Both free and complexed trypsin inhibitors stimulate pancreatic secretion and change duodenal enzyme levels.

Secretion of pancreatic digestive enzymes was measured in pancreatic cannulated rats after duodenal stimulation with Kunitz or Bowman-Birk protease inhibitors or their complexes with trypsin and/or chymotrypsin. Free and complexed inhibitors were bound by the duodenal epithelium, stimulated the discharge of cholecystokinin, and significantly increased secretion rates of alpha-amylase, trypsinogen, and chymotrypsinogen. Inasmuch as secretion rates returned to basal levels with cholecystokinin-A receptor antagonists, the stimulation was likely to be mediated by cholecystokinin. Soya factors also influenced the duodenal concentration of pancreatic enzymes under simulated feeding conditions. Thus the level of alpha-amylase increased while the trypsin concentration decreased in rats gavaged with free or complexed inhibitors. The same was true for chymotrypsin when the Bowman-Birk inhibitor was used, but the Kunitz inhibitor and its trypsin complex actually raised the luminal concentration of chymotrypsin. Accordingly, because soya inhibitors remained effective in stimulating pancreatic secretion after elimination of their inhibitory activity by complex formation, it is questionable whether the signal for cholecystokinin secretion was solely due to lowering of duodenal protease levels.