Serotonin 5-HT4 receptors are present in human atrial myocytes and have been proposed to contribute to the generation of atrial fibrillation (AF). Here, we quantified 5-HT4 receptors as well as other key genes involved in cardiac rhythm and contraction in right atrial appendages of patients with chronic AF (CAF) and acute AF (AAF). Right atrial appendages were obtained from eleven patients in sinus rhythm (SR), five with AAF and six with CAF (>12 months). TaqMan real time quantitative RT-PCR was performed on total RNA. Results were normalised to the average of three housekeeping genes, cyclophilin, GADPH and RL-19. The rank order of expression of h5-HT4 receptors variants was (b) > (a) > (g) > (c) in the group of patients in SR. In AAF, we found a strong decrease in h5-HT4(b), h5-HT4(c), and h5-HT4(g) transcripts. In CAF patients, the mRNA expression level of the h5-HT4(b) isoform significantly increased two fold versus SR. A similar increase was reported for beta(1)-adrenergic receptor, connexin 43 and the L-type Ca2+ channel CaC-NA1C subunit. Interestingly, CAF was associated with a strong increase in the expression of Na+/Ca2+ exchanger and the voltage-dependent Na+ channel SCN5A subunit. Our results indicate that h5-HT4(b) is the dot minant cardiac isoform of human 5-HT4 receptors and its expression is increased in CAF. These data support the involvemen of 5-HT4 receptors in atrial arrhythmia. (c) 2007 Elsevier Inc. All rights reserved.