Two components of delayed rectifier current in canine atrium and ventricle. Does IKs play a role in the reverse rate dependence of class III agents?

Because the number and characteristics of delayed rectifier K+ current (IK) components vary between species, the role of each component in the action potential and modulation by class III agents is uncertain. To address these issues, IK was assessed in adult isolated canine ventricular and and atrial myocytes by using whole-cell and perforated-patch techniques. IK components were characterized by using two complementary approaches: a kinetic approach (based on biexponential fits to deactivating tail currents) and a pharmacological approach approach (using the methanesulfonanilide compound E-4031). In ventricular myocytes, two exponential tail current components were distinguished; these components differed in the voltage and time dependence of activation and the effect of lower (K+). Both kinetic components contributed equally to peak tail current amplitude (measured at -35 mV) after a single 300-ms pulse to 5 mV, simulating an action potential. By use of E-4031, rapidly and slowly activating components described kinetically were identified. The activation kinetics and rectification properties of canine IKr and IKs are qualitatively similar to those described previously for guinea pigs. In contrast, canine IKr and IKs deactivation kinetics differed markedly from those found in guinea pigs, with canine IKr deactivating slowly (time constant tau, 2 to 3 s near -35 mV) and IKs deactivating rapidly (tau, 150 ms near -35 mV and decreasing to 30 ms near -85 mV). E-4031 elicited reverse rate-dependent effects (greater drug-induced prolongation of the action potential at slower stimulation rates); this effect is inconsistent with the hypothesis attributing reverse rate dependence to incomplete IKs deactivation during rapid stimulation (due to rapid deactivation of canine IKs). Two IK components with characteristics comparable to those found in ventricular myocytes were also observed in atrial myocytes. In conclusion, (1) IKr- and IKs-like components of IK are present in canine atrial and ventricular myocytes, with deactivation kinetics strikingly different from those found in guinea pigs, and (2) the rapid deactivation kinetics of canine IKs do not support its role in reverse rate dependence with class III agents in this species.

The cardiac electrophysiological effects of sparteine and its analogue BRB-I-28 in the rat

This study compares the cardiovascular and antiarrhythmic effects of sparteine and a 3,7-diheterobicyclo[3.3.1]nonane analogue of sparteine, BRB-I-28, in pentobarbitone-anaesthetized rats subjected to left-ventricle electrical stimulation and occlusion of the left anterior descending coronary artery. Sparteine and BRB-I-28 produced a dose-dependent reduction in heart rate and blood pressure over the dose range 1-64 mumol/kg/min. As well, the P-R and Q-aT intervals of the electrocardiogram (ECG) were prolonged. The thresholds for induction of premature beats and ventricular fibrillation were dose-dependently increased and both drugs increased refractoriness. While sparteine and BRB-I-28 (at 16 and 64 mumol/kg/min, respectively) did not change the incidence of premature beats or ventricular tachycardia with coronary occlusion, both drugs equally reduced the incidence of ventricular fibrillation. We characterized the actions of sparteine and BRB-I-28 on cardiac Na+, transient outward and sustained outward plateau K+ currents of rat myocytes using the whole-cell patch-clamp. Sparteine and BRB-I-28 produced a concentration-dependent reduction in Na+ current with EC50 values of 110 and 230 microM, respectively. Both drugs produced hyperpolarizing shifts of 8 and 11 mV, respectively, for Na+ channel inactivation while neither produced a change in channel activation. Both drugs produced a concentration-dependent block of the sustained plateau K+ current and increased the rate of decay of the transient outward K+ current. Thus, sparteine and BRB-I-28 possess Na+ and K+ channel blocking properties which may account for their antiarrhythmic actions against electrical and ischaemic arrhythmias.

Molecular and functional characterization of a rat brain Kv beta 3 potassium channel subunit

A novel potassium channel beta-subunit (Kv beta 3) was cloned from rat brain being the third member of a Kv beta subunit gene family. It is a protein of 403 amino acid residues with a 68% amino acid sequence homology to Kv beta 1.1. Kv beta 3 is primarily expressed in rat brain having a distribution distinct to those of Kv beta 1.1 and Kv beta 2. This subunit also has a long N-terminal structure and induces inactivation in N-terminal deleted Kv1.4 but not in other members of the Kv1 channel family. Similarly to Kv beta 1.1, the Kv beta 3-induced inactivation is regulated by the intracellular redox potential.

Delayed rectifier channels in human ventricular myocytes

BACKGROUND

Previous studies have shown that in heart there are two kinetically distinct components of delayed rectifier current: a rapidly activating component (IKr) and a more slowly activating component (IKs). The presence of IKr and/or IKs appears to be species dependent. We studied the nature of the delayed rectifier current in human ventricle in whole-cell and single-channel experiments.

METHODS AND RESULTS

Ventricular myocytes were obtained from hearts of patients with ischemic or dilated cardiomyopathy. Single-channel currents and whole-cell tail currents were recorded at negative potentials directly after return from a depolarizing step. Single-channel currents were measured in the cell-attached patch configuration with 140 mmol/L K+ in the pipette. In the present study, we identified a voltage-dependent channel with a single-channel conductance of 12.9 +/- 0.8 pS (mean +/- SEM, n = 5) and a reversal potential near to the K+ equilibrium potential, suggesting that the channel is selective to K+ ions. Channel activity was observed only after a depolarizing step and increased with the duration and amplitude of the depolarization, indicating time- and voltage-dependent activation. Activation at +30 mV was complete within 300 milliseconds, and the time constant of activation, determined in the whole-cell configuration, was 101 +/- 25 milliseconds (mean +/- SEM, n = 4). The voltage dependence of activation could be described by a Boltzmann equation with a half-activation potential of -29.9 mV and a slope factor of 9.5 mV. The addition of the class III antiarrhythmic drug E-4031 completely blocked channel activity in one patch. No indications for the presence of IKs were found in these experiments.

CONCLUSIONS

The conformity between the properties of IKr and those of the K+ channel in the present study strongly suggests that IKr is present in human ventricle.

Cloned human inward rectifier K+ channel as a target for class III methanesulfonanilides

Methanesulfonanilide derivatives such as dofetilide are members of the widely used Class III group of cardiac antiarrhythmic drugs. A methanesulfonanilide-sensitive cardiac current has been identified as IKr, the rapidly activating component of the repolarizing outward cardiac K+ current, IK. IKr may be encoded by the human ether-related gene (hERG), which belongs to the family of voltage-dependent K+ (Kv) channels having six putative transmembrane segments. The hERG also expresses an inwardly rectifying, methanesulfonanilide-sensitive K+ current. Here we show that hIRK, a member of the two-transmembrane-segment family of inward K+ rectifiers that we have cloned from human heart, is a target for dofetilide. hIRK currents, expressed heterologously in Xenopus oocytes, are blocked by dofetilide at submicromolar concentrations (IC50 = 533 nmol/L at 40 mV and 20 degrees C). The drug has no significant blocking effect on the human cardiac Kv channels hKv1.2, hKv1.4, hKv1.5, or hKv2.1. The block is voltage dependent, use dependent, and shortens open times in a manner consistent with open-channel block. While steady state block is strongest at depolarized potentials, recovery from block is very slow even at hyperpolarized potentials (tau = 1.17 seconds at -80 mV). Thus, block of hIRK may persist during diastole and might thereby affect cardiac excitability.

beta-Adrenergic modulation of the inwardly rectifying potassium channel in isolated human ventricular myocytes. Alteration in channel response to beta-adrenergic stimulation in failing human hearts

The beta-adrenergic modulation of the inwardly-rectifying K+ channel (IK1) was examined in isolated human ventricular myocytes using patch-clamp techniques. Isoproterenol (ISO) reversibly depolarized the resting membrane potential and prolonged the action potential duration. Under the whole-cell C1- -free condition, ISO applied via the bath solution reversibly inhibited macroscopic IdK1. The reversal potential of the ISO-sensitive current was shifted by approximately 60 mV per 10-fold change in the external K+ concentration and was sensitive to Ba2+. The ISO-induced inhibition of IK1 was mimicked by forskolin and dibutyrl cAMP, and was prevented by including a cAMP-dependent protein kinase (PKA) inhibitor (PKI) in the pipette solution. In single-channel recordings from cell-attached patches, bath applied ISO could suppress IK1 channels by decreasing open state probability. Bath application of the purified catalytic sub-unit of PKA to inside-out patches also inhibited IK1 and the inhibition could be antagonized by alkaline phosphatase. When beta-adrenergic modulation of IK1 was compared between ventricular myocytes isolated from the failing and the nonfailing heart, channel response to ISO and PKA was significantly reduced in myocytes from the failing heart. Although ISO inhibited IK1 in a concentration-dependent fashion in both groups, a half-maximal concentration was greater in failing (0.12 microM) than in nonfailing hearts (0.023 microM). These results suggest that IK1 in human ventricular myocytes can be inhibited by a PKA-mediated phosphorylation and the modulation is significantly reduced in ventricular myocytes from the failing heart compared to the nonfailing heart.

A novel K+ channel beta-subunit (hKv beta 1.3) is produced via alternative mRNA splicing

Voltage-gated K+ channels can form multimeric complexes with accessory beta-subunits. We report here a novel K+ channel beta-subunit cloned from human heart, hKv beta 1.3, that has 74-83% overall identity with previously cloned beta-subunits. Comparison of hKv beta 1.3 with the previously cloned hKv beta 3 and rKv beta 1 proteins indicates that the carboxyl-terminal 328 amino acids are identical, while unique variable length amino termini exist. Analysis of human beta-subunit cDNA and genomic nucleotide sequences confirm that these three beta-subunits are alternatively spliced from a common beta-subunit gene. Co-expression of hKv beta 1.3 in Xenopus oocytes with the delayed rectifier hKv1.5 indicated that hKv beta 1.3 has unique functional effects. This novel beta-subunit induced a time-dependent inactivation during membrane voltage steps to positive potentials, induced a 13-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 13 +/- 0.5 ms versus 35 +/- 1.7 ms at -40 mV). Most notably, hKv beta 1.3 converted the Kv1.5 outwardly rectifying current voltage relationship to one showing strong inward rectification. These data suggest that Kv channel current diversity may arise from association with alternatively spliced Kv beta-subunits. A simplified nomenclature for the K+ channel beta-subunit subfamilies is suggested.

Anti-minK antisense decreases the amplitude of the rapidly activating cardiac delayed rectifier K+ current

The rapidly and slowly activating delayed rectifier K+ currents (IKr and IKs, respectively), which have different physiological properties have been identified in cardiac cells from several species, including humans. Although expression of the minimal K+ channel protein (minK) cDNA in some systems results in a current resembling IKs, the role of this gene product in channel function remains controversial. In atrial tumor myocytes (AT-1 cells), no IKs is recorded, but minK mRNA is detected, raising the possibility that expression of the minK gene serves an as-yet-unidentified function. In these experiments, AT-1 cells were exposed to antisense oligonucleotides targeting the 5' translation start site of the minK cDNA cloned from an AT-1 library. Cell size, IKr, and L-type and T-type Ca2+ currents were measured 24 to 48 hours after exposure and compared with data in cells exposed to the corresponding sense oligonucleotide or grown in medium only. Antisense oligonucleotide significantly reduced IKr compared with sense and medium-only control cells in 0 of 2 experiments (n = 3 to 6 cells per treatment in each experiment) at 50 nmol/L, 1 of 2 at 250 nmol/L, 6 of 6 at 1000 nmol/L, and 2 of 2 at 10,000 nmol/L. At 1000 nmol/L, maximum tail current in antisense-exposed cells was 2.5 +/- 0.1 pA/pF (mean +/- SEM, n = 28, 6 separate experiment), 6.6 +/- 0.4 pA/pF in sense-exposed cells (n = 27), 5.4 +/- 0.6 pA/pF in medium-only cells (n = 21), and 5.8 +/- 0.7 pA/pF in cells exposed to a random oligonucleotide (n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative description of the E-4031-sensitive repolarization current in rabbit ventricular myocytes

We have measured the E-4031-sensitive repolarization current (IKr) in single ventricular myocytes isolated from rabbit hearts. The primary goal of this analysis was a description of the IKr kinetic and ion transfer properties. Surprisingly, the maximum time constant of this component was 0.8 s at 33-34 degrees C, which is significantly greater than the value of 0.18 s previously reported under similar conditions in the original measurements of IKr from guinea pig ventricular myocytes. The primary, novel feature of our analysis concerns the relationship of the bell-shaped curve that describes the voltage dependence of the kinetics and the sigmoidal curve that describes the activation of IKr. The midpoint of the latter occurred at approximately +10 mV on the voltage axis, as compared to -30 mV for the point on the voltage axis at which the maximum time constant occurred. Moreover, the voltage dependence of the kinetics was much broader than the steepness of the activation curve would predict. Taken together, these results comprise a gating current paradox that is not resolved by the incorporation of a fast inactivated state in the analysis. The fully activated current-voltage relation for IKr exhibited strong inward-going rectification, so much so that the current was essentially nil at +30 mV, even though the channel opens rapidly in this voltage range. This result is consistent with the lack of effect of E-4031 on the early part of the plateau phase of the action potential. Surprisingly, the reversal potential Of /Kr was ~15 mV positive to the potassium ion equilibrium potential,which indicates that this channel carries inward current during the latter part of the repolarization phase of the action potential.

Thyroid status and diabetes modulate regional differences in potassium currents in rat ventricle

1. The rate dependence and recovery kinetics of the Ca(2+)-independent transient (I(t)) and steady-state or 'pedestal' (Iss) outward potassium (K+) currents were studied in single myocytes isolated from epicardial and endocardial regions of rat left ventricles. The whole-cell, suction microelectrode method was used to measure baseline (fully reactivated) I(t), as well as its rate-dependent attenuation. Results from a group of control animals were compared with data from three other groups having an experimentally altered hormonal status. 2. I(t) was significantly smaller in endocardial cells than in epicardial cells, in part due to a very large difference in the recovery kinetics of this current in endocardial cells. This was reflected in a pronounced rate-dependent prolongation of endocardial action potentials. In contrast, the non-inactivating 'pedestal' current, Iss, was very similar in magnitude and showed comparable rate dependence in cells from both epicardium and endocardium. 3. Changing the thyroid status had selective, differential actions on the amplitude and rate dependence of It in epicardial and endocardial cells. Under hypothyroid conditions there was a more pronounced reduction of baseline I(t) in epicardial than in endocardial cells. Moreover, a slowing of the recovery kinetics in epicardial cells resulted in an enhanced attenuation of this current at high rates. Changing thyroid status had no effect on the magnitude or rate dependence of Iss in cells from either region of the left ventricle. 4. Following establishment of hyperthyroid conditions, there was no significant change in I(t) magnitude at baseline. However, when compared with control data, the recovery of I(t) was considerably faster in endocardial cells, and marginally faster in epicardial cells. 5. Streptozotocin-induced diabetic conditions resulted in a much greater attenuation of I(t) in epicardial cells than in endocardial cells. Epicardial action potentials in these conditions showed prominent rate-dependent prolongation. Iss was reduced to a similar extent in cells from these two regions. 6. Our findings demonstrate that altered hormonal status can selectively change the amplitude and kinetics of It in the epi- and endocardium of rat left ventricle. These changes can reduce the epicardial-endocardial gradients in the magnitude and recovery kinetics of It and hence diminish the intrinsic differences in both action potential duration and refractoriness.

Newer developments in the management of atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia encountered in clinical practice. Unlike reentrant supraventricular tachycardia and malignant ventricular tachyarrhythmias, for which highly effective and safe nonpharmacologic therapies are available, the treatment of AF remains controversial and often problematic. Whereas electrical cardioversion restores sinus rhythm in most patients with AF, the maintenance of sinus rhythm often requires membrane-active antiarrhythmic drugs that may increase mortality by inducing ventricular proarrhythmia. The control of ventricular response rate, often associated with oral anticoagulation to prevent thromboembolic complications, is an alternative strategy in AF management. The relative efficacy and risks of these strategies and their respective role in different patient subgroups remain to be established. This article focuses on newer developments in the management of AF, including prospects for improved methods to maintain sinus rhythm, newer approaches to rate control, controversies regarding the use of oral anticoagulation, and novel nonpharmacologic therapies. These newer developments may lead over the next 10 years to a revolution in the management of AF as profound as that produced over the last 10 years by nonpharmacologic therapy of other arrhythmias.

Assembly of voltage-gated potassium channels. Conserved hydrophilic motifs determine subfamily-specific interactions between the alpha-subunits

Voltage-gated potassium (K+) channels are assembled by four identical or homologous alpha-subunits to form a tetrameric complex with a central conduction pore for potassium ions. Most of the cloned genes for the alpha-subunits are classified into four subfamilies: Kv1 (Shaker), Kv2 (Shab), Kv3 (Shaw), and Kv4 (Shal). Subfamily-specific assembly of heteromeric K+ channel complexes has been observed in vitro and in vivo, which contributes to the diversity of K+ currents. However, the molecular codes that mediate the subfamily-specific association remain unknown. To understand the molecular basis of the subfamily-specific assembly, we tested the protein-protein interactions of different regions of alpha-subunits. We report here that the cytoplasmic NH2-terminal domains of Kv1, Kv2, Kv3, and Kv4 subfamilies each associate to form homomultimers. Using the yeast two-hybrid system and eight K+ channel genes, two genes (one isolated from rat and one from Drosophila) from each subfamily, we demonstrated that the associations to form heteromultimers by the NH2-terminal domains are strictly subfamily-specific. These subfamily-specific associations suggest a molecular basis for the selective formation of heteromultimeric channels in vivo.

Efficacy and proarrhythmic hazards of pharmacologic cardioversion of atrial fibrillation: prospective comparison of sotalol versus quinidine

OBJECTIVES

This study compared the efficacy and safety of sotalol and quinidine for conversion and prevention of recurrent atrial fibrillation.

BACKGROUND

Atrial fibrillation is the most common arrhythmia. Pharmacologic therapy has been advocated for both immediate restoration of sinus rhythm and prevention of recurrent atrial fibrillation. Quinidine is the therapeutic mainstay for both purposes, but its safety has recently been questioned. Although sotalol has been used successfully to maintain sinus rhythm after direct current cardioversion, its efficacy in pharmacologically reverting atrial fibrillation has not been examined.

METHODS

Fifty consecutive patients with persistent atrial fibrillation were randomized to receive quinidine or sotalol for up to 7 days to restore sinus rhythm. Patients were followed up for 6 months.

RESULTS

Quinidine was more effective than sotalol in terminating atrial fibrillation (60% vs. 20%, p = 0.009). When nonresponders to drug therapy underwent subsequent direct current cardioversion, total conversion rates in the quinidine and sotalol groups were comparable (88% vs. 68%, p = 0.17), as was the efficacy of the two drugs in preventing recurrent atrial fibrillation. Side effects necessitating drug discontinuation were more often observed with quinidine. No patient receiving sotalol but four patients receiving quinidine had drug-associated arrhythmia (torsade de pointes in three patients, sustained ventricular tachycardia in one patient). Precordial QT dispersion determined on the surface electrocardiogram (ECG) increased with quinidine (mean +/- SD 34 +/- 9 vs. 44 +/- 16 ms, p = 0.02), indicating enhanced inhomogeneity in ventricular repolarization. There was no change in QT dispersion in patients receiving sotalol (36 +/- 18 vs. 40 +/- 17 ms, p = 0.44).

CONCLUSIONS

Quinidine is more effective than sotalol in terminating atrial fibrillation but is associated with more side effects. The proarrhythmic risk may be related to quinidine's propensity to increase disparity in ventricular repolarization. This risk warrants careful ECG monitoring during the 1st 4 to 7 days of therapy. Because most proarrhythmic effects occurred shortly after restoration of sinus rhythm, observation should continue > or = 2 to 3 days after sinus rhythm is reestablished.

The voltage-dependent K+ channel (Kv1.5) cloned from rabbit heart and facilitation of inactivation of the delayed rectifier current by the rat beta subunit

We have isolated a cDNA coding for a delayed rectifier K+ channel (RBKV1.5) from rabbit heart. The amino acid sequence of RBKV1.5 displays a homology to that of other K+ channels of Kv1.5 class. Overall amino acid identity between RBKV1.5 channel and Kv1.5 channel of other species is about 85%. RNA blot analysis revealed the expression of the primary transcript in various rabbit tissues, at the highest level in both the atrium and ventricle. When expressed in Xenopus oocytes, RBKV1.5 current showed a delayed rectifier type characteristics, which was converted to rapidly inactivating currents upon coexpression with a beta subunit.

Basic concepts in cellular cardiac electrophysiology: Part II: Block of ion channels by antiarrhythmic drugs

Antiarrhythmic drugs have relative specificity for blocking each of the major classes of ion channels that control the action potential. The kinetics of block is determined by the state of the channel. Those channel states occupied at depolarized potentials generally have greater affinity for the blocking drugs. The kinetics of the drug-channel interaction is important in determining the blocking profile observed clinically. The increased mortality resulting from drug treatment in CAST and several atrial fibrillation trials has resulted in a shift in antiarrhythmic drug development from the Na+ channel blocking (Class I) drugs to the K+ channel blocking (Class III) drugs. While both Classes of drugs have a proarrhythmic potential, this may be less for the Class III agents. Their lack of negative inotropy also make them more attractive. It is important that the potential advantages of these agents be evaluated in controlled clinical trials. In several laboratories, the techniques of molecular biology and biophysics are being combined to determine the block site of available drugs. This information will aid in the future development of agents with greater specificity, and hopefully greater efficacy and safety than those currently in clinical use.

Alternative splicing of the human Shaker K+ channel beta 1 gene and functional expression of the beta 2 gene product

Mammalian voltage-activated Shaker K+ channels associate with at least three cytoplasmic proteins: Kv beta 1, Kv beta 2 and Kv beta 3. These beta subunits contain variable N-termini, which can modulate the inactivation of Shaker alpha subunits, but are homologous throughout an aldo-keto reductase core. Human and ferret beta 3 proteins are identical with rat beta 1 throughout the core while beta 2 proteins are not; beta 2 also contains a shorter N-terminus and has no reported physiological role. We report that human beta 1 and beta 3 are derived from the same gene and that beta 2 modulates the inactivation properties of Kv1.4 alpha subunits.

Stereoselective block of a human cardiac potassium channel (Kv1.5) by bupivacaine enantiomers

Stereoselective drug-channel interactions may help to elucidate the molecular basis of voltage-gated potassium channel block by local anesthetic drugs. We studied the effects of the enantiomers of bupivacaine on a cloned human cardiac potassium channel (hKv1.5). This channel was stably expressed in a mouse Ltk- cell line and studied using the whole-cell configuration of the patch-clamp technique. Both enantiomers modified the time course of this delayed rectifier current. Exposure to 20 microM of either S(-)-bupivacaine or R(+)-bupivacaine did not modify the activation time constant of the current, but reduced the peak outward current and induced a subsequent exponential decline of current with time constants of 18.7 +/- 1.1 and 10.0 +/- 0.9 ms, respectively. Steady-state levels of block (assessed with 250-ms depolarizing pulses to +60 mV) averaged 30.8 +/- 2.5% (n = 6) and 79.5 +/- 3.2% (n = 6) (p < 0.001), for S(-)- and R(+)-bupivacaine, respectively. The concentration dependence of hKv1.5 inhibition revealed apparent KD values of 27.3 +/- 2.8 and 4.1 +/- 0.7 microM for S(-)-bupivacaine and R(+)-bupivacaine, respectively, with Hill coefficients close to unity, suggesting that binding of one enantiomer molecule per channel was sufficient to block potassium permeation. Analysis of the rate constants of association (k) and dissociation (l) yielded similar values for l (24.9 s-1 vs. 23.6 s-1 for S(-)- and R(+)-bupivacaine, respectively) but different association rate constants (1.0 x 10(6) vs. 4.7 x 10(6) M-1 s-1 for S(-)- and R(+)-bupivacaine, respectively). Block induced by either enantiomer displayed a shallow voltage dependence in the voltage range positive to 0 mV, i.e., where the channel is fully open, consistent with an equivalent electrical distance delta of 0.16 +/- 0.01. This suggested that at the binding site, both enantiomers of bupivacaine experienced 16% of the applied transmembrane electrical field, referenced to the inner surface. Both bupivacaine enantiomers reduced the tail current amplitude recorded on return to -40 mV and slowed their time course relative to control, resulting in a "crossover" phenomenon. These data indicate 1) the charged form of both bupivacaine enantiomers block the hKv1.5 channel after it opens, 2) binding occurs within the transmembrane electrical field, 3) unbinding is required before the channel can close, 4) block of hKv1.5 channels by bupivacaine is markedly stereoselective, with the R(+)-enantiomer being the more potent one, 5) this stereoselective block was associated with a 1.11 -kcal/mol difference in binding energy between both enantiomers, and 6) the stereoselectivity derives mainly from a difference in the association rate constants, suggesting that the S(-)-enantiomer is less likely to access the binding site in an optimal configuration.

Characterization of a voltage-gated K+ channel beta subunit expressed in human heart

Voltage-gated K+ channels are important modulators of the cardiac action potential. However, the correlation of endogenous myocyte currents with K+ channels cloned from human heart is complicated by the possibility that heterotetrameric alpha-subunit combinations and function-altering beta subunits exist in native tissue. Therefore, a variety of subunit interactions may generate cardiac K+ channel diversity. We report here the cloning of a voltage-gated K+ channel beta subunit, hKv beta 3, from adult human left ventricle that shows 84% and 74% amino acid sequence identity with the previously cloned rat Kv beta 1 and Kv beta 2 subunits, respectively. Together these three Kv beta subunits share > 82% identity in the carboxyl-terminal 329 aa and show low identity in the amino-terminal 79 aa. RNA analysis indicated that hKv beta 3 message is 2-fold more abundant in human ventricle than in atrium and is expressed in both healthy and diseased human hearts. Coinjection of hKv beta 3 with a human cardiac delayed rectifier, hKv1.5, in Xenopus oocytes increased inactivation, induced an 18-mV hyperpolarizing shift in the activation curve, and slowed deactivation (tau = 8.0 msec vs. 35.4 msec at -50 mV). hKv beta 3 was localized to human chromosome 3 by using a human/rodent cell hybrid mapping panel. These data confirm the presence of functionally important K+ channel beta subunits in human heart and indicate that beta-subunit composition must be accounted for when comparing cloned channels with endogenous cardiac currents.

Idiopathic atrial fibrillation in dogs: electrophysiologic determinants and mechanisms of antiarrhythmic action of flecainide

OBJECTIVES

This study sought to determine the mechanisms of idiopathic atrial fibrillation and the atrial antifibrillatory action of flecainide in dogs.

BACKGROUND

In a small subset of dogs, sustained atrial fibrillation can be readily induced in the absence of vagal tone. The electrophysiologic mechanisms underlying this ability to sustain atrial fibrillation, and of flecainide action on the arrhythmia, are unknown.

METHODS

Six dogs with inducible sustained atrial fibrillation were studied before and after flecainide administration and compared with a control group of 10 dogs.

RESULTS

Dogs with atrial fibrillation differed in displaying more shortening of the atrial refractory period with increased rate, resulting in a significantly shorter refractory period and wavelength for reentry at rapid rates, and in increased regional dispersion in refractoriness. Activation maps during sustained fibrillation showed a mean (+/- SE) of 6.3 +/- 0.4 coexistent zones of reentry, compatible with short wavelengths, whereas in control dogs activation during self-limited atrial fibrillation was better organized, and the number of reentrant circuits was smaller. Quantitative analysis demonstrated significantly greater inhomogeneity of activation during atrial fibrillation in dogs with atrial fibrillation than in control animals. Flecainide terminated atrial fibrillation by increasing the duration and homogeneity of atrial refractoriness at rapid rates, thereby reducing the number of reentry circuits and the heterogeneity of activation.

CONCLUSIONS

The ability of atrial fibrillation to sustain itself resulted from enhanced rate-dependent shortening of atrial refractoriness and increased regional heterogeneity. Flecainide reversed these changes and restored sinus rhythm. These results suggest potential mechanisms of idiopathic atrial fibrillation and are pertinent to understanding the clinical actions of flecainide.

Localization of the Kv1.5 K+ channel protein in explanted cardiac tissue

The cloned Kv1.5 K+ channel displays similar kinetics and pharmacology to a delayed rectifier channel found in atrial myocytes. To determine whether the Kv1.5 isoform plays a role in the cardiac action potential, it is necessary to confirm the expression of this channel in cardiac myocytes. Using antibodies directed against two distinct channel epitopes, the Kv1.5 isoform was localized in human atrium and ventricle. Kv1.5 was highly localized at intercalated disk regions as determined by colocalization with connexin and N-cadherin specific antibodies. While both antichannel antibodies localized the Kv1.5 protein in cardiac myocytes, only the NH2-terminal antibodies stained vascular smooth muscle. The selective staining of vasculature by this antiserum suggests that epitope accessibility, and perhaps channel structure, varies between cardiac and vascular myocytes. Kv1.5 expression was localized less in newborn tissue, with punctate antibody staining dispersed on the myocyte surface. This increasing organization with age was similar to that observed for connexin. Future work will address whether altered K+ channel localization is associated with cardiac disease in addition to changing with development.

Effects of sematilide, a novel class III antiarrhythmic agent, on action potential in guinea pig atrium

Electrophysiological effects of sematilide, a novel class III antiarrhythmic agent, were examined and compared with those of (+/-)sotalol in guinea pig left atrium by a conventional microelectrode technique. Application of 0.1-1000 microM sematilide or 1-1000 microM (+/-)sotalol concentration-dependently prolonged the duration of action potentials (APD) that were elicited by electrical stimulation at 1 Hz. Other parameters of action potentials such as the maximum upstroke velocity of phase 0 depolarization, action potential amplitude and resting membrane potential were not affected significantly by these drugs in the concentration ranges employed. The prolongation of APD by sematilide or (+/-)sotalol was accompanied by a corresponding increase in the effective refractory period (ERP). Approximately a 30% increase in ERP was obtained by the treatment with 5 microM sematilide or 100 microM (+/-)sotalol, suggesting that sematilide as a class III antiarrhythmic agent is approximately 20 times more potent than (+/-)sotalol on a molecular basis. When the stimulation rate was increased stepwise from 0.2 to 2 Hz, the relative increase in APD at 90% repolarization by the treatment with sematilide and (+/-)sotalol was slightly larger at 2 Hz than at 0.2 Hz, indicating that "reverse rate-dependence" was not observed under these conditions. These results may suggest a possibility that sematilide effectively blocks atrial arrhythmia.

Mechanism of block of a human cardiac potassium channel by terfenadine racemate and enantiomers

1. The cardiac toxicity of racemic terfenadine (marked QT prolongation and polymorphic ventricular arrhythmias) is probably due to potassium channel blockade. To test whether one of its enantiomers would be a less efficient potassium channel blocker, we compared the mechanism of action of the racemate with that of the individual enantiomers. 2. We synthesized the individual enantiomers of terfenadine and examined under whole cell voltage-clamp conditions the mechanism of action of the racemate, both enantiomers and a major metabolite on a cloned human cardiac potassium channel, hKv1.5. This delayed rectifier is sensitive to quinidine, clofilium and other 'class III' antiarrhythmic drugs at clinically relevant concentrations. 3. Upon depolarization, racemic terfenadine and its enantiomers induced a fast decline of hKv1.5 current towards a reduced steady state current level. During subsequent repolarization the tail currents deactivated more slowly than the control, resulting in a 'crossover' phenomenon. 4. The voltage-dependence of block was biphasic with a steep increase in block over the voltage range of channel opening (-30 to 0 mV), and a more shallow phase positive to 0 mV (where the channel is fully open). The latter was consistent with a binding reaction sensing 21% of the transmembrane electrical field (with reference to the cell interior). 5. The EC50 for hKv1.5 block by racemic terfenadine was 0.88 microM, while the values for R- and S-terfenadine were 1.19 microM and 1.16 microM, respectively. In contrast, the acid metabolite reduced hKv1.5 current by only 5% at a concentration of 50 microM. 6. These findings suggest that terfenadine blocks the hKvl.5 channel after it opens by entering into the internal mouth of the channel. We have previously shown that quinidine blocks hKvl.5 in a similar manner but with an apparent affinity of ~6 micro M. Thus, terfenadine and its enantiomers are approximately equipotent open state blockers of this human K+ channel and about 6 times more potent than quinidine. The similar state-, time-, and voltage-dependence of hKvl.5 block by both enantiomers also indicates that the chiral centre does not significantly constrain the orientation of critical binding determinants of terfenadine with respect to the receptor site.

Electrophysiologic mechanisms of the long QT interval syndromes and torsade de pointes

PURPOSE

To review the current understanding of the mechanisms and treatment of the long QT interval syndromes and torsade de pointes.

DATA SOURCES

Personal databases of the authors and a search of the MEDLINE database from 1966 to 1994.

STUDY SELECTION

Experimental and clinical studies and topical reviews on the electrophysiologic mechanisms and treatment of torsade de pointes were analyzed.

RESULTS

The long QT interval syndromes have been classified into acquired and hereditary forms, both of which are associated with a characteristic type of life-threatening polymorphic ventricular tachycardia called torsade de pointes. The acquired form is caused by various agents and conditions that reduce the magnitude of outward repolarizing K+ currents, enhance inward depolarizing Na+ or Ca2+ currents, or both, thereby triggering the development of early afterdepolarizations that initiate the tachyarrhythmia. The hereditary form appears to result from an abnormal response to adrenergic or sympathetic nervous system stimulation. At least some cases of the hereditary long QT interval syndromes may result from a single gene defect that alters the intracellular regulatory proteins responsible for the modulation of K+ channel function. Treatment of the acquired form is primarily directed at identifying and withdrawing the offending agent, although emergent therapy using maneuvers and agents that favorably modulate transmembrane ion currents can be lifesaving. In torsade de pointes associated with the hereditary long QT interval syndromes, early diagnosis leading to treatments designed to both shorten the QT interval and block the beta-adrenergic-induced instability of the QT interval is essential.

CONCLUSIONS

The long QT interval syndromes and torsade de pointes are potentially life-threatening conditions caused by various agents, conditions, and genetic defects. The mechanisms responsible for these conditions and available treatment options for them are reviewed.

Molecular cloning and functional expression of a novel potassium channel beta-subunit from human atrium

We report the cloning and functional expression of a novel K+ channel beta-subunit from human atrium, hKv beta 3. hKv beta 3 is highly homologous to the two beta-subunits cloned from rat brain, Kv beta 1 and Kv beta 2, but has an essentially unique stretch of 79 N-terminal residues. Upon expression in Xenopus oocytes, hKv beta 3 accelerates the inactivation of co-injected hKv1.4 currents and induces fast inactivation of non-inactivating co-injected hKv1.5 currents. By contrast, hKv beta 3 had no effect on hKv1.1, hKv1.2, or hKv2.1 currents. Thus, hKv beta 3 represents a third type of K+ channel beta-subunit which modulates the kinetics of a unique subset of channels in the Kv1 subfamily.

Modulation of an inactivating human cardiac K+ channel by protein kinase C

The transient outward current (ITO) is an important repolarizing component of the cardiac action potential. In native cardiac myocytes, ITO is modulated after activation of protein kinase C, although the molecular nature of this effect is not well understood. A channel recently cloned from human ventricular myocardium (Kv1.4, HK1) produces a rapidly inactivating K+ current, which has phenotypic similarities to the 4-aminopyridine-sensitive component of ITO. Therefore, we examined whether this recombinant channel was also modulated by protein kinase C activation by investigating the effects of the diacylglycerol analogue phorbol 12-myristate 13-acetate (PMA) on Kv1.4 K+ current expressed in Xenopus oocytes. At a concentration of 10 nmol/L, PMA caused a biphasic response with an initial increase (14 +/- 4%, mean +/- SEM) in current, which peaked in 14 minutes. This was followed by a significant reduction (40 +/- 11%) in the current within 30 minutes. There was no significant change in cell membrane electrical capacitance with 10 nmol/L PMA (1 +/- 1% decline in 30 minutes), demonstrating that loss of cell membrane surface area did not explain the reduction in K+ current, although cell capacitance did decrease when using a higher concentration of PMA (81 nmol/L). The inactive stereoisomer, 4 alpha-PMA, had no effect on Kv1.4 current, whereas preincubation with the protein kinase inhibitor staurosporine or protein kinase C-selective chelerythrine prevented the effects of PMA. When purified from a stably transfected mammalian cell line by using immunoprecipitation, the channel protein was readily phosphorylated in vitro by purified protein kinase C.(ABSTRACT TRUNCATED AT 250 WORDS)

Flecainide

Flecainide is a Class IC antiarrhythmic agent whose primary electrophysiologic effect is a slowing of conduction in a wide range of cardiac tissues. It is well absorbed and effective in suppressing isolated premature ventricular contractions (PVCs) or nonsustained ventricular arrhythmia but has only a modest efficacy when electrophysiologic testing is used as an endpoint. Its adverse effect on mortality in the CAST trial suggested a propensity to proarrhythmia--a phenomenon to which the Class IC agents appear particularly prone. Despite the applicability of the CAST study only to patients with a prior myocardial infarction, there has been a shift away from flecainide in ventricular arrhythmia, but the low noncardiac side effect profile of the agent allows for its continued use in a wide variety of supraventricular arrhythmias.

Effects of lisinopril on cardiac contractility and ionic currents

1. The effects of lisinopril, an angiotensin-converting enzyme inhibitor, were studied on cardiac contractile force, action potential characteristics and membrane ionic currents. 2. In guinea-pig atria, lisinopril (0.001-1 microM) exerted a negative inotropic effect which was accompanied by a shortening of the time to peak tension and time for total contraction. However, it did not modify atrial rate or the characteristics of the ventricular action potentials recorded either in normally polarized or in depolarized papillary muscles. 3. In isolated guinea-pig ventricular myocytes, lisinopril had no effect on the inward L-type Ca2+ (ICa,L), the inward rectifier (IK1) or the delayed rectifier K+ currents (IK), but abolished the stimulation-dependent facilitation of the ICa,L. Furthermore, it did not alter a cloned human cardiac K+ current (hKv1.5) expressed in a mouse L cell line (Ltk-). 4. The absence of negative inotropic effects in patients with congestive heart failure can be explained by the potent arterial vasodilator action of lisinopril which reduced left ventricular afterload overriding the expected direct cardiodepressant effects of the drug.

Computer aided development of antiarrhythmic agents with class IIIa properties

Most antiarrhythmic agents were discovered accidentally. In the last decade, the understanding of the mechanisms of action of agents with electrophysiologic activity has progressed greatly. As a result, it was possible to compute, before the CAST trial, that the agents selected for the trial would not be effective against tachycardias and that the drugs would be unsafe. Extension of these computations to existing Class I agents indicated that they were all poor suppressors of ventricular tachycardia. Furthermore, a Class I agent with an optimal electrophysiologic profile still computes to be a two-edged sword, possessing both antiarrhythmic and proarrhythmic properties. Fortunately, it is possible to conceive of drug profiles that would be purer antiarrhythmic agents. For example, a drug that only upon the development of a tachycardia lengthens action potential duration in a use-dependent manner until the refractory period exceeds the tachycardia cycle length will render continuation of the tachycardia impossible. Recognition of chemicals that have Class IIIa properties with the appropriate kinetics is a challenging task. However, today's microprocessors have become powerful enough to characterize the Class III kinetics. A system that fully automatically screens for effective antiarrhythmic agents is described. It is expected that chemicals selected for optimal basic electrophysiologic properties will yield safer and more effective antiarrhythmic agents.

Observations from intraatrial recordings on the termination of electrically induced atrial fibrillation in humans

BACKGROUND

The circulating wavelet hypothesis suggests that atrial fibrillation could terminate by either progressive fusion or simultaneous block of all wavelets.

METHODS

Intraatrial recordings from the right atrial free wall were made during procainamide induced (n = 8) or spontaneous (n = 7) termination of electrically induced atrial fibrillation in 14 patients. Atrial rate, mean magnitude squared coherence, and direction of activation during sequential electrograms were measured. Rate and coherence were calculated from the earliest point within 5 minutes prior to termination as well as from the 4-second interval just prior to termination.

RESULTS

Termination was directly to sinus rhythm (13 episodes) or to atrial flutter (2 episodes). For the eight procainamide induced terminations, rate decreased between the first measurement and the measurement just prior to termination, from 443 +/- 127 beats/min to 322 +/- 119 beats/min. For the seven spontaneous terminations, rate also decreased from 373 +/- 119 beats/min to 323 +/- 88 beats/min; however, a slight increase in atrial rate prior to termination was observed in three episodes. No specific patterns of atrial cycle lengths were seen during the final few seconds of fibrillation. No increase in coherence was observed. In seven episodes, recordings were made using orthogonal bipoles in the x, y, and z directions, allowing direction of activation of wavefronts to be measured. Three episodes showed multiple instances where direction of activation remained similar over several electrograms as we have previously reported for chronic fibrillation. However, no such instances precipitated termination in any of the seven episodes.

CONCLUSIONS

Atrial fibrillation usually terminates directly to sinus rhythm and does so abruptly and without forewarning. While we and others have previously reported that the rate of atrial fibrillation decreases with procainamide infusion, a decrease in the rate of atrial fibrillation is not required for the rhythm to terminate and consequently may not be a part of the termination process at all. Coherence does not demonstrate a progressive increase in the organization of atrial fibrillation prior to termination. Lack of stabilization in the direction of activation of wavefronts in the final few seconds also fails to support fusion of wavefronts as the mechanism of termination of atrial fibrillation. Simultaneous block of all wavelets is consistent with, but not proven by, our observations.

Adrenergic Modulation of Potassium Currents in Isolated Human Atrial Myocytes

The adrenergic modulation of inwardly rectifying and depolarization-activated outward potassium currents was studied in single cardiac myocytes obtained from the human atrium. Membrane currents were recorded in enzymatically dissociated cells using the whole-cell voltage-clamp technique. It was observed that, in the presence or absence of atenolol (or 1 &mgr;M propranolol), 30 &mgr;M phenylephrine attenuated inwardly rectifying and depolarization-activated outward potassium currents including both transient and late-activated current. This suppressant effect of phenylephrine could be prevented by pretreatment with an alpha-adrenoceptor antagonist. Isoproterenol (30 &mgr;M) increased the late outward potassium current and net transient outward current. It is concluded that, in human atrial myocytes, alpha-adrenergic activation reduces depolarization-activated transient and late outward potassium current and inwardly rectifying background potassium current. beta-Adrenergic activation resulted in an increase in the depolarization-activated transient and late outward potassium current. Copyright 1994 S. Karger AG, Basel

Molecular biology of voltage-gated K+ channels in heart

The recent cloning of numerous voltage-activated K+ channels provides new information concerning the architecture of K+ channel proteins. The combination of molecular genetic and biophysical methods gives us a new insight into the molecular mechanisms of K+ channel pharmacology.

K+ currents expressed from the guinea pig cardiac IsK protein are enhanced by activators of protein kinase C

We have isolated cardiac cDNA and genomic clones encoding the guinea pig IsK protein. The deduced amino acid sequence is approximately 78% identical to the rat, mouse, and human variants of this channel, and the structure of the gene encoding the protein is also similar to that in other species. For example, the gene is present only once in the haploid genome, the protein-coding sequence is present on a single uninterrupted exon, an intron exists in the 5' untranslated domain, and multiple alternative polyadenylation sites are used in processing the transcript. Expression of the guinea pig protein in Xenopus oocytes results in a slowly activating, voltage-dependent K+ current, IsK, similar to those expressed previously from the rat, mouse, and human genes. However, in sharp contrast to the rat and mouse currents, activation of protein kinase C with phorbol esters increases the amplitude of the guinea pig IsK current, analogous to its effects on the endogenous IKs current in guinea pig cardiac myocytes. Mutagenesis of the guinea pig cDNA to alter four cytoplasmic amino acid residues alters the phenotype of the current response to protein kinase C from enhancement to inhibition, mimicking that of rat and mouse IsK currents. This mutation is consistent with reports that phosphorylation of Ser-102 by protein kinase C decreases the current amplitude. These data explain previously reported differences in the regulatory properties between recombinant rat or mouse IsK channels and native guinea pig IKs channels and provide further evidence that the IsK protein forms the channels that underlie the IKs current in the heart.

A controlled study on oral propafenone versus digoxin plus quinidine in converting recent onset atrial fibrillation to sinus rhythm

Eighty-seven patients with recent onset atrial fibrillation (< or = 8 days) without clinical signs of heart failure were randomly allocated to one of the following treatments: (i) oral propafenone (600 mg as a loading dose followed after 8 h by 300 mg t.i.d.); (ii) intravenous digoxin as acute scheme (up to 1.125 mg/24 h) followed after 6 h by hydroquinidine chlorhydrate (total dose, 1350 mg); or (iii) placebo. The patients were submitted to Holter monitoring for 48 h.

RESULTS

propafenone achieved higher successful conversion rates at 6, 12 and 24 h compared either with placebo (62% vs. 17%, 83% vs. 34%; 86% vs. 55%; P < 0.01, respectively) or with digoxin at 6 h (62% vs. 38%; P < 0.05) and digoxin plus quinidine at 12 h (83% vs. 48%; P < 0.05). At 48 h, a placebo conversion rate of 76% was observed with consequent lack of any significant difference with the active treatments. Mean conversion times within 48 h were 267 +/- 238 min for propafenone, 648 +/- 631 min for digoxin plus quinidine (P < 0.01 vs. propafenone) and 893 +/- 622 min for placebo (P < 0.001 vs. propafenone). Propafenone and digoxin plasma levels were within the therapeutic range. Asymptomatic phases of atrial flutter with > or = 2:1 atrio-ventricular conduction ratio were observed during Holter monitoring, before conversion to sinus rhythm, in four patients treated with propafenone, in one patient taking digoxin plus quinidine and in four patients with placebo.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiarrhythmic drug classifications and the clinician: a gambit in the land of chaos

Several classifications of antiarrhythmic drugs have appeared and all suffer considerable limitations. Recently a Task Force of the Working Group on Arrhythmias of the European Society of Cardiology proposed a novel classification of antiarrhythmic drugs (the so-called Sicilian Gambit) based on their action on the most vulnerable parameter of an arrhythmogenic mechanism. The present article attempts a critical reappraisal of the antiarrhythmic drug actions and the relationship of vulnerable parameters with cellular mechanisms such as ion channels. The clinical applicability of these concepts, the implications of the new classification in the pharmacologic therapy of arrhythmias, and its potential limitations are discussed.

Effects of disopyramide and flecainide on the kinetics of inward rectifier potassium channels in rabbit heart muscle

1. Standard patch-clamp techniques were used to study the interaction of therapeutic concentrations of flecainide and disopyramide with single inwardly-rectifying potassium channels in cell-attached membrane patches from rabbit ventricular myocytes. 2. Under drug-free conditions, the potassium channels had a conductance of 31 +/- 2 pS (n = 13), a mean open time of 230 +/- 6 ms (n = 11) recorded at the resting cell potential, and an open probability of 0.66 +/- 0.20 (n = 39). The resting potential of the cells studied was -68.5 +/- 3.6 mV (n = 32). 3. Disopyramide did not reduce the open probability of the channel when the cell was voltage-clamped at the resting cell potential. However, disopyramide increased the mean open time of the channel, recorded at the resting cell potential, by 15% at 5 microM and by 29% at 20 microM. The action potential prolonging actions of disopyramide in therapeutic concentrations appear not to be due to blocking the inward rectifier K+ channel. 4. Flecainide (3.0 microM, but not at 0.5 microM) decreased the open probability without changing the conductance of the channel, at 3 microM (51.0 +/- 7.2%, n = 6, P = 0.03) at the resting cell potential. Flecainide increased the mean open time of the channel, recorded at the resting cell potential, by 12% at 3.0 microM. 5. We propose that flecainide stabilized the inward rectifier K+ channel in an inactivated state, without plugging the conducting pore. In addition, it appeared to bind to an open conformation of the channel,since some of the reduction in open probability could be accounted for by the lengthening of the mean open time. The changes in open-state kinetics suggest that this binding may be in the region of the activation gate.

Why do some patients have high defibrillation thresholds at defibrillator implantation? Answers from basic research

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients.

Endocardial mapping of reentry around an anatomical barrier in the canine right atrium: observations during the action of the Class IC agent, flecainide

INTRODUCTION

Flecainide is effective in terminating stable atrial flutter in the conscious dog with a Y-shaped right atrial lesion. In this model, flutter is due to circus movement of the impulse around a fixed anatomical barrier.

METHODS AND RESULTS

To investigate the mechanism of flecainide-induced termination of this type of reentry, we determined the pattern of endocardial activation of the right and left atria before and during administration of flecainide by recording simultaneously from 192 electrode pairs in the isolated blood perfused heart. At least five consecutive flutter beats were analyzed before and during flecainide for each of eight termination episodes in five hearts. In all, flecainide increased flutter cycle length (164 +/- 24 msec) by 89% to 309 +/- 77 msec (P < 0.05) before termination. Atrial refractory period and conduction time during paced beats were also increased by flecainide. In five episodes, termination was due to conduction block of the impulse at critical sites within the reentrant circuit (mode 1). Cycle length oscillations (+/- 30 msec) at sites proximal to site of block preceded termination in three of these episodes. In three other episodes, interruption of the original circuit occurred when there was failure of a lateral boundary, giving rise to an impulse that reset the original circuit (mode 2). In these episodes, long-short cycle length oscillations led to return reexcitation by the impulse within the primary path and subsequent termination.

CONCLUSION

In summary, similar to our previous findings with the Class III agent, d-sotalol, two different modes of termination of atrial reentry were observed with flecainide.

Choice and chance in drug therapy of cardiac arrhythmias: technique versus drug-specific responses in evaluation of efficacy

Numerous recent advances in pharmacotherapy for arrhythmia have necessitated a reorientation in terms of choice of specific agents, techniques for predicting drug effects, and the endpoints for judging therapeutic efficacy. For the management of ventricular arrhythmias and preventing mortality, several trends are becoming clear. It is unlikely that sodium channel blockers will continue to play a major role, except in patients with structurally normal hearts. Emphasis is shifting way from class I agents to those that act by prolonging repolarization without effect on conduction. These latter agents have been termed pure class III agents and have been developed because of the clinical experience with sotalol and amiodarone. On the other hand, there is compelling evidence that sympathetic inhibition per se (as exemplified by beta blockers) or as an integral component of more complex molecules (e.g., sotalol, amiodarone) is a critical feature of desirable antifibrillatory agents. Thus, compared with D,L-sotalol or amiodarone, pure class III agents are likely to be much less effective and may need to be used in combination with antiadrenergic compounds. Compared with amiodarone, they are likely to induce a higher incidence of torsades de pointes, especially in the case of concomitant diuretic therapy. Therapy guided by programmed electrical stimulation or Holter monitoring is likely to play a diminishing role in the development of antiarrhythmic drug regimens, and thus an antiarrhythmic agent's effectiveness may need to be evaluated against the background of implantable cardioverter-defibrillators or against amiodarone therapy. There is increasing evidence that "guided" therapy may simply identify responders from nonresponders and objective endpoints of therapy may be influenced more by drug-specific responses than by the techniques used for their selection. The data raise the issue whether in the future, therapy for ventricular tachycardia or fibrillation might be chosen empirically but from a limited range of compounds, such as beta blockers, amiodarone, sotalol, and possibly certain pure class III agents that are presently under development. Although it is reasonably certain that there is a need to shift from delaying conduction as a means for treating arrhythmias to one that entails prolongation of repolarization, it remains to be determined what might be the characteristics of an ideal antifibrillatory compound. The greatest promise is the area of complex molecules with a diversity of electrophysiologic actions, as exemplified by amiodarone and similar compounds that have the property of blunting sympathetic excitation. The complexity of their electrophysiologic and pharmacodynamic properties might provide a more favorable match with the vulnerable substrate for reducing electrical instability, thereby preventing ventricular fibrillation.

Electrophysiologic basis for the antiarrhythmic actions of sotalol and comparison with other agents

Although synthesized as a nonselective beta-adrenergic blocking compound, sotalol has emerged as the prototype of the so-called class III antiarrhythmic compounds. It delays cardiac repolarization by inhibiting the delayed rectifier potassium current, having a lesser effect on the inward rectifying potassium current with little or no effect on the inward calcium or sodium currents. This property of prolonging repolarization with an accompanying increase in the effective refractory period is not due to blockade of the beta-adrenergic receptors. The major electrophysiologic profile of sotalol constitutes the summed effects of beta blockade and prolonged repolarization. Sotalol exerts a potent antifibrillatory action modulated by its antiadrenergic effects. It suppresses premature ventricular contractions and nonsustained ventricular tachycardia while preventing inducible ventricular tachycardia and fibrillation in patients with advanced structural heart disease. The compound is therefore likely to exert a broad spectrum of antiarrhythmic actions in ventricular arrhythmias.

Delayed rectifier outward current and repolarization in human atrial myocytes

Previous work has suggested that the primary time-dependent repolarizing current in human atrium is the transient outward current (Ito), but interventions known to alter the magnitude of the delayed rectifier current (IK) affect atrial electrophysiology and arrhythmias in humans. To explore the potential role of IK in human atrial tissue, we used the whole-cell configuration of the patch-clamp technique to record action potentials and ionic currents in isolated myocytes from human atrium. A delayed outward current was present in the majority of myocytes, activating with a time constant ranging from 348 +/- 61 msec (mean +/- SEM) at -20 mV to 129 +/- 25 msec at +60 mV. The reversal potential of tail currents was linearly related to log [K+]o with a slope of 55 mV per decade, and fully activated tail currents showed inward rectification. The potassium selectivity, kinetics, and voltage dependence were similar to those reported for IK in other cardiac preparations. In cells with both Ito and IK, IK greatly exceeded both components of Ito (Ito1 and Ito2) within 50 msec of a voltage step from -70 to +20 mV. Based on the relative magnitude of Ito and IK, three types of cells could be distinguished: type 1 (58% [73/126] of the cells) displayed a large Ito together with a clear IK, type 2 (13% [17/126] of the cells) displayed only IK, and type 3 (29% [36/126] of the cells) was characterized by a prominent Ito and negligible IK. Consistent differences in action potential morphology were observed, with type 2 cells having a higher plateau and steeper phase 3 slope and type 3 cells showing a triangular action potential and lesser phase 3 slope compared with type 1 cells. We conclude that IK is present in a majority of human atrial myocytes and may play a significant role in their repolarization and that previously observed variability in human atrial action potential morphology may be partially due to differences in the relative magnitude of time-dependent outward currents.

Direct and autonomically mediated effects of oral quinidine on RR/QT relation after an abrupt increase in heart rate

OBJECTIVES

This study evaluates the direct and autonomically mediated effects of oral quinidine on ventricular repolarization in humans.

BACKGROUND

Interactions between quinidine-related vagolytic properties and autonomic modulation on ventricular repolarization are unknown. The relative role of the two components, if present, might improve our understanding of the therapeutic and proarrhythmic mechanisms of quinidine on the ventricular tissue.

METHODS

Rate-related changes in the QT interval were investigated after an abrupt increase in heart rate in 15 patients during atrial pacing. In the control study, the QT interval was measured at six paced cycle lengths (600, 540, 500, 460, 430 and 400 ms) both in the basal state and after autonomic blockade (intravenous propranolol, 0.2 mg/kg, and intravenous atropine, 0.04 mg/kg); oral quinidine was then administered at a daily dosage of 1,200 mg for 3 to 4 days, after which the QT duration was reassessed using the same method in a second study.

RESULTS

During the control study, the mean slope of the regression curve estimating the correlation between pacing cycle length and QT duration was significantly lower after autonomic blockade (0.14 +/- 0.05) than in the basal state (0.27 +/- 0.10, p < 0.05). Quinidine exhibited a prominent but opposite effect on the mean slope of the regression curves in basal conditions (from 0.27 +/- 0.10 to 0.20 +/- 0.07, p < 0.05) and after withdrawal of autonomic modulation (from 0.14 +/- 0.05 to 0.19 +/- 0.05, p < 0.05), thus annulling the differences observed between the two states in the control study.

CONCLUSIONS

A quinidine-induced increase in QT duration as cycle length is prolonged is consistent with a reverse use dependence effect on ventricular repolarization. This effect is not evident in the basal state owing to interaction of quinidine-related vagolytic effect with the autonomic tone. Reverse use dependence and vagolytic activity on ventricular tissue indicate two potentially undesirable effects that could play a role in the lack of efficacy or proarrhythmic effect of quinidine.

Electrophysiological effects of flecainide acetate on stretched guinea pig left atrial muscle fibers

The electrophysiological effects of flecainide acetate (3 x 10(-6) M) on stretched atrial tissue were investigated using guinea-pig left atrial muscle fibers. Before stretching, the resting membrane potential was not affected by flecainide at 1 Hz, although the overshoot potential (Eov) and the action potential duration at 50% repolarization (APD50) were slightly but significantly decreased by 2 +/- 1 mV and 2 +/- 1 msec, respectively. The effective refractory period (ERP) was increased by 3 +/- 1 msec. The reduction of Vmax was 20.6 +/- 1.2%. The half-maximum potential (Vh) of the relationship between Vmax and the resting potential was shifted to become more negative by flecainide (from -60.6 +/- 2.1 mV to -63.2 +/- 1.7 mV). After 90-120 min of washout with drug-free Tyrode's solution, the tissue was mechanically stretched to 150% of its slack length. Stretching significantly decreased the Vmax by 16.9 +/- 3.1%, along with a slight but significant increase in ERP (3 +/- 1 msec) and shifted Vh to become more negative (from -60.6 +/- 2.1 to -63.1 +/- 1.8 mV). In the presence of flecainide, Vmax further decreased by 20.2 +/- 2.6%, and Vh shifted from -63.1 +/- 1.8 to -65.0 +/- 1.5 mV. Comparison with the control unstretched fibers showed that flecainide significantly decreased Vmax by 34.0 +/- 2.7%, reduced the resting membrane potential by 3 +/- 1 mV, decreased Eov by 4 +/- 1 mV, and shifted Vh from -60.6 +/- 2.1 to -65.0 +/- 1.5 mV, while the APD50 and ERP did not change. In conclusion, the reduction of Vmax in the presence of flecainide was much greater in the stretched atrial muscle fibers than in the unstretched fibers, because the Vmax-resting potential relationship was shifted towards more negative potentials by both flecainide and stretching. These results suggest that flecainide exerts a stronger antiarrhythmic action on stretched atrial muscle fibers than on normal fibers.

Propafenone versus sotalol for suppression of recurrent symptomatic atrial fibrillation

Because conventional antiarrhythmic therapy is often ineffective in maintaining sinus rhythm or is associated with adverse side effects in patients with atrial fibrillation (AF), there is a clinical need to test newer agents. One hundred patients with AF who had unsuccessful therapy with 1.9 +/- 1.0 type IA antiarrhythmic agents were randomized to receive either propafenone (n = 50) or sotalol (n = 50). Patients were stratified into 4 groups based on AF pattern (chronic vs paroxysmal) and left atrial size (large [> or = 4.5 cm] vs small [< 4.5]). The proportion of patients remaining in sinus rhythm on each agent was calculated for each group by the Kaplan-Meier method. For patients randomized to propafenone, 46 +/- 8%, 41 +/- 8% and 30 +/- 8% remained in sinus rhythm at 3, 6 and 12 months, respectively, after cardioversion. A similar proportion of patients treated with sotalol remained in sinus rhythm at follow-up (49 +/- 7%, 46 +/- 8% and 37 +/- 8% at 3, 6 and 12 months, respectively; p = NS). The proportion of patients remaining in sinus rhythm on propafenone and sotalol was not dependent on arrhythmia pattern or left atrial dimension. Except for constipation that occurred more frequently in patients treated with propafenone, adverse side effects were equally distributed between the 2 therapies. Two patients receiving sotalol died during follow-up. Propafenone and sotalol, 2 new antiarrhythmic agents, were found to be equally effective in maintaining sinus rhythm in 100 patients with recurrent AF. Response rates were not affected by arrhythmia pattern, left atrial size or unsuccessful prior drug therapy.(ABSTRACT TRUNCATED AT 250 WORDS)

Rate-dependence of class III actions in the heart

The pharmacodynamics of many antiarrhythmic drugs are altered by heart rate. The ability of sodium channel blockers to decrease conduction velocity (class I action) is more pronounced with rapid heart rates. Drugs with class III action increase action potential duration and refractoriness in the heart. Most drugs with class III actions, currently being developed, produce their action by blocking one or several of the potassium channels responsible for repolarization. In vitro and in vivo studies have shown that their ability to increase repolarization time is less pronounced, or even disappears, at rapid pacing or heart rates. This so called 'inverse' rate-dependence of class III action is a characteristic of all drugs currently used in man except amiodarone, for which prolongation of repolarization time persists to a limited extent with rapid heart rates. It has been suggested that one possible mechanism of the inverse rate-dependence of class III action is related to the preferential binding of drugs to the potassium channels in the closed, polarized state. An inverse rate-dependence of class III action has also been found on prolongation of refractoriness. However, preliminary studies suggest that the positive inotropism of class III drugs not only persists but may increase with rapid heart rates. The clinical consequences of this phenomenon remain unclear, especially in view of the fact that the rate-dependence of class III action on dispersion of repolarization has not been specifically studied and that class III actions tend to decrease in ischemic tissues. However, the increase of action prolongation at slow heart rates may contribute to the bradycardia-dependent development of torsades de pointes arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of quinidine and flecainide on plateau duration of human atrial action potential

Quinidine and flecainide, two class-I antiarrhythmics increase action potential duration (APD) at 90% repolarization and cellular refractory period in human atrial fibers without significant change in resting potential. On the other hand, quinidine decreases APD at 50%, whereas flecainide slightly increases, which suggests different effects on Ca2+ current. Using isolation cell procedure and whole cell recording, we found that 10 microM quinidine (34.77 +/- 6.5%, n = 5) and 0.5 microM flecainide (50.46 +/- 6.2%, n = 4) decrease calcium current in human atrium. It is concluded that, at therapeutical concentrations, quinidine and flecainide modify the action potential plateau phase in a different manner, which is not only related to the calcium current decrease.