Action Potential Duration Values Research Articles

ActionPytential: An open source tool for analyzing and visualizing cardiac action potential data

The action potential forms the basis of cardiac pacemaking, conduction, and contraction. Action potentials can be recorded from numerous preparation types, including ventricular or atrial trabecules, Purkinje fibers, isolated cardiac myocytes. Numerous techniques are also available as well, such as the conventional microelectrode and the single-cell current clamp techniques, optical mapping, or in silico modeling. With such a vast array of electrophysiological methods comes an array of available hardware and software solutions. In this work, we present a software with an intuitive graphical user interface, ActionPytential, that enables the analysis of any type of cardiac action potential, regardless of acquisition method or tissue type.In most available software tools, the analysis of continuous (gap-free) recordings often requires manual user interaction to segment the individual action potentials. We provide an automated solution for this, both for slow-response and for externally paced action potentials. As of now, ActionPytential calculates 34 parameters from each action potential. The most often utilized ones, including amplitude, maximal rate of depolarization, and action potential duration values, were validated on 1200 action potentials from human, dog, rabbit, guinea pig, and rat cardiac preparations. We also provide new parameters that were previously only measurable manually, including the position and the depth of the notch in potentials showing a spike-and-dome morphology. Further notable features include a Butterworth-type low-pass filter, the averaging of multiple potentials, automated corrections for baseline drifting, aided manual analysis, high-quality plots, and batch processing for any number of potentials.ActionPytential is available for all major platforms (Windows, MacOS, GNU + Linux, BSD).

Scenarios for Increasing, Decreasing and Stability of Tpe/QT Ratio (Simulation Study)

The objective of this simulation was to study the physiological meaning of the novel index for arrhythmic risk stratification – Tpe/QT ratio (the interval from the peak to the end of the T-wave (Tpe) on electrocardiogram divided by QT interval). Methods and Results: The role of two factors determining Tpe/QT ratio – action potential duration (APD) and dispersion of repolarization (DOR) – was studied in silico at two levels: ventricular wall segment and the whole heart ventricles. The simulations performed in the framework of both the segment and the entire ventricles’ models showed that Tpe/QT magnitude reflects the dynamic relationship between the longest and the shortest APD rather than the magnitude of DOR, as interval Tpe does. Tpe/QT ratio remained unchanged even at large DOR, provided that the longer the ventricular APD, the greater the gap between the longest and the shortest of them. The imbalance between the longest and the shortest APD values led to the increased or decreased Tpe/QT. Conclusion: Simulations showed that Tpe/QT is not just a corrected Tpe interval, but it reflects the balance between the longest and the shortest APD in the heart ventricles.

Suppression-Replacement KCNQ1 Gene Therapy for Type 1 Long QT Syndrome.

Type 1 long QT syndrome (LQT1) is caused by loss-of-function variants in the KCNQ1-encoded Kv7.1 potassium channel α-subunit that is essential for cardiac repolarization, providing the slow delayed rectifier current. No current therapies target the molecular cause of LQT1. A dual-component suppression-and-replacement (SupRep) KCNQ1 gene therapy was created by cloning a KCNQ1 short hairpin RNA and a short hairpin RNA-immune KCNQ1 cDNA modified with synonymous variants in the short hairpin RNA target site, into a single construct. The ability of KCNQ1-SupRep gene therapy to suppress and replace LQT1-causative variants in KCNQ1 was evaluated by means of heterologous expression in TSA201 cells. For a human in vitro cardiac model, induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from 4 patients with LQT1 (KCNQ1-Y171X, -V254M, -I567S, and -A344A/spl) and an unrelated healthy control. CRISPR-Cas9 corrected isogenic control iPSC-CMs were made for 2 LQT1 lines (correction of KCNQ1-V254M and KCNQ1-A344A/spl). FluoVolt voltage dye was used to measure the cardiac action potential duration (APD) in iPSC-CMs treated with KCNQ1-SupRep. In TSA201 cells, KCNQ1-SupRep achieved mutation-independent suppression of wild-type KCNQ1 and 3 LQT1-causative variants (KCNQ1-Y171X, -V254M, and -I567S) with simultaneous replacement of short hairpin RNA-immune KCNQ1 as measured by allele-specific quantitative reverse transcription polymerase chain reaction and Western blot. Using FluoVolt voltage dye to measure the cardiac APD in the 4 LQT1 patient-derived iPSC-CMs, treatment with KCNQ1-SupRep resulted in shortening of the pathologically prolonged APD at both 90% and 50% repolarization, resulting in APD values similar to those of the 2 isogenic controls. This study provides the first proof-of-principle gene therapy for complete correction of long QT syndrome. As a dual-component gene therapy vector, KCNQ1-SupRep successfully suppressed and replaced KCNQ1 to normal wild-type levels. In TSA201 cells, cotransfection of LQT1-causative variants and KCNQ1-SupRep caused mutation-independent suppression and replacement of KCNQ1. In LQT1 iPSC-CMs, KCNQ1-SupRep gene therapy shortened the APD, thereby eliminating the pathognomonic feature of LQT1.

Kvβ2 Subunit Regulates Cardiac Levels of SLC41a3 a Sodium‐Magnesium Transporter.

BackgroundClinical evidence has demonstrated that administration of magnesium can provide cardioprotective benefits during acute MI as well as suppression of cardiac arrhythmias. Further, meta‐analysis data has uncovered an inverse relation to circulatory levels of magnesium and risk for cardiac injury. Magnesium influx/efflux behaviors have been recorded in isolated cardiomyocytes; further the addition of magnesium during and prior to cardiac injury such as ischemia reperfusion was demonstrated to significantly reduce ischemic injury. While clinical evidence highlights magnesium's cardiovascular benefits, identification and subsequent regulation remain largely unknown of the magnesium channels responsible. Recent research has identified one of the key magnesium/sodium exchange channels as SLC41a1‐3 family. Murine cardiac expression levels demonstrate significant mRNA levels of SLC41a3 in particular.Methods and ResultsA recent microarray study performed in hearts from Kvβ2 knockout mice demonstrated a significant decrease in SLC41a3 mRNA expression compared with control. Protein expression validated the decrease in expression levels of SLC41a3. These findings may indicate the Kvβ2 subunit acts as a novel regulator to SLC41a3, previous research identified Kvβ2 having protein‐protein interactions with SLC39a1 another member of the solute carrier family (SLC). Cardiac function measured by monophasic action potential (AP) recordings presented with a significant reduction in AP duration values in control hearts perfused with magnesium (MgSO4) while Kvβ2 KO hearts demonstrate little to no alterations in AP duration post magnesium exposure. Further functionality will be assessed in isolated cardiomyocytes testing the influx and efflux of magnesium to understand Kvβ2's role in channel activity.HypothesisBased upon our preliminary data we hypothesis that the Kvβ2 subunit plays significant roles in SLC41a3 expression as well as function within the heart.ConclusionThese novel roles of Kvβ2 may shed new light on the molecular understanding of how administration of magnesium can benefit both cardiac injury and arrhythmias.Support or Funding InformationResearch Supported: This Research work was funded by National Institutes of Health (NIH) Grant R01‐HL‐102171 (S. M. Tipparaju).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Mechanism of doxorubicin cardiotoxicity evaluated by integrating multiple molecular effects into a biophysical model.

Doxorubicin (DOX) is an effective cancer therapeutic agent but causes therapy-limiting cardiotoxicity. The effects of DOX and its metabolite doxorubicinol (DOXL) on individual channels have been well characterized in isolation. However, it is unknown how the action and interaction of affected channels combine to generate the phenotypic cardiotoxic outcome. We sought to develop an in silico model that links drug effects on channels to action potential duration (APD) and intracellular Ca2+ concentration in order to address this gap in knowledge. We first propose two methods to obtain, from published values, consensus drug effects on the currents of individual channels, transporters and pumps. Separately, we obtained equivalent values for APD and Ca2+ concentration (the readouts used as surrogates for cardiotoxicity). Once derived, the consensus effects on the currents were incorporated into established biophysical models of the cardiac myocyte and were refined adjusting the sarcoplasmic reticulum Ca2+ leak current (ILeak ) until the consensus effects on APD and Ca2+ dynamics were replicated. Using factorial analysis, we then quantified the relative contribution of each channel to DOX and DOXL cardiotoxicity. The factorial analysis identified the rapid delayed rectifying K+ current, the L-type Ca2+ current and the sarcoplasmic reticulum ILeak as the targets primarily responsible for the cardiotoxic effects on APD and Ca2+ dynamics. This study provides insight into the mechanisms of DOX-induced cardiotoxicity and a framework for the development of future diagnostic and therapeutic strategies.

Procainamide and lidocaine produce dissimilar changes in ventricular repolarization and arrhythmogenicity in guinea‐pig

Procainamide is class Ia Na(+) channel blocker that may prolong ventricular repolarization secondary to inhibition of IK r , the rapid component of the delayed rectifier K(+) current. In contrast to selective IN a blockers such as lidocaine, procainamide was shown to produce arrhythmogenic effects in the clinical setting. This study examined whether pro-arrhythmic responses to procainamide may be accounted for by drug-induced repolarization abnormalities including impaired electrical restitution kinetics, spatial gradients in action potential duration (APD), and activation-to-repolarization coupling. In perfused guinea-pig hearts, procainamide was found to prolong the QT interval on ECG and left ventricular (LV) epicardial monophasic APD, increased the maximum slope of electrical restitution, enhanced transepicardial APD variability, and eliminated the inverse correlation between the local APD and activation time values determined at distinct epicardial recording sites prior to drug infusion. In contrast, lidocaine had no effect on electrical restitution, the degree of transepicardial repolarization heterogeneities, and activation-to-repolarization coupling. Spontaneous episodes of monomorphic ventricular tachycardia were observed in 57% of procainamide-treated heart preparations. No arrhythmia was induced by lidocaine. In summary, this study suggests that abnormal changes in repolarization may contribute to pro-arrhythmic effects of procainamide.

Fibroblast KATP currents modulate myocyte electrophysiology in infarcted hearts

Cardiac metabolism remains altered for an extended period of time after myocardial infarction. Studies have shown fibroblasts from normal hearts express KATP channels in culture. It is unknown whether fibroblasts from infarcted hearts express KATP channels and whether these channels contribute to scar and border zone electrophysiology. KATP channel subunit expression levels were determined in fibroblasts isolated from normal hearts (Fb), and scar (sMI-Fb) and remote (rMI-Fb) regions of left anterior descending coronary artery (LAD) ligated rat hearts. Whole cell KATP current density was determined with patch clamp. Action potential duration (APD) was measured with optical mapping in myocyte-only cultures and heterocellular cultures with fibroblasts with and without 100 μmol/l pinacidil. Whole heart optical mapping was used to assess KATP channel activity following LAD ligation. Pinacidil activated a potassium current (35.4 ± 7.5 pA/pF at 50 mV) in sMI-Fb that was inhibited with 10 μmol/l glibenclamide. Kir6.2 and SUR2 transcript levels were elevated in sMI-Fb. Treatment with Kir6.2 short interfering RNA decreased KATP currents (87%) in sMI-Fb. Treatment with pinacidil decreased APD (26%) in co-cultures with sMI-Fb. APD values were prolonged in LAD ligated hearts after perfusion with glibenclamide. KATP channels are present in fibroblasts from the scar and border zones of infarcted hearts. Activation of fibroblast KATP channels could modulate the electrophysiological substrate beyond the acute ischemic event. Targeting fibroblast KATP channels could represent a novel therapeutic approach to modify border zone electrophysiology after cardiac injury.

Impact of Multiple Ionic Changes in Arrhythmic Risk Biomarkers in Human Ventricular Electrophysiology

Electrophysiological variability represents an important challenge in evaluating cardiac response to disease and drug action. Our goal is to quantitatively evaluate the sensitivity of biomarkers to the simultaneous alteration of several ionic current in human cardiomyocytes. Computer simulations of human ventricular electrophysiology are conducted using an action potential (AP) model proposed by Carro et al.Ionic current conductances and kinetics were simultaneously varied by +/-30%. Model output was evaluated by quantifying biomarkers including AP duration (APD) and triangulation. Statistical techniques based on experiments design along with a second order response surface model were used.Figure 1. A) AP duration, B) AP triangulation. Marked area indicates physiological rangeView Large Image | View Hi-Res Image | Download PowerPoint SlideResults reveal that simultaneous alteration of several ionic properties results in nonlinear effects in biomarkers response. APD is more sensitive than triangulation to changes in ionic properties. 30% changes in sodium/potassium pump and in calcium inactivation result in APD values outside the physiological range (287-350ms; Fig. 1A). In contrast, 30% changes in most ionic properties result in triangulation within physiological range (70-86ms). However, simultaneous reduction of GKr and GCaL by 30% significantly increases triangulation (Fig. 1B).

Mechanism of Reverse Rate-Dependent Action of Cardioactive Agents

Class 3 antiarrhythmic agents exhibit reverse rate-dependent lengthening of the action potential duration (APD), i.e. changes in APD are greater at longer than at shorter cycle lengths. In spite of the several theories developed to explain this reverse rate-dependency, its mechanism has been clarified only recently. The aim of the present study is to elucidate the mechanisms responsible for reverse rate-dependency in mammalian ventricular myocardium. Action potentials were recorded using conventional sharp microelectrodes from human, canine, rabbit, guinea pig, and rat ventricular myocardium in a rate-dependent manner. Rate-dependent drug-effects of various origin were studied using agents known to lengthen or shorten action potentials allowing thus to determine the drug-induced changes in APD as a function of the cycle length. Both drug-induced lengthening and shortening of action potentials displayed reverse rate-dependency in human, canine, and guinea pig preparations, but not in rabbit and rat myocardium. Similar results were obtained when repolarization was modified by injection of inward or outward current pulses in isolated canine cardiomyocytes. In contrast to reverse rate-dependence, drug-induced changes in APD well correlated with baseline APD values (i.e. that measured before the superfusion of drug or injection of current) in all of the preparations studied. Since the net membrane current (I(net)), determined from the action potential waveform at the middle of the plateau, was inversely proportional to APD, and consequently to cycle length, it is concluded that that reverse rate-dependency may simply reflect the inverse relationship linking I(net) to APD. In summary, reverse rate-dependency is an intrinsic property of drug action in the hearts of species showing positive APD - cycle length relationship, including humans. This implies that development of a pure K(+) channel blocking agent without reverse rate-dependent effects is not likely to be successful.

004 Dispersion of refractoriness promotes arrhythmogenesis in a murine model of Brugada syndrome

<h3>Introduction</h3> Recent experiments have implicated accentuated right ventricular (RV) transmural repolarization gradients in arrhythmogenesis in murine models of Brugada Syndrome. The present experiments investigated for the first time the involvement of heterogeneities in the corresponding ventricular effective refractory periods (VERP), hitherto assumed to vary in concert with action potential duration (APD). <h3>Methods</h3> VERPs and APDs were compared in the right ventricular (RV) and left ventricular (LV), epicardial and endocardial, regions of Langendorff-perfused wild-type (WT) and loss of function <i>Scn5a+/−</i> murine hearts. APDs were obtained during regular pacing and VERPs from a programmed S1S2 protocol, before and following addition of flecainide or quinidine. VERP dispersions were calculated from the difference in VERP between neighbouring cardiac regions. <h3>Results</h3> In contrast to APD values, <i>Scn5a+/−</i> hearts showed longer VERPs than WT. Both showed shorter VERPs in RV than LV epicardium and in the epicardia than endocardia of both LV and RV (Abstract 004 Figure 1). VERP/APD ratios were consistently larger in <i>Scn5a+/−</i> than WT but did not vary significantly between regions, contrasting with expectations from re-entrant arrhythmogenic mechanisms suggested previously for the gain of function, <i>Scn5aΔ/+</i> genotype. However, <i>Scn5a+/−</i> hearts showed greater VERP dispersions, particularly across the RV, than WT. Both flecainide and quinidine increased the regional VERP values particularly in <i>Scn5a+/−</i>. However, flecainide increased but quinidine decreased the VERP dispersions, particularly across the RV in the Scn5a+/− (Abstract 004 Figure 2). This is in direct parallel with the established pro- and anti-arrhythmic effects of flecainide and quinidine experimentally and clinically. <h3>Conclusions</h3> These findings implicate for the first time an involvement of VERP dispersions in the RV in the arrhythmogenesis found in murine <i>Scn5a+/−</i> hearts.

Reverse rate-dependent changes are determined by baseline action potential duration in mammalian and human ventricular preparations

Class III antiarrhythmic agents exhibit reverse rate-dependent lengthening of the action potential duration (APD). In spite of the several theories developed so far to explain this reverse rate-dependency (RRD), its mechanism has not yet been clarified. The aim of the present work was to further elucidate the mechanisms responsible for RRD in mammalian ventricular myocardium. Action potentials were recorded using conventional sharp microelectrodes from human, canine, rabbit and guinea pig ventricular myocardium in a rate-dependent manner varying the cycle length (CL) between 0.3 and 5 s. Rate-dependent drug effects were studied using agents known to lengthen or shorten action potentials, and these drug-induced changes in APD were correlated with baseline APD values. Both drug-induced lengthening (by dofetilide, sotalol, E-4031, BaCl(2), veratrine, BAY K 8644) and shortening (by mexiletine, tetrodotoxin, lemakalim) of action potentials displayed RRD, i.e., changes in APD were greater at longer than at shorter CLs. In rabbit, where APD is a biphasic function of CL, the drug-induced APD changes were proportional to baseline APD values but not to CL. Similar results were obtained when repolarization was modified by injection of inward or outward current pulses in isolated canine cardiomyocytes. In each case the change in APD was proportional to baseline APD (i.e., that measured before the superfusion of drug or injection of current). Also, the net membrane current (I (net)), determined from the action potential waveform at the middle of the plateau, was inversely proportional to APD and consequently with to CL. The results indicate that RRD is a common characteristic of all the drugs tested regardless of the modified ion current species. Thus, drug-induced RRD can be considered as an intrinsic property of cardiac membranes based on the inverse relationship between I (net) and APD.

Arrhythmogenic actions of the Ca2+ channel agonist FPL‐64716 in Langendorff‐perfused murine hearts

The experiments explored the extent to which alterations in L-type Ca2+ channel-mediated Ca2+ entry triggers Ca2+-mediated arrhythmogenesis in Langendorff-perfused murine hearts through use of the specific L-type Ca2+ channel modulator FPL-64716 (FPL). Introduction of FPL (1 μm) resulted in a gradual development (>10 min) of diastolic electrical events and alternans in spontaneously beating hearts from which monophasic action potentials were recorded. In regularly paced hearts, they additionally led to non-sustained and sustained ventricular tachycardia (nsVT and sVT). Programmed electrical stimulation (PES) resulted in nsVT and sVT after 5–10 and >10 min perfusion, respectively. Pretreatments with nifedipine, diltiazem and cyclopiazonic acid abolished arrhythmogenic tendency induced by subsequent introduction of FPL, consistent with its dependence upon both extracellular Ca2+ entry and the degree of filling of the sarcoplasmic reticular Ca2+ store. Values for action potential duration at 90% repolarization when any of these agents were applied to FPL-treated hearts became indistinguishable from those shown by untreated control hearts, in contrast to earlier reports of their altering in long QT syndrome type 3 and hypokalaemic murine models for re-entrant arrhythmogenesis. These arrhythmic effects instead correlated with alterations in Ca2+ homeostasis at the single-cell level found in investigations of the effects of both FPL and the same agents in regularly stimulated fluo−3 loaded myocytes. These findings are compatible with a prolonged extracellular Ca2+ entry that potentially results in an intracellular Ca2+ overload and produces the cardiac arrhythmogenecity following addition of FPL.

Two-Term Asymptotic Approximation of a Cardiac Restitution Curve

If spatial extent is neglected, ionic models of cardiac cells consist of systems of ordinary differential equations (ODEs) which have the property of excitability, i.e., a brief stimulus produces a prolonged evolution (called an action potential in the cardiac context) before the eventual return to equilibrium. Under repeated stimulation, or pacing, cardiac tissue exhibits electrical restitution: the steady-state action potential duration (APD) at a given pacing period B shortens as B is decreased. Independent of ionic models, restitution is often modeled phenomenologically by a one-dimensional mapping of the form APD(next) = f(B - APD(previous)). Under some circumstances, a restitution function f can be derived as an asymptotic approximation to the behavior of an ionic model.In this paper, extending previous work, we derive the next term in such an asymptotic approximation for a particular ionic model consisting of two ODEs. The two-term approximation exhibits excellent quantitative agreement with the actual restitution curve, whereas the leading-order approximation significantly underestimates actual APD values.

Effects of halothane on action potential configuration in sub-endocardial and sub-epicardial myocytes from normotensive and hypertensive rat left ventricle

Halothane shortens ventricular action potential duration (APD), as a consequence of its inhibitory effects on a variety of membrane currents, an effect that is greater in sub-endocardial than sub-epicardial myocytes. In hypertrophied ventricle, APD is prolonged as a consequence of electrical remodelling. In this study, we compared the effects of halothane on transmural APD in myocytes from normal and hypertrophied ventricle. Myocytes were isolated from the sub-endocardium and sub-epicardium of the left ventricle of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Action potentials were recorded before, during, and after a 1-min exposure to 0.6 mM halothane and APD measured from the peak of the action potential to repolarization at -50 mV (APD(-50 mV)). Data are presented as mean (SEM). In WKY myocytes, halothane reduced APD(-50 mV) from 21 (2) to 18 (2) ms (P<0.001, n=15) in sub-epicardial myocytes but abbreviated APD(-50 mV) to a greater extent in sub-endocardial myocytes (37 (4) to 28 (3) ms; P<0.001, n=14). In SHR myocytes, APD(-50 mV) values were prolonged compared with WKY and APD(-50 mV) was reduced by halothane from 36 (6) to 27 (4) ms (P<0.016) and from 77 (10) to 38 (4) ms (P<0.001) in sub-epicardial and sub-endocardial myocytes, respectively. In the SHR, hypertrophic remodelling was not homogeneous; APD(-50 mV) was prolonged to a greater extent in sub-endocardial than sub-epicardial cells. Halothane reduced APD to a greater extent in sub-endocardium than sub-epicardium in both WKY and SHR but this effect was larger proportionately in SHR myocytes. The transmural gradient of repolarization was reduced in WKY and effectively abolished in SHR by halothane, which might disturb normal ventricular repolarization.

Effects of K+ channel blockers on the action potential of hypoxic rabbit myocardium.

1. In order to assess the role of different ionic currents in hypoxia-induced action potential shortening, we investigated the effects of blockers of voltage-dependent and ATP-sensitive K(+)-channel on the membrane potential of hypoxic rabbit hearts and papillary muscles. The response to blocking of the inward rectifier was studied at three external K+ concentration: 2.5, 5, and 7.5 mM. 2. Hypoxia produced a progressive decline in action potential duration (APD) that levelled off after 15 to 20 min. Steady state APD values at 25% and 95% repolarization (APD25 and APD95) were 26.0 +/- 1.9% and 42.2 +/- 2.4% of controls respectively. 3. Tetraethylammonium (TEA, 10 mM) delayed but did not reduce APD shortening at the steady state. 4. Blocking of IK1 with a mixture of 0.2 mM Ba2+ and 4 mM Cs+ lengthened APD in normoxia and prevented APD95 shortening in hypoxia. The APD25 shortening was significantly attenuated at all [K]o. 5. Glibenclamide (Glib, 30 microM) did not prevent APD shortening, but produced a progressive action potential (AP) lengthening after 15 min of hypoxia. Steady levels of 48 +/- 3.5% and 62 +/- 5.0% of controls for APD25 and APD95 respectively were reached after 45 min. 6. The relation between APD25 and pacing rate was determined in normoxic and hypoxic papillary muscles and the effects of 2 mM 4-aminopyridine (4-AP) were examined. Hypoxia attenuated the APD25 shortening currently observed when the stimulation rate was lowered from 1 to 0.1 Hz without altering the plateau reduction occurring at frequencies above 2 Hz. These effects were potentiated by 4-AP.7. Our data suggest that the accelerated AP repolarization in hypoxic rabbit myocardium represents a delicate balance of several outward currents: IKI, IK-ATP. and at least one yet unidentified current component rather insensitive to changes in [K]o and to K+ channel blockers.

Monophasic Action Potential Duration During Programmed Electrical Stimulation

To examine changes in monophasic action potential duration (APD) with a pacing protocol similar to that used during electrophysiological testing, action potentials were recorded in vivo from the left ventricular apical endocardium of 12 normal mongrel dogs. The atrioventricular node was ablated and the dogs paced from the anterior right ventricle at a baseline cycle length of 1000 ms between interventions. Mean steady-state APD (APDss) was 266 +/- 7 ms at a pacing cycle length (PCL) of 1000 ms. Two pacing protocols were used. The first consisted of a sudden acceleration in pacing from a cycle length of 1000 ms to one between 300 and 600 ms. The second consisted of an 8-beat train at a cycle length of 400 ms followed by a premature beat at a coupling interval of 280 ms followed by a pause. The inter-train pause varied between 1 second and 32 seconds. With a sudden acceleration in pacing rate, steady-state values for APD at the faster PCLs were significantly smaller than APDss at 1000 ms with a change to cycle lengths of 600 ms (247 +/- 29 ms), 500 ms (229 +/- 21 ms), 400 ms (220 +/- 17 ms), and 300 ms (203 +/- 31 ms; P less than 0.01 for all comparisons). The time constant of the change in APD was shorter at a PCL of 300 ms (14.9 +/- 0.8 s) than 600 ms (20.3 +/- 4.7 s; P less than 0.05). With drive train pacing and incorporating an inter-train pause, the percent drop in steady-state APD compared to APD for the first train ranged from 10.1% with a 1-second inter-train pause to 2.1% with a 32-second pause. The difference in APD between the first drive train and drive trains after at least 3 minutes of pacing when APD had stabilized was not significant for an inter-train pause exceeding 8 seconds. (1) with a sudden acceleration in pacing rate, endocardial APD in vivo decreases exponentially. The faster the new rate, the shorter the new steady-state APD and the shorter the time constant. (2) When pacing using an 8-beat drive train and an inter-train pause, there is a decremental shortening in APD for pause lengths shorter than 16 seconds. Thus, while performing programmed stimulation using a pause, a conditioning period of at least 2 minutes should be used prior to diastole scanning to allow APD to achieve a steady state.

Effects of Propranolol on Premature Action Potentials in Canine Purkinje and Ventricular Muscle

We compared the effects of a low (0.09 microgram/ml) concentration of propranolol expected to produce only beta-adrenoceptor blockade and a high concentration (0.9 microgram/ml) expected to produce additional direct local anesthetic-like electrophysiological effects on basic and premature action potentials. Both isolated dog cardiac Purkinje and ventricular muscle fibers were examined using conventional microelectrode techniques. The low concentration of propranolol produced no significant electrophysiological change in either fiber type. The high concentration of propranolol shortened the action potential duration and refractoriness while decreasing the maximal upstroke velocity (Vmax) in both Purkinje and ventricular muscle fibers at a constant basic cycle length. In Purkinje fibers, the high concentration also slowed the kinetics of restitution of the action potential duration (tau c from 124.6 +/- 6.5 to 201.4 +/- 16.0 ms, p less than 0.01, n = 7), slowed the recovery kinetics of Vmax, and shifted the early portion of the normalized restitution curve toward longer action potential duration values in both fiber types. The range of premature action potential durations, defined as the difference between action potential durations during the first 100 ms of restitution, was decreased by the high concentration of propranolol in both Purkinje and ventricular muscle fibers by 39.5% and 33.9%, respectively. These findings indicate that (a) low concentrations of propranolol produced no direct electrophysiological effect, and (b) high concentrations of propranolol produced several potential antiarrhythmic effects in addition to the previously reported effects on Vmax and the action potential duration.

Effects of Antiarrhythmic Drugs on Premature Action Potential Duration in Canine Ventricular Muscle Fibers

The range of premature action potential duration (APDs) during the first 100 ms of electrical restitution was determined in canine ventricular muscle fibers (VM) in the presence of lidocaine (4 and 8 micrograms/ml), mexiletine (8 micrograms/ml), flecainide (2 and 4 micrograms/ml), procainamide (50 micrograms/ml), quinidine (10 micrograms/ml), and disopyramide (10 micrograms/ml). The drug effects on the characteristics of action potential at the basic cycle lengths (BCLs) of 500 and 1,000 ms were similar to those reported previously. At control, the range of premature APDs was approximately 40 ms at BCL of 1,000 ms and approximately 30 ms at BCL of 500 ms. It was decreased 26-52% by lidocaine, mexiletine, and flecainide, not changed significantly by procainamide, and increased 17-53% by quinidine and disopyramide. The range of premature APDs at control and in the presence of drugs in ventricular muscle was smaller than in Purkinje fibers at the same or lower drug concentrations (Varro et al., J Pharmacol Exp Ther 1985;233:304). The four factors influencing the premature APD range in the Purkinje fibers operated in the VM as follows: The difference between the duration of effective refractory period (ERP) and the APD at BCL (APDb) (i.e., the ERP-APDb interval) was increased by lidocaine, mexiletine, and flecainide; normalized restitution curve was shifted toward longer premature APD values only by flecainide; the kinetics of restitution were slowed only by procainamide; and APD at BCL of 1,000 ms was shortened by lidocaine and mexiletine, and prolonged at BCLs of 1,000 and 500 ms by the other four drugs.(ABSTRACT TRUNCATED AT 250 WORDS)