5-HT4 and 5-HT2 receptors antagonistically influence gap junctional coupling between rat auricular myocytes

5-hydroxytryptamine-4 (5-HT₄) receptors have been proposed to contribute to the generation of atrial fibrillation in human atrial myocytes, but it is unclear if these receptors are present in the hearts of small laboratory animals (e.g. rat). In this study, we examined presence and functionality of 5-HT₄ receptors in auricular myocytes of newborn rats and their possible involvement in regulation of gap junctional intercellular communication (GJIC, responsible for the cell-to-cell propagation of the cardiac excitation). Western-blotting assays showed that 5-HT₄ receptors were present and real-time RT-PCR analysis revealed that 5-HT_4b was the predominant isoform. Serotonin (1 μM) significantly reduced cAMP concentration unless a selective 5-HT₄ inhibitor (GR113808 or ML10375, both 1 μM) was present. Serotonin also reduced the amplitude of L-type calcium currents and influenced the strength of GJIC without modifying the phosphorylation profiles of the different channel-forming proteins or connexins (Cxs), namely Cx40, Cx43 and Cx45. GJIC was markedly increased when serotonin exposure occurred in presence of a 5-HT₄ inhibitor but strongly reduced when 5-HT_2A and 5-HT_2B receptors were inhibited, showing that activation of these receptors antagonistically regulated GJIC. The serotoninergic response was completely abolished when 5-HT₄, 5-HT_2A and 5-HT_2B were simultaneously inhibited. A 24 h serotonin exposure strongly reduced Cx40 expression whereas Cx45 was less affected and Cx43 still less. In conclusion, this study revealed that 5-HT₄ (mainly 5-HT_4b), 5-HT_2A and 5-HT_2B receptors coexisted in auricular myocytes of newborn rat, that 5-HT₄ activation reduced cAMP concentration, I_CaL and intercellular coupling whereas 5-HT_2A or 5-HT_2B activation conversely enhanced GJIC.