Difference between the inhibitory actions of prostaglandin E1 and verapamil in canine small intestine

The effect of prostaglandin (PG) E1 on the contractile responses of canine separated longitudinal and circular muscles to field stimulation, which produced the release of acetylcholine (ACh), was examined and compared with the effect of verapamil. PGE1 inhibited the contractile response of circular but not of longitudinal muscle to field stimulation. However, it did not affect or only slightly inhibited ACh-induced contraction in both muscles. Verapamil inhibited the contractile responses of both muscles to field stimulation, and it also inhibited ACh-induced contraction in both muscles. These results suggest that PGE1 mainly acts on the cholinergic nerve terminals of the circular muscle layer, whereas verapamil influences the smooth muscle cells of both muscle layers. The inhibitory effects of PGE1 and verapamil were partly neutralized by an increase in the external calcium concentration.