Mechanisms mediating the effects of cholecystokinin on avian small intestine longitudinal smooth muscle.

The aims of this study were (1) to define the effects of CCK-8s and related peptides on chicken ileum longitudinal smooth muscle and (2) to explore the mechanisms by which such effects occur. The effects of CCK-8s were assayed in vitro on chicken longitudinal ileal strips. CCK-8s produced contraction of ileal strips (EC50 8.8.10(-9) M). CCK-8ns and CCK-4 did not have remarkable contractile effects even when added at concentrations 200-times higher than the EC50 for CCK-8s. L365,260 slightly inhibited the effects of CCK-8s whereas L364,718 was ineffective. Tetrodotoxin (10(-6) M) markedly decreased the effects of CCK-8s. Atropine (10(-6) M) did not modify the neurally mediated effects of CCK-8s, whereas ketanserin (10(-5) M) decreased the response to CCK-8s. Substance P-desensitized preparations exhibited reduced responses to CCK-8s. Our results indicate that CCK receptors present in chicken ileum behave similarly but not identically to the CCK-A receptor described in mammals. Most of these CCK receptors are neurally located but a minor proportion is also present on smooth muscle. The neurally mediated response to CCK-8s does not involve cholinergic mechanisms, but serotonin and substance P releasing neurons.