Isolation of the serotoninergic 5-HT4(e) receptor from human heart and comparative analysis of its pharmacological profile in C6-glial and CHO cell lines

1 RT-PCR technique was used to clone the human 5-HT4(e) receptor (h5-HT4(e)) from heart atrium. We showed that this h5-HT4(e) receptor splice variant is restricted to brain and heart atrium. 2 Recombinant h5-HT4(e) receptor was stably expressed in CHO and C6-glial cell lines at 347 and 88 fmol mg(-1) protein, respectively. Expression of h5-HT4(e) receptors at the cell membrane was confirmed by immunoblotting. 3 The receptor binding profile, determined by competition with [H-3]-GR113808 of a number of 5-HT4 ligands, was consistent with that previously reported for other 5-HT4 receptor isoforms. Surprisingly, we found that the rank order of potencies (EC50) of 5-HT4 agonists obtained from adenylyl cyclase functional assays was inversely correlated to their rank order of affinities (K-i) obtained from binding assays. Furthermore, EC50 values for 5-HT, renzapride and cisapride were 2 fold lower in C6-glial cells than in CHO cells. 4 ML10302 and renzapride behaved like partial agonists on the h5-HT4(e) receptor. These results are in agreement with the reported low efficacy of the these two compounds on L-type Ca2+ currents and myocyte contractility in human atrium. 5 A constitutive activity of the h5-HT4(e) receptor was observed in CHO cells in the absence of any 5-HT4 ligand and two 5-HT4 antagonists, GR113808 and ML10375, behaved as inverse agonists. 6 These data show that the h5-HT4(e) receptor has a pharmacological profile which is close to the native h5-HT4 receptor in human atrium with a. Functional potency which is dependent on the cellular context in which the receptor is expressed.