Abstract-Gene-expression changes in atrial fibrillation patients reflect both underlying heart-disease substrates and changes because of atrial fibrillation-induced atrial-tachycardia remodeling. These are difficult to separate in clinical investigations. This study assessed time-dependent mRNA expression-changes in canine models of atrial-tachycardia remodeling and congestive heart failure. Five experimental groups (5 dogs/group) were submitted to atrial (ATP, 400 bpm ϫ24 hours, 1 or 6 weeks) or ventricular (VTP, 240 bpm ϫ24 hours or 2 weeks) tachypacing. The expression of Ϸ21,700 transcripts was analyzed by microarray in isolated left-atrial cardiomyocytes and (for 18 genes) by real-time RT-PCR. Protein-expression changes were assessed by Western blot. In VTP, a large number of significant mRNA-expression changes occurred after both 24 hours (2209) and 2 weeks (2720). In ATP, fewer changes occurred at 24 hours (242) and fewer still (87) at 1 week, with no statistically-significant alterations at 6 weeks. Expression changes in VTP varied over time in complex ways. Extracellular matrix-related transcripts were strongly upregulated by VTP consistent with its pathophysiology, with 8 collagen-genes upregulated Ͼ10-fold, fibrillin-1 8-fold and MMP2 4.5-fold at 2 weeks (time of fibrosis) but unchanged at 24 hours. Other extracellular matrix genes (eg, fibronectin, lysine oxidase-like 2) increased at both time-points (Ϸ10, Ϸ5-fold respectively). In ATP, mRNA-changes almost exclusively represented downregulation and were quantitatively smaller. This study shows that VTP-induced congestive heart failure and ATP produce qualitatively different temporally-evolving patterns of gene-expression change, and that specific transcriptomal responses associated with atrial fibrillation versus underlying heart disease substrates must be considered in assessing gene-expression changes in man. Key Words: arrhythmia Ⅲ remodeling Ⅲ genomic A trial fibrillation (AF) is the most common sustained cardiac rhythm disorder, and with the aging of the population both the prevalence and economic impact of AF are increasing progressively. 1 Although the mechanistic basis of AF remains incompletely understood, active research promises to provide new insights that may lead to improved therapeutic options. 2,3 A variety of animal models have been used to assess AF pathophysiology under controlled conditions. Atrial tachyarrhythmias, including AF itself, alter atrial electrophysiology in ways that promote AF vulnerability. [4][5][6] Experimentally-induced congestive heart failure (CHF) also creates a substrate for AF maintenance, but by quite different mechanisms. 7 The atrial-tachycardia remodeling paradigm shows prominent changes in ion-channel function that lead to action-potential abbreviation and the promotion of atrial reentry. 8,9 CHF-induced ionic-current changes do not promote reentry but may favor ectopic-impulse formation, 10 and CHF-induced fibrosis promotes reentry by interfering with intraatrial conduction. 7 The molecular basis of AF ...