Inhibitory Activity of Antibodies Against the Human Atrial 5-HT4Receptor

Antibodies directed against the second extracellular loop of G protein-coupled receptors have been shown to exert “agonist-like” activities. In order to test the hypothesis that this is a general phenomenon, antibodies were raised in rabbits against a synthetic peptide corresponding to the second extracellular loop of the newly sequenced human cardiac 5-HT4receptor. The antibodies were affinity-purified and shown to recognize the 5-HT4receptor in immunoblots of Chinese hamster ovary (CHO) cells expressing the receptor. The antibodies had no intrinsic effect but could depress the activation of L-type calcium channel induced by serotonin in human atrial cells. This antagonist-like effect was exerted both by intact IgG and by Fab fragments. These results are physiologically important since it has been shown that the 5-HT4receptor could be a target for autoantibodies in mothers at risk of giving birth to children with neonatal atrio-ventricular block.