Jose Luis Valverde Piedra
The role of bile and digestive enzymes of pancreas in regulation of secretion of pancreatic juice in pigs

Pancreatic exocrine secretion in rats, dogs and humans is mediated by the neural pathways and gut regulatory peptides, however, a luminal feedback regulation of secretion which depends on luminal protease activity is believed to play a substantial role as well. A decrease in protease activity results in pancreatic hypersecretion. In pigs, there is a controversy concerning the existence of feedback regulation. Bile has been known to affect feedback regulation in animal species other than pigs and in humans, but the mechanism remains unclear. Some researchers suggested that bile acids may stabilize luminal protease activity resulting in an inhibition of CCK, and that the inhibition of plasma CCK is achieved due to enhanced somatostatin release by the bile. In conscious minipigs, the presence of pancreatic feedback regulation was observed only during the interdigestive period. This mechanism was, however, not related to duodenal enzymatic activity and plasma CCK but to duodenal acidity and plasma secretin. Recently, the existence of local duodenal, CCK-related mechanism controlling the secretion of pancreatic juice in conscious pigs was confirmed.
The first aim of the present study was to clarify the influence of pancreatic juice and exogenous pancreatic enzyme (Pancreatin) administration into the duodenum or ileum (at 20, 100 and 200% of the mean prandial trypsin activity in pancreatic juice) on the regulation of pancreatic juice secretion during the interdigestive and digestive period in conscious weaned pigs (i.e. to study the duodenal and ileal feedback mechanism). The second aim was to evaluate the influence of low pH of the chyme (with the proton pump inhibitor, omeprazole) entering the duodenum, and the involvement of duodenal CCK 1 receptors (with tarazepide, a CCK 1 receptor antagonist) in controlling the secretion of pancreatic juice. The third aim was to evaluate the involvement of bile during pancreatic juice/bile reintroduction or diversion in the exocrine pancreatic secretion in conscious pigs.
A unique experimental model was developed to allow collection, and reintroduction or diversion of pancreatic juice or both pancreatic juice and bile simultaneously. Weaned pigs were prepared with silicone pancreatic duct, bile duct and ileal catheters and duodenal cannulae. Animals were fed with a standard pig diet twice a day with free access to tap water. Morning pre-, prandial and postprandial collections of pancreatic juice and/or bile with and without its reintroduction and/or Pancreatin administration were performed on freely-moving pigs starting from one week after surgery.
During the preprandial period, the diversion of pancreatic juice led to a significant rise in pancreatic protein output, whereas during the prandial period the pancreatic juice diversion significantly increased both the pancreatic protein and trypsin outputs. Duodenal and ileal administration of Pancreatin reduced pancreatic response to food in a similar way. CCK blood plasma levels did not change significantly, however, the tendencies toward higher CCK level were observed in the pancreatic juice diverted pigs. These results demonstrate the existence of the feedback regulation of pancreatic juice secretion during the interdigestive as well as the digestive periods, furthermore, the regulation may occur at the level of the duodenum as well as ileum.
The diversion of bile alone or both bile and pancreatic juice resulted in a higher protein and trypsin outflow compared to the reintroduction of both pancreatic juice and bile. Feeding after 1-hour-absence of bile and both bile and pancreatic juice in the intestine evoked a higher response in bile diverted pigs than in the control. Intraduodenal administration of bicarbonates instead of bile did not prevent hypersecretion of pancreatic juice. Intravenous administration of omeprazole, in pancreatic juice and bile reintroduced pigs, tended to diminish pancreatic juice volume during the digestive period. Intraduodenal tarazepide administration tended to decrease prandial protein outflow and trypsin activity in reintroduced pigs. These results support the importance of the acidic chyme entering the duodenum in the feedback regulation of the pancreatic juice secretion but the role of bile is crucial in the regulation of this mechanism in the pig. This is more likely due to bile acids than due to the buffering properties of bile bicarbonates, since the administration of bicarbonates did not prevent pancreas stimulation and omeprazole administration only reduced the volume of pancreatic juice. On the other hand, the effect of tarazepide administration on protein output and trypsin activity demonstrates the involvement of local duodenal CCK 1 receptors in the pancreatic feedback regulation in pigs.
Overnight bile diversion strongly stimulated pancreatic juice secretion and decreased bile outflow as compared to short-term bile diversion in the interdigestive and digestive period. Intraduodenal administration of exogenous bile salt solution of various molar concentration (0.02, 0.04 and 0.08 M ) during the prandial hour and one postprandial hour inhibited pancreatic juice hyperstimulation in a dose dependent manner. However, two hours after completing bile salt infusion, an increase in pancreatic juice volume was observed. Bile outflow decreased after overnight diversion and increased after intraduodenal administration of bile salts in a dose dependent manner. These results support the results obtained in short-term bile diverted pigs and prove the crucial role of bile/bile acids in the feedback regulation of pancreatic secretion in pigs.
Interestingly, leptin was present in bile, and its concentration was several-fold higher than in plasma. Leptin concentration in bile decreased when bile was diverted over night. In control pigs bile leptin outflow increased during the prandial period whilst in bile diverted pigs it decreased. It remains unknown if a leptin enterohepatic circulation exists and could play a role in the feedback regulation of exocrine pancreatic secretion.
In conclusion, the exocrine pancreatic secretion is regulated by a complex feedback mechanism, in which the nervous system and gut regulatory peptides are involved. However, the stimuli initiating this mechanism are different and seem to be in some hierarchy, in which a higher level of regulation masks or eliminates the lower level of regulation. It can be concluded that there is a feedback mechanism dependent on: a) the presence of bile/bile acids in the small intestine, b) the presence of pancreatic juice enzymes both in the duodenum and ileum, c) the presence of acidic chyme in the duodenum.