Action Potential Characteristics Research Articles

Computer simulation for localization and extensiveness of myocardial ischemia

Features of genesis of the electrocardiosignal in conditions of the myocardial ischemia under changing localization and extensiveness of the ischemia foci, as well as an impact of this process on the repolarization parameters of the electrocardiosignal were investigated using a system for 3D simulation of the heart electrical activity. The model of the ventricles of the heart is a cellular automaton with the matrix about of 10(5) elements of the "Myocardium", "His", and "Purkinje" types and with prescribed rules of their interaction. In process of the study a developed earlier method of segmentation of the modeled ventricles was used. For estimating the contribution of the "ischemic" segments to the total signal of different leads of the electrocardiograms the previously suggested method of the partial curves was used. The ischemia foci were simulated by changing the characteristics of the transmembrane action potential in selected areas of the ventricles, such as the resting potential, transmembrane action potential amplitude, duration of the plateau phase, and the steepness of the final phase of rapid repolarization in accordance with literature data. Electrocardiosignals generated by the model were processed to obtain previously selected parameters, characterizing repolarization process. The influence of size and localization of the myocardial ischemia on the parameters obtained are examined, as well-as some hypotheses about their relation with an electrophysiological state of the heart muscle are discussed.

Distinct subclassification of DRG neurons innervating the distal colon and glans penis/distal urethra based on the electrophysiological current signature.

Spinal sensory neurons innervating visceral and mucocutaneous tissues have unique microanatomic distribution, peripheral modality, and physiological, pharmacological, and biophysical characteristics compared with those neurons that innervate muscle and cutaneous tissues. In previous patch-clamp electrophysiological studies, we have demonstrated that small- and medium-diameter dorsal root ganglion (DRG) neurons can be subclassified on the basis of their patterns of voltage-activated currents (VAC). These VAC-based subclasses were highly consistent in their action potential characteristics, responses to algesic compounds, immunocytochemical expression patterns, and responses to thermal stimuli. For this study, we examined the VAC of neurons retrogradely traced from the distal colon and the glans penis/distal urethra in the adult male rat. The afferent population from the distal colon contained at least two previously characterized cell types observed in somatic tissues (types 5 and 8), as well as four novel cell types (types 15, 16, 17, and 18). In the glans penis/distal urethra, two previously described cell types (types 6 and 8) and three novel cell types (types 7, 14, and 15) were identified. Other characteristics, including action potential profiles, responses to algesic compounds (acetylcholine, capsaicin, ATP, and pH 5.0 solution), and neurochemistry (expression of substance P, CGRP, neurofilament, TRPV1, TRPV2, and isolectin B4 binding) were consistent for each VAC-defined subgroup. With identification of distinct DRG cell types that innervate the distal colon and glans penis/distal urethra, future in vitro studies related to the gastrointestinal and urogenital sensory function in normal as well as abnormal/pathological conditions may be benefitted.

Energy demands of diverse spiking cells from the neocortex, hippocampus, and thalamus.

It has long been known that neurons in the brain are not physiologically homogeneous. In response to current stimulus, they can fire several distinct patterns of action potentials that are associated with different physiological classes ranging from regular-spiking cells, fast-spiking cells, intrinsically bursting cells, and low-threshold cells. In this work we show that the high degree of variability in firing characteristics of action potentials among these cells is accompanied with a significant variability in the energy demands required to restore the concentration gradients after an action potential. The values of the metabolic energy were calculated for a wide range of cell temperatures and stimulus intensities following two different approaches. The first one is based on the amount of Na+ load crossing the membrane during a single action potential, while the second one focuses on the electrochemical energy functions deduced from the dynamics of the computational neuron models. The results show that the thalamocortical relay neuron is the most energy-efficient cell consuming between 7 and 18 nJ/cm2 for each spike generated, while both the regular and fast spiking cells from somatosensory cortex and the intrinsically-bursting cell from a cat visual cortex are the least energy-efficient, and can consume up to 100 nJ/cm2 per spike. The lowest values of these energy demands were achieved at higher temperatures and high external stimuli.

A device for emulating cuff recordings of action potentials propagating along peripheral nerves.

This paper describes a device that emulates propagation of action potentials along a peripheral nerve, suitable for reproducible testing of bio-potential recording systems using nerve cuff electrodes. The system is a microcontroller-based stand-alone instrument which uses established nerve and electrode models to represent neural activity of real nerves recorded with a nerve cuff interface, taking into consideration electrode impedance, voltages picked up by the electrodes, and action potential propagation characteristics. The system emulates different scenarios including compound action potentials with selectable propagation velocities and naturally occurring nerve traffic from different velocity fiber populations. Measured results from a prototype implementation are reported and compared with in vitro recordings from Xenopus Laevis frog sciatic nerve, demonstrating that the electrophysiological setting is represented to a satisfactory degree, useful for the development, optimization and characterization of future recording systems.

Effects of endogenous cannabinoid anandamide on excitation–contraction coupling in rat ventricular myocytes

A role for anandamide (N-arachidonoyl ethanolamide; AEA), a major endocannabinoid, in the cardiovascular system in various pathological conditions has been reported in earlier reports. In the present study, the effects of AEA on contractility, Ca2+ signaling, and action potential (AP) characteristics were investigated in rat ventricular myocytes. Video edge detection was used to measure myocyte shortening. Intracellular Ca2+ was measured in cells loaded with the fluorescent indicator fura-2 AM. AEA (1μM) caused a significant decrease in the amplitudes of electrically evoked myocyte shortening and Ca2+ transients. However, the amplitudes of caffeine-evoked Ca2+ transients and the rate of recovery of electrically evoked Ca2+ transients following caffeine application were not altered. Biochemical studies in sarcoplasmic reticulum (SR) vesicles from rat ventricles indicated that AEA affected Ca2+-uptake and Ca2+-ATPase activity in a biphasic manner. [3H]-ryanodine binding and passive Ca2+ release from SR vesicles were not altered by 10μM AEA. Whole-cell patch-clamp technique was employed to investigate the effect of AEA on the characteristics of APs. AEA (1μM) significantly decreased the duration of AP. The effect of AEA on myocyte shortening and AP characteristics was not altered in the presence of pertussis toxin (PTX, 2μg/ml for 4h), AM251 and SR141716 (cannabinoid type 1 receptor antagonists; 0.3μM) or AM630 and SR 144528 (cannabinoid type 2 receptor antagonists; 0.3μM). The results suggest that AEA depresses ventricular myocyte contractility by decreasing the action potential duration (APD) in a manner independent of CB1 and CB2 receptors.

Increased Activity of Nav1.9 Sodium Channels Causes Loss of Pain Perception

Nociceptive sensory neurons transmit painful stimuli from the periphery to the central nervous system. Voltage-gated sodium (NaV) channels are instrumental for the generation of the corresponding electrical signals, but - so far - the sensory neuron-specific NaV1.9 only appeared to play a minor role. Recently, we identified a de novo heterozygous mutation in the SCN11A gene encoding for NaV1.9 and demonstrated the implication of NaV1.9 in human pain perception (Leipold et al., Nature Genetics, DOI 10.1038/ng.2767). Surprisingly, in affected individuals the mutation (L811P) leads to an inability to experience pain by conferring gain-of-function (GOF) properties to NaV1.9. Mutant channels activate at hyperpolarized voltages and display a slow-down of channel inactivation and deactivation, thereby causing sustained depolarization of nociceptor cells and alterations of action potential characteristics. This new channelopathy is different from other pain-related NaV channel disorders: a GOF of homologous NaV1.7 channels is associated with chronic pain, and loss of functional NaV1.7 channels causes congenital indifference to pain. To gain further insight into the mechanisms underlying the mutation we introduced the homologous L-to-P mutation into channel isoforms NaV1.4 (L802P), NaV1.7 (L957P) and NaV1.8 (L890P) and compared the functional parameters of wild-type and mutant channels by means of the whole-cell patch-clamp technique. The L-to-P mutation shifted the activation in all channel subtypes to hyperpolarized potentials in the order NaV1.9 > NaV1.7 > NaV1.4 > NaV1.8. Fast channel inactivation was slowed down most prominently in NaV1.9 and NaV1.8, followed by NaV1.7 and NaV1.4. These results show that a homologous L-to-P mutation affects channel activation and inactivation in a subtype-specific manner.

Spreading depression triggers ictaform activity in partially disinhibited neuronal tissues

There is unequivocal electrophysiological evidence that spreading depression (SD) can trigger epileptiform field potentials. In vitro experiments on human brain tissues indicated that γ-aminobutyric acid (GABA)-mediated inhibition prevented this process. Intra- and extracellular recordings of bioelectrical activities were performed in the rodent neocortex, hippocampus and amygdala after perfusion of low concentrations of the GABAA antagonist bicuculline and induction of SD by KCl application. Induction of SD in combined amygdala–hippocampus–cortex slices pre-treated with low concentration of bicuculline triggered epileptiform burst discharges in cortical as well as subcortical brain structures. Propagation of SD significantly depolarized the membrane, decreased the amplitude and duration of action potentials (APs) and after-hyperpolarization as well as the neuronal membrane input resistance and the amplitude of threshold potentials. Ten to twenty minutes after induction of SD, the pattern of APs changed from regular firing to a series of APs riding on an underlying paroxysmal depolarization shift before the appearance of typical ictaform activities. Changes of characteristic features of APs occurred after SD persisted during the appearance of epileptiform activities. These results indicate that SD increases neuronal excitability and facilitates synchronization of neuronal discharges in the presence of partial disinhibition of neuronal tissues. Our findings might explain the occurrence of seizures in neurological disorders with partial impairment of inhibitory tone, such as brain ischemia and epilepsy.

Effect of long-chain triglyceride lipid emulsion on bupivacaine-induced changes in electrophysiological parameters of rabbit Purkinje cells

Lipid emulsions are used in the reversal of local anesthetic toxicity. The aim of this study was to investigate the cellular electrophysiological effects of long-chain triglyceride lipid emulsion (LCTE) on cardiac action potential characteristics and conduction disturbances induced by bupivacaine. Purkinje fibers were dissected from the left ventricle of New Zealand white rabbit hearts and superfused with either Tyrode's solution during 30 min (control group), with bupivacaine 10(-6) M, 10(-5) M, and 5.10(-5) M alone, or in the presence of LCTE 0.5%, in addition, LCTE at 0.1%, 0.5%, and 1% was perfused alone. Electrophysiological parameters were recorded using the conventional microelectrode technique (37 °C, 1 Hz frequency). Bupivacaine 5.10(-5) M-induced conduction blocks (8/8 preparations): LCTE 0.5% suppressed the bupivacaine 5.10(-5) M-induced conduction blocks (1/8 preparations). Exposure to bupivacaine 10(-6) M, 10(-5) M, and 5.10(-5) M resulted in a significant decrease in the maximal rate of depolarization (Vmax) (respectively, 25%, 55%, 75%; P < 0.002 vs. control group). In the presence of LCTE 0.5%, bupivacaine 10(-6) M did not significantly decreased Vmax (13%; P = 0.10 vs. control group). The decrease in Vmax resulting from bupivacaine 10(-5) M alone was significantly less in the presence of LCTE 0.5% (P < 0.01 vs. bupivacaine 10(-5) M alone). Exposure to bupivacaine 10(-6) M, 10(-5) M, and 5.10(-5) M alone or in the presence of LCTE 0.5% resulted in a significant decrease in action potential duration measured at 50% and 90% repolarization (APD50 and APD90; P < 0.01 vs. control group). LCTE inhibited the Purkinje fibers conduction blocks induced by bupivacaine. Moreover, LCTE 0.5% attenuates the decrease in Vmax induced by bupivacaine 10(-6) M and 10(-5) M.

Functional, Anatomical, and Molecular Investigation of the Cardiac Conduction System and Arrhythmogenic Atrioventricular Ring Tissue in the Rat Heart

BackgroundThe cardiac conduction system consists of the sinus node, nodal extensions, atrioventricular (AV) node, penetrating bundle, bundle branches, and Purkinje fibers. Node‐like AV ring tissue also exists at the AV junctions, and the right and left rings unite at the retroaortic node. The study aims were to (1) construct a 3‐dimensional anatomical model of the AV rings and retroaortic node, (2) map electrical activation in the right ring and study its action potential characteristics, and (3) examine gene expression in the right ring and retroaortic node.Methods and ResultsThree‐dimensional reconstruction (based on magnetic resonance imaging, histology, and immunohistochemistry) showed the extent and organization of the specialized tissues (eg, how the AV rings form the right and left nodal extensions into the AV node). Multiextracellular electrode array and microelectrode mapping of isolated right ring preparations revealed robust spontaneous activity with characteristic diastolic depolarization. Using laser microdissection gene expression measured at the mRNA level (using quantitative PCR) and protein level (using immunohistochemistry and Western blotting) showed that the right ring and retroaortic node, like the sinus node and AV node but, unlike ventricular muscle, had statistically significant higher expression of key transcription factors (including Tbx3, Msx2, and Id2) and ion channels (including HCN4, Cav3.1, Cav3.2, Kv1.5, SK1, Kir3.1, and Kir3.4) and lower expression of other key ion channels (Nav1.5 and Kir2.1).ConclusionsThe AV rings and retroaortic node possess gene expression profiles similar to that of the AV node. Ion channel expression and electrophysiological recordings show the AV rings could act as ectopic pacemakers and a source of atrial tachycardia.

Atrium-Specific Kir3.x Determines Inducibility, Dynamics, and Termination of Fibrillation by Regulating Restitution-Driven Alternans

Atrial fibrillation is the most common cardiac arrhythmia. Ventricular proarrhythmia hinders pharmacological atrial fibrillation treatment. Modulation of atrium-specific Kir3.x channels, which generate a constitutively active current (I(K,ACh-c)) after atrial remodeling, might circumvent this problem. However, it is unknown whether and how I(K,ACh-c) contributes to atrial fibrillation induction, dynamics, and termination. Therefore, we investigated the effects of I(K,ACh-c) blockade and Kir3.x downregulation on atrial fibrillation. Neonatal rat atrial cardiomyocyte cultures and intact atria were burst paced to induce reentry. To study the effects of Kir3.x on action potential characteristics and propagation patterns, cultures were treated with tertiapin or transduced with lentiviral vectors encoding Kcnj3- or Kcnj5-specific shRNAs. Kir3.1 and Kir3.4 were expressed in atrial but not in ventricular cardiomyocyte cultures. Tertiapin prolonged action potential duration (APD; 54.7±24.0 to 128.8±16.9 milliseconds; P<0.0001) in atrial cultures during reentry, indicating the presence of I(K,ACh-c). Furthermore, tertiapin decreased rotor frequency (14.4±7.4 to 6.6±2.0 Hz; P<0.05) and complexity (6.6±7.7 to 0.6±0.8 phase singularities; P<0.0001). Knockdown of Kcnj3 or Kcnj5 gave similar results. Blockade of I(K,ACh-c) prevented/terminated reentry by prolonging APD and changing APD and conduction velocity restitution slopes, thereby altering the probability of APD alternans and rotor destabilization. Whole-heart mapping experiments confirmed key findings (e.g., >50% reduction in atrial fibrillation inducibility after I(K,ACh-c) blockade). Atrium-specific Kir3.x controls the induction, dynamics, and termination of fibrillation by modulating APD and APD/conduction velocity restitution slopes in atrial tissue with I(K,ACh-c). This study provides new molecular and mechanistic insights into atrial tachyarrhythmias and identifies Kir3.x as a promising atrium-specific target for antiarrhythmic strategies.

Conductance estimation of a conductance-based neuron model by the differential evolution algorithm

In the studies by a biologically detailed computational model of a neural circuit, it is an important step to construct the conductance-based model of a single neuron that reproduces electrophysiological data. Fitting model parameters such as the maximal conductances of ionic currents can be formulated as an optimization problem in which we should minimize the error between the data experimentally recorded and simulated by the model neuron. We here applied the differential evolution (DE) algorithm [1,2], one of evolutionary computation algorithms, to the parameter fitting problem based on the objective function defined by characteristics of an action potential profile. We applied the DE algorithm to the conductance fitting to the synthetic data generated by a conductance-based model neuron. The model neuron consisted of spike-generating sodium, delayed rectifier potassium, A-type potassium and muscarinic potassium channels, conductance of which should be estimated. The objective function to be minimized was defined as a sum of six scores (distances to the target values) on action potential characteristics: firing rate, action potential peak, action potential width, depth of afterhyperpolarization, a latency to the first spike and degree of spike frequency adaptation [3]. We then compared its performance with that of the real-coded genetic algorithm (RCGA) as a benchmark under the same condition of population size and the number of generations. Figure ​Figure11 shows the time courses of values of the objective function, Evaluation values. Figure ​Figure1A1A illustrates averages and standard deviations over 10 trials at every 10 generations obtained by the DE with the population size of 100, 150 and 200. The inset of Figure ​Figure1A1A magnifies them in the lower range of the evaluation values. Figure ​Figure1B1B is similar to 1A, but by the RCGA. As indicated in the figures, the DE could minimize the evaluation value much more quickly than the RCGA. The DE reached the best value of 0.57 at 100th generation and the value was achieved in all the trials. On the other hand, the best value by the RCGA was 29.29 only once out of the 10 trials. Thus, the results suggested that the DE had much better performance for the conductance fitting than the RCGA. Figure 1 Performances of conductance estimation by the DE and the RCGA.

A flexible software tool for fitting the parameters of neuronal models

Multi-compartmental conductance-based neuronal models have the potential to imitate the behavior of real neurons with great accuracy. However, these models have many parameters, often poorly constrained by the available data. One alternative is to utilize much simpler (e.g., reduced compartmental or integrate-and-fire type) model neurons, whose non-biophysical parameters need to be set such that the behavior of the model cell best approximates that of the real neuron. For both simple and detailed models, the relationship between the values of the parameters and the behavior of the model is nonlinear and complex, and the task of finding the optimal parameter values is highly non-trivial. Thus, a reliable automated parameter-constraining procedure which could handle the different scenarios relevant for model fitting and model simplification would be of great value, and the goal of our research was to develop such a flexible framework. The result of our efforts is the software called Optimizer, implemented in the Python programming language. The program consists of five independent modules, which are connected via a sixth core module. These modules are responsible for reading and handling the target data, interfacing with the simulator, storing the settings specified by the user, as well as implementing the error functions and the optimization algorithms. The program can run optimizations based on one or more traces, and currently implements nine commonly used error functions based on either point-by-point comparisons or extracted features, including mean squared error, spike count, and action potential characteristics. Arbitrary linear combinations of these error functions are also supported. For optimization, the program relies on the inspyred package, which implements a wide selection of powerful nonlinear optimization algorithms; our default choice is a canonical genetic algorithm. The software can be easily modified, expanded, and customized to suit individual needs. For example, the user can implement his/her own specific error function simply by creating the function in the appropriate file and adding the name to the list of the functions. The program currently supports the Neuron simulator, but it can be easily extended to support other simulation software. Optimizer provides two different interfaces. One is a graphical interface, which allows the user to set the optimization parameters and observe, analyze the results in a convenient way. The other interface is command-line based, which reads the settings from a text file, and thus allows the optimization to run on platforms that do not support graphical interactions. So far, we tested the program on four different types of problems: we used it to determine conductance densities in squid axon and in a multi compartmental CA1 pyramidal cell; we fitted the passive parameters of a reconstructed neuron; we fitted a nonlinear integrate-and-fire model to complex biological data; and we successfully applied the software in two different cases of systematic model simplification. In all tests, Optimizer showed equally good or better performance than software solutions described in the literature. Its simplicity and remarkable flexibility make the software applicable to a wide range of problems, making it an extremely useful research tool.

Subepicardial Action Potential Characteristics Are a Function of Depth and Activation Sequence in Isolated Rabbit Hearts

Electric excitability in the ventricular wall is influenced by cellular electrophysiology and passive electric properties of the myocardium. Action potential (AP) rise time, an indicator of myocardial excitability, is influenced by conduction pattern and distance from the epicardial surface. This study examined AP rise times and conduction velocity as the depolarizing wavefront approaches the epicardial surface. Two-photon excitation of di-4-aminonaphthenyl-pyridinum-propylsulfonate was used to measure electric activity at discrete epicardial layers of isolated Langendorff-perfused rabbit hearts to a depth of 500 μm. Endo-to-epicardial wavefronts were studied during right atrial or ventricular endocardial pacing. Similar measurements were made with epi-to-endocardial, transverse, and longitudinal pacing protocols. Results were compared with data from a bidomain model of 3-dimensional (3D) electric propagation within ventricular myocardium. During right atrial and endocardial pacing, AP rise time (10%-90% of upstroke) decreased by ≈50% between 500 and 50 μm from the epicardial surface, whereas conduction velocity increased and AP duration was only slightly shorter (≈4%). These differences were not observed with other conduction patterns. The depth-dependent changes in rise time were larger at higher pacing rates. Modeling data qualitatively reproduced the behavior seen experimentally and demonstrated a parallel reduction in peak I(Na) and electrotonic load as the wavefront approaches the epicardial surface. Decreased electrotonic load at the epicardial surface results in more rapid AP upstrokes and higher conduction velocities compared with the bulk myocardium. Combined effects of tissue depth and pacing rate on AP rise time reduce conduction safety and myocardial excitability within the ventricular wall.

Functional interaction between charged nanoparticles and cardiac tissue: a new paradigm for cardiac arrhythmia?

To investigate the effect of surface charge of therapeutic nanoparticles on sarcolemmal ionic homeostasis and the initiation of arrhythmias. Cultured neonatal rat myocytes were exposed to 50 nm-charged polystyrene latex nanoparticles and examined using a combination of hopping probe scanning ion conductance microscopy, optical recording of action potential characteristics and patch clamp. Positively charged, amine-modified polystyrene latex nanoparticles showed cytotoxic effects and induced large-scale damage to cardiomyocyte membranes leading to calcium alternans and cell death. By contrast, negatively charged, carboxyl-modified polystyrene latex nanoparticles (NegNPs) were not overtly cytotoxic but triggered formation of 50-250-nm nanopores in the membrane. Cells exposed to NegNPs revealed pro-arrhythmic events, such as delayed afterdepolarizations, reduction in conduction velocity and pathological increment of action potential duration together with an increase in ionic current throughout the membrane, carried by the nanopores. The utilization of charged nanoparticles is a novel concept for targeting cardiac excitability. However, this unique nanoscopic investigation reveals an altered electrophysiological substrate, which sensitized the heart cells towards arrhythmias.

Augmented response of renal sympathetic nerve single units to chemoreceptor stimuli in rabbits with angiotensin induced hypertension

We examined the effect of angiotensin (AngII) hypertension on the activity of post‐ganglionic renal sympathetic units to determine whether elevated whole renal nerve activity is due to recruitment or increased firing frequency. Rabbits were treated with AngII for 4 weeks (50ng/kg/min, high dose) or for 8 weeks (20ng/kg/min, low dose) and then chloralose‐urethane anesthetized to test unitary responses to altered chemoreceptor activity. Units were detected from multi‐unit recordings using a novel algorithm that separated units by action potential shape characteristics using templates that matched spikes within a prescribed SD. Resting firing frequency was similar in sham (1.0±0.04 spikes/s, n=144) and high AngII rabbits (1.1±0.04 spikes/s, n=112) but was lower with low AngII (0.65±0.04 spikes/s, n=149). Firing frequency increased markedly to hypoxia (8% O2) in the high AngII group (+103%) but only by +39% in low AngII and +82% in sham group. The increase in firing rate with hypercapnia (8% CO2) was also greatest in the high AngII group whilst 100% O2 suppressed unitary firing in high AngII and sham groups. This study suggests that increases in sympathetic outflow in low dose AngII hypertension are most likely due to recruitment of individual neurons but high dose AngII markedly increases firing frequency to chemoreceptor stimuli. Supported by grants from National Health and Medical Research Council of Australia.

Stem Cell–Derived Cardiomyocytes as a Tool for Studying Proarrhythmia

Proarrhythmia is a leading cause of hospitalization for adverse drug events, and screening for proarrhythmic potential resulting from QT interval prolongation has become a major component of the development process for any new drug candidate. Conventional wisdom, now embedded in regulatory guidelines from the Food and Drug Administration and other regulatory agencies, holds that drug block of the rapid component of the cardiac delayed-rectifier potassium current, I Kr, is the major proximate mechanism underlying prolongation of cardiac action potentials, an effect that leads to early afterdepolarizations in vitro and is manifest on the surface ECG as QT prolongation and occasionally torsades de pointes.1,2 As a result, regulatory guidance suggests an early look at I Kr blocking potency of new drug entities, even recognizing that other mechanisms, some newly described,3,4 may contribute. Article see p 1677 However, the correlation between I Kr block and QT interval prolongation is imperfect. Multiple mechanisms may underlie this “disconnect,” and a reasonable generalization is that I Kr is but one (albeit important) component of normal cardiac repolarization, and abnormalities in expression or function of other components of repolarization may, in turn, modulate the net effect of I Kr block on action potentials in cardiac myocytes and on QT interval and arrhythmia susceptibility in the whole heart. The logical extension of this line of reasoning is that screening for proarrhythmic potential should include the evaluation of not only drug effects on individual ionic currents such as I Kr contributing to repolarization but also the integrated effect of drugs on action potentials in vitro and on the QT interval in animal models and in humans. In conventional drug development programs, this is commonly accomplished by comparing the effects of a candidate drug molecule on QT intervals in human subjects with …

The Characteristics of Action Potentials in Primo Vessels and the Effects of Acetylcholine Injection to the Action Potentials

In a previous study, we found that Primo vessels generate different action potentials in smooth muscles, but this study compared the pulse shape to distinguish the two tissues. Thus, a more sophisticated extracellular experiment was performed in this study using an acetylcholine injection; we then observed changes in the amplitude, FWHM (full width at half maximum), and period to explore Primo vessel function. A third type of pulse was recorded for Primo vessels. We observed fast depolarizing and repolarizing phases for this pulse. Further, its FWHM was 30 ms between smooth muscles and neurons. Acetylcholine affected only the period. The amplitude and FWHM were consistent after injection. Primo-vessels generated action potentials at twice the frequency after injection. From the results, we speculate that Primo-vessels perform a role in transferring signals in a different manner, which may be relevant for acupuncture treatment.

Neuropeptide Substance-P Modulates Electrical Characteristics of Rabbit Atrial Myocytes

Background: The cardiac nervous system plays an important role in the initiation and maintenance of atrial fibrillation (AF). Besides the classical neurotransmitters, the nerves innervating the atria also release a wide range of neuropeptides that may play a role in the pathophysiology of AF. Since the direct effect of these neuropeptides on atrial myocytes is largely unknown, we investigated the electrophysiological response of atrial myocytes to several neuropeptides, importantly substance-P (Sub-P).Methods and Results: Single left atrial myocytes from rabbit hearts were obtained by enzymatic dissociation and the effects of Sub-P, neuropeptide-Y (NPY), somatostatin-14 (SOM-14) and vasoactive intestinal peptide (VIP), were studied using patch-clamp and indo-1 fluorescence methodologies. NPY, SOM-14 and VIP (1 μM) did not affect atrial action potential (AP) characteristics. Sub-P however, reduced both resting membrane potential (RMP) and action potential amplitude, and increased action potential duration at 90% of repolarisation (APD90) by 40%. The effects on APD90 were dose-dependent and occurred from a concentration of 10 nM. Voltage-clamp analysis revealed that Sub-P significantly diminished L-type calcium current, inward rectifier K+ current (IK1) and an unknown steady-state outward current. Transient outward current, Ca2+- activated chloride current and Na+-Ca2+-exchanger current were unaffected, as was the intracellular calcium handling. The reduction in RMP and increase in APD90 are likely due to the decrease in IK1 and steady-state outward current, respectively. Our experiments indicate that the latter is probably carried by background-like potassium channels.Conclusion: Atrial myocyte electrophysiology is significantly modulated by the neuropeptide Sub-P, mainly characterized by a substantial AP prolongation due to inhibition of a background potassium current. Since lengthening of atrial repolarization is potentially anti-arrhythmic, we hypothesize that release of Sub-P may be beneficial to the prevention and/or termination of AF.

Neurophysiological characteristics of human leg muscle action potentials evoked by transcutaneous magnetic stimulation of the spine

The objectives of this study were to establish the neurophysiological properties of the compound muscle action potentials (CMAPs) evoked by transcutaneous magnetic stimulation of the spine (tsMSS) and the effects of tsMSS on the soleus H-reflex. In semi-prone seated subjects with trunk semi-flexed, the epicenter of a figure-of-eight magnetic coil was placed at Thoracic 10 with the handle on the midline of the vertebral column. The magnetic stimulator was triggered by monophasic single pulses of 1 ms, and the intensity ranged from 90% to 100% of the stimulator output across subjects. CMAPs were recorded bilaterally from ankle and knee muscles at the interstimulus intervals of 1, 3, 5, 8, and 10 s. The CMAPs evoked were also conditioned by posterior tibial and common peroneal nerve stimulation at a conditioning-test (C-T) interval of 50 ms. The soleus H-reflex was conditioned by tsMSS at the C-T intervals of 50, 20, -20, and -50 ms. The amplitude of the CMAPs was not decreased when evoked at low stimulation frequencies, excitation of group I afferents from mixed peripheral nerves in the leg affected the CMAPs in a non-somatotopical neural organization pattern, and tsMSS depressed soleus H-reflex excitability. These CMAPs are likely due to orthodromic excitation of nerve motor fibers and antidromic depolarization of different types of afferents. The latency of these CMAPs may be utilized to establish the spine-to-muscle conduction time in central and peripheral nervous system disorders in humans. tsMSS may constitute a non-invasive modality to decrease spinal reflex hyperexcitability and treat hypertonia in neurological disorders.

Chronic atrial fibrillation up-regulates β1-Adrenoceptors affecting repolarizing currents and action potential duration

β-adrenergic stimulation has profound influence in the genesis and maintenance of atrial fibrillation (AF). However, the effects of β-Adrenoceptor stimulation on repolarizing currents and action potential (AP) characteristics in human atrial myocytes from left (LAA) and right atrial appendages (RAA) obtained from sinus rhythm (SR) and chronic atrial fibrillation (CAF) patients have not been compared yet. Currents and APs were recorded using whole-cell patch-clamp in RAA and LAA myocytes from SR and CAF patients. Isoproterenol concentration-dependently decreased the Ca(2+)-independent 4-aminopyridine-sensitive component of the transient outward current (I(to1)) and the inward rectifying current (I(K1)). CAF significantly enhanced this inhibition, this effect being more marked in the left than in the right atria. CAF dramatically enhanced β-Adrenoceptor-mediated increase in the slow component of the delayed rectifier current (I(Ks)), whose density was already markedly increased by CAF. Conversely, the ultrarapid component of the delayed rectifier current (I(Kur)) of both SR and CAF myocytes was insensitive to low isoproterenol concentrations. As a consequence, stimulation of β1-Adrenoceptors in SR myocytes lengthened, whereas in CAF myocytes shortened, the AP duration. Quantitative PCR revealed that CAF up-regulated β1-Adrenoceptor expression, preferentially in the left atria. The present results demonstrate that CAF increases the effects of β1-Adrenoceptor stimulation on repolarizing currents by means of a chamber-specific up-regulation of the receptors. This, together with the ion channel derangements produced by CAF, could contribute to the long-term stabilization of the arrhythmia by shortening the AP duration.