The effects of serotonin on the electrophysiological properties of atrioventricular node during an experimental atrial fibrillation.

A few studies explored the atrioventricular (AV) nodal effects of 5-hydroxytyptamine (serotonin, 5-HT) during supraventricular tachyarrhythmia. The aims of the present study are to investigate (i) 5-HT effects on the rate-dependent electrophysiological functions of AV node during atrial fibrillation (AF) and (ii) the potential contribution of various 5-HT receptors and the role of the autonomic nervous system on 5-HT effects on AV nodal properties. The specific stimulation protocols were applied to detect the electrophysiological parameters of AV node in seven groups of isolated rabbit AV nodal preparations (N = 75) in the presence of 5-HT (0.5, 1, 5, 10, and 20 μM) and its receptor antagonists, nadolol and atropine. The simulated AF protocol was executed in a separate group, and specific indices, including mean His-His interval, a zone of concealment (ZOC), and concealed beats recorded. 5-HT (10-20 μM) increased significantly functional refractory period, Wenckebach cycle length, and excitability index (p < 0.05). The percentage of gap and echo beats was significantly decreased with increasing 5-HT concentrations (p < 0.05). Ketanserin and tropisetron increased significantly atrial-His conduction time, effective refractory period, and Wenckebach cycle length (p < 0.05). 5-HT effects on functional refractory period and Wenckebach cycle length were abrogated by tropisetron and nadolol (p < 0.05). 5-HT elicited prolongation of ZOC and nodal refractoriness (p < 0.05). We conclude that 5-HT elicited prolongation of the nodal refractoriness more than atrial-His conduction time leads to increase in the excitability index and ZOC without significant reduction of the ventricular rates during AF.