Cholecystokinin-octapeptide (CCK-OP) evoked contracion and 3H-acetylcholine (3H-ACh) release of the muscle strip of the guinea pig gallbladder were studied. Although hexamethonium (10-5M) or tetrodotoxin (10-6M) had no effect on CCK-OP (10-8M) evoked contraction and 3H-ACh release, Ca-free medium almost completely abolished CCK-OP (10-8M) evoked 3H-ACh release. In Ca-free medium contraction evoked by CCK-OP (10-8M) was reduced to about 84%, which fitted well with the calculated reduction

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... d in the presence of atropine (10-6M). Dibutyryl cyclic GMP (10-3M), which significantly reduced CCK-OP (10-8M) evoked contraction to about 40%, had no effect on CCK-OP (10-8M) evoked 3H-ACh release. CCK-OP receptors on the intramural cholinergic neurons might be suggested to be different from those on the muscle cells of the guinea pig gallbladder.