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

Heart failure as a substrate and trigger for ventricular tachycardia

Heart failure (HF) is a major cause of morbidity and mortality with more than 5.1 million individuals affected in the USA. Ventricular tachyarrhythmias (VAs) including ventricular tachycardia and ventricular fibrillation are common in patients with heart failure. The pathophysiology of these mechanisms as well as the contribution of heart failure to the genesis of these arrhythmias is complex and multifaceted. Myocardial hypertrophy and stretch with increased preload and afterload lead to shortening of the action potential at early repolarization and lengthening of the action potential at final repolarization which can result in re-entrant ventricular tachycardia. Myocardial fibrosis and scar can create the substrate for re-entrant ventricular tachycardia. Altered calcium handling in the failing heart can lead to the development of proarrhythmic early and delayed after depolarizations. Various medications used in the treatment of HF such as loop diuretics and angiotensin converting enzyme inhibitors have not demonstrated a reduction in sudden cardiac death (SCD); however, beta-blockers (BB) are effective in reducing mortality and SCD. Amongst patients who have HF with reduced ejection fraction, the angiotensin receptor-neprilysin inhibitor (sacubitril/valsartan) has been shown to reduce cardiovascular mortality, specifically by reducing SCD, as well as death due to worsening HF. Implantable cardioverter-defibrillator (ICD) implantation in HF patients reduces the risk of SCD; however, subsequent mortality is increased in those who receive ICD shocks. Prophylactic ICD implantation reduces death from arrhythmia but does not reduce overall mortality during the acute post-myocardial infarction (MI) period (less than 40 days), for those with reduced ejection fraction and impaired autonomic dysfunction. Furthermore, although death from arrhythmias is reduced, this is offset by an increase in the mortality from non-arrhythmic causes. This article provides a review of the aforementioned mechanisms of arrhythmogenesis in heart failure; the role and impact of HF therapy such as cardiac resynchronization therapy (CRT), including the role, if any, of CRT-P and CRT-D in preventing VAs; the utility of both non-invasive parameters as well as multiple implant-based parameters for telemonitoring in HF; and the effect of left ventricular assist device implantation on VAs.

Mitochondria-derived ROS bursts disturb Ca²⁺ cycling and induce abnormal automaticity in guinea pig cardiomyocytes: a theoretical study

Mitochondria are in close proximity to the redox-sensitive sarcoplasmic reticulum (SR) Ca(2+) release [ryanodine receptors (RyRs)] and uptake [Ca(2+)-ATPase (SERCA)] channels. Thus mitochondria-derived reactive oxygen species (mdROS) could play a crucial role in modulating Ca(2+) cycling in the cardiomyocytes. However, whether mdROS-mediated Ca(2+) dysregulation translates to abnormal electrical activities under pathological conditions, and if yes what are the underlying ionic mechanisms, have not been fully elucidated. We hypothesize that pathological mdROS induce Ca(2+) elevation by modulating SR Ca(2+) handling, which activates other Ca(2+) channels and further exacerbates Ca(2+) dysregulation, leading to abnormal action potential (AP). We also propose that the morphologies of elicited AP abnormality rely on the time of mdROS induction, interaction between mitochondria and SR, and intensity of mitochondrial oxidative stress. To test the hypotheses, we developed a multiscale guinea pig cardiomyocyte model that incorporates excitation-contraction coupling, local Ca(2+) control, mitochondrial energetics, and ROS-induced ROS release. This model, for the first time, includes mitochondria-SR microdomain and modulations of mdROS on RyR and SERCA activities. Simulations show that mdROS bursts increase cytosolic Ca(2+) by stimulating RyRs and inhibiting SERCA, which activates the Na(+)/Ca(2+) exchanger, Ca(2+)-sensitive nonspecific cationic channels, and Ca(2+)-induced Ca(2+) release, eliciting abnormal AP. The morphologies of AP abnormality are largely influenced by the time interval among mdROS burst induction and AP firing, dosage and diffusion of mdROS, and SR-mitochondria distance. This study defines the role of mdROS in Ca(2+) overload-mediated cardiac arrhythmogenesis and underscores the importance of considering mitochondrial targets in designing new antiarrhythmic therapies.

Explaining calcium-dependent gating of anoctamin-1 chloride channels requires a revised topology

RATIONALE

Ca2+ -activated Cl channels play pivotal roles in the cardiovascular system. They regulate vascular smooth muscle tone and participate in cardiac action potential repolarization in some species. Ca2+ -activated Cl channels were recently discovered to be encoded by members of the anoctamin (Ano, also called Tmem16) superfamily, but the mechanisms of Ano1 gating by Ca2+ remain enigmatic.

OBJECTIVE

The objective was to identify regions of Ano1 involved in channel gating by Ca2+.

METHODS AND RESULTS

The Ca2+ sensitivity of Ano1 was estimated from rates of current activation, and deactivation in excised patches rapidly switched between zero and high Ca2+ on the cytoplasmic side. Mutation of glutamates E702 and E705 dramatically altered Ca2+ sensitivity. E702 and E705 are predicted to be in an extracellular loop, but antigenic epitopes introduced into this loop are not accessible to extracellular antibodies, suggesting this loop is intracellular. Cytoplasmically applied membrane-impermeant sulfhydryl reagents alter the Ca2+ sensitivity of Ano1 E702C and E705C as expected if E702 and E705 are intracellular. Substituted cysteine accessibility mutagenesis of the putative re-entrant loop suggests that E702 and E705 are located adjacent to the Cl conduction pathway.

CONCLUSIONS

We propose an alternative model of Ano1 topology based on mutagenesis, epitope accessibility, and cysteine-scanning accessibility. These data contradict the popular re-entrant loop model by showing that the putative fourth extracellular loop (ECL 4) is intracellular and may contain a Ca2+ binding site. These studies provide new perspectives on regulation of Ano1 by Ca2+.

Heart failure enhanced pulmonary vein arrhythmogenesis and dysregulated sodium and calcium homeostasis with increased calcium sparks

Late sodium currents and intracellular Ca(2+) (Ca(2+) (i)) dynamics play an important role in arrhythmogenesis of pulmonary vein (PV) and heart failure (HF). It is not clear whether HF enhances PV arrhythmogenesis through modulation of Ca(2+) homeostasis and increased late sodium currents. The aim of this study was to investigate the sodium and calcium homeostasis in PV cardiomyocytes with HF.

METHODS AND RESULTS

Whole-cell patch clamp was used to investigate the action potentials and ionic currents in isolated rabbit single PV cardiomyocytes with and without rapid pacing induced HF. The Ca(2+) (i) dynamics were evaluated through fluorescence and confocal microscopy. As compared to control PV cardiomyocytes (n = 18), HF PV cardiomyocytes (n = 13) had a higher incidence of delayed afterdepolarization (45% vs 13%, P < 0.05) and faster spontaneous activity (3.0 ± 0.2 vs 2.1 ± 0.2 Hz, P < 0.05). HF PV cardiomyocytes had increased late Na(+) currents, Na(+) /Ca(2+) exchanger currents, and transient inward currents, but had decreased Na(+) currents or L-type calcium currents. HF PV cardiomyocytes with pacemaker activity had larger Ca(2+) (i) transients (R410/485, 0.18 ± 0.04 vs 0.11 ± 0.02, P < 0.05), and sarcoplasmic reticulum Ca(2+) stores. Moreover, HF PV cardiomyocytes with pacemaker activity (n = 18) had higher incidence (95% vs 70%, P < 0.05), frequency (7.8 ± 3.1 vs 2.3 ± 1.2 spark/mm/s, P < 0.05), amplitude (F/F(0) , 3.2 ± 0.8 vs 1.9 ± 0.5, P < 0.05), and longer decay time (65 ± 3 vs 48 ± 4 ms, P < 0.05) of Ca(2+) sparks than control PV cardiomyocytes with pacemaker activity (n = 18).

CONCLUSIONS

Dysregulated sodium and calcium homeostasis, and enhanced calcium sparks promote arrhythmogenesis of PV cardiomyocytes in HF, which may play an important role in the development of atrial fibrillation.

Inhibition of the calcium-activated chloride current in cardiac ventricular myocytes by N-(p-amylcinnamoyl)anthranilic acid (ACA)

N-(p-amylcinnamoyl)anthranilic acid (ACA), a phospholipase A(2) (PLA(2)) inhibitor, is structurally-related to non-steroidal anti-inflammatory drugs (NSAIDs) of the fenamate group and may also modulate various ion channels. We used the whole-cell, patch-clamp technique at room temperature to investigate the effects of ACA on the Ca(2+)-activated chloride current (I(Cl(Ca))) and other chloride currents in isolated pig cardiac ventricular myocytes. ACA reversibly inhibited I(Cl(Ca)) in a concentration-dependent manner (IC(50)=4.2 μM, n(Hill)=1.1), without affecting the L-type Ca(2+) current. Unlike ACA, the non-selective PLA(2) inhibitor bromophenacyl bromide (BPB; 50 μM) had no effect on I(Cl(Ca)). In addition, the analgesic NSAID structurally-related to ACA, diclofenac (50 μM) also had no effect on I(Cl(Ca)), whereas the current in the same cells could be suppressed by chloride channel blockers flufenamic acid (FFA; 100 μM) or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS;100 μM). Besides I(Cl(Ca)), ACA (50 μM) also suppressed the cAMP-activated chloride current, but to a lesser extent. It is proposed that the inhibitory effects of ACA on I(Cl(Ca)) are PLA(2)-independent and that the drug may serve as a useful tool in understanding the nature and function of cardiac anion channels.

Calcium and arrhythmogenesis

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

Would modulation of intracellular Ca2+ be antiarrhythmic?

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

Trapidil enhances the slowly activating delayed rectifier potassium current and suppresses the transient inward current induced by catecholamine in Guinea pig ventricular myocytes

We examined the electrophysiological effects of trapidil on the ionic currents influencing the repolarization and on the transient inward current (ITi) that can cause triggered arrhythmia using the whole-cell patch-clamp technique in guinea pig ventricular myocytes. Trapidil shortened the action potential duration (APD) and increased the delayed rectifier potassium current (IK) in a concentration-dependent manner. The effect of trapidil on the rapidly and slowly activating components of IK (IKr and IKs, respectively) was studied by the envelope of tails test. Trapidil failed to affect IKr and selectively enhanced IKs. Trapidil increased the amplitude of the L-type Ca2+ current (ICa,L), with an acceleration of its inactivation, whereas isoproterenol, a beta-adrenoceptor agonist, increased the amplitude of the ICa,L in a different manner. Isoproterenol activated ITi; however, trapidil not only failed to facilitate ITi but also suppressed isoproterenol-induced ITi. The inhibitory effect of trapidil on isoproterenol-induced ITi is at least partly via a reduction of Ca2+ overload through an acceleration of ICa,L inactivation and/or a sarcoplasmic reticulum (SR) Ca channel modulation. These results suggest that trapidil does not prolong the QT interval and has an antiarrhythmic effect on arrhythmias elicited by triggered activity secondary to Ca2+ overload at much higher concentrations than clinical concentration.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Calcium and cardiac arrhythmias: DADs, EADs, and alternans

Rapid progress has been made in understanding the molecular mechanisms by which calcium ions mediate certain cardiac arrhythmias. Principal advances include imaging of cytosolic calcium in isolated cells and in intact tissues, use of fluorescent indicators and monophasic action potentials to record membrane potentials in isolated tissue, and sequencing of the genes that encode critical ion channel proteins. In this review, five types of arrhythmias are discussed where calcium ion currents, or currents controlled by calcium, appear to be responsible for arrythmogenesis. These include: (1) the delayed afterpotential that occurs in conditions of intracellular calcium overload such as digitalis toxicity; (2) the early afterdepolarization that occurs when action potential duration is prolonged; (3) the slowly conducted calcium-dependent action potential (the slow response) in the SA and AV nodes; (4) the phenomenon of calcium transient alternans during ischemia, which is related to action potential duration alternans and t-wave alternans; (5) catecholamine-induced cardiac arrhythmias in families with mutations of the sarcoplasmic reticulum calcium-release channel. For each type of arrhythmia, the clinical implications of emerging knowledge are discussed. An especially important issue is whether ventricular fibrillation during acute coronary artery occlusion is due to calcium transient alternans. Ventricular fibrillation due to acute ischemia is an important subset of the 400,000 sudden cardiac deaths that occur annually in the U.S. Certain drugs, including beta blockers, fish oils, verapamil, and diltiazem, seem to specifically prevent ventricular fibrillation in this setting, and in most cases an effect of the drug on cytosolic calicum appears to be involved.

Effects of analogues of inorganic phosphate and sodium ion on mineralization of matrix vesicles isolated from growth plate cartilage of normal rapidly growing chickens

The mechanism of matrix vesicle (MV) mineralization was studied using MVs isolated from normal growth plate tissue, as well as several putative intermediates in the MV mineralization pathway--amorphous calcium phosphate (ACP), calcium phosphate phosphatidylserine complex (CPLX) and hydroxyapatite (HAP). Radionuclide uptake and increase in turbidity were used to monitor mineral formation during incubation in synthetic cartilage lymph (SCL). Inhibitors of phosphate (Pi) metabolism, as well as replacing Na(+) with various cations, were used to study MV Pi transport, which had been thought to be Na(+)-dependent. MVs induced rapid mineralization approximately 3 h after addition to SCL; CPLX and HAP caused almost immediate induction; ACP required approximately 1 h. Phosphonoformate (PFA), a Pi analog, potently delayed the onset and reduced the rate of mineral formation of MV and the intermediates with IC(50)'s of 3-6 microM and approximately 10 microM, respectively. PFA:Pi molar ratios required to reduce the rate of rapid mineralization by 50% were approximately 1:30 for ACP, approximately 1:20 for HAP, approximately 1:3.3 for CPLX, and approximately 1:2.0 for MVs. MV mineralization was not found to be strictly Na(+)-dependent: substitution of Li(+) or K(+) for Na(+) had minimal effect; while N-methyl D-glucamine (NMG(+)) was totally inhibitory, choline(+) was clearly stimulatory. Na(+) substitutions had minimal effect on HAP- and CPLX-seeded mineral formation. However with ACP, NMG(+) totally blocked and choline(+) stimulated, just as they did MV mineralization. Thus, kinetic analyses indicate that ACP is a key intermediate, nevertheless, formation of CPLX appears to be the rate-limiting factor in MV mineralization.

Presence of a calcium-activated chloride current in mouse ventricular myocytes

The properties of several components of outward K(+) currents, including the pharmacological and kinetics profiles as well as the respective molecular correlates, have been identified in mouse cardiac myocytes. Surprisingly little is known with regard to the Ca(2+)-activated ionic currents. We studied the Ca(2+)-activated transient outward currents in mouse ventricular myocytes. We have identified a 4-aminopyridine (4-AP)- and tetraethyl ammonium-resistant transient outward current that is Ca(2+) dependent. The current is carried by Cl(-) and is critically dependent on Ca(2+) influx via voltage-gated Ca(2+) channels and the sarcoplasmic reticulum Ca(2+) store. The current can be blocked by the anion transport blockers niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Single channel recordings reveal small conductance channels (approximately 1 pS in 140 mM Cl(-)) that can be blocked by anion transport blockers. Ensemble-averaged current faithfully mirrors the transient kinetics observed at the whole level. Niflumic acid (in the presence of 4-AP) leads to prolongation of the early repolarization. Thus this current may contribute to early repolarization of action potentials in mouse ventricular myocytes.

Ionic mechanism of delayed afterdepolarizations in ventricular cells isolated from human end-stage failing hearts

BACKGROUND

Animal studies have shown that the Ca(2+)-activated Cl(-) current (I(Cl(Ca))) and the Na(+)/Ca(2+) exchange current (I(Na/Ca)) contribute to the transient inward current (I(ti)). I(ti) is responsible for the proarrhythmic delayed afterdepolarizations (DADs). We investigated the ionic mechanism of I(ti) and DADs in human cardiac cells.

METHODS AND RESULTS

Human ventricular cells were enzymatically isolated from explanted hearts of patients with end-stage heart failure and studied with patch-clamp methodology. I(ti)s were elicited in the presence of 1 micromol/L norepinephrine by trains of repetitive depolarizations from -80 to +50 mV. DADs were induced in the presence of 1 micromol/L norepinephrine at a stimulus frequency of 1 Hz. I(ti) currents were inwardly directed over the voltage range between -110 and + 50 mV. Neither the Cl(-) channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid nor changes in [Cl(-)](i) affected I(ti) or DAD amplitude. This excludes an important role for I(Cl(Ca)). Blockade of Na(+)/Ca(2+) exchange by substitution of all extracellular Na(+) by Li(+), conversely, completely inhibited I(ti). In rabbit, I(Cl(Ca)) density in ventricular cells isolated from control hearts did not differ significantly from that in ventricular cells isolated from failing hearts.

CONCLUSIONS

In contrast to many animal species, I(ti) and DADs in human ventricular cells from failing hearts consist only of I(Na/Ca). In rabbits, heart failure per se does not alter I(Cl(Ca)) density, suggesting that I(Cl(Ca)) may also be absent during DADs in nonfailing human ventricular cells.

Mechanisms underlying delayed afterdepolarizations in hypertrophied left ventricular myocytes of rats

Cardiac hypertrophy was induced in rats by daily injection of isoproterenol (5 mg/kg ip) for 7 days. Membrane voltage and currents were recorded using the whole cell patch-clamp technique in left ventricular myocytes from control and hypertrophied hearts. Ryanodine-sensitive delayed afterdepolarizations (DADs) and transient inward current (I(ti)) appeared in hypertrophied cells more often and were of larger amplitude than in control cells. DADs and I(ti) are carried principally by Na/Ca exchange with smaller contributions from a nonselective cation channel and from a Cl- channel. The latter is expressed only in hypertrophied myocytes. In hypertrophy, the density of caffeine-induced Na/Ca exchange current (I(Na/Ca)) was increased by 26%, sarcoplasmic reticulum (SR) Ca2+ content as assessed from the integral of I(Na/Ca) was increased by 30%, the density of Na-pump current (I(pump)) was reduced by 40%, and the intracellular Na+ content, measured by Na+-selective microelectrodes was increased by 55%. The results indicate that DADs and I(ti) are generated by spontaneous Ca2+ release from an overloaded SR caused by a downregulated Na pump and an upregulated Na/Ca exchange. These findings may explain the propensity for arrhythmias seen in this model of hypertrophy.

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.

Early and delayed afterdepolarizations in rabbit heart Purkinje cells viewed by confocal microscopy

We investigated action potentials and Ca(2+) transients in rabbit Purkinje myocytes using whole cell patch clamp recordings and a confocal microscope. Purkinje cells were loaded with 5 microM Fluo-3/AM for 30min. Action potentials were elicited by application of a stimulus delivered through the recording pipettes. When Purkinje cells were stimulated in 2.0mM Ca(2+), transverse XT line scans revealed a symmetrical 'U'-shaped Ca(2+) transient demonstrating that the transient was initiated at the cell periphery. When Purkinje cells were superfused with 1 microM isoprenaline, both early and delayed afterdepolarizations were induced. XT line scans of cells exhibiting early afterdepolarizations showed a second symmetrical 'U'-shaped transient. This Ca(2+) transient was initiated at the cell periphery suggesting reactivation of the Ca(2+) current. In contrast, in Purkinje cells exhibiting delayed afterdepolarizations and a corresponding transient inward current, XT line scans revealed a heterogenous rise in Ca(2+) at both peripheral and central regions of the cell. Immunofluorescence staining of Purkinje cells with an antibody to ryanodine receptors (RyRs) revealed that RyRs are located at regularly spaced intervals throughout the interior of Purkinje cells. These results suggest that, although RyRs are located throughout Purkinje cells, only peripheral RyRs are activated to produce transients, sparks and early afterdepolarizations. During delayed afterdepolarizations, we observed a heterogenous rise in Ca(2+) at both peripheral and central regions of the cell as well as large central increases in Ca(2+). Although the latter may result from central release, we cannot exclude the possibility that it reflects Ca(2+) diffusion from subsarcolemmal sites.

Increased susceptibility to development of triggered activity in myocytes from mice with targeted disruption of endothelial nitric oxide synthase

Nitric oxide generated by cardiac myocytes or delivered by drugs has been shown to regulate cardiac contractile function and has been implicated in suppressing some cardiac arrhythmias, although this remains controversial. We examined the ability of the soluble cardiac glycoside, ouabain, to trigger arrhythmic contractions in ventricular myocytes isolated from mice lacking a functional endothelial nitric oxide synthase gene (eNOS(null)). Arrhythmic activity, defined as aftercontractions, was induced with ouabain (50 micromol/L) and recorded using a video-motion detector in isolated, electrically driven single ventricular myocytes from adult eNOS(null)or from their wild-type (WT) littermates. The rate of ouabain-induced arrhythmic contractions was significantly higher in eNOS(null)myocytes than in WT myocytes. Application of the NO donor S-nitroso-acetylcysteine (SNAC) significantly diminished the frequency of arrhythmic contractions in eNOS(null)myocytes. The antiarrhythmic effect of NO, whether generated by eNOS in WT cells or by SNAC, could be partially reversed by 1H-[1,2,4]oxadiazolo-[4, 3-a]- quinoxalin-1-one (ODQ), a specific soluble guanylyl cyclase inhibitor. Ouabain significantly increased intracellular cGMP in WT but not eNOS(null)hearts, and this cGMP response was blocked by ODQ. Since cardiac glycoside- induced aftercontractions are activated by the transient inward current (I(ti)), the role of NO in ouabain (100 micromol/L)- induced I(ti)was examined using the nystatin-perforated patch-clamp technique. The frequency of ouabain-induced I(ti)was significantly higher in eNOS(null)myocytes than in WT myocytes, and this could be suppressed by SNAC. These data demonstrate that NO derived from myocyte eNOS activation suppresses ouabain-induced arrhythmic contractions by a mechanism that might involve activation of guanylyl cyclase and elevation of cGMP.

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.

Anion transport in heart

Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.

Ca2+-activated non-selective cation current in rabbit ventricular myocytes

Oscillatory currents (OCs) were studied in isolated rabbit ventricular myocytes with whole cell mode voltage clamp using Na+-free intracellular and extracellular solutions under conditions where K+ currents were anticipated to be eliminated or minimized. All OCs were dependent on release of Ca2+ from the sarcoplasmic reticulum (SR) because they were associated with intracellular Ca2+ ([Ca2+]i) transients, and were suppressed by high concentrations of BAPTA (20 mmol l-1) or pretreatment with the SR antagonist agents ryanodine (10 micromol l-1) or thapsigargin (1 micromol l-1). The reversal potential (Vrev) for OCs shifted with changes in the calculated Vrev for Cl- (ECl) but was between ECl and the calculated Vrev for elemental monovalent cations (ECat), indicating that more than one Ca2+-activated current contributed to OCs. Addition of the Ca2+-activated Cl- current (ICl(Ca)) antagonist, niflumic acid, shifted the OC Vrev to ECat, suggesting that ICl(Ca) and a Ca2+-activated non-selective cation current (ICAN) contributed to the observed OCs. A reduced niflumic acid-insensitive Ca2+-activated OC persisted following marked symmetrical reduction of Cl- in the intracellular and extracellular solutions. Subsequent removal of all extracellular monovalent cations, by N-methyl-D-glucamine (NMDG) substitution, eliminated OCs and the inward holding current suggesting that ICAN and ICl(Ca) accounted for all or most of the Ca2+-activated OC in the absence of Na+. The OC Vrev was equal to ECl in the absence of monovalent elemental cations. Under these conditions niflumic acid eliminated all OCs. Macroscopic OC is partially due to ICAN in rabbit ventricular myocytes.

Bimodal control of a Ca(2+)-activated Cl(-) channel by different Ca(2+) signals

Ca(2+)-activated Cl(-) channels play important roles in a variety of physiological processes, including epithelial secretion, maintenance of smooth muscle tone, and repolarization of the cardiac action potential. It remains unclear, however, exactly how these channels are controlled by Ca(2+) and voltage. Excised inside-out patches containing many Ca(2+)-activated Cl(-) channels from Xenopus oocytes were used to study channel regulation. The currents were mediated by a single type of Cl(-) channel that exhibited an anionic selectivity of I(-) > Br(-) > Cl(-) (3.6:1.9:1.0), irrespective of the direction of the current flow or [Ca(2+)]. However, depending on the amplitude of the Ca(2+) signal, this channel exhibited qualitatively different behaviors. At [Ca(2+)] < 1 microM, the currents activated slowly upon depolarization and deactivated upon hyperpolarization and the steady state current-voltage relationship was strongly outwardly rectifying. At higher [Ca(2+)], the currents did not rectify and were time independent. This difference in behavior at different [Ca(2+)] was explained by an apparent voltage-dependent Ca(2+) sensitivity of the channel. At +120 mV, the EC(50) for channel activation by Ca(2+) was approximately fourfold less than at -120 mV (0.9 vs. 4 microM). Thus, at [Ca(2+)] < 1 microM, inward current was smaller than outward current and the currents were time dependent as a consequence of voltage-dependent changes in Ca(2+) binding. The voltage-dependent Ca(2+) sensitivity was explained by a kinetic gating scheme in which channel activation was Ca(2+) dependent and channel closing was voltage sensitive. This scheme was supported by the observation that deactivation time constants of currents produced by rapid Ca(2+) concentration jumps were voltage sensitive, but that the activation time constants were Ca(2+) sensitive. The deactivation time constants increased linearly with the log of membrane potential. The qualitatively different behaviors of this channel in response to different Ca(2+) concentrations adds a new dimension to Ca(2+) signaling: the same channel can mediate either excitatory or inhibitory responses, depending on the amplitude of the cellular Ca(2+) signal.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Calmodulin kinase inhibition prevents development of the arrhythmogenic transient inward current

Although it is widely accepted that afterdepolarizations initiate arrhythmias when action potentials are prolonged, the underlying mechanisms are unclear. In this study, we tested the hypothesis that action potential prolongation would raise intracellular calcium and thereby activate the arrhythmogenic transient inward current (Iti). Furthermore, given that Iti can be activated by sarcoplasmic reticulum Ca2+ release, we tested the hypothesis that inhibition of calmodulin (CaM) kinase would prevent Iti. Isolated rabbit ventricular myocytes were studied with whole-cell-mode voltage clamp. Stimulation with a prolonged action potential clamp, under near-physiological conditions, increased [Ca2+]i. Iti was reproducibly induced in 60 of 60 cells, but Iti was not seen with the use of a shorter action potential waveform (n=12). Iti was associated with a secondary elevation in [Ca2+]i. When [Ca2+]i buffering was enhanced by dialysis with BAPTA (20 mmol/L, n=9), no Iti was present. The Na+/Ca2+ exchanger was likely responsible for Iti, because Iti was inhibited by the Na+/Ca2+ exchanger inhibitory peptide XIP (10 micromol/L, n=6), but not by an inactive scrambled peptide (10 micromol/L, n=5) or by the Cl- current antagonist niflumic acid (10 to 40 micromol/L, n=9). Activator Ca2+ from the sarcoplasmic reticulum was essential for development of Iti, because it was prevented by pretreatment with ryanodine (10 micromol/L, n=6) or thapsigargin (1 micromol/L, n=6). Two different CaM kinase inhibitory peptides (n=16) and a CaM inhibitory peptide (n=4) completely suppressed Iti. These results are consistent with the hypothesis that CaM kinase plays a role in arrhythmias related to increased [Ca2+]i.

Pharmacological tests of the mechanism of the periodic rhythm caused by veratramine in the sinoatrial node of the guinea pig

1. We investigated the effects of several drugs and extracellular ions on the periodic sinoatrial node rhythm caused by high concentrations of veratramine (>2 microM) in isolated guinea pig sinus atria. 2. During the active phase of this rhythm, pacemaker activity appeared to be due to transient afterdepolarizations resembling the delayed afterdepolarizations attributed to Ca++-induced Ca++ release in cardiac tissue. 3. Ryanodine (200-2200 nM) did not decrease the transient afterdepolarizations, and instead increased the heart rate during the active phase, prolonged the active phase, and sometimes caused conversion to regular rhythm. 4. Dichlorobenzamyl (10-110 microM), a blocker of electrogenic Na+-Ca++ exchange, did not slow or stop beating during periodic rhythm, but rather increased average heart rate and, at a higher concentration, caused conversion to regular rhythm. 5. Ouabain (0.1 microM), an inhibitor of the sodium pump and electrogenic Na+-K+ exchange, had little effect on veratramine periodic rhythm, but at higher concentrations it caused increased average heart rate and conversion to regular rhythm. 6. The chronotropic effect of Ca++ was normally weakly positive; however, in the presence of veratramine, and before the appearance of periodic rhythm, the chronotropic effect of Ca++ was weakly negative, and was associated with destabilization of the heart rate, leading to frequency oscillations or periodic rhythm. 7. Veratramine changed the chronotropic effect of K+ from weakly negative to moderately positive. 8. When half the Na+ or Cl- in the bathing medium was replaced by an impermeant ion, in the absence of veratramine the average heart rate was slightly decreased, whereas, in the presence of veratramine and periodic rhythm the average rate was increased, although the increase was not statistically significant in the case of low Na+. 9. These observations indicate that Ca++-induced Ca++ release, Na+-Ca++ exchange, and probably electrogenic Na+-K+ exchange play no important role in generation of periodic rhythm. The increased K+ dependence suggests an altered pacemaker mechanism.

INaCa and ICl(Ca) contribute to isoproterenol-induced delayed after depolarizations in midmyocardial cells

The contributions of electrogenic sodium/calcium exchange current (INaCa), calcium-activated chloride conductance [ICl(Ca)], and calcium-activated nonselective cation conductance to delayed afterdepolarizations (DAD) were examined. Nonselective cation channels were absent in canine M cells, since inhibition of INaCa and ICl(Ca) eliminated all calcium-activated currents without abolishing cell shortening. After the cells were treated with isoproterenol and ouabain to increase calcium loading, INaCa was 168 +/- 30 x 10(-3) pC/pF and ICl(Ca) was 114 +/- 24 x 10(-3) pC/pF. Transient overlapping inward and outward currents were evoked positive to the chloride reversal potential (ECl). Outward current was chloride sensitive, and inward current was blocked by replacement of external sodium with lithium. When ECl was -50 mV, triggered activity occurred in normal external sodium and persisted after inhibition of INaCa. Steps to -80 mV revealed oscillating inward currents in normal sodium and chloride, which persisted after inhibition of INaCa. When ECl was equal to -113 mV, ICl(Ca) opposed INaCa at the resting potential. DAD occurred in normal sodium, and inhibition of outward ICl(Ca) provoked triggered activity. We conclude that INaCa represents approximately 60% of the total calcium-activated current at resting potentials but that both INaCa and ICl(Ca) work in concert to cause DAD in calcium-overloaded cells.

Facilitation of epinephrine-induced afterdepolarizations by class III antiarrhythmic drugs

The electrophysiologic actions of epinephrine (10(-9) M, 10(-8) M, and 10(-7) M) were evaluated in canine Purkinje fibers pretreated with the class III antiarrhythmic drugs clofilium (10(-7) M) or d,l-sotalol (10(-6) M). Clofilium and d,l-sotalol prolonged action potential duration at 50% and 90% of repolarization without provoking early afterdepolarization (EAD) or delayed afterdepolarization (DAD). Subsequent administration of epinephrine provoked both bradycardia-dependent EADs and tachycardia-dependent DADs in clofilium-treated Purkinje fibers, with predominantly EADs observed in d,l-sotalol-treated Purkinje fibers. A temporary increase in Ca0(+2) from 1.35 mM to 5 mM suppressed both EADs and DADs. The data demonstrate facilitation of epinephrine-induced EADs and DADs by class III antiarrhythmic drugs. The acute suppression of both EADs and DADs observed following an acute increase in Ca0(+2) suggests inward Na(+)-Ca0(+2) exchange current as a basis for both EADs and DADs observed in the presence of class III antiarrhythmic drugs and epinephrine.

Early afterdepolarizations produced by d,l-sotalol and clofilium

INTRODUCTION

The roles for L-type calcium current and Na-Ca exchange in early afterdepolarizations (EADs) attending d,l-sotalol and clofilium were examined in canine Purkinje fibers and in enzymatically dispersed myocytes from canine subepicardium.

METHODS AND RESULTS

Spontaneous EADs were compared to EAD formation potentiated by stimulation of Na-Ca exchange and facilitation of ICa-L (Bay K8644). Bay K8644 (10(-8) M) and stimulation of Na-Ca exchange potentiated bradycardia-dependent EADs. Stimulation of Na-Ca exchange in Purkinje fibers pretreated with d,l-sotalol (10(-5) M) and clofilium (10(-7) M) induced EADs at takeoff potentials negative (-63 +/- 4 and -62 +/- 4 mV, respectively) to EADs potentiated by Bay K8644 (10(-8) M) (-33 +/- 2 and -34 +/- 2 mV, respectively, P < 0.05), or EADs induced by Bay K8644 alone (10(-6) M) (-31 +/- 5 mV). In myocytes, Bay K8644 (10(-8) M) potentiated EADs in d,l-sotalol- (10(-6) to 10(-4) M) or clofilium-treated (10(-9) to 10(-7) M) cells at reduced potentials (-10 +/- 3 and -10 +/- 4 mV, respectively) compared to EADs elicited by clofilium or d,l-sotalol alone (-25 +/- 3 and -24 +/- 3 mV, respectively), or stimulation of Na-Ca exchange in the presence of d,l-sotalol or clofilium (-26 +/- 4 and -26 +/- 4 mV, respectively). Spontaneous EADs or EADs elicited by stimulation of Na-Ca exchange coincident with drug treatment were suppressed by increasing Ca02+ but were not suppressed by nifedipine (10(-7) M).

CONCLUSION

EADs elicited by d,l-sotalol and clofilium in canine Purkinje tissue and epicardial myocytes are dependent upon Na-Ca exchange rather than ICa-L "window current."

Role of intracellular sodium overload in the genesis of cardiac arrhythmias

A number of clinical cardiac disorders may be associated with a rise of the intracellular Na concentration (Na(i)) in heart muscle. A clear example is digitalis toxicity, in which excessive inhibition of the Na/K pump causes the Na(i) concentration to become raised above the normal level. Especially in digitalis toxicity, but also in many other situations, the rise of Na(i) may be an important (or contributory) cause of increased cardiac arrhythmias. In this review, we consider the mechanisms by which a raised Na(i) may cause cardiac arrhythmias. First, we describe the factors that regulate Na(i), and we demonstrate that the equilibrium level of Na(i) is determined by a balance between Na entry into the cell, and Na extrusion from the cell. A number of mechanisms are responsible for Na entry into the cell, whereas the Na/K pump appears to be the main mechanism for Na extrusion. We then consider the processes by which an increased level of Nai might contribute to cardiac arrhythmias. A rise of Na(i) is well known to result in an increase of intracellular Ca, via the important and influential Na/Ca exchange mechanism in the cell membrane of cardiac muscle cells. A rise of intracellular Ca modulates the activity of a number of sarcolemmal ion channels and affects release of intracellular Ca from the sarcoplasmic reticulum, all of which might be involved in causing arrhythmia. It is possible that the increase in contractile force that results from the rise of intracellular Ca may initiate or exacerbate arrhythmia, since this will increase wall stress and energy demands in the ventricle, and an increase in wall stress may be arrhythmogenic. In addition, the rise of Na(i) is anticipated to modulate directly a number of ion channels and to affect the regulation of intracellular pH, which also may be involved in causing arrhythmia. We also present experiments in this review, carried out on the working rat heart preparation, which suggest that a rise of Na(i) causes an increase of wall stress-induced arrhythmia in this model. In addition, we have investigated the effect on wall stress-induced arrhythmia of maneuvers that might be anticipated to change intracellular Ca, and this has allowed identification of some of the factors involved in causing arrhythmia in the working rat heart.

Cellular electrophysiologic basis of cardiac arrhythmias

During normal sinus rhythm, the cardiac impulse originates in the sinus node at a rate appropriate to the age and activity of the animal and spreads in an orderly fashion throughout the atria, the atrioventricular (AV) node, the His-Purkinje system, and then throughout the ventricles. An arrhythmia is an abnormality in the rate, regularity, or site of origin of the cardiac impulse or a disturbance in conduction of the impulse so that the normal sequence of activation of atria and ventricles is altered. Cardiac arrhythmias and conduction disturbances occur in every region of the heart and are caused by numerous factors. In particular, some are aligned with certain disease states. In the final analysis, however, all arrhythmias and conduction disturbances--regardless of their pathoelectrophysiologic cause--result from critical alterations, either acute or chronic, in the electrical activity of the cardiac myocyte. This review will provide basic information on how normal cardiac electrophysiology can be changed by disease and how these changes can lead to conduction disturbances and cardiac arrhythmias.

Unitary Cl- channels activated by cytoplasmic Ca2+ in canine ventricular myocytes

Recent whole-cell studies have shown that Ca(2+)-activated Cl- currents contribute to the Ca(2+)-dependent 4-aminopyridine-insensitive component of the transient outward current and to the arrhythmogenic transient inward current in rabbit and canine cardiac cells. These Cl(-)-sensitive currents are activated by Ca2+ release from the sarcoplasmic reticulum and are inhibited by anion transport blockers; however, the unitary single channels responsible have yet to be identified. We used inside-out patches from canine ventricular myocytes and conditions under which the only likely permeant ion is Cl- to identify 4-aminopyridine-resistant unitary Ca(2+)-activated Cl- channels, Ca2+ applied to the cytoplasmic surface of membrane patches activated small-conductance (1.0 to 1.3 pS) channels. These channels were Cl- selective, with rectification properties that could be described by the Goldman-Hodgkin-Katz current equation. Channel activity exhibited time independence when cytoplasmic Ca2+ was held constant and was blocked by the anion transport blockers, DIDS and niflumic acid. Ca2+ (ranging from pCa > or = 6 to pCa 3) applied to the cytoplasmic surface of inside-out patches increased, in a dose-dependent manner, NPo, where N is the number of channels opened and Po is open probability. At negative membrane potentials (-60 to -130 mV), an estimate of the dependence of NPo on cytoplasmic Ca2+ yielded an apparent Kd of 150.2 mumol/L. At pCa 3, an average channel density of approximately equal to 3 microns-2 was estimated. Calculations based on these estimates of cytoplasmic Ca2+ sensitivity and channel current amplitude and density suggest that these small-conductance Cl- channels contribute significant whole-cell membrane current in response to changes in intracellular Ca2+ within the physiological range. We suggest that these small-conductance Ca(2+)-activated Cl- channels underlie the transient Ca(2+)-activated 4-aminopyridine-insensitive current, which contributes to phase-1 repolarization, and under conditions of Ca2+ overload, these channels may generate transient inward currents, contributing to the development of triggered cardiac arrhythmias.

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.

Contribution of Na(+)-Ca2+ exchange to stimulation of transient inward current by isoproterenol in rabbit cardiac Purkinje fibers

Cellular mechanisms underlying beta-adrenergic stimulation of the arrhythmogenic transient inward current (TI) were investigated by using a two-microelectrode voltage-clamp technique in rabbit cardiac Purkinje fibers. TI induced by elevating [Ca2+]o to 30 mmol/L and substituting [Na+]o with N-methyl-D-glucamine (NMG) chloride had a distinct reversal potential (EREV) of -25 mV, suggesting that Na(+)-Ca2+ exchange was not the charge carrier for TI. In the absence of [Na+]o, isoproterenol (ISO, 0.01 to 5.0 mumol/L) had no effect on either inward or outward TI or on the current-voltage relation of TI. However, ISO (0.1 mumol/L) significantly increased both inward and outward TIs without affecting the EREV of TI, if [Na+]o was present. Pretreatment with propranolol (0.2 mumol/L) or atenolol (0.2 mumol/L) abolished the stimulatory effects of ISO. Addition of propranolol (0.2 to 0.5 mumol/L) after the effects of ISO had developed caused only partial reversal of TI stimulation. This indicates persistence of stimulatory effects downstream from the initial agonist-receptor interaction. Forskolin (1 mumol/L), a direct adenylate cyclase activator, also strongly increased both inward and outward TI in the presence of [Na+]o. These effects also were abolished when [Na+]o was substituted by NMG. Inward and outward TIs enhanced by either ISO or forskolin were reversed by two putative Na(+)-Ca2+ exchange blockers, dodecylamine (20 mumol/L) and quinacrine (20 mumol/L). These results suggest that beta-adrenergic stimulation of TI is mediated by the Na(+)-Ca2+ exchange; stimulation likely involves phosphorylation of the exchanger or some factor that modulates exchanger activity.

Two components of [Ca2+]i-activated Cl- current during large [Ca2+]i transients in single rabbit heart Purkinje cells

1. Single Purkinje cells, enzymatically isolated from rabbit ventricle, were studied under whole-cell voltage clamp conditions and internally perfused with the fluorescent Ca2+ indicator fura-2(100 microM). 2. Ca2+ release from the sarcoplasmic reticulum was either induced by external application of caffeine or occurred spontaneously in Ca2+i-overloaded cells. Membrane currents accompanying these Ca(2+)-release signals were studied at steady membrane potentials. 3. [Ca2+]i transients were accompanied by transient membrane currents. In the absence of Na(+)-Ca2+ exchange, two current components could be observed. The first component peaked well before the [Ca2+]i transient (Ifast) and relaxed before peak [Ca2+]i. The second component, on the other hand, peaked at the time when [Ca2+]i was maximal (Islow). 4. In symmetrical Cl- solutions both current components had a reversal potential close to O mV. A reduction of external or internal [Cl-] shifted this reversal potential in accordance with the change of the Cl- equilibrium potential. 5. Each [Ca2+]i transient was accompanied by Ifast. Properties of Ifast suggest that this current component is the [Ca2+]i-dependent Cl- current, ICl(Ca), previously observed during depolarizing pulses. 6. Islow was only detected in cells that displayed a large [Ca2+]i transient with or without elevated resting [Ca2+]i. 7. It is concluded that during large [Ca2+]i transients a slow component of ICl(Ca) can be activated. This second component may arise from the same channel population as the previously described fast component and be related to the presence of spatial and temporal inhomogeneities of [Ca2+]i. Alternatively, this current component may arise from a different Cl- channel population with a different Ca2+ sensitivity.

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)