What Is a Secretagogue? Growth Hormone, Insulin and Other Secretagogues Explained

Secretagogues are substances that make a cell or gland secrete something: a hormone, a digestive enzyme or stomach acid. The word turns up in endocrinology papers, pharmacology textbooks and peptide product pages, often without explanation. Anyone designing a secretion assay or reading a label should know what it claims and what it leaves out.

This guide covers the word’s meaning, its history from secretin to ghrelin, the main classes, how secretagogue activity is measured, and how a secretagogue differs from a hormone, an agonist and a mimetic.

Secretagogue meaning and word origin

Merriam-Webster’s medical dictionary defines a secretagogue as “a substance stimulating secretion (as by the stomach or pancreas)”, built from secretion plus -agogue. The suffix came through French and Late Latin from the Greek agōgós, leading or drawing forth, so a secretagogue is literally something that draws out a secretion. The word’s first known use was in 1919. Three points follow:

• It is functional. A gut hormone (secretin), a neurotransmitter (acetylcholine), a sugar (glucose) and a class of medicines (sulfonylureas) all qualify by what they do.
• It needs a target. “Insulin secretagogue” is precise; “a secretagogue” on its own is not.
• It names the stimulus, not the product. When ghrelin releases growth hormone, ghrelin is the secretagogue and growth hormone the secretion.

From secretin to ghrelin: a short history

The idea is older than the word. In 1902 Bayliss and Starling put weak hydrochloric acid into a loop of an anaesthetised dog’s intestine whose nerves had been cut, leaving only its blood supply, and the pancreas still secreted. A filtered acid extract of intestinal lining, introduced into a vein, produced an even larger secretion: the signal travelled in the blood. That substance, secretin, is regarded as the first hormone discovered. Starling introduced the word hormone, proposed by W. B. Hardy from a Greek verb meaning to set in motion, in his Croonian Lectures to the Royal College of Physicians.

Two Montreal-trained scientists carried the idea into the brain. Roger Guillemin earned his PhD in 1953 in Hans Selye’s Montreal program, and Andrew Schally completed his McGill University doctorate in 1957, after showing with M. Saffran in 1955 that hypothalamic tissue held corticotropin-releasing activity. In 1971 Schally’s group in New Orleans reported that one polypeptide, GnRH, regulates the secretion of two pituitary hormones, and in 1977 the two men shared the Nobel Prize in Physiology or Medicine “for their discoveries concerning the peptide hormone production of the brain”. Our article on Canada’s century of peptide discovery tells more.

The growth hormone story ran backwards. In 1984 Cyril Bowers and colleagues reported a synthetic hexapeptide, now called growth hormone-releasing peptide-6 (GHRP-6), that released growth hormone (GH) from the pituitary specifically, through a receptor no one had identified. In 1996 a team including Roy Smith cloned that receptor from pituitary and hypothalamus and suggested the synthetic compounds imitated an undiscovered hormone. In 1999 Masayasu Kojima, Kenji Kangawa and colleagues found it in the stomach: ghrelin. Smith later called this route from synthetic ligand to receptor to natural hormone reverse pharmacology, and a 2005 International Union of Pharmacology review recommended renaming the growth hormone secretagogue receptor type 1a (GHS-R1a) the ghrelin receptor. Our GHRP-6, GHRP-2 and hexarelin comparison and ghrelin explainer follow both threads.

The main classes of secretagogues

Growth hormone: the GHRH and ghrelin pathways

The classic growth hormone-releasing peptides act through two separate receptors. GHRH acts at the GHRH receptor on somatotrophs, the GH-producing cells of the anterior pituitary, as does sermorelin, which the US National Library of Medicine defines as GHRH(1-29)-amide, the biologically active fragment of human GHRH. The ghrelin receptor is a family A G protein-coupled receptor with seven transmembrane domains, a family covered in our GPCR explainer.

Rat pituitary cell studies showed the two pathways are separate. In 1989 GHRP-6 released GH without changing cyclic AMP, while GHRH raised it about threefold, and together they released more GH than either alone. In 1991 cells desensitised to one peptide still responded to the other, and somatostatin blocked either peptide on its own. Our guides to cyclic AMP and protein kinase A and somatostatin explain those signals.

The ghrelin-receptor group includes GHRP-6, GHRP-2 (pralmorelin), hexarelin and ipamorelin, a pentapeptide described in 1998 as the first selective growth hormone secretagogue. Our comparison of sermorelin and ipamorelin as growth hormone secretagogues sets the two pathways side by side, and our growth hormone peptides research map covers the wider group.

Insulin secretagogues

Glucose is the beta cell’s physiological secretagogue. Its metabolism raises ATP, which closes the ATP-sensitive potassium (K-ATP) channel; the membrane depolarises, calcium enters through voltage-gated channels and insulin granules are released. The beta-cell channel has four Kir6.2 and four SUR1 subunits. Sulfonylurea drugs bind the channel and close it directly, bypassing metabolism, and glinides (meglitinides) such as repaglinide block it too. Selectivity differs: tolbutamide and gliclazide block only SUR1-type channels, while glibenclamide and repaglinide also block the SUR2 types found in heart and muscle.

Incretins work differently. Glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are released from gut endocrine cells within minutes of eating and enhance the insulin response to a meal. In 1973 a team from McGill University and the University of British Columbia showed that purified GIP, given with glucose to healthy volunteers, increased the rise in insulin; a 1978 study found that GIP’s effect needs at least some rise in blood glucose. Our GIP explainer tells the full story.

Gonadotropin secretagogues

GnRH regulates pituitary secretion of luteinising hormone (LH) and follicle-stimulating hormone (FSH), and kisspeptin acts one step higher. In 2005 kisspeptin raised LH and FSH in normal mice but not in mice lacking its receptor, GPR54, and in sheep it sharply raised GnRH in cerebrospinal fluid, evidence that it stimulates GnRH release directly. Our kisspeptin research guide covers the later work.

Gastric and pancreatic secretagogues

Secretin stimulates pancreatic secretion. Cholecystokinin (CCK), found in 1928 as a gallbladder-contracting intestinal factor, drives pancreatic enzyme secretion and potentiates secretin. Parietal cells in the stomach secrete acid in response to histamine, acetylcholine and gastrin, which act at receptors on the cells’ basolateral (blood-facing) membrane; stimulation moves the H⁺/K⁺-ATPase proton pump into the apical membrane, on the stomach side. Gastrin shares a C-terminal sequence with CCK, and the gastrin receptor is the CCK2 receptor.

How secretagogue activity is measured in the lab

A secretagogue assay exposes living cells to the candidate and measures what they release against a baseline and a known secretagogue. Released insulin is measured by immunoassay; radioimmunoassay of peptide hormones earned Rosalyn Yalow a share of the 1977 Nobel Prize.

Pituitary cells: static and perifusion assays

The 1989, 1991 and 1998 studies cited here used rat pituitary cells. In a static assay the cells are incubated with the compound and the GH in the medium is measured, as in the 1989 cyclic AMP study; in primary rat pituitary cells, ipamorelin released GH with potency and efficacy similar to GHRP-6. Perifusion passes fresh medium continuously over the cells, so the onset, duration and fading of a response can be followed; the 1991 study used it to show that GHRP-6’s burst of GH was shorter-lived than GHRH’s. Antagonists assign the receptor: profiling with GHRP and GHRH antagonists showed that ipamorelin, like GHRP-6, acts through a GHRP-type receptor.

Islets and beta-cell lines: glucose-stimulated insulin secretion

Insulin secretagogues are tested on isolated islets or beta-cell lines. In a static glucose-stimulated insulin secretion (GSIS) assay, islets are held at about 3 mM glucose to measure basal release, then moved to 11.1 mM for a half-maximal response or above 28 mM for a maximal one. Stimulated over basal release gives the stimulation index, about 2 to 20 in healthy rodent islets. Perifusion measures the kinetics: glucose-stimulated release is biphasic, a rapid first-phase spike followed by a sustained second-phase plateau. Cell lines need checking: of two mouse insulinoma lines described in 1990, MIN6 secreted insulin in response to glucose comparably with cultured normal islet cells, but MIN7 did not.

A convincing result needs a positive control, such as GHRH or high glucose, to show the cells can respond; a way to assign the receptor, such as an antagonist; and checks on other hormones, as the ipamorelin study made for adrenocorticotropic hormone (ACTH), cortisol and four pituitary hormones.

Secretagogue, hormone, agonist or mimetic?

Each of these overlapping words is defined by a different property.

One molecule can carry several labels. GHRP-6 is a ghrelin-receptor agonist, a ghrelin mimetic (studies in mice lacking the receptor confirmed that synthetic growth hormone secretagogues act this way) and a growth hormone secretagogue at once, while glucose is an insulin secretagogue that works through metabolism rather than a receptor.

Why “secretagogue” says nothing about safety or approval

Read “secretagogue” as “triggered secretion in the system tested”. The word does not tell you:

• How selective it is. In the 1998 animal comparison, GHRP-6 and GHRP-2 raised ACTH and cortisol as well as GH; ipamorelin did not.
• What else it binds. Hexarelin also binds CD36 in rat heart membranes; a Université de Montréal-led team found that its effect on coronary perfusion pressure was absent in CD36-deficient mouse hearts, an action unrelated to secretion.
• Whether a result carries over to people. The GHRP-6, ipamorelin and hexarelin studies cited here used rat cells, perfused hearts and animals, so for human physiology they are preliminary.
• Whether it is approved. The word covers natural messengers such as histamine, medicines such as sulfonylureas and peptides studied mainly in laboratories; approval is a regulatory decision about a specific product.

What’s established and what isn’t

Sourcing secretagogue peptides for research in Canada

Secretagogue experiments compare responses across concentrations, so the material matters as much as the cells. Our guide to choosing a peptide supplier in Canada has the full checklist; for this work, three questions stand out:

• Confirm identity against the sequence. GHRP-6 and GHRP-2 share their last four residues and differ only in the first two, so ask how identity was confirmed and read the report with our guide to reading a peptide COA.
• Separate purity from peptide content. An HPLC purity figure and the amount of peptide in a vial are different numbers, and a potency comparison depends on both; our HPLC peptide testing guide explains the first.
• Match the batch. The vial’s lot number should match the report.

Secretagogue research peptides in our catalogue

GHRP-6The hexapeptide Bowers reported in 1984, twelve years before its receptor was clonedView product GHRP-2Pralmorelin, which differs from GHRP-6 only in its first two residuesView product HexarelinA GHRP-family peptide that also binds CD36 in rat heart membranesView product SermorelinGHRH(1-29)-amide, the active fragment of human GHRHView product

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