5-HT4(a) and 5-HT4(b) receptors have nearly identical pharmacology and areboth expressed in human atrium and ventricle

5-Hydroxytryptamine (5-HT) increases human heart rate and atrial contractil e force and hastens atrial relaxation through 5-HT4 receptors. Moreover, 5- HT may be arrhythmogenic and give rise to atrial fibrillation. It is not cl ear which splice variant(s) of the 5-HT4 receptor is expressed and mediates these effects of 5-HT in the human heart. Previous studies have indicated different pharmacological properties of 5-HT, receptors in human heart and mouse colliculi neurones, possibly due to expression of different splice va riants. We therefore cloned the human 5-HT4(b) receptor and compared its ph armacological properties with those of the cloned human 5-HT4(a) receptor a nd searched for the corresponding mRNA in human tissues. The primary struct ures of the two human 5-HT4 receptor splice variants are identical except f or divergent C-terminal tails of 28 and 29 amino acids in the 5-HT4(a) and 5-HT4(b), receptors, respectively. Reverse transcription-polymerase chain r eaction (RT-PCR) analysis showed that both variants were coexpressed in var ious tissues, including cardiac atrium and ventricle. Additional bands sugg ested the presence of more than two human 5-HT4 receptor splice variants. W ith cloned receptors stably expressed in HEK293 cells or transiently expres sed in COS-7 cells, [H-3]GR113808 consistently showed slightly higher bindi ng affinity to h5-HT4(b) than to h5-HT4(a) receptor (pK(d) 0.1-0.2 log unit s higher). Competition of agonists, partial agonists and antagonists for [3 H]GR113808 binding revealed no significant differences between the two rece ptors. For 5-HT4 receptor agonists and antagonists, their potencies in stim ulating or inhibiting, respectively, 5-HT-stimulated adenylyl cyclase activ ity correlated well with their binding affinities. Tropisetron and SB207710 showed partial agonist activity at high receptor expression levels for bot h isoforms. Cisapride and renzapride were both partial agonists at moderate receptor levels and full agonists at high receptor levels. Cisapride was m ore potent than renzapride while the converse was the case in human atrium, for which cisapride had lower affinity and agonist potency than at the rec ombinant receptors. The binding affinities and agonist potencies of ligands for both 5-HT4(a) and 5-HT4(b) receptors correlated with the corresponding affinities and potencies in human atrium. The agonist potency of SB207710 was around 10 times lower than its binding and blocking affinity for both s plice variants, suggesting that activation of adenylyl cyclase and blockade of 5-HT responses are mediated through different conformational states. Th e similar pharmacological properties of the two human 5-HT4 receptor splice variants together with their expression in human atrium would be consisten t with mediation of the cardiostimulant effects of 5-HT through both varian ts. However, the effects of cisapride appear either mediated through non-a and non-b splice variants of the 5-HT4 receptor or 5-HT4(,) and 5-HT4(b), r eceptor expression in human atrial cells alters somewhat their pharmacologi cal profile through still unknown mechanisms.