and nanoscales. It also serves as a model system for nonlinear active matter.

Automated quantification study of human cardiomyocyte synchronization using holographic imaging

This paper investigates the rhythm strip and parameters of synchronization of human induced pluripotent stem cell (iPS) derived cardiomyocytes. The synchronization is evaluated from quantitative phase images of beating cardiomyocytes which are obtained using the time-lapse digital holographic imaging method. By quantitatively monitoring the dry mass redistribution, digital holography provides the physical contraction-relaxation signal caused by autonomous cardiac action potential. In order to analyze the synchronicity at the cell-to-cell level, we extracted single cardiac muscle cells, which contain the nuclei, from the phase images of cardiomyocytes containing multiple cells resulting from the fusion of k-means clustering and watershed segmentation algorithms. We demonstrate that mature cardiomyocyte cell synchronization can be automatically evaluated by time-lapse microscopic holographic imaging. Our proposed method can be applied for studies on cardiomyocyte disorders and drug safety testing systems.

Community effect of cardiomyocytes in beating rhythms is determined by stable cells

The community effect of cardiomyocytes was investigated in silico by the change in number and features of cells, as well as configurations of networks. The theoretical model was based on experimental data and accurately reproduced recently published experimental results regarding coupled cultured cardiomyocytes. We showed that the synchronised beating of two coupled cells was tuned not to the cell with a faster beating rate, but to the cell with a more stable rhythm. In a network of cardiomyocytes, a cell with low fluctuation, but not a hight frequency, became a pacemaker and stabilised the beating rhythm. Fluctuation in beating rapidly decreased with an increase in the number of cells (N), almost irrespective of the configuration of the network, and a cell comes to have natural and stable beating rhythms, even for N of approximately 10. The universality of this community effect lies in the fluctuation-dissipation theorem in statistical mechanics.

Integrate and fire model with refractory period for synchronization of two cardiomyocytes

We investigate an integrate and fire model for two cardiomyocytes interacting with each other. A feature of the model is to incorporate the refractory periods of the cardiomyocytes as well as the influence of firing of adjacent cells. The present model predicts that, if refractory periods of the two cells are nearly equal, the beating rhythms of the two cells always synchronize and their beating rate is tuned to the faster rate between the two cells. On the other hand, if their refractory periods significantly differ, they exhibit various kinds of harmonious beating rhythms. These results successfully explain the well known characteristics of synchronized beating of cultured cardiomyocytes. We also discuss effects of a delay time of cell-to-cell interaction, that gives further complicated phase diagrams for the beating rhythms.

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.

Symmetry-broken states on networks of coupled oscillators

When identical oscillators are coupled together in a network, dynamical steady states are often assumed to reflect network symmetries. Here, we show that alternative persistent states may also exist that break the symmetries of the underlying coupling network. We further show that these symmetry-broken coexistent states are analogous to those dubbed "chimera states," which can occur when identical oscillators are coupled to one another in identical ways.

Morphological Modifications in Myofibrils by Suppressing Tropomyosin 4α in Chicken Cardiac Myocytes

Tropomyosin (TPM) localizes along F-actin and, together with troponin T (TnT) and other components, controls calcium-sensitive muscle contraction. The role of the TPM isoform (TPM4α) that is expressed in embryonic and adult cardiac muscle cells in chicken is poorly understood. To analyze the function of TPM4α in myofibrils, the effects of TPM4α-suppression were examined in embryonic cardiomyocytes by small interference RNA transfection. Localization of myofibril proteins such as TPM, actin, TnT, α-actinin, myosin and connectin was examined by immunofluorescence microscopy on day 5 when almost complete TPM4α-suppression occurred in culture. A unique large structure was detected, consisting of an actin aggregate bulging from the actin bundle, and many curved filaments projecting from the aggregate. TPM, TnT and actin were detected on the large structure, but myosin, connectin, α-actinin and obvious myofibril striations were undetectable. It is possible that TPM4α-suppressed actin filaments are sorted and excluded at the place of the large structure. This suggests that TPM4α-suppression significantly affects actin filament, and that TPM4α plays an important role in constructing and maintaining sarcomeres and myofibrils in cardiac muscle.

Cell-specific cardiac electrophysiology models

The traditional cardiac model-building paradigm involves constructing a composite model using data collected from many cells. Equations are derived for each relevant cellular component (e.g., ion channel, exchanger) independently. After the equations for all components are combined to form the composite model, a subset of parameters is tuned, often arbitrarily and by hand, until the model output matches a target objective, such as an action potential. Unfortunately, such models often fail to accurately simulate behavior that is dynamically dissimilar (e.g., arrhythmia) to the simple target objective to which the model was fit. In this study, we develop a new approach in which data are collected via a series of complex electrophysiology protocols from single cardiac myocytes and then used to tune model parameters via a parallel fitting method known as a genetic algorithm (GA). The dynamical complexity of the electrophysiological data, which can only be fit by an automated method such as a GA, leads to more accurately parameterized models that can simulate rich cardiac dynamics. The feasibility of the method is first validated computationally, after which it is used to develop models of isolated guinea pig ventricular myocytes that simulate the electrophysiological dynamics significantly better than does a standard guinea pig model. In addition to improving model fidelity generally, this approach can be used to generate a cell-specific model. By so doing, the approach may be useful in applications ranging from studying the implications of cell-to-cell variability to the prediction of intersubject differences in response to pharmacological treatment.

Mathematical models of cardiac cell electrophysiology are widely used as predictive and illuminatory tools, but have been developed for decades using a suboptimal process. The models are typically constructed by manual adjustment of parameters to fit simple data and therefore often underperform when used to predict complex behavior such as arrhythmias. We present a novel method of model parameterization using automated optimization and dynamically rich fitting data and then demonstrate that this approach is better at finding the “real” model of a cell. Application of the method to cardiac myocytes leads to cell-specific models, which may enable well-controlled studies of both cellular- and subject-level population heterogeneity in disease propensity and response to therapies.

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.

Dependence of the community effect of cultured cardiomyocytes on the cell network pattern

To elucidate the role of the community effect in cardiomyocytes, we developed an on-chip single-cell-based culture system with which the dynamics of the change of beat rate and beat rate fluctuation of cultured cardiomyocytes can be measured at the single-cell level before and after the formation of a cell network. We examined the dependence of the community effect on cell network pattern by culturing cardiomyocytes in differently shaped microchambers and investigated the relation between the network pattern and the stability of the beat rate. We found that beat rate fluctuation tends to decrease as cell-community size increases, irrespective of cell network pattern. This indicates that on-chip single-cell-based cardiomyocyte communities might be able to model a heart tissue accurately enough to be used in practical applications such as drug screening.

Role of the community effect of cardiomyocyte in the entrainment and reestablishment of stable beating rhythms

To investigate the roles that the community effect and entrainment function of cultured cardiomyocyte play in decreasing beating fluctuation and reestablishing synchronized beating, we developed a single-cell-based two-dimensional network culture assay to measure and compare the dynamics of beating rhythm synchronization of individual cells before and after they form networks. Studying the formation of two-cell networks, we found that their synchronized beating tended to be determined by the cardiomyocyte whose beat rate fluctuated less than that of the other cardiomyocyte. We further found that the strength of this tendency increased with the number of cells in the network. These results indicate that (1) beating fluctuation is one of the important factors influencing the reestablishment of a stable synchronous beating rhythm, (2) the larger networks reduce fluctuation, and (3) the formation of a spatial network can itself stabilize cardiomyocyte beat rates.

Stability of beating frequency in cardiac myocytes by their community effect measured by agarose microchamber chip

To understand the contribution of community effect on the stability of beating frequency in cardiac myocyte cell groups, the stepwise network formation of cells as the reconstructive approach using the on-chip agarose microchamber cell microcultivation system with photo-thermal etching method was applied. In the system, the shapes of agarose microstructures were changed step by step with photo-thermal etching of agarose-layer of the chip using a 1064-nm infrared focused laser beam to increase the interaction of cardiac myocyte cells during cultivation. First, individual rat cardiac myocyte in each microstructure were cultivated under isolated condition, and then connected them one by one through newly-created microchannels by photo-thermal etching to compare the contribution of community size for the magnitude of beating stability of the cell groups. Though the isolated individual cells have 50% fluctuation of beating frequency, their stability increased as the number of connected cells increased. And finally when the number reached to eight cells, they stabilized around the 10% fluctuation, which was the same magnitude of the tissue model cultivated on the dish. The result indicates the importance of the community size of cells to stabilize their performance for making cell-network model for using cells for monitoring their functions like the tissue model.

An ionic model for rhythmic activity in small clusters of embryonic chick ventricular cells

We recorded transmembrane potential in whole cell recording mode from small clusters (2-4 cells) of spontaneously beating 7-day embryonic chick ventricular cells after 1-3 days in culture and investigated effects of the blockers D-600, diltiazem, almokalant, and Ba2+. Electrical activity in small clusters is very different from that in reaggregates of several hundred embryonic chick ventricular cells, e.g., TTX-sensitive fast upstrokes in reaggregates vs. TTX-insensitive slow upstrokes in small clusters (maximum upstroke velocity approximately 100 V/s vs. approximately 10 V/s). On the basis of our voltage- and current-clamp results and data from the literature, we formulated a Hodgkin-Huxley-type ionic model for the electrical activity in these small clusters. The model contains a Ca2+ current (ICa), three K+ currents (IKs, IKr, and IK1), a background current, and a seal-leak current. ICa generates the slow upstroke, whereas IKs, IKr, and IK1 contribute to repolarization. All the currents contribute to spontaneous diastolic depolarization, e.g., removal of the seal-leak current increases the interbeat interval from 392 to 535 ms. The model replicates the spontaneous activity in the clusters as well as the experimental results of application of blockers. Bifurcation analysis and simulations with the model predict that annihilation and single-pulse triggering should occur with partial block of ICa. Embryonic chick ventricular cells have been used as an experimental model to investigate various aspects of spontaneous beating of cardiac cells, e.g., mutual synchronization, regularity of beating, and spontaneous initiation and termination of reentrant rhythms; our model allows investigation of these topics through numerical simulation.

A novel method of cultivating cardiac myocytes in agarose microchamber chips for studying cell synchronization

We have developed a new method that enables agar microstructures to be used to cultivate cardiac myocyte cells in a manner that allows their connection patterns to be controlled. Non-contact three-dimensional photo-thermal etching with a 1064-nm infrared focused laser beam was used to form the shapes of agar microstructures. This wavelength was selected as it is not absorbed by water or agar. Identical rat cardiac myocytes were cultured in adjacent microstructures connected by microchannels and the interactions of asynchronous beating cardiac myocyte cells observed. Two isolated and independently beating cardiac myocytes were shown to form contacts through the narrow microchannels and by 90 minutes had synchronized their oscillations. This occurred by one of the two cells stopping their oscillation and following the pattern of the other cell. In contrast, when two sets of synchronized beating cells came into contact, those two sets synchronized without any observable interruptions to their rhythms. The results indicate that the synchronization process of cardiac myocytes may be dependent on the community size and network pattern of these cells.

Multiple oscillators provide metastability in rhythm generation

Biological rhythms such as cardiac and circadian rhythms arise from activity of multiple oscillators with dispersed intrinsic frequencies. It has been proposed that a stable population rhythm, fundamental to normal physiological processes, can be achieved in these systems by synchronization, through mutual entrainment, of individual oscillators. Mutual entrainment, however, is unlikely to be the mechanism underlying the generation of a stable rhythm in a population of multiple weakly coupled or uncoupled oscillators. We have recently identified such a population that is involved in the sympathetic regulation of vascular tone in a thermoregulatory circulation. In this paper, we investigate the stability of the output rhythm of these sympathetic oscillators by subjecting the system to a periodic driving force (the lung inflation cycle-related activity). We show that a population rhythm coupled to the drive can remain stable over a much wider driving frequency range compared with that of any one of its constituent oscillators. This population rhythmicity still exists despite the fact that the dominant frequencies of individual oscillators are not necessarily 1:1 frequency-locked to the drive. We provide evidence to show that this population metastability is achieved through linear and nonlinear dynamic interactions between the driving force and single sympathetic oscillators. Our study suggests that the generation of a stable population rhythm can exist even in the absence of mutual entrainment of its constituents, and this allows the population to generate a stable and flexible patterned response.

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.

Gap junction formation and functional interaction between neonatal rat cardiocytes in culture: a correlative physiological and ultrastructural study

The time course of gap junction formation and growth, following contraction synchronization of cardiac myocytes in culture, has been studied in a combined (electro) physiological and ultrastructural study. In cultures of collagenase-dissociated neonatal rat cardiocytes, pairs of spontaneously beating myocytes synchronized their contractions within one beat interval within 2-20 min after they apparently had grown into contact. 45 sec after the first synchronized beat an appreciable junctional region containing several small gap junctions was already present. In the following 30 min, neither the area of individual gap junctions nor their total area increased. 75 min after synchronization both the area of individual gap junctions and their total area had increased by a factor of 10-15 with respect to what was found in the first half hour. In the period between 75 and 300 min again no further increase in gap junctional area was found. In double voltage-clamp experiments, gap junctions between well-coupled cells behaved like ohmic conductors. In poorly coupled cells, in which the number of functional gap-junctional channels was greatly reduced, the remaining channels showed voltage-dependent gating. Their single-channel conductance was 40-50 pS. The electrophysiologically measured junctional conductance agreed well with the conductance calculated from the morphometrically determined gap-junctional area. It is concluded that a rapid initial gap junction formation occurs during the 2-20 min period prior to synchronization by assembly of functional channels from existing channel precursors already present in the cell membranes. It then takes at least another 30 min before the gap-junctional area increases possibly by de novo synthesis or by recruitment from intracellular stores or from nonjunctional membranes, a process completed in the next 45 min.

The 43-kD polypeptide of heart gap junctions: immunolocalization, topology, and functional domains

Analysis by SDS-PAGE of gap junction fractions isolated from heart suggests that the junctions are comprised of a protein with an Mr 43,000. Antibodies against the electroeluted protein and a peptide representing the 20 amino terminal residues bind specifically on immunoblots to the 43-kD protein and to the major products arising from proteolysis during isolation. By immunocytochemistry, the protein is found in ventricle and atrium in patterns consistent with the known distribution of gap junctions. Both antibodies bind exclusively to gap junctions in fractions from heart examined by EM after gold labeling. Since only domains of the protein exposed at the cytoplasmic surface should be accessible to antibody, we conclude that the 43-kD protein is assembled in gap junctions with the amino terminus of the molecule exposed on the cytoplasmic side of the bilayer, that is, on the same side as the carboxy terminus as determined previously. By combining proteolysis experiments with data from immunoblotting, we can identify a third cytoplasmic region, a loop of some 4 kD between membrane protected domains. This loop carries an antibody binding site. The protein, if transmembrane, is therefore likely to cross the membrane four times. We have used the same antisera to ascertain if the 43-kD protein is involved in cell-cell communication. The antiserum against the amino terminus blocked dye coupling in 90% of cell pairs tested; the antiserum recognizing epitopes in the cytoplasmic loop and cytoplasmic tail blocked coupling in 75% of cell pairs tested. Preimmune serum and control antibodies (one against MIP and another binding to a cardiac G protein) had no or little effect on dye transfer. Our experimental evidence thus indicates that, in spite of the differences in amino acid sequence, the gap junction proteins in heart and liver share a general organizational plan and that there may be several domains (including the amino terminus) of the molecule that are involved in the control of junctional permeability.

SEM observations on the effect of anthracycline drugs on cultured newborn rat cardiomyocytes

The effect of two anthracyclines-doxorubicin hydrochloride (adriamycin) and 4'-epidoxorubicin (epirubicin) and an anthracenedione (novantrone) on the contractibility and surface ultrastructure of newborn rat cardiomyocytes cultured for five days was examined. While the beating rate of the cells was affected only by the anthracyclines, an alteration of the sarcolemma, disruption of the slender processes and swelling of the nuclei and/or the cells was observed following incubation with each of the three drugs for two hours. However, the damage induced by adriamycin was more pronounced than that induced by the other two drugs, when doses extrapolated from those accepted as therapeutic were compared.

Pathophysiology of cardiomyocytes

Isolated cardiomyocytes lend themselves very well to the quantification of pathological damage and to the determination of reversible versus irreversible changes. These single cells were used to study the cellular response to a variety of pathologic stimuli that impair structure and function. Degenerative alterations are accompanied by hyperactivation and irreversible rounding up of otherwise quiescent rod-shaped cells. Stereotypic degenerative changes and loss of sarcolemma-bound Ca2+ were seen during prolonged severe hypoxia, exposure to either depolarizing concentrations of potassium, veratrine, acylcarnitines, cationic amphiphiles, free-radical-generating systems, cardiac glycosides, or uncouplers of oxidative phosphorylation. Since the presence of extracellular Ca2+ is a prerequisite to obtain cell degeneration in most of these aggressive insults and since cellular Ca2+ overload parallels the damage, we studied the influence of representative compounds of the various subclasses of Ca2+ antagonists: verapamil, nifedipine, nicardipine, and diltiazem (Ca2+ blockers with high affinity for cardiac slow Ca2+ channels), cinnarizine, flunarizine, lidoflazine, and mioflazine (Ca2+ blockers with no affinity for cardiac slow Ca2+ channels). The non-slow-channel-blocking drugs were generally superior in protection against the imposed insults suggesting that prevention of Ca2+ overload is not correlated with slow channel blockade. For the latter group of drugs, other (hitherto not elucidated) mechanisms of membrane-drug interactions seem to be responsible for the preservation of Ca2+ homeostasis during the induction of pathological Ca2+ influx.

The development of beat-rate synchronization of rat myocyte pairs in cell culture

When two spontaneously beating neonatal rat heart cells in tissue culture were allowed to grow together they synchronized their originally independent beats to a common rhythm, as measured with an opto-electronic technique. Both single isolated cells and cell pairs exhibited a highly irregular beating pattern. Beating irregularity was strongly and positively correlated with mean interbeat interval. Synchronization of beating occurred in 50% of the pairs studied within one beating interval. In the remaining cell pairs, the first synchronized beat was followed by a 4-65 s period of partial synchronization. The time difference between contraction moments of two cells in a pair respective to each other (latency) changed upon synchronization from a random value to a fixed value. In a few cases the latency decreased during 20 to 30 s after the first synchronized beat before a steady-state value was reached. The mean interbeat interval (IBI) of the synchronized cell pairs was governed by the mean IBI of the originally faster beating cells. In 83% of the cases the mean IBI of the cell pairs was between that of the originally isolated beating cells. We conclude from the experiments described that physical coupling (i.e. gap junction formation) is virtually complete before beating synchronization occurs.

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.

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.

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

The mechanisms of synchronization between sino-atrial pace-maker cells were studied in biological preparations from rabbit hearts, and in computer simulations of the Hodgkin & Huxley type. For biological experiments, thin strips of sino-atrial node were placed in a three-compartment bath. The electrical properties of the tissue in the middle segment (the 'gap') were manipulated pharmacologically to alter electrical coupling and/or excitability of cells in that segment, and to study the patterns of interaction between two pace-maker centres in the external segments. Superfusion of the gap segment with either verapamil (2 microM) or acetylcholine (10 microM) produced a loss of 1:1 synchrony (entrainment) of spontaneous discharges generated by the external pace-makers but subharmonic (i.e. 3:2; 5:4; 9:8; etc.) entrainment was always maintained. When the gap segment was superfused with heptanol (3.5 mM), which is known to increase intercellular resistance, the pace-maker centres in the external chambers beat independently of one another. Progressive loss of synchrony paralleled reductions in amplitude of electrotonic responses to current pulses applied across the gap. Gap superfusion with hypertonic Tyrode solution (600 mosM) produced a major reduction in the degree of synchronization between the external pace-makers, even though the cells in the central compartment maintained their excitability. Under these conditions, as many as three independent pace-maker centres, one in each chamber, coexisted in a given preparation. Using computerized simulations based on equations of time- and voltage-dependent membrane currents, three 'cells', each capable of maintaining spontaneous activity, were connected in a linear array through ohmic resistances. When selective parameters (e.g. membrane conductances, coupling resistance) were modified appropriately, the mathematical simulations reproduced very closely the interaction patterns observed in the experimental preparations. Our results show that synchronization in the sinus node results from mutual interactions and entrainment between all the cells in this region. These interactions are of the kind expected for a population of coupled, self-sustained oscillators, and are mediated through electrotonic propagation of current across low-resistance junctions.

An ultrastructural topographical study on myofibrillogenesis in the heart of the chick embryo during pulsation onset period

Ultrathin sections of the chick embryonic heart at the 8-, 9- and 10-somite stage were cut serially at an interval of 20 micron and mounted for transmission electron microscopic examination on a copper grid with a sufficiently large hole to survey the entire section area. The grid was supported by a formvar film. Thick filaments were first found to assemble into well-defined bundles in several cells composing the caudal region of the newly formed heart just before onset of the pulsation at the 8-somite stage. Then, at the 9-somite stage when pulsation commences, the cells possessing nascent myofibril(s) increase in number, slightly more in the right side of ventricular region. At the 10-somite stage, the rhythmical contraction is established and striated myofibrils become distinctly discernible. Right side dominance is more conspicuous at this stage than previously. Then, myofibrillogenesis gradually progresses toward the cranial or bulbar region.

Effects of calcium on electrical propagation in early embryonic precontractile heart as revealed by multiple-site optical recording of action potentials

The effects of Ca2+ on electrical propagation in early embryonic precontractile chick hearts were studied optically using a voltage-sensitive merocyanine-rhodanine dye. Spontaneous optical signals, corresponding to action potentials, were recorded simultaneously from 25 separate regions of the eight-to-nine-somite embryonic primitive heart, using a square photodiode array. Electrical propagation was assessed by analyzing the timing of the signals obtained from different regions. Electrical propagation in the heart was suppressed by either lowering or raising extracellular Ca2+. Similar effects were produced by a Ca2+ ionophore (A23187). We have also found that electrical propagation across the primordial fusion line at the midline of the heart was enhanced by increasing, and depressed by lowering, external Ca2+. One possible interpretation is that intercellular communication in the embryonic precontractile heart is regulated by the level of the intracellular Ca2+ concentration, and it is suggested that intercellular communication across the primordial fusion line strongly depends on external Ca2+.

Synchronization in chains of pacemaker cells by phase resetting action potential effects

Interactions between pacemaker cells in a chain were calculated according to a "phase-reset" model. It is based on effects of action potentials in the cells on the cycle lengths of neighbouring cells. These effects were defined for each cell by a latency-phase curve (LPC), giving the latency time (L) until the onset of the next action potential in that cell, as a function of the phase (phi) at which a neighbour cell fired an action potential. Neighbour cells with simultaneous action potentials did not influence each others cycle length. We investigated how stable synchronization depends on the shape of the LPC's of the pacemaker cells and on chain length. Three types of interactive behaviour were distinguished. First, anti-phase synchrony, in which neighbouring cells fired with large phase differences with respect to the synchronized period Ps. Second, asynchrony, in which the periods of the cells did not become equal and constant. Third, in-phase synchrony, in which the phase differences between the neighbouring cells were zero or much smaller than the synchronized period Ps, depending on the differences between the intrinsic periods. Asynchrony and anti-phase synchrony may be seen as cardiophysiological arrhythmias, while in-phase synchrony represents the physiological type of synchrony in the heart. In-phase synchrony appeared to be strongly favoured by LPC's, which have a no-effect (refractory) part at early phases, a lengthened latency (or phase delay) part at intermediate phases and a shortened latency (or phase advance) part at late phases in the cycle. Such LPC-shapes are commonly found in preparations of cardiac pacemaker cells. When the pacemaker cells were identical, the synchronized period Ps during in-phase synchrony was equal to their intrinsic period P*i. For different intrinsic periods, Ps was equal to the intrinsic period of the fastest cell if the LPC's contained a sufficiently long initial no-effect period at early phases and a shortened latency part at late phases. When, on the other hand, such cell chains had a linear gradient in their intrinsic periods, "action potentials" started from the fast end and traveled along the chain. The propagation of an action potential wave slowed down as it reached the slower cells. When the gradient in the intrinsic periods was too steep, only the intrinsically fast end of the chain developed synchrony.(ABSTRACT TRUNCATED AT 400 WORDS)

Model of bidirectional interaction between myocardial pacemakers based on the phase response curve

As a basis for the study of sinus rhythm determination, a model is proposed of bidirectionally-coupled oscillators as a system of difference equations based on the phase response curve of sinoatrial pacemaker cells. Solutions corresponding to the one-to-one synchronization of the two pacemakers are obtained, and the relation among those solutions is examined: It is revealed that two different solutions with different cycle length coexist, and the synchronized frequency can be higher or lower than the original intrinsic frequencies of the two pacemaker cells. The experimental results of the cultured cells of cardiac pacemakers are interpreted by the analytical result of the model.

Induction of cell--cell channel formation by mRNA

Intercellular junctional communication is very common in normal organized tissue. It provides a pathway for transmission of electrical signals, especially in heart muscle, and may be important in differentiation and growth control. The hydrophilic channels which enable cell--cell communication have been well characterized by biophysical methods, and there is now good evidence that they are contained in the nexus (gap junctions). Little, however, is known about the molecular mechanism of biosynthesis of junctional channels. Knowledge in this area has been obtained almost exclusively from experiments with reaggregated cells, a system complicated by the fact that de novo synthesis of channel proteins is obscured by reassembly of pre-existing subunits or utilization of precirsors. To avoid these problems, we have now isolated mRNA from cells that are in the process of making new intercellular nexus with high efficiency, incorporated it via liposomes into communication-defective cells and have shown that the recipient cells established junctional communication.

Action potential synchrony in embryonic precontractile chick heart: optical monitoring with potentiometric dyes

1. Using an optical method for monitoring membrane potential, we recorded spontaneous action potentials simultaneously in several different areas of the 7-9 somite embryonic chick hearts. 2. Absorption signals resembling spontaneous action potentials were well synchronized among the different areas in the prebeating embryonic heart during the 7-9 somite stages of development, and the synchronization spread over the entire area of the heart. From these experimental results, it is evident that there is electrical coupling among embryonic chick heart cells even in the early stages of cardiogenesis. 3. When an embryonic heart was separated into right and left or anterior and posterior parts, the action potential synchrony between the two halves was completely blocked; however, the synchrony and the intrinsic rhythmicity in action potential recurrence remained in each part.

Cell junctions and intercellular communication

We have compared intercellular communication in normal and regenerating rat liver. Gap junctions are greatly reduced in size and numbers 29 to 35 hr after hepatectomy, but we still find some 90% of hepatocytes coupled by electrophysiological criteria. The spread of dyes such as carboxyfluorescein however is very limited in the regenerating organs as compared to the situation in the controls. We show how the apparent discrepancies between morphological and physiological data can be reconciled. We also present a summary of preliminary findings on the biosynthesis of gap junction protein and some of the conclusions one can draw from the sequence of 58 amino acids at the amino terminal of the protein.

De novo construction of cell-to-cell channels

Nexus (gap junctions), which are considered to contain cell-to-cell channels, are newly formed in uterine smooth muscle during parturition or in response to estrogen treatment of virginal animals. A mRNA preparation was isolated from estrogen-dominated rat myometria and was encapsulated into liposomes. Subsequently the liposomes were fused with cultured cells of a mouse cell line CL-1D. It is established that these tumor cells normally are neither electrically coupled nor do they contain nexus. The cells, however, become electrically coupled a few hours after being loaded with the mRNA preparation. This de novo expression of cell coupling persisted for a litte more than 24 hr after a single loading procedure. Freeze-fracture electron microscopy revealed small nexus-like particle aggregates at the time coupling was present. In control experiments the cells remained noncoupling when the RNA preparation was pretreated with ribonuclease, when cycloheximide was applied to the cells, or when liposomes filled with buffer solution only were used. These data suggest that the de novo expression of cell-to-cell coupling is accomplished by mRNA-induced protein biosynthesis resulting in the formation of cell-to-cell channels.