Secretin

Secretin is a 27-amino acid peptide hormone with the distinction of being the first substance identified as a hormone, discovered by William Bayliss and Ernest Starling in 1902. Naturally produced by S-cells in the duodenal mucosa in response to acidic chyme, secretin stimulates the pancreas to secrete bicarbonate-rich fluid, the liver to produce bile, and the stomach to reduce acid output. The discovery of secretin fundamentally changed biology by establishing the concept of hormonal (chemical) regulation of organ function.

Synthetic human secretin is used clinically as a diagnostic agent in several important applications. The secretin stimulation test evaluates exocrine pancreatic function by measuring the volume, bicarbonate concentration, and enzyme content of pancreatic secretions collected via duodenal aspiration after secretin administration. This test remains the gold standard for diagnosing chronic pancreatitis and exocrine pancreatic insufficiency, though its complexity has limited routine clinical use.

In gastroenterology and oncology, secretin is used as a provocative test for gastrinoma (Zollinger-Ellison syndrome). In normal physiology, secretin inhibits gastrin release; however, gastrinoma cells paradoxically respond to secretin with a dramatic increase in gastrin secretion. A rise in serum gastrin of >120 pg/mL (or >200 pg/mL in some protocols) following intravenous secretin administration is highly specific for the diagnosis of gastrinoma.

More recently, secretin has found an important role in enhancing magnetic resonance cholangiopancreatography (MRCP). Secretin-enhanced MRCP improves visualization of the pancreatic duct by stimulating pancreatic fluid secretion, which distends the duct and makes it easier to visualize. This has become a valuable tool for detecting subtle pancreatic duct abnormalities, evaluating chronic pancreatitis, and assessing pancreaticobiliary anatomy.