Secretin

Overview

Secretin holds a singular place in endocrinology as the first substance to be formally designated a hormone. In 1902, Bayliss and Starling demonstrated that an acidic extract of duodenal mucosa could stimulate pancreatic secretion when injected intravenously, establishing the concept of chemical signaling between organs. Secretin is produced by S-cells in the duodenal and jejunal epithelium in response to luminal acidification, particularly when gastric chyme with a pH below 4.5 enters the duodenum.

Structure

Secretin is a 27-amino acid linear peptide with a molecular weight of approximately 3055 Da. It shares approximately 50% sequence identity with glucagon and belongs to the secretin superfamily of G-protein-coupled receptor ligands. The N-terminal region is essential for receptor activation, while the C-terminal portion mediates receptor binding. Unlike several other GI peptides, secretin does not undergo tyrosine sulfation, and all naturally occurring forms appear to be unsulfated.

Mechanism of Action

Secretin exerts its primary effects through the secretin receptor (S receptor), a class B G-protein-coupled receptor coupled to adenylyl cyclase. Upon binding, secretin activates intracellular cAMP signaling pathways in pancreatic ductal cells, stimulating the secretion of a bicarbonate-rich fluid. This alkaline pancreatic juice neutralizes gastric acid entering the duodenum, raising luminal pH to levels optimal for pancreatic enzyme activity.

The S receptor is expressed on pancreatic duct epithelial cells, gastric parietal cells (where it inhibits acid secretion), cholangiocytes, and certain central neurons. Secretin also potentiates the actions of CCK on pancreatic enzyme secretion, creating a coordinated response to nutrient entry.

Physiological Functions

The principal physiological role of secretin is the regulation of duodenal pH. By stimulating bicarbonate secretion and simultaneously inhibiting gastric acid production, secretin ensures the intestinal environment supports enzymatic digestion and mucosal integrity. Secretin also stimulates bile bicarbonate secretion from the liver and enhances the water and electrolyte output of the pancreas.

Clinical Applications

Secretin has been used diagnostically to assess pancreatic exocrine function, though this application has largely been supplanted by endoscopic and imaging modalities. Synthetic secretin is employed in the evaluation of suspected pancreatic insufficiency and in certain research protocols studying gut-brain axis signaling. Secretin receptor imaging has emerged as a potential tool for detecting neuroendocrine tumors expressing S receptors.