The rabbit motilin receptor: molecular characterisation and pharmacology

1 Following identification of the human motilin receptor, we isolated the rabbit orthologue by PCR amplification and found this to be 85% identical to the open reading frame of the human receptor. The protein encoded was 84% identical to the human polypeptide. 2 In HEK293T cells transfected with the rabbit receptor, motilin concentration-dependently increased intracellular calcium mobilisation (pEC(50) = 9.25). After transfection with G(o1)alpha, motilin similarly stimulated [S-35]GTPgammaS binding (pEC(50) = 8.87). Using both systems, similar values were obtained with the human receptor, with rank-order potencies of motilin =[Nle(13)]-motilin > erythromycin; ghrelin was ineffective. 3 In circular muscle preparations of rabbit gastric antrum, [Nle(13)]-motilin 0.1-30 nM concentration-dependently increased the amplitude of electrically-evoked, neuronally-mediated contractions (pEC(50) = 8.3); higher concentrations increased the muscle tension (30-3000 nM). Both responses to [Nle(13)]-motilin faded rapidly during its continual presence. Rat or human ghrelin 0.01-10 muM were without activity. 4 Erythromycin 30-3000 nM and 10 muM, respectively, increased neuronal activity and muscle tension in rabbit stomach. Unlike [Nle(13)]-motilin, the increase in neuronal activity did not fade during continual presence of submaximally-effective concentrations of erythromycin; some fade was observed at higher concentrations. 5 We conclude that the pharmacology of the rabbit motilin receptor is similar to the human orthologue and, when expressed as a recombinant, comparable to the native receptor. However, in terms of their ability to increase neuronal activity in rabbit stomach, [Nle(13)]-motilin and erythromycin are distinguished by different response kinetics, reflecting different rates of ligand degradation and/or interaction with the receptor.