Cardiac arrhythmias remain a major cause of morbidity and mortality. Some arrhythmias result from rare congenital conditions, while most are caused by damage to the heart and its electrical system from ischemia, infarction, cardiac surgery, and assorted non-cardiac diseases. All of these conditions show enhanced dispersion of repolarization (DR) and of refractory periods (DRP) and this may be a unifying mechanism, central to the initiation of arrhythmias and the ensuing pathophysiological process. The mechanisms by which prolonged action potential duration (APD) contributes to ventricular tachycardia (VT), fibrillation (VF), and sudden death are not well understood. New insights of arrhythmogenic mechanisms have emerged from patients with the rare congenital long QT syndrome (LQTS) that is caused by mutations that prolong the cardiac APD and produce polymorphic (VT). This proposal seeks to identify how spatial heterogeneity of channel expression in myocardial tissue generates spatial and temporal heterogeneity in repolarization and refractoriness. It will use a combination transgenic mouse models, measurements of spatio-temporal changes in repolarization and computer modeling to elucidate basic mechanisms by which reentrant arrhythmias are initiated, sustained and self-terminated. To clarify the role of K+ channels and K+ currents in health and disease, we have engineered mice with specific K+ channel and K+ current defects. i) A dominant negative transgenic mouse that expresses an N- terminal fragment of Kv1.1 and has a prolonged QT interval, with spontaneous and inducible VT. ii) A dominant negative transgenic that expresses a mutant Kv4.2 alpha subunit and has a prolonged QT and brief inducible VT. iii) A mouse with a targeted mutation of Merg1, the mouse homolog of HERG has a normal QT but is highly susceptible to PVT. Voltage-sensitive dyes and imaging techniques will be used on perfused intact mouse hearts to map spatio-temporal characteristics of Aps, DR and AP restitution kinetics as well as vulnerability to arrhythmia by programmed stimulation. Enhanced DR at the level of the intact heart will be correlated with cellular variations of channel expression and ionic currents (immuno-histochemistry and single cell voltage clamp) to test the underlying mechanism for DR in these mouse models.
The specific aims will be to test the following hypotheses: 1) Enhanced DR and/or DRP are required for uni-directional propagation of extra-Aps and initiation of VT. 2) Spatial heterogeneities of cardiac ionic currents are the basis of altered DR. Cells isolated from various regions will be used to correlate repolarization heterogeneities in intact hearts to cellular properties using voltage-clamp. 3) Spatio-temporal modifications of [Ca2+]i transient underlie electrical instabilities, enhanced DR and promote VT. 4) Develop mathematical models of repolarization to predict the measured repolarization abnormalities from genetic alterations in ionic channels. These studies will provide new insights on the molecular basis of spatio-temporal heterogeneities of repolarization and refractoriness and their role in maintaining a normal cardiac rhythm or in producing conditions that provoke arrhythmias.
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