Fast inactivation causes rectification of the IKr channel

Characterization and functional consequences of delayed rectifier current transient in ventricular repolarization

Although inactivation of the rapidly activating delayed rectifier current (I(Kr)) limits outward current on depolarization, the role of I(Kr) (and recovery from inactivation) during repolarization is uncertain. To characterize I(Kr) during ventricular repolarization (and compare with the inward rectifier current, I(K1)), voltage-clamp waveforms simulating the action potential were applied to canine ventricular, atrial, and Purkinje myocytes. In ventricular myocytes, I(Kr) was minimal at plateau potentials but transiently increased during repolarizing ramps. The I(Kr) transient was unaffected by repolarization rate and maximal after 150-ms depolarizations (+25 mV). Action potential clamps revealed the I(Kr) transient terminating the plateau. Although peak I(Kr) transient density was relatively uniform among myocytes, potentials characterizing the peak transients were widely dispersed. In contrast, peak inward rectifier current (I(K1)) density during repolarization was dispersed, whereas potentials characterizing I(K1) defined a narrower (more negative) voltage range. In summary, rapidly activating I(Kr) provides a delayed voltage-dependent (and functionally time-independent) outward transient during ventricular repolarization, consistent with rapid recovery from inactivation. The heterogeneous voltage dependence of I(Kr) provides a novel means for modulating the contribution of this current during repolarization.

Trapping of a methanesulfonanilide by closure of the HERG potassium channel activation gate

Deactivation of voltage-gated potassium (K(+)) channels can slow or prevent the recovery from block by charged organic compounds, a phenomenon attributed to trapping of the compound within the inner vestibule by closure of the activation gate. Unbinding and exit from the channel vestibule of a positively charged organic compound should be favored by membrane hyperpolarization if not impeded by the closed gate. MK-499, a methanesulfonanilide compound, is a potent blocker (IC(50) = 32 nM) of HERG K(+) channels. This bulky compound (7 x 20 A) is positively charged at physiological pH. Recovery from block of HERG channels by MK-499 and other methanesulfonanilides is extremely slow (Carmeliet 1992; Ficker et al. 1998), suggesting a trapping mechanism. We used a mutant HERG (D540K) channel expressed in Xenopus oocytes to test the trapping hypothesis. D540K HERG has the unusual property of opening in response to hyperpolarization, in addition to relatively normal gating and channel opening in response to depolarization (Sanguinetti and Xu 1999). The hyperpolarization-activated state of HERG was characterized by long bursts of single channel reopening. Channel reopening allowed recovery from block by 2 microM MK-499 to occur with time constants of 10.5 and 52.7 s at -160 mV. In contrast, wild-type HERG channels opened only briefly after membrane hyperpolarization, and thus did not permit recovery from block by MK-499. These findings provide direct evidence that the mechanism of slow recovery from HERG channel block by methanesulfonanilides is due to trapping of the compound in the inner vestibule by closure of the activation gate. The ability of HERG channels to trap MK-499, despite its large size, suggests that the vestibule of this channel is larger than the well studied Shaker K(+) channel.

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

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

Ultrafast inactivation causes inward rectification in a voltage-gated K(+) channel from Caenorhabditis elegans

The exp-2 gene in the nematode Caenorhabditis elegans influences the shape and duration of the action potential of pharyngeal muscle cells. Several loss-of-function mutations in exp-2 lead to broadening of the action potential and to a concomitant slowing of the pumping action of the pharynx. In contrast, a gain-of-function mutation leads to narrow action potentials and shallow pumping. We cloned and functionally characterized the exp-2 gene. The exp-2 gene is homologous to genes of the family of voltage-gated K(+) channels (Kv type). The Xenopus oocyte-expressed EXP-2 channel, although structurally closely related to Kv-type channels, is functionally distinct and very similar to the human ether-à-gogo-related gene (HERG) K(+) channel. In response to depolarization, EXP-2 activates slowly and inactivates very rapidly. On repolarization, recovery from inactivation is also rapid and strongly voltage-dependent. These kinetic properties make the Kv-type EXP-2 channel an inward rectifier that resembles the structurally unrelated HERG channel. Apart from many similarities to HERG, however, the molecular mechanism of fast inactivation appears to be different. Moreover, the single-channel conductance is 5- to 10-fold larger than that of HERG and most Kv-type K(+) channels. It appears that the inward rectification mechanism by rapid inactivation has evolved independently in two distinct classes of structurally unrelated, voltage-gated K(+) channels.

Protein kinase C enhances the rapidly activating delayed rectifier potassium current, IKr, through a reduction in C-type inactivation in guinea-pig ventricular myocytes

1. The rapidly activating delayed rectifier potassium current, IKr, was studied in guinea-pig ventricular myocytes in the presence of thiopentone, which blocks the more slowly activating component of the delayed rectifier potassium current, IKs, and using whole cell perforated patch clamp or switched voltage clamp with sharp electrodes to minimise intracellular dialysis. 2. Activation of protein kinase A (PKA) by isoprenaline or forskolin caused an increase in IKr tail currents. Following a 300 ms depolarising step to +20 mV, mean tail current amplitude was increased 47 +/- 12% by isoprenaline, and 73 +/- 13% by forskolin. No increase in IKr was observed when IKr was studied using whole cell ruptured patch clamp and there was no change in the reversal potential of IKr in the presence of isoprenaline. 3. The rectification of the current sensitive to E4031, a selective IKr blocker, was markedly reduced in the presence of isoprenaline and the region of negative slope was absent. This is consistent with a reduction in the inactivation of IKr and was supported by the finding that IKr, in the presence of isoprenaline, was somewhat less sensitive to block. E4031 (5 microM) blocked only 81 +/- 5% of IKr in the presence of isoprenaline compared to 100 +/- 0% in control. 4. The forskolin- and isoprenaline-induced increases in IKr were inhibited by staurosporine and by the selective protein kinase C (PKC) inhibitor bisindolymaleimide I. Direct activation of PKC by phorbol dibutyrate increased IKr tail currents by 24 +/- 5%. Both the isoprenaline- and forskolin-induced increases in IKr were inhibited when calcium entry was reduced by block of ICa with nifedipine or when myocytes were pre-incubated in BAPTA-AM. 5. The selective PKA inhibitor KT5720 prevented the isoprenaline-induced increase in IKr only when the increase in ICa was also suppressed. 6. These data show a novel mechanism of regulation of IKr by PKC and this kinase was activated by beta-adrenoceptor stimulation. IKr seems to be enhanced through a reduction in the C-type inactivation which underlies the rectification of the channel and such a mechanism may occur in other channels with this type of inactivation.

Ultrafast inactivation causes inward rectification in a voltage-gated K(+) channel from Caenorhabditis elegans

The exp-2 gene in the nematode Caenorhabditis elegans influences the shape and duration of the action potential of pharyngeal muscle cells. Several loss-of-function mutations in exp-2 lead to broadening of the action potential and to a concomitant slowing of the pumping action of the pharynx. In contrast, a gain-of-function mutation leads to narrow action potentials and shallow pumping. We cloned and functionally characterized the exp-2 gene. The exp-2 gene is homologous to genes of the family of voltage-gated K(+) channels (Kv type). The Xenopus oocyte-expressed EXP-2 channel, although structurally closely related to Kv-type channels, is functionally distinct and very similar to the human ether-à-gogo-related gene (HERG) K(+) channel. In response to depolarization, EXP-2 activates slowly and inactivates very rapidly. On repolarization, recovery from inactivation is also rapid and strongly voltage-dependent. These kinetic properties make the Kv-type EXP-2 channel an inward rectifier that resembles the structurally unrelated HERG channel. Apart from many similarities to HERG, however, the molecular mechanism of fast inactivation appears to be different. Moreover, the single-channel conductance is 5- to 10-fold larger than that of HERG and most Kv-type K(+) channels. It appears that the inward rectification mechanism by rapid inactivation has evolved independently in two distinct classes of structurally unrelated, voltage-gated K(+) channels.

Effects of external acidosis on HERG current expressed in Xenopus oocytes

We investigated effects of external acidosis on HERG current expressed in Xenopus oocytes. HERG current was rapidly and reversibly suppressed by external acidosis in a voltage-independent manner. The slope conductance was decreased from 143 +/- 11 to 93.4 +/- 6.8 microS by changing external pH (pH(o)) from 7.6 to 6.0 (P<0.05). Steady-state activation was shifted by about 20 mV in a depolarized direction with a change from pH(o) 7.6 to 6.0, while steady-state inactivation was not significantly changed. Activation time constants were increased, deactivation and recovery time constants were decreased, while those of inactivation showed no significant change. When external K(+) concentration ([K(+)](o)) was increased from 2 mM to 10 mM, a ratio of slope conductance at pH(o) 6.0 to pH(o) 7.6 was significantly smaller in 2 mM (pH(o) 6.0/pH(o) 7.6 = 0.65 +/- 0.04) than in 10 mM[K(+)](o) (0.83 +/- 0.06, P<0.05). The changes in activation, deactivation and recovery from inactivation were not affected by change in [K(+)](o). The results indicated that external acidosis suppressed HERG current mainly by shifting the voltage-dependence of the activation and deactivation kinetics, and partly by decreasing slope conductance. Moreover, the reduction of HERG current could be partly antagonized with increasing [K(+)](o).

Enhancement of HERG K+ currents by Cd2+ destabilization of the inactivated state

We have studied the functional effects of extracellular Cd(2+) on human ether-a-go-go-related gene (HERG) encoded K(+) channels. Low concentrations (10-200 microM) of extracellular Cd(2+) increased outward currents through HERG channels; 200 microM Cd(2+) more than doubled HERG currents and altered current kinetics. Cd(2+) concentrations up to 200 microM did not change the voltage dependence of channel activation, but shifted the voltage dependence of inactivation to more depolarized membrane potentials. Cd(2+) concentrations >or=500 microM shifted the voltage dependence of channel activation to more positive potentials. These results are consistent with a somewhat specific ability of Cd(2+) to destabilize the inactivated state. We tested the hypothesis that channel inactivation is essential for Cd(2+)-induced increases in HERG K(+) currents, using a double point mutation (G628C/S631C) that diminishes HERG inactivation (Smith, P. L., T. Baukrowitz, and G. Yellen. 1996. Nature (Lond.). 379:833-836). This inactivation-removed mutant is insensitive to low concentrations of Cd(2+). Thus, Cd(2+) had two distinct effects on HERG K(+) channels. Low concentrations of Cd(2+) caused relatively selective effects on inactivation, resulting in a reduction of the apparent rectification of the channel and thereby increasing HERG K(+) currents. Higher Cd(2+) concentrations affected activation gating as well, possibly by a surface charge screening mechanism or by association with a lower affinity site.

Mechanism for the effects of extracellular acidification on HERG-channel function

Human ether-à-go-go-related gene (HERG) encodes a K channel similar to the rapid delayed rectifier channel current (I(Kr)) in cardiac myocytes. Modulation of I(Kr) by extracellular acidosis under pathological conditions may impact on cardiac electrical activity. Therefore, we studied the effects of extracellular acidification on I(Kr) function and the underlying mechanism, using HERG expressed in Xenopus oocytes as a model. Acidification [extracellular pH (pH(o)) 8.5-6.5] accelerated HERG deactivation (at -80 mV, the time constant tau of the major component of deactivation was 253 +/- 17, 158 +/- 10, and 65 +/- 5 ms at pH(o) 8.5, 7.5, and 6.5, respectively; n = 7-10 each), with no effects on other gating kinetics except a modest acceleration of recovery from inactivation (at -80 mV, tau of recovery was 4.7 +/- 0.3, 3.8 +/- 0.3, and 1.3 +/- 0.2 ms at pH(o) 8. 5, 7.5, and 6.5, respectively; n = 4-7 each). The following were ruled out as the underlying mechanisms: 1) voltage shift in channel activation, 2) pore blockade by protons, 3) protonation of histidines on the extracellular domain of HERG, 4) acceleration of recovery from C-type inactivation, and 5) interaction between an external H(+) binding site and the cytoplasmic NH(2)-terminal domain (a key determinant of HERG deactivation rate). Extracellular application of diethylpyrocarbonate caused an irreversible acceleration of HERG deactivation and prevented further acceleration by external acidification. Our data suggest that side chains accessible to the extracellular solution mediated the effects of elevating extracellular H(+) concentration on channel deactivation.

Inactivation of the ERG current in NG108-15 cells

Differentiated NG108-15 neuroblastoma x glioma hybrid cells were whole-cell voltage clamped. The rate of inactivation of ERG (ether-à-go-go related gene) potassium channels was measured with a three-pulse protocol. Contamination with delayed rectifier current at positive potentials was avoided by using the selective ERG channel blocker E-4031. The curve relating time constant of inactivation tau to membrane potential V could be fitted by a Gauss curve. In a bath with 40 mM K(+), the curve peaked at V = -36 mV. Lowering [K(+)](o) decreased tau. At V = -20 mV, the average tau was 25.4 ms in 40 mM K(+), 20.6 ms in 6.5 mM K(+), and 15.0 ms in 0 mM K(+). This resembles the relation between tau and [K(+)](o) in ERG channels expressed in Xenopus oocytes.

A small domain in the N terminus of the regulatory alpha-subunit Kv2. 3 modulates Kv2.1 potassium channel gating

Recent work has demonstrated the existence of regulatory K(+) channel alpha-subunits that are electrically silent but capable of forming heterotetramers with other pore-forming subunits to modify their function. We have investigated the molecular determinant of the modulatory effects of Kv2.3, a silent K(+) channel alpha-subunit specific of brain. This subunit induces on Kv2.1 channels a marked deceleration of activation, inactivation, and closing kinetics. We constructed chimeras of the Kv2.1 and Kv2.3 proteins and analyzed the K(+) currents resulting from the coexpression of the chimeras with Kv2.1. The data indicate that a region of 59 amino acids in the N terminus, adjacent to the first transmembrane segment, is the major structural element responsible for the regulatory function of Kv2.3. The sequence of this domain of Kv2.3 is highly divergent compared with the same region in the other channels of the Kv2 family. Replacement of the regulatory fragment of Kv2.3 by the equivalent of Kv2.1 leads to loss of modulatory function, whereas gain of modulatory function is observed when the Kv2.3 fragment is transferred to Kv2.1. Thus, this study identifies a N-terminus domain involved in Kv2.1 channel gating and in the modulation of this channel by a regulatory alpha-subunit.

Inhibition of the human ether-a-go-go-related gene (HERG) potassium channel by cisapride: affinity for open and inactivated states

1 Cisapride is a prokinetic agent which has been associated with QT prolongation, torsades de pointes and cardiac arrest. The cellular mechanism for these observations is high affinity blockade of IKr (encoded by HERG). 2 In a chronic transfection model using CHO-K1 cells, cisapride inhibited HERG tail currents after a step to +25 mV with similar potency at room and physiological temperatures (IC50 16. 4 nM at 20-22 degrees C and 23.6 nM at 37 degrees C). 3 Channel inhibition exhibited time-, voltage- and frequency-dependence. In an envelope of tails test, channel blockade increased from 27+/-8% after a 120 ms depolarizing step to 50+/-4% after a 1.0 s step. These findings suggested affinity for open and/or inactivated channel states. 4 Inactivation was significantly accelerated by cisapride in a concentration-dependent manner and there was a small (-7 mV) shift in the voltage dependence of steady state inactivation. 5 Channel blockade by cisapride was modulated by [K+]o, with a 26% reduction in the potency of channel blockade when [K+]o was increased from 1 to 10 mM. 6 In conclusion, HERG channel inhibition by cisapride exhibits features consistent with open and inactivated state binding and is sensitive to external potassium concentration. These features may have significant clinical implications with regard to the mechanism and treatment of cisapride-induced proarrhythmia.

Inhibition of the current of heterologously expressed HERG potassium channels by imipramine and amitriptyline

1 Tricyclic antidepressants (TCAs) are associated with cardiovascular side effects including prolongation of the QT interval of the ECG. In this report we studied the effects of two TCAs (imipramine and amitriptyline) on ionic current mediated by cloned HERG potassium channels. 2 Voltage clamp measurements of HERG currents were made from CHO cells transiently transfected with HERG cDNA. HERG-encoded potassium channels were inhibited in a reversible manner by both imipramine and amitriptyline. HERG tail currents (IHERG) following test pulses to +20 mV were inhibited by imipramine with an IC50 of 3.4+/-0.4 microM (mean+/-s.e.mean) and a Hill coefficient of 1.17+/-0.03 (n = 5). 3 microM amitriptyline inhibited IHERG by 34+/-6% (n = 3). The inhibition showed only weak voltage dependence. 3 Using an 'envelope of tails' comprised of pulses to +20 mV of varying durations, the tau of activation was found to be 155+/-30 ms for control and 132+/-26 ms for 3 microM imipramine (n = 5). Once maximal channel activation was achieved after 320 ms (as demonstrated by maximal tail currents), further prolongation of depolarization did not increase imipramine-mediated HERG channel inhibition. 4 Taking current measurements every second during a 10 s depolarizing pulse from -80 mV to 0 mV, block was observed during the first pulse in the presence of imipramine and the level of IHERG block was similar throughout the pulse (n=5). 5 A three pulse protocol (two depolarizing pulses to +20 mV separated by 20 ms at -80 mV) revealed that imipramine did not significantly alter the kinetics of IHERG inactivation. The tau of inactivation was 8+/-2 ms and 5.6+/-0.4 ms (n = 5) in the absence and presence of 3 microM imipramine, respectively, and currents inactivated to a similar extent. 6 Our data are consistent with TCAs causing components of block of the HERG channel in both the closed and open states. Any component of open channel block occurs rapidly upon depolarization. Inhibition of IHERG by the prototype TCAs imipramine and amitriptyline may suggest a mechanism for QT prolongation associated with risks of arrhythmia and sudden death that accompany high concentrations of TCAs following overdose.

Inactivation gating determines nicotine blockade of human HERG channels

We have previously found that nicotine blocked multiple K+ currents, including the rapid component of delayed rectifier K+ currents (IKr), by interacting directly with the channels. To shed some light on the mechanisms of interaction between nicotine and channels, we performed detailed analysis on the human ether-à-go-go-related gene (HERG) channels, which are believed to be equivalent to the native I(Kr) when expressed in Xenopus oocytes. Nicotine suppressed the HERG channels in a concentration-dependent manner with greater potency with voltage protocols, which favor channel inactivation. Nicotine caused dramatic shifts of the voltage-dependent inactivation curve to more negative potentials and accelerated the inactivation process. Conversely, maneuvers that weakened the channel inactivation gating considerably relieved the blockade. Elevating the extracellular K+ concentration from 5 to 20 mM increased the nicotine concentration (by approximately 100-fold) needed to achieve the same degree of inhibition. Moreover, nicotine lost its ability to block the HERG channels when a single mutation was introduced to a residue located after transmembrane domain 6 (S631A) to remove the rapid channel inactivation. Our data suggest that the inactivation gating determines nicotine blockade of the HERG channels.

A small domain in the N terminus of the regulatory alpha-subunit Kv2. 3 modulates Kv2.1 potassium channel gating

Recent work has demonstrated the existence of regulatory K(+) channel alpha-subunits that are electrically silent but capable of forming heterotetramers with other pore-forming subunits to modify their function. We have investigated the molecular determinant of the modulatory effects of Kv2.3, a silent K(+) channel alpha-subunit specific of brain. This subunit induces on Kv2.1 channels a marked deceleration of activation, inactivation, and closing kinetics. We constructed chimeras of the Kv2.1 and Kv2.3 proteins and analyzed the K(+) currents resulting from the coexpression of the chimeras with Kv2.1. The data indicate that a region of 59 amino acids in the N terminus, adjacent to the first transmembrane segment, is the major structural element responsible for the regulatory function of Kv2.3. The sequence of this domain of Kv2.3 is highly divergent compared with the same region in the other channels of the Kv2 family. Replacement of the regulatory fragment of Kv2.3 by the equivalent of Kv2.1 leads to loss of modulatory function, whereas gain of modulatory function is observed when the Kv2.3 fragment is transferred to Kv2.1. Thus, this study identifies a N-terminus domain involved in Kv2.1 channel gating and in the modulation of this channel by a regulatory alpha-subunit.

Inactivation block of the HERG human cardiac K+ channels by RP58866

1. RP58866 possesses a unique electrophysiological property: highly effective against various types of arrhythmias including ventricular fibrillation in animal models, noticeably those occurring during ischaemia with depolarized membrane due to elevated extracellular K+ concentrations. To understand the potential ionic mechanisms, we performed detailed studies on the effects of RP58866 on the HERG channels expressed in Xenopus oocytes, which are believed to be important compositions of the rapid component of delayed rectifier K+ current in the hearts. 2. RP58866 significantly inhibited the HERG channels in a concentration-dependent manner, with approximately 50% decrease in the current amplitude at a concentration of 1 microM. RP58866 produced more pronounced inhibition with voltage protocols which favoured inactivation of the HERG channels. It caused substantial negative shift of the inactivation curves but did not alter the activation properties. The inhibition was considerably relieved by elevating [K+]o from 5 - 20 mM, which weakened the channel inactivation. More importantly, the potency was reduced by approximately 100 fold on the mutated HERG channels (S631A) in which the C-type inactivation was substantially weakened. 4. We conclude that blockade of the HERG channels by RP58866 is mainly associated with the binding of the drugs to the inactivated channels. This unique property of HERG blockade might explain some previously reported but unexplained observations: RP58866 maintains its efficacy in APD prolongation with depolarized membrane potential and in arrhythmias during ischaemia with manifested membrane depolarization.

Long QT syndrome-associated mutations in the S4-S5 linker of KvLQT1 potassium channels modify gating and interaction with minK subunits

Long QT syndrome is an inherited disorder of cardiac repolarization caused by mutations in cardiac ion channel genes, including KVLQT1. In this study, the functional consequences of three long QT-associated missense mutations in KvLQT1 (R243C, W248R, E261K) were characterized using the Xenopus oocyte heterologous expression system and two-microelectrode voltage clamp techniques. These mutations are located in or near the intracellular linker between the S4 and S5 transmembrane domains, a region implicated in activation gating of potassium channels. The E261K mutation caused loss of function and did not interact with wild-type KvLQT1 subunits. R243C or W248R KvLQT1 subunits formed functional channels, but compared with wild-type KvLQT1 current, the rate of activation was slower, and the voltage dependence of activation and inactivation was shifted to more positive potentials. Co expression of minK and KvLQT1 channel subunits induces a slow delayed rectifier K(+) current, I(Ks), characterized by slow activation and a markedly increased magnitude compared with current induced by KvLQT1 subunits alone. Coexpression of minK with R243C or W248R KvLQT1 subunits suppressed current, suggesting that coassembly of mutant subunits with minK prevented normal channel gating. The decrease in I(Ks) caused by loss of function or altered gating properties explains the prolonged QT interval and increased risk of arrhythmia and sudden death associated with these mutations in KVLQT1.

Pathways of HERG inactivation

The rapid, repolarizing K(+) current in cardiomyocytes (I(Kr)) has unique inwardly rectifying properties that contribute importantly to the downstroke of the cardiac action potential. The human ether-à-go-go-related gene (HERG) expresses a macroscopic current virtually identical to I(Kr), but a description of the single-channel properties that cause rectification is lacking. For this reason we measured single-channel and macropatch currents heterologously expressed by HERG in Xenopus oocytes. Our experiments had two main findings. First, the single-channel current-voltage relation showed inward rectification, and conductance was 9.7 pS at -100 mV and 3.9 pS at 100 mV when measured in symmetrical 100 mM K(+) solutions. Second, single channels frequently showed no openings during depolarization but nevertheless revealed bursts of openings during repolarization. This type of gating may explain the inward rectification of HERG currents. To test this hypothesis, we used a three-closed state kinetics model and obtained rate constants from fits to macropatch data. Results from the model are consistent with rapid inactivation from closed states as a significant source of HERG rectification.

Proton and zinc effects on HERG currents

The proton and Zn2+ effects on the human ether-a-go-go related gene (HERG) channels were studied after expression in Xenopus oocytes and stable transfection in the mammalian L929 cell line. Experiments were carried out using the two-electrode voltage clamp at room temperature (oocytes) or the whole-cell patch clamp technique at 35 degrees C (L929 cells). In oocytes, during moderate extracellular acidification (pHo = 6.4), current activation was not shifted on the voltage axis, the time course of current activation was unchanged, but tail current deactivation was dramatically accelerated. At pHo < 6.4, in addition to accelerating deactivation, the time course of activation was slower and the midpoint voltage of current activation was shifted to more positive values. Protons and Zn2+ accelerated the kinetics of deactivation with apparent Kd values about one order of magnitude lower than for tail current inhibition. For protons, the Kd values for the effect on tail current amplitude versus kinetics were, respectively, 1.8 microM (pKa = 5.8) and 0.1 microM (pKa = 7.0). In the presence of Zn2+, the corresponding Kd values were, respectively, 1.2 mM and 169 microM. In L929 cells, acidification to pHo = 6.4 did not shift the midpoint voltage of current activation and had no effect on the time course of current activation. Furthermore, the onset and recovery of inactivation were not affected. However, the acidification significantly accelerated tail current deactivation. We conclude that protons and Zn2+ directly interact with HERG channels and that the interaction results, preferentially, in the regulation of channel deactivation mechanism.

Concomitant Block of the Rapid (I(Kr)) and Slow (I(Ks)) Components of the Delayed Rectifier Potassium Current is Associated With Additional Drug Effects on Lengthening of Cardiac Repolarization

BACKGROUND: The delayed rectifier potassium current, which comprises both a rapid (I(Kr)) and as slow (I(Ks)) component, is a major outward current involved in repolarization of cardiac myocytes. I(Kr) is the target of most drugs that prolong repolarization, whereas electrophysiological effects resulting from combined block of I(Kr) and I(Ks) still need to be characterized. METHODS AND RESULTS: Studies in isolated, buffer-perfused guinea pig hearts were undertaken to compare lengthening of cardiac repolarization under conditions of I(Kr) block alone, I(Ks) Block alone, or combined block of I(Kr) and I(Ks). In protocol A, isolated perfusion with N-acetylprocainamide (NAPA) (I(Kr) block), indapamide (I(Ks) block), or combined NAPA/indapamide was performed at a pacing cycle length of 250 msec. Increases in monophasic action potential duration measured at 90% polarization (MAPD(90)) from baseline after perfusion with NAPA 100 µmol/L (IC(50) for block of I(Kr)) was 19 +/- 6 msed (P <.05), after indapamide 100 µmol/L (EC(50) for block of I(Ks)) 13 +/- 2 msec (P <.05), but 42 +/- 5 msec after combined NAPA 100 µmol/L and indapamide 100 µmol/L (P <.05 vs. baseline and isolated administrations), suggesting the possibility of excessive lengthening of cardiac repolarization by blocking both I(Kr) and I(Ks). As well, in protocol B where sequential perfusions with dofetilide (I(Kr) blocker), dofetilide/indapamide, and indapamide in the same hearts were used, combined dofetilide/indapamide infusion showed a greater increase in MAPD(90) during all pacing cycles studied (250 to 150 msec). CONCLUSIONS: Combined I(Kr) and I(Ks) block may lead to excessive lengthening of cardiac repolarization. This may predispose patients to proarrhythmia during coadministration of drugs.

Effects of outer mouth mutations on hERG channel function: a comparison with similar mutations in the Shaker channel

The fast-inactivation process in the hERG channel can be affected by mutations in the pore or S6 domain, similar to the C-type inactivation in the Shaker channel. However, differences in the kinetics and voltage dependence of inactivation between these two channels suggest that different structural determinants may be involved. To explore this possibility, we mutated a serine in the outer mouth region of hERG (S631) to residues of different physicochemical properties and compared the resulting changes in the channel's inactivation process with those resulting from mutations of an equivalent position in the Shaker channel (T449). The most dramatic differences are seen when this position is occupied by a charged residue: S631K and S631E disrupted C-type inactivation in hERG, whereas T449K and T449E facilitate C-type inactivation in Shaker. S631K and S631E also disrupted the K selectivity of hERG pore, a change not seen in T449K or T449E of Shaker. To further study why there are such differences, we replaced S631 with cysteine. This allowed us to manipulate the properties of thiol groups at position 631 and correlate side-chain properties here with changes in channel function. S631C behaved like the wild-type channel when the thiol groups were in the reduced state. Oxidizing thiol groups with H2O2 or modifying them with MTSET or MTSES disrupted C-type inactivation and K selectivity, similar to the phenotype of S631K and S631E. The same thiol-modifying maneuvers did not affect the wild-type channel function. Our results suggest differences in the outer mouth structure between hERG and Shaker, and we propose a "molecular spring" hypothesis to explain these differences.

Mechanism of block and identification of the verapamil binding domain to HERG potassium channels

Calcium channel antagonists have diverse effects on cardiac electrophysiology. We studied the effects of verapamil, diltiazem, and nifedipine on HERG K+ channels that encode IKr in native heart cells. In our experiments, verapamil caused high-affinity block of HERG current (IC50=143.0 nmol/L), a value close to those reported for verapamil block of L-type Ca2+ channels, whereas diltiazem weakly blocked HERG current (IC50=17.3 micromol/L), and nifedipine did not block HERG current. Verapamil block of HERG channels was use and frequency dependent, and verapamil unbound from HERG channels at voltages near the normal cardiac cell resting potential or with drug washout. Block of HERG current by verapamil was reduced by lowering pHO, which decreases the proportion of drug in the membrane-permeable neutral form. N-methyl-verapamil, a membrane-impermeable, permanently charged verapamil analogue, blocked HERG channels only when applied intracellularly. Verapamil antagonized dofetilide block of HERG channels, which suggests that they may share a common binding site. The C-type inactivation-deficient mutations, Ser620Thr and Ser631Ala, reduced verapamil block, which is consistent with a role for C-type inactivation in high-affinity drug block, although the Ser620Thr mutation decreased verapamil block 20-fold more than the Ser631Ala mutation. Our findings suggest that verapamil enters the cell membrane in the neutral form to act at a site within the pore accessible from the intracellular side of the cell membrane, possibly involving the serine at position 620. Thus, verapamil shares high-affinity HERG channel blocking properties with other class III antiarrhythmic drugs, and this may contribute to its antiarrhythmic mechanism.

Inhibition of HERG channels stably expressed in a mammalian cell line by the antianginal agent perhexiline maleate

Perhexiline has been used as an anti-anginal agent for over 25 years, and is known to cause QT prolongation and torsades de pointes. We hypothesized that the cellular basis for these effects was blockade of I(Kr). A stable transfection of HERG into a CHO-K1 cell line produced a delayed rectifier, potassium channel with similar properties to those reported for transient expression in Xenopus oocytes. Perhexiline caused voltage- and frequency-dependent block of HERG (IC50 7.8 microM). The rate of inactivation was increased and there was a 10 mV hyperpolarizing shift in the voltage-dependence of steady-state inactivation, suggestive of binding to the inactivated state. In conclusion, perhexiline potently inhibits transfected HERG channels and this is the probable mechanism for QT prolongation and torsades de pointes. Channel blockade shows greatest affinity for the inactivated state.

Effects of mitoxantrone on action potential and membrane currents in isolated cardiac myocytes

1. The effects of mitoxantrone (MTO), an anticancer drug, on the membrane electrical properties of cardiac myocytes were investigated using the whole-cell clamp technique. 2. In isolated guinea-pig ventricular myocytes, 30 microM MTO induced a time-dependent prolongation of action potential duration (APD) which was occasionally accompanied by early afterdepolarizations. APD prolongation was preserved in the presence of 10 microM tetrodotoxin and showed reverse rate-dependence. 3. Both the inward rectifier K+ current (I(KI)) and the delayed rectifier K+ current (I(K)) of guinea-pig ventricular myocytes were significantly depressed by 30 microM MTO. The rapidly activating component of I(k) (I(Kr)) seemed to be preferentially blocked by MTO. The transient outward current was not affected by MTO in rat ventricular myocytes. 4. Thirty microM MTO had no direct effect on the L-type Ca2+ current (I(Ca(L))), but reversed the inhibitory effect of 1 microM carbamylcholine but not the A1-adenosine receptor agonist (-)-N6-phenylisopropyladenosine (1 microM) on I(Ca(L)) enhanced by 50 nM isoprenaline in guinea-pig ventricular myocytes. In guinea-pig atrial mycotyes, 30 microM MTO inhibited by 93% the muscarinic receptor gated K+ current (I(K,ACh)) evoked by 1 microM carbamylcholine, whereas I(K,ACh) elicited by 100 microM GTPgammaS, a nonhydrolysable GTP analogue, was only decreased by 12%. 5. The specific binding of [3H]QNB, a muscarinic receptor ligand, to human atrial membranes was concentration-dependently displaced by MTO (1-1000 microM). 6. In conclusion, MTO blocks cardiac muscarinic receptors and prolongs APD by inhibition of I(KI) and I(Kr). The occasionally observed early afterdepolarizations may signify a potential cardiac hazard of the drug.

Dysfunction of delayed rectifier potassium channels in an inherited cardiac arrhythmia

The rapid (IKr) and slow (IKs) delayed rectifier K+ currents are key regulators of cardiac repolarization. HERG encodes the Kr channel, and KVLQT1 and hminK encode subunits that coassemble to form Ks channels. Mutations in any one of these genes cause Romano-Ward syndrome, an autosomal dominant form of long QT syndrome (LQT). Mutations in KVLQT1 and HERG are the most common cause of LQT. Not all missense mutations of HERG or KVLQT1 have the same effect on K+ channel function. Most mutations result in a dominant-negative effect, but the severity of the resulting phenotype varies widely, as judged by reduction of current induced by coexpression of wild-type and mutant subunits in heterologous expression systems. Mutations in hminK (S74L, D76N) reduce IKs by shifting the voltage dependence of activation and accelerating channel deactivation. A recessive form of LQT is caused by mutations in either KVLQT1 or hminK. The functional consequences of mutations in delayed rectifier K+ channel subunits are delayed cardiac repolarization, lengthened QT interval, and an increased risk of torsade de pointes and sudden death.

The eag family of K+ channels in Drosophila and mammals

Mutations of eag, first identified in Drosophila on the basis of their leg-shaking phenotype, cause repetitive firing and enhanced transmitter release in motor neurons. The encoded EAG polypeptide is related both to voltage-gated K+ channels and to cyclic nucleotide-gated cation channels. Homology screens identified a family of eag-related channel polypeptides, highly conserved from nematodes to humans, comprising three subfamilies: EAG, ELK, and ERG. When expressed in frog oocytes, EAG channels behave as voltage-dependent, outwardly rectifying K(+)-selective channels. Mutations of the human eag-related gene (HERG) result in a form of cardiac arrhythmia that can lead to ventricular fibrillation and sudden death. Electrophysiological and pharmacological studies have provided evidence that HERG channels specify one component of the delayed rectifier, IKr, that contributes to the repolarization phase of cardiac action potentials. An important role for HERG channels in neuronal excitability is also suggested by the expression of these channels in brain tissue. Moreover, mutations of ERG-type channels in the Drosophila sei mutant cause temperature-induced convulsive seizures associated with aberrant bursting activity in the flight motor pathway. The in vivo function of ELK channels has not yet been established, but when these channels are expressed in frog oocytes, they display properties intermediate between those of EAG- and ERG-type channels. Coexpression of the K(+)-channel beta subunit encoded by Hk with EAG in oocytes dramatically increases current amplitude and also affects the gating and modulation of these currents. Biochemical evidence indicates a direct physical interaction between EAG and HK proteins. Overall, these studies highlight the diverse properties of the eag family of K+ channels, which are likely to subserve diverse functions in vivo.

Functional analysis of a mouse brain Elk-type K+ channel

Members of the Ether à go-go (Eag) K+ channel subfamilies Eag, Erg, and Elk are widely expressed in the nervous system, but their neural functions in vivo remain largely unknown. The biophysical properties of channels from the Eag and Erg subfamilies have been described, and based on their characteristic features and expression patterns, Erg channels have been associated with native currents in the heart. Little is known about the properties of channels from the Elk subfamily. We have identified a mouse gene, Melk2, that encodes a predicted polypeptide with 48% amino acid identity to Drosophila Elk but only 40 and 36% identity with mouse Erg (Merg) and Eag (Meag), respectively. Melk2 RNA appears to be expressed at high levels only in brain tissue. Functional expression of Melk2 in Xenopus oocytes reveals large, transient peaks of current at the onset of depolarization. Like Meag currents, Melk2 currents activate relatively quickly, but they lack the nonsuperimposable Cole-Moore shift characteristic of the Eag subfamily. Melk2 currents are insensitive to E-4031, a class III antiarrhythmic compound that blocks the Human Ether-à-go-go-Related Gene (HERG) channel and its counterpart in native tissues, IKr. Melk2 channels exhibit inward rectification because of a fast C-type inactivation mechanism, but the slower rate of inactivation and the faster rate of activation results in less inward rectification than that observed in HERG channels. This characterization of Melk currents should aid in identification of native counterparts to the Elk subfamily of channels in the nervous system.

MiRP1 forms IKr potassium channels with HERG and is associated with cardiac arrhythmia

A novel potassium channel gene has been cloned, characterized, and associated with cardiac arrhythmia. The gene encodes MinK-related peptide 1 (MiRP1), a small integral membrane subunit that assembles with HERG, a pore-forming protein, to alter its function. Unlike channels formed only with HERG, mixed complexes resemble native cardiac IKr channels in their gating, unitary conductance, regulation by potassium, and distinctive biphasic inhibition by the class III antiarrhythmic E-4031. Three missense mutations associated with long QT syndrome and ventricular fibrillation are identified in the gene for MiRP1. Mutants form channels that open slowly and close rapidly, thereby diminishing potassium currents. One variant, associated with clarithromycin-induced arrhythmia, increases channel blockade by the antibiotic. A mechanism for acquired arrhythmia is revealed: genetically based reduction in potassium currents that remains clinically silent until combined with additional stressors.

Functional analysis of a mouse brain Elk-type K+ channel

Members of the Ether à go-go (Eag) K+ channel subfamilies Eag, Erg, and Elk are widely expressed in the nervous system, but their neural functions in vivo remain largely unknown. The biophysical properties of channels from the Eag and Erg subfamilies have been described, and based on their characteristic features and expression patterns, Erg channels have been associated with native currents in the heart. Little is known about the properties of channels from the Elk subfamily. We have identified a mouse gene, Melk2, that encodes a predicted polypeptide with 48% amino acid identity to Drosophila Elk but only 40 and 36% identity with mouse Erg (Merg) and Eag (Meag), respectively. Melk2 RNA appears to be expressed at high levels only in brain tissue. Functional expression of Melk2 in Xenopus oocytes reveals large, transient peaks of current at the onset of depolarization. Like Meag currents, Melk2 currents activate relatively quickly, but they lack the nonsuperimposable Cole-Moore shift characteristic of the Eag subfamily. Melk2 currents are insensitive to E-4031, a class III antiarrhythmic compound that blocks the Human Ether-à-go-go-Related Gene (HERG) channel and its counterpart in native tissues, IKr. Melk2 channels exhibit inward rectification because of a fast C-type inactivation mechanism, but the slower rate of inactivation and the faster rate of activation results in less inward rectification than that observed in HERG channels. This characterization of Melk currents should aid in identification of native counterparts to the Elk subfamily of channels in the nervous system.

Long QT syndrome-associated mutations in the Per-Arnt-Sim (PAS) domain of HERG potassium channels accelerate channel deactivation

Mutations in the human ether-a-go-go-related gene (HERG) cause long QT syndrome, an inherited disorder of cardiac repolarization that predisposes affected individuals to life-threatening arrhythmias. HERG encodes the cardiac rapid delayed rectifier potassium channel that mediates repolarization of ventricular action potentials. In this study, we used the oocyte expression system and voltage clamp techniques to determine the functional consequences of eight long QT syndrome-associated mutations located in the amino-terminal region of HERG (F29L, N33T, G53R, R56Q, C66G, H70R, A78P, and L86R). Mutant subunits formed functional channels with altered gating properties when expressed alone in oocytes. Deactivation was accelerated by all mutations. Some mutants shifted the voltage dependence of channel availability to more positive potentials. Voltage ramps indicated that fast deactivation of mutant channels would reduce outward current during the repolarization phase of the cardiac action potential and cause prolongation of the corrected QT interval, QTc. The amino-terminal region of HERG was recently crystallized and shown to possess a Per-Arnt-Sim (PAS) domain. The location of these mutations suggests they may disrupt the PAS domain and interfere with its interaction with the S4-S5 linker of the HERG channel.

Human ether-à-go-go-related gene K+ channel gating probed with extracellular ca2+. Evidence for two distinct voltage sensors

Human ether-à-go-go-related gene (HERG) encoded K+ channels were expressed in Chinese hamster ovary (CHO-K1) cells and studied by whole-cell voltage clamp in the presence of varied extracellular Ca2+ concentrations and physiological external K+. Elevation of external Ca2+ from 1.8 to 10 mM resulted in a reduction of whole-cell K+ current amplitude, slowed activation kinetics, and an increased rate of deactivation. The midpoint of the voltage dependence of activation was also shifted +22.3 +/- 2.5 mV to more depolarized potentials. In contrast, the kinetics and voltage dependence of channel inactivation were hardly affected by increased extracellular Ca2+. Neither Ca2+ screening of diffuse membrane surface charges nor open channel block could explain these changes. However, selective changes in the voltage-dependent activation, but not inactivation gating, account for the effects of Ca2+ on Human ether-à-go-go-related gene current amplitude and kinetics. The differential effects of extracellular Ca2+ on the activation and inactivation gating indicate that these processes have distinct voltage-sensing mechanisms. Thus, Ca2+ appears to directly interact with externally accessible channel residues to alter the membrane potential detected by the activation voltage sensor, yet Ca2+ binding to this site is ineffective in modifying the inactivation gating machinery.

Blockade of HERG channels expressed in Xenopus laevis oocytes by external divalent cations

We have investigated actions of various divalent cations (Ba2+, Sr2+, Mn2+, Co2+, Ni2+, Zn2+) on human ether-a-go-go related gene (HERG) channels expressed in Xenopus laevis oocytes using the voltage clamp technique. All divalent cations inhibited HERG current dose-dependently in a voltage-dependent manner. The concentration for half-maximum inhibition (Ki) decreased at more negative potentials, indicating block is facilitated by hyperpolarization. Ki at 0 mV for Zn2+, Ni2+, Co2+, Ba2+, Mn2+, and Sr2+ was 0.19, 0.36, 0. 50, 0.58, 2.36, and 6.47 mM, respectively. The effects were manifested in four ways: 1) right shift of voltage dependence of activation, 2) decrease of maximum conductance, 3) acceleration of current decay, and 4) slowing of activation. However, each parameter was not affected by each cation to the same extent. The potency for the shift of voltage dependence of activation was in the order Zn2+ > Ni2+ >/= Co2+ > Ba2+ > Mn2+ > Sr2+, whereas the potency for the decrease of maximum conductance was Zn2+ > Ba2+ > Sr2+ > Co2+ > Mn2+. The kinetics of activation and deactivation were also affected, but the two parameters are not affected to the same extent. Slowing of activation by Ba2+ was most distinct, causing a marked initial delay of current onset. From these results we concluded that HERG channels are nonselectively blocked by most divalent cations from the external side, and several different mechanism are involved in their actions. There exist at least two distinct binding sites for their action: one for the voltage-dependent effect and the other for reducing maximum conductance.

Mutations of the S4-S5 linker alter activation properties of HERG potassium channels expressed in Xenopus oocytes

1. The structural basis for the activation gate of voltage-dependent K+ channels is not known, but indirect evidence has implicated the S4-S5 linker, the cytoplasmic region between the fourth and fifth transmembrane domains of the channel subunit. We have studied the effects of mutations in the S4-S5 linker of HERG (human ether-á-go-go-related gene), a human delayed rectifier K+ channel, in Xenopus oocytes. 2. Mutation of acidic residues (D540, E544) in the S4-S5 linker of HERG channels to neutral (Ala) or basic (Lys) residues accelerated the rate of channel deactivation. Most mutations greatly accelerated the rate of activation. However, E544K HERG channels activated more slowly than wild-type HERG channels. 3. Mutation of residues in the S4-S5 linker had little or no effect on fast inactivation, consistent with independence of HERG channel activation and inactivation 4. In response to large hyperpolarizations, D540K HERG channels can reopen into a state that is distinct from the normal depolarization-induced open state. It is proposed that substitution of a negatively charged Asp with the positively charged Lys disrupts a subunit interaction that normally stabilizes the channel in a closed state at negative transmembrane potentials. 5. The results indicate that the S4-S5 linker is a crucial component of the activation gate of HERG channels.

Voltage-dependent gating of single wild-type and S4 mutant KAT1 inward rectifier potassium channels

The voltage-dependent gating mechanism of KAT1 inward rectifier potassium channels was studied using single channel current recordings from Xenopus oocytes injected with KAT1 mRNA. The inward rectification properties of KAT1 result from an intrinsic gating mechanism in the KAT1 channel protein, not from pore block by an extrinsic cation species. KAT1 channels activate with hyperpolarizing potentials from -110 through -190 mV with a slow voltage-dependent time course. Transitions before first opening are voltage dependent and account for much of the voltage dependence of activation, while transitions after first opening are only slightly voltage dependent. Using burst analysis, transitions near the open state were analyzed in detail. A kinetic model with multiple closed states before first opening, a single open state, a single closed state after first opening, and a closed-state inactivation pathway accurately describes the single channel and macroscopic data. Two mutations neutralizing charged residues in the S4 region (R177Q and R176L) were introduced, and their effects on single channel gating properties were examined. Both mutations resulted in depolarizing shifts in the steady state conductance-voltage relationship, shortened first latencies to opening, decreased probability of terminating bursts, and increased burst durations. These effects on gating were well described by changes in the rate constants in the kinetic model describing KAT1 channel gating. All transitions before the open state were affected by the mutations, while the transitions after the open state were unaffected, implying that the S4 region contributes to the early steps in gating for KAT1 channels.

Crystal structure and functional analysis of the HERG potassium channel N terminus: a eukaryotic PAS domain

The HERG voltage-dependent K+ channel plays a role in cardiac electrical excitability, and when defective, it underlies one form of the long QT syndrome. We have determined the crystal structure of the HERG K+ channel N-terminal domain and studied its role as a modifier of gating using electrophysiological methods. The domain is similar in structure to a bacterial light sensor photoactive yellow protein and provides the first three-dimensional model of a eukaryotic PAS domain. Scanning mutagenesis of the domain surface has allowed the identification of a hydrophobic "hot spot" forming a putative interface with the body of the K+ channel to which it tightly binds. The presence of the domain attached to the channel slows the rate of deactivation. Given the roles of PAS domains in biology, we propose that the HERG N-terminal domain has a regulatory function.

Cell cycle-related changes in the conducting properties of r-eag K+ channels

Release from arrest in G2 phase of the cell cycle causes profound changes in rat ether-à-go-go (r-eag) K+ channels heterologously expressed in Xenopus oocytes. The most evident consequence of the onset of maturation is the appearance of rectification in the r-eag current. The trigger for these changes is located downstream of the activation of mitosis-promoting factor (MPF). We demonstrate here that the rectification is due to a voltage-dependent block by intracellular Na+ ions. Manipulation of the intracellular Na+ concentration indicates that the site of Na+ block is located approximately 45% into the electrical distance of the pore and is only present in oocytes undergoing maturation. Since the currents through excised patches from immature oocytes exhibited a fast rundown, we studied CHO-K1 cells permanently transfected with r-eag. These cells displayed currents with a variable degree of block by Na+ and variable permeability to Cs+. Partial synchronization of the cultures in G0/G1 or M phases of the cell cycle greatly reduced the variability. The combined data obtained from mammalian cells and oocytes strongly suggest that the permeability properties of r-eag K+ channels are modulated during cell cycle-related processes.

Regulation of deactivation by an amino terminal domain in human ether-à-go-go-related gene potassium channels

Abnormalities in repolarization of the cardiac ventricular action potential can lead to life-threatening arrhythmias associated with long QT syndrome. The repolarization process depends upon the gating properties of potassium channels encoded by the human ether-à-go-go-related gene (HERG), especially those governing the rate of recovery from inactivation and the rate of deactivation. Previous studies have demonstrated that deletion of the NH2 terminus increases the deactivation rate, but the mechanism by which the NH2 terminus regulates deactivation in wild-type channels has not been elucidated. We tested the hypothesis that the HERG NH2 terminus slows deactivation by a mechanism similar to N-type inactivation in Shaker channels, where it binds to the internal mouth of the pore and prevents channel closure. We found that the regulation of deactivation by the HERG NH2 terminus bears similarity to Shaker N-type inactivation in three respects: (a) deletion of the NH2 terminus slows C-type inactivation; (b) the action of the NH2 terminus is sensitive to elevated concentrations of external K+, as if its binding along the permeation pathway is disrupted by K+ influx; and (c) N-ethylmaleimide, covalently linked to an aphenotypic cysteine introduced within the S4-S5 linker, mimics the N deletion phenotype, as if the binding of the NH2 terminus to its receptor site were hindered. In contrast to N-type inactivation in Shaker, however, there was no indication that the NH2 terminus blocks the HERG pore. In addition, we discovered that separate domains within the NH2 terminus mediate the slowing of deactivation and the promotion of C-type inactivation. These results suggest that the NH2 terminus stabilizes the open state and, by a separate mechanism, promotes C-type inactivation.

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.

Cloning of a mammalian elk potassium channel gene and EAG mRNA distribution in rat sympathetic ganglia

1. Three new members of the EAG potassium channel gene family were identified in rat and the complete coding sequence of one of these genes (elk1) was determined by cDNA cloning. 2. The elk1 gene, when expressed in Xenopus oocytes, encodes a slowly activating and slowly deactivating potassium channel. 3. The elk1 gene is expressed in sympathetic ganglia and is also expressed in sciatic nerve. 4. Six of the seven known EAG genes were found to be expressed in rat sympathetic ganglia, suggesting an important functional role for these channels in the sympathetic nervous system.

Inwardly rectifying potassium (IRK) currents are correlated with IRK subunit expression in rat nucleus accumbens medium spiny neurons

Inwardly rectifying K+ (IRK) channels are critical for shaping cell excitability. Whole-cell patch-clamp and single-cell RT-PCR techniques were used to characterize the inwardly rectifying K+ currents found in projection neurons of the rat nucleus accumbens. Inwardly rectifying currents were highly selective for K+ and blocked by low millimolar concentrations of Cs+ or Ba2+. In a subset of neurons, the inwardly rectifying current appeared to inactivate at hyperpolarized membrane potentials. In an attempt to identify this subset, neurons were profiled using single-cell RT-PCR. Neurons expressing substance P mRNA exhibited noninactivating inward rectifier currents, whereas neurons expressing enkephalin mRNA exhibited inactivating inward rectifier currents. The inactivation of the inward rectifier was correlated with the expression of IRK1 mRNA. These results demonstrate a clear physiological difference in the properties of medium spiny neurons and suggest that this difference could influence active state transitions driven by cortical and hippocampal excitatory input.

Modulation of human erg K+ channel gating by activation of a G protein-coupled receptor and protein kinase C

1. Modulation of the human ether-à-go-go-related gene (HERG) K+ channel was studied in two-electrode voltage-clamped Xenopus oocytes co-expressing the channel protein and the thyrotropin-releasing hormone (TRH) receptor. 2. Addition of TRH caused clear modifications of HERG channel gating kinetics. These variations consisted of an acceleration of deactivation, as shown by a faster decay of hyperpolarization-induced tail currents, and a slower time course of activation, measured using an envelope of tails protocol. The voltage dependence for activation was also shifted by nearly 20 mV in the depolarizing direction. Neither the inactivation nor the inactivation recovery rates were altered by TRH. 3. The alterations in activation gating parameters induced by TRH were demonstrated in a direct way by looking at the increased outward K+ currents elicited in extracellular solutions in which K+ was replaced by Cs+. 4. The effects of TRH were mimicked by direct pharmacological activation of protein kinase C (PKC) with beta-phorbol 12-myristate, 13-acetate (PMA). The TRH-induced effects were antagonized by GF109203X, a highly specific inhibitor of PKC that also abolished the PMA-dependent regulation of the channels. 5. It is concluded that a PKC-dependent pathway links G protein-coupled receptors that activate phospholipase C to modulation of HERG channel gating. This provides a mechanism for the physiological regulation of cardiac function by phospholipase C-activating receptors, and for modulation of adenohypophysial neurosecretion in response to TRH.

Inwardly rectifying potassium (IRK) currents are correlated with IRK subunit expression in rat nucleus accumbens medium spiny neurons

Inwardly rectifying K+ (IRK) channels are critical for shaping cell excitability. Whole-cell patch-clamp and single-cell RT-PCR techniques were used to characterize the inwardly rectifying K+ currents found in projection neurons of the rat nucleus accumbens. Inwardly rectifying currents were highly selective for K+ and blocked by low millimolar concentrations of Cs+ or Ba2+. In a subset of neurons, the inwardly rectifying current appeared to inactivate at hyperpolarized membrane potentials. In an attempt to identify this subset, neurons were profiled using single-cell RT-PCR. Neurons expressing substance P mRNA exhibited noninactivating inward rectifier currents, whereas neurons expressing enkephalin mRNA exhibited inactivating inward rectifier currents. The inactivation of the inward rectifier was correlated with the expression of IRK1 mRNA. These results demonstrate a clear physiological difference in the properties of medium spiny neurons and suggest that this difference could influence active state transitions driven by cortical and hippocampal excitatory input.

Transfer of rapid inactivation and sensitivity to the class III antiarrhythmic drug E-4031 from HERG to M-eag channels

The gating behaviour and pharmacological sensitivity of HERG are remarkably different from the corresponding properties of M-eag, a structurally similar member of the Eag family of potassium channels. In contrast to HERG, M-eag exhibits no apparent inactivation and little rectification, and is insensitive to the class III antiarrhythmic drug E-4031. We generated chimeric channels of HERG and M-eag sequences and made point mutations to identify the region necessary for rapid inactivation in HERG. This region includes the P region and half of the S6 putative transmembrane domain, including sites not previously associated with inactivation and rectification in HERG. Transfer of a small segment of the HERG polypeptide to M-eag, consisting largely of the P region and part of the S6 transmembrane domain, is sufficient to confer rapid inactivation and E-4031 sensitivity to M-eag. This region differs from the corresponding region in M-eag by only fifteen residues. Previous hypotheses that rapid inactivation of HERG channels occurs by a C-type inactivation mechanism are supported by the parallel effects on rates of HERG inactivation and Shaker C-type inactivation by a series of mutations at two equivalent sites in the polypeptide sequences. In addition to sites homologous to those previously described for C-type inactivation in Shaker, inactivation in HERG involves a residue in the upstream P region not previously associated with C-type inactivation. Although this site is equivalent to one implicated in P-type inactivation in Kv2.1 channels, our data are most consistent with a single, C-type inactivation mechanism.

Activation and inactivation of homomeric KvLQT1 potassium channels

The voltage-gated potassium channel protein KvLQT1 (Wang et al., 1996. Nature Genet. 12:17-23) is believed to underlie the delayed rectifier potassium current of cardiac muscle together with the small membrane protein minK (also named IsK) as an essential auxiliary subunit (Barhanin et al., 1996. Nature. 384:78-80; Sanguinetti et al., 1996. Nature. 384:80-83) Using the Xenopus oocyte expression system, we analyzed in detail the gating characteristics of homomeric KvLQT1 channels and of heteromeric KvLQT1/minK channels using two-electrode voltage-clamp recordings. Activation of homomeric KvLQT1 at positive voltages is accompanied by an inactivation process that is revealed by a transient increase in conductance after membrane repolarization to negative values. We studied the recovery from inactivation and the deactivation of the channels during tail repolarizations at -120 mV after conditioning pulses of variable amplitude and duration. Most measurements were made in high extracellular potassium to increase the size of inward tail currents. However, experiments in normal low-potassium solutions showed that, in contrast to classical C-type inactivation, the inactivation of KvLQT1 is independent of extracellular potassium. At +40 mV inactivation develops with a delay of 100 ms. At the same potential, the activation estimated from the amplitude of the late exponential decay of the tail currents follows a less sigmoidal time course, with a late time constant of 300 ms. Inactivation of KvLQT1 is not complete, even at the most positive voltages. The delayed, voltage-dependent onset and the incompleteness of inactivation suggest a sequential gating scheme containing at least two open states and ending with an inactivating step that is voltage independent. In coexpression experiments of KvLQT1 with minK, inactivation seems to be largely absent, although biphasic tails are also observed that could be related to similar phenomena.

Differential effects of bupivacaine on cardiac K channels: role of channel inactivation and subunit composition in drug-channel interaction

INTRODUCTION

We examined the effects of a nonspecific ion channel blocker, bupivacaine, on K channels encoded by hERG, rKv1.4, rKv4.3, and hKvLQT1 along with hIsK. Their native counterparts in the heart are important for the function of I(Kr), I(to) and I(Ks) and, thus, play an important role in repolarization.

METHODS AND RESULTS

To elucidate the mechanisms and sites of bupivacaine's actions, we correlated the voltage and time dependencies of drug effects with those of channel gating. We also studied the effects of altering the C-type (hERG) or N-type (rKv1.4) inactivation process or the subunit composition (hKvLQT1 with or without hIsK) on bupivacaine's actions. The results suggest that, except for hKvLQT1 co-expressed with hIsK, bupivacaine binding occurred at depolarized voltages coinciding with channel activation. With hKvLQT1 co-expressed with hIsK, bupivacaine bound preferentially at negative voltages when channels were in the closed state, and unbound at depolarized voltages when channels opened. The C-type inactivation of hERG enhanced, whereas the N-type inactivation of rKv1.4 hindered, bupivacaine's effects.

CONCLUSION

We propose that bupivacaine's actions on these K channels can be described as a nonspecific pore blockade in the inner mouth region. However, the apparent binding affinity and voltage dependence of binding can be differentially influenced by the inactivation processes occurring at two ends of the pore (C-type inactivation at the outer end and N-type inactivation at the inner end), or by the interaction between hIsK and hKvLQT1 subunits.

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.

Voltage-dependent inactivation of the human K+ channel KvLQT1 is eliminated by association with minimal K+ channel (minK) subunits

1. The time course and voltage dependence of inactivation of KvLQT1 channels expressed in Xenopus oocytes were studied using two-microelectrode voltage-clamp techniques. 2. Tail current analysis was used to characterize the kinetics of channel inactivation and deactivation. The time constant for recovery from channel inactivation was voltage dependent and varied from 30 +/- 2 ms at -90 mV to 36 +/- 1 ms at -30 mV. The time constant for deactivation varied from 186 +/- 21 to 986 +/- 43 ms over the same voltage range. 3. Inactivation of KvLQT1 channels was incomplete, reducing fully activated current by 35 % at +40 mV. Inactivation of KvLQT1 channels was half-maximal at -18 +/- 2 mV. 4. The onset of KvLQT1 channel inactivation during a single depolarization to +20 mV was exponential (tau = 130 +/- 10 ms), and developed after a delay of approximately 75 ms. In contrast, when inactivation was reinduced following transient recovery of channels to the open state(s), the onset of inactivation was immediate and 10 times faster. These findings suggest multiple open states, and a sequential gating model for KvLQT1 channel activation and inactivation (C1<==> Cn<==> O1<==> O2<==>I). 5. Delayed rectifier K+ (IKs) channels formed by heteromultimeric coassembly of KvLQT1 and minimal K+ channel (minK) subunits did not inactivate. Thus, minK subunits eliminate, or greatly slow, the gating associated with channel inactivation when coassembled with KvLQT1.

Idiosyncratic gating of HERG-like K+ channels in microglia

A simple kinetic model is presented to explain the gating of a HERG-like voltage-gated K+ conductance described in the accompanying paper (Zhou, W., F.S. Cayabyab, P.S. Pennefather, L.C. Schlichter, and T.E. DeCoursey. 1998. J. Gen. Physiol. 111:781-794). The model proposes two kinetically distinct closing pathways, a rapid one favored by depolarization (deactivation) and a slow one favored by hyperpolarization (inactivation). The overlap of these two processes leads to a window current between -50 and +20 mV with a peak at -36 mV of approximately 12% maximal conductance. The near absence of depolarization-activated outward current in microglia, compared with HERG channels expressed in oocytes or cardiac myocytes, can be explained if activation is shifted negatively in microglia. As seen with experimental data, availability predicted by the model was more steeply voltage dependent, and the midpoint more positive when determined by making the holding potential progressively more positive at intervals of 20 s (starting at -120 mV), rather than progressively more negative (starting at 40 mV). In the model, this hysteresis was generated by postulating slow and ultra-slow components of inactivation. The ultra-slow component takes minutes to equilibrate at -40 mV but is steeply voltage dependent, leading to protocol-dependent modulation of the HERG-like current. The data suggest that "deactivation" and "inactivation" are coupled through the open state. This is particularly evident in isotonic Cs+, where a delayed and transient outward current develops on depolarization with a decay time constant more voltage dependent and slower than the deactivation process observed at the same potential after a brief hyperpolarization.