Abstract-Catecholaminergic polymorphic ventricular tachycardia (VT) is a lethal familial disease characterized by bidirectional VT, polymorphic VT, and ventricular fibrillation. Catecholaminergic polymorphic VT is caused by enhanced Ca 2ϩ release through defective ryanodine receptor (RyR2) channels. We used epicardial and endocardial optical mapping, chemical subendocardial ablation with Lugol's solution, and patch clamping in a knockin (RyR2/RyR2 R4496C ) mouse model to investigate the arrhythmogenic mechanisms in catecholaminergic polymorphic VT. In isolated hearts, spontaneous ventricular arrhythmias occurred in 54% of 13 RyR2/RyR2 R4496C and in 9% of 11 wild-type (Pϭ0.03) littermates perfused with Ca 2ϩ and isoproterenol; 66% of 12 RyR2/RyR2 R4496C and 20% of 10 wild-type hearts perfused with caffeine and epinephrine showed arrhythmias (Pϭ0.04). Epicardial mapping showed that monomorphic VT, bidirectional VT, and polymorphic VT manifested as concentric epicardial breakthrough patterns, suggesting a focal origin in the His-Purkinje networks of either or both ventricles. Monomorphic VT was clearly unifocal, whereas bidirectional VT was bifocal. Polymorphic VT was initially multifocal but eventually became reentrant and degenerated into ventricular fibrillation. Endocardial mapping confirmed the Purkinje fiber origin of the focal arrhythmias. Chemical ablation of the right ventricular endocardial cavity with Lugol's solution induced complete right bundle branch block and converted the bidirectional VT into monomorphic VT in 4 anesthetized RyR2/RyR2 R4496C mice. Under current clamp, single Purkinje cells from RyR2/RyR2 R4496C mouse hearts generated delayed afterdepolarization-induced triggered activity at lower frequencies and level of adrenergic stimulation than wild-type. Overall, the data demonstrate that the His-Purkinje system is an important source of focal arrhythmias in catecholaminergic polymorphic VT. 604772) is an inherited disease leading to arrhythmias and sudden cardiac death. 1 The autosomal dominant form has been linked to ryanodine receptor gene (RyR2) mutations, leading to increased spontaneous Ca 2ϩ release from the sarcoplasmic reticulum. 2 Typical arrhythmias are bidirectional ventricular tachycardia (BVT) and polymorphic ventricular tachycardia (PVT) that can degenerate into ventricular fibrillation (VF) and thus sudden cardiac death. 3 BVT is infrequent, characterized by beat-to-beat 180°alternation of the QRS of the ECG and occurs in CPVT, as well as in digitalis toxicity; thus, it has been inferred that arrhythmogenesis in CPVT is mediated by delayed afterdepolarization (DAD)-induced triggered activity (TA).Mice heterozygous for the R4496C mutation (RyR2/ RyR2 R4496C ) recapitulate the human phenotype of CPVT by developing BVT, PVT, and/or VF under adrenergic stimulation. 4 Recently, Liu et al 5 have demonstrated DADs in RyR2/RyR2 R4496C mouse ventricular myocytes both in control and in the presence of isoproterenol. However, it remains to be demonstrated whether the arrhythmia origin...

Summary BACKGROUND The intrinsic neural plexus of the mouse heart has not been adequately investigated despite the extensive use of this species in experimental cardiology. OBJECTIVE We determined the distribution of cholinergic, adrenergic and sensory neural components in whole-mount mouse heart preparations using double immunohistochemical labeling. METHODS AND RESULTS Intrinsic neurons were concentrated within 19±3 ganglia (n = 20 mice) of varying size, scattered on the medial side of the inferior caval (caudal) vein on the right atrium and close to the pulmonary veins on the left atrium. Of a total of 1082±160 neurons, most somata (83%) were choline acetyltransferase (ChAT)-immunoreactive, while 4% were tyrosine hydroxylase (TH)-immunoreactive; 14% of ganglionic cells were biphenotypic for ChAT and TH. The most intense ChAT staining was observed in axonal varicosities. ChAT was evident in nerve fibers interconnecting intrinsic ganglia. Both ChAT and TH immunoreactivity were abundant within the nerves accessing the heart. However, epicardial TH-immunoreactive nerve fibers were predominant on the dorsal and ventral left atrium, whereas most ChAT-positive axons proceeded on the heart base toward the large intrinsic ganglia and on the epicardium of the root of the right cranial vein. Substance P-positive and calcitonin gene-related peptide-immunoreactive nerve fibers were abundant on the epicardium and within ganglia adjacent to the heart hilum. Small intensely fluorescent cells were grouped into clusters of 3–8 and dispersed within large ganglia or separately on the atrial and ventricular walls. CONCLUSIONS While some nerves and neuronal bundles of the mouse epicardial plexus are mixed, most express either adrenergic or cholinergic markers. Therefore, selective stimulation and/or ablation of the functionally distinct intrinsic neural pathways should allow the study of specific effects on cardiac function.

We describe a mutation (E299V) in KCNJ2, the gene that encodes the strong inward rectifier K + channel protein (Kir2.1), in an 11-y-old boy. The unique short QT syndrome type-3 phenotype is associated with an extremely abbreviated QT interval (200 ms) on ECG and paroxysmal atrial fibrillation. Genetic screening identified an A896T substitution in a highly conserved region of KCNJ2 that resulted in a de novo mutation E299V. Whole-cell patch-clamp experiments showed that E299V presents an abnormally large outward I K1 at potentials above −55 mV (P < 0.001 versus wild type) due to a lack of inward rectification. Coexpression of wild-type and mutant channels to mimic the heterozygous condition still resulted in a large outward current. Coimmunoprecipitation and kinetic analysis showed that E299V and wild-type isoforms may heteromerize and that their interaction impairs function. The homomeric assembly of E299V mutant proteins actually results in gain of function. Computer simulations of ventricular excitation and propagation using both the homozygous and heterozygous conditions at three different levels of integration (single cell, 2D, and 3D) accurately reproduced the electrocardiographic phenotype of the proband, including an exceedingly short QT interval with merging of the QRS and the T wave, absence of ST segment, and peaked T waves. Numerical experiments predict that, in addition to the short QT interval, absence of inward rectification in the E299V mutation should result in atrial fibrillation. In addition, as predicted by simulations using a geometrically accurate three-dimensional ventricular model that included the His-Purkinje network, a slight reduction in ventricular excitability via 20% reduction of the sodium current should increase vulnerability to life-threatening ventricular tachyarrhythmia. cellular electrophysiology | computer models | genetics | ion channels | channelopathies T he short QT syndrome (SQTS) is an inherited arrhythmogenic disorder characterized by a remarkably abbreviated repolarization and a predisposition to supraventricular and ventricular arrhythmias in the absence of detectable structural heart disease (1). Mutations found in SQTS patients in the genes encoding potassium channels cause "gain of function," whereas mutations found in the alpha 1C subunit of the voltage-dependent L-type Ca + channel (CACNA1C), the beta 2b subunit of the voltage dependent Ca 2+ channel (CACNB2B) and the alpha2/delta subunit 1 of the voltage dependent Ca 2+ channel (CACNA2D1) cause "loss of function" (1, 2). In 2005, we reported a mutation (D172N) in the strong inward rectifier K + channel protein (Kir2.1), which is coded by KCNJ2, in an SQTS patient: Functional characterization revealed that D172N shows an increased outward component of the inward rectifier current I K1 (3). Computer simulation demonstrated that D172N leads to a shortening of the QT interval and predisposes the heart to develop reentrant arrhythmias. Here we report a different KCNJ2 mutation (E299V) identified in a child with a re...

Previous studies have suggested an important role for the inward rectifier K + current (I K1 ) in stabilizing rotors responsible for ventricular tachycardia (VT) and fibrillation (VF). To test this hypothesis, we used a line of transgenic mice (TG) overexpressing Kir 2.1-green fluorescent protein (GFP) fusion protein in a cardiac-specific manner. Optical mapping of the epicardial surface in ventricles showed that the Langendorff-perfused TG hearts were able to sustain stable VT/VF for 350 ± 1181 s at a very high dominant frequency (DF) of 44.6 ± 4.3 Hz. In contrast, tachyarrhythmias in wild-type hearts (WT) were short-lived (3 ± 9 s), and the DF was 26.3 ± 5.2 Hz. The stable, high frequency, reentrant activity in TG hearts slowed down, and eventually terminated in the presence of 10 µM Ba 2+ , suggesting an important role for I K1 . Moreover, by increasing I K1 density in a two-dimensional computer model having realistic mouse ionic and action potential properties, a highly stable, fast rotor (≈45 Hz) could be induced. Simulations suggested that the TG hearts allowed such a fast and stable rotor because of both greater outward conductance at the core and shortened action potential duration in the core vicinity, as well as increased excitability, in part due to faster recovery of Na + current. The latter resulted in a larger rate of increase in the local conduction velocity as a function of the distance from the core in TG compared to WT hearts, in both simulations and experiments. Finally, simulations showed that rotor frequencies were more sensitive to changes (doubling) in I K1 , compared to other K + currents. In combination, these results provide the first direct evidence that I K1 up-regulation in the mouse heart is a substrate for stable and very fast rotors.