Isolation, sequence, and bioactivity of chicken motilin

Motilin was isolated from acid extracts of the small intestine of chickens by a combination of gel filtration chromatography, ion-exchange, and reverse-phase HPLC. The purification was monitored using a radioreceptor assay. The sequence of chicken motilin is FVPFFTQSDIQKMQEK-ERNKGQ. Although the six residues differing from porcine motilin (4, 7-10, and 12) are mostly in the pharmacophore of porcine motilin, the affinity of chicken motilin and of the (1-14) fragment of chicken motilin for the motilin receptor of rabbit antral smooth muscle is not much reduced (pK(d)s of 8.90 and 8.45), compared with the affinity of [Nle(13)]porcine motilin (pK(d) 9.12). With smooth muscle tissue of the chicken, however, receptors could not be demonstrated with binding studies. In the tissue bath chicken motilin induced a dose-dependent tonic contraction, which was most pronounced with muscle strips prepared from chicken jejunum. This response was blocked by the Ca2+ antagonist verapamil, but atropine, TTX, L-NNA, guanethidine, prazosin, and yohimbine had no effect. The pEC(50) for chicken motilin in the chicken jejunum was 7.41. Motilins from other species had lower potencies, and [Phe(3),Leu(13)]porcine motilin, an antagonist in the rabbit, was an agonist in the chicken. The motilin agonists erythromycin A and EM-523 were almost without effect. Tested against rabbit duodenum, chicken motilin had a smaller potency than mammalian motilins. Thus, chicken motilin and the chicken motilin receptor differ from their mammalian counterparts.