Salt and water transport and its control Flashcards
Transcellular
The movement of ions/ water through the cell between each membrane.
Transport establishes concentration gradients and can work against electrochemical gradients.
Paracellular
The movement of ions/water between cells in the interstitial space.
This transport it regulated by tight junctions and dictated mainly by an electrochemical gradient.
Tight junctions
Structure between cells in the intestinal epithelium.
Regulates the permeability of the intestines to water and solutes.
Primary active transport
ATP driven transcellular transport that moves ions against an electrochemical gradient.
Example: Na-ATPase
Secondary active transport
ATP driven transcelluar transport.
Co-transports ions.
Example:
Na-GLUC transporter
Na/K+ ATPase
Basolateral antiporter on enterocytes that drives Na+ absorption.
Na+ transport is against concentration gradient as Na+ is already low inside cell
Na+ coupled transport
Apical symporter on enterocytes.
Na/K-ATPase created an electrochemical gradient for Na+ to move into the cell.
Na+ brings glucose and amino acids as it moves into cell.
This causes the lumen to become more negative- driving Cl- absorption into the cell.
Na+/ H+ exchanger
Present on the luminar membrane of the enterocyte.
Drives Na+ absorption and H+ secretion.
Na/K-ATPase establishes gradient for this exchanger
HCO3-/ Cl- exchange
Present on the luminar membrane of the enterocyte.
Cl- movement into the cell is driven by increasing negativity of the lumen- caused by Na+ absorption.
Na/K/2Cl symporter
Present on the basolateral membrane of the enterocyte.
Drives electrochemical gradient for transport of Cl- into the lumen via the CFTR chloride channel.
CFTR chloride channel
Channel present on the luminar membrane of the enterocyte.
Allows flow of Cl- out of the cell into the lumen.
This channel is expressed most in the colon and ileum.
Water absorption
Na+ is aborbed into the cell via: Na coupled transport Na/H+ exchanger. - Then into the blood via Na/K-ATPase
Cl- is absorbed into the cell via Cl-/HCO3- exchanger.
This drives the paracellular transport of water into the blood- via tight junctions
Secretion of water
Cl- is secreted into the lumen via CFTR.
This drags and Na+ and H2O paracellularly across tight junctions into the lumen.
Jejunum ion transport
Contains Na+, Cl-, K+, HCO3- and water absorption.
- Highest Na+ absorption (coupled)
- Solvent drag the most important mechanism of transport.
Apical:
- Na coupled with glucose, a.a and fructose
- Na/H exchange
Basolateral:
- Glucose, fructose and a,a channels.
- K+ channels
- Na/K- ATPase
- Bicarbonate channel
Solvent drag
Water moves to obtain osmotic equilibrium.
This causes solvents to be pulled with water transport.
Occurs where right junctions are the leakiest (upper small intestines)
Ileum ions transport
Site has the highest absorption of NaCl
Mechanism are similar to jejunum transport.
Apical:
- Na coupled
- Na/H
- HCO3/Cl
Basolateral:
- Glucose, a.a and fructose channel.
- K channel
- Na/K
- Cl channel
Colon ion transport
Does not contain tight junctions for H2O absorption.
Apical:
- Na+ channels
- Na/H
- Cl/HCO3
Basolateral:
- K channel
- Na/K-ATPase
Aldosterone
Mineralocorticoid that increases the synthesis of Na+ channels.
Stimulates more K+ secretion into lumen.
Can cause hypokalemia.
Oral rehydration therapy
Solution of glucose, sodium (salt) and amino acids.
The facilitates absorption of Na+ into the cell which also drives Cl- absorption.
Cholera toxin on intestinal juice secretion
Toxin is secreted by Vibrio cholerae.
- stimulates excess production of cAMP in crypt cells.
This drives Cl- out of the cell as CFTR is regulated by cAMP.
This draws out Na+ and H2O paracellularly- can lead to dehydration through secretory diarrhea.
Treatment- oral rehydration therapy.
Lactose intolerance
Lactase deficiency means that lactose is not broken down to release glucose.
Lactose drives water out paracellularly and causes osmotic diarrhea.
Bacterial enterotoxins that cause secretory diarrhea
Cholera
Heat-liable E.coli
Salmonella
Campilobacter
Heat stable E.coli
Yersinia
4 pathophysiological changes of transepithelial transport
- Stimulates of net fluid and electrolyte secretion.
- Mucosal destruction—-> Increased permeability
- Increased propulsive muscle contractions.
- Nutrient malabsorption
Colon water and electrolyte absorption
Na+ enters cell on apical membrane:
- Na+ channels
- Na/H antiport
Na/K-ATPase drives Na+ entry into cells
K+ leaves into lumen via channels and is also absorbed in the blood.
Cl-/HCO3- exchange release bicarbonate to neutralise acid and removes Cl- from lumen.
NaCl- into cells creates osmotic gradient for transcellular water movement.
Ion transport in Jejunum
- Na+
- K+
- Cl-
- HCO3-
Na- actively absorbed [ Na+/H+- ATPase]
- especially coupled transport with glucose, amino ands
K- passively absorbed through K+ channels when water is absorbed .
Cl- absorbed
HCO3- absorbed, so it can be secreted in ileum and colon.
Ion transport in Ileum
- Na+
- K+
- Cl-
- HCO3-
Na- actively absorbed using sodium pump and coupled transport.
K- passively absorbed through channels
Cl- absorbed, some using exchange with HCO3-
HCO3- Secreted, some in exchange with Cl-
Ion transport in colon
- Na+
- K+
- Cl-
- HCO3-
Na- Actively absorbed
K- secreted when concentration is <25 mM
Cl- absorbed, some in exchange for Cl-
HCO3- secreted, some in exchange for Cl-
