Thesis (doctoral)--Rijksuniversiteit te Utrecht.

Thesis (doctoral)--Rijksuniversiteit te Utrecht.

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The IKs current constitutes part of the repolarisation reserve in the human myocardium, and whilst it does not play a major role at resting heart rates, it becomes a crucial component of repolarisation in the setting of increased sympathetic tone and high heart rates. The formation of the IKs current requires the KCNQ1 α-subunit and the KCNE1 β-subunit. Mutations in either of these subunits can lead to long QT syndrome types 1 and 5, respectively. Loss-of-function mutations in the IKs channel can reduce the repolarisation reserve and lead to action potential prolongation, predisposing to lethal cardiac arrhythmias such as torsades de pointes and ventricular fibrillation. It is widely recognised that the IKs channel requires the minor membrane phospholipid PIP2 for its function, and previous work in this laboratory found that mutations in a PIP2-binding region in KCNQ1 led to retention of the channel in the endoplasmic reticulum, suggesting that PIP2 may play a role in anterograde trafficking. Here, the rapamycin-inducible dimerisation system was used to manipulate levels of PIP2 and/or PI4P at the plasma membrane or Golgi, and the effect of this on IKs channel trafficking and function was investigated using molecular biology, confocal microscopy and electrophysiology. Despite difficulties with optimising the rapamycin-induced dimerisation system, it was observed that the IKs channel does not require PIP2 for anterograde trafficking, but is heavily reliant on PIP2 for channel opening. In addition, activation of the β1-adrenergic receptor (β 1-AR) led to an increase in the IKs current amplitude. The potential interplay between β1-AR and PIP2 signalling was also explored by depleting PIP2 during β1-AR stimulation. PIP2 depletion was less effective at inhibiting the IKs current during β1-AR stimulation, but this requires further investigation. In conclusion, the results suggest that the IKs channel is reliant on PIP2 for function, but not anterograde trafficking.

Heart failure patients have dysfunction of the cardiac conduction system that contributes to a high burden of arrhythmias including atrial fibrillation and sudden cardiac death. Heart failure has been associated with the inflammatory response, but it is unknown if inflammation is playing a role in the remodelling of the cardiac conduction system in heart failure. Inflammation has been shown to be present in the myocardium from failing hearts and it is known to have detrimental effects on cardiac function, inducing fibrosis, remodelling of ion channels and even arrhythmias. However, the effect of inflammation on the cardiac conduction system has not been investigated. The aims of this study were to determine if there is an increase of pro-inflammatory cytokines and inflammatory cells in the cardiac conduction system in heart failure. In addition, to identify if there is possible inflammation-associated fibrosis and apoptosis in the cardiac conduction system in heart failure. To test these aims, three models of heart failure were used: a rat model of pulmonary arterial hypertension, a rabbit model of congestive heart failure and a rat model of myocardial infarction. In the rat model of pulmonary arterial hypertension there was a bradycardia, a prolongation of the QT interval, and an increase in the atrioventricular and ventricular refractory periods, suggesting electrical remodelling in these animals. The rats with pulmonary arterial hypertension displayed an increase in pro-inflammatory cytokines such as interleukins 1β and TGFβ in the right side of the heart, including the sinoatrial node and right Purkinje fibres of the cardiac conduction system. In addition, in these areas, there was an increase in components of the extracellular matrix, including fibronectin, collagen I and vimentin. Histology revealed regions of non-myocyte nuclei, only in the right ventricle of the rats with pulmonary arterial hypertension. Immunohistochemistry demonstrated patches of CD68 and vimentin expression (markers for macrophages and fibroblasts, respectively) in the right side of the heart in these animals. TUNEL staining also revealed an increase in apoptosis in the right side of the heart. In the rabbit model of congestive heart failure, the region most affected by inflammation was the right atrium, while few changes were measured in the ventricles or cardiac conduction system. Although these results are surprising, it is suggested that the atria could be more sensitive to the physical stretch produced in this model. In the rat model of myocardial infarction, there were regions of non-myocyte nuclei in the border zone. This region also had increases in pro-inflammatory and fibrosis markers. In conclusion, this work has presented the novel finding that there can be inflammation in the cardiac conduction system in heart failure. This could be contributing to the arrhythmias seen in heart failure patients. This could possibly lead the way to anti-inflammatories as a possible novel therapeutic for heart failure patients.

Photocopy of typescript. / Digitized by Kansas Correctional Industries

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on September 25, 2009). Thesis advisor: Dr. Luis Polo Parada. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1969. / Typescript. Vita. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1970. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The specialized myocytes of the ventricular conduction system (VCS) coordinate ventricular contraction and are critical for efficient pumping by the heart. Impaired VCS conduction is characteristic of inherited forms of cardiac conduction disorders. Here we show that the Iroquois homeobox 3 (Irx3) transcription factor is preferentially expressed in the developing and mature VCS. Loss of Irx3 in mice results in slowed VCS conduction and prolonged QRS duration with right bundle branch block, caused by reduction (42%) in VCS-specific connexin 40 (Cx40) expression and VCS fiber hypoplasia, absent in littermate controls. Therefore, we show that the role of Irx3 in the heart is two-fold, whereby Irx3 (1) indirectly regulates Cx40 gene expression, by repressing a repressor of Cx40 transcript, and (2) controls VCS maturation, possibly in an Nkx2-5-dependent manner. To our knowledge, this is the first report of a role for Irx3 in regulating the development and function of the VCS.