Infarcted Canine Heart Research Articles

Slow antegrade excitation and delayed retrograde activation through an "inexcitable zone": A basis for arrhythmia formation in infarcted myocardium ex vivo.

The electrophysiologic mechanism for rate-dependent PVBs associated with double potentials (DPs) was investigated in infarcted canine hearts using bipolar and intracellular microelectrode recordings. Dogs exhibiting rate-related ventricular ectopic beats (coupling interval, 390 ± 54 milliseconds) during sinus rhythm or atrial pacing were studied 4-5 days (N = 63) or 25 days (N = 16) following anterior descending coronary artery ligation. Sites of DP and rate-dependent arrhythmia formation were identified in vivo using bipolar recordings for subsequent ex vivo studies. Rate-dependent conduction delays with increasing duration isoelectric intervals representing very slow conduction were observed at sites of DP formation, frequently provoking both manifest and concealed reentry (non-stimulated beats) over a narrow range of paced cycle lengths. Both slow antegrade and retrograde activation across an inexcitable gap (reflection) were integral components of extrasystole formation. Retrograde reflection to a region of very slow conduction (mid-potential) during antegrade activation was routinely observed at 4-5 days (42 of 63 preparations, 67%) and 25 days (22 of 26 preparations, 85%) postcoronary artery ligation. Reflection and premature re-activation of the proximal site was then observed in 6 of 63 (9%), and 3 of 26 preparations (12%). The present experiments demonstrate DP formation and rate-dependent constant-coupled late epicardial premature beats in infarcted dog hearts. Microelectrode recordings at DP sites demonstrating prolonged isoelectric intervals display very slow conduction preceding distal activation and "reentrant" re-activation of more proximal sites, representing reflection as an arrhythmia mechanism in ischemically injured epicardium during subacute myocardial infarction.

Efficacy of cardiac resynchronization in acutely infarcted canine hearts with electromechanical dyssynchrony.

Patients with acute myocardial infarction (MI), left bundle branch block (LBBB), and marked left ventricular (LV) decompensation suffer from nearly 50% early mortality. Whether cardiac resynchronization therapy (CRT) improves hemodynamic status in this condition is unknown. We tested CRT in this setting by using a canine model of delayed lateral wall (LW) activation combined with 2 hours of coronary artery occlusion-reperfusion. This study aimed to evaluate the acute hemodynamic effects of CRT during and immediately after MI. Adult dogs (n = 8) underwent open-chest 2-hour mid-left anterior descending artery occlusion followed by 1-hour reperfusion. Four pacing modes were compared: right atrial pacing, pseudo-left bundle block (right ventricular pacing), and CRT with the LV lead positioned at either the LW (LW-CRT) or the peri-infarct zone (peri-infarct zone-CRT). Continuous LV pressure-volume data, regional segment length, and proximal left anterior descending flow rates were recorded. At baseline, both right ventricular pacing and peri-infarct zone CRT reduced anterior wall regional work by ~50% (vs right atrial pacing). During coronary occlusion, this territory became dyskinetic, and dyskinesis rose further with both CRT modes as compared to pseudo-LBBB. Global cardiac output, stroke work, and ejection fraction all still improved by 11%-23%. After reperfusion, both CRT modes elevated infarct zone regional work and blood flow by ~10% as compared to pseudo-LBBB, as well as improved global function. CRT improves global chamber systolic function in left ventricles with delayed LW activation during and after sustained coronary occlusion. It does so while modestly augmenting infarct zone dyskinesis during occlusion and improving regional function and blood flow after reperfusion. These findings support CRT in the setting of early post-MI dyssynchronous heart failure.

Ankyrin-G Participates in INa Remodeling in Myocytes from the Border Zones of Infarcted Canine Heart

Cardiac Na channel remodeling provides a critical substrate for generation of reentrant arrhythmias in border zones of the infarcted canine heart. Recent studies show that Nav1.5 assembly and function are linked to ankyrin-G, gap, and mechanical junction proteins. In this study our objective is to expound the status of the cardiac Na channel, its interacting protein ankyrinG and the mechanical and gap junction proteins at two different times post infarction when arrhythmias are known to occur; that is, 48 hr and 5 day post coronary occlusion. Previous studies have shown the origins of arrhythmic events come from the subendocardial Purkinje and epicardial border zone. Our Purkinje cell (Pcell) voltage clamp study shows that INa and its kinetic parameters do not differ between Pcells from the subendocardium of the 48hr infarcted heart (IZPCs) and control non-infarcted Pcells (NZPCs). Immunostaining studies revealed that disturbances of Nav1.5 protein location with ankyrin-G are modest in 48 hr IZPCs. Therefore, Na current remodeling does not contribute to the abnormal conduction in the subendocardial border zone 48 hr post myocardial infarction as previously defined. In addition, immunohistochemical data show that Cx40/Cx43 co-localize at the intercalated disc (IDs) of control NZPCs but separate in IZPCs. At the same time, Purkinje cell desmoplakin and desmoglein2 immunostaining become diffuse while plakophilin2 and plakoglobin increase in abundance at IDs. In the epicardial border zone 5 days post myocardial infarction, immunoblot and immunocytochemical analyses showed that ankyrin-G protein expression is increased and re-localized to submembrane cell regions at a time when Nav1.5 function is decreased. Thus, Nav1.5 and ankyrin-G remodeling occur later after myocardial infarction compared to that of gap and mechanical junctional proteins. Gap and mechanical junctional proteins remodel in IZPCs early, perhaps to help maintain Nav1.5 subcellular location position and preserve its function soon after myocardial infarction.

Function of Ca 2+ Release Channels in Purkinje Cells That Survive in the Infarcted Canine Heart

Background— Triggered Purkinje ectopy can lead to the initiation of serious ventricular arrhythmias in post–myocardial infarction patients. In the canine model, Purkinje cells from the subendocardial border of the healing infarcted heart can initiate ventricular arrhythmias. Intracellular Ca 2+ abnormalities underlie these arrhythmias, yet the subcellular reasons for these abnormalities remain unknown. Methods and Results— Using 2D confocal microscopy, we directly quantify and compare typical spontaneous Ca 2+ events in specific subcellular regions of normal Purkinje cells with those Purkinje cells from the subendocardium of the 48-hour infarcted canine heart (IZPCs). The Ca 2+ event rate was higher in the subsarcolemmal region of IZPCs when compared with normal Purkinje cells; IZPC amplitudes were higher, yet the spatial extents of these events were similar. The amplitude of caffeine-releasable Ca 2+ in either the subsarcolemmal or core regions of IZPCs did not differ from normal Purkinje cells, suggesting that Ca 2+ overload was not related to the frequency change. In permeabilized Purkinje cells from both groups, the event rate was related to free [Ca 2+ ] in both subsarcolemmal and core, but in IZPCs, this event rate was significantly increased at each free Ca 2+ , suggesting an enhanced sensitivity to Ca 2+ release. Furthermore, decays of wide long lasting Ca 2+ release events in IZPC’s core were significantly accelerated compared with those in normal Purkinje cells. JTV519 (K201) suppressed IZPC cell wide Ca 2+ waves as well as normalized the enhanced event rate and its response to free Ca 2+ . Conclusions— Increased spontaneous Ca 2+ release events in IZPCs are due to uniform regionally increased Ca 2+ release channel sensitivity to Ca 2+ without a change in sarcoplasmic reticulum content. In addition, Ca 2+ reuptake in IZPCs is accelerated. These properties would lower the threshold of Ca 2+ release channels, setting the stage for the highly frequent arrhythmogenic cell wide Ca 2+ waves observed in IZPCs.

Building maps of local apparent conductivity of the epicardium with a 2-D electrophysiological model of the heart

In this paper, we address the problem of estimating the parameters of an electrophysiological model of the heart from a set of electrical recordings. The chosen model is the reaction-diffusion model on the transmembrane potential proposed by Aliev and Panfilov. For this model of the transmembrane, we estimate a local apparent two-dimensional conductivity from a measured depolarization time distribution. First, we perform an initial adjustment including the choice of initial conditions and of a set of global parameters. We then propose a local estimation by minimizing the quadratic error between the depolarization time computed by the model and the measures. As a first step we address the problem on the epicardial surface in the case of an isotropic version of the Aliev and Panfilov model. The minimization is performed using Brent method without computing the derivative of the error. The feasibility of the approach is demonstrated on synthetic electrophysiological measurements. A proof of concept is obtained on real electrophysiological measures of normal and infarcted canine hearts.

Remodeling in Cells From Different Regions of the Reentrant Circuit During Ventricular Tachycardia

Anisotropic reentrant excitation occurs in the remodeled substrate of the epicardial border zone (EBZ) of the 5-day infarcted canine heart. Reentry is stabilized because of the formation of functional lines of block. We hypothesized that regional differences of ionic currents in cells of the EBZ form these lines of block. Therefore, we first mapped reentrant circuits of sustained tachycardias, then dispersed cells (infarct zone cells, IZs) from the central common pathway of the circuit (IZc) as well as from the other side of the line of block (outer pathway, IZo) for study. We mapped reentrant circuits in the EBZ of infarcted hearts during sustained ventricular tachycardias (>30 seconds, n=17 episodes, cycle lengths=218+/-7.9 ms). INa density was reduced in both IZc and IZo, and the kinetic properties of IZc INa were markedly altered versus IZo. Structural remodeling of the sodium channel protein Nav1.5 occurred in IZs, with cell surface localization differing from normal cells. Both IZc and IZo have similar but reduced ICaL, whereas IZc showed changes in Ca2+ current kinetics with an acceleration of current decay. Computer simulations of the 2D EBZ showed that incorporating only differences between INa in IZc and IZo prevented stability of the reentrant circuit. Incorporating only differences between ICaL in the IZc and IZo cells also prevented stability of the circuit. However, incorporating both INa and ICaL current differences stabilized the simulated reentrant circuit, and lines of block formed between the 2 distinct regions. Despite differences in INa and ICaL properties in cells of the center and outer pathways of a reentrant circuit, the resulting changes in effective refractory periods tend to stabilize reentry in this remodeled substrate.

Can PKA activators rescue Na+ channel function in epicardial border zone cells that survive in the infarcted canine heart?

In this study, we investigated the effects of a PKA stimulating cocktail on sodium currents from normal epicardial cells (NZs) and on those from cells dispersed from the epicardial zone of the 5-day infarcted canine heart (IZs). To do so, we used whole-cell voltage-clamp techniques. During superfusion with the PKA activator cocktail, peak sodium current (I(Na)) density significantly increased by 32+/-5.3% (NZs) and 17+/-5.4% (IZs). However, despite this increase, IZ peak I(Na) still was not fully restored to NZ values. In both cell types, the density effect was accompanied by a shift in I/I(max) curves, as well as a slowing in recovery from inactivation. Inactivation from a closed state was accelerated. Furthermore, in the presence of chloroquine, which is known to interrupt intracellular vesicular traffic, PKA activator effects to augment I(Na) were only partially inhibited in NZs but abolished in IZs. To understand whether the phosphorylation status of basal Na(+) channels in the two cell groups differed, the effects of okadaic acid and PP2A1 were studied. Results suggest that in IZs, Na(+) channels in the basal state are already phosphorylated. PKA stimulation of I(Na) of the remodeled IZ does augment current density possibly by augmenting the trafficking of channels to an active site on the membrane. However, the resulting I(Na), while partially rescued, is not similar to the potentiated I(Na) of NZs. Specific kinetic changes also occur with the PKA stimulation of IZs and results with okadaic acid and PP2A1 suggest that in their remodeled state, Na(+) channels in IZs are already phosphorylated.

2APB- and JTV519(K201)-sensitive micro Ca 2+ waves in arrhythmogenic Purkinje cells that survive in infarcted canine heart

Objectives/background Studies from several laboratories have implicated intracellular Ca 2+ dynamics in the modulation of electrical activity. We have reported that abnormal Ca 2+ wave activity is the underlying cause of afterdepolarization-induced electrical activity in subendocardial Purkinje cells that survive in the 48-hour infarcted canine heart. These cells form the focus of arrhythmias at this time postcoronary artery occlusion. Methods We studied the effects of agonists and antagonists on the abnormal Ca 2+ release activity of Purkinje cell aggregates dispersed from the subendocardium 48 hours postcoronary artery occlusion (IZPCs). Studies were completed using epifluorescent microscopy of Fluo-3 loaded Purkinje cells. Results Similar to our previous report, highly frequent traveling micro Ca 2+ transients(μCaiTs) and cell-wide Ca 2+waves were seen in IZPCs in the absence of any drug. Isoproterenol (ISO) increased μCaiTs and cell-wide Ca 2+waves in Purkinje cells dispersed from the normal heart (NZPCs). In IZPCs, ISO increased cell-wide wave frequency but had no effect on the already highly frequent micro Ca 2+ wave transient activity, suggesting that ISO lowers the threshold of cell-wide generators responding to micro Ca 2+ transients. Drugs that block inward sodium or calcium currents (verapamil, tetrodotoxin) had no effect on Ca 2+activity in Purkinje cells. Antagonists of intracellular Ca 2+release channels [ryanodine, JTV519(K201)] greatly suppressed spontaneous Ca 2+ release events in IZPCs. 2APB, an agent that blocks IP 3 receptors, greatly reduced the frequency of Ca 2+events in IZPCs. Conclusions In arrhythmogenic Purkinje cells that survive in the infarcted heart, agents that block or inhibit intracellular Ca 2+release channel activity reduced Ca 2+waves and could be antiarrhythmic.

Protein tyrosine kinases and L-type Ca2+ currents in cells that have survived in epicardial border zone of canine infarcted heart.

Previously a reduction was shown in the density of the L-type Ca currents in cells that have survived in the epicardial border zone of the 5-day infarcted canine heart (IZ). A hyporesponsiveness of I(CaL) to beta-adrenergic stimulation in IZs versus cells from the noninfarcted heart (NZs) was also shown. To determine the role of protein tyrosine kinase (PTK) activity in this altered adrenergic response as well as in the reduced basal current function in IZs, the effects of genistein and T23, specific inhibitors of PTK, on basal I(CaL) in the absence and presence of isoproterenol (5 nM ) were studied using whole-cell patch-clamp techniques. Genistein reduction of I(CaL) was similar in NZs and IZs and was not mimicked by daidzein, an inactive analogue of genistein. Submaximal isoproterenol produced a small response in both cell types that was potentiated in the presence of genistein. T23 also reduced I(CaL) in both NZs and IZs; however, submaximal isoproterenol was not potentiated in its presence. In sum, basal I(CaL) is sensitive to genistein and T23, suggesting that persistent PTK activity contributes to I(CaL) in both NZs and IZs. With genistein but not with T23, there is an enhanced sensitivity of I(CaL) to isoproterenol in both NZs and IZs but peak I(CaL) is not fully restored in IZs. Thus, dysregulation of PTK activity cannot account for the reduced basal Ca currents or hyporesponsiveness of I(CaL) to isoproterenol in the cells that have survived in the infarcted heart.

Effects of mibefradil, a T-type calcium current antagonist, on electrophysiology of Purkinje fibers that survived in the infarcted canine heart.

We studied the effects of mibefradil (MIB), a nondihydropyridine T-type Ca2+ channel antagonist, on T- and L-type Ca2+ (I(CaT), I(CaL)) currents in Purkinje myocytes dispersed from the subendocardium of the left ventricle of normal (NZPC) and 48-hour infarcted (IZPC) hearts. Currents were recorded with Cs+- and EGTA-rich pipettes and in Na+-K+-free external solutions to eliminate overlapping currents. In all cells, I(Ca) was reduced by MIB (0.1 to 10 microM). No change in the time course of decay of peak I(Ca) was noted. Average peak T/L ratio decreased in NZPCs but not IZPCs with 1 microM MIB. Steady-state availability of I(CaL) was altered with 1 microM MIB in both cell types (mean +/- SEM) (V0.5 = -22 +/- 4 mV for NZPC and -25 +/- 5 mV for IZPC before drug; -63 +/- 9 mV for NZPC and -67 +/- 6 mV for IZPC after drug; P < 0.05). For I(CaT), V0.5 (-50 +/- 3 mV for NZPC and -52 +/- 1 mV for IZPC before drug) shifted to -60 +/- 2 mV (NZPC) and -62 +/- 3 mV (IZPC) (P < 0.05) after drug. We also determined the effects of MIB on spontaneously beating Purkinje normal fibers and on depolarized abnormally automatic fibers from the infarcted heart using standard microelectrode techniques. When NZPC and IZPC fibers were superfused with [K+]o = 2.7 mM, MIB 3 microM and 10 microM had no effect on rate or the maximum diastolic potential, but action potential plateau shifted to more negative values, the slope of repolarization phase 3 decreased, and action potential duration increased. MIB blocks L- and T-type Ca2+ currents in Purkinje myocytes but lacks an effect on either normal or abnormal automaticity in Purkinje fibers.

Location of diastolic potentials in reentrant circuits causing sustained ventricular tachycardia in the infarcted canine heart: relationship to predicted critical ablation sites.

The complete reentrant circuit for ablation of reentrant ventricular tachycardia (VT) in humans can rarely be localized by mapping. As a result, surrogate markers, such as diastolic electrical activity, subsequently confirmed by entrainment, have been used. However, ablation at those sites has had variable efficacy. The reasons for this variability are not clear. We correlated activation maps of reentrant circuits in the epicardial border zone of 4-day old infarcted dog hearts with the corresponding ECGs for 45 VTs to determine the regions of the reentrant circuits activated during diastole. In VTs with a figure-8 reentrant pattern, the center point of the central common pathway, the part of the circuit critical for the maintenance of reentry, was activated in early diastole in 32 of 35 VTs (91.4%), in late diastole in 1 (2.9%), and in systole in 2 (5.7%). Regions outside the circuit were rarely activated in diastole. In 10 VTs, the reentrant circuit was characterized by a single reentrant loop. In these circuits, no one region was predicted to be critical for maintenance of reentry, and a segment of the circuits was activated during diastole. However, regions peripheral to the circuit were also activated during diastole. The pattern of reentrant activation determines the specificity of diastolic activity for locating critical sites for ablation of VT.

Ca(2+)-dependent outward currents in myocytes from epicardial border zone of 5-day infarcted canine heart.

Myocytes from the epicardial border zone (EBZ) of the 5-day infarcted canine heart (IZ) have abnormal transmembrane action potentials, reduced L-type Ca2+ currents (ICa,L) and altered intracellular Ca2+ (Cai) transients compared with those of normal epicardial myocytes (NZ). We hypothesized that altered Cai cycling might be reflected in differences in Cai-dependent outward currents (Ito2). We recorded Ito2 in NZ and IZ using whole cell patch-clamp techniques. Ito2 was defined as the amplitude of the 4-aminopyridine-resistant transient outward current that was blocked by 200 microM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or DIDS+ ryanodine (2 microM). Ito2 were present in both NZ and IZ, but peak density was significantly reduced in IZ, particularly at positive plateau voltages. Time course of decay of Ito2 was biexponential and similar in NZ and IZ. A given peak ICa,L was usually associated with a smaller peak Ito2 in IZ. These differences were exaggerated when Ito2 and Cai transients were determined in rapidly paced cells. In summary, myocytes surviving in the EBZ of the infarcted heart have Ito2, yet they are reduced in density and can vary, particularly at fast pacing rates.

Altered Pharmacologic Responsiveness of Reduced L‐Type Calcium Currents in Myocytes Surviving in the Infarcted Heart

The pharmacologic responses of macroscopic L-type calcium channel currents to the dihydropyridine agonist, Bay K 8644, and beta-adrenergic receptor stimulation by isoproterenol were studied in myocytes enzymatically dissociated from the epicardial border zone of the arrhythmic 5-day infarcted canine heart (IZs). Calcium currents were recorded at 36 degrees to 37 degrees C using the whole cell, patch clamp method and elicited by applying step depolarizations from a holding potential of -40 mV to various test potentials for 250-msec duration at 8-second intervals. A Cs+ -rich and 10 mM EGTA-containing pipette solution and a Na+ -and K+ -free external solutions were used to isolate calcium currents from other contaminating currents. During control, peak ICa,L density was found to be significantly less in IZs (4.0 +/- 1.1 pA/pF) than in myocytes dispersed from the epicardium of the normal noninfarcted heart (NZs; 6.5 +/- 1.8 pA/pF). Bay K 8644 (1 micro M) significantly increased peak ICa,L density 3.5-fold above control levels in both NZs (to 22.5 +/- 6.2 pA/pF; n = 7) and IZs (to 12.8 +/- 3.0 pA/pF; n = 5), yet peak ICa,L density in the presence of drug was significantly less in IZs than NZs. The effects of Bay K 8644 on kinetics of current decay and steady-state inactivation relations of peak ICa,L were similar in the two cell types. In contrast, the response of peak L-type current density to isoproterenol (1 micro M) was significantly diminished in IZs compared to NZs regardless of whether Ba2+ or Ca2+ ions carried the current. Thus, these results indicate an altered responsiveness to beta-adrenergic stimulation in cells that survive in the infarcted heart. Furthermore, application of forskolin (1 micro M and 10 micro M) or intracellular cAMP (200 micro M), agents known to act downstream of the beta-receptor, also produced a smaller increase in peak IBa density in IZs versus NZs, suggesting that multiple defects exist in the beta-adrenergic signaling pathway of IZs. In conclusion, these studies illustrate that reduced macroscopic calcium currents of cells in the infarcted heart exhibit an altered pharmacologic profile that has important implications in the development of drugs for the diseased heart.

Transient outward currents in subendocardial Purkinje myocytes surviving in the infarcted heart.

Altered electrical activity of subendocardial Purkinje fibers contributes to arrhythmias in the 48-hour infarcted canine heart. Changes in the transmembrane action potentials of these fibers include marked action potential prolongation. The ionic basis for these changes is unknown. We used whole-cell voltage-clamp techniques to study 4-aminopyridine (4-AP)-sensitive voltage-dependent transient outward currents (Ito1) in Purkinje myocytes isolated from LV subendocardial (n = 14) and free-running (n = 15) bundles of the normal canine heart. Ito1 in these two groups of control cells (normal-zone Purkinje cells [NZPCS]) did not differ. NZPCS Ito1 was then compared with Ito1 of Purkinje myocytes dispersed from subendocardium of infarcted hearts 48 hours after total coronary artery occlusion (IZPC48, n = 14). Ito1 amplitude and current density were significantly reduced (P < .01) in IZPC48s (1650 +/- 389 pA, 9 +/- 2 pA/pF) compared with NZPCS (2917 +/- 267 pA, 20.2 +/- 2 pA/pF) at Vt = +55 mV, Vh = -60 mV, where Vt is test potential and Vh is holding potential. Decay of Ito1 was biexponential in all NZPCS but monoexponential in 71% of IZPC48s. Both NZPCS and IZPC48s have a sustained 4-AP-sensitive component (at 250 ms, Vt = +55 mV: 4 +/- 1 pA/pF, 3 +/- 1 pA/pF, respectively). Ito1 voltage dependence of inactivation did not differ between groups. In IZPC48s, recovery of Ito1 from inactivation was slowed significantly. Furthermore, significantly more Ito1 was seen with rapid pacing in NZPCS (cycle length [CL] 5000 ms = 100%, CL 1300 ms = 73%, CL 330 ms = 46%) than in IZPC48s (CL 5000 ms = 100%, CL 1300 ms = 58%, CL 330 ms = 31%). In three IZPC48s, no Ito1 was seen at CL 330 ms. Ito1 plays a major role in normal Purkinje myocyte electrophysiology, contributing both a large transient and a sustained component that are 4-AP-sensitive. In subendocardial Purkinje myocytes that survive in the 48-hour infarcted heart, density of Ito1 is markedly reduced and the remaining Ito1 showed specific changes in kinetics. The alterations observed in both Ito1 density and function could contribute to abnormally long transmembrane action potentials of these arrhythmogenic Purkinje myocytes of the infarcted heart.

Characteristics of the β-Adrenergic Receptor Complex in the Epicardial Border Zone of the 5-Day Infarcted Canine Heart

The effect of isoproterenol on increasing the peak amplitude of the L-type calcium current is reduced in myocytes dispersed from the epicardial border zone (EBZ) of the 5-day infarcted canine heart when compared with control cells from noninfarcted hearts. This suggests that specific alterations in the beta-adrenergic receptor complex develop in this setting. The present study is an examination of individual components of the beta-adrenergic receptor complex with the aim of elucidating the biochemical defect(s) that might be responsible for the diminished beta-adrenergic receptor responsiveness in the myocytes that survive in the infarcted heart. We compared components of the beta-adrenergic receptor signaling pathway in membranes prepared from the EBZ of the 5-day infarcted heart and a remote, noninfarcted region (RZ) of the same ventricle as well as the corresponding regions of noninfarcted ventricles. Defects in multiple components of the beta-adrenergic receptor complex were confined to the EBZ of the 5-day infarcted heart. These include a decrease in beta-adrenergic receptor density; diminished basal, guanine nucleotide-, isoproterenol-, forskolin-, and manganese-dependent adenylyl cyclase activities; an increase in the EC50 for isoproterenol-dependent activation of adenylyl cyclase; a diminished level of the alpha-subunit of the Gs protein. and an elevated level of the alpha-subunit of the Gi protein. Defects in multiple components of the membrane beta-adrenergic receptor complex were identified in the EBZ of the 5-day infarcted canine heart. This constellation of abnormalities would be predicted to impair functional beta-adrenergic responsiveness and contribute to the defect in isoproterenol-dependent stimulation of the L-type calcium current in myocytes isolated from this tissue.

Electrophysiological Effects of Flecainide on Anisotropic Conduction and Reentry in Infarcted Canine Hearts

The class IC antiarrhythmic drug flecainide has been shown to be ineffective for the treatment of ventricular arrhythmias in some patients who have had a prior myocardial infarction and sometimes even provoke arrhythmias (proarrhythmic effect). Since some ventricular tachycardias may be caused by anisotropic reentry, we determined the effects of flecainide on this mechanism for reentry in infarcted canine hearts in order to determine possible causes for its clinical effects. The effects of flecainide were determined on ventricular tachycardia induced by programmed electrical stimulation in dogs with healing myocardial infarction 4 days after coronary artery occlusion. Activation in the reentrant circuits causing tachycardia was mapped with a 196-channel computerized mapping system. We found that flecainide converted inducible unsustained ventricular tachycardia to inducible sustained ventricular tachycardia by modifying conduction in the reentrant circuit. In general, by slowing conduction, the reentrant wave front did not block after flecainide, leading to perpetuation of reentrant excitation. When sustained ventricular tachycardia could be induced before the drug, flecainide prolonged the coupling interval of premature impulses necessary to induce tachycardia by lengthening the line of block and slowing conduction around it. Flecainide also slowed the rate of the tachycardia but did not terminate it. The anisotropic reentrant circuits were modified so that the central common pathway of "figure-of-eight" circuits was narrowed and lengthened due to extension of the lines of block that bounded the pathways. Extension of the lines of block resulted from depression of conduction in the direction transverse to the long axis of the myocardial fiber bundles caused by flecainide. Flecainide also slowed conduction in the longitudinal direction in part of the circuits. The depressant effects of flecainide on both longitudinal and transverse anisotropic conduction were quantified by pacing from the center of the electrode array and it was found, contrary to predictions, that transverse conduction was depressed as much as longitudinal conduction. Flecainide slows conduction in both the longitudinal and transverse direction relative to the orientation of the myocardial fibers. This enables sustained reentry to occur more easily. Flecainide does not cause conduction block in crucial regions of reentrant circuits (central common pathway) and therefore does not prevent reentrant tachycardia in healing infarcts.

Actions of Volatile Anesthetics on Ischemic and Nonischemic Purkinje Fibers in the Infarcted Canine Heart

The effects of halothane, isoflurane, and enflurane on proximal (false tendon) and distal (apical) Purkinje fibers were measured in vitro in infarcted canine hearts to assess their effects on action potentials of fibers located within the nonischemic and ischemic regions, respectively. High- and low-dose anesthetic effects were evaluated in three groups of eight preparations and compared to changes occurring at identical times in eight infarcted control preparations. Under control conditions in all groups, the action potential duration at 90% repolarization (APD90, mean +/- SEM) of ischemic distal fibers (396 +/- 9 ms) was longer (P < or = 0.01) than that of nonischemic proximal fibers (344 +/- 5 ms) and the ischemic fibers exhibited (P < or = 0.05) reduced maximum diastolic potential, amplitude, and Vmax relative to nonischemic fibers. Halothane (0.25 and 0.6 mM), isoflurane (0.4 and 0.8 mM), and enflurane (0.8 and 1.6 mM) produced dose-dependent decreases of nonischemic fiber APD90 with less decrease (P < or = 0.01) of ischemic fiber APD90 and thereby accentuated (P < or = 0.05) regional differences of APD90 at high dose. The decreases of nonischemic fiber APD90 were greater (P < or = 0.01) for 0.8 mM (2.9 minimum alveolar anesthetic concentration [MAC]) isoflurane (-95 +/- 5 ms) and 1.6 mM (2.5 MAC) enflurane (-79 +/- 12 ms) than for 0.6 mM (2.2 MAC) halothane (-41 +/- 3 ms). Isoflurane increased the pathologic difference (ischemic > nonischemic) between the repolarization times (APD90) of Purkinje fibers in the infarcted heart more (P < or = 0.05) than halothane.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of Halothane and Isoflurane on Purkinje Fibers in the Infarcted Canine Heart

The actions of halothane (HAL) and isoflurane (ISO) on conduction and regional refractoriness were studied in infarcted canine hearts to compare their effects on reentry in vitro. In two anesthetic groups of 8 hearts, high and low dose effects were assessed using action potentials recorded from Purkinje fibers located in the nonischemic and ischemic regions. An extrastimulus technique was used to determine the relationship between delay of conduction of premature impulses into the more refractory ischemic region and induction of reentrant responses. At high doses (HAL 0.60 mM and ISO 0.64 mM, approximately 2.3 minimum alveolar anesthetic concentration [MAC]) both anesthetics decreased (P < or = 0.05) the effective refractory period for direct intracellular stimulation of nonischemic fibers (local ERP, initial control: 294 +/- 8 ms); the decrease with HAL (-29 +/- 6 ms) was smaller (P < or = 0.05) than with ISO (-50 +/- 7 ms). HAL and ISO also decreased (P < or = 0.05) the coupling interval of the earliest premature impulse which conducted into the infarct (system effective refractory period [SERP], control: 301 +/- 7 ms) by -31 +/- 11 and -44 +/- 8 ms, respectively. In contrast, the functional refractory period (FRP) in the ischemic region (control:354 +/- 4 ms) was increased by HAL (26 +/- 8 ms; P < or = 0.05) but decreased by ISO (-14 +/- 4 ms, P < or = 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular Defibrillation in Canines with Chronic Infarction, and Effects of Lidocaine and Procainamide

Prior studies in dogs with normal hearts have demonstrated that lidocaine increases but procainamide does not change the energy required for successful defibrillation. Because many postinfarct patients receiving implantable cardioverter defibrillator devices require adjunctive antiarrhythmic therapy, we have studied the effects of lidocaine and procainamide on the relationship between delivered voltage and defibrillation success in mongrel dogs 21 +/- 3 days following ligation of the left anterior descending and first diagonal coronary arteries. Internal defibrillation testing using a patch-patch electrode configuration was performed before and during the administration of saline controls (n = 10), lidocaine (n = 10) and procainamide (n = 10). The mean infarct size as determined by staining with tetrazolium was 13.4% +/- 8.3% of right and left ventricles, and did not differ significantly between groups. The 50% effective defibrillation (ED50) voltage increased with infusions of saline (16% +/- 15%), lidocaine (40% +/- 22%), and procainamide (13% +/- 15%) and the ED50 energy increased 41% +/- 44%, 104% +/- 62%, and 35% +/- 36%, respectively. However, the increase in ED50 voltages and energies were significantly greater in animals receiving lidocaine compared to those receiving either saline control or procainamide (P < 0.01). There were trends toward change of hemodynamic parameters in all animals following baseline defibrillation testing; stroke volume declined 21% +/- 16%; and mean pulmonary artery and aortic pressure increased by 22% +/- 25% and 11% +/- 15%, respectively. In conclusion, unlike our previous studies in dogs with normal hearts, in this model hemodynamic deterioration occurred with repeated fibrillation and defibrillation, and defibrillation voltage requirements increased in the control series. Taking into consideration the increase in defibrillation voltage requirements over the duration of the experiments, lidocaine increases and procainamide does not change ED50; thus, their effects are similar in normal and infarcted canine hearts.

Electrophysiologic effects of selective B 1 adrenergic stimulation in the late phase of myocardial infarct healing

Nonspecific adrenergic stimulation plays an important role in the genesis of ventricular tachyarrhythmias in the postinfarction period. However, the role of selective B 1 adrenergic stimulation in the aggravation of reentrant ventricular tachycardia is still poorly understood. The purpose of this investigation was to study the regional electrophysiologic actions and arrhythmogenic effects of the B 1 adrenergic agonist dobutamine in a postinfarction canine model. Eleven dogs with 4-week-old anterior wall myocardial infarctions, chronic indwelling intramyocardial electrodes and no inducible sustained ventricular tachycardia at baseline, were studied in the closed-chest state at baseline (control) and during intravenous infusion of dobutamine 10 μg/kg/min. Dobutamine caused reductions in the effective and absolute refractory periods in the infarct, peri-infarct and noninfarcted regions of the myocardium which were similar in magnitude at each region. With dobutamine, only 1 dog had inducible sustained ventricular tachycardia (polymorphic). No spontaneous arrhythmias were seen. In summary, selective B 1 adrenergic stimulation alone does not cause dispersion of myocardial refractoriness and does not cause significant proarrhythmia in the chronically infarcted canine heart. Additional metabolic, electrolyte or ischemic abnormalities may be required for B 1 adrenergic stimulation to aggravate arrhythmias in this model.