Secretin

Secretin is released by duodenal S-cells when acidic chyme arrives, and it was the molecule whose 1902 discovery by Bayliss and Starling created the concept of a hormone. Acting through the class B GPCR secretin receptor, it raises cAMP in pancreatic ductal epithelium, driving CFTR-mediated bicarbonate and water secretion that neutralises duodenal acid. It also stimulates biliary bicarbonate output and inhibits gastrin release and gastric acid production. Diagnostically, all three properties are exploited: pancreatic function testing measures the bicarbonate response, secretin-enhanced MRCP uses the fluid surge to distend and visualise the pancreatic duct, and the gastrinoma test relies on the paradoxical gastrin rise that only gastrinoma tissue produces.

Targets: Secretin receptor (SCTR), Pancreatic ductal CFTR, Gastrin-producing G cells

• · The label dose is 0.2 mcg/kg — approximately 14 mcg for a 70 kg adult. A test dose is recommended in patients with prior exposure or allergy history.
• · 0.4 mcg/kg — around 28 mcg for a 70 kg adult. A gastrin rise above roughly 110 pg/mL over baseline supports the diagnosis.

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• commonTransient nausea— Usually resolves within minutes.
• commonFlushing and a sensation of warmth
• uncommonAbdominal discomfort
• rareHypersensitivity or anaphylactoid reaction— Risk is higher with repeat exposure; the label suggests a test dose in previously exposed or allergy-prone patients.

• Acute pancreatitis — wait until the episode has resolved before testing.
• Known hypersensitivity to secretin.
• Patients on anticholinergics may give false results; hold them before testing.
• Uncontrolled acid hypersecretion or recent vagotomy can confound the gastrin test.

• conflictanticholinergics — Blunt the pancreatic secretory response and invalidate the test; hold before the procedure.
• conflictproton-pump-inhibitors — Must be withheld before the gastrinoma test because they independently raise baseline gastrin.
• synergysincalide — Sometimes given in sequence during combined pancreatic function testing — secretin for ductal bicarbonate, sincalide for acinar enzyme output.

• · Duodenal bicarbonate concentration during pancreatic function testing.
• · Serial serum gastrin at 0, 2, 5, 10, 15 and 30 minutes for gastrinoma testing.
• · Observe for hypersensitivity for a short period after injection.

FDA-approved diagnostic agent, prescription-only, administered in clinical settings. Not available for self-administration.

• ChiRhoStim (human secretin) US prescribing information (label)
• Bayliss and Starling 1902, the mechanism of pancreatic secretion (other)
• Consensus recommendations on secretin-enhanced MRCP in pancreatic imaging (guideline)

Secretin is the molecule that created endocrinology. Bayliss and Starling showed in 1902 that acid in a denervated loop of duodenum still triggered pancreatic secretion, which meant a blood-borne chemical messenger existed. Starling coined the word 'hormone' for it. Mechanistically it is a class B GPCR agonist. The secretin receptor on pancreatic ductal epithelium is Gs-coupled; activation raises cAMP, activates protein kinase A, and opens CFTR on the ductal apical membrane. Chloride exits, is exchanged for bicarbonate through the SLC26 anion exchangers, and a large volume of bicarbonate-rich fluid is secreted. Physiologically this neutralises acidic chyme arriving from the stomach, protecting the duodenal mucosa and creating the alkaline pH that pancreatic enzymes need. The same receptor drives biliary bicarbonate secretion from cholangiocytes. The third effect is inhibitory: secretin suppresses gastrin release from antral G cells and reduces gastric acid secretion. This is a negative feedback loop — acid triggers secretin, secretin shuts down acid. The diagnostic uses exploit all three, and understanding which effect each test uses makes the tests make sense. Pancreatic exocrine function testing measures the bicarbonate concentration in duodenal fluid after a secretin bolus. It is the most sensitive direct test of exocrine pancreatic function and detects insufficiency earlier than faecal elastase does. Secretin-enhanced MRCP uses the fluid surge itself as contrast: the pancreatic duct distends transiently, which reveals strictures, divisum anatomy and side-branch IPMN communications that a standard MRCP misses. The gastrinoma test uses the paradoxical response. Normal G cells are inhibited by secretin. Gastrinoma cells do the opposite and release gastrin. That inversion is the diagnostic signal, and it is specific in a way few endocrine tests are — a rise of at least 110 pg/mL over basal strongly supports the diagnosis per the label. There is no therapeutic use. The 45-minute half-life and intravenous-only route settle that. Secretin has periodically appeared in autism treatment claims; controlled trials did not support it.

The classic failure is a gastrinoma test performed on someone still taking a proton pump inhibitor. The basal gastrin is already elevated by the drug, the interpretation collapses, and the patient goes through the whole procedure for nothing. Getting the PPI stopped safely in advance is the hard part of this test, not the injection. The second is hypersensitivity on repeat exposure. Secretin is a foreign-context peptide given intravenously, and risk rises with prior exposure. A test dose in previously exposed or atopic patients is the label's own suggestion and it is not a formality. The third is a genuinely unusual handling requirement: ChiRhoStim is stored frozen at -20°C, which is uncommon among peptide products and catches pharmacies out. Reconstitution is with 0.9% saline, and unusually the product requires vigorous shaking to dissolve — the opposite of the gentle swirling drilled into anyone who handles peptides. Use immediately and discard the remainder. The fourth is doing the test during acute pancreatitis, which is contraindicated. Wait for the episode to resolve. The fifth, and the reason this entry exists in a peptide corpus at all: secretin periodically resurfaces in wellness contexts as a 'gut healing' or autism treatment. It is a 45-minute diagnostic hormone administered intravenously in a hospital. There is no evidence base for any therapeutic use, and controlled trials in autism were negative.

1. 14 mcgSingle procedure — 0.2 mcg/kg intravenously — about 14 mcg for a 70 kg adult — given as a slow bolus over roughly 1 minute, for pancreatic exocrine function testing and secretin-enhanced MRCP.
2. 28 mcgSingle procedure — 0.4 mcg/kg — about 28 mcg for a 70 kg adult — for gastrinoma diagnosis. Double the functional testing dose, because you are looking for a paradoxical response rather than a maximal secretory one.
3. —Not applicable — There is no titration, no repeat dosing regimen and no cycle. This is a single-use diagnostic agent. The only dose adjustment that exists is the test dose recommended in patients with prior exposure or allergy history.

• Secretin receptor (SCTR), class B GPCR — Not resolved to a published Ki or EC50 in this session.

  Gs-coupled, raising cAMP and activating protein kinase A on pancreatic ductal cells and cholangiocytes. This is the entire pharmacology.

• Pancreatic ductal CFTR — Not a direct target.

  Opened downstream via PKA, driving chloride efflux which is exchanged for bicarbonate. This is why cystic fibrosis produces pancreatic insufficiency — no functional CFTR means no bicarbonate response to secretin.

• Cholangiocyte bicarbonate secretion — Same receptor.

  Increased biliary bicarbonate output, contributing to the 'biliary bicarbonate umbrella' that protects cholangiocytes from bile acid injury.

• Antral G cells (gastrin-producing) — Same receptor.

  Inhibition of gastrin release in normal tissue — but paradoxical stimulation in gastrinoma. This inversion is the entire basis of the Zollinger-Ellison diagnostic test.

• FDA approval of ChiRhoStim (human secretin) for stimulation of pancreatic secretions, gastrin secretion in gastrinoma diagnosis, and facilitation of pancreatic duct cannulation at ERCP Approved

  Approved diagnostic indications, with pharmacokinetics (45-minute half-life, 2.7 L volume of distribution, 580.9 mL/min clearance) and the 110 pg/mL gastrin cut-off documented in the label.

Pancreatic secretion begins within about 5 minutes of the bolus and peaks at 15-30 minutes. Plasma concentrations return to baseline in 90-120 minutes. The gastrin sampling window for gastrinoma diagnosis is 30 minutes. Everything about this molecule happens inside a single procedure.

Secretin and sincalide are genuinely used in sequence during combined pancreatic function testing: secretin for the ductal bicarbonate response, sincalide for the acinar enzyme response. That is a real, deliberate pairing rather than a stack in any recreational sense. The things that must be withheld matter more than the things that can be added. Anticholinergics blunt the pancreatic secretory response and will invalidate a functional test. Proton pump inhibitors and H2 blockers independently raise basal gastrin and must be stopped before the gastrinoma test — this is the most common cause of an uninterpretable result. Recent vagotomy or uncontrolled acid hypersecretion will confound the gastrin test in the other direction. There is no meaningful concept of stacking secretin with anything else, because there is no therapeutic use and no repeat administration.

Secretin is in this corpus for completeness and for one useful contrast: it is the oldest hormone known and one of the few peptides here with a fully characterised human pharmacokinetic profile in an FDA label. Compare that to BPC-157, where thirty years of research has produced no human plasma curve at all. The difference is not the age of the molecule; it is whether anyone was required to measure it. Against sincalide, the two are complementary rather than competing: secretin tests ductal bicarbonate function, sincalide tests acinar enzyme output and gallbladder contraction. Combined pancreatic function testing uses both. Against faecal elastase, which is the routine non-invasive test for pancreatic insufficiency, secretin-stimulated testing is considerably more sensitive for early disease but requires duodenal intubation or MRCP and is available in relatively few centres. Within the peptide-hormone family, secretin's N-terminal His-Ser-Asp motif places it in the same superfamily as glucagon, GLP-1 and GLP-2 — which means teduglutide and secretin are structural cousins acting on related class B receptors, one therapeutically and one diagnostically.

Not applicable. This is a single-use hospital diagnostic billed per procedure, not a monthly therapy. No monthly cost concept exists.

• ?No published receptor affinity (Ki or EC50) at the secretin receptor was resolved in this session, so the affinity fields are qualitative.
• ?Protein binding is not stated in the label and was not resolved elsewhere.
• ?Tmax is entered as 0.25 hours based on the label's statement that pancreatic secretion peaks around 15-30 minutes; this is a pharmacodynamic peak rather than a measured plasma Tmax.
• ?The clearance value of 580.9 mL/min exceeds renal plasma flow, which implies substantial extrarenal clearance — this is my inference from the label figure rather than a stated mechanism.
• ?Blood-brain barrier penetration is marked unknown. Secretin receptors are expressed in the CNS and central secretin signalling exists, but whether peripherally administered secretin reaches them meaningfully was not established.
• ?The Bayliss and Starling 1902 paper and the secretin-enhanced MRCP consensus recommendations referenced in the Core record were not resolved to citable records in this session.
• ?Reference thresholds for duodenal bicarbonate concentration are laboratory-specific and no generic cut-off is asserted here.

• CHIRHOSTIM (human secretin) for injection, US prescribing information ChiRhoClin, Inc., DailyMed / FDA label

  Source for the full amidated 27-residue sequence, the empirical formula C130H220N44O39 and 3039.44 molecular weight, the pharmacokinetics (45-minute half-life, 580.9 ± 51.3 mL/min clearance, 2.7 L volume of distribution, return to baseline in 90-120 minutes after 0.4 mcg/kg), and the ≥110 pg/mL gastrin rise criterion for gastrinoma.