Gut secretions and reflexes Flashcards
1
Q
Describe the features and contents of saliva and describe its function
A
- Approximately 1.5 L secreted/day (pH 7 (basal) - pH 8 (stimulated i.e. when eating))
- Contains two digestive enzymes: lingual lipase (glands on tongue ) and salivary amylase (Salivary glands)
- Also contains mucins to lubricate food and facilitate swallowing, lysozyme which breaks down bacterial cell wall, lactoferrin binds iron and is bacteriostatic, and proline-rich proteins - protect teeth and bind tannins
- Acts as a solvent and lubricant, facilitating taste and speech respectively
2
Q
List the salivary glands, their relative contribution and type of secretion
A
- Parotid glands (25% watery secretion)
- Submandibular glands (70% mixed secretion: mucous and water)
- Sublingual glands (5% viscous secretion)
3
Q
Describe the salivon
A
- salivon unit that secretes
- acinus secreted into
- divided by intercalated duct
- straited duct carries out into mouth - can change content of saliva along length eg more water, K HCO3, less Na Cl
- serous cell: watery secretion - electrolytes and salivary amylase
- mucuous cell: mucin
4
Q
Describe the key transports of the acinus duct
A
- key transporters include - in duct not acinus:
- H/K ATP-ase - K out, H in - regulated by iCa and M3 (Ach)
- CL/HCO3 exchanger - REABSORB cL - reabsorbed via Cl channels, HCO3 secretion
- NA- h atpase -> Na/K ATPase (reabsorb Na)
- note lumen is impermeable to water
5
Q
Describe the electrolyte composition of saliva and how this changes
A
- osmolarity increases with rate of secretion
- Na, HCO CL up
- k GOES DOWN
- not enough time for Na Cl to be absorbed
- HCO3 going up to bring pH up ?
6
Q
Briefly describe oesophageal secretion and its function
A
- Mucous only, provides lubrication for passage of bolus down esophagus
7
Q
Describe stomach secretions and the key cell types involved
A
- Stomach glands secrete 2 L/day of isotonic fluid
- Cell types include parietal cells, chief cells, mucous neck cells, and endocrine cells
- parietal cells chief cells; mucous neck and endocrine cells (in small quantities): are found in body mucosa pits i.e. gastric gland
- parietals release protons and intrinsic factors
- chiefs release pepsinogens
- chief and endocrine cells, no parietal cells in antral portion
- endocrine cells include G cells and D cells that produce gastrin and somatostatin respectively - which are endocrine and paracrine regulators of proton secretion
- parietal cells chief cells; mucous neck and endocrine cells (in small quantities): are found in body mucosa pits i.e. gastric gland
8
Q
Describe the parietal cell
A
- found in resting and active conditions
- tubulovesicule network in resting
- enables canaliculus formation in active
- increase surface area of apical side by 50-100%
- also abundant mitochondria: secretion is expensive process
9
Q
Describe intrinsic factor
A
- A 49 kDa glycoprotein crucial for the absorption of vitamin B12 in the small intestine
- ie binds cyanocobalamin
- Deficiency leads to pernicious anemia and requires injections of cyanocobalamin
- note that symptoms may take years to develop
10
Q
Describe acid secretion
A
- Mediated by HK ATPase: P-TYPE ATPases (H swapped for K, from splitting water, so OH and Co2 makes HCO3 swapped out for Cl; Cl goes into lumen along gradient; K along gradient in or out)
- juice: Cl and H increases with secretory rate; K (related to pump) and Na goes down
11
Q
Describe acid secretion regulation
A
- Regulated by three secretagogues: acetylcholine, gastrin, and histamine
- Ach and gastrin act directly and indirectly
- Histamine acts directly
- Ach binds M3- induces histamine release from ECL cells to promote H, and Ach binding M3 directly induces H from parietal cells
- Gastrin promotes Histamine release from ECL cells and H directly from parietal cells
- note omeprazole directly binds HK ATPase
- antihistamines can bind H2 to reduce acid secretion
- Somatostatin acts as a major inhibitory mechanism
- made by D cells in corpus and antrum
- 2 forms, SS-14 and 28
- Direct pathway: binds SS receptors on parietal cells to inhibit the stimulatory effect of histamine; acts endocrine via D CELLS OF ANTRUM OR paracrine via D cells in corpus
- Indirect pathway: Paracrine; acts in corpus to reduce histamine release from ECL cells, and in antrum reduces gastrin release from G cells
12
Q
List and describe the phases of acid secretion
A
- Acid secretion follows basal state and three phases during a meal: cephalic, gastric, and intestinal
- Basal state: Inter-digestive phase, H+ secretion takes place 24/7
- follows circadian rhythm: lowest in morning highest in evening
- H+ secretion is associated with parietal cell number
- high varaibility in basal state pH (3-7)
- Cephalic phase: Activated by sight, smell, taste, thought, and swallowing of food. Vagus plays main role. 30% of acid secretion
- starts happening before food reches stomach
- four distinct phases
- Direct stimulation of parietal cells(acetylcholine mediated)
- Release of histamine from ECL cells(acetylcholine mediated)
- Release of gastrin from G-cells(gastrin-releasing peptide mediated)
- Decrease in somatostatin release from D-cells (acetylcholine mediated)
- Gastric phase: Stimulated by distention of gastric wall and partially digested proteins. Gastrin release in main role. 50-60% of acid secretion
- Entry of food in stomach causes two stimuli for H+ secretion
- Distention of gastric wall (corpus and antrum)
- Activates Vagovagal reflex(same response as that seen in cephalic phase)
- Activates local ENS reflex (Ach mediated activation of parietal cells)
- Partially digested proteins (peptones) stimulate G-cells in antrum to release more gastrin
- Distention of gastric wall (corpus and antrum)
- Entry of food in stomach causes two stimuli for H+ secretion
- Intestinal phase: Stimulated by peptones and amino acids in proximal small intestine. 5-10% of acid secretion
- Peptones stimulate G-cells in duodenum - gastrin release
- An ‘entero-oxyntin’ stimulates parietal cells
- Absorbed amino acids stimulate acid secretion by an unknown mechanism
-
13
Q
List the intestinal factors that reduce proton secretion
A
Intestinal chyme during gastric phase inhibits gastric acid secretion- Reverse Enterogastric Reflex
- Purpose is to slow down release of chyme from stomach
1. Transmitted through ENS and Vagus nerve
- Initiated by: stretch, pH, peptones
2. Mediated by intestinal hormones
- Initiated by: pH, fat, peptones & osmolarity
- Mainly Secretin but also GIP, VIP, and SS
14
Q
Describe the process of pepsinogen secretion
A
- Multiple pepsinogens are secreted by Chief cells
- Initiate protein digestion as aspartic proteases
- Released as zymogens by ‘compound exocytosis’ (i.e. vesicles fuse and then exocytose)
- secretin and other compounds act via cAMP
- Ach, gastrin/CCK act via increased intracellular Ca concentration
15
Q
Describe pepsin activation
A
- Small N-terminal fragment needs to be cleaved to become active
- pH < 5 - note at lower pepsin catalyses reaction to make more pepsin
- Optimal activity pH: 1.8 - 3.5
- pH > 7.2 irreversibly inactivates the enzyme - a protective feature to ensure SI cells are protected from degradation
