Potentiation between 13-Nle-motilin and acetylcholine on rabbit pyloric muscle in vitro

The interaction of 13-nle-motilin-a synthetic analogue of the natural polypeptide-with acetylcholine was studied on rabbit isolated pyloric sphincter in vitro. Adding subthreshold doses of 13-nle-motilin reduced the calculated acetylcholine dose for half maximal contraction from 3.68 to 1.33 X 10(-6) g/ml and increased the calculated maximal response by 15.3%. Thus 13-nle-motilin caused potentiation of the acetylcholine response. Since 13-nle-motilin induced increase of endogenous acetylcholine levels could be excluded, a direct action of motilin on or in the smooth muscle cell is suggested. The results are compatible with the view that in vivo subthreshold doses of motilin may affect cholinergic driven pyloric motor activity.