Abstract 17445: Overexpression of Adenylyl Cyclase Type 5 (AC5) in the Heart Predisposes to Cardiac Arrhythmias
Adenylyl cyclase type 5 (AC5) is one of two major AC isoforms in the heart. It is controversial whether AC5 overexpression (AC5 OE) in the heart is protective or deleterious, particularly related to arrhythmogenesis. Accordingly, we determined the electrophysiological and intracellular calcium ion (Cai2+) handling properties in AC5 OE transgenic mouse hearts. Cai2+ was imaged using a CCD-based system in the fluo-4 AM loaded ventricular myocytes. The SR Ca2+ contents (F/F0: 4.2 ± 0.2 vs. 3.4 ± 0.2), fractional Ca2+ releases (84.0 ± 2.3 % vs. 68.3 ± 3.4%), and twitch Ca2+ transients (F/F0: 3.4 ± 0.1 vs. 2.4 ± 0.1) were significantly higher in the AC5 OE vs. WT myocytes (p < 0.05 in each case), indicating Ca2+ overload in AC5 OE myocytes. We also recorded action potentials (APs) by using patch clamping technique and showed that AP duration at 90% repolarization (APD90) was significantly longer in AC5 OE (65.2 ± 7.6 ms) than in WT myocytes (46.7 ± 6.9 ms) (p < 0.05). Blocking Na+-Ca2+ exchanger current (INCX) by replacing Na+ with Li+ in the Tyrode's perfusate shortened the APD90, further confirm the Ca2+ overload in AC5OE myocytes. Moreover, AC5 OE myocytes developed spontaneous Ca waves in a significant larger fraction compared with WT myocytes, particularly when cells were exposed to isoproterenol (61.9% vs. 42.9%). The Ca2+ waves further induced early (EADs) and/or delayed afterdepolarizations (DADs) and triggered APs, reminiscent of the cellular mechanisms for catecholaminergic polymorphic ventricular tachycardia (CPVT). Western blotting analysis showed increased levels (∼ 2 folds) of SR Ca2+ ATPase (SERCA2a) in AC5OE hearts, which may contribute for the increased SR Ca2+ load. The standard lead II ECG was recorded in anaesthetized animals. While arrhythmic events were not detected in either AC5 OE or WT mice under control conditions, catecholamine challenge (4 mg/kg isoproterenol, i.p.) induced frequent premature ventricular contractions, and polymorphic ventricular tachycardia in AC5OE (4/5), but not in WT (0/8) mice. Thus AC5 OE exerts an adverse effect on arrhythmogenesis, by inducing SR Ca2+ overload and spontaneous Ca2+ release/Ca2+ waves, which in turn increase the propensity for EADs, DADs in myocytes and CPVT in vivo.
- © 2010 by American Heart Association, Inc.