Intracellular calcium dynamics and acceleration of sinus rhythm by beta-adrenergic stimulation

BACKGROUND

Recent evidence indicates that membrane voltage and Ca2+ clocks jointly regulate sinoatrial node (SAN) automaticity. Here we test the hypothesis that sinus rate acceleration by beta-adrenergic stimulation involves synergistic interactions between these clock mechanisms.

METHODS AND RESULTS

We simultaneously mapped intracellular calcium (Ca(i)) and membrane potential in 25 isolated canine right atrium, using previously described criteria of the timing of late diastolic Ca(i) elevation (LDCAE) relative to the action potential upstroke to detect the Ca2+ clock. Before isoproterenol, the earliest pacemaking site occurred in the inferior SAN, and LDCAE was observed in only 4 of 25 preparations. Isoproterenol infusion (1 micromol/L) increased sinus rate and shifted pacemaking site to superior SAN, concomitant with the appearance of LDCAE preceding the action potential upstroke by 98+/-31 ms. Caffeine had similar effects, whereas sarcoplasmic reticulum Ca2+ depletion with ryanodine and thapsigargin prevented isoproterenol-induced LDCAE and blunted sinus rate acceleration. Ca(i) transient relaxation time during isoproterenol was shorter in superior SAN (124+/-34 ms) than inferior SAN (138+/-24 ms; P=0.01) or right atrium (164+/-33 ms; P=0.001) and was associated with a lower sarcoplasmic reticulum Ca2+ ATPase pump to phospholamban protein ratio in SAN than in right atrium. Hyperpolarization-activated pacemaker current (I(f)) blockade with ZD 7288 modestly blunted but did not prevent LDCAE or sinus rate acceleration by isoproterenol.

CONCLUSIONS

Acceleration of the Ca2+ clock in the superior SAN plays an important role in sinus acceleration during beta-adrenergic stimulation, interacting synergistically with the voltage clock to increase sinus rate.

Oxidative-stress-induced afterdepolarizations and calmodulin kinase II signaling

In the heart, oxidative stress caused by exogenous H(2)O(2) has been shown to induce early afterdepolarizations (EADs) and triggered activity by impairing Na current (I(Na)) inactivation. Because H(2)O(2) activates Ca(2+)/calmodulin kinase (CaMK)II, which also impairs I(Na) inactivation and promotes EADs, we hypothesized that CaMKII activation may be an important factor in EADs caused by oxidative stress. Using the patch-clamp and intracellular Ca (Ca(i)) imaging in Fluo-4 AM-loaded rabbit ventricular myocytes, we found that exposure to H(2)O(2) (0.2 to 1 mmol/L) for 5 to 15 minutes consistently induced EADs that were suppressed by the I(Na) blocker tetrodotoxin (10 micromol/L), as well as the I(Ca,L) blocker nifedipine. H(2)O(2) enhanced both peak and late I(Ca,L), consistent with CaMKII-mediated facilitation. By prolonging the action potential plateau and increasing Ca influx via I(Ca,L), H(2)O(2)-induced EADs were also frequently followed by DADs in response to spontaneous (ie, non-I(Ca,L)-gated) sarcoplasmic reticulum Ca release after repolarization. The CaMKII inhibitor KN-93 (1 micromol/L; n=4), but not its inactive analog KN-92 (1 micromol/L, n=5), prevented H(2)O(2)-induced EADs and DADs, and the selective CaMKII peptide inhibitor AIP (autocamtide-2-related inhibitory peptide) (2 micromol/L) significantly delayed their onset. In conclusion, H(2)O(2)-induced afterdepolarizations depend on both impaired I(Na) inactivation to reduce repolarization reserve and enhancement of I(Ca,L) to reverse repolarization, which are both facilitated by CaMKII activation. Our observations support a link between increased oxidative stress, CaMKII activation, and afterdepolarizations as triggers of lethal ventricular arrhythmias in diseased hearts.

Electrophysiological consequences of acute regional ischemia/reperfusion in neonatal rat ventricular myocyte monolayers

BACKGROUND

Electrophysiological changes promoting arrhythmias during acute regional ischemia/reperfusion are challenging to study in intact cardiac tissue because of complex 3-dimensional myocardial and vascular geometry. We characterized electrophysiological alterations and arrhythmias during regional ischemia/reperfusion in a simpler 2-dimensional geometry of cultured neonatal rat ventricular myocyte monolayers.

METHODS AND RESULTS

Optical mapping of intracellular Ca (Ca(i)) and voltage was performed with the use of Rhod 2-AM and Rh-237, respectively. Regional ischemia was mimicked by covering the central portion of monolayer with a glass coverslip, and reperfusion was mimicked by removing the coverslip. Monolayers were stained with fluorescent antibodies to detect total and dephosphorylated connexin-43 at various time points. During coverslip ischemia, action potential duration shortened, Ca(i) transient duration was prolonged, and local conduction velocity (CV) slowed progressively, with loss of excitability after 10.6 +/- 3.6 minutes. CV slowing was accompanied by connexin-43 dephosphorylation. During ischemia, spontaneous reentry occurred in 5 of 11 monolayers, initiated by extrasystoles arising from the border zone or unidirectional conduction block of paced beats. On reperfusion, excitability recovered within 1.0 +/- 0.8 minutes, but CV remained depressed for 9.0 +/- 3.0 minutes, promoting reentry in the reperfused zone. As connexin-43 phosphorylation recovered in the reperfused zone, CV normalized, and arrhythmias resolved.

CONCLUSIONS

Acute regional ischemia/reperfusion in neonatal rat ventricular myocyte monolayers recapitulates electrophysiological alterations and arrhythmias similar to those observed during acute coronary occlusion/reperfusion in intact hearts. During early reperfusion, slow recovery from connexin-43 dephosphorylation leads to persistent CV slowing, creating a highly arrhythmogenic substrate.

Glycolytic oscillations in isolated rabbit ventricular myocytes

Previous studies have shown that glycolysis can oscillate periodically, driven by feedback loops in regulation of key glycolytic enzymes by free ADP and other metabolites. Here we show both theoretically and experimentally in cardiac myocytes that when the capacity of oxidative phosphorylation and the creatine kinase system to buffer the cellular ATP/ADP ratio is suppressed, glycolysis can cause large scale periodic oscillations in cellular ATP levels (0.02-0.067 Hz), monitored from glibenclamide-sensitive changes in action potential duration or intracellular free Mg2+. Action potential duration oscillations originate primarily from glycolysis, since they 1) occur in the presence of cyanide or rotenone, 2) are suppressed by iodoacetate, 3) are accompanied by at most very small mitochondrial membrane potential oscillations, and 4) exhibit an anti-phase relationship to NADH fluorescence. By uncoupling energy supply-demand balance, glycolytic oscillations may promote injury and electrophysiological heterogeneity during acute metabolic stresses, such as acute myocardial ischemia in which both oxidative phosphorylation and creatine kinase activity are inhibited.

Systems approach to understanding electromechanical activity in the human heart: a national heart, lung, and blood institute workshop summary

The National Heart, Lung, and Blood Institute (NHLBI) convened a workshop of cardiologists, cardiac electrophysiologists, cell biophysicists, and computational modelers on August 20 and 21, 2007, in Washington, DC, to advise the NHLBI on new research directions needed to develop integrative approaches to elucidate human cardiac function. The workshop strove to identify limitations in the use of data from nonhuman animal species for elucidation of human electromechanical function/activity and to identify what specific information on ion channel kinetics, calcium handling, and dynamic changes in the intracellular/extracellular milieu is needed from human cardiac tissues to develop more robust computational models of human cardiac electromechanical activity. This article summarizes the workshop discussions and recommendations on the following topics: (1) limitations of animal models and differences from human electrophysiology, (2) modeling ion channel structure/function in the context of whole-cell electrophysiology, (3) excitation-contraction coupling and regulatory pathways, (4) whole-heart simulations of human electromechanical activity, and (5) what human data are currently needed and how to obtain them. The recommendations can be found on the NHLBI Web site at http://www.nhlbi.nih.gov/meetings/workshops/electro.htm.

Tachycardia-induced early afterdepolarizations: insights into potential ionic mechanisms from computer simulations

Although early afterdepolarizations (EADs) are classically thought to occur at slow heart rates, mounting evidence suggests that EADs may also occur at rapid heart rates produced by tachyarrhythmias, due to Ca overload of the sarcoplasmic reticulum (SR) leading to spontaneous SR Ca release. We hypothesized that the mechanism of tachycardia-induced EADs depends on the spatial and temporal morphology of spontaneous SR Ca release, and tested this hypothesis in computer simulations using a ventricular action potential mathematical model. Using two previously suggested spontaneous release morphologies, we found two distinct tachycardia-induced EAD mechanisms: one mechanistically similar to bradycardia-induced EADs, the other to delayed afterdepolarizations (DADs).

Superiority of biphasic over monophasic defibrillation shocks is attributable to less intracellular calcium transient heterogeneity

OBJECTIVES

The purpose of this study was to test the hypothesis that superiority of biphasic waveform (BW) over monophasic waveform (MW) defibrillation shocks is attributable to less intracellular calcium (Ca(i)) transient heterogeneity.

BACKGROUND

The mechanism by which BW shocks have a higher defibrillation efficacy than MW shocks remains unclear.

METHODS

We simultaneously mapped epicardial membrane potential (Vm) and Ca(i) during 6-ms MW and 3-ms/3-ms BW shocks in 19 Langendorff-perfused rabbit ventricles. After shock, the percentage of depolarized area was plotted over time. The maximum (peak) post-shock values (VmP and Ca(i)P, respectively) were used to measure heterogeneity. Higher VmP and Ca(i)P imply less heterogeneity.

RESULTS

The defibrillation thresholds for BW and MW shocks were 288 +/- 99 V and 399 +/- 155 V, respectively (p = 0.0005). Successful BW shocks had higher VmP (88 +/- 9%) and Ca(i)P (70 +/- 13%) than unsuccessful MW shocks (VmP 76 +/- 10%, p < 0.001; Ca(i)P 57 +/- 8%, p < 0.001) of the same shock strength. In contrast, for unsuccessful BW and MW shocks of the same shock strengths, the VmP and Ca(i)P were not significantly different. The MW shocks more frequently created regions of low Ca(i) surrounded by regions of high Ca(i) (post-shock Ca(i) sinkholes). The defibrillation threshold for MW and BW shocks became similar after disabling the sarcoplasmic reticulum (SR) with thapsigargin and ryanodine.

CONCLUSIONS

The greater efficacy of BW shocks is directly related to their less heterogeneous effects on shock-induced SR Ca release and Ca(i) transients. Less heterogeneous Ca(i) transients reduces the probability of Ca(i) sinkhole formation, thereby preventing the post-shock reinitiation of ventricular fibrillation.

Reactive oxygen species production in energized cardiac mitochondria during hypoxia/reoxygenation: modulation by nitric oxide

Mitochondria are an important source of reactive oxygen species (ROS), implicated in ischemia/reperfusion injury. When isolated from ischemic myocardium, mitochondria demonstrate increased ROS production as a result of damage to electron transport complexes. To investigate the mechanisms, we studied effects of hypoxia/reoxygenation on ROS production by isolated energized heart mitochondria. ROS production, tracked using Fe(2+)-catalyzed, H(2)O(2)-dependent H(2)DCF oxidation or Amplex Red, was similar during normoxia and hypoxia but markedly increased during reoxygenation, in proportion to the duration of hypoxia. In contrast, if mitochondria were rapidly converted from normoxia to near-anoxia ([O(2)], <1 micromol/L), the increase in H(2)DCF oxidation rate during reoxygenation was markedly blunted. To elicit the robust increase in H(2)DCF oxidation rate during reoxygenation, hypoxia had to be severe enough to cause partial, but not complete, respiratory chain inhibition (as shown by partial dissipation of membrane potential and increased NADH autofluorescence). Consistent with its cardioprotective actions, nitric oxide ( O) abrogated increased H(2)DCF oxidation under these conditions, as well as attenuating ROS-induced increases in matrix [Fe(2+)] and aconitase inhibition caused by antimycin. Collectively, these results suggest that (1) hypoxia that is sufficient to cause partial respiratory inhibition is more damaging to mitochondria than near-anoxia; and (2) O suppresses ROS-induced damage to electron transport complexes, probably by forming O-Fe(2+) complexes in the presence of glutathione, which inhibit hydroxyl radical formation.

Principles of calcium signaling. Salisbury Cove, Maine, June 28-30, 2007

A three-day International Symposium entitled "Principles of Calcium Signaling" organized by James N. Weiss, Yale E. Goldman, Stéphane Hatem, Lars Cleemann and Nikolai M. Soldatov in honor of the research contributions of Professor Martin Morad was held at the Mount Desert Island Biological Laboratory, Salisbury Cove, Maine. Support for this meeting was provided in part by GlaxoSmithKline, Leica Microsystems, Nikon Corp., St. Jude Medical, Inc., UCLA Cardiac Arrhythmia Center, Dr. Donald S. Orkand, Bob Hillis Family and OML, and Mount Desert Island Biological Laboratory. The symposium featured sessions on Cardiac physiology, Ion channels and Calcium signaling.

Virtual electrodes and the induction of fibrillation in Langendorff-perfused rabbit ventricles: the role of intracellular calcium

A strong premature electrical stimulus (S(2)) induces both virtual anodes and virtual cathodes. The effects of virtual electrodes on intracellular Ca(2+) concentration ([Ca(2+)](i)) transients and ventricular fibrillation thresholds (VFTs) are unclear. We studied 16 isolated, Langendorff-perfused rabbit hearts with simultaneous voltage and [Ca(2+)](i) optical mapping and for vulnerable window determination. After baseline pacing (S(1)), a monophasic (10 ms anodal or cathodal) or biphasic (5 ms-5 ms) S(2) was applied to the left ventricular epicardium. Virtual electrode polarizations and [Ca(2+)](i) varied depending on the S(2) polarity. Relative to the level of [Ca(2+)](i) during the S(1) beat, the [Ca(2+)](i) level 40 ms after the onset of monophasic S(2) increased by 36+/-8% at virtual anodes and 20+/-5% at virtual cathodes (P<0.01), compared with 25+/-5% at both virtual cathode-anode and anode-cathode sites for biphasic S(2). The VFT was significantly higher and the vulnerable window significantly narrower for biphasic S(2) than for either anodal or cathodal S(2) (n=7, P<0.01). Treatment with thapsigargin and ryanodine (n=6) significantly prolonged the action potential duration compared with control (255+/-22 vs. 189+/-6 ms, P<0.05) and eliminated the difference in VFT between monophasic and biphasic S(2), although VFT was lower for both cases. We conclude that virtual anodes caused a greater increase in [Ca(2+)](i) than virtual cathodes. Monophasic S(2) is associated with lower VFT than biphasic S(2), but this difference was eliminated by the inhibition of the sarcoplasmic reticulum function and the prolongation of the action potential duration. However, the inhibition of the sarcoplasmic reticulum function also reduced VFT, indicating that the [Ca(2+)](i) dynamics modulate, but are not essential, to ventricular vulnerability.

Deducing topology of protein-protein interaction networks from experimentally measured sub-networks

Background

Protein-protein interaction networks are commonly sampled using yeast two hybrid approaches. However, whether topological information reaped from these experimentally-measured sub-networks can be extrapolated to complete protein-protein interaction networks is unclear.

Results

By analyzing various experimental protein-protein interaction datasets, we found that they are not random samples of the parent networks. Based on the experimental bait-prey behaviors, our computer simulations show that these non-random sampling features may affect the topological information. We tested the hypothesis that a core sub-network exists within the experimentally sampled network that better maintains the topological characteristics of the parent protein-protein interaction network. We developed a method to filter the experimentally sampled network to result in a core sub-network that more accurately reflects the topology of the parent network. These findings have fundamental implications for large-scale protein interaction studies and for our understanding of the behavior of cellular networks.

Conclusion

The topological information from experimental measured networks network as is may not be the correct source for topological information about the parent protein-protein interaction network. We define a core sub-network that more accurately reflects the topology of the parent network.

Proteomic insights into cardiac cell death and survival

Cardiovascular disease is the leading cause of death and disability in the developed world. To design novel therapeutic strategies to treat and prevent this disease, better understanding of cardiac cell function is necessary. In addition to (and, indeed, in combination with) genetics, physiology and molecular biology, proteomics plays a critical role in our understanding of cardiovascular systems at multiple scales. The purpose of this review is to examine recent developments in the field of myocardial injury and protection, examining how proteomics has informed investigations into organelles, signaling complexes, and cardiac phenotype.

Altered proteome biology of cardiac mitochondria under stress conditions

Myocardial ischemia-reperfusion induces mitochondrial dysfunction and, depending upon the degree of injury, may lead to cardiac cell death. However, our ability to understand mitochondrial dysfunction has been hindered by an absence of molecular markers defining the various degrees of injury. To address this paucity of knowledge, we sought to characterize the impact of ischemic damage on mitochondrial proteome biology. We hypothesized that ischemic injury induces differential alterations in various mitochondrial subcompartments, that these proteomic changes are specific to the severity of injury, and that they are important to subsequent cellular adaptations to myocardial ischemic injury. Accordingly, an in vitro model of cardiac mitochondria injury in mice was established to examine two stress conditions: reversible injury (induced by mild calcium overload) and irreversible injury (induced by hypotonic stimuli). Both forms of injury had a drastic impact on the proteome biology of cardiac mitochondria. Altered mitochondrial function was concomitant with significant protein loss/shedding from the injured organelles. In the setting of mild calcium overload, mitochondria retained functionality despite the release of numerous proteins, and the majority of mitochondria remained intact. In contrast, hypotonic stimuli caused severe damage to mitochondrial structure and function, induced increased oxidative modification of mitochondrial proteins, and brought about detrimental changes to the subproteomes of the inner mitochondrial membrane and matrix. Using an established in vivo murine model of regional myocardial ischemic injury, we validated key observations made by the in vitro model. This preclinical investigation provides function and suborganelle location information on a repertoire of cardiac mitochondrial proteins sensitive to ischemia reperfusion stress and highlights protein clusters potentially involved in mitochondrial dysfunction in the setting of ischemic injury.

Systematic characterization of the murine mitochondrial proteome using functionally validated cardiac mitochondria

Mitochondria play essential roles in cardiac pathophysiology and the murine model has been extensively used to investigate cardiovascular diseases. In the present study, we characterized murine cardiac mitochondria using an LC/MS/MS approach. We extracted and purified cardiac mitochondria; validated their functionality to ensure the final preparation contains necessary components to sustain their normal function; and subjected these validated organelles to LC/MS/MS-based protein identification. A total of 940 distinct proteins were identified from murine cardiac mitochondria, among which, 480 proteins were not previously identified by major proteomic profiling studies. The 940 proteins consist of functional clusters known to support oxidative phosphorylation, metabolism, and biogenesis. In addition, there are several other clusters, including proteolysis, protein folding, and reduction/oxidation signaling, which ostensibly represent previously under-appreciated tasks of cardiac mitochondria. Moreover, many identified proteins were found to occupy other subcellular locations, including cytoplasm, ER, and golgi, in addition to their presence in the mitochondria. These results provide a comprehensive picture of the murine cardiac mitochondrial proteome and underscore tissue- and species-specification. Moreover, the use of functionally intact mitochondria insures that the proteomic observations in this organelle are relevant to its normal biology and facilitates decoding the interplay between mitochondria and other organelles.

Phosphatidylinositol-4,5-bisphosphate (PIP2) regulation of strong inward rectifier Kir2.1 channels: multilevel positive cooperativity

Inwardly rectifying potassium (Kir) channels are gated by the interaction of their cytoplasmic regions with membrane-bound phosphatidylinositol-4,5-bisphosphate (PIP(2)). In the present study, we examined how PIP(2) interaction regulates channel availability and channel openings to various subconductance levels (sublevels) as well as the fully open state in the strong inward rectifier Kir2.1 channel. Various Kir2.1 channel constructs were expressed in Xenopus oocytes and single channel or macroscopic currents were recorded from inside-out patches. The wild-type (WT) channel rarely visited the subconductance levels under control conditions. However, upon reducing Kir2.1 channel interaction with PIP(2) by a variety of interventions, including PIP(2) antibodies, screening PIP(2) with neomycin, or mutating PIP(2) binding sites (e.g. K188Q), visitation to the sublevels was markedly increased before channels were converted to an unavailable mode in which they did not open. No channel activity was detected in channels with the double mutation K188A/R189A, a mutant which exhibits extremely weak interaction with PIP(2). By linking subunits together in tandem dimers or tetramers containing mixtures of WT and K188A/R189A subunits, we demonstrate that one functional PIP(2)-interacting WT subunit is sufficient to convert channels from the unavailable to the available mode with a high open probability dominated by the fully open state, with similar kinetics as tetrameric WT channels. Occasional openings to sublevels become progressively less frequent as the number of WT subunits increases. Quantitative analysis reveals that the interaction of PIP(2) with WT subunits exerts strong positive cooperativity in both converting the channels from the unavailable to the available mode, and in promoting the fully open state over sublevels. We conclude that the interaction of PIP(2) with only one Kir2.1 subunit is sufficient for the channel to become available and to open to its full conductance state. Interaction with additional subunits exerts positive cooperativity at multiple levels to further enhance channel availability and promote the fully open state.

Spatially discordant alternans in cardiomyocyte monolayers

Repolarization alternans is a harbinger of sudden cardiac death, particularly when it becomes spatially discordant. Alternans, a beat-to-beat alternation in the action potential duration (APD) and intracellular Ca (Cai), can arise from either tissue heterogeneities or dynamic factors. Distinguishing between these mechanisms in normal cardiac tissue is difficult because of inherent complex three-dimensional tissue heterogeneities. To evaluate repolarization alternans in a simpler two-dimensional cardiac substrate, we optically recorded voltage and/or Cai in monolayers of cultured neonatal rat ventricular myocytes during rapid pacing, before and after exposure to BAY K 8644 to enhance dynamic factors promoting alternans. Under control conditions (n = 37), rapid pacing caused detectable APD alternans in 81% of monolayers, and Cai transient alternans in all monolayers, becoming spatially discordant in 62%. After BAY K 8644 (n = 28), conduction velocity restitution became more prominent, and APD and Cai alternans developed and became spatially discordant in all monolayers, with an increased number of nodal lines separating out-of-phase alternating regions. Nodal lines moved closer to the pacing site with faster pacing rates and changed orientation when the pacing site was moved, as predicted for the dynamically generated, but not heterogeneity-based, alternans. Spatial APD gradients during spatially discordant alternans were sufficiently steep to induce conduction block and reentry. These findings indicate that spatially discordant alternans severe enough to initiate reentry can be readily induced by pacing in two-dimensional cardiac tissue and behaves according to predictions for a predominantly dynamically generated mechanism.

Dynamic modulation of intracellular glucose imaged in single cells using a FRET-based glucose nanosensor

To study intracellular glucose homeostasis, the glucose nanosensor FLIPglu-600 microM, which undergoes changes in fluorescence resonance energy transfer (FRET) upon interaction with glucose, was expressed in four mammalian cell lines: COS-7, CHO, HEK293, and C2C12. Upon addition of extracellular glucose, the intracellular FRET ratio decreased rapidly as intracellular glucose increased. The kinetics were fast (tau=5 to 15 s) in COS and C2C12 cells and slow (tau=20 to 40 s) in HEK and CHO cells. Upon removal of extracellular glucose, the FRET ratio returned to its initial value at similar rates (tau=15 to 40 s) in all cell types. In all cell types, the glucose uptake FRET signal was blocked by the glucose transporter (GLUTx) inhibitor cytochalasin B and was not affected by the Na/glucose transporter inhibitor phlorizin. Glucose clearance was inhibited by the glycolytic inhibitor iodoacetate. Using beta-escin to permeabilize the cell, we found that the glucose gradient across the membrane was strongly dependent on the rates of glucose uptake versus glucose clearance. With 10 mM extracellular glucose and a high rate of glucose clearance, intracellular glucose level fell below 100 muM when glucose uptake rate was low, whereas it exceeded 0.5 mM when glucose uptake was high. Cells cultured in high glucose maintained lower basal intracellular glucose levels than cells cultured in low glucose, attributed to "reciprocal regulation" of glycolysis and gluconeogenesis. Basal glucose level also increased with elevated temperatures. Experiments performed with C2C12 cells demonstrated a shift from fast glucose uptake to slow glucose uptake in the absence of insulin during differentiation.

Vulnerability to re-entry in simulated two-dimensional cardiac tissue: effects of electrical restitution and stimulation sequence

Ventricular fibrillation is a lethal arrhythmia characterized by multiple wavelets usually starting from a single or figure-of-eight re-entrant circuit. Understanding the factors regulating vulnerability to the re-entry is essential for developing effective therapeutic strategies to prevent ventricular fibrillation. In this study, we investigated how pre-existing tissue heterogeneities and electrical restitution properties affect the initiation of re-entry by premature extrastimuli in two-dimensional cardiac tissue models. We studied two pacing protocols for inducing re-entry following the "sinus" rhythm (S1) beat: (1) a single premature (S2) extrastimulus in heterogeneous tissue; (2) two premature extrastimuli (S2 and S3) in homogeneous tissue. In the first case, the vulnerable window of re-entry is determined by the spatial dimension and extent of the heterogeneity, and is also affected by electrical restitution properties and the location of the premature stimulus. The vulnerable window first increases as the action potential duration (APD) difference between the inside and outside of the heterogeneous region increases, but then decreases as this difference increases further. Steeper APD restitution reduces the vulnerable window of re-entry. In the second case, electrical restitution plays an essential role. When APD restitution is flat, no re-entry can be induced. When APD restitution is steep, re-entry can be induced by an S3 over a range of S1S2 intervals, which is also affected by conduction velocity restitution. When APD restitution is even steeper, the vulnerable window is reduced due to collision of the spiral tips.

Cardiac alternans in embryonic mouse ventricles

T-wave alternans, an important arrhythmogenic factor, has recently been described in human fetuses. Here we sought to determine whether alternans can be induced in the embryonic mouse hearts, despite its underdeveloped sarcoplasmic reticulum (SR) and, if so, to analyze the response to pharmacological and autonomic interventions. Immunohistochemistry confirmed minimal sarcoplasmic-endoplasmic reticulum Ca-ATPase 2a expression in embryonic mouse hearts at embryonic day (E) 10.5 to E12.5, compared with neonatal or adult mouse hearts. We optically mapped voltage and/or intracellular Ca (Ca(i)) in 99 embryonic mouse hearts (dual mapping in 64 hearts) at these ages. Under control conditions, ventricular action potential duration (APD) and Ca(i) transient alternans occurred during rapid pacing at an average cycle length of 212 +/- 34 ms in 57% (n = 15/26) of E10.5-E12.5 hearts. Maximum APD restitution slope was steeper in hearts developing alternans than those that did not (2.2 +/- 0.6 vs. 0.8 +/- 0.4; P < 0.001). Disabling SR Ca(i) cycling with thapsigargin plus ryanodine did not significantly reduce alternans incidence (44%, n = 8/18, P = 0.5), whereas isoproterenol (n = 14) increased the incidence to 100% (P < 0.05), coincident with steepening APD restitution slope. Verapamil abolished Ca(i) transients (n = 9). Thapsigargin plus ryanodine had no major effects on Ca(i)-transient amplitude or its half time of recovery in E10.5 hearts, but significantly depressed Ca(i)-transient amplitude (by 47 +/- 8%) and prolonged its half time of recovery (by 18 +/- 3%) in E11.5 and older hearts. Embryonic mouse ventricles can develop cardiac alternans, which generally is well correlated with APD restitution slope and does not depend on fully functional SR Ca(i) cycling.

Dispersion of refractoriness and induction of reentry due to chaos synchronization in a model of cardiac tissue

Ventricular fibrillation is a lethal condition caused by multiple chaotically wandering electrical wavelets in the heart, reentering their own and each other's territories. The development of effective therapies requires a detailed understanding of how these reentrant waves are initiated. In this Letter, we demonstrate a novel mechanism for inducing reentry, in which chaos synchronization causes large-scale heterogeneities of refractoriness transverse to the direction of propagation. These regions of increased refractoriness create localized conduction block, which induces spiral wave reentry.

The pinwheel experiment revisited: effects of cellular electrophysiological properties on vulnerability to cardiac reentry

In normal heart, ventricular fibrillation can be induced by a single properly timed strong electrical or mechanical stimulus. A mechanism first proposed by Winfree and coined the “pinwheel experiment” emphasizes the timing and strength of the stimulus in inducing figure-of-eight reentry. However, the effects of cellular electrophysiological properties on vulnerability to reentry in the pinwheel scenario have not been investigated. In this study, we extend Winfree's pinwheel experiment to show how the vulnerability to reentry is affected by the graded action potential responses induced by a strong premature stimulus, action potential duration (APD), and APD restitution in simulated monodomain homogeneous two-dimensional tissue. We find that a larger graded response, longer APD, or steeper APD restitution slope reduces the vulnerable window of reentry. Strong graded responses and long APD promote tip-tip interactions at long coupling intervals, causing the two initiated spiral wave tips to annihilate. Steep APD restitution promotes wave front-wave back interaction, causing conduction block in the central common pathway of figure-of-eight reentry. We derive an analytical treatment that shows good agreement with numerical simulation results.

Calcium transient dynamics and the mechanisms of ventricular vulnerability to single premature electrical stimulation in Langendorff-perfused rabbit ventricles

BACKGROUND

Single strong premature electrical stimulation (S(2)) may induce figure-eight reentry. We hypothesize that Ca current-mediated slow-response action potentials (APs) play a key role in the propagation in the central common pathway (CCP) of the reentry.

METHODS

We simultaneously mapped optical membrane potential (V(m)) and intracellular Ca (Ca(i)) transients in isolated Langendorff-perfused rabbit ventricles. Baseline pacing (S(1)) and a cathodal S(2) (40-80 mA) were given at different epicardial sites with a coupling interval of 135 +/- 20 ms.

RESULTS

In all 6 hearts, S(2) induced graded responses around the S(2) site. These graded responses propagated locally toward the S(1) site and initiated fast APs from recovered tissues. The wavefront then circled around the refractory tissue near the site of S(2). At the side of S(2) opposite to the S(1), the graded responses prolonged AP duration while the Ca(i) continued to decline, resulting in a Ca(i) sinkhole (an area of low Ca(i)). The Ca(i) in the sinkhole then spontaneously increased, followed by a slow V(m) depolarization with a take-off potential of -40 +/- 3.9 mV, which was confirmed with microelectrode recordings in 3 hearts. These slow-response APs then propagated through CCP to complete a figure-eight reentry.

CONCLUSION

We conclude that a strong premature stimulus can induce a Ca(i) sinkhole at the entrance of the CCP. Spontaneous Ca(i) elevation in the Ca(i) sinkhole precedes the V(m) depolarization, leading to Ca current-mediated slow propagation in the CCP. The slow propagation allows more time for tissues at the other side of CCP to recover and be excited to complete figure-eight reentry.

Effects of metabolic inhibition on conduction, Ca transients, and arrhythmia vulnerability in embryonic mouse hearts

Developing myocardium is more dependent on glycolysis than adult myocardium, yet the effects of selectively inhibiting glycolysis versus oxidative phosphorylation on embryonic heart function have not been well characterized. Accordingly, we investigated how selective metabolic inhibition affects membrane voltage and intracellular Ca (Ca(i)) transients in embryonic mouse hearts, including their susceptibility to arrhythmias. A total of 136 isolated embryonic mouse hearts were exposed to either 1) 2-deoxyglucose (2DG; 10 mM) or iodoacetate (IAA; 0.1 mM) with 10 mM pyruvate in place of glucose to selectively inhibit glycolysis or 2) the mitochondrial uncoupler protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP; 500 nM) with 10 mM glucose present to selectively inhibit oxidative phosphorylation. Using confocal imaging, we found that mitochondrial membrane potential monitored with tetramethylrhodamine methyl ester (200 nM) remained stable with 2DG or IAA but depolarized within 5 min after exposure to FCCP. IAA and FCCP decreased heart rate, inhibited Ca(i) transient amplitude, shortened action potential duration at 80% repolarization (APD(80)), and prolonged atrioventricular conduction time to similar extents. Although 2DG decreased heart rate and Ca(i) transient amplitude, it did not significantly affect APD(80) and AV conduction time. In addition, spontaneous arrhythmias occurred in 77 of 136 embryonic hearts (57%) after exposure to IAA (28/53) or FCCP (49/83). There were no significant differences in the types or incidence of arrhythmias induced by IAA and FCCP. These data support the idea that both glycolysis and oxidative phosphorylation play critical metabolic roles in regulating cardiac function in the embryonic mouse heart.