Pharmacological characterization of the human 5‐HT 4(d) receptor splice variant stably expressed in Chinese hamster ovary cells

The recently identified C‐terminal splice variant of the human 5‐HT 4 receptor, the h5‐HT 4(d) receptor, was stably expressed in a CHO cell line at 493±25 fmol mg −1 protein. We analysed its pharmacological properties by measuring binding affinities and 5‐HT 4 ligand‐induced cyclic AMP production. The pharmacological binding profile determined in competition studies with the specific antagonist [ 3 H]‐GR113808 revealed a rank order of affinity of 5‐HT 4 ligands for the h5‐HT 4(d) receptor that was consistent with those previously reported for other 5‐HT 4 receptor isoforms. In adenylyl cyclase functional assays, the h5‐HT 4(d) receptor displayed equipotent coupling for all 5‐HT 4 agonists tested (EC 50 in the range of 1–6 n M ). EC 50 values were lower than those previously obtained with the 5‐HT 4(e) receptor stably expressed in CHO cells indicating that the 5‐HT 4(d) receptor was more efficiently coupled to its effector than the 5‐HT 4(e) receptor isoform. Moreover, in terms of agonist efficacy (E max ), the benzamide derivative, renzapride displayed full agonist properties at the h5‐HT 4(d) receptor (same E max as 5‐HT) whereas it was previously shown to be a partial agonist at the h5‐HT 4(e) receptor. A constitutive activity of the h5‐HT 4(d) receptor was observed in CHO cells in the absence of any 5‐HT 4 ligand. Surprisingly, two 5‐HT 4 ligands, SB204070 and RS39604 which are described as highly potent antagonists in various biological models, revealed partial agonist properties at the h5‐HT 4(d) receptor. We conclude that C‐terminal tails of 5‐HT 4 receptor isoforms may directly influence their functional properties.British Journal of Pharmacology (2000) 131 , 827–835; doi: 10.1038/sj.bjp.0703641