Fast Response Action Potential Research Articles

Antiarrhythmic effects of dehydroevodiamine in isolated human myocardium and cardiomyocytes

Ethnopharmacological relevanceDehydroevodiamine alkaloid (DeHE), a bioactive component of the Chinese herbal medicine Wu-Chu-Yu (Evodiae frutus), exerted antiarrhythmic effect in guinea-pig ventricular myocytes. We further characterize the electromechanical effects of DeHE in the human atrial and ventricular tissues obtained from hearts of patients undergoing corrective cardiac surgery or heart transplantation. Materials and methodsThe transmembrane potentials of human myocardia were recorded with a traditional microelectrode technique while sarcolemmal Na+ and Ca2+ currents in single human cardiomyocytes were measured by a whole-cell patch-clamp technique. The intracellular pH (pHi) and Na+–H+ exchanger (NHE) activity were determined using BCECF-fluorescence in human atria. ResultsIn human atria, DeHE (0.1–0.3μM) depressed upstroke velocity, amplitude of action potential, and contractile force, both in slow and fast response action potential. Moreover, the similar depressant effects of DeHE were found in human ventricular myocardium. Both in isolated human atrial and ventricular myocytes, DeHE (0.1–1μM) reversibly, concentration-dependently decreased the Na+ and Ca2+currents. Moreover, DeHE (0.1 and 0.3μM) suppressed delayed afterdepolarizations and aftercontractions, induced by epinephrine and high [Ca2+]o in atria. In human ventricular myocardium, the strophanthidin-induced triggered activities were attenuated by pretreating DeHE (0.3μM). The resting pHi and NHE activity were also significantly increased by DeHE (0.1–0.3μM). ConclusionsWe concluded for the first time that, in the human hearts, DeHE could antagonize triggered arrhythmias induced by cardiotonic agents through a general reduction of the Na+ and Ca2+ inward currents, while increase of resting pHi and NHE activity.

Diagnostic criteria for the Brugada syndrome: Can they be improved?

IntroductionDiagnosis of Brugada syndrome (BS) currently requires documentation of a characteristic repolarization pattern (type 1 Brugada ECG). Mutations in the SCN5A gene, which codes for sodium channel NaV 1.5, are found in 38% of familial cases of BS. Sodium current dysfunction negatively affects the cardiac fast response action potential, particularly in atrial and ventricular myocytes and in the fast-conducting Purkinje system. ObjectivesTo detect carriers of SCN5A mutations without using the characteristic repolarization pattern (type 1 Brugada ECG). MethodsOf a total of 141 members of three different families including 55 carriers of two nonsense SCN5A mutations causing BS, all those aged over 16 (113 individuals, 42 carriers) were studied. The PR interval (PR) and QT dispersion (QTd) between leads V1 and V3 were measured on conventional ECG. Using signal-averaged ECG the total duration of the filtered QRS complex (fQRS), the root-mean-square (RMS40) and the low-amplitude signal (LAS) were measured. The following procedures were developed to detect carriers: (1) a screening test (ScreenTest) with PS (PR+fQRS)≥250 (250ms is 80% of the theoretical maximum in healthy individuals); and (2) a diagnostic test (DiagTest) for the simultaneous fulfillment of four conditions: PS≥250 and QTd≥10 and LAS>26 and RMS40 ≤29 (the latter two cut-offs are approximately 70% of the theoretical maximum in healthy carriers). ResultsSignificant differences in PR, QTd, fQRS, RMS40 and LAS were found between carriers and non-carriers. The SCN5A gene was associated with all these variables, the strongest association being with PR. Both tests were applied to 63 family members (38 carriers). The ScreenTest was positive in 38 of 38 carriers, with eight false positives in 27 non-carriers (sensitivity [SE]=100% and specificity [SP]=66.67%). From ROC curve analysis a cut-off of PS=252.5 shows SE=100% and SP=76% and a cut-off of PS=260 shows SE=94.7% and SP=84%. The DiagTest was positive in 36 of 38 carriers, with three false positives: SE=94.74% and SP=88.89%. From ROC curve analysis a multivariate logistic model identifies a cut-off with SE=92% and SP=92%. In the same group the SE and SP of the characteristic spontaneous repolarization pattern (type 1 Brugada ECG) to detect carriers were 52.4% and 97.2%, respectively, and the difference between the SE of the DiagTest and of the typical repolarization pattern is statistically significant. ConclusionsThe ScreenTest and DiagTest are more effective tools than the characteristic repolarization pattern to discriminate between carriers and non-carriers of these two nonsense SCN5A mutations. We suggest their use in first-degree relatives of Brugada patients when the results of genetic testing are not available, in a score of disease probability in individuals with idiopathic Brugada ECG, and in patients with arrhythmias or other Brugada-related symptoms presenting type 2 or type 3 Brugada ECG.

Critérios de diagnóstico da Síndrome de Brugada. Podemos melhorar?

IntroductionDiagnosis of Brugada syndrome (BS) currently requires documentation of a characteristic repolarization pattern (type 1 Brugada ECG). Mutations in the SCN5A gene, which codes for sodium channel NaV 1.5, are found in 38% of familial cases of BS. Sodium current dysfunction negatively affects the cardiac fast response action potential, particularly in atrial and ventricular myocytes and in the fast-conducting Purkinje system. ObjectivesTo detect carriers of SCN5A mutations without using the characteristic repolarization pattern (type 1 Brugada ECG). MethodsOf a total of 141 members of three different families including 55 carriers of two nonsense SCN5A mutations causing BS, all those aged over 16 (113 individuals, 42 carriers) were studied. The PR interval (PR) and QT dispersion (QTd) between leads V1 and V3 were measured on conventional ECG. Using signal-averaged ECG the total duration of the filtered QRS complex (fQRS), the root-mean-square (RMS40) and the low-amplitude signal (LAS) were measured. The following procedures were developed to detect carriers/To detect carriers the following procedures were developed: (1) a screening test (ScreenTest) with PS (PR+fQRS) ≥250 (250ms is 80% of the theoretical maximum in healthy individuals); and (2) a diagnostic test (DiagTest) for the simultaneous fulfillment of four conditions: PS ≥250 and QTd ≥10 and LAS >26 and RMS40 ≤29 (the latter two cut-offs are approximately 70% of the theoretical maximum in healthy carriers). ResultsSignificant differences in PR, QTd, QRSf, RMS40 and LAS were found between carriers and non-carriers. The SCN5A gene was associated with all these variables, the strongest association being with PR. Both tests were applied to 63 family members (38 carriers). The ScreenTest was positive in 38 of 38 carriers, with eight false positives in 27 non-carriers (sensitivity [SE]=100% and specificity [SP]=66.67%). From ROC curve analysis a cut-off of PS=252.5 shows SE=100% and SP=76% and a cut-off of PS=260 shows SE=94.7% and SP=84%. The DiagTest was positive in 36 of 38 carriers, with three false positives: SE=94.74% and SP=88.89%. From ROC curve analysis a multivariate logistic model identifies a cut-off with SE=92% and SP=92%. In the same group the SE and SP of the characteristic spontaneous repolarization pattern (type 1 Brugada ECG) to detect carriers were 52.4% and 97.2%, respectively, and the difference between the SE of the DiagTest and of the typical repolarization pattern is statistically significant. ConclusionsThe ScreenTest and DiagTest are more effective tools than the characteristic repolarization pattern to discriminate between carriers and non-carriers of these two nonsense SCN5A mutations. We suggest their use in first-degree relatives of Brugada patients when the results of genetic testing are not available, in a score of disease probability in individuals with idiopathic Brugada ECG, and in patients with arrhythmias or other Brugada-related symptoms presenting type 2 or type 3 Brugada ECG.

Homogenous distribution of fast response action potentials in canine pulmonary vein sleeves: a contradictory report

Pulmonary veins may serve as source of ectopic focus (or foci) in initiating atrial tachyarrhythmias in human beings. However, the animal model for such focal atrial fibrillation is still lacking and cellular mechanism for arrhythmias remains to be studied. Recently, a series of reports of cellular electrophysiological characterization of pulmonary vein sleeves demonstrated an extremely high incidence of automaticity (varied from 40 to 76%) and triggered activity (from 0 to 44%) in normal healthy control dogs and rabbits. The present study was therefore designed to re-investigate the cellular electrophysiological properties of canine pulmonary veins. Intracellular action potentials were characterized in pulmonary vein sleeves in 50 normal healthy dogs. Conventional glass microelectrode recording technique was used. Experiments were focused on the incidence of automaticity and triggered activity in pulmonary vein sleeve tissues. Surprisingly, our results showed that all pulmonary vein sleeves tissues in these dogs displayed fast-response action potentials under the well-controlled experimental condition (100%, n=50). No spontaneous pacemaking activities, early or delayed afterdepolarisations were observed (0%, n=50). No high-frequency spikes or irregular rhythm could be recorded in all experiments (0%, n=50). Both the frequency response and membrane responsiveness of the pulmonary vein action potentials were characterized. No electrophysiological inhomogeneity between the distal and the proximal part of the sleeves was observed. In conclusion, canine pulmonary vein sleeves do not display arrhythmogenic activities under normal physiological conditions. The possible explanations for the controversy in pulmonary veins electrophysiology were discussed.

Electrophysiology of single cardiomyocytes isolated from rabbit pulmonary veins: implication in initiation of focal atrial fibrillation.

Pulmonary veins (PVs) are important foci in initiation of paroxysmal atrial fibrillation. However, the mechanisms of the high arrhythmogenic activity of PVs are unclear. This study aimed to isolate single cardiomyocytes from PVs and evaluate their electrophysiological characteristics and arrhythmogenic potential. Cardiomyocytes of rabbit PVs were isolated by retrograde perfusion with digestive enzymes from aorta via left ventricle and left atrium. The action potentials and ionic currents were investigated in isolated single PV cardiomyocytes using the whole-cell clamp technique. Dissociation of PVs yielded single pacemaker cardiomyocytes (76%) and non-pacemaker cardiomyocytes with a fast response action potential. Both the pacemaker and non-pacemaker cardiomyocytes had similar inward Ca2+ currents and transient outward K+ currents. However, the pacemaker cardiomyocytes had a smaller inward rectifier K+ current (1.50 +/- 0.22 versus 4.21 +/- 1.15 pA/pF, P < 0.005) and a larger delayed rectifier K+ current (0.60 +/- 0.05 versus 0.24 +/- 0.05 pA/pF, P < 0.005) than non-pacemaker cardiomyocytes. Acetylcholine induced hyperpolarization and inhibited the spontaneous action potential. Isoproterenol (10 nM) accelerated the spontaneous activity and induced early or delayed afterdepolarization, which could be suppressed by nifedipine. The PV cardiomyocytes with early afterdepolarization have a greater prolongation of action potential duration (deltaAPD, +67 +/- 17 versus -109 +/- 20 ms, P < 0.0001) and a greater increase of inward Ca2+ current (0.90 +/- 0.23 versus 0.38 +/- 0.08 pA/pF, P < 0.05) after isoproterenol than those with out early afterdepolarization. These findings suggest that PV cardiomyocytes have distinct action potentials and ionic current profiles, which may be responsible for the high arrhythmogenic activity of the PVs.

Electrical interactions between a rabbit atrial cell and a nodal cell model.

Atrial activation involves interactions between cells with automaticity and slow-response action potentials with cells that are intrinsically quiescent with fast-response action potentials. Understanding normal and abnormal atrial activity requires an understanding of this process. We studied interactions of a cell with spontaneous activity, represented by a "real-time" simulation of a model of the rabbit sinoatrial (SA) node cell, simultaneously being electrically coupled via our "coupling clamp" circuit to a real, isolated atrial myocyte with variations in coupling conductance (Gc) or stimulus frequency. The atrial cells were able to be driven at a regular rate by a single SA node model (SAN model) cell. Critical Gc for entrainment of the SAN model cell to a nonstimulated atrial cell was 0.55 +/- 0.05 nS (n = 7), and the critical Gc that allowed entrainment when the atrial cell was directly paced at a basic cycle length of 300 ms was 0.32 +/- 0.01 nS (n = 7). For each atrial cell we found periodic phenomena of synchronization other than 1:1 entrainment when Gc was between 0.1 and 0.3 nS, below the value required for frequency entrainment, when the atrial cell was directly driven at a basic cycle length of either 300 or 600 ms. In conclusion, the high input resistance of the atrial cells allows successful entrainment of nodal and atrial cells at low values of Gc, but further uncoupling produces arrhythmic interactions.

Depressant Effects of Prostacyclin in Human Atrial Fibers and Cardiomyocytes.

The aim of this study was to characterize the electropharmacological effects of prostacyclin (PGI(2)) in human atrial fibers and cardiomyocytes. Atrial tissues obtained from the hearts of 28 patients undergoing corrective cardiac surgery were used. Transmembrane action potentials were recorded using a conventional microelectrode technique, and twitch force by a transducer. Effects of PGI(2) (1 nM-10 &mgr;M) on action potential characteristics and contraction of atrial fibers were evaluated in normal [K](o) (4 mM) and high [K](o) (27 mM) in the absence and presence of cardiotonic agents. In addition, atrial and ventricular myocytes were isolated enzymatically from atrial tissues and hearts of 4 patients undergoing cardiac transplant. The effects of PGI(2) on Na- and Ca-dependent inward currents (I(Na) and I(Ca)) of cardiomyocytes were tested. In 9 human atrial fibers showing fast-response action potentials (mean dV/dt(max) = 101 +/- 15 Vs(-1)) in 4 mM [K](o), PGI(2) did not influence dV/dt(max) of phase 0 depolarization even at 1 &mgr;M. However, at a concentration as low as 10 nM, PGI(2) depressed spontaneous rhythms or slow-response action potentials in high-K-depolarized fibers. PGI(2) also depressed delayed afterdepolarizations and aftercontractions induced by cardiotonic agents. In isolated cardiomyocytes, PGI(2) reduced I(Ca) but not I(Na). The present findings show that, in human atrial fibers and cardiomyocytes, PGI(2) induces greater depressant effects on the slow-response action potential, I(Ca) and triggered activity than on the fast-response action potential. It is suggested that PGI(2) may act through a selective reduction of transmembrane Ca influx. Copyright 1994 S. Karger AG, Basel

Effects of Ritanserin on Transmembrane Action Potentials in Canine Purkinje Fibers

Ritanserin has been reported to be a potential antiarrhythmic. We studied the cellular electrophysiologic effects of ritanserin in canine Purkinje fibers. Ritanserin produced significant depressant effects on transmembrane action potentials elicited in canine Purkinje fibers. At concentrations of 10 and 40 mg/liter, ritanserin decreased Vmax (the upstroke velocity) of action potential in a dose-dependent fashion and shortened the duration of fast response action potential. These concentrations of ritanserin also reduced the amplitude and duration of the slow response action potentials induced in Purkinje fibers treated with isoproterenol (10(-5) M) and high K+ (22 mM). These in vitro results suggest that the cellular electrophysiologic actions of ritanserin may be due to its direct actions on cardiac sodium and calcium channels, which, in turn, may account for its antiarrhythmic effects.

Electropharmacological effects of sandostatin in human atrial fibers

We studied the actions of sandostatin (0.1–1000 nM), an analogue of somatostatin, on human atrial tissues obtained from hearts of 20 patients undergoing corrective cardiac surgery. In 3 preparations showing fast response action potential in normal [K] 0 Tyrode solution, sandostatin induced little effect, even at the highest concentration (1 μM). In 10 preparations showing a slow rate of phase-0 depolarization when atrial fibers were depolarized (maximum diastolic potential near −40 mV) in high [K] 0 (27 mM), sandostatin at concentrations as low as 1 nM decreased significantly the velocity of the upstroke, and the amplitude of slow response of the action potential as well as the force of contraction. In 6 experiments on spontaneously active atrial fibers (maximum diastolic potential = −53.8 ± 2.7 mV), sandostatin increased the spontaneous cycle length in a fashion dependent upon concentration. The decrease in spontaneous rate of firing was associated with an inhibition of the late diastolic slope, a change also induced by somatostatin. A longer period of washout, however, (30 min or longer) was required for complete recovery from the depressant effects. Sandostatin (0.1–100 nM) also depressed triggered activity induced by cardiotonic agents. The present findings indicate that sandostatin induces a prolonged action in human atrial cells. Sandostatin may depress abnormal automatic rhythms through an inhibition of transmembrane influx of calcium.

Depressant effect of adenosine in isolated human and canine atrial fibres

STUDY OBJECTIVE0--he aim was to explore the cellular mechanisms responsible for the depressant effects of adenosine in human atrial tissues. Conventional microelectrode technique was used to record transmembrane action potential of human atrial tissues obtained at cardiac surgery. Effects of adenosine (0.1-100 microM) on action potential characteristics and contractile force of human atrial fibres in the absence and presence of an adenosine receptor antagonist (DPSPX) were evaluated. Results were then compared with those obtained from the canine atrial tissues. Atrial tissues obtained from hearts of 25 patients undergoing corrective cardiac surgery were used. Seven mongrel dogs were anaesthetised and strands of atrial muscle were removed and used for comparison. In human atrial fibres showing fast response action potential (mean dV/dtmax around 100 V.s-1) in normal [K]o (4 mM) Tyrode solution, adenosine (1 nM-10 microM) did not induce consistent effects on the action potential characteristics. When the fibres were depolarised in high [K]o (27 mM) or in atrial fibres showing slow response action potential (dV/dtmax less than 50 V.s-1), however, 10 microM adenosine reduced the upstroke velocity and the amplitude of action potential significantly and markedly depressed the contractile force. In atrial fibres spontaneously active in normal Tyrode solution (maximum diastolic potential around -50 mV), adenosine inhibited rate of spontaneous discharge in a concentration dependent manner. Delayed afterdepolarisation and aftercontraction induced by adrenaline or/and high [Ca]o were also suppressed. The depressant effects of adenosine were blocked after pretreatment with 50 microM DPSPX, a specific antagonist for adenosine receptor. These findings show that adenosine may abolish abnormal automatic rhythms and triggered activity in human atria.

Cardiac electrophysiologic effects of AHR 5360C

We used microelectrode and blood superfusion techniques to study the cardiac electrophysiologic effects of a new drug, AHR 5360C, which has antihypertensive and calcium channel blocking properties in several experimental models. AHR 5360C, 10 −7 M significantly depressed the amplitude of the slow response action potential in canine Purkinje fibers. The fast response action potential was also depressed in a dose-dependent fashion, but with a threshold concentration of 5 × 10 −6 M. AHR 5360C decreased normal automaticity and barium-induced abnormal automaticity at concentrations of 5 × 10 −6 and 10 −5 M respectively, as well as ouabain-induced delayed afterdepolarizations at a threshold concentration of 10 −6 M. In blood superfusion studies, i.v. administration of AHR 5360C, 0.3 mg/kg, significantly reduced blood pressure. Doses of 1.0 mg/kg induced a high degree of A-V block, further reduction of blood pressure, and no physiological changes in heart rate and in the blood superfused fibers. In conclusion, AHR 5360C has calcium blocking properties that depress A-V conduction at concentrations that do not affect the sodium-dependent fast response action potential.

Suppressive effects of somatostatin in dog Purkinje fibres.

1. The effects of somatostatin (SS, 1 nM-3 microM) on the electrical and mechanical activities of isolated Purkinje fibres of the dog were studied. 2. In most Purkinje fibres driven electrically in normal [K]o Tyrode solution, SS decreased the force of contraction slightly and had very little effect on the fast response action potential. However, in sensitive fibres SS induced a moderate reduction of action potential duration and contractile force in normal [K]o and depressed the slow response action potentials in high [K]o. 3. In spontaneously beating Purkinje fibres, SS decreased the regular rhythms slightly but abolished bursts of fast rhythms at a concentration as low as 1 nM. 4. When the fibres were depolarized in the presence of 0.2 mM barium or in Na-free solution, SS suppressed the Ca-dependent slow response action potentials. 5. These findings suggest that SS may suppress abnormal automatic activity of dog Purkinje fibres through a reduction of transmembrane Ca influx or a modulation of intracellular calcium.

Circus movement in the canine atrium around the tricuspid ring during experimental atrial flutter and during reentry in vitro.

A Y-shaped lesion in the right atrium allows induction of atrial flutter in dogs. We recorded the activation sequence during this tachycardia from 96 endocardial bipolar electrodes using intracavitary electrode arrays during 12 separate episodes in three isolated perfused hearts. In each case a reentrant impulse circulated around the tricuspid valve orifice in either a clockwise or counter-clockwise direction. Cutting the pathway terminated the rhythm and prevented its reinduction. There was no discrete segment of markedly slow conduction in the reentrant circuit. The tachycardia cycle length was decreased by methacholine and increased by lidocaine. Reentry was also induced in atrial tissue around the tricuspid orifice when this structure was isolated and superfused in vitro. Tachycardia cycle lengths varied from 205 to 399 msec, depending on the circumference of the ring and temperature. Induction of tachycardia by premature stimulation depended on differences in the duration of the effective refractory period among parts of the ring. Conduction velocity was relatively uniform and was slower during tachycardias than during pacing at long cycle lengths. Analysis of the response to premature stimuli that reset the tachycardia provided evidence for incomplete recovery of excitability between depolarizations during the tachycardia. Fast-response action potentials were recorded throughout the pathway and up to six to eight cell layers deep. Histologic studies showed the supravalvular lamina, a circumferential band of fibers several cell layers below the endocardial surface, to be continuous around the tricuspid orifice. Propagation through this layer best explains the conduction velocities observed in the intact heart during flutter in this preparation.

Somatostatin effects in isolated human atrial fibres

Effects of hypothalamic hormone somatostatin on the action potential and contractile force of 54 human atrial preparations obtained at cardiac surgery were studied with standard microelectrode techniques. In the atrial fibres responding to electrical stimuli with fast response action potentials in 4 mM [K]0 Tyrode solution (maximum rate of phase 0 depolarization greater than 50 V/s), somatostatin (10(-7) to 10(-6) M) reduced slightly the duration of action potential and decreased twitch force dose-dependently. In spontaneously active atrial fibres, somatostatin (10(-7) to 10(-6) M) abolished the action potentials in the preparations with low maximal diastolic potential (MDP, -48 +/- 2.8 mV), but induced only mild suppressive effect in the preparations with high MDP (-70 +/- 2.4 mV). When the fibres were depolarized in 27 mM [K]0 Tyrode solution, somatostatin decreased the maximum rate of depolarization and amplitude of the slow response action potentials induced by electrical stimuli. The delayed afterdepolarizations or triggered action potentials induced by high-frequency electrical drive in the presence of epinephrine were also suppressed by somatostatin. The above findings suggest that somatostatin may suppress the abnormal automaticity and the triggered activity in human atrial fibres through a reduction of cellular calcium.