Ion Transport and Water Absorption Flashcards
Where in the intestine is the net rate of of Na absorption highest after eating
jejunum
Which Na transporter in the intestine is used after eating?
Na/Glucose (SGLT) or Na/amino acid transporter
Na passes down its electrochemical gradient into the enterocyte which provides energy for it to take a molecule of glucose/aa with it
Na gradient is set up by Na/K ATPase on basal and lateral cell membranes
Which Na transporter in the intestine is used when intestinal lumen pH is high?
Na-H exchanger
mostly in the jejunum, some in the duodenum
Which Na transporter in the intestine is used between meal in the fasted state?
Parallel N/H exchanger and Cl/HCO3 exchanger (electroneutral NaCl absorption)
Na in/H out
Cl in/HCO3 out
- electroneutral transport
- increase in cAMP, cGMP, and intracellular Ca reduces NaCl absorption
- Occurs in ilium and colon
- HCO3 in cell generated by carbonic anhydrase
Which Na transporter is in the distal colon?
Epithelial Na channels (ENaC)
capable of absorbing Na against a large gradient
effected by aldosterone
The 3 ways Cl is absorbed
- Passive absorption via paracellular or transcellular route
- Parallel Na/H and Cl/HCO3 exchangers
3, Cl/HCO3 exchanger in isolation in the ileum and colon
Aldosterone and angiotensin are absorptagogues (promote absorption) of Na - how do they achieve this effect?
Angiotensin in the SI increases NaCl absorption by uprgulating the Na-H exchanger
Aldosterone in the colon stimulates Na absorption through increasing opening, synthesis, and insertion into membrane of ENaC
How do secretogogues (molecules promoting secretion) cause diarrhoea?
- Secondary messenger system makes Ca or cAMP increase
- Inhibits electroneutral transport of NaCl and Increase Cl secretion into lumen
- Water moves into lumen
The 4 types of secretagogues which cause diarrhoea, with examples, are…
- Bacterial enterotoxins
- cholera toxin, C. diff toxin - Hormones and neurotransmitters
- vasoactive intestinal peptide (VIP) in Verner-Morrison syndrome, Ach - Immune mediated
- histamine and prostaglandins (IBD) - Laxatives
How can you treat secretory diarrohea as caused by cholera toxin?
oral rehydration solution contraining both Na and glucose - the sodium/glucose transporters still work
such as dioralyte or homemade solution of salt, sugar and water
Examples of anti-diarrhoeal drugs
loperamide
somatostatin analogue
How do bulk laxatives work?
They act as dietary fibre which increases osmotic load of faeces so it retains more water. Stool bulk causes lumen to swell > enteric NS stimulates peristalsis, accelerating exit
How do osmotic laxatives work?
Create a hyperosmotic gradient in lumen - water drawn from cellular space into gut
eg lactulose is converted to hyperosmotic particles in the lumen by bacteria
How do stimulant laxatives work?
enhance intestinal motility through enteric NS eg Senna
The net absorption of calcium per day in the gut is
175mg/day
500mg/day secreted and 325mg/day absorbed
