Istaroxime treatment ameliorates calcium dysregulation in a zebrafish model of phospholamban R14del cardiomyopathy.

The heterozygous Phospholamban p.Arg14del mutation is found in patients with dilated or arrhythmogenic cardiomyopathy. This mutation triggers cardiac contractile dysfunction and arrhythmogenesis by affecting intracellular Ca2+ dynamics. Little is known about the physiological processes preceding induced cardiomyopathy, which is characterized by sub-epicardial accumulation of fibrofatty tissue, and a specific drug treatment is currently lacking. Here, we address these issues using a knock-in Phospholamban p.Arg14del zebrafish model. Hearts from adult zebrafish with this mutation display age-related remodeling with sub-epicardial inflammation and fibrosis. Echocardiography reveals contractile variations before overt structural changes occur, which correlates at the cellular level with action potential duration alternans. These functional alterations are preceded by diminished Ca2+ transient amplitudes in embryonic hearts as well as an increase in diastolic Ca2+ level, slower Ca2+ transient decay and longer Ca2+ transients in cells of adult hearts. We find that istaroxime treatment ameliorates the in vivo Ca2+ dysregulation, rescues the cellular action potential duration alternans, while it improves cardiac relaxation. Thus, we present insight into the pathophysiology of Phospholamban p.Arg14del cardiomyopathy.

Fever-triggered ventricular arrhythmias in Brugada syndrome and type 2 long-QT syndrome.

The risk for lethal ventricular arrhythmias is increased in individuals who carry mutations in genes that encode cardiac ion channels. Loss-of-function mutations in SCN5A, the gene encoding the cardiac sodium channel, are linked to Brugada syndrome (BrS). Arrhythmias in BrS are often preceded by coved-type ST-segment elevation in the right-precordial leads V1 and V2. Loss-of-function mutations in KCNH2, the gene encoding the cardiac ion channel that is responsible for the rapidly activating delayed rectifying potassium current, are linked to long-QT syndrome type 2 (LQT-2). LQT-2 is characterised by delayed cardiac repolarisation and rate-corrected QT interval (QTc) prolongation. Here, we report that the risk for ventricular arrhythmias in BrS and LQT-2 is further increased during fever. Moreover, we demonstrate that fever may aggravate coved-type ST-segment elevation in BrS, and cause QTc lengthening in LQT-2. Finally, we describe molecular mechanisms that may underlie the proarrhythmic effects of fever in BrS and LQT-2. (Neth Heart J 2010;18:165-9.).

The cardiac sodium channel displays differential distribution in the conduction system and transmural heterogeneity in the murine ventricular myocardium.

Cardiac sodium channels are responsible for conduction in the normal and diseased heart. We aimed to investigate regional and transmural distribution of sodium channel expression and function in the myocardium. Sodium channel Scn5a mRNA and Na(v)1.5 protein distribution was investigated in adult and embryonic mouse heart through immunohistochemistry and in situ hybridization. Functional sodium channel availability in subepicardial and subendocardial myocytes was assessed using patch-clamp technique. Adult and embryonic (ED14.5) mouse heart sections showed low expression of Na(v)1.5 in the HCN4-positive sinoatrial and atrioventricular nodes. In contrast, high expression levels of Na(v)1.5 were observed in the HCN4-positive and Cx43-negative AV or His bundle, bundle branches and Purkinje fibers. In both ventricles, a transmural gradient was observed, with a low Na(v)1.5 labeling intensity in the subepicardium as compared to the subendocardium. Similar Scn5a mRNA expression patterns were observed on in situ hybridization of embryonic and adult tissue. Maximal action potential upstroke velocity was significantly lower in subepicardial myocytes (mean +/- SEM 309 +/- 32 V/s; n = 14) compared to subendocardial myocytes (394 +/- 32 V/s; n = 11; P < 0.05), indicating decreased sodium channel availability in subepicardium compared to subendocardium. Scn5a and Na(v)1.5 show heterogeneous distribution patterns within the cardiac conduction system and across the ventricular wall. This differential distribution of the cardiac sodium channel may have profound consequences for conduction disease phenotypes and arrhythmogenesis in the setting of sodium channel disease.

Chronic inhibition of the Na+/H+ - exchanger causes regression of hypertrophy, heart failure, and ionic and electrophysiological remodelling.

BACKGROUND AND PURPOSE: Increased activity of the Na+/H+ -exchanger (NHE-1) in heart failure underlies raised [Na+]i causing disturbances of calcium handling. Inhibition of NHE-1, initiated at the onset of pressure/volume overload, prevents development of hypertrophy, heart failure and remodelling. We hypothesized that chronic inhibition of NHE-1, initiated at a later stage, would induce regression of hypertrophy, heart failure, and ionic and electrophysiological remodelling. EXPERIMENTAL APPROACH: Development of heart failure in rabbits was monitored electrocardiographically and echocardiographically, after one or three months. Cardiac myocytes were also isolated. One group of animals were treated with cariporide (inhibitor of NHE-1) in the diet after one month. Cytoplasmic calcium, sodium and action potentials were measured with fluorescent markers and sarcoplasmic reticulum calcium content by rapid cooling. Calcium after-transients were elicited after rapid pacing. Sodium channel current (INa) was measured using patch-clamp techniques. KEY RESULTS: Hypertrophy and heart failure developed after one month and progressed during the next two months. After one month, dietary treatment with cariporide was initiated. Two months of treatment reduced hypertrophy and heart failure, duration of action potential QT-interval and QRS, and restored sodium and calcium handling and the incidence of calcium after-transients. In cardiac myocytes, parameters of INa were not changed by cariporide. CONCLUSION AND IMPLICATIONS: In rabbit hearts with hypertrophy and signs of heart failure one month after induction of pressure/volume overload, two months of dietary treatment with the NHE-1 inhibitor cariporide caused regression of hypertrophy, heart failure and ionic and electrophysiological remodelling.

Gender disparities in torsade de pointes ventricular tachycardia.

BACKGROUND: Gender disparities in the incidence of torsade de pointes (TdP) ventricular tachycardia exist, but the mechanisms in humans are unresolved. We addressed this issue using a mathematical model of a human ventricular cell. METHODS: We implemented gender differences in the Priebe-Beuckelmann model cell by modifying the amplitudes of the L-type Ca(2+) current (I(Ca,L)), transient outward K(+) current (I(to)), and rapid component of the delayed rectifier K(+)current (I(Kr)), according to experimental data from animal male and female hearts. Gender disparities in electrical heterogeneity between transmural layers (subepicardium, midmyocardium, subendocardium) were implemented by modifying various ion currents according to experimental data. RESULTS: Action potentials in female cells have longer durations and steeper duration versus frequency relationships than male cells. In the female cells, electrical heterogeneity between transmural layers is larger and the susceptibility to early afterdepolarisations is higher than in male cells. CONCLUSION: Gender-related differences in I(Ca,L), I(to), and I(Kr )may explain the gender disparities in human cardiac electrophysiology. Female cells have an increased susceptibility to early afterdepolarisations following mild reductions in net repolarising forces. Combined with their greater electrical heterogeneity, this renders them more vulnerable to TdP. (Neth Heart J 2007;15:405-11.).

Cellular basis of sex disparities in human cardiac electrophysiology.

AIM: Sex disparities in electrocardiogram variables and dysrhythmia susceptibility exist, notably in long QT syndrome (LQTS) and Brugada syndrome, but the underlying mechanisms in man are unknown. We studied the cellular basis of sex distinctions in human cardiac electrophysiology and dysrhythmia susceptibility using mathematical models of human ventricular myocytes. METHODS: We implemented sex differences in the Priebe-Beuckelmann and ten Tusscher-Noble-Noble-Panfilov human ventricular cell models by modifying densities of the L-type Ca(2+) current (I(Ca,L)), transient outward K(+) current (I(to)), and rapid delayed rectifier K(+) current (I(Kr)), according to experimental data from male and female hearts of various species. Sex disparities in transmural repolarization were studied in transmural strands of cells with ion current densities based on canine experimental data. RESULTS: Female cells have longer action potential duration (APD), steeper APD-heart rate relationship, larger transmural APD heterogeneity, and a greater susceptibility to pro-dysrhythmogenic early afterdepolarizations (EADs) than male cells. Conversely, male cells have more prominent phase-1 repolarization and are more susceptible to all-or-none repolarization. CONCLUSION: Sex differences in I(Ca,L), I(to) and I(Kr) densities may explain sex disparities in human cardiac electrophysiology. Female cells exhibit a limited 'repolarization reserve' as demonstrated by their larger susceptibility to EADs, which, combined with their larger transmural electrical heterogeneity, renders them more vulnerable to tachydysrhythmias in LQTS. Conversely, male cells have a limited 'depolarization reserve', as shown by their larger susceptibility to all-or-none repolarization, which facilitates tachydysrhythmias in Brugada syndrome. These general principles may also apply to dysrhythmia susceptibility in common disease.

Novel Brugada syndrome-causing mutation in ion-conducting pore of cardiac Na+ channel does not affect ion selectivity properties.

AIM: Brugada syndrome is an inherited cardiac disease with an increased risk of sudden cardiac death. Thus far Brugada syndrome has been linked only to mutations in SCN5A, the gene encoding the alpha-subunit of cardiac Na+ channel. In this study, a novel SCN5A gene mutation (D1714G) is reported, which has been found in a 57-year-old male patient. Since the mutation is located in a segment of the ion-conducting pore of the cardiac Na+ channel, which putatively determines ion selectivity, it may affect ion selectivity properties. METHODS: HEK-293 cells were transfected with wild-type (WT) or D1714G alpha-subunit and beta-subunit cDNA. Whole-cell configuration of the patch-clamp technique was used to study biophysical properties at room temperature (21 degrees C) and physiological temperature (36 degrees C). This study represents the first measurements of human Na+ channel kinetics at 36 degrees C. Ion selectivity, current density, and gating properties of WT and D1714G channel were studied. RESULTS: D1714G channel yielded nearly 80% reduction of Na+ current density at 21 and 36 degrees C. At both temperatures, no significant changes were observed in V(1/2) values and slope factors for voltage-dependent activation and inactivation. At 36 degrees C, but not at 21 degrees C, D1714G channel exhibited more slow inactivation compared with WT channel. Ion selectivity properties were not affected by the mutation at both temperatures, as assessed by either current or permeability ratio. CONCLUSION: This study shows no changes in ion selectivity properties of D1714G channel. However, the profoundly decreased current density associated with the D1714G mutation may explain the Brugada syndrome phenotype in our patient.

Identification of swelling-activated Cl(-) current in rabbit cardiac Purkinje cells.

The presence and functional role of the swelling-activated Cl(-) current (I(Cl(swell))) in rabbit cardiac Purkinje cells was examined using patch-clamp methodology. Extracellular hypotonicity (210 or 135 mOsm) activated an outwardly rectifying, time-independent current with a reversal potential close to the calculated Cl(-) equilibrium potential (E(Cl)). The magnitude of this current was related to tonicity of the superfusate. The current was blocked by 0.5 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). These features are comparable to those of I(Cl(swell)) found in sinoatrial nodal, atrial, and ventricular myocytes. I(Cl(swell)) activation at 210 and 135 mOsm depolarized the resting membrane potential with 6 and 10 mV and shortened the action potential by approximately 18 and approximately 33%, respectively. DIDS partially reversed I(Cl(swell))-induced action potential changes. We conclude that I(Cl(swell)) is present in Purkinje cells and its activation leads to action potential shortening and resting membrane potential depolarization, both of which can promote the development of reentrant arrhythmias.

Ca2+-activated Cl- current reduces transmural electrical heterogeneity within the rabbit left ventricle.

OBJECTIVE: Various cationic membrane channels contribute to the heterogeneity of action potential configuration between the transmural layers of the left ventricle. The role of anionic membrane channels is less intensively studied. We investigated the role of the Ca2+-activated Cl- current, ICl(Ca), in transmural electrical heterogeneity. METHODS AND RESULTS: We determined the density of ICl(Ca) and its physiological role in subepicardial and subendocardial ventricular myocytes of rabbit using the patch-clamp technique. ICl(Ca) was measured as the 4,4'diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) sensitive current. The current-voltage relationships and the densities of ICl(Ca) were similar in subepicardial and subendocardial myocytes. However, the functional role of ICl(Ca) exhibited striking differences. In subendocardial myocytes, blockade of ICl(Ca) by DIDS increased action potential duration (APD) significantly at all measured stimulus frequencies (3.33-0.2 Hz). In subepicardial myocytes, ICl(Ca) blockade increased APD only at 3.33 Hz, but not at the lower stimulus frequencies. At 1 Hz, ICl(Ca) blockade in subepicardial myocytes only caused an APD increase when the transient outward K+ current, Ito1, was blocked. CONCLUSIONS: The densities and gating properties of ICl(Ca) are similar in subepicardial and subendocardial myocytes. ICl(Ca) contributes to APD shortening in subendocardial, but not in subepicardial myocytes except at 3.33 Hz. These differences in functional expression of ICl(Ca) reduce the electrical heterogeneity in rabbit left ventricle.

Role of Ca2+-activated Cl- current during proarrhythmic early afterdepolarizations in sheep and human ventricular myocytes.

OBJECTIVES: The proarrhythmic early afterdepolarizations (EADs) during phase-2 of the cardiac action potential (phase-2 EADs) are associated with secondary Ca2+-release of the sarcoplasmic reticulum. This makes it probable that the Ca2+-activated Cl- current [ICl(Ca)] is present during phase-2 EADs. Activation of ICl(Ca) during phase-2 of the action potential will result in an outwardly directed, repolarizing current and may thus be expected to prevent excessive depolarization of phase-2 EADs. The present study was designed to test this hypothesis. METHODS AND RESULTS: The contribution of ICl(Ca) during phase-2 EADs was studied in enzymatically isolated sheep and human ventricular myocytes using the patch-clamp methodology. EADs were induced by a combination of a low stimulus frequency (0.5 Hz) and exposure to 1 microm noradrenaline. In sheep myocytes, the ICl(Ca) blocker 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS, 0.5 mm) abolished phase-1 repolarization of the action potential in all myocytes tested. This indicates that ICl(Ca) is present in all sheep myocytes. However, DIDS had no effect on phase-2 EAD characteristics. In human myocytes, DIDS neither affected phase-1 repolarization nor phase-2 EAD characteristics. CONCLUSION: In sheep ventricular myocytes, but not in human ventricular myocytes, ICl(Ca) contributes to phase-1 repolarization of the action potential. In both sheep and human myocytes, ICl(Ca) plays a limited role during phase-2 EADs.

Limited role of Ca2+-activated Cl- current in early afterdepolarisations.

OBJECTIVES: The proarrhythmic, early afterdepolarisations during phase two of the action potential (phase-2 EADs) are associated with secondary Ca2+ release from the sarcoplasmic reticulum. This makes it probable that the Ca2+-activated Cl- current (ICl(Ca)) may contribute to phase-2 EADs. Activation of ICl(Ca) during phase two of the action potential will result in a repolarising current and may thus be expected to prevent excessive depolarisation of phase-2 EADs. The present study was designed to test this hypothesis. METHODS: The contribution of ICl(Ca) during phase-2 EADs was studied in enzymatically isolated sheep ventricular myocytes using the patch-clamp methodology. EADs were induced at a stimulus frequency of 0.5 Hz by exposure of the myocytes to 1 µM noradrenaline. RESULTS: The ICl(Ca) blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS, 0.5 mM) abolished phase-1 repolarisation of the action potential in all myocytes tested. This indicates that ICl(Ca) is present in all myocytes. However, DIDS had no effect on phase-2 EAD characteristics. CONCLUSION: In sheep ventricular myocytes, ICl(Ca) contributes to phase-1 repolarisation of the action potential, but plays a limited role in phase-2 EADs.

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.

Norepinephrine induces action potential prolongation and early afterdepolarizations in ventricular myocytes isolated from human end-stage failing hearts.

AIMS: Congestive heart failure is characterized by high levels of norepinephrine which is considered to be arrhythmogenic. It is unclear whether increased norepinephrine is only a marker of the severity of heart failure or whether it directly triggers ventricular arrhythmias. METHODS AND RESULTS: Ventricular myocytes were isolated from eight explanted hearts of patients with end-stage heart failure (ischaemic or dilated cardiomyopathy). With the whole-cell configuration of the patch-clamp technique the effect of 1 micromol x l(-1)norepinephrine on action potentials and membrane currents was studied. The cells had a membrane capacitance of 256 +/- 25 pF (n = 26) and action potential duration (APD90) during control conditions was 620 +/- 45 ms at 1 Hz (n = 14). Norepinephrine induced action potential prolongation in all cells and early afterdepolarizations in 50% of them. Norepinephrine significantly increased the calcium current but had no effect on the delayed rectifier current, the inward rectifier current or the transient outward current. Norepinephrine also significantly increased the steady-state calcium window-current measured between -40 and 0 mV. CONCLUSIONS: In contrast to many animal species, norepinephrine induces action potential prolongation in ventricular myocytes from human failing hearts, as well as early afterdepolarization, by an increase in both the calcium peak current and window current. Thus norepinephrine seems to be an important arrhythmogenic factor in congestive heart failure.

Role of Ca(2+)-activated Cl(-) current in ventricular action potentials of sheep during adrenoceptor stimulation.

Adrenoceptor stimulation enhances repolarising and depolarising membrane currents to different extents in cardiac myocytes. We investigated the opposing effects of the repolarising Ca(2+)-activated Cl(-) current (I(Cl(Ca))) and depolarising L-type Ca(2+) current (I(Ca,L)) on the action potential configuration of sheep ventricular myocytes stimulated with noradrenaline. Whole-cell current-clamp recordings revealed that noradrenaline accelerated and prolonged phase-1 repolarisation. We define the minimal potential at the end of phase-1 repolarisation as "notch level". Noradrenaline (1 microM) caused the notch level to fall from 14 +/- 2.6 to 7.8 +/- 2.8 mV (n = 24), but left action potential duration, resting membrane potential or action potential amplitude unaffected. Whole-cell voltage-clamp recordings showed that 1 microM noradrenaline increased both I(Ca,L) and I(Cl(Ca)), but it had no significant effect on the principal K(+) currents. Blockage of I(Cl(Ca)) by 0.5 mM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) in both the absence and the presence of noradrenaline abolished phase-1 repolarisation. In the presence of noradrenaline, DIDS caused elevation of the plateau phase amplitude and an increase in the action potential duration. In conclusion, elevation of the plateau phase amplitude and action potential prolongation associated with an increased I(Ca,L) upon adrenoceptor stimulation is prevented by an increased I(Cl(Ca)) in sheep ventricular myocytes. Experimental Physiology (2001) 86.2, 151-159.

Effects of cell-to-cell uncoupling and catecholamines on Purkinje and ventricular action potentials: implications for phase-1b arrhythmias.

OBJECTIVE: The delayed phase of ventricular arrhythmias during acute ischemia (phase-1b arrhythmia) is associated with depletion of catecholamines and cell-to-cell uncoupling between depressed depolarized intramural ischemic region and surviving cells in subepicardium and subendocardium. In the present study we determined the effects of uncoupling and catecholamines on development of proarrhythmic afterdepolarizations. METHODS: Depressed depolarized ischemic region was simulated by a passive electronic circuit with a potential of -73, -53, -33 or -13 mV. Using patch-clamp methodology, single sheep Purkinje and ventricular cells were coupled to the simulated ischemic region via a variable conductance. By varying coupling conductance, we were able to selectively study the effects of various degrees of uncoupling. RESULTS: At strong coupling, cells were inexcitable and depolarized to potentials near those of the simulated ischemic region. Excitability, action potential duration and resting potential increased with progressive uncoupling. In a critical range of uncoupling, ventricular and "high-plateau" Purkinje cells developed early afterdepolarizations when the potential of the simulated ischemic region was -13 mV. Norepinephrine (1 microM) frequently induced early and delayed afterdepolarizations in both ventricular and Purkinje cells, but these afterdepolarizations were only present during uncoupling when the potential of the simulated ischemic region was -33 mV or more positive. CONCLUSIONS: In a critical range of uncoupling, afterdepolarizations were present when the potential of the simulated ischemic region was -33 or -13 mV, suggesting that triggered activity plays a role in phase-1b arrhythmias when surviving layers uncouple from a highly depolarized intramural ischemic region.

Cl- current blockade reduces triggered activity based on delayed afterdepolarisations.

OBJECTIVES: Increasing evidence suggests that a Ca2+-activated Cl- current (ICl(Ca)) contributes to the transient inward current (Iti), the current responsible for proarrhythmic delayed after-depolarisations (DADs). Because the equilibrium potential for Cl- ions (ECl) in myocytes is around - 50 mV, activation of the ICl(Ca) results in an inward depolarising current at resting membrane potential and ICl(Ca) may thus be responsible for a part of the depolarisation during a DAD. In this study, we investigated the ionic nature of Iti and the effects of Cl- current blockade on DADs. METHODS AND RESULTS: The ionic mechanisms of Iti 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 Iti were elicited by repetitive depolarisations from -93 mV to +37 mV in the presence of the drug. The current-voltage relation of Iti reversed in sign around -20 mV. The outward Iti was completely blocked by the anion current blocker 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), whereas the inward Iti was only slightly affected. The DIDS-sensitive component of Iti was outwardly rectifying with a reversal potential close to ECl. The DIDS-insensitive component of Iti 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, Iti 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.

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.

Injury current modulates afterdepolarizations in single human ventricular cells.

OBJECTIVE: Injury current (I(injury)) and afterdepolarizations are thought to play an important role in arrhythmias that occur during acute ischemia. However, little is known about the effects of I(injury) on afterdepolarizations. The present study was designed to study the effect of I(injury) on afterdepolarizations and action potentials in single human ventricular cells. METHODS: The patch-clamp technique was used to record action potentials and to apply I(injury) to human ventricular cells. In these cells, early and delayed afterdepolarizations (EADs and DADs) were induced by 1 microM norepinephrine. I(injury) was simulated by coupling cells via a variable coupling resistance to a passive resistance circuit with a potential of 0, -20, or -40 mV, mimicking a depolarized ischemic region. RESULTS: At all potentials, I(injury) induced depolarization of the resting membrane potential and action potential shortening. Flowing from 0 mV, I(injury) induced EADs by itself and aggravated the EADs and DADs that were induced by norepinephrine. Flowing from -40 mV, I(injury) abolished the noradrenaline-induced EADs and DADs. CONCLUSIONS: Our results demonstrate that I(injury) may either prevent or promote the occurrence of afterdepolarizations in human ventricle. The latter holds if conduction is slowed to such an extent that it permits flow of current from depolarized ischemic cells at plateau level to cells in phase 3 or phase 4.