The composition of pancreatic secretions varies widely depending on the rate of secretion and the secretagogues involved. Secretin is the primary stimulant of fluid and electrolyte secretion and CCK is the primary stimulant of enzyme secretion by the exocrine pancreatic cells . Release of ACh from parasympathetic nerves also stimulates secretion of enzyme-rich fluid.
Sympathetic stimulation decreases secretion, in part by reducing blood flow to the pancreas.
Gastrin, from the gastric mucosa, also plays a significant role in the stimulation of enzyme secretion by the pancreas. Because gastrin acts as a hormone, all of the gastric reflexes involving secretion of gastrin will influence secretion of fluid and enzymes from both the pancreas and gastric mucosa. The appearance of both amino acids and lipid digestion products in the duodenum initially activates a vagal reflex that stimulates secretion of an enzyme-rich, pancreatic fluid. The same digestion products stimulate a slower, sustained release of CCK from the intestinal mucosa which stimulates primarily enzyme secretion by the pancreas. As the pH of the duodenal chyme falls due to gastric emptying, chemoreceptors respond by releasing secretin which stimulates primarily an i ncrease in the volume and bicarbonate content of the pancreatic fluid.