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 1.8 x 10^-6 for gastrin. The maximal response to gastrin averaged 66% of 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.