Antiarrhythmic effects of beta3-adrenergic receptor stimulation in a canine model of ventricular tachycardia.

BACKGROUND: Beta3-adrenergic receptor (beta3-AR) stimulation inhibits cardiac contractility. OBJECTIVE: This study sought to test the hypothesis that beta3-AR stimulation is antiarrhythmic. METHODS: We implanted a radio transmitter for continuous electrocardiogram monitoring in 18 dogs with a tendency for high incidence of spontaneous ventricular tachycardia (VT). Ten of 18 had subcutaneous continuous BRL37344 (beta3-AR agonist) infusion (experimental group) for 1 month. The other dogs were controls. Western blotting studies were performed on tissues sampled from the noninfarcted left ventricular free wall of all dogs that survived the 60-day follow-up period. RESULTS: Phase 2 VT appeared significantly later in the experimental group than in the control group (P <.05). The number of VT episodes in the experimental group was significantly lower than in the control group during both the first month (0.5 +/- 0.95 episodes/day vs. 2.6 +/- 2.3 episodes/day) and the second month (0.2 +/- 0.2 episode/day vs. 1.2 +/- 1.1 episodes/day, P <.05 for both). The experimental group had shorter QTc than control (P <.002). The experimental group had decreased protein levels for sodium calcium exchanger and dihydropyridine receptor, increased beta3-AR expression, without changes in beta1-AR, beta2-AR. The average heart weight and the left ventricular free wall thickness in the experimental group (226 +/- 17 g and 15.1 +/- 1.2 mm, respectively) was significantly lower than in the control group (265 +/- 21 g and 17.4 +/- 2.5 mm, respectively, P <.05 for both). There was no difference in the incidences of sudden cardiac death in these 2 groups of dogs. CONCLUSION: Beta3-AR stimulation significantly reduces the occurrence of ventricular tachycardia.

Remodelling of action potential and intracellular calcium cycling dynamics during subacute myocardial infarction promotes ventricular arrhythmias in Langendorff-perfused rabbit hearts.

We hypothesize that remodelling of action potential and intracellular calcium (Ca(i)) dynamics in the peri-infarct zone contributes to ventricular arrhythmogenesis in the postmyocardial infarction setting. To test this hypothesis, we performed simultaneous optical mapping of Ca(i) and membrane potential (V(m)) in the left ventricle in 15 rabbit hearts with myocardial infarction for 1 week. Ventricular premature beats frequently originated from the peri-infarct zone, and 37% showed elevation of Ca(i) prior to V(m) depolarization, suggesting reverse excitation-contraction coupling as their aetiology. During electrically induced ventricular fibrillation, the highest dominant frequency was in the peri-infarct zone in 61 of 70 episodes. The site of highest dominant frequency had steeper action potential duration restitution and was more susceptible to pacing-induced Ca(i) alternans than sites remote from infarct. Wavebreaks during ventricular fibrillation tended to occur at sites of persistently elevated Ca(i). Infusion of propranolol flattened action potential duration restitution, reduced wavebreaks and converted ventricular fibrillation to ventricular tachycardia. We conclude that in the subacute phase of myocardial infarction, the peri-infarct zone exhibits regions with steep action potential duration restitution slope and unstable Ca(i) dynamics. These changes may promote ventricular extrasystoles and increase the incidence of wavebreaks during ventricular fibrillation. Whereas increased tissue heterogeneity after subacute myocardial infarction creates a highly arrhythmogenic substrate, dynamic action potential and Ca(i) cycling remodelling also contribute to the initiation and maintenance of ventricular fibrillation in this setting.

Dynamic origin of spatially discordant alternans in cardiac tissue.

Alternans, a condition in which there is a beat-to-beat alternation in the electromechanical response of a periodically stimulated cardiac cell, has been linked to the genesis of life-threatening ventricular arrhythmias. Optical mapping of membrane voltage (V(m)) and intracellular calcium (Ca(i)) on the surface of animal hearts reveals complex spatial patterns of alternans. In particular, spatially discordant alternans has been observed in which regions with a large-small-large action potential duration (APD) alternate out-of-phase adjacent to regions of small-large-small APD. However, the underlying mechanisms that lead to the initiation of discordant alternans and govern its spatiotemporal properties are not well understood. Using mathematical modeling, we show that dynamic changes in the spatial distribution of discordant alternans can be used to pinpoint the underlying mechanisms. Optical mapping of V(m) and Ca(i) in paced rabbit hearts revealed that spatially discordant alternans induced by rapid pacing exhibits properties consistent with a purely dynamical mechanism as shown in theoretical studies. Our results support the viewpoint that spatially discordant alternans in the heart can be formed via a dynamical pattern formation process which does not require tissue heterogeneity.

Optical recording-guided pacing to create functional line of block during ventricular fibrillation.

Low-energy defibrillation is very desirable in cardiac rhythm management. We previously reported that ventricular fibrillation (VF) can be synchronized with a novel synchronized pacing technique (SyncP) using low-energy pacing pulses. This study sought to create a line of block during VF using SyncP. SyncP was performed in six isolated rabbit hearts during VF using optical recording to control the delivery of pacing pulses in real time. Four pacing electrodes with interelectrode distances of 5 mm were configured in a line along and across the myocardial fiber direction. The electrodes were controlled independently (independent mode) or fired together (simultaneous mode). Significant wavefront synchronization was observed along the electrode line as indicated by a decrease in variance. With the independent SyncP protocol, the decrease in the variance was 19.3 and 13.7% (P<0.001) for the along-, and across-fiber configurations, respectively. With the simultaneous SyncP protocol, the variance was reduced by 24.2 and 10.7% (P<0.001) in the along- and across-fiber configurations. The effect of synchronization dropped off with distance from the line of pacing. We conclude that SyncP can effectively create a line of functional block that isolates regions of VF propagation. Further optimization of this technique may prove useful for low-energy ventricular defibrillation.

Electrical connections between left superior pulmonary vein, left atrium, and ligament of Marshall: implications for mechanisms of atrial fibrillation.

The importance of the ligament of Marshall (LOM) to rapid activations within the left superior pulmonary vein (LSPV) during atrial fibrillation (AF) remains poorly understood. We aimed to characterize the importance of electrical coupling between the LSPV with the left atrium (LA) and the LOM in the generation of high-frequency activations within this PV. We performed high-density mapping of the LSPV-LA-LOM junction in eight dogs, using 1,344 electrodes with a 1-mm resolution before and after posterior ostial ablation to diminish PV-LA electrical connections. A LOM potential was recordable up to 6.5 mm (SD 2.2) into the LSPV in all dogs during sinus rhythm (SR) and LA pacing. Functional LOM-LSPV electrical connections bypassing the PV-LA junction were present in five of eight dogs. Direct LOM-LSPV connections contributed to 46.5% (SD 16.0) of LSPV activations during AF, resulting in a greater propensity to develop focal activations (P < 0.05) and a higher activation rate during AF of LSPVs with direct LOM connections compared with those without (P < 0.03). Posterior LSPV ostial ablation without damaging the anterior wall or LOM slowed residual LA-PV conduction (P < 0.001). This diminished PV-LA coupling prevented the reinduction of LSPV focal activations in all dogs. However, persistent LOM focal activations in two dogs continued to activate the LSPV rapidly [cycle length 151.8 ms (SD 4.8)] via direct LOM-LSPV connections. LOM-LSPV connection forms an accessory pathway that contributes to the electrical coupling between LSPV and LA during SR and AF. This pathway may contribute to rapid activations within the LSPV during AF.

High-density mapping of pulmonary veins and left atrium during ibutilide administration in a canine model of sustained atrial fibrillation.

Ibutilide can prolong refractory period and terminate reentry. Whether ibutilide has the same effects on pulmonary vein (PV) focal discharge (FD) is unclear. We induced sustained atrial fibrillation (AF) in seven dogs by rapid left atrial (LA) pacing for 74 +/- 46 days. Ibutilide was repeatedly infused until it terminated AF (0.02 +/- 0.01 mg/kg) or when a cumulative dose was reached (0.04 mg/kg). High-resolution computerized epicardial mapping was performed. We found intermittent FD at the PVs and reentry at the PV-LA junction during AF. Ibutilide increased the cycle length of consecutive reentry from 97 +/- 13 to 112 +/- 18 ms and increased FD from 96 +/- 7 to 113 +/- 9 ms. In four dogs with both FD and reentry at the PVs, the incidence of reentry decreased from 3.5 +/- 1.9/s at baseline to 2.2 +/- 1.8/s after ibutilide administration. However, the incidence of FD remained unchanged. The conducted wave fronts between PV and LA were significantly reduced by ibutilide (10.4 +/- 2.0/s vs. 8.0 +/- 1.6/s). The ibutilide dose needed to terminate AF correlated negatively with the baseline effective refractory period of PV and LA. We conclude that ibutilide reduces reentrant wave fronts but not PV FD in a canine model of pacing-induced sustained AF. These findings suggest that the PV FD during AF is due to nonreentrant mechanisms. High doses of ibutilide may completely terminate all reentrant activity, converting AF to PV tachycardia before the resumption of sinus rhythm.

Intracellular calcium dynamics and anisotropic reentry in isolated canine pulmonary veins and left atrium.

BACKGROUND: Rapid activations due to either focal discharge or reentry are often present during atrial fibrillation (AF) in the pulmonary veins (PVs). The mechanisms of these rapid activations are unclear. METHODS AND RESULTS: We studied 7 isolated, Langendorff-perfused canine left atrial (LA) and PV preparations and used 2 cameras to map membrane potential alone (Vm, n=3) or Vm and intracellular calcium simultaneously (Ca(i), n=4). Rapid atrial pacing induced 26 episodes of focal discharge from the proximal PVs in 5 dogs. The cycle lengths were 223+/-52 ms during ryanodine infusion (n=13) and 133+/-59 ms during ryanodine plus isoproterenol infusion (n=13). The rise of Ca(i) preceded Vm activation at the sites of focal discharge in 6 episodes of 2 preparations, compatible with voltage-independent spontaneous Ca(i) release. Phase singularities during pacing-induced reentry clustered specifically at the PV-LA junction. Periodic acid-Schiff (PAS) stain identified large cells with pale cytoplasm along the endocardium of PV muscle sleeves. There were abrupt changes in myocardial fiber orientation and increased interstitial fibrosis in the PV and at the PV-LA junction. CONCLUSIONS: PV muscle sleeves may develop voltage-independent Ca(i) release, resulting in focal discharge. Focal discharge may also be facilitated by the presence of PAS-positive cells that are compatible with node-like cells. During reentry, phase singularities clustered preferentially at sites of increased anisotropy such as the PV-LA junction. These findings suggest that focal discharge caused by spontaneous calcium release and anisotropic reentry both contribute to rapid activations in the PVs during AF.

Decreased vagal control over heart rate in rats with right-sided congestive heart failure: downregulation of neuronal nitric oxide synthase.

BACKGROUND: Parasympathetic drive is attenuated in heart failure, and resulting autonomic imbalance may increase the risk of sudden cardiac death. The anatomic site(s) and molecular mechanisms underlying this parasympathetic withdrawal are unknown. METHODS AND RESULTS: We examined the effects of pre- and post-ganglionic vagal nerve stimulation (VS) and acetylcholine (ACh) application on the heart rate of rats with right-sided congestive heart failure (CHF) induced by monocrotaline. Heart rate reduction in response to pre-ganglionic VS in CHF rats in vivo was significantly less than in controls. The suppression of spontaneous beating of isolated right atria including the whole sinoatrial (SA) node in response to post-ganglionic VS was significantly attenuated in CHF rats as well. In contrast, ACh application to the right atria resulted in a significantly larger suppression of spontaneous beating in CHF rats than controls. Proteins of neuronal nitric oxide synthase (nNOS) in the right atria were significantly decreased, whereas muscarinic (M2) receptor was significantly increased in CHF rats compared with controls. CONCLUSIONS: Both pre-and post-ganglionic vagal nerve functions are diminished in CHF rats, whereas M2 receptor-mediated regulation of the SA node is upregulated. Downregulation of nNOS may be involved in this parasympathetic withdrawal.