Affinity of mosapride citrate, a new gastroprokinetic agent, for 5-HT4 receptors in guinea pig ileum

We examined the binding affinity of mosapride citrate (mosapride) (4-amino-5-chloro-2-ethoxy-N-{[4-(4-fluorobenzyl)-2-morpholinyl]methyl}benzamide citrate), a novel gastroprokinetic agent, for the 5-hydroxytryptamine (5-HT) 4 receptors in guinea pig ileum using a selective 5-HT4-receptor radioligand, [H-3]GR113808. In membrane preparations from longitudinal muscle with myenteric plexus in guinea pig ileum, specific [H-3]GR113808 binding revealed a single saturable site of high affinity (K-d=0.28+/-0.02 nM, B-max=45+/-3 fmol/mg protein). Mosapride and other 5-HT4-receptor agonists inhibited the specific binding of [H-3]GR113808 in guinea pig ileum. The 5-HT4 agonists examined displayed the following inhibition potency order: BIMU-8 > cisapride > mosapride > renzapride > 5-HT > zacopride > metoclopramide. Mosapride exhibited monophasic inhibition of the specific [H-3]GR113808 binding in the ileum (K-i value: 84.2 nM). The presence of mosapride (30 nM) significantly increased the K-d value to 0.44+/-0.05 nM in the Scatchard analysis of [H-3]GR113808 binding. B-max of [H-3]GR113808, however, was not affected (48+/-4 fmol/mg protein) by mosapride. As for the affinity of mosapride, the addition of GppNHp (100 mu M) slightly increased the K-i value to 104 nM. These results indicate that mosapride has an affinity for 5-HT4 receptors in guinea pig ileum in the radioligand binding study.