Different effects of class Ic and III antiarrhythmic drugs on vagotonic atrial fibrillation in the canine heart

Effects of class Ic drug pilsicainide and class III drug MS-551 were determined in the canine model of atrial fibrillation (AF) induced under vagal stimulation. Pilsicainide injected intravenously at a dose of 1.0 mg/kg over 3 min terminated AF in six of six dogs. After pilsicainide injection, the effective refractory period (ERP) of the right atrium (RA) increased (104 +/- 22 to 122 +/- 31 ms; p < 0.05), and intraatrial conduction time (CT) increased (24%; p < 0.05) in the RA during vagal stimulation. Wavelength index (WLI; ERP/CT), an estimate of the wavelength for reentry, was decreased slightly but significantly (-2%; p < 0.05) in the RA after pilsicainide. MS-551 injected intravenously at a dose of 0.5 mg/kg over a 3-min period terminated AF in three of eight dogs. An additional dose of 0.5 mg/kg of MS-551 terminated AF in three of the remaining five dogs. After MS-551 injection, ERP increased (100 +/- 30 to 143 +/- 28 ms; p < 0.05), but CT remained unchanged in the RA, and therefore WLI was increased significantly (48%; p < 0.01). Immediately before termination of AF with test drugs, mean AF intervals (FF intervals) increased, whereas the standard deviation of FF intervals did not change significantly. In conclusion, both pilsicainide and MS-551 effectively terminated vagotonic AF after an increase in FF intervals. However, changes in WLI were different between the two test drugs. Vagotonic AF could, therefore, be terminated either by prolongation of ERP or suppression of conduction with antiarrhythmic drugs.

Properties of HERG channels stably expressed in HEK 293 cells studied at physiological temperature

We have established stably transfected HEK 293 cell lines expressing high levels of functional human ether-a go-go-related gene (HERG) channels. We used these cells to study biochemical characteristics of HERG protein, and to study electrophysiological and pharmacological properties of HERG channel current at 35 degrees C. HERG-transfected cells expressed an mRNA band at 4.0 kb. Western blot analysis showed two protein bands (155 and 135 kDa) slightly larger than the predicted molecular mass (127 kDa). Treatment with N-glycosidase F converted both bands to smaller molecular mass, suggesting that both are glycosylated, but at different levels. HERG current activated at voltages positive to -50 mV, maximum current was reached with depolarizing steps to -10 mV, and the current amplitude declined at more positive voltages, similar to HERG channel current expressed in other heterologous systems. Current density at 35 degrees C, compared with 23 degrees C, was increased by more than twofold to a maximum of 53.4 +/- 6.5 pA/pF. Activation, inactivation, recovery from inactivation, and deactivation kinetics were rapid at 35 degrees C, and more closely resemble values reported for the rapidly activating delayed rectifier K+ current (I(Kr)) at physiological temperatures. HERG channels were highly selective for K+. When we used an action potential clamp technique, HERG current activation began shortly after the upstroke of the action potential waveform. HERG current increased during repolarization to reach a maximum amplitude during phases 2 and 3 of the cardiac action potential. HERG contributed current throughout the return of the membrane to the resting potential, and deactivation of HERG current could participate in phase 4 depolarization. HERG current was blocked by low concentrations of E-4031 (IC50 7.7 nM), a value close to that reported for I(Kr) in native cardiac myocytes. Our data support the postulate that HERG encodes a major constituent of I(Kr) and suggest that at physiological temperatures HERG contributes current throughout most of the action potential and into the postrepolarization period.

Molecular cloning and tissue distribution of an alternatively spliced variant of an A-type K+ channel alpha-subunit, Kv4.3 in the rat

We describe here (1) the heterogeneous expression of Ca2+-independent transient (A-type) K+ channel alpha-subunits (Kv1.4, Kv3.3, Kv3.4, Kv4.2 and Kv4.3) in rat smooth muscle, heart and brain, (2) the molecular cloning and tissue distribution of a novel alternatively spliced variant of an A-type K+ channel alpha-subunit, Kv4.3, and (3) the functional expression of A-type K+ channels in HEK293 cells by the transfection with the novel splice variant of Kv4.3. A cDNA encoding this splice variant was identified from rat vas deferens by RT-PCR cloning. This cDNA clone contains a 1965 bp open reading frame that encodes for a protein of 655 amino acids. It has a 19 amino acid insertion in comparison with Kv4.3 previously reported in rat brain. RT-PCR analyses showed that the mRNAs of this longer variant are abundantly expressed in a number of smooth muscles of the rat, and that the mRNAs of the previously reported clones are absent. The longer splice variant is very weakly expressed in brain, but is the major product in heart.

The real life of voltage-gated K+ channels: more than model behaviour

The past ten years have provided an embarrassment of riches for those interested in cloned voltage-gated K+ (Kv) channels. Details of their physiology and pharmacology in expression systems, and their precise cellular location abound, making them excellent targets for pharmacologists. However, there is still a considerable and important gap in our knowledge between the behaviour of expressed Kv channels and K+ currents in vivo. In this review Brian Robertson focuses on a few of the recent developments in the field of Kv channels, namely modulation of their behaviour by accessory subunits, their control, and localization of identified Kv subunits.

High extracellular glucose impairs cardiac E-C coupling in a glycosylation-dependent manner

Hyperglycemia is a major manifestation of all forms of diabetes mellitus and is associated with increased risk of cardiovascular disease. It is well established that cardiac excitation-contraction (E-C) coupling is adversely affected in diabetic animals such that ventricular myocyte action potential duration is prolonged and intracellular Ca2+ clearing and mechanical relaxation are slowed. We now report that ventricular myocytes incubated in a culture medium containing high extracellular glucose (25.5 mM) also exhibit these same changes in E-C coupling. These effects are not manifested for approximately 24 h after exposure. Furthermore, in the presence of normal glucose (5.5 mM), relaxation is also prolonged by fructose (20 mM), yet is unaffected by equimolar concentrations of nonmetabolizable sugars such as L-glucose and mannitol, implying that the high glucose effects require glucose entry into the cell and metabolic processing. The prolonged relaxation can also be produced by 5 mM glucosamine (an intermediate of glycosylation) and is blocked by 0.5 microgram/ml tunicamycin (an inhibitor of N-linked glycoprotein synthesis). Culturing myocytes with an inhibitor of glycation (10 mM aminoguanidine) does not prevent the high extracellular glucose concentration effects. Thus our data indicate that high extracellular glucose impairs cellular mechanisms contributing to myocardial relaxation and that this impairment may involve glycosylation of nascent proteins.

Signalling via the G protein-activated K+ channels

The inwardly rectifying K+ channels of the GIRK (Kir3) family, members of the superfamily of inwardly rectifying K+ channels (Kir), are important physiological tools to regulate excitability in heart and brain by neurotransmitters, and the only ion channels conclusively shown to be activated by a direct interaction with heterotrimeric G protein subunits. During the last decade, especially since their cloning in 1993, remarkable progress has been made in understanding the structure, mechanisms of gating, activation by G proteins, and modulation of these channels. However, much of the molecular details of structure and of gating by G protein subunits and other factors, mechanisms of modulation and desensitization, and determinants of specificity of coupling to G proteins, remain unknown. This review summarizes both the recent advances and the unresolved questions now on the agenda in GIRK studies.

Association and colocalization of the Kvbeta1 and Kvbeta2 beta-subunits with Kv1 alpha-subunits in mammalian brain K+ channel complexes

The differential expression and association of cytoplasmic beta-subunits with pore-forming alpha-subunits may contribute significantly to the complexity and heterogeneity of voltage-gated K+ channels in excitable cells. Here we examined the association and colocalization of two mammalian beta-subunits, Kvbeta1 and Kvbeta2, with the K+ channel alpha-subunits Kv1.1, Kv1.2, Kv1.4, Kv1.6, and Kv2.1 in adult rat brain. Reciprocal coimmunoprecipitation experiments using subunit-specific antibodies indicated that Kvbeta1 and Kvbeta2 associate with all the Kv1 alpha-subunits examined, and with each other, but not with Kv2.1. A much larger portion of the total brain pool of Kv1-containing channel complexes was found associated with Kvbeta2 than with Kvbeta1. Single- and multiple-label immunohistochemical staining indicated that Kvbeta1 codistributes extensively with Kv1.1 and Kv1.4 in cortical interneurons, in the hippocampal perforant path and mossy fiber pathways, and in the globus pallidus and substantia nigra. Kvbeta2 codistributes extensively with Kv1.1 and Kv1.2 in all brain regions examined and was strikingly colocalized with these alpha-subunits in the juxtaparanodal region of nodes of Ranvier as well as in the axons and terminals of cerebellar basket cells. Taken together, these data provide a direct demonstration that Kvbeta1 and Kvbeta2 associate and colocalize with Kv1 alpha-subunits in native tissues and provide a biochemical and neuroanatomical basis for the differential contribution of Kv1 alpha- and beta-subunits to electrophysiologically diverse neuronal K+ currents.

Interactions among inactivating and noninactivating Kvbeta subunits, and Kvalpha1.2, produce potassium currents with intermediate inactivation

Experiments were carried out to determine whether coinjection of Kvalpha1.2 with inactivating and noninactivating Kvbeta subunits would produce currents with intermediate kinetics and channel complexes containing a mixture of these subunits. Upon coexpression with a saturating amount of Kvbeta1.2 and increasing levels of a noninactivating deletion mutant of Kvbeta1.2, we show that macroscopic Kvalpha1.2 currents have levels of fractional inactivation and inactivation time constants that are intermediate between those obtained with either the inactivating Kvbeta1.2 or the noninactivating Kvbeta1.2 mutant. We also find that coexpression of Kvalpha1.2 with saturating amounts of Kvbeta1.2 and the deletion mutant produces a population of single channels with properties intermediate to either the inactivating or noninactivating parental phenotype. Our data can best be explained by the presence of an intermediate population of heterooligomeric channels consisting of Kvalpha1.2 with different combinations of both types of subunits. Since Kvalpha1.2 subunits coexist in cells with inactivating and noninactivating Kvbeta subunits, our findings suggest that heterooligomeric assembly of these subunits occurs to increase the range of K+ current kinetics and expression levels.

Modulation of HERG affinity for E-4031 by [K+]o and C-type inactivation

Rectification of HERG is due to a rapid inactivation process that has been labeled C-type inactivation and is believed to be due to closure of the external mouth of the pore. We examined the effects of mutation of extracellular residues that remove C-type inactivation on binding of the intracellularly acting methanesulfonanilide drug E-4031. Removal of inactivation through mutation reduced drug affinity by more than an order of magnitude. Elevation of [K+]o in the wild-type channel reduces channel affinity for E-4031. Elevation of [K+]o also interferes with the extracellular pore mouth closure associated with C-type inactivation through a 'foot in the door' mechanism. We examined the possibility that [K+]o elevation reduces drug binding through inhibition of C-type inactivation by comparing drug block in the wild-type and inactivation-removed mutant channels. Elevation of [K+]o decreased affinity in both channel constructs by a roughly equal amount. These results suggest that [K+]o alters drug binding affinity independently of its effects on C-type inactivation. They further suggest that inhibition of pore mouth closure by elevated [K+]o does not have same effect on drug affinity as mutations removing C-type inactivation.

Localization of the ROMK protein on apical membranes of rat kidney nephron segments

The ATP-sensitive, inwardly rectifying K+ channel, ROMK, has been suggested to be the low-conductance ATP-sensitive K+ channel identified in apical membranes of mammalian renal thick ascending limb (TAL) and cortical collecting duct (CCD). Mutations in the human ROMK gene (KIR 1.2) have been identified in kindreds with neonatal Bartter's syndrome. In the present study, we generated polyclonal antibodies raised against both a COOH-terminal (amino acids 252-391) ROMK-maltose binding protein (MBP) fusion protein and an NH2-terminal (amino acids 34-49) ROMK peptide. Affinity-purified anti-ROMK COOH-terminal antibody detected the 45-kDa ROMK protein in kidney tissues and HEK-293 cells transfected with ROMK1 cDNA. The antibody also recognized 85- to 90-kDa proteins in kidney tissue; these higher molecular weight proteins were abolished by immunoabsorption with ROMK-MBP fusion protein and were also detected on Western blots using anti-ROMK NH2-terminal antibody. Immunofluoresence studies using anti-ROMK COOH-terminal antibody showed intense apical staining along the loop of Henle and distal nephron; staining with preimmune and immunoabsorbed serum was negative. When colocalized with distal nephron markers [the thiazide-sensitive cotransporter (rTSC1), the bumetanide-sensitive cotransporter (rBSC1), the vacuolar type H(+)-ATPase, and neuronal nitric oxide synthase (NOS I)], the ROMK protein was found primarily at the apical border of cells in the TAL, macula densa, distal convoluted tubule, and connecting tubule. Within the CCD, the ROMK protein was expressed in principal cells and was absent from intercalated cells. The tubule localization and polarity of ROMK staining are consistent with the distribution of ROMK mRNA and provide more support for ROMK being the low-conductance K+ secretory channel in the rat distal nephron.

Effect of descarboethoxyloratadine, the major metabolite of loratadine, on the human cardiac potassium channel Kv1.5

The effects of descarboethoxyloratadine (DCL), the major metabolite of loratadine, were studied on a human cardiac K+ channel (hKv1.5) cloned from human ventricle and stably expressed in a mouse cell line by means of the patch-clamp technique. DCL (1-100 microM) inhibited hKv1.5 current in a concentration-dependent manner with an apparent affinity constant of 12.5+/-1.2 microM. The blockade increased steeply over the voltage range of channel opening, which indicated that DCL binds preferentially to the open state of the channel. At more depolarized potentials a weaker voltage-dependence was observed consistent with a binding reaction sensing approximately 20% of the transmembrane electrical field. DCL, 20 microM, increased the time constant of deactivation of tail currents, thus inducing a 'crossover' phenomenon. The present results demonstrated that DCL blocked hKv1.5 channels in a concentration-, voltage-, and time-dependent manner.

Hypoxia inhibits gene expression of voltage-gated K+ channel alpha subunits in pulmonary artery smooth muscle cells

Activity of voltage-gated K+ channels (KV) in pulmonary arterial smooth muscle cells (PASMC) is pivotal in controlling membrane potential, cytoplasmic free Ca2+ concentration ([Ca2+]cyt, and pulmonary vasomotor tone. Acute hypoxia selectively inhibits KV channels, depolarizes PASMC, raises [Ca2+]cyt, and causes pulmonary vasoconstriction and vascular remodeling. Prolonged hypoxia (24-60 h) decreased significantly the mRNA levels of KV channel alpha subunits, KV1.2 and KV1.5. Consistently, the protein levels of KV1.2 and KV1.5 were also decreased significantly by hypoxia (48-72 h). Nevertheless, hypoxia affected negligibly the mRNA levels of KV channel beta subunits (KVbeta1, KVbeta2, and KVbeta3). The native K+ channels are composed of pore-forming alpha and auxiliary beta subunits. Assembly of KV beta subunits with alpha subunits confers rapid inactivation on the slowly or non-inactivating delayed rectifier KV channels. KV beta subunits also function as an open-channel blocker of KV channels. Thus, the diminished transcription and expression of KV alpha subunits may reduce the number of KV channels and decrease KC currents. Unchanged transcription of KV beta subunits may increase the fraction of the KV channel alpha subunits that are associated with beta subunits and further reduce the total KV currents. These data demonstrate a novel mechanism by which chronic hypoxia may cause pulmonary vasoconstriction and hypertension.

A rapidly activating sustained K+ current modulates repolarization and excitation-contraction coupling in adult mouse ventricle

1. The K+ currents which control repolarization in adult mouse ventricle, and the effects of changes in action potential duration on excitation-contraction coupling in this tissue, have been studied with electrophysiological methods using single cell preparations and by recording mechanical parameters from an in vitro working heart preparation. 2. Under conditions where Ca(2+)-dependent currents were eliminated by buffering intracellular Ca2+ with EGTA, depolarizing voltage steps elicited two rapidly activating outward K+ currents: (i) a transient outward current, and (ii) a slowly inactivating or 'sustained' delayed rectifier. 3. These two currents were separated pharmacologically by the K+ channel blocker 4-amino-pyridine (4-AP). 4-AP at concentrations between 3 and 200 microM resulted in (i) a marked increase in action potential duration and a large decrease in the sustained K+ current at plateau potentials, as well as (ii) a significant increase in left ventricular systolic pressure in the working heart preparation. 4. The current-voltage (I-V) relation, kinetics, and block by low concentrations of 4-AP strongly suggest that the rapid delayed rectifier in adult mouse ventricles is the same K+ current (Kv1.5) that has been characterized in detail in human and canine atria. 5. These results show that the 4-AP-sensitive rapid delayed rectifier is a very important repolarizing current in mouse ventricle. The enhanced contractility produced by 4-AP (50 microM) in the working heart preparation demonstrates that modulation of the action potential duration, by blocking a K+ current, is a very significant inotropic variable.

The pacemaker current I(f) in single human atrial myocytes and the effect of beta-adrenoceptor and A1-adenosine receptor stimulation

1. We used single human atrial myocytes to study I(f) occurrence, properties and pharmacological modulation. Cells were obtained by chunk enzymatic digestion from samples of right atrial appendages of patients undergoing corrective cardiac surgery. 2. Patch-clamped cells in the whole-cell configuration were superfused with a modified Tyrode solution to reduce contamination by interfering currents and to amplify I(f). The average cell membrane capacitance was 85.06+/-2.41 pF (n=531). Data were consistent with the geometrical dimensions of the cells (length 94.2+/-1.89 microm, width 17.9+/-0.42 microm, n=126). 3. When hyperpolarizing to -120 mV from a holding potential of -40 mV, 252 of 306 tested cells (82%) expressed a hyperpolarization-activated inward current (I(f) density=3.77+/-0.25 pA pF(-1)); the current was considered to be present in a given cell if its density at -120 mV was larger than 0.5 pA pF(-1). 4. Current activation was sigmoidal and fitted a Boltzmann model; the average activation curve (n=25) showed a maximum current amplitude of 205.97+/-19.94 pA, corresponding to 3.87+/-0.63 pA pF(-1), voltage of half-maximal activation (V(1/2)) at -86.68+/-2.19 mV and a slope of -11.39+/-0.69 mV. The reversal potential of I(f) measured by tail-current analysis was -13.07+/-1.92 mV (n=6). The addition of CsCl (5 mM) fully and reversibly blocked the current. 5. In the presence of the beta-adrenoceptor agonist isoprenaline (Iso, 1 microM), V(1/2) was significantly shifted toward less negative potentials by 6.06+/-1.96 mV (n=16, P=0.0039). The selective A1-adenosine receptor agonist cyclopentyladenosine (CPA, 1 microM) caused a statistically significant shift of V(1/2) toward more negative potentials with respect to the control curve, both in the absence (-7.37+/-1.83 mV, P=0.0005, n=11) and in the presence of 1 microM Iso (-4.97+/-1.78, P=0.031, n=6). 6. These results demonstrate that a current with the properties of I(f) described in cardiac primary and secondary pacemakers occurs in the majority of human atrial cells. While the pathophysiological relevance of I(f) in human atrial tissue remains to be defined, our data clearly show that it is modulated through stimulation of beta-adrenoceptors and A1-adenosine receptors.

Atrial fibrillation: antiarrhythmic therapy versus rate control with antithrombotic therapy

Atrial fibrillation is a major health problem in the United States, but the best strategies for treating it have not been rigorously determined in clinical studies. Specifically, there is a paucity of data comparing the approach of maintaining sinus rhythm using prophylactic antiarrhythmic drug therapy with the approach of controlling the ventricular response to atrial fibrillation while reducing embolic events with concomitant antithrombotic therapy. Until ongoing randomized trials are completed, which patients benefit most from a specific approach cannot be determined with certainty. In general, the most reasonable strategies include (1) the restoration of sinus rhythm (without prophylactic antiarrhythmic therapy) after the patient's first episode of atrial fibrillation; and (2) the maintenance of sinus rhythm (including the use of prophylactic antiarrhythmic therapy) in patients who remain symptomatic despite adequate rate control, and who are not at high risk for proarrhythmia and/or are unlikely to maintain sinus rhythm. The risks and benefits need to be carefully weighed in patients with truly asymptomatic atrial fibrillation. Many patients may require multiple attempts to maintain sinus rhythm. Current investigative treatment modalities (e.g., ablation techniques, atrial implantable cardioverter-defibrillators, new antiarrhythmic agents) are likely to alter the current approaches to atrial fibrillation.

Separable Kvbeta subunit domains alter expression and gating of potassium channels

Kvbeta subunits have been shown to affect kinetic properties of voltage-gated K+ channel Kv1alpha subunits and increase the number of cell surface dendrotoxin-binding sites when coexpressed with Kv1. 2. Here, we show that Kvbeta1.2 alters both current expression and gating of Kvalpha1 channels and that each effect is mediated by a distinct Kvbeta1.2 domain. The Kvbeta1.2 N terminus or Kvalpha1-blocking domain introduced steady state current block, an apparent negative shift in steady state activation, and a slowing of deactivation along with a dramatic reduction in single channel open probability. N-terminal deletions of Kvbeta1.2 no longer altered channel kinetics but promoted dramatic increases in Kv1.2 current. The conserved Kvbeta1 C terminus or Kvalpha1 expression domain alone was sufficient to increase the number of functional channels. The same effect was observed with the normally noninactivating subunit, Kvbeta2. By contrast, Kv1.5 currents were reduced when coexpressed with either the Kvbeta1 C terminus or Kvbeta2, indicating that the Kvalpha1 expression domain has Kvalpha1 isoform-specific effects. Our results demonstrate that Kvbeta subunits consist of two domains that are separable on the basis of both primary structure and functional modulation of voltage-gated K+ channels.

Mapping the kidney potassium channel ROMK1. Glycosylation of the pore signature sequence and the COOH terminus

ROMK1, also known as Kir 1.1, is an inwardly rectifying K+ channel and is the prototypical member of the large Kir gene family. The accepted model of Kir topology predicts intracellular NH2 and COOH termini, and two membrane-spanning segments, M1 and M2, connected by an intramembranous pore-forming segment, H5. The sequence of H5 is similar in voltage-dependent K+ channels and features a strictly conserved GY/FG in its mid-region, which has been proposed as the selectivity filter of the pore. We have been using N-glycosylation substitution mutants to map the extracellular topology of ROMK1 biochemically and have described several loci in H5 that were glycosylated. We now report glycosylation at loci Tyr144 and Phe146, which indicates that the signature GYG sequence (143-145) rather than being intramembranous is extracellular. The COOH terminus was predicted to begin at position 178, but contrary to the model, we observed that position 257 was glycosylated and surrounding positions at 199, 222, and 298 were unglycosylated. N-Glycosylation sequon substitution at the latter three positions abolished K+/Na+ selectivity. Our results suggest a major revision of the topology of ROMK1 with H5 and the pore signature sequence now completely extracellular. The COOH terminus appears to form two additional membrane-spanning segments and to contribute to the ion conduction pathway.

Decreased expression of Kv4.2 and novel Kv4.3 K+ channel subunit mRNAs in ventricles of renovascular hypertensive rats

Hypertension-induced cardiac hypertrophy is associated with alterations in ventricular action potentials. To understand molecular mechanisms underlying this electrical abnormality, expression of cardiac voltage-gated K+ channel subunit genes was examined in ventricles of renovascular hypertensive rats. While generating a rat Kv4.3 probe, we discovered a previously unreported 19-amino acid insertion in the C-terminal intracellular region of the channel subunit. RNase protection assays indicated that this novel isoform is predominant in rat lung and heart. Effects of renovascular hypertension were then determined by using renal artery clipping models: two-kidney, one clip (2K-1C) rats, a model of high-renin hypertension with a normal plasma volume, and one-kidney, one clip (1K-1C) rats, a model of normal renin with a raised plasma volume. Expression of Kv4.2 and Kv4.3 mRNAs was diminished by > 50% in ventricles of 2K-1C rats; however, no changes in the expression of Kv1.2, Kv1.4, Kv1.5, Kv2.1, or KvLQT1 mRNAs were detected. Similar downregulation of Kv4.2 and Kv4.3 mRNAs was detected in 1K-1C rats. Chronic administration of captopril, an angiotensin-converting enzyme inhibitor, blocked the development of hypertension and the suppression of Kv4 subfamily channel mRNA expression in 2K-1C rats. Furthermore, captopril administration to sham-operated rats significantly increased Kv4.2 mRNA. These results indicate that renovascular hypertension causes specific reductions in Kv4 subfamily channel mRNA expression and that this effect is likely to be mediated primarily by an increase in cardiac afterload.

Ionic remodeling underlying action potential changes in a canine model of atrial fibrillation

Rapid electrical activation, as occurs during atrial fibrillation (AF), is known to cause reductions in atrial refractoriness and in adaptation to heart rate of the atrial refractory period, which promote the maintenance of AF, but the underlying ionic mechanisms are unknown. In order to determine the cellular and ionic changes caused by chronic atrial tachycardia, we studied right atrial myocytes from dogs subjected to 1, 7, or 42 days of atrial pacing at 400/min and compared them with myocytes from sham-operated dogs (pacemaker inserted but not activated). Rapid pacing led to progressive increases in the duration of AF induced by bursts of 10-Hz stimuli (from 3 +/- 2 seconds in sham-operated dogs to 3060 +/- 707 seconds in dogs after 42 days of pacing, P < .001) and reduced atrial refractoriness and adaptation to rate of the atrial refractory period. Voltage-clamp studies showed that chronic rapid pacing did not alter inward rectifier K+ current, rapid or slow components of the delayed rectifier current, the ultrarapid delayed rectifier current, T-type Ca2+ current, or Ca(2+)-dependent Cl- current. In contrast, the densities of transient outward current (Ito) and L-type Ca2+ current (ICa) were progressively reduced as the duration of rapid pacing increased, without concomitant changes in kinetics or voltage dependence. In keeping with in vivo changes in refractoriness, action potential duration (APD) and APD adaptation to rate were decreased by rapid pacing. The response of the action potential and ionic currents flowing during the action potential (as exposed by action-potential voltage clamp) to nifedipine in normal canine cells and in cells from rapidly paced dogs suggested that the APD changes in paced dogs were largely due to reductions in ICa. We conclude that sustained atrial tachycardia reduces Ito and ICa, that the reduced ICa decreases APD and APD adaptation to rate, and that these cellular changes likely account for the alterations in atrial refractoriness associated with enhanced ability to maintain AF in the model.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

Autonomic influences in atrial ischemia: vagally mediated atrial conduction improvement

To investigate the effects of autonomic nerve activation on electrophysiological properties of ischemic atrial myocardium, experiments were performed in 10 open chest adult dogs anesthetized with xylazine and alpha-chloralose. Ischemia was created in the right atrial free wall by ligation of one or more branches of the right coronary artery. Bipolar electrograms were recorded from multiple sites in the ischemic and non-ischemic zones. The atria were paced at 400 ms and 180 ms to assess conduction properties. One hour after ligation, delayed activation, electrogram fractionation, and electrogram alternans were observed in the ischemic zone. All local conduction abnormalities were heart rate dependent in that they were only observed at a pacing cycle length of 180 ms. The average duration of ischemic zone electrograms was significantly prolonged from 17.7+/-1.6 ms to 26.4+/-1.6 ms (P<0.001). Right and left vagal stimulation significantly shortened the electrogram duration in the ischemic zone from 26.4+/-1.6 ms to 19.7+/-1.1 ms (P<0.01) and 20.0+/-1.1 ms (P<0.01), respectively. Ischemia-induced electrogram alternans was eliminated completely. During right and left stellate stimulation, electrogram duration was not altered and alternans was still present. In conclusion, vagal stimulation in this canine model improves local conduction in ischemic myocardium in the right atrium. This effect may be mediated by a reversal of the ischemia-induced membrane depolarization and a shortening of refractoriness in the atrium during vagal activation.

Nonlinear analysis of the pharmacological conversion of sustained atrial fibrillation in conscious goats by the class Ic drug cibenzoline

Methods from nonlinear dynamics were applied to test the hypothesis that the dynamics of sustained atrial fibrillation (AF) is modified by the class Ic drug cibenzoline during pharmacological conversion. The experiments were performed in conscious goats in which sustained AF was induced by continuous maintenance of AF via programmed electrical stimulation. Data were collected from electrophysiological experiments in five goats to terminate sustained AF by continuous infusion of cibenzoline. Sets of five unipolar epicardial electrograms of one minute duration were recorded from the left and right atrial free wall during sustained AF (control), and at three episodes during infusion of cibenzoline, when the mean AF interval had been prolonged to 25%, 50% and 85% with respect to control. Ventricular far-field potentials were removed from atrial electrograms by a coherent averaging procedure. Using the Grassberger-Procaccia method, the dynamics of the local atrial electrograms was investigated by estimating the (coarse-grained) correlation dimension and correlation entropy from the correlation integral. The results were related to a recently proposed classification (types I-III) of AF based on the degree of complexity of atrial activation patterns. The coarse-grained correlation dimension D(cg) and entropy K(cg) indicated that sustained AF corresponded to type II. During drug administration the coarse-grained parameters were not significantly different from control. Scaling regions in the correlation integral were observed after infusion of cibenzoline (3 out of 5 goats) suggesting that the drug introduced low-dimensional features (type I) in the dynamics of AF (correlation dimension D ranging from 2.8 to 4.4 and correlation entropy K from 1.6 to 6.2 nats/s). Sinus rhythm recorded shortly after cardioversion was very regular (D<2 and K<3 nats/s). The hypothesis that the electrograms during AF and sinus rhythm were generated by a static transformation of a linear Gaussian random process was rejected using a test for time reversibility. The nonlinear analysis revealed that cibenzoline does not significantly alter the dynamics of sustained AF during pharmacological conversion other than a slowing down of the atrial activation and a somewhat increasing global organization of the atrial activation pattern. The sudden change in the dynamical behavior at cardioversion suggests a mechanism that is reminiscent of a bifurcation. (c) 1997 American Institute of Physics.

Tachycardia-induced atrial myopathy: an important mechanism in the pathophysiology of atrial fibrillation?

The atrial myocardium of patients with chronic atrial fibrillation (AF) is often abnormal in its histologic features and in its electrophysiologic properties. These abnormalities have been interpreted in some cases as the cause of AF and in others as a consequence of AF. We believe that both are the case. We will review the features of this atrial myopathy and discuss the likely mechanisms and consequences of the process.

The antiarrhythmic agent bertosamil induces inactivation of the sustained outward K+ current in human atrial myocytes

1. In whole-cell patch-clamped human atrial myocytes, the antiarrhythmic agent bertosamil (10 microM) inhibited the sustained component, Isus (38.6 +/- 3.1%), and enhanced the inactivating component, I(t) (9.1 +/- 6.1%), of the outward K+ current elicited by 750 ms test pulses from -60 mV to +50 mV. Higher concentrations of bertosamil (> 10 microM) inhibited both I(t) and Isus. 2. Suppression of Isus and stimulation of I(t) by 10 microM bertosamil was observed on renewed stimulation following a 2 min rest period during which the drug was applied and persisted after washout, indicating a rest-dependent effect of bertosamil on the outward K+ current. 3. Cell dialysis with an internal solution containing 10 microM bertosamil increased both I(t) (78.0 +/- 14.7%) and Itotal (26.7 +/- 8.4%) and inhibited Isus (28.9 +/- 6.3%, n = 6). In the presence of intracellular bertosamil, external application of the drug inhibited I(t) and Isus in a concentration-dependent and use-dependent manner. 4. Following the suppression of Isus by 200 microM 4-aminopyridine (4-AP), bertosamil (10 microM) inhibited I(t). Washout of 4-AP was associated with a larger I(t) amplitude than that observed in control conditions. In myocytes characterized by a prominent Isus and lack of I(t), bertosamil (10 microM) induced a rapid and partial inactivation of the current, together with inward rectification of the current measured at the end of the test pulse. 5. In the presence of bertosamil the activation/voltage relationships, steady-state inactivation and recovery from inactivation of I(t) were markedly modified, pointing to changes in the conductance underlying I(t). 6. We conclude that bertosamil induces rapid inactivation of sustained outward current which leads to an apparent increase in I(t) and decrease in Isus. This effect, which was distinct from the use-dependent inhibition of the outward K+ current, could represent a new antiarrhythmic mechanism.

Direct-current cardioversion of atrial tachyarrhythmias under oral flecainide therapy

Results of direct-current cardioversion of 48 patients with atrial arrhythmia taking oral flecainide were compared with those of 96 control-matched patients. After completion of 3 shocks, 98% of flecainide patients were converted versus 78% of control patients (p <0.01). Thus, despite an experimental increase in defibrillation threshold, flecainide given before cardioversion seems to have a beneficial effect.

Selective enhancement of the slow component of delayed rectifier K+ current in guinea-pig atrial cells by external ATP

1. The effects of external ATP on the rapidly and slowly activating components (IKr and IKs, respectively) of the delayed rectifier K+ current (IK) in guinea-pig atrial myocytes were determined using the whole-cell configuration of the patch-clamp technique. 2. An envelope of tails test was conducted by applying depolarizing pulses to +40 mV from a holding potential of -40 mV for various durations between 50 ms and 2 s under control conditions and during exposure to 50 microM ATP. The ATP-induced IK, obtained by digital subtraction, exhibited a constant ratio (0.37) of the tail current to time-dependent current, regardless of the pulse duration. This current ratio was compatible with the predicted ratio of the driving force at +40 and -40 mV for a non-rectifying K+ conductance, suggesting that the ATP-induced IK is due primarily to IKs. 3. The amplitude of IKr isolated from the IK enhanced by ATP, determined as an E-4031 (5 microM)-sensitive current, was similar to the control magnitude of IKr, thus showing that external ATP did not cause an increase in IKr. 4. The voltage-dependent activation of the ATP-induced IK during 500 ms depolarizing test pulses could be described by a Boltzmann equation with a half-activation voltage (V1/2) of 11.5 mV and slope factor (k) of 12.0 mV, which were close to those of IKs (V1/2 of 12.1 mV and k of 12.3 mV), determined as an E-4031-resistant IK, under the same isochronal (500 ms) activation conditions. 5. These results provide evidence to suggest that extracellular ATP selectively potentiates the slow component of IK (IKs), with no measurable effects on IKr, in guinea-pig atrial myocytes.

Allosteric effects of permeating cations on gating currents during K+ channel deactivation

K+ channel gating currents are usually measured in the absence of permeating ions, when a common feature of channel closing is a rising phase of off-gating current and slow subsequent decay. Current models of gating invoke a concerted rearrangement of subunits just before the open state to explain this very slow charge return from opening potentials. We have measured gating currents from the voltage-gated K+ channel, Kv1.5, highly overexpressed in human embryonic kidney cells. In the presence of permeating K+ or Cs+, we show, by comparison with data obtained in the absence of permeant ions, that there is a rapid return of charge after depolarizations. Measurement of off-gating currents on repolarization before and after K+ dialysis from cells allowed a comparison of off-gating current amplitudes and time course in the same cells. Parallel experiments utilizing the low permeability of Cs+ through Kv1.5 revealed similar rapid charge return during measurements of off-gating currents at ECs. Such effects could not be reproduced in a nonconducting mutant (W472F) of Kv1.5, in which, by definition, ion permeation was macroscopically absent. This preservation of a fast kinetic structure of off-gating currents on return from potentials at which channels open suggests an allosteric modulation by permeant cations. This may arise from a direct action on a slow step late in the activation pathway, or via a retardation in the rate of C-type inactivation. The activation energy barrier for K+ channel closing is reduced, which may be important during repetitive action potential spiking where ion channels characteristically undergo continuous cyclical activation and deactivation.

Block of human cardiac Kv1.5 channels by loratadine: voltage-, time- and use-dependent block at concentrations above therapeutic levels

OBJECTIVE

The aim of this study was to analyze the effects of loratadine on a human cardiac K+ channel (hKv1.5) cloned from human ventricle and stably expressed in a mouse cell line.

METHODS

Currents were studied using the whole-cell configuration of the patch-clamp technique in Ltk- cells transfected with the gene encoding hKv1.5 channels.

RESULTS

Loratadine inhibited in a concentration-dependent manner the hKv1.5 current, the apparent affinity being 1.2 +/- 0.2 microM. The blockade increased steeply between -30 and 0 mV which corresponded with the voltage range for channel opening, thus suggesting that the drug binds preferentially to the open state of the channel. The apparent association and dissociation rate constants were (3.6 +/- 0.5) x 10(6).M-1.s-1 and 3.7 +/- 1.6.s-1, respectively. Loratadine, 1 microM, increased the time constant of deactivation of tail currents elicited on return to -40 mV after 500 ms depolarizing pulses to +60 mV from 36.2 +/- 3.4 to 64.9 +/- 3.6 ms (n = 6, P < 0.01), thus inducing a 'crossover' phenomenon. Application of trains of pulses at 1 Hz lead to a progressive increase in the blockade reaching a final value of 48.6 +/- 4.3%. Recovery from loratadine-induced block at -80 mV exhibited a time constant of 743.0 +/- 78.0 ms. Finally, the results of a mathematical stimulation of the effects of loratadine, based on an open-channel block model, reproduced fairly well the main effects of the drug.

CONCLUSIONS

The present results demonstrated that loratadine blocked hKv1.5 channels in a concentration-, voltage-, time- and use-dependent manner but only at concentrations much higher than therapeutic plasma levels in man.

Effect of phenylephrine infusion on atrial electrophysiological properties

OBJECTIVE

To determine the effect of changes in autonomic tone induced by phenylephrine infusion on atrial refractoriness and conduction.

DESIGN

Left and right atrial electrophysiological properties were measured before and after a constant phenylephrine infusion designed to increase sinus cycle length by 25%.

SUBJECTS

20 patients, aged 53 (SD 6) years, undergoing electrophysiological study for investigation of idiopathic paroxysmal atrial fibrillation (seven patients) or for routine follow up after successful catheter ablation of supraventricular tachycardia (13 patients).

MAIN OUTCOME MEASURES

Changes in left and right atrial effective refractory periods, atrial activation times, and frequency of induction of atrial fibrillation.

RESULTS

Phenylephrine (mean dose 69 (SD 18) mg/min) increased mean blood pressure by 22 (12) mm Hg (range 7 to 44) and lengthened sinus cycle length by 223 (94) ms (20 to 430). Left atrial effective refractory period lengthened following phenylephrine infusion from 250 (25) to 264 (21) ms (P < 0.001) but there was no significant change in right atrial effective refractory period: 200 (20) v 206 (29), P = 0.11. There was a significant relation between the effect of phenylephrine on sinus cycle length and on right atrial refractoriness (r = 0.6, P = 0.005) with shortening of right atrial refractoriness in patients with the greatest prolongation in sinus cycle length. During phenylephrine infusion, the right atrial stimulus to left atrial activation time at the basic pacing cycle length of 600 ms was unchanged, at 130 (18) v 131 (17) ms, but activation delay with a premature extrastimulus increased: 212 (28) v 227 (38) ms, P = 0.002. Atrial fibrillation was induced by two of 58 refractory period measurements at baseline and by 12 of 61 measurements during phenylephrine infusion (P < 0.01). Phenylephrine increased the difference between left and right atrial refractory periods by 22.8 (19.4) ms in the five patients with induced atrial fibrillation after phenylephrine compared to 0.9 (16.2) ms in the 13 patients without induced atrial fibrillation after phenylephrine infusion (P = 0.02).

CONCLUSIONS

Phenylephrine infusion increased left atrial refractoriness and intra-atrial conduction delay following a premature right atrial extrastimulus. Induction of atrial fibrillation during phenylephrine infusion was associated with non-uniform changes in atrial refractoriness. These data support the concept that changes in autonomic tone may precipitate atrial fibrillation in susceptible individuals.

Subunit combinations defined for K+ channel Kv1 subtypes in synaptic membranes from bovine brain

Voltage-dependent Shaker-related (Kv1) K+ channels are composed of transmembrane alpha subunits and peripheral Kv beta proteins that exist as octomers with (alpha)4(beta)4 stoichiometry. Although several alpha (designated Kv1.X) and three Kv beta subunits are known to be expressed in brain, their oligomeric combinations in neurons have yet to be deciphered. Herein, the subunits comprising a number of neuronal K+ channels from bovine brain cortex were deduced by immunoprecipitation and Western blotting, using antibodies specific for Kv1.X and Kv beta subtypes. Only a subset of the theoretically possible oligomers was detected, showing that the synthesis and/or assembly of these multisubunit K+ proteins is controlled to yield a limited variety of K+ channels. Except for a small population of Kv1.4 containing K+ channels, all the recognizable species contained Kv1.2 and beta2 subunits. Furthermore, several subpopulations were identified including a fully defined complex of Kv1.2/1.3/1.4/1.6 and Kv beta2, plus oligomers containing three or two assigned alpha subunits. Kv1.2 was also shown to occur in the absence of these other subunits as a putative homo-oligomer. Thus, for the first time, the complete subunit combination of an authentic K+ channel has been elucidated; also, the strategy employed to establish this can now be applied to closely related members of other K+ channel families.

Lymphocyte-specific inducible expression of potassium channel beta subunits

Many studies have shown that voltage-gated potassium (Kv) channel activity is essential for T-lymphocyte proliferation. The IL-2-inducible neuroimmune gene, I2rf5 is the mouse homologue of the rat Kv beta 2 subunit. In this study we show that in addition to constitutive expression in adult murine brain, expression of Kv channel subunits beta 1.1 and beta 2.1 is inducible in a cloned T-helper cell line stimulated with IL-2 and in normal murine splenocytes stimulated with Con A or LPS. This expression pattern appears to be lymphocyte specific, because stimulated fibroblasts and vascular smooth muscle cells do not express Kv beta channel subunit mRNA. These observations suggest that Kv beta subunit expression is tissue specific and inducible in stimulated lymphocytes. Because Kv beta subunits modulate K+ channel activity, the inducible and variable expression of these subunits in lymphocytes may represent an additional regulatory mechanism for lymphocyte proliferation.

A quantitative analysis of the activation and inactivation kinetics of HERG expressed in Xenopus oocytes

1. The human ether à-go-go-related gene (HERG) encodes a K+ channel that is believed to be the basis of the delayed rectified current, IKr, in cardiac muscle. We studied HERG expressed in Xenopus oocytes using a two-electrode and cut-open oocyte clamp technique with [K+]0 of 2 and 98 mM. 2. The time course of activation of the channel was measured using an envelope of tails protocol and demonstrated that activation of the heterologously expressed HERG current (IHERG) was sigmoidal in onset. At least three closed states were required to reproduce the sigmoid time course. 3. The voltage dependence of the activation process and its saturation at positive voltages suggested the existence of at least one relatively voltage-insensitive step. A three closed state activation model with a single voltage-insensitive intermediate closed state was able to reproduce the time and voltage dependence of activation, deactivation and steady-state activation. Activation was insensitive to changes in [K+]0. 4. Both inactivation and recovery time constants increased with a change of [K+]0 from 2 to 98 mM. Steady-state inactivation shifted by approximately 30 mV in the depolarized direction with a change from 2 to 98 mM K+0. 5. Simulations showed that modulation of inactivation is a minimal component of the increase of this current by [K+]0, and that a large increase in total conductance must also occur.

Electrophysiologic Effects of the New Class III Antiarrhythmic Drug Dofetilide in an Experimental Canine Model of Pacing-induced Atrial Fibrillation

BACKGROUND: Dofetilide is a new class III antiarrhythmic drug currently under investigation for the treatment of supraventricular arrhythmias in humans. Dofetilide have been previously shown to be highly effective in terminating and suppressing reentrant atrial flutter in the experimental canine crush-injury model, in which its antiarrhythmic efficacy was correlated with prolongation of wavelength and reduction in dispersion of refractoriness, effects not produced by the class IA antiarrhythmic drug quinidine. The purpose of this study was to evaluate the antiarrhythmic efficacy and mechanisms of action of dofetilide in an experimental model of atrial fibrillation. METHODS AND RESULTS: Dofetilide was administered intravenously to seven open-chest dogs with acute sustained atrial fibrillation induced by rapid atrial pacing for up to 4 hours. Mean atrial effective refractory period (ERP), dispersion of ERP, conduction velocity and wave-length were determined by multipoint right atrial programmed stimulation and activation mappin gusing a 56-electrode mapping plaque on the right atrial free wall. Dofetilide prolonged average ERP by 22% from 104 +/- 13 to 127 +/- 15 ms (P <.001), prolonged maximum ERP by 11% from 129 +/- 7 to 143 +/- 10 (P <.003), had no effect on conduction velocity at 200 ms pacing cycle length, slowed conduction velocity by 16% from 0.89 +/- 12 to 0.75 +/-.17 ms at 150 ms pacing cycle length, slowed conduction velocity by 16% from 0.89 +/- 12 to 0.75 +/-.17 ms at 150 ms pacing cycle length (P <.001), increased wavelength by 20% from 93 +/- 7 to 112 +/- 9 mm (P <.01), reduced dispersion of ERP by 24% from 11.4 +/- 2.9 to 8.7 +/- 2.3 (P =.016), and reduced the number of adjacent electrodes with ERP difference >/=20 ms by 67% from 18.4 +/- 7.1 to 6.1 +/- 4.2 (P <.001). Dofetilide reduced the number of excitation wavelets (total over three beats) entering the region of the mapping plaque by 38% from 5.0 +/-.8 to 3.1 +/-.4 (P <.002). Dofetilide terminated atrial fibrillation in all seven dogs at a mean of 3.4 +/- 2.2 minutes into the loading infusion and prevented reinduction of atrial fibrillation in all seven dogs after completion of the loading infusion, while on maintenance infusion. Time to termination of atrial fibrillation correlated closely with change in ERP (r = 0.78, P =.036). CONCLUSIONS: Dofetilide was highly effective in terminating and suppressing sustained pacing induced atrial fibrillation in this canine model. Time to termination of atrial fibrillation correlated with the degree of change in ERP produced by dofetilide. The mechanism of termination of atrial fibrillation by dofetilide appeared to be a progressive reduction and eventual extinction of re-entrant wavelets. The predominant electrophysiologic effects of dofetilide were prolongation of ERP and wavelength and a reduction in dispersion of refractoriness. Dofetilide had little effect on conduction velocity in this model, except at very short pacing cycle lengths.

Regional localization of ERG, the channel protein responsible for the rapid component of the delayed rectifier, K+ current in the ferret heart

Repolarization of the cardiac action potential varies widely throughout the heart. This could be due to the differential distribution of ion channels responsible for repolarization, especially the K+ channels. We have therefore studied the cardiac localization of ERG, a channel protein known to play an important role in generation of the rapid component of the delayed rectifier K+ current (IKr), an important determinant of the repolarization waveform, Cryosections of the ferret atrium and ventricle were prepared to determine the localization of ERG by fluorescence in situ hybridization (FISH) and immunofluorescence. We found that in the ferret, ERG transcript and protein expression was most abundant in the epicardial cell layers throughout most of the ventricle, except at the base. In the atrium, we found that ERG is most abundant in the medial right atrium, especially in the trabeculae and the crista terminalis of the right atrial appendage. It also is present in areas within the sinoatrial node. In all regions studied, FISH and immunofluorescence showed concordant localization patterns. These data suggest that repolarization mediated by IKr is not uniform throughout the ferret heart and provide a molecular explanation for heterogeneity in action potential repolarization throughout the mammalian heart.

Purification, characterization, and synthesis of an inward-rectifier K+ channel inhibitor from scorpion venom

We have purified a protein inhibitor of an inward-rectifier K+ channel, ROMK1, from the venom of the scorpion Leiurus quinquestriatus var. hebraeus. The inhibitor is Lq2, a previously discovered blocker of voltage- and Ca2+-activated K+ channels. Mutations were made on the channel and the inhibitor, and the resulting effects were examined using an electrophysiological assay. The data show that Lq2 blocks the pore of ROMK1, and that the interaction surface on Lq2 is the same for binding to inward-rectifier, voltage-activated, or Ca2+-activated K+ channels. These findings support the notion that different classes of K+ channels have different gates but a similar K+-selective pore structure.

Rapid inactivation determines the rectification and [K+]o dependence of the rapid component of the delayed rectifier K+ current in cardiac cells

Two characteristic features of the rapid component of the cardiac delayed rectifier current (IKr) are prominent inward rectification and an unexpected reduction in activating current with decreased [K+]o. Similar features are observed with heterologous expression of HERG, the gene thought to encode the channel carrying IKr, moreover, recent studies indicate that the mechanism underlying rectification of HERG current is the inactivation that channels rapidly undergo during depolarizing pulses. The present studies were designed to determine the mechanism of IKr rectification and [K+]o sensitivity in the mouse atrial myocyte cell line, AT-1 cells. Reducing [Mg2+]i to 0, which reverses inward rectification of some K+ channels, did not alter IKr current-voltage relationships, although it did decrease sensitivity to the IKr blockers dofetilide and quinidine 2- to 5-fold. To determine the presence and extent of fast inactivation of IKr in AT-1 cells, a brief hyperpolarizing pulse (20 ms to -120 mV) was applied during long depolarizations. Immediately after this pulse, a very large outward current that decayed rapidly to the previous activating current baseline was observed. This outward current component was blocked by the IKr-specific inhibitor dofetilide, indicating that it represented recovery from fast inactivation during the hyperpolarizing step, with fast reinactivation during the return to depolarized potential. With removal of inactivation using this approach, current-voltage relationships for IKr ([K+]o, 1 to 20 mmol/L) were linar and reversed close to the predicted Nernst potential for K+. In addition, decreased [K+]o decreased the time constants for open-->inactivated and inactivated-->open transitions. Thus, in these cardiac myocytes, as with heterologously expressed HERG, IKr undergoes fast inactivation that determines its characteristic inward rectification. These studies demonstrate that the mechanism underlying decreased activating current observed at low [K+]o is more extensive fast inactivation.

Comparative effects of nonsedating histamine H1 receptor antagonists, ebastine and terfenadine, on human Kv1.5 channels

The effects of ebastine and terfenadine, long-acting nonsedating histamine H1 receptor antagonists, were studied on hKv1.5 channels using the whole-cell voltage-clamp configuration of the patch-clamp technique in Ltk- cells transfected with the gene encoding the hKv1.5 channel. Upon depolarization to +60 mV, terfenadine, 1 microM and 3 microM, inhibited the hKv1.5 current by 42.4 +/- 6.4% and 69.3 +/- 4.2% (P < 0.01). In contrast, at the same range of concentrations, ebastine-induced inhibition of this K+ current averaged 6.5 +/- 2.0% and 13.0 +/- 2.0 (P < 0.05). At the highest concentration tested (3 microM) neither terfenadine carboxylate nor carebastine significantly modified hKv1.5 current. All these results suggest that ebastine could represent a safer alternative to terfenadine in the clinical practice.

Kvbeta2 inhibits the Kvbeta1-mediated inactivation of K+ channels in transfected mammalian cells

Cloned auxiliary beta-subunits (e.g. Kvbeta1) modulate the kinetic properties of the pore-forming alpha-subunits of a subset of Shaker-like potassium channels. Coexpression of the alpha-subunit and Kvbeta2, however, induces little change in channel properties. Since more than one beta-subunit has been found in individual K+ channel complexes and expression patterns of different beta-subunits overlap in vivo, it is important to test the possible physical and/or functional interaction(s) between different beta-subunits. In this report, we show that both Kvbeta2 and Kvbeta1 recognize the same region on the pore-forming alpha-subunits of the Kv1 Shaker-like potassium channels. In the absence of alpha-subunits the Kvbeta2 polypeptide interacts with additional beta-subunit(s) to form either a homomultimer with Kvbeta2 or a heteromultimer with Kvbeta1. When coexpressing alpha-subunits and Kvbeta1 in the presence of Kvbeta2, we find that Kvbeta2 is capable of inhibiting the Kvbeta1-mediated inactivation. Using deletion analysis, we have localized the minimal interaction region that is sufficient for Kvbeta2 to associate with both alpha-subunits and Kvbeta1. This mapped minimal interaction region is necessary and sufficient for inhibiting the Kvbeta1-mediated inactivation, consistent with the notion that the inhibitory activity of Kvbeta2 results from the coassembly of Kvbeta2 with compatible alpha-subunits and possibly with Kvbeta1. Together, these results provide biochemical evidence that Kvbeta2 may profoundly alter the inactivation activity of another beta-subunit by either differential subunit assembly or by competing for binding sites on alpha-subunits, which indicates that Kvbeta2 is capable of serving as an important determinant in regulating the kinetic properties of K+ currents.

KvLQT1, a voltage-gated potassium channel responsible for human cardiac arrhythmias

The clinical features of long QT syndrome result from episodic life-threatening cardiac arrhythmias, specifically the polymorphic ventricular tachycardia torsades de pointes. KVLQT1 has been established as the human chromosome 11-linked gene responsible for more than 50% of inherited long QT syndrome. Here we describe the cloning of a full-length KVLQT1 cDNA and its functional expression. KVLQT1 encodes a 676-amino acid polypeptide with structural characteristics similar to voltage-gated potassium channels. Expression of KvLQT1 in Xenopus oocytes and in human embryonic kidney cells elicits a rapidly activating, K+-selective outward current. The I(Kr)-specific blockers, E-4031 and dofetilide, do not inhibit KvLQT1, whereas clofilium, a class III antiarrhythmic agent with the propensity to induce torsades de pointes, substantially inhibits the current. Elevation of cAMP levels in oocytes nearly doubles the amplitude of KvLQT1 currents. Coexpression of minK with KvLQT1 results in a conductance with pharmacological and biophysical properties more similar to I(Ks) than other known delayed rectifier K+ currents in the heart.

Restoring Sinus Rhythm in Patients With Atrial Flutter and Fibrillation: Pharmacologic or Electrical Cardioversion?

Atrial fibrillation and atrial flutter, the most frequently encountered tachyarrhythmias requiring treatment, have become a major focus for clinical and basic research in recent years. Restoration and maintenance of sinus rhythmn, having been shown to improve exercise capacity, alleviate symptoms, and reduce the incidence of thromboembolic events, may be the optimal management strategy. Identification of the safest, most efficacious and cost-effective means of restoring sinus rhythm is necessary prior to the institution of optimal antiarrhythmic therapy to maintain sinus rhythm. Potential advantages of pharmacologic compared with electrical cardioversion include lack of need for general anesthesia and likely lower cost. Pharmacologic conversion include lack of need for general anesthesia and likely lower cost. Pharmacologic conversion has been accomplished with drugs that prolong atrial refractorinerss, including class Ia (quinidine, procainamide, disopyramide), class Ic (flecainide, propafenone), and class II (sotalol, amiodarone) compounds. The so-called pure class III agents were created to overcome the blocker side effects of sotalol and the complex pharmacodynamic profile of amiodarone. Two such agents are dofetilide, which selectively blocks the rapid component of the delayed rectifier current (Ikr) and ibutilide, which augments the slow inward sodium current, with a smaller component of action mediated by the block of Ikr. Reported overall conversion rates for recent onset atrial fibrillation and atrial flutter were 31% and 54% for difetilide, respectively, and 29-31% and 38-63%, respectively, for ibutilide. Proarrhythmia, manifested as polymorphic ventricular tachycardia requiring cardioversion, was a significant early side effect of both agents. Data from clinical trtials with these new agents, combined with increasing nowledge of the electrophysiologic substrate for these arrhythmias, has renewed initerest in the development of safer, more efficacious class IIIdrugs for atrial fibrillation and atrial flutter conversion.

Insights into mechanisms of antiarrhythmic drug action from experimental models of atrial fibrillation

Atrial fibrillation (AF) remains a challenge to medical therapy. Over the past several years, a variety of experimental models of AF have been developed. These have provided insights into mechanisms underlying AF and antiarrhythmic drug action against the arrhythmia. A variety of drugs effective against clinical AF, including flecainide, propafenone, procainamide, and sotalol, have been found to terminate experimental AF. All of these agents appear to act by prolonging the wavelength for atrial reentry at rapid rates, thereby increasing the size and decreasing the number of functional circuits maintaining the arrhythmia. While the ability to terminate AF is determined by refractoriness prolongation at rapid rates, refractoriness prolongation at slow rates (e.g., sinus rhythm) can prevent AF induction by premature beats. Thus, drugs with strong reverse use-dependence (like sotalol) may be much more effective in preventing than in terminating AF. Spacial heterogeneity in refractoriness is an important contributor to AF occurrence in some models, particularly vagal AF, and is reduced by some (but not all) drugs that terminate AF. New insights are being gained into mechanisms of electrical remodeling, which promotes AF maintenance when rapid atrial rates are maintained, such as during AF. This electrical remodeling may be an interesting novel target for therapy of AF. Insights into AF mechanisms obtained in experimental models of AF should help in the development of new and improved therapeutic approaches.

Comparisons of oral propafenone and sotalol as an initial treatment in patients with symptomatic paroxysmal atrial fibrillation

The main goal of this study is to evaluate the safety and efficacy of propafenone versus sotalol as an initial choice of treatment in patients with symptomatic paroxysmal atrial fibrillation (AF), according to a double-blind randomized system. In the oral propafenone group (n = 41), 2 patients (5%) discontinued therapy because of gastrointestinal discomfort in 1 and dizziness in the other. Thirty-one (79%) of the 39 patients who continued the treatment had effective response to oral propafenone (>75% reduction of symptomatic arrhythmic attacks) on a mean dose of 663 +/- 99 mg/day with a decrease in attack frequency from 10 +/- 3 to 2 +/- 1 times per week. In the oral sotalol group (n = 38), 4 patients (11%) discontinued treatment because of dizziness in 2 and symptomatic bradycardia in 2. Twenty-six of the 34 patients (76%) who continued the treatment had effective response to oral sotalol on a mean dose of 200 +/- 57 mg/day with a decrease in attack frequency from 11 +/- 3 to 2 +/- 1 times per week. Comparisons of the results between propafenone and sotalol groups showed a similar incidence of intolerable (2 of 41 vs 4 of 38, p = 0.42) and tolerable side effects (10 of 39 vs 8 of 34, p = 1.0). The attack frequency at baseline (11 +/- 3 vs 10 +/- 4 times per week, p = 0.23) and after treatment (3 +/- 1 vs 3 +/- 2 times per week, p = 0.85) did not differ significantly between the 2 groups. The incidence of effective response to drugs was also similar (31 of 39 vs 26 of 34, p = 0.78). Furthermore, the decrease of symptom scores (-32 +/- 8% vs -29 +/- 7%, p = 0.18) and percentage decrease of ventricular rate (-15 +/- 4% vs -18 +/- 4%, p = 0.10) during AF were also similar between the 2 groups. In conclusion, oral propafenone and sotalol are equally effective and safe in preventing attacks and alleviating symptoms of paroxysmal AF.

Antisense oligodeoxynucleotides directed against Kv1.5 mRNA specifically inhibit ultrarapid delayed rectifier K+ current in cultured adult human atrial myocytes

Several cloned K+ channel subunits are candidates to underlie macroscopic currents in the human heart, but direct evidence bearing on their role is lacking. The Kv1.5 K+ channel subunit has been suggested to play a potential role in human cardiac ultrarapid delayed rectifier (IKur) and transient outward (Ito) currents. To evaluate the role of proteins encoded by the Kv1.5 gene, we incubated cultured human atrial myocytes for 48 hours in medium containing antisense phosphorothioate oligodeoxynucleotides directed against octodecameric segments of the Kv1.5 mRNA coding sequence, the same concentration of homologous oligodeoxynucleotides with four mismatch mutations, or vehicle (control group). Cells exposed to antisense showed a highly significant (approximately 50%) reduction in IKur whether measured by step current at the end of a 400-millisecond depolarizing pulse, tail current at -20 mV, or current sensitive to a concentration of 4-aminopyridine (50 mumol/L) that is highly selective for IKur compared with control cells or cells exposed to mismatch oligodeoxynucleotides. In contrast, Ito was not different among the three experimental groups. When cultured human ventricular myocytes were exposed to Kv1.5 antisense oligodeoxynucleotides with the same controls, no changes occurred in either Ito or the sustained current at the end of a depolarizing pulse. We conclude that Kv1.5 channel subunits are essential to the expression of IKur and do not play a role in Ito in cultured human atrial myocytes. These studies provide the first direct evidence with an antisense approach for the equivalence between a macroscopic cardiac K+ current and a cloned K+ channel subunit and offer insights into the molecular electrophysiology of the human heart.

Gating charge and ionic currents associated with quinidine block of human Kv1.5 delayed rectifier channels

1. The mechanism of quinidine-induced ionic and gating current inhibition was studied in human Kv1.5 (hKv1.5) delayed rectifier channels expressed in human embryonic kidney cells. In the steady state, quinidine produced a voltage-dependent block between +30 and +120 mV (Kd at +60 mV = 7.2 microM) with an equivalent electrical distance, delta, of 0.29 +/- 0.06 and 0.26 +/- 0.05 at 10 and 50 microM quinidine, respectively. The apparent affinity at 0 mV (Kd) was 25 microM at 10 microM quinidine and 38 microM at 50 microM quinidine. The data suggested a quinidine binding site that sensed 20-30% of the transmembrane electrical field, from the inside. Non-steady-state measurements indicated rapid open channel block with mean time constants of 2.1 +/- 0.9 and 1.2 +/- 0.2 ms at 10 and 50 microM quinidine, respectively. 2. 'On' gating current (on-Ig) was unaffected over a wide range of potentials and between 10 and 100 microM quinidine. On-gating charge (Qon) was similarly conserved in the steady state between -100 and +50 mV. On return to -100 mV, quinidine slowed the off-gating current (off-Ig) after depolarizations more positive than -25 mV. After depolarizations to +50 mV, only 59 +/- 3.4% (10 microM quinidine) and 6.6 +/- 9.5% (100 microM quinidine) of the charge returned within 25 ms, compared with 100% in control. Due to the conservation of Qon in subsequent pulses, the remaining charge must have returned during the subsequent 10 s interpulse interval. 3. A threshold for quinidine action on off-Ig was established positive to -25 mV. The voltage dependence of Qoff immobilization at more positive potentials than +20 mV had an equivalent electrical distance of 0.32 +/- 0.04 (10 microM quinidine) and 0.20 +/- 0.32 (100 microM quinidine) with calculated Kd values of 21.6 +/- 4.6 and 16.2 +/- 8.4 microM at 10 and 100 microM quinidine, respectively. These characteristics of block are in good agreement with values obtained from ionic data. 4. Simultaneous measurements of ionic and gating currents confirmed, after subtraction, an ionic current threshold at -21.8 +/- 1.8 mV. The gating current data confirm directly that ionic current block by quinidine occurs by binding at a site on the hKv1.5 channel that becomes accessible when the channel opens. There was no evidence for action of quinidine on kinetic states prior to the open state at concentrations of quinidine up to 100 microM.

4-aminopyridine activates potassium currents by activation of a muscarinic receptor in feline atrial myocytes

1. The effects of 4-aminopyridine (4-AP) on action potentials, macroscopic membrane currents and single-channel recording from cardiac left atrial myocytes of the adult cat were studied using the whole-cell and cell-attached configurations of the patch-clamp technique. 2. 4-AP (1 mM) produced a hyperpolarization of the resting membrane potential and a shortening of action potential duration. Under voltage-clamp conditions, we have found that 4-AP increased a background current and a delayed rectifier outward current. These effects were antagonized by atropine. In addition, both effects seemed to be mediated through a pertussis toxin-sensitive G protein. 3. The background current induced by 4-AP displayed properties that are highly similar to those of the inwardly rectifying potassium current activated by acetylcholine (IK(ACh)). The time-dependent potassium current activated by 4-AP has kinetic and pharmacological properties different from those of the delayed rectifier potassium current previously identified in cardiac myocytes. 4. The activation of the delayed rectifier-like potassium current could be explained by the activation of a novel muscarinic receptor subtype in which acetylcholine acts as the antagonist. Another possibility is that 4-AP activates IK(ACh) in a time- and voltage-dependent manner.