To characterize directly the ability of cholecystokinin (CCK) to interact with receptors on the sphincter of Oddi (SO), we measured binding of 125I-labeled Bolton-Hunter-labeled COOH-terminal octapeptide of cholecystokinin (125I-BH-CCK-8) to tissue sections from the guinea pig SO. Autoradiography localized binding of 125I-BH-CCK-8 over the SO smooth muscle layer. Binding was saturable, specific, dependent on time, pH, and temperature, and was reversible. Binding of 125I-BH-CCK-8 was inhibited by various CCK receptor agonists with the following potencies: CCK-8 >> des(SO3)CCK-8 >> gastrin-17-I and by various CCK receptor antagonists with the following potencies: L-364,718 > proglumide analogue 10 >> carbobenzoxy-Tyr(SO3H)-Met-Gly-Trp-Met-Asp-NH2 > N2,O2' dibutyryl guanosine 3',5'-cyclic monophosphate. The potencies of agonists in stimulating and of antagonists in inhibiting CCK-8-stimulated SO contractions correlated closely with their abilities to inhibit binding of 125I-BH-CCK-8. Analysis of binding of 125I-BH-CCK-8 to SO tissue sections revealed two classes of CCK binding sites: a high-affinity site [dissociation constant (K(d)) 0.2 nM] and a low-affinity site (K(d) 70 nM). Atropine or tetrodotoxin (TTX) caused a similar rightward shift of the CCK-8 dose-response curve for stimulation of SO contraction. Comparison of receptor occupation to CCK-8-induced contraction suggested that CCK-8 occupation of the high-affinity binding site correlated with contraction in the absence of atropine and the low-affinity CCK binding with contraction in the presence of atropine or TTX. These results suggest the possibility that high-affinity CCK binding is to neural elements, which mediate atropine- and TTX-sensitive SO contraction, whereas low-affinity binding is to muscle, which mediates atropine- and TTX-insensitive SO contraction. Like CCK receptors on the gallbladder muscle, these CCK receptors on the SO had a high affinity for CCK and for the CCK receptor antagonist L-364,718 and a low affinity for gastrin.
|American Journal of Physiology - Gastrointestinal and Liver Physiology
|Published - 1990
- Bile flow
- Biliary tract
- Cholecystokinin receptor antagonists
ASJC Scopus subject areas
- Physiology (medical)