Transient inward current in guinea-pig atrial myocytes reflects a change of sodium-calcium exchange current

Exploring the role of TRPM4 in calcium-dependent triggered activity and cardiac arrhythmias

Cardiac arrhythmias pose a major threat to a patient's health, yet prove to be often difficult to predict, prevent and treat. A key mechanism in the occurrence of arrhythmias is disturbed Ca2+ homeostasis in cardiac muscle cells. As a Ca2+-activated non-selective cation channel, TRPM4 has been linked to Ca2+-induced arrhythmias, potentially contributing to translating an increase in intracellular Ca2+ concentration into membrane depolarisation and an increase in cellular excitability. Indeed, evidence from genetically modified mice, analysis of mutations in human patients and the identification of a TRPM4 blocking compound that can be applied in vivo further underscore this hypothesis. Here, we provide an overview of these data in the context of our current understanding of Ca2+-dependent arrhythmias.

TRPM4 inhibition by meclofenamate suppresses Ca2+-dependent triggered arrhythmias

AIMS

Cardiac arrhythmias are a major factor in the occurrence of morbidity and sudden death in patients with cardiovascular disease. Disturbances of Ca2+ homeostasis in the heart contribute to the initiation and maintenance of cardiac arrhythmias. Extrasystolic increases in intracellular Ca2+ lead to delayed afterdepolarizations and triggered activity, which can result in heart rhythm abnormalities. It is being suggested that the Ca2+-activated nonselective cation channel TRPM4 is involved in the aetiology of triggered activity, but the exact contribution and in vivo significance are still unclear.

METHODS AND RESULTS

In vitro electrophysiological and calcium imaging technique as well as in vivo intracardiac and telemetric electrocardiogram measurements in physiological and pathophysiological conditions were performed. In two distinct Ca2+-dependent proarrhythmic models, freely moving Trpm4-/- mice displayed a reduced burden of cardiac arrhythmias. Looking further into the specific contribution of TRPM4 to the cellular mechanism of arrhythmias, TRPM4 was found to contribute to a long-lasting Ca2+ overload-induced background current, thereby regulating cell excitability in Ca2+ overload conditions. To expand these results, a compound screening revealed meclofenamate as a potent antagonist of TRPM4. In line with the findings from Trpm4-/- mice, 10 µM meclofenamate inhibited the Ca2+ overload-induced background current in ventricular cardiomyocytes and 15 mg/kg meclofenamate suppressed catecholaminergic polymorphic ventricular tachycardia-associated arrhythmias in a TRPM4-dependent manner.

CONCLUSION

The presented data establish that TRPM4 represents a novel target in the prevention and treatment of Ca2+-dependent triggered arrhythmias.

Increasing SERCA function promotes initiation of calcium sparks and breakup of calcium waves

KEY POINTS

Increasing sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) pump activity enhances sarcoplasmic reticulum calcium (Ca) load, which increases both ryanodine receptor opening and driving force of Ca release flux. Both of these effects promote Ca spark formation and wave propagation. However, increasing SERCA activity also accelerates local cytosolic Ca decay as the wave front travels to the next cluster, which limits wave propagation. As a result, increasing SERCA pump activity has a biphasic effect on the propensity of arrhythmogenic Ca waves, but a monotonic effect to increase Ca spark frequency and amplitude.

ABSTRACT

Waves of sarcoplasmic reticulum (SR) calcium (Ca) release can cause arrhythmogenic afterdepolarizations in cardiac myocytes. Ca waves propagate when Ca sparks at one Ca release unit (CRU) recruit new Ca sparks in neighbouring CRUs. Under normal conditions, Ca sparks are too small to recruit neighbouring Ca sparks where Ca sensitivity is also low. However, under pathological conditions such as a Ca overload or ryanodine receptor (RyR) sensitization, Ca sparks can be larger and propagate more readily as macro-sparks or full Ca waves. Increasing SERCA pump activity promotes SR Ca load, which promotes RyR opening and increases driving force of the Ca release flux from SR to cytosol, promoting Ca waves. However, high sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) activity can also decrease local cytosolic [Ca] as it approaches the next CRU, thereby reducing wave appearance and propagation. In this study, we use a physiologically detailed model of subcellular Ca cycling and experiments in phospholamban-knockout mice, to show how Ca waves are initiated and propagate and how different conditions contribute to the generation and propagation of Ca waves. We show that reducing diffusive coupling between Ca sparks by increasing SERCA activity prevents Ca waves by reducing [Ca] at the next CRU, as do Ca buffers, low intra-SR Ca diffusion and distance between CRUs. Increasing SR Ca uptake rate has a biphasic effect on Ca wave propagation; initially it enhances Ca spark probability and amplitude and CRU coupling, thereby promoting arrhythmogenic Ca wave propagation, but at higher levels SR Ca uptake can abort those arrhythmogenic Ca waves.

New role of TRPM4 channel in the cardiac excitation-contraction coupling in response to physiological and pathological hypertrophy in mouse

The transient receptor potential Melastatin 4 (TRPM4) channel is a calcium-activated non-selective cation channel expressed widely. In the heart, using a knock-out mouse model, the TRPM4 channel has been shown to be involved in multiple processes, including β-adrenergic regulation, cardiac conduction, action potential duration and hypertrophic adaptations. This channel was recently shown to be involved in stress-induced cardiac arrhythmias in a mouse model overexpressing TRPM4 in ventricular cardiomyocytes. However, the link between TRPM4 channel expression in ventricular cardiomyocytes, the hypertrophic response to stress and/or cellular arrhythmias has yet to be elucidated. In this present study, we induced pathological hypertrophy in response to myocardial infarction using a mouse model of Trpm4 gene invalidation, and demonstrate that TRPM4 is essential for survival. We also demonstrate that the TRPM4 is required to activate both the Akt and Calcineurin pathways. Finally, using two hypertrophy models, either a physiological response to endurance training or a pathological response to myocardial infarction, we show that TRPM4 plays a role in regulating transient calcium amplitudes and leads to the development of cellular arrhythmias potentially in cooperation with the Sodium-calcium exchange (NCX). Here, we report two functions of the TRPM4 channel: first its role in adaptive hypertrophy, and second its association with NCX could mediate transient calcium amplitudes which trigger cellular arrhythmias.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Reducing ryanodine receptor open probability as a means to abolish spontaneous Ca2+ release and increase Ca2+ transient amplitude in adult ventricular myocytes

The aim of this work was to investigate whether it is possible to remove arrhythmogenic Ca2+ release from the sarcoplasmic reticulum that occurs in calcium overload without compromising normal systolic release. Exposure of rat ventricular myocytes to isoproterenol (1 micromol/L) resulted in an increased amplitude of the systolic Ca2+ transient and the appearance of waves of diastolic Ca2+ release. Application of tetracaine (25 to 50 micromol/L) decreased the frequency or abolished the diastolic Ca2+ release. This was accompanied by an increase in the amplitude of the systolic Ca2+ transient. Cellular Ca2+ flux balance was investigated by integrating Ca2+ entry (on the L-type Ca2+ current) and efflux (on Na-Ca2+ exchange). Isoproterenol increased Ca2+ influx but failed to increase Ca2+ efflux during systole (because of the abbreviation of the duration of the Ca2+ transient). To match this increased influx the bulk of Ca2+ efflux occurred via Na-Ca2+ exchange during a diastolic Ca2+ wave. Subsequent application of tetracaine increased systolic Ca2+ efflux and abolished the diastolic efflux. The increase of systolic efflux in tetracaine resulted from both increased amplitude and duration of the systolic Ca2+ transient. In the presence of isoproterenol, those Ca2+ transients preceded by diastolic release were smaller than those where no diastolic release had occurred. When tetracaine was added, the amplitude of the Ca2+ transient was similar to those in isoproterenol with no diastolic release and larger than those preceded by diastolic release. We conclude that tetracaine increases the amplitude of the systolic Ca2+ transient by removing the inhibitory effect of diastolic Ca2+ release.

Na(+) current through KATP channels: consequences for Na(+) and K(+) fluxes during early myocardial ischemia

During early myocardial ischemia, the myocytes are loaded with Na(+), which in turn leads to Ca(2+) overload and cell death. The pathway of the Na(+) influx has not been fully elucidated. The aim of the study was to quantify the Na(+) inward current through sarcolemmal KATP channels (IKATP,Na) in anoxic isolated cardiomyocytes at the actual reversal potential (Vrev) and to estimate the contribution of this current to the Na(+) influx in the ischemic myocardium. IKATP,Na was determined in excised single channel patches of mouse ventricular myocytes and macropatches of Xenopus laevis oocytes expressing SUR2A/Kir6.2 channels. In the presence of K+ ions, the respective permeability ratios for Na(+) to K(+) ions, PNa/PK, were close to 0.01. Only in the presence of Na(+) ions on both sides of the membrane was IKATP,Na similarly large to that calculated from the permeability ratio PNa/PK, indicative of a Na(+) influx that is largely independent of the K+ efflux at Vrev. With the use of a peak KATP channel conductance in anoxic cardiomyocytes of 410 nS, model simulations for a myocyte within the ischemic myocardium showed that the amplitude of the Na(+) influx and K(+) efflux is even larger than the respective fluxes by the Na(+) - K(+) pump and all other background fluxes. These results suggest that during early ischemia the Na(+) influx through KATP channels essentially contributes to the total Na+ influx and that it also balances the K(+) efflux through KATP channels.

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.

Differential effects of organic calcium-channel blockers on diastolic SR calcium-handling in the frog heart

1. Gradual loss of sarcoplasmic reticular (SR) calcium during a rest-period is responsible for the rest-induced decay (RID) of force in mammalian myocardium. Effect of verapamil and diltiazem on a similar RID in the frog myocardium suggests a new mechanism of action of these drugs. 2. Strips of frog-ventricle were paced at 0.2 Hz and the rhythm was interrupted by varying rest-periods ranging from 10 to 180 s. In control conditions, the amplitude of the post-rest beat was significantly lower than that of the pre-rest beat for rest-periods more than 40 s (RID). 3. Verapamil and diltiazem (which are organic calcium-channel blockers (OCCB)) changed the pattern of RID in the control solution to a 'rest-induced potentiation' (RIP) in the same preparation while another OCCB nifedipine and the inorganic calcium-channel blocker cadmium did not alter the post-rest phenomenon. 4. We propose that verapamil and diltiazem produce an RIP due to either blockade of SR calcium-leak during rest or enhancement of SR calcium-uptake during rest.

Stem cell-derived cardiomyocytes demonstrate arrhythmic potential

BACKGROUND

Cardiomyocytes (CMs) derived from pluripotent embryonic stem cells (ESCs) and embryonal carcinoma cells (ECCs) have some but not all characteristics of adult myocytes. ESCs have shown the ability to engraft in areas of myocardial damage, which suggests their use in cell transplantation therapy for cardiomyopathy. We studied the arrhythmogenic properties of CMs differentiated from mouse ESCs and ECCs.

METHODS AND RESULTS

CMs derived in vitro were studied in the whole-cell patch-clamp mode. CMs from both sources showed action potential (AP) morphology heterogeneity, with reduced maximum upstroke velocities (dV/dt) and prolonged AP durations. CMs demonstrated prolonged, spontaneous electrical activity in culture. Frequent triggered activity was observed with and without pharmacological enhancement. Phase 2 or 3 early afterdepolarizations could be induced easily by Bay K8644 plus tetraethylammonium chloride (TEA) or [TEA]o after Cs+ replacement for [K+]i, respectively. A combination of bradycardic stimulation, hypokalemia, and quinidine resulted in early afterdepolarizations. Delayed afterdepolarizations could be induced easily and reversibly by hypercalcemia or isoproterenol.

CONCLUSIONS

ESCs or ECCs differentiated into at least 3 AP phenotypes. CMs showed spontaneous activity, low dV/dt, prolonged AP duration, and easily inducible triggered arrhythmias. These findings raise caution about the use of totipotent ESCs in cell transplantation therapy, because they may act as an unanticipated arrhythmogenic source from any of the 3 classic mechanisms (reentry, automaticity, or triggered activity).

The role of inositol 1,4,5-trisphosphate receptors in Ca(2+) signalling and the generation of arrhythmias in rat atrial myocytes

Various cardio-active stimuli, including endothelin-1 (ET-1), exhibit potent arrhythmogenicity, but the underlying cellular mechanisms of their actions are largely unclear. We used isolated rat atrial myocytes and related changes in their subcellular Ca(2+) signalling to the ability of various stimuli to induce diastolic, premature extra Ca(2+) transients (ECTs). For this, we recorded global and spatially resolved Ca(2+) signals in indo-1- and fluo-4-loaded atrial myocytes during electrical pacing. ET-1 exhibited a higher arrhythmogenicity (arrhythmogenic index; ratio of number of ECTs over fold-increase in Ca(2+) response, 8.60; n = 8 cells) when compared with concentrations of cardiac glycosides (arrhythmogenic index, 4.10; n = 8 cells) or the beta-adrenergic agonist isoproterenol (arrhythmogenic index, 0.11; n = 6 cells) that gave similar increases in the global Ca(2+) responses. Seventy-five percent of the ET-1-induced arrhythmogenic Ca(2+) transients were accompanied by premature action potentials, while for digoxin this proportion was 25 %. The beta-adrenergic agonist failed to elicit a significant number of ECTs. Direct activation of inositol 1,4,5-trisphosphate (InsP(3)) receptors with a membrane-permeable InsP(3) ester (InsP(3) BM) mimicked the effect of ET-1 (arrhythmogenic index, 14.70; n = 6 cells). Inhibition of InsP(3) receptors using 2 microM 2-aminoethoxydiphenyl borate, which did not display any effects on Ca(2+) signalling under control conditions, specifically suppressed the arrhythmogenic action of ET-1 and InsP(3) BM. Immunocytochemistry indicated a co-localisation of peripheral, junctional ryanodine receptors with InsP(3)Rs. Thus, the pronounced arrhythmogenic potency of ET-1 is due to the spatially specific recruitment of Ca(2+) sparks by subsarcolemmal InsP(3)Rs. Summation of such sparks efficiently generates delayed after depolarisations that trigger premature action potentials. We conclude that the particular spatial profile of cellular Ca(2+) signals is a major, previously unrecognised, determinant for arrhythmogenic potency and that the InsP(3) signalling cassette might therefore be a promising new target for understanding and managing atrial arrhythmia.

Inhibition of Na+/Ca2+ exchange by KB-R7943: transport mode selectivity and antiarrhythmic consequences

The Na+/Ca2+ exchanger plays a prominent role in regulating intracellular Ca2+ levels in cardiac myocytes and can serve as both a Ca2+ influx and efflux pathway. A novel inhibitor, KB-R7943, has been reported to selectively inhibit the reverse mode (i.e., Ca2+ entry) of Na+/Ca2+ exchange transport, although many aspects of its inhibitory properties remain controversial. We evaluated the inhibitory effects of KB-R7943 on Na+/Ca2+ exchange currents using the giant excised patch-clamp technique. Membrane patches were obtained from Xenopus laevis oocytes expressing the cloned cardiac Na+/Ca2+ exchanger NCX1.1, and outward, inward, and combined inward-outward currents were studied. KB-R7943 preferentially inhibited outward (i.e., reverse) Na+/Ca2+ exchange currents. The inhibitory mechanism consists of direct effects on the transport machinery of the exchanger, with additional influences on ionic regulatory properties. Competitive interactions between KB-R7943 and the transported ions were not observed. The antiarrhythmic effects of KB-R7943 were then evaluated in an ischemia-reperfusion model of cardiac injury in Langendorff-perfused whole rabbit hearts using electrocardiography and measurements of left ventricular pressure. When 3 microM KB-R7943 was applied for 10 min before a 30-min global ischemic period, ventricular arrhythmias (tachycardia and fibrillation) associated with both ischemia and reperfusion were almost completely suppressed. The observed electrophysiological profile of KB-R7943 and its protective effects on ischemia-reperfusion-induced ventricular arrhythmias support the notion of a prominent role of Ca2+ entry via reverse Na+/Ca2+ exchange in this process.

Cl- current blockade reduces triggered activity based on delayed afterdepolarisations

OBJECTIVES

Increasing evidence suggests that a Ca²⁺-activated Cl^- current (I_Cl(Ca)) contributes to the transient inward current (I_ti), the current responsible for proarrhythmic delayed after-depolarisations (DADs). Because the equilibrium potential for Cl^- ions (E_Cl) in myocytes is around - 50 mV, activation of the I_Cl(Ca) results in an inward depolarising current at resting membrane potential and I_Cl(Ca) may thus be responsible for a part of the depolarisation during a DAD. In this study, we investigated the ionic nature of I_ti and the effects of Cl^- current blockade on DADs.

METHODS AND RESULTS

The ionic mechanisms of I_ti and underlying DADs were studied in sheep ventricular myocytes using the patch-clamp methodology. The DADs were induced in the myocytes by exposure to 1 μM noradrenaline and the I_ti were elicited by repetitive depolarisations from -93 mV to +37 mV in the presence of the drug. The current-voltage relation of I_ti reversed in sign around -20 mV. The outward I_ti was completely blocked by the anion current blocker 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), whereas the inward I_ti was only slightly affected. The DIDS-sensitive component of I_ti was outwardly rectifying with a reversal potential close to E_Cl. The DIDS-insensitive component of I_ti was abolished by blockade of the Na⁺-Ca²⁺ exchanger by substitution of extracellular Na⁺ by equimolar Li⁺. Interestingly, DIDS reduced the DAD amplitude and triggered activity based on DADs.

CONCLUSION

In sheep ventricular myocytes, I_ti consists of two ionic mechanisms: a Cl^- current and a Na⁺-Ca²⁺ exchange current. Blockade of the Cl^- current may be potentially antiarrhythmic by lowering DAD amplitude and triggered activity based on DADs.

Allosteric regulation of Na/Ca exchange current by cytosolic Ca in intact cardiac myocytes

The cardiac sarcolemmal Na-Ca exchanger (NCX) is allosterically regulated by [Ca](i) such that when [Ca](i) is low, NCX current (I(NCX)) deactivates. In this study, we used membrane potential (E(m)) and I(NCX) to control Ca entry into and Ca efflux from intact cardiac myocytes to investigate whether this allosteric regulation (Ca activation) occurs with [Ca](i) in the physiological range. In the absence of Ca activation, the electrochemical effect of increasing [Ca](i) would be to increase inward I(NCX) (Ca efflux) and to decrease outward I(NCX). On the other hand, Ca activation would increase I(NCX) in both directions. Thus, we attributed [Ca](i)-dependent increases in outward I(NCX) to allosteric regulation. Ca activation of I(NCX) was observed in ferret myocytes but not in wild-type mouse myocytes, suggesting that Ca regulation of NCX may be species dependent. We also studied transgenic mouse myocytes overexpressing either normal canine NCX or this same canine NCX lacking Ca regulation (Delta680-685). Animals with the normal canine NCX transgene showed Ca activation, whereas animals with the mutant transgene did not, confirming the role of this region in the process. In native ferret cells and in mice with expressed canine NCX, allosteric regulation by Ca occurs under physiological conditions (K(mCaAct) = 125 +/- 16 nM SEM approximately resting [Ca](i)). This, along with the observation that no delay was observed between measured [Ca](i) and activation of I(NCX) under our conditions, suggests that beat to beat changes in NCX function can occur in vivo. These changes in the I(NCX) activation state may influence SR Ca load and resting [Ca](i), helping to fine tune Ca influx and efflux from cells under both normal and pathophysiological conditions. Our failure to observe Ca activation in mouse myocytes may be due to either the extent of Ca regulation or to a difference in K(mCaAct) from other species. Model predictions for Ca activation, on which our estimates of K(mCaAct) are based, confirm that Ca activation strongly influences outward I(NCX), explaining why it increases rather than declines with increasing [Ca](i).

Sarcoplasmic reticulum Ca(2+) release causes myocyte depolarization. Underlying mechanism and threshold for triggered action potentials

Spontaneous sarcoplasmic reticulum (SR) Ca(2+) release causes delayed afterdepolarizations (DADs) via Ca(2+)-induced transient inward currents (I:(ti)). However, no quantitative data exists regarding (1) Ca(2+) dependence of DADs, (2) Ca(2+) required to depolarize the cell to threshold and trigger an action potential (AP), or (3) relative contributions of Ca(2+)-activated currents to DADs. To address these points, we evoked SR Ca(2+) release by rapid application of caffeine in indo 1-AM-loaded rabbit ventricular myocytes and measured caffeine-induced DADs (cDADs) with whole-cell current clamp. The SR Ca(2+) load of the myocyte was varied by different AP frequencies. The cDAD amplitude doubled for every 88+/-8 nmol/L of Delta[Ca(2+)](i) (simple exponential), and the Delta[Ca(2+)](i) threshold of 424+/-58 nmol/L was sufficient to trigger an AP. Blocking Na(+)-Ca(2+) exchange current (I(Na/Ca)) by removal of [Na](o) and [Ca(2+)](o) (or with 5 mmol/L Ni(2+)) reduced cDADs by >90%, for the same Delta[Ca(2+)](i). In contrast, blockade of Ca(2+)-activated Cl(-) current (I(Cl(Ca))) with 50 micromol/L niflumate did not significantly alter cDADs. We conclude that DADs are almost entirely due to I(Na/Ca), not I(Cl(Ca)) or Ca(2+)-activated nonselective cation current. To trigger an AP requires 30 to 40 micromol/L cytosolic Ca(2+) or a [Ca(2+)](i) transient of 424 nmol/L. Current injection, simulating I(ti)s with different time courses, revealed that faster I:(ti)s require less charge for AP triggering. Given that spontaneous SR Ca(2+) release occurs in waves, which are slower than cDADs or fast I(ti)s, the true Delta[Ca(2+)](i) threshold for AP activation may be approximately 3-fold higher in normal myocytes. This provides a safety margin against arrhythmia in normal ventricular myocytes.

Calcium-activated Cl(-) current contributes to delayed afterdepolarizations in single Purkinje and ventricular myocytes

BACKGROUND

The ionic mechanism underlying the transient inward current (I(ti)), the current responsible for delayed afterdepolarizations (DADs), appears to be different in ventricular myocytes and Purkinje fibers. In ventricular myocytes, I(ti) was ascribed to a Na(+)-Ca(2+) exchange current, whereas in Purkinje fibers, it was additionally ascribed to a Cl(-) current and a nonselective cation current. If Cl(-) current contributes to I(ti) and thus to DADs, Cl(-) current blockade may be potentially antiarrhythmogenic. In this study, we investigated the ionic nature of I(ti) in single sheep Purkinje and ventricular myocytes and the effects of Cl(-) current blockade on DADs.

METHODS AND RESULTS

In whole-cell patch-clamp experiments, I(ti) was induced by repetitive depolarizations from -93 to +37 mV in the presence of 1 micromol/L norepinephrine. In both Purkinje and ventricular myocytes, I(ti) was inward at negative potentials and outward at positive potentials. The anion blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) blocked outward I(ti) completely but inward I(ti) only slightly. The DIDS-sensitive component of I(ti) was outwardly rectifying, with a reversal close to the reversal potential of Cl(-) currents. Blockade of Na(+)-Ca(2+) exchange by substitution of extracellular Na(+) by equimolar Li(+) abolished the DIDS-insensitive component of I(ti). DIDS reduced both DAD amplitude and triggered activity based on DADs. Conclusions-In both Purkinje and ventricular myocytes, I(ti) consists of 2 ionic mechanisms: a Cl(-) current and a Na(+)-Ca(2+) exchange current. Blockade of the Cl(-) current may be potentially antiarrhythmogenic by lowering DAD amplitude and triggered activity based on DADs.

Calcium extrusion during aftercontractions in cardiac myocytes: the role of the sodium-calcium exchanger in the generation of the transient inward current

Spontaneous release of calcium from the sarcoplasmic reticulum leads to delayed afterdepolarizations which may represent an arrhythmogenic mechanism in the intact heart. The current underlying delayed afterdepolarizations is the transient inward current, but how this is triggered by a spontaneous rise in cytoplasmic calcium concentration is a matter of debate. We have investigated this by rapid application of caffeine to isolated guinea-pig cardiac myocytes, before and after drive train-induced aftercontractions. Mean (+/- s.e.m.) sarcoplasmic reticulum content reduced from 85 +/- 11 micromol/l accessible cell volume to 53 +/- 9 micromol/l accessible cell volume (n=11) during the course of the aftercontraction. The charge movement expected to result from extrusion of this calcium via the sodium-calcium exchanger was 70.1 +/- 5.4 pC, compared with charge measured during the transient inward current of 70.1 +/- 10.8 pC in the same cells (P=0.9969). Rapid inhibition of the sodium-calcium exchanger, by replacement of the superfusate with a sodium and calcium free solution between the end of the drive train and the aftercontraction, completely abolished the transient inward current (from 90.4 +/- 10.2 pA inward current to 23.8 +/- 14.9 pA outward current, P<0.001). We conclude that the transient inward current in this species is explained entirely by sodium-calcium exchange current without the need to invoke other calcium-activated conductances.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Antiarrhythmic effects of magnesium on rat papillary muscle and guinea pig ventricular myocytes

1. Despite widespread use of magnesium ion (Mg2+) for antiarrhythmic purposes, little direct information is available regarding its antiarrhythmic mechanisms. To elucidate the possible cellular mechanism, the effects of Mg2+ on early afterdepolarization (EAD), delayed afterdepolarization (DAD), triggered activity (TA), transient inward current (TI) and aftercontraction (AC) were examined in various cardiac preparations. The effects of Mg2+ on myoplasmic Ca2+ concentration were also studied. 2. The effects of Mg2+ on AC, induced by overdrive stimulation, were studied in isolated rat ventricular papillary muscle superfused with low K+ solution. In enzymatically isolated guinea pig myocytes, EAD, DAD and/or TA were induced after overdrive stimulation under conditions of superfusion with low K+ solution, using the whole-cell current-clamp method, and TI was also induced by the whole-cell voltage-clamp method. 3. Immediately after changing the solutions, containing varying concentrations of Mg2+, the effects of Mg2+ were examined. In addition, effects of Mg2+ on Ca transient were studied, using fura-2. 4. We found that: (1) in the rat papillary muscle, 10 mM Mg2+ effectively inhibited AC, which was produced after stimulation at both 3.3 Hz and 5 Hz, although 5 mM Mg2+ was without effect in the case of AC induced after 5-Hz stimulation; (2) in the myocytes, 5 mM Mg2+ did not inhibit DADs, EADs and TA, but 10 mM Mg2+ inhibited them completely; (3) the amplitude and frequency of TI decreased significantly in the presence of 10 mM Mg2+; and finally (4) 10 mM Mg2+ inhibited the Ca transient underlying DAD and/or TA. 5. The findings suggest, but do not prove unequivocally, that Mg's actions are probably due to a combination of a shift of the threshold of various ion channels to less negative potentials, a decrease in Ca2+ influx via Ca channels, a block of several K channels, and/or a block of Na-Ca exchanger. In conclusion, the present study indicates that extracellular Mg2+, via whatever mechanism, exerts antiarrhythmic activities.

Withdrawal of acetylcholine elicits Ca2+-induced delayed afterdepolarizations in cat atrial myocytes

BACKGROUND

Recent experiments in atrial myocytes indicate that withdrawal of cholinergic agonist can directly increase Ca2+ influx via L-type Ca2+ current and stimulate Ca2+ uptake into the sarcoplasmic reticulum (SR), thereby increasing intracellular Ca2+. Overload of cellular Ca2+ within the SR can initiate various types of atrial dysrhythmias. The present study was designed to determine whether withdrawal of acetylcholine (ACh) can elicit Ca2+-induced delayed afterdepolarizations (DADs) in atrial myocytes.

METHODS AND RESULTS

A nystatin perforated-patch whole-cell method and fluorescence microscopy (indo 1) were used to measure electrical activities and intracellular free Ca2+ ([Ca2+]i), respectively. Withdrawal of ACh (1 micromol/L) increased action potential duration, shifted plateau voltage toward positive, and generated DADs that initiated spontaneous action potentials. Voltage-clamp analysis revealed that withdrawal of ACh elicited a rebound stimulation of L-type Ca2+ current (I(Ca,L)) (+45%) and Na/Ca exchange current (I(NaCa)) (+16%) and the appearance of transient inward current (I(ti)) and spontaneous [Ca2+]i transients. Each of these changes induced by withdrawal of ACh was abolished by Rp-cAMPs (50 to 100 micromol/L) or H-89 (2 micromol/L), inhibitors of cAMP-dependent protein kinase A. Ryanodine (1 micromol/L) abolished I(NaCa) and the appearance of I(ti) without decreasing the rebound stimulation of I(Ca,L) elicited by withdrawal of ACh.

CONCLUSIONS

Withdrawal of ACh can elicit cAMP-mediated stimulation of Ca2+ influx via I(Ca,L) and uptake of SR Ca2+. As a result, cellular Ca2+ overload causes enhanced SR Ca2+ release and the initiation of DADs. These mechanisms may generate triggered and/or spontaneous atrial depolarizations elicited by withdrawal of vagal nerve activity.

Sodium-calcium exchange: recent advances

Na-Ca exchange proteins are involved in Ca homeostasis in a wide variety of tissues. Unique Na-Ca exchangers have been identified by molecular biological approaches and it appears that these may represent a superfamily of ion transporters, similar to that identified for ion channels. Major advances in our understanding of these transporters have occurred in the past decade by combining molecular approaches with electrophysiological analyses. The regulatory and transport properties of Na-Ca exchangers are beginning to become understood in molecular detail. It also appears that the physiological roles of Na-Ca exchange may be quite complex. This brief review highlights some recent advances in Na-Ca exchange research obtained through the combination of molecular biological and electrophysiological approaches.

The heart and development

The perspective from which the developing heart is viewed can lead to differing conclusions about the effects of development on cardiac function. The hearts of the embryo, fetus and adult, viewed from a global perspective, sustain the circulation through the same basic mechanisms of developing pressure and ejecting blood. The failure of the embryonic heart to perform these tasks results in growth failure, edema, and embryonic death, just as in the infant and adult such failure results in premature death. Furthermore, from the viewpoint of gross anatomy, following embryonic morphogenesis, the developing and adult hearts appear in general to be structurally similar, differing only in size and mass. However, a closer view shows, in the molecular and structural makeup of the myocardium, richly complex changes that can modulate the basic physiological properties of the cardiac myocyte. This article focuses on how these changes and the effects of birth and development alter ventricular function.

Cardiac mechanical and electrophysiologic modulations of guinea-pig by caffeine and thapsigargin

1. The effects of caffeine and thapsigargin on the contractile force and the action potential in guinea-pig papillary muscles were examined. 2. Caffeine (1 to 10 mM) initially increased contractile force in a concentration-dependent manner. Subsequently, 1 mM caffeine decreased it as compared with precaffeine level (but not significantly). At 5 mM or 10 mM, caffeine also decreased contractile force, but the decrease was still positive as compared with control level. 3. Exchange to low [Ca]o (0.9 mM) or high [K]o (8 mM) decreased steady-state value during exposure to 1 mM caffeine. Addition of 1 microM thapsigargin (TG) decreased the steady-state value during exposure to 1 mM caffeine, but enhanced it with 5 mM and 10 mM caffeine. TG (1 microM) alone increased the force. 4. In electrophysiologic, studies, caffeine shortened the action potential duration (APD) in a concentration-dependent manner. In the presence of caffeine (1 mM), high [K]o shortened APD and decreased the action potential amplitude and resting potential. 5. These results suggest that in the presence of caffeine and/or thapsigargin calcium overload might not occur in the left ventricular papillary muscles of the guinea-pig heart.

Ionic mechanisms of ischemia-related ventricular arrhythmias

The aim of this review is the utmost simplification of the cellular electrophysiologic background of ischemia-related arrhythmias. In the acute and subacute phase of myocardial infarction, arrhythmias can be caused by an abnormal impulse generation, abnormal automaticity or triggered activity caused by early or delayed afterdepolarizations (EAD and DAD), or by abnormalities of impulse conduction (i.e., reentry). This paper addresses therapeutic intervention aimed at preventing the depolarization of "pathologic" slow fibers, counteracting the inward calcium (Ca) influx that takes place through the L-type channels (Ca antagonists), or hyperpolarizing the diastolic membrane action potential, increasing potassium (K) efflux (K-channel openers) in arrhythmias generated by an abnormal automaticity (ectopic tachycardias or accelerated idioventricular rhythms). If the cause enhanced impulse generation is related to triggered activity, and since both EAD and DAD are dependent on calcium currents that can appear during a delayed repolarization, the therapeutic options are to shorten the repolarization phase through K-channel openers or Ca antagonists, or to suppress the inward currents directly responsible for the afterdepolarization with Ca blockers. Magnesium seems to represent a reasonable choice, as it is able to shorten the action potential duration and to function as a Ca antagonist. Abnormalities of impulse conduction (re-entry) account for the remainder of arrhythmias that occur in the acute and subacute phase of ischemia and for most dysrhythmias that develop during the chronic phase. Reentrant circuits due to ischemia are usually Na channel-dependent. Drug choice will depend on the length of the excitable gap: in case of a short gap (ventricular fibrillation, polymorphic ventricular tachycardia, etc.), the refractory period has been identified as the most vulnerable parameter, and therefore a correct therapeutic approach will be based on drugs able to prolong the effective refractory period (K-channel blockers, such as class III antiarrhythmic drugs); on the other hand, for those arrhythmias characterized by a long excitable gap (most of the monomorphic ventricular tachycardias), the most appropriate therapeutic intervention consists of depressing ventricular excit-ability and conduction by use of sodium-channel blockers such as mexiletine and lidocaine. Compared with other class I antiarrhythmic agents, these drugs minimally affect refractoriness and exhibit a use-dependent effect and a voltage dependent action (i.e., more pronounced on the ischemic tissue because of its partial depolarization).

Ca2+-induced current oscillations in rabbit ventricular myocytes

Transient currents are activated by spontaneous Ca2+ oscillations in rabbit ventricular myocytes. We investigated the ionic basis for these transient currents under conditions in which K+ currents would be expected to be blocked. Holding cells under voltage clamp at positive potentials leads to a rise in intracellular Ca2+ via reversal of the Na+-Ca2+ exchanger and subsequently to the initiation of spontaneous Ca2+ transients, presumably from a Ca2+-overloaded sarcoplasmic reticulum. The current transients associated with these Ca2+ transients reversed at about +10 to +15 mV under conditions of approximately symmetrical Cl-. In the absence of Cl-, this current was inward at all potentials examined over the range from -88 to +72 mV, consistent with a Na+-Ca2+ exchanger current. In the absence of Na+, the repetitive spontaneous Ca2+ transients could be initiated by a brief train of depolarizations to activate the inward Ca2+ current. Under such conditions, the current was found to reverse at -3 mV when the equilibrium potential of Cl- (ECl) was -2 mV, and the reversal potential shifted to -32 mV when internal Cl- was lowered, to make ECl -33 mV. Thus, in the absence of Na+, it appears that the current is exclusively a Ca2+-activated Cl- current. There is no evidence to indicate the presence of a Ca2+-activated cationic conductance. Further, our results demonstrate that the Ca2+-activated Cl- conductance can carry inward current at potentials more negative to ECl in rabbit ventricular myocytes and is therefore likely to contribute to the arrhythmogenic delayed afterdepolarizations that occur in Ca2+-overloaded cells.

Specific inhibition of Na-Ca exchange function by antisense oligodeoxynucleotides

The Na-Ca exchanger is essential for the Ca2+ homeostasis in many cell types. This transporter has been difficult to investigate because no specific inhibitor is available. We have synthesized an antisense oligodeoxynucleotide directed against the rat cardiac Na-Ca exchanger mRNA. To estimate the activity of the Na-Ca exchange in single cultured myocytes, the exchange current (INaCa) was measured with the voltage-clamp technique while the intracellular Ca2+ concentration ([Ca2+]i) was simultaneously recorded. Most cells exposed to antisense oligodeoxynucleotide showed neither an INaCa nor an increase of [Ca2+]i upon extracellular Na+ removal. Liberation of Ca2+ by flashphotolysis of caged Ca2+ was not followed by a decay of [Ca2+]i in cells exposed to the antisense oligonucleotide, whereas in control cells resting [Ca2+]i was reached 6 s after the flash. Control experiments with non-sense and mismatched oligonucleotides were performed to exclude unspecific inhibitory effects. These results demonstrate that the Na-Ca exchange was specifically and completely suppressed and that antisense oligodeoxynucleotides represent a useful tool to investigate the cellular and molecular properties of the Na-Ca exchanger.

Characteristics of tetanic contractions in caffeine-treated rat myocardium

Skinned fiber preparations are used to obtain the maximal contractile activation of isolated myocardial preparations. Tetanic contractions elicited in the presence of sarcoplasmic reticulum inhibitors have also been used as an alternative method to produce maximal active tension in the intact myocardium. In this work our purpose was to define the best conditions to obtain tetanic contractions in the rat myocardium and to compare the influence of muscle length and inotropic interventions (Ca2+ and Bay K 8644) in the tension produced in twitches and tetanic contractures. Papillary muscles were mounted in a perfusion chamber to record isometric force. Tetanic contractions were elicited by using suprathreshold stimulation with rectangular pulses (10 ms duration) at 5 Hz in the presence of 2.5 mM caffeine. Caffeine depressed the twitch tension but the tetanic tension was similar to that produced under steady-state stimulation (0.5 Hz) in control conditions. Tetanic and twitch tensions were similar along the whole extension of the length-tension curve and under the positive inotropic effects produced by Ca2+ (0.25 to 3.75 mM) or by the Ca(2+)-channel agonist Bay K 8644 (1 microM). During long tetanic stimuli (60 s) a time-dependent tension decay was observed. This decay was prolonged by reducing the extracellular K+ from 5.4 to 1.0 microM, suggesting that Ca2+ extrusion through the Na-Ca exchanger seems to occur during tetanic stimulation. Since tetanic tension was never higher than the tension obtained in twitches elicited at the same Ca2+ concentration (0.5 Hz), we conclude that tetanic contractures represent a useful tool to investigate the contractile response of intact myocardial preparations with a nonfunctional sarcoplasmic reticulum.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracellular citrate induces regenerative calcium release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum was studied in voltage-clamped guinea-pig atrial myocytes. Cells were dialysed with a pipette solution containing the Ca2+ indicator 1- [2-amino-5-(6-carboxyindol-2-yl) phenoxy]-2-(2'-amino-5'-methylphenoxy) ethane-N,N,N',N'-tetraacetic acid] (Indo-1, 100 microM) and as main anion either chloride or the low-affinity Ca2+ buffer citrate. Intracellular Ca2+ transients (Cai transients) were elicited by depolarizations from a holding potential of -50 mV. In chloride-dialysed cells, Cai transients showed a bell-shaped dependence on the amplitude of the depolarizing pulse. In citrate-dialysed cells, membrane depolarizations were associated with a small rise in [Ca2+]i. These small changes in [Ca2+]i were either followed by a large Cai transient or failed to induce large changes in [Ca2+]i. The peak amplitude of the large Cai transient did not vary with the amplitude of the depolarizing pulse. These results demonstrate that in the presence of intracellular chloride, Ca2+ release in atrial cells is a graded process triggered by Ca2+ influx. Using citrate as the main intracellular anoin, Ca2+ release triggered by Ca2+ entry was no longer graded but occurred in a regenerative manner. The results are discussed in terms of two models in which citrate, affects the spatial distribution of [Ca2+]i or the loading state of the sarcoplasmic reticulum.

Voltage dependence of Na-Ca exchanger conformational currents

Properties of a transient current (Icont) believed to reflect a conformational change of the Na-Ca exchanger molecules after Ca2+ binding were investigated. Intracellular Ca2+ concentration jumps in isolated cardiac myocytes were generated with flash photolysis of caged Ca2+ dimethoxynitrophenamine, and membrane currents were simultaneously measured using the whole-cell variant of the patch-clamp technique. A previously unresolved shallow voltage dependence of Icont was revealed after developing an experimental protocol designed to compensate for the photoconsumption of the caged compound. This voltage dependence can be interpreted to reflect the distribution of Na-Ca exchanger conformational states with the Ca2+ binding site exposed to the inside of the cell immediately before the flash. Analysis performed by fitting a Boltzmann distribution to the observed data suggests that under control conditions most exchanger molecules reside in states with the Ca2+ binding site facing the outside of the cell. Dialysis of the cytosol with 3',4'-dichlorobenzamil, an organic inhibitor of the Na-Ca exchange, increased the magnitude of Icont and changed the voltage dependence, consistent with a parallel shift of the charge/voltage curve. This shift may result from intracellular DCB interfering with an Na(+)-binding or Na(+)-translocating step. These observations are consistent with Icont arising from a charge movement mediated by the Na-Ca exchanger molecules after binding of Ca2+.

Effects of caffeine on intracellular sodium activity in cardiac Purkinje fibres: relation to force

1. An increase in cytoplasmic calcium by caffeine would lead to Ca extrusion via the Na/Ca exchange. The hypotheses were investigated that, as a consequence, caffeine might increase intracellular sodium activity (aiNa) and that the relation between aiNa and force might be conditioned by the Ca load. 2. Action potential, aiNa and contractile force were recorded in sheep Purkinje fibres during exposure to caffeine under conditions that decrease or increase the Ca load by different mechanisms. 3. In Tyrode solution, caffeine (8 mM) increased aiNa from 8.05 +/- 0.20 to 10.52 +/- 0.40 mM (+30.5%) and had a triphasic effect on force: an initial transient increase (+93.6%), a subsequent decrease (-37.1%) (negative inotropy) and slow partial recovery (+8.9%). 4. Decreasing the Ca load by means of manganese (1 mM) decreased aiNa and force. Adding caffeine re-increased aiNa and no longer caused a negative inotropic action. Cadmium (0.2 mM) also decreased aiNa, and caffeine reincreased it although far less than in Tyrode solution. 5. High [K]o (10 mM) and tetrodotoxin (5 microM) decreased aiNa as well as force. In their presence, caffeine re-increased aiNa and no longer had a negative inotropic action. 6. Increasing the Ca load by means of high [Ca]o (8.1 mM) increased force (+195%) and decreased aiNa, (-20.3%). Adding caffeine re-increased aiNa (+28.1%), but immediately decreased force (-32.3%). 7. Addition of pyruvate (10 mM) to caffeine increased force, as it does in the presence of Ca overload. 8. Noradrenaline (0.1-1 microM) decreased aiNa and increased contractile force. In its presence, caffeine decreased aiNa further and increased force. 9. It is concluded that caffeine increases aiNa, even during the negative inotropic effect. The decrease in force appears to depend on Ca load. Thus, caffeine no longer decreases force under conditions that decrease Ca load (Mn, high [K]0, TTX) and immediately decreases force when the Ca load is increased(high [Ca]0). However, in the presence of noradrenaline, caffeine decreases aiNa and markedly increases force, as the Ca load is increased, but Ca can be removed from the cytoplasm into the SR.

A dynamic model of the cardiac ventricular action potential. II. Afterdepolarizations, triggered activity, and potentiation

The action potential model presented in our accompanying article in this journal is used to investigate phenomena that involve dynamic changes of [Ca2+]i, as described below. Delayed afterdepolarizations (DADs) are induced by spontaneous Ca2+ release from the sarcoplasmic reticulum (SR), which, in turn, activates both the Na(+)-Ca2+ exchanger (INaCa) and a nonspecific Ca(2+)-activated current (Ins(Ca)). The relative contributions of INaCa and of Ins(Ca) to the generation of DADs are different under different degrees of Ca2+ overload. Early afterdepolarizations (EADs) can be categorized into two types: (1) plateau EADs, resulting from a secondary activation of the L-type Ca2+ current during the plateau of an action potential, and (2) phase-3 EADs, resulting from activation of INaCa and Ins(Ca) by increased [Ca2+]i due to spontaneous Ca2+ release from the SR during the late repolarization phase. Spontaneous rhythmic activity and triggered activity are caused by spontaneous Ca2+ release from the SR under conditions of Ca2+ overload. Postextrasystolic potentiation reflects the time delay associated with translocation of Ca2+ from network SR to junctional SR. The cell is paced at high frequencies to investigate the long-term effects on the intracellular ionic concentrations.

A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes

A mathematical model of the cardiac ventricular action potential is presented. In our previous work, the membrane Na+ current and K+ currents were formulated. The present article focuses on processes that regulate intracellular Ca2+ and depend on its concentration. The model presented here for the mammalian ventricular action potential is based mostly on the guinea pig ventricular cell. However, it provides the framework for modeling other types of ventricular cells with appropriate modifications made to account for species differences. The following processes are formulated: Ca2+ current through the L-type channel (ICa), the Na(+)-Ca2+ exchanger, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), buffering of Ca2+ in the SR and in the myoplasm, a Ca2+ pump in the sarcolemma, the Na(+)-K+ pump, and a nonspecific Ca(2+)-activated membrane current. Activation of ICa is an order of magnitude faster than in previous models. Inactivation of ICa depends on both the membrane voltage and [Ca2+]i. SR is divided into two subcompartments, a network SR (NSR) and a junctional SR (JSR). Functionally, Ca2+ enters the NSR and translocates to the JSR following a monoexponential function. Release of Ca2+ occurs at JSR and can be triggered by two different mechanisms, Ca(2+)-induced Ca2+ release and spontaneous release. The model provides the basis for the study of arrhythmogenic activity of the single myocyte including afterdepolarizations and triggered activity. It can simulate cellular responses under different degrees of Ca2+ overload. Such simulations are presented in our accompanying article in this issue of Circulation Research.

Modulation of Ca2+ release in cultured neonatal rat cardiac myocytes. Insight from subcellular release patterns revealed by confocal microscopy

It is well established that in heart muscle the influx of Ca2+ through Ca2+ channels during the action potential is the main trigger for Ca2+ release from the sarcoplasmic reticulum (SR), but intact cardiac tissue and single myocytes are also known to exhibit spontaneous Ca2+ release from the SR under a variety of circumstances. Although conditions favoring spontaneous activity have been examined extensively, mechanisms modulating or regulating spontaneous as well as triggered Ca2+ release are still largely unknown. Using the high spatial and temporal resolution of laser-scanning confocal microscopy, we investigated subcellular aspects of spontaneous and triggered Ca2+ release in isolated rat neonatal myocytes loaded with the Ca(2+)-sensitive fluorescent dye fluo 3. Three distinct patterns of spontaneous Ca2+ release were identified: (1) a homogeneous Ca2+ release, presumably corresponding to Ca2+ release during a spontaneous action potential, (2) a focal or spatially restricted Ca2+ release with no or only limited subcellular propagation, and (3) a Ca2+ release propagating as a wave throughout the entire cell. Pharmacologic tools that interfere with the SR revealed that all release types were critically dependent on the Ca2+ release and uptake function of the SR. From our results we conclude that the probability, extent, and pattern of Ca2+ release are modulated on the subcellular level. The observed spectrum of release patterns can be explained by a space- and time-dependent variability in the positive feedback of the Ca(2+)-induced Ca(2+)-release mechanism within an individual myocyte. Presumably, this variability depends on the existence of subcellular functional elements of the SR. The actual degree of positive feedback may be modulated locally by the Ca(2+)-loading state of each SR element.

Voltage-dependent kinetics of Na-Ca exchange current in Ca(2+)-loaded guinea pig heart cells

Voltage-dependent properties of Na-Ca exchange current were revealed with the patch-clamp technique in Ca(2+)-overloaded guinea pig ventricular myocytes in the whole cell configuration. With the assumption that the transient inward current (Iti) is mediated by the Na-Ca exchanger, oscillations of internal Ca2+ concentration ([Ca2+]i) were used to investigate voltage-dependent kinetics of exchange current differences at two [Ca2+]i values. After Iti was elicited by clamping from -45 mV to basic pulses of +10 mV, pairs of equipotential short test pulses were applied during the basic pulse at both the phase of low [Ca2+]i (between two neighboring Iti values) and the phase of high [Ca2+]i (at the peak of Iti). The test pulses were short enough to leave the time course of Iti during the basic pulse approximately unchanged, which allowed study of the voltage dependence of the respective current differences without disturbing the underlying oscillation of [Ca2+]i. The current differences were inward at all potentials between -140 and +70 mV, started from an equal initial value, and obeyed characteristic voltage-dependent time courses: hyperpolarization to potentials negative to -70 mV caused an initial current increase, which was followed by a decay to very small amplitudes or zero with a decay time constant decreasing toward hyperpolarization e-fold per 45.6 mV. Depolarizing pulses caused a decay of the current differences to smaller levels. Respective current differences formed during a slowly decaying current component, following the Ca current spike, showed equal voltage-dependent properties. This indicates that the slowly decaying current component is preferentially also carried by the Na-Ca exchanger.(ABSTRACT TRUNCATED AT 250 WORDS)

Buffering of calcium influx by sarcoplasmic reticulum during the action potential in guinea-pig ventricular myocytes

1. Intracellular [Ca2+] ([Ca2+]i) transients, monitored by the fluorescent Ca2+ indicator, indo-1, and twitch contractions elicited by action potentials, by voltage clamp pulses or by rapid, brief pulses of caffeine, were measured in guinea-pig single ventricular myocytes. Experiments were designed to determine whether and to what extent the trans-sarcolemmal Ca2+ influx is immediately sequestered by the sarcoplasmic reticulum (SR). 2. Rapid, brief (100-200 ms) pulses of caffeine onto a rested myocyte elicited a [Ca2+]i transient and a contraction. Following exposure to specific SR inhibitors, ryanodine (100 nM) or thapsigargin (200 nM), the rapid application of caffeine onto a rested myocyte failed to elicit changes in [Ca2+]i or in cell length, indicating that caffeine increases [Ca2+]i by specifically discharging Ca2+ from the SR. In the absence of these inhibitors, a second pulse of caffeine, within 3 min following a prior pulse, failed to elicit a [Ca2+]i transient or contraction, indicating that a caffeine pulse depletes the SR releasable Ca2+ pool. 3. Following Ca2+ depletion of the SR by double caffeine pulses at rest, an electrical stimulation elicited a slow increase in [Ca2+]i, and, after a delay, a small, slow twitch contraction. The simultaneous application of caffeine and electrical stimulation of cells in which the SR was Ca2+ depleted elicited [Ca2+]i transients with an increased rate of rise and a larger amplitude (53 +/- 8 and 63 +/- 9% respectively; mean +/- S.E.M., n = 21) than those elicited by electrical stimulation alone. 4. Whether caffeine affected the L-type calcium current (ICa) elicited by electrical stimulation was determined under whole-cell voltage clamp. A caffeine pulse delivered at the onset of a depolarizing voltage clamp step also increased the rates of rise and the amplitudes of the [Ca2+]i transients and twitch contractions in cells in which the SR was depleted of Ca2+. However, Ca2+ influx via ICa decreased when caffeine was pulsed in conjunction with the voltage clamp, as the peak ICa was either unchanged or decreased while its inactivation was consistently accelerated. 5. Because the stimulation-dependent trans-sarcolemmal Ca2+ influx via ICa is not increased by a caffeine pulse, the augmentation of the rates of rise and the amplitudes of the electrically stimulated [Ca2+]i transients by caffeine pulsed in conjunction with the electrical stimulation in cells in which the SR had been depleted of Ca2+ indicates that a portion of Ca2+ influx during depolarization in the absence of caffeine is rapidly buffered by the SR.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium sparks: elementary events underlying excitation-contraction coupling in heart muscle

Spontaneous local increases in the concentration of intracellular calcium, called "calcium sparks," were detected in quiescent rat heart cells with a laser scanning confocal microscope and the fluorescent calcium indicator fluo-3. Estimates of calcium flux associated with the sparks suggest that calcium sparks result from spontaneous openings of single sarcoplasmic reticulum (SR) calcium-release channels, a finding supported by ryanodine-dependent changes of spark kinetics. At resting intracellular calcium concentrations, these SR calcium-release channels had a low rate of opening (approximately 0.0001 per second). An increase in the calcium content of the SR, however, was associated with a fourfold increase in opening rate and resulted in some sparks triggering propagating waves of increased intracellular calcium concentration. The calcium spark is the consequence of elementary events underlying excitation-contraction coupling and provides an explanation for both spontaneous and triggered changes in the intracellular calcium concentration in the mammalian heart.

Activation of Na-Ca exchange current by photolysis of "caged calcium"

Intracellular photorelease of Ca2+ from "caged calcium" (DM-nitrophen) was used to investigate the Ca(2+)-activated currents in ventricular myocytes isolated from guinea pig hearts. The patch-clamp technique was applied in the whole-cell configuration to measure membrane current and to dialyze the cytosol with a pipette solution containing the caged compound. In the presence of inhibitors for Ca2+, K+, and Na+ channels, concentration jumps of [Ca2+]i induced a rapidly activating inward Na-Ca exchange current which then decayed slowly (tau approximately 500 ms). The initial peak of the inward current and the time-course of current decay were voltage-dependent, and no reversal of the current direction was found between -100 and +100 mV. The observed shallow voltage dependence can be described in terms of the movement of an apparently fractional elementary charge (+0.44e-) across an energy barrier located symmetrically in the electrical field of the membrane. The currents were dependent on extracellular Na+ with a half-maximal activation at 73 mM and a Hill coefficient of 2.8. No change of membrane conductance was activated by the Ca2+ concentration jump when extracellular Na+ was completely replaced by Li+ or N-methyl-D-glucamine (NMG) or when the Na-Ca exchange was inhibited by extracellular Ni2+, La3+, or dichlorobenzamil (DCB). The velocity of relengthening after a twitch induced by photorelease of Ca2+ was only reduced drastically when both the sarcoplasmic reticulum and the Na-Ca exchange were inhibited suggesting that all other Ca2+ removing mechanisms have a low transport capacity under these conditions. In conclusion, we have used a novel approach to study Na-Ca exchange activity with photolysis of "caged" calcium. We found that in guinea pig heart muscle cells the Na-Ca exchange is a potent mechanism for Ca2+ extrusion, is weakly voltage-dependent (118 mV for e-fold change) and can be studied without contamination with other Ca(2+)-activated currents.

Ratiometric confocal Ca(2+)-measurements with visible wavelength indicators in isolated cardiac myocytes

We present a new method for ratiometric Ca2+ measurements using indicators with excitation spectra in the visible range of wavelengths. Laser-scanning confocal microscopy was used to record intracellular Ca(2+)-signals with high temporal and spatial resolution in single cardiac myocytes. The patch-clamp technique was applied to load the cells with the fluorescent Ca(2+)-indicators and to follow the membrane currents with the fluorescence signals simultaneously. Intracellular free Ca(2+)-concentration ([Ca2+]i) was estimated with a ratiometric method. An in vitro calibration procedure was used to convert the fluorescence ratio obtained with two different Ca(2+)-indicators (Fluo-3 and Fura-Red) into Ca(2+)-concentrations. Fluo-3 showed an increase in fluorescence upon a rise in intracellular Ca(2+)-concentration, while the Fura-Red fluorescence decreased. Since the fluorescence of Fluo-3 was around 2-fold brighter than the Fura-Red signal the cells were loaded with a 1:2 mixture of the two indicators. The large increase of the fluorescence ratio during a rise in [Ca2+]i (up to 4-fold) allowed us to record time-resolved signals with this mixture even when monitored in a very small subcellular volume (around 1 micron3). Long lasting continuous recordings of the fluorescence were possible because the dye-mixture exhibited no detectable bleaching with illumination periods of up to 30 s. The use of the Fluo-3/Fura-Red ratio method should significantly facilitate and improve quantitative measurements of [Ca2+]i with high temporal and spatial resolution. Moreover, this approach is especially valuable when used with confocal microscopes which are usually equipped with lasers in the visible light range. Furthermore, it may be possible to use the same approach with mixtures of other indicators to estimate the concentration of other biologically important ions/compounds with a ratiometric calibration.

Ionic mechanisms of transient inward current in the absence of Na(+)-Ca2+ exchange in rabbit cardiac Purkinje fibres

1. Membrane currents were measured with a two-microelectrode technique in voltage clamped rabbit cardiac Purkinje fibres under conditions known to cause intracellular calcium overload and to eliminate or minimize Na(+)-Ca2+ exchange. 2. Increasing [Ca2+]o from 2.5 to 5 mM or above and substituting external sodium with either sucrose, choline or Li+ induced an oscillatory transient inward current (TI) which peaked 200-300 ms after repolarization from a previous depolarizing pulse. The TI quickly disappeared upon return to normal Tyrode solution. Both the rate and configuration of action potentials of Purkinje fibres also returned to control upon return to Tyrode solution after 30 min of high Ca2+ exposure, if the Ca2+ concentration was 30 mM or less. 3. The TI in Na(+)-free solution was Ca2+ dependent. Either zero or low (2.5 mM) [Ca2+]o, or replacement of [Ca2+]o by BaCl prevented induction of the TI current upon repolarization from a previous depolarizing pulse. 4. In the presence of 30 mM-CaCl2 and with choline chloride as the substitute for NaCl, TI had a distinct reversal potential (Erev) of -25 mV. The time-to-peak TI, either inward or outward, did not shift significantly with change in voltage. Both inward and outward TI were simultaneously abolished by exposure to 1 microM-ryanodine, suggesting they were both activated by transient release of Ca2+ from the sarcoplasmic reticulum. The occurrence of TI in the absence of [Na+]o is not compatible with an electrogenic Na(+)-Ca2+ exchange mechanism. The existence of a clear-cut reversal potential suggests that an ionic channel may be responsible for the TI under these conditions. 5. Both the magnitude of peak TI and the Erev were affected by changes of CaCl2 concentration. (i) Under steady-state conditions, peak inward TI was significantly increased when the [Ca2+]o was elevated from 5 to 15 mM. The peak TI in the outward direction was significantly increased when [Ca2+]o was elevated from 15 to 30 mM; however, the difference in peak inward TI at 15 and 30 mM [Ca2+]o was small. (ii) Clear-cut reversals of TI were found at Ca2+ concentrations of 10 mM (Erev = -19.5 mV) or greater, and elevation of [Ca2+]o to 20, 30, 50 and 105 mM shifted the Erev to more negative potentials. (iii) In the presence of 5 mM [Ca2+]o the inward TI declined to zero at about -30 mV, and test voltages between -55 and +5 mV failed to reveal a distinct outward TI.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium transients caused by calcium entry are influenced by the sarcoplasmic reticulum in guinea-pig atrial myocytes

1. Single atrial myocytes obtained by enzyme perfusion from hearts of adult guinea-pigs were investigated using whole-cell voltage clamp and Indo-1 micro-fluorometry. 2. In myocytes loaded with a solution containing citrate as a low-affinity, non-saturable Ca2+ chelator, two types of [Ca2+]i transients could be recorded during repetitive activation of L-type Ca2+ current. Both large and small [Ca2+]i transients occurred; large transients reached peak values of about 1 microM, and small transients were about 100 nM or less in amplitude. 3. In the case of the large transients, peak [Ca2+]i was usually reached with a variable delay after repolarization from a voltage step that activated calcium current (ICa). For the small transients the rise in [Ca2+]i paralleled ICa. Upon repolarization [Ca2+]i started to decay. 4. The small transients reflect entry of Ca2+ through Ca2+ channels (entry transients), whereas the large transients are due to entry and release from the sarcoplasmic reticulum (release transients). 5. The entry transients displayed a positive staircase pattern during trains of depolarizing voltage steps despite constant or even decreasing amplitude of ICa. The steepness of the staircase was increased by elevation of [Ca2+]o. Entry transients were always smallest immediately after a release transient. 6. After functional removal of the sarcoplasmic reticulum by caffeine (1-5 mM) the staircase pattern of the transients reflecting Ca2+ entry was abolished. 7. It is concluded that the staircase pattern is due to rapid uptake by the sarcoplasmic reticulum of Ca2+ entering the cell, resulting in an attenuation of the signal. The attenuation is strongest shortly after a release signal, when the rate of sequestration of Ca2+ by the SR should be highest. 8. Evidence is provided that a compartment of the SR is involved in attenuation of the entry transients. This compartment has been identified recently as a peripheral release compartment.

Mitochondrial and sarcolemmal Ca2+ transport reduce [Ca2+]i during caffeine contractures in rabbit cardiac myocytes

1. Contraction and intracellular Ca2+ (Ca2+i) transients were measured in isolated rabbit ventricular myocytes during twitches and contractures induced by rapid application of 10 mM-caffeine. 2. The amplitude of caffeine-induced contractures and the accompanying Ca2+i transients were larger than during normal twitches and also declined more slowly. This may be because only a fraction of sarcoplasmic reticulum (SR) Ca2+ is released during a normal twitch, or because of a temporal overlap of SR Ca2+ release and uptake during the twitch. 3. When a caffeine contracture was initiated in Na(+)-free, Ca(2+)-free medium (to prevent sarcolemmal Na(+)-Ca2+ exchange) the contracture and Ca2+i transient were larger and decreased much more slowly. Thus, Ca2+ extrusion via Na(+)-Ca2+ exchange may limit the amplitude of caffeine-induced contractures. 4. Relaxation half-time (t1/2) for the twitch (0.17 +/- 0.03 s) was increased to 0.54 +/- 0.07 s for caffeine contractures in control solution and 8.8 +/- 1 s for caffeine-induced contractures in Na(+)-free, Ca(2+)-free solution. These results confirm that the SR Ca2+ pump and Na(+)-Ca2+ exchange are the predominant mechanisms for cytoplasmic Ca2+ removal during relaxation. However slower mechanisms can still reduce intracellular [Ca2+]. 5. Relaxation of caffeine contractures in Na(+)-free solution was further slowed when (a) mitochondrial Ca2+ uptake was inhibited with the oxidative phosphorylation uncoupler, FCCP (t1/2 = 19.7 +/- 3.2 s), or (b) the sarcolemmal Ca(2+)-ATPase pumping ability was depressed by a large transmembrane [Ca2+] gradient (t1/2 = 27.5 +/- 6.9 s). 6. When the four Ca2+ transport systems were simultaneously inhibited (i.e. SR Ca2+ pump, Na(+)-Ca2+ exchange, mitochondrial Ca2+ uptake and sarcolemmal Ca2+ pump), relaxation was practically abolished, but the cell could recover quickly when Na+ was reintroduced and caffeine removed. 7. We conclude that, under our experimental conditions, the sarcolemmal Ca2+ pump and mitochondria are approximately 37- and 50-fold slower than the Na(+)-Ca2+ exchange at removing Ca2+ from the cytoplasm. Additionally, the SR Ca2+ pump is about 3-4 times faster than Na(+)-Ca2+ exchange.

Effects of intracellular Ca2+ chelating compounds on inward currents caused by Ca2+ release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum (SR) of mammalian cardiac myocytes occurring either due to activation by a depolarization or the resulting transmembrane Ca2+ current (ICa), or spontaneously due to Ca2+ overload has been shown to cause inward current(s) at negative membrane potentials. In this study, the effects of different intracellular Ca2+ chelating compounds on ICa-evoked or spontaneous Ca(2+)-release-dependent inward currents were examined in dialysed atrial myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. As compared to dialysis with solutions containing only a low concentration of a high affinity ethylene glycol-bis(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) like chelator (50-200 microM), inward membrane currents (at -50 mV) due to evoked Ca2+ release, spontaneous Ca2+ release or Ca2+ overload following long-lasting depolarizations to very positive membrane potentials are prolonged if tne dialysing fluid contains a high concentration of a low affinity Ca2+ chelating compound such as citrate or free adenosine 5'-triphosphate (ATP). Without such a non-saturable Ca2+ chelator in the dialysing fluid, Ca(2+)-release-dependent inward currents are often oscillatory and show an irregular amplitude. With a low affinity chelator in a non-saturable concentration, discrete inward currents with constant properties can be recorded. We conclude that the variability in Ca(2+)-release-dependent inward current seen in single cells arises from spatial inhomogeneities of intracellular Ca2+ concentration ([Ca2+]i) due to localized saturation of endogenous and exogenous high affinity Ca2+ buffers (e.g.). This can be avoided experimentally by addition of a non-saturable buffer to the intracellular solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Ba2+ current oscillation evoked by bradykinin in ras-transformed fibroblasts

By voltage-clamp recording, we show a novel inward current which oscillates after activation with bradykinin or serum in v-Ki-ras-transformed NIH/3T3 cells. The current oscillation was infrequently observed in control NIH/3T3 fibroblasts. The same stimulation evokes Ca2+ oscillations in the ras-transformed cells but not in parental cells (Fu et al., FEBS Lett. 281, 263-266, 1991). The results suggest that the oscillatory currents are generated by influxes of divalent cations to maintain Ca2+ oscillations in ras-transformed NIH/3T3 cells.