IKs, a Slow and Intriguing Cardiac K+ Channel and Its Associated Long QT Diseases

β-adrenergic stimulation augments transmural dispersion of repolarization via modulation of delayed rectifier currents IKs and IKr in the human ventricle

Long QT syndrome (LQTS) is an inherited or drug induced condition associated with delayed repolarization and sudden cardiac death. The cardiac potassium channel, I_Kr, and the adrenergic-sensitive cardiac potassium current, I_Ks, are two primary contributors to cardiac repolarization. This study aimed to elucidate the role of β-adrenergic (β-AR) stimulation in mediating the contributions of I_Kr and I_Ks to repolarizing the human left ventricle (n = 18). Optical mapping was used to measure action potential durations (APDs) in the presence of the I_Ks blocker JNJ-303 and the I_Kr blocker E-4031. We found that JNJ-303 alone did not increase APD. However, under isoprenaline (ISO), both the application of JNJ-303 and additional E-4031 significantly increased APD. With JNJ-303, ISO decreased APD significantly more in the epicardium as compared to the endocardium, with subsequent application E-4031 increasing mid- and endocardial APD80 more significantly than in the epicardium. We found that β-AR stimulation significantly augmented the effect of I_Ks blocker JNJ-303, in contrast to the reduced effect of I_Kr blocker E-4031. We also observed synergistic augmentation of transmural repolarization gradient by the combination of ISO and E-4031. Our results suggest β-AR-mediated increase of transmural dispersion of repolarization, which could pose arrhythmogenic risk in LQTS patients.

Comparison of electrophysiological effects of calcium channel blockers on cardiac repolarization

Dihydropyridine (DHP) calcium channel blockers (CCBs) have been widely used to treat of several cardiovascular diseases. An excessive shortening of action potential duration (APD) due to the reduction of Ca²⁺ channel current (I_Ca) might increase the risk of arrhythmia. In this study we investigated the electrophysiological effects of nicardipine (NIC), isradipine (ISR), and amlodipine (AML) on the cardiac APD in rabbit Purkinje fibers, voltage-gated K⁺ channel currents (I_Kr, I_Ks) and voltage-gated Na⁺ channel current (I_Na). The concentration-dependent inhibition of Ca²⁺ channel currents (I_Ca) was examined in rat cardiomyocytes; these CCBs have similar potency on I_Ca channel blocking with IC₅₀ (the half-maximum inhibiting concentration) values of 0.142, 0.229, and 0.227 nM on NIC, ISR, and AML, respectively. However, ISR shortened both APD₅₀ and APD₉₀ already at 1 µM whereas NIC and AML shortened APD₅₀ but not APD₉₀ up to 30 µM. According to ion channel studies, NIC and AML concentration-dependently inhibited I_Kr and I_Ks while ISR had only partial inhibitory effects (<50% at 30 µM). Inhibition of I_Na was similarly observed in the three CCBs. Since the I_Kr and I_Ks mainly contribute to cardiac repolarization, their inhibition by NIC and AML could compensate for the AP shortening effects due to the block of I_Ca.

Role of slow delayed rectifying potassium current in dynamics of repolarization and electrical memory in swine ventricles

Dynamics of repolarization, quantified as restitution and electrical memory, impact conduction stability. Relatively less is known about role of slow delayed rectifying potassium current, I(Ks), in dynamics of repolarization and memory compared to the rapidly activating current I(Kr). Trans-membrane potentials were recorded from right ventricular tissues from pigs during reduction (chromanol 293B) and increases in I(Ks) (mefenamic acid). A novel pacing protocol was used to explicitly control diastolic intervals to quantify memory. Restitution hysteresis, a consequence of memory, increased after chromanol 293B (loop thickness and area increased 27 and 38 %) and decreased after mefenamic acid (52 and 53 %). Standard and dynamic restitutions showed an increase in average slope after chromanol 293B and a decrease after mefenamic acid. Increase in slope and memory are hypothesized to have opposite effects on electrical stability; therefore, these results suggest that reduction and enhancement of I(Ks) likely also have offsetting components that affect stability.

Characterization of binding site of closed-state KCNQ1 potassium channel by homology modeling, molecular docking, and pharmacophore identification

This investigation was performed to assess the importance of interaction in the binding of blockers to KCNQ1 potassium using molecular modeling. This work could be considered made up by three main steps: (1) the construction of closed-state structure of KCNQ1 through homology modeling; (2) the automated docking of three blockers: IKS-142, L-735821, and BMS-IKS, using DOCK program; (3) the generation and validation of pharmacophore for KCNQ1 ligands using Catalyst/HypoGen. The obtained results highlight the hydrophobic or aromatic residues involved in S6 transmembrane domain and the base of the pore helix of KCNQ1, confirming the mutagenesis data and pharmacophore model, and giving new suggestions for the rational design of novel KCNQ1 ligands.

p11, an annexin II subunit, an auxiliary protein associated with the background K+ channel, TASK-1

TASK-1 belongs to the 2P domain K+ channel family and is the prototype of background K+ channels that set the resting membrane potential and tune action potential duration. Its activity is highly regulated by hormones and neurotransmitters. Although numerous auxiliary proteins have been described to modify biophysical, pharmacological and expression properties of different voltage- and Ca2+-sensitive K+ channels, none of them is known to modulate 2P domain K+ channel activity. We show here that p11 interacts specifically with the TASK-1 K+ channel. p11 is a subunit of annexin II, a cytoplasmic protein thought to bind and organize specialized membrane cytoskeleton compartments. This association with p11 requires the integrity of the last three C-terminal amino acids, Ser-Ser-Val, in TASK-1. Using series of C-terminal TASK-1 deletion mutants and several TASK-1-GFP chimeras, we demonstrate that association with p11 is essential for trafficking of TASK-1 to the plasma membrane. p11 association with the TASK-1 channel masks an endoplasmic reticulum retention signal identified as Lys-Arg-Arg that precedes the Ser-Ser-Val sequence.

Altered potassium balance and aldosterone secretion in a mouse model of human congenital long QT syndrome

The voltage-dependent K(+) channel responsible for the slowly activating delayed K(+) current I(Ks) is composed of pore-forming KCNQ1 and regulatory KCNE1 subunits, which are mutated in familial forms of cardiac long QT syndrome. Because KCNQ1 and KCNE1 genes also are expressed in epithelial tissues, such as the kidneys and the intestine, we have investigated the adaptation of KCNE1-deficient mice to different K(+) and Na(+) intakes. On a normal K(+) diet, homozygous kcne1(-/-) mice exhibit signs of chronic volume depletion associated with fecal Na(+) and K(+) wasting and have lower plasma K(+) concentration and higher levels of aldosterone than wild-type mice. Although plasma aldosterone can be suppressed by low K(+) diets or stimulated by low Na(+) diets, a high K(+) diet provokes a tremendous increase of plasma aldosterone levels in kcne1(-/-) mice as compared with wild-type mice (7.1-fold vs. 1.8-fold) despite lower plasma K(+) in kcne1(-/-) mice. This exacerbated aldosterone production in kcne1(-/-) mice is accompanied by an abnormally high plasma renin concentration, which could partly explain the hyperaldosteronism. In addition, we found that KCNE1 and KCNQ1 mRNAs are expressed in the zona glomerulosa of adrenal glands where I(Ks) may directly participate in the control of aldosterone production by plasma K(+). These results, which show that KCNE1 and I(Ks) are involved in K(+) homeostasis, might have important implications for patients with I(Ks)-related long QT syndrome, because hypokalemia is a well known risk factor for the occurrence of torsades de pointes ventricular arrhythmia.

Slow delayed rectifier current and repolarization in canine cardiac Purkinje cells

Although cardiac Purkinje cells (PCs) are believed to be the source of early afterdepolarizations generating ventricular tachyarrhythmias in long Q-T syndromes (LQTS), the ionic determinants of PC repolarization are incompletely known. To evaluate the role of the slow delayed rectifier current (I(Ks)) in PC repolarization, we studied PCs from canine ventricular false tendons with whole cell patch clamp (37 degrees C). Typical I(Ks) voltage- and time-dependent properties were noted. Isoproterenol enhanced I(Ks) in a concentration-dependent fashion (EC(50) approximately 30 nM), negatively shifted I(Ks) activation voltage dependence, and accelerated I(Ks) activation. Block of I(Ks) with 293B did not alter PC action potential duration (APD) in the absence of isoproterenol; however, in the presence of isoproterenol, 293B significantly prolonged APD. We conclude that, without beta-adrenergic stimulation, I(Ks) contributes little to PC repolarization; however, beta-adrenergic stimulation increases the contribution of I(Ks) by increasing current amplitude, accelerating I(Ks) activation, and shifting activation voltage toward the PC plateau voltage range. I(Ks) may therefore provide an important "braking" function to limit PC APD prolongation in the presence of beta-adrenergic stimulation.

KCNQ1/KCNE1 potassium channels in mammalian vestibular dark cells

The high [K(+)] in the inner ear endolymph is essential for mechanosensory transduction in hearing and balance. Several ion channels, including a slowly activating, voltage-dependent, outwardly conducting K(+) channel composed of the KCNQ1 (KvLQT1) and KCNE1 (IsK/minK) subunits, are expressed at the apical surface of vestibular dark cells. We investigated the underlying molecular mechanisms of this conductance using in situ hybridization, RT-PCR, and immunocytochemistry and by tracking the ultrastructural changes of vestibular structures in kcne1(-/-) mice. In the wild type mice, the KCNE1 and KCNQ1 proteins are expressed specifically at the apical membrane of dark cells, as early as gestational day (GD) 17 for KCNE1 while KCNQ1 mRNAs can be detected at GD 18. This is the first demonstration that the two protein components of this potassium channel co-localize in a polarized fashion at the cellular level. Although the vestibular end-organs are normal at birth in kcne1(-/-) mice, they begin to show modifications during postnatal development: we observed an increase in the height of the dark cells, in their number of mitochondria, and in basolateral membrane infoldings. Subsequently, the epithelium degenerates and the endolymphatic space collapses. Similar changes are known to occur in the cardio-auditory Jervell--Lange-Nielsen syndrome which is caused by mutations in the same channel.

Pharmacogenetics of cardiac K(+) channels

A number of commonly prescribed drugs belonging to various therapeutic classes (antiarrhythmic, antibiotic, antifungal, antihistamine, antipsychotic, prokinetic drugs...) possess, in common, the adverse property to prolong cardiac repolarization [prolonged QT interval duration on surface electrocardiogram (ECG)], exposing patients to a risk of torsade-de-pointes arrhythmias, syncope, and sudden death. Arrhythmias related to drug-induced QT prolongation do not occur in every patient treated with these drugs but most likely occur in a subset of susceptible patients. These patients have a high risk of recurrence of arrhythmias upon exposure to any of the other drugs that broaden the QT interval. It is currently suspected (though not yet proven) that susceptible individuals carry a silent mutation in one of the genes responsible for the congenital long QT syndrome. Indeed, it appears more and more clear that a large proportion of congenital long QT syndrome gene carriers, have a normal QT interval and a normal phenotype and therefore, remain undiagnosed. Therefore, a much larger than previously thought proportion of the general population may be affected by asymptomatic mutations in cardiac ion channel encoding genes. No routine technology is currently available in identifying these patients preventively.

KCNE2 confers background current characteristics to the cardiac KCNQ1 potassium channel

Mutations in HERG and KCNQ1 (or KVLQT1) genes cause the life-threatening Long QT syndrome. These genes encode K(+) channel pore-forming subunits that associate with ancillary subunits from the KCNE family to underlie the two components, I(Kr) and I(Ks), of the human cardiac delayed rectifier current I(K). The KCNE family comprises at least three members. KCNE1 (IsK or MinK) recapitulates I(Ks) when associated with KCNQ1, whereas it augments the amplitude of an I(Kr)-like current when co-expressed with HERG. KCNE3 markedly changes KCNQ1 as well as HERG current properties. So far, KCNE2 (MirP1) has only been shown to modulate HERG current. Here we demonstrate the interaction of KCNE2 with the KCNQ1 subunit, which results in a drastic change of KCNQ1 current amplitude and gating properties. Furthermore, KCNE2 mutations also reveal their specific functional consequences on KCNQ1 currents. KCNQ1 and HERG appear to share unique interactions with KCNE1, 2 and 3 subunits. With the exception of KCNE3, mutations in all these partner subunits have been found to lead to an increased propensity for cardiac arrhythmias.

M-type KCNQ2-KCNQ3 potassium channels are modulated by the KCNE2 subunit

KCNQ2 and KCNQ3 subunits belong to the six transmembrane domain K+ channel family and loss of function mutations are associated with benign familial neonatal convulsions. KCNE2 (MirP1) is a single transmembrane domain subunit first described to be a modulator of the HERG potassium channel in the heart. Here, we show that KCNE2 is present in brain, in areas which also express KCNQ2 and KCNQ3 channels. We demonstrate that KCNE2 associates with KCNQ2 and/or KCNQ3 subunits. In transiently transfected COS cells, KCNE2 expression produces an acceleration of deactivation kinetics of KCNQ2 and of the KCNQ2-KCNQ3 complex. Effects of two previously identified arrhythmogenic mutations of KCNE2 have also been analyzed.

The genetic basis for cardiac dysrhythmias and the long QT syndrome

Cardiac muscle excitation is the result of ion fluxes through cellular membrane channels. Any alterations in channel proteins that produce abnormal ionic fluxes will change the cardiac action potential and the pattern of electrical firing within the heart. The idiopathic long QT syndrome (LQTS) is an inherited cardiac pathology localized to mutated genes encoding for myocardial, voltage-activated sodium and potassium ion channels. The expression of abnormal sodium and potassium channels results in aberrant ionic fluxes that produce a prolonged ventricular repolarization. This prolonged time to repolarization is the electrophysiologic basis for prolongation of the QT interval. Individuals with LQTS are at significant risk for developing lethal ventricular dysrhythmias due to an abnormal pattern of cardiac excitation. Identification of a genetic basis for LQTS has had significant implications for genetic counseling, the development of effective antidysrhythmic drug therapies, and nursing interventions.