In response to the presence of hydrochloric acid (HCl), the hormone secretin is synthesized in the duodenum. It is a peptide hormone and encoded by the SCT gene in humans. The hormone is synthesized as a 120 amino acid precursor peptide known as prosecretin. The mature secretin peptide hormone is composed of 27 amino acids (MW 3055).
Secretin is produced in cytoplasmic secretory granules of S-cells in the mucosa of the duodenum. Secretin is released into circulation in response to low duodenal pH (2 and 4.5) due to the hydrochloric acid (HCl) in the chyme (partly digested food that is expelled from the stomach) that enters the duodenum from the stomach. The primary function of Secretin is to neutralize the pH in the duodenum by inhibiting gastric gland secretion and gastric motility. Secretin also modulates the water and electrolyte transport in pancreatic duct cells.