Gastro-Intestinal Secretions Flashcards
How is saliva formed through salivary ducts? What is the pH and tonicity of saliva?
The acinar cells secrete proteins into the lumen and water and salts are moved into the lumen trans- and paracellularly. The ductal cells reabsorbed 3 Na+ for every 2 K+ secreted, absorb chloride and secrete bicarbonate; final product is hypotonic and alkaline
What are the important organic constituents of saliva?
water bicarbonate mucins amylase lysozyme, lactoferrin, IgA epidermal & nerve growth factor
Which branch of the ANS is the primary activator of salivatory glands?
Parasympathetics
How do parasympathetics mediate production of saliva?
Substance P, ACh, or norepinephrine (binding to alpha) => IP3=> inc. intracellular Ca2+
What is the effect of sympathetic stimulation on salivary glands?
SNS increases cAMP levels and increases amylase levels in saliva; Mediated by norepinephrine binding to beta receptors.
What product released by parasympathetics increases blood flow to glands?
Vasoactive Intestinal Peptide
True or False: Modifications of the acinar secretions only happen with PNS
True
Through what receptor does ACh mediate salivary duct transporter activity?
Muscarinic 3 of intercalated duct cells
How is sodium transported through the apical and basolateral ends of salivary intercalated duct cells
Absorbed apically through ENaC channel and Na+-H+ exchanger;
secreted BL by Na+/K+ ATPase
How is potassium transported through the apical and basolateral ends of salivary intercalated duct cells
Absorbed BL through Na+/K+ ATPase; Secreted apically by K+-H+ exchanger
How is bicarbonate transported through the apical and basolateral ends of salivary intercalated duct cells
Only secreted apically through Cl-/HCO3- exchanger
How is chloride transported through the apical and basolateral ends of salivary intercalated duct cells?
Absorbed apically through Cl-/HCO3- and secreted basolaterally and apically CFTR channel
Describe the composition of parasympathetically stimulated production/secretion of saliva?
Copious amount of water saliva that is protein poor, low in potassium and high bicarbonate; Sustained secretion
Describe the composition of sympathetically stimulated production/secretion of saliva?
Scant amount of viscous saliva that is protein-rich, high potassium, and low bicarbonate
What is the simple salivary reflex?
Pressure- and chemoreceptors in the mouth project to the salivary center in the medulla generating an autonomic response that activates the salivary glands to increase the production and secretion of saliva
What is the conditioned salivary reflex?
When other inputs (smelling food, seeing food) into the cerebral cortex that can affect salivation
What are the three divisions of the stomach based on function?
LES and cardia; fundus and body; antrum and pylorus
What is produced in the LES/cardia region of the stomach?
Mucus and HCO3-
What is secreted by the fundus/body region of the stomach?
H+, intrinsic factor, mucus, HCO3-, pepsinogens, and lipase
What portion of the stomach serves as a reservoir and provides tonic force during gastric emptying?
Fundus/Body
What is secreted by the antrum and pylorus?
Mucus and HCO3-
What are gastric pits? What cells are included? What are the major lytic secretions?
Indentations in the mucosal surface of the stomach that contain surface epithelial cells, mucous neck cells, parietal cells, and chief cells;
major lytic enzymes released are HCl, pepsinogen, and gastric lipase
What is secreted by surface epithelial cells/ mucous neck cells? What is the function?
Secrete mucus, HCO3-, Trefoil peptides; Function- lubrication and protection
What is secreted by parietal cells? What is the function?
Make H+ and secrete intrinsic factor; H+ important for digestion (pepsinogen-> pepsin) and intrinsic factor necessary for absorption of B12
What is secreted by chief cells? What is the function?
Makes pepsinogen and gastric lipase; Pepsinogen is important for protein digestion; lipase important for signaling to the duodenum in triglyceride digestion
What is secreted by the gastric endocrine cells?
Gastrin, Histamine, and Somatostatin
What is the main stimulus for an effector response in the gastric phase? What is triggered
Mechanical stimulation- the stomach senses distension and activates intrinsic and extrinsic nerve pathway, which triggers secretion of H+, pepsinogens, mucus, IF, gastrin, lipase, and bicarb
What chemical stimuli trigger the effector response in the gastric phase and what is the result?
Oligopeptides, amino acids, and H+; activate release of gastrin and somatostatin and intrinsic/extrinsic nerve pathways causing motor responses of receptive relaxation, antral peristalsis, and blood flow
How is the Vago-Vagal triggered and mediated?
Distension is sensed by vagal afferents and transmitted to dorsal vagal complex which transmits signals to the stomach and pancreas via efferent vagal nerves
What are the stomach responses to the Vago-Vagal reflex?
Receptive relaxation; secretion of H+ by parietal cells, pepsinogen from chief cells, and gastrin from G cells; antral contraction
What are the pancreatic responses to the Vago-Vagal reflex?
Secretion of enzymes, slight relaxation of the sphincter of Oddi
What is the most potent relaxer of the sphincter of Oddi?
CCK
Describe the positive feedback exhibited with pepsinogen and gastrin in the gastric phase?
Distension stimulates the vago-vagal reflex, resulting in secretion of H+, pepsinogen, and gastrin.
H+ cleaves pepsinogen to pepsin which breaks protein into oligopeptides. The oligopeptides cause an increased release of gastrin which then increases H+ and pepsinogen
What is the optimal pH range for amylase activity? How is the activity maintained in the stomach?
Optimal range is pH= 7-8, which is the approximate range of salivary pH. When food boli drop into the stomach, the low pH is buffered by neg. charged proteins/ molecules that actually raises the pH to 5-6.5 such that amylase is still about 50% active
What is the process by which parietal cells produce gastric acid?
Carbonic anhydrase produces H+ and HCO3-.
H+ is exported into the lumen via Na+/K+ ATPase. HCO3- is transported into the lumen in exchange for Cl-, which moves into the cell down its concentration gradient. Cl- moves into the lumen through the CFTR channel. Now there is H+ and Cl- in the lumen which makes HCl
What type of receptor is the histamine receptor on the parietal cells? What is the downstream effect?
GsPCR; Enhances K+/H+ ATPase to move more H+ into stomach
What type of receptors are the somatostatin/prostaglandin receptors on the parietal cells? What is the downstream effect?
GiPCR; Decreased PKA decreases the K+/H+ pump
What is the gastrin receptor on parietal cells called? What type of receptor is it? What is the downstream effect?
CCKb; GqPCR; Increased calcium enhances K+/H+ ATPase resulting in more H+ pumped into the stomach
What is the acetylcholine receptor on parietal cells called? What type of receptor is it? What is the downstream effect?
Muscarinic 3 receptor; GqPCR; Increased calcium increases K+/H+ ATPase activity, pumping more H+ into the stomach
What is potentiation and what is its relevance in the gastric phase?
Potentiation occurs when the response to simultaneous administration of 2 stimulants is greater than the sum of responses to either agent given alone; Gastrin, Histamine, and ACh all activate the same pathway to yield HCl secretion
How does the vagus mediate gastric secretion?
ACh binds to M3 on parietal cells to secrete HCl, chief cells to secrete pepsinogen, enterochromaffin cells to secrete histamine, and vagal stimulation by GRP onto G cells stimulates secretion of gastrin into the blood stream
What stimulates the release of somatostatin? What cells produce it? What are the effects of somatostatin?
Once pH in the antrum drops below 3, D cells release somatostatin, which shuts off gastrin production in the G cell and inhibits release of H+ from the parietal cell
How is pepsin produced and activated and what is its function?
Pepsinogen is stored in zymogen granules in chief cells and are released upon stimulation by vagus n. and gastrin.
Pepsinogen is converted to pepsin by low pH, and pepsin can also catalyze the conversion of pepsinogen to pepsin.
Active pepsin cleaves proteins into oligopeptides
What is the preferential site of cleavage for pepsin?
Between hydrophobic and aromatic amino acids
What is duodenal-gastric feedback?
Contents move from stomach to duodenum, causing distention and hypertonicity that stimulates secretion of enterogastrones (decrease gastric emptying) and activates neural receptors (also inhibit gastric emptying)
How is pH in the small intestines maintained?
Open endocrine cells sense the change of pH of duodenal chyme; the more acidic the pH is from 7.4, the more secretin is produced by S cells, which will stimulate the production of bicarbonate from the pancreatic duct, bile duct, and duodenal mucosa, as well as inhibiting H+ secretion by gastric parietal cells
When, where, and through what receptors stimulate acid secretion?
CCK binds to the parietal cell in the gastric phase to stimulate acid production and binds to CCKb in the stomach to stimulate a small amount of acid secretion
What is the duodenal cluster?
Stomach, duodenum, liver, common bile duct, gallbladder, and pancreas
What structures does secretin act on yield secretion of bicarbonate into the duodenum?
Brunner’s glands and ductular cells
What cell produce cholecystokinin?
I cells of the duodenum
What are the stimuli and series of events leading up to the production of CCK?
Free fatty acids (minorly amino acids) bind to enterocytes and trigger the release of CCK-releasing peptide which binds to I cell to generate CCK
To what receptor does CCK bind in the stomach? In the small intestines? Which has a high affinity for gastrin?
CCKb in stomach (high affinity for gastrin); CCKa in intestines
What is the function of CCK in the gallbladder?
Contraction and release of bile
What is the function of CCK in the pancreas?
Acinar secretion of enzymes
What is the function of CCK in the stomach?
Reduces the rate of emptying and produces stomach acid
What is the function of CCK on the sphincter of Oddi?
Relaxation
