New derivatives of arylpiperazine 9 were designed from ML 10302, a potent 5-HT(4) receptor agonist in the gastrointestinal system. Compounds were synthesized by condensation of a number of available arylpiperazines or heteroarylpiperazines with 2-bromoethyl 4-amino-5-chloro-2-methoxybenzoate. They were evaluated in binding assays on the recently cloned human 5-HT(4(e)) isoform stably expressed in C6 glial cells with [(3)H]GR 113808 as the radioligand. The affinity values (K(i)) depended upon the substituent on the aromatic ring. A chlorine atom produced a marked drop in activity (K(i) > 100 nM), while a m-methoxy group gave a compound with nanomolar affinity (K(i) = 3 nM). The most potent compounds were the heterocyclic derivatives with pyrimidine, pyrazine, pyridazine, or pyridine moieties (compounds 9r, 9t, 9u, 9x, respectively). K(i) values for 9a and 9r were determined for the 5-HT(4(a)), 5-HT(4(b)), 5-HT(4(c)), and 5-HT(4(d)) receptor isoforms transiently expressed in COS cells. The results indicated that the compounds were not selective. They produced an inhibition of the 5-HT-stimulated cyclic AMP synthesis in the C6 glial cells stably expressing the 5-HT(4(e)) receptor and shifted the 5-HT concentration-effect curve on adenylyl cyclase activity with pK(D) values of 7.44 and 8.47, respectively. In isolated human atrial myocytes, 9r antagonized the stimulatory effect of 5-HT on the L-type calcium current (I(Ca)) with a K(D) value of 0.7 nM.