Gastrointestinal Hormone Overview

• Gastrin , produced by G cells primarily in the antrum of the stomach and the duodenum, is released in response to stomach distension, the presence of peptides and amino acids in the stomach, vagal stimulation via Gastrin-Releasing Peptide (GRP), and an increase in gastric pH. Its release is inhibited by somatostatin, high gastric acidity (pH < 2), secretin, and gastric inhibitory peptide (GIP). Gastrin's primary actions include stimulating gastric acid secretion by parietal cells, increasing gastric motility, promoting growth of the gastric mucosa, stimulating pepsinogen secretion by chief cells, and increasing lower esophageal sphincter (LES) tone.
• Cholecystokinin (CCK) is secreted by I cells located in the duodenum and jejunum. The major stimuli for CCK release include fatty acids, peptides, and amino acids in the duodenum, as well as the presence of acid. Somatostatin, pancreatic polypeptide (PP), and vagal inhibition suppress its release. CCK plays a crucial role in stimulating gallbladder contraction and bile release, promoting pancreatic enzyme secretion, potentiating secretin's action on bicarbonate secretion, inhibiting gastric emptying, promoting satiety, and stimulating growth of the exocrine pancreas.
• Secretin , produced by S cells in the duodenum, is released primarily in response to acid in the duodenum (pH < 4.5) and the presence of fatty acids. Its release is inhibited by somatostatin and a negative feedback loop from increased duodenal pH. Secretin's key functions are to stimulate pancreatic bicarbonate secretion to neutralize acid, stimulate bile secretion, inhibit gastric acid secretion, stimulate pepsinogen secretion, promote growth of the exocrine pancreas, inhibit gastric emptying, and increase LES tone.
• Gastric Inhibitory Peptide (GIP), also known as Glucose-dependent Insulinotropic Peptide, is secreted by K cells located in the duodenum and jejunum. Glucose, fatty acids, and amino acids in the duodenum are the primary stimuli for GIP release, which is inhibited by somatostatin. GIP's main actions involve stimulating insulin release in a glucose-dependent manner, inhibiting gastric acid secretion, inhibiting gastric motility, and stimulating intestinal fluid and electrolyte secretion.
• Motilin , produced by M cells in the duodenum and jejunum, is released during fasting and in response to an alkaline pH. Its release is inhibited by eating and somatostatin. Motilin stimulates gastric motility (specifically, the migrating motor complex - MMC), stimulates pepsin secretion, and increases LES tone.
• Histamine , synthesized and released by Enterochromaffin-like (ECL) cells in the stomach (primarily the fundus), is stimulated by gastrin and vagal stimulation. Somatostatin and prostaglandins inhibit its release. Histamine acts as a paracrine mediator to stimulate gastric acid secretion by parietal cells through H2 receptors and potentiates the effects of gastrin and acetylcholine.
• Somatostatin , produced by D cells in the stomach, duodenum, and pancreas (delta cells), is released in response to acid in the stomach and fatty acids in the duodenum. Vagal stimulation and decreased gastric pH inhibit its release. Somatostatin functions as a broad inhibitor, suppressing the release of most GI hormones (gastrin, CCK, secretin, GIP, motilin, VIP), inhibiting gastric acid secretion, inhibiting pancreatic enzyme secretion, inhibiting gallbladder contraction, inhibiting intestinal motility, and inhibiting insulin and glucagon secretion.
• Vasoactive Intestinal Polypeptide (VIP) is released from neurons throughout the GI tract, acting via neurocrine and paracrine signaling. Vagal stimulation stimulates its release, while somatostatin and adrenergic stimulation inhibit it. VIP relaxes smooth muscle (e.g., LES, stomach, gallbladder), stimulates intestinal secretion of water and electrolytes, inhibits gastric acid secretion, and stimulates pancreatic bicarbonate secretion.
• Gastrin-Releasing Peptide (GRP), also known as Bombesin, is released from neurons in the stomach, acting via neurocrine signaling. Vagal stimulation is the main stimulus for release. GRP stimulates gastrin release from G cells, stimulates pancreatic enzyme secretion, and stimulates gastric motility.
• Enkephalins, produced by neurons and enteroendocrine cells throughout the GI tract, act via neurocrine, paracrine, and endocrine signaling. Various stimuli, including inflammation and stress, trigger their release. Enkephalins generally decrease intestinal motility, inhibit intestinal secretion, increase sphincter tone, reduce the perception of pain, and enhance the absorption of water and electrolytes.
• Substance P, released from neurons throughout the GI tract via neurocrine and paracrine signaling, is stimulated by local irritation and inflammation. Enkephalins inhibit its release. Substance P stimulates smooth muscle contraction (promoting peristalsis), increases intestinal secretion, promotes inflammation, and causes vasodilation.
• Neurokinin A (Substance K), similar to Substance P, is released from neurons throughout the GI tract via neurocrine and paracrine signaling, with local irritation and inflammation acting as stimuli, and enkephalins inhibiting its release. Its actions are also similar to Substance P, stimulating smooth muscle contraction, increasing intestinal secretion, promoting inflammation, and vasodilation.
• Dynorphin, released from neurons throughout the GI tract via neurocrine and paracrine signaling, is stimulated by stress and inflammation. It is generally inhibitory, decreasing intestinal motility, modulating pain perception, and influencing secretion in complex and context-dependent ways.
• Glucagon-like Peptide-1 (GLP-1) is produced by L cells primarily in the ileum and colon, and in lower levels in the duodenum, is released in response to nutrients (glucose and lipids). DPP-4 degrades GLP-1 and thus inhibits its action. GLP-1 stimulates insulin secretion (glucose-dependent), inhibits glucagon secretion, increases satiety, slows gastric emptying, and increases beta cell mass.
• Guanylin, produced by intestinal cells in the small intestine and colon, acts via paracrine signaling. Increased osmolarity stimulates its release. Guanylin binds to guanylate cyclase C (GC-C) receptors on intestinal epithelial cells, increasing cGMP production. This stimulates secretion of chloride and water, inhibiting sodium absorption and promoting intestinal motility, ultimately regulating fluid and electrolyte balance.
• Pancreatic Polypeptide (PP), produced by PP cells in the pancreas, is released in response to fat in the ileum, protein digestion products, and vagal stimulation. Somatostatin inhibits its release. PP inhibits pancreatic enzyme secretion, inhibits gallbladder contraction, stimulates gastric emptying, and reduces appetite.
• Oxyntomodulin, produced by L cells in the small intestine and colon, is released in response to glucose and fat. It inhibits gastric acid secretion, slows gastric emptying, stimulates insulin secretion, increases energy expenditure, and reduces appetite.
• Urogastrone (EGF) , produced by Brunner's glands of the duodenum and salivary glands, acts via paracrine and autocrine signaling. It is stimulated by various inflammatory and growth factors. It primarily suppresses gastric acid secretion and promotes cellular proliferation, cell differentiation, and cell survival, which is involved in healing and repairing damage to the intestinal mucosa.
• Ghrelin, produced by P/D1 cells primarily in the fundus of the stomach, but also in the small intestine and brain, is released during fasting and hypoglycemia. Food intake, somatostatin, insulin, and glucose inhibit its release. Ghrelin stimulates appetite, increases gastric motility, stimulates gastric acid secretion, and promotes growth hormone release.

Explanation of Columns:

• Hormone: The name of the GI hormone.

• Cell Type: The specific cell in the GI tract that produces and releases the hormone.

• Location: The primary location within the GI tract where the hormone-producing cells are found.

• Signaling Mode(s):

• Endocrine: Hormone is released into the bloodstream and travels to distant target cells.

• Paracrine: Hormone acts on nearby cells within the same tissue.

• Autocrine: Hormone acts on the same cell that released it.

• Neurocrine: Hormone is released from a neuron and acts on a target cell (another neuron, muscle cell, or gland).

• Stimulus: The trigger that causes the release of the hormone.

• Inhibition: Factors that suppress the release of the hormone.

• Action: The primary physiological effects of the hormone on target tissues.

Additional Notes and Nuances:

• Overlap and Interactions: Many GI hormones have overlapping functions, and their actions are tightly coordinated. For example, CCK and secretin often work together to regulate pancreatic secretion.

• Receptor Specificity: Hormones exert their effects by binding to specific receptors on target cells. The distribution of these receptors determines the hormone's range of action.

• Regulation: GI hormone release is regulated by a complex interplay of neural, hormonal, and nutrient-related factors. Feedback loops are common.

• Clinical Significance: Dysregulation of GI hormone secretion can contribute to a variety of GI disorders, including peptic ulcer disease, irritable bowel syndrome (IBS), and pancreatic insufficiency. Some GI hormones are also targets for therapeutic interventions (e.g., GLP-1 receptor agonists for diabetes).

• Gut-Brain Axis: Many gastrointestinal hormones like GLP-1, peptide YY, and ghrelin also act on the brain to regulate appetite, satiety, and other functions.

• Diurnal rhythms Many of these hormones are released in a circadian rhythm.

• Receptors There can be multiple receptors that the same hormone can activate. Therefore actions of the hormone may differ.

• Enkephalins, Substance P, Substance K, Dynorphin (Opioid Peptides and Neuropeptides): These neuropeptides play important roles in GI motility, secretion, and pain modulation. Their effects are complex and often context-dependent. Enkephalins generally inhibit motility and secretion, while Substance P and Neurokinin A promote them. Dynorphin has both inhibitory and excitatory effects depending on the specific receptor subtypes involved.

• GLP-1 (Glucagon-like Peptide-1): While traditionally known for its role in glucose homeostasis, GLP-1 also significantly impacts GI function by slowing gastric emptying and promoting satiety.

• Guanylin: This peptide is crucial for regulating intestinal fluid and electrolyte balance. It's particularly relevant in the context of diarrheal diseases.

• Pancreatic Polypeptide (PP): This hormone is released by the pancreas and influences pancreatic exocrine function, gallbladder contraction, and appetite.

• Oxyntomodulin: This hormone, also produced by L cells, has effects on both GI function and energy balance.