The electrical activity of embryonic chick heart cells isolated in tissue culture singly or in interconnected cell sheets

Interpretation of field and LEAP potentials recorded from cardiomyocyte monolayers

Multielectrode arrays (MEAs) are the method of choice for electrophysiological characterization of cardiomyocyte monolayers. The field potentials recorded using an MEA are like extracellular electrograms recorded from the myocardium using conventional electrodes. Nevertheless, different criteria are used to interpret field potentials and extracellular electrograms, which hamper correct interpretation and translation to the patient. To validate the criteria for interpretation of field potentials, we used neonatal rat cardiomyocytes to generate monolayers. We recorded field potentials using an MEA and simultaneously recorded action potentials using sharp microelectrodes. In parallel, we recreated our experimental setting in silico and performed simulations. We show that the amplitude of the local RS complex of a field potential correlated with conduction velocity in silico but not in vitro. The peak time of the T wave in field potentials exhibited a strong correlation with APD90 while the steepest upslope correlated well with APD50. However, this relationship only holds when the T wave displayed a biphasic pattern. Next, we simulated local extracellular action potentials (LEAPs). The shape of the LEAP differed markedly from the shape of the local action potential, but the final duration of the LEAP coincided with APD90. Criteria for interpretation of extracellular electrograms should be applied to field potentials. This will provide a strong basis for the analysis of heterogeneity in conduction velocity and repolarization in cultured monolayers of cardiomyocytes. Finally, a LEAP is not a recording of the local action potential but is generated by intracellular current provided by neighboring cardiomyocytes and is superior to field potential duration in estimating APD90.NEW & NOTEWORTHY We present a physiological basis for the interpretation of multielectrode array-derived, extracellular, electrical signals.

Multicellular In vitro Models of Cardiac Arrhythmias: Focus on Atrial Fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia in clinical practice with a large socioeconomic impact due to its associated morbidity, mortality, reduction in quality of life and health care costs. Currently, antiarrhythmic drug therapy is the first line of treatment for most symptomatic AF patients, despite its limited efficacy, the risk of inducing potentially life-threating ventricular tachyarrhythmias as well as other side effects. Alternative, in-hospital treatment modalities consisting of electrical cardioversion and invasive catheter ablation improve patients' symptoms, but often have to be repeated and are still associated with serious complications and only suitable for specific subgroups of AF patients. The development and progression of AF generally results from the interplay of multiple disease pathways and is accompanied by structural and functional (e.g., electrical) tissue remodeling. Rational development of novel treatment modalities for AF, with its many different etiologies, requires a comprehensive insight into the complex pathophysiological mechanisms. Monolayers of atrial cells represent a simplified surrogate of atrial tissue well-suited to investigate atrial arrhythmia mechanisms, since they can easily be used in a standardized, systematic and controllable manner to study the role of specific pathways and processes in the genesis, perpetuation and termination of atrial arrhythmias. In this review, we provide an overview of the currently available two- and three-dimensional multicellular in vitro systems for investigating the initiation, maintenance and termination of atrial arrhythmias and AF. This encompasses cultures of primary (animal-derived) atrial cardiomyocytes (CMs), pluripotent stem cell-derived atrial-like CMs and (conditionally) immortalized atrial CMs. The strengths and weaknesses of each of these model systems for studying atrial arrhythmias will be discussed as well as their implications for future studies.

Cardiotrophic Growth Factor-Driven Induction of Human Muse Cells Into Cardiomyocyte-Like Phenotype

Multilineage-differentiating stress-enduring (Muse) cells are endogenous nontumorigenic stem cells collectable as stage-specific embryonic antigen 3 (SSEA-3) + from various organs including the bone marrow and are pluripotent-like. The potential of human bone marrow-derived Muse cells to commit to cardiac lineage cells was evaluated. We found that (1) initial treatment of Muse cells with 5'-azacytidine in suspension culture successfully accelerated demethylation of cardiac marker Nkx2.5 promoter; (2) then transferring the cells onto adherent culture and treatment with early cardiac differentiation factors including wingless-int (Wnt)-3a, bone morphogenetic proteins (BMP)-2/4, and transforming growth factor (TGF) β1; and (3) further treatment with late cardiac differentiation cytokines including cardiotrophin-1 converted Muse cells into cardiomyocyte-like cells that expressed α-actinin and troponin-I with a striation-like pattern. MLC2a expression in the final step suggested differentiation of the cells into an atrial subtype. MLC2v, a marker for a mature ventricular subtype, was expressed when cells were treated with Dickkopf-related protein 1 (DKK-1) and Noggin, inhibitors of Wnt3a and BMP-4, respectively, between steps (2) and (3). None of the steps included exogenous gene transfection, making induced cells feasible for future clinical application.

Intrinsic cellular chirality regulates left-right symmetry breaking during cardiac looping

The vertebrate body plan is overall symmetrical but left-right (LR) asymmetric in the shape and positioning of internal organs. Although several theories have been proposed, the biophysical mechanisms underlying LR asymmetry are still unclear, especially the role of cell chirality, the LR asymmetry at the cellular level, on organ asymmetry. Here with developing chicken embryos, we examine whether intrinsic cell chirality or handedness regulates cardiac C looping. Using a recently established biomaterial-based 3D culture platform, we demonstrate that chick cardiac cells before and during C looping are intrinsically chiral and exhibit dominant clockwise rotation in vitro. We further show that cells in the developing myocardium are chiral as evident by a rightward bias of cell alignment and a rightward polarization of the Golgi complex, correlating with the direction of cardiac tube rotation. In addition, there is an LR polarized distribution of N-cadherin and myosin II in the myocardium before the onset of cardiac looping. More interestingly, the reversal of cell chirality via activation of the protein kinase C signaling pathway reverses the directionality of cardiac looping, accompanied by a reversal in cellular biases on the cardiac tube. Our results suggest that myocardial cell chirality regulates cellular LR symmetry breaking in the heart tube and the resultant directionality of cardiac looping. Our study provides evidence of an intrinsic cellular chiral bias leading to LR symmetry breaking during directional tissue rotation in vertebrate development.

Electrical coupling and its channels

As the physiology of synapses began to be explored in the 1950s, it became clear that electrical communication between neurons could not always be explained by chemical transmission. Instead, careful studies pointed to a direct intercellular pathway of current flow and to the anatomical structure that was (eventually) called the gap junction. The mechanism of intercellular current flow was simple compared with chemical transmission, but the consequences of electrical signaling in excitable tissues were not. With the recognition that channels were a means of passive ion movement across membranes, the character and behavior of gap junction channels came under scrutiny. It became evident that these gated channels mediated intercellular transfer of small molecules as well as atomic ions, thereby mediating chemical, as well as electrical, signaling. Members of the responsible protein family in vertebrates-connexins-were cloned and their channels studied by many of the increasingly biophysical techniques that were being applied to other channels. As described here, much of the evolution of the field, from electrical coupling to channel structure-function, has appeared in the pages of the Journal of General Physiology.

Optimization of direct fibroblast reprogramming to cardiomyocytes using calcium activity as a functional measure of success

Direct conversion of fibroblasts to induced cardiomyocytes (iCMs) has great potential for regenerative medicine. Recent publications have reported significant progress, but the evaluation of reprogramming has relied upon non-functional measures such as flow cytometry for cardiomyocyte markers or GFP expression driven by a cardiomyocyte-specific promoter. The issue is one of practicality: the most stringent measures - electrophysiology to detect cell excitation and the presence of spontaneously contracting myocytes - are not readily quantifiable in the large numbers of cells screened in reprogramming experiments. However, excitation and contraction are linked by a third functional characteristic of cardiomyocytes: the rhythmic oscillation of intracellular calcium levels. We set out to optimize direct conversion of fibroblasts to iCMs with a quantifiable calcium reporter to rapidly assess functional transdifferentiation. We constructed a reporter system in which the calcium indicator GCaMP is driven by the cardiomyocyte-specific Troponin T promoter. Using calcium activity as our primary outcome measure, we compared several published combinations of transcription factors along with novel combinations in mouse embryonic fibroblasts. The most effective combination consisted of Hand2, Nkx2.5, Gata4, Mef2c, and Tbx5 (HNGMT). This combination is >50-fold more efficient than GMT alone and produces iCMs with cardiomyocyte marker expression, robust calcium oscillation, and spontaneous beating that persist for weeks following inactivation of reprogramming factors. HNGMT is also significantly more effective than previously published factor combinations for the transdifferentiation of adult mouse cardiac fibroblasts to iCMs. Quantification of calcium function is a convenient and effective means for the identification and evaluation of cardiomyocytes generated by direct reprogramming. Using this stringent outcome measure, we conclude that HNGMT produces iCMs more efficiently than previously published methods.

Induced regeneration--the progress and promise of direct reprogramming for heart repair

Regeneration of cardiac tissue has the potential to transform cardiovascular medicine. Recent advances in stem cell biology and direct reprogramming, or transdifferentiation, have produced powerful new tools to advance this goal. In this Review we examine key developments in the generation of new cardiomyocytes in vitro as well as the exciting progress that has been made toward in vivo reprogramming of cardiac tissue. We also address controversies and hurdles that challenge the field.

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.

Mechanism of spontaneous excitability in human embryonic stem cell derived cardiomyocytes

Human embryonic stem cell-derived cardiomyocytes (hES-CMs) are thought to recapitulate the embryonic development of heart cells. Given the exciting potential of hES-CMs as replacement tissue in diseased hearts, we investigated the pharmacological sensitivity and ionic current of mid-stage hES-CMs (20-35 days post plating). A high-resolution microelectrode array was used to assess conduction in multicellular preparations of hES-CMs in spontaneously contracting embryoid bodies (EBs). TTX (10 microm) dramatically slowed conduction velocity from 5.1 to 3.2 cm s(-1) while 100 microm TTX caused complete cessation of spontaneous electrical activity in all EBs studied. In contrast, the Ca2+channel blockers nifedipine or diltiazem (1 microm) had a negligible effect on conduction. These results suggested a prominent Na+ channel current, and therefore we patch-clamped isolated cells to record Na+ current and action potentials (APs). We found for isolated hES-CMs a prominent Na+ current (244 +/- 42 pA pF(-1) at 0 mV; n=19), and a hyperpolarization-activated current (HCN), but no inward rectifier K+ current. In cell clusters, 3 microm TTX induced longer AP interpulse intervals and 10 microm TTX caused cessation of spontaneous APs. In contrast nifedipine (Ca2+ channel block) and 2 mm Cs+ (HCN complete block) induced shorter AP interpulse intervals. In single cells, APs stimulated by current pulses had a maximum upstroke velocity (dV/dtmax) of 118 +/- 14 V s(-1) in control conditions; in contrast, partial block of Na+ current significantly reduced stimulated dV/dtmax (38 +/- 15 V s(-1)). RT-PCR revealed NaV1.5, CaV1.2, and HCN-2 expression but we could not detect Kir2.1. We conclude that hES-CMs at mid-range development express prominent Na+ current. The absence of background K+ current creates conditions for spontaneous activity that is sensitive to TTX in the same range of partial block of NaV1.5; thus, the NaV1.5 Na+ channel is important for initiating spontaneous excitability in hES-derived heart cells.

Relationship of muscle growth in vitro to sodium pump activity and transmembrane potential

Serum stimulates embryonic avian skeletal muscle growth in vitro and the growth-related processes of amino acid transport and protein synthesis. Serum also stimulates myotube Na pump activity (measured as ouabain-sensitive rubidium-86 uptake) for at least 2 h after serum addition. Serum-stimulated growth depends on this Na pump activity since ouabain added at the same time as serum totally inhibits the growth responses. The relationship of myotube growth, Na pump activity, and transmembrane potential was studied to determine whether serum-stimulated Na pump activation and growth are coupled by long-term membrane hyperpolarization. When myotube amino acid transport and protein synthesis are prestimulated by serum, ouabain was found to have little inhibitory effect, indicating that the already stimulated growth-related processes are not tightly coupled to continued Na pump activity. Serum-stimulated protein synthesis is tightly coupled to Na pump activity, but only during the first 5-10 min after serum addition. When myotube transmembrane potentials were measured using the lipophilic cation tetraphenylphosphonium, serum at concentrations that stimulate myotube growth and Na pump activity was found to have little effect on the cell's transmembrane potential. Furthermore, partial depolarization of the myotubes with 12- to 55-mM extracellular potassium does not prevent serum stimulation of myotube growth. Monensin was found to hyperpolarize the myotubes, but causes myotube atrophy. These results indicate that although Na pump activity is associated with initiation of serum-stimulated myotube growth, continued Na pump activity is not essential, and there is little relationship between myotube growth and the myotube's transmembrane potential.

Tetrodotoxin-sensitive Na+ channels in isolated single cultured rat myocardial cells

We studied the existence of tetrodotoxin (TTX)-sensitive fast Na+ channels in isolated single (verified by dye injection) myocardial cells compared with small multiple-cell groups from 1- to 3-day-old rat ventricles in cell culture. For single cells, average values were -63 mV maximum membrane potential, 32 mV overshoot, and 65 V/s maximum rate of rise of the action potentials (+Vmax). These values were comparable to values from groups of multiple (2-10) cells. TTX strongly depressed +Vmax dose dependently, with no difference between single and multiple cells. +Vmax was also decreased by lowering extracellular Na+ concentration but not by D 600 or lowering extracellular Ca2+ concentration. These results suggest that 1) isolated single cells possess TTX-sensitive fast Na+ channels, 2) culturing per se does not alter TTX sensitivity, and 3) TTX sensitivity is not modified by cell density.

Optical indications of pace-maker potential and rhythm generation in early embryonic chick heart

1. Pace-maker type action potentials and rhythm generation in very early embryonic chick hearts have been monitored using a voltage-sensitive merocyanine-rhodanine dye. 2. Rhythmicity in recurrence of the spontaneous action potentials was evident at the 7 somite developmental stage, and the rhythm was completely organized by the early period of the 9 somite stage; just before the first contraction. The rhythmic recurrence of action potentials was increased in frequency as development proceeded from the 7 to the 9 somite stage, and presumably gives rise to the rhythm of the initial contractions. 3. Optical signals resembling the pace-maker type action potential with a diastolic depolarization phase were first detected in embryonic hearts at the 8 somite stage. At this stage, pace-maker type action potentials were detected from various regions, such as the ventricle and the unfused primordia at the atrium level. 4. Regionalization of pace-maker type action signals was exhibited at the early period of the 9 somite stage. At this stage, the pace-maker type signals were often evident at the atrium level, while the cardiac type signals were detected in the ventricular region. 5. Hence it is concluded (i) that the rhythmicity has already been generated at the 7 somite developmental stage, (ii) that the pace-making cells are widely distributed in the embryonic hearts at the 7-8 somite stages and (iii) that the appearance of the pace-maker potential is initially localized to the atrium level at about the 9-10 somite stages.

Electrical activity of rat myocardial cells in culture: La3+-induced alterations

Intracellular analysis of neonatal rat (1-2 day old) ventricular cells in culture shows that contracting myocardial cells exhibit an array of different patterns of spontaneous electrical activity. Resting membrane potentials varied between -40 mV and -98 mV. Our results indicate that some cultured cells show resting membrane potentials, overshoot, and total spike amplitude values comparable to those normally found in neonatal and adult rat heart. A low ratio of pacemaker (40%) to nonpacemaker cells (60%) and low incidence of hyperpolarizing after-potentials (35%) were found. La3+ application (0.1-4.0 mM) induced progressive cell depolarization, concomitant diminution in discharge frequency, and marked alteration of action potential configuration. A parallel decline in frequency and strength of rhythmic contractions was observed. Abolition of contractility occurred only in association with depolarization and complete disappearance of action potentials. Recovery of electrical and contractile activity followed medium replacement. Our results indicate that La3+ does not act as a specific excitation-contraction (E-C) uncoupler in the cultured cells but has multiple effects upon their normal electrical characteristics.

Electrophysiological properties of tissue cultured heart cells grown in a linear array

Embryonic chick heart cells were grown in tissue culture on an oriented substrate (channels cut in an agar coated slide), so that they formed narrow(5-100mu) strands of arbitrary length. The electrical properties of these strands were examined using intracellular microelectrodes. ac and dc cable studies were performed to determine the passive cable parameters. Quantitative histology, using light and electronmicroscopy, permitted calculation of intrinsic capacitances and resistivities. Electrical coupling between polarizing and recording electrodes was ubiquitous, falling off exponentially with distance. It was concluded that individual cells were electrically connected, since coupling was observed at distances greater than 3 mm, and the maximum cell length was estimated to be less that 300 mu. The strands were usually spontaneously active, with phase 4 depolarization (pacemaker potential) occurring almost simultaneously in all cells of a strand. The passive electrical properties determined during phase 4 were: core resistivity (cytoplasm plus cell-to-cell resistance), 245 ohm/cm; membrane capacitance, 1.46 muF/CM2. The membrane resistance increased from 16 to 136 kohm/cm2 during phase 4. The space and time constants showed commensurate changes, from 0.95 to 3.2 mm, and from 29 to 269 msec, respectively. The input resistance also increased, from 1.1 to 3.8 Mohm.

Ultraviolet-induced alterations of beat rate and electrical properties of embryonic chick heart cell aggregates

Embryonic heart cell aggregates were irradiated with ultraviolet light at wavelengths between 260 and 310 nm. Spontaneous beat rate was monitored with the aid of a closed-circuit TV camera and, in separate experiments, electrophysiological changes were assayed by intracellular recording. The characteristic response of 7-day aggregates was an increase in spontaneous beat rate to a maximum plateau level, followed by a rather abrupt cessation of beating. Intracellular recordings during irradiation showed a marked decline in the maximum rate of rise, overshoot, and repolarization phase of the action potential, and a significant change in threshold toward zero. The action spectrum for the termination of beating peaked between 290 and 295 nm; it fell off sharply at longer wavelengths and more slowly at shorter wavelengths. The maximum increase in beat rate was increasingly greater for shorter wavelengths and exhibited no peak in the wavelength range investigated. The sensitivity of aggregates to 295-nm light, as measured by the inverse of irradiation time required to terminate beating, decreased with increasing aggregate size and external potassium concentration, was relatively independent of temperature, and increased with embryonic age. The ultraviolet-induced increase in beat rate and termination of beating are attributed to separate complementary processes, a depolarization of the membrane, and a decline in "fast" sodium conductance.

A synthetic strand of cardiac muscle: its passive electrical properties

The passive electrical properties of synthetic strands of cardiac muscle, grown in tissue culture, were studied using two intracellular microelectrodes: one to inject a rectangular pulse of current and the other to record the resultant displacement of membrane potential at various distances from the current source. In all preparations, the potential displacement, instead of approaching a steady value as would be expected for a cell with constant electrical properties, increased slowly with time throughout the current step. In such circumstances, the specific electrical constants for the membrane and cytoplasm must not be obtained by applying the usual methods, which are based on the analytical solution of the partial differential equation describing a one-dimensional cell with constant electrical properties. A satisfactory fit of the potential waveforms was, however, obtained with numerical solutions of a modified form of this equation in which the membrane resistance increased linearly with time. Best fits of the waveforms from 12 preparations gave the following values for the membrane resistance times unit length, membrane capacitance per unit length, and for the myoplasmic resistance: 1.22 plus or minus 0.13 x 10-5 omegacm, 0.224 plus or minus 0.023 uF with cm-minus 1, and 1.37 plus or minus 0.13 x 10-7 omegacm-minus 1, respectively. The value of membrane capacitance per unit length was close to that obtained from the time constant of the foot of the action potential and was in keeping with the generally satisfactory fit of the recorded waveforms with solutions of the cable equation in which the membrane impedance is that of a single capacitor and resistor in parallel. The area of membrane per unit length and the cross-sectional area of myoplasm at any given length of the preparation were determined from light and composite electron micrographs, and these were used to calculate the following values for the specific electrical membrane resistance, membrane capacitance, and the resistivity of the cytoplasm: 20.5 plus or minus 3.0 x 10-3 omegacm-2, l.54 plus or minus 0.24 uFWITHcm-minus 2, and 180 plus or minus 34 omegacm, respectively.

Membrane response to current pulses in spheroidal aggregates of embryonic heart cells

Hearts from chick embryos aged 4,7, or 14 days were dissociated into their component cells, and the cells allowed to reassociate in the form of smooth-surfaced spheroidal aggregates on a gyratory shaker. Records from intracellular electrodes inserted into two widely spaced cells in a spontaneously beating aggregate indicated that the action potentials occurred virtually simultaneously. In aggregates made quiescent with tetrodotoxin, the voltage response to a current pulse injected in one cell could be noted by recording with a second microelectrode at various distance from the current source. The magnitude of the response was found not to vary with distance. It is concluded that the component cells in an aggregate are normally tightly coupled electrically; the cell boundaries do not constitute an appreciable resistive barrier. Such ag-regates behave as virtually isopential systems, with properties similar to those of single spherical cells, as modeled by Eisenberg and Engel (1970. J. Gen. Physiol. 55:736-757). Passive membrane time constant ranged from 11 to 31 ms, with a mean value of 17 ms; this value did not vary with aggregate size. Input resistance (V/I) varied inversely with aggregate size, as predicted, but with much scatter in the measured values. Specific membrane resistance was calculated as either 13,000 or 800 ohm-cm2 depending on whether input resistance was attributed to the total cell surface membrane area or to the outer surface of the sphere alone. No systematic difference in passive electrical properties of aggregates composed of 4-, 7-, and 14-day cells was seen. It is concluded that these aggregates may be suitable for voltage clamp analysis of their excitable membrane properties.

Electrical activity in embryonic heart cell aggregates. Developmental aspects

Action potential parameters were measured in beating heart cell aggregates which were formed from trypsin-dissociated cells of embryonic chick heats aged 2 1/2, 4 or 7 days. 1. In aggregates composed of cells from the whole heart there was an increase in the maximum diastolic potential, overshoot, maximum rate of rise of the action potential (V max), and action potential duration between days 2 1/2 and 7. 2. Action potential parameters from 4- or 7-day aggregates composed exclusively of atrial or ventricular cells were similar to those in whole heart aggregates of the same age with the exception of the action potential duration in which atrial less than whole heart less than ventricular. Between days 4 and 7 the increases in duration were approximately 14% in atrial, 35% in whole heart, and 50% in ventricular aggregates. Differences in action potential duration, within or between ages, were not due solely to differences in the rate of beating. 3. Action potentials in whole heart aggregates aged 2 1/2 days were insensivitive to TTX (10-5 g/ml) but abolished by D600 (1 MUG/ML). Conversely, at 7 days activity was suppressed by TTX (2 X 10-8 G/ML) WHILE D600 (1 mug/ml) shortened the action potential duration and reduced the overshoot without influencing V max. 4. Adrenaline (1 mug/ml) restored the action potential overshoot and duration in 7-day aggregates treated with D600. 5. Action potential development in embryonic heart cells appears to be characterized by the functional appearance of fast inward channels. The slow channel mechanism, previously utilized in action potential generation, may gradually assume its adult role of carrying inward current during the plateau phase. 6. In contrast to monolayer cultures, embryonic heart cells cultured in aggregate form seem to have membrane properties similar to those of intact tissue.

Electrical activity in embryonic heart cell aggregates. Pacemaker oscillations

Aggregates were formed from dissociated heart cells of 7-day chick embryos. When spontaneous action potentials were blocked with 10-8 to 10-7 g/ml tetrodotoxin (TTX) oscillatory pacemaker potentials were sometimes seen. The emergence of these pacemaker potentials was critically dependent on the external potassium concentration. In 1.3 mM potassium medium TTX suppression of action potential generation always led to a stable resting potential close to the threshold level (-55 to -50 mV). In 4.3 mM potassium TTX suppression was followed by a train of pacemaker potentials which usually gave way to a stable resting potential of about -70 mV. Raising the calcium concentration from 1.8 to 5 mM often induced long lasting (3 hrs) pacemaker oscillations of 20 to 30 mV peak to peak amplitude. These were abolished by raising the potassium concentration to 8.3 mM or upon the addition of 1.5 mM Mn2 plus. The responses of TTX-treated aggregates are discussed in terms of Noble and Tsien's pacemaker theory for Purkinje fibers. The results are well described by assuming the existence of an ik2-like potassium current in embryonic heart cells. The role of calcium is unclear but it may help provide the inward current against which the outward potassium current can function.

Tetrodotoxin desensitization in aggregates of embryonic chick heart cells

Spontaneous beating of heart-cell aggregates from 4-day chick embryos was initially blocked by 10(-5) g/ml tetrodotoxin (TTX). With continued exposure to the drug, the fraction of blocked aggregates decreased from about 80% at 15 min to about 25% at 2-3 h, at which time, beating aggregates had become desensitized to the toxin, showing no response to a fresh dose. Aggregates from 5-day hearts were more sensitive to TTX, but fewer became desensitized in its presence. Desensitization to TTX was not seen in 6- and 7-day aggregates. Inhibition of protein synthesis by cycloheximide did not affect beating or initial sensitivity to TTX of 4-day aggregates, but desensitization failed to occur. Before TTX, the mean value of maximal upstroke velocity (V(max)) of the action potentials in 4-day aggregates was 33 V/s. After desensitization V(max) was 12 V/s. Activity of desensitized aggregates in the presence of TTX was augmented by elevated calcium levels, and suppressed by presumed inhibitors of slow inward current (manganese, D600). Desensitization was reversible; upon removal of TTX 10(-5) g/ml, aggregates regained their responsiveness to a fresh dose of the drug with a 2-3 h time-course similar to that of desensitization. This was prevented by continued exposure to TTX at concentrations as low as 10(-8) g/ml. These data suggest that (a) desensitization involves a change in the mode of action-potential generating from one involving Na-specific, TTX-sensitive channels to one utilizing slower Mn-sensitive channels; (b) the process of desensitization occurs over a period of 2-3 h and is dependent upon the products of protein synthesis; and (c) desensitization is reversible after removal of TTX over a 2-3 h time-course similar to its onset.

Ion levels and membrane potential in chick heart tissue and cultured cells

Intracellular concentrations of sodium and potassium as well as resting potentials and overshoots have been determined in heart tissue from chick embryos aged 2-18 days. Intracellular potassium declined from 167 mM at day 2 to 117-119 mM at days 14-18. Intracellular sodium remained nearly constant at 30-35 mM during the same period. The mean resting potential increased from -61.8 mV at day 3 to about -80 mV at days 14-18. The mean overshoot during the same period increased from 12 to 30 mV. P(Na)/P(K) calculated from the ion data and resting potentials declined from 0.08 at day 3 to 0.01 at days 14-18. Thus, the development of embryonic chick heart during days 2-14 is characterized by a declining intracellular potassium concentration and an increasing resting potential and overshoot. Heart cells from 7- to 8-day embryos, cultured either in monolayer or reassociated into aggregates, were compared with intact tissue of the same age. The intracellular concentrations of sodium and potassium were similar in the three preparations and cultured cells responded to incubation in low potassium medium or treatment with ouabain in a manner similar to that of intact tissue. Resting potentials and overshoots were also similar in the three preparations.

Mechanism initiating ventricular fibrillation demonstrated in cultured ventricular muscle tissue

Ventricular muscle strips of the rat embryo heart were used for tissue culture preparations without adding trypsin. Fibrillation-like arrhythmia was induced by adding aconitine or strophanthin, and the potentials were recorded with one or two microelectrodes. The tracing by one microelectrode was no different than tracings from fibrillating adult mammalian hearts. The size and shape of the action potential varied beat by beat, and its time of appearance was quite irregular and rapid. But, when tracings obtained simultaneously by two microelectrodes were compared, most of the action potentials were roughly synchronous and thus unlike adult cardiac fibrillation. This ruled out the possibility of multiple reentry or multifocal origin in this preparation.

Because of the small size of the cultured tissue, reentry of the excitation wave probably could not occur, and the conclusion that fibrillation originated from a single focus is thus supported. Three varieties of the onset of fibrillation, i.e., gradual increase of tachycardia with progressively steeper slow diastolic depolarization, a prominent positive afterpotential followed by a negative afterpotential, and abortive action potentials superimposed on the repolarization of the preceding action potentials or on the negative afterpotential, supported the unifocal onset.