Altered cardiac sarcoplasmic reticulum function of intact myocytes of rat ventricle during metabolic inhibition

The role of iron overload and ferroptosis in arrhythmia pathogenesis

Ferroptosis is a newly discovered form of programmed cell death triggered by intracellular iron overload, which leads to the accumulation of lipid peroxides in various cells. It has been implicated in the pathogenesis and progression of various diseases, including tumors, neurological disorders, and cardiovascular diseases. The intricate mechanism underlying ferroptosis involves an imbalance between the oxidation and antioxidant systems, disturbances in iron metabolism, membrane lipid peroxidation, and dysregulation of amino acid metabolism. We highlight the key molecular mechanisms governing iron overload and ferroptosis, and discuss potential molecular pathways linking ferroptosis with arrhythmias.

Myocardial ATP depletion detected noninvasively predicts sudden cardiac death risk in heart failure patients

BACKGROUND

Sudden cardiac death (SCD) remains a worldwide public health problem in need of better noninvasive predictive tools. Current guidelines for primary preventive SCD therapies, such as implantable cardioverter defibrillators (ICDs), are based on left ventricular ejection fraction (LVEF), but these guidelines are imprecise: fewer than 5% of ICDs deliver lifesaving therapy per year. Impaired cardiac metabolism and ATP depletion cause arrhythmias in experimental models, but to our knowledge a link between arrhythmias and cardiac energetic abnormalities in people has not been explored, nor has the potential for metabolically predicting clinical SCD risk.

METHODS

We prospectively measured myocardial energy metabolism noninvasively with phosphorus magnetic resonance spectroscopy in patients with no history of significant arrhythmias prior to scheduled ICD implantation for primary prevention in the setting of reduced LVEF (≤35%).

RESULTS

By 2 different analyses, low myocardial ATP significantly predicted the composite of subsequent appropriate ICD firings for life-threatening arrhythmias and cardiac death over approximately 10 years. Life-threatening arrhythmia risk was approximately 3-fold higher in patients with low ATP and independent of established risk factors, including LVEF. In patients with normal ATP, rates of appropriate ICD firings were several-fold lower than reported rates of ICD complications and inappropriate firings.

CONCLUSION

To the best of our knowledge, these are the first data linking in vivo myocardial ATP depletion and subsequent significant arrhythmic events in people, suggesting an energetic component to clinical life-threatening ventricular arrhythmogenesis. The findings support investigation of metabolic strategies that limit ATP loss to treat or prevent life-threatening cardiac arrhythmias and herald noninvasive metabolic imaging as a complementary SCD risk stratification tool.

TRIAL REGISTRATION

ClinicalTrials.gov NCT00181233.

FUNDING

This work was supported by the DW Reynolds Foundation, the NIH (grants HL61912, HL056882, HL103812, HL132181, HL140034), and Russell H. Morgan and Clarence Doodeman endowments at Johns Hopkins.

Sodium-glucose co-transporter 2 inhibition as a mitochondrial therapy for atrial fibrillation in patients with diabetes?

While patients with type 2 diabetes mellitus (T2DM) are at increased risk to develop atrial fibrillation (AF), the mechanistic link between T2DM and AF-susceptibility remains unclear. Common co-morbidities of T2DM, particularly hypertension, may drive AF in the setting of T2DM. But direct mechanisms may also explain this relation, at least in part. In this regard, recent evidence suggests that mitochondrial dysfunction drives structural, electrical and contractile remodelling of atrial tissue in patients T2DM. Mitochondrial dysfunction may therefore be the mechanistic link between T2DM and AF and could also serve as a therapeutic target. An elegant series of experiments published in Cardiovascular Diabetology provide compelling new evidence to support this hypothesis. Using a model of high fat diet (HFD) and low-dose streptozotocin (STZ) injection, Shao et al. provide data that demonstrate a direct association between mitochondrial dysfunction and the susceptibility to develop AF. But the authors also demonstrated that the sodium-glucose co-transporter 2 inhibitors (SGLT2i) empagliflozin has the capacity to restore mitochondrial function, ameliorate electrical and structural remodelling and prevent AF. These findings provide a new horizon in which mitochondrial targeted therapies could serve as a new class of antiarrhythmic drugs.

Metabolic Inhibition Induces Transient Increase of L-type Ca2+ Current in Human and Rat Cardiac Myocytes

Metabolic inhibition is a common condition observed during ischemic heart disease and heart failure. It is usually accompanied by a reduction in L-type Ca²⁺ channel (LTCC) activity. In this study, however, we show that metabolic inhibition results in a biphasic effect on LTCC current (I_CaL) in human and rat cardiac myocytes: an initial increase of I_CaL is observed in the early phase of metabolic inhibition which is followed by the more classical and strong inhibition. We studied the mechanism of the initial increase of I_CaL in cardiac myocytes during β-adrenergic stimulation by isoprenaline, a non-selective agonist of β-adrenergic receptors. The whole-cell patch⁻clamp technique was used to record the I_CaL in single cardiac myocytes. The initial increase of I_CaL was induced by a wide range of metabolic inhibitors (FCCP, 2,4-DNP, rotenone, antimycin A). In rat cardiomyocytes, the initial increase of I_CaL was eliminated when the cells were pre-treated with thapsigargin leading to the depletion of Ca²⁺ from the sarcoplasmic reticulum (SR). Similar results were obtained when Ca²⁺ release from the SR was blocked with ryanodine. These data suggest that the increase of I_CaL in the early phase of metabolic inhibition is due to a reduced calcium dependent inactivation (CDI) of LTCCs. This was further confirmed in human atrial myocytes where FCCP failed to induce the initial stimulation of I_CaL when Ca²⁺ was replaced by Ba²⁺, eliminating CDI of LTCCs. We conclude that the initial increase in I_CaL observed during the metabolic inhibition in human and rat cardiomyocytes is a consequence of an acute reduction of Ca²⁺ release from SR resulting in reduced CDI of LTCCs.

Basic Electrophysiologic Mechanisms of Sudden Cardiac Death Caused by Acute Myocardial Ischemia and Infarction

Sudden cardiac death caused by acute ischemia results from electrophysiologic changes in myocardium deprived of its blood supply. These changes include a reduction in resting potential and phase 0 depolarization and an increase in intercellular resistivity that slow conduction, cause conduction block, and lead to reentrant excitation and ventricular fibrillation. Reperfusion of a coronary artery after a short period of occlusion leads to similar changes.

Metabolic inhibition reduces cardiac L-type Ca2+ channel current due to acidification caused by ATP hydrolysis

Metabolic stress evoked by myocardial ischemia leads to impairment of cardiac excitation and contractility. We studied the mechanisms by which metabolic inhibition affects the activity of L-type Ca²⁺ channels (LTCCs) in frog ventricular myocytes. Metabolic inhibition induced by the protonophore FCCP (as well as by 2,4- dinitrophenol, sodium azide or antimycin A) resulted in a dose-dependent reduction of LTCC current (I_Ca,L) which was more pronounced during β-adrenergic stimulation with isoprenaline. I_Ca,L was still reduced by metabolic inhibition even in the presence of 3 mM intracellular ATP, or when the cell was dialysed with cAMP or ATP-γ-S to induce irreversible thiophosphorylation of LTCCs, indicating that reduction in I_Ca,L is not due to ATP depletion and/or reduced phosphorylation of the channels. However, the effect of metabolic inhibition on I_Ca,L was strongly attenuated when the mitochondrial F₁F₀-ATP-synthase was blocked by oligomycin or when the cells were dialysed with the non-hydrolysable ATP analogue AMP-PCP. Moreover, increasing the intracellular pH buffering capacity or intracellular dialysis of the myocytes with an alkaline solution strongly attenuated the inhibitory effect of FCCP on I_Ca,L. Thus, our data demonstrate that metabolic inhibition leads to excessive ATP hydrolysis by the mitochondrial F₁F₀-ATP-synthase operating in the reverse mode and this results in intracellular acidosis causing the suppression of I_Ca,L. Limiting ATP break-down by F₁F₀-ATP-synthase and the consecutive development of intracellular acidosis might thus represent a potential therapeutic approach for maintaining a normal cardiac function during ischemia.

Mechanisms of sudden cardiac death: oxidants and metabolism

Ventricular arrhythmia is the leading cause of sudden cardiac death (SCD). Deranged cardiac metabolism and abnormal redox state during cardiac diseases foment arrhythmogenic substrates through direct or indirect modulation of cardiac ion channel/transporter function. This review presents current evidence on the mechanisms linking metabolic derangement and excessive oxidative stress to ion channel/transporter dysfunction that predisposes to ventricular arrhythmias and SCD. Because conventional antiarrhythmic agents aiming at ion channels have proven challenging to use, targeting arrhythmogenic metabolic changes and redox imbalance may provide novel therapeutics to treat or prevent life-threatening arrhythmias and SCD.

The effect of 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBQ) on intracellular Ca2+ handling in rat ventricular myocytes

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The suitability of TBQ as a specific inhibitor of SERCA was investigated.

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TBQ decreased SERCA activity in a concentration dependent manner.

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TBQ inhibited the calcium current.

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TBQ activated an outward current consistent with an ATP-dependent potassium channel.

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TBQ cannot be used as a specific inhibitor of SERCA in rat cardiac myocytes.

2,5-Di-(tert-butyl)-1,4-benzohydroquinone (TBQ) is a reversible inhibitor of SERCA, potentially making it a useful tool to study the effects of SERCA inhibition in cardiac cells. However, it is unknown if TBQ also has effects on other components of ventricular Ca handling. The aim of these experiments was to characterise the effects of TBQ on Ca handling in rat ventricular myocytes and assess its suitability as a specific inhibitor of SERCA. This was achieved by voltage clamp via perforated patch and [Ca²⁺]_i measurement using Fluo-3 AM. TBQ produced a fully reversible, concentration dependent decrease in the rate of systolic Ca decay. 10 μM TBQ decreased the amplitude of the systolic Ca transient by 48 ± 5% and the rate of decay by 54 ± 6%. SR Ca content was also reduced by 62 ± 4%. However, 10 μM TBQ also decreased the peak L-type Ca current by 23 ± 7%. At higher concentrations (100 μM), TBQ also activated an outward current with a current–voltage relationship consistent with a potassium current. This outward current was abolished by Glibenclamide (100 μM). These data show that TBQ can be used to reversibly inhibit SERCA. However, at concentrations that decrease SERCA activity, TBQ also decreases the L-type Ca current and (at higher concentrations) activates an outward current which appears to be an ATP dependent potassium current. We conclude that TBQ cannot be used as a specific inhibitor of SERCA in rat ventricular myocytes.

Effects of dantrolene on arrhythmogenicity in isolated regional ischemia-reperfusion rabbit hearts with or without pacing-induced heart failure

Dantrolene was reported to suppress ventricular fibrillation (VF) in failing hearts with acute myocardial infarction, but its antiarrhythmic efficacy in regional ischemia-reperfusion (IR) hearts remains debatable. Heart failure (HF) was induced by right ventricular pacing. The IR rabbit model was created by coronary artery ligation for 30 min, followed by reperfusion for 15 min in vivo in both HF and non-HF groups (n = 9 in each group). Simultaneous voltage and intracellular Ca²⁺ (Ca_i) optical mapping was then performed in isolated Langendorff-perfused hearts. Electrophysiological studies were conducted and VF inducibility was evaluated by dynamic pacing. Dantrolene (10 μM) was administered after baseline studies. The HF group had a higher VF inducibility than the control group. Dantrolene had both antiarrhythmic (prolonged action potential duration (APD) and effective refractory period) and proarrhythmic effects (slowed conduction velocity, steepened APD restitution slope, and enhanced arrhythmogenic alternans induction) but had no significant effects on ventricular premature beat (VPB) suppression and VF inducibility in both groups. A higher VF conversion rate in the non-HF group was likely due to greater APD prolonging effects in smaller hearts compared to the HF group. The lack of significant effects on VPB suppression by dantrolene suggests that triggered activity might not be the dominant mechanism responsible for VPB induction in the IR model.

Mitochondria and arrhythmias

Mitochondria are essential to providing ATP, thereby satisfying the energy demand of the incessant electrical activity and contractile action of cardiac muscle. Emerging evidence indicates that mitochondrial dysfunction can adversely affect cardiac electrical functioning by impairing the intracellular ion homeostasis and membrane excitability through reduced ATP production and excessive reactive oxygen species (ROS) generation, resulting in increased propensity to cardiac arrhythmias. In this review, the molecular mechanisms linking mitochondrial dysfunction to cardiac arrhythmias are discussed with an emphasis on the impact of increased mitochondrial ROS on the cardiac ion channels and transporters that are critical to maintaining normal electromechanical functioning of the cardiomyocytes. The potential of using mitochondria-targeted antioxidants as a novel antiarrhythmia therapy is highlighted.

Emerging mechanisms of T-tubule remodelling in heart failure

Cardiac excitation-contraction coupling occurs primarily at the sites of transverse (T)-tubule/sarcoplasmic reticulum junctions. The orderly T-tubule network guarantees the instantaneous excitation and synchronous activation of nearly all Ca(2+) release sites throughout the large ventricular myocyte. Because of the critical roles played by T-tubules and the array of channels and transporters localized to the T-tubule membrane network, T-tubule architecture has recently become an area of considerable research interest in the cardiovascular field. This review will focus on the current knowledge regarding normal T-tubule structure and function in the heart, T-tubule remodelling in the transition from compensated hypertrophy to heart failure, and the impact of T-tubule remodelling on myocyte Ca(2+) handling function. In the last section, we discuss the molecular mechanisms underlying T-tubule remodelling in heart disease.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

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.

Effect of metabolic inhibition on couplon behavior in rabbit ventricular myocytes

We investigated the effect of combined inhibition of oxidative and glycolytic metabolism on L-type Ca(2+) channels (LCCs) and Ca(2+) spikes in isolated patch-clamped rabbit ventricular myocytes. Metabolic inhibition (MI) reduced LCC open probability, increased null probability, increased first latency, and decreased open time but left conductance unchanged. These results explain the reduction in macroscopic Ca(2+) current observed during MI. MI also produced a gradual reduction in action potential duration at 90% repolarization (APD(90)), a clear decline in spike probability, and an increase in spike latency and variance. These effects are consistent with the changes we observed in LCC activity. MI had no effect on the amplitude or time to peak of Ca(2+) spikes until APD(90) reached 10% of control, suggesting preserved sarcoplasmic reticulum Ca(2+) stores and ryanodine receptor (RyR) conductance in those couplons that remained functioning. Ca(2+) spikes disappeared completely when APD(90) reached <2% of control, although in two cells, spikes were reactivated in a highly synchronized fashion by very short action potentials. This reactivation is probably due to the increased driving force for Ca(2+) entry through a reduced number of LCCs that remain open during early repolarization. The enlarged single channel flux produced by rapid repolarization is apparently sufficient to trigger RyRs whose Ca(2+) sensitivity is likely reduced by MI. We suggest that loss of coupling fidelity during MI is explained by loss of LCC activity (possibly mediated by Ca(2+)-calmodulin kinase II activity). In addition, the results are consistent with loss of RyR activity, which can be mitigated under conditions likely to enlarge the trigger.

Three distinct phases of VF during global ischemia in the isolated blood-perfused pig heart

Changes in ventricular fibrillation (VF) organization occurring after the onset of global ischemia are relevant to defibrillation and survival but remain poorly understood. We hypothesized that ischemia-specific dynamic instability of the action potential (AP) causes a loss of spatiotemporal periodicity of propagation and broadening of the electrocardiogram (ECG) frequency spectrum during VF in the ischemic myocardium. We recorded voltage-sensitive fluorescence of di-4-ANEPPS (anterior left ventricle, 35 x 35 mm, 64 x 64 pixels) and the volume-conducted ECG in six blood-perfused hearts during 10 min of VF and global ischemia. We used coefficient of variation (CV) to estimate variability of AP amplitude, AP duration, and diastolic interval (CV-APA, CV-APD, and CV-DI, respectively). We computed excitation median frequency (Median_F), spectral width of the AP and ECG (SpW-AP and SpW-ECG, respectively), wavebreak incidence (WBI), and recurrence of propagation direction (RPD). We found three distinct phases of local VF dynamics: "relatively periodic" (<or=1 min, high Median_F, moderate AP variability, high WBI, low RPD), "highly periodic" (1-2 min, reduced Median_F, low AP variability, low WBI, high RPD), and "aperiodic" (3-10 min, low Median_F, high AP variability, high WBI, low RPD). In one experiment, spontaneous conversion from the aperiodic to the highly periodic phase occurred after 5 min of ischemia. The SpW-ECG was correlated with SpW-AP, CV-APD, and CV-APA. We conclude that 1) at least three distinct phases of VF dynamics are present in our model, and 2) the newly described aperiodic phase is related to ischemia-specific dynamic instability of the AP shape, which underlies broadening of the ECG spectrum during VF evolution.

Protein kinase A is activated by the n-3 polyunsaturated fatty acid eicosapentaenoic acid in rat ventricular muscle

During cardiac ischaemia antiarrhythmic n-3 polyunsaturated fatty acids (PUFAs) are released following activation of phospholipase A2, if they are in the diet prior to ischaemia. Here we show a positive lusitropic effect of one such PUFA, eicosapentaenoic acid (EPA) in the antiarrhythmic concentration range in Langendorff hearts and isolated rat ventricular myocytes due to activation of protein kinase A (PKA). Several different approaches indicated activation of PKA by EPA (5-10 micromol l(-1)): the time constant of decay of the systolic Ca2+ transient decreased to 65.3 +/- 5.0% of control, Western blot analysis showed a fourfold increase in phospholamban phosphorylation, and PKA activity increased by 21.0 +/- 7.3%. In addition myofilament Ca2+ sensitivity was reduced in EPA; this too may have resulted from PKA activation. We also found that EPA inhibited L-type Ca2+ current by 38.7 +/- 3.9% but this increased to 63.3 +/- 3.4% in 10 micromol l(-1) H89 (to inhibit PKA), providing further evidence of activation of PKA by EPA. PKA inhibition also prevented the lusitropic effect of EPA on the systolic Ca2+ transient and contraction. Our measurements show, however, PKA activation in EPA cannot be explained by increased cAMP levels and alternative mechanisms for PKA activation are discussed. The combined lusitropic effect and inhibition of contraction by EPA may, respectively, combat diastolic dysfunction in ischaemic cardiac muscle and promote cell survival by preserving ATP. This is a further level of protection for the heart in addition to the well-documented antiarrhythmic qualities of these fatty acids.

The effect of isoflurane during reoxygenation on the sarcoplasmic reticulum and cellular injury in isolated ventricular myocytes

In contrast to pretreatment with isoflurane its benefit when applied during reperfusion in rat hearts was only modest. As cellular injury during reoxygenation is greatly determined by sarcoplasmic reticulum (SR) calcium [Ca2+] handling we investigated the effect of isoflurane after simulated ischemia in rat ventricular myocytes. Hypoxic metabolic inhibition was induced by exposure to an acidic medium (pH: 6.3) containing deoxyglucose. Ambient pO2 was reduced to <15 mm Hg. After 30 min, cells were reoxygenated for 30 min with a glucose containing medium (pH: 7.4) in air (Air) or in the presence of isoflurane (Iso), or two SR blockers, i.e. either 3 microM ryanodine (Rya) or 10 microM of cyclopiazonic acid (CPA). During inhibition, diastolic cytosolic calcium ([Ca2+]i) increased and systolic cell shortening decreased. [Ca2+]i further increased in all groups towards the end of reoxygenation. However, [Ca2+]i in the Iso and the Rya group climbed twice as high as in the Air and the CPA group (P < 0.05). Hypercontracture occurred in 23% and 18% in the Iso and the Rya and in 10% and 9% in the Air and the CPA group, respectively (P < 0.05). Cell relengthening and shortening was impaired in Iso, Rya, and CPA treated cells (P < 0.05 vs. Air). Isoflurane given solely during reoxygenation appears to augment cellular injury. Its action seems to be blockade of SR Ca2+ release and Ca2+ efflux. SR Ca2+ overload induces spontaneous Ca2+ oscillations that cause hypercontracture. However, [Ca2+]i does not independently govern cellular systolic and diastolic dysfunction.

Roles of KATP channels in delayed cardioprotection and intracellular Ca(2+) in the rat heart as revealed by kappa-opioid receptor stimulation with U50488H

The effect of preconditioning with U50488 H (UP), a selective kappa-opioid receptor (kappa-OR) agonist, on infarct size and intracellular Ca2+ ([Ca2+]i) in the heart subjected to ischaemic insults were studied and evaluated. U50488 H administered intravenously reduced the infarct size 18-48 h after administration in isolated hearts subjected to regional ischaemia/reperfusion (I/R). The effect was dose dependent. A peak effect was reached at 10 mg x kg-1 U50488 H and at 24 h after administration. The effect of 10 mg x kg-1 U50488 H at 24 h after administration was abolished by nor-binaltorphimine (nor-BNI), a selective kappa-OR antagonist, indicating the effect was kappa-OR mediated. The infarct reducing effect of U50488 H was attenuated when a selective blocker of mitochondrial (5-hydroxydecanoic acid, 5-HD) or sarcolemmal (HRM-1098) ATP-sensitive potassium channel (KATP) was coadministered with U50488 H 24 h before ischaemia or when 5-HD was administered just before ischaemia. U50488 H also attenuated the elevation in [Ca2+]i and reduction in electrically induced [Ca2+]i transient in cardiomyocytes subjected to ischaemic insults. The effects were reversed by blockade of KATP channel, which abolished the protective effect of preconditioning with U50488 H. The results indicated that mitochondrial KATP channel serves as both a trigger and a mediator, while sarcolemmal KATP channel as a trigger only, of delayed cardioprotection of kappa-OR stimulation. The effects of these channels may result from prevention/attenuation of [Ca2+]i overload induced by ischaemic insults.

Physiological and pathological modulation of ryanodine receptor function in cardiac muscle

Calcium release from the sarcoplasmic reticulum (SR) in cardiac muscle occurs through a specialised release channel, the ryanodine receptor, RyR, via the process of Ca-induced Ca release (CICR). The open probability of the RyR is increased by elevation of cytoplasmic Ca concentration ([Ca(2+)](i)). However, in addition to Ca, other modulators affect the RyR open probability. Agents which increase the RyR opening during systole produce a transient increase of systolic [Ca(2+)](i) followed by a return to the initial level due to a compensating decrease of SR Ca content. Increasing RyR opening during diastole decreases SR Ca content and thereby decreases systolic [Ca(2+)](i). We therefore conclude that potentiation of RyR opening will, if anything, decrease systolic [Ca(2+)](i). The effects of specific examples of modulators of the RyR, such as phosphorylation, metabolic changes, heart failure and polyunsaturated fatty acids, are discussed.

Interplay between SERCA and sarcolemmal Ca2+ efflux pathways controls spontaneous release of Ca2+ from the sarcoplasmic reticulum in rat ventricular myocytes

Waves of calcium-induced calcium release occur in a variety of cell types and have been implicated in the origin of cardiac arrhythmias. We have investigated the effects of inhibiting the SR Ca(2+)-ATPase (SERCA) with the reversible inhibitor 2',5'-di(tert-butyl)-1,4-benzohydroquinone (TBQ) on the properties of these waves. Cardiac myocytes were voltage clamped at a constant potential between -65 and -40 mV and spontaneous waves evoked by increasing external Ca(2+) concentration to 4 mm. Application of 100 microm TBQ decreased the frequency of waves. This was associated with increases of resting [Ca(2+)](i), the time constant of decay of [Ca(2+)](i) and the integral of the accompanying Na(+)-Ca(2+) exchange current. There was also a decrease in propagation velocity of the waves. There was an increase of the calculated Ca(2+) efflux per wave. The SR Ca(2+) content when a wave was about to propagate decreased to 91.7 +/- 3.2%. The period between waves increased in direct proportion to the Ca(2+) efflux per wave meaning that TBQ had no effect on the Ca(2+) efflux per unit time. We conclude that (i) decreased wave frequency is not a direct consequence of decreased Ca(2+) pumping by SERCA between waves but, rather, to more Ca(2+) loss on each wave; (ii) inhibiting SERCA increases the chance of spontaneous Ca(2+) release propagating at a given SR content.

Abrogation of ventricular arrhythmias in a model of ischemia and reperfusion by targeting myocardial calcium cycling

Abnormal intracellular Ca(2+) cycling plays an important role in cardiac dysfunction and ventricular arrhythmias in the setting of heart failure and transient cardiac ischemia followed by reperfusion (I/R). We hypothesized that overexpression of the sarcoplasmic reticulum Ca(2+) ATPase pump (SERCA2a) may improve both contractile dysfunction and ventricular arrhythmias. Continuous ECG recordings were obtained in 46 conscious rats after adenoviral gene transfer of either SERCA2a or the reporter gene beta-galactosidase (beta gal) or parvalbumin (PV), as early as 48 h before and 48 h after 30 min ligation of the left anterior descending artery by using an implantable telemetry system. Sham-operated animals were used for comparison for hemodynamic measurements, whereas within-animal baseline was used for electrocardiographic and echocardiographic parameters. All episodes of nonsustained ventricular tachycardia (VT) and ventricular fibrillation (VF) were counted, and their durations were summed by telemetry. I/R decreased regional cardiac wall thickening as well as the maximal rate of left ventricular pressure rise (+dP/dt) and ventricular pressure fall (-dP/dt). SERCA2a restored regional wall thickening and +dP/dt and -dP/dt to levels seen preoperatively. Regional-wall motion and anterior-wall thickening were improved in the SERCA2a animals, as assessed by echocardiography and piezoelectric crystals. To assess whether these effects are SERCA2a specific, we overexpressed a skeletal-muscle protein, PV, to examine whether Ca(2+) buffering alone can mitigate ventricular arrhythmias. During the first hour after I/R, the rate of nonsustained VT plus VF was 16 +/- 5 episodes per h (n = 6) in the Ad.beta gal group, 22 +/- 6 in the Ad.PV group, and 4 +/- 2(n = 6, P < 0.01) in the Ad.SERCA2a group. The decrease in VT plus VF in the Ad.SERCA2a group was consistent throughout the 48 h of monitoring. These results show that improving intracellular Ca(2+) handling by overexpression of SERCA2a restores contractile function and reduces ventricular arrhythmias during I/R.

pH-dependent and -independent effects inhibit Ca(2+)-induced Ca2+ release during metabolic blockade in rat ventricular myocytes

We have investigated the role of changes of intracellular pH (pHi) in the effects of metabolic blockade (cyanide plus 2-deoxyglucose) on Ca2+ release from the sarcoplasmic reticulum (SR) in rat ventricular myocytes. pHi and cell length were measured simultaneously. Metabolic blockade decreased the frequency of Ca2+ waves, an effect previously shown to be due to inhibition of Ca2+ release from the SR. This was accompanied by an intracellular acidification. Intracellular acidification was produced in the absence of metabolic inhibition by application of sodium butyrate. A maintained intracellular acidosis produced a decrease of wave frequency. A hysteresis between pHi and wave frequency was observed such that during the onset of the acidification the wave frequency decreased more than in the steady state. Comparison of the steady state relationship between pHi and wave frequency showed that the decrease of wave frequency produced by metabolic blockade was greater than could be accounted for simply by the accompanying decrease of pHi. In other experiments the buffering power of the solution was increased. Under these conditions, metabolic blockade produced no change of pHi but the decrease of wave frequency persisted. We conclude that, although intracellular acidification occurs during metabolic blockade, it is not responsible for most of the inhibition of Ca2+ release from the SR.

In situ visualization of the intracellular Ca2+ dynamics at the border of the acute myocardial infarct

Ischemic insult to the heart produces myocyte Ca2+ ([Ca2+]i) overload. However, little is known about spatiotemporal changes in [Ca2+]i within the ischemic heart in situ at the cellular level. Using real-time confocal microscopy, we successfully visualized [Ca2+]i dynamics at the border zone on the subepicardial myocardium of the heart 2 h after coronary ligations followed by loading with fluo 3/AM. Three distinct regions were identified in the acute infarcted heart. In intact regions, the myocytes showed spatially uniform Ca2+ transients synchronously to QRS complex in the electrocardiogram. The myocytes at the infarcted regions showed no fluorescence intensity (FI). At the border zones between the intact and infarcted regions, Ca2+ waves emerged sporadically and randomly, instead of Ca2+ transients, at a mean frequency of 11.5 +/- 8.5 min/cell with a propagation velocity of 151.0 +/- 35.7 microm/sec along the longitudinal axis of the individual myocytes. In addition, some myocytes within the border zone exhibited homogeneously high static FI, indicating severe Ca2+ overload. In summary, we provided the first direct evidence of abnormal [Ca2+]i dynamics in acute infarcted hearts at the cellular level. The observed diversity in spatiotemporal [Ca2+]i dynamics at the border zone may contribute to the arrhythmias or contractile failure in acute myocardial infarction.

Early effects of metabolic inhibition on intracellular Ca2+ in toad pacemaker cells: involvement of Ca2+ stores

The early effects of metabolic inhibition on intracellular Ca(2+) concentration ([Ca(2+)](i)), Ca(2+) current, and sarcoplasmic reticulum (SR) Ca(2+) content were studied in single pacemaker cells from the sinus venosus of the cane toad. The amplitude of the spontaneous elevations of systolic [Ca(2+)](i) (Ca(2+) transients) was reduced after 5-min exposure to 2 mM NaCN from 338 +/- 30 to 189 +/- 37 nM (P < 0.005, n = 9), and the spontaneous firing rate was reduced from 27 +/- 2 to 12 +/- 4 beats/min (P < 0.002, n = 9). It has been proposed that CN(-) acts by inhibition of cytochrome P-450, resulting in a reduction of cAMP and Ca(2+) current. To test this proposal, we used clotrimazole, a cytochrome P-450 inhibitor, which also decreased the Ca(2+) transients and firing rate. CN(-) caused an insignificant fall of Ca(2+) current (23 +/- 11%) but a substantial reduction of SR Ca(2+) content (by 65 +/- 5%), whereas clotrimazole produced a larger reduction of Ca(2+) current and did not affect the SR Ca(2+) content. Thus the main effect of CN(-) does not seem to be through inhibition of cytochrome P-450. In conclusion, CN(-) appears to reduce Ca(2+) release from the SR mainly by reducing SR Ca(2+) content. A likely cause of the decreased SR content is reduced Ca(2+) uptake by the SR pump.

Subcellular Ca2+ alternans represents a novel mechanism for the generation of arrhythmogenic Ca2+ waves in cat atrial myocytes

Ca(2+) alternans is a potentially arrhythmogenic beat-to-beat alternation of the amplitude of the action potential-induced [Ca(2+)](i) transient in cardiac myocytes. Despite its pathophysiological significance the cellular mechanisms underlying Ca(2+) alternans are poorly understood. Recent evidence, however, points to the modulation of Ca(2+)-induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) by localized alterations in energy metabolism as an important determinant of Ca(2+) alternans. We therefore studied the subcellular properties of Ca(2+) alternans in field-stimulated cat atrial myocytes employing fast two-dimensional fluorescence confocal microscopy. Ca(2+) alternans was elicited by an increase in stimulation frequency or by metabolic interventions targeting glycolysis. Marked subcellular variations in the time of onset, the magnitude, and the phase of alternans were observed. Longitudinal and transverse gradients of Ca(2+) alternans were found as well as neighbouring subcellular regions alternating out-of-phase. Moreover, focal inhibition of glycolysis resulted in spatially restricted Ca(2+) alternans. When two adjacent regions within a myocyte alternated out-of-phase, steep [Ca(2+)](i) gradients developed at their border giving rise to delayed propagating Ca(2+) waves. The results demonstrate that Ca(2+) alternans is a subcellular phenomenon caused by modulation of SR Ca(2+) release, which is mediated, at least in part, by local inhibition of energy metabolism. The generation of arrhythmogenic Ca(2+) waves by subcellular variations in the phase of Ca(2+) alternans represents a novel mechanism for the development of atrial disrhythmias.

Effects of pharmacological preconditioning with U50488H on calcium homeostasis in rat ventricular myocytes subjected to metabolic inhibition and anoxia

1. The effects of pharmacological preconditioning with U50488H (U(50)), a selective kappa-opioid receptor agonist, on Ca(2+) homeostasis in rat ventricular myocytes subjected for 9 min to metabolic inhibition (MI) and anoxia (A), consequences of ischaemia, were studied and compared with those of preconditioning with brief periods of MI/A. 2. Precondition with 30 micro M of U(50) for three cycles of 1 min each cycle separated by 3 min of recovery (UP) significantly increased the percentage of non-blue cells following MI/A. The effect of UP is the same as that of preconditioning with an inhibitor of glycolysis and an oxygen scavenger for three 1-min cycles separated by three-minute recovery (MI/AP). The results indicate that like MI/AP, UP also confers cardioprotection. 3. MI/A increased intracellular Ca(2+) ([Ca(2+)](i)) and reduced the amplitude of caffeine-induced [Ca(2+)](i) transients, an indication of Ca(2+) content in the sarcoplasmic reticulum (SR). MI/A also reduced the electrically-induced [Ca(2+)](i) transient, that indicates Ca(2+)-release during excitation-contraction coupling, and Ca(2+) sparks in unstimulated myocytes, that indicates spontaneous Ca(2+)-release from SR. It also prolonged the decline of the electrically-induced [Ca(2+)](i) transient and slowed down the recovery of the electrically-induced [Ca(2+)](i) transient after administration of caffeine. In addition, MI/A prolonged the decline of caffeine induced [Ca(2+)](i) transient, an indication of Na(+)-Ca(2+) exchange activity, and UP prevented it. So UP, that confers cardioprotection, prevented the changes induced by MI/A. With the exception of Ca(2+)-spark, which was not studied, the effects of MI/AP are the same as those of UP. 4. It is concluded that pharmacological preconditioning with U(50), that confers immediate cardioprotection, prevents changes of Ca(2+) homeostasis altered by MI/A in the rat heart. This may be responsible, at least partly, for the cardioprotective action. 5. The study also provided evidence that MI/A causes mobilization of Ca(2+) from SR to cytoplasm causing Ca(2+)-overload which may be due to reduced Ca(2+)-uptake by SR. MI/A also reduces spontaneous and electrically induced Ca(2+) release from SR.

A comparison of the effects of ATP and tetracaine on spontaneous Ca(2+) release from rat permeabilised cardiac myocytes

1. Fluo-3 fluorescence measurements were made in isolated beta-escin permeablised rat cardiac myocytes using confocal microscopy. Perfusion of a mock intracellular solution containing 0.22-0.23 microM Ca(2+) and 5 mM ATP elicited regular waves of Ca(2+) (approximately every 5 s) due to spontaneous release of Ca(2+) from the sarcoplasmic reticulum (SR). 2. An approximately linear relationship was noted between Ca(2+) wave velocity (v) and amplitude (sigma). Under the control conditions the ratio of velocity to amplitude (v/sigma) varied little and was 99.8 +/- 2.5 m s(-1) microM(-1) (n = 78). 3. Reduction of [ATP] in the bathing solution to 0.5 and 0.2 mM ATP progressively decreased Ca(2+) wave frequency and propagation velocity while increasing the amplitude. The changes in Ca(2+) wave characteristics in 0.5 mM ATP were similar to those observed during perfusion with 50 microM tetracaine. In 0.2 mM ATP the decline of [Ca(2+)] during a Ca(2+) wave was slowed suggesting a lowered rate of Ca(2+) re-uptake by the SR Ca(2+) pump. 4. Reduction of [ATP] to 0.1 mM abolished Ca(2+) waves after 15-20 s. Returning the [ATP] to 5 mM caused a burst of high frequency and large amplitude waves. Mean velocity of the first wave on returning to 5 mM ATP was larger than normal but the v/sigma value was 32 +/- 6 % of control (n = 6). In the similar burst on removal of 100 microM tetracaine v/sigma was higher than control (166 +/- 9 %, n = 6). 5. Rapid application of caffeine (10 mM) was used to assess the SR Ca(2+) content. This showed that SR Ca(2+) increased as [ATP] was reduced or [tetracaine] was increased. The highest SR Ca(2+) content was observed after perfusion with 0.1 mM ATP, which was 245 +/- 15 % of control values. 6. Returning [ATP] from 0.1 mM to 5 mM caused a burst of high frequency, large amplitude Ca(2+) waves. But recovery after incubation with 300 microM tetracaine resulted in SR Ca(2+) release with no coherent wave pattern. The reason for this discrepancy is discussed.