Interactions of Cholecystokinin (CCK-PZ) and Gastrin on Motor Activity of Isolated Guinea-pig Antrum and Fundus

Pressure responses to cholecystokinin (CCK-PZ, approximately 500 U/mg) and gastrin (human synthetic gastrin) were investigated in isolated guinea-pig antral and fundal pouches. Both drugs stimulated motor activity, in the antrum mainly by increasing amplitudes and rhythmic activity, in the fundus by increasing basal tension. Antral responses to gastrin were markedly smaller than to CCK-PZ, while the differences were less pronounced in the fundus. Prestimulation with gastrin, being an agonist by itself, significantly reduced the antral responses to submaximal doses of CCK-PZ, while no inhibition was found in the fundus. When pre-exposed to gastrin, the antral dose-response curve to CCK-PZ was flattened, with reduced maximal response, simulating a noncompetitive interaction. It seems that gastrin behaves like a partial agonist to CCK-PZ on guinea-pig antral smooth muscle.