Interaction between octapeptide-cholecystokinin, gastrin, and secretin on cat gallbladder in vitro

Effects of OP-CCK, gastrin, and secretin were studied on isometric tension development in strips of cat gallbladder. Effective molar concentrations were 2.2 X 10(-10) to 5.3 X 10(-9) for OP-CCK, and 1.13 X 10(-7) to 1.5 X 10(-6) for gastrin. The maximal response to gastrin averaged 66% the maximal response to OP-CCK and effects were not blocked by atropine. Secretin was weakly stimulatory or ineffective by itself. Prior addition of submaximal doses of gastrin shifted the dose-response curve of OP-CCK to the right, but neither the slope nor the calculated maximal response (CMR) was significantly changed. This suggests that gastrin and OP-CCK compete for a common receptor on cat gallbladder. On the other hand, a background dose of secretin shifted the dose-response curves for both OP-CCK and gastrin to the left and increased the slopes significantly with increase in the respective CMRs. The combined action of OP-CCK (or gastrin) and secretin are supra-additive. These experiments suggest that OP-CCK and gastrin act at a common receptor site which is different from the secretin-receptor site.