GIT Hormones and Digestion
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What is primarily responsible for stimulating pancreatic secretion during the intestinal phase?

  • Acid and long chain fatty acids (correct)
  • Glycerol
  • Gastrin
  • Distension of the stomach

    • Which two hormones play a significant role in the stimulation of pancreatic secretion?

  • Gastrin and secretin
  • Secretin and CCK (correct)
  • ACh and norepinephrine
  • Insulin and glucagon

    • What role does secretin play in pancreatic secretion?

  • Stimulates water and bicarbonate secretion (correct)
  • Constricts the sphincter of Oddi
  • Inhibits enzyme secretion
  • Stimulates acid production

    • What initiates the vagovagal reflexes that stimulate pancreatic secretion during the gastric phase?

    <p>Distension of the stomach wall</p> Signup and view all the answers

    How does CCK affect the gall bladder during digestion?

    <p>Relaxes the sphincter of Oddi and constricts the gall bladder</p> Signup and view all the answers

    Which component of bile is most crucial for fat digestion?

    <p>Bile acids</p> Signup and view all the answers

    What is the primary source of CCK secretion?

    <p>Digestion of fats and proteins</p> Signup and view all the answers

    Which of the following statements about gastrin's role in pancreatic secretion is accurate?

    <p>Gastrin has no significant role in pancreatic stimulation.</p> Signup and view all the answers

    What effect does neuropeptide Y (NPY) have on metabolism?

    <p>It inhibits metabolism.</p> Signup and view all the answers

    Which peptide acts as an antagonist of the MC4 receptor?

    <p>Agouti-related peptide (AgRP)</p> Signup and view all the answers

    What is the relationship between the POMC pathway and the NPY system?

    <p>POMC pathway inhibits NPY system.</p> Signup and view all the answers

    What role do peptides that stimulate the melanocortin system play in relation to the NPY system?

    <p>They inhibit the NPY system.</p> Signup and view all the answers

    Which pathway is activated for increased food intake?

    <p>NPY pathway</p> Signup and view all the answers

    What initiates the cephalic phase of gastric secretion?

    <p>Thoughts or sight of food</p> Signup and view all the answers

    During which phase does the stimulation of peristalsis occur?

    <p>Gastric phase</p> Signup and view all the answers

    What is the function of cholecystokinin (CCK) in the intestinal phase?

    <p>Inhibits gastric secretion</p> Signup and view all the answers

    Which sequence of amino acids is crucial for the activity of both gastrin and CCK?

    <p>Gly – Trp – Met – Asp – Phe</p> Signup and view all the answers

    Where is the densest collection of gastrin-containing cells located?

    <p>Antrum of the stomach</p> Signup and view all the answers

    What stimulates the release of gastrin during the gastric phase?

    <p>Vagal stimulation via gastrin releasing hormone (GRP)</p> Signup and view all the answers

    Which type of receptor does CCK predominantly act upon in the gallbladder?

    <p>CCK-1 receptor</p> Signup and view all the answers

    What is the primary stimulus for gastrin release during the intestinal phase?

    <p>Breakdown of protein into amino acids</p> Signup and view all the answers

    What receptor does gastrin primarily act on to increase acid secretion in the stomach?

    <p>CCK-2 receptors</p> Signup and view all the answers

    Which hormone increases blood flow and stimulates gastric acid secretion through overlapping actions with gastrin?

    <p>Cholecystokinin (CCK)</p> Signup and view all the answers

    What is the primary action of secretin?

    <p>Inhibition of gastric secretions</p> Signup and view all the answers

    Which statement about motilin is correct?

    <p>Motilin triggers the migrating motor complex.</p> Signup and view all the answers

    What effect does gastric inhibitory peptide (GIP) primarily have on insulin secretion?

    <p>Stimulates insulin secretion</p> Signup and view all the answers

    Which hormone is released from I cells in the upper small intestine?

    <p>Cholecystokinin (CCK)</p> Signup and view all the answers

    Which of these hormones is known for its inhibitory effects on gastrointestinal motility and secretions?

    <p>Somatostatin</p> Signup and view all the answers

    What is a common stimulus for the secretion of cholecystokinin (CCK)?

    <p>Protein and fats</p> Signup and view all the answers

    Which hormone's release is primarily triggered by increased blood glucose and amino acids?

    <p>Somatostatin</p> Signup and view all the answers

    What role does serotonin (5-HT) play in gastrointestinal physiology?

    <p>Is involved in the vomiting response</p> Signup and view all the answers

    Which of the following hormones enter the systemic circulation?

    <p>Motilin</p> Signup and view all the answers

    Secretin primarily influences the secretion of which substance?

    <p>Bicarbonate</p> Signup and view all the answers

    Where are enteroendocrine cells predominantly located?

    <p>Scattered throughout the mucosa</p> Signup and view all the answers

    What type of hormonal communication occurs between neighboring cells?

    <p>Paracrine</p> Signup and view all the answers

    Which gastrointestinal hormone is primarily responsible for stimulating gastric acid secretion?

    <p>Gastrin</p> Signup and view all the answers

    Which hormone is NOT primarily considered a gastrointestinal hormone?

    <p>Epinephrine</p> Signup and view all the answers

    Which hormones are involved in the regulation of food intake?

    <p>All of the above</p> Signup and view all the answers

    Where is the highest release of gastrin typically observed?

    <p>Antrum of the stomach</p> Signup and view all the answers

    Which phase of gastric secretion is triggered by the sight, smell, or thoughts of food?

    <p>Cephalic phase</p> Signup and view all the answers

    What is the primary type of receptor that CCK predominantly acts upon in the gallbladder?

    <p>CCK-1 receptor</p> Signup and view all the answers

    The secretion of gastrin during the gastric phase is triggered by which of the following stimuli?

    <p>Vagovagal stimulation</p> Signup and view all the answers

    Which characteristic is not true about the structural relationship between gastrin and CCK?

    <p>CCK is shorter than gastrin.</p> Signup and view all the answers

    During which phase is gastric secretion inhibited by CCK and secretin due to activation of receptors in the small intestines?

    <p>Intestinal phase</p> Signup and view all the answers

    Which amino acids sequence is crucial for the biological activity of both gastrin and CCK?

    <p>Gly – Trp – Met – Asp – Phe</p> Signup and view all the answers

    Where in the digestive system is the densest collection of gastrin-containing cells located?

    <p>Antrum of the stomach</p> Signup and view all the answers

    In which situation is gastrin most likely released from G cells?

    <p>During protein breakdown to amino acids</p> Signup and view all the answers

    What is primarily secreted by pancreatic acinar cells?

    <p>Inactive digestive enzyme precursors</p> Signup and view all the answers

    During which phase does the vagus nerve stimulate pancreatic secretion primarily through acetylcholine?

    <p>Cephalic phase</p> Signup and view all the answers

    Which component of pancreatic secretion helps to neutralize gastric acid in the small intestine?

    <p>Na+ and HCO3-</p> Signup and view all the answers

    What stimulates the release of pancreatic enzymes during the intestinal phase?

    <p>Cholecystokinin (CCK)</p> Signup and view all the answers

    Which substance is NOT typically a component of exocrine pancreatic secretions?

    <p>Hormones</p> Signup and view all the answers

    What overall effect does the parasympathetic nervous system have on pancreatic secretion?

    <p>Stimulation of both acinar and ductal secretion</p> Signup and view all the answers

    What is the primary purpose of the basal secretion of pancreatic enzymes?

    <p>To aid in digestion during fasting</p> Signup and view all the answers

    What initiates the cephalic phase in relation to pancreatic function?

    <p>Conditioned reflexes triggered by sensory stimuli</p> Signup and view all the answers

    During the intestinal phase, what primarily accounts for 70-80% of the stimulation of pancreatic secretion?

    <p>Presence of digestion products and H+</p> Signup and view all the answers

    What is the role of secretin in pancreatic secretion?

    <p>Stimulates ductal cells to increase aqueous solution</p> Signup and view all the answers

    Which hormone is predominantly responsible for enzyme secretion during the intestinal phase?

    <p>Cholecystokinin (CCK)</p> Signup and view all the answers

    How does CCK facilitate bile release?

    <p>By constricting the sphincter of Oddi and contracting the gallbladder</p> Signup and view all the answers

    Which statement about the gastric phase's effect on pancreatic secretion is correct?

    <p>Stomach wall distension initiates vagovagal reflexes to the pancreas.</p> Signup and view all the answers

    What type of interactions are indicated by the dashed lines in the described mechanisms?

    <p>Potentiative interactions</p> Signup and view all the answers

    Which component of bile is crucial for its function in fat digestion?

    <p>Bile salts</p> Signup and view all the answers

    What primarily stimulates the release of secretin?

    <p>Acid present in the small intestine</p> Signup and view all the answers

    What is the effect of blocking the vagal afferent fibers on satiety?

    <p>It eliminates the feeling of satiety.</p> Signup and view all the answers

    Which hormone directly acts on the hypothalamus to induce satiety after a meal?

    <p>Leptin</p> Signup and view all the answers

    How does CCK contribute to the inhibition of feeding?

    <p>By stimulating vagal afferents and insulin release.</p> Signup and view all the answers

    What role does Ghrelin play in food intake regulation?

    <p>It stimulates orexigenic NPY neurons.</p> Signup and view all the answers

    What is the primary action of Leptin in terms of appetite regulation?

    <p>It induces satiety by stimulating POMC and inhibiting NPY pathways.</p> Signup and view all the answers

    What is the role of Peptide YY in regulating appetite?

    <p>It inhibits NPY orexigenic nerves to promote satiety.</p> Signup and view all the answers

    Which peptide is known to be released by the enteroendocrine cells of the ileum and colon in response to fat digestion?

    <p>Peptide YY</p> Signup and view all the answers

    What effect does the stimulation of the vagus nerve have on eating behavior?

    <p>It induces feelings of satiety and inhibits feeding.</p> Signup and view all the answers

    How do the mechanisms of action differ between fasted and fed states in relation to ghrelin?

    <p>Ghrelin levels increase in the fasted state.</p> Signup and view all the answers

    Which hormone is primarily responsible for stimulating insulin release after carbohydrate intake?

    <p>GIP</p> Signup and view all the answers

    Which hormone governs gastric motility during states of acid secretion?

    <p>Gastrin</p> Signup and view all the answers

    What physiological effect does GIP have on food intake regulation in comparison to leptin?

    <p>GIP enhances appetite while leptin inhibits it.</p> Signup and view all the answers

    Which of the following is true regarding the role of motilin within the gastrointestinal tract?

    <p>Motilin promotes gastric emptying during fasting.</p> Signup and view all the answers

    Which step is crucial in determining whether a substance is classified as a gastrointestinal hormone?

    <p>Isolating a substance that mimics the effect of the original stimulus.</p> Signup and view all the answers

    What impact does glucose and amino acid presence have on gastrointestinal hormone release?

    <p>They trigger insulin release from the pancreas.</p> Signup and view all the answers

    What is the key effect of leptin on food intake in relation to NPY and POMC?

    <p>Leptin inhibits both NPY and stimulates POMC.</p> Signup and view all the answers

    Study Notes

    ### GIT Hormones

    • Autocrine communication is within a single cell.
    • Paracrine communication is between neighbouring cells.
    • Endocrine communication is between distant cells via hormones in the bloodstream.
    • The term hormone can refer to all three types of signaling.

    ### Types of GIT hormones

    • There are many GI peptides/amines with various functions.
    • Only five are considered to be true hormones (enter circulation): gastrin, cholecystokinin (CCK), secretin, glucose-dependent insulinotropic peptide (GIP), and motilin.
    • Some peptides are endocrine, some paracrine, and some neural.
    • These peptides are released from enteroendocrine cells distributed throughout the mucosa, not organized in glands.
    • The duodenum and jejunum are the primary sites for hormone release.

    Gastric Phases of Digestion

    • Cephalic Phase: Begins with sight, smell, or thought of food. Triggers gastrin release and secretion of mucus, HCl, and pepsinogen.
    • Gastric Phase: Stomach stretching activates stretch receptors, stimulating peristalsis (mixing) and gastric emptying
    • Intestinal Phase: Begins with activation of receptors in the small intestine. Inhibition of gastric secretion by CCK and secretin slows gastric emptying.

    ### Gastrin and CCK

    • Gastrin and CCK are structurally related peptides that act on CCK receptors.
    • Both have a shared C-terminal sequence (Gly-Trp-Met-Asp-Phe), essential for activity.
    • Gastrin (17 amino acids) and CCK (33 amino acids) can both act on each other's receptors.
    • CCK-1 receptors are primarily found in the gallbladder (CCK).
    • CCK-2 (or gastrin) receptors are primarily found in the stomach (gastrin).

    ### Gastrin Release

    • G cells in the antrum of the stomach contain the highest concentration of gastrin.
    • G cells are also found in the duodenum.
    • Stimuli for Gastrin Release:
      • Cephalic Phase: Gastrin-releasing peptide (GRP) is the neurotransmitter released via vagovagal stimulation.
      • Gastric Phase: Stomach stretching stimulates GRP release.
      • Gastric and Intestinal Phase: Breakdown of proteins to amino acids also stimulates gastrin release.
    • Gastrin Actions:
      • Increases acid secretion from oxyntic/parietal cells via CCK-2 receptors.
      • Stimulates histamine release from ECL cells, leading to further acid secretion.
      • Acts as a trophic agent, promoting growth of stomach mucosa.
      • Stimulates splanchnic blood flow.

    ### Cholecystokinin (CCK) Release

    • I cells located in the upper small intestine (duodenum and jejunum) release CCK.
    • CCK release is stimulated by proteins and fats (peptides, single amino acids, fatty acids, monoglycerides).
    • Sensory afferents (nerve fibers) are activated, and I cells may directly sense substrates during absorption.

    ### Cholecystokinin (CCK) Actions

    • Most actions are on the gallbladder and pancreas.
    • Effects on Gastric Acid Secretion:
      • Stimulates acid secretion via CCK-2 receptors.
      • Inhibits acid secretion via CCK-1 receptors, stimulating somatostatin release from D cells.
    • Increases splanchnic blood flow.

    ### Secretin

    • Secretin shares amino acid residues with GIP, VIP, and glucagon.
    • S cells in the duodenum release secretin.
    • Stimuli for Secretin Release:
      • Low pH and, to a lesser extent, fatty acids.
      • Secretin-releasing peptide activates sensory afferent fibers.
      • S cells may directly sense pH and fatty acids.
    • Secretin Actions:
      • Stimulates pancreatic secretions (aqueous solution, bicarbonate).
      • Stimulates insulin release.
      • Decreases gastric acid secretion and gastric motility (via somatostatin and vagal pathways).
      • Increases splanchnic blood flow.

    ### Other Hormones

    • Somatostatin is released from delta (D) cells in the pancreas and stomach.

      • Stimulated by CCK and ACh (released in response to increased blood glucose and amino acids).
      • Acts as an inhibitor, decreasing motility, acid secretion, and blood flow.

    • Motilin is released from enteroendocrine (“Mo”) cells in the mucosa of the upper GI tract.

      • Released in 90-minute cycles, and inhibited by food intake.
      • Responsible for the migrating motor complex, which assists in clearing foreign bodies.

    • Gastric Inhibitory Peptide (GIP), now called glucose-dependent insulinotropic polypeptide (GIP), is released from K cells in the duodenum and jejunum.

      • Stimulated by the presence of food in the upper small intestine.
      • Inhibits gastric secretions and motility.
      • Primarily stimulates insulin secretion.

    • Serotonin (5-HT) is released from enterochromaffin cells.

      • Involved in vomiting.
      • Some anti-emetics (e.g., ondansetron) block 5-HT3 receptors on sensory afferent fibers.

    Pancreatic Secretion

    • Gastric Phase: Stomach distension activates vagovagal reflexes, stimulating pancreatic secretion. Gastrin plays a minimal role.
    • Intestinal Phase: The presence of digestion products (70-80% of stimulation) and H+ in the small intestine stimulates pancreatic secretion.
      • Secretin released by acid stimulates ductal cells, increasing aqueous volume and pH (due to HCO3-).
      • CCK released by fat and protein digestion stimulates acinar cells to increase enzyme secretion.
      • CCK acts indirectly via vagal afferent fibers.

    ### Bile Secretion

    • Bile is produced by hepatocytes (liver cells), stored in the gallbladder, and released into the duodenum.
      • Bile is crucial for fat digestion and consists of: water, bilirubin, cholesterol, bile salts, and other fats.
      • Secretin stimulates water and bicarbonate secretion from bile ducts.
      • CCK acts on CCK-1 receptors in the gallbladder to contract it and relax the sphincter of Oddi, releasing bile acids into the small intestine.

    ### Central Control of Food Intake

    • The hypothalamus plays a crucial role in regulating food intake.
    • Neuropeptide Y (NPY) and proopiomelanocortin (POMC) systems:
      • NPY system: stimulates food intake.
      • POMC system: inhibits food intake.
      • These systems have antagonistic effects.
      • CCK inhibits food intake by activating POMC neurons.
    • CCK also acts on the vagus nerve, contributing to satiety.

    Neurohumoral Signals

    • The cephalic phase of gastric secretion is initiated by sight, smell or thoughts of food, releasing gastrin into the blood and stimulating secretion of mucus, HCl and pepsinogen.
    • During the gastric phase, stretching of the stomach activates stretch receptors, stimulating peristalsis (mixing) and gastric emptying.
    • The intestinal phase begins with activation of receptors in the small intestines, inhibiting gastric secretion by CCK and secretin, and slowing gastric emptying.

    Gastrin and Cholecystokinin (CCK)

    • Gastrin and CCK are structurally related peptide hormones that act at the CCK receptors.
    • They share the C-terminal sequence, essential for activity: Gly – Trp – Met – Asp – Phe.
    • CCK-1 receptor is found in the gallbladder, predominantly activated by CCK.
    • CCK-2 (or gastrin) receptor is found in the stomach, predominantly activated by gastrin.

    Gastrin Release

    • Gastrin-containing cells (G cells) are densest in the antrum of the stomach, with some in the duodenum.
    • Gastrin release is triggered by different stimuli across each gastric phase:
      • Cephalic and gastric phases: vagovagal stimulation, gastrin releasing hormone (GRP) as neurotransmitter.
      • Gastric and intestinal phases: breakdown of protein to amino acids also stimulates gastrin release.

    Pancreatic Secretions

    • The pancreas has both endocrine and exocrine secretions.
    • Endocrine secretions (glucagon and insulin) are not the focus here.
    • Exocrine secretions consist of:
      • An aqueous component (mainly Na+ and HCO3- secreted by ductal epithelial cells).
      • A proteinaceous/enzymatic component (inactive precursors of digestive enzymes secreted by acinar cells).

    Control of Pancreatic Secretion

    • The pancreas produces a basal secretion of enzymes and fluid during the interdigestive period, coordinated by the parasympathetic nervous system, aided by CCK.
    • Pancreatic juice secretion increases up to 20 times during a meal, driven by stimulatory and inhibitory mechanisms across cephalic, gastric, and intestinal phases.

    Cephalic Phase of Pancreatic Secretion

    • Stimuli: conditioned reflexes, smell, taste, chewing, and swallowing.
    • Afferent impulses travel to the vagal nucleus.
    • Vagal ACh stimulates both acinar and ductal secretions.
    • Gastrin (released by the vagus) stimulates acinar cells, the role in humans is unclear.

    Gastric Phase of Pancreatic Secretion

    • Stimulation is mediated by the same mechanisms as the cephalic phase.
    • Distension of the stomach wall initiates vagovagal reflexes to the pancreas.
    • Gastrin plays little to no role in stimulating the pancreas.

    Intestinal Phase of Pancreatic Secretion

    • Presence of digestion products and H+ in the small intestine accounts for 70-80% of pancreatic secretion stimulation.
    • Secretin and CCK account for almost all hormonal stimulation:
      • Secretin release, stimulated by acid and long chain fatty acids, stimulates the ductal cells to increase the aqueous solution, increasing volume and pH.
      • CCK, stimulated by fat and protein digestion, stimulates the acinar cells to increase enzyme secretion.

    Bile

    • Secreted by liver cells (hepatocytes), stored in the gall bladder, and released into the duodenum.
    • Crucial for fat digestion, consisting of water, bilirubin, cholesterol, bile salts, and other fats.
    • Secretin stimulates water and bicarbonate secretion from bile ducts.
    • CCK constricts the gall bladder and relaxes the sphincter of Oddi, releasing bile acids.

    Food Intake - Peripheral Regulation

    • The vagus nerve carries afferent fibers relaying information to the nucleus tractus solitarius in the brain.
    • Input can also cause efferent signals by vagal nerves, resulting in changes in gut function: "vagovagal reflex."
    • Vagal stimulation induces satiety and inhibits feeding.
    • Blocking of vagal afferents eliminates satiety.
    • Insulin, released from pancreatic β-cells after a meal, acts directly on the hypothalamus to induce satiety.
    • Leptin, released from adipocytes, stimulates the POMC pathway, inhibits the NPY pathway, inducing satiety.

    Food Intake - Gastric (Inhibitors)

    • Local gastric stimuli pass information back to the hypothalamus.
    • Distension of the stomach stimulates vagal afferents and inhibits feeding.
    • CCK, released due to food presence in the small intestine, stimulates satiety and inhibits feeding, by stimulating vagal afferents and insulin release.
    • Peptide YY, released by enteroendocrine cells of the ileum and colon by fat digestion products, stimulates satiety, directly inhibiting NPY orexigenic nerves.

    Food Intake - Stimulants

    • Ghrelin is released from oxyntic glands in the stomach, also released from extra-gastric sites.
    • Stimulates release of growth hormone and directly stimulates orexigenic NPY neurons (increased food intake).
    • Ghrelin release is not affected by protein intake or distention.
    • Dopaminergic neurons from the ventral tegmental area (VTA) of the midbrain (reward pathways) influence the hypothalamus, affected by ghrelin, leptin etc.

    Summary

    • Gastrin: stimulated by acid, protein, distension, nervous input.
    • CCK: stimulated by fats, carbohydrates, distension, nervous input.
    • Secretin: stimulated by acid, carbohydrates, distension, nervous input.
    • GIP: stimulated by carbohydrates, distension, nervous input.
    • Motilin: stimulated by fats, carbohydrates, distension, nervous input.

    Summary

    • Gastrin: decreases gastric emptying, increases gastric motility, increases acid secretion.
    • CCK: decreases gastric emptying, increases pancreatic secretion, increases gallbladder contraction.
    • Secretin: increases pancreatic secretion, increases bicarbonate secretion, increases bile duct secretion.
    • GIP: increases insulin release.
    • Motilin: increases intestinal motility.

    GI Hormone Criteria

    • Step 1: Does a meal or other physiological event provide a stimulus for one part of the GI tract to affect another?
    • Step 2: Does this affect persist after nervous communication between those parts is removed?
    • Step 3: Can you isolate a substance from the site of the stimulus that when injected mimics the effect of the stimulus?
    • Step 4: Can you identify the substance chemically and synthesise it?

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