The aromatic cluster in KCHIP1b affects Kv4 inactivation gating

The KChIP1b splice variant has been shown to induce slow recovery from inactivation for Kv4.2 whereas KChIP1a enhanced the recovery. Both splice variants differ only by the insertion of the exon1b, rich in aromatic residues (5/11). We analysed in detail the modifications of Kv4.2 gating induced by the KChIP1b splice variant and the role for the aromatic cluster in KChIP1b in inducing these changes. By substituting alanine for the aromatic residues individually or in combination, we could convert the KChIP1b recovery behaviour into that of KChIP1a. The replacement of one or two aromatic residues resulted in a partial restitution of the KChIP1a recovery behaviour. When three aromatic residues were replaced in the exon1b, the recovery from inactivation was fast with time constants that were similar to those obtained with KChIP1a. Moreover, similar findings were observed for closed state inactivation and for the voltage dependence of inactivation. Thus, reduction of the side chain bulkiness in exon1b resulted in the conversion of the KChIP1b phenotype into the KChIP1a phenotype. These results indicate that the aromatic cluster in exon1b modulates the transitions towards and from the closed inactivated states and the steady state distribution over the respective states.

A single hERG mutation underlying a spectrum of acquired and congenital long QT syndrome phenotypes

The long QT syndrome (LQTS) is a multi-factorial disorder that predisposes to life-threatening arrhythmias. Both hereditary and acquired subforms have been identified. Here, we present clinical and biophysical evidence that the hERG mutation c.1039 C>T (p.Pro347Ser or P347S) is responsible for both the acquired and the congenital phenotype. In one case the genotype remained silent for years until the administration of several QT-prolonging drugs resulted into a full-blown phenotype, that was reversible upon cessation of these compounds. On the other hand the mutation was responsible for a symptomatic congenital LQTS in a Dutch family, displaying a substantial heterogeneity of the clinical symptoms. Biophysical characterization of the p.Pro347Ser potassium channels using whole-cell patch clamp experiments revealed a novel pathogenic mechanism of reciprocal changes in the inactivation kinetics combined with a dominant-negative reduction of the functional expression in the heterozygous situation, yielding a modest genetic predisposition for LQTS. Our data show that in the context of the multi-factorial aetiology underlying LQTS a modest reduction of the repolarizing power can give rise to a spectrum of phenotypes originating from one mutation. This observation increases the complexity of genotype-phenotype correlations in more lenient manifestations of the disease and underscores the difficulty of predicting the expressivity of the LQTS especially for mutations with a more subtle impact such as p.Pro347Ser.

Modulation of HERG gating by a charge cluster in the N-terminal proximal domain

Human ether-a-go-go-related gene (HERG) potassium channels contribute to the repolarization of the cardiac action potential and display unique gating properties with slow activation and fast inactivation kinetics. Deletions in the N-terminal 'proximal' domain (residues 135-366) have been shown to induce hyperpolarizing shifts in the voltage dependence of activation, suggesting that it modulates activation. However, we did not observe a hyperpolarizing shift with a subtotal deletion designed to preserve the local charge distribution, and other deletions narrowed the region to the KIKER containing sequence 362-372. Replacing the positively charged residues of this sequence by negative ones (EIEEE) resulted in a -45 mV shift of the voltage dependence of activation. The shifts were intermediate for individual charge reversals, whereas E365R resulted in a positive shift. Furthermore, the shifts in the voltage dependence were strongly correlated with the net charge of the KIKER region. The apparent speeding of the activation was attributable to the shifted voltage dependence of activation. Additionally, the introduction of negative charges accelerated the intermediate voltage-independent forward rate constant. We propose that the modulatory effects of the proximal domain on HERG gating are largely electrostatic, localized to the charged KIKER sequence.

Obligatory heterotetramerization of three previously uncharacterized Kv channel alpha-subunits identified in the human genome

Voltage-gated K(+) channels control excitability in neuronal and various other tissues. We identified three unique alpha-subunits of voltage-gated K(+)-channels in the human genome. Analysis of the full-length sequences indicated that one represents a previously uncharacterized member of the Kv6 subfamily, Kv6.3, whereas the others are the first members of two unique subfamilies, Kv10.1 and Kv11.1. Although they have all of the hallmarks of voltage-gated K(+) channel subunits, they did not produce K(+) currents when expressed in mammalian cells. Confocal microscopy showed that Kv6.3, Kv10.1, and Kv11.1 alone did not reach the plasma membrane, but were retained in the endoplasmic reticulum. Yeast two-hybrid experiments failed to show homotetrameric interactions, but showed interactions with Kv2.1, Kv3.1, and Kv5.1. Co-expression of each of the previously uncharacterized subunits with Kv2.1 resulted in plasma membrane localization with currents that differed from typical Kv2.1 currents. This heteromerization was confirmed by co-immunoprecipitation. The Kv2 subfamily consists of only two members and uses interaction with "silent subunits" to diversify its function. Including the subunits described here, the "silent subunits" represent one-third of all Kv subunits, suggesting that obligatory heterotetramer formation is more widespread than previously thought.

Effects of antiarrhythmic drugs on cloned cardiac voltage-gated potassium channels expressed in Xenopus oocytes

The effects of 17 commonly used antiarrhythmic drugs on the rapidly activating cardiac voltage-gated potassium channels (Kv1.1, Kv1.2, Kv1.4, Kv1.5, Kv2.1 and Kv4.2) were studied in the expression system of the Xenopus oocyte. A systematic overview on basic properties was obtained using a simple and restricted experimental protocol (command potentials 10 mV and 50 mV positive to the threshold potential; concentration of 100 micromol/l each). The study revealed that 8 of 17 drugs yielded significant effects (changes >10% of control) on at least one type of potassium channel in the oocyte expression system. These drugs were ajmaline, diltiazem, flecainide, phenytoin, propafenone, propranolol, quinidine and verapamil, whereas the effects of adenosine, amiodarone, bretylium, disopyramide, lidocaine, mexiletine, procainamide, sotalol and tocainide were negligible. The drug effects were characterized by reductions of the potassium currents (except for quinidine and ajmaline). A voltage-dependence of drug effect was found for quinidine, verapamil and diltiazem. The different effect of the drugs was not related to the fast or slow current inactivation of the potassium channels (except for verapamil). Profiles of the individual drug effects at the different potassium channel types were identical for propafenone and flecainide and differed for all other substances. The study demonstrates marked differences in sensitivity to antiarrhythmic drugs within the group of voltage-operated cardiac potassium channel types. Taking the restrictions of the oocyte system into consideration, the findings suggest that several antiarrhythmic drugs exert significant effects at rapidly activating cardiac potassium channels.

Effects of a quaternary bupivacaine derivative on delayed rectifier K(+) currents

Block of hKv1.5 channels by R-bupivacaine has been attributed to the interaction of the charged form of the drug with an intracellular receptor. However, bupivacaine is present as a mixture of neutral and charged forms both extra- and intracellularly. We have studied the effects produced by the R(+) enantiomer of a quaternary bupivacaine derivative, N-methyl-bupivacaine, (RB(+)1C) on hKv1.5 channels stably expressed in Ltk(-) cells using the whole-cell configuration of the patch-clamp technique. When applied from the intracellular side of the membrane, RB(+)1C induced a time- and voltage-dependent block similar to that induced by R-bupivacaine. External application of 50 microM RB(+)1C reduced the current at +60 mV by 24+/-2% (n=10), but this block displayed neither time- nor voltage-dependence. External RB(+)1C partially relieved block induced by R-bupivacaine (61+/-2% vs 56+/-3%, n=4, P<0.05), but it did not relieve block induced by internal RB(+)1C. In addition, it did not induce use-dependent block, but when applied in combination with internal RB(+)1C a use-dependent block that increased with pulse duration was observed. These results indicate that RB(+)1C induces different effects on hKv1.5 channels when applied from the intra or the extracellular side of the membrane, suggesting that the actions of bupivacaine are the resulting of those induced on the external and the internal side of hKv1.5 channels.

Structure and function of cardiac potassium channels

Recent advances in molecular biology have had a major impact on our understanding of the biophysical and molecular properties of ion channels. This review is focused on cardiac potassium channels which, in general, serve to control and limit cardiac excitability. Approximately 60 K+ channel subunits have been cloned to date. The (evolutionary) oldest potassium channel subunits consist of two transmembrane (Tm) segments with an intervening pore-loop (P). Channels formed by four 2Tm-1P subunits generally function as inwardly rectifying K(+)-selective channels (KirX.Y): they conduct substantial current near the resting potential but carry little or no current at depolarized potentials. The inward rectifier IK1 and the ligand-gated KATP and KACh channels are composed of such subunits. The second major class of K+ channel subunits consists of six transmembrane segments (S1-S6). The S5-P-S6 section resembles the 2Tm-1P subunit, and the additional membrane-spanning segments (especially the charged S4 segment) endow these 6Tm-1P channels with voltage-dependent gating. For both major families, four subunits assemble into a homo- or heterotetrameric channel, subject to specific subunit-subunit interactions. The 6Tm-1P channels are closed at the resting potential, but activate at different rates upon depolarization to carry sustained or transient outward currents (the latter due to inactivation by different mechanisms). Cardiac cells typically display at least one transient outward current and several delayed rectifiers to control the duration of the action potential. The molecular basis for each of these currents is formed by subunits that belong to different Kvx.y subfamilies and alternative splicing can contribute further to the diversity in native cells. These subunits display distinct pharmacological properties and drug-binding sites have been identified. Additional subunits have evolved by concatenation of two 2Tm-1P subunits (4Tm-2P); dimers of such subunits yield voltage-independent leak channels. A special class of 6Tm-1P subunits encodes the 'funny' pacemaker current which activates upon hyperpolarization and carries both Na+ and K+ ions. The regional heterogeneity of K+ currents and action potential duration is explained by the heterogeneity of subunit expression, and significant changes in expression occur in cardiac disease, most frequently a reduction. This electrical remodelling may also be important for novel antiarrhythmic therapeutic strategies. The recent crystallization of a 2Tm-1P channel enhances the outlook for more refined molecular approaches.

Functional expression of an inactivating potassium channel (Kv4.3) in a mammalian cell line

OBJECTIVE

The goal of this study was to characterize the electrophysiological properties of the Kv4.3 channels expressed in a mammalian cell line.

METHODS

Currents were recorded using the whole-cell voltage clamp technique.

RESULTS

The threshold for activation of the expressed Kv4.3 current was approximately -30 mV. The dominant time constant for activation was 1.71 +/- 0.16 ms (n = 10) at +60 mV. The current inactivated, this process being incomplete, resulting in a sustained level which contributed 15 +/- 2% (n = 25) of the total current. The time course of inactivation was fit by a biexponential function, the fast component contributing 74 +/- 5% (n = 9) to the overall inactivation. The fast time constant was voltage-dependent [27.6 +/- 2.0 ms at +60 mV (n = 10) versus 64.0 +/- 3.6 ms at 0 mV (n = 10); P < 0.01], whereas the slow was voltage-independent [142 +/- 15 ms at +60 mV (n = 10) versus 129 +/- 33 ms at 0 mV (n = 6) P > 0.05]. The voltage-dependence of inactivation exhibited midpoint and slope values of -26.9 +/- 1.5 mV and 5.9 +/- 0.3 mV (n = 21). Recovery from inactivation was faster at more negative membrane potentials [203 +/- 17 ms (n = 13) and 170 +/- 19 ms (n = 4), at -90 and -100 mV]. Bupivacaine block of Kv4.3 channels was not stereoselective (KD approximately 31 microM).

CONCLUSIONS

The functional profile of Kv4.3 channels expressed in Ltk- cells corresponds closely to rat ITO, although differences in recovery do not rule out association with accessory subunits. Nevertheless, the sustained component needs to be considered with respect to native ITO.

A K+ channel splice variant common in human heart lacks a C-terminal domain required for expression of rapidly activating delayed rectifier current

We have cloned HERG USO, a C-terminal splice variant of the human ether-à-go-go-related gene (HERG), the gene encoding the rapid component of the delayed rectifier (IKr), from human heart, and we find that its mRNA is approximately 2-fold more abundant than that for HERG1 (the originally described cDNA). After transfection of HERG USO in Ltk- cells, no current was observed. However, coexpression of HERG USO with HERG1 modified IKr by decreasing its amplitude, accelerating its activation, and shifting the voltage dependence of activation 8.8 mV negative. As with HERG USO, HERGDeltaC (a HERG1 construct lacking the C-terminal 462 amino acids) also produced no current in transfected cells. However, IKr was rescued by ligation of 104 amino acids from the C terminus of HERG1 to the C terminus of HERGDeltaC, indicating that the C terminus of HERG1 includes a domain (</=104 amino acids) that is critical for faithful recapitulation of IKr. The lack of this C-terminal domain not only explains the finding that HERG USO does not generate IKr but also indicates a similar mechanism for hitherto-uncharacterized long QT syndrome HERG mutations that disrupt the splice site or the C-terminal. We suggest that the amplitude and gating of cardiac IKr depends on expression of both HERG1 and HERG USO.

Evidence for multiple open and inactivated states of the hKv1.5 delayed rectifier

The kinetic properties of hKv1.5, a Shaker-related cardiac delayed rectifier, expressed in Ltk- cells were studied. hKv1.5 currents elicited by membrane depolarizations exhibited a delay followed by biphasic activation. The biphasic activation remained after 5-s prepulses to membrane potentials between -80 and -30 mV; however, the relative amplitude of the slow component increased as the prepulse potential approached the threshold of channel activation, suggesting that the second component did not reflect activation from a hesitant state. The decay of tail currents at potentials between -80 and -30 mV was adequately described with a biexponential. The time course of deactivation slowed as the duration of the depolarizing pulse increased. This was due to a relative increase in the slowly decaying component, despite similar initial amplitudes reflecting a similar open probability after 50- and 500-ms prepulses. To further investigate transitions after the initial activated state, we examined the temperature dependence of inactivation. The time constants of slow inactivation displayed little temperature and voltage dependence, but the degree of the inactivation increased substantially with increased temperature. Recovery from inactivation proceeded with a biexponential time course, but long prepulses at depolarized potentials slowed the apparent rate of recovery from inactivation. These data strongly indicate that hKv1.5 has both multiple open states and multiple inactivated states.

Distinct domains of the voltage-gated K+ channel Kv beta 1.3 beta-subunit affect voltage-dependent gating

The Kvbeta1.3 subunit confers a voltage-dependent, partial inactivation (time constant = 5.76 +/- 0.14 ms at +50 mV), an enhanced slow inactivation, a hyperpolarizing shift in the activation midpoint, and an increase in the deactivation time constant of the Kv1.5 delayed rectifier. Removal of the first 10 amino acids from Kvbeta1.3 eliminated the effects on fast and slow inactivation but not the voltage shift in activation. Addition of the first 87 amino acids of Kvbeta1.3 to the amino terminus of Kv1.5 reconstituted fast and slow inactivation without altering the midpoint of activation. Although an internal pore mutation that alters quinidine block (V512A) did not affect Kvbeta1.3-mediated inactivation, a mutation of the external mouth of the pore (R485Y) increased the extent of fast inactivation while preventing the enhancement of slow inactivation. These data suggest that 1) Kvbeta1.3-mediated effects involve at least two distinct domains of this beta-subunit, 2) inactivation involves open channel block that is allosterically linked to the external pore, and 3) the Kvbeta1.3-induced shift in the activation midpoint is functionally distinct from inactivation.

Molecular determinants of stereoselective bupivacaine block of hKv1.5 channels

Enantiomers of local anesthetics are useful probes of ion channel structure that can reveal three-dimensional relations for drug binding in the channel pore and may have important clinical consequences. Bupivacaine block of open hKv1.5 channels is stereoselective, with the R(+)-enantiomer being 7-fold more potent than the S(-)-enantiomer (Kd = 4.1 mumol/L versus 27.3 mumol/L). Using whole-cell voltage clamp of hKv1.5 channels and site-directed mutants stably expressed in Ltk- cells, we have identified a set of amino acids that determine the stereoselectivity of bupivacaine block. Replacement of threonine 505 by hydrophobic amino acids (isoleucine, valine, or alanine) abolished stereoselective block, whereas a serine substitution preserved it [Kd = 60 mumol/L and 7.4 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. A similar substitution at the internal tetraethylammonium binding site (T477S) reduced the affinity for both enantiomers similarly, thus preserving the stereoselectivity [Kd = 45.5 mumol/L and 7.8 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. Replacement of L508 or V512 by a methionine (L508M and V512M) abolished stereoselective block, whereas substitution of V512 by an alanine (V512A) preserved it. Block of Kv2.1 channels, which carry valine, leucine, and isoleucine residues at T505, L508, and V512 equivalent sites, respectively, was not stereoselective [Kd = 8.3 mumol/L and 13 mumol/L for S(-)- and R(+)-bupivacaine, respectively]. These results suggest that (1) the bupivacaine binding site is located in the inner mouth of the pore, (2) stereoselective block displays subfamily selectivity, and (3) a polar interaction with T505 combined with hydrophobic interactions with L508 and V512 are required for stereoselective block.

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.

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.

Inhibition of cardiac potassium currents by the vesnarinone analog OPC-18790: comparison with quinidine and dofetilide

OPC-18790 is a vesnarinone analog currently in clinical trials for treatment of heart failure. In vitro studies have shown that, in addition to its positive inotropic actions, OPC-18790 prolongs cardiac action potentials. Therefore, in this study, the effects of OPC-18790 on cardiac potassium currents were compared with those we previously observed for the blockers quinidine and dofetilide in two test systems, i.e., L-cells stably transfected with mammalian cardiac potassium channel clones (Kv1.4, Kv1.5 and Kv2.1) and mouse AT-1 cells, in which the rapidly inactivating component of the cardiac delayed rectifier (I(Kr)) is the major repolarizing current. In L-cells, 10 to 100 microM OPC-18790 reduced Kv1.4, Kv1.5 and Kv2.1 currents by <30%, whereas quinidine was a more potent blocker (EC50 < 10 microM) and the I(Kr)-specific blocker dofetilide was without effect. In contrast, in AT-1 cells, OPC-18790 blocked I(Kr) with an EC50 (0.96 +/- 0.12 microM, n = 10) similar to that of quinidine (0.9 +/- 0.2 microM). For both drugs, block was voltage dependent, increasing at positive potentials. OPC-18790 and quinidine showed no frequency dependence, implying block of resting channels and/or very rapid block of open channels; this is in contrast to dofetilide, which displayed slow onset kinetics of block. Thus, we conclude that, 1) unlike quinidine, OPC-18790 does not significantly inhibit currents obtained by expression of the cardiac potassium channel clones Kv1.4, Kv1.5 and Kv2.1; 2) like quinidine and dofetilide, OPC-18790 blocks I(Kr) in AT-1 cells, but the kinetics of block onset more closely resemble those of quinidine than dofetilide; and 3) block of I(Kr) appears to be an important mechanism underlying the action potential-prolonging properties of OPC-18790.

Electrophysiological and pharmacological correspondence between Kv4.2 current and rat cardiac transient outward current

OBJECTIVE

The transient outward current (ITO) plays an important role in early repolarization and overall time course of the cardiac action potential. At least two K+ channel alpha-subunits cloned from cardiac tissue (Kv1.4 and Kv4.2) encode rapidly inactivating channels. The goal of this study was to determine functional and pharmacological properties of Kv4.2 expressed in mammalian cells, especially those that would differentiate between both isoforms in comparison to native ITO.

METHODS

Both Kv4.2 and Kv1.4 isoforms were stably expressed in mouse L-cell lines, and expressed currents were studied using whole-cell voltage clamp techniques.

RESULTS

The expressed Kv4.2 currents displayed fast inactivation with a half-inactivation potential of -41 mV. Recovery from inactivation was rapid (tau recov = 160 ms at -90 mV) and strongly voltage-dependent. Flecainide (10 microM) had minimal effects on Kv1.4 currents, but reduced Kv4.2 peak current by 53% and increased the apparent rate of inactivation consistent with open channel block. Quinidine (10-20 microM) reduced the peak current and accelerated the apparent rate of inactivation in both isoforms. The Kv4.2 current displayed use-dependent unblock in the presence of 4-AP.

CONCLUSIONS

The functional properties of Kv4.2, especially the flecainide sensitivity, resemble those of ITO in rat (and human) myocytes better than those of Kv1.4. These results provide the necessary functional support for the hypothesis that Kv4.2 is a major isoform contributing to cardiac ITO, consistent with independent biochemical and molecular evidence that indicates that Kv4.2 is readily detected in rat myocytes.

Effects of ropivacaine on a potassium channel (hKv1.5) cloned from human ventricle

BACKGROUND

Ropivacaine, a new amide local anesthetic agent chemically related to bupivacaine, is able to induce early after depolarizations in isolated cardiac preparations. The underlying mechanism by which ropivacaine induces this effect has not been explored, but it is likely to involve K+ channel block.

METHODS

Cloned human cardiac K+ channels (hKv1.5) were stably transfected in Ltk cells, and the effects of ropivacaine on the expressed hKv1.5 currents were assessed using the whole-cell configuration of the patch-clamp technique.

RESULTS

Ropivacaine (100 microM) did not modify the initial activation time course of the current, but induced a fast subsequent decline to a lower steady-state current level with a time constant of 12.2 +/- 0.6 ms. Ropivacaine inhibited hKv1.5 with an apparent KD of 80 +/- 4 microM. Block displayed an intrinsic voltage-dependent, consistent with an electrical distance for the binding site of 0.153 +/- 0.007 (n = 6) (from the cytoplasmic side). Ropivacaine reduced the tail current amplitude recorded at -40 mV, and slowed the deactivation time course, resulting in a "crossover" phenomenon when control and ropivacaine tail currents were superimposed.

CONCLUSIONS

These results indicate that: (1) ropivacaine is an open channel blocker of hKv1.5; (2) binding occurs in the internal mouth of the ion pore; and (3) unbinding is required before the channel can close. These effects explain the ropivacaine availability of induction early after depolarizations and could be clinically relevant.

Class III antiarrhythmic effects of zatebradine. Time-, state-, use-, and voltage-dependent block of hKv1.5 channels

BACKGROUND

Zatebradine is a bradycardic agent that inhibits the hyperpolarization-activated current (I(f)) in the rabbit sinoatrial node. It also prolongs action potential duration in papillary muscles in guinea pigs and in Purkinje fibers in rabbits. The underlying mechanism by which zatebradine induces this effect has not been explored, but it is likely to involve K+ channel block.

METHODS AND RESULTS

Cloned human cardiac K+ delayed rectifer currents (hKv1.5) were recorded in Ltk- cells transfected with their coding sequence. Zatebradine 10 mumol/L did not modify the initial activation time course of the current but induced a subsequent decline to a lower steady-state current level with a time constant of 109 +/- 16 ms. Zatebradine inhibited hKv1.5 with an apparent KD of 1.86 +/- 0.14 mumol/L. Block was voltage dependent (electrical distance delta = 0.177 +/- 0.003) and accumulated in a use-dependent manner during 0.5- and 1-Hz pulse trains because of slower recovery kinetics in the presence of the drug. Zatebradine reduced the tail current amplitude, recorded at -30 mV, and slowed the deactivation time course, which resulted in a "crossover" phenomenon when control and zatebradine tail currents were superimposed.

CONCLUSIONS

These results indicate that (1) zatebradine is an open-channel blocker of hKv 1.5, (2) binding occurs in the internal mouth of the ion pore, (3) unbinding is required before the channel can close, and (4) zatebradine-induced block is use dependent because of slower recovery kinetics in the presence of the drug. These effects may explain the prolongation of the cardiac action potential and could be clinically relevant.

Molecular analysis of a binding site for quinidine in a human cardiac delayed rectifier K+ channel. Role of S6 in antiarrhythmic drug binding

The antiarrhythmic agent quinidine blocks the human cardiac hKv1.5 channel expressed in mammalian cells at therapeutically relevant concentrations (EC50, 6.2 mumol/L). Mechanistic analysis has suggested that quinidine acts as a cationic open-channel blocker at a site in the internal mouth of the ionic pore and that binding is stabilized by hydrophobic interactions. We tested these hypotheses using site-directed mutagenesis of residues proposed to line the internal mouth of the channel or of nearby residues. Amino acid substitutions in the midsection of S6 (T505I, T505V, T505S, and V512A) reduced the dissociation rate for quinidine, increased the affinity (0.7, 1.5, 3.4, and 1.4 mumol/L, respectively), and preserved both the voltage-dependent open channel-block mechanism and the electrical binding distance (0.19 to 0.22). In contrast, smaller or nonsignificant effects were observed for: deletion of the intracellular C-terminal domain, charge neutralizations in the region immediately C-terminal to S6, elimination of aromatic residues in S6, and mutations at the putative internal turn of the P loop, at the external entrance of the pore, and at sites in the S4S5 linker. The approximately 10-fold increase in affinity with T505I and the reduction of the dissociation rate constant with the mutations that increased affinity are consistent with a hydrophobic stabilization of binding. Moreover, the T505 and V512 residues align on the same side of the putative alpha-helical S6 segment. Taken together, these results localize the hydrophobic binding site for this antiarrhythmic drug in the internal mouth of this human K+ channel and provide molecular support for the open channel-block model and the role of S6 in contributing to the inner pore.

High affinity open channel block by dofetilide of HERG expressed in a human cell line

In the long QT syndrome, excessive prolongation of the cardiac action potential leads to polymorphic ventricular tachycardia (torsades de pointes) and sudden death. Mutations in HERG have been identified as one of the causes of the chromosome 7-linked form of congenital long QT syndrome. The biophysical properties of currents recorded from HERG expressing Xenopus oocytes are similar to those of a cardiac K+ current, I(Kr), but the characteristic nanomolar methanesulfonanilide sensitivity has not been demonstrated. To determine the biophysical and pharmacological properties of HERG under experimental conditions similar to those used to study native cardiac currents, we examined currents expressed after expression of HERG in a human cell line, human embryonic kidney 293. Transfected cells display K+-selective outward currents that activated at membrane potentials positive to -50 mV with strongly voltage-dependent kinetics [time constant (tau) = 2 sec at -20 mV and 188 msec at +20 mV]. Marked inward rectification was observed for depolarizations positive to +0 mV, which was due to rapid channel inactivation (tau = 6 msec at +50 mV). The subsequent tail currents at -40 mV displayed an initial rising phase with tau = 10 msec, followed by a slow multiexponential decline. The EC50 for the methanesulfonanilide I(Kr) blocker dofetilide was 12 +/- 2 nM. Induction of block depended on depolarization beyond the threshold for channel opening. Time-dependent block developed slowly, with tau = 5.2 +/- 0.6 sec (300 nM) at +10 mV, and was delayed by stronger depolarizations. This pattern suggested that dofetilide preferentially blocks open (or activated) channels and that the fast inactivation may competitively slow the binding kinetics. The latter occurrence was further supported by a simplified mathematical model that addressed the impact on binding kinetics of fast inactivation. These results indicate that the HERG gene product encodes an alpha subunit that, when expressed in mammalian cells, displays both the major functional and pharmacological properties of native I(Kr). Dofetilide acts as a slow-onset/slow-offset open channel blocker of this current at nanomolar concentrations.

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.

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.

Stereoselective block of cardiac sodium channels by bupivacaine in guinea pig ventricular myocytes

BACKGROUND

Bupivacaine is a potent local anesthetic widely used for prolonged local and regional anesthesia. However, accidental intravascular injection of bupivacaine can produce severe arrhythmias and cardiac depression. Although used clinically as a racemic mixture, S(-)-bupivacaine appears less toxic than the R(+)-enantiomer despite at least equal potency for local anesthesia. If the R(+)-enantiomer is more potent in blocking cardiac sodium channels, then the S(-)-enantiomer could be used with less chance of cardiovascular toxicity. Therefore, we tested whether such stereoselectivity existed in the bupivacaine affinity for the cardiac sodium channel.

METHODS AND RESULTS

The inhibitory effects on the cardiac sodium current (INa) of 10 mumol/L R(+)- and S(-)-bupivacaine were investigated by use of the whole-cell voltage clamp technique in isolated guinea pig ventricular myocytes. Both enantiomers produced similar but limited levels of tonic block (6% and 8%). During long depolarizations (5 seconds at 0 mV), R(+)-bupivacaine induced a significantly larger inhibition of INa: 72 +/- 2% versus 58 +/- 3% for the S(-)-enantiomer (P < .01). Development of block was slow, but its rate was faster for R(+)-bupivacaine [time constant, 1.84 +/- 0.16 versus 2.56 +/- 0.26 seconds for the S(-)-enantiomer, P < .05]. The voltage dependence of the availability of the Na+ current was shifted to more hyperpolarizing potentials compared with the control; R(+)-bupivacaine induced a larger shift than S(-)-bupivacaine (37 +/- 2 versus 30 +/- 2 mV, P < .05). These data indicate stereoselective interactions with the inactivated state. In addition, both enantiomers induced substantial use-dependent block during 2.5-Hz pulse trains with medium (100-ms) and short (10-ms) depolarizations but without stereoselective difference. A stepwise approach was used to model these experimental results and to derive apparent affinities and rate constants. We initially assumed that bupivacaine interacted only with the rested and inactivated states of the Na+ channel. The apparent affinities of the inactivated state for S(-)- and R(+)-bupivacaine were 4.8 and 2.9 mumol/L, respectively. With the derived binding and unbinding rate constants, this model reproduced the stereoselective block during long depolarizations but failed to predict the use-dependent block induced by trains of short (10-ms) depolarizations. To account for the observed use-dependent interactions, it was necessary to include interactions with the activated state, which resulted in adequate reproduction of the experimental results. The apparent affinities of the activated or open state for S(-)- and R(+)-bupivacaine were 4.3 and 3.3 mumol/L, respectively.

CONCLUSIONS

Both the large level of pulse-dependent block and the failure of the pure inactivated-state block model indicate that bupivacaine interacts with the activated (or open) state of the cardiac sodium channel in addition to its block of the inactivated state. The bupivacaine-induced block of the inactivated state of the Na+ channel displayed stereoselectivity, with R(+)-bupivacaine interacting faster and more potently. Both enantiomers also bind with high affinity to the activated or open state of the channel, but this interaction did not display stereoselectivity, although the binding to the activated or open state was faster for S(-)- than for R(+)-bupivacaine. The higher potency of R(+)-bupivacaine to block the inactivated state of the cardiac Na+ channel may explain its higher toxicity because of the large contribution of the inactivated-state block during the plateau phase of the cardiac action potential. These results would support the use of the S(-)-enantiomer to reduce cardiac toxicity.

Determinants of antiarrhythmic drug action. Electrostatic and hydrophobic components of block of the human cardiac hKv1.5 channel

The molecular basis of antiarrhythmic drug action is still poorly understood. We recently reported that block of the human cardiac hKv1.5 channel by quinidine displayed similarity with internal quaternary ammonium block of squid and Shaker potassium channels. To gain further insight into the molecular determinants of the affinity and the stereoselectivity of antiarrhythmic drug action, we studied the effects of quinine (a diastereomer of quinidine), clofilium (a quaternary ammonium class III agent), and tetrapentylammonium (TPeA, a biophysical reference probe for the internal quaternary ammonium binding site). For all compounds, block was voltage dependent, with a steep increase over the voltage range of channel opening and a superimposed weaker voltage dependence at more positive potentials. The latter electrostatic component was similar for all drugs, consistent with a binding reaction sensing approximately 20% of the transmembrane electrical field. Clofilium and TPeA displayed a higher apparent affinity (0.15 and 0.28 mumol/L, respectively), and quinine displayed a lower one (21 mumol/L) compared with quinidine (6.2 mumol/L). Block development upon depolarization was time dependent for clofilium and TPeA but slow compared with quinidine. A time-dependent component was difficult to resolve for quinine, but the time course of deactivating tail currents was slower than in the control condition. The resulting crossover phenomenon was also observed for the quaternary drugs. Compared with TPeA alone, the combined application of quinine and TPeA resulted in a reduced current that decayed slower, consistent with competition.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Ibutilide, a methanesulfonanilide antiarrhythmic, is a potent blocker of the rapidly activating delayed rectifier K+ current (IKr) in AT-1 cells. Concentration-, time-, voltage-, and use-dependent effects

BACKGROUND

Ibutilide is an action potential-prolonging antiarrhythmic currently in clinical trials. The drug shares structural similarities with E-4031 and dofetilide, specific blockers of the rapidly activating delayed rectifier K+ current (IKr). However, previous in vitro studies in guinea pig myocytes have indicated that ibutilide does not block IKr but rather increases a slow inward sodium current.

METHODS AND RESULTS

In this study, we compared the effects of ibutilide with those of dofetilide on outward current in mouse atrial tumor myocytes (AT-1 cells), a preparation in which, unlike guinea pig, a typical IKr is the major delayed rectifier and can be readily recorded in isolation from other currents. In AT-1 cells, ibutilide and dofetilide were both potent IKr blockers, with EC50 values of 20 (n = 12) and 12 (n = 8) nmol/L, respectively, at +20 mV. The time and voltage dependence of IKr inhibition by the two compounds were virtually identical. The following characteristics were most consistent with open channel block: (1) block increased with depolarizing pulses; (2) block increased with longer pulses; (3) currents deactivated more slowly in the presence of drug, resulting in a "crossover" typical of open channel block; and (4) with repetitive pulsing after drug wash-in, use-dependent block was observed.

CONCLUSIONS

These data suggest that the clinical actions of ibutilide are mediated at least in part by block of IKr; an effect on inward currents is not excluded. AT-1 cells are a useful model system for the study of drug block of this important repolarizing current.

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

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

K+ currents and K+ channel mRNA in cultured atrial cardiac myocytes (AT-1 cells)

Atrial tumor myocytes derived from transgenic mice (AT-1 cells) maintain a well-differentiated cardiac biochemical and histological phenotype. In addition, they beat spontaneously in culture and exhibit long action potentials whose repolarization resembles that observed in native mammalian myocytes. In this study, we identified the major depolarization-activated outward currents in AT-1 cells; also, the presence of mRNAs that encode outwardly conducting ion channels was determined by cloning from an AT-1 cDNA library or by Northern hybridization. Among K+ channel isoforms, Kv2.1, minK, and Kv1.4 were readily detected in tumors and at 1 day in culture. Their abundance remained relatively stable (twofold or less change) after 14 days. The major outward current in AT-1 cells is a delayed rectifier that displays prominent inward rectification, activates rapidly (eg, 182 +/- 27 milliseconds [mean +/- SEM] at + 20 mV, n = 12), exhibits biexponential deactivation kinetics, and is extremely sensitive to the methanesulfonanilide dofetilide (IC50, 12 nmol/L). These characteristics identify this current as IKr, a delayed rectifier observed only in cardiac cells. IKr in AT-1 cells displayed slow inactivation: dofetilide-sensitive deactivating tails were greater after 1-second than after 5-second pulses. When IKr was blocked by > or = 0.5 mumol/L dofetilide, time-independent current was usually recorded (50 of 65 experiments); rapidly inactivating (6 of 65) or slowly inactivating (9 of 65) outward currents were occasionally observed. We conclude that AT-1 cells express mRNAs encoding cardiac K+ channels and display a cardiac electrophysiological phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential expression of Isk mRNAs in mouse tissue during development and pregnancy

The molecular isoform of the cDNA clone Isk present in the AT-1 atrial tumor cell line was characterized by molecular cloning of Isk cDNA. Since Isk mRNA was found in mouse heart, kidney, and uterus, a complete study of its expression during development in the heart and kidney was performed, in addition to its expression in the uterus during pregnancy. In the heart, Isk showed a 4-fold upregulation during the perinatal period followed by a 20-fold decrease between birth and the adult state. Furthermore, the two 0.9- and 3.4-kb transcripts were differentially regulated after birth. In the kidney, Isk progressively increased 10-fold, reaching steady-state adult values at 21 days. Isk mRNA levels in the uterus increased threefold at late pregnancy and decreased sixfold rapidly after birth. The Isk gene is differentially expressed during development in kidney and cardiac tissue, and both Isk transcripts appeared to be differentially regulated. Furthermore, the drastic changes in transcript levels before delivery and after birth suggest that Isk plays a significant role in myometrium during late pregnancy and delivery.

Deletion of highly conserved C-terminal sequences in the Kv1 K+ channel sub-family does not prevent expression of currents with wild-type characteristics

The C-terminal regions of Kv1 K+ channels show little conservation between isoforms except for the last four C-terminal residues, (E/L)TDV, which are well conserved from Drosophila to man. Deletions of the 4, 16, and 57 C-terminal residues of the human Kv1.5 channel did not affect whole cell current amplitude, midpoint of activation, degree of inactivation, or activation kinetics following expression in mouse L-cells. Similar results were obtained with the rat Kv1.1 channel. Therefore, the conserved (E/L)TDV motif, and most of the C-terminal amino acids, are not required for Kv1 channel expression.

Influence of cloned voltage-gated K+ channel expression on alanine transport, Rb+ uptake, and cell volume

Voltage-gated K+ channels are involved in regulation of action potential duration and in setting the resting membrane potential in nerve and muscle. To determine the effects of voltage-gated K+ channel expression on processes not associated with electrically excitable cells, we studied cell volume, membrane potential, Na(+)-K(+)-ATPase activity, and alanine transport after the stable expression of the Kv1.4 and Kv1.5 human K+ channels in Ltk- mouse fibroblasts (L-cells). The fast-activating noninactivating Kv1.5 channel, but not the rapidly inactivating Kv1.4 channel, prevented dexamethasone-induced increases in intracellular volume and inhibited Na(+)-K(+)-ATPase activity by 25%, as measured by 86Rb+ uptake. Alanine transport, measured separately by systems A and ASC, was lower in Kv1.5-expressing cells, indicating that the expression of this channel modified the Na(+)-dependent amino acid transport of both systems. Expression of the Kv1.4 channel did not alter alanine transport relative to wild-type or sham-transfected cells. The changes specific to Kv1.5 expression may be related to the resting membrane potential induced by this channel (-30 mV) in contrast to that measured in wild-type sham-transfected, or Kv1.4-transfected cells (-2 to 0 mV). Blocking of the Kv1.5 channel by 60 microM quinidine negated the effects of Kv1.5 expression on intracellular volume, Na(+)-K(+)-ATPase, and Na(+)-dependent alanine transport. These results indicate that delayed rectifier channels such as Kv1.5 can play a key role in the control of cell membrane potential, cell volume, Na(+)-K(+)-ATPase activity, and electrogenic alanine transport across the plasma membrane of electrically unexcitable cells.

Molecular and functional diversity of cloned cardiac potassium channels

Action potential duration is an important determinant of refractoriness in cardiac tissue and thus of the ability to propagate electrical impulses. Action potential duration is controlled in part by activation of K+ currents. Block of K+ channels and the resultant prolongation of action potential duration has become an increasingly attractive mode of anti-arrhythmic intervention. Detailed investigation of individual cardiac K+ channels has been hampered by the presence of multiple types of K+ channels in cardiac cells and the difficulty of isolating individual currents. We have approached this problem by employing a combination molecular cloning technology, heterologous channel expression systems, and biophysical analysis of expressed channels. We have focused on six different channels cloned from the rat and human cardiovascular systems. Each channel has unique functional and pharmacological characteristics, and as a group they comprise a series of mammalian K+ channel isoforms that can account for some of the diversity of channels in the mammalian heart. Each channel appears to be encoded by a different gene with little or no evidence for alternate splicing of RNA transcripts to account for the differences in primary amino acid sequence. In addition to the unique kinetic properties of these channel isoforms when expressed as homotetrameric assemblies, the formation of heterotetrameric K+ channels is also observed. The formation of heterotetrameric channels from the different gene products to create new channels with unique kinetic and pharmacological properties might further account for cardiac K+ channel diversity.

Heteromultimeric assembly of human potassium channels. Molecular basis of a transient outward current?

To gain insight into the molecular basis of cardiac repolarization, we have expressed K+ channels cloned from ventricular myocardium in Xenopus oocytes. A recently identified human cardiac K+ channel isoform (human Kv1.4) has properties similar to the 4-aminopyridine-sensitive calcium-insensitive component of the cardiac transient outward current. However, these channels recovered from inactivation much slower than native channels. Hybrid channels consisting of subunits from different K+ channel clones (delayed rectifier channels [Kv1.1, Kv1.2, and Kv1.5] and Kv1.4) were created by coinjection of cRNAs in oocytes. Multimeric channels consisting of Kv1.4:Kv1.1, Kv1.4:Kv1.2, and Kv1.4:Kv1.5 were expressed and compared. The hybrid channels displayed characteristics of heterotetrameric channels with kinetics that more closely resembled a native cardiac transient outward current. The inactivation and recovery from inactivation of the heteromeric channels indicated that the presence of a single inactivating subunit (Kv1.4) was probably sufficient to cause channel inactivation. The results demonstrate that expression of different K+ channel genes can produce channel protein subunits that assemble as heteromultimers with unique properties. It is shown that certain combinations of voltage-gated K+ channels probably do not contribute to native transient outward current. However, one combination of subunits could not be excluded. Therefore, this mechanism of channel assembly may underlie some of the functional diversity of potassium channels found in the cardiovascular system.

A rapidly activating and slowly inactivating potassium channel cloned from human heart. Functional analysis after stable mammalian cell culture expression

The electrophysiological properties of HK2 (Kv1.5), a K+ channel cloned from human ventricle, were investigated after stable expression in a mouse Ltk- cell line. Cell lines that expressed HK2 mRNA displayed a current with delayed rectifier properties at 23 degrees C, while sham transfected cell lines showed neither specific HK2 mRNA hybridization nor voltage-activated currents under whole cell conditions. The expression of the HK2 current has been stable for over two years. The dependence of the reversal potential of this current on the external K+ concentration (55 mV/decade) confirmed K+ selectivity, and the tail envelope test was satisfied, indicating expression of a single population of K+ channels. The activation time course was fast and sigmoidal (time constants declined from 10 ms to < 2 ms between 0 and +60 mV). The midpoint and slope factor of the activation curve were Eh = -14 +/- 5 mV and k = 5.9 +/- 0.9 (n = 31), respectively. Slow partial inactivation was observed especially at large depolarizations (20 +/- 2% after 250 ms at +60 mV, n = 32), and was incomplete in 5 s (69 +/- 3%, n = 14). This slow inactivation appeared to be a genuine gating process and not due to K+ accumulation, because it was present regardless of the size of the current and was observed even with 140 mM external K+ concentration. Slow inactivation had a biexponential time course with largely voltage-independent time constants of approximately 240 and 2,700 ms between -10 and +60 mV. The voltage dependence of slow inactivation overlapped with that of activation: Eh = -25 +/- 4 mV and k = 3.7 +/- 0.7 (n = 14). The fully activated current-voltage relationship displayed outward rectification in 4 mM external K+ concentration, but was more linear at higher external K+ concentrations, changes that could be explained in part on the basis of constant field (Goldman-Hodgkin-Katz) rectification. Activation and inactivation kinetics displayed a marked temperature dependence, resulting in faster activation and enhanced inactivation at higher temperature. The current was sensitive to low concentrations of 4-aminopyridine, but relatively insensitive to external TEA and to high concentrations of dendrotoxin. The expressed current did not resemble either the rapid or the slow components of delayed rectification described in guinea pig myocytes. However, this channel has many similarities to the rapidly activating delayed rectifying currents described in adult rat atrial and neonatal canine epicardial myocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Molecular biology of the voltage-gated potassium channels of the cardiovascular system

K+ channels represent the most diverse class of voltage-gated ion channels in terms of function and structure. Voltage-gated K+ channels in the heart establish the resting membrane K+ permeability, modulate the frequency and duration of action potentials, and are targets of several antiarrhythmic drugs. Consequently, an understanding of K+ channel structure-function relationships and pharmacology is of great practical interest. However, the presence of multiple overlapping currents in native cardiac myocytes complicates the study of basic K+ channel function and drug-channel interactions in these cells. The application of molecular cloning technology to cardiovascular K+ channels has identified the primary structure of these proteins, and heterologous expression systems have allowed a detailed analysis of channel function and pharmacology without contaminating currents. To date six different K+ channels have been cloned from rat and human heart, and all have been functionally characterized in either Xenopus oocytes or mammalian tissue culture systems. This initial research is an important step toward understanding the molecular basis of the action potential in the heart. An important challenge for the future is to determine the cell-specific expression and relative contribution of these cloned channels to cardiac excitability.

Functional expression of an inactivating potassium channel cloned from human heart

Recently a putative K+ channel with homology to the Shaker family of potassium channels has been cloned from human ventricular myocardium. However, proof that the cDNA encodes a K+ channel requires appropriate translation and expression of a functional ion-selective channel. Therefore, expression of this putative human K+ channel DNA was attempted by cytoplasmic injections of in vitro transcribed cRNA into Xenopus laevis oocytes and screening by two-electrode voltage-clamp methods. This resulted in expression of voltage-gated channels that rapidly inactivated (time constant of inactivation, 47.6 +/- 3.6 msec; 0 mV; n = 10) and were at least 50 times more selective for K+ than Na+ (Na+/K+ permeability ratio of 0.02). The channels showed voltage-dependent activation (half-maximal voltage, -34 +/- 0.7 mV; n = 5), and 50% of the channels were inactivated within 2 seconds when the membrane potential was clamped near -60 mV (half-maximal voltage, -62 +/- 7 mV; n = 10). The expressed protein resulted in a K+ current that had many properties similar to the 4-aminopyridine-sensitive calcium-insensitive component of the cardiac transient outward current that is observed in native cardiac myocytes and thus may serve as one molecular substrate for this current.

Effects of quinidine on the sodium current of guinea pig ventricular myocytes. Evidence for a drug-associated rested state with altered kinetics

In guinea pig cardiac myocytes quinidine (20 microM) caused less than 10% tonic block reduction of the sodium current at -120 mV, but a fast pulse train reduced it more than 90%. Recovery from use-dependent block was time and voltage dependent, and was always slow (tau = 34 +/- 10 seconds at -160 mV; tau = 90 +/- 35 seconds at -120 mV; n = 15, mean +/- SD, p less than 0.001, paired t test). However, in association with repeated activation a fast component of recovery from block was observed: use-dependent unblocking. Availability of sodium channels for use-dependent unblocking was enhanced by hyperpolarization until a plateau was reached near -160 mV. Compared with the availability of drug-free sodium channels (h-curve), the voltage dependence of availability for use-dependent unblocking (h'-curve) was shifted by about 30 mV to more negative potentials, and its slope was reduced 2.5-fold. At -160 mV, the kinetics of development of availability of sodium channels for use-dependent unblocking were rapid (tau less than 10 msec). Depolarization to -120 mV reduced the availability of sodium channels for fast unblocking with a time constant of 191 +/- 46 msec (n = 14). Finally, block established by frequent brief depolarizations (activations) declined during prolonged inactivation. From these results we concluded that the time and voltage dependence of the availability of sodium channels for unblocking are considerably different from the availability for activation of drug-free channels, that rested drug-associated channels do exist, and that drug-associated channels do not conduct (or at least have a greatly reduced conductance) upon activation unless they first unblock. Furthermore, activated and inactivated channels have a different affinity for quinidine, and since quinidine can occupy the channel receptor even when "guarded," our results are incompatible with the guarded receptor hypothesis but can be explained within the framework of the modulated receptor hypothesis.

Class III antiarrhythmic agents have a lot of potential but a long way to go. Reduced effectiveness and dangers of reverse use dependence

With regard to currently available class III agents, although their class III effect may reduce the likelihood of tachycardia initiation, their reverse use-dependent prolongation of action potential duration reduces their effectiveness during tachycardias and may even render them proarrhythmic, especially after long diastolic intervals. In contrast, agents that exhibit normal use-dependent prolongation of refractoriness hold great promise: While having relatively less effects on the normal heart beat, they could induce self-termination of a tachycardia. Prolongation of refractoriness can be achieved by lengthening of action potential duration and delaying recovery of excitability. Combination of these drug actions may yield important clinical applications.

Temperature and voltage dependence of sodium channel blocking and unblocking by O-demethyl encainide in isolated guinea pig myocytes

O-demethyl encainide (ODE) is a metabolite of encainide with potent antiarrhythmic effects. We studied block of sodium channels by ODE in isolated guinea pig ventricular myocytes, using both direct measurement of whole cell sodium current and Vmax. Specifically, we examined whether block by ODE was use-dependent, whether ODE blocked activated or inactivated channels, and whether ODE's effect depended on membrane potential. At cold temperatures (15-20 degrees C), ODE was a potent blocker of activated cardiac sodium channels, with little or no effect on inactivated channels. At these temperatures, there was no detectable diastolic recovery from block. Repeated depolarizations produced use-dependent unblocking, which increased as the holding potential was made more negative. At warmer temperatures, use-dependent unblocking was increased, the availability curve of sodium channels for use-dependent unblocking was less shifted toward negative potentials, and diastolic recovery from block became detectable, albeit slow (tau approximately 25 s). Our findings demonstrate that electrophysiologic effects of agents such as ODE may be both quantitatively and qualitatively different at cold temperatures than at normal body temperature. One should be cautious about extrapolating electrophysiologic data obtained at cold temperatures to clinical situations.

Quinidine delays IK activation in guinea pig ventricular myocytes

A major action of the antiarrhythmic agent quinidine is prolongation of cardiac repolarization. In these experiments, the time-dependent effects of quinidine on the delayed rectifier potassium current, IK, a current contributing to cardiac repolarization, were investigated in acutely disaggregated guinea pig ventricular myocytes using the whole-cell recording configuration of the patch-clamp method. The effect of quinidine on IK was dependent on the duration of depolarization. After long (2,000 msec) pulses, IK was reduced by 30 +/- 27% (SD; n = 8, paired) by 10 microM quinidine; in contrast, after short (100 msec) pulses, the drug decreased IK 65 +/- 35% (p less than 0.05). This effect was found both in paired experiments as well as when quinidine-pretreated cells were compared to non-pretreated cells. Quinidine significantly delayed IK activation (9 +/- 20 msec at baseline vs. 44 +/- 25 msec in drug, p less than 0.05), but did not alter the subsequent time course of activation (time constant 659 +/- 118 msec). These findings are consistent with the hypothesis that quinidine promotes occupancy of a channel state from which opening does not occur.

Alinidine modifies the pacemaker current in sheep Purkinje fibers

(1) The "specific bradycardic agent" alinidine reduces the slope of the diastolic depolarization in sinoatrial tissue and Purkinje fibers. In short Purkinje fibers of sheep, alinidine (28 microM) decreased the pacemaker current by a dual action. The voltage dependence of if activation was shifted in the hyperpolarizing direction by 7.8 +/- 0.6 mV (n = 18, p less than 0.001) and the conductance of the fully activated if current was reduced to 73 +/- 2% (n = 18, p less than 0.001) of its control value. These effects were reversible and dose-dependent. (2) Ionophoretic injections of alinidine caused reversible reductions of the diastolic depolarization rate and simultaneous transient hyperpolarizing shifts of the if activation range. (3) Some prolongation of the action potential duration was observed at 28 microM and more pronounced at higher concentration. This was presumably the consequence of a reduction by alinidine of outward repolarizing current carried by the background inward rectifier and plateau current ix. (4) The action of alinidine on if resulted in a slower activation of a reduced fraction of the pacemaker current at the maximal diastolic potential level. This explains the decrease of the diastolic depolarization rate observed in Purkinje fibers.

Effects of 4-aminopyridine on inward rectifying and pacemaker currents of cardiac Purkinje fibres

1. 4-Aminopyridine (4-AP), in a concentration of 1-5 mM, prolongs the action potential duration, induces spontaneous activity and depolarizes sheep cardiac Purkinje strands. These effects are different from those obtained with 0.1 mM and are reversible. 2. Voltage clamp experiments demonstrate that the higher drug concentrations affect membrane currents measured in the potential range between -100 and -40 mV, in addition to the reduction of the transient outward current already described for small amounts of the drug (0.1--0.5 mM). 3. The analysis of membrane current modifications by 4-AP in the presence of cesium and barium ions indicates that 4-AP, in the higher concentration range, reduces the inward rectifying time independent potassium current iK1 and modified the voltage dependence of the time and voltage dependent pacemaker current. The steady-state activation curve of the pacemaker current is shifted towards less negative potentials and is less steeply voltage dependent. The time constant (tau) curve has an increased maximum, displaced towards less negative potentials. 4. The modifications by 4-AP of the iK1 and pacemaker currents explain the changes in resting potential, action potential duration and the induction of spontaneous activity. The latter effect is not the result of an indirect effect of 4-AP through increased release of neurotransmitters from sympathetic nerve endings. A possible action of 4-AP at the inside of the membrane, explaining the multiple actions, is discussed.