Regulation of pancreatic secretion

Nervous regulation

The autonomic nervous system exerts multidirectional effects on the function of pancreocytes.

Parasympathetic division The main stimulator is the vagus nerve (n. vagus). Cholinergic nerve fibers release acetylcholine, which binds to M-cholinergic receptors on the glandular cells. This triggers a cascade of intracellular reactions: the release of calcium ions and the activation of a second messenger (guanylyl cyclase–cGMP). As a result, enzymes and bicarbonates are actively secreted. It has been proven that transection or pharmacological blockade of the vagus nerve (vagotomy) sharply reduces juice secretion.

Sympathetic division Acts as a brake. Inhibition of secretion occurs via two pathways:

• Direct: action on $\alpha$-adrenergic receptors of the glandular cells themselves.
• Vascular: activation of vascular $\alpha$-adrenergic receptors leads to their vasoconstriction, which reduces the blood supply to the organ.

Sympathetic inhibition is engaged during pain, emotional stress, intense mental work, and during sleep.

Humoral regulation: secretin and cholecystokinin

Gastrointestinal hormones play a key role in digestion. A historically significant milestone was the discovery of secretin by W. Bayliss and E. Starling in 1902.

Secretin Produced in the mucosal cells of the duodenum as inactive prosecretin. It is activated by hydrochloric acid (HCl) entering from the stomach. The main task of secretin is to stimulate the release of a large volume of pancreatic juice that is rich in bicarbonates but low in enzymes. This is necessary to neutralize the acidic chyme. An interesting anatomical detail: a direct vascular collateral runs from the portal vein to the gland. Thanks to this, secretin manages to act on the organ before being degraded in the liver.

Cholecystokinin (pancreozymin) Synthesized by CCK cells of the small intestine in response to the intake of fatty food. Its effect is opposite to that of secretin in terms of composition: it causes the release of juice extremely rich in enzymes. It operates on a feedback principle—the accumulation of trypsin in the intestinal lumen inhibits the further release of cholecystokinin.

Other hormones and autoregulation

In addition to the main hormones, dozens of other substances influence the pancreas.

• Stimulate: gastrin, serotonin, insulin (enhances the synthesis of amylase and trypsinogen), bombesin, bile acids, chymodenin (specific for chymotrypsinogen).
• Inhibit: glucagon (reduces enzyme production), somatostatin, vasopressin, gastric inhibitory peptide (GIP), calcitonin, and others.
• Dual action: VIP (vasoactive intestinal peptide) can either activate or suppress secretion.

Mechanisms of autoregulation and adaptation The gland is able to adapt to current needs (short-term and long-term adaptation to dietary patterns) using neural, hormonal, and paracrine signals. Duodenal contents play an important role: a large volume of chyme inhibits secretion, whereas high acidity stimulates it. There are also protective mechanisms preventing the gland from autodigestion: if pressure rises in the ducts or the blood level of pancreatic enzymes increases, secretion is automatically blocked.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of activation of second messengers (cGMP)
• The effect of insulin and glucagon on the enzymatic composition of juice
• Anatomy of portal vein vascular collaterals and the pancreas
• Short-term and long-term adaptation of the gland to various types of diets
• Paracrine interactions in the islets of Langerhans

Other topics in the "Digestion" section

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