A pharmacological model for calcium overload-induced tachycardia in isolated rat left atria Paul E. Wolkowicz a, ⁎ , Hernan E. Grenett a , Jian Huang a , Hsien Chin Wu b , David D. Ku b , Ferdinand Urthaler a a The Department of Medicine, BBRB 806, The University of Alabama at Birmingham, Birmingham, Alabama 35294, USA b The Department of Pharmacology, The University of Alabama at Birmingham, Birmingham, Alabama 35294, USA Received 1 April 2007; received in revised form 30 July 2007; accepted 6 August 2007 Available online 14 August 2007 Abstract Few experimental models produce spontaneous tachycardia in normal left atria to allow the study of the cellular mechanisms underlying this contributor to atrial fibrillation. We reported 2-aminoethoxydiphenyl borate (2-APB) that provokes sporadic spontaneous mechanical activity and calcium leak in isolated rat left atria. Since sarcoplasmic reticulum calcium leak in the presence of high calcium load may trigger tachyarrhythmias, we tested how conditions that increase calcium load affect 2-APB-induced ectopic activity. Exposing superfused rat left atria to (i) 30 nM isoproterenol, (ii) 3 μM forskolin, (iii) 300 nM (-)BayK 8644 ((4S)-1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluormethyl)phenyl]-3- pyridinecarboxylic acid methyl ester), (iv) 300 nM FPL-64176 (2,5-Dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester) or (v) 120 μM ouabain increases their force of contraction, evidence of calcium loading, but does not produce ectopic activity. Spontaneous mechanical activity occurs in left atria superfused with 20 μM 2-APB at 47 ± 6 contractions/min in the absence of pacing. Any of these five agents increase rates of 2-APB-induced spontaneous mechanical activity to N 200 contractions/min in the absence of pacing. Washing tachycardic left atria with superfusate lacking 2-APB restores normal function, demonstrating the reversibility of these effects. Decreasing superfusate sodium reverses this tachycardia and two hyperpolarization-activated current (I f ) inhibitors blunt this ectopic activity. Thus conditions that increase atrial calcium load increase the frequency of spontaneous mechanical activity. Decreasing extracellular sodium and I f inhibitors suppress this spontaneous tachycardia suggesting forward-mode sodium–calcium exchange and I f -like activities underlie this activity. This model may help define cell pathways that trigger atrial tachyarrhythmias. © 2007 Elsevier B.V. All rights reserved. Keywords: 2-APB; I f ; Na/Ca-exchanger; Atrial tachycardia; Atrial fibrillation 1. Introduction Spontaneous left atrial tachycardia is a recognized cause of atrial fibrillation (Jaïs et al., 1997; Shah et al., 2002; Nattel, 2005; Rostock et al., 2006), but precisely how tachycardia arises in left atria remains unresolved. Studies in isolated ventricular myocytes suggest a calcium-activated inward depolarizing current (I ti ) carried by the sodium–calcium exchanger (NCX) may trigger after-depolarizations in vivo (Pogwizd and Bers, 2004). Increased sarcoplasmic reticulum calcium load in the presence of sarcoplasmic reticulum calcium leak is hypothesized to exacerbate I ti and produce triggered ventricular tachyarrhythmias (Pogwizd and Bers, 2004; Vassalle and Lin, 2004). However, whether I ti triggers ectopic activity in intact atrial muscle and whether increased calcium load underlies ectopic left atrial tachycardia are open questions. A lack of experimental models that acutely create tachycardia in isolated left atria in the absence of tissue remodeling slows the resolution of these questions (Friedrichs, 2000). Calcium leaked through the cardiac ryanodine receptor calcium release channel is proposed as the trigger for I ti ectopic activity (Marx et al., 2000; Ai et al., 2005). However, recent work suggests that cells contain multiple calcium leak path- ways, including calcium leaked through the inositol 1,4,5- trisphosphate (IP3) receptor, and shows that enhanced calcium Available online at www.sciencedirect.com European Journal of Pharmacology 576 (2007) 122 – 131 www.elsevier.com/locate/ejphar ⁎ Corresponding author. Tel.: +1 205 975 5252; fax: +1 205 934 7121. E-mail address: wolk@uab.edu (P.E. Wolkowicz). 0014-2999/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2007.08.004