Full and partial relaxing CCK-C-terminal fragments in hog duodenal circular muscle

The essential active moieties in the cholecystokinin (CCK) ligand for relaxing duodenal circular muscles of pigs were investigated. The decrease in isotonic tension from the normal tone was expressed as a percentage of the relaxation obtained with Ca2+-free EGTA (2 mM) solution. Inhibition was expressed as a percentage of the relaxation induced by CCK-C-terminal tetrapeptide (CCK-4, 4.23 microM) without antagonists. Amino acid sequences from CCK-(27-28) to CCK-(27-31) increased the potency. CCK-(27-30) or CCK-(27-31) attained the same extent of relative intrinsic activity as CCK-8. The amino acid sequence from Tyr27, CCK-(27), to Trp30, CCK-(30), is another prerequisite for relaxation, in addition to the sequence of CCK-4. Newly synthesized CCK-(27-30)-R derivatives with increasingly bulky substituents (R = H, ethyl, phenyl, n-butyl and allyl) were changed from full agonists to partial agonists. Phenyl, n-butyl and allyl derivatives of CCK-(27-30)-R, in particular, had a mixed agonist and antagonist action. CCK-(27-30)-allyl antagonized competitively the CCK-4-induced relaxation (IC50 = 123 (75.9-200) microM).