Restitution Curve Research Articles

Idiopathic Ventricular Fibrillation Characterized by Spatial Heterogeneity of Action Potential Duration and Its Restitution Kinetics

A 44-year-old man who had suffered multiple episodes of syncope presented with ventricular fibrillation (VF). Structural heart disease was ruled out. Programmed stimulation induced VF at the right ventricular apex (RVA) but not at the outflow tract (RVOT). Monophasic action potential duration (MAPD) at a basic cycle length of 400 msec was shorter at the RVA than at the RVOT (208 versus 231 ms). The maximum slope of the MAPD restitution curve at the 400-msec cycle length was much steeper at the RVA than at the RVOT (1.4 versus 1.0). Such spatial heterogeneity of the MAPD and of its restitution may facilitate wavebreak and functional reentry, predisposing to VF.

Investigation of pacing site-related changes in global restitution dynamics by non-contact mapping

The determination of dynamic changes in ventricular repolarization may provide insight into arrhythmogenic mechanisms as a consequence of pacing site. This study investigated acute pacing site effects on global characteristics of electrical restitution using high resolution, non-contact mapping (NCM). Activation-recovery intervals (ARIs) were determined from reconstructed left ventricular electrograms by the NCM system and were analysed during pacing from the right atrial appendage (RAA, intrinsic), right ventricular apex (RVA), and right ventricular septum (RVS) with extrasystoles delivered at intermediate and short coupling intervals in anesthetized swine (n = 5). Electrical restitution curves were determined by the S1-S2 pacing protocol. Activation-recovery interval restitution slopes were determined by the overlapping linear segments regression method. Global distribution of repolarization was defined as the coefficient of variation of the ARIs during restitution. The maximum ARI slopes yielded by RVA pacing were significantly greater than RAA pacing (0.44 vs. 0.32; P < 0.05) and RVS pacing (0.44 vs. 0.37; P = 0.05). There was no significant difference between RAA and RVS pacing (0.32 vs. 0.37). The global distribution of ARIs during restitution from RVA pacing was significantly greater than RAA pacing (12.0 vs. 8.1%; P < 0.05). Right ventricular apex pacing is associated with impaired global repolarization patterns compared to RAA and RVS. These observations support the hypothesis that RVA pacing may be associated with increased risk of ventricular arrhythmias compared to RVS pacing.

Effect of Pacing Site on Systolic Mechanical Restitution Curves in the In Vivo Canine Model

Pacing site is known to influence the contractile state of the ventricle. Non-physiologic pacing sites such as the right ventricular apex (RVA) or left ventricular freewall (LVFW) have been shown to decrease the contractile state of normal myocardium, due to abnormal electrical propagation. The impact of pacing at these sites may alter mechanical restitution (MR), a fundamental cardiac property involving the electro-mechanical regulation of contraction. This, in turn, may affect cardiac function. The present study was conducted to determine if pacing site alters the time constant of MR: tau. Anesthetized canines (n = 6) were acutely paced at four sites: right atrium (RA), RVA, right ventricular septum (RVS), and LVFW. MR data was captured by the S1-S2 pacing protocol and used to create MR curves, generating a restitution time constant, tau, at each site. No significant difference in tau was found between pacing sites. A linear regression analysis of MR curves revealed that there was no significant difference in slope between pacing sites. Although pacing site has been found to influence the contractile state of the ventricle, this is the first known study to demonstrate no change in tau in an in vivo preparation. This suggests that alteration of electro-mechanical coupling described by MR is not sufficiently robust to provide insight into pacing site and cardiac function in healthy hearts.

Electrophysiologic Actions of d,l-Sotalol and GLG-V-13 in Ischemically Injured Canine Epicardium

The electrophysiologic actions of the class III antiarrhythmic agents, GLG-V-13 and d,l-sotalol, were examined in superfused normal and ischemically injured epicardium. Both drugs produced concentration and reverse-use dependent prolongation of the action potential duration in normal myocardium without altering resting potential, action potential amplitude, or Vmax. Both drugs increased the slope of restitution curves in normal epicardium but prevented action potential alternans at short cycle lengths. The response of superfused ischemically injured left ventricular epicardium to drug 4 days after coronary artery ligation was determined by the extent of ischemic injury, with no electrophysiologic changes produced within epicardial cells characterized by prominent action potential shortening and no further action potential shortening with pacing. Cells demonstrating less severe injury (as evidenced by less severely depressed action potential amplitudes, Vmax, and action potential durations) retained a limited ability to respond to drug administration with action potential prolongation. A concentration-dependent, increased disparity of action potential duration was observed concurrent with the ability of single premature stimuli to induce monomorphic tachycardia. The present data demonstrate a variable response of ischemically injured canine epicardial cells to action potential prolongation with GLG-V-13 and d,l-sotalol, facilitating localized reentry in vitro, despite a failure of the same drugs to facilitate reentrant tachycardia in vivo.

Dynamics of sustained reentry in a loop model with discrete gap junction resistances

The dynamics of reentry is studied in a one-dimensional loop of model cardiac cells with discrete intercellular gap junction resistance (R). Each cell is represented by a continuous cable with ionic current given by a modified Beeler-Reuter formulation. For R below a limiting value, propagation is found to change from period-1 to quasiperiodic (QP) at a critical loop length (L(crit)) that decreases with R. Quasiperiodic reentry exists from L(crit) to a minimum length (L(min)), which also shortens with R. The decrease of L(crit) (R) is not a simple scaling, but the bifurcation can still be predicted from the slope of the restitution curve giving the duration of the action potential as a function of the diastolic interval. However, the shape of the restitution curve changes with R. An increase of R does not seem to increase the number of possible QP solutions since, as in the continuous cable, only two QP modes of propagation were found despite an extensive search through alternative initial conditions.

Nitric oxide mediates the vagal protective effect on ventricular fibrillation via effects on action potential duration restitution in the rabbit heart

We have previously shown that direct vagus nerve stimulation (VNS) reduces the slope of action potential duration (APD) restitution while simultaneously protecting the heart against induction of ventricular fibrillation (VF) in the absence of any sympathetic activity or tone. In the current study we have examined the role of nitric oxide (NO) in the effect of VNS. Monophasic action potentials were recorded from a left ventricular epicardial site on innervated, isolated rabbit hearts (n = 7). Standard restitution, effective refractory period (ERP) and VF threshold (VFT) were measured at baseline and during VNS in the presence of the NO synthase inhibitor N(G)-nitro-L-arginine (L-NA, 200 microm) and during reversing NO blockade with L-arginine (L-Arg, 1 mm). Data represent the mean +/- S.E.M. The restitution curve was shifted upwards and became less steep with VNS when compared to baseline. L-NA blocked the effect of VNS whereas L-Arg restored the effect of VNS. The maximum slope of restitution was reduced from 1.17 +/- 0.14 to 0.60 +/- 0.09 (50 +/- 5%, P < 0.0001) during control, from 0.98 +/- 0.14 to 0.93 +/- 0.12 (2 +/- 10%, P = NS) in the presence of L-NA and from 1.16 +/- 0.17 to 0.50 +/- 0.10 (41 +/- 9%, P = 0.003) with L-Arg plus L-NA. ERP was increased by VNS in control from 119 +/- 6 ms to 130 +/- 6 ms (10 +/- 5%, P = 0.045) and this increase was not affected by L-NA (120 +/- 4 to 133 +/- 4 ms, 11 +/- 3%, P = 0.0019) or L-Arg with L-NA (114 +/- 4 to 123 +/- 4 ms, 8 +/- 2%, P = 0.006). VFT was increased from 3.0 +/- 0.3 to 5.8 +/- 0.5 mA (98 +/- 12%, P = 0.0017) in control, 3.4 +/- 0.4 to 3.8 +/- 0.5 mA (13 +/- 12%, P = 0.6) during perfusion with L-NA and 2.5 +/- 0.4 to 6.0 +/- 0.7 mA (175 +/- 50%, P = 0.0017) during perfusion with L-Arg plus L-NA. Direct VNS increased VFT and flattened the slope of APD restitution curve in this isolated rabbit heart preparation with intact autonomic nerves. These effects were blocked using L-NA and reversed by replenishing the substrate for NO production with L-Arg. This is the first study to demonstrate that NO plays an important role in the anti-fibrillatory effect of VNS on the rabbit ventricle, possibly via effects on APD restitution.

Restitution analysis of alternans and its relationship to arrhythmogenicity in hypokalaemic Langendorff-perfused murine hearts

Alternans and arrhythmogenicity were studied in hypokalaemic (3.0 mM K+) Langendorff-perfused murine hearts paced at high rates. Epicardial and endocardial monophasic action potentials were recorded and durations quantified at 90% repolarization. Alternans and arrhythmia occurred in hypokalaemic, but not normokalaemic (5.2 mM K+) hearts (P < 0.01): this was prevented by treatment with lidocaine (10 μM, P < 0.01). Fourier analysis then confirmed transition from monomorphic to polymorphic waveforms for the first time in the murine heart. Alternans and arrhythmia were associated with increases in the slopes of restitution curves, obtained for the first time in the murine heart, while the anti-arrhythmic effect of lidocaine was associated with decreased slopes. Thus, hypokalaemia significantly increased (P < 0.05) maximal gradients (from 0.55 ± 0.14 to 2.35 ± 0.67 in the epicardium and from 0.67 ± 0.13 to 1.87 ± 0.28 in the endocardium) and critical diastolic intervals (DIs) at which gradients equalled unity (from −2.14 ± 0.52 ms to 50.93 ± 14.45 ms in the epicardium and from 8.14 ± 1.49 ms to 44.64 ± 5 ms in the endocardium). While treatment of normokalaemic hearts with lidocaine had no significant effect (P > 0.05) on either maximal gradients (0.78 ± 0.27 in the epicardium and 0.83 ± 0.45 in the endocardium) or critical DIs (6.06 ± 2.10 ms and 7.04 ± 3.82 ms in the endocardium), treatment of hypokalaemic hearts with lidocaine reduced (P < 0.05) both these parameters (1.05 ± 0.30 in the epicardium and 0.89 ± 0.36 in the endocardium and 30.38 ± 8.88 ms in the epicardium and 31.65 ± 4.78 ms in the endocardium, respectively). We thus demonstrate that alternans contributes a dynamic component to arrhythmic substrate during hypokalaemia, that restitution may furnish an underlying mechanism and that these phenomena are abolished by lidocaine, both recapitulating and clarifying clinical findings.

Influence of channel subunit composition on L-type Ca2+ current kinetics and cardiac wave stability

Previous studies have demonstrated that the slope of the function relating the action potential duration (APD) and the diastolic interval, known as the APD restitution curve, plays an important role in the initiation and maintenance of ventricular fibrillation. Since the APD restitution slope critically depends on the kinetics of the L-type Ca(2+) current, we hypothesized that manipulation of the subunit composition of these channels may represent a powerful strategy to control cardiac arrhythmias. We studied the kinetic properties of the human L-type Ca(2+) channel (Ca(v)1.2) coexpressed with the alpha(2)delta-subunit alone (alpha(1C) + alpha(2)delta) or in combination with beta(2a), beta(2b), or beta(3) subunits (alpha(1C) + alpha(2)delta + beta), using Ca(2+) as the charge carrier. We then incorporated the kinetic properties observed experimentally into the L-type Ca(2+) current mathematical model of the cardiac action potential to demonstrate that the APD restitution slope can be selectively controlled by altering the subunit composition of the Ca(2+) channel. Assuming that beta(2b) most closely resembles the native cardiac L-type Ca(2+) current, the absence of beta, as well as the coexpression of beta(2a), was found to flatten restitution slope and stabilize spiral waves. These results imply that subunit modification of L-type Ca(2+) channels can potentially be used as an antifibrillatory strategy.

Mechanism Underlying Initiation of Paroxysmal Atrial Flutter/Atrial Fibrillation by Ectopic Foci

The mechanisms underlying paroxysmal atrial flutter/atrial fibrillation initiation by ectopic foci from various locations are unclear. We used parallel computational techniques to study an anatomically accurate 3-dimensional atrial structure incorporating a detailed ionic-current model of an atrial myocyte. At the single-cell level, upregulation of the L-type Ca2+ current I(Ca,L) steepened restitution curves of action potential duration and conduction velocity compared with the control. Spontaneous firings of ectopic foci, coupled with sinus activity, produced dynamic spatial dispersions of repolarization, including discordant alternans, which caused conduction block and reentry only for the elevated I(Ca,L) case. For each foci location, a vulnerable window for atrial flutter/atrial fibrillation induction was identified as a function of the coupling interval and focus cycle length. For ectopic foci in the pulmonary veins and left atrium, the site of conduction block and reentry gradually shifted, as a function of coupling interval, from the right atrium to the interatrial area and finally to the left atrium. The size of the vulnerable window was largest for pulmonary vein foci, becoming markedly smaller for right atrial foci, especially those near the sinoatrial node. These findings suggest that a mechanism of dynamically induced repolarization dispersion, especially discordant alternans, underlies the induction of atrial flutter/atrial fibrillation by atrial ectopic foci. The sites and likelihood of reentry induction varied according to ectopic focus location and timing, with the largest vulnerable window corresponding to the pulmonary vein region.

Autonomic modulation of electrical restitution, alternans and ventricular fibrillation initiation in the isolated heart

Abnormal autonomic nerve activity is a strong prognostic marker for ventricular arrhythmias but the mechanisms underlying the autonomic modulation of ventricular fibrillation (VF) initiation are poorly understood. We examined the effects of direct sympathetic (SS) and vagus (VS) nerve stimulation on electrical restitution, alternans and VF threshold (VFT) in a novel isolated rabbit heart preparation with intact dual autonomic innervation. Monophasic Action Potentials (MAPs) were recorded from a left ventricular epicardial site on innervated, isolated rabbit hearts (n=16). Standard restitution, effective refractory period (ERP), electrical alternans and VFT were measured at baseline and during SS and VS separately. The restitution curve was shifted downwards and made steeper with SS whilst VS caused an upward shift and a flattening of the curve. The maximum slope of restitution was increased from 1.30+/-0.10 at baseline to 1.86+/-0.17 (by 45+/-12%, P<0.01) with SS and decreased to 0.69+/-0.10 (by 51+/-6%, P<0.001) with VS. ERP was decreased from 127.3+/-2.5 ms to 111.8+/-1.8 ms with SS (by 12+/-2%, P<0.001) and increased to 144.0+/-2.2 ms with VS (by 13+/-2%, P<0.001). VFT was decreased from 4.7+/-0.6 mA to 1.9+/-0.5 mA with SS (by 64+/-5%, P<0.001) and increased to 8.7+/-1.1 mA with VS (by 89+/-14%, P<0.0005). There was a significant inverse relationship between the maximum slope of restitution and VFT (r=-0.63, P<0.0001). When compared with baseline, SS caused electrical alternans at longer pacing cycle lengths (139.0+/-8.4 vs. 123.0+/-7.8 ms, P<0.01) with greater degree of alternans (32.5+/-9.9 vs. 15.4+/-3.2%, P<0.05). It also caused a wider range of cycle lengths where alternans occurred (53.0+/-6.2 vs. 41.0+/-7.0 ms, P<0.05) whilst vagus nerve stimulation shortened this range (33.0+/-7.3 ms, P<0.001). Sympathetic stimulation increased maximum slope of restitution and electrical alternans but decreased ERP and VF threshold whilst vagus nerve stimulation had opposite effects. The interaction between action potential duration and beat-to-beat interval may play an important role in the autonomic modulation of VF initiation.

Theory of action potential wave block at-a-distance in the heart

Propagation failure of an action potential wave at a finite distance from its source (so-called type-II block) may cause spiral wave formation or wave breakup in the heart, phenomena that are believed to underlie lethal and nonlethal heart rhythm disorders. In this study, we develop a sufficient condition for this type of block in a homogeneous, spatially one-dimensional system. Using a topological argument, we find that type-II block of a wave will always occur when launched within a finite range of times if the velocity of the trailing edge of the preceding wave, as measured at the stimulus site, is smaller than the velocity of a wave launched with the minimum diastolic interval (DI) for which propagation is possible. This "blocking condition" is robust, remaining valid even when memory and waveback electrotonic effects are included. The condition suggests that type-II block is greatly facilitated when waves are initiated at irregular intervals in time such that (1) the velocities of consecutive waves are as different as possible and (2) the DIs preceding each wave fall on the steeply sloped portion of the action potential duration restitution curve as often as possible. The set of timing intervals between stimuli that are predicted by the blocking condition to produce block are found to be consistent with these guidelines, and also to agree well with a coupled-maps computer simulation model, for the case of waves launched by four rapidly and irregularly timed stimuli.

Mechanical restitution curves — a possible load independent assessment of contractile function

The time constant of mechanical restitution (T((MRC))), proposed to reflect changes in calcium release and uptake, has been shown to increase in left ventricular (LV) failure, and might have a potential as an index of contractile function. However, in vivo studies of the effect on T((MRC)) of changing loading conditions in the normal and failing heart have not been reported. Consequently, in this study, we tested the hypothesis that the increase in T((MRC)) in vivo is independent of preload and afterload. Left ventricular pressure-volume loops were assessed at baseline in eight open chest pigs using the combined pressure-volume conductance catheter technique during right atrial pacing at 120b/min. Mechanical restitution curves (MRC) were constructed during four different loading conditions in all eight animals: uninfluenced load, reduced preload (balloon catheter in v. cava inferior), increased afterload (balloon catheter in descending aorta), and increased preload combined with reduced afterload (aortocaval shunting). Acute LV failure was then induced by microembolization through the left main coronary artery, and the experimental protocol was repeated. Contractile response was defined as the maximal first derivative of pressure (dP/dt(max)), and T((MRC)) was calculated using a least square approximation algorithm. Hemodynamic data 30min after microembolization showed decreased mean arterial pressure (98+/-14-67+/-10mmHg, (mean+/-SD) P<0.0001) and dP/dt(max) (1482+/-193-1001+/-125mmHg/s, P=0.001). Stroke volume decreased from 30+/-5 to 20+/-5ml (P<0.0001) compared to baseline, and preload recruitable stroke work decreased from 52+/-7 to 31+/-10mmHg (P=0.002). T((MRC)) increased in all eight animals after induction of LV failure at all loading conditions. There was no difference between the different loading conditions at baseline, nor at LV heart failure, but T((MRC)) increased significantly after the induction of heart failure (ANOVA, two ways). We have shown that the left ventricular T((MRC)) increases after developed heart failure. The increase in T((MRC)) was independent on loading conditions and thus have a potential for a contractility index.

The Rate- and Species-Dependence of Short-Term Memory in Cardiac Myocytes

Short-term memory is an intrinsic property of paced cardiac myocytes that reflects the influence of pacing history, and not just the immediately preceding diastolic interval (DI), on the action potential duration (APD). Although it is recognized that short-term memory affects the dynamics of cardiac myocytes in general, and the onset of irregular cardiac rhythm in particular, its has never been adequately quantified or measured directly in experiments or numerical simulations, mainly due to the absence of appropriate techniques. As a result, very little is known about the rate- and species dependent behavior of short-term memory. In this study, we introduce a new approach that allows one to estimate how much short-term memory, M(S), is present in the cardiac myocyte at different pacing rates. The new quantification is based on the fact that pacing history affects not only the APD, but the entire dynamics of paced cardiac myocytes, in particular the restitution curve. Using the patch clamp technique and numerical simulations, we measured short-term memory restitution-the dependence of M(S) on the cycle length-in isolated rabbit and guinea pig ventricular myocytes. In both species, M(S) is rate- and species-dependent, displaying a biphasic behavior as a function of cycle length. Moreover, our results indicate that there is a significant difference in M(S) measured between both species at small cycle lengths. Numerical simulations suggest that the kinetics of the rapidly activating delayed rectifier potassium current I(Kr) is partially responsible for this difference.

An ionically based mapping model with memory for cardiac restitution.

Many features of the sequence of action potentials produced by repeated stimulation of a patch of cardiac muscle can be modeled by a 1D mapping, but not the full behavior included in the restitution portrait. Specifically, recent experiments have found that (i) the dynamic and S1-S2 restitution curves are different (rate dependence) and (ii) the approach to steady state, which requires many action potentials (accommodation), occurs along a curve distinct from either restitution curve. Neither behavior can be produced by a 1D mapping. To address these shortcomings, ad hoc 2D mappings, where the second variable is a "memory" variable, have been proposed; these models exhibit qualitative features of the relevant behavior, but a quantitative fit is not possible. In this paper we introduce a new 2D mapping and determine a set of parameters for it that gives a quantitatively accurate description of the full restitution portrait measured from a bullfrog ventricle. The mapping can be derived as an asymptotic limit of an idealized ionic model in which a generalized concentration acts as a memory variable. This ionic basis clarifies how the present model differs from previous models. The ionic basis also provides the foundation for more extensive cardiac modeling: e.g., constructing a PDE model that may be used to study the effect of memory on propagation. The fitting procedure for the mapping is straightforward and can easily be applied to obtain a mathematical model for data from other experiments, including experiments on different species.

Effects of Pacing Rate on Mechanical Restitution Within the In vivo Canine Heart: Study of the Force–Frequency Relationship

It is known that as stimulation frequency is increased in a healthy heart, a corresponding increase in LV contractile function (dP/dt(max)) is observed, i.e., force-frequency relationship. The impact of this relationship on systolic and diastolic mechanical restitution in an ejecting, in vivo preparation has yet to be explored. Understanding this relationship may lead to further insight on the cellular processes that govern the contraction and relaxation of the heart, in addition to providing a safer, more feasible clinical diagnostic tool. Anesthetized canines (n = 8) were paced from the RA at rates of 130, 150, and 180 bpm. At each rate, extrasystoles were delivered at varying intervals. The LV dP/dt(max) and dP/dt(min) associated with the extrasystolic beat were expressed as a percentage of steady-state levels and plotted as a function of the extrasystolic interval to obtain mechanical restitution curves. The systolic restitution time constant length decreased significantly with all increases in heart rate, P < 0.05. In the diastolic case, significant decreases in restitution time constants were seen when heart rate was increased from 130 bpm to 180 bpm, and from 150 bpm to 180 bpm, P < 0.05. This study was the first to quantify the finding that the time constant of restitution significantly and consistently decreased with a consistent increase in heart rate. The identification of such behavior may be employed to develop stimulation protocols and chronic diagnostic tools to more safely and sensitively identify and optimize the clinical status of patients receiving pacing therapy.

Spatial heterogeneity of the restitution portrait in rabbit epicardium.

Spatial heterogeneity of repolarization can provide a substrate for reentry to occur in myocardium. This heterogeneity may result from spatial differences in action potential duration (APD) restitution. The restitution portrait (RP) measures many aspects of rate-dependent restitution: the dynamic restitution curve (RC), S1-S2 RC, and short-term memory response. We used the RP to characterize epicardial patterns of spatial heterogeneity of restitution that were repeatable across animals. New Zealand White rabbit ventricles were paced from the epicardial apex, midventricle, or base, and optical action potentials were recorded from the same three regions. A perturbed downsweep pacing protocol was applied that measured the RP over a range of cycle lengths from 1,000 to 140 ms. The time constant of short-term memory measured close to the stimulus was dependent on location. In the midventricle the mean time constant was 19.1 +/- 1.1 s, but it was 39% longer at the apex (P < 0.01) and 23% longer at the base (P = 0.03). The S1-S2 RC slope was dependent on pacing site (P = 0.015), with steeper slope when pacing from the apex than from the base. There were no significant repeatable spatial patterns in steady-state APD at all cycle lengths or in dynamic RC slope. These results indicate that transient patterns of epicardial heterogeneity of APD may occur after a change in pacing rate. Thus it may affect cardiac electrical stability at the onset of a tachycardia or during a series of ectopic beats. Differences in restitution with respect to pacing site suggest that vulnerability may be affected by the location of reentry or ectopic foci.

Abnormal Intracellular Calcium Handling Underlying T-Wave Alternans and Its Hysteresis

Aims: To investigate the mechanism underlying T-wave alternans (TWA) and its hysteresis under ischemia conditions. Methods: Transmembrane action potential (AP) from endocardial, M, and epicardial cells and monophasic AP (MAP) from four epicardial sites were recorded in ventricular wedge preparation and in isolated intact rabbit heart, respectively. The AP/MAP duration (APD/ MAPD), effective refractory period (ERP), activation time, and APD/MAPD restitution were determined under control and ischemia conditions. The effects of ryanodine (0.01 and 1 µmol·l^–1) on TWA, and the effects of low extracellular Ca²⁺ and 4-aminopyridine on its hysteresis were studied. Results: Ischemia shortened the APD/MAPD and effective refractory period of all recording sites symmetrically, except the APD of M cells, which shortened markedly. In the ischemia group, TWA was induced within a cycle length (CL) range from 160 to 250 ms, which corresponded to a diastolic interval region of 0–70 ms. In this diastolic interval region, the repolarization restitution curve was the steepest (slope > 1.0). All TWA were accompanied by repolarization alternations. Low concentration ryanodine (0.01 µmol·l^–1) facilitated TWA, high concentration (1 µmol·l^–1) abolished it. Alternans of calcium transient were observed in myocytes purfused with ischemia solution during rapid stimulation. Ryanodine (0.1 µmol·l^–1) abolished alternans of calcium transient, and ryanodine (0.01 µmol·l^–1) facilitated them. After 60 min pacing at a CL of 200 ms, TWA persisted until the initial several beats at a CL of 300 ms at which a TWA was exceptional. The suppression of hysteresis by low extracellular Ca²⁺ and 4-aminopyridine indicated an underlying role of the intracellular Ca²⁺ overload and transient outward current (I_to). Conclusion: TWA is principally due to repolarization alternans, which is secondary to steep APD/MAPD restitution, and relates to intracellular calcium cycling. Hysteresis relates to intracellular Ca²⁺ overload and I_to.

Action Potential Duration Restitution Portraits of Mammalian Ventricular Myocytes: Role of Calcium Current

Construction of the action potential duration (APD) restitution portrait allows visualization of multiple aspects of the dynamics of periodically paced myocytes at various basic cycle lengths (BCLs). For the first time, we obtained the restitution portrait of isolated rabbit and guinea pig cardiac ventricular myocytes and analyzed the time constant, τ, of APD accommodation and the slopes of different types of restitution curves, S dyn and S 12, measured at varying BCLs. Our results indicate that both τ and the individual slopes are species and pacing dependent. In contrast, the mutual relationship between slopes S dyn and S 12 does not depend on pacing history, being a generic feature of the species. In addition, the maximum slope S 12, measured in the restitution portrait at the lowest BCL, predicts the onset of alternans. Further, we investigated the role of the L-type calcium current, I Ca-L, in the restitution portrait. We found that I Ca-L dramatically affects APD accommodation, as well as the individual slopes S dyn and S 12 measured in the restitution portrait. However, peak calcium current plays a role only at small values of BCL. In conclusion, the results demonstrate that the restitution portrait is a powerful technique to investigate restitution properties of periodically paced cardiac myocytes and the onset of alternans, in particular. Moreover, the data also show that I Ca-L plays a crucial role in multiple aspects of cardiac dynamics measured through the restitution portrait.

Action potential characterization in intact mouse heart: steady-state cycle length dependence and electrical restitution

Transgenic mice have been increasingly utilized to investigate the molecular mechanisms of cardiac arrhythmias, yet the rate dependence of the murine action potential duration and the electrical restitution curve (ERC) remain undefined. In the present study, 21 isolated, Langendorff-perfused, and atrioventricular node-ablated mouse hearts were studied. Left ventricular and left atrial action potentials were recorded using a validated miniaturized monophasic action potential probe. Murine action potentials (AP) were measured at 30, 50, 70, and 90% repolarization (APD(30)-APD(90)) during steady-state pacing and varied coupling intervals to determine ERCs. Murine APD showed rate adaptation as well as restitution properties. The ERC time course differed dramatically between early and late repolarization: APD(30) shortened with increasing S1-S2 intervals, whereas APD(90) was prolonged. When fitted with a monoexponential function, APD(30) reached plateau values significantly faster than APD(90) (tau = 29 +/- 2 vs. 78 +/- 6 ms, P < 0.01, n = 12). The slope of early APD(90) restitution was significantly <1 (0.16 +/- 0.02). Atrial myocardium had shorter final repolarization and significantly faster ERCs that were shifted leftward compared with ventricular myocardium. Recovery kinetics of intracellular Ca(2+) transients recorded from isolated ventricular myocytes at 37 degrees C (tau = 93 +/- 4 ms, n = 18) resembled the APD(90) ERC kinetics. We conclude that mouse myocardium shows AP cycle length dependence and electrical restitution properties that are surprisingly similar to those of larger mammals and humans.

Electrical pacing counteracts intrinsic shortening of action potential duration of neonatal rat ventricular cells in culture

Previous studies have demonstrated the relationship between the functional electrophysiological properties of cultured neonatal rat ventricular myocytes (NRVMs) and the ability of the substrate to induce and sustain arrhythmia. The goal of this study was to examine the effects of chronic pacing at a constant rate akin to that in vivo, on the functional electrophysiological properties of NRVM monolayers. Confluent NRVM monolayers grown on 20 mm diameter cover slips were left either unpaced or were stimulated at 3 Hz for the duration of the culture, and were optically mapped on days 4, 6, or 8. Action potential duration at 80% repolarization (APD 80), conduction velocity (CV), and Kv4.3 ( I to) and NCX protein expression were measured. The effects of the excitation–contraction uncoupler 2,3-butadione monoxime (BDM) were also investigated. The 2 Hz APD 80 of non-paced monolayers decreased significantly on days 6 (137.1 ± 13.9 ms) and 8 (109.8 ± 9.0 ms) compared with day 4 (197.0 ± 11.8 ms), while that of paced monolayers did not (206.8 ± 9.7, 209.1 ± 9.2, and 210.6 ± 9.9 ms, respectively). The 2 Hz CV of non-paced monolayers increased significantly on days 6 (26.0 ± 1.6 cm/s) and 8 (26.5 ± 1.0 cm/s) compared with day 4 (20.0 ± 1.0 cm/s), while that of paced monolayers did not change significantly (26.0 ± 2.0, 26.0 ± 1.0, and 23.8 ± 1.2 cm/s, respectively). The restitution curves of APD 80 and CV of paced monolayers were also unchanging from days 4 through 8. Despite the unchanging APD 80 and CV, a decrease in Kv4.3 expression and an increase in NCX expression were observed in paced compared with non-paced monolayers. Cessation of pacing or administration of BDM caused a reversal of phenotype back to that of non-paced monolayers. In summary, chronic electrical stimulation of confluent NRVM monolayers results in stabilization of APD 80 and an advancement of the developmental rise of CV that is mediated by electromechanical coupling. These effects produce a steadier functional phenotype that may be beneficial for electrophysiological studies.