Calcium transient alternans in blood-perfused ischemic hearts: observations with fluorescent indicator fura red

A cross species thermoelectric and spatiotemporal analysis of alternans in live explanted hearts using dual voltage-calcium fluorescence optical mapping

Objective.Temperature plays a crucial role in influencing the spatiotemporal dynamics of the heart. Electrical instabilities due to specific thermal conditions typically lead to early period-doubling bifurcations and beat-to-beat alternans. These pro-arrhythmic phenomena manifest in voltage and calcium traces, resulting in compromised contractile behaviors. In such intricate scenario, dual optical mapping technique was used to uncover unexplored multi-scale and nonlinear couplings, essential for early detection and understanding of cardiac arrhythmia.Approach.We propose a methodological analysis of synchronized voltage-calcium signals for detecting alternans, restitution curves, and spatiotemporal alternans patterns under different thermal conditions, based on integral features calculation. To validate our approach, we conducted a cross-species investigation involving rabbit and guinea pig epicardial ventricular surfaces and human endocardial tissue under pacing-down protocols.Main results.We show that the proposed integral feature, as the area under the curve, could be an easily applicable indicator that may enhance the predictability of the onset and progression of cardiac alternans. Insights into spatiotemporal correlation analysis of characteristic spatial lengths across different heart species were further provided.Significance.Exploring cross-species thermoelectric features contributes to understanding temperature-dependent proarrhythmic regimes and their implications on coupled spatiotemporal voltage-calcium dynamics. The findings provide preliminary insights and potential strategies for enhancing arrhythmia detection and treatment.

Role of Mitochondrial ROS for Calcium Alternans in Atrial Myocytes

Atrial calcium transient (CaT) alternans is defined as beat-to-beat alternations in CaT amplitude and is causally linked to atrial fibrillation (AF). Mitochondria play a significant role in cardiac excitation-contraction coupling and Ca signaling through redox environment regulation. In isolated rabbit atrial myocytes, ROS production is enhanced during CaT alternans, measured by fluorescence microscopy. Exogenous ROS (tert-butyl hydroperoxide) enhanced CaT alternans, whereas ROS scavengers (dithiothreitol, MnTBAP, quercetin, tempol) alleviated CaT alternans. While the inhibition of cellular NADPH oxidases had no effect on CaT alternans, interference with mitochondrial ROS (ROSm) production had profound effects: (1) the superoxide dismutase mimetic MitoTempo diminished CaT alternans and shifted the pacing threshold to higher frequencies; (2) the inhibition of cyt c peroxidase by SS-31, and inhibitors of ROSm production by complexes of the electron transport chain S1QEL1.1 and S3QEL2, decreased the severity of CaT alternans; however (3) the impairment of mitochondrial antioxidant defense by the inhibition of nicotinamide nucleotide transhydrogenase with NBD-Cl and thioredoxin reductase-2 with auranofin enhanced CaT alternans. Our results suggest that intact mitochondrial antioxidant defense provides crucial protection against pro-arrhythmic CaT alternans. Thus, modulating the mitochondrial redox state represents a potential therapeutic approach for alternans-associated arrhythmias, including AF.

Innovative Characterization of Alternans Onset and Development in Dual Voltage-Calcium Whole-Heart Optical Mapping Signals at Multiple Thermal States

Cardiac electrical dynamics show complex space-time instabilities, like period-doubling bifurcation and beat-to-beat alternans, known to occur as pro-arrhythmic phenomena and linked to membrane voltage and intracellular calcium kinetics. Besides, cellular ionic dynamics are critically affected by temperature oscillations, further enhancing the complexity of such arrhythmias precursors that lead to irregular cardiac contraction. In this complex scenario, fluorescence dual optical mapping techniques allow the unveiling of nonlinear and multi-scale couplings. In this contribution, we propose a novel methodological analysis of synchronous dual voltage-calcium traces obtained from whole rabbit hearts for (i) detecting alternans onset and evolution, (ii) characterizing novel restitution curves, and (iii) defining spatio-temporal alternans patterns at four thermal states. We validate our approach against well-accepted analyses considering complete pacing-down restitution protocols. The proposed methodology computes integral features, e.g., area under the curve, suggesting that a novel, easy-to-use indicator, may advance predictability on alternans onset and evolution, further providing insights into spatio-temporal cardiac analyses.Clinical Relevance- This work introduces new methods for the early detection of cardiac alternans onset and development as precursors of arrhythmias and fibrillation at different temperatures.

Investigating two kinds of cellular alternans and corresponding TWA induced by impaired calcium cycling in myocardial ischemia

BACKGROUND

The utility of T wave alternans (TWA) in identifying arrhythmia risk has been demonstrated. During myocardial ischemia (MI), TWA could be induced by cellular alternans. However, the relationship between cellular alternans patterns and TWA patterns in MI has not been investigated thoroughly.

METHODS

We set MI conditions to simulate alternans. Either prolonging Ca²⁺ release or increasing spark-induced sparks (secondary sparks) can give rise to different patterns of APD alternans and TWA. In addition, different ischemic zones and reduced conduction velocity are also considered in one dimensional simulation.

RESULTS

Delay of Ca²⁺ release can produce discordant Ca²⁺-driven alternans in single cell simulation. Increasing secondary sparks leads to concordant alternans. Correspondingly, morphology and magnitude of TWA vary in two different cellular alternans. Epi ischemia results in alternans concentrating in the first half of T wave. Endo and transmural ischemia lead to fluctuations in the second half of T wave. In addition, slowing conduction velocity has no effect on TWA magnitude.

CONCLUSION

Specific ionic channel dysfunction and ischemic zones affect TWA patterns.

Clinical Potential of Beat-to-Beat Diastolic Interval Control in Preventing Cardiac Arrhythmias

Life‐threatening ventricular arrhythmias and sudden cardiac death are often preceded by cardiac alternans, a beat‐to‐beat oscillation in the T‐wave morphology or duration. However, given the spatiotemporal and structural complexity of the human heart, designing algorithms to effectively suppress alternans and prevent fatal rhythms is challenging. Recently, an antiarrhythmic constant diastolic interval pacing protocol was proposed and shown to be effective in suppressing alternans in 0‐, 1‐, and 2‐dimensional in silico studies as well as in ex vivo whole heart experiments. Herein, we provide a systematic review of the electrophysiological conditions and mechanisms that enable constant diastolic interval pacing to be an effective antiarrhythmic pacing strategy. We also demonstrate a successful translation of the constant diastolic interval pacing protocol into an ECG‐based real‐time control system capable of modulating beat‐to‐beat cardiac electrical activity and preventing alternans. Furthermore, we present evidence of the clinical utility of real‐time alternans suppression in reducing arrhythmia susceptibility in vivo. We provide a comprehensive overview of this promising pacing technique, which can potentially be translated into a clinically viable device that could radically improve the quality of life of patients experiencing abnormal cardiac rhythms.

Mitochondrial depolarization promotes calcium alternans: Mechanistic insights from a ventricular myocyte model

Mitochondria are vital organelles inside the cell and contribute to intracellular calcium (Ca2+) dynamics directly and indirectly via calcium exchange, ATP generation, and production of reactive oxygen species (ROS). Arrhythmogenic Ca2+ alternans in cardiac myocytes has been observed in experiments under abnormal mitochondrial depolarization. However, complex signaling pathways and Ca2+ cycling between mitochondria and cytosol make it difficult in experiments to reveal the underlying mechanisms of Ca2+ alternans under abnormal mitochondrial depolarization. In this study, we use a newly developed spatiotemporal ventricular myocyte computer model that integrates mitochondrial Ca2+ cycling and complex signaling pathways to investigate the mechanisms of Ca2+ alternans during mitochondrial depolarization. We find that elevation of ROS in response to mitochondrial depolarization plays a critical role in promoting Ca2+ alternans. Further examination reveals that the redox effect of ROS on ryanodine receptors and sarco/endoplasmic reticulum Ca2+-ATPase synergistically promote alternans. Upregulation of mitochondrial Ca2+ uniporter promotes Ca2+ alternans via Ca2+-dependent mitochondrial permeability transition pore opening. Due to their relatively slow kinetics, oxidized Ca2+/calmodulin-dependent protein kinase II activation and ATP do not play significant roles acutely in the genesis of Ca2+ alternans after mitochondrial depolarization, but their roles can be significant in the long term, mainly through their effects on sarco/endoplasmic reticulum Ca2+-ATPase activity. In conclusion, mitochondrial depolarization promotes Ca2+ alternans acutely via the redox effect of ROS and chronically by ATP reduction. It suppresses Ca2+ alternans chronically through oxidized Ca2+/calmodulin-dependent protein kinase II activation.

Mitochondrial calcium uniporter complex activation protects against calcium alternans in atrial myocytes

Cardiac alternans, defined as beat-to-beat alternations in action potential duration, cytosolic Ca transient (CaT) amplitude, and cardiac contraction is associated with atrial fibrillation (AF) and sudden cardiac death. At the cellular level, cardiac alternans is linked to abnormal intracellular calcium handling during excitation-contraction coupling. We investigated how pharmacological activation or inhibition of cytosolic Ca sequestration via mitochondrial Ca uptake and mitochondrial Ca retention affects the occurrence of pacing-induced CaT alternans in isolated rabbit atrial myocytes. Cytosolic CaTs were recorded using Fluo-4 fluorescence microscopy. Alternans was quantified as the alternans ratio (AR = 1 - CaT_small/CaT_large, where CaT_small and CaT_large are the amplitudes of the small and large CaTs of a pair of alternating CaTs). Inhibition of mitochondrial Ca sequestration via mitochondrial Ca uniporter complex (MCUC) with Ru360 enhanced the severity of CaT alternans (AR increase) and lowered the pacing frequency threshold for alternans. In contrast, stimulation of MCUC mediated mitochondrial Ca uptake with spermine-rescued alternans (AR decrease) and increased the alternans pacing threshold. Direct measurement of mitochondrial [Ca] in membrane permeabilized myocytes with Fluo-4 loaded mitochondria revealed that spermine enhanced and accelerated mitochondrial Ca uptake. Stimulation of mitochondrial Ca retention by preventing mitochondrial Ca efflux through the mitochondrial permeability transition pore with cyclosporin A also protected from alternans and increased the alternans pacing threshold. Pharmacological manipulation of MCUC activity did not affect sarcoplasmic reticulum Ca load. Our results suggest that activation of Ca sequestration by mitochondria protects from CaT alternans and could be a potential therapeutic target for cardiac alternans and AF prevention.NEW & NOTEWORTHY This study provides conclusive evidence that mitochondrial Ca uptake and retention protects from Ca alternans, whereas uptake inhibition enhances Ca alternans. The data suggest pharmacological mitochondrial Ca cycling modulation as a potential therapeutic strategy for alternans-related cardiac arrhythmia prevention.

Real-Time Closed-Loop Suppression of Repolarization Alternans Reduces Arrhythmia Susceptibility In Vivo

BACKGROUND

Repolarization alternans (RA) has been implicated in the pathogenesis of ventricular arrhythmias and sudden cardiac death.

METHODS

We have developed a real-time, closed-loop system to record and analyze RA from multiple intracardiac leads, and deliver dynamically R-wave triggered pacing stimuli during the absolute refractory period. We have evaluated the ability of this system to control RA and reduce arrhythmia susceptibility, in vivo.

RESULTS

R-wave triggered pacing can induce RA, the magnitude of which can be modulated by varying the amplitude, pulse width, and size of the pacing vector. Using a swine model (n=9), we demonstrate that to induce a 1 µV change in the alternans voltage on the body surface, coronary sinus and left ventricle leads, requires a delivered charge of 0.04±0.02, 0.05±0.025, and 0.06±0.033 µC, respectively, while to induce a one unit change of the K_score, requires a delivered charge of 0.93±0.73, 0.32±0.29, and 0.33±0.37 µC, respectively. For all body surface and intracardiac leads, both Δ(alternans voltage) and ΔK_score between baseline and R-wave triggered paced beats increases consistently with an increase in the pacing pulse amplitude, pulse width, and vector spacing. Additionally, we show that the proposed method can be used to suppress spontaneously occurring alternans (n=7), in the presence of myocardial ischemia. Suppression of RA by pacing during the absolute refractory period results in a significant reduction in arrhythmia susceptibility, evidenced by a lower S_rank score during programmed ventricular stimulation compared with baseline before ischemia.

CONCLUSIONS

We have developed and evaluated a novel closed-loop method to dynamically modulate RA in a swine model. Our data suggest that suppression of RA directly reduces arrhythmia susceptibility and reinforces the concept that RA plays a critical role in the pathophysiology of arrhythmogenesis.

The roles of mid-myocardial and epicardial cells in T-wave alternans development: a simulation study

Background

The occurrence of T-wave alternans in electrocardiographic signals was recently linked to susceptibility to ventricular arrhythmias and sudden cardiac death. Thus, by detecting and comprehending the origins of T-wave alternans, it might be possible to prevent such events.

Results

Here, we simulated T-wave alternans in a computer-generated human heart model by modulating the action potential duration and amplitude during the first part of the repolarization phase. We hypothesized that changes in the intracardiac alternans patterns of action potential properties would differentially influence T-wave alternans measurements at the body surface. Specifically, changes were simulated globally in the whole left and right ventricles to simulate concordant T-wave alternans, and locally in selected regions to simulate discordant and regional discordant, hereinafter referred to as “regional”, T-wave alternans. Body surface potential maps and 12-lead electrocardiographic signals were then computed. In depth discrimination, the influence of epicardial layers on T-wave alternans development was significantly higher than that of mid-myocardial cells. Meanwhile, spatial discrimination revealed that discordant and regional action potential property changes had a higher influence on T-wave alternans amplitude than concordant changes. Notably, varying T-wave alternans sources yielded distinct body surface potential map patterns for T-wave alternans amplitude, which can be used for location of regions within hearts exhibiting impaired repolarization. The highest ability for T-wave alternans detection was achieved in lead V1. Ultimately, we proposed new parameters Vector Magnitude Alternans and Vector Angle Alternans, with higher ability for T-wave alternans detection when using multi-lead electrocardiographic signals processing than for single leads. Finally, QT alternans was found to be associated with the process of T-wave alternans generation.

Conclusions

The distributions of the body surface T-wave alternans amplitude have been shown to have unique patterns depending on the type of alternans (concordant, discordant or regional) and the location of the disturbance in the heart. The influence of epicardial cells on T-wave alternans development is significantly higher than that of mid-myocardial cells, among which the sub-endocardial layer exerted the highest influence. QT interval alternans is identified as a phenomenon that correlate with T-wave alternans.

Sympathetic Nervous Regulation of Calcium and Action Potential Alternans in the Intact Heart

Rationale: Arrhythmogenic cardiac alternans are thought to be an important determinant for the initiation of ventricular fibrillation. There is limited information on the effects of sympathetic nerve stimulation (SNS) on alternans in the intact heart and the conclusions of existing studies, focused on investigating electrical alternans, are conflicted. Meanwhile, several lines of evidence implicate instabilities in Ca handling, not electrical restitution, as the primary mechanism underpinning alternans. Despite this, there have been no studies on Ca alternans and SNS in the intact heart. The present study sought to address this, by application of voltage and Ca optical mapping for the simultaneous study of APD and Ca alternans in the intact guinea pig heart during direct SNS.

Objective: To determine the effects of SNS on APD and Ca alternans in the intact guinea pig heart and to examine the mechanism(s) by which the effects of SNS are mediated.

Methods and Results: Studies utilized simultaneous voltage and Ca optical mapping in isolated guinea pig hearts with intact innervation. Alternans were induced using a rapid dynamic pacing protocol. SNS was associated with rate-independent shortening of action potential duration (APD) and the suppression of APD and Ca alternans, as indicated by a shift in the alternans threshold to faster pacing rates. Qualitatively similar results were observed with exogenous noradrenaline perfusion. In contrast with previous reports, both SNS and noradrenaline acted to flatten the slope of the electrical restitution curve. Pharmacological block of the slow delayed rectifying potassium current (I_Ks), sufficient to abolish I_Ks-mediated APD-adaptation, partially reversed the effects of SNS on pacing-induced alternans. Treatment with cyclopiazonic acid, an inhibitor of the sarco(endo)plasmic reticulum ATPase, had opposite effects to that of SNS, acting to increase susceptibility to alternans, and suggesting that accelerated Ca reuptake into the sarcoplasmic reticulum is a major mechanism by which SNS suppresses alternans in the guinea pig heart.

Conclusions: SNS suppresses calcium and action potential alternans in the intact guinea pig heart by an action mediated through accelerated Ca handling and via increased I_Ks.

Altered Calcium Handling and Ventricular Arrhythmias in Acute Ischemia

Acute ischemia results in deadly cardiac arrhythmias that are a major contributor to sudden cardiac death (SCD). The electrophysiological changes involved have been extensively studied, yet the mechanisms of ventricular arrhythmias during acute ischemia remain unclear. What is known is that during acute ischemia both focal (ectopic excitation) and nonfocal (reentry) arrhythmias occur, due to an interaction of altered electrical, mechanical, and biochemical properties of the myocardium. There is particular interest in the role that alterations in intracellular calcium handling, which cause changes in intracellular calcium concentration and to the calcium transient, play in ischemia-induced arrhythmias. In this review, we briefly summarize the known contributors to ventricular arrhythmias during acute ischemia, followed by an in-depth examination of the potential contribution of altered intracellular calcium handling, which may include novel targets for antiarrhythmic therapy.

A technical review of optical mapping of intracellular calcium within myocardial tissue

Optical mapping of Ca(2+)-sensitive fluorescence probes has become an extremely useful approach and adopted by many cardiovascular research laboratories to study a spectrum of myocardial physiology and disease conditions. Optical mapping data are often displayed as detailed pseudocolor images, providing unique insight for interpreting mechanisms of ectopic activity, action potential and Ca(2+) transient alternans, tachycardia, and fibrillation. Ca(2+)-sensitive fluorescent probes and optical mapping systems continue to evolve in the ongoing effort to improve therapies that ease the growing worldwide burden of cardiovascular disease. In this technical review we provide an updated overview of conventional approaches for optical mapping of Cai (2+) within intact myocardium. In doing so, a brief history of Cai (2+) probes is provided, and nonratiometric and ratiometric Ca(2+) probes are discussed, including probes for imaging sarcoplasmic reticulum Ca(2+) and probes compatible with potentiometric dyes for dual optical mapping. Typical measurements derived from optical Cai (2+) signals are explained, and the analytics used to compute them are presented. Last, recent studies using Cai (2+) optical mapping to study arrhythmias, heart failure, and metabolic perturbations are summarized.

Cardiac myocyte alternans in intact heart: Influence of cell-cell coupling and β-adrenergic stimulation

BACKGROUND

Cardiac alternans are proarrhythmic and mechanistically link cardiac mechanical dysfunction and sudden cardiac death. Beat-to-beat alternans occur when beats with large Ca(2+) transients and long action potential duration (APD) alternate with the converse. APD alternans are typically driven by Ca(2+) alternans and sarcoplasmic reticulum (SR) Ca(2+) release alternans. But the effect of intercellular communication via gap junctions (GJ) on alternans in the intact heart remains unknown.

OBJECTIVE

We assessed the effects of cell-to-cell coupling on local alternans in intact Langendorff-perfused mouse hearts, measuring single myocyte [Ca(2+)] alternans synchronization among neighboring cells, and effects of β-adrenergic receptor (β-AR) activation and reduced GJ coupling.

METHODS AND RESULTS

Mouse hearts (C57BL/6) were retrogradely perfused and loaded with Fluo8-AM to record cardiac myocyte [Ca(2+)] in situ with confocal microscopy. Single cell resolution allowed analysis of alternans within the intact organ during alternans induction. Carbenoxolone (25 μM), a GJ inhibitor, significantly increased the occurrence and amplitude of alternans in single cells within the intact heart. Alternans were concordant between neighboring cells throughout the field of view, except transiently during onset. β-AR stimulation only reduced Ca(2+) alternans in tissue that had reduced GJ coupling, matching effects seen in isolated myocytes.

CONCLUSIONS

Ca(2+) alternans among neighboring myocytes is predominantly concordant, likely because of electrical coupling between cells. Consistent with this, partial GJ uncoupling increased propensity and amplitude of Ca(2+) alternans, and made them more sensitive to reversal by β-AR activation, as in isolated myocytes. Electrical coupling between myocytes may thus limit the alternans initiation, but also allow alternans to be more stable once established.

T-wave alternans as an arrhythmic risk stratifier: state of the art

Microvolt level T-wave alternans (MTWA), a phenomenon of beat-to-beat variability in the repolarization phase of the ventricles, has been closely associated with an increased risk of ventricular tachyarrhythmic events (VTE) and sudden cardiac death (SCD) during medium- and long-term follow-up. Recent observations also suggest that heightened MTWA magnitude may be closely associated with short-term risk of impending VTE. At the subcellular and cellular level, perturbations in calcium transport processes likely play a primary role in the genesis of alternans, which then secondarily lead to alternans of action potential morphology and duration (APD). As such, MTWA may play a role not only in risk stratification but also more fundamentally in the pathogenesis of VTE. In this paper, we outline recent advances in understanding the pathogenesis of MTWA and also the utility of T-wave alternans testing for clinical risk stratification. We also highlight emerging clinical applications for MTWA.

Inhibition of the late sodium current slows t-tubule disruption during the progression of hypertensive heart disease in the rat

The treatment of heart failure (HF) is challenging and morbidity and mortality are high. The goal of this study was to determine if inhibition of the late Na(+) current with ranolazine during early hypertensive heart disease might slow or stop disease progression. Spontaneously hypertensive rats (aged 7 mo) were subjected to echocardiographic study and then fed either control chow (CON) or chow containing 0.5% ranolazine (RAN) for 3 mo. Animals were then restudied, and each heart was removed for measurements of t-tubule organization and Ca(2+) transients using confocal microscopy of the intact heart. RAN halted left ventricular hypertrophy as determined from both echocardiographic and cell dimension (length but not width) measurements. RAN reduced the number of myocytes with t-tubule disruption and the proportion of myocytes with defects in intracellular Ca(2+) cycling. RAN also prevented the slowing of the rate of restitution of Ca(2+) release and the increased vulnerability to rate-induced Ca(2+) alternans. Differences between CON- and RAN-treated animals were not a result of different expression levels of voltage-dependent Ca(2+) channel 1.2, sarco(endo)plasmic reticulum Ca(2+)-ATPase 2a, ryanodine receptor type 2, Na(+)/Ca(2+) exchanger-1, or voltage-gated Na(+) channel 1.5. Furthermore, myocytes with defective Ca(2+) transients in CON rats showed improved Ca(2+) cycling immediately upon acute exposure to RAN. Increased late Na(+) current likely plays a role in the progression of cardiac hypertrophy, a key pathological step in the development of HF. Early, chronic inhibition of this current slows both hypertrophy and development of ultrastructural and physiological defects associated with the progression to HF.

Uncoupling the mitochondria facilitates alternans formation in the isolated rabbit heart

Alternans of action potential duration (APD) and intracellular calcium ([Ca²⁺]i) transients in the whole heart are thought to be markers of increased propensity to ventricular fibrillation during ischemia-reperfusion injuries. During ischemia, ATP production is affected and the mitochondria become uncoupled, which may affect alternans formation in the heart. The aim of our study was to investigate the role of mitochondria on the formation of APD and [Ca²⁺]i alternans in the isolated rabbit heart. We performed dual voltage and [Ca²⁺]i optical mapping of isolated rabbit hearts under control conditions, global no-flow ischemia (n = 6), and after treatment with 50 nM of the mitochondrial uncoupler FCCP (n = 6). We investigated the formation of alternans of APD, [Ca²⁺]i amplitude (CaA), and [Ca²⁺]i duration (CaD) under different rates of pacing. We found that treatment with FCCP leads to the early occurrence of APD, CaD, and CaA alternans; an increase of intraventricular APD but not CaD heterogeneity; and significant reduction in conduction velocity compared with that of control. Furthermore, we demonstrated that FCCP and global ischemia have similar effects on the prolongation of [Ca²⁺]i transients, whereas ischemia induces a significantly larger reduction of APD compared with that in FCCP treatment. In conclusion, our results demonstrate that uncoupling of mitochondria leads to an earlier occurrence of alternans in the heart. Thus, in conditions of mitochondrial stress, as seen during myocardial ischemia, uncoupled mitochondria may be responsible for the formation of both APD and [Ca²⁺]i alternans in the heart, which in turn creates a substrate for ventricular arrhythmias.

Irregular Ca(2+) oscillations regulate transcription via cumulative spike duration and spike amplitude

BACKGROUND

[Ca(2+)](i) oscillations are irregular and heterogeneous.

RESULTS

The correlations between NFκB/STAT3-GFP transcription and [Ca(2+)](i) spike amplitude/cumulative spike duration are revealed by simultaneous monitoring in single cells and validated in cell population.

CONCLUSION

[Ca(2+)](i) oscillations regulate transcription through [Ca(2+)](i) spike amplitude and cumulative spike duration.

SIGNIFICANCE

How irregular [Ca(2+)](i) oscillations control transcription is crucial for understanding biological [Ca(2+)](i) signal-regulated events. Agonist-stimulated [Ca(2+)](i) oscillations are universally irregular in their kinetics. How irregular [Ca(2+)](i) oscillations dynamically regulate agonist-stimulated downstream events has not been studied. To overcome the obstacles of irregularity and heterogeneity of [Ca(2+)](i) oscillations, agonist-stimulated [Ca(2+)](i) signaling and NFκB/STAT3-GFP nuclear translocation were simultaneously monitored in each single cell examined. The cause-effect relationship between [Ca(2+)](i) oscillation parameters and transcriptional activities was validated in cell populations through irregular [Ca(2+)](i) oscillations with varied parameters. The time duration of cumulative [Ca(2+)](i) elevations reaching the threshold [Ca(2+)](i) level for a transcriptional factor activation and [Ca(2+)](i) spike amplitude was found to control agonist-stimulated transcription and gene expression.

Regulation of cardiac alternans by β-adrenergic signaling pathways

In cat atrial myocytes, β-adrenergic receptor (β-AR) stimulation exerts profound effects on excitation-contraction coupling and cellular Ca(2+) cycling that are mediated by β(1)- and β(2)-AR subtypes coupled to G proteins (G(s) and G(i)). In this study, we determined the effects of β-AR stimulation on pacing-induced Ca(2+) alternans. Ca(2+) alternans was recorded from single cat atrial myocytes with the fluorescent Ca(2+) indicator indo-1. Stable Ca(2+) alternans occurred at an average pacing frequency of 1.7 Hz at room temperature with a mean alternans ratio of 0.43. Nonselective β-AR stimulation as well as selective stimulation of β(1)/G(s), β(2)/G(s) + G(i), and β(2)/G(s) coupled pathways all abolished pacing-induced Ca(2+) alternans. β(1)-AR stimulation abolished alternans through stimulation of PKA and Ca(2+)/calmodulin-dependent protein kinase II, whereas β(2)-AR stimulation exclusively involved PKA and was mediated via G(s), whereas a known second pathway in cat atrial myocytes acting through G(i) and nitric oxide production was not involved in alternans regulation. Inhibition of various mitochondrial functions (dissipation of the mitochondrial membrane potential or inhibition of mitochondrial F(1)/F(0)-ATP synthase, mitochondrial Ca(2+) uptake via the mitochondrial Ca(2+) uniporter, and Ca(2+) extrusion via mitochondrial Na(+)/Ca(2+) exchange) enhanced Ca(2+) alternans; however, β-AR stimulation still abrogated alternans, provided that sufficient cellular ATP was available. Selective inhibition of mitochondrial or glycolytic ATP production did not prevent β-AR stimulation from abolishing Ca(2+) alternans. However, when both ATP sources were depleted, β-AR stimulation failed to decrease Ca(2+) alternans. These results indicate that in atrial myocytes, β-AR stimulation protects against pacing-induced alternans by acting through parallel and complementary signaling pathways.

Refractoriness of sarcoplasmic reticulum Ca2+ release determines Ca2+ alternans in atrial myocytes

Cardiac alternans is a recognized risk factor for cardiac arrhythmia and sudden cardiac death. At the cellular level, Ca(2+) alternans appears as cytosolic Ca(2+) transients of alternating amplitude at regular beating frequency. Cardiac alternans is a multifactorial process but has been linked to disturbances in intracellular Ca(2+) regulation. In atrial myocytes, we tested the role of voltage-gated Ca(2+) current, sarcoplasmic reticulum (SR) Ca(2+) load, and restitution properties of SR Ca(2+) release for the occurrence of pacing-induced Ca(2+) alternans. Voltage-clamp experiments revealed that peak Ca(2+) current was not affected during alternans, and alternans of end-diastolic SR Ca(2+) load, evaluated by application of caffeine or measured directly with an intra-SR fluorescent Ca(2+) indicator (fluo-5N), were not a requirement for cytosolic Ca(2+) alternans. Restitution properties and kinetics of refractoriness of Ca(2+) release after activation during alternans were evaluated by four different approaches: measurements of 1) the delay (latency) of occurrence of spontaneous global Ca(2+) releases and 2) Ca(2+) spark frequency, both during rest after a large and small alternans Ca(2+) transient; 3) the magnitude of premature action potential-induced Ca(2+) transients after a large and small beat; and 4) the efficacy of a photolytically induced Ca(2+) signal (Ca(2+) uncaging from DM-nitrophen) to trigger additional Ca(2+) release during alternans. The results showed that the latency of global spontaneous Ca(2+) release was prolonged and Ca(2+) spark frequency was decreased after the large Ca(2+) transient during alternans. Furthermore, the restitution curve of the Ca(2+) transient elicited by premature action potentials or by photolysis-induced Ca(2+) release from the SR lagged behind after a large-amplitude transient during alternans compared with the small-amplitude transient. The data demonstrate that beat-to-beat alternation of the time-dependent restitution properties and refractory kinetics of the SR Ca(2+) release mechanism represents a key mechanism underlying cardiac alternans.

The transient receptor potential (TRP) channel TRPC3 TRP domain and AMP-activated protein kinase binding site are required for TRPC3 activation by erythropoietin

Modulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an important signaling pathway controlling erythroid proliferation and differentiation. Transient receptor potential (TRP) channels TRPC3 and homologous TRPC6 are expressed on normal human erythroid precursors, but Epo stimulates an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Here, the role of specific domains in the different responsiveness of TRPC3 and TRPC6 to erythropoietin was explored. TRPC3 and TRPC6 TRP domains differ in seven amino acids. Substitution of five amino acids (DDKPS) in the TRPC3 TRP domain with those of TRPC6 (EERVN) abolished the Epo-stimulated increase in [Ca(2+)](i). Substitution of EERVN in TRPC6 TRP domain with DDKPS in TRPC3 did not confer Epo responsiveness. However, substitution of TRPC6 TRP with DDKPS from TRPC3 TRP, as well as swapping the TRPC6 distal C terminus (C2) with that of TRPC3, resulted in a chimeric TRPC6 channel with Epo responsiveness similar to TRPC3. Substitution of TRPC6 with TRPC3 TRP and the putative TRPC3 C-terminal AMP-activated protein kinase (AMPK) binding site straddling TRPC3 C1/C2 also resulted in TRPC6 activation. In contrast, substitution of the TRPC3 C-terminal leucine zipper motif or TRPC3 phosphorylation sites Ser-681, Ser-708, or Ser-764 with TRPC6 sequence did not affect TRPC3 Epo responsiveness. TRPC3, but not TRPC6, and TRPC6 chimeras expressing TRPC3 C2 showed significantly increased plasma membrane insertion following Epo stimulation and substantial cytoskeletal association. The TRPC3 TRP domain, distal C terminus (C2), and AMPK binding site are critical elements that confer Epo responsiveness. In particular, the TRPC3 C2 and AMPK site are essential for association of TRPC3 with the cytoskeleton and increased channel translocation to the cell surface in response to Epo stimulation.

A novel lead configuration for optimal spatio-temporal detection of intracardiac repolarization alternans

BACKGROUND

Electric alternans is a pattern of variation in the shape of ECG waveform that occurs every other beat. In humans, alternation in ventricular repolarization, known as repolarization alternans (RA), has been associated with increased vulnerability to ventricular tachycardia/fibrillation and sudden cardiac death.

METHODS AND RESULTS

This study investigates the spatio-temporal variability of intracardiac RA and its relationship to body surface RA in an acute myocardial ischemia model in swine. We developed a real-time multichannel repolarization signal acquisition, display, and analysis system to record ECG signals from catheters in the right ventricle, coronary sinus, left ventricle, and epicardial surface before and after circumflex coronary artery balloon occlusion. We found that RA is detectable within 4 minutes after the onset ischemia and is most prominently seen during the first half of the repolarization interval. Ischemia-induced RA was detectable on unipolar and bipolar leads (both in near- and far-field configurations) and on body surface leads. Far-field bipolar intracardiac leads were more sensitive for RA detection than body surface leads, with the probability of body surface RA detection increasing as the number of intracardiac leads detecting RA increased, approaching 100% when at least three intracardiac leads detected RA. We developed a novel, clinically applicable intracardiac lead system based on a triangular arrangement of leads spanning the right ventricular and coronary sinus catheters, which provided the highest sensitivity for intracardiac RA detection when compared with any other far-field bipolar sensing configurations.

CONCLUSIONS

In conclusion, intracardiac alternans, a complex spatio-temporal phenomenon associated with arrhythmia susceptibility and sudden cardiac death, can be reliably detected through a novel triangular right ventricular-coronary sinus lead configuration.

The role of mitochondria for the regulation of cardiac alternans

Electro-mechanical and Ca alternans is a beat-to-beat alternation of action potential duration, contraction strength and Ca transient amplitude observed in cardiac myocytes at regular stimulation frequency. Ca alternans is a multifactorial process that is causally linked to cardiac arrhythmias. At the cellular level, conditions that increase fractional release from the sarcoplasmic reticulum or reduce diastolic Ca sequestration favor the occurrence of alternans. Mitochondria play a significant role in cardiac excitation–contraction coupling and Ca signaling by providing the energy for contraction and ATP-dependent processes and possibly by serving as Ca buffering organelles. Here we tested the hypothesis that impairment of mitochondrial function generates conditions that favor the occurrence of Ca alternans. Alternans were elicited by electrical pacing (>1 Hz) in single cat atrial myocytes and intracellular Ca ([Ca]_i) was measured with the fluorescent Ca indicator Indo-1. The degree of alternans was quantified as the alternans ratio (AR = 1 − S/L, where S/L is the ratio of the small to the large amplitude of a pair of alternating Ca transients). Dissipation of mitochondrial membrane potential (with FCCP) as well as inhibition of mitochondrial F₁/F₀-ATP synthase (oligomycin), electron transport chain (rotenone, antimycin, CN⁻), Ca-dependent dehydrogenases and mitochondrial Ca uptake or extrusion, all enhanced AR and lowered the threshold for the occurrence of Ca alternans. The data indicate that impairment of mitochondrial function adversely affects cardiac Ca cycling leading to proarrhythmic Ca alternans.

High-precision recording of the action potential in isolated cardiomyocytes using the near-infrared fluorescent dye di-4-ANBDQBS

The use of voltage-sensitive fluorescent dyes (VSD) for noninvasive measurement of the action potential (AP) in isolated cells has been hindered by low-photon yield of the preparation, dye toxicity, and photodynamic damage. Here we used a new red-shifted VSD, di-4-ANBDQBS, and a fast electron-multiplied charge-coupled device camera for optical AP (OAP) recording in guinea pig cardiac myocytes. Loading di-4-ANBDQBS did not alter APs recorded with micropipette. With short laser exposures (just enough to record one OAP every 1-5 min), di-4-ANBDQBS yielded fluorescent signals with very high signal-to-background ratios (change in fluorescence on depolarization/fluorescence at resting potential: 19.2 ± 4.1%) and signal-to-noise ratios (40 ± 13.2). Quantum chemical calculations comparing the ANBDQ chromophore to the conventional ANEP chromophore showed that the higher wavelength and the greater voltage sensitivity of the former have the same electro-optical origin: a longer path for electron redistribution in the excited state. OAP closely tracked simultaneously recorded electrical APs, permitting measurement of AP duration within 1% error. Prolonged laser exposure caused progressive AP duration prolongation and instability. However, these effects were alleviated or abolished by reducing the dye concentration and by perfusion with antioxidants. Thus the presented technique provides a unique opportunity for noninvasive AP recording in single cardiomyocytes.

Early development of intracellular calcium cycling defects in intact hearts of spontaneously hypertensive rats

Defects in excitation-contraction coupling have been reported in failing hearts, but little is known about the relationship between these defects and the development of heart failure (HF). We compared the early changes in intracellular Ca(2+) cycling to those that underlie overt pump dysfunction and arrhythmogenesis found later in HF. Laser-scanning confocal microscopy was used to measure Ca(2+) transients in myocytes of intact hearts in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs) at different ages. Early compensatory mechanisms include a positive inotropic effect in SHRs at 7.5-9 mo compared with 6 mo. Ca(2+) transient duration increased at 9 mo in SHRs, indicating changes in Ca(2+) reuptake during decompensation. Cell-to-cell variability in Ca(2+) transient duration increased at 7.5 mo, decreased at 9 mo, and increased again at 22 mo (overt HF), indicating extensive intercellular variability in Ca(2+) transient kinetics during disease progression. Vulnerability to intercellular concordant Ca(2+) alternans increased at 9-22 mo in SHRs and was mirrored by a slowing in Ca(2+) transient restitution, suggesting that repolarization alternans and the resulting repolarization gradients might promote reentrant arrhythmias early in disease development. Intercellular discordant and subcellular Ca(2+) alternans increased as early as 7.5 mo in SHRs and may also promote arrhythmias during the compensated phase. The incidence of spontaneous and triggered Ca(2+) waves was increased in SHRs at all ages, suggesting a higher likelihood of triggered arrhythmias in SHRs compared with WKY rats well before HF develops. Thus serious and progressive defects in Ca(2+) cycling develop in SHRs long before symptoms of HF occur. Defective Ca(2+) cycling develops early and affects a small number of myocytes, and this number grows with age and causes the transition from asymptomatic to overt HF. These defects may also underlie the progressive susceptibility to Ca(2+) alternans and Ca(2+) wave activity, thus increasing the propensity for arrhythmogenesis in HF.

Regulation of thromboxane receptor signaling at multiple levels by oxidative stress-induced stabilization, relocation and enhanced responsiveness

BACKGROUND

Thromboxane A(2) (TxA(2)) is a major, unstable arachidonic acid metabolite, and plays a key role in normal physiology and control of vascular tone. The human thromboxane receptor (TPβ), expressed in COS-7 cells, is located predominantly in the endoplasmic reticulum (ER). Brief hydrogen peroxide exposure increases the efficiency of translocation of TPβ from the ER into the Golgi complex, inducing maturation and stabilization of TPβ. However, the ultimate fate of this post-ER TPβ pool is not known, nor is its capacity to initiate signal transduction. Here we specifically assessed if functional TPβ was transported to the plasma membrane following H(2)O(2) exposure.

RESULTS

We demonstrate, by biotinylation and confocal microscopy, that exposure to H(2)O(2) results in rapid delivery of a cohort of TPβ to the cell surface, which is stable for at least eight hours. Surface delivery is brefeldin A-sensitive, indicating that translocation of this receptor cohort is from internal pools and via the Golgi complex. H(2)O(2) treatment results in potentiation of the increase to intracellular calcium concentrations in response to TPβ agonists U46619 and 8-iso PGF(2α) and also in the loss of ligand-dependent receptor internalization. Further there is increased responsiveness to a second application of the agonist. Finally we demonstrate that the effect of H(2)O(2) on stimulating surface delivery is shared with the FP prostanoid receptor but not the EP3 or EP4 receptors.

CONCLUSIONS/SIGNIFICANCE

In summary, brief exposure to H(2)O(2) results in an immediate and sustained increase in the surface pool of thromboxane receptor that is capable of mediating a persistent hyper-responsiveness of the cell and suggests a highly sophisticated mechanism for rapidly regulating thromboxane signaling.

Acidosis and ischemia increase cellular Ca2+ transient alternans and repolarization alternans susceptibility in the intact rat heart

Cardiac cellular Ca(2+) transient (CaT) alternans and electrocardiographic T-wave alternans (TWA) often develop in myocardial ischemia, but the mechanisms for this relationship have not been elucidated. Acidosis is a major component of ischemia, but there is no direct evidence linking acidosis-induced cellular CaT alternans to ischemia-induced CaT alternans and TWA in whole heart. We used laser-scanning confocal microscopy to measure intracellular Ca(2+) (Ca(i)(2+)) cycling in individual myocytes of fluo-4 AM-loaded rat hearts and simultaneously recorded pseudo-ECGs to investigate changes in CaTs and late-phase repolarization, respectively, during baseline and rapid pacing under control and either globally acidic or globally ischemic conditions. Acidosis (hypercapnia; pH 6.6) increased diastolic Ca(i)(2+) levels, prolonged CaT duration, and shifted to slower heart rates both the development of pacing-induced acidosis-induced CaT alternans (both concordant and discordant) and of repolarization alternans (RPA, a measure of TWA in rat ECGs). The magnitudes of these shifts were equivalent for both CaT alternans and RPA, suggesting a close association between them. Nearly identical results were found in low-flow global ischemia. Additionally, ischemic preconditioning reduced the increased propensity for CaT alternans and RPA development and was mimicked by preconditioning by acidosis alone. Our results demonstrate that global acidosis or ischemia modifies Ca(i)(2+) cycling in myocytes such that the diastolic Ca(i)(2+) rises and the cellular CaT duration is prolonged, causing spatially concordant as well as spatially discordant cellular CaT alternans to develop at slower heart rates than in controls. Since RPA also developed at slower heart rates, our results suggest that acidosis is a major contributor to CaT alternans, which underlies the proarrhythmic state induced by myocardial ischemia and therefore may play a role in its modulation and prevention.

TRPC3 activation by erythropoietin is modulated by TRPC6

Regulation of intracellular calcium ([Ca(2+)](i)) by erythropoietin (Epo) is an essential part of signaling pathways controlling proliferation and differentiation of erythroid progenitors, but regulatory mechanisms are largely unknown. TRPC3 and the homologous TRPC6 are two members of the transient receptor potential channel (TRPC) superfamily that are expressed on normal human erythroid precursors. Here we show that TRPC3 expression increases but TRPC6 decreases during erythroid differentiation. This is associated with a significantly greater increase in [Ca(2+)](i) in response to Epo stimulation, suggesting that the ratio of TRPC3/TRPC6 is physiologically important. In HEK 293T cells heterologously expressing TRPC and erythropoietin receptor (Epo-R), Epo stimulated an increase in [Ca(2+)](i) through TRPC3 but not TRPC6. Replacement of the C terminus of TRPC3 with the TRPC6 C terminus (TRPC3-C6C) resulted in loss of activation by Epo. In contrast, substitution of the C terminus of TRPC6 with that of TRPC3 (TRPC6-C3C) resulted in an increase in [Ca(2+)](i) in response to Epo. Substitution of the N termini had no effect. Domains in the TRPC3 C terminus between amino acids 671 and 746 are critical for the response to Epo. Epo-R and phospholipase Cgamma associated with TRPC3, and these interactions were significantly reduced with TRPC6 and TRPC3-C6C chimeras. TRPC3 and TRPC6 form heterotetramers. Coexpression of TRPC6 or C3/C6 chimeras with TRPC3 and Epo-R inhibited the Epo-stimulated increase in [Ca(2+)](i). In a heterologous expression system, Epo stimulation increased cell surface expression of TRPC3, which was inhibited by TRPC6. However, in primary erythroblasts, an increase in TRPC3 cell surface expression was not observed in erythroblasts in which Epo stimulated an increase in [Ca(2+)](i), demonstrating that increased membrane insertion of TRPC3 is not required. These data demonstrate that TRPC6 regulates TRPC3 activation by Epo. Endogenously, regulation of TRPC3 by TRPC6 may primarily be through modulation of signaling mechanisms, including reduced interaction of TRPC6 with phospholipase Cgamma and Epo-R.

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

BACKGROUND

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Characteristics of intracellular Ca2+ cycling in intact rat heart: a comparison of sex differences

Males and females show distinct differences in action potential waveform, ion channel expression patterns, and ECG characteristics. However, it is not known how sex-based differences in Ca2+ cycling might contribute to these differences in electrophysiological activity. The goal of this study was to investigate the differences in cellular Ca2+ transients in males and females and to examine how these might contribute to electrophysiological function. Ca2+ transients were measured in individual myocytes within microscopic regions of the fluo-4 AM-loaded left ventricular epicardium of intact rat heart of both sexes (3 to 5 mo old). Pacing protocols were used to measure transient characteristics at a basic cycle length of 500 ms and during 10-s trains of rapid pacing delivered to the left ventricular apex. Ca2+ transients were smaller in magnitude and longer in duration in females than in males. More importantly, the variability in Ca2+ transient characteristics between myocytes in a microscopic recording site (heterogeneity index) was greater for females than males for characteristics related to transient duration. The rate sensitivity of Ca2+ alternans development in individual myocytes was greater in females than in males, but there was also a greater heterogeneity in cellular responses to the rate dependence of alternans development in females. The longer Ca2+ transients in females were also associated with slower restitution, which was likely to be responsible for the development of Ca2+ and repolarization alternans at slower heart rates. These results demonstrate that there are distinct differences in cellular Ca2+ cycling in male and female rat hearts. Not only is there slower reuptake of Ca2+ in female rats, but there is greater local variability in Ca2+ cycling at the microscopic level. These sex-based differences in Ca2+ cycling could contribute to differences in ECG morphology and in arrhythmia sensitivity in males and females.

Simulated ischemia-induced preconditioning of isolated ventricular myocytes from young adult and aged Fischer-344 rat hearts

The impact of ischemic preconditioning (IPC) on contraction, Ca(2+) homeostasis, and cell survival was compared in isolated ventricular myocytes from young adult ( approximately 3 mo) and aged ( approximately 24 mo) male Fischer-344 rats. Myocytes were field stimulated at 4 Hz (37 degrees C). Contraction (edge detector) and intracellular Ca(2+) (fura-2) were measured simultaneously. Viability was assessed with trypan blue. All cells were exposed to 30 min of simulated ischemia followed by reperfusion. Some cells were preconditioned by exposure to 5 min of simulated ischemia before prolonged ischemia. Pretreatment with IPC abolished postischemic contractile depression, inhibited diastolic contracture, and increased Ca(2+) transient amplitudes in reperfusion in young adult and aged cells. IPC did not affect the modest rise in diastolic Ca(2+) in ischemia in young adult myocytes. However, IPC abolished the marked rise in diastolic Ca(2+) observed in ischemia and early reperfusion in aged myocytes. IPC also suppressed mechanical alternans in ischemia in aged cells, but younger myocytes showed little evidence of mechanical alternans whether or not cells were preconditioned. IPC markedly improved cell viability in reperfusion in young adult but not aged cells. These results suggest that IPC augments the recovery of contractile function in reperfusion by increasing Ca(2+) transient amplitudes in ventricular myocytes from young adult and aged rats. IPC reduced diastolic Ca(2+) accumulation in ischemia in aged myocytes, which may diminish the severity of mechanical alternans in aged cells. Nonetheless, the efficacy of IPC is compromised in aging, as IPC did not improve survival of aged myocytes exposed to ischemia and reperfusion.

Alternans of cardiac calcium cycling in a cluster of ryanodine receptors: a simulation study

Mechanical alternans in cardiac muscle is associated with intracellular Ca(2+) alternans. Mechanisms underlying intracellular Ca(2+) alternans are unclear. In previous experimental studies, we produced alternans of systolic Ca(2+) under voltage clamp, either by partially inhibiting the Ca(2+) release mechanism, or by applying small depolarizing pulses. In each case, alternans relied on propagating waves of Ca(2+) release. The aim of this study is to investigate by computer modeling how alternans of systolic Ca(2+) is produced. A mathematical model of a cardiac cell with 75 coupled elements is developed, with each element contains L-type Ca(2+) current, a subspace into which Ca release takes place, a cytoplasmic space, sarcoplasmic reticulum (SR) release channels [ryanodine receptor (RyR)], and uptake sites (SERCA). Interelement coupling is via Ca(2+) diffusion between neighboring subspaces via cytoplasmic spaces and network SR spaces. Small depolarizing pulses were simulated by step changes of cell membrane potential (20 mV) with random block of L-type channels. Partial inhibition of the release mechanism is mimicked by applying a reduction of RyR open probability in response to full stimulation by L-type channels. In both cases, systolic alternans follow, consistent with our experimental observations, being generated by propagating waves of Ca(2+) release and sustained through alternation of SR Ca(2+) content. This study provides novel and fundamental insights to understand mechanisms that may underlie intracellular Ca(2+) alternans without the need for refractoriness of L-type Ca or RyR channels under rapid pacing.

Effects of ischemia and reperfusion on isolated ventricular myocytes from young adult and aged Fischer 344 rat hearts

This study examined the impact of age on contractile function, Ca2+homeostasis, and cell viability in isolated myocytes exposed to simulated ischemia and reperfusion. Ventricular myocytes were isolated from anesthetized young adult (3 mo) and aged (24 mo) male Fischer 344 rats. Cells were field-stimulated at 4 Hz (37°C), exposed to simulated ischemia, and reperfused with Tyrode solution. Cell shortening and intracellular Ca2+were measured simultaneously with an edge detector and fura-2. Cell viability was assessed by Trypan blue exclusion. Ischemia (20–45 min) depressed amplitudes of contraction equally in isolated myocytes from young adult and aged animals. The degree of postischemic contractile depression (stunning) was comparable in both groups. Ca2+transient amplitudes were depressed in early reperfusion in young adult and aged cells and then recovered to preischemic levels in both groups. Cell viability also declined equally in reperfusion in both groups. In short, some cellular responses to simulated ischemia and reperfusion were similar in both groups. Even so, aged myocytes exhibited a much greater and more prolonged accumulation of diastolic Ca2+in ischemia and in early reperfusion compared with myocytes from younger animals. In addition, the degree of mechanical alternans in ischemia increased significantly with age. The observation that there is an age-related increase in accumulation of diastolic Ca2+in ischemia and early reperfusion may account for the increased sensitivity to ischemia and reperfusion injury in the aging heart. The occurrence of mechanical alternans in ischemia may contribute to contractile dysfunction in ischemia in the aging heart.

TRPC3 is the erythropoietin-regulated calcium channel in human erythroid cells

Erythropoietin (Epo) stimulates a significant increase in the intracellular calcium concentration ([Ca(2+)](i)) through activation of the murine transient receptor potential channel TRPC2, but TRPC2 is a pseudogene in humans. TRPC3 expression increases on normal human erythroid progenitors during differentiation. Here, we determined that erythropoietin regulates calcium influx through TRPC3. Epo stimulation of HEK 293T cells transfected with Epo receptor and TRPC3 resulted in a dose-dependent increase in [Ca(2+)](i), which required extracellular calcium influx. Treatment with the phospholipase C (PLC) inhibitor U-73122 or down-regulation of PLCgamma1 by RNA interference inhibited the Epo-stimulated increase in [Ca(2+)](i) in TRPC3-transfected HEK 293T cells and in primary human erythroid precursors, demonstrating a requirement for PLC. TRPC3 associated with PLCgamma, and substitution of predicted PLCgamma Src homology 2 binding sites (Y226F, Y555F, Y648F, and Y674F) on TRPC3 reduced the interaction of TRPC3 with PLCgamma and inhibited the rise in [Ca(2+)](i). Substitution of Tyr(226) alone with phenylalanine significantly reduced the Epo-stimulated increase in [Ca(2+)](i) but not the association of PLCgamma with TRPC3. PLC activation results in production of inositol 1,4,5-trisphosphate (IP(3)). To determine whether IP(3) is involved in Epo activation of TRPC3, TRPC3 mutants were prepared with substitution or deletion of COOH-terminal IP(3) receptor (IP(3)R) binding domains. In cells expressing TRPC3 with mutant IP(3)R binding sites and Epo receptor, interaction of IP(3)R with TRPC3 was abolished, and Epo-modulated increase in [Ca(2+)](i) was reduced. Our data demonstrate that Epo modulates TRPC3 activation through a PLCgamma-mediated process that requires interaction of PLCgamma and IP(3)R with TRPC3. They also show that TRPC3 Tyr(226) is critical in Epo-dependent activation of TRPC3. These data demonstrate a redundancy of TRPC channel activation mechanisms by widely different agonists.

Mechanisms of calcium transient and action potential alternans in cardiac cells and tissues

Alternation of cardiac action potential duration (APD) from beat to beat and concurrent alternation of the amplitude of the calcium transient are regarded as important arrhythmia mechanisms. These phenomena are causally interrelated and can be reliably evoked by an increase in beat frequency or by ischemia. The first part of this historical review deals with the physiology of APD alternans. Sections recounting the evolution of knowledge about calcium-activated ion currents and calcium transient alternans are interspersed among sections describing the growth of the so-called “restitution hypothesis,” which involves time-dependent recovery of potassium channels (including their passage through pre-open states) as a function of diastolic interval. Major developments are generally in chronological order, but it is necessary to move back and forth between the two theories to respect the overall time line, which runs from about l965 to the present. The concluding two sections deal with the pathophysiology of calcium transient and APD alternans during ischemia, which may be the basis for out-of-hospital cardiac arrest during the initial stages of acute myocardial infarction.

Temporal variations in microvolt T-wave alternans testing after acute myocardial infarction

BACKGROUND

Microvolt T-wave alternans (TWA) have been accepted as a tool for assessing vulnerability to ventricular tachyarrhythmias. There is lack of data concerning prospective temporal variations in TWA measurements after acute myocardial infarction (AMI). We analysed the temporal patterns of TWA in post-AMI patients.

METHODS AND RESULTS

TWA tests were performed <1 month (TWA_early) and 6 months (TWA_late) after AMI in 51 consecutive patients treated with successful percutaneous coronary intervention (PCI). Twenty seven patients (53%) had anterior wall infarctions and 24 (47%) had inferior/lateral wall infarctions. TWA was measured during a treadmill manual exercise protocol and defined as positive, negative and indeterminate. Group A included patients with TWA negative in both tests and Group B included those with TWA abnormal (positive or indeterminate) at first or second determinations. TWA_early was negative, positive and indeterminate in 38 (74.6%), 8 (15.6%) and 5 (9.8%) patients, respectively, whereas TWA_late was negative, positive and indeterminate in 30 (58.9%), 14 (27.4%) and 7 (13.7%) patients, respectively. TWA_early was classified as normal in 74.6% and abnormal in 25.4% of the cases, and TWA_late was normal in 58.9% and abnormal in 41.1%. TWA tests were concordant in 36 patients (70.6%) and discordant 15 patients (29.4%). At TWA_late, 10 patients (19.6%) changed from TWA negative to TWA abnormal and 4 patients (7.8%) from TWA abnormal to TWA negative. After TWA_early+TWA_late, 45% of the patients had 1 test classified as abnormal. Left ventricular ejection fraction was <50% in 22% of Group A and 52% of Group B (P = 0.037).

CONCLUSIONS

In the era of primary PCI, temporal changes in TWA measurements may occur frequently during the first 6 months post-AMI. These findings should be considered in risk stratification strategies following AMI.

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.

Limiting sarcolemmal Na+ entry during resuscitation from ventricular fibrillation prevents excess mitochondrial Ca2+ accumulation and attenuates myocardial injury

BACKGROUND

intracellular Na+ accumulation during ischemia and reperfusion leads to cytosolic Ca2+ overload through reverse-mode operation of the sarcolemmal Na+ -Ca2+ exchanger. Cytosolic Ca2+ accumulation promotes mitochondrial Ca2+ (Ca2+ m) overload, leading to mitochondrial injury. We investigated whether limiting sarcolemmal Na+ entry during resuscitation from ventricular fibrillation (VF) attenuates Ca2+ m overload and lessens myocardial dysfunction in a rat model of VF and closed-chest resuscitation.

METHODS

hearts were harvested from 10 groups of 6 rats each representing baseline, 15 min of untreated VF, 15 min of VF with chest compression given for the last 5 min (VF/CC), and 60 min postresuscitation (PR). VF/CC and PR included four groups each randomized to receive before starting chest compression the new NHE-1 inhibitor AVE4454B (1.0 mg/kg), the Na+ channel blocker lidocaine (5.0 mg/kg), their combination, or vehicle control. The left ventricle was processed for intracellular Na+ and Ca2+ m measurements.

RESULTS

limiting sarcolemmal Na+ entry attenuated cytosolic Na+ increase during VF/CC and the PR phase and prevented Ca2+ m overload yielding levels that corresponded to 77% and 71% of control hearts at VF/CC and PR, without differences among specific Na+ -limiting interventions. Limiting sarcolemmal Na+ entry attenuated reductions in left ventricular compliance during VF and prompted higher mean aortic pressure (110 +/- 7 vs. 95 +/- 11 mmHg, P < 0.001) and higher cardiac work index (159 +/- 34 vs. 126 +/- 29 g x m x min(-1) x kg(-1), P < 0.05) with lesser increases in circulating cardiac troponin I at 60 min PR.

CONCLUSIONS

Na+ -limiting interventions prevented excess Ca2+ m accumulation induced by ischemia and reperfusion and ameliorated myocardial injury and dysfunction.

Regulation of TRP channel TRPM2 by the tyrosine phosphatase PTPL1

TRPM2, a member of the transient receptor potential (TRP) superfamily, is a Ca(2+)-permeable channel, which mediates susceptibility to cell death following activation by oxidative stress, TNFalpha, or beta-amyloid peptide. We determined that TRPM2 is rapidly tyrosine phosphorylated after stimulation with H(2)O(2) or TNFalpha. Inhibition of tyrosine phosphorylation with the tyrosine kinase inhibitors genistein or PP2 significantly reduced the increase in [Ca(2+)](i) observed after H(2)O(2) or TNFalpha treatment in TRPM2-expressing cells, suggesting that phosphorylation is important in TRPM2 activation. Utilizing a TransSignal PDZ domain array blot to identify proteins which interact with TRPM2, we identified PTPL1 as a potential binding protein. PTPL1 is a widely expressed tyrosine phosphatase, which has a role in cell survival and tumorigenesis. Immunoprecipitation and glutathione-S-transferase pull-down assays confirmed that TRPM2 and PTPL1 interact. To examine the ability of PTPL1 to modulate phosphorylation or activation of TRPM2, PTPL1 was coexpressed with TRPM2 in human embryonic kidney-293T cells. This resulted in significantly reduced TRPM2 tyrosine phosphorylation, and inhibited the rise in [Ca(2+)](i) and the loss of cell viability, which follow H(2)O(2) or TNFalpha treatment. Consistent with these findings, reduction in endogenous PTPL1 expression with small interfering RNA resulted in increased TRPM2 tyrosine phosphorylation, a significantly greater rise in [Ca(2+)](i) following H(2)O(2) treatment, and enhanced susceptibility to H(2)O(2)-induced cell death. Endogenous TRPM2 and PTPL1 was associated in U937-ecoR cells, confirming the physiological relevance of this interaction. These data demonstrate that tyrosine phosphorylation of TRPM2 is important in its activation and function and that inhibition of TRPM2 tyrosine phosphorylation reduces Ca(2+) influx and protects cell viability. They also suggest that modulation of TRPM2 tyrosine phosphorylation is a mechanism through which PTPL1 may mediate resistance to cell death.

Calcium instabilities in mammalian cardiomyocyte networks

The degeneration of a regular heart rhythm into fibrillation (a chaotic or chaos-like sequence) can proceed via several classical routes described by nonlinear dynamics: period-doubling, quasiperiodicity, or intermittency. In this study, we experimentally examine one aspect of cardiac excitation dynamics, the long-term evolution of intracellular calcium signals in cultured cardiomyocyte networks subjected to increasingly faster pacing rates via field stimulation. In this spatially extended system, we observed alternans and higher-order periodicities, extra beats, and skipped beats or blocks. Calcium instabilities evolved nonmonotonically with the prevalence of phase-locking or Wenckebach rhythm, low-frequency magnitude modulations (signature of quasiperiodicity), and switches between patterns with occasional bursts (signature of intermittency), but period-doubling bifurcations were rare. Six ventricular-fibrillation-resembling episodes were pace-induced, for which significantly higher complexity was confirmed by approximate entropy calculations. The progressive destabilization of the heart rhythm by coexistent frequencies, seen in this study, can be related to theoretically predicted competition of control variables (voltage and calcium) at the single-cell level, or to competition of excitation and recovery at the cell network level. Optical maps of the response revealed multiple local spatiotemporal patterns, and the emergence of longer-period global rhythms as a result of wavebreak-induced reentries.

TRPM2 is an ion channel that modulates hematopoietic cell death through activation of caspases and PARP cleavage

TRPM2 is a Ca(2+)-permeable channel activated by oxidative stress or TNF-alpha, and TRPM2 activation confers susceptibility to cell death. The mechanisms were examined here in human monocytic U937-ecoR cells. This cell line expresses full-length TRPM2 (TRPM2-L) and several isoforms including a short splice variant lacking the Ca(2+)-permeable pore region (TRPM2-S), which functions as a dominant negative. Treatment with H(2)O(2), a model of oxidative stress, or TNF-alpha results in reduced cell viability. Expression of TRPM2-L and TRPM2-S was modulated by retroviral infection. U937-ecoR cells expressing increased levels of TRPM2-L were treated with H(2)O(2) or TNF-alpha, and these cells exhibited significantly increased intracellular calcium concentration ([Ca(2+)](i)), decreased viability, and increased apoptosis. A dramatic increase in cleavage of caspases-8, -9, -3, and -7 and poly(ADP-ribose)polymerase (PARP) was observed, demonstrating a downstream mechanism through which cell death is mediated. Bcl-2 levels were unchanged. Inhibition of the [Ca(2+)](i) rise with the intracellular Ca(2+) chelator BAPTA blocked caspase/PARP cleavage and cell death induced after activation of TRPM2-L, demonstrating the critical role of [Ca(2+)](i) in mediating these effects. Downregulation of endogenous TRPM2 by RNA interference or increased expression of TRPM2-S inhibited the rise in [Ca(2+)](i), enhanced cell viability, and reduced numbers of apoptotic cells after exposure to oxidative stress or TNF-alpha, demonstrating the physiological importance of TRPM2. Our data show that one mechanism through which oxidative stress or TNF-alpha mediates cell death is activation of TRPM2, resulting in increased [Ca(2+)](i), followed by caspase activation and PARP cleavage. Inhibition of TRPM2-L function by reduction in TRPM2 levels, interaction with TRPM2-S, or Ca(2+) chelation antagonizes this important cell death pathway.

Erythropoietin-modulated calcium influx through TRPC2 is mediated by phospholipase Cgamma and IP3R

In the present study, we examined the mechanisms through which erythropoietin (Epo) activates the calcium-permeable transient receptor potential protein channel (TRPC)2. Erythroblasts were isolated from the spleens of phenylhydrazine-treated mice, and Epo stimulation resulted in a significant and dose-dependent increase in intracellular calcium concentration ([Ca(2+)](i)). This increase in [Ca(2+)](i) was inhibited by pretreatment with the phospholipase C (PLC) inhibitor U-73122 but not by the inactive analog U-73343, demonstrating the requirement for PLC activity in Epo-modulated Ca(2+) influx in primary erythroid cells. To determine whether PLC is involved in the activation of TRPC2 by Epo, cell models were used to examine this interaction. Single CHO-S cells that expressed transfected Epo receptor (Epo-R) and TRPC2 were identified, and [Ca(2+)](i) was quantitated. Epo-induced Ca(2+) influx through TRPC2 was inhibited by pretreatment with U-73122 or by downregulation of PLCgamma1 by RNA interference. PLC activation results in the production of inositol 1,4,5-trisphosphate (IP(3)), and TRPC2 has IP(3) receptor (IP(3)R) binding sites. To determine whether IP(3)R is involved in Epo-R signaling, TRPC2 mutants were prepared with partial or complete deletions of the COOH-terminal IP(3)R binding domains. In cells expressing TRPC2 IP(3)R binding mutants and Epo-R, no significant increase in [Ca(2+)](i) was observed after Epo stimulation. TRPC2 coassociated with Epo-R, PLCgamma, and IP(3)R, and the association between TRPC2 and IP(3)R was disrupted in these mutants. Our data demonstrate that Epo-R modulates TRPC2 activation through PLCgamma; that interaction of IP(3)R with TRPC2 is required; and that Epo-R, TRPC2, PLCgamma, and IP(3)R interact to form a signaling complex.

A simulation study of the effects of cardiac anatomy in ventricular fibrillation

In ventricular fibrillation (VF), the principal cause of sudden cardiac death, waves of electrical excitation break up into turbulent and incoherent fragments. The causes of this breakup have been intensely debated. Breakup can be caused by fixed anatomical properties of the tissue, such as the biventricular geometry and the inherent anisotropy of cardiac conduction. However, wavebreak can also be caused purely by instabilities in wave conduction that arise from ion channel dynamics, which represent potential targets for drug action. To study the interaction between these two wave-breaking mechanisms, we used a physiologically based mathematical model of the ventricular cell, together with a realistic three-dimensional computer model of cardiac anatomy, including the distribution of fiber angles throughout the myocardium. We find that dynamical instabilities remain a major cause of the wavebreak that drives VF, even in an anatomically realistic heart. With cell physiology in its usual operating regime, dynamics and anatomical features interact to promote wavebreak and VF. However, if dynamical instability is reduced, for example by modeling of certain pharmacologic interventions, electrical waves do not break up into fibrillation, despite anatomical complexity. Thus, interventions that promote dynamical wave stability show promise as an antifibrillatory strategy in this more realistic setting.

Tracking cardiac electrical instability by computing interlead heterogeneity of T-wave morphology

Oscillations in T-wave morphology, particularly T-wave alternans (TWA), have been fundamentally linked to increased susceptibility to ventricular fibrillation (VF). We investigated whether the escalation in complexity of T-wave oscillations before VF is attributable to increased spatial heterogeneity of repolarization. Peak interlead T-wave heterogeneity (TWH) was measured by second central moment analysis of T-wave morphology in epicardial electrograms in dogs during left anterior descending coronary artery occlusion. TWH differentiated cases in which myocardial ischemia provoked VF from those without VF (563 +/- 56 vs. 139 +/- 36 microV, P < 0.01). In the former group, progressive, significant increases in TWH above preocclusion baseline (70 +/- 8 microV) began at 2.25 min after the start of occlusion and were associated successively with TWA (at 155 +/- 19 microV), T-wave multupling (at 386 +/- 100 microV), complex oscillatory T-wave forms (at 560 +/- 76 microV), discordant TWA (at 572 +/- 98 microV), and VF at 4.36 +/- 0.14 min. TWH in precordial ECGs in 12 pigs during angioplasty-balloon-induced myocardial ischemia also discriminated animals that experienced VF (from 90 +/- 14 at baseline to 382 +/- 39 microV, P < 0.05) from those without VF (from 96 +/- 17 at baseline to 199 +/- 61 microV, NS). Ischemia-induced changes in ST segment and T-wave amplitude did not predict VF. Heightened spatial heterogeneity of repolarization, as assessed by second central moment analysis of TWH, underlies TWA and increased risk for ischemia-induced VF. Monitoring spatial TWH from precordial leads could prove useful in stratifying risk for life-threatening arrhythmias.

Spatial heterogeneity of action potential alternans during global ischemia in the rabbit heart

Cardiac ischemia causes beat-to-beat fluctuation in action potential duration (APD) alternans, which leads to T wave alternans and arrhythmias. Occurrence of APD alternans that is out of phase at two sites is especially important, but most APD alternans studies have involved rapid pacing of normal myocardium rather than ischemia. To determine the spatial features of APD alternans during ischemia, blood-perfused rabbit hearts were stained with 4-[beta-[2(di-n-butylamino)-6-napthyl]vinyl]pyridinium (di-4-ANEPPS) and imaged with a high-resolution camera. Hearts were perfused with oxygenated Tyrode solution at 37 degrees C for staining and then switched to a 50:50% blood/Tyrode mixture. Hearts were paced from the right ventricle at 3/s, and made ischemic by stopping flow for 6 min. Images of 10,000 pixels were obtained at 300 frames/s. Motion artifact was controlled by immobilization and by manual selection of undistorted single-pixel records. Upstroke propagation and conduction isochrones were displayed by computerized image processing. APD alternans was demonstrated in six of seven hearts, and was out of phase in different regions of the image in three hearts. The largest spatial variation in the onset of depolarization to 50% repolarization (APD50) was 155%. This caused beat-to-beat reversal of repolarization. An alternans map could be constructed for well-immobilized portions of the image. There were discrete regions of APD alternans separated by a boundary, as occurs with intracellular Ca2+ concentration alternans. Pixels as close together as 1.1 mm showed an APD alternans that was out of phase. The out-of-phase APD alternans was not due to conduction alternans, as shown by upstroke intervals and conduction isochrones. This contrasts with rapid pacing, where a causal relationship appears to exist. These new observations suggest distinct mechanisms for the genesis of arrhythmias during ischemia.

How Inotropic Drugs Alter Dynamic and Static Indices of Cyclic Myoplasmic [Ca2+] to Contractility Relationships in Intact Hearts

The authors examined effects of positive (dopamine and digoxin) and negative (nifedipine and lidocaine) inotropic interventions on the instantaneous cyclic relationship between myoplasmic [Ca2+] and simultaneously developed left ventricular pressure (LVP) in intact guinea pig hearts. Novel indices were developed to quantify this relationship based on (1) transient [Ca2+] and LVP signal morphology, ie, maxima and minima, peak derivatives, beat areas, durations, and ratios of indices of LVP to [Ca2+]; (2) temporal delay; and (3) LVP versus [Ca2+] loop morphology, ie, orientation, size, hysteresis, position, shape, and duration. These analyses were used to assess the cost of phasic [Ca2+] for contraction and relaxation over one beat after inotropic intervention. It was found that dopamine and digoxin increased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike dopamine, digoxin did not decrease relaxation response time. Nifedipine and lidocaine decreased contractile and relaxation responsiveness to phasic [Ca2+], cumulative Ca2+, and net Ca2+ flux. Unlike lidocaine, nifedipine decreased net available Ca2+ and Ca2+ influx. Positive inotropic agents increased [Ca2+]-LVP loop area and hysteresis and resulted in a more vertically oriented loop. Nifedipine and lidocaine decreased these loop indices and lidocaine exhibited greater loop hysteresis than did nifedipine. These novel indices provide a quantitative assessment of myoplasmic [Ca2+] handling for cardiac contractile function.

Time related changes in calcium handling in the isolated ischemic and reperfused rat heart

The main aim of this study was to assess the kinetics of intracellular free calcium (Ca(2+)i) handling by isolated rat hearts rendered ischemic for 30 min followed by 30 min of reperfusion analyzing the upstroke and downslope of the Ca(2+)i transient. Changes in mechanical performance and degradation of membrane phospholipids--estimated by tissue arachidonic acid content--were correlated with Ca(2+)i levels of the heart. The fluorescence ratio technique was applied to estimate Ca(2+)i. The disappearance of mechanical activity of the heart preceded that of the Ca(2+)i transient in the first 2 min of ischemia. The slope of upstroke of the Ca(2+)i transient, reflecting Ca2+ release, decreased by 60%, while the duration of the downslope of the transient, reflecting Ca2+ sequestration, expressed a significant prolongation (105 +/- 17 vs. 149 +/- 39 msec) during the first 3 min of ischemia. At about 20 min of ischemia end-diastolic pressure expressed a 3.5-fold increase (contracture) when the fluorescence ratio showed a 2-fold elevation. Reperfusion was accompanied with a further precipitous increase in end-diastolic pressure, while resting Ca(2+)i remained at end-ischemic levels. Increases in the arachidonic acid (AA) content of the ischemic and postischemic hearts were proportional to Ca(2+)i levels. In summary, the present findings indicate that both calcium release and removal are hampered during the early phase of ischemia. Moreover, a critical level of Ca(2+)i and a critical duration of ischemia may exist to provoke contracture of the heart. Upon reperfusion the hearts show membrane phospholipid degradation and signs of stunning exemplified by elevated AA levels, partial recovery of Ca(2+)i handling and sustained depression of mechanical performance.

Two-photon molecular excitation imaging of Ca2+ transients in Langendorff-perfused mouse hearts

The ability to image calcium signals at subcellular levels within the intact depolarizing heart could provide valuable information toward a more integrated understanding of cardiac function. Accordingly, a system combining two-photon excitation with laser-scanning microscopy was developed to monitor electrically evoked [Ca(2+)](i) transients in individual cardiomyocytes within noncontracting Langendorff-perfused mouse hearts. [Ca(2+)](i) transients were recorded at depths </=100 microm from the epicardial surface with the fluorescent indicators rhod-2 or fura-2 in the presence of the excitation-contraction uncoupler cytochalasin D. Evoked [Ca(2+)](i) transients were highly synchronized among neighboring cardiomyocytes. At 1 Hz, the times from 90 to 50% (t(90-50%)) and from 50 to 10% (t(50-10%)) of the peak [Ca(2+)](i) were (means +/- SE) 73 +/- 4 and 126 +/- 10 ms, respectively, and at 2 Hz, 62 +/- 3 and 94 +/- 6 ms (n = 19, P < 0.05 vs. 1 Hz) in rhod-2-loaded cardiomyocytes. [Ca(2+)](i) decay was markedly slower in fura-2-loaded hearts (t(90-50%) at 1 Hz, 128 +/- 9 ms and at 2 Hz, 88 +/- 5 ms; t(50-10%) at 1 Hz, 214 +/- 18 ms and at 2 Hz, 163 +/- 7 ms; n = 19, P < 0.05 vs. rhod-2). Fura-2-induced deceleration of [Ca(2+)](i) decline resulted from increased cytosolic Ca(2+) buffering, because the kinetics of rhod-2 decay resembled those obtained with fura-2 after incorporation of the Ca(2+) chelator BAPTA. Propagating calcium waves and [Ca(2+)](i) amplitude alternans were readily detected in paced hearts. This approach should be of general utility to monitor the consequences of genetic and/or functional heterogeneity in cellular calcium signaling within whole mouse hearts at tissue depths that have been inaccessible to single-photon imaging.

Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes

Subcellular Ca(2+) signalling during normal excitation-contraction (E-C) coupling and during Ca(2+) alternans was studied in atrial myocytes using fast confocal microscopy and measurement of Ca(2+) currents (I(Ca)). Ca(2+) alternans, a beat-to-beat alternation in the amplitude of the [Ca(2+)](i) transient, causes electromechanical alternans, which has been implicated in the generation of cardiac fibrillation and sudden cardiac death. Cat atrial myocytes lack transverse tubules and contain sarcoplasmic reticulum (SR) of the junctional (j-SR) and non-junctional (nj-SR) types, both of which have ryanodine-receptor calcium release channels. During E-C coupling, Ca(2+) entering through voltage-gated membrane Ca(2+) channels (I(Ca)) triggers Ca(2+) release at discrete peripheral j-SR release sites. The discrete Ca(2+) spark-like increases of [Ca(2+)](i) then fuse into a peripheral 'ring' of elevated [Ca(2+)](i), followed by propagation (via calcium-induced Ca(2+) release, CICR) to the cell centre, resulting in contraction. Interrupting I(Ca) instantaneously terminates j-SR Ca(2+) release, whereas nj-SR Ca(2+) release continues. Increasing the stimulation frequency or inhibition of glycolysis elicits Ca(2+) alternans. The spatiotemporal [Ca(2+)](i) pattern during alternans shows marked subcellular heterogeneities including longitudinal and transverse gradients of [Ca(2+)](i) and neighbouring subcellular regions alternating out of phase. Moreover, focal inhibition of glycolysis causes spatially restricted Ca(2+) alternans, further emphasising the local character of this phenomenon. When two adjacent regions within a myocyte alternate out of phase, delayed propagating Ca(2+) waves develop at their border. In conclusion, the results demonstrate that (1) during normal E-C coupling the atrial [Ca(2+)](i) transient is the result of the spatiotemporal summation of Ca(2+) release from individual release sites of the peripheral j-SR and the central nj-SR, activated in a centripetal fashion by CICR via I(Ca) and Ca(2+) release from j-SR, respectively, (2) Ca(2+) alternans is caused by subcellular alterations of SR Ca(2+) release mediated, at least in part, by local inhibition of energy metabolism, and (3) the generation of arrhythmogenic Ca(2+) waves resulting from heterogeneities in subcellular Ca(2+) alternans may constitute a novel mechanism for the development of cardiac dysrhythmias.