Cardiac myocytes normally initiate action potentials in response to a current stimulus that depolarizes the membrane above an excitation threshold. Aberrant excitation can also occur due to spontaneous calcium (Ca(2+)) release (SCR) from... more
Cardiac myocytes normally initiate action potentials in response to a current stimulus that depolarizes the membrane above an excitation threshold. Aberrant excitation can also occur due to spontaneous calcium (Ca(2+)) release (SCR) from intracellular stores after the end of a preceding action potential. SCR drives the Na(+)/Ca(2+) exchange current inducing a "delayed afterdepolarization" that can in turn trigger an action potential if the excitation threshold is reached. This "triggered activity" is known to cause arrhythmias, but how it is initiated and terminated is not understood. Using computer simulations of a ventricular myocyte model, we show that initiation and termination are inherently random events. We determine the probability of those events from statistical measurements of the number of beats before initiation and before termination, respectively, which follow geometric distributions. Moreover, we elucidate the origin of randomness by a statistical...
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Background Remodeling of cardiac repolarizing currents, such as the downregulation of slowly activat- ing K+ channels (IKs), could underlie ventricular fibrillation (VF) in heart failure (HF). We evaluated the role of Iks remodeling in VF... more
Background
Remodeling of cardiac repolarizing currents, such as the downregulation of slowly activat- ing K+ channels (IKs), could underlie ventricular fibrillation (VF) in heart failure (HF). We evaluated the role of Iks remodeling in VF susceptibility using a tachypacing HF model of transgenic rabbits with Long QT Type 1 (LQT1) syndrome.
Methods and Results
LQT1 and littermate control (LMC) rabbits underwent three weeks of tachypacing to induce cardiac myopathy (TICM). In vivo telemetry demonstrated steepening of the QT/RR slope in LQT1 with TICM (LQT1-TICM; pre: 0.26±0.04, post: 0.52±0.01, P<0.05). In vivo electro- physiology showed that LQT1-TICM had higher incidence of VF than LMC-TICM (6 of 11 vs. 3 of 11, respectively). Optical mapping revealed larger APD dispersion (16±4 vs. 38±6 ms, p<0.05) and steep APD restitution in LQT1-TICM compared to LQT1-sham (0.53±0.12 vs. 1.17±0.13, p<0.05). LQT1-TICM developed spatially discordant alternans (DA), which caused conduction block and higher-frequency VF (15±1 Hz in LQT1-TICM vs. 13±1 Hz in LMC-TICM, p<0.05). Ca2+ DA was highly dynamic and preceded voltage DA in LQT1- TICM. Ryanodine abolished DA in 5 out of 8 LQT1-TICM rabbits, demonstrating the impor- tance of Ca2+ in complex DA formation. Computer simulations suggested that HF remodel- ing caused Ca2+-driven alternans, which was further potentiated in LQT1-TICM due to the lack of IKs.
Conclusions
Compared with LMC-TICM, LQT1-TICM rabbits exhibit steepened APD restitution and com- plex DA modulated by Ca2+. Our results strongly support the contention that the downregu- lation of IKs in HF increases Ca2+ dependent alternans and thereby the risk of VF.
Remodeling of cardiac repolarizing currents, such as the downregulation of slowly activat- ing K+ channels (IKs), could underlie ventricular fibrillation (VF) in heart failure (HF). We evaluated the role of Iks remodeling in VF susceptibility using a tachypacing HF model of transgenic rabbits with Long QT Type 1 (LQT1) syndrome.
Methods and Results
LQT1 and littermate control (LMC) rabbits underwent three weeks of tachypacing to induce cardiac myopathy (TICM). In vivo telemetry demonstrated steepening of the QT/RR slope in LQT1 with TICM (LQT1-TICM; pre: 0.26±0.04, post: 0.52±0.01, P<0.05). In vivo electro- physiology showed that LQT1-TICM had higher incidence of VF than LMC-TICM (6 of 11 vs. 3 of 11, respectively). Optical mapping revealed larger APD dispersion (16±4 vs. 38±6 ms, p<0.05) and steep APD restitution in LQT1-TICM compared to LQT1-sham (0.53±0.12 vs. 1.17±0.13, p<0.05). LQT1-TICM developed spatially discordant alternans (DA), which caused conduction block and higher-frequency VF (15±1 Hz in LQT1-TICM vs. 13±1 Hz in LMC-TICM, p<0.05). Ca2+ DA was highly dynamic and preceded voltage DA in LQT1- TICM. Ryanodine abolished DA in 5 out of 8 LQT1-TICM rabbits, demonstrating the impor- tance of Ca2+ in complex DA formation. Computer simulations suggested that HF remodel- ing caused Ca2+-driven alternans, which was further potentiated in LQT1-TICM due to the lack of IKs.
Conclusions
Compared with LMC-TICM, LQT1-TICM rabbits exhibit steepened APD restitution and com- plex DA modulated by Ca2+. Our results strongly support the contention that the downregu- lation of IKs in HF increases Ca2+ dependent alternans and thereby the risk of VF.
Research Interests:
Phospholamban (PLB) inhibits the activity of cardiac sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a). Phos- phorylation of PLB during sympathetic activation reverses SERCA2a inhibition, increasing SR Ca2+ uptake. How- ever, sympathetic... more
Phospholamban (PLB) inhibits the activity of cardiac sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2a). Phos- phorylation of PLB during sympathetic activation reverses SERCA2a inhibition, increasing SR Ca2+ uptake. How- ever, sympathetic activation also modulates multiple other intracellular targets in ventricular myocytes (VMs), making it impossible to determine the specific effects of the reversal of PLB inhibition on the spontaneous SR Ca2 + release. Therefore, it remains unclear how PLB regulates rhythmic activity in VMs.
Here, we used the Fab fragment of 2D12, a monoclonal anti-PLB antibody, to test how acute reversal of PLB inhi- bition affects the spontaneous SR Ca2 + release in normal VMs. Ca2 + sparks and spontaneous Ca2 + waves (SCWs) were recorded in the line-scan mode of confocal microscopy using the Ca2+ fluorescent dye Fluo-4 in isolated permeabilized mouse VMs. Fab, which reverses PLB inhibition, significantly increased the frequency, amplitude, and spatial/temporal spread of Ca2+ sparks in VMs exposed to 50 nM free [Ca2+]. At physiological diastolic free [Ca2+] (100–200 nM), Fab facilitated the formation of whole-cell propagating SCWs. At higher free [Ca2+], Fab increased the frequency and velocity, but decreased the decay time of the SCWs. cAMP had little additional effect on the frequency or morphology of Ca2 + sparks or SCWs after Fab addition. These findings were complemented by computer simulations. In conclusion, acute reversal of PLB inhibition alone significantly increased the sponta- neous SR Ca2 + release, leading to the facilitation and organization of whole-cell propagating SCWs in normal VMs. PLB thus plays a key role in subcellular Ca2+ dynamics and rhythmic activity of VMs.
Here, we used the Fab fragment of 2D12, a monoclonal anti-PLB antibody, to test how acute reversal of PLB inhi- bition affects the spontaneous SR Ca2 + release in normal VMs. Ca2 + sparks and spontaneous Ca2 + waves (SCWs) were recorded in the line-scan mode of confocal microscopy using the Ca2+ fluorescent dye Fluo-4 in isolated permeabilized mouse VMs. Fab, which reverses PLB inhibition, significantly increased the frequency, amplitude, and spatial/temporal spread of Ca2+ sparks in VMs exposed to 50 nM free [Ca2+]. At physiological diastolic free [Ca2+] (100–200 nM), Fab facilitated the formation of whole-cell propagating SCWs. At higher free [Ca2+], Fab increased the frequency and velocity, but decreased the decay time of the SCWs. cAMP had little additional effect on the frequency or morphology of Ca2 + sparks or SCWs after Fab addition. These findings were complemented by computer simulations. In conclusion, acute reversal of PLB inhibition alone significantly increased the sponta- neous SR Ca2 + release, leading to the facilitation and organization of whole-cell propagating SCWs in normal VMs. PLB thus plays a key role in subcellular Ca2+ dynamics and rhythmic activity of VMs.
Research Interests:
Background—Computer simulations have predicted that the balance of various electrogenic sarcolemmal ion currents may control the amplitude and phase of beat-to-beat alternans of membrane potential (Vm). However, experimental evidence for... more
Background—Computer simulations have predicted that the balance of various electrogenic sarcolemmal ion currents may control the amplitude and phase of beat-to-beat alternans of membrane potential (Vm). However, experimental evidence for the mechanism by which alternans of calcium transients produces alternation of Vm (Vm-ALT) is lacking.
Objective—We sought to provide experimental evidence that Ca-to-Vm coupling during alternans is determined by the balanced influence of two Ca-sensitive electrogenic sarcolemmal ionic currents, INCX and ICa.
Methods and Results—Vm-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n=41) using perforated patch and Indo-1AM fluorescence, respectively. There were three study groups: 1) Control, 2) INCX predominance created by adenoviral-induced NCX overexpression, and 3) ICa predominance created by INCX inhibition (SEA-0400) or enhanced ICa (As2O3). During alternans, 14 of 14 control myocytes demonstrated positive Ca-to- Vm coupling, consistent with INCX, but not ICa as the major electrogenic current in modulating action potential duration. Positive Ca-to-Vm coupling was maintained during INCX predominance in 8 of 8 experiments with concurrent increase in Ca-to-Vm gain (p<0.05), reaffirming the role of increased forward mode electrogenic INCX. Conversely, ICa predominance produced negative Ca- to-Vm coupling in 14 of 19 myocytes (p<0.05) and decreased Ca-to-Vm gain compared to control (p<0.05). Furthermore, computer simulation demonstrated that Ca-to-Vm coupling changes from negative to positive was due to a shift from ICa to INCX predominance with increasing pacing rate.
Objective—We sought to provide experimental evidence that Ca-to-Vm coupling during alternans is determined by the balanced influence of two Ca-sensitive electrogenic sarcolemmal ionic currents, INCX and ICa.
Methods and Results—Vm-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n=41) using perforated patch and Indo-1AM fluorescence, respectively. There were three study groups: 1) Control, 2) INCX predominance created by adenoviral-induced NCX overexpression, and 3) ICa predominance created by INCX inhibition (SEA-0400) or enhanced ICa (As2O3). During alternans, 14 of 14 control myocytes demonstrated positive Ca-to- Vm coupling, consistent with INCX, but not ICa as the major electrogenic current in modulating action potential duration. Positive Ca-to-Vm coupling was maintained during INCX predominance in 8 of 8 experiments with concurrent increase in Ca-to-Vm gain (p<0.05), reaffirming the role of increased forward mode electrogenic INCX. Conversely, ICa predominance produced negative Ca- to-Vm coupling in 14 of 19 myocytes (p<0.05) and decreased Ca-to-Vm gain compared to control (p<0.05). Furthermore, computer simulation demonstrated that Ca-to-Vm coupling changes from negative to positive was due to a shift from ICa to INCX predominance with increasing pacing rate.
Research Interests:
In heart failure (HF), T-tubule (TT) disruption contributes to dyssynchronous calcium (Ca) release and impaired contraction, but its role in arrhythmogenesis remains unclear. In this study, we investigate the effects of TT dis- ruption... more
In heart failure (HF), T-tubule (TT) disruption contributes to dyssynchronous calcium (Ca) release and impaired contraction, but its role in arrhythmogenesis remains unclear. In this study, we investigate the effects of TT dis- ruption and other HF remodeling factors on Ca alternans in ventricular myocytes using computer modeling. A ventricular myocyte model with detailed spatiotemporal Ca cycling modeled by a coupled Ca release unit (CRU) network was used, in which the L-type Ca channels and the ryanodine receptor (RyR) channels were sim- ulated by random Markov transitions. TT disruption, which removes the L-type Ca channels from the associated CRUs, results in “orphaned” RyR clusters and thus provides increased opportunity for spark-induced Ca sparks to occur. This effect combined with other HF remodeling factors promoted alternans by two distinct mechanisms: 1) for normal sarco-endoplasmic reticulum Ca ATPase (SERCA) activity, alternans was caused by both CRU refrac- toriness and coupling. The increased opportunity for spark-induced sparks by TT disruption combined with the enhanced CRU coupling by Ca elevation in the presence or absence of increased RyR leakiness facilitated spark synchronization on alternate beats to promote Ca alternans; 2) for down-regulated SERCA, alternans was caused by the sarcoplasmic reticulum (SR) Ca load-dependent mechanism, independent of CRU refractoriness. TT dis- ruption and increased RyR leakiness shifted and steepened the SR Ca release-load relationship, which combines with down-regulated SERCA to promote Ca alternans. In conclusion, the mechanisms of Ca alternans for normal and down-regulated SERCA are different, and TT disruption promotes Ca alternans by both mechanisms, which may contribute to alternans at different stages of HF.
Research Interests:
Early afterdepolarizations (EADs) and delayed afterdepolarizations (DADs) are voltage oscillations known to cause cardiac arrhythmias. EADs are mainly driven by voltage oscillations in the repolarizing phase of the action potential (AP),... more
Early afterdepolarizations (EADs) and delayed afterdepolarizations (DADs) are voltage oscillations known to cause cardiac arrhythmias. EADs are mainly driven by voltage oscillations in the repolarizing phase of the action potential (AP), while DADs are driven by spontaneous calcium (Ca) release during diastole. Because voltage and Ca are bidirectionally coupled, they modulate each other’s behaviors, and new AP and Ca cycling dynamics can emerge from this coupling. In this study, we performed computer simulations using an AP model with detailed spatiotemporal Ca cycling incorporating stochastic openings of Ca channels and ryanodine receptors to investigate the effects of Ca-voltage coupling on EAD and DAD dynamics. Simulations were complemented by experiments in mouse ventricular myocytes. We show that: 1) alteration of the Ca transient due to increased ryanodine receptor leakiness and/or sarco/endoplasmic reticulum Ca ATPase activity can either promote or suppress EADs due to the complex effects of Ca on ionic current properties; 2) spontaneous Ca waves also exhibit complex effects on EADs, but cannot induce EADs of significant amplitude without the participation of ICa,L; 3) lengthening AP duration and the occurrence of EADs promote DADs by increasing intracellular Ca loading, and two mechanisms of DADs are identified, i.e., Ca-wave-dependent and Ca-wave-independent; and 4) Ca-voltage coupling promotes complex EAD patterns such as EAD alternans that are not observed for solely voltage-driven EADs. In conclusion, Ca-voltage coupling combined with the nonlinear dynamical behaviors of voltage and Ca cycling play a key role in generating complex EAD and DAD dynamics observed exper- imentally in cardiac myocytes, whose mechanisms are complex but analyzable.
Research Interests:
Background—Hypokalemia is known to promote ventricular arrhythmias, especially in combination with Class III antiarrhythmic drugs like dofetilide. Here we evaluated the underlying molecular mechanisms. Methods and Results—Arrhythmias... more
Background—Hypokalemia is known to promote ventricular arrhythmias, especially in combination with Class III antiarrhythmic drugs like dofetilide. Here we evaluated the underlying molecular mechanisms.
Methods and Results—Arrhythmias were recorded in isolated rabbit and rat hearts or patch-clamped ventricular myocytes exposed to hypokalemia (1.0-3.5 mmol/l) in the absence or presence of dofetilide (1 µmol/l). Spontaneous early afterdepolarizations (EADs) and ventricular tachycardia/fibrillation (VF/VF) occurred in 50% of hearts at 2.7 mmol/l [K] in the absence of dofetilide, and 3.3 mmol/l [K] in its presence. Pre-treatment with the CaMKII inhibitor KN-93, but not its inactive analogue KN-92, abolished EADs and hypokalemia-induced VT/VF, as did the selective late Na current (INa) blocker GS-967. In intact hearts, moderate hypokalemia (2.7 mmol/l) significantly increased tissue CaMKII activity. Computer modeling revealed that EAD generation by hypokalemia (with or without dofetilide) required Na-K pump inhibition to induce intracellular Na and Ca overload with consequent CaMKII activation enhancing late INa and the L-type Ca current. K current suppression by hypokalemia and/or dofetilide alone in the absence of CaMKII activation were ineffective at causing EADs.
Conclusions—We conclude that Na-K pump inhibition by even moderate hypokalemia plays a critical role in promoting EAD-mediated arrhythmias by inducing a positive feedback cycle activating CaMKII and enhancing late INa. Class III antiarrhythmic drugs like dofetilide sensitize the heart to this positive feedback loop.
Methods and Results—Arrhythmias were recorded in isolated rabbit and rat hearts or patch-clamped ventricular myocytes exposed to hypokalemia (1.0-3.5 mmol/l) in the absence or presence of dofetilide (1 µmol/l). Spontaneous early afterdepolarizations (EADs) and ventricular tachycardia/fibrillation (VF/VF) occurred in 50% of hearts at 2.7 mmol/l [K] in the absence of dofetilide, and 3.3 mmol/l [K] in its presence. Pre-treatment with the CaMKII inhibitor KN-93, but not its inactive analogue KN-92, abolished EADs and hypokalemia-induced VT/VF, as did the selective late Na current (INa) blocker GS-967. In intact hearts, moderate hypokalemia (2.7 mmol/l) significantly increased tissue CaMKII activity. Computer modeling revealed that EAD generation by hypokalemia (with or without dofetilide) required Na-K pump inhibition to induce intracellular Na and Ca overload with consequent CaMKII activation enhancing late INa and the L-type Ca current. K current suppression by hypokalemia and/or dofetilide alone in the absence of CaMKII activation were ineffective at causing EADs.
Conclusions—We conclude that Na-K pump inhibition by even moderate hypokalemia plays a critical role in promoting EAD-mediated arrhythmias by inducing a positive feedback cycle activating CaMKII and enhancing late INa. Class III antiarrhythmic drugs like dofetilide sensitize the heart to this positive feedback loop.