Interaction between gastrin, CCK, and secretin on canine antral smooth muscle in vitro

This study compared the actions and interactions of human synthetic gastrin, octapeptide-cholecystokinin (OP-CCK), cholecystokinin (CCK), and secretin on the amplitude of isometric tension developed in strips of dog antral smooth muscle in vitro. Cholecystokinin, OP-CCK, and gastrin produced maximal stimulatory effects at 7.5 x 10(-9), 4.5 x 10(-9), and 3.5 x 10(-9) M, respectively. Secretin alone was ineffective up to 2.5 x 10(-8) M. Observed maximal responses to gastrin, OP-CCK, and CCK tested alone were not significantly different. A submaximal gastrin dose added with OP-CCK, shifted the OP-CCK dose-response curve to the left and significantly reduced the D50, but the calculated maximal response (CMR) did not change. Also, submaximal OP-CCK plus gastrin shifted the gastrin dose-response curve to the left and significantly lowered the D50 with no change in CMR. Secretin decreased CMR but did not change the D50 for gastrin. Responses obtained to gastrin and OP-CCK tested alone were not affected by tetrodotoxin (1 x 10(-5) M), hexamethonium bromide (4 x 10(-5) M), or atropine (1 x 10(-7) M). Larger doses of atropine (5 x 10(-6) M) reduced peptide responses an average 30%. The results indicate that OP-CCK, CCK, and gastrin share a common noncholinergic receptor site. Secretin acts at a different receptor site.