Antagonistic properties of [Phe 3,Leu 13]porcine motilin

Abstract

We describe the antagonistic properties due to the replacement of Pro 3 by phenylalanine in porcine motilin. The analogue, [Phe 3,Leu 13]porcine motilin (OHM-11526), displaces iodinated [Nle 13]porcine motilin bound to a homogenate of rabbit antral smooth muscle tissue. The dissociation constant (p K d) was 9.26 ± 0.04, versus 9.11 ± 0.01 for motilin and 8.24 ± 0.06 for ANQ-11125, the (1–14) fragment of OHM-11526. The Hill coefficient was close to one and Schild plot analysis confirmed the competitive nature of the interaction. In the tissue bath OHM-11526 was unable to induce contractions of segments of rabbit duodenum. At a concentration of 10 −6 M, OHM-11526 inhibited the effect of maximally effective doses of porcine motilin and of the erythromycin derivative, EM-523, but was without effect on contractions induced by acetylcholine, substance P and serotonin. Increasing doses of OHM-11526 shifted the dose-response curves of motilin and EM-523 to the right, but caused a depression of the maximal response as well. From the motilin curves, and assuming a dual competitive and non-competitive interaction, the pA 2 was 7.79 ± 0.08, the pD′ 2 6.91 ± 0.08. The EM-523 curves yielded comparable data (pA 2 = 8.10 ± 0.12 and pD′ 2 = 7.06 ± 0.13). OHM-11526 also blocked the motilin responses observed with smooth muscle strips from the rabbit and human antrum. However, in a preparation of the chicken small intestine, OHM-11526 was a full agonist with a potency (pD 2 = 6.84) comparable to that of porcine motilin (pD 2 = 6.71). Our data confirm the interaction of motilides with the motilin receptor. Due to its increased affinity for the motilin receptor, OHM-11526 will be a valuable tool for studying the physiology of motilin and the pharmacology of motilin and motilides.