What Do the Delta Cells of the Pancreas Secrete?

Pancreatic delta cells secrete somatostatin, a hormone that acts as a powerful brake on the other hormone-producing cells in the pancreatic islets. Though delta cells make up only about 5% of the islet cell population, their output keeps insulin and glucagon secretion in check and helps fine-tune blood sugar regulation from moment to moment.¹ That small percentage belies the outsized influence delta cells have on pancreatic function, and recent research has revealed surprising complexity in how they work, what else they may produce, and what happens when they go wrong.

Somatostatin and Its Job Inside the Islet

Somatostatin is a peptide hormone that exists in two main forms throughout the body: a 14-amino-acid version and a longer 28-amino-acid version. The pancreatic delta cells produce both, though SST-14 predominates in the islets. Regardless of form, the hormone’s central role in the pancreas is inhibitory. Once released, somatostatin dials down the secretion of insulin from neighboring beta cells, glucagon from alpha cells, and even somatostatin itself from other delta cells.¹ Think of it as a local volume knob: when blood sugar rises and beta cells ramp up insulin output, delta cells simultaneously release somatostatin to keep the insulin response from overshooting. And when alpha cells pump out glucagon, somatostatin restrains that too.

This kind of local, cell-to-cell chemical signaling is called paracrine signaling. Somatostatin does not need to travel through the bloodstream to do its work. It diffuses across the tiny gaps between islet cells and binds to somatostatin receptors on their surfaces. The effect is almost immediate: once somatostatin latches onto a receptor, it triggers a chain of events inside the target cell that suppresses both electrical activity and hormone release.¹

How Glucose Triggers Somatostatin Release

Delta cells respond directly to blood glucose. At low glucose levels, certain potassium channels on the delta cell membrane stay open, keeping the cell electrically quiet and somatostatin release low. When glucose rises, those channels close. This allows the cell membrane to depolarize, calcium floods in, and the cell begins actively secreting somatostatin.¹ It is a mechanism broadly similar to how beta cells sense glucose and release insulin, which makes sense given that these cells share a developmental lineage.

Live imaging of delta cells in mice has revealed two functionally distinct populations. About 94% of delta cells respond to glucose with fast, spiky bursts of calcium activity. The remaining roughly 6% show slower, more gradual calcium dynamics. Both groups respond to glucose, but the speed and pattern of their response differ, and the fast-spiking cells calm down within minutes, entering a recovery phase even while glucose stays high.² This built-in timeout prevents somatostatin from permanently silencing the rest of the islet and allows insulin and glucagon secretion to proceed once the initial burst of inhibition has done its job.

Glucose alone, however, is not the only thing controlling somatostatin release. Signals from neighboring alpha and beta cells amplify the glucose response. So delta cells are not acting as solo glucose sensors; they are integrating information from the entire islet neighborhood, making their output context-dependent rather than purely mechanical.

Reaching Across the Islet With Filopodia

One of the more striking features of delta cells is their shape. Unlike the relatively round beta and alpha cells, delta cells are elongated and extend thin, arm-like projections called filopodia. These structures can stretch remarkable distances, ranging from about 2 to 27 micrometers in living tissue, allowing a single delta cell to make physical contact with alpha, beta, and even other delta cells that are several cell bodies away.³

This architectural trick compensates for the fact that delta cells are relatively rare. If they simply dumped somatostatin from a stationary round cell body, the hormone would not reach distant targets before being diluted or degraded. By stretching out toward target cells, delta cells deliver somatostatin right where it is needed. Researchers have confirmed that somatostatin-positive filopodia make close contact with both alpha and beta cells in mouse and primate islets.³ In pre-diabetic mice, filopodia become even more elongated and delta cell shape changes, which may alter how effectively paracrine signaling works as the disease progresses.⁴

Which Receptors Does Somatostatin Act Through?

There are five known somatostatin receptor subtypes, and different islet cells favor different ones. In human islets, alpha cells are richest in SSTR2, beta cells in SSTR1 and SSTR5, and delta cells themselves primarily express SSTR5.⁵ The picture in rodents differs somewhat: mice and rats show broader expression of multiple receptor subtypes across islet cell types.⁶

These receptor differences are not just trivia. They explain why somatostatin can have selective effects on different neighbors. Because alpha cells rely heavily on SSTR2, drugs that block SSTR2 selectively release glucagon from alpha cells without necessarily disinhibiting insulin secretion from beta cells. And because delta cells express SSTR5, somatostatin feeds back on them too, creating a self-limiting loop that prevents runaway inhibition.

Somatostatin’s Role in Restraining Glucagon

One of somatostatin’s most critical jobs is keeping glucagon in check. In isolated rat pancreas preparations, somatostatin acts as a powerful and constant inhibitor of glucagon secretion. When researchers blocked somatostatin’s action, glucose could no longer suppress glucagon release the way it normally does, suggesting that much of the glucose-mediated suppression of glucagon actually works through somatostatin as an intermediary rather than through a direct glucose effect on alpha cells.⁷

The gut hormone GLP-1 (the same hormone that drugs like semaglutide are designed to mimic) also partly relies on this somatostatin pathway. In perfused rat pancreas studies, GLP-1 suppressed glucagon secretion substantially, but when researchers blocked the SSTR2 receptor specifically, the glucagon suppression vanished entirely.⁸ In other words, GLP-1 may suppress glucagon not by directly silencing alpha cells but by boosting somatostatin release from delta cells, which then does the silencing.

Ghrelin, the Hunger Hormone, Activates Delta Cells

Ghrelin, best known as the “hunger hormone” produced mainly by the stomach, also talks to delta cells. When mouse islets were stimulated with ghrelin, the result was a reduction in glucose-stimulated insulin secretion. That might seem like ghrelin is directly suppressing beta cells, but the actual mechanism runs through delta cells. Ghrelin activates delta cells, prompting them to release somatostatin, which then inhibits insulin secretion via the SSTR3 receptor on beta cells. Blocking SSTR3 completely abolished ghrelin’s insulin-suppressing effect.⁹

This is a good example of delta cells acting as intermediaries in a hormonal conversation they did not start. The gut tells the pancreas that the body is hungry (via ghrelin), the delta cell receives the message, and it relays it onward by dampening insulin release. It makes metabolic sense: when you are hungry and fasting, you do not want insulin driving your blood sugar down further.

The Nervous System Gets a Vote Too

Somatostatin secretion is not controlled entirely by glucose and hormones. The autonomic nervous system also exerts direct control over delta cells. Classic experiments in dogs showed that stimulating the vagus nerve, the major parasympathetic nerve to the gut, actually decreased somatostatin output to about 57% of baseline levels. Stimulating the splanchnic (sympathetic) nerve had a similar inhibitory effect, dropping somatostatin to about 68% of baseline. Both arms of the autonomic nervous system, then, tend to suppress somatostatin release, though a secondary beta-adrenergic mechanism can boost it under certain conditions.¹⁰

Studies of the isolated human pancreas have confirmed similar dynamics. Stimulating the celiac nerve bundle cut glucose-stimulated somatostatin release by about two-thirds. Blocking alpha-adrenergic receptors resulted in a roughly 90% boost in somatostatin release, while blocking cholinergic receptors suppressed it by about 40%.¹¹ The practical upshot: stress, digestion, and nervous system state all shape how much somatostatin delta cells produce at any given moment.

Do Delta Cells Produce Anything Besides Somatostatin?

The textbook answer has always been that delta cells make somatostatin and nothing else of note. But a 2023 study challenged that picture. Researchers found that delta cells contain a protein product derived from the insulin gene, though not insulin itself. Through alternative splicing, the insulin gene produces a variant transcript, and the resulting protein ends up in delta cell secretory granules but not in beta cells. This alternatively spliced product contains portions of the insulin molecule, including the signal peptide and the B chain, but it is not functional insulin.¹²

Why does this matter? The finding has implications for type 1 diabetes, where the immune system mistakenly attacks insulin-producing cells. If delta cells also carry insulin-derived proteins that the immune system can recognize, delta cells could become unintended targets of the autoimmune attack. The research is still early, but it opens a line of inquiry into whether delta cell dysfunction in type 1 diabetes is partly driven by immune targeting rather than just collateral damage from inflammation.

Somatostatin Beyond the Pancreas

It is worth noting that delta cells are not the body’s only source of somatostatin, and the pancreas is not even the main one. The gastrointestinal tract is actually the largest producer and target of somatostatin in the body. GI somatostatin was discovered after the hormone was first identified in the brain’s hypothalamus, and it turns out to be a potent inhibitor of many gut functions: it slows peristalsis, suppresses gastric acid production, tamps down the release of various GI hormones, and has anti-inflammatory effects that help maintain the intestinal barrier.¹³ Pancreatic delta cells, then, are part of a broader somatostatin system distributed across the gut and brain, all serving the general theme of putting the brakes on secretion and activity.

What Happens When Delta Cells Go Wrong

Tumors arising from delta cells, called somatostatinomas, are exceedingly rare but illustrative. Because these tumors pour out massive amounts of somatostatin, they produce a recognizable syndrome: diabetes (from suppressed insulin), fatty diarrhea or steatorrhea (from impaired fat digestion), and gallstones (from inhibited gallbladder contraction). Patients may also experience weight loss, anemia, and low stomach acid.¹⁴ Early case reports found that the tumor cells were indistinguishable from normal islet delta cells and that their extracts potently inhibited both insulin and glucagon secretion.¹⁵

Somatostatinomas are a natural experiment that shows what happens when the braking system runs unchecked. Every symptom traces back to somatostatin’s inhibitory actions being cranked up far beyond normal levels: too much inhibition of insulin causes high blood sugar, too much inhibition of digestive enzymes causes malabsorption, and too much inhibition of bile flow leads to gallstone formation.

Targeting Somatostatin Signaling as Therapy

Because somatostatin from delta cells suppresses glucagon, and because people with type 1 diabetes often lose the ability to mount an adequate glucagon response to low blood sugar (a dangerous condition called hypoglycemia-associated autonomic failure), researchers have explored whether blocking somatostatin’s action could restore protective glucagon surges during hypoglycemia. The logic is straightforward: if somatostatin is holding glucagon back, removing that restraint should let glucagon rise when blood sugar drops.

In diabetic rats, prolonged treatment with a drug that blocks the SSTR2 receptor led to a stronger glucagon response during induced hypoglycemia. The treated rats reached a higher blood sugar nadir and spent less time in the dangerously low range compared to untreated animals.¹⁶ A 2025 phase 1 clinical trial in people with long-standing type 1 diabetes tested a similar approach. When participants received an SSTR2 antagonist (called ZT-01) before being subjected to controlled hypoglycemia, their glucagon levels rose during low blood sugar, while placebo-treated participants showed little glucagon response until blood sugar dropped much lower. Both dose levels of the drug produced more frequent and larger glucagon increases compared to placebo.¹⁷

This line of research is still early-stage but represents a genuinely novel approach to a stubborn clinical problem. Severe hypoglycemia remains one of the most feared complications for people on insulin therapy, and current tools for preventing it are limited. If blocking somatostatin’s grip on alpha cells can safely restore the body’s natural glucagon defense, it would be a meaningful advance. The fact that a five-percent minority cell in the pancreatic islet controls a receptor pathway important enough to build drugs around underscores how much influence delta cells wield despite their small numbers.

Epigenetic Identity of the Delta Cell

Scientists have begun mapping the genetic and epigenetic features that make a delta cell a delta cell rather than an alpha or beta cell. A recent study examining chromatin accessibility across purified human islet cell types found distinct patterns of open and closed DNA regions in delta cells compared to their neighbors. Certain transcription factor binding sites were preferentially associated with delta cell-specific enhancer regions, offering clues about why the somatostatin gene is activated in these cells and kept silent in others.¹⁸ This kind of work matters for regenerative medicine. If researchers ever want to convert one islet cell type into another, or grow delta cells from stem cells, they will need to know which genetic switches to flip. The delta cell’s unique chromatin landscape is essentially the instruction manual for building its identity.