ANTAGONISTIC PROPERTIES OF [PHE(3),LEU(13)]PORCINE MOTILIN

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 (pK(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 +/- 10.08. 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.