Cardiovascular pharmacology Inhibitors of arachidonate-regulated calcium channel signaling suppress triggered activity induced by the late sodium current Paul Wolkowicz a,n , Patrick K. Umeda b , Oleg F. Sharifov b , C. Roger White b , Jian Huang b , Harry Mahtani c , Ferdinand Urthaler b a KOR Therapies, LLC, The University of Alabama at Birmingham, Birmingham, AL 35294, USA b The Department of Medicine, The University of Alabama at Birmingham, Birmingham, AL 35294, USA c The Department of Anesthesiology, The University of Alabama at Birmingham, Birmingham, AL 35294, USA article info Article history: Received 20 May 2013 Received in revised form 11 December 2013 Accepted 12 December 2013 Available online 19 December 2013 Keywords: ARC channel Orai ATXII SKF-96365 LOE-908 Triggered activity Early afterdepolarization Late sodium current Store operated calcium channel abstract Disturbances in myocyte calcium homeostasis are hypothesized to be one cause for cardiac arrhythmia. The full development of this hypothesis requires (i) the identification of all sources of arrhythmogenic calcium and (ii) an understanding of the mechanism(s) through which calcium initiates arrhythmia. To these ends we superfused rat left atria with the late sodium current activator type II Anemonia sulcata toxin (ATXII). This toxin prolonged atrial action potentials, induced early afterdepolarization, and provoked triggered activity. The calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93 (N-[2-[[[3-(4-chlorophenyl)-2-prope- nyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphon-amide) suppressed ATXII triggered activity but its inactive congener KN-92 (2-[N-(4-methoxy benzenesulfonyl)]amino-N-(4-chlor- ocinnamyl)-N-methylbenzylamine) did not. Neither drug affected normal atrial contractility. Calcium entry via L-type channels or calcium leakage from sarcoplasmic reticulum stores are not critical for this type of ectopy as neither verapamil ((RS)-2-(3,4-dimethoxyphenyl)-5-{[2-(3,4-dimethoxyphenyl)ethyl]-(methyl) amino}-2-prop-2-ylpentanenitrile) nor ryanodine affected ATXII triggered activity. By contrast, inhibitors of the voltage independent arachidonate-regulated calcium (ARC) channel and the store-operated calcium channel specifically suppressed ATXII triggered activity without normalizing action potentials or affecting atrial contractility. Inhibitors of cytosolic calcium-dependent phospholipase A 2 also suppressed triggered activity suggesting that this lipase, which generates free arachidonate, plays a key role in ATXII ectopy. Thus, increased left atrial late sodium current appears to activate atrial Orai-linked ARC and store operated calcium channels, and these voltage-independent channels may be unexpected sources for the arrhythmogenic calcium that underlies triggered activity. & 2013 Elsevier B.V. All rights reserved. 1. Introduction At least 15% of all deaths in post-industrial societies result from disturbances in heart electrical activity (Huikuri et al., 2001). Most hypotheses to explain this major cause of premature death focus on electrical phenomena such as impulse reentry or on abnormalities in the biophysical properties of voltage-dependent ion channels (January and Riddle, 1989; Keating and Sanguinetti, 2001; Moe and Ablidskov, 1959; Panfilov and Pertsov, 2001). Abnormal myocyte calcium homeostasis also is arrhythmogenic as Anderson et al. (1998) demonstrated that calmodulin-dependent protein kinase II (CaMKII) inhibitors suppress early afterdepolarization and triggered activity. This work and other data demonstrate that electrical instabilities like afterdepolarization, tachycardia, and fibrillation require arrhythmogenic calcium, which arises from one or more cell sources (Anderson et al., 1998; Choi et al., 2002, Lehnart et al., 2008; Shannon et al., 2003). Calcium performs multiple, well-defined functions in heart phy- siology and pathology. It promotes repolarization during phase 2 of the cardiac action potential and calcium-induced sarcoplasmic reti- culum (SR) calcium release coupled with calcium binding to troponin C is necessary for heart contraction (Bers, 2002; Fabiato, 1983; Ringer, 1883). Calcium transporters and release channels that respond to the electrical potential across the plasma membrane carry out these voltage-dependent functions. Calcium also is critical for cardiac pathological adaptations, as hypertrophy and heart failure require altered calcium homeostasis and signaling (Gwathmey et al., 1987; Molketin et al., 1998). Voltage-independent calcium transporters and calcium release channels that respond to inositol-1,4,5-trisphosphate mediate many of these adaptive responses (Lipp et al., 2000). While considered functionally distinct, cross-talk can occur between muscle voltage-dependent and voltage-independent calcium pools and signaling pathways (Dirksen, 2009; Stiber et al., 2008; Urthaler et al., 1989). Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/ejphar European Journal of Pharmacology 0014-2999/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ejphar.2013.12.020 n Correspondence to: KOR Therapies, LLC, 2008 Bridgelake Drive, Hoover, Alabama 35244, USA. Tel.: þ1 205 253 5838. E-mail address: kor.cures@gmail.com (P. Wolkowicz). European Journal of Pharmacology 724 (2014) 92–101