We performed a structure-activity study with the human motilin receptor, which was recently cloned from thyroid tissue. N-terminal fragments, Ala-analogs of motilin, and motilides were tested in a cell line that expresses the cloned human motilin receptor and apoaequorin. Full potency to induce calcium fluxes was obtained with N-terminal fragments of 14 amino acids, Motilin fragments 1-14 in which residues 1 (Phe), 4 (Ile), and 7 (Tyr) were replaced by Ala showed the largest reduction in potency. Only motilides with an enol configuration had markedly higher potencies compared to erythromycin A. The potencies to induce Ca2+ fluxes correlated strongly with rabbit binding and contractility data, suggesting that the cloned receptor is indeed the motilin receptor, responsible for contractile effects, Conservation of the motilin pharmacophore in evolution indicates an important physiological role of motilin. (C) 2002 Elsevier Science (USA). All rights reserved.