Human pancreatic and biliary responses to physiological concentrations of cholecystokinin octapeptide.

To determine the functional significance of physiological plasma concentrations of cholecystokinin, five volunteers each received graded doses of intravenous infusions of cholecystokinin octapeptide (CCK-8). At each dose plasma concentrations of CCK-8 were determined and pancreatic and biliary outputs were measured. Threshold plasma concentrations of CCK-8 for augmenting pancreatic trypsin secretion were undetectable (less than 3 pmol/l), and maximal trypsin output of 21.9 +/- 1.95 k-i.u./30 min was produced by 17.1 +/- 6.4 pmol of CCK-8/1. Calculated halfmaximal output was produced by 4.7 pmol of CCK-8/1. Maximal trypsin output during infusions of CCK-8 was significantly less than that after a combination of the CCK-like peptide, caerulein, and secretin (32.95 +/- 2.16 k-i.u./30 min, P less than 0.001). Biliary bile acid and bilirubin outputs were significantly augmented only when plasma concentrations of CCK-8 were greater than 5 pmol/l. Plasma concentrations of CCK-8 in the low picomolar range exert significant effects on pancreatic and biliary secretion. CCK-8 fulfills the criteria for a circulating hormone.