The rabbit motilin receptor: molecular characterisation and pharmacology

Abstract

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. In HEK293T cells transfected with the rabbit receptor, motilin concentration‐dependently increased intracellular calcium mobilisation (pEC₅₀=9.25). After transfection with G_o1α, motilin similarly stimulated [³⁵S]GTPγS binding (pEC₅₀=8.87). Using both systems, similar values were obtained with the human receptor, with rank‐order potencies of motilin=[Nle¹³]‐motilin>erythromycin; ghrelin was ineffective. In circular muscle preparations of rabbit gastric antrum, [Nle¹³]‐motilin 0.1–30 nM concentration‐dependently increased the amplitude of electrically‐evoked, neuronally‐mediated contractions (pEC₅₀=8.3); higher concentrations increased the muscle tension (30–3000 nM). Both responses to [Nle¹³]‐motilin faded rapidly during its continual presence. Rat or human ghrelin 0.01–10 μM were without activity. Erythromycin 30–3000 nM and 10 μM, respectively, increased neuronal activity and muscle tension in rabbit stomach. Unlike [Nle¹³]‐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. 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, [Nle¹³]‐motilin and erythromycin are distinguished by different response kinetics, reflecting different rates of ligand degradation and/or interaction with the receptor. British Journal of Pharmacology (2003) 140, 948–954. doi:10.1038/sj.bjp.0705505