It has been suggested that the gut hormone cholecystokinin (CCK), by modulating insulin output from pancreatic β-cells, plays an important role in the enteroinsular axis. To investigate this hypothesis, eight rats were studied on two different occasions: after injection of L 364718, a specific antagonist of CCK binding to its membrane receptor, and after vehicle injection. In both studies a mixture of casein (11%) and glucose (9%) was infused through a chronic indwelling intraduodenal catheter to evoke CCK secretion. Plasma was analyzed for insulin, glucose, glucagon, and tyrosine many times during the procedure. Prior administration of the CCK antagonist significantly attenuated the increase in plasma insulin and glucagon after casein infusion. These results support the concept that cholecystokinin plays an important physiologic role in the in vivo regulation of postprandial plasma insulin and glucagon concentrations after protein ingestion.

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