Proglumide peptides and CCK antagonistic action in hog duodenal circular muscle

New proglumide peptides were synthesized and their inhibitory activity against the cholecystokinin (CCK)-induced relaxation was tested in duodenal circular muscles of hogs in order to locate the active moiety. Isotonic tension is reported as a percentage of the relaxation induced by CCK-4 (4.23 microM) or by Ca-free EGTA (2 mM). In R1-CONHCH(CH2CH2CO-R2)CON(CH2CH2CH3)2, P2-II(R1 = PhCH2O, R2 = Phe33-NH2), its D isomer and P3-I (R1 = Ph, R2 = Asp32-Phe33-NH2) had potent antagonistic activity (IC50 = 47.9 (32.4-72.4) microM, 25.1 (10.0-61.7) microM and 186 (141-240) microM, respectively). Benzotript, from another group of CCK antagonists had an IC50 of 52.5 (40.7-67.6) microM. P2-II (49 microM) or its D isomer (19.6 microM) antagonized non-competitively, and P3-I (168 microM) and benzotript (58.4 microM) antagonized competitively the CCK-4-induced relaxation. P3-I (84 or 168 microM) also antagonized competitively the CCK-8-induced relaxation. The amino acids of Tyr27, Trp30 and Phe33, separately, play important roles for agonist activity for relaxation. Tyr27 is the main relaxing group, Trp30 an antagonistic one and Phe33 is another relaxing one. In the proglumide skeleton, N,N-dipropylglutaramic acid acts like Trp30. Agonist, partial agonist and antagonist activities are exerted by these three groups and bulky substituents.