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T Harada, T Katsuragi and T Furukawa
Mechanisms involved in the action of cholecystokinin octapeptide (CCK- 8) on the isolated circular muscle of the guinea pig sphincter of Oddi were investigated. CCK-8 (10(-9) to 10(-6) M) caused a concentration- dependent contraction of the sphincter. Angiotensin II, bethanechol and serotonin also contracted this tissue. CCK-8 exerted the most potent effect. However, bradykinin, porcine motilin and norepinephrine failed to elicit any contraction. The CCK-8-induced contraction was inhibited by about 60% by atropine (3 X 10(-7) M) or tetrodotoxin (3 X 10(-7) M) but was not affected by hexamethonium (3 X 10(-5) to 10(-4) M) or phentolamine (3 X 10(-6) to 10(-5) M). Proglumide (3 X 10(-4) to 3 X 10(-3) M), a derivative of glutaramic acid, inhibited competitively the contraction by CCK-8. However, proglumide influenced neither the electrically elicited twitch contraction nor the bethanechol-induced contraction. CCK-8 evoked a concentration-related release of [3H]acetylcholine (ACh) from previously labeled stores. The CCK-8- evoked release of [3H]ACh was eliminated by tetrodotoxin and was inhibited, in a concentration-dependent manner, by proglumide. These results suggest that the contractile response to CCK-8 of the guinea pig sphincter of Oddi consists of a direct effect on the smooth muscle and an indirect effect mediated by ACh release from postganglionic parasympathetic neurons.
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