Neural Regulation
The autonomic nervous system exerts diverse effects on exocrine pancreatic cells.
Parasympathetic Division The primary stimulator is the vagus nerve (n. vagus). Cholinergic nerve fibers release acetylcholine, which binds to muscarinic receptors ($M_3$) on pancreatic acinar and ductal cells. This triggers intracellular signaling cascades: calcium ion release and secondary messenger activation (guanylyl cyclase–cGMP). As a result, digestive enzymes and bicarbonates are actively secreted. Transection or pharmacological blockade of the vagus nerve (vagotomy) sharply reduces pancreatic juice output.
Sympathetic Division Acts as an inhibitory control. Inhibition of secretion occurs via two pathways:
- Direct: Action on $\alpha$-adrenergic receptors on the glandular cells.
- Vascular: Activation of vascular $\alpha$-adrenergic receptors leads to vasoconstriction, decreasing organ blood flow.
Sympathetic inhibition is activated during pain, emotional stress, intense mental work, and sleep.
Humoral Regulation: Secretin and Cholecystokinin
Gastrointestinal hormones play a pivotal role in digestive regulation. A historical milestone was the discovery of secretin by W. Bayliss and E. Starling in 1902.
Secretin Produced by S-cells in the duodenal mucosa as inactive prosecretin. It is activated by hydrochloric acid (HCl) entering from the stomach. Secretin's main function is to stimulate the release of a large volume of pancreatic juice rich in bicarbonates but low in enzymes. This is essential for neutralizing acidic chyme. An important anatomical detail: a direct vascular collateral runs from the portal vein to the pancreas, allowing secretin to act on the gland before being metabolized by the liver.
Cholecystokinin (Pancreozymin) Synthesized by I-cells of the small intestine in response to dietary fats and amino acids. Its effect contrasts with secretin: it stimulates the release of pancreatic juice extremely rich in digestive enzymes. It operates via negative feedback—the accumulation of trypsin in the intestinal lumen inhibits further cholecystokinin release.
Other Hormones and Autoregulation
In addition to major hormones, dozens of other signaling molecules influence the pancreas.
- Stimulatory: Gastrin, serotonin, insulin (enhances amylase and trypsinogen synthesis), bombesin, bile acids, chymodenin (specific for chymotrypsinogen).
- Inhibitory: Glucagon (reduces enzyme production), somatostatin, vasopressin, gastric inhibitory polypeptide (GIP), calcitonin, and others.
- Dual effect: VIP ( vasoactive intestinal peptide) can both stimulate and inhibit secretion.
Autoregulation and Adaptation Mechanisms The gland adapts to current demands (short- and long-term adaptation to dietary patterns) via neural, hormonal, and paracrine signals. Duodenal content plays a key role: a large volume of chyme inhibits secretion, whereas high acidity stimulates it. Protective mechanisms also shield the gland from autodigestion: if intraductal pressure rises or blood levels of active pancreatic enzymes increase, secretion is automatically suppressed.