Dynamics of the inward rectifier K+ current during the action potential of guinea pig ventricular myocytes

In silico models of the macromolecular NaV1.5-KIR2.1 complex

In cardiac cells, the expression of the cardiac voltage-gated Na+ channel (NaV1.5) is reciprocally regulated with the inward rectifying K+ channel (KIR2.1). These channels can form macromolecular complexes that pre-assemble early during forward trafficking (transport to the cell membrane). In this study, we present in silico 3D models of NaV1.5-KIR2.1, generated by rigid-body protein-protein docking programs and deep learning-based AlphaFold-Multimer software. Modeling revealed that the two channels could physically interact with each other along the entire transmembrane region. Structural mapping of disease-associated mutations revealed a hotspot at this interface with several trafficking-deficient variants in close proximity. Thus, examining the role of disease-causing variants is important not only in isolated channels but also in the context of macromolecular complexes. These findings may contribute to a better understanding of the life-threatening cardiovascular diseases underlying KIR2.1 and NaV1.5 malfunctions.

Hypochlorite-Modified LDL Induces Arrhythmia and Contractile Dysfunction in Cardiomyocytes

Neutrophil-derived myeloperoxidase (MPO) and its potent oxidant, hypochlorous acid (HOCl), gained attention as important oxidative mediators in cardiac damage and dysfunction. As cardiomyocytes generate low-density lipoprotein (LDL)-like particles, we aimed to identify the footprints of proatherogenic HOCl-LDL, which adversely affects cellular signalling cascades in various cell types, in the human infarcted myocardium. We performed immunohistochemistry for MPO and HOCl-LDL in human myocardial tissue, investigated the impact of HOCl-LDL on electrophysiology and contractility in primary cardiomyocytes, and explored underlying mechanisms in HL-1 cardiomyocytes and human atrial appendages using immunoblot analysis, qPCR, and silencing experiments. HOCl-LDL reduced ICa,L and IK1, and increased INaL, leading to altered action potential characteristics and arrhythmic events including early- and delayed-afterdepolarizations. HOCl-LDL altered the expression and function of CaV1.2, RyR2, NCX1, and SERCA2a, resulting in impaired contractility and Ca2+ homeostasis. Elevated superoxide anion levels and oxidation of CaMKII were mediated via LOX-1 signaling in HL-1 cardiomyocytes. Furthermore, HOCl-LDL-mediated alterations of cardiac contractility and electrophysiology, including arrhythmic events, were ameliorated by the CaMKII inhibitor KN93 and the INaL blocker, ranolazine. This study provides an explanatory framework for the detrimental effects of HOCl-LDL compared to native LDL and cardiac remodeling in patients with high MPO levels during the progression of cardiovascular disease.

Canine Myocytes Represent a Good Model for Human Ventricular Cells Regarding Their Electrophysiological Properties

Due to the limited availability of healthy human ventricular tissues, the most suitable animal model has to be applied for electrophysiological and pharmacological studies. This can be best identified by studying the properties of ion currents shaping the action potential in the frequently used laboratory animals, such as dogs, rabbits, guinea pigs, or rats, and comparing them to those of human cardiomyocytes. The authors of this article with the experience of three decades of electrophysiological studies, performed in mammalian and human ventricular tissues and isolated cardiomyocytes, summarize their results obtained regarding the major canine and human cardiac ion currents. Accordingly, L-type Ca²⁺ current (I_Ca), late Na⁺ current (I_Na-late), rapid and slow components of the delayed rectifier K⁺ current (I_Kr and I_Ks, respectively), inward rectifier K⁺ current (I_K1), transient outward K⁺ current (I_to1), and Na⁺/Ca²⁺ exchange current (I_NCX) were characterized and compared. Importantly, many of these measurements were performed using the action potential voltage clamp technique allowing for visualization of the actual current profiles flowing during the ventricular action potential. Densities and shapes of these ion currents, as well as the action potential configuration, were similar in human and canine ventricular cells, except for the density of I_K1 and the recovery kinetics of I_to. I_K1 displayed a largely four-fold larger density in canine than human myocytes, and I_to recovery from inactivation displayed a somewhat different time course in the two species. On the basis of these results, it is concluded that canine ventricular cells represent a reasonably good model for human myocytes for electrophysiological studies, however, it must be borne in mind that due to their stronger I_K1, the repolarization reserve is more pronounced in canine cells, and moderate differences in the frequency-dependent repolarization patterns can also be anticipated.

Electrophysiology of hiPSC-Cardiomyocytes Co-Cultured with HEK Cells Expressing the Inward Rectifier Channel

The immature electrophysiology of human-induced pluripotent stem cell-derived cardiomyocytes (hiCMs) complicates their use for therapeutic and pharmacological purposes. An insufficient inward rectifying current (I_K1) and the presence of a funny current (if) cause spontaneous electrical activity. This study tests the hypothesis that the co-culturing of hiCMs with a human embryonic kidney (HEK) cell-line expressing the Kir2.1 channel (HEK-I_K1) can generate an electrical syncytium with an adult-like cardiac electrophysiology. The mechanical activity of co-cultures using different HEK-I_K1:hiCM ratios was compared with co-cultures using wildtype (HEK–WT:hiCM) or hiCM alone on days 3–8 after plating. Only ratios of 1:3 and 1:1 showed a significant reduction in spontaneous rate at days 4 and 6, suggesting that I_K1 was influencing the electrophysiology. Detailed analysis at day 4 revealed an increased incidence of quiescent wells or sub-areas. Electrical activity showed a decreased action potential duration (APD) at 20% and 50%, but not at 90%, alongside a reduced amplitude of the aggregate AP signal. A computational model of the 1:1 co-culture replicates the electrophysiological effects of HEK–WT. The addition of the I_K1 conductance reduced the spontaneous rate and APD20, 50 and 90, and minor variation in the intercellular conductance caused quiescence. In conclusion, a 1:1 co-culture HEK-I_K1:hiCM caused changes in electrophysiology and spontaneous activity consistent with the integration of I_K1 into the electrical syncytium. However, the additional electrical effects of the HEK cell at 1:1 increased the possibility of electrical quiescence before sufficient I_K1 was integrated into the syncytium.

Ion current profiles in canine ventricular myocytes obtained by the "onion peeling" technique

The profiles of ion currents during the cardiac action potential can be visualized by the action potential voltage clamp technique. To obtain multiple ion current data from the same cell, the "onion peeling" technique, based on sequential pharmacological dissection of ion currents, has to be applied. Combination of the two methods allows recording of several ion current profiles from the same myocyte under largely physiological conditions. Using this approach, we have studied the densities and integrals of the major cardiac inward (I_Ca, I_NCX, I_Na-late) and outward (I_Kr, I_Ks, I_K1) currents in canine ventricular cells and studied the correlation between them. For this purpose, canine ventricular cardiomyocytes were chosen because their electrophysiological properties are similar to those of human ones. Significant positive correlation was observed between the density and integral of I_Ca and I_Kr, and positive correlation was found also between the integral of I_Ca and I_NCX. No further correlations were detected. The Ca²⁺-sensitivity of K⁺ currents was studied by comparing their parameters in the case of normal calcium homeostasis and following blockade of I_Ca. Out of the three K⁺ currents studied, only I_Ks was Ca²⁺-sensitive. The density and integral of I_Ks was significantly greater, while its time-to-peak value was shorter at normal Ca²⁺ cycling than following I_Ca blockade. No differences were detected for I_Kr or I_K1 in this regard. Present results indicate that the positive correlation between I_Ca and I_Kr prominently contribute to the balance between inward and outward fluxes during the action potential plateau in canine myocytes. The results also suggest that the profiles of cardiac ion currents have to be studied under physiological conditions, since their behavior may strongly be influenced by the intracellular Ca²⁺ homeostasis and the applied membrane potential protocol.

Silica Nanoparticles Disturb Ion Channels and Transmembrane Potentials of Cardiomyocytes and Induce Lethal Arrhythmias in Mice

Background

The toxicity of silica nanoparticles (SiNPs) on cardiac electrophysiology has seldom been evaluated.

Methods

Patch-clamp was used to investigate the acute effects of SiNP-100 (100 nm) and SiNP-20 (20 nm) on the transmembrane potentials (TMPs) and ion channels in cultured neonatal mouse ventricular myocytes. Calcium mobilization in vitro, cardiomyocyte ROS generation, and LDH leakage after exposure to SiNPs in vitro and in vivo were measured using a microplate reader. Surface electrocardiograms were recorded in adult mice to evaluate the arrhythmogenic effects of SiNPs in vivo. SiNP endocytosis was observed using transmission electron microscopy.

Results

Within 30 min, both SiNPs (10^-8-10^-6 g/mL) did not affect the resting potential and I_K1 channels. SiNP-100 increased the action potential amplitude (APA) and the I_Na current density, but SiNP-20 decreased APA and I_Na density. SiNP-100 prolonged the action potential duration (APD) and decreased the I_to current density, while SiNP-20 prolonged or shortened the APD, depending on exposure concentrations and increased I_to density. Both SiNPs (10^-6 g/mL) induced calcium mobilization but did not increase ROS and LDH levels and were not endocytosed within 10 min in cardiomyocytes in vitro. In vivo, SiNP-100 (4-10 mg/kg) and SiNP-20 (4-30 mg/kg) did not elevate myocardial ROS but increased LDH levels depending on dose and exposure time. The same higher dose of SiNPs (intravenously injected) induced tachyarrhythmias and lethal bradyarrhythmias within 90 min in adult mice.

Conclusion

SiNPs (i) exert rapid toxic effects on the TMPs of cardiomyocytes in vitro largely owing to their direct interfering effects on the I_Na and I_to channels and Ca²⁺ homeostasis but not I_K1 channels and ROS levels, and (ii) induce tachyarrhythmias and lethal bradyarrhythmias in vivo. SiNP-100 is more toxic than SiNP-20 on cardiac electrophysiology, and the toxicity mechanism is likely more complicated in vivo.

Action potential clamp characterization of the S631A hERG mutation associated with short QT syndrome

The hERG potassium channel is critical to normal repolarization of cardiac action potentials (APs) and loss- and gain-of-function hERG mutations are associated, respectively, with long and short QT syndromes, pathological conditions that can lead to arrhythmias and sudden death. hERG current (IhERG ) exhibits uniquely fast inactivation involving conformational changes to the channel pore. The S631A hERG pore mutation was originally engineered to interrogate hERG channel inactivation, but has very recently been found in a family with short QT syndrome (SQTS). Accordingly, this study characterized the effects of the S631A mutation on IhERG profile during ventricular, atrial, and Purkinje fiber (PF) AP waveforms, using patch clamp recording from hERG expressing HEK 293 cells at 37°C. Under conventional voltage clamp, the current-voltage (I-V) relation for IhERG exhibited a marked right-ward shift in the region of negative slope at positive membrane potentials. Under ventricular AP clamp, the S631A mutation resulted in augmented IhERG , which also peaked much earlier during the AP plateau than did wild-type (WT) IhERG . Instantaneous I-V relations showed a marked positive shift in peak repolarizing current during the ventricular AP in the S631A setting, while the instantaneous conductance-voltage relation showed an earlier and more sustained rise in S631A compared to WT IhERG conductance during ventricular repolarization. Experiments with atrial and PF APs in each case also showed augmented and positively shifted IhERG in the S631A setting, indicating that the S631A mutation is likely to accelerate repolarization in all three cardiac regions. Ventricular AP clamp experiments showed retained effectiveness of the class Ia antiarrhythmic drug quinidine (1 μmol/L) against S631A IhERG . Quinidine is thus likely to be effective in reducing excessively fast repolarization in SQTS resulting from the S631A hERG mutation.

A Singular Role of IK1 Promoting the Development of Cardiac Automaticity during Cardiomyocyte Differentiation by IK1 -Induced Activation of Pacemaker Current

The inward rectifier potassium current (I_K1) is generally thought to suppress cardiac automaticity by hyperpolarizing membrane potential (MP). We recently observed that I_K1 could promote the spontaneously-firing automaticity induced by upregulation of pacemaker funny current (I_f) in adult ventricular cardiomyocytes (CMs). However, the intriguing ability of I_K1 to activate I_f and thereby promote automaticity has not been explored. In this study, we combined mathematical and experimental assays and found that only I_K1 and I_f, at a proper-ratio of densities, were sufficient to generate rhythmic MP-oscillations even in unexcitable cells (i.e. HEK293T cells and undifferentiated mouse embryonic stem cells [ESCs]). We termed this effect I_K1-induced I_f activation. Consistent with previous findings, our electrophysiological recordings observed that around 50% of mouse (m) and human (h) ESC-differentiated CMs could spontaneously fire action potentials (APs). We found that spontaneously-firing ESC-CMs displayed more hyperpolarized maximum diastolic potential and more outward I_K1 current than quiescent-yet-excitable m/hESC-CMs. Rather than classical depolarization pacing, quiescent mESC-CMs were able to fire APs spontaneously with an electrode-injected small outward-current that hyperpolarizes MP. The automaticity to spontaneously fire APs was also promoted in quiescent hESC-CMs by an I_K1-specific agonist zacopride. In addition, we found that the number of spontaneously-firing m/hESC-CMs was significantly decreased when I_f was acutely upregulated by Ad-CGI-HCN infection. Our study reveals a novel role of I_K1 promoting the development of cardiac automaticity in m/hESC-CMs through a mechanism of I_K1-induced I_f activation and demonstrates a synergistic interaction between I_K1 and I_f that regulates cardiac automaticity.

Soluble epoxide hydrolase inhibitors might prevent ischemic arrhythmias via microRNA-1 repression in primary neonatal mouse ventricular myocytes

Ischemic arrhythmias are the main causes of sudden cardiac death.

Electrophysiologic effects of the IK1 inhibitor PA-6 are modulated by extracellular potassium in isolated guinea pig hearts

The pentamidine analog PA-6 was developed as a specific inward rectifier potassium current (IK1) antagonist, because established inhibitors either lack specificity or have side effects that prohibit their use in vivo. We previously demonstrated that BaCl2, an established IK1 inhibitor, could prolong action potential duration (APD) and increase cardiac conduction velocity (CV). However, few studies have addressed whether targeted IK1 inhibition similarly affects ventricular electrophysiology. The aim of this study was to determine the effects of PA-6 on cardiac repolarization and conduction in Langendorff-perfused guinea pig hearts. PA-6 (200 nm) or vehicle was perfused into ex-vivo guinea pig hearts for 60 min. Hearts were optically mapped with di-4-ANEPPS to quantify CV and APD at 90% repolarization (APD90). Ventricular APD90 was significantly prolonged in hearts treated with PA-6 (115 ± 2% of baseline; P < 0.05), but not vehicle (105 ± 2% of baseline). PA-6 slightly, but significantly, increased transverse CV by 7%. PA-6 significantly prolonged APD90 during hypokalemia (2 mmol/L [K+]o), although to a lesser degree than observed at 4.56 mmol/L [K+]o In contrast, the effect of PA-6 on CV was more pronounced during hypokalemia, where transverse CV with PA-6 (24 ± 2 cm/sec) was significantly faster than with vehicle (13 ± 3 cm/sec, P < 0.05). These results show that under normokalemic conditions, PA-6 significantly prolonged APD90, whereas its effect on CV was modest. During hypokalemia, PA-6 prolonged APD90 to a lesser degree, but profoundly increased CV Thus, in intact guinea pig hearts, the electrophysiologic effects of the IK1 inhibitor, PA-6, are [K+]o-dependent.

Pharmacological inhibition of IK1 by PA-6 in isolated rat hearts affects ventricular repolarization and refractoriness

The inwardly rectifying potassium current (I_K1) conducted through K_ir2.X channels contribute to repolarization of the cardiac action potential and to stabilization of the resting membrane potential in cardiomyocytes. Our aim was to investigate the effect of the recently discovered I_K1 inhibitor PA‐6 on action potential repolarization and refractoriness in isolated rat hearts. Transiently transfected HEK‐293 cells expressing I_K1 were voltage‐clamped with ramp protocols. Langendorff‐perfused heart experiments were performed on male Sprague–Dawley rats, effective refractory period, Wenckebach cycle length, and ventricular effective refractory period were determined following 200 nmol/L PA‐6 perfusion. 200 nmol/L PA‐6 resulted in a significant time‐latency in drug effect on the I_K1 current expressed in HEK‐293 cells, giving rise to a maximal effect at 20 min. In the Langendorff‐perfused heart experiments, PA‐6 prolonged the ventricular action potential duration at 90% repolarization (from 41.8 ± 6.5 msec to 72.6 ± 21.1 msec, 74% compared to baseline, P < 0.01, n = 6). In parallel, PA‐6 significantly prolonged the ventricular effective refractory period compared to baseline (from 34.8 ± 4.6 msec to 58.1 ± 14.7 msec, 67%, P < 0.01, n = 6). PA‐6 increased the short‐term beat‐to‐beat variability and ventricular fibrillation was observed in two of six hearts. Neither atrial ERP nor duration of atrial fibrillation was altered following PA‐6 application. The results show that pharmacological inhibition of cardiac I_K1 affects ventricular action potential repolarization and refractoriness and increases the risk of ventricular arrhythmia in isolated rat hearts.

Engineering biosynthetic excitable tissues from unexcitable cells for electrophysiological and cell therapy studies

Patch-clamp recordings in single-cell expression systems have been traditionally used to study the function of ion channels. However, this experimental setting does not enable assessment of tissue-level function such as action potential (AP) conduction. Here we introduce a biosynthetic system that permits studies of both channel activity in single cells and electrical conduction in multicellular networks. We convert unexcitable somatic cells into an autonomous source of electrically excitable and conducting cells by stably expressing only three membrane channels. The specific roles that these expressed channels have on AP shape and conduction are revealed by different pharmacological and pacing protocols. Furthermore, we demonstrate that biosynthetic excitable cells and tissues can repair large conduction defects within primary 2- and 3-dimensional cardiac cell cultures. This approach enables novel studies of ion channel function in a reproducible tissue-level setting and may stimulate the development of new cell-based therapies for excitable tissue repair.

Decreased connexin43 expression in the mouse heart potentiates pacing-induced remodeling of repolarizing currents

Gap junction redistribution and reduced expression, a phenomenon termed gap junction remodeling (GJR), is often seen in diseased hearts and may predispose toward arrhythmias. We have recently shown that short-term pacing in the mouse is associated with changes in connexin43 (Cx43) expression and localization but not with increased inducibility into sustained arrhythmias. We hypothesized that short-term pacing, if imposed on murine hearts with decreased Cx43 abundance, could serve as a model for evaluating the electrophysiological effects of GJR. We paced wild-type (normal Cx43 abundance) and heterozygous Cx43 knockout (Cx43^+/−; 66% mean reduction in Cx43) mice for 6 h at 10–15% above their average sinus rate. We investigated the electrophysiological effects of pacing on the whole animal using programmed electrical stimulation and in isolated ventricular myocytes with patch-clamp studies. Cx43^+/− myocytes had significantly shorter action potential durations (APD) and increased steady-state (I_ss) and inward rectifier (I_K1) potassium currents compared with those of wild-type littermate cells. In Cx43^+/− hearts, pacing resulted in a significant prolongation of ventricular effective refractory period and APD and significant diminution of I_ss compared with unpaced Cx43^+/− hearts. However, these changes were not seen in paced wild-type mice. These data suggest that Cx43 abundance plays a critical role in regulating currents involved in myocardial repolarization and their response to pacing. Our study may aid in understanding how dyssynchronous activation of diseased, Cx43-deficient myocardial tissue can lead to electrophysiological changes, which may contribute to the worsened prognosis often associated with pacing in the failing heart.

Electrical remodeling contributes to complex tachyarrhythmias in connexin43-deficient mouse hearts

Loss of connexin43 (Cx43) gap junction channels in the heart results in a marked increase in the incidence of spontaneous and inducible polymorphic ventricular tachyarrhythmias (PVTs). The mechanisms resulting in this phenotype remain unclear. We hypothesized that uncoupling promotes regional ion channel remodeling, thereby increasing electrical heterogeneity and facilitating the development of PVT. In isolated-perfused control hearts, programmed electrical stimulation elicited infrequent monomorphic ventricular tachyarrhythmias (MVT), and dominant frequencies (DFs) during MVT were similar in the right ventricle (RV) and left ventricle (LV). Moreover, conduction properties, action potential durations (APDs), and repolarizing current densities were similar in RV and LV myocytes. In contrast, PVT was common in Cx43 conditional knockout (OCKO) hearts, and arrhythmias were characterized by significantly higher DFs in the RV compared to the LV. APDs in OCKO myocytes were significantly shorter than those from chamber-matched controls, with RV OCKO myocytes being most affected. APD shortening was associated with higher levels of sustained current in myocytes from both chambers as well as higher levels of the inward rectifier current only in RV myocytes. Thus, alterations in cell-cell coupling lead to regional changes in potassium current expression, which in this case facilitates the development of reentrant arrhythmias. We propose a new mechanistic link between electrical uncoupling and ion channel remodeling. These findings may be relevant not only in cardiac tissue but also to other organ systems where gap junction remodeling is known to occur.

Action potential clamp fingerprints of K+ currents in canine cardiomyocytes: their role in ventricular repolarization

AIM

The aim of the present study was to give a parametric description of the most important K(+) currents flowing during canine ventricular action potential.

METHODS

Inward rectifier K(+) current (I(K1)), rapid delayed rectifier K(+) current (I(Kr)), and transient outward K(+) current (I(to)) were dissected under action potential clamp conditions using BaCl(2), E-4031, and 4-aminopyridine, respectively.

RESULTS

The maximum amplitude of I(to) was 3.0 +/- 0.23 pA/pF and its integral was 29.7 +/- 2.5 fC/pF. The current peaked 4.4 +/- 0.7 ms after the action potential upstroke and rapidly decayed to zero with a time constant of 7.4 +/- 0.6 ms. I(Kr) gradually increased during the plateau, peaked 7 ms before the time of maximum rate of repolarization (V(max)(-)) at -54.2 +/- 1.7 mV, had peak amplitude of 0.62 +/- 0.08 pA/pF, and integral of 57.6 +/- 6.7 fC/pF. I(K1) began to rise from -22.4 +/- 0.8 mV, peaked 1 ms after the time of V(max)(-) at -58.3 +/- 0.6 mV, had peak amplitude of 1.8 +/- 0.1 pA/pF, and integral of 61.6 +/- 6.2 fC/pF. Good correlation was observed between peak I(K1) and V(max)(-) (r = 0.93) but none between I(Kr) and V(max)(-). Neither I(K1) nor I(Kr) was frequency-dependent between 0.2 and 1.66 Hz. Congruently, I(Kr) failed to accumulate in canine myocytes at fast driving rates.

CONCLUSION

Terminal repolarization is dominated by I(K1), but action potential duration is influenced by several ion currents simultaneously. As I(to) was not active during the plateau, and neither I(K1) nor I(Kr) was frequency-dependent, other currents must be responsible for the frequency dependence of action potential duration at normal and slow heart rates in canine ventricular cells.

Calcium and arrhythmogenesis

Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.

Inward-rectifier K+ current in guinea-pig ventricular myocytes exposed to hyperosmotic solutions

Superfusion of heart cells with hyperosmotic solution causes cell shrinkage and inhibition of membrane ionic currents, including delayed-rectifer K+ currents. To determine whether osmotic shrinkage also inhibits inwardly-rectifying K+ current (I(K1)), guinea-pig ventricular myocytes in the perforated-patch or ruptured-patch configuration were superfused with a Tyrode's solution whose osmolarity (T) relative to isosmotic (1T) solution was increased to 1.3-2.2T by addition of sucrose. Hyperosmotic superfusate caused a rapid shrinkage that was accompanied by a negative shift in the reversal potential of Ba(2+)-sensitive I(K1), an increase in the amplitude of outward I(K1), and a steepening of the slope of the inward I(K1)-voltage (V) relation. The magnitude of these effects increased with external osmolarity. To evaluate the underlying changes in chord conductance (G(K1)) and rectification, G(K1)-V data were fitted with Boltzmann functions to determine maximal G(K1) (G(K1)max) and voltage at one-half G(K1)max (V(0.5)). Superfusion with hyperosmotic sucrose solutions led to significant increases in G(K1)max (e.g., 28 +/- 2% with 1.8T), and significant negative shifts in V(0.5) (e.g., -6.7 +/- 0.6 mV with 1.8T). Data from myocytes investigated under hyperosmotic conditions that do not induce shrinkage indicate that G(K1)max and V(0.5) were insensitive to hyperosmotic stress per se but sensitive to elevation of intracellular K+. We conclude that the effects of hyperosmotic sucrose solutions on I(K1) are related to shrinkage-induced concentrating of intracellular K+.

The inward rectifier current (IK1) controls cardiac excitability and is involved in arrhythmogenesis

The cardiac inwardly rectifying potassium current (I(K1)) stabilizes the resting membrane potential and is responsible for shaping the initial depolarization and final repolarization of the action potential. The inwardly rectifying potassium channel (Kir2.x) subfamily members primarily mediate cardiac I(K1), but other inward rectifiers, including the acetylcholine-sensitive (Kir3.x) and ATP-sensitive (Kir6.x) inward rectifiers, also may modulate cardiac excitability. Studies suggest I(K1) plays a role in ventricular arrhythmias, highlighted by the recently described Andersen's syndrome and studies in the guinea pig heart model of ventricular fibrillation. This article describes the salient properties of cardiac I(K1) and discusses the role of this current in the cardiac action potential and in underlying regional differences in cardiac excitability. The mechanism of channel block, assembly, and structure are reviewed. The article discusses the role of I(K1) in ventricular fibrillation and speculates on modulation of I(K1) as a preventative antiarrhythmic mechanism.

Transient outward current carried by inwardly rectifying K+channels in guinea pig ventricular myocytes dialyzed with low-K+solution

There have been periodic reports of nonclassic (4-aminopyridine insensitive) transient outward K+current in guinea pig ventricular myocytes, with the most recent one describing a novel voltage-gated inwardly rectifying type. In the present study, we have investigated a transient outward current that overlaps inward Ca2+current ( ICa,L) in myocytes dialyzed with 10 mM K+solution and superfused with Tyrode’s solution. Although depolarizations from holding potential ( Vhp) −40 to 0 mV elicited relatively small inward ICa,Lin these myocytes, removal of external K+or addition of 0.2 mM Ba2+more than doubled the amplitude of the current. The basis of the enhancement of ICa,Lwas the suppression of a large transient outward K+current. Similar enhancement was observed when Vhpwas moved to −80 mV and test depolarizations were preceded by short prepulses to −40 mV. Investigation of the time and voltage properties of the outward K+transient indicated that it was inwardly rectifying and unlikely to be carried by voltage-gated channels. The outward transient was attenuated in myocytes dialyzed with high-Mg2+solution, accelerated in myocytes dialyzed with 100 μM spermine solution, and abolished with time in myocytes dialyzed with ATP-free solution. These and other findings suggest that the outward transient is a component of classic “time-independent” inwardly rectifying K+current.

Reopening of L-type calcium channels in human ventricular myocytes during applied epicardial action potentials

AIMS

Present study was performed to compare the dynamics of human L-type calcium current (ICa,L) flowing during rectangular voltage pulses, voltage ramps, and action potentials (APs) recorded from epicardiac and endocardiac canine ventricular cells.

METHODS

ICa,L was recorded in single myocytes isolated from undiseased human hearts using the whole cell voltage clamp technique.

RESULTS

The decay of ICa,L was monotonic when using rectangular pulses or endocardial APs as voltage commands, whereas the current became double-peaked (displaying a second rise and fall) during epicardial (EPI) APs or voltage ramps used to mimic EPI APs. These ICa,L profiles were associated with single-hooked and double-hooked phase-plane trajectories, respectively. No sustained current was observed during the AP commands. Kinetics of deactivation and recovery from inactivation of human ICa,L were determined using twin-pulse voltage protocols and voltage ramps, and the results were similar to those obtained previously in canine cells under identical experimental conditions.

CONCLUSIONS

ICa,L can inactivate partially before and deactivate during the phase-1 repolarization of the epicardiac AP, and reopening of these channels seems to be associated with formation of the dome.

Differential effects of extracellular cesium on early afterdepolarizations in ventricular myocytes and arrhythmogenesis in isolated hearts of rats and guinea pigs

CsCl has been shown to be arrhythmogenic in-vivo and to cause early afterdepolarizations (EADs) in isolated cardiac preparations, but the underlying electrophysiological mechanisms are ill-defined. To elucidate these actions further, the effects of extracellular solutions containing 3 mM CsCl and either 2 mM KCl (Cs2K solution) or 5 mM KCl (Cs5K solution) on membrane potential and ionic currents in rat and guinea-pig ventricular myocytes were compared. Cs2K solution rapidly and reversibly inhibited outward I(K1), and reduced other K(+) currents by about 20%. Current-clamped myocytes were rapidly hyperpolarized by this solution and action potentials were prolonged, but EADs were not observed. In contrast, EADs were triggered by E-4031, H(2)O(2), and the pyrethroid tefluthrin. Membrane-potential changes reversed after replacing Cs2K with Cs5K solution, with the recovery of 50% of outward I(K1). These results suggest that Cs2K solution inhibited I(K1) and caused a late prolongation of the action-potential duration, but the affected membrane potentials were too negative to elicit EAD mechanisms. In isolated hearts perfused with modified Tyrode's, Cs2K, and Cs5K solutions, bradycardia and arrhythmias were evoked by both CsCl-containing solutions. A comparison of such results with the effects of these solutions on myocytes suggests that I(K1) inhibition and EADs in ventricular myocytes are unlikely to be involved in arrhythmogenesis under our conditions.

Effects of ACh and adenosine mediated by Kir3.1 and Kir3.4 on ferret ventricular cells

The inotropic effects of ACh and adenosine on ferret ventricular cells were investigated with the action potential-clamp technique. Under current clamp, both agonists resulted in action potential shortening and a decrease in contraction. Under action potential clamp, both agonists failed to decrease contraction substantially. In the absence of agonist, application of the short action potential waveform (recorded previously in the presence of agonist) also resulted in a decrease in contraction. Under action potential clamp, application of ACh resulted in a Ba(2+)-sensitive outward current with the characteristics of muscarinic K+ current (I(K,ACh)); the presence of the muscarinic K+ channel was confirmed by PCR and immunocytochemistry. In the absence of agonist, on application of the short ACh action potential waveform, the decrease in contraction was accompanied by loss of the inward Na(+)/Ca(2+) exchange current (I(NaCa)). ACh also inhibited the background inward K+ current (I(K,1)). It is concluded that ACh activates I(K,ACh), inhibits I(K,1), and indirectly inhibits I(NaCa); this results in action potential shortening, decrease in contraction, and, as a result of the inhibition of I(K,1), minimum decrease in excitability.

Nongenomic actions of thyroid hormone on the heart

Extranuclear or nongenomic actions of thyroid hormone do not require formation of a nuclear complex between the hormone and its traditional 3,5,3'-triiodo-L-thyronine (T3) receptor (TR). Among nongenomic actions of iodothyronines that are relevant to the heart are those on membrane ion channels or pumps. These include stimulation of the sarcolemmal Na+ channel, inward-rectifying K+ channel, voltage-activated potassium channels, and calcium pump (Ca2+-adenosine triphosphatases [ATPases]) and have been shown in intact cells or isolated membranes. Because circulating levels of thyroid hormone are relatively stable, actions on channels or pumps may contribute to setting of basal activity of these transport functions. The mechanism of certain of these membrane effects may involve actions of the hormone on signal transducing protein kinases that modulate levels of activity of plasma membrane channels. Thyroid hormone nongenomically enhances myocardial contractility in isolated myocardial cells, in the isolated perfused rat heart and in human subjects. Iodothyronines also decrease vasomotor tone in a variety of models and in man by a mechanism independent of cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), or nitric oxide generation. Acutely increased myocardial mitochondrial respiration has been demonstrated in isolated organelles exposed to thyroid hormone. Genomic and nongenomic actions of thyroid hormone can interface, e.g., at the level of sarcoplasmic reticulum Ca2+-ATPase, where gene expression is regulated by the TR-T3 complex and activity of the enzyme can be modulated nongenomically. The relevance of nongenomic actions of thyroid hormone on the heart has been demonstrated in acute effects of the hormone on cardiac output and systemic vascular resistance in human subjects.

The consequences of disrupting cardiac inwardly rectifying K(+) current (I(K1)) as revealed by the targeted deletion of the murine Kir2.1 and Kir2.2 genes

1. Ventricular myocytes demonstrate a steeply inwardly rectifying K(+) current termed I(K1). We investigated the molecular basis for murine I(K1) by removing the genes encoding Kir2.1 and Kir2.2. The physiological consequences of the loss of these genes were studied in newborn animals because mice lacking Kir2.1 have a cleft palate and die shortly after birth. 2. Kir2.1 (-/-) ventricular myocytes lack detectable I(K1) in whole-cell recordings in 4 mM external K(+). In 60 mM external K(+) a small, slower, residual current is observed. Thus Kir2.1 is the major determinant of I(K1). Sustained outward K(+) currents and Ba(2+) currents through L- and T-type channels were not significantly altered by the mutation. A 50 % reduction in I(K1) was observed in Kir2.2 (-/-) mice, raising the possibility that Kir2.2 can also contribute to the native I(K1). 3. Kir2.1 (-/-) myocytes showed significantly broader action potentials and more frequent spontaneous action potentials than wild-type myocytes. 4. In electrocardiograms of Kir2.1 (-/-) neonates, neither ectopic beats nor re-entry arrhythmias were observed. Thus the increased automaticity and prolonged action potential of the mutant ventricular myocytes were not sufficiently severe to disrupt the sinus pacing of the heart. The Kir2.1 (-/-) mice, however, had consistently slower heart rates and this phenotype is likely to arise indirectly from the influence of Kir2.1 outside the heart. 5. Thus Kir2.1 is the major component of murine I(K1) and the Kir2.1 (-/-) mouse provides a model in which the functional consequences of removing I(K1) can be studied at both cellular and organismal levels.

Existence of a transient outward K(+) current in guinea pig cardiac myocytes

A novel transient outward K(+) current that exhibits inward-going rectification (I(to.ir)) was identified in guinea pig atrial and ventricular myocytes. I(to.ir) was insensitive to 4-aminopyridine (4-AP) but was blocked by 200 micromol/l Ba(2+) or removal of external K(+). The zero current potential shifted 51-53 mV/decade change in external K(+). I(to.ir) density was twofold greater in ventricular than in atrial myocytes, and biexponential inactivation occurs in both types of myocytes. At -20 mV, the fast inactivation time constants were 7.7 +/- 1.8 and 6.1 +/- 1.2 ms and the slow inactivation time constants were 85.1 +/- 14.8 and 77.3 +/- 10.4 ms in ventricular and atrial cells, respectively. The midpoints for steady-state inactivation were -36.4 +/- 0.3 and -51.6 +/- 0.4 mV, and recovery from inactivation was rapid near the resting potential (time constants = 7.9 +/- 1.9 and 8.8 +/- 2.1 ms, respectively). I(to.ir) was detected in Na(+)-containing and Na(+)-free solutions and was not blocked by 20 nmol/l saxitoxin. Action potential clamp revealed that I(to.ir) contributed an outward current that activated rapidly on depolarization and inactivated by early phase 2 in both tissues. Although it is well known that 4-AP-sensitive transient outward current is absent in guinea pig, this Ba(2+)-sensitive and 4-AP-insensitive K(+) current has been overlooked.

Beat-to-beat repolarization variability in ventricular myocytes and its suppression by electrical coupling

Single ventricular myocytes paced at a constant rate and held at a constant temperature exhibit beat-to-beat variations in action potential duration (APD). In this study we sought to quantify this variability, assess its mechanism, and determine its responsiveness to electrotonic interactions with another myocyte. Interbeat APD(90) (90% repolarization) of single cells was normally distributed. We thus quantified APD(90) variability as the coefficient of variability, CV = (SD/mean APD(90)) x 100. The mean +/- SD of the CV in normal solution was 2.3 +/- 0.9 (132 cells). Extracellular TTX (13 microM) and intracellular EGTA (14 mM) both significantly reduced the CV by 44 and 26%, respectively. When applied in combination the CV fell by 54%. In contrast, inhibition of the rapid delayed rectifier current with L-691,121 (100 nM) increased the CV by 300%. The CV was also significantly reduced by 35% when two normal myocytes were electrically connected with a junctional resistance (R(j)) of 100 MOmega. Electrical coupling (R(j) = 100 MOmega) of a normal myocyte to one producing early afterdepolarization (EAD) completely blocked EAD formation. These results indicate that beat-to-beat APD variability is likely mediated by stochastic behavior of ion channels and that electrotonic interactions act to limit temporal dispersion of refractoriness, a major contributor to arrhythmogenesis.

Connexins and impulse propagation in the mouse heart

Gap junction channels are essential for normal cardiac impulse propagation. Three gap junction proteins, known as connexins, are expressed in the heart: Cx40, Cx43, and Cx45. Each of these proteins forms channels with unique biophysical and electrophysiologic properties, as well as spatial distribution of expression throughout the heart. However, the specific functional role of the individual connexins in normal and abnormal propagation is unknown. The availability of genetically engineered mouse models, together with new developments in optical mapping technology, makes it possible to integrate knowledge about molecular mechanisms of intercellular communication and its regulation with our growing understanding of the microscopic and global dynamics of electrical impulse propagation during normal and abnormal cardiac rhythms. This article reviews knowledge on the mechanisms of cardiac impulse propagation, with particular focus on the role of cardiac connexins in electrical communication between cells. It summarizes results of recent studies on the electrophysiologic consequences of defects in the functional expression of specific gap junction channels in mice lacking either the Cx43 or Cx40 gene. It also reviews data obtained in a transgenic mouse model in which cell loss and remodeling of gap junction distribution leads to increased susceptibility to arrhythmias and sudden cardiac death. Overall, the results demonstrate that these are potentially powerful strategies for studying fundamental mechanisms of cardiac electrical activity and for testing the hypothesis that certain cardiac arrhythmias involve gap junction or other membrane channel dysfunction. These new approaches, which permit one to manipulate electrical wave propagation at the molecular level, should provide new insight into the detailed mechanisms of initiation, maintenance, and termination of cardiac arrhythmias, and may lead to more effective means to treat arrhythmias and prevent sudden cardiac death.

Extracellular K+ dependence of inward rectification kinetics in human left ventricular cardiomyocytes

BACKGROUND

In human ventricular cells, the inwardly rectifying K+ current (IK1) is very similar to that of other mammalian species, but detailed knowledge about the K+-dependent distribution of open and blocked states during rectification and about the K+-dependent modulation of inactivation on hyperpolarization is currently lacking.

METHODS AND RESULTS

We used the whole-cell patch-clamp technique to record IK1 in myocytes isolated from subendocardial layers of left ventricular septum from patients with nonfailing hearts with aortic stenosis and cardiac hypertrophy who were undergoing open-heart surgery. Outward currents were very small at voltages positive to the reversal potential but increased at high external [K+]. Chord conductance measurements and kinetic analyses allowed us to estimate the proportion of channels in the open state and of those showing either slow unblock or instantaneous unblock (the so-called slow or instantaneous "activation") on hyperpolarization: the distribution in the individual states was dependent on external [K+]. The proportion of channels unblocking slowly was greater than that of channels unblocking instantaneously on hyperpolarization from the plateau voltage range. Hence, because of the previously reported link between the presence of highly protonated blocking molecules and slow unblock kinetics, it is suggested that high cellular concentrations of spermine may account for the low outward current density recorded in these cells. The current decrease observed on extended hyperpolarization was significantly relieved by an increase in external [K+].

CONCLUSIONS

The pattern of IK1 current alterations observed in the present model of human ventricular hypertrophy might favor enhanced excitability and underlie ventricular arrhythmias, possibly via increased intracellular polyamine levels.

Regression of LV hypertrophy with captopril normalizes membrane currents in rabbits

Recent studies indicate that regression of left ventricular hypertrophy (LVH) normalizes the in situ electrophysiological abnormalities of the left ventricle. This study was designed to determine whether regression of LVH also normalizes the abnormalities of individual membrane currents. LVH was induced in rabbits by renal artery banding. Single ventricular myocytes from rabbits with LVH at 3 mo after renal artery banding demonstrated increased cell membrane capacitance, prolonged action potential duration, decreased inward rectifier K+ current density, and increased transient outward K+ current density compared with myocytes from age-matched controls. Additional rabbits were randomized at 3 mo after banding to treatment with either vehicle or captopril for an additional 3 mo. Myocytes from LVH rabbits treated with vehicle showed persistent membrane current abnormalities. However, myocytes isolated from LVH rabbits treated with captopril had normal cell membrane capacitance, action potential duration, and membrane current densities. Captopril had no direct effect on membrane currents of either control or LVH myocytes. These data support the hypothesis that the action potential prolongation and membrane current abnormalities of LVH are reversed by regression. Normalization of membrane currents probably explains the reduced vulnerability to ventricular arrhythmia observed in this LVH model after treatment with captopril.

A repolarization-induced transient increase in the outward current of the inward rectifier K+ channel in guinea-pig cardiac myocytes

1. Outward currents of the inwardly rectifying K+ current (IKir) in guinea-pig ventricular myocytes were studied in the presence of 1 mM intracellular free Mg2+ using the whole-cell patch-clamp technique. 2. During repolarizing voltage steps following a large depolarizing pulse (> 0 mV), outward IKir increased transiently at voltages positive to the K+ equilibrium potential (EK, -84 mV for 5.4 mM extracellular [K+]). The rising phase was almost instantaneous, while the decay was exponential. The decay rate was faster at voltages closer to EK (time constants, 33.9 +/- 9.8 and 4.8 +/- 1.4 ms at -30 and -50 mV, respectively). 3. The transient outward IKir was absent when the preceding depolarization was applied from -40 mV. Larger transient currents developed as the voltage before the depolarization was shifted to more hyperpolarized levels. 4. Shift of the depolarizing voltage from > 0 mV to more negative ranges diminished the amplitudes of transient outward IKir and instantaneous inward IKir during the subsequent repolarizing steps positive and negative to EK, respectively. Since blockage of IKir by internal Mg2+ occurs upon large depolarization, and the block is instantaneously relieved at voltages negative to EK, the rising phase of the transient outward IKir was attributed to the relief of Mg2+ block at voltages positive to EK. Transient outward IKir was absent when intracellular [Mg2+] was reduced to 10 microM or lower. 5. Prolongation of the repolarizing voltage step increased the amplitude of time-dependent inward IKir during the subsequent hyperpolarization, indicating the progress of a gating process (presumably the channel block by intracellular polyamine) during the decaying phase of outward IKir. 6. Progressive prolongation of the depolarizing pulse (> 0 mV) from 100 to 460 ms decreased the transient outward IKir amplitude during the subsequent repolarizing step due to slow progress of the gating (polyamine block) at > 0 mV. 7. Current-voltage relations measured using repolarizing ramp pulses (-3.4 mV ms-1) showed an outward hump at around -50 mV, the magnitude of which increased as the voltage before the conditioning depolarization (10 mV) was shifted to more negative levels. With slower ramp speeds (-1.5 and -0.6 mV ms-1), the hump was depressed at voltages near EK. 8. Our study suggests that the relief of Mg2+ block may increase outward IKir during repolarization of cardiac action potentials, and that the resting potential, the level/duration of action potential plateau and the speed of repolarization influence the outward IKir amplitude. 9. A kinetic model incorporating a competition between polyamine block and Mg2+ block was able to account for the time dependence of outward IKir.

Spiral waves in two-dimensional models of ventricular muscle: formation of a stationary core

Previous experimental studies have clearly demonstrated the existence of drifting and stationary electrical spiral waves in cardiac muscle and their involvement in cardiac arrhythmias. Here we present results of a study of reentrant excitation in computer simulations based on a membrane model of the ventricular cell. We have explored in detail the parameter space of the model, using tools derived from previous numerical studies in excitation-dynamics models. We have found appropriate parametric conditions for sustained stable spiral wave dynamics (1 s of activity or approximately 10 rotations) in simulations of an anisotropic (ratio in velocity 4:1) cardiac sheet of 2 cm x 2 cm. Initially, we used a model that reproduced well the characteristics of planar electrical waves exhibited by thin sheets of sheep ventricular epicardial muscle during rapid pacing at a cycle length of 300 ms. Under these conditions, the refractory period was 147 ms; the action potential duration (APD) was 120 ms; the propagation velocity along fibers was 33 cm/s; and the wavelength along fibers was 4.85 cm. Using cross-field stimulation in this model, we obtained a stable self-sustaining spiral wave rotating around an unexcited core of 1.75 mm x 7 mm at a period of 115 ms, which reproduced well the experimental results. Thus the data demonstrate that stable spiral wave activity can occur in small cardiac sheets whose wavelength during planar wave excitation in the longitudinal direction is larger than the size of the sheet. Analysis of the mechanism of this observation demonstrates that, during rotating activity, the core exerts a strong electrotonic influence that effectively abbreviates APD (and thus wavelength) in its immediate surroundings and is responsible for the stabilization and perpetuation of the activity. We conclude that appropriate adjustments in the kinetics of the activation front (i.e., threshold for activation and upstroke velocity of the initiating beat) of currently available models of the cardiac cell allow accurate reproduction of experimentally observed self-sustaining spiral wave activity. As such, the results set the stage for an understanding of functional reentry in terms of ionic mechanisms.

Characterization of a transient outward K+ current with inward rectification in canine ventricular myocytes

The threshold potential for the classical depolarization-activated transient outward K+ current and Cl- current is positive to -30 mV. With the whole cell patch technique, a transient outward current was elicited in the presence of 5 mM 4-aminopyridine (4-AP) and 5 microM ryanodine at voltages positive to the K+ equilibrium potential in canine ventricular myocytes. The current was abolished by 200 microM Ba2+ or omission of external K+ (K+o) and showed biexponential inactivation. The current-voltage relation for the peak of the transient outward component showed moderate inward rectification. The transient outward current demonstrated voltage-dependent inactivation (half-inactivation voltage: -43.5 +/- 3.2 mV) and rapid, monoexponential recovery from inactivation (time constant: 13.2 +/- 2.5 ms). The reversal potential responded to the changes in K+o concentration. Action potential clamp revealed two phases of Ba2(+)-sensitive current during the action potential, including a large early transient component after the upstroke and a later outward component during phase 3 repolarization. The present study demonstrates that depolarization may elicit a Ba2(+)- and K(+o)-sensitive, 4-AP-insensitive, transient outward current with inward rectification in canine ventricular myocytes. The properties of this K+ current suggest that it may carry a significant early outward current upon depolarization that may play a role in determining membrane excitability and action potential morphology.

Time-dependent outward currents through the inward rectifier potassium channel IRK1. The role of weak blocking molecules

Outward currents through the inward rectifier K+ channel contribute to repolarization of the cardiac action potential. The properties of the IRK1 channel expressed in murine fibroblast (L) cells closely resemble those of the native cardiac inward rectifier. In this study, we added Mg2+ (0.44-1.1 mM) or putrescine (approximately 0.4 mM) to the intracellular milieu where endogenous polyamines remained, and then examined outward IRK1 currents using the whole-cell patch-clamp method at 5.4 mM external K+. Without internal Mg2+, small outward currents flowed only at potentials between -80 (the reversal potential) and approximately -40 mV during voltage steps applied from -110 mV. The strong inward rectification was mainly caused by the closed state of the activation gating, which was recently reinterpreted as the endogenous-spermine blocked state. With internal Mg2+, small outward currents flowed over a wider range of potentials during the voltage steps. The outward currents at potentials between -40 and 0 mV were concurrent with the contribution of Mg2+ to blocking channels at these potentials, judging from instantaneous inward currents in the following hyperpolarization. Furthermore, when the membrane was repolarized to -50 mV after short depolarizing steps (> 0 mV), a transient increase appeared in outward currents at -50 mV. Since the peak amplitude depended on the fraction of Mg(2+)-blocked channels in the preceding depolarization, the transient increase was attributed to the relief of Mg2+ block, followed by a re-block of channels by spermine. Shift in the holding potential (-110 to -80 mV), or prolongation of depolarization, increased the number of spermine-blocked channels and decreased that of Mg(2+)-blocked channels in depolarization, which in turn decreased outward currents in the subsequent repolarization. Putrescine caused the same effects as Mg2+. When both spermine (1 microM, an estimated free spermine level during whole-cell recordings) and putrescine (300 microM) were applied to the inside-out patch membrane, the findings in whole-cell IRK1 were reproduced. Our study indicates that blockage of IRK1 by molecules with distinct affinities, spermine and Mg2+ (putrescine), elicits a transient increase in the outward IRK1, which may contribute to repolarization of the cardiac action potential.

Mechanism of action potential prolongation by RP 58866 and its active enantiomer, terikalant. Block of the rapidly activating delayed rectifier K+ current, IKr

BACKGROUND

The class III antiarrhythmic agent RP 58866 and its active enantiomer, terikalant, are reported to selectively block the inward rectifier K+ current, IK1. These drugs have demonstrated efficacy in animal models of cardiac arrhythmias, suggesting that block of IK1 may be a useful antiarrhythmic mechanism. The symmetrical action potential (AP)-prolonging and bradycardic effects of these drugs, however, are inconsistent with a sole effect on IK1.

METHODS AND RESULTS

We studied the effects of RP 58866 and terikalant on AP and outward K+ currents in guinea pig ventricular myocytes. RP 58866 and terikalant potently blocked the rapidly activating delayed rectifier K+ current, IKr, with IC50S of 22 and 31 nmol/L, respectively. Block of IK1 was approximately 250-fold less potent; IC50S were 8 and 6 mumol/L, respectively. No significant block of the slowly activating delayed rectifier, IK1, was observed at < or = 10 mumol/L. The phenotypical IKr currents in mouse AT-1 cells and Xenopus oocytes expressing HERG were also blocked 50% by 200 to 250 nmol/L RP 58866 or terikalant, providing further conclusive evidence for potent block of IKr. RP 58866 < or = 1 mumol/L and dofetilide increased AP duration symmetrically, consistent with selective block of IKr. Only higher concentrations (> or = 10 mumol/L) of RP 58866 slowed the rate of AP repolarization and decreased resting membrane potential, consistent with an additional but substantially less potent block of IK1.

CONCLUSIONS

These data demonstrate that RP 58866 and terikalant are potent blockers of IKr and prompt a reinterpretation of previous studies that assumed specific block of IK1 by these drugs.

Reconstruction of retinal horizontal cell responses by the ionic current model

An ionic current model of the retinal horizontal cell is constructed. The horizontal cell models are interconnected by gap junctions to form a horizontal cell layer. The light response properties of the L-type horizontal cell are analyzed using this model. We demonstrate the functional role of each ionic current and the role of the feedback loop between cones and horizontal cells. The present study provides insight into the dynamic relationships between characteristics on the cellular level and on the multi-cellular level for producing the light response in horizontal cells.

Regulation of potassium channels by nonsedating antihistamines

BACKGROUND

Terfenadine and astemizole are widely prescribed nonsedating antihistamines that have been associated with QT-interval prolongation and ventricular arrhythmias. Since potassium channels are intrinsically involved in repolarization, this study was designed to evaluate the effect of the nonsedating antihistamines on potassium channel modulation.

METHODS AND RESULTS

The whole-cell patch-clamp technique was used to study K+ currents in enzymatically isolated rat and guinea pig ventricular myocytes. Three distinct K+ channels were examined: the inward rectifier (IK1), the delayed rectifier (IK), and the transient outward (I(to)) currents. The dialyzing pipette solution was buffered with EGTA, and ionic channels other than potassium were pharmacologically inhibited or electrically inactivated. Both astemizole and terfenadine suppressed the IK1 channel by 17% to 50% in a voltage-dependent manner in rat and guinea pig myocytes. Ito was evaluated in rat ventricular myocytes. Both drugs also inhibited the maintained component of I(to) to a lesser extent, by 23%, in a dose-dependent, reversible manner. IK was examined mainly in guinea pig myocytes. Terfenadine but not astemizole slightly inhibited IK, by 9%, and only at higher drug concentrations. The medications had dose-dependent inhibitory actions, with specific K+ channel suppression evident only beginning at concentrations > 0.1 mumol/L.

CONCLUSIONS

These findings suggest that the mechanism of action of the rare proarrhythmic effects of the nonsedating antihistamines appears to be secondary to potassium channel blockade. A significant voltage-dependent blockade of the IK1 channel was demonstrated, as well as additional inhibitory effects on I(to) and IK channels. These actions lead to delayed repolarization, QT interval prolongation, and enhanced susceptibility to the development of premature ventricular depolarizations. Caution is advised in the prescription of nonsedating antihistamines, particularly in patients at risk of elevated serum levels of the antihistamine or patients with existing repolarization abnormalities.

Cloning a novel human brain inward rectifier potassium channel and its functional expression in Xenopus oocytes

We have cloned a novel inward rectifier K+ channel (hIRK2) from a human frontal cortex cDNA library. The amino acid sequence of hIRK2 shares 60% and 40% identity with the mouse IRK1 and the rat ROMK1 channels, respectively. Xenopus oocytes injected with hIRK2 cRNA showed an inwardly rectifying K+ current that had a prominent 'N-shape' I-V curve and was blocked by extracellular Ba2+. The hIRK2 channel has two unique features: (a) an 18 amino acid insertion between the first transmembrane region and the pore, and (b) restricted mRNA distribution found only in human brain and heart.

Role of the Inward K Rectifier in the Repetitive Activity at the Depolarized Level in Single Ventricular Myocytes

The role of the inward K(+) rectifier in the repetitive activity at depolarized levels was studied in guinea pig single ventricular myocytes by voltage- and current-clamp methods. In action potentials arrested at the plateau by a depolarizing current, small superimposed hyperpolarizing currents caused much larger voltage displacements than at the resting potential and sometimes induced a regenerative repolarization. Around -20 mV, sub- and suprathreshold repetitive inward currents were found. In the same voltage range, small hyperpolarizing currents reversed their polarity. During depolarizing voltage-clamp ramps, around -20 mV there was a sudden decrease in the outward current (I(ns): current underlying the negative slope in the inward K(+) rectifier steady state I-V relation). During repolarizing ramps, the reincrease in outward current was smaller and slower. During depolarizing and repolarizing current ramps, sudden voltage displacements showed a similar asymmetry. Repetitive I(ns) could continue as long as the potential was kept at the level at which they appeared. Depolarizing voltage-clamp steps also caused repetitive I(ns) and depolarizing current steps induced repetitive slow responses. Cadmium and verapamil reduced I(ns) amplitude during the depolarizing ramp. BRL 34915 (cromakalim), an opener of the ATP-sensitive K(+) channel, eliminated the negative slope and I(ns), whereas barium increased I(ns) frequency (an effect abolished by adding BRL). Depolarization-induced slow responses persisted in an NaCl- Ca-free solution. Thus, the mechanism of repetitive activity at the depolarized level appears to be related to the presence of the negative slope in the inward K(+) rectifier I-V relation. Copyright 1994 S. Karger AG, Basel

Calcium and potassium currents in ventricular myocytes isolated from diabetic rats

1. The whole-cell voltage-clamp technique was applied to ventricular myocytes isolated from normal and streptozotocin-induced diabetic rat hearts to investigate the contribution of the calcium current and of the calcium-independent potassium currents to diabetes-induced alterations of the action potential. 2. In single calcium-tolerant isolated myocytes diabetes induced a lengthening of the action potential similar to that previously described in intact ventricular muscles. 3. Only L-type calcium current was present both in normal and diabetic cells. Inactivation of ICa was described in both preparations by two exponentials, whose time constants were not modified by diabetes. 4. Calcium current density-voltage relationships and steady-state inactivation curves were not significantly affected by diabetes. 5. Potassium background inward rectifier current was not modified by diabetes. 6. Calcium-independent outward potassium current inactivated, in both cell types, according to a biexponential process whose time constants were not affected by diabetes. 7. The transient outward potassium current density was significantly reduced by diabetes whereas neither the voltage dependence of the inactivation nor the time dependence of recovery from inactivation was modified. 8. A 4-aminopyridine-insensitive potassium current was also reduced by diabetes. 9. Our results show that in isolated ventricular myocytes the lengthening of the action potential induced by diabetes results mainly from a decrease of the transmembrane calcium-independent potassium permeability.