Original-experimental geneticFunctional expression of “cardiac-type” Nav1.5 sodium channel in canine intracardiac ganglia
Introduction
Voltage-dependent sodium (Nav) channels play a critical role in membrane electrogenesis and repetitive firing of excitable cells. In the heart, the sodium current is carried largely by the Nav1.5 (or SCN5A) isoform, although the presence of “neuronal-type” sodium currents has been reported.1, 2 Nav1.5 gene mutations are linked to a wide diversity of arrhythmic cardiac syndromes, including long QT syndrome type 3 (LQT3), Brugada syndrome, and progressive cardiac conduction disease. These syndromes are often associated with the development of life-threatening cardiac arrhythmias.3, 4 LQT3 is often associated with sinus bradycardia, sinus pauses, and atrial standstill, which are not readily explained by the gain of function of the sodium channel responsible for QT interval prolongation.3 Although autonomic dysfunction has been invoked to explain various aspects of arrhythmogenesis attending LQT3 and Brugada syndrome,5, 6 a direct link to defects in Nav1.5 affecting autonomic regulation has not been previously considered. Nav1.5 channels have been found in other noncardiac tissues, including neonatal dorsal root ganglion neurons (DRG),7 human intestinal smooth muscle and Cajal cells,8, 9 and neurons from distinct regions of the brain.10, 11 Direct evidence for the presence of this channel isoform in intracardiac neurons has not been provided. The present study provides evidence for the functional expression of Nav1.5 channels in intracardiac neurons of the canine heart. This evidence is supported by pharmacologic and biophysical characterization of the sodium current in isolated canine intracardiac neurons, by reverse transcription-polymerase chain reaction (RT-PCR) and direct sequencing of the channel, and by immunohistochemical studies conducted in freshly dissected ganglia.
Section snippets
Neuron dissociation
Principal neurons from the atrial ganglionated plexuses of the dog were obtained by standard enzymatic dissociation procedures. Dogs weighing 20 to 25 kg were anticoagulated with heparin and anesthetized with pentobarbital (30–35 mg/kg IV). The chest was opened via left thoracotomy. The heart was excised, placed in a cardioplegic solution consisting of cold (4°C) Tyrode solution containing 8.5 mM [K+]o, and transported to a dissection tray. The fat pads on the ventral, lateral, and dorsal
TTX-R sodium channels underlie TTX-R action potentials in isolated canine intracardiac neurons
Isolated intracardiac neurons display morphologic characteristics of parasympathetic cardiac neurons similar to those previously described in fat pad slices (Figure 1A).13, 14, 15 Electrophysiologic recordings were obtained 16 to 24 hours after dissociation from neurons lacking cellular processes. Figure 1B illustrates current clamp recordings showing action potential (AP) activity in control conditions (2.5 mM CaCl2, 140 mM NaCl) and in the presence of 300 nM TTX. Block of neuronal TTX-S
Discussion
Intracardiac ganglia function as signal integrating centers within the heart. The final output to the heart results from the AP activity of the postganglionic neurons, determined by a combination of the intrinsic electrical properties of the neurons and neurotransmitter-induced modulation of membrane ion channel conductances.25 Sodium currents are critical determinants of the electrical activity of excitable cells. Previous voltage clamp studies involving rat and guinea pig intracardiac neurons
Acknowledgments
We thank Dr. Hali Hartmann for critical review of the data; Ryan Pfeiffer for assistance in sequencing; Robert Goodrow and Andrew Pitoniak for technical support; and Judy Hefferon for assistance with the illustrations.
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Atrial arrhythmias in inherited arrhythmogenic disorders
2016, Journal of ArrhythmiaConcomitant abnormalities in Brugada syndrome
2013, Journal of ArrhythmiaCitation Excerpt :Although these findings may be controversial [37], it is important to consider the coexistence of NMS in BS patients. A recent basic study showed that sodium channel, voltage-gated, type V, alpha subunit (SCN5A), a major responsible gene in BS, is expressed in both myocardial cells and intracardiac autonomic ganglia [50]. Furthermore, Makita et al. also demonstrated a novel nonsense mutation in the SCN5A gene in a BS patient who had been diagnosed with NMS [51].
Augmented ST-segment elevation during recovery from exercise predicts cardiac events in patients with brugada syndrome
2010, Journal of the American College of CardiologyCitation Excerpt :Conversely, the SCN5Amutation was more frequently identified in group 1. Scornik et al. (19) reported that SCN5Amutation can accentuate parasympathetic activity toward the heart directly. It was also reported that specific mutations in the SCN5Agene may lead to augmentation of J-point amplitude or ST-segment amplitude during beta-adrenergic stimulation (20,21).
Structure and function of splice variants of the cardiac voltage-gated sodium channel Na<inf>v</inf>1.5
2010, Journal of Molecular and Cellular CardiologyImpact of Genetic Discoveries on the Classification of Lone Atrial Fibrillation
2010, Journal of the American College of CardiologyCitation Excerpt :A complementary mechanism may involve the intracardiac ganglia that densely innervate this region of the heart (46). Recent work revealed that Nav1.5 channels are highly expressed within intracardiac ganglia, alluding to the possibility that hyperexcitability of surrounding neurons may be the driving force behind the rapidly firing ectopic foci (47). The distinct electrophysiological features of an AF driven by cellular hyperexcitability again serve to reinforce the clinical relevance of a subclassification system for lone AF.
Gain-of-function mutation of Na<inf>v</inf>1.5 in atrial fibrillation enhances cellular excitability and lowers the threshold for action potential firing
2009, Biochemical and Biophysical Research CommunicationsCitation Excerpt :In the context of atrial fibrillation, rapid focal atrial activity, commonly observed from pulmonary myocardial sleeves, may be the result of repetitive myocyte AP firing. Furthermore, a link between sodium channel activity and autonomic mediated AF has been suggested in a study by Scornik et al., which identified a high expression of Nav1.5 channels in intracardiac ganglia [30]. These investigators demonstrated Nav1.5-induced AP activity from these neurons and speculated that a gain-of-function of the Nav1.5 channel might enhance AP activity of intracardiac neurons, augmenting acetylcholine release.
This work was supported in part by American Heart Association–Northeast Affiliate Grant 0335446T to Dr. Scornik, and National Institutes of Health Grant HL073161 to Dr. Pérez, Grant HL47678 to Dr. Antzelevitch, and Grant HL61669 to Dr. Brugada.