Multiple Ionic Currents Research Articles

Reconstructing ventricular cardiomyocyte dynamics and parameter estimation using data-assimilation.

Cardiac ventricular myocyte action potential dynamics are regulated by intricate and nonlinear interactions between the cell transmembrane potential and ionic currents and concentrations. Present technology limits the ability to measure transmembrane potential and multiple ionic currents simultaneously, which narrows the scope of experiments to provide a complete snapshot of the cardiac myocyte state. This limitation presents an obstacle for understanding how perturbations can trigger arrhythmias and more broadly how the myocyte responds to different conditions, such as changes in pacing rate or responses to drug treatment. In this study, we demonstrate that a data-assimilation approach can successfully reconstruct and predict the dynamics of a heterogeneous virtual cardiac ventricular myocyte population in the presence of parameter uncertainty. A population of heterogeneous cardiac ventricular myocytes is generated by varying ionic current conductance parameters, and additional observational uncertainty is mimicked by the addition of Gaussian noise to the transmembrane potential. We demonstrate that the data-assimilation approach accurately reconstructs transmembrane potential, with error less than the magnitude of the noise. Further, the data-assimilation approach successfully estimates the conductances of ionic currents generally with high accuracy and requiring low computational time. As a proof of concept, we apply the data-assimilation approach to reconstruct action potential dynamics from optical mapping experiments in an ex vivo isolated guinea pig heart. Critically, we demonstrate that the ionic conductance parameters estimated from a recording at one pacing frequency can accurately predict action potential dynamics at different rates.

Single shot detection of alterations across multiple ionic currents from assimilation of cell membrane dynamics

The dysfunction of ion channels is a causative factor in a variety of neurological diseases, thereby defining the implicated channels as key drug targets. The detection of functional changes in multiple specific ionic currents currently presents a challenge, particularly when the neurological causes are either a priori unknown, or are unexpected. Traditional patch clamp electrophysiology is a powerful tool in this regard but is low throughput. Here, we introduce a single-shot method for detecting alterations amongst a range of ion channel types from subtle changes in membrane voltage in response to a short chaotically driven current clamp protocol. We used data assimilation to estimate the parameters of individual ion channels and from these we reconstructed ionic currents which exhibit significantly lower error than the parameter estimates. Such reconstructed currents thereby become sensitive predictors of functional alterations in biological ion channels. The technique correctly predicted which ionic current was altered, and by approximately how much, following pharmacological blockade of BK, SK, A-type K+ and HCN channels in hippocampal CA1 neurons. We anticipate this assay technique could aid in the detection of functional changes in specific ionic currents during drug screening, as well as in research targeting ion channel dysfunction.

Ion channels and the diversity of spontaneous firing in anterior pituitary corticotrophs: A dynamical analysis

Pituitary cells release hormones based on firing patterns, mediated by multiple ionic currents. Analyzing a single channel’s effect on firing is insufficient. We used a four-dimensional Hodgkin–Huxley-type corticotroph model to analyze its dynamics from a geometric perspective. We conducted a two-parameter bifurcation analysis of the system and found that activation levels of BK, NS, Kir, Ca, and LCa channels are crucial in generating and transitioning between different firing patterns. Chaotic firing can arise during the transition between firing patterns. We have employed the Poincaré map method and sequence reconstructions to construct return maps to characterize chaos-firing. The membrane potential time series is affected by fluctuations in intracellular calcium ([Ca2+]i). We studied the variable c using fast–slow analysis techniques and phase portraits to explore its relationship with phase portraits. The study revealed that the system exhibits bistability, which is dependent on the initial values of c or V. The findings indicate that firing activities can arise from either a supercritical or subcritical Hopf bifurcation. Analyses show the interaction between the membrane potential and [Ca2+]i at various BK channel activation levels.

Caveolin-3 and Caveolae regulate ventricular repolarization

RationaleFlask-shaped invaginations of the cardiomyocyte sarcolemma called caveolae require the structural protein caveolin-3 (Cav-3) and host a variety of ion channels, transporters, and signaling molecules. Reduced Cav-3 expression has been reported in models of heart failure, and variants in CAV3 have been associated with the inherited long-QT arrhythmia syndrome. Yet, it remains unclear whether alterations in Cav-3 levels alone are sufficient to drive aberrant repolarization and increased arrhythmia risk. ObjectiveTo determine the impact of cardiac-specific Cav-3 ablation on the electrophysiological properties of the adult mouse heart. Methods and resultsCardiac-specific, inducible Cav3 homozygous knockout (Cav-3KO) mice demonstrated a marked reduction in Cav-3 expression by Western blot and loss of caveolae by electron microscopy. However, there was no change in macroscopic cardiac structure or contractile function. The QTc interval was increased in Cav-3KO mice, and there was an increased propensity for ventricular arrhythmias. Ventricular myocytes isolated from Cav-3KO mice exhibited a prolonged action potential duration (APD) that was due to reductions in outward potassium currents (Ito, Iss) and changes in inward currents including slowed inactivation of ICa,L and increased INa,L. Mathematical modeling demonstrated that the changes in the studied ionic currents were adequate to explain the prolongation of the mouse ventricular action potential. Results from human iPSC-derived cardiomyocytes showed that shRNA knockdown of Cav-3 similarly prolonged APD. ConclusionWe demonstrate that Cav-3 and caveolae regulate cardiac repolarization and arrhythmia risk via the integrated modulation of multiple ionic currents.

Reconstructing action potential and ionic current dynamics of a mammalian ventricular cell using data assimilation.

The dynamics of the cardiac ventricular myocyte action potential (AP) is regulated by intricate and nonlinear interactions between the cell transmembrane potential, and ionic currents and concentrations. Present technology limits the ability to simultaneously measuring multiple ionic currents and concentration during an AP in in vitro single cell experiments, which limits the scope of experimental data to provide a complete snapshot of the cardiac myocyte system state. This limitation presents an obstacle for understanding how cellular perturbations can trigger arrhythmogenic conditions. In this study, we applied a data assimilation approach, which combines limited noisy observations with predictions from a computational model, paired with a parameter estimation to minimize the effects of parameter uncertainty and model error. We performed a series of in silico experiments, in which a forecasting system was tasked with reconstructing AP dynamics of a virtual ventricular myocyte in the presence varying degrees of parameter and model error. With appropriate protocol design, we find that a data assimilation approach can successfully reconstruct and predict the dynamics of not only the directly observed state variables (such as the transmembrane potential and a selected ionic current), but also unobserved state variables, such as unmeasured ionic currents and concentrations. As an extension of the results of these series of in silico experiments, we generated simulations to assess which ionic currents, when measured, offer greater predictive ability using the data assimilation algorithm. This can offer valuable information for the design of single cardiac in vitro experiments, as it is not feasible to measure all ionic currents and concentrations across time within the same cell during an AP. Future work will investigate the longer-term accuracy of this approach predicting potentially pro-arrhythmic perturbations for arrhythmia risk assessment.

Effective Perturbations by Phenobarbital on INa, IK(erg), IK(M) and IK(DR) during Pulse Train Stimulation in Neuroblastoma Neuro-2a Cells.

Phenobarbital (PHB, Luminal Sodium®) is a medication of the barbiturate and has long been recognized to be an anticonvulsant and a hypnotic because it can facilitate synaptic inhibition in the central nervous system through acting on the γ-aminobutyric acid (GABA) type A (GABAA) receptors. However, to what extent PHB could directly perturb the magnitude and gating of different plasmalemmal ionic currents is not thoroughly explored. In neuroblastoma Neuro-2a cells, we found that PHB effectively suppressed the magnitude of voltage-gated Na+ current (INa) in a concentration-dependent fashion, with an effective IC50 value of 83 µM. The cumulative inhibition of INa, evoked by pulse train stimulation, was enhanced by PHB. However, tefluthrin, an activator of INa, could attenuate PHB-induced reduction in the decaying time constant of INa inhibition evoked by pulse train stimuli. In addition, the erg (ether-à-go-go-related gene)-mediated K+ current (IK(erg)) was also blocked by PHB. The PHB-mediated inhibition on IK(erg) could not be overcome by flumazenil (GABA antagonist) or chlorotoxin (chloride channel blocker). The PHB reduced the recovery of IK(erg) by a two-step voltage protocol with a geometrics-based progression, but it increased the decaying rate of IK(erg), evoked by the envelope-of-tail method. About the M-type K+ currents (IK(M)), PHB caused a reduction of its amplitude, which could not be counteracted by flumazenil or chlorotoxin, and PHB could enhance its cumulative inhibition during pulse train stimulation. Moreover, the magnitude of delayed-rectifier K+ current (IK(DR)) was inhibited by PHB, while the cumulative inhibition of IK(DR) during 10 s of repetitive stimulation was enhanced. Multiple ionic currents during pulse train stimulation were subject to PHB, and neither GABA antagonist nor chloride channel blocker could counteract these PHB-induced reductions. It suggests that these actions might conceivably participate in different functional activities of excitable cells and be independent of GABAA receptors.

Zingerone Modulates Neuronal Voltage-Gated Na+ and L-Type Ca2+ Currents.

Zingerone (ZO), a nontoxic methoxyphenol, has been demonstrated to exert various important biological effects. However, its action on varying types of ionic currents and how they concert in neuronal cells remain incompletely understood. With the aid of patch clamp technology, we investigated the effects of ZO on the amplitude, gating, and hysteresis of plasmalemmal ionic currents from both pituitary tumor (GH3) cells and hippocampal (mHippoE-14) neurons. The exposure of the GH3 cells to ZO differentially diminished the peak and late components of the INa. Using a double ramp pulse, the amplitude of the INa(P) was measured, and the appearance of a hysteresis loop was observed. Moreover, ZO reversed the tefluthrin-mediated augmentation of the hysteretic strength of the INa(P) and led to a reduction in the ICa,L. As a double ramp pulse was applied, two types of voltage-dependent hysteresis loops were identified in the ICa,L, and the replacement with BaCl2-attenuated hysteresis of the ICa,L enhanced the ICa,L amplitude along with the current amplitude (i.e., the IBa). The hysteretic magnitude of the ICa,L activated by the double pulse was attenuated by ZO. The peak and late INa in the hippocampal mHippoE-14 neurons was also differentially inhibited by ZO. In addition to acting on the production of reactive oxygen species, ZO produced effects on multiple ionic currents demonstrated herein that, considered together, may significantly impact the functional activities of neuronal cells.

Integrated electrophysiologic analyses of amiodarone in human iPSC-derived cardiomyocytes

Abnormalities in cardiac rhythm, or arrhythmia, affect more than 2% of individuals and can cause sudden cardiac death. Amiodarone, the most frequently prescribed antiarrhythmic drug, is effective yet exhibits significant toxicity. Amiodarone is extensively metabolized and displays a prolonged elimination half-life, lipophilic partitioning, and blockade of multiple ionic currents that form the cardiac action potential, complicating characterization of its therapeutic mechanisms. Further constraining this effort is the limited availability of clinical samples of healthy human cardiac cells and tissues.

Use of hiPSC-Derived Cardiomyocytes to Rule Out Proarrhythmic Effects of Drugs: The Case of Hydroxychloroquine in COVID-19.

In the early phases of the COVID-19 pandemic, drug repurposing was widely used to identify compounds that could improve the prognosis of symptomatic patients infected by SARS-CoV-2. Hydroxychloroquine (HCQ) was one of the first drugs used to treat COVID-19 due to its supposed capacity of inhibiting SARS-CoV-2 infection and replication in vitro. While its efficacy is debated, HCQ has been associated with QT interval prolongation and potentially Torsades de Pointes, especially in patients predisposed to developing drug-induced Long QT Syndrome (LQTS) as silent carriers of variants associated with congenital LQTS. If confirmed, these effects represent a limitation to the at-home use of HCQ for COVID-19 infection as adequate ECG monitoring is challenging. We investigated the proarrhythmic profile of HCQ with Multi-Electrode Arrays after exposure of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) from two healthy donors, one asymptomatic and two symptomatic LQTS patients. We demonstrated that: I) HCQ induced a concentration-dependent Field Potential Duration (FPD) prolongation and halted the beating at high concentration due to the combined effect of HCQ on multiple ion currents. II) hiPSC-CMs from healthy or asymptomatic carriers tolerated higher concentrations of HCQ and showed lower susceptibility to HCQ-induced electrical abnormalities regardless of baseline FPD. These findings agree with the clinical safety records of HCQ and demonstrated that hiPSC-CMs potentially discriminates symptomatic vs. asymptomatic mutation carriers through pharmacological interventions. Disease-specific cohorts of hiPSC-CMs may be a valid preliminary addition to assess drug safety in vulnerable populations, offering rapid preclinical results with valuable translational relevance for precision medicine.

Cardiac Toxicity of Cadmium Involves Complex Interactions Among Multiple Ion Currents in Rainbow Trout (Oncorhynchus mykiss) Ventricular Myocytes.

Cadmium (Cd2+ ) is cardiotoxic to fish, but its effect on the electrical excitability of cardiac myocytes is largely unknown. To this end, we used the whole-cell patch-clamp method to investigate the effects of Cd2+ on ventricular action potentials (APs) and major ion currents in rainbow trout (Oncorhynchus mykiss) ventricular myocytes. Trout were acclimated to +4 °C, and APs were measured at the acclimated temperature and elevated temperature (+18 °C). Cd2+ (10, 20, and 100 µM) altered the shape of the ventricular AP in a complex manner. The early plateau fell to less positive membrane voltages, and the total duration of AP prolonged. These effects were obvious at both +4 °C and +18 °C. The depression of the early plateau is due to the strong Cd2+ -induced inhibition of the L-type calcium (Ca2+ ) current (ICaL ), whereas the prolongation of the AP is an indirect consequence of the ICaL inhibition: at low voltages of the early plateau, the delayed rectifier potassium (K+ ) current (IKr ) remains small, delaying repolarization of AP. Cd2+ reduced the density and slowed the kinetics of the Na+ current (INa ) but left the inward rectifier K+ current (IK1 ) intact. These altered cellular and molecular functions can explain several Cd2+ -induced changes in impulse conduction of the fish heart, for example, slowed propagation of the AP in atrial and ventricular myocardia (inhibition of INa ), delayed relaxation of the ventricle (prolongation of ventricular AP duration), bradycardia, and atrioventricular block (inhibition of ICaL ). These findings indicate that the cardiotoxicity of Cd2+ in fish involves multiple ion currents that are directly and indirectly altered by Cd2+ . Through these mechanisms, Cd2+ may trigger cardiac arrhythmias and impair myocardial contraction. Elevated temperature (+18 °C) slightly increases Cd2+ toxicity in trout ventricular myocytes. Environ Toxicol Chem 2021;40:2874-2885. © 2021 SETAC.

Comodulation of h- and Na+/K+ Pump Currents Expands the Range of Functional Bursting in a Central Pattern Generator by Navigating between Dysfunctional Regimes.

Central pattern generators (CPGs), specialized oscillatory neuronal networks controlling rhythmic motor behaviors such as breathing and locomotion, must adjust their patterns of activity to a variable environment and changing behavioral goals. Neuromodulation adjusts these patterns by orchestrating changes in multiple ionic currents. In the medicinal leech, the endogenous neuromodulator myomodulin speeds up the heartbeat CPG by reducing the electrogenic Na+/K+ pump current and increasing h-current in pairs of mutually inhibitory leech heart interneurons (HNs), which form half-center oscillators (HN HCOs). Here we investigate whether the comodulation of two currents could have advantages over a single current in the control of functional bursting patterns of a CPG. We use a conductance-based biophysical model of an HN HCO to explain the experimental effects of myomodulin. We demonstrate that, in the model, comodulation of the Na+/K+ pump current and h-current expands the range of functional bursting activity by avoiding transitions into nonfunctional regimes, such as asymmetric bursting and plateau-containing seizure-like activity. We validate the model by finding parameters that reproduce temporal bursting characteristics matching experimental recordings from HN HCOs under control, three different myomodulin concentrations, and Cs+ treated conditions. The matching cases are located along the border of an asymmetric regime away from the border with more dangerous seizure-like activity. We found a simple comodulation mechanism with an inverse relation between the pump and h-currents makes a good fit of the matching cases and comprises a general mechanism for the robust and flexible control of oscillatory neuronal networks.SIGNIFICANCE STATEMENT Rhythm-generating neuronal circuits adjust their oscillatory patterns to accommodate a changing environment through neuromodulation. In different species, chemical messengers participating in such processes may target two or more membrane currents. In medicinal leeches, the neuromodulator myomodulin speeds up the heartbeat central pattern generator by reducing Na+/K+ pump current and increasing h-current. In a computational model, we show that this comodulation expands the range of central pattern generator's functional activity by navigating the circuit between dysfunctional regimes resulting in a much wider range of cycle period. This control would not be attainable by modulating only one current, emphasizing the synergy of combined effects. Given the prevalence of h-current and Na+/K+ pump current in neurons, similar comodulation mechanisms may exist across species.

Effects of Sesamin, the Major Furofuran Lignan of Sesame Oil, on the Amplitude and Gating of Voltage-Gated Na+ and K+ Currents.

Sesamin (SSM) and sesamolin (SesA) are the two major furofuran lignans of sesame oil and they have been previously noticed to exert various biological actions. However, their modulatory actions on different types of ionic currents in electrically excitable cells remain largely unresolved. The present experiments were undertaken to explore the possible perturbations of SSM and SesA on different types of ionic currents, e.g., voltage-gated Na+ currents (INa), erg-mediated K+ currents (IK(erg)), M-type K+ currents (IK(M)), delayed-rectifier K+ currents (IK(DR)) and hyperpolarization-activated cation currents (Ih) identified from pituitary tumor (GH3) cells. The exposure to SSM or SesA depressed the transient and late components of INa with different potencies. The IC50 value of SSM needed to lessen the peak or sustained INa was calculated to be 7.2 or 0.6 μM, while that of SesA was 9.8 or 2.5 μM, respectively. The dissociation constant of SSM-perturbed inhibition on INa, based on the first-order reaction scheme, was measured to be 0.93 μM, a value very similar to the IC50 for its depressant action on sustained INa. The addition of SSM was also effective at suppressing the amplitude of resurgent INa. The addition of SSM could concentration-dependently inhibit the IK(M) amplitude with an IC50 value of 4.8 μM. SSM at a concentration of 30 μM could suppress the amplitude of IK(erg), while at 10 μM, it mildly decreased the IK(DR) amplitude. However, the addition of neither SSM (10 μM) nor SesA (10 μM) altered the amplitude or kinetics of Ih in response to long-lasting hyperpolarization. Additionally, in this study, a modified Markovian model designed for SCN8A-encoded (or NaV1.6) channels was implemented to evaluate the plausible modifications of SSM on the gating kinetics of NaV channels. The model demonstrated herein was well suited to predict that the SSM-mediated decrease in peak INa, followed by increased current inactivation, which could largely account for its favorable decrease in the probability of the open-blocked over open state of NaV channels. Collectively, our study provides evidence that highlights the notion that SSM or SesA could block multiple ion currents, such as INa and IK(M), and suggests that these actions are potentially important and may participate in the functional activities of various electrically excitable cells in vivo.

MicroRNAs Contribute to Angiotensin II‐Mediated Regulation of Transmural Electrical Remodeling in the Heart

Activation of the local renin angiotensin system (RAS) by Angiotensin II (Ang II) in the heart induces electrical remodeling, including inhibition of transient outward potassium current Ito, prolongation of the action potential, and increased contractility. Moreover, the long‐term, persistent action of Ang II causes cardiac hypertrophy and heart failure (anatomic remodeling). The transmural gradient of multiple ionic currents, including Ito, is involved in Ang II‐mediated electrical remodeling. Differential expression of miRNAs may play a critical role in the signaling cascade for Ang II‐induced electrical remodeling. To investigate this hypothesis, a detailed analysis of potential miRNAs involved in Ang II‐mediated modulation was undertaken using miRNA microarray analysis of endocardium and epicardium tissue from the left ventricle of a Sprague‐Dawley rat. The left ventricular tissue samples were treated with either Ang II (2mM) or Losartan (2mM) to further screen for differences in miRNA expression between EPI and ENDO. Although forty‐nine miRNAs were expressed in a significant gradient between EPI and ENDO, only miR‐385‐5p and miR‐301a‐3p were found to exhibit similar expression patterns as Ito when modulated by Ang II. Taken together, these data suggest that miR‐382‐5p and miR‐301a‐3p could contribute to Ang II‐mediated transmural electrical remodeling and may provide a potential target for the study of heart failure mechanisms and therapeutic application.Expression of miR‐382‐5p and miR‐301a‐3p in EPI and ENDO.A. Left: miR‐382‐5p is upregulated in EPI compared to ENDO. Middle: miR‐382‐5p is upregulated in EPI compared to EPI with Ang II. Right: miR‐382‐5p is downregulated in ENDO compared to ENDO with Losartan.B. Left: miR‐301a‐3p is downregulated in EPI compared to ENDO. Middle: miR‐301a‐3p is downregulated in EPI compared to EPI with Ang II. Right: miR‐301a‐3p is upregulated in ENDO compared to ENDO with Losartan.Figure 1Schematic diagram of Ang II‐mediated anatomic and electrical remodeling in the heart. The transmural distribution of Ang II causes the transmural gradient of ionic currents including Ito, Ip, ICaL and ICaTL (electrical remodeling). Under long term imbalances between volume load and maximum contractile strength, the heart undergoes hypertrophy and heart failure (anatomic remodeling). We hypothesize that miR‐382‐5p and miR‐301a‐3p contribute to Ang‐II mediated remodeling of the heart.Figure 2

T vector velocity: A new ECG biomarker for identifying drug effects on cardiac ventricular repolarization.

BackgroundWe present a new family of ECG biomarkers for assessing drug effects on ventricular repolarization. We show that drugs blocking inward (depolarizing) ion currents cause a relative increase of the T vector velocity (TVV) and accelerate repolarization, while drugs blocking outward ion currents cause a relative decrease of the TVV and delay repolarization. The results suggest a link between the TVV and the instantaneous change of the cellular action potentials that may contribute to bridge the gap between the surface ECG and myocardial cellular processes.MethodsWe measure TVV as the time required to reach X% of the total Trajectory length of the T vector loop, denoted as TrX. Applied to data from two FDA funded studies (22+22 subjects, 5232+4208 ECGs) which target ECG effects of various ion-channel blocking drugs, the TrX effect profiles indicate increasingly delayed electrical activity over the entire repolarization process for drugs solely reducing outward potassium current (dofetilide, moxifloxacin). For drugs eliciting block of the inward sodium or calcium currents (mexiletine, lidocaine), the TrX effect profiles were consistent with accelerated electrical activity in the initial repolarization phase. For multichannel blocking drugs (ranolazine) or drug combinations blocking multiple ion currents (dofetilide + mexiletine, dofetilide + lidocaine), the overall TrX effect profiles indicate a superposition of the individual TrX effect profiles.ResultsThe parameter Tr40c differentiates pure potassium channel blocking drugs from multichannel blocking drugs with an area under the ROC curve (AUC) of 0.90, CI = [0.88 to 0.92]. This is significantly better than the performance of J-Tpeakc (0.81, CI = [0.78 to 0.84]) identified as the best parameter in the second FDA study. Combining the ten parameters Tr10c to Tr100c in a logistic regression model further improved the AUC to 0.94, CI = [0.92 to 0.96].ConclusionsTVV analysis substantially improves assessment of drug effects on cardiac repolarization, providing a plausible and improved mechanistic link between drug effects on ionic currents and overall ventricular repolarization reflected in the body surface ECG. TVV contributes to an enhanced appraisal of the proarrhythmic risk of drugs beyond QTc prolongation and J-Tpeakc.

Long QT syndrome caveolin-3 mutations differentially modulate Kv 4 and Cav 1.2 channels to contribute to action potential prolongation.

Mutations in the caveolae scaffolding protein, caveolin-3 (Cav3), have been linked to the long QT type 9 inherited arrhythmia syndrome (LQT9) and the cause of underlying action potential duration prolongation is incompletely understood. In the present study, we show that LQT9 Cav3 mutations, F97C and S141R, cause mutation-specific gain of function effects on Cav 1.2-encoded L-type Ca2+ channels responsible for ICa,L and also cause loss of function effects on heterologously expressed Kv 4.2 and Kv 4.3 channels responsible for Ito . A computational model of the human ventricular myocyte action potential suggests that the major ionic current change causing action potential duration prolongation in the presence of Cav3-F97C is the slowly inactivating ICa,L but, for Cav3-S141R, both increased ICa,L and increased late Na+ current contribute equally to action potential duration prolongation. Overall, the LQT9 Cav3-F97C and Cav3-S141R mutations differentially impact multiple ionic currents, highlighting the complexity of Cav3 regulation of cardiac excitability and suggesting mutation-specific therapeutic approaches. Mutations in the CAV3 gene encoding caveolin-3 (Cav3), a scaffolding protein integral to caveolae in cardiomyocytes, have been associated with the congenital long-QT syndrome (LQT9). Initial studies demonstrated that LQT9-associated Cav3 mutations, F97C and S141R, increase late sodium current as a potential mechanism to prolong action potential duration (APD) and cause LQT9. Whether these Cav3 LQT9 mutations impact other caveolae related ion channels remains unknown. We used the whole-cell, patch clamp technique to characterize the effect of Cav3-F97C and Cav3-S141R mutations on heterologously expressed Cav 1.2+Cav β2cN4 channels, as well as Kv 4.2 and Kv 4.3 channels, in HEK 293 cells. Expression of Cav3-S141R increased ICa,L density without changes in gating properties, whereas expression of Cav3-F97C reduced Ca2+ -dependent inactivation of ICa,L without changing current density. The Cav3-F97C mutation reduced current density and altered the kinetics of IKv4.2 and IKv4.3 and also slowed recovery from inactivation. Cav3-S141R decreased current density and also slowed activation kinetics and recovery from inactivation of IKv4.2 but had no effect on IKv4.3 . Using the O'Hara-Rudy computational model of the human ventricular myocyte action potential, the Cav3 mutation-induced changes in Ito are predicted to have negligible effect on APD, whereas blunted Ca2+ -dependent inactivation of ICa,L by Cav3-F97C is predicted to be primarily responsible for APD prolongation, although increased ICa,L and late INa by Cav3-S141R contribute equally to APD prolongation. Thus, LQT9 Cav3-associated mutations, F97C and S141R, produce mutation-specific changes in multiple ionic currents leading to different primary causes of APD prolongation, which suggests the use of mutation-specific therapeutic approaches in the future.

Real Patient and its Virtual Twin: Application of Quantitative Systems Toxicology Modelling in the Cardiac Safety Assessment of Citalopram

A quantitative systems toxicology (QST) model for citalopram was established to simulate, in silico, a ‘virtual twin’ of a real patient to predict the occurrence of cardiotoxic events previously reported in patients under various clinical conditions. The QST model considers the effects of citalopram and its most notable electrophysiologically active primary (desmethylcitalopram) and secondary (didesmethylcitalopram) metabolites, on cardiac electrophysiology. The in vitro cardiac ion channel current inhibition data was coupled with the biophysically detailed model of human cardiac electrophysiology to investigate the impact of (i) the inhibition of multiple ion currents (IKr, IKs, ICaL); (ii) the inclusion of metabolites in the QST model; and (iii) unbound or total plasma as the operating drug concentration, in predicting clinically observed QT prolongation. The inclusion of multiple ion channel current inhibition and metabolites in the simulation with unbound plasma citalopram concentration provided the lowest prediction error. The predictive performance of the model was verified with three additional therapeutic and supra-therapeutic drug exposure clinical cases. The results indicate that considering only the hERG ion channel inhibition of only the parent drug is potentially misleading, and the inclusion of active metabolite data and the influence of other ion channel currents should be considered to improve the prediction of potential cardiac toxicity. Mechanistic modelling can help bridge the gaps existing in the quantitative translation from preclinical cardiac safety assessment to clinical toxicology. Moreover, this study shows that the QST models, in combination with appropriate drug and systems parameters, can pave the way towards personalised safety assessment.

Electrophysiological heterogeneity of pacemaker cells in the rabbit intercaval region, including the SA node: insights from recording multiple ion currents in each cell.

Cardiac pacemaker cells, including cells of the sinoatrial node, are heterogeneous in size, morphology, and electrophysiological characteristics. The exact extent to which these cells differ electrophysiologically is unclear yet is critical to understanding their functioning. We examined major ionic currents in individual intercaval pacemaker cells (IPCs) sampled from the paracristal, intercaval region (including the sinoatrial node) that were spontaneously beating after enzymatic isolation from rabbit hearts. The beating rate was measured at baseline and after inhibition of the Ca2+ pump with cyclopiazonic acid. Thereafter, in each cell, we consecutively measured the density of funny current ( If), delayed rectifier K+ current ( IK) (a surrogate of repolarization capacity), and L-type Ca2+ current ( ICa,L) using whole cell patch clamp. The ionic current densities varied to a greater extent than previously appreciated, with some IPCs demonstrating very small or zero If . The density of none of the currents was correlated with cell size, while ICa,L and If densities were related to baseline beating rates. If density was correlated with IK density but not with that of ICa,L. Inhibition of Ca2+ cycling had a greater beating rate slowing effect in IPCs with lower If densities. Our numerical model simulation indicated that 1) IPCs with small (or zero) If or small ICa,L can operate via a major contribution of Ca2+ clock, 2) If-Ca2+-clock interplay could be important for robust pacemaking function, and 3) coupled If- IK function could regulate maximum diastolic potential. Thus, we have demonstrated marked electrophysiological heterogeneity of IPCs. This heterogeneity is manifested in basal beating rate and response to interference of Ca2+ cycling, which is linked to If. NEW & NOTEWORTHY In the present study, a hitherto unrecognized range of heterogeneity of ion currents in pacemaker cells from the intercaval region is demonstrated. Relationships between basal beating rate and L-type Ca2+ current and funny current ( If) density are uncovered, along with a positive relationship between If and delayed rectifier K+ current. Links are shown between the response to Ca2+ cycling blockade and If density.

Mechanisms of generation of membrane potential resonance in a neuron with multiple resonant ionic currents.

Neuronal membrane potential resonance (MPR) is associated with subthreshold and network oscillations. A number of voltage-gated ionic currents can contribute to the generation or amplification of MPR, but how the interaction of these currents with linear currents contributes to MPR is not well understood. We explored this in the pacemaker PD neurons of the crab pyloric network. The PD neuron MPR is sensitive to blockers of H- (IH) and calcium-currents (ICa). We used the impedance profile of the biological PD neuron, measured in voltage clamp, to constrain parameter values of a conductance-based model using a genetic algorithm and obtained many optimal parameter combinations. Unlike most cases of MPR, in these optimal models, the values of resonant- (fres) and phasonant- (fϕ = 0) frequencies were almost identical. Taking advantage of this fact, we linked the peak phase of ionic currents to their amplitude, in order to provide a mechanistic explanation the dependence of MPR on the ICa gating variable time constants. Additionally, we found that distinct pairwise correlations between ICa parameters contributed to the maintenance of fres and resonance power (QZ). Measurements of the PD neuron MPR at more hyperpolarized voltages resulted in a reduction of fres but no change in QZ. Constraining the optimal models using these data unmasked a positive correlation between the maximal conductances of IH and ICa. Thus, although IH is not necessary for MPR in this neuron type, it contributes indirectly by constraining the parameters of ICa.

Modification of multiple ion channel functions in vivo by pharmacological inhibition: observation by threshold tracking and modeling.

Maintenance of axonal excitability relies on complex balance by multiple ion currents, but its evaluation is limited by in vitro single channel neurophysiological study on overall behavior. We sought to evaluate behaviors of multiple ion currents by pharmacological blockade. The threshold tracking technique was used to measure multiple excitability indices on tail sensory nerve of normal male mice before and after administration of either BaCl2 or ivabradine. Mathematical modeling was used to identify the interval changes of the channel parameters. After administration of BaCl2 and ivabradine, the following changes were present: greater threshold changes of both depolarizing and hyperpolarizing threshold electrotonus by both; additionally, reduced S2 accommodation, reduced late subexcitability and increased superexcitability by BaCl2, increased S3 accommodation by ivabradine. Mathematical modelling implied reduction of slow K+ conductance, along with reduction of H conductance (Ih) by BaCl2; and reduction of Ih while augmentation of K+ conductances by ivabradine. Pharmacological blockade of a selective ion channel may be compensated by other ion channels. Unintended effects by ion channel modification could be caused by secondary current alteration by multiple ion channels. J. Med. Invest. 64: 30-38, February, 2017.

Cardiac Arrhythmias: Update on Mechanisms and Clinical Managements.

Cardiac Arrhythmias: Update on Mechanisms and Clinical Managements.