Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

Intravenous propafenone: efficacy and safety in the conversion to sinus rhythm of recent onset atrial fibrillation--a single-blind placebo-controlled study

The effectiveness of intravenous propafenone for conversion to sinus rhythm (SR) of paroxysmal atrial fibrillation (AF), lasting less than 7 days, was evaluated with a single-blind, randomized, placebo-controlled study, given the possible spontaneous conversion of this arrhythmia. Group 1 (98 patients) received intravenous propafenone (2 mg/kg iv over 10 minutes followed by 0.007 mg/kg/min); and group 2 (84 patients) received intravenous placebo (0.9% saline solution). The infusion was continued until restoration of SR but no longer than 24 hours. Eight-nine patients (90.8%) received propafenone and 27 patients (32%) receiving placebo were converted to SR (p < 0.005). The mean conversion time was 2.46 +/- 2.59 hours in group 1 and 17.15 +/- 5.78 hours in group 2 (p < 0.005). In patients treated with propafenone, conversion of SR mostly occurred in the first 4 hours (86.5%), considered to be the optimal infusion time in our experience. In both groups, the left atrial size was significantly larger in nonconverted than in converted patients. Similarly, the duration of the arrhythmia was significantly longer in nonconverted patients. In nonconverted patients, the mean ventricular rate decreased from 143 +/- 16 beats/min to 101 +/- 18 beats/ min after propafenone and from 135 +/- 19 beats/min to 119 +/- 16 beats/min after placebo (group 1 vs. group 2: p < 0.005). Two episodes of sinus standstill (3.4 and 3.8, seconds, respectively) occurred at SR restoration obtained with propafenone. Intravenous propafenone is an effective, safe, and usually rapid drug for AF treatment. Moreover, it produces a real and significant reduction in the mean ventricular rate in nonconverted patients.

Adrenergic modulation of ultrarapid delayed rectifier K+ current in human atrial myocytes

The ultrarapid delayed rectifier K+ current (IKur) in human atrial cells appears to correspond to Kv1.5 cloned channels and to play an important role in human atrial repolarization. Kv1.5 channels have consensus sites for phosphorylation by protein kinase A and C, suggesting possible modulation by adrenergic stimulation. The present study was designed to assess the adrenergic regulation of IKur in human atrial myocytes. Isoproterenol increased IKur in a concentration-dependent manner, with significant effects at concentrations as low as 10 nmol/L. The effects of isoproterenol were reversible by washout or by the addition of propranolol (1 mumol/L). Isoproterenol's effects were mimicked by the direct adenylate cyclase stimulator, forskolin, and by the membrane-permeable form of cAMP, 8-bromo cAMP. Isoproterenol had no effect on IKur when the protein kinase A inhibitor peptide, PKI(6-22)amide, was included in the pipette solution; in a separate set of experiments in which isoproterenol alone increased IKur by 45 +/- 9% relative to control, subsequent superfusion with isoproterenol in the presence of the protein kinase inhibitor H-7 failed to alter IKur. In contrast to isoproterenol, phenylephrine (in the presence of propranolol to block beta-adrenegic effects) induced a concentration-dependent inhibition of IKur, with significant effects observed at concentrations as low as 10 mumol/L. The inhibitory actions of phenylephrine were reversed by the addition of prazosin and prevented by coadministration with a highly selective inhibitor of protein kinase C, bisindolylmaleimide. These results indicate that beta-adrenergic stimulation enhances, whereas alpha-adrenergic stimulation inhibits, IKur and suggest that these actions are mediated by protein kinase A and protein kinase C, respectively. The modulation of IKur by adrenergic influences is a potentially novel control mechanism for human atrial repolarization and arrhythmias.

Fast inactivation causes rectification of the IKr channel

The mechanism of rectification of HERG, the human cardiac delayed rectifier K+ channel, was studied after heterologous expression in Xenopus oocytes. Currents were measured using two-microelectrode and macropatch voltage clamp techniques. The fully activated current-voltage (I-V) relationship for HERG inwardly rectified. Rectification was not altered by exposing the cytoplasmic side of a macropatch to a divalent-free solution, indicating this property was not caused by voltage-dependent block of outward current by Mg2+ or other soluble cytosolic molecules. The instantaneous I-V relationship for HERG was linear after removal of fast inactivation by a brief hyperpolarization. The time constants for the onset of and recovery from inactivation were a bell-shaped function of membrane potential. The time constants of inactivation varied from 1.8 ms at +50 mV to 16 ms at -20 mV; recovery from inactivation varied from 4.7 ms at -120 mV to 15 ms at -50 mV. Truncation of the NH2-terminal region of HERG shifted the voltage dependence of activation and inactivation by +20 to +30 mV. In addition, the rate of deactivation of the truncated channel was much faster than wild-type HERG. The mechanism of HERG rectification is voltage-gated fast inactivation. Inactivation of channels proceeds at a much faster rate than activation, such that no outward current is observed upon depolarization to very high membrane potentials. Fast inactivation of HERG and the resulting rectification are partly responsible for the prolonged plateau phase typical of ventricular action potentials.

Mode of Inhibition of Transient Outward Current by 1-Benzyl-1,2,3,4-Tetrahydroisoquinoline in Rat Ventricular Cells

In this study, we examined the effects of 1-benzyl-1,2,3,4-tetrahydroisoquinoline (S49) on a transient outward current (I(to)) in rat ventricular myocytes using the whole-cell patch-clamp technique. Depolarization of ventricular myocytes not only activated I(to), but sustained outward currents as well. S49 dose-dependently inhibited the amplitude or integral of I(to), with a 50% inhibitory concentration (IC(50)) of 4.3 and 2.7 &mgr;M, respectively. The inhibition of I(to) by S49 was associated with an acceleration of I(to) decay. However, S49 had no effect on half inactivation voltage (V(0.5)) and slope factor of the voltage-dependent steady-state inactivation curve of I(to). Time-course analysis revealed that S49 developed a block during the depolarizing voltage-clamp step in a monoexponential manner. The rate and magnitude of block were concentration dependent. The equilibrium dissociation constant (K(d)) used to inhibit I(to) induced by S49, as calculated from the time constant of developing block, was 3.4 &mgr;M. The time constant of recovery of I(to) from the inactivation state was prolonged by S49. Following treatment with quinidine, the process of I(to) recovery was divided into rapid and extremely slow recovery components. Also, the relief from quinidine- or S49-induced block was assessed by comparing the recovery processes of I(to) with or without drugs. That comparison revealed the relief from the block of I(to) channels by S49 to be more rapid. In summary, the inhibition of I(to) by S49 was dose dependent, time dependent but voltage independent. The mechanisms of action might be an open-state block. Copyright 1996 S. Karger AG, Basel

Evidence for two components of delayed rectifier K+ current in human ventricular myocytes

Previous voltage-clamp studies have suggested that the delayed rectifier current (IK) is small or absent in the human ventricle and, when present, consists only of the rapid component (IKr); however, molecular studies suggest the presence of functionally important IK in the human heart, specific IKr blockers are known to delay ventricular repolarization and cause the long QT syndrome in humans, and we have shown that the expression of IK is strongly influenced by cell isolation techniques. The present experiments were designed to assess the expression of IK in myocytes obtained by arterial perfusion of right ventricular tissue from explanted human hearts. Of 35 cells from three hearts, 33 (94%) showed time-dependent currents typical of IK. The envelope-of-tails test was not satisfied under control conditions but became satisfied in the presence of the benzenesulfonamide E-4031 (5 micromol/L). E-4031 suppressed a portion of IK in 32 of 33 cells, with properties of the drug-sensitive and -resistant components consistent with previous descriptions of IKr and the slow component (IKs), respectively. Action potential duration to 95% repolarization at 1 Hz was prolonged by E-4031 from 336+/-16 (mean +/- SEM) to 421 +/- 19ms (n = 5, P < .01), indicating a functional role for IK. Indapamide, a diuretic agent previously shown to inhibit IKs selectively, suppressed E-4031-resistant current. The presence of a third type of delayed rectifier, the ultrarapid delayed rectifier current (IKur), was evaluated with the use of depolarizing prepulses and low concentrations (50 micromol/L) of 4-aminopyridine. Although these techniques revealed clear IKur in five of five human atrial cells, no corresponding component was observed in any of five human ventricular myocytes. We conclude that a functionally significant IK, with components corresponding to IKr and IKs, is present in human ventricular cells, whereas IKur appears to be absent. These findings are important for understanding the molecular, physiological, and pharmacological determinants of human ventricular repolarization and arrhythmias.

Beta subunits promote K+ channel surface expression through effects early in biosynthesis

Voltage-gated K+ channels are protein complexes composed of ion-conducting integral membrane alpha subunits and cytoplasmic beta subunits. Here, we show that, in transfected mammalian cells, the predominant beta subunit isoform in brain, Kv beta 2, associates with the Kv1.2 alpha subunit early in channel biosynthesis and that Kv beta 2 exerts multiple chaperone-like effects on associated Kv1.2 including promotion of cotranslational N-linked glycosylation of the nascent Kv1.2 polypeptide, increased stability of Kv beta 2/Kv1.2 complexes, and increased efficiency of cell surface expression of Kv1.2. Taken together, these results indicate that while some cytoplasmic K+ channel beta subunits affect the inactivation kinetics of alpha subunits, a more general, and perhaps more fundamental, role is to mediate the biosynthetic maturation and surface expression of voltage-gated K+ channel complexes. These findings provide a molecular basis for recent genetic studies indicating that beta subunits are key determinants of neuronal excitability.

Rate-dependent effects of the class III antiarrhythmic drug almokalant on refractoriness in the pig

The electrophysiologic effects of intravenously administered almokalant, a new class III antiarrhythmic drug, in 7 isoflurane-anesthetized pigs after low and high dose were investigated. Low-dose almokalant included bolus infusion of 0.05 mumol/kg/min for 5 min followed by a continuous infusion of 0.0025 mumol/kg/min for 40 min. Thereafter, a high dose of 0.2 mumol/kg/min for 5 min and 0.01 mumol/kg/min for 40 min was given. PR, QRS, AH, and HV intervals did not change during almokalant administration. The QT interval increased dose dependently from 337 +/- 17 to 442 +/- 20 ms at high dose (p < 0.05). Atrial refractory periods (AERP) were prolonged dose dependently at a 500-ms pacing cycle length from 178 +/- 15 at baseline to 227 +/- 27 and 253 +/- 23 ms during low- and high-dose almokalant infusion, respectively. For pacing cycle lengths of 400 and 300 ms, these values were 180 +/- 11, 207 +/- 25, and 259 +/- 34 and 157 +/- 12, 193 +/- 21, and 234 +/- 28 ms, respectively. At a pacing cycle length of 500 ms, mean ventricular effective refractory period (VERP) was 270 +/- 25 ms as compared with 306 +/- 24 and 337 +/- 17 during low and high dose, respectively. A similar pattern of VERP changes during both low- and high-dose infusion was noted at the shorter pacing cycle lengths, with an increase from 240 +/- 23 to 274 +/- 22 and 279 +/- 24 ms during a 400-ms cycle length and from 210 +/- 17 to 235 +/- 19 and 234 +/- 21 ms during a 300-ms cycle length. The ratio of the VERP and ventricular monophasic action potential duration (VAPD) did not change significantly. The Wenckebach cycle length increased by 36 +/- 36 and 83 +/- 37 ms with low- and high-dose almokalant infusion, respectively. The percent increase of AERP at pacing cycle lengths of 500, 400, and 300 ms during high-dose almokalant was 42, 44, and 49%, respectively; these increases for VERP were 25, 16, and 11%, respectively. In conclusion, prolongation of refractoriness by almokalant was more pronounced at the atrial than the ventricular level. Prolongation of refractoriness was maintained at short pacing cycle lengths especially in the atrium, indicating absence of reverse-use dependence of almokalant in the porcine heart. The marked atrial effects, paralleled by atrioventricular conduction slowing, and the absence of reverse use-dependence all contribute to the feasibility of use of almokalant, in particular in the treatment of supraventricular tachyarrhythmias.

Sudden Death During Flecainide Therapy for Atrial Fibrillation Complicating Wolff-Parkinson-White Syndrome

The Wolff-Parkinson-White syndrome can be complicated by atrial fibrillation that may increase morbidity and mortality. Different pharmacologic therapy, includes class IA, IC, and III agents, has been used in such cases with variable success. We now use less pharmacologic intervention with development of an electrode catheter ablation for accessory pathways. However, antiarrhythmic agents are still being used, especially when an electrode catheter ablation is unavailable or if a patient refuses such a procedure. Therefore, it is prudent that one understands each antiarrhythmic agents' electropharmacologic properties as well as its potential proarrhythmic effect in order to accurately assess each drug's risk-benefit ratio. We present a case that illustrates electropharmacologic properties of quinidine, flecainide, sotalol, and amiodarone on various cardiac tissues, as well as possible proarrhythmic effect of flecainide on a structurally normal heart.

Selective interaction of voltage-gated K+ channel beta-subunits with alpha-subunits

To begin to study the molecular bases that determine the selective interaction of the beta-subunits of voltage-gated K+ channels with alpha-subunits observed in situ, we have expressed these polypeptides in transfected mammalian cells. Analysis of the specificity of alpha/bet a-subunit interaction indicates that both the Kvbeta1 and Kvbeta2 beta-subunits display robust and selective interaction with the five members of the Shaker-related (Kv1) alpha-subunit subfamily tested. The interaction of these beta-subunits with Kv1 alpha-subunits does not require the beta-subunit N-terminal domains. Thus, the previously observed failure of N-terminal mutants of Kv beta1 to modulate inactivation kinetics of Kv1 family members is not simply due to a lack of subunit interaction. Interaction of these beta-subunits with members of two other subfamilies (Shab- and Shaw-related) could not be detected. Somewhat surprisingly, a member of the Shal-related subfamily was found to interact with beta-subunits; however, this interaction had biochemical characteristics distinct from the beta-subunit interaction with Kv1 family members. In all cases, Kvbeta1 and Kvbeta2 exhibited indistinguishable alpha-subunit selectivity. These studies point to a selective interaction between K+ channel alpha- and beta-subunits mediated through conserved domains in the respective subunits.

Mechanisms of block of a human cloned potassium channel by the enantiomers of a new bradycardic agent: S-16257-2 and S-16260-2

1. The effects of S-16257-2 (S57) and S-16260-2 (R60), the two enantiomers of a new bradycardic agent, were studied on human cloned K+ channels (hKv1.5) stably expressed in a mouse L cell line using the whole-cell configuration of the patch-clamp technique. 2. S57 and R60 did not modify the sigmoidal activation time course of the current but reduced the amplitude and increased the rate of the decay of the current during the application of depolarizing pulses. Both, S57 and R60 produced a concentration-dependent block of hKv1.5 channels with apparent KD values of 29.0 +/- 1.9 microM and 40.9 +/- 4.0 microM, respectively. Thus, S57 was 1.4 fold more potent than R60 in blocking hKv1.5 channels. 3. The blockade produced by S57 and R60 was voltage-dependent and increased steeply between -30 and 0 mV, which corresponded with the voltage range for channel opening. This result indicated that both enantiomers block the hKv1.5 channels, preferentially, when they are in the open state. Between 0 and +60 mV the blockade exhibited a shallow voltage-dependence which was described by an electrical distance of 0.18 +/- 0.002 and 0.19 +/- 0.004 for S57 and R60, respectively. 4. S57 and R60 also increased the rate of decline of the current during the application of depolarizing pulses. The time constant of such decline (tau Block) was faster in the presence of R60 than in the presence of S57 (16.2 +/- 1.5 ms vs. 24.0 +/- 2.6 ms; P < 0.01). The apparent association rate constants (k) were similar for S57 and R60 ((0.52 +/- 0.13) x 10(6) M-1 s-1 and (0.66 +/- 0.13) x 10(6) M-1 s-1, respectively), whereas the dissociation rate constant (l) was faster for R60 than for S57 (25.8 +/- 1.8 s-1 and 13.0 +/- 2.4 s-1, respectively). 5. Both enantiomers slowed the deactivation of the tail currents elicited upon repolarization to -40 mV, thus inducing a 'crossover' phenomenon. These results suggested that drug unbinding is required before hKv1.5 channels can close. 6. It is concluded that R60 and S57 produced a similar time- voltage- and state-dependent block of hKv1.5 channels that can be interpreted as open channel block by the charged form of each enantiomer. The main difference between R60 and S57 were linked to the apparent dissociation rate constants.

Characteristics of the delayed rectifier K current compared in myocytes isolated from the atrioventricular node and ventricle of the rabbit heart

The delayed rectifier potassium current (IK) is known to be important in action potential repolarisation and may contribute to the diastolic pacemaker depolarisation in pacemaker cells from the heart. In this study, using whole-cell patch clamp, we investigated the characteristics of IK in morphologically normal cells from the atrioventricular node (AVN) and ventricle of the rabbit heart. Cells were held at -40 mV and 5 microM external nifedipine was used to block L-type calcium current (ICa,L). Significant IK was observed with pulses to potentials more positive than -30 mV. The steady-state activation curve in both cell types showed maximal activation at between + 10 and + 20 mV. Half-maximal activation of IK occurred at -4.9 and -4.1 mV with slope factors of 8.3 and 12.4 mV in ventricular and AVN cells, respectively. Using pulses of increasing duration, significant IK tails after repolarisation from + 40 mV were observed with pulses of 20 ms and increased with pulses up to 100-120 ms in both cell types. Pulses of longer duration did not activate further IK and this suggested that only the rapid component of IK, called IKr, was present in either cell type. Moreover, IK tails after pulses to all potentials were blocked completely by E-4031, a selective blocker of IKr. The reversal potential of IK varied with the concentration of external K. Superfusion of AVN cells with medium containing 4, 15 and 40 mM [K+]o resulted in reversal potentials of -81, -56 and -32 mV, respectively, which are close to values predicted if the IK channel were highly selective for K. The time constants for deactivation of IK in ventricle and AVN on return to -40 mV after a 500-ms activating pulse to + 60 mV were 480 ms and 230 ms, respectively. The faster deactivation of IK in AVN cells was a distinguishing feature and suggests that there may be differences in the IKr channel protein between ventricular and AVN cells.

Differences between outward currents of human atrial and subepicardial ventricular myocytes

1. Outward currents were studied in myocytes isolated from human atrial and subepicardial ventricular myocardium using the whole-cell voltage clamp technique at 22 degrees C. The Na+ current was inactivated with prepulses to -40 mV and the Ca2+ current was eliminated by both reducing extracellular [Ca2+] to 0.5 mM and addition of 100 microM CdCl2 to the bath solution. 2. In human myocytes, three different outward currents were observed. A slowly inactivating sustained outward current, I(so), was found in atrial but not ventricular myocytes. A rapidly inactivating outward current, I(to), of similar current density was observed in cells from the two tissues. An additional uncharacterized non-inactivating background current of similar size was observed in atrial and in ventricular myocytes. 3. I(to) and I(so) could be differentiated in atrial myocytes by their different kinetics and potential dependence of inactivation, and their different sensitivities to block by 4-amino-pyridine, suggesting that two individual channel types were involved. 4. In atrial cells, inactivation of I(to) was more rapid and steady-state inactivation occurred at more negative membrane potentials than in ventricular cells. Furthermore, the recovery of I(to) from inactivation was slower and without overshoot in atrial myocytes. In addition, 4-aminopyridine-induced block of I(to) was more efficient in atrial than in ventricular cells. These observations suggest that the channels responsible for atrial and ventricular I(to) were not identical. 5. We conclude that the differences in outward currents substantially contribute to the particular shapes of human atrial and ventricular action potentials. The existence of I(so) in atrial cells only provides a clinically interesting target for anti-arrhythmic drug action, since blockers of I(so) would selectively prolong the atrial refractory period, leaving ventricular refractoriness unaltered.

Characterization of a beta-adrenergically inhibited K+ current in rat cardiac ventricular cells

1. The electrophysiological properties and beta-adrenergic regulation of a non-inactivating K+ current were studied using the whole-cell patch-clamp technique (22 +/- 2 degrees C) in adult rat ventricular cells. 2. In the presence of 4-aminopyridine, an inhibitor of the rapidly inactivating current, the depolarization-activated current consisted only of a slowly decaying outward current (IK). The presence of a non-inactivating current (ISS) was revealed when analysing inactivation curves. 3. IK and ISS were both sensitive to 50 mM tetraethylammonium and 10 mM 4-aminopyridine inhibition. IK was totally blocked by 100 microM clofilium, while ISS was not inhibited but rather enhanced by this class III anti-arrhythmic agent. 4. Unlike IK, ISS was only slightly decreased by depolarizing prepulses and it did not show time-dependent inactivation when measured during 500 ms depolarizations. 5. ISS was decreased by the beta-adrenergic agonist isoprenaline (1 microM). Forskolin (10 microM) mimicked the effects of isoprenaline. The non-specific beta-adrenergic antagonist, propranolol (3 microM), and a specific beta 1-adrenergic antagonist, CGP 20712A (0.3 microM), both prevented the effects of isoprenaline. Cell perfusion with 100 microM PKI6-22, a peptide inhibitor of the cyclic AMP-dependent protein kinase, reduced or abolished the effects of isoprenaline. 6. The dose-response curve for the inhibition of ISS by isoprenaline was positioned to the left of that for the calcium current. The threshold dose and the dose giving 50% of the maximal effect were, respectively, 0.1 and 0.21 nM for ISS and 1 and 4.3 nM for ICa. 7. In view of the high sensitivity of ISS to isoprenaline, its possible physiological effect was evaluated on action potential duration during beta-adrenergic stimulation. At 1 nM, a concentration that did not increase ICa, isoprenaline induced a significant prolongation of action potential duration as a consequence of ISS inhibition. With 1 microM isoprenaline, the action potential was further prolonged, due largely to an evoked increase in ICa. 8. In conclusion, a K+ current displaying a weak voltage-dependent inactivation is present in rat ventricular cells. It is inhibited by stimulation of beta 1-adrenergic receptors and is highly sensitive to phosphorylation by protein kinase A. This current may play an important role in the neuromodulation of excitation-contraction coupling.

Functional differences in Kv1.5 currents expressed in mammalian cell lines are due to the presence of endogenous Kv beta 2.1 subunits

The voltage-sensitive currents observed following hKv1.5 alpha subunit expression in HEK 293 and mouse L-cells differ in the kinetics and voltage dependence of activation and slow inactivation. Molecular cloning, immunopurification, and Western blot analysis demonstrated that an endogenous L-cell Kv beta 2.1 subunit assembled with transfected hKv 1.5 protein. In contrast, both mRNA and protein analysis failed to detect a beta subunit in the HEK 293 cells, suggesting that functional differences observed between these two systems are due to endogenous L-cell Kv beta 2.1 expression. In the absence of Kv beta 2.1, midpoints for activation and inactivation of hKv1.5 in HEK 293 cells were -0.2 +/- 2.0 and -9.6 +/- 1.8 mV, respectively. In the presence of Kv beta 2.1 these values were -14.1 +/- 1.8 and -22.1 +/- 3.7 mV, respectively. The beta subunit also caused a 1.5-fold increase in the extent of slow inactivation at 50 mV, thus completely reconstituting the L-cell current phenotype in the HEK 293 cells. These results indicate that 1) the Kv beta 2.1 subunit can alter Kv1.5 alpha subunit function, 2) beta subunits are not required for alpha subunit expression, and 3) endogenous beta subunits are expressed in heterologous expression systems used to study K+ channel function.

Antiarrhythmic and electrophysiologic effects of intravenous ibutilide and sotalol in the canine sterile pericarditis model

INTRODUCTION

Atrial arrhythmias are a frequent clinical complication following open heart surgery. We compared the Class III agents d,l-so-talol and ibutilide fumarate in an intravenous cross-over study using the canine atrial sterile pericarditis model.

METHODS AND RESULTS

We studied pacing-induced sustained atrial flutter over a 7-day post-surgical period in conscious dogs, alternating analysis of ibutilide (1.0 to 30.0 micrograms/kg) and d,l-sotalol (0.1 to 3.0 mg/kg). Ibutilide significantly increased atrial flutter cycle length (AFL CL) 11 +/- 2 msec and atrial effective refractory period (AERP) 13 +/- 2 msec, and terminated atrial flutter in all cases (n = 12) following a mean dose of 6 +/- 2 micrograms/kg. Plasma concentrations of ibutilide were 53 +/- 13 ng/mL. Ventricular effective refractory period (VERP) was not significantly affected (4 +/- 2 msec). Following termination with ibutilide, atrial flutter could be reinitiated in 1 of 12 trials, and was nonsustained (40-sec duration). Sotalol significantly increased AFL CL 23 +/- 3 msec and terminated atrial flutter in 8 of 12 trials following a mean dose of 1.5 +/- 0.4 mg/kg. AERP and VERP were significantly increased 20 +/- 6 and 12 +/- 2 msec, respectively. The incidence of reinduced atrial flutter was 9 of 12 trials (P < or = 0.05 vs ibutilide) (7 nonsustained 57 +/- 7 sec duration, and 2 sustained). Sotalol failed to terminate atrial flutter in two dogs on days 1 and 5, despite increases in AFL CL (21 +/- 8 msec) and AERP (16 +/- 9 msec), whereas on day 3, ibutilide (20 +/- 7 micrograms/kg) terminated atrial flutter in those two dogs while increasing AFL CL and AERP 18 +/- 6 and 15 +/- 0 msec, respectively.

CONCLUSION

Both sotalol and ibutilide terminate atrial flutter in this model. Ibutilide converted atrial flutter in dogs in which sotalol was not successful. Following atrial flutter termination, ibutilide had a lower incidence of reinduced arrhythmias compared to sotalol. Ibutilide produced atrial antiarrhythmic effects while having no significant electrophysiologic effects on the ventricle.

The cardiac ion channels: relevance to management of arrhythmias

The electrical activity of cardiac tissue is determined by the highly regulated flow of ions across the cell membrane during the cardiac action potential. Ion channels are pore-forming proteins through which these electric currents flow. In this review, the ion currents that underlie the action potential are first described. Then, the way in which expression of individual ion-channel genes results in such ion currents is discussed. Finally, the concept that arrhythmias may be due to abnormalities of structure, function, or number of ion channels, or the way in which they respond to abnormalities in their environment (such as acute ischemia), is reviewed. Further understanding of the molecular mechanisms underlying normal and abnormal cardiac electrophysiologic behavior should allow the development of safer and more effective antiarrhythmic interventions.

Dual function of the voltage-dependent Ca2+ channel alpha 2 delta subunit in current stimulation and subunit interaction

Voltage-dependent Ca2+ channels are modulated by complex interactions with the alpha 2 delta subunit. In vitro translation was used to demonstrate a single transmembrane topology of the alpha 2 delta subunit in which all but the transmembrane sequence and 5 carboxy-terminal amino acids are extracellular. The glycosylated extra-cellular domain is required for current stimulation, as shown by coexpression of truncated alpha 2 delta subunits with alpha 1A and beta 4 subunits in Xenopus oocytes and deglycosylation with peptide-N-glycosidase F. However, coexpression of the transmembrane domain-containing delta subunit reduced the stimulatory effects of full-length alpha 2 delta subunits and substitution of a different transmembrane domain resulted in a loss of current stimulation. These results support a model whereby the alpha 2 delta transmembrane domain mediates subunit interactions and the glycosylated extracellular domain enhances current amplitude.

A new method for analysis of atrial activation during chronic atrial fibrillation in man

To further clarify the mechanisms maintaining chronic atrial fibrillation (CAF), a method identifying preferable activation patterns of the atria during fibrillation, by time averaging of multiple discrete excitation vectors, was developed. Repeated recordings, each of 56 atrial bipolar electrograms simultaneously acquired during 8 s, were made at multiple sites in the right atrial free wall and the left atrial appendage in 16 patients with CAF using a 2.17 x 3.54 cm electrode array. The local activation times (LAT's) in each recording were estimated as the median activation time at the respective measurement point. By calculating the time difference between the LAT's at adjacent measurement points in two spatial dimensions, a direction vector was created for each activation wave passing each set of measurement points, a total of 42 sets. By time averaging of the individual direction vectors (typically n = 55) at each set of measurement points, preferable activation patterns were determined. Three types of activation patterns were found: 1) inconsistent activation (n = 5), 2) consistent activation with preferential propagation directions (n = 7) and 3) consistent activation with impulses originating from a localizable site within the recording area (n = 4). All activation patterns were reproducible and the two latter patterns were proven significant using statistical tests. We conclude that this new method is useful in further clarification of the mechanisms involved in the maintenance of atrial fibrillation.

Kv beta 1 subunit binding specific for shaker-related potassium channel alpha subunits

Voltage-activated potassium (Kv) channels from mammalian brain are hetero-oligomers containing alpha and beta subunits. Coexpression of Kv1 alpha and Kv beta 1 subunits confers rapid A-type inactivation on noninactivating potassium channels (delayed rectifiers) in expression systems in vitro. We have delineated a Kv1.5 aminoterminal region of up to 90 amino acids (residues 112-201) that is sufficient for interactions of Kv1.5 alpha and Kv beta 1 subunits. Within this region of the Kv1.5 amino terminus (residues 193-201), a Kv beta 1 interaction site necessary for Kv beta 1-mediated rapid inactivation of Kv1.5 currents was detected. This interaction site motif (FYE/QLGE/DEAM/L) is found exclusively in the Shaker-related subfamily (Kv1). The results show that hetero-oligomerization between alpha and Kv beta 1 subunits is restricted to Shaker-related potassium channel alpha subunits.

Extracellular potassium modulation of drug block of IKr. Implications for torsade de pointes and reverse use-dependence

BACKGROUND

Torsade de pointes often occurs with underlying hypokalemia and bradycardia. A common effect of many drugs producing torsade de pointes is block of the rapidly activating component of the cardiac delayed rectifier (IKr). In this study, we evaluated the effect of changing extracellular potassium ([K+]o) on IKr block by the nonspecific agent quinidine and by the specific IKr blocker dofetilide.

METHODS AND RESULTS

IKr was measured in AT-1 cells, where contaminating outward currents are absent. The drug concentration producing 50% inhibition of IKr tails (IC50) was strikingly [K+]o-dependent. Elevating [K+]o from 1 to 8 mmol/L increased the IC50 for dofetilide block from 2.7 +/- 0.9 to 79 +/- 32 nmol/L and for quinidine block from 0.4 +/- 0.1 to 3.8 +/- 1.2 mumol/L.

CONCLUSIONS

(1) The increase in drug block with low [K+]o provides a mechanism to explain the link between hypokalemia and torsade de pointes. (2) Elevations in [K+]o occur with myocardial ischemia and with rapid pacing. Possible consequences of blunted drug block with high [K+]o include loss of drug efficacy with ischemia and with rapid pacing; the latter may contribute to "reverse use-dependent" action potential prolongation. Extracellular potassium is a critical determinant of drug block of IKr, with substantial clinical implications.

Drug therapy for prevention of atrial fibrillation

Atrial fibrillation is not a homogeneous entity, and many factors are responsible for a number of different behaviors, clinical consequences, and reactions to therapy. Therefore, the conventional evaluation of preventive treatments is not really adapted to provide the correct answers to difficult problems of therapeutic indications, as the 2 components of the benefit-risk ratio are not really known. Like ventricular fibrillation, atrial fibrillation may be primary or secondary to organized tachyarrhythmias, and reentrant flutter or automatic atrial tachycardia may well form the actual target for treatment. The automatic nervous system is never absent as a determinant of the onset of arrhythmia, and the vagal as well as the sympathetic action may predominate and explain why a treatment may or may not be effective in situations that are identical only in appearance. The electrophysiologic milieu formed by the atrial tissue probably accounts for the perpetuation of the process of atrial fibrillation or its self-termination, and drugs themselves may contribute to modify the milieu in a way that in the end may be favorable or not. Finally, the presence or the absence of heart disease and heart failure largely contributes to the state of the vagosympathetic balance, to the hemodynamic consequences of atrial fibrillation, and ultimately to the proper toxic effects of drugs. The overall consequence of these complex situations is that any precise therapeutic decision algorithm for atrial fibrillation is always simplistic and that any global evaluation of drug efficacy or toxicity is not really meaningful as long as the category of patients treated is not precisely determined: no drug appears better or worse than others, but simply more or less adapted to various situations.

Electrophysiologic mechanisms of perpetuation of atrial fibrillation

The presence of an excitable gap during atrial fibrillation (AF), although short and variable, may be of potential importance for the development of alternative techniques for termination of AF by rapid pacing. Also the notion that perpetuation of AF may be partly dependent on macroreentry around the natural atrial orifices, may provide a new therapeutic option for the permanent cure of AF by interrupting the anatomical circular pathways in the atria by radiofrequency ablation. In our opinion the rapidly growing understanding of the electrophysiologic mechanisms of AF certainly warrants some optimism about the possibility of cure of AF in the near future without causing too much discomfort and without carrying on unacceptable risk.

A randomized comparison of flecainide versus verapamil in paroxysmal supraventricular tachycardia. The Flecainide Multicenter Investigators Group

Reentrant paroxysmal supraventricular tachycardia (PSVT) are frequently encountered in clinical practice. Verapamil and flecainide have both been successfully used as chronic oral therapy to prevent PSVT recurrences. This open-label, randomized, multicenter study was designed to compare the efficacy and adverse effects of verapamil (median dose, 240 mg/day) versus flecainide (median dose, 200 mg/day) in patients with frequent and symptomatic attacks of PSVT (other than atrial fibrillation or flutter). A total of 121 patients receiving flecainide (n = 63) or verapamil (n = 58) were followed for 8.1 +/- 5.1 and 7.5 +/- 5.4 months, respectively. Response was judged clinically as effective or not by the treating physician. By life table analysis, 11% discontinued flecainide and 19% discontinued verapamil for inefficacy at 1 year (difference not significant). Both groups showed a marked reduction in the frequency of attacks of PSVT. Before therapy, 71% of flecainide patients and 73% of verapamil patients had > or = 2 attacks per month. During therapy, 86% of all flecainide patient-months and 73% of all verapamil patient-months occurred with 0 or 1 attack; 19 (30%) patients on flecainide completed the trial ( > 270 days) without symptomatic attacks versus 7 (13%) of the patients on verapamil (p = 0.026). Both drugs were well tolerated; 19% of the flecainide group discontinued primarily because of adverse effects, compared with 24% discontinuing verapamil for this reason (difference not significant). Both flecainide and verapamil are effective and well tolerated for the prevention of recurrences of PSVT. For patients in whom radiofrequency ablation procedures cannot be performed or are not indicated, either therapy is a reasonable choice for long-term prophylaxis.

The modulation of the rate of inactivation of the mKv1.1 K+ channel by the beta subunit, Kv beta 1 and lack of effect of a Kv beta 1 N-terminal peptide

The coexpression of the rat Kv beta 1 subunit with the mouse Kv1.1 (mKv1.1) K+ channel in Chinese hamster ovary cells caused an increase in the rate of inactivation of whole-cell current. Current decayed in a bi-exponential fashion with a fast voltage-dependent and a slower voltage-independent component. The inactivating current component accounted for around 40% of the total outward current. In contrast to previous studies using K+ channel alpha subunits, peptides based on the N-terminal of the Kv beta 1 subunit were unable to mimic the action of the entire subunit. The findings indicate differences between the inactivation induced by the Kv beta 1 subunit and the N-type inactivation mechanism associated with certain rapidly-inactivating cloned K+ channel alpha subunits.

Induced termination of fibrillation

A previous communication in the Creative Musings section of this Journal summarized additions to the wavelet hypothesis related to the initiation of cardiac fibrillation. That hypothesis is also relevant to the termination of fibrillation, and further additions related to that event are presented in this report. Findings in both reports were obtained with a computer model based on the wavelet hypothesis, and results concerning initiation and termination of fibrillation were closely related. Refractory period (RP) conditions that terminated fibrillation were the inverse of those that increased vulnerability to the initiation of fibrillation. Increased RP range or decreased RP duration increased vulnerability to the initiation of fibrillation and decreased RP range or increased RP duration were capable of terminating fibrillation. Slow propagation increased vulnerability to initiation of fibrillation and acted to sustain fibrillation when instituted during fibrillation. The combination of increased duration and decreased range of RPs was more effective in terminating fibrillation than either alone. The magnitude of increased RP duration or decreased RP range required to terminate fibrillation and the effects of slow propagation on the maintenance of fibrillation depended on RP duration and range present during fibrillation. The findings extend the wavelet hypothesis of the nature of fibrillation to the prediction of conditions required to terminate fibrillation.

Comparison of direct negative chronotropic and positive inotropic effects of sematilide to those of E-4031 and MS-551 and the reverse frequency-dependent prolongation of cardiac refractoriness of sematilide

Direct cardiac effects of sematilide, a new class III antiarrhythmic drug, were compared with those of E-4031 and MS-551 in canine isolated blood-perfused heart preparations. Doses of sematilide, E-4031, and MS-551 causing a 10% decrease in the spontaneous sinoatrial beating rate were 58 +/- 15, 9 +/- 5, and 84 +/- 10 micrograms (n = 5); those causing a 10% increase in developed tension of the papillary muscle were 485 +/- 49, 17 +/- 2, and 267 +/- 50 micrograms (n = 6); and those causing a 10% prolongation of effective refractory period (ERP) of the atrioventricular node were 68 +/- 10, 11 +/- 2, and 53 +/- 15 micrograms (n = 5), respectively. There were few effects on atrio-His or His-ventricular intervals. Also, in in situ open-chest dog hearts, the percent increases in ERP of the atrioventricular conduction system caused by 1 mg/kg of sematilide were 21 +/- 3, 16 +/- 2 and 9 +/- 1% at cycle lengths of 800, 600, and 400 ms, respectively (p < 0.01; n = 8). These results indicate that (a) sematilide, as well as E-4031 and MS-551, has direct negative chronotropic and positive inotropic effects and prolongs cardiac refractoriness without affecting conduction velocities; (b) quantitatively, the cardiac effects of sematilide were almost identical to those of MS-551 and five to ten times less potent than those of E-4031; (c) and prolongation of ERP of the atrioventricular conduction system by sematilide occurred in a reverse frequency-dependent manner.

Effects of the class III antiarrhythmic drug ambasilide on outward currents in human atrial myocytes

We have studied the inhibitory influence of the class III antiarrhythmic drug ambasilide (LU 47110) on the transient outward current Ito1 and the sustained current Iso following inactivation of Ito1, in human atrial myocytes. The two currents are separated by a mathematical procedure based on the amplitudes and time constants of the biexponential inactivation of the total outward current. The frequency dependence, the recovery from inactivation and the kinetics of activation and inactivation are described. Ambasilide reversibly and concentration dependently inhibited Ito1, Iso and the sodium current INa. Concentration required for half maximal inhibition (IC50) for the effects on Ito1 and Iso were 23.3 mumol/l and 45.7 mumol/l respectively, concentrations shown by others to be effective in terminating and preventing fibrillation in a dog atrial arrhythmia model. Ambasilide not only reduced the amplitude of Ito1 and Iso but also accelerated the time course of inactivation from 14.22 to 6.69 ms and from 202.3 to 87.9 ms respectively. The amplitude of Ito1 showed only a small dependence on stimulation frequency characteristic for human atrial myocytes, whereas Iso was reduced significantly at higher stimulation frequencies. Ambasilide did not change these relationships (0.1-4 Hz) and therefore did not show the reverse use-dependence known from other class III antiarrhythmic agents and which is an important property for a prospective antiarrhythmic drug. The lack of an effect of ambasilide on both steady-state activation and inactivation of Ito1, and the time constant of recovery from inactivation, suggests that ambasilide acts by changing conductance rather than by influencing the gating mechanism. The described characteristics make ambasilide an interesting substance in the group of class III antiarrhythmic drugs.

Structural and functional characterization of human potassium channel subunit beta 1 (KCNA1B)

Voltage-activated Shaker-related potassium channels (kv1) consist of alpha and beta subunits. We have analysed the structure of the human KCNA1B (hKv beta 1) gene. KCNA1B is > 250 kb in size and encodes at least three Kv beta 1 splice variants. The Kv beta 1 open reading frame is divided into 14 exons. In contrast, genes coding for family members of KCNA (Kv 1 alpha) subunits are markedly smaller and have intronless open reading frames. The expression of Kv 1 alpha and Kv beta mRNA was compared in Northern blots of poly(A+) RNA isolated from various human brain tissues. The results suggest an intricate and cell-specific regulation of Kv 1 alpha and Kv beta mRNA synthesis such that distinct combinations of alpha and beta subunits would occur in different nuclei of the brain. The splice variants hKv beta 1.1 and hKv beta 1.2 were functionally characterized in coexpression studies with hKv 1.5 alpha subunits in 293 cells. It is shown that the confer rapid inactivation on hKv 1.5 channels with different potencies. This may be due to differences in their amino terminal sequences and/or inactivating domains. It is also shown that the amino terminal Kv beta 1.1 and Kv 1.4 alpha inactivating domains compete with each other, probably for the binding to the same receptor site(s) on Kv 1 alpha-subunits.

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.

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.

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.