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

Biology of the cardiac myocyte in heart disease

Cardiac hypertrophy is a major risk factor for heart failure, and it has been shown that this increase in size occurs at the level of the cardiac myocyte. Cardiac myocyte model systems have been developed to study this process. Here we focus on cell culture tools, including primary cells, immortalized cell lines, human stem cells, and their morphological and molecular responses to pathological stimuli. For each cell type, we discuss commonly used methods for inducing hypertrophy, markers of pathological hypertrophy, advantages for each model, and disadvantages to using a particular cell type over other in vitro model systems. Where applicable, we discuss how each system is used to model human disease and how these models may be applicable to current drug therapeutic strategies. Finally, we discuss the increasing use of biomaterials to mimic healthy and diseased hearts and how these matrices can contribute to in vitro model systems of cardiac cell biology.

Requisite Role of Kv1.5 Channels in Coronary Metabolic Dilation

RATIONALE

In the working heart, coronary blood flow is linked to the production of metabolites, which modulate tone of smooth muscle in a redox-dependent manner. Voltage-gated potassium channels (Kv), which play a role in controlling membrane potential in vascular smooth muscle, have certain members that are redox-sensitive.

OBJECTIVE

To determine the role of redox-sensitive Kv1.5 channels in coronary metabolic flow regulation.

METHODS AND RESULTS

In mice (wild-type [WT], Kv1.5 null [Kv1.5(-/-)], and Kv1.5(-/-) and WT with inducible, smooth muscle-specific expression of Kv1.5 channels), we measured mean arterial pressure, myocardial blood flow, myocardial tissue oxygen tension, and ejection fraction before and after inducing cardiac stress with norepinephrine. Cardiac work was estimated as the product of mean arterial pressure and heart rate. Isolated arteries were studied to establish whether genetic alterations modified vascular reactivity. Despite higher levels of cardiac work in the Kv1.5(-/-) mice (versus WT mice at baseline and all doses of norepinephrine), myocardial blood flow was lower in Kv1.5(-/-) mice than in WT mice. At high levels of cardiac work, tissue oxygen tension dropped significantly along with ejection fraction. Expression of Kv1.5 channels in smooth muscle in the null background rescued this phenotype of impaired metabolic dilation. In isolated vessels from Kv1.5(-/-) mice, relaxation to H2O2 was impaired, but responses to adenosine and acetylcholine were normal compared with those from WT mice.

CONCLUSIONS

Kv1.5 channels in vascular smooth muscle play a critical role in coupling myocardial blood flow to cardiac metabolism. Absence of these channels disassociates metabolism from flow, resulting in cardiac pump dysfunction and tissue hypoxia.

Isolevuglandin adducts in disease

SIGNIFICANCE

A diverse family of lipid-derived levulinaldehydes, isolevuglandins (isoLGs), is produced by rearrangement of endoperoxide intermediates generated through both cyclooxygenase (COX) and free radical-induced cyclooxygenation of polyunsaturated fatty acids and their phospholipid esters. The formation and reactions of isoLGs with other biomolecules has been linked to alcoholic liver disease, Alzheimer's disease, age-related macular degeneration, atherosclerosis, cardiac arythmias, cancer, end-stage renal disease, glaucoma, inflammation of allergies and infection, mitochondrial dysfunction, multiple sclerosis, and thrombosis. This review chronicles progress in understanding the chemistry of isoLGs, detecting their production in vivo and understanding their biological consequences.

CRITICAL ISSUES

IsoLGs have never been isolated from biological sources, because they form adducts with primary amino groups of other biomolecules within seconds. Chemical synthesis enabled investigation of isoLG chemistry and detection of isoLG adducts present in vivo.

RECENT ADVANCES

The first peptide mapping and sequencing of an isoLG-modified protein present in human retina identified the modification of a specific lysyl residue of the sterol C27-hydroxylase Cyp27A1. This residue is preferentially modified by iso[4]LGE2 in vitro, causing loss of function. Adduction of less than one equivalent of isoLG can induce COX-associated oligomerization of the amyloid peptide Aβ1-42. Adduction of isoLGE2 to phosphatidylethanolamines causes gain of function, converting them into proinflammatory isoLGE2-PE agonists that foster monocyte adhesion to endothelial cells.

FUTURE DIRECTIONS

Among the remaining questions on the biochemistry of isoLGs are the dependence of biological activity on isoLG isomer structure, the structures and mechanism of isoLG-derived protein-protein and DNA-protein cross-link formation, and its biological consequences.

Ionic mechanisms of pacemaker activity in spontaneously contracting atrial HL-1 cells

Although normally absent, spontaneous pacemaker activity can develop in human atrium to promote tachyarrhythmias. HL-1 cells are immortalized atrial cardiomyocytes that contract spontaneously in culture, providing a model system of atrial cell automaticity. Using electrophysiologic recordings and selective pharmacologic blockers, we investigated the ionic basis of automaticity in atrial HL-1 cells. Both the sarcoplasmic reticulum Ca release channel inhibitor ryanodine and the sarcoplasmic reticulum Ca ATPase inhibitor thapsigargin slowed automaticity, supporting a role for intracellular Ca release in pacemaker activity. Additional experiments were performed to examine the effects of ionic currents activating in the voltage range of diastolic depolarization. Inhibition of the hyperpolarization-activated pacemaker current, If, by ivabradine significantly suppressed diastolic depolarization, with modest slowing of automaticity. Block of inward Na currents also reduced automaticity, whereas inhibition of T- and L-type Ca currents caused milder effects to slow beat rate. The major outward current in HL-1 cells is the rapidly activating delayed rectifier, IKr. Inhibition of IKr using dofetilide caused marked prolongation of action potential duration and thus spontaneous cycle length. These results demonstrate a mutual role for both intracellular Ca release and sarcolemmal ionic currents in controlling automaticity in atrial HL-1 cells. Given that similar internal and membrane-based mechanisms also play a role in sinoatrial nodal cell pacemaker activity, our findings provide evidence for generalized conservation of pacemaker mechanisms among different types of cardiomyocytes.

Regulation of the Kv2.1 potassium channel by MinK and MiRP1

Kv2.1 is a voltage-gated potassium (Kv) channel alpha-subunit expressed in mammalian heart and brain. MinK-related peptides (MiRPs), encoded by KCNE genes, are single-transmembrane domain ancillary subunits that form complexes with Kv channel alpha-subunits to modify their function. Mutations in human MinK (KCNE1) and MiRP1 (KCNE2) are associated with inherited and acquired forms of long QT syndrome (LQTS). Here, coimmunoprecipitations from rat heart tissue suggested that both MinK and MiRP1 form native cardiac complexes with Kv2.1. In whole-cell voltage-clamp studies of subunits expressed in CHO cells, rat MinK and MiRP1 reduced Kv2.1 current density three- and twofold, respectively; slowed Kv2.1 activation (at +60 mV) two- and threefold, respectively; and slowed Kv2.1 deactivation less than twofold. Human MinK slowed Kv2.1 activation 25%, while human MiRP1 slowed Kv2.1 activation and deactivation twofold. Inherited mutations in human MinK and MiRP1, previously associated with LQTS, were also evaluated. D76N-MinK and S74L-MinK reduced Kv2.1 current density (threefold and 40%, respectively) and slowed deactivation (60% and 80%, respectively). Compared to wild-type human MiRP1-Kv2.1 complexes, channels formed with M54T- or I57T-MiRP1 showed greatly slowed activation (tenfold and fivefold, respectively). The data broaden the potential roles of MinK and MiRP1 in cardiac physiology and support the possibility that inherited mutations in either subunit could contribute to cardiac arrhythmia by multiple mechanisms.

Computationally efficient strategy for modeling the effect of ion current modifiers

Electrophysiological studies often seek to relate changes in ion current properties caused by a chemical modifier to changes in cellular properties. Therefore, quantifying concentration-dependent effects of modifiers on ion currents is a topic of importance. In this paper, we sought a mathematical method for using ion current data to predict the effect of several theoretical ion current modifiers on cellular and tissue properties that is computationally efficient without compromising predictive power. We focused on the K+ current I(K,r) as an example case due to its link to long QT syndrome and arrhythmias, but these methods should be generally applicable to other electrophysiological studies. We compared predictions using a Markov model with mass action binding of the modifiers to specific conformational states of the channel to predictions generated by two simplified models. We investigated scaling I(K,r) conductance, and found that although this method produced predictions that agreed qualitatively with the more complicated model, it did not generate quantitatively consistent predictions for all modifiers tested. Our simulations showed that a more computationally efficient Hodgkin-Huxley model that incorporates the effect of modifiers through functional changes in the current produced quantitatively consistent predictions of concentration-dependent changes in cell and tissue properties for all modifiers tested.

Evaluation of functional and binding assays in cells expressing either recombinant or endogenous hERG channel

INTRODUCTION

The hERG (human ether-a-go-go related gene) potassium channel is required for normal cardiac repolarization, is susceptible to inhibition by a wide variety of compounds, and its blockage can lead to cardiac QT interval prolongation and life threatening arrhythmias. The present report examines the ability of hERG binding and functional assays to identify compounds with potential cardiovascular liabilities at the earliest stages of drug discovery.

METHODS

Competitive binding assays were developed using (3)H-dofetilide and membranes from HEK293EBNA cells stably expressing recombinant hERG (HEK293-hERG) and IMR-32 cells expressing hERG endogenously. hERG functional assays were also developed using membrane potential indicator dye and rubidium efflux. The ability of these assays to identify compounds with potential adverse cardiac effects was examined using drugs with known cardiac effects ranging from those with no known adverse effects to drugs that were withdrawn from the market due to increased risk of sudden death associated with Torsades de Points.

RESULTS

Binding assays using HEK293-hERG membranes and (3)H-dofetilide were robust (Z'=0.69+/-0.015, mean+/-S.E.M.), highly reproducible (test-retest slope=1.04, r(2)=0.98), and correlated well with IC(50) values obtained by patch clamp (slope=0.98, r(2)=0.89). Binding assays using IMR-32 membranes were less sensitive (Z'=0.4+/-0.03, mean+/-S.E.M., false negative rate=0.4) but still correlated well with patch clamp data (slope=1.06, r(2)=0.83). The hERG membrane potential assay could detect potent hERG inhibitors (defined by hERG patch clamp IC(50)<0.1 muM) using HEK293-hERG cells, but were prone to generate false-negative results with less potent inhibitors (false negative rate=0.5). Finally, the rubidium efflux assay gave highly reproducible results (Z'=0.80+/-0.02, mean+/-S.E.M.) that correlated with patch clamp IC(50) values (slope=0.87, r(2)=0.73).

DISCUSSION

The hERG binding and rubidium efflux assays are robust, predictive of patch clamp results, and can be used at the earliest stages of drug discovery.

Dofetilide: first of a new generation of class III agents

Dofetilide (Pfizer, Sandwich, Kent, UK) is a novel, highly specific class III methanesulfonanilide anti-arrhythmic drug. At nanomolar concentrations this agent prolongs both the atrial and ventricular effective refractory periods and action potential duration. Dofetilide's mechanism of action relies on potent blockade of the rapidly activating, inwardly rectifying component of the delayed rectifier potassium current (Ikr), the main current responsible for cardiac repolarisation. Dofetilide does not appear to interact with other cardiac ionic channels, and this explains its minimal effects upon conduction velocity, myocardial contractility and systemic haemodynamics. Dofetilide's mechanism of action makes it suitable for the termination of supraventricular and ventricular tachyarrhythmias. Small scale clinical trials have provided encouraging results, with preliminary data confirming its efficacy in the termination of atrial fibrillation and atrial flutter, and in increasing the electrical threshold for inducible ventricular tachycardia/fibrillation. The results of large scale, randomised, placebo-controlled trials are awaited in order to establish dofetilide's role in clinical practice. Due to its very specific mode of action, dofetilide has very few systemic side-effects. Dofetilide represents a novel and promising new class III agent.

Levuglandins and isolevuglandins: stealthy toxins of oxidative injury

Inspired by a reaction discovered through basic research on the chemistry of the bicyclic peroxide nucleus of the prostaglandin endoperoxide PGH2, we postulated that levulinaldehyde derivatives with prostaglandin side chains, levuglandins (LGs), and structurally isomeric analogues, isolevuglandins (iso[n]LGs), would be generated by nonenzymatic rearrangements of prostanoid and isoprostanoid endoperoxides. Two decades of subsequent studies culminated in our discoveries of the LG and isoLG pathways, branches of the cyclooxygenase and isoprostane pathways, respectively. In cells, PGH2 rearranges nonenzymatically to LGs even in the presence of enzymes that use PGH2 as a substrate. IsoLGs, also known as isoketals or neuroketals, are generated in vivo through free radical-induced autoxidation of polyunsaturated phospholipid esters. Hydrolysis occurs after rapid adduction of isoLG phospholipids to proteins. The proclivity of these reactive species to avidly bind covalently with and cross-link proteins and nucleic acids complicated the hunt for LGs and isoLGs in vivo. The extraordinary reactivity of these "stealthy toxins" underlies much, if not all, of the biological consequences of LG and isoLG generation. They interfere with protein function and are among the most potent neurotoxic products of lipid oxidation known. Because they can accumulate over the lifetimes of proteins, iso[n]LG-protein adducts represent a convenient dosimeter of oxidative stress.

Cardiac IKr Channels Minimally Comprise hERG 1a and 1b Subunits

Previous studies suggest native cardiac IKr channels are composed of alpha subunits encoded solely by the 1a transcript of the ERG1 gene. Using isoform-specific ERG1 antibodies, we have new evidence that subunits encoded by an alternate transcript, ERG1b, are also expressed in rat, canine, and human heart. The ERG1a and -1b subunits associate in vivo where they localize to the T tubules of ventricular myocytes. These data indicate native ventricular IKr channels are heteromers containing two alpha subunit types, ERG1a and -1b. The hERG1b-specific exon thus represents a novel target to screen for mutations causing type 2 long QT syndrome. These findings also suggest phenotypic analyses of existing type 2 long QT syndrome mutations, especially those exclusive to the hERG1a amino terminus, should be carried out in systems expressing both subunits.

Modification of proteins by isoketal-containing oxidized phospholipids

Oxidative stress frequently leads to altered function of membrane proteins. Isoketals are highly reactive products of the isoprostane pathway of free radical-induced lipid peroxidation that rapidly form covalent protein adducts and exhibit a remarkable proclivity to form protein cross links in vitro. Examination of isoketal adducts from an animal model of oxidative injury revealed that initial adducts were formed by isoketals esterified in phospholipids, representing a novel oxidative injury-associated modification of proteins by phospholipids. Maturation of adducts involved cleavage from phospholipids and conversion of adducts to a more stable chemical form that can be detected for extended periods. Because initial adducts were formed by phospholipid-esterified isoketals, the functional consequence of isoketal adduction was examined using a model membrane protein (a cardiac K(+) channel). These studies revealed that isoketal adduction profoundly altered protein function, inhibiting potassium current in a dose-dependent manner. These findings indicate that phospholipid-esterified isoketals rapidly adduct membrane proteins and that such modification can alter protein function, suggesting a generalized cellular mechanism for alteration of membrane function as a consequence of oxidative stress.

Contrasting roles of a novel K+ channel blocker and a K+ channel opener on electro-mechanical activity in canine heart tissue

We tested the effects of a potassium channel opener diazoxide on the action potential duration (APD) and contractile force changes in canine Purkinje tissue induced by a novel class III anti-arrhythmic agent (C3A), KCB-328 (0.5 microM) with 3,4-dimethoxyphenethyl ring structure (0.5 microM). KCB-328 shortened APD(25) by 8.3+/-2.1%, prolonged APD(50) and APD(90) by 31.2+/-5.3 and 50.0+/-7.1%, respectively. Diazoxide (0.1 mM) shortened APD at all levels by 58.3+/-8.1, 54.1+/-6.1, and 42.8+/-5.8%, respectively. In the presence of diazoxide, KCB-328 still prolonged APD(50) and APD(90) (12.5+/-3.8 and 26.8+/-5.9%, respectively). KCB-328 increased force of contraction in a dose-dependent manner. KCB-328 increased force less in the presence of diazoxide. Administration of diazoxide only, reduced force of contraction. We conclude that APD prolongation by KCB-328 may occur even in the presence of diazoxide. It is not sufficient for the restoration of already diminished contractile force and that such an APD prolongation may be unrelated to the restoration of force of contraction even though both are most often seen to occur simultaneously.

Abnormal calcium and protein kinase C-epsilon signaling in hypertrophied atrial tumor myocytes (AT-1 cells)

Cardiac hypertrophy leads to contractile dysfunction and altered hormone responsiveness through incompletely understood mechanisms. Atrial tumor (AT-1) myocytes (AT-1 cells) are a cardiomyocyte lineage that proliferates but hypertrophies when proliferation is prevented with mitomycin C. Because both states maintain a highly differentiated phenotype, AT-1 cells were used to explore the signaling pathways that accompany and/or contribute to hypertrophic cardiomyocyte growth. Mitomycin C-induced AT-1 cell enlargement is associated with a pronounced increase in the amplitude and the duration of both electrically stimulated calcium transients and endothelin receptor-dependent calcium responses. Studies with caffeine indicate that the intracellular pool of releasable calcium is similar in control and hypertrophied AT-1 cells. This agrees with the results of Northern analyses that show similar steady-state levels of transcripts encoding the sarcoplasmic reticulum Ca-ATPase (and higher levels of transcripts encoding the Na+/Ca2+ exchanger) in hypertrophied AT-1 cells, relative to proliferating control cultures. However, immunoblot analyses reveal a marked increase in the expression of protein kinase C (PKC)-epsilon (a critical intermediate in the signaling pathway for endothelin receptor-dependent modulation of intracellular calcium) during AT-1 cell hypertrophy; the abundance of other PKC isoforms is not changed. Collectively, these results identify reciprocal regulation between calcium/PKC signaling and hypertrophic growth. The evidence that AT-1 cell hypertrophy leads to abnormalities in calcium regulation and specific changes in PKC-epsilon expression that alter endothelin receptor responsiveness supports the notion that pathophysiological changes in PKC-epsilon abundance lead to functionally important changes in hormonal modulation of cardiomyocyte function.

AE anion exchangers in atrial tumor cells

Intracellular pH homeostasis and intracellular Cl(-) concentration in cardiac myocytes are regulated by anion exchange mechanisms. In physiological extracellular Cl(-) concentrations, Cl(-)/HCO(3)(-) exchange promotes intracellular acidification and Cl(-) loading sensitive to inhibition by stilbene disulfonates. We investigated the expression of AE anion exchangers in the AT-1 mouse atrial tumor cell line. Cultured AT-1 cells exhibited a substantial basal Na(+)-independent Cl(-)/HCO(3)(-) (but not Cl(-)/OH(-)) exchange activity that was inhibited by DIDS but not by dibenzamidostilbene disulfonic acid (DBDS). AT-1 cell Cl(-)/HCO(3)(-) activity was stimulated two- to threefold by extracellular ATP and ANG II. AE mRNAs detected by RT-PCR in AT-1 cells included brain AE3 (bAE3), cardiac AE3 (cAE3), AE2a, AE2b, AE2c1, AE2c2, and erythroid AE1 (eAE1), but not kidney AE1 (kAE1). Cultured AT-1 cells expressed AE2, cAE3, and bAE3 polypeptides, which were detected by immunoblot and immunocytochemistry. An AE1-like epitope was detected by immunocytochemistry but not by immunoblot. Both bAE3 and cAE3 were present in intact AT-1 tumors. Cultured AT-1 cells provide a useful system for the study of mediators and regulators of Cl(-)/HCO(3)(-) exchange activity in an atrial cell type.

Heregulin-dependent trafficking and cleavage of ErbB-4

Heregulin was shown to promote the proteolytic cleavage of its receptor, ErbB-4, in several cell lines. The growth factor also rapidly promoted the transient translocation of ErbB-4 to a detergent-insoluble fraction, in which the receptor was hyper-tyrosine-phosphorylated compared with the receptor present in the detergent-soluble pool. However, an 80-kDa proteolytic fragment of ErbB-4 was found in the detergent-soluble fraction, but not in the detergent-insoluble fraction. Although the heregulin-induced cleavage of ErbB-4 produced a fragment of ErbB-4 very similar to that induced by 12-O-tetradecanoylphorbol-13-acetate or pervanadate (each of which is blocked by metalloprotease inhibitors), the growth factor-induced cleavage was not sensitive to these inhibitors under the same conditions. The heregulin-induced cleavage of ErbB-4 could be blocked by conditions that prevent clathrin-coated pit formation, suggesting that heregulin-mediated ErbB-4 cleavage occurs subsequent to internalization. When reagents that prevent acidification of endosomes were employed, heregulin-induced ErbB-4 cleavage was sensitive to metalloprotease inhibitors. The results imply that during ligand-dependent receptor trafficking, activated ErbB-4 receptors are subject to proteolytic cleavage involving an intracellular metalloprotease.

Indapamide blocks the rapid component of the delayed rectifier current in atrial tumor cells (AT-1 cells)

We studied the effects of a well known blocker (indapamide) of the slow component (I(ks)) of the delayed rectifier (I(k)) on K(+) currents in atrial tumor myocytes derived from transgenic mice (AT-1 cells) using one electrode voltage clamp method. These cells have been shown to express mRNAs encoding cardiac K(+) channels and display a cardiac electrophysiological phenotype. The major K(+) current is the rapid component (I(kr)) of the delayed rectifier current (I(k)). The purpose of this study was to show that a diuretic agent, indapamide, which was shown to be a selective blocker of the slow component (I(ks)) of delayed rectifier, also blocks I(kr) in a dose dependent manner. The steady state current at the end of a 1s pulse (I(1s), step to +40 mV from a holding potential of -40 mV) was 1070.4+/-202.2 pA (n=5) and the tail current (I(tail)) was 416.3+/-112.9 pA. Indapamide (750 microM) reduced I(1s) and I(tail) to 254.5+/-62.3 pA and 42.2+/-37.7 pA respectively. Indapamide induced block was partially reversible for higher concentrations (> or =750 microM).

Developmental differences in delayed rectifying outward current in feline ventricular myocytes

In the present work, we found that the delayed rectifying outward potassium current (I(K)) in adult and neonatal cat ventricular myocytes consists of both rapid and slow components, I(Kr) and I(Ks), respectively, which can be isolated pharmacologically. Thus after complete blockade of I(Kr) with dofetilide, the remaining I(Ks) current is homogeneous, as shown by an envelope of tails test. I(Kr) maximum tail current density, measured at -40 mV, was similar in adult and neonatal myocytes. I(Ks) maximum tail current density in neonatal myocytes, measured at -40 mV, was significantly smaller than in adult myocytes. Activation kinetics of I(Kr) and I(Ks) was similar in both age groups. However, the I(Kr) deactivation time course was significantly faster in neonatal than in adult myocytes. Developmental differences in the subunit composition of I(Kr) that display distinctly different deactivation kinetics are suggested.

The erg-like potassium current in rat lactotrophs

1. The ether-à-go-go-related gene (erg)-like K+ current in rat lactotrophs from primary culture was characterized and compared with that in clonal rat pituitary cells (GH3/B6). The class III antiarrhythmic E-4031 known to block specifically erg K+ channels was used to isolate the erg-like current as the E-4031-sensitive current. The experiments were performed in 150 mM K+ external solution using the patch-clamp technique. 2. The erg-like K+ current elicited with hyperpolarizing pulses negative to -100 mV consisted of a fast and a pronounced slowly deactivating current component. The contribution of the slow component to the total current amplitude was potential dependent and varied from cell to cell. At -100 mV it ranged from 50 to 85% and at -140 mV from 21 to 45%. 3. The potential-dependent channel availability curves determined with 2 s prepulses were fitted with the sum of two Boltzmann functions. The function related to the slowly deactivating component of the erg-like current was shifted by more than 40 mV to more negative membrane potentials compared with that of the fast component. 4. In contrast to that of native lactotrophs studied under identical conditions, the erg-like K+ current of GH3/B6 cells was characterized by a predominant fast deactivating current component, with similar kinetic and steady-state properties to the fast deactivating current component of native lactotrophs. 5. Thyrotrophin-releasing hormone reduced the erg-like current in native lactotrophs via an intracellular signal cascade which seemed to involve a pathway independent from protein kinase A and protein kinase C. 6. RT-PCR studies on cytoplasm from single lactotrophs revealed the presence of mRNA of the rat homologue of the human ether-à-go-go-related gene HERG (r-erg1) as well as mRNA of the two other cloned r-erg cDNAs (r-erg2 and r-erg3) in different combinations. In GH3/B6 cells, only the transcripts of r-erg1 and r-erg2 were found.

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.

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.

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.

Kir 4.1 channel expression in neuroblastomaxglioma hybrid NG108-15 cell line

To study a possible involvement of inwardly rectifying K+ 4.1 (Kir 4. 1) channels in neural cell development, RT-PCR, immunocytochemistry and whole-cell patch-clamp techniques were used to assess expression of Kir 4.1 channels in proliferating and differentiated NG108-15 cells. RT-PCR revealed co-expression of Kir 4.1 and rat ether-a-go-go-related gene (R-ERG) mRNAs in both proliferating and differentiated cells. The relative Kir 4.1 mRNA concentration increased markedly as cells progressed from undifferentiated to differentiated cells. Kir 4.1-immunoreactivity was barely detectable in undifferentiated cells, but clearly detected in differentiated cells, indicating that Kir 4.1 gene and protein expressions are developmentally regulated. However, corresponding Kir 4.1 current could not be detected in differentiated cells using whole-cell patch-clamp recording. The 'silent' channel/receptor, often found in tumor cells, may carry genetic defects, which prevent functional expression of the channel. NG108-15 may serve as unique model for studying the relationship between the expression of an ion channel gene and the electrophysiological phenotype it encodes.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

A novel benzodiazepine that activates cardiac slow delayed rectifier K+ currents

The slowly activating delayed rectifier K+ current, IKs, is an important modulator of cardiac action potential repolarization. Here, we describe a novel benzodiazepine, [L-364,373 [(3-R)-1, 3-dihydro-5-(2-fluorophenyl)-3-(1H-indol-3-ylmethyl)-1-methyl-2H- 1,4-benzodiazepin-2-one] (R-L3), that activates IKs and shortens action potentials in guinea pig cardiac myocytes. These effects were additive to isoproterenol, indicating that channel activation by R-L3 was independent of beta-adrenergic receptor stimulation. The increase of IKs by R-L3 was stereospecific; the S-enantiomer, S-L3, blocked IKs at all concentrations examined. The increase in IKs by R-L3 was greatest at voltages near the threshold for normal channel activation, caused by a shift in the voltage dependence of IKs activation. R-L3 slowed the rate of IKs deactivation and shifted the half-point of the isochronal (7.5 sec) activation curve for IKs by -16 mV at 0.1 microM and -24 mV at 1 microM. R-L3 had similar effects on cloned KvLQT1 channels expressed in Xenopus laevis oocytes but did not affect channels formed by coassembly of KvLQT1 and hminK subunits. These findings indicate that the association of minK with KvLQT1 interferes with the binding of R-L3 or prevents its action once bound to KvLQT1 subunits.

HERG-like K+ channels in microglia

A voltage-gated K+ conductance resembling that of the human ether-à-go-go-related gene product (HERG) was studied using whole-cell voltage-clamp recording, and found to be the predominant conductance at hyperpolarized potentials in a cell line (MLS-9) derived from primary cultures of rat microglia. Its behavior differed markedly from the classical inward rectifier K+ currents described previously in microglia, but closely resembled HERG currents in cardiac muscle and neuronal tissue. The HERG-like channels opened rapidly on hyperpolarization from 0 mV, and then decayed slowly into an absorbing closed state. The peak K+ conductance-voltage relation was half maximal at -59 mV with a slope factor of 18.6 mV. Availability, assessed by a hyperpolarizing test pulse from different holding potentials, was more steeply voltage dependent, and the midpoint was more positive (-14 vs. -39 mV) when determined by making the holding potential progressively more positive than more negative. The origin of this hysteresis is explored in a companion paper (Pennefather, P.S., W. Zhou, and T.E. DeCoursey. 1998. J. Gen. Physiol. 111:795-805). The pharmacological profile of the current differed from classical inward rectifier but closely resembled HERG. Block by Cs+ or Ba2+ occurred only at millimolar concentrations, La3+ blocked with Ki = approximately 40 microM, and the HERG-selective blocker, E-4031, blocked with Ki = 37 nM. Implications of the presence of HERG-like K+ channels for the ontogeny of microglia are discussed.

Blockade of HERG channels by the class III antiarrhythmic azimilide: mode of action

1. The class III antiarrhythmic azimilide has previously been shown to inhibit I(Ks) and I(Kr) in guinea-pig cardiac myocytes and I(Ks) (minK) channels expressed in Xenopus oocytes. Because HERG channels underly the conductance I(Kr), in human heart, the effects of azimilide on HERG channels expressed in Xenopus oocytes were the focus of the present study. 2. In contrast to other well characterized HERG channel blockers, azimilide blockade was reverse use-dependent, i.e., the relative block and apparent affinity of azimilide decreased with an increase in channel activation frequency. Azimilide blocked HERG channels at 0.1 and 1 Hz with IC50s of 1.4 microM and 5.2 microM respectively. 3. In an envelope of tail test, HERG channel blockade increased with increasing channel activation, indicating binding of azimilide to open channels. 4. Azimilide blockade of HERG channels expressed in Xenopus oocytes and I(Kr) in mouse AT-1 cells was decreased under conditions of high [K+]e, whereas block of slowly activating I(Ks) channels was not affected by changes in [K+]e. 5. In summary, azimilide is a blocker of cardiac delayed rectifier channels, I(Ks) and HERG. Because of the distinct effects of stimulation frequency and [K+]e on azimilide block of I(Kr) and I(Ks) channels, we conclude that the relative contribution of block of each of these cardiac delayed rectifier channels depends on heart frequency. [K+]e and regulatory status of the respective channels.

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

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

Constitutive proteolysis of the ErbB-4 receptor tyrosine kinase by a unique, sequential mechanism

The heregulin receptor tyrosine kinase ErbB-4 is constitutively cleaved, in the presence or absence of ligand, by an exofacial proteolytic activity producing a membrane-anchored cytoplasmic domain fragment of 80 kD. Based on selective sensitivity to inhibitors, the proteolytic activity is identified as that of a metalloprotease. The 80-kD product is tyrosine phosphorylated and retains tyrosine kinase activity. Importantly, the levels of this fragment are controlled by proteasome function. When proteasome activity is inhibited for 6 h, the kinase-active 80-kD ErbB-4 fragment accumulates to a level equivalent to 60% of the initial amount of native ErbB-4 (approximately 10(6) receptors per cell). Hence, proteasome activity is essential to prevent the accumulation of a significant level of ligand-independent, active ErbB-4 tyrosine kinase generated by metalloprotease activity. Proteasome activity, however, does not act on the native ErbB-4 receptor before the metalloprotease-mediated cleavage, as no ErbB-4 fragments accumulate when metalloprotease activity is blocked. Although no ubiquitination of the native ErbB-4 is detected, the 80-kD fragment is polyubiquitinated. The data, therefore, describe a unique pathway for the processing of growth factor receptors, which involves the sequential function of an exofacial metalloprotease and the cytoplasmic proteasome.

Modulation of the Electrophysiologic Actions of E-4031 and Dofetilide by Hyperkalemia and Acidosis in Rabbit Ventricular Myocytes

BACKGROUND: E-4031 and dofetilide are new class III antiarrhythmic agents that inhibit the rapid component of the delayed rectifier potassium channel (I(Kr)); however, the effectiveness of many antiarrhythmic drugs in ischemic conditions is uncertain. METHODS AND RESULTS: We modeled two components of ischemia, hyperkalemia (9.6 mM) and acidosis (pH 6.8), in voltage-clamped single rabbit ventricular myocytes to help determine the effect of ischemia on the action of these two drugs. In physiologic solution both E-4031 and dofetilide blocked I(Kr) and significantly reduced total outward current. In hyperkalemic solution, both E-4031 and dofetilide showed significantly reduced blockade of I(Kr), while in acidotic solution dofetilide showed significantly reduced blockade of I(Kr) and E-4031 showed a trend to reduced blockade. Neither drug significantly reduced total outward current in hyperkalemic or acidotic solutions. CONCLUSIONS: In these conditions, E-4031 and dofetilide demonstrate reduced blockade of I(Kr), resulting in loss of class III effect. Furthermore, the complete loss of blocking effect on total outward current during simulated ischemia suggests increases of other repolarizing currents also contribute to loss of class III effect.

A HERG-like K+ channel in rat F-11 DRG cell line: pharmacological identification and biophysical characterization

1. The relationships between the K+ inward rectifier current present in neuroblastoma cells (IIR) and the current encoded by the human ether-á-go-go-related gene (HERG), IHERG, and the rapidly activating repolarizing cardiac current IK(r), were investigated in a rat dorsal root ganglion (DRG) x mouse neuroblastoma hybrid cell line (F-11) using pharmacological and biophysical treatments. 2. IIR shared the pharmacological features described for IK(r), including the sensitivity to the antiarrhythmic drugs E4301 and WAY-123,398, whilst responding to Cs+, Ba2+ and La3+ in a similar way to IHERG. 3. The voltage-dependent gating properties of IIR were similar to those of IK(r) and IHERG, although IIR outward currents were negligible in comparison. 4. In high K+ extracellular solutions devoid of divalent cations, IIR deactivation kinetics were removed resulting in long-lasting currents apparently activated in hyperpolarization, with a marked (2.7-fold) increase in conductance, as recorded from the instantaneous linear current-voltage relationship at -120 mV. Re-addition of Ca2+ restored the original closure of the channel whereas re-addition of Mg2+ reduced the peak current. 5. The IIR described here, the heart IK(r) and the IHERG could be successfully predicted by a unique kinetic model where the voltage dependencies of the activation/inactivation gates were properly voltage shifted. On the whole, IIR seems to be the first example of a HERG-type current constitutively expressed and operating in mammalian cells of the neuronal lineage.

Recent advances in understanding the molecular mechanisms of the long QT syndrome

Competing theories to explain the congenital long QT syndrome have included an imbalance in sympathetic innervation of the heart or a defect in repolarizing ion currents. Recent studies have identified at least four chromosomal loci at which mutations cause the congenital long QT syndrome in different families. The specific genes mutated in affected individuals have been identified at two of these loci, and both encode cardiac ion channels. The affected genes are SCN5A, the cardiac sodium channel gene, and HERG, whose protein product likely underlies IKr, the rapidly activating delayed rectifier. Thus, currently available evidence indicates that the congenital long QT syndrome is a primary disease of cardiac ion channels. Abnormalities in either inward or outward currents can cause the disease. Ongoing studies are evaluating the function of the mutant ion channels and the relationship between individual mutations and the clinical manifestations of the syndrome.

A quantitative description of the E-4031-sensitive repolarization current in rabbit ventricular myocytes

We have measured the E-4031-sensitive repolarization current (IKr) in single ventricular myocytes isolated from rabbit hearts. The primary goal of this analysis was a description of the IKr kinetic and ion transfer properties. Surprisingly, the maximum time constant of this component was 0.8 s at 33-34 degrees C, which is significantly greater than the value of 0.18 s previously reported under similar conditions in the original measurements of IKr from guinea pig ventricular myocytes. The primary, novel feature of our analysis concerns the relationship of the bell-shaped curve that describes the voltage dependence of the kinetics and the sigmoidal curve that describes the activation of IKr. The midpoint of the latter occurred at approximately +10 mV on the voltage axis, as compared to -30 mV for the point on the voltage axis at which the maximum time constant occurred. Moreover, the voltage dependence of the kinetics was much broader than the steepness of the activation curve would predict. Taken together, these results comprise a gating current paradox that is not resolved by the incorporation of a fast inactivated state in the analysis. The fully activated current-voltage relation for IKr exhibited strong inward-going rectification, so much so that the current was essentially nil at +30 mV, even though the channel opens rapidly in this voltage range. This result is consistent with the lack of effect of E-4031 on the early part of the plateau phase of the action potential. Surprisingly, the reversal potential Of /Kr was ~15 mV positive to the potassium ion equilibrium potential,which indicates that this channel carries inward current during the latter part of the repolarization phase of the action potential.

A mechanistic link between an inherited and an acquired cardiac arrhythmia: HERG encodes the IKr potassium channel

Mutations in HERG cause an inherited cardiac arrhythmia, long QT syndrome (LQT). To define the function of HERG, we expressed the protein in Xenopus oocytes. The biophysical properties of expressed HERG are nearly identical to the rapidly activating delayed rectifier K+ current (IKr) in cardiac myocytes. HERG current is K+ selective, declines with depolarizations above 0 mV, is activated by extracellular K+, and is blocked by lanthanum. Interestingly, HERG current is not blocked by drugs that specifically block IKr in cardiac myocytes. These data indicate that HERG proteins form IKr channels, but that an additional subunit may be required for drug sensitivity. Since block of IKr is a known mechanism for drug-induced cardiac arrhythmias, the finding that HERG encodes IKr channels provides a mechanistic link between certain forms of inherited and acquired LQT.