BLOCKADE OF HUMAN ATRIAL 5-HT(4) RECEPTORS BY GR-113808

1 The mode of antagonism of 5-hydroxytryptamine (5-HT)-induced positiv e inotropic effects by the highly selective 5-HT4 receptor antagonist GR 113808 ({1-[2-methlylsulphonylamino ethyl]-4-piperidinyl}methyl 1-m ethyl-1H-indole-3-carboxylate) was investigated on isolated preparatio ns of human right atrium. 2 GR 113808 caused concentration-dependent ( 2-100 nM) surmountable antagonism of the effects of 5-HT with a pK(B) (M) of 8.8. 3 The affinity of GR 113808 for human atrial 5-HT4 recepto rs, together with its high selectivity for 5-HT4 receptors comprise us eful properties for investigating the question of 5-HT4 receptor subty pes.