Sarcoplasmic reticulum Ca2+ refilling controls recovery from Ca2+-induced Ca2+ release refractoriness in heart muscle

Predicting local SR Ca(2+) dynamics during Ca(2+) wave propagation in ventricular myocytes

Of the many ongoing controversies regarding the workings of the sarcoplasmic reticulum (SR) in cardiac myocytes, two unresolved and interconnected topics are 1), mechanisms of calcium (Ca(2+)) wave propagation, and 2), speed of Ca(2+) diffusion within the SR. Ca(2+) waves are initiated when a spontaneous local SR Ca(2+) release event triggers additional release from neighboring clusters of SR release channels (ryanodine receptors (RyRs)). A lack of consensus regarding the effective Ca(2+) diffusion constant in the SR (D(Ca,SR)) severely complicates our understanding of whether dynamic local changes in SR [Ca(2+)] can influence wave propagation. To address this problem, we have implemented a computational model of cytosolic and SR [Ca(2+)] during Ca(2+) waves. Simulations have investigated how dynamic local changes in SR [Ca(2+)] are influenced by 1), D(Ca,SR); 2), the distance between RyR clusters; 3), partial inhibition or stimulation of SR Ca(2+) pumps; 4), SR Ca(2+) pump dependence on cytosolic [Ca(2+)]; and 5), the rate of transfer between network and junctional SR. Of these factors, D(Ca,SR) is the primary determinant of how release from one RyR cluster alters SR [Ca(2+)] in nearby regions. Specifically, our results show that local increases in SR [Ca(2+)] ahead of the wave can potentially facilitate Ca(2+) wave propagation, but only if SR diffusion is relatively slow. These simulations help to delineate what changes in [Ca(2+)] are possible during SR Ca(2+)release, and they broaden our understanding of the regulatory role played by dynamic changes in [Ca(2+)](SR).

Models of cardiac excitation-contraction coupling in ventricular myocytes

Mathematical and computational modeling of cardiac excitation-contraction coupling has produced considerable insights into how the heart muscle contracts. With the increase in biophysical and physiological data available, the modeling has become more sophisticated with investigations spanning in scale from molecular components to whole cells. These modeling efforts have provided insight into cardiac excitation-contraction coupling that advanced and complemented experimental studies. One goal is to extend these detailed cellular models to model the whole heart. While this has been done with mechanical and electrophysiological models, the complexity and fast time course of calcium dynamics have made inclusion of detailed calcium dynamics in whole heart models impractical. Novel methods such as the probability density approach and moment closure technique which increase computational efficiency might make this tractable.

Frequency and release flux of calcium sparks in rat cardiac myocytes: a relation to RYR gating

Cytosolic calcium concentration in resting cardiac myocytes locally fluctuates as a result of spontaneous microscopic Ca²⁺ releases or abruptly rises as a result of an external trigger. These processes, observed as calcium sparks, are fundamental for proper function of cardiac muscle. In this study, we analyze how the characteristics of spontaneous and triggered calcium sparks are related to cardiac ryanodine receptor (RYR) gating. We show that the frequency of spontaneous sparks and the probability distribution of calcium release flux quanta of triggered sparks correspond quantitatively to predictions of an allosteric homotetrameric model of RYR gating. This model includes competitive binding of Ca²⁺ and Mg²⁺ ions to the RYR activation sites and allosteric interaction between divalent ion binding and channel opening. It turns out that at rest, RYRs are almost fully occupied by Mg²⁺. Therefore, spontaneous sparks are most frequently evoked by random openings of the highly populated but rarely opening Mg₄RYR and CaMg₃RYR forms, whereas triggered sparks are most frequently evoked by random openings of the less populated but much more readily opening Ca₂Mg₂RYR and Ca₃MgRYR forms. In both the spontaneous and the triggered sparks, only a small fraction of RYRs in the calcium release unit manages to open during the spark because of the limited rate of Mg²⁺ unbinding. This mechanism clarifies the unexpectedly low calcium release flux during elementary release events and unifies the theory of calcium signaling in resting and contracting cardiac myocytes.

Hypoxia-induced cytosolic calcium decrease is mediated primarily by the forward mode of Na(+)/Ca(2+) exchanger in smooth muscle cells of fetal ductus arteriosus

Closure of the ductus arteriosus (DA) after birth, essential for postnatal adaptation, is initiated by the transition from hypoxia to normoxia. The current study investigated how hypoxia affects the level of cytosolic calcium ([Ca(2+)](i)) in fetal lamb DA smooth muscle cells (DASMCs) and the role of calcium pumps in this process. The [Ca(2+)](i) variation in response to acute hypoxia was determined spectrofluorometrically with fura-3-AM in cultured fetal DASMCs. Interventions using chemicals or solutions including thapsigargin, vanadate, KB-R7943, alkaline PH9.0 solution, or Na(+)-free medium were administered when samples were exposed to acute hypoxia. The results show that [Ca(2+)](i) decreased dramatically under acute hypoxia. This decrease was not attenuated completely by an inhibitor of sarcoplasmic/endoplasmic reticulum Ca(2+) adenosine triphosphatase (ATPase) (SERCA), a blocker of plasma membrane Ca(2+) ATPase (PMCA), or an inhibitor and activator of the reserve mode of the Na(+)/Ca(2+) exchanger (NCX). In contrast, KT-R9743, an inhibitor of the forward mode of NCX at a high concentration (30 microm), greatly diminished the hypoxia-induced [Ca(2+)](i) decrease in fetal DASMCs. These results suggest that a hypoxia-induced Ca(2+) decrease in fetal DASMCs results from cytosolic Ca(2+) efflux mediated primarily by the forward mode of NCX.

Cardiac calsequestrin: the new kid on the block in arrhythmias

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare inherited disease characterized by physical or emotional stress-induced ventricular arrhythmias in the absence of any structural heart disease or QT prolongation. Thus far, mutations in genes encoding the sarcoplasmic reticulum Ca(2+) release channel (RYR2) and the sarcoplasmic reticulum Ca(2+) binding protein cardiac calsequestrin (CASQ2) have been identified in CPVT patients. Here, we review the role of cardiac calsequestrin in health and disease, with a particular focus on how calsequestrin deficiency can cause arrhythmia susceptibility. Clinical implications and a promising new drug therapy for CPVT are discussed.

Modulation of the local SR Ca2+ release by intracellular Mg2+ in cardiac myocytes

In cardiac muscle, Ca²⁺-induced Ca²⁺ release (CICR) from the sarcoplasmic reticulum (SR) defines the amplitude and time course of the Ca²⁺ transient. The global elevation of the intracellular Ca²⁺ concentration arises from the spatial and temporal summation of elementary Ca²⁺ release events, Ca²⁺ sparks. Ca²⁺ sparks represent the concerted opening of a group of ryanodine receptors (RYRs), which are under the control of several modulatory proteins and diffusible cytoplasmic factors (e.g., Ca²⁺, Mg²⁺, and ATP). Here, we examined by which mechanism the free intracellular Mg²⁺ ([Mg²⁺]_free) affects various Ca²⁺ spark parameters in permeabilized mouse ventricular myocytes, such as spark frequency, duration, rise time, and full width, at half magnitude and half maximal duration. Varying the levels of free ATP and Mg²⁺ in specifically designed solutions allowed us to separate the inhibition of RYRs by Mg²⁺ from the possible activation by ATP and Mg²⁺-ATP via the adenine binding site of the channel. Changes in [Mg²⁺]_free generally led to biphasic alterations of the Ca²⁺ spark frequency. For example, lowering [Mg²⁺]_free resulted in an abrupt increase of spark frequency, which slowly recovered toward the initial level, presumably as a result of SR Ca²⁺ depletion. Fitting the Ca²⁺ spark inhibition by [Mg²⁺]_free with a Hill equation revealed a K_i of 0.1 mM. In conclusion, our results support the notion that local Ca²⁺ release and Ca²⁺ sparks are modulated by Mg²⁺ in the intracellular environment. This seems to occur predominantly by hindering Ca²⁺-dependent activation of the RYRs through competitive Mg²⁺ occupancy of the high-affinity activation site of the channels. These findings help to characterize CICR in cardiac muscle under normal and pathological conditions, where the levels of Mg²⁺ and ATP can change.

Studies of RyR function in situ

The ryanodine receptors (RyRs) are intracellular Ca2+ release channels of the sarcoplasmic reticulum (SR) involved in many cellular responses, including muscle excitation-contraction coupling. Multiple biochemical and biophysical methods are available to study RyR functions. However, most of them are somewhat limited because they can only be used to examine channels which are purified from the SR and no longer in their natural environment. In this review we discuss optical methods for studying RyR functions in situ. We describe several techniques for the investigation of local (microscopic) intracellular Ca2+ signals (a.k.a Ca2+ sparks) by means of confocal microscopy and flash photolysis of caged compounds. We discuss how these studies can and will continue to contribute to our understanding of RyR function in physiological and pathological conditions.

Molecular basis of catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a malignant arrhythmia syndrome linked to mutations in the cardiac ryanodine receptor (RyR2) and calsequestrin (CASQ2). RyR2 and CASQ2 are parts of the multimolecular Ca(2+) release channel complex that is present on the sarcoplasmic reticulum (SR) to support myocyte Ca(2+) cycling and contractile activity. Whereas RyR2 operates as a Ca(2+) release channel, the SR Ca(2+) binding protein CASQ2 plays a dual role by serving as a SR Ca(2+) buffer and by regulating RyR2 function. Essential to stable Ca(2+) cycling, SR luminal Ca(2+)-dependent control of RyR2 activity by CASQ2 contributes to RyR2 deactivation and to the development of a temporary refractory state that occurs after each Ca(2+) release. Accumulating evidence suggests that the CPVT mutations act by reducing the extent and shortening the duration of Ca(2+) signaling refractoriness, thereby promoting untimely SR Ca(2+) release and arrhythmogenic delayed afterdepolarizations in cardiac myocytes. Similar mechanisms may apply to arrhythmias during various conditions, including heart failure and ischemic heart disease, associated with acquired defects in components of the Ca(2+) release channel complex.

Dysregulated sarcoplasmic reticulum calcium release: potential pharmacological target in cardiac disease

In the heart, Ca(2+) released from the intracellular Ca(2+) storage site, the sarcoplasmic reticulum (SR), is the principal determinant of cardiac contractility. SR Ca(2+) release is controlled by dedicated molecular machinery, composed of the cardiac ryanodine receptor (RyR2) and a number of accessory proteins, including FKBP12.6, calsequestrin (CASQ2), triadin (TRD) and junctin (JN). Acquired and genetic defects in the components of the release channel complex result in a spectrum of abnormal Ca(2+) release phenotypes ranging from arrhythmogenic spontaneous Ca(2+) releases and Ca(2+) alternans to the uniformly diminished systolic Ca(2+) release characteristic of heart failure. In this article, we will present an overview of the structure and molecular components of the SR and Ca(2+) release machinery and its modulation by different intracellular factors, such as Ca(2+) levels inside the SR as well as phosphorylation and redox modification of RyR2s. We will also discuss the relationships between abnormal SR Ca(2+) release and various cardiac disease phenotypes, including, arrhythmias and heart failure, and consider SR Ca(2+) release as a potential therapeutic target.

Moment closure for local control models of calcium-induced calcium release in cardiac myocytes

In prior work, we introduced a probability density approach to modeling local control of Ca2+-induced Ca2+ release in cardiac myocytes, where we derived coupled advection-reaction equations for the time-dependent bivariate probability density of subsarcolemmal subspace and junctional sarcoplasmic reticulum (SR) [Ca2+] conditioned on Ca2+ release unit (CaRU) state. When coupled to ordinary differential equations (ODEs) for the bulk myoplasmic and network SR [Ca2+], a realistic but minimal model of cardiac excitation-contraction coupling was produced that avoids the computationally demanding task of resolving spatial aspects of global Ca2+ signaling, while accurately representing heterogeneous local Ca2+ signals in a population of diadic subspaces and junctional SR depletion domains. Here we introduce a computationally efficient method for simulating such whole cell models when the dynamics of subspace [Ca2+] are much faster than those of junctional SR [Ca2+]. The method begins with the derivation of a system of ODEs describing the time-evolution of the moments of the univariate probability density functions for junctional SR [Ca2+] jointly distributed with CaRU state. This open system of ODEs is then closed using an algebraic relationship that expresses the third moment of junctional SR [Ca2+] in terms of the first and second moments. In simulated voltage-clamp protocols using 12-state CaRUs that respond to the dynamics of both subspace and junctional SR [Ca2+], this moment-closure approach to simulating local control of excitation-contraction coupling produces high-gain Ca2+ release that is graded with changes in membrane potential, a phenomenon not exhibited by common pool models. Benchmark simulations indicate that the moment-closure approach is nearly 10,000-times more computationally efficient than corresponding Monte Carlo simulations while leading to nearly identical results. We conclude by applying the moment-closure approach to study the restitution of Ca2+-induced Ca2+ release during simulated two-pulse voltage-clamp protocols.

Modulation of SR Ca release by luminal Ca and calsequestrin in cardiac myocytes: effects of CASQ2 mutations linked to sudden cardiac death

Cardiac calsequestrin (CASQ2) is an intrasarcoplasmic reticulum (SR) low-affinity Ca-binding protein, with mutations that are associated with catecholamine-induced polymorphic ventricular tachycardia (CPVT). To better understand how CASQ2 mutants cause CPVT, we expressed two CPVT-linked CASQ2 mutants, a truncated protein (at G112+5X, CASQ2(DEL)) or CASQ2 containing a point mutation (CASQ2(R33Q)), in canine ventricular myocytes and assessed their effects on Ca handling. We also measured CASQ2-CASQ2 variant interactions using fluorescence resonance transfer in a heterologous expression system, and evaluated CASQ2 interaction with triadin. We found that expression of CASQ2(DEL) or CASQ2(R33Q) altered myocyte Ca signaling through two different mechanisms. Overexpressing CASQ2(DEL) disrupted the CASQ2 polymerization required for high capacity Ca binding, whereas CASQ2(R33Q) compromised the ability of CASQ2 to control ryanodine receptor (RyR2) channel activity. Despite profound differences in SR Ca buffering strengths, local Ca release terminated at the same free luminal [Ca] in control cells, cells overexpressing wild-type CASQ2 and CASQ2(DEL)-expressing myocytes, suggesting that a decline in [Ca](SR) is a signal for RyR2 closure. Importantly, disrupting interactions between the RyR2 channel and CASQ2 by expressing CASQ2(R33Q) markedly lowered the [Ca](SR) threshold for Ca release termination. We conclude that CASQ2 in the SR determines the magnitude and duration of Ca release from each SR terminal by providing both a local source of releasable Ca and by effects on luminal Ca-dependent RyR2 gating. Furthermore, two CPVT-inducing CASQ2 mutations, which cause mechanistically different defects in CASQ2 and RyR2 function, lead to increased diastolic SR Ca release events and exhibit a similar CPVT disease phenotype.

Unraveling the mechanisms of catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a heritable arrhythmia unmasked by exertion or stress, characterized by triggered activity and sudden cardiac death in affected patients. In this study we used a mathematical model to simulate two mutations linked toCPVT, in cardiac calsequestrin (CSQN2) and the ryanodine receptor (RyR2). The aim of the present study is to characterize the mutations responsible for CPVT and establish the mechanistic basis for spontaneous Ca2+ release events that lead to delayed afterdepolarizations (DADs) and triggered arrhythmias. Simulated calcium transients in the mutant CSQN2 model recapitulated the smaller amplitude and time to peak, as well as accelerated recovery from inactivation seen in experiments. When simulated CSQN2-mutant myocytes were paced in current-clamp mode, DADs were observed, suggesting that accelerated recovery of RyR2 induced by impaired luminal Ca2(+) sensing can lead to the triggered activity observed in the mutant CSQN2. Simulations of mutant RyR2 suggest that the hyperactive, "leaky" receptors characteristic of reduced FKBP12.6 function may be centrally involved in triggering DADs. These results provide plausible mechanisms by which defects in RyR2 gating may lead to the cellular triggers of CPVT, with implications for the development of targeted therapies.

Dystrophic cardiomyopathy: amplification of cellular damage by Ca2+ signalling and reactive oxygen species-generating pathways

AIMS

Cardiac myopathies are the second leading cause of death in patients with Duchenne and Becker muscular dystrophy, the two most common and severe forms of a disabling striated muscle disease. Although the genetic defect has been identified as mutations of the dystrophin gene, very little is known about the molecular and cellular events leading to progressive cardiac muscle damage. Dystrophin is a protein linking the cytoskeleton to a complex of transmembrane proteins that interact with the extracellular matrix. The fragility of the cell membrane resulting from the lack of dystrophin is thought to cause an excessive susceptibility to mechanical stress. Here, we examined cellular mechanisms linking the initial membrane damage to the dysfunction of dystrophic heart.

METHODS AND RESULTS

Cardiac ventricular myocytes were enzymatically isolated from 5- to 9-month-old dystrophic mdx and wild-type (WT) mice. Cells were exposed to mechanical stress, applied as osmotic shock. Stress-induced cytosolic and mitochondrial Ca(2+) signals, production of reactive oxygen species (ROS), and mitochondrial membrane potential were monitored with confocal microscopy and fluorescent indicators. Pharmacological tools were used to scavenge ROS and to identify their possible sources. Osmotic shock triggered excessive cytosolic Ca(2+) signals, often lasting for several minutes, in 82% of mdx cells. In contrast, only 47% of the WT cardiomyocytes responded with transient and moderate intracellular Ca(2+) signals. On average, the reaction was 6-fold larger in mdx cells. Removal of extracellular Ca(2+) abolished these responses, implicating Ca(2+) influx as a trigger for abnormal Ca(2+) signalling. Our further experiments revealed that osmotic stress in mdx cells produced an increase in ROS production and mitochondrial Ca(2+) overload. The latter was followed by collapse of the mitochondrial membrane potential, an early sign of cell death.

CONCLUSION

Overall, our findings reveal that excessive intracellular Ca(2+) signals and ROS generation link the initial sarcolemmal injury to mitochondrial dysfunctions. The latter possibly contribute to the loss of functional cardiac myocytes and heart failure in dystrophy. Understanding the sequence of events of dystrophic cell damage and the deleterious amplification systems involved, including several positive feed-back loops, may allow for a rational development of novel therapeutic strategies.

AKAP complex regulates Ca2+ re-uptake into heart sarcoplasmic reticulum

The beta-adrenergic receptor/cyclic AMP/protein kinase A (PKA) signalling pathway regulates heart rate and contractility. Here, we identified a supramolecular complex consisting of the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2), its negative regulator phospholamban (PLN), the A-kinase anchoring protein AKAP18delta and PKA. We show that AKAP18delta acts as a scaffold that coordinates PKA phosphorylation of PLN and the adrenergic effect on Ca(2+) re-uptake. Inhibition of the compartmentalization of this cAMP signalling complex by specific molecular disruptors interferes with the phosphorylation of PLN. This prevents the subsequent release of PLN from SERCA2, thereby affecting the Ca(2+) re-uptake into the sarcoplasmic reticulum induced by adrenergic stimuli.

Regulation of Ca2+ and electrical alternans in cardiac myocytes: role of CAMKII and repolarizing currents

Alternans of cardiac repolarization is associated with arrhythmias and sudden death. At the cellular level, alternans involves beat-to-beat oscillation of the action potential (AP) and possibly Ca(2+) transient (CaT). Because of experimental difficulty in independently controlling the Ca(2+) and electrical subsystems, mathematical modeling provides additional insights into mechanisms and causality. Pacing protocols were conducted in a canine ventricular myocyte model with the following results: 1) CaT alternans results from refractoriness of the sarcoplasmic reticulum Ca(2+) release system; alternation of the L-type calcium current has a negligible effect; 2) CaT-AP coupling during late AP occurs through the sodium-calcium exchanger and underlies AP duration (APD) alternans; 3) increased Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity extends the range of CaT and APD alternans to slower frequencies and increases alternans magnitude; its decrease suppresses CaT and APD alternans, exerting an antiarrhythmic effect; and 4) increase of the rapid delayed rectifier current (I(Kr)) also suppresses APD alternans but without suppressing CaT alternans. Thus CaMKII inhibition eliminates APD alternans by eliminating its cause (CaT alternans) while I(Kr) enhancement does so by weakening CaT-APD coupling. The simulations identify combined CaMKII inhibition and I(Kr) enhancement as a possible antiarrhythmic intervention.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Ca2+ release from the sarcoplasmic reticulum activated by the low affinity Ca2+ chelator TPEN in ventricular myocytes

The Ca2+ content of the sarcoplasmic reticulum (SR) of cardiac myocytes is thought to play a role in the regulation and termination of SR Ca2+ release through the ryanodine receptors (RyRs). Experimentally altering the amount of Ca2+ within the SR with the membrane-permeant low affinity Ca2+ chelator TPEN could improve our understanding of the mechanism(s) by which SR Ca2+ content and SR Ca2+ depletion can influence Ca2+ release sensitivity and termination. We applied laser-scanning confocal microscopy to examine SR Ca2+ release in freshly isolated ventricular myocytes loaded with fluo-3, while simultaneously recording membrane currents using the whole-cell patch-clamp technique. Following application of TPEN, local spontaneous Ca2+ releases increased in frequency and developed into cell-wide Ca2+ waves. SR Ca2+ load after TPEN application was found to be reduced to about 60% of control. Isolated cardiac RyRs reconstituted into lipid bilayers exhibited a two-fold increase of their open probability. At the low concentration used (20-40microTPEN did not significantly inhibit the SR-Ca2+-ATPase in SR vesicles. These results indicate that TPEN, traditionally used as a low affinity Ca2+ chelator in intracellular Ca2+ stores, may also act directly on the RyRs inducing an increase in their open probability. This in turn results in an increased Ca2+ leak from the SR leading to its Ca2+ depletion. Lowering of SR Ca2+ content may be a mechanism underlying the recently reported cardioprotective and antiarrhythmic features of TPEN.

Mechanisms of abnormal calcium homeostasis in mutations responsible for catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia is a heritable arrhythmia unmasked by exertion or stress and is characterized by triggered activity and sudden cardiac death. In this study, we simulated mutations in 2 genes linked to catecholaminergic polymorphic ventricular tachycardia, the first located in calsequestrin (CSQN2) and the second in the ryanodine receptor (RyR2). The aim of the study was to investigate the mechanistic basis for spontaneous Ca2+ release events that lead to delayed afterdepolarizations in affected patients. Sarcoplasmic reticulum (SR) luminal Ca2+ sensing was incorporated into a model of the human ventricular myocyte, and CSQN2 mutations were modeled by simulating disrupted RyR2 luminal Ca2+ sensing. In voltage-clamp mode, the mutant CSQN2 model recapitulated the smaller calcium transients, smaller time to peak calcium transient, and accelerated recovery from inactivation seen in experiments. In current clamp mode, in the presence of beta stimulation, we observed delayed afterdepolarizations, suggesting that accelerated recovery of RyR2 induced by impaired luminal Ca2+ sensing underlies the triggered activity observed in mutant CSQN2-expressing myocytes. In current-clamp mode, in a model of mutant RyR2 that is characterized by reduced FKBP12.6 binding to the RyR2 on beta stimulation, the impaired coupled gating characteristic of these mutations was modeled by reducing cooperativity of RyR2 activation. In current-clamp mode, the mutant RyR2 model exhibited increased diastolic RyR2 open probability that resulted in formation of delayed afterdepolarizations. In conclusion, these minimal order models of mutant CSQN2 and RyR2 provide plausible mechanisms by which defects in RyR2 gating may lead to the cellular triggers for arrhythmia, with implications for the development of targeted therapy.

Kinetic properties of the cardiac L-type Ca2+ channel and its role in myocyte electrophysiology: a theoretical investigation

The L-type Ca(2+) channel (Ca(V)1.2) plays an important role in action potential (AP) generation, morphology, and duration (APD) and is the primary source of triggering Ca(2+) for the initiation of Ca(2+)-induced Ca(2+)-release in cardiac myocytes. In this article we present: 1), a detailed kinetic model of Ca(V)1.2, which is incorporated into a model of the ventricular mycoyte where it interacts with a kinetic model of the ryanodine receptor in a restricted subcellular space; 2), evaluation of the contribution of voltage-dependent inactivation (VDI) and Ca(2+)-dependent inactivation (CDI) to total inactivation of Ca(V)1.2; and 3), description of dynamic Ca(V)1.2 and ryanodine receptor channel-state occupancy during the AP. Results are: 1), the Ca(V)1.2 model reproduces experimental single-channel and macroscopic-current data; 2), the model reproduces rate dependence of APD, [Na(+)](i), and the Ca(2+)-transient (CaT), and restitution of APD and CaT during premature stimuli; 3), CDI of Ca(V)1.2 is sensitive to Ca(2+) that enters the subspace through the channel and from SR release. The relative contributions of these Ca(2+) sources to total CDI during the AP vary with time after depolarization, switching from early SR dominance to late Ca(V)1.2 dominance. 4), The relative contribution of CDI to total inactivation of Ca(V)1.2 is greater at negative potentials, when VDI is weak; and 5), loss of VDI due to the Ca(V)1.2 mutation G406R (linked to the Timothy syndrome) results in APD prolongation and increased CaT.

Restitution of Ca(2+) release and vulnerability to arrhythmias

New information has recently been obtained along two essentially parallel lines of research: investigations into the fundamental mechanisms of Ca(2+)-induced Ca(2+) release (CICR) in heart cells, and analyses of the factors that control the development of unstable rhythms such as repolarization alternans. These lines of research are starting to converge such that we can begin to understand unstable and potentially arrhythmogenic cardiac dynamics in terms of the underlying mechanisms governing not only membrane depolarization and repolarization but also the complex bidirectional interactions between electrical and Ca(2+) signaling in heart cells. In this brief review, we discuss the progress that has recently been made in understanding the factors that control the beat-to-beat regulation of cardiac Ca(2+) release and attempt to place these results within a larger context. In particular, we discuss factors that may contribute to unstable Ca(2+) release and speculate about how instability in CICR may contribute to the development of arrhythmias under pathological conditions.

Ca2+ microdomains in smooth muscle

In smooth muscle, Ca(2+) controls diverse activities including cell division, contraction and cell death. Of particular significance in enabling Ca(2+) to perform these multiple functions is the cell's ability to localize Ca(2+) signals to certain regions by creating high local concentrations of Ca(2+) (microdomains), which differ from the cytoplasmic average. Microdomains arise from Ca(2+) influx across the plasma membrane or release from the sarcoplasmic reticulum (SR) Ca(2+) store. A single Ca(2+) channel can create a microdomain of several micromolar near (approximately 200 nm) the channel. This concentration declines quickly with peak rates of several thousand micromolar per second when influx ends. The high [Ca(2+)] and the rapid rates of decline target Ca(2+) signals to effectors in the microdomain with rapid kinetics and enable the selective activation of cellular processes. Several elements within the cell combine to enable microdomains to develop. These include the brief open time of ion channels, localization of Ca(2+) by buffering, the clustering of ion channels to certain regions of the cell and the presence of membrane barriers, which restrict the free diffusion of Ca(2+). In this review, the generation of microdomains arising from Ca(2+) influx across the plasma membrane and the release of the ion from the SR Ca(2+) store will be discussed and the contribution of mitochondria and the Golgi apparatus as well as endogenous modulators (e.g. cADPR and channel binding proteins) will be considered.

Integration of calcium with the signaling network in cardiac myocytes

Calcium has evolved as global intracellular messenger for signal transduction in the millisecond time range by reversibly binding to calcium-sensing proteins. In the cardiomyocyte, ion pumps, ion exchangers and channels keep the cytoplasmic calcium level at rest around approximately 100 nM which is more than 10,000-fold lower than outside the cell. Intracellularly, calcium is mainly stored in the sarcoplasmic reticulum, which comprises the bulk of calcium available for the heartbeat. Regulation of cardiac function including contractility and energy production relies on a three-tiered control system, (i) immediate and fast feedback in response to mechanical load on a beat-to-beat basis (Frank-Starling relation), (ii) more sustained regulation involving transmitters and hormones as primary messengers, and (iii) long-term adaptation by changes in the gene expression profile. Calcium signaling over largely different time scales requires its integration with the protein kinase signaling network which is governed by G-protein-coupled receptors, growth factor and cytokine receptors at the surface membrane. Short-term regulation is dominated by the beta-adrenergic system, while long-term regulation with phenotypic remodeling depends on sustained signaling by growth factors, cytokines and calcium. Mechanisms and new developments in intracellular calcium handling and its interrelation with the MAPK signaling pathways are discussed in detail.

Analysis of puff dynamics in oocytes: interdependence of puff amplitude and interpuff interval

Puffs are localized Ca(2+) signals that arise in oocytes in response to inositol 1,4,5-trisphosphate (IP(3)). They are analogous to the sparks of myocytes and are believed to be the result of the liberation of Ca(2+) from the endoplasmic reticulum through the coordinated opening of IP(3) receptor/channels clustered at a functional release site. In this article, we analyze sequences of puffs that occur at the same site to help elucidate the mechanisms underlying puff dynamics. In particular, we show a dependence of the interpuff time on the amplitude of the preceding puff, and of the amplitude of the following puff on the preceding interval. These relationships can be accounted for by an inhibitory role of the Ca(2+) that is liberated during puffs. We construct a stochastic model for a cluster of IP(3) receptor/channels that quantitatively replicates the observed behavior, and we determine that the characteristic time for a channel to escape from the inhibitory state is of the order of seconds.

Would modulation of intracellular Ca2+ be antiarrhythmic?

Under several types of conditions, reversal of steps of excitation-contraction coupling (RECC) can give rise to nondriven electrical activity. In this review we explore those conditions for several cardiac cell types (SA, atrial, Purkinje, ventricular cells). We find that abnormal spontaneous Ca2+ release from intracellular Ca2+ stores, aberrant Ca2+ influx from sarcolemmal channels or abnormal Ca2+ surges in nonuniform muscle can be the initiators of the RECC. Often, with such increases in Ca2+, spontaneous Ca2+ waves occur and lead to membrane depolarizations. Because the change in membrane voltage is produced by Ca2+-dependent changes in ion channel function, we also review here what is known about the molecular interaction of Ca2+ and several Ca2+-dependent processes, including the intracellular Ca2+ release channels implicated in the genetic basis of some forms of human arrhythmias. Finally, we review what is known about the effectiveness of several agents in modifying such Ca2+-dependent arrhythmias.

Synergistic interactions between Ca2+ entries through L-type Ca2+ channels and Na+-Ca2+ exchanger in normal and failing rat heart

We used confocal Ca2+ imaging and the patch-clamp technique to investigate the interplay between Ca2+ entries through L-type Ca2+ channels (LCCs) and reverse-mode Na+-Ca2+ exchange (NCX) in activating Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) in cardiac myocytes from normal and failing rat hearts. In normal myocytes exposed to N(6),2'-O-dibutyryl adenosine-3',5'-cyclic monophosphate (db-cAMP, membrane-permeable form of cAMP), the bell-shaped voltage dependence of cytosolic Ca2+ transients was dramatically broadened due to activation of SR Ca2+ release at high membrane potentials (30-120 mV). This broadening of Ca2+-transient voltage dependence could be prevented by KB-R7943, an inhibitor of the reverse-mode NCX. Trans-sarcolemmal Ca2+ entries were measured fluorometrically in myocytes during depolarizing steps to high membrane potentials. The total Ca2+ entry (deltaF(Tot)) was separated into two Ca2+ entry components, LCC-mediated (deltaF(LCC)) and NCX-mediated (deltaF(NCX)), by exposing the cells to the specific inhibitors of LCCs and reverse-mode NCX, nifedipine and KB-R7943, respectively. In the absence of protein kinase A (PKA) stimulation the amplitude of the Ca2+-inflow signal (deltaF(Tot)) corresponded to the arithmetic sum of the amplitudes of the KB-R7943- and nifedipine-resistant components (deltaF(Tot)=deltaF(LCC)+deltaF(NCX)). PKA activation resulted in significant increases in deltaF(Tot) and deltaF(LCC). Paradoxically, deltaF(Tot) became approximately threefold larger than the sum of the deltaF(NCX) and deltaF(LCC) components. In myocytes from failing hearts, stimulation of PKA failed to induce a shift in Ca2+ release voltage dependence toward more positive membrane potentials. Although the total and NCX-mediated Ca2+ entries were increased again, deltaF(Tot) did not significantly exceed the sum of deltaF(LCC) and deltaF(NCX). We conclude that the LCC and NCX Ca2+-entry pathways interact synergistically to trigger SR Ca2+ release on depolarization to positive membrane potentials in PKA-stimulated cardiac muscle. In heart failure, this new form of Ca2+ release is diminished and may potentially account for the compromised contractile performance and reduced functional reserve in failing hearts.

Local recovery of Ca2+ release in rat ventricular myocytes

Excitation-contraction coupling in the heart depends on the positive feedback process of Ca2+-induced Ca2+ release (CICR). While CICR provides for robust triggering of Ca2+ sparks, the mechanisms underlying their termination remain unknown. At present, it is unclear how a cluster of Ca2+ release channels (ryanodine receptors or RyRs) can be made to turn off when their activity is sustained by the Ca2+ release itself. We use a novel experimental approach to investigate indirectly this issue by exploring restitution of Ca2+ sparks. We exploit the fact that ryanodine can bind, nearly irreversibly, to an RyR subunit (monomer) and increase the open probability of the homotetrameric channel. By applying low concentrations of ryanodine to rat ventricular myocytes, we observe repeated activations of individual Ca2+ spark sites. Examination of these repetitive Ca2+ sparks reveals that spark amplitude recovers with a time constant of 91 ms whereas the sigmoidal recovery of triggering probability lags behind amplitude recovery by approximately 80 ms. We conclude that restitution of Ca2+ sparks depends on local refilling of SR stores after depletion and may also depend on another time-dependent process such as recovery from inactivation or a slow conformational change after rebinding of Ca2+ to SR regulatory proteins.

Ca2+-induced Ca2+ Release in Sensory Neurons

Ca(2+)-induced Ca(2+) release (CICR) is a ubiquitous mechanism by which Ca(2+) release from the endoplasmic reticulum amplifies the trigger Ca(2+) entry and generates propagating Ca(2+) waves. To elucidate the mechanisms that control this positive feedback, we investigated the spatial and temporal kinetics and measured the gain function of CICR in small sensory neurons from mammalian dorsal root ganglions (DRGs). We found that subsurface Ca(2+) release units (CRUs) are under tight local control by Ca(2+) entry, whereas medullar CRUs as a "common pool" system are recruited by inwardly propagating CICR. Active CRUs often displayed repetitive Ca(2+) sparks, conferring the ability to encode a "memory" of neuronal activity well beyond the duration of an action potential. Store Ca(2+) reserve was able to support all CRUs each to fire approximately 15 sparks, excluding use-dependent inactivation or store depletion as the major CICR termination mechanism. Importantly, CICR in DRG neurons operated in a low gain, linear regime (gain = 0.54), which conferred intrinsic stability to CICR. Combined with high Ca(2+) current density (-156 pA/pF at -10 mV), such a low gain CICR system generated large intracellular Ca(2+) transients without jeopardizing the stability. These findings provide the first demonstration that CICR operating in a low gain regime can be harnessed to provide a robust and graded amplification of Ca(2+) signal in the absence of counteracting inhibitory mechanism.

Paradoxical SR Ca2+ release in guinea-pig cardiac myocytes after beta-adrenergic stimulation revealed by two-photon photolysis of caged Ca2+

In heart muscle the amplification and shaping of Ca(2+) signals governing contraction are orchestrated by recruiting a variable number of Ca(2+) sparks. Sparks reflect Ca(2+) release from the sarcoplasmic reticulum (SR) via Ca(2+) release channels (ryanodine receptors, RyRs). RyRs are activated by Ca(2+) influx via L-type Ca(2+) channels with a specific probability that may depend on regulatory mechanisms (e.g. beta-adrenergic stimulation) or diseased states (e.g. heart failure). Changes of RyR phosphorylation may be critical for both regulation and impaired function in disease. Using UV flash photolysis of caged Ca(2+) and short applications of caffeine in guinea-pig ventricular myocytes, we found that Ca(2+) release signals on the cellular level were largely governed by global SR content. During beta-adrenergic stimulation resting myocytes exhibited smaller SR Ca(2+) release signals when activated by photolysis (62.3% of control), resulting from reduced SR Ca(2+) content under these conditions (58.6% of control). In contrast, local signals triggered with diffraction limited two-photon photolysis displayed the opposite behaviour, exhibiting a larger Ca(2+) release (164% of control) despite reduced global and local SR Ca(2+) content. This apparent paradox implies changes of RyR open probabilities after beta-adrenergic stimulation, enhancing local regenerativity and reliability of Ca(2+) signalling. Thus, our results underscore the importance of phosphorylation of RyRs (or of a related protein), as a regulatory physiological mechanism that may also provide new therapeutic avenues to recover impaired Ca(2+) signalling during cardiac disease.