SM_130b-131b: Salivary Gland & Stomach Secretion, Stomach & Exocrine Pancreas Secretion Flashcards
Saliva contains ____, ____, and ____
Saliva contains water, electrolytes, proteins
- Composition of saliva depends on secretory rate but always hypotonic with respect to plasma
- Major electrolytes re Na+, K+, Ca2+, Cl-, HCO-, HPO3-
- Major proteins are amylase and glycoproteins
Saliva is ____ and protects against ____
Saliva is alkaline and protects against destructive effects of bacterial acid
Due to buffering of bicarbonate
(also has antimicrobial and immunological functions)
Saliva ___ and ___ the oral cavity
Saliva lubricates and washes the oral cavity
Saliva contains ____ which breaks the ____
Saliva contains alpha-amylase which breaks the alpha-1,4-glycosidic bonds in starch
(optimum pH is 6.9, produces maltose / maltotriose / dextrins)
For salivary secretion, sympathetic preganglionic fibers originate in _____ and are _____
For salivary secretion, sympathetic preganglionic fibers originate in T1-T4 and are cholinergic
For salivary secretion, postganglionic sympathetic fibers originate in _____ and follow _____ to innervate _____, _____, _____, and _____
For salivary secretion, postganglionic sympathetic fibers originate in superior cervical ganglion and follow blood vessels to innervate acinar, duct, and myoepithelial cells and blood vessels
Sympathetic stimulation for salivary secretion causes ____ and ____
Sympathetic stimulation for salivary secretion causes vasoconstriction and secretion of a small volume of thick viscous secretion (protein)
Parasympathetic innervation for salivary secretion produces ____ and ____
Parasympathetic innervation for salivary secretion produces vasodilation in glands and a large increase in serous secretion
Salivary secretion is entirely dependent on ____
Salivary secretion is entirely dependent on nerves
(no baseline secretion)
Acinar cells are ____
Acinar cells are electrotonically coupled
(stimulaiton spreads from cell to cell)
Parasympathetic stimulation for salivation causes ____
Parasympathetic stimulation for salivation causes increase in serous and mucous cell secretion, myoepithelial contraction, and vasodilation
- Myoepithelial contraction and secretion are due to muscarinic stimulation - blocked by atropine
- Vasodilation is deu to VIP co-localized with ACh in parasympathetic postganglionic nerves
Sympathetic stimulation for salivation leads to ____ via ____ and ____ by ____
Sympathetic stimulation for salivation leads to vasoconstriction via alpha receptors and stimulation of acinar secretion by beta receptors
- Mediated by different groups of sympathetic neurons activated by different CNS pathways
- Myoepithelial cells are stimulated by alpha adrenergic and muscarinic receptors
Phases of gastric activity are ____, ____, ____, and ____
Phases of gastric activity are cephalic phase, gastric phase, intestinal, and interdigestive phase
Describe mechanisms of secretion
Mechanisms of secretion
- In acinar cells, more Na+ actively transported out than K+ in by Na+/K+ ATPase on basal surface
- Electrochemical gradient
- Cl- enters cell using coupled transporters
- Building of Cl- in cell creates a gradient for it to leave via apical Cl- channels
- Secretion of Cl- creates an electrical gradient for Na+ to enter the lumen between the cells
- Na+/K+ ATPase drives secretion of NaCl
- Water secreted by osmotic forces
- In ducts, more a+ actively transported out than K+ in by Na+/K+ ATPase
- Fluid becomes hypotonic because duct cells are relatively impermeable to water
- HCO3- actively transported into acini and ducts
- Cl- restributes via a Cl-/HCO3- exchange
Alpha amylase is stored in ____ and is released via ____ upon stimulation
Alpha amylase is stored in zymogen granules and is released via exocytosis upon stimulation
Describe the functions of HCl in the stomach
Functions of HCl in the stomach
- Actives pepsinogens to active pepsins
- Creates an acid environment favorable for enzyme actions
- Acts to denature proteins and make them more accessible to proteolytic attack
- Solubilizes Ca2+ and Fe3+ so that they are more readily absorbed from the small intestine
- Destroys some bacteria swallowed with food and thus prevents their entry into the small intestine
Composition of gastric juice varies with ____
Composition of gastric juice varies with rate of secretion
- Water and electrolytes secreted by parietal and non-parietal cells
- Organic substances: mucus, intrinsic factor, and enzymes (chiefly pepsins)
Gastric juice pH depends on _____
Gastric juice pH depends on parietal cell secretion
- pH from 0.8-7.67
- Parietal [H+] secretion is constant
- Acid output is dependent on the number of parietal cells secreting
- Gastric juice is isotonic or slightly hypotonic
Describe mechanism of H+ secretion by parietal cells
Mechanism of H+ secretion by parietal cells
- H+ produced by hydrolysis of water through cytochrome using glucose and O2
- H+ actively pumped out of cell into lumen in exchange for K+ via H+/K+ ATPase
- Cl- leaves the cell via a Cl- channel (electrochemical gradient maintained by basolateral Na+/K+ ATPase)
- K+ high in cell (due to Na+/K+ ATPase) and diffuses into lumen following the Cl-
- OH- produced by hydrolysis combines with CO2 from metabolism and plasma to form HCO3-
- HCO3- diffuses out of the cell
- Alkaline gastric venous blood during acid secretion (alkaline tide)
Gastric juice is almost isotonic because ____ while ____
Gastric juice is almost isotonic because mucosa is fairly impermeable to Na+ while H2O diffuses passively
Non-parietal cells are ____
Non-parietal cells are surface epithelial cells that secrete a slightly alkaline isotonic fluid at a constant composition and volume
Bicarbonate actively transported
Intrinsic factor is ____ and ____
Intrinsic factor is a glycoprotein secreted by parietal cells that binds to B12 and transports it to the ileum
Binds to ileal mucosa, facilitating absorption of B12
____ results in the absence of intrinsic factor
Pernicious anemia results in the absence of intrinsic factor
(intrinsic factor required for B12 absorption)
Pepsinogens are stored as inactive proenzymes in zymogen granules in ____ and are converted to ____ in the lumen at a pH < 5
Pepsinogens are stored as inactive proenzymes in zymogen granules in chief cells and are converted to pepsins in the lumen at a pH < 5
Pepsins are ____
Pepsins are proteolytic
- Digest intermediary proteins, polypeptides, and amino acids
- Rarely complete
- Increased by ACh and secretin directly and gastrin and histamine indirectly
HCl secretion stimulates ____, ____, and ____ secretion
HCl secretion stimulates pepsinogen, mucus, and intrinsic factor secretion
- HCl secretion stimulated by histamine, ACh, and gastrin
In the interdigestive period, gastric juice is ____ in acid and pepsin primarily consists of ____ cell secretion
In the interdigestive period, gastric juice is low in acid and pepsin primarily consists of non-parietal cell secretion
- Follows a circadian pattern and influenced by emotional factors
- Average basal acid output is 1-5 mM/hr in normal individuals and is decreased by vagotomy or atropine and abolished by vagotomy and antrectomy due to removal of gastrin-secreting cells
In the cephalic phase, gastric secretory responses are excited by the ____
In the cephalic phase, gastric secretory responses are excited by the vagus nerve
- Vagal activity excites parietal cells via muscarinic receptors
- Vagal activity excites G cells by GRP
- Secretory activity depends on type, strength, and duration of stimuli
In the gastric phase, ____, ____, and ____ is increased by ____ and ____
In the gastric phase, gastric juice volume, acid, and pepsinogen secretion are increased by distention and chemical stimulation of mucus by chyme
In the gastric phase, distention stimulates ____ reflexes and ____ reflexes
In the gastric phase, distention stimulates vago-vagal reflexes and intrinsic reflexes
- Vagal reflexes cause cholinergic activation of parietal cells
- Intrinsic reflexes activate parietal cells by ACh and stimulate release of gastrin
- Chemical stimulation of mucosa by chyme
- Protein digests stimulate gastrin release
- Ca2+ stimulates gastrin release and acid secretion
In the intestinal phase, gastric secretion is inhibited by ____ and ____
In the intestinal phase, gastric secretion is inhibited by acid and digests of fats / proteins / carbohydrates in small intestine
- Acid delivered to duodenum inhibits gastric acid secretion: acid stimulates release of secretin -> inhibits gastric secretion, acid stimulates local inhibitory reflex (enterogastric reflex)
GIP reduces ____
GIP reduces gastrin stimulated secretion
- Also reduces histamine stimulated secretion
- Fatty acids, monoglycerides, and simple sugars cause release of GIP
*
Fatty acids and amino acids cause release of ____, binding to ____ and blocking binding of ____
Fatty acids and amino acids cause release of cholecystokinin, binding to gastrin receptor and blocking binding of gastrin
Describe protective mechanisms preventing the stomach from digesting itself
Protective mechanisms preventing the stomach from digesting itself
- HCl secretion is very low in interdigestive period - food has considerable buffering capacity when HCl secreted in digestive periods
- Gastric mucosal barrier: tight junctions, cells impermeablke to HCl
- High blood flow to stomach submucosa: any HCl which penetrates mucosa is quickly washed away
- Insoluble mucus secreted by surface epithelial cells forms a layer covering the stomach surface: acid diffuses poorly through this layer
- Insoluble mucus traps unstirred layer of alkaline fluid secreted by surface epithelium: buffers HCl
- Pepsin secreted in inactive pepsinogen form: cells forming gastric mucosa replaced rapidly
- Surface mucous secretion forms a protective fluid layer
Pancreatic juice is ____ which is important to neutralize ____
Pancreatic juice is alkaline which is important to neutralize gastric acid
Pancreatic juice composition varies with ____
Pancreatic juice composition varies with rate of secretion
- Anions HCO3- and Cl- vary reciprocally
- Cations are constant
- Pancreatic juice contains Na+ and K+ iso-osmotic with plasma
- At low secretion rates, Cl- and HCO3- are roughly iso-osmotic with plasma
- At high secretion rates, HCO3- secretion is high and [Cl-] is low
Describe composition of pancreatic juice
Composition of pancreatic juice
- Na+ and K+
- HCO3- and Cl-
- Amylase: splits 1,4-glycosidic bonds in carbohydrates
- Lipolytic enzymes: pancreatic lipase, phospholipase A2, cholesterol esterase
- Proteolytic enzymes secreted as inactive proenzymes: trypsinogen and several chymotrypsinogens
All attack interior of peptide chains
Descibe mechanisms of fluid and electrolyte secretion
Mechanisms of fluid and electrolyte secretion
- Primary driving force is Na+/K+ ATPase on the basal surface of duct of centroacinar cells: provides electrochemical gradient for Na+ to enter cell as H+ is pumped out
- H+ derived from H2CO3 produced by CA from H2O and CO2
- Na+ diffuses passively into lumen and K+ does too down its electrochemical gradient
- HCO3- enters lumen by a HCO3-/Cl- exchange
- Cl- enters lumen through channels
- H2O follows electrolytes passively (relatively leaky epithelium)
Explain the inverse relationship between Cl- and HCO3- in fluid/electrolye secretion
Inverse relationship between Cl- and HCO3- in fluid/electrolye secretion
- At low secretion rates, pancreatic fluid is mainly composed of a fluid iso-osmotic with plasma secreted by a cell type
- During high secretion, fluid mainly represents duct and centroacinar cell secretion that is high in HCO3-
- Cl- distributes passively
Describe mechanisms of enzyme secretion (acinar cells)
Mechanisms of enzyme secretion (acinar cells)
- Proteins synthesized in RER
- Stored in zymogen granules after passing through Golgi smooth vesicles and condensing vacuoles
- Secreted by fusion of zymogen membrane with cell membrane
- Enzymes can be maintained by diet
Basal electrolye and enzyme secretion from pancreas is ___
Basal electrolye and enzyme secretion from pancreas is low
Describe cephalic phase secretion from pancreas
Cephalic phase secretion from pancreas
- Stimuli from food activates vagal innervation of pancreas
- Vagal postganglionic fibers release ACh
- ACh excites acinar cells to secrete enzymes and duct and centroacinar cells to secrete fluid and electrolytes
- ACh also sensitizes pancreatic response to secretin and CCK
Describe gastric phase secretion from pancreas
Gastric phase secretion from pancreas
- Vagally mediated due to distention of stomach
- Vagus activates pancreas as in cephalic phase
- Some stimulation by gastrin from stomach
70-80% of pancreatic secretion occurs in the ____ phase
70-80% of pancreatic secretion occurs in the intestinal phase
Due to presence of acid and digestion products in intestine
Describe intestinal phase of pancreatic secretion
Intestinal phase of pancreatic secretion
- Gastric acid stimulates S cells in duodenum and jejunum to release secretin -> secretin stimulates duct and centroacinar cells to secrete HCO- and fluid
- Fatty acids and protein digests stimulate I cells in duodenum to release CCK -> stimulates acinar cells to secrete enzymes
- Acid and fatty acids -> evoke vagovagal reflexes to stimulate pancreas
Acid load, not concentration is the important variable
Salivary composition varies with ____
Salivary composition varies with secretory rate
Describe regulation of gastric acid secretion
Regulation of gastric acid secretion
- Direct pathway: ACh, gastrin, and histamine stimulate parietal cells -> secretion of H+ into the lumen
- Indirect pathway: ACh and gastrin stimulate ECL cell -> secretion of histamine -> acts on parietal cell
ACh, histamine, and gastrin are ____
ACh, histamine, and gastrin are synergistic
Summarize gastric secretion
Gastric secretion
Summarize the role of CCK
Role of CCK
Summarize endocrine and paracrine transmitters from acinar and duct cells
Endocrine and paracrine transmitters from acinar and duct cells
