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SummaryMutations altering the scaffolding protein Shank are linked to several psychiatric disorders, and to synaptic and behavioral defects in mice. Among its many binding partners, Shank directly binds CaV1 voltage activated calcium channels. Here we show that the C. elegans SHN-1/Shank promotes CaV1 coupling to calcium activated potassium channels. Mutations inactivating SHN-1, and those preventing SHN-1 binding to EGL-19/CaV1 all increase action potential durations in body muscles. Action potential repolarization is mediated by two classes of potassium channels: SHK-1/KCNA and SLO-1 and SLO-2 BK channels. BK channels are calcium-dependent, and their activation requires tight coupling to EGL-19/CaV1 channels. SHN-1’s effects on AP duration are mediated by changes in BK channels. In shn-1 mutants, SLO-2 currents and channel clustering are significantly decreased in both body muscles and neurons. Finally, increased and decreased shn-1 gene copy number produce similar changes in AP width and SLO-2 current. Collectively, these results suggest that an important function of Shank is to promote nanodomain coupling of BK with CaV1.

Although plasma electrolyte levels are quickly and precisely regulated in the mammalian cardiovascular system, even small transient changes in K+, Na+, Ca2+, and/or Mg2+ can significantly alter physiological responses in the heart, blood vessels, and intrinsic (intracardiac) autonomic nervous system. We have used mathematical models of the human atrial action potential (AP) to explore the electrophysiological mechanisms that underlie changes in resting potential (Vr) and the AP following decreases in plasma K+, [K+]o, that were selected to mimic clinical hypokalemia. Such changes may be associated with arrhythmias and are commonly encountered in patients (i) in therapy for hypertension and heart failure; (ii) undergoing renal dialysis; (iii) with any disease with acid-base imbalance; or (iv) post-operatively. Our study emphasizes clinically-relevant hypokalemic conditions, corresponding to [K+]o reductions of approximately 1.5 mM from the normal value of 4 to 4.5 mM. We show how the resulting electrophysiological responses in human atrial myocytes progress within two distinct time frames:(i) Immediately after [K+]o is reduced, the K+-sensing mechanism of the background inward rectifier current (IK1) responds. Specifically, its highly non-linear current-voltage relationship changes significantly as judged by the voltage dependence of its region of outward current. This rapidly alters, and sometimes even depolarizes, Vr and can also markedly prolong the final repolarization phase of the AP, thus modulating excitability and refractoriness.(ii) A second much slower electrophysiological response (developing 5–10 minutes after [K+]o is reduced) results from alterations in the intracellular electrolyte balance. A progressive shift in intracellular [Na+]i causes a change in the outward electrogenic current generated by the Na+/K+ pump, thereby modifying Vr and AP repolarization and changing the human atrial electrophysiological substrate.In this study, these two effects were investigated quantitatively, using seven published models of the human atrial AP. This highlighted the important role of IK1 rectification when analyzing both the mechanisms by which [K+]o regulates Vr and how the AP waveform may contribute to “trigger” mechanisms within the proarrhythmic substrate. Our simulations complement and extend previous studies aimed at understanding key factors by which decreases in [K+]o can produce effects that are known to promote atrial arrhythmias in human hearts.

Cardiac arrhythmias are the most common cause of sudden cardiac death worldwide. Lengthening the ventricular action potential duration (APD) either congenitally or via pathologic or pharmacologic means, predisposes to a life-threatening ventricular arrhythmia, Torsade de Pointes. IKs, a slowly activating K+ current plays a role in action potential repolarization. In this study, we screened a chemical library in silico by docking compounds to the voltage sensing domain (VSD) of the IKs channel. Here we show that C28 specifically shifted IKs VSD activation in ventricle to more negative voltages and reversed drug-induced lengthening of APD. At the same dosage, C28 did not cause significant changes of the normal APD in either ventricle or atrium. This study provides evidence in support of a computational prediction of IKs VSD activation as a potential therapeutic approach for all forms of APD prolongation. This outcome could expand the therapeutic efficacy of a myriad of currently approved drugs that may trigger arrhythmias.Significance statementC28, identified by in silico screening, specifically facilitated voltage dependent activation of a cardiac potassium ion channel, IKs. C28 reversed drug-induced prolongation of action potentials, but minimally affected the normal action potential at the same dosage. This outcome supports a computational prediction of modulating IKs activation as a potential therapy for all forms of action potential prolongation, and could expand therapeutic efficacy of many currently approved drugs that may trigger arrhythmias.

Regulation of cardiac electrical and mechanical activities are highly dependent on L-type calcium current (I CaL ). Moreover, the phenomenon of I CaL . inactivation plays a crucial role in adjusting action potential repolarization. This paper investigates the relationship between extracellular calcium concentration, [Ca+2]o, and action potential (AP). In particular, calcium-dependent inactivation (CDI) of I CaL has been analyzed to quantify its dependence on [Ca+2] o and its impact on AP duration. We have analyzed three human atrial cell models (hAM) by testing benchmarks like AP, I CaL , and I Ncx (sodium-calcium exchange current) with the variation of [Ca+2]o. The role of CDI was assessed by simulating the AP-clamp technique followed by 1) running the baseline model under normal CDI mechanism and 2) running the model without the CDI mechanism. The comparison of three hAMs shows that besides the contribution of CDI, driving force (DF), and I NCX current should also be considered while studying the relationship between AP duration (APD) and [Ca+2] o. None of the hAMs reproduced in a physiologically correct way the effects of [Ca+2]o on APD, therefore our results call for a new hAM able to reproduce correctly the [Ca+2]o effect

Voltage-gated potassium channels (KV) mainly response in action potential repolarization in excitable cells and also participate in regulating resting potentials in non-excitable cells, involved in diverse physiological processes. This review focuses on potential drug developments targeting the KV1.1 and KV1.3 channels. KV1.1 mainly existing in the nervous system plays key roles in controlling neuronal excitability; while the distribution of KV1.3 in different types of cells contributes to a variety of cellular processes. This article seeks to review the distributions of two channels, their roles in diseases, phenotypes of knockout mice, human channelopathies and selective modulators.

ObjectiveTo observe the effects of angiotensin Ⅱ(Ang Ⅱ) perfusion on transmural heterogeneity of Cx43 expression in the rabbit model with acute myocardial ischemia reperfusion (MIR), and investigate the role of rennin-angiotensin system in malignant ventricular arrhythmia induced by MIR.MethodsTwenty rabbits were randomly divided into MIR group (n = 10) and Ang Ⅱ group (n = 10). MIR model was produced with traditional ligation and opening of the anterior descending coronary artery in all animal. The hearts in vitro in the MIR group and the Ang Ⅱ group were perfused with simply improved Tyrode's solution and containing Ang Ⅱ Tyrode's solution respectively. 90% monophasic action potential repolarization duration, transmural dispersion of repolarization, Cx43 protein (Cx43-pro) and mRNA (Cx43-Cq) expression in subepicardial, midmyocardial and subendocardial myocardium were measured in both groups. The greatest differences of Cx43-pro and Cx43-Cq among three myocardial layers were calculated and shown with ΔCx43-pro and ΔCx43-Cq respectively.ResultsAfter Ang Ⅱ perfusion, 90% monophasic action potential repolarization duration among three myocardial layer were significantly prolonged (P

Potassium channels play a critical role in modulating cardiac excitability and action potential repolarization. They include the voltage-gated, inwardly rectifying (Kir), small conductance calcium-activated, two-pore domain K+ channels. In this chapter, we will focus on Kir channels, in light of advancements made in the structural biology of these proteins and our increased understanding of their regulation by pharmacological agents.

Mitral and tufted cells, the two classes of principal neurons in the mammalian main olfactory bulb, exhibit morphological differences but remain widely viewed as functionally equivalent. Results from several recent studies, however, suggest that these two cell classes may encode complementary olfactory information in their distinct patterns of afferent-evoked activity. To understand how these differences in activity arise, we have performed the first systematic comparison of synaptic and intrinsic properties between mitral and tufted cells. Consistent with previous studies, we found that tufted cells fire with higher probability and rates and shorter latencies than mitral cells in response to physiological afferent stimulation. This stronger response of tufted cells could be partially attributed to synaptic differences, as tufted cells received stronger afferent-evoked excitation than mitral cells. However, differences in intrinsic excitability also contributed to the differences between mitral and tufted cell activity. Compared to mitral cells, tufted cells exhibited twofold greater excitability and peak instantaneous firing rates. These differences in excitability probably arise from differential expression of voltage-gated potassium currents, as tufted cells exhibited faster action potential repolarization and afterhyperpolarizations than mitral cells. Surprisingly, mitral and tufted cells also showed firing mode differences. While both cell classes exhibited regular firing and irregular stuttering of action potential clusters, tufted cells demonstrated a greater propensity to stutter than mitral cells. Collectively, stronger afferent-evoked excitation, greater intrinsic excitability and more irregular firing in tufted cells can combine to drive distinct responses of mitral and tufted cells to afferent-evoked input.

ObjectiveTo observe effects of hypokalemia on transmural heterogeneity of ventricular repolarization in left ventricular myocardium of rabbit, and explore the role of hypokalemia in malignant ventricular arrhythmia (MVA).MethodsA total of 20 rabbits were randomly divided into control group and hypokalemic group. Isolated hearts in the control group were simply perfused with modified Tyrode's solution, and were perfused with hypokalemic Tyrode's solution in hypokalemic group. Ventricular fibrillation threshold (VFT), 90% monophasic action potential repolarization duration (APD90) of subepicardial, midmyocardial and subendocardial myocardium, transmural dispersion of repolarization (TDR) and C×43 protein expression in three layers of myocardium were measured in both groups.ResultsVFT in the control group and the hypokalemic group were (13.40±2.95) V, and (7.00±1.49) V, respectively. There was a significant difference between two groups (P

Rapidly activating and rapidly inactivating voltage-gated A-type K+ currents, IA, are key determinants of neuronal excitability and several studies suggest a critical role for the Kv4.2 pore-forming α subunit in the generation of IA channels in hippocampal and cortical pyramidal neurons. The experiments here demonstrate that Kv4.2, Kv4.3 and Kv1.4 all contribute to the generation of IA channels in mature cortical pyramidal (CP) neurons and that Kv4.2-, Kv4.3- and Kv1.4-encoded IA channels play distinct roles in regulating the intrinsic excitability and the firing properties of mature CP neurons. In vivo loss of Kv4.2, for example, alters the input resistances, current thresholds for action potential generation and action potential repolarization of mature CP neurons. Elimination of Kv4.3 also prolongs action potential duration, whereas the input resistances and the current thresholds for action potential generation in Kv4.3−/− and WT CP neurons are indistinguishable. In addition, although increased repetitive firing was observed in both Kv4.2−/− and Kv4.3−/− CP neurons, the increases in Kv4.2−/− CP neurons were observed in response to small, but not large, amplitude depolarizing current injections, whereas firing rates were higher in Kv4.3−/− CP neurons only with large amplitude current injections. In vivo loss of Kv1.4, in contrast, had minimal effects on the intrinsic excitability and the firing properties of mature CP neurons. Comparison of the effects of pharmacological blockade of Kv4-encoded currents in Kv1.4−/− and WT CP neurons, however, revealed that Kv1.4-encoded IA channels do contribute to controlling resting membrane potentials, the regulation of current thresholds for action potential generation and repetitive firing rates in mature CP neurons.

Small conductance calcium-activated potassium channels (SK channels) play a critical role in action potential repolarization in cardiomyocytes. Recently, the potential anti-arrhythmic effect of metformin in diabetic patients has been recognized, yet the underlying mechanism remains elusive.

At cellular level, changes in the cardiac action potential (AP) duration (APD) are relevant proarrhythmic markers. The assessment of single current contributions to APD changes allows the investigation of the complex interplay of ionic mechanisms underlying such repolarization changes. In this paper, we present a new method to quantify the contributions of each membrane current to AP D changes due to a perturbation from the basal to a different condition. To achieve our goal, we used a piecewiselinear approximation of the AP. We tested our method on the OHara-Rudy model in case of rate adaptation: from the basal condition (pacing at 60 bpm), two different pacing rates are used as perturbations: 30 bpm, which prolongs APD by 21 ms, and 120 bpm, which shortens APD by -37 ms. At steady state, the most significant current contributions (30 bpm/120 bpm) are: INaK (68/-73 ms), ICaL (-58/51 ms), INaCa (-10/25 ms), IKs (13/-7 ms) and INaL (7/-23 ms). Our method allows also quantifying the dynamic adaptation to rate changes from the perturbation until the steady state. In conclusion, our method enables the quantification of the adaptive and compensatory mechanisms implemented by the (in silico model of) cell in response to a perturbation, such as the pacing rate change.

The work presented in this thesis centres on the in silico investigation of arrhythmia substrates in an inherited cardiac condition: the short QT syndrome (SQTS).

The K + channel encoded by the human ether-a-go-go related gene ( HERG ) is one of many ion channels that are crucial for normal action potential repolarization in cardiac myocytes. HERG encodes the pore-forming subunit of the rapid component of the delayed rectifier K + channel, I K(Vr) . HERG K + channels are of considerable pharmaceutical interest as possible therapeutic targets for anti-arrhythmic agents and as the molecular target responsible for the cardiac toxicity of a wide range of pharmaceutical agents. Recent studies of the molecular basis of the promiscuity of HERG K + channel drug binding has not only started to shed light on this tricky pharmaceutical problem but has also provided further insights into the structure and function of HERG K + channels.

The cardiac I(Ks) current is involved in action potential repolarization, where its primary function is to limit action potential prolongation during sympathetic stimulation. The I(Ks) channel is mainly composed of K(V)7.1 ion channels associated with KCNE1 auxiliary subunits. The availability of KCNE1 solution structure by nuclear magnetic resonance spectroscopy in conjunction with biochemical assays addressing K(V)7.1-KCNE1 residue interactions has provided new insights into the structural basis for K(V)7.1 modulation by KCNE1. Recent evidence further suggests that KCNE2 may associate with the K(V)7.1-KCNE1 channel complex and modulate its current amplitude. Here we review recent studies in this area and discuss potential roles for multiple KCNE(x) subunits in I(Ks) generation and modulation as well as the clinical relevance of the new information.

In addition to the well-understood role in action potential repolarization and control of brain homeostasis, potassium channels also play a fundamental role in controlling cellular proliferation. Here we wish to summarize evidence supporting potassium channels’ involvement in tumorigenesis. We will first introduce basic concepts in cell cycle in the central nervous system, and then describe the multiple roles of potassium channels and their modulators in the central nervous system. Finally, we will focus on how brain tumors are controlled by potassium channel regulation, linking tumorigenesis process to cell cycle re-entry. We also introduce a novel possible modulator of potassium channels, and thus of tumorigenesis.

Mudpuppy parasympathetic cardiac neurons generate spontaneous miniature outward currents (SMOCs) that result from the simultaneous activation of large conductance calcium activated (Kca) channels by Ca 2+ released from caffeineand ryanodine-sensitive intraceliular stores (Satin and Adams 1987; Merriam et ah, 1999; Scornik et al, 2001). The present article summarizes recent findings on SMOC regulation and on the role of these currents on action potential (AP) generation. The data presented in this article suggest that spatial distribution determines differential roles of Kca channels, in the generation and repolarization of AP in parasympathetic neurons from the mudpuppy cardiac ganglia. Parasympathetic cardiac neurons, like other autonomic neurons, express a number of different voltageand ligand-gated potassium channels in their plasma membranes that are important determinants of membrane potential and neuronal excitability (Brown 1988; Rudy 1988; Adams and Harper 1995). Among these different potassium conductances, Kca channels have been shown particularly, to play a key role in determining resting membrane potential, action potential repolarization and post action potential hyperpolarizations (Rudy 1988; Kaczorowski etal, 1996; Sah 1996). Activation of Kca channels is very dynamic because it is gated by elevation of the Ca concentration at the intraceliular side of the channel. Very localized elevations of Ca, which do not significantly elevate global intraceliular Ca, can activate Kca channels. Thus, physiological activation of Kca channels may be coupled to highly localized changes of ceil Ca. Although spontaneous miniature outward currents (SMOCs) were described in neurons over two decades ago, their physiological role remained unclear.

Nonpenetrating mechanical stimulation of the precordial chest is particularly likely to instantaneously induce sustained rhythm disturbances if timed to coincide with ventricular repolarization. A number of possible mechanisms have been proposed, including mechanoelectric feedback acting via stretch-activated ion channels. The cellular effects of such channel activation have been studied and mathematically modeled in great detail. In this study, we investigate their dynamic interaction with the trailing wave of action potential repolarization in a two-dimensional model of ventricular tissue. The model identifies how stretch activation of cation-nonselective ion channels causes ectopic excitation in fully repolarized tissue and functional block of conduction at the intersection of the mechanical stimulus and the repolarization wave end, which may give rise to both trigger and sustaining mechanisms of ventricular arrhythmia. Simulation of stretch activation of K+-selective ion channels alone is insufficient in causing instantaneous arrhythmia, although it may, via action potential shortening, contribute to its sustenance.

Potassium channels are crucial regulators of neuronal excitability, setting resting membrane potentials and firing thresholds, repolarizing action potentials and limiting excitability. Although most of our understanding of K+ channels is based on somatic recordings, there is good evidence that these channels are present in synaptic terminals. In recent years the improved access to presynaptic compartments afforded by direct recording techniques has indicated diverse roles for native K+ channels, from suppression of aberrant firing to action potential repolarization and activity-dependent modulation of synaptic activity. This article reviews the growing evidence for multiple roles and discrete localization of distinct K+ channels at presynaptic terminals.

Fast spiking (FS), GABAergic neurons of the reticular thalamic nucleus (RTN) are capable of firing high-frequency trains of brief action potentials, with little adaptation. Studies in recombinant systems have shown that high-voltage-activated K+ channels containing the Kv3.1 and/or Kv3.2 subunits display biophysical properties that may contribute to the FS phenotype. Given that RTN expresses high levels of Kv3.1, with little or no Kv3.2, we tested whether this subunit was required for the fast action potential repolarization mechanism essential to the FS phenotype. Single- and multiple-action potentials were recorded using whole-cell current clamp in RTN neurons from brain slices of wild-type and Kv3.1-deficient mice. At 23°C, action potentials recorded from homozygous Kv3.1 deficient mice (Kv3.1−/−) compared with their wild-type (Kv3.1+/+) counterparts had reduced amplitudes (−6%) and fast after-hyperpolarizations (−16%). At 34°C, action potentials in Kv3.1−/− mice had increased duration (21%) due to a reduced rate of repolarization (−30%) when compared with wild-type controls. Action potential trains in Kv3.1−/− were associated with a significantly greater spike decrement and broadening and a diminished firing frequency versus injected current relationship ( F/I) at 34°C. There was no change in either spike count or maximum instantaneous frequency during low-threshold Ca2+ bursts in Kv3.1−/− RTN neurons at either temperature tested. Our findings show that Kv3.1 is not solely responsible for fast spikes or high-frequency firing in RTN neurons. This suggests genetic redundancy in the system, possibly in the form of other Kv3 members, which may suffice to maintain the FS phenotype in RTN neurons in the absence of Kv3.1.

Whole-cell patch clamp recordings were performed on hypoglossal motoneurons (HMs) in a brain stem slice preparation from the neonatal rat. The medium afterhyperpolarization (mAHP) was the only afterpotential always present after single or multiple spikes, making it suitable for studying its role in firing behavior. At resting membrane potential (−68.8 ± 0.7 mV), mAHP (23 ± 2 ms rise-time and 150 ± 10 ms decay) had 9.5 ± 0.7 mV amplitude, was suppressed in Ca2+-free medium or by 100 nM apamin, and reversed at −94 mV membrane potential. These observations suggest that mAHP was due to activation of Ca2+-dependent, SK-type K+ channels. Carbachol (10–100 μM) reversibly and dose dependently blocked the mAHP and depolarized HMs (both effects prevented by 10 μM atropine). Similar mAHP block was produced by muscarine (50 μM). In control solution a constant current pulse (1 s) induced HM repetitive firing with small spike frequency adaptation. When the mAHP was blocked by apamin, the same current pulse evoked much higher frequency firing with strong spike frequency adaptation. Carbachol also elicited faster firing and adapting behavior. Voltage clamp experiments demonstrated a slowly deactivating, apamin-sensitive K+ current ( I AHP) which could account for the mAHP. I AHP reversed at −94 mV membrane potential, was activated by depolarization as short as 1 ms, decayed with a time constant of 154 ± 9 ms at −50 mV, and was also blocked by 50 μM carbachol. These data suggest that mAHP had an important role in controlling firing behavior as clearly demonstrated after its pharmacological block and was potently modulated by muscarinic receptor activity.

Afterdepolarizations represent a unique mechanism of abnormal impulse formations in which they are induced by initiating or triggering action potential. There are two types of afterdepolarizations, early and delayed afterdepolarizations. Early afterdepolarizations (EADs) occur during the course of preceding action potential repolarization, while delayed afterdepolarizations (DADs) develop after complete repolarization. DADs, which also have different terms such as transient depolarization, oscillatory afterpotential and so on, are damped membrane oscillations with amplitudes either less than reaching threshold or large enough to reach threshold. In the former case, DADs may appear as several membrane oscillations to subside gradually or may become large enough to propagate as subthreshold depolarization. In the latter case, they reach threshold to induce either a single spontaneous excitation, continuous rhythmic activity or runs of tachycardia.

Background K+ channels are diverse; both in terms of their function and their molecular composition. Shal subunits were first described in Drosophila. There are three mammalian orthologs, which are members of the Kv4 subfamily. They are involved in neuronal firing patterns as well as control of the cardiac action potential duration. Results Here, we report the biophysical and pharmacological characterization of zShal3, which is the ortholog of the mammalian Kv4.3 subunit, which in mammals is involved in action potential repolarization and gives rise to neuronal A-type K+ currents involved in somatodendretic signal integration. Conclusion We demonstrate that zShal has similar functional and pharmacological characteristics compared to Kv4.3 and it is similarly regulated by pharmacological agents and by the Kv4 accessory subunit, NCS-1.

Introduction: In the hereditary long QT syndrome, arrhythmia risk changes with age despite the presence of an ion channel mutation throughout development. Age-dependent changes in the transmural dispersion of repolarization may modulate this vulnerability. We recorded cardiac action potentials in infant, periadolescent, and adult rabbit myocardium to determine if transmural heterogeneities in repolarization are developmentally determined. Methods and Results: Arterially perfused ventricular preparations were studied from 2-week (n = 7), 7-week (n = 7), and adult (n = 6) NZW rabbits. Action potentials were recorded with microelectrodes in five regions: epicardium (epi), subepicardium (subepi), midwall (mid), subendocardium (subendo), and endocardium (endo) during endocardial S1 pacing at cycle lengths of 2,000, 1,000, and 500 ms. At 2 weeks, the transmural APD90 profile was flat. With age, APD prolongation from subepi to endo created a transmural repolarization gradient. At 7 weeks, APD90 was significantly longer at subendo [204 ± 2 ms (mean ± SE) 2,000-ms cycle length, P < 0.05] vs both endo (193 ± 2 ms) and epi (172 ± 2 ms), causing a heterogeneous transmural APD90 gradient. In adults, the transmural gradient was a smooth continuum such that APD was shortest in epicardium and longest in endocardium. Conclusion: The transmural distribution of APD is developmentally determined. Tissue-specific age-dependent changes in APD can result in transmural repolarization heterogeneity. These age-related effects may modulate arrhythmia vulnerability during development. (J Cardiovasc Electrophysiol, Vol. 15, pp. 795-801, July 2004)

A model of one-dimensional action potential propagation was used to compare activation times and recovery times measured from simulated unipolar and bipolar electrograms with the activation and recovery times measured from simulated transmembrane action potentials. Theory predicts that the intrinsic deflection--the time of the maximum negative slope of the unipolar electrogram QRS complex--corresponds to the time of maximum positive slope of action potential depolarization. Similarly, the time of the maximum positive slope of the unipolar electrogram T wave corresponds to the time of maximum negative slope of action potential repolarization. This study showed that the difference between the unipolar electrogram activation time and the action potential activation time and the difference between the unipolar electrogram recovery time and the action potential recovery time were small during ideal conditions of uniform propagation in a long cable. Nonideal conditions, however, were associated with activation time differences in excess of 1.8 msec and recovery time differences in excess of 30 msec (243 msec in certain conditions). Nonideal conditions that had a major influence were changes in activation sequence, propagation in a short cable, and propagation through regions of nonuniform coupling resistance and/or nonuniform membrane properties. Nonideal conditions that had a smaller influence were variations in distance from the measurement site to the simulated tissue surface, nonzero reference potentials, and the addition of distant events. Recovery time differences were more sensitive to the nonideal conditions than were activation time differences, and both depended on the action potential shape. When distant events significantly contributed to the unipolar electrogram waveform, the time differences when bipolar electrograms were used were less than those when unipolar electrograms were used; however, under other conditions, the time differences were comparable. Results showed that activation times and especially recovery times measured from electrograms can be greatly affected by conditions independent of changes in the underlying action potential waveforms.

A suction electrode recording technique was used to study action potential characteristics of rat optic nerve fibers. Two pharmacologically distinct potassium channels are described. One is sensitive to 4-aminopyridine (4-AP) and the other to tetraethylammonium (TEA). 4-AP application leads to a substantial broadening of the optic nerve action potential, but TEA does not. 4-AP application also elicits a TEA-sensitive post-spike positivity, i.e. an intracellular hyperpolarization. From these results we suggest that the 4-AP-sensitive channel, not the TEA-sensitive channel, is primarily responsible for action potential repolarization of mammalian optic nerve fibers.

Ibogaine is a plant alkaloid used as anti-addiction drug in dozens of alternative medicine clinics worldwide. Recently, alarming reports of life-threatening cardiac arrhythmias and cases of sudden death associated with the ingestion of ibogaine have accumulated. Using whole-cell patch clamp recordings, we assessed the effects of ibogaine and its main metabolite noribogaine on action potentials in human ventricular-like cardiomyocytes derived from induced pluripotent stem cells. Therapeutic concentrations of ibogaine and its long-lived active metabolite noribogaine significantly retarded action potential repolarization in human cardiomyocytes. These findings represent the first experimental proof that ibogaine application entails a cardiac arrhythmia risk for humans. In addition, they explain the clinically observed delayed incidence of cardiac adverse events several days after ibogaine intake. We conclude that therapeutic concentrations of ibogaine retard action potential repolarization in the human heart. This may give rise to a prolongation of the QT interval in the electrocardiogram and cardiac arrhythmias.

Current clamp experiments performed in rat ventricular fibres revealed the presence of a resistance Rs (between 14.5 and 15.5 KΩ) in series with the membrane capacity. Rs behaved as a lumped resistance and its value remained constant throught the action potential repolarization phase.

Repolarization of the action potential in squid axon1 and several types of neurones2–4 involves a voltage-activated potassium (K+) current. Voltage clamp analysis has demonstrated that this current has rapid activation kinetics1,3–5. In several neuronal types, the same technique has also revealed a slowly activated K+ current that is calcium (Ca2+)-sensitive3,5–10. This slow Ca2+-activated K+ current is the major current underlying the late, slower portion of the after-hyperpolarization following an action potential11–14. In several muscle types, fast, transient Ca2+-dependent K+ currents have been described15–17 which may contribute to repolarization of the action potential. Rapidly activating, Ca2+-dependent K+ currents have been observed in sympathetic neurones of the bullfrog and it has been suggested that they contribute to action potential repolarization of those neurones8,9,18. We have studied the membrane currents in bullfrog sympathetic neurones using voltage clamp methods and report here a transient outward current that appears to be composed of two separate currents. One of those currents is a transient, Ca2+-sensitive outward current as indicated by a significant reduction of the current by treatments that reduce or block Ca2+ entry (Mn2+, Cd2+, Co2+, Mg2+ or Ca2+ -free Ringer). Such treatments also decreased the rate of action potential repolarization. The results suggest that this current is involved in repolarization of the action potential and consequently may regulate Ca2+ entry into the neurone during spike activity.

International audience; In CinC2015 we have shown that the dynamics of the action potential (AP) repolarization could be tracked throughout the stimulation course. Despite some valuable outcomes, the populations of interest (control, Ctrl, and streptozotocin-induced, STZ, diabetic mice) could not be significantly distinguished in term of dynamics because of the global extracted feature. In this study, the computation of new features for each repolarization percentage allows an accurate and meaningful characterization of the two groups leading to a significant classification. APs in isolated left ventricular cardiomyocytes obtained from Ctrl and STZ mice were measured by patch-clamp. The progressive changes in AP repolarization for individual cells were tested on a set of 100 consecutive excita-tions at 2Hz pacing rate. The corresponding repolariza-tions are stacked in a matrix decomposed with a new approach. Observations are modeled as a sum of vectors multiplied by specific polynomial functions. This approach is similar to the Singular Value Decomposition (SVD), but the corresponding scalars are replaced by these functions. Model unknowns are estimated by using an alternated least square algorithm. Finally, the mean of the polynomial first derivative is computed for each repolarization percentage as a representative feature. A Wilcoxon signed rank test (p

The heart rate in horses can vary between 30 and 250 beats per minute. This suggests an important role for a dynamic regulation of the repolarizing potassium currents.Methods and Results: Kv7.1 and KCNE1 that underlie the slow delayed rectifying current (IKs) as well as Kv11.1 and KCNE2 that underlie the rapid delayed rectifying current (IKr) were cloned from equine hearts. All subunits exhibit a high degree of homology with their human variants. Co-expression of equine Kv7.1 and KCNE1 in Xenopus laevis oocytes revealed a current with overall properties similar to the human Kv7.1/KCNE1 complex, however, the voltage dependence of activation was significantly right shifted (V1/2, equine=51.7 mV and V1/2, human=26.4 mV) and the deactivation was slower (Tauequine=1288 and Tauhuman=613 ms). Equine Kv11.1 and KCNE2 current kinetics were similar to those of the human variants and the channel complex was susceptible to pharmacological block by terfenadine. To address the functional role of IKr and IKs in equine hearts, we recorded action potentials from arterially-perfused wedge preparations from the right ventricular wall at different pacing rates. In the presence of 10 μM terfenadine a marked prolongation of the action potential duration (from 420 ms to 495 ms at 90% of repolarization (APD90) at a basic cycle length of 2000 ms) was observed.Conclusions: The expression of Kv7.1 and Kv11.1 in equine hearts suggests as in humans they are important for cardiac repolarization and we have demonstrated a functional role for IKr in equine hearts. The slower activation of equine IK7.1/KCNE1 may be an adaption to long action potential duration at rest and the slower deactivation may lead to current accumulation during fast rates and thus be important for decreasing action potential duration at faster heart rates.