An overview of the noradrenergic system, including the identification of norepinephrine (NE) in animal tissue, its synthesis and metabolism, adrenoceptor classification, peripheral and central actions, uptake and storage, and mechanisms of NE release are presented. After characterizing the kinetic, ion dependence and inhibitor sensitivity of the norepinephrine transporter (NET) expressed in a recombinant cell line (LLC-NET cells), the influence of catecholamine (CA) metabolizing enzymes on studies of transport was assessed. Inhibitors of catechol-O-methyltransferase (COMT) potentiated the apparent uptake and retention of [3H]NE and [3H]DA. COMT inhibition had a greater influence on [3H]DA than [3H]NE uptake and retention, which corresponds to the higher spontaneous loss of radiolabel from cells exposed to [3H]DA than [3H]NE ([3H]methoxytyramine, is more lipophilic than [3H]normetanephrine). The monoamine oxidase inhibitor, pargyline, had no augmentary action on [3H]CA uptake, but actually inhibited substrate influx by blocking the NET. [3H]substrate specific differences were demonstrated for [3H]NE, [3H]DA and [3H]MPP+. For a given length of exposure to low Na+ or tyramine, [3H]NE release was the lowest, but most sensitive to NET inhibitors. Disparities in the kinetics of each [3H]substrate for the inwardly facing NET may account for this. Inhibitors of the NET were found to stimulate the efflux of [3H]substrates from preloaded cells incubated in a physiological HEPES buffer. Efflux was NET-dependent and differed greatly for each [3H]substrate. Inhibitor-induced release was greatest for [3H]MPP+ and least for [3H]NE. Finally, a functional model of carrier-mediated NE release in myocardial ischemia, was developed in this study. Release of [3H]MPP+ was stimulated by Na+-H+ exchanger (NHE) activation and modulated by inhibitors of the NET, NHE, Na+,K+-ATPase, and via a receptor-operated pathway. Excessive NE release contributes to severe myocardial arrhythmias, therefore an improved understanding of the carrier-mediated NE release process will ultimately enhance our ability to intervene and prevent the deleterious effects of excessive NE release.

Published Article / Magnesium is the second most abundant intracellular cation with many control and regulatory functions. It regulates energy production and utilization and modulates activity of membrane ionic channels. Magnesium has direct control effects on cardiac myocyte ion channels making it useful in certain arrhythmias. Calcium is responsible for pacemaker excitation and for excitation-contraction coupling in myocytes but increased intracellular calcium produces early and late afterdepolarisations initiating arrhythmias. Magnesium regulates calcium channel activity preventing raised intracellular levels. Potassium channel activity is enhanced by magnesium hyperpolarizing the cell reducing arrhythmia generation. Magnesium is effective against long QT Torsade de Pointes. In rapid atrial fibrillation magnesium produces rate control slowing AV nodal conduction. Magnesium prevents digitalis toxicity due to associated hypomagnesemia.

Cardiac disease is one of the leading causes of death in the world, with an increase in cardiac arrhythmias in recent years. In addition, myocardial ischemia, which arises from the lack of blood in the cardiac tissue, can lead to cardiac arrhythmias and even sudden cardiac death. Cardiac arrhythmias, such as atrial fibrillation, are characterised by abnormal wave excitation and repolarization patterns in the myocardial tissue. These abnormal patterns are usually diagnosed through non-invasive electrical measurements on the surface of the body, i.e., the electrocardiogram (ECG). However, the most common lead configuration of the ECG, the 12-lead ECG, has its limitations in providing sufficient information to identify and locate the origin of cardiac arrhythmias. Therefore, there is an increasing need to develop novel methods to diagnose and find the origin of arrhythmic excitation, which will increase the efficacy of the treatment and diagnosis of cardiac arrhythmias. The objective of this research was to develop a family of multi-scale computational models of the human heart and thorax to simulate and investigate the effect of arrhythmic electrical activity in the heart on the electric and magnetic activities on the surface of the body. Based on these simulations, new theoretical algorithms were developed to non-invasively diagnose the origins of cardiac arrhythmias, such as the location of ectopic activities in the atria or ischemic regions within the ventricles, which are challenging to the clinician. These non-invasive diagnose methods were based on the implementation of multi-lead ECG systems, magnetocardiograms (MCGs) and electrocardiographic imaging.

No description available.

Atrial arrhythmias are very common clinically relevant conditions that are strongly associated with aging and parasympathetic tone. Additionally, ATP-sensitive K+ (KATP) channel activation has been reported to facilitate the development of re-entrant atrial arrhythmias. Since KATP channels are direct effectors of Gαi/o and RGS4 is an inhibitor of Gαi/o-signaling, we here investigate whether KATP channel activity is increased under decreased RGS4 activity in a manner that enhances susceptibility to AF. We show that loss of RGS4 facilitates the induction of atrial arrhythmias under parasympathetic challenge both in whole animals and isolated atrial tissues. Furthermore, using both genetic disruption (Kir6.2 ablation) and pharmacologic blockade (tolbutamide), we show that loss of functional KATP channels decreases the incidence of pacing-induced re-entry and prolongs repolarization in RGS4-deficient atria. Our findings are consistent with the conclusion that enhanced KATP channel activity may contribute to pacing-induced re-entrant rotors in the RGS4-deficient mouse model.

Atrial arrhythmias are very common clinically relevant conditions that are strongly associated with aging and parasympathetic tone. Additionally, ATP-sensitive K+ (KATP) channel activation has been reported to facilitate the development of re-entrant atrial arrhythmias. Since KATP channels are direct effectors of Gαi/o and RGS4 is an inhibitor of Gαi/o-signaling, we here investigate whether KATP channel activity is increased under decreased RGS4 activity in a manner that enhances susceptibility to AF. We show that loss of RGS4 facilitates the induction of atrial arrhythmias under parasympathetic challenge both in whole animals and isolated atrial tissues. Furthermore, using both genetic disruption (Kir6.2 ablation) and pharmacologic blockade (tolbutamide), we show that loss of functional KATP channels decreases the incidence of pacing-induced re-entry and prolongs repolarization in RGS4-deficient atria. Our findings are consistent with the conclusion that enhanced KATP channel activity may contribute to pacing-induced re-entrant rotors in the RGS4-deficient mouse model.

In patients with normal hearts, increased vagal tone is associated with the onset of paroxysmal atrial fibrillation (AF). Vagal denervation of the atria renders AF less inducible. Circumferential pulmonary vein ablation (CPVA), with or without isolation (CPVI), is effective for treating paroxysmal AF, and has been shown to impact HRV indices, in turn reflecting vagal denervation. We examined the impact of CPVI on HRV indices over time, and evaluated the relationship between vagal modification and rate of recurrence of AF. High resolution ECG recordings were collected from 64 patients (49 male, 15 female, mean age 57.1±9.7) undergoing CPVI for paroxysmal (n=46) or persistent (n=18) AF. Recordings were made pre-procedure, and at intervals up to 12 months. Success was defined as no recurrence. After CPVI, 27 patients presented recurrence. Pre-procedure HRV variables did not differ from controls in patients with a subsequent successful procedure. However, patients with recurrence demonstrated significantly-reduced pre-procedure HRV compared both with controls, and with patients having successful procedures (39.6±23.4 & 33.7±19.2 vs 21.8±11.8, P =0.01 & P=0.04). Following the procedure, HRV was reduced vs pre-procedure in patients with successful procedures (33.7±19.2 vs 18.6±15.8, P=0.01), and did not differ from unsuccessful procedures over a 12 month FU. Both groups were reduced compared with a control value. There was no significant difference in HRV between patients who experienced recurring AF (n=9), and those who experienced AT or flutter (n=18). Our data suggests that patients experiencing recurrence after one procedure have reduced HRV that is not changed by CPVI; whereas patients with a successful single procedure experience a change in HRV variables that is sustained over a long period, but is no different post-procedure from patients experiencing recurrence. These data suggest that denervation associated with CPVI may benefit patients with normal vagal tone prior to the procedure, but that sustained denervation is not a critical factor in successful outcome after CPVI. / Thesis (Master, Computing) -- Queen's University, 2010-12-07 08:32:15.066

Cardiac conduction abnormalities can often lead to heart failure, stroke and sudden cardiac death. Heart disease stands as the leading cause of mortality and morbidity in the United States, accounting for 30% of all deaths. Early detection of malfunctions such as arrhythmias and systolic heart failure, the two heart conditions studied in this dissertation, would definitely help reduce the burden cardiovascular diseases have on public health and overcome the current clinical challenges. The imaging techniques currently available to doctors for cardiac activation sequence mapping are invasive, ionizing, time-consuming and costly. Thus, there is an undeniable urgent need for a non-invasive and reliable imaging tool, which could play a crucial role in the early diagnosis of conduction diseases and allow physicians to choose the best course of action. The 12-lead electrocardiogram (ECG) is the current non-invasive clinical tool routinely used to diagnose and localize cardiac arrhythmias prior to intracardiac catheter ablation. However, it has limited accuracy and can be subject to operator bias. Besides, QRS complex narrowing on the clinical ECG after pacing device implantation is also used for response assessment in patients undergoing Cardiac Resynchronization Therapy (CRT). The latter is an established treatment for systolic heart failure patients who have Left Bundle Branch Block as well as a reduced ejection fraction and prolonged QRS duration. Yet, it is still not well understood why 30 to 40 % of CRT recipients do not respond. Echocardiography, due to its portability and ease-of-use, is the most frequently used imaging modality in clinical cardiology. In this dissertation, we assess the clinical performance of Electromechanical Wave Imaging (EWI) as a high frame rate ultrasound-based functional modality that can non-invasively map the electromechanical activation of the heart, i.e., the transient deformations immediately following the electrical activation. The objective of this dissertation is to demonstrate the potential clinical value of EWI for both arrhythmia detection and CRT characterization applications. The first step in translating EWI to the clinic was ensuring that the technique could reli- ably and reproducibly measure the electromechanical activation sequence independently of the probe angle and imaging view in healthy human volunteers (n=7). This dissertation then demonstrated the accuracy of EWI for localizing a variety of ventricular and atrial arrhythmias (accessory pathways in Wolff-Parkinson-White (WPW) syndrome, premature ventricular contractions, focal atrial tachycardia and macro-reentrant atrial flutter) in pediatric (n=14) and adult (n=55) patients prior to catheter ablation more accurately than 12-lead ECG predictions, as validated against electroanatomical mapping. Additionally, 3D-rendered EWI isochrones were illustrated to be capable of significantly distinguishing different biventricular pacing conditions (p≤0.05) with the RWAT and LWAT metrics, assessing the ventricular dyssynchrony change in heart failure patients (n=16) undergoing CRT, and visualizing it in 3D. EWI also provided quantification of %𝘙𝘔𝘓𝘝 in CRT patients (n=38): the amount of left-ventricular resynchronized myocardium, which was found to be a reliable response predictor at 3-, 6-, or 9-month clinical follow-up through its post-CRT values by significantly identifying super-responders from non-responders within 24 hours of implantation (p≤0.05). Furthermore, 3D-rendered isochrones successfully characterized the ventricular activation resulting from His Bundle pacing for the first time (n=4), which was undistinguishable from true physiological activation in sinus rhythm healthy volunteers with the EWI-based activation time distribution dispersion metric. The dispersion was, however, reported to significantly discriminate novel His pacing from other more conventional biventricular pacing schemes (p≤0.01). Finally, we developed and optimized a fully automated zero-crossing algorithm towards a faster, more robust and less observer dependent EWI isochrone generation process. The support vector machine (SVM) and Random Forest machine learning models were both shown capable of successfully identifying the accessory pathway in WPW patients and the pacing electrode location in paced canines. Nevertheless, the best performing algorithm was hereby proven to be the Random Forest classifier with n=200 trees with a precision rising to 97%, and a predictivity that was not impacted by the type of testing dataset it was applied to (human or canine). Overall, in this dissertation, we established the clinical potential of EWI as a viable assisting visual feedback tool, that could not only be used for diagnosis and treatment planning prior to surgical procedures, but also for monitoring during, and assessing long-term resolution of arrhythmia after catheter ablation or heart failure after a CRT implant.

Cardiovascular disease is the leading cause of death world-wide, with 42% of sudden cardiac death in young adults caused by myocarditis. Viruses represent the main cause of myocarditis, with adenovirus being a leading pathogen. However, it is not understood how adenoviruses cause sudden cardiac arrest. Myocarditis is defined by two phases, acute and chronic. The acute phase involves viral-mediated remodeling of subcellular structures in the myocardium, which is thought to contribute to arrhythmogenesis. The chronic phase is immune response-mediated, where the host immune system causes damage that induces gross remodeling of the heart, which can result in cardiac arrest or heart failure. Electrical impulses of the heart are propagated by cardiomyocytes, via gap junctions, ion channels, and intracellular junctions, creating the healthy heartbeat. Cx43, the primary gap junction protein in the myocardium, not only propagates electrical signals, but also anti-viral molecules. Viral targeting of gap junction function leads to reduced anti-viral responses in neighboring cells. However, reduced cellular communication would dangerously alter cardiac conduction. Using a cardiotropic adenovirus, MAdV-3, we find that viral genomes are significantly enriched in the heart, with a decrease of gap junction and ion channel mRNA in infected hearts, however, their protein levels were unchanged. Phosphorylation of Cx43 at serine 368, known to reduce gap junction open probability, was increased in infected hearts. Ex vivo optical mapping illustrated decreased conduction velocity in the infected heart and patch clamping of isolated cardiomyocytes revealed prolonged action potential duration, along with decreased potassium current density during infection. Pairing mouse work with human induced pluripotent stem cell-derived cardiomyocytes, we found that human adenovirus type-5 infection increased pCx43-Ser368 and perturbation of intercellular coupling, as we observed with in vivo MAdV-3 infection. Allowing adenovirus infection to progress in vivo, we find myocardium remodeling and immune cell infiltration. Together, these data demonstrate the complexity of cardiac infection from viral-infection induced subcellular alterations in electrophysiology to immune-mediated cardiomyopathy of cardiac adenoviral infection. Our data describe virally induced mechanisms of arrhythmogenesis, which could lead to the development of new diagnostic tools and therapies, to help protect patients from arrhythmia following infection. / Doctor of Philosophy / Viral infection has long thought to be a cause of unexplained sudden cardiac death, especially in young adults. Viruses have been identified to cause many cases of deleterious remodeling of the heart, which can result in heart failure. The heart relies on electrical signaling that moves in a coordinated fashion to contract and pump blood throughout the body. The cells within the heart that do this are called cardiomyocytes, and they join end-to-end to communicate with each other via gap junctions. Gap junctions are tunnels that allow for ions that create electrical impulses to pass, and molecules, such as ones that are important in immune responses to infection. In addition to gap junctions in the heart, ion channels, which are highly selective to allow only one ion flow, unlike gap junctions, create the healthy heartbeat. The most common gap junction in the heart comprises Cx43 proteins. If a virus were to alter how Cx43 connects to a neighboring cell, this would cause a better environment for the virus, as this would keep anti-viral surveillance low, however, this would change how the electrical signal moves throughout the heart, creating arrhythmias. Adenoviruses are a common cold virus, but have been found in the hearts of many cardiac arrest patients. However, little is known on how adenoviruses may cause cardiac arrest, because human adenoviruses are only successful in humans, and mouse adenoviruses are only successful in mice. This creates a challenge when studying the dynamic heart, which does not translate well to cells in a dish. A mouse adenovirus, called Mouse Adenovirus Type-3 (MAdV-3) was reported to favor infecting the heart in mice, but no research has been published on if this virus can answer how adenoviruses change the heart. Because of this virus, and our prior research that adenoviruses can decrease Cx43 within skin cells in a dish, we used MAdV-3 to understand if, how adenoviruses could cause sudden cardiac arrest, and if longer infection could change the overall structure of the heart. We find that MAdV-3 infection prefers the heart to other organs, and that early stages, reduce both the speed of the electrical signal moves through heart and, looking within a cardiomyocyte, how it creates that electrical signal. These changes are arrhythmogenic and accompany modification of Cx43 that would close the gap junction between two cells, changing how ions and molecules move between cells. Using a human adenovirus infection in human cardiomyocytes created from stem cells, this result is also observed. If infection is allowed to continue in the mouse to cause chronic infection, the heart itself changes shape and is diseased. Together, this work shows that adenoviruses create a diseased heart, first the virus changes how the electrical signal moves and then later, causes thinning of the heart muscle. These data illustrate the role viruses play in causing cardiac arrest and could lead to diagnostic or drug targets.

No description available.