Heterogeneity within the ventricular wall. Electrophysiology and pharmacology of epicardial, endocardial, and M cells

Three-dimensional electromechanical mapping: imaging in the operating room of the future

BACKGROUND

Three-dimensional electromechanical mapping has previously been shown to be a clinically important tool for cardiac imaging and intervention. We hypothesized that this technology may be beneficial as an intraoperative modality for assessing cardiac hemodynamics and viability during cardiac surgery. We report here the use of this technology as an imaging modality for intraoperative cardiac surgery.

METHODS

The tip of a locatable catheter connected to an endocardial mapping and navigating system is accurately localized while simultaneously recording local electrical and mechanical functions. Thus the three-dimensional geometry of the beating cardiac chamber is reconstructed in real time. The system was tested on 6 goats that underwent acute dynamic cardiomyoplasty and on 5 dogs that underwent left anterior descending (LAD) coronary artery ligation.

RESULTS

The electromechanical mapping system provided an accurate three-dimensional reconstruction of the beating left ventricle during cardiomyoplasty. After the wrapping procedure, significant end-diastolic area reduction was noted in the base and mid parts of the heart (948 +/- 194 mm2 vs 1245 +/- 33 mm2, p = 0.021; and 779 +/- 200 mm2 vs 1011 +/- 80 mm2, p = 0.016). The area of the cross-section of the apex did not change during the operation. Acute infarcted tissue was characterized 3 days after LAD ligation by concomitant deterioration in both electrical and mechanical function.

CONCLUSIONS

By providing both a clear view of the anatomical changes that occur during cardiac surgery, and an accurate assessment of tissue viability, electroanatomic mapping may serve as an important adjunct tool for imaging and analysis of the heart during cardiac surgery

Regulation of KChIP2 potassium channel beta subunit gene expression underlies the gradient of transient outward current in canine and human ventricle

Expression of four members of the KChIP family of potassium channel beta subunits was examined in canine heart. Only one member of the gene family, KChIP2, was expressed in heart. There was a steep gradient of KChIP2 mRNA expression across the canine ventricular free wall. KChIP2 mRNA was 25-fold more abundant in the epicardium than in the endocardium, and this gradient paralleled the gradient in transient outward current (Ito) expression. In contrast, Kv4.3 potassium channel alpha subunit mRNA was expressed at equal levels across the ventricular wall. There was no difference in the pharmacological sensitivity of epicardial and endocardial Ito channels to flecainide, suggesting that the current is produced by the same channel in the two tissues. A similar gradient of KChIP2 expression was found across the ventricular wall of human heart, but not rat heart. It is concluded that transcriptional regulation of the KChIP2 beta subunit gene, rather than the Kv4.3 [alpha] subunit gene, is the primary determinant regulating the transmural gradient of Ito expression in the ventricular free wall of canine and human heart.

Computational models of normal and abnormal action potential propagation in cardiac tissue: linking experimental and clinical cardiology

Computational models have the potential to make a huge impact on our understanding of normal and abnormal cardiac function. The aim of this article is to review tools that have been developed to simulate the electrophysiology of cardiac cells and tissue, and to show how computational models have been used to gain insight into normal and abnormal action potential propagation. Some of the practical problems experienced in the development and application of these models are described, and examples are given.

Larger late sodium conductance in M cells contributes to electrical heterogeneity in canine ventricle

Action potentials and whole cell sodium current were recorded in canine epicardial, midmyocardial, and endocardial myocytes in normal sodium at 37 degrees C. Tetrodotoxin (TTX) reduced the action potential duration of midmyocardial cells to a greater degree than either epicardial or endocardial cells. Whole cell recordings in potassium-free and very-low-chloride solutions revealed a slowly decaying current that was completely inhibited by 5 microM TTX or replacement of external and internal sodium with the impermeant cation N-methyl-D-glucamine. Late sodium current density at 0 mV was 47% greater in midmyocardial cells and averaged -0.532 +/- 0.058 pA/pF in endocardial, -0.463 +/- 0.068 pA/pF in epicardial, and -0.785 +/- 0.070 pA/pF in midmyocardial cells. Neither the frequency dependence of late sodium current nor its recovery from inactivation exhibited transmural differences. After a 4.5-s pulse to -30 mV, late sodium current recovered with a single time constant of 140 ms. We conclude that a larger late sodium conductance in midmyocardial cells will favor longer action potentials in these cells. More importantly, drugs that slow inactivation of sodium channels will produce a nonuniform response across the ventricular wall that is proarrhythmic.

The molecular physiology of the cardiac transient outward potassium current (I(to)) in normal and diseased myocardium

G. Y. Oudit, Z. Kassiri, R. Sah, R. J. Ramirez, C. Zobel and P. H. Backx. The Molecular Physiology of the Cardiac Transient Outward Potassium Current (I(to)) in Normal and Diseased Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 851-872. The Ca(2+)-independent transient outward potassium current (I(to)) plays an important role in early repolarization of the cardiac action potential. I(to)has been clearly demonstrated in myocytes from different cardiac regions and species. Two kinetic variants of cardiac I(to)have been identified: fast I(to), called I(to,f), and slow I(to), called I(to,s). Recent findings suggest that I(to,f)is formed by assembly of K(v4.2)and/or K(v4.3)alpha pore-forming voltage-gated subunits while I(to,s)is comprised of K(v1.4)and possibly K(v1.7)subunits. In addition, several regulatory subunits and pathways modulating the level and biophysical properties of cardiac I(to)have been identified. Experimental findings and data from computer modeling of cardiac action potentials have conclusively established an important physiological role of I(to)in rodents, with its role in large mammals being less well defined due to complex interplay between a multitude of cardiac ionic currents. A central and consistent electrophysiological change in cardiac disease is the reduction in I(to)density with a loss of heterogeneity of I(to)expression and associated action potential prolongation. Alterations of I(to)in rodent cardiac disease have been linked to repolarization abnormalities and alterations in intracellular Ca(2+)homeostasis, while in larger mammals the link with functional changes is far less certain. We review the current literature on the molecular basis for cardiac I(to)and the functional consequences of changes in I(to)that occur in cardiovascular disease.

Coronary vasomotor and cardiac electrophysiologic effects of diadenosine polyphosphates and nonhydrolyzable analogs in the guinea pig

Platelet activation in heart disease is important owing to the effects of platelet-derived compounds on myocardial perfusion and cardiac electrophysiology. Diadenosine polyphosphates are secreted from platelets and present in the myocardium, but their electrophysiologic and vasomotor effects are incompletely understood. We used isolated guinea-pig hearts to study the effects of diadenosine triphosphate (Ap3A), tetraphosphate (Ap4A), pentaphosphate (Ap5A), and hexaphosphate (Ap6A) (10 pM-0.1 mM), comparing their actions to those of adenosine, adenosine triphosphate, and non-hydrolyzable Ap4A and Ap5A analogs. Diadenosine polyphosphates (0.1 nM-0.1 microM) transiently reduced coronary perfusion pressure, which recovered during the continued presence of the compounds. At concentrations greater than 0.1 microM effects were maximal and sustained (perfusion pressure decreased from 36.5+/-3.4 to 18.6+/-2.5 mm Hg, p < 0.001, with 1 microM Ap4A). The changes in action potential duration and refractory period developed slowly but were maintained (0.1 nM-1 microM). With 1 nM Ap4A, action potential duration increased from 170.6+/-2.6 to 187.3+/-3.8 ms, p < 0.05, and refractory period increased from 138.5+/-1.6 to 147.9+/-2.0 ms, p < 0.05. Ap4A and its analog reduced QRS duration (from 24.7+/-1.1 to 13.9+/-1.6 ms with 1 microM Ap4A, p < 0.05). P2-purinergic (adenosine triphosphate) receptor antagonism (suramin) reduced perfusion pressure but was without electrophysiologic effect. Other changes in coronary perfusion pressure and electrophysiologic variables associated with Ap4A were not seen in the presence of suramin. P1-(adenosine) antagonism (8-[p-sulfophenyl]theophylline) attenuated the electrophysiologic effects only. Diadenosine polyphosphates have potent cardiac electrophysiologic and coronary vasomotor effects via purinergic receptors, suggesting an important role during platelet activation in acute coronary syndromes.

Late sodium current is a novel target for amiodarone: studies in failing human myocardium

V. A. Maltsev, H. N. Sabbah and A. I. Undrovinas. Late Sodium Current is a Novel Target for Amiodarone: Studies in Failing Human Myocardium. Journal of Molecular and Cellular Cardiology (2001) 33, 923-932. The authors recently reported the existence of a novel late Na(+)current (I(NaL)) in ventricular cardiomyocytes (VC) isolated from both normal and failing human hearts. Both in failing human and canine VC, partial block of I(NaL)normalized action potential (AP) duration and abolished early after depolarizations (EADs). The most recent computer simulation studies indicate a significant contribution of the persistent Na(+)current into the ion current balance on the plateau of VC AP as well as its important role in the dispersion of AP duration across the ventricular wall. The data thus indicate a possibility for I(NaL)to be a new therapeutic target. The present study tested a hypothesis that I(naL)could be a novel target for amiodarone (AMIO). Midmyocardial VC isolated from left ventricle of explanted failing human hearts were measured by a whole-cell clamp. I(NaL)was effectively blocked by AMIO in therapeutic concentrations, with IC(50)being 6.7+/-1.1 microM (mean+/-S.E.M., n=16 cells). At the same time, AMIO (5 microM ) produced almost no effect on the transient Na(+)current (IC(50)=87+/-28 microM, n=8). AMIO significantly shifted the steady-state inactivation (SSI) curve of I(NaL)towards more negative potentials and accelerated decay time course in a dose-dependent manner. At 5 microM, AMIO shifted SSI by 21+/-3 mV (n=7) and decreased the decay time constant from 0.67+/-0.05 s to 0.37+/-0.04 s (n=5, P<0.004). Evaluation of AMIO binding to different Na(+)channel (NaCh) states by means of mathematical models describing dose-dependent SSI shift and decay acceleration was consistent with an action that AMIO blocks NaCh preferentially in inactivated and activated states rather than in resting state. The authors conclude that the late Na(+)current is effectively blocked by AMIO and represents a new target for the drug in patients with chronic heart failure (HF).

The Brugada syndrome: clinical, genetic, cellular, and molecular abnormalities

The Brugada syndrome is an arrhythmic syndrome characterized by a right bundle branch block pattern and ST segment elevation in the right precordial leads of the electrocardiogram in conjunction with a high incidence of sudden death secondary to ventricular tachyarrhythmias. No evidence of structural heart disease is noted during diagnostic evaluation of these patients. In 25% of families, there appears to be an autosomal dominant mode of transmission with variable expression of the abnormal gene. Mutations have been identified in the gene that encodes the alpha subunit of the sodium channel (SCN5A) on chromosome 3. This genetic defect causes a reduction in the density of the sodium current and explains the worsening of the above electrocardiographic abnormalities when patients are treated with sodium channel blocking antiarrhythmic agents, which further diminish the already reduced sodium current. The prognosis is poor with up to a 10% per year mortality. Antiarrhythmic drugs including beta-blockers and amiodarone have no benefit in prolonging survival. The treatment of choice is the insertion of an implantable cardioverter-defibrillator.

Extrasystolic beats affect transmural electrical dispersion during programmed electrical stimulation

BACKGROUND

Experimental studies suggest that the electrocardiographic Tpeak-Tend (TpTe) interval reflects transmural dispersion of repolarization (TDR). The genesis and role of the TpTe interval in a clinical setting have not been established. This study aimed to assess the clinical usefulness of the TpTe interval as an index of TDR and a pro-arrhythmic marker.

MATERIALS AND METHODS

Endocardial monophasic action potential (MAP) duration and electrocardiographic QTp, QTe and TpTe intervals were assessed in 13 patients undergoing an electrophysiological study. Surface electrocardiograms were recorded during right ventricular pacing (Basic Cycle Length = 600 ms) before and after single extrastimuli.

RESULTS

Ventricular arrhythmia was induced in six patients. During ventricular pacing, MAP duration and QTp intervals shortened in response to extrastimuli applied at progressively shorter coupling intervals. In contrast, QTe intervals increased in response to premature stimulation and QTe dispersion increased at short coupling intervals. During sinus rhythm, the TpTe interval was greater in the inducible group in leads V3-V4. Premature stimulation increased the duration of TpTe intervals, suggesting an increase in TDR. The maximum TpTe interval was greater in the inducible than in the noninducible group, both during baseline ventricular drive pacing (163 +/- 22 vs. 130 +/- 27 ms, respectively, P < 0.03) and after application of shortly coupled extrastimuli (263 +/- 66 vs. 200 +/- 47 ms, respectively, P < 0.05).

CONCLUSIONS

The TpTe interval of surface ECG is likely to represent TDR. TDR is increased by premature ventricular stimulation and the magnitude of the maximum TpTe interval (i.e. maximum TDR) during ventricular pacing is greater in patients with inducible arrhythmias.

Effects of simulated ischemia on spiral wave stability

Regional hyperkalemia during acute myocardial ischemia is a major factor promoting electrophysiological abnormalities leading to ventricular fibrillation (VF). However, steep action potential duration restitution, recently proposed to be a major determinant of VF, is typically decreased rather than increased by hyperkalemia and acute ischemia. To investigate this apparent contradiction, we simulated the effects of regional hyperkalemia and other ischemic components (anoxia and acidosis) on the stability of spiral wave reentry in simulated two-dimensional cardiac tissue by use of the Luo-Rudy ventricular action potential model. We found that the hyperkalemic "ischemic" area promotes wavebreak in the surrounding normal tissue by accelerating the rate of spiral wave reentry, even after the depolarized ischemic area itself has become unexcitable. Furthermore, wavebreak and fibrillation can be prevented if the dynamical instability of the normal tissue is reduced significantly by targeting electrical restitution properties, suggesting a novel therapeutic approach.

Coexistence of multiple spiral waves with independent frequencies in a heterogeneous excitable medium

We studied the interactions and coexistence of stable spiral waves with independent frequencies in a heterogeneous excitable medium, using numerical simulations of a spatial system based on the FitzHugh-Nagumo cell model. When the heterogeneity of the medium exceeded a critical value, a transition took place from a single dominant spiral wave to a coexistence of multiple spiral waves with independent frequencies and n:n-1 wave conduction blocks. In this case, multiple spiral waves could coexist because they are "insulated" from each other by chaotic regions.

hKv4.3 channel characterization and regulation by calcium channel antagonists

Relative expression pattern of short and long isoforms of hKv4.3 channels was evaluated by RT-PCR and RPA. Electrophysiological studies were performed in HEK293 cells transfected with short or long hKv4.3 cDNA. The long variant L-hKv4.3 was the only form present in lung, pancreas, and small intestine. The short variant S-hKv4.3 was predominant in brain whereas expression levels of the two isoforms were similar in cardiac and skeletal muscles. Properties of the ionic channels encoded by L-hKv4.3 and S-hKv4.3 cDNAs were essentially similar. Cadmium chloride and verapamil inhibited hKv4.3 current (with EC50s of 0.110 +/- 0.004 mM and 492.9 +/- 15.1 microM, respectively). Verapamil also accelerated current inactivation. Another calcium channel antagonist nicardipine was found inactive. In conclusion, this study confirms that both isoforms underlie the transient outward potassium current. Moreover, calcium channel inhibitors markedly affect hKv4.3 current, an effect which must be considered when evaluating transient outward potassium channel properties in native tissues.

Electrophysiological heterogeneity and stability of reentry in simulated cardiac tissue

Generation of wave break is a characteristic feature of cardiac fibrillation. In this study, we investigated how dynamic factors and fixed electrophysiological heterogeneity interact to promote wave break in simulated two-dimensional cardiac tissue, by using the Luo-Rudy (LR1) ventricular action potential model. The degree of dynamic instability of the action potential model was controlled by varying the maximal amplitude of the slow inward Ca(2+) current to produce spiral waves in homogeneous tissue that were either nearly stable, meandering, hypermeandering, or in breakup regimes. Fixed electrophysiological heterogeneity was modeled by randomly varying action potential duration over different spatial scales to create dispersion of refractoriness. We found that the degree of dispersion of refractoriness required to induce wave break decreased markedly as dynamic instability of the cardiac model increased. These findings suggest that reducing the dynamic instability of cardiac cells by interventions, such as decreasing the steepness of action potential duration restitution, may still have merit as an antifibrillatory strategy.

Basic mechanisms of reentrant arrhythmias

The mechanisms responsible for active cardiac arrhythmias are generally divided into two major categories: (1) enhanced or abnormal impulse formation and (2) reentry. Reentry can be subdivided into three subcategories: (1) circus movement, (2) reflection, and (3) Phase 2 reentry. Reentry occurs when a propagating impulse fails to die out after normal activation of the heart and persists to re-excite the heart after expiration of the refractory period. Evidence implicating reentry as a mechanism of cardiac arrhythmias stems back to the turn of the century. Amplification of intrinsic electrical heterogeneities provides the substrate responsible for developing Phase 2 and circus movement reentry, which underlie ventricular tachycardia in the long QT and Brugada syndromes.

Simultaneous maps of optical action potentials and calcium transients in guinea-pig hearts: mechanisms underlying concordant alternans

1. The mechanisms underlying electro-mechanical alternans caused by faster heart rates were investigated in perfused guinea-pig hearts stained with RH237 and Rhod-2 AM to simultaneously map optical action potentials (APs) and intracellular free Ca2+ (Ca2+i). 2. Fluorescence images of the heart were focused on two 16 x 16 photodiode arrays to map Ca2+i (emission wavelength (lamdda;em) = 585 +/- 20 nm) and APs (lamdda;em > 715 nm) from 252 sites. Spatial resolution was 0.8 mm x 0.8 mm per diode and temporal resolution 4000 frames s-1. 3. The mean time-to-peak for APs and [Ca2+]i was spatially homogeneous (8.8 +/- 0.5 and 25.6 +/- 5.0 ms, respectively; n = 6). The durations of APs (APDs) and Ca2+i transients were shorter at the apex and progressively longer towards the base, indicating a gradient of ventricular relaxation. 4. Restitution kinetics revealed increasingly longer delays between AP and Ca2+i upstrokes (9.5 +/- 0.4 to 11.3 +/- 0.4 ms) with increasingly shorter S1-S2 intervals, particularly at the base, despite nearly normal peak [Ca2+]i. 5. Alternans of APs and Ca2+i transients were induced by a decrease++ in cycle length (CL), if the shorter CL captured at the pacing site and was shorter than refractory periods (RPs) near the base, creating heterogeneities of conduction velocity. 6. Rate-induced alternans in normoxic hearts were concordant (long APD with large [Ca2+]i) across the epicardium, with a magnitude (difference between odd-even signals) that varied with the local RP. Alternans were initiated by gradients of RP, producing alternans of conduction that terminated spontaneously without progressing to fibrillation.

Early repolarization syndrome: clinical characteristics and possible cellular and ionic mechanisms

Early repolarization syndrome (ERS) has traditionally been regarded as benign. In the electrocardiogram (ECG), it is characterized by a diffuse upward ST-segment concavity ending in a positive T wave in leads V2-V4 (5). Clinical interest in this ECG phenomenon has recently been rekindled because of similarities with the electrocardiographic manifestations of the highly arrhythmogenic Brugada syndrome and the potential for misdiagnosis. This article addresses the clinical characteristics and cellular and ionic basis for ERS. In experimental models, the ECG signature of ERS can be converted to that of the Brugada syndrome, raising the possibility that ERS may not be as benign as generally thought, and that under certain conditions known to predispose to ST-segment elevation, patients with ERS may be at greater risk. Further clinical and experimental data are clearly required to test these hypotheses, and the characteristics of ERS need to be more fully delineated within the framework of what has been learned about the Brugada syndrome in recent years.

An altered repolarizing potassium current in rat cardiac myocytes after subtotal nephrectomy

Renal failure in humans is associated with electrocardiographic changes including altered QT interval dispersion, which suggests that cardiac myocyte repolarization is abnormal and which appears to correlate with cardiac prognosis. In this study, cardiac myocyte repolarizing currents have been studied in isolated cells from rats 8 wk after subtotal nephrectomy (SNx), using sham-operated animals as controls. In addition, monophasic cardiac action potentials were recorded from the epicardial surface of the left ventricle (LV) apex, LV base, and the right ventricle of isolated perfused hearts paced at 320/min. SNx was associated with cardiac hypertrophy and histologic evidence of myocardial fibrosis, but SNx rats were not hypertensive. Repolarizing K(+) currents were measured using whole-cell patch-clamp, and 4-aminopyridine (4-AP)-sensitive transient outward (I(to)) and 4-AP-insensitive sustained outward (I(so)) components were quantified. After SNx, I(to) was increased by two to threefold at voltages from -30 to +60 mV and showed increased heterogeneity. For example, at 0 mV voltage clamp pulse, the median I(to) was increased from 3.23 pA/pF in control myocytes (interquartile range 3.20 pA/pF, n = 24) to 5.86 pA/pF in SNx myocytes (interquartile range 7.32 pA/pF, n = 21, P: < 0.005). The kinetics of inactivation of I(to) were altered after SNx with slowing both of the onset and the recovery from inactivation. The mean time constant of inactivation at +30 mV after SNx was 14.2 +/- 1.6 ms (n = 20) compared with control values of 9.8 +/- 0.6 ms (n = 23, P: < 0.05). Neither I(so) nor inward rectifier K(+) currents were altered after SNx. The action potential duration (APD(50)) at the left ventricular base was approximately 20% shorter (P: < 0.02) in hearts from SNx rats compared with controls. 4-AP (2 mM) prolonged the APD(50) in all regions in hearts from both SNx and control rats and abolished the APD(50) shortening in SNx. These results indicate that abnormalities of the cardiac transient outward K(+) current contribute to alterations in the cardiac action potential in renal failure and warrant further investigation because they may contribute to altered repolarization and arrythmogenesis.

Effects of halothane on the transient outward K(+) current in rat ventricular myocytes

1. Halothane has been shown to affect several membrane currents in cardiac tissue including the L-type calcium current (I(Ca)), sodium current and a variety of potassium currents. However, little is known about the effects of halothane on the transient outward K(+) current (I(to)). 2. Single ventricular myocytes from rat hearts were voltage clamped using the whole cell patch configuration and an EGTA-containing pipette solution to record the Ca(2+)-independent, 4-aminopyridine sensitive component of I(to). 300 microM Cd(2+) or 10 microM nifedipine was used to block I(Ca). 3. At +80 mV, I(to) (peak current minus current at the end of the pulse) was 1.8+/-0.2 nA under control conditions which was reduced to 1.3+/-0.2 nA by 1 mM halothane (P:<0.001, mean+/-s.e.mean, n=9). The inhibition of I(to) by halothane was concentration-dependent (K(0.5), 1.1+/-0.2 mM). 4. One mM halothane led to a 16 mV shift in the steady-state inactivation curve towards negative membrane potentials (P:=0.005, n=8) but had no significant effect on the activation-voltage relationship (P:=0. 724). One mM halothane also increased the rate of inactivation of I(to); the dominant time constant of inactivation was reduced from 14+/-1 to 9+/-1 ms (P:=0.017, mean+/-s.e.mean, n=6). 5. These data show that halothane reduced I(to); 0.3 mM, close to the MAC(50) value for halothane, inhibited the current by 15% and as such, the inhibition of I(to) will be relevant to the clinical situation. Halothane induced a shift in the steady-state inactivation curve and accelerated the inactivation process of I(to) which could be responsible for its inhibitory effect. 6. Due to the differential transmural expression of I(to) in ventricular tissue, inhibition of I(to) would reduce the transmural dispersion of refractoriness which could contribute to the arrhythmogenic properties of halothane.

Diastolic (dys-)function and electrophysiology

Cross-talk between cardiac electrical and mechanical function is a bidirectional process: The origin and spread of electric excitation govern cardiac contraction and relaxation, while the mechanic environment provides feedback information to the heart's electric behavior. The latter tends to be unduly disregarded by the medical community. This article reviews experimental findings on the effects of diastolic mechanics on cardiac electrophysiology, and describes physiological correlates, clinical manifestations, and therapeutic utility of cardiac mechanic stimulation in humans.

Modulation of arrhythmias by isoproterenol in a rabbit heart model of d-sotalol-induced long Q-T intervals

Sympathetic influences have been implicated in arrhythmias associated with both congenital and acquired long Q-T intervals. We recorded epicardial electrograms, a left ventricular endocardial monophasic action potential (MAP), and a bipolar electrocardiogram in 23 isolated rabbit hearts. Spontaneous focal arrhythmias appeared within 8-18 min following 92 microM d-sotalol in 15 of 23 hearts. The epicardial activation-recovery interval was shorter at baseline and increased to a significantly greater degree after d-sotalol administration in the hearts that developed focal activity. The standard deviation of the activation-recovery interval of the epicardial sites also increased. With the addition of 0.01 microM isoproterenol, the incidence of focal activity increased, and its mean cycle length was shortened by 7%. Also, myocardial recovery time in the epicardium was shortened to a greater degree than the endocardial MAP duration. It did not alter local epicardial heterogeneity of recovery but did increase the regional dispersion between epicardial recovery times, and the endocardial MAP duration. Therefore, beta-adrenergic stimulation in the presence of d-sotalol favors the appearance of arrhythmias by increasing the propensity for closely coupled focal activity and the temporal dispersion of recovery.

Chronic amiodarone effects on epicardial conduction and repolarization in the isolated porcine heart

Amiodarone is a potent antiarrhythmic agent with complex chronic effects, notably on repolarization and conduction, that are not fully understood. Its low arrhythmogenic potential has been related to a lack of increase in repolarization dispersion. Since its effects are not documented in pigs we conducted a mapping study of activation and repolarization in isolated perfused porcine hearts. Amio20 female pigs (n = 7) received amiodarone 20 mg/kg per day over 4 weeks while Amio50 female pigs (n = 7) received 50 mg/kg per day over 4 weeks. Concentrations of the drug encompassed values found in clinical studies. Then, activation patterns and activation-to-recovery intervals (ARI) were mapped epicardially from 128 unipolar electrograms in isolated perfused hearts in corroboration of epicardial action potential recordings. Mean ARI was longer in Amio20 experiments compared to the seven control hearts (325 +/- 11 ms vs 288 +/- 5 ms at 1,000 ms), whereas ARI dispersion was not different, being comprised between 7 and 11 ms and generating smooth gradients. In Amio50 experiments, mean ARI was further prolonged (390 +/- 10 ms at 1,500 ms) with an exaggerated reverse rate dependence concomitant with a depressant effect on the plateau of the action potential. Again, ARI dispersion did not differ from controls. Finally, the drug depressed the maximal rate of depolarization (Vmax) and slowed conduction in a rate dependent and concentration dependent fashion. In conclusion, chronic amiodarone induces Class I and Class III antiarrhythmic effects in ventricular porcine epicardium that are concentration dependent but does not affect dispersion of repolarization. This may partly explain its low arrhythmogenic potential.

Scroll wave dynamics in a three-dimensional cardiac tissue model: roles of restitution, thickness, and fiber rotation

Scroll wave (vortex) breakup is hypothesized to underlie ventricular fibrillation, the leading cause of sudden cardiac death. We simulated scroll wave behaviors in a three-dimensional cardiac tissue model, using phase I of the Luo-Rudy (LR1) action potential model. The effects of action potential duration (APD) restitution, tissue thickness, filament twist, and fiber rotation were studied. We found that APD restitution is the major determinant of scroll wave behavior and that instabilities arising from APD restitution are the main determinants of scroll wave breakup in this cardiac model. We did not see a "thickness-induced instability" in the LR1 model, but a minimum thickness is required for scroll breakup in the presence of fiber rotation. The major effect of fiber rotation is to maintain twist in a scroll wave, promoting filament bending and thus scroll breakup. In addition, fiber rotation induces curvature in the scroll wave, which weakens conduction and further facilitates wave break.

Differential expression of KvLQT1 isoforms across the human ventricular wall

Long Q-T mutant (KvLQT1) K(+) channels associate with their regulatory subunit IsK to produce the slow component of the delayed rectifier potassium (I(Ks)) cardiac current. The amplitude of KvLQT1 current depends on the expression of a KvLQT1 splice variant (isoform 2) that exerts strong dominant negative effects on the full-length KvLQT1 protein (isoform 1). We used RNase protection assays to determine the relative expression of KvLQT1 isoforms 1 and 2 and IsK mRNAs in human ventricular layers. Overall expression of KvLQT1 and IsK genes was similar in the three layers. However, there was a significant difference in the ratio between KvLQT1 isoforms 1 and 2. Isoform 2 represented 25.2 +/- 2.3%, 31.7 +/- 1.2%, and 24.9 +/- 1.7% of total KvLQT1 expression in left ventricular endocardial, midmyocardial, and epicardial tissues, respectively. Similar data were obtained from right ventricular samples. COS-7 cells were intranuclearly injected with KvLQT1 isoforms 1 or 2 plus IsK cDNAs, using two different isoform 2-to-isoform 1 ratios. Cells injected with an isoform 2-to-isoform 1 ratio mimicking that in the midmyocardium showed a K(+) current with approximately 75% reduced amplitude compared with those injected with a ratio mimicking that in the epicardium. Our results suggest that differential expression of KvLQT1 isoform 2 in endocardial, midmyocardial, and epicardial tissues is responsible for differential I(Ks) amplitude and contributes to the regional action potential heterogeneity observed across the ventricular wall.

Length-tension relationships of sub-epicardial and sub-endocardial single ventricular myocytes from rat and ferret hearts

In vivo the sub-epicardial myocardium (EPI) and sub-endocardial myocardium (ENDO) operate over different ranges of sarcomere length (SL). However, it has not been previously shown whether EPI and ENDO work upon different ranges of the same or differing length-tension curves. We have compared the SL-tension relationship of intact, single ventricular EPI and ENDO myocytes from rat and ferret hearts. Cells were attached to carbon fibres of known compliance in order to stretch them and to record force at rest (passive tension) and during contractions (active tension). In both species, ENDO cells were significantly stiffer (i.e. had steeper SL-passive tension relationships) than EPI cells. Ferret ENDO cells had significantly steeper SL-active tension relationships than EPI cells; rat cells tended to behave similarly but no significant regional differences in active properties were observed. There were no inter-species differences in the active and passive properties of EPI cells, but ferret ENDO cells displayed significantly steeper passive and active SL-tension relationships than rat ENDO. We conclude that in vivo, ferret EPI and ENDO myocytes will function over different ranges of different SL-tension curves. There is a close relationship between SL and active tension (the Frank-Starling law of the heart), and our observations suggest that regional differences in the response to ventricular dilation will depend on both the change in SL and differing regional slopes of the SL-active tension curves.

Molecular dissection of cardiac repolarization by in vivo Kv4.3 gene transfer

Heart failure leads to marked suppression of the Ca(2+)-independent transient outward current (I(to1)), but it is not clear whether I(to1) downregulation suffices to explain the concomitant action potential prolongation. To investigate the role of I(to1) in cardiac repolarization while circumventing culture-related action potential alterations, we injected adenovirus vectors in vivo to overexpress or to suppress I(to1) in guinea pigs and rats, respectively. Myocytes were isolated 72 hours after intramyocardial injection and stimulation of the ecdysone-inducible vectors with intraperitoneal injection of an ecdysone analog. Kv4.3-infected guinea pig myocytes exhibited robust transient outward currents. Increasing density of I(to1) progressively depressed the plateau potential in Kv4. 3-infected guinea pig myocytes and abbreviated action potential duration (APD). In vivo infection with a dominant-negative Kv4. 3-W362F construct suppressed peak I(to1) in rat ventriculocytes, elevated the plateau height, significantly prolonged the APD, and resulted in a prolongation by about 30% of the QT interval in surface electrocardiogram recordings. These results indicate that I(to1) plays a crucial role in setting the plateau potential and overall APD, supporting a causative role for suppression of this current in the electrophysiological alterations of heart failure. The electrocardiographic findings indicate that somatic gene transfer can be used to create gene-specific animal models of the long QT syndrome.

Inhomogeneous transmural conduction during early ischaemia in patients with coronary artery disease

Electrical inhomogeneity and conduction slowing are critical factors in the initiation and maintenance of ventricular arrhythmias during early ischaemia. Studies in animal models have shown delay in epicardial activation compared to endocardial activation. Epicardial activation delay has been attributed to either enhanced sensitivity of epicardium to ischaemia or to mid-myocardial conduction delay. No information is available in humans and in particular in patients with chronic ischaemia due to coronary artery disease who may have altered electrophysiological properties. Twenty-three patients undergoing routine coronary surgery were studied. All had severe two or three vessel coronary artery disease and a documented history of angina for a mean of 2.4 years. On cardiopulmonary bypass a 3 min period of ischaemia was created by cross clamping the aorta between the input from the pump oxygenator and the coronary arteries. During atrial pacing (normal endocardial to epicardial activation) intramyocardial activation time within the left ventricular free wall between subendocardial and subepicardial plunge electrode terminals, increased from 12.7+/-1.5 ms (control) to 28.2+/-3.2 ms after 3 min ischaemia at the base. At the apex, the activation time increase (over the same distance) was less (19.5+/-2 ms at 3 min ischaemia). This difference in increase in activation time at the base and apex was significant (P<0.05). At the apex the ischaemia induced activation delay occurred primarily over the endocardial half of the wall, whereas the opposite was observed at the base of the heart. Using an epicardial electrode array stimulation along the long axis of the epicardial fibres showed minimal conduction delay during ischaemia whereas stimulation transverse to the epicardial fibres resulted in substantial conduction time prolongation, as was the case with intramural conduction. Intramural conduction during ischaemia was similar in non-infarcted regions of infarcted hearts compared to hearts with no previous MI. To conclude, in patients with coronary artery disease epicardial activation delay early during ischaemia is caused primarily by intramural delay and not by delay along the epicardium. Moreover, the ischaemia-induced transmural activation delay is inhomogeneous.

Rapid ventricular repolarization in rodents: electrocardiographic manifestations, molecular mechanisms, and clinical insights

This article examines specific electrocardiographic (ECG) and electrophysiological features of ventricular repolarization in rats and mice, and the role of depolarization-activated potassium currents in mediating the unique features of ECG recordings in these rodents. This article describes the currents that underlie ventricular repolarization in these rodents, identifies terminology that appropriately describes the unique features of murine ECG recordings, and correlates these unique findings with selected human ECG ventricular repolarization abnormalities. The absence of a distinct isoelectric interval between the QRS complex and the T wave, accompanied by a relatively short QT interval, are common features of ECG recordings in mice and rats, but not in ECGs in guinea pigs. The murine ECG morphology is apparently attributable to the presence of large outward K+ currents that dominate the early phase of ventricular repolarization. In rats and mice, the predominant current underlying the early phase of repolarization appears to be the rapidly activating and inactivating 4-aminopyridine-sensitive transient outward current (ie, I(to)). Importantly, the density of I(to) in rats and mice is high, whereas this current is not evident in the ventricular myocytes of guinea pigs. The high density of I(to) appears to underlie the prominent J wave or downsloping ST-segment elevation seen in rats and mice, whereas the ST-segment is isoelectric in guinea pigs. The unusual J wave and ST-segment pattern in murine ECGs, however, does bear some resemblance to ECG features observed in humans with Brugada syndrome, and with hypothermia and ischemia. These patterns in rats and mice might, therefore, serve as an experimental model for the idiopathic J wave.

Cellular basis for long QT, transmural dispersion of repolarization, and torsade de pointes in the long QT syndrome

Genetic studies have identified four forms of congenital long QT syndrome (LQTS) caused by mutations in ion channel genes located on chromosomes 3 (LQT3), 7 (LQT2), 11 (LQT1), and 21 (LQT5). Preliminary clinical studies have reported different phenotypic electrocardiographic patterns and different sensitivity to pacing or pharmacological therapy for each genotype. A transmural electrocardiogram and transmembrane action potentials from epicardial, M, and endocardial cells were simultaneously recorded from an arterially perfused wedge of canine left ventricle. Isoproterenol (100 nmol/L) in the presence of chromanol 293B (30 micromol/L), an I(Ks) blocker (LQT1 model), produced a preferential prolongation of M-cell action potential duration (APD), resulting in an increase in transmural dispersion of repolarization (TDR) and a broad-based T wave, as commonly seen in LQT1 patients. D-Sotalol (100 micromol/L), an I(Kr) blocker (LQT2 model), and ATX-II (20 nmol/L), an agent that augments late I(Na) (LQT3 model), also produced a preferential prolongation of M-cell APD, an increase in TDR, and low-amplitude T wave with a bifurcated appearance (LQT2), and late-appearing T wave (LQT3), respectively. APD-, QT-, and TDR-rate relations were much steeper in the LQT3 model than in either the LQT1 or LQT2 model, whereas the rate relations in the LQT1 and LQT2 models were both steeper than those under control conditions. Spontaneous and programmed electrical stimulation-induced torsade de pointes (TdP) were observed in all 3 models. Propranolol (1 micromol/L), a beta blocker, completely prevented the effect of isoproterenol to persistently or transiently increase TDR and to induce TdP in the LQT1 and LQT2 models, but facilitated TdP in the LQT3 model. Mexiletine, a class IB Na+ channel blocker, dose-dependently (2-20 micromol/L) abbreviated the QT and APD more in the LQT3 model, but decreased TDR and suppressed TdP in the 3 models.

New body surface isopotential map evaluation method to detect minor potential losses in non-Q-wave myocardial infarction

BACKGROUND

Potential losses caused by stable non-Q-wave myocardial infarction (MI) are too small to diagnose with the use of standard ECG. The aim of the present study was to obtain accurate diagnostic criteria for this prognostically important disease with the help of body surface mapping.

METHODS AND RESULTS

Body surface potentials were recorded with the use of 63 unipolar leads in 45 patients with a non-Q-wave MI (41 to 75 years old); 24 healthy adults, 42 patients with unstable angina, and 70 patients with Q-wave MI served as reference groups. Qualitative pathological features of the isopotential maps, such as onset time and site and magnitude of the first right-anterior/anterior minimum, as well as pathological negativities at that time, were defined in non-Q-wave MI cases. These features, which account for the activation sequence and the body surface projections of specific cardiac regions (Selvester classification), showed a 91% sensitivity and an 88% specificity for the detection of non-Q-wave MI. In comparison, the different departure maps (first third QRS, QRS, and QRST isoarea) resulted in less favorable specificities (50% to 58%). Concordance between the isopotential maps and the acute-phase ECG (90%), hypokinesis (64%), fixed perfusion defects (59%), and significant stenosis of the infarct-related coronary artery (87%) supported the concept that these isopotential map changes correspond to the supposed sites of MI. There were pathological features in 69% of patients with unstable angina, with similar concordances as in non-Q-wave MI.

CONCLUSIONS

Isopotential maps revealed characteristic features that were suitable for the detection and localization of non-Q-wave MI in the clinical setting of unstable coronary artery disease.

Enhanced dispersion of repolarization and refractoriness in transgenic mouse hearts promotes reentrant ventricular tachycardia

The heterogeneous distribution of ion channels in ventricular muscle gives rise to spatial variations in action potential (AP) duration (APD) and contributes to the repolarization sequence in healthy hearts. It has been proposed that enhanced dispersion of repolarization may underlie arrhythmias in diseases with markedly different causes. We engineered dominant negative transgenic mice that have prolonged QT intervals and arrhythmias due to the loss of a slowly inactivating K(+) current. Optical techniques are now applied to map APs and investigate the mechanisms underlying these arrhythmias. Hearts from transgenic and control mice were isolated, perfused, stained with di-4-ANEPPS, and paced at multiple sites to optically map APs, activation, and repolarization sequences at baseline and during arrhythmias. Transgenic hearts exhibited a 2-fold prolongation of APD, less shortening (8% versus 40%) of APDs with decreasing cycle length, altered restitution kinetics, and greater gradients of refractoriness from apex to base compared with control hearts. A premature impulse applied at the apex of transgenic hearts produced sustained reentrant ventricular tachycardia (n=14 of 15 hearts) that did not occur with stimulation at the base (n=8) or at any location in control hearts (n=12). In transgenic hearts, premature impulses initiated reentry by encountering functional lines of conduction block caused by enhanced dispersion of refractoriness. Reentrant VT had stable (>30 minutes) alternating long/short APDs associated with long/short cycle lengths and T wave alternans. Thus, optical mapping of genetically engineered mice may help elucidate some electrophysiological mechanisms that underlie arrhythmias and sudden death in human cardiac disorders.

Isoproterenol specifically modulates reverse rate-dependent effects of d,l-sotalol, d-sotalol, and dofetilide

The modulation of antiarrhythmic and proarrhythmic properties of antiarrhythmic compounds by increased sympathetic activity is of experimental and clinical interest. However, the interaction of adrenergic stimulation with the rate-response pattern of class III antiarrhythmic agents is not well established. Using standard microelectrode techniques, we evaluated the effects of isoproterenol (iso) on the action of d,l-sotalol (d,l-sot), d-sotalol (d-sot), and dofetilide (dof) on action-potential parameters recorded from isolated canine cardiomyocytes. The cell-isolation procedure was performed from the endocardial layers of left ventricular myocardium from healthy beagle dogs. The following electrophysiologic parameters were recorded: resting membrane potential (RMP), action-potential amplitude (APA), action-potential duration at 90% repolarization (APD 90), and effective refractory period (ERP). After exposure to iso, the class III activity of d,l-sot was well maintained over the entire range of frequencies studied. In contrast, iso differentially antagonized the action of d-sot and dof. In comparison to dof, the class III action of d-sot was particularly sensitive to iso, predominantly at faster stimulation rates. Our observations demonstrate specific rate regulation of the class III action of d,l-sot, d-sot, and dof in response to adrenergic stimulation. The unfavorable rate-response pattern of d-sot compared with d,l-sot and dof might prove disadvantageous in high-catecholamine states.

Transmural heterogeneity of ventricular repolarization under baseline and long QT conditions in the canine heart in vivo: torsades de pointes develops with halothane but not pentobarbital anesthesia

INTRODUCTION

In vitro studies have provided evidence for the existence of M cells. The present study examines the contribution of the M cell to transmural dispersion of repolarization (TDR) and to the development of torsades de pointes (TdP) in the canine heart in vivo in animals anesthetized with either pentobarbital or halothane.

METHODS AND RESULTS

Monophasic action potentials (MAPs) were recorded from 4 to 7 transmural sites, before and after d-sotalol. Cells displaying the longest MAP duration (MAPD) generally were localized to the deep subendocardium to mid-myocardium (M region) in the anterior wall of the left ventricle. d-Sotalol preferentially prolonged the MAPD of the M region, increasing TDR significantly more (P < 0.05) in animals anesthetized with halothane (31+/-5 to 88+/-17 msec) than in those receiving pentobarbital (24+/-9 to 53+/-7 msec; basic cycle length 1,500 msec). In halothane-anesthetized dogs, a remarkable transient increase in M cell MAPD followed interpolation of one or more extrasystole(s), leading to a transient increase in TDR and TdP. TdP was never observed with pentobarbital anesthesia.

CONCLUSION

Our results demonstrate that transmural heterogeneity of repolarization is amplified under acquired long QT conditions and that the increase in TDR underlies the development of TdP in halothane- but not pentobarbital-anesthetized dogs. The data support an important contribution of M cells to TDR and to the development of TdP in the canine heart in vivo. Our data also highlight the importance of acceleration-induced prolongation of MAPD (a phenomena observed principally in M cells) in the development of TdP.

Measurement and interpretation of QT dispersion

QT dispersion was proposed as an index of the spatial inhomogeneity of ventricular recovery times. The results of studies that found significant correlation between dispersion of ventricular recovery times measured with monophasic action potentials and QT dispersion were interpreted as proof of the direct link between QT dispersion and the dispersion of ventricular recovery times. Later it was shown that QT dispersion is not a direct reflection of the spatial variation of the recovery times and cannot be used for quantification of this variation. The interlead variability of the QT intervals is a result of different projections of the spatial T-wave loop into the various electrocardiographic leads. The reliability of both manual and automatic measurement of QT dispersion is low and is often of the order of the differences of Qt dispersion between different patient groups. The measurement reliability is influenced by intrinsic factors (e.g., amplitude of the T wave) and extrinsic factors (e.g., noise, paper speed of recording, instruments for manual measurements, and type of algorithm and interalgorithmic settings for automatic measurement). There is very little to choose between the different indices of expression of QT dispersion, as well as between the different lead configurations used for its measurement. QT dispersion is not simply a result of measurement error, but a crude measure of abnormalities during the whole course of repolarization. Only grossly prolonged QT dispersion (e.g., > or =100 ms), must be interpreted simply as a sign of the abnormal course of the repolarization, and inferences about the actual dispersion of the ventricular recovery times should not be made. Newer concepts of assessment of the morphology of the T wave are already emerging and will probably be of higher clinical value.

Electrophysiological basis of QT dispersion measurements

Dispersion of ventricular repolarization is a now widely used term describing nonhomogeneous recovery of excitability or heterogeneity of ventricular repolarization. It is usually expressed as the difference or the range of various repolarization measurements obtained from a heart. Experimentally, an increased dispersion of ventricular repolarization was found to be tightly associated with increased propensity for ventricular arrhythmias, and, therefore, is considered an important arrhythmogenic mechanism. Noninvasively, this arrhythmogenic substrate was approached using multilead body surface potential mapping, but also QT interval dispersion (QTd) and similar electrocardiogram (ECG) variables from the 12-lead surface ECG. Standard QTd from the ECG correlates significantly with dispersion of repolarization measured from the myocardium. A causal relationship is, however, still unclear, and there are 2 main hypotheses to explain the electrophysiological basis of QTd. The local hypothesis explaining QTd with spatial differences in action potential duration mirrored in the various QT intervals competes with the global hypothesis explaining the variation in surface ECG measurements with different projections of a common T-wave vector. Notwithstanding the final explanation for QTd, and particularly for technical reasons, new markers like advanced T-wave loop variables may best reflect the abnormal repolarization substrate on the surface ECG.

The role of the delayed rectifier component IKs in dog ventricular muscle and Purkinje fibre repolarization

1. The relative contributions of the rapid and slow components of the delayed rectifier potassium current (IKr and IKs, respectively) to dog cardiac action potential configuration were compared in ventricular myocytes and in multicellular right ventricular papillary muscle and Purkinje fibre preparations. Whole-cell patch-clamp techniques, conventional microelectrode and in vivo ECG measurements were made at 37C. 2. Action potential duration (APD) was minimally increased (less than 7%) by chromanol 293B (10 microM) and L-735,821 (100 nM), selective blockers of IKs, over a range of pacing cycle lengths (300-5000 ms) in both dog right ventricular papillary muscles and Purkinje fibre strands. D-Sotalol (30 microM) and E-4031 (1 microM), selective blockers of IKr, in the same preparations markedly (20-80%) lengthened APD in a reverse frequency-dependent manner. 3. In vivo ECG recordings in intact anaesthetized dogs indicated no significant chromanol 293B (1 mg kg-1 i.v.) effect on the QTc interval (332.9 +/- 16.1 ms before versus 330.5 +/- 11.2 ms, n = 6, after chromanol 293B), while D-sotalol (1 mg kg-1 i.v.) significantly increased the QTc interval (323.9 +/- 7.3 ms before versus 346.5 +/- 6.4 ms, n = 5, after D-sotalol, P < 0.05). 4. The current density estimated during the normal ventricular muscle action potential (i.e. after a 200 ms square pulse to +30 mV or during a 250 ms long 'action potential-like' test pulse) indicates that substantially more current is conducted through IKr channels than through IKs channels. However, if the duration of the square test pulse or the 'action potential-like' test pulse was lengthened to 500 ms the relative contribution of IKs significantly increased. 5. When APD was pharmacologically prolonged in papillary muscle (1 microM E-4031 and 1 microg ml-1 veratrine), 100 nM L-735,821 and 10 microM chromanol 293B lengthened repolarization substantially by 14.4 +/- 3.4 and 18. 0 +/- 3.4% (n = 8), respectively. 6. We conclude that in this study IKs plays little role in normal dog ventricular muscle and Purkinje fibre action potential repolarization and that IKr is the major source of outward current responsible for initiation of final action potential repolarization. Thus, when APD is abnormally increased, the role of IKs in final repolarization increases to provide an important safety mechanism that reduces arrhythmia risk.

SCN5A mutation (T1620M) causing Brugada syndrome exhibits different phenotypes when expressed in Xenopus oocytes and mammalian cells

Brugada syndrome is a hereditary cardiac disease causing abnormal ST segment elevation in the ECG, right bundle branch block, ventricular fibrillation and sudden death. In this study we characterized a new mutation in the SCN5A gene (T1620M), causing the Brugada syndrome. The mutated channels were expressed in both Xenopus leavis oocytes and in mammalian tsA201 cells with and without the beta-subunit and studied using the patch clamp technique. Opposite phenotypes were observed depending on the expression system. T1620M mutation led to a faster recovery from inactivation and a shift of steady-state inactivation to more positive voltages when expressed in Xenopus oocytes. However, using the mammalian expression system no effect on steady-state inactivation was observed, but this mutation led to a slower recovery from inactivation. Our finding supports the idea that the slower recovery from inactivation of the cardiac sodium channels seen in our mammalian expression system could decrease the density of sodium channels during the cardiac cycle explaining the in vivo arrhythmogenesis in patients with Brugada syndrome.

The role of cromakalim and a nitric oxide synthase blocker in cardiac arrhythmia in the intact baboon model

The arrhythmogenic effect of adenosine triphosphate (ATP)-sensitive potassium channel openers is controversial and may be dependent on the type of animal model used. Information on the effect of these drugs in the normal primate model is limited. The purpose of this study was first to determine the arrhythmogenic properties of cromakalim in the baboon and second to determine whether N-omega-nitro-L-arginine methyl ester (L-NAME) has any effect on the induced arrhythmia. Adult (2-4 years old) baboons (Papio ursinus) were anesthetized with a continuous i.v. infusion of ketamine (100 mg/ ml), diazepam (5 mg/ml), and saline (ratio 2:2:50) at a rate of 40-60 ml/h. Sympathetic responses were inhibited by administration of propranolol (1 mg/kg) before the start of the experiments. Cromakalim (30 microg/kg) was administered before and after L-NAME (7.5 mg/kg), and the parameters were monitored for 15 min after each intervention. A Millar double-tipped microcatheter was used to record left ventricular and aortic pressures. Lead II of the ECG was monitored. During a 15-min period after administration of cromakalim, 22.3 +/- 6.0 abnormal ventricular complexes were recorded. L-NAME administration significantly reduced these abnormal complexes to 4.5 +/- 2 (paired t test, p < or = 0.05). We therefore conclude that cromakalim has arrhythmogenic properties in the baboon and that these can be attenuated by L-NAME.

[Sudden death (VI). The Brugada syndrome and right myocardiopathies as a cause of sudden death. The differences and similarities]

In 1992 we described a new syndrome characterized by syncopal or sudden death episodes in patients with a structurally normal heart and a characteristic electrocardiogram 9 showing a pattern of right bundle branch block and ST segment elevation in right precordial leads V1 to V3. The disease is genetically determined with and autosomic dominant pattern of transmission. Until now three mutations and one polymorphism in the sodium cardiac channel gene have been identified in two families and one sporadic patient. As in many other genetically determined diseases, the disease is heterogeneous, caused by more than one gene. The syndrome has been identified in almost all countries in the world. Its incidence is difficult to evaluate, but it seems to be responsible for 4 to 10 sudden deaths per year per 10,000 inhabitants in areas like Laos or Thailand, and it represents the most frequent cause of death in young male adults in these countries. Up to 50% of all sudden deaths in patients with structurally normal heart are caused by the disease. The diagnosis can be easily made thanks to the characteristic electrocardiographic pattern. In some patients, the presence of concealed and intermittent forms might make the diagnosis more difficult. The electrocardiogram can be modulated by autonomic changes and administration of antiarrhythmic drugs. Beta-adrenergic stimulation normalizes the electrocardiogram, whereas ajmaline, flecainide or procainamide administration increase ST segment elevation. These drugs allow the unmasking of concealed or intermittent forms of the disease. Prognosis of patients with the syndrome is poor without an implantable defibrillator and antiarrhythmic drugs like amiodarone or betablockers do not protect against sudden death. The poor prognosis is similar in patients with a history of aborted sudden death or syncope and in asymptomatic patients in whom the abnormal electrocardiogram characteristic of the syndrome, was identified during a routine examination.

Effect of acidosis on transient outward potassium current in isolated rat ventricular myocytes

The effect of acidosis on the transient outward K(+) current (I(to)) of rat ventricular myocytes has been investigated using the perforated patch-clamp technique. When the holding potential was -80 mV, depolarizing pulses to potentials positive to -20 mV activated I(to) in subepicardial cells but activated little I(to) in subendocardial cells. Exposure to an acid solution (pH 6.5) had no significant effect on I(to) activated from this holding potential in either subepicardial or subendocardial cells. When the holding potential was -40 mV, acidosis significantly increased I(to) at potentials positive to -20 mV in subepicardial cells but had little effect on I(to) in subendocardial cells. The increase in I(to) in subepicardial cells was inhibited by 10 mM 4-aminopyridine. In subepicardial cells, acidosis caused a +8.57-mV shift in the steady-state inactivation curve. It is concluded that in subepicardial rat ventricular myocytes acidosis increases the amplitude of I(to) as a consequence of a depolarizing shift in the voltage dependence of inactivation.

Electrophysiological modeling of cardiac ventricular function: from cell to organ

Three topics of importance to modeling the integrative function of the heart are reviewed. The first is modeling of the ventricular myocyte. Emphasis is placed on excitation-contraction coupling and intracellular Ca2+ handling, and the interpretation of experimental data regarding interval-force relationships. Second, data on use of diffusion tensor magnetic resonance (DTMR) imaging for measuring the anatomical structure of the cardiac ventricles are presented. A method for the semi-automated reconstruction of the ventricles using a combination of gradient recalled acquisition in the steady state (GRASS) and DTMR images is described. Third, we describe how these anatomically and biophysically based models of the cardiac ventricles can be implemented on parallel computers.

Associations between different status of myocardial ischemia and ischemia-related negative or positive epicardial U-waves: observations during coronary angioplasty

This study examined the relationships between the polarity of the U wave on intracoronary electrocardiogram (ECG) and the status of myocardial ischemia during angioplasty. The ECG features of ischemia-related U waves were also evaluated. Among 63 patients with intracoronary ECGs adequate for analysis of U waves, there were 26 patients showing a change of the U wave to a negative direction and 18 patients showing a change to a positive direction from baseline to coronary occlusion. Among these patients, 10 of the former showed a distinct change in polarity of the U wave from positive to negative (group A), and 7 of the latter patients showed the opposite change (group B). Patients in group B had a higher incidence of prior myocardial infarction (86% vs 30%; P < .05), presence of an abnormal Q wave on intracoronary ECG (71% vs 20%; P < .05), poor wall motion in the angioplasty-related area (100% vs 30%; P < .01), and lower left ventricular ejection fraction (55.7% +/-8.1% vs 66.6% +/- 4.5%; P < .01) than patients in group A. The remaining patients (other than groups A and B) showing U wave change in a negative (n = 16) or positive (n = 11) direction presented with similar features to those in groups A or B, respectively. The ECG features of several types of ischemia-related U wave were determined by analysis of intracoronary ECG obtained from the patients in groups A and B. In group A, the Bazett-corrected Q (positive U) interval measured at baseline (myocardial state; near normal) was significantly shorter than the Q-(negative U) interval measured during coronary occlusion (acute ischemia) (0.518 +/- 0.031 s vs 0.579 +/- 0.046 s; P < .01). In group B, the Q-(negative U) interval measured at baseline (chronic ischemia) was longer than the Q-(positive U) interval measured during angioplasty (acute-on-chronic ischemia) (0.582 +/- 0.034 s vs 0.501 +/- 0.027 s; P < .001). Thus, intracoronary ECG recorded during angioplasty in the present study revealed physiologic U wave, two types ("acute" and "chronic") of ischemia-related negative and one type ("pseudonormal") of ischemia-related positive U waves, each of which appeared in a different status of myocardial ischemia and possessed characteristic ECG features in its appearance.

The M cell: its contribution to the ECG and to normal and abnormal electrical function of the heart

The discovery and characterization of the M cell, a unique cell type residing in the deep layers of the ventricular myocardium, has opened a new door in our understanding of the electrophysiology and pharmacology of the heart in both health and disease. The hallmark of the M cell is the ability of its action potential to prolong much more than that of other ventricular myocardial cells in response to a slowing of rate and/or in response to agents that act to prolong action potential duration. Our goal in this review is to provide a comprehensive characterization of the M cell, its contribution to transmural heterogeneity, and its role in the normal electrical function of the heart, in the inscription of the ECG (particularly the T wave), and in the development of QT dispersion, T wave alternans, long QT intervals, and cardiac arrhythmias, such as torsades de pointes. Our secondary goal is to address the controversy that has arisen relative to the functional importance of the M cell in the normal heart. The controversy derives largely from the failure of some investigators to demonstrate transmural heterogeneity of repolarization in the dog in vivo under control conditions and after administration of quinidine. The inability to demonstrate transmural heterogeneity under these conditions may be due to the use of bipolar recording techniques that, in our experience, seriously underestimate transmural dispersion of repolarization (TDR). The use of sodium pentobarbital and alpha-chloralose as anesthesia also is problematic, because these agents reduce or eliminate TDR by affecting a variety of ion channel currents. Finally, attempts to amplify transmural dispersion of repolarization with an agent such as quinidine must take into account that relatively high concentrations can result in effects opposite to those desired due to drug inhibition of multiple ion channels. These observations may explain the inability of earlier studies to detect the M cell.

Four different components contribute to outward current in rat ventricular myocytes

In rat ventricle, two Ca(2+)-insensitive components of K(+) current have been distinguished kinetically and pharmacologically, the transient, 4-aminopyridine (4-AP)-sensitive I(to) and the sustained, tetraethylammonium (TEA)-sensitive I(K). However, a much greater diversity of depolarization-activated K(+) channels has been reported on the level of mRNA and protein. In the search for electrophysiological evidence of further current components, the whole cell voltage-clamp technique was used to analyze steady-state inactivation of outward currents by conditioning potentials in a wide voltage range. Peak (I(peak)) and late (I(late)) currents during the test pulse were analyzed by Boltzmann curve fitting, producing three fractions each. Fractions a and b had different potentials of half-maximum inactivation (V(0.5)); the third residual fraction, r, did not inactivate. Fractions a for I(peak) and I(late) had similar relative amplitudes and V(0.5) values, whereas size and V(0.5) of fractions b differed significantly between I(peak) and I(late). Only b of I(peak) was transient, suggesting a relation with I(to), whereas a, b, and r of I(late) appeared to be three different sustained currents. Therefore, four individual outward current components were distinguished: I(to) (b of I(peak)), I(K) (a), the steady-state current I(ss) (r), and the novel current I(Kx) (b of I(late)). This was further supported by differential sensitivity to TEA, 4-AP, clofilium, quinidine, dendrotoxin, heteropodatoxin, and hanatoxin. With the exception of I(to), none of the currents exhibited a marked transmural gradient. Availability of I(K) was low at resting potential; nevertheless, I(K) contributed to action potential shortening in hyperpolarized subendocardial myocytes. In conclusion, on the basis of electrophysiological and pharmacological evidence, at least four components contribute to outward current in rat ventricular myocytes.

The autonomic control of the transmural dispersion of ventricular repolarization in anesthetized dogs

INTRODUCTION

The initiation of ventricular arrhythmias is in part associated with autonomic nervous tone. We investigated the effects of sympathetic and parasympathetic stimulation on the monophasic action potentials (MAPs) of the epicardium (EPi), mid-myocardial (M) region, and endocardium (Endo) in vivo.

METHODS AND RESULTS

In 12 mongrel open chest anesthetized dogs, both sides of the cervical vagus nerves and stellate ganglia were crushed with a tight ligature. Right atrial pacing at 600 msec cycle length was begun after the sinus nodal area had been crushed. MAPs from the M region were measured by two needle electrodes that were supported by a W-shaped plastic frame. The epicardial, M region, and endocardial MAP durations at 90% repolarization (MAPD90) were 287+/-7, 315+/-7, and 290+/-8 msec, respectively. The MAPD90 from M cells was longer than that from Epi or Endo. Sympathetic stimulation shortened MAPD90 more in the M region (53+/-4 msec) than that in the Epi (27+/-3 msec) or Endo (26+/-4 msec). The transmural dispersion of repolarization during sympathetic stimulation was shorter than that of the control. Parasympathetic stimulation did not significantly affect any of the MAPD90 values. Simultaneous sympathetic and parasympathetic stimulation produced changes not significantly to those produced by sympathetic stimulation alone.

CONCLUSION

Our results suggest that sympathetic activity can reduce transmural dispersion of repolarization under autonomic control in the canine heart under baseline conditions.

Electrophysiological effects of cetirizine, astemizole and D-sotalol in a canine model of long QT syndrome

Observations of torsades de pointes during therapy with terfenadine and astemizole has raised concern about the cardiac safety of non-sedating H1-antagonist agents. We compared cetirizine, another compound of that class, to D-sotalol and to astemizole in a model of acquired long QT syndrome. Open-chest surgery was performed in adult beagle dogs anaesthetized with halothane and thiopental. Bradycardia was produced with beta-adrenergic blockade and sinus node crush. Four left ventricular intramyocardial unipolar monophasic action potentials (MAP) were recorded during atrial pacing at basic cycle lengths (BCL) 400-1500 msec, before and during three successive 1-h drug infusions (0.14, 0.45 and 1.4 mg/kg/h for astemizole and cetirizine and 1.1, 2.2 and 4.5 mg/kg/h for D-sotalol). Dose- and bradycardia-dependent prolongations of MAP duration (MAPD) were produced by D-sotalol (P < 0.001) and astemizole (P < 0.001) but not by cetirizine. At BCL 1500 ms, the three infusions of astemizole prolonged endocardial MAPD from 323 +/- 8 msec (mean +/- SE) at baseline to 343 +/- 10, 379 +/- 13 and 468 +/- 26 msec, respectively (n = 9). Sotalol prolonged that MAPD from 339 +/- 6 msec to 377 +/- 7, 444 +/- 15 and 485 +/- 24 msec (n = 7). In contrast, cetirizine did not prolong MAPD: 341 +/- 8 msec at baseline Vs 330 +/- 8, 324 +/- 9 and 323 +/- 11 msec (n = 9). Drug-induced increase in transmural dispersion reached +79 +/- 19 msec after astemizole, +59 +/- 21 msec after D-sotalol and only +7 +/- 11 msec after cetirizine. Runs of ventricular tachycardias and torsades de pointes occurred during dose three of astemizole (5/9 dogs) and D-sotalol (4/7 dogs) but never during cetirizine. In the present model, astemizole and D-sotalol but not cetirizine prolonged MAPD and transmural dispersions of repolarization and produced torsades de pointes. These results suggest that the halothane-anaesthetized bradycardic dog could be a valuable model to discriminate drugs for their class III effects and proarrhythmic potencies.

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.