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2017 Cardiovascular Research Day Abstract Book

17 Cardiac Cycle Affects

17 Cardiac Cycle Affects Ultrasound Measurements of Ascending Aortic Diameter in a Marfan Mouse Model Zheying Chen 1 • Hisashi Sawada, MD, PhD 1 • Debra Rateri 1 • Alan Daugherty 1 • Mary Sheppard, MD 1 1Saha Cardiovascular Research Center, University of Kentucky Graduate Student Objective: Ultrasound measurements of aortic diameter are a common endpoint in preclinical studies. However, there is a lack of standardization in both image capture and analysis. For our study, we developed a standardized protocol for measuring ascending aortic diameter and examined effects of cardiac cycle in wild type and fibrillin-1 hypomorphic (FBN1mgR/mgR) mice. Methods and Results: Twelve week old male and female FBN1mgR/mgR mice were anesthetized and maintained at a heart rate of 450-550 beats per minute. Ultrasound images were captured using a Vevo 2100 system with a 40MHz tranducer. Images captured were standardized according to two anatomical landmarks: the innominate artery branchpoint and aortic valves. The largest luminal ascending aortic diameter between the sinotubular junction and the innominate artery were measured in mid-systole and end-diastole by two blinded, independent observers. Aortic diameters were significantly different (p

18 A Novel Approach for Modifying Intrinsic Heart Rate: RRad knock-down Bryana Levitan 1 • Mihir Shah 2 • Joshua Rutland, MD 1 • Brooke Ahern 3 • Douglas Andres, PhD 4 • Jonathan Satin, PhD 3 1Cardiology, University of Kentucky • 2 University of Kentucky • 3 Physiology, University of Kentucky • 4 Molecular and Cellular Biochemistry, University of Kentucky Staff Introduction: About 3 million people worldwide are dependent on pacemakers. Each year 600,000 pacemakers are implanted worldwide, for the primary indication of symptomatic bradycardia. Biological pacemakers represent an emerging technology for bradyarrhythmias that require a pacemaker such as sinoatrial nodal failure due to congenital disease or aging. Leading edge biological pacemaker approaches include implantation of induced, pluripotent stem cell-derived cardiomyocytes (iPSC-CMs); however, intrinsic rates of implanted cells might be insufficient to meet physiologic demands, and cell implantation carries a variety of obstacles. In the present report, we introduce a novel molecular mechanism for increasing heart rate. Rad is a monomeric G-protein and constituent of the L-type calcium channel macromolecular complex (LTCC). Rad inhibits LTCC inward calcium current. LTCC activity contributes to pacemaker depolarization. Therefore, we tested the hypothesis that reduction of cardiac Rad will result in a stable increased heart rate (HR). We suggest that knockdown of cardiac Rad could facilitate the development of rate support alone or in combination with cells as a biologic pacemaker. Methods & Experimental Design: Cardiac-restricted, inducible Rad knockout (cRadKO) female mice were implanted with radio-telemeters (model F10A, DSI, Minneapolis MN). Baseline recordings (continuous 72h) were followed by induction of Rad knockout (RadKO). Continuous recordings followed induction of RadKO at times specified in results. Intrinsic heart rate was measured after complete autonomic blockade with injection of propranolol (1 mg/kg) and atropine (1 mg/kg). Peak heart rate was achieved by injection of isoproterenol (ISO) at diurnal peak. Startle response was performed using a series of air jet challenges to provoke an increase in native adrenergic stimulation. Results: Baseline measures demonstrated normal diurnal rhythms of HR (daytime trough, night-time peak) without evidence of significant arrhythmias. Trough HR increased without evidence of significant arrhythmias 4 days post cRadKO, while peak HR remained unchanged. The increase in trough HR was sustained 1 month after cRadKO. During trough periods episodes of sinus bradycardia without SA node or AV node block were noted consistent with high vagal tone. Intrinsic HR was also significantly increased post-cRadKO over baseline. ISO challenge induced sinus tachycardia with a blunted response as compared to controls. During recovery 1-8 hours after ISO challenge both cRadKO and control mice exhibited transient episodes of bradyarrhythmias. All animals returned to pre-ISO rhythm without incident. The startle response did not elicit arrhythmias. Conclusion: Cardiac Rad deletion results in a stable increase in intrinsic heart rate with retained physiologic autonomic responsiveness. Rad depletion did not induce any significant arrhythmias under physiological stressors. We conclude that inhibition of Rad can be used to increase the intrinsic heart rate. This could represent a novel mechanism for tuning iPSC-CMs to more physiologic heart rates. 34

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